Fermi arcs in the superconducting clustered state for underdoped cuprate superconductors

The one-particle spectral function of a state formed by superconducting (SC) clusters is studied via Monte Carlo techniques. The clusters have similar SC amplitudes but randomly distributed phases. This state is stabilized by competition with the antiferromagnetism expected to be present in the cuprates and after quenched disorder is introduced. A Fermi surface composed of disconnected segments, i.e., Fermi arcs, is observed between the critical temperature T_(c) and the cluster formation temperature scale T*.     

Oxyanion derivatives of cuprate superconductors superconducting 123 phosphates

While YSr₂Cu₃O₇ cannot be prepared under ambient conditions, partial substitution of the phosphate group for copper, as in YSr₂Cu_2.8(PO₄)_0.2O_y, stabilizes this phase in the orthorhombic structure, but the material is not superconducting. Superconductivity in YSr₂Cu_2.8(PO₄)_0.2O_y is obtained by increasing the hole concentration through partial substitution of Y by Ca, as in Y_0.7Ca_0.3Sr₂Cu_2.8(PO₄)_0.2O_y (T_c≈40 K). By incorporating the phosphate group in orthorhombic YBaSrCu₃O ₇, a stable tetragonal derivative of the formula YBaSrCu_2.8(PO₄)_0.2O_y (T_c≈ 47 K) has been prepared; the T_c increases to 70 K by partial substitution of Y by Ca as in Y_0.7Ca_0.3BaSrCu_2.8(PO₄)_0.2O_y.     

Superconducting gap in the hubbard model and the two-gap energy scales of high-T{c} cuprate superconductors

Recent experiments (angle-resolved photoemission spectroscopy and Raman) suggest the presence of two distinct energy gaps in high-temperature superconductors (HTSC), exhibiting different doping dependences. The results of a variational cluster approach to the superconducting state of the two-dimensional Hubbard model are presented which show that this model qualitatively describes this gap dichotomy. The antinodal gap increases with less doping, a behavior long considered as reflecting the general gap behavior of the HTSC. On the other hand, the near-nodal gap does even slightly decrease with underdoping. An explanation of this unexpected behavior is given which emphasizes the crucial role of spin fluctuations in the pairing mechanism.     

Interplay between superconductivity and pseudogap state in bilayer cuprate superconductors

The interplay between the superconducting gap and normal-state pseudogap in the bilayer cuprate superconductors is studied based on the kinetic energy driven superconducting mechanism. It is shown that the charge carrier interaction directly from the interlayer coherent hopping in the kinetic energy by exchanging spin excitations does not provide the contribution to the normal-state pseudogap in the particle–hole channel and superconducting gap in the particle–particle channel, while only the charge carrier interaction directly from the intralayer hopping in the kinetic energy by exchanging spin excitations induces the normal-state pseudogap in the particle–hole channel and superconducting gap in the particle–particle channel, and then the two-gap behavior is a universal feature for the single layer and bilayer cuprate superconductors.     

Theory of superconductivity in strongly correlated materials: Application to cuprate superconductors

A microscopic theory of superconductivity in systems with strong electron correlations is considered within the Hubbard model. The Dyson equation for the matrix Green function in terms of the Hubbard operators is derived and solved in the noncrossing approximation for the self-energy. Two channels of superconducting pairing are revealed: mediated by antiferromagnetic (AFM) exchange and spin-fluctuations. It is proved that AFM exchange interaction results in pairing of all electrons in the conduction band and high T _c proportional to the Fermi energy. T _c dependence on lattice constants (or pressure) and an oxygen isotope shift of T _c are explained. The text was submitted by the author in English.     

Time-resolved photoexcitation of the superconducting two-gap state in MgB2 thin films

Femtosecond pump-probe studies show that carrier dynamics in MgB2 films is governed by the sub-ps electron-phonon (e-ph) relaxation present at all temperatures, the few-ps e-ph process well pronounced below 70 K, and the sub-ns superconducting relaxation below T(c). The amplitude of the superconducting component versus temperature follows the superposition of the isotropic dirty gap and the three-dimensional pi gap dependences, closing at two different T(c) values. The time constant of the few-ps relaxation exhibits a double divergence at temperatures corresponding to the T(c)'s of the two gaps.     

