Search

Experimental studies of the endpoint of the superconducting dome for (Bi,Pb)2Sr2CuO6+δ (Bi2201) and La2−xSrxCuO4 (LSCO) have raised questions regarding the nature of the cuprate pairing mechanism. Motivated by these experiments, we examine the results of dynamic cluster approximation (DCA) calculations and find that a spin-fluctuation interaction can provide a framework for understanding this end point behavior.

The nature and mechanism of superconductivity in the extremely electron-doped FeSe based superconductors continues to be a matter of debate. In these systems, the hole-like band has moved below the Fermi energy, and various spin-fluctuation theories involving pairing between states near the electron Fermi surface and states of this incipient band have been proposed. Here, using a dynamic cluster quantum Monte Carlo calculation for a bilayer Hubbard model we show that the pairing in these systems can be understood in terms of an effective retarded attractive interaction between electrons near the electron Fermi surface., 5 pages, 5 figures

It is notoriously difficult to make quantitative theoretical predictions of the superconducting transition temperature, Tc, either from first-principles or even from a knowledge of normal state properties. Ultimately, this reflects the fact that the energy scales involved in the superconducting state are extremely small in natural units, and that Tc depends exponentially on a subtle interplay between different interactions so that small uncertainties in microscopic processes can lead to order one effects on Tc. However, in some circumstances, it may be possible to determine (approximate) bounds on Tc. Here we propose such a bound for the conventional phonon-mediated mechanism of pairing with strongly retarded interactions, i.e. in the case in which $$\hbar \bar \omega \ll E_F$$ ℏ ω ¯ ≪ E F , where $$\bar \omega$$ ω ¯ is an appropriate characteristic phonon frequency and EF is the Fermi energy. Specifically, drawing on both empirical results (shown in Fig. 2 below) and recent results1 of determinant quantum Monte Carlo (DQMC) studies of the paradigmatic Holstein model, we propose that $$k_BT_c \le A_{\max }\hbar \bar \omega$$ k B T c ≤ A max ℏ ω ¯ , where Amax is a dimensionless number of order one that we estimate to be Amax ≈ 1/10.

A phenomenological spin-fluctuation analysis [Dahm et al., Nat. Phys. 5, 217 (2009)], based upon inelastic neutron scattering (INS) and angular resolved photoemission spectroscopy (ARPES) data for ${\mathrm{YBCO}}_{6.6}\phantom{\rule{4pt}{0ex}}({T}_{c}=61\phantom{\rule{4pt}{0ex}}\text{K})$, is used to calculate the functional derivative of the $d$-wave eigenvalue ${\ensuremath{\lambda}}_{d}$ of the linearized gap equation with respect to the imaginary part of the spin susceptibility ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}(q,\ensuremath{\omega})$ at 70 K. For temperatures near ${T}_{c}$, the variation of ${T}_{c}$ with respect to ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}(q,\ensuremath{\omega})$ is proportional to this functional derivative. We find that above an energy $\ensuremath{\sim}4{T}_{c}$ the functional derivative becomes positive so that adding spin-fluctuation spectral weight at higher frequencies leads to an increase in ${T}_{c}$. The strongest pairing occurs for large momentum transfers, and small momentum spin-fluctuations suppress the pairing.

The superconducting (SC) and charge-density-wave (CDW) susceptibilities of the two dimensional Holstein model are computed using determinant quantum Monte Carlo (DQMC), and compared with results computed using the Migdal-Eliashberg (ME) approach. We access temperatures as low as 25 times less than the Fermi energy, $E_F$, which are still above the SC transition. We find that the SC susceptibility at low $T$ agrees quantitatively with the ME theory up to a dimensionless electron-phonon coupling $\lambda_0 \approx 0.4$ but deviates dramatically for larger $\lambda_0$. We find that for large $\lambda_0$ and small phonon frequency $\omega_0 \ll E_F$ CDW ordering is favored and the preferred CDW ordering vector is uncorrelated with any obvious feature of the Fermi surface., Comment: 7 pages, 7 figures. v2: discussion improved, added supplemental material

While both the hole and electron doped cuprates can exhibit ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave superconductivity, the local distribution of the doped carriers is known to be significantly different with the doped holes going primarily on the O sites while the doped electrons go on the Cu sites. Here we report the results of density-matrix-renormalization-group calculations for a three-orbital model of a ${\mathrm{CuO}}_{2}$ lattice. In addition to the asymmetric dependence of the intra-unit-cell occupation of the Cu and O for hole and electron doping, we find important differences in the longer range spin and charge correlations. As expected, the pair-field response has a ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-like structure for both the hole and electron doped systems.

