Theory of magnetic phases of hexagonal rare earth manganites

The magnetic phases of hexagonal perovskites RMnO_3 (R=Ho, Er, Tm, Yb, Sc, Y) are analysed using group theory and the Landau theory of phase transitions. The competition between various magnetic order parameters is discussed in the context of antiferromagnetic interactions. A phenomenological model based on four one-dimensional magnetic order parameters is developed and studied numerically. It is shown that coupling of the various order parameters leads to a complex magnetic field-temperature phase diagram and the results are compared to experiment.Comment: 11 pages, 14 figures. Manuscript with higher quality figures can be obtained here: http://www.physics.mun.ca/~curnoe/papers/RMnO3.submit.pd

Vortex Fractionalization in a Josephson Ladder

We show numerically that, in a Josephson ladder with periodic boundary conditions and subject to a suitable transverse magnetic field, a vortex excitation can spontaneously break up into two or more fractional excitations. If the ladder has N plaquettes, and N is divisible by an integer q, then in an applied transverse field of 1/q flux quanta per plaquette the ground state is a regular pattern of one fluxon every q plaquettes. When one additional fluxon is added to the ladder, it breaks up into q fractional fluxons, each carrying 1/q units of vorticity. The fractional fluxons are basically walls between different domains of the ground state of the underlying 1/q lattice. The fractional fluxons are all depinned at the same applied current and move as a unit. For certain applied fields and ladder lengths, we show that there are isolated fractional fluxons. It is shown that the fractional fluxons would produce a time-averaged voltage related in a characteristic way to the ac voltage frequency.Comment: 13 Figures. 10 page

Length-scale-dependent phase transition in BSCCO single crystals

Electrical transport measurements using a multiterminal configuration are presented, which prove that in BSCCO single crystals near the transition temperature in zero external magnetic field the secondary voltage is induced by thermally activated vortex loop unbinding. The phase transition between the bound and unbound states of the vortex loops was found to be below the temperature where the phase coherence of the superconducting order parameter extends over the whole volume of the sample. We show experimentally that 3D/2D phase transition in vortex dimensionality is a length-scale-dependent layer decoupling process and takes place simultaneously with the 3D/2D phase transition in superconductivity at the same temperature.Comment: 14 pages, 4 figures, to be published in Philos. Ma

Rocking ratchets in 2D Josephson networks: collective effects and current reversal

A detailed numerical study on the directed motion of ac-driven vortices and antivortices in 2D Josephson junction arrays (JJA) with an asymmetric periodic pinning potential is reported. Dc-voltage rectification shows a strong dependence on vortex density as well as an inversion of the vortex flow direction with ac amplitude for a wide range of vortex density around $f$=1/2 ($f$=$Ha^2 / \Phi_0$), in good agreement with recent experiments by Shal\'om and Pastoriza [Phys. Rev. Lett. {\bf 94}, 177001 (2005)]. The study of vortex structures, spatial and temporal correlations, and vortex-antivortex pairs formation gives insight into a purely collective mechanism behind the current reversal effect.Comment: 4 pages, 5 figures. Accepted for publication in Phys. Rev. Let

Analytical Results for Cold Asymmetrical Fermion Superfluids at the Mean-Field Level

We present the analytical results at the mean-field level for the asymmetrical fermion system with attractive contact interaction at the zero temperature. The results can be expressed in terms of linear combinations of the elliptic integrals of the first and second kinds. In the limit of small gap parameter, we discuss how the asymmetry in fermion species affects the phases of the ground state. In the limit of large gap parameter, we show that two candidate phases are competing for the system's ground state. The Sarma phase containing a pure Fermi fluid and a mixed condensate is favored at large degree of asymmetry. The separated phase consisting of a pure Fermi fluid and a boson condensate supports the system at smaller degree of asymmetry. The two phases are degenerate in the limit of infinite pairing gap.Comment: 23 pages, no figur

Josephson effect in ballistic graphene

We solve the Dirac-Bogoliubov-De-Gennes equation in an impurity-free superconductor-normal-superconductor (SNS) junction, to determine the maximal supercurrent that can flow through an undoped strip of graphene with heavily doped superconducting electrodes. The result is determined by the superconducting gap and by the aspect ratio of the junction (length L, small relative to the width W and to the superconducting coherence length). Moving away from the Dirac point of zero doping, we recover the usual ballistic result in which the Fermi wave length takes over from L. The product of critical current and normal-state resistance retains its universal value (up to a numerical prefactor) on approaching the Dirac point.Comment: 4 pages, 2 figure

Two-bands effect on the superconducting fluctuating diamagnetism in MgB&#8322

The field dependence of the magnetization above the transition temperature Tc in MgB₂ is shown to evidence a diamagnetic contribution consistent with superconducting fluctuations reflecting both the σ and π bands. In particular, the upturn field Hup in the magnetization curve, related to the incipient effect of the magnetic field in quenching the fluctuating pairs, displays a double structure, in correspondence to two correlation lengths. The experimental findings are satisfactorily described by the extension to the diamagnetism of a recent theory for paraconductivity, in the framework of a zero-dimensional model for the fluctuating superconducting droplets above Tc

Collective pinning of the vortex lattice by columnar defects in layered superconductors

The mixed phase of layered superconductors with no magnetic screening is studied through a partial duality analysis of the corresponding frustrated XY model in the presence of random columnar pins. A small fraction of pinned vortex lines is assumed. Thermally induced plastic creep of the vortex lattice within isolated layers results in an intermediate Bose glass phase that exhibits weak superconductivity across layers in the limit of weak Josephson coupling. The correlation volume of the vortex lattice is estimated in the strongly-coupled Bose-glass regime at lower temperature. In the absence of additional point pins, no peak effect in the critical current density is predicted to occur on this basis as a function of the Josephson coupling. Also, the phase transition observed recently inside of the vortex-liquid phase of high-temperature superconductors pierced by sparse columnar defects is argued to be a sign of dimensional cross-over.Comment: 16 pages, 1 figure, account of transition to ``nanoliquid'' in BSCCO, to appear in PR

Incoherent multiple Andreev reflection in an array of SNS junctions

Last years many interesting effects related to incoherent MAR have been experimentally found, but only few of them were theoretically explained. It was shown, for example, that if the voltage at the edges of a linear array is $V$ then subgarmonic structures in the current -voltage characteristics appear not only at usual for nonstationary Josephson effect positions, $V_n=2\Delta/n$, where $n$ is integer, but also at voltages other than $V_n$. A step towards description of electron transport in a dirty array of SNS junctions is done in this letter. It is shown that subgarmonic structures may indeed appear at ``unusual'' voltages

Manipulating superconductivity through the domain structure of a ferromagnet: experimental aspects and theoretical implications

In the present work we study experimentally the influence that the domain structure of a fer- romagnet (FM) has on the properties of a superconductor (SC) in bilayers and multilayers of La0.60Ca0.40MnO3/Nb and FePt/Nb proximity hybrids. Specific experimental protocols that were employed in the performed magnetization measurements enabled us to directly uncover a generic property of FM/SC hybrids: in the absence of an external magnetic field, the multidomain struc- ture of the FM promotes the nucleation of superconductivity, while its monodomain state strongly suppresses it. Our experimental findings support recent theoretical studies proposing that when an inhomogeneous exchange field is offered by the FM to the SC the superconducting pairs are not susceptible to pair-breaking.Comment: 4 pages, 4 figure