120 research outputs found
Impurity spin textures across conventional and deconfined quantum critical points of two-dimensional antiferromagnets
We describe the spin distribution in the vicinity of a non-magnetic impurity
in a two-dimensional antiferromagnet undergoing a transition from a
magnetically ordered Neel state to a paramagnet with a spin gap. The quantum
critical ground state in a finite system has total spin S=1/2 (if the system
without the impurity had an even number of S=1/2 spins), and recent numerical
studies in a double layer antiferromagnet (K. H.Hoglund et al.,
cond-mat/0611418) have shown that the spin has a universal spatial form
delocalized across the entire sample. We present the field theory describing
the uniform and staggered magnetizations in this spin texture for two classes
of antiferromagnets: (i) the transition from a Neel state to a paramagnet with
local spin singlets, in models with an even number of S=1/2 spins per unit
cell, which are described by a O(3) Landau-Ginzburg-Wilson field theory; and
(ii) the transition from a Neel state to a valence bond solid, in
antiferromagnets with a single S=1/2 spin per unit cell, which are described by
a deconfined field theory of spinons.Comment: 30 pages, 9 figure
Anomalous Axion Interactions and Topological Currents in Dense Matter
Recently an effective Lagrangian for the interactions of photons,
Nambu-Goldstone bosons and superfluid phonons in dense quark matter has been
derived using anomaly matching arguments. In this paper we illuminate the
nature of certain anomalous terms in this Lagrangian by an explicit microscopic
calculation. We also generalize the corresponding construction to introduce the
axion field. We derive an anomalous axion effective Lagrangian describing the
interactions of axions with photons and superfluid phonons in the dense matter
background. This effective Lagrangian, among other things, implies that an
axion current will be induced in the presence of magnetic field. We speculate
that this current may be responsible for the explanation of neutron star kicks.Comment: 10 page
Antiferromagnetism in metals: from the cuprate superconductors to the heavy fermion materials
The critical theory of the onset of antiferromagnetism in metals, with
concomitant Fermi surface reconstruction, has recently been shown to be
strongly coupled in two spatial dimensions. The onset of unconventional
superconductivity near this critical point is reviewed: it involves a subtle
interplay between the breakdown of fermionic quasiparticle excitations on the
Fermi surface, and the strong pairing glue provided by the antiferromagnetic
fluctuations. The net result is a logarithm-squared enhancement of the pairing
vertex for generic Fermi surfaces, with a universal dimensionless co-efficient
independent of the strength of interactions, which is expected to lead to
superconductivity at the scale of the Fermi energy. We also discuss the
possibility that the antiferromagnetic critical point can be replaced by an
intermediate `fractionalized Fermi liquid' phase, in which there is Fermi
surface reconstruction but no long-range antiferromagnetic order. We discuss
the relevance of this phase to the underdoped cuprates and the heavy-fermion
materials.Comment: Talk at SCES 2011; 19 pages, 12 figures; (v2) corrected typo
Sign-problem-free quantum Monte Carlo of the onset of antiferromagnetism in metals
The quantum theory of antiferromagnetism in metals is necessary for our
understanding of numerous intermetallic compounds of widespread interest. In
these systems, a quantum critical point emerges as external parameters (such as
chemical doping) are varied. Because of the strong coupling nature of this
critical point, and the "sign problem" plaguing numerical quantum Monte Carlo
(QMC) methods, its theoretical understanding is still incomplete. Here, we show
that the universal low-energy theory for the onset of antiferromagnetism in a
metal can be realized in lattice models, which are free from the sign problem
and hence can be simulated efficiently with QMC. Our simulations show Fermi
surface reconstruction and unconventional spin-singlet superconductivity across
the critical point.Comment: 17 pages, 4 figures; (v2) revised presentatio
Topological Objects in Two-component Bose-Einstein Condensates
We study the topological objects in two-component Bose-Einstein condensates.
