Exact Kohn-Sham potential of strongly correlated finite systems

The dissociation of molecules, even the most simple hydrogen molecule, cannot be described accurately within density functional theory because none of the currently available functionals accounts for strong on-site correlation. This problem has led to a discussion of properties that the local Kohn-Sham potential has to satisfy in order to correctly describe strongly correlated systems. We derive an analytic expression for this potential at the dissociation limit and show that the numerical calculations for a one-dimensional two electron model system indeed approach and reach this limit. It is shown that the functional form of the potential is universal, i.e. independent of the details of the system.Comment: 17 pages, 3 figures, submitted to JC

Enhancements to the GW space-time method

We describe the following new features which significantly enhance the power of the recently developed real-space imaginary-time GW scheme (Rieger et al., Comp. Phys. Commun. 117, 211 (1999)) for the calculation of self-energies and related quantities of solids: (i) to fit the smoothly decaying time/energy tails of the dynamically screened Coulomb interaction and other quantities to model functions, treating only the remaining time/energy region close to zero numerically and performing the Fourier transformation from time to energy and vice versa by a combination of analytic integration of the tails and Gauss-Legendre quadrature of the remaining part and (ii) to accelerate the convergence of the band sum in the calculation of the Green's function by replacing higher unoccupied eigenstates by free electron states (plane waves). These improvements make the calculation of larger systems (surfaces, clusters, defects etc.) accessible.Comment: 10 pages, 6 figure

Orbital magneto-optical response of periodic insulators from first principles

Magneto-optical response, i.e. optical response in the presence of a magnetic field, is commonly used for characterization of materials and in optical communications. However, quantum mechanical description of electric and magnetic fields in crystals is not straightforward as the position operator is ill defined. We present a reformulation of the density matrix perturbation theory for time-dependent electromagnetic fields under periodic boundary conditions, which allows us to treat the orbital magneto-optical response of solids at the ab initio level. The efficiency of the computational scheme proposed is comparable to standard linear-response calculations of absorption spectra and the results of tests for molecules and solids agree with the available experimental data. A clear signature of the valley Zeeman effect is revealed in the continuum magneto-optical spectrum of a single layer of hexagonal boron nitride. The present formalism opens the path towards the study of magneto-optical effects in strongly driven low-dimensional systems

A subradiant optical mirror formed by a single structured atomic layer

Efficient and versatile interfaces for the interaction of light with matter are an essential cornerstone for quantum science. A fundamentally new avenue of controlling light-matter interactions has been recently proposed based on the rich interplay of photon-mediated dipole-dipole interactions in structured subwavelength arrays of quantum emitters. Here we report on the direct observation of the cooperative subradiant response of a two-dimensional (2d) square array of atoms in an optical lattice. We observe a spectral narrowing of the collective atomic response well below the quantum-limited decay of individual atoms into free space. Through spatially resolved spectroscopic measurements, we show that the array acts as an efficient mirror formed by only a single monolayer of a few hundred atoms. By tuning the atom density in the array and by changing the ordering of the particles, we are able to control the cooperative response of the array and elucidate the interplay of spatial order and dipolar interactions for the collective properties of the ensemble. Bloch oscillations of the atoms out of the array enable us to dynamically control the reflectivity of the atomic mirror. Our work demonstrates efficient optical metamaterial engineering based on structured ensembles of atoms and paves the way towards the controlled many-body physics with light and novel light-matter interfaces at the single quantum level.Comment: 8 pages, 5 figures + 12 pages Supplementary Infomatio

Texture Evolution of AZ31 Magnesium Alloy Sheet at High Strain Rates

In the current contribution the mechanical behaviour at high strain rates of AZ31 magnesium alloy sheet is studied. Uniaxial deformation properties were studied by means of tensile split Hopkinson pressure bar (SHPB) at different temperatures. The influence of the strain rate and temperature on the deformation mechanisms was investigated by means of electron backscatter diffraction (EBSD) and neutron diffraction. It is shown that twinning plays an important role on high strain rate deformation of this alloy, even at elevated temperatures. Significant evidence of prismatic slip as a deformation mechanism is observed, also at warm temperatures, leading to the alignment of directions with the tensile axis and to a spread of the intensities of the basal pole figure towards the in-plane direction perpendicular to the tensile axis. The rate of decrease of the CRSS of non-basal systems is observed to be slower than at quasi-static rates. Secondary twinning and pyramidal slip were also outlined for some conditions. At warm temperatures, in contrast to quasi-static range, a generalized dynamic recrystallization is not observed. Moreover, the activation of rotational recrystallization mechanisms is reporte

Twinning and grain subdivision during dynamic deformation of a Mg AZ31 sheet alloy at room temperature

