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The normal-state magnetotransport properties of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{CuO}}_{4}$ single crystals with $x=0.095$ are measured; at this composition, a structural transition to a low-temperature tetragonal (LTT) phase occurs without suppression of superconductivity. None of the measured properties (in-plane and out-of-plane resistivity, magnetoresistance, and Hall coefficient) shows any sudden change at the LTT phase transition, indicating that the occurrence of the LTT phase does not necessarily cause an immediate change in the electronic state such as the charge-stripe stabilization.

We report on vector magnetization studies of a La{sub 1.9}Sr{sub 0.1}CuO{sub 4} single crystal, cut in the form of a disk with the anisotropy (c) axis parallel to the disk plane. We observe a sharp threshold in the angular dependences of the isothermal remanent magnetization. When the magnetic field is applied within a threshold angle to the ab plane, the remanent moment is strictly parallel to ab, and also the c-axis component of magnetization is reversible. The threshold angle scales with temperature in the same manner as the lower critical field parallel to the c axis, H{sub c1}{sup c}. These observations give direct support to the presence of a vortex lock-in transition to the CuO planes in our crystal. {copyright} {ital 1997} {ital The American Physical Society}

Ultrasonic and specific-heat measurements have been performed on single crystals of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ (LSCO) with x=0.09, 0.14, and 0.19 across the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$. We observed jumplike decreases in the longitudinal elastic moduli at ${\mathit{T}}_{\mathit{c}}$, as are seen in conventional superconductors. Anisotropic strain dependence of ${\mathit{T}}_{\mathit{c}}$ was estimated from both the jump in the elastic moduli and specific heat at ${\mathit{T}}_{\mathit{c}}$ via the Ehrenfest relation. With further lowering temperature below ${\mathit{T}}_{\mathit{c}}$, we found a pronounced lattice stiffening in the elastic moduli of LSCO, which contrasts with the continuous softening observed in most of the conventional superconductors. Thermodynamic analysis revealed that the stiffening originates from the change in the superconducting condensation energy under the lattice strain. This unusual lattice stiffening in the superconducting state is possibly a common character of the high-${\mathit{T}}_{\mathit{c}}$ copper oxides.

Phonons and magnetic excitations were studied as a function of temperature in the spin-Peierls phase of an inorganic material ${\mathrm{CuGeO}}_{3}$ by Raman scattering. All of the phonon Raman peaks observed at room temperature can be assigned on condition that the crystal symmetry is ${\mathit{D}}_{2\mathit{h}}^{5}$(Pbmm). Five peaks intrinsic to the spin-Peierls state were observed at 30, 105, 228, 370, and 818 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the spin-Peierls phase. The temperature dependence of the frequency of the 30-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ peak agrees approximately with 2\ensuremath{\Delta}(T), where \ensuremath{\Delta}(T) is the spin-Peierls gap. The Raman spectrum between 0 and 300 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ was compared with the density of states of the magnetic excitons. The 105- and 228-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ peaks were assigned to two-magnetic-exciton Raman scattering. The peaks at 370 and 818 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ were assigned to one-phonon Raman scattering of the folded ${\mathit{A}}_{\mathit{g}}$ modes, which occurs due to the formation of a dimerization in the spin-Peierls phase.

We present resonant ultrasound spectroscopy measurements of the complete elastic moduli of La[sub 2[minus][ital x]]Sr[sub [ital x]]CuO[sub 4] ([ital x]=0.00 and 0.14). The [ital x] dependence of both the absolute values of the moduli in the tetragonal phase and the temperature dependences of the moduli near the structural phase transition is weak. In both materials [ital c][sub 44] and 1/2 ([ital c][sub 11][minus][ital c][sub 12]) are unaffected by the transition, [ital c][sub 33] shows a steplike drop broadened by fluctuations, and [ital c][sub 11] and [ital c][sub 66] decrease as 1/([ital T][minus][ital T][sub SPT]) for [ital T][gt][ital T][sub SPT] (where [ital T][sub SPT] is the structural phase transition temperature). We also present measurements of the anisotropic thermal expansion coefficients in La[sub 1.86]Sr[sub 0.14]CuO[sub 4] using a high-resolution capacitance dilatometer. These data when taken together raise serious questions about the standard interpretation of the structural phase transition in La[sub 2[minus][ital x]]Sr[sub [ital x]]CuO[sub 4].

