133 research outputs found
Unresolved problems in superconductivity of CaC6
We discuss the current status of the theory of the "high-temperature"
superconductivity in intercalated graphites YbC6 and CaC6. We emphasize that
while the general picture of conventional, phonon-driven superconductivity has
already emerged and is generally accepted, there are still interesting problems
with this picture, such as weak-coupling regime inferred from specific heat
suggesting coupling exclusively with high-energy carbon phonons coming in
direct contradiction with the isotope effect measurements suggesting coupling
exclusively with the low-energy intercalant modes. At the same time, the first
principle calculations, while explaining Tc, contradict both of the experiments
above by predicting equal coupling with both groups of phonons.Comment: Contribution to the Proceedings of the M2S Conference in Dresden,
200
Magnetic Phase Diagram of Ca2-xSrxRuO4 Governed by Structural Distortions
We constructed, by the first-principles calculations, a magnetic phase
diagram of SrRuO in the space spanned by structural distortions. Our
phase diagram can qualitatively explain the experimental one for
CaSrRuO. We found that the rotation and the tilting of RuO
octahedron are responsible for the ferro- and antiferro-magnetism,
respectively, while the flattening of RuO is the key factor to stabilize
those magnetic ground states. Our results imply that the magnetic and the
structural instabilities in SrRuO are closely correlated cooperatively
rather than competitively.Comment: 3 figures; accepted by PRB as rapid communicatio
Effects of metallic spacer in layered superconducting Sr2(MgTi)O3FeAs
The highly two-dimensional superconducting system
Sr2(MgTi)O3FeAs, recently synthesized in the range of 0.2 < y <
0.5, shows an Mg concentration-dependent . Reducing the Mg concentration
from y=0.5 leads to a sudden increase in , with a maximum ~40 K at
y=0.2. Using first principles calculations, the unsynthesized stoichiometric
y=0 and the substoichiometric y=0.5 compounds have been investigated. For the
50% Mg-doped phase (y=0.5), Sr2(MgTi)O3 layers are completely
insulating spacers between FeAs layers, leading to the fermiology such as that
found for other Fe pnictides. At y=0, representing a phase with metallic
Sr2TiO3 layers, the -centered Fe-derived Fermi surfaces (FSs)
considerably shrink or disappear. Instead, three -centered Ti FSs
appear, and in particular two of them have similar size, like in MgB2.
Interestingly, FSs have very low Fermi velocity in large fractions: the lowest
being 0.6 cm/s. Furthermore, our fixed spin moment calculations
suggest the possibility of magnetic ordering, with magnetic Ti and nearly
nonmagnetic Fe ions. These results indicate a crucial role of
Sr2(MgTi)O3 layers in this superconductivity.Comment: 7 pages; Proceedings of ICSM-201
On the possibility of superconductivity in PrBa2Cu3O7
Recent reports about observations of superconductivity in PrBa2Cu3O7 raise a
number of questions: (i) of various theories striving to explain the Tc
suppression in PrxY{1-x}Ba2Cu3O7, are there any compatible with possible
superconductivity in stoichiometric PrBa2Cu3O7? (ii) if this superconductivity
is not an experimental artifact, are the superconducting carriers (holes) of
the same character as in the other high-Tc cuprates, or do they represent
another electronic subsystem? (iii) is the underlying mechanism the same as in
other high-Tc superconductors? I present an answer to the first two questions,
while leaving the last one open.Comment: 4 pages 4 eps fig
Orbital Dependent Phase Control in Ca2-xSrxRuO4
We present first-principles studies on the orbital states of the layered
perovskites CaSrRuO. The crossover from antiferromagnetic (AF)
Mott insulator for to nearly ferromagnetic (FM) metal at is
characterized by the systematic change of the orbital occupation. For the
AF side (), we present firm evidence for the ferro-orbital
ordering. It is found that the degeneracy of (or ) states is
lifted robustly due to the two-dimensional (2D) crystal-structure, even without
the Jahn-Teller distortion of RuO. This effect dominates, and the
cooperative occupation of orbital is concluded. In contrast to recent
proposals, the resulting electronic structure explains well both the observed
X-ray absorption spectra and the double peak structure of optical conductivity.
For the FM side (), however, the orbital with half filling opens a
pseudo-gap in the FM state and contributes to the spin =1/2 moment (rather
than =1 for =0.0 case) dominantly, while states are itinerant
with very small spin polarization, explaining the recent neutron data
consistently.Comment: 17 pages, 5 figure
Surface electronic structure of Sr2RuO4
We have addressed the possibility of surface ferromagnetism in Sr2RuO4 by
investigating its surface electronic states by angle-resolved photoemission
spectroscopy (ARPES). By cleaving samples under different conditions and using
various photon energies, we have isolated the surface from the bulk states. A
comparison with band structure calculations indicates that the ARPES data are
most readily explained by a nonmagnetic surface reconstruction.Comment: 4 pages, 4 figures, RevTex, submitted to Phys. Rev.
Effect of magnetic and non-magnetic impurities on highly anisotropic superconductivity
We generalize Abrikosov-Gor'kov solution of the problem of weakly coupled
superconductor with impurities on the case of a multiband superconductor with
arbitrary interband order parameter anisotropy, including interband sign
reversal of the order parameter. The solution is given in terms of the
effective (renormalized) coupling matrix and describes not only
suppression but also renormalization of the superconducting gap basically at
all temperatures. In many limiting cases we find analytical solutions for the
critical temperature suppression. We illustrate our results by numerical
calculations for two-band model systems.Comment: 18 pages (12pt) RevTeX, 4 postscript figure
Anisotropy in the Antiferromagnetic Spin Fluctuations of Sr2RuO4
It has been proposed that Sr_2RuO_4 exhibits spin triplet superconductivity
mediated by ferromagnetic fluctuations. So far neutron scattering experiments
have failed to detect any clear evidence of ferromagnetic spin fluctuations
but, instead, this type of experiments has been successful in confirming the
existence of incommensurate spin fluctuations near q=(1/3 1/3 0). For this
reason there have been many efforts to associate the contributions of such
incommensurate fluctuations to the mechanism of its superconductivity. Our
unpolarized inelastic neutron scattering measurements revealed that these
incommensurate spin fluctuations possess c-axis anisotropy with an anisotropic
factor \chi''_{c}/\chi''_{a,b} of \sim 2.8. This result is consistent with some
theoretical ideas that the incommensurate spin fluctuations with a c-axis
anisotropy can be a origin of p-wave superconductivity of this material.Comment: 5 pages, 3 figures; accepted for publication in PR
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