680 research outputs found
Meteoroid and debris special investigation group data acquisition procedures
The entire LDEF spacecraft was examined by M&D SIG for impact (i.e., craters greater than or = 0.5 mm and penetrations greater than or = 0.3 mm in diameter) and related features (e.g., debris, secondaries). During the various detailed surveys conducted at NASA Kennedy, approx. 5,000 impact related features were photodocumented, and their locations measured and recorded; an additional approx. 30,000 smaller features were counted. The equipment and techniques used by the M&D SIG permitted the determination and recording of the locations and diameters of the 5,000 imaged features. A variety of experimental and LDEF structural hardware was acquired by the M&D SIG and is presently being examined and curated at NASA Johnson
Octet baryon electromagnetic form factors in nuclear medium
We study the octet baryon electromagnetic form factors in nuclear matter
using the covariant spectator quark model extended to the nuclear matter
regime. The parameters of the model in vacuum are fixed by the study of the
octet baryon electromagnetic form factors. In nuclear matter the changes in
hadron properties are calculated by including the relevant hadron masses and
the modification of the pion-baryon coupling constants calculated in the
quark-meson coupling model. In nuclear matter the magnetic form factors of the
octet baryons are enhanced in the low region, while the electric form
factors show a more rapid variation with . The results are compared with
the modification of the bound proton electromagnetic form factors observed at
Jefferson Lab. In addition, the corresponding changes for the bound neutron are
predicted.Comment: Version accepted for publication in J.Phys. G. Few changes. 40 pages,
14 figures and 8 table
Engineering the spin polarization of one-dimensional electrons
We present results of magneto-focusing on the controlled monitoring of spin polarization within a one-dimensional (1D) channel, and its subsequent effect on modulating the spin-orbit interaction (SOI) in a 2D GaAs electron gas. We demonstrate that electrons within a 1D channel can be partially spin polarized as the effective length of the 1D channel is varied in agreement with the theoretical prediction. Such polarized 1D electrons when injected into a 2D region result in a split in the odd-focusing peaks, whereas the even peaks remain unaffected (single peak). On the other hand, the unpolarized electrons, achieved by reducing the effective length of the 1D channel, do not affect the focusing spectrum and the odd and even peaks remain as single peaks, respectively. The split in odd-focusing peaks is evidence of direct measurement of spin polarization within a 1D channel, where each sub-peak represents the population of a particular spin state. Confirmation of the spin splitting is determined by a selective modulation of the focusing peaks due to the Zeeman energy in the presence of an in-plane magnetic field. We suggest that the SOI in the 2D regime is enhanced by a stream of polarized 1D electrons. The spatial control of spin states of injected 1D electrons and the possibility of tuning the SOI may open up a new regime of spin-engineering with application in future quantum information schemes
Direct observation of exchange-driven spin interactions in one-dimensional system
We present experimental results of transverse electron focusing measurements performed on an ntype
GaAs based mesoscopic device consisting of one-dimensional (1D) quantum wires as injector
and detector. We show that non-adiabatic injection of 1D electrons at a conductance of e2/
h results in
a single first focusing peak, which transforms into two asymmetric sub-peaks with a gradual
increase in the injector conductance up to 2e2/
h , each sub-peak representing the population of spinstate
arising from the spatially separated spins in the injector. Further increasing the conductance
flips the spin-states in the 1D channel, thus reversing the asymmetry in the sub-peaks. On applying
a source-drain bias, the spin-gap, so obtained, can be resolved, thus providing evidence of exchange
interaction induced spin polarization in the 1D systems. V
Unconventional Josephson Junctions with Topological Kondo Insulator Weak Links