We present results from a neutron-scattering study of lattice anomalies associated with the transition to the gapped spin-singlet state observed in the spin-1/2 linear-chain compound ${\mathrm{CuGeO}}_{3}$. Instead of the lattice dimerization along the chain direction (c axis) expected for a spin-Peierls system, we observe a remarkable softening of the longitudinal acoustic phonons in a direction (along the b axis) perpendicular to the chains. A spontaneous strain appears in the same direction below the magnetic transition temperature of 14 K.

We have measured and calculated the electron-positron momentum distribution of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ samples for Sr concentrations of 0, 0.1, 0.13, and 0.2. Measured distributions were obtained at room temperature with high statistical precision, greater than 4\ifmmode\times\else\texttimes\fi{}${10}^{8}$ events, in the Lawrence Livermore National Laboratory positron-annihilation angular correlation spectrometer on single-crystal samples fabricated using the traveling solvent floating zone technique. Corresponding theoretical momentum-density calculations were performed using the linear muffin-tin-orbital method. The momentum distribution of all samples contained features derived from the overlap of the positron distribution with the valence electrons. In addition, discontinuities typical of a Fermi surface are seen in the doped samples. The form and position of these features are in general agreement with the Fermi surface and overall momentum distributions as predicted by band theory. However, the evolution of the Fermi surface with doping differed significantly from expectations based on single electron band theories.

Neutron-scattering experiments have been performed to study the dynamic spin properties of La{sub 1.85}Sr{sub 0.15}CuO{sub 4} ({ital T}{sub {ital c}}=33 K) in the temperature range 1.9 K{le}{ital T}{le}150 K and energy range 1.5 meV{le}{omega}{le}20 meV; these span, respectively, {ital T}{sub {ital c}} and the weak-coupling BCS gap energy 2{Delta}{similar_to}10 meV. The width of {ital S}({bold q},{omega}) in momentum space decreases with decreasing temperature at fixed energy as well as with decreasing energy at fixed {ital T}, becoming very sharp at both low {omega} and low temperature. It is found that both in the normal state at {ital T}=35 K and in the superconducting state at {ital T}=10 K the integrated generalized susceptibility {chi}{prime}{prime}({omega})={integral}{sub ({pi},{pi})} d{ital q}{sub 2D}{chi}{prime}{prime}({bold q},{omega}) is constant above {omega}{similar_to}10 meV with an amplitude about three times that in La{sub 1.98}Sr{sub 0.02}CuO{sub 4} and begins to decrease with decreasing {omega} at {omega}{similar_to}10 meV, which corresponds to the BCS superconducting gap energy 2{Delta}. However, we observe nonzero magnetic scattering well below the pseudogap energy at temperatures as low as 2 K. These results are compared with those in La{sub 1.98}Sr{sub 0.02}CuO{sub 4} and YBa{sub 2}Cu{sub 3}O{sub 6.6}.

Capacitance-dilatometer measurements of the anisotropic thermal expansion on a superconducting single crystal of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ (x\ensuremath{\sim}0.13) show large anomalies at the structural transition from the high-temperature tetragonal to the low-temperature orthorhombic phase followed by smaller anomalies of the opposite sign at the superconducting phase transition. Neutron-diffraction experiments on the same crystal indicate that at ${\mathit{T}}_{\mathit{c}}$ the tilt of the ${\mathrm{CuO}}_{6}$ octahedra related to the structural phase transition is frozen in.

Neutron-scattering experiments with markedly improved signal-to-noise ratios have been performed on single-crystal superconducting La[sub 1.85]Sr[sub 0.15]CuO[sub 4] ([ital T][sub [ital c]]=33 K) at energies down to 1.5 meV. The measurements provide quantitative information on the temperature dependence of [chi][prime][prime]([bold q],[omega]). Surprisingly, [chi][prime][prime]([bold q][sub peak],[omega]) is approximately constant below [ital T][sub [ital c]] for the measured energies 1.5 meV[le][omega][le]6 meV, that is, for energies much less than the mean-field BCS gap energy of [similar to]10 meV, hence implying other than conventional [ital s]-wave superconductivity.

Measurements are reported of the static, instantaneous, and dynamic spin correlations in single crystals of ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$. The as-grown crystals show antiferromagnetic long-range order. However, the N\'eel temperatures and spin-stiffness constants are greatly reduced from their respective values in ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$. After reduction and annealing, the crystals superconduct with ${\mathit{T}}_{\mathit{c}}$=23 K, but inclusions of the antiferromagnetic phase, albeit with reduced N\'eel temperatures, are always present. Based on these data, we suggest that the disorder due to excess oxygen in as-grown samples stabilizes the magnetic correlations. This leads to a speculative model for the effects of deoxygenation in which the removal of the excess oxygen causes the magnetic correlations to diminish and hence superconductivity to become possible.

