arXiv subjects

J. G. G. S. Ramos

Publications and source records attributed to J. G. G. S. Ramos.

28 records · Page 2Linked to original sources

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.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

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

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

arXiv2011

Conductance peaks in open quantum dots

We present a simple measure of the conductance fluctuations in open ballistic chaotic quantum dots, extending the number of maxima method originally proposed for the statistical analysis of compound nuclear reactions. The average number of extreme points (maxima and minima) in the dimensionless conductance, $T$, as a function of an arbitrary external parameter $Z$, is directly related to the autocorrelation function of $T(Z)$. The parameter $Z$ can be associated to an applied gate voltage causing shape deformation in quantum dot, an external magnetic field, the Fermi energy, etc.. The average density of maxima is found to be $<ρ_{Z}> = α_{Z}/Z_c$, where $α_{Z}$ is a universal constant and $Z_c$ is the conductance autocorrelation length, which is system specific. The analysis of $<ρ_{Z}>$ does not require large statistic samples, providing a quite amenable way to access information about parametric correlations, such as $Z_c$.

cond-mat.mes-hall↗

arXiv2011

Presence of asymmetric noise in multi-terminal chaotic cavities

This work deals with chaotic quantum dot connected to two and four leads. We use standard diagrammatic procedure to integrate on the unitary group, to study the main term in the semiclassical expansion of the noise in the three pure Wigner-Dyson ensembles at finite frequency and temperature, in the noninteracting and interacting regimes. We investigate several limits, related to the temperature and the potential difference in the leads, in the presence of a capacitive environment. At the thermal crossover regime, we obtain general expressions described in terms of parameters that can be controlled experimentally. As an interesting result, we show the appearance of asymmetries in the noise, controlled by the topology of the cavity and the number of open channels in the corresponding terminals.

cond-mat.mes-hall↗

arXiv2011

Localized D-dimensional global k-defects

We explicitly demonstrate the existence of static global defect solutions of arbitrary dimensionality whose energy does not diverge at spatial infinity, by considering maximally symmetric solutions described by an action with non-standard kinetic terms in a D+1 dimensional Minkowski space-time. We analytically determine the defect profile both at small and large distances from the defect centre. We verify the stability of such solutions and discuss possible implications of our findings, in particular for dark matter and charge fractionalization in graphene.

hep-th↗

arXiv2011

Crossover of thermal to shot noise in chaotic cavities

We study the crossover between thermal and shot-noise power in a chaotic quantum dot in the presence of non-ideal contacts at finite temperature. The result explicitly demonstrates that the temperature affect the suppression-amplification effect present in the main quantum noise. In particular, the weak localization contribution to the noise has an anomalous thermal behavior when one let the barriers vary, indicating the presence of a critical point related to specific value of the tunneling barriers. We also show how to get to the opaque limit of the quantum dot at finite temperature.

cond-mat.mes-hall↗