Broad peak in the dx2-y2 superconducting correlation length as a function of hole concentration in the two-dimensional t-J model

We have calculated high temperature series to 12th order in inverse temperature for singlet superconducting correlation functions of the 2D t-J model with s, dx2-y2, and dxy symmetry pairs. Our calculations differ from previous work by removing disconnected pieces from the original four-point correlator and by treating the resulting pairing correlator as a matrix. We find the correlation length for dx2-y2 pairing grows significantly with decreasing temperature and develops a broad peak as a function of doping around delta=0.25 for T/J=0.25 at J/t=0.4. The correlation lengths for s and dxy symmetry remain small and do not display peaks. Antiferromagnetic spin correlations at low doping act to suppress the dx2-y2 and dxy superconducting correlation lengths.     

Femtosecond mid-IR pump-probe spectroscopy of photoinduced insulator to metal transition in dimer Mott insulator κ-(BEDT-TTF)2X

Ultrafast dynamics of the photoinduced insulator (I) to metal (M) transition were investigated using femtosecond mid-infrared pump-probe spectroscopy in two-dimensional organic Mott insulators [bis (ethylenedithio)]-tetrathiafulvalene (BEDT-TTF) salts κ-(BEDT-TTF)₂X (κ-(ET)₂X, where X denotes anion). In κ-(d-ET)₂Cu[N(CN)₂]Br, a metallic state was photogenerated using a phonon-mediated mechanism: the effective bandwidth increases through the photoinduced molecular rearrangement. The mechanism differs fundamentally from the previously reported photoinduced filling control in one-dimensional Mott insulators.     

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.     

Edge currents in superconductors with a broken time-reversal symmetry

We analyze edge currents and edge bands at the surface of a time-reversal symmetry breaking dx2-y2 + id(xy) superconductor. We show that the currents have large Friedel oscillations with two interfering frequencies: square root of 2kF from subgap states, and 2kF from the continuum. The results are based independently on a self-consistent slave-boson mean-field theory for the t-J model on a triangular lattice, and on a T-matrix scattering theory calculation. The shape of the edge-state band, as well as the particular frequency square root of 2kF of the Friedel oscillations, are attributes unique for the dx2-y2 + id(xy) case, and may be used as a fingerprint for its identification. Extensions to different time-reversal symmetry breaking superconductors can be achieved within the same approach.     

Direct observation of a nonmonotonic dx2-y2- wave superconducting gap in the electron-doped high-Tc superconductor Pr0.89LaCe0.11CuO4

We performed high-resolution angle-resolved photoemission spectroscopy on electron-doped high-Tc superconductor Pr0.89LaCe0.11CuO4 to study the anisotropy of the superconducting gap. The observed momentum dependence is basically consistent with the dx2-y2- wave symmetry, but obviously deviates from the monotonic dx2-y2- gap function. The maximum gap is observed not at the zone boundary, but at the hot spot where the antiferromagnetic spin fluctuation strongly couples to the electrons on the Fermi surface. The present experimental results suggest the spin-mediated pairing mechanism in electron-doped high-Tc superconductors.     

Magnetic-field dependence of the maximum supercurrent of La2-xCexCuO4-y interferometers: evidence for a predominant dx2-y2 superconducting order parameter

We performed a phase-sensitive test of the symmetry of the superconducting order parameter of the electron doped cuprate La(2-x)Ce(x)CuO(4-y) using a superconducting quantum interferometer with spatially distributed Josephson junctions. The studies were made on a thin film grown on a SrTiO3 tetracrystal substrate. The superconducting transition temperature was about 29 K which indicates that the sample is close to optimal doping. The magnetic field dependence of the critical current gives strong evidence for a predominant dx(2)(-y(2)) order parameter symmetry of the sample measured. It also gives upper limits for the s-wave component in a mixed order parameter of the type s+idx(2)(-y(2)).     

Temperature-dependent orbital degree of freedom of a bilayer manganite by magnetic compton scattering

We have measured temperature-dependent magnetic Compton profiles (MCPs) from a single crystal of La1.2Sr1.8Mn2O7. The MCPs, which involved the scattering of circularly polarized x rays, are in general related to the momentum density of all the unpaired spins in the system. Nevertheless, we show that when the x-ray scattering vector lies along the [110] direction, the number of magnetic electrons of a specific symmetry, i.e., d electrons of x(2)-y(2) symmetry, yield a distinct signature in the MCP, allowing us to monitor substantial changes in the occupancy of the dx(2)(-y(2)) states over the investigated temperature range of 5-200 K. This study indicates that magnetic Compton scattering can provide a powerful window on the properties of specific magnetic electrons in complex materials.     

