68 research outputs found
Probing beyond ETH at large
We study probe corrections to the Eigenstate Thermalization Hypothesis (ETH)
in the context of 2D CFTs with large central charge and a sparse spectrum of
low dimension operators. In particular, we focus on observables in the form of
non-local composite operators
with . As a
light probe, is constrained by ETH and satisfies
for a high energy energy eigenstate
. In the CFTs of interests, is related to a Heavy-Heavy-Light-Light (HL)
correlator, and can be approximated by the vacuum Virasoro block, which we
focus on computing. A sharp consequence of ETH for is
the so called "forbidden singularities", arising from the emergent thermal
periodicity in imaginary time. Using the monodromy method, we show that finite
probe corrections of the form drastically alter both sides
of the ETH equality, replacing each thermal singularity with a pair of
branch-cuts. Via the branch-cuts, the vacuum blocks are connected to infinitely
many additional "saddles". We discuss and verify how such violent modification
in analytic structure leads to a natural guess for the blocks at finite : a
series of zeros that condense into branch cuts as . We also discuss
some interesting evidences connecting these to the Stoke's phenomena, which are
non-perturbative effects. As a related aspect of these probe
modifications, we also compute the Renyi-entropy in high energy
eigenstates on a circle. For subsystems much larger than the thermal length, we
obtain a WKB solution to the monodromy problem, and deduce from this the
entanglement spectrum.Comment: 35 pages, 40 figures, additional results and comments adde
Quantum critical metals in dimensions
We study the quantum theory of a Fermi surface coupled to a gapless boson
scalar in spacetime dimensions as a simple model for non-Fermi
liquids (NFL) near a quantum phase transition. Our analysis takes into account
the full backreaction from Landau damping of the boson, and obtains an RG flow
that proceeds through three distinct stages. Above the scale of Landau damping
the Fermi velocity flows to zero, while the coupling evolves according to its
classical dimension. Once damping becomes important, its backreaction leads to
a crossover regime where dynamic and static damping effects compete and the
fermion self-energy does not respect scaling. Below this crossover and having
tuned the boson to criticality, the theory flows to a scalar interacting
with a NFL. By increasing the number of bosonic flavors, the phase diagram near
the quantum critical point interpolates between a superconducting dome fully
covering the NFL behavior, and a phase where NFL effects become important
first, before the onset of superconductivity. A generic prediction of the
theory is that the Fermi velocity and quasiparticle residue vanish with a
power-law as the fixed point is approached. These features
may be useful for understanding some of the phenomenology of high
materials in a systematic --expansion.Comment: 38 pages, 6 figures. v2: comments and references added; version
published in PR
Black branes in flux compactifications
We construct charged black branes in type IIA flux compactifications that are
dual to (2+1)-dimensional field theories at finite density. The internal space
is a general Calabi-Yau manifold with fluxes, with internal dimensions much
smaller than the AdS radius. Gauge fields descend from the 3-form RR potential
evaluated on harmonic forms of the Calabi-Yau, and Kaluza-Klein modes decouple.
Black branes are described by a four-dimensional effective field theory that
includes only a few light fields and is valid over a parametrically large range
of scales. This effective theory determines the low energy dynamics, stability
and thermodynamic properties. Tools from flux compactifications are also used
to construct holographic CFTs with no relevant scalar operators, that can lead
to symmetric phases of condensed matter systems stable to very low
temperatures. The general formalism is illustrated with simple examples such as
toroidal compactifications and manifolds with a single size modulus. We
initiate the classification of holographic phases of matter described by flux
compactifications, which include generalized Reissner-Nordstrom branes,
nonsupersymmetric and hyperscaling violating solutions.Comment: 37 pages, 2 figures, typos corrected and comments adde
A modular toolkit for bulk reconstruction
We introduce new tools for studying modular flow in AdS/CFT. These tools
allow us to efficiently extract bulk information related to causality and
locality. For example, we discuss the relation between analyticity in modular
time and entanglement wedge nesting which can then be used to extract the
location of the Ryu-Takayanagi (RT) surface directly from the boundary theory.
Probing the RT surface close to the boundary our results reduce to the recent
proof of the Quantum Null Energy Condition. We focus on heavy probe operators
whose correlation functions are determined by spacelike geodesics. These
geodesics interplay with the RT surface via a set of rules that we conjecture
and give evidence for using the replica trick.Comment: 31 pages, 10 figures, v2: typos fixed and references adde
Metallic quantum critical points with finite BCS couplings
We study the fate of superconductivity in the vicinity of a class of metallic
quantum critical points obtained by coupling a Fermi surface to a critical
boson. In such systems there is a competition between the enhanced pairing
tendency due to the presence of long-range attractive interactions near
criticality, and the suppression of superconductivity due to the destruction of
the Landau quasiparticles. We show that there are regimes in which these two
effects offset one another, resulting in a novel non-Fermi liquid fixed point
with finite, scale invariant, BCS coupling. While these interactions lead to
substantial superconducting fluctuations, they do not drive the system into a
superconducting ground state. The metallic quantum critical fixed points are
connected to the superconducting regime by a continuous phase transition. These
results are established using a controlled expansion in the deviation from d=3
spatial dimensions, as well as in a large number N of internal flavors. We
discuss the possible relevance of our findings to the phenomenon of
superconducting domes condensing out of a non-Fermi liquid normal state near
quantum critical points.Comment: 28 pages, 7 figure
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