120 research outputs found
Phase coherence and the Nernst effect at magic angles in organic conductors
A giant Nernst signal was recently observed for fields near crystallographic
directions in (TMTSF)PF. Such large Nernst signals are most naturally
associated with the motion of pancake vortices. We propose a model in which
phase coherence is destroyed throughout the sample except in planes closely
aligned with the applied field . A small tilt above or below the plane
changes the direction and density of the penetrating vortices and leads to a
Nernst signal that varies with the tilt angle of as observed. The
resistance notches at magic angles are understood in terms of flux-flow
dissipation from field-induced vortices.Comment: 4 pages, 4 figure
Planar defects as a way to account for explicit anharmonicity in high temperature thermodynamic properties of silicon
Silicon is indispensable in semiconductor industry. Understanding its
high-temperature thermodynamic properties is essential both for theory and
applications. However, first-principle description of high-temperature
thermodynamic properties of silicon (thermal expansion coefficient and specific
heat) is still incomplete. Strong deviation of its specific heat at high
temperatures from the Dulong-Petit law suggests substantial contribution of
anharmonicity effects. We demonstrate, that anharmonicity is mostly due to two
transverse phonon modes, propagating in (111) and (100) directions, and can be
quantitatively described with formation of the certain type of nanostructured
planar defects of the crystal structure. Calculation of these defects'
formation energy enabled us to determine their input into the specific heat and
thermal expansion coefficient. This contribution turns out to be significantly
greater than the one calculated in quasi-harmonic approximation
Orbital quantization in the high magnetic field state of a charge-density-wave system
A superposition of the Pauli and orbital coupling of a high magnetic field to
charge carriers in a charge-density-wave (CDW) system is proposed to give rise
to transitions between subphases with quantized values of the CDW wavevector.
By contrast to the purely orbital field-induced density-wave effects which
require a strongly imperfect nesting of the Fermi surface, the new transitions
can occur even if the Fermi surface is well nested at zero field. We suggest
that such transitions are observed in the organic metal
-(BEDT-TTF)KHg(SCN) under a strongly tilted magnetic field.Comment: 14 pages including 4 figure
Conceptual aspects of QCD factorization in hadronic B decays
I review the meaning of ``QCD factorization'' in hadronic two-body B decays
and then discuss recent results of theoretical (rather than phenomenological)
nature: the proof of factorization at two loops; the identification of
``chirally enhanced'' power corrections; and the role of annihilation
contributions.Comment: 10 pages, LaTeX. Based on talks presented at the UK Phenomenology
Workshop on Heavy Flavour and CP Violation, 17 - 22 September 2000, Durham,
proceedings to appear in J. Phys. G; the 5th International Symposium on
Radiative Corrections (RADCOR2000), Carmel, California, September 11 - 15,
2000; the 4th Workshop on Continuous Advances in QCD, Minneapolis, 12-14 May
2000; the Vth International Workshop on Heavy Quark Physics, Dubna, 6-8 April
200
Baryon Charge Radii and Quadrupole Moments in the 1/N_c Expansion: The 3-Flavor Case
We develop a straightforward method to compute charge radii and quadrupole
moments for baryons both with and without strangeness, when the number of QCD
color charges is N_c. The minimal assumption of the single-photon exchange
ansatz implies that only two operators are required to describe these baryon
observables. Our results are presented so that SU(3) flavor and isospin
symmetry breaking can be introduced according to any desired specification,
although we also present results obtained from two patterns suggested by the
quark model with gluon exchange interactions. The method also permits to
extract a number of model-independent relations; a sample is r^2_Lambda / r_n^2
= 3/(N_c+3), independent of SU(3) symmetry breaking.Comment: 30 pages, no figures, REVTeX
Possible Triplet Electron Pairing and an Anisotropic Spin Susceptibility in Organic Superconductors (TMTSF)_2 X
We argue that (TMTSF)_2 PF_6 compound under pressure is likely a triplet
superconductor with a vector order parameter d(k) \equiv (d_a(k) \neq 0, d_c(k)
= ?, d_{b'}(k) = 0); |d_a(k)| > |d_c(k)|. It corresponds to an anisotropic spin
susceptibility at T=0: \chi_{b'} = \chi_0, \chi_a \ll \chi_0, where \chi_0 is
its value in a metallic phase. [The spin quantization axis, z, is parallel to a
so-called b'-axis]. We show that the suggested order parameter explains why the
upper critical field along the b'-axis exceeds all paramagnetic limiting
fields, including that for a nonuniform superconducting state, whereas the
upper critical field along the a-axis (a \perp b') is limited by the Pauli
paramagnetic effects [I. J. Lee, M. J. Naughton, G. M. Danner and P. M.
Chaikin, Phys. Rev. Lett. 78, 3555 (1997)]. The triplet order parameter is in
agreement with the recent Knight shift measurements by I. J. Lee et al. as well
as with the early results on a destruction of superconductivity by nonmagnetic
impurities and on the absence of the Hebel-Slichter peak in the NMR relaxation
rate.Comment: 4 pages, 1 eps figur
Electrotransport and magnetic properies of Cr-GaSb spintronic materials synthesized under high pressure
Electrotarnsport and magnetic properties of new phases in the system Cr-GaSb
were studied. The samples were prepared by high-pressure (P=6-8 GPa)
high-temperature treatment and identified by x-ray diffraction and scanning
electron microscopy (SEM). One of the CrGaSb phases with an
orthorhombic structure has a combination of ferromagnetic and
semiconductor properties and is potentially promising for spintronic
applications. Another high-temperature phase is paramagnetic and identified as
tetragonal
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