arXiv subjects

Gerson J. Ferreira

Publications and source records attributed to Gerson J. Ferreira.

At least 19 recordsLinked to original sources

arXiv2026

Physical Pictures for Quasisymmetry in Crystals

Quasisymmetry (QS) provides a novel route to understand and control near-degeneracies, Berry curvature, optical selection rules, and symmetry-protected phenomena in quantum materials. Here we give physical interpretations of the emergence of QS operators across multiple material families. Using density functional theory and the $\mathbf{\mathit{k}}\cdot\mathbf{\mathit{p}}$ formalism, we identify QS subspaces and calculate their representation matrices, quantifying the quasisymmetry via a metric $ε$ that measures subspace invariance. For Sn/SiC and transition-metal dichalcogenide monolayers, QS corresponds to an emergent mirror symmetry, whereas in wurtzite crystals it manifests as an emergent spatial inversion. By contrast, for AgLa the QS appearing in avoided crossings is inherited from a nearby high-symmetry point rather than being an emergent lattice symmetry. Combining group-theoretical analysis and $\mathbf{\mathit{k}}\cdot\mathbf{\mathit{p}}$ modeling, our results establish concrete physical pictures for QS and provide practical criteria to diagnose it in first-principles calculations.

cond-mat.mtrl-sci↗

arXiv2026

Jittery Quantum Boomerang Effect

We study the dynamics of a spin-polarized wave packet in a disordered Rashba two-dimensional electron gas and identify a jittery quantum boomerang effect in which longitudinal and transverse motion return to the origin through fundamentally distinct mechanisms. Starting from an initial state with finite momentum along $x$ and spin polarized along $z$, we calculate the time evolution by combining a Chebyshev expansion of the time-evolution operator with a disorder ensemble average. In the weak-scattering regime, equations of motion derived from the quantum kinetic equation reproduce the numerical trends and show that impurity scattering acts as a viscous damping mechanism that suppresses the transient Zitterbewegung and drives the transverse displacement back to $y=0$ at long times. In contrast, the longitudinal dynamics show a Drude-like saturation at weak disorder. These results are consistent with the vanishing intrinsic spin Hall conductivity in the disordered Rashba model and with experimental observations of a transient intrinsic spin Hall effect in the time-domain. As disorder increases, the longitudinal dynamics evolve to a partial return toward the origin, which signals a transition from weak antilocalization to Anderson localization in 2D.

cond-mat.dis-nn↗

arXiv2025

An Unusual Dresselhaus Spin-Orbit Contribution of Even Order in Momentum

The spin-orbit (SO) coupling is conventionally known to manifest as \emph{odd} functions of momentum. Here, through both model calculations and symmetry analysis along with the method of invariants, we reveal that, in ordinary semiconductor heterostructures, a \emph{quadratic} Dresselhaus SO term -- inheriting from its bulk crystal form -- emerges via the interband effect, while complying with time-reversal and spatial symmetries. Furthermore, we observe that this unusual SO term gives rise to a range of striking quantum phenomena, including hybridized swirling texture, anisotropic energy dispersion, avoided band crossing, longitudinal \emph{Zitterbewegung}, and opposite spin evolution between different bands in quantum dynamics. These stand in stark contrast to those associated with the usual \emph{linear} SO terms. Our findings uncover a previously overlooked route for exploiting interband effects and open new avenues for spintronic functionalities that leverage unusual SO terms of \emph{even} orders in momentum.

cond-mat.mes-hall↗

arXiv2024

Phase transitions and scale invariance in topological Anderson insulators

We investigate disordered-driven transitions between trivial and topological insulator (TI) phases in two-dimensional (2D) systems. Our study primarily focuses on the BHZ model with Anderson disorder, while other standard 2DTI models exhibit equivalent features. The analysis is based on the local Chern marker (LCM), a local quantity that allows for the characterization of topological transitions in finite and disordered systems. Our simulations indicate that disorder-driven trivial to topological insulator transitions are nicely characterized by $\mathcal{C}_0$, the disorder averaged LCM near the central cell of the system. We show that $\mathcal{C}_0$ is characterized by a single-parameter scaling, namely, $\mathcal{C}_0(M, W, L) \equiv \mathcal{C}_0(z)$ with $z = [W^μ-W_c^μ(M)]L$, where $M$ is the Dirac mass, $W$ is the disorder strength and $L$ is the system size, while $W_c(M) \propto \sqrt{M}$ and $μ\approx 2$ stand for the critical disorder strength and critical exponent, respectively. Our numerical results are in agreement with a theoretical prediction based on a first-order Born approximation (1BA) analysis. These observations lead us to speculate that the universal scaling function we have found is rather general for amorphous and disorder-driven topological phase transitions.

