arXiv subjects

L. Diago-Cisneros

Publications and source records attributed to L. Diago-Cisneros.

15 recordsLinked to original sources

arXiv2021

Time-Energy Uncertainty Limit for spin-related wavepacket evolution

In this report we study the quantum transport of charge carriers for low dimensional systems with spin-orbit coupling by means of Heisenberg's inequalities. To develop our analysis, an accurate \emph{gendanken} experiment was carefully put together, mainly based on the spin-field effect transistor phenomenology and taking into account several wide accepted approaches on quantum mechanical limited determinism. While verifying the applicability of time-energy uncertainty relation (TEUR) during electronic wavepacket's evolution through a semiconductor quantum wire, some qualitative information related the dynamic behavior of the system is found. The problem is also approached in the framework of the stationary phase method to guarantee robust coherence for wavepacket's evolution, which lately implies certain restrictions on the incident energy values for the envisioned cases. Tailoring different input parameters such as: incoming particles spin polarization, barrier thickness and Rashba's spin-orbit interaction (R-SOI) strength, we have observed appealing effects while the packet evolves through a quasi-1D heterostructure. Hopefully some of them could be of help in spintronics device designing. We have obtained most of the numerical simulation results, within the so-called conservation zone. However, for certain barrier thicknesses, some values drop into the forbidden area regarding the clear theoretical limit imposed by the TEUR. As an expected bonus throughout our theoretical validation of the TEUR, spin splitting due to finite values of R-SOI was nicely noticed.

cond-mat.mes-hall↗

arXiv2021

Surveying the Multicomponent Scattering Matrix: Unitarity and Symmetries

Multicomponent-multiband fluxes of spim-charge carriers, whose components propagate mixed and synchronously, with \emph{a priori} nonzero incoming amplitudes, do not obey the standard unitarity condition on the scattering matrix for an arbitrary basis set. For such cases, we have derived a robust theoretical procedure, which is fundamental in quantum-transport problems for unitarity preservation and we have named after \emph{structured unitarity condition}. Our approach deals with $(N \times N)$ interacting components (for $N \geq 2$), within the envelope function approximation (EFA), and yet the standard unitary properties of the ($N = 1$) scattering matrix are recovered. Rather arbitrary conditions to the basis-set and/or to the output scattering coefficients, are not longer required, if the \emph{eigen}-functions are orthonormalized in both the configuration and the spinorial spaces. We expect the present model to be workable, for different kind of multiband-multicomponent physical systems described by Hermitian Hamiltonians within the EFA, with small transformations if any. We foretell the interplay for the state-vector transfer matrix, together with the large values of its condition number, as a novel complementary tools for a more accurate definition of the threshold for tunnelling channels in a scattering experiment.

quant-ph↗

arXiv2020

A new phenomenon in graphene: The pseudospinorial Zitterbewegung

We foretell a new pseudospin-dependent phenomenon in mono-layer graphene (MLG), which is numerically simulated \emph{via} an innovator nano-spintronic device. We proposed a novel theoretical procedure for describing the dynamics of Dirac fermions, departing from classic theoretical modelling. More importantly, we have found appealing evidences of wiggling anti-phase oscillations in the probability density time-distribution for each sub-lattice state, which we called pseudospinorial Zitterbewegung effect (PZBE). The PZBE undergoes modulated by a robust transient character, with decay time of femtoseconds. Interestingly, several features of the PZBE become tunable, even up to fully vanishing it at the vicinity of the Dirac points, as well as for a symmetric pseudospin configuration. We have observed evidences of perfect Klein tunneling and perfect anti-Klein backscattering in a single simulation, which is unprecedented for Q1D-MLG, as far as we know.

