Showing 1–50 of 50 results for author: Sanz, A

Human-AI Synergy in Adaptive Active Learning for Continuous Lithium Carbonate Crystallization Optimization

Authors: Shayan S. Mousavi Masouleh, Corey A. Sanz, Ryan P. Jansonius, Cara Cronin, Jason E. Hein, Jason Hattrick-Simpers

Abstract: As demand for high-purity lithium surges with the growth of the electric vehicle (EV) industry, cost-effective extraction from lower-grade North American sources like the Smackover Formation is critical. These resources, unlike high-purity South American brines, require innovative purification techniques to be economically viable. Continuous crystallization is a promising method for producing batt… ▽ More As demand for high-purity lithium surges with the growth of the electric vehicle (EV) industry, cost-effective extraction from lower-grade North American sources like the Smackover Formation is critical. These resources, unlike high-purity South American brines, require innovative purification techniques to be economically viable. Continuous crystallization is a promising method for producing battery-grade lithium carbonate, but its optimization is challenged by a complex parameter space and limited data. This study introduces a Human-in-the-Loop (HITL) assisted active learning framework to optimize the continuous crystallization of lithium carbonate. By integrating human expertise with data-driven insights, our approach accelerates the optimization of lithium extraction from challenging sources. Our results demonstrate the framework's ability to rapidly adapt to new data, significantly improving the process's tolerance to critical impurities like magnesium from the industry standard of a few hundred ppm to as high as 6000 ppm. This breakthrough makes the exploitation of low-grade, impurity-rich lithium resources feasible, potentially reducing the need for extensive pre-refinement processes. By leveraging artificial intelligence, we have refined operational parameters and demonstrated that lower-grade materials can be used without sacrificing product quality. This advancement is a significant step towards economically harnessing North America's vast lithium reserves, such as those in the Smackover Formation, and enhancing the sustainability of the global lithium supply chain. △ Less

Submitted 25 July, 2025; originally announced July 2025.

Lower and upper bounds for configurations of points on a sphere

Authors: Paolo Amore, Ricardo A. Sáenz

Abstract: We present a new proof (based on spectral decomposition) of a bound originally proved by Sidelnikov~\, for the frame potentials $\sum_{ij} \left( {\bf P}_i \cdot {\bf P}_j \right)^\ell $ on a unit--sphere in $d$ dimensions. Sidelnikov's bound is a special case of the lower bound for the weighted sums $\sum_{ij} f_i f_j \left( {\bf P}_i \cdot {\bf P}_j \right)^\ell$, where $f_i>0$ are scalar quanti… ▽ More We present a new proof (based on spectral decomposition) of a bound originally proved by Sidelnikov~\, for the frame potentials $\sum_{ij} \left( {\bf P}_i \cdot {\bf P}_j \right)^\ell $ on a unit--sphere in $d$ dimensions. Sidelnikov's bound is a special case of the lower bound for the weighted sums $\sum_{ij} f_i f_j \left( {\bf P}_i \cdot {\bf P}_j \right)^\ell$, where $f_i>0$ are scalar quantities associated to each point on the sphere, which we also prove using spectral decomposition. Moreover, in three dimensions, again using spectral decomposition, we find a sharp upper bound for $\sum_{ijk}^N \left[ \left( {\bf P}_i \times {\bf P}_j\right) \cdot {\bf P}_k \right]^2$. We explore two applications of these bounds: first, we examine configurations of points corresponding to the local minima of the Thomson problem for $N=972$; second, we analyze various distributions of points within a three-dimensional volume, where a suitable weighted sum is defined to satisfy a specific bound. △ Less

Submitted 7 December, 2024; originally announced December 2024.

Comments: 22 pages, 6 figures

Artificial Intelligence-Enabled Optimization of Battery-Grade Lithium Carbonate Production

Authors: S. Shayan Mousavi Masouleh, Corey A. Sanz, Ryan P. Jansonius, Samuel Shi, Maria J. Gendron Romero, Jason E. Hein, Jason Hattrick-Simpers

Abstract: By 2035, the need for battery-grade lithium is expected to quadruple. About half of this lithium is currently sourced from brines and must be converted from a chloride into lithium carbonate (Li2CO3) through a process called softening. Conventional softening methods using sodium or potassium salts contribute to carbon emissions during reagent mining and battery manufacturing, exacerbating global w… ▽ More By 2035, the need for battery-grade lithium is expected to quadruple. About half of this lithium is currently sourced from brines and must be converted from a chloride into lithium carbonate (Li2CO3) through a process called softening. Conventional softening methods using sodium or potassium salts contribute to carbon emissions during reagent mining and battery manufacturing, exacerbating global warming. This study introduces an alternative approach using carbon dioxide (CO2(g)) as the carbonating reagent in the lithium softening process, offering a carbon capture solution. We employed an active learning-driven high-throughput method to rapidly capture CO2(g) and convert it to lithium carbonate. The model was simplified by focusing on the elemental concentrations of C, Li, and N for practical measurement and tracking, avoiding the complexities of ion speciation equilibria. This approach led to an optimized lithium carbonate process that capitalizes on CO2(g) capture and improves the battery metal supply chain's carbon efficiency. △ Less

Submitted 20 February, 2024; v1 submitted 10 February, 2024; originally announced February 2024.

Multi-scale energy homogenization for 3D printed microstructures with a Diritchlet boundary condition relaxation under plastic deformation

Authors: Antonio Tabanera, Luis Saucedo-Mora, Miguel Angel Sanz, Francisco J. Montans

Abstract: The present work is a proof of concept of the capabilities of paralellization in the calculation of metamaterials in a non-linear regime. In this work we subdivided the bulk material into subregions where the mechanical properties are homogenized energetically. We demonstrate that the calculation can be subdivided to save RAM memory and fit the local non-linear behaviour of the metamaterial. This… ▽ More The present work is a proof of concept of the capabilities of paralellization in the calculation of metamaterials in a non-linear regime. In this work we subdivided the bulk material into subregions where the mechanical properties are homogenized energetically. We demonstrate that the calculation can be subdivided to save RAM memory and fit the local non-linear behaviour of the metamaterial. This methodology has the potentiality to be implemented in the parallelization of those calculations, where the right estimation of the energy of the local processes at every step is important. △ Less

Submitted 24 November, 2023; originally announced November 2023.

Comments: 5 pages, 1 figure

Observation of confinement-induced resonances in a 3D lattice

Authors: Deborah Capecchi, Camilo Cantillano, Manfred J. Mark, Florian Meinert, Andreas Schindewolf, Manuele Landini, Alejandro Saenz, Fabio Revuelta, Hanns-Christoph Nägerl

Abstract: We report on the observation of confinement-induced resonances for strong three-dimensional (3D) confinement in a lattice potential. Starting from a Mott-insulator state with predominantly single-site occupancy, we detect loss and heating features at specific values for the confinement length and the 3D scattering length. Two independent models, based on the coupling between the center-of-mass and… ▽ More We report on the observation of confinement-induced resonances for strong three-dimensional (3D) confinement in a lattice potential. Starting from a Mott-insulator state with predominantly single-site occupancy, we detect loss and heating features at specific values for the confinement length and the 3D scattering length. Two independent models, based on the coupling between the center-of-mass and the relative motion of the particles as mediated by the lattice, predict the resonance positions to a good approximation, suggesting a universal behavior. Our results extend confinement-induced resonances to any dimensionality and open up an alternative method for interaction tuning and controlled molecule formation under strong 3D confinement. △ Less

Submitted 26 September, 2022; originally announced September 2022.

Comments: 9 pages, 5 figures

Journal ref: Phys. Rev. Lett. 131, 213002 (2023)

Challenges of interpreting dielectric dilatometry for the study of pressure densification

Authors: Alejandro Sanz, Jeppe C. Dyre, Kristine Niss

Abstract: We report an experimental study documenting the challenge of employing dielectric dilatometry for the study of pressure densification in glass-forming materials. An influence of the dielectric cell geometry on the resulting capacitance of 5-poly-phenyl-ether upon vitrification under different thermobaric pathways is documented. The capacitive response is studied for two different multilayer capaci… ▽ More We report an experimental study documenting the challenge of employing dielectric dilatometry for the study of pressure densification in glass-forming materials. An influence of the dielectric cell geometry on the resulting capacitance of 5-poly-phenyl-ether upon vitrification under different thermobaric pathways is documented. The capacitive response is studied for two different multilayer capacitors: one with, in principle, fixed plate distance and one with Kapton spacers allowing for contraction/expansion. A combination of changes in the dielectric permittivity of the material and modifications of the capacitor geometry determines the final capacitance. We conclude that, in order to convert the measured capacitance to material density, it is of paramount importance to understand the geometry. The data presented do not make it possible to conclude on whether or not simple glass formers such as 5-poly-phenyl-ether can be pressure densified, but our work highlights the challenge of utilizing dielectric spectroscopy to tackle this problem effectively. △ Less

Submitted 27 February, 2019; originally announced February 2019.

