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http://hdl.handle.net/2117/394
Thu, 02 Jul 2015 05:29:06 GMT2015-07-02T05:29:06Zwebmaster.bupc@upc.eduUniversitat Politècnica de Catalunya. Servei de Biblioteques i DocumentaciónoGeneralized equipartition theorem and confining walls
http://hdl.handle.net/2117/28219
Title: Generalized equipartition theorem and confining walls
Authors: Rey Oriol, Rosendo
Abstract: It is shown that the generalized equipartition theorem as applied to the momentum may fail, even though the usual conditions of validity are met. This failure is linked to the nature of the confining walls, with ideal walls constituting a singular case. The corresponding analysis illustrates the use of proper sets of canonical coordinates, and the utility of canonical transformations other than point transformations.Mon, 08 Jun 2015 14:52:26 GMThttp://hdl.handle.net/2117/282192015-06-08T14:52:26ZRey Oriol, RosendonoOscillators, Free oscillations, Boundary value problems, Signal generators, Statistical analysisIt is shown that the generalized equipartition theorem as applied to the momentum may fail, even though the usual conditions of validity are met. This failure is linked to the nature of the confining walls, with ideal walls constituting a singular case. The corresponding analysis illustrates the use of proper sets of canonical coordinates, and the utility of canonical transformations other than point transformations.Lifespan method as a tool to study criticality in absorbing-state phase transitions
http://hdl.handle.net/2117/28182
Title: Lifespan method as a tool to study criticality in absorbing-state phase transitions
Authors: de Souza Mata, Angelica; Boguñá, Marian; Castellano, Claudio; Pastor Satorras, Romualdo
Abstract: In a recent work, a new numerical method (the lifespan method) has been introduced to study the critical properties of epidemic processes on complex networks [M. Boguna, C. Castellano, and R. Pastor-Satorras, Phys. Rev. Lett. 111, 068701 (2013)]. Here, we present a detailed analysis of the viability of this method for the study of the critical properties of generic absorbing-state phase transitions in lattices. Focusing on the well-understood case of the contact process, we develop a finite-size scaling theory to measure the critical point and its associated critical exponents. We show the validity of the method by studying numerically the contact process on a one-dimensional lattice and comparing the findings of the lifespan method with the standard quasistationary method. We find that the lifespan method gives results that are perfectly compatible with those of quasistationary simulations and with analytical results. Our observations confirm that the lifespan method is a fully legitimate tool for the study of the critical properties of absorbing phase transitions in regular lattices.Thu, 04 Jun 2015 16:32:04 GMThttp://hdl.handle.net/2117/281822015-06-04T16:32:04Zde Souza Mata, Angelica; Boguñá, Marian; Castellano, Claudio; Pastor Satorras, RomualdonoCoupled chemical-reactions, Density series expansions, Directed percolation, Lattice, SquareIn a recent work, a new numerical method (the lifespan method) has been introduced to study the critical properties of epidemic processes on complex networks [M. Boguna, C. Castellano, and R. Pastor-Satorras, Phys. Rev. Lett. 111, 068701 (2013)]. Here, we present a detailed analysis of the viability of this method for the study of the critical properties of generic absorbing-state phase transitions in lattices. Focusing on the well-understood case of the contact process, we develop a finite-size scaling theory to measure the critical point and its associated critical exponents. We show the validity of the method by studying numerically the contact process on a one-dimensional lattice and comparing the findings of the lifespan method with the standard quasistationary method. We find that the lifespan method gives results that are perfectly compatible with those of quasistationary simulations and with analytical results. Our observations confirm that the lifespan method is a fully legitimate tool for the study of the critical properties of absorbing phase transitions in regular lattices.Characterizing ordering in liquids: An information theoretic approach
http://hdl.handle.net/2117/27961
Title: Characterizing ordering in liquids: An information theoretic approach
Authors: Pardo Soto, Luis Carlos; Henao Aristizábal, Andrés; Vispa, Alessandro
