Search

The Multi Unit Spectroscopic Explorer (MUSE) is an integral field spectrograph on the Very Large Telescope Unit Telescope 4, capable of laser guide star assisted and tomographic adaptive optics using the GALACSI module. Its observing capabilities include a wide field (1 square arcmin), ground layer AO mode (WFM-AO) and a narrow field (7.5"x7.5"), laser tomography AO mode (NFM-AO). The latter has had several upgrades in the 4 years since commissioning, including an optimisation of the control matrices for the AO system and a new sub-electron noise detector for its infra-red low order wavefront sensor. We set out to quantify the NFM-AO system performance by analysing $\sim$230 spectrophotometric standard star observations taken over the last 3 years. To this end we expand upon previous work, designed to facilitate analysis of the WFM-AO system performance. We briefly describe the framework that will provide a user friendly, semi-automated way for system performance monitoring during science operations. We provide the results of our performance analysis, chiefly through the measured Strehl ratio and full width at half maximum (FWHM) of the core of the point spread function (PSF) using two PSF models, and correlations with atmospheric conditions. These results will feed into a range of applications, including providing a more accurate prediction of the system performance as implemented in the exposure time calculator, and the associated optimization of the scientific output for a given set of limiting atmospheric conditions., Comment: SPIE proceedings (2022), Observatory Operations: Strategies, Processes, and Systems IX

Structural, electronic and magnetic properties of bulk ilmenite CoTiO$_3$ are analyzed in the framework of density functional theory (DFT), using the generalized gradient approximation (GGA) and Hubbard-corrected approaches. We find that the G-type antiferromagnetic (G-AFM) structure, which consists of antiferromagnetically coupled ferromagnetic $ab$ planes, is the ground-state of the system, in agreement with experiments. Furthermore, cobalt titanates present two critical temperatures related to the breaking of the inter- and intra-layer magnetic ordering. This would result in the individual planes remaining ferromagnetic even at temperatures above the N\'{e}el temperature. When spin-orbit coupling is included in our calculations, we find an out-of-plane magnetic anisotropy, which can be converted to an in-plane anisotropy with a small doping of electrons corresponding to about 2.5% Ti substitution for Co, consistent with experimental expectations. We thus present a disorder-dependent study of the magnetic anisotropy in bulk $\text{CoTiO}_3$, which will determine its magnon properties, including topological aspects.

In bilayer CrI3, experimental and theoretical studies suggest that the magnetic order is closely related to the layer staking configuration. In this work, we study the effect of dynamical lattice distortions, induced by non-linear phonon coupling, in the magnetic order of the bilayer system. We use density functional theory to determine the phonon properties and group theory to obtain the allowed phonon-phonon interactions. We find that the bilayer structure possesses low-frequency Raman modes that can be non-linearly activated upon the coherent photo-excitation of a suitable infrared phonon mode. This transient lattice modification, in turn, inverts the sign of the interlayer spin interaction for parameters accessible in experiments, indicating a low-frequency light-induced antiferromagnet-to-ferromagnet transition., Comment: 7 pages, 4 figures

Motivated by the quest for experimentally accessible dynamical probes of Floquet topological insulators, we formulate the linear response theory of a periodically driven system. We illustrate the applications of this formalism by giving general expressions for optical conductivity of Floquet systems, including its homodyne and heterodyne components and beyond. We obtain the Floquet optical conductivity of specific driven models, including two-dimensional Dirac material such as the surface of a topological insulator, graphene, and the Haldane model irradiated with circularly or linearly polarized laser, as well as semiconductor quantum well driven by an ac potential. We obtain approximate analytical expressions and perform numerically exact calculations of the Floquet optical conductivity in different scenarios of the occupation of the Floquet bands, in particular, the diagonal Floquet distribution and the distribution obtained after a quench. We comment on experimental signatures and detection of Floquet topological phases using optical probes., Comment: 16 pages, 10 figures

The electrophysiological source imagine reconstruction is sensitive to the head model construction, which depends on the accuracy of the anatomical landmarks locations knowns as fiducials. This work describes how to perform automatic fiducials detection, towards development of an application for automatic electrodes placement (digitization), over a three-dimensional surface of a subject head, scanned with the Occipital Inc. structure sensor ST01. We offer a wide description of the proposed algorithm to explore the three-dimensional object to features detection, by means of: dimensional reduction with perspective projection from 3D to 2D, object detection with custom detectors, robotic control of mouse motion and clicks events and reprojection from 2D to 3D to get spatial coordinates. This is done taking into account the characteristics of the scanner information, the training process of detectors with Computer-Vision Toolbox resources of MATLAB R2018b, the integration of FieldTrip Toolbox and the main properties of several ways to represents pixels; putting together all those things to automatically find the fiducials landmarks to generate the subject's coordinate system. All this result is presented as the initial state of a project focused on developing one application for automatic electrode digitization., Comment: 10 pages 15 figures 2 tables

