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Ultrahigh-mobility graphene devices from chemical vapor deposition on reusable copper

A novel dry transfer technique opens the door to large-scale CVD graphene with carrier mobilities of up to several 100,000 cm2 V−1 s−1. Graphene research has prospered impressively in the past few years, and promising applications such as high-frequency transistors, magnetic field sensors, and flexible optoelectronics are just waiting for a scalable and cost-efficient fabrication technology to produce high-mobility graphene. Although significant progress has been made in chemical vapor deposition (CVD) and epitaxial growth of graphene, the carrier mobility obtained with these techniques is still significantly lower than what is achieved using exfoliated graphene. We show that the quality of CVD-grown graphene depends critically on the used transfer process, and we report on an advanced transfer technique that allows both reusing the copper substrate of the CVD growth and making devices with mobilities as high as 350,000 cm2 V–1 s–1, thus rivaling exfoliated graphene.

Probing relaxation times in graphene quantum dots

Graphene quantum dots are attractive candidates for solid-state quantum bits. In fact, the predicted weak spin-orbit and hyperfine interaction promise spin qubits with long coherence times. Graphene quantum dots have been extensively investigated with respect to their excitation spectrum, spin-filling sequence and electron-hole crossover. However, their relaxation dynamics remain largely unexplored. This is mainly due to challenges in device fabrication, in particular concerning the control of carrier confinement and the tunability of the tunnelling barriers, both crucial to experimentally investigate decoherence times. Here we report pulsed-gate transient current spectroscopy and relaxation time measurements of excited states in graphene quantum dots. This is achieved by an advanced device design that allows to individually tune the tunnelling barriers down to the low megahertz regime, while monitoring their asymmetry. Measuring transient currents through electronic excited states, we estimate a lower bound for charge relaxation times on the order of 60–100 ns.

Thermoelectric performance of a driven double quantum-dot

In this paper we investigate the thermoelectric performance of a double-dot device driven by timedependently modulated gate voltages. We show that if the modulation frequency is sufficiently small, not only quantized charge pumping can be realized, but also the heat current flowing in the leads is quantized and exhibits plateaux in units of 2π kBT ln2. The factor ln2 stems from the degeneracy of the double-dot states involved into transport. This opens the possibility of using the pumping cycle to transfer heat against a temperature gradient or to extract work from a hot reservoir with Carnot efficiency. However, the performance of a realistic device is limited by dissipative effects due to leakage currents and finite-frequency operation, which we take into account rigorously by means of a real-time diagrammatic approach in the regime where the double dot is weakly coupled to the leads. We show that despite these effects, the efficiency ofa double-dot charge pump performing work against a dc-source can reach of up to 70% of the ideal value.

Current noise in molecular junctions : Effects of the electron-phonon interaction

We study inelastic effects on the electronic current noise in molecular junctions, due to the coupling between transport electrons and vibrational degrees of freedom. Using a full counting statistics approach based on the generalized Keldysh Greens function technique, we calculate in an unified manner both the mean current and the zero-frequency current noise. For multilevel junctions with weak electron-phonon coupling, we give analytical formulas for the lowest order inelastic corrections to the noise in terms of universal temperature- and voltage-dependent functions and junction-dependent prefactors, which can be evaluated microscopically, e.g. with ab-initio methodologies. We identify distinct terms corresponding to the mean-field contribution to noise and to the vertex corrections, and we show that the latter contribute substantially to the inelastic noise. Finally, we illustrate our results by a simple model of two electronic levels which are mutually coupled by the electron-phonon interaction and show that the inelastic noise spectroscopy is a sensitive diagnostic tool.

Sub-Poissonian phononic population in a nanoelectromechanical system

The properties of the phononic distribution of a mechanical oscillator coupled to a single-electron transistor are investigated in the sequential tunnelling regime. It is shown that for not too strong electron?phonon interaction the electrical current may induce a distribution of phonons with sub-Poissonian statistics, which is characterized by a selective population of few phonon states. Depending on the choice of parameters, such a sub-Poissonian phonon distribution can be accompanied either by a super- or a sub-Poissonian electronic Fano factor.

Anomalous suppression of the shot noise in a nanoelectromechanical system

In this paper we report a relaxation-induced suppression of the noise for a single level quantum dot coupled to an oscillator with incoherent dynamics in the sequential tunneling regime. It is shown that relaxation induces qualitative changes in the transport properties of the dot, depending on the strength of the electron-phonon coupling and on the applied voltage. In particular, critical thresholds in voltage and relaxation are found such that a suppression below 1 / 2 of the Fano factor is possible. Additionally, the current is either enhanced or suppressed by increasing relaxation, depending on bias being greater or smaller than the above threshold. These results exist for any strength of the electron-phonon coupling and are confirmed by a four states toy model.

Fermion-parity duality and energy relaxation in interacting open systems

We study the transient heat current out of a confined electron system into a weakly coupled electrode in response to a voltage switch. We show that the decay of the Coulomb interaction energy for this repulsive system exhibits signatures of electron-electron attraction, and is governed by an interaction-independent rate. This can only be understood from a general duality that relates the non-unitary evolution of a quantum system to that of a dual model with inverted energies. Deriving from the fermion-parity superselection postulate, this duality applies to a large class of open systems.

Nonequilibrium phonon backaction on the current noise in atomic-sized junctions

Fachbereich Physik, Universita¨t Konstanz, D-78457 Konstanz, Germany(Dated: May 20, 2011)We study back-action effects of phonon heating due to tunneling electrons on the current noise inatomic-sized junctions. Deriving a generalized kinetic approximation within the extended KeldyshGreen’s functions technique, we demonstrate the existence of a characteristic back-action contri-bution to the noise in case of low external phonon damping. We provide a physically intuitiveinterpretation of this contribution at large voltage in terms of slow fluctuations of the phonon oc-cupation, and show that it generally gives a significant correction to the noise above the phononemission threshold.

Correlations between charge and energy current in ac-driven coherent conductors

We study transport in coherent conductors driven by a time-periodic bias voltage. We present results of the charge and energy noise and complement them by a study of the mixed noise, namely the zero-frequency correlator between charge and energy current. The mixed noise presents interference contributions and transport contributions, showing features different from those of charge and energy noise. The mixed noise can be accessed by measuring the correlator between the fluctuations of the power provided to the system and the charge current.

Shot noise in charge and magnetization currents of a quantum ring

The shot noise in a quantum ring, connected to leads, is studied in the presence of electron interactions in the sequential tunneling regime. Two qualitatively different noise correlations with distinctly different behaviors are identified and studied in a large range of parameters. Noise in the total current is due to the discreteness of the electron charge and can become super-Poissonian as result of electron interaction. The noise in the magnetization current is comparatively insensitive to the interaction but can be greatly enhanced if population inversion of the angular states is assumed. The characteristic time scales are studied by a Monte-Carlo simulation. PACS numbers: 73.50.Td,71.10.Pm,73.23.-b