A Triumph for Theoretical Physics Medallists recognise Dirac's undeniable influence in the development of topological insulator theories (l-r) Charles L. Kane, Shoucheng Zhang and F. Duncan M. Haldane 04/07/2013 - Trieste The concept of topological insulators, one of the hottest topics in condensed matter physics today, has spurred extensive research in the realms of theoretical and experimental physics alike. The three condensed matter physicists, F. Duncan M. Haldane (Princeton University, USA), Charles L. Kane (University of Pennsylvania, USA) and Shoucheng Zhang (Stanford University, USA), whose research paved the way for advancing knowledge about topological insulators were presented their Dirac Medals at an award ceremony held on 4 July 2013 at ICTP. Topological insulators are materials that can conduct electricity at the surface but act as insulators in the interior. One fascinating aspect of the topological insulator research is that the materials were theoretically predicted before being experimentally discovered. Zhang, who theoretically predicted the first topological insulator material, mercury telluride, says, Initially, I was deeply attracted by the mathematical beauty of topological insulators. But very quickly I realized that this beautiful mathematical structure could be found in nature, in real materials. This was my 'Eureka!' moment. Paul Wiegmann (University of Chicago), who introduced the medallists to the audience at the ceremony, pointed out that the theoretical prediction preceding the experiments is an undeniable triumph for theoretical physics. Topological insulators are expected to find applications in computing and communications technology. Topological protection in real devices could solve many technological problems such as heat dissipation in semiconductor chips, making computers ever more powerful, explains Zhang. Kane, in his talk, pointed out another theoretical and exciting application--using topological insulators to create the elusive Majorana fermion--a particle that is identical to its own antiparticle; Majorana fermions have been discussed in particle physics for decades, but have so far evaded experimental verification. The foundation for the development of topological insulators was Haldane's work done in the late 1980's. I am very happy for the recognition of my old 1988 work on the two-dimensional 'zero-field' or anomalous quantum Hall effect without Landau levels, says Haldane. He adds that Kane's subsequent research with Eugene Mele on this topic lead to the explosion of theoretical and experimental interest in all things 'topological', and the fundamentally-new, three-dimensional topological insulating systems. Kane gave the audience a historic perspective on topological insulators and walked them through the story of topological insulators. I have spent a good part of my career following trails that Duncan has blazed, he said. He pointed out that receiving an honour in the name of Dirac for research on topological insulators was fitting. Dirac's profound impact on theoretical physics shows up in almost every aspect of the theory of topological insulators. Haldane drives home the point saying, Now, condensed matter physics has become invigorated with the discovery of quantum effects stemming in part from the fundamentally topological nature of the Dirac spectrum in one, two, three, and even higher spatial dimensions. Topological insulator combines beautiful mathematical structures with experimentally accessible natural phenomena. This is very fundamental for science, and is something Dirac would have liked very much, says Zhang. ICTP's Dirac Medal, first awarded in 1985, is given in honour of P.A.M. Dirac, one of the greatest physicists of the 20th century and a staunch friend of the Centre. It is awarded annually on Dirac's birthday, 8 August, to scientists who have made significant contributions to theoretical physics. The Medallists also receive a prize of US$ 5,000. The Dirac Medal is not awarded to Nobel Laureates, Fields Medallists, or Wolf Foundation Prize winners, although many Dirac Medallists have proceeded to win these prestigious prizes. For more details about the Dirac Medal, visit its webpage .
