1,263 research outputs found
Magnetic anisotropy and spin-spiral wave in V, Cr and Mn atomic chains on Cu(001) surface: First principles calculations
Recent ab intio studies of the magnetic properties of all 3d transition
metal(TM) freestanding atomic chains predicted that these nanowires could have
a giant magnetic anisotropy energy (MAE) and might support a spin-spiral
structure, thereby suggesting that these nanowires would have technological
applicationsin, e.g., high density magnetic data storages. In order to
investigate how the substrates may affect the magnetic properties of the
nanowires, here we systematically study the V, Cr and Mn linear atomic chains
on the Cu(001) surface based on the density functional theory with the
generalized gradient approximation. We find that V, Cr, and Mn linear chains on
the Cu(001) surface still have a stable or metastable ferromagnetic state.
However, the ferromagnetic state is unstable against formation of a
noncollinear spin-spiral structure in the Mn linear chains and also the V
linear chain on the atop sites on the Cu(001) surface, due to the frustrated
magnetic interactions in these systems. Nonetheless, the presence of the
Cu(001) substrate does destabilize the spin-spiral state already present in the
freestanding V linear chain and stabilizes the ferromagnetic state in the V
linear chain on the hollow sites on Cu(001). When spin-orbit coupling (SOC) is
included, the spin magnetic moments remain almost unchanged, due to the
weakness of SOC in 3d TM chains. Furthermore, both the orbital magnetic moments
and MAEs for the V, Cr and Mn are small, in comparison with both the
corresponding freestanding nanowires and also the Fe, Co and Ni linear chains
on the Cu (001) surface.Comment: Accepted for publication in J. Phys. D: Applied Physic
Collective synchronization in spatially extended systems of coupled oscillators with random frequencies
We study collective behavior of locally coupled limit-cycle oscillators with
random intrinsic frequencies, spatially extended over -dimensional
hypercubic lattices. Phase synchronization as well as frequency entrainment are
explored analytically in the linear (strong-coupling) regime and numerically in
the nonlinear (weak-coupling) regime. Our analysis shows that the oscillator
phases are always desynchronized up to , which implies the lower critical
dimension for phase synchronization. On the other hand, the
oscillators behave collectively in frequency (phase velocity) even in three
dimensions (), indicating that the lower critical dimension for frequency
entrainment is . Nonlinear effects due to periodic nature of
limit-cycle oscillators are found to become significant in the weak-coupling
regime: So-called {\em runaway oscillators} destroy the synchronized (ordered)
phase and there emerges a fully random (disordered) phase. Critical behavior
near the synchronization transition into the fully random phase is unveiled via
numerical investigation. Collective behavior of globally-coupled oscillators is
also examined and compared with that of locally coupled oscillators.Comment: 18 pages, 18 figure
Anisotropic dynamics of a vicinal surface under the meandering step instability
We investigate the nonlinear evolution of the Bales-Zangwill instability,
responsible for the meandering of atomic steps on a growing vicinal surface. We
develop an asymptotic method to derive, in the continuous limit, an evolution
equation for the two-dimensional step flow. The dynamics of the crystal surface
is greatly influenced by the anisotropy inherent to its geometry, and is
characterized by the coarsening of undulations along the step direction and by
the elastic relaxation in the mean slope direction. We demonstrate, using
similarity arguments, that the coalescence of meanders and the step flow follow
simple scaling laws, and deduce the exponents of the characteristic length
scales and height amplitude. The relevance of these results to experiments is
discussed.Comment: 10 pages, 7 figures; submitted to Phys. Rev.
Medical Student Burnout in a Small-Sized Medical School
Introduction: Burnout is an occupational condition characterized by emotional exhaustion, depersonalization, and a low sense of personal accomplishment. While medical students begin schooling with mental health profiles similar to or better than peers who pursue other careers, there is a downward trajectory throughout school suggesting this phenomenon often originates in medical school. For physicians and residents, burnout has been linked to poor outcomes such as patient safety, might contribute to suicidal ideation and substance abuse, and may undermine professional development. Furthermore, there is a lack of surveillance of the prevalence of medical student burnout in a small-sized school setting.
Methods: The Maslach Burnout Inventory (MBI), a 22-question survey, is largely accepted as the gold standard for assessment; however, we utilized the 7-question, Well-Being Index (WBI), which has been shown equal efficacy as the full MBI. Eligible participants were currently enrolled in their respective class at the East Tennessee State University Quillen College of Medicine. Each year, a participant was given a WBI survey during the winter season (overall response rate 83%, n = 239).
