1,550 research outputs found
Localization and Mobility Gap in Topological Anderson Insulator
It has been proposed that disorder may lead to a new type of topological
insulator, called topological Anderson insulator (TAI). Here we examine the
physical origin of this phenomenon. We calculate the topological invariants and
density of states of disordered model in a super-cell of 2-dimensional
HgTe/CdTe quantum well. The topologically non-trivial phase is triggered by a
band touching as the disorder strength increases. The TAI is protected by a
mobility gap, in contrast to the band gap in conventional quantum spin Hall
systems. The mobility gap in the TAI consists of a cluster of non-trivial
subgaps separated by almost flat and localized bands.Comment: 8 pages, 7 figure
Transport through the intertube link between two parallel carbon nanotubes
Quantum transport through the junction between two metallic carbon nanotubes
connected by intertube links has been studied within the TB method and Landauer
formula. It is found that the conductance oscillates with both of the coupling
strength and length. The corresponding local density of states (LDOS) is
clearly shown and can be used to explain the reason why there are such kinds of
oscillations of the conductances, which should be noted in the design of
nanotube-based devices.Comment: 6 pages, 4 figure
Ab-initio GMR and current-induced torques in Au/Cr multilayers
We report on an {\em ab-initio} study of giant magnetoresistance (GMR) and
current-induced-torques (CITs) in Cr/Au multilayers that is based on
non-equilibrium Green's functions and spin density functional theory. We find
substantial GMR due primarily to a spin-dependent resonance centered at the
Cr/Au interface and predict that the CITs are strong enough to switch the
antiferromagnetic order parameter at current-densities times smaller
than typical ferromagnetic metal circuit switching densities.Comment: 8 pages, 6 figure
The Origin of Carbon-Bearing Volatiles in a Continental Hydrothermal System in the Great Basin: Water Chemistry and Isotope Characterizations
Hydrothermal systems on Earth are active centers in the crust where organic molecules can be synthesized biotically or abiotically under a wide range of physical and chemical conditions [1-3]. Not only are volatile species (CO, CO2, H2, and hydrocarbons) a reflection of deep-seated hydrothermal alteration processes, but they also form an important component of biological systems. Studying carbon-bearing fluids from hydrothermal systems is of specific importance to understanding (bio-)geochemical processes within these systems. With recent detection of methane in the martian atmosphere [4-7] and the possibility of its hydrothermal origin [8, 9], understanding the formation mechanisms of methane may provide constraints on the history of the martian aqueous environments and climate
Coexistence of the topological state and a two-dimensional electron gas on the surface of Bi2Se3
Topological insulators are a recently discovered class of materials with
fascinating properties: While the inside of the solid is insulating,
fundamental symmetry considerations require the surfaces to be metallic. The
metallic surface states show an unconventional spin texture, electron dynamics
and stability. Recently, surfaces with only a single Dirac cone dispersion have
received particular attention. These are predicted to play host to a number of
novel physical phenomena such as Majorana fermions, magnetic monopoles and
unconventional superconductivity. Such effects will mostly occur when the
topological surface state lies in close proximity to a magnetic or electric
field, a (superconducting) metal, or if the material is in a confined geometry.
Here we show that a band bending near to the surface of the topological
insulator BiSe gives rise to the formation of a two-dimensional
electron gas (2DEG). The 2DEG, renowned from semiconductor surfaces and
interfaces where it forms the basis of the integer and fractional quantum Hall
effects, two-dimensional superconductivity, and a plethora of practical
applications, coexists with the topological surface state in BiSe. This
leads to the unique situation where a topological and a non-topological, easily
tunable and potentially superconducting, metallic state are confined to the
same region of space.Comment: 12 pages, 3 figure
Current-Induced Torques in Magnetic Metals: Beyond Spin Transfer
Current-induced torques on ferromagnetic nanoparticles and on domain walls in
ferromagnetic nanowires are normally understood in terms of transfer of
conserved spin angular momentum between spin-polarized currents and the
magnetic condensate. In a series of recent articles we have discussed a
microscopic picture of current-induced torques in which they are viewed as
following from exchange fields produced by the misaligned spins of current
carrying quasiparticles. This picture has the advantage that it can be applied
to systems in which spin is not approximately conserved. More importantly, this
point of view makes it clear that current-induced torques can also act on the
order parameter of an antiferromagnetic metal, even though this quantity is not
related to total spin. In this informal and intentionally provocative review we
explain this picture and discuss its application to antiferromagnets.Comment: 5 figures, to appear in Journal of Magnetism and
Correlated Hierarchy, Dirac Masses and Large Mixing Angles
We introduce a new parametrization of the MNS lepton mixing matrix which
separates the hierarchical Grand Unified relations among quarks and leptons. We
argue that one large angle stems from the charged leptons, the other from the
seesaw structure of the neutral lepton mass matrix. We show how two large
mixing angles can arise naturally provided there are special requirements on
the Dirac () and Majorana () masses.
