On the theory of the Kondo effect in two-dimensional metals

The Kondo effect in a (quasi-)two-dimensional metal is studied. The special feature of the two-dimensionality is the Van Hove singularity in the electron density of states. For the band filling choosen such, that the Fermi level is close to the saddle points of the band spectrum, the Van Hove singularity comes into play and changes the usual Kondo log to the log². It turnes out to be possible to carry out the first order parquet summation and to obtain the conditions for the Kondo antiferromagnetic resonance for an arbitrary geometry of the band spectrum. The connection with the Orthogonality Catastrophe is traced and it is shown, that the weak coupling Kondo problem just corresponds to the intermediate asymptotics of the metal's relaxation in a time-dependent external potential.     

Spontaneous deformation of the fermi surface due to strong correlation in the two-dimensional t- J model

The Fermi surface of the two-dimensional t- J model is studied using the variational Monte Carlo method. We study the Gutzwiller-projected d-wave superconducting state with an additional variational parameter t(')(v) corresponding to the next-nearest-neighbor hopping term. It is found that the finite t(')(v)<0 gives the lowest variational energy in the wide range of hole-doping rates. The obtained momentum distribution function shows that the Fermi surface deforms spontaneously. It is also shown that the Van Hove singularity is always located very close to the Fermi energy. Using the Gutzwiller approximation, we show that this deformation is due to the Gutzwiller projection operator or the strong correlation.     

Kinematics of electrons near a van hove singularity

A two-dimensional electronic system, where the Fermi surface is close to a Van Hove singularity, shows a variety of weak coupling instabilities. It is a convenient model to study the interplay between antiferromagnetism and anisotropic superconductivity. We present a detailed analysis of the kinematics of electron scattering in this model. The similarities and differences between a standard renormalization group approach and previous work based on parquet summations of log (2) divergences are analyzed, with emphasis on the underlying physical processes. General properties of the phase diagram are discussed.     

Robustness of the Van Hove scenario for high-T(c) superconductors

The pinning of the Fermi level to the Van Hove singularity and the formation of flat bands in the two-dimensional t-t' Hubbard model is investigated by the renormalization group technique. The "Van Hove" scenario of non-Fermi-liquid behavior for high-T(c) compounds can take place in a broad enough range of the hole concentrations. The results are in qualitative agreement with the recent angle-resolved photoemission spectroscopy data on La 2CuO (4).     

Doping and temperature dependence of the spin susceptibility in the p-d model

The spin magnetic susceptibility of the p-d model is calculated by means of a perturbation theory in the hybridization term V through a generalized cumulant expansion (GCE). The analysis is approached from the paramagnetic metallic phase. The results qualitatively reproduce some unusual magnetic properties in the normal state of the hole-doped cuprates, supporting the scenario of a Van Hove singularity near the Fermi level. Received 15 October 1998 and Received in final form 24 March 1999     

Photoemission quasiparticle spectra of Sr2RuO4

Multiband quasiparticle calculations based on perturbation theory and dynamical mean-field methods show that the creation of a photoemission hole state in Sr2RuO4 is associated with a highly anisotropic self-energy. Since the narrow Ru-derived d(xz,yz) bands are more strongly distorted by Coulomb correlations than the wide d(xy) band, charge is partially transferred from d(xz,yz) to d(xy), thereby shifting the d(xy) Van Hove singularity close to the Fermi level.     

One- and two-channel Kondo model with logarithmic Van Hove singularity: A numerical renormalization group solution

Simple scaling consideration and NRG solution of the one- and two-channel Kondo model in the presence of a logarithmic Van Hove singularity at the Fermi level is given. The temperature dependences of local and impurity magnetic susceptibility and impurity entropy are calculated. The low-temperature behavior of the impurity susceptibility and impurity entropy turns out to be non-universal in the Kondo sense and independent of the s–d coupling J. The resonant level model solution in the strong coupling regime confirms the NRG results. In the two-channel case the local susceptibility demonstrates a non-Fermi-liquid power-law behavior.     

