arXiv subjects

M. Moreno

Publications and source records attributed to M. Moreno.

17 recordsLinked to original sources

arXiv2022

Structural changes in Ti1-xAlxN coatings during turning: A XANES and EXAFS study of worn tools

Structural changes in Ti1-xAlxN coated tool inserts used for turning in 316L stainless steel were investigated by XANES, EXAFS, EDS, and STEM. For coarse-grained fcc-structured Ti1-xAlxN coatings, with 0 \leq x \leq 0.62, the XANES spectrum changes with Al-content. XANES Ti 1s line-scans across the rake face of the worn samples reveals that TiN-enriched domains have formed during turning in Ti0.47Al0.53N and Ti0.38Al0.62N samples as a result of spinodal decomposition. The XANES spectra reveal the locations on the tool in which the most TiN-rich domains have formed, indicating which part of the tool-chip contact area that experienced the highest temperature during turning. Changes in the pre-edge features in the XANES spectra reveal that structural changes occur also in the w-TiAlN phase in fine-grained Ti0.38Al0.62N during turning. EDS shows that Cr and Fe from the steel adhere to the tool rake face during machining. Cr 1s and Fe 1s XANES show that Cr is oxidized in the end of the contact length while the adhered Fe retains in the same fcc-structure as that of the 316L stainless steel.

cond-mat.mtrl-sci↗

arXiv2022

Strong Quantum Turbulence in Bose Einstein Condensates

By combining experiments and numerical simulations which model the dynamics of shaken atomic Bose-Einstein condensates, we reveal the surprising nature of quantum turbulence in these systems. Unlike the tangles of vortex lines described in the superfluid helium literature, we find that our turbulent atomic condensate contains a mixture of strong fragmented density fluctuations and small random vortex loops which are not homogeneously distributed. This unusual form of turbulence, with its own properties and scaling behaviour, which we call strong quantum turbulence, is significantly different from the turbulence which is observed in either classical or other quantum systems, thus posing a new challenge in turbulence research.

cond-mat.quant-gas↗

arXiv2021

Microscopic metallic air-bridge arrays for connecting quantum devices

We present a single-exposure fabrication technique for a very large array of microscopic air-bridges using a tri-layer resist process with electron-beam lithography. The technique is capable of forming air-bridges with strong metal-metal or metal-substrate connections. This was demonstrated by its application in an electron tunnelling device consisting of 400 identical surface gates for defining quantum wires, where the air-bridges are used as suspended connections for the surface gates. This technique enables us to create a large array of uniform one-dimensional channels that are open at both ends. In this article, we outline the details of the fabrication process, together with a study and the solution of the challenges present in the development of the technique, which includes the use of water-IPA (isopropyl alcohol) developer, calibration of resist thickness and numerical simulation of the development.

cond-mat.mes-hall↗

arXiv2021

Observing separate spin and charge Fermi seas in a strongly correlated one-dimensional conductor

An electron is usually considered to have only one form of kinetic energy, but could it have more, for its spin and charge, by exciting other electrons? In one dimension (1D), the physics of interacting electrons is captured well at low energies by the Tomonaga-Luttinger model, yet little has been observed experimentally beyond this linear regime. Here, we report on measurements of many-body modes in 1D gated-wires using tunnelling spectroscopy. We observe two parabolic dispersions, indicative of separate Fermi seas at high energies, associated with spin and charge excitations, together with the emergence of two additional 1D 'replica' modes that strengthen with decreasing wire length. The effective interaction strength is varied by changing the amount of 1D inter-subband screening by over 45%. Our findings demonstrate the existence of spin-charge separation in the whole energy band outside the low-energy limit of validity of the Tomonaga-Luttinger model, and also set a constraint on the validity of the newer nonlinear Tomonaga-Luttinger theory.

cond-mat.mes-hall↗

arXiv2020

Managing Data Lineage of O&G Machine Learning Models: The Sweet Spot for Shale Use Case