Numerical calculations are being used to explore both the properties of strongly-correlated electron systems and the interactions which give rise to these properties. Here, we review what has been learned about the two-dimensional (2D) Hubbard model and comment on its relationship to the high T c cuprate problem.

We have carried out density-matrix-renormalization group (DMRG) calculations for the problem of one doped hole in a two-leg $t-J$ ladder. Recent studies have concluded that exotic "Mott" physics --- arising from the projection onto the space of no double-occupied sites --- is manifest in this model system, leading to charge localization and a new mechanism for charge modulation. In contrast, we show that there is no localization and that the charge density modulation arises when the minimum in the quasiparticle dispersion moves away from $\pi$. Although singular changes in the quasiparticle dispersion do occur as a function of model parameters, all the DMRG results can be qualitatively understood from a non-interacting "band-structure" perspective., Comment: 4 pages, 3 figures, plus another 4 pages of supplementary material, with 6 figures

The intermodulation power and quality factor Q of high-T-c superconducting microstrip resonators with different shapes is calculated. Acute corners increase the losses and the intermodulation power. The influence of normal conducting regions is investigated. The case of two microstrips carrying in phase or out of phase currents is also studied. The losses and the intermodulation power are significantly reduced if the currents are in phase in the two microstrips compared to the mode with out of phase currents. (C) 1997 American Institute of Physics.

Results of a calculation of the c-axis infrared conductivity {sigma}{sub c} for a d{sub x{sup 2}{minus}y{sup 2}}-wave superconductor, including both elastic impurity and inelastic spin-fluctuation scattering are presented and compared with the ab-plane conductivity {sigma}{sub ab} in the same model. In this model, the interlayer c-axis coupling is taken to be weak and diffusive. While in clean systems, inelastic scattering leads to a peak at {omega}=4{Delta}{sub 0} in {sigma}{sub ab} for T{lt}T{sub c}, it has little effect on the corresponding {sigma}{sub c}, which exhibits structure only at {omega}{approx_equal}2{Delta}{sub 0} and is directly related to the single-particle density of states N({omega}). The c-axis penetration depth {lambda}{sub c} in the same model is predicted to vary as T{sup 3} at low temperatures in clean samples. We discuss recent optical experiments on the cuprates and compare with these predictions. {copyright} {ital 1997} {ital The American Physical Society}

Superconductivity has been recently observed in Sr_{0.4}Ca_{13.6}Cu_{24}O_{41.84} which contains quasi-one-dimensional Cu_{2}O_{3} 2-leg ladders. If, as suggested by some theories, the superconductivity arises from these 2-leg ladders, it will be important to determine the structure of the superconducting gap. In particular, does the gap in a 2-leg ladder change sign when one goes from the bonding to antibonding fermi surface points? Here we carry out phenomenological calculations of nuclear relaxation rates and inelastic neutron scattering intensity in order to provide estimates of the experimental resolution that will be required to determine the structure of the superconducting gap associated with an array of weakly coupled 2-leg ladders.

Angle resolved photoemission spectroscopy studies of the overdoped cuprate superconductor La${}_{2\ensuremath{-}x}$Sr${}_{x}$CuO${}_{4}$ find only small changes in the near nodal electron self-energy over a spectral range of several hundred meV as the doping increases from $x=0.2$ to $x=0.3$ and the superconducting transition temperature ${T}_{c}$ decreases from 32 to 0 K. These measurements put constraints on the structure of the electron-electron interaction. Here we show that a spin-fluctuation interaction leads to behavior which is consistent with these experimental results.

