1,861 research outputs found
Health Reforms in Mexico City. 2000-2006
Objectives: To describe the reform applied to the health system of Mexico City between 2000 and 2006 and to present their results
A flow equation approach to periodically driven quantum systems
We present a theoretical method to generate a highly accurate {\em
time-independent} Hamiltonian governing the finite-time behavior of a
time-periodic system. The method exploits infinitesimal unitary transformation
steps, from which renormalization group-like flow equations are derived to
produce the effective Hamiltonian. Our tractable method has a range of validity
reaching into frequency regimes that are usually inaccessible via high
frequency expansions in the parameter , where is the
upper limit for the strength of local interactions. We demonstrate our approach
on both interacting and non-interacting many-body Hamiltonians where it offers
an improvement over the more well-known Magnus expansion and other high
frequency expansions. For the interacting models, we compare our approximate
results to those found via exact diagonalization. While the approximation
generally performs better globally than other high frequency approximations,
the improvement is especially pronounced in the regime of lower frequencies and
strong external driving. This regime is of special interest because of its
proximity to the resonant regime where the effect of a periodic drive is the
most dramatic. Our results open a new route towards identifying novel
non-equilibrium regimes and behaviors in driven quantum many-particle systems.Comment: 25 pages, 14 figure
Magnon thermal Hall effect in kagome antiferromagnets with Dzyaloshinskii-Moriya interactions
We theoretically study magnetic and topological properties of
antiferromagnetic kagome spin systems in the presence of both in- and
out-of-plane Dzyaloshinskii-Moriya interactions. In materials such as the iron
jarosites, the in-plane interactions stabilize a canted noncollinear "umbrella"
magnetic configuration with finite scalar spin chirality. We derive expressions
for the canting angle, and use the resulting order as a starting point for a
spin-wave analysis. We find topological magnon bands, characterized by non-zero
Chern numbers. We calculate the magnon thermal Hall conductivity, and propose
the iron jarosites as a promising candidate system for observing the magnon
thermal Hall effect in a noncollinear spin configuration. We also show that the
thermal conductivity can be tuned by varying an applied magnetic field, or the
in-plane Dzyaloshinskii-Moriya strength. In contrast with previous studies of
topological magnon bands, the effect is found to be large even in the limit of
small canting.Comment: 11 pages, 11 figure
Near- to mid-infrared picosecond optical parametric oscillator based on periodically poled RbTiOAsO4
We describe a Ti:sapphire-pumped picosecond optical parametric oscillator based on periodically poled RbTiOAsO4 that is broadly tunable in the near to mid infrared. A 4.5-mm single-grating crystal at room temperature in combination with pump wavelength tuning provided access to a continuous-tuning range from 3.35 to 5 mu m, and a pump power threshold of 90 mW was measured. Average mid-infrared output powers in excess of 100 mW and total output powers of 400 mW in similar to 1-ps pulses were obtained at 33% extraction efficiency. (C) 1998 Optical Society of America.</p
Momentum-space entanglement after a quench in one-dimensional disordered fermionic systems
We numerically investigate the momentum-space entanglement entropy and
entanglement spectrum of the random-dimer model and its generalizations, which
circumvent Anderson localization, after a quench in the Hamiltonian parameters.
The type of dynamics that occurs depends on whether or not the Fermi level of
the initial state is near the energy of the delocalized states present in these
models. If the Fermi level of the initial state is near the energy of the
delocalized states, we observe an interesting slow logarithmic-like growth of
the momentum-space entanglement entropy followed by an eventual saturation.
Otherwise, the momentum-space entanglement entropy is found to rapidly
saturate. We also find that the momentum-space entanglement spectrum reveals
the presence of delocalized states in these models for long times after the
quench and the many-body entanglement gap decays logarithmically in time when
the Fermi level is near the energy of the delocalized states.Comment: 4+e pages, 3 figure
Analogue of Hamilton-Jacobi theory for the time-evolution operator
In this paper we develop an analogue of Hamilton-Jacobi theory for the
time-evolution operator of a quantum many-particle system. The theory offers a
useful approach to develop approximations to the time-evolution operator, and
also provides a unified framework and starting point for many well-known
approximations to the time-evolution operator. In the important special case of
periodically driven systems at stroboscopic times, we find relatively simple
equations for the coupling constants of the Floquet Hamiltonian, where a
straightforward truncation of the couplings leads to a powerful class of
approximations. Using our theory, we construct a flow chart that illustrates
the connection between various common approximations, which also highlights
some missing connections and associated approximation schemes. These missing
connections turn out to imply an analytically accessible approximation that is
the "inverse" of a rotating frame approximation and thus has a range of
validity complementary to it. We numerically test the various methods on the
one-dimensional Ising model to confirm the ranges of validity that one would
expect from the approximations used. The theory provides a map of the relations
between the growing number of approximations for the time-evolution operator.
We describe these relations in a table showing the limitations and advantages
of many common approximations, as well as the new approximations introduced in
this paper.Comment: 17 pages, 5 figures, 1 tabl
A Nordic survey of the management of palliative care in patients with head and neck cancer
Background The five Nordic countries with a population of 27M people form a rather homogenous region in terms of health care. The management of Head and Neck Cancer (HNC) is centralized to the 21 university hospitals in these countries. Our aim was to survey the current status of organization of palliative care for patients with HNC in the Nordic countries as the field is rapidly developing. Materials and methods A structured web-based questionnaire was sent to all the Departments of Otorhinolaryngology-Head and Neck Surgery and Oncology managing HNC in the Nordic countries. Results All 21 (100%) Nordic university hospitals responded to the survey. A majority (over 90%) of the patients are discussed at diagnosis in a multidisciplinary tumor board (MDT), but the presence of a palliative care specialist is lacking in 95% of these MDT's. The patients have access to specialized palliative care units (n = 14, 67%), teams (n = 10, 48%), and consultants (n = 4, 19%) in the majority of the hospitals. Conclusion The present results show that specialized palliative care services are available at the Nordic university hospitals. A major finding was that the collaboration between head and neck surgeons, oncologists and palliative care specialists is not well structured and the palliative care pathway of patients with HNC is not systematically organized. We suggest that early integrated palliative care needs to be included as an addition to the already existing HNC care pathways in the Nordic countries.Peer reviewe
Ultrasonography and colour Doppler in juvenile idiopathic arthritis: Diagnosis and follow-up of ultrasound-guided steroid injection in the wrist
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