546 research outputs found
Electron-phonon coupling and phonon self-energy in MgB: do we really understand MgB Raman spectra ?
We consider a model Hamiltonian fitted on the ab-initio band structure to
describe the electron-phonon coupling between the electronic bands and
the phonon E mode in MgB. The model allows for analytical
calculations and numerical treatments using very large k-point grids. We
calculate the phonon self-energy of the E mode along two high symmetry
directions in the Brillouin zone. We demonstrate that the contribution of the
bands to the Raman linewidth of the E mode via the
electron-phonon coupling is zero. As a consequence the large resonance seen in
Raman experiments cannot be interpreted as originated from the mode at
. We examine in details the effects of Fermi surface singularities in
the phonon spectrum and linewidth and we determine the magnitude of finite
temperature effects in the the phonon self-energy. From our findings we suggest
several possible effects which might be responsible for the MgB Raman
spectra.Comment: 10 pages, 9 figure
Properties of the superconducting state in a two-band model
Eliashberg theory is used to investigate the range of thermodynamic
properties possible within a two-band model for s-wave superconductivity and to
identify signatures of its two-band nature. We emphasize dimensionless BCS
ratios (those for the energy gaps, the specific heat jump and the negative of
its slope near Tc, the thermodynamic critical field Hc(0), and the normalized
slopes of the critical field and the penetration depth near Tc), which are no
longer universal even in weak coupling. We also give results for
temperature-dependent quantities, such as the penetration depth and the energy
gap. Results are presented both for microscopic parameters appropriate to MgB2
and for variations away from these. Strong coupling corrections are identified
and found to be significant. Analytic formulas are provided which show the role
played by the anisotropy in coupling in some special limits. Particular
emphasis is placed on small interband coupling and on the opposite limit of no
diagonal coupling. The effect of impurity scattering is considered,
particularly for the interband case.Comment: 20 pages, 14 figures, final version accepted in PR
Effect of nitrification inhibitors on the growth and activity of Nitrosotalea devanaterra in culture and soil
Peer reviewedPublisher PD
Optical absorption in the strong coupling limit of Eliashberg theory
We calculate the optical conductivity of superconductors in the
strong-coupling limit. In this anomalous limit the typical energy scale is set
by the coupling energy, and other energy scales such as the energy of the
bosons mediating the attraction are negligibly small. We find a universal
frequency dependence of the optical absorption which is dominated by bound
states and differs significantly from the weak coupling results. A comparison
with absorption spectra of superconductors with enhanced electron-phonon
coupling shows that typical features of the strong-coupling limit are already
present at intermediate coupling.Comment: 10 pages, revtex, 4 uuencoded figure
Book Reviews
Book reviews of:
Bound in Wedlock: Slave and Free Black Marriage in the Nineteenth Century. By Tera W. Hunter. (Cambridge: The Belknap Press of Harvard University Press, 2017. 404 pp. 28 cloth. ISBN 9781496817457.)
Let the People See: The Story of Emmett Till. By Elliott J. Gorn. (New York: Oxford University Press, 2018. Acknowledgements, source notes, bibliography, index, Pp. xi, 400. 40.00 cloth. ISBN: 978-1-4696-1757-2.)
Stepdaughters of History: Southern Women and the American Civil War. By Catherine Clinton. (Baton Rouge: Louisiana State University Press, 2016. 39.95 Cloth. ISBN: 9781469640471.)
Vicksburg, Grant’s Campaign that Broke the Confederacy. By Donald L. Miller. (New York: Simon & Schuster, 2019. Notes, illustrations, index. Pp. 663. $35.00.
Urine lipoarabinomannan testing for diagnosis of pulmonary tuberculosis in children: a prospective study
Background Urine tests for mycobacterial lipoarabinomannan might be useful for point-of-care diagnosis of
tuberculosis in adults with advanced HIV infection, but have not been assessed in children. We assessed the accuracy
of urine lipoarabinomannan testing for the diagnosis of pulmonary tuberculosis in HIV-positive and HIV-negative
children.
Methods We prospectively recruited children (aged ≤15 years) who presented with suspected tuberculosis at a primary
health-care clinic and paediatric referral hospital in South Africa, between March 1, 2009, and April 30, 2012. We assessed
the diagnostic accuracy of urine lipoarabinomannan testing with lateral fl ow assay and ELISA, with mycobacterial culture
of two induced sputum samples as the reference standard. Positive cultures were identifi ed by acid-fast staining and
tested to confi rm Mycobacterium tuberculosis and establish susceptibility to rifampicin and isoniazid.
