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The Anderson-Mott Transition

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The Anderson-Mott Transition The Anderson-Mott transition D. Belitz Department of Physics, and Materials Science Institute, University of Oregon, Eugene, Oregon 97409 T. R. Kirkpatrick lnsti tute for Physical Science and Technology, and Department of Physics, University of Maryland, College Park, Maryland 20742 The interacting disordered electron problem is reviewed with emphasis on the quantum phase transitions that occur in a model system and on the field-theoretic methods used to describe them. An elementary discussion of conservation laws and diffusive dynamics is followed by a detai1ed derivation of the extended nonlinear sigma model, which serves as an effective field theory for the problem. A general scaling theory of metal-insulator and related transitions is developed, and explicit renormalization-group calculations for the various universality classes are reviewed and compared with experimental results. A discussion of per- tinent physical ideas and phenomenological approaches to the metal-insulator transition not contained in the sigma-model approach is given, and phase-transition aspects of related problems, like disordered su- perconductors and the quantum Hall effect, are discussed. The review concludes with a list of open prob- lems. CONTENTS 4. Symmetry considerations 298 a. Spontaneously broken symmetry 298 b. Externally broken symmetry 299 I. Introduction 262 c. Remaining soft modes and universality II. Diffusive Electrons 266 classes 300 A. Diffusion poles 266 C. Summary of results for noninteracting electrons 301 1 ~ The quasiclassical approximation for electron 1. Renormalization of the nonlinear sigma model 301 transport 267 2. Composite operators, ultrasonic attenuation, a. A phenomenological argument for diffusion 267 and the HalI conductivity 302 b. Linear response and the Boltzmann equation 268 3. Conductance fluctuations 303 c. Diagrammatic derivation of the diffusive den- IV. Scaling Scenarios for the Disordered Electron Problem 303 sity response 270 A. Metal-insulator phase-transition scenarios 304 d. Beyond the quasiclassical approximation 272 1. The noninteracting problem 304 2. Disorder renormalization of electron-electron 2. The interacting problem 306 and electron-phonon interactions 273 B. Possible magnetic phase transitions 308 a. Dynamical screening of diffusive electrons 273 V. Metal-Insulator Transitions in Systems with Spin-Flip b. Coupling of phonons to diffusive electrons 274 Mechanisms 310 B. Perturbation theory for interacting electrons; early A. Theory 310 scaling ideas 276 1. Systems with magnetic impurities 310 III. Field-Theoretic Description 276 2. Systems in external magnetic fields 313 A. Field theories for fermions 276 3. Systems with spin-orbit scattering 315 1. Grassmann variables 277 a. Asymptotic critical behavior 315 2. Fermion coherent states 278 b. Logarithmic corrections to scaling 317 3. The partition function for many-fermion systems 278 B. Experiments 319 B. Sigma-model approach to disordered electronic sys- 1. Transport properties 320 tems 279 2. Thermodynamic properties and the tunneling 1. The model in matrix formulation 279 density of states 323 a. The model in terms of Grassmann fields 279 3. Nuclear-spin relaxation 324 b. Spinor notation and separation in phase VI. Phase Transitions in the Absence of Spin-Flip Mecha- space 281 nisms 325 c. Decoupling of the four-fermion terms 284 A. Theory 326 d. The model in terms of classical matrix fields 286 1. The loop expansion 326 2. Saddle-point solutions 289 a. One-loop results 326 a. The saddle-point solution in the noninteract- b. Absence of disorder renormalizations 328 ing case and Hartree-Fock theory 289 c. Two-loop results 329 b. BCS-Gorkov theory as a saddle-point solu- d. Resummation of leading singularities 330 tion of the field theory 291 2. The pseudomagnetic phase transition 332 3 ~ The generalized nonlinear sigma model 292 a. Numerical solution 332 a. An effective theory for diffusion modes 292 b. Analytic scaling solution 332 b. Cxaussian theory 293 c. Renormalization-group analysis 333 c. Loop expansion and the renormalization d. Summary 335 group 295 3. The metal-insulator transition 337 d. Fermi-liquid corrections and identification of B. Experiments 338 observables 296 1. Transport properties 338 e. Long-range interaction 298 2. Thermodynamic properties 340 Reviews of Modern Physics, Vol. 66, No. 2, April 1994 0034-6861/94/66(2) /261 (120)/$1 7.00 1994 The American Physical Society 261 262 D. Belitz and T. R. Kirkpatrick: The Anderson-Mott transition VII. Destruction of Conventional Superconductivity by Dis- at zero temperature, so that the Auctuations determining order 341 the critical behavior are quantum mechanical rather than A. Disorder and superconductivity: A brief overview 341 thermal in nature. B. Theories for homogeneous disordered superconduc- Metal-insulator transitions divided tors 343 can be into two 1. Generalizations of BCS and Eliashberg theory 343 categories (see, for