14,355 research outputs found
Magnetic field tuning of antiferromagnetic YbPt
We present measurements of the specific heat, magnetization, magnetocaloric
effect and magnetic neutron diffraction carried out on single crystals of
antiferromagnetic YbPt, where highly localized Yb moments order at
K in zero field. The antiferromagnetic order was suppressed to
by applying a field of 1.85 T in the plane.
Magnetocaloric effect measurements show that the antiferromagnetic phase
transition is always continuous for , although a pronounced step
in the magnetization is observed at the critical field in both neutron
diffraction and magnetization measurements. These steps sharpen with decreasing
temperature, but the related divergences in the magnetic susceptibility are cut
off at the lowest temperatures, where the phase line itself becomes vertical in
the field-temperature plane. As , the antiferromagnetic
transition is increasingly influenced by a quantum critical endpoint, where
ultimately vanishes in a first order phase transition.Comment: 9 pages, 6 figure
X-ray Observations of XSS J12270-4859 in a New Low State: A Transformation to a Disk-Free Rotation-Powered Pulsar Binary
We present XMM-Newton and Chandra observations of the low-mass X-ray binary
XSS J12270--4859, which experienced a dramatic decline in optical/X-ray
brightness at the end of 2012, indicative of the disappearance of its accretion
disk. In this new state, the system exhibits previously absent
orbital-phase-dependent, large-amplitude X-ray modulations with a decline in
flux at superior conjunction. The X-ray emission remains predominantly
non-thermal but with an order of magnitude lower mean luminosity and
significantly harder spectrum relative to the previous high flux state. This
phenomenology is identical to the behavior of the radio millisecond pulsar
binary PSR J1023+0038 in the absence of an accretion disk, where the X-ray
emission is produced in an intra-binary shock driven by the pulsar wind. This
further demonstrates that XSS J12270-4859 no longer has an accretion disk and
has transformed to a full-fledged eclipsing "redback" system that hosts an
active rotation-powered millisecond pulsar. There is no evidence for diffuse
X-ray emission associated with the binary that may arise due to outflows or a
wind nebula. An extended source situated 1.5' from XSS J12270--4859 is unlikely
to be associated, and is probably a previously uncatalogued galaxy cluster.Comment: 8 pages, 6 figures; accepted for publication in the Astrophysical
Journa
Optical transmittance of multilayer graphene
We study the optical transmittance of multilayer graphene films up to 65
layers thick. By combing large-scale tight-binding simulation and optical
measurement on CVD multilayer graphene, the optical transmission through
graphene films in the visible region is found to be solely determined by the
number of graphene layers. We argue that the optical transmittance measurement
is more reliable in the determination of the number of layers than the commonly
used Raman Spectroscopy. Moreover, optical transmittance measurement can be
applied also to other 2D materials with weak van der Waals interlayer
interaction.Comment: Europhysics Letters (2014
Gravitational wave astronomy with the SKA
On a time scale of years to decades, gravitational wave (GW) astronomy will
become a reality. Low frequency (nanoHz) GWs are detectable through long-term
timing observations of the most stable pulsars. Radio observatories worldwide
are currently carrying out observing programmes to detect GWs, with data sets
being shared through the International Pulsar Timing Array project. One of the
most likely sources of low frequency GWs are supermassive black hole binaries
(SMBHBs), detectable as a background due to a large number of binaries, or as
continuous or burst emission from individual sources. No GW signal has yet been
detected, but stringent constraints are already being placed on galaxy
evolution models. The SKA will bring this research to fruition.
In this chapter, we describe how timing observations using SKA1 will
contribute to detecting GWs, or can confirm a detection if a first signal
already has been identified when SKA1 commences observations. We describe how
SKA observations will identify the source(s) of a GW signal, search for
anisotropies in the background, improve models of galaxy evolution, test
theories of gravity, and characterise the early inspiral phase of a SMBHB
system.
We describe the impact of the large number of millisecond pulsars to be
discovered by the SKA; and the observing cadence, observation durations, and
instrumentation required to reach the necessary sensitivity. We describe the
noise processes that will influence the achievable precision with the SKA. We
assume a long-term timing programme using the SKA1-MID array and consider the
implications of modifications to the current design. We describe the possible
benefits from observations using SKA1-LOW. Finally, we describe GW detection
prospects with SKA1 and SKA2, and end with a description of the expectations of
GW astronomy.Comment: 19 pages, 3 figures, to be published in: "Advancing Astrophysics with
the Square Kilometre Array", Proceedings of Science, PoS(AASKA14)03
Condensation vs. phase-ordering in the dynamics of first order transitions
The origin of the non commutativity of the limits and in the dynamics of first order transitions is investigated. In the
large-N model, i.e. taken first, the low temperature phase is
characterized by condensation of the large wave length fluctuations rather than
by genuine phase-ordering as when is taken first. A detailed
study of the scaling properties of the structure factor in the large-N model is
carried out for quenches above, at and below T_c. Preasymptotic scaling is
found and crossover phenomena are related to the existence of components in the
order parameter with different scaling properties. Implications for
phase-ordering in realistic systems are discussed.Comment: 15 pages, 13 figures. To be published in Phys. Rev.
Spreading with immunization in high dimensions
We investigate a model of epidemic spreading with partial immunization which
is controlled by two probabilities, namely, for first infections, , and
reinfections, . When the two probabilities are equal, the model reduces to
directed percolation, while for perfect immunization one obtains the general
epidemic process belonging to the universality class of dynamical percolation.
We focus on the critical behavior in the vicinity of the directed percolation
point, especially in high dimensions . It is argued that the clusters of
immune sites are compact for . This observation implies that a
recently introduced scaling argument, suggesting a stretched exponential decay
of the survival probability for , in one spatial dimension,
where denotes the critical threshold for directed percolation, should
apply in any dimension and maybe for as well. Moreover, we
show that the phase transition line, connecting the critical points of directed
percolation and of dynamical percolation, terminates in the critical point of
directed percolation with vanishing slope for and with finite slope for
. Furthermore, an exponent is identified for the temporal correlation
length for the case of and , , which
is different from the exponent of directed percolation. We also
improve numerical estimates of several critical parameters and exponents,
especially for dynamical percolation in .Comment: LaTeX, IOP-style, 18 pages, 9 eps figures, minor changes, additional
reference
Curved branes from string dualities
We describe a simple method for generating new string solutions for which the
brane worldvolume is a curved space. As a starting point we use solutions with
NS-NS charges combined with 2-d CFT's representing different parts of
space-time. We illustrate our method with many examples, some of which are
associated with conformally invariant sigma models. Using U-duality, we also
obtain supergravity solutions with RR charges which can be interpreted as
D-branes with non-trivial worldvolume geometry. In particular, we discuss the
case of a D5-brane wrapped on AdS_3 x S^3, a solution interpolating between
AdS_3 x S^3 x R^5 and AdS_3 x S^3 x S^3 x R, and a D3-brane wrapped over S^3 x
R or AdS_2 x S^2. Another class of solutions we discuss involves NS5-branes
intersecting over a 3-space and NS5-branes intersecting over a line. These
solutions are similar to D7-brane or cosmic string backgrounds.Comment: 21 pages, harvmac; misprint correcte
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