702 research outputs found
Break-Junction Tunneling on MgB_2
Tunneling data on magnesium diboride, MgB_2, are reviewed with a particular
focus on superconductor-insulator-superconductor (SIS) junctions formed by a
break-junction method. The collective tunneling literature reveals two distinct
energy scales, a large gap, Delta_L~7.2 meV, close to the expected BCS value,
and a small gap, Delta_S~2.4 meV. The SIS break junctions show clearly that the
small gap closes near the bulk critical temperature, T_c=39 K. The SIS spectra
allow proximity effects to be ruled out as the cause for the small gap and
therefore make a strong case that MgB_2 is a coupled, two-band superconductor.
While the break junctions sometimes reveal parallel contributions to the
conductance from both bands, it is more often found that Delta_S dominates the
spectra. In these cases, a subtle feature is observed near Delta_S+Delta_L that
is reminiscent of strong-coupling effects. This feature is consistent with
quasiparticle scattering contributions to the interband coupling which provides
an important insight into the nature of two-band superconductivity in MgB_2.Comment: 9 pages, 10 pictures, accepted for publication in a special issue of
Physica C on MgB2, minor change
Dephasing in sequential tunneling through a double-dot interferometer
We analyze dephasing in a model system where electrons tunnel sequentially
through a symmetric interference setup consisting of two single-level quantum
dots. Depending on the phase difference between the two tunneling paths, this
may result in perfect destructive interference. However, if the dots are
coupled to a bath, it may act as a which-way detector, leading to partial
suppression of the phase-coherence and the reappearance of a finite tunneling
current. In our approach, the tunneling is treated in leading order whereas
coupling to the bath is kept to all orders (using P(E) theory). We discuss the
influence of different bath spectra on the visibility of the interference
pattern, including the distinction between "mere renormalization effects" and
"true dephasing".Comment: 18 pages, 8 figures; For a tutorial introduction to dephasing see
http://iff.physik.unibas.ch/~florian/dephasing/dephasing.htm
Stability of dark solitons in a Bose-Einstein condensate trapped in an optical lattice
We investigate the stability of dark solitons (DSs) in an effectively
one-dimensional Bose-Einstein condensate in the presence of the magnetic
parabolic trap and an optical lattice (OL). The analysis is based on both the
full Gross-Pitaevskii equation and its tight-binding approximation counterpart
(discrete nonlinear Schr{\"o}dinger equation). We find that DSs are subject to
weak instabilities with an onset of instability mainly governed by the period
and amplitude of the OL. The instability, if present, sets in at large times
and it is characterized by quasi-periodic oscillations of the DS about the
minimum of the parabolic trap.Comment: Typo fixed in Eq. (1): cos^2 -> sin^
Material screening and selection for XENON100
Results of the extensive radioactivity screening campaign to identify
materials for the construction of XENON100 are reported. This Dark Matter
search experiment is operated underground at Laboratori Nazionali del Gran
Sasso (LNGS), Italy. Several ultra sensitive High Purity Germanium detectors
(HPGe) have been used for gamma ray spectrometry. Mass spectrometry has been
applied for a few low mass plastic samples. Detailed tables with the
radioactive contaminations of all screened samples are presented, together with
the implications for XENON100.Comment: 8 pages, 1 figur
Stabilities of nanohydrated thymine radical cations: insights from multiphoton ionization experiments and ab initio calculations
Multi-photon ionization experiments have been carried out on thymine-water clusters in the gas phase. Metastable H2O loss from T+(H2O)n was observed at n ≥ 3 only. Ab initio quantum-chemical calculations of a large range of optimized T+(H2O)n conformers have been performed up to n = 4, enabling binding energies of water to be derived. These decrease smoothly with n, consistent with the general trend of increasing metastable H2O loss in the experimental data. The lowest-energy conformers of T+(H2O)3 and T+(H2O)4 feature intermolecular bonding via charge-dipole interactions, in contrast with the purely hydrogen-bonded neutrals. We found no evidence for a closed hydration shell at n = 4, also contrasting with studies of neutral clusters
Dust Devil Tracks
