862 research outputs found
Statistical mechanical description of liquid systems in electric field
We formulate the statistical mechanical description of liquid systems for
both polarizable and polar systems in an electric field in the
-ensemble, which is the pendant to the thermodynamic description in
terms of the free energy at constant potential. The contribution of the
electric field to the configurational integral in
the -ensemble is given in an exact form as a factor in the
integrand of . We calculate the contribution of the
electric field to the Ornstein-Zernike formula for the scattering function in
the -ensemble. As an application we determine the field induced
shift of the critical temperature for polarizable and polar liquids, and show
that the shift is upward for polarizable liquids and downward for polar
liquids.Comment: 6 page
Loop quantum gravity without the Hamiltonian constraint
We show that under certain technical assumptions, including the existence of
a constant mean curvature (CMC) slice and strict positivity of the scalar
field, general relativity conformally coupled to a scalar field can be
quantised on a partially reduced phase space, meaning reduced only with respect
to the Hamiltonian constraint and a proper gauge fixing. More precisely, we
introduce, in close analogy to shape dynamics, the generator of a local
conformal transformation acting on both, the metric and the scalar field, which
coincides with the CMC gauge condition. A new metric, which is invariant under
this transformation, is constructed and used to define connection variables
which can be quantised by standard loop quantum gravity methods. While it is
hard to address dynamical problems in this framework (due to the complicated
'time' function), it seems, due to good accessibility properties of the CMC
gauge, to be well suited for problems such as the computation of black hole
entropy, where actual physical states can be counted and the dynamics is only
of indirect importance. The corresponding calculation yields the surprising
result that the usual prescription of fixing the Barbero-Immirzi parameter beta
to a constant value in order to obtain the well-known formula S = a(Phi) A/(4G)
does not work for the black holes under consideration, while a recently
proposed prescription involving an analytic continuation of beta to the case of
a self-dual space-time connection yields the correct result. Also, the
interpretation of the geometric operators gets an interesting twist, which
exemplifies the deep relationship between observables and the choice of a time
function and has consequences for loop quantum cosmology.Comment: 8 pages. v2: Journal version. Black hole state counting based on
physical states added. Applications to loop quantum cosmology discussed.
Gauge condition used shown to coincide with CMC gauge. Minor clarifications.
v3: Erroneous topology dependence of the entropy in journal version
corrected, conclusions fixed accordingly. Main results unaffecte
New Variables for Classical and Quantum Gravity in all Dimensions II. Lagrangian Analysis
We rederive the results of our companion paper, for matching spacetime and
internal signature, by applying in detail the Dirac algorithm to the Palatini
action. While the constraint set of the Palatini action contains second class
constraints, by an appeal to the method of gauge unfixing, we map the second
class system to an equivalent first class system which turns out to be
identical to the first class constraint system obtained via the extension of
the ADM phase space performed in our companion paper. Central to our analysis
is again the appropriate treatment of the simplicity constraint. Remarkably,
the simplicity constraint invariant extension of the Hamiltonian constraint,
that is a necessary step in the gauge unfixing procedure, involves a correction
term which is precisely the one found in the companion paper and which makes
sure that the Hamiltonian constraint derived from the Palatini Lagrangian
coincides with the ADM Hamiltonian constraint when Gauss and simplicity
constraints are satisfied. We therefore have rederived our new connection
formulation of General Relativity from an independent starting point, thus
confirming the consistency of this framework.Comment: 42 pages. v2: Journal version. Some nonessential sign errors in
section 2 corrected. Minor clarification
On a partially reduced phase space quantisation of general relativity conformally coupled to a scalar field
The purpose of this paper is twofold: On the one hand, after a thorough
review of the matter free case, we supplement the derivations in our companion
paper on 'loop quantum gravity without the Hamiltonian constraint' with
calculational details and extend the results to standard model matter, a
cosmological constant, and non-compact spatial slices. On the other hand, we
provide a discussion on the role of observables, focussed on the situation of a
symmetry exchange, which is key to our derivation. Furthermore, we comment on
the relation of our model to reduced phase space quantisations based on
deparametrisation.Comment: 51 pages, 5 figures. v2: Gauge condition used shown to coincide with
CMC gauge. Minor clarifications and correction
Determination of the Michel Parameters rho, xi, and delta in tau-Lepton Decays with tau --> rho nu Tags
Using the ARGUS detector at the storage ring DORIS II, we have
measured the Michel parameters , , and for
decays in -pair events produced at
center of mass energies in the region of the resonances. Using
as spin analyzing tags, we find , , , , and . In addition, we report
the combined ARGUS results on , , and using this work
und previous measurements.Comment: 10 pages, well formatted postscript can be found at
http://pktw06.phy.tu-dresden.de/iktp/pub/desy97-194.p
Field-induced effects in the spin liquid candidate PbCuTeO
PbCuTeO is considered as one of the rare candidate materials for a
three-dimensional quantum spin liquid (QSL). This assessment was based on the
results of various magnetic experiments, performed mainly on polycrystalline
material. More recent measurements on single crystals revealed an even more
