74 research outputs found
Testing the validity of THz reflection spectra by dispersion relations
Complex response function obtained in reflection spectroscopy at terahertz
range is examined with algorithms based on dispersion relations for integer
powers of complex reflection coefficient, which emerge as a powerful and yet
uncommon tools in examining the consistency of the spectroscopic data. It is
shown that these algorithms can be used in particular for checking the success
of correction of the spectra by the methods of Vartiainen et al [1] and
Lucarini et al [2] to remove the negative misplacement error in the terahertz
time-domain spectroscopy.Comment: 17 pages, 4 figure
A Global Analog of Cheshire Charge
It is shown that a model with a spontaneously broken global symmetry can
support defects analogous to Alice strings, and a process analogous to Cheshire
charge exchange can take place. A possible realization in superfluid He-3 is
pointed out.Comment: 24 pages (figures 1-4 included as uu-encoded tar files), CALT-68-1865
(Revised version: an expression (eq. 17) for global charge density is
corrected; some typos and sign mismatches are removed.
Internal Frame Dragging and a Global Analog of the Aharonov-Bohm Effect
It is shown that the breakdown of a {\it global} symmetry group to a discrete
subgroup can lead to analogues of the Aharonov-Bohm effect. At sufficiently low
momentum, the cross-section for scattering of a particle with nontrivial
charge off a global vortex is almost equal to (but definitely different from)
maximal Aharonov-Bohm scattering; the effect goes away at large momentum. The
scattering of a spin-1/2 particle off a magnetic vortex provides an amusing
experimentally realizable example.Comment: (14 pp
How to create Alice string (half-quantum vortex) in a vector Bose-Einstein condensate
We suggest a procedure how to prepare the vortex with N=1/2 winding number --
the counterpart of the Alice string -- in a Bose--Einstein condensate with
hyperfine spin F=1. Other possible vortices in Bose-condensates are also
discussed.Comment: RevTex file, 3 pages, no figures, extended version submitted to JETP
Letter
Fermions on half-quantum vortex
The spectrum of the fermion zero modes in the vicinity of the vortex with
fractional winding number is discussed. This is inspired by the observation of
the 1/2 vortex in high-temperature superconductors (Kirtley, et al, Phys. Rev.
Lett. 76 (1996) 1336). The fractional value of the winding number leads to the
fractional value of the invariant, which describes the topology of the energy
spectrum of fermions. This results in the phenomenon of the "half-crossing":
the spectrum approaches zero but does not cross it, being captured at the zero
energy level. The similarity with the phenomenon of the fermion condensation is
discussed.Comment: In revised version the discussion is extended and 4 references are
added. The paper is accepted for publication in JETP Letters. 10 pages, LaTeX
file, 3 figures are available at
ftp://boojum.hut.fi/pub/publications/lowtemp/LTL-96004.p
Evidence of a subenergy gap in the overdoped regime of Y_{1-x}Ca_{x}Ba_{2}Cu_{3}O_{7-\delta} thin films from THz Spectroscopy
We measured the terahertz (THz) complex conductivity of Ca doped
YBa_{2}Cu_{3}O_{7-\delta} thin films in the frequency range of 0.1 to 3 THz (3
to 100 cm^{-1}) and at a temperature range of 20 to 300 K. The films were
measured using both time domain and frequency domain THz methods. We showed
evidence for the existence of a sub-gap in overdoped
Y_{1-x}Ca_{x}Ba_{2}Cu_{3}O_{7-\delta} samples doped with 5% and 10% Ca.
Evidence for the opening of this sub-gap appears as a sharp decrease in the
spectrum of the real part of conductivity at frequencies equivalent to a gap
energy of 1 meV and is more prominent with increased doping. This decrease in
conductivity can be explained by using d-wave pairing symmetry with an
imaginary part of is or id_{xy} which suggests node removal.Comment: 7 pages, 7 figure
Spin Dynamics and Spin Transport
Spin-orbit (SO) interaction critically influences electron spin dynamics and
spin transport in bulk semiconductors and semiconductor microstructures. This
interaction couples electron spin to dc and ac electric fields. Spin coupling
to ac electric fields allows efficient spin manipulating by the electric
component of electromagnetic field through the electric dipole spin resonance
(EDSR) mechanism. Usually, it is much more efficient than the magnetic
manipulation due to a larger coupling constant and the easier access to spins
at a nanometer scale. The dependence of the EDSR intensity on the magnetic
field direction allows measuring the relative strengths of the competing SO
coupling mechanisms in quantum wells. Spin coupling to an in-plane electric
field is much stronger than to a perpendicular field. Because electron bands in
microstructures are spin split by SO interaction, electron spin is not
conserved and spin transport in them is controlled by a number of competing
parameters, hence, it is rather nontrivial. The relation between spin
transport, spin currents, and spin populations is critically discussed.
