368 research outputs found
Superconductivity near the vibrational mode instability in MgCNi3
To understand the role of electron-phonon interaction in superconducting
MgCNi we have performed density functional based linear response
calculations of its lattice dynamical properties. A large coupling constant = 1.51 is predicted and contributing phonons are identified as
displacements of Ni atoms towards octahedral interstitials of the perovskite
lattice. Instabilities found for some vibrational modes emphasize the role of
anharmonic effects in resolving experimental controversies.Comment: 4 pages, 4 eps figures, replaces the older versio
Comment on "Experimental determination of superconducting parameters for the intermetallic perovskite superconductor MgCNi"
In a recent paper (Phys. Rev. {\bf B 67}, 094502 (2003)) Mao et al.
investigated the bias-dependent conductance of mechanical junctions between
superconducting MgCNi and a sharp W tip. They interpreted their results in
terms of 'single-particle tunneling'. We show it is more likely that current
transport through those junctions is determined by thermal effects due to the
huge normal-state resistivity of MgCNi. Therefore no conclusion can be
drawn about the possible unconventional pairing or strong-coupling
superconductivity in MgCNi.Comment: 2 pages, 1 Fig. Comment on Z. Q. Mao et al. (Phys. Rev. {\bf B 67},
094502 (2003)
Superconducting Properties of MgCNi3 Films
We report the magnetotransport properties of thin polycrystalline films of
the recently discovered non-oxide perovskite superconductor MgCNi3. CNi3
precursor films were deposited onto sapphire substrates and subsequently
exposed to Mg vapor at 700 C. We report transition temperatures (Tc) and
critical field values (Hc2) of MgCNi3 films ranging in thickness from 7.5 nm to
100 nm. Films thicker than ~40 nm have a Tc ~ 8 K, and an upper critical field
Hc2 ~ 14 T, which are both comparable to that of polycrystalline powders. Hall
measurements in the normal state give a carrier density, n =-4.2 x 10^22 cm^-3,
that is approximately 4 times that reported for bulk samples.Comment: submitted to PR
Pressure-dependence of electron-phonon coupling and the superconducting phase in hcp Fe - a linear response study
A recent experiment by Shimizu et al. has provided evidence of a
superconducting phase in hcp Fe under pressure. To study the
pressure-dependence of this superconducting phase we have calculated the phonon
frequencies and the electron-phonon coupling in hcp Fe as a function of the
lattice parameter, using the linear response (LR) scheme and the full potential
linear muffin-tin orbital (FP-LMTO) method. Calculated phonon spectra and the
Eliashberg functions indicate that conventional s-wave
electron-phonon coupling can definitely account for the appearance of the
superconducting phase in hcp Fe. However, the observed change in the transition
temperature with increasing pressure is far too rapid compared with the
calculated results. For comparison with the linear response results, we have
computed the electron-phonon coupling also by using the rigid muffin-tin (RMT)
approximation. From both the LR and the RMT results it appears that
electron-phonon interaction alone cannot explain the small range of volume over
which superconductivity is observed. It is shown that
ferromagnetic/antiferromagnetic spin fluctuations as well as scattering from
magnetic impurities (spin-ordered clusters) can account for the observed values
of the transition temperatures but cannot substantially improve the agreeemnt
between the calculated and observed presure/volume range of the superconducting
phase. A simplified treatment of p-wave pairing leads to extremely small ( K) transition temperatures. Thus our calculations seem to rule out
both - and - wave superconductivity in hcp Fe.Comment: 12 pages, submitted to PR
Influence of Carbon Concentration on the Superconductivity in MgCxNi3
The influence of carbon concentration on the superconductivity (SC) in
MgCNi has been investigated by measuring the low temperature specific
heat combined with first principles electronic structure calculation. It is
found that the specific heat coefficient of the
superconducting sample () in normal state is twice that of the
non-superconducting one (). The comparison of measured
