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S. Moroni

Publications and source records attributed to S. Moroni.

18 recordsLinked to original sources
arXiv2015

Imaginary Time Correlations for a High-Density two-dimensional Electron Gas

We evaluate imaginary time density-density correlation functions for a two-dimensional homogeneous electron gas using the phaseless auxiliary field quantum Monte Carlo method. We show that such methodology, once equipped with suitable numerical stabilization techniques necessary to deal with exponentials, products and inversions of large matrices, gives access to the calculation of imaginary time correlation functions for medium-sized systems; we present simulations of a number up to 42 correlated fermions in the continuum, using up to 300 plane waves as basis set elements. We discuss the numerical stabilization techniques and the computational complexity of the methodology. We perform the inverse Laplace transform of the obtained density-density correlation functions, assessing the ability of the phaseless auxiliary field quantum Monte Carlo method to evaluate dynamical properties of many-fermion systems.

cond-mat.str-el↗
arXiv2015

Conditions for entangled photon emission from (111)B site-controlled Pyramidal quantum dots

A study of highly symmetric site-controlled Pyramidal In0.25Ga0.75As quantum dots (QDs) is presented. It is discussed that polarization-entangled photons can be also obtained from Pyramidal QDs of different designs from the one already reported in Juska et al. (Nat. Phot. 7, 527, 2013). Moreover, some of the limitations for a higher density of entangled photon emitters are addressed. Among these issues are (1) a remaining small fine-structure splitting and (2) an effective QD charging under non-resonant excitation conditions, which strongly reduce the number of useful biexciton-exciton recombination events. A possible solution of the charging problem is investigated exploiting a dual-wavelength excitation technique, which allows a gradual QD charge tuning from strongly negative to positive and, eventually, efficient detection of entangled photons from QDs, which would be otherwise ineffective under a single-wavelength (non-resonant) excitation.

quant-ph↗
arXiv2013

Imaginary Time Correlations and the phaseless Auxiliary Field Quantum Monte Carlo

The phaseless Auxiliary Field Quantum Monte Carlo method provides a well established approximation scheme for accurate calculations of ground state energies of many-fermions systems. Here we apply the method to the calculation of imaginary time correlation functions. We give a detailed description of the technique and we test the quality of the results for static and dynamic properties against exact values for small systems.

cond-mat.str-el↗
arXiv2013

Dynamic structure factor for 3He in two-dimensions

Recent neutron scattering experiments on 3He films have observed a zero-sound mode, its dispersion relation and its merging with -and possibly emerging from- the particle-hole continuum. Here we address the study of the excitations in the system via quantum Monte Carlo methods: we suggest a practical scheme to calculate imaginary time correlation functions for moderate-size fermionic systems. Combined with an efficient method for analytic continuation, this scheme affords an extremely convincing description of the experimental findings.

cond-mat.other↗
arXiv2012

Equation of state of two--dimensional $^3$He at zero temperature

We have performed a Quantum Monte Carlo study of a two-dimensional bulk sample of interacting 1/2-spin structureless fermions, a model of $^3$He adsorbed on a variety of preplated graphite substrates. We have computed the equation of state and the polarization energy using both the standard fixed-node approximate technique and a formally exact methodology, relying on bosonic imaginary-time correlation functions of operators suitably chosen in order to extract fermionic energies. As the density increases, the fixed-node approximation predicts a transition to an itinerant ferromagnetic fluid, whereas the unbiased methodology indicates that the paramagnetic fluid is the stable phase until crystallization takes place. We find that two-dimensional $^3$He at zero temperature crystallizes from the paramagnetic fluid at a density of 0.061 Å$^{-2}$ with a narrow coexistence region of about 0.002 Å$^{-2}$. Remarkably, the spin susceptibility turns out in very good agreement with experiments.

cond-mat.stat-mech↗
arXiv2011

Phase diagram of soft-core bosons in two dimensions

The low temperature phase diagram of Bose soft disks in two dimensions is studied by numerical simulations. It is shown that a supersolid cluster phase exists, within a range of the model parameters, analogous to that recently observed for a system of aligned dipoles interacting via a softened potential at short distance. These findings indicate that a long-range tail of the interaction is unneeded to obtain such a phase, and that the soft-core repulsive interaction is the minimal model for supersolidity.

cond-mat.quant-gas↗
arXiv2009

Correlation Energy and the Spin Susceptibility of the Two-Valley Two-dimensional Electron Gas

We find that the spin susceptibility of a two-dimensional electron system with valley degeneracy does not grow critically at low densities, at variance with experimental results [A. Shashkin et al., Phys. Rev. Lett. 96, 036403 (2006)]. We ascribe this apparent discrepancy to the weak disorder present in experimental samples. Our prediction is obtained from accurate correlation energies computed with state of-the-art diffusion Monte Carlo simulations and fitted with an analytical expression which also provides a local spin density functional for the system under investigation.

cond-mat.str-el↗
arXiv2007

Spin Susceptibility of Interacting Two-dimensional Electrons with Anisotropic Effective Mass

We report measurements of the spin susceptibility in dilute (rs up to 10) AlAs two-dimensional (2D) electrons occupying a single conduction-band valley with an anisotropic in-plane Fermi contour, characterized by longitudinal and transverse effective masses, ml and mt. As the density is decreased, the spin susceptibility is significantly enhanced over its band value, reflecting the role of interaction. Yet the enhancement is suppressed compared to the results of quantum Monte Carlo based calculations that take the finite thickness of the electron layer into account but assume an isotropic effective mass equal to sqrt(ml.mt). Proper treatment of an interacting 2D system with an anisotropic effective mass therefore remains a theoretical challenge.

