László Oroszlány

Adsorbate-Limited Conductivity of Graphene

We present a theory of electronic transport in graphene in the presence of randomly placed adsorbates. Our analysis predicts a marked asymmetry of the conductivity about the Dirac point, as well as a negative weak-localization magnetoresistivity. In the region of strong scattering, renormalization group corrections drive the system further towards insulating behavior. These results explain key features of recent experiments, and are validated by numerical transport computations.

Theory of snake states in graphene

We study the dynamics of the electrons in a non-uniform magnetic field applied perpendicular to a graphene sheet in the low energy limit when the excitation states can be described by a Dirac type Hamiltonian. We show that as compared to the two-dimensional electron gas (2DEG) snake states in graphene exibit peculiar properties related to the underlying dynamics of the Dirac fermions. The current carried by snake states is locally uncompensated even if the Fermi energy lies between the first non-zero energy Landau levels of the conduction and valence bands. The nature of these states is studied by calculating the current density distribution. It is shown that besides the snake states in finite samples surface states also exist.

Bound states in inhomogeneous magnetic field in graphene: Semiclassical approach

We derive semiclassical quantization equations for graphene monolayer and bilayer systems where the excitations are confined by the applied inhomogeneous magnetic field. The importance of a semiclassical phase, a consequence of the spinor nature of the excitations, is pointed out. The semiclassical eigenenergies show good agreement with the results of quantum-mechanical calculations based on the Dirac equation of graphene and with numerical tight-binding calculations.

Nonthermal broadening in the conductance of double quantum dot structures

We study the transport properties of a double quantum dot (DQD) molecule at zero and at finite temperature. The properties of the zero-temperature conductance depend on whether the level attraction between the symmetric and antisymmetric states of the DQD, produced by the coupling to the leads, exceeds or not the interdot tunneling. For finite temperature, we find a remarkable nonthermal broadening effect of the conductance resonance when the energy levels of the individual dots are detuned.

Gap Generation in Topological Insulator Surface States by non-Ferromagnetic Magnets

It is shown that, contrary to the naive expectation, single particle spectral gaps can be opened on the surface states of three dimensional topological insulators by using commensurate out- and in-plane antiferromagnetic or ferrimagnetic insulating thin films.

Fast and slow edges in bilayer graphene nanoribbons: Tuning the transition from band to Mott insulator

We show that gated bilayer graphene zigzag ribbons possess a fast and a slow edge, characterized by edge-state velocities that differ due to non-negligible next-nearest-neighbor hopping elements. By applying bosonization and renormalization group methods, we find that the slow edge can acquire a sizable interaction-induced gap, which is tunable via an external gate voltage Vg. In contrast to the gate-induced gap in the bulk, the interaction-induced gap depends nonmonotonously on the on-site potential V.

Carbon Nanotube Archimedes Screws

X iv :0 90 2. 07 53 v2 [ co nd -m at .m es -h al l] 2 7 Ju n 20 09 Carbon nanotube quantum pumps L. Oroszlány1, V. Zólyomi1,2, and C. J. Lambert1 1Physi s Department, Lan aster University, LA1 4YB, Lan aster, United Kingdom and 2Resear h Institute for Solid State Physi s and Opti s of the Hungarian A ademy of S ien es, P. O. B. 49, H-1525, Budapest, Hungary Re ently nanome hani al devi es omposed of a long stationary inner arbon nanotube and a shorter, slowly-rotating outer tube have been fabri ated. In this Letter, we study the possibility of using su h devi es as adiabati quantum pumps. Using the Brouwer formula, we employ a Greens fun tion te hnique to determine the pumped harge from one end of the inner tube to the other, driven by the rotation of a hiral outer nanotube. We show that there is virtually no pumping if the hiral angle of the two nanotubes is the same, but for optimal hiralities the pumped harge an be a signi ant fra tion of a theoreti al upper bound. PACS numbers: 72.10.-d, 73.23.-b Quantum pumps are time-dependent ele tron s atterers, whi h are able to transport ele trons between two external reservoirs. They are adiabati if the frequen y of the pump y le is smaller than the inverse of the hara teristi times ale of the s atterer, namely the Wigner delay time [1℄. Re ent experimental [2, 3℄ and theoreti al [4, 5, 6, 7, 8, 9, 10℄ studies of adiabati quantum pumps have examined the onditions for optimal pumping and the e e ts of noise and dissipation. All of these devi es are based on ele tri al pumping. In this work, we propose and analyze a novel realization of a me hani ally-driven quantum pump. The signi an e of me hani ally-driven quantum pumps lies in their ability to onvert me hani al energy to ele tri al energy, whi h ould be used for energy s avenging, via the onversion of ambient vibrational energy to ele tri al energy (see for example Ref. [11℄). The pumped urrent ould be used to power or ontrol nanos ale ele troni devi es, making it a useful omponent in NEMS devi es. As it will be shown below, the proposed nanomehani al pump an operate at 30-40% of the theoreti al upper limit, whi h makes it highly attra tive as an energy s avenger. Our analysis was stimulated by re ent experiments [12, 13℄, whi h demonstrate that it is possible to engineer a double-walled arbon nanotube in su h a way that the inner tube is xed, and the outer is aused to rotate around it by an external for e. In this paper we demonstrate that su h a devi e an also be used as a quantum pump. The basi idea is that if the two nanotubes have di erent hirality, the rotation of one of the tubes will produ e a time-dependent potential that indu es ele tron ow in the other. Su h ow is learly allowed by symmetry, but the question of whether or not the pumped harge is signi ant must be answered by a quantitative al ulations based on a realisti Hamiltonian. In what follows the results of su h a al ulation are presented. We al ulate the adiabati ally-pumped harge in the double-walled, arbon-nanotube, shuttle geometry shown in Figure 1, whi h mimi ks the experimental setup of Ref. L

Exfoliation of single layer BiTeI flakes

Spin orbit interaction can be strongly boosted when a heavy element is embedded into an inversion asymmetric crystal field. A simple structure to realize this concept in a 2D crystal contains three atomic layers, a middle one built up from heavy elements generating strong atomic spin-orbit interaction and two neighboring atomic layers with different electron negativity. BiTeI is a promising candidate for such a 2D crystal, since it contains heavy Bi layer between Te and I layers. Recently the bulk form of BiTeI attracted considerable attention due to its giant Rashba interaction, however, 2D form of this crystal was not yet created. In this work we report the first exfoliation of single layer BiTeI using a recently developed exfoliation technique on stripped gold. Our combined scanning probe studies and first principles calculations show that SL BiTeI flakes with sizes of 100

Modeling ultrafast all-optical switching in synthetic ferrimagnets

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Exchange interactions from a nonorthogonal basis: applications to 3d ferromagnets and graphene based systems.

m were achieved which are stable at ambient conditions. The giant Rashba splitting and spin-momentum locking of this new member of 2D crystals open the way towards novel spintronic applications and synthetic topological heterostructures.