Anomalous peak in the superconducting condensate density of cuprate high- T(c) superconductors at a unique doping state

The doping dependence of the ratio of the superconducting condensate density to the effective mass, n(o)(s)/m(*)(ab), was studied in detail by muon-spin rotation for Y(0.8)Ca(0.2)Ba(2)(Cu(1-z)Zn(z))(3)O(7-delta) and Tl(0.5-y)Pb(0.5+y)Sr(2)Ca(1-x)Y(x)Cu(2)O(7). Our data show that n(o)(s)/m(*)(ab) exhibits a peak at a unique doping state in the overdoped regime. Its position coincides with the critical doping state, where the normal state pseudogap was reported to appear and to deplete the electronic density of states. This finding implies that the pseudogap primarily arises from a change in the electronic ground state rather than from thermal fluctuations.     

Application of a lindhard like model for normal state transport properties of the cuprate superconductors

We account for the transport properties of the cuprates and of organic superconductors in the normal state by a model in which, following Lindhard's theory, the velocity near the Fermi surface is increased by reduced screening. We show that the conductivity due to elastic scattering becomes temperature dependent. We account for the conductivity anisotropy, the thermoelectric power, and the Hall constant. An analysis of the London penetration depth, which takes into account the low dimensionality of the conduction, gives large Fermi velocities and small effective masses, consistent with the analysis of the transport properties.     

Rotational symmetry breaking in the ground state of sodium-doped cuprate superconductors

We use an extended t-J model to study a single hole bound to a Na+ acceptor in Ca2-xNaxCuO2Cl2. For parameters suitable to cuprates, the ground state has a twofold degeneracy, corresponding to even (odd) reflection symmetry around the x (y) axes. The conductance pattern of the broken symmetry state is anisotropic as the tip of a tunneling microscope scans above the Cu-O-Cu bonds along the x (y) axes. This anisotropy is pronounced at lower voltages but reduced at higher voltages. Our theory agrees qualitatively with recent data of scanning tunneling microscopy showing broken local rotational symmetry.     

Normal state magnetism of the high Tc cuprate superconductors

Magnetic measurements of various types have played an essential role in establishing the novel normal state characteristics of high transition temperature (T_c) superconductors with T_c > 23 K. Among these materials, the highest T_c's ( 125 K) are exhibited by the layered cuprates. In this paper, the normal state magnetic susceptibilities of the cuprates are reviewed and interpreted in the context of magnetic neutron scattering and other magnetic measurements, using the La_2−xM_xCuO₄-type and YBa₂Cu₃O_6+x-type materials as prototypical examples. The evolution of the magnetism upon doping the insulating antiferromagnetic “parent” compounds with x = 0 to form the high temperature superconductors is described. A recurrent property which differentiates these materials from conventional superconductors is the existence of strong antiferromagnetic correlations in the metallic state on the same sublattice of the structure in which the itinerant carriers reside.     

Relevance of 3d multiplet structure in nickelate and cuprate superconductors

The recent discovery of superconductivity in doped rare-earth infinite-layer nickelates RNiO_2, R = Nd, Pr as a new family of unconventional superconductors has inspired extensive research on their intriguing properties. One of the major motivation to explore the nickelate superconductors originated from their similarities with and differences from the cuprate superconductors, which have been extensively studied over the last decades but are still lack of the thorough understanding.In this short review, we summarized our recent investigation of the relevance of Ni/Cu-3 d multiplet structure on the hole doped spin states in cuprate and recently discovered nickelate superconductors via an impurity model incorporating all the 3d orbitals. Further plausible explorations to be conducted are outlined as well. Our presented work provides an insightful framework for the investigation of the strongly correlated electronic systems in terms of the multiplet structure of transition metal compounds.     

