204 research outputs found
Experimental evidence for the formation of stripe phases in Si/SiGe
We observe pronounced transport anisotropies in magneto-transport experiments
performed in the two-dimensional electron system of a Si/SiGe heterostructure.
They occur when an in-plane field is used to tune two Landau levels with
opposite spin to energetic coincidence. The observed anisotropies disappear
drastically for temperatures above 1 K. We propose that our experimental
findings may be caused by the formation of a unidirectional stripe phase
oriented perpendicular to the in-plane field.Comment: 4 pages, 3 figure
Valley splitting of Si/SiGe heterostructures in tilted magnetic fields
We have investigated the valley splitting of two-dimensional electrons in
high quality Si/SiGe heterostructures under tilted magnetic fields.
For all the samples in our study, the valley splitting at filling factor
() is significantly different before and after the
coincidence angle, at which energy levels cross at the Fermi level. On both
sides of the coincidence, a linear density dependence of on the
electron density was observed, while the slope of these two configurations
differs by more than a factor of two. We argue that screening of the Coulomb
interaction from the low-lying filled levels, which also explains the observed
spin-dependent resistivity, is responsible for the large difference of
before and after the coincidence.Comment: REVTEX 4 pages, 4 figure
Efficient room-temperature light-emitters based on partly amorphised Ge quantum dots in crystalline Si
Semiconductor light emitters compatible with standard Si integration
technology (SIT) are of particular interest for overcoming limitations in the
operating speed of microelectronic devices 1-3. Light sources based on group-IV
elements would be SIT compatible but suffer from the poor optoelectronic
properties of bulk Si and Ge. Here, we demonstrate that epitaxially grown Ge
quantum dots (QDs) in a fully coherent Si matrix show extraordinary optical
properties if partially amorphised by Ge-ion bombardment (GIB). The GIB-QDs
exhibit a quasi-direct-band gap and show, in contrast to conventional SiGe
nanostructures, almost no thermal quenching of the photoluminescence (PL) up to
room-temperature (RT). Microdisk resonators with embedded GIB-QDs exhibit
threshold-behaviour and super-linear increase of the integrated PL-intensity
(IPL) with increasing excitation power Pexc which indicates light amplification
by stimulated emission in a fully SIT-compatible group-IV nano-system
Screening Breakdown on the Route toward the Metal-Insulator Transition in Modulation Doped Si/SiGe Quantum Wells
Exploiting the spin resonance of two-dimensional (2D) electrons in SiGe/Si
quantum wells we determine the carrier-density-dependence of the magnetic
susceptibility. Assuming weak interaction we evaluate the density of states at
the Fermi level D(E_F), and the screening wave vector, q_TF. Both are constant
at higher carrier densities n, as for an ideal 2D carrier gas. For n < 3e11
cm-2, they decrease and extrapolate to zero at n = 7e10 cm-2. Calculating the
mobility from q_TF yields good agreement with experimental values justifying
the approach. The decrease in D(E_F) is explained by potential fluctuations
which lead to tail states that make screening less efficient and - in a
positive feedback - cause an increase of the potential fluctuations. Even in
our high mobility samples the fluctuations exceed the electron-electron
interaction leading to the formation of puddles of mobile carriers with at
least 1 micrometer diameter.Comment: 4 pages, 3 figure
Interaction effects at the magnetic-field induced metal-insulator transition in Si/SiGe superlattices
A metal-insulator transition was induced by in-plane magnetic fields up to 27
T in homogeneously Sb-doped Si/SiGe superlattice structures. The localisation
is not observed for perpendicular magnetic fields. A comparison with
magnetoconductivity investigations in the weakly localised regime shows that
the delocalising effect originates from the interaction-induced spin-triplet
term in the particle-hole diffusion channel. It is expected that this term,
possibly together with the singlet particle-particle contribution, is of
general importance in disordered n-type Si bulk and heterostructures.Comment: 5 pages, 3 figures, Solid State Communications, in prin
Double quantum dot with integrated charge sensor based on Ge/Si heterostructure nanowires
Coupled electron spins in semiconductor double quantum dots hold promise as
the basis for solid-state qubits. To date, most experiments have used III-V
materials, in which coherence is limited by hyperfine interactions. Ge/Si
heterostructure nanowires seem ideally suited to overcome this limitation: the
predominance of spin-zero nuclei suppresses the hyperfine interaction and
chemical synthesis creates a clean and defect-free system with highly
controllable properties. Here we present a top gate-defined double quantum dot
based on Ge/Si heterostructure nanowires with fully tunable coupling between
the dots and to the leads. We also demonstrate a novel approach to charge
sensing in a one-dimensional nanostructure by capacitively coupling the double
dot to a single dot on an adjacent nanowire. The double quantum dot and
integrated charge sensor serve as an essential building block required to form
a solid-state spin qubit free of nuclear spin.Comment: Related work at http://marcuslab.harvard.edu and
http://cmliris.harvard.ed
Emerging technologies revolutionise insect ecology and monitoring
Insects are the most diverse group of animals on Earth, but their small size and
high diversity have always made them challenging to study. Recent technologi-
cal advances have the potential to revolutionise insect ecology and monitoring.
