384 research outputs found
Non-equilibrium structural phase transitions of the vortex lattice in MgB2
We have studied non-equilibrium phase transitions in the vortex lattice in
superconducting MgB2, where metastable states are observed in connection with
an intrinsically continuous rotation transition. Using small-angle neutron
scattering and a stop-motion technique, we investigated the manner in which the
metastable vortex lattice returns to the equilibrium state under the influence
of an ac magnetic field. This shows a qualitative difference between the
supercooled case which undergoes a discontinuous transition, and the
superheated case where the transition to the equilibrium state is continuous.
In both cases the transition may be described by an an activated process, with
an activation barrier that increases as the metastable state is suppressed, as
previously reported for the supercooled vortex lattice [E. R. Louden et al.,
Phys. Rev. B 99, 060502(R) (2019)]. Separate preparations of superheated
metastable vortex lattices with different domain populations showed an
identical transition towards the equilibrium state. This provides further
evidence that the vortex lattice metastability, and the kinetics associated
with the transition to the equilibrium state, is governed by nucleation and
growth of domains and the associated domain boundaries.Comment: 27 pages, 10 figures. arXiv admin note: text overlap with
arXiv:1812.0597
Structural Transition Kinetics and Activated Behavior in the Superconducting Vortex Lattice
Using small-angle neutron scattering, we investigated the behavior of a
metastable vortex lattice state in MgB2 as it is driven towards equilibrium by
an AC magnetic field. This shows an activated behavior, where the AC field
amplitude and cycle count are equivalent to, respectively, an effective
"temperature" and "time". The activation barrier increases as the metastable
state is suppressed, corresponding to an aging of the vortex lattice.
Furthermore, we find a cross-over from a partial to a complete suppression of
metastable domains depending on the AC field amplitude, which may empirically
be described by a single free parameter. This represents a novel kind of
collective vortex behavior, most likely governed by the nucleation and growth
of equilibrium vortex lattice domains.Comment: 5 pages plus 3 pages of supplemental materia
The Iliad’s big swoon: a case of innovation within the epic tradition
In book 5 of the Iliad Sarpedon suffers so greatly from a wound that his ‘‘ψυχή leaves him’. Rather than dying, however, Sarpedon lives to fight another day. This paper investigates the phrase τὸν δὲ λίπε ψυχή in extant archaic Greek poetry to gain a sense of its traditional referentiality and better assess the meaning of Sarpedon’s swoon. Finding that all other instances of the ψυχή leaving the body signify death, it suggests that the Iliad exploits a traditional unit of utterance to flag up the importance of Sarpedon to this version of the Troy story
Precision study of 6p 2Pj - 8s 2S1/2 relative transition matrix elements in atomic Cs
A combined experimental and theoretical study of transition matrix elements
of the 6p 2Pj - 8s 2S1/2 transition in atomic Cs is reported. Measurements of
the polarization-dependent two-photon excitation spectrum associated with the
transition were made in an approximately 200 cm-1 range on the low frequency
side of the 6s 2S1/2 - 6p 2P3/2 resonance. The measurements depend
parametrically on the relative transition matrix elements, but also are
sensitive to far-off-resonance 6s 2S1/2 - np 2Pj - 8s 2S1/2 transitions. In the
past, this dependence has yielded a generalized sum rule, the value of which is
dependent on sums of relative two-photon transition matrix elements. In the
present case, best available determinations from other experiments are combined
with theoretical matrix elements to extract the ratio of transition matrix
elements for the 6p 2Pj - 8s 2S1/2 (j = 1/2,3/2) transition. The resulting
experimental value of 1.423(2) is in excellent agreement with the theoretical
value, calculated using a relativistic all-order method, of 1.425(2)
Structural studies of metastable and equilibrium vortex lattice domains in MgB2
The vortex lattice in MgB2 is characterized by the presence of long-lived
metastable states, which arise from cooling or heating across the equilibrium
phase boundaries. A return to the equilibrium configuration can be achieved by
inducing vortex motion. Here we report on small-angle neutron scattering
studies of MgB2, focusing on the structural properties of the vortex lattice as
it is gradually driven from metastable to equilibrium states by an AC magnetic
field. Measurements were performed using initial metastable states obtained
either by cooling or heating across the equilibrium phase transition. In all
cases, the longitudinal correlation length remains constant and comparable to
