447 research outputs found
Ocean time-series near Bermuda: Hydrostation S and the US JGOFS Bermuda Atlantic time-series study
Bermuda is the site of two ocean time-series programs. At Hydrostation S, the ongoing biweekly profiles of temperature, salinity and oxygen now span 37 years. This is one of the longest open-ocean time-series data sets and provides a view of decadal scale variability in ocean processes. In 1988, the U.S. JGOFS Bermuda Atlantic Time-series Study began a wide range of measurements at a frequency of 14-18 cruises each year to understand temporal variability in ocean biogeochemistry. On each cruise, the data range from chemical analyses of discrete water samples to data from electronic packages of hydrographic and optics sensors. In addition, a range of biological and geochemical rate measurements are conducted that integrate over time-periods of minutes to days. This sampling strategy yields a reasonable resolution of the major seasonal patterns and of decadal scale variability. The Sargasso Sea also has a variety of episodic production events on scales of days to weeks and these are only poorly resolved. In addition, there is a substantial amount of mesoscale variability in this region and some of the perceived temporal patterns are caused by the intersection of the biweekly sampling with the natural spatial variability. In the Bermuda time-series programs, we have added a series of additional cruises to begin to assess these other sources of variation and their impacts on the interpretation of the main time-series record. However, the adequate resolution of higher frequency temporal patterns will probably require the introduction of new sampling strategies and some emerging technologies such as biogeochemical moorings and autonomous underwater vehicles
Angle-resolved photoemission spectroscopy with quantum gas microscopes
Quantum gas microscopes are a promising tool to study interacting quantum
many-body systems and bridge the gap between theoretical models and real
materials. So far they were limited to measurements of instantaneous
correlation functions of the form , even though
extensions to frequency-resolved response functions would provide important information about the elementary
excitations in a many-body system. For example, single particle spectral
functions, which are usually measured using photoemission experiments in
electron systems, contain direct information about fractionalization and the
quasiparticle excitation spectrum. Here, we propose a measurement scheme to
experimentally access the momentum and energy resolved spectral function in a
quantum gas microscope with currently available techniques. As an example for
possible applications, we numerically calculate the spectrum of a single hole
excitation in one-dimensional models with isotropic and anisotropic
antiferromagnetic couplings. A sharp asymmetry in the distribution of spectral
weight appears when a hole is created in an isotropic Heisenberg spin chain.
This effect slowly vanishes for anisotropic spin interactions and disappears
completely in the case of pure Ising interactions. The asymmetry strongly
depends on the total magnetization of the spin chain, which can be tuned in
experiments with quantum gas microscopes. An intuitive picture for the observed
behavior is provided by a slave-fermion mean field theory. The key properties
of the spectra are visible at currently accessible temperatures.Comment: 16+7 pages, 10+2 figure
Pressure and temperature driven phase transitions in HgTe quantum wells
We present theoretical investigations of pressure and temperature driven
phase transitions in HgTe quantum wells grown on CdTe buffer. Using the 8-band
\textbf{kp} Hamiltonian we calculate evolution of energy band structure
at different quantum well width with hydrostatic pressure up to 20 kBar and
temperature ranging up 300 K. In particular, we show that in addition to
temperature, tuning of hydrostatic pressure allows to drive transitions between
semimetal, band insulator and topological insulator phases. Our realistic band
structure calculations reveal that the band inversion under hydrostatic
pressure and temperature may be accompanied by non-local overlapping between
conduction and valence bands. The pressure and temperature phase diagrams are
presented.Comment: 9 pages, 8 figures + Supplemental material (5 pages
Phase transitions in two tunnel-coupled HgTe quantum wells. Bilayer graphene analogy and beyond
HgTe quantum wells possess remarkable physical properties as for instance the
quantum spin Hall state and the 'single-valley' analog of graphene, depending
on their layer thicknesses and barrier composition. However, double HgTe
quantum wells yet contain more fascinating and still unrevealed features. Here
we report on the study of the quantum phase transitions in tunnel-coupled HgTe
layers separated by CdTe barrier. We demonstrate that this system has a 3/2
pseudo spin degree of freedom, which features a number of particular properties
associated with the spin-dependent coupling between HgTe layers. We discover a
specific metal phase arising in a wide range of HgTe and CdTe layer
thicknesses, in which a gapless bulk and a pair of helical edge states coexist.
This phase holds some properties of bilayer graphene such as an unconventional
quantum Hall effect and an electrically-tunable band gap. In this 'bilayer
graphene' phase, electric field opens the band gap and drives the system into
the quantum spin Hall state. Furthermore, we discover a new type of quantum
phase transition arising from a mutual inversion between second electron- and
hole-like subbands. This work paves the way towards novel materials based on
multi-layered topological insulators
Submerged entry nozzle clogging during continuous casting of Al-killed steel
Nozzle clogging is a common problem in the production of continuously cast Al-killed steels. Clogging occurs when there are solid inclusions in molten steel at casting temperatures. SENs (Submerged entry nozzles) from continuous casting of Al-killed low alloy steel grades with increased content of sulfur (0,020 to 0,035 % S) were examined. The examinations revealed that the deposits are mainly alumina based, with spinel and sulfur inclusions and some entrapped steel melt. It was concluded that the process of clogging begins when the steel melt infiltrates the refractory and removes the protective zirconia surface, thus allowing the adhesion of fine solid aluminates, which form the deposits
Field Effect Transistors for Terahertz Detection: Physics and First Imaging Applications
Resonant frequencies of the two-dimensional plasma in FETs increase with the
reduction of the channel dimensions and can reach the THz range for sub-micron
gate lengths. Nonlinear properties of the electron plasma in the transistor
channel can be used for the detection and mixing of THz frequencies. At
cryogenic temperatures resonant and gate voltage tunable detection related to
plasma waves resonances, is observed. At room temperature, when plasma
oscillations are overdamped, the FET can operate as an efficient broadband THz
detector. We present the main theoretical and experimental results on THz
detection by FETs in the context of their possible application for THz imaging.Comment: 22 pages, 12 figures, review pape
Terahertz Radiation Detection by Field Effect Transistor in Magnetic Field
We report on terahertz radiation detection with InGaAs/InAlAs Field Effect
Transistors in quantizing magnetic field. The photovoltaic detection signal is
investigated at 4.2 K as a function of the gate voltage and magnetic field.
Oscillations analogous to the Shubnikov-de Haas oscillations, as well as their
strong enhancement at the cyclotron resonance, are observed. The results are
quantitatively described by a recent theory, showing that the detection is due
to rectification of the terahertz radiation by plasma waves related
nonlinearities in the gated part of the channel.Comment: 4 pages, 3 figure
High intensity study of THz detectors based on field effect transistors
Terahertz power dependence of the photoresponse of field effect transistors,
operating at frequencies from 0.1 to 3 THz for incident radiation power density
up to 100 kW/cm^2 was studied for Si metal-oxide-semiconductor field-effect
transistors and InGaAs high electron mobility transistors. The photoresponse
increased linearly with increasing radiation power up to kW/cm^2 range. The
saturation of the photoresponse was observed for all investigated field effect
transistors for intensities above several kW/cm^2. The observed signal
saturation is explained by drain photocurrent saturation similar to saturation
in direct currents output characteristics. The theoretical model of terahertz
field effect transistor photoresponse at high intensity was developed. The
model explains quantitatively experimental data both in linear and nonlinear
(saturation) range. Our results show that dynamic range of field effect
transistors is very high and can extend over more than six orderd of magnitudes
of power densities (from 0.5 mW/cm^2 to 5 kW/cm^2)
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