arXiv subjects

Efstratios Manousakis

Publications and source records attributed to Efstratios Manousakis.

At least 55 records · Page 3Linked to original sources

arXiv2012

Ab initio studies of opto-electronic excitations in VO2

We study the optical response of VO2 in the M1 insulating phase using methods based on density functional theory in its most recent developments. We start from a hybrid functional approach which may be a good starting point to carry out many-body perturbation theory since, as we show, it gives a qualitatively and to some degree quantitatively correct description of the insulating phase. In order to calculate the dielectric function, first, using hybrid density functional wavefunctions we have added GW corrections on top of the hybrid density functional calculations and we solved the so-called Bethe Salpeter equation to include effects of correlations of the electron-hole pair created upon photon absorption. We find that the effects of electron correlations are very important and they show up as a strong contribution of the electron-hole interaction in calculating the effects of the Bethe-Salpeter equation in the electron/hole pair excitation on top of the interacting group state.

cond-mat.str-el↗

arXiv2011

Hole spectral functions in lightly doped quantum antiferromagnets

We study the hole and magnon spectral functions as a function of hole doping in the two-dimensional (2D) t-J and t-t'-t"-J models working within the limits of the spin-wave theory, by linearizing the hole-spin-deviation interaction and by adapting the non-crossing approximation (NCA). We find that the staggered magnetization decreases rather rapidly with doping and it goes to zero at a few percent of hole concentration in both t-J and the t-t'-t"-J model. We find that with doping, the residue of the quasiparticle peak at G=(pi/2,pi/2) decreases rapidly with doping and the spectral function is in agreement with high resolution angle-resolved photo-emission spectroscopy (ARPES) studies of the copper-oxide superconductors. The observed large shift of the chemical potential inside the Mott gap is found to be a result of broadening of the quasiparticle peak. We find pockets centered at G, similar to those observed by quantum oscillation measurements, (i) with an elliptical shape with large eccentricity along the anti-nodal direction in the case of the t-J model, and (ii) with an almost circular shape in the case of the t-t'-t"-J model. We also find that the spectral intensity distribution in the doped antiferromagnet has a waterfall-like patten along the nodal direction of the Brillouin zone, a feature that is also seen in ARPES measurements.

cond-mat.str-el↗

arXiv2011

Simulation of melting of two dimensional Lennard-Jones solids

We study the nature of melting of a two dimensional (2D) Lennard-Jones solid using large scale Monte Carlo simulation. We use systems of up to 102,400 particles to capture the decay of the correlation functions associated with translational order (TO) as well as the bond-orientational (BO) order. We study the role of dislocations and disclinations and their distribution functions. We computed the temperature dependence of the second moment of the two order parameters, i.e., of the TO order parameter as well as of the order parameter associated with BO order and using finite-size scaling we determined the corresponding two anomalous dimension critical exponents eta. We also computed the temperature dependent distribution of these two order parameters on the complex plane which support a two stage melting with a hexatic phase as an intermediate phase. The analysis of our results leads to a consistent picture strongly supporting a two stage melting scenario as predicted by the Kosterlitz, Thouless, Halperin, Nelson, and Young (KTHNY) theory. We find that the two correlation lengths associated with translational and bond-orientational order have a distinctly different temperature dependence each diverging at different temperature and that their finite size scaling properties are consistent with the KTHNY theory. We also used the temperature dependence of etas and their theoretical bounds to provide estimates for the two critical temperatures, which are found to be in agreement with those determined from the divergent correlation lengths and from the Binder ratio.

cond-mat.stat-mech↗

arXiv2010

When perceptual time stands still: Long stable memory in binocular rivalry

We have carried out binocular rivalry experiments with a large number of subjects to obtain high quality statistics on probability distribution of dominance duration (PDDD) for two cases where (a) the rival stimulus is continuously presented and (b) the rival stimulus is periodically removed, with stimulus-on and stimulus-off intervals Ton and Toff respectively. It is shown that the PDDD obtained for the latter case can be reproduced to a reasonable degree of approximation by simply using the PDDD of part (a) and slicing it at pieces of time extent Ton and by introducing intervals of length Toff between the on-intervals where the PDDD is set to zero. This suggests that the variables representing the perceptual state do not change significantly during long blank intervals. We argue that these findings impose challenges to theoretical models which aim at describing visual perception.

q-bio.NC↗

arXiv2010

Quantum simulation of 3He impurities and of 4He interstitials in solid 4He

We have studied the role of an atomic 3He impurity and an interstitial 4He atom in two- and three-dimensional solid 4He using path integral Monte Carlo (PIMC) simulation. We find that when a substitutional 3He impurity is introduced, the impurity becomes localized and occupies an ideal lattice site. When an interstitial 3He impurity is introduced in the 4He solid, we find that the impurity becomes localized at a substitutional position and, thus, promotes the extra 4He atom to the interstitial space. As a consequence we find that the one-body density matrix (OBDM) and the superfluid fraction, for the case of a 4He solid with an interstitial impurity, are very similar to those calculated for a 4He solid with a 4He interstitial atom. Namely, while the off-diagonal OBDM approaches zero exponentially with increasing particle displacement for the "pure" solid, an interstitial 4He atom or a 3He impurity appear to enhance it at long distances. Finally, the effective mass of the 3He impurity quasiparticle in 2D and 3D crystalline 4He is estimated.

