Current-induced synchronized switching of magnetization

We investigate current-induced magnetization switching for a multilayer structure that allows a reduced switching current while maintaining high thermal stability of the magnetization. The structure consists of a perpendicular polarizer, a perpendicular free-layer, and an additional free-layer having in-plane magnetization. When the current runs perpendicular to the structure, the in-plane free-layer undergoes a precession and supplies an internal rf field to the perpendicular free-layer, resulting in a reduced switching current for one current polarity. For the other polarity, the in-plane free-layer almost saturates perpendicular to the plane and acts as another perpendicular polarizer, which also reduces the switching current.Comment: 18 pages, 4 figure

Thermoelectric Conductivities at Finite Magnetic Field and the Nernst Effect

We study the thermoelectric conductivities of a strongly correlated system in the presence of a magnetic field by the gauge/gravity duality. We consider a class of Einstein-Maxwell-Dilaton theories with axion fields imposing momentum relaxation. General analytic formulas for the direct current(DC) conductivities and the Nernst signal are derived in terms of the black hole horizon data. For an explicit model study, we analyse in detail the dyonic black hole modified by momentum relaxation. In this model, for small momentum relaxation, the Nernst signal shows a bell-shaped dependence on the magnetic field, which is a feature of the normal phase of cuprates. We compute all alternating current(AC) electric, thermoelectric, and thermal conductivities by numerical analysis and confirm that their zero frequency limits precisely reproduce our analytic DC formulas, which is a non-trivial consistency check of our methods. We discuss the momentum relaxation effects on the conductivities including cyclotron resonance poles.Comment: v3: Minor chages, discussions clarified, version accepted in JHE

Gauge Invariance and Holographic Renormalization

We study the gauge invariance of physical observables in holographic theories under the local diffeomorphism. We find that gauge invariance is intimately related to the holographic renormalisation: the local counter terms defined in the boundary cancel most of gauge dependences of the on-shell action as well as the divergences. There is a mismatch in the degrees of freedom between the bulk theory and the boundary one. We resolve this problem by noticing that there is a residual gauge symmetry(RGS). By extending the RGS such that it satisfies infalling boundary condition at the horizon, we can understand the problem in the context of general holographic embedding of a global symmetry at the boundary into the local gauge symmetry in the bulk.Comment: 14 pages, v2: minor changes, typos corrected, references adde

Character of Matter in Holography: Spin-Orbit Interaction

Gauge/Gravity duality as a theory of matter needs a systematic way to characterise a system. We suggest a `dimensional lifting' of the least irrelevant interaction to the bulk theory. As an example, we consider the spin-orbit interaction, which causes magneto-electric interaction term. We show that its lifting is an axionic coupling. We present an exact and analytic solution describing diamagnetic response. Experimental data on annealed graphite shows a remarkable similarity to our theoretical result. We also find an analytic formulas of DC transport coefficients, according to which, the anomalous Hall coefficient interpolates between the coherent metallic regime with $\rho_{xx}^{2}$ and incoherent metallic regime with $\rho_{xx}$ as we increase the disorder parameter $\beta$. The strength of the spin-orbit interaction also interpolates between the two scaling regimes.Comment: 15pages, 3 figure

Coherent/incoherent metal transition in a holographic model

We study AC electric($\sigma$), thermoelectric($\alpha$), and thermal($\bar{\kappa}$) conductivities in a holographic model, which is based on 3+1 dimensional Einstein-Maxwell-scalar action. There is momentum relaxation due to massless scalar fields linear to spatial coordinate. The model has three field theory parameters: temperature($T$), chemical potential($\mu$), and effective impurity($\beta$). At low frequencies, if $\beta < \mu$, all three AC conductivities($\sigma, \alpha, \bar{\kappa}$) exhibit a Drude peak modified by pair creation contribution(coherent metal). The parameters of this modified Drude peak are obtained analytically. In particular, if $\beta \ll \mu$ the relaxation time of electric conductivity approaches to $2\sqrt{3} \mu/\beta^2$ and the modified Drude peak becomes a standard Drude peak. If $\beta > \mu$ the shape of peak deviates from the Drude form(incoherent metal). At intermediate frequencies($T<\omega<\mu$), we have analysed numerical data of three conductivities($\sigma, \alpha, \bar{\kappa}$) for a wide variety of parameters, searching for scaling laws, which are expected from either experimental results on cuprates superconductors or some holographic models. In the model we study, we find no clear signs of scaling behaviour.Comment: 27 pages, 9 figures, v2,v3: minor changes, typos corrected, reference adde