Finite temperature neutral-ionic transition and lattice dimerization in charge-transfer complexes: QMC study

We numerically investigate the neutral-ionic (NI) phase transition on the basis of the ionic extended Hubbard model with electron-lattice coupling as well as inter-chain Coulomb interaction. Finite-temperature (T) phase transitions are examined by a quantum Monte Carlo method, within adiabatic approximation for the lattice displacement together with inter-chain mean-field treatment. The NI transition either with or without the electron-lattice coupling is of first-order at low-T and transforms into a cross-over regime with increasing T. We confirm there exist two phases in the ionic region: with and without lattice dimerization, as suggested in recent experiments. The former is ferroelectric, which is rooted in the Coulomb interactions and a spin-Peierls mechanism.     

Phase diagram of the Kohn-Luttinger superconducting state for bilayer graphene

The effect of Coulomb interaction between Dirac fermions on the formation of the Kohn-Luttinger superconducting state in bilayer doped graphene is studied disregarding of the effect of the van der Waals potential of the substrate and impurities. The phase diagram determining the boundaries of superconductive domains with different types of symmetry of the order parameter is built using the extended Hubbard model in the Born weak-coupling approximation with allowance for the intratomic, interatomic, and interlayer Coulomb interactions between electrons. It is shown that the Kohn-Luttinger polarization contributions up to the second order of perturbation theory in the Coulomb interaction inclusively and an account for the long-range intraplane Coulomb interactions significantly affect the competition between the superconducting phases with the f-, p + ip-, and d + id-wave symmetries of the order parameter. It is demonstrated that the account for the interlayer Coulomb interaction enhances the critical temperature of the transition to the superconducting phase.     

Equilibrium properties of two-component classical plasmas

Two-component overall neutral classical Coulomb Gas is considered in the canonical ensemble for any value of the space dimensionality ν. The equilibrium properties, i.e. pair correlations and thermodynamic functions are investigated in two complementary ways. The first one is adequate in considering the low temperature range and uses the “molecular” interaction within a pair of unlike charges as a zero order starting point. On the other hand, the high-temperature fully ionized and translation-invariant plasma is considered within the nodal expression with respect to the classical plasma parameter. These two ways are possible through the use of effective temperature-dependent classical interaction for ν > 2. As a by-product, we obtain a unified treatment of the Coulomb Gas thermal properties with respect to dimensionality (integer or real). We also obtain a contrasting comparison with corresponding properties of the one component plasma model which are already known. In this analysis the ν = 2 two-component Coulomb Gas seems to be a landmark for the other TCP'₈. I do not consider degeneracy effects. I consider diffraction corrections in a first order expansion with respect to the Coulomb interaction, in the high-temperature range. The “Hydrogen atom” spectrum is explained for all ν. The long-range hypernetted chain resummation of the pair correlation functions asymptotic behavior does not hold for symmetrical (Z₁ = ?Z₂) plasmas; the corresponding onset of short-range order disappears when the plasma parameter increases. The modified long- and short-range behaviors of the pair correlation functions are then displayed with the canonical thermodynamics.     

Surface-induced weak orientational order and role of isotropic-nematic interface fluctuations in the appearance of an induced nematic film

Recently the nontrivial spatial and temperature dependence of the surface-induced weak planar orientational order parameter Q(z, T) was determined just above the isotropic-nematic (IN) phase transition point (Ji-H. Lee et al., Phys. Rev. Lett. 102, 167801 (2009)). In this paper we present a theoretical explanation of the observed behaviour. We obtain expressions for the short-range and long-range contributions to the interface potential of the induced nematic film and specify the repulsive character of the interaction between the soft IN interface and the external bounding substrate. It is shown that the small value of the IN interfacial tension results in the renormalization of the repulsive interaction potential due to the thermal fluctuations of the soft IN interface. This leads to an increase of the equilibrium thickness of the induced nematic film and the appearance of a step-like orientational order parameter profile. We find that only renormalized short-range and thermal pseudo-Casimir interactions are essential for the appearance of the induced nematic film, which provide the observed thickness, h ?? 30 nm, of this film. The long-range van der Waals interaction is shown to be negligibly small and the dominant role is played by the renormalized short-range repulsion. Fitting of the experimental order parameter profiles (Ji-H. Lee et al. (2009)) with the expressions based on these interactions makes it possible to determine the material parameters of the system, including the amplitudes of the surface interaction, the IN interfacial tension and the interfacial coherence length. The agreement between theory and experiment confirms the importance of the interface fluctuation renormalization of the interface potentials for soft interfaces.      

