Equilibrium magnetisation structures in ferromagnetic nanorings

The ground state of the ring-shape magnetic nanoparticle is studied. Depending on the geometrical and magnetic parameters of the nanoring, there exist different magnetisation configurations (magnetic phases): two phases with homogeneous magnetisation (easy-axis and easy-plane phases) and two inhomogeneous (planar vortex phase and out-of-plane one). The existence of a new intermediate out-of-plane vortex phase, where the inner magnetisation is not strongly parallel to the easy axis, is predicted. Possible transitions between different phases are analysed using the combination of analytical calculations and micromagnetic simulations.Comment: LaTeX, 19 pages, 11 figure

Antiferromagnetic superconductors with effective mass anisotropy in magnetic fields

We derive critical field H_c2 equations for antiferromagnetic \textit{s}-wave, d_{x^2-y^2}-wave, and d_{xy}-wave superconductors with effective mass anisotropy in three dimensions, where we take into account (i) the Jaccarino-Peter mechanism of magnetic-field-induced superconductivity (FISC) at high fields, (ii) an extended Jaccarino-Peter mechanism that reduces the Pauli paramagnetic pair-breaking effect at low fields where superconductivity and an antiferromagnetic long-range order with a canted spin structure coexist, and (iii) the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO or LOFF) state. As an example, experimental phase diagrams observed in organic superconductor kappa-(BETS)_2FeBr_4 are theoretically reproduced. In particular, the upper critical field of low-field superconductivity is well reproduced without any additional fitting parameter other than those determined from the critical field curves of the FISC at high fields. Therefore, the extended Jaccarino-Peter mechanism seems to occur actually in the present compound. It is predicted that the FFLO state does not occur in the FISC at high fields in contrast to the compound lambda-(BETS)_2FeCl_4, but it may occur in low-field superconductivity for s-wave and d_{x^2-y^2}-wave pairings. We also briefly discuss a possibility of compounds that exhibit unconventional behaviors of upper critical fields.Comment: 11 pages, 9 figures, revtex

Anisotropic scattering in angular-dependent magnetoresistance oscillations of quasi-2D and quasi-1D metals: beyond the relaxation-time approximation

The electrical resistivity for a current moving perpendicular to layers (chains) in quasi-2D (quasi-1D) metals under an applied magnetic field of varying orientation is studied using Boltzmann transport theory. We consider the simplest non-trivial quasi-2D and quasi-1D Fermi surfaces but allow for an arbitrary elastic collision integral (i.e., a scattering probability with arbitrary dependence on momentum-transfer) and obtain an expression for the resistivity which generalizes that previously found using a single relaxation-time approximation. The dependence of the resistivity on the angle between the magnetic field and current changes depending on the momentum-dependence of the scattering probability. So, whereas zero-field intra-layer transport is sensitive only to the momentum-averaged scattering probability (the transport relaxation rate) the resistivity perpendicular to layers measured in a tilted magnetic field provides detailed information about the momentum-dependence of interlayer scattering. These results help clarify the meaning of the relaxation rate determined from fits of angular-dependent magnetoresistance oscillations (AMRO) experimental data to theoretical expressions. Furthermore, we suggest how AMRO might be used to probe the dominant scattering mechanism.Comment: 12 pages, 7 figure

77^{77}Se NMR evidence for the Jaccarino-Peter mechanism in the field induced superconductor, λ\lambda(BETS)2_2FeCl4_4}

