arXiv subjects

Efstratios Manousakis

Publications and source records attributed to Efstratios Manousakis.

72 records · Page 4Linked to original sources

arXiv2003

Heat capacity scaling function for confined superfluids

We study the specific heat scaling function of superfluids confined in cubic geometry and in parallel-plate (film) geometry with open boundary conditions (BC) along the finite dimensions using Monte Carlo simulation. For the case of cubic geometry for the superfluid order parameter we apply open BC in all three directions. We also calculated the specific heat scaling function for the parallel-plate confinement using open BC along the finite dimension and periodic BC along the other two dimensions and we find it to be very close to the earlier calculated using Dirichlet instead of open BC. We find that the specific heat scaling function is significantly different for the two different geometries studied. In addition, we generally find that the scaling function for a fixed given geometry when calculated with open BC is quite close to that calculated with Dirichlet BC, while when calculated with periodic BC is quite different. Our results for both the scaling functions obtained for the parallel-plate geometry and for cubic geometry with open BC along the finite dimensions are in very good agreement with the recent very high quality experimental measurements with no free parameters.

cond-mat.str-el↗

arXiv2002

Commensurate-incommensurate transitions in quantum films: Submonolayer molecular hydrogen on graphite

We have used the Path Integral Monte Carlo method to simulate a monolayer of molecular hydrogen on graphite above 1/3 submonolayer coverage. We find that at low temperature and as the coverage increases the system undergoes a series of transformations starting from the $\sqrt{3} \times \sqrt{3}$ commensurate solid near 1/3 coverage. First, a phase is formed which is characterized by a uniaxially compressed incommensurate solid with additional mass density modulations along the same direction which can be viewed as an ordered domain-wall solid with a characteristic domain-wall distance which depends on the surface coverage. At low temperature and higher coverage there is a transition to an incommensurate solid which is rotated relative to the substrate commensurate lattice. As a function of temperature the domain-wall ordering first melts into a fluid of domain-walls and at higher temperature the solid melts into a uniform fluid. Regardless of the large amplitude of quantum fluctuations, these phase transitions are analogous to those in classical monolayer films. Our calculated values for the surface coverage and temperature where these transitions occur, the calculated structure factors and specific heat are in general agreement with the available experimental results with no adjustable parameteters.

cond-mat.str-el↗

arXiv2002

Hole-pair hopping in arrangements of hole-rich/hole-poor domains in a quantum antiferromagnet

We study the motion of holes in a doped quantum antiferromagnet in the presence of arrangements of hole-rich and hole-poor domains such as the stripe-phase in high-$T_C$ cuprates. When these structures form, it becomes energetically favorable for single holes, pairs of holes or small bound-hole clusters to hop from one hole-rich domain to another due to quantum fluctuations. However, we find that at temperature of approximately 100 K, the probability for bound hole-pair exchange between neighboring hole-rich regions in the stripe phase, is one or two orders of magnitude larger than single-hole or multi-hole droplet exchange. As a result holes in a given hole-rich domain penetrate further into the antiferromagnetically aligned domains when they do it in pairs. At temperature of about 100 K and below bound pairs of holes hop from one hole-rich domain to another with high probability. Therefore our main finding is that the presence of the antiferromagnetic hole-poor domains act as a filter which selects, from the hole-rich domains (where holes form a self-bound liquid), hole pairs which can be exchanged throughout the system. This fluid of bound hole pairs can undergo a superfluid phase ordering at the above mentioned temperature scale.

cond-mat.str-el↗

arXiv2002

A Quantum-Dot Array as Model for Copper-Oxide Superconductors: A Dedicated Quantum Simulator for the Many-Fermion Problem

Quantum systems with a large number of fermionic degrees of freedom are intractable by quantum simulations. In this paper we introduce the concept of a dedicated quantum simulator(DQS) which is an artificial system of quantum dots whose Hamiltonian maps exactly to the original many fermion problem. While the universal quantum simulator (UQS) introduced by Feynman in 1982 can simulate any quantum mechanical many-body problem, a DQS can only solve a particular many body problem. Our concept of the dedicated quantum simulator is not a quantum computer but rather a quantum "analog" device, dedicated to a particular quantum computation. As an example, we consider the system of the $CuO$ plane in the copper-oxide superconductors and we propose an array of electrostatically confined quantum dots to be used as its dedicated quantum simulator. We show that this dedicated device can be used to image stripe formation as a function of the electron doping using electric force microscopy. We argue that such a dedicated quantum simulator may be easier to realize in the future compared to a general purpose quantum computer.

