Nonlinear transport and oscillating magnetoresistance in double quantum wells

We study the evolution of low-temperature magnetoresistance in double quantum wells in the region below 1 Tesla as the applied current density increases. A flip of the magneto-intersubband oscillation peaks, which occurs as a result of the current-induced inversion of the quantum component of resistivity, is observed. We also see splitting of these peaks as another manifestation of nonlinear behavior, specific for the two-subband electron systems. The experimental results are quantitatively explained by the theory based on the kinetic equation for the isotropic non-equilibrium part of electron distribution function. The inelastic scattering time is determined from the dependence of the inversion magnetic field on the current.Comment: 20 pages, 10 figure

MPI-CUDA parallel linear solvers for block-tridiagonal matrices in the context of SLEPc's eigensolvers

[EN] We consider the computation of a few eigenpairs of a generalized eigenvalue problem Ax = lambda Bx with block-tridiagonal matrices, not necessarily symmetric, in the context of Krylov methods. In this kind of computation, it is often necessary to solve a linear system of equations in each iteration of the eigensolver, for instance when B is not the identity matrix or when computing interior eigenvalues with the shift-and-invert spectral transformation. In this work, we aim to compare different direct linear solvers that can exploit the block-tridiagonal structure. Block cyclic reduction and the Spike algorithm are considered. A parallel implementation based on MPI is developed in the context of the SLEPc library. The use of GPU devices to accelerate local computations shows to be competitive for large block sizes.This work was supported by Agencia Estatal de Investigacion (AEI) under grant TIN2016-75985-P, which includes European Commission ERDF funds. Alejandro Lamas Davina was supported by the Spanish Ministry of Education, Culture and Sport through a grant with reference FPU13-06655.Lamas Daviña, A.; Roman, JE. (2018). MPI-CUDA parallel linear solvers for block-tridiagonal matrices in the context of SLEPc's eigensolvers. Parallel Computing. 74:118-135. https://doi.org/10.1016/j.parco.2017.11.006S1181357

Beyond the LTV ratio: new macroprudential lessons from Spain

Incrementos excesivos de los precios de la vivienda han estado históricamente correlacionados con el deterioro de los estándares crediticios de las hipotecas. No obstante, la evidencia en España muestra que la ratio préstamo-valor (LTV, por sus siglas en inglés) no refleja apropiadamente estos estándares. Utilizando dos extensas bases de datos que incluyen información de más de cinco millones de operaciones hipotecarias en España durante un período que cubre el pasado ciclo financiero, encontramos evidencia de que el indicador LTV presenta importantes limitaciones para el análisis de vulnerabilidades. Hallamos que la identificación de los riesgos mejora cuando se incluyen otros indicadores. En particular, reconocemos que la ratio préstamo-precio de transmisión (LTP, por sus siglas en inglés), así como ratios que incorporan el ingreso de los prestatarios, son determinantes muy relevantes de los impagos de hipotecas. Adicionalmente, identificamos importantes efectos no lineales de los estándares de crédito sobre el riesgo de impago. Por último, encontramos que la relación entre los estándares de crédito y la probabilidad de impago es dinámica y cambia a lo largo del ciclo financiero. En general, nuestros resultados proporcionan un mejor entendimiento de los efectos que puede tener la implementación de medidas macroprudenciales.Booming house prices have been historically correlated with the loosening of banks’ lending standards. Nonetheless, the evidence in Spain shows that the deterioration of lending policies may not be fully captured by the popular loan-to-value (LTV) ratio. Drawing on two large datasets comprising more than five million mortgage operations that cover the last financial cycle, we show that the LTV indicator may exhibit a misleading picture of actual mortgage credit imbalances and risk. In turn, risk identification improves when other metrics are considered. In particular, we show that loan-to-price (LTP) as well as ratios that consider the income of borrowers are major determinants of mortgage defaults. Moreover, we identify relevant non-linear effects of lending standards on default risk. Finally, we document that the relationship between lending standards and default rates changes over the cycle. Overall, the findings provide useful insights for the design of the macroprudential policy mix and, in particular, for the implementation of borrower-based measures

Experimental Polarization State Tomography using Optimal Polarimeters

We report on the experimental implementation of a polarimeter based on a scheme known to be optimal for obtaining the polarization vector of ensembles of spin-1/2 quantum systems, and the alignment procedure for this polarimeter is discussed. We also show how to use this polarimeter to estimate the polarization state for identically prepared ensembles of single photons and photon pairs and extend the method to obtain the density matrix for generic multi-photon states. State reconstruction and performance of the polarimeter is illustrated by actual measurements on identically prepared ensembles of single photons and polarization entangled photon pairs

Roots and recourse mortgages: handing back the keys

En este estudio identificamos diferencias en las condiciones de las hipotecas asociadas al arraigo de los prestatarios tras separar este efecto de otros factores que explican dichas condiciones. Para esto usamos datos de préstamos hipotecarios en España de la Central de Información de Riesgos, complementados con datos de hipotecas titulizadas para un ciclo financiero completo. Identificamos que los prestatarios extranjeros con poco arraigo pagan tipos de interés más altos que aquellos con mayores vínculos en el país. También encontramos que un bajo nivel de arraigo está asociado con tasas más altas de impago y con mayores incentivos a no continuar pagando la deuda hipotecaria ante situaciones de patrimonio neto negativo. En general, identificamos que el arraigo es un factor muy relevante que explica las diferencias en las condiciones de las hipotecas en el momento de su concesión, así como las decisiones de impago. Desde el punto de vista de la política, nuestros resultados tienen importantes implicaciones para el entendimiento de las consecuencias de transitar hacia un régimen de dación en pago, y para la efectividad de la política macroprudencial.In this study we disentangle the effect of roots from other confounding factors to explain differences in immigrants’ outcomes in the mortgage market. Using loan-level data from the Spanish Credit Register complemented with data on securitized mortgages over a complete financial cycle, we identify that foreign-born borrowers with shallow roots to the host country pay higher mortgage rates at origination than similar debtors that are better-settled. We also find that weak roots are associated with higher default rates and with greater incentives to go into default in negative equity situations. Overall, we show that rootedness explains differential loan conditions at origination and default behavior in mortgages. From a policy perspective, our results have important implications for understanding the potential consequences of moving away from recourse mortgage regimes, and for the effectiveness of macroprudential policy

Roots and recourse mortgages: handing back the keys

Summary of Banco de España Working Paper no. 220

Characterisation and numerical modelling of the geometry of rock joints

This paper describes the measurement and analysis of the roughness of natural rock joints with the aim of proceeding to their characterisation and comparison. Eight rock joints (corresponding to sixteen surfaces) were scanned with a three-dimensional scanner. This was followed by the description and characterisation of the surface roughness, as well as its reproduction and generation using techniques such as fractal models and Fourier transforms. Shear tests under constant normal load were performed, the joint surfaces were again scanned, and the resulting roughness surfaces were compared allowing the evaluation of asperity breakage and surface wear. One of the joint specimens was tested under several normal stresses, which allowed the estimation of its mechanical characteristics, and was scanned after each test to evaluate the influence of the normal and shear load on the variation of roughness. The scans of the rock joints produced accurate numerical descriptions of their topologies, allowing countless possibilities of future studies, such as the study of the matching of rock joints by the numerical adjustment of bottom and top surfaces, the statistical and geostatistical characterisation of rock joints or the degradation of roughness after normal and shear tests. This paper presents an overview of the research work that is still under way.info:eu-repo/semantics/publishedVersio