On the nature of the spin-polarized hole states in a quasi-two-dimensional GaMnAs ferromagn
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版内有这方面的内容顺磁,意味进行non-spin polarized的计算,也就是ISPIN=1。
铁磁,意味进行spin-polarized的计算,ISPIN=2,而且每个磁性原子的初始磁矩设置为一样的值,也就是磁性原子的MAGMOM设置为一样的值。
对非磁性原子也可以设置成一样的非零值(与磁性原子的一样)或零,最后收敛的结果,非磁性原子的local磁矩很小,快接近0,很小的情况,很可能意味着真的是非磁性原子也会被极化而出现很小的local磁矩。
反铁磁,也意味着要进行spin-polarized的计算,ISPIN=2,这是需采用反铁磁的磁胞来进行计算,意味着此时计算所采用的晶胞不再是铁磁计算时的最小原胞。
比如对铁晶体的铁磁状态,你可以采用bcc的原胞来计算,但是在进行反铁磁的Fe计算,这是你需要采用sc的结构来计算,计算的晶胞中包括两个原子,你要设置一个原子的MAGMOM为正的,另一个原子的MAGMOM设置为负,但是它们的绝对值一样。
因此在进行反铁磁的计算时,应该确定好反铁磁的磁胞,以及磁序,要判断哪种磁序和磁胞是最可能的反铁磁状态,那只能是先做好各种可能的排列组合,然后分别计算这些可能组合的情况,最后比较它们的总能,总能最低的就是可能的磁序。
同样也可以与它们同铁磁或顺磁的进行比较。
了解到该材料究竟是铁磁的、还是顺磁或反铁磁的。
亚铁磁,也意味要进行spin-polarized的计算,ISPIN=2,与反铁磁的计算类似,不同的是原子正负磁矩的绝对值不是样大。
非共线的磁性,那需采用专门的non-collinear的来进行计算,除了要设置ISPIN,MAGMOM的设置还需要指定每个原子在x,y,z方向上的大小。
这种情况会复杂一些。
举个例子来说,对于Mn-Cu(001)c(2x2)这种体系,原胞里面有2个Mn原子,那么你直接让两个Mn原子的MAGMOM的绝对值一样,符号相反就可以了,再加上ISPIN=2。
INCAR各个参数是什么意思INCAR是决定how to do的文件限于能力,只对部分最基本的一些参数(>,没有这个标志的参数都是可以不出现的)详细说明,在这里只是简单介绍这些参数的设置,详细的问题在后文具体示例中展开。
部分可能会干扰VASP运行的参数在这里被刻意隐去了,需要的同学还是请查看VASP自带的帮助文档原文。
参数列表如下:>SYSTEM name of System任务的名字***>NWRITE verbosity write-flag (how much is written)输出内容详细程度0-3缺省2如果是做长时间动力学计算的话最好选0或1(首末步/每步核运动输出)据说也可以结合shell的tail或grep命令手动输出>ISTART startjob:restart选项0-3缺省0/1 for无/有前次计算的WAVECAR(波函数)1 'restart with constant energy cut-off'2 'restart with constant basis set'3 'full restart including wave function and charge prediction'ICHARG charge: 1-file 2-atom 10-const Default:if ISTART=0 2 else 0ISPIN spin polarized calculation (2-yes 1-no) default 2MAGMOM initial mag moment / atom Default NIONS*1INIWAV initial electr wf. : 0-lowe 1-randDefault 1 only used for start jobs (ISTART=0)IDIPOL calculate monopole/dipole and quadrupolecorrections1-3只计算第一/二/三晶矢方向适于slab的计算4全部计算尤其适于就算孤立分子>PREC precession: medium, high or low(VASP.4.5+ also: normal, accurate)Default: Medium VASP4.5+采用了优化的accurate来替代high,所以一般不推荐使用high。
在线说明书整理出来的非线性磁矩和自旋轨道耦以下是从VASP合的计算说明。
非线性磁矩计算:和CHGCAR文件。
1)计算非磁性基态产生WAVECAR)然后INCAR中加上2ISPIN=2文件和CHGCAR11 !读取WAVECAR ICHARG=1 或LNONCOLLINEAR=.TRUE. MAGMOM=注意:①对于非线性磁矩计算,要在x, y 和 z方向分别加上磁矩,如MAGMOM = 1 0 0 0 1 0 !表示第一个原子在x方向,第二个原子的y方向有磁矩②在任何时候,指定MAGMOM值的前提是ICHARG=2(没有WAVECAR和CHGCAR文件)或者ICHARG=1 或11(有WAVECAR和CHGCAR文件),但是前一步的计算是非磁性的(ISPIN=1)。
磁各向异性能(自旋轨道耦合)计算:注意: LSORBIT=.TRUE. 会自动打开LNONCOLLINEAR= .TRUE.选项,且自旋轨道计算只适用于PAW赝势,不适于超软赝势。
.自旋轨道耦合效应就意味着能量对磁矩的方向存在依赖,即存在磁各向异性能(MAE),所以要定义初始磁矩的方向。
如下:LSORBIT = .TRUE.SAXIS = s_x s_y s_z (quantisation axis for spin)默认值: SAXIS=(0+,0,1),即x方向有正的无限小的磁矩,Z方向有磁矩。
要使初始的磁矩方向平行于选定方向,有以下两种方法:MAGMOM = x y z ! local magnetic moment in x,y,zSAXIS = 0 0 1 ! quantisation axis parallel to zorMAGMOM = 0 0 total_magnetic_moment ! local magnetic moment parallel to SAXIS (注意每个原子分别指定)SAXIS = x y z ! quantisation axis parallel to vector (x,y,z),如 0 0 1两种方法原则上应该是等价的,但是实际上第二种方法更精确。
一、阅读理解1. 请认真阅读下列短文,并根据所读内容在文章后表格中的空格里填入一个最恰当的单词。
注意:请将答案写在答题卡上相应题号的横线上。
每个空格只填1个单词。
Anyone who’s ever made room for a big milestone of adult life----a job, a marriage, a move----has likely shoved a friendship to the side. After all, there is no contract locking us to the other person, as in marriage, and there are no blood bonds, as in family. We choose our friends, and our friends choose us. That’s a really distinctive attribute of friendships.But modern life can become so busy that people forget to keep choosing each other. That’s when friendships fade, and there’s reason to believe it’s happening more than ever. Loneliness is on the rise, and feeling lonely has been found to increase a person’s risk of dying early by 26%----and to be even worse for the body than