57 research outputs found

### The nature and strength of inter-layer binding in graphite

We computed the inter-layer bonding properties of graphite using an ab-initio
many body theory. We carried out variational and diffusion quantum Monte Carlo
calculations and found an equilibrium inter-layer binding energy in good
agreement with most recent experiments. We also analyzed the behavior of the
total energy as a function of interlayer separation at large distances
comparing the results with the predictions of the random phase approximation.Comment: 5 pages; to appear in Phys. Rev. Let

### Finite compressibility in the low-doping region of the two-dimensional $t{-}J$ model

We revisit the important issue of charge fluctuations in the two-dimensional
$t{-}J$ model by using an improved variational method based on a wave function
that contains both the antiferromagnetic and the d-wave superconducting order
parameters. In particular, we generalize the wave function introduced some time
ago by J.P. Bouchaud, A. Georges, and C. Lhuillier [J. de Physique {\bf 49},
553 (1988)] by considering also a {\it long-range} spin-spin Jastrow factor, in
order to correctly reproduce the small-$q$ behavior of the spin fluctuations.
We mainly focus our attention on the physically relevant region $J/t \sim 0.4$
and find that, contrary to previous variational ansatz, this state is stable
against phase separation for small hole doping. Moreover, by performing
projection Monte Carlo methods based on the so-called fixed-node approach, we
obtain a clear evidence that the $t{-}J$ model does not phase separate for $J/t
\lesssim 0.7$ and that the compressibility remains finite close to the
antiferromagnetic insulating state.Comment: 10 page

### Theoretical constraints for the magnetic-dimer transition in two-dimensional spin models

From general arguments, that are valid for spin models with sufficiently
short-range interactions, we derive strong constraints on the excitation
spectrum across a continuous phase transition at zero temperature between a
magnetic and a dimerized phase, that breaks the translational symmetry. From
the different symmetries of the two phases, it is possible to predict, at the
quantum critical point, a branch of gapless excitations, not described by
standard semi-classical approaches. By using these arguments, supported by
intensive numerical calculations, we obtain a rather convincing evidence in
favor of a first-order transition from the ferromagnetic to the dimerized phase
in the two-dimensional spin-half model with four-spin ring-exchange
interaction, recently introduced by A.W. Sandvik et al. [Phys. Rev. Lett. 89,
247201 (2002)].Comment: 7 pages and 5 figure

### Ab initio investigation of the melting line of nitrogen at high pressure

Understanding the behavior of molecular systems under pressure is a
fundamental problem in condensed matter physics. In the case of nitrogen, the
determination of the phase diagram and in particular of the melting line, are
largely open problems. Two independent experiments have reported the presence
of a maximum in the nitrogen melting curve, below 90 GPa, however the position
and the interpretation of the origin of such maximum differ. By means of ab
initio molecular dynamics simulations based on density functional theory and
thermodynamic integration techniques, we have determined the phase diagram of
nitrogen in the range between 20 and 100 GPa. We find a maximum in the melting
line, related to a transformation in the liquid, from molecular N_2 to
polymeric nitrogen accompanied by an insulator-to-metal transition

### Ab-initio calculations for the beta-tin diamond transition in Silicon: comparing theories with experiments

We investigate the pressure-induced metal-insulator transition from diamond
to beta-tin in bulk Silicon, using quantum Monte Carlo (QMC) and density
functional theory (DFT) approaches. We show that it is possible to efficiently
describe many-body effects, using a variational wave function with an optimized
Jastrow factor and a Slater determinant. Variational results are obtained with
a small computational cost and are further improved by performing diffusion
Monte Carlo calculations and an explicit optimization of molecular orbitals in
the determinant. Finite temperature corrections and zero point motion effects
are included by calculating phonon dispersions in both phases at the DFT level.
Our results indicate that the theoretical QMC (DFT) transition pressure is
significantly larger (smaller) than the accepted experimental value. We discuss
the limitation of DFT approaches due to the choice of the exchange and
correlation functionals and the difficulty to determine consistent
pseudopotentials within the QMC framework, a limitation that may significantly
affect the accuracy of the technique.Comment: 13 pages, 9 figures, submitted to the Physical Review B on October 2

