The previous consideration of the irreversible magnetic self-field entry into a current-carrying type-II superconductor (SC) [Phys. Rev. B 49, 6950 (1994)] is extended to the case of an applied longitudinal magnetic field. The structure of the helical Abrikosov vortex is found exactly in the framework of the London approximation. The field-dependent critical current of a spontaneous entry of the right-handed flux helix against the surface Bean-Livingston barrier into the SC cylinder of arbitrary radius R is calculated, as well as the critical current of the flux-line instability with respect to the left-handed helical expansion. On the basis of these two mechanisms, a diagram of the resistive state in current-field coordinates is evaluated. The direction of the optimal helix for which the surface barrier is the least and which, hence, is the first to enter the sample turns out to coincide with that of the full magnetic field at the surface only in the case of a thick cylinder with R≫λ, where λ is the magnetic penetration depth. For the latter case, the generalized Silsbee’s rule of a breakdown of nondissipative state is formulated.

A theory of a Josephson vortex motion under a temperature gradient is presented. Microscopic calculations of the temperature dependence of a Josephson vortex transport entropy, ${\mathit{S}}_{\mathit{c}\mathit{p}\mathit{h}\mathit{i}}$, are done in the whole temperature range. It is shown that ${\mathit{S}}_{\mathit{c}\mathit{p}\mathit{h}\mathit{i}}$→0 as T→0 and ${\mathit{T}}_{\mathit{c}}$. Results of the theory are compared with our experimental data for superconductor–normal-metal–superconductor (SNS) and superconductor-insulator-superconductor Josephson junctions. The value of a transport entropy obtained from our experiments is in good agreement with theoretical estimates. An additional maximum of the transport entropy has been found experimentally in long SNS junctions with ${\mathit{d}}_{\mathit{N}}$/${\xi }_{\mathit{N}}$≫1. This result is explained in the framework of the microscopic model presented in the paper.

The nuclear and magnetic structures of ${\mathrm{HoNi}}_2$${\mathrm{B}}_2$C have been investigated by neutron-powder-diffraction methods at room temperature and at 10, 5.1, and 2.2 K. The compound crystallizes with the symmetry of space group I4/mmm and lattice parameters a=3.5170(1) and c=10.5217(3) Å, at room temperature. The structure may be regarded as formed of (HoC) layers with the rock salt configuration alternating with (B)(${\mathrm{Ni}}_2$)(B) blocks having a fluorite-type arrangement of the atoms. No phase transition of the nuclear structure has been observed down to a temperature of 2.2 K. Magnetic peaks begin to appear at about 8 K. The magnetic structure is the superposition of two configurations, one in which ferromagnetic sheets of holmium spins parallel to the a,b plane are coupled antiferromagnetically along the c axis (magnetic space group Pm’mm’), and another in which ferromagnetic planes are rotated away from the antiparallel configuration to give an incommensurate helicoidal structure with a period approximately equal to 12 times the length of the c axis.

The space-time tunneling bosonization has been introduced to study N-coupled Luttinger liquids. The interaction is separated into a forward exchange and a cooperon part. At T=0 the cooperon part is described by tunneling events which form a Coulomb gas in the plasma phase. At T≠0 we ignore the cooperon part and show that the correlation functions show a Luttinger-liquid behavior. In the limit N→∞, we extract the two-dimensional behavior and show that for a certain range of parameters the Luttinger liquid survives.

The phase diagram of a thin superconducting film with a circular hole in axial magnetic field is presented. The result is obtained by solving numerically the nonlinear Ginzburg-Landau (GL) equation in the limit of a thin film with large ${\mathrm{\kappa }}_{\mathit{e}\mathit{f}\mathit{f}}$=${\lambda }_{\mathit{e}\mathit{f}\mathit{f}}$/ξ (where ${\lambda }_{\mathit{e}\mathit{f}\mathit{f}}$=${\lambda }^2$/d is the effective screening length for a film of thickness d). First-order phase transitions between localized (around the hole) superconducting states with different orbital momenta are predicted. Corresponding jumps in the magnetic moment, latent heat, and specific heat are presented in the universal form. The same problem is solved analytically for a thin-wire superconducting loop. All the results obtained for a film with a circular hole are valid for high-${\mathit{T}}_{\mathit{c}}$ superconductors with columnar defects because in such compounds the GL parameter κ∼100 so the screening effect is negligible: a relative distortion of the phase diagram after taking into account the screening effect is proportional to 1/${\mathrm{\kappa }}^2$ in the three-dimensional case and 1/${\mathrm{\kappa }}_{\mathit{e}\mathit{f}\mathit{f}}$ in the case of a thin film.

