Václav Janiš

The Czech Academy of Sciences | AVCR · Institute of Physics

Symmetry-breaking phases of many-fermion systems are characterized by anomalous functions representing transient processes during which some characteristics of free particles, such as spin or charge, are not conserved. Matching the low-temperature symmetry-breaking phase with the high-temperature one consistently with nontrivial quantum dynamics is...

We disclose a serious deficiency of the Baym-Kadanoff construction of thermodynamically consistent conserving approximations. There are two vertices in this scheme: dynamical and conserving. The divergence of each indicates a phase instability. We show that each leads to incomplete and qualitatively different behavior at different critical points....

We present an improved model for studying the interactions between dipole moments of water molecules confined in beryl crystals inspired by recent NMR experiments. Our model is based on a local crystal potential with dihexagonal symmetry for the rotations of water dipole moments, leading to deflection from the ab hexagonal crystallographic plane. T...

In agreement with recent NMR experiments on beryl crystals containing confined water molecules, we developed an improved model for studying the interactions among the water molecules' dipole moments. The model is based on a local crystal potential with a dihexagonal symmetry for the rotations of the water dipole moments, leading to their deflection...

The zero-temperature physics of interacting quantum dots attached to superconducting leads is now well understood. The overall qualitative picture is obtained from the static mean-field approximation. The situation drastically changes at non-zero temperatures. No reliable solutions apart from numerical simulations exist there. We show that any stat...

We apply a two-particle semianalytic approach to a single Anderson impurity attached to two biased metallic leads. The theory is based on reduced parquet equations justified in critical regions of singularities in the Bethe-Salpeter equations. It offers a way to treat one-particle and two-particle thermodynamic and spectral quantities on the same f...

The zero-temperature physics of interacting quantum dots attached to superconducting leads is now well understood. The overall qualitative picture is obtained from the static mean-field approximation. The situation drastically changes at non-zero temperatures. No reliable solutions apart from numerical simulations exist there. We show that any stat...

We apply a two-particle semi-analytic approach to a single Anderson impurity attached to two biased metallic leads. The theory is based on reduced parquet equations justified in critical regions of singularities in the Bethe-Salpeter equations. It offers a way to treat one-particle and two-particle thermodynamic and spectral quantities on the same...

I review the quantum theory of the electron moving in a random environment. First, the quantum mechanics of individual particles scattered on a random potential is discussed. The quantum-mechanical description is extended to many-body systems by using many-body Green functions. The many-body approach is used to derive the coherent-potential approxi...

I review the way the many-body Green functions are used to renormalize the perturbation theory of correlated fermions. The Green functions are introduced to implement systematically dynamical corrections to the static mean-field theory. The renormalizations enter the perturbation theory via self-consistent evaluations of one-particle and two-partic...

We develop the general many-body perturbation theory for a superconducting quantum dot represented by a single-impurity Anderson model attached to superconducting leads. We build our approach on a thermodynamically consistent mean-field approximation with a two-particle self-consistency of the parquet type. The two-particle self-consistency leading...

We develop the general many-body perturbation theory for a superconducting quantum dot represented by a single-impurity Anderson model attached to superconducting leads. We build our approach on a thermodynamically consistent mean-field approximation with a two-particle self-consistency of the parquet type. The two-particle self-consistency leading...

We study the low-temperature critical behavior of the one-dimensional Hubbard model near half filling caused by enhanced antiferromagnetic fluctuations. We use a mean-field-type approximation with a two-particle self-consistency renormalizing the bare interaction. It allows us to control a transition from high to low temperatures as well as from we...

We study the low-temperature critical behavior of the one-dimensional Hubbard model near half filling caused by enhanced antiferromagnetic fluctuations. We use a mean-field-type approximation with a two-particle self-consistency renormalizing the bare interaction. It allows us to control a transition from high to low temperatures as well as from we...

The genesis of the Curie-Weiss magnetic response observed in most transition metals that are Fermi liquids at low temperatures has been an enigma for decades and has not yet been fully explained from microscopic principles. We show on the single-impurity Anderson model how the quantum dynamics of strong electron correlations leads to the Curie-Weis...

The genesis of the Curie-Weiss magnetic response observed in most transition metals that are Fermi liquids at low temperatures has been an enigma for decades and has not yet been fully explained from microscopic principles. We show on the single-impurity Anderson model how the quantum dynamics of strong electron correlations leads to the Curie-Weis...

