arXiv subjects

A. L. R. Barbosa

Publications and source records attributed to A. L. R. Barbosa.

At least 19 recordsLinked to original sources

arXiv2021

Multifractal Magnetoconductance Fluctuations in Mesoscopic Systems

We perform a multifractal detrended fluctuation analysis of the magnetoconductance data of two standard types of mesoscopic systems: a disordered nanowire and a ballistic chaotic billiard, with two different lattice structures. We observe in all cases that multifractality is generally present and that it becomes stronger in the quantum regime of conduction, i.e., when the number of open scattering channels is small. We argue that this behavior originates from correlations induced by the magnetic field, which can be characterized through the distribution of conductance increments in the corresponding "stochastic time series," with the magnetic field playing the role of a fictitious time. More specifically, we show that the distributions of conductance increments are well fitted by $q$ Gaussians and that the value of the parameter $q$ is a useful quantitative measure of multifractality in magnetoconductance fluctuations.

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arXiv2020

Conductance Peak Density in Disordered Graphene Topological Insulators

We investigate the universal properties of quantum transport in graphene nanowires that engender subtle universal conductance fluctuations. We present results for three of the main microscopic models that describe the sublattice of graphene and generate, as we shall show, all the chiral universal symmetries. The results are robust and demonstrate the widely sought sign of chirality even in the regime of many open channels. The fingerprints paves the way to distinguish systems with sublattice symmetry such as topological insulators from ordinary ones by an order of magnitude. The experimental realization requires a single measurement of the chaotic fluctuations of the associated valleytronics conductante. Through the phase coherence length, our theoretical predictions are confirmed with the data from traditional measurements in the literature concerning quantum magnetotransport.

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arXiv2020

Spin Hall angle fluctuations in a disorder device

We investigate a disorderly mesoscopic device that supports spin-orbit interaction. The systemis connected to four semi-infinite leads embedded in the Landauer-Buttiker setup for quantumtransport and, according to our analysis, exhibits spin Hall angle fluctuations. We show analyticallyand numerically the fingerprint of the universal fluctuation of the polarization mediated by theconversion of charge current into spin current. Our investigation shows the complete compatibilityof our analytical and numerical results with the most recent experiments. Furthermore, we shownonzero and universal features of spin Hall effect in Rashba 2DEG with disorder. All the resultsshow the relevance of microscopic parameters for electronic transport with charge-spin conversionand, in many cases, inevitably lead to universal numbers.

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arXiv2019

Klein Paradox in Chaotic Dirac Billiards

We investigate the Schrödinger (non-relativistic) and the Dirac (``relativistic") billiards in the universal regime. The study is based on a non-ideal quantum resonant scattering numerical simulation. We show universal results that reveal anomalous behavior on the conductance, on the shot-noise power and on the respective eigenvalues distributions. In particular, we demonstrate the Klein's paradox in the graphene and tunable suppression/amplification transitions on the typical observables of the quantum dots.

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arXiv2018

Conductance Peak Density in Nanowires

We present a complete numerical calculation and an experimental data analysis of the universal conductance fluctuations in quasi-one-dimension nanowires. The conductance peak density model, introduced in nanodevice research on [Phys. Rev. Lett. 107, 176807 (2011)], is applied successfully to obtain the coherence length of InAs nanowire magnetoconductance and we prove its equivalence with correlation methods. We show the efficiency of the method and therefore a prominent alternative to obtain the phase-coherence length. The peak density model can be similarly applied to spintronic setups, graphene and topological isolator where phase-coherence length is a relevant experimental parameter.

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arXiv2018

Tunable ${\cal χ/PT}$ Symmetry in Noisy Graphene

We investigate the resonant regime of a mesoscopic cavity made of graphene or a doped beam splitter. Using Non-Hermitian Quantum Mechanics, we consider the Bender-Boettcher assumption that a system must obey parity and time reversal symmetry. Therefore, we describe such system by coupling chirality, parity and time reversal symmetries through the scattering matrix formalism and apply it in the shot noise functions, also derived here. Finally we show how to achieve the resonant regime only by setting properly the parameters concerning the chirality and the PT symmetry.