cond-mat.stat-mech↗

arXiv2010

Photovoltaic effect for narrow-gap Mott insulators

We discuss the photovoltaic effect at a p-n heterojunction, in which the illuminated side is a doped Mott insulator, using the simplest description of a Mott insulator within the Hubbard model. We find that the internal quantum efficiency of such a device, if we choose an appropriate narrow-gap Mott insulator, can be significantly enhanced due to impact ionization caused by the photoexcited ``hot'' electron/hole pairs. Namely, the photoexcited electron and/or hole can convert its excess energy beyond the Mott-Hubbard gap to additional electrical energy by creating multiple electron/hole pairs in a time scale which can be shorter than the time characterizing other relaxation processes.

cond-mat.str-el↗

arXiv2009

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.

q-bio.NC↗

arXiv2009

Quantum critical behavior driven by Hund's rule coupling in quantum antiferromagnets

When localized spins on different d orbitals prefer different types of antiferromagnetic ordering, the Hund's rule coupling creates frustration. Using spin-wave theory we study the case of two such orbitals on a square lattice coupled through Hund's rule, such that the first one couples antiferromagnetically (AF) more strongly to its nearest neighbors, while the second couples more strongly to its next nearest neighbors. We find that the zero temperature phase diagram has four regions, one characterized by the familiar $(π,π)$ AF order, a second by the columnar $(π,0)$ order, a third by a {\it canted} order and a fourth region where a quantum-disordered state emerges. We comment on the possible relevance of these findings for the case of Fe-pnictide based antiferromagnets.

cond-mat.str-el↗

arXiv2008

Effective Hamiltonian for FeAs based superconductors

The recently discovered FeAs-based superconductors show intriguing behavior and unusual dynamics of electrons and holes which occupy the Fe $d$-orbitals and As $4s$ and $4p$ orbitals. Starting from the atomic limit, we carry out a strong coupling expansion to derive an effective hamiltonian that describes the electron and hole behavior. The hopping and the hybridization parameters between the Fe $d$ and As $s$ and $p$-orbitals are obtained by fitting the results of our density-functional-theory calculations to a tight-binding model with nearest-neighbor interactions and a minimal orbital basis. We find that the effective hamiltonian, in the strong on-site Coulomb repulsion limit, operates on three distinct sub-spaces coupled through Hund's rule. The three sub-spaces describe different components (or subsystems): (a) one spanned by the $d_{x^2-y^2}$ Fe orbital; (b) one spanned by the degenerate atomic Fe orbitals $d_{xz}$ and $d_{yz}$; and (c) one spanned by the atomic Fe orbitals $d_{xy}$ and $d_{z^2}$. Each of these Hamiltonians is an extended t-t'-J-J' model and is characterized by different coupling constants and filling factors. For the case of the undoped material the second subspace alone prefers a ground state characterized by a spin-density-wave order similar to that observed in recent experimental studies, while the other two subspaces prefer an antiferromagnetic order. We argue that the observed spin-density-wave order minimizes the ground state energy of the total hamiltonian.

cond-mat.str-el↗

arXiv2008

Variational Monte-Carlo calculation of the nematic state of the two-dimensional electron gas in a magnetic field

We use a Jastrow-Slater wave function with an elliptical Fermi sea to describe the nematic state of the two-dimensional electron gas in a magnetic field and the Monte Carlo method to calculate a variational energy upper bound. These energy upper bounds are compared with other upper bounds describing stripe-ordered ground states which are obtained from optimized Hartree-Fock calculations and with those which correspond to an isotropic ground state. Our findings support the conclusions drawn in our previous study where the Fermi-hypernetted chain approximation was used instead of the Monte Carlo method. Namely, the nematic state becomes energetically favorable relative to the stripe-ordered Wigner crystal phase for the second excited Landau level and below a critical value of the layer ``thickness'' parameter which is very close to its value in the actual materials.

cond-mat.mes-hall↗

arXiv2008

Finite temperature spectral function of a hole in a quantum antiferromagnet and role of phonons

We study thermal broadening of the hole spectral function of the two-dimensional t-J model (and its extensions) within the non-crossing approximation with and without the contribution of optical phonons. We find that phonons at finite temperature broaden the lowest energy quasiparticle peak, however, the string excitations survive even for relatively strong electron-phonon coupling. Experimental angle resolved photo-emission spectroscopy(ARPES) results compare well with our calculations at finite temperature when we use strong electron-phonon coupling without any adhoc broadening. In addition, we have studied the role of vertex corrections and we find that their contribution allows us achieve the same overall agreement with the ARPES experimental results but using smaller values for the electron-phonon coupling.

