Effects of dimensionality and anisotropy on the Holstein polaron

We apply weak-coupling perturbation theory and strong-coupling perturbation theory to the Holstein molecular crystal model in order to elucidate the effects of anisotropy on polaron properties in D dimensions. The ground state energy is considered as a primary criterion through which to study the effects of anisotropy on the self-trapping transition, the self-trapping line associated with this transition, and the adiabatic critical point. The effects of dimensionality and anisotropy on electron-phonon correlations and polaronic mass enhancement are studied, with particular attention given to the polaron radius and the characteristics of quasi-1D and quasi-2D structures. Perturbative results are confirmed by selected comparisons with variational calculations and quantum Monte Carlo data

Efecto de la impregnación al vacío sobre los cambios físicos durante la elaboración de aceitunas de mesa

Among the benefits which vacuum impregnation (VI) may provide to fruits and vegetables, this study focused on weight and texture changes during the processing of table olives. VI applied to Manzanilla olives led to around 10% weight gain, which was maintained after their packing as black olives. However, this weight gain was only around 4 % for Hojiblanca olives. Likewise, the use of calcium chloride was recommended to maintain the firmness of the olives, in particular those of the softer Manzanilla cultivar. With regard to the Spanish-style, the Hojiblanca cultivar achieved around 4% weight gain during processing but the use of VI for Manzanilla olives was ruled out due to softening of the fruit. In addition. the black and green color of olives and their flavor were not modified by the application of VI. This technology could be very useful to reduce weight loss during table olive processing.Entre los beneficios que la aplicación de la impregnación al vacío puede originar en frutas y verduras, este estudio se ha centrado sobre los cambios en la textura y el peso de las aceitunas de mesa durante su procesamiento. La impregnación al vacío de aceitunas Manzanilla dio lugar a un aumento cercano al 10 % de peso y ello se mantuvo después de su envasado como aceitunas negras, mientras que esta ganancia fue de sólo el 4& para aceitunas Hojiblanca. Asimismo, se vio necesario el empleo de cloruro cálcico para el mantenimiento de la textura de los frutos, en particular para aquellos de la variedad Manzanilla. Con respecto al procesamiento como verdes estilo español, la variedad Hojiblanca consiguió un aumento del 4% en peso debido al empleo de impregnación al vacío y este tratamiento se descartó para la Manzanilla debido al ablandamiento de la aceituna. Además, se debe indicar que el color verde y negro de las aceitunas no se vio afectado por el hecho de emplear impregnación al vacío. Esta tecnología podría ser muy útil para reducir las pérdidas de peso que se producen durante la elaboración de aceitunas de mesa

Nature of segregation of reactants in diffusion controlled A+B reactions: Role of mobility in forming compact clusters

We investigate the A+B=0 bimolecular chemical reaction taking place in low-dimensional spaces when the mobilities of the two reacting species are not equal. While the case of different reactant mobilities has been previously reported as not affecting the scaling of the reactant densities with time, but only the pre-exponential factor, the mechanism for this had not been explained before. By using Monte-Carlo simulations we show that the nature of segregation is very different when compared to the normal case of equal reactant mobilities. The clusters of the mobile species are statistically homogeneous and randomly distributed in space, but the clusters of the less mobile species are much more compact and restricted in space. Due to the asymmetric mobilities, the initial symmetric random density fluctuations in time turn into asymmetric density fluctuations. We explain this trend by calculating the correlation functions for the positions of particles for the several different cases

Noncommutative Quantum Cosmology

We consider noncommutative quantum cosmology in the case of the low-energy string effective theory. Exacts solutions are found and compared with the commutative case.The Noncommutative quantum cosmology is considered in the case of the low-energy string effective theory. Exacts solutions are found and compared with the commutative case.Comment: Revtex4, 3 pages, 2 figures, to appear in Gen.Rel.Gra

