Effect of spin fluctuations on the superconducting phase of Hubbard fermions in the t-t′-t″-J* model

The effect of spin-fluctuation scattering processes on the region of the superconducting phase in strongly correlated electrons (Hubbard fermions) is investigated by the diagram technique for Hubbard operators. Modified Gor’kov equations in the form of an infinitely large system of integral equations are derived taking into account contributions of anomalous components $ P_{0\sigma ,\bar \sigma 0} The effect of spin-fluctuation scattering processes on the region of the superconducting phase in strongly correlated electrons (Hubbard fermions) is investigated by the diagram technique for Hubbard operators. Modified Gor’kov equations in the form of an infinitely large system of integral equations are derived taking into account contributions of anomalous components of strength operator . It is shown that spinfluctuation scattering processes in the one-loop approximation for the t-t′-t″-J* model taking into account long-range hoppings and three-center interactions are reflected by normal (P _{0σ, 0σ}) and anomalous () components of the strength operator. Three-center interactions result in different renormalizations of the kernels of the integral equations for the superconducting d phase in the expressions for the self-energy and strength operators. In this approximation for the d-type symmetry of the order parameter for the superconducting phase, the system of integral equations is reduced to a system of nonhomogeneous equations for amplitudes. The resultant dependences of critical temperature on the electron concentrations show that joint effect of long-range hoppings, three-center interactions, and spin-fluctuation processes leads to strong renormalization of the superconducting phase region. Original Russian Text ? V.V. Val’kov, A.A. Golovnya, 2008, published in Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2008, Vol. 134, No. 6, pp. 1167–1180.     

Excitation gap in the normal and superconducting state of underdoped Bi2Sr2Ca1-xDyxCu2O8+\delta thin film and single crystals

We summarize photoemission data from underdoped Bi₂Sr₂Ca_1-xDy_xCu₂O_8+\delta that revealed anomalous properties: i) We observed an excitation gap in the normal state of underdoped samples, and this normal state gap closes in overdoped samples; ii) The normal state gap has similar magnitude and momentum dependence as the superconducting gap, which is consistent with a d_x ² _-y ² order parameter; iii) The normal state gap persists to a temperature range much higher than T_c; iv) The superconducting gap in the underdoped regime does not scale with T_c. These results are consistent with theoretical models that suggest the underdoped regime can be characterized by two temperatures, a mean‐field temperature below which there is pairing, and a lower superconducting transition temperature at which the pairs become phase coherent. This revised version was published online in August 2006 with corrections to the Cover Date.     

Coulomb repulsion-induced hyperbolic pairing as a mechanism of high-T c superconductivity

The hyperbolic metric of the dispersion law (the effective mass tensor components of carriers are opposite in sign) in the vicinity of the Fermi contour in high-T _c superconducting cuprates in the case of repulsive interaction gives rise to a superconducting state characterized by the condensate of pairs with a large total momentum (hyperbolic pairing). The gain in the energy of the superconducting state over the normal state is due to the fact that a change in the kinetic energy of pairs (because of the negative light component of the effective mass) dominates over the change in the potential energy (corresponding to energy loss). The shift of the chemical potential upon the transition to the superconducting phase is substantial in this case. With increasing repulsive interaction, the superconducting gap δ_K increases and the resulting gain in energy changes to an energy loss at a certain critical value of the repulsive potential. The low temperature T _c of the superconducting transition and the large value of δ _K in this region of potential values are the reasons for the high value of the 2δ_K/T _c ratio and for the developed quantum fluctuations that are observed in underdoped cuprate superconductors.     

A pseudopotential approach to the superconducting state properties of Cu<Subscript>C</Subscript>Zr<Subscript>100-C</Subscript> binary metallic glasses

The theoretical investigation of the superconducting state properties viz. electron-phonon coupling strength λ, Coulomb pseudopotential μ*, transition temperature T _C, isotope effect exponent α and effective interaction strength N ₀ V of ten binary Cu_CZr_100-C (C = 25–60 at%) metallic glasses is performed, using Ashcroft’s empty core model potential. Five local-field correction functions proposed by Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used to study the screening influence on the aforesaid properties. It is shown that the electronphonon coupling strength λ and the transition temperature T _C are quite sensitive to the selection of the local-field correction functions, whereas the Coulomb pseudopotential μ*, isotope effect exponent α and effective interaction strength N ₀ V show a weak dependence on the local-field correction functions. The values of T _C obtained from the H-local-field correction function are found to be in qualitative agreement with available theoretical or experimental data and show almost linear behavior with respect to the concentration C of Cu. The present results are shown to be in good agreement with other available theoretical or experimental data. The obtained results confirm the existence of the superconducting phase in the metallic glasses.     