Abstract: The determination of special molecular arrangements in disordered phases such as liquids is inherently difficult due to its lack of periodicity, in contrast to the crystalline solids. We have already settled a general method to study molecular liquids capable to unveil the details of the molecular ordering from small molecules to systems as big as a protein. However it would be desirable to extract some general features of a liquid phase without going into such details. In this work we propose a method to achieve this challenge by analyzing the probability distributions describing position and orientational molecular ordering within the framework of information theory. (C) 2014 Elsevier B.V. All rights reserved.Tue, 19 May 2015 10:05:55 GMThttp://hdl.handle.net/2117/279612015-05-19T10:05:55ZPardo Soto, Luis Carlos; Henao Aristizábal, Andrés; Vispa, AlessandronoShort range order, Structure, Liquid, Information theory, Reverse-Monte-Carlo, Ocorrelations, Carbon tetrachloride, Molecular liquids, Glass-transitionThe determination of special molecular arrangements in disordered phases such as liquids is inherently difficult due to its lack of periodicity, in contrast to the crystalline solids. We have already settled a general method to study molecular liquids capable to unveil the details of the molecular ordering from small molecules to systems as big as a protein. However it would be desirable to extract some general features of a liquid phase without going into such details. In this work we propose a method to achieve this challenge by analyzing the probability distributions describing position and orientational molecular ordering within the framework of information theory. (C) 2014 Elsevier B.V. All rights reserved.Burstiness and aging in social temporal networks
http://hdl.handle.net/2117/27534
Title: Burstiness and aging in social temporal networks
Authors: Moinet, Antoine; Starnini, Michele; Pastor Satorras, Romualdo
Abstract: The presence of burstiness in temporal social networks, revealed by a power-law form of the waiting time distribution of consecutive interactions, is expected to produce aging effects in the corresponding time-integrated network. Here, we propose an analytically tractable model, in which interactions among the agents are ruled by a renewal process, that is able to reproduce this aging behavior. We develop an analytic solution for the topological properties of the integrated network produced by the model, finding that the time translation invariance of the degree distribution is broken. We validate our predictions against numerical simulations, and we check for the presence of aging effects in a empirical temporal network, ruled by bursty social interactions.Wed, 22 Apr 2015 17:43:58 GMThttp://hdl.handle.net/2117/275342015-04-22T17:43:58ZMoinet, Antoine; Starnini, Michele; Pastor Satorras, RomualdonoTime-varying networks, Human dynamics, PatternsThe presence of burstiness in temporal social networks, revealed by a power-law form of the waiting time distribution of consecutive interactions, is expected to produce aging effects in the corresponding time-integrated network. Here, we propose an analytically tractable model, in which interactions among the agents are ruled by a renewal process, that is able to reproduce this aging behavior. We develop an analytic solution for the topological properties of the integrated network produced by the model, finding that the time translation invariance of the degree distribution is broken. We validate our predictions against numerical simulations, and we check for the presence of aging effects in a empirical temporal network, ruled by bursty social interactions.Quantum Monte Carlo estimation of complex-time correlations for the study of the ground-state dynamic structure function
http://hdl.handle.net/2117/27527
Title: Quantum Monte Carlo estimation of complex-time correlations for the study of the ground-state dynamic structure function
Authors: Rota, R; Casulleras Ambrós, Joaquín; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi
Abstract: We present a method based on the path integral Monte Carlo formalism for the calculation of ground-state time correlation functions in quantum systems. The key point of the method is the consideration of time as a complex variable whose phase d acts as an adjustable parameter. By using high-order approximations for the quantum propagator, it is possible to obtain Monte Carlo data all the way from purely imaginary time to d values near the limit of real time. As a consequence, it is possible to infer accurately the spectral functions using simple inversion algorithms. We test this approach in the calculation of the dynamic structure function S(q, omega) of two one-dimensional model systems, harmonic and quartic oscillators, for which S(q, omega) can be exactly calculated. We notice a clear improvement in the calculation of the dynamic response with respect to the common approach based on the inverse Laplace transform of the imaginary-time correlation function. (C) 2015 AIP Publishing LLC.Wed, 22 Apr 2015 15:28:44 GMThttp://hdl.handle.net/2117/275272015-04-22T15:28:44ZRota, R; Casulleras Ambrós, Joaquín; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, JordinoAnalytic continuation, Path-integrals, Maximum-entropy, Rate constants, Systems, SimulationsWe present a method based on the path integral Monte Carlo formalism for the calculation of ground-state time correlation functions in quantum systems. The key point of the method is the consideration of time as a complex variable whose phase