Proximity-induced superconductivity in three-dimensional (3D) topological insulators forms a new quantum phase of matter and accommodates exotic quasiparticles such as Majorana bound states. One of the biggest drawbacks of the commonly studied 3D topological insulators is the presence of conducting bulk that obscures both surface states and low energy bound states. Introducing superconductivity in topological Kondo insulators such as SmB6, however, is promising due to their true insulating bulk at low temperatures. In this work, we develop an unconventional Josephson junction by coupling superconducting Nb leads to the surface states of a SmB6 crystal. We observe a robust critical current at low temperatures that responds to the application of an out-of-plane magnetic field with significant deviations from usual Fraunhofer patterns. The appearance of Shaphiro steps under microwave radiation gives further evidence of a Josephson effect. Moreover, we explore the effects of Kondo breakdown in our devices, such as ferromagnetism at the surface and anomalous temperature dependence of supercurrent. Particularly, the magnetic diffraction patterns show an anomalous hysteresis with the field sweep direction suggesting the coexistence of magnetism with superconductivity at the SmB6 surface. The experimental work will advance the current understanding of topologically nontrivial superconductors and emergent states associated with such unconventional superconducting phases
A limiting velocity for quarkonium propagation in a strongly coupled plasma via AdS/CFT
We study the dispersion relations of mesons in a particular hot strongly
coupled supersymmetric gauge theory plasma. We find that at large momentum k
the dispersion relations become omega = v_0 k + a + b/k + ..., where the
limiting velocity v_0 is the same for mesons with any quantum numbers and
depends only on the ratio of the temperature to the quark mass T/m_q. We
compute a and b in terms of the meson quantum numbers and T/m_q. The limiting
meson velocity v_0 becomes much smaller than the speed of light at temperatures
below but close to T_diss, the temperature above which no meson bound states at
rest in the plasma are found. From our result for v_0, we find that the
temperature above which no meson bound states with velocity v exist is
T_diss(v) \simeq (1-v^2)^(1/4) T_diss, up to few percent corrections.We thus
confirm by direct calculation of meson dispersion relations a result inferred
indirectly in previous work via analysis of the screening length between a
static quark and antiquark in a moving plasma. Although we do not do our
calculations in QCD, we argue that the qualitative features of the dispersion
relation we compute, including in particular the relation between dissociation
temperature and meson velocity, may apply to bottomonium and charmonium mesons
propagating in the strongly coupled plasma of QCD. We discuss how our results
can contribute to understanding quarkonium physics in heavy ion collisions.Comment: 57 pages, 12 figures; references adde
Non-Perturbative Corrections and Modularity in N=1 Type IIB Compactifications
Non-perturbative corrections and modular properties of four-dimensional type
IIB Calabi-Yau orientifolds are discussed. It is shown that certain
non-perturbative alpha' corrections survive in the large volume limit of the
orientifold and periodically correct the Kahler potential. These corrections
depend on the NS-NS two form and have to be completed by D-instanton
contributions to transform covariantely under symmetries of the type IIB
orientifold background. It is shown that generically also the D-instanton
superpotential depends on the two-form moduli as well as on the complex
dilaton. These contributions can arise through theta-functions with the dilaton
as modular parameter. An orientifold of the Enriques Calabi-Yau allows to
illustrate these general considerations. It is shown that this compactification
leads to a controlled four-dimensional N=1 effective theory due to the absence
of various quantum corrections. Making contact to the underlying topological
string theory the D-instanton superpotential is proposed to be related to a
specific modular form counting D3, D1, D(-1) degeneracies on the Enriques
Calabi-Yau.Comment: 35 page
The Corporate Veil Doctrine Revisited: A Comparative Study of the English and the U.S. Corporate Veil Doctrines
The Particle Spectrum of Heterotic Compactifications
Techniques are presented for computing the cohomology of stable, holomorphic
vector bundles over elliptically fibered Calabi-Yau threefolds. These
cohomology groups explicitly determine the spectrum of the low energy,
four-dimensional theory. Generic points in vector bundle moduli space manifest
an identical spectrum. However, it is shown that on subsets of moduli space of
co-dimension one or higher, the spectrum can abruptly jump to many different
values. Both analytic and numerical data illustrating this phenomenon are
presented. This result opens the possibility of tunneling or phase transitions
between different particle spectra in the same heterotic compactification. In
the course of this discussion, a classification of SU(5) GUT theories within a
specific context is presented.Comment: 77 pages, 3 figure
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