arXiv subjects

Pablo Serra

Publications and source records attributed to Pablo Serra.

32 records · Page 2Linked to original sources

arXiv2015

Electron Localization and Energy Levels' Oscillations Induced by Controlled Deformation

Manipulating energy levels while controlling the electron localization is an essential step for many applications of confined systems. In this paper we demonstrate how to achieve electron localization and induce energy level oscillation in one-dimensional quantum systems by externally controlling the deformation of the system. From a practical point of view, the one-dimensional potentials can be realized using layered structures. In the analysis, we considered three different examples. The first one is a graded quantum well between confining infinite walls where the deformation is modeled by varying slightly the graded well. The second systems is a symmetric multiple quantum well between infinite walls under the effect of biasing voltage. The third system is a layered 2D hybrid perovskites where pressure is used to induce deformation. The calculations are conducted both numerically and analytically using the perturbation theory. It is shown that the obtained oscillations are associated with level avoided crossings and that the deformation results in changing the spatial localization of the electrons.

cond-mat.mes-hall↗

arXiv2014

Self-avoiding walks on a bilayer Bethe lattice

We propose and study a model of polymer chains in a bilayer. Each chain is confined in one of the layers and polymer bonds on first neighbor edges in different layers interact. We also define and comment results for a model with interactions between monomers on first neighbor sites of different layers. The thermodynamic properties of the model are studied in the grand-canonical formalism and both layers are considered to be Cayley trees. In the core region of the trees, which we may call a bilayer Bethe lattice, we find a very rich phase diagram in the parameter space defined by the two activities of monomers and the Boltzmann factor associated to the interlayer interaction between bonds or monomers. Beside critical and coexistence surfaces, there are tricritical, bicritical and critical endpoint lines, as well as higher order multicritical points.

cond-mat.stat-mech↗

arXiv2013

Molecular kinetics of solid and liquid CHCl$_3$

We present a detailed analysis of the molecular kinetics of CHCl$_3$ in a range of temperatures covering the solid and liquid phases. Using nuclear quadrupolar resonance we determine the relaxation times for the molecular rotations in solid at pre-melting conditions. Molecular dynamics simulations are used to characterize the rotational dynamics in the solid and liquid phases and to study the local structure of the liquid in terms of the molecular relative orientations. We find that in the pre-melting regime the molecules rotate about the C-H bond, but the rotations are isotropic in the liquid, even at supercooled conditions.

physics.chem-ph↗

arXiv2013

The Plastic and Liquid Phases of CCl$_3$Br Studied by Molecular Dynamics Simulations

We present a molecular dynamics study of the liquid and plastic crystalline phases of CCl$_3$Br. We investigated the short-range orientational order using a recently developed classification method and we found that both phases behave in a very similar way. The only differences occur at very short molecular separations, which are shown to be very rare. The rotational dynamics was explored using time correlation functions of the molecular bonds. We found that the relaxation dynamics corresponds to an isotropic diffusive mode for the liquid phase, but departs from this behavior as the temperature is decreased and the system transitions into the plastic phase.

physics.chem-ph↗

arXiv2013

Quantum Confinement and Negative Heat Capacity

Thermodynamics dictates that the specific heat of a system is strictly non-negative. However, in finite classical systems there are well known theoretical and experimental cases where this rule is violated, in particular finite atomic clusters. Here, we show for the first time that negative heat capacity can also occur in finite quantum systems. The physical scenario on which this effect might be experimentally observed is discussed. Observing such an effect might lead to the design of new light harvesting nano devices, in particular a solar nano refrigerator.

cond-mat.stat-mech↗

arXiv2013

Energy and lifetime of resonant states with real basis sets

Using a probabilistic interpretation of resonant states, we propose a formula useful to calculate the lifetime of a resonance using square-integrable real basis-set expansion techniques. Our approach does not require an estimation of the density of states. The method is illustrated with calculations of $s$ and $p$ resonant-state energies and lifetimes.

quant-ph↗

arXiv2013

Quantum control of a model qubit based on a multi-layered quantum dot

In this work we present a model qubit whose basis states are eigenstates of a multi-layered quantum dot. We show that the proper design of the quantum dot results in qubit states that have excellent dynamical properties when a time-dependent driving is applied to it. In particular, it is shown that a simple sinusoidal driving is sufficient to obtain good quality Rabi oscillations between the qubit states. Moreover, the switching between states can be performed with very low leakage, even under off-resonance conditions. In this sense, the quantum control of the qubit is robust under some perturbations and achieved with simple means.

quant-ph↗

arXiv2012

Ground-state stability and criticality of two-electron atoms with screened Coulomb potentials using the B-splines basis set

