arXiv subjects

M. C. Gordillo

Publications and source records attributed to M. C. Gordillo.

At least 37 records · Page 2Linked to original sources

arXiv2020

Diffusion Monte Carlo calculations of fully-heavy (multiquark) bound states

We use a diffusion Monte Carlo method to solve the many-body Schrödinger equation describing fully-heavy tetraquark systems. This approach allows to reduce the uncertainty of the numerical calculation at the percent level, accounts for multi-particle correlations in the physical observables, and avoids the usual quark-clustering assumed in other theoretical techniques applied to the same problem. The interaction between particles was modeled by the most general and accepted potential, i.e. a pairwise interaction including Coulomb, linear-confining and hyperfine spin-spin terms. This means that, in principle, our analysis should provide some rigorous statements about the mass location of the all-heavy tetraquark ground states, which is particularly timely due to the very recent observation made by the LHCb collaboration of some enhancements in the invariant mass spectra of $J/ψ$-pairs. Our main results are: (i) the $cc\bar c\bar c$, $cc\bar b\bar b$ ($bb\bar c\bar c$) and $bb\bar b \bar b$ lowest-lying states are located well above their corresponding meson-meson thresholds; (ii) the $J^{PC}=0^{++}$ $cc\bar c\bar c$ ground state with preferred quark-antiquark pair configurations is compatible with the enhancement(s) observed by the LHCb collaboration; (iii) our results for the $cc\bar c\bar b$ and $bb\bar c\bar b$ sectors seem to indicate that the $0^+$ and $1^+$ ground states are almost degenerate with the $2^+$ located around $100\,\text{MeV}$ above them; (iv) smaller mass splittings for the $cb\bar c\bar b$ system are predicted, with absolute mass values in reasonable agreement with other theoretical works; (v) the $1^{++}$ $cb\bar c\bar b$ tetraquark ground state lies at its lowest $S$-wave meson-meson threshold and it is compatible with a molecular configuration.

hep-ph↗

arXiv2020

Superfluid and supersolid phases of 4He on the second layer of graphite

We revisited the phase diagram of the second layer of 4He on top of graphite using quantum Monte Carlo methods. Our aim was to explore the existence of the novel phases suggested recently in experimental works, and determine their properties and stability limits. We found evidence of a superfluid quantum phase with hexatic correlations, induced by the corrugation of the first Helium layer, and a quasi-two-dimensional supersolid corresponding to a 7/12 registered phase. The 4/7 commensurate solid was found to be unstable, while the triangular incommensurate crystals, stable at large densities, were normal.

cond-mat.other↗

arXiv2018

Fluid and registered phases in the second layer of 3He on graphite

A quantum Monte Carlo approach, considering all the corrugation effects, was used to calculate the complete phase diagram of the second $^3$He layer adsorbed on graphite. We found that a first-layer triangular solid was in equilibrium with a gas in the second layer. At a surface density $0.166 \pm 0.001$ Å$^{-2}$, this fluid changes into two first-layer registered phases: 4/7 and 7/12 solids. The 7/12 arrangement transforms into an incommensurate triangular structure of $ρ= 0.189 \pm 0.001$ Å$^{-2}$ upon further helium loading. A recently proposed hexatic phase was found to be unstable with respect to those commensurate solids.

cond-mat.other↗

arXiv2017

Luttinger parameter of quasi-one-dimensional para-H2

We have studied the ground-state properties of para-hydrogen in one dimension and in quasi-one-dimensional configurations using the path integral ground state Monte Carlo method. This method produces zero-temperature exact results for a given interaction and geometry. The quasi-one-dimensional setup has been implemented in two forms: the inner channel inside a carbon nanotube coated with H$_2$ and a harmonic confinement of variable strength. Our main result is the dependence of the Luttinger parameter on the density within the stable regime. Going from one dimension to quasi-one dimension, keeping the linear density constant, produces a systematic increase of the Luttinger parameter. This increase is however not enough to reach the superfluid regime and the system always remain in the quasi-crystal regime, according to Luttinger liquid theory.

cond-mat.other↗

arXiv2016

3He on preplated graphite

By using the diffusion Monte Carlo method, we obtained the full phase diagram of $^3$He on top of graphite preplated with a solid layer of $^4$He. All the $^4$He atoms of the substrate were explicitly considered and allowed to move during the simulation. We found that the ground state is a liquid of density 0.007 $\pm$ 0.001 Å$^{-2}$, in good agreement with available experimental data. This is significantly different from the case of $^3$He on clean graphite, in which both theory and experiment agree on the existence of a gas-liquid transition at low densities. Upon an increase in $^3$He density, we predict a first-order phase transition between a dense liquid and a registered 7/12 phase, the 4/7 phase being found metastable in our calculations. At larger second-layer densities, a final transition is produced to an incommensurate triangular phase.

