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Thomas Schaefer

Publications and source records attributed to Thomas Schaefer.

At least 91 records · Page 5Linked to original sources
arXiv2008

Density Functional Theory for non-relativistic Fermions in the Unitarity Limit

We derive an energy density functional for non-relativistic spin one-half fermions in the limit of a divergent two-body scattering length. Using an epsilon expansion around d=4-epsilon spatial dimensions we compute the coefficient of the leading correction beyond the local density approximation (LDA). In the case of N fermionic atoms trapped in a harmonic potential this correction has the form E=E_(LDA)(1+c_s (3N)^(-2/3)), where E_(LDA) is the total energy in LDA approximation. At next-to-leading order in the epsilon expansion we find c_s=1.68, which is significantly larger than the result for non-interacting fermions, c_s=0.5.

nucl-th↗
arXiv2008

From Equilibrium to Transport Properties of Strongly Correlated Fermi Liquids

We summarize recent results regarding the equilibrium and non-equilibrium behavior of cold dilute atomic gases in the limit in which the two body scattering length a goes to infinity. In this limit the system is described by a Galilean invariant (non-relativistic) conformal field theory. We discuss the low energy effective lagrangian appropriate to the limit a->infinity, and compute low energy coefficients using an epsilon-expansion. We also show how to combine the effective lagrangian with kinetic theory in order to compute the shear viscosity, and compare the kinetic theory predictions to experimental results extracted from the damping of collective modes in trapped Fermi gases.

nucl-th↗
arXiv2008

Euclidean Correlation Functions in a Holographic Model of QCD

We compute euclidean coordinate space correlation functions in a holographic model of QCD. We concentrate, in particular, on channels that are related to the U(1)_A problem, the flavor-singlet axialvector, pseudoscalar meson, and pseudoscalar glueball (topological charge) correlator. We find that even a very simple holographic model defined on a slice of AdS_5 provides a qualitatively correct description of QCD correlation functions. We study the role of anomaly terms, and show that both euclidean positivity and low energy theorems based on the axial anomaly relation are correctly implemented. We compare the results with expectations from an instanton model of the QCD vacuum.

hep-ph↗
arXiv2008

The Shear Viscosity to Entropy Density Ratio of Trapped Fermions in the Unitarity Limit

We extract the shear viscosity to entropy density ratio η/s of cold fermionic atoms in the unitarity limit from experimental data on the damping of collective excitations. We find that near the critical temperature η/s is roughly equal to 1/2 in units of \hbar/k_B. With the possible exception of the quark gluon plasma, this value is closer to the conjectured lower bound 1/(4π) than any other known liquid.

cond-mat.other↗
arXiv2008

Color superconductivity in dense quark matter

Matter at high density and low temperature is expected to be a color superconductor, which is a degenerate Fermi gas of quarks with a condensate of Cooper pairs near the Fermi surface that induces color Meissner effects. At the highest densities, where the QCD coupling is weak, rigorous calculations are possible, and the ground state is a particularly symmetric state, the color-flavor locked (CFL) phase. The CFL phase is a superfluid, an electromagnetic insulator, and breaks chiral symmetry. The effective theory of the low-energy excitations in the CFL phase is known and can be used, even at more moderate densities, to describe its physical properties. At lower densities the CFL phase may be disfavored by stresses that seek to separate the Fermi surfaces of the different flavors, and comparison with the competing alternative phases, which may break translation and/or rotation invariance, is done using phenomenological models. We review the calculations that underlie these results, and then discuss transport properties of several color-superconducting phases and their consequences for signatures of color superconductivity in neutron stars.

hep-ph↗
arXiv2007

Bulk viscosity due to kaons in color-flavor-locked quark matter

We calculate the bulk viscosity of color-superconducting quark matter in the color-flavor-locked (CFL) phase. We assume that the lightest bosons are the superfluid mode H and the kaons K0 and K+, and that there is no kaon condensate. We calculate the rate of strangeness-equilibrating processes that convert kaons into superfluid modes, and the resultant bulk viscosity. We find that for oscillations with a timescale of milliseconds, at temperatures T << 1 MeV, the CFL bulk viscosity is much less than that of unpaired quark matter, but at higher temperatures the bulk viscosity of CFL matter can become larger.

nucl-th↗
arXiv2007

Shear viscosity of a superfluid Fermi gas in the unitarity limit

We compute the shear viscosity of a superfluid atomic Fermi gas in the unitarity limit. The unitarity limit is characterized by a divergent scattering length between the atoms, and it has been argued that this will result in a very small viscosity. We show that in the low temperature T limit the shear viscosity scales as xi^5/T^5, where the universal parameter 'xi' relates the chemical potential and the Fermi energy, mu=xi E_F. Combined with the high temperature expansions of the viscosity our results suggest that the viscosity has a minimum near the critical temperature T_c. A naive extrapolation indicates that the minimum value of the ratio of viscosity over entropy density is within a factor of ~ 5 of the proposed lower bound hbar/(4πk_B).

cond-mat.other↗
arXiv2007

Perfect Fluidity in Atomic Physics

Experimental results obtained at the Relativistic Heavy Ion Collider (RHIC) have been interpreted in terms of a strongly interacting quark gluon plasma. The strongly interacting plasma is characterized by ``perfect fluidity'', i.e. a ratio of shear viscosity to entropy density that saturates a proposed lower bound. In this contribution we explore the possibility that a similar phenomenon takes place in a strongly coupled non-relativistic Fermi liquid in which the scattering length between the Fermions is infinitely large.

