Reconstructing the Initial Relaxation Time of Young Star Clusters in the Large Magellanic C

RANDOM WALK TO GRAPHENENobel Lecture, December 8, 2010byANDRE K. GEIMSchool of Phys i cs and Astronomy, The Un i vers i ty of Manchester, Oxford Road, Manchester M13 9PL, Un i ted K i ngdom.If one wants to understand the beaut i ful phys i cs of graphene, they w i ll be spo i led for cho i ce w i th so many rev i ews and popular sc i ence art i cles now ava i lable. I hope that the reader w i ll excuse me i f on th i s occas i on I recommend my own wr i t i ngs [1–3]. Instead of repeat i ng myself here, I have chosen to descr i be my tw i sty sc i ent ific road that eventually led to the Nobel Pr i ze. Most parts of th i s story are not descr i bed anywhere else, and i ts t i me-l i ne covers the per i od from my PhD i n 1987 to the moment when our 2004 paper, recogn i sed by the Nobel Comm i ttee, was accepted for publ i cat i on. The story naturally gets denser i n events and explanat i ons towards the end. Also, i t prov i des a deta i led rev i ew of pre-2004 l i terature and, w i th the benefit of h i nds i ght, attempts to analyse why graphene has attracted so much i nter-est. I have tr i ed my best to make th i s art i cle not only i nformat i ve but also easy to read, even for non-phys i c i sts.ZOMBIE MANAGEMENTMy PhD thes i s was called “Invest i gat i on of mechan i sms of transport relaxa-t i on i n metals by a hel i con resonance method”. All I can say i s that the stuff was as i nterest i ng at that t i me as i t sounds to the reader today. I publ i shed five journal papers and fin i shed the thes i s i n five years, the offic i al durat i on for a PhD at my i nst i tut i on, the Inst i tute of Sol i d State Phys i cs.Web of Sc i ence so-berly reveals that the papers were c i ted tw i ce, by co-authors only. The subject was dead a decade before I even started my PhD. However, every cloud has i ts s i lver l i n i ng, and what I un i quely learned from that exper i ence was that I should never torture research students by offer i ng them “zomb i e” projects. After my PhD, I worked as a staff sc i ent i st at the Inst i tute of M i cro-electron i cs Technology, Chernogolovka, wh i ch belongs to the Russ i an Academy of Sc i ences. The Sov i et system allowed and even encouraged jun i or staff to choose the i r own l i ne of research. After a year of pok i ng i n d i fferent d i rect i ons, I separated research-w i se from my former PhD superv i sor, V i ctor Petrashov, and started develop i ng my own n i che. It was an exper i mental system that was both new and doable, wh i ch was nearly an oxymoron, tak i ng i nto account the scarce resources ava i lable at the t i me at Sov i et researchi nst i tutes. I fabr i cated a sandw i ch cons i st i ng of a th i n metal film and a super-conductor separated by a th i n i nsulator. The superconductor served only to condense an external magnet i c field i nto an array of vort i ces, and th i s h i ghly i nhomogeneous magnet i c field was projected onto the film under i nvest i ga-t i on. Electron transport i n such a m i croscop i cally i nhomogeneous field (vary i ng on a subm i cron scale) was new research terr i tory, and I publ i shed the first exper i mental report on the subject [4], wh i ch was closely followed by an i ndependent paper from S i mon Bend i ng [5]. It was an i nterest i ng and reasonably i mportant n i che, and I cont i nued study i ng the subject for the next few years, i nclud i ng a spell at the Un i vers i ty of Bath i n 1991 as a postdoctoral researcher work i ng w i th S i mon.Th i s exper i ence taught me an i mportant lesson: that i ntroduc i ng a new exper i mental system i s generally more reward i ng than try i ng to find new phenomena w i th i n crowded areas. The chances of success are much h i gher where the field i s new. Of course, the fantast i c results one or i g i nally hopes for are unl i kely to mater i al i se, but, i n the process of study i ng any new system, someth i ng or i g i nal i nev i tably shows up.ONE MAN’S JUNK, ANOTHER MAN’S GOLDIn 1990, thanks to V i taly Ar i stov, d i rector of my Inst i tute i n Chernogolovka at the t i me, I rece i ved a s i x month v i s i t i ng fellowsh i p from the Br i t i sh Royal Soc i ety. Laurence Eaves and Peter Ma i n from Nott i ngham Un i vers i ty k i ndly agreed to accept me as a v i s i tor. S i x months i s a very short per i od for exper i mental work, and c i rcumstances d i ctated that I could only study de-v i ces read i ly ava i lable i n the host laboratory. Ava i lable were subm i cron GaAs w i res left over from prev i ous exper i ments, all done and dusted a few years earl i er. Under the c i rcumstances, my exper i ence of work i ng i n a poverty-str i cken Sov i et academy was helpful. The samples that my hosts cons i dered pract i cally exhausted looked l i ke a gold ve i n to me, and I started work i ng 100 hours per week to explo i t i t. Th i s short v i s i t led to two Phys. Rev. Letters of decent qual i ty [6,7], and I often use th i s exper i ence to tease my younger colleagues. When th i ngs do not go as planned and people start compla i n i ng, I provoke them by procla i m i ng ‘there i s no such th i ng as bad samples; there are only bad postdocs/students’. Search carefully and you w i ll always find someth i ng new. Of course, i t i s better to avo i d such exper i ences and explore new terr i tor i es, but even i f