145 research outputs found
Spatially resolved femtosecond pump-probe study of topological insulator Bi2Se3
Carrier and phonon dynamics in Bi2Se3 crystals are studied by a spatially
resolved ultrafast pump-probe technique. Pronounced oscillations in
differential reflection are observed with two distinct frequencies, and are
attributed to coherent optical and acoustic phonons, respectively. The rising
time of the signal indicates that the thermalization and energy relaxation of
hot carriers are both sub-ps in this material. We found that the thermalization
and relaxation time decreases with the carrier density. The expansion of the
differential reflection profile allows us to estimate an ambipolar carrier
diffusion coefficient on the order of 500 square centimeters per second. A
long-term slow expansion of the profile shows a thermal diffusion coefficient
of 1.2 square centimeters per second.Comment: 8 pages, 6 figure
Magnetothermoelectric properties of Bi2Se3
We present a study of entropy transport in Bi2Se3 at low temperatures and
high magnetic fields. In the zero-temperature limit, the magnitude of the
Seebeck coefficient quantitatively tracks the Fermi temperature of the 3D Fermi
surface at \Gamma-point as the carrier concentration changes by two orders of
magnitude (10 to 10cm). In high magnetic fields, the
Nernst response displays giant quantum oscillations indicating that this
feature is not exclusive to compensated semi-metals. A comprehensive analysis
of the Landau Level spectrum firmly establishes a large -factor in this
material and a substantial decrease of the Fermi energy with increasing
magnetic field across the quantum limit. Thus, the presence of bulk carriers
significantly affects the spectrum of the intensively debated surface states in
Bi2Se3 and related materials.Comment: 10 pages, 9 figure
Declining magma supply to a poroelastic magma mush explains long-term deformation at Soufrière Hills Volcano
This is the final version. Available on open access from Elsevier via the DOI in this record. Data availability:
The data supporting this study can be acquired from the Montserrat Volcano Observatory (MVO) upon a collaboration agreement.Volcano deformation studies traditionally consider melt-dominated magma reservoirs, often overlooking the significant role of poroelastic mush in modifying surface deformation. Here, we analyze the deformation of Soufrière Hills Volcano (SHV) with a focus on a mush-dominated, poroelastic magma reservoir, drawing on temporal deformation data from 14 continuous GPS stations during the ongoing intra-eruptive unrest period. We implemented a 3D Finite Element model and optimization to simulate the observed deformation. Our results reveal that the deformation is likely driven by ongoing, melt injection (Q = 1.1 m³ sec⁻¹) into a low permeability (k = 4.7 × 10⁻¹⁰ m²) reservoir with the injection to the base of the reservoir at 16.5 km depth below sea level. Our findings highlight temporal variations in the melt injection rate to fit the decreasing GPS-recorded deformation rates. An initial injection rate Qi = 1.9 m³ sec⁻¹ inferred at the start of our study period decreases to Qf = 0.3 m³ sec⁻¹ over a 12-year period (2010–2022). Exploratory forward modeling suggests that if current trends continue, the end of magma supply to our modelled reservoir could occur around June 2024 ± 2 years. However, this does not imply the end of associated volcanic hazards at SHV, as the poroelastic diffusion of melt will continue, causing redistribution of melt, surface deformation, and potentially initiating reservoir rupture. While our models offer new insights, inherent limitations in our simulations include interdependencies among our explored model parameters that would benefit from further refinement. Despite these limitations, the study offers crucial guidance on understanding and forecasting volcano deformation dynamics, particularly for volcanoes like SHV with crystal-rich magma reservoirs.University of Exete
Relações quantidade-intensidade de potássio em solos do Rio Grande do Sul
The Q/I approach was applied to evaluate the K status of eleven representative soils from Rio Grande do Sul (Brazil). The Q/I parameters, labile K (Kc), solution K (ARko) and potential buffering capacity (BCk) were correlated respectively with exchangeable K, % K-saturation and cation exchange capacity determined with 1M NH4Cl. When 0.002M CaCl2 was used as the equilibration solution some of the Ca displaced exchangeable Al from the very acid soils which resulted in an undesirable displacement of the Q/I curve. As a consequence, the Q/I parameters were not comparable among soils.O sistema Q/I foi usado para avaliar o estado do K em 11 solos representativos do Rio Grande do Sul. Os parâmetros Q/I, K+ equilibrável (Kc), K+ em solução (ARko) e capacidade tampão (BCk) estão correlacionados respectivamente com K+ trocável, % K-saturação e capacidade de troca de cátions determinados com 1M NH4Cl. Quando 0,002M CaCl2 foi usado como solução de referência, parte do Ca2+ deslocou Al3+ trocável dos solos muito ácidos, o que resultou em um indesejável deslocamento da curva Q/I. Como consequência, os parâmetros Q/I não são comparáveis entre solos
High pressure transport properties of the topological insulator Bi2Se3
We report x-ray diffraction, electrical resistivity, and magnetoresistance
measurements on Bi2Se3 under high pressure and low temperature conditions.