Comments: 7 figures

Isochronal superposition and density scaling of the $α$-relaxation from pico- to millisecond

Authors: Henriette Wase Hansen, Bernhard Frick, Simone Capaccioli, Alejandro Sanz, Kristine Niss

Abstract: The relaxation dynamics in two van der Waals bonded and one hydrogen-bonding molecular liquids is studied as a function of pressure and temperature by incoherent neutron scattering using simultaneous dielectric spectroscopy. The dynamics is studied in a range of alpha relaxation times from nano- to milliseconds, primarily in the equilibrium liquid state. In this range we find that isochronal super… ▽ More The relaxation dynamics in two van der Waals bonded and one hydrogen-bonding molecular liquids is studied as a function of pressure and temperature by incoherent neutron scattering using simultaneous dielectric spectroscopy. The dynamics is studied in a range of alpha relaxation times from nano- to milliseconds, primarily in the equilibrium liquid state. In this range we find that isochronal superposition and density scaling work not only for the two van der Waals liquids, but also for the hydrogen-bonding liquid, though the density scaling exponent is much smaller for the latter. Density scaling and isochronal superposition are seen to break down for intra-molecular dynamics when it is separated in time from the $α$-relaxation in close agreement with previous observations from molecular dynamics simulations. △ Less

Submitted 7 September, 2018; originally announced September 2018.

Comments: 10 pages, 12 figures

Experimental evidence of a state-point dependent scaling exponent of liquid dynamics

Authors: Alejandro Sanz, Tina Hecksher, Henriette Wase Hansen, Kristine Niss, Ulf R. Pedersen

Abstract: A large class of liquids have hidden scale invariance characterized by a scaling exponent. In this letter we present experimental evidence that the scaling exponent of liquid dynamics is state-point dependent for the glass-forming silicone oil tetramethyl-tetraphenyl-trisiloxane (DC704) and 5-polyphenyl ether (5PPE). From dynamic and thermodynamic properties at equilibrium, we use a method to esti… ▽ More A large class of liquids have hidden scale invariance characterized by a scaling exponent. In this letter we present experimental evidence that the scaling exponent of liquid dynamics is state-point dependent for the glass-forming silicone oil tetramethyl-tetraphenyl-trisiloxane (DC704) and 5-polyphenyl ether (5PPE). From dynamic and thermodynamic properties at equilibrium, we use a method to estimate the value of $γ$ at any state point of the pressure-temperature plane, both in the supercooled and normal liquid regimes. We find agreement between the average exponents and the value obtained by superposition of relaxation times over a large range of state-points. We confirm the state-point dependence of $γ$ by reanalyzing data of 20 metallic liquids and two model liquids. △ Less

Submitted 24 August, 2018; v1 submitted 31 July, 2018; originally announced July 2018.

Journal ref: Phys. Rev. Lett. 122, 055501 (2019)

Wave-packet numerical investigation of thermal diffuse scattering: A time-dependent quantum approach to the Debye method

Authors: S. Rudinsky, A. S. Sanz, R. Gauvin

Abstract: The effects of thermal diffuse scattering on the transmission and eventual diffraction of highly accelerated electrons are investigated with a method that incorporates the frozen phonon approximation to the exact numerical solution of the time-dependent Schrödinger equation. Unlike other methods in the related literature, in this approach the attenuation of diffraction features arises in a natural… ▽ More The effects of thermal diffuse scattering on the transmission and eventual diffraction of highly accelerated electrons are investigated with a method that incorporates the frozen phonon approximation to the exact numerical solution of the time-dependent Schrödinger equation. Unlike other methods in the related literature, in this approach the attenuation of diffraction features arises in a natural way by averaging over a number of wave-packet realizations, thus avoiding any additional experimentally obtained Debye-Waller factors or artificial modulations. Without loss of generality, the method has been applied to analyze the transmission of an electron beam through a thin Al film in two dimensions, making use of Einstein's model to determine the phonon configuration for each realization at a given temperature. It is shown that, as temperature and hence atomic vibration amplitudes increase, incoherence among different electron wave-function realizations gradually increases, blurring the well-defined diffraction features characterizing the zero-temperature intensity. △ Less

Submitted 17 January, 2018; originally announced January 2018.

Bohmian pathways into chemistry: A brief overview

Authors: A. S. Sanz

Abstract: Perhaps because of the popularity that trajectory-based methodologies have always had in Chemistry and the important role they have played, Bohmian mechanics has been increasingly accepted within this community, particularly in those areas of the theoretical chemistry based on quantum mechanics, e.g., quantum chemistry, chemical physics, or physical chemistry. From a historical perspective, this e… ▽ More Perhaps because of the popularity that trajectory-based methodologies have always had in Chemistry and the important role they have played, Bohmian mechanics has been increasingly accepted within this community, particularly in those areas of the theoretical chemistry based on quantum mechanics, e.g., quantum chemistry, chemical physics, or physical chemistry. From a historical perspective, this evolution is remarkably interesting, particularly when the scarce applications of Madelung's former hydrodynamical formulation, dating back to the late 1960s and the 1970s, are compared with the many different applications available at present. As also happens with classical methodologies, Bohmian trajectories are essentially used to described and analyze the evolution of chemical systems, to design and implement new computational propagation techniques, or a combination of both. In the first case, Bohmian trajectories have the advantage that they avoid invoking typical quantum-classical correspondence to interpret the corresponding phenomenon or process, while in the second case quantum-mechanical effects appear by themselves, without the necessity to include artificially quantization conditions. Rather than providing an exhaustive revision and analysis of all these applications (excellent monographs on the issue are available in the literature for the interested reader, which can be consulted in the bibliography here supplied), this Chapter has been prepared in a way that it may serve the reader to acquire a general view (or impression) on how Bohmian mechanics has permeated the different traditional levels or pathways to approach molecular systems in Chemistry: electronic structure, molecular dynamics and statistical mechanics. This is done with the aid of some illustrative examples -- theoretical developments in some cases and numerical simulations in other cases. △ Less

Submitted 15 January, 2018; originally announced January 2018.

Comments: 50 pages, 5 figures

High-pressure cell for simultaneous dielectric and neutron spectroscopy

Authors: Alejandro Sanz, Henriette Wase Hansen, Bo Jakobsen, Ib H. Pedersen, Simone Capaccioli, Karolina Adrjanowicz, Marian Paluch, Julien Gonthier, Bernhard Frick, Eddy Lelièvre-Berna, Judith Peters, Kristine Niss

Abstract: In this article we report on the design, manufacture and testing of a high-pressure cell for doing simultaneous dielectric and neutron spectroscopy. This cell is a unique tool for studying dynamics on different timescales, from kilo- to picoseconds, covering universal features such as the alpha relaxation and fast vibrations at the same time. The cell, constructed in cylindrical geometry, is made… ▽ More In this article we report on the design, manufacture and testing of a high-pressure cell for doing simultaneous dielectric and neutron spectroscopy. This cell is a unique tool for studying dynamics on different timescales, from kilo- to picoseconds, covering universal features such as the alpha relaxation and fast vibrations at the same time. The cell, constructed in cylindrical geometry, is made in high-strength aluminum alloy and operates up to 500 MPa in a temperature range between roughly 2 and 320 K. In order to measure the scattered neutron intensity and the sample capacitance simultaneously, a cylindrical capacitor is positioned within the bore of the high-pressure container. The capacitor consists of two concentric electrodes separated by insulating spacers. The performance of this setup has been successfully verified by collecting simultaneous dielectric and neutron spectroscopy data on dipropylene glycol, using both backscattering and time-of-flight instruments. We have carried out the experiments at different combinations of temperature and pressure in both supercooled liquid and glassy state. △ Less

Submitted 28 September, 2017; originally announced September 2017.

