Nonphonon mechanism of superconductivity in compounds with quasi-two-dimensional complexes RuO2

The characteristics of nonphonon pairing of hybridized p and d electrons in planar complexes RuO₂ are studied in the presence of a strong short-range Hubbard repulsion. The phase diagram of superconductivity as a function of the degree of underfilling of the 2p ⁶ and 4d ⁶ shells in the RuO₂ complexes is calculated in a generalized Hubbard model. Fiz. Tverd. Tela (St. Petersburg) 40, 980–983 (June 1998)     

Bases for qudits from a nonstandard approach to <Emphasis Type="Italic">SU</Emphasis>(2)

Bases of finite-dimensional Hilbert spaces (in dimension d) of relevance for quantum information and quantum computation are constructed from angular momentum theory and su(2) Lie algebraic methods. We report on a formula for deriving in one step the (1 + p)p qupits (i.e., qudits with d = p a prime integer) of a complete set of 1 + p mutually unbiased bases in C ^p . Repeated application of the formula can be used for generating mutually unbiased bases in C ^d with d = p ^e (e ≥ 2) a power of a prime integer. A connection between mutually unbiased bases and the unitary group SU(d) is briefly discussed in the case d = p ^e .     

Possible phases of the two-dimensional Hubbard model

We present a stability analysis of the 2D t - t' Hubbard model on a square lattice for various values of the next-nearest-neighbor hopping t' and electron concentration. Using the free energy expression, derived by means of the flow equations method, we have performed numerical calculation for the various representations under the point group C _4ν in order to determine at which temperature symmetry broken phases become more favorable than the symmetric phase. A surprisingly large number of phases has been observed. Some of them have an order parameter with many nodes in -space. Commonly discussed types of order found by us are antiferromagnetism, d _{x2 - y2} -wave singlet superconductivity, d-wave Pomeranchuk instability and flux phase. A few instabilities newly observed are a triplet analog of the flux phase, a particle-hole instability of p-type symmetry in the triplet channel which gives rise to a phase of magnetic currents, an s^*-magnetic phase, a g-wave Pomeranchuk instability and the band splitting phase with p-wave character. Other weaker instabilities are found also. A comparison with experiments is made. Received 25 July 2002 / Received in final form 28 November 2002 Published online 14 February 2003 RID="a" ID="a"Current address: Département de physique and Centre de recherche sur les propriétés électroniques de matériaux avancés, Université de Sherbrooke, Sherbrooke, Québec, Canada J1K 2R1 e-mail: vaha@physique.usherb.ca     

Phase diagram of the Mott transition in a two-band Hubbard model in infinite dimensions

The Mott metal-insulator transition in the two-band Hubbard model in infinite dimensions is studied by using the linearized dynamical mean-field theory recently developed by Bulla and Potthoff. The phase boundary of the metal-insulator transition is obtained analytically as a function of the on-site Coulomb interaction at the d-orbital, the charge-transfer energy between the d- and p-orbitals and the hopping integrals between p-d, d-d and p-p orbitals. The result is in good agreement with the numerical results obtained from the exact diagonalization method. Received 5 October 2000 and Received in final form 8 December 2000     

Ferromagnetism of binary compounds with cubic symmetry

Based on the idea of a strong interaction within the same unit cell, the possible existence of a ferromagnetic instability in a system with jumps from transition element cations to non-transition element anions and vice versa is established. A phase diagram is constructed for the ferromagnetic ordering as a function of the degree of filling, n _p and n _d , of the p ⁶-and d ¹⁰-shells of non-transition and transition elements, respectively. Zh. éksp. Teor. Fiz. 115, 1393–1410 (April 1999)     

Electronic spectrum in a microscopical model for the Zn-doped CuO2 plane

We consider a microscopical model for the Zn-doped CuO₂ plane with Zn impurities being described as vacancies for the d-states on Cu sites. A reduction of the original p-d model to an effective one-band model results in the t-J model with vacancies for the spin 1/2 d-states at the Zn-sites. By employing the T-matrix formalism for the Green functions in terms of the Hubbard operators the density of electronic states (DOS) is calculated. Symmetry analysis of the perturbation matrix shows that in the system with d-type electronic wave functions additional DOS of d-, p- and s-types appear due to the perturbation of local energy levels and the interaction between nearest neighbors around the vacancy. The local and resonant state formation caused by Zn impurities is analyzed. Received 24 May 2000 and Received in final form 1 August 2000     