The Kohn-Luttinger mechanism and phase diagram of the superconducting state in the Shubin-Vonsovsky model

Using the Shubin-Vonsovsky model in the weak-coupling regime W > U > V (W is the bandwidth, U is the Hubbard onsite repulsion, and V is the Coulomb interaction at neighboring sites) based on the Kohn-Luttinger mechanism, we determined the regions of the existence of the superconducting phases with the d _xy , p, s, and $d_{x^2 - y^2 } $ symmetry types of the order parameter. It is shown that the effective interaction in the Cooper channel considerably depends not only on single-site but also on intersite Coulomb correlations. This is demonstrated by the example of the qualitative change and complication of the phase diagram of the superconducting state. The superconducting (SC) phase induction mechanism is determined taking into account polarization contributions in the second-order perturbation theory in the Coulomb interaction. The results obtained for the angular dependence of the superconducting gap in different channels are compared with angule-resolved photoemission spectroscopy (ARPES) results. The influence of long-range hops in the phase diagram and critical superconducting transition temperature in different channels is analyzed. The conditions for the appearance of the Kohn-Luttinger superconductivity with the $d_{x^2 - y^2 } $ symmetry and high critical temperatures T _c ～ 100 K near the half-filling are determined.     

Pressure dependent elastic properties of diluted magnetic semiconductors: Hg1−xMnxS (x=0.02 and 0.07)

A theoretical study of the elastic properties in diluted magnetic semiconductors Hg_1−xMn_xS (x=0.02 and 0.07) using an effective interionic interaction potential (EIoIP) in which long-range Coulomb interactions, charge transfer mechanism (three body interaction) and the Hafemeister and Flygare type short-range overlap repulsion extending up to the second neighbor ions and the van der Waals (vdW) interaction is considered. Particular attention is devoted to evaluate Poisson's ratio ν, the ratio R_S/B of S (Voigt averaged shear modulus) over B (bulk modulus), elastic anisotropy parameter, elastic wave velocity, average wave velocity and thermodynamic property as Debye temperature is calculated. By analyzing Poisson's ratio ν and the ratio R_S/B we conclude that Hg_1−xMn_xS is brittle in zinc blende (B3). To our knowledge this is the first quantitative theoretical prediction of the pressure dependence of ductile (brittle) nature of Hg_1−xMn_xS compounds and still awaits experimental confirmations.     

Unconventional superconductivity in low density electron systems and conventional superconductivity in hydrogen metallic alloys

In this short review, we first discuss the results, which are mainly devoted to the generalizations of the famous Kohn–Luttinger mechanism of superconductivity in purely repulsive fermion systems at low electron densities. In the context of repulsive-U Hubbard model and Shubin–Vonsovsky model we consider briefly the superconducting phase diagrams and the symmetries of the order parameter in novel strongly correlated electron systems including idealized monolayer and bilayer graphene. We stress that purely repulsive fermion systems are mainly the subject of unconventional low-temperature superconductivity. To get the high temperature superconductivity in cuprates (with T_C of the order of 100 K) we should proceed to the t–J model with the van der Waals interaction potential and the competition between short-range repulsion and long-range attraction. Finally we note that to describe superconductivity in metallic hydrogen alloys under pressure (with T_C of the order of 200 K) it is reasonable to reexamine more conventional mechanisms connected with electron–phonon interaction. These mechanisms arise in the attractive-U Hubbard model with static onsite or intersite attractive potential or in more realistic theories (which include retardation effects) such as Migdal–Eliashberg strong coupling theory or even Fermi–Bose mixture theory of Ranninger et al. and its generalizations.     

Anisotropic structure of the gap in high-T c superconductors: Competition between s-and d-type symmetry

We show that the anisotropic structure, observed in angle-resolved photoemission spectroscopy experiments, of the gap in high-T _c superconductors based on layered cuprate metal-oxide compounds is the result of the strong anisotropy of the electronic spectrum in the plane of the layers, an anisotropy caused by the hybridization between the overlapping broad and anomalously narrow bands. Depending on the values of the electron-phonon coupling constants and the Coulomb repulsion, which in certain conditions is balanced almost perfectly by the attraction caused by the electron-plasmon interaction, either the or the s _xy symmetry is realized in the superconducting order parameter. When the initial C _v4 symmetry of the band spectrum of the Bi(2212) single crystal with a superlattice or the Y(123) single crystal with one-dimensional chains is broken, an anomalous temperature dependence of the anisotropic-gap width Δ is observed, and this dependence differs dramatically from the standard Δ (T) dependence of the BCS theory. Zh. éksp. Teor. Fiz. 111, 298–317 (January 1997)     

