Novel neutron resonance mode in dx2-y2-wave superconductors

We show that a new resonant magnetic excitation at incommensurate momenta, observed recently by inelastic neutron scattering experiments on YBa2Cu3O6.85 and YBa2Cu3O6.6, is a spin exciton. Its location in the Brillouin zone and its frequency are determined by the momentum dependence of the particle-hole continuum. We identify several features that distinguish this novel mode from the previous resonance mode observed near Q=(pi,pi).     

Observation of band renormalization effects in hole-doped high-Tc Superconductors

We report a systematic high-resolution angle-resolved photoemission spectroscopy on high-T(c) superconductors Bi(2)Sr(2)Ca(n-1)Cu(n)O(2n+4) (n=1-3) to study the origin of many-body interactions responsible for superconductivity. For n=2 and 3, a sudden change in the energy dispersion, so called "kink", becomes pronounced on approaching (pi,0) in the superconducting state, while a kink appears only around the nodal direction in the normal state. For n=1, the kink shows no significant temperature dependence even across T(c). This could suggest that the coupling of electrons with Q=(pi,pi) magnetic mode is dominant in the superconducting state for multilayered cuprates, while the interactions at the normal state and that of single-layered cuprates have a different origin.     

Coincidence of checkerboard charge order and antinodal state decoherence in strongly underdoped superconducting Bi2Sr2CaCu2O8 + delta)

The doping dependence of nanoscale electronic structure in superconducting Bi(2)Sr(2)CaCu(2)O(8 + delta) is studied by scanning tunneling microscopy. At all dopings, the low energy density-of-states modulations are analyzed according to a simple model of quasiparticle interference and found to be consistent with Fermi-arc superconductivity. The superconducting coherence peaks, ubiquitous in near-optimal tunneling spectra, are destroyed with strong underdoping and a new spectral type appears. Exclusively in regions exhibiting this new spectrum, we find local "checkerboard" charge ordering of high energy states, with a wave vector of Q = (+/- 2pi/4.5a(0),0); (0, +/- 2pi/4.5a(0)) +/- 15%. Surprisingly, this spatial ordering of high energy states coexists harmoniously with the low energy Bogoliubov quasiparticle states.     

Temperature dependent scattering rates at the fermi surface of optimally doped Bi(2)Sr(2)CaCu(2)O(8+delta)

For optimally doped Bi(2)Sr(2)CaCu(2)O(8+delta), scattering rates in the normal state are found to have a linear temperature dependence over most of the Fermi surface. In the immediate vicinity of the (pi, 0) point, the scattering rates are nearly constant in the normal state, consistent with models in which scattering at this point determines the c-axis transport. In the superconducting state, the scattering rates away from the nodal direction appear to level off and become temperature independent.     

Doubling of the bands in overdoped Bi2Sr2CaCu2O(8+delta): evidence for c-axis bilayer coupling

We present high resolution angle resolved photoemission data of the bilayer superconductor Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212) showing a clear doubling of the near E(F) bands. This splitting approaches zero along the (0,0)-->(pi,pi) nodal line and is not observed in single layer Bi(2)Sr(2)CuO(6+delta) (Bi2201), indicating that the splitting is due to the long sought after bilayer splitting effect. The splitting has a magnitude of approximately 75 meV near the middle of the zone, extrapolating to about 110 meV near the (pi,0) point. The existence of these two bands also helps to clear up the recent controversy concerning the topology of the Fermi surface.     

Valence-bond spin-liquid state in two-dimensional frustrated spin-1/2 Heisenberg antiferromagnets

Fermionic valence-bond approach in terms of SU(4) representation is proposed to describe the J1-J2 frustrated Heisenberg antiferromagnetic (AF) model on a bipartite square lattice. A uniform mean field solution without breaking the translational and rotational symmetries describes a valence-bond spin-liquid state, interpolating the two different AF ordered states in the large J1 and large J2 limits, respectively. This novel spin-liquid state is gapless with the vanishing density of states at the Fermi nodal points. Moreover, a sharp resonance peak in the dynamic structure factor is predicted for momenta q=(0,0) and (pi,pi) in the strongly frustrated limit J(2)/J(1) approximately 1/2, which can be checked by neutron scattering experiments.     