the sample thickness. Correspondingly, the vortex lattice may be considered as
a system of straight rods, where the formation and growth of equilibrium state
domains only occurs in the two-dimensional plane perpendicular to the applied
field direction. Spatially resolved raster scans of the sample were performed
with apertures as small as 80 microns, corresponding to only 1.2*10^6 vortices
for an applied field of 0.5 T. These revealed spatial variations in the
metastable and equilibrium vortex lattice populations, but individual domains
were not directly resolved. A statistical analysis of the data indicates an
upper limit on the average domain size of approximately 50 microns.Comment: 13 pages, 9 figure
LRG-BEASTS: Ground-based Detection of Sodium and a Steep Optical Slope in the Atmosphere of the Highly Inflated Hot-Saturn WASP-21b
We present the optical transmission spectrum of the highly inflated
Saturn-mass exoplanet WASP-21b, using three transits obtained with the ACAM
instrument on the William Herschel Telescope through the LRG-BEASTS survey (Low
Resolution Ground-Based Exoplanet Atmosphere Survey using Transmission
Spectroscopy). Our transmission spectrum covers a wavelength range of 4635-9000
Angstrom, achieving an average transit depth precision of 197ppm compared to
one atmospheric scale height at 246ppm. We detect Na I absorption in a bin
width of 30 Angstrom, at >4 confidence, which extends over 100
Angstrom. We see no evidence of absorption from K I. Atmospheric retrieval
analysis of the scattering slope indicates it is too steep for Rayleigh
scattering from H, but is very similar to that of HD 189733b. The features
observed in our transmission spectrum cannot be caused by stellar activity
alone, with photometric monitoring of WASP-21 showing it to be an inactive
star. We therefore conclude that aerosols in the atmosphere of WASP-21b are
giving rise to the steep slope that we observe, and that WASP-21b is an
excellent target for infra-red observations to constrain its atmospheric
metallicity.Comment: Accepted for publication in MNRAS. 21 pages, 10 tables, 16 figure
MOVES – I. The evolving magnetic field of the planet-hosting star HD189733
HD189733 is an active K dwarf that is, with its transiting hot Jupiter, among the most studied exoplanetary systems. In this first paper of the Multiwavelength Observations of an eVaporating Exoplanet and its Star (MOVES) programme, we present a 2-yr monitoring of the large-scale magnetic field of HD189733. The magnetic maps are reconstructed for five epochs of observations, namely 2013 June–July, 2013 August, 2013 September, 2014 September and 2015 July, using Zeeman–Doppler imaging. We show that the field evolves along the five epochs, with mean values of the total magnetic field of 36, 41, 42, 32 and 37 G, respectively. All epochs show a toroidally dominated field. Using previously published data of Moutou et al. and Fares et al., we are able to study the evolution of the magnetic field over 9 yr, one of the longest monitoring campaigns for a given star. While the field evolved during the observed epochs, no polarity switch of the poles was observed. We calculate the stellar magnetic field value at the position of the planet using the potential field source surface extrapolation technique. We show that the planetary magnetic environment is not homogeneous over the orbit, and that it varies between observing epochs, due to the evolution of the stellar magnetic field. This result underlines the importance of contemporaneous multiwavelength observations to characterize exoplanetary systems. Our reconstructed maps are a crucial input for the interpretation and modelling of our MOVES multiwavelength observations.Publisher PDFPeer reviewe
Transmission spectroscopy of the inflated exoplanet WASP-52b, and evidence for a bright region on the stellar surface
We have measured the transmission spectrum of the extremely inflated hot Jupiter WASP-52b using simultaneous photometric observations in Sloan Digital Sky Survey u΄, g΄ and a filter centred on the sodium doublet (Na i) with the ULTRACAM instrument mounted on the 4.2-m William Herschel Telescope. We find that Rayleigh scattering is not the dominant source of opacity within the planetary atmosphere and find a transmission spectrum more consistent with wavelength-independent opacity such as from clouds. We detect an in-transit anomaly that we attribute to the presence of stellar activity and find that this feature can be more simply modelled as a bright region on the stellar surface akin to solar faculae rather than spots. A spot model requires a significantly larger planet/star radius ratio than that found in previous studies. Our results highlight the precision that can be achieved by ground-based photometry with errors in the scaled planetary radii of less than one atmospheric scale height, comparable to Hubble Space Telescope observations
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