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Target mass number dependence of subthreshold antiproton production in proton-, deuteron- a

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a r X i v :n u c l -t h /0502071v 1 23 F eb 2005Target mass number dependence of subthreshold antiproton production in proton-,deuteron-and alpha-particle-induced reactionsH M¨u ller 1and V I Komarov 21Institut f¨u r Kern-und Hadronenphysik,Forschungszentrum Rossendorf,Postfach 510119,D-01314Dresden,Germany2Joint Institute for Nuclear Research,LNP,141980Dubna,Russia E-mail:H.Mueller@fz-rossendorf.deAbstract.Data from KEK on subthreshold ¯p as well as on π±and K ±production in proton-,deuteron-and α-induced reactions at energies between 2.0and 12.0A GeV for C,Cu and Pb targets are described within a unified approach.We use a model which considers a nuclear reaction as an incoherent sum over collisions of varying numbers of projectile and target nucleons.It samples complete events and thus allows for the simultaneous consideration of all final particles including the decay products of the nuclear residues.The enormous enhancement of the ¯p cross section,as well as the moderate increase of meson production in deuteron and αinduced compared to proton-induced reactions,is well reproduced for all target nuclei.In our approach,the observed enhancement near the production threshold is mainly due to the contributions from the interactions of few-nucleon clusters by simultaneously considering fragmentation processes of the nuclear residues.The ability of the model to reproduce the target mass dependence may be considered as a further proof of the validity of the cluster concept.Submitted to:J.Phys.G:Nucl.Phys.1.IntroductionIn two recent papers [1,2]we discussed data [3]on ¯p ,π±and K ±production in the reactions pC,dC and αC at energies between 3.5and 12.0A GeV within the Rossendorf collision (ROC)model.Special emphasis was on subthreshold production of antiprotons the enormous enhancement of which in dC and αC compared to pC reactions could be well reproduced.Here,we consider the target mass number dependence of this effect by confronting the KEK data [3]on hadron production in pCu,dCu,αCu and dPb reactions with ROC-model calculations and make predictions for the reactions pPb and αPb not measured so far.2.The modelThe ROC model considers a nuclear reaction within the spectator-participant picture as an incoherent sum over collisions of varying numbers of projectile and target nucleons.Partial cross sections for these interactions are derived from a probabilistic interpretation of the Glauber theory[4]in close analogy to the cooperative model [5–12].They depend strongly on the mass numbers via the wave functions of the nuclei.For light nuclei they drop more rapidly with increasing number of participants than in case of heavier nuclei.Particle production is proposed to proceed via intermediate states calledfireballs (FBs).This assumption yields a natural explanation of particle correlations observed at higher energies(see the discussion in[13]).Two parameters,temperature and radius,define number and relative kinetic energy of thefinal hadrons the FBs decay into.Resonances among thefinal hadrons decay later on into stable particles observed in the experiment.The nuclear residue is assumed to become excited during the reaction due to the distortion of the nuclear structure by the separation of the participants from the spectators and due to the passage of the reaction products through the spectator system.In this way,final-state interactions are taken into account without making special assumptions concerning re-absorption,re-scattering, self-energies,potentials etc for each particle type separately.Obviously,a comparison of model calculations with data for heavier nuclei should be much more sensitive to the treatment of such secondary effects.As thoroughly discussed in[1],the consideration of the fragmentation of the nuclear residues is decisive for reproducing subthreshold cross sections.The model describes fragmentation in analogy to the decay of the excited FBs also by a temperature and a radius parameter.Obviously,the main ingredients of the model (partial cross sections,FB excitation,final-state interactions and fragmentation)may influence the total mass number dependence of hadron production cross sections quite differently.Thus,the consideration of the mass number dependence is an important check to verify the model assumptions.The mathematical formulation of the above picture is given in[2].In the following we employ the same parameter set as used in[1,2]for the C target also for the heavier target nuclei except the maximum temperature of the nuclear residues(see section3).parison with experimental dataAs discussed in[1,15],one of the outstanding features of the ROC model is the unified treatment of particle production and fragmentation processes.The interplay of these two energy consuming processes is of special importance at subthreshold energies where the available energy is decisive for particle production cross sections. Therefore,we start our consideration of mass number dependence of(subthreshold) particle production byfixing the parameters of the nuclear residues from independent data[14,16–19]for multifragmentation in proton-nucleus interactions.For simplicity, the radius parameter R R=1.7fm and the value of¯a=0.5(equation(17)in[2]) defining the dependence of the temperature parameterΘR from the number of participants a[1−exp(−a/¯a A1/3)]ΘR(a,A)=ΘmaxRFigure1.Energy spectra[14]of the nuclear fragments(symbols)indicated in thefigure from p+Al(upper panel)and p+Ag(lower panel)reactions at4.9GeVcompared with ROC-model calculations(histograms)usingΘmaxR =10MeV forAl andΘmaxR=6.8MeV for Ag.are taken as constant.A reasonable overall reproduction of the data could be achievedwithΘmaxR taken as a function of the target mass number,which strongly decreases atlight and becomes constant at heavy target masses.The fragment spectra result from a rather complicated superposition of contributions with different values ofΘR(a,A),which are for small numbers of participants much smaller thanΘmaxR .The highervalues ofΘmaxR at low residue mass numbers do not necessarily imply a higher’effective’ΘR contributing to the spectra,because for light nuclei the partial cross sections drop more rapidly with increasing number of participants than in case of heavier nuclei. Anyway,decisive for the intended discussion of hadron production is the reproduction of existing data to get a realistic estimate of the energy consumed by fragmentation.As example,figure1demonstrates that the basic features of fragment spectraare reasonably reproduced.In[2],we usedΘmaxR =12MeV for light nuclei up to C,hereΘmaxR =10MeV is taken for Al andΘmaxR=6.8MeV for Ag(seefigure1).Formass numbers above100the temperature is assumed to be constant and we takeΘmaxR =6.8MeV also for Pb.Since in the region around Cu there are no data availablewe(nonlinearly‡)interpolate and getΘmaxR =7.5MeV for Cu.It should be stressedthat for the following considerations all other model parameters are the same as used in[1,2]for describing the experimental results of[3]for interactions with carbon.Infigure2,the momentum spectra ofπ±,K±and¯p from pCu,dCu andαCu reactions obtained at KEK[3]are displayed.This experiment was motivated by‡All values ofΘmaxR used are approximately described byΘmaxR/MeV=6.8[1+1.2exp(−0.036A)]with A being the target mass number.Figure 2.Momentum spectra ofπ±,K±and¯p[3](symbols)from pCu, dCu andαCu reactions compared with ROC-model calculations(histograms). Experimental and calculated results forπ+and K−mesons are multiplied by the factors indicated in the legend.The scale extends from2×10−8to 105mb sr−1GeV−1c for energies up to4.0A GeV,while the lower limit is2×10−6 for5.0A GeV and2×10−2mb sr−1GeV−1c for12.0A GeV.measurements of large cross sections of subthreshold¯p production in nucleus-nucleus reactions at LBL-BEVALAC[20,21]and at GSI[22].The authors[3]claimed that the use of light-ion beams for investigating subthreshold¯p production should be useful to verify theoretical models.Secondary effects like re-scattering,re-absorption,self-energies,potentials etc,which hide and influence features of the primary production process,should be smaller than in heavy-ion reactions.Thus,it is the outstanding feature of the data that in d-andα-induced reactions an enormous enhancement of ¯p production by nearly two and three orders of magnitude compared to p-induced interactions could be observed.This enhancement makes light-ion-induced reactions a candidate for the key to a deeper understanding of subthreshold¯p production in nucleus-nucleus reactions.The KEK group[3]interprets their¯p data by using the‘first-chance NN collision