### Lattice effects on the spin dynamics in antiferromagnetic molecular rings

We investigate spin dynamics in antiferromagnetic (AF) molecular rings at
finite temperature in the presence of spin-phonon (s-p) interaction. We derive
a general expression for the spin susceptibility in the weak s-p coupling limit
and then we focus on the low-frequency behavior, in order to discuss a possible
microscopic mechanism for nuclear relaxation in this class of magnetic
materials. To lowest order in a perturbative expansion, we find that the
susceptibility takes a Lorentzian profile and all spin operators ($S^x$, $S^y,
S^z$) contribute to spin dynamics at wave vectors $q\ne 0$. Spin anisotropies
and local s-p coupling play a key role in the proposed mechanism. Our results
prove that small changes in the spatial symmetry of the ring induce qualitative
changes in the spin dynamics at the nuclear frequency, providing a novel
mechanism for nuclear relaxation. Possible experiments are proposed.Comment: 4 pages, 2 figures. to appear in PR

### Theoretical investigation of methane under pressure

We present computer simulations of liquid and solid phases of condensed
methane at pressures below 25 GPa, between 150 and 300 K, where no appreciable
molecular dissociation occurs. We used molecular dynamics (MD) and metadynamics
techniques, and empirical potentials in the rigid molecule approximation, whose
validity was confirmed a posteriori by carrying out it ab initio MD simulations
for selected pressure and temperature conditions. Our results for the melting
line are in satisfactory agreement with existing measurements. We find that the
fcc crystal transforms into a hcp structure with 4 molecules per unit cell (B
phase) at about 10 GPa and 150 K, and that the B phase transforms into a
monoclinic high pressure phase above 20 GPa. Our results for solid/solid phase
transitions are consistent with those of Raman studies but the phase boundaries
estimated in our calculations are at higher pressure than those inferred from
spectroscopic data.Comment: to appear on Journ. Chem. Phy

### Magnetism and superconductivity in the $t{-}t^\prime{-}J$ model

We present a systematic study of the phase diagram of the $t{-}t^\prime{-}J$
model by using the Green's function Monte Carlo (GFMC) technique, implemented
within the fixed-node (FN) approximation and a wave function that contains both
antiferromagnetic and d-wave pairing. This enables us to study the interplay
between these two kinds of order and compare the GFMC results with the ones
obtained by the simple variational approach. By using a generalization of the
forward-walking technique, we are able to calculate true FN ground-state
expectation values of the pair-pair correlation functions. In the case of
$t^\prime=0$, there is a large region with a coexistence of superconductivity
and antiferromagnetism, that survives up to $\delta_c \sim 0.10$ for $J/t=0.2$
and $\delta_c \sim 0.13$ for $J/t=0.4$. The presence of a finite $t^\prime/t<0$
induces a strong suppression of both magnetic (with $\delta_c \lesssim 0.03$,
for $J/t=0.2$ and $t^\prime/t=-0.2$) and pairing correlations. In particular,
the latter ones are depressed both in the low-doping regime and around $\delta
\sim 0.25$, where strong size effects are present.Comment: 10 pages, 9 figure

### Weakly frustrated two-dimensional Heisenberg antiferromagnets: thermodynamic properties from a non-perturbative approach

We analyze the thermodynamic properties of the spin-S two-dimensional quantum
Heisenberg antiferromagnet on a square lattice with nearest and next-nearest
neighbor couplings in the Neel phase (J_2/J_1<0.4) employing the quantum
hierarchical reference theory (QHRT), a non-perturbative implementation of the
renormalization group method to quantum systems. We investigate the staggered
susceptibility, the structure factors and the correlation length at finite
temperature and for different values of the frustration ratio. From the finite
temperature results, we also extrapolate ground state properties, such as spin
stiffness and spontaneous staggered magnetization, providing an estimate of the
extent of quantum corrections. The behavior of these quantities as a function
of frustration may provide some hint on the breakdown of the Neel phase at zero
temperature for larger values of J_2

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