6,471 research outputs found
Revised Results for Non-thermal Recombination Flare Hard X-Ray Emission
Brown and Mallik (BM) recently showed that, for hot sources, recombination of
non-thermal electrons (NTR) onto highly ionised heavy ions is not negligible
compared to non-thermal bremsstrahlung (NTB) as a source of flare hard X-rays
(HXRs) and so should be included in modelling non-thermal HXR flare emission.
In view of major discrepancies between BM results for the THERMAL continua and
those of the Chianti code and of RHESSI solar data, we critically re-examine
and correct the BM analysis and modify the conclusions concerning the
importance of NTR. Although the analytic Kramers expression used by BM is
correct for the purely hydrogenic recombination cross section, the heuristic
expressions used by BM to extend the Kramers expression beyond the `bare
nucleus' case to which it applies had serious errors. BM results have therefore
been recalculated using corrected expressions, which have been validated
against the results of detailed calculations. At T ~ 10-30 MK the dominant ions
are Fe 22+, 23+, 24+ for which BM erroneously overestimated NTR emission by
around an order of magnitude. Contrary to the BM claim, NTR in hot flare
plasmas does NOT dominate over NTB, although in some cases it can be comparable
and so still very important in inversions of photon spectra to derive electron
spectra, especially as NTR includes sharp edge features. The BM claim of
dominance of NTR over NTB in deka-keV emission is incorrect due to a serious
error in their analysis. However, the NTR contribution can still be large
enough to demand inclusion in spectral fitting, the spectral edges having
potentially serious effects on inversion of HXR spectra to infer fast electron
spectra.Comment: 6 pages, 8 figures, 1 tabl
The vertical metal insulator semiconductor tunnel transistor: A proposed Fowler-Nordheim tunneling device
We propose a new field-effect transistor, the vertical metal insulator semiconductor tunnel transistor (VMISTT) which operates using gate modulation of the Fowler-Nordheim tunneling current through a metal insulator semiconductor (M-I-S) diode. The VMISTT has significant advantages over the metal-oxide-semiconductor field-effect transistor in device scaling. In order to allow room-temperature operation of the VMISTT, the tunnel oxide has to be optimized for the metal-to-insulator barrier height and the current-voltage characteristics. We have grown TiO2 layers as the tunnel insulator by oxidizing 7 and 10 nm thick Ti metal films vacuum-evaporated on silicon substrates, and characterized the films by current-voltage and capacitance-voltage techniques. The quality of the oxide films showed variations, depending on the oxidation temperatures in the range of 450-550 degrees C. Fowler-Nordheim tunneling was observed at low temperatures at bias voltage of 2 V and above and a barrier height of approximately 0.4 eV was calculated. Leakage currents present were due Schottky-barrier emission at room-temperature, and hopping at liquid nitrogen temperature
Magnetic anisotropy, first-order-like metamagnetic transitions and large negative magnetoresistance in the single crystal of GdPdSi
Electrical resistivity (), magnetoresistance (MR), magnetization,
thermopower and Hall effect measurements on the single crystal
GdPdSi, crystallizing in an AlB-derived hexagonal structure are
reported. The well-defined minimum in at a temperature above N\'eel
temperature (T= 21 K) and large negative MR below 3T, reported
earlier for the polycrystals, are reproducible even in single crystals. Such
features are generally uncharacteristic of Gd alloys. In addition, we also
found interesting features in other data, e.g., two-step first-order-like
metamagnetic transitions for the magnetic field along [0001] direction. The
alloy exhibits anisotropy in all these properties, though Gd is a S-state ion.Comment: RevTeX, 5 pages, 6 encapsulated postscript figures; scheduled to be
published in Phy. Rev. B (01 November 1999, B1
Magnetic behaviour of Eu_2CuSi_3: Large negative magnetoresistance above Curie temperature
We report here the results of magnetic susceptibility,
electrical-resistivity, magnetoresistance (MR), heat-capacity and ^{151}Eu
Mossbauer effect measurements on the compound, Eu_2CuSi_3, crystallizing in an
AlB_2-derived hexagonal structure. The results establish that Eu ions are
divalent, undergoing long-range ferromagnetic-ordering below (T_C=) 37 K. An
interesting observation is that the sign of MR is negative even at temperatures
close to 3T_C, with increasing magnitude with decreasing temperature exhibiting
a peak at T_C. This observation, being made for a Cu containing magnetic
rare-earth compound for the first time, is of relevance to the field of
collosal magnetoresistance.Comment: To appear in PRB, RevTex, 4 pages text + 6 psFigs. Related to our
earlier work on Gd systems (see cond-mat/9811382, cond-mat/9811387,
cond-mat/9812069, cond-mat/9812365
On the nucleon self-energy in nuclear matter
We consider the nucleon self-energy in nuclear matter in the absence of Pauli
blocking. It is evaluated using the partial-wave analysis of scattering
data. Our results are compared with that of a realistic calculation to estimate
the effect of this blocking. It is also possible to use our results as a check
on the realistic calculations.Comment: 6 pages, 2 figure
PT-symmetric supersymmetry in a solvable short-range model
The simplest purely imaginary and piecewise constant -symmetric
potential located inside a larger box is studied. Unless its strength exceeds a
certain critical value, all the spectrum of its bound states remains real and
discrete. We interpret such a model as an initial element of the generalized
non-Hermitian Witten's hierarchy of solvable Hamiltonians and construct its
first supersymmetric (SUSY) partner in closed form.Comment: 3 figures, 1 tabl
PT-symmetric square well and the associated SUSY hierarchies
The PT-symmetric square well problem is considered in a SUSY framework. When
the coupling strength lies below the critical value
where PT symmetry becomes spontaneously broken, we find a hierarchy of SUSY
partner potentials, depicting an unbroken SUSY situation and reducing to the
family of -like potentials in the limit. For above
, there is a rich diversity of SUSY hierarchies, including
some with PT-symmetry breaking and some with partial PT-symmetry restoration.Comment: LaTeX, 18 pages, no figure; broken PT-symmetry case added (Sec. 6
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