840 research outputs found
Andreev rectifier: a nonlocal conductance signature of topological phase transitions
The proximity effect in hybrid superconductor-semiconductor structures,
crucial for realizing Majorana edge modes, is complicated to control due to its
dependence on many unknown microscopic parameters. In addition, defects can
spoil the induced superconductivity locally in the proximitised system which
complicates measuring global properties with a local probe. We show how to use
the nonlocal conductance between two spatially separated leads to probe three
global properties of a proximitised system: the bulk superconducting gap, the
induced gap, and the induced coherence length. Unlike local conductance
spectroscopy, nonlocal conductance measurements distinguish between
non-topological zero-energy modes localized around potential inhomogeneities,
and true Majorana edge modes that emerge in the topological phase. In addition,
we find that the nonlocal conductance is an odd function of bias at the
topological phase transition, acting as a current rectifier in the low-bias
limit. More generally, we identify conditions for crossed Andreev reflection to
dominate the nonlocal conductance and show how to design a Cooper pair splitter
in the open regime.Comment: 11 pages, 13 figure
The evolution of the galactic morphological types in clusters
The morphological types of galaxies in nine clusters in the redshift range
0.1<z<0.25 are derived from very good seeing images taken at the NOT and the La
Silla Danish telescopes. With the purpose of investigating the evolution of the
fraction of different morphological types with redshift, we compare our results
with the morphological content of nine distant clusters studied by the MORPHS
group, five clusters observed with HST-WFPC2 at redshift z = 0.2-0.3, and
Dressler's (1980) large sample of nearby clusters. After having checked the
reliability of our morphological classification both in an absolute sense and
relative to the MORPHS scheme, we analyze the relative occurrence of
elliptical, S0 and spiral galaxies as a function of the cluster properties and
redshift. We find a large intrinsic scatter in the S0/E ratio, mostly related
to the cluster morphology. In particular, in our cluster sample, clusters with
a high concentration of ellipticals display a low S0/E ratio and, vice-versa,
low concentration clusters have a high S0/E. At the same time, the trend of the
morphological fractions and ratios with redshift clearly points to a
morphological evolution: as the redshift decreases, the S0 population tends to
grow at the expense of the spiral population, whereas the frequency of Es
remains almost constant. We also analyze the morphology-density (MD) relation
in our clusters and find that -similarly to higher redshift clusters- a good MD
relation exists in the high-concentration clusters, while it is absent in the
less concentrated clusters. Finally, the comparison of the MD relation in our
clusters with that of the D97 sample suggests that the transformation of
spirals into S0 galaxies becomes more efficient with decreasing local density.Comment: 24 pages including 11 figures and 4 tables, accepted for publication
in Ap
Photoionization and Photoelectric Loading of Barium Ion Traps
Simple and effective techniques for loading barium ions into linear Paul
traps are demonstrated. Two-step photoionization of neutral barium is achieved
using a weak intercombination line (6s2 1S0 6s6p 3P1, 791 nm) followed by
excitation above the ionization threshold using a nitrogen gas laser (337 nm).
Isotopic selectivity is achieved by using a near Doppler-free geometry for
excitation of the triplet 6s6p 3P1 state. Additionally, we report a
particularly simple and efficient trap loading technique that employs an
in-expensive UV epoxy curing lamp to generate photoelectrons.Comment: 5 pages, Accepted to PRA 3/20/2007 -fixed typo -clarified figure 3
caption -added reference [15
Evolution of Galaxy morphologies in Clusters
We have studied the evolution of galaxian morphologies from ground-based,
good-seeing images of 9 clusters at z=0.09-0.25. The comparison of our data
with those relative to higher redshift clusters (Dressler et al. 1997) allowed
us to trace for the first time the evolution of the morphological mix at a
look-back time of 2-4 Gyr, finding a dependence of the observed evolutionary
trends on the cluster properties.Comment: 4 pages with 2 figures in Latex-Kluwer style. To be published in the
proceedings of the Granada Euroconference "The Evolution of
Galaxies.I-Observational Clues
Superconducting, Insulating, and Anomalous Metallic Regimes in a Gated Two-Dimensional Semiconductor-Superconductor Array
The superconductor-insulator transition in two dimensions has been widely
investigated as a paradigmatic quantum phase transition. The topic remains
controversial, however, because many experiments exhibit a metallic regime with
saturating low-temperature resistance, at odds with conventional theory. Here,
we explore this transition in a novel, highly controllable system, a
semiconductor heterostructure with epitaxial Al, patterned to form a regular
array of superconducting islands connected by a gateable quantum well. Spanning
nine orders of magnitude in resistance, the system exhibits regimes of
superconducting, metallic, and insulating behavior, along with signatures of
flux commensurability and vortex penetration. An in-plane magnetic field
eliminates the metallic regime, restoring the direct superconductor-insulator
transition, and improves scaling, while strongly altering the scaling exponent
Zero-Energy Modes from Coalescing Andreev States in a Two-Dimensional Semiconductor-Superconductor Hybrid Platform
We investigate zero-bias conductance peaks that arise from coalescing subgap
Andreev states, consistent with emerging Majorana zero modes, in hybrid
semiconductor-superconductor wires defined in a two-dimensional InAs/Al
heterostructure using top-down lithography and gating. The measurements
indicate a hard superconducting gap, ballistic tunneling contact, and in-plane
critical fields up to ~T. Top-down lithography allows complex geometries,
branched structures, and straightforward scaling to multicomponent devices
compared to structures made from assembled nanowires.Comment: Includes Supplementary Materia
The WINGS Survey: a progress report
A two-band (B and V) wide-field imaging survey of a complete, all-sky X-ray
selected sample of 78 clusters in the redshift range z=0.04-0.07 is presented.