cond-mat.dis-nn↗

arXiv2024

DFT2kp: effective kp models from ab-initio data

The $\mathbf{k}\cdot\mathbf{p}$ method, combined with group theory, is an efficient approach to obtain the low energy effective Hamiltonians of crystalline materials. Although the Hamiltonian coefficients are written as matrix elements of the generalized momentum operator $\mathbfπ=\mathbf{p}+\mathbf{p}_{\rm SOC}$ (including spin-orbit coupling corrections), their numerical values must be determined from outside sources, such as experiments or ab initio methods. Here, we develop a code to explicitly calculate the Kane (linear in crystal momentum) and Luttinger (quadratic in crystal momentum) parameters of $\mathbf{k}\cdot\mathbf{p}$ effective Hamiltonians directly from ab initio wavefunctions provided by Quantum ESPRESSO. Additionally, the code analyzes the symmetry transformations of the wavefunctions to optimize the final Hamiltonian. This is an optional step in the code, where it numerically finds the unitary transformation $U$ that rotates the basis towards an optimal symmetry-adapted representation informed by the user. Throughout the paper, we present the methodology in detail and illustrate the capabilities of the code applying it to a selection of relevant materials. Particularly, we show a "hands-on" example of how to run the code for graphene (with and without spin-orbit coupling). The code is open source and available at https://gitlab.com/dft2kp/dft2kp.

cond-mat.mes-hall↗

arXiv2023

Charge-Spin Conversion in Two-Subband Quantum Wells with Conventional and Unconventional Rashba Spin-Orbit Coupling

The reciprocal interconversion between spin polarization and charge current (CSC) is the focus of intensive theoretical and experimental investigation in spintronics research. Its physical origin stems from the Rashba spin-orbit coupling (SOC) induced by the breaking of the structure inversion symmetry. The steady-state interconversion efficiency is the result of the non-trivial spin textures of the electric-field distorted Fermi surface. Its full understanding and evaluation requires the consideration of disorder-induced relaxation effects in the presence of spin-orbit induced band splitting. In this paper the additional effect of the orbital degree of freedom is analyzed in a two-subband quantum well with both conventional and unconventional Rashba SOC in the presence of disorder impurity scattering. The latter is treated at the level of the Born approximation in the Green's function self-energy and with the inclusion of vertex corrections in the linear response functions for the charge current and the spin polarization. By explicitly considering the symmetry properties of the Hamiltonian the matrix structure of the correlation functions is shown to decompose in independent blocks of symmetry-related physical observables. We find that the inclusion of vertex corrections is important for the correct estimate of the CSC efficiency, which also depends on the position of the Fermi level. We also find that the relative sign of the Rashba SOC in the two subbands plays a key role in determining the behavior of the CSC. Finally, we point out how the two-subband model compares with the standard single-band two-dimensional electron gas.

cond-mat.mes-hall↗

arXiv2019

Topological flat band, Dirac fermions and quantum spin Hall phase in 2D Archimedean lattices

Materials with designed properties arises in a synergy between theoretical and experimental approaches. In this study we explore the set of Archimedean lattices forming a guidance to its electronic properties and topological phases. Within these lattices, rich electronic structure emerge forming type-I and II Dirac fermions, topological flat bands and high-degeneracy points with linear and flat dispersions. Employing a tight-binding model, with spin-orbit coupling, we characterize a quantum spin Hall (QSH) phase in all Archimedean lattices. Our discussion is validated within density functional theory calculations, where we show the characteristic bands of the studied lattices arising in 2D carbon allotropes.

cond-mat.mes-hall↗

arXiv2019

Orbital pseudospin-momentum locking in two-dimensional chiral borophene

Recently, orbital-textures have been found in Rashba and topological insulator (TI) surface states as a result of the spin-orbit coupling (SOC). Here, we predict a $p_x/p_y$ orbital texture, in linear dispersive Dirac bands, arising at the K/K' points of $χ$-$h_0$ borophene chiral monolayer. Combining first-principles calculations with effective hamiltonians, we show that the orbital pseudospin has its direction locked with the momentum in a similar way as TIs' spin-textures. Additionally, considering a layer pseudospin degree of freedom, this lattice allows stackings of layers with equivalent or opposite chiralities. In turn, we show a control of the orbital textures and layer localization through the designed stacking and external electric field. For instance, for the opposite chirality stacking, the electric field allows for an on/off switch of the orbital-textured Dirac cone.

cond-mat.mes-hall↗

arXiv2018

Zitterbewegung and bulk-edge Landau-Zener tunneling in topological insulators

We investigate the ballistic zitterbewegung dynamics and the Landau-Zener tunneling between edge and bulk states of wave packets in two-dimensional topological insulators. In bulk, we use the Ehrenfest theorem to show that an external in-plane electric field not only drifts the packet longitudinally, but also induces a transverse finite side-jump for both trivial and topological regimes. For finite ribbons of width $W$, we show that the Landau-Zener tunneling between bulk and edge states vanishes for large $W$ as their electric field-induced coupling decays with $W^{-3/2}$. This is demonstrated by expanding the time-dependent Schrödinger equation in terms of Houston states. Hence we cannot picture the quantum spin Hall states as arising from the zitterbewegung bulk trajectories `leaking' into the edge states, as proposed in Phys. Rev. B 87, 161115 (2013).

