arXiv subjects

N. Garcia

Publications and source records attributed to N. Garcia.

At least 19 recordsLinked to original sources

arXiv2023

First light of VLT/HiRISE: High-resolution spectroscopy of young giant exoplanets

A major endeavor of this decade is the direct characterization of young giant exoplanets at high spectral resolution to determine the composition of their atmosphere and infer their formation processes and evolution. Such a goal represents a major challenge owing to their small angular separation and luminosity contrast with respect to their parent stars. Instead of designing and implementing completely new facilities, it has been proposed to leverage the capabilities of existing instruments that offer either high contrast imaging or high dispersion spectroscopy, by coupling them using optical fibers. In this work we present the implementation and first on-sky results of the HiRISE instrument at the very large telescope (VLT), which combines the exoplanet imager SPHERE with the recently upgraded high resolution spectrograph CRIRES using single-mode fibers. The goal of HiRISE is to enable the characterization of known companions in the $H$ band, at a spectral resolution of the order of $R = λ/Δλ= 100\,000$, in a few hours of observing time. We present the main design choices and the technical implementation of the system, which is constituted of three major parts: the fiber injection module inside of SPHERE, the fiber bundle around the telescope, and the fiber extraction module at the entrance of CRIRES. We also detail the specific calibrations required for HiRISE and the operations of the instrument for science observations. Finally, we detail the performance of the system in terms of astrometry, temporal stability, optical aberrations, and transmission, for which we report a peak value of $\sim$3.9% based on sky measurements in median observing conditions. Finally, we report on the first astrophysical detection of HiRISE to illustrate its potential.

astro-ph.IM↗

arXiv2011

Spin Transfer from a Ferromagnet into a Semiconductor through an Oxide barrier

We present results on the magnetoresistance of the system Ni/Al203/n-doped Si/Al2O3/Ni in fabricated nanostructures. The results at temperature of 14K reveal a 75% magnetoresistance that decreases in value up to approximately 30K where the effect disappears. We observe minimum resistance in the antiparallel configurations of the source and drain of Ni. As a possibility, it seems to indicate the existence of a magnetic state at the Si/oxide interface. The average spin diffusion length obtained is of 650 nm approximately. Results are compared to the window of resistances that seems to exist between the tunnel barrier resistance and two threshold resistances but the spin transfer seems to work in the range and outside the two thresholds.

cond-mat.mtrl-sci↗

arXiv2009

Andreev reflection measurements in nanostructures with amorphous WC$_x$ superconducting contacts

Point-contact Andreev reflection measurements of Co/ and Cu/tungsten-carbide (WC$_x$) contacts are presented. Metallic thin films were patterned by e-beam-lithography and lift-off; tungsten carbide superconducting tips were grown directly on the pre-patterned samples by decomposition of a metallo-organic vapour (tungsten hexacarbonyl) under a focused Ga$^+$-ion beam (FIB). Current-voltage measurements as a function of temperature and magnetic field clearly showed the signatures of Andreev reflection. The experimental conductance-voltage curves were analyzed within the Blonder-Tinkham-Klapwijk theory. The results highlight the possibilities, advantages and disadvantages of using FIB-produced amorphous WC$_x$ tips for point-contact spectroscopy in metallic nanostructures.

cond-mat.supr-con↗

arXiv2007

Theory of New Quantum Oscillations in the Magnetoresistance of Graphene Layers

We present a theory presenting new quantum oscillations in the magnetoresistance that are revealed as fine structures superimposed to the Schubnikov-de-Haas oscillations. They may be observed in experiments on graphene layers as fine structures that until now seem to have been overseen or considered to be noise. These oscillations appear also in the behaviour of the resistance as a function of the gate voltage that changes the number of carriers or Fermi level. Experimental studies of these resonances should give information of the uniformity and defects of the samples and represent a new fine structure spectroscopy. Also the lateral sample size and quantum effects may explain the absence of magnetoresistance in a few grapheme layers. Experiments are proposed.

cond-mat.mes-hall↗

arXiv2007

Theory of Electromagnetic Wave Transmission through Metallic Gratings of Subwavelength Slits

We present FDTD calculations for transmission of light and other electromagnetic waves through periodic arrays of slits in a metallic slab. The results show resonant, frequency dependent, transmittance peaks for subwavelength widths of the slits which can be up to a factor of ten with respect to those out of resonance. Although our conclusions agree with previous work by Lezec and Thio as regards both the magnitude of the enhancement and the lack of contribution of surface plasmon polaritons of the metal surface to this effect, we derive an interpretation from a theory that deals with emerging beam- Rayleigh anomalies of the grating, and with Fabry-Perot resonances of the perforated slab considered as an effective medium.

cond-mat.mes-hall↗

arXiv2007

Are Graphene Sheets Stable?