cond-mat.str-el↗

arXiv2007

Possible role of 3He impurities in solid 4He

We use a quantum lattice gas model to describe essential aspects of the motion of 4He atoms and of 3He impurities in solid 4He. This study suggests that 3He impurities bind to defects and promote 4He atoms to interstitial sites which can turn the bosonic quantum disordered crystal into a metastable supersolid. It is suggested that defects and interstitial atoms are produced during the solid 4He nucleation process where the role of 3He impurities (in addition to the cooling rate) is known to be important even at very small (1 ppm) impurity concentration. It is also proposed that such defects can form a glass phase during the 4He solid growth by rapid cooling.

cond-mat.str-el↗

arXiv2007

String excitations of a hole in a quantum antiferromagnet and photoelectron spectrospopy

The aim of the present paper is twofold. The first goal is to show that high resolution angle-resolved photoelectron spectra from cuprates indicate the presence of string-like excitations of the quasihole excitation in a quantum antiferromagnet. In order to compare with the experimental intensity plots, we calculate the spectral function of the t-J and the t-t'-t"-J models within the self-consistent Born approximation for widely accepted values of the parameters of these models. The main features of the high resolution photoelectron spectra are in general agreement with the results based on the above models and can be understood by considering not only the lowest energy quasiparticle peak but, also, the higher energy string-like excitations of the hole. The second goal of the present paper is to make the case that a proper analysis of both the numerical results obtained from these models and of the experimental results, suggests a theoretical picture for the internal structure of the hole-quasiparticle and a string-exchange pairing mechanism due to strong antiferromagnetic correlations among the background spins. We find that, using a simple model in which the Hilbert space is restricted to states of only unbroken strings attached to the holes and in which the holes are connected with unbroken strings, we can provide a good quantitative description of the most accurate numerical results. We find that the holes experience an effective interaction due to a string-exchange mechanism which is characterized by a rather large string tension and this can provide pairing energy scales much larger that those suggested by spin-fluctuation mediated pairing models. In addition, it is argued that such string-exchange interaction tends to bind holes in a bound state with the $d_{x^2-y^2}$ symmetry.

cond-mat.str-el↗

arXiv2007

Quantum nematic as ground state of a two-dimensional electron gas in a magnetic field

We study the ground state of a nematic phase of the two-dimensional electron gas at filling fraction $ν= 1/2$ using a variational wavefunction having Jastrow pair-correlations of the form $Π_{i < j}(z_i-z_j)^2$ and an elliptical Fermi sea. Using the Fermi hypernetted chain approximation we find that below a critical value of the broken symmetry parameter, the nematic phase is energetically favorable as compared to the isotropic state for the second excited Landau level. We also find that below a critical value of the layer ``thickness'' parameter $λ$ (and in the actual materials) the quantum nematic is energetically favorable relative to the stripe ordered Wigner crystal phase.

cond-mat.mes-hall↗

arXiv2006

Possible observation of "string excitations" of a hole in a quantum antiferromagnet

We argue that recently reported high resolution angle-resolved photoelectron spectra from cuprates, where an anomalous high-energy dispersion was identified, reveal the internal structure of the hole quasiparticle in quantum antiferromagnets and more importantly it is evidence for the existence of ``string-excitations'' which validate early predictions based on the $t-J$ model. Their energy-momentum despersion as well as the manner in which the spectral weight is transfered to higher energy string excitations as well as the vanishing of the quasiparticle spectral weight near the $Γ$ point, are all in agreement with predictions without adjusting any parameters.

cond-mat.str-el↗

arXiv2006

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.

quant-ph↗

arXiv2005

Monolayer charged quantum films: A quantum simulation study

We use path-integral Monte Carlo (PIMC) to study the effects of adding a long-range repulsive Coulomb interaction to the usual Van der Waals interaction between two atoms of a submonolayer quantum film such as helium on graphite or a pure two-dimensional superfluid. Such interactions frustrate or compete with the natural tendency of the system for phase separation namely to form a macroscopic liquid or solid phase. We find that as a function of the relative strength of the long-range repulsion, surface coverage and temperature, the system undergoes a series of transformations, including a triangular Wigner-like crystal of clusters, a charge stripe-ordered phase and a fluid phase. The goal of these studies is to understand the role of quantum fluctuations when such competing interactions appear together with formation of preexisting electron pairs as might be the case in cuprate superconductors.

cond-mat.stat-mech↗

arXiv2004

Collective charge excitations along cell membranes

A significant part of the thin layers of counter-ions adjacent to the exterior and interior surfaces of a cell membrane form quasi-two-dimensional (2D) layers of mobile charge. Collective charge density oscillations, known as plasmon modes, in these 2D charged systems of counter-ions are predicted in the present paper. This is based on a calculation of the self-consistent response of this system to a fast electric field fluctuation. The possibility that the membrane channels might be using these excitations to carry out fast communication is suggested and experiments are proposed to reveal the existence of such excitations.

q-bio.SC↗