Mass Renormalization in the Su-Schrieffer-Heeger Model

This study of the one dimensional Su-Schrieffer-Heeger model in a weak coupling perturbative regime points out the effective mass behavior as a function of the adiabatic parameter $\omega_{\pi}/J$, $\omega_{\pi}$ is the zone boundary phonon energy and $J$ is the electron band hopping integral. Computation of low order diagrams shows that two phonons scattering processes become appreciable in the intermediate regime in which zone boundary phonons energetically compete with band electrons. Consistently, in the intermediate (and also moderately antiadiabatic) range the relevant mass renormalization signals the onset of a polaronic crossover whereas the electrons are essentially undressed in the fully adiabatic and antiadiabatic systems. The effective mass is roughly twice as much the bare band value in the intermediate regime while an abrupt increase (mainly related to the peculiar 1D dispersion relations) is obtained at $\omega_{\pi}\sim \sqrt{2}J$.Comment: To be published in Phys.Rev.B - 3 figure

Ab initio calculations of the hydrogen bond

Recent x-ray Compton scattering experiments in ice have provided useful information about the quantum nature of the interaction between H$_2$O monomers. The hydrogen bond is characterized by a certain amount of charge transfer which could be determined in a Compton experiment. We use ab-initio simulations to investigate the hydrogen bond in H$_2$O structures by calculating the Compton profile and related quantities in three different systems, namely the water dimer, a cluster containing 12 water molecules and the ice crystal. We show how to extract estimates of the charge transfer from the Compton profiles.Comment: 16 pages, 7 figures, to appear in Phys. Rev.

Insights into the magnetic dead layer in La0.7Sr0.3MnO3 thin films from temperature, magnetic field and thickness dependence of their magnetization

Experimental investigations of the magnetic dead layer in 7.6 nm thick film of La0.7Sr0.3MnO3 (LSMO) are reported. The dc magnetization (M) measurements for a sample cooled to T = 5 K in applied field H = 0 reveal the presence of negative remanent magnetization (NRM) in the M vs. H (magnetic field) measurements as well as in the M vs. T measurements in H = 50 Oe and 100 Oe. The M vs. T data in ZFC (zero-field-cooled) and FC (field-cooled) protocols are used to determine the blocking temperature TB in different H. Isothermal hysteresis loops at differ- ent T are used to determine the temperature dependence of saturation magnetization (MS), remanence (MR) and coercivity HC. The MS vs. T data are fit to the Bloch law,MS (T)=M0 (1–BT 3/2),showingagoodfitforT \u3c100Kandyieldingthe nearest-neighbor exchange constant J/kB 18 K. The variations of TB vs. H andHC vs. T are well described by the model often used for randomly oriented mag- netic nanoparticles with magnetic domain diameter ≈ 9 nm present in the dead-layer of thickness d =1.4 nm. Finally, the data available from literature on the thickness (D) variation of Curie temperature (TC) and MS of LSMO films grown under 200, 150, and 0.38 mTorr pressures of O2 are analyzed in terms of the finite-size scaling, with MS vs. D data fit to MS (D) = MS(b)(1-d/D) yielding the dead layer thickness d = 1.1 nm, 1.4 nm and 2.4 nm respectively

Calculation of excited polaron states in the Holstein model

An exact diagonalization technique is used to investigate the low-lying excited polaron states in the Holstein model for the infinite one-dimensional lattice. For moderate values of the adiabatic ratio, a new and comprehensive picture, involving three excited (coherent) polaron bands below the phonon threshold, is obtained. The coherent contribution of the excited states to both the single-electron spectral density and the optical conductivity is evaluated and, due to the invariance of the Hamiltonian under the space inversion, the two are shown to contain complementary information about the single-electron system at zero temperature. The chosen method reveals the connection between the excited bands and the renormalized local phonon excitations of the adiabatic theory, as well as the regime of parameters for which the electron self-energy has notable non-local contributions. Finally, it is shown that the hybridization of two polaron states allows a simple description of the ground and first excited state in the crossover regime.Comment: 12 pages, 9 figures, submitted to PR

Layered topological semimetal GaGeTe: New polytype with non-centrosymmetric structure

