317 research outputs found
STM imaging of electronic waves on the surface of BiTe: topologically protected surface states and hexagonal warping effects
Scanning tunneling spectroscopy studies on high-quality BiTe crystals
exhibit perfect correspondence to ARPES data, hence enabling identification of
different regimes measured in the local density of states (LDOS). Oscillations
of LDOS near a step are analyzed. Within the main part of the surface band
oscillations are strongly damped, supporting the hypothesis of topological
protection. At higher energies, as the surface band becomes concave,
oscillations appear which disperse with a particular wave-vector that may
result from an unconventional hexagonal warping term.Comment: 4 pages, 4 figures. Revised manuscript with improved analysis and
figure
Direct Optical Coupling to an Unoccupied Dirac Surface State in the Topological Insulator BiSe
We characterize the occupied and unoccupied electronic structure of the
topological insulator BiSe by one-photon and two-photon angle-resolved
photoemission spectroscopy and slab band structure calculations. We reveal a
second, unoccupied Dirac surface state with similar electronic structure and
physical origin to the well-known topological surface state. This state is
energetically located 1.5 eV above the conduction band, which permits it to be
directly excited by the output of a Ti:Sapphire laser. This discovery
demonstrates the feasibility of direct ultrafast optical coupling to a
topologically protected, spin-textured surface state.Comment: Accepted to Physical Review Letter
Cosmological Constant and Axions in String Theory
String theory axions appear to be promising candidates for explaining
cosmological constant via quintessence. In this paper, we study conditions on
the string compactifications under which axion quintessence can happen. For
sufficiently large number of axions, cosmological constant can be accounted for
as the potential energy of axions that have not yet relaxed to their minima. In
compactifications that incorporate unified models of particle physics, the
height of the axion potential can naturally fall close to the observed value of
cosmological constant.Comment: 22 page
Singularities and Closed String Tachyons
A basic problem in gravitational physics is the resolution of spacetime
singularities where general relativity breaks down. The simplest such
singularities are conical singularities arising from orbifold identifications
of flat space, and the most challenging are spacelike singularities inside
black holes (and in cosmology). Topology changing processes also require
evolution through classically singular spacetimes. I briefly review how a phase
of closed string tachyon condensate replaces, and helps to resolve, basic
singularities of each of these types. Finally I discuss some interesting
features of singularities arising in the small volume limit of compact
negatively curved spaces and the emerging zoology of spacelike singularities.Comment: 8 pages latex, based on comments at Solvay meetin
Reheating Metastable O'Raifeartaigh Models
In theories with multiple vacua, reheating to a temperature greater than the
height of a barrier can stimulate transitions from a desirable metastable
vacuum to a lower energy state. We discuss the constraints this places on
various theories and demonstrate that in a class of supersymmetric models this
transition does not occur even for arbitrarily high reheating temperature.Comment: 21 pages, 1 figure. Typos corrected and some references adde
Magnetic fields above the surface of a superconductor with internal magnetism
The author presents a method for calculating the magnetic fields near a
planar surface of a superconductor with a given intrinsic magnetization in the
London limit. He computes solutions for various magnetic domain boundary
configurations and derives relations between the spectral densities of the
magnetization and the resulting field in the vacuum half space, which are
useful if the magnetization can be considered as a statistical quantity and its
features are too small to be resolved individually. The results are useful for
analyzing and designing magnetic scanning experiments. Application to existing
data from such experiments on SrRuO show that a domain wall would have
been detectable, but the magnetic field of randomly oriented small domains and
small defects may have been smaller than the experimental noise level.Comment: 8 pages, 2 figures. Journal version. Added one figure, some
discussion. A few typos correcte
It's On: Early Interpretations of ATLAS Results in Jets and Missing Energy Searches
The first search for supersymmetry from ATLAS with 70/nb of integrated
luminosity extends the Tevatron' s reach for colored particles that decay into
jets plus missing transverse energy. For gluinos that decay directly or through
a one step cascade into the LSP and two jets, the mass range m_g < 205 GeV is
disfavored by the ATLAS searches, regardless of the mass of the LSP. In some
cases the coverage extends up to m_g ~ 295 GeV, already surpassing the
Tevatron's reach for compressed supersymmetry spectra.Comment: 5 pages, 3 figures, 1 table; references and figure added; Physics
Letters B (2011
Discovering New Light States at Neutrino Experiments
Experiments designed to measure neutrino oscillations also provide major
opportunities for discovering very weakly coupled states. In order to produce
neutrinos, experiments such as LSND collide thousands of Coulombs of protons
into fixed targets, while MINOS and MiniBooNE also focus and then dump beams of
muons. The neutrino detectors beyond these beam dumps are therefore an
excellent arena in which to look for long-lived pseudoscalars or for vector
bosons that kinetically mix with the photon. We show that these experiments
have significant sensitivity beyond previous beam dumps, and are able to
partially close the gap between laboratory experiments and supernovae
constraints on pseudoscalars. Future upgrades to the NuMI beamline and Project
X will lead to even greater opportunities for discovery. We also discuss thin
target experiments with muon beams, such as those available in COMPASS, and
show that they constitute a powerful probe for leptophilic PNGBs.Comment: 32 text pages, 5 figures, 4 table
A Stringy Test of Flux-Induced Isometry Gauging
Supergravity analysis suggests that the effect of fluxes in string theory
compactifications is to gauge isometries of the scalar manifold. However,
isometries are generically broken by brane instanton effects. Here we
demonstrate how fluxes protect exactly those isometries from quantum
corrections which are gauged according to the classical supergravity analysis.
We also argue that all other isometries are generically broken.Comment: 14 pages, 1 figure; v2: references adde
Correlation tuned cross-over between thermal and nonthermal states following ultrafast transient pumping
We examine electron-electron mediated relaxation following excitation of a
correlated system by an ultrafast electric field pump pulse. The results reveal
a dichotomy in the temporal evolution as one tunes through a Mott
metal-to-insulator transition: in the metallic regime relaxation can be
characterized by evolution toward a steady-state electronic distribution well
described by Fermi-Dirac statistics with an increased effective temperature;
however, in the insulating regime this quasithermal paradigm breaks down with
relaxation toward a nonthermal state with a more complicated electronic
distribution that does not vary monotonically as a function of energy. We
characterize the behavior by studying changes in the energy, photoemission
response, and electronic distribution as functions of time. Qualitatively these
results should be observable on short enough time scales that the electrons
behave like an isolated system not in contact with additional degrees of
freedom which can act as a thermal bath. Importantly, proper modeling used to
analyze experimental findings should account for this behavior, especially when
using strong driving fields or studying materials whose physics may manifest
the effects of strong correlations.Comment: Main Text: 5 pages, 4 figures; Supplementary Material: 3 pages, 5
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