Showing papers on "Mott transition published in 1977"

Investigation of Exciton-Plasma Mott Transition in Si

Abstract: Measurements of luminescence spectra in pure Si demonstrate the dissociation of excitons with increasing excitation intensity. The density at which the dissociation begins and its temperature variation agree well with Mott's predictions. We have also determined a phase diagram of Si, including the effects of electron-hole-liquid and Mott transition.

Screened donor impurities in many-valley semiconductors with anisotropic masses

Abstract: The binding energy of a conduction electron bound to a donor impurity in a doped semiconductor, using a Lindhard dielectric function, is calculated as a function of the free-carrier concentration, taking into full account the effects of the nonisotropic mass of the bound electron on both the kinetic energy and the screened Coulomb potential. A variational approach is adopted in which the trial wave function is chosen with a form similar to the solution of the problem of an electron bound in the Hulth\'en potential. The wave function is appropriately warped to account for the asymmetry caused by the nonisotropic masses. This calculation, in the limit of zero carrier concentration, is equivalent to work of earlier authors (Kohn and Luttinger and others) where a hydrogenic trial wave function was used. The method thus obtained is specifically applied to silicon and germanium and the results are compared to calculations done in the isotropic-mass approximation. The electron density where the Mott transition takes place is found to be lowered by introducing the nonisotropy. For germanium where the mass ratio is approximately 20, this effect is quite large. A similar calculation using the simpler Thomas-Fermi dielectric function is included for completeness.

Metallic condensation of Wannier-Mott impurity states in lithium-hexamethylphosphoramide glasses

Abstract: A metal to non-metal (MNM) transition observed in frozen solutions of lithium in hexamethylphosphoramide (HMPA) was tentatively interpreted as a Mott transition in which localized Wannier-type impurity states were the source of electrons in the metallic state. In this paper this assertion is examined in greater detail by calculating critical densities (nc) on the basis of a scaled (variational) form of Mott's original criterion for the onset of localization in a dielectrically screened Coulomb potential, and also on the basis of the Hubbard tight-binding model. Mott's model for the transition is based upon the screening properties of a freely propagating gas of metallic electrons. In the Hubbard regime, however, the phenomenon is viewed from the tight-binding limit; the transition from localized to delocalized states occurs when the bandwidth (Δ) of a regular lattice of isolated centres exceeds the value of the intra-atomic Coulombic repulsion integral (U) associated with electron correlation.Both electro...

Conductivity transitions in itinerant-electron systems: Experimental evidence for an antiferromagnetic metallic phase

Abstract: Approximate solutions of the simplest model of correlated electrons in a narrow band predicts the appearance of an antiferromagnetic metallic phase after the Mott transition as Δ/U is increased at T = 0. It lies between the antiferromagnetic insulator and the non-magnetic metal. This phase is absent from the observed electronic phase diagram of V2O3. NiS, however, is a semimetallic antiferromagnet below its first order conductivity transition and its properties, particularly those requiring an itinerant-electron model for the magnetism, are summarized. In the system (Fe1−xNix)S, itinerant and localized moments coexist, and an antiferromagnetic insulating phase is not obtained. The recently-reported electronic phase diagrams of solid solutions of NiS2 (an antiferromagnetic semiconsuctor) and NiSe2 or NiAs2 (non-magnetic metals) show a region where the systems are antiferromagnetic metals. Among the transition metal compounds these systems may be the ones which best correspond to the Mott-Hubbard model.