Importance of transients and sharp gradients for generating spin magnetization
by electric fields and for ballistic spin transport is clarified.Comment: Invited talk at the 3rd Intern. Conf. on Physics and Applications of
Spin-Related Phenomena in Semiconductors, Santa Barbara (CA), July 21 - 23.
To be published in the Journal of Superconductivity. 7 pages, 2 figure
The Fermi Bubble as a Source of Cosmic Rays in the Energy Range > 10E15 eV
The {\it Fermi} Large Area Telescope has recently discovered two giant
gamma-ray bubbles which extend north and south of the Galactic center with
diameters and heights of the order of kpc. We suggest that the
periodic star capture processes by the Galactic supermassive black hole Sgr
A, with a capture rate of
yr and an energy release of erg per capture, can
result in hot plasma injecting into the Galactic halo at a wind velocity of
cm s. The periodic injection of hot plasma can produce a
series of shocks. Energetic protons in the bubble are re-accelerated when they
interact with these shocks. We show that for energy larger than
eV, the acceleration process can be better described by the stochastic
second-order Fermi acceleration.
We propose that hadronic cosmic rays (CRs) within the ``knee'' of the
observed CR spectrum are produced by Galactic supernova remnants distributed in
the Galactic disk. Re-acceleration of these particles in the Fermi Bubble
produces CRs beyond the knee. With a mean CR diffusion coefficient in this
energy range in the bubble cm s, we can
reproduce the spectral index of the spectrum beyond the knee and within. The
conversion efficiency from shock energy of the bubble into CR energy is about
10\%. This model provides a natural explanation of the observed CR flux,
spectral indices, and matching of spectra at the knee.Comment: 43 pages, 8 figues, to be published in the Astrophysical Journal;
version 2, 45 pages, 8 figures, added references and corrected typo
Cosmology, Particle Physics and Superfluid 3He
Many direct parallels connect superfluid 3He with the field theories
describing the physical vacuum, gauge fields and elementary fermions.
Superfluid He exhibits a variety of topological defects which can be
detected with single-defect sensitivity. Modern scenarios of defect-mediated
baryogenesis can be simulated by the interaction of the 3He vortices and domain
walls with fermionic quasiparticles. Formation of defects in a
symmetry-breaking phase transition in the early Universe, which could be
responsible for large-scale structure formation and for microwave-background
anisotropy, also may be modelled in the laboratory. This is supported by the
recent observation of vortex formation in neutron-irradiated 3He-B where the
"primordial fireball" is formed in an exothermic nuclear reaction.Comment: Invited talk at LT-21 Conference, 20 pages, 3 figures available at
request, compressed ps file of the camera-ready format with 3 figures is at
ftp://boojum.hut.fi/pub/publications/lowtemp/LTL-96006.ps.g
Identification of a Vitamin-D Receptor Antagonist, MeTC7, which Inhibits the Growth of Xenograft and Transgenic Tumors In Vivo
Vitamin-D receptor (VDR) mRNA is overexpressed in neuroblastoma and carcinomas of lung, pancreas, and ovaries and predicts poor prognoses. VDR antagonists may be able to inhibit tumors that overexpress VDR. However, the current antagonists are arduous to synthesize and are only partial antagonists, limiting their use. Here, we show that the VDR antagonist MeTC7 (5), which can be synthesized from 7-dehydrocholesterol (6) in two steps, inhibits VDR selectively, suppresses the viability of cancer cell-lines, and reduces the growth of the spontaneous transgenic TH-MYCN neuroblastoma and xenografts in vivo. The VDR selectivity of 5 against RXRα and PPAR-γ was confirmed, and docking studies using VDR-LBD indicated that 5 induces major changes in the binding motifs, which potentially result in VDR antagonistic effects. These data highlight the therapeutic benefits of targeting VDR for the treatment of malignancies and demonstrate the creation of selective VDR antagonists that are easy to synthesize
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