Self-organized electronic superlattices in layered materials

We show that in layered systems with electronic phase separation tendency, the long-range Coulomb interaction can drive the spontaneous formation of unidirectional superlattices of electronic charge in a completely homogeneous crystalline background. In this self-organized electronic heterostructure, the ratio among the number of crystalline planes in the minority and majority electronic phases corresponds to Farey fractions with the superlattice period controlled by the background charge density and the frustrating Coulomb interaction strength. The phase diagram displays Arnold tongues obeying a modified Farey tree hierarchy and a devil's staircase typical of systems with frustration among different scales. We further discuss the competition of these electronic superlattices, recently observed in iron-based superconductors and mixed valence compounds, with in-plane electronically modulated phases.Comment: 5 pages, 4 figure

Estudio del problema elastoplástico con deformaciones finitas mediante ecuaciones integrales de contorno y de dominio

En este estudio se aborda el problema de análisis de sólidos metálicos sometidos simultáneamente a no-linealidades materiales y geométricas. El comportamiento plástico elegido está basado en el criterio de plastificación de von Mises con ley de flujo asociada y endurecimiento isótropo, pudiendo ser las deformaciones plásticas finitas. La base del método numérico empleado es el Método de los Elementos de Contorno aunque, al no poderse trasladar todas las integrales que intervienen al contorno, es necesario discretizar también el dominio. Se ha adoptado una formulación material junto con un planteamiento lagrangiano actualizado y se utiliza el algoritmo de retorno generalizado para el cálculo de las deformaciones plásticas. Los gradientes de desplazamiento se obtienen indirectamente mediante derivación polinómica del campo de desplazamientos en el dominio, evitando así las singularidades que se presentan. El método global obtenido es incremental y en cada incremento se necesita un proceso iterativo. Se presenta un ejemplo que muestra la aplicabilidad del método propuesto.This paper presents a formulation of the static problem of metallic solids undergoing both material and geometrical nonlinearities. The plastic constitutive relations are based on the von Mises yield criterion with associated flow rule and isotropic hardening. The plastic strains can be large. The numerical approach is based on the Boundary Element Method but, as it is no possible to take al1 the integrals to the boundary, domain discretization is needed as well as boundary discretization. A material description is adopted together with an updated Lagrangian approach. The generalized midpoint algorithm is used for the computation of the large scale plastic strains. The displacement gradients are obtained, in order to avoid singularities, from polynomial differentiation of the displacement field in each domain element from the nodal values. The resulting method is incremental and iterations are needed in each increment. A example is presented, showing the applicability of the proposed method.Peer Reviewe

Viscoelasticity and metastability limit in supercooled liquids

A supercooled liquid is said to have a kinetic spinodal if a temperature Tsp exists below which the liquid relaxation time exceeds the crystal nucleation time. We revisit classical nucleation theory taking into account the viscoelastic response of the liquid to the formation of crystal nuclei and find that the kinetic spinodal is strongly influenced by elastic effects. We introduce a dimensionless parameter \lambda, which is essentially the ratio between the infinite frequency shear modulus and the enthalpy of fusion of the crystal. In systems where \lambda is larger than a critical value \lambda_c the metastability limit is totally suppressed, independently of the surface tension. On the other hand, if \lambda < \lambda_c a kinetic spinodal is present and the time needed to experimentally observe it scales as exp[\omega/(\lambda_c-\lambda)^2], where \omega is roughly the ratio between surface tension and enthalpy of fusion

Static, modal and dynamic behaviour of a stress ribbon footbridge : experimental and computational results

Congreso celebrado en la Escuela de Arquitectura de la Universidad de Sevilla desde el 24 hasta el 26 de junio de 2015.Response for the static, modal and dynamic problem corresponding to a stress ribbon footbridge is studied. The equilibrium equations describing the problem are coupled nonlinear differential equations which are numerically solved using the finite element method. The objective of this work is to present a proper computational model for such a structure and to check its applicability in predicting not only the static behaviour but also modal parameters and estimate its dynamic response. As the footbridge is continuously monitored, it has been possible to measure the sag and to identify natural modes. This experimental data has been used for updating the finite element model

