arXiv subjects

E. Manousakis

Publications and source records attributed to E. Manousakis.

25 records · Page 2Linked to original sources

arXiv1998

Critical behavior of the planar magnet model in three dimensions

We use a hybrid Monte Carlo algorithm in which a single-cluster update is combined with the over-relaxation and Metropolis spin re-orientation algorithm. Periodic boundary conditions were applied in all directions. We have calculated the fourth-order cumulant in finite size lattices using the single-histogram re-weighting method. Using finite-size scaling theory, we obtained the critical temperature which is very different from that of the usual XY model. At the critical temperature, we calculated the susceptibility and the magnetization on lattices of size up to $42^3$. Using finite-size scaling theory we accurately determine the critical exponents of the model and find that $ν$=0.670(7), $γ/ν$=1.9696(37), and $β/ν$=0.515(2). Thus, we conclude that the model belongs to the same universality class with the XY model, as expected.

cond-mat.stat-mech↗

arXiv1998

Monolayer Solid Helium-4 Clusters on Graphite

In order to resolve the controversy about the low density region of the phase diagram of the 4He monolayer on graphite, we have undertaken a path integral Monte Carlo study of the system. We provide direct evidence that the low density monolayer possesses solid clusters and a low density vapor as opposed to the most recent proposal that the system is in a superfluid phase. We further establish that the rounded heat capacity peaks observed at low densities are caused by melting of such solid clusters and are not associated with the suggested superfluid transition.

cond-mat.mtrl-sci↗

arXiv1997

Phase separation at all interaction strengths in the t-J model

We investigate the phase diagram of the two-dimensional t-J model using a recently developed Green's Function Monte Carlo method for lattice fermions. We use the technique to calculate exact ground-state energies of the model on large lattices. In contrast to many previous studies, we find the model phase separates for all values of J/t. In particular, it is unstable at the hole dopings and interaction strengths at which the model was thought to describe the cuprate superconductors.

cond-mat.str-el↗

arXiv1996

Boundary effects on the scaling of the superfluid density

We study numerically the influence of the substrate (boundary conditions) on the finite--size scaling properties of the superfluid density $ρ_s$ in superfluid films of thickness $H$ within the XY model employing the Monte Carlo method. Our results suggest that the jump $ρ_s H/T_c$ at the Kosterlitz--Thouless transition temperature $T_c$ depends on the boundary conditions.

cond-mat↗

arXiv1995

Scaling of the specific heat in superfluid films

We study the specific heat of the $x-y$ model on lattices $L \times L \times H$ with $L \gg H$ (i.e. on lattices representing a film geometry) using the Cluster Monte--Carlo method. In the $H$--direction we apply Dirichlet boundary conditions so that the order parameter in the top and bottom layers is zero. We find that our results for the specific heat of various thickness size $H$ collapse on the same universal scaling function. The extracted scaling function of the specific heat is in good agreement with the experimentally determined universal scaling function using no free parameters.

cond-mat↗

arXiv1995

The specific heat of superfluids near the transition temperature

The specific heat of the $x-y$ model is studied on cubic lattices of sizes $L \times L \times L$ and on lattices $L \times L \times H$ with $L \gg H$ (i.e. on lattices representing a film geometry) using the Cluster Monte Carlo method. Periodic boundary conditions were applied in all directions. In the cubic case we obtained the ratio of the critical exponents $α/ν$ from the size dependence of the energy density at the critical temperature $T_λ$. Using finite--size scaling theory, we find that while for both geometries our results scale to universal functions, these functions differ for the different geometries. We compare our findings to experimental results and results of renormalization group calculations.

cond-mat↗

arXiv1994

Crossover from Two- to Three-Dimensional Behavior in Superfluids

We have studied the superfluid density $ρ_{s}$ on various size-lattices in the geometry $L \times L \times H$ by numerical simulation of the $x-y$ model using the Cluster Monte Carlo method. Applying the Kosterlitz-Thouless-Nelson renormalization group equations for the superfluid density we have been able to extrapolate to the $L \to \infty$ limit for a given value of $H$. In the superfluid phase we find that the superfluid density faithfully obeys the expected scaling law with $H$, using the experimental value for the critical exponent $ν=0.6705$. For the sizes of film thickness studied here the critical temperature $T_{c}$ and the coefficient $b$ entering the equation $T/(ρ_{s} H) \propto 1-b(1-T/T_{c})^{1/2}$ are in agreement with the expected $H$-dependence deduced from general scaling ideas.

cond-mat↗