A two-particle self-consistency is rarely part of mean-field theories. It is, however, essential for avoiding spurious critical transitions and unphysical behavior. We present a general scheme for constructing analytically controllable approximations with self-consistent equations for the two-particle vertices based on the parquet equations. We exp...

Kondo temperature is standardly defined from the local zero-temperature susceptibility in the regime of strong electron correlations as a new scale controlling the low-temperature asymptotics of thermodynamic quantities. We show by using a two-particle self-consistent theory that the Kondo temperature can be identified as a crossover temperature at...

A two-particle self-consistency is rarely part of mean-field theories. It is, however, essential for avoiding spurious critical transitions and unphysical behavior. We present a general scheme for constructing analytically controllable approximations with self-consistent equations for the two-particle vertices based on the parquet equations. We exp...

All lecture notes are available online at https://www.cond-mat.de/events/correl17/

We extend the spin-polarized effective-interaction approximation of the parquet renormalization scheme from Refs. [1,2] applied on the symmetric Anderson model by adding the low-temperature asymptotics of the total energy and the specific heat. We calculate numerically the Wilson ratio and determine analytically its asymptotic value in the strong-c...

Mean-field approximation offers a first qualitative picture of the behavior of interacting systems in particular in the critical region of singularities in the response functions. The static Hartree approximation fails in strong coupling and the dynamical mean-field approximation, as the limit to infinite dimensions, is not analytically solvable in...

We study the subgap spectrum of the interacting single-level quantum dot coupled between two superconducting reservoirs, forming the Josephson-type circuit, and additionally hybridized with a metallic normal lead. This system allows for the phase-tunable interplay between the correlation effects and the proximity-induced electron pairing resulting...

Behavior of Andreev gap states in a quantum dot with Coulomb repulsion symmetrically attached to superconducting leads is studied via the perturbation expansion in the interaction strength. We find the exact asymptotic form of the spin-symmetric solution for the Andreev states continuously approaching the Fermi level. We thereby derive a critical i...

One- and two-electron Green functions are simultaneously needed to determine the response functions of the electron gas in a random potential. Reliable approximations must retain consistency between the two types of Green functions expressed via Ward identities so that their output is compliant with macroscopic symmetries and conservation laws. Suc...

Quantum criticality emerges in proximity of a low-temperature singularity in the two-particle Green function. A consistent description of the quantum critical behavior demands that the two-particle divergence is simultaneously accompanied by a symmetry breaking in the one-particle self-energy. This cannot be easily achieved in models of correlated...

Self-consistent perturbation expansion up to the second order in the interaction strength is used to study a single-level quantum dot with local Coulomb repulsion attached asymmetrically to two generally different superconducting leads. At zero temperature and wide range of other parameters the spin-symmetric version of the expansion yields excelle...

The book can be downloaded free online at the address http://www.cond-mat.de/events/correl15/manuscripts/

We discuss the mean-field theory of spin-glass models with frustrated long-range random spin exchange. We analyze the reasons for breakdown of the simple mean-field theory of Sherrington and Kirkpatrick. We relate the replica-symmetry breaking to ergodicity breaking and use the concept of real replicas to restore thermodynamic homogeneity of the eq...

We address the problem of vanishing of diffusion in noninteracting disordered electron systems and its description by means of averaged Green functions. Since vanishing of diffusion, Anderson localization, cannot be identified by means of one-electron quantities, one must appropriately approximate two-particle functions. We show how to construct no...

A single-level quantum dot with Coulomb repulsion attached to two superconducting leads is studied via the perturbation expansion in the interaction strength. We use the Nambu formalism and the standard many-body diagrammatic representation of the impurity Green functions to formulate the Matsubara self-consistent perturbation expansion. We show th...

We discuss ergodicity breaking in frustrated disordered systems with no apparent broken symmetry of the Hamiltonian and present a way how to amend it in the low-temperature phase. We demonstrate this phenomenon on mean-field models of spin glasses. We use replicas of the spin variables to test thermodynamic homogeneity of ergodic equilibrium system...

We study spectral properties of a quantum dot attached to two superconductors with nonzero phase difference. The system is described as a single-impurity Anderson model coupled to BCS superconducting leads. We utilize diagrammatic perturbation expansion in the Coulomb interaction to capture relevant physical phenomena, particularly the effect of th...

