The Madelung energy of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{1+\mathit{x}}$${\mathrm{Cu}}_2$${\mathrm{O}}_{6\pm \mathrm{\delta }}$ system, whose properties change from semiconductor to superconductor under high-oxygen-pressure annealing, is calculated. We find the increase in Δ${\mathit{V}}_{\mathit{A}}$, the difference in Madelung site potential between the apex and the ${\mathrm{CuO}}_2$ planes’ oxygen, promotes the superconducting transition in this system. Slight intercalation of oxygen between two ${\mathrm{CuO}}_2$ planes is found to be of primary importance in determining Δ${\mathit{V}}_{\mathit{A}}$.

The formation of bound states of holes in an antiferromagnetic spin-1/2 background is studied using numerical techniques applied to the t-J Hamiltonian on clusters with up to 26 sites. An analysis of the binding energy as a function of cluster size suggests that a two-hole bound state is formed for couplings larger than a ‘‘critical’’ value J/t${\mathrm{\Vert }}_{\mathit{c}}$ The symmetry of the bound state is ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$. We also observed that its ‘‘quasiparticle’’ weight ${\mathit{Z}}_{2\mathit{h}}$ (defined in the text), is finite for coupling J/t≥J/t${\mathrm{\Vert }}_{\mathit{c}}$. Thus, in the region J/t≥J/t${\mathrm{\Vert }}_{\mathit{c}}$ the bound state of two holes behaves like a quasiparticle with charge Q=2e, spin S=0, and ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$ internal symmetry. The relation with recent ideas that have suggested the possibility of d-wave pairing in theories of the high-${\mathit{T}}_{\mathit{c}}$ cuprates is briefly discussed.

High volume fractions of the n=2 and n=3 members of a cuprate system of general formula ${\mathrm{Hg}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Pb}}_{\mathit{x}}$${\mathrm{Ba}}_2$${\mathrm{Ca}}_{\mathit{n}\mathrm{-}1}$${\mathrm{Cu}}_{\mathit{n}}$${\mathrm{O}}_{2\mathit{n}\mathrm{-}2+\mathrm{\delta }}$ (with an average value of x at 0.33) have been synthesized via a relatively facile process using low oxygen pressures. For as-synthesized n=3 (Hg,Pb-1:2:2:3) bulk superconducting critical temperatures (${\mathit{T}}_{\mathit{c}}$) of up to 133 K have been observed. For n=2 (Hg,Pb-1:2:1:2), which is obtained with some rare-earth-ion substitution at the calcium site, ${\mathit{T}}_{\mathit{c}}$ is 125 K after annealing in a reducing atmosphere. High-resolution lattice imaging coupled with structural simulations show that the Pb ions go into the Hg layer, leading to a chemical pressure-induced compositional modulation in this plane. Strong flux pinning and an irreversibility field of 1 T at 80 K are observed for Hg,Pb-1:2:2:3.

We report detailed magnetic measurements in H parallel to the c-axis direction on a single crystal of ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{Cu}}_2$${\mathrm{O}}_{4\mathrm{-}\mathit{y}}$. The magnetization data indicate that the entrance field is determined by surface barriers. At low temperatures, the penetration field ${\mathit{H}}_{\mathit{p}}$(T) can be fit with ${\mathit{H}}_{\mathit{p}}$=${\mathit{H}}_{\mathit{c}}$exp(-T/${\mathit{T}}_0$); at temperature close to ${\mathit{T}}_{\mathit{c}}$, ${\mathit{H}}_{\mathit{p}}$(T) is linear in T. The date can be qualitatively understood by a model where two-dimensional point vortices are thermally nucleated near the surface in weakly coupled layered superconductors.

We propose an experiment to test for broken time-reversal symmetry in the high-temperature superconductors. We configure a superconducting quantum interference device (SQUID) in a manner similar to that used recently to test for ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$ symmetry, but where one junction is on a crystal face of arbitrary angle. Assuming a ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$+iε${\mathit{d}}_{\mathit{x}\mathit{y}}$ symmetry, we calculate the magnetic diffraction patterns for the SQUID for various values of ε and angle. For any nonzero ε, i.e., time-reversal symmetry breaking, we find a nonzero circulating current spontaneously arises in the SQUID even in zero applied magnetic field and zero bias current.