The microstructural evolution of an AZ31 rolled sheet during dynamic deformation at strain rates of ∼103 s−1 has been investigated by electron backscatter diffraction, X-ray and neutron diffraction. The influence of orientation on the predominant deformation mechanisms and on the recovery processes taking place during deformation has been systematically examined. The results have been compared with those corresponding to the same alloy tested quasi-statically under equivalent conditions. It has been found that strain rate enhances the activation of extension twinning dramatically, while contraction and secondary twinning are not significantly influenced. The polarity of extension twinning is even reversed in some grains under selected testing conditions. Significant grain subdivision by the formation of geometrically necessary boundaries (GNBs) takes place during both quasi-static and dynamic deformation of this AZ31 alloy. It is remarkable that GNBs of high misorientations form even at the highest strain rates. The phenomenon of recovery has been found to be orientation dependen

Benchmarking the Variational Reduced Density Matrix Theory in the Doubly Occupied Configuration Interaction Space with Integrable Pairing Models

The variational reduced density matrix theory has been recently applied with great success to models within the truncated doubly occupied configuration interaction space, which corresponds to the seniority zero subspace. Conservation of the seniority quantum number restricts the Hamiltonians to be based on the SU(2) algebra. Among them there is a whole family of exactly solvable Richardson-Gaudin pairing Hamiltonians. We benchmark the variational theory against two different exactly solvable models, the Richardson-Gaudin-Kitaev and the reduced BCS Hamiltonians. We obtain exact numerical results for the so-called PQGT N-representability conditions in both cases for systems that go from 10 to 100 particles. However, when random single-particle energies as appropriate for small superconducting grains are considered, the exactness is lost but still a high accuracy is obtained.Fil: Rubio García, A.. Instituto de Estructura de la Materia; España. Consejo Superior de Investigaciones Científicas; EspañaFil: Alcoba, Diego Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Capuzzi, Pablo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Dukelsky, J.. Consejo Superior de Investigaciones Científicas; España. Instituto de Estructura de la Materia; Españ

Floquet Prethermalization in a Bose-Hubbard System

Periodic driving has emerged as a powerful tool in the quest to engineer new and exotic quantum phases. While driven many-body systems are generically expected to absorb energy indefinitely and reach an infinite-temperature state, the rate of heating can be exponentially suppressed when the drive frequency is large compared to the local energy scales of the system -- leading to long-lived 'prethermal' regimes. In this work, we experimentally study a bosonic cloud of ultracold atoms in a driven optical lattice and identify such a prethermal regime in the Bose-Hubbard model. By measuring the energy absorption of the cloud as the driving frequency is increased, we observe an exponential-in-frequency reduction of the heating rate persisting over more than 2 orders of magnitude. The tunability of the lattice potentials allows us to explore one- and two-dimensional systems in a range of different interacting regimes. Alongside the exponential decrease, the dependence of the heating rate on the frequency displays features characteristic of the phase diagram of the Bose-Hubbard model, whose understanding is additionally supported by numerical simulations in one dimension. Our results show experimental evidence of the phenomenon of Floquet prethermalization, and provide insight into the characterization of heating for driven bosonic systems

Circulating Exosomal Mir21 And Mir320 In Obstructive Sleep Apnea

Rational. Epidemiological studies indicate that there may be an association between obstructive sleep apnea (OSA) and cardiovascular and metabolic diseases. Some pro-inflammatory miRs (miR-21, miR320) critical for the immune response or hypoxia are often overexpressed in cancers and atherosclerosis. Aim. To examine the expression of miR-21& miR320 in circulating exosomes from patients with OSA. Methods: From a Sleep Unit and in the frame of a long-term longitudinal cohort study we selected 65 non-smokers OSA patients (apnea-hypopnea index -AHI- 30 events/ti) and 26 age, gender and BMI-matched controls (AHI 0.85 mm. Plasma-derived exosomes were isolated by precipitation using miRCURY, , , Exosome Isolation Kit. Exosomes were characterized by transmission electron microscopy, dynamic light scattering assay and Western Blot analysis using CD63 and HSP70. Exosome total RNA was obtained using miRCURY"* RNA isolation kit. miR-21 -5p and miR-320-3p were analysed by real time quantitative PCR (RT-qPCR) using miRCURY LNA~ technology..

Onset of dissipation in ballistic atomic wires

Electronic transport at finite voltages in free-standing gold atomic chains of up to 7 atoms in length is studied at low temperatures using a scanning tunneling microscope (STM). The conductance vs voltage curves show that transport in these single-mode ballistic atomic wires is non-dissipative up to a finite voltage threshold of the order of several mV. The onset of dissipation and resistance within the wire corresponds to the excitation of the atomic vibrations by the electrons traversing the wire and is very sensitive to strain.Comment: Revtex4, 4 pages, 3 fig