Comment on "Is the nonlinear Meissner effect unobservable?"

In a recent Letter (Phys. Rev. Lett. 81, p.5640 (1998), cond-mat/9808249 v3), it was suggested that nonlocal effects may prevent observation of the nonlinear Meissner effect in YBCO. We argue that this claim is incorrect with regards to measurements of the nonlinear transverse magnetic moment, and that the most likely reason for a null result lies elsewhere.Comment: 1 pag

Discovery of a Wide Substellar Companion to a Nearby Low-Mass Star

We report the discovery of a wide (135+/-25 AU), unusually blue L5 companion 2MASS J17114559+4028578 to the nearby M4.5 dwarf G 203-50 as a result of a targeted search for common proper motion pairs in the Sloan Digital Sky Survey and the Two Micron All Sky Survey. Adaptive Optics imaging with Subaru indicates that neither component is a nearly equal mass binary with separation > 0.18", and places limits on the existence of additional faint companions. An examination of TiO and CaH features in the primary's spectrum is consistent with solar metallicity and provides no evidence that G 203-50 is metal poor. We estimate an age for the primary of 1-5 Gyr based on activity. Assuming coevality of the companion, its age, gravity and metallicity can be constrained from properties of the primary, making it a suitable benchmark object for the calibration of evolutionary models and for determining the atmospheric properties of peculiar blue L dwarfs. The low total mass (M_tot=0.21+/-0.03 M_sun), intermediate mass ratio (q=0.45+/-0.14), and wide separation of this system demonstrate that the star formation process is capable of forming wide, weakly bound binary systems with low mass and BD components. Based on the sensitivity of our search we find that no more than 2.2% of early-to-mid M dwarfs (9.0 0.06 M_sun.Comment: 24 pages, 5 figures, accepted for publication in Ap

Quantum motion of a neutron in a wave-guide in the gravitational field

We study theoretically the quantum motion of a neutron in a horizontal wave-guide in the gravitational field of the Earth. The wave-guide in question is equipped with a mirror below and a rough absorber above. We show that such a system acts as a quantum filter, i.e. it effectively absorbs quantum states with sufficiently high transversal energy but transmits low-energy states. The states transmitted are mainly determined by the potential well formed by the gravitational field of the Earth and the mirror. The formalism developed for quantum motion in an absorbing wave-guide is applied to the description of the recent experiment on the observation of the quantum states of neutrons in the Earth's gravitational field

Effect of Fibonacci Modulation On Superconductivity

We have studied finite-sized single band models with short range pairing interactions between electrons in presence of diagonal Fibonacci modulation in one dimension. Two models, namely the attractive Hubbard model and the Penson-Kolb model, have been investigated at half-filling at zero temperature by solving the Bogoliubov-de Gennes equations in real space within a mean field approximation. The competition between ``disorder'' and the pairing interaction leads to a suppression of superconductivity (of usual pairs with zero centre-of-mass momenta) in the strong-coupling limit while an enhancement of the pairing correlation is observed in the weak-coupling regime for both the models. However, the dissimilarity of the pairing mechanisms in these two models brings about notable difference in the results. The extent to which the bond ordered wave and the $\eta$-paired (of pairs with centre-of-mass momenta = $\pi$) phases of the Penson-Kolb model are affected by the disorder has also been studied in the present calculation. Some finite size effects are also identified.Comment: 14 pages, 13 figure

Lifetime of metastable states in resonant tunneling structures

We investigate the transport of electrons through a double-barrier resonant-tunneling structure in the regime where the current-voltage characteristics exhibit bistability. In this regime one of the states is metastable, and the system eventually switches from it to the stable state. We show that the mean switching time grows exponentially as the voltage across the device is tuned from the its boundary value into the bistable region. In samples of small area we find that the logarithm of the lifetime is proportional to the voltage (measured from its boundary value) to the 3/2 power, while in larger samples the logarithm of the lifetime is linearly proportional to the voltage.Comment: REVTeX 4, 5 pages, 3 EPS-figure

Effects of resonant tunneling in electromagnetic wave propagation through a polariton gap

We consider tunneling of electromagnetic waves through a polariton band gap of a 1-D chain of atoms. We analytically show that a defect embedded in the structure gives rise to the resonance transmission at the frequency of a local polariton state associated with the defect. Numerical Monte-Carlo simulations are used to examine properties of the electromagnetic band arising inside the polariton gap due to finite concentration of defects.Comment: 12 pages, 6 figures, RevTe