cond-mat.mes-hall↗

arXiv2017

Tuning the topological states in metal-organic bilayers

We have investigated the energetic stability and the electronic properties of metal-organic topological insulators bilayers (BLs), $(MC_4S_4)_3$-BL, with M=Ni and Pt, using first-principles calculations and tight-binding model. Our findings show that $(MC_4S_4)_3$-BL is an appealing platform to perform electronic band structure engineering, based on the topologically protected chiral edge states. The energetic stability of the BLs is ruled by van der Waals interactions; being the AA stacking the energetically most stable one. The electronic band structure is characterized by a combination of bonding and anti-bonding kagome band sets (KBSs), revealing that $(NiC_4S_4)_3$-BL presents a Z$_2$-metallic phase, whereas $(PtC_4S_4)_3$-BL may present both Z$_2$-metallic phase or quantum spin Hall phase. Those non-trivial topological states were confirmed by the formation of chiral edge states in $(MC_4S_4)_3$-BL nanoribbons. We show that the localization of the edge states can be controlled with a normal external electric field, breaking the mirror symmetry. Hence, the sign of electric field selects in which layer each set of edge states are located. Such a control on the (layer) localization, of the topological edge states, bring us an additional and interesting degree of freedom to control the transport properties in layered metal-organic topological insulator.

cond-mat.mtrl-sci↗

arXiv2017

Conductance and Kondo Interference beyond Proportional Coupling

The transport properties of nanostructured systems are deeply affected by the geometry of the effective connections to metallic leads. In this work we derive a conductance expression for interacting systems whose connectivity geometries do not meet the Meir-Wingreen proportional coupling condition. As an interesting application, we consider a quantum dot connected coherently to tunable electronic cavity modes. The structure is shown to exhibit a well-defined Kondo effect over a wide range of coupling strengths between the two subsystems. In agreement with recent experimental results, the calculated conductance curves exhibit strong modulations and asymmetric behavior as different cavity modes are swept through the Fermi level. These conductance modulations occur, however, while maintaining robust Kondo singlet correlations of the dot with the electronic reservoir, a direct consequence of the lopsided nature of the device.

cond-mat.mes-hall↗

arXiv2017

Spin drift and diffusion in one- and two-subband helical systems

The theory of spin drift and diffusion in two-dimensional electron gases is developed in terms of a random walk model incorporating Rashba, linear and cubic Dresselhaus, and intersubband spin-orbit couplings. The additional subband degree of freedom introduces new characteristics to the persistent spin helix (PSH) dynamics. As has been described before, for negligible intersubband scattering rates, the sum of the magnetization of independent subbands leads to a checkerboard pattern of crossed PSHs with long spin lifetime. For strong intersubband scattering we model the fast subband dynamics as a new random variable, yielding a dynamics set by averaged spin-orbit couplings of both subbands. In this case the crossed PSH becomes isotropic, rendering circular (Bessel) patterns with short spin lifetime. Additionally, a finite drift velocity breaks the symmetry between parallel and transverse directions, distorting and dragging the patterns. We find that the maximum spin lifetime shifts away from the PSH regime with increasing drift velocity. We present approximate analytical solutions for these cases and define their domain of validity. Effects of magnetic fields and initial package broadening are also discussed.

cond-mat.mes-hall↗

arXiv2016

Topological Nonsymmorphic Ribbons out of Symmorphic Bulk

States of matter with nontrivial topology have been classified by their bulk symmetry properties. However, by cutting the topological insulator into ribbons, the symmetry of the system is reduced. By constructing effective Hamiltonians containing the proper symmetry of the ribbon, we find that the nature of topological states is dependent on the reduced symmetry of the ribbon and the appropriate boundary conditions. We apply our model to the recently discovered two-dimensional topological crystalline insulators composed by IV-VI monolayers, where we verify that the edge terminations play a major role on the Dirac crossings. Particularly, we find that some bulk cuts lead to nonsymmorphic ribbons, even though the bulk material is symmorphic. The nonsymmorphism yields a new topological protection, where the Dirac cone is preserved for arbitrary ribbon width. The effective Hamiltonians are in good agreement with ab initio calculations.