Results: Overall the self-reported burnout rate over the two-year study period was 65.2% and was significantly higher in those reporting as female (71%). There was also variation tracking the class from one year to the next. The second year at this institution showed the highest reported amount of burnout (75%, n=145) while the lowest amount of burnout reported was during the fourth year at 47%.
Conclusions: Burnout experienced at this institution was reportedly higher than national average. There are limitations to this study as the periods in which medical students were asked to answer the survey were consistently at the same time in the calendar year, but the host institution’s curriculum had been changed so that it might not match up accordingly. Furthermore, class sizes changed from year to year and might skew the data. This information suggests that burnout prevalence is higher at Quillen College of Medicine and intervention strategies to address burnout should be pursued
Extremely Correlated Quantum Liquids
We formulate the theory of an extremely correlated electron liquid,
generalizing the standard Fermi liquid. This quantum liquid has specific
signatures in various physical properties, such as the Fermi surface volume and
the narrowing of electronic bands by spin and density correlation functions.
We use Schwinger's source field idea to generate equations for the Greens
function for the Hubbard operators. A local (matrix) scale transformation in
the time domain to a quasiparticle Greens function, is found to be optimal.
This transformation allows us to generate vertex functions that are guaranteed
to reduce to the bare values for high frequencies, i.e. are ``asymptotically
free''. The quasiparticles are fractionally charged objects, and we find an
exact Schwinger Dyson equation for their Greens function. We find a hierarchy
of equations for the vertex functions, and further we obtain Ward identities so
that systematic approximations are feasible.
An expansion in terms of the density of holes measured from the Mott Hubbard
insulating state follows from the nature of the theory. A systematic
presentation of the formalism is followed by some preliminary explicit
calculations.Comment: 40 pages, typos remove
Molecular dynamics simulations of lead clusters
Molecular dynamics simulations of nanometer-sized lead clusters have been
performed using the Lim, Ong and Ercolessi glue potential (Surf. Sci. {\bf
269/270}, 1109 (1992)). The binding energies of clusters forming crystalline
(fcc), decahedron and icosahedron structures are compared, showing that fcc
cuboctahedra are the most energetically favoured of these polyhedral model
structures. However, simulations of the freezing of liquid droplets produced a
characteristic form of ``shaved'' icosahedron, in which atoms are absent at the
edges and apexes of the polyhedron. This arrangement is energetically favoured
for 600-4000 atom clusters. Larger clusters favour crystalline structures.
Indeed, simulated freezing of a 6525-atom liquid droplet produced an imperfect
fcc Wulff particle, containing a number of parallel stacking faults. The
effects of temperature on the preferred structure of crystalline clusters below
the melting point have been considered. The implications of these results for
the interpretation of experimental data is discussed.Comment: 11 pages, 18 figues, new section added and one figure added, other
minor changes for publicatio
Formation of atomic tritium clusters and condensates
We present an extensive study of the static and dynamic properties of systems
of spin-polarized tritium atoms. In particular, we calculate the two-body
|F,m_F>=|0,0> s-wave scattering length and show that it can be manipulated via
a Feshbach resonance at a field strength of about 870G. Such a resonance might
be exploited to make and control a Bose-Einstein condensate of tritium in the
|0,0> state. It is further shown that the quartet tritium trimer is the only
bound hydrogen isotope and that its single vibrational bound state is a
Borromean state. The ground state properties of larger spin-polarized tritium
clusters are also presented and compared with those of helium clusters.Comment: 5 pages, 3 figure
Magnons in real materials from density-functional theory
We present an implementation of the adiabatic spin-wave dynamics of Niu and
Kleinman. This technique allows to decouple the spin and charge excitations of
a many-electron system using a generalization of the adiabatic approximation.
The only input for the spin-wave equations of motion are the energies and Berry
curvatures of many-electron states describing frozen spin spirals. The latter
are computed using a newly developed technique based on constrained
density-functional theory, within the local spin density approximation and the
pseudo-potential plane-wave method. Calculations for iron show an excellent
agreement with experiments.Comment: 1 LaTeX file and 1 postscript figur
Massless Dirac Fermions, Gauge Fields, and Underdoped Cuprates
We study 2+1 dimensional massless Dirac fermions and bosons coupled to a U(1)
gauge field as a model for underdoped cuprates. We find that the uniform
susceptibility and the specific heat coefficient are logarithmically enhanced
(compared to linear-in-T behavior) due to the fluctuation of transverse gauge
field which is the only massless mode at finite boson density. We analyze
existing data, and find good agreement in the spin gap phase. Within our
picture, the drop of the susceptibility below the superconducting T_c arises
from the suppression of gauge fluctuations.Comment: 4 pages, REVTEX, 1 eps figur
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