We applied the finite-size scaling method using the B-splines basis set to construct the stability diagram for two-electron atoms with a screened Coulomb potential. The results of this method for two electron atoms are very accurate in comparison with previous calculations based on Gaussian, Hylleraas, and finite-element basis sets. The stability diagram for the screened two-electron atoms shows three distinct regions: a two-electron region, a one-electron region, and a zero-electron region, which correspond to stable, ionized and double ionized atoms. In previous studies, it was difficult to extend the finite size scaling calculations to large molecules and extended systems because of the computational cost and the lack of a simple way to increase the number of Gaussian basis elements in a systematic way. Motivated by recent studies showing how one can use B-splines to solve Hartree-Fock and Kohn-Sham equations, this combined finite size scaling using the B-splines basis set, might provide an effective systematic way to treat criticality of large molecules and extended systems. As benchmark calculations, the two-electron systems show the feasibility of this combined approach and provide an accurate reference for comparison.

physics.atom-ph↗

arXiv2012

Near-threshold properties of the electronic density of layered quantum-dots

We present a way to manipulate an electron trapped in a layered quantum dot based on near-threshold properties of one-body potentials. We show that potentials with a simple global parameter allows the manipulation of the wave function changing its spatial extent. This phenomenon seems to be fairly general and could be implemented using current quantum-dot quantum wells technologies and materials if a proper layered quantum dot is designed. The layered quantum dot under consideration is similar to a quantum-dot quantum well device, i.e. consists of a spherical core surrounded by successive layers of different materials. The number of layers and the constituent material are chosen to highlight the near-threshold properties. In particular we show that the near-threshold phenomena can be observed using an effective mass approximation model that describes the layered quantum dot which is consistent with actual experimental parameters.

cond-mat.mes-hall↗

arXiv2011

Real stabilization of resonance states employing two parameters: basis set size and coordinate scaling

The resonance states of one- and two-particle Hamiltonians are studied using variational expansions with real basis-set functions. The resonance energies, $E_r$, and widths, $Γ$, are calculated using the density of states and an ${\mathcal L}^2$ golden rule-like formula. We present a recipe to select adequately some solutions of the variational problem. The set of approximate energies obtained shows a very regular behaviour with the basis-set size, $N$. Indeed, these particular variational eigenvalues show a quite simple scaling behaviour and convergence when $N\rightarrow \infty$. Following the same prescription to choose particular solutions of the variational problem we obtain a set of approximate widths. Using the scaling function that characterizes the behaviour of the approximate energies as a guide, it is possible to find a very good approximation to the actual value of the resonance width.

physics.atom-ph↗

arXiv2010

The scaling of the density of states in systems with resonance states

Resonance states of a two-electron quantum dot are studied using a variational expansion with both real basis-set functions and complex scaling methods. We present numerical evidence about the critical behavior of the density of states in the region where there are resonances. The critical behavior is signaled by a strong dependence of some features of the density of states with the basis-set size used to calculate it. The resonance energy and lifetime are obtained using the scaling properties of the density of states

cond-mat.other↗

arXiv2010

Entropy, fidelity, and double orthogonality for resonance states in two-electron quantum dots

Resonance states of a two-electron quantum dot are studied using a variational expansion with both real basis-set functions and complex scaling methods. The two-electron entanglement (linear entropy) is calculated as a function of the electron repulsion at both sides of the critical value, where the ground (bound) state becomes a resonance (unbound) state. The linear entropy and fidelity and double orthogonality functions are compared as methods for the determination of the real part of the energy of a resonance. The complex linear entropy of a resonance state is introduced using complex scaling formalism.

quant-ph↗

arXiv2006

Solution of a model of self-avoiding walks with multiple monomers per site on the Bethe lattice

We solve a model of self-avoiding walks with up to two monomers per site on the Bethe lattice. This model, inspired on the Domb-Joyce model, was recently proposed to describe the collapse transition observed in interacting polymers [J. Krawczyk et al, Phys. Rev. Lett. 96, 240603 (2006)]. When immediate self-reversals are allowed (RA model), the solution displays a phase diagram with a polymerized phase and a non-polymerized phase, separated by a phase transition which is of first order for a non-vanishing statistical weight of doubly occupied sites. If the configurations are restricted forbidding immediate self-reversals (RF model), a richer phase diagram is found, displaying a tricritical point and a critical endpoint.

cond-mat.stat-mech↗

arXiv2004

Polymers with attractive interactions on the Husimi tree

We obtain the solution of models of self-avoiding walks with attractive interactions on Husimi lattices built with squares. Two attractive interactions are considered: between monomers on first-neighbor sites and not consecutive along a walk and between bonds located on opposite edges of elementary squares. For coordination numbers q>4, two phases, one polymerized the other non-polymerized, are present in the phase diagram. For small values of the attractive interaction the transition between those phases is continuous, but for higher values a first-order transition is found. Both regimes are separated by a tricritical point. For q=4 a richer phase diagram is found, with an additional (dense) polymerized phase, which is stable for for sufficiently strong interactions between bonds. The phase diagram of the model in the three-dimensional parameter space displays surfaces of continuous and discontinuous phase transitions and lines of tricritical points, critical endpoints and triple points.

cond-mat.stat-mech↗