2,066 research outputs found
GdRhSi: An exemplary tetragonal system for antiferromagnetic order with weak in-plane anisotropy
The anisotropy of magnetic properties commonly is introduced in textbooks
using the case of an antiferromagnetic system with Ising type anisotropy. This
model presents huge anisotropic magnetization and a pronounced metamagnetic
transition and is well-known and well-documented both, in experiments and
theory. In contrast, the case of an antiferromagnetic - system with weak
in-plane anisotropy is only poorly documented. We studied the anisotropic
magnetization of the compound GdRhSi and found that it is a perfect
model system for such a weak-anisotropy setting because the Gd ions in
GdRhSi have a pure spin moment of S=7/2 which orders in a simple AFM
structure with . We observed experimentally in a
continuous spin-flop transition and domain effects for field applied along the
- and the -direction, respectively. We applied a mean field model
for the free energy to describe our data and combine it with an Ising chain
model to account for domain effects. Our calculations reproduce the
experimental data very well. In addition, we performed magnetic X-ray
scattering and X-ray magnetic circular dichroism measurements, which confirm
the AFM propagation vector to be and indicate the absence of
polarization on the rhodium atoms
Paramagnon dispersion in -FeSe observed by Fe -edge resonant inelastic x-ray scattering
We report an Fe -edge resonant inelastic x-ray scattering (RIXS) study of
the unusual superconductor -FeSe. The high energy resolution of this
RIXS experiment (55meV FWHM) made it possible to resolve
low-energy excitations of the Fe manifold. These include a broad peak
which shows dispersive trends between 100-200meV along the and
directions of the one-Fe square reciprocal lattice, and which can
be attributed to paramagnon excitations. The multi-band valence state of FeSe
is among the most metallic in which such excitations have been discerned by
soft x-ray RIXS
High-energy magnetic excitations in overdoped LaSrCuO studied by neutron and resonant inelastic X-ray scattering
We have performed neutron inelastic scattering and resonant inelastic X-ray
scattering (RIXS) at the Cu- edge to study high-energy magnetic
excitations at energy transfers of more than 100 meV for overdoped
LaSrCuO with ( K) and
(non-superconducting) using identical single crystal samples for the two
techniques. From constant-energy slices of neutron scattering cross-sections,
we have identified magnetic excitations up to ~250 meV for . Although
the width in the momentum direction is large, the peak positions along the (pi,
pi) direction agree with the dispersion relation of the spin-wave in the
non-doped LaCuO (LCO), which is consistent with the previous RIXS
results of cuprate superconductors. Using RIXS at the Cu- edge, we have
measured the dispersion relations of the so-called paramagnon mode along both
(pi, pi) and (pi, 0) directions. Although in both directions the neutron and
RIXS data connect with each other and the paramagnon along (pi, 0) agrees well
with the LCO spin-wave dispersion, the paramagnon in the (pi, pi) direction
probed by RIXS appears to be less dispersive and the excitation energy is lower
than the spin-wave of LCO near (pi/2, pi/2). Thus, our results indicate
consistency between neutron inelastic scattering and RIXS, and elucidate the
entire magnetic excitation in the (pi, pi) direction by the complementary use
of two probes. The polarization dependence of the RIXS profiles indicates that
appreciable charge excitations exist in the same energy range of magnetic
excitations, reflecting the itinerant character of the overdoped sample. A
possible anisotropy in the charge excitation intensity might explain the
apparent differences in the paramagnon dispersion in the (pi, pi) direction as
detected by the X-ray scattering.Comment: 7 pages, 7 figure
High-energy spin and charge excitations in electron-doped copper oxide superconductors
The evolution of electronic (spin and charge) excitations upon carrier doping
is an extremely important issue in superconducting layered cuprates and the
knowledge of its asymmetry between electron- and hole-dopings is still
fragmentary. Here we combine x-ray and neutron inelastic scattering
measurements to track the doping dependence of both spin and charge excitations
in electron-doped materials. Copper L3 resonant inelastic x-ray scattering
spectra show that magnetic excitations shift to higher energy upon doping.
