We report a detailed measurement of the magnetization for a high-quality ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ single crystal as a function of magnetic field (H) and the angle between H and the c-axis, near the transition temperature ${\mathit{T}}_{\mathit{c}}$. The fluctuation magnetization for H parallel to the c-axis was found to be in excellent agreement with the recently developed vortex-fluctuation theory by Bulaevskii et al. and the order-parameter fluctuation theory by Tešanović et al. However, our fluctuation magnetization data for the fields, applied at a large angle with respect to the c-axis, were found inconsistent with the expectations from the above theories.

We have measured the high-magnetic-field dependences (up to 18 T) of the in-plane and out-of-plane transport properties of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ (Bi-2:2:1:2) single crystals. From I-V curves of c-axis transport we extract the magnetic field and temperature dependences of the interlayer critical current and define a decoupling line ${\mathit{H}}_{\mathit{D}}$(T) above which ${\mathit{J}}_{\mathit{c},}$c vanishes. The temperature dependence of ${\mathit{J}}_{\mathit{c},}$c supports a first-order decoupling transition. The decoupling line lies close to the irreversibility line in the field interval 0.5–5 T. The field dependence of the in-plane vortex activation energy is ∼${\mathit{B}}^{\mathrm{-}0.5\pm 0.1}$.

The temperature dependence of the resistivity of single-crystal ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ below 260 K of Crespi et al. is analyzed utilizing the results of Raman scattering. It is found that coupling to the two lowest intramolecular ${\mathit{H}}_{\mathit{g}}$ modes gives an excellent fit to the temperature variation of the resistivity with the inclusion of weak lower frequency acoustic or librational scattering. The relative coupling strength of ${\mathit{H}}_{\mathit{g}}$(1) to ${\mathit{H}}_{\mathit{g}}$(2) is found to be similar to that estimated from Raman studies of thin films. The weaker low-frequency contribution is found to have a value consistent with neutron-scattering studies.

The reflection spectrum and excitation spectra of self-trapped exciton emissions have been measured in the spectral region from 6.5 to 30 eV for a freshly cleaved KBr crystal at liquid-helium temperature. The following mechanisms of multiplication of electronic excitations have been distinguished by using luminescence methods: (i) formation of secondary excitons by hot photoelectrons; (ii) formation of secondary electron-hole pairs by hot photoelectrons; (iii) formation of electron-hole pairs due to nonradiative Auger decay of cation electronic excitations. The analysis of the spectrum of the intensity ratio of π and σ emissions of self-trapped excitons allowed us to obtain quantitative data on exciton formation by photoelectrons. The excitation spectra for the T and X Jahn-Teller components of ${\mathrm{Tl}}^{+}$-center emission have been measured in a KBr:Tl crystal at liquid-helium temperature in the region from 6.5 to 30 eV. A sharp maximum at 15 eV in the intensity ratio of T and X emissions is connected with the excitation of ${\mathrm{Tl}}^{+}$ centers by hot photoelectrons.

The observed broadening of the line shape of the emission spectra of the luminescence centers in crystals with high electron-lattice coupling has been explained in terms of the ion-motion spatial confinement in the lattice. For this, a simple and analytically solvable model for the potential of the electronic energy sheets has been proposed. As an example of application the emission spectrum of ${\mathrm{BiO}}_6^{9\mathrm{-}}$ complex has been analyzed.

We experimentally show that in the inorganic spin-Peierls compound ${\mathrm{CuGeO}}_3$ the inelastic-neutron-scattering intensity with ΔE=1 meV at (0,1,0.5) exponentially decreases below ${\mathit{T}}_{\mathrm{SP}}$(=14 K) due to the formation of the spin-Peierls gap. We also found that the magnetic excitation spectrum exhibits a pronounced energy dispersion, the intrachain and interchain exchange parameters ${\mathit{J}}_{\mathit{c}}$≊10.4 meV, ${\mathit{J}}_{\mathit{b}}$≊0.1${\mathit{J}}_{\mathit{c}}$, and ${\mathit{J}}_{\mathit{a}}$≊-0.01${\mathit{J}}_{\mathit{c}}$, and the gap energy at T=0 K is 2.1 meV, suggesting that ${\mathit{J}}_{\mathit{b}}$/${\mathit{J}}_{\mathit{c}}$≊0.1 is rather large compared to the organic spin-Peierls system.

