Construction of Many-Body Eigenstates with Displacement Transformations

Many-body eigenstates beyond the gaussian approximation can be constructed in terms of local integrals of motion (IOM), although their actual computation has been until now a daunting task. We present a new practical computation of IOMS based on displacement transformations. It represents a general and systematic way to extend Hartree-Fock and configuration interaction theories to higher order. Our method combines minimization of energy and energy variance of a reference state with exact diagonalization. We show that our implementation is able to perform ground state calculations with high precision for relatively large systems. Since it keeps track of the IMO's forming a reference state, our method is particularly efficient dealing with excited states, both in accuracy and the number of different states that can be constructed

Delocalization by Disorder in Layered Systems

Motivated by anomalously large conductivity anisotropy in layered materials, we propose a simple model of randomly spaced potential barriers (mimicking stacking faults) with isotropic impurities in between the barriers. We solve this model both numerically and analytically, by utilizing an exact solution for the conductivity of a one-dimensional (1D) disordered system. In the absence of bulk disorder, electron motion in the out-of-plane direction is localized. Bulk disorder destroys 1D localization. As a result, the out-of-plane conductivity is finite and scales linearly with the scattering rate by bulk impurities until planar and bulk disorder become comparable. The \emph{ac} out-of-plane conductivity is of a manifestly non-Drude form, with a maximum at the frequency corresponding to the scattering rate by potential barriers.Comment: 4 pages, 4 figure

Transiciones de fase en cristales liquidos

1 v.Centro de Informacion y Documentacion Cientifica (CINDOC). C/Joaquin Costa, 22. 28002 Madrid. SPAIN / CINDOC - Centro de Informaciòn y Documentaciòn CientìficaSIGLEESSpai

Many‐body localization from the perspective of Integrals of Motion

We study many-body localization (MBL) and delocalization from the perspective of integrals of motion (IOMs). MBL can be understood phenomenologically through the existence of macroscopically many localized IOMs. However, IOMs exist for all many-body systems, and non-localized IOMs determine properties on the ergodic side of the MBL transition too. Here we explore their properties using our method of displacement transformations. We show how different quantities can be calculated using the IOMs as an expansion in the number of operators. For all values of disorder the typical IOMs are localized, suggesting the importance of rare fluctuations in understanding the delocalization transition

Conducting polymers as electron glasses: Surface charge domains and slow relaxation

The surface potential of conducting polymers has been studied with scanning Kelvin probe microscopy. The results show that this technique can become an excellent tool to really 'see' interesting surface charge interaction effects at the nanoscale. The electron glass model, which assumes that charges are localized by the disorder and that interactions between them are relevant, is employed to understand the complex behavior of conducting polymers. At equilibrium, we find surface potential domains with a typical lateral size of 50 nm, basically uncorrelated with the topography and strongly fluctuating in time. These fluctuations are about three times larger than thermal energy. The charge dynamics is characterized by an exponentially broad time distribution. When the conducting polymers are excited with light the surface potential relaxes logarithmically with time, as usually observed in electron glasses. In addition, the relaxation for different illumination times can be scaled within the full aging model.This work has been funded by the Ministerio de Economia y Competitividad and (MINECO, Spain) FEDER (EU)through the projects FIS2012-38206, CSD2010-00024 and ENE2013-48816-C5-1-R -C02-01 and by the Fundacion Seneca 15324/PI/10 and 19907/GERM/15. EE and ELE thanks the MINECO programs MAT 2006-12970-C02-01, CSD2010-00024 for the financial support. EPL thanks the Ramon y Cajal program RyC2010.Peer reviewe

Graphene Oxide: The spring-dance of electrons ("Una Jota Aragonesa")

Resumen del póster presentado en el congreso ChemOnTubes 2022, celebrado en Donostia-San Sebastián (España), del 24 al 28 de abril.Graphene oxide is a unique nanoscale object with tunable electronic properties, largely influenced by the presence of oxygen functional groups located on the basal plane [1-3]. In this work shine light on the nanoscale charge density and dynamics of isolated GO nanosheets, probed by Kelvin Probe Microscopy [4]. We visualize the existence of charge-domains with respective charge interactions. We further reveal short range fluctuations of jumping electrons and the existence of very long-term relaxation times. The experimental findings are consistent with theoretical simulations, underlining that graphene oxide needs to be understood in terms of an electron glass revealing hopping transport between charge-domains.Spanish MICINN/AEI: Projects PID2019-104272RB-C51/AEI/10.13039/501100011033 and PID2019-104272RB-C52/AEI/10.13039/501100011033; Government of Aragón: Grupos Reconocidos DGA T03_20R