Comments: A.S. and H.W.H. contributed equally to this work

Evidence of a one-dimensional thermodynamic phase diagram for simple glass-formers

Authors: Henriette Wase Hansen, Alejandro Sanz, Karolina Adrjanowicz, Bernhard Frick, Kristine Niss

Abstract: The glass transition plays a central role in nature as well as in industry, ranging from biological systems such as proteins and DNA to polymers and metals. Yet the fundamental understanding of the glass transition which is a prerequisite for optimized application of glass formers is still lacking. Glass formers show motional processes over an extremely broad range of timescales, covering more tha… ▽ More The glass transition plays a central role in nature as well as in industry, ranging from biological systems such as proteins and DNA to polymers and metals. Yet the fundamental understanding of the glass transition which is a prerequisite for optimized application of glass formers is still lacking. Glass formers show motional processes over an extremely broad range of timescales, covering more than ten orders of magnitude, meaning that a full understanding of the glass transition needs to comprise this tremendous range in timescales. Here we report on first-time simultaneous neutron and dielectric spectroscopy investigations of three glass-forming liquids, probing in a single experiment the full range of dynamics. For two van der Waals liquids we locate in the pressure-temperature phase diagram lines of identical dynamics of the molecules on both second and picosecond timescales. This confirms predictions of the isomorph theory and effectively reduces the phase diagram from two to one dimension. The implication is that dynamics on widely different timescales are governed by the same underlying mechanisms. △ Less

Submitted 26 September, 2017; originally announced September 2017.

Non-equilibrium Structure Affects Ferroelectric Behaviour in Confined Polymers

Authors: D. E. Marinez-Tong, A. Sanz, J. Martín, T. E. Ezquerra, A. Nogales

Abstract: The effect of interfaces and confinement in polymer ferroelectric structured is discussed. Results on confinement under different geometries are presented and the comparison of all of them allows to evidence that the presence of an interface in particular cases stabilizes a ferroelectric phase that is not spontaneously formed under normal bulk processing conditions The effect of interfaces and confinement in polymer ferroelectric structured is discussed. Results on confinement under different geometries are presented and the comparison of all of them allows to evidence that the presence of an interface in particular cases stabilizes a ferroelectric phase that is not spontaneously formed under normal bulk processing conditions △ Less

Submitted 25 June, 2017; originally announced June 2017.

A novel quantum dynamical approach in electron microscopy combining wave-packet propagation with Bohmian trajectories

Authors: Samantha Rudinsky, Angel S. Sanz, Raynald Gauvin

Abstract: The numerical analysis of the diffraction features rendered by transmission electron microscopy (TEM) typically relies either on classical approximations (Monte Carlo simulations) or quantum paraxial tomography (the multislice method and any of its variants). Although numerically advan- tageous (relatively simple implementations and low computational costs), they involve important approximations a… ▽ More The numerical analysis of the diffraction features rendered by transmission electron microscopy (TEM) typically relies either on classical approximations (Monte Carlo simulations) or quantum paraxial tomography (the multislice method and any of its variants). Although numerically advan- tageous (relatively simple implementations and low computational costs), they involve important approximations and thus their range of applicability is limited. To overcome such limitations, an alternative, more general approach is proposed, based on an optimal combination of wave-packet propagation with the on-the-fly computation of associated Bohmian trajectories. For the sake of clarity, but without loss of generality, the approach is used to analyze the diffraction of an electron beam by a thin aluminum slab as a function of three different incidence (work) conditions which are of interest in electron microscopy: the probe width, the tilting angle, and the beam energy. Specifically, it is shown that, because there is a dependence on particular thresholds of the beam energy, this approach provides a clear description of the diffraction process at any energy, revealing at the same time any diversion of the beam inside the material towards directions that cannot be accounted for by other conventional methods, which is of much interest when dealing with relatively low energies and/or relatively large tilting angles. △ Less

Submitted 30 January, 2017; originally announced January 2017.

Journal ref: J. Chem. Phys. 146, 104702 (2017)

Liquid dynamics in partially crystalline glycerol

Authors: Alejandro Sanz, Kristine Niss

Abstract: We present a dielectric study on the dynamics of supercooled glycerol during crystallization. We explore the transformation into a solid phase in real time by monitoring the temporal evolution of the amplitude of the dielectric signal. Neither the initial nucleation or the crystal growth influence the liquid dynamics visibly. For one of the samples studied, a tiny fraction of glycerol remained in… ▽ More We present a dielectric study on the dynamics of supercooled glycerol during crystallization. We explore the transformation into a solid phase in real time by monitoring the temporal evolution of the amplitude of the dielectric signal. Neither the initial nucleation or the crystal growth influence the liquid dynamics visibly. For one of the samples studied, a tiny fraction of glycerol remained in the disordered state after the end of the transition. We examined the nature of the alpha relaxation in this frustrated crystal and find that it is virtually identical to the bulk dynamics. In addition to that, we have found no evidence that supercooled glycerol transforms into a peculiar phase where either a new solid amorphous state or nano-crystals dispersed in a liquid matrix are formed. △ Less

Submitted 28 February, 2017; v1 submitted 13 October, 2016; originally announced October 2016.

Comments: 7 pages, 7 figures

Journal ref: The Journal of Chemical Physics 2017, 146, 044502

Thermalization Calorimetry: A simple method for investigating glass transition and crystallization of supercooled liquids

Authors: Bo Jakobsen, Alejandro Sanz, Kristine Niss, Tina Hecksher, Ib H. Pedersen, Torben Rasmussen, Tage Christensen, Niels Boye Olsen, Jeppe C. Dyre

Abstract: We present a simple method for fast and cheap thermal analysis on supercooled glass-forming liquids. This "Thermalization Calorimetry" technique is based on monitoring the temperature and its rate of change during heating or cooling of a sample for which the thermal power input comes from heat conduction through an insulating material, i.e., is proportional to the temperature difference between sa… ▽ More We present a simple method for fast and cheap thermal analysis on supercooled glass-forming liquids. This "Thermalization Calorimetry" technique is based on monitoring the temperature and its rate of change during heating or cooling of a sample for which the thermal power input comes from heat conduction through an insulating material, i.e., is proportional to the temperature difference between sample and surroundings. The monitored signal reflects the sample's specific heat and is sensitive to exo- and endothermic processes. The technique is useful for studying supercooled liquids and their crystallization, e.g., for locating the glass transition and melting point(s), as well as for investigating the stability against crystallization and estimating the relative change in specific heat between the solid and liquid phases at the glass transition. △ Less

Submitted 11 July, 2016; v1 submitted 30 November, 2015; originally announced November 2015.

Comments: 17 pages, 9 figures, minor changes

Journal ref: AIP Advances, 6, 055019, 2016

The completeness of the Bethe ansatz for the periodic ASEP

Authors: Eric Brattain, Norman Do, Axel Saenz

Abstract: The asymmetric simple exclusion process (ASEP) for N particles on a ring with L sites may be analyzed using the Bethe ansatz. In this paper, we provide a rigorous proof that the Bethe ansatz is complete for the periodic ASEP. More precisely, we show that for all but finitely many values of the hopping rate, the solutions of the Bethe ansatz equations do indeed yield all L choose N eigenstates. The… ▽ More The asymmetric simple exclusion process (ASEP) for N particles on a ring with L sites may be analyzed using the Bethe ansatz. In this paper, we provide a rigorous proof that the Bethe ansatz is complete for the periodic ASEP. More precisely, we show that for all but finitely many values of the hopping rate, the solutions of the Bethe ansatz equations do indeed yield all L choose N eigenstates. The proof follows ideas of Langlands and Saint-Aubin, which draw upon a range of techniques from algebraic geometry, topology and enumerative combinatorics. △ Less

Submitted 3 December, 2017; v1 submitted 11 November, 2015; originally announced November 2015.