We calculate the dynamical response of two-layer Josephson arrays to a bias current applied into only one layer (primary layer), in a perpendicular magnetic field. The pancake vortices in the two layers sometimes form into flux lines which move as a unit under the influence of the bias current. The lines break apart into independent pancakes in sufficiently high currents, sufficiently weak interlayer coupling, or possibly sufficiently high fields. We discuss the relevance of these results to recent experiments in high-${\mathit{T}}_{\mathit{c}}$ superconductors in the flux transformer geometry.

We show that electron hopping in a lattice of molecules possessing a Berry phase naturally leads to pairing. Our building block is a simple molecular site model inspired by ${\mathrm{C}}_{60}$, but realized in closer similarity with ${\mathrm{Na}}_3$. In the resulting model electron hopping must be accompanied by orbital operators, whose function is to switch on and off the Berry phase as the electron number changes. The effective Hamiltonians (electron-rotor and electron-pseudospin) obtained in this way are then shown to exhibit a strong pairing phenomenon, by means of one-dimensional linear-chain case studies. This emerges naturally from numerical studies of small-N-site rings, as well as from a BCS-like mean-field theory formulation. The pairing may be explained as resulting from the exchange of singlet pairs of orbital excitations, and is intimately connected with the extra degeneracy implied by the Berry phase when the electron number is odd. The relevance of this model to fullerides, to other molecular superconductors, as well as to present and future experiments, is discussed.

A microscopic model for describing a superconducting mesoscopic weak link is presented. We consider a model geometry consisting of a narrow channel coupled to wider superconducting electrodes which act as reservoirs fixing the asymptotic values of the complex order parameter. For this model, the Bogoliubov–de Gennes equations are discretized and solved self-consistently using a nonequilibrium Green-function formalism. The transport properties and the electronic excitation spectra of this system are studied for the different regimes that can be reached by varying parameters like coherence length, constriction length, normal transmission coefficient, and temperature. We study in detail the transition from the point-contact limit to the infinite channel length case, analyzing the maximum Josephson current that can be sustained by the weak link as a function of its transmission coefficient and length. It is also shown that for a constriction size ranging from zero to several times the coherence length, most of the current is carried, inside the constriction region, by bound states within the superconducting energy gap. These states correspond to Cooper pairs with binding energies smaller than the superconducting gap and which are spatially extended along the channel region, decaying exponentially inside the reservoirs. The importance of the self-consistent determination of the order parameter along the weak link is illustrated by analyzing different profiles obtained for channel lengths of the order of the coherence length. For temperatures not very close to ${\mathit{T}}_{\mathit{c}}$, our microscopic calculation predicts the appearance of features which cannot be obtained from Ginzburg-Landau theory.

Using the Bogoliubov–de Gennes equation, we study superconducting current in a ballistic double superconducting–normal-metal–superconducting (SNSNS) structure. There are two contributions to the current: current ${\mathit{I}}_1$ due to the discrete part of the excitation spectrum and current ${\mathit{I}}_2$ due to the continuous part of the spectrum. Bound states (Andreev levels) are associated to each normal region. If the length L of the middle S region is comparable to (or smaller than) the coherence length ${\xi }_0$, the Andreev levels in different regions interact; if L≫${\xi }_0$ the Andreev levels are noninteracting. The interaction between the Andreev levels influences ${\mathit{I}}_1$. Moreover, ${\mathit{I}}_2$ is negligible only in the limits L≪${\xi }_0$ and L≫${\xi }_0$. We present curves for the dependence of the critical current ${\mathit{I}}_{\mathit{c}}$ on L and on the pair potential of the middle S region. The pair potential of the middle S region prevents reduction of ${\mathit{I}}_{\mathit{c}}$. The effects we study will not be present in a transfer Hamiltonian approach.