cond-mat.mes-hall↗
arXiv2005

Computational spectroscopy of helium-solvated molecules: effective inertia, from small He clusters toward the nano-droplet regime

Accurate computer simulations of the rotational dynamics of linear molecules solvated in He clusters indicate that the large-size (nano-droplet) regime is attained quickly for light rotors (HCN, CO) and slowly for heavy ones (OCS, N$_2$O, CO$_2$), thus challenging previously reported results. Those results spurred the view that the different behavior of light rotors with respect to heavy ones - including a smaller reduction of inertia upon solvation of the former - would result from the lack of adiabatic following of the He density upon molecular rotation. We have performed computer experiments in which the rotational dynamics of OCS and HCN molecules was simulated using a fictitious inertia appropriate to the other molecule. These experiments indicate that the approach to the nano-droplet regime, as well as the reduction of the molecular inertia upon solvation, is determined by the anistropy of the potential, more than by the molecular weight. Our findings are in agreement with recent infrared and/or microwave experimental data which, however, are not yet totally conclusive by themselves.

cond-mat.other↗
arXiv2004

Effects of thickness on the spin susceptibility of the 2D electron gas

Using available quantum Monte Carlo predictions for a strictly 2D electron gas, we have estimated the spin susceptibility of electrons in actual devices taking into account the effect of the finite transverse thickness and finding a very good agreement with experiments. A weak disorder, as found in very clean devices and/or at densities not too low, just brings about a minor enhancement of the susceptibility.

cond-mat.str-el↗
arXiv2003

Monte Carlo simulations of two-dimensional charged bosons

Quantum Monte Carlo methods are used to calculate various ground state properties of charged bosons in two dimensions, throughout the whole density range where the fluid phase is stable. Wigner crystallization is predicted at $r_s\simeq 60$. Results for the ground state energy and the momentum distribution are summarized in analytic interpolation formulas embodying known asymptotic behaviors. Near freezing, the condensate fraction is less than 1%. The static structure factor $S(k)$ and susceptibility $χ(k)$ are obtained from the density-density correlation function in imaginary time, $F({\bf k},τ)$. An estimate of the energy of elementary excitations, given in terms of an upper bound involving $S(k)$ and $χ(k)$, is compared with the result obtained via analytic continuation from $F({\bf k},τ)$.

cond-mat↗
arXiv2001

Derivatives of the fixed-node energy

We present quantum Monte Carlo calculations of total energy derivatives, consistently performed in the fixed-node approximation. Contributions from nodal displacements, neglected or approximated in previous investigations, are properly taken into account. Their impact on the efficiency is discussed.

cond-mat↗
arXiv2000

Spin-Polarization transition in the two dimensional electron gas

We present a numerical study of magnetic phases of the 2D electron gas near freezing. The calculations are performed by diffusion Monte Carlo in the fixed node approximation. At variance with the 3D case we find no evidence for the stability of a partially polarized phase. With plane wave nodes in the trial function, the polarization transition takes place at Rs=20, whereas the best available estimates locate Wigner crystallization around Rs=35. Using an improved nodal structure, featuring optimized backflow correlations, we confirm the existence of a stability range for the polarized phase, although somewhat shrunk, at densities achievable nowadays in 2 dimensional hole gases in semiconductor heterostructures . The spin susceptibility of the unpolarized phase at the magnetic transition is approximately 30 times the Pauli susceptibility.

cond-mat.str-el↗
arXiv1999

Anisotropic Condensation of Helium in Nanotube Bundles

Helium atoms are strongly attracted to the interstitial channels within a bundle of carbon nanotubes. The strong corrugation of the axial potential within a channel can produce a lattice gas system where the weak mutual attraction between atoms in neighboring channels of a bundle induces condensation into a remarkably anisotropic phase with very low binding energy. We estimate the binding energy and critical temperature for 4He in this novel quasi-one-dimensional condensed state. At low temperatures, the specific heat of the adsorbate phase (fewer than 2% of the total number of atoms) greatly exceeds that of the host material.

cond-mat.stat-mech↗
arXiv1998

Reptation Quantum Monte Carlo

We present an elementary and self-contained account of the analogies existing between classical diffusion and the imaginary-time evolution of quantum systems. These analogies are used to develop a new quantum simulation method which allows to calculate the ground-state expectation values of local observables without any mixed estimates nor population-control bias, as well as static and dynamic (in imaginary time) response functions. This method, which we name Reptation Quantum Monte Carlo, is demonstrated with a few case applications to 4He, including the calculation of total and potential energies, static and imaginary-time dependent density response functions, and low-lying excitation energies. Finally, we discuss the relations of our technique with other simulation schemes.

cond-mat↗
arXiv1995

Bounds for the phonon-roton dispersion in superfluid 4He

The sum rule approach is used to derive upper bounds for the dispersion law $ω_0(q)$ of the elementary excitations of a Bose superfluid. Bounds are explicitly calculated for the phonon-roton dispersion in superfluid $^4$He, both at equilibrium ($ρ=0.02186$ Å$^{-3}$) and close to freezing ($ρ=0.02622$ Å$^{-3}$). The bound $ω_0(q) \le 2S(q)\midχ(q)\mid^{-1}$, where $S(q)$ and $χ(q)$ are the static structure factor and density response respectively, is calculated microscopically for several values of the wavevector $q$. The results provide a significant improvement with respect to the Feynman approximation $ω_F(q)= q^2(2mS(q))^{-1}$. A further, stronger bound, requiring the additional knowledge of the current correlation function is also investigated. New results for the current correlation function are presented.

cond-mat↗