Nodal quasiparticle lifetime in the superconducting state of Bi(2)Sr(2)CaCu(2)O(8+delta)

We have measured the complex conductivity sigma of a Bi(2)Sr(2)CaCu(2)O(8+delta) thin film between 0.2 and 0.8 THz. We find sigma in the superconducting state to be well described as the sum of contributions from quasiparticles, condensate, and order parameter fluctuations which draw 30% of the spectral weight from the condensate. An analysis based on this decomposition yields a quasiparticle scattering rate on the order of k(B)T/Planck's over 2pi for temperatures below T(c).     

Bond stretching phonon softening and kinks in the angle-resolved photoemission spectra of optimally doped Bi2Sr1.6La0.4Cu2O6+delta superconductors

We report the first measurement of the Cu-O bond stretching phonon dispersion in optimally doped Bi2Sr1.6La0.4Cu2O6+delta using inelastic x-ray scattering. We found a softening of this phonon at q=( approximately 0.25,0,0) from 76 to 60 meV, similar to the one reported in other cuprates. A comparison with angle-resolved photoemission data on the same sample revealed an excellent agreement in terms of energy and momentum between the angle-resolved photoemission nodal kink and the soft part of the bond stretching phonon. Indeed, we find that the momentum space where a 63+/-5 meV kink is observed can be connected with a vector q=(xi,0,0) with xi > or =0.22, corresponding exactly to the soft part of the bond stretching phonon.     

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.     

Renormalization of spectral line shape and dispersion below Tc in Bi2Sr2CaCu2O8+delta

Angle-resolved photoemission data in the superconducting state of Bi2Sr2CaCu2O8+delta show a kink in the dispersion along the zone diagonal, which is related via a Kramers-Kr?nig analysis to a drop in the low energy scattering rate. As one moves towards (pi,0), this kink evolves into a spectral dip. The occurrence of these anomalies in the dispersion and line shape throughout the zone indicates the presence of a new energy scale in the superconducting state.     

Quasiparticles in the superconducting state of Bi(2)Sr(2)CaCu(2)O(8+delta)

Recent improvements in momentum resolution lead to qualitatively new angle-resolved photoemission spectroscopy results on the spectra of Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212) along the (pi,pi) direction, where there is a node in the superconducting gap. We now see the intrinsic line shape, which indicates the presence of true quasiparticles at all Fermi momenta in the superconducting state, and lack thereof in the normal state. The region of momentum space probed here is relevant for charge transport, motivating a comparison of our results to conductivity measurements by infrared reflectivity.     

Direct observation of quadrupolar excitons in the heavy-fermion superconductor PrOs4Sb12

We report inelastic neutron scattering experiments performed to investigate the low energy magnetic excitations on single crystals of the heavy-fermion superconductor PrOs(4)Sb(12). The observed excitation clearly softens at a wave vector Q=(1,0,0), which is the same as the modulation vector of the field-induced antiferro-quadrupolar ordering, and its intensity at Q=(1,0,0) is smaller than that around the zone center. This result directly evidences that this excitonic behavior is derived mainly from nonmagnetic quadrupolar interactions. Furthermore, the narrowing of the linewidths of the excitations below the superconducting transition temperature indicates the close connection between the superconductivity and the excitons.     

Anomalous enhancement of the coupling to the magnetic resonance mode in underdoped Pb-Bi2212

High-resolution angle-resolved photoemission with variable excitation energies is used to disentangle bilayer splitting effects and intrinsic (self-energy) effects in the electronic spectral function near the (pi,0) point of differently doped (Pb,Bi)(2)Sr(2)CaCu(2)O(8+delta). In contrast to overdoped samples, where intrinsic effects at the (pi,0) point are virtually absent, we find in underdoped samples intrinsic effects in the superconducting-state (pi,0) spectra of the antibonding band. This intrinsic effect is present only below the critical temperature and weakens considerably with doping. Our results give strong support for models which involve a strong coupling of electronic excitations with the resonance mode seen in inelastic neutron scattering experiments.     

Raising Bi-O bands above the Fermi energy level of hole-doped Bi2Sr2CaCu2O8+delta and other cuprate superconductors

The Fermi surface (FS) of Bi2Sr2CaCu2O8+delta (Bi2212) predicted by band theory displays Bi-related pockets around the (pi, 0) point, which have never been observed experimentally. We show that when the effects of hole doping either by substituting Pb for Bi or by adding excess O in Bi2212 are included, the Bi-O bands are lifted above the Fermi energy (E(F)) and the resulting first-principles FS is in remarkable accord with measurements. With decreasing hole doping the Bi-O bands drop below and the system self-dopes below a critical hole concentration. Computations on other Bi- as well as Tl- and Hg-based compounds indicate that lifting of the cation-derived band with hole doping is a general property of the electronic structures of the cuprates.     