obesity and air pollution. Loneliness damages health in many ways, particularly because it removes the safety net of social support. “When we perceive our world as threatening, that can be associated with an increase in heart rate and blood pressure.”The solution is simple: friendship. It helps protect the brain and body from stress, anxiety and depression. “Being around trusted others, in essence, signals safety and security,” says Holt-Lunstad. A study last year found that friendships are especially beneficial later in life. Having supportive friends in old age is a stronger predictor of well-being than family ties ----suggesting that the friends you pick may be at least as important as the family you’re born into.Easy as the fix may sound, it can be difficult to keep and make friends as an adult. But research suggests that you only need between four and five close pals. If you’ve ever had a good one, you know hat you’re looking for. “The expectations of friends, once you have a mature understanding of friendship, don’t really change across the life course,” Rawlins says. “People want their close friends to be someone they can talk to and someone they can depend upon.”If you’re trying to fill a dried-up friendship pool, start by looking inward. Think back to how you met some of your very favorite friends.V olunteering on a political campaign or in a favorite spin class? Playing in a band? “Friendships are always about something,” says Rawlins. Common passions help people bond at a personal level, and they bridge people of different ages and life experiences.Whatever you’re into, someone else is too. Let your passion guide you toward people. V olunteer, for example, take a new course or join a committee at your community centers. If you like yoga, start going to classes regularly. Fellow dog lovers tend to gather at dog runs. Using apps and social media----like Facebook to find a local book club----is also a good way to find easy-going folks.Once you meet a potential future friend, then comes the scary part: inviting them to do something. “Y ou do have to put yourself out there,” says Janice McCabe, associate professor of sociology at Dartmouth College and a friendship researcher. “There’s a chance that the person will say no. But there’s also the chance they’ll say yes, and something really great could happen.”The process takes time, and you may experience false starts. Not everyone will want to put in the effort necessary to be a good friend.It’s never too late to start being a better pal. The work you put into friendships----both new and old --- will be well worth it for your health and happiness.文章大意:本文是一篇说明文。
Unit 4 The Nature of Scientific Reasoning1.Evidence for ArgumentsRead the following evidence and write down the arguments they are supporting: Argument: ________________________1)It is absolutely impossible for Copernicus to go out and record the fact that theearth moves around the sun.2)It is impossible for Kepler to work out his laws by taking enough readings andthen squaring and cubing everything in sight.3)The Royal Society will not take the notebooks with recordings of one’s lifelongobservation.Answer: Science is not a large collection of facts.2.Context and Other Clues1)Meaning: clear or deep perception of a situationContext and clue: To be able to see things clearly requires a power of mind, such as imagination or creation.Word: insight2)Meaning: record by writingContext and clue: Like those writers, scientists do not just record what they see.Word: fix3)Meaning: the earlier stage ofContext and clue: A historian does not know how a scientist starts his discovery.In this