1,547 research outputs found
Origin of electron-hole asymmetry in the scanning tunneling spectrum of
We have developed a material specific theoretical framework for modelling
scanning tunneling spectroscopy (STS) of high temperature superconducting
materials in the normal as well as the superconducting state. Results for
(Bi2212) show clearly that the tunneling process
strongly modifies the STS spectrum from the local density of states (LDOS) of
the orbital of Cu. The dominant tunneling channel to the surface
Bi involves the orbitals of the four neighbouring Cu atoms. In
accord with experimental observations, the computed spectrum displays a
remarkable asymmetry between the processes of electron injection and
extraction, which arises from contributions of Cu and other orbitals
to the tunneling current.Comment: 5 pages, 4 figures, published in PR
Pinned Balseiro-Falicov Model of Tunneling and Photoemission in the Cuprates
The smooth evolution of the tunneling gap of Bi_2Sr_2CaCu_2O_8 with doping
from a pseudogap state in the underdoped cuprates to a superconducting state at
optimal and overdoping, has been interpreted as evidence that the pseudogap
must be due to precursor pairing. We suggest an alternative explanation, that
the smoothness reflects a hidden SO(N) symmetry near the (pi,0) points of the
Brillouin zone (with N = 3, 4, 5, or 6). Because of this symmetry, the
pseudogap could actually be due to any of a number of nesting instabilities,
including charge or spin density waves or more exotic phases. We present a
detailed analysis of this competition for one particular model: the pinned
Balseiro-Falicov model of competing charge density wave and (s-wave)
superconductivity. We show that most of the anomalous features of both
tunneling and photoemission follow naturally from the model, including the
smooth crossover, the general shape of the pseudogap phase diagram, the
shrinking Fermi surface of the pseudogap phase, and the asymmetry of the
tunneling gap away from optimal doping. Below T_c, the sharp peak at Delta_1
and the dip seen in the tunneling and photoemission near 2Delta_1 cannot be
described in detail by this model, but we suggest a simple generalization to
account for inhomogeneity, which does provide an adequate description. We show
that it should be possible, with a combination of photoemission and tunneling,
to demonstrate the extent of pinning of the Fermi level to the Van Hove
singularity. A preliminary analysis of the data suggests pinning in the
underdoped, but not in the overdoped regime.Comment: 18 pages LaTeX, 26 ps. figure
Theory of non-Fermi liquid and pairing in electron-doped cuprates
We apply the spin-fermion model to study the normal state and pairing
instability in electron-doped cuprates near the antiferromagnetic QCP. Peculiar
frequency dependencies of the normal state properties are shown to emerge from
the self-consistent equations on the fermionic and bosonic self-energies, and
are in agreement with experimentally observed ones. We argue that the pairing
instability is in the channel, as in hole-doped cuprates, but
theoretical is much lower than in the hole-doped case. For the same
hopping integrals and the interaction strength as in hole-doped materials, we
obtain K at the end point of the antiferromagnetic phase. We argue
that a strong reduction of in electron-doped cuprates compared to
hole-doped ones is due to critical role of the Fermi surface curvature for
electron-doped materials. The -pairing gap
is strongly non-monotonic along the Fermi surface.
The position of the gap maxima, however, does not coincide with the hot spots,
as the non-monotonic gap persists even at doping when the hot
spots merge on the Brillouin zone diagonals.Comment: 16 page
Closed-form expressions for particle relative velocities induced by turbulence
In this note we present complete, closed-form expressions for random relative
velocities between colliding particles of arbitrary size in nebula turbulence.