model’from[24]where the internal nucleon momenta were extracted from backward proton production[25]as a superposition of two Gaussian distributions. Momentum spectra and the incidence-energy dependence of p-induced reactions could be successfully reproduced by adapting one normalization parameter.However,the application of this model to d-induced reactions severely underestimates the¯p cross sections at subthreshold energies.Thus,according to[3],the effect cannot be explained by the internal motion of the nucleons in the deuteron.In contrast,in[26],¯p production in p+A and d+A reactions is analysed within a phase-space model incorporating the self-energies of the baryons.It is claimed that the internal momentum distribution of the deuteron provides a natural explanation of the large enhancement under discussion.In[2]we reproduced the observed increase of subthreshold¯p production in dC andαC reactions compared to pC interactions and demonstrated that within the ROC model the number of participating nucleons is the key quantity for understanding subthreshold particle production.In addition,the whole set of experimental data for all ejectiles measured was well described.This is of special importance since there is not only the enhancement of¯p production,but also the increase of the K−cross sections with increasing energy and mass number of the projectile around the elementary production threshold at2.6GeV.Also the completely different energy and projectile mass dependence of the pion production cross sections far above the threshold could be reproduced.Here,we repeat these considerations for the heavier target nuclei Cu and Pb and come to rather similar consequences.Infigure2,the results of the ROC model for pCu, dCu andαCu reactions are compared to the data[3].Again,the overall agreement is quite satisfactory in view of the different projectile types,the large region of incidence energies,the variety of ejectile species and the huge differences of many orders of magnitude in the considered cross section values.In case of the Pb target(seefigure 3)only data for d-induced reactions up to5A MeV and for p-induced reactions at 12GeV are available.Also for such a heavy target the reproduction of the data is satisfactory.For12GeV protons the ROC calculations tend to underestimate the K±and ¯p data(seefigures2and3).Infigure4a similar data set[23]is compared with ROC-model calculations.The spectra are measured at a slightly lower energy and at a similar angle but in a much wider momentum region compared to the KEK data.In this case the tendency of the ROC results goes in the opposite direction and overestimates the cross sections for the heavier ejectiles K±and¯p.Also for the higher ejectile momenta the description of the whole data set for targets betweenFigure3.The same asfigure2for Pb target.The scale extends from4×10−8 to5×105mb sr−1GeV−1c for energies up to4.0A GeV,while the lower limit is 4×10−6for5.0A GeV and4×10−2mb sr−1GeV−1c for12.0A GeV.Figure4.Momentum spectra ofπ±,K±and¯p[23](symbols)compared to ROC-model calculations(histograms).Table 1.Calculated cross sections and statistical errorsσp,σd andσαfor ¯p production in p-,d-andα-induced interactions,respectively,with Cu at3.5.A GeV for different temperature parametersΘmaxR7.0 5.2±0.20.57±0.03 1.7±0.17.5 2.7±0.10.39±0.05 1.5±0.28.0 1.9±0.20.27±0.02 1.2±0.2that at a given incidence energy per nucleon an increasing projectile mass diminishes the dependence of the subthreshold production cross section on the temperatureparameterΘmaxR of the residual nucleus,too.This can be explained by the largereffective number of participants in case of the heavier projectile,which leads to an increase of the energy available for particle production.A striking feature of the obtained results is the consistent quality of data reproduction for all types offinal particles independent of the target mass.With regard to the fact that the cross sections for the interaction of the variousfinal particles with nucleons differ considerably one should,especially for heavy target nuclei,expect distinct deviations of the calculated spectra from the measured ones, since individual properties of the various outgoing particles like the mentioned cross sections are not(yet)incorporated in the model.In the light of this result the question of the treatment offinal-state interactions is worth being discussed.It is the excitation of the residual nucleus which diminishes the energy available for particle production in the model calculations.This decreases the corresponding cross sections especially in the threshold region independent of the type of secondary particles.It seems that individual interactions of the outgoing particles with the target nucleons do not play a decisive role.A rigorous explanation would be the assumption of afinite life time of intermediate states called FBs in the ROC model.If such a life time would be comparable to the time the FBs need to leave the residual nucleus,then the individual properties of the various particles would wash-out.We are aware of the difficulty to articulate such a point of view regarding the fact that a major topic of present research activities consists in investigating medium modifications of individual particle properties.On the other hand,although the deviations between our calculations and the data are usually less than a factor of2or so,there remains room for not too large effects not yet included in the ROC model.As shown in[2]and supported by the present results,the main contribution for subthreshold particle production in conditions of the experiments[3]comes,according to the ROC model,from the excitation of FBs with baryon numbers from2to4.This means that physical properties of such FBs may be directly studied in collisions of proton,deuteron and helium-ion beams with the lightest nuclear targets,2H,3He and4He.In this case such complicating effects like varying numbers of participating nucleons,loss of energy for the excitation of the residual nucleus,final-state absorption and rescattering in the residue,are significantly diminished or removed at all.The model reveals a complete analogy of the interaction of few-nucleon groups to the interaction of single nucleons in the subthreshold production processes.This may be treated as an indication that properties of few-nucleon groups in conditions of high energy-momentum transfer are governed by quark-gluon degrees of freedom.That makes experiments on subthreshold production at light target nuclei a promising tool for the study of the transition from hadronic to quark-gluon properties.4.SummarySubthreshold particle production is a collective phenomenon which is far from being completely understood.From the viewpoint of the ROC model,data on subthreshold particle production can be reproduced by considering the interaction of few-nucleon groups in complete analogy to the interaction of single nucleons,also with regard to high-momentum transfer processes.It has been demonstrated here that in a wide range of target mass numbers the cluster concept yields a quite natural explanationof the enhancement of subthreshold particle production due to the energy gain in the interaction of few-nucleon groups compared to NN interactions.Especially at energies below or near the NN threshold the consideration of the interplay between hadron production and fragmentation processes is important for the reproduction of the cross sections.This concept should be applicable not only in proton-or light-ion-induced reactions,but also for heavy-ion interactions,although in the latter case the number of partial processes increases tremendously.In this sense,the ROC model can be considered as a promising approach to a unified description of particle production processes in a large variety of different types of nuclear reactions. AcknowledgmentsOne of the authors(H.M.)would like to thank W.Enghardt for the promotion of this study.References[1]Komarov V I,M¨u ller H and Sibirtsev A2004J.Phys.G.30921./nucl-th/0312087[2]M¨u ller H and Komarov V I2004J.Phys.G.301379./nucl-th/0402009[3]Sugaya Y et al.1998Nucl.Phys.A634115[4]Glauber R J and Mathiae J1970Nucl.Phys.B21135[5]Knoll J1979Phys.Rev.C20773[6]Knoll J1980Nucl.Phys.A343511[7]Bohrmann S and Knoll J1981Nucl.Phys.A356498[8]Shyam R and Knoll J1984Nucl.Phys.A426606[9]Shyam R and Knoll J1986Nucl.Phys.A448322[10]Knoll J and Shyam R1988Nucl.Phys.A483711[11]Ghosh B and Shyam R1990Phys.Lett.B234248[12]Ghosh B1992Phys.Rev.C45R518[13]M¨u ller H2001Eur.Phys.J.C18563./hep-ph/0011350[14]Westfall G et al.1978Phys.Rev.C171368[15]M¨u ller H and Sistemich K1992Z.Phys.A344197[16]KatkoffS,Fickel H R and Wyttenbach A1968Phys.Rev.1661147[17]English G and Porile N1974Phys.Rev.C102268[18]Kaufman S,Weisfield M,Steinberg E,Wilkins B and Henderson D1976Phys.Rev.C141121[19]Kaufman S and Steinberg E1980Phys.Rev.C22167[20]Caroll J et al.1989Phys.Rev.Lett.621829[21]Shor A et al.1989Phys.Rev.Lett.632192[22]Schr¨o ter A et al.1993Nucl.Phys.A553775c[23]Vorontsov I A,Safronov G A,Sibirtsev A A,Smirnov G N and Trebukhovsky Y V1988ITEP-11-1988[24]Shor A,Perez-Mendez V and Ganezer K1990Nucl.Phys.A514717[25]Geaga J V et al.1980Phys.Rev.Lett.451993[26]Cassing W,Lykasov G and Teis S1994Z.Phys.A348247。

