55 research outputs found
Thermodynamics of the BCS-BEC crossover
We present a self-consistent theory for the thermodynamics of the BCS-BEC
crossover in the normal and superfluid phase which is both conserving and
gapless. It is based on the variational many-body formalism developed by
Luttinger and Ward and by DeDominicis and Martin. Truncating the exact
functional for the entropy to that obtained within a ladder approximation, the
resulting self-consistent integral equations for the normal and anomalous Green
functions are solved numerically for arbitrary coupling. The critical
temperature, the equation of state and the entropy are determined as a function
of the dimensionless parameter , which controls the crossover from the
BCS-regime of extended pairs to the BEC-regime of tightly bound molecules. The
tightly bound pairs turn out to be described by a Popov-type approximation for
a dilute, repulsive Bose gas. Even though our approximation does not capture
the critical behaviour near the continuous superfluid transition, our results
provide a consistent picture for the complete crossover thermodynamics which
compare well with recent numerical and field-theoretic approaches at the
unitarity point.Comment: published versio
Comment on electron spectral function and algebraic spin liquid for the normal state of underdoped high Tc superconductors (multiple letters
In a recent Letter [1], Rantner and Wen made a theoretical prediction of the power-law behavior of the electron spectral function in the pseudogap phase of underdoped cuprates, reminiscent of that in the one-dimensional Luttinger liquid
Effect of gauge boson mass on the phase structure of QED
Dynamical chiral symmetry breaking (DCSB) in QED with finite gauge
boson mass is
studied in the framework of the rainbow approximation of Dyson-Schwinger
equations.
By adopting a simple gauge boson propagator ansatz at finite temperature, we
first numerically solve the
Dyson-Schwinger equation for the fermion self-energy to
determine the chiral phase diagram of QED with finite gauge boson mass
at finite chemical potential and finite temperature, then we study the
effect of the finite gauge mass on the phase diagram of QED. It is found
that the gauge boson mass suppresses the occurrence of
DCSB. The area of the region in the chiral phase diagram corresponding to
DCSB phase decreases as
the gauge boson mass increases. In
particular, chiral symmetry gets restored when is above a
certain critical value. In this paper, we use DCSB to describe the
antiferromagnetic order and use the gauge boson mass to describe the
superconducting order. Our results give qualitatively a physical
picture on the competition and coexistence between antiferromagnetic
order and superconducting orders in high temperature cuprate superconductors.Comment: 10 pages, 2 figure
Entanglement Entropy of 3-d Conformal Gauge Theories with Many Flavors
Three-dimensional conformal field theories (CFTs) of deconfined gauge fields
coupled to gapless flavors of fermionic and bosonic matter describe quantum
critical points of condensed matter systems in two spatial dimensions. An
important characteristic of these CFTs is the finite part of the entanglement
entropy across a circle. The negative of this quantity is equal to the finite
part of the free energy of the Euclidean CFT on the three-sphere, and it has
been proposed to satisfy the so called F-theorem, which states that it
decreases under RG flow and is stationary at RG fixed points. We calculate the
three-sphere free energy of non-supersymmetric gauge theory with a large number
N_F of bosonic and/or fermionic flavors to the first subleading order in 1/N_F.
We also calculate the exact free energies of the analogous chiral and
non-chiral {\cal N} = 2 supersymmetric theories using localization, and find
agreement with the 1/N_F expansion. We analyze some RG flows of supersymmetric
theories, providing further evidence for the F-theorem.Comment: 31 pages, 2 figures; v2 refs added, minor change
A gauge invariant and string independent fermion correlator in the Schwinger model
We introduce a gauge invariant and string independent two-point fermion
correlator which is analyzed in the context of the Schwinger model (QED_2). We
also derive an effective infrared worldline action for this correlator, thus
enabling the computation of its infrared behavior. Finally, we briefly discuss
possible perspectives for the string independent correlator in the QED_3
effective models for the normal state of HTc superconductors.Comment: 14 pages, LaTe
Electron spectral function and algebraic spin liquid for the normal state of underdoped high superconductors
We propose to describe the spin fluctuations in the normal state of
underdoped high superconductors as a manifestation of an algebraic spin
liquid. We have performed calculations within the slave-boson model to support
our proposal. Under the spin-charge separation picture, the normal state (the
spin-pseudogap phase) is described by massless Dirac fermions, charged bosons,
and a gauge field. We find that the gauge interaction is a marginal
perturbation and drives the mean-field free-spinon fixed point to a more
complicated spin-quantum-fixed-point -- the algebraic spin liquid, where
gapless excitations interact at low energies. The electron spectral function in
the normal state was found to have a Luttinger-liquid-like line shape as
observed in experiments. The spectral function obtained in the superconducting
state shows how a coherent quasiparticle peak appears from the incoherent
background as spin and charge recombine.Comment: 4 pages, 3 figures. published versio
High Superconductivity, Skyrmions and the Berry Phase
It is here pointed out that the antiferromagnetic spin fluctuation may be
associated with a gauge field which gives rise to the antiferromagnetic ground
state chirality. This is associated with the chiral anomaly and Berry phase
when we consider the two dimensional spin system on the surface of a 3D sphere
with a monopole at the centre. This realizes the RVB state where spinons and
holons can be understood as chargeless spins and spinless holes attached with
magnetic flux. The attachment of the magnetic flux of the charge carrier
suggest, that this may be viewed as a skyrmion. The interaction of a massless
fermion representing a neutral spin with a gauge field along with the
interaction of a spinless hole with the gauge field enhances the
antiferromagnetic correlation along with the pseudogap at the underdoped
region. As the doping increases the antiferromagnetic long range order
disappears for the critical doping parameter . In this framework,
the superconducting pairing may be viewed as caused by skyrmion-skyrmion bound
states.Comment: 10 pages, accepted in Phys. Rev.
Effect of gauge boson mass on chiral symmetry breaking in QED
In three-dimensional quantum electrodynamics (QED) with massive gauge
boson, we investigate the Dyson-Schwinger equation for the fermion self-energy
in the Landau gauge and find that chiral symmetry breaking (CSB) occurs when
the gauge boson mass is smaller than a finite critical value
but is suppressed when . We further show that the critical
value does not qualitatively change after considering higher order
corrections from the wave function renormalization and vertex function. Based
on the relation between CSB and the gauge boson mass , we give a field
theoretical description of the competing antiferromagnetic and superconducting
orders and, in particular, the coexistence of these two orders in high
temperature superconductors. When the gauge boson mass is generated via
instanton effect in a compact QED of massless fermions, our result shows
that CSB coexists with instanton effect in a wide region of , which can be
used to study the confinement-deconfinement phase transition.Comment: 34 pagess, 2 figure
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