cond-mat.mes-hall↗

arXiv2013

Magnetically Defined Qubits on 3D Topological Insulators

We explore potentials that break time-reversal symmetry to confine the surface states of 3D topological insulators into quantum wires and quantum dots. A magnetic domain wall on a ferromagnet insulator cap layer provides interfacial states predicted to show the quantum anomalous Hall effect (QAHE). Here we show that confinement can also occur at magnetic domain heterostructures, with states extended in the inner domain, as well as interfacial QAHE states at the surrounding domain walls. The proposed geometry allows the isolation of the wire and dot from spurious circumventing surface states. For the quantum dots we find that highly spin-polarized quantized QAHE states at the dot edge constitute a promising candidate for quantum computing qubits.

cond-mat.mes-hall↗

arXiv2012

Helical States in Curved Bilayer Graphene

We study spin effects of quantum wires formed in bilayer graphene by electrostatic confinement. With a proper choice of the confinement direction, we show that in the presence of magnetic field, spin-orbit interaction induced by curvature, and intervalley scattering, bound states emerge that are helical. The localization length of these helical states can be modulated by the gate voltage which enables the control of the tunnel coupling between two parallel wires. Allowing for proximity effect via an s-wave superconductor, we show that the helical modes give rise to Majorana fermions in bilayer graphene.

cond-mat.mes-hall↗

arXiv2011

Low Bias Negative Differential Resistance in Graphene Nanoribbon Superlattices

We theoretically investigate negative differential resistance (NDR) for ballistic transport in semiconducting armchair graphene nanoribbon (aGNR) superlattices (5 to 20 barriers) at low bias voltages V_SD < 500 mV. We combine the graphene Dirac Hamiltonian with the Landauer-Büttiker formalism to calculate the current I_SD through the system. We find three distinct transport regimes in which NDR occurs: (i) a "classical" regime for wide layers, through which the transport across band gaps is strongly suppressed, leading to alternating regions of nearly unity and zero transmission probabilities as a function of V_SD due to crossing of band gaps from different layers; (ii) a quantum regime dominated by superlattice miniband conduction, with current suppression arising from the misalignment of miniband states with increasing V_SD; and (iii) a Wannier-Stark ladder regime with current peaks occurring at the crossings of Wannier-Stark rungs from distinct ladders. We observe NDR at voltage biases as low as 10 mV with a high current density, making the aGNR superlattices attractive for device applications.

cond-mat.mes-hall↗

arXiv2010

Collapse of $ρ_{xx}$ ringlike structures in 2DEGs under tilted magnetic fields

In the quantum Hall regime, the longitudinal resistivity $ρ_{xx}$ plotted as a density--magnetic-field ($n_{2D}-B$) diagram displays ringlike structures due to the crossings of two sets of spin split Landau levels from different subbands [e.g., Zhang \textit{et al.}, Phys. Rev. Lett. \textbf{95}, 216801 (2005)]. For tilted magnetic fields, some of these ringlike structures "shrink" as the tilt angle is increased and fully collapse at $θ_c \approx 6^\circ$. Here we theoretically investigate the topology of these structures via a non-interacting model for the 2DEG. We account for the inter Landau-level coupling induced by the tilted magnetic field via perturbation theory. This coupling results in anti-crossings of Landau levels with parallel spins. With the new energy spectrum, we calculate the corresponding $n_{2D}-B$ diagram of the density of states (DOS) near the Fermi level. We argue that the DOS displays the same topology as $ρ_{xx}$ in the $n_{2D}-B$ diagram. For the ring with filling factor $ν=4$, we find that the anti-crossings make it shrink for increasing tilt angles and collapse at a large enough angle. Using effective parameters to fit the $θ= 0^\circ$ data, we find a collapsing angle $θ_c \approx 3.6^\circ$. Despite this factor-of-two discrepancy with the experimental data, our model captures the essential mechanism underlying the ring collapse.

cond-mat.mes-hall↗

arXiv2010

Many-body effects on the $ρ_{xx}$ ringlike structures in two-subband wells

The longitudinal resistivity $ρ_{xx}$ of two-dimensional electron gases formed in wells with two subbands displays ringlike structures when plotted in a density--magnetic-field diagram, due to the crossings of spin-split Landau levels (LLs) from distinct subbands. Using spin density functional theory and linear response, we investigate the shape and spin polarization of these structures as a function of temperature and magnetic-field tilt angle. We find that (i) some of the rings "break" at sufficiently low temperatures due to a quantum Hall ferromagnetic phase transition, thus exhibiting a high degree of spin polarization ($\sim 50 $%) within, consistent with the NMR data of Zhang \textit{et al.} [Phys. Rev. Lett. {\bf 98}, 246802 (2007)], and (ii) for increasing tilting angles the interplay between the anticrossings due to inter-LL couplings and the exchange-correlation (XC) effects leads to a collapse of the rings at some critical angle $θ_c$, in agreement with the data of Guo \textit{et al.} [Phys. Rev. B {\bf 78}, 233305 (2008)].

cond-mat.mes-hall↗