Histogram Monte Carlo simulation results are presented for the magnetic-field–temperature phase diagram of the XY model on a stacked triangular lattice with antiferromagnetic intraplane and ferromagnetic interplane interactions. Finite-size scaling results at the various transition boundaries are consistent with expectations based on symmetry arguments. Although a molecular-field treatment of the Hamiltonian fails to reproduce the correct structure for the phase diagram, it is demonstrated that a phenomenological Landau-type free-energy model contains all the essential features. These results serve to complement and extend our earlier work [Phys. Rev. B 48, 3840 (1993)].

We consider the anisotropic quantum Heisenberg antiferromagnetic (with anistropy λ) on a square lattice using a Chern-Simons (or Wigner-Jordan) approach. We show that the average field approximation (AFA) yields a phase diagram with two phases: a Neèl state for λ>${\lambda }_{\mathit{c}}$ and a flux phase for λ<${\lambda }_{\mathit{c}}$ separated by a second-order transition at ${\lambda }_{\mathit{c}}$<1. We show that this phase diagram does not describe the XY regime of the antiferromagnet. Fluctuations around the AFA induce relevant operators which yield the correct phase diagram. We find an equivalence between the antiferromagnet and a relativistic field theory of two self-interacting Dirac fermions coupled to a Chern-Simons gauge field. The field theory has a phase diagram with the correct number of Goldstone modes in each regime and a phase transition at a critical coupling ${\lambda }^{\mathrm{*}}$>${\lambda }_{\mathit{c}}$. We identify this transition with the isotropic Heisenberg point. It has a nonvanishing Neèl order parameter, which drops to zero discontinuously for λ<${\lambda }^{\mathrm{*}}$.

The Anderson model of local state–conduction electron mixing is applied to the problem of interlayer magnetic coupling in metallic multilayered structures. Two types of coupling, Ruderman-Kittel-Kasuya-Yosida (RKKY) and superexchange arise from this interaction. The RKKY coupling comes only from intermediate states which correspond to spin excitations of the Fermi sea, that is, states corresponding to electron-hole pair production in the Fermi sea with an attendant spin flip. The superexchange coupling arises from virtual-charge excitations in which electrons from local states are promoted above the Fermi sea (one from each layer); this provides a second contribution to the coupling, that is in addition to the RKKY coupling. Moreover, the RKKY coupling is largely influenced by the topology of the Fermi surface and contains oscillatory modes of the coupling while superexchange coupling does not contain much Fermi-surface information. The RKKY coupling is ferromagnetically biased, while the superexchange coupling is antiferromagnetically biased. The total interlayer coupling (RKKY plus superexchange) resembles the oscillatory coupling of magnetic impurities in a free electron gas (the conventional RKKY coupling or Lindhard range function), but is antiferromagnetically biased if the density of states has a relatively large peak just above the Fermi energy. These characteristics are visible in two ways in experimental data: (i) the spin polarization pattern in the spacer layer between magnetic ones can be different from that of the interlayer magnetic coupling, and (ii) some metallic multilayered structures could have relatively large antiferromagnetic biases in their interlayer magnetic coupling; for example, Fe/Cr systems.

Low-temperature ac and dc magnetic susceptibility, high-field magnetization, and specific-heat measurements have been carried out on CePdSb. The magnetic measurements confirm the ferromagnetic transition at ${\mathit{T}}_{\mathit{C}}$=17 K as previously reported. However, no anomaly in the specific heat is observed at that temperature, but only the onset of the magnetic contribution, ${\mathit{C}}_{\mathit{M}}$(T). The maximum of ${\mathit{C}}_{\mathit{M}}$(T) is observed at ${\mathit{T}}_{\max }$=9.7 K. The lack of a ${\mathit{C}}_{\mathit{M}}$(T) jump at ${\mathit{T}}_{\mathit{C}}$ is discussed in terms of the comparison of the exchange integrals between first- and second-nearest magnetic neighbors. Depending on their relative values, the magnetic structure can behave as low dimensional or incommensurate. The entropy of the magnetic phase (R ln2) and the low density of states (γ=11 mJ/mol ${\mathrm{K}}^2$) excludes a Kondo-lattice character for this compound.

We investigate the phase transitions of the frustrated XY model in the square lattice with frustration parameter f=1/2. The system has doubly degenerate ground states in addition to those associated with U(1) symmetry. We study the system via a high-precision Monte Carlo technique. It is found that the system has two separate transitions. At the lower temperature 0.440(2)J/${\mathit{k}}_{\mathit{B}}$, we find a Kosterlitz-Thouless transition with a larger-than-universal jump in the helicity modulus. The critical index η(${\mathit{T}}_{\mathrm{KT}}$) of the spin-spin correlation function is 0.220(2), less than the universal value of 1/4. At higher temperature 0.454(2)J/${\mathit{k}}_{\mathit{B}}$, we find a second-order vortex-lattice melting transition with critical indices α=0.363(10), β=0.089(8), γ=1.448(24), and ν=0.813(5).

