Disentangling the effects of spin-orbit and hyperfine interactions on spin blockade

We have achieved the few-electron regime in InAs nanowire double quantum dots. Spin blockade is observed for the first two half-filled orbitals, where the transport cycle is interrupted by forbidden transitions between triplet and singlet states. Partial lifting of spin blockade is explained by spin-orbit and hyperfine mechanisms that enable triplet to singlet transitions. The measurements over a wide range of interdot coupling and tunneling rates to the leads are well reproduced by a simple transport model. This allows us to separate and quantify the contributions of the spin-orbit and hyperfine interactions.Comment: 5 pages, 4 figure

Analysis of strain and stacking faults in single nanowires using Bragg coherent diffraction imaging

Coherent diffraction imaging (CDI) on Bragg reflections is a promising technique for the study of three-dimensional (3D) composition and strain fields in nanostructures, which can be recovered directly from the coherent diffraction data recorded on single objects. In this article we report results obtained for single homogeneous and heterogeneous nanowires with a diameter smaller than 100 nm, for which we used CDI to retrieve information about deformation and faults existing in these wires. The article also discusses the influence of stacking faults, which can create artefacts during the reconstruction of the nanowire shape and deformation.Comment: 18 pages, 6 figures Submitted to New Journal of Physic

Order to disorder transition in the XY-like quantum magnet Cs2CoCl4 induced by noncommuting applied fields

We explore the effects of noncommuting applied fields on the ground-state ordering of the quasi-one-dimensional spin-1/2 XY-like antiferromagnet Cs2CoCl4 using single-crystal neutron diffraction. In zero field interchain couplings cause long-range order below T_N=217(5) mK with chains ordered antiferromagnetically along their length and moments confined to the (b,c) plane. Magnetic fields applied at an angle to the XY planes are found to initially stabilize the order by promoting a spin-flop phase with an increased perpendicular antiferromagnetic moment. In higher fields the antiferromagnetic order becomes unstable and a transition occurs to a phase with no long-range order in the (b,c) plane, proposed to be a spin liquid phase that arises when the quantum fluctuations induced by the noncommuting field become strong enough to overcome ordering tendencies. Magnetization measurements confirm that saturation occurs at much higher fields and that the proposed spin-liquid state exists in the region 2.10 < H_SL < 2.52 T || a. The observed phase diagram is discussed in terms of known results on XY-like chains in coexisting longitudinal and transverse fields.Comment: revtex, 14 figures, 2 tables, to appear in Phys. Rev.

Many Body Theory of Charge Transfer in Hyperthermal Atomic Scattering

We use the Newns-Anderson Hamiltonian to describe many-body electronic processes that occur when hyperthermal alkali atoms scatter off metallic surfaces. Following Brako and Newns, we expand the electronic many-body wavefunction in the number of particle-hole pairs (we keep terms up to and including a single particle-hole pair). We extend their earlier work by including level crossings, excited neutrals and negative ions. The full set of equations of motion are integrated numerically, without further approximations, to obtain the many-body amplitudes as a function of time. The velocity and work-function dependence of final state quantities such as the distribution of ion charges and excited atomic occupancies are compared with experiment. In particular, experiments that scatter alkali ions off clean Cu(001) surfaces in the energy range 5 to 1600 eV constrain the theory quantitatively. The neutralization probability of Na$^+$ ions shows a minimum at intermediate velocity in agreement with the theory. This behavior contrasts with that of K$^+$, which shows ... (7 figures, not included. Figure requests: [email protected])Comment: 43 pages, plain TeX, BUP-JBM-

Avalanche amplification of a single exciton in a semiconductor nanowire

Interfacing single photons and electrons is a crucial ingredient for sharing quantum information between remote solid-state qubits. Semiconductor nanowires offer the unique possibility to combine optical quantum dots with avalanche photodiodes, thus enabling the conversion of an incoming single photon into a macroscopic current for efficient electrical detection. Currently, millions of excitation events are required to perform electrical read-out of an exciton qubit state. Here we demonstrate multiplication of carriers from only a single exciton generated in a quantum dot after tunneling into a nanowire avalanche photodiode. Due to the large amplification of both electrons and holes (> 10^4), we reduce by four orders of magnitude the number of excitation events required to electrically detect a single exciton generated in a quantum dot. This work represents a significant step towards single-shot electrical read-out and offers a new functionality for on-chip quantum information circuits

From Children to Adults: Motor Performance across the Life-Span

The life-span approach to development provides a theoretical framework to examine the general principles of life-long development. This study aims to investigate motor performance across the life span. It also aims to investigate if the correlations between motor tasks increase with aging. A cross-sectional design was used to describe the effects of aging on motor performance across age groups representing individuals from childhood to young adult to old age. Five different motor tasks were used to study changes in motor performance within 338 participants (7–79 yrs). Results showed that motor performance increases from childhood (7–9) to young adulthood (19–25) and decreases from young adulthood (19–25) to old age (66–80). These results are mirroring results from cognitive research. Correlation increased with increasing age between two fine motor tasks and two gross motor tasks. We suggest that the findings might be explained, in part, by the structural changes that have been reported to occur in the developing and aging brain and that the theory of Neural Darwinism can be used as a framework to explain why these changes occur

