Imaginary-time time-dependent density functional theory for periodic systems.

Imaginary-time time-dependent density functional theory (it-TDDFT) has been proposed as an alternative method for obtaining the ground state within density functional theory (DFT) which avoids some of the difficulties with convergence encountered by the self-consistent-field (SCF) iterative method. It-TDDFT was previously applied to clusters of atoms where it was demonstrated to converge in select cases where SCF had difficulty with convergence. In the present work we implement it-TDDFT propagation forperiodic systemsby modifying the Quantum ESPRESSO (QE) package, which uses a plane-wave basis with multiplekpoints, and has the options of non-collinear and DFT + U calculations using ultra-soft or norm-conserving pseudo potentials. We demonstrate that our implementation of it-TDDFT propagation with multiplekpoints is correct for DFT + U non-collinear calculations and for DFT + U calculations with ultra-soft pseudo potentials. Our implementation of it-TDDFT propagation converges to the exact SCF energy (up to the decimal guaranteed by double precision) in all but one case where it converged to a slightly lower value than SCF, suggesting a useful alternative for systems where SCF has difficulty reaching the Kohn-Sham (KS) ground state. In addition, we demonstrate that more rapid convergence can be achieved if we use adaptive-size imaginary-time-steps for different kinetic-energy plane-waves.

Topological phase transition at the interface of a topological with a conventional insulator.

We study the topological states which appear at the interface between a topological insulator (TI) and a conventional insulator (CI) using effective Hamiltonians which accurately describe the band structure of the Bi2Se3 family. Due to the hybridization between the TI and the CI states, the band-gap that appears in the interface Dirac cone decreases and ultimately vanishes by tuning the interface-hopping amplitude or by selecting a CI of appropriate band effective mass. More importantly, we find that a topologically trivial TI slab can be made non-trivial and vice-versa by tuning of such an interface-hopping amplitude or by tuning the CI band effective-mass; namely, a topological phase transition can be induced in such heterostructures indicated by the presence or absence of gapless linear edge modes. We discuss the relevance and realization of our results and conclusions in future experiments.

Imaginary-Time Time-Dependent Density Functional Theory and Its Application for Robust Convergence of Electronic States.

Reliable and robust convergence to the electronic ground state within density functional theory (DFT) Kohn-Sham (KS) calculations remains a thorny issue in many systems of interest. In such cases, charge sloshing can delay or completely hinder the convergence. Here, we use an approach based on transforming the time-dependent DFT equations to imaginary time, followed by imaginary-time evolution, as a reliable alternative to the self-consistent field (SCF) procedure for determining the KS ground state. We discuss the theoretical and technical aspects of this approach and show that the KS ground state should be expected to be the long-imaginary-time output of the evolution, independent of the exchange-correlation functional or the level of theory used to simulate the system. By maintaining self-consistency between the single-particle wave functions (orbitals) and the electronic density throughout the determination of the stationary state, our method avoids the typical difficulties encountered in SCF. To demonstrate dependability of our approach, we apply it to selected systems which struggle to converge with SCF schemes. In addition, through the van Leeuwen theorem, we affirm the physical meaningfulness of imaginary-time TDDFT, justifying its use in certain topics of statistical mechanics such as in computing imaginary-time path integrals.

Optimizing the role of impact ionization in conventional insulators.

A mechanism for multiple carrier generation through impact ionization (IA) proposed earlier for bulk systems of strongly correlated insulators is generalized to the case of conventional insulators that contain localized bands a few eV above and below the highest occupied band. Specifically, we study the case of hybridization of localized orbitals with more dispersive bands near the Fermi level, where the generated multiple carriers, which ultimately decay to the edges of the dispersive bands by means of IA processes, acquire lighter mass and this could allow their more efficient separation before recombination. We argue that this may be applicable to the case of halide perovskites and it could be one of the reasons for their observed photovoltaic efficiency. We discuss the criteria one should use to uncover the appropriate material in order to harvest the optimum effect of IA for the spectrum of the solar photon energy distribution.

Metal to Insulator Quantum-Phase Transition in Few-Layered ReS2.

In ReS2, a layer-independent direct band gap of 1.5 eV implies a potential for its use in optoelectronic applications. ReS2 crystallizes in the 1T'-structure, which leads to anisotropic physical properties and whose concomitant electronic structure might host a nontrivial topology. Here, we report an overall evaluation of the anisotropic Raman response and the transport properties of few-layered ReS2 field-effect transistors. We find that ReS2 exfoliated on SiO2 behaves as an n-type semiconductor with an intrinsic carrier mobility surpassing µ(i) ∼ 30 cm(2)/(V s) at T = 300 K, which increases up to ∼350 cm(2)/(V s) at 2 K. Semiconducting behavior is observed at low electron densities n, but at high values of n the resistivity decreases by a factor of >7 upon cooling to 2 K and displays a metallic T(2)-dependence. This suggests that the band structure of 1T'-ReS2 is quite susceptible to an electric field applied perpendicularly to the layers. The electric-field induced metallic state observed in transition metal dichalcogenides was recently claimed to result from a percolation type of transition. Instead, through a scaling analysis of the conductivity as a function of T and n, we find that the metallic state of ReS2 results from a second-order metal-to-insulator transition driven by electronic correlations. This gate-induced metallic state offers an alternative to phase engineering for producing ohmic contacts and metallic interconnects in devices based on transition metal dichalcogenides.

Flat histogram diagrammatic Monte Carlo method: calculation of the Green's function in imaginary time.

The diagrammatic Monte Carlo (DiagMC) method is a numerical technique which samples the entire diagrammatic series of the Green's function in quantum many-body systems. In this work, we incorporate the flat histogram principle in the diagrammatic Monte Carlo method, and we term the improved version the "flat histogram diagrammatic Monte Carlo" method. We demonstrate the superiority of this method over the standard DiagMC in extracting the long-imaginary-time behavior of the Green's function, without incorporating any a priori knowledge about this function, by applying the technique to the polaron problem.

When perceptual time stands still: long percept-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 T(on) and T(off) respectively. In the present study we have chosen to study the regime of relatively long stimulus-on time, i.e., T(on)> 1s, where the stimulus presentation duration is significantly longer than the human reaction and recognition time. In the case of periodically removed stimulus, the total probability for percept reversal during each of the successive stimulus-on intervals T(on) can be predicted using the PDDD for continuous viewing. More importantly, this total probability for percept reversal during any stimulus-on interval is independent of the length T(off) of the preceding blank time, which can be quite long. We argue that this suggests that, in the regime of long T(on) and T(off) considered here, the variables representing the perceptual state do not change significantly during long blank intervals. We discuss that these findings impose challenges to theoretical models which aim at describing visual perception.

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.