On the nature of the Mott transition in multiorbital systems

We analyze the nature of Mott metal-insulator transition in multiorbital systems using dynamical mean-field theory (DMFT). The auxiliary multiorbital quantum impurity problem is solved using continuous time quantum Monte Carlo (CTQMC) and the rotationally invariant slave-boson (RISB) mean field approximation. We focus our analysis on the Kanamori Hamiltonian and find that there are two markedly different regimes determined by the nature of the lowest energy excitations of the atomic Hamiltonian. The RISB results at $T\to0$ suggest the following rule of thumb for the order of the transition at zero temperature: a second order transition is to be expected if the lowest lying excitations of the atomic Hamiltonian are charge excitations, while the transition tends to be first order if the lowest lying excitations are in the same charge sector as the atomic ground state. At finite temperatures the transition is first order and its strength, as measured e.g. by the jump in the quasiparticle weight at the transition, is stronger in the parameter regime where the RISB method predicts a first order transition at zero temperature. Interestingly, these results seem to apply to a wide variety of models and parameter regimes.Comment: Accepted for publication in Physical Review

Lattice specific heat for the RMIn5_5 (R = Gd, La, Y, M = Co, Rh) compounds: non-magnetic contribution subtraction

We analyze theoretically a common experimental process used to obtain the magnetic contribution to the specific heat of a given magnetic material. In the procedure, the specific heat of a non-magnetic analog is measured and used to subtract the non-magnetic contributions, which are generally dominated by the lattice degrees of freedom in a wide range of temperatures. We calculate the lattice contribution to the specific heat for the magnetic compounds GdMIn$_5$ (M = Co, Rh) and for the non-magnetic YMIn$_5$ and LaMIn$_5$ (M = Co, Rh), using density functional theory based methods. We find that the best non-magnetic analog for the subtraction depends on the magnetic material and on the range of temperatures. While the phonon specific heat contribution of YRhIn$_5$ is an excellent approximation to the one of GdCoIn$_5$ in the full temperature range, for GdRhIn$_5$ we find a better agreement with LaCoIn$_5$, in both cases, as a result of an optimum compensation effect between masses and volumes. We present measurements of the specific heat of the compounds GdMIn$_5$ (M = Co, Rh) up to room temperature where it surpasses the value expected from the Dulong-Petit law. We obtain a good agreement between theory and experiment when we include anharmonic effects in the calculations

State-of-the-art techniques for calculating spectral functions in models for correlated materials

The dynamical mean field theory (DMFT) has become a standard technique for the study of strongly correlated models and materials overcoming some of the limitations of density functional approaches based on local approximations. An important step in this method involves the calculation of response functions of a multiorbital impurity problem which is related to the original model. Recently there has been considerable progress in the development of techniques based on the density matrix renormalization group (DMRG) and related matrix product states (MPS) implying a substantial improvement to previous methods. In this article we review some of the standard algorithms and compare them to the newly developed techniques, showing examples for the particular case of the half-filled two-band Hubbard model.Comment: 8 pages, 4 figures, to be published in EPL Perspective

Vertisol Soil fertility Indicators inRotational grazing on the Cauto Valley.

The behavior of some Vertisol soil fertility indicators was randomly studied for four years, at a milk production unit on the Cauto River Valley, Cuba. Brachiaria humidicola cv CIAT 679 pasture was used without irrigation and fertilization. Grazing intensity had mean variation values, of 243 UGM/ha in the rainy season and 190 UGM/ha in the dry season. The daily grazing stripe depended on pasture availability. The occupation time was 2 days. No signif-icant variations were observed in time, or associated to grazing intensity in terms of physical properties of the soil. The apparent density decreased according to the season (P < 0.05) and the highest value was observed in the dry sea-son (1.42 and 1.18 g/cm3 ). The grazing stripe variable brought benefits to the balance of exchangeable ions, with a higher ratio of Ca2+/Mg2+and Mg2+/K+. Interaction between season and grazing was produced in mobile forms of soil nitrogen and NO3 -, NH4 + and NFH, higher during the rainy season in the grazing areas. Stripe grazing does not diminish the main soil fertility indicators; however, it does favor nitrogen mobility and mineralization, depending on the most suitable grazing stripe variable for this kind of soil

