L. Vranješ Markić

Quantum Monte Carlo simulation of spin-polarized tritium

The ground-state properties of spin-polarized tritium T↓ at zero temperature are obtained by means of diffusion Monte Carlo calculations. Using an accurate ab initio T↓-T↓ interatomic potential we have studied its liquid phase, from the spinodal point until densities above its freezing point. The equilibrium density of the liquid is significantly higher and the equilibrium energy of −3.664(6) K significantly lower than in previous approximate descriptions. The solid phase has also been studied for three lattices up to high pressures and we find that hcp lattice is slightly preferred. The liquid-solid phase transition has been determined using the double-tangent Maxwell construction; at zero temperature, bulk tritium freezes at a pressure of P=9(1) bar.

Quantum Monte Carlo study of small pure and mixed spin-polarized tritium clusters.

We have investigated the stability limits of small spin-polarized clusters consisting of up to ten spin-polarized tritium T downward arrow atoms and the mixtures of T downward arrow with spin-polarized deuterium D downward arrow and hydrogen H downward arrow atoms. All of our calculations have been performed using the variational and diffusion Monte Carlo methods. For clusters with D downward arrow atoms, the released node procedure is used in cases where the wave function has nodes. In addition to the energy, we have also calculated the structure of small clusters using unbiased estimators. Results obtained for pure T downward arrow clusters are in good accordance with previous calculations, confirming that the trimer is the smallest spin-polarized tritium cluster. Our results show that mixed T downward arrow-H downward arrow clusters having up to ten atoms are unstable and that it takes at least three tritium atoms to bind one, two, or three D downward arrow atoms. Among all the considered clusters, we have found no other Borromean states except the ground state of the T downward arrow trimer.

Ground state of small mixed helium and spin-polarized tritium clusters: A quantum Monte Carlo study

We report results for the ground-state energy and structural properties of small (4)He-T↓ clusters consisting of up to four T↓ and eight (4)He atoms. These results have been obtained using very well-known (4)He-(4)He and T↓- T↓ interaction potentials and several models for the (4)He- T↓ interatomic potential. All the calculations have been performed with variational and diffusion Monte Carlo methods. It takes at least three atoms to form a mixed bound state. In particular, for small clusters the binding energies are significantly affected by the precise form of the (4)He- T↓ interatomic potential but the stability limits remain unchanged. The only exception is the (4)He(2)T↓ trimer whose stability in the case of the weakest (4)He- T↓ interaction potential is uncertain while it seems stable for other potentials. The mixed trimer (4)He(T↓)(2), a candidate for the Borromean state, is not bound. All other studied clusters are stable. Some of the weakest bound clusters can be classified as quantum halo as a consequence of having high probability of being in a classically forbidden region.

Quantum Monte Carlo study of large spin-polarized tritium clusters

This work expands recent investigations in the field of spin-polarized tritium (T downward arrow) clusters. We report the results for the ground-state energy and structural properties of large T downward arrow clusters consisting of up to 320 atoms. All calculations have been performed with variational and diffusion Monte Carlo methods, using an accurate ab initio interatomic potential. Our results for N < or = 40 are in good agreement with results obtained by other groups. Using a liquid-drop expression for the energy per particle, we estimate the liquid equilibrium density, which is in good agreement with our recently obtained results for bulk T downward arrow. In addition, the calculations of the energy for large clusters have allowed for an estimation of the surface tension. From the mean-square radius of the drop, determined using unbiased estimators, we determine the dependence of the radii on the size of the cluster and extract the unit radius of the T downward arrow liquid.

Spin-polarized hydrogen and its isotopes: A rich class of quantum phases (Review Article)

We review the recent activity in the theoretical description of spin-polarized atomic hydrogen and its isotopes at very low temperatures. Spin-polarized hydrogen is the only system in nature that remains stable in the gas phase even in the zero temperature limit due to its small mass and weak interatomic interaction. Hydrogen and its heavier isotope tritium are bosons, the heavier mass of tritium producing a self-bound (liquid) system at zero temperature. The other isotope, deuterium, is a fermion with nuclear spin one making possible the study of three different quantum systems depending on the population of the three degenerate spin states. From the theoretical point of view, spin-polarized hydrogen is specially appealing because its interatomic potential is very accurately known making possible its precise quantum many-body study. The experimental study of atomic hydrogen has been very difficult due to its high recombination rate, but it finally led to its Bose-Einstein condensate state in 1998. Degene...

Stability limits of mixed spin-polarised tritium clusters

We present variational and diffusion Monte Carlo studies of pure and mixed spin-polarised tritium clusters. Our results for pure clusters, consisting of only tritium atoms T↓, are in good agreement with previous calculations. We show that mixed clusters, consisting of up to 60 T↓ atoms and one spin-polarized deuterium D↓ atom are stable. On the other hand, we verified that tritium clusters of up to N = 60 atoms do not bind even one H↓ atom; the ground-state energy of the considered mixed T↓-H↓ clusters is within the errorbars equal to or higher than the energy of the pure tritium cluster. In addition to the energy, we present results of the structure of the clusters, which has been determined using unbiased estimators. For larger N, a D↓ atom forms an Andreev state on the surface of a tritium cluster core. The H↓ atoms stay further away from the center of the cluster than the D↓ atoms and slowly increase their separation as the simulation proceeds, indicating instability.

Quantum Monte Carlo study of spin-polarized deuterium

The ground-state properties of spin-polarized deuterium (D#) at zero temperature are obtained by means of diffusion Monte Carlo calculations within the fixed-node approximation. Three D# species have been investigated (D#1, D#2, D#3), corresponding respectively to one, two, and three equally occupied nuclear-spin states. The influence of the backflow correlations on the ground-state energy of the systems is explored. The equations of state of liquid D#2 and D#3 are obtained and compared with the ones obtained in previous approximate predictions. The density and pressure at which D#1 experiences a gas-liquid transition at T=0 are obtained. PACS numbers: 67.63.Gh, 02.70.Ss

Ground-state properties of weakly bound helium-alkali trimers

Weakly bound triatomic molecules consisting of two helium atoms and one alkali metal atom are studied by means of the diffusion Monte Carlo method. We determined the stability of 4He2A, 4He3HeA, and 3He2A, where A is one of the alkali atoms Li, Na, K, Rb, or Cs. Some of the trimers with 3He are predicted to be self-bound for the first time, but this is observed to be dependent on the He-A interaction potential model. In addition to the ground-state energy of the trimers, we determined their density, radial, and angular distributions. Many of them are spatially very extended, which qualifies them as quantum halo states.

One dimensional 1H, 2H and 3H

The ground-state properties of one-dimensional electron-spin-polarized hydrogen

Harmonically trapped Bose–Bose mixtures: a quantum Monte Carlo study

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