one i s fortunate enough to find an exper i mental system as new and exc i t i ng as graphene, met i culousness and perseverance allow one to progress much further.The pace of research at Nott i ngham was so relentless and, at the same t i me so i nsp i r i ng, that a return to Russ i a was not an opt i on. Sw i mm i ng through Sov i et treacle seemed no less than wast i ng the rest of my l i fe. So at the age of th i rty-three and w i th an h-i ndex of 1 (latest papers not yet publ i shed), I entered the Western job market for postdocs. Dur i ng the next four years I moved between d i fferent un i vers i t i es, from Nott i ngham to Copenhagen to Bath and back to Nott i ngham. Each move allowed me to get acqua i nted w i thyet another top i c or two, s i gn ificantly broaden i ng my research hor i zons. The phys i cs I stud i ed i n those years could be broadly descr i bed as mesoscop i c and i nvolved such systems and phenomena as two-d i mens i onal electron gases (2DEGs), quantum po i nt contacts, resonant tunnell i ng and the quantum Hall effect (QHE), to name but a few. In add i t i on, I became fam i l i ar w i th GaAlAs heterostructures grown by molecular beam ep i taxy (MBE) and i mproved my expert i se i n m i crofabr i cat i on and electron-beam l i thography, technolog i es I had started learn i ng i n Russ i a. All these elements came together to form the foundat i on for the successful work on graphene a decade later.DUTCH COMFORTBy 1994 I had publ i shed enough qual i ty papers and attended enough con-ferences to hope for a permanent academ i c pos i t i on. When I was offered an assoc i ate professorsh i p at the Un i vers i ty of N i jmegen, I i nstantly se i zed upon the chance of hav i ng some secur i ty i n my new post-Sov i et l i fe. The first task i n N i jmegen was of course to establ i sh myself. To th i s end, there was no start-up and no m i crofabr i cat i on to cont i nue any of my prev i ous l i nes of re-search. As resources, I was offered access to magnets, cryostats and electron i c equ i pment ava i lable at N i jmegen’s H i gh F i eld Magnet Laboratory, led by Jan Kees Maan. He was also my formal boss and i n charge of all the money. Even when I was awarded grants as the pr i nc i pal i nvest i gator (the Dutch fund i ng agency FOM was generous dur i ng my stay i n N i jmegen), I could not spend the money as I w i shed. All funds were d i str i buted through so-called ‘work i ng groups’ led by full professors. In add i t i on, PhD students i n the Netherlands could formally be superv i sed only by full professors. Although th i s probably sounds strange to many, th i s was the Dutch academ i c system of the 1990s. It was tough for me then. For a couple of years, I really struggled to adjust to the system, wh i ch was such a contrast to my joyful and product i ve years at Nott i ngham. In add i t i on, the s i tuat i on was a b i t surreal because outs i de the un i vers i ty walls I rece i ved a warm-hearted welcome from everyone around, i nclud i ng Jan Kees and other academ i cs.St i ll, the research opportun i t i es i n N i jmegen were much better than i n Russ i a and, eventually, I managed to surv i ve sc i ent ifically, thanks to help from abroad. Nott i ngham colleagues (i n part i cular Mohamed Hen i n i) prov i ded me w i th 2DEGs that were sent to Chernogolovka, where Sergey Dubonos, a close colleague and fr i end from the 1980s, m i crofabr i cated requested dev i ces. The research top i c I eventually found and later focused on can be referred to as mesoscop i c superconduct i v i ty. Sergey and I used m i cron-s i zed Hall bars made from a 2DEG as local probes of the magnet i c field around small superconduct i ng samples. Th i s allowed measurements of the i r magnet i sat i on w i th accuracy suffic i ent to detect not only the entry and ex i t of i nd i v i dual vort i ces but also much more subtle changes. Th i s was a new exper i mental n i che, made poss i ble by the development of an or i g i nal techn i que of ball i st i c Hall m i cromagnetometry [8]. Dur i ng the next fewyears, we explo i ted th i s n i che area and publ i shed several papers i n Nature and Phys. Rev. Letters wh i ch reported a paramagnet i c Me i ssner effect, vort i ces carry i ng fract i onal flux, vortex configurat i ons i n confined geometr i es and so on. My w i fe Ir i na Gr i gor i eva, an expert i n vortex phys i cs [9], could not find a job i n the Netherlands and therefore had plenty of t i me to help me w i th conquer i ng the subject and wr i t i ng papers. Also, Sergey not only made the dev i ces but also v i s i ted N i jmegen to help w i th measurements. We establ i shed a very product i ve modus operand i where he collected data and I analysed them w i th i n an hour on my computer next door to dec i de what should be done next.A SPELL OF LEVITYThe first results on mesoscop i c superconduct i v i ty started emerg i ng i n 1996, wh i ch made me feel safer w i th i n the Dutch system and also more i nqu i s i-t i ve. I started look i ng around for new areas to explore. The major fac i l i ty at N i jmegen’s H i gh F i eld Lab was powerful electromagnets. They were a major headache, too. These magnets could prov i de fields up to 20 T, wh i ch was somewhat h i gher than 16 to 18 T ava i lable w i th the superconduct i ng magnets that many of our compet i tors had. On the other hand, the elec-tromagnets were so expens i ve to run that we