There has been a proof by Sewell that the hypothesis of off-diagonal long-range order in the reduced density matrix ${\mathrm{\rho }}_2$ implies the Meissner effect. We present in this paper an elementary and straightforward proof that not only the Meissner effect but also the property of magnetic flux quantization follows from the hypothesis. It is explicitly shown that the two phenomena are closely related, and phase coherence is the origin for both.

A detailed study is reported by the Bitter decoration technique of the mixed vortex chain–vortex lattice phase found in tilted magnetic fields in single crystals of the highly anisotropic ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{x}}$ (BSCCO). From the field and tilt-angle dependencies of the vortex periods in this structure we conclude that the vortex chains in BSCCO are qualitatively different from those in the moderately anisotropic and well-studied ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$. In particular, the intrachain vortex period is independent of the tilt angle cphi for a given perpendicular component of the field while the vortex period in the intervening flux-line lattice (FLL) shows a maximum vs cphi. In addition, the ratio of the number of the FLL vortices to the total number of vortices decreases rapidly for cphi>70°. Proceeding from these dependencies, we show that the observed mixed vortex phase can be explained consistently by the chain and the FLL vortices having different orientations, neither parallel to the applied field, as suggested by recent theoretical calculations.

The optical reflectivity and microwave conductivity are calculated for a metal that becomes superconducting below a transition temperature ${\mathit{T}}_{\mathit{c}}$. Electron-electron scattering on a nested Fermi surface with an isotropic weak-coupling energy gap is found to yield a reflectance at low frequencies that is compatible with measurements on superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ crystals. The computed microwave surface resistance ${\mathit{R}}_{\mathit{s}}$ drops dramatically near ${\mathit{T}}_{\mathit{c}}$ in accord with cuprate experiments, although the data at very low temperatures remain above the isotropic gap prediction. Our nested Fermi-liquid analysis also gives a conductivity peak below ${\mathit{T}}_{\mathit{c}}$. The temperature variation of the measured ${\mathit{R}}_{\mathit{s}}$, the conductivity line-shape data, and the reflectivity spectra are all different from the corresponding results obtained with conventional phonon damping.

Field- and temperature-dependent surface resistance measurements at 12.9 GHz are reported for polycrystalline Hg-Ba-Ca-Cu-O, which was found to consist primarily of the ${\mathrm{HgBa}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{6+\mathrm{\delta }}$ and ${\mathrm{HgBa}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{8+\mathrm{\delta }}$ phases. A small amount of a third phase, most likely ${\mathrm{HgBa}}_2$${\mathrm{Ca}}_3$${\mathrm{Cu}}_4$${\mathrm{O}}_{10+\mathrm{\delta }}$, was also detected. With variation of the angle Ψ between the applied field and current density, the low-temperature data exhibit large flux-flow resistivity that varies approximately as ${\sin }^2$(Ψ), while at high temperatures, the dominant field- and temperature-dependent response is attributed to isotropic phase-slip losses in intragranular defects. For temperatures just below the transition temperature of the two-layer phase, very small flux flow is evidenced. This result is consistent with an effective ${\mathit{H}}_{\mathit{c}2}$ of unusual size, suggesting a dirty-limit value for ${\mathit{H}}_{\mathit{c}2}$.

Cu 2p core-level spectra have been obtained on as-grown and reduced ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{\mathit{y}}$. The intensity ratio of the Cu 2p satellite to main peaks (${\mathit{I}}_{\mathit{s}}$/${\mathit{I}}_{\mathit{m}}$) is lower compared with the case of p-type cuprates. This characteristic is discussed on the basis of the crystal-chemical parameters. The bond valence sums (BVS) of each ion are calculated. It is found that the effective oxidation state of Cu is, in contrast to the p-type cuprate superconductors, less than 2+. The relationship among the BVS, the Madelung site potential difference δ${\mathit{V}}_{\mathit{M}}$ between the in-plane O and Cu, and the Cu-O charge-transfer gap Δ is discussed. It is pointed out that the relatively lower ${\mathit{I}}_{\mathit{s}}$/${\mathit{I}}_{\mathit{m}}$ of Cu 2p XPS spectra reflects the lower valence state of Cu, or relatively smaller values of Δ as well as δ${\mathit{V}}_{\mathit{M}}$.