We study singularities in the self-energy and a two-particle irreducible vertex connected with the metal-insulator transition of the disordered Falicov-Kimball model. We resort to the dynamical mean-field approximation. We set general conditions for the existence of a critical metal-insulator transition caused by divergence of the imaginary part of...

A quantum dot with Coulomb repulsion attached to left and right superconducting leads is studied via the perturbation expansion in the interaction strength. We use the Nambu formalism and the standard many-body diagrammatic representation of the impurity Green functions. We formulate the perturbation expansion in the spectral representation so that...

The mean-field theory for noninteracting disordered electron systems is widely and successfully used to describe equilibrium properties of alloys over the whole range of disorder strengths. However, it fails to take into account the effects of quantum coherence and localizing backscattering effects when applied to transport phenomena. Vertex correc...

The full mean-field solution of spin glass models with a continuous order-parameter function is not directly available and approximate schemes must be used to assess its properties. One of the authors recently proposed a representation of the free energy generating this solution via an evolution operator parametrized by attainable values of overlap...

We propose a renormalization scheme of the Kubo formula for the electrical conductivity with multiple backscatterings contributing to the electron-hole irreducible vertex derived from the asymptotic limit to high spatial dimensions. We use this vertex to represent the two-particle Green function via a symmetrized Bethe-Salpeter equation in momentum...

The full mean-field solution of spin glass models with a continuous order-parameter function is not directly available and approximate schemes must be used to assess its properties. The averaged physical quantities are to be represented via the replica trick and the limit to zero number of replicas is to be performed for each of them. To avoid this...

We propose a renormalization scheme of the Kubo formula for the electrical conductivity with multiple backscatterings contributing to the electron-hole irreducible vertex. The asymptotic limit to high spatial dimensions is used to determine this vertex via a symmetrized Bethe-Salpeter equation in momentum space. We further utilize the dominance of...

We address the problem of fulfilling consistency conditions in solutions for disordered noninteracting electrons. We prove that if we assume the existence of the diffusion pole in an electron–hole symmetric theory we cannot achieve a solution with a causal self-energy that would fully fit the Ward identity. Since the self-energy must be causal, we...

We apply the Wiener–Hopf method of solving convolutive integral equations on a semi-infinite interval to the X-ray edge problem. Dyson equations for basic Green functions from the X-ray problem are rewritten as convolutive integral equations on a time-interval [0,t] with t→∞. The long-time asymptotics of solutions to these equations is derived with...

We report electronic structure calculations of an iron impurity in a gold host. The spin, orbital, and dipole magnetic moments were investigated using the local density approximation (LDA) + U correlated band theory. We show that the around-mean-field LDA + U reproduces the x-ray magnetic circular dichroism (XMCD) experimental data well and does no...

We expand asymptotically mean-field solutions of the p<4 Potts glass with various levels of replica-symmetry breaking below the transition temperature to the glassy phase. We find that the ordered phase is degenerate and solutions with one hierarchy of spin replicas and with the full continuous replica-symmetry breaking coexist for p>p*≈2.82. The l...

We study effects of quantum coherence due to scatterings of electrons on impurities on two-particle vertex functions. We use the diagrammatic perturbation expansion in the impurity potential and analyze possible ways how to introduce renormalizations on the two-particle level. We discuss the differences between the low-dimensional self-consistent t...

We investigate the low-temperature equilibrium state of the p-state mean-field Potts glass. It is the presently believed that there is a temperature interval T(2) < T < T(c) within which the equilibrium state for p > p(*) approximately 2.82 is described by the cavity method corresponding to the first level of replica-symmetry breaking. We demonstra...

We extend an approximation that we developed earlier for the single-impurity Anderson model to a full-size impurity solver for models of interacting electrons with multiple orbitals. The approximation is based on parquet equations simplified by separating small and large energy fluctuations justified in the critical region of a pole in the two-part...

The correlated band theory picture (LSDA+U) has been applied to UGe 2, in which superconductivity has been found to coexist with robust ferromagnetism. Over a range of volumes (i.e. pressures), two nearly degenerate states are obtained, which differ most strikingly in their orbital character (on uranium). The calculated moment, and its separation i...

Quantum coherence of elastically scattered lattice fermions is studied. We calculate vertex corrections to the electrical conductivity of electrons scattered either on thermally equilibrated or statically distributed random impu-rities and we demonstrate that the sign of the vertex corrections to the Drude conductivity is in both cases negative. PA...