sense, the word is usually used in its plural form.Word: beginnings4)Meaning: the outside qualitiesContext and clue: Things appear differently. Only by thinking hard, using metaphors or analogies, can a scientist find the likeness. Appearance refers to natural phenomenon.Word: appearance5)Meaning: rise as if with a jumpContext and clue: He suddenly had a wild imagination.Word: leap6)Meaning: search forContext and clue: Kepler tried to work out his laws by using metaphors.Word: feel for7)Meaning: powerfully, with strong persuasivenessContext and clue: This point has been explained clearly in a fable by Karl Popper.Word: forcefully8)Meaning: narratingContext and clue: The schoolbooks do not tell the whole story.Word: account9)Meaning: occur to one’s mindContext and clue: At the sight of the fall of an apple, Newton realized that the same force of gravity might go on reaching out beyond the earth.Word: strike10)Meaning: matchContext and clue: Newton had already found the likeness, for the two things went together.Word: agree3. Vocabulary studyFill in the blanks with appropriate words.1) A better understanding of these phenomena may completely alter our ________of the nature of the universe.2)However, even solid foods will pass down the tube with the aid of _______.3)To understand galaxy formation we would lik e to think of ______.4)The _______ of free immigration was magnificent, the reality inevitably less so. Answer: conception, gravity, gravitation, concept4. True or FalseRead the following statements and decide whether they are true or false. Write down T for True and F for False.1)The readers of Balzac and Zola think that the two writers are more honest thanother writers. _____2)Science should be based on experiments, so it is not imaginative. ____3)The purpose of this passage is to reveal to the readers the essentialcharacteristics of scientific thinking. _____4)The English bishops would not like to open Joanna Southcott’s box. ____5)It can be inferred from the passage that a scientist is more imaginative thanpeople of other professions.。
大自然启发的发明英语作文Nature's Ingenious Innovations: Inventions Inspired by the Natural World.Throughout history, nature has served as an inexhaustible source of inspiration for human ingenuity. From the earliest examples of biomimicry to modern-day technological advancements, the natural world has played a pivotal role in shaping the course of human innovation.Flight.One of the most iconic examples of nature-inspired invention is the airplane. The Wright brothers' pioneering flight in 1903 was heavily influenced by their observations of bird wings. The curvature of the wings, the aerodynamic shape, and the principles of lift and drag were all carefully studied and emulated in the design of their aircraft. Today, modern passenger jets continue to draw inspiration from the flight patterns of birds, resulting inimproved aerodynamics and fuel efficiency.Adhesion.Geckoes, with their remarkable ability to cling to smooth surfaces, have inspired the development of adhesive materials. The tiny hairs on their feet, called setae, have inspired the creation of synthetic adhesives that are lightweight, durable, and reusable. These gecko-inspired adhesives have applications in various fields, including robotics, medicine, and construction.Water Repellency.The lotus leaf, with its water-repellent properties, has been a source of inspiration for raincoats, tents, and other waterproof materials. The self-cleaning surface of the lotus leaf, where water droplets form spherical beads and roll off, has been replicated in nanostructured surfaces. These surfaces exhibit