Machine Learning (ML) has increased its role, becoming essential in several industries. However, questions around training data lineage, such as "where has the dataset used to train this model come from?"; the introduction of several new data protection legislation; and, the need for data governance requirements, have hindered the adoption of ML models in the real world. In this paper, we discuss how data lineage can be leveraged to benefit the ML lifecycle to build ML models to discover sweet-spots for shale oil and gas production, a major application in the Oil and Gas O&G Industry.

cs.DB↗

arXiv2018

Momentum-dependent power law measured in an interacting quantum wire beyond the Luttinger limit

Power laws in physics have until now always been associated with a scale invariance originating from the absence of a length scale. Recently, an emergent invariance even in the presence of a length scale has been predicted by the newly-developed nonlinear-Luttinger-liquid theory for a one-dimensional (1D) quantum fluid at finite energy and momentum, at which the particle's wavelength provides the length scale. We present the first experimental example of this new type of power law in the spectral function of interacting electrons in a quantum wire using a transport-spectroscopy technique. The observed momentum dependence of the power law in the high-energy region matches the theoretical predictions, supporting not only the 1D theory of interacting particles beyond the linear regime but also the existence of a new type of universality that emerges at finite energy and momentum.

cond-mat.mes-hall↗

arXiv2016

Nonlinear spectra of spinons and holons in short GaAs quantum wires

One-dimensional electronic fluids are peculiar conducting systems, where the fundamental role of interactions leads to exotic, emergent phenomena, such as spin-charge (spinon-holon) separation. The distinct low-energy properties of these 1D metals are successfully described within the theory of linear Luttinger liquids, but the challenging task of describing their high-energy nonlinear properties has long remained elusive. Recently, novel theoretical approaches accounting for nonlinearity have been developed, yet the rich phenomenology that they predict remains barely explored experimentally. Here, we probe the nonlinear spectral characteristics of short GaAs quantum wires by tunnelling spectroscopy, using an advanced device consisting of 6000 wires. We find evidence for the existence of an inverted (spinon) shadow band in the main region of the particle sector, one of the central predictions of the new nonlinear theories. A (holon) band with reduced effective mass is clearly visible in the particle sector at high energies.

cond-mat.mes-hall↗

arXiv2015

Nature of the many-body excitations in a quantum wire: theory and experiment

The natural excitations of an interacting one-dimensional system at low energy are hydrodynamic modes of Luttinger liquid, protected by the Lorentz invariance of the linear dispersion. We show that beyond low energies, where quadratic dispersion reduces the symmetry to Galilean, the main character of the many-body excitations changes into a hierarchy: calculations of dynamic correlation functions for fermions (without spin) show that the spectral weights of the excitations are proportional to powers of $\mathcal{R}^{2}/L^{2}$, where $\mathcal{R}$ is a length-scale related to interactions and $L$ is the system length. Thus only small numbers of excitations carry the principal spectral power in representative regions on the energy-momentum planes. We have analysed the spectral function in detail and have shown that the first-level (strongest) excitations form a mode with parabolic dispersion, like that of a renormalised single particle. The second-level excitations produce a singular power-law line shape to the first-level mode and multiple power-laws at the spectral edge. We have illustrated crossover to Luttinger liquid at low energy by calculating the local density of state through all energy scales: from linear to non-linear, and to above the chemical potential energies. In order to test this model, we have carried out experiments to measure momentum-resolved tunnelling of electrons (fermions with spin) from/to a wire formed within a GaAs heterostructure. We observe well-resolved spin-charge separation at low energy with appreciable interaction strength and only a parabolic dispersion of the first-level mode at higher energies. We find structure resembling the second-level excitations, which dies away rapidly at high momentum in line with the theoretical predictions here.

cond-mat.str-el↗

arXiv2014

Hierarchy of modes in an interacting system

Studying interacting fermions in 1D at high energy, we find a hierarchy in the spectral weights of the excitations theoretically and we observe evidence for second-level excitations experimentally. Diagonalising a model of fermions (without spin), we show that levels of the hierarchy are separated by powers of $\mathcal{R}^{2}/L^{2}$, where $\mathcal{R}$ is a length-scale related to interactions and $L$ is the system length. The first-level (strongest) excitations form a mode with parabolic dispersion, like that of a renormalised single particle. The second-level excitations produce a singular power-law line shape to the first-level mode and multiple power-laws at the spectral edge. We measure momentum-resolved tunnelling of electrons (fermions with spin) from/to a wire formed within a GaAs heterostructure, which shows parabolic dispersion of the first-level mode and well-resolved spin-charge separation at low energy with appreciable interaction strength. We find structure resembling the second-level excitations, which dies away quite rapidly at high momentum.