We study quantum coherence of elastically scattered lattice fermions. We calculate vertex corrections to the electrical conductivity of electrons scattered either on thermally equilibrated or statically distributed random impurities. We demonstrate that the sign of the vertex corrections to the Drude conductivity is in both cases negative. Quantum...

We investigate the p-state mean-field model of the Potts glass ($2\le p \le 4$) below the continuous phase transition to a glassy phase. We find that apart from a solution with a first hierarchical level of replica-symmetry breaking (1RSB), locally stable close to the transition point, there is a continuous full replica-symmetry breaking (FRSB) sol...

We discuss restrictions on the existence of the diffusion pole in the translationally invariant diagrammatic treatment of disordered electron systems. We analyze Bethe-Salpeter equations for the two-particle vertex in the electron-hole and the electron-electron scattering channels and derive for systems with electron-hole symmetry a nonlinear integ...

We study the strong-coupling regime of the single impurity Anderson model. We calculate the two-particle vertex and the spectral function using a resummation of the diagrammatic expansion in the interaction strength. The earlier developed parquet resummation is extended to include all three representations of Bethe-Salpeter equations. In an applied...

We study the single impurity Anderson model in an external magnetic field. There are no exact results for the spectral function in this situation. Using a resummation of the diagrammatic expansion we demonstrate that the strong coupling regime in a weak magnetic field is Kondo-like with a quasiparticle resonant peak split into two. We find two expo...

The Parisi formula for the free energy of the Sherrington-Kirkpatrick model is completed to a closed-form generating functional. We first find an integral representation for a solution of the Parisi differential equation and represent the free energy as a functional of order parameters. Then, we set stationarity equations for local maxima of the fr...

We study hierarchies of replica-symmetry-breaking solutions of the Sherrington-Kirkpatrick model. Stationarity equations for order parameters of solutions with an arbitrary number of hierarchies are set and the limit to infinite number of hierarchical levels is discussed. In particular, we demonstrate how the continuous replica-symmetry breaking sc...

The Parisi formula for the free energy of the Sherrington-Kirkpatrick model is completed to a closed-form generating functional. We first find an integral representation for a solution of the Parisi differential equation and represent the free energy as a functional of order parameters. Then we set stationarity equations for local maxima of the fre...

The low-temperature behavior of the asymmetric single-impurity Anderson model is studied by diagrammatic methods resulting in analytically controllable approximations. We first discuss the ways one can simplify parquet equations in critical regions of singularities in the two-particle vertex. The scale vanishing at the critical point defines the Ko...

The influence of local disorder on the thermodynamics of interacting electrons is studied within the infinite-dimensional disordered Hubbard model. Using a finite-temperature quantum Monte Carlo method, the averaged local moment and staggered susceptibility are calculated and the magnetic phase diagram at half-filling is constructed. From the avera...

We study the single-impurity Anderson model with diagrammatic techniques. We employ the parquet approach to determine the electron-hole and electron-electron irreducible vertices self-consistently. We demonstrate that when the dominant contributions from the critical region of the singularity driven by multiple electron-hole scatterings are properl...

We present an analytic universal impurity solver for strongly correlated electrons. We extend the many-body perturbation expansion via suitable two-particle renormalizations from the Fermi-liquid regime to the critical region of the metal-insulator transition. The reliability of the approximation in the strong-coupling limit is demonstrated by repr...

We analyze the replica-symmetry-breaking construction in the Sherrington-Kirkpatrick model of a spin glass. We present a general scheme for deriving an exact asymptotic behavior near the critical temperature of the solution with an arbitrary number of discrete hierarchies of the broken replica symmetry. We show that all solutions with finitely many...

Actinide elements and their chemistry have a significant number of applications. Bringing together contributions from the leading experts in the field, Recent Advances in Actinide Science covers six main topics: * Analysis, the environment and biotransformations * Coordination and organometallic chemistry * Heavy elements * Nuclear fuels, materials...

We analyze the low-temperature behavior of mean-field equations of Thouless, Anderson, and Palmer (TAP). We demonstrate that degeneracy in free energy makes the low-temperature TAP states unstable. Different solutions of the TAP equations, independent in the TAP approach, become coupled if an infinitesimal interaction between them is introduced. By...