三年多前,出版社拿来一本大书 Mathematicians: an outer view of the inner world (Princeton University Press, 2009) ,是一本图文对照本——摄影家 Mariana Cook 为 92 个当代数学家拍的肖像加每个数学家自己写的一页“自白”——我没说“图文并茂”,是因为有些数学家写的自白文字实在无趣;也因为这一点,我觉得自己偶尔读几页还好,中译本可能不会有多少观众。(那会儿说得更艺术一点:“自述文字都不长,像刚出校门的同学写的简历,质朴而可爱。”)当然,也有着实有趣的片段,我曾偷过几段在博文里,题曰“心语”( http://blog.sciencenet.cn/blog-279992-249801.html )。 现在有出版社买了版权,样书要还给人家了。于是趁还书之前又偷看了几眼,就看见苏联 - 俄罗斯数学家 Israel Moiseevich Gelfand ( 1913-2009 ,与 Polya 一样的长寿数学家)讲的小故事: 30 年代, Pauli 在他著名的量子力学课本里批判了狄拉克方程,罪状是 3 条: 1) 假定存在谁也没见过的带正电荷的粒子; 2) 电子遇到正电子时会湮灭生成两个光子; 3) 两个光子能倒回去变成电子 - 正电子对。 而最后一条,简直是疯了。(泡老师似乎还客气,没说它“连错都算不上”。)遗憾的是,没过几天,正电子就被发现了。于是,泡老师改变了态度。我们现在看到的根据 1958 年版英译的 General Principle of Quantum Mechanics 已经找不到那三点影子了。(看他的书名,还是压过了老狄。) 有趣的是,泡老师的这种“现实主义”态度,似乎帮他发现了他的不相容原理——玩笑而已,没有考证过。他没见过猪跑,就说猪不存在;没见过正电子,就说老狄的方程有问题。所以,当他没见过两个相同的电子同居一处时,便自然得出结论说它们本来就不能在一起。这会儿,现实主义胜利了。 G 老回忆说,他曾有幸与老狄在匈牙利玩儿过几天,学会了很多东西。他问狄老师,狄兄啊,听了那些流言,你咋没丢了你的方程而另寻结果呢? ( Paul, why, in spite of these comments, did you not abandon your equations and continue to pursue your results? ) 狄拉克的回答很简单也很响亮: because they are beautiful. G 老自述的中心思想就是,数学是美的。数学是美丽、简单和精确的组合,而那种组合也来自古典音乐和诗歌。所以,好多数学家喜欢严肃音乐( serious music )。 从泡利与狄拉克的这个例子,我为昨天的话题再补充两点: 现实主义科学的态度是,没见过的就是不存在的; 浪漫主义科学的态度是,美的就是真的。
在实空间中粒子的波函数遵循狄拉克方程: \ 设虚空间中的波函数为:ψ(x ν )也同样遵循狄拉克方程: \ 设从实空间进入虚空间的转换矩阵为S,则有: ψ’(Λx)= Sψ(x) 代入第1个方程,并同第2个方程比较,可以得到: \ 考虑到无限次洛伦茨变换,可以获得: \ 由此可以获得对于有限次数洛伦茨变换的公式: \ 最后如果证明在虚空间中下式成立,则虚空间中狄拉克方程也是成立的,因为它符合洛伦茨协变性的要求。 \ 其中: \ \ \begin{array}{*{20}{c}} \alpha \\ \beta \\ \end{array}\] 参考文献: Srednicki M A. Quantum Field Theory . Cambridge University Press, 2007. 麦克斯韦尔方程在洛仑兹变换下的协变性及伽利略变换下的不协变_相对论吧_百度贴吧 . . http://tieba.baidu.com/p/572278131. Szabó L E. On the meaning of Lorentz covariance . (2003-08) . http://philsci-archive.pitt.edu/1322/. Nair V P. Quantum Field Theory: A Modern Perspective . Springer, 2005.
