Efstratios Manousakis

Classical Phase Fluctuations in High Temperature Superconductors

Phase fluctuations of the superconducting order parameter play a larger role in the cuprates than in conventional BCS superconductors because of the low superfluid density {rho}{sub s} of a doped insulator. In this paper, we analyze an XY model of classical phase fluctuations in the high temperature superconductors using a low temperature expansion and Monte Carlo simulations. In agreement with experiment, the value of {rho}{sub s} at temperature T=0 is a quite robust predictor of T{sub c} , and the evolution of {rho}{sub s} with T , including its T -linear behavior at low temperature, is insensitive to microscopic details. {copyright} {ital 1999} {ital The American Physical Society }

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 Greens 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.

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

The two-dimensional electron gas (2DEG) in moderate magnetic fields in ultraclean 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) approximately 65 mK, and is slightly rounded by a small background microscopic anisotropy. We propose a light scattering experiment to measure the critical susceptibility.

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.

Path integral Monte Carlo applications to quantum fluids in confined geometries

Path integral Monte Carlo is an exact simulation method for calculating thermodynamic properties of bosonic systems. Properties such as superfluidity and bose condensation are directly related to multiparticle exchange cycles of individual particle paths. Such calculations of bosonic systems in confined geometries, such as helium and hydrogen on surfaces and in droplets are reviewed.

Founding Quantum Theory on the Basis of Consciousness

In the present work, quantum theory is founded on the framework of consciousness, in contrast to earlier suggestions that consciousness might be understood starting from quantum theory. The notion of streams of consciousness, usually restricted to conscious beings, is extended to the notion of a Universal/Global stream of conscious flow of ordered events. The streams of conscious events which we experience constitute sub-streams of the Universal stream. Our postulated ontological character of consciousness also consists of an operator which acts on a state of potential consciousness to create or modify the likelihoods for later events to occur and become part of the Universal conscious flow. A generalized process of measurement-perception is introduced, where the operation of consciousness brings into existence, from a state of potentiality, the event in consciousness. This is mathematically represented by (a) an operator acting on the state of potential consciousness before an actual event arises in consciousness and (b) the reflecting of the result of this operation back onto the state of potential consciousness for comparison in order for the event to arise in consciousness. Beginning from our postulated ontology that consciousness is primary and from the most elementary conscious contents, such as perception of periodic change and motion, quantum theory follows naturally as the description of the conscious experience.

Role of spin-orbit coupling and evolution of the electronic structure of WTe 2 under an external magnetic field

Here, we present a detailed study on the temperature and angular dependence of the Shubnikovde-Haas (SdH) effect in the semi-metal WTe2. This compound was recently shown to display a very large non-saturating magnetoresistance which was attributed to nearly perfectly compensated densities of electrons and holes. We observe four fundamental SdH frequencies and attribute them to spin-orbit split, electron- and hole-like, Fermi surface (FS) cross-sectional areas. Their angular dependence is mildly consistent with ellipsoidal FSs with volumes implying an excess of � 7 % in the density of electrons with respect to that of the holes. Nevertheless, we show that density functional theory (DFT) calculations can reasonably describe the experimentally determined electron FSs but fail to accurately describe the hole FSs. When their cross-sectional areas are adjusted to reflect the experimental data, the resulting volumes of the electron/hole FSs obtained from the DFT would imply a strong imbalance between the densities of electrons and holes. We observe a severe fieldinduced renormalization of the effective masses suggesting that the electronic structure of WTe2 is particularly sensitive to the Zeeman-effect. By combining the results of our DFT calculations with our analysis of the experimental results we conclude that WTe2 is unlikely to remain compensated under an external field.

PATH-INTEGRAL MONTE CARLO SIMULATION OF THE SECOND LAYER OF 4HE 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.

Quantum formalism to describe binocular rivalry

On the basis of the general character and operation of the process of perception, a formalism is sought to mathematically describe the subjective or abstract/mental process of perception. It is shown that the formalism of orthodox quantum theory of measurement, where the observer plays a key role, is a broader mathematical foundation which can be adopted to describe the dynamics of the subjective experience. The mathematical formalism describes the psychophysical dynamics of the subjective or cognitive experience as communicated to us by the subject. Subsequently, the formalism is used to describe simple perception processes and, in particular, to describe the probability distribution of dominance duration obtained from the testimony of subjects experiencing binocular rivalry. Using this theory and parameters based on known values of neuronal oscillation frequencies and firing rates, the calculated probability distribution of dominance duration of rival states in binocular rivalry under various conditions is found to be in good agreement with available experimental data. This theory naturally explains an observed marked increase in dominance duration in binocular rivalry upon periodic interruption of stimulus and yields testable predictions for the distribution of perceptual alteration in time.

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.