393 research outputs found
Ehrenfest relations and magnetoelastic effects in field-induced ordered phases
Magnetoelastic properties in field-induced magnetic ordered phases are
studied theoretically based on a Ginzburg-Landau theory. A critical field for
the field-induced ordered phase is obtained as a function of temperature and
pressure, which determine the phase diagram. It is found that magnetic field
dependence of elastic constant decreases discontinuously at the critical field,
Hc, and that it decreases linearly with field in the ordered phase (H>Hc). We
found an Ehrenfest relation between the field dependence of the elastic
constant and the pressure dependence of critical field. Our theory provides the
theoretical form for magnetoelastic properties in field- and pressure-induced
ordered phases.Comment: 7 pages, 3 figure
Competition of crystal field splitting and Hund's rule coupling in two-orbital magnetic metal-insulator transitions
Competition of crystal field splitting and Hund's rule coupling in magnetic
metal-insulator transitions of half-filled two-orbital Hubbard model is
investigated by multi-orbital slave-boson mean field theory. We show that with
the increase of Coulomb correlation, the system firstly transits from a
paramagnetic (PM) metal to a {\it N\'{e}el} antiferromagnetic (AFM) Mott
insulator, or a nonmagnetic orbital insulator, depending on the competition of
crystal field splitting and the Hund's rule coupling. The different AFM Mott
insulator, PM metal and orbital insulating phase are none, partially and fully
orbital polarized, respectively. For a small and a finite crystal
field, the orbital insulator is robust. Although the system is nonmagnetic, the
phase boundary of the orbital insulator transition obviously shifts to the
small regime after the magnetic correlations is taken into account. These
results demonstrate that large crystal field splitting favors the formation of
the orbital insulating phase, while large Hund's rule coupling tends to destroy
it, driving the low-spin to high-spin transition.Comment: 4 pages, 4 figure
Magnetic order and exchange couplings in the frustrated diamond lattice antiferromagnet MnScSe
We report the magnetic properties of -site spinel compound MnScSe.
The macroscopic magnetic measurements uncovers successive magnetic transitions
at = 2.04 K, followed by two further transitions at
=1.8 K and =1.6 K. Neutron powder diffraction reveals
that both, and , orders are
associated with the propagation vector =(3/4 3/4 0), while the magnetic
structures are collinear amplitude modulated and helical, respectively. Using
neutron powder spectroscopy we demonstrated the effect of substitution of S by
Se on the magnetic exchange. The energy range of the spin-wave excitations is
supressed due to the chemical pressure of the - ion in MnSc (=S,
Se) spinels.Comment: Accepted in Phys. Rev.
Informed consent in critically ill adults participating to a randomized trial.
The 2014 update of the Swiss law on research increases patients' protection; it adds specific requirements for emergency situations, implying an active search for patients' wishes regarding research participation; the possibility of consent waivers is not clearly stated. We explored its practical impact in a RCT on critically ill adults.
We considered prospectively collected consents of a multicenter trial addressing the impact of continuous EEG on survival. We assessed the proportions of consents obtained strictly according to the law, of specific waivers for this study obtained from the IRB (early death; relatives' unavailability despite repeated attempts), and the yield of retrieving statements on willingness to research participation. We compared the proportion of consent refusals with those of recent trials in similar environments, and estimated the potential impact on study results.
Of 402 recruited patients, six had double inclusions, one died before intervention, and 27 (6.7%, alive on long-term) were excluded following consent refusal or withdrawal, leaving 368 analyzable patients. Specific waivers allowed inclusion of 134 (36.4%) patients, while informed consents were obtained for all others. A statement of willingness to research participation was found in only 14.1%. In recent trials, consent refusal oscillated between 0%-23%, according to different waiver policies.
Consent waivers should be specifically foreseen to prevent losing a potentially relevant proportion of patients reaching endpoints, and ensure results generalizability. The yield of looking for willingness to research participation seems low; this questions its current usefulness and calls for a public awareness campaign
Pressure-Induced Magnetic Quantum Phase Transitions from Gapped Ground State in TlCuCl3
Magnetization maesurements under hydrostatic pressure were performed on an
S=1/2 coupled spin system TlCuCl3 with a gapped ground state under magnetic
field H parallel to the [2,0,1] direction. With increasing applied pressure P,
the gap decreases and closes completely at Pc=0.42 kbar. For P>Pc, TlCuCl3
undergoes antiferromagnetic ordering. A spin-flop transition was observed at
Hsf=0.7T. The spin-flop field is approximately independent of pressure,
although the sublattice magnetization increases with pressure. The gap and Neel
temperature are presented as function is attributed to to the relative
enhancement of the interdimer exchange interactions compared with the
intradimer exchange interaction.Comment: 4pages,3figures To be published in J. Phys. Soc. Jpn. Vol.73 No.1
Dynamics of an anisotropic Haldane antiferromagnet in strong magnetic field
We report the results of elastic and inelastic neutron scattering experiments
on the Haldane gap quantum antiferromagnet Ni(C5D14N2)2N3(PF6) performed at mK
temperatures in a wide range of magnetic field applied parallel to the S = 1
spin chains. Even though this geometry is closest to an ideal axially symmetric
configuration, the Haldane gap closes at the critical field Hc~4T, but reopens
again at higher fields. The field dependence of the two lowest magnon modes is
experimentally studied and the results are compared with the predictions of
several theoretical models. We conclude that of several existing theories, only
the recently proposed model [Zheludev et al., cond-mat/0301424 ] is able to
reproduce all the features observed experimentally for different field
orientations.Comment: 11 pages 8 figures submitted to Phys. Rev.
Field-Induced Quantum Criticality of Systems with Ferromagnetically Coupled Structural Spin Units
The field-induced quantum criticality of compounds with ferromagnetically
coupled structural spin units (as dimers and ladders) is explored by applying
Wilson's renormalization group framework to an appropriate effective action. We
determine the low-temperature phase boundary and the behavior of relevant
quantities decreasing the temperature with the applied magnetic field fixed at
its quantum critical point value. In this context, a plausible interpretation
of some recent experimental results is also suggested.Comment: to be published in Physics Letters
The Out-of-Equilibrium Time-Dependent Gutzwiller Approximation
We review the recently proposed extension of the Gutzwiller approximation, M.
Schiro' and M. Fabrizio, Phys. Rev. Lett. 105, 076401 (2010), designed to
describe the out-of-equilibrium time-evolution of a Gutzwiller-type variational
wave function for correlated electrons. The method, which is strictly
variational in the limit of infinite lattice-coordination, is quite general and
flexible, and it is applicable to generic non-equilibrium conditions, even far
beyond the linear response regime. As an application, we discuss the quench
dynamics of a single-band Hubbard model at half-filling, where the method
predicts a dynamical phase transition above a critical quench that resembles
the sharp crossover observed by time-dependent dynamical mean field theory. We
next show that one can actually define in some cases a multi-configurational
wave function combination of a whole set of mutually orthogonal Gutzwiller wave
functions. The Hamiltonian projected in that subspace can be exactly evaluated
and is equivalent to a model of auxiliary spins coupled to non-interacting
electrons, closely related to the slave-spin theories for correlated electron
models. The Gutzwiller approximation turns out to be nothing but the mean-field
approximation applied to that spin-fermion model, which displays, for any
number of bands and integer fillings, a spontaneous symmetry breaking
that can be identified as the Mott insulator-to-metal transition.Comment: 25 pages. Proceedings of the Hvar 2011 Workshop on 'New materials for
thermoelectric applications: theory and experiment
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