Their dispersion becomes steeper near the magnetic zone center and deeply mix
with charge excitations, indicating that electrons acquire a highly itinerant
character in the doped metallic state. Moreover, above the magnetic
excitations, an additional dispersing feature is observed near the
{\Gamma}-point, and we ascribe it to particle-hole charge excitations. These
properties are in stark contrast with the more localized spin-excitations
(paramagnons) recently observed in hole-doped compounds even at high
doping-levels.Comment: 20 page
Universal conservation law and modified Noether symmetry in 2d models of gravity with matter
It is well-known that all 2d models of gravity---including theories with
nonvanishing torsion and dilaton theories---can be solved exactly, if matter
interactions are absent. An absolutely (in space and time) conserved quantity
determines the global classification of all (classical) solutions. For the
special case of spherically reduced Einstein gravity it coincides with the mass
in the Schwarzschild solution. The corresponding Noether symmetry has been
derived previously by P. Widerin and one of the authors (W.K.) for a specific
2d model with nonvanishing torsion. In the present paper this is generalized to
all covariant 2d theories, including interactions with matter. The related
Noether-like symmetry differs from the usual one. The parameters for the
symmetry transformation of the geometric part and those of the matterfields are
distinct. The total conservation law (a zero-form current) results from a two
stage argument which also involves a consistency condition expressed by the
conservation of a one-form matter ``current''. The black hole is treated as a
special case.Comment: 3
On the Consistency of the Exact Renormalization Group Approach Applied to Gauge Theories in Algebraic Non-Covariant Gauges
We study a class of Wilsonian formulations of non-Abelian gauge theories in
algebraic non-covariant gauges where the Wilsonian infrared cutoff is
inserted as a mass term for the propagating fields. In this way the
Ward-Takahashi identities are preserved to all scales. Nevertheless
BRST-invariance in broken and the theory is gauge-dependent and unphysical at
. Then we discuss the infrared limit . We show that
the singularities of the axial gauge choice are avoided in planar gauge and
light-cone gauge. In addition the issue of infrared divergences is addressed in
some explicit example. Finally the rectangular Wilson loop of size is evaluated at lowest order in perturbation theory and a non commutativity
between the limits and is pointed out.Comment: Latex2e, 49 pages, 2 EPS figures. Misprints corrected. Version to be
published on IJMP
How chemical pressure affects the fundamental properties of rare-earth pnictides: an ARPES view
Angle-resolved photoelectron spectroscopy, supplemented by theoretical
calculations has been applied to study the electronic structure of
heavy-fermion material CeFePO, a homologue to the Fe-based high-temperature
superconductors, and CeFeAs_0.7P_0.3O, where the applied chemical pressure
results in a ferromagnetic order of the 4f moments. A comparative analysis
reveals characteristic differences in the Fe-derived band structure for these
materials, implying a rather different hybridization of valence electrons to
the localized 4f orbitals. In particular, our results suggest that the
ferromagnetism of Ce moments in CeFeAs_0.7P_0.3O is mediated mainly by Fe
3d_xz/yz orbitals, while the Kondo screening in CeFePO is instead due to a
strong interaction of Fe 3d_3z^2-r^2 orbitals.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. B (Rapid
Magnetic excitations in stripe-ordered LaBaCuO studied using resonant inelastic x-ray scattering
The charge and spin correlations in LaBaCuO (LBCO
1/8) are studied using Cu edge resonant inelastic x-ray scattering
(RIXS). The static charge order (CO) is observed at a wavevector of
and its charge nature confirmed by measuring the dependence of this peak on the
incident x-ray polarization. The paramagnon excitation in LBCO 1/8 is then
measured as it disperses through the CO wavevector. Within the experimental
uncertainty no changes are observed in the paramagnon due to the static CO, and
the paramagnon seems to be similar to that measured in other cuprates, which
have no static CO. Given that the stripe correlation modulates both the charge
and spin degrees of freedom, it is likely that subtle changes do occur in the
paramagnon due to CO. Consequently, we propose that future RIXS measurements,
realized with higher energy resolution and sensitivity, should be performed to
test for these effects.Comment: 5 pages, 4 figure
Quantum Gauge Equivalence in QED
We discuss gauge transformations in QED coupled to a charged spinor field,
and examine whether we can gauge-transform the entire formulation of the theory
from one gauge to another, so that not only the gauge and spinor fields, but
also the forms of the operator-valued Hamiltonians are transformed. The
discussion includes the covariant gauge, in which the gauge condition and
Gauss's law are not primary constraints on operator-valued quantities; it also
includes the Coulomb gauge, and the spatial axial gauge, in which the
constraints are imposed on operator-valued fields by applying the
Dirac-Bergmann procedure. We show how to transform the covariant, Coulomb and
spatial axial gauges to what we call
``common form,'' in which all particle excitation modes have identical
properties. We also show that, once that common form has been reached, QED in
different gauges has a common time-evolution operator that defines
time-translation for states that represent systems of electrons and photons.
By combining gauge transformations with changes of representation from
standard to common form, the entire apparatus of a gauge theory can be
transformed from one gauge to another.Comment: Contribution for a special issue of Foundations of Physics honoring
Fritz Rohrlich; edited by Larry P. Horwitz, Tel-Aviv University, and Alwyn
van der Merwe, University of Denver (Plenum Publishing, New York); 40 pages,
REVTEX, Preprint UCONN-93-3, 1 figure available upon request from author
Production and characterization of monoclonal antibodies raised against recombinant human granzymes A and B and showing cross reactions with the natural proteins
The human serine proteases granzymes A and B are expressed in cytotoplasmic granules of activated cytotoxic T lymphocytes and natural killer cells. Recombinant granzyme A and granzyme B proteins were produced in bacteria, purified and then used to raise specific mouse monoclonal antibodies. Seven monoclonal antibodies (mAb) were raised against granzyme A, which all recognized the same or overlapping epitopes. They reacted specifically in an immunoblot of interleukin-2 (IL-2) stimulated PBMNC with a disulfide-linked homodimer of 43 kDa consisting of 28 kDa subunits. Seven mAb against granzyme B were obtained, which could be divided into two groups, each recognizing a different epitope. On an immunoblot, all mAb reacted with a monomer of 33 kDa protein. By immunohistochemistry, these mAb could be used to detect granzymes A and B expression in activated CTL and NK cells. The availability of these mAb may facilitate studies on the role of human cytotoxic cells in various immune reactions and may contribute to a better understanding of the role of granzmes A and B in the cytotoxic response in vivo
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