178 research outputs found
Superconductivity in an organic insulator at very high magnetic fields
We investigate by electrical transport the field-induced superconducting
state (FISC) in the organic conductor -(BETS)FeCl. Below 4 K,
antiferromagnetic-insulator, metallic, and eventually superconducting (FISC)
ground states are observed with increasing in-plane magnetic field. The FISC
state survives between 18 and 41 T, and can be interpreted in terms of the
Jaccarino-Peter effect, where the external magnetic field {\em compensates} the
exchange field of aligned Fe ions. We further argue that the Fe
moments are essential to stabilize the resulting singlet, two-dimensional
superconducting stateComment: 9 pages 3 figure
Enhancement of the upper critical field and a field-induced superconductivity in antiferromagnetic conductors
We propose a mechanism by which the paramagnetic pair-breaking effect is
largely reduced in superconductors with coexisting antiferromagnetic long-
range and short-range orders. The mechanism is an extension of the Jaccarino
and Peter mechanism to antiferromagnetic conductors, but the resultant phase
diagram is quite different. In order to illustrate the mechanism, we examine a
model which consists of mobile electrons and antiferromagnetically correlated
localized spins with Kondo coupling between them. It is found that for weak
Kondo coupling, the superconductivity occurs over an extraordinarily wide
region of the magnetic field including zero field. The critical field exceeds
the Chandrasekhar and Clogston limit, but there is no lower limit in contrast
to the Jaccarino and Peter mechanism. On the other hand, for strong Kondo
coupling, both the low-field superconductivity and a field-induced
superconductivity occur. Possibilities in hybrid ruthenate cuprate
superconductors and some organic superconductors are discussed.Comment: 5 pages, 1 figure, revtex.sty, to be published in J.Phys.Soc.Jpn.
Vol.71, No.3 (2002
Temperature dependence of the upper critical field of an anisotropic singlet superconductivity in a square lattice tight-binding model in parallel magnetic fields
Upper critical field parallel to the conducting layer is studied in
anisotropic type-II superconductors on square lattices. We assume enough
separation of the adjacent layers, for which the orbital pair-breaking effect
is suppressed for exactly aligned parallel magnetic field. In particular, we
examine the temperature dependence of the critical field H_c(T) of the
superconductivity including the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO or LOFF)
state, in which the Cooper pairs have non-zero center-of-mass momentum q. In
the system with the cylindrically symmetric Fermi-surface, it is known that
H_c(T) of the d-wave FFLO state exhibits a kink at a low temperature due to a
change of the direction of q in contrast to observations in organic
superconductors. It is shown that the kink disappears when the Fermi-surface is
anisotropic to some extent, since the direction of q is locked in an optimum
direction independent of the temperature.Comment: 5 pages, 5 figures, revtex.sty, submitted to J.Phys.Soc.Jp
The transverse magnetoresistance of the two-dimensional chiral metal
We consider the two-dimensional chiral metal, which exists at the surface of
a layered, three-dimensional sample exhibiting the integer quantum Hall effect.
We calculate its magnetoresistance in response to a component of magnetic field
perpendicular to the sample surface, in the low temperature, but macroscopic,
regime where inelastic scattering may be neglected. The magnetoresistance is
positive, following a Drude form with a field scale,
, given by the transverse field strength at which
one quantum of flux, , passes through a rectangle with sides set by the
layer-spacing, , and the elastic mean free path, .
