1,086 research outputs found
Tuning impurity states in bilayer graphene
We study the impurity states in bilayer graphene in the unitary limit using
Green's function method. Unlike in single layer graphene, the presence of
impurities at two non-equivalent sites in bilayer graphene produce different
impurity states which is understood as the change in the band structure due to
interlayer hopping of electrons. The impurity states can also be tuned by
changing the band structure of bilayer grahene through external electric field
bias.Comment: 7 pages, 9 figures, sumbitted to PR
Spatial dependence of the superexchange interactions for transition-metal trimers in graphene
This study examines the magnetic interactions between spatially-variable
manganese and chromium trimers substituted into a graphene superlattice. Using
density functional theory, we calculate the electronic band structure and
magnetic populations for the determination of the electronic and magnetic
properties of the system. To explore the super-exchange coupling between the
transition-metal atoms, we establish the magnetic magnetic ground states
through a comparison of multiple magnetic and spatial configurations. Through
an analysis of the electronic and magnetic properties, we conclude that the
presence of transition-metal atoms can induce a distinct magnetic moment in the
surrounding carbon atoms as well as produce an RKKY-like super-exchange
coupling. It hoped that these simulations can lead to the realization of
spintronic applications in graphene through electronic control of the magnetic
clusters.Comment: 6 pages, 5 Figur
Casimir effect for the massless Dirac field in two-dimensional Reissner-Nordstr\"{o}m spacetime
In this paper, the two-dimensional Reissner-Nordstr\"{o}m black hole is
considered as a system of the Casimir type. In this background the Casimir
effect for the massless Dirac field is discussed. The massless Dirac field is
confined between two ``parallel plates'' separated by a distance and there
is no particle current drilling through the boundaries. The vacuum expectation
values of the stress tensor of the massless Dirac field at infinity are
calculated separately in the Boulware state, the Hartle-Hawking state and the
Unruh state.Comment: 10 pages, no figure. Accepted for publication in IJMP
Ultrafast Spin-To-Charge Conversion at the Surface of Topological Insulator Thin Films
Strong spin-orbit coupling, resulting in the formation of
spin-momentum-locked surface states, endows topological insulators with
superior spin-to-charge conversion characteristics, though the dynamics that
govern it have remained elusive. Here, we present an all-optical method that
enables unprecedented tracking of the ultrafast dynamics of spin-to-charge
conversion in a prototypical topological insulator BiSe/ferromagnetic
Co heterostructure, down to the sub-picosecond timescale. Compared to pure
BiSe or Co, we observe a giant terahertz emission in the
heterostructure than originates from spin-to-charge conversion, in which the
topological surface states play a crucial role. We identify a 0.12-picosecond
timescale that sets a technological speed limit of spin-to-charge conversion
processes in topological insulators. In addition, we show that the
spin-to-charge conversion efficiency is temperature independent in BiSe
as expected from the nature of the surface states, paving the way for designing
next-generation high-speed opto-spintronic devices based on topological
insulators at room temperature.Comment: 19 pages, 4 figure
Field induced d_x^2-y^2+id_xy state in d-density-wave metals
We argue that the d_{xy} component of the order parameter can be generated to
form the d_x^2-y^2+id_xy-density wave state by the external magnetic field. The
driving force for this transition is the coupling of the magnetic field with
the orbital magnetism. The fully gapped particle spectrum and the magnetically
active collective mode of the condensate are discussed as a possible signature
of the d+id' density wave state.Comment: 5 pages, 2 color figure
In-plane Tunneling Spectrum into a [110]-Oriented High- Superconductor in the Pseudogap Regime
Both the differential tunneling conductance and the surface local density of
states (LDOS) of a [110]-oriented high-temperature superconductor in the
pseudogap (PG) regime are studied theoretically. As a competing candidate for
the mechanism of PG state, the charge-density wave (CDW), spin-density wave
(SDW), -density wave (DDW), and d-wave superconducting (DSC) orderings show
distinct features in the tunneling conductance. For the CDW, SDW, and DSC
orderings, the tunneling conductance approaches the surface LDOS as the barrier
potential is increased. For the DDW ordering, we show for the first time that
there exist midgap states at the [110] surface, manifesting themselves as a
sharp zero-energy peak in the LDOS, as in the case of DSC ordering. However,
due to the particle-hole pair nature of the DDW state, these states do not
carry current, and consequently the one-to-one correspondence between the
tunneling conductance and the surface LDOS is absent.Comment: 5 pages, 4 figures embedded in the tex
Theory of Magnetic Field Induced Spin Density Wave in High Temperature Superconductors
The induction of spin density wave (SDW) and charge density wave (CDW)
orderings in the mixed state of high superconductors (HTS) is
investigated by using the self-consistent Bogoliubov-de Gennes equations based
upon an effective model Hamiltonian with competing SDW and d-wave
superconductivity interactions. For optimized doping sample, the modulation of
the induced SDW and its associated CDW is determined by the vortex lattice and
their patterns obey the four-fold symmetry. By deceasing doping level, both SDW
and CDW show quasi-one dimensional like behavior, and the CDW has a period just
half that of the SDW along one direction. From the calculation of the local
density of states (LDOS), we found that the majority of the quasi-particles
inside the vortex core are localized. All these results are consistent with
several recent experiments on HTS
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