Antisymmetric magnetoresistance in magnetic multilayers with perpendicular anisotropy

While magnetoresistance (MR) has generally been found to be symmetric in applied field in non-magnetic or magnetic metals, we have observed antisymmetric MR in Co/Pt multilayers. Simultaneous domain imaging and transport measurements show that the antisymmetric MR is due to the appearance of domain walls that run perpendicular to both the magnetization and the current, a geometry existing only in materials with perpendicular magnetic anisotropy. As a result, the extraordinary Hall effect (EHE) gives rise to circulating currents in the vicinity of the domain walls that contributes to the MR. The antisymmetric MR and EHE have been quantitatively accounted for by a theoretical model.Comment: 17 pages, 4 figure

Unusual Magnetization Reversal in [Co/Pt]\u3csub\u3e4\u3c/sub\u3e Multilayers with Perpendicular Anisotropy

Unusual magnetization reversal of [Co(4 Å)/Pt(10 Å)]4 multilayers with perpendicular magnetic anisotropy has been revealed macroscopically by magnetometry measurements and microscopically by magneto-optical Kerr effect microscopy and magnetic force microscopy (MFM) imaging. During the first-order reversal process, the magnetization first decreases, then reaches a plateau, and finally rises back to saturation, corresponding to expanding bubble domains, stationary domains, and fading contrast but unchanged boundary domains, respectively. MFM imaging reveals the existence of many submicron-scaled unreversed channels within the boundary of the “bubble” domains. The magnetization reversal behavior can be accounted for by the evolution of the unusual domain structures in different field regimes

Extended Impurity Potential in a d_{x^2-y^2} Superconductor

We investigate the role of a finite potential range of a nonmagnetic impurity for the local density of states in a d_{x^2-y^2} superconductor. Impurity induced subgap resonances are modified by the appearance of further scattering channels beyond the $s$--wave scattering limit. The structure of the local density of states (DOS) in the vicinity of the impurity is significantly enhanced and therefore improves the possibility for observing the characteristic anisotropic spatial modulation of the local DOS in a d_{x^2-y^2} superconductor by scanning tunneling microscopy.Comment: 4 pages, Revtex, with 4 embedded eps figures. Submitted to Phys. Rev. Let

Unusual Magnetization Reversal in [Co/Pt]\u3csub\u3e4\u3c/sub\u3e Multilayers with Perpendicular Anisotropy

Unusual magnetization reversal of [Co(4 Å)/Pt(10 Å)]4 multilayers with perpendicular magnetic anisotropy has been revealed macroscopically by magnetometry measurements and microscopically by magneto-optical Kerr effect microscopy and magnetic force microscopy (MFM) imaging. During the first-order reversal process, the magnetization first decreases, then reaches a plateau, and finally rises back to saturation, corresponding to expanding bubble domains, stationary domains, and fading contrast but unchanged boundary domains, respectively. MFM imaging reveals the existence of many submicron-scaled unreversed channels within the boundary of the “bubble” domains. The magnetization reversal behavior can be accounted for by the evolution of the unusual domain structures in different field regimes

Quantum critical effects on transition temperature of magnetically mediated p-wave superconductivity

We determine the behavior of the critical temperature of magnetically mediated p-wave superconductivity near a ferromagnetic quantum critical point in three dimensions, distinguishing universal and non-universal aspects of the result. We find that the transition temperature is non-zero at the critical point, raising the possibility of superconductivity in the ferromagnetic phase.Comment: 4 pages, 4 figure

Fluctuation Exchange Analysis of Superconductivity in the Standard Three-Band CuO2 Model

The fluctuation exchange, or FLEX, approximation for interacting electrons is applied to study instabilities in the standard three-band model for CuO2 layers in the high-temperature superconductors. Both intra-orbital and near-neigbor Coulomb interactions are retained. The filling dependence of the d(x2-y2) transition temperature is studied in both the "hole-doped" and "electron-doped" regimes using parameters derived from constrained-occupancy density-functional theory for La2CuO4. The agreement with experiment on the overdoped hole side of the phase diagram is remarkably good, i.e., transitions emerge in the 40 K range with no free parameters. In addition the importance of the "orbital antiferromagnetic," or flux phase, charge density channel is emphasized for an understanding of the underdoped regime.Comment: REVTex and PostScript, 31 pages, 26 figures; to appear in Phys. Rev. B (1998); only revised EPS figures 3, 4, 6a, 6b, 6c, 7 and 8 to correct disappearance of some labels due to technical problem

Vortex Pull by an External Current

In the context of a dynamical Ginzburg-Landau model it is shown numerically that under the influence of a homogeneous external current J the vortex drifts against the current with velocity $V= -J$ in agreement to earlier analytical predictions. In the presence of dissipation the vortex undergoes skew deflection at an angle $90^{\circ} < \delta < 180^{\circ}$ with respect to the external current. It is shown analytically and verified numerically that the angle $\delta$ and the speed of the vortex are linked through a simple mathematical relation.Comment: 19 pages, LATEX, 6 Postscript figures included in separate compressed fil

On Existence and Properties of Approximate Pure Nash Equilibria in Bandwidth Allocation Games

In \emph{bandwidth allocation games} (BAGs), the strategy of a player consists of various demands on different resources. The player's utility is at most the sum of these demands, provided they are fully satisfied. Every resource has a limited capacity and if it is exceeded by the total demand, it has to be split between the players. Since these games generally do not have pure Nash equilibria, we consider approximate pure Nash equilibria, in which no player can improve her utility by more than some fixed factor $\alpha$ through unilateral strategy changes. There is a threshold $\alpha_\delta$ (where $\delta$ is a parameter that limits the demand of each player on a specific resource) such that $\alpha$-approximate pure Nash equilibria always exist for $\alpha \geq \alpha_\delta$, but not for $\alpha < \alpha_\delta$. We give both upper and lower bounds on this threshold $\alpha_\delta$ and show that the corresponding decision problem is ${\sf NP}$-hard. We also show that the $\alpha$-approximate price of anarchy for BAGs is $\alpha+1$. For a restricted version of the game, where demands of players only differ slightly from each other (e.g. symmetric games), we show that approximate Nash equilibria can be reached (and thus also be computed) in polynomial time using the best-response dynamic. Finally, we show that a broader class of utility-maximization games (which includes BAGs) converges quickly towards states whose social welfare is close to the optimum

Quantum transport in ultracold atoms

Ultracold atoms confined by engineered magnetic or optical potentials are ideal systems for studying phenomena otherwise difficult to realize or probe in the solid state because their atomic interaction strength, number of species, density, and geometry can be independently controlled. This review focuses on quantum transport phenomena in atomic gases that mirror and oftentimes either better elucidate or show fundamental differences with those observed in mesoscopic and nanoscopic systems. We discuss significant progress in performing transport experiments in atomic gases, contrast similarities and differences between transport in cold atoms and in condensed matter systems, and survey inspiring theoretical predictions that are difficult to verify in conventional setups. These results further demonstrate the versatility offered by atomic systems in the study of nonequilibrium phenomena and their promise for novel applications.Comment: 24 pages, 7 figures. A revie