Spin fluctuations and ferromagnetic order in two-dimensional itinerant systems with Van Hove singularities

The quasistatic approach is used to analyze the criterion of ferromagnetism for two-dimensional (2D) systems with the Fermi level near Van Hove (VH) singularities of the electron spectrum. It is shown that the spectrum of spin excitations (paramagnons) is positively defined when the interaction between electrons and paramagnons, determined by the Hubbard on-site repulsion U, is sufficiently large. Due to incommensurate spin fluctuations near the ferromagnetic quantum phase transition, the critical interaction U_c remains finite at VH filling and exceeds considerably its value obtained from the Stoner criterion. A comparison with the functional renormalization group results and mean-field approximation which yields a phase separation is also performed.     

Electronic structure and Fermi surface topology of the infinite-layered superconductor Sr1−xCaxCuO2

Superconductivity reported at 110 K, and recently at 150–170 K, in the infinite-layered (Sr_xCa_1−x)CuO₂ system is investigated by means of the full potential linear muffin-tin orbital (FLMTO) method. As in other high-T_c cuprates, the electronic structure of the parent compounds, CaCuO₂ and SrCuO₂, and of the separately calculated composition Sr_0.7Ca_0.3CuO₂ using the experimental lattice parameters, displays strong 2D features including a low density of states at E_F (lower than in the other cuprates) and a simple 2D Fermi surface (rounded square) with strong nesting due to a single hybridized Cu d-O p band. As in La₂CuO₄, a major van Hove saddle-point singularity exists near E_F. The drastic changes of the Fermi surface when Ca/Sr vacancies shift the Fermi energy to the van Hove singularity may have a strong influence on the superconducting properties of the compounds and indicate the need for Sr/Ca vacancies in inducing the high T_c.     

What is the highest T c for phonon-mediated superconductivity?

We suggest that the high-temperature superconductivity can be attributed to the director-roles of the van Hove singularity between an electron-electron interaction and an electron-phonon interaction. The difference between the critical temperature and the pairing temperature is presented, and the Fermi arc, the d-wave symmetry and the poor conductivity, etc., are discussed. In particular, the non-s-wave symmetry is predicted to have the highest T _c for superconductors.     

Magnetic fluctuations and itinerant ferromagnetism in two-dimensional systems with Van Hove singularities

A criterion for ferromagnetism in two-dimensional systems with the Fermi level near Van Hove singularities (VHSs) is analyzed. In the quasistatic approximation applied to a spin-fermion model, it is shown that the spectrum of spin excitations (paramagnons) is positively defined when the interaction I between the electronic and spin degrees of freedom is sufficiently large (I > I _c). The critical interaction I _c is much greater than its value determined from the Stoner criterion; hence, the latter criterion is not an adequate criterion for ferromagnetism in the presence of Van Hove singularities in the electronic spectrum. By combining the quasistatic approximation with the method of equations of motion, the electronic self-energy is obtained in the first order in the inverse number of spin components.     

s+d混合波对称性下联合模型的超导电性

提出一个联合模型，在ｓ＋ｄ混合波对称性下，合并考虑电子间配对相互作用的各向异性及二维电子结构上Ｖａｎ Ｈｏｖｅ奇异性对超导电性的影响．理论结果表明：Ｖａｎ Ｈｏｖｅ奇异性及配对相互作用的各向异性都是使Ｔ_c提高的重要因素；各向异性的电子配对相互作用自然导致序参量的ｄ波成分，当此各向异性增强时，ｄ波成分也增大．高温超导体的较高２Δ（０）／ｋ_ＢＴ_c值可能预示着在这些材料中ｓ波的权重远小于ｄ波权重．联合效应模型下的Ｔ_c处比热跳跃行为与经典的ＢＣＳ理论也完全不同 关键词：     

On the Fermi surface of some organic superconductors with sizeable critical temperatures

Recent studies of the Fermi surface of quasi-two-dimensional organic conductors comfort the idea that the presence of a van Hove anomaly near the Fermi level is a favourable factor for superconductivity. Received: 28 April 1998 / Revised and accepted: 13 May 1998     

Impurity-induced anisotropic semiconductor-semimetal transition in monolayer biased black phosphorus

Taking into account the electron-impurity interaction within the continuum approximation of tight-binding model, the Born approximation, and the Green's function method, the main features of anisotropic electronic phase transition are investigated in monolayer biased black phosphorus (BP). To this end, we concentrated on the disordered electronic density of states (DOS), which gives useful information for electro-optical devices. Increasing the impurity concentration in both unbiased and biased impurity-infected single-layer BP, in addition to the decrease of the band gap, independent of the direction, leads to the midgap states and an extra Van Hove singularity inside and outside of the band gap, respectively. Furthermore, strong impurity scattering potentials lead to a semiconductor-semimetal transition and one more Van Hove singularity in x-direction of unbiased BP and surprisingly, this transition does not occur in biased BP. We found that there is no phase transition in y-direction. Since real applications require structures with modulated band gaps, we have studied the influence of different bias voltages on the disordered DOS in both directions, resulting in the increase of the band gap.     