d acts as an adjustable parameter. By using high-order approximations for the quantum propagator, it is possible to obtain Monte Carlo data all the way from purely imaginary time to d values near the limit of real time. As a consequence, it is possible to infer accurately the spectral functions using simple inversion algorithms. We test this approach in the calculation of the dynamic structure function S(q, omega) of two one-dimensional model systems, harmonic and quartic oscillators, for which S(q, omega) can be exactly calculated. We notice a clear improvement in the calculation of the dynamic response with respect to the common approach based on the inverse Laplace transform of the imaginary-time correlation function. (C) 2015 AIP Publishing LLC.1H nuclear spin relaxation of liquid water from molecular dynamics simulations
http://hdl.handle.net/2117/27329
Title: 1H nuclear spin relaxation of liquid water from molecular dynamics simulations
Authors: Calero Borrallo, Carles; Martí Rabassa, Jordi; Guàrdia Manuel, Elvira
Abstract: We have investigated the nuclear spin relaxation properties of 1H in liquid water with the help of molecular dynamics simulations. We have computed the 1H nuclear spin relaxation times T1 and T2 and determined the contribution of the different interactions to the relaxation at different temperatures and for different classical water models (SPC/E, TIP3P, TIP4P, and TIP4P/2005). Among the water models considered, the TIP4P/2005 model exhibits the best agreement with the experiment. The same analysis was performed with Car–Parrinello ab initio molecular dynamics simulations of bulk water at T = 330 K, which provided results close to the experimental values at room temperature. To complete the study, we have successfully accounted for the temperature-dependence of T1 and T2 in terms of a simplified model, which considers the reorientation in finite angle jumps and the diffusive translation of water molecules.Tue, 14 Apr 2015 14:49:29 GMThttp://hdl.handle.net/2117/273292015-04-14T14:49:29ZCalero Borrallo, Carles; Martí Rabassa, Jordi; Guàrdia Manuel, ElviranoMolecular Dynamics SimulationsWe have investigated the nuclear spin relaxation properties of 1H in liquid water with the help of molecular dynamics simulations. We have computed the 1H nuclear spin relaxation times T1 and T2 and determined the contribution of the different interactions to the relaxation at different temperatures and for different classical water models (SPC/E, TIP3P, TIP4P, and TIP4P/2005). Among the water models considered, the TIP4P/2005 model exhibits the best agreement with the experiment. The same analysis was performed with Car–Parrinello ab initio molecular dynamics simulations of bulk water at T = 330 K, which provided results close to the experimental values at room temperature. To complete the study, we have successfully accounted for the temperature-dependence of T1 and T2 in terms of a simplified model, which considers the reorientation in finite angle jumps and the diffusive translation of water molecules.Sharp crossover from composite fermionization to phase separation in microscopic mixtures of ultracold bosons
http://hdl.handle.net/2117/27031
Title: Sharp crossover from composite fermionization to phase separation in microscopic mixtures of ultracold bosons
Authors: Garcia March, Miguel Angel; Julia Diaz, Bruno; Astrakharchik, Grigori; Busch, T; Boronat Medico, Jordi; Rios Polls, Artur
Abstract: We show that a two-component mixture of a few repulsively interacting ultracold atoms in a one-dimensional trap possesses very diverse quantum regimes and that the crossover between them can be induced by tuning the interactions in one of the species. Starting from the composite fermionization regime, in which the interactions between both components are large and neither gas is phase coherent, our results show that a phase-separated state can be reached by increasing the interaction in one of the species. In this regime, the weakly interacting component stays at the center of the trap and becomes almost fully phase coherent, while the strongly interacting one is expelled to the edges of the trap. The crossover is sharp, as can be witnessed in the system's energy and in the occupation of the lowest natural orbital of the weakly interacting species. We show that such a transition is a few-atom effect which disappears for a large population imbalance.Wed, 25 Mar 2015 16:00:33 GMThttp://hdl.handle.net/2117/270312015-03-25T16:00:33ZGarcia March, Miguel Angel; Julia Diaz, Bruno; Astrakharchik, Grigori; Busch, T; Boronat Medico, Jordi; Rios Polls, ArturnoInteracting species, Large population, Natural orbitals, Phase coherent, Quantum regimes, Two-component mixtures, Ultracold atoms, Ultracold bosonsWe show that a two-component mixture of a few repulsively interacting ultracold atoms in a one-dimensional trap possesses very diverse quantum regimes and that the crossover between them can be induced by tuning the interactions in one of the species. Starting from the composite fermionization regime, in which the interactions between both components are large and neither gas is phase coherent, our results show that a phase-separated state can be reached by increasing the interaction in one of the species. In this regime, the weakly interacting component stays at the center of the trap and becomes almost fully phase coherent, while the strongly interacting one is expelled to the edges of the trap. The crossover is sharp, as can be witnessed in the system's energy and in the occupation of the lowest natural orbital of the weakly interacting species. We show that such a transition is a few-atom effect which disappears for a large population imbalance.Temporal percolation in activity-driven networks
