Temperature-induced crossovers in the static roughness of a one-dimensional interface (Editors' Suggestion in PRB)

Interfaces in nature are never flat, e.g. the outline of a maritime coast or the imbibing front of water propagating in a napkin can be seen as lines which are always rough, even though this feature depends a priori on how close one looks at them. What both complicates and makes interesting the study of such systems is the presence of disorder, i.e. the inhomogeneities or impurities always present in real-life materials. Starting from a model suited in particular to interfaces in magnetic storage devices (namely ferromagnetic domain walls), we focus in this work on the role of temperature in the roughness of such lines confined to a plane. We explore specifically the implications of the very physical assumption that they have a non-zero width, e.g. typically a few nanometers in magnets. Interestingly, at sufficiently low temperatures this additional ingredient is crucial for capturing the equilibrium behavior at small lengthscales, i.e. when one zooms close on the interface. These results are of interest in the understanding of the dynamics of interfaces, that move by a succession of local avalanches on several orders of magnitude in size.n.

Depinning of domain walls with an integral degree of freedom

Taking into account the coupling between the position of the wall and an internal degree of freedom, namely, its phase, we examine, in the rigid-wall approximation, the dynamics of a magnetic domain wall subject to a weak pinning potential. We determine the corresponding force-velocity characteristics, which display several unusual features when compared to the standard depinning behavior. At zero temperature, there exists a bistable regime for low forces, with a logarithmic behavior close to the transition. For weak pinning, there occurs a succession of bistable transitions corresponding to different topological modes of the phase evolution. At finite temperature, the force-velocity characteristics become nonmonotonous. We compare our results to recent experiments on permalloy nanowires. n.

Dissipative phase fluctuations in superconducting wires capacitively coupled to diffusive metals (Editor's Suggestion in PRB)

The effects of the environment on the properties of low-dimensional superconductors are at present not completely understood. In particular for superconducting wires, the understanding of environment-induced dissipation is crucial for potential applications, e.g., miniaturization of superconducting circuits. In this paper we analyze theoretically the properties of an ultrathin superconducting wire placed near a diffusive metal. Intuitively, the presence of the metal should favor superconductivity reducing the repulsion among electrons in the wire (brought about by the Coulomb interaction). At the same time, dissipation (inherited from the diffusive motion of electrons in the disordered metal) will tend to destroy superconductivity. The outcome of this delicate interplay is far from being obvious, and our results show that superconductivity could be controlled by the properties of the screening metal. In particular, its dimensionality plays a major role, yielding a good superconductor with low resistance when the screening metal is two-dimensional, while it would turn the wire in the normal state in the one-dimensional case. n.

Imaging the Essential Role of Spin Fluctuations in High-T_c Superconductivity

We have used scanning tunneling spectroscopy to investigate short-length electronic correlations in three-layer Bi₂Sr₂Ca₂Cu₃O_10+δ (Bi-2223). We show that the superconducting gap and the energy Ω_dip, defined as the difference between the dip minimum and the gap, are both modulated in space following the lattice superstructure and are locally anticorrelated. Based on fits of our data to a microscopic strong-coupling model, we show that Ω_dip is an accurate measure of the collective-mode energy in Bi-2223. We conclude that the collective mode responsible for the dip is a local excitation with a doping dependent energy and is most likely the (π, π) spin resonance.n.

Skyrmion in spinor condensates and its stability in trap potentials

A necessary condition for the existence of a skyrmion in two-component Bose-Einstein condensates with SU(2) symmetry was recently provided by two of the authors [I. F. Herbut and M. Oshikawa, Phys. Rev. Lett. 97, 080403 (2006)], by mapping the problem to a classical particle in a potential subject to time-dependent dissipation. Here we further elaborate this approach. For two classes of models, we demonstrate the existence of the critical dissipation strength above which the skyrmion solution does not exist. Furthermore, we discuss the local stability of the skyrmion solution by considering the second-order variation. A sufficient condition for the local stability is given in terms of the ground-state energy of a one-dimensional quantum-mechanical Hamiltonian. This condition requires a minimum number of bosons, for a certain class of the trap potential. In the optimal case, the minimum number of bosons can be as small $\sim 10^4$. n.

Dissipation-driven phase transitions in superconducting wires

We report on the reinforcement of superconductivity in a system consisting of a narrow superconducting wire weakly coupled to a diffusive metallic film. We analyze the effective phase-only action of the system by a perturbative renormalization group and a self-consistent variational approach to obtain the critical points and phases at T=0. We predict a quantum phase transition toward a superconducting phase with long-range order as a function of the wire stiffness and coupling to the metal. We discuss implications for the dc resistivity of the wire. n.

Kondo effect in transport through Aharonov-Bohm and Aharonov-Casher interferometers

We derive the extension of the Hubbard model to include Rashba spin–orbit coupling that correctly describes Aharonov-Bohm and Aharonov-Casher phases in a ring under applied magnetic and electric fields. When the ring is connected to conducting leads, we develop a formalism that is able to describe both, Kondo and interference effects. We find that in the Kondo regime, the spin–orbit coupling reduces strongly the conductance from the unitary limit. This effect in combination with the magnetic flux, can be used to produce spin polarized carriers. n.

Field-Controlled Magnetic Order in the Quantum Spin-Ladder System (Hpip)2CuBr4

Neutron diffraction is used to investigate the field-induced, antiferromagnetically ordered state in the two-leg spin-ladder material (Hpip)2CuBr4. This “classical” phase, a consequence of weak interladder coupling, is nevertheless highly unconventional: its properties are influenced strongly by the spin Luttinger-liquid state of the ladder subunits. We determine directly the order parameter (transverse magnetization), the ordering temperature, the spin structure, and the critical exponents around the transition. We introduce a minimal microscopic model for the interladder coupling and calculate the quantum fluctuation corrections to the mean-field interaction. n.