2,490 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
The thermopower as a fingerprint of the Kondo breakdown quantum critical point
We propose that the thermoelectric power distinguishes two competing
scenarios for quantum phase transitions in heavy fermions : the
spin-density-wave (SDW) theory and breakdown of the Kondo effect. In the Kondo
breakdown scenario, the Seebeck coefficient turns out to collapse from the
temperature scale , associated with quantum fluctuations of the Fermi
surface reconfiguration. This feature differs radically from the physics of the
SDW theory, where no reconstruction of the Fermi surface occurs, and can be
considered as the hallmark of the Kondo breakdown theory. We test these ideas,
upon experimental results for YbRhSi
Device for preventing high voltage arcing in electron beam welding Patent
Development of device to prevent high voltage arcing in electron beam weldin
Electron-phonon heat transfer in monolayer and bilayer graphene
We calculate the heat transfer between electrons to acoustic and optical
phonons in monolayer and bilayer graphene (MLG and BLG) within the
quasiequilibrium approximation. For acoustic phonons, we show how the
temperature-power laws of the electron-phonon heat current for BLG differ from
those previously derived for MLG and note that the high-temperature
(neutral-regime) power laws for MLG and BLG are also different, with a weaker
dependence on the electronic temperature in the latter. In the general case we
evaluate the heat current numerically. We suggest that a measurement of the
heat current could be used for an experimental determination of the
electron-acoustic phonon coupling constants, which are not accurately known.
However, in a typical experiment heat dissipation by electrons at very low
temperatures is dominated by diffusion, and we estimate the crossover
temperature at which acoustic-phonon coupling takes over in a sample with Joule
heating. At even higher temperatures optical phonons begin to dominate. We
study some examples of potentially relevant types of optical modes, including
in particular the intrinsic in-plane modes, and additionally the remote surface
phonons of a possible dielectric substrate.Comment: 13 pages, 8 figures; moved details to appendixes, added discussion of
remote phonon
Reconstructing the electron in a fractionalized quantum fluid
The low energy physics of the fractional Hall liquid is described in terms
quasiparticles that are qualitatively distinct from electrons. We show,
however, that a long-lived electron-like quasiparticle also exists in the
excitation spectrum: the state obtained by the application of an electron
creation operator to a fractional quantum Hall ground state has a non-zero
overlap with a complex, high energy bound state containing an odd number of
composite-fermion quasiparticles. The electron annihilation operator similarly
couples to a bound complex of composite-fermion holes. We predict that these
bound states can be observed through a conductance resonance in experiments
involving a tunneling of an external electron into the fractional quantum Hall
liquid. A comment is made on the origin of the breakdown of the Fermi liquid
paradigm in the fractional hall liquid.Comment: 5 pages, 2 figure
Novel theoretical approach in photoemission spectroscopy: application to isotope effect and boron-doped diamond
A new path-integral theory is developed to calculate the photoemission
spectra (PES) of correlated many-electron systems. The application to the study
on Bi2Sr2CaCu2O8 (Bi2212) and boron-doped diamond (BDD) is discussed in
details. It is found that the isotopic shift in the angle-resolved
photoemission spectra of Bi2212 is due to the off-diagonal quadratic
electron-phonon (e-ph) coupling, whereas the presence of electron-electron
repulsion partially suppresses this effect. For the BDD, a semiconductor-metal
phase transition, which is induced by increasing the e-ph coupling and dopant
concentration, is reproduced by our theory. Additionally, the presence of Fermi
edge and phonon step-like structure in PES is found to be due to a co-existence
of itinerant and localized electronic states in BDD.Comment: 6 pages, 4 figures, Procs. of LEHTSC 2007, submitted to J. Phys.:
Conf. Se
Phonon-induced dephasing of singlet-triplet superpositions in double quantum dots without spin-orbit coupling
We show that singlet-triplet superpositions of two-electron spin states in a
double quantum dot undergo a phonon-induced pure dephasing which relies only on
the tunnel coupling between the dots and on the Pauli exclusion principle. As
such, this dephasing process is independent of spin-orbit coupling or hyperfine
interactions. The physical mechanism behind the dephasing is elastic phonon
scattering, which persists to much lower temperatures than real phonon-induced
transitions. Quantitative calculations performed for a lateral GaAs/AlGaAs
gate-defined double quantum dot yield micro-second dephasing times at
sub-Kelvin temperatures, which is consistent with experimental observations.Comment: Extended versio
Comment on "Density and Spin response of a strongly-interacting Fermi gas in the attractive and quasi-repulsive regime"
This is a comment on Phys. Rev. Lett. 108, 080401 (2012) by Palestini et al.
We pointed out that the diagrammatic method in that article violates gauge
invariance. As a consequence, there will a Meissner effect in the normal phase
and the contribution from collective modes are not mentioned in the
symmetry-broken phase.Comment: 1 page, no figur
Ba Quadrupole Polarizabilities: Theory versus Experiment
Three different measurements have been reported for the ground state
quadrupole polarizability in the singly ionized barium (Ba) which disagree
with each other. Our calculation of this quantity using the relativistic
coupled-cluster method disagrees with two of the experimental values and is
within the error bars of the other. We discuss the issues related to the
accuracy of our calculations and emphasize the need for further experiments to
measure the quadrupole polarizability for this state and/or the 5D states.Comment: 6 pages, 3 table
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