387 research outputs found
Purity-bounded uncertainty relations in multidimensional space -- generalized purity
Uncertainty relations for mixed quantum states (precisely, purity-bounded
position-momentum relations, developed by Bastiaans and then by Man'ko and
Dodonov) are studied in general multi-dimensional case. An expression for
family of mixed states at the lower bound of uncertainty relation is obtained.
It is shown, that in case of entropy-bounded uncertainty relations, lower-bound
state is thermal, and a transition from one-dimensional problem to
multi-dimensional one is trivial. Results of numerical calculation of the
relation lower bound for different types of generalized purity are presented.
Analytical expressions for general purity-bounded relations for highly mixed
states are obtained.Comment: 12 pages, 2 figures. draft version, to appear in J. Phys. A Partially
based on a poster "Multidimensional uncertainty relations for states with
given generalized purity" presented on X Intl. Conf. on Quantum Optics'2004
(Minsk, Belarus, May 30 -- June 3, 2004) More actual report is to be
presented on ICSSUR-2005, Besan\c{c}on, France and on EQEC'05, Munich. V. 5:
amended article after referees' remark
Robust procedure for creating and characterizing the atomic structure of scanning tunneling microscope tips
Scanning tunneling microscopes (STM) are used extensively for studying and
manipulating matter at the atomic scale. In spite of the critical role of the
STM tip, the control of the atomic-scale shape of STM tips remains a poorly
solved problem. Here, we present a method for preparing tips {\it in-situ} and
for ensuring the crystalline structure and reproducibly preparing tip structure
up to the second atomic layer. We demonstrate a controlled evolution of such
tips starting from undefined tip shapes.Comment: 12 pages preprint-style; 5 figure
A strongly inhomogeneous superfluid in an iron-based superconductor
Among the mysteries surrounding unconventional, strongly correlated
superconductors is the possibility of spatial variations in their superfluid
density. We use atomic-resolution Josephson scanning tunneling microscopy to
reveal a strongly inhomogeneous superfluid in the iron-based superconductor
FeTe0.55Se0.45. By simultaneously measuring the topographic and electronic
properties, we find that this inhomogeneity in the superfluid density is not
caused by structural disorder or strong inter-pocket scattering, and does not
correlate with variations in Cooper pair-breaking gap. Instead, we see a clear
spatial correlation between superfluid density and quasiparticle strength,
putting the iron-based superconductors on equal footing with the cuprates and
demonstrating that locally, the quasiparticles are sharpest when the
superconductivity is strongest. When repeated at different temperatures, our
technique could further help elucidate what local and global mechanisms limit
the critical temperature in unconventional superconductors
Identification of the het-r vegetative incompatibility gene of Podospora anserina as a member of the fast evolving HNWD gene family
In fungi, vegetative incompatibility is a conspecific non-self recognition mechanism that restricts formation of viable heterokaryons when incompatible alleles of specific het loci interact. In Podospora anserina, three non-allelic incompatibility systems have been genetically defined involving interactions between het-c and het-d, het-c and het-e, het-r and het-v. het-d and het-e are paralogues belonging to the HNWD gene family that encode proteins of the STAND class. HET-D and HET-E proteins comprise an N-terminal HET effector domain, a central GTP binding site and a C-terminal WD repeat domain constituted of tandem repeats of highly conserved WD40 repeat units that define the specificity of alleles during incompatibility. The WD40 repeat units of the members of this HNWD family are undergoing concerted evolution. By combining genetic analysis and gain of function experiments, we demonstrate that an additional member of this family, HNWD2, corresponds to the het-r non-allelic incompatibility gene. As for het-d and het-e, allele specificity at the het-r locus is determined by the WD repeat domain. Natural isolates show allelic variation for het-
Amplifier for scanning tunneling microscopy at MHz frequencies
Conventional scanning tunneling microscopy (STM) is limited to a bandwidth of
circa 1kHz around DC. Here, we develop, build and test a novel amplifier
circuit capable of measuring the tunneling current in the MHz regime while
simultaneously performing conventional STM measurements. This is achieved with
an amplifier circuit including a LC tank with a quality factor exceeding 600
and a home-built, low-noise high electron mobility transistor (HEMT). The
amplifier circuit functions while simultaneously scanning with atomic
resolution in the tunneling regime, i.e. at junction resistances in the range
of giga-ohms, and down towards point contact spectroscopy. To enable high
signal-to-noise and meet all technical requirements for the inclusion in a