could use them only for a few hours at n i ght, when electr i c i ty was cheaper. My work on mesoscop i c super-conduct i v i ty requ i red only t i ny fields (< 0.01T), and I d i d not use the electro-magnets. Th i s made me feel gu i lty as well as respons i ble for com i ng up w i th exper i ments that would just i fy the fac i l i ty’s ex i stence. The only compet i t i ve edge I could see i n the electromagnets was the i r room temperature (T) bore. Th i s was often cons i dered as an extra d i sadvantage because research i n condensed matter phys i cs typ i cally requ i res low, l i qu i d-hel i um T. The con-trad i ct i on prompted me, as well as other researchers work i ng i n the lab, to ponder on h i gh-field phenomena at room T. Unfortunately, there were few to choose from.Eventually, I stumbled across the mystery of so-called magnet i c water. It i s cla i med that putt i ng a small magnet around a hot water p i pe prevents format i on of scale i ns i de the p i pe. Or i nstall such a magnet on a water tap, and your kettle w i ll never suffer from chalky depos i ts. These magnets are ava i lable i n a great var i ety i n many shops and on the i nternet. There are also hundreds of art i cles wr i tten on th i s phenomenon, but the phys i cs beh i nd i t rema i ns unclear, and many researchers are scept i cal about the very ex i stence of the effect [10]. Over the last fifteen years I have made several attempts to i nvest i gate “magnet i c water” but they were i nconclus i ve, and I st i ll have noth i ng to add to the argument. However, the ava i lab i l i ty of ultra-h i gh fields i n a room T env i ronment i nv i ted lateral th i nk i ng about water. Bas i cally, i f magnet i c water ex i sted, I thought, then the effect should be clearer i n 20 T rather than i n typ i cal fields of <0.1 T created by standard magnets.W i th th i s i dea i n m i nd and, allegedly, on a Fr i day n i ght, I poured water i ns i de the lab’s electromagnet when i t was at i ts max i mum power. Pour i ngwater i n one's equ i pment i s certa i nly not a standard sc i ent ific approach, and I cannot recall why I behaved so ‘unprofess i onally’. Apparently, no one had tr i ed such a s i lly th i ng before, although s i m i lar fac i l i t i es ex i sted i n several places around the world for decades. To my surpr i se, water d i d not end up on the floor but got stuck i n the vert i cal bore of the magnet. Humberto Carmona, a v i s i t i ng student from Nott i ngham, and I played for an hour w i th the water by break i ng the blockage w i th a wooden st i ck and chang i ng the field strength. As a result, we saw balls of lev i tat i ng water (F i gure 1). Th i s was awesome. It took l i ttle t i me to real i se that the phys i cs beh i nd was good old d i amagnet i sm. It took much longer to adjust my i ntu i t i on to the fact that the feeble magnet i c response of water (~10–5), b i ll i ons of t i mes weaker than that of i ron, was suffic i ent to compensate the earth’s grav i ty. Many colleagues, i nclud i ng those who worked w i th h i gh magnet i c fields all the i r l i ves, were flabbergasted, and some of them even argued that th i s was a hoax.I spent the next few months demonstrat i ng magnet i c lev i tat i on to colleagues and v i s i tors, as well as try i ng to make a ‘non-boffin’i llustrat i on for th i s beaut i ful phenomenon. Out of the many objects that we had float i ng i ns i de the magnet, i t was the i mage of a lev i tat i ng frog (F i gure 1) that started the med i a hype. More i mportantly, though, beh i nd all the med i a no i se, th i s i mage found i ts way i nto many textbooks. However qu i rky, i t has become a beaut i ful symbol of ever-present d i amagnet i sm, wh i ch i s no longer perce i ved to be extremely feeble. Somet i mes I am stopped at conferences by people excla i m i ng “I know you! Sorry, i t i s not about graphene. I start my lectures w i th show i ng your frog. Students always want to learn how i t could fly.” The frog story, w i th some i ntr i cate phys i cs beh i nd the stab i l i ty of d i amagnet i c lev i tat i on, i s descr i bed i n my rev i ew i n Phys i cs Today [11].F i gure 1. Lev i tat i ng moments i n N i jmegen. Left – Ball of water (about 5 cm i n d i ameter) freely floats i ns i de the vert i cal bore of an electromagnet. R i ght – The frog that learned to fly. Th i s i mage cont i nues to serve as a symbol show i ng that magnet i sm of ‘nonmagnet i c th i ngs’, i nclud i ng humans, i s not so negl i g i ble. Th i s exper i ment earned M i chael Berry and me the 2000 Ig Nobel Pr i ze. We were asked first whether we dared to accept th i s pr i ze, and I take pr i de i n our sense of humour and self-deprecat i on that we d i d.FRIDAY NIGHT EXPERIMENTSThe lev i tat i on exper i ence was both i nterest i ng and add i ct i ve. It taught me the i mportant lesson that pok i ng i n d i rect i ons far away from my i mmed i ate area of expert i se could lead to i nterest i ng results, even i f the i n i t i al i deas were extremely bas i c. Th i s i n turn i nfluenced my research style, as I started mak i ng s i m i lar exploratory detours that somehow acqu i red the name ‘Fr i day n i ght exper i ments’. The term i s of course i naccurate. No ser i ous work can be accompl i shed i n just one n i ght. It usually requ i res many months of lateral th i nk i ng and d i gg i ng through i rrelevant l i terature w i thout any clear i dea i n s i