nucl-th↗
arXiv2007

What atomic liquids can teach us about quark liquids

We discuss some aspects of cold atomic Fermi gases in the unitarity limit that are of interest in connection with the physics of quark matter and the quark gluon plasma. We consider, in particular, the equation of state, transport properties, the critical temperature for pair condensation, and the response to a pair breaking stress.

hep-ph↗
arXiv2006

Goldstone boson currents in a kaon condensed CFL phase

We study the stability of the kaon condensed color-flavor locked (CFL) phase of dense quark matter with regard to the formation of a non-zero Goldstone boson current. In the kaon condensed phase there is an electrically charged fermion which becomes gapless near μ_s^(1) \simeq 1.35Δand a neutral fermion which becomes gapless near μ_s^(2)\simeq 1.61Δ. Here, μ_s=m_s^2/(2p_F) is the shift in the Fermi energy due to the strange quark mass m_s and Δis the gap in the chiral limit. The transition to the gapless phase is continuous at μ_s^(1) and first order at μ_s^(2). We find that the magnetic screening masses are real in the regime μ_s< μ_s^(2), but some screening masses are imaginary for μ_s> μ_s^(2). We show that there is a very weak current instability for μ_s>μ_s^(1) and a more robust instability in a small window near μ_s^(2). We also show that in the Goldstone boson current phase all components of the magnetic screening mass are real. There is a range of values of μ_s below 2Δin which the magnetic gluon screening masses are imaginary but the phase is stable with respect to electrically neutral fluctuations of the gauge field.

hep-ph↗
arXiv2006

Polarized fermions in the unitarity limit

We consider a polarized Fermi gas in the unitarity limit. Results are calculated analytically up to next-to-leading order in an expansion about d=4 spatial dimensions. We find a first order transition from superfluid to normal phase. The critical chemical potential asymmetry for this phase transition is delta_mu_c= 2/(mu epsilon)*(1-0.467ε), where epsilon=4-d is the expansion parameter and 'mu' is the average chemical potential of the two fermion species. Stability of the superfluid phase in the presence of supercurrents is also studied.

cond-mat.other↗
arXiv2006

From Trapped Atoms to Liberated Quarks

We discuss some aspects of cold atomic gases in the unitarity limit that are of interest in connection with the physics of dense hadronic matter. We consider, in particular, the equation of state at zero temperature, the magnitude of the pairing gap, and the phase diagram at non-zero polarization.

nucl-th↗
arXiv2006

Effective Theories of Dense and Very Dense Matter

In these lectures we discuss the uses of effective field theory in studying dense hadronic matter. We focus on two regimes in the phase diagram. At low baryon density nucleons can be described as non-relativistic point particles interacting via short range forces. Interesting effects arise because of the large nucleon-nucleon scattering length. At very large density QCD matter can be described in terms of quarks and gluons. Unusual many body effects arise because of long range gluons fields, and because of the way color and flavor get entangled in the superconducting state.

nucl-th↗
arXiv2006

The Kohn-Luttinger Effect in Gauge Theories

Kohn and Luttinger showed that a many body system of fermions interacting via short range forces becomes superfluid even if the interaction is repulsive in all partial waves. In gauge theories such as QCD the interaction between fermions is long range and the assumptions of Kohn and Luttinger are not satisfied. We show that in a U(1) gauge theory the Kohn-Luttinger phenomenon does not take place. In QCD attractive channels always exist, but there are cases in which the primary pairing channel leaves some fermions ungapped. As an example we consider the unpaired fermion in the 2SC phase of QCD with two flavors. We show that it acquires a very small gap via a mechanism analogous to the Kohn-Luttinger effect. The gap is too small to be phenomenologically relevant.

hep-ph↗
arXiv2006

Non-Fermi Liquid Effective Field Theory of Dense QCD Matter

We review an effective field theory for the non-Fermi liquid regime of dense QCD matter. Non-Fermi liquid effects arise due the presence of unscreened magnetic gluon exchanges. We show that there is a systematic low energy expansion in fractional powers and logarithms of energy. We discuss the validity of some standard theorems of Fermi liquid theory.

hep-ph↗
arXiv2006

The CFL phase and m_s: An effective field theory approach

We study the phase diagram of dense quark matter with an emphasis on the role of the strange quark mass. Our approach is based on two effective field theories (EFTs). The first is an EFT that describes quark quasi-particles near the Fermi surface. This EFT is valid at energies small compared to the chemical potential. The second is an EFT for the Goldstone modes in the paired phase. We find that in response to a non-zero strange quark mass the CFL phase first undergoes a transition to a kaon condensed phase, and then to a gapless phase with a non-zero Goldstone boson current.

nucl-th↗
arXiv2006

Meson Supercurrent State in High Density QCD

We study the effect of a non-zero strange quark mass on the color-flavor-locked (CFL) phase of high density quark matter. We have previously shown that for a strange quark mass $m_s\sim m_u^{1/3}Δ^{2/3}$ the CFL state becomes unstable toward the formation of a neutral kaon condensate. Recently, several authors discovered that for $m_s\sim (2Δp_F)^{1/2}$ the CFL state contains gapless fermions, and that the gapless modes lead to an instability in current-current correlation functions. Using an effective theory of the CFL state we demonstrate that this instability is resolved by the formation of an inhomogeneous meson condensate, analogous to Migdal's p-wave pion condensate. This state has a non-zero meson current which is canceled by a backflow of gapless fermions.

hep-ph↗