Time-resolved charge fractionalization in inhomogeneous Luttinger liquids

The recent observation of charge fractionalization in single Tomanga-Luttinger liquids (TLLs) [Kamata et al., Nature Nanotech., 9 177 (2014)] opens new routes for a systematic investigation of this exotic quantum phenomenon. In this Letter we perform measurements on two adjacent TLLs and put forward an accurate theoretical framework to address the experiments. The theory is based on the plasmon scattering approach and can deal with injected charge pulses of arbitrary shape in TLL regions. We accurately reproduce and interpret the time-resolved multiple fractionalization events in both single and double TLLs. The effect of inter-correlations between the two TLLs is also discussed.Comment: 5 pages + Supplementary Material. To appear in Phys. Rev. B: Rapid. Com

Identification of nanoindentation-induced phase changes in silicon by in situ electrical characterization

In situ electrical measurements during nanoindentation of Czochralski grown p-type crystalline silicon (100) have been performed using a conducting diamond Berkovich indenter tip. Through-tip current monitoring with a sensitivity of ∼10pA and extraction of current-voltage curves at various points on the complete load-unload cycle have been used to track the phase transformations of silicon during the loading and unloading cycle. Postindent current-voltage curves prove to be extremely sensitive to phase changes during indentation, as well as to the final phase composition within the indented volume. For example, differences in the final structure are detected by current-voltage measurements even in an unloading regime in which only amorphous silicon is expected to form. The electrical measurements are interpreted with the aid of previously reported transmission electron microscopy and Raman microspectroscopy measurements.This work was funded by the Australian Research Council and WRiota Pty Ltd

Charge qubits and limitations of electrostatic quantum gates

We investigate the characteristics of purely electrostatic interactions with external gates in constructing full single qubit manipulations. The quantum bit is naturally encoded in the spatial wave function of the electron system. Single-electron{transistor arrays based on quantum dots or insulating interfaces typically allow for electrostatic controls where the inter-island tunneling is considered constant, e.g. determined by the thickness of an insulating layer. A representative array of 3x3 quantum dots with two mobile electrons is analyzed using a Hubbard Hamiltonian and a capacitance matrix formalism. Our study shows that it is easy to realize the first quantum gate for single qubit operations, but that a second quantum gate only comes at the cost of compromising the low-energy two-level system needed to encode the qubit. We use perturbative arguments and the Feshbach formalism to show that the compromising of the two-level system is a rather general feature for electrostatically interacting qubits and is not just related to the specific details of the system chosen. We show further that full implementation requires tunable tunneling or external magnetic fields.Comment: 7 pages, 5 figures, submitted to PR

Relativistic Beaming and Flux Variability in Active Galactic Nuclei

We discuss the impact of special relativistic effects on the observed light curves and variability duty cycles of AGNs. We model the properties of AGN light curves at radio wavelengths using a simulated shot noise process in which the occurrence of major flaring events in a relativistic jet is governed by Poisson statistics. We show that flaring sources whose radiation is highly beamed toward us are able to reach very high flux levels, but will in fact spend most of their time in relatively low flaring states due to relativistic contraction of flare time scales in the observer frame. The fact that highly beamed AGNs do not return to a steady-state quiescent level between flares implies that their weakly beamed counterparts should have highly stable flux densities that result from a superposition of many long-term, low-amplitude flares. The ``apparent'' quiescent flux levels of these weakly beamed AGNs (identified in many unified models as radio galaxies) will be significantly higher than their ''true'' quiescent (i.e., non-flaring) levels. We use Monte Carlo simulations to investigate flux variability bias in the selection statistics of flat-spectrum AGN samples. In the case of the Caltech-Jodrell Flat-spectrum survey, the predicted orientation bias towards jets seen end-on is weakened if the parent population is variable, since the highly beamed sources have a stronger tendency to be found in low flaring states. This effect is small, however, since highly beamed sources are relatively rare, and their fluxes tend to be boosted sufficiently above the survey limit such that they are selected regardless of their flaring level. We find that for larger flat-spectrum AGN surveys with fainter flux cutoffs, variability should not be an appreciable source of selection bias.Comment: Accepted for publication in the Astrophysical Journa