ght. Eventually, you get a feel i ng – rather than an i dea – about what could be i nterest i ng to explore. Next, you g i ve i t a try, and normally you fa i l. Then, you may or may not try aga i n. In any case, at some moment you must dec i de (and th i s i s the most d i fficult part) whether to cont i nue further efforts or cut losses and start th i nk i ng of another exper i ment. All th i s happens aga i nst the backdrop of your ma i n research and occup i es only a small part of your t i me and bra i n.Already i n N i jmegen, I started us i ng lateral i deas as under- and post-graduate projects, and students were always exc i ted to buy a p i g i n a poke. Kostya Novoselov, who came to N i jmegen as a PhD student i n 1999, took part i n many of these projects. They never lasted for more than a few months, i n order not to jeopard i se a thes i s or career progress i on. Although the enthus i asm i nev i tably van i shed towards the end, when the pred i ctable fa i lures mater i al i sed, some students later confided that those exploratory detours were i nvaluable exper i ences.Most surpr i s i ngly, fa i lures somet i mes fa i led to mater i al i se. Gecko tape i s one such example. Acc i dentally or not, I read a paper descr i b i ng the mechan i sm beh i nd the amaz i ng cl i mb i ng ab i l i ty of geckos [12]. The phys i cs i s rather stra i ghtforward. Gecko’s toes are covered w i th t i ny ha i rs. Each ha i r attaches to the oppos i te surface w i th a m i nute van der Waals force (i n the nN range), but b i ll i ons of ha i rs work together to create a form i dable attract i on suffic i ent to keep geckos attached to any surface, even a glass ce i l i ng. In part i cular, my attent i on was attracted by the spat i al scale of the i r ha i rs. They were subm i cron i n d i ameter, the standard s i ze i n research on mesoscop i c phys i cs. After toy i ng w i th the i dea for a year or so, Sergey Dubonos and I came up w i th procedures to make a mater i al that m i m i cked a gecko’s ha i ry feet. He fabr i cated a square cm of th i s tape, and i t exh i b i ted notable adhes i on [13]. Unfortunately, the mater i al d i d not work as well as a gecko’s feet, deter i orat i ng completely after a couple of attachments. St i ll, i t was an i mportant proof-of-concept exper i ment that i nsp i red further work i n the field. Hopefully, one day someone w i ll develop a way to repl i cate the h i erarch i cal structure of gecko’s setae and i ts self-clean i ng mechan i sm. Then gecko tape can go on sale.BETTER TO BE WRONG THAN BORINGWh i le prepar i ng for my lecture i n Stockholm, I comp i led a l i st of my Fr i day n i ght exper i ments. Only then d i d I real i se a stunn i ng fact. There were two dozen or so exper i ments over a per i od of approx i mately fifteen years and, as expected, most of them fa i led m i serably. But there were three h i ts: lev i tat i on, gecko tape and graphene. Th i s i mpl i es an extraord i nary success rate: more than 10%. Moreover, there were probably near-m i sses, too. For example, I once read a paper [14] about g i ant d i amagnet i sm i n FeGeSeAs alloys, wh i ch was i nterpreted as a s i gn of h i gh-T superconduct i v i ty. I asked Lamarches for samples and got them. Kostya and I employed ball i st i c Hall magnetometry to check for g i ant d i amagnet i sm but found noth i ng, even at 1 K. Th i s happened i n 2003, well before the d i scovery of i ron pn i ct i de superconduct i v-i ty, and I st i ll wonder whether there were any small i nclus i ons of a supercon-duct i ng mater i al wh i ch we m i ssed w i th our approach. Another m i ss was an attempt to detect “heartbeats” of i nd i v i dual l i v i ng cells. The i dea was to use 2DEG Hall crosses as ultrasens i t i ve electrometers to detect electr i cal s i gnals due to phys i olog i cal act i v i ty of i nd i v i dual cells. Even though no heartbeats were detected wh i le a cell was al i ve, our sensor recorded huge voltage sp i kes at i ts “last gasp” when the cell was treated w i th excess alcohol [15]. Now I attr i bute th i s near-m i ss to the unw i se use of yeast, a very dormant m i cro-organ i sm. Four years later, s i m i lar exper i ments were done us i ng embryon i c heart cells and – what a surpr i se – graphene sensors, and they were successful i n detect i ng such b i oelectr i cal act i v i ty [16].Frankly, I do not bel i eve that the above success rate can be expla i ned by my lateral i deas be i ng part i cularly good. More l i kely, th i s tells us that pok i ng i n new d i rect i ons, even randomly, i s more reward i ng than i s generally perce i ved. We are probably d i gg i ng too deep w i th i n establ i shed areas, leav i ng plenty of unexplored stuff under the surface, just one poke away. When one dares to try, rewards are not guaranteed, but at least i t i s an adventure.THE MANCUNIAN WAYBy 2000, w i th mesoscop i c superconduct i v i ty, d i amagnet i c lev i tat i on and four Nature papers under my belt, I was well placed to apply for a full professorsh i p. Colleagues were rather surpr i sed when I chose the Un i vers i ty of Manchester, decl i n i ng a number of seem i ngly more prest i g i ous offers. The reason was s i mple.M i ke Moore, cha i rman of the search comm i ttee, knew my w i fe Ir i na when she was a very successful postdoc i n Br i stol rather than my co-author and a part-t i me teach i ng lab techn i c i an i n N i jmegen. He