科学上的所谓 “ 圣经 ”,指极具权威、 谬误极少的专著。 物理学中 公认的 “ 圣经 ” 只有两本 : 一本是牛顿的《原理》,另一本是狄拉克的《原理》。 对狄拉克的物理学, 杨振宁 先生评论为,干净而完整;除了感叹 “ 秋水文章不染尘 ” 之外,找不出问题。彭桓武先生早年在英国和狄拉克有过交往,晚年回忆往事时,彭先生对周围的人说:怀疑狄拉克的结果常常是徒劳的 ! 他拿出一个观点之前,考虑得非常广泛而且深入,至少有一麻袋的草稿纸支持他的观点。 一、一桩公案 不过狄拉克在《原理》中引入了 球坐标下的径向动量算符 p r 这一最基本的物理学概念,导致了至少有二十篇论文讨论之,结论都认为狄拉克错了。 有一点很是怪异。最早的批判出现在 1960 年 ( R. Dicke and J. Wittke ) ,然后发生在 1966 年 (A. Messiah) ,再在 1973 年 (R. L. Liboff, I. Nebenzahl, H. H. Fleischmann) ,后来就更多了。狄拉克 1980 年代还发表了不少文章, 1984 年才辞世, 1960 年代正值壮年, 不可能不知道物理学界对他的批评。如果真是一个错误,他能至若惘然 ? 这是一桩公案 ! ———— Paul Dirac Anecdotes : “ He was making a statement ” When Paul Dirac made a rare error in an equation on the blackboard during a lecture one day, a courageous student raised his hand: "Professor Dirac," he declared, "I do not understand equation 2." When Dirac continued writing, the student, assuming that he had not been heard, raised his hand again and repeated his remark. Again Dirac merely continued writing... "Professor Dirac," another student finally interjected, "that man is asking a question." "Oh?" Dirac replied. "I thought he was making a statement." ———— 二、狄拉克的观点 如果细读狄拉克的《原理》,会发现他一定具有丰富和细腻的情感。我读的是第四版。 狄拉克非常重视一个形容词“ physical ” ( 物理的 ) ,却没有定义“ physical quantity ”或者“ physical variable ”。可能是为了给当时图像不甚清晰的量子力学理论体系提供一个完整而严谨的概念基础,狄拉克居然没有参考 von Neumann ,而是创造性地定义或引入了一些物理概念。例如: 1 、变量 (variable) :例如正则变量。 2 、动力学量 (dynamical variable) :所有经典力学量,量子力学中由线性算符表示。 3 、实动力学量 (real dynamical variable) :用自伴算符表示,但本征函数集合未必完备。 4 、可观变量 (observable) :具有谱表示的实动力学量,具有理论上的可观察性。 5 、厄米矩阵 (Hermitian matrix) 。 6 、真实的 ( 量 )(true) :狄拉克只用了一次,即他认为径向动量 p r 是和 r 共轭的“真实的动量”。 ( “ true momentum conjugate to r ” ) 。 7 、物理的 ( 量 )(physical) 。 等等。 因此,狄拉克明白无误地写下了,径向动量 p r 不仅是“ true ”还是“ real ”的动力学量。根据狄拉克的定义,它是一个自伴算符,不过本征函数的集合未必完备。 按照 目前物理学界公认的说法,狄拉克的这一观点是错误的。 我注意到,狄拉克没有说,径向动量 p r 是一个可观测量。 三、狄拉克未错之处 目前数学物理学界对径向动量算符的研究已经有些偏离物理学,更多的是数学构造。例如,有文章研究 ( p r ) 2 的 平方根算符,发现和 p r 极不相同。 我的问题是,是否狄拉克在这个问题上是否真的错了 ? 我相信,狄拉克可能并没有大错。注意到如下三个事实。 它们保证了狄拉克引入径向动量的合理性 ! 1 , 1928 年, Podolsky 就指出:动能算符可以写为 其中 为所谓的广义动量。球坐标下的结果如下: 其中 , , . 2 ,考察梯度算符在球坐标下的分解: 这意味着 ??? 嗯 ? 有问题 ! 当然,第一眼的问题,往往在第二眼时就不存在了。为什么 ? 第二眼要深刻一点。第二眼看上去就是,由于不对易性,要考虑进一步的对称化 。结果为, 这说明径向动量和 其它两个角度方向的动量有关联 ! 3 ,注意算符 p r 具有确定的期望值和不确定度。也就是它在数学上的行为还算良好 ! 四、结论 1 、 p r 不具有物理上的充分合理性。 2 、 p r 具有一定的合理性。如果不赋予物理学上的含义,仅仅作为一个数学符号,运算起来没有问题。 3 、如果要发掘 p r 的合理性,不可罔顾 三个事实 !