Electron-phonon interactions and phase transitions : [lectures presented at the NATO Advanced Study Institute on Electron-Phonon Interactions and Phase Transitions held in Geilo, Norway, April 1977]

Abstract: Phase Transitions and Electron-Phonon Couplings in Perfect Crystals. Modulated Structures. An Introduction.- I. The Framework.- A. Phase changes in perfect crystals.- B. Interatomic forces.- II. Electron-Phonon Couplings (for Delocalized Electrons).- A. Soft modes at OK. Adiabatic and kinematic approximations.- 1. General (crystals).- 2. Energy change for the phonon.- 3. Classical examples.- B. Discussion of the approximations.- 1. The meaning of v.- 2. Self consistency (to first order).- 3. Other correlation effects.- 4. Degeneracy of electronic states.- 5. Anharmonic terms.- 6. Adiabatic approximation.- 7. Entropy at finite temperature.- Appendix A. Short Range Order due to Dispersion Forces.- Appendix B. LCAO Studies of the Band Structures of Metals and Covalents.- Appendix C. Phase Stability for Nearly Free Electrons.- Appendix D. Cohesion in Transitional Metals.- Neutron Scattering Studies of Electron-Phonon Interactions.- I. Phonon Dispersion in Metals.- II. Kohn Singularities.- III. Neutron Spectroscopy of Superconductors.- IV. Magnetic Field Effects.- V. Charge Density Wave Instabilities.- Phase Transitions in Quasi One-Dimensional Metals (TTF-TCNQ and KCP).- 1. Introduction.- 2. Interchain Coupling.- 3. Landau-Ginzburg Theory of Structural Phase Transformations and Charge Density Waves in TTF-TCNQ.- 3.1 The 54K transition.- 3.2 The 47K transition.- 3.3 The 38K transition.- 3.4 The 4kF anomaly.- 3.5 Critical behaviour.- 4. Impurities.- 4.1 One-dimensional random systems.- 4.2 Combined effects of random impurities and interchain coupling.- Solitons and Charge Density Waves.- 1. Abstract.- 2. Introduction.- 3. Change Density Waves and the Sine-Gordon Equation.- 4. Solitons and Charge Density Waves.- 5. Solitons in Disordered Systems.- 6. Classical Solitons in Two Dimension.- 7. Three Dimensional Ordering.- Charge Density Waves in Layered Compounds.- Landau Theory of the Charge Density Waves.- A. The Landau Free Energy.- B. Phase Transitions.- C. Fluctuation Modes.- D. Impurity Effects l.- E. CDW Dislocations l.- F. Discommensurations.- Microscopic Model of CDW in 2H-TaSe2.- Light Scattering by Charge Density Wave Modes in KCP and 2H-TaSe2.- Symmetry Classification of Modulated Structures.- 1. Definition of Modulated Structures.- 2. Symmetry Operations and -Translations.- 3. Properties of MS-Space Group Operations.- 4. Reduced Form of Point Group Operations.- 5. Equivalence and Invariance of k-vectors.- 6. Point Groups.- 7. Rational and Irrational Non-Zero Components of k l.- 8. Necessity of Introduction of Bravais Lattice Types with Improper Translations.- 9. Two-Dimensional Example of Improper Translations.- 10. Enumeration of Lattice Types.- Superspace Groups for the Classification of Modulated Crystals.- I. Introduction.- II. Superspace Groups.- III. Equivalence Classes.- IV. Examples.- V. Conclusions.- Structural Phase Transitions and Superconductivity in A-15 Compounds.- I. Introduction.- II. Instabilities and Transformation Effects on the Physical Behaviour.- III. More on the Relation of Structural Instability and High Temperature Superconductivity.- IV. Instabilities, Unstable Phases, and Superconductivity.- V. Defects, Instabilities, and Superconductivity.- Superconductivity and Martensitic Transformations in A-15 Compounds.