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arXiv2017

Spin-to-Charge Conversion in 2D Electron Gas and Single-layer Graphene Devices

We investigate the spin-to-charge conversion emerging from a mesoscopic device connected to multiple terminals. We obtain analytical expressions to the characteristic coefficient of spin-to-charge conversion which are applied in two kinds of ballistic chaotic quantum dots at low temperature. We perform analytical diagrammatic calculations in the universal regime for two-dimensional electron gas and single-layer graphene with strong spin-orbit interaction in the universal regime. Furthermore, our analytical results are confirmed by numerical simulations. Finally, we connect our analytical finds to recent experimental measures giving a conceptual explanation about the apparent discrepancies between them.

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arXiv2017

Entanglement Distribution Statistic in Andreev Billiards

We investigate statistical aspects of the entanglement production for open chaotic mesoscopic billiards in contact with superconducting parts, known as Andreev billiards. The complete distributions of concurrence and entanglement of formation are obtained by using the Altland-Zirnbauer symmetry classes of circular ensembles of scattering matrices, which complements previous studies in chaotic universal billiards belonging to other classes of random matrix theory. Our results show a unique and very peculiar behavior: the realization of entanglement in a Andreev billiard always results in non-separable state, regardless of the time reversal symmetry. The analytical calculations are supported by a numerical Monte Carlo simulation.

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arXiv2017

Parity and Time Reversal Symmetry in Hanbury Brown-Twiss Effect

The current manuscript employs the parity and time reversal symmetry in the Hanbury Brown-Twiss experiment. For this purpose, we develop a general scattering matrix framework founded on the concatenation of many individual compounded scattering processes on the setup. In this way, we derive the general scattering matrix of a parity and time reversal symmetric Hanbury Brown-Twiss experiment (HBT-PT). Within such scattering formulation, we propose a theoretical framework which provides how to measure the symmetry of the system through the correlation function of a pair of particles transmitted through the leads. The correlation function naturally reveal the quantum statistics of both bosons and fermions and demonstrate a very preponderant role of PT symmetry on the HBT experiment. We indicate the formation of both quantum and classical universal Turing machine depending on controllable parameters of the apparatus.

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arXiv2016

Time-Reversal Symmetry Breaking and Decoherence in Chaotic Dirac Billiards

In this work, we perform a statistical study on Dirac Billiards in the extreme quantum limit (a single open channel on the leads). Our numerical analysis uses a large ensemble of random matrices and demonstrates the preponderant role of dephasing mechanisms in such chaotic billiards. Physical implementations of these billiards range from quantum dots of graphene to topological insulators structures. We show, in particular, that the role of finite crossover fields between the universal symmetries quickly leaves the conductance to the asymptotic limit of unitary ensembles. Furthermore, we show that the dephasing mechanisms strikingly lead Dirac billiards from the extreme quantum regime to the semiclassical Gaussian regime.

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arXiv2016

Universal Spin Hall Conductance Fluctuations in Chaotic Dirac Quantum Dots

We present complete analytical and numerical results that demonstrate the anomalous universal fluctuations of the spin-Hall conductance in chiral materials such as graphene and topological insulators. We investigated both the corresponding fluctuations, the Universal Fractionated and the Universal Quantized, and also the open channels orbital number crossover between the two regimes. In particular, we show that the Wigner-Dyson symmetries does not properly describe such conductances and the preponderant role of the chiral classes on the Dirac quantum dots. The results are analytical and solve outstanding issues.

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arXiv2015

Correlation functions and correlation widths in Quantum-Chaotic Scattering for mesoscopic systems and nuclei

We derive analytical expressions for the correlation functions of the electronic conductance fluctuations of an open quantum dot under several conditions. Both the variation of energy and that of an external parameter such as an applied perpendicular or parallel magnetic fields are considered in the general case of partial openness.. These expressions are then used to obtain the ensemble averaged density of maxima, a measure recently suggested to contain invaluable information concerning the chaoticity of the system. The correlation width is then calculated for the case of energy variation and a significant deviation from the Weisskopf estimate is found in the case of two terminals. The results are extended to more than two terminals. All our results are analytical.The use of these results in other fields, such as nuclei, where the system can only be studied through a variation of the energy, is then discussed.