High Resolution VSOP Imaging of a Southern Blazar PKS 1921-293 at 1.6 GHz

We present a high resolution 1.6 GHz VSOP image of the southern blazar PKS 1921-293. The image shows a typical core-jet morphology, consistent with ground-based VLBI images. However, the addition of data from the space antenna has greatly improved the angular resolution (especially along the north-south direction for this source), and thus allowed us to clearly identify the core. Model fitting reveals an inner jet component ~1.5 mas north of the core. This jet feature may be moving on a common curved path connecting the jet within a few parsecs to the 10-parsec-scale jet. The compact core has a brightness temperature of 2.6*10**12 K (in the rest frame of the quasar), an indication of relativistic beaming. We analyzed the source in terms of three models, involving the inverse Compton catastrophe, an inhomogeneous relativistic jet, and the equipartition of energy between the radiating particles and the magnetic field. Our analysis of this gamma-ray-quiet blazar shows no preference to any particular one of these models.Comment: 7 pages including 2 figures and 1 table, PASJLaTeX, accepted for publication in PAS

Phonon-induced relaxation of a two-state system in solids

We study phonon-induced relaxation of quantum states of a particle (e.g., electron or proton) in a rigid double-well potential in a solid. Relaxation rate due to Raman two-phonon processes have been computed. We show that in a two-state limit, symmetry arguments allow one to express these rates in terms of independently measurable parameters. In general, the two-phonon processes dominate relaxation at higher temperature. Due to parity effect in a biased two-state system, their rate can be controlled by the bias.Comment: 5 PR pages, 1 figur

Semaphorin-6A controls guidance of corticospinal tract axons at multiple choice points

<p>Abstract</p> <p>Background</p> <p>The trajectory of corticospinal tract (CST) axons from cortex to spinal cord involves a succession of choice points, each of which is controlled by multiple guidance molecules. To assess the involvement of transmembrane semaphorins and their plexin receptors in the guidance of CST axons, we have examined this tract in mutants of <it>Semaphorin-6A </it>(<it>Sema6A</it>), <it>Plexin-A2 </it>(<it>PlxnA2</it>) and <it>Plexin-A4 </it>(<it>PlxnA4</it>).</p> <p>Results</p> <p>We describe defects in CST guidance in <it>Sema6A </it>mutants at choice points at the mid-hindbrain boundary (MHB) and in navigation through the pons that dramatically affect how many axons arrive to the hindbrain and spinal cord and result in hypoplasia of the CST. We also observe defects in guidance within the hindbrain where a proportion of axons aberrantly adopt a ventrolateral position and fail to decussate. This function in the hindbrain seems to be mediated by the known Sema6A receptor PlxnA4, which is expressed by CST axons. Guidance at the MHB, however, appears independent of this and of the other known receptor, PlxnA2, and may depend instead on Sema6A expression on CST axons themselves at embryonic stages.</p> <p>Conclusion</p> <p>These data identify Sema6A as a major contributor to the guidance of CST axons at multiple choice points. They highlight the active control of guidance at the MHB and also implicate the inferior olive as an important structure in the guidance of CST axons within the hindbrain. They also suggest that Sema6A, which is strongly expressed by oligodendrocytes, may affect CST regeneration in adults.</p

Dephasing of coupled spin qubit system during gate operations due to background charge fluctuations

It has been proposed that a quantum computer can be constructed based on electron spins in quantum dots or based on a superconducting nanocircuit. During two-qubit operations, the fluctuation of the coupling parameters is a critical factor. One source of such fluctuation is the stirring of the background charges. We focused on the influence of this fluctuation on a coupled spin qubit system. The induced fluctuation in exchange coupling changes the amount of entanglement, fidelity, and purity. In our previous study, the background charge fluctuations were found to be an important channel of dephasing for a single Josephson qubit.Comment: 10 pages, 7 figure. to be publishe

The tunnel magnetoresistance in chains of quantum dots weakly coupled to external leads

We analyze numerically the spin-dependent transport through coherent chains of three coupled quantum dots weakly connected to external magnetic leads. In particular, using the diagrammatic technique on the Keldysh contour, we calculate the conductance, shot noise and tunnel magnetoresistance (TMR) in the sequential and cotunneling regimes. We show that transport characteristics greatly depend on the strength of the interdot Coulomb correlations, which determines the spacial distribution of electron wave function in the chain. When the correlations are relatively strong, depending on the transport regime, we find both negative TMR as well as TMR enhanced above the Julliere value, accompanied with negative differential conductance (NDC) and super-Poissonian shot noise. This nontrivial behavior of tunnel magnetoresistance is associated with selection rules that govern tunneling processes and various high-spin states of the chain that are relevant for transport. For weak interdot correlations, on the other hand, the TMR is always positive and not larger than the Julliere TMR, although super-Poissonian shot noise and NDC can still be observed