24,899 research outputs found
Photoemission Spectroscopy of Magnetic and Non-magnetic Impurities on the Surface of the BiSe Topological Insulator
Dirac-like surface states on surfaces of topological insulators have a chiral
spin structure that suppresses back-scattering and protects the coherence of
these states in the presence of non-magnetic scatterers. In contrast, magnetic
scatterers should open the back- scattering channel via the spin-flip processes
and degrade the state's coherence. We present angle-resolved photoemission
spectroscopy studies of the electronic structure and the scattering rates upon
adsorption of various magnetic and non-magnetic impurities on the surface of
BiSe, a model topological insulator. We reveal a remarkable
insensitivity of the topological surface state to both non-magnetic and
magnetic impurities in the low impurity concentration regime. Scattering
channels open up with the emergence of hexagonal warping in the high-doping
regime, irrespective of the impurity's magnetic moment.Comment: 5 pages, 4 figure
Coherent resonant tunneling in ac fields
We have analyzed the tunneling transmission probability and electronic
current density through resonant heterostructures in the presence of an
external electromagnetic field. In this work, we compare two different models
for a double barrier : In the first case the effect of the external field is
taken into account by spatially dependent AC voltages and in the second one the
electromagnetic field is described in terms of a photon field that irradiates
homogeneously the whole sample. While in the first description the tunneling
takes place mainly through photo sidebands in the case of homogeneous
illumination the main effective tunneling channels correspond to the coupling
between different electronic states due to photon absorption and emission. The
difference of tunneling mechanisms between these configurations is strongly
reflected in the transmission and current density which present very different
features in both cases. In order to analyze these effects we have obtained,
within the Transfer Hamiltonian framework, a general expression for the
transition probability for coherent resonant tunneling in terms of the Green's
function of the system.Comment: 16 pages,Figures available upon request,to appear in Phys.Rev B (15
April 1996
Extensions of operators
We introduce the concept of the extension spectrum of a Hilbert space operator. This is a natural subset of the spectrum which plays an essential role in dealing with certain extension properties of operators. We prove that it has spectral-like properties and satisfies a holomorphic version of the Spectral Mapping Theorem. We establish structural theorems for algebraic extensions of triangular operators which use the extension spectrum in a natural way. The extension spectrum has some properties in common with the Kato spectrum, and in the final section we show how they are different and we examine their inclusion relationships
On the Reconstructed Fermi Surface in the Underdoped Cuprates
The Fermi surface topologies of underdoped samples the high-Tc superconductor
Bi2212 have been measured with angle resolved photoemission. By examining
thermally excited states above the Fermi level, we show that the Fermi surfaces
in the pseudogap phase of underdoped samples are actually composed of fully
enclosed hole pockets. The spectral weight of these pockets is vanishingly
small at the anti-ferromagnetic zone boundary, which creates the illusion of
Fermi "arcs" in standard photoemission measurements. The area of the pockets as
measured in this study is consistent with the doping level, and hence carrier
density, of the samples measured. Furthermore, the shape and area of the
pockets is well reproduced by a phenomenological model of the pseudogap phase
as a spin liquid.Comment: 4 pages, 4 figures. Submitted to Physics Review Letter
High-energy kink in high-temperature superconductors
In conventional metals, electron-phonon coupling, or the phonon-mediated
interaction between electrons, has long been known to be the pairing
interaction responsible for the superconductivity. The strength of this
interaction essentially determines the superconducting transition temperature
TC. One manifestation of electron-phonon coupling is a mass renormalization of
the electronic dispersion at the energy scale associated with the phonons. This
renormalization is directly observable in photoemission experiments. In
contrast, there remains little consensus on the pairing mechanism in cuprate
high temperature superconductors. The recent observation of similar
renormalization effects in cuprates has raised the hope that the mechanism of
high temperature superconductivity may finally be resolved. The focus has been
on the low energy renormalization and associated "kink" in the dispersion at
around 50 meV. However at that energy scale, there are multiple candidates
including phonon branches, structure in the spin-fluctuation spectrum, and the
superconducting gap itself, making the unique identification of the excitation
responsible for the kink difficult. Here we show that the low-energy
renormalization at ~50 meV is only a small component of the total
renormalization, the majority of which occurs at an order of magnitude higher
energy (~350 meV). This high energy kink poses a new challenge for the physics
of the cuprates. Its role in superconductivity and relation to the low-energy
kink remains to be determined.Comment: 13 pages, 4 figure
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