Charge ordering,superconductivity, and stripes in doped La<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

The special features of the phase diagrams of La_2?xSr_xCuO₄ are considered in terms of the high-temperature superconductivity model according to which the mechanism responsible for the anomalous properties of these compounds is the interaction of electrons with diatomic negative U-centers. A microstructural model that assumes the coexistence of domains with different types of dopant ion ordering is suggested for La_2?xSr_xCuO₄. According to this model, the main characteristics of the experimental phase diagrams of La_2?xSr_xCuO₄ only reflect square lattice geometric relations and competition between different dopant ordering types. Close agreement between the calculated and experimental “superconducting” and “magnetic” phase diagrams is an important argument in favor of the suggested high-temperature superconductivity model.     

亚稳中间相的电导和其反常超导相变的机理

我们系统地测得了在低温凝聚InSb膜亚稳中间相的不同结构下其σ(T_a)与温度T的关系,发现相应于某些特定σ(T_a)的相变区,样品超导转变的R(T)曲线出现奇异的反常现象。本文提出一个类半导体相和亚稳金属相的混合态模型来解释这些现象,并且按此模型做了理论计算,理论曲线和实验数值较好地符合。 关键词：     

Semiconductor–superconductor phase transition of the compound LaO0.89F0.11FeSb

We report the synthesis and superconductivity in iron-based oxypnictide LaO_0.89F_0.11FeSb, with the phase transition in the resistivity at 78.6 K and the onset transition temperature at 20.3 K. This compound showed an anomalous change in the resistivity slope, where a transition between the semiconducting and superconducting state has occurred.     

Normal phase and superconducting instability in the attractive Hubbard model: a DMFT(NRG) study

We study the normal (nonsuperconducting) phase of the attractive Hubbard model within the dynamical mean field theory (DMFT) using the numerical renormalization group (NRG) as an impurity solver. A wide range of attractive potentials U is considered, from the weak-coupling limit, where superconducting instability is well described by the BCS approximation, to the strong-coupling region, where the superconducting transition is described by Bose condensation of compact Cooper pairs, which are formed at temperatures much exceeding the superconducting transition temperature. We calculate the density of states, the spectral density, and the optical conductivity in the normal phase for this wide range of U, including the disorder effects. We also present the results on superconducting instability of the normal state dependence on the attraction strength U and the degree of disorder. The disorder influence on the critical temperature T _c is rather weak, suggesting in fact the validity of Anderson’s theorem, with the account of the general widening of the conduction band due to disorder.     

Pressure dependence of the superconducting state parameters of a binary Ca<Subscript>70</Subscript>Mg<Subscript>30</Subscript> metallic glass superconductor

Theoretical computation of the pressure dependence of superconducting state parameters of a binary Ca₇₀Mg₃₀ metallic glass has been performed using the model potential formalism. Explicit expressions have been derived for the volume dependence of the electron-phonon coupling strength λ and the Coulomb pseudopotential μ*, considering the variation of the Fermi momentum k _F and Debye temperature θ_D with volume. Well-known Ashcroft’s empty core model pseudopotential and five different types of the local-field correction functions, namely, Hartree, Taylor, Ichimaru-Utsumi, Farid et al. and Sarkar et al. have been used for obtaining pressure dependence of transition temperature T _C and the logarithmic volume derivative Φ of the effective interaction strength N ₀ V for the metallic glass superconductor. It has been obtained that T _C of Ca₇₀Mg₃₀ metallic glass decreases rapidly with increasing pressure up to 60% decrease in the volume, for which the μ* and Φ curves show a linear nature. The superconducting phase disappears at about 60% decrease in the volume.     