cond-mat.str-el↗

arXiv2012

Comment on "Identification of different electron screening behavior between the bulk and surface of (Ga,Mn)As [Phys. Rev. Lett. 107, 187203 (2011)]"

In a recent Letter [Phys. Rev. Lett. 107, 187203 (2011)], Fujii et al. reported Mn 2p photoelectron emission spectra for (Ga,Mn)As recorded using hard x-rays. Due to the enhanced bulk sensitivity, hard-x-ray spectra reveal an extra "low-binding-energy peak", which is absent in surface-sensitive spectra recorded using soft x-rays. Based on Anderson-impurity-model calculations, Fujii et al. assigned the low-binding-energy peak to a cd6L2 final state, and related the variations in its intensity to variations in the As 4p-Mn 3d hybridization strength V. We show here that the definition of the charge-transfer energy considered by Fujii et al. is different from that considered in the Zaanen-Sawatzky-Allen diagram. We note that the Anderson impurity model is insufficient to describe low-binding-energy peaks in hard-x-ray core-level photoemission for transition-metal compounds on the verge of a metal-insulator transition. We propose a more plausible origin for the (Ga,Mn)As low-binding-energy peak, related to the nature of its metal-insulator transition.

cond-mat.mes-hall↗

arXiv2011

Phase-separated high-temperature-annealed (Ga,Mn)As: A negative charge-transfer-energy material

The approximate location in the Zaanen-Sawatzky-Allen diagram of the phase-separated (Ga,Mn)As material, consisting of MnAs nanoclusters embedded in GaAs, is determined on the basis of configuration-interaction (CI) cluster-model analysis of their Mn 2p core-level photoemission. The composite material is found to belong to the special class of materials with negative charge-transfer energy (delta). As such, its metallic or insulating/semiconducting behavior depends on the strength of the p-d hybridization (affected by strain) relative to the (size-dependent) p-bandwidth. Whereas internal strain in the embedded clusters counteracts gap opening, a metal-to-semiconductor transition is expected to occur for decreasing cluster size, associated to the opening of a small gap of p-p type (covalent gap). The electronic properties of homogeneous and phase-separated (Ga,Mn)As materials are analyzed, with emphasis on the nature of their metal-insulator transitions.

cond-mat.mes-hall↗

arXiv2011

Enhanced electron correlations, local moments, and Curie temperature in strained MnAs nanocrystals embedded in GaAs

We have studied the electronic structure of hexagonal MnAs, as epitaxial continuous film on GaAs(001) and as nanocrystals embedded in GaAs, by Mn 2p core-level photoemission spectroscopy. Configuration-interaction analyses based on a cluster model show that the ground state of the embedded MnAs nanocrystals is dominated by a d5 configuration that maximizes the local Mn moment. Nanoscaling and strain significantly alter the properties of MnAs. Internal strain in the nanocrystals results in reduced p-d hybridization and enhanced ionic character of the Mn-As bonding interactions. The spatial confinement and reduced p-d hybridization in the nanocrystals lead to enhanced d-electron localization, triggering d-d electron correlations and enhancing local Mn moments. These changes in the electronic structure of MnAs have an advantageous effect on the Curie temperature of the nanocrystals, which is measured to be remarkably higher than that of bulk MnAs.

cond-mat.mes-hall↗

arXiv2008

MgAl2O4:Cr3+ and emerald display a different colour but the local symmetry is the same: Microscopic origin

