Asymmetric field dependence of magnetoresistance in magnetic films

We study an asymmetric in field magnetoresistance that is frequently observed in magnetic films and, in particular, the odd longitudinal voltage peaks that appear during magnetization reversal in ferromagnetic films, with out-of-plane magnetic anisotropy. We argue that the anomalous signals result from small variation of magnetization and Hall resistivity along the sample. Experimental data can be well described by a simple circuit model, the latter being supported by analytic and numerical calculations of current and electric field distribution in films with a gradual variation of the magnetization and Hall resistance.Comment: 7 pages, 5 figures, to be published in Phys. Rev.

Unconventional Anomalous Hall Effect in the Metallic Triangular-Lattice Magnet PdCrO2

We experimentally reveal an unconventional anomalous Hall effect (UAHE) in a quasi-two-dimensional triangular-lattice antiferromagnet PdCrO2. Using high quality single crystals of PdCrO2, we found that the Hall resistivity deviates from the conventional behavior below T* = 20 K, noticeably lower than TN = 37.5 K, at which Cr^{3+} (S=3/2) spins order in a 120 degree structure. In view of the theoretical expectation that the spin chirality cancels out in the simplest 120 degree spin structure, we discuss required conditions for the emergence of UAHE within Berry-phase mechanisms.Comment: 4 pages, 5 figures, accepted for publication in PR

Superconducting d-wave junctions: The disappearance of the odd ac components

We study voltage-biased superconducting planar d-wave junctions for arbitrary transmission and arbitrary orientation of the order parameters of the superconductors. For a certain orientation of the superconductors the odd ac components disappear, resulting in a doubling of the Josephson frequency. We study the sensitivity of this disappearance to orientation and compare with experiments on grain boundary junctions. We also discuss the possibility of a current flow parallel to the junction.Comment: 5 pages, 3 figure

Tunneling spectra for (dx2−y2+isd_{x^2-y^2}+is)-wave superconductors versus tunneling spectra for (dx2−y2+idxyd_{x^2-y^2}+id_{xy})-wave superconductors

The tunneling conductance spectra of a normal metal / insulator / singlet superconductor is calculated from the reflection amplitudes using the Blonder-Tinkham-Klapwijk (BTK) formulation. The pairing symmetry of the superconductor is assumed to be $d_{x^2-y^2}+is$, or $d_{x^2-y^2}+id_{xy}$. It is found that in the ($d_{x^2-y^2}+is$)-wave case there is a well defined conductance peak in the conductance spectra, in the amplitude of the secondary s-wave component. In the ($d_{x^2-y^2}+id_{xy}$)-wave case the tunneling conductance has residual values within the gap, due to the formation of bound states. The bound state energies depend on the angle of the incident quasiparticles, and also on the boundary orientation. On the basis of this observation an electron focusing experiment is proposed to probe the ($d_{x^2-y^2}+id_{xy}$)-wave state.Comment: 17 pages with 9 figure

Intrinsic vs. extrinsic anomalous Hall effect in ferromagnets

A unified theory of the anomalous Hall effect (AHE) is presented for multi-band ferromagnetic metallic systems with dilute impurities. In the clean limit, the AHE is mostly due to the extrinsic skew-scattering. When the Fermi level is located around anti-crossing of band dispersions split by spin-orbit interaction, the intrinsic AHE to be calculated ab initio is resonantly enhanced by its non-perturbative nature, revealing the extrinsic-to-intrinsic crossover which occurs when the relaxation rate is comparable to the spin-orbit interaction energy.Comment: 5 pages including 4 figures, RevTex; minor changes, to appaer in Phys. Rev. Let

Topological Hall effect and Berry phase in magnetic nanostructures

We discuss the anomalous Hall effect in a two-dimensional electron gas subject to a spatially varying magnetization. This topological Hall effect (THE) does not require any spin-orbit coupling, and arises solely from Berry phase acquired by an electron moving in a smoothly varying magnetization. We propose an experiment with a structure containing 2D electrons or holes of diluted magnetic semiconductor subject to the stray field of a lattice of magnetic nanocylinders. The striking behavior predicted for such a system (of which all relevant parameters are well known) allows to observe unambiguously the THE and to distinguish it from other mechanisms.Comment: 5 pages with 4 figure