Symbiotic Fusion Through Single-Crystal Li6D

A symbiotic fusion scheme is introduced. In the first phase of the process, a low-density current of deuterons propagates in a circular tunnel in the interior of a block of single-crystal Li⁶D with cubic crystal structure. Energy of the deuterons is limited by the condition that they not be neutralized by collisions with the tunnel wall. Deuteron current is driven through a rapidly rising magnetic field normal to the plane of the loop. Because of the coupling of the beam to the periodic array of molecules in the host, the beam goes into extended states. At the critical temperature (T _c 9.07 K) it becomes superconducting and with sufficient wave function overlap in this phase, it is proposed that fusion takes place. A superconducting product wave function is given by Gaussian space components and spin-1 functions polarized in the direction of the applied magnetic field. The Landau—Ginzberg equation is employed to calculate the coherence length for this process which is found to be of the order of the spread of the Gaussian per period of the wave function. This value of coherence length is consistent with significant wavefunction overlap. In the second phase, for a cubic fuel sample of edge length 15 cm, emitted particles interact with the host nuclei in a chain reaction. Assuming a probability of 0.01 that deuterons in the superconducting loop fuse, in 1 ms the device produces a yield 0.2 GJ. Injection and magnetic rise-time intervals are described and the interval over which the beam goessimultaneously to a superconducting phase from an extended state that fills the current tube. A model is described for this process that gives a criterion for the onset of fusion. A study of deuteron interaction in the superconducting state leads to a requirement for the tensile strength of the tunnel material. Another study derives the property that the Coulomb singularity between deuterons is reduced in the superconducting phase.     

Quasiparticles in the superconducting state of high-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> metals

We consider the behavior of quasiparticles in the superconducting state of high-T_c metals within the framework of the theory of the superconducting state based on the fermion condensation quantum phase transition. We show that the behavior coincides with the behavior of Bogoliubov quasiparticles, whereas the maximum value of the superconducting gap and other exotic properties are determined by the presence of the fermion condensate. If at low temperatures the normal state is recovered by the application of a magnetic field suppressing the superconductivity, the induced state can be viewed as a Landau-Fermi liquid. These observations are in good agreement with recent experimental facts.     

Phase transitions in hybrid SFS structures with thin superconducting layers

Features of a phase transition between 0 and π states in superconductor/ferromagnet/superconductor (SFS) Josephson structures with thin superconducting layers and a ferromagnetic barrier are studied experimentally and theoretically. The dependence of the critical temperature T_c of a transition of the hybrid structure to a superconducting state on the thickness of superconducting layers d_s is analyzed by a local method involving measurements of the nonlinear microwave response of the system by a near-field probe. An anomalous increase in the measured temperature T_c at the reduction of the thickness d_s is detected and is attributed to the 0-π transition.     

Double-stage continuous-discontinuous superconducting phase transition in the Pauli paramagnetic limit of a 3D superconductor: the URu<Subscript>2</Subscript>Si<Subscript>2</Subscript> case

The sharp suppression of the de-Haas van-Alphen oscillations observed in the mixed superconducting (SC) state of the heavy fermion compound URu₂Si₂ is shown to confirm a theoretical prediction of a narrow double-stage SC phase transition, smeared by fluctuations, in a 3D paramagnetically-limited superconductor. The predicted scenario of a second order transition to a nonuniform (FFLO) state followed by a first order transition to a uniform SC state, obtained by using a non-perturbative approach, is also found to be consistent with recent thermal conductivity measurements performed on this material.     

Vibrational and electronic properties of a Cu<Subscript>90</Subscript>Nb<Subscript>10</Subscript> nanocrystalline composite

The vibrational, electronic, and superconducting properties of a Cu₉₀Nb₁₀ nanocrystalline composite undergoing a transition from a coarse-grained state to a nanocrystalline state are investigated using neutron scattering and low-temperature heat capacity measurements. It is found that, compared to a coarse-grained sample, the nanocomposite is characterized by a higher density of low-frequency excitations and a decrease both in the density of states and in the superconducting transition temperature due to the size effect.     

