412 research outputs found
Magnetotunnelling in resonant tunnelling structures with spin-orbit interaction
Magnetotunnelling spectroscopy of resonant tunnelling structures provides
information on the nature of the two-dimensional electron gas in the well. We
describe a model based on nonequilibrium Green's functions that allows for a
comprehensive study of the density of states, tunnelling currents and current
spin polarization. The investigated effects include the electron-phonon
interaction, interface roughness scattering, Zeeman effect and the Rashba
spin-orbit interaction. A qualitative agreement with experimental data is found
regarding the satellite peaks. The spin polarization is predicted to be larger
than ten percent for magnetic fields above 2 Tesla and having a structure even
at the satellite peaks. The Rashba effect is confirmed to be observable as a
beating pattern in the density of states but found to be too small to affect
the tunnelling current.Comment: 31 pages, 11 figure
Chern semi-metal and Quantized Anomalous Hall Effect in HgCr2Se4
In three dimensional (3D) momentum space of solid crystal, a topological
phase boundary separating the Chern insulating layers from normal insulating
layers may exist, where the gap must be closed, resulting in a "Chern
semi-metal" state with topologically unavoidable band-crossings at fermi level.
This state, if found to exist, is a condensed-matter realization of chiral
fermions (or called Weyl fermions) in (3+1)D, and should exhibit remarkable
features, like magnetic monopoles in the bulk and fermi arcs on the surface.
Here we predict, based on first-principles calculations, that such novel
quantum state can be realized in a known ferromagnetic compound HgCr2Se4, with
a single pair of Weyl fermions separated in momentum space. The characteristic
feature of this state in HgCr2Se4 is the presence of quantum Hall effect
without external magnetic field in its quantum-well structure.Comment: Published on Phys. Rev. Lett. [5 pages, 4 figures
Group-IV graphene- and graphane-like nanosheets
We performed a first principles investigation on the structural and
electronic properties of group-IV (C, SiC, Si, Ge, and Sn) graphene-like sheets
in flat and buckled configurations and the respective hydrogenated or
fluorinated graphane-like ones. The analysis on the energetics, associated with
the formation of those structures, showed that fluorinated graphane-like sheets
are very stable, and should be easily synthesized in laboratory. We also
studied the changes on the properties of the graphene-like sheets, as result of
hydrogenation or fluorination. The interatomic distances in those graphane-like
sheets are consistent with the respective crystalline ones, a property that may
facilitate integration of those sheets within three-dimensional nanodevices
Response theory for time-resolved second-harmonic generation and two-photon photoemission
A unified response theory for the time-resolved nonlinear light generation
and two-photon photoemission (2PPE) from metal surfaces is presented. The
theory allows to describe the dependence of the nonlinear optical response and
the photoelectron yield, respectively, on the time dependence of the exciting
light field. Quantum-mechanical interference effects affect the results
significantly. Contributions to 2PPE due to the optical nonlinearity of the
surface region are derived and shown to be relevant close to a plasmon
resonance. The interplay between pulse shape, relaxation times of excited
electrons, and band structure is analyzed directly in the time domain. While
our theory works for arbitrary pulse shapes, we mainly focus on the case of two
pulses of the same mean frequency. Difficulties in extracting relaxation rates
from pump-probe experiments are discussed, for example due to the effect of
detuning of intermediate states on the interference. The theory also allows to
determine the range of validity of the optical Bloch equations and of
semiclassical rate equations, respectively. Finally, we discuss how collective
plasma excitations affect the nonlinear optical response and 2PPE.Comment: 27 pages, including 11 figures, version as publishe
Effects of STN and GPi Deep Brain Stimulation on Impulse Control Disorders and Dopamine Dysregulation Syndrome
Impulse control disorders (ICDs) and dopamine dysregulation syndrome (DDS) are important behavioral problems that affect a subpopulation of patients with Parkinson's disease (PD) and typically result in markedly diminished quality of life for patients and their caregivers. We aimed to investigate the effects of subthalamic nucleus (STN) and internal globus pallidus (GPi) deep brain stimulation (DBS) on ICD/DDS frequency and dopaminergic medication usage.A retrospective chart review was performed on 159 individuals who underwent unilateral or bilateral PD DBS surgery in either STN or GPi. According to published criteria, pre- and post-operative records were reviewed to categorize patients both pre- and post-operatively as having ICD, DDS, both ICD and DDS, or neither ICD nor DDS. Group differences in patient demographics, clinical presentations, levodopa equivalent dose (LED), and change in diagnosis following unilateral/bilateral by brain target (STN or GPi DBS placement) were examined.28 patients met diagnostic criteria for ICD or DDS pre- or post-operatively. ICD or DDS classification did not differ by GPi or STN target stimulation. There was no change in DDS diagnosis after unilateral or bilateral stimulation. For ICD, diagnosis resolved in 2 of 7 individuals after unilateral or bilateral DBS. Post-operative development of these syndromes was significant; 17 patients developed ICD diagnoses post-operatively with 2 patients with pre-operative ICD developing DDS post-operatively.Unilateral or bilateral DBS did not significantly treat DDS or ICD in our sample, even though a few cases of ICD resolved post-operatively. Rather, our study provides preliminary evidence that DDS and ICD diagnoses may emerge following DBS surgery
Consequences of local gauge symmetry in empirical tight-binding theory
A method for incorporating electromagnetic fields into empirical
tight-binding theory is derived from the principle of local gauge symmetry.
Gauge invariance is shown to be incompatible with empirical tight-binding
theory unless a representation exists in which the coordinate operator is
diagonal. The present approach takes this basis as fundamental and uses group
theory to construct symmetrized linear combinations of discrete coordinate
eigenkets. This produces orthogonal atomic-like "orbitals" that may be used as
a tight-binding basis. The coordinate matrix in the latter basis includes
intra-atomic matrix elements between different orbitals on the same atom.