Magnetic relaxation and hysteresis studies have been performed for a c-axis-oriented single crystal of ${\mathrm{La}}_{1.86}$${\mathrm{Sr}}_{0.14}$${\mathrm{CuO}}_{4}$. We demonstrate that the effective activation energy for flux creep, ${\mathit{U}}_{\mathrm{eff}}$, is a strongly nonlinear function of the current density J (as given by the irreversible magnetization). For fixed fields of 0.5, 1.0, 2.0, and 3.0 T and temperatures between 4 and 15 K (as consistent with full field penetration of the sample), ${\mathit{U}}_{\mathrm{eff}}$(J) is seen to vary approximately logarithmically with J. A scaling relationship, ${\mathit{U}}_{\mathrm{eff}}$(J)=[-${\mathit{U}}_{1}$g(T)/${\mathit{H}}^{\mathit{n}}$]ln(J/${\mathit{J}}_{\mathit{c}}$), is shown to be obeyed for this crystal with n\ensuremath{\simeq}0.9 (H in T), ${\mathit{U}}_{1}$\ensuremath{\simeq}180${\mathit{k}}_{\mathit{B}}$, and ${\mathit{J}}_{\mathit{c}}$(1 T)\ensuremath{\simeq}3.2\ifmmode\times\else\texttimes\fi{}${10}^{5}$ A/${\mathrm{cm}}^{2}$. Several functional forms of g(T) have been explored and shown to give reasonable fits if monotonically decreasing in temperature over the range where flux creep is observed. We also explore the ramifications of this functional dependence in explaining an observed exponential temperature dependence of the hysterically determined critical current density. Finally, we show that comparison of the temperature dependence of logarithmic relaxation rate A=dM/d ln(t) and the derivative of the magnetization with respect to ln(T) at fixed time, ${\mathit{dM}}_{0}$/d ln(T), can be used to set a scale of the attempt frequency for flux motion consistent with the other measurements.

Linewidths of some transverse acoustic phonons have been studied as a function of temperature on a superconducting single-crystal sample of ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4}$ (${\mathit{T}}_{\mathit{c}}$=33 K) with the goal of measuring the electron-phonon interaction. The phonons studied have energies in the range \ensuremath{\omega}\ensuremath{\sim}6.0--9.5 meV and observed full widths at half-maximum in the range 2${\ensuremath{\gamma}}_{\mathrm{obs}}$\ensuremath{\sim}0.25--0.5 meV. No temperature-dependent changes were observed in any of the linewidths near the transition temperature. This null result is consistent with a low electronic density of states at the Fermi surface, and an electron-phonon coupling parameter \ensuremath{\lambda} that is not unusually high compared with conventional superconductors.

The anisotropic lower critical fields ${\mathit{H}}_{\mathit{c}1}$ are determined from a novel procedure based on the Bean's critical-state model to pick up the onset of flux penetration in broad zero-field-cooled magnetization curves in (${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${)}_{2}$${\mathrm{CuO}}_{4}$ (x\ensuremath{\approxeq}0.07) single crystals. The values of ${\mathit{H}}_{\mathit{c}1}$ at T=0 K are estimated to be 70 G for H\ensuremath{\parallel} layer and 300 G for H\ensuremath{\perp} layer. The temperature dependence of ${\mathit{H}}_{\mathit{c}1}$ is such that it tends to saturate towards the lowest temperature measured (2 K) in contrast to the other recent results which report the abnormal upturn of the ${\mathit{H}}_{\mathit{c}1}$-T curve at low temperatures. Instead the results indicate that the critical current density in plane has a strong temperature dependence even in the temperature range well below ${\mathit{T}}_{\mathit{c}}$ and becomes \ensuremath{\sim}${10}^{6}$ A/${\mathrm{cm}}^{2}$ at 2 K.

The temperature dependence of the sound velocity and the sound-attenuation coefficient in single-crystalline La 1.85 Sr 0.15 CuO 4 have been measured in the mixed state with an ultrasonic technique. An increase in the sound velocity and an attenuation peak, due to the elasticity of the flux-line lattice and the flux-pinning effect, are observed at a temperature lower than the superconducting transition temperature T c . The compression and tilt moduli of the flux-line lattice are found to be softened in the case of H⊥c