[EN] GaGeTe is a layered van der Waals material composed of germanene and GaTe sublayers that has been recently predicted to be a basic Z2 topological semimetal. To date, only one polytype of GaGeTe is known with trigonal centrosymmetric structure (a phase, space group R-3m, No. 166). Here we show that asgrown samples of GaGeTe show traces of at least another polytype with hexagonal noncentrosymmetric structure (f3 phase, space group P63mc, No. 186). Moreover, we suggest that another bulk hexagonal polytype (g phase, space group P-3m1, No. 164) could also be found near room conditions. Both a and f3 polytypes have been identified and characterized by means of X-ray diffraction and Raman scattering measurements with the support of ab initio calculations. We provide the vibrational properties of both polytypes and show that the Raman spectrum reported for GaGeTe almost forty years ago and attributed to the a phase, was, in fact, that of the secondary f3 phase. Additionally, we show that a Fermi resonance occurs in a-GaGeTe under non-resonant excitation conditions, but not under resonant excitation conditions. Theoretical calculations show that bulk f3-GaGeTe is a non-centrosymmetric weak topological semimetal with even smaller lattice thermal conductivity than centrosymmetric bulk aGaGeTe. In perspective, our work paves the way for the control and engineering of GaGeTe polytypes to design and implement complex van der Waals heterostructures formed by a combination of centrosymmetric and non-centrosymmetric layers of up to three different polytypes in a single material, suitable for a number of fundamental studies and technological applications.This publication is part of the project MALTA Consolider Team network (RED2018-102612-T) , financed by MINECO/AEI/10.13039/501100003329; by I ? D ? i projects PID2019-106383 GB -41/42/43 financed by MCIN/AEI/10.13039/501100011033; and by project PROMETEO/2018/123 (EFIMAT) financed by Generalitat Valenciana. E.B. would like to thank the Universitat Politecnica de Valencia for his postdoctoral contract (Ref. PAID -10-21) . AHR was supported by the U.S. Department of Energy (DOE) , Office of Science, Basic Energy Sciences under award DE-SC0021375. We also acknowledge the computational resources awarded by XSEDE, a project supported by National Science Foundation grant number ACI-1053575. The authors also acknowledge the support from the Texas Advances Computer Center (with the Stampede2 and Bridges supercom- puters) . E.L.d.S would like to acknowledge the Network of Extreme Conditions Laboratories (NECL) , financed by FCT and co -financed by NORTE 2020, through the program Portugal 2020 and FEDER; the High Performance Computing Chair-a R & D infrastructure (based at the University of ? Evora; PI: M. Avillez) ; and for the computational support provided by the HPC center OBLIVION -U. ? Evora to perform the lattice thermal conductivity calculations. A.L. and D.E. would like to thank the Generalitat Valenciana for the Ph.D. Fellowship no. GRISOLIAP/2019/025.Gallego-Parra, S.; Bandiello, E.; Liang, A.; Da Silva, EL.; Rodriguez-Hernandez, P.; Muñoz, A.; Radescu, S.... (2022). Layered topological semimetal GaGeTe: New polytype with non-centrosymmetric structure. Materials Today Advances. 16:1-16. https://doi.org/10.1016/j.mtadv.2022.1003091161

Quantum Monte Carlo and variational approaches to the Holstein model

Based on the canonical Lang-Firsov transformation of the Hamiltonian we develop a very efficient quantum Monte Carlo algorithm for the Holstein model with one electron. Separation of the fermionic degrees of freedom by a reweighting of the probability distribution leads to a dramatic reduction in computational effort. A principal component representation of the phonon degrees of freedom allows to sample completely uncorrelated phonon configurations. The combination of these elements enables us to perform efficient simulations for a wide range of temperature, phonon frequency and electron-phonon coupling on clusters large enough to avoid finite-size effects. The algorithm is tested in one dimension and the data are compared with exact-diagonalization results and with existing work. Moreover, the ideas presented here can also be applied to the many-electron case. In the one-electron case considered here, the physics of the Holstein model can be described by a simple variational approach.Comment: 18 pages, 11 Figures, v2: one typo correcte