Population inversion and dynamical phase transitions in a driven superconductor

We consider a superconductor in which the density of states at the Fermi level or the pairing interaction is driven periodically with a frequency larger than the superconducting gap in the collisionless regime. We show by numerical and analytical computations that a subset of quasiparticle excitations enter into resonance and perform synchronous Rabi oscillations leading to cyclic population inversion with a frequency that depends on the amplitude of the drive. As a consequence a different "Rabi-Higgs" mode emerges. Turning off the drive at different times and modulating the strength allows access to all known dynamical phases of the order parameter: persistent oscillations, oscillations with damping and overdamped dynamics. We discuss physical realizations of the drive and methods to detect the dynamics.Fil: Ojeda Collado, Hector Pablo. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Lorenzana, José. Università degli studi di Roma "La Sapienza"; ItaliaFil: Usaj, Gonzalo. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Balseiro, Carlos Antonio. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentin

Thermal properties of vortices on curved surfaces

We use Monte Carlo simulations to study the finite temperature behavior of vortices in the XY model for tangent vector order on curved backgrounds. Contrary to naive expectations, we show that the underlying geometry does not affect the proliferation of vortices with temperature respect to what is observed on a flat surface. Long-range order in these systems is analyzed by using two-point correlation functions. As expected, in the case of slightly curved substrates these correlations behave similarly to the plane. However, for high curvatures, the presence of geometry-induced unbounded vortices at low temperatures produces the rapid decay of correlations and an apparent lack of long-range order. Our results shed light on the finite-temperature physics of soft-matter systems and anisotropic magnets deposited on curved substrates.Fil: Leopoldo R. Gómez. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; ArgentinaFil: García, Nicolás A.. Institut Laue Langevin; FranciaFil: Vega, Daniel Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; ArgentinaFil: Lorenzana, José. Università degli studi di Roma "La Sapienza"; Itali

Dynamic characterization, modelling and model updating of a lively footbridge

Congreso celebrado en la Escuela de Arquitectura de la Universidad de Sevilla desde el 24 hasta el 26 de junio de 2015.This paper presents finite element analyses, experimental measurements and finite element model updating of a lively footbridge consisting on an arched main span 40m long and several shorter access spans. Firstly, a finite element model of the bridge is created in a commercial CAE software and static and modal response are numerically estimated. Then, experimental measurements using static loading test and ambient vibration tests are performed. Initial finite element model is adjusted to match with the static response by fitting some selected parameters. Modal parameters (natural frequencies, mode shapes and modal damping) are extracted and after that the current finite element model is updated. Among the selected parameters, semi-rigid connections in some joints, concrete Young’s modulus and mass density of the concrete deck (to account on pavement mass) are selected to minimize the differences between numerical and experimental structural response. Sensitivity of the modal response to these parameters is also shown. At the end of the study, good agreement between analytical and experimental results is achieved, revealing the suitability of the entire process

Fate of dynamical phases of a BCS superconductor beyond the dissipationless regime

The BCS model of an isolated superconductor initially prepared in a nonequilibrium state predicts the existence of interesting dynamical phenomena in the time-dependent order parameter as decaying oscillations, persistent oscillations, and overdamped dynamics. To make contact with real systems remains an open challenge as one needs to introduce dissipation due to the environment in a self-consistent computation. Here, we reach this goal with the use of the Keldysh formalism to treat the effect of a thermal bath. We show that, contrary to the dissipationless case, all dynamical phases reach the equilibrium order parameter in a characteristic time that depends on the coupling with the bath. Remarkably, as time evolves, the overdamped phase shows a fast crossover where the superconducting order parameter recovers to reach a state with a well-developed long range order that tends towards equilibrium with the damped Higgs mode oscillations. Our results provide a benchmark for the description of the dynamics of real out-of-equilibrium superconductors relevant for quantum technological applications.Fil: Ojeda Collado, Hector Pablo. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Usaj, Gonzalo. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Lorenzana, José. Università degli studi di Roma "La Sapienza"; ItaliaFil: Balseiro, Carlos Antonio. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentin