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At least 37 records · Page 2Linked to original sources
arXiv2007

Persistence of the 0.7 anomaly of quantum point contacts in high magnetic fields

The spin degeneracy of the lowest subband that carries one-dimensional electron transport in quantum point contacts appears to be spontaneously lifted in zero magnetic field due to a phenomenon that is known as the 0.7 anomaly. We measured this energy splitting, and studied how it evolves into a splitting that is the sum of the Zeeman effect and a field-independent exchange contribution when applying a magnetic field. While this exchange contribution shows sample-to-sample fluctuations, it is for all QPCs correlated with the zero-field splitting of the 0.7 anomaly. This provides evidence that the splitting of the 0.7 anomaly is dominated by this field-independent exchange splitting.

cond-mat.mes-hall↗
arXiv2007

Two-photon coherent polarization flipping of confined excitons

A two-photon process leading to coherent transitions between the two circularly polarized exciton states in a quantum dot is studied. It is shown that optical flipping of the exciton polarization is possible with picosecond laser pulses. The process is closely related to two-photon Rabi oscillations of a biexciton but it is much more stable against shifts of the laser frequency.

cond-mat.mes-hall↗
arXiv2007

Phonon-assisted excitation transfer in quantum dot molecules

We derive a quantum-kinetic description of phonon-assisted F{ö}rster transfer between two coupled quantum dots (a quantum dot molecule). We show that the exciton state decays to the ground state of the QDM via a combination of the Rabi rotation and exponential decay. For moderately spaced dots this process takes place on a picosecond time scale.

cond-mat.mes-hall↗
arXiv2007

Quasiparticle Energies and Band Gaps of Graphene Nanoribbons

We present calculations of the quasiparticle energies and band gaps of graphene nanoribbons (GNRs) carried out using a first-principles many-electron Green's function approach within the GW approximation. Because of the quasi-one-dimension nature of a GNR, electron-electron interaction effects due to the enhanced screened Coulomb interaction and confinement geometry greatly influence the quasiparticle band gap. Compared with previous tight-binding and density functional theory studies, our calculated quasiparticle band gaps show significant self-energy corrections for both armchair and zigzag GNRs, in the range of 0.5-3.0 eV for ribbons of width 2.4-0.4 nm. The quasiparticle band gaps found here suggest that use of GNRs for electronic device components in ambient conditions may be viable.

cond-mat.mes-hall↗
arXiv2007

Effect of Two Level System Saturation on Charge Noise in Josephson Junction Qubits

We show that charge noise $S_Q$ in Josephson qubits can be produced by fluctuating two level systems (TLS) with electric dipole moments in the substrate using a flat density of states. At high frequencies the frequency and temperature dependence of the charge noise depends on the ratio $J/J_c$ of the electromagnetic flux $J$ to the critical flux $J_c$. It is not widely appreciated that TLS in small qubits can easily be strongly saturated with $J/J_c\gg 1$. Our results are consistent with experimental conclusions that $S_Q\sim 1/f$ at low frequencies and $S_Q\sim f$ at high frequencies.

cond-mat.mes-hall↗
arXiv2008

TDDFT and Strongly Correlated Systems: Insight From Numerical Studies

We illustrate the scope of Time Dependent Density Functional Theory (TDDFT) for strongly correlated (lattice) models out of equilibrium. Using the exact many body time evolution, we reverse engineer the exact exchange correlation (xc) potential $v_{xc}$ for small Hubbard chains exposed to time-dependent fields. We introduce an adiabatic local density approximation (ALDA) to $v_{xc}$ for the 1D Hubbard model and compare it to exact results, to gain insight about approximate xc potentials. Finally, we provide some remarks on the v-representability for the 1D Hubbard model.

cond-mat.mes-hall↗
arXiv2007

Energy Measurements and Preparation of Canonical Phase States of a Nano-Mechanical Resonator

We show that a continuous quantum non-demolition measurement of the energy of a nanomechanical resonator can be achieved by monitoring the resonator with a quantum point contact via a Cooper-pair box. This technique can further be used to prepare highly non-classical states of two resonators, such as canonical phase-reference states, and so-called "noon" states.

quant-ph↗
arXiv2009

Effect of Microwaves on the Current-Phase-Relation in SNS Josephson Junctions

We investigate the current-phase-relation (CPR) of long diffusive superconductor-normal metal-superconductor (SNS) Josephson junctions in thermodynamic equilibrium and under microwave irradiation. While in equilibrium good agreement with the predictions of quasi-classical theory is found, we observe that the shape of the CPR can be strongly affected by microwave irradiation. Close to a Josephson-phase difference of pi, the supercurrent can be strongly suppressed when increasing the rf-power. Our results can be understood in terms of microwave excitation of low-lying Andreev bound states across the mini-gap in the junction. In the frequency interval studied, this mechanism becomes important when the mini-gap closes at a phase difference of pi.

cond-mat.mes-hall↗
arXiv2007

Ignition and Propagation of Magnetic Avalanches in Mn$_{12}$-Acetate: the effect of quantum tunneling

Using a wire heater to ignite magnetic avalanches in fixed magnetic field applied along the easy axis of single crystals of the molecular magnet Mn$_{12}$-acetate, we report fast local measurements of the temperature and time-resolved measurements of the local magnetization as a function of magnetic field. In addition to confirming maxima in the velocity of propagation, we find that avalanches trigger at a threshold temperature which exhibits pronounced minima at resonant magnetic fields, demonstrating that thermally assisted quantum tunneling plays an important role in the ignition as well as the propagation of magnetic avalanches in molecular magnets.

cond-mat.mes-hall↗
arXiv2008

Spin blockade and lifetime-enhanced transport in a few-electron Si/SiGe double quantum dot

Spin blockade occurs when an electron is unable to access an energetically favorable path through a quantum dot due to spin conservation, resulting in a blockade of the current through the dot. Spin blockade is the basis of a number of recent advances in spintronics, including the measurement and the manipulation of individual electron spins. We report measurements of the spin blockade regime in a silicon double quantum dot, revealing a complementary phenomenon: lifetime-enhanced transport. We argue that our observations arise because the decay times for electron spins in silicon are long, enabling the electron to maintain its spin throughout its transit across the quantum dot and access fast paths that exist in some spin channels but not in others. Such long spin lifetimes are important for applications such as quantum computation and, more generally, spintronics.

cond-mat.mes-hall↗
arXiv2007

Generation and detection of non-Abelian matrix Berry phases through manipulation of electric confinement potential of a semiconductor quantum dot

A matrix Berry phase can be generated and detected by {\it all electric means} in II-VI or III-V n-type semiconductor quantum dots by changing the shape of the confinement potential. This follows from general symmetry considerations in the presence of spin-orbit coupling terms. The resulting 2 x 2 matrix Berry phase can be characterized by two numbers of geometric origin. We investigate how these parameters depend on the shape and area of closed adiabatic paths. We suggest how the matrix Berry phase may be detected in transport measurements.

cond-mat.mes-hall↗
arXiv2007

Fano resonances at light scattering by an obstacle

It is shown that elastic resonance scattering of light by a finite-size obstacle with weak dissipation is analogous to quantum scattering by a potential with quasi-discrete levels and exhibits Fano resonances. Localized plasmons (polaritons), exited in the obstacle by the incident light, are equivalent to the quasi-discrete levels, while the radiative decay of these excitations plays exactly the same role as tunnelling from the quasi-discrete levels for the quantum problem. Mie scattering of light by a spherical particle and an exactly solvable discrete model with nonlocal coupling simulating wave scattering in systems with reduced spatial dimensionality are discussed as examples.

