442 research outputs found
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
Antiferromagnetic Vortex Core of Tl_2Ba_2CuO_{6+x} Studied by Nuclear Magnetic Resonance
Spatially-resolved NMR is used to probe the magnetism in and around vortex
cores of nearly optimally-doped Tl_2Ba_2CuO_{6+x} (Tc=85 K). The NMR relaxation
rate 1/T_1 at Tl site provides a direct evidence that the AF spin correlation
is significantly enhanced in the vortex core region. In the core region Cu
spins show a local AF ordering with moments parallel to the layers at T_N=20K.
Above T_N the core region is in the paramagnetic state which is a reminiscence
of the state above the pseudogap temperature (T*~120 K), indicating that the
pseudogap disappears within cores.Comment: 4 pages, 4 figure
Magnetic Coherence as a Universal Feature of Cuprate Superconductors
Recent inelastic neutron scattering (INS) experiments on
LaSrCuO have established the existence of a {\it magnetic
coherence effect}, i.e., strong frequency and momentum dependent changes of the
spin susceptibility, , in the superconducting phase. We show, using the
spin-fermion model for incommensurate antiferromagnetic spin fluctuations, that
the magnetic coherence effect establishes the ability of INS experiments to
probe the electronic spectrum of the cuprates, in that the effect arises from
the interplay of an incommensurate magnetic response, the form of the
underlying Fermi surface, and the opening of the d-wave gap in the fermionic
spectrum. In particular, we find that the magnetic coherence effect observed in
INS experiments on LaSrCuO requires that the Fermi surface be
closed around up to optimal doping. We present several predictions
for the form of the magnetic coherence effect in YBaCuO in
which an incommensurate magnetic response has been observed in the
superconducting state.Comment: 9 pages, 12 figures; extended version of Phys. Rev B, R6483 (2000
Direct Evidence for a Magnetic f-electron Mediated Cooper Pairing Mechanism of Heavy Fermion Superconductivity in CeCoIn5
To identify the microscopic mechanism of heavy-fermion Cooper pairing is an
unresolved challenge in quantum matter studies; it may also relate closely to
finding the pairing mechanism of high temperature superconductivity.
Magnetically mediated Cooper pairing has long been the conjectured basis of
heavy-fermion superconductivity but no direct verification of this hypothesis
was achievable. Here, we use a novel approach based on precision measurements
of the heavy-fermion band structure using quasiparticle interference (QPI)
imaging, to reveal quantitatively the momentum-space (k-space) structure of the
f-electron magnetic interactions of CeCoIn5. Then, by solving the
superconducting gap equations on the two heavy-fermion bands
with these magnetic interactions as mediators of the
Cooper pairing, we derive a series of quantitative predictions about the
superconductive state. The agreement found between these diverse predictions
and the measured characteristics of superconducting CeCoIn5, then provides
direct evidence that the heavy-fermion Cooper pairing is indeed mediated by the
f-electron magnetism.Comment: 19 pages, 4 figures, Supplementary Information: 31 pages, 5 figure
NMR relaxation time around a vortex in stripe superconductors
Site-dependent NMR relaxation time is calculated in the vortex
state using the Bogoliubov-de Gennes theory, taking account of possible
"field-induced stripe'' states in which the magnetism arises locally around a
vortex core in d-wave superconductivity. The recently observed huge enhancement
below at a core site in TlBaCuO is
explained. The field-induced stripe picture explains consistently other
relevant STM and neutron experiments.Comment: 4 pages, 4 figure
Impurities, Quantum Interference and Quantum Phase Transitions in s-wave superconductors
We study the effects of quantum interference in impurity structures
consisting of two or three magnetic impurities that are located on the surface
of an s-wave superconductor. By using a self-consistent Bogoliubov-de Gennes
formalism, we show that quantum interference leads to characteristic signatures
not only in the local density of states (LDOS), but also in the spatial form of
the superconducting order parameter. We demonstrate that the signatures of
quantum interference in the LDOS are qualitatively, and to a large extent
quantitatively unaffected by the suppression of the superconducting order
parameter near impurities, which illustrates the robustness of quantum
interference phenomena. Moreover, we show that by changing the interimpurity
distance, or the impurities' scattering strength, the s-wave superconductor can
be tuned through a series of first order quantum phase transitions in which the
spin polarization of its ground state changes. In contrast to the single
impurity case, this transition is not necessarily accompanied by a -phase
shift of the order parameter, and can in certain cases even lead to its
enhancement. Our results demonstrate that the superconductor's LDOS, its spin
state, and the spatial form of the superconducting order parameter are
determined by a subtle interplay between the relative positions of the
impurities and their scattering strength
Resonant Impurity States in the D-Density-Wave Phase
We study the electronic structure near impurities in the d-density-wave (DDW)
state, a possible candidate phase for the pseudo-gap region of the
high-temperature superconductors. We show that the local DOS near a
non-magnetic impurity in the DDW state is {\it qualitatively} different from
that in a superconductor with -symmetry. Since this result is a
robust feature of the DDW phase, it can help to identify the nature of the two
different phases recently observed by scanning tunneling microscopy experiments
in the superconducting state of underdoped Bi-2212 compounds
Identifying Collective Modes via Impurities in the Cuprate Superconductors
We show that the pinning of collective charge and spin modes by impurities in
the cuprate superconductors leads to qualitatively different fingerprints in
the local density of states (LDOS). In particular, in a pinned (static) spin
droplet, the creation of a resonant impurity state is suppressed, the
spin-resolved LDOS exhibits a characteristic spatial pattern, and the LDOS
undergoes significant changes with increasing magnetic field. Since all of
these fingerprints are absent in a charge droplet, impurities are a new probe
for identifying the nature and relative strength of collective modes.Comment: 4 pages, 4 figure
Two nonmagnetic impurities in the DSC and DDW state of the cuprate superconductors as a probe for the pseudogap
The quantum interference between two nonmagnetic impurities is studied
numerically in both the d-wave superconducting (DSC) and the d-density wave
(DDW) state. In all calculations we include the tunnelling through excited
states from the CuO planes to the BiO layer probed by the STM tip. Compared
to the single impurity case, a systematic study of the modulations in the
two-impurity local density of states can distinguish between the DSC or DDW
states. This is important if the origin of the pseudogap phase is caused by
preformed pairs or DDW order. Furthermore, in the DSC state the study of the
LDOS around two nonmagnetic impurities provide further tests for the potential
scattering model versus more strongly correlated models.Comment: 6 pages, 6 figure
Luttinger theorem for a spin-density-wave state
We obtained the analog of the Luttinger relation for a commensurate
spin-density-wave state. We show that while the relation between the area of
the occupied states and the density of particles gets modified in a simple and
predictable way when the system becomes ordered, a perturbative consideration
of the Luttinger theorem does not work due to the presence of an anomaly
similar to the chiral anomaly in quantum electrodynamics.Comment: 4 pages, RevTeX, 1 figure embedded in the text, ps-file is also
available at http://lifshitz.physics.wisc.edu/www/morr/morr_homepage.htm
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