and the calculated electronic density of states (DOS) shows that the
effective mass renormalization changes remarkably as the carbon concentration
changes. The large mass renormalization for the superconducting sample and the
low (7K) indicate that more than one kind of boson mediated
electron-electron interactions exist in MgCNi.Comment: 4 pages, 4 figure
The evolution of microstructure and electrical performance in doped Mn-Co and Cu-Mn oxide layers with the extended oxidation time
Mn-Co and Cu-Mn based alloy coatings are deposited on 430 SS by high energy micro-arc alloying process, and then alloy coatings are oxidized at 750 â in air to form the spinel oxides. Some composite oxides, such as Co3O4, Mn2O3 and (Mn,Co)3O4, are formed at the initial stage of oxidation. The single-phase spinel (pure MnCo2O4) without impurity phases is only found on the surface of Co-33Mn-17Cu oxide layer with the extended oxidation time, its area-specific resistance values at 500 â to 800 â are low and close to that of Mn-35Cu oxide layer which mainly composed of Cu1.2Mn1.8O4 spinel
Measurement of the Ds lifetime
We report precise measurement of the Ds meson lifetime. The data were taken
by the SELEX experiment (E781) spectrometer using 600 GeV/c Sigma-, pi- and p
beams. The measurement has been done using 918 reconstructed Ds. The lifetime
of the Ds is measured to be 472.5 +- 17.2 +- 6.6 fs, using K*(892)0K+- and phi
pi+- decay modes. The lifetime ratio of Ds to D0 is 1.145+-0.049.Comment: 5 pages, 2 figures submitted to Phys. Lett.
Impact force identification with pseudo-inverse method on a lightweight structure for under-determined, even-determined and over-determined cases
Force identification using inverse technique is important especially when direct measurement through force transducer is not possible. Considering the effects of impact excitation force on the integrity of a lightweight structure, impact force identification has become the subject of several studies. A methodology utilising Operating Deflection Shape (ODS) analysis, Frequency Response Function (FRF) measurement and pseudo-inverse method to evaluate the dynamic force is presented. A rectangular plate with four ground supports was used as a test rig to simulate the motions of a simple vehicle body. By using the measured responses at remote points that are away from impact locations and measured FRFs of the test rig, unknown force locations and their time histories can be recovered by the proposed method. The performance of this approach in various cases such as under-determined, even-determined and over-determined cases was experimentally demonstrated. Good and bad combinations of response locations were selected based on the condition number of FRF matrix. This force identification method was examined under different response combinations and various numbers of response locations. It shows that in the over-determined case, good combination of response locations (i.e. low average of condition number of FRF matrix) and high number of response locations give the best accuracy of force identification result compared to under-determined and even-determined cases
Confirmation of the Double Charm Baryon Xi_cc+ via its Decay to p D+ K-
We observes a signal for the double charm baryon Xi_cc+ in the charged decay
mode Xi_cc+ -> p D+ K- to complement the previously reported decay Xi_cc+ ->
Lambda_c K- pi+ in data from SELEX, the charm hadro-production experiment
(E781) at Fermilab. In this new decay mode we observe an excess of 5.62 events
over an expected background estimated by event mixing to be 1.38+/-0.13 events.
The Poisson probability that a background fluctuation can produce the apparent
signal is less than 6.4E-4. The observed mass of this state is
(3518+/-3)MeV/c^2, consistent with the published result. Averaging the two
results gives a mass of (3518.7+/-1.7)MeV/c^2. The observation of this new weak
decay mode confirms the previous SELEX suggestion that this state is a double
charm baryon. The relative branching ratio Gamma(Xi_cc+ -> pD+K-)/Gamma(Xi_cc+
-> Lambda_c K- pi+) = 0.36+/-0.21.Comment: 11 pages, 6 included eps figures. v2 includes improved statistical
method to determine significance of observation. Submitted to PL
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