3,940 research outputs found
Mechanism of hopping transport in disordered Mott insulators
By using a combination of detailed experimental studies and simple
theoretical arguments, we identify a novel mechanism characterizing the hopping
transport in the Mott insulating phase of CaSrRuO near the
metal-insulator transition. The hopping exponent shows a systematic
evolution from a value of deeper in the insulator to the
conventional Mott value closer to the transition. This behavior,
which we argue to be a universal feature of disordered Mott systems close to
the metal-insulator transition, is shown to reflect the gradual emergence of
disorder-induced localized electronic states populating the Mott-Hubbard gap.Comment: 5 pages, 3 figures, To be published in Physical Review Letter
Upper Critical Field of the 3 Kelvin Phase in Sr2RuO4
The inhomogeneous 3 Kelvin phase is most likely a superconducting state
nucleating at the interface between micrometer-sized Ru-metal inclusions and
Sr2RuO4 above the bulk onset of superconductivity. This filamentary
superconducting state yields a characteristic temperature dependence of the
upper critical field which is sublinear, i.e., H_{c2} (T) \propto (T^* -
T)^{\gamma} with 0.5 \leq \gamma < 1 (T^*: nucleation temperature). The
Ginzburg-Landau theory is used to analyze the behavior of the nucleated
spin-triplet phase in a field and the characteristic features of H_{c2}
observed in the experiment are explained based on a two-component order
parameter in the presence of a filament of enhanced superconductivity with a
finite width.Comment: 4 pages, 3 figure
Cyclotron Resonance in the Layered Perovskite Superconductor Sr2RuO4
We have measured the cyclotron masses in Sr2RuO4 through the observation of
periodic-orbit-resonances - a magnetic resonance technique closely related to
cyclotron resonance. We obtain values for the alpha, beta and gamma Fermi
surfaces of (4.33+/-0.05)me, (5.81+/-0.03)me and (9.71+/-0.11)me respectively.
The appreciable differences between these results and those obtained from de
Haas- van Alphen measurements are attributable to strong electron-electron
interactions in this system. Our findings appear to be consistent with
predictions for a strongly interacting Fermi liquid; indeed, semi-quantitative
agreement is obtained for the electron pockets beta and gamma.Comment: 4 pages + 3 figure
Effects of In-Plane Impurity Substitution in Sr2RuO4
We report comparative substitution effects of nonmagnetic Ti^(4+) and
magnetic Ir^(4+) impurities for Ru^(4+) in the spin-triplet superconductor
Sr2RuO4. We found that both impurities suppress the superconductivity
completely at a concentration of approximately 0.15%, reflecting the high
sensitivity to translational symmetry breaking in Sr2RuO4. In addition, a rapid
enhancement of residual resistivity is in quantitative agreement with
unitarity-limit scattering. Our result suggests that both nonmagnetic and
magnetic impurities in Sr2RuO4 act as strong potential scatterers, similar to
the nonmagnetic Zn^(2+) impurity in the high-Tc cuprates.Comment: 4 pages, 2 figures. submitted to Journal of the Physical Society of
Japa
Metamagnetic Transition in NaCoO Single Crystals
We report the magnetization, specific heat and transport measurements of high
quality NaCoO single crystals in applied magnetic fields up to
14T. In high temperatures, the system is in a paramagnetic phase. It undergoes
a magnetic phase transition below about 20K. When the field is applied along
the c-axis, the measurement data of magnetization, specific heat and
magnetoresistance reveal a metamagnetic transition from an antiferromagnetic
state to a quasi-ferromagnetic state at about 8T in low temperatures. However,
no transition is observed in the magnetization measurements up to 14T when the
field is applied perpendicular to the c-axis. The low temperature magnetic
phase diagram of NaCoO is determined.Comment: 4 pages, 5 figure
The ground state of Sr3Ru2O7 revisited; Fermi liquid close to a ferromagnetic instability
We show that single-crystalline Sr3Ru2O7 grown by a floating-zone technique
is an isotropic paramagnet and a quasi-two dimensional metal as spin-triplet
superconducting Sr2RuO4 is. The ground state is Fermi liquid with very low
residual resistivity (3 micro ohm cm for in-plane currents) and a nearly
ferromagnetic metal with the largest Wilson ratio Rw>10 among paramagnets so
far. This contrasts with the ferromagnetic order at Tc=104 K reported on single
crystals grown by a flux method [Cao et al., Phys. Rev. B 55, R672 (1997)]. We
have also found a dramatic changeover from paramagnetism to ferromagnetism
under applied pressure. This suggests the existence of a substantial
ferromagnetic instability on the verge of a quantum phase transition in the
Fermi liquid state.Comment: 5 pages, 4 figures, to be published in Phys. Rev. B : Rapid co
Robust paramagnetism in Bi2-xMxRu2O7 (M=Mn,Fe,Co,Ni,Cu) pyrochlore
We report physical property characterization of Bi2-xMxRu2O7 pyrochlores,
including magnetic suseptibility, resistivity, and Seebeck coefficients. The
solid solution exists up to x=0.5 for (M=Cu,Ni,Co) and up to x=0.1 for
(M=Fe,Mn). None of the doped materials exhibit ferromagnetism or any localized
ruthenium moment behavior. Instead we find the Ru-O and Bi-O sublattices to be
essentially independent, with any magnetism resulting from the unpaired
transition metal dopant spins. Cobalt substitution for bismuth results in
localized Co{2+}, and low temperature spin-glass transitions in several cases.