- p-d Hybridization, Incipient Lattice Instabilities and Superconductivity in Transition METAL Compounds.- Pseudo-Spin Approach to Structural Phase Transitions.- Abstract.- 1. Introduction.- 2. Models.- (i) Spin-phonon systems.- (ii) Jahn-Teller systems.- (iii) Order-disorder and tunnelling ferroelectrics.- (iv) Displacive ferroelectrics 2l.- (v) Hamiltonians.- 3. Properties of Models.- (i) Statics.- (ii) Formalism for dynamics.- (iii) Dynamic properties.- (iv) Damping.- 4. Mixed and Dilute Systems.- (i) Statics.- (ii) Dynamics.- (iii) Generalisations.- 5. The Central Peak Critical Behaviour.- (i) The central peak.- (ii) Critical behaviour.- Theory of Jahn-Teller Transitions.- 1. Introduction.- 2. Dynamics of JT-Systems.- 2.1 Electronic configuration.- 2.2 Vibronic coupling.- 2.3 Collective behaviour, mean-field approximation (MFA).- 2.4 Coupling to eleastic strain.- 3. Specific Cases.- 3.1 E x ? coupling.- 3.2 E x ? coupling.- 3.3 T ? ? coupling.- 3.4 T ? t2 coupling.- 3.5 (A+B) ? ? pseudo-JT coupling.- Local Jahn-Teller Effect at a Structural Phase Transition.- Abstract.- 1. Introduction.- 2. Multimode JT-Effect.- 3. Critical Enhancement.- Optical Studies of Jahn-Teller Transitions.- 1. Introduction.- 2. 3d-Transition Metal Ions.- Cu2+:CaO.- Ti3+:A12O3.- 3. Cooperative Jahn-Teller Effects in Rare-Earth Crystals.- 4. Optical Studies of Complicated Jahn-Teller Transitions.- 5. Conclusion.- Electric Susceptibility Studies of Cooperative Jahn-Teller Ordering in Rare-Earth CRYSTALS.- Neutron Scattering Studies of the Cooperative Jahn-Teller Effect.- Abstract.- 1. Introduction.- 2. The Neutron Probe.- 3. Symmetries and Crystal Fields.- 4. Theory.- 5. Static and Critical Properties.- 6. Normal and Mixed Modes.- 7. Discussion 3l.- Gamma-Ray Diffraction Studies of the Mosaic Distribution in TmAsO4 Near the Cooperative Jahn-Teller Transition at 6 K.- The Central Peak in TbVO4.- The Nature of the Eigenfunctions in a Strongly Coupled Jahn-Teller Problem.- I. Introduction.- II. The Physical Setting.- III. The Hamiltonian.- IV. Absorption Spectrum.- V. Summary and Conclusions.- Cooperative Pseudo Jahn-Teller Model of the Sequence of Ferroelastic Transitions in Barium Sodium Niobate.- Single Ion and Cooperative Jahn-Teller Effect for a Nearly Degenerate E Doublet.- Abstract.- 1. Physical System and Model.- 2. Calculations, Results and Discussion.- 3. Conclusions.- Study of the Mott Transition in n.Type CdS by Spin Flip Raman Scattering and Faraday Rotation.- I. Introduction.- II. Basic Properties of Cadmium Sulfide.- Band structure.- Impurity levels.- Free exciton.- Bound exciton.- III. Spin Flip Scattering.- IV. Study of the SFRS Linewidth.- Mott transition in CdS.- Analysis of experimental results.- V. Measurement of X0 Rotation.- VI. Discussion of Results.- Electron Phonon Interactions and Charge Ordering in Insulators.- I. Charge Ordering in Insulators.- II. Second Grade Ordering: Jahn-Teller Ordering in K2PbCu(NO2)6..- II.1 Introduction.- II. 2 Successive Phase Transitions.- II.3 Phase III: Canted pseudospin structure, antiferrodistortive phase.- II.4 Phase III: 'Fan' spin structure, incommensurate phase.- II.5 Summary and discussions.- III. Zeroth Grade Ordering in Fe3O4.- III.1 Introduction.- III.2 Symmetry property of the phonon field and the charge density field.- III.3 Pseudospin-phonon formalism and neutron scattering cross sections.- III.4 Summary.- The Verwey Transition in Magnetite.- I. Introduction.- II. Crystal and Symmetry.- III. Critical Scattering.- IV. Structure below TV.- Participants.