435 research outputs found
Interplay between interferences and electron-electron interactions in epitaxial graphene
We separate localization and interaction effects in epitaxial graphene
devices grown on the C-face of a 4H-SiC substrate by analyzing the low
temperature conductivities. Weak localization and antilocalization are
extracted at low magnetic fields, after elimination of a geometric
magnetoresistance and subtraction of the magnetic field dependent Drude
conductivity. The electron electron interaction correction is extracted at
higher magnetic fields, where localization effects disappear. Both phenomena
are weak but sizable and of the same order of magnitude. If compared to
graphene on silicon dioxide, electron electron interaction on epitaxial
graphene are not significantly reduced by the larger dielectric constant of the
SiC substrate
Growth of monolayer graphene on 8deg off-axis 4H-SiC (000-1) substrates with application to quantum transport devices
Using high temperature annealing conditions with a graphite cap covering the
C-face of an 8deg off-axis 4H-SiC sample, large and homogeneous single
epitaxial graphene layers have been grown. Raman spectroscopy shows evidence of
the almost free-standing character of these monolayer graphene sheets, which
was confirmed by magneto-transport measurements. We find a moderate p-type
doping, high carrier mobility and half integer Quantum Hall effect typical of
high quality graphene samples. This opens the way to a fully compatible
integration of graphene with SiC devices on the wafers that constitute the
standard in today's SiC industry.Comment: 11 pages, 4 figures , Submitted in AP
Scheimpflug-based analysis of the reflectivity of the cornea in Marfan Syndrome
Purpose:We sought to investigate corneal reflectivity inMarfan syndrome (MFS) on the basis of Scheimpflug light intensity distribution. Methods: In a retrospective case-control analysis, the left eyes of 40 MFS patients and 40 age- and refraction-matched healthy controls were investigated. Patients with MFS meeting the Ghent II diagnostic criteria and with genetic confirmation of disease were included. Exclusion criteria were the following: Coexisting corneal, conjunctival, or scleral pathology, use of medication known to affect corneal transparency, history of ocular surgery, and insufficient data. Scheimpflug tomography images were exported to analyze corneal transparency in different corneal layers and regions. Each corneal image was automatically segmented, after which the corresponding pixel intensities in the defined regions of interest were statistically modeled using a Weibull probability density function from which parameters a (transparency) and ß (homogeneity) were derived. Results: The cornea in MFS showed significantly higher light reflectivity (overall cornea, a = 71 ± 17 arbitrary units (a.u.)) than in the control group (overall cornea, a = 59 ± 15 a.u.) (t test, P = 0.003). The a parameter was significantly higher in MFS eyes in all examined layers and regions (P 0.05). The difference in a did not correlate with ocular biometric properties (corneal thickness and curvature) or ectopia lentis (P> 0.05). Conclusions: The cornea in MFS shows significantly higher reflectivity than healthy controls with similar levels of homogeneity. Translational Relevance: The proposed methodology detects corneal reflectivity changes in MFS not available from regular slit-lamp examination
Temperature-induced topological phase transition in HgTe quantum wells
We report a direct observation of temperature-induced topological phase
transition between trivial and topological insulator in HgTe quantum well. By
using a gated Hall bar device, we measure and represent Landau levels in fan
charts at different temperatures and we follow the temperature evolution of a
peculiar pair of "zero-mode" Landau levels, which split from the edge of
electron-like and hole-like subbands. Their crossing at critical magnetic field
is a characteristic of inverted band structure in the quantum well. By
measuring the temperature dependence of , we directly extract the critical
temperature , at which the bulk band-gap vanishes and the topological
phase transition occurs. Above this critical temperature, the opening of a
trivial gap is clearly observed.Comment: 5 pages + Supplemental Materials; Phys. Rev. Lett. (accepted
Anterior eye surface changes following miniscleral contact lens wear
Purpose To quantify the effect of short-term miniscleral contact lens wear on the anterior eye surface of healthy eyes, including cornea, corneo-scleral junction and sclero-conjuctival area. Methods Twelve healthy subjects (29.9 ± 5.7 years) wore a highly gas-permeable miniscleral contact lens of 16.5 mm diameter during a 5-hour period. Corneo-scleral height profilometry was captured before, immediately following lens removal and 3 h after lens removal. Topography based corneo-scleral limbal radius estimates were derived from height measurements. In addition, elevation differences in corneal and scleral region were calculated with custom-written software. Sclero-conjuctival flattening within different sectors was analysed. Results Short-term miniscleral lens wear significantly modifies the anterior eye surface. Significant limbal radius increment (mean ± standard deviation) of 146 ± 80 μm, (p = 0.004) and flattening of −122 ± 90 μm in the sclero-conjuctival area, (p << 0.001) were observed immediately following lens removal. These changes did not recede to baseline levels 3 h after lens removal. The greatest anterior eye surface flattening was observed in the superior sector. No statistically significant corneal shape change was observed immediately following lens removal or during the recovery period. Conclusions Short-term miniscleral contact lens wear in healthy eyes does not produce significant corneal shape changes measured with profilometry but alters sclero-conjuctival topography. In addition, sclero-conjuctival flattening was not uniformly distributed across the anterior eye
The siliciclastic Permian-Triassic deposits in Central and Northeastern Iberian Peninsula (Iberian, Ebro and Catalan Basins): a proposal for correlation
The siliciclastic deposits of the Iberian, Ebro and Catalan Basins have been described for more than a century, but facies similarities and a lack of biostratigaphic data have difficulted the correlation of the local stratigraphic units in a general framework up to now. Combining pollen and spores data, the identification of the regional unconformities and hiatuses and the quantitative analysis of the subsidence by backstripping methods, a new correlation scheme for these facies is proposed
Room Temperature Coherent and Voltage Tunable Terahertz Emission from Nanometer-Sized Field Effect Transistors
We report on reflective electro-optic sampling measurements of TeraHertz
emission from nanometer-gate-length InGaAs-based high electron mobility
transistors. The room temperature coherent gate-voltage tunable emission is
demonstrated. We establish that the physical mechanism of the coherent
TeraHertz emission is related to the plasma waves driven by simultaneous
current and optical excitation. A significant shift of the plasma frequency and
the narrowing of the emission with increasing channel's current are observed
and explained as due to the increase of the carriers density and drift
velocity.Comment: 3 figure
Temperature-driven single-valley Dirac fermions in HgTe quantum wells
We report on temperature-dependent magnetospectroscopy of two HgTe/CdHgTe
quantum wells below and above the critical well thickness . Our results,
obtained in magnetic fields up to 16 T and temperature range from 2 K to 150 K,
clearly indicate a change of the band-gap energy with temperature. The quantum
well wider than evidences a temperature-driven transition from
topological insulator to semiconductor phases. At the critical temperature of
90 K, the merging of inter- and intra-band transitions in weak magnetic fields
clearly specifies the formation of gapless state, revealing the appearance of
single-valley massless Dirac fermions with velocity of
ms. For both quantum wells, the energies extracted from
experimental data are in good agreement with calculations on the basis of the
8-band Kane Hamiltonian with temperature-dependent parameters.Comment: 5 pages, 3 figures and Supplemental Materials (4 pages
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