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     

1,3(n-π*) excited states of benzaldehyde

Geometries of the first triplet and first singletn-π* excited states of benzaldehyde have been optimized using thesindo 1 molecular orbital wave-function (with CI) and the Newton-Raphson method. The triplet excited state geometry of the molecule is found to be appreciably non-planar whereas that of the singlet excited state is planar. A crossing of molecular orbitals occurs in going from the ground state equilibrium geometry to the triplet and singletn-π* excited state equilibrium geometries. Existence of the para-directing effect of the singletn-π* transition for electronic charges found in an earlier work is confirmed by the present work. The tripletn-π* excitation rearranges electronic charges mainly on the CHO group. It is found that the dipole moment of the molecule would appreciably increase following the singletn-π* excitation whereas the same would appreciably decrease following the tripletn-π* excitation.     

Superconductivity of strongly correlated electrons in copper and ruthenium oxides within the t-J-I model

We propose a t-J-I model with direct ferromagnetic exchange I to explain the superconductivity of copper oxides and the ruthenate Sr₂RuO₄ on the basis of the analysis of the electronic structure of these substances. We analyze the possible p-and d-type superconducting solutions. Solutions of the s type with singlet pairings are impossible in the strong-electron-correlations regime, and p-type solutions correspond to triplet superconductivity and is formed near the ferromagnetic instability threshold in ruthenates. The solution with the symmetry near the antiferromagnetic instability threshold corresponds to copper oxides. We also discuss the reason for the high values of the superconducting transition temperature (T _c ∼100 K) in copper oxides and the low values (T _c ∼1 K) in ruthenates. Zh. éksp. Teor. Fiz. 116, 655–670 (August 1999)     

Competition mechanism between singlet and triplet superconductivity in the tight-binding model with anisotropic attractive potential

Based upon the tight-binding formalism a model of a high-T_c superconductor with isotropic and anisotropic attractive interactions is considered analytically. Symmetry facets of the group C_4v are included within a method of successive transformations of the reciprocal space. Complete sets of basis functions of C_4v irreducible representations are given. Plausible spin-singlet and spin-triplet superconducting states are classified with regard to the chosen basis functions. It is displayed that pairing interaction coefficients and the dispersion relation, which can be characterized by the parameter η= 2t₁/t₀, have a diverse and mutually competing influence on the value of the transition temperature. It is also shown that in the case of a nearly half-filled conduction band and an anisotropic pairing interaction the spin-singlet d-wave symmetry superconducting state is realized for small values of the parameter η, whereas in the opposite limit, for sufficiently large values, the spin-triplet p-wave symmetry superconducting state has to be formed. This result cannot be obtained within the Van Hove scenario or BCS-type approaches, where the p-wave symmetry superconducting state absolutely dominates. The specific heat jump and the isotope shift as functions of the parameter η are assessed and discussed for the d-wave symmetry singlet and the p-wave symmetry triplet states.     

Electronic properties of nano-structured anisotropic superconductors

Using the Bogoliubov-de Gennes equation on the tight-binding electron model, we have investigated nano-structured anisotropic superconductors. For a π/4-rotated square d-wave superconducting plate, the superconducting symmetry becomes s+id, not only at the surfaces, but over the whole superconductor. Also we have investigated quasi-particle structures of this superconducting state.     

On rescattering effects in the reaction π?d → π?d

Rescattering corrections to the impulse approximation for the processes γd → π ⁰ d and π ⁻ d → π ⁻ d are discussed. It is shown that the rescattering effects give a nonnegligible contribution to the real part of these amplitudes. At the same time, the contributions from the imaginary parts of impulse and rescattering corrections drastically cancel each other. This cancellation means that the processes π ⁻ d → π ⁰ nn and γd → π ⁺ nn/π ⁻ pp, when the nucleon pair is in the spin triplet state, are strongly suppressed near threshold as required by the Pauli principle. From Yadernaya Fizika, Vol. 67, No. 4, 2004, pp. 764–768. Original English Text Copyright ? 2004 by Baru, Kudryavtsev, Tarasov. This article was submitted by the authors in English.     