Resonant tunneling through a small quantum dot coupled to superconducting leads

We address the problem of non-linear transport through discrete electronic levels in a small quantum dot coupled to superconducting electrodes. In our approach the low temperature I-V characteristics can be calculated including all multiple quasi-particle and Andreev processes. The limit of very weak coupling to the leads and large charging energies is briefly analyzed comparing the calculated lineshapes of the I-V curves with recent experimental results. When the coupling to the leads increases and Coulomb blockade effects can be neglected, the combination of multiple Andreev processes and resonant transmission gives rise to a rich subgap structure which largely differs from the one found in the more studied S-N-S systems. We show how multiple processes can be included within a simple sequential tunneling picture qualitatively explaining the subgap structure. We suggest an experimental set-up where the predicted effects could be observed.Comment: 11 pages, 4 postscript figures, to be published in Phys. Rev. B (rapid communications

Berry's phase contribution to the anomalous Hall effect of gadolinium

When conduction electrons are forced to follow the local spin texture, the resulting Berry phase can induce an anomalous Hall effect (AHE). In gadolinium, as in double-exchange magnets, the exchange interaction is mediated by the conduction electrons and the AHE may therefore resemble that of chromium dioxide and other metallic double-exchange ferromagnets. The Hall resistivity, magnetoresistance, and magnetization of single crystal gadolinium were measured in fields up to 30 T. Measurements between 2 K and 400 K are consistent with previously reported data. A scaling analysis for the Hall resistivity as a function of the magnetization suggests the presence of a Berry's-phase contribution to the anomalous Hall effect.Comment: 6 pages, 7 figures, submitted to Phys. Rev.

Orbital magnetization and its effects in spin-chiral ferromagnetic Kagome lattice

Recently, Berry phase in the semiclassical dynamical of Bloch electrons has been found to make a correction to the phase-space density of states and a general multi-band formula for finite-temperature orbital magnetization has been given [Phys. Rev. Lett. \textbf{97}, 026603 (2006)], where the orbital magnetization $\mathcal{M}$ consists of two parts, i.e., the conventional part $M_{c}$ and the Berry-phase correction part $M_{\Omega}$. Using this general formula, we theoretically investigate the orbital magnetization and its effects on thermoelectric transport and magnetic susceptibility properties of the two-dimensional \textit{kagom\'{e}} lattice with spin anisotropies included. The study in this paper is highly interesting by the occurrence of nonzero Chern number in the lattice. The spin chirality parameter $\phi$ (see text) results in profound effects on the orbital magnetization properties. It is found that the two parts in orbital magnetization opposite each other. In particular, we show that $M_{c}$ and $M_{\Omega}$ yield the paramagnetic and diamagnetic responses, respectively. It is further shown that the orbital magnetization displays fully different behavior in the metallic and insulating regions, which is due to the different roles $M_{c}$ and $M_{\Omega}$ play in these two regions. The anomalous Nernst conductivity is also calculated, which displays a peak-valley structure as a function of the electron Fermi energy.Comment: 9 pages, 7 figure

A precursor state to unconventional superconductivity in CeIrIn5{_5}

We present sensitive measurements of the Hall effect and magnetoresistance in CeIrIn${_5}$ down to temperatures of 50 mK and magnetic fields up to 15 T. The presence of a low temperature coherent Kondo state is established. Deviations from Kohler's rule and a quadratic temperature dependence of the cotangent of the Hall angle are reminiscent of properties observed in the high temperature superconducting cuprates. The most striking observation pertains to the presence of a \textit{precursor} state--characterized by a change in the Hall mobility--that appears to precede the superconductivity in this material, in similarity to the pseudogap in the cuprate high $T_c$ superconductors.Comment: 4 figure