arXiv subjects

Carmine Ortix

Publications and source records attributed to Carmine Ortix.

At least 91 records · Page 5Linked to original sources

arXiv2015

Edge states and topological insulating phases generated by curving a nanowire with Rashba spin-orbit coupling

We prove that curvature effects in low-dimensional nanomaterials can promote the generation of topological states of matter by considering the paradigmatic example of quantum wires with Rashba spin-orbit coupling, which are periodically corrugated at the nanometer scale. The effect of the periodic curvature generally results in the appearance of insulating phases with a corresponding novel butterfly spectrum characterized by the formation of fine measure complex regions of forbidden energies. When the Fermi energy lies in the gaps, the system displays localized end states protected by topology. We further show that for certain corrugation periods the system possesses topologically non-trivial insulating phases at half-filling. Our results suggest that the local curvature and the topology of the electronic states are inextricably intertwined in geometrically deformed nanomaterials.

cond-mat.mes-hall↗

arXiv2015

Quantum mechanics of a spin-orbit coupled electron constrained to a space curve

We derive the effective one-dimensional Schrodinger-Pauli equation for electrons constrained to move on a space curve. The electrons are confined using a double thin-wall quantization procedure with adiabatic separation of fast and slow quantum degrees of freedom. This procedure is capable of yielding a correct Hermitian one-dimensional Schrodinger-Pauli operator. We find that the torsion of the space curve generates an additional quantum geometric potential, adding to the well-known curvature-induced one. Finally, we derive an analytic form of the one-dimensional Hamiltonian for spin-orbit coupled electrons in a nanoscale helical wire.

cond-mat.mes-hall↗

arXiv2015

Energy-tunable sources of entangled photons: a viable concept for solid-state-based quantum relays

We propose a new method of generating triggered entangled photon pairs with wavelength on demand. The method uses a micro-structured semiconductor-piezoelectric device capable of dynamically reshaping the electronic properties of self-assembled quantum dots (QDs) via anisotropic strain-engineering. Theoretical models based on kp theory in combination with finite-element calculations show that the energy of the polarization-entangled photons emitted by QDs can be tuned in a range larger than 100 meV without affecting the degree of entanglement of the quantum source. These results pave the way towards the deterministic implementation of QD entanglement resources in all-electrically-controlled solid-state-based quantum relays.

cond-mat.mtrl-sci↗

arXiv2015

Topological classification of k.p Hamiltonians for Chern insulators

We proof the existence of two different topological classes of low-energy k.p Hamiltonians for Chern insulators. Using the paradigmatic example of single-valley two-band models, we show that k.p Hamiltonians that we dub local have a topological invariant corresponding precisely to the Hall conductivity and linearly dispersing chiral midgap edge states at the expansion point. Non-local k.p Hamiltonians have a topological invariant that is twice the Hall conductivity of the system. This class is characterized by a non-local bulk-edge correspondence with midgap edge states appearing away from the high-symmetry k.p expansion point.

cond-mat.mes-hall↗

arXiv2015

One-dimensional Dirac electrons on the surface of weak topological insulators

We show that a class of weak three-dimensional topological insulators feature one-dimensional Dirac electrons on their surfaces. Their hallmark is a line-like energy dispersion along certain directions of the surface Brillouin zone. These one-dimensional Dirac line degeneracies are topologically protected by a symmetry that we refer to as in-plane time-reversal invariance. We show how this invariance leads to Dirac lines in the surface spectrum of stacked Kane-Mele systems and more general models for weak three-dimensional topological insulators.

cond-mat.mes-hall↗

arXiv2014

Kekule' textures, pseudo-spin one Dirac cones and quadratic band crossings in a graphene-hexagonal indium chalcogenide bilayer

Using density-functional theory, we calculate the electronic bandstructure of single-layer graphene on top of hexagonal In_2Te_2 monolayers. The geometric configuration with In and Te atoms at centers of carbon hexagons leads to a Kekule' texture with an ensuing bandgap of 20 meV. The alternative structure, nearly degenerate in energy, with the In and Te atoms on top of carbon sites is characterized instead by gapless spectrum with the original Dirac cones of graphene reshaped, depending on the graphene-indium chalcogenide distance, either in the form of an undoubled pseudo-spin one Dirac cone or in a quadratic band crossing point at the Fermi level. These electronic phases harbor charge fractionalization and topological Mott insulating states of matter.

cond-mat.mtrl-sci↗

arXiv2014

Strongly anisotropic ballistic magnetoresistance in compact three-dimensional semiconducting nanoarchitectures