Distribution of voltage fluctuations in a current-biased conductor

We calculate the fluctuating voltage V(t) over a conductor driven out of equilibrium by a current source. This is the dual of the shot noise problem of current fluctuations I(t) in a voltage-biased circuit. In the single-channel case the distribution of the accumulated phase Phi=(e/ variant Planck's over 2pi ) integral Vdt is the Pascal (or binomial waiting-time) distribution-distinct from the binomial distribution of transferred charge Q= integral Idt. The weak-coupling limit of a Poissonian P(Phi) is reached in the limit of a ballistic conductor, while in the tunneling limit P(Phi) has the chi-square form.     

Fermi surface of Bi2Sr2CaCu2O8

We study the Fermi surface of Bi2Sr2CaCu2O8 using angle resolved photoemission spectroscopy (ARPES) with a momentum resolution of approximately 0.01 of the Brillouin zone. We show that, contrary to recent suggestions, the ARPES derived Fermi surface is a large hole barrel centered at (pi,pi), independent of the incident photon energy. We caution that the photon energy and k dependence of the matrix elements, if not properly accounted for, can lead to misinterpretation of ARPES intensities.     

Determination of the pion charge form factor at Q2=1.60 and 2.45 (GeV/c)2

The 1H(e,e'pi+)n cross section was measured at four-momentum transfers of Q2=1.60 and 2.45 GeV2 at an invariant mass of the photon nucleon system of W=2.22 GeV. The charged pion form factor (F(pi)) was extracted from the data by comparing the separated longitudinal pion electroproduction cross section to a Regge model prediction in which F(pi) is a free parameter. The results indicate that the pion form factor deviates from the charge-radius constrained monopole form at these values of Q2 by one sigma, but is still far from its perturbative quantum chromodynamics prediction.     

Q2 evolution of the neutron spin structure moments using a 3He target

We have measured the spin structure functions g(1) and g(2) of 3He in a double-spin experiment by inclusively scattering polarized electrons at energies ranging from 0.862 to 5.058 GeV off a polarized 3He target at a 15.5 degrees scattering angle. Excitation energies covered the resonance and the onset of the deep inelastic regions. We have determined for the first time the Q2 evolution of Gamma(1)(Q2)= integral (1)(0)g(1)(x,Q2)dx, Gamma(2)(Q2)= integral (1)(0)g(2)(x,Q2)dx, and d(2)(Q2)= integral (1)(0)x(2)[2g(1)(x,Q2)+3g(2)(x,Q2)]dx for the neutron in the range 0.1< or =Q2< or =0.9 GeV2 with good precision. Gamma(1)(Q2) displays a smooth variation from high to low Q2. The Burkhardt-Cottingham sum rule holds within uncertainties and d(2) is nonzero over the measured range.     

Unconventional magnetic correlations in DyB2C and HoB2C

Layered borocarbides RB2C (R=Dy, Ho, and Er) have been studied by powder neutron diffraction at 2-30 K. ErB2C has two-sublattice antiferromagnetic order below T(N)=16.3 K, but DyB2C and HoB2C show a coexistence of a conventional canted k=(000) ferromagnetic structure and unconventional magnetic correlations. The k=(000) phase orders at T(c)=8.5 K (DyB2C) and 7.1 K (HoB2C), but low-Q diffraction peaks from the unconventional correlations appear above T(c) with different critical temperatures for different peaks: at 8, 10.5, and 15.7 K for HoB2C. This scattering is fitted as diffraction from a Warren-type random magnetic layer lattice and may result from quadrupolar interactions between R3+ spins.     

Absence of a loss of in-plane infrared spectral weight in the pseudogap regime of Bi(2)Sr(2)CaCu(2)O(8+delta)

The ab-plane reflectance of Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi-2212) thin films was measured in the 30-25 000 cm(-1) range for one underdoped ( T(c) = 70 K), and one overdoped sample ( T(c) = 63 K) down to 10 K. We find similar behaviors in the temperature dependence of the normal-state infrared response of both samples. Above T(c), the effective spectral weight, obtained from the integrated conductivity, does not decrease when T decreases, so that no opening of an optical pseudogap is seen. We suggest that these are consequences of the pseudogap opening in the k = (0,pi) direction and of the in-plane infrared conductivity being mostly sensitive to the k = (pi,pi) direction.