23 research outputs found
Conformal Truncation of Chern-Simons Theory at Large
We set up and analyze the lightcone Hamiltonian for an abelian Chern-Simons
field coupled to fermions in the limit of large using conformal
truncation, i.e. with a truncated space of states corresponding to primary
operators with dimension below a maximum cutoff . In both the
Chern-Simons theory, and in the model at infinite , we compute the
current spectral functions analytically as a function of and
reproduce previous results in the limit that the truncation
is taken to . Along the way, we determine how to preserve gauge
invariance and how to choose an optimal discrete basis for the momenta of
states in the truncation space.Comment: 32+25 pages, 8 figures. v2: updated ref
Thermalization, Viscosity and the Averaged Null Energy Condition
We explore the implications of the averaged null energy condition for thermal
states of relativistic quantum field theories. A key property of such thermal
states is the thermalization length. This lengthscale generalizes the notion of
a mean free path beyond weak coupling, and allows finite size regions to
independently thermalize. Using the eigenstate thermalization hypothesis, we
show that thermal fluctuations in finite size `fireballs' can produce states
that violate the averaged null energy condition if the thermalization length is
too short or if the shear viscosity is too large. These bounds become very weak
with a large number N of degrees of freedom but can constrain real-world
systems, such as the quark-gluon plasma.Comment: 28 pages, 3 figure
Hydrodynamic theory of quantum fluctuating superconductivity
A hydrodynamic theory of transport in quantum mechanically phase-disordered
superconductors is possible when supercurrent relaxation can be treated as a
slow process. We obtain general results for the frequency-dependent
conductivity of such a regime. With time-reversal invariance, the conductivity
is characterized by a Drude-like peak, with width given by the supercurrent
relaxation rate. Using the memory matrix formalism, we obtain a formula for
this width (and hence also the dc resistivity) when the supercurrent is relaxed
by short range Coulomb interactions. This leads to a new -- effective field
theoretic and fully quantum -- derivation of a classic result on flux flow
resistance. With strong breaking of time-reversal invariance, the optical
conductivity exhibits what we call a `hydrodynamic supercyclotron' resonance.
We obtain the frequency and decay rate of this resonance for the case of
supercurrent relaxation due to an emergent Chern-Simons gauge field. The
supercurrent decay rate in this `topologically ordered superfluid vortex
liquid' is determined by the conductivities of the normal component of the
liquid. Our work gives a controlled framework for low temperature metallic
phases arising from phase-disordered superconductivity.Comment: 1 + 44 pages. 2 figures. v2 discussion improved in places. v3 sign
errors fixed in section
Superfluids as Higher-form Anomalies
We recast superfluid hydrodynamics as the hydrodynamic theory of a system
with an emergent anomalous higher-form symmetry. The higher-form charge counts
the winding planes of the superfluid -- its constitutive relation replaces the
Josephson relation of conventional superfluid hydrodynamics. This formulation
puts all hydrodynamic equations on equal footing. The anomalous Ward identity
can be used as an alternative starting point to prove the existence of a
Goldstone boson, without reference to spontaneous symmetry breaking. This
provides an alternative characterization of Landau phase transitions in terms
of higher-form symmetries and their anomalies instead of how the symmetries are
realized. This treatment is more general and, in particular, includes the case
of BKT transitions. As an application of this formalism we construct the
hydrodynamic theories of conventional (0-form) and 1-form superfluids.Comment: 29 pages; v3 corrected Eq. (1.25), published versio
Damping of Pseudo-Goldstone Fields
International audienceApproximate symmetries abound in nature. If these symmetries are also spontaneously broken, the would-be Goldstone modes acquire a small mass, or inverse correlation length, and are referred to as pseudo-Goldstones. At nonzero temperature, the effects of dissipation can be captured by hydrodynamics at sufficiently long scales compared to the local equilibrium. Here, we show that, in the limit of weak explicit breaking, locality of hydrodynamics implies that the damping of pseudo-Goldstones is completely determined by their mass and diffusive transport coefficients. We present many applications: superfluids, QCD in the chiral limit, Wigner crystal and density wave phases in the presence of an external magnetic field or not, nematic phases, and (anti)ferromagnets. For electronic density wave phases, pseudo-Goldstone damping generates a contribution to the resistivity independent of the strength of disorder, which can have a linear temperature dependence provided the associated diffusivity saturates a bound. This is reminiscent of the phenomenology of strange metal high-Tc superconductors, where charge density waves are observed across the phase diagram