suggested that Ir i na could apply for the lecturesh i p that was there to support the professorsh i p. After s i x years i n the Netherlands, the i dea that a husband and w i fe could offic i ally work together had not even crossed my m i nd. Th i s was the dec i s i ve factor. We apprec i ated not only the poss i b i l i ty of sort i ng out our dual career problems but also felttouched that our future colleagues cared. We have never regretted the move.So i n early 2001, I took charge of several d i lap i dated rooms stor i ng anc i ent equ i pment of no value, and a start-up grant of £100K. There were no central fac i l i t i es that I could explo i t, except for a hel i um l i quefier. No problem. I followed the same rout i ne as i n N i jmegen, comb i n i ng help from other places, espec i ally Sergey Dubonos. The lab started shap i ng up surpr i s i ngly qu i ckly. W i th i n half a year, I rece i ved my first grant of £500K, wh i ch allowed us to acqu i re essent i al equ i pment. Desp i te be i ng consumed w i th our one year old daughter, Ir i na also got her start i ng grant a few months later. We i nv i ted Kostya to jo i n us as a research fellow (he cont i nued to be offic i ally reg i stered i n N i jmegen as a PhD student unt i l 2004 when he defended h i s thes i s there). And our group started generat i ng results that led to more grants that i n turn led to more results.By 2003 we publ i shed several good-qual i ty papers i nclud i ng Nature, Nature Mater i als and Phys. Rev. Letters, and we cont i nued beefing up the labora-tory w i th new equ i pment. Moreover, thanks to a grant of £1.4M (research i nfrastructure fund i ng scheme masterm i nded by the then sc i ence m i n i ster Dav i d Sa i nsbury), Ern i e H i ll from the Department of Computer Sc i ences and I managed to set up the Manchester Centre for Mesosc i ence and Nanotechnology. Instead of pour i ng the w i ndfall money i nto br i cks-and-mortar, we ut i l i sed the ex i st i ng clean room areas (~250 m2) i n Computer Sc i ences. Those rooms conta i ned obsolete equ i pment, and i t was thrown away and replaced w i th state-of-the-art m i crofabr i cat i on fac i l i t i es, i nclud i ng a new electron-beam l i thography system. The fact that Ern i e and I are most proud of i s that many groups around the world have more expens i ve fac i l i t i es but our Centre has cont i nuously, s i nce 2003, been produc i ng new structures and dev i ces. We do not have a posh horse here that i s for show, but rather a draft horse that has been work i ng really hard.Whenever I descr i be th i s exper i ence to my colleagues abroad, they find i t d i fficult to bel i eve that i t i s poss i ble to establ i sh a fully funct i onal labora-tory and a m i crofabr i cat i on fac i l i ty i n less than three years and w i thout an astronom i cal start-up grant. If not for my own exper i ence, I would not bel i eve i t e i ther. Th i ngs progressed unbel i evably qu i ckly. The Un i vers i ty was support i ve, but my greatest thanks are reserved spec ifically for the respons i ve mode of the UK Eng i neer i ng and Phys i cal Sc i ences Research Counc i l (EPSRC). The fund i ng system i s democrat i c and non-xenophob i c. Your pos i t i on i n an academ i c h i erarchy or an old-boys network counts for l i ttle. Also, ‘v i s i onary i deas’ and grand prom i ses to ‘address soc i al and econom i c needs’ play l i ttle role when i t comes to the peer rev i ew. In truth, the respons i ve mode d i str i butes i ts money on the bas i s of a recent track record, whatever that means i n d i fferent subjects, and the fund i ng normally goes to researchers who work both effic i ently and hard. Of course, no system i s perfect, and one can always hope for a better one. However, paraphras i ng W i nston Church i ll, the UK has the worst research fund i ng system, except for all the others that I am aware of.THREE LITTLE CLOUDSAs our laboratory and Nanotech Centre were shap i ng up, I got some spare t i me for th i nk i ng of new research detours. Gecko tape and the fa i led attempts w i th yeast and quas i-pn i ct i des took place dur i ng that t i me. Also, Serge Morozov, a sen i or fellow from Chernogolovka, who later became a regular v i s i-tor and i nvaluable collaborator, wasted h i s first two v i s i ts on study i ng magnet i c water. In the autumn of 2002, our first Manchester PhD student, Da J i ang, arr i ved, and I needed to i nvent a PhD project for h i m. It was clear that for the first few months he needed to spend h i s t i me learn i ng Engl i sh and gett i ng acqua i nted w i th the lab. Accord i ngly, as a starter, I suggested to h i m a new lateral exper i ment. It was to make films of graph i te ‘as th i n as poss i ble’ and, i f successful, I prom i sed we would then study the i r ‘mesoscop i c’ propert i es. Recently, try i ng to analyse how th i s i dea emerged, I recalled three badly shaped thought clouds.One cloud was a concept of ‘metall i c electron i cs’. If an external electr i c field i s appl i ed to a metal, the number of charge carr i ers near i ts surface changes, so that one may expect that i ts surface propert i es change, too. Th i s i s how modern sem i conductor electron i cs works. Why not use a metal i nstead of s i l i con? As an undergraduate student, I wanted to use electr i c field effect (EFE) and X-ray analys i s to i nduce and detect changes i n the latt i ce constant. It was naïve