We describe the state of the art of four technologies (computer vision, acoustic
monitoring, radar, and molecular methods), and assess their advantages, current
limitations, and future potential. We discuss how these technologies can adhere
to modern standards of data curation and transparency, their implications for
citizen science, and their potential for integration among different monitoring
programmes and technologies. We argue that they provide unprecedented
possibilities for insect ecology and monitoring, but it will be important to foster
international standards via collaborationpublishedVersio
A Novel Inhibitor of Human La Protein with Anti-HBV Activity Discovered by Structure-Based Virtual Screening and In Vitro Evaluation
Background: Over 350 million people worldwide are infected with hepatitis B virus (HBV), a major cause of liver failure and hepatocellular carcinoma. Current therapeutic agents are highly effective, but are also associated with development of viral resistance. Therefore, strategies for identifying other anti-HBV agents with specific, but distinctive mechanisms of action are needed. The human La (hLa) protein, which forms a stabilizing complex with HBV RNA ribonucleoprotein to promote HBV replication, is a promising target of molecular therapy. Aims: This study aimed to discover novel inhibitors of hLa that could inhibit HBV replication and expression. Methods: A multistage molecular docking approach was used to screen a Specs database and an in-house library against hLa binding sites. Sequential in vitro evaluations were performed to detect potential compounds with high scores in HepG2.2.15 cells. Results: Of the 26 potential compounds with high scores chosen for experimental verification, 12 had HBV DNA inhibition ratios of less than 50 % with P,0.05. Six had significant inhibition of HBV e antigen (HBeAg) levels, and 13 had significant inhibition of HBV surface antigen (HBsAg) levels by in vitro assays. Compounds HBSC-11, HBSC-15 and HBSC-34 (HBSC is system prefix for active compounds screened by the library) were selected for evaluation. HBSC-11 was found to have an obvious inhibitory effect on hLa transcription and expression
Generalization of auditory sensory and cognitive learning in typically developing children
Despite the well-established involvement of both sensory (“bottom-up”) and cognitive (“top-down”) processes in literacy, the extent to which auditory or cognitive (memory or attention) learning transfers to phonological and reading skills remains unclear. Most research has demonstrated learning of the trained task or even learning transfer to a closely related task. However, few studies have reported “far-transfer” to a different domain, such as the improvement of phonological and reading skills following auditory or cognitive training. This study assessed the effectiveness of auditory, memory or attention training on far-transfer measures involving phonological and reading skills in typically developing children. Mid-transfer was also assessed through untrained auditory, attention and memory tasks. Sixty 5- to 8-year-old children with normal hearing were quasi-randomly assigned to one of five training groups: attention group (AG), memory group (MG), auditory sensory group (SG), placebo group (PG; drawing, painting), and a control, untrained group (CG). Compliance, mid-transfer and far-transfer measures were evaluated before and after training. All trained groups received 12 x 45-min training sessions over 12 weeks. The CG did not receive any intervention. All trained groups, especially older children, exhibited significant learning of the trained task. On pre- to post-training measures (test-retest), most groups exhibited improvements on most tasks. There was significant mid-transfer for a visual digit span task, with highest span in the MG, relative to other groups. These results show that both sensory and cognitive (memory or attention) training can lead to learning in the trained task and to mid-transfer learning on a task (visual digit span) within the same domain as the trained tasks. However, learning did not transfer to measures of language (reading and phonological awareness), as the PG and CG improved as much as the other trained groups. Further research is required to investigate the effects of various stimuli and lengths of training on the generalization of sensory and cognitive learning to literacy skills
Spin dynamics in semiconductors
This article reviews the current status of spin dynamics in semiconductors
which has achieved a lot of progress in the past years due to the fast growing
field of semiconductor spintronics. The primary focus is the theoretical and
experimental developments of spin relaxation and dephasing in both spin
precession in time domain and spin diffusion and transport in spacial domain. A
fully microscopic many-body investigation on spin dynamics based on the kinetic
spin Bloch equation approach is reviewed comprehensively.Comment: a review article with 193 pages and 1103 references. To be published
in Physics Reports
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