The lack of nesting of the electron and hole Fermi-surface sheets in the Fe-based superconductor LiFeAs, with a critical temperature of 18 K, has led to questions as to whether the origin of superconductivity in this material might be different from other Fe-based superconductors. Both angle-resolved photoemission and quasiparticle interference experiments have reported fully gapped superconducting order parameters with significant anisotropy. The system is also of interest because relatively strong correlations seem to be responsible for significant renormalization of the hole bands. Here we present calculations of the superconducting gap and pairing in the random-phase approximation using Fermi surfaces derived from measured photoemission spectra. The qualitative features of the gaps obtained in these calculations are shown to be different from previous two-dimensional theoretical works, and in good agreement with experiment on the main Fermi surface pockets. We analyze the contributions to the pairing vertex thus obtained and show that the scattering processes between electron and hole pockets that are believed to dominate the pairing in other Fe-based superconductors continue to do so in LiFeAs despite the lack of nesting, leading to gaps with anisotropic $s_\pm$ structure. Some interesting differences relating to the enhanced $d_{xy}$ orbital content of the LiFeAs Fermi surface are noted., 13 pages, 7 figures

Superconductivity in alkali-intercalated iron selenide, with T_c's of 30K and above, may have a different origin than that of the other Fe-based superconductors, since it appears that the Fermi surface does not have any holelike sheets centered around the Gamma point. Here we investigate the symmetry of the superconducting gap in the framework of spin-fluctuation pairing calculations using density functional theory bands downfolded onto a three-dimensional (3D), ten-orbital tight-binding model, treating the interactions in the random-phase approximation (RPA). We find a leading instability towards a state with d-wave symmetry, but show that the details of the gap function depend sensitively on electronic structure. As required by crystal symmetry, quasi-nodes on electron pockets always occur, but are shown to be either horizontal, looplike or vertical depending on details. A variety of other 3D gap structures, including bonding-antibonding s-symmetry states which change sign between inner and outer electron pockets are found to be subdominant. We then investigate the possibility that spin-orbit coupling effects on the one-electron band structure, which lead to enhanced splitting of the two M-centered electron pockets in the 2-Fe zone, may stabilize the bonding-antibonding s_+/- wave states. Finally, we discuss our results in the context of current phenomenological theories and experiments.

Calculations are presented of the in-plane far-infrared conductivity of a ${\mathit{d}}_{{\mathit{x}}^{2}\mathrm{\ensuremath{-}}{\mathit{y}}^{2}}$-wave superconductor, incorporating elastic scattering due to impurities and inelastic scattering due to spin fluctuations. The impurity scattering is modeled by short-range potential scattering with arbitrary phase shift, while scattering due to spin fluctuations is calculated within a weak-coupling Hubbard model picture. The conductivity is characterized by a low-temperature residual Drude feature whose height and weight are controlled by impurity scattering, as well as a broad peak centered at 4${\mathrm{\ensuremath{\Delta}}}_{0}$ arising from clean-limit inelastic processes. Results are in qualitative agreement with experiment despite missing spectral weight at high energies. \textcopyright{} 1996 The American Physical Society.

We calculate the spin susceptibility at and below ${\mathit{T}}_{\mathit{c}}$ for a ${\mathit{d}}_{\mathit{x}}^{2}$-${\mathit{y}}^{2}$-wave superconductor with resonant impurity scattering and Coulomb correlations. Both the impurity scattering and the Coulomb correlations act to maintain peaks in the spin susceptibility, as a function of momentum, at the Brillouin zone edge. These peaks would otherwise be suppressed by the superconducting gap. At the highest impurity concentration considered here, the predicted amount of suppression of the imaginary part of the spin susceptibility in the superconducting state compared to the normal state is in qualitative agreement with results from recent magnetic neutron-scattering experiments on ${\mathrm{La}}_{1.86}$${\mathrm{Sr}}_{0.14}$${\mathrm{CuO}}_{4}$ for momentum values at the zone edge and along the zone diagonal. The predicted peak widths in the superconducting state, however, are narrower than those in the normal state, a narrowing which has not been observed experimentally.

Recent experiments on the alkali-intercalated iron selenides have raised questions about the symmetry of the superconducting phase. Random phase approximation calculations of the leading pairing eigenstate for a tight-binding 5-orbital Hubbard-Hund model of $A$Fe${}_{2}$Se${}_{2}$ find that a $d$-wave (${B}_{1g}$) state evolves into an extended ${s}^{\ifmmode\pm\else\textpm\fi{}}$ (${A}_{1g}$) state as the system is hole-doped. However, over a range of doping these two states are nearly degenerate. Here, we calculate the imaginary part of the magnetic spin susceptibility ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}(q,\ensuremath{\omega})$ for these gaps and discuss how the evolution of neutron scattering resonances can distinguish between them.