Pseudogap and symmetry of superconducting order parameter in cuprates

The phase diagram, nature of the normal state pseudogap, type of the Fermi surface, and behavior of the superconducting gap in various cuprates are discussed in terms of a correlated state with valence bonds. The variational correlated state, which is a band analogue of the Anderson (RVB) states, is constructed using local unitary transformations. Formation of valence bonds causes attraction between holes in the d-channel and corresponding superconductivity compatible with antiferromagnetic spin order. Our calculations indicate that there is a fairly wide range of doping with antiferromagnetic order in isolated CuO₂ planes. The shape of the Fermi surface and phase transition curve are sensitive to the value and sign of the hopping interaction t′ between diagonal neighboring sites. In underdoped samples, the dielectrization of various sections of the Fermi boundary, depending on the sign of t′, gives rise to a pseudogap detected in photoemission spectra for various quasimomentum directions. In particular, in bismuth-and yttrium-based ceramics (t′>0), the transition from the normal state of overdoped samples to the pseudogap state of underdoped samples corresponds to the onset of dielectrization on the Brillouin zone boundary near k=(0,π) and transition from “large” to “small” Fermi surfaces. The hypothesis about s-wave superconductivity of La-and Nd-based ceramics has been revised: a situation is predicted when, notwithstanding the d-wave symmetry of the superconducting order parameter, the excitation energy on the Fermi surface does not vanish at all points of the phase space owing to the dielectrization of the Fermi boundary at k _x=± k _y. The model with orthorhombic distortions and two peaks on the curve of T _c versus doping is discussed in connection with experimental data for the yttrium-based ceramic. Zh. éksp. Teor. Fiz. 115, 649–674 (February 1999)     

Coherent and incoherent π0 photoproduction from the deuteron

Differential cross-sections for the reactions d (γ,π⁰)d and d (γ,π⁰)pn have been measured at MAMI with the TAPS detector setup in the energy range 140 MeV < E _γ < 306 MeV. By use of the Glasgow tagging spectrometer an 0.8 MeV energy resolution for photons incident on the target was achieved. The π⁰ missing energy resolution was sufficient for a reliable separation of coherent and incoherent channels. The data for the break-up channel exhibit very strong final state interaction effects, whereas the observed angular dependence of the inclusive process d (γ,π⁰)X is in quantitative agreement with predictions for a quasi-free process. The observed absolute d (γ,π⁰)X cross-sections, on the other hand, are significantly smaller than predicted by the quasi-free process for E _γ >∼ 250 MeV. Associating this failure with the π⁰ photoproduction on the neutron would suggest that its cross-section is up to 25% below the presently believed value. Received: 13 February 2001 / Accepted: 13 April 2001     

Remnant Fermi surface in a pseudogap regime of the two-dimensional Hubbard model at finite temperature

A precursor effect on the Fermi surface in the two-dimensional Hubbard model at finite temperatures near the antiferromagnetic instability is studied using three different itinerant approaches: the second order perturbation theory, the paramagnon theory (PT), and the two-particle self-consistent (TPSC) approach. In general, at finite temperature, the Fermi surface of the interacting electron systems is not sharply defined due to the broadening effects of the self-energy. In order to take account of those effects we consider the single-particle spectral function A(, 0) at the Fermi level, to describe the counterpart of the Fermi surface at T = 0. We find that the Fermi surface is destroyed close to the pseudogap regime due to the spin-fluctuation effects in both PT and TPSC approaches. Moreover, the top of the effective valence band is located around = (π/2,π/2) in agreement with earlier investigations on the single-hole motion in the antiferromagnetic background. A crossover behavior from the Fermi-liquid regime to the pseudogap regime is observed in the electron concentration dependence of the spectral function and the self-energy. Received 8 September 2000 and Received in final form 20 December 2000     

Two-dimensional t-J model at moderate doping

Using the method which retains the rotation symmetry of spin components in the paramagnetic state and has no preset magnetic ordering, spectral and magnetic properties of the two-dimensional t-J model in the normal state are investigated for the ranges of hole concentrations 0 ⩽ x ⩽ 0.16 and temperatures 0.01t ⩽ T ⩽ 0.2t. The used hopping t and exchange J parameters of the model correspond to hole-doped cuprates. The obtained solutions are homogeneous which indicates that stripes and other types of phase separation are not connected with the strong electron correlations described by the model. A series of nearly equidistant maxima in the hole spectral function calculated for low T and x is connected with hole vibrations in the region of the perturbed short-range antiferromagnetic order. The hole spectrum has a pseudogap in the vicinity of (0,π) and (π, 0). For x ≈ 0.05 the shape of the hole Fermi surface is transformed from four small ellipses around (±π/2,±π/2) to two large rhombuses centered at (0, 0) and (π,π). The calculated temperature and concentration dependencies of the spin correlation length and the magnetic susceptibility are close to those observed in cuprate perovskites. These results offer explanations for the observed scaling of the static uniform susceptibility and for the changes in the spin-lattice relaxation and spin-echo decay rates in terms of the temperature and doping variations in the spin excitation spectrum of the model. Received 14 November 2002 Published online 1st April 2003 RID="a" ID="a"e-mail: alexei@fi.tartu.ee     