Resonant impurity states in the d-density-wave phase

We study the electronic structure near impurities in the d-density-wave (DDW) state, a possible candidate phase for the pseudogap region of the high-temperature superconductors. We show that the density of states near a nonmagnetic impurity in the DDW state is qualitatively different from that in a superconductor with dx(2)(-y(2)) symmetry. Thus, the electronic structure near impurities can provide insight into the nature of the two phases recently observed by scanning tunneling microscopy experiments in the superconducting state of underdoped Bi-2212 compounds.     

Study of a spin-3/2 system by a quadrupolar-echo sequence: suppression of spurious signals.

The density matrix describing the evolution of a spin-3/2 system excited by a quadrupolar-echo sequence consisting of two rf pulses in quadrature phase, (X)-tau 2-(Y)-tau 4-[acquisition(y)], is calculated from the equilibrium state to the acquisition period. The interactions involved are the first-order quadrupolar interaction throughout the experiment and a local heteronuclear magnetic dipolar term between the two pulses and during the acquisition period. Three echoes, one due to a satellite transition at tau 4 = tau 2 and two due to the central transition at tau 4 = tau 2 and tau 4 = 3 tau 2, are predicted. They have similar expressions than those obtained with two rf pulses of the same phase, (X)-tau 2-(X)-tau 4-[acquisition(y)], except the signs. Moreover, it is shown experimentally that a combination of these two sequences, namely: (X)-tau 2-(X)-tau 4-[acquisition(y)]-recycle delay-(X)-tau 2-(-X)-tau 4-[acquisition(y)]-recycle delay-(X)-tau 2-(Y)-tau 4-[acquisition(-y)]-recycle delay-(X)-tau 2-(-Y)-tau 4-[acquisition(-y)]-recycle delay, cancels the spurious piezo-electric signals when studying a ferro-electric material in the single-crystal form.     

Observation of splintered Josephson vortices at grain boundaries in YBa(2)Cu(3)O(7-delta)

We have directly observed well-separated Josephson vortex splinters with unquantized magnetic flux at asymmetric 45 degrees grain boundaries in YBa(2)Cu(3)O(7-delta) films by imaging magnetic flux with scanning SQUID microscopy. The existence of these splinter vortices has been predicted and is well described by a model based on dx(2)(-y(2)) pairing symmetry and facetting of the grain boundary on a length scale shorter than the Josephson penetration depth.     

Photoinduced transition of [Fe(CN)5NO] nitroprusside anions to metastable states and electron localization in the (BEDT-TTF)4K[Fe(CN)5NO]2 molecular metal

The crystals of (BEDT-TTF)₄K[Fe(CN)₅NO]₂, representing a quasi-two-dimensional organic metal with conducting layers of bis(ethylenedithio)tetrathiofulvalene (BEDT-TTF) and nonconducting layers containing pho-tochromic nitroprusside anions [Fe(CN)₅NO]^2?, were studied by the method of electron spin resonance. Illuminated by light with a wavelength of 514.5 nm, the organic metal crystals feature the formation of localized paramagnetic centers in the conducting cation layers of BEDT-TTF. The phenomenon of electron localization in the BEDT-TTF layers is related to the light-induced formation of long-lived metastable states of nitroprusside anions.     

Magnetic resonance in the spin excitation spectrum of electron-doped cuprate superconductors

We study the emergence of a magnetic resonance in the superconducting state of the electron-doped cuprate superconductors. We show that the recently observed resonance peak in the electron-doped superconductor Pr0.88LaCe0.12CuO4-delta is consistent with an overdamped spin exciton located near the particle-hole continuum. We present predictions for the magnetic-field dependence of the resonance mode as well as its temperature evolution in those parts of the phase diagram where dx2-y2-wave superconductivity may coexist with an antiferromagnetic spin-density wave.     

Tunneling Conductance in dx2-y2 ＋idly Mixed Wave Superconductor Graphene Junctions

Using the extended Blonder-Tinkham-Klapwijk formalism, we investigate the conductance spectra of normal metal/dx2-y2 ＋ idxy mixed wave superconductor graphene junctions. It is found that the conductance spectra vary strongly with the orientation of the gap and the amplitude ratio （Δ1/Δ0） of two components for dx2-y2 ＋ idxy mixed wave. The zero bias conductance is nearly 2 and the conductance peak vanishes in doped graphene for a = 0 and Δ1/Δ0 = 1. The conductance increases with increasing the amplitude ratio of two components for α =π/4 and Δ1/Δ0 -- 1. The ZBCP becomes observable wide with 1 〈 EF/Δ0 〈 100 for α= π/4 and Δ1/Δ0 = 1. This property is different from that in normal metal/dx2-y2 wave superconductor graphene junctions.     