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arXiv2015

Fluctuation Phenomena in Chaotic Dirac Quantum Dots: Artificial Atoms on Graphene Flakes

We develop the stub model for the Dirac Quantum Dot, an electron confining device on a grapheme surface. Analytical results for the average conductance and the correlation functions are obtained and found in agreement with those found previously using semiclassical calculation. Comparison with available data are presented. The results reported here demonstrate the applicability of Random Matrix Theory in the case of Dirac electrons confined in a stadium.

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arXiv2015

Anomalous Entanglement in Chaotic Dirac Billiards

We present analytical and numerical results that demonstrate the presence of anomalous entanglement behavior on the Dirac Billiards. We investigate the statistical distribution of the characteristic entangled measures, focusing on the mean, on the variance and on the quantum interference terms. We show a quite distinct behavior of the Dirac Billiard compared with the non-relativist (Schrodinger) ones. Particularly, we show a very plausible Bell state and a sharp amplitude of quantum interference term on entangled electrons left from the Dirac Billiards. The results have remarkable relevance to the novel quantum dots build of materials like graphene or topological insulators.

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arXiv2014

Universal Braess paradox in open quantum dots

We present analytical and numerical results that demonstrate the presence of the Braess paradox in chaotic quantum dots. The paradox that we identify, originally perceived in classical networks, shows that the addition of more capacity to the network can suppress the current flow in the universal regime. We investigate the weak localization term, showing that it presents the paradox encoded in a saturation minimum of the conductance, under the presence of hyperflow in the external leads. In addition, we demonstrate that the weak localization suffers a transition signal depending on the overcapacity lead and presents an echo on the magnetic crossover before going to zero due to the full time-reversal symmetry breaking.We also show that the quantum interference contribution can dominate the Ohm term in the presence of constrictions and that the corresponding Fano factor engenders an anomalous behavior

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arXiv2013

Anti-correlation for Conductance Fluctuations in Chaotic Quantum Dots

We investigate the correlation functions of mesoscopic electronic transport in open chaotic quantum dots with finite tunnel barriers in the crossover between Wigner-Dyson ensembles. Using an analytical stub formalism, we show the emergence of a depletion/amplification of conductance fluctuations as a function of tunnel barriers, for both parametric variations of electron energy or magnetoconductance fields. Furthermore, even for pure Dyson ensembles, correlation functions of conductance fluctuations in chaotic quantum dots can exhibit anti-correlation. Experimental support to our findings is pointed out.

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arXiv2012

Generalized correlation functions for conductance fluctuations and the mesoscopic spin Hall effect

We study the spin-Hall conductance fluctuations in ballistic mesoscopic systems. We obtain universal expressions for the spin and charge current fluctuations, cast in terms of current-current autocorrelation functions. We show that the latter are conveniently parametrized as deformed Lorentzian shape lines, functions of an external applied magnetic field and the Fermi energy. We find that the charge current fluctuations show quite unique statistical features at the symplectic-unitary crossover regime. Our findings are based on an evaluation of the generalized transmission coefficients correlation functions within the stub model and are amenable to experimental test.

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arXiv2012

Spin Accumulation Encoded in Electronic Noise for Mesoscopic Billiards with Finite Tunneling Rates

We study the effects of spin accumulation (inside reservoirs) on electronic transport with tunneling and reflections at the gates of a quantum dot. Within the stub model, the calculation focus on the current-current correlation function for the flux of electrons injected into the quantum dot. The linear response theory used allows to obtain the noise power in the regime of thermal crossover as a function of parameters that reveal the spin polarization at the reservoirs. The calculation is performed employing diagrammatic integration within the universal groups (ensembles of Dyson) for a non-ideal, non-equilibrium chaotic quantum dot. We show that changes in the spin distribution determines significant alteration in noise behavior at values of the tunneling rates close to zero, in the regime of strong reflection at the gates.

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