321 research outputs found
Concepts in thermal physics
Stephen J. Blundell and Katherine M. Blundell.xviii, 493 p. : ill. ; 26 cm
An analytical treatment of in-plane magnetotransport in the Falicov-Sievert model
We derive an analytical expression which allows efficient computation of the
effect of all the Fermi surface trajectories induced by a combination of Bragg
scattering and magnetic breakdown on the in-plane components of the resistivity
tensor. The particular network of coupled orbits which we consider was first
formulated by Falicov and Sievert, who studied the problem numerically. Our
approach, based upon a method used previously to derive an analytical solution
for interlayer transport, allows us to show that the conductivity tensor can be
written as a sum of a matrix representing the effect of total magnetic
breakdown and one representing a combination of complex electronic
trajectories, and we find a compact expression for the in-plane components of
the resistivity tensor that can be evaluated straightforwardly.Comment: 4 pages, 4 figure
Will spin-relaxation times in molecular magnets permit quantum information processing?
Using X-band pulsed electron spin resonance, we report the intrinsic
spin-lattice () and phase coherence () relaxation times in molecular
nanomagnets for the first time. In Cr heterometallic wheels, with = Ni
and Mn, phase coherence relaxation is dominated by the coupling of the electron
spin to protons within the molecule. In deuterated samples reaches 3
s at low temperatures, which is several orders of magnitude longer than
the duration of spin manipulations, satisfying a prerequisite for the
deployment of molecular nanomagnets in quantum information applications.Comment: 4 pages, 3 figures, in press at Physical Review Letter
Control via electron count of the competition between magnetism and superconductivity in cobalt and nickel doped NaFeAs
Using a combination of neutron, muon and synchrotron techniques we show how
the magnetic state in NaFeAs can be tuned into superconductivity by replacing
Fe by either Co or Ni. Electron count is the dominant factor, since Ni-doping
has double the effect of Co-doping for the same doping level. We follow the
structural, magnetic and superconducting properties as a function of doping to
show how the superconducting state evolves, concluding that the addition of 0.1
electrons per Fe atom is sufficient to traverse the superconducting domain, and
that magnetic order coexists with superconductivity at doping levels less than
0.025 electrons per Fe atom.Comment: 4 pages, 6 figure
Two-gap superconductivity with line nodes in CsCaFeAsF
We report the results of a muon-spin rotation (SR) experiment to
determine the superconducting ground state of the iron-based superconductor
CsCaFeAsF with K. This compound is
related to the fully-gapped superconductor CaCsFeAs, but here the
Ca-containing spacer layer is replaced with one containing CaF. The
temperature evolution of the penetration depth strongly suggests the presence
of line nodes and is best modelled by a system consisting of both an - and a
-wave gap. We also find a potentially magnetic phase which appears below
K but does not appear to compete with the superconductivity. This
compound contains the largest alkali atom in this family of superconductors and
our results yield a value for the in-plane penetration depth of
nm.Comment: 6 pages, 2 figure
Magnetic Monopole Noise
Magnetic monopoles are hypothetical elementary particles exhibiting quantized
magnetic charge and quantized magnetic flux . A classic proposal for detecting such magnetic charges is to measure the
quantized jump in magnetic flux threading the loop of a superconducting
quantum interference device (SQUID) when a monopole passes through it.
Naturally, with the theoretical discovery that a plasma of emergent magnetic
charges should exist in several lanthanide-pyrochlore magnetic insulators,
including DyTiO, this SQUID technique was proposed for their direct
detection. Experimentally, this has proven extremely challenging because of the
high number density, and the generation-recombination (GR) fluctuations, of the
monopole plasma. Recently, however, theoretical advances have allowed the
spectral density of magnetic-flux noise due to GR
fluctuations of magnetic charge pairs to be determined. These
theories present a sequence of strikingly clear predictions for the
magnetic-flux noise signature of emergent magnetic monopoles. Here we report
development of a high-sensitivity, SQUID based flux-noise spectrometer, and
consequent measurements of the frequency and temperature dependence of
for DyTiO samples. Virtually all the elements
of predicted for a magnetic monopole plasma, including the
existence of intense magnetization noise and its characteristic frequency and
temperature dependence, are detected directly. Moreover, comparisons of
simulated and measured correlation functions of the magnetic-flux
noise imply that the motion of magnetic charges is strongly
correlated because traversal of the same trajectory by two magnetic charges of
same sign is forbidden
Exploring a New Population of Compact Objects: X-ray and IR Observations of the Galactic Centre
I describe the IR and X-ray observational campaign we have undertaken for the
purpose of determining the nature of the faint discrete X-ray source population
discovered by Chandra in the Galactic Center (GC). Data obtained for this
project includes a deep Chandra survey of the Galactic Bulge; deep, high
resolution IR imaging from VLT/ISAAC, CTIO/ISPI, and the UKIDSS Galactic Plane
Survey (GPS); and IR spectroscopy from VLT/ISAAC and IRTF/SpeX. By
cross-correlating the GC X-ray imaging from Chandra with our IR surveys, we
identify candidate counterparts to the X-ray sources via astrometry. Using a
detailed IR extinction map, we are deriving magnitudes and colors for all the
candidates. Having thus established a target list, we will use the multi-object
IR spectrograph FLAMINGOS-2 on Gemini-South to carry out a spectroscopic survey
of the candidate counterparts, to search for emission line signatures which are
a hallmark of accreting binaries. By determining the nature of these X-ray
sources, this FLAMINGOS-2 Galactic Center Survey will have a dramatic impact on
our knowledge of the Galactic accreting binary population.Comment: 4 pages, 2 figures, to appear in the Proceedings of The Second
Kolkata Conference on Observational Evidence for Black Holes in the
Universe'', ed. S. Charkrabarti, Kolkata, India; AIP Conf. Serie
The radio luminosity function from the low-frequency 3CRR, 6CE & 7CRS complete samples
We measure the radio luminosity function (RLF) of steep-spectrum radio
sources using three redshift surveys of flux-limited samples selected at low
(151 & 178 MHz) radio frequency, low-frequency source counts and the local RLF.
The redshift surveys used are the new 7C Redshift Survey (7CRS) and the
brighter 3CRR and 6CE surveys totalling 356 sources with virtually complete
redshift information. This yields unprecedented coverage of the radio
luminosity versus z plane for steep-spectrum sources, and hence the most
accurate measurements of the steep-spectrum RLF yet made. We find that a simple
dual-population model for the RLF fits the data well, requiring differential
density evolution (with z) for the two populations. The low-luminosity
population can be associated with radio galaxies with weak emission lines, and
includes sources with both FRI and FRII radio structures; its comoving space
density rises by about one dex between z~0 and z~1 but cannot yet be
meaningfully constrained at higher redshifts. The high-luminosity population
can be associated with FRII radio galaxies and quasars with strong emission
lines; its rises by nearly three dex between z~0 and z~2. These results
mirror the situation seen in X-ray and optically-selected AGN. The integrated
radio luminosity density of the combination of the two populations is
controlled by the value of at the low-luminosity end of the RLF of the
high-luminosity population, a quantity which has been directly measured at z~1
by the 7CRS. We argue that robust determination of this quantity at higher
redshifts requires a new redshift survey based on a large (~1000 source) sample
about five times fainter than the 7CRS.Comment: 20 pages, 16 figures, accepted for publication in MNRA
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