DOCSLIB.ORG

Introduction to Superconductivity Theory

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Introduction to Superconductivity Theory Ecole GDR MICO June, 2010 Introduction to Superconductivity Theory Indranil Paul [email protected] www.neel.cnrs.fr Free Electron System εεε k 2 µµµ Hamiltonian H = ∑∑∑i pi /(2m) - N, i=1,...,N . µµµ is the chemical potential. N is the total particle number. 0 k k Momentum k and spin σσσ are good quantum numbers. F εεε H = 2 ∑∑∑k k nk . The factor 2 is due to spin degeneracy. εεε 2 µµµ k = ( k) /(2m) - , is the single particle spectrum. nk T=0 εεεk/(k BT) nk is the Fermi-Dirac distribution. nk = 1/[e + 1] 1 T ≠≠≠ 0 The ground state is a filled Fermi sea (Pauli exclusion). µµµ 1/2 Fermi wave-vector kF = (2 m ) . 0 kF k Particle- and hole- excitations around kF have vanishingly 2 2 low energies. Excitation spectrum EK = |k – kF |/(2m). Finite density of states at the Fermi level. Ek 0 k kF Fermi Sea hole excitation particle excitation Metals: Nearly Free “Electrons” The electrons in a metal interact with one another with a short range repulsive potential (screened Coulomb). The phenomenological theory for metals was developed by L. Landau in 1956 (Landau Fermi liquid theory). This system of interacting electrons is adiabatically connected to a system of free electrons. There is a one-to-one correspondence between the energy eigenstates and the energy eigenfunctions of the two systems. Thus, for all practical purposes we will think of the electrons in a metal as non-interacting fermions with renormalized parameters, such as m →→→ m*. (remember Thierry’s lecture) CV (i) Specific heat (C V): At finite-T the volume of πππ 2∆∆∆ 2 ∆∆∆ excitations ∼∼∼ 4 kF k, where Ek ∼∼∼ ( kF/m) k ∼∼∼ kBT. This gives free energy F ∝∝∝ –T2. γγγ CV = T. T χχχ (ii) Landau Diamagnetism ( L): A uniform magnetic field H affects the orbital 2 motion of the electrons (Lorentz force). H = ∑∑∑i (p i – eA/c) /(2m). χχχ χχχ 2 πππ2 2 M = L H, L = - (e kF)/(12 mc ). M is anti-parallel to H. Real metals are weakly diamagnetic . ρρρ (iii) Finite resistivity (ρρρ): Metals carry current following (T) Ohm’s law E = ρρρ J. There is a corresponding voltage drop V. Usually ρρρ increases with temperature . T (remember Kamran’s lecture) A quick reminder: magnets, paramagnets & diamagnets B = H + 4 πππM (constitutive relation). H is the external magnetic field. M is the magnetization in a material in response to H. χχχ (dM/dH) is the magnetic susceptibility ( m). B is the “net” magnetic field in the system. Also called magnetic field induction. In a non-magnetic material (such as vacuum, M=0 no matter what) B = H. (a) Magnet: M ≠≠≠ 0 even when H is zero. E.g.: Fe, Ni (ferromagnet) and Cr (antiferromagnet). χχχ (b) Paramagnet: when m is positive. In response to H there is a M in the same direction. Most metals are here. χχχ (c) Diamagnet: when m is negative. In response to H there is a M in the opposite direction. Eg: Bismuth. Electrons have two sources of M: (i) electron spin [Zeeman term H(n ↑↑↑ – n⇓⇓⇓)] (ii) orbital contribution: p →→→ (p – eA/c) (remember Lorentz force). Discovery of Superconductivity Nobel Prize 1913 In 1908 Heike Kamerlingh Onnes liquified He. In 1911 he discovered superconductivity in Hg. For temperatures below Tc ≈≈≈ 4K the resistance goes to zero. Can we think of a superconductor as an ideal conductor for which ρρρ =0? The answer is NO. The response of a superconductor to a magnetic field (Meissner effect) is different from that of an ideal conductor. Maxwell’s Equations 1. ∇∇∇·∇···E = 4 πππ n (Gauss’s law) 2. ∇∇∇××× E = - ∂∂∂ B/(c ∂∂∂ t) (Faraday’s law of induction) 3. ∇∇∇·∇···B = 0 (no magnetic-monopole) 4. ∇∇∇××× B = (4 πππ/c) J + ∂∂∂ E/(c ∂∂∂ t) (Ampere’s law + Maxwell’s correction) Conductors in a Magnetic Field A real conductor in a magnetic field A magnetic field induces a screening current [Faraday’s’ law, ∇∇∇××× E = - ∂∂∂ B/(c ∂∂∂ t) ]. In a real conductor the screening current decays, and in equilibrium the magnetic field penetrates almost entirely into the metal (weak diamagnetism). An ideal conductor in a magnetic field E = ρρρ J = 0. Thus, ∂∂∂ B/(c ∂∂∂ t) = - ∇∇∇××× E =0. B = constant, inside an ideal conductor. The final state depends on whether the system was cooled below Tc in the presence/absence of a