http://hdl.handle.net/2117/26924
Title: Temporal percolation in activity-driven networks
Authors: Starnini, Michele; Pastor Satorras, Romualdo
Abstract: We study the temporal percolation properties of temporal networks by taking as a representative example the recently proposed activity-driven-network model [N. Perra et al., Sci. Rep. 2, 469 ( 2012)]. Building upon an analytical framework based on a mapping to hidden variables networks, we provide expressions for the percolation time T-p marking the onset of a giant connected component in the integrated network. In particular, we consider both the generating function formalism, valid for degree-uncorrelated networks, and the general case of networks with degree correlations. We discuss the different limits of the two approaches, indicating the parameter regions where the correlated threshold collapses onto the uncorrelated case. Our analytical predictions are confirmed by numerical simulations of the model. The temporal percolation concept can be fruitfully applied to study epidemic spreading on temporal networks. We show in particular how the susceptible-infected-removed model on an activity-driven network can be mapped to the percolation problem up to a time given by the spreading rate of the epidemic process. This mapping allows us to obtain additional information on this process, not available for previous approaches.Mon, 23 Mar 2015 09:53:58 GMThttp://hdl.handle.net/2117/269242015-03-23T09:53:58ZStarnini, Michele; Pastor Satorras, RomualdonoTime-varying networks, Random graphsWe study the temporal percolation properties of temporal networks by taking as a representative example the recently proposed activity-driven-network model [N. Perra et al., Sci. Rep. 2, 469 ( 2012)]. Building upon an analytical framework based on a mapping to hidden variables networks, we provide expressions for the percolation time T-p marking the onset of a giant connected component in the integrated network. In particular, we consider both the generating function formalism, valid for degree-uncorrelated networks, and the general case of networks with degree correlations. We discuss the different limits of the two approaches, indicating the parameter regions where the correlated threshold collapses onto the uncorrelated case. Our analytical predictions are confirmed by numerical simulations of the model. The temporal percolation concept can be fruitfully applied to study epidemic spreading on temporal networks. We show in particular how the susceptible-infected-removed model on an activity-driven network can be mapped to the percolation problem up to a time given by the spreading rate of the epidemic process. This mapping allows us to obtain additional information on this process, not available for previous approaches.Stopping criteria in contrastive divergence: Alternatives to the reconstruction error
http://hdl.handle.net/2117/26625
Title: Stopping criteria in contrastive divergence: Alternatives to the reconstruction error
Authors: Buchaca, David; Romero Merino, Enrique; Mazzanti Castrillejo, Fernando Pablo; Delgado Pin, Jordi
Abstract: Restricted Boltzmann Machines (RBMs) are general unsupervised learning devices to ascertain generative models of data distributions.
RBMs are often trained using the Contrastive Divergence learning algorithm (CD), an approximation to the gradient of the data log-likelihood.
A simple reconstruction error is often used to decide whether the approximation provided by the CD algorithm is good enough, though several authors (Schulz et al., 2010; Fischer & Igel, 2010) have raised doubts concerning the feasibility of this procedure. However, not many alternatives to the reconstruction error have been used in the literature. In this manuscript we investigate simple alternatives to the reconstruction error in order to detect as soon as possible the decrease in the log-likelihood during learning.Mon, 09 Mar 2015 12:28:53 GMThttp://hdl.handle.net/2117/266252015-03-09T12:28:53ZBuchaca, David; Romero Merino, Enrique; Mazzanti Castrillejo, Fernando Pablo; Delgado Pin, JordinoRestricted Boltzmann Machines, RBMs: Contrastive divergence learning algorithmRestricted Boltzmann Machines (RBMs) are general unsupervised learning devices to ascertain generative models of data distributions.