We study the electron and spin transport in a van der Waals system formed by one layer with strong spin-orbit coupling and a second layer without spin-orbit coupling, in the regime when the interlayer tunneling is random. We find that in the layer without intrinsic spin-orbit coupling spin-charge coupled transport can be induced by two distinct mechanisms. First, the gapless diffusion modes of the two isolated layers hybridize in the presence of tunneling, which constitutes a source of spin-charge coupled transport in the second layer. Second, the random tunneling introduces spin-orbit coupling in the effective disorder-averaged single-particle Hamiltonian of the second layer. This results in non-trivial spin transport and, for sufficiently strong tunneling, in spin-charge coupling. As an example, we consider a van der Waals system formed by a two-dimensional electron gas (2DEG)--such as graphene--and the surface of a topological insulator (TI) and show that the proximity of the TI induces a coupling of the spin and charge transport in the 2DEG. In addition, we show that such coupling can be tuned by varying the doping of the TI's surface. We then obtain, for a simple geometry, the current-induced non-equilibrium spin accumulation (Edelstein effect) caused in the 2DEG by the coupling of charge and spin transport., Comment: 5+1 pages, 1 figure

We develop a low-frequency perturbation theory in the extended Floquet Hilbert space of a periodically driven quantum systems, which puts the high- and low-frequency approximations to the Floquet theory on the same footing. It captures adiabatic perturbation theories recently discussed in the literature as well as diabatic deviation due to Floquet resonances. For illustration, we apply our Floquet perturbation theory to a driven two-level system as in the Schwinger-Rabi and the Landau-Zener-St\"uckelberg-Majorana models. We reproduce some known expressions for transition probabilities in a simple and systematic way and clarify and extend their regime of applicability. We then apply the theory to a periodically-driven system of fermions on the lattice and obtain the spectral properties and the low-frequency dynamics of the system., Comment: v2: 28 single-column pages, 5 figures; various typos fixed; some notation and connection to other perturbation schemes clarified; new, more descriptive title and abstract. Published version

We study quantum noise in a nonequilibrium, periodically driven, open system attached to static leads. Using a Floquet Green's function formalism we show, both analytically and numerically, that local voltage noise spectra can detect the rich structure of Floquet topological phases unambiguously. Remarkably, both regular and anomalous Floquet topological bound states can be detected, and distinguished, via peak structures of noise spectra at the edge around zero-, half-, and full-drive-frequency. We also show that the topological features of local noise are robust against moderate disorder. Thus, local noise measurements are sensitive detectors of Floquet topological phases., Comment: 4.5 pages + supplemental material; v2: improved presentation and new and updated references

We study the low-frequency dynamics of periodically driven one-dimensional systems hosting Floquet topological phases. We show, both analytically and numerically, in the low-frequency limit $\Omega\to0$, the topological invariants of a chirally-symmetric driven system exhibit universal fluctuations. While the topological invariants in this limit nearly vanish on average over a small range of frequencies, we find that they follow a universal Gaussian distribution with a width that scales as $1/\sqrt{\Omega}$. We explain this scaling based on a diffusive structure of the winding numbers of the Floquet-Bloch evolution operator at low frequency. We also find that the maximum quasienergy gap remains finite and scales as $\Omega^2$. Thus, we argue that the adiabatic limit of a Floquet topological insulator is highly structured, with universal fluctuations persisting down to very low frequencies., Comment: In this version we present additional numerical results for the topological invariants of the SSH, Kitaev, and SSH-Kitaev models in the low-frequency regime. The draft has 5 pages, and 11 figures

The control of a ferromagnet's magnetization via only electric currents requires the efficient generation of current-driven spin-torques. In magnetic structures based on topological insulators (TIs) current-induced spin-orbit torques can be generated. Here we show that the addition of graphene, or bilayer graphene, to a TI-based magnetic structure greatly enhances the current-induced spin density accumulation and significantly reduces the amount of power dissipated. We find that this enhancement can be as high as a factor of 100, giving rise to a giant Edelstein effect. Such a large enhancement is due to the high mobility of graphene (bilayer graphene) and to the fact that the graphene (bilayer graphene) sheet very effectively screens charge impurities, the dominant source of disorder in topological insulators. Our results show that the integration of graphene in spintronics devices can greatly enhance their performance and functionalities., Comment: 8 pages, 10 figures

