6,974 research outputs found
Unconventional scanning tunneling conductance spectra for graphene
We compute the tunneling conductance of graphene as measured by a scanning
tunneling microscope (STM) with a normal/superconducting tip. We demonstrate
that for undoped graphene with zero Fermi energy, the first derivative of the
tunneling conductance with respect to the applied voltage is proportional to
the density of states of the STM tip. We also show that the shape of the STM
spectra for graphene doped with impurities depends qualitatively on the
position of the impurity atom in the graphene matrix and relate this
unconventional phenomenon to the pseudopsin symmetry of the Dirac
quasiparticles in graphene. We suggest experiments to test our theory.Comment: 6 pages, 3 figure
Magnetotransport of Dirac Fermions on the surface of a topological insulator
We study the properties of Dirac fermions on the surface of a topological
insulator in the presence of crossed electric and magnetic fields. We provide
an exact solution to this problem and demonstrate that, in contrast to their
counterparts in graphene, these Dirac fermions allow relative tuning of the
orbital and Zeeman effects of an applied magnetic field by a crossed electric
field along the surface. We also elaborate and extend our earlier results on
normal metal-magnetic film-normal metal (NMN) and normal metal-barrier-magnetic
film (NBM) junctions of topological insulators [Phys. Rev. Lett. {\bf 104},
046403 (2010)]. For NMN junctions, we show that for Dirac fermions with Fermi
velocity , the transport can be controlled using the exchange field
of a ferromagnetic film over a region of width . The
conductance of such a junction changes from oscillatory to a monotonically
decreasing function of beyond a critical which leads to the
possible realization of magnetic switches using these junctions. For NBM
junctions with a potential barrier of width and potential , we find
that beyond a critical , the criteria of conductance maxima
changes from to for
integer . Finally, we compute the subgap tunneling conductance of a normal
metal-magnetic film-superconductor (NMS) junctions on the surface of a
topological insulator and show that the position of the peaks of the zero-bias
tunneling conductance can be tuned using the magnetization of the ferromagnetic
film. We point out that these phenomena have no analogs in either conventional
two-dimensional materials or Dirac electrons in graphene and suggest
experiments to test our theory.Comment: 11 pages, 12 figures; v
Tuning the conductance of Dirac fermions on the surface of a topological insulator
We study the transport properties of the Dirac fermions with Fermi velocity
on the surface of a topological insulator across a ferromagnetic strip
providing an exchange field over a region of width . We show
that the conductance of such a junction changes from oscillatory to a
monotonically decreasing function of beyond a critical . This
leads to the possible realization of a magnetic switch using these junctions.
We also study the conductance of these Dirac fermions across a potential
barrier of width and potential in the presence of such a
ferromagnetic strip and show that beyond a critical , the
criteria of conductance maxima changes from
to for integer . We point out that these novel phenomena
have no analogs in graphene and suggest experiments which can probe them.Comment: v1 4 pages 5 fig
Topological Confinement and Superconductivity
We derive a Kondo Lattice model with a correlated conduction band from a
two-band Hubbard Hamiltonian. This mapping allows us to describe the emergence
of a robust pairing mechanism in a model that only contains repulsive
interactions. The mechanism is due to topological confinement and results from
the interplay between antiferromagnetism and delocalization. By using
Density-Matrix-Renormalization-Group (DMRG), we demonstrate that this mechanism
leads to dominant superconducting correlations in a 1D-system.Comment: 4 pages, 4 figure
Langevin Dynamics simulations of a 2-dimensional colloidal crystal under confinement and shear
Langevin Dynamics simulations are used to study the effect of shear on a
two-dimensional colloidal crystal confined by structured parallel walls. When
walls are sheared very slowly, only two or three crystalline layers next to the
walls move along with them, while the inner layers of the crystal are only
slightly tilted. At higher shear velocities, this inner part of the crystal
breaks into several pieces with different orientations. The velocity profile
across the slit is reminiscent of shear-banding in flowing soft materials,
where liquid and solid regions coexist; the difference, however, is that in the
latter case the solid regions are glassy while here they are crystalline. At
even higher shear velocities, the effect of the shearing becomes smaller again.
Also the effective temperature near the walls (deduced from the velocity
distributions of the particles) decreases again when the wall velocity gets
very large. When the walls are placed closer together, thereby introducing a
misfit, a structure containing a soliton staircase arises in simulations
without shear. Introducing shear increases the disorder in these systems until
no solitons are visible any more. Instead, similar structures like in the case
without misfit result. At high shear rates, configurations where the
incommensurability of the crystalline structure is compensated by the creation
of holes become relevant
Oscillator Strengths of the Allowed no-n'd and nd-n'f Transitions in the Helium Isoelectronic Sequence
Soft Breakdown of Zener Diode
Zener diodes are found to show breakdown properties at much lower bias voltages compared to the so called Zener breakdown voltages. The soft breakdown becomes conspicuous from the occurrence of a point of inflexion near the origin of the I-V characteristics. The phenomenon has been explained by interband tunneling of carriers taking place under small reverse bias voltages
Heavy Fermion superconductor CeCuSi under high pressure: multiprobing the valence crossover
The first heavy fermion superconductor CeCuSi has not revealed all
its striking mysteries yet. At high pressures, superconductivity is supposed to
be mediated by valence fluctuations, in contrast to ambient pressure, where
spin fluctuations most likely act as pairing glue. We have carried out a
multiprobe (electric transport, thermopower, ac specific heat, Hall and Nernst
effects) experiment up to on a high quality CeCuSi
single crystal. Reliable resistivity data reveal for the first time a scaling
behavior close to the supposed valence transition, and allow to locate the
critical end point at and a slightly negative
temperature. In the same pressure region, remarkable features have also been
detected in the other physical properties, acting as further signatures of the
Ce valence crossover and the associated critical fluctuations.Comment: 13 pages, 14 figure
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