Lattice gauge theory is then used to define discrete electromagnetic fields and
their interaction with electrons. Local gauge symmetry is shown to impose
strong restrictions limiting the range of the Hamiltonian in the coordinate
basis. The theory is applied to the semiconductors Ge and Si, for which it is
shown that a basis of 15 orbitals per atom provides a satisfactory description
of the valence bands and the lowest conduction bands. Calculations of the
dielectric function demonstrate that this model yields an accurate joint
density of states, but underestimates the oscillator strength by about 20% in
comparison to a nonlocal empirical pseudopotential calculation.Comment: 23 pages, 7 figures, RevTeX4; submitted to Phys. Rev.
Risk-taking in disorders of natural and drug rewards: neural correlates and effects of probability, valence, and magnitude.
Pathological behaviors toward drugs and food rewards have underlying commonalities. Risk-taking has a fourfold pattern varying as a function of probability and valence leading to the nonlinearity of probability weighting with overweighting of small probabilities and underweighting of large probabilities. Here we assess these influences on risk-taking in patients with pathological behaviors toward drug and food rewards and examine structural neural correlates of nonlinearity of probability weighting in healthy volunteers. In the anticipation of rewards, subjects with binge eating disorder show greater risk-taking, similar to substance-use disorders. Methamphetamine-dependent subjects had greater nonlinearity of probability weighting along with impaired subjective discrimination of probability and reward magnitude. Ex-smokers also had lower risk-taking to rewards compared with non-smokers. In the anticipation of losses, obesity without binge eating had a similar pattern to other substance-use disorders. Obese subjects with binge eating also have impaired discrimination of subjective value similar to that of the methamphetamine-dependent subjects. Nonlinearity of probability weighting was associated with lower gray matter volume in dorsolateral and ventromedial prefrontal cortex and orbitofrontal cortex in healthy volunteers. Our findings support a distinct subtype of binge eating disorder in obesity with similarities in risk-taking in the reward domain to substance use disorders. The results dovetail with the current approach of defining mechanistically based dimensional approaches rather than categorical approaches to psychiatric disorders. The relationship to risk probability and valence may underlie the propensity toward pathological behaviors toward different types of rewards.This is the final version. It was first published by NPG at http://www.nature.com/npp/journal/v40/n4/full/npp2014242a.htm
Helical structure motifs made searchable for functional peptide design
The systematic design of functional peptides has technological and therapeutic applications.
However, there is a need for pattern-based search engines that help locate desired functional
motifs in primary sequences regardless of their evolutionary conservation. Existing databases
such as The Protein Secondary Structure database (PSS) no longer serves the community,
while the Dictionary of Protein Secondary Structure (DSSP) annotates the secondary
structures when tertiary structures of proteins are provided. Here, we extract 1.7 million
helices from the PDB and compile them into a database (Therapeutic Peptide Design database; TP-DB) that allows queries of compounded patterns to facilitate the identification of
sequence motifs of helical structures. We show how TP-DB helps us identify a known
purification-tag-specific antibody that can be repurposed into a diagnostic kit for Helicobacter
pylori. We also show how the database can be used to design a new antimicrobial peptide
that shows better Candida albicans clearance and lower hemolysis than its template homologs. Finally, we demonstrate how TP-DB can suggest point mutations in helical peptide
blockers to prevent a targeted tumorigenic protein-protein interaction. TP-DB is made
available at http://dyn.life.nthu.edu.tw/design/
You turn me cold: evidence for temperature contagion
Introduction
During social interactions, our own physiological responses influence those of others. Synchronization of physiological (and behavioural) responses can facilitate emotional understanding and group coherence through inter-subjectivity. Here we investigate if observing cues indicating a change in another's body temperature results in a corresponding temperature change in the observer.
Methods
Thirty-six healthy participants (age; 22.9±3.1 yrs) each observed, then rated, eight purpose-made videos (3 min duration) that depicted actors with either their right or left hand in visibly warm (warm videos) or cold water (cold videos). Four control videos with the actors' hand in front of the water were also shown. Temperature of participant observers' right and left hands was concurrently measured using a thermistor within a Wheatstone bridge with a theoretical temperature sensitivity of <0.0001°C. Temperature data were analysed in a repeated measures ANOVA (temperature × actor's hand × observer's hand).
Results
Participants rated the videos showing hands immersed in cold water as being significantly cooler than hands immersed in warm water, F(1,34) = 256.67, p0.1). There was however no evidence of left-right mirroring of these temperature effects p>0.1). Sensitivity to temperature contagion was also predicted by inter-individual differences in self-report empathy.
Conclusions
We illustrate physiological contagion of temperature in healthy individuals, suggesting that empathetic understanding for primary low-level physiological challenges (as well as more complex emotions) are grounded in somatic simulation
Electronic States and Light Absorption in a Cylindrical Quantum Dot Having Thin Falciform Cross Section
Energy level structure and direct light absorption in a cylindrical quantum dot (CQD), having thin falciform cross section, are studied within the framework of the adiabatic approximation. An analytical expression for the energy spectrum of the particle is obtained. For the one-dimensional “fast” subsystem, an oscillatory dependence of the wave function amplitude on the cross section parameters is revealed. For treatment of the “slow” subsystem, parabolic and modified Pöschl-Teller effective potentials are used. It is shown that the low-energy levels of the spectrum are equidistant. In the strong quantization regime, the absorption coefficient and edge frequencies are calculated. Selection rules for the corresponding quantum transitions are obtained
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