magnetic field. Superconductor in a Magnetic Field: Meissner Effect Irrespective of whether the system was cooled below Tc in the presence/absence of a magnetic field, B = 0 inside a superconductor (a perfect diamagnet). This remarkable property of a superconductor was discovered by W. Meissner and R. Ochsenfeld in 1933. Later we will understand it as a consequence of phase rigidity in a superconductor . Thermodynamic property: Specific heat γγγ (i) For T > T C, C V ≈≈≈ T. (ii) At T C the specific heat jumps (typical signature of a mean field type phase transition). - ∆∆∆/(k T) ∆∆∆ As T →→→ 0, C V ≈≈≈ A e B , with ≈≈≈ 1.44 kBTC. Evidence for an energy gap between the ground state and the excited states of a superconductor (very different from a metal). TC Specific heat of Al with T C ≈≈≈ 1.2 K. N. E. Phillips, Phys. Rev. 114, 676 (1959) Note: A magnetic field H = 0.03 T suppresses T C and the appearance of the superconducting state. Sufficiently large magnetic fields destroy superconductivity and brings back metallicity . Summary 1. A superconductor is a zero resistance state (a resolution limited statement). 2. It is a perfect diamagnet. B=0 in the bulk irrespective of how the state was prepared. Different from an ideal metal (perfect diamagnet only if there is field-after-cooling); and certainly very different from any realistic metal (weak diamagnets). 3. Evidence of an energy gap between the ground state and the excited states of a superconductor. A superconductor is a new phase of matter compared to a metal. London equations & the two-fluid model (1935) For T < T C, the total density of electrons n = n s + nn. n ns = density of superconducting electrons; nn = density of normal electrons. ns For T →→→ 0, n s →→→ n; and for T →→→ TC, n s →→→ 0. The normal electrons conduct with finite resistance, while the superconducting electrons have TC T dissipationless flow. nd m (dv s)/(dt) = - eE (Newton’s 2 law). Since J = - en svs, 2 st dJ/dt = n se /m E (1 London equation) ∂∂∂ ∂∂∂ ∂∂∂ ∂∂∂ 2 Combined with Maxwell eqn ∇∇∇××× E = - B/(c t) gives / t [ ∇∇∇××× J + n se /(mc) B] =0 . Trivially satisfied for static B and J. Thus, it does not necessarily imply B=0 inside a superconductor (Meissner effect). 2 nd ∇∇∇××× J + n se /(mc) B =0 (2 London equation) The 2 nd London equation implies Meissner effect. Does a perfect diamagnet imply an ideal conductor? Derive 1 st London eqn from the 2 nd London eqn. B-field expulsion: London penetration depth 2 πππ The London eqn ∇∇∇××× J = - nse /(mc)B combined with Maxwell eqn ∇∇∇××× B = (4 /c)J gives 2 ΛΛΛ -2 2 ΛΛΛ -2 ΛΛΛ 2 πππ 2 1/2 ∇∇∇ B = L B, ∇∇∇ J = L J, L = [mc /(4 ns e )] (London penetration depth) . 2 2 ΛΛΛ -2 Eqn: d B/(dx ) = L B(x). -x / ΛΛΛ B(x) = B 0 e L . B0 Solution: πππΛΛΛ -x / ΛΛΛ J(x) = c B 0/( 4 L) e L . B(x) B0 Meissner effect : In response to B 0, the superconductor develops a supercurrent J at the surface which screens the B-field. ΛΛΛ L ΛΛΛ Currents & B-fields exist only within a boundary layer of thickness L. How do we justify the 2 nd London equation? Ginzburg Landau theory ...... Landau’s theory of phase transitions A phase transition between a symmetrical high-T phase & a symmetry-broken low-T phase is described by an order parameter (OP). The OP is zero in the symmetrical phase and is non-zero in the symmetry-broken phase. Near T C the free energy can be expressed in powers of the OP, keeping only those terms that are allowed by the symmetries of the system. The equilibrium value of the OP is obtained by minimizing the free energy with respect to the OP. Example: paramagnet-ferromagnet transition Let us assume that there is strong magnetic anisotropy and the magnetic moments order along the z-direction (easy axis). In this case the relevant symmetry is time reversal symmetry . The order parameter is Mz (magnetization along z-direction). 2 4 3 F[M z] = a M z /2 + b M z /4 + LLL. Why isn’t M z allowed? T > Tc T < Tc At the phase transition “a” changes F ′′′ sign: a = a (T – TC). Below T C a spontaneous magnetization M 0 0 1/2 z M Mz = (-a/b) develops. z Ginzburg Landau theory The order parameter to describe the phase transition between a high-T metallic phase and a low-T superconducting state is a complex-valued function ΨΨΨ(r). Physical meaning of ΨΨΨ(r) We will learn later that the basic building blocks of a superconductor are bound pairs of electrons (Cooper pairs). ΨΨΨ(r) is the wave-function describing the centre of mass motion of a Cooper pair. All the pairs are in the same quantum state (a condensate). A single wave-function describes the superconducting electrons. ΨΨΨ 1/2 i ΦΦΦ ΨΨΨ 2 = (n s/2) e (| | gives density of superconducting electrons). Symmetry : In a superconductor U(1) symmetry is broken. This is associated with particle number conservation. F[ ΨΨΨ] must be invariant under ΨΨΨ →→→ ΨΨΨ ei ααα .