cond-mat.str-el↗

arXiv2001

Scaling of thermal conductivity of helium confined in pores

We have studied the thermal conductivity of confined superfluids on a bar-like geometry. We use the planar magnet lattice model on a lattice $H\times H\times L$ with $L \gg H$. We have applied open boundary conditions on the bar sides (the confined directions of length $H$) and periodic along the long direction. We have adopted a hybrid Monte Carlo algorithm to efficiently deal with the critical slowing down and in order to solve the dynamical equations of motion we use a discretization technique which introduces errors only $O((δt)^6)$ in the time step $δt$. Our results demonstrate the validity of scaling using known values of the critical exponents and we obtained the scaling function of the thermal resistivity. We find that our results for the thermal resistivity scaling function are in very good agreement with the available experimental results for pores using the temp

cond-mat.stat-mech↗

arXiv2000

Quantum Films Adsorbed on Graphite: Third and Fourth Helium Layers

Using a path-integral Monte Carlo method for simulating superfluid quantum films, we investigate helium layers adsorbed on a substrate consisting of graphite plus two solid helium layers. Our results for the promotion densities and the dependence of the superfluid density on coverage are in agreement with experiment. We can also explain certain features of the measured heat capacity as a function of temperature and coverage.

cond-mat.stat-mech↗

arXiv1999

The Role of Substrate Corrugations in Helium Monolayer Solidification

We investigate the first layer of helium adsorbed on graphite with path-integral Monte Carlo, examining the role of substrate corrugations on the phase diagram. When no corrugations are present, the equilibrium state of the system is a liquid phase, with solidification occurring only under compression but before layer promotion. We determine the solid-liquid coexistence region and compare our results to recent Green's function Monte Carlo calculations on the same system. When substrate corrugations are included, we find that the equilibrium phase is the $\sqrt 3 \times \sqrt3$ commensurate solid phase that is well known from experiment. The melting behavior, heat capacity, and single particle binding energy are determined and compared to experiment. We further find that for densities below the commensurate coverage, the low temperature phase of the system consists of solid clusters in coexistence with coalesced vacancies. We find no first layer liquid phase and so no superfluidity in this layer, in contrast to some rather recent suggestions.

cond-mat.stat-mech↗

arXiv1999

Green's Function Monte Carlo for Lattice Fermions: Application to the t-J Model

We develop a general numerical method to study the zero temperature properties of strongly correlated electron models on large lattices. The technique, which resembles Green's Function Monte Carlo, projects the ground state component from a trial wave function with no approximations. We use this method to determine the phase diagram of the two-dimensional t-J model, using the Maxwell construction to investigate electronic phase separation. The shell effects of fermions on finite-sized periodic lattices are minimized by keeping the number of electrons fixed at a closed-shell configuration and varying the size of the lattice. Results obtained for various electron numbers corresponding to different closed-shells indicate that the finite-size effects in our calculation are small. For any value of interaction strength, we find that there is always a value of the electron density above which the system can lower its energy by forming a two-component phase separated state. Our results are compared with other calculations on the t-J model. We find that the most accurate results are consistent with phase separation at all interaction strengths.

cond-mat.str-el↗

arXiv1999

Nematic phase of the two-dimensional electron gas in a magnetic field

The two dimensional electron gas (2DEG) in moderate magnetic fields in ultra-clean AlAs-GaAs heterojunctions exhibits transport anomalies suggestive of a compressible, anisotropic metallic state. Using scaling arguments and Monte Carlo simulations, we develop an order parameter theory of an electron nematic phase. The observed temperature dependence of the resistivity anisotropy behaves like the orientational order parameter if the transition to the nematic state occurs at a finite temperature, $T_c \sim 65 mK$, and is slightly rounded by a small background microscopic anisotropy. We propose a light scattering experiment to measure the critical susceptibility.

cond-mat.mes-hall↗

arXiv1998

Stripes and the t-J model

We investigate the two-dimensional t-J model at a hole doping of x = 1/8 and J/t = 0.35 with exact diagonalization. The low-energy states are uniform (not striped). We find numerous excited states with charge density wave structures, which may be interpreted as striped phases. Some of these are consistent with neutron scattering data on the cuprates and nickelates.

cond-mat.str-el↗

arXiv1998

Scaling of the specific heat of superfluids confined in pores

We investigate the scaling properties of the specific heat of the XY model on lattices H x H x L with L >> H (i.e. in a bar-like geometry) with respect to the thickness H of the bar, using the Cluster Monte Carlo method. We study the effect of the geometry and boundary conditions on the shape of the universal scaling function of the specific heat by comparing the scaling functions obtained for cubic, film, and bar-like geometry. In the presence of physical boundary conditions applied along the sides of the bars we find good agreement between our Monte Carlo results and the most recent experimental data for superfluid helium confined in pores.

cond-mat.stat-mech↗

arXiv1998

Path integral Monte Carlo simulation of the second layer of helium-4 adsorbed on graphite