The answer to the title question is yes and the sheets exhibit diffraction peaks but may not have long range crystalline order. This is not a trivial question and answer and is immersing in the very active field now days of the study of properties of graphite and specially of graphene. This note is motivated in fact by a recent paper by Meyer et al (1) on the structure of suspended graphene sheets.

cond-mat.mes-hall↗

arXiv2006

The One Dimensional Approachissimo Quantum Harmonic Oscillator: The Hilbert-Polya Hamiltonian for the Primes and the Zeros of the Riemann Function

I have made an ample study of one dimensional quantum oscillators, ranging from logarithmic to exponential potentials. I have found that the eigenvalues of the hamiltonian of the oscillator with the limiting (approachissimo) harmonic potential (~ p(x)2) maps the zeros of the Riemann function height up in the Riemann line. This is the potential created by the field of J(x) that is the Riemann generator of the prime number counting function, p(x), that in turn can be defined by an integral transformation of the Riemann zeta function. This plays the role of the spring strength of the quantum limiting harmonic oscillator. The number theory meaning of this result is that the roots height up of the zeta function are the eigenvalues of a Hamiltonian whose potential is the number of primes squared up to a given x. Therefore this may prove the never published Hilbert-Polya conjecture. The conjecture is true but does not imply the truth of the Riemann hypothesis. We can have complex conjugated zeros off the Riemman line and map them with another hermitic operator and a general expression is given for that. The zeros off the line affect the fluctuation of the eigenvalues but not their mean values.

quant-ph↗

arXiv2006

Theory of Transmission of Light by Sub-wavelength Cylindrical Holes in Metallic Films

This paper presents theory and finite-difference time-domain (FDTD) calculations for a single and arrays of sub-wavelength cylindrical holes in metallic films presenting large transmission. These calculations are in excellent agreement with experimental measurements. This effect has to be understood in terms of the properties exhibited by the dielectric constant of metals which cannot be treated as ideal metals for the purpose of transmission and diffraction of light. We discuss the cases of well-differentiated metals silver and tungsten. It is found that the effect of surface plasmons or other surface wave excitations due to a periodical set of holes or other roughness at the surface is marginal. The effect can enhance but also can depress the transmission of the arrays as shown by theory and experiments. The peak structure observed in experiments is a consequence of the interference of the wavefronts transmitted by each hole and is determined by the surface array period independently of the material. Without large transmission through a single hole there is no large transmission through the array. We found that in the case of Ag which at the discussed frequencies is a metal there are cylindrical plasmons at the wall of the hole, as reported by Economu et al 30 years ago, that enhanced the transmission. But it turns out, as will be explained, that for the case of W which behaves as a dielectric, there is also a large transmission when compared with that of an ideal metal waveguide. To deal with this problem one has to use the measured dielectric function of the metals. We discuss thoroughly all these cases and compare with the data.

physics.optics↗

arXiv2006

Electrostatic Force Microscopy on Oriented Graphite Surfaces: Where Insulating and Conducting Behaviors Coexist

We present measurements of the electric potential fluctuations on the surface of highly oriented pyrolytic graphite using electrostatic force and atomic force microscopy. Micrometric domain-like potential distributions are observed even when the sample is grounded. Such potential distributions are unexpected given the good metallic conductivity of graphite because the surface should be an equipotential. Our results indicate the coexistence of regions with metallic and insulating behaviors showing large potential fluctuations of the order of 0.25V. We discuss the implications of these measurements in the disorder structure of graphite.

cond-mat.mes-hall↗

arXiv2006

Is there Ballistic Conductance Quantization in Real Life Metals Nanocontacts?

Theory and a vast set of experimental work in metals, since a century, appear to show that the mean free path of conduction electrons in a real metal is about the min (bulk mean free path, smallest transversal size of the metal), a result that was already proposed by J. J. Thomson in 1901. This establishes, as discussed in this work, serious difficulties to justify conductance quantization and ballistic transport in atomic/nanocontacts or nanoconstrictions of real life metals. The ohmic resistance of the leads proves to be as important as the ballistic one of the constriction.

cond-mat.mes-hall↗

arXiv2006

Revision of the quasiclassical boundary resistance of the metal/superconductor interface

Transparency of the metal/superconductor interface is discussed in a frame of the Kupryanov-Lukichev (K-L) quasiclassical boundary conditions. It is shown that the original K-L boundary transparency is expressed via the single-channel Landauer conductance. It is a source of dramatic discrepancy between the theory and experiments on the superconducting proximity effect. The improved multi-channel derivation is proposed for the boundary conditions, which rehabilitates consistency of the quasiclassical approach to the boundary transparency problem, and eliminates at least an order of magnitude of discrepancy between the theory and the experiment. The influence of the interface roughness and wave function symmetry mismatch at the boundary on the interface transparency is discussed. Warning is made to theories that use the quasiclassical boundary conditions with the interface transparencies close to unity.

cond-mat.supr-con↗

arXiv2005

Light Collimation and Focussing by a Thin Flat Metallic Slab

We present experimental and theoretical work showing that a flat metallic slab can collimate and focus light impinging on the slab from a punctual source. The effect is optimised when the radiation is around the bulk, not at the surface, plasma frequency. And the smaller the imaginary part of the permittivity is, the better the collimation. Experiments for Ag in the visible as well as calculations are presented. We also discuss the interesting case of the Aluminium whose imaginary part of the permittivity is very small at the plasma frequency in UV radiation. Generalization to other materials and radiations are also discussed.