Here, based upon density matrix renormalization group calculations, we discuss the structure of the stripes found in the doped $t$-$t^\prime$-$J$ model and the physics that underlies their formation., 26 pages

Here we argue that the dominant quasi-particle scattering mechanism in the cuprate superconductors arises from antiferromagnetic spin-fluctuations and that therefore the one-band Hubbard model provides the appropriate starting point for understanding their properties. Then using Monte Carlo and diagrammatic calculations we show that antiferromagnetic spin fluctuations lead to an attractive pairing interaction in the dx2−y2 singlet channel. We conclude with a discussion of experiments which provide information on the symmetry of the gap.

We develop a simple theory of the electromagnetic response of a d-wave superconductor in the presence of potential scatterers of arbitrary s-wave scattering strength and inelastic scattering by antiferromagnetic spin fluctuations. In the clean London limit, the conductivity of such a system may be expressed in ``Drude'' form, in terms of a frequency-averaged relaxation time. We compare predictions of the theory with recent data on Y-Ba-Cu-O and Bi-Sr-Si-Cu-O crystals and on Y-Ba-Cu-O films. While fits to penetration-depth measurements are promising, the low-temperature behavior of the measured microwave conductivity appears to be in disagreement with our results. We discuss implications for d-wave-pairing scenarios in the cuprate superconductors.

Results of a conserving fluctuation exchange calculation for the superconducting state of a two-dimensional Hubbard model are presented. Self-consistent solutions for the full momentum- and frequency-dependent gap, renormalization, and frequency shift parameters have been obtained. The resulting frequency- and momentum-dependent gap is found to have ${\mathit{d}}_{\mathit{x}}^{2}$-${\mathit{y}}^{2}$ symmetry. Below ${\mathit{T}}_{\mathit{c}}$, the maximum gap opens rapidly to a low temperature 2\ensuremath{\Delta}(0)/${\mathit{kT}}_{\mathit{c}}$ value of order 10. We find that the effective interaction arising from the exchange of spin and charge fluctuations is stabilized by self-energy feedback effects as the superconducting gap opens.

The bilayer Hubbard model with an intralayer hopping $t$ and an interlayer hopping ${t}_{\ensuremath{\perp}}$ provides an interesting testing ground for several aspects of what has been called unconventional superconductivity. One can study the type of pair structures which arise when there are multiple Fermi surfaces. One can also examine the pairing for a system in which the structure of the spin-fluctuation spectral weight can be changed. Using a dynamic cluster quantum Monte Carlo approximation, we find that near half filling, if the splitting between the bonding and antibonding bands ${t}_{\ensuremath{\perp}}/t$ is small, the gap has ${B}_{1g}$ (${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave) symmetry, but when the splitting becomes larger, ${A}_{1g}$ (${s}^{\ifmmode\pm\else\textpm\fi{}}$-wave) pairing is favored. We also find that in the ${s}^{\ifmmode\pm\else\textpm\fi{}}$ pairing region, the pairing is driven by interlayer spin fluctuations and that ${T}_{c}$ is enhanced.

Recently, neutron scattering spin-echo measurements have provided high-resolution data on the temperature dependence of the linewidth $\ensuremath{\Gamma}(\mathbf{q},T)$ of acoustic phonons in conventional superconductors Pb and Nb [see P. Aynajian et al., Science 319, 1509 (2008)]. At low temperatures, the merging of the $2\ensuremath{\Delta}(T)$ structure in the linewidth with a peak associated with a low-lying $\ensuremath{\hbar}{\ensuremath{\omega}}_{{\mathbf{q}}_{KA}}$ Kohn anomaly suggested a coincidence between $2\ensuremath{\Delta}(0)$ and $\ensuremath{\hbar}{\ensuremath{\omega}}_{{\mathbf{q}}_{KA}}$ in Pb and Nb. Here we carry out a standard BCS calculation of the phonon linewidth to examine its temperature evolution and explore how close $2\ensuremath{\Delta}(0)/\ensuremath{\hbar}{\ensuremath{\omega}}_{{\mathbf{q}}_{KA}}$ must be to unity in order to be consistent with the neutron data.