Load more

Recommended publications
  • Lecture Notes: BCS Theory of Superconductivity
    Lecture Notes: BCS theory of superconductivity Prof. Rafael M. Fernandes Here we will discuss a new ground state of the interacting electron gas: the superconducting state. In this macroscopic quantum state, the electrons form coherent bound states called Cooper pairs, which dramatically change the macroscopic properties of the system, giving rise to perfect conductivity and perfect diamagnetism. We will mostly focus on conventional superconductors, where the Cooper pairs originate from a small attractive electron-electron interaction mediated by phonons. However, in the so- called unconventional superconductors - a topic of intense research in current solid state physics - the pairing can originate even from purely repulsive interactions. 1 Phenomenology Superconductivity was discovered by Kamerlingh-Onnes in 1911, when he was studying the transport properties of Hg (mercury) at low temperatures. He found that below the liquifying temperature of helium, at around 4:2 K, the resistivity of Hg would suddenly drop to zero. Although at the time there was not a well established model for the low-temperature behavior of transport in metals, the result was quite surprising, as the expectations were that the resistivity would either go to zero or diverge at T = 0, but not vanish at a finite temperature. In a metal the resistivity at low temperatures has a constant contribution from impurity scattering, a T 2 contribution from electron-electron scattering, and a T 5 contribution from phonon scattering. Thus, the vanishing of the resistivity at low temperatures is a clear indication of a new ground state. Another key property of the superconductor was discovered in 1933 by Meissner.
    [Show full text]
  • The Superconductor-Metal Quantum Phase Transition in Ultra-Narrow Wires
    The superconductor-metal quantum phase transition in ultra-narrow wires Adissertationpresented by Adrian Giuseppe Del Maestro to The Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the subject of Physics Harvard University Cambridge, Massachusetts May 2008 c 2008 - Adrian Giuseppe Del Maestro ! All rights reserved. Thesis advisor Author Subir Sachdev Adrian Giuseppe Del Maestro The superconductor-metal quantum phase transition in ultra- narrow wires Abstract We present a complete description of a zero temperature phasetransitionbetween superconducting and diffusive metallic states in very thin wires due to a Cooper pair breaking mechanism originating from a number of possible sources. These include impurities localized to the surface of the wire, a magnetic field orientated parallel to the wire or, disorder in an unconventional superconductor. The order parameter describing pairing is strongly overdamped by its coupling toaneffectivelyinfinite bath of unpaired electrons imagined to reside in the transverse conduction channels of the wire. The dissipative critical theory thus contains current reducing fluctuations in the guise of both quantum and thermally activated phase slips. A full cross-over phase diagram is computed via an expansion in the inverse number of complex com- ponents of the superconducting order parameter (equal to oneinthephysicalcase). The fluctuation corrections to the electrical and thermal conductivities are deter- mined, and we find that the zero frequency electrical transport has a non-monotonic temperature dependence when moving from the quantum critical to low tempera- ture metallic phase, which may be consistent with recent experimental results on ultra-narrow MoGe wires. Near criticality, the ratio of the thermal to electrical con- ductivity displays a linear temperature dependence and thustheWiedemann-Franz law is obeyed.