We have performed $^{77}$Se NMR on a single crystal sample of the field induced superconductor $\lambda$-(BETS)$_{2}$FeCl$_{4}$. Our results obtained in the paramagnetic state provide a microscopic insight on the exchange interaction $J$ between the spins \textbf{s} of the BETS $\pi$ conduction electrons and the Fe localized $d$ spins \textbf{S}. The absolute value of the Knight shift \textbf{K} decreases when the polarization of the Fe spins increases. This reflects the ``negative'' spin polarization of the $\pi$ electrons through the exchange interaction $J$. The value of $J$ has been estimated from the temperature and the magnetic field dependence of \textbf{K} and found in good agreement with that deduced from transport measurements (L. Balicas \textit{et al}. Phys. Rev. Lett. \textbf{87}, 067002 (2001)). This provides a direct microscopic evidence that the field induced superconductivity is due to the compensation effect predicted by Jaccarino and Peter (Phys. Rev. Lett. \textbf{9}, 290 (1962)). Furthermore, an anomalous broadening of the NMR line has been observed at low temperature, which suggests the existence of charge disproportionation in the metallic state neighboring the superconducting phase

Suppression of superconductivity by non-magnetic disorder in organic superconductor κ\kappa-(BEDT-TTF)2_{2}Cu(NCS)2_{2}

The suppression of superconductivity by nonmagnetic disorder is investigated systematically in the organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$. We introduce a nonmagnetic disorder arising from molecule substitution in part with deuterated BEDT-TTF or BMDT-TTF for BEDT-TTF molecules and molecular defects introduced by X-ray irradiation. A quantitative evaluation of the scattering time $\tau_{\rm dHvA}$ is carried out by de Haas-van Alphen (dHvA) effect measurement. A large reduction in $T_{\rm c}$ with a linear dependence on $1/\tau_{\rm dHvA}$ is found in the small-disorder region below $1/\tau_{\rm dHvA} \simeq$ 1 $\times$ 10$^{12}$ s$^{-1}$ in both the BMDT-TTF molecule-substituted and X-ray-irradiated samples. The observed linear relation between $T_{\rm c}$ and $1/\tau_{\rm dHvA}$ is in agreement with the Abrikosov-Gorkov (AG) formula, at least in the small-disorder region. This observation is reasonably consistent with the unconventional superconductivity proposed thus far for the present organic superconductor. A deviation from the AG formula, however, is observed in the large-disorder region above $1/\tau_{\rm dHvA} \simeq$ 1 $\times$ 10$^{12}$ s$^{-1}$, which reproduces the previous transport study (J. G. Analytis {\it et al.}: Phys. Rev. Lett. {\bf 96} (2006) 177002). We present some interpretations of this deviation from the viewpoints of superconductivity and the inherent difficulties in the evaluation of scattering time.Comment: 11 pages, 6 figure

Hemispheric differences between left and right supramarginal gyrus for pitch and rhythm memory

Functional brain imaging studies and non-invasive brain stimulation methods have shown the importance of the left supramarginal gyrus (SMG) for pitch memory. The extent to which this brain region plays a crucial role in memory for other auditory material remains unclear. Here, we sought to investigate the role of the left and right SMG in pitch and rhythm memory in non-musicians. Anodal or sham transcranial direct current stimulation (tDCS) was applied over the left SMG (Experiment 1) and right SMG (Experiment 2) in two different sessions. In each session participants completed a pitch and rhythm recognition memory task immediately after tDCS. A significant facilitation of pitch memory was revealed when anodal stimulation was applied over the left SMG. No significant effects on pitch memory were found for anodal tDCS over the right SMG or sham condition. For rhythm memory the opposite pattern was found; anodal tDCS over the right SMG led to an improvement in performance, but anodal tDCS over the left SMG had no significant effect. These results highlight a different hemispheric involvement of the SMG in auditory memory processing depending on auditory material that is encoded

On-chip dynamic time reversal of light in a coupled-cavity system

We theoretically and experimentally demonstrate dynamic, all-linear time-reversal of infrared light in planar optical circuits for the first time. We propose that the oscillatory motion of the light stored in cavities can be time-reversed by fast nonadiabatic tuning of the frequency of eigenmodes of a coupled cavity system and experimentally demonstrate it using a system consisting of distant high-Q-factor two-dimensional photonic crystal cavities between which effective direct couplings are formed via line-defect waveguides. We also analyze the loss and methods to reduce the loss, as well as a theory that expands our system to realize general time-reversal operation for any input light