We establish theoretically that in nonmagnetic semiconducting bilayer or multilayer thin film systems rolled up into compact quasi-one-dimensional nanoarchitectures, the ballistic magnetoresistance is very anisotropic: conductances depend strongly on the direction of an externally applied magnetic field. This phenomenon originates from the curved open geometry of rolled-up nanotubes, which leads to a tunability of the number of quasi-one-dimensional magnetic subbands crossing the Fermi energy. The experimental significance of this phenomenon is illustrated by a sizable anisotropy that scales with the inverse of the winding number, and persists up to a critical temperature that can be strongly enhanced by increasing the strength of the external magnetic field or the characteristic radius of curvature, and can reach room temperature.

cond-mat.mes-hall↗

arXiv2014

Self-organized electronic superlattices in layered materials

We show that in layered systems with electronic phase separation tendency, the long-range Coulomb interaction can drive the spontaneous formation of unidirectional superlattices of electronic charge in a completely homogeneous crystalline background. In this self-organized electronic heterostructure, the ratio among the number of crystalline planes in the minority and majority electronic phases corresponds to Farey fractions with the superlattice period controlled by the background charge density and the frustrating Coulomb interaction strength. The phase diagram displays Arnold tongues obeying a modified Farey tree hierarchy and a devil's staircase typical of systems with frustration among different scales. We further discuss the competition of these electronic superlattices, recently observed in iron-based superconductors and mixed valence compounds, with in-plane electronically modulated phases.

cond-mat.str-el↗

arXiv2013

Curvature-induced Rashba spin-orbit interaction in strain-driven nanostructures

We derive the effective dimensionally reduced Schrödinger equation with spin-orbit interaction in low-dimensional electronic strain driven nanostructures. A method of adiabatic separation among fast normal quantum degrees of freedom and slow tangential quantum degrees of freedom is used to show the emergence of a strain-induced Rashba-like spin-orbit interaction (SOI). By applying this analysis to one-dimensional curved quantum wires we demonstrate that the curvature-induced Rashba SOI leads to enhanced spin-orbit effects.

cond-mat.mes-hall↗

arXiv2013

Absence of helical surface states in bulk semimetals with broken inversion symmetry

Whereas the concept of topological band-structures was developed originally for insulators with a bulk bandgap, it has become increasingly clear that the prime consequences of a non-trivial topology -- spin-momentum locking of surface states -- can also be encountered in gapless systems. Concentrating on the paradigmatic example of mercury chalcogenides HgX (X = Te, Se, S), we show that the existence of helical semimetals, i.e. semimetals with topological surface states, critically depends on the presence of crystal inversion symmetry. An infinitesimally small broken inversion symmetry (BIS) renders the helical semimetallic state unstable. The BIS is also very important in the fully gapped regime, renormalizing the surface Dirac cones in an anisotropic manner. As a consequence the handedness of the Dirac cones can be flipped by a biaxial stress field.

cond-mat.mes-hall↗

arXiv2013

Stacked topological insulator built from bismuth-based graphene sheet analogues

Commonly materials are classified as either electrical conductors or insulators. The theoretical discovery of topological insulators (TIs) in 2005 has fundamentally challenged this dichotomy. In a TI, spin-orbit interaction generates a non-trivial topology of the electronic band-structure dictating that its bulk is perfectly insulating, while its surface is fully conducting. The first TI candidate material put forward -graphene- is of limited practical use since its weak spin-orbit interactions produce a band-gap of ~0.01K. Recent reinvestigation of Bi2Se3 and Bi2Te3, however, have firmly categorized these materials as strong three-dimensional TI's. We have synthesized the first bulk material belonging to an entirely different, weak, topological class, built from stacks of two-dimensional TI's: Bi14Rh3I9. Its Bi-Rh sheets are graphene analogs, but with a honeycomb net composed of RhBi8-cubes rather than carbon atoms. The strong bismuth-related spin-orbit interaction renders each graphene-like layer a TI with a 2400K band-gap.

cond-mat.mtrl-sci↗

arXiv2013

The fundamental differences between Quantum Spin Hall edge-states at zig-zag and armchair terminations of honeycomb and ruby nets

Combining an analytical and numerical approach we investigate the dispersion of the topologically protected spin-filtered edge-states of the Quantum Spin Hall state on honeycomb and ruby nets with zigzag (ZZ) and armchair (AC) edges. We show that the Fermi velocity of the helical edge-states on ZZ edges increases linearly with the strength of the spin-orbit coupling (SOC) whereas for AC edges the Fermi velocity is independent of the SOC. Also the decay length of edge states into the bulk is dramatically different for AC and ZZ edges, displaying an inverse functional dependence on the SOC.