superhydrophobicity, making them highly resistant to water and dirt.Energy Generation.Solar panels are another example of nature-inspired invention. The ability of plants to convert sunlight into energy through photosynthesis has inspired the development of photovoltaic cells that convert solar energy into electricity. These cells are used in solar panels to generate clean and sustainable energy.Biomaterials.Nature offers a treasure trove of materials that have inspired the development of biomedical innovations. Collagen, a protein found in animal connective tissue, has been used to create artificial skin and blood vessels. Spider silk, known for its strength and elasticity, has inspired the development of biocompatible sutures and wound dressings.Drug Discovery.The natural world is a rich source of medicinalcompounds. Penicillin, one of the most important antibiotics, was discovered by observing the ability of a mold to inhibit the growth of bacteria. Aspirin, a widely used pain reliever, was inspired by the active ingredient in willow bark. Nature continues to provide valuable leads for the development of new drugs and therapies.Sustainable Design.Nature's principles of sustainability have inspired the development of eco-friendly and sustainable technologies. Wind turbines, inspired by the flight of birds, generate clean energy from the wind. Biodegradable materials, such as polylactic acid derived from corn starch, offer alternatives to traditional plastics.Conclusion.Nature's ingenious designs have played a profound role in shaping the course of human innovation. From the flight of airplanes to the development of water-repellent materials, the natural world has served as a constantsource of inspiration and guidance. By embracing the principles and lessons from nature, we can continue to create innovative and sustainable solutions for our planet and ourselves.。
a r X i v :c o n d -m a t /0702053v 1 [c o n d -m a t .m t r l -s c i ] 2 F eb 2007On the nature of the spin-polarized hole states in a quasi-two-dimensional GaMnAsferromagnetic layerE.Dias CabralInstitute de F´ısica,Universidade do Estado do Rio de Janeiro,20.500-013Rio de Janeiro,R.J.,BrazilM. A.BoselliDepartamento de F´ısica,Universidade Federal de Ouro Preto,35400-000Ouro Preto,M.G.,BrazilA.T.da Cunha LimaUniversidade Veiga de Almeida,28905-970Cabo Frio,RJ,BrazilA.Ghazali (in memoriam)Institut des NanoSciences de Paris,75015Paris,FranceI.C.da Cunha LimaInstituto de F´ısica,Universidade do Estado do Rio de Janeiro,20.500-013Rio de Janeiro,R.J.,Brazil,and Departamento de F´ısica,Universidade Federal de Ouro Preto,35400-000Ouro Preto,M.G.,Brazil(Dated:February 6,2008)A self-consistent calculation of the density of states and the spectral density function is performed in a two-dimensional spin-polarized hole system based on a multiple-scattering ing parameters corresponding to GaMnAs thin layers,a wide range of Mn concentrations and hole densities have been explored to understand the nature,localized or extended,of the spin-polarized holes at the Fermi level for several values of the average magnetization of the Mn system.We show that,for a certain interval of Mn and hole densities,an increase on the magnetic order of the Mn ions come together with a change of the nature of the states at the Fermi level.This fact provides a delocalization of spin-polarized extended states anti-aligned to the average Mn magnetization,and a higher spin-polarization of the hole gas.These results are consistent with the occurrence of ferromagnetism with relatively high transition temperatures observed in some thin film samples and multilayered structures of