204 research outputs found
Dynamical spin susceptibility and the resonance peak in the pseudogap region of the underdoped cuprate superconductors
We present a study of the dynamical spin susceptibility in the pseudogap
region of the high-T cuprate superconductors. We analyze and compare the
formation of the so-called resonance peak, in three different ordered states:
the -wave superconducting (DSC) phase, the -density wave (DDW)
state, and a phase with coexisting DDW and DSC order. An analysis of the
resonance's frequency and momentum dependence in all three states reveals
significant differences between them. In particular, in the DDW state, we find
that a nearly dispersionless resonance excitation exists only in a narrow
region around . At the same time, in the coexisting DDW and
DSC state, the dispersion of the resonance peak near is significantly
changed from that in the pure DSC state. Away from , however, we
find that the form and dispersion of the resonance excitation in the coexisting
DDW and DSC state and pure DSC state are quite similar. Our results demonstrate
that a detailed experimental measurement of the resonance's dispersion allows
one to distinguish between the underlying phases - a DDW state, a DSC state, or
a coexisting DDW and DSC state - in which the resonance peak emerges.Comment: 9 pages, 9 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
Striped superconductors in the extended Hubbard model
We present a minimal model of a doped Mott insulator that simultaneously
supports antiferromagnetic stripes and d-wave superconductivity. We explore the
implications for the global phase diagram of the superconducting cuprates. At
the unrestricted mean-field level, the various phases of the cuprates,
including weak and strong pseudogap phases, and two different types of
superconductivity in the underdoped and the overdoped regimes, find a natural
interpretation. We argue that on the underdoped side, the superconductor is
intrinsically inhomogeneous -- striped coexistence of of superconductivity and
magnetism -- and global phase coherence is achieved through Josephson-like
coupling of the superconducting stripes. On the overdoped side, the state is
overall homogeneous and the superconductivity is of the classical BCS type.Comment: 5 pages, 3 eps figures. Effect of t' on stripe filling + new
references are adde
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
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
Impurity-induced spin polarization and NMR line broadening in underdoped cuprates
We present a theory of magnetic (S=1) Ni and nonmagnetic Zn impurities in
underdoped cuprates. Both types of impurities are shown to induce S=1/2 moments
on Cu sites in the proximity of the impurity, a process which is intimately
related to the spin gap phenomenon in cuprates. Below a characteristic Kondo
temperature, the Ni spin is partially screened by the Cu moments, resulting in
an effective impurity spin S=1/2. We further analyze the
Ruderman-Kittel-Kasiya-Yosida-type response of planar Cu spins to a
polarization of the effective impurity moments and derive expressions for the
corresponding ^{17}O NMR line broadening. The peculiar aspects of recent
experimental NMR data can be traced back to different spatial characteristics
of Ni and Zn moments as well as to an inherent temperature dependence of local
antiferromagnetic correlations.Comment: PRB B1 01June9
Robust Upward Dispersion of the Neutron Spin Resonance in the Heavy Fermion Superconductor CeYbCoIn
The neutron spin resonance is a collective magnetic excitation that appears
in copper oxide, iron pnictide, and heavy fermion unconventional
superconductors. Although the resonance is commonly associated with a
spin-exciton due to the ()-wave symmetry of the superconducting
order parameter, it has also been proposed to be a magnon-like excitation
appearing in the superconducting state. Here we use inelastic neutron
scattering to demonstrate that the resonance in the heavy fermion
superconductor CeYbCoIn with has a ring-like
upward dispersion that is robust against Yb-doping. By comparing our
experimental data with random phase approximation calculation using the
electronic structure and the momentum dependence of the -wave
superconducting gap determined from scanning tunneling microscopy for
CeCoIn, we conclude the robust upward dispersing resonance mode in
CeYbCoIn is inconsistent with the downward dispersion
predicted within the spin-exciton scenario.Comment: Supplementary Information available upon reques
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
Quasiparticle excitation in and around the vortex core of underdoped YBa_2Cu_4O_8 studied by site-selective NMR
We report a site-selective ^{17}O spin-lattice relaxation rate T_1^{-1} in
the vortex state of underdoped YBa_2Cu_4O_8. We found that T_1^{-1} at the
planar sites exhibits an unusual nonmonotonic NMR frequency dependence. In the
region well outside the vortex core, T_1^{-1} cannot be simply explained by the
density of states of the Doppler-shifted quasiparticles in the d-wave
superconductor. Based on T_1^{-1} in the vortex core region, we establish
strong evidence that the local density of states within the vortex core is
strongly reduced.Comment: 5 pages, 3 figure
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