Angle-resolved photoemission spectroscopy measurements on the recently discovered superconductors in the KFe${}_{2}$Se${}_{2}$ family with critical temperatures up to $~33$ K suggest that no Fermi pockets of hole character centered on the $\ensuremath{\Gamma}$ point of the Brillouin zone are present, in contrast to all other known ferropnictide and ferrochalcogenide superconductors. Using a fluctuation exchange approximation and a five-orbital tight-binding description of the band structure, we calculate the effective pairing interaction. We find that the pairing state in this system is most likely to have $d$-wave symmetry due to pair scattering between the remaining electron Fermi pockets at wave vector $\mathbf{q}~(\ensuremath{\pi},\ensuremath{\pi})$, but without any symmetry-imposed nodes for the given Fermi surface. We propose experimental tests of this result, including the form of the resonance spectrum probed by inelastic neutron scattering.

We study a diagonal two-orbital ladder model of the Fe-based superconductors using the density-matrix renormalization-group method. At half filling, we find a close competition between a ``spin-striped'' state and a noncollinear ``spin-checkerboard'' state, as well as significant nematic correlations. Upon finite hole or electron doping, the dominant pairing correlations are found to have ${\text{A}}_{1g}$ ($S$-wave) symmetry.

We study the phase diagram of the Hubbard model in the limit where U, the onsite repulsive interaction, is much smaller than the bandwidth. We present an asymptotically exact expression for T$_c$, the superconducting transition temperature, in terms of the correlation functions of the non-interacting system which is valid for arbitrary densities so long as the interactions are sufficiently small. Our strategy for computing T$_c$ involves first integrating out all degrees of freedom having energy higher than an unphysical initial cutoff $\Omega_0$. Then, the renormalization group (RG) flows of the resulting effective action are computed and T$_c$ is obtained by determining the scale below which the RG flows in the Cooper channel diverge. We prove that T$_c$ is independent of $\Omega_0$. Using this method, we find a variety of unconventional superconducting ground states in two and three dimensional lattice systems and present explicit results for T$_c$ and pairing symmetries as a function of the electron concentration., Comment: 18 pages, 17 figures

Calculations of the pairing interaction in multi-band models of the Fe superconductors show that it is attractive in both the $A_{1g}$ (s-wave) and $B_{1g}$ (d-wave) channels. This raises the possibility that these materials may have collective excitonic modes. Here, assuming an s-wave groundstate, we investigate the d-wave collective excitonic mode and its coupling to the Raman scattering.

Results are given from a density matrix renormalization group study of pairing on a striped $t$-$t'$-$J$ lattice in the presence of boundary magnetic and pair fields. We find that pairing on a stripe depends sensitively on both $J/t$ and $t'/t$. In the strong-pairing model-parameter regime the stripes are easily coupled by the pair field, and have a uniform phase. There is a small but measurable energy cost to create anti-phase superconducting domain walls.

The existence of a neutron scattering resonance at a wavevector q* implies a sign change of the gap between two Fermi surface regions separated by wavevector q* . For the Fe pnictides, a resonance has been observed for a wavevector q* which connects a hole Fermi surface around the $\Gamma$ point with an electron Fermi surface around the X or Y points of the 1 Fe/unit cell Brillouin zone. Here we study the neutron scattering resonance for a five orbital model within an RPA-BCS approximation. Our results show that both sign-switched and extended s-wave gaps are consistent with the present data for q* near ($\pi$, 0) and that scattering at other momentum transfers can be useful in distinguishing between gap structures., Comment: 5 pages, 4 figures

An important issue in the study of the iron-arsenic based superconductors is the symmetry of the superconducting gap, a problem complicated by multiple gaps on different Fermi surface sheets. Electronic Raman scattering is a flexible bulk probe which allows one in principle to determine gap magnitudes and test for gap nodes in different regions of the Brillouin zone by employing different photon polarization states. Here we calculate the clean Raman intensity for $A_{1g}$, $B_{1g}$ and $B_{2g}$ polarizations, and discuss the peak structures and low-energy power laws which might be expected for several popular models of the superconducting gap in these systems., 9 pages, 9 figures

We show, using a simple model, how ordinary disorder can gap an extended-$s$ ($A_{1g}$) symmetry superconducting state with nodes. The concommitant crossover of thermodynamic properties, particularly the $T$-dependence of the superfluid density, from pure power law behavior to an activated one is exhibited. We discuss applications of this scenario to experiments on the ferropnictide superconductors., Comment: 9 pages