Superconducting state parameters of La<Subscript>100-<Emphasis Type="Italic">C</Emphasis></Subscript>Ga<Subscript><Emphasis Type="Italic">C</Emphasis></Subscript> binary metallic glasses

The theoretical investigations of the superconducting state parameters (SSP) viz. electron-phonon coupling strength λ, Coulomb pseudopotential μ*, transition temperature T _C , isotope effect exponent α and effective interaction strength N _O V of six binary La_100-C Ga _C (C = 16, 20, 22, 24, 26 and 28 at. %) metallic glasses have been reported using Ashcroft’s empty core (EMC) model potential for the first time. Five local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used in the present investigation to study the screening influence on the aforesaid properties. It is observed that the electron-phonon coupling strength λ and the transition temperature T _C are quite sensitive to the selection of the local field correction functions, whereas the Coulomb pseudopotential μ*, isotope effect exponent α and effective interaction strength N _O V show weak dependences on the local field correction functions. The T _C obtained from H-local field correction function are found in qualitative agreement with available experimental data and show almost linear nature with the concentration (C) of ‘Ga’ element. A linear T _C equation is proposed by fitting the present outcomes for H-local field correction function, which is in conformity with other results for the experimental data. Also, the present results are found to be in qualitative agreement with other such earlier reported data, which confirms the superconducting phase in the metallic glasses.      

Anomalous behavior of the structural parameters of YBa2Cu4O8 single crystals at the transition to the superconducting state

High-precision x-ray crystallographic studies of YBa₂Cu₄O₈ single crystals (T _c =70 K) are performed at eight temperatures in the interval 20–295 K. It is found that a number of structural parameters exhibit anomalous behavior near the superconducting transition of the crystal. A characteristic effect near the phase transition is the displacement of the O1 bridge atom that joins the Cu1 atom of the cuprate chain to the Cu2 atom of the cuprate plane. The shift of this oxygen toward the Cu2 atom is indicative of a change in the Cu2-O1 chemical bond and of charge transfer to the cuprate plane in the process of the transition of the crystal to the superconducting state. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 7, 502–506 (10 October 1997)     

Resonance enhancement of electron-phonon interaction

A superconducting state near the phase transition curve from paramagnetic phase to superconducting phase for perovskites La_{2 ? x} Sr _x CuO₄ and YBa₂Cu₃O₆ is considered. Expressions for effective parameters of electron-spin-phonon interaction are obtained. It is shown that the critical temperature of the phase transition from paramagnetic phase to superconducting phase T _c is determined by the enhancement of electron-phonon interaction by spin fluctuations of exchange type.     

Low-temperature anomalies in the specific heat and thermal conductivity of MgB<Subscript>2</Subscript>

The temperature dependences of the specific heat C(T) and thermal conductivity K(T) of MgB₂ were measured at low temperatures and in the neighborhood of T _c . In addition to the well-known superconducting transition at T _c ≈40 K, this compound was found to exhibit anomalous behavior of both the specific heat and thermal conductivity at lower temperatures, T≈10–12 K. Note that the anomalous behavior of C(T) and K(T) is observed in the same temperature region where MgB₂ was found to undergo negative thermal expansion. All the observed low-temperature anomalies are assigned to the existence in MgB₂ of a second group of carriers and its transition to the superconducting state at T_c2≈10?12 K.     

Magnetic susceptibility of fine crystalline high-temperature YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.93</Subscript> superconductors. The role of nanoscale structural disordering

Static magnetic susceptibility χ(T) in the normal state (T_c ≤ T ≤ 400 K) and specific heat C(T) near temperature T_c of the transition to the superconducting state are experimentally studied for a series of fine crystalline samples of high-temperature YBa₂Cu₃O_y superconductor, having y and T_c close to optimal but differing in the degree of nanoscale structural disordering. It is shown that under the influence of structural disordering, there is enhancement of anomalous pseudogap behavior of the studied characteristics and a significant increase in the width of the pseudogap.     

Theoretical studies of superconductivity in doped BaCoSO

We investigate superconductivity that may exist in the doped BaCoSO, a multi-orbital Mott insulator with a strong antiferromagnetic ground state. The superconductivity is studied in both t-J type and Hubbard type multi-orbital models by mean field approach and random phase approximation (RPA) analysis. Even if there is no C₄ rotational symmetry, it is found that the system still carries a d-wave like pairing symmetry state with gapless nodes and sign changed superconducting order parameters on Fermi surfaces. The results are largely doping insensitive. In this superconducting state, the three \({t_{{2_g}}}\) orbitals have very different superconducting form factors in momentum space. In particular, the intra-orbital pairing of the \({d_{{x^2} - {y^2}}}\) orbital has an s-wave like pairing form factor. The two methods also predict very different pairing strength on different parts of Fermi surfaces. These results suggest that BaCoSO and related materials can be a new ground to test and establish fundamental principles for unconventional high temperature superconductivity.