46,009 research outputs found

### Molecular Dynamics Simulation of Collisions between Hydrogen and Graphite

Hydrogen adsorption by graphite is examined by classical molecular dynamics
simulation using a modified Brenner REBO potential. Such interactions are
typical in chemical sputtering experiments, and knowledge of the fundamental
behavior of hydrogen and graphene in collisional conditions is essential for
modeling the sputtering mechanism. The hydrogen adsorption rate is found to be
dependent on the incident hydrogen energy and not on graphene temperature.
Rather than destroying the graphene, hydrogen incidence at energies of less
than 100 eV can be classified into three regimes of adsorption, reflection and
penetration through one or more graphene layers. Incidence at the lowest
energies is shown to distort the graphene structure.Comment: 4 pages, 4 figures. Accepted for publication by j. Plasma Phys.
Proccedings for the joint conference of 19th International Conference on
Numerical Simulation of Plasmas and 7th Asia Pacific Plasma Theor

### Cluster vs Single-Spin Algorithms -- Which are More Efficient?

A comparison between single-cluster and single-spin algorithms is made for
the Ising model in 2 and 3 dimensions. We compare the amount of computer time
needed to achieve a given level of statistical accuracy, rather than the speed
in terms of site updates per second or the dynamical critical exponents. Our
main result is that the cluster algorithms become more efficient when the
system size, $L^d$, exceeds, $L\sim 70$--$300$ for $d=2$ and $L\sim 80$--$200$
for $d=3$. The exact value of the crossover is dependent upon the computer
being used. The lower end of the crossover range is typical of workstations
while the higher end is typical of vector computers. Hence, even for
workstations, the system sizes needed for efficient use of the cluster
algorithm is relatively large.Comment: 13pages, postscript file, HLRZ 21/9

### Neutrino mass constraints on beta decay

Using the general connection between the upper limit on the neutrino mass and
the upper limits on certain types of non-Standard Model interaction that can
generate loop corrections to the neutrino mass, we derive constraints on some
non-Standard Model $d\to ue^-\bar{\nu}$ interactions. When cast into limits on
$n\to pe^-\bar{\nu}$ coupling constants, our results yield constraints on
scalar and tensor weak interactions improved by more than an order of magnitude
over the current experimental limits. When combined with the existing limits,
our results yield |C_S/C_V|\alt 5\times 10^{-3}, |C'_S/C_V|\alt 5\times
10^{-3}, |C_T/C_A| \alt 1.2\times 10^{-2} and |C'_T/C_A| \alt 1.2\times
10^{-2}.Comment: 4 pages, 3 figures; revised to reflect referee comments; accepted for
publication in Physical Review Letter

### Magnetoelastic Coupling in the Spin-Dimer System TlCuCl$_3$

We present high-resolution measurements of the thermal expansion and the
magnetostriction of TlCuCl$_{3}$ which shows field-induced antiferromagnetic
order. We find pronounced anomalies in the field and temperature dependence of
different directions of the lattice signaling a large magnetoelastic coupling.
The phase boundary is extremely sensitive to pressure, e.g. the transition
field would change by about +/- 185$%/GPa under uniaxial pressure applied along
certain directions. This drastic effect can unambiguously be traced back to
changes of the intradimer coupling under uniaxial pressure. The interdimer
couplings remain essentially unchanged under pressure, but strongly change when
Tl is replaced by K.Comment: 4 pages with 4 figures include

### Reconfiguration on sparse graphs

A vertex-subset graph problem Q defines which subsets of the vertices of an
input graph are feasible solutions. A reconfiguration variant of a
vertex-subset problem asks, given two feasible solutions S and T of size k,
whether it is possible to transform S into T by a sequence of vertex additions
and deletions such that each intermediate set is also a feasible solution of
size bounded by k. We study reconfiguration variants of two classical
vertex-subset problems, namely Independent Set and Dominating Set. We denote
the former by ISR and the latter by DSR. Both ISR and DSR are PSPACE-complete
on graphs of bounded bandwidth and W[1]-hard parameterized by k on general
graphs. We show that ISR is fixed-parameter tractable parameterized by k when
the input graph is of bounded degeneracy or nowhere-dense. As a corollary, we
answer positively an open question concerning the parameterized complexity of
the problem on graphs of bounded treewidth. Moreover, our techniques generalize
recent results showing that ISR is fixed-parameter tractable on planar graphs
and graphs of bounded degree. For DSR, we show the problem fixed-parameter
tractable parameterized by k when the input graph does not contain large
bicliques, a class of graphs which includes graphs of bounded degeneracy and
nowhere-dense graphs

### Dynamical breakdown of the Ising spin-glass order under a magnetic field

The dynamical magnetic properties of an Ising spin glass
Fe$_{0.55}$Mn$_{0.45}$TiO$_3$ are studied under various magnetic fields. Having
determined the temperature and static field dependent relaxation time
$\tau(T;H)$ from ac magnetization measurements under a dc bias field by a
general method, we first demonstrate that these data provide evidence for a
spin-glass (SG) phase transition only in zero field. We next argue that the
data $\tau(T;H)$ of finite $H$ can be well interpreted by the droplet theory
which predicts the absence of a SG phase transition in finite fields.Comment: 4 pages, 5 figure

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### Spectral cues are necessary to encode azimuthal auditory space in the mouse superior colliculus.

Sound localization plays a critical role in animal survival. Three cues can be used to compute sound direction: interaural timing differences (ITDs), interaural level differences (ILDs) and the direction-dependent spectral filtering by the head and pinnae (spectral cues). Little is known about how spectral cues contribute to the neural encoding of auditory space. Here we report on auditory space encoding in the mouse superior colliculus (SC). We show that the mouse SC contains neurons with spatially-restricted receptive fields (RFs) that form an azimuthal topographic map. We found that frontal RFs require spectral cues and lateral RFs require ILDs. The neurons with frontal RFs have frequency tunings that match the spectral structure of the specific head and pinna filter for sound coming from the front. These results demonstrate that patterned spectral cues in combination with ILDs give rise to the topographic map of azimuthal auditory space

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