Quantum ice: a quantum Monte Carlo study

Ice states, in which frustrated interactions lead to a macroscopic ground-state degeneracy, occur in water ice, in problems of frustrated charge order on the pyrochlore lattice, and in the family of rare-earth magnets collectively known as spin ice. Of particular interest at the moment are "quantum spin-ice" materials, where large quantum fluctuations may permit tunnelling between a macroscopic number of different classical ground states. Here we use zero-temperature quantum Monte Carlo simulations to show how such tunnelling can lift the degeneracy of a spin or charge ice, stabilizing a unique "quantum-ice" ground state-a quantum liquid with excitations described by the Maxwell action of (3+1)-dimensional quantum electrodynamics. We further identify a competing ordered squiggle state, and show how both squiggle and quantum-ice states might be distinguished in neutron scattering experiments on a spin-ice material.     

Electron emission from diamondoids: a diffusion quantum Monte Carlo study

We present density-functional theory (DFT) and quantum Monte Carlo (QMC) calculations designed to resolve experimental and theoretical controversies over the optical properties of H-terminated C nanoparticles (diamondoids). The QMC results follow the trends of well-converged plane-wave DFT calculations for the size dependence of the optical gap, but they predict gaps that are 1-2 eV higher. They confirm that quantum confinement effects disappear in diamondoids larger than 1 nm, which have gaps below that of bulk diamond. Our QMC calculations predict a small exciton binding energy and a negative electron affinity (NEA) for diamondoids up to 1 nm, resulting from the delocalized nature of the lowest unoccupied molecular orbital. The NEA suggests a range of possible applications of diamondoids as low-voltage electron emitters.     

High-temperature superconductivity in the apex-oxygen model: a quantum Monte Carlo study

The Projector Quantum Monte Carlo Method is applied to examine the existence of superconductivity in the Apex-Oxygen-Model which describes the physics of the correlated carriers in the CuO₂-planes coupled to the anharmonic vibrations of the apex oxygen in the hightemperature superconductors. The simulations show clear evidence for extendeds-wave superconductivity in the 100 K temperature range for realistic choice of coupling parameters.     

Monte Carlo study of hot electrons in quantum wells

We have performed an ensemble Monte Carlo calculation of the high field response at 77 K of two different two-dimensional systems, one a modulation doped (MD) single square well of GaAs/AlGaAs and the other a δ-doping layer formed by a sheet of donor impurities embedded in bulk GaAs. Results of this calculation show a reduced low field mobility and a decrease in the transient overshoot velocity in the δ-doping versus MD system due to the effect of impurity scattering. At high fields, however, the steady state saturated drift velocities appear to be similar in the two systems. Because of the higher carrier density, the performance of short-channel devices based on the δ-layer will be advantageous where high output currents are necessary.     

Correlated adatom trimer on a metal surface: a continuous-time quantum Monte Carlo study

The problem of three interacting Kondo impurities is solved within a numerically exact continuous-time quantum Monte Carlo scheme. A suppression of the Kondo resonance by interatomic exchange interactions for different cluster geometries is investigated. It is shown that a drastic difference between the Heisenberg and Ising cases appears for antiferromagnetically coupled adatoms. The effects of magnetic frustrations in the adatom trimer are investigated, and possible connections with available experimental data are discussed.     

Quantum dynamics at finite temperature: Time-dependent quantum Monte Carlo study

In this work we investigate the ground state and the dissipative quantum dynamics of interacting charged particles in an external potential at finite temperature. The recently devised time-dependent quantum Monte Carlo (TDQMC) method allows a self-consistent treatment of the system of particles together with bath oscillators first for imaginary-time propagation of Schrödinger type of equations where both the system and the bath converge to their finite temperature ground state, and next for real time calculation where the dissipative dynamics is demonstrated. In that context the application of TDQMC appears as promising alternative to the path-integral related techniques where the real time propagation can be a challenge.     

Dipolar interaction and incoherent quantum tunneling: a Monte Carlo study of magnetic relaxation

We study the magnetic relaxation of a system of localized spins interacting through weak dipole interactions, at a temperature large with respect to the ordering temperature but low with respect to the crystal field level splitting. The relaxation results from quantum spin tunneling but is only allowed on sites where the dipole field is very small. At low times, the magnetization decrease is proportional to as predicted by Prokofiev and Stamp, and at long times the relaxation can be described as an extension of a relaxed zone. The results can be directly compared with very recent experimental data on Fe₈ molecular clusters. Received 9 February 1999     

Fourier path integral Monte Carlo study of a two-dimensional model quantum monolayer

Adsorption isotherms have been constructed for a 2-dimensional ²⁰Ne fluid that represents a quantum monolayer. A quantum distribution function theory is presented and implemented in the computation of the chemical potential as a function of the density of the adsorbed material. The quantum partition function in the canonical ensemble is written in its path integral representation with paths expanded in a Fourier series (Fourier path integral). The multidimensional integrals obtained in this representation are solved using the j-walking Monte Carlo integration technique. The results obtained suggest that as the quantum contributions increase the amount of adsorbed material decreases, compared with classical results. An increment in internal and kinetic energies due to quantum effects is responsible for the reduction in the amount of adsorbed material. As expected, quantum effects are much larger at low temperatures.     

Multireference quantum Monte Carlo study of the O4 molecule

Fixed-node diffusion quantum Monte Carlo (FN-DMC) calculations are performed to obtain the most accurate dissociation barrier and heat of formation with respect to dissociation into molecular oxygen for the chemically bound tetraoxygen molecule. Multireference trial wave functions were used and built from truncated CASSCF(16,12) through a weight-consistent scheme allowing to control the fixed-node error. Results are compared with the previous ab initio benchmark Complete Active Space SCF Averaged Coupled Pair Functional/aug-cc-pVQZ (CASSCF-ACPF/AVQZ) results. The FN-DMC barriers to dissociation and heat of formation obtained are 11.6+/-1.6 kcal/mol and 98.5+/-1.9 kcal/mol, respectively. These thermochemical energies should be taken as the theoretical references when discussing the relevance of tetraoxygen in a variety of experiments and atmospheric chemical processes.