example, Mott, 1990). In the first a. Enhancement and degradation of the mean- category, some change in the ionic lattice, such as a field T, 343 structural phase transition, leads to a splitting of the elec- b. Breakdown of mean-field theory 345 tronic conduction band and hence to a metal-insulator 2. Field-theoretic treatments 345 transition. In the second category the transition is purely a. The Landau-Ginzburg-Wilson functional 345 electronic in origin and can be described by models in b. The nonlinear-sigma-model approach 349 which the lattice is either fixed or altogether absent as in c. Other approaches 350 "jellium" C. Experiments 350 models of the type. It is this second category 1. Bulk systems 350 which forms the subject of the present article. Histori- 2. Thin films 351 cally, the second category has again been divided into VIII. Disorder-Induced Unconventional Superconductivity 352 two classes, one in which the transition is .triggered by A. A mechanism for even-parity spin-triplet supercon- electronic correlations and one in which it is triggered by ductivi. ty 352 disorder. The first case is known as a Mott or Mott- B. Mean-field theory 354 Hubbard transition, the second as an Anderson transi- 1. The gap equation 3S4 2. The charge-fluctuation mechanism 355 tion. 3. The spin-fluctuation mechanism 356 Mott's original idea (Mott, 1949, 1990) of the C. Is this mechanism realized in nature? 3S7 correlation-induced transition was intended to explain Other Theoretical Approaches and Related Topics 357 why certain materials with one electron per unit cell, e.g., A. Results in one dimension 357 NiO, are insulators. Mott imagined a crystalline array of B. Local magnetic-moment effects 360 atomic potentials with one electron per atom and a 1. Local moments in the insulator 360 phase Coulomb interaction between the electrons. For 2. The disordered Kondo problem 361 3. Formation of local moments 363 sufficiently small 1attice spacing, or high electron density, 4. Two-fluid model 365 the ion cores will be screened, and the system will be me- C. Delocalization transition in the quantum Hall effect 366 tallic. Mott argued that for lattice spacing larger than a D. Superconductor-insulator transition in two- critical value the screening will break down, and the sys- dimensional films 368 tem will undergo a erst order transiti-on to an insulator. X. Conclusions 369 This argument depended on the long-range nature of the A. Summary of results 369 Coulomb interaction. A related, albeit continuous, B. Open problems 370 1. Problems concerning local moments 371 metal-insulator transition is believed to occur in a tight- 2. Description of known or expected phase transi- binding model with a short-ranged electron-electron in- tions 371 teraction known as the Hubbard model (Anderson, a. Phase diagram for the generic universality 1959a; Gutzwiller, 1963; Hubbard, 1963). The model class 371 Hamiltonian is b. Critical-exponent problems 372 c. The superconductor-metal transition 372 (l.la) d. The quantum Hall effect 372 3. Problems with the nonlinear sigma model 372 a. Nonperturbative aspects and completeness 373 where a, and 8; are creation and annihilation opera- b. Symmetry considerations and renormalizabil- tors, respectively, for electrons with spin cr at site i, the ity 373 summation in the tight-binding term is over nearest 4. Alternatives to the nonlinear sigma model 374 neighbors, and 5. The 2-d ground-state problem 374 Ackno wledgments 374 (1.1b) Refer ences 374 Here t is the hopping matrix element, and U is an on-site repulsion energy ( U) 0). Despite its simplicity, remark- I ~ INTRODUCTION ably little is known about the Hubbard model. In one di- mension (1 —d) it has been solved exactly by Lieb and In the field of continuous phase transitions, metal- Wu (1968), who showed that at half filling the ground insulator transitions play a special role. In the first place, state is an antiferromagnetic insulator for any U) 0. In they are not nearly so well understood, either experimen- higher dimensions, various approximations have suggest- tally or theoretically, as the classic examples of liquid-gas ed that the model with one electron per site shows a con- critical point, Curie point, A, point in He, etc. In the tinuous metal-insulator transition at zero temperature as second place, one important subclass of metal-insulator a function of U (see Mott, 1990). This is generally be- transition consists of what are now called quantum phase lieved to be true, but has not been firmly established. Re- transitions, i.e., continuous phase transitions that occur cently the Hubbard model has received renewed atten- Rev. Mod. Phys. , Vol. 66, No. 2, April 1994 D. Belitz and T. R. Kirkpatrick: The Anderson-Mott transition 263 tion, mostly because models closely related to it have 1.0 0 been suggested to contain the relevant physics for an un- 0 — o derstanding of high- T, superconductivity (Anderson, 0.8 This renewed interest 0 1987). has spawned new approxi- 0 6— mation schemes as well as new rigorous results.
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