We compare two competing theories of two-component Bose-Einstein condensate,
the popular Gross-Pitaevskii theory and the recently proposed gauge theory of
two-component Bose-Einstein condensate which has an induced vorticity
interaction. We show that two theories produce very similar topological
objects, in spite of the obvious differences in dynamics. Furthermore we show
that the gauge theory of two-component Bose-Einstein condensate, with the U(1)
gauge symmetry, is remarkably similar to the Skyrme theory. Just like the
Skyrme theory the theory admits the non-Abelian vortex, the helical vortex, and
the vorticity knot. We construct the lightest knot solution in two-component
Bose-Einstein condensate numerically, and discuss how the knot can be
constructed in the spin-1/2 condensate of atoms.Comment: 18 pages, 15 figures, Phys. Rev. A in pres
Hidden Fermi surfaces in compressible states of gauge-gravity duality
General scaling arguments, and the behavior of the thermal entropy density,
are shown to lead to an infrared metric holographically representing a
compressible state with hidden Fermi surfaces. This metric is characterized by
a general dynamic critical exponent, z, and a specific hyperscaling violation
exponent, \theta. The same metric exhibits a logarithmic violation of the area
law of entanglement entropy, as shown recently by Ogawa et al.
(arXiv:1111.1023). We study the dependence of the entanglement entropy on the
shape of the entangling region(s), on the total charge density, on temperature,
and on the presence of additional visible Fermi surfaces of gauge-neutral
fermions; for the latter computations, we realize the needed metric in an
Einstein-Maxwell-dilaton theory. All our results support the proposal that the
holographic theory describes a metallic state with hidden Fermi surfaces of
fermions carrying gauge charges of deconfined gauge fields.Comment: 33 pages, 5 figures; (v2) added refs, corrected typos, and modified
figure; (v3) added table summarizing result
Stellar spectroscopy: Fermions and holographic Lifshitz criticality
Electron stars are fluids of charged fermions in Anti-de Sitter spacetime.
They are candidate holographic duals for gauge theories at finite charge
density and exhibit emergent Lifshitz scaling at low energies. This paper
computes in detail the field theory Green's function G^R(w,k) of the
gauge-invariant fermionic operators making up the star. The Green's function
contains a large number of closely spaced Fermi surfaces, the volumes of which
add up to the total charge density in accordance with the Luttinger count.
Excitations of the Fermi surfaces are long lived for w <~ k^z. Beyond w ~ k^z
the fermionic quasiparticles dissipate strongly into the critical Lifshitz
sector. Fermions near this critical dispersion relation give interesting
contributions to the optical conductivity.Comment: 38 pages + appendices. 9 figure
Entanglement spectra of critical and near-critical systems in one dimension
The entanglement spectrum of a pure state of a bipartite system is the full
set of eigenvalues of the reduced density matrix obtained from tracing out one
part. Such spectra are known in several cases to contain important information
beyond that in the entanglement entropy. This paper studies the entanglement
spectrum for a variety of critical and near-critical quantum lattice models in
one dimension, chiefly by the iTEBD numerical method, which enables both
integrable and non-integrable models to be studied. We find that the
distribution of eigenvalues in the entanglement spectra agrees with an
approximate result derived by Calabrese and Lefevre to an accuracy of a few
percent for all models studied. This result applies whether the correlation
length is intrinsic or generated by the finite matrix size accessible in iTEBD.
For the transverse Ising model, the known exact results for the entanglement
spectrum are used to confirm the validity of the iTEBD approach. For more
general models, no exact result is available but the iTEBD results directly
test the hypothesis that all moments of the reduced density matrix are
determined by a single parameter.Comment: 6 pages, 5 figure
Quantum Criticality
This is a review of the basic theoretical ideas of quantum criticality, and
of their connection to numerous experiments on correlated electron compounds. A
shortened, modified, and edited version appeared in Physics Today. This arxiv
version has additional citations to the literature.Comment: 17 pages, 7 figures; (v2) added ref
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