199 research outputs found
Zero Sound in Strange Metallic Holography
One way to model the strange metal phase of certain materials is via a
holographic description in terms of probe D-branes in a Lifshitz spacetime,
characterised by a dynamical exponent z. The background geometry is dual to a
strongly-interacting quantum critical theory while the probe D-branes are dual
to a finite density of charge carriers that can exhibit the characteristic
properties of strange metals. We compute holographically the low-frequency and
low-momentum form of the charge density and current retarded Green's functions
in these systems for massless charge carriers. The results reveal a
quasi-particle excitation when z<2, which in analogy with Landau Fermi liquids
we call zero sound. The real part of the dispersion relation depends on
momentum k linearly, while the imaginary part goes as k^2/z. When z is greater
than or equal to 2 the zero sound is not a well-defined quasi-particle. We also
compute the frequency-dependent conductivity in arbitrary spacetime dimensions.
Using that as a measure of the charge current spectral function, we find that
the zero sound appears only when the spectral function consists of a single
delta function at zero frequency.Comment: 20 pages, v2 minor corrections, extended discussion in sections 5 and
6, added one footnote and four references, version published in JHE
Towards strange metallic holography
We initiate a holographic model building approach to `strange metallic'
phenomenology. Our model couples a neutral Lifshitz-invariant quantum critical
theory, dual to a bulk gravitational background, to a finite density of gapped
probe charge carriers, dually described by D-branes. In the physical regime of
temperature much lower than the charge density and gap, we exhibit anomalous
scalings of the temperature and frequency dependent conductivity. Choosing the
dynamical critical exponent appropriately we can match the non-Fermi liquid
scalings, such as linear resistivity, observed in strange metal regimes. As
part of our investigation we outline three distinct string theory realizations
of Lifshitz geometries: from F theory, from polarised branes, and from a
gravitating charged Fermi gas. We also identify general features of
renormalisation group flow in Lifshitz theories, such as the appearance of
relevant charge-charge interactions when . We outline a program to
extend this model building approach to other anomalous observables of interest
such as the Hall conductivity.Comment: 71 pages, 8 figure
Novel Solutions of Finite-Density D3/D5 Probe Brane System and Their Implications for Stability
In this article we present a novel set of solutions of the probe brane system
consisting of Nf-D5 probe branes embedded in the near-horizon geometry
generated by Nc-D3 branes, with the D5 worldvolume U(1) gauge fields turned on.
Our system is holographically dual to a supersymmetric defect field theory at
finite density in non-trivial vacua. We find that a large class of vacua turns
out to satisfy a no-force condition, even with supersymmetry explicitly broken
by the finite density; our solutions include configuration in which charge
separates from the horizon and is instead carried by probe branes outside the
horizon. The free energy is lowered in this process. Whether this corresponds
to a genuine instability of the finite-density probe brane system remains to be
seen.Comment: 11 pages, 1 figure
Zero Sound in Effective Holographic Theories
We investigate zero sound in -dimensional effective holographic theories,
whose action is given by Einstein-Maxwell-Dilaton terms. The bulk spacetimes
include both zero temperature backgrounds with anisotropic scaling symmetry and
their near-extremal counterparts obtained in 1006.2124 [hep-th], while the
massless charge carriers are described by probe D-branes. We discuss
thermodynamics of the probe D-branes analytically. In particular, we clarify
the conditions under which the specific heat is linear in the temperature,
which is a characteristic feature of Fermi liquids. We also compute the
retarded Green's functions in the limit of low frequency and low momentum and
find quasi-particle excitations in certain regime of the parameters. The
retarded Green's functions are plotted at specific values of parameters in
, where the specific heat is linear in the temperature and the
quasi-particle excitation exists. We also calculate the AC conductivity in
-dimensions as a by-product.Comment: 29 pages, 1 figur
Probes on D3-D7 Quark-Gluon Plasmas
We study the holographic dual model of quenched flavors immersed in a
quark-gluon plasma with massless dynamical quarks in the Veneziano limit. This
is modeled by embedding a probe D7 brane in a background where the backreaction
of massless D7 branes has been taken into account. The background, and hence
the effects, are perturbative in the Veneziano parameter N_f/N_c, therefore
giving small shifts of all magnitudes like the constituent mass, the quark
condensate, and several transport coefficients. We provide qualitative results
for the effect of flavor degrees of freedom on the probes. For example, the
meson melting temperature is enhanced, while the screening length is
diminished. The drag force is also enhanced.Comment: 31 pages, 17 figure
Thermodynamics of Holographic Defects
Using the AdS/CFT correspondence, we study the thermodynamic properties and
the phase diagram of matter fields on (2+1)-dimensional defects coupled to a
(3+1)-dimensional N=4 SYM "heat bath". Considering a background magnetic field,
(net) quark density, defect "magnitude" and the mass of the
matter, we study the defect contribution to the thermodynamic potentials and
their first and second derivatives to map the phases and study their physical
properties.