RBMs are often trained using the Contrastive Divergence learning algorithm (CD), an approximation to the gradient of the data log-likelihood.
A simple reconstruction error is often used to decide whether the approximation provided by the CD algorithm is good enough, though several authors (Schulz et al., 2010; Fischer & Igel, 2010) have raised doubts concerning the feasibility of this procedure. However, not many alternatives to the reconstruction error have been used in the literature. In this manuscript we investigate simple alternatives to the reconstruction error in order to detect as soon as possible the decrease in the log-likelihood during learning.Short range order of CCl4: RMC and MD methods
http://hdl.handle.net/2117/26574
Title: Short range order of CCl4: RMC and MD methods
Authors: Silva-Santisteban López, Alvaro; Henao Aristizábal, Andrés; Pothoczki, Szilvia; Bermejo, F. Javier; Tamarit Mur, José Luis; Guàrdia Manuel, Elvira; Cuello, Gabriel Julio; Pardo Soto, Luis Carlos
Abstract: The main objective of this paper is to present a method to fully determine the six
degrees of freedom regarding position and orientation of a neighboring molecule around a central
one, i.e. the g(rCM,¿) distribution function. This is accomplished by completely determining the
short range structure of liquid carbon tetrachloride, while employing results from two different
methods, Molecular Dynamics (MD) [1] and Reverse Monte Carlo (RMC) [2]. Exclusively, the
structural ordering of the first four molecules will be detailed.Wed, 04 Mar 2015 12:11:45 GMThttp://hdl.handle.net/2117/265742015-03-04T12:11:45ZSilva-Santisteban López, Alvaro; Henao Aristizábal, Andrés; Pothoczki, Szilvia; Bermejo, F. Javier; Tamarit Mur, José Luis; Guàrdia Manuel, Elvira; Cuello, Gabriel Julio; Pardo Soto, Luis CarlosnoThe main objective of this paper is to present a method to fully determine the six
degrees of freedom regarding position and orientation of a neighboring molecule around a central
one, i.e. the g(rCM,¿) distribution function. This is accomplished by completely determining the
short range structure of liquid carbon tetrachloride, while employing results from two different
methods, Molecular Dynamics (MD) [1] and Reverse Monte Carlo (RMC) [2]. Exclusively, the
structural ordering of the first four molecules will be detailed.Dynamic structure function of a cold Fermi gas at unitarity
http://hdl.handle.net/2117/26519
Title: Dynamic structure function of a cold Fermi gas at unitarity
Authors: Astrakharchik, Grigori; Boronat Medico, Jordi; Krotscheck, Eckhard; Lichtenegger, Thomas
Abstract: We present a theoretical study of the dynamic structure function of a resonantly interacting two-component Fermi gas at zero temperature. Our approach is based on dynamic many-body theory able to describe excitations in strongly correlated Fermi systems. The fixed-node diffusion Monte Carlo method is used to produce the ground-state correlation functions which are used as an input for the excitation theory. Our approach reproduces recent Bragg scattering data in both the density and the spin channel. In the BCS regime, the response is close to that of the ideal Fermi gas. On the BEC side, the Bose peak associated with the formation of dimers dominates the density channel of the dynamic response. When the fraction of dimers is large our theory departs from the experimental data, mainly in the spin channel.Thu, 26 Feb 2015 08:58:44 GMThttp://hdl.handle.net/2117/265192015-02-26T08:58:44ZAstrakharchik, Grigori; Boronat Medico, Jordi; Krotscheck, Eckhard; Lichtenegger, ThomasnoDynamic response, Electron gas, Monte Carlo methods, Diffusion Monte Carlo method, Dynamic structure, Excitation theory, Ground-state correlation, Many-body theory, Theoretical study, Two-component fermi gas, Zero temperatures, FermionsWe present a theoretical study of the dynamic structure function of a resonantly interacting two-component Fermi gas at zero temperature. Our approach is based on dynamic many-body theory able to describe excitations in strongly correlated Fermi systems. The fixed-node diffusion Monte Carlo method is used to produce the ground-state correlation functions which are used as an input for the excitation theory. Our approach reproduces recent Bragg scattering data in both the density and the spin channel. In the BCS regime, the response is close to that of the ideal Fermi gas. On the BEC side, the Bose peak associated with the formation of dimers dominates the density channel of the dynamic response. When the fraction of dimers is large our theory departs from the experimental data, mainly in the spin channel.Universality in molecular halo clusters
