The Quantum Monte Carlo (QMC) method can yield the imaginary-time dependence of a correlation function $C(tau)$ of an operator $hat O$. The analytic continuation to real-time proceeds by means of a numerical inversion of these data to find the respon
se function or spectral density $A(omega)$ corresponding to $hat O$. Such a technique is very sensitive to the statistical errors in $C(tau)$ especially for large values of $tau$, when we are interested in the low-energy excitations. In this paper, we find that if we use the flat histogram technique in the QMC method, in such a way to make the {it histogram of} $C(tau)$ flat, the results of the analytic continuation for low-energy excitations improve using the same amount of computational time. To demonstrate the idea we select an exactly soluble version of the single-hole motion in the $t-J$ model and the diagrammatic Monte Carlo technique.
Solar cells based on conventional semiconductors have low efficiency in converting solar energy into electricity because the excess energy beyond the gap of an incident solar photon is converted into heat by phonons. Here we show by ab initio methods
that the presence of strong Coulomb interactions in strongly correlated insulators (SCI) causes the highly photo-excited electron-hole pair to decay fast into multiple electron-hole pairs via impact ionization (II). We show that the II rate in the insulating $M_1$ phase of vanadium dioxide (chosen for this study as it is considered a prototypical SCI) is two orders of magnitude higher than in Si and much higher than the rate of hot electron/hole decay due to phonons. Our results indicate that a rather broad class of materials may be harnessed for an efficient solar-to-electrical energy conversion that has been not considered before.
The diagrammatic Monte Carlo (Diag-MC) method is a numerical technique which samples the entire diagrammatic series of the Greens function in quantum many-body systems. In this work, we incorporate the flat histogram principle in the diagrammatic Mon
te method and we term the improved version Flat Histogram Diagrammatic Monte Carlo method. We demonstrate the superiority of the method over the standard Diag-MC in extracting the long-imaginary-time behavior of the Greens function, without incorporating any a priori knowledge about this function, by applying the technique to the polaron problem
During the last decade, ab initio methods to calculate electronic structure of materials based on hybrid functionals are increasingly becoming widely popular. In this Letter, we show that, in the case of small gap transition metal oxides, such as VO2
, with rather subtle physics in the vicinity of the Fermi-surface, such hybrid functional schemes without the inclusion of expensive fully self-consistent GW corrections fail to yield this physics and incorrectly describe the features of the wave function of states near the Fermi-surface. While a fully self-consistent GW on top of hybrid functional approach does correct these wave functions as expected, and is found to be in general agreement with the results of a fully self-consistent GW approach based on semilocal functionals, it is much more computationally demanding as compared to the latter approach for the benefit of essentially the same results.
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 m
easurement, 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.
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 exp
ansion 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 Hunds 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.
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 boun
ds 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.
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 bro
aden 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.