Effects of a k⃗-dependent Hybridisation on the Fermi surface of an extended d–p Hubbard model

ABSTRACT

The topology of the Fermi surface of an extended d–p Hubbard model is investigated using Green's function technique in a n-pole approximation. The effects of the d–p hybridisation on the Fermi surface are the main focus in the present work. Nevertheless, the effects of doping, Coulomb interaction and hopping to second-nearest-neighbours on the Fermi surface, are also studied. Particularly, it is shown that the crossover from hole-like to electron-like Fermi surface (Lifshitz transition) is deeply affected by the d–p hybridisation. Moreover, the pseudogap present in the low doping regime is also affected by the hybridisation. The results show that both the doping and the hybridisation act in the sense of suppresses the pseudogap. Therefore, the systematic investigation of the Fermi surface topology, shows that not only the doping but also the hybridisation can be considered as a control parameter for both the pseudogap and the Lifshitz transition. Assuming that the hybridisation is sensitive to external pressure, the present results agree qualitatively with recent experimental data for the cuprate Nd-LSCO.     

Antiferromagnetism in the Hubbard model

The energy spectrum of the two-sublattice Hubbard model is obtained in the static-fluctuation approximation. It is shown how the structure of the energy spectrum is modified as the parameters of the Hubbard model are varied. The ground state of the simple Hubbard model of dimension d=2 is the dielectric antiferromagnetic state. The author derives a consistency equation for the magnetization, which has an antiferromagnetic solution. Fiz. Tverd. Tela (St. Petersburg) 39, 1594–1599 (September 1997)     

Evidence for strong Coulomb correlations in the metallic phase of vanadium dioxide

The influence of Coulomb correlations on magnetic and spectral properties in the metallic rutile phase of vanadium dioxide is studied by state of the art LDA + DMFT method. Calculation results in strongly correlated metallic state with an effective mass renormalization m*/m ≈ 2. Uniform magnetic susceptibility shows Curie-Weiss temperature dependence with effective magnetic moment, p _eff^theor = 1.54μ_B, in a good agreement with the experimental value p _eff^exp = 1.53μ_B that is close to ideal value for V⁴⁺ ion with the spin S = 1/2, p _eff = 1.73μ_B. Calculated spectral function shows well developed Hubbard bands observable in the recent experimental photoemission spectra. We conclude that VO₂ in rutile phase is strongly correlated metal with local magnetic moments formed by vanadium d-electrons.     

Effective Hubbard model for Zn-doped CuO2 plane

In the framework of the cell-perturbation method for the original p-d model an effective two-band Hubbard model for the CuO₂ plane with Zn impurities is derived. Zn impurities are modelled by Wannir oxygen one-hole states at vacant Cu sites. The model is based on the results of band structure calculations carried out within the local-density approximation. Further reduction to an extended t-J model shows a large ferromagnetic superexchange interaction between the Cu spin with the nearest virtual oxygen spin in the Zn cell. Received 17 November 1998     

Nuclear fusion in the mesic molecule d μ 3He

A new scheme is presented for the physical processes leading to the nuclear fusion reaction d(³He, ⁴He)p catalyzed by a negatively charged muon μ⁻. It is shown that the observable rate and yield of the nuclear reaction depend on a chain of ion-molecular reactions involving the participation of the dμ ³He molecule. New calculations of the nuclear fusion rates in the dμ³He molecule are presented. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 2, 89–94 (25 January 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

d-Symmetry superconductivity as a consequence of valence-bond type correlations

It is shown that d _x ²−y ² symmetry of superconducting order due to valence bond (VB) type correlations is possible. The VB correlations are compatible with antiferromagnetic (AF) spin order. For the two-dimensional Hubbard model with arbitrary doping, the variational method of local unitary transformations is used to construct explicitly a uniform state with VB structure. The d-channel attraction of holes is a consequence of the modulation of hops by the populations of centers accompanying VB formation, and the parameters of the modulation are determined variationally. The increase in the density of states at the Fermi level accompanying AF splitting of the band, which is absent in the paramagnetic state, is important for the gap width. The gap width and its ratio to T _c are of the order of 2Δ≃0.1t and 2Δ/kT _c≃4–4.5 with U/t≃8. The agreement between the phase diagram found and experiment is discussed. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 5, 350–355 (10 March 1998)     

X-ray photoelectron investigation of nanostructures in modified cast irons and steels

A comparative study of industrial samples of classic cast iron, stainless steel, and nanomodified samples was performed. It was shown that the high strength of the obtained materials could be explained by the formation of strong hybridized 3d(Fe) and 3p(Al) bonds, and by the presence of C-C complexes with diamond-like sp ³ hybridization of electrons.     

