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*.     

Protected nodes and the collapse of Fermi arcs in high-T{c} cuprate superconductors

Angle resolved photoemission on underdoped Bi2Sr2CaCu2O8 reveals that the magnitude and d-wave anisotropy of the superconducting state energy gap are independent of temperature all the way up to T{c}. This lack of T variation of the entire k-dependent gap is in marked contrast to mean field theory. At T{c} the point nodes of the d-wave gap abruptly expand into finite length "Fermi arcs." This change occurs within the width of the resistive transition, and thus the Fermi arcs are not simply thermally broadened nodes but rather a unique signature of the pseudogap phase.     

Slow quantum oscillations without fine-grained Fermi surface reconstruction in cuprate superconductors

The Fourier transform of the observed magnetic quantum oscillations (MQOs) in YBa₂Cu₃O_6+δ high-temperature superconductors has a prominent low-frequency peak with two smaller neighboring peaks. The separation and positions of these three peaks are almost independent of doping. This pattern has been explained previously by rather special, exquisitely detailed, Fermi-surface reconstruction. We propose that these MQOs have a different origin, and their frequencies are related to the bilayer and inter-bilayer electron hopping rather than directly to the areas of tiny Fermi-surface pockets. Such so-called “slow oscillations” explain more naturally many features of the observed oscillations and allow us to estimate the inter-layer transfer integrals and in-plane Fermi momentum.     

Possible higher temperature superconductivity in the modulation-doped superlattice structure of cuprate superconductors

We design a new structure for a cuprate superconductor indicating the possibility of higher temperature superconductivity using our recently proposed composite fermions theory. It is constructed with modulation-doped superlattice structures, which are often used in the design of semiconductor superlattice devices. The superconductive critical temperature ($T_c$) was calculated in the superlattice structures of the superconductor in which the optimal doped CuO₂ layer was sandwiched between two less-doped CuO₂ layers. We find that if these structures could be realized in a cuprate superconductor such as Bi₂Sr₂Ca₂Cu₃O₁₀ or HgBa₂Ca₂Cu₃O₉, the highest $T_c$ could attain the level of 300 K at atmospheric pressure.     

Dimensional crossover in cuprate superconductors

A central concern in understanding the mechanism for the occurrence of superconductivity in cuprates is the interaction driving the phase transition and their dimensionality. As physical systems near a phase transition have a marked dependence on dimensionality, this can be explored with symples where one of the physical dimensions is reduced and becomes comparable to the correlation length. Recently, it became possible to fabricate sufficiently thin cuprate slabs, revealing a fall ofT _c with reduced thickness, becoming pronounced for slabs a few unit cells thick. Related effects have been observed in the YBCO bulk compounds 123, 124 and 247. We analyze the experimental data by invoking finite size scaling and a Ginzburg-Landau treatment. The main conclusions include the following: the fall ofT _c with decreasing thickness corresponds to a dimensional crossover, revealing the three-dimensional nature of the interaction mediating superconductivity; there is a predominance of two-dimensional fluctuations and boundaries with reduced thickness; there are crossover phenomena reminiscent of⁴He films and thin slabs of conventional super-conductors.     

Electron-phonon renormalization in cuprate superconductors

Electron-phonon (e-ph) renormalization effects in a model cuprate system CaCuO2 are studied by employing density functional theory based methods. Whereas calculations based on the local spin-density approximation (LSDA) predicts negligible e-ph coupling effects of the half-breathing Cu-O bond stretching mode, the inclusion of a screened on-site Coulomb interaction (U) in the LSDA+U calculations greatly enhances the e-ph coupling strength of this mode. The full-breathing mode, on the other hand, shows a much weaker e-ph renormalization effect.     

Fluctuation conductivity in cuprate superconductors

We have measured the in-plane resistivity of Bi₂Sr₂CaCu₂O_8+δ and Tl₂Ba₂ CaCu₂O_8+δ single crystals in the temperature range 70–300 K. The thermodynamic fluctuations in the conductivity of both the samples start around ∼ 125 K. We find the Lawrence and Doniach [1] model to be inadequate to describe the fluctuation conductivity in these materials. The modification suggested by Ramallo et al [4] where by the conductivity is enhanced due to the presence of two superconducting layers in each unit cell is also not adequate. We suggest the fluctuation conductivity to be reduced due to the reduction in the density of states (DOS) of the quasiparticles which results due to the formation of Cooper pairs at the onset of the fluctuations. The data agrees with the theory proposed by Dorin et al [5] which takes into account this reduction in DOS.     

Spin-lattice relaxation in cuprate superconductors

This presentation gives a personal review of nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) spin-lattice relaxation studies in cuprate superconductors mainly dealing with the YBa₂Cu₄O₈ compound with many examples from the Zürich laboratory. The studies were performed in both the normal and the superconducting state with various NMR isotopes (e.g.,¹⁷O,^63,65Cu,^135,137Ba). The relatively broad signals were mostly obtained by a phase-alternating add-subtract spin-echo technique. We will discuss the general behavior of spin-lattice relaxation in the normal state and the calculation of the dynamic spin including an approach (on the basis of thet-J model) to calculate the relaxation for plane copper, oxygen, and yttrium. An application of the Luttingerliquid model to the relaxation of chain copper in YBa₂Cu₃O₇ and YBa₂Cu₄O₈ is also given. We then will deal with characteristic features of the YBa₂Cu₄O₈ structure: the spin gap, an electronic crossover in the normal state, the single-spin fluid model, and the d-wave pairing.     