Nickel moments on the pyrochlore lattice display properties intermediate to
localized and itinerant. Finally, copper doping results in only an enhancement
of the Pauli metallic density of states.Comment: submitted, Phys. Rev.
Superconductivity in Sr2RuO4-Sr3Ru2O7 eutectic crystals
Superconducting behavior has been observed in the Sr2RuO4-Sr3Ru2O7 eutectic
system as grown by the flux-feeding floating zone technique. A supercurrent
flows across a single interface between Sr2RuO4 and Sr3Ru2O7 areas at distances
that are far beyond those expected in a conventional proximity scenario. The
current-voltage characteristics within the Sr3Ru2O7 macrodomain, as extracted
from the eutectic, exhibit signatures of superconductivity in the bilayered
ruthenate. Detailed microstructural, morphological and compositional analyses
address issues on the concentration and the size of Sr2RuO4 inclusions within
the Sr3Ru2O7 matrix. We speculate on the possibility of inhomogeneous
superconductivity in the eutectic Sr3Ru2O7 and exotic pairing induced by the
Sr2RuO4 inclusions.Comment: Pages 4, figures 3, submitted to Phys. Rev. Let
Fourth Order Perturbation Theory for Normal Selfenergy in Repulsive Hubbard Model
We investigate the normal selfenergy and the mass enhancement factor in the
Hubbard model on the two-dimensional square lattice. Our purpose in this paper
is to evaluate the mass enhancement factor more quantitatively than the
conventional third order perturbation theory. We calculate it by expanding
perturbatively up to the fourth order with respect to the on-site repulsion
. We consider the cases that the system is near the half-filling, which are
similar situations to high- cuprates. As results of the calculations, we
obtain the large mass enhancement on the Fermi surface by introducing the
fourth order terms. This is mainly originated from the fourth order
particle-hole and particle-particle diagrams. Although the other fourth order
terms have effect of reducing the effective mass, this effect does not cancel
out the former mass enhancement completely and there remains still a large mass
enhancement effect. In addition, we find that the mass enhancement factor
becomes large with increasing the on-site repulsion and the density of
state (DOS) at the Fermi energy . According to many current reseaches,
such large and enhance the effective interaction between
quasiparticles, therefore the superconducting transition temperature
increases. On the other hand, the large mass enhancement leads the reduction of
the energy scale of quasiparticles, as a result, is reduced. When we
discuss , we have to estimate these two competitive effects.Comment: 6pages,8figure
Tunneling properties at the interface between superconducting Sr2RuO4 and a Ru micro-inclusion
We have investigated the magnetic field and temperature dependence of the
tunneling spectra of the eutectic system Sr2RuO4-Ru. Electric contacts to
individual Ru lamellae embedded in Sr2RuO4 enable the tunneling spectra at the
interface between ruthenate and a Ru microinclusion to be measured. A zero bias
conductance peak (ZBCP) was observed in the bias voltage dependence of the
differential conductance, suggesting that Andreev bound states are present at
the interface. The ZBCP starts to appear at a temperature well below the
superconducting transition temperature. The onset magnetic field of the ZBCP is
also considerably smaller than the upper critical field when the magnetic field
is parallel to the ab-plane. We propose that the difference between the onset
of the ZBCP and the onset of superconductivity can be understood in terms of
the existence of the single-component state predicted by Sigrist and Monien.Comment: 4 pages, 4 figures, to appear in J. Phys. Soc. Jpn. vol. 74 no.
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