Phenomenological approach to the non-spin-wave behavior of cuprate magnetic susceptibility in the superconducting state

Experimental data on the non-spin-wave behavior of the cuprate magnetic susceptibility χ(q, ω) in the superconducting state are reconstructed on the basis of a spherically symmetric self-consistent approach within the framework of the frustrated Heisenberg model. For the Green’s function of the spin-wave excitations, a polarization operator resonant with the superconducting state is introduced phenomenologically. This gives rise to an additional (compared to the normal state) branch of the incommensurate peaks and to a pronounced non-spin-wave behavior of the susceptibility χ(q, ω) in the frequency range near 80 meV (the so-called “dark region”). The renormalization of the real part of the polarization operator plays a crucial role in the theory.     

Local structure study of T′ cuprate superconductors

Undoped T′-type cuprates are believed to be charge transfer (Mott-Hubbard) insulators. Recent observation of undoped superconducting T′-type cuprates suggests half-filling weakly correlated metallic electron states (Tsukada et al.), which is against the fundamental picture. Here we report the local structures of undoped T′-(La³⁺,Y³⁺)₂CuO₄ and doped T′-(La³⁺,Ce⁴⁺)₂CuO₄ thin-film single crystals by polarized extended X-ray absorption fine structure (EXAS). The in-plane (E//ab) polarized Cu K-EXAFS data shows a remarkable Cu-O bond distance displacement. The broadened Cu-O distribution suggests a large local lattice distortion in T′-type structures, which might influence the electronic structure.     

Evidence for mixed-charge state from the normal-phase transport properties of cuprate superconductors

We derive the expressions for the resistivity, Hall coefficient and Hall angle for a cuprate superconductor crystal having semi-metal normal state characteristics and show that the normal-state transport properties can be explained in terms of the mixed-charge behavior of the system which, for highly mobile hole systems, leads to a R_H scaling rule [Phys. Rev. Lett. 72 (1994) 2636]. This is consistent with the mixed charge condensation model proposed earlier [Phys. Lett. A 208 (1995) 171].     

Gauge invariance and spontaneous symmetry breaking in two-gap superconductors

The gauge symmetry of the Ginzburg–Landau theory for two-gap superconductors is analyzed in this letter. We argue that the existence of two different phases, associated with the two independent scalar Higgs fields, explicitly breaks the gauge symmetry of the Ginzburg–Landau Hamiltonian, unless a new additional vector field is included. Furthermore, the interference term, or Josephson coupling, holding a direct dependence with the phase difference, also explicitly breaks down the gauge symmetry. We show that a solution for the problem is achieved by adding an additional kinetic coupling term between the two vector fields, which generates the desired terms through a spontaneous symmetry breaking mechanism. Finally, the electrodynamics of the system is also presented in terms of the supercurrents inside the superconducting region.     

Spectral measurement of the Hall angle response in normal state cuprate superconductors

We measure the temperature and frequency dependence of the complex Hall angle for normal state YBa(2)Cu(3)O(7) films from dc to far-infrared frequencies (20-250 cm(-1)) using a new modulated polarization technique. We determine that the functional dependence of the Hall angle on scattering does not fit the expected Lorentzian response. We find spectral evidence supporting models of the Hall effect where the scattering Gamma(H) is linear in T, suggesting that a single relaxation rate, linear in temperature, governs transport in the cuprates.     

Commensurate spin dynamics in the superconducting state of an electron-doped cuprate superconductor

We report neutron scattering studies on two single crystal samples of the electron-doped (n-type) superconducting (SC) cuprate Nd2-xCexCuO4 (x=0.15) with T(c)=18 and 25 K. Unlike the hole-doped (p-type) SC cuprates, where incommensurate magnetic fluctuations commonly exist, the n-type cuprate shows commensurate magnetic fluctuations at the tetragonal (1/2 1/2 0) reciprocal points both in the SC and in the normal state. A spin gap opens up when the n-type cuprate becomes SC, as in the optimally doped p-type La2-xSrxCuO4. The gap energy, however, increases gradually up to about 4 meV as T decreases from T(c) to 2 K, which contrasts with the spin pseudogap behavior with a T-independent gap energy in the SC state of p-type cuprates.