Pressure induces profound changes in both the room temperature value of the
electrical resistivity as well as the temperature dependence of the
resistivity. Initially, pressure drives Bi2Se3 towards increasingly insulating
behavior and then, at higher pressures, the sample appears to enter a fully
metallic state coincident with a change in the crystal structure. Within the
low pressure phase, Bi2Se3 exhibits an unusual field dependence of the
transverse magnetoresistance that is positive at low fields and becomes
negative at higher fields. Our results demonstrate that pressures below 8 GPa
provide a non-chemical means to controllably reduce the bulk conductivity of
Bi2Se3
Topological insulator quantum dot with tunable barriers
Thin (6-7 quintuple layer) topological insulator Bi2Se3 quantum dot devices
are demonstrated using ultrathin (2~4 quintuple layer) Bi2Se3 regions to
realize semiconducting barriers which may be tuned from Ohmic to tunneling
conduction via gate voltage. Transport spectroscopy shows Coulomb blockade with
large charging energy >5 meV, with additional features implying excited states
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The INtegrated CAtchment model of phosphorus dynamics (INCA-P): description and demonstration of new model structure and equations
INCA-P is a dynamic, catchment-scale phosphorus model which has been widely applied during the last decade. Since its original release in 2002, the model structure and equations have been significantly altered during several development phases. Here, we provide the first full model description since 2002 and then test the latest version of the model (v1.4.4) in a small rural catchment in northeast Scotland. The particulate phosphorus simulation was much improved compared to previous model versions, whilst the latest sorption equations allowed us to explore the potential time lags between reductions in terrestrial inputs and improvements in surface water quality, an issue of key policy relevance. The model is particularly suitable for use as a research tool, but should only be used to inform policy and land management in data-rich areas, where parameters and processes can be well-constrained. More long-term data is needed to parameterise dynamic models and test their predictions
Möbius Kondo insulators
Heavy fermion materials have recently attracted attention for their potential
to combine topological protection with strongly correlated electron physics. To
date, the ideas of topological protection have been restricted to the heavy
fermion or "Kondo" insulators with the simplest point-group symmetries. Here we
argue that the presence of nonsymmorphic crystal symmetries in many heavy
fermion materials opens up a new family of topologically protected heavy
electron systems. Re-examination of archival resistivity measurements in
nonsymmorphic heavy fermion insulators CeBiPt and CeNiSn reveals
the presence of low temperature conductivity plateau, making them candidate
members of the new class of material. We illustrate our ideas with a specific
model for CeNiSn, showing how glide symmetries generate surface states with a
novel Mobius braiding that can be detected by ARPES or non-local conductivity
measurements. One of the interesting effects of strong correlation, is the
development of partially localization or "Kondo breakdown" on the surfaces,
which transforms Mobius surface states into quasi-one dimensional conductors,
with the potential for novel electronic phase transitions.Comment: Revised version. 24 pages, 9 figures, accepted by Nature Physic
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