1,104 research outputs found
Metamagnetism of itinerant electrons in multi-layer ruthenates
The problem of quantum criticality in the context of itinerant ferro- or
metamagnetism has received considerable attention [S. A. Grigera et. al.,
Science 294, 329 (2001); C. Pfleiderer et. al., Nature, 414, 427 (2001)]. It
has been proposed that a new kind of quantum criticality is realised in
materials such as MnSi or Sr_3Ru_2O_7. We show based on a mean-field theory
that the low-temperature behaviour of the n-layer ruthenates
Sr_{n+1}Ru_nO_{3n+1} can be understood as a result of a Van Hove singularity
(VHS). We consider a single band whose Fermi energy, E_F, is close to the VHS
and deduce a complex phase diagram for the magnetism as a function of
temperature, magnetic field and E_F. The location of E_F with respect to the
VHS depends on the number of layers or can be tuned by pressure. We find that
the ferromagnetic quantum phase transition in this case is not of second but of
first order, with a metamagnetic quantum critical endpoint at high magnetic
field. Despite its simplicity this model describes well the properties of the
uniform magnetism in the single, double and triple layer ruthenates. We would
like to emphasise that the origin of this behaviour lies in the band structure.Comment: 7 pages, 3 figures, typos corrected and acknowledgement added, to
appear in the Europhysics Letter
Magnetic domain formation in itinerant metamagnets
We examine the effects of long-range dipolar forces on metamagnetic
transitions and generalize the theory of Condon domains to the case of an
itinerant electron system undergoing a first-order metamagnetic transition. We
demonstrate that within a finite range of the applied field, dipolar
interactions induce a spatial modulation of the magnetization in the form of
stripes or bubbles. Our findings are consistent with recent observations in the
bilayer ruthenate SrRuO.Comment: 4 pages, 3 figures, minor changes, references adde
Fractional vortices on grain boundaries --- the case for broken time reversal symmetry in high temperature superconductors
We discuss the problem of broken time reversal symmetry near grain boundaries
in a d-wave superconductor based on a Ginzburg-Landau theory. It is shown that
such a state can lead to fractional vortices on the grain boundary. Both
analytical and numerical results show the structure of this type of state.Comment: 9 pages, RevTeX, 5 postscript figures include
Josephson interferometer in a ring topology as a symmetry prove of Sr_2RuO_4
The Josephson effect is theoretically studied in two types of SQUIDs
consisting of wave superconductor and SrRuO. Results show various
response of the critical Josephson current to applied magnetic fields depending
on the type of SQUID and on the pairing symmetries. In the case of a
wave symmetry, the critical current in a corner SQUID becomes an asymmetric
function of magnetic fields near the critical temperatures. Our results well
explain a recent experimental finding [Nelson et. al, Science \textbf{306},
1151 (2004)]. We also discuss effects of chiral domains on the critical
current.Comment: 7 page
One-dimensional Kondo lattice at partial band filling
An effective Hamiltonian for the localized spins in the one-dimensional Kondo
lattice model is derived via a unitary transformation involving a bosonization
of delocalized conduction electrons. The effective Hamiltonian is shown to
reproduce all the features of the model as identified in various numerical
simulations, and provides much new information on the ferro- to paramagnetic
phase transition and the paramagnetic phase.Comment: 11 pages Revtex, 1 Postscript figure. To appear in Phys. Rev. Let
Half-quantum vortices on c-axis domain walls in chiral p-wave superconductors
Chiral superconductors are two-fold degenerate and domains of opposite
chirality can form, separated by domain walls. There are indications of such
domain formation in the quasi two-dimensional putative chiral -wave
superconductor SrRuO, yet no experiment has explicitly resolved
individual domains in this material. In this work, -axis domain walls lying
parallel to the layers in chiral -wave superconductors are explored from a
theoretical point of view. First, using both a phenomenological Ginzburg-Landau
and a quasiclassical Bogoliubov-deGennes approach, a consistent qualitative
description of the domain wall structure is obtained. While these domains are
decoupled in the isotropic limit, there is a finite coupling in anisotropic
systems and the domain wall can be treated as an effective Josephson junction.
In the second part, the formation and structure of half-quantum vortices (HQV)
on such -axis domain walls are discussed.Comment: 14 pages, 12 figures; to be submitted to NJ
Plaquette bond order wave in the quarter-filled extended Hubbard model on the checkerboard lattice
An extended Hubbard model (including nearest-neighbor repulsion and
antiferromagnetic spin exchange) is investigated on the frustrated checkerboard
lattice, a two-dimensional analog of the pyrochlore lattice. Combining
Gutzwiller renormalized mean-field (MF) calculations, exact diagonalization
(ED) techniques, and a weak-coupling renormalization group (RG) analysis we
provide strong evidence for a crystalline valence bond plaquette phase at
quarter-filling. The ground state is twofold degenerate and breaks translation
symmetry. The bond energies show a staggering while the charge distribution
remains uniform.Comment: 8 pages, 6 figures, published versio
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