A nested Fermi surface with nearly parallel orbit segments is found to yield a singlet d-wave superconducitng state at high temperatures for a restricted range of the on-site Coulomb repulsion that avoids the competing spin-density-wave instability. The computed superconducting transition temperature drops dramatically as the nesting vector is decreased, in accord with recent photoemission data on the Bi2212 and Bi2201 cuprates. Superconducting transition temperatures in the 100 K range are produced by the nesting mechanism in the leading order pairing interaction caused by exchange of spin fluctuations.

An anomalously broad foot structure is observed in the zero-field resistive transition for the ab plane of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{y}}$ single crystals, which are found to have a high density of dislocations. However, this anomalous behavior is not apparent for resistive measurements along the c axis. The foot structure can be treated as a superposition of a long resistance tail and a dissipation peak, in which the long resistance tail obeys the theory of KT scaling behavior of resistivity. The dissipation peak is related to the fluxon transition in Josephson-coupled layered superconductors as detailed by Horovitz [Phys. Rev. Lett. 72, 1569 (1994)].

The electron-phonon contribution to the normal-state resistivity, ρ(T), has been calculated for the following conventional and oxide superconductors: Nb, V, VN, ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_3$, and ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_4$ (NCCO). The resistivity formula developed from the Boltzmann equation by Ziman and Allen is used, incorporating measured physical constants. The calculations are novel in that they incorporate the electron-phonon spectral function, ${\mathrm{\alpha }}^2$F(ω), for each material as obtained from superconducting tunneling measurements, the same function that yields the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$. New tunneling and transport data are presented for VN. Fits to the experimental ρ(T) are obtained over a wide temperature range with only two parameters: the plasma frequency ${\mathrm{\omega }}_{\mathit{p}}$, which is in good agreement with independent measurements and a saturation resistivity, ${\mathrm{\rho }}_{\mathit{s}}$, which manifests itself at high temperatures. We find that the T-dependent normal state ρ(T) is completely accounted for by phonon scattering in all materials except NCCO, where an additional electron-electron scattering term appears to be present. Apart from developing a consistent description of superconductivity and normal-state transport, several important issues are raised by the present work including indications of a conventional Fermi-liquid normal state in NCCO, insight into the importance of the shape of the Fermi surface in determining ρ(T), and the role of high-energy optical phonons in oxide and nitride superconductors.

We report clear and systematic evidence of quantum interference effects on Y-based high-${\mathit{T}}_{\mathit{c}}$ polycrystalline superconductors. Reliable direct measurements of the critical transport current, at different temperatures and fixed low magnetic fields, stepwisely increased, exhibit on top of the smeared Fraunhofer-like gross structure, a pattern of almost regular oscillations of ${\mathit{J}}_{\mathit{c}}$(B).

Relaxation of the remanent (${\mathit{M}}_{\mathit{R}}$) and isothermoremanent (${\mathit{M}}_{\mathrm{IR}}$) magnetic moment, having the same initial value, is investigated in small superconducting Y-Ba-Cu-O particles in magnetic fields below the first critical field ${\mathit{H}}_{\mathit{c}1}$. It is found that the relaxation rate depends strongly on the cooling procedure. A noticeable relaxation of ${\mathit{M}}_{\mathrm{IR}}$ is observed after cooling the sample in zero magnetic field and then applying the magnetizing field. Instead, the value of ${\mathit{M}}_{\mathrm{R}}$ obtained after cooling the sample in a field proved to be very stable. This result indicates that the structure of the magnetic flux trapped in superconducting particles is different in these two cases. We attribute ${\mathit{M}}_{\mathrm{R}}$ to a single or a few vortices trapped in the center of superconducting particles, while ${\mathit{M}}_{\mathrm{IR}}$ consists of many short interacting vortices trapped near the surface.

We have performed high-energy-resolution angle-resolved photoemission studies of the normal-state band structure of oxygen-overdoped ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{x}}$. We find that there is an extended saddle-point singularity in the density of states along the Γ-M¯-Z direction. The data also indicate that there is an asymmetry in the Fermi surface for both the Γ-M¯-Z and perpendicular directions.