this material.The possibility of having diluted magnetic semicon-ductor (DMS)nanostructures based on GaAs opens a wide range of potential applications such as integrated magneto-optoelectronic devices [1].In the Ga 1−x Mn x As alloy Mn is,in fact,a strong p dopant,the free hole con-centration reaching even 1020−21cm −3.The 3d level is half filled with five electrons,in such a way that it car-ries a spin 5¯h /2.At small Mn concentrations,the alloy is a paramagnetic insulator.As x increases it becomes ferromagnetic,going through a non-metal-to-metal tran-sition for higher concentrations,and keeping its ferro-magnetic phase.Above 7%,the alloy becomes a ferro-magnetic insulator.[2]In the metallic phase,depending on the value of x ,the temperature of the ferromagnetic transition is observed in the range of 30-160K.The oc-currence of ferromagnetism in (Ga,Mn)As thin films and (Ga,Mn)As/GaAs superlattices [3]made it clear that a deeper theoretical investigation of such nanostructures is required.Recent calculations [4,5]performed in GaM-nAs/GaAs multilayered structures show the interplay of magnetic order and spin-polarization of free carries oc-currying in these systems.In this work we study the roles of disorder and spin-polarization on determining the nature of the spin-polarized states at the Fermi level in a 2D hole system.We aim to obtain information about the nature of the states of carriers in the metallic and ferromagnetic phaseof a thin layer of GaMnAs.We obtain the density of states (DOS)by a multiple-scattering approximation in the Klauder approach [6]developed by Serre and Ghazali [7],and we consider the Zeeman splitting (ZS)equiva-lent to the separation of the spin aligned and spin anti-aligned subbands resulting of the Kondo-like interaction with the localized magnetic moments.We show that by analyzing the spectral density function (SDF)we can in-fer,for a given average magnetization,the nature of the spin-polarized states at the Fermi level.Details of the multiple-scattering treatment used by Serre and Ghazali can be found in Ref.7.The scattering potential due to a system of N i impu-rities per unit volume at sites R i is given by U (q )=ρimp (q )v (q ),where we used the Fourier transforms ofthe impurity density ρimp (r )=i δ(r −R i ),and the screened Coulomb potential due to the ionized impurity v (r ).The expansion implies in performing averages on products of impurity densities,using the technique by Kohn and Luttinger [8].The multiple scattering approx-imation consists in selecting from the self-energy inser-tions those terms consisting of the scattering which occur several times by the same impurity.This is different from the Born approximation used in systems with low con-centration of impurities,where only double scattering is considered.The multiple-scattering approach reproducesthe correct result even for the very diluted limit.After performing the configuration average the Green’s functionG(p,E)=G0(p,E)+Σ(p,E)(2π)dd d q′v(q′−q)(2π)dd d q′1v(q1−q′1)(2π)d d d q′1v(q′−q)G(p,E),at theseenergies and we observed that they have typical shapescorresponding to localized and extended states.Next we take into account the localized magnetic mo-ments at the Mn sites.We treat them assuming a ho-mogeneous magnetization.Then,we obtain an effectivemagnetic potential[4]V mag=−xD e n s i t y o f s t a t e s (a .u .)Energy (Ry*)0.00.51.01.52.00.00.10.27635421(b)S p e c t r a l D e n s i t y F u n c t i o n (a .u .)k (1/a 0*)FIG.2:Results for n s =5×1011cm −2and N i =2×1012cm −2.In part (a)curve 1represents the DOS of the anti-aligned spins,held fixed.Curves 2,3,and 4represent the shifted