These results are exact for very small particles (those with stopping times
much shorter than the large eddy overturn time) and are also surprisingly
accurate in complete generality (that is, also apply for particles with
stopping times comparable to, or much longer than, the large eddy overturn
time). We note that some previous studies may have adopted previous simple
expressions, which we find to be in error regarding the size dependence in the
large particle regime.Comment: 8 pages, accepted as Research Note by A&
Limb imaging of the Venus O2 visible nightglow with the Venus Monitoring Camera
We investigated the Venus O2 visible nightglow with imagery from the Venus
Monitoring Camera on Venus Express. Drawing from data collected between April
2007 and January 2011, we study the global distribution of this emission,
discovered in the late 70s by the Venera 9 and 10 missions. The inferred
limb-viewing intensities are on the order of 150 kiloRayleighs at the lower
latitudes and seem to drop somewhat towards the poles. The emission is
generally stable, although there are episodes when the intensities rise up to
500 kR. We compare a set of Venus Monitoring Camera observations with
coincident measurements of the O2 nightglow at 1.27 {\mu}m made with the
Visible and Infrared Thermal Imaging Spectrometer, also on Venus Express. From
the evidence gathered in this and past works, we suggest a direct correlation
between the instantaneous emissions from the two O2 nightglow systems. Possible
implications regarding the uncertain origin of the atomic oxygen green line at
557.7 nm are noted.Comment: 7 pages, 3 figure
Perfect State Transfer without State Initialization and Remote Collaboration
We present a perfect state transfer protocol via a qubit chain with the
evolution governed by the Hamiltonian. In contrast to the recent protocol
announced in [Phys. Rev. Lett. {\bf 101}, 230502 (2008)], our method does not
demand any remote-cooperated initialization and sending classical information
about measurement outcomes. We achieve the perfect state transfer only with the
assumption of access to two spins at each end of the chain, while the initial
state of the whole chain is irrelevant
Strong Enhancement of Superconducting Correlation in a Two-Component Fermion Gas
We study high-density electron-hole (e-h) systems with the electron density
slightly larger than the hole density. We find a new superconducting phase, in
which the excess electrons form Cooper pairs moving in an e-h BCS phase. The
coexistence of the e-h and e-e orders is possible because e and h have opposite
charges, whereas analogous phases are impossible in the case of two fermion
species that have the same charge or are neutral. Most strikingly, the e-h
order enhances the superconducting e-h order parameter by more than one order
of magnitude as compared with that given by the BCS formula, for the same value
of the effective e-e attractive potential \lambda^{ee}. This new phase should
be observable in an e-h system created by photoexcitation in doped
semiconductors at low temperatures.Comment: 5 pages including 5 PostScript figure
Helicopter tail rotor thrust and main rotor wake coupling in crosswind flight
The tail rotor of a helicopter with a single main rotor configuration can experience a significant reduction in thrust when the aircraft operates in crosswind flight. Brown’s vorticity transport model has been used to simulate a main rotor and tail rotor system translating at a sideslip angle that causes the tail rotor to interact with the main rotor tip vortices as they propagate downstream at the lateral extremities of the wake. The tail rotor is shown to exhibit a distinct directionally dependent mode during which tail rotors that are configured so that the blades travel forward at the top of the disk develop less thrust than tail rotors with the reverse sense of rotation. The range of flight speeds over which this mode exists is shown to vary considerably with the vertical location of the tail rotor. At low flight speeds, the directionally dependent mode occurs because the tail rotor is immersed within not only the downwash from the main rotor but also the rotational flow associated with clusters of largely disorganized vorticity within the main rotor wake. At higher flight speeds, however, the tail rotor is immersed within a coherent supervortex that strongly influences the velocity field surrounding the tail rotor
Stripes, Pseudogaps, and Van Hove Nesting in the Three-band tJ Model
Slave boson calculations have been carried out in the three-band tJ model for
the high-T_c cuprates, with the inclusion of coupling to oxygen breathing mode
phonons. Phonon-induced Van Hove nesting leads to a phase separation between a
hole-doped domain and a (magnetic) domain near half filling, with long-range
Coulomb forces limiting the separation to a nanoscopic scale. Strong
correlation effects pin the Fermi level close to, but not precisely at the Van
Hove singularity (VHS), which can enhance the tendency to phase separation. The
resulting dispersions have been calculated, both in the uniform phases and in
the phase separated regime. In the latter case, distinctly different
dispersions are found for large, random domains and for regular (static)
striped arrays, and a hypothetical form is presented for dynamic striped
arrays. The doping dependence of the latter is found to provide an excellent
description of photoemission and thermodynamic experiments on pseudogap
formation in underdoped cuprates. In particular, the multiplicity of observed
gaps is explained as a combination of flux phase plus charge density wave (CDW)
gaps along with a superconducting gap. The largest gap is associated with VHS
nesting. The apparent smooth evolution of this gap with doping masks a
crossover from CDW-like effects near optimal doping to magnetic effects (flux
phase) near half filling. A crossover from large Fermi surface to hole pockets
with increased underdoping is found. In the weakly overdoped regime, the CDW
undergoes a quantum phase transition (), which could be obscured
by phase separation.Comment: 15 pages, Latex, 18 PS figures Corrects a sign error: major changes,
esp. in Sect. 3, Figs 1-4,6 replace
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