Using the cluster-variation method we study the phase diagram of the Blume-Emergy-Griffiths (BEG) model on simple cubic and face-centered cubic lattices. For the simple cubic lattice the main attention is paid to reentrant phenomena and ferrimagnetic phases occurring in a certain range of coupling constants. The results are in close agreement with Monte-Carlo data, available for parts of the phase diagram. Several ferrimagnetic phases are obtained in the vicinity of the line in parameter space, at which the model reduces to the antiferromagnetic three-state Potts model. Our results imply the existence of three phase transitions in the antiferromagnetic Potts model on the simple-cubic lattice. The phase diagrams for the BEG model on the face-centered cubic lattice are obtained in the region of antiquadrupolar ordering. Also the several ordered phases of the antiferromagnetic Potts model on this lattice are discussed.

We present exact results for the dynamical structure function, i.e., the density-density correlations for the 1/${\mathit{r}}^2$ system of interacting particles at three special values of the coupling constant. The results are interpreted in terms of exact excitations of the model which are available from Bethe’s Ansatz, thereby throwing light on the quasiparticle content of the elementary excitations. We also obtain the first moment of the discrete version of the model, which provides a nontrivial check on its structure function. We show that the property of spectrum saturation is a common feature of both versions.

We investigate the one-dimensional quantum XXZ model in the presence of diagonal disorder. Recently the model has been analyzed with the help of field-theoretical renormalization-group methods, and a phase diagram has been predicted. We study the model with exact diagonalization techniques up to chain lengths of 16 sites. Using finite-size scaling methods we estimate critical exponents and the phase diagram and find reasonably good agreement with the field-theoretical results, namely, that any amount of disorder destroys the superfluidity for XXZ anisotropy Δ between -1/2 and 1, while the superfluidity persists to finite disorder strength for -1<Δ<-1/2 and then undergoes a Kosterlitz-Thouless-type transition.

We show that the addition of a weak periodic drive can stabilize the in-phase state of a current-biased series array of N Josephson junctions. The stabilization can be predicted by a local analysis of an effective one-junction problem regardless of N. In the parameter regime considered, the stabilization appears to be global.

We report on dc-magnetization measurements on high-quality single crystals of the organic superconductors κ-(BEDT-TTF${)}_2$ Cu(NCS${)}_2$ and κ-(BEDT-TTF${)}_2$Cu[N(CN${)}_2$]Br for fields parallel and perpendicular to the conducting planes. For H perpendicular to the planes, we find clear indications for strong superconducting fluctuations, i.e., a pronounced rounding of the transition with increasing field and field-induced diamagnetism above ${\mathit{T}}_{\mathit{c}}$. Careful experiments near ${\mathit{T}}_{\mathit{c}}$ reveal the existence of a field-independent magnetization at ${\mathit{M}}^{\mathrm{*}}$(${\mathit{T}}^{\mathrm{*}}$,H) for H≤10 kOe. The anisotropy of superconducting parameters such as the upper critical field and coherence length, along with a scaling form of the high-field magnetization data demonstrate the highly anisotropic character for the present superconductors. Over an extended range in the H-T plane the mixed-state properties are governed by an entirely reversible vortex motion. The lower boundary of this range, the so-called irreversibility line, is found to be anisotropic and, for H perpendicular to the planes, to follow an exponential T dependence. Cooling below the irreversibility line is accompanied by the occurrence of magnetic relaxation effects, indicating a metastable vortex state.

The thickness dependence of the superconducting properties of Nb films has been studied by resistance and ac magnetic susceptibility measurements. The film thickness was varied from 825 to 200 Å by anodizing a single dc sputtered film in order to minimize any sample-to-sample preparation variations. For films 400 Å thick and greater, the normal-state resistivity at 10 K is constant and the suppression of the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ with decreasing thickness is explained in terms of both the proximity effect and weak localization. The magnetic susceptibility results show that the diamagnetic transition broadens with decreasing thickness and increasing ac fields as a result of the effective fields becoming greater than the critical field ${\mathrm{H}}_{\mathit{c}1}$ of Nb.

Candidates for the ground state of the anisotropic t-${\mathit{J}}_{\mathit{z}}$ model at low hole doping are studied within two related lattices: the Cayley tree and the square lattice. The Cayley tree allows for the evaluation of the single hole as well as the hole-pair ground states to arbitrary precision. Threshold values for the formation of bound hole pairs appear to be similar for both lattices. We show that the main contributions to the binding of a hole pair on the Cayley tree comes from the excess number of frustrated bonds (‘‘spinons’’). Considering systems with a larger number of holes, we establish high stability of configurations with holes forming straight domain walls between nonequivalent antiferromagnetic domains. In particular, for the Cayley tree we find that domain-wall-like configurations have lower energy than separate hole pairs. For the square lattice, however, there exists at low but finite doping a parameter interval 0.18<J/t<0.56 where, in the ground state, holes are paired.

The critical behavior of type-II superconductors near the upper critical magnetic field ${\mathit{H}}_{\mathit{c}2}$(T) is studied. The thermal fluctuations are shown to forbid a spontaneous breakdown of the U(1) symmetry in d<4 dimensions within the 1/N expansion, where N is the complex order parameter component number. The lattice effects such as Harper’s broadening and splitting of Landau levels are found to restore the continuous phase transition in the conventional mixed state described by critical exponents which are identical to those of the Ginzburg-Landau model with H=0.