commercial low temperature, ultra-high vacuum STM, we use superconducting
cross-wound inductors and choose materials and circuit elements with low heat
load. We demonstrate the high performance of the amplifier by spatially mapping
the Poissonian noise of tunneling electrons on an atomically clean Au(111)
surface. We also show differential conductance spectroscopy measurements at
3MHz, demonstrating superior performance over conventional spectroscopy
techniques. Further, our technology could be used to perform impedance matched
spin resonance and distinguish Majorana modes from more conventional edge
states
To glide or to swim? A reinvestigation of the enigmatic Wapitisaurus problematicus (Reptilia) from the Early Triassic of British Columbia, Canada
Wapitisaurus problematicus was initially described as a member of the Weigeltisauridae, a clade of Late Permian gliding reptiles from Eurasia and Madagascar. However, the poor preservation of the holotype and only known specimen, from the lower Sulphur Mountain Formation at Ganoid Ridge (British Columbia, Canada), raised doubts about this assignment. Here, we redescribe W. problematicus and reassess its systematic position among diapsid reptiles. Comparison with all known weigeltisaurids, as well as contemporaneous reptiles from the Sulphur Mountain Formation, indicates that the taxon instead represents a thalattosauroid thalattosauriform, with noted similarities to Thalattosaurus and Paralonectes. This reidentification restricts weigeltisaurids to the Late Permian, with no occurrence in North America. Wapitisaurus problematicus potentially represents one of the oldest thalattosauriforms and increases our understanding of their diversity and disparity during the late Early and Middle Triassic. The close morphological similarities with later (thalattosauroid) thalattosauriforms and their high abundance in (shallow) marine settings may indicate an earlier invasion of this realm than previously assumed. This parallels observations in early ichthyopterygians with widespread opportunistic trophic niche diversification occurring relatively rapidly after the end-Permian mass extinction event
Nanofabricated tips for device-based scanning tunneling microscopy
We report on the fabrication and performance of a new kind of tip for
scanning tunneling microscopy. By fully incorporating a metallic tip on a
silicon chip using modern micromachining and nanofabrication techniques, we
realize so-called smart tips and show the possibility of device-based STM tips.
Contrary to conventional etched metal wire tips, these can be integrated into
lithographically defined electrical circuits. We describe a new fabrication
method to create a defined apex on a silicon chip and experimentally
demonstrate the high performance of the smart tips, both in stability and
resolution. In situ tip preparation methods are possible and we verify that
they can resolve the herringbone reconstruction and Friedel oscillations on
Au(111) surfaces. We further present an overview of possible applications
Loss of coherence in double-slit diffraction experiments
7 pågs.; 3 figs.; PACS numberssd: 03.65.Yz, 03.65.Ta, 03.75.DgThe effects of incoherence and decoherence in a double-slit experiment are studied using both optical and quantum-phenomenological models. The results are compared with experimental data obtained with cold neutrons. ©2005 American Physical SocietyThis work was supported in part by MCyT Spaind under Contracts No.
BFM2000-347 and No. BQU2003-8212. A.S.S. gratefully
acknowledges partial support from the ConsejerĂa de EducaciĂłn
y Cultura of the Comunidad AutĂłnoma de Madrid.Peer Reviewe
Poor electronic screening in lightly doped Mott insulators observed with scanning tunneling microscopy
The effective Mott gap measured by scanning tunneling microscopy (STM) in the
lightly doped Mott insulator differs
greatly from values reported by photoemission and optical experiments. Here, we
show that this is a consequence of the poor electronic screening of the
tip-induced electric field in this material. Such effects are well known from
STM experiments on semiconductors, and go under the name of tip-induced band
bending (TIBB). We show that this phenomenon also exists in the lightly doped
Mott insulator and that, at doping
concentrations of , it causes the measured energy gap in the sample
density of states to be bigger than the one measured with other techniques. We
develop a model able to retrieve the intrinsic energy gap leading to a value
which is in rough agreement with other experiments, bridging the apparent
contradiction. At doping we further observe circular features
in the conductance layers that point to the emergence of a significant density
of free carriers in this doping range, and to the presence of a small
concentration of donor atoms. We illustrate the importance of considering the
presence of TIBB when doing STM experiments on correlated-electron systems and
discuss the similarities and differences between STM measurements on
semiconductors and lightly doped Mott insulators.Comment: 9 pages, 5 figure
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