A Derivative Formula for the Free Energy Function

We consider bond percolation on the Z ^d lattice. Let M _n be the number of open clusters in B(n)=[−n,n] ^d . It is well known that E _p M _n /(2n+1) ^d converges to the free energy function κ(p) at the zero field. In this paper, we show that s²_p(M_n)/(2n+1)^d\sigma^{2}_{p}(M_{n})/(2n+1)^{d} converges to −p(1−p)κ′(p).     

Piezoelectric properties of non-uniform polymeric structures under static load

ABSTRACT

Piezo-electrical properties of structures containing ‘soft’ and ‘hard’ dielectric layers with charge stored on the interfaces are described in the paper. The piezo-activation process of structures containing layers with gas voids by partial discharges is described. The influence of the mechanical properties of the ‘soft’ layer on the piezoelectric parameter d₃₃ value and its dependence on the static pressure p are also discussed. It was found experimentally, that for the fibrous type of ‘soft’ dielectric layer, the dependence of the piezoelectric parameter d₃₃ (p) can be described by the function d₃₃ ÷ p^?n, where n ≤ 1.     

Ising models on hyperbolic graphs

We consider Ising models on a hyperbolic graph which, loosely speaking, is a discretization of the hyperbolic planeH ² in the same sense asZ ^d is a discretization ofR ^d . We prove that the models exhibit multiple phase transitions. Analogous results for Potts models can be obtained in the same way.     

Frustration of antiferromagnetism in the t-t′-Hubbard model at weak coupling

The perfect-nesting instability towards antiferromagnetism of the Hubbard model is suppressed by next-nearest neighbor hopping t′. The asymptotic behavior of the critical coupling U_c(t′) at small t′ is calculated in dimensions d = 2,3, ∞ using Hartree theory; this yields the exact result at least in d > 2. The order of the transition is also determined. A region of stability of a metallic antiferromagnetic phase in d = 3 is identified.     

Flow to strong coupling in the two-dimensional Hubbard model

We extend the analysis of the renormalization group flow in the two-dimensional Hubbard model close to half-filling using the recently developed temperature flow formalism. We investigate the interplay of d-density wave and Fermi surface deformation tendencies with those towards d-wave pairing and antiferromagnetism. For a ratio of next nearest to nearest neighbor hoppings, t'/t = - 0.25, and band fillings where the Fermi surface is inside the Umklapp surface, only the d-pairing susceptibility diverges at low temperatures. When the Fermi surface intersects the Umklapp surface close to the saddle points, d-wave pairing, d-density wave, antiferromagnetic and, to a weaker extent, d-wave Fermi surface deformation susceptibilities grow together when the interactions flow to strong coupling. We interpret these findings as indications for a non-trivial strongly coupled phase with short-ranged superconducting and antiferromagnetic correlations, in close analogy with the spin liquid ground state in the well-understood two-leg Hubbard ladder. Received 23 January 2002     

Precision measurement of vector and tensor analyzing powers in elastic deuteron-proton scattering

High-precision vector and tensor analyzing powers of elastic deuteron-proton ( d + p) scattering have been measured at intermediate energies to investigate effects of three-nucleon forces. Angular distributions in the range of 70^°-120^° in the center-of mass frame for incident-deuteron energies E _d ^lab = 130 and 180 MeV were obtained using the RIKEN facility. The beam polarization was unambiguously determined by measuring the ^12C (d, α)^10B(2⁺) reaction at 0^°. Results of the measurements are compared with state-of-the-art three-nucleon calculations. The present modeling of nucleon-nucleon forces and its extension to the three-nucleon system is not sufficient to describe the high-precision data consistently and requires, therefore, further investigation.     

Phase diagrams of (d x 2−y 2+id xy) superconductors on a square lattice

A model of strongly coupled electrons on a square lattice with attraction of the electrons to nearest-neighbor and next-nearest neighbor lattice sites is studied. For this model, the phase diagrams containing d _x ²−y ², d _xy, and (d _x ²−y ²+id _xy) states are constructed in the variables temperature versus chemical potential for different ratios of the corresponding potentials. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 5, 356–360 (10 March 1998)