Comments: 36 pages. In version 2, the main difference is that lemma 3.2 was reworked and promoted to proposition 3.2, and some corresponding remarks were added along the paper

MSC Class: 82C22; 82C05; 60K35; 60J27

Theory of inelastic confinement-induced resonances due to the coupling of center-of-mass and relative motion

Authors: Simon Sala, Alejandro Saenz

Abstract: A detailed study of the anharmonicity-induced resonances caused by the coupling of center-of-mass and relative motion is presented for a system of two ultracold atoms in single-well potentials. As has been confirmed experimentally, these inelastic confinement-induced resonances are of interest, since they can lead to coherent molecule formation, losses, and heating in ultracold atomic gases. A per… ▽ More A detailed study of the anharmonicity-induced resonances caused by the coupling of center-of-mass and relative motion is presented for a system of two ultracold atoms in single-well potentials. As has been confirmed experimentally, these inelastic confinement-induced resonances are of interest, since they can lead to coherent molecule formation, losses, and heating in ultracold atomic gases. A perturbative model is introduced to describe the resonance positions and the coupling strengths. The validity of the model and the behavior of the resonances for different confinement geometries are analyzed in comparison with exact numerical ab initio calculations. While such resonances have so far only been detected for large positive values of the $s$-wave scattering length, it is found that they are present also for negative $s$-wave scattering lengths, i. e. for attractive interactions. The possibility to coherently tune the resonances by a variation of the external confinement geometry might pave the way for coherent molecule association where magnetic Feshbach resonances are inaccessible. △ Less

Submitted 18 September, 2015; originally announced September 2015.

Comments: 14 pages, 10 figures

Journal ref: Phys. Rev. A 94, 022713 (2016)

Inelastic Confinement-Induced Resonances in Quantum Dots

Authors: Maria Troppenz, Simon Sala, Philipp-Immanuel Schneider, Alejandro Saenz

Abstract: Recently, it was shown that the coupling of center-of-mass and relative motion in atomic systems leads to inelastic confinement-induced resonances (ICIRs) [Phys. Rev. Lett. 109, 073201 (2012)]. In the present work, the possible occurrence of ICIRs in quantum dots is investigated. Particularly, electron-hole and electron-electron two-body systems with long-range Coulomb interaction are considered u… ▽ More Recently, it was shown that the coupling of center-of-mass and relative motion in atomic systems leads to inelastic confinement-induced resonances (ICIRs) [Phys. Rev. Lett. 109, 073201 (2012)]. In the present work, the possible occurrence of ICIRs in quantum dots is investigated. Particularly, electron-hole and electron-electron two-body systems with long-range Coulomb interaction are considered using the material parameters of GaAs. ICIRs are identified for the electron-hole system verifying the universal nature of the ICIR and, additionally, resonances due to the coupling of center-of-mass and relative motion are found also for the electron-electron system. In analogy to the coherent molecule formation appearing at ICIR in atomic systems a significant change in the mean distance between electrons and holes at the resonance is observed. By using the redistribution of the particle densities at the resonance position in modern quantum-dot experiments, the ICIR can provide a new technique for the control of the electron distribution in quantum dots and for the generation of single photons on demand. △ Less

Submitted 3 September, 2015; originally announced September 2015.

Comments: 11 pages, 9 figures

Resonances in ultracold dipolar atomic and molecular gases

Authors: Bruno Schulz, Simon Sala, Alejandro Saenz

Abstract: A previously developed approach for the numerical treatment of two particles that are confined in a finite optical-lattice potential and interact via an arbitrary isotropic interaction potential has been extended to incorporate an additional anisotropic dipole-dipole interaction. The interplay of a model but realistic short-range Born-Oppenheimer potential and the dipole-dipole interaction for two… ▽ More A previously developed approach for the numerical treatment of two particles that are confined in a finite optical-lattice potential and interact via an arbitrary isotropic interaction potential has been extended to incorporate an additional anisotropic dipole-dipole interaction. The interplay of a model but realistic short-range Born-Oppenheimer potential and the dipole-dipole interaction for two confined particles is investigated. A variation of the strength of the dipole-dipole interaction leads to diverse resonance phenomena. In a harmonic confinement potential some resonances show similarities to $s$-wave scattering resonances while in an anharmonic trapping potential like the one of an optical lattice inelastic confinement-induced dipolar resonances occur. The latter are due to a coupling of the relative and center-of-mass motion caused by the anharmonicity of the external confinement. △ Less

Submitted 9 December, 2014; originally announced December 2014.

Comments: 24 pages, 12 figures

MSC Class: 81V45; 81V55

Ultracold-atom quantum simulator for attosecond science

Authors: Simon Sala, Johann Förster, Alejandro Saenz

Abstract: A quantum simulator based on ultracold optically trapped atoms for simulating the physics of atoms and molecules in ultrashort intense laser fields is introduced. The slowing down by about 13 orders of magnitude allows to watch in slow motion the tunneling and recollision processes that form the heart of attosecond science. The extreme flexibility of the simulator promises a deeper understanding o… ▽ More A quantum simulator based on ultracold optically trapped atoms for simulating the physics of atoms and molecules in ultrashort intense laser fields is introduced. The slowing down by about 13 orders of magnitude allows to watch in slow motion the tunneling and recollision processes that form the heart of attosecond science. The extreme flexibility of the simulator promises a deeper understanding of strong-field physics, especially for many-body systems beyond the reach of classical computers. The quantum simulator can experimentally straightforwardly be realized and is shown to recover the ionization characteristics of atoms in the different regimes of laser-matter interaction. △ Less

Submitted 10 November, 2013; originally announced November 2013.

Comments: 7 pages, 2 figures

Journal ref: Phys. Rev. A 95, 011403 (2017)

How does a magnetic trap work?

Authors: J. Pérez-Ríos, A. S. Sanz

Abstract: Magnetic trapping is a cornerstone for modern ultracold physics and its applications (e.g., quantum information processing, quantum metrology, quantum optics, or high-resolution spectroscopies). Here a comprehensive analysis and discussion of the basic physics behind the most commonly used magnetic traps in Bose-Einstein condensation is presented. This analysis includes the quadrupole trap, the ti… ▽ More Magnetic trapping is a cornerstone for modern ultracold physics and its applications (e.g., quantum information processing, quantum metrology, quantum optics, or high-resolution spectroscopies). Here a comprehensive analysis and discussion of the basic physics behind the most commonly used magnetic traps in Bose-Einstein condensation is presented. This analysis includes the quadrupole trap, the time-averaged orbiting potential trap, and the Ioffe-Pritchard trap. It is shown how the trapping conditions and efficiency of these devices can be determined from simple derivations based on classical electromagnetism, even though they operate on quantum objects. △ Less

Submitted 22 October, 2013; originally announced October 2013.

Comments: 24 pages, 5 figures

Journal ref: Am. J. Phys. 81, 836 (2013)

A study of one-dimensional transport of Bose-Einstein condensates using exterior complex scaling

Authors: Julien Dujardin, Alejandro Saenz, Peter Schlagheck

Abstract: We numerically investigate the one-dimensional transport of Bose-Einstein condensates in the context of guided atom lasers using a mean-field description of the condensate in terms of a spatially discretized Gross-Pitaevskii equation. We specifically consider a waveguide configuration in which spatial inhomogeneities and nonvanishing atom-atom interactions are restricted to a spatially localized s… ▽ More We numerically investigate the one-dimensional transport of Bose-Einstein condensates in the context of guided atom lasers using a mean-field description of the condensate in terms of a spatially discretized Gross-Pitaevskii equation. We specifically consider a waveguide configuration in which spatial inhomogeneities and nonvanishing atom-atom interactions are restricted to a spatially localized scattering region of finite extent. We show how the method of smooth exterior complex scaling can be implemented for this particular configuration in order to efficiently absorb the outgoing flux within the waveguide. A numerical comparison with the introduction of a complex absorbing potential as well as with the analytically exact elimination of the dynamics of the free non-interacting motion outside the scattering region, giving rise to transparent boundary conditions, clearly confirms the accuracy and efficiency of the smooth exterior complex scaling method. △ Less

Submitted 11 December, 2014; v1 submitted 14 October, 2013; originally announced October 2013.

Comments: 9 pages, 6 figures

Journal ref: Appl. Phys. B 117, 765 (2014)

Exact band structures for 1D superlattices beyond the tight-binding approximation

Authors: Olaf Krüger, Alejandro Saenz

Abstract: The band structures describing non-interacting particles in one-dimensional superlattices of arbitrary periodicity are obtained by an analytical diagonalization of the Hamiltonian without adopting the popular tight-binding approximation. The results are compared with those of the tight-binding approximation. In this way, a quantitative prediction of the validity and failure of the tight-binding ap… ▽ More The band structures describing non-interacting particles in one-dimensional superlattices of arbitrary periodicity are obtained by an analytical diagonalization of the Hamiltonian without adopting the popular tight-binding approximation. The results are compared with those of the tight-binding approximation. In this way, a quantitative prediction of the validity and failure of the tight-binding approximation becomes possible. In particular, it is demonstrated that in contrast to the prediction of the tight-binding approximation the central energy bands do not touch for periodicities $τ$ of the lattice where $τ=4n$ and $n$ is an integer. △ Less

Submitted 16 June, 2013; originally announced June 2013.

