1,643 research outputs found
Electron-Phonon Interaction in NbB_2 : A Comparison with MgB_2
We present a comparison of electron-phonon interaction in NbB_2 and MgB_2,
calculated using full-potential, density-functional-based methods in P6/mmm
crystal structure. Our results, described in terms of (i) electronic structure,
(ii) phonon density of states F(\omega), (iii) Eliashberg function \alpha
^2F(\omega), and (iv) the solutions of the isotropic Eliashberg gap equation,
clearly show significant differences in the electron-phonon interaction in
NbB_2 and MgB_2. We find that the average electron-phonon coupling constant
\lambda is equal to 0.59 for MgB_2 and 0.43 for NbB_2, leading to
superconducting transition temperature T_c of around 22 K for MgB_2 and 3 K for
NbB_2
Nonradiative interaction and entanglement between distant atoms
We show that nonradiative interactions between atomic dipoles placed in a
waveguide can give rise to deterministic entanglement at ranges much larger
than their resonant wavelength. The range increases as the dipole-resonance
approaches the waveguide's cutoff frequency, caused by the giant density of
photon modes near cutoff, a regime where the standard (perturbative) Markov
approximation fails. We provide analytical theories for both the Markovian and
non-Markovian regimes, supported by numerical simulations, and discuss possible
experimental realizations.Comment: 9 pages, 2 figure
Hole-doped, High-Temperature Superconductors Li_{x}BC, Na_{x}BC and C_{x} : A Coherent-Potential-Based Prediction
Using density-functional-based methods, we show that the hole-doped
Li_{x}BC and Na_{x}BC in P6_{3}/mmc crystal structure and C_{x} in graphite
structure are capable of showing superconductivity, possibly with a T_{c} much
higher than that of
MgB_{2}. We use full-potential methods to obtain the optimized lattice
constants a and c, coherent-potential approximation to describe the effects of
disorder, Gaspari-Gyorffy formalism to obtain the electron-phonon coupling
constant , and Allen-Dynes equation to calculate T_{c} as a function
of hole concentration in these alloys.Comment: 4 pages, 5 figure
Extreme Electron-Phonon Coupling in Boron-based Layered Superconductors
The phonon-mode decomposition of the electron-phonon coupling in the
MgB2-like system Li_{1-x}BC is explored using first principles calculations. It
is found that the high temperature superconductivity of such systems results
from extremely strong coupling to only ~2% of the phonon modes. Novel
characteristics of E_2g branches include (1) ``mode lambda'' values of 25 and
greater compared to a mean of for other modes, (2) a precipitous
Kohn anomaly, and (3) E_2g phonon linewidths within a factor of ~2 of the
frequency itself, indicating impending breakdown of linear electron-phonon
theory. This behavior in borne out by recent inelastic x-ray scattering studies
of MgB2 by Shukla et al.Comment: 4 two-column pages, 4 figures. Equations simplified. Figure 4
changed. Comparison with new data include
Acoustical-Mode-Driven Electron-Phonon Coupling in Transition-Metal Diborides
We show that the electron-phonon coupling in the transition-metal diborides
NbB2 and TaB2 is dominated by the longitudinal acoustical (LA) mode, in
contrast to the optical E_{2g} mode dominated coupling in MgB2. Our ab initio
results, described in terms of phonon dispersion, linewidth, and partial
electron-phonon coupling along Gamma to A, also show that (i) NbB2 and TaB2
have a relatively weak electron-phonon coupling, (ii) the E_{2g} linewidth is
an order of magnitude larger in MgB2 than in NbB2 or TaB2, (iii) the E_{2g}
frequency in NbB2 and TaB2 is considerably higher than in MgB2, and (iv) the LA
frequency at A for TaB2 is almost half of that of MgB2 or NbB2.Comment: 4 pages, 4 figures, and 1 tabl
Wolf-Rayet Galaxies in the Sloan Digital Sky Survey: the metallicity dependence of the initial mass function
We use a large sample of 174 Wolf-Rayet (WR) galaxies drawn from the Sloan
Digital Sky Survey to study whether and how the slope of the stellar initial
mass function depends on metallicity. We calculate for each object its oxygen
abundance according to which we divide our sample into four metallicity
subsamples. For each subsample, we then measure three quantities: the
equivalent width of \hb emission line, the equivalent width of WR bump around
4650\AA, and the WR bump-to-\hb intensity ratio, and compare to the predictions
of the same quantities by evolutionary synthesis models of Schaerer & Vacca.
Such comparisons lead to a clear dependence of the slope of initial mass
function () on metallicity in that galaxies at higher metallicities
tend to have steeper initial mass functions, with the slope index ranging from
1.00 for the lowest metallicity of to 3.30
for the highest metallicity . We have carefully examined the possible
sources of systematic error either in models or in our observational
measurements and shown that these sources do not change this result.Comment: 12 pages, 6 figures, ApJ accepte
8-Methoxy-3,3,5-trimethyl-3,11-dihydropyrano[3,2-a]carbazole
In the title compound, C19H19NO2, commonly called koenimbine, the pyran ring adopts a sofa conformation. The carbazole ring system is planar [r.m.s. deviation = 0.063 (1) Å]. A C(10) zigzag chain running along the b axis is formed through intermolecular C—H⋯O hydrogen bonds. The chains are linked via weak C—H⋯π and N—H⋯π interactions
First-Principles Calculation of the Superconducting Transition in MgB2 within the Anisotropic Eliashberg Formalism
We present a study of the superconducting transition in MgB2 using the
ab-initio pseudopotential density functional method and the fully anisotropic
Eliashberg equation. Our study shows that the anisotropic Eliashberg equation,
constructed with ab-initio calculated momentum-dependent electron-phonon
interaction and anharmonic phonon frequencies, yields an average
electron-phonon coupling constant lambda = 0.61, a transition temperature Tc =
39 K, and a boron isotope-effect exponent alphaB = 0.31 with a reasonable
assumption of mu* = 0.12. The calculated values for Tc, lambda, and alphaB are
in excellent agreement with transport, specific heat, and isotope effect
measurements respectively. The individual values of the electron-phonon
coupling lambda(k,k') on the various pieces of the Fermi surface however vary
from 0.1 to 2.5. The observed Tc is a result of both the raising effect of
anisotropy in the electron-phonon couplings and the lowering effect of
anharmonicity in the relevant phonon modes.Comment: 4 pages, 3 figures, 1 tabl
Implications of reflectance measurements on the mechanism for superconductivity in MgB
Recent optical studies in c-axis oriented superconducting MgB films
indicate that the electron-phonon coupling is weak [tu01]. We reinforce this
conclusion by examining the raw reflectance data; its frequency dependence is
incompatible with strong electron-phonon scattering. This is further
strengthened by analysis of the real part of the conductivity, and by the
temperature dependence of the effective Drude scattering rate. Using a
realistic electron-phonon spectral shape [kong01], we find , in agreement with Tu et al. [tu01]. To the extent that
, this disagrees sharply with model
calculations [kong01,kortus01,an01], and is far too low to provide the means
for K. A simple model is constructed with coupling to a high
frequency excitation, which is consistent with both the low frequency optical
data and the high .Comment: 4 pages, 4 figure
Constraints from and the isotope effect for MgB
With the constraint that K, as observed for MgB, we use the
Eliashberg equations to compute possible allowed values of the isotope
coefficient, . We find that while the observed value can
be obtained in principle, it is difficult to reconcile a recently calculated
spectral function with such a low observed value
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