Research trends in electron-doped cuprate superconductors

In this review, we look back on some intriguing and puzzling issues in electron-doped cuprate superconductors, such as electron-hole asymmetry, two types of carriers, quantum critical points, order-parameter symmetry, etc. The necessity of study on this family is invoked in comparison with the hole-doped counterparts from several aspects. The related progress, especially in last few years, has been outlined point to point, as well as other hot topics like the discovery of ambipolar superconductors, the applications in superconducting electronics, and the emergency of superconductivity in parent compounds. In perspective, the utilization of blooming advanced techniques, electric double layer transistor and combinatorial film deposition, will bring some new insights into the mechanism such as electron-doped cuprate superconductors.     

Superfluid response in electron-doped cuprate superconductors

We propose a weakly coupled two-band model with dx(2)(-y(2)) pairing symmetry to account for the anomalous temperature dependence of superfluid density rho(s) in electron-doped cuprate superconductors. This model gives a unified explanation to the presence of an upward curvature in rho(s) near T(c) and a weak temperature dependence of rho(s) in low temperatures. Our work resolves a discrepancy in the interpretation of different experimental measurements and suggests that the pairing in electron-doped cuprates has predominately dx(2)(-y(2)) symmetry in the whole doping range.     

Electron spectrum in high-temperature cuprate superconductors

A microscopic theory for the electron spectrum of the CuO₂ plane within an effective p-d Hubbard model is proposed. The Dyson equation for the single-electron Green’s function in terms of the Hubbard operators is derived and solved self-consistently for the self-energy evaluated in the noncrossing approximation. Electron scattering on spin fluctuations induced by the kinematic interaction is described by a dynamical spin susceptibility with a continuous spectrum. The doping and temperature dependence of electron dispersions, spectral functions, the Fermi surface, and the coupling constant λ are studied in the hole-doped case. At low doping, an arc-type Fermi surface and a pseudogap in the spectral function close to the Brillouin zone boundary are observed. The text was submitted by the authors in English.     

Terahertz oscillations in mercury cuprate superconductors

It has been recently reported that the three-dimensional Bose-Einstein condensation of the quasi-particles is valid for the mercury cuprates at liquid helium temperature. In this study, the validity of the interlayer theory in three dimensions has been investigated for optimally oxygen-doped mercury cuprates at the temperature interval of 0–15 K. Furthermore, some thermodynamic and electrodynamics parameters of mercury cuprates have been calculated for both the under-doped and the over-doped samples at the vicinity of 4.2 K. Moreover, it has been determined that the superconducting system behaves as a terahertz wave cavity regardless of the oxygen doping concentration.     

Disorder-induced static antiferromagnetism in cuprate superconductors

Using model calculations of a disordered d-wave superconductor with on-site Hubbard repulsion, we show how dopant disorder can stabilize novel states with antiferromagnetic order. We find that the critical strength of correlations or impurity potential necessary to create an ordered magnetic state in the presence of finite disorder is reduced compared to that required to create a single isolated magnetic droplet. This may explain why, in cuprates such as La2-xSrxCuO4, low-energy probes have identified a static magnetic component which persists well into the superconducting state, whereas, in cleaner systems such as YBa(2)Cu(3)O(6+delta), it is absent or minimal.     

Giant proximity effect in cuprate superconductors

Using an advanced molecular beam epitaxy system, we have reproducibly synthesized atomically smooth films of high-temperature superconductors and uniform trilayer junctions with virtually perfect interfaces. We found that supercurrent runs through very thick barriers. We can rule out pinholes and microshorts; this "giant proximity effect" (GPE) is intrinsic. It defies the conventional explanation; it might originate in resonant tunneling through pair states in an almost-superconducting barrier. GPE may also be significant for superconducting electronics, since thick barriers are easier to fabricate.     

Dopant-modulated pair interaction in cuprate superconductors

A comparison of recent experimental STM data with single-impurity and many-impurity Bogoliubov-de Gennes calculations strongly suggests that random out-of-plane dopant atoms in cuprates modulate the pair interaction locally. This type of disorder is crucial to understanding the nanoscale electronic inhomogeneity observed in BSCCO-2212, and can reproduce observed correlations between the positions of impurity atoms and various aspects of the local density of states such as the gap magnitude and the height of the coherence peaks. Our results imply that each dopant atom modulates the pair interaction on a length scale of order one lattice constant.     

Delocalized fermions in underdoped cuprate superconductors

Low-temperature heat transport was used to investigate the ground state of high-purity single crystals of the lightly doped cuprate YBa2Cu3O6.33. Samples were measured with doping concentrations on either side of the superconducting phase boundary. We report the observation of delocalized fermionic excitations at zero energy in the nonsuperconducting state, which shows that the ground state of underdoped cuprates is a thermal metal. Its low-energy spectrum appears to be similar to that of the d-wave superconductor, i.e., nodal. The insulating ground state observed in underdoped La2-xSrxCuO4 is attributed to the competing spin-density-wave order.