Superconducting transitions were studied in single-crystal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ with magnetic fields in the range 0.02–8 T applied along the c axis and six contacts mounted on two ab-plane surfaces to measure voltages on opposite sides of the crystal. In the beginning of the transition curves, a change was observed at about 0.3 T from a high-field region, where vortices are preserved over the length of the crystal, to a low-field behavior where vortices move independently in different layers. In the high-field region a peak was observed in ${\mathit{V}}_{\mathrm{bot}}$, which can be qualitatively understood in terms of breaking of long vortices by increased thermal fluctuations. At about 0.3 T we also observed a crossover in the magnetic-field dependence of an apparent activation energy from a 1/B behavior at high fields to -lnB at low fields. From the data in the low-field region new quantitative support is obtained for a Kosterlitz-Thouless-type transition where vortex-antivortex pairs dissociate and move independently in different layers of the crystal. The correlation length of these vortices is estimated to be 650±150 Å.

A response is given to a criticism in a paper by Kim and Overhauser of an earlier work by Abrikosov and Gor’kov on superconductors with impurities. The criticism is based on a primitive misunderstanding of the nature of the attraction of electrons achieved by phonons. As it was explained by the present authors in several publications, there exists a complete equivalence of their results with the later work by Anderson.

That the Abrikosov-Gor’kov theory of an impure superconductor seriously violates Anderson’s theorem is reaffirmed. The contrary assertion, together with other statements in the Comment, are shown to be incorrect physically, mathematically, and historically.

In a recent paper Misra and Felsteiner calculate the low-temperature ordered states of certain R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ compounds, using the Luttinger-Tisza method. Based on the results of these calculations the authors conclude that dipolar interactions alone can account for all the observed orderings of these materials. However, the calculations and the conclusions drawn from these calculations are based on assumptions that we believe are inconsistent with many crystal electric field measurements in the R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ compounds.

It is explained that the points raised by Whitehead, De’Bell, and Noakes in their Comment on our paper in Phys. Rev. B 46, 11 033 (1992) are really not applicable.

Oscillatory structure is found in the atomic background absorption in x-ray-absorption fine-structure (XAFS) measurements. This atomic XAFS (AXAFS) arises from scattering within an embedded atom, and is analogous to the Ramsauer-Townsend effect. Calculations and measurements confirm the existence of AXAFS and show that it can dominate contributions such as multielectron excitations. The structure is sensitive to chemical effects and thus provides a probe of bonding and exchange effects on the scattering potential.

A real-space, Green-function method is proposed to examine the electronic structure and site preference of substitutional ternary additions to ${\mathrm{Ni}}_3$Al (γ’) using the tight-binding method of linear muffin-tin orbitals (TB-LMTO). Pairs of atoms are embedded in a random ${\mathrm{Ni}}_{0.75}$${\mathrm{Al}}_{0.25}$ medium (of approximately 1000 atoms), one-electron energies are obtained by performing recursion on the TB-LMTO Hamiltonian, and the effective pair interactions (EPI’s) determined by configurational averaging. Depending on the relative values of the three EPI’s, ${\mathit{W}}_{\mathrm{Ni}\mathrm{-}\mathrm{Al}}$, ${\mathit{W}}_{\mathrm{Ni}\mathrm{-}}$X, and ${\mathit{W}}_{\mathrm{Al}\mathrm{-}}$X, the ternary addition may occupy either the Ni site or the Al site exclusively, or the site preference may be a function of alloy stoichiometry. The site-substitution behavior in both stoichiometric and nonstoichiometric ${\mathrm{Ni}}_3$Al alloys is computed for six ternary additions, X (X=Co,Cu,Zn,Rh,Pd,Si). The theoretical predictions are in excellent agreement with experimental data, where available.

We describe a quantum Monte Carlo technique suitable for generically complex problems, such as systems coupled to external magnetic fields or anyons in two spatial dimensions. We find that the gauge plays a nontrivial role in the simulation, and that a judicious choice of gauge considerably reduces the statistical error. One may similarly reduce the statistical error in simulations of particles in external magnetic fields by arranging an approximate cancellation between the (gauge-dependent) phases of the statistical flux and the external magnetic flux.