The aim of this survey is to provide the astronomical community with a complete
set of homogeneous, CCD-based surface photometry and morphological data of
nearby cluster galaxies located within 1.5 Mpc from the cluster center. The
data collection has been completed in seven observing runs at the INT and
ESO-2.2m telescopes. For each cluster, photometric data of about 2500 galaxies
(down to V~23) and detailed morphological information of about 600 galaxies
(down to V~21) are obtained by using specially designed automatic tools.
As a natural follow up of the photometric survey, we also illustrate a long
term spectroscopic program we are carrying out with the WHT-WYFFOS and AAT-2dF
multifiber spectrographs. Star formation rates and histories, as well as
metallicity estimates will be derived for about 350 galaxies per cluster from
the line indices and equivalent widths measurements, allowing us to explore the
link between the spectral properties and the morphological evolution in high-
to low-density environments, and across a wide range in cluster X-ray
luminosities and optical properties.Comment: 12 pages, 10 eps figures, Proceedings of the SAIt Conference 200
Quantized conductance doubling and hard gap in a two-dimensional semiconductor-superconductor heterostructure
The prospect of coupling a two-dimensional (2D) semiconductor heterostructure
to a superconductor opens new research and technology opportunities, including
fundamental problems in mesoscopic superconductivity, scalable superconducting
electronics, and new topological states of matter. For instance, one route
toward realizing topological matter is by coupling a 2D electron gas (2DEG)
with strong spin-orbit interaction to an s-wave superconductor. Previous
efforts along these lines have been hindered by interface disorder and unstable
gating. Here, we report measurements on a gateable InGaAs/InAs 2DEG with
patterned epitaxial Al, yielding multilayer devices with atomically pristine
interfaces between semiconductor and superconductor. Using surface gates to
form a quantum point contact (QPC), we find a hard superconducting gap in the
tunneling regime, overcoming the soft-gap problem in 2D
superconductor-semiconductor hybrid systems. With the QPC in the open regime,
we observe a first conductance plateau at 4e^2/h, as expected theoretically for
a normal-QPC-superconductor structure. The realization of a hard-gap
semiconductor-superconductor system that is amenable to top-down processing
provides a means of fabricating scalable multicomponent hybrid systems for
applications in low-dissipation electronics and topological quantum
information.Comment: includes main text, supplementary information and code for
simulations. Published versio
Two-dimensional epitaxial superconductor-semiconductor heterostructures: A platform for topological superconducting networks
Progress in the emergent field of topological superconductivity relies on
synthesis of new material combinations, combining superconductivity, low
density, and spin-orbit coupling (SOC). For example, theory [1-4] indicates
that the interface between a one-dimensional (1D) semiconductor (Sm) with
strong SOC and a superconductor (S) hosts Majorana modes with nontrivial
topological properties [5-8]. Recently, epitaxial growth of Al on InAs
nanowires was shown to yield a high quality S-Sm system with uniformly
transparent interfaces [9] and a hard induced gap, indicted by strongly
suppressed sub gap tunneling conductance [10]. Here we report the realization
of a two-dimensional (2D) InAs/InGaAs heterostructure with epitaxial Al,
yielding a planar S-Sm system with structural and transport characteristics as
good as the epitaxial wires. The realization of 2D epitaxial S-Sm systems
represent a significant advance over wires, allowing extended networks via
top-down processing. Among numerous potential applications, this new material
system can serve as a platform for complex networks of topological
superconductors with gate-controlled Majorana zero modes [1-4]. We demonstrate
gateable Josephson junctions and a highly transparent 2D S-Sm interface based
on the product of excess current and normal state resistance
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