    [Show full text]
  • The Development of the Science of Superconductivity and Superfluidity
    Universal Journal of Physics and Application 1(4): 392-407, 2013 DOI: 10.13189/ujpa.2013.010405 http://www.hrpub.org Superconductivity and Superfluidity-Part I: The development of the science of superconductivity and superfluidity in the 20th century Boris V.Vasiliev ∗Corresponding Author: [email protected] Copyright ⃝c 2013 Horizon Research Publishing All rights reserved. Abstract Currently there is a common belief that the explanation of superconductivity phenomenon lies in understanding the mechanism of the formation of electron pairs. Paired electrons, however, cannot form a super- conducting condensate spontaneously. These paired electrons perform disorderly zero-point oscillations and there are no force of attraction in their ensemble. In order to create a unified ensemble of particles, the pairs must order their zero-point fluctuations so that an attraction between the particles appears. As a result of this ordering of zero-point oscillations in the electron gas, superconductivity arises. This model of condensation of zero-point oscillations creates the possibility of being able to obtain estimates for the critical parameters of elementary super- conductors, which are in satisfactory agreement with the measured data. On the another hand, the phenomenon of superfluidity in He-4 and He-3 can be similarly explained, due to the ordering of zero-point fluctuations. It is therefore established that both related phenomena are based on the same physical mechanism. Keywords superconductivity superfluidity zero-point oscillations 1 Introduction 1.1 Superconductivity and public Superconductivity is a beautiful and unique natural phenomenon that was discovered in the early 20th century. Its unique nature comes from the fact that superconductivity is the result of quantum laws that act on a macroscopic ensemble of particles as a whole.
    [Show full text]
  • ANTIMATTER a Review of Its Role in the Universe and Its Applications
    A review of its role in the ANTIMATTER universe and its applications THE DISCOVERY OF NATURE’S SYMMETRIES ntimatter plays an intrinsic role in our Aunderstanding of the subatomic world THE UNIVERSE THROUGH THE LOOKING-GLASS C.D. Anderson, Anderson, Emilio VisualSegrè Archives C.D. The beginning of the 20th century or vice versa, it absorbed or emitted saw a cascade of brilliant insights into quanta of electromagnetic radiation the nature of matter and energy. The of definite energy, giving rise to a first was Max Planck’s realisation that characteristic spectrum of bright or energy (in the form of electromagnetic dark lines at specific wavelengths. radiation i.e. light) had discrete values The Austrian physicist, Erwin – it was quantised. The second was Schrödinger laid down a more precise that energy and mass were equivalent, mathematical formulation of this as described by Einstein’s special behaviour based on wave theory and theory of relativity and his iconic probability – quantum mechanics. The first image of a positron track found in cosmic rays equation, E = mc2, where c is the The Schrödinger wave equation could speed of light in a vacuum; the theory predict the spectrum of the simplest or positron; when an electron also predicted that objects behave atom, hydrogen, which consists of met a positron, they would annihilate somewhat differently when moving a single electron orbiting a positive according to Einstein’s equation, proton. However, the spectrum generating two gamma rays in the featured additional lines that were not process. The concept of antimatter explained. In 1928, the British physicist was born.
