39 research outputs found
Electrostatic Modulation of the Electronic Properties of Dirac Semimetal Na3Bi
Large-area thin films of topological Dirac semimetal NaBi are grown on
amorphous SiO:Si substrates to realise a field-effect transistor with the
doped Si acting as back gate. As-grown films show charge carrier mobilities
exceeding 7,000 cm/Vs and carrier densities below 3 10
cm, comparable to the best thin-film NaBi. An ambipolar field effect
and minimum conductivity are observed, characteristic of Dirac electronic
systems. The results are quantitatively understood within a model of
disorder-induced charge inhomogeneity in topological Dirac semimetals. Due to
the inverted band structure, the hole mobility is significantly larger than the
electron mobility in NaBi, and when present, these holes dominate the
transport properties.Comment: 5 pages, 4 figures; minor corrections and revisions for readabilit
Prevalence of and Factors Associated with Negative Microscopic Diagnosis of Cutaneous Leishmaniasis in Rural Peru.
Cutaneous leishmaniasis is endemic to South America where diagnosis is most commonly conducted via microscopy. Patients with suspected leishmaniasis were referred for enrollment by the Ministry of Health (MoH) in Lima, Iquitos, Puerto Maldonado, and several rural areas of Peru. A 43-question survey requesting age, gender, occupation, characterization of the lesion(s), history of leishmaniasis, and insect-deterrent behaviors was administered. Polymerase chain reaction (PCR) was conducted on lesion materials at the Naval Medical Research Unit No. 6 in Lima, and the results were compared with those obtained by the MoH using microscopy. Factors associated with negative microscopy and positive PCR results were identified using
Diel variations of H2O2 in Greenland: A discussion of the cause and effect relationship
Atmospheric hydrogen peroxide (H2O2) measurements at Summit, Greenland, in May–June, 1993 exhibited a diel variation, with afternoon highs typically 1–2 parts per billion by volume (ppbv) and nighttime lows about 0.5 ppbv lower. This variation closely followed that for temperature; specific humidity exhibited the same general trend. During a 17-day snowfall-free period, surface snow was accumulating H2O2, apparently from nighttime cocondensation of H2O and H2O2. Previous photochemical modeling (Neftel et al., 1995) suggests that daytime H2O2 should be about 1 ppbv, significantly lower than our measured values. Previous equilibrium partitioning measurements between ice and gas phase (Conklin et al., 1993) suggest that air in equilibrium with H2O2 concentrations measured in surface snow (15–18 μM) should have an H2O2 concentration 2–3 times what we measured 0.2–3.5 m above the snow surface. A simple eddy diffusion model, with vertical eddy diffusion coefficients calculated from balloon soundings, suggested that atmospheric H2O2 concentrations should be affected by any H2O2 degassed from surface snow. However, field measurements showed the absence of either high concentrations of H2O2 or a measurable concentration gradient between inlets 0.2 and 3 m above the snow. A surface resistance to degassing, that is, slow release of H2O2 from the ice matrix, is a plausible explanation for the differences between observations and modeled atmospheric profiles. Degassing of H2O2 at a rate below our detection limit would still influence measured atmospheric concentrations and help explain the difference between measurements and photochemical modeling. The cumulative evidence suggests that surface snow adjusts slowly to drops in atmospheric H2O2 concentration, over timescales of at least weeks. The H2O2 losses previously observed in pits sampled over more than 1 year are thought to have occurred later in the summer or fall, after the May–July field season
Observation of Effective Pseudospin Scattering in ZrSiS
3D Dirac semimetals are an emerging class of materials that possess
topological electronic states with a Dirac dispersion in their bulk. In
nodal-line Dirac semimetals, the conductance and valence bands connect along a
closed path in momentum space, leading to the prediction of pseudospin vortex
rings and pseudospin skyrmions. Here, we use Fourier transform scanning
tunneling spectroscopy (FT-STS) at 4.5 K to resolve quasiparticle interference
(QPI) patterns at single defect centers on the surface of the line nodal
semimetal zirconium silicon sulfide (ZrSiS). Our QPI measurements show
pseudospin conservation at energies close to the line node. In addition, we
determine the Fermi velocity to be eV {\AA} in the
{\Gamma}-M direction ~300 meV above the Fermi energy , and the line node
to be ~140 meV above . More importantly, we find that certain scatterers
can introduce energy-dependent non-preservation of pseudospins, giving rise to
effective scattering between states with opposite valley pseudospin deep inside
valence and conduction bands. Further investigations of quasiparticle
interference at the atomic level will aid defect engineering at the synthesis
level, needed for the development of lower-power electronics via
dissipationless electronic transport in the future
Electronic bandstructure of in-plane ferroelectric van der Waals
Layered indium selenides () have recently been discovered to
host robust out-of-plane and in-plane ferroelectricity in the and
' phases, respectively. In this work, we utilise angle-resolved
photoelectron spectroscopy to directly measure the electronic bandstructure of
, and compare to hybrid density functional theory (DFT)
calculations. In agreement with DFT, we find the band structure is highly
two-dimensional, with negligible dispersion along the c-axis. Due to n-type
doping we are able to observe the conduction band minima, and directly measure
the minimum indirect (0.97 eV) and direct (1.46 eV) bandgaps. We find the Fermi
surface in the conduction band is characterized by anisotropic electron pockets
with sharp in-plane dispersion about the points, yielding
effective masses of 0.21 along and 0.33 along
. The measured band structure is well supported by hybrid
density functional theory calculations. The highly two-dimensional (2D)
bandstructure with moderate bandgap and small effective mass suggest that
is a potentially useful new van der Waals semiconductor.