通过(键驰豫)理论, (DFT, TB)计算,与(XPS, workfunction, Raman,vacuum melting, TEM, AFM elasticity measuremts 等)实验的结合,我们获得如下系列自洽的结果和认知,仅供有共同志趣的同仁切磋分享: 1。 C-C 键随原子配位数的降低而自发变短(by =30%) 和增强(by =160%)。 2。 键长和键能的驰豫导致电荷、能量和质量的局域致密和钉扎以及边界非(悬)键单电子的极化. 3。 单键力常数可达1000 eV/m; 德拜温度从金刚石的2000K降至600K。 4. 体弹模量由金刚石的1.0 TPa增至 2.6 TPa;熔化温度从金刚石的3800K降至1593K.单层有效厚度为 0.142 nm。 5。 扶手椅形和重构锯齿形(5,7 原子环)边界的石墨烯的半导体特性源于准双键在最近邻(长度为d)的边界原子间的产生。 6. 锯齿形边界的石墨烯和原子空位的金属特性及狄拉克费米子的选择性产生源于边界等距(长度为sqrt(3)d)悬键电子的极化。 7. 狄拉克费米子具有非零自旋(未对电子),弱结合能(极化杂质态)、极小的有效质量和极大的群速度。 8. 由于弱作用,狄拉克费米子既不显著贡献哈密顿量又不占据哈密顿量所确立的色散关系而是狄拉克色散。 9. 氢原子与悬键电子结合成键淹没狄拉克费米子。 10. 纳米碳管可近似为无边界的石墨烯。 11. 所有这些皆起源于泡令-歌德施密特的“原子半径随配位数减少而收缩”的原理及其拓展 - 键弛豫理论。 12. 有关拉曼和选区XPS研究正在深入,结果待续。 尤其值得关注的是: 13. 边界极化态可能带来更多让人费解的新奇特性,有如超导,热电,拓扑绝缘体,等 - 期待中 。。。 14. 从键与非键的形成,断裂,振动,弛豫以及相应的电子转移,极化,局域化和致密化的动力学过程以及对材料物性的角度出发进行材料科学研究可能成为必然。 主要参考文献: http://www3.ntu.edu.sg/home/ecqsun Underneath the fascinations of carbon nanotubes and graphene nanoribbons. Energy Environmental Science, 2011; 4: 627-655. Discriminative generation and hydrogen modulation of the Dirac-Fermi polarons at graphene edges and atomic vacancies. Carbon, 2011; doi:10.1016/j.carbon.2011.04.064. Graphene nanoribbon band-gap expansion: Broken-bond-induced edge strain and quantum entrapment. Nanoscale, 2010; 2: 2160-2163. Dominance of Broken Bonds and Unpaired Nonbonding pi-Electrons in the Band Gap Expansion and Edge States Generation in Graphene Nanoribbons. J Chem Phys C, 2008; 112: 18927-18934. Coordination-Resolved C-C Bond Length and the C 1s Binding Energy of Carbon Allotropes and the Effective Atomic Coordination of the Few-Layer Graphene. J Chem Phys C, 2009; 113: 16464-16467. Dimension, strength, and chemical and thermal stability of a single C-C bond in carbon nanotubes. J. Phys. Chem. B, 2003; 107: 7544-7546.
《高等量子力学》课件 费因曼路径积分(11月29日课件) http://blog.sina.com.cn/s/blog_605a02a90100mz5a.html 貌似在本科生印象中,费因曼比狄拉克更伟大,至少名气大得多。 1999年,美国人弄的十大物理学家排行榜 The top physicists of all time The Physics World faxed and e-mailed a list of seven questions to over 250 physicists around the world. In the end we received some 130 replies. 估计回邮件的都是蛋疼的,居然有30多人不投牛顿,居然把法拉第(爱因斯坦心中的前三)给投出了十大,居然费曼在狄拉克前面,玻尔取代了普朗克高居第四,呵呵。 1 Albert Einstein 1879-1955 German/Swiss/American 119/130 votes 2 Isaac Newton 1642-1727 British 96 votes 3 James Clerk Maxwell 1831-1879 British 67 votes 4 Niels Bohr 1885-1962 Danish 47 votes 5 Werner Heisenberg 1901-1976 German 30 votes 6 Galileo Galilei 1564-1642 Italian 27 votes 7 Richard Feynman 1918-1988 American 23 votes 8= Paul Dirac 1902-1984 British 22 votes 8= Erwin Schrdinger 1887-1961 Austrian 22 votes 10 Ernest Rutherford 1871-1937 New Zealander 20 votes 11= Ludwig Boltzmann 1844-1906 Austrian, Michael Faraday 1791-1867 British, Max Planck 1858-1947 German: 16 votes each http://physicsworld.com 看到这个榜,咱也给弄迷糊了,到底谁更大呢 ?