    [Show full text]
  • 3: Applications of Superconductivity
    Chapter 3 Applications of Superconductivity CONTENTS 31 31 31 32 37 41 45 49 52 54 55 56 56 56 56 57 57 57 57 57 58 Page 3-A. Magnetic Separation of Impurities From Kaolin Clay . 43 3-B. Magnetic Resonance Imaging . .. ... 47 Tables Page 3-1. Applications in the Electric Power Sector . 34 3-2. Applications in the Transportation Sector . 38 3-3. Applications in the Industrial Sector . 42 3-4. Applications in the Medical Sector . 45 3-5. Applications in the Electronics and Communications Sectors . 50 3-6. Applications in the Defense and Space Sectors . 53 Chapter 3 Applications of Superconductivity INTRODUCTION particle accelerator magnets for high-energy physics (HEP) research.l The purpose of this chapter is to assess the significance of high-temperature superconductors Accelerators require huge amounts of supercon- (HTS) to the U.S. economy and to forecast the ducting wire. The Superconducting Super Collider timing of potential markets. Accordingly, it exam- will require an estimated 2,000 tons of NbTi wire, ines the major present and potential applications of worth several hundred million dollars.2 Accelerators superconductors in seven different sectors: high- represent by far the largest market for supercon- energy physics, electric power, transportation, indus- ducting wire, dwarfing commercial markets such as trial equipment, medicine, electronics/communica- MRI, tions, and defense/space. Superconductors are used in magnets that bend OTA has made no attempt to carry out an and focus the particle beam, as well as in detectors independent analysis of the feasibility of using that separate the collision fragments in the target superconductors in various applications.
    [Show full text]
  • 1- Basic Connection Between Superconductivity and Superfluidity
    -1- Basic Connection between Superconductivity and Superfluidity Mario Rabinowitz Electric Power Research Institute and Armor Research 715 Lakemead Way, Redwood City, CA 94062-3922 E-mail: [email protected] Abstract A basic and inherently simple connection is shown to exist between superconductivity and superfluidity. It is shown here that the author's previously derived general equation which agrees well with the superconducting transition temperatures for the heavy- electron superconductors, metallic superconductors, oxide supercon- ductors, metallic hydrogen, and neutron stars, also works well for the superfluid transition temperature of 2.6 mK for liquid 3He. Reasonable estimates are made from 10-3 K to 109K -- a range of 12 orders of magnitude. The same paradigm applies to the superfluid transition temperature of liquid 4He, but results in a slightly different equation. The superfluid transition temperature for dilute solutions of 3He in superfluid 4He is estimated to be ~ 1 to 10µK. This paradigm works well in detail for metallic, cuprate, and organic superconductors. -2- 1. INTRODUCTION The experimental discovery in 1972 of superfluidity in liquid 3He (L3He) at 2.6mK was long preceded by predictions of the critical < temperature Tc for this transition at ~ 0.1K which was then well below the lower limit of the known experimental data. These predictions were based on the BCS theory (Bardeen, Cooper, and Schrieffer, 1957) where the sensitive exponential dependence of Tc makes it hard to make accurate predictions of Tc . Theoretical papers (Pitaevski ,1959; Brueckner et al, 1960; Emery and Sessler, 1960) incorporated pairing of 3He Fermi atoms to make Bosons by analogy with the Cooper pairing of Fermi electrons in metals.
    [Show full text]
  • Introduction to Superconductivity
    Introduction to Superconductivity Clare Yu University of California, Irvine Introduction to Superconductivity Superconductivity was discovered in 1911 by Kamerlingh Onnes. • Zero electrical resistance Meissner Effect • Magnetic field expelled. Superconducting surface current ensures B=0 inside the superconductor. Flux Quantization where the “flux quantum” Φo is given by Explanation of Superconductivity • Ginzburg-Landau Order Parameter Think of this as a wavefunction describing all the electrons. Phase φ wants to be spatially uniform (“phase rigidity”). T C s BCS Theory • Electrons are paired. Josephson Effect If we put 2 superconductors next to each other separated by a thin insulating layer, the phase difference (θ2-θ1) between the 2 superconductors will cause a current of superconducting Cooper pairs to flow between the superconductors. Current flow without batteries! This is the Josephson effect. High Temperature Superconductors The high temperature superconductors have high transition temperatures. YBa2Cu3O7-δ (YBCO) TC=92 K Bi2Sr2CaCu2O8 (BSCCO) TC=90 K The basis of the high temperature superconductors are copper-oxygen planes. These planes are separated from other copper-oxygen planes by junk. The superconducting current flows more easily in the planes than between the planes. High Temperature Superconduc3vity Phase Diagram Cuprates Type I Superconductors Type I superconductors expel the magnetic field totally, but if the field is too big, the superconductivity is destroyed. Type II Superconductors For intermediate field strengths, there is partial field penetration in the form of vortex lines of magnetic flux. Vortices in Layered Superconductors In a layered superconductor the planes are superconducting and the vortex lines are correlated stacks of pancakes (H || ĉ). A pancake is a vortex in a plane.