We have developed a path integral Monte Carlo method for simulating helium films and apply it to the second layer of helium adsorbed on graphite. We use helium-helium and helium-graphite interactions that are found from potentials which realistically describe the interatomic interactions. The Monte Carlo sampling is over both particle positions and permutations of particle labels. From the particle configurations and static structure factor calculations, we find that this layer possesses, in order of increasing density, a superfluid liquid phase, a sqrt(7) x sqrt(7) commensurate solid phase that is registered with respect to the first layer, and an incommensurate solid phases. By applying the Maxwell construction to the dependence of the low-temperature total energy on the coverage, we are able to identify coexistence regions between the phases. From these, we deduce an effectively zero-temperature phase diagram. Our phase boundaries are in agreement with heat capacity and torsional oscillator measurements, and demonstrate that the experimentally observed disruption of the superfluid phase is caused by the growth of the commensurate phase. We further observe that the superfluid phase has a transition temperature consistent with the two-dimensional value. Promotion to the third layer occurs for densities above 0.212 atom/A^2, in good agreement with experiment. Finally, we calculate the specific heat for each phase and obtain peaks at temperatures in general agreement with experiment.

cond-mat↗

arXiv1998

Phase diagram of second layer of helium adsorbed on graphite

Using realistic helium-helium and helium-graphite interactions and the path integral Monte Carlo method, we are able to identify the gas, superfluid liquid, commensurate-solid, and incommensurate-solid phases, and the coexistence regions between them, for the second layer of helium on graphite. The phase boundaries and the specific heat that we determine are in good agreement with experiment. The appearance and disappearance of superfluidity with increasing coverage can be explained by the growth of coexistence phases, as was observed by torsional oscillator experiments.

cond-mat↗

arXiv1997

Boundary effects in superfluid films

We have studied the superfluid density and the specific heat of the XY model on lattices L x L x H with L >> H (i.e. on lattices representing a film geometry) using the Cluster Monte Carlo method. In the H-direction we applied staggered boundary conditions so that the order parameter on the top and bottom layers is zero, whereas periodic boundary conditions were applied in the L-directions. We find that the system exhibits a Kosterlitz-Thouless phase transition at the H-dependent temperature T_{c}^{2D} below the critical temperature T_λ of the bulk system. However, right at the critical temperature the ratio of the areal superfluid density to the critical temperature is H-dependent in the range of film thicknesses considered here. We do not find satisfactory finite-size scaling of the superfluid density with respect to H for the sizes of H studied. However, our numerical results can be collapsed onto a single curve by introducing an effective thickness H_{eff} = H + D (where D is a constant) into the corresponding scaling relations. We argue that the effective thickness depends on the type of boundary conditions. Scaling of the specific heat does not require an effective thickness (within error bars) and we find good agreement between the scaling function f_{1} calculated from our Monte Carlo results, f_{1} calculated by renormalization group methods, and the experimentally determined function f_1.

cond-mat.stat-mech↗

arXiv1996

Scaling of the superfluid density in superfluid films

We study scaling of the superfluid density with respect to the film thickness by simulating the $x-y$ model on films of size $L \times L \times H$ ($L >> H$) using the cluster Monte Carlo. While periodic boundary conditions where used in the planar ($L$) directions, Dirichlet boundary conditions where used along the film thickness. We find that our results can be scaled on a universal curve by introducing an effective thickness. In the limit of large $H$ our scaling relations reduce to the conventional scaling forms. Using the same idea we find scaling in the experimental results using the same value of $ν= 0.6705$.

cond-mat↗

arXiv1994

Two-Dimensional t-J Model at Low Electron Density

The phase diagram of the two-dimensional t-J model is determined accurately at low electron density by a combination of analytic and numerical techniques. The ground state exhibits three phases in the limit of zero electron density: an unpaired state at small J/t, a gas of s-wave pairs for 2 < J/t < 3.4367, and a phase separated state at larger interaction strengths. Bound states of larger clusters are never realized, and the instabilities present at small densities are discussed.

cond-mat↗

arXiv1994

Three-dimensional $x-y$ model with the Chern-Simons term

We investigate the influence of the Chern-Simons term coupled to the three-dimensional $x-y$ model. This term endows vortices with an internal angular momentum and thus gives them arbitrary statistics. The Chern-Simons term for the $x-y$ model takes an integer value which can be written as a sum over all vortex lines of the product of the vortex charge and the winding number of the internal phase angle along that vortex line. We have used the Monte-Carlo method to study the three-dimensional $x-y$ model with the Chern-Simons term. Our findings suggest that this model belongs to the $x-y$ universality class with the critical temperature growing with increasing internal angular momentum.

hep-lat↗

arXiv1993

Finite-Size Scaling in Two-Dimensional Superfluids

Using the $x-y$ model and a non-local updating scheme called cluster Monte Carlo, we calculate the superfluid density of a two dimensional superfluid on large-size square lattices $L \times L$ up to $400\times 400$. This technique allows us to approach temperatures close to the critical point, and by studying a wide range of $L$ values and applying finite-size scaling theory we are able to extract the critical properties of the system. We calculate the superfluid density and from that we extract the renormalization group beta function. We derive finite-size scaling expressions using the Kosterlitz-Thouless-Nelson Renormalization Group equations and show that they are in very good agreement with our numerical results. This allows us to extrapolate our results to the infinite-size limit. We also find that the universal discontinuity of the superfluid density at the critical temperature is in very good agreement with the Kosterlitz-Thouless-Nelson calculation and experiments.

cond-mat↗