Genetic risk factors for ischaemic stroke and its subtypes (the METASTROKE Collaboration): a meta-analysis of genome-wide association studies

&lt;p&gt;Background - Various genome-wide association studies (GWAS) have been done in ischaemic stroke, identifying a few loci associated with the disease, but sample sizes have been 3500 cases or less. We established the METASTROKE collaboration with the aim of validating associations from previous GWAS and identifying novel genetic associations through meta-analysis of GWAS datasets for ischaemic stroke and its subtypes.&lt;/p&gt; &lt;p&gt;Methods - We meta-analysed data from 15 ischaemic stroke cohorts with a total of 12 389 individuals with ischaemic stroke and 62 004 controls, all of European ancestry. For the associations reaching genome-wide significance in METASTROKE, we did a further analysis, conditioning on the lead single nucleotide polymorphism in every associated region. Replication of novel suggestive signals was done in 13 347 cases and 29 083 controls.&lt;/p&gt; &lt;p&gt;Findings - We verified previous associations for cardioembolic stroke near PITX2 (p=2·8×10−16) and ZFHX3 (p=2·28×10−8), and for large-vessel stroke at a 9p21 locus (p=3·32×10−5) and HDAC9 (p=2·03×10−12). Additionally, we verified that all associations were subtype specific. Conditional analysis in the three regions for which the associations reached genome-wide significance (PITX2, ZFHX3, and HDAC9) indicated that all the signal in each region could be attributed to one risk haplotype. We also identified 12 potentially novel loci at p&#60;5×10−6. However, we were unable to replicate any of these novel associations in the replication cohort.&lt;/p&gt; &lt;p&gt;Interpretation - Our results show that, although genetic variants can be detected in patients with ischaemic stroke when compared with controls, all associations we were able to confirm are specific to a stroke subtype. This finding has two implications. First, to maximise success of genetic studies in ischaemic stroke, detailed stroke subtyping is required. Second, different genetic pathophysiological mechanisms seem to be associated with different stroke subtypes.&lt;/p&gt

Field-Induced Two-Step Phase Transitions in the Singlet Ground State Triangular Antiferromagnet CsFeBr3_3

The ground state of the stacked triangular antiferromagnet CsFeBr$_3$ is a spin singlet due to the large single ion anisotropy $D(S^z)^2$. The field-induced magnetic ordering in this compound was investigated by the magnetic susceptibility, the magnetization process and specific heat measurements for an external field parallel to the $c$-axis. Unexpectedly, two phase transitions were observed in the magnetic field $H$ higher than 3 T. The phase diagram for temperature versus magnetic field was obtained. The mechanism leading to the successive phase transitions is discussed.Comment: 8 pages, 9 figures, 10 eps files, jpsj styl

Unintentional high density p-type modulation doping of a GaAs/AlAs core-multi-shell nanowire

Achieving significant doping in GaAs/AlAs core/shell nanowires (NWs) is of considerable technological importance but remains a challenge due to the amphoteric behavior of the dopant atoms. Here we show that placing a narrow GaAs quantum well in the AlAs shell effectively getters residual carbon acceptors leading to an \emph{unintentional} p-type doping. Magneto-optical studies of such a GaAs/AlAs core multi-shell NW reveal quantum confined emission. Theoretical calculations of NW electronic structure confirm quantum confinement of carriers at the core/shell interface due to the presence of ionized carbon acceptors in the 1~nm GaAs layer in the shell. Micro-photoluminescence in high magnetic field shows a clear signature of avoided crossings of the $n=0$ Landau level emission line with the $n=2$ Landau level TO phonon replica. The coupling is caused by the resonant hole-phonon interaction, which points to a large 2D hole density in the structure.Comment: just published in Nano Letters (http://pubs.acs.org/doi/full/10.1021/nl500818k

Embodiment and the origin of interval timing: kinematic and electromyographic data

Recent evidence suggests that interval timing (the judgment of durations lasting from approximately 500 ms. to a few minutes) is closely coupled to the action control system. We used surface electromyography (EMG) and motion capture technology to explore the emergence of this coupling in 4-, 6-, and 8-month-olds. We engaged infants in an active and socially relevant arm-raising task with 7 cycles and response period. In one condition cycles were slow (every 4 seconds) in another they were fast (every 2 seconds). In the slow condition, we found evidence of time locked sub-threshold EMG activity even in the absence of any observed overt motor responses at all 3 ages. This study shows that EMGs can be a more sensitive measure of interval timing in early development than overt behavior