    [Show full text]
  • The Thermodynamics of Quantum Critical Points Zachary Fisk Science 325, 1348 (2009); DOI: 10.1126/Science.1179046
    PERSPECTIVES zon synchronized in-phase across the sky in substantial advances are coming. Much of galaxy evolution, determine the mass of the a series of acoustic oscillations of the tem- what we know today has come from combin- light neutrinos, and test the Gaussianity and perature anisotropy. Clear evidence of this is ing WMAP’s CMB measurements with the power spectrum of infl ation. seen in the peaks and troughs of the tempera- Sloan Digital Sky Survey’s galaxy redshift ture power spectrum and in the superhorizon survey. The recently launched Planck satel- References and Notes 1. E. Hubble, Proc. Natl. Acad. Sci. U.S.A. 15, 168 (1929). temperature-polarization cross-correlation; lite will map the full sky with greater angular 2. F. Zwicky, Helv. Phys. Acta 6, 110 (1933). (v) there should be statistical isotropy with resolution and sensitivity. More than a dozen 3. A. A. Penzias, R. W. Wilson, Astrophys. J. 142, 419 (1965). fl uctuations having Gaussian-distributed ran- ground and balloon-borne measurements are 4. J. C. Mather et al., Astrophys. J. 354, L37 (1990). 5. D. J. Fixsen et al., Astrophys. J. 581, 817 (2002). dom phases. Although some small excep- also in various stages of development and/or 6. A. H. Guth, D. I. Kaiser, Science 307, 884 (2005). tions have been claimed, observations to date observations ( 15). These experiments cover 7. W. Percival et al., Mon. Not. R. Astron. Soc. 381, 1053 match this prediction; and (vi) the generation a wide range of frequencies, vary in angular (2007). 8. W. Freedman et al., Astrophys.
    [Show full text]
  • Bose-Einstein Condensates As Universal Quantum Matter C S Unnikrishnan
    Bose-Einstein Condensates as Universal Quantum Matter C S Unnikrishnan To cite this version: C S Unnikrishnan. Bose-Einstein Condensates as Universal Quantum Matter. 2018. hal-01949624 HAL Id: hal-01949624 https://hal.archives-ouvertes.fr/hal-01949624 Preprint submitted on 10 Dec 2018 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Bose-Einstein Condensates as Universal Quantum Matter C. S. Unnikrishnan Tata Institute of Fundamental Research, Mumbai 400005. Abstract S. N. Bose gave a new identity to the quanta of light, introduced into physics by Planck and Einstein, and derived Planck’s formula for the radiation spectrum. Chance events led to Einstein’s insightful use of Bose’s result to build a quantum theory of atomic gases, with the remarkable prediction of a new type of condensation without interactions. Though the spectacular phenomena of superfluidity and superconductivity were identified as the consequences of Bose-Einstein Condensation, the phenomenon in its near pure form, as foresaw by Einstein, was observed 70 years later in atomic gases. The quest for atomic gas BEC was behind much of the developments of laser cooling and trapping of atoms.
    [Show full text]
  • 9 Superconductivity
    9 Superconductivity 9.1 Introduction So far in this lecture we have considered materials as many-body “mix” of electrons and ions. The key approximations that we have made are the Born-Oppenheimer approximation that allowed us to separate electronic and • ionic degrees of freedom and to concentrate on the solution of the electronic many- body Hamiltonian with the nuclei only entering parameterically. the independent-electron approximation, which greatly simplifies the theoretical de- • scription and facilitates our understanding of matter. For density-functional theory we have shown in Chapter 3 that the independent-electron approximation is always possible for the many-electron ground state. that the independent quasiparticle can be seen as a simple extension of the independent- • electron approximation (e.g. photoemission, Fermi liquid theory for metals, etc.). We have also considered cases in which these approximations break down. The treatment of electron-phonon (or electron-nuclei) coupling, for example, requires a more sophisti- cated approach, as we saw. Also most magnetic orderings are many-body phenomena. In Chapter 8 we studied, for example, how spin coupling is introduced through the many- body form of the wave function. In 1911 H. Kamerlingh Onnes discovered superconductivity in Leiden. He observed that the electrical resistance of various metals such as mercury, lead and tin disappeared com- pletely at low enough temperatures (see Fig. 9.1). Instead of dropping off as T 5 to a finite Figure 9.1: The resistivity of a normal (left) and a superconductor (right) as a function of temperature. 255 Figure 9.2: Superconducting elements in the periodic table.