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全国翻译专业资格(水平)考试：2021三级笔译真题模拟及答案(5)

全国翻译专业资格(水平)考试：2021三级笔译真题模拟及答案(5)

全国翻译专业资格（水平）考试：2021三级笔译真题模拟及答案(5)共640道题1、______（单选题）A. inB. forC. aboutD. within试题答案：B2、It began to rain so they called off the match and didn't announce when it would be held.（单选题）A. counted onB. estimatedC. explained aboutD. canceled试题答案：D3、The reason why turtles are imported into China lies in that _________.（单选题）A. turtles are famous only in ChinaB. people in other parts of the world don’t have the habit of eating turtlesC. the demand is greater than supply in ChinaD. these countries are rich in wild turtles试题答案：C4、He told the doctor that he’d not rather having the operation unless it was absolutely necessary.（单选题）A. rather not haveB. rather not to haveC. not rather haveD. rather not having试题答案：A5、Which of the following is NOT mentioned in the last paragraph?（单选题）A. Biologists began to study new species of turtles for Chinese food markets.B. Biologists discovered a few rare species of turtles as a result of search previouslyC. Some species of turtles will soon be extinct if they are caught and killed at the present speed.D. Turtles will soon be a target for rare collectors as dear as pearl and diamond.试题答案：D6、______（单选题）A. factorsB. ingredientsC. composersD. facts试题答案：B7、People had been conscious of the problem before, but the new book made them aware of its importance.（单选题）A. altitudeB. multitudeC. magnitudeD. aptitude试题答案：C8、The word “sturdy” underlined in Paragraph 1 means______.（单选题）A. robustB. ornateC. softD. hard试题答案：D9、______（单选题）A. bestB. justiceC. fairD. worst试题答案：D10、The mass production of goods from the Industrial Revolution in the 1800’s made person-to-person selling less efficient than mass distribution.（单选题）A. influentialB. effectiveC. sensibleD. frequent试题答案：B11、______（单选题）A. forB. withC. withoutD. except试题答案：A12、Variations in the color of sea water from blue to green seem to be caused by high or low concentrations of salt.（单选题）A. ChangesB. DescriptionsC. MeasuresD. Clarity试题答案：A13、Mary happened to meet her best friend in the middle school while she was in Paris on business.（单选题）A. ran overB. ran afterC. ran intoD. ran down试题答案：C14、______（单选题）A. In other wordsB. To sum upC. On the contraryD. Generally speaking试题答案：A15、______（单选题）A. residenceB. inhabitanceC. occupantD. people试题答案：C16、______（单选题）A. gotB. alternatedC. alteredD. taken试题答案：C17、All mammals have hair, but not always evident.（单选题）A. but it is notB. but it isC. but they are notD. but they are试题答案：A18、______（单选题）A. nearB. underC. onD. above试题答案：B19、Ten o’clock is a very late hour for a boy of his age to stay up.（单选题）A. too late an hourB. a too late hourC. very much late an hourD. a so late hour试题答案：A20、What does the author mean by the statement “children’s literature began to bloom” underlined in Paragraph 2?（单选题）A. Quality illustrations and attractive formats made books more appealing.B. Illustrations often included flowers.C. It became more varied and attractive.D. It became more numerous and popular.试题答案：D21、Diplomatic misunderstandings can often be traced back to blunders in translation.（单选题）A. mistakesB. featsC. evaluationsD. duplicates试题答案：A22、______（单选题）A. in effectB. as a resultC. for exampleD. in a sense试题答案：C23、The mother didn't know who is blamed for the broken glass.（单选题）A. blamedB. would blameC. to blameD. be blamed试题答案：C24、With the change of red fights, there are long queues of vehicles that wait while a few people who walk across the zebra.（单选题）A. wait...walkB. waiting...walkingC. waiting...walkD. wait...walking试题答案：C25、______（单选题）A. thoughtB. provedC. realizedD. got试题答案：A26、______（单选题）A. cashB. profitC. moneyD. gain试题答案：B27、The Navajo people of Arizona and New Mexico build their hogans, cone-shaped dwellings made of logs and earth, during the summer.（单选题）A. stadiumsB. residencesC. stacksD. stalls试题答案：B28、______（单选题）A. driveB. bringC. dragD. push试题答案：A29、______（单选题）A. strengthenB. accommodateC. stimulateD. enhance试题答案：B30、I urged all the students to take the initiative in their own hands to determine their goals for further pursuit rather than to depend on their teachers and their parents.（单选题）A. determineB. determiningC. in determiningD. for determining试题答案：C31、______（单选题）A. stoutB. powerfulC. wonderfulD. hopeful试题答案：B32、______（单选题）A. supportB. supplyC. compensateD. outearn试题答案：D33、The example of the pupils’ learning about Mexico shows that __________.（单选题）A. a child usually learns the right things from their teachersB. a teacher can correct a pupil’s wrong ideasC. a teacher’s attitude can influence a child’s attitude by teachingD. a child’s attitude is very changeable试题答案：C34、These observations indicates the extreme difficulty in cultivating this precious thing called democracy.（单选题）A. elaborateB. treasonableC. valuableD. deliberate试题答案：C35、______（单选题）A. outB. onC. forwardD. through试题答案：C36、I find that most children’s books are about boys, men and male animals, and most deal about male adventures.（单选题）A. deal withB. are dealing withC. deal inD. are dealing in试题答案：A37、Algae vary in their capacity to tolerate salinity changes.（单选题）A. indicationB. abilityC. effortD. intellect试题答案：B38、______（单选题）A. numberB. amountC. quantityD. numbers试题答案：D39、The pie had been in the oven for twenty minutes，with ten another pies to go，and Kasia was reading to Jim，when a knock sounded on the apartment door.（单选题）A. otherB. ten otherC. another tenD. more ten试题答案：C40、If a businessperson wants to be promoted, the most important thing that is required of him or her is __________.（单选题）A. overseas experienceB. high-tech communications equipmentC. a foreign languageD. English试题答案：A41、______（单选题）A. debtB. incredibilityD. losses试题答案：D42、______（单选题）A. preventB. inhibitC. keepD. motivate试题答案：B43、My parents want very much to see you, and as we live in a very large house, we could offer you spacious accommodations.（单选题）A. criminalB. pumpC. psychologyD. lodgings试题答案：D44、The author implies that __________.（单选题）A. the teacher should guide all discussions by revealing her own attitudeB. in some aspects of social studies a greater variety of methods can be used in the upper grades than in the lower gradesC. people usually act on the basis of reasoning rather than emotionD. children’s attitudes often come from those of other children试题答案：B45、______（单选题）A. displayingC. creatingD. exchanging试题答案：A46、According to the passage, “Alice in Wonderland” was an example of______.（单选题）A. a fantasy storyB. a German folktaleC. a book celebrating childhoodD. a book of instruction试题答案：C47、The ice cream cone, the hamburger, and iced tea were all introduced at the 1904 Louisiana Purchase Exposition informally known as the St. Louis Fair.（单选题）A. previouslyB. unofficiallyC. visiblyD. experimentally试题答案：B48、Niagara Falls is a great tourist attraction, luring millions of visitors each year.（单选题）A. servingB. attractingC. entertainingD. receiving试题答案：B49、Diplomatic misunderstandings can often be traced back to blunders in translation.（单选题）A. mistakesB. featsC. evaluationsD. duplicates试题答案：A50、There is something suspect about unanimous praise in this context.（单选题）A. coincidentalB. unacceptableC. concordantD. Formidable试题答案：C51、______（单选题）A. fromB. toC. ofD. on试题答案：D52、______（单选题）A. premierB. primaryC. premiumD. precision试题答案：C53、I urged all the students to take the initiative in their own hands to determine their goals for further pursuit rather than to depend on their teachers and their parents.（单选题）A. determineB. determiningC. in determiningD. for determining试题答案：C54、We tried moving the piano，but it looked better while it was originally.（单选题）A. whereB. whenC. asD. than试题答案：A55、The problems themselves are still not truly strategic because the operation of the global environment is not affected and the survival of civilization is not at stake.（单选题）A. at riskB. at issueC. pent upD. adhere to试题答案：A56、Elworth’s barn burnt down，but they saved what cattle which were in the barn.（单选题）A. thereB. thatC. theyD. where试题答案：A57、Which of the following is NOT true of online shopping?（单选题）A. Customers can save time by online shopping.B. Online shopping helps reduce retailers’ cost.C. Online shopping saves customers’ energy.D. Customers are sure to buy high-quality goods online.试题答案：D58、______（单选题）A. butB. notC. orD. nor试题答案：C59、______（单选题）A. spacesB. aspectsC. directionsD. places试题答案：D60、It is not so much the language that the cultural background that makes the book difficult to understand.