We study the thermal relaxation dynamics of VO$_2$ films after the ultrafast photo-induced metal-insulator transition for two VO$_2$ film samples grown on Al$_2$O$_3$ and TiO$_2$ substrates. We find two orders of magnitude difference in the recovery time (a few ns for the VO$_2$/Al$_2$O$_3$ sample vs. hundreds of ns for the VO$_2$/TiO$_2$ sample). We present a theoretical model that accurately describes the MIT thermal properties and interpret the experimental measurements. We obtain quantitative results that show how the microstructure of the VO$_2$ film and the thermal conductivity of the interface between the VO$_2$ film and the substrate affect long time-scale recovery dynamics. We also obtain a simple analytic relationship between the recovery time-scale and some of the film parameters., Comment: 10 pages, 15 figures

We discuss the conditions under which the predicted (but not yet observed) zero-field interlayer excitonic condensation in double layer graphene has a critical temperature high enough to allow detection. Crucially, disorder arising from charged impurities and corrugation in the lattice structure --- invariably present in all real samples --- affects the formation of the condensate via the induced charge inhomogeneity. In the former case, we use a numerical Thomas-Fermi-Dirac theory to describe the local fluctuations in the electronic density in double layer graphene devices and estimate the effect these realistic fluctuations have on the formation of the condensate. To make this estimate, we calculate the critical temperature for the interlayer excitonic superfluid transition within the mean-field BCS theory for both optimistic (unscreened) and conservative (statically screened) approximations for the screening of the interlayer Coulomb interaction. We also estimate the effect of allowing dynamic contributions to the interlayer screening. We then conduct similar calculations for double quadratic bilayer graphene, showing that the quadratic nature of the low-energy bands produces pairing with critical temperature of the same order of magnitude as the linear bands of double monolayer graphene., Comment: 10 pages, 7 figures, RevTeX, final published version

In this work we present an explicit relation between the number of points on a family of algebraic curves over $\F_{q}$ and sums of values of certain hypergeometric functions over $\F_{q}$. Moreover, we show that these hypergeometric functions can be explicitly related to the roots of the zeta function of the curve over $\F_{q}$ in some particular cases. A general conjecture relating these last two is presented and advances toward its proof are shown in the last section., Comment: 24 pages

The half-life, 3.8755(12) s, and superallowed branching ratio, 0.5315(12), for ^{22}Mg beta-decay have been measured with high precision. The latter depended on gamma-ray intensities being measured with an HPGe detector calibrated for relative efficiencies to an unprecedented 0.15%. Previous precise measurements of 0^+ to 0^+ transitions have been restricted to the nine that populate stable daughter nuclei. No more such cases exist, and any improvement in a critical CKM unitarity test must depend on precise measurements of more exotic nuclei. With this branching-ratio measurement, we show those to be possible for T_z = -1 parents. We obtain a corrected Ft-value of 3071(9) s, in good agreement with expectations., Comment: 4 pages, 2 figures, revtex4

The latest experimental data on nuclei at $^{132}$Sn permit us for the first time to determine the spin-orbit splittings of neutrons and protons in identical orbits in this neutron-rich doubly-magic region and compare the case to that of $^{208}$Pb. Using the new results, which are now consistent for the two neutron-rich doubly magic regions, a theoretical analysis defines the isotopic dependence of the mean field spin-orbit potential and leads to a simple explicit expression for the difference between the spin-orbit splittings of neutrons and protons. The isotopic dependence is explained in the framework of different theoretical approaches., Comment: 8 pages, revtex

By the use of the latest experimental data on the spectra of $^{133}$Sb and $^{131}$Sn and on the analysis of properties of other odd nuclei adjacent to doubly magic closed shells the isospin dependence of a mean spin-orbit potential is defined. Such a dependence received the explanation in the framework of different theoretical approaches., Comment: 52 pages, Revtex, no figures

We have studied structural, magnetic and transport properties as a function of temperature and magnetic field in the electron doped manganite YxCa1-xMnO3, for 00. With a simple model where antiferromagnetic (AF) superexchange and ferromagnetic (FM) double exchange (DE) compete, we fit the experimental \chi(x, T) obtaining parameter values which indicate that the FM-DE interaction is about twice as intense as the AF interaction. In the ordered phase, the H-dependence of the magnetization M(x,T) is explained in terms of magnetic polarons. We propose that the displacement of the eg electrons (in the G-type AF background) causes the alignement of the polaron with H. Signatures of polaronic behavior were also found in the x and T dependence of the electric resistivity., Comment: 8 pages, 10 figures, updated 07/05/00