Spin gaps in coupled t-J ladders are investigated by exact diagonalization of small clusters up to 4×8 sites. At half-filling, the numerical results for the triplet excitation spectrum are in very good agreement with a second-order perturbation expansion in terms of small interladder and intraladder exchange couplings between rungs (J/J’<0.25). The band of local triplet excitations moving coherently along the ladder (with momenta close to π) is split by the interladder coupling. For intermediate couplings, finite-size scaling is used to estimate the spin gap. In the isotropic infinite four-chain system (two coupled ladders) we find a spin gap of 0.245J, roughly half of the single-ladder spin gap. When the system is hole doped, bonding and antibonding bound pairs of holes can propagate coherently along the chains and the spin gap remains finite.

The recent experimental observation of steplike structure in the superfluid density of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films on graphite is interpreted in terms of passage close to Mott insulating lobes in the Bose-Hubbard model. Plateaus develop in the superfluid density near the completion of each layer because of the He-He repulsion. We modify the Bose-Hubbard Hamiltonian, using a density-dependent He-He interaction term to model the He-substrate potential. Quantum Monte Carlo simulations yield results that agree well with the experimental data.

We have calculated the largest eigenvalues of the reduced density matrix of the singlet superconducting correlation function and pair-field susceptibility in two-dimensional and two-chain Hubbard models with quantum Monte Carlo methods. A singlet superconducting channel opens upon doping but is otherwise closed at half-filling when U/t=4. The interaction vertex contribution to the largest eigenvalue of the singlet pair-field susceptibility grows with a power law as temperature is decreased. With these two observations, we revise our previous understanding of the interaction vertex in the two-dimensional Hubbard model. We observe that some phases reside in the two-chain Hubbard model. An analysis of the 2${\mathit{k}}_{\mathit{F}}$ charge correlation up to 32×2 sites suggests that the 2${\mathit{k}}_{\mathit{F}}$ correlation is dominant over the superconducting correlation (0.25≤${\mathit{K}}_{\mathrm{\rho }}$≤1.00) in the two-component Luttinger-liquid phase (U/t=2 and ${\mathit{t}}_{\mathit{v}}$/t=1.4). We conclude the reduced-density-matrix analysis discussed here is needed to unveil the nature of Hubbard and related models.

With powder-x-ray-diffraction methods, the ternary compound LaNiSi has been identified to have a LaPtSi-type crystal structure with four formula units in a unit cell of the space group I$4_1$md with dimensions a=4.181(1) Å, c=14.069(8) Å. The agreement of the transition temperature ${\mathit{T}}_{\mathit{c}}$=1.20∼1.26 K (10∼90 % values) measured by ac-susceptibility and heat-capacity techniques provides clear evidence of bulk superconductivity in LaNiSi. The normal-state specific-heat data can be fit to the expression ${\mathit{C}}_{\mathit{n}}$=γT+β${\mathit{T}}^3$ by a least-squares analysis, where γ=8.89 mJ/mol ${\mathrm{K}}^2$ and β=0.487 mJ/mol ${\mathrm{K}}^4$, resulting in a Debye temperature ${\mathrm{\Theta }}_{\mathit{D}}$=229 K. Below ${\mathit{T}}_{\mathit{c}}$, the specific-heat data have a dominant low-temperature behavior of the form exp[-Δ(0)/${\mathit{k}}_{\mathit{BT}}$], where the order parameter 2Δ(0)=3.5${\mathit{k}}_{\mathit{BT}\mathit{c}}$. In addition, the measured heat-capacity jump ΔC at the transition point is found to be equal to 1.46γ${\mathit{T}}_{\mathit{c}}$, implying that LaNiSi is a weakly coupled superconductor.