（单选题）A. butB. norC. asD. like试题答案：C61、______（单选题）A. injuredB. hinderedC. destroyedD. immersed试题答案：C62、Although the Moon has not held great prominence in the history of religion, the veneration of the Moon by some societies has been practiced since early times.（单选题）A. magic ofB. symbol forC. worship ofD. dependence on试题答案：C63、______（单选题）A. opportunitiesB. experienceC. skillsD. supply试题答案：B64、The sentence “Our planet has shrunk” underlined in Paragraph 1 means that __________.（单选题）A. the earth has become physically smallerB. the more advanced ways of traveling has made the distance between countries shorterC. the traditional concept of our planet has become out-of-dateD. modern means of communication has made it much easier for people to communicate with each other from different parts of the world试题答案：D65、______（单选题）A. lengthB. timeC. typeD. history试题答案：A66、______（单选题）A. toB. inC. fromD. beyond试题答案：D67、A female mantis does not hesitate to devour her own mate if she is hungry.（单选题）A. ignoreB. fightC. consumeD. hurt试题答案：C68、He was such a good friend that he was practically a member of the family.（单选题）A. virtuallyB. theoreticallyC. skillfullyD. seasonally试题答案：A69、I should say Henry is not very much a writer as a reporter.（单选题）A. thatB. soC. thisD. as试题答案：B70、The author believes before long, machines will __________.（单选题）A. actually replace unskilled workersB. have learned to think for usC. be shaped like robotsD. no longer be needed试题答案：A71、Which of the following is NOT true about the Hawthorne study?（单选题）A. It was the first documented evidence of the psychological effects on doing work.B. The Hawthorne study continued for five years.C. They found that the workers responded not to the level of lighting but to the fact that other work conditions were not favorable.D. The study changed the focus from economics to a multifaceted approach.试题答案：C72、Eating too much fat can devote to heart disease and cause high blood pressure.（单选题）A. attribute toB. attend toC. distribute toD. contribute to试题答案：D73、______（单选题）A. useB. mixtureC. intakeD. pollution试题答案：D74、______（单选题）A. breakB. shakeC. touchD. push试题答案：B75、______（单选题）A. resultB. leadC. bringD. stir试题答案：A76、______（单选题）A. thisB. thatC. whatD. it试题答案：D77、______（单选题）A. activelyB. peacefullyC. warmlyD. positively试题答案：A78、Luther Whitney, the perfectionist cat burglar played by Clint Eastwood in Absolute Power, is as meticulous in his craftsmanship as Vermeer, as adept at disguise as Alec, as elusive as Houdini.（单选题）A. absurdB. proficientC. obviousD. hilarious试题答案：B79、______（单选题）A. ariseB. deriveC. raiseD. originate试题答案：A80、The word “despot” underlined in Paragraph 2 means a person __________.（单选题）A. who enjoys a high reputationB. who is very successful in politicsC. with unlimited powersD. who deposits a large sum of money in a bank试题答案：C81、______（单选题）A. globalizationB. revolutionC. industrializationD. evolution试题答案：A82、______（单选题）A. teachingB. lessonC. educationD. course试题答案：C83、______（单选题）A. fromB. apartC. beyondD. above试题答案：C84、Online retailer Dressmart _______.（单选题）A. proved right to do too much in a short time periodB. carried out the plan of doing its business at home firstC. made great profits by expanding its business abroadD. contracted its business from abroad before complete failure试题答案：D85、The breakup of the habitats resulted in the following except __________.（单选题）A. a lack of food for tamarins to live onB. potential threats to tamarins coming nearbyC. tamarins’ hardly coming down trees to avoid predatorsD. a good fortune for many other threatened animals试题答案：D86、The artist captured the girl’s personality in his photograph.（单选题）A. distortedB. caughtC. maskedD. flattered试题答案：B87、______（单选题）A. createB. conserveC. harnessD. reserve试题答案：B88、“You are very selfish. It’s high time you will realize that you are not the most important person in the world.” Edgar said to his boss angrily.（单选题）A. realizedB. have realizedC. realizeD. should realized试题答案：A89、______（单选题）A. obliviousB. obviousC. obliqueD. obscure试题答案：B90、The word “sturdy” underlined in Paragraph 1 means______.（单选题）A. robustB. ornateC. softD. hard试题答案：D91、______（单选题）A. dangerB. perilC. threatD. disparity试题答案：C92、Camels have extra, almost transparent, eyelids through which they can see in sandstorms.（单选题）A. additionalB. invaluableC. protectiveD. sensitive试题答案：A93、______（单选题）A. looksB. seemsC. saysD. is试题答案：B94、Useful drugs are made from inorganic substances or are plant and animal by-products.（单选题）A. materialsB. derivativesC. chemicalsD. ingredients试题答案：B95、The phrase “call-outs” in the passage is used to____________.（单选题）A. cancelled NHSB. emergent visits to his patients at any timeC. night visits to the home of his patientsD. night walks out of his home试题答案：C96、The scenery on the way was truly spectacular , with beautiful mountains, rivers and valleys, and I took a lot of pictures from the window.（单选题）A. notoriousB. amazingC. nobleD. diplomatic试题答案：B97、______（单选题）A. alwaysB. almostC. entirelyD. surely试题答案：B98、Scientific evidence from different fields of study demonstrates that in most humans the left hemisphere of the brain controls language.（单选题）A. spectacularsB. disciplinesC. spectrumsD. instructions试题答案：B99、The most significant finding of the original research was __________.（单选题）A. lighting had no consistent effect on productionB. production sometimes increased when lighting was reduced to the level of ordinary moonlightC. that lighting was no more important than the other factors of temperature and humidityD. the results were unexpected and confusing试题答案：A100、According to Paragraph 1, the tamarin story can be retold chronologically as follows:Ⅰ. Tamarins were declared extinct.Ⅱ. No one saw a tamarin in the wild.Ⅲ. Tamarins used to live in forests in southern Brazil.Ⅳ. The researchers have been doing everything they can to save tamarins.Ⅴ. Some researchers found by chance a small set of tamarins.（单选题）A. Ⅳ—Ⅱ—Ⅰ—Ⅲ—ⅤB. Ⅲ—Ⅰ—Ⅱ—Ⅴ—ⅣC. Ⅱ—Ⅰ—Ⅲ—Ⅴ—ⅣD. Ⅰ—Ⅱ—Ⅲ—Ⅳ—Ⅴ试题答案：B101、He meant telling us about it, but he forgot to tell us.（单选题）A. to be tellingB. tellingC. to have toldD. having told试题答案：C102、______（单选题）A. everythingB. anythingC. nothingD. something试题答案：D103、______（单选题）A. durableB. excessiveC. surplusD. multiple试题答案：D104、Of course, there have been times, as a young man, when I got tired from study and devoted my time to playing.（单选题）A. tired ofB. got tired ofC. tired upD. used to tire of试题答案：B105、How I wish John knew how to apply grammatical rules properly and recognizes the fact that he is nearly always in the wrong.（单选题）A. recognizeB. recognizedC. to recognizeD. recognizing试题答案：B106、______（单选题）A. inB. onC. withD. to试题答案：A107、______（单选题）A. bookB. villaC. propertyD. equipment试题答案：C108、______（单选题）A. licensedB. allowedC. followedD. neglected试题答案：C109、Wang’s zeal was contagious; soon all his fellow students were busily making posters, inspired by his ardent enthusiasm for the cause.（单选题）A. eagerB. enthusiasmC. zealotD. fanatic试题答案：B110、They soon moved back to the country because they did’t used to city life.（单选题）A. usedn’t toB. wouldn’t used toC. hadn’t used toD. couldn’t get used to试题答案：D111、Mary's daughter, although only six years old, showed ingenuity in making a dress for her doll.（单选题）A. thoughtfulnessB. clevernessC. sincerityD. truthfulness试题答案：B112、Which of the following can be concluded from the author’s statement “Any beneficial substances from the turtles could also be synthesized chemically, so as to prevent the killing of turtles” in Paragraph 3 ?（单选题）A. The author feels sorry for killing turtles massively.B. Turtles can be used to substitute many other medicines.C. Many other kinds of herbs can be as useful as turtles.D. International market feels happy for Chinese businessmen to buy.试题答案：A113、Our plan didn’t get up the ground because no one could come.（单选题）A. get overB. get onC. get offD. get through试题答案：C114、The mother didn't know who is blamed for the broken glass.（单选题）A. blamedB. would blameC. to blameD. be blamed试题答案：C115、We seem oblivious of the fragility of the earth’s natural system.（单选题）A. frailtyB. solitudeC. prevalenceD. fission试题答案：A116、______（单选题）A. brightnessB. foreseeingC. foresightD. prediction试题答案：C117、______（单选题）A. OnB. InC. ToD. For试题答案：B118、A good dictionary will help you up in your reading.（单选题）A. inB. outC. downD. through试题答案：B119、The ability to communicate ideas and instruction was all necessary for the incredible development of the frontal brain lobe in human beings.（单选题）A. all that was necessaryB. necessary all thatC. all necessary thatD. that all was necessary试题答案：A120、______（单选题）A. strangeB. OrientalC. orthodoxD. Western试题答案：D121、Lack government support, they had to approach sponsors, organizers, and musicians on their own — at first, she claims, in her country.