Comments: 5 pages, 5 figures

Two-channel Bose-Hubbard model of atoms at a Feshbach resonance

Authors: Philipp-Immanuel Schneider, Alejandro Saenz

Abstract: Based on the analytic model of Feshbach resonances in harmonic traps described in Phys. Rev. A 83, 030701 (2011) a Bose-Hubbard model is introduced that provides an accurate description of two atoms in an optical lattice at a Feshbach resonance with only a small number of Bloch bands. The approach circumvents the problem that the eigenenergies in the presence of a delta-like coupling do not conver… ▽ More Based on the analytic model of Feshbach resonances in harmonic traps described in Phys. Rev. A 83, 030701 (2011) a Bose-Hubbard model is introduced that provides an accurate description of two atoms in an optical lattice at a Feshbach resonance with only a small number of Bloch bands. The approach circumvents the problem that the eigenenergies in the presence of a delta-like coupling do not converge to the correct energies, if an uncorrelated basis is used. The predictions of the Bose-Hubbard model are compared to non-perturbative calculations for both the stationary states and the time-dependent wavefunction during an acceleration of the lattice potential. For this purpose, a square-well interaction potential is introduced, which allows for a realistic description of Feshbach resonances within non-perturbative single-channel calculations. △ Less

Submitted 19 March, 2013; originally announced March 2013.

Comments: 15 pages, 12 figures

Coherent molecule formation in anharmonic potentials near confinement-induced resonances

Authors: S. Sala, G. Zürn, T. Lompe, A. N. Wenz, S. Murmann, F. Serwane, S. Jochim, A. Saenz

Abstract: We perform a theoretical and experimental study of a system of two ultracold atoms with tunable interaction in an elongated trapping potential. We show that the coupling of center-of-mass and relative motion due to an anharmonicity of the trapping potential leads to a coherent coupling of a state of an unbound atom pair and a molecule with a center of mass excitation. By performing the experiment… ▽ More We perform a theoretical and experimental study of a system of two ultracold atoms with tunable interaction in an elongated trapping potential. We show that the coupling of center-of-mass and relative motion due to an anharmonicity of the trapping potential leads to a coherent coupling of a state of an unbound atom pair and a molecule with a center of mass excitation. By performing the experiment with exactly two particles we exclude three-body losses and can therefore directly observe coherent molecule formation. We find quantitative agreement between our theory of inelastic confinement-induced resonances and the experimental results. This shows that the effects of center-of-mass to relative motion coupling can have a significant impact on the physics of quasi-1D quantum systems. △ Less

Submitted 7 March, 2013; originally announced March 2013.

Comments: 7 pages, 4 figures

Non-perturbative theoretical description of two atoms in an optical lattice with time-dependent perturbations

Authors: Philipp-Immanuel Schneider, Sergey Grishkevich, Alejandro Saenz

Abstract: A theoretical approach for a non-perturbative dynamical description of two interacting atoms in an optical lattice potential is introduced. The approach builds upon the stationary eigenstates found by a procedure described in Grishkevich et al. [Phys. Rev. A 84, 062710 (2011)]. It allows presently to treat any time-dependent external perturbation of the lattice potential up to quadratic order. Exa… ▽ More A theoretical approach for a non-perturbative dynamical description of two interacting atoms in an optical lattice potential is introduced. The approach builds upon the stationary eigenstates found by a procedure described in Grishkevich et al. [Phys. Rev. A 84, 062710 (2011)]. It allows presently to treat any time-dependent external perturbation of the lattice potential up to quadratic order. Example calculations of the experimentally relevant cases of an acceleration of the lattice and the turning-on of an additional harmonic confinement are presented. △ Less

Submitted 2 September, 2012; originally announced September 2012.

Comments: 8 pages, 6 figures

Adsorbate surface diffusion: The role of incoherent tunneling in light particle motion

Authors: A. S. Sanz, R. Martinez-Casado, S. Miret-Artes

Abstract: The role of incoherent tunneling in the diffusion of light atoms on surfaces is investigated. With this purpose, a Chudley-Elliot master equation constrained to nearest neighbors is considered within the Grabert-Weiss approach to quantum diffusion in periodic lattices. This model is applied to recent measurements of atomic H and D on Pt(111), rendering friction coefficients that are in the range o… ▽ More The role of incoherent tunneling in the diffusion of light atoms on surfaces is investigated. With this purpose, a Chudley-Elliot master equation constrained to nearest neighbors is considered within the Grabert-Weiss approach to quantum diffusion in periodic lattices. This model is applied to recent measurements of atomic H and D on Pt(111), rendering friction coefficients that are in the range of those available in the literature for other species of adsorbates. A simple extension of the model has also been considered to evaluate the relationship between coverage and tunneling, and therefore the feasibility of the approach. An increase of the tunneling rate has been observed as the surface coverage decreases. △ Less

Submitted 9 September, 2013; v1 submitted 13 July, 2012; originally announced July 2012.

Comments: 7 pages, 2 figures; important reorganization of the work (including title changes)

Journal ref: Surf. Sci. 617, 229 (2013)

Transmission properties in waveguides: An optical streamline analysis

Authors: A. S. Sanz, J. Campos-Martinez, S. Miret-Artes

Abstract: A novel approach to study transmission through waveguides in terms of optical streamlines is presented. This theoretical framework combines the computational performance of beam propagation methods with the possibility to monitor the passage of light through the guiding medium by means of these sampler paths. In this way, not only the optical flow along the waveguide can be followed in detail, but… ▽ More A novel approach to study transmission through waveguides in terms of optical streamlines is presented. This theoretical framework combines the computational performance of beam propagation methods with the possibility to monitor the passage of light through the guiding medium by means of these sampler paths. In this way, not only the optical flow along the waveguide can be followed in detail, but also a fair estimate of the transmitted light (intensity) can be accounted for by counting streamline arrivals with starting points statistically distributed according to the input pulse. Furthermore, this approach allows to elucidate the mechanism leading to energy losses, namely a vortical dynamics, which can be advantageously exploited in optimal waveguide design. △ Less

Submitted 11 April, 2012; v1 submitted 24 August, 2011; originally announced August 2011.

Comments: 8 pages, 4 figures

Journal ref: J. Opt. Soc. Am. A 29, 695 (2012)

Theoretical description of two ultracold atoms in finite 3D optical lattices using realistic interatomic interaction potentials

Authors: Sergey Grishkevich, Simon Sala, Alejandro Saenz

Abstract: A theoretical approach is described for an exact numerical treatment of a pair of ultracold atoms interacting via a central potential that are trapped in a finite three-dimensional optical lattice. The coupling of center-of-mass and relative-motion coordinates is treated using an exact diagonalization (configuration-interaction) approach. The orthorhombic symmetry of an optical lattice with three… ▽ More A theoretical approach is described for an exact numerical treatment of a pair of ultracold atoms interacting via a central potential that are trapped in a finite three-dimensional optical lattice. The coupling of center-of-mass and relative-motion coordinates is treated using an exact diagonalization (configuration-interaction) approach. The orthorhombic symmetry of an optical lattice with three different but orthogonal lattice vectors is explicitly considered as is the Fermionic or Bosonic symmetry in the case of indistinguishable particles. △ Less

Submitted 24 August, 2011; v1 submitted 14 July, 2011; originally announced July 2011.

Comments: 19 pages, 5 figures

Inelastic Confinement-Induced Resonances in Low-Dimensional Quantum Systems

Authors: Simon Sala, Philipp-Immanuel Schneider, Alejandro Saenz

Abstract: A theoretical model is presented describing the confinement-induced resonances observed in the recent loss experiment of Haller et al. [Phys. Rev. Lett. 104, 153203 (2010)]. These resonances originate from possible molecule formation due to the coupling of center-of-mass and relative motion. A corresponding model is verified by ab initio calculations and predicts the resonance positions in 1D as w… ▽ More A theoretical model is presented describing the confinement-induced resonances observed in the recent loss experiment of Haller et al. [Phys. Rev. Lett. 104, 153203 (2010)]. These resonances originate from possible molecule formation due to the coupling of center-of-mass and relative motion. A corresponding model is verified by ab initio calculations and predicts the resonance positions in 1D as well as in 2D confinement in agreement with the experiment. This resolves the contradiction of the experimental observations to previous theoretical predictions. △ Less

Submitted 13 October, 2011; v1 submitted 8 April, 2011; originally announced April 2011.