cond-mat.other↗

arXiv2016

Liquid and solid phases of 3He on graphite

Recent heat-capacity experiments show quite unambiguously the existence of a liquid $^3$He phase adsorbed on graphite. This liquid is stable at an extremely low density, possibly one of the lowest found in Nature. Previous theoretical calculations of the same system, and in strictly two dimensions, agree with the result that this liquid phase is not stable and the system is in the gas phase. We calculated the phase diagram of normal $^3$He adsorbed on graphite at $T=0$ using quantum Monte Carlo methods. Considering a fully corrugated substrate we observe that at densities lower that 0.006 Å$^{-2}$ the system is a very dilute gas, that at that density is in equilibrium with a liquid of density 0.014 Å$^{-2}$. Our prediction matches very well the recent experimental findings on the same system. On the contrary, when a flat substrate is considered, no gas-liquid coexistence is found, in agreement with previous calculations. We also report results on the different solid structures, and the corresponding phase transitions that appear at higher densities.

cond-mat.other↗

arXiv2015

Bosonic hard spheres in quasi-one dimensional bichromatic optical lattices

We calculated the phase diagram of a continuous system of hard spheres loaded in a quasi-one dimensional bichromatic optical lattice. The wavelengths of both lattice-defining lasers were chosen to model an incommensurate arrangement. Densities of one particle and half a particle per potential well were considered. Our results can be compared directly to those of the experimental system [Fallani et al. PRL, {\bf 98} 130404 (2007)] from which our initial parameters were taken. The phase diagrams for both densities are significatively different to those obtained by describing the same experimental setup with a Bose-Hubbard model.

cond-mat.quant-gas↗

arXiv2014

State diagram for continuous quasi-one dimensional systems in optical lattices

We studied the appearance of Mott insulator domains of hard sphere bosons on quasi one-dimensional optical lattices when an harmonic trap was superimposed along the main axis of the system. Instead of the standard approximation represented by the Bose-Hubbard model, we described those arrangements by continuous Hamiltonians that depended on the same parameters as the experimental setups. We found that for a given trap the optical potential depth, $V_0$, needed to create a single connected Mott domain decreased with the number of atoms loaded on the lattice. If the confinement was large enough, it reached a minimum when, in absence of any optical lattice, the atom density at the center of the trap was the equivalent of one particle per optical well. For larger densities, the creation of that single domain proceeded via an intermediate shell structure in which Mott domains alternated with superfluid ones.

cond-mat.quant-gas↗

arXiv2014

A continuous model for bosonic hard spheres in quasi one-dimensional optical lattices

By means of diffusion Monte Carlo calculations, we investigated the quantum phase transition between a superfluid and a Mott insulator for a system of hard-sphere bosons in a quasi one-dimensional optical lattice. For this continuous hamiltonian, we studied how the stability limits of the Mott phase changed with the optical lattice depth and the transverse confinement width. A comparison of these results to those of a one-dimensional homogeneous Bose-Hubbard model indicates that this last model describes accurately the phase diagram only in the limit of deep lattices. For shallow ones, our results are comparable to those of the sine-Gordon model in its limit of application. We provide an estimate of the critical parameters when none of those models are realistic descriptions of a quasi one-dimensional optical lattice.

cond-mat.quant-gas↗

arXiv2014

Zero-temperature phase diagram of hard sphere bosons in asymmetric three dimensional optical lattices

We studied the superfluid-to-Mott insulator transition for bosonic hard spheres loaded in asymmetric three-dimensional optical lattices by means of diffusion Monte Carlo calculations. The onset of the transition was monitored through the change in the chemical potential around the density corresponding to one particle per potential well. With this method, we were able to reproduce the results given in the literature for three-dimensional symmetric lattices and for systems whose asymmetry makes them equivalent to a set of quasi-one dimensional tubes. The location of the same transition for asymmetric systems akin to a stack of quasi-two dimensional lattices will be also given. Our results were checked against those given by a Bose-Hubbard model for similar arrangements.

cond-mat.quant-gas↗

arXiv2013

Second layer of H2 and D2 adsorbed on graphene

We report diffusion Monte Carlo calculations on the phase diagrams of para-H2 and ortho-D2 adsorbed on top of a first layer of the same substances on graphene. We found that the ground state of the second layer is a triangular incommensurate solid for both isotopes. The densities for promotion to a second layer and for the onset of a two-dimensional solid on that second layer compare favorably to available experimental data in both cases.

cond-mat.mtrl-sci↗

arXiv2012

4He adsorbed outside a single carbon nanotube

The phase diagrams of $^4$He adsorbed on the external surfaces of single armchair carbon nanotubes with radii in the range 3.42 -- 10.85 Å\ are calculated using the diffusion Monte Carlo method. For nanotubes narrower than a (10,10) one, the ground state is an incommensurate solid similar to the one found for H$_2$ on the same substrates. For wider nanotubes, the phase with the minimum energy per particle is a liquid layer. Curved $\sqrt 3 \times \sqrt 3$ registered solids similar to the ones found on graphene and graphite were unstable for all the tubes considered.