cond-mat.mes-hall↗

arXiv2019

Electrical tuning of helical edge states in topological multilayers

Mainstream among topological insulators, GaSb/InAs quantum wells present a broken gap alignment for the energy bands which supports the quantum spin Hall insulator phase and forms an important building block in the search of exotic states of matter. Such structures allow the band-gap inversion with electrons and holes confined in adjacent layers, providing a fertile ground to tune the corresponding topological properties. Using a full 3D 8-band ${\bf k}\cdot{\bf p}$ method we investigate the inverted band structure of GaSb/InAs/GaSb and InAs/GaSb/InAs multilayers and the behavior of the helical edge states, under the influence of an electric field applied along the growth direction. By tuning the electric field modulus, we induce the change of the energy levels of both conduction and valence bands, resulting in a quantum spin Hall insulator phase where the helical edge states are predominantly confined in the GaSb layer. In particular, we found that InAs/GaSb/InAs has a large hybridization gap of about $12\,\textrm{meV}$ and, therefore, are promising to observe massless Dirac fermions with a large Fermi velocity. Our comprehensive characterization of GaSb/InAs multilayers creates a basis platform upon which further optimization of III-V heterostructures can be contrasted.

cond-mat.mes-hall↗

arXiv2019

A bandmixing treatment for multiband-coupled systems via nonlinear-eigenvalue scenario

We present a numeric-computational procedure to deal with the intricate bandmixing phenomenology in the framework of the quadratic eigenvalue problem (QEP), which is derived from a physical system described by N-coupled components Sturm-Liouville matrix boundary-equation. The modeling retrieves the generalized Schur decomposition and the root-locus-like techniques to describe the dynamics of heavy holes (hh), light holes (lh) and spin-split holes (sh) in layered semiconductor heterostructures. By exercising the extended (N = 6) Kohn Lüttinger model, our approach successfully overcomes the medium-intensity regime for quasi-particle coupling of previous theoretical studies. As a bonus, the sufficient conditions for a generalized QEP have been refined. The sh-related off -diagonal elements in the QEP mass-matrix, becomes a competitor of the bandmixing parameter, leading the hh-sh and lh-sh spectral distribution to change, then they can not be disregarded or zeroed, as was assumed in previous theoretical studies. Thereby, we unambiguously predict that several of the new features detected for hh-lh-sh spectral properties and propagating modes, become directly influenced by the metamorphosis of the effective band-offset scattering profile due sub-bandmixing effects strongly modulated with the assistance of sh, even at low-intensity mixing regime.

cond-mat.mes-hall↗

arXiv2015

Quantum transport properties in Datta-Das tuned opacity spin-transistors

We studied the spin-dependent quantum transport properties using a simple modelling of a Datta-Das spin transistor. We refine previous results by accounting the propagation medium changes of opacity felt by itinerant electrons, when the gate-voltage is switched on and modelling them via the transversal energy levels mismatch. Monitoring the topological-dependent conductance, we are able to identify the device operating points. If the incoming electrons energy approaches the biased-induced barriers height, the spin-resolved conductance oscillations become significant. In a zero temperature picture, our computations of the spin-dependent conductance as function of the electric field at the region below the gate electrode suggest the feasibility of the modeled device. Although we demonstrate that phase time may not be spin-resolved, our simulation allows us to evaluate the time that takes an electron to experience a spin-flip process, resulting in an order of magnitude lower than typical values of the spin relaxation times.

cond-mat.mes-hall↗

arXiv2015

Rashba-coupling modelling for two-dimensional and high-order Rashba Hamiltonian for one-dimensional confined heavy holes

Based on standard k.p (8 x 8) multiband Hamiltonian, we have deduced an explicit analytical expression for the Rashba-coupling parameter which clarifies its anomalous behavior for heavy holes (hh), gated in quasi-two-dimensional (Q2D) systems, by letting grow the density. Our modelling remarkable better agrees with experimental results in comparison with earlier theoretical models, while recovers the expected cubic dependence on the quasi-momentum. For quasi-one-dimensional (Q1D) hh systems, we have formally derived an effective Rashba Hamiltonian with two competitive terms on the quasi-momentum, a linear term and a cubic one as predicted from suitable approximations to the Q2D scope. The Rashba-coupling parameters also behave anomalously and qualitatively support recent experiments in core/shell nanowires. Furthermore, they exhibit an essential asymptotic discontinuity in the low density regime as a function of the lateral confinement length. For hh, we present closed schemes to accurately quote the Rashba-coupling parameters both for the Q2D and Q1D systems, which become unprecedented for holes.