Comments: 5 pages, 4 figures

Setting up tunneling conditions by means of Bohmian mechanics

Authors: A. S. Sanz, S. Miret-Artes

Abstract: Usually tunneling is established after imposing some matching conditions on the (time-independent) wave function and its first derivative at the boundaries of a barrier. Here an alternative scheme is proposed to determine tunneling and estimate transmission probabilities in time-dependent problems, which takes advantage of the trajectory picture provided by Bohmian mechanics. From this theory a ge… ▽ More Usually tunneling is established after imposing some matching conditions on the (time-independent) wave function and its first derivative at the boundaries of a barrier. Here an alternative scheme is proposed to determine tunneling and estimate transmission probabilities in time-dependent problems, which takes advantage of the trajectory picture provided by Bohmian mechanics. From this theory a general functional expression for the transmission probability in terms of the system initial state can be reached. This expression is used here to analyze tunneling properties and estimate transmissions in the case of initial Gaussian wave packets colliding with ramp-like barriers. △ Less

Submitted 9 November, 2011; v1 submitted 7 April, 2011; originally announced April 2011.

Comments: 18 pages, 4 figures

Journal ref: J. Phys. A: Math. Theor. 44, 485301 (2011)

Quantum Zeno and anti-Zeno effects in surface diffusion of interacting adsorbates

Authors: H. C. Pennate-Rodriguez, R. Martinez-Casado, G. Rojas-Lorenzo, A. S. Sanz, S. Miret-Artes

Abstract: Surface diffusion of interacting adsorbates is here analyzed within the context of two fundamental phenomena of quantum dynamics, namely the quantum Zeno effect and the anti-Zeno effect. The physical implications of these effects are introduced here in a rather simple and general manner within the framework of non-selective measurements and for two (surface) temperature regimes: high and very low… ▽ More Surface diffusion of interacting adsorbates is here analyzed within the context of two fundamental phenomena of quantum dynamics, namely the quantum Zeno effect and the anti-Zeno effect. The physical implications of these effects are introduced here in a rather simple and general manner within the framework of non-selective measurements and for two (surface) temperature regimes: high and very low (including zero temperature). The quantum intermediate scattering function describing the adsorbate diffusion process is then evaluated for flat surfaces, since it is fully analytical in this case. Finally, a generalization to corrugated surfaces is also discussed. In this regard, it is found that, considering a Markovian framework and high surface temperatures, the anti-Zeno effect has already been observed, though not recognized as such. △ Less

Submitted 23 February, 2012; v1 submitted 7 April, 2011; originally announced April 2011.

Comments: 17 pages, 1 figure

Journal ref: J. Phys.: Condens. Matter 24, 104013 (2012)

Quantum computation with ultracold atoms in a driven optical lattice

Authors: Philipp-Immanuel Schneider, Alejandro Saenz

Abstract: We propose a scheme for quantum computation in optical lattices. The qubits are encoded in the spacial wavefunction of the atoms such that spin decoherence does not influence the computation. Quantum operations are steered by shaking the lattice while qubit addressability can be provided with experimentally available techniques of changing the lattice with single-site resolution. Numerical calcula… ▽ More We propose a scheme for quantum computation in optical lattices. The qubits are encoded in the spacial wavefunction of the atoms such that spin decoherence does not influence the computation. Quantum operations are steered by shaking the lattice while qubit addressability can be provided with experimentally available techniques of changing the lattice with single-site resolution. Numerical calculations show possible fidelities above 99% with gate times on the order of milliseconds. △ Less

Submitted 14 March, 2012; v1 submitted 25 March, 2011; originally announced March 2011.

Comments: 4 pages, 5 figures

Feshbach resonances of harmonically trapped atoms

Authors: Philipp-Immanuel Schneider, Yulian V. Vanne, Alejandro Saenz

Abstract: Employing a short-range two-channel description we derive an analytic model of atoms in isotropic and anisotropic harmonic traps at a Feshbach resonance. On this basis we obtain a new parameterization of the energy-dependent scattering length which differs from the one previously employed. We validate the model by comparison to full numerical calculations for Li-Rb and explain quantitatively the e… ▽ More Employing a short-range two-channel description we derive an analytic model of atoms in isotropic and anisotropic harmonic traps at a Feshbach resonance. On this basis we obtain a new parameterization of the energy-dependent scattering length which differs from the one previously employed. We validate the model by comparison to full numerical calculations for Li-Rb and explain quantitatively the experimental observation of a resonance shift and trap-induced molecules in exited bands. Finally, we analyze the bound state admixture and Landau-Zener transition probabilities. △ Less

Submitted 15 December, 2010; v1 submitted 28 May, 2010; originally announced May 2010.

Comments: 4 pages, 2 figures; revised version with extension to anisotropic traps and new paragraph on trap-induced molecules in excited bands

Frustrated collisions and unconventional pairing on a quantum superlattice

Authors: Manuel Valiente, Matthias Kuster, Alejandro Saenz

Abstract: We solve the problem of scattering and binding of two spin-1/2 fermions on a one-dimensional superlattice with a period of twice the lattice spacing analytically. We find the exact bound states and the scattering states, consisting of a generalized Bethe ansatz augmented with an extra scattering product due to "asymptotic" degeneracy. If a Bloch band is doubly occupied, the extra wave can be a bou… ▽ More We solve the problem of scattering and binding of two spin-1/2 fermions on a one-dimensional superlattice with a period of twice the lattice spacing analytically. We find the exact bound states and the scattering states, consisting of a generalized Bethe ansatz augmented with an extra scattering product due to "asymptotic" degeneracy. If a Bloch band is doubly occupied, the extra wave can be a bound state in the continuum corresponding to a single-particle interband transition. In all other cases, it corresponds to a quasi-momentum changing, frustrated collision. △ Less

Submitted 20 May, 2010; v1 submitted 26 April, 2010; originally announced April 2010.

Comments: 4 pages, 2 figures

Journal ref: EPL 92, 10001 (2010)

Phonon lineshapes in atom-surface scattering

Authors: R. Martinez-Casado, A. S. Sanz, S. Miret-Artes

Abstract: Phonon lineshapes in atom-surface scattering are obtained from a simple stochastic model based on the so-called Caldeira-Leggett Hamiltonian. In this single-bath model, the excited phonon resulting from a creation or annihilation event is coupled to a thermal bath consisting of an infinite number of harmonic oscillators, namely the bath phonons. The diagonalization of the corresponding Hamiltonian… ▽ More Phonon lineshapes in atom-surface scattering are obtained from a simple stochastic model based on the so-called Caldeira-Leggett Hamiltonian. In this single-bath model, the excited phonon resulting from a creation or annihilation event is coupled to a thermal bath consisting of an infinite number of harmonic oscillators, namely the bath phonons. The diagonalization of the corresponding Hamiltonian leads to a renormalization of the phonon frequencies in terms of the phonon friction or damping coefficient. Moreover, when there are adsorbates on the surface, this single-bath model can be extended to a two-bath model accounting for the effect induced by the adsorbates on the phonon lineshapes as well as their corresponding lineshapes. △ Less

Submitted 24 July, 2010; v1 submitted 3 March, 2010; originally announced March 2010.

Comments: 14 pages, 2 figures

Journal ref: J. Phys.: Condens. Matter 22, 304017 (2010)

Two-channel model of photoassociation in the vicinity of a Feshbach resonance

Authors: Philipp-Immanuel Schneider, Alejandro Saenz

Abstract: We derive the two-channel (TC) description of the photoassociation (PA) process in the presence of a magnetic Feshbach resonance and compare to full coupled multi-channel calculations for the scattering of $^{6}$Li-$^{87}$Rb. Previously derived results [P. Pellegrini et al., Phys. Rev. Lett. 101, 053201 (2008)] are corrected. The PA process is shown to be fully described by two parameters: the m… ▽ More We derive the two-channel (TC) description of the photoassociation (PA) process in the presence of a magnetic Feshbach resonance and compare to full coupled multi-channel calculations for the scattering of $^{6}$Li-$^{87}$Rb. Previously derived results [P. Pellegrini et al., Phys. Rev. Lett. 101, 053201 (2008)] are corrected. The PA process is shown to be fully described by two parameters: the maximal transition rate and the point of vanishing transition rate. The TC approximation reproduces excellently the PA transition rates of the full multi-channel calculation and reveals, e.g., that the enhancement of the rate at a resonance is directly connected to the position of vanishing rate. For the description of two independent resonances it was found that only three parameters completely characterize the PA process. △ Less

Submitted 24 September, 2009; originally announced September 2009.