（单选题）A. LacksB. LackedC. LackingD. A lack of试题答案：C122、Though in no means rich, he was better off than at any other period in his life.（单选题）A. by any meansB. by some meansC. by all meansD. by no means试题答案：D123、______（单选题）A. AnywhereB. NowhereC. WhereverD. Whereto试题答案：B124、This volume comprises samples from the works of one hundred authors in the past fifty years.（单选题）A. containsB. spraysC. splitsD. spear试题答案：A125、______（单选题）A. availableB. enoughC. sufficientD. convenient试题答案：A126、______（单选题）A. inB. toC. ofD. for试题答案：A127、______（单选题）A. he shouldB. should heC. had heD. he had试题答案：B128、Incredible as it sounds, that is now the policy of a certain faction of Parliament.（单选题）A. IncredulousB. DiscreditableC. IndicativeD. Unbelievable试题答案：D129、If only I am enough vitality，I could probably do without my one-o’clock nap.（单选题）A. had enough hadB. had enoughC. has enoughD. has had enough试题答案：B130、______（单选题）A. lengthB. timeC. typeD. history试题答案：A131、Which of the following is NOT true about the Hawthorne study?（单选题）A. It was the first documented evidence of the psychological effects on doing work.B. The Hawthorne study continued for five years.C. They found that the workers responded not to the level of lighting but to the fact that other work conditions were not favorable.D. The study changed the focus from economics to a multifaceted approach.试题答案：C132、From the passage we learn that _________.（单选题）A. turtles are the only source used to make medicine to cure a number of ailmentsB. some chemical substances may have the same effect as turtlesC. turtles are nearly extinct in ChinaD. turtle dishes are the only expensive delicacy in Chinese restaurants试题答案：B133、I have a tinge of regret that I didn’t accept her offer.（单选题）A. taintB. signC. shadeD. mark试题答案：C134、What is the name of the place where the doctor’s NHS practice is done?（单选题）A. In Ealing.B. In the downtown of London.C. In one of London’s suburbs.D. In Heston near the Gym.试题答案：C135、There are actually too many opportunities to learn more, to see more, and to understand more, and they all require us to change, at least a little bit, in order to make progress.（单选题）A. very manyB. too muchC. so manyD. extremely many试题答案：C136、The pie had been in the oven for twenty minutes，with ten another pies to go，and Kasia was reading to Jim，when a knock sounded on the apartment door.（单选题）A. otherB. ten otherC. another tenD. more ten试题答案：C137、Leaving the market-place, the fat man looked pleased with himself in having discovered a bargain.（单选题）A. at…forB. at…atC. with…forD. with...at试题答案：D138、“You are very selfish. It’s high time you will realize that you are not the most important person in the world.” Edgar said to his boss angrily.（单选题）A. realizedB. have realizedC. realizeD. should realized试题答案：A139、______（单选题）A. endureB. stayC. liveD. suffer试题答案：B140、What impact did color printing have on children’s books?（单选题）A. Sales increased by the middle of the 19th century.B. Rough illustrations were now more appealing.C. Illustrations told more of the story and were more appealing.D. Books became more amusing.试题答案：C141、______（单选题）A. visionB. viewpointC. sightseeingD. prospect试题答案：B142、When he fails his final examination, he is sure of a university place.（单选题）A. IfB. In caseC. Even whenD. Even if试题答案：D143、That play has puzzled authorities for generations, but to my father has the clarity and the simplicity of a medieval moral play.（单选题）A. perplexedB. disorderedC. anguishedD. seduced试题答案：A144、______（单选题）A. studyB. discoveryC. researchD. development试题答案：C145、No adults would like to admit that they are weaker than children to solve sticky and puzzling problems.（单选题）A. to solve aboutB. solveC. in solving aboutD. in solving试题答案：D146、Such essentially different stimulants as cold, drugs, and grief can provoke identical chemical reactions in the body.（单选题）A. illnessB. weatherC. sorrowD. travel试题答案：C147、“The turtle — along with a few other Cuora species — was once imported into China by the pet trade.” in Paragraph 1 means _________.（单选题）A. the turtle is a wonderful animal in Chinese historyB. it is a logical result that Chinese people treat turtles as petsC. long time ago, the turtle and some other Cuora species were brought into China as petsD. turtle is a delicate food treated by Chinese like a wonder试题答案：C148、______（单选题）A. conceivedB. thoughtC. believedD. perceived试题答案：A149、The experience of foreign countries is worth learning from and taking for reference.（单选题）A. learning and referenceB. our learning and our referenceC. our learning from and taking for referenceD. our learning and make reference to试题答案：C150、The most significant finding of the original research was __________.（单选题）A. lighting had no consistent effect on productionB. production sometimes increased when lighting was reduced to the level of ordinary moonlightC. that lighting was no more important than the other factors of temperature and humidityD. the results were unexpected and confusing试题答案：A151、According to the passage, what changed children’s publishing in 1774?（单选题）A. The Newberry Medal.B. Some books began to be produced mainly for children to enjoy.C. Some books had attractive formats, quality illustrations and sturdy bindings.D. Some books no longer contained religious instruction rules of behavior, ethical messages and moral platitudes.试题答案：B152、This was but an additional testimony of the superiority of the socialist economic system over the capitalist economic system.（单选题）A. proofB. witnessC. evidenceD. validity试题答案：A153、______（单选题）A. enhancedB. decreasedC. broadenedD. enlarged试题答案：A154、A land ethic of course cannot prevent the conversion, management, and use of the resources of soil, waters, plants and animals.（单选题）A. altercationB. alternationC. alterationD. allocation试题答案：C155、______（单选题）A. thingB. realityC. factD. conclusion试题答案：C156、______（单选题）A. capableB. cheapC. practicalD. affordable试题答案：C157、As much is known about what occurs during sleep, the precise function of sleep and its different stages remains largely in the realm of assumption.（单选题）A. BecauseB. SinceC. ForD. While试题答案：D158、______（单选题）A. ThereforeB. NeverthelessC. On the contraryD. Similarly试题答案：B159、Though in no means rich, he was better off than at any other period in his life.（单选题）A. by any meansB. by some meansC. by all meansD. by no means试题答案：D160、Many people prefer to have their tax forms completed by a professional rather than having done it themselves.（单选题）A. doingB. doC. to doD. did试题答案：B161、The critics tended to speculate to who had the greatest influence on the development of that writer’s novels.（单选题）A. as toB. so as toC. thatD. such that试题答案：A162、Mr. Smith was occupied in preparing for his lessons and marking students’compositions.（单选题）A. was absorbed inB. was freed fromC. was busy withD. was attracted by试题答案：C163、Such essentially different stimulants as cold, drugs, and grief can provoke identical chemical reactions in the body.（单选题）A. illnessB. weatherC. sorrowD. travel试题答案：C164、______（单选题）A. thatB. whichC. in order thatD. for试题答案：B165、After months of fighting the rebels were subdued in that country.（单选题）A. surrenderedB. attackedC. conqueredD. elevated试题答案：C166、A newly developed plastic, prized by engineers for its low weight and its durability , has been developed.（单选题）A. low costB. flexibilityC. long lifeD. versatility试题答案：C。

$phi$ meson production in Au - Au collisions at $sqrt{s_{NN}}$ = 200 GeV

$phi$ meson production in Au - Au collisions at $sqrt{s_{NN}}$ = 200 GeV

a r X i v :n u c l -e x /0209028v 1 28 S e p 20021φmeson production in Au -Au collisions at√s NN =200GeV at mid-rapidity by the PHENIXexperiment.The preliminary minimum bias yields dN φ/dy in the kaon and electronchannels are 2.01±0.22(stat.)