Deconfined criticality, runaway flow in the two-component scalar electrodynamics and weak first-order superfluid-solid transitions

We perform a comparative Monte Carlo study of the easy-plane deconfined critical point (DCP) action and its short-range counterpart to reveal close similarities between the two models for intermediate and strong coupling regimes. For weak coupling, the structure of the phase diagram depends on the interaction range: while the short-range model features a tricritical point and a continuous U(1) × U(1) transition, the long-range DCP action is characterized by the runaway renormalization flow of coupling into a first (I) order phase transition. We develop a “numerical flowgram” method for high precision studies of the runaway effect, weakly I-order transitions, and polycritical points. We prove that the easy-plane DCP action is the field theory of a weakly I-order phase transition between the valence bond solid and the easy-plane antiferromagnet (or superfluid, in particle language) for any value of the weak coupling strength. Our analysis also solves the long standing problem of what is the ultimate fate of the runaway flow to strong coupling in the theory of scalar electrodynamics in three dimensions with U(1) × U(1) symmetry of quartic interactions.     

Phase transformation and elastic behavior of MgX (X=S, Se, Te) alkaline earth chalcogenides

Pressure induced structural aspects of NaCl-type (B1) to CsCl-type (B2) structure in alkaline earth chalcogenides (AECs) magnesium chalcogenides (MgX; X=S, Se, and Te) are presented. An effective interionic interaction potential (EIoIP) with long-range Coulomb interactions and the Hafemeister and Flygare type short-range overlap repulsion extending up to the second neighbor ions and the van der Waals (vdW) interaction is developed. The vdW coefficients are evaluated following the Slater-Kirkwood variational method, as both the ions are polarizable. The present calculations have revealed reasonably good agreement with the available experimental data on structural transition (B1-B2 structure), the phase transition pressures P_t of 167 (MgS), 170 (MgSe), and 176 (MgTe) GPa as well the elastic properties. The calculated values of the volume collapses [ΔV(P)/V(0)] are also closer to their observed data. Further, the variations of the second and third order elastic constants with pressure have followed a systematic trend, which are almost identical to those exhibited by the observed data measured for other semiconducting compounds with rocksalt (B1) type crystal structure. The Born and relative stability criteria is valid in Mg monochalcogenides.     

Coulomb repul sion of hole s and competition between $${d_{{x^2} - {y^2}}}$$ -wave and s-wave paring s in cuprate superconductor s

The effect of the Coulomb repulsion of holes on the Cooper instability in an ensemble of spin–polaron quasiparticles has been analyzed, taking into account the peculiarities of the crystallographic structure of the CuO₂ plane, which are associated with the presence of two oxygen ions and one copper ion in the unit cell, as well as the strong spin–fermion coupling. The investigation of the possibility of implementation of superconducting phases with d-wave and s-wave of the order parameter symmetry has shown that in the entire doping region only the d-wave pairing satisfies the self-consistency equations, while there is no solution for the s-wave pairing. This result completely corresponds to the experimental data on cuprate HTSC. It has been demonstrated analytically that the intersite Coulomb interaction does not affect the superconducting d-wave pairing, because its Fourier transform V _q does not appear in the kernel of the corresponding integral equation.     

Second-order phase transition with coexistence of short-range and long-range ferromagnetic interactions in Ba1.7La0.3FeMoO6 compound

In this work, the magnetic properties and critical behavior around ferromagnetic–paramagnetic (FM–PM) phase transition in Ba_1.7La_0.3FeMoO₆ compound have been investigated in detail. This compound exhibits a second-order magnetic phase transition with Curie temperature T_C = 345 K. The critical exponents β, γ, and δ that are determined by using the modified Arrott plots (MAP), the Kouvel–Fisher (KF) and the critical isotherm analysis agree very well. Our results indicate a coexistence of short-range and long-range ferromagnetic (FM) interactions in Ba_1.7La_0.3FeMoO₆ compound. The existence of long-range FM interactions in this compound can be associated with the crystal structure of materials with long-range Fe/Mo ordering parameter and strength of double-exchange interaction, whereas the existence of the short-range FM interactions can be explained by magnetic inhomogeneity and FM clusters.     

Theory of the radial distribution functions in α-AgI

The three partial radial distribution functions g_Ag-Ag(r), g_Ag-I(r) and g_I-I(r) in α-AgI are calculated on the basis of the fact that I-ions form a bcc lattice and Ag-ions are randomly distributed over the 12(d) sites. The short-range order between the Ag-ions is described in terms of the microcrystalline model which has been recently divised by the present author on the occasion of the study on the superionic transition in CuI. The calculated results are in good agreement with those of the computer simulation of Vashishta and Rahman. We have an evidence that the Ag-ions are correlated in order to decrease the Coulomb interaction between them (not core-core repulsion) and the microcrystalline model is suitable to represent the correlated region. We discuss the relation between the microcrystalline model and the caterpillar mechanism.     

