4,443 research outputs found
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
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 and incoherent metallic regime with as we
increase the disorder parameter . 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(), thermoelectric(), and
thermal() 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(), chemical potential(), and
effective impurity(). At low frequencies, if , all three AC
conductivities() exhibit a Drude peak modified by
pair creation contribution(coherent metal). The parameters of this modified
Drude peak are obtained analytically. In particular, if the
relaxation time of electric conductivity approaches to
and the modified Drude peak becomes a standard Drude peak. If the
shape of peak deviates from the Drude form(incoherent metal). At intermediate
frequencies(), we have analysed numerical data of three
conductivities() 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
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
Flow Characteristics Around Step-Up Street Canyons with Various Building Aspect Ratios
We investigate the flow characteristics around step-up street canyons with various building aspect ratios (ratio of along-canyon building length to street-canyon width, and upwind building height to downwind building height) using a computational fluid dynamics (CFD) model. Simulated results are validated against experimental wind-tunnel results, with the CFD simulations conducted under the same building configurations as those in the wind-tunnel experiments. The CFD model reproduces the measured in-canyon vortex, rooftop recirculation zone above the downwind building, and stagnation point position reasonably well. We analyze the flow characteristics, focusing on the structural change of the in-canyon flows and the interaction between the in- and around-canyon flows with the increase of building-length ratio. The in-canyon flows undergo development and mature stages as the building-length ratio increases. In the development stage (i.e., small building-length ratios), the position of the primary vortex wanders, and the incoming flow closely follows both the upstream and downstream building sidewalls. As a result, increasing momentum transfer from the upper layer contributes to a momentum increase in the in-canyon region, and the vorticity in the in-canyon region also increases. In the mature stage (i.e., large building-length ratios), the primary vortex stabilizes in position, and the incoming flow no longer follows the building sidewalls. This causes momentum loss through the street-canyon lateral boundaries. As the building-length ratio increases, momentum transfer from the upper layer slightly decreases, and the reverse flow, updraft, and streamwise flow in the in-canyon region also slightly decrease, resulting in vorticity reduction
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