Theories based on the coupling between spin fluctuations and fermionic quasiparticles are among the leading contenders to explain the origin of high-temperature superconductivity, but estimates of the strength of this interaction differ widely. Here we analyze the charge- and spin-excitation spectra determined by angle-resolved photoemission and inelastic neutron scattering, respectively, on the same crystals of the high-temperature superconductor YBa2Cu3O6.6. We show that a self-consistent description of both spectra can be obtained by adjusting a single parameter, the spin-fermion coupling constant. In particular, we find a quantitative link between two spectral features that have been established as universal for the cuprates, namely high-energy spin excitations and "kinks" in the fermionic band dispersions along the nodal direction. The superconducting transition temperature computed with this coupling constant exceeds 150 K, demonstrating that spin fluctuations have sufficient strength to mediate high-temperature superconductivity., Comment: 25 pages, 7 figures, including supplementary information, accepted for publication in Nature Physics

We discuss the large gap anisotropy found for the A1g (s-wave) state in RPA spin-fluctuation and functional renormalization group calculations and show how the simple arguments leading to isotropic sign-switched s-wave states in these systems need to be supplemented by a consideration of pair scattering within Fermi surface sheets and between the individual electron sheets as well. In addition, accounting for the orbital makeup of the states on the Fermi surface is found to be crucial., Comment: 6 pages, 7 figures

The importance of the electronic spin fluctuations in the CuO{sub 2} planes in the theory of the transverse nuclear relaxation for YBa{sub 2}Cu{sub 3}O{sub 7} has been demonstrated by Pennington and Slichter. We present the predictions of a RPA-like theory for the transverse nuclear relaxation time, {tau}, in the superconducting state. {ital s}- and {ital d}-wave gap symmetries yield distinctively different results for {tau}{sup {minus}1}; for an {ital s}-wave gap {tau}{sup {minus}1} decays rapidly below {ital T}{sub {ital c}}, while for a {ital d}-wave gap it remains nearly constant. Measurements of {tau}{sup {minus}1} below {ital T}{sub {ital c}} could provide valuable information about the symmetry of the superconducting state. Results for the temperature dependence of {tau}{sup {minus}1} in the normal state are also given.

On the basis of exact diagonalizations, a comparative study of two-particle optical and magnetic, as well as single-particle, excitations is presented for a two-dimensional (2D) multiorbital Hubbard model. For reasonable parameter sets appropriate for the cuprate superconductors, the single-particle excitations display strongly correlated states related to the Zhang-Rice Cu-O singlet construction. These states define the gap (to the upper Hubbard band) at half-filling and become partially occupied by doping holes in our 2\ifmmode\times\else\texttimes\fi{}2 unit-cell system. The optical results, which are the first quantitative calculations performed for realistic parameters of the three-band Hubbard model, clearly show three allowed optical transitions: (i) itinerant motion of the Cu-O singlets, having (for doping concentrations x\ensuremath{\ne}0) a spectral Drude distribution around \ensuremath{\omega}=0 with spectral weight proportional to x; (ii) unbinding of the O hole from the Cu spin in the singlet. This gives, in particular, a strong absorption peak due to singlet\ensuremath{\rightarrow}nonbonding oxygen transitions, again with relative weight \ensuremath{\sim}x.It is roughly centered at \ensuremath{\omega}\ensuremath{\sim}${\mathit{J}}_{\mathrm{Kondo}}$, where ${\mathit{J}}_{\mathrm{Kondo}}$ is the Kondo coupling and \ensuremath{\Delta} the bare Cu-O charge-transfer energy. It is this singlet unbinding that results in the by far dominant absorption structure between Drude and higher-energy Cu-O transitions and not the often discussed ``mid-infrared'' absorption due to transitions between different singlet states. (iii) Cu-O charge-transfer processes at energy \ensuremath{\sim}\ensuremath{\Delta}+${\mathit{U}}_{\mathit{p}\mathit{d}}$. They show a pronounced excitonic effect due to the p-d interaction ${\mathit{U}}_{\mathit{p}\mathit{d}}$ and have a reduced spectral weight shifted to higher energies for increased dopings. Findings (i)--(iii) are in general accordance with recent experimental data. Our study of the low-energy absorption is complemented with a numerical scaling analysis of the Drude weight in 1D, where, in particular, we find an interesting violation of Lenz's law for 4n-site Hubbard rings. Finally, the magnetic structure factor is calculated for the 2D case. For finite doping it contains a peak at 2${\mathit{J}}_{\mathrm{Kondo}}$, which should be detectable in experiment.