http://hdl.handle.net/2117/26455
Title: Universality in molecular halo clusters
Authors: Stipanovic, P.; Markic, L. Vranjes; Beslic, Ivana; Boronat Medico, Jordi
Abstract: The ground state of weakly bound dimers and trimers with a radius extending well into the classically forbidden region is explored, with the goal to test the predicted universality of quantum halo states. The focus of the study is molecules consisting of T down arrow, D down arrow, He-3, He-4, and alkali atoms, where the interaction between particles is much better known than in the case of nuclei, which are traditional examples of quantum halos. The study of realistic systems is supplemented by model calculations in order to analyze how low-energy properties depend on the interaction potential. The use of variational and diffusion Monte Carlo methods enabled a very precise calculation of both the size and binding energy of the trimers. In the quantum halo regime, and for large values of scaled binding energies, all clusters follow almost the same universal line. As the scaled binding energy decreases, Borromean states separate from tango trimers.Fri, 20 Feb 2015 16:17:53 GMThttp://hdl.handle.net/2117/264552015-02-20T16:17:53ZStipanovic, P.; Markic, L. Vranjes; Beslic, Ivana; Boronat Medico, JordinoInteraction cross-sections, Surface integral method, Quantum halos, Ground-state, Perturbation-theory, Systems, Potentials, Radii, HE, ScatteringThe ground state of weakly bound dimers and trimers with a radius extending well into the classically forbidden region is explored, with the goal to test the predicted universality of quantum halo states. The focus of the study is molecules consisting of T down arrow, D down arrow, He-3, He-4, and alkali atoms, where the interaction between particles is much better known than in the case of nuclei, which are traditional examples of quantum halos. The study of realistic systems is supplemented by model calculations in order to analyze how low-energy properties depend on the interaction potential. The use of variational and diffusion Monte Carlo methods enabled a very precise calculation of both the size and binding energy of the trimers. In the quantum halo regime, and for large values of scaled binding energies, all clusters follow almost the same universal line. As the scaled binding energy decreases, Borromean states separate from tango trimers.First-principles modeling of quantum nuclear effects and atomic interactions in solid He-4 at high pressure
http://hdl.handle.net/2117/26453
Title: First-principles modeling of quantum nuclear effects and atomic interactions in solid He-4 at high pressure
Authors: Cazorla Silva, Claudio; Boronat Medico, Jordi
Abstract: We present a first-principles computational study of solid He-4 at T = 0 K and pressures up to similar to 160 GPa. Our computational strategy consists in using van der Waals density functional theory (DFT-vdW) to describe the electronic degrees of freedom in this material, and the diffusion Monte Carlo (DMC) method to solve the Schrodinger equation describing the behavior of the quantum nuclei. For this, we construct an analytical interaction function based on the pairwise Aziz potential that closely matches the volume variation of the cohesive energy calculated with DFT-vdW in dense helium. Interestingly, we find that the kinetic energy of solid He-4 does not increase appreciably with compression for P >= 85 GPa. Also, we show that the Lindemann ratio in dense solid He-4 amounts to 0.10 almost independently of pressure. The reliability of customary quasiharmonic DFT (QH DFT) approaches in describing quantum nuclear effects in solids is also studied. We find that QH DFT simulations, although provide a reasonable equation of state in agreement with experiments, are not able to reproduce correctly these critical effects in compressed He-4. In particular, we disclose huge discrepancies of at least similar to 50% in the calculated He-4 kinetic energies using both the QH DFT and present DFT-DMC methods.Fri, 20 Feb 2015 15:30:21 GMThttp://hdl.handle.net/2117/264532015-02-20T15:30:21ZCazorla Silva, Claudio; Boronat Medico, JordinoMonte-Carlo algorithms, Augmented-wave method, X-Ray-diffraction, Equation-of-state, Ground-state, Molecular-hydrogen, Megabar pressures, Single-crystal, Dense helium, TemperatureWe present a first-principles computational study of solid He-4 at T = 0 K and pressures up to similar to 160 GPa. Our computational strategy consists in using van der Waals density functional theory (DFT-vdW) to describe the electronic degrees of freedom in this material, and the diffusion Monte Carlo (DMC) method to solve the