Study of compact QED with light fermions

We present a numerical study of compact QED including dynamical fermions of mass = 0.10 (lattice units) on a 6⁴ lattice. Staggered lattice fermions and the Wilson action and mixed gauge actions were studied. The simulations used two modified hybrid methods: one with stochastic fields and the other with deterministic ones. Working with a variable number of flavors we found that the chiral transition which separates the strong and weak coupling phases is of first order in all cases. The strength of these discontinuous transitions increase with the number of flavors.     

First-order chiral phase transition in lattice QCD

The chiral phase transition in lattice QCD has been studied for light fermions of mass ma=0.025 on lattices of size 4⁴ and 8³×4 using the hybrid algorithm. We find evidence for a first-order chiral phase transition with a large latent heat. A comparison with 10³×6 data shows violations of asymptotic scaling for T_ch which are similar in magnitude to those observed in the pure gauge sector.     

A numerical study of lattice QED with dynamical fermions

Using the pseudo-fermion method U(1) lattice gauge theory with dynamical staggered fermions is studied. The plaqette energy, the chiral order parameter and the Polyakov line have been measured. With fermions of mass 0.20 and 0.25 a phase transition is observed, separating the strong coupling regime from a phase where chiral symmetry is restored.     

Charge polarization in compact lattice QED

We investigate the polarization cloud around static electric charges in the framework of compact lattice QED. We calculate the correlations of Polyakov loops with the electric charge densityψ ^? ψ normalized to the chiral condensate. It turns out that virtual anticharges screen static charges as expected from continuum QED. Charge polarization is more pronounced at strong couplings and extends to a longer range on the lattice in the weak coupling phase. In both phases the correlations decrease with larger fermion mass.     

Phase transition in four-dimensional compact QED

The energy and the specific heat of the four-dimensional U(1) lattice gauge model is evaluated by Monte Carlo simulations on lattices of size L⁴, where L = 4, 5 and 6, evidence is presented for the occurence of a second-order phase transition. A finite size scaling analysis of our results gives the critical value of the coupling constant e²_c = 0.995 and a correlation length exponent $\text{v ≈}\text{1}\text{3}$.     

Confinement and vacuum structure in compact QED on the lattice

Using an effective action and assuming that the physical ground state has no structure, we demonstrate that the spectrum of the compact U(1) gauge model has massless (vector and scalar) bosons.     

Finite temperature behavior of topological excitations in lattice compact QED

We study the deconfining phase transition at finite temperature in compact QED in lagrangian formulation. Expressing the partition function in terms of topological excitations we explicitly show that at large temperature these become the vortices of a system in one less dimension. An intuitive picture of the phase transition is discussed.     

First-order phase transitions in large entropy lattice models

We show the existence of a first-order phase transition in thev-dimensional Potts model forv≧2, when the number of states of a single spin is big enough. Low-temperature pure phases are proved to survive up to the critical temperature. Also the existence of a first-order transition in thev-dimensional Potts gauge model,v≧3, is obtained if the underlying gauge group is finite but large.     

First-order reorientation transition of the flux-line lattice in CaAlSi

The flux-line lattice in CaAlSi has been studied by small-angle neutron scattering. A well-defined hexagonal flux-line lattice is seen just above H(c1) in an applied field of only 54 Oe. A 30° reorientation of this vortex lattice has been observed in a very low field of 200 Oe. This reorientation transition appears to be first-order and could be explained by nonlocal effects. The magnetic field dependence of the form factor is well-described by a single penetration depth of λ=1496(1) ? and a single coherence length of ξ=307(1) ? at 2 K. At 1.5 K, the penetration depth anisotropy is γ(λ)=2.7(1), with the field applied perpendicular to the c axis, and agrees with the coherence length anisotropy determined from critical field measurements.     

First-order superconducting phase transition in CeCoIn5

The superconducting phase transition in heavy fermion CeCoIn5 (T(c)=2.3 K in zero field) becomes first order when the magnetic field H parallel [001] is greater than 4.7 T, and the transition temperature is below T0 approximately 0.31T(c). The change from second order at lower fields is reflected in strong sharpening of both specific heat and thermal expansion anomalies associated with the phase transition, a strong magnetocaloric effect, and a steplike change in the sample volume. This effect is due to Pauli limiting in a type-II superconductor, and was predicted theoretically in the mid-1960s.