because s i mple est i mates show that the effect would be negl i g i ble. Indeed, no d i electr i c allows fields much h i gher than 1V/nm, wh i ch translates i nto max i mum changes i n charge carr i er concentrat i on n at the metal surface of about 1014 per cm2. In compar i son, a typ i cal metal (e.g., Au) conta i ns ~1023 electrons per cm3 and, even for a 1 nm th i ck film, th i s y i elds relat i ve changes i n n and conduct i v i ty of ~1%, leav i ng as i de much smaller changes i n the latt i ce constant.Prev i ously, many researchers asp i red to detect the field effect i n metals. The first ment i on i s as far back as 1902, shortly after the d i scovery of the electron. J. J. Thomson (1906 Nobel Pr i ze i n Phys i cs) suggested to Charles Mott, the father of Nev i ll Mott (1977 Nobel Pr i ze i n Phys i cs), to look for the EFE i n a th i n metal film, but noth i ng was found [17]. The first attempt to measure the EFE i n a metal was recorded i n sc i ent ific l i terature i n 1906 [18]. Instead of a normal metal, one could also th i nk of sem i metals such as b i smuth, graph i te or ant i mony wh i ch have a lot fewer carr i ers. Over the last century, many researchers used B i films (n ~1018 cm–3) but observed only small changes i n the i r conduct i v i ty [19,20]. Aware of th i s research area and w i th exper i ence i n GaAlAs heterostructures, I was cont i nuously, albe i t casually, look i ng for other cand i dates, espec i ally ultra-th i n films of superconductors i n wh i ch the field effect can be ampl ified i n prox i m i ty to the superconduct i ng trans i t i on [21,22]. In N i jmegen, my enthus i asm was once sparked by learn i ng about nm-th i ck Al films grown by MBE on top of GaAlAs heterostructures but, after est i mat i ng poss i ble effects, I dec i ded that the chances of success were so poor i t was not worth try i ng.Carbon nanotubes were the second cloud hang i ng around i n the late。

回弹效应英语You are correct, I am ChatGPT, a large language model trained by OpenAI, based on the GPT-3.5 architecture. I will do my best to write a high-quality 700-word article on the topic of the rebound effect in English, following your instructions.The Rebound Effect: Exploring the Paradox of Efficiency。

Efficiency is often touted as the holy grail of modern society, a panacea for the world's problems. We strive to optimize our processes, streamline our operations, and squeeze every last drop of productivity out of our resources. However, the pursuit of efficiency can sometimes lead to unintended consequences, a phenomenon known as the rebound effect.The rebound effect, also known as the Jevons paradox, is a concept that challenges the assumption that increased efficiency automatically leads to reduced resource consumption. In fact, the opposite can often be true. As efficiency improvements lower the cost or increase the availability of a resource, the demand for that resource may actually increase, offsetting or even negating the expected savings.This counterintuitive effect was first observed by the English economist William Stanley Jevons in the 19th century. Jevons noted that as the efficiency of coal-fired steam engines improved, the overall consumption of coal increased, rather than decreased as one might have expected. This was because the improved efficiency made coal-powered technologies more accessible and affordable, leading to a surge in their adoption and usage.The rebound effect can manifest in various forms, from direct rebound effects, where the increased efficiency of a specific product or service leads to increased consumption of that product or service, to indirect rebound effects, where the savings from one area are redirected to increased consumption in other areas.For example, consider the case of fuel-efficient vehicles. When cars become more fuel-efficient, the cost of driving per mile decreases, potentially leading to an increase inthe number of miles driven, as the perceived cost of driving is lower. This direct rebound effect can partially or even fully offset the expected fuel savings from the improved efficiency.Similarly, the indirect rebound effect can come into play when the money saved from fuel-efficient vehicles is used to purchase other goods or services, which in turn consume resources and generate emissions, negating the initial environmental benefits.The rebound effect is not limited to the realm of energy and resource consumption. It can also be observed in other areas, such as the adoption of more energy-efficient appliances or the implementation of water-saving technologies. In each case, the increased efficiency can lead to a surge in demand, undermining the intended environmental or resource-saving goals.Recognizing and addressing the rebound effect is crucial for policymakers, businesses, and individuals who seek to achieve meaningful and lasting sustainability. Simply improving efficiency is not enough; we must also consider the broader systemic implications and find ways to mitigate the rebound effect.One approach is to couple efficiency improvements with other policy measures, such as pricing mechanisms, regulations, or behavioral interventions, to ensure that the gains from efficiency are not entirely offset by increased consumption. For example, a carbon tax or cap-and-trade system can help maintain the incentive for efficiency while also discouraging excessive consumption.Additionally, a shift in mindset from a focus on efficiency to a focus on sufficiency, where the emphasis is on meeting our needs with the least possible resource use, can help address the rebound effect. This may involve rethinking our consumption patterns, embracing more sustainable lifestyles, and finding ways to satisfy our needs without relying solely on technological solutions.In conclusion, the rebound effect serves as a cautionary tale, reminding us that the pursuit of efficiency alone is not a panacea for addressing environmental and resource challenges. By understanding and addressing the rebound effect, we can strive for a moreholistic and effective approach to sustainability, one that recognizes the complex interplay between efficiency, consumption, and the broader systems in which we operate.。