+1.01−0.52(syst.)and 5.4±2.5(stat )+3.4−2.8(syst.),respectively.The centrality dependence of the yield in the K +K −channel is presented.1.IntroductionThe φmeson is an important probe for studying relativistic heavy ion collisions.Since the mass of the φmeson is close to twice the kaon mass,any medium modification of its spectral shape (mass and/or width)[1]as chiral symmetry restoration is approached may induce a change in its branching ratio in the kaon channel.The simultaneous measurement of φdecay into K +K −and e +e −is a very powerful tool in the search for such in-medium modifications.Consisting of s ¯s ,the φmeson is also a sensitive probe of strangeness production [2].The PHENIX experiment at RHIC,with its excellent mass resolution and particle identification capability,(comparable to or better than the natural width of the φmeson)has the unique capability to measure the φmeson through both K +K −and e +e −decay channels at mid-rapidity.We present the preliminary results of the φmeson measurement made during the 2001RHIC run.2.Experimental setup and data analysisThe results presented here were obtained using the two central arms of the PHENIX spectrometer [3].The φ→K +K −analysis used the drift chamber (DC),two sets of multiwire proportional chambers with pixel-pad readout (PC1and PC3),and the time of flight (TOF)module of the east arm.The acceptance is defined by the TOF module which covers the pseudorapidity range |η|<0.35and an azimuthal range of ∆φ≈30◦.The kaons were identified via reconstructed momentum combined with a TOF measurement with a time resolution ∼120ps.With a 2σmomentum dependent cut in the mass squared distributions,π/K were well separated up to p ∼2.0GeV/c.The φ→e +e −measurements were performed with the DC,PC1,PC3,the Ring Imag-ing Cerenkov Detector (RICH)and the electromagnetic calorimeter (EMCAL)of both the east and west arm,each one covering ∆φ=90◦and |η|<0.35.Electrons were identified2primarily by the RICH.Further identification was provided by requiring the energy in the EMCAL to match the measured momentum of the tracks.Charged particle tracks and their momenta were reconstructed using the DC and PC1. Tracks were confirmed by a matching hit in PC3and TOF in the case of kaons,and in the EMCAL for electrons.The beam-beam counters(BBC)and the zero-degree calorimeters(ZDC)provided the trigger and were used to determine the event centrality.The BBC were also used to determine the z-coordinate of the collision vertex(z vertex).The analysis used27.4×106 (for kaons)and25.8×106(for electrons)minimum bias events with a vertex position within|z vertex|<30cm.3.Analysis procedure3.1.DataAll identified kaon tracks in a given event were combined to form the invariant mass distributions of the like sign(N++,N−−)and unlike sign pairs.The large combinatorial background inherent to this procedure was estimated by an event mixing method in which all K+tracks from one event were combined with K−tracks of ten other events of the same centrality and vertex class.The mixed event invariant mass distribution was then normalized to the measured2√s NN=130GeV[4].The generatedφ’s were then propagated through the PHENIX detector simulation and the pair acceptance and reconstruction efficiency was calculated by:ǫ=N generated φdy =N Signalφ×ǫ×ǫ2mult34.Results4.1.Invariant mass distributionsFig.1shows the minimum bias invariant mass distributions forφ→K+K−(panel (a))andφ→e+e−(panel(c)).The kaon channel results for the40%-80%centrality range are shown in Fig.1(b),where we see a considerable improvement in the signal to background ratio compared with minimum bias spectrum.Since the combinatorial background increases quadratically with centrality,this means that the signal increases atFigure1:Invariant mass distributions ofφ→K+K−for(a)minimum bias events andb)40-80%central events.The mass spectrum ofφ→e+e−for minimum bias is shown in(c).The points in the upper panels represent the data and thefilled histograms show the combinatorial background whereas the lower panels exhibit the subtracted mass spectra.4.2.dNφ/dyThe rapidity density(dNφ/dy)values calculated using Eq.(2)for the kaon and elec-tron channels are shown in Table1.The systematic errors in both cases originate from the correction factors in the Monte Carlo simulation and the analysis method.In the φ→K+K−analysis,the main systematic uncertainty originates from the uncertainty (assumed to be±20%)in the temperature used in the Monte Carlo calculations.Since the TOF array has no acceptance for low pair p T,the correction factorǫis very sensitive to the slope of the parent p T distribution.Forφ→e+e−,the analysis procedure is the major source of the systematic error.Within the present large statistical and systematic errors,the values of dNφ/dy for both cases are consistent with each other.φmesons from dimuon and kaon channels were also observed by thefixed target NA50and NA49ex-periments respectively at a beam energy of158AGeV at CERN.These two independent experimental results showed a factor of5difference in the dNφ/dy values for the muon channel(dNφ/dy∼13)as compared to the kaon channel(dNφ/dy∼2.35)[5].Higher statistics together with a better understanding of the systematics will enable us to see whether or not such a large difference persists at RHIC energies.4.3.Centrality dependenceIn Fig.2(a),we plot dNφ/dy as a function of centrality(expressed in percentiles) for the kaon channel.The variation of the rapidity density per participant with the4Table1:φ-meson rapidity density in Au-Au at√dNφ/dy Systematic error2.01+1.01−0.525.4+3.4−2.8。

Two-Dimensional Gas of Massless Dirac Fermions in Graphene

Two-Dimensional Gas of Massless Dirac Fermions in Graphene

Two-Dimensional Gas of Massless Dirac Fermions in Graphene K.S. Novoselov1, A.K. Geim1, S.V. Morozov2, D. Jiang1, M.I. Katsnelson3, I.V. Grigorieva1, S.V. Dubonos2, A.A. Firsov21Manchester Centre for Mesoscience and Nanotechnology, University of Manchester, Manchester, M13 9PL, UK2Institute for Microelectronics Technology, 142432, Chernogolovka, Russia3Institute for Molecules and Materials, Radboud University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, the NetherlandsElectronic properties of materials are commonly described by quasiparticles that behave as nonrelativistic electrons with a finite mass and obey the Schrödinger equation. Here we report a condensed matter system where electron transport is essentially governed by the Dirac equation and charge carriers mimic relativistic particles with zero mass and an effective “speed of light” c∗ ≈106m/s. Our studies of graphene – a single atomic layer of carbon – have revealed a variety of unusual phenomena characteristic of two-dimensional (2D) Dirac fermions. In particular, we have observed that a) the integer quantum Hall effect in graphene is anomalous in that it occurs at halfinteger filling factors; b) graphene’s conductivity never falls below a minimum value corresponding to the conductance quantum e2/h, even when carrier concentrations tend to zero; c) the cyclotron mass mc of massless carriers with energy E in graphene is described by equation E =mcc∗2; and d) Shubnikov-de Haas oscillations in graphene exhibit a phase shift of π due to Berry’s phase.Graphene is a monolayer of carbon atoms packed into a dense honeycomb crystal structure that can be viewed as either an individual atomic plane extracted from graphite or unrolled single-wall carbon nanotubes or as a giant flat fullerene molecule. This material was not studied experimentally before and, until recently [1,2], presumed not to exist. To obtain graphene samples, we used the original procedures described in [1], which involve micromechanical cleavage of graphite followed by identification and selection of monolayers using a combination of optical, scanning-electron and atomic-force microscopies. The selected graphene films were further processed into multi-terminal devices such as the one shown in Fig. 1, following standard microfabrication procedures [2]. Despite being only one atom thick and unprotected from the environment, our graphene devices remain stable under ambient conditions and exhibit high mobility of charge carriers. Below we focus on the physics of “ideal” (single-layer) graphene which has a different electronic structure and exhibits properties qualitatively different from those characteristic of either ultra-thin graphite films (which are semimetals and whose material properties were studied recently [2-5]) or even of our other devices consisting of just two layers of graphene (see further). Figure 1 shows the electric field effect [2-4] in graphene. Its conductivity σ increases linearly with increasing gate voltage Vg for both polarities and the Hall effect changes its sign at Vg ≈0. This behaviour shows that substantial concentrations of electrons (holes) are induced by positive (negative) gate voltages. Away from the transition region Vg ≈0, Hall coefficient RH = 1/ne varies as 1/Vg where n is the concentration of electrons or holes and e the electron charge. The linear dependence 1/RH ∝Vg yields n =α·Vg with α ≈7.3·1010cm-2/V, in agreement with the theoretical estimate n/Vg ≈7.2·1010cm-2/V for the surface charge density induced by the field effect (see Fig. 1’s caption). The agreement indicates that all the induced carriers are mobile and there are no trapped charges in graphene. From the linear dependence σ(Vg) we found carrier mobilities µ =σ/ne, whichreached up to 5,000 cm2/Vs for both electrons and holes, were independent of temperature T between 10 and 100K and probably still limited by defects in parent graphite. To characterise graphene further, we studied Shubnikov-de Haas oscillations (SdHO). Figure 2 shows examples of these oscillations for different magnetic fields B, gate voltages and temperatures. Unlike ultra-thin graphite [2], graphene exhibits only one set of SdHO for both electrons and holes. By using standard fan