arXiv subjects
Publications and source records attributed to G. Koutsoumbas.
We study the Abelian Higgs Model using an improved form of the action in the scalar sector. The subleading corrections are carefully analysed and the connection between lattice and continuous parameters is worked out. The simulation shows a remarkable improvement of the numerical performance.
In this paper, we discuss the role of an external magnetic field on the dynamically generated fermion mass in even-flavor QED in three space-time dimensions. Based on some reasonable approximations, we present analytic arguments on the fact that, for weak fields, the magnetically-induced mass increases quadratically with increasing field, while at strong fields one crosses over to a mass scaling logarithmically with the external field. We also confirm this type of scaling behavior through quenched lattice calculations using the non-compact version for the gauge field. Both the zero and finite temperature cases are examined. A preliminary study of the fermion condensate in the presence of magnetic flux tubes on the lattice is also included.
The magnetically induced fermionic condensate is studied at zero and finite temperatures. The effects of a non-homogeneous external magnetic fields is briefly considered.
The enhancement of the fermionic condensate due to the presence of both homogeneous and non-homogeneous external magnetic fields is studied for three-dimensional QED.
We perform Monte Carlo simulations of a four-dimensional gauge invariant spin system which describes random surfaces with gonihedric action. We develop the analogy between the flat-crumpled phase transition of the lattice surface model and the liquid-gas phase transition of non-ideal gases, and identify the self-intersection coupling constant $k$ of the surface model with the pressure $P$. As $k$ increases the system moves to a critical point in complete analogy with the situation for non-ideal gases, where the liquid and the gas phases approach each other with increasing $P$. We measure vacuum expectation values of various operators and the corresponding critical indices.
We suggest that a spin-charge separating ansatz, leading to non-Abelian $SU(2) \otimes U_S(1)$ gauge symmetries in doped antiferromagnets, proposed earlier as a way of describing Kosterlitz-Thouless superconducting gaps at the nodes of the gap of d-wave (high-T_c) superconductors, may also lead to a pseudogap phase, characterised by the formation of (non-superconducting) pairing and the absence of phase coherence. The crucial assumption is again the presence of electrically charged Dirac fermionic excitations (holons) about the points of the (putative) fermi surface in the pertinent phase of the superconductor. We present arguments in support of the rôle of non-perturbative effects (instantons) on the onset of the pseudogap phase. As a means of probing such gauge interactions experimentally, we perform a study of the scaling of the thermal conductivity with an externally-applied magnetic field, in certain effective models involving gauge and/or four-fermion (contact) interactions.
We perform a lattice study, in the quenched approximation, of dynamical mass generation in a system of relativistic (Dirac) fermions, coupled to an Abelian gauge field in (2+1)-dimensions, in the presence of an external (constant) magnetic field, perpendicular to the spatial planes. It is shown that a strong magnetic field catalyzes chiral symmetry breaking, in agreement with results in the continuum. The rôle of the higher-Landau poles in inducing a critical temperature above which the phenomenon disappears is pointed out. We also discuss the implications of this model on the opening of a gap in doped antiferromagnetic superconductors.
We report on a numerical investigation of the SU(2) gauge theory with gluinos. The low-lying spectrum in bosonic and fermionic channels is determined. Improvements of the multi-bosonic algorithm are discussed.
We perform Monte Carlo simulations of a gauge invariant spin system which describes random surfaces with gonihedric action in four dimensions. The Hamiltonian is a mixture of one-plaquette and additional two- and three-plaquette interaction terms with specially adjusted coupling constants. For the system with the large self-intersection coupling constant $k$ we observe the second-order phase transition at temperature $β_{c}\simeq 1.75$. The string tension is generated by quantum fluctuations as it was expected theoretically. This result suggests the existence of a noncritical string in four dimensions. For smaller values of $k$ the system undergoes the first order phase transition and for $k$ close to zero exhibits a smooth crossover.
We study the non-compact version of the U(1) gauge-Higgs model in three dimensions for $m_H = 30 GeV.$ We found that, using this formulation, rather modest lattices approach quite well the infinite volume behaviour.The phase transition is first order, as expected for this Higgs mass. The latent heat (in units of $T_{cr}^4$) is compatible with the predictions of the two-loop effective potential; it is an order of magnitude less than the corresponding SU(2) value. The transition temperature and $<φ^* φ>$ in units of the critical temperature are also compatible with the perturbative results.
As a preparation for the numerical study of the SU(2) gauge theory with gluinos, the spectral properties of the fermion matrix and the masses of pseudoscalar and scalar states are investigated in the quenched approximation. The behaviour of the disconnected fermion diagrams on small lattices is also studied.
We analyse the curvature representation of the gonihedric action $A(M)$ for the cases when the dependence on the dihedral angle is arbitrary.
We study the behaviour of lattice momentum-space gluon propagators for a pure $SU(2)$ gauge theory at finite temperature. We find out that the magnetic mass is $0.26 g^2(T) T$; we have repeated the same calculations in three dimensions.
We examine an extension of the ideas of quantum cosmology and, in particular, the proposal of Hartle and Hawking for the boundary conditions of the Universe, to models which incorporate Yang-Mills fields. Inhomogeneous perturbations about a homogeneous, isotropic minisuperspace background model are considered, by expanding the Yang-Mills fields in harmonics of the spatial directions which are taken to be three-spheres. The expansions are made explicit for $SO(N)$ gauge fields thereby obtaining formulae compatible with the formalism conventionally used in quantum cosmology. We apply these results to the gauge group $SO(3)$ and derive the Lagrangian and the semi-classical wave function for this special case.
We study the spectrum of the Dirac Hamiltonian in the presence of high temperature sphaleron-like fluctuations of the electroweak gauge and Higgs fields, relevant for the conditions prevailing in the early universe. The fluctuations are created by numerical lattice simulations. It is shown that a change in Chern-Simons number by one unit is accompanied by eigenvalues crossing zero and a change of sign of the generalized chirality $\tGf= (-1)^{2T+1} \gf$ which labels these modes. This provides further evidence that the sphaleron-like configurations observed in lattice simulations may be viewed as representing continuum configurations.