11月02日课件 http://blog.sina.com.cn/s/blog_605a02a90100m93c.html 朱子曰:问渠那得清如许,为有源头活水来。 LJ译: Originality originates from the origin。 今天在课堂上用狄拉克确立的量子力学的数学体系证明了定态薛定谔方程,完事以后,想起了
狄拉克确立的量子力学的理论体系是建立在量子态的重叠原理的基础上的。对重叠原理没有深刻的认识,对量子力学就只能够当门外汉,或者槛外人了。 因为本学期南京大学研究生院、物理学院的《高等量子力学》课程由超星学术视频全程录像(公布时的名称《狄拉克量子力学原理教程》),将来要面对全世界的物理学专家,所以本人在讲授量子力学的基本原理时, 唯狄拉克马首是瞻 ,不敢自己胡言乱语。 通过多次仔细阅读狄拉克原著并与别的量子力学教材比较,深感讲清楚量子力学的基本原理是对所有教师的艰巨挑战。狄拉克本人在剑桥大学讲授量子力学课程时,干脆用朗读自己的原著来代替讲课,这当然不是成功的课堂教学行为。 有鉴于自己对我国理科教学的现状的了解,今年本人先期课程中概述了古希腊首创的科学传统(主要是爱因斯坦强调的 形式逻辑体系 )、文化重生与科学革命( Renaissance and scientific revolution )时期完成的科学方法的创造与历次科学革命中的 范式转变 ( Paradigm shift )。本人确认,没有这些前期准备,不具备 r eason、objectivity、logic 这三种主要科学品格和思维能力的基础,是不可能弄懂量子力学的。 课件地址: http://blog.sina.com.cn/s/articlelist_1616511657_0_1.html 课件10月19日 量子力学没有测不准原理 http://blog.sina.com.cn/s/blog_605a02a90100lx51.html 博主十分希望与科学网的各位网友交流学习量子力学的心得,欢迎批评指正。 2010年 研究生院、物理学院双语课 《狄拉克量子力学原理教程》 授课大纲 Lecture Notes on Dirac's Principles of Quantum Mechanics Chapter One Recipe to comprehend and command Quantum Mechanics: Paradigm shifts Section 1.1 Brief history of quantum physics A. Expeimental discoveries leading to quantum mechanics B. Theoretical innovations in quantum era Section 1.2 The Scientific Method A.Definition of science as given by Einstein B.Greek philosophers :From Thales to Aristotle C. Euclidean geometry and formal logical system D. Renaissance and scientific revolution: From Copernicus to Newton E. Descartes Method of Science: The four precepts F. The Cartesian geometry Section 1.3 Review of Classical Mechanics A. Einsteins critical review of Newtonian mechanics based on Descartes four precepts B. The Lagrangian mechanics C. The Hamiltonian mechanics Section 1.4 Paradigm and paradigm shifts in scientific revolutions A. Paradigm in science B. Paradigm shifts in scientific revolutions Section 1.5 Paradigm shifts: the recipe to comprehend and command QuantumMechanics A. Example one of paradigm shifts in quantum physics: Planck oscillator, from c-number to q-number and from visible physical space to abstract mathematical space B. Example two of paradigm shifts in quantum physics: The SternGerlach experiment and spin, paradigm of quantum measurement and Pauli matrix approach to two-level system Chapter Two Dirac's four axioms of Quantum Mechanics: Superposition, Observables, Canonical quantization and Equation of motion Section 2.1 Axiom I:Principle of superposition A. Definition of quantum states and the general principle of superposition B. Mathematical formulation of the principle C. Dirac's notation for vectors: the ket D. Dirac's introduction of inner product function and bra vectors E. The dual relationship between ket and bra Section 2.2 Axiom II:Principle of observables A. Linear operators (q-numbers) B. Operator operating on the bra vectors C. Conjugate relations D. Eigenvalues,eigenvectors and eigenspace E. The eigenvalue problem of Hermitian operators F. Axioms of observables in quantum mechanics and explanation of the Stern-Gerlach experiment Section 2.3 Axiom III:Quantization conditions A. Sequential Stern-Gerlach experiment again B. Commutability and compatibility C. Uncertainty relation D. Axiom of quantization conditions: Dirac canonical quantization E. Heisenberg uncertainty relation between x and p Section 2.4 Axiom IV:Equation of motion A. The Heisenberg equation of motion B. The Schrdinger equation of motion Chapter Three Dirac's three rules of manipulations in Quantum Mechanics: Representations, Transformations and Pictures Section 3.1 Representations of discrete eigenvalue spectra - matrix A. The basis of a linear vector space and the basis vectors B. The eigenvectors of Hermitian operators as orthonormal basis of Hilbert space C. The discrete eigenvalue spectra and the matrix representation or matrix mechanics D. Matrix (energy or Heisenberg) representation of Planck oscillator E. Matrix representation of spin one half and the Stern-Gerlach experiment again