Findings 535 children (median age 42·5 months, IQR 19·1–66·3) had urine and two induced specimens available for
testing. 89 (17%) had culture-confi rmed tuberculosis and 106 (20%) had HIV. The lateral fl ow lipoarabinomannan test
showed poor accuracy against the reference standard, with sensitivity of 48·3% (95% CI 37·6–59·2), specifi city of
60·8% (56·1–65·3), and an area under the receiver operating characteristic curve of 0·53 (0·46–0·60) for children
without HIV and 0·64 (0·51–0·76) for children with HIV. ELISA had poor sensitivity in children without HIV
(sensitivity 3·0%, 95% CI 0·4–10·5) and children with HIV (0%, 0·0–14·3); overall specifi city was 95·7% (93·4–97·4).
Interpretation Urine lipoarabinomannan tests have insuffi cient sensitivity and specifi city to diagnose HIV-positive
and HIV-negative children with tuberculosis and should not be used in this patient population
Functional brain networks before the onset of psychosis : a prospective fMRI study with graph theoretical analysis
Individuals with an at-risk mental state (ARMS) have a risk of developing a psychotic disorder significantly greater than the general population. However, it is not currently possible to predict which ARMS individuals will develop psychosis from clinical assessment alone. Comparison of ARMS subjects who do, and do not, develop psychosis can reveal which factors are critical for the onset of illness. In the present study, 37 patients with an ARMS were followed clinically at least 24 months subsequent to initial referral. Functional MRI data were collected at the beginning of the follow-up period during performance of an executive task known to recruit frontal lobe networks and to be impaired in psychosis. Graph theoretical analysis was used to compare the organization of a functional brain network in ARMS patients who developed a psychotic disorder following the scan (ARMS-T) to those who did not become ill during the same follow-up period (ARMS-NT) and aged-matched controls. The global properties of each group's representative network were studied (density, efficiency, global average path length) as well as regionally-specific contributions of network nodes to the organization of the system (degree, farness-centrality, betweenness-centrality). We focused our analysis on the dorsal anterior cingulate cortex (ACC), a region known to support executive function that is structurally and functionally impaired in ARMS patients. In the absence of between-group differences in global network organization, we report a significant reduction in the topological centrality of the ACC in the ARMS-T group relative to both ARMS-NT and controls. These results provide evidence that abnormalities in the functional organization of the brain predate the onset of psychosis, and suggest that loss of ACC topological centrality is a potential biomarker for transition to psychosis
Comparison of s- and d-wave gap symmetry in nonequilibrium superconductivity
Recent application of ultrafast pump/probe optical techniques to
superconductors has renewed interest in nonequilibrium superconductivity and
the predictions that would be available for novel superconductors, such as the
high-Tc cuprates. We have reexamined two of the classical models which have
been used in the past to interpret nonequilibrium experiments with some
success: the mu* model of Owen and Scalapino and the T* model of Parker.
Predictions depend on pairing symmetry. For instance, the gap suppression due
to excess quasiparticle density n in the mu* model, varies as n^{3/2} in d-wave
as opposed to n for s-wave. Finally, we consider these models in the context of
S-I-N tunneling and optical excitation experiments. While we confirm that
recent pump/probe experiments in YBCO, as presently interpreted, are in
conflict with d-wave pairing, we refute the further claim that they agree with
s-wave.Comment: 14 pages, 11 figure
What are the experimentally observable effects of vertex corrections in superconductors?
We calculate the effects of vertex corrections, of non-constant density of
states and of a (self-consistently determined) phonon self-energy for the
Holstein model on a 3D cubic lattice. We replace vertex corrections with a
Coulomb pseudopotential, mu*, adjusted to give the same Tc, and repeat the
calculations, to see which effects are a distinct feature of vertex
corrections. This allows us to determine directly observable effects ofvertex
corrections on a variety of thermodynamic properties of superconductors. To
this end, we employ conserving approximations (in the local approximation) to
calculate the superconducting critical temperatures, isotope coefficients,
superconducting gaps, free-energy differences and thermodynamic critical fields
for a range of parameters. We find that the dressed value of lambda is
significantly larger than the bare value. While vertex corrections can cause
significant changes in all the above quantities (even whenthe bare
electron-phonon coupling is small), the changes can usually be well-modeled by
an appropriate Coulomb pseudopotential. The isotope coefficient proves to be
the quantity that most clearly shows effects of vertex corrections that can not
be mimicked by a mu*.Comment: 28 pages, 12 figure
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