Nanoscale charge density and dynamics in graphene oxide

4 figures.-- Supplementary information available.Graphene oxide (GO) is widely used as a component in thin film optoelectronic device structures for practical reasons because its electronic and optical properties can be controlled. Progress critically depends on elucidating the nanoscale electronic structure of GO. However, direct experimental access is challenging because of its disordered and nonconductive character. Here, we quantitatively mapped the nanoscopic charge distribution and charge dynamics of an individual GO sheet by using Kelvin probe force microscopy (KPFM). Charge domains are identified, presenting important charge interactions below distances of 20 nm. Charge dynamics with very long relaxation times of at least several hours and a logarithmic decay of the time correlation function are in excellent agreement with Monte Carlo simulations, revealing an universal hopping transport mechanism best described by Efros–Shklovskii’s law.This research was financed by the Ministerio de Ciencia e Innovación and the Agencia Estatal de Investigación (MICINN/AEI, Spain) and associated Funds of the European Union through the projects “Nano and Meso Scales: Modelling, Structure and Characterization” (PID2019-104272RB-C52/AEI/10.13039/501100011033 and “Photoelectrochemical hydrogen production by optimized graphene-based interfaces” (PID2019-104272RB-C51/AEI/10.13039/501100011033) and the Fundación Séneca through the projects 19907/GERM/15 and 20860/PI/18, as well as the Gobierno de Aragón (Grupo Reconocido DGA-T03_20R).E.C. acknowledges funding of his predoctoral contract by Spanish MINEICO and associated European Social Funds (BES2017-080020).Peer reviewe

Nanoscale charge dynamics in Graphene Oxide and other low dimensional materials studied by electrostatic scanning force microscopy

1 figure.-- Abstract of the work presented at the NanoteC19 conference, International Conference on Carbon Nanosciene and Nanotechnology, 27th-30th august 2019, Zaragoza (Spain).Charge transport in ordered system is generally well understood, but correct modelling of conductivity in highly disordered system often presents an important theoretical as well as experimental challenge. Slow conductance relaxation has been studied in many disordered insulators using field effect measurements. The use of local probe techniques, such as Scanning Kelvin probe microscopy (SKPM), presents two advantages as compared with the conductance measurements performed so far: i) it allows a study of the phenomena at the nanometer scale, and ii) samples with larger resistances can be measured. In the present work Dynamic Atomic Force Microscopy (DAFM) is used to characterize the electronic properties of Graphene Oxide islands (see figure 1) as well as other high resistance nanoscale systems. Their time evolution is studied using ¿movies¿ where topography and surface potential are acquired simultaneously. Our DAFM studies suggest evidence of the formation of an electron glass in the materials studied. The fluctuations of the surface potential are compatible with variations of the Coulomb energy of a single charge over the distance between domains. At the same time, the fact that the fluctuations are larger than kT and that time correlations are dominated by a broad distribution of characteristic times can be naturally explained within the electron glass model.Peer reviewe

Disclosing the nature of vacancy defects in α-Ag2WO4

Defects at semiconductors with electron acceptor and donor sites govern the electronic and optoelectronic applications due to their unique electronic properties. This work provides deep insight into the nature of defects and the conduction mechanism in α-Ag2WO4. To this aim, a detailed analysis of the results of XRD with Rietveld refinements, FE-SEM images, and measurements of different spectroscopies (impedance, positron annihilation lifetime, and photoluminescence) are carried out on α-Ag2WO4 samples synthesized by a simple co-precipitation method. Two types of vacancy defects: cationic O-vacancies, and anionic Ag or Ag–O vacancy complexes are elucidated with a Schottky p-type potential barrier. The results indicate that the Ag vacancies remain constant during thermal treatment, while an opposite effect is found for the oxygen vacancies. This behavior governs the multifunctional properties of α-Ag2WO4 semiconductors via a tunneling plus thermionic conduction mechanism