Production of φ mesons in near-threshold πN and NN reactions

We analyze the production of φ mesons in πN and NN reactions in the near-threshold region, using throughout the conventional “non-strange” dynamics based on such processes which are allowed by the non-ideal ω-φ mixing. We show that the occurrence of the direct φNN interaction may show up in different unpolarized and polarization observables in πN→Nφ reactions. We find a strong non-trivial difference between observables in the reactions pp→ppφ and pn→pnφ caused by the different role of the spin singlet and triplet states in the entrance channel. A series of predictions for the experimental study of this effect is presented. Received: 27 January 2000     

A non-perturbative open-shell theory for atomic and molecular systems: Application to transbutadiene

A non-perturbative theory is proposed in this article in which an energy independent effective Hamiltonian is obtained for open-shell systems. We have given a diagrammatic version of theory to facilitate the analysis of the problem. The theory has been applied to a model 4-π electron problem, for calculating the lowestπ-π* singlet and triplet energy levels of transbutadiene. Comparison with full Cl calculation indicates the excellent workability of the theory.     

On the mechanism of dissipation in the internal Josephson effect in layered superconductors at low temperatures

The current-voltage characteristics (IVCs) of a layered superconductor with singlet d pairing at low temperatures are calculated in the internal Josephson effect (IJE) regime. Coherent electron tunneling between layers is assumed. A finite resistance of the superconductor in the resistive state arises because of quasiparticle transitions through the superconducting gap near nodes. Because of charge effects the interaction of the Josephson junctions formed by the layers does not lead to substantial differences in the shapes of different branches of the IVCs. The model describes the basic qualitative features of the effect in high-temperature superconductors for voltages which are low compared with the amplitude of the superconducting gap. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 516–521 (25 October 1999)     

The phase diagram of the superconducting state of superconductor-band-antiferromagnet superlattices

The formation of the superconducting phase in short-period proximity-effect layered superlattices of the superconductor-band-antiferromagnetic-metal (SC/AF) type is studied. The exact solution of the Usadel equations is used to discuss the possibility of formation in such structures of a ground state in which the order parameters of the adjacent superconducting layers have opposite signs (the “π-phase”). The dependence of the superconducting transition temperature and the upper critical field normal to the layers on the lattice period, the intensity of magnetic interaction in the antiferromagnetic layer, and the state of the interface between the layers is examined. It is found that there exists a nonlinear dependence of the conditions for the appearance of the superconducting state in a layered SC/AF system on the system’s parameters. Finally, the conditions for the appearance of the superconducting phase in proximity-effect superlattices consisting of a superconductor with nonmagnetic, ferromagnetic, and antiferromagnetic metals are compared. Zh. éksp. Teor. Fiz. 111, 547–561 (February 1997)     

Spin susceptibilities for the Hubbard model in a band approach

The properties of spin excitations superposed on a uniform ground state with antiferromagnetic (or spiral) spin structure are studied in a 2D Hubbard model. Expressions are derived for the spin susceptibility in the random phase approximation (RPA) using split Hubbard bands as a zeroth approximation. The calculated collective modes with dispersion ω(Q)=c|Q−(π, π)| near Q∼(π, π) reproduce well the characteristics of the spin excitations observed in undoped cuprates. For doped systems with an antiferromagnetic structure of the ground state, calculating X″(Q,ω→0) gives the same mode with a peak at Q∼(π, π), regardless of the type of Fermi surface. It is shown that in doped systems with a spiral ground state spin structure, X″(Q,ω→0) peaks occur with incommensurate quasimomenta Q that are coupled to the spirality vector. Zh. éksp. Teor. Fiz. 116, 1058–1080 (September 1999)     