One possibility is a correlated hierarchy between them, the other is that the
Majorana mass has a specific texture; it is Dirac-like for
two of the three families.Comment: 15 pages, no fig, some refs. adde
The Origin of Carbon-bearing Volatiles in Surprise Valley Hot Springs in the Great Basin: Carbon Isotope and Water Chemistry Characterizations
There are numerous hydrothermal fields within the Great Basin of North America, some of which have been exploited for geothermal resources. With methane and other carbon-bearing compounds being observed, in some cases with high concentrations, however, their origins and formation conditions remain unknown. Thus, studying hydrothermal springs in this area provides us an opportunity to expand our knowledge of subsurface (bio)chemical processes that generate organic compounds in hydrothermal systems, and aid in future development and exploration of potential energy resources as well. While isotope measurement has long been used for recognition of their origins, there are several secondary processes that may generate variations in isotopic compositions: oxidation, re-equilibration of methane and other alkanes with CO2, mixing with compounds of other sources, etc. Therefore, in addition to isotopic analysis, other evidence, including water chemistry and rock compositions, are necessary to identify volatile compounds of different sources. Surprise Valley Hot Springs (SVHS, 41 deg 32'N, 120 deg 5'W), located in a typical basin and range province valley in northeastern California, is a terrestrial hydrothermal spring system of the Great Basin. Previous geophysical studies indicated the presence of clay-rich volcanic and sedimentary rocks of Tertiary age beneath the lava flows in late Tertiary and Quaternary. Water and gas samples were collected for a variety of chemical and isotope composition analyses, including in-situ pH, alkalinity, conductivity, oxidation reduction potential (ORP), major and trace elements, and C and H isotope measurements. Fluids issuing from SVHS can be classified as Na-(Cl)-SO4 type, with the major cation and anion being Na+ and SO4(2-), respectively. Thermodynamic calculation using ORP and major element data indicated that sulfate is the most dominant sulfur species, which is consistent with anion analysis results. Aquifer temperatures at depth estimated by both dissolved SiO2 and Na-K-Ca geothermometers are in the range of 125.0 to 135.4 C, and higher than the values measured at orifices (77.3 to 90.0 C). CO2 and homologs of straight chain alkanes (C1-C5) were identified in gas samples. Carbon isotope values of alkanes increase with carbon numbers. The C-13 fractionation between CO2 and dissolved inorganic carbon suggests they are out of carbon isotope equilibrium. The hypothesis regarding the formation of carbon-bearing compounds in SVHS may involve two processes: 1) Under high heat flow conditions which are caused by regional faulting and crustal extension, original high molecular weight organic compounds (kerogens) in clay-rich rocks decomposed to generate methane and other alkane homologs. 2) The SVHS area is associated with outflow structures, and distant from the heat source. Anaerobic oxidation of methane (AOM) with sulfate at shallow depth (< 90 C) is suggested as being responsible for the generation of CO2 in SVHS
Enhancement of non-equilibrium thermal quantum discord and entanglement of a three-spin XX chain by multi-spin interaction and external magnetic field
We investigate the non-equilibrium thermal quantum discord and entanglement
of a three-spin chain whose two end spins are respectively coupled to two
thermal reservoirs at different temperatures. In the three-spin chain, besides
the XX-type nearest-neighbor two-spin interaction, a multi-spin interaction is
also considered and a homogenous magnetic field is applied to each spin. We
show that the extreme steady-state quantum discord and entanglement of the two
end spins can always be created by holding both a large magnetic field and a
strong multi-spin interaction. The results are explained by the thermal
excitation depression due to switching a large energy gap between the ground
state and the first excited state. The present investigation may provide a
useful approach to control coupling between a quantum system and its
environment.Comment: 16 pages, 10 figure
The Origin of Carbon-bearing Volatiles in Surprise Valley Hot Springs in the Great Basin: Carbon Isotope aud Water Chemistry Characterizations
There are numerous hydrothermal fields within the Great Basin of North America, some of which have been exploited for geothermal resources. With methane and other carbon-bearing compounds being observed, in some cases with high concentrations, however, their origins and formation conditions remain unknown. Thus, studying hydrothermal springs in this area provides us an opportunity to expand our knowledge of subsurface (bio)chemical processes that generate organic compounds in hydrothermal systems, and aid in future development and exploration of potential energy resources as well. While isotope measurement has long been used for recognition of their origins, there are several secondary processes that may generate variations in isotopic compositions: oxidation, re-equilibration of methane and other alkanes with CO2, mixing with compounds of other sources, etc. Therefore, in addition to isotopic analysis, other evidence, including water chemistry and rock compositions, are necessary to identify volatile compounds of different sources. Surprise Valley Hot Springs (SVHS, 4132'N, 1205'W), located in a typical basin and range province valley in northeastern California, is a terrestrial hydrothermal spring system of the Great Basin. Previous geophysical studies indicated the presence of clay-rich volcanic and sedimentary rocks of Tertiary age beneath the lava flows in late Tertiary and Quaternary. Water and gas samples were collected for a variety of chemical and isotope composition analyses, including in-situ pH, alkalinity, conductivity, oxidation reduction potential (ORP), major and trace elements, and C and H isotope measurements. Fluids issuing from SVHS can be classified as Na-(Cl)-SO4 type, with the major cation and anion being Na+ and SO4 2-, respectively. Thermodynamic calculation using ORP and major element data indicated that sulfate is the most dominant sulfur species, which is consistent with anion analysis results. Aquifer temperatures at depth estimated by both dissolved SiO2 and Na-K-Ca geothermometers are in the range of 125.0 to 135.4 oC, and higher than the values measured at orifices (77.3 to 90.0 oC). CO2 and homologs of straight chain alkanes (C1-C5) were identified in gas samples. Carbon isotope values of alkanes increase with carbon numbers. The 13C fractionation between CO2 and dissolved inorganic carbon suggests they are out of carbon isotope equilibrium. The hypothesis regarding the formation of carbon-bearing compounds in SVHS may involve two processes: 1) Under high heat flow conditions which are caused by regional faulting and crustal extension, original high molecular weight organic compounds (kerogens) in clay-rich rocks decomposed to generate methane and other alkane homologs. 2) The SVHS area is associated with outflow structures, and distant from the heat source. Anaerobic oxidation of methane (AOM) with sulfate at shallow depth (< 90 oC) is suggested as being responsible for the generation of CO2 in SVHS
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