cond-mat.mes-hall↗

arXiv2015

Exploring the transport properties of polytypic and twin-plane nanowires: from tunneling phase-time to spin-orbit interaction effects

The variety of nanowire crystal structures gave rise to unique and novel transport phenomena. In particular, we have explored the superlattice profile generated by strain field modulation in twinplane nanowires for the tuning of transport channels and the built-in spin-orbit potential profile of polytypic nanowires, in order to realize a spin filter. The Multicomponent Scattering Approach has been used in terms of the Transfer Matrix Method to describe the phase-time of charge carriers. This system showed advantages for attaining conditions for the propagation of wave packets with negative group velocity. Moreover, the spin transport effect of a potential profile with volumetric spin-orbit bulk inversion asymmetry, as present on polytypic nanowires, was described through the Reverse Runge-Kutta Method. Using the peculiar symmetry of the excited states we have characterized a dominant spin dependence on structural parameters that results in effective spin filtering.

cond-mat.mes-hall↗

arXiv2014

Permutations of the transverse momentum dependent effective valence-band potential for layered heterostructures. Pseudomorphic strain effects

The evolution of transverse-momentum-dependent effective band offset ($V_{\mathrm{eff}}$) profile for heavy (\emph{hh})- and light-holes (\emph{lh}), is detailed studied. Several new features in the metamorphosis of the standardized fixed-height $V_{\mathrm{eff}}$ profile for holes, in the presence of gradually increasing valence-band mixing and pseudomorphic strain, are presented. In some $III-V$ unstrained semiconducting layered heterostructures a fixed-height potential, is not longer valid for \emph{lh}. Indeed, we found ---as predicted for electrons---, permutations of the $V_{\mathrm{eff}}$ character for \emph{lh}, that resemble a "\emph{keyboard}", together with bandgap changes, whenever the valence-band mixing varies from low to large intensity. Strain is able to diminish the \emph{keyboard} effect on $V_{\mathrm{eff}}$, and also makes it emerge or vanish occasionally. We found that multiband-mixing effects and stress induced events, are competitors mechanisms that can not be universally neglected by assuming a fixed-height rectangular spatial distribution for fixed-character potential energy, as a reliable test-run input for heterostructures. Prior to the present report, neither direct transport-domain measurements, nor theoretical calculations addressed to these $V_{\mathrm{eff}}$ evolutions and permutations, has been reported for holes, as far as we know. Our results may be of relevance for promising heterostructure's design guided by valence-band structure modeling to enhance the hole mobility in $III-V$ materials.

cond-mat.mes-hall↗

arXiv2013

Localization in the ground state of a triple quantum well

A model is presented, consisting of a single structureless particle on the line subject to a potential with three minima, with an exactly soluble ground level. In this model the ground level probability density becomes more sensitive to the global shape of the potential as the distance between the minima increases, so that for big enough distances small variations in the potential bring a qualitative change in the probability density, taking it from a unimodal, localized, distribution, to a bimodal one. We conjecture that this effect, of which we have not found any precedent in the literature, may be relevant in the design and characterization of mesoscopic devices such as triple quantum well systems.

math-ph↗

arXiv2013

Valence-band effective-potential evolution for coupled holes

We present the metamorphosis in the effective-potential profile of layered heterostructures, for several III-V semiconductor binary compounds, when the band mixing of light and heavy holes increases. A root-locus-like procedure, is directly applied to an eigenvalue quadratic problem obtained from a multichannel system of coupled modes, in the context of multiband effective mass approximation. By letting grow valence-band mixing, it is shown the standard fixed-height rectangular potential-energy for the scatterer distribution, to be a reliable test-run input for heavy holes. On the contrary, this scheme is no longer valid for light holes and a mutable effective \emph{band offset} profile has to be considered instead, whenever the in-plane kinetic energy changes.