    [Show full text]
  • Superconductivity the Structure Scale of the Universe
    i Superconductivity The Structure Scale of the Universe Twenty Fourth Edition – December 08, 2018 Richard D. Saam 525 Louisiana Ave Corpus Christi, Texas 78404 USA e-mail: [email protected] http://xxx.lanl.gov/abs/physics/9905007 PREVIOUS PUBLICATIONS First Edition, October 15, 1996 Twelfth Edition, December 6, 2007 http://xxx.lanl.gov/abs/physics/9705007 http://xxx.lanl.gov/abs/physics/9905007, version 19 Second Edition, May 1, 1999 Thirteenth Edition, January 15, 2008 http://xxx.lanl.gov/abs/physics/9905007, version 6 http://xxx.lanl.gov/abs/physics/9905007, version 20 Third Edition, February 23, 2002 Fourteenth Edition, March 25, 2008 http://xxx.lanl.gov/abs/physics/9905007, version 7 http://xxx.lanl.gov/abs/physics/9905007, version 21 Fourth Edition, February 23, 2005 Fifteenth Edition, February 02, 2009 http://xxx.lanl.gov/abs/physics/9905007, version 9 http://xxx.lanl.gov/abs/physics/9905007, version 22 Fifth Edition, April 15, 2005 Sixteenth Edition, May 12, 2009 http://xxx.lanl.gov/abs/physics/9905007, version 11 http://xxx.lanl.gov/abs/physics/9905007, version 23 Sixth Edition, June 01, 2005 Seventeenth Edition, March 15, 2010 http://xxx.lanl.gov/abs/physics/9905007, version 13 http://xxx.lanl.gov/abs/physics/9905007, version 24 Seventh Edition, August 01, 2005 Eighteenth Edition, November 24, 2010 http://xxx.lanl.gov/abs/physics/9905007, version 14 http://xxx.lanl.gov/abs/physics/9905007, version 25 Eighth Edition, March 15, 2006 Nineteenth Edition, May 10, 2011 http://xxx.lanl.gov/abs/physics/9905007, version 15 http://xxx.lanl.gov/abs/physics/9905007,
    [Show full text]
  • Key Factors of Superconductivity
    Key Factors of Superconductivity Now, in independent studies reported in Science and Nature, scientists from the Department of Energy's SLAC National Accelerator Laboratory and Stanford University report two important advances: They measured collective vibrations of electrons for the first time and showed how collective interactions of the electrons with other factors appear to boost superconductivity. [35] At the Joint Quantum Institute (JQI), a group, led by Jimmy Williams, is working to develop new circuitry that could host such exotic states. [34] The effect appears in compounds of lanthanum and hydrogen squeezed to extremely high pressures. [33] University of Wisconsin-Madison engineers have added a new dimension to our understanding of why straining a particular group of materials, called Ruddlesden- Popper oxides, tampers with their superconducting properties. [32] Nuclear techniques have played an important role in determining the crystal structure of a rare type of intermetallic alloy that exhibits superconductivity. [31] A potential new state of matter is being reported in the journal Nature, with research showing that among superconducting materials in high magnetic fields, the phenomenon of electronic symmetry breaking is common. [30] Researchers from the University of Geneva (UNIGE) in Switzerland and the Technical University Munich in Germany have lifted the veil on the electronic characteristics of high-temperature superconductors. Their research, published in Nature Communications, shows that the electronic densities measured in these superconductors are a combination of two separate effects. As a result, they propose a new model that suggests the existence of two coexisting states rather than competing ones postulated for the past thirty years, a small revolution in the world of superconductivity.
    [Show full text]