cond-mat.mtrl-sci↗

arXiv2012

Zero-temperature phase diagram of the second layer of $^{\bf 4}$He adsorbed on graphene

The phase diagram at zero temperature of $^4$He adsorbed on an helium incommensurate triangular solid on top of a single graphene sheet has been obtained using the diffusion Monte Carlo method. We have found that, in accordance with previous experimental and simulation results for graphite, the ground state of $^4$He on this setup is a liquid that, upon compression, transforms into a triangular solid. To define the stability limits of both liquid and solid phases, we considered not only the adsorption energies of the atoms located on the second layer but the average energy of the atoms in both layers. Our results show that the lower density limit for a stable liquid in the second layer is 0.163 $\pm$ 0.005 Å$^{-2}$ and that the lower limit for the existence of an incommensurate solid on the second layer is 0.186 $\pm$ 0.003 Å$^{-2}$. Both values are in overall agreement with the results of torsional oscillator experiments and heat capacity measurements on graphite. The 4/7 and 7/12 registered solids are found to be metastable with respect to triangular incommensurate arrangements of the same density.

cond-mat.other↗

arXiv2011

Phase transitions of H2 adsorbed on the surface of single carbon nanotubes

By means of Diffusion Monte Carlo calculations, we obtained the complete phase diagrams of H$_2$ adsorbed on the outer surface of isolated armchair carbon nanotubes of radii ranging from 3.42 to 10.85 Å. We only considered density ranges corresponding to the filling of the first adsorption layer in these curved structures. In all cases, the zero-temperature ground state was found to be an incommensurate solid, except in the widest tube, in which the structure with lowest energy is an analogous of the $\sqrt{3} \times \sqrt{3}$ phase found in planar substrates. Those incommensurate solids result form the arrangement of the hydrogen molecules in circumferences whose plane is perpendicular to the main axis of the carbon nanotube. For each tube, there is only one of such phases stable in the density range considered, except in the case of the (5,5) and (6,6) tubes, in which two of these incommensurate solids are separated by novel first order phase transitions.

cond-mat.mes-hall↗

arXiv2011

Supersolidity in quantum films adsorbed on graphene and graphite

Using quantum Monte Carlo we have studied the superfluid density of the first layer of $^4$He and H$_2$ adsorbed on graphene and graphite. Our main focus has been on the equilibrium ground state of the system, which corresponds to a registered $\sqrt3 \times \sqrt3$ phase. The perfect solid phase of H$_2$ shows no superfluid signal whereas $^4$He has a finite but small superfluid fraction (0.67%). The introduction of vacancies in the crystal makes the superfluidity increase, showing values as large as 14% in $^4$He without destroying the spatial solid order.

cond-mat.other↗

arXiv2010

Phase diagram of H2 adsorbed on graphene

The phase diagram of the first layer of H$_2$ adsorbed on top of a single graphene sheet has been calculated by means of a series of diffusion Monte Carlo (DMC) simulations. We have found that, as in the case of $^4$He, the ground state of molecular hydrogen is a $\sqrt3 \times \sqrt3$ commensurate structure, followed, upon a pressure increase, by an incommensurate triangular solid. A striped phase of intermediate density was also considered, and found lying on top of the equilibrium curve separating both commensurate and incommensurate solids.

cond-mat.mtrl-sci↗

arXiv2009

Quantum Fluids in Nanotubes: a Quantum Monte Carlo Approach

We review quantum Monte Carlo results on energetic and structure properties of quantum fluids adsorbed in a bundle of carbon nanotubes. Using realistic interatomic interactions the different adsorption sites that a bundle offer are accurately studied and compared in some cases with strictly one-dimensional geometries. The study is performed quite extensively for $^4$He and restricted to the inner part of a single nanotube for H$_2$ and D$_2$. From a theoretical point of view, nanotubes open the real possibility of a quasi-one-dimensional confinement where to study quantum fluids in extremely reduced dimensionality. The results obtained show that in the narrowest configurations the system is nearly one-dimensional reinforcing the interest on the physics of one-dimensional quantum fluids. Experimental confirmation of the theoretical results obtained is still not in a satisfactory situation due to the difficulties on extracting from the data the dominant adsorption sites.

cond-mat.mtrl-sci↗

arXiv2009

4He on a single graphene sheet

The phase diagram of the first layer of $^4$He adsorbed on a single graphene sheet has been calculated by a series of diffusion Monte Carlo calculations including corrugation effects. As the number of C-He interactions is reduced with respect to graphite the binding energy of $^4$He atoms to graphene is approximately 13.4 K/He atom smaller. Our results indicate that the phase diagram is qualitatively similar to that of helium on top of graphite. A two-dimensional liquid film on graphene is predicted to be metastable with respect to the commensurate solid but the difference in energy between both phases is very small, opening the possibility of such a liquid film to be experimentally observed.

cond-mat.other↗