cond-mat.other↗
arXiv2008

Charge transport in arrays of PbSe nanocrystals

We report electrical transport measurements of arrays of PbSe nanocrystals forming the channels of field effect transistors. We measure the current in these devices as a function of source-drain voltage, gate voltage and temperature. Annealing is necessary to observe measurable current after which a simple model of hopping between intrinsic localized states describes the transport properties of the nanocrystal solid. We find that the majority carriers are holes, which are thermally released from acceptor states. At low source-drain voltages, the activation energy for the conductivity is given by the energy required to generate holes plus the activation over barriers resulting from site disorder. At high source-drain voltages the activation energy is given by the former only. The thermal activation energy of the zero-bias conductance indicates that the Fermi energy is close to the highest-occupied valence level, the 1Sh state, and this is confirmed by field-effect measurements, which give a density of states of approximately eight per nanocrystal as expected from the degeneracy of the 1Sh state.

cond-mat.mes-hall↗
arXiv2007

Temperature Coefficients of the Raman Peaks for the Single-Layer and Bi-Layer Graphene

We carried out micro-Raman spectroscopy of graphene layers over the temperature range from approximately 80 K to 370 K. The number of layers was independently confirmed by the quantum Hall measurements and atomic force microscopy. The measured values of the temperature coefficients for the G and 2D-band frequencies of the single-layer graphene are -0.016 1/(cm K) and -0.034 1/(cm K), respectively. The G peak temperature coefficient of the bi-layer graphene and bulk graphite are -0.015 1/(cm K) and -0.011 1/(cm K), respectively.

cond-mat.mtrl-sci↗
arXiv2007

Fractionally Quantized Hall Effect: Liquid-Crystal Ground-State and it Excitations

It is shown that the ground-state and the lowest excited-states of two-dimensional electron system (2DES), with ion jellium background, correspond to partial crystal-like (with the period $L_{x}^{\square}=\sqrt{2 m π} \ell_{0}$) correlation order among $N$ electrons of the main region (MR; $L_{x} \times L_{y}$). Many-body variational ground-state wave function of 2DES is presented at the fractional and the integral filling factors $ν=1/m$; $m=2\ell+1$ and $\ell=0, 1, 2,...$. The ground-state manifests the broken symmetry liquid-crystal state with 2DES density that is periodic along the $y-$ direction, with the period $L_{x}^{\square}/m$, and independent of $x$. At $m=3, 5$, the ground-state has essentially lower energy per electron than the Laughlin, uniform liquid, ground-state; the same holds at $m=1$. At $m \geq 3$, the compound form of the many-body ground-state wave function leads to the compound structure of each electron within the main strip (MS; $L_{x}^{\square} \times L_{y}$). Obtained compound exciton and compound spin-exciton states show finite excitation gaps. The excited compound electron (hole) is composed, within MS, from $m$ strongly correlated quasielectrons (quasiholes) of the total charge $e/m$ ($-e/m$) each. The activation gap is obtained: it is given by the excitation gap of relevant compound exciton, at $m \geq 3$, and by the gap of pertinent compound spin-exciton, at $m=1$. Quantized Hall conductance $σ_{H}=e^{2}/(2 m π\hbar)$ is obtained. The theory is in good agreement with experiments.

cond-mat.mes-hall↗
arXiv2007

Probing the doping in metallic and semiconducting carbon nanotubes by Raman and transport measurements

In-situ Raman experiments together with transport measurements have been carried out on carbon nanotubes as a function of gate voltage. In metallic tubes, a large increase in the Raman frequency of the $G^-$ band, accompanied by a substantial decrease of its line-width, is observed with electron or hole doping. In addition, we see an increase in Raman frequency of the $G^+$ band in semiconducting tubes. These results are quantitatively explained using ab-initio calculations that take into account effects beyond the adiabatic approximation. Our results imply that Raman spectroscopy can be used as an accurate measure of the doping of both metallic and semiconducting nanotubes.

cond-mat.mes-hall↗