Schrodinger equation describing the behavior of the quantum nuclei. For this, we construct an analytical interaction function based on the pairwise Aziz potential that closely matches the volume variation of the cohesive energy calculated with DFT-vdW in dense helium. Interestingly, we find that the kinetic energy of solid He-4 does not increase appreciably with compression for P >= 85 GPa. Also, we show that the Lindemann ratio in dense solid He-4 amounts to 0.10 almost independently of pressure. The reliability of customary quasiharmonic DFT (QH DFT) approaches in describing quantum nuclear effects in solids is also studied. We find that QH DFT simulations, although provide a reasonable equation of state in agreement with experiments, are not able to reproduce correctly these critical effects in compressed He-4. In particular, we disclose huge discrepancies of at least similar to 50% in the calculated He-4 kinetic energies using both the QH DFT and present DFT-DMC methods.Beam focalization in reflection from flat dielectric subwavelength gratings
http://hdl.handle.net/2117/26401
Title: Beam focalization in reflection from flat dielectric subwavelength gratings
Authors: Cheng, Yu Chieh; Zeng, H; Trullàs Simó, Joaquim; Cojocaru, Crina; Malinauskas, Mangirdas; Jukna, T; Wiersma, D.S.; Staliunas, Kestutis
Abstract: We experimentally demonstrate the recently predicted effect of near-field focusing for light beams from flat dielectric subwavelength gratings (SWGs). This SWGs were designed for visible light 532 nm and fabricated by direct laser writing in a negative photoresist, with the refractive index n = 1.5 and the period d = 314 nm. The laterally invariant gratings can focus light beams without any optical axis to achieve the transversal invariance. We show that focal distances can be obtained up to 13 mu m at normal reflection for TE polarization.Wed, 18 Feb 2015 07:43:19 GMThttp://hdl.handle.net/2117/264012015-02-18T07:43:19ZCheng, Yu Chieh; Zeng, H; Trullàs Simó, Joaquim; Cojocaru, Crina; Malinauskas, Mangirdas; Jukna, T; Wiersma, D.S.; Staliunas, KestutisnoNegative refraction, Photonic crystals, Transmission, Laser, LensWe experimentally demonstrate the recently predicted effect of near-field focusing for light beams from flat dielectric subwavelength gratings (SWGs). This SWGs were designed for visible light 532 nm and fabricated by direct laser writing in a negative photoresist, with the refractive index n = 1.5 and the period d = 314 nm. The laterally invariant gratings can focus light beams without any optical axis to achieve the transversal invariance. We show that focal distances can be obtained up to 13 mu m at normal reflection for TE polarization.Luttinger-liquid behavior of one-dimensional He-3
http://hdl.handle.net/2117/26200
Title: Luttinger-liquid behavior of one-dimensional He-3
Authors: Astrakharchik, Grigori; Boronat Medico, Jordi
Abstract: The ground-state properties of one-dimensional He-3 are studied using quantum Monte Carlo methods. The equation of state is calculated in a wide range of physically relevant densities and is well interpolated by a power-series fit. The Luttinger liquid theory is found to describe the long-range properties of the correlation functions. The density dependence of the Luttinger parameter is explicitly found, and interestingly it shows a nonmonotonic behavior. Depending on the density, the static structure factor can be a smooth function of the momentum or might contain a peak of a finite or infinite height. Although no phase transitions are present in the system, we identify a number of physically different regimes, including an ideal Fermi gas, aTue, 03 Feb 2015 18:41:49 GMThttp://hdl.handle.net/2117/262002015-02-03T18:41:49ZAstrakharchik, Grigori; Boronat Medico, JordinoGround-state, Electron-gas, Anderson localization, Single-barrier, Quantum fluids, Monte-Carlo, Bose-gas, Transport, Crystal, SystemsThe ground-state properties of one-dimensional He-3 are studied using quantum Monte Carlo methods. The equation of state is calculated in a wide range of physically relevant densities and is well interpolated by a power-series fit. The Luttinger liquid theory is found to describe the long-range properties of the correlation functions. The density dependence of the Luttinger parameter is explicitly found, and interestingly it shows a nonmonotonic behavior. Depending on the density, the static structure factor can be a smooth function of the momentum or might contain a peak of a finite or infinite height. Although no phase transitions are present in the system, we identify a number of physically different regimes, including an ideal Fermi gas, a