We find some features that are qualitatively similar to other systems e.g. in
(3+1) dimensions and a number of features that are particular to the defect
nature, such as its magnetic properties, unexpected properties at T->0 and
finite density; and the finite effects, e.g. a diverging
susceptibility and vanishing density of states at small temperatures, a
physically consistent negative heat capacity and new types of consistent
phases.Comment: 33 pages, 16 figures (jpg and pdf), typos fixed and references added,
final version published in JHE
Probe Branes, Time-dependent Couplings and Thermalization in AdS/CFT
We present holographic descriptions of thermalization in conformal field
theories using probe D-branes in AdS X S space-times. We find that the induced
metrics on Dp-brane worldvolumes which are rotating in an internal sphere
direction have horizons with characteristic Hawking temperatures even if there
is no black hole in the bulk AdS. The AdS/CFT correspondence applied to such
systems indeed reveals thermal properties such as Brownian motions and AC
conductivities in the dual conformal field theories. We also use this framework
to holographically analyze time-dependent systems undergoing a quantum quench,
where parameters in quantum field theories, such as a mass or a coupling
constant, are suddenly changed. We confirm that this leads to thermal behavior
by demonstrating the formation of apparent horizons in the induced metric after
a certain time.Comment: LaTeX, 47 pages, 14 figures; Typos corrected and references added
(v2); minor corrections, references added(v3
Baryon charge from embedding topology and a continuous meson spectrum in a new holographic gauge theory
We study a new holographic gauge theory based on probe D4-branes in the
background dual to D4-branes on a circle with antiperiodic boundary conditions
for fermions. Field theory configurations with baryons correspond to smooth
embeddings of the probe D4-branes with nontrivial winding around an S^4 in the
geometry. As a consequence, physics of baryons and nuclei can be studied
reliably in this model using the abelian Born-Infeld action. However,
surprisingly, we find that the meson spectrum is not discrete. This is related
to a curious result that the action governing small fluctuations of the gauge
field on the probe brane is the five-dimensional Maxwell action in Minkowski
space despite the non-trivial embedding of the probe brane in the curved
background geometry.Comment: 24 pages, LaTeX, 10 figures, v4: previously ignored effects of
coupling to RR-fields included, meson spectrum qualitatively changed, v5:
journal versio
Holographic Conductivity in Disordered Systems
The main purpose of this paper is to holographically study the behavior of
conductivity in 2+1 dimensional disordered systems. We analyze probe D-brane
systems in AdS/CFT with random closed string and open string background fields.
We give a prescription of calculating the DC conductivity holographically in
disordered systems. In particular, we find an analytical formula of the
conductivity in the presence of codimension one randomness. We also
systematically study the AC conductivity in various probe brane setups without
disorder and find analogues of Mott insulators.Comment: 43 pages, 28 figures, latex, references added, minor correction
Heavy quark density in N=4 SYM: from hedgehog to Lifshitz spacetimes
We study the effect of an order N^2 density of heavy quarks in strongly
coupled N=4 SUSY Yang-Mills theory in the large N limit. This is achieved in
the type IIB supergravity dual by introducing a uniformly smeared density of
macroscopic string sources stretching to the boundary of AdS_5 x S^5. The
backreacted system exhibits a flow from an AdS_5 "hedgehog" geometry to a
scaling Lifshitz-like solution Lif_5 x S^5 with dynamical critical exponent
z=7, wherein the scaling symmetry is broken by a logarithmic running dilaton.
We find an exact black brane solution within the scaling regime which describes
the low temperature thermodynamics of the system.Comment: 20 pages, 2 figures, references adde
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