DOS for aligned spins due to Zeeman splittings 50,100and 150meV.The corresponding Fermi levels are indicated.In part (b),curve 1corresponds to either aligned or anti-aligned SDF at the Fermi level with zero magnetization,curves 2and 3to anti-aligned and aligned spins with a splitting of 50meV,4and 5idem for 100meV,and 6and 7for 150meV.that the Fermi level increases with the separation.The shape of the spin-dependent SDF does not change con-siderably as the average magnetization increases in the sample with a lower impurity concentration.However the sample with higher Mn concentration appearing in Fig.3shows a change in the nature of the states at the Fermi level,even for a splitting of 50meV.Curve 1corresponds to zero splitting,where the two SDF coin-cide.Curve 2corresponds to the splitting of 50meV for spin anti-aligned,curve 3for spin aligned.The former shows an extended character,the latter is localized.As the splitting increases,the SDF for anti-aligned spins be-comes sharper,while for aligned spins it spreads in the k-space,with the maximum approaching k =0.Therefore,if the impurity concentration is below a cer-tain threshold,the increase of the magnetization does not lead to a significant change in the localized character of the spin-polarized states.Above this threshold we reach conditions allowing the occurrence of extended states at the Fermi level -more properly extended states in a verydirty metal -which are very sensitive to the average mag-netization.As the average magnetization increases,a sudden change of the aligned spin states occur,becom-ing localized,the extended character of the anti-aligned spins becomes more and more pronounced,and the spin polarization of the gas of holes increases.To conclude,this work approaches the problem of the occurrence of ferromagnetism in GaMnAs thin films pointing to the entaglement of the average magnetization with the extended or localized characters of the spin-polarized hole states.We do not try to explain the oc--4-3-2-101230.000.020.040.060.08(a)43214321D e n s i t y o f s t a t e s (a .u .)Energy (Ry*)0.00.51.0 1.52.00.00.51.01.52.02.57654321(b)S p e c t r a l D e n s i t y F u n c t i o n (a .u .)k (1/a 0*)FIG.3:Same as above for n s =5×1012cm −2and N i =1×1013cm −2currence of the ferromagnetic order.But,on the other hand,we demonstrate that for a convenient range of Mn concentration the increase of the magnetic order goes to-gether with the increase of the extended character of the majority of the spin-polarized carriers.This is in agree-ment with results obtained in GaMnAs/GaAs thin quan-tum wells and multilayered structures [4,5]showing that in the metallic phase a strong spin-polarization of the “free carriers”in the regions where the Mn impurities are located is responsible for the magnetic order in that samples with relatively high transition temperatures.This work was partially supported by CNPq (ESN support and research grant),FAPEMIG,FAPESP and FAPERJ.ICCL is grateful for the hospitality of Prof.M.W.Wu group at the USTC,Hefei,Anhui,China.[1]Y.Ohno,D.K.Young,B.Beschoten,F.Matsukura,H.Ohno and D.D.Awschalom,Nature 402,790(1999).[2]F.Matsukura,H.Ohno,A.Shen,and Y.Sugawara,Phys.Rev.B 57,R2037(1998).[3]R.Mathieu,P.Svendlindh,J.Sadowski,K.Swiatek,M.Karlsteen,J.Kanski,and L.Ilver,App.Phys.Lett.81,3013(2002).[4]M.A.Boselli,I.C.da Cunha Lima,and A.Ghazali,Phys.Rev.B 68,085319(2003).[5]S.C.P.Rodrigues,L.M.R.Scolfaro,J.R.Leite,I.C.da Cunha Lima,G.M.Sipahi,M.A.Boselli,Phys.Rev.B 70,165308(2004).[6]J.R.Klauder,Ann.Phys.(N.Y.)14,43(1961).[7]J.Serre and A.Ghazali,Phys.Rev.B 28,4704(1983).[8]W.Kohn,J.M.Luttinger,Phys.Rev.108,590(1957).。