We analyze thermodynamic behavior of general $n$-component mean-field spin glass models in order to identify origin of the hierarchical structure of the order parameters from the replica-symmetry breaking solution. We derive a configurationally dependent free energy with local magnetizations and averaged local susceptibilities as order parameters....

The Anderson model of noninteracting disordered electrons is studied in high spatial dimensions. In this limit the coupled Bethe-Salpeter equations determining two-particle vertices (parquet equations) reduce to a single algebraic equation for a local vertex. We find a disorder-driven bifurcation point in this equation signaling vanishing of electr...

The ways of introducing and handling renormalizations in the many-body perturbation theory are reviewed. We stress the indispensable role the technique of Green functions plays in extrapolating the weak-coupling perturbative approaches to intermediate and strong couplings. We separately discuss mass and charge renor-malizations. The former is incor...

We discuss conditions to be put on mean-field-like theories to be able to describe fundamental physical phenomena in disordered electron systems. In particular, we investigate options for a consistent mean-field theory of electron localization and for a reliable description of transport properties. We argue that a mean-field theory for the Anderson...

We use real replicas within the Thouless, Anderson and Palmer construction to investigate stabil-ity of solutions with respect to uniform scalings in the phase space of the Sherrington-Kirkpatrick model. We show that the demand of homogeneity of thermodynamic potentials leads in a natural way to a thermodynamically dependent ultrametric hierarchy o...

We investigate the effect of strong Coulomb correlations on the electronic structure of the Pu-based superconductor PuCoGa5 by employing the relativistic local spin density approximation+ Hubbard U (LSDA+U) method. The inclusion of intra-atomic Coulomb U and exchange J parameters leads to a significant reconstruction of the f states electronic stru...

The Anderson model of noninteracting disordered electrons is studied in high spatial dimensions. We find that off-diagonal one- and two-particle propagators behave as Gaussian random variables with respect to momentum summations. With this simplification and with the electron-hole symmetry we reduce the parquet equations for two-particle irreducibl...

We study the two-band degenerate Hubbard model using the fluctuation exchange approximation (FLEX) and compare the results with quantum Monte Carlo (QMC) calculations. Both the self-consistent and the non-self-consistent versions of the FLEX scheme are investigated. We find that, unlike in the one-band case, in the multiband case, good agreement wi...

We use real replicas to investigate stability of thermodynamichomogeneity of the free energy of the Sherrington-Kirkpatrick (SK) model of spin glasses. Within the replica trick with the replica symmetric ansatz we show that the averaged free energy at low temperatures is not thermodynamically homogeneous. The demand of minimization of the inhomogen...

The correlated band theory picture (LSDA+U) has been applied to UGe2, in which superconductivity has been found to coexist with robust ferromagnetism. Over a range of volumes (i.e., pressures), two nearly degenerate states, which differ most strikingly in their orbital character (on uranium), are obtained. The calculated moment (and its separation...

We discuss conditions for the existence of the diffusion pole and its consequences in disordered noninteracting electron systems. Using only nonperturbative and exact arguments we find against expectations that the diffusion pole can exist only in the diffusive (metallic) regime. We demonstrate that the diffusion pole in the Anderson localization p...

We discuss conservation of probability in noninteracting disordered electron systems. We argue that although the norm of the electron wave function is conserved in individual realizations of the random potential, we cannot extend this conservation law easily to configurationally averaged systems in the thermodynamic limit. A direct generalization o...

Slovance 2, CZ-18221 Praha 8, Czech Republic * (Dated: February 2, 2008) Anderson model of noninteracting disordered electrons is studied in high spatial dimensions. We find that off-diagonal one-and two-particle propagators behave as gaussian random variables w.r.t. momentum summations. With this simplification and with the electron-hole symmetry...

We analyse two-particle renormalizations within the many-fermion perturbation expansion. We show that present diagrammatic theories suffer from a lack of direct diagrammatic control over the physical two-particle functions. To rectify this, we introduce and prove a Ward identity enabling an explicit construction of the self-energy from a given two-...

We study noninteracting quantum charged particles (electron gas) subject to a strong random potential and perturbed by a weak classical electromagnetic field. We examine consequences of gauge invariance and charge conservation in the space of Bloch waves. We use two specific forms of the Ward identity between the one- and two-particle averaged Gree...