The difference in colour between emerald (Be3Si6Al2O18:Cr3+, green) and the Cr3+-doped spinel MgAl2O4 (red) is striking, considering that in both systems colour is due to CrO69- complexes with D3 symmetry and the measured Cr3+-O2- distance is practically the same (1.98 and 1.97 angstroms, respectively). By means of density functional calculations it is shown that this surprising difference can reasonably be explained once the electric field, ER, which all lattice ions lying outside the CrO69- complex exert on localized electrons, is taken into consideration. The origin of the different shape of ER in the two host lattices is analysed in detail. It is shown that ER raises (decreases) the 2p(O) levels for Be3Si6Al2O18:Cr3+ (MgAl2O4:Cr3+) along the trigonal axis thus favouring a decrease (increase) of 10Dq. The present work demonstrates the key role played by ER (not considered in the traditional ligand field theory) for understanding the differences exhibited by the same complex embedded in host lattices which do not have the same crystal structure. Some remarks on the colour of Cr2O3 pure compound are also reported.

cond-mat.mtrl-sci↗

arXiv2002

Classical limit for the scattering of Dirac particles in a magnetic field

We present a relativistic quantum calculation at first order in perturbation theory of the differential cross section for a Dirac particle scattered by a solenoidal magnetic field. The resulting cross section is symmetric in the scattering angle as those obtained by Aharonov and Bohm (AB) in the string limit and by Landau and Lifshitz (LL) for the non relativistic case. We show that taking pr_0\|sin(θ/2)|/\hbar<<1 in our expression of the differential cross section it reduces to the one reported by AB, and if additionally we assume θ<< 1 our result becomes the one obtained by LL. However, these limits are explicitly singular in \hbar as opposed to our initial result. We analyze the singular behavior in \hbar and show that the perturbative Planck's limit (\hbar -> 0) is consistent, contrarily to those of the AB and LL expressions. We also discuss the scattering in a uniform and constant magnetic field, which resembles some features of QCD.

quant-ph↗

arXiv2002

Dirac Neutrino Anapole Moment

We calculate the Dirac neutrino anapole moment (a_{ν_l}) in the context of the Standard Model (SM) making use of the Dirac form factor F_D(q^2) introduced recently by J. Bernabeu, L. G. Cabral-Rosetti, J. Papavassiliou y J. Vidal by using the Pinch Technique (PT) formalism, working in two different gauge-fixing schemes (R_ξguage and the electroweak BFM), at the one loop level. We show that the neutrino anapole form factor F_A(q^2) and Dirac form factor F_D(q^2) are related as follows: F_A(q^2) = \frac{1}{q^2} F_D(q^2). Hence, the Dirac neutrino charge radius < r^2_{ν_l} > and the anapole moment satisfy the simple relation a_{ν_l} = {1/6} < r^2_{ν_l} >. Therefore, we show that the anapole moment (as the charge radius) of the neutrino is a physical quantity, which only gets contribution from the proper neutrino electromagnetic vertex (in electroweak BFM), and that a_{ν_l} is of the order 10^{-34} cm^2.

hep-ph↗

arXiv2001

Compactification with U(1) magnetic field within Dirac supersymmetry

We consider Dirac-supersymmetric interactions, which produce CP-conserving separation of positive and negative energy solutions in the Dirac equation in order to investigate an alternative to the Kaluza-Klein mechanism. We review conditions under which separation is possible into free particle and compactified behaviors in different dimensions, with attention to spin degrees of freedom. We show a U(1) constant magnetic field produces such kind of behavior; an explicit treatment is given to the 6-d to 4-d and 4-d to 2-d breaking cases and the spectrum is obtained. A dynamical mass-creation mechanism is suggested from the procedure.

hep-th↗

arXiv1994

Exotic and conventional superconductivity in a Dirac supersymmetric scheme

A new pairing theory for many-fermion systems is obtained via the Dirac supersymmetry framework recently introduced to describe Dirac particles in external potentials. It is shown that the standard Bogoliubov-Valatin canonical transformation treatment of the quasi-particle BCS singlet pairing mechanism naturally falls within this framework. Straightforward generalizations in which the fermions can be ascribed $ ν$ {\it components} are shown to lead to enhanced gap energies and critical temperatures as in the case of cuprate superconductors without invoking a stronger electron-boson coupling. The new $T_{c}^{max}$ limit is $T_c^{max} = νT_c^{BCS}$, with $T_c^{BCS} \approx 40^0 K$.

cond-mat↗