Dust devils that leave dark- or light-toned tracks are common on Mars and they can also be found on the Earth’s surface. Dust devil tracks (hereinafter DDTs) are ephemeral surface features with mostly sub-annual lifetimes. Regarding their size, DDT widths can range between ∼1 m and ∼1 km, depending on the diameter of dust devil that created the track, and DDT lengths range from a few tens of meters to several kilometers, limited by the duration and horizontal ground speed of dust devils. DDTs can be classified into three main types based on their morphology and albedo in contrast to their surroundings; all are found on both planets: (a) dark continuous DDTs, (b) dark cycloidal DDTs, and (c) bright DDTs. Dark continuous DDTs are the most common type on Mars. They are characterized by their relatively homogenous and continuous low albedo surface tracks. Based on terrestrial and martian in situ studies, these DDTs most likely form when surficial dust layers are removed to expose larger-grained substrate material (coarse sands of ≥500 μm in diameter). The exposure of larger-grained materials changes the photometric properties of the surface; hence leading to lower albedo tracks because grain size is photometrically inversely proportional to the surface reflectance. However, although not observed so far, compositional differences (i.e., color differences) might also lead to albedo contrasts when dust is removed to expose substrate materials with mineralogical differences. For dark continuous DDTs, albedo drop measurements are around 2.5 % in the wavelength range of 550–850 nm on Mars and around 0.5 % in the wavelength range from 300–1100 nm on Earth. The removal of an equivalent layer thickness around 1 μm is sufficient for the formation of visible dark continuous DDTs on Mars and Earth. The next type of DDTs, dark cycloidal DDTs, are characterized by their low albedo pattern of overlapping scallops. Terrestrial in situ studies imply that they are formed when sand-sized material that is eroded from the outer vortex area of a dust devil is redeposited in annular patterns in the central vortex region. This type of DDT can also be found in on Mars in orbital image data, and although in situ studies are lacking, terrestrial analog studies, laboratory work, and numerical modeling suggest they have the same formation mechanism as those on Earth. Finally, bright DDTs are characterized by their continuous track pattern and high albedo compared to their undisturbed surroundings. They are found on both planets, but to date they have only been analyzed in situ on Earth. Here, the destruction of aggregates of dust, silt and sand by dust devils leads to smooth surfaces in contrast to the undisturbed rough surfaces surrounding the track. The resulting change in photometric properties occurs because the smoother surfaces have a higher reflectance compared to the surrounding rough surface, leading to bright DDTs. On Mars, the destruction of surficial dust-aggregates may also lead to bright DDTs. However, higher reflective surfaces may be produced by other formation mechanisms, such as dust compaction by passing dust devils, as this may also cause changes in photometric properties. On Mars, DDTs in general are found at all elevations and on a global scale, except on the permanent polar caps. DDT maximum areal densities occur during spring and summer in both hemispheres produced by an increase in dust devil activity caused by maximum insolation. Regionally, dust devil densities vary spatially likely controlled by changes in dust cover thicknesses and substrate materials. This variability makes it difficult to infer dust devil activity from DDT frequencies. Furthermore, only a fraction of dust devils leave tracks. However, DDTs can be used as proxies for dust devil lifetimes and wind directions and speeds, and they can also be used to predict lander or rover solar panel clearing events. Overall, the high DDT frequency in many areas on Mars leads to drastic albedo changes that affect large-scale weather patterns
A novel mechanism for generating auxetic behaviour in reticulated foams: missing rib foam model
Spectra of prompt electrons from decays of B+ and B0 mesons and ratio of inclusive semielectronic branching fractions
We present spectra of prompt electrons from decays of neutral and charged B
mesons. The results are based on 140 /fb of data collected by the Belle
detector on the Upsilon(4S) resonance at the KEKB e+e- asymmetric collider. We
tag Upsilon(4S) -> B \bar{B} events by reconstructing a B meson in one of
several hadronic decay modes; the semileptonic decay of the other B meson is
inferred from the presence of an identified electron. We obtain for charged and
neutral B mesons the partial rates of semileptonic decay, to electrons with
momentum greater than 0.6 GeV/c in the B rest frame, and their ratio b_+/b_0 =
1.08 +- 0.05 +- 0.02, where the first and second errors are statistical and
systematic, respectively.Comment: 16 pages, 2 figure files, submitted to Phys. Lett.
Search for Resonant Decays at Belle
We report measurements and searches for resonant decays where is a
meson or
the X(3872) particle.Comment: accepted by Physics Letters
Moments of the Hadronic Invariant Mass Spectrum in B --> X_c l nu Decays at Belle
We present a measurement of the hadronic invariant mass squared (M^2_X)
spectrum in charmed semileptonic B meson decays B --> X_c l nu based on 140
fb^-1 of Belle data collected near the Y(4S) resonance. We determine the first,
the second central and the second non-central moments of this spectrum for
lepton energy thresholds ranging between 0.7 and 1.9 GeV. Full correlations
between these measurements are evaluated.Comment: published version of the paper (one figure added, minor changes in
the text); 16 pages, 3 figures, 10 table
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