Comments: 4 pages, 2 figures

Linear response theory of activated surface diffusion with interacting adsorbates

Authors: R. Martinez-Casado, A. S. Sanz, J. L. Vega, G. Rojas-Lorenzo, S. Miret-Artes

Abstract: Activated surface diffusion with interacting adsorbates is analyzed within the Linear Response Theory framework. The so-called interacting single adsorbate model is justified by means of a two-bath model, where one harmonic bath takes into account the interaction with the surface phonons, while the other one describes the surface coverage, this leading to defining a collisional friction. Here, the… ▽ More Activated surface diffusion with interacting adsorbates is analyzed within the Linear Response Theory framework. The so-called interacting single adsorbate model is justified by means of a two-bath model, where one harmonic bath takes into account the interaction with the surface phonons, while the other one describes the surface coverage, this leading to defining a collisional friction. Here, the corresponding theory is applied to simple systems, such as diffusion on flat surfaces and the frustrated translational motion in a harmonic potential. Classical and quantum closed formulas are obtained. Furthermore, a more realistic problem, such as atomic Na diffusion on the corrugated Cu(001) surface, is presented and discussed within the classical context as well as within the framework of Kramer's theory. Quantum corrections to the classical results are also analyzed and discussed. △ Less

Submitted 10 May, 2010; v1 submitted 3 September, 2009; originally announced September 2009.

Comments: 40 pages, 4 figures

Journal ref: Chem. Phys. 370, 180 (2010)

Three-body bound states in a lattice

Authors: Manuel Valiente, David Petrosyan, Alejandro Saenz

Abstract: We pursue three-body bound states in a one-dimensional tight-binding lattice described by the Bose-Hubbard model with strong on-site interaction. Apart from the simple strongly-bound "trimer" state corresponding to all three particles occupying the same lattice site, we find two novel kinds of weakly-bound trimers with energies below and above the continuum of scattering states of a single parti… ▽ More We pursue three-body bound states in a one-dimensional tight-binding lattice described by the Bose-Hubbard model with strong on-site interaction. Apart from the simple strongly-bound "trimer" state corresponding to all three particles occupying the same lattice site, we find two novel kinds of weakly-bound trimers with energies below and above the continuum of scattering states of a single particle ("monomer") and a bound particle pair ("dimer"). The corresponding binding mechanism can be inferred from an effective Hamiltonian in the strong-coupling regime which contains an exchange interaction between the monomer and dimer. In the limit of very strong on-site interaction, the exchange-bound trimers attain a universal value of the binding energy. These phenomena can be observed with cold atoms in optical lattices. △ Less

Submitted 17 July, 2009; originally announced July 2009.

Journal ref: Phys. Rev. A 81, 011601(R) (2010)

Exact theoretical description of two ultracold atoms in a single site of a 3D optical lattice using realistic interatomic interaction potentials

Authors: Sergey Grishkevich, Alejandro Saenz

Abstract: A theoretical approach was developed for an exact numerical description of a pair of ultracold atoms interacting via a central potential that are trapped in a three-dimensional optical lattice. The coupling of center-of-mass and relative-motion coordinates is explicitly considered using a configuration-interaction (exact-diagonalization) technique. Deviations from the harmonic approximation are… ▽ More A theoretical approach was developed for an exact numerical description of a pair of ultracold atoms interacting via a central potential that are trapped in a three-dimensional optical lattice. The coupling of center-of-mass and relative-motion coordinates is explicitly considered using a configuration-interaction (exact-diagonalization) technique. Deviations from the harmonic approximation are discussed for several heteronuclear alkali-metal atom pairs trapped in a single site of an optical lattice. The consequences are discussed for the analysis of a recent experiment [C. Ospelkaus et al, Phys. Rev. Lett. 97, 120402 (2006)] in which radio-frequency association was used to create diatomic molecules from a fermionic and a bosonic atom and to measure their binding energies close to a magnetic Feshbach resonance. △ Less

Submitted 12 May, 2009; v1 submitted 16 April, 2009; originally announced April 2009.

Comments: 18 pages, 10 figures

Two-bath model for activated surface diffusion of interacting adsorbates

Authors: R. Martinez-Casado, A. S. Sanz, G. Rojas-Lorenzo, S. Miret-Artes

Abstract: The diffusion and low vibrational motions of adsorbates on surfaces can be well described by a purely stochastic model, the so-called interacting single adsorbate model, for low-moderate coverages (θ\lesssim 0.12). Within this model, the effects of thermal surface phonons and adsorbate-adsorbate collisions are accounted for by two uncorrelated noise functions which arise in a natural way from a… ▽ More The diffusion and low vibrational motions of adsorbates on surfaces can be well described by a purely stochastic model, the so-called interacting single adsorbate model, for low-moderate coverages (θ\lesssim 0.12). Within this model, the effects of thermal surface phonons and adsorbate-adsorbate collisions are accounted for by two uncorrelated noise functions which arise in a natural way from a two-bath model based on a generalization of the one-bath Caldeira-Leggett Hamiltonian. As an illustration, the model is applied to the diffusion of Na atoms on a Cu(001) surface with different coverages. △ Less

Submitted 5 February, 2010; v1 submitted 10 October, 2008; originally announced October 2008.

Comments: 7 pages, 3 figures

Journal ref: J. Chem. Phys. 132, 054704 (2010)

Feshbach resonances in an ultracold $^7$Li and $^{87}$Rb mixture

Authors: C. Marzok, B. Deh, C. Zimmermann, Ph. W. Courteille, E. Tiemann, Y. V. Vanne, A. Saenz

Abstract: We report on the observation of five Feshbach resonances in collisions between ultracold $^7$Li and $^{87}$Rb atoms in the absolute ground state mixture where both species are in their $|f,m_f>=|1,1>$ hyperfine states. The resonances appear as trap losses for the $^7$Li cloud induced by inelastic heteronuclear three-body collisions. The magnetic field values where they occur are important quanti… ▽ More We report on the observation of five Feshbach resonances in collisions between ultracold $^7$Li and $^{87}$Rb atoms in the absolute ground state mixture where both species are in their $|f,m_f>=|1,1>$ hyperfine states. The resonances appear as trap losses for the $^7$Li cloud induced by inelastic heteronuclear three-body collisions. The magnetic field values where they occur are important quantities for an accurate determination of the interspecies interaction potentials. Results of coupled channels calculations based on the observed resonances are presented and refined potential parameters are given. A very broad Feshbach resonance centered around 649 G should allow for fine tuning of the interaction strength in future experiments. △ Less

Submitted 28 August, 2008; originally announced August 2008.

Comments: 7 pages, 3 figures, 4 tables

Quantum Markovian activated surface diffusion of interacting adsorbates

Authors: R. Martinez-Casado, A. S. Sanz, S. Miret-Artes

Abstract: A quantum Markovian activated atom-surface diffusion model with interacting adsorbates is proposed for the intermediate scattering function, which is shown to be complex-valued and factorizable into a classical-like and a quantum-mechanical factor. Applications to the diffusion of Na atoms on flat (weakly corrugated) and corrugated-Cu(001) surfaces at different coverages and surface temperatures… ▽ More A quantum Markovian activated atom-surface diffusion model with interacting adsorbates is proposed for the intermediate scattering function, which is shown to be complex-valued and factorizable into a classical-like and a quantum-mechanical factor. Applications to the diffusion of Na atoms on flat (weakly corrugated) and corrugated-Cu(001) surfaces at different coverages and surface temperatures are analyzed. Quantum effects are relevant to diffusion at low surface temperatures and coverages even for relatively heavy particles, such as Na atoms, where transport by tunneling is absent. △ Less

Submitted 14 November, 2008; v1 submitted 4 March, 2008; originally announced March 2008.