cond-mat.other↗

arXiv2005

Huge Ballistic Magnetoresistance in Multiple Nanocontacts Devices

In this paper we report an exhaustive experimental work on magnetoresistance effects found in a system in which a large number of nanocontacts are produced between oxidized Fe fine particles. We have obtained the following performances: i) Huge low field room temperature magnetoresistance (over 1000%). ii) Non-linear I-V at different applied fields and temperatures. iii) Large thermal stability and reproducible resistance value under thermal cycles from room temperature down to 5 K. iv) Easy to fabricate with an almost 100% success. v) Heavy duty and transportable samples with reproducibility tested in several laboratories. We realized that the extraordinary effect found is related to the oxygen content at the particles surface

cond-mat.mes-hall↗

arXiv2004

Spin Screening and Antiscreening in a Ferromagnet/Superconductor Heterojunction

We present a theoretical study of spin screening effects in a ferromagnet/superconductor (F/S) heterojunction. It is shown that the magnetic moment of the ferromagnet is screened or antiscreened, depending on the polarization of the electrons at the Fermi level. If the polarization is determined by the electrons of the majority (minority) spin band then the magnetic moment of the ferromagnet is screened (antiscreened) by the electrons in the superconductor. We propose experiments that may confirm our theory: for ferromagnetic alloys with certain concentration of Fe or Ni ions there will be screening or antiscreening respectively. Different configurations for the density of states are also discussed.

cond-mat.supr-con↗

arXiv2004

Reply to "Comment on 'Theory for tailoring sonic devices: Diffraction dominates over refraction' "

In their comment, A. Hakansson, J. Sanchez-Dehesa, F. Cervera, F. Meseguer, L. Sanchis, and J. Llinares say that our conclusion stating that diffraction prevails over refraction in acoustic lenses whose aperture is of several wavelengths, such as those addressed in our calculations [N. Garcia, M. Nieto-Vesperinas, E. V. Ponizovskaya, and M.Torres, Phys. Rev. E 67, 046606 (2003)] and in their experiments [F.Cervera, L. Sanchis, J. V. Sanchez-Perez, R. Martinez-Sala, C. Rubio, F.Meseguer, C. Lopez, D. Caballero, and J. Sanchez-Dehesa, Phys. Rev. Lett.88, 023902 (2003)], is misleading because the size of their lenses is larger than ours. They state that diffraction effects are negligible at the scale of their experiments. In this reply we calculate the propagation of a plane wave through both a lens and a slab of aluminum cylinders, identical to those presented by such authors in previous experiments, by using a finite difference time domain (FDTD) method. We then compare our results to the experiments previously reported by the authors of the comment and significant differences are found. Our present calculations show that refraction and diffraction are intrinsically interwoven also at the scale of their experiments.

cond-mat.mtrl-sci↗

arXiv2004

Low Losses Left Handed Materials Using Metallic Magnetic Cylinders

We discuss materials based on arrays of metallic magnetic cylindrical structures near ferromagnetic resonance with applied magnetic fields at microwave frequencies. We have found that the materials have a negative refraction index when the appropriate structure is chosen. Numerical FDTD simulations were performed, after a very large number of geometries were swept. The simulations reveal that only ferromagnetic cylinders, with diameters of 0.1 cm and 0.5 cm apart, and with periodic or random configurations, are left-handed materials with very small losses; i.e. with transmitivity practically unity or no losses.

cond-mat.mtrl-sci↗

arXiv2003

Measuring Magnetostriction with an Atomic Force Microscope: Application to Wires in Ballistic Magnetoresistance

In this study we present a new method of measuring magnetostriction with an atomic force microscope adapted for the application magnetic fields. The experiment allows us to visualise, in an elegant and educational way how the lateral magnetoelastic shape changes take place on the sample surface when a magnetic field is applied. We have, furthermore, used this technique to observe magnetically induced strains as small as 5*10-8, and have measured Ni, permalloy and commercial Cu wires and films, as well as pure Cu and Pt wires, where results are in agreement with other methods of measurement. The applications are, moreover, relevant to studies of ballistic magnetoresistance, where we can draw conclusions involving the effect of the magnetically induced strains on magnetoresistance measured at the same time as magnestostriction.

cond-mat.mtrl-sci↗

arXiv2003

Quantum toys for quantum computing: persistent currents controled by spin chirality

Quantum devices and computers will need operational units in different architectural configurations for their functioning. The unit should be a simple ``quantum toy'', easy to handle superposition states. Here a novel such unit of quantum mechanical flux state (or persistent current) in a conducting ring with three ferromagnetic quantum dots is presented. The state is labeled by the two direction of the persistent current, which is driven by the spin chirality of the dots, and is controled by the spin. It is demosntrated that by use of two rings connected, one can carry out unitary transformations on the input flux state by controling one spin in one of the rings, unabling us to prepare superposition states. The flux is shown to be a quantum XOR operation gate, and may be useful in quantum computing.

cond-mat.mes-hall↗