Variational Monte Carlo method is used to calculate ground state properties of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ droplets, containing 70, 112, 168, 240, 330, and 728 particles. The resulting particle and kinetic energy densities are used as an input in the Feynman-Lekner theory for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ impurities. The kinetic energy density of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ atoms and the energy of the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ surface states are compared with the results of previous phenomenological calculations.

Using a transfer-matrix method, we study the Josephson current in a normal, clean, one-dimensional quantum channel in contact with two superconducting electrodes characterized by a finite value of the ratio between the gap Δ and the Fermi energy μ. Going beyond the ‘‘Andreev approximation,’’ gaps open up in the quasiparticle level spectrum, the critical current is reduced, and the relationship between current and superconducting phase difference φ becomes more Josephson-like without any discontinuous jump at φ=π. The mechanism behind these effects is the normal reflection at the interfaces due to a finite value of Δ/μ. The gap at φ=π goes to zero only at transmission resonances ${\mathit{k}}_{\mathit{F}}$L=(2n+1)π/2.

We report on a high-temperature perturbation expansion study of the superfluid-density spatial correlation function of a Ginzburg-Landau-model superconducting film in a magnetic field. We have derived a closed form which expresses the contribution to the correlation function from each graph of the perturbation theory in terms of the number of Euler paths around appropriate subgraphs. We have enumerated all graphs appearing out to 12th order in the expansion and have evaluated their contributions to the correlation function. Low-temperature correlation functions, obtained using Padé approximants, are in good agreement with Monte Carlo simulation results and show that the vortex liquid becomes strongly correlated at temperatures well above the vortex solidification temperature. We have also evaluated the high-temperature expansion for the free energy of the Ginzburg-Landau model to 13th order, two orders further than in previous work.

Point-contact spectroscopy has been used to study the temperature evolution of the spin-density-wave (SDW) behavior of a ${\mathrm{URu}}_2$${\mathrm{Si}}_2$ single crystal. In the c direction at a temperature of 19 K, we observe the onset of an asymmetric structure in the dV/dI versus V characteristic, and this has been correlated with the development of a SDW gap. We observe that the data follow a BCS behavior. In addition, we have performed tunneling experiments that indicate that only portions of the Fermi surface are being gapped. From these data we have measured the energy gap, 2Δ=11.7 meV, and the ratio 2Δ/${\mathit{K}}_{\mathit{B}}$${\mathit{T}}_{\mathit{N}}$ which gives a value of the order of 7.2.

The elastic properties of a randomly pinned flux-line lattice in a type-II superconductor on a small scale are investigated using the replica method. The nonlocal effects of the system are considered. The renormalized elastic matrix of the flux-line lattice in a random-pinning potential is calculated. The thermal and pinning-caused fluctuations are evaluated.

The hyperfine interactions at the europium lattice sites in samples of the homologous (Fe,Cu)${\mathrm{Sr}}_2$(Eu,Ce${)}_{\mathit{n}}$${\mathrm{Cu}}_2$${\mathrm{O}}_{4+2\mathit{n}+\mathit{z}}$ (n=2,3) series were studied by ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ Mössbauer spectroscopy. The work was motivated by the search for new superconducting phases. This homologous series is based on the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (1:2:3) structure. The samples used in the Mössbauer measurements consisted of crystallites with random orientation and grain oriented crystallites. The texture of oriented samples was analyzed by x-ray diffraction. The complete quadrupole Hamiltonian of the 21.5-keV γ-transition of ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ was successfully applied in the analyses of all the Mössbauer spectra. In samples having n≤2 the europium atoms occupy a single lattice site, whereas the spectra of the n=3 samples exhibit hyperfine interactions of the two different europium sites. Analyzing the hyperfine parameters of the latter samples was made possible by simultaneous fitting of three spectra, corresponding to three different crystal orientations of the same specimen. This fitting scheme also enables more precise determination of the hyperfine parameters in the n=2 samples. In these samples an electric field gradient (EFG), with a large negative-valued main component (${\mathit{V}}_{\mathit{z}\mathit{z}}$) parallel with the crystal c axis, was found. In the n=3 samples, the two EFG’s of the europium sites were found to have ${\mathit{V}}_{\mathit{z}\mathit{z}}$ components of opposite signs. The negative ${\mathit{V}}_{\mathit{z}\mathit{z}}$ value was attributed to the rare-earth site adjacent to the CuO layer. This site, found in all samples of the series, corresponds to the rare-earth site of the 1:2:3 system.

The experimental results of resistivity and thermoelectric power (TEP) of two ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$/${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (YBCO/PrBCO) superlattice samples with different thickness of YBCO layers in the temperature range from 80 to 270 K are reported. The contribution of interface scattering to the resistivity of YBCO layers is estimated. Compared with slightly oxygen-deficient YBCO films, the TEP of superlattice samples has higher positive value and the slope of the linearly temperature-dependent part changes from negative to positive. A TEP peak just above ${\mathit{T}}_{\mathit{c}}$ was also observed.