755 research outputs found
Magnetic response of nonmagnetic impurities in cuprates
A theory of the local magnetic response of a nonmagnetic impurity in a doped
antiferromagnet, as relevant to the normal state in cuprates, is presented. It
is based on the assumption of the overdamped collective mode in the bulk system
and on the evidence, that equal-time spin correlations are only weakly
renormalized in the vicinity of the impurity. The theory relates the Kondo-like
behavior of the local susceptibility to the anomalous temperature dependence of
the bulk magnetic susceptibility, where the observed increase of the Kondo
temperature with doping reflects the crossover to the Fermi liquid regime and
the spatial distribution of the magnetization is given by bulk
antiferromagnetic correlations.Comment: 5 pages, 3 figure
Chirality sensitive effect on surface states in chiral p-wave superconductors
We study the local density of states at the surface of a chiral p-wave
superconductor in the presence of a weak magnetic field. As a result, the
formation of low-energy Andreev bound states is either suppressed or enhanced
by an applied magnetic field, depending on its orientation with respect to the
chirality of the p-wave superconductor. Similarly, an Abrikosov vortex, which
is situated not too far from the surface, leads to a zero-energy peak of the
density of states, if its chirality is the same as that of the superconductor,
and to a gap structure for the opposite case. We explain the underlying
principle of this effect and propose a chirality sensitive test on
unconventional superconductors.Comment: 4 pages, 2 figure
Spin, charge and orbital fluctuations in a multi-orbital Mott insulator
The two-orbital degenerate Hubbard model with distinct hopping integrals is
studied by combining dynamical mean-field theory with quantum Monte Carlo
simulations. The role of orbital fluctuations for the nature of the Mott
transition is elucidated by examining the temperature dependence of spin,
charge and orbital susceptibilities as well as the one-particle spectral
function. We also consider the effect of the hybridization between the two
orbitals, which is important particularly close to the Mott transition points.
The introduction of the hybridization induces orbital fluctuations, resulting
in the formation of a Kondo-like heavy-fermion behavior, similarly to
electron systems, but involving electrons in bands of comparable width.Comment: 8 pages, 9 figure
Competitions in layered ruthenates: ferro- vs. antiferromagnetism and triplet vs. singlet pairing
Ru based perovskites demonstrate an amazing richness in their magnetic
properties, including 3D and quasi-2D ferromagnetism, antiferromagnetism, and
unconventional superconductivity. Tendency to ferromagnetism, stemming from the
unusually large involvement of O in magnetism in ruthenates, leads to
ferromagnetic spin fluctuations in Sr2RuO4 and eventually to p-wave
superconductivity. A related compound Ca2RuO4 was measured to be
antiferromagnetic, suggesting a possibility of antiferromagnetic fluctuations
in Sr2RuO4 as well. Here we report first principles calculations that
demonstrate that in both compounds the ferro- and antiferromagnetic
fluctuations coexist, leading to an actual instability in Ca2RuO4 and to a
close competition between p-wave and d-wave superconducting symmetries in
Sr2RuO4. The antiferromagnetism in this system appears to be mostly related
with the nesting, which is the strongest at Q=(2pi/3,2pi/3,0). Surprisingly,
for the Fermiology of Sr2RuO4 the p-wave state wins over the d-wave one
everywhere except in close vicinity of the antiferromagnetic instability. The
most stable state within the d-wave channel has vanishing order parameter at
one out of three Fermi surfaces in Sr2RuO4, while in the p channel its
amplitude is comparable at all three of them.Comment: 4 Revtex pages with 4 embedded postscript figure. Some figures are
color, but should look OK in B&W as wel
On the Bloch Theorem Concerning Spontaneous Electric Current
We study the Bloch theorem which states absence of the spontaneous current in
interacting electron systems. This theorem is shown to be still applicable to
the system with the magnetic field induced by the electric current. Application
to the spontaneous surface current is also examined in detail. Our result
excludes the possibility of the recently proposed -wave superconductivity
having the surface flow and finite total current.Comment: 12 pages, LaTeX, 3 Postscript figure
Phenomenological theory of the 3 Kelvin phase in Sr2RuO4
We model the 3K-phase of Sr2RuO4 with Ru-metal inclusion as interface state
with locally enhanced transition temperatures. The resulting 3K-phase must have
a different pairing symmetry than the bulk phase of Sr2RuO4, because the
symmetry at the interface is lower than in the bulk. It is invariant under time
reversal and a second transition, in general, above the onset of bulk
superconductivity is expected where time reversal symmetry is broken. The
nucleation of the 3K-phase exhibits a ``capillary effect'' which can lead to
frustration phenomena for the superconducting states on different
Ru-inclusions. Furthermore, the phase structure of the pair wave function gives
rise to zero-energy quasiparticle states which would be visible in
quasiparticle tunneling spectra. Additional characteristic properties are
associated with the upper critical field Hc2. The 3K-phase has a weaker
anisotropy of Hc2 between the inplane and z-axis orientation than the bulk
superconducting phase. This is connected with the more isotropic nature
Ru-metal which yields a stronger orbital depairing effect for the inplane
magnetic field than in the strongly layered Sr$_2RuO4. An anomalous temperature
dependence for the z-axis critical field is found due to the coupling of the
magnetic field to the order parameter texture at the interface. Various other
experiments are discussed and new measurements are suggested.Comment: 10 pages, 5 figure
Orbital-selective Mott transitions in the anisotropic two-band Hubbard model at finite temperatures
The anisotropic degenerate two-orbital Hubbard model is studied within
dynamical mean-field theory at low temperatures. High-precision calculations on
the basis of a refined quantum Monte Carlo (QMC) method reveal that two
distinct orbital-selective Mott transitions occur for a bandwidth ratio of 2
even in the absence of spin-flip contributions to the Hund exchange. The second
transition -- not seen in earlier studies using QMC, iterative perturbation
theory, and exact diagonalization -- is clearly exposed in a low-frequency
analysis of the self-energy and in local spectra.Comment: 4 pages, 5 figure
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