We have measured longitudinal magnetoresistance on a high-quality single crystal of ${\mathrm{UBe}}_{13}$. We were able to scale all the magnetoresistance results for fields between 0 and 14 T and for temperatures between 1.4 and 15 K using the following scaling relation (T-0.75 K${)}^{0.6}$/H. This scaling is inconsistent with both a Kondo model and a quadrupolar Kondo model and suggests existence of ferromagnetic correlations in ${\mathrm{UBe}}_{13}$.

Bechgaard salts like (TMTSF${)}_2$X with X=${\mathrm{PF}}_6$, ${\mathrm{NO}}_3$, and ${\mathrm{ClO}}_4$ exhibit Shubnikov–de Haas–like oscillations, which are not described within the standard model. In addition (TMTSF${)}_2$${\mathrm{NO}}_3$ exhibits slow oscillations with characteristic periods rather similar to those corresponding, to the field-induced spin-density waves found in (TMTSF${)}_2$${\mathrm{ClO}}_4$ and (TMTSF${)}_2$${\mathrm{PF}}_6$ under pressure. I propose a simple theory which describes these phenomena.

The ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sn}$ high-pressure Mössbauer-effect technique on 0.5 at. % ${}_{}{}^{119}{}_{}{}^{}\mathrm{doped}$ ${\mathrm{YMn}}_2$ has been used to investigate, on a microscopic level, the nature of the pressure-induced magnetic-to-nonmagnetic phase transition in the itinerant-electron antiferromagnet ${\mathrm{YMn}}_2$. We find, in contrast to the temperature-induced first-order magnetic phase transition at ${\mathit{T}}_{\mathit{N}}$ and at ambient pressure, a strong but continuous reduction of the ordered Mn local magnetic moment in the range 0≤p<0.3 GPa, which reveals a pressure-induced second-order phase transition. In the same pressure range, we observe a change of the nature of the spin fluctuations from transversal (0≤p<0.3 GPa) to longitudinal at higher pressures. We further show evidence for short-range magnetic order in the pressure-induced nonmagnetic state (0.3≤p≤4 GPa).

NMR data are presented for the conducting delafossites ${\mathrm{Y}}_{0.99}$${\mathrm{Ca}}_{0.01}$${\mathrm{CuO}}_{2.65}$ and ${\mathrm{LaCuO}}_{2.64}$. These materials exhibit a planar structure with triangular arrays of exchange-coupled S=1/2(${\mathrm{Cu}}^{2+}$) spins. The two compounds examined show strikingly different quadrupole-broadened NMR powder patterns, which is evidence for different in-plane arrangements of ${\mathrm{O}}^{2\mathrm{-}}$ ions. Their doped-hole carrier densities are comparable to those of the superconducting cuprates. In consequence, they have Korringa-like ${\mathit{T}}_1$ processes. We also find evidence for dynamic Jahn-Teller motion and ferromagnetic Benhancement of the spin paramagnetism for ${\mathrm{Y}}_{0.99}$${\mathrm{Ca}}_{0.01}$${\mathrm{CuO}}_{2.65}$. ${\mathit{T}}_2$ studies indicate that the strong antiferromagnetic fluctuation effects found in the superconducting cuprates are absent.

The two-dimensional Hubbard model with a single spin-up electron interacting with a finite density of spin-down electrons is studied by use of the quantum Monte Carlo technique, the standard second-order perturbation theory and a very good variational wave function introduced recently by Edwards. We also present a new conjugate gradient scheme for the evaluation of this variational state. We performed simulations up to 242 sites at U/t=4 reaching the zero-temperature properties with no ‘‘fermion-sign problem’’ and found a surprisingly good accuracy of the Edwards state at low density or low doping. The conjugate gradient method was then applied to the system up to 3698 sites and infinite U. Fermi-liquid theory seems to remain stable in two dimensions for all cases studied with the exception of the half-filling case where a Luttinger-like behavior survives in the Hubbard model, yielding a vanishing quasiparticle weight in the thermodynamic limit.

The experimental properties of multilayer ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ films adsorbed on graphite are presently described by a model assuming the existence of several structural phases in the second layer and a continuous growth of uniform liquid beyond second-layer coverages. We propose an alternative model, where second-layer ferromagnetism is induced by a condensed third-layer liquid, which provides a simple description of experimental data.

In previous works the mass of a vortex line in superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ was argued to arise from the tiny core of the vortex. We show that due to gauge-symmetry breaking, the condensate compressibility contributes to a vortex mass that far exceeds the core mass. This large vortex mass should play an important role in some problems of vortex dynamics.