This together with its ferroelectricity makes it a viable material for
high-mobility ferroelectric-photovoltaic devices, with applications in
non-volatile memory switching and renewable energy technologies.Comment: 19 pages, 4 + 1 figures; typos corrected;added references; updated
figures & discussion to reflect changes in mode
Electric Field-Tuned Topological Phase Transition in Ultra-Thin Na3Bi - Towards a Topological Transistor
The electric field induced quantum phase transition from topological to
conventional insulator has been proposed as the basis of a topological field
effect transistor [1-4]. In this scheme an electric field can switch 'on' the
ballistic flow of charge and spin along dissipationless edges of the
two-dimensional (2D) quantum spin Hall insulator [5-9], and when 'off' is a
conventional insulator with no conductive channels. Such as topological
transistor is promising for low-energy logic circuits [4], which would
necessitate electric field-switched materials with conventional and topological
bandgaps much greater than room temperature, significantly greater than
proposed to date [6-8]. Topological Dirac semimetals(TDS) are promising systems
in which to look for topological field-effect switching, as they lie at the
boundary between conventional and topological phases [3,10-16]. Here we use
scanning probe microscopy/spectroscopy (STM/STS) and angle-resolved
photoelectron spectroscopy (ARPES) to show that mono- and bilayer films of TDS
Na3Bi [3,17] are 2D topological insulators with bulk bandgaps >400 meV in the
absence of electric field. Upon application of electric field by doping with
potassium or by close approach of the STM tip, the bandgap can be completely
closed then re-opened with conventional gap greater than 100 meV. The large
bandgaps in both the conventional and quantum spin Hall phases, much greater
than the thermal energy kT = 25 meV at room temperature, suggest that ultrathin
Na3Bi is suitable for room temperature topological transistor operation
Relationships between synoptic-scale transport and interannual variability of inorganic cations in surface snow at Summit, Greenland: 1992-1996
Version of RecordTo fully utilize the long-term chemical records retrieved from central Greenland ice cores, specific relationships between atmospheric circulation and the variability of chemical species in the records need to be better understood. This research examines associations between the variability of surface snow inorganic cation chemistry at Summit, Greenland (collected during 1992-1996 summer field seasons) and changes in air mass transport pathways and source regions, as well as variations in aerosol source strength. Transport patterns and source regions are determined through 10-day isentropic backward air mass trajectories during a 1 month (late May to late June) common season over the 5 years. Changes in the extent of exposed continental surfaces in source regions are evaluated to estimate aerosol-associated calcium and magnesium ion source strength, while forest fire activity in the circumpolar north is investigated to estimate aerosol ammonium ion source strength. During the 1995 common season, 3 times more calcium and magnesium accumulated in the snowpack than the other study years. Also, an increasing trend of ammonium concentration was noted throughout the 5 years. Anomalous transport pathways and velocities were observed during 1995, which likely contributed to the high levels of calcium and magnesium. Increased forest fire activity in North America was concurrent with increased levels of ammonium and potassium, except for 1996, when ion levels were above average and forest fire activity was below average. Because of the ubiquitous nature of soluble ions, we conclude that it is very difficult to establish a quantitative link between the ion content of snow and firn at Summit and changes in aerosol source regions and source strength.Slater, J. F., Dibb, J. E., Keim, B. D., & Kahl, J. D. w. (2001). Relationships between synoptic-scale transport and interannual variability of inorganic cations in surface snow at Summit, Greenland: 1992-1996. Journal of Geophysical Research 106(D18), 20,897-20,91
Introspective physicalism as an approach to the science of consciousness
Most theories of consciousness are based on vague speculations about the properties of conscious experience. We aim to provide a more solid basis for a science of consciousness. We argue that a theory of consciousness should provide an account of the very processes that allow us to acquire and use information about our own mental states the processes underlying introspection. This can be achieved through the construction of information processing models that can account for Type-C processes. Type-C processes can be specified experimentally by identifying paradigms in which awareness of the stimulus is necessary for an intentional action. The Shallice (1988b) framework is put forward as providing an initial account of Type-C processes, which can relate perceptual consciousness to consciously performed actions. Further, we suggest that this framework may be refined through the investigation of the functions of prefrontal cortex. The formulation of our approach requires us to consider fundamental conceptual and methodological issues associated with consciousness. The most significant of these issues concerns the scientific use of introspective evidence. We outline and justify a conservative methodological approach to the use of introspective evidence, with attention to the difficulties historically associated with its use in psychology
Temperature-dependent n-p transition in a three-dimensional Dirac semimetal Na3Bi thin film
We study the temperature dependence ( K - K) of the longitudinal
resistivity and Hall coefficient of thin films (thickness nm) of three
dimensional topological Dirac semimetal NaBi grown via molecular beam
epitaxy (MBE). The temperature-dependent Hall coefficient is electron-like at
low temperature, but transitions to hole-like transport around K. We
develop a model of a Dirac band with electron-hole asymmetry in Fermi velocity
and mobility (assumed proportional to the square of Fermi velocity) which
explains well the magnitude and temperature dependence of the Hall resistivity.
We find that the hole mobility is about times larger than the electron
mobility. In addition, we find that the electron mobility decreases
significantly with increasing temperature, suggesting electron-phonon
scattering strongly limits the room temperature mobility.Comment: 12 page