Full time nonexponential decay in double-barrier quantum structures

We examine an analytical expression for the survival probability for the time evolution of quantum decay to discuss a regime where quantum decay is nonexponential at all times. We find that the interference between the exponential and nonexponential terms of the survival amplitude modifies the usual exponential decay regime in systems where the ratio of the resonance energy to the decay width, is less than 0.3. We suggest that such regime could be observed in semiconductor double-barrier resonant quantum structures with appropriate parameters.Comment: 6 pages, 5 figure

Equivalence between the real time Feynman histories and the quantum shutter approaches for the "passage time" in tunneling

We show the equivalence of the functions $G_{\rm p}(t)$ and $|\Psi(d,t)|^2$ for the ``passage time'' in tunneling. The former, obtained within the framework of the real time Feynman histories approach to the tunneling time problem, using the Gell-Mann and Hartle's decoherence functional, and the latter involving an exact analytical solution to the time-dependent Schr\"{o}dinger equation for cutoff initial waves

Quantum shutter approach to tunneling time scales with wave packets

The quantum shutter approach to tunneling time scales (G. Garc\'{\i }a-Calder\'{o}n and A. Rubio, Phys. Rev. A \textbf{55}, 3361 (1997)), which uses a cutoff plane wave as the initial condition, is extended in such a way that a certain type of wave packet can be used as the initial condition. An analytical expression for the time evolved wave function is derived. The time-domain resonance, the peaked structure of the probability density (as the function of time) at the exit of the barrier, originally found with the cutoff plane wave initial condition, is studied with the wave packet initial conditions. It is found that the time-domain resonance is not very sensitive to the width of the packet when the transmission process is in the tunneling regime.Comment: 6 page

Dynamical description of the buildup process in resonant tunneling: Evidence of exponential and non-exponential contributions

The buildup process of the probability density inside the quantum well of a double-barrier resonant structure is studied by considering the analytic solution of the time dependent Schr\"{o}dinger equation with the initial condition of a cutoff plane wave. For one level systems at resonance condition we show that the buildup of the probability density obeys a simple charging up law, $| \Psi (\tau) / \phi | =1-e^{-\tau /\tau_0},$ where $\phi$ is the stationary wave function and the transient time constant $\tau_0$ is exactly two lifetimes. We illustrate that the above formula holds both for symmetrical and asymmetrical potential profiles with typical parameters, and even for incidence at different resonance energies. Theoretical evidence of a crossover to non-exponential buildup is also discussed.Comment: 4 pages, 2 figure

Wheel shape optimization approaches to reduce railway rolling noise

[EN] A wheel shape optimization of a railway wheel cross section by means of Genetic Algorithms (GAs) is presented with the aim of minimizing rolling noise radiation. Two different approaches have been implemented with this purpose, one centred on direct Sound poWer Level (SWL) minimization, calculated using TWINS methodology, and another one emphasizing computational efficiency, focused on natural frequencies maximization. Numerical simulations are carried out with a Finite Element Method (FEM) model using general axisymmetric elements. The design space is defined by a geometric parametrization of the wheel cross section with four parameters: wheel radius, a web thickness factor, fillet radius and web offset. For all wheel candidates a high-cycle fatigue analysis has been performed according to actual standards, in order to assure structural feasibility. Rolling noise reductions have been achieved, with a decrease of up to 5 dB(A) when considering the wheel component. Response surfaces have been also computed to study the dependency of the objective functions on the geometric parameters and to test the adequacy of the optimization algorithm applied.This study was financially supported by Ministerio de Ciencia, Innovacion y Universidades - Agencia Estatal de Investigacion, European Regional Development Fund (project TRA2017-84701-R), and Conselleria d'Educacio, Investigacio, Cultura i Esport (Generalitat Valenciana, project Prometeo/2016/007).García-Andrés, FX.; Gutiérrez-Gil, J.; Martínez Casas, J.; Denia, FD. (2020). Wheel shape optimization approaches to reduce railway rolling noise. Structural and Multidisciplinary Optimization. 62(5):2555-2570. https://doi.org/10.1007/s00158-020-02700-6S25552570625Beranek LL (2007) Basic acoustical quantities: levels and decibels, chapter 1 pp 1–24, John Wiley & Sons, LtdBouvet P, Vincent N, Coblentz A, Demilly F (2000) Optimization of resilient wheels for rolling noise control. 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