exotic behavior, yielding ferroelectric order below , accompanied by distinct lattice distortions, and a somewhat
modified magnetic response which is still consistent with a QSL. Here we report
on low-temperature measurements of various thermodynamic, magnetic and
dielectric properties of single crystalline PbCuTeO in magnetic fields
. The combination of these various probes allows us to
construct a detailed - phase diagram including a ferroelectric phase for
and a -induced magnetic phase at
. These phases are preceded by or coincide with a structural
transition from a cubic high-temperature phase into a distorted non-cubic
low-temperature state. The phase diagram discloses two quantum critical points
(QCPs) in the accessible field range, a ferroelectric QCP at =
and a magnetic QCP at = . Field-induced
lattice distortions, observed in the state at and which are
assigned to the effect of spin-orbit interaction of the Cu-ions, are
considered as the key mechanism by which the magnetic field couples to the
dielectric degrees of freedom in this material
Coherent X-ray Scattering from Manganite Charge and Orbital Domains
We report coherent x-ray scattering studies of charge and orbital domains in
manganite systems. The experiments were carried out on LaMnO_3 and
Pr_{0.6}Ca_{0.4}MnO_3, with the incident photon energy tuned near the Mn K
edge. At room temperature, the orbital speckle pattern of LaMnO_3 was observed
to be constant over a timescale of at least minutes, which is indicative of
static orbital domains on this timescale. For Pr_{0.6}Ca_{0.4}MnO_3, both
charge and orbital speckle patterns were observed. The observation of the
latter rules out the presence of fast orbital fluctuations, while long time
series data-- on the order of several minutes-- were suggestive of slow dynamic
behavior. In contrast, the charge order speckle patterns were static.Comment: 6 pages, 4 figure
Interfaces in Diblocks: A Study of Miktoarm Star Copolymers
We study AB miktoarm star block copolymers in the strong segregation
limit, focussing on the role that the AB interface plays in determining the
phase behavior. We develop an extension of the kinked-path approach which
allows us to explore the energetic dependence on interfacial shape. We consider
a one-parameter family of interfaces to study the columnar to lamellar
transition in asymmetric stars. We compare with recent experimental results. We
discuss the stability of the A15 lattice of sphere-like micelles in the context
of interfacial energy minimization. We corroborate our theory by implementing a
numerically exact self-consistent field theory to probe the phase diagram and
the shape of the AB interface.Comment: 12 pages, 11 included figure
Spin liquid and ferroelectricity close to a quantum critical point in PbCuTe2O6
Geometrical frustration among interacting spins combined with strong quantum
fluctuations destabilize long-range magnetic order in favour of more exotic
states such as spin liquids. By following this guiding principle, a number of
spin liquid candidate systems were identified in quasi-two-dimensional
(quasi-2D) systems. For 3D, however, the situation is less favourable as
quantum fluctuations are reduced and competing states become more relevant.
Here we report a comprehensive study of thermodynamic, magnetic and dielectric
properties on single crystalline and pressed-powder samples of PbCuTeO,
a candidate material for a 3D frustrated quantum spin liquid featuring a
hyperkagome lattice. Whereas the low-temperature properties of the powder
samples are consistent with the recently proposed quantum spin liquid state, an
even more exotic behaviour is revealed for the single crystals. These crystals
show ferroelectric order at , accompanied by
strong lattice distortions, and a modified magnetic response -- still
consistent with a quantum spin liquid -- but with clear indications for quantum
critical behaviour.Comment: 59 pages, 15 figures, This version of the article has been accepted
for publication, after peer review but is not the Version of Record and does
not reflect post-acceptance improvements, or any corrections. The Version of
Record is available onlin
Intimal and medial arterial changes defined by ultra-high-frequency ultrasound: Response to changing risk factors in children with chronic kidney disease
BACKGROUND: Patients with chronic kidney disease (CKD) are exposed to both traditional 'Framingham' and uremia related cardiovascular risk factors that drive atherosclerotic and arteriosclerotic disease, but these cannot be differentiated using conventional ultrasound. We used ultra-high-frequency ultrasound (UHFUS) to differentiate medial thickness (MT) from intimal thickness (IT) in CKD patients, identify their determinants and monitor their progression. METHODS: Fifty-four children and adolescents with CKD and 12 healthy controls underwent UHFUS measurements using 55-70MHz transducers in common carotid and dorsal pedal arteries. Annual follow-up imaging was performed in 31 patients. RESULTS: CKD patients had higher carotid MT and dorsal pedal IT and MT compared to controls. The carotid MT in CKD correlated with serum phosphate (p<0.001, r = 0.42), PTH (p = 0.03, r = 0.36) and mean arterial pressure (p = 0.03, r = 0.34). Following multivariable analysis, being on dialysis, serum phosphate levels and mean arterial pressure remained the only independent predictors of carotid MT (R2 64%). Transplanted children had lower carotid and dorsal pedal MT compared to CKD and dialysis patients (p = 0.02 and p = 0.01 respectively). At 1-year follow-up, transplanted children had a decrease in carotid MT (p = 0.01), but an increase in dorsal pedal IT (p = 0.04) that independently correlated with annualized change in BMI. CONCLUSIONS: Using UHFUS, we have shown that CKD is associated with exclusively medial arterial changes that attenuate when the uremic milieu is ameliorated after transplantation. In contrast, after transplantation intimal disease develops as hypertension and obesity become prevalent, representing rapid vascular remodeling in response to a changing cardiovascular risk factor profile
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