The many-body part of the multiband Hubbard model was solved using more advanced single channel approximations (FLEX) of the canonical perturbation theory. Approximation of the many-body part due to a local interaction matrix with direct and exchange term was done within the dynamical mean-field theory with multiple two-particle scattering. The res...

Mean-field limit (d=∞) of the Hubbard model at half filling is investigated within the parquet approach. Simplified parquet equations are explicitly solved at intermediate coupling where nonself-consistent approximations of the FLEX type become numerically unstable. A new solution with anomalous vertex functions (complex effective interaction) was...

The density functional theory (DFT)1 supplemented by the local density approximation (LDA) or generalized gradient approximation (GGA) in which the electron-correlation part is treated on the basis of the electron gas model is a highly reliable method for evaluation of the ground state properties of molecules and solids2. However, the DFT fails in...

We discuss phase transitions and critical behavior deviating from the standard scheme based on a mean-field theory renormalizing only the mass of the critical excitations completed with a perturbative scaling renormalization of the interaction strength. On examples of mean-field theories for spin glasses and for quantum phase transitions we show th...

A diagrammatic technique for two-particle vertex functions is used to describe systematically the influence of spatial quantum coherence and backscattering effects on transport properties of noninteracting electrons in a random potential. In analogy with many-body theory we construct parquet equations for topologically distinct nonlocal irreducible...

Coulomb repulsion drives correlated electrons toward a critical point where bound electron-hole pairs condense. However, before the critical point is reached quantum fluctuations dominate the critical region and can lead to formation of resonant pair states. We use a two-particle parquet approach to describe quantum criticality. We show with a simp...

A tight-binding version of the itinerant spin-fluctuation theory is used to describe the electronic structure of MnTe in both magnetically ordered and disordered phases. The static model of disordered local moments is shown to be sufficiently accurate to form the basis for a quantitative description of semiconducting behaviour of paramagnetic MnTe....

The electrical {\em dc}-conductivity of disordered, non-interacting electrons is calculated in the asymptotic limit of high lattice dimensions $d\to \infty$. To go beyond the lowest-order contribution in the expansion parameter $1/d$ of the single bubble diagram, vertex corrections are calculated from an asymptotic expression for the two-particle v...

We present a general scheme describing correlation and disorder effects consistently on the same footing in multiband, tight-binding Hamiltonians constructed from linear muffin-tin orbital (LMTO) calculations. The method is applied to determine electronic properties of (disordered) solids with correlated d electrons in the weak-coupling case, U/w<1...

We present a general framework for investigating the stability of solutions being a parquet-type extension of the Baym-Kadanoff construction of conserving approximations. To obtain a consistent description of one- and two-particle quantities, needed for the stability analysis, we use explicit equations for one- and two-particle Green functions simu...

The question of the thermodynamic consistence of mean-field theories for lattice itinerant models defined from the asymptotic limit of high spatial dimensions is addressed. We show that the local ansatz for the self-energy and the thermodynamic potential generating all physical quantities via functional derivatives does not lead to the complete hig...

We consider a model for a metal-insulator transition of correlated electrons in an external magnetic field. We find a broad region in interaction and magnetic field where metallic and insulating (fully magnetized) solutions coexist and the system undergoes a first-order metal-insulator transition. A global instability of the magnetically saturated...

Increasing external magnetic field B gradually forces the electron spins to align in the direction of the applied field. The Hartree solution becomes exact for B>=Bs(U). A new small parameter ΔB=Bs-B enables one to control the transition between weak- and strong-coupling regimes and the metal-insulator transition. Necessity for dynamical vertex cor...

The author investigates the instability of the high-temperature phase of the Sherrington-Kirkpatrick model at finite magnetic fields. He finds that the relevant function for this analysis is q(12)(h(1),h(2)) identical to (mi(h(1))mi(h (2)))av with two external fields h(1) and h(2) and reveals its non-analytic structure at the critical points. The a...

The model for X-ray scattering known as the Mahan-Nozieres-De Dominicis model is extended to a finite concentration of randomly distributed core electrons. A systematic, renormalized expansion for the phase shift of the imaginary-time Green function of the deep-level electrons is derived in the limit of high dimensions, i.e. within a dynamical mean...

We analyse the behaviour of correlated electrons described by Hubbard-like models at intermediate coupling. We argue that with increasing interaction a pole in a generic two-particle Green function is approached. The pole signals condensation of electron-hole pairs and a metal-insulator transition at half-filling. The two-particle singularity calls...