122 research outputs found
Fermi Surface of Metallic VO from Angle-Resolved Photoemission: Mid-level Filling of Bands
Using angle resolved photoemission spectroscopy (ARPES) we report the first
band dispersions and distinct features of the bulk Fermi surface (FS) in the
paramagnetic metallic phase of the prototypical metal-insulator transition
material VO. Along the -axis we observe both an electron pocket and
a triangular hole-like FS topology, showing that both V 3 and
states contribute to the FS. These results challenge the existing
correlation-enhanced crystal field splitting theoretical explanation for the
transition mechanism and pave the way for the solution of this mystery.Comment: 5 pages, 4 figures plus supplement 12 pages, 3 figures, 1 tabl
Theory of Dynamic Stripe Induced Superconductivity
Since the recently reported giant isotope effect on T* [1] could be
consistently explained within an anharmonic spin-charge-phonon interaction
model, we consider here the role played by stripe formation on the
superconducting properties within the same model. This is a two-component
scenario and we recast its basic elements into a BCS effective Hamiltonian. We
find that the stripe formation is vital to high-Tc superconductivity since it
provides the glue between the two components to enhance Tc to the unexpectedly
large values observed experimentally.Comment: 7 pages, 2 figure
Bilayer Splitting in the Electronic Structure of Heavily Overdoped Bi2Sr2CaCu2O8+d
The electronic structure of heavily overdoped
BiSrCaCuO is investigated by angle-resolved
photoemission spectroscopy. The long-sought bilayer band splitting in this
two-plane system is observed in both normal and superconducting states, which
qualitatively agrees with the bilayer Hubbard model calculations. The maximum
bilayer energy splitting is about 88 meV for the normal state feature, while it
is only about 20 meV for the superconducting peak. This anomalous behavior
cannot be reconciled with the quasiparticle picture.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Effect of an Electron-phonon Interaction on the One-electron Spectral Weight of a d-wave Superconductor
We analyze the effects of an electron-phonon interaction on the one-electron
spectral weight A(k,omega) of a d_{x^2-y^2} superconductor. We study the case
of an Einstein phonon mode with various momentum-dependent electron-phonon
couplings and compare the structure produced in A(k,omega) with that obtained
from coupling to the magnetic pi-resonant mode. We find that if the strength of
the interactions are adjusted to give the same renormalization at the nodal
point, the differences in A(k,omega) are generally small but possibly
observable near k=(pi,0).Comment: 10 pages, 14 figures (color versions of Figs. 2,4,10,11,12 available
upon request
Interplay between electron-phonon and Coulomb interactions in cuprates
Evidence for strong electron-phonon coupling in high-Tc cuprates is reviewed,
with emphasis on the electron and phonon spectral functions. Effects due to the
interplay between the Coulomb and electron-phonon interactions are studied. For
weakly doped cuprates, the phonon self-energy is strongly reduced due to
correlation effects, while there is no corresponding strong reduction for the
electron self-energy. Polaron formation is studied, focusing on effects of
Coulomb interaction and antiferromagnetic correlations. It is argued that
experimental indications of polaron formation in undoped cuprates are due to a
strong electron-phonon interaction for these systems.Comment: 43 pages and 22 figure
Dual Nature of the Electronic Structure of the Stripe Phase
High resolution angle-resolved photoemission measurements have been carried
out on (La_1.4-xNd_0.6Sr_x)CuO_4, a model system with static stripes, and
(La_1.85Sr_0.15)CuO_4, a high temperature superconductor (T_c=40K) with dynamic
stripes. In addition to the straight segments near (pi, 0) and (0, pi)
antinodal regions, we have identified the existence of nodal spectral weight
and its associated Fermi surface in the electronic structure of both systems.
The ARPES spectra in the nodal region show well-defined Fermi cut-off,
indicating a metallic character of this charge-ordered state. This observation
of nodal spectral weight, together with the straight segments near antinodal
regions, reveals dual nature of the electronic structure of the stripes due to
the competition of order and disorder
Muon Spin Relaxation Study of (La, Ca)MnO3
We report predominantly zero field muon spin relaxation measurements in a
series of Ca-doped LaMnO_3 compounds which includes the colossal
magnetoresistive manganites. Our principal result is a systematic study of the
spin-lattice relaxation rates 1/T_1 and magnetic order parameters in the series
La_{1-x}Ca_xMnO_3, x = 0.0, 0.06, 0.18, 0.33, 0.67 and 1.0. In LaMnO_3 and
CaMnO_3 we find very narrow critical regions near the Neel temperatures T_N and
temperature independent 1/T_1 values above T_N. From the 1/T_1 in LaMnO_3 we
derive an exchange integral J = 0.83 meV which is consistent with the mean
field expression for T_N. All of the doped manganites except CaMnO_3 display
anomalously slow, spatially inhomogeneous spin-lattice relaxation below their
ordering temperatures. In the ferromagnetic (FM) insulating
La_{0.82}Ca_{0.18}MnO_3 and ferromagnetic conducting La_{0.67}Ca_{0.33}MnO_3
systems we show that there exists a bi-modal distribution of \muSR rates
\lambda_f and \lambda_s associated with relatively 'fast' and 'slow' Mn
fluctuation rates, respectively. A physical picture is hypothesized for these
FM phases in which the fast Mn rates are due to overdamped spin waves
characteristic of a disordered FM, and the slower Mn relaxation rates derive
from distinct, relatively insulating regions in the sample. Finally, likely
muon sites are identified, and evidence for muon diffusion in these materials
is discussed.Comment: 21 pages, 17 figure
Symmetry Breaking in Few Layer Graphene Films
Recently, it was demonstrated that the quasiparticle dynamics, the
layer-dependent charge and potential, and the c-axis screening coefficient
could be extracted from measurements of the spectral function of few layer
graphene films grown epitaxially on SiC using angle-resolved photoemission
spectroscopy (ARPES). In this article we review these findings, and present
detailed methodology for extracting such parameters from ARPES. We also present
detailed arguments against the possibility of an energy gap at the Dirac
crossing ED.Comment: 23 pages, 13 figures, Conference Proceedings of DPG Meeting Mar 2007
Regensburg Submitted to New Journal of Physic
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