cond-mat.mes-hall↗

arXiv2012

Quantum collapses and revivals of a matter wave in the dynamics of symmetry breaking

Using the remarkable mathematical construct of Eugene Wigner to visualize quantum trajectories in phase space, quantum processes can be described in terms of a quasi-probability distribution analogous to the phase space probability distribution of the classical realm. In contrast to the incomplete glimpse of the wave function that is achievable in a single shot experiment, the Wigner distribution, accessible by quantum state tomography, reflects the full quantum state. We show that during the fundamental symmetry-breaking process of a generic quantum system - with a symmetry breaking field driving the quantum system far from equilibrium - the Wigner distribution evolves continuously with the system undergoing a sequence of revivals into the symmetry unbroken state, followed by collapses onto a quasi-classical state akin the one realised in infinite size systems. We show that generically this state is completely delocalised both in momentum and in real space.

quant-ph↗

arXiv2012

Microscopic origin of large negative magneto-electric coupling in Sr$_{1/2}$Ba$_{1/2}$MnO$_{3}$

With a combined ab initio density functional and model Hamiltonian approach we establish that in the recently discovered multiferroic phase of the manganite Sr$_{1/2}$Ba$_{1/2}$MnO$_{3}$ the polar distortion of Mn and O ions is stabilized via enhanced in-plane Mn-O hybridizations. The magnetic superexchange interaction is very sensitive to the polar bond-bending distortion, and we find that this dependence directly causes a strong magnetoelectric coupling. This novel mechanism for multiferroicity is consistent with the experimentally observed reduced ferroelectric polarization upon the onset of magnetic ordering.

cond-mat.mtrl-sci↗

arXiv2011

Graphene on incommensurate substrates: trigonal warping and emerging Dirac cone replicas with halved group velocity

The adhesion of graphene on slightly lattice-mismatched surfaces, for instance of hexagonal boron nitride (hBN) or Ir(111), gives rise to a complex landscape of sublattice symmetry-breaking potentials for the Dirac fermions. Whereas a gap at the Dirac point opens for perfectly lattice-matched graphene on hBN, we show that the small lattice incommensurability prevents the opening of this gap and rather leads to a renormalized Dirac dispersion with a trigonal warping. This warping breaks the effective time reversal symmetry in a single valley. On top of this a new set of massless Dirac fermions is generated, which are characterized by a group velocity that is half the one of pristine graphene.

cond-mat.mtrl-sci↗

arXiv2011

Defect Formation Preempts Dynamical Symmetry Breaking in Closed Quantum Systems

We show that no matter how slowly a quantum-to-classical symmetry breaking process is driven, the adiabatic limit can never be reached in a macroscopic body. Massive defect formation preempts an adiabatic quantum-classical crossover and triggers the appearance of a symmetric non-equilibrium state that recursively collapses into the classical state, breaking the symmetry at punctured times. The presence of this state allows the quantum-classical transition to be investigated and controlled in mesoscopic devices by supplying externally the proper dynamical symmetry breaking perturbation.

quant-ph↗

arXiv2010

Absence of anomalous interactions in the quantum theory of constrained charged particles in presence of electrical currents

The experimental progress in synthesizing low-dimensional nanostructures where carriers are confined to bent surfaces has boosted the interest in the theory of quantum mechanics on curved two-dimensional manifolds. It was recently asserted that constrained electrically charged particles couple to a term linear in A_3 M, where A_3 is the transversal component of the electromagnetic vector potential and M the surface mean curvature, thereby making a dimensional reduction procedure impracticable in the presence of fields. Here we resolve this apparent paradox by providing a consistent general framework of the thin-wall quantization procedure. We also show that the separability of the equation of motions is not endangered by the particular choice of the constraint imposed on the transversal fluctuations of the wavefunction, which renders the thin-wall quantization procedure well-founded. It can be applied without restrictions.

cond-mat.mes-hall↗

arXiv2010

Effect of Curvature on the Electronic Structure and Bound State Formation in Rolled-up Nanotubes

We analyze the electronic properties of a two-dimensional electron gas rolled-up into a nanotube by both numerical and analytical techniques. The nature and the energy dispersion of the electronic quantum states strongly depend upon the geometric parameters of the nanotube: the typical radius of curvature and the number of windings. The effect of the curvature results in the appearance of atomic-like bound states localized near the points of maximum curvature. For a two-dimensional sheet rolled up into an Archimedean spiral we find that the number of bound states is equal to the number of windings of the spiral.

cond-mat.mes-hall↗