diagrams [2,3], we have determined the fundamental SdHO frequency BF for various Vg. The resulting dependence of BF as a function of n is plotted in Fig. 3a. Both carriers exhibit the same linear dependence BF = β·n with β ≈1.04·10-15 T·m2 (±2%). Theoretically, for any 2D system β is defined only by its degeneracy f so that BF =φ0n/f, where φ0 =4.14·10-15 T·m2 is the flux quantum. Comparison with the experiment yields f =4, in agreement with the double-spin and double-valley degeneracy expected for graphene [6,7] (cf. caption of Fig. 2). Note however an anomalous feature of SdHO in graphene, which is their phase. In contrast to conventional metals, graphene’s longitudinal resistance ρxx(B) exhibits maxima rather than minima at integer values of the Landau filling factor ν (Fig. 2a). Fig. 3b emphasizes this fact by comparing the phase of SdHO in graphene with that in a thin graphite film [2]. The origin of the “odd” phase is explained below. Another unusual feature of 2D transport in graphene clearly reveals itself in the T-dependence of SdHO (Fig. 2b). Indeed, with increasing T the oscillations at high Vg (high n) decay more rapidly. One can see that the last oscillation (Vg ≈100V) becomes practically invisible already at 80K whereas the first one (Vg <10V) clearly survives at 140K and, in fact, remains notable even at room temperature. To quantify this behaviour we measured the T-dependence of SdHO’s amplitude at various gate voltages and magnetic fields. The results could be fitted accurately (Fig. 3c) by the standard expression T/sinh(2π2kBTmc/heB), which yielded mc varying between ≈ 0.02 and 0.07m0 (m0 is the free electron mass). Changes in mc are well described by a square-root dependence mc ∝n1/2 (Fig. 3d). To explain the observed behaviour of mc, we refer to the semiclassical expressions BF = (h/2πe)S(E) and mc =(h2/2π)∂S(E)/∂E where S(E) =πk2 is the area in k-space of the orbits at the Fermi energy E(k) [8]. Combining these expressions with the experimentally-found dependences mc ∝n1/2 and BF =(h/4e)n it is straightforward to show that S must be proportional to E2 which yields E ∝k. Hence, the data in Fig. 3 unambiguously prove the linear dispersion E =hkc∗ for both electrons and holes with a common origin at E =0 [6,7]. Furthermore, the above equations also imply mc =E/c∗2 =(h2n/4πc∗2)1/2 and the best fit to our data yields c∗ ≈1⋅106 m/s, in agreement with band structure calculations [6,7]. The employed semiclassical model is fully justified by a recent theory for graphene [9], which shows that SdHO’s amplitude can indeed be described by the above expression T/sinh(2π2kBTmc/heB) with mc =E/c∗2. Note that, even though the linear spectrum of fermions in graphene (Fig. 3e) implies zero rest mass, their cyclotron mass is not zero. The unusual response of massless fermions to magnetic field is highlighted further by their behaviour in the high-field limit where SdHO evolve into the quantum Hall effect (QHE). Figure 4 shows Hall conductivity σxy of graphene plotted as a function of electron and hole concentrations in a constant field B. Pronounced QHE plateaux are clearly seen but, surprisingly, they do not occur in the expected sequence σxy =(4e2/h)N where N is integer. On the contrary, the plateaux correspond to half-integer ν so that the first plateau occurs at 2e2/h and the sequence is (4e2/h)(N + ½). Note that the transition from the lowest hole (ν =–½) to lowest electron (ν =+½) Landau level (LL) in graphene requires the same number of carriers (∆n =4B/φ0 ≈1.2·1012cm-2) as the transition between other nearest levels (cf. distances between minima in ρxx). This results in a ladder of equidistant steps in σxy which are not interrupted when passing through zero. To emphasize this highly unusual behaviour, Fig. 4 also shows σxy for a graphite film consisting of only two graphene layers where the sequence of plateaux returns to normal and the first plateau is at 4e2/h, as in the conventional QHE. We attribute this qualitative transition between graphene and its two-layer counterpart to the fact that fermions in the latter exhibit a finite mass near n ≈0 (as found experimentally; to be published elsewhere) and can no longer be described as massless Dirac particles. 2The half-integer QHE in graphene has recently been suggested by two theory groups [10,11], stimulated by our work on thin graphite films [2] but unaware of the present experiment. The effect is single-particle and intimately related to subtle properties of massless Dirac fermions, in particular, to the existence of both electron- and hole-like Landau states at exactly zero energy [912]. The latter can be viewed as a direct consequence of the Atiyah-Singer index theorem that plays an important role in quantum field theory and the theory of superstrings [13,14]. For the case of 2D massless Dirac fermions, the theorem guarantees the existence of Landau states at E=0 by relating the difference in the number of such states with opposite chiralities to the total flux through the system (note that magnetic field can also be inhomogeneous). To explain the half-integer QHE qualitatively, we invoke the formal expression [9-12] for the energy of massless relativistic fermions in quantized fields, EN =[2ehc∗2B(N +½ ±½)]1/2. In QED, sign ± describes two spins whereas in the case of graphene it refers to “pseudospins”. The latter have nothing to do with the real spin but are “built in” the Dirac-like spectrum of graphene, and their origin can be traced to the presence of two carbon sublattices. The above formula shows that the lowest LL (N =0) appears at E =0 (in agreement with the index theorem) and accommodates fermions with only one (minus) projection of the pseudospin. All other levels N ≥1 are occupied by fermions with both (±) pseudospins. This implies that for N =0 the degeneracy is half of that for any other N. Alternatively, one can say that all LL have the same “compound” degeneracy but zeroenergy LL is shared equally by electrons and holes. As a result the first Hall plateau occurs at half the normal filling and, oddly, both ν = –½ and +½ correspond to the same LL (N =0). All other levels have normal degeneracy 4B/φ0 and, therefore, remain shifted by the same ½ from the standard sequence. This explains the QHE at ν =N + ½ and, at the same time, the “odd” phase of SdHO (minima in ρxx correspond to plateaux in ρxy and, hence, occur at half-integer ν; see Figs. 2&3), in agreement with theory [9-12]. Note however that from another perspective the phase shift can be viewed as the direct manifestation of Berry’s phase acquired by Dirac fermions moving in magnetic field [15,16]. Finally, we return to zero-field behaviour and discuss another feature related to graphene’s relativistic-like spectrum. The spectrum implies vanishing concentrations of both carriers near the Dirac point E =0 (Fig. 3e), which suggests that low-T resistivity of the zero-gap semiconductor should diverge at Vg ≈0. However, neither of our devices showed such behaviour. On the contrary, in the transition region between holes and electrons graphene’s conductivity never falls below a well-defined value, practically independent of T between 4 and 100K. Fig. 1c plots values of the maximum resistivity ρmax(B =0) found in 15 different devices, which within an experimental error of ≈15% all exhibit ρmax ≈6.5kΩ, independent of their mobility that varies by a factor of 10. Given the quadruple degeneracy f, it is obvious to associate ρmax with h/fe2 =6.45kΩ where h/e2 is the resistance quantum. We emphasize that it is the resistivity (or conductivity) rather than resistance (or conductance), which is quantized in graphene (i.e., resistance R measured experimentally was not quantized but scaled in the usual manner as R =ρL/w with changing length L and width w of our devices). Thus, the effect is completely different from the conductance quantization observed previously in quantum transport experiments. However surprising, the minimum conductivity is an intrinsic property of electronic systems described by the Dirac equation [17-20]. It is due to the fact that, in the presence of disorder, localization effects in such systems are strongly suppressed and emerge only at exponentially large length scales. Assuming the absence of localization, the observed minimum conductivity can be explained qualitatively by invoking Mott’s argument [21] that mean-free-path l of charge carriers in a metal can never be shorter that their wavelength λF. Then, σ =neµ can be re-written as σ = (e2/h)kFl and, hence, σ cannot be smaller than ≈e2/h per each type of carriers. This argument is known to have failed for 2D systems with a parabolic spectrum where disorder leads to localization and eventually to insulating behaviour [17,18]. For the case of 2D Dirac fermions, no localization is expected [17-20] and, accordingly, Mott’s argument can be used. Although there is a broad theoretical consensus [18-23,10,11] that a 2D gas of Dirac fermions should exhibit a minimum 3conductivity of about e2/h, this quantization was not expected to be accurate and most theories suggest a value of ≈e2/πh, in disagreement with the experiment. In conclusion, graphene exhibits electronic properties distinctive for a 2D gas of particles described by the Dirac rather than Schrödinger equation. This 2D system is not only interesting in itself but also allows one to access – in a condensed matter experiment – the subtle and rich physics of quantum electrodynamics [24-27] and provides a bench-top setting for studies of phenomena relevant to cosmology and astrophysics [27,28].1. Novoselov, K.S. et al. PNAS 102, 10451 (2005). 