In the modern world,where the hustle and bustle of life often leaves little room for solitude,there are those who find solace and joy in the embrace of loneliness.This essay explores the reasons why some individuals relish the quiet moments of being alone and the benefits they derive from it.The Appeal of Solitude1.Introspection and SelfDiscovery:For many,solitude is a time for deep reflection.It allows individuals to delve into their thoughts,evaluate their life choices,and understand their true desires.This introspection can lead to personal growth and a clearer sense of self.2.Creativity and Inspiration:Solitude can be a fertile ground for creativity.Many artists, writers,and musicians find that being alone allows them to tap into their imagination and produce their best work.The absence of distractions can lead to a heightened state of focus and inspiration.3.Recharging and Relaxation:In a society that often demands constant interaction, solitude provides a muchneeded break.Its an opportunity to recharge ones mental and emotional batteries,to relax and rejuvenate without the pressure of social expectations.4.Freedom and Autonomy:Being alone means not having to conform to the expectations or desires of others.It offers a sense of freedom,allowing individuals to make decisions based solely on their own preferences and to pursue their interests without judgment.The Benefits of Enjoying Loneliness1.Improved Mental Health:Solitude can be therapeutic,helping to reduce stress and anxiety.It provides a safe space for individuals to process their emotions and develop coping mechanisms for dealing with lifes challenges.2.Enhanced Productivity:Without the interruptions that come with social interactions, individuals can be more productive when they are alone.They can focus on tasks without distraction,leading to better results and a sense of accomplishment.3.Strengthened Relationships:Paradoxically,enjoying time alone can actually strengthen relationships.It allows individuals to develop a strong sense of self,which can lead to healthier,more balanced interactions with others.4.Increased SelfReliance:Solitude fosters selfreliance and independence.It teachesindividuals to rely on their own resources and to find satisfaction in their own company, which can be empowering and liberating.ConclusionWhile the concept of enjoying loneliness may seem counterintuitive in a world that often equates happiness with social engagement,it is a testament to the diverse ways in which individuals find contentment.For some,the quiet moments of solitude are not just a respite from the world but a source of joy and fulfillment.It is a reminder that happiness can be found in the most unexpected places,and that sometimes,the company of ones own thoughts can be the most enriching of all.。

澄清你的需求英语作文Title: Clarifying Your Requirements。

In the process of communicating your needs, clarity is paramount. Whether you're outlining project specifications, articulating expectations, or detailing instructions, precision in language ensures mutual understanding and effective execution. Crafting a comprehensive explanation not only fosters clear communication but also minimizes the potential for misunderstandings or errors. Here, we delve into strategies for enhancing clarity in expressing your requirements.Firstly, it's essential to delineate the core objectives. Clearly stating what you aim to achieve provides a foundational framework for subsequent discussions. Whether it's a business proposal, academic assignment, or personal endeavor, articulating the primary goals sets the direction for all involved parties. For instance, if you're commissioning a marketing campaign,specifying targets such as increased brand awareness, customer engagement, or sales conversion clarifies the overarching purpose.Moreover, specificity breeds understanding. Instead of vague assertions, precise descriptions elucidate thedesired outcomes. Whether it pertains to technical specifications, project milestones, or performance metrics, providing granular details leaves little room for ambiguity. For example, instead of requesting "high-quality content," delineate criteria such as word count, tone, style, and key messaging points to convey your expectations effectively.Furthermore, establishing clear parameters is imperative. Defining constraints regarding budget, timeline, resources, and scope delineates the boundaries within which the project operates. Explicitly outlining limitations prevents scope creep, budget overruns, or timeline extensions. For instance, specifying a deadline ensures timely delivery, while delineating budgetary constraints informs decision-making regarding