We have measured the surface impedance Zs = Rs + iXs in the normal and superconducting state of κ(BEDT-TTF)2Cu(NCS)2. In the superconducting state the electrodynamics is in agreement with calculations based on a BCS ground state, with strongly anisotropic parameters.

Inelastic neutron scattering provides a probe for studying the spin and momentum structure of the superconducting gap. Here, using a two-orbital model for the Fe-pnictide superconductors and a random-phase approximation--BCS approximation for the dynamic spin susceptibility, we explore the scattering response for various gaps that have been proposed.

The question of whether one should speak of a "pairing glue" in the Hubbard and t-J models is basically a question about the dynamics of the pairing interaction. If the dynamics of the pairing interaction arises from virtual states, whose energies correspond to the Mott gap, and give rise to the exchange coupling J, the interaction is instantaneous on the relative time scales of interest. In this case, while one might speak of an "instantaneous glue," this interaction differs from the traditional picture of a retarded pairing interaction. However, as we will show, the dominant contribution to the pairing interaction for both of these models arises from energies reflecting the spectrum seen in the dynamic spin susceptibility. In this case, the basic interaction is retarded, and one speaks of a spin-fluctuation glue which mediates the d-wave pairing.

We have measured the millimeter-wave (100-GHz) surface resistance of high-quality laser-deposited YBa{sub 2}Cu{sub 3}O{sub 7} films on SrTiO{sub 3} and LaAlO{sub 3} substrates. Due to finite film thickness, radiation losses are important in the normal state and in the superconducting state near {ital T}{sub {ital c}}. These effects are calculated, and {ital R}{sub {ital s}} characteristics of the ohmic losses in the film are extracted from the data. {ital R}{sub {ital s}} drops rapidly at {Tc}, and a detailed comparison with calculations that include finite mean-free-path effects suggests a gap that opens more rapidly as {ital T}/{Tc} decreases and attains a larger 2{Delta}(0)/{ital k}{sub {ital B}}{ital T}{sub {ital c}} ratio than the standard BCS behavior.

We have measured the millimeter-wave (100 GHz) surface resistance of high-quality laserdeposited YBa2Cu3O7 films on SrTiO3 and LaA103 substrates. Due to finite film thickness, radiation losses are important in the normal state and in the superconducting state nearT c. These effects are calculated andR s characteristic of the ohmics losses in the film are extracted from the data. The surface resistanceR s drops rapidly atT c, and a detailed comparison with calculations which include finite mean free path effects suggests a gap which exceeds the weak coupling BCS limit.

Dynamical theory for the polarization of bound vortex-antivortex pairs near the Kosterlitz-Thouless transition ({ital T}{sub KT}=88.4 K) has been applied to thin films of YBa{sub 2}Cu{sub 3}O{sub 7}. Calculations show that the correct order of magnitude is predicted for the loss function {omega}{ital G}/{ital c}{sup 2} at {ital T}{sub KT}, but the temperature dependence below the transition is wrong. The theoretical value drops much more rapidly with decreasing temperature than observed experimentally. Similar disagreement is found for the penetration depth {lambda}({ital T}). Estimates of the loss function at microwave frequencies show rather large effects near the critical temperature, but these become negligible by 80 K. The performance of microwave devices operating at liquid-N{sub 2} temperature should not be degraded by vortex-antivortex pairs.

We perform the numerical equivalent of a phase sensitive experiment on doped 2-leg t - J ladders. We apply proximity effect fields with different complex phases at both ends of an open system and we study the transport of Cooper pairs. Measuring the response of the system and the induced Josephson current, density matrix renormalization group calculations show how, depending on the doping fraction, the rung-leg parity of the pair field changes from minus to plus as the density of holes is increased. The Josephson current exhibits a phase transition as a function of J/t.

In this Letter, we analyze, using scanning tunneling spectroscopy, the density of electronic states in nearly optimally doped Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} in zero magnetic field. Focusing on the superconducting gap, we find patches of what appear to be two different phases in a background of some average gap, one with a relatively small gap and sharp large coherence peaks and one characterized by a large gap with broad weak coherence peaks. We compare these spectra with calculations of the local density of states for a simple phenomenological model in which a 2{zeta}{sub 0} x 2{zeta}{sub 0} patch with an enhanced or suppressed d-wave gap amplitude is embedded in a region with a uniform average d-wave gap.