We report extensive measurements of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ (Bi 2:2:1:2) and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ (Y 1:2:3) single crystals with high-resolution electron-energy-loss spectroscopy (HREELS). Both as-grown and oxygen-annealed Bi 2:2:1:2 samples were studied. In all cases, peaks due to surface optical phonons were observed at loss energies from 24 to 80 meV. We see no evidence for the weak feature near 60 meV which has been previously reported and attributed to the superconducting energy gap. Our results demonstrate that the optical conductivity of high-temperature superconductors deduced from HREELS, like that deduced from infrared spectroscopy, does not exhibit the gaplike structure expected for a BCS superconductor.

We report measurements of the dynamic resistance ${\mathit{R}}_{\mathit{D}}$ of a dc SQUID formed by two contacts between two different edges of a single crystal of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.9}$ and a Nb block. The phase of the field-induced ${\mathit{R}}_{\mathit{D}}$ oscillations was compared with those of two adjacent SQUID’s, each formed by two Nb-Nb contacts. We find a phase difference of 160±30° between the oscillations of the composite SQUID and those of the SQUID’s formed by Nb-Nb junctions. A possible explanation for this reproducible effect is that this cuprate superconductor has an unconventional order parameter.

We have investigated the anisotropic resistivities of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ using detwinned crystals with various oxygen contents (6.68 ≤7-y≤ 6.93). The out-of-plane resistivity ${\mathrm{\rho }}_{\mathit{c}}$ shows a crossover from high-T metallic to low-T semiconducting behavior while the in-plane resistivity ${\mathrm{\rho }}_{\mathit{a}}$ deviates in the low-T region from T-linear dependence. We find that the crossover in ${\mathrm{\rho }}_{\mathit{c}}$ is linked with the onset of nonlinearity in ${\mathrm{\rho }}_{\mathit{a}}$, which seems to be associated with the ‘‘spin gap’’ suggested by neutron and NMR studies.

X-ray-absorption spectroscopy (XAS) is used to explore the systematic variations in the 4d/5d transition-metal {T=Au, Pt, Ir, Re, Pd, Ag} d-orbital occupancy in binary T-X compounds {X=Al, Ga, In, Si, Ge, Sn}. Specifically, the strength of the white line (WL) feature at the T-${\mathit{L}}_{2,3}$ edges is used to quantify the changes in the d-orbital occupancy induced by T-X bond formation. Systematic chemical trends in bonding-induced d-orbital-occupancy changes, evidenced by the data, are discussed. A charge-transfer scale (C scale) is developed to approximately summarize the number of T-d holes created in all of the ${\mathit{T}}_{1\mathrm{-}\mathit{x}}$${\mathit{X}}_{\mathit{x}}$ compounds studied. Comparison with the electronegativity scale emphasizes that, although the C scale deals with a more restricted type of charge transfer, it provides insight into the T-X bonding that is not contained in the traditional scale.

This work reports the behavior of the luminescence bands of the self-trapped exciton (STE) in KI under hydrostatic pressure. The STE was created by populating the upper branches of the transverse exciton-polaritons using multiphoton excitation. The polariton can then scatter to the lower branches and relax to a ‘‘bottleneck’’ from which it will eventually become self-trapped. The σ and π bands of the STE experience different blue shifts under pressure and show significant changes in their intensity and bandwidth. A qualitative interpretation of the phenomena is given in terms of the nearest-neighbor F-H pair representation of the off-center STE configuration and how they are affected by the lattice compression.

A first-principles computation of tetragonal distortion, structural stability, and equation of state has been carried out for indium, using the linear muffin-tin orbital electron-band-theory technique. Results on the variation of tetragonal distortion (c/a) with compression show a broad maximum around V/${\mathit{V}}_0$≊0.8 (19 GPa) in agreement with experimental data. We do not find 5s5p-to-5d electron transfer as speculated to be the cause of the maximum in c/a. However, our results reveal that the core overlap resulting from broadening of the 4d states is the probable reason for the turnover in the c/a ratio. Within the atomic-sphere approximation we find the face-centered orthorhombic phase to be marginally stable beyond 56 GPa in accord with the experiments of Takemura et al.