Superconducting state in strong coupling

We consider the behavior of the critical temperature in the limit of strong electron-phonon coupling. The peculiar behavior of the order parameter in this limit allows one to evaluate T_c analytically and directly from Eliashberg's equation. We also consider the effect of Coulomb interaction and the contribution of low phonon modes, the role of lattice instabilities and the relative contribution of phonon and non-phonon interactions.     

On the theory of superconductivity in the extended Hubbard model

A microscopic theory of superconductivity in the extended Hubbard model which takes into account the intersite Coulomb repulsion and electron-phonon interaction is developed in the limit of strong correlations. The Dyson equation for normal and pair Green functions expressed in terms of the Hubbard operators is derived. The self-energy is obtained in the noncrossing approximation. In the normal state, antiferromagnetic short-range correlations result in the electronic spectrum with a narrow bandwidth. We calculate superconducting T _c by taking into account the pairing mediated by charge and spin fluctuations and phonons. We found the d-wave pairing with high-T _c mediated by spin fluctuations induced by the strong kinematic interaction for the Hubbard operators. Contributions to the d-wave pairing coming from the intersite Coulomb repulsion and phonons turned out to be small.     

Collective modes in superconductors near Tc

A phonon-like critical mode is obtained in superconductors near T_c as a result of screening due to electron-lattice ion interaction.     

Pressure induced phase transitions in Ga1 − x In x P

We have theoretically investigated the effect of pressure on the structural stability of GaP?:?InP mixed system. The three-body-potential (TBP) model has been used. The TBP model consists of long-range as well as short-range interactions; the long-range part includes the modified Coulomb force as well as a three-body term; the short-range part in TBP defines the van der Waals and overlap repulsive interactions. We observe a pressure-induced structural phase transformation from ZnS (B3) to NaCl (B1) type phase in Ga _1?x In _x P. Our calculated transition pressures for the initial GaP and final InP compound semiconductors are in good agreement with other reported data.     

Molecular dynamics study of a classical two-dimensional electron system: positional and orientational orders

Molecular dynamics simulation is used to investigate the crystallization of a classical two-dimensional electron system, in which electrons interact with the Coulomb repulsion. From the positional and the orientational correlation functions, we have found an indication that the solid phase has a quasi-long-range (power-law correlated) positional order and a long-range orientational order. This implies that the long-range 1/r system shares the absence of the true long-range crystalline order at finite temperatures with short-range ones to which Mermin's theorem applies. We also discuss the existence of the “hexatic” phase predicted by the Kosterlitz–Thouless–Halperin–Nelson–Young theory.     

Pressure-induced structural phase transition and elastic properties of rare earth Pr chalcogenides and pnictides

Pressure-induced structural aspects and elastic properties of NaCl-type (B1) to CsCl-type (B2) structure in praseodymium chalcogenides and pnictides are presented. Ground-state properties are numerically computed by considering long-range Coulomb interactions, Hafemeister and Flygare type short-range overlap repulsion, and van der Waals interaction in the interionic potential. From the elastic constants, Poisson's ratio ν, the ratio R_G/B of G (shear modulus) over B (bulk modulus), anisotropy parameter, shear and Young's moduli, Lamé's constant, Kleinman parameter, elastic wave velocity and thermodynamical property such as Debye temperature are calculated. Poisson's ratio ν and the ratio R_G/B indicate that PrX and PrY are brittle in B1 phase and ductile in B2 phase. To our knowledge, this is the first quantitative theoretical prediction of the ductile (brittle) nature of praseodymium chalcogenides and pnictides and still awaits experimental confirmation.     

Critical behavior at finite-temperature confinement transitions

Confinement in a pure gauge theory at non-zero temperature may be discussed in terms of an order parameter which transforms under a global symmetry group, the center of the gauge group. Integrating out all degrees of freedom except this order parameter generates an effective scalar field theory for the order parameter, globally invariant under the center symmetry. We argue that the effective theory possesses only short-range couplings, and hence that the finite-temperature confinement phase transition (when continuous) is accompanied by long-range fluctuations only in the order parameter. Universality ideas then lead to predictions for the critical properties of U(1), Z(N), and SU(N) gauge theories for all dimensionalities of space-time. An explicit renormalization-group calculation is presented for the U(1) gauge theory in (2 + 1) dimensions, the results of which fit the general picture.