Experimental measurement of this magnetoresistance may therefore provide a
direct determination of the elastic mean free path in the chiral metal.Comment: submitted to Phys Rev
Reduction of Pauli paramagnetic pair-breaking effect in antiferromagnetic superconductors
Antiferromagnetic superconductors in a magnetic field are studied. We examine
a mechanism which significantly reduces the Pauli paramagnetic pair-breaking
effect. The mechanism is realized even in the presence of the orbital
pair-breaking effect. We illustrate it using a three-dimensional model with an
intercalated magnetic subsystem. The upper critical field is calculated for
various parameters. It is shown that the upper critical field can reach several
times the pure Pauli paramagnetic limit. The possible relevance to the large
upper critical field observed in the heavy fermion antiferromagnetic
superconductor CePt_3Si discovered recently is briefly discussed. We try to
understand the large upper critical field in the compound CePt_3Si and
field-induced superconductivity in the compound CePb_3 within a unified
framework.Comment: 5 pages, 2 figures, revtex4, minor correction
New superconducting phases in field-induced organic superconductor lambda-(BETS)2FeCl4
We derive the parallel upper critical field, Hc2, as a function of the
temperature T in quasi-2D organic compound lambda-(BETS)2FeCl4, accounting for
the formation of the nonuniform LOFF state. To further check the 2D LOFF model
we propose to study the Hc2(T) curve at low T in tilted fields, where the
vortex state is described by the high Landau level functions characterized by
the index n. We predict a cascade of first order transitions between vortex
phases with different n, between phases with different types of the symmetry at
given n and the change of the superconducting transition from the second order
to the first order as FeCl4 ions are replaced partly by GaCl4 ions.Comment: 4 pages, 3 figures, to be published in PR
d-Wave Spin Density Wave phase in the Attractive Hubbard Model with Spin Polarization
We investigate the possibility of unconventional spin density wave (SDW) in
the attractive Hubbard model with finite spin polarization. We show that
pairing and density fluctuations induce the transverse d-wave SDW near the
half-filling. This novel SDW is related to the d-wave superfluidity induced by
antiferromagnetic spin fluctuations, in the sense that they are connected with
each other through Shiba's attraction-repulsion transformation. Our results
predict the d-wave SDW in real systems, such as cold Fermi atom gases with
population imbalance and compounds involving valence skipper elements
Graphite and Graphene Fairy Circles:A Bottom-Up Approach for the Formation of Nanocorrals
A convenient covalent functionalization approach and nanopatterning method of graphite and graphene is developed. In contrast to expectations, electrochemically activated dediazotization of a mixture of two aryl diazonium compounds in aqueous media leads to a spatially inhomogeneous functionalization of graphitic surfaces, creating covalently modified surfaces with quasi-uniform spaced islands of pristine graphite or graphene, coined nanocorrals. Cyclic voltammetry and chronoamperometry approaches are compared. The average diameter (45-130 nm) and surface density (20-125 corrals/μm 2 ) of these nanocorrals are tunable. These chemically modified nanostructured graphitic (CMNG) surfaces are characterized by atomic force microscopy, scanning tunneling microscopy, Raman spectroscopy and microscopy, and X-ray photoelectron spectroscopy. Mechanisms leading to the formation of these CMNG surfaces are discussed. The potential of these surfaces to investigate supramolecular self-assembly and on-surface reactions under nanoconfinement conditions is demonstrated. © 2019 American Chemical Society
de Haas-van Alphen Effect in the Two-Dimensional and the Quasi-Two-Dimensional Systems
We study the de Haas-van Alphen (dHvA) oscillation in two-dimensional and
quasi-two-dimensional systems. We give a general formula of the dHvA
oscillation in two-dimensional multi-band systems. By using this formula, the
dHvA oscillation and its temperature-dependence for the two-band system are
shown. By introducing the interlayer hopping , we examine the crossover
from the two-dimension, where the oscillation of the chemical potential plays
an important role in the magnetization oscillation, to the three-dimension,
where the oscillation of the chemical potential can be neglected as is well
know as the Lifshitz and Kosevich formula. The crossover is seen at , where a and b are lattice constants, is the flux
quantum and 8t is the width of the total energy band. We also study the dHvA
oscillation in quasi-two-dimensional magnetic breakdown systems. The quantum
interference oscillations such as oscillation as well as the
fundamental oscillations are suppressed by the interlayer hopping , while
the oscillation gradually increases as increases and it
has a maximum at . This interesting dependence on the
dimensionality can be observed in the quasi-two-dimensional organic conductors
with uniaxial pressure.Comment: 11 pages, 14 figure
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