cond-mat.mes-hall↗

arXiv2013

Giant conductance and phase time anomalous events of hole quantum transport

Events of giant conductance and anomalies of the phase transmission time for holes, are theoretically investigated within the multicomponent scattering approach. Based on this model, new analytical expressions for unitarity relations in the uncoupled hole transport are obtained and directly applied to study the behavior of the conductance and the phase transmission time in a double barrier resonant tunneling (DBRT) and a superlattice $GaAs$-cladding layer$/(AlAs/GaAs)^{n}/GaAs$-cladding layer. Clear-signature evidences of giant conductance phenomena for hole transmission without valence-band mixing through a DBRT and a superlattice were found. The giant conductance effect losses robustness by manipulating the number of superlattice layers and by including the valence-band particles coupling as well. Phase time through the heterostructure exhibits extremal dependencies in the gaps and in the barriers, as those reported before for electrons. We have detected an earlier arrival phase time for the propagation of both flavors of holes within the barrier, in the order of few tenths of picoseconds. An appealing filter-like effect is presented, whenever a selective confinement strength arises independently for both flavors of holes in the uncoupled regime. Our results also prescribe noticeable evidences for both uncoupled and coupled hole fluxes, similar to those foretold by Hartman, upon transmission of electrons through opaque barriers.

cond-mat.mes-hall↗

arXiv2007

The Stationary Phase Method for a Wave Packet in a Semiconductor Layered System. The applicability of the method

Using the formal analysis made by Bohm in his book, {\em "Quantum theory"}, Dover Publications Inc. New York (1979), to calculate approximately the phase time for a transmitted and the reflected wave packets through a potential barrier, we calculate the phase time for a semiconductor system formed by different mesoscopic layers. The transmitted and the reflected wave packets are analyzed and the applicability of this procedure, based on the stationary phase of a wave packet, is considered in different conditions. For the applicability of the stationary phase method an expression is obtained in the case of the transmitted wave depending only on the derivatives of the phase, up to third order. This condition indicates whether the parameters of the system allow to define the wave packet by its leading term. The case of a multiple barrier systems is shown as an illustration of the results. This formalism includes the use of the Transfer Matrix to describe the central stratum, whether it is formed by one layer (the single barrier case), or two barriers and an inner well (the DBRT system), but one can assume that this stratum can be comprise of any number or any kind of semiconductor layers.

cond-mat.mes-hall↗

arXiv2004

Interesting coupling phenomena of heavy and light holes in a (GaAs/AlAs) superlattice

An appropriate combination of the scattering theory and the transfer matrix formalism, for the solution of a (4 x 4) Kohn-Luttinger model, allow us to study the multichannel-multiband transmission process of heavy and light holes through a (GaAs/AlAs) superlattice. Appealing effects and interesting channel coupling phenomena, mediated by quasi-bond states, are clearly foreseen.

cond-mat.mes-hall↗

arXiv2004

Symmetries and General Principies in the Multiband Effective Mass Theory: A Transfer Matrix Study

We study the time reversal and space inversion symmetry properties of those transfer matrices mostly used in the calculation of energy spectra and transport-process. We study the time reversal and space inversion symmetry properties of those transfer matrices mostly used in the calculation of energy spectra and transport-process quantities. We determine the unitary transformation relating transfer matrices. We consider the Kohn-Luttinger model for a quasi-2D system and show that even though the system studied in the (4 x 4) scheme satisfies all the symmetry requirements, the (2 x 2)subspaces do not fulfill such constrains, except in the Gamma point of the Brillouin Zone. We find new exchange properties between the (2\times 2)subspace quantities.

cond-mat.mes-hall↗