Gerson J. Ferreira
ContactDepartamento de Física e InformáticaInstituto de Física de São Carlos Universidade de São Paulo, Brazil

Short CV
 2013Present: Postdoc with Prof. Dr. J. Carlos Egues @ University of São Paulo (USP/IFSC/Brazil)
 20122013: Postdoc with Prof. Dr. Daniel Loss @ University of Basel
 20062011: PhD in Physics @ University of São Paulo (USP/IFSC/Brazil), with Prof. Dr. J. Carlos Egues
 20022005: BSc in Physics, emphasis in Computational Physics @ University of São Paulo (USP/IFSC/Brazil)
Google Scholar Citations
Interesting inititative by Google that help us keep track of citations to our work: [click here]Publications
Show all abstracts.1.  MagneticallyDefined Qubits on 3D Topological Insulators 
Gerson J. Ferreira and Daniel Loss. arXiv:1305.5003
We explore timereversalsymmetrybreaking potentials 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 spinpolarized quantized QAHE states at the dot edge
constitute a promising candidate for quantum computing qubits.
 
2.  Helical States in Curved Bilayer Graphene 
Jelena Klinovaja, Gerson J. Ferreira, and Daniel Loss. Phys. Rev. B 86, 235416 (2012); arXiv:1208.2601.
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, spinorbit 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 swave superconductor, we show that the helical modes give rise to Majorana fermions in bilayer graphene.
 
3.  Low Bias Negative Differential Resistance in Graphene Nanoribbon Superlattices 
Gerson J. Ferreira, Michael N. Leuenberger, Daniel Loss, and J. Carlos Egues. Phys. Rev. B 84, 125453 (2011); arXiv:1105.4850.
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 LandauerB\"uttiker 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 WannierStark ladder regime with current peaks occurring at
the crossings of WannierStark 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.
 
4.  Manybody effects on the $\rho_{xx}$ ringlike structures in twosubband wells 
Gerson J. Ferreira, Henrique J. P. Freire, and J. Carlos Egues. Phys. Rev. Lett. 104, 066803 (2010); arXiv:0909.2175.
The longitudinal resistivity $\rho_{xx}$ of twodimensional electron gases
formed in wells with two subbands displays ringlike structures when plotted in
a densitymagneticfield diagram, due to the crossings of spinsplit 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 magneticfield 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
interLL couplings and the exchangecorrelation (XC) effects leads to a
collapse of the rings at some critical angle $\theta_c$, in agreement with the
data of Guo \textit{et al.} [Phys. Rev. B {\bf 78}, 233305 (2008)].
 
5.  Collapse of $\rho_{xx}$ ringlike structures in 2DEGs under tilted magnetic fields 
Gerson J. Ferreira and J. Carlos Egues. J. Supercond. Novel Magnetism 23, 19 (2010); arXiv:1003.1761.
In the quantum Hall regime, the longitudinal resistivity $\rho_{xx}$ plotted
as a densitymagneticfield ($n_{2D}B$) 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 $\theta_c \approx
6^\circ$. Here we theoretically investigate the topology of these structures
via a noninteracting model for the 2DEG. We account for the inter Landaulevel
coupling induced by the tilted magnetic field via perturbation theory. This
coupling results in anticrossings 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 $\rho_{xx}$ in the $n_{2D}B$ diagram. For the
ring with filling factor $\nu=4$, we find that the anticrossings make it
shrink for increasing tilt angles and collapse at a large enough angle. Using
effective parameters to fit the $\theta = 0^\circ$ data, we find a collapsing
angle $\theta_c \approx 3.6^\circ$. Despite this factoroftwo discrepancy with
the experimental data, our model captures the essential mechanism underlying
the ring collapse.
 
6.  Ringlike structures in the densitymagneticfield $\rho_{xx}$ diagram of twosubband quantum Hall systems 
Gerson J. Ferreira, Henrique J. P. Freire, and J. Carlos Egues. Phys. Stat. Sol. (c) 3, 4364 (2006); arXiv:condmat/0607456.
Motivated by recent experiments [Zhang \textit{et al.}, Phys. Rev. Lett.
\textbf{95}, 216801 (2005) and Ellenberger \textit{et al.}, condmat/0602271]
reporting novel ringlike structures in the densitymagneticfield
($n_{2D}$\emph{B}) diagrams of the longitudinal resistivity $\rho_{xx}$ of
quantum wells with two subbands, we investigate theoretically here the
magnetotransport properties of these quantumHall systems. We determine
$\rho_{xx}$ via both the Hartree and the KohnSham selfconsistent schemes plus
the Kubo formula. While the Hartree calculation yields diamondshaped
structures in the $n_{2D}$\emph{B} diagram, the calculation including
exchange and correlation effects (KohnSham) more closely reproduces the
ringlike structures in the experiments.
 
7.  Preface: Proceedings of the 20th Latin American Symposium on Solid State Physics (SLAFES XX) held in March 2011 in Maragogi/AL, Brazil. 
Poliana H. Penteado, Gerson J. Ferreira, Guilheme M. Sipahi, and J. Carlos Egues. J. Supercond. Novel Magnetism (online first)  
8.  Preface: Proceedings of the PASPS V Conference Held in August 2008 in Foz do Iguaçu, Brazil 
Gerson J. Ferreira, Guilherme M. Sipahi, Yara G. Gobato, and J. Carlos Egues. J. Supercond. Novel Magnetism 23, 1 (2010)
From the issue entitled "5th International Conference on Physics and Applications of Spinrelated Phenomena in Semiconductors (PASPS V), Foz do Iguaçu, Brazil, August 36, 2008. Guest Editors: Gerson J. Ferreira, Guilherme M. Sipahi, Yara G. Gobato, and J. Carlos Egues