2. Novoselov, K.S. et al. Science 306, 666 (2004); cond-mat/0505319. 3. Zhang, Y., Small, J.P., Amori, M.E.S. & Kim, P. Phys. Rev. Lett. 94, 176803 (2005). 4. Berger, C. et al. J. Phys. Chem. B, 108, 19912 (2004). 5. Bunch, J.S., Yaish, Y., Brink, M., Bolotin, K. & McEuen, P.L. Nanoletters 5, 287 (2005). 6. Dresselhaus, M.S. & Dresselhaus, G. Adv. Phys. 51, 1 (2002). 7. Brandt, N.B., Chudinov, S.M. & Ponomarev, Y.G. Semimetals 1: Graphite and Its Compounds (North-Holland, Amsterdam, 1988). 8. Vonsovsky, S.V. and Katsnelson, M.I. Quantum Solid State Physics (Springer, New York, 1989). 9. Gusynin, V.P. & Sharapov, S.G. Phys. Rev. B 71, 125124 (2005). 10. Gusynin, V.P. & Sharapov, S.G. cond-mat/0506575. 11. Peres, N.M.R., Guinea, F. & Castro Neto, A.H. cond-mat/0506709. 12. Zheng, Y. & Ando, T. Phys. Rev. B 65, 245420 (2002). 13. Kaku, M. Introduction to Superstrings (Springer, New York, 1988). 14. Nakahara, M. Geometry, Topology and Physics (IOP Publishing, Bristol, 1990). 15. Mikitik, G. P. & Sharlai, Yu.V. Phys. Rev. Lett. 82, 2147 (1999). 16. Luk’yanchuk, I.A. & Kopelevich, Y. Phys. Rev. Lett. 93, 166402 (2004). 17. Abrahams, E., Anderson, P.W., Licciardello, D.C. & Ramakrishnan, T.V. Phys. Rev. Lett. 42, 673 (1979). 18. Fradkin, E. Phys. Rev. B 33, 3263 (1986). 19. Lee, P.A. Phys. Rev. Lett. 71, 1887 (1993). 20. Ziegler, K. Phys. Rev. Lett. 80, 3113 (1998). 21. Mott, N.F. & Davis, E.A. Electron Processes in Non-Crystalline Materials (Clarendon Press, Oxford, 1979). 22. Morita, Y. & Hatsugai, Y. Phys. Rev. Lett. 79, 3728 (1997). 23. Nersesyan, A.A., Tsvelik, A.M. & Wenger, F. Phys. Rev. Lett. 72, 2628 (1997). 24. Rose, M.E. Relativistic Electron Theory (John Wiley, New York, 1961). 25. Berestetskii, V.B., Lifshitz, E.M. & Pitaevskii, L.P. Relativistic Quantum Theory (Pergamon Press, Oxford, 1971). 26. Lai, D. Rev. Mod. Phys. 73, 629 (2001). 27. Fradkin, E. Field Theories of Condensed Matter Systems (Westview Press, Oxford, 1997). 28. Volovik, G.E. The Universe in a Helium Droplet (Clarendon Press, Oxford, 2003).Acknowledgements This research was supported by the EPSRC (UK). We are most grateful to L. Glazman, V. Falko, S. Sharapov and A. Castro Netto for helpful discussions. K.S.N. was supported by Leverhulme Trust. S.V.M., S.V.D. and A.A.F. acknowledge support from the Russian Academy of Science and INTAS.43µ (m2/Vs)0.8c4P0.4 22 σ (1/kΩ)10K0 0 1/RH(T/kΩ) 1 2ρmax (h/4e2)1-5010 Vg (V) 50 -10ab 0 -100-500 Vg (V)50100Figure 1. Electric field effect in graphene. a, Scanning electron microscope image of one of our experimental devices (width of the central wire is 0.2µm). False colours are chosen to match real colours as seen in an optical microscope for larger areas of the same materials. Changes in graphene’s conductivity σ (main panel) and Hall coefficient RH (b) as a function of gate voltage Vg. σ and RH were measured in magnetic fields B =0 and 2T, respectively. The induced carrier concentrations n are described by [2] n/Vg =ε0ε/te where ε0 and ε are permittivities of free space and SiO2, respectively, and t ≈300 nm is the thickness of SiO2 on top of the Si wafer used as a substrate. RH = 1/ne is inverted to emphasize the linear dependence n ∝Vg. 1/RH diverges at small n because the Hall effect changes its sign around Vg =0 indicating a transition between electrons and holes. Note that the transition region (RH ≈ 0) was often shifted from zero Vg due to chemical doping [2] but annealing of our devices in vacuum normally allowed us to eliminate the shift. The extrapolation of the linear slopes σ(Vg) for electrons and holes results in their intersection at a value of σ indistinguishable from zero. c, Maximum values of resistivity ρ =1/σ (circles) exhibited by devices with different mobilites µ (left y-axis). The histogram (orange background) shows the number P of devices exhibiting ρmax within 10% intervals around the average value of ≈h/4e2. Several of the devices shown were made from 2 or 3 layers of graphene indicating that the quantized minimum conductivity is a robust effect and does not require “ideal” graphene.ρxx (kΩ)0.60 aVg = -60V4B (T)810K12∆σxx (1/kΩ)0.4 1ν=4 140K 80K B =12T0 b 0 25 50 Vg (V) 7520K100Figure 2. Quantum oscillations in graphene. SdHO at constant gate voltage Vg as a function of magnetic field B (a) and at constant B as a function of Vg (b). Because µ does not change much with Vg, the constant-B measurements (at a constant ωcτ =µB) were found more informative. Panel b illustrates that SdHO in graphene are more sensitive to T at high carrier concentrations. The ∆σxx-curves were obtained by subtracting a smooth (nearly linear) increase in σ with increasing Vg and are shifted for clarity. SdHO periodicity ∆Vg in a constant B is determined by the density of states at each Landau level (α∆Vg = fB/φ0) which for the observed periodicity of ≈15.8V at B =12T yields a quadruple degeneracy. Arrows in a indicate integer ν (e.g., ν =4 corresponds to 10.9T) as found from SdHO frequency BF ≈43.5T. Note the absence of any significant contribution of universal conductance fluctuations (see also Fig. 1) and weak localization magnetoresistance, which are normally intrinsic for 2D materials with so high resistivity.75 BF (T) 500.2 0.11/B (1/T)b5 10 N 1/2025 a 0 0.061dmc /m00.04∆0.02 0c0 0 T (K) 150n =0e-6-3036Figure 3. Dirac fermions of graphene. a, Dependence of BF on carrier concentration n (positive n correspond to electrons; negative to holes). b, Examples of fan diagrams used in our analysis [2] to find BF. N is the number associated with different minima of oscillations. Lower and upper curves are for graphene (sample of Fig. 2a) and a 5-nm-thick film of graphite with a similar value of BF, respectively. Note that the curves extrapolate to different origins; namely, to N = ½ and 0. In graphene, curves for all n extrapolate to N = ½ (cf. [2]). This indicates a phase shift of π with respect to the conventional Landau quantization in metals. The shift is due to Berry’s phase [9,15]. c, Examples of the behaviour of SdHO amplitude ∆ (symbols) as a function of T for mc ≈0.069 and 0.023m0; solid curves are best fits. d, Cyclotron mass mc of electrons and holes as a function of their concentration. Symbols are experimental data, solid curves the best fit to theory. e, Electronic spectrum of graphene, as inferred experimentally and in agreement with theory. This is the spectrum of a zero-gap 2D semiconductor that describes massless Dirac fermions with c∗ 300 times less than the speed of light.n (1012 cm-2)σxy (4e2/h)4 3 2 -2 1 -1 -2 -3 2 44Kn7/ 5/ 3/ 1/2 2 2 210 ρxx (kΩ)-4σxy (4e2/h)0-1/2 -3/2 -5/2514T0-7/2 -4 -2 0 2 4 n (1012 cm-2)Figure 4. Quantum Hall effect for massless Dirac fermions. Hall conductivity σxy and longitudinal resistivity ρxx of graphene as a function of their concentration at B =14T. σxy =(4e2/h)ν is calculated from the measured dependences of ρxy(Vg) and ρxx(Vg) as σxy = ρxy/(ρxy + ρxx)2. The behaviour of 1/ρxy is similar but exhibits a discontinuity at Vg ≈0, which is avoided by plotting σxy. Inset: σxy in “two-layer graphene” where the quantization sequence is normal and occurs at integer ν. The latter shows that the half-integer QHE is exclusive to “ideal” graphene.。

新版超声医学英语-新版-精选.pdf

新版超声医学英语-新版-精选.pdf


模 糊 hazy ， indistinct ， blurred ，
ill-defined ，obscured ， silhouette out
(sth) ；不规则 irregular ；毛刺状、针
状
spiculated ； 分 叶 的
lobulated,multilobulated ；
八 、 密 度 density(dense) ， densitometry ， attenuation （ X 线成 像）： 透亮 lucency(lucent) ， transparent ； 病灶 lesion ：阴影 shadow ；不透光 haziness ， opacification ， opacity ， opaque ；致密 density(dense) ； 低密度 hypodense ，low density ；高 密度 hyperdense ， high density ；混
二、范围 extent ： 局 限 localized ， regional ； 弥 散 diffuse ；
三、分布 distribution ：
单侧
unilateral ； 双 侧
bilateral,(in)both(lung fields) ； 对 称
symmetric ；不对称 asymmetric ；孤
homogen 均 匀 增 强 nonhomegeneous
enhancement
环状增强 circular enhancement
结节状增强 nodular enhancement
片状增强 patchy enhancement
脑回样增强 gyriform enhancement
立 solitary ；散 在 scattered ； 融 合