resource allocation.Additionally, fostering open dialogue cultivates a shared understanding. Encouraging questions, seeking feedback, and addressing concerns promote collaborative engagement. Effective communication is not a one-way street but a reciprocal exchange where both parties contribute insights and perspectives. By soliciting input and actively listening to stakeholders, you gain valuable insights that refine and enrich your requirements.Furthermore, leveraging visual aids enhances comprehension. Whether it's flowcharts, diagrams, wireframes, or prototypes, visual representations elucidate complex concepts and facilitate alignment. Visual aids transcend language barriers and cater to diverse learning styles, fostering a deeper understanding of the requirements. Integrating visual elements into your communication arsenal enhances clarity and fosters consensus among stakeholders.Moreover, documenting requirements formalizes agreements and mitigates disputes. Transcribing discussions, decisions, and action items into written form provides areference point for all parties involved. Detailed documentation serves as a roadmap guiding the project from conception to completion. Additionally, it serves as a safeguard against misunderstandings or discrepancies, ensuring alignment with the agreed-upon objectives.Lastly, embracing adaptability acknowledges the fluid nature of requirements. Recognizing that circumstances may evolve necessitates flexibility in approach. While clarity is essential, rigid adherence to initial specifications may impede progress or overlook emerging opportunities. Embracing agility enables you to pivot in response to changing dynamics, ensuring that your requirements remain relevant and responsive to evolving needs.In conclusion, clarity in expressing your requirements is fundamental to effective communication and successful outcomes. By delineating objectives, providing specificity, establishing parameters, fostering dialogue, leveraging visual aids, documenting agreements, and embracing adaptability, you enhance understanding and alignment among stakeholders. Clear requirements serve as the cornerstoneof collaborative endeavors, guiding projects towards fruition and fostering mutual satisfaction.。

具体技术-restart⽂献阅读EvaluatingCDCLRestartSchemes Evaluating CDCL Restart SchemesPublished: March 15, 2019and本⽂可以看做是对之前重启策略的⼀个回顾、确认及展望。

AbstractModern CDCL (conflict-driven clause learning) SAT solvers are used for many practical applications. One of thekey ingredients of state-of-the-art CDCL solvers are efficient restart schemes. The main contribution of this workis an extensive empirical evaluation of various restart strategies.译⽂：对各种重启策略进⾏⼴泛的实证评估。

We show that optimal static restart intervals are not only correlated with the satisfiability status of a certaininstance, but also with the more specific problem class of the given benchmark. We further compare uniformrestart intervals with the performance of non-uniform restart schemes, such as Luby restarts. Finally, we revisitthe dynamic restart strategy used in Glucose and propose a new variant thereof, which is based on the conceptof exponential moving averages. The resulting implementation in Lingeling improves state-of-the-artperformance in SAT solving.ConclusionIn this paper, we provided an extensive empirical evaluation of difffferent restart strategies in the context ofmodern CDCL solvers.We fifirst looked at static restart schemes.Our results show that, for uniform restart policies, the optimal interval size not only depends on thesatisfifiability status of a given instance, but also on the specifific problem class. In particular, oursolver version static-256 was the best performing one, regarding intervals with fifixed size.Comparing those results with non-uniform restart schemes, it turned out that previously most successfulstrategies, such as Luby restarts, did not give an additional benefifit in combination with the current state-of-the-art solver Lingeling on recent competition benchmarks. Our best non-uniform version, luby-02, solvedexactly as many instances as the best uniform one. Interestingly, the equally well-known inner/outerscheme actually performed worse. This emphasizes the need for occasional re-evaluation of well-knowntechniques in consideration of the steady changes made in modern CDCL solvers due to the ongoingdevelopment.In a second part, we revisited the Glucose restart scheme, being the currently most successful dynamicstrategy.Since solvers that used Glucose restarts were able to solve a substantial number of instances in the SATcompetition 2014 that Lingeling version sc14ayv was not able to solve, we implemented a similar strategyin our new version average. Our experimental results show, that this version of Lingeling signifificantlyoutperforms all versions with static restart schemes, as well as the SAT competition version of Glucose.We also gave a formalization of the Glucose restart strategy in the context of moving averages, widely usedin statistics. We argued that the original implementation of Glucose restarts is actually a combination ofsimple moving averages with cumulative moving averages. We then proposed to use exponential movingaverages, because of several desirable properties. In particular, exponential moving averages do notrequire the implementation of a queue and, more important, gradually smooth the inflfluence of earliervalues. 译⽂：指数移动平均不需要队列的实现，更重要的是，逐渐平滑早期值的影响。