Optical confinement of spatial frequencies in the photonic crystal waveguide

The optical confinement of spatial frequencies in the photonic crystal waveguide has been investigated theoretically and simulated numerically. It is found that the enhanced gap confinement is at frequency close to the upper band edge, in contrary to the conventional concept that the strongest optical confinement is found at frequencies near the mid-gap. The anomalous phenomenon may be attributed to a Van Hove saddle point singularity in a band adjacent to a photonic crystal band gap. In general, the saddle point favors the appearance of a very flat band, which in turn causes an enhanced confinement at band-gap frequencies.     

Possible spin-tripletf-wave pairing due to disconnected fermi surfaces in NaxCoO2.yH2O

We propose that the spin-triplet pairing mechanism due to disconnected Fermi surfaces proposed in our previous study [Phys. Rev. B 63, 174507 (2001)]] may be at work in a recently discovered superconductor NaxCoO2.yH2O. We introduce a single band effective model that takes into account the pocketlike Fermi surfaces along with the van Hove singularity near the K point found in the band calculation results. Applying the fluctuation exchange method and solving the linearized Eliashberg equation, the most dominant pairing is found to have spin-triplet f-wave symmetry, where the nodes of the gap function do not intersect the pocket Fermi surfaces. The presence of finite Tc is suggested in sharp contrast to cases when the gap nodes intersect the Fermi surface.     

On the enhancement of superconductivity due to Van Hove singularity

We have rigorously solved the standard BCS gap equation with the density of states of Van Hove type. The result shows that the enhancement ofT _c is not as effective as previously estimated. It seems unlikely that Van Hove singularity inN(E) can be theT _c -enhancement mechanism for the high-T _c Cu oxides.     

Two-dimensional Van Hove singularity in TaS2 band structure

The transverse optical properties of a number of transition metal dichalcogenides have been studied in the 2 eV – 12 eV range. For TaS₂, a two-dimensional Van Hove singularity clearly appears at 7 eV in good agreement with the saddle point case.     

高Tc氧化物超导体的电子结构

本文综述了高Tc氧化物超导体(La_2CuO_4及YBa_2Cu_3O_(7-8)的(计算)电子结构。本文基于Freeman教授研究组的工作。 (1) 体心四方结构的La_2CuO_4能带结构计算结果表明:La_2CuO_4的电子结构由在正方形Cu-O平面内的Cu3d-O2p相瓦作用所控制。存在一个沿[110]方向|q|=2k_F(强的)迭套的Fermi表面,它导致了电子驱动的结构失稳,以及在电子状态密度上的VanHove对数奇性。此种Fermi面的迭套被认为是引起了(Peierls 2k_F)失稳性,从而解释了La_(2-x)M_xCuO_4由四方结构到正交结构的相变。 (2) 如同在La_(2-x)M_xCuO_4中的情况一样,具正交结构的YBa_2Cu_3O_(7-8)的电子结构由Cu-O间的相互作用所支配;同时存在着二维及一维的Cu3d-O2p相互作用为YBa_2Cu_3O_(7-8)电子结构的特征。YBa2Cu_3O_(7-8)在E_F处的状态密度值低于La_(2-x)M_xCuO_4的相应值,在YBa_2Cu_3O_(7-8)中的Y或Ba原子作用如同一电子的施主。     

Fermi surface, surface states, and surface reconstruction in Sr2RuO4

The electronic structure of Sr2RuO4 is investigated by high angular resolution ARPES at several incident photon energies. We address the controversial issues of the Fermi surface (FS) topology and the van Hove singularity at the M point, showing that a surface state and the replica of the primary FS due to sqrt[2]xsqrt[2] surface reconstruction are responsible for previous conflicting interpretations. The FS thus determined by ARPES is consistent with the de Haas-van Alphen results, and it provides additional information on the detailed shape of the alpha, beta, and gamma sheets.