Studying the ω mass in-medium in γ + A →π0γ + X reactions

Simulations based on a coupled-channel transport model have been performed to analyze the feasibility to study the in-medium ω mass exploiting the process γ + A→π⁰γ + X for C, Ca and Nb nuclei. The distortions due to final-state interactions of the π⁰ and background contributions from the reaction γ + A→π⁰π⁰ + X are found to be small in the mass range of interest ( 0.6 < M _π0γ < 0.8 GeV). Furthermore, the effect of the detector resolution on the π⁰γ mass determination is discussed. Received: 5 December 2000 / Accepted: 18 May 2001     

Intersystem crossing in oligothiophenes studied by fs time-resolved spectroscopy

High triplet quantum yields of more than 90% for bithiophene and terthiophene have to be connected with very fast and effective formation of triplets after excitation. We studied these processes with fs pump–probe spectroscopy. The time behaviour of transient optical spectra within the singlet and triplet manifold was examined for bi- and terthiophene (2T and 3T) in solution. For 2T we used two-photon absorption for excitation. We found transient spectra of the excited singlet state, the triplet state and that of radical cations. The kinetics of the excited-state absorption was described by a bi-exponential function. Additionally we observed formation and recombination of radical cations. The recombination is connected with triplet formation. Both processes could be described by a time constant of 62 ps±9 ps. For 3T we found a dependence of the processes on excitation energy using one-photon absorption. The triplet quantum yield increased with higher excitation energy. The kinetics becomes bi-exponential with increasing amplitude of the short time constant of 2 ps at increasing excitation energy. The main reasons for the effective intersystem crossing (ISC) in both oligothiophenes are – besides the high spin-orbit coupling factor introduced by the sulphur atom – the almost isoenergetic positions of the S ₁ and T ₂ states, detected by PD-PES [1]. At higher photon excitation energy for 3T above the band gap an additional channel for ISC was detected. We believe that during the geometric change from the non-relaxed non-planar to the relaxed planar excited state S ₁, ultrafast intersystem crossing takes place. Received: 6 December 1999 / Published online: 2 August 2000     

Superconductor order parameter symmetry in UPd Al

The heavy fermion compound UPd₂Al₃ has attracted much interest on account of the coexistence of antiferromagnetism and superconductivity at temperatures below 2 K. The antiferromagnetic fluctuations provide, principally via inelastic neutron scattering, a window on the low frequency dynamics in this material. By an analysis of neutron scattering data, and taking into consideration results from other experimental probes, it is suggested which sheet(s) of the f-electron Fermi surface may play an active role in forming the superconducting state in UPd₂Al₃. The proposed scheme sheds new light on previously reported anomalies in this material. Received 16 July 1999     

Coexistence of superconductivity and ferromagnetism in ferromagnet/superconductor proximity structures

The Nambu spinor Green's function approach is applied to calculating the density of states (DOS) and superconducting order parameter in normal-metal/insulator/ferromagnet/superconductor (NM/I/FM/SC) junctions. It is found that the s-wave superconductivity and ferromagnetism can coexist near the FM/SC interface, which is induced by proximity effect. On the SC side, the spin-dependent DOS appears both within and without the energy gap. On the FM side, the superconducting order parameter displays a damped oscillation and the DOS exhibits some superconducting behavior. The calculated result for the DOS in FM for “0 state” and “π state” can reproduce recent tunneling spectra in Al/Al₂O₃/PdNi/Nb tunnel junctions. Received 1st July 2002 Published online 19 November 2002     

The reaction pp→pΛK + near threshold

We analyze the recent total cross section data for pp→pΛK ⁺ near threshold measured at COSY. Using an effective range approximation for the on-shell pΛ S-wave final state interaction we extract from these data the combination ?= (2|K _s|²+|K _t|²)^−1/2= 0.38 fm⁴ of the singlet (K _s) and triplet (K _t) threshold transition amplitudes. We present an exploratory calculation of various (tree-level) vector and pseudoscalar meson exchange diagrams. Pointlike ω-exchange alone and the combined (ρ⁰,ω,K^*+)-exchange can explain the experimental value of ?. The pseudoscalar meson exchanges based on a SU(3) chiral Lagrangian turn out to be too large. However, when adding π⁰-exchange in combination with the resonant πN→S ₁₁(1650) →KΛ transition and introducing monopole form factors with a cut-off Λ_c= 1.5 GeV one is again able to reproduce the experimental value of ?. More exclusive measurements are necessary to reveal the details of the pp→pΛK ⁺ production mechanism. Received: 28 October 1998 / Revised version: 12 January 1999