46,748 research outputs found
Temperature dependence of the impurity-induced resonant state in Zn-doped Bi_2Sr_2CaCu_2O by Scanning Tunneling Spectroscopy
We report on the temperature dependence of the impurity-induced resonant
state in Zn-doped Bi_2Sr_2CaCu_2O by scanning tunneling
spectroscopy at 30 mK < T < 52 K. It is known that a Zn impurity induces a
sharp resonant peak in tunnel spectrum at an energy close to the Fermi level.
We observed that the resonant peak survives up to 52 K. The peak broadens with
increasing temperature, which is explained by the thermal effect. This result
provides information to understand the origin of the resonant peak.Comment: 4 pages, 3 figures, to appear in Phys. Rev.
The solution of special squeeze film gas bearing problems by an improved numerical technique
Computer program for solving squeeze film gas bearing problem
Local density of states and Friedel oscillations around a non-magnetic impurity in unconventional density wave
We present a mean-field theoretical study on the effect of a single
non-magnetic impurity in quasi-one dimensional unconventional density wave. The
local scattering potential is treated within the self-consistent -matrix
approximation. The local density of states around the impurity shows the
presence of resonant states in the vicinity of the Fermi level, much the same
way as in -density waves or unconventional superconductors. The assumption
for different forward and backscattering, characteristic to quasi-one
dimensional systems in general, leads to a resonance state that is double
peaked in the pseudogap. The Friedel oscillations around the impurity are also
explored in great detail, both within and beyond the density wave coherence
length . Beyond we find power law behavior as opposed to the
exponential decay of conventional density wave. The entropy and specific heat
contribution of the impurity are also calculated for arbitrary scattering
strengths.Comment: 13 pages, 4 figure
Quantum interference in dirty d-wave superconductors
The local differential tunneling conductance on a Zn impurity in a disordered
d-wave superconductors is studied. Quantum interference between many impurities
leads to definitive quasiparticle spectra. We suggest that an elaborate
analysis on impurity-induced spectra with quantum interference effect included
may be able to pin down the sign and strength of the scattering potential of a
Zn impurity in low density limit. Numerical simulations calculated with
appropriately determined impurity parameters are in satisfactory agreement with
the observations from scanning tunneling microscopy (STM) experiments even in
subtle details
Josephson scanning tunneling microscopy
We propose a set of scanning tunneling microscopy experiments in which the
surface of superconductor is scanned by a superconducting tip. Potential
capabilities of such experimental setup are discussed. Most important
anticipated results of such an experiment include the position-resolved
measurement of the superconducting order parameter and the possibility to
determine the nature of the secondary component of the order parameter at the
surface. The theoretical description based on the tunneling Hamiltonian
formalism is presented.Comment: 6 pages, 7 figures, submitted to Phys. Rev.
Impurity spin textures across conventional and deconfined quantum critical points of two-dimensional antiferromagnets
We describe the spin distribution in the vicinity of a non-magnetic impurity
in a two-dimensional antiferromagnet undergoing a transition from a
magnetically ordered Neel state to a paramagnet with a spin gap. The quantum
critical ground state in a finite system has total spin S=1/2 (if the system
without the impurity had an even number of S=1/2 spins), and recent numerical
studies in a double layer antiferromagnet (K. H.Hoglund et al.,
cond-mat/0611418) have shown that the spin has a universal spatial form
delocalized across the entire sample. We present the field theory describing
the uniform and staggered magnetizations in this spin texture for two classes
of antiferromagnets: (i) the transition from a Neel state to a paramagnet with
local spin singlets, in models with an even number of S=1/2 spins per unit
cell, which are described by a O(3) Landau-Ginzburg-Wilson field theory; and
(ii) the transition from a Neel state to a valence bond solid, in
antiferromagnets with a single S=1/2 spin per unit cell, which are described by
a deconfined field theory of spinons.Comment: 30 pages, 9 figure
Valence bond solid order near impurities in two-dimensional quantum antiferromagnets
Recent scanning tunnelling microscopy (STM) experiments on underdoped
cuprates have displayed modulations in the local electronic density of states
which are centered on a Cu-O-Cu bond (Kohsaka et. al., cond-mat/0703309). As a
paradigm of the pinning of such bond-centered ordering in strongly correlated
systems, we present the theory of valence bond solid (VBS) correlations near a
single impurity in a square lattice antiferromagnet. The antiferromagnet is
assumed to be in the vicinity of a quantum transition from a magnetically
ordered Neel state to a spin-gap state with long-range VBS order. We identify
two distinct classes of impurities: i) local modulation in the exchange
constants, and ii) a missing or additional spin, for which the impurity
perturbation is represented by an uncompensated Berry phase. The `boundary'
critical theory for these classes is developed: in the second class we find a
`VBS pinwheel' around the impurity, accompanied by a suppression in the VBS
susceptibility. Implications for numerical studies of quantum antiferromagnets
and for STM experiments on the cuprates are noted.Comment: 41 pages, 6 figures; (v2) Minor changes in terminology, added
reference
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
Nanoscale Impurity Structures on the Surface of -wave Superconductors
We study the effects of nanoscale impurity structures on the local electronic
structure of -wave superconductors. We show that the interplay
between the momentum dependence of the superconducting gap, the geometry of the
nanostructure and its orientation gives rise to a series of interesting quantum
effects. Among these are the emergence of a zero bias conductance peak in the
superconductor's density of states and the suppression of impurity states for
certain nanostructures. The latter effect can be used to screen impurity
resonances in the superconducting state.Comment: 4 pages, 5 figure
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