Comments: 6 pages, 4 figures

Journal ref: J. Chem. Phys. 129, 184704 (2008)

Stochastic theory of lineshape broadening in quasielastic He atom scattering with interacting adsorbates

Authors: R. Martinez-Casado, J. L. Vega, A. S. Sanz, S. Miret-Artes

Abstract: The activated surface diffusion of interacting adsorbates is described in terms of the so-called interacting single adsorbate approximation, which is applied to the diffusion of Na atoms on Cu(001) for coverages up to 20% in quasielastic He atom scattering experiments. This approximation essentially consists of solving the standard Langevin equation with two noise sources and frictions: a Gaussi… ▽ More The activated surface diffusion of interacting adsorbates is described in terms of the so-called interacting single adsorbate approximation, which is applied to the diffusion of Na atoms on Cu(001) for coverages up to 20% in quasielastic He atom scattering experiments. This approximation essentially consists of solving the standard Langevin equation with two noise sources and frictions: a Gaussian white noise accounting for the friction with the substrate, and a white shot noise characterized by a collisional friction simulating the adsorbate-adsorbate collisions. The broadenings undergone by the quasielastic peak are found to be in very good agreement with the experimental data reported at two surface temperatures 200 and 300 K. △ Less

Submitted 11 March, 2008; v1 submitted 17 October, 2007; originally announced October 2007.

Comments: 6 pages, 3 figures

Journal ref: Phys. Rev. B 77, 115414 (2008)

Quasielastic He atom scattering from surfaces: A stochastic description of the dynamics of interacting adsorbates

Authors: R. Martinez-Casado, J. L. Vega, A. S. Sanz, S Miret-Artes

Abstract: The study of diffusion and low frequency vibrational motions of particles on metal surfaces is of paramount importance; it provides valuable information on the nature of the adsorbate-substrate and the substrate-substrate interactions. In particular, the experimental broadening observed in the diffusive peak with increasing coverage is usually interpreted in terms of a dipole-dipole like interac… ▽ More The study of diffusion and low frequency vibrational motions of particles on metal surfaces is of paramount importance; it provides valuable information on the nature of the adsorbate-substrate and the substrate-substrate interactions. In particular, the experimental broadening observed in the diffusive peak with increasing coverage is usually interpreted in terms of a dipole-dipole like interaction among adsorbates via extensive molecular dynamics calculations within the Langevin framework. Here we present an alternative way to interpret this broadening by means of a purely stochastic description, namely the interacting single adsorbate approximation, where two noise sources are considered: (1) a Gaussian white noise accounting for the surface friction and temperature, and (2) a white shot noise replacing the interaction potential between adsorbates. Standard Langevin numerical simulations for flat and corrugated surfaces (with a separable potential) illustrate the dynamics of Na atoms on a Cu(100) surface which fit fairly well to the analytical expressions issued from simple models (free particle and anharmonic oscillator) when the Gaussian approximation is assumed. A similar broadening is also expected for the frustrated translational mode peaks. △ Less

Submitted 16 July, 2007; originally announced July 2007.

Comments: 32 pages, 7 figures

Journal ref: J. Phys.: Condens. Matter 19, 305002 (2007)

Line Shape Broadening in Surface Diffusion of Interacting Adsorbates with Quasielastic He Atom Scattering

Authors: R. Martinez-Casado, J. L. Vega, A. S. Sanz, S. Miret-Artes

Abstract: The experimental line shape broadening observed in adsorbate diffusion on metal surfaces with increasing coverage is usually related to the nature of the adsorbate-adsorbate interaction. Here we show that this broadening can also be understood in terms of a fully stochastic model just considering two noise sources: (i) a Gaussian white noise accounting for the surface friction, and (ii) a shot n… ▽ More The experimental line shape broadening observed in adsorbate diffusion on metal surfaces with increasing coverage is usually related to the nature of the adsorbate-adsorbate interaction. Here we show that this broadening can also be understood in terms of a fully stochastic model just considering two noise sources: (i) a Gaussian white noise accounting for the surface friction, and (ii) a shot noise replacing the physical adsorbate-adsorbate interaction potential. Furthermore, contrary to what could be expected, for relatively weak adsorbate-substrate interactions the opposite effect is predicted: line shapes get narrower with increasing coverage. △ Less

Submitted 24 May, 2007; v1 submitted 8 February, 2007; originally announced February 2007.

Comments: 4 pages, 2 figures (slightly revised version)

Journal ref: Phys. Rev. Lett. 98, 216102 (2007)

A generalized Chudley-Elliott vibration-jump model in activated atom surface diffusion

Authors: R. Martinez-Casado, J. L. Vega, A. S. Sanz, S. Miret-Artes

Abstract: Here the authors provide a generalized Chudley-Elliott expression for activated atom surface diffusion which takes into account the coupling between both low-frequency vibrational motion (namely, the frustrated translational modes) and diffusion. This expression is derived within the Gaussian approximation framework for the intermediate scattering function at low coverage. Moreover, inelastic co… ▽ More Here the authors provide a generalized Chudley-Elliott expression for activated atom surface diffusion which takes into account the coupling between both low-frequency vibrational motion (namely, the frustrated translational modes) and diffusion. This expression is derived within the Gaussian approximation framework for the intermediate scattering function at low coverage. Moreover, inelastic contributions (arising from creation and annihilation processes) to the full width at half maximum of the quasi-elastic peak are also obtained. △ Less

Submitted 21 May, 2007; v1 submitted 11 September, 2006; originally announced September 2006.

Comments: (5 pages, 2 figures; revised version)

Journal ref: J. Chem. Phys. 126, 194711 (2007)

Quasi-elastic peak lineshapes in adsorbate diffusion on nearly flat surfaces at low coverages: the motional narrowing effect in Xe on Pt(111)

Authors: R. Martinez-Casado, J. L. Vega, A. S. Sanz, S. Miret-Artes

Abstract: Quasi-elastic helium atom scattering measurements have provided clear evidence for a two-dimensional free gas of Xe atoms on Pt(111) at low coverages. Increasing the friction due to the surface, a gradual change of the shape of the quasi-elastic peak is predicted and analyzed for this system in terms of the so-called motional narrowing effect. The type of analysis presented here for the quasi-el… ▽ More Quasi-elastic helium atom scattering measurements have provided clear evidence for a two-dimensional free gas of Xe atoms on Pt(111) at low coverages. Increasing the friction due to the surface, a gradual change of the shape of the quasi-elastic peak is predicted and analyzed for this system in terms of the so-called motional narrowing effect. The type of analysis presented here for the quasi-elastic peak should be prior to any deconvolution procedure carried out in order to better extract information from the process, e.g. diffusion coefficients and jump distributions. Moreover, this analysis also provides conditions for the free gas regime different than those reported earlier. △ Less

Submitted 12 April, 2007; v1 submitted 31 August, 2006; originally announced August 2006.

Comments: 12 pages, 4 figures (revised version)

Journal ref: J. Phys.: Condensed. Matter 19, 176006 (2007)

Surface diffusion and low vibrational motion with interacting adsorbates: A shot noise description

Authors: R. Martinez-Casado, J. L. Vega, A. S. Sanz, S. Miret-Artes

Abstract: Here, an approach in terms of shot noise is proposed to study and characterize surface diffusion and low vibrational motion when having interacting adsorbates on surfaces. In what we call statistical limit, that is, at long times and high number of collisions, one expects that diffusing particles display an essential Markovian behavior. Accordingly, the action of the pairwise potentials accounti… ▽ More Here, an approach in terms of shot noise is proposed to study and characterize surface diffusion and low vibrational motion when having interacting adsorbates on surfaces. In what we call statistical limit, that is, at long times and high number of collisions, one expects that diffusing particles display an essential Markovian behavior. Accordingly, the action of the pairwise potentials accounting for particle-particle collisions is equivalent to considering a shot noise acting on a single particle. We call this approach the interacting single adsorbate approximation, which gathers three important advantages: (i) the dynamics underlying surface diffusion and low vibrational motion can be easily understood in terms of relatively simple stochastic processes; (ii) from our model, appropriate (and well justified) working formulas are easily obtained, which explain the results arising from more complicated (but commonly used) molecular dynamics simulations within the Langevin formulation; and (iii), at the same time, it is less demanding computationally than the latter type of calculations. In order to illustrate the application of this model, numerical results are presented. Specially, our model reproduces the experimental observation regarding the broadening of the quasielastic peak ruling surface diffusion. △ Less

Submitted 5 June, 2007; v1 submitted 31 August, 2006; originally announced August 2006.

Comments: (13 pages, 7 figues; slightly revised version)

Journal ref: Phys. Rev. E 75, 051128 (2007)