Vortex state in NaxCoO2.yH2O: px+/-ipy-wave versus d(x2-y2)+/-id xy-wave pairing

Based on an effective Hamiltonian specified in the triangular lattice with possible p(x)+/-ip(y)- or dx(2)(-y(2))+/-id(xy)-wave pairing, which has close relevance to the newly discovered Na0.35CoO2.yH(2)O, the electronic structure of the vortex state is studied by solving the Bogoliubov-de Gennes equations. It is found that p(x)+/-ip(y) wave is favored for the electron doping as the hopping integral t<0. The lowest-lying vortex bound states are found to have, respectively, zero and positive energies for p(x)+/-ip(y)- and dx(2)(-y(2))+/-id(xy)-wave superconductors, whose vortex structures exhibit the intriguing sixfold symmetry. In the presence of strong on-site repulsion, the antiferromagnetic order and local ferromagnetic moment are induced around the vortex cores for the former and the latter, respectively, both of which cause the splitting of the local density of states peaks due to the lifting of spin degeneracy.     

Robust dx2-y2 pairing symmetry in hole-doped cuprate superconductors

Although initially quite controversial, it is now widely accepted that the Cooper pairs in optimally doped cuprate superconductors have predominantly dx2-y2 wave function symmetry, and the controversy has now shifted to whether the pairing symmetry changes away from optimal doping. Here we present phase-sensitive tricrystal experiments on three cuprate systems: Y(0.7)Ca(0.3)Ba(2)Cu(3)O(7-delta) (Ca-doped Y-123), La2-xSrxCuO4 (La-214), and Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi-2212), with doping levels covering the underdoped, optimal, and overdoped regions. Our work implies that predominantly d x2-y2 pairing symmetry is robust over a large variation in doping.     

Vortex dynamics and the Fulde-Ferrell-Larkin-Ovchinnikov state in a magnetic-field-induced organic superconductor

Under special conditions, a superconducting state where the order parameter oscillates in real space, the so-called FFLO state, is theoretically predicted to exist near the upper critical field, as first proposed by Fulde and Ferrell, and Larkin and Ovchinnikov. We report systematic measurements of the interlayer resistance in high magnetic fields to 45 T in the two-dimensional magnetic-field-induced organic superconductor lambda-(BETS)2FeCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene. The resistance is found to show characteristic dip structures in the superconducting state. The results are consistent with pinning interactions between the vortices penetrating the insulating layers and the order parameter of the FFLO state. This gives strong evidence for an oscillating order parameter in real space.     

Band structure of β-(BEDT-TTF) 2PF6. One-dimensional metal along the side-by-side molecular array

The energy-band structures of isostructural organic conductors, β-(BEDT-TTF) ₂PF₆ and (BEDT-TTF) ₂AsF₆ are calculated (BEDT-TTF: bis (ethylenedithio) tetrathiafulvalene). The strong dimerization of the face-to-face array makes the Fermi surface open perpendicularly to the side-by-side array. This dimer model indicates that the phase transition around room temperature is associated with a 2k_F charge density wave.     

Sharp signature of a dx2-y2 quantum critical point in the Hall coefficient of cuprate superconductors

We study the behavior of the Hall coefficient, R(H), in a system exhibiting dx(2)(-y(2)) density-wave order in a regime in which the carrier concentration, x, is tuned to approach a quantum critical point at which the order is destroyed. At the mean-field level, we find that n(Hall)=1/R(H) evinces a sharp signature of the transition. There is a kink in n(Hall) at the critical value of the carrier concentration, x(c); as the critical point is approached from the ordered side, the slope of n(Hall) diverges. Hall transport experiments in the cuprates, at high magnetic fields sufficient to destroy superconductivity, should reveal this effect.     

Structure of the pairing interaction in the two-dimensional Hubbard model

Dynamic cluster Monte Carlo calculations for the doped two-dimensional Hubbard model are used to study the irreducible particle-particle vertex responsible for dx2-y2 pairing in this model. This vertex increases with increasing momentum transfer and decreases when the energy transfer exceeds a scale associated with the Q=(pi, pi) spin susceptibility. Using an exact decomposition of this vertex into a fully irreducible two-fermion vertex and charge and magnetic exchange channels, the dominant part of the effective pairing interaction is found to come from the magnetic, spin S=1 exchange channel.     

Critical behavior of a filling-controlled Mott-transition observed at an organic field-effect-transistor interface

An organic Mott-insulator κ-(BEDT-TTF)₂Cu[N(CN)₂] Cl(κ-Cl) on SiO₂/Si substrate showed an ambipolar field-effect-transistor (FET) characteristics without any hysteresis, which means a continuous Mott-transition at filling-controlled regime (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene). In order to extract the critical exponent in the vicinity of the Mott-insulating phase, an analysis based on Efros-Shklovskii approximation is performed. The model fitting well reproduces the device characteristics over wide range of temperature and gate voltage. In this analysis, Coulomb gap temperature that characterizes the ground state of doped Mott-insulator shows inversely proportional critical behavior against the doping concentration.