Szabolcs Csonka

Cooper pair splitter realized in a two-quantum-dot Y-junction

Non-locality is a fundamental property of quantum mechanics that manifests itself as correlations between spatially separated parts of a quantum system. A fundamental route for the exploration of such phenomena is the generation of Einstein–Podolsky–Rosen (EPR) pairs of quantum-entangled objects for the test of so-called Bell inequalities. Whereas such experimental tests of non-locality have been successfully conducted with pairwise entangled photons, it has not yet been possible to realize an electronic analogue of it in the solid state, where spin-1/2 mobile electrons are the natural quantum objects. The difficulty stems from the fact that electrons are immersed in a macroscopic ground state—the Fermi sea—which prevents the straightforward generation and splitting of entangled pairs of electrons on demand. A superconductor, however, could act as a source of EPR pairs of electrons, because its ground-state is composed of Cooper pairs in a spin-singlet state. These Cooper pairs can be extracted from a superconductor by tunnelling, but, to obtain an efficient EPR source of entangled electrons, the splitting of the Cooper pairs into separate electrons has to be enforced. This can be achieved by having the electrons ‘repel’ each other by Coulomb interaction. Controlled Cooper pair splitting can thereby be realized by coupling of the superconductor to two normal metal drain contacts by means of individually tunable quantum dots. Here we demonstrate the first experimental realization of such a tunable Cooper pair splitter, which shows a surprisingly high efficiency. Our findings open a route towards a first test of the EPR paradox and Bell inequalities in the solid state.

Finite-bias Cooper pair splitting.

In a device with a superconductor coupled to two parallel quantum dots (QDs) the electrical tunability of the QD levels can be used to exploit nonclassical current correlations due to the splitting of Cooper pairs. We experimentally investigate the effect of a finite potential difference across one quantum dot on the conductance through the other completely grounded QD in a Cooper pair splitter fabricated on an InAs nanowire. We demonstrate that the nonlocal electrical transport through the device can be tuned by electrical means and that the energy dependence of the effective density of states in the QDs is relevant for the rates of Cooper pair splitting (CPS) and elastic cotunneling. Such experimental tools are necessary to understand and develop CPS-based sources of entangled electrons in solid-state devices.

Giant fluctuations and gate control of the g-factor in InAs nanowire quantum dots

We study the g-factor of discrete electron states in InAs nanowire based quantum dots. The g values are determined from the magnetic field splitting of the zero bias anomaly due to the spin 1/2 Kondo effect. Unlike to previous studies based on 2DEG quantum dots, the g-factors of neighboring electron states show a surprisingly large fluctuation: g can scatter between 2 and 18. Furthermore electric gate tunability of the g-factor is demonstrated.

Ferromagnetic Proximity Effect in a Ferromagnet–Quantum-Dot–Superconductor Device

L. Hofstetter, S. Csonka, A. Geresdi, M. Aagesen, J. Nyg̊ard and C. Schönenberger 1 Department of Physics, University of Basel, Klingelbergstr. 82, CH-4056 Basel, Switzerland 2 Department of Physics, Budapest University of Technology and Economics, Budafoki u. 6, 1111 Budapest, Hungary and 3 Niels Bohr Institute, Univ. of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark ∗ These authors contributed equally to this work (Dated: November 3, 2010)

Local electrical tuning of the nonlocal signals in a Cooper pair splitter

A Cooper pair splitter consists of a central superconducting contact, S, from which electrons are injected into two parallel, spatially separated quantum dots (QDs). This geometry and electron interactions can lead to correlated electrical currents due to the spatial separation of spin-singlet Cooper pairs from S. We present experiments on such a device with a series of bottom gates, which allows for spatially resolved tuning of the tunnel couplings between the QDs and the electrical contacts and between the QDs. Our main findings are gate-induced transitions between positive conductance correlation in the QDs due to Cooper pair splitting and negative correlations due to QD dynamics. Using a semi-classical rate equation model we show that the experimental findings are consistent with in-situ electrical tuning of the local and nonlocal quantum transport processes. In particular, we illustrate how the competition between Cooper pair splitting and local processes can be optimized in such hybrid nanostructures.

Electrical tuning of Rashba spin-orbit interaction in multigated InAs nanowires

Indium arsenide nanowires (NWs) are a promising platform to fabricate quantum electronic devices, among other advantages they have strong spin-orbit interaction (SOI). The controlled tuning of the SOI is desired in spin-based quantum devices. In this study we investigate the possibility of tuning the SOI by electrostatic fields generated by a back gate and two side gates placed on the opposite sides of the NW. The strength of the SOI is analyzed by weak anti-localization effect. We demonstrate that the strength of the SOI can be strongly tuned up to a factor of 2 with the electric field across the NW, while the average electron density is kept constant. Furthermore, a simple electrostatic model is introduced to calculate the expected change of the SOI. Good agreement is found between the experimental results and the estimated Rashba-type SOI generated by the gate-induced electric field.

Pulling platinum atomic chains by carbon monoxide molecules

The interaction of carbon monoxide molecules with atomic-scale platinum nanojunctions is investigated by low temperature mechanically controllable break junction experiments. Combining plateau length analysis, two-dimensional conductance-displacement histograms and conditional correlation analysis a comprehensive microscopic picture is proposed about the formation and evolution of Pt-CO-Pt single-molecule configurations. Our analysis implies that before pure Pt monoatomic chains are formed a CO molecule infiltrates the junction, first in a configuration that is perpendicular to the contact axis. This molecular junction is strong enough to pull a monoatomic platinum chain with the molecule being incorporated in the chain. Along the chain formation the molecule can either stay in the perpendicular configuration, or rotate to a parallel configuration. The evolution of the single-molecule configurations along the junction displacement shows quantitative agreement with theoretical predictions, justifying the interpretation in terms of perpendicular and parallel molecular alignment. Our analysis demonstrates that the combination of two-dimensional conductance-displacement histograms with conditional correlation analysis is a useful tool to analyze separately fundamentally different types of junction trajectories in single molecule break junction experiments.

Emergence of bound states in ballistic magnetotransport of graphene antidots

An experimental method for detection of bound states around an antidot formed by a hole in a graphene sheet is proposed via measuring the ballistic two-terminal conductance. In particular, we consider the effect of bound states formed by magnetic field on the two terminal conductance and show that one can observe Breit-Wigner like resonances in the conductance as a function of the Fermi level close to the energies of the bound states. In addition, we develop a new numeri- cal method utilizing a reduced computational effort compared to the existing numerical recursive Green’s function methods.

Kondo effect and spin-active scattering in ferromagnet-superconductor junctions

We study the interplay of superconducting and ferromagnetic correlations on charge transport in different geometries with a focus on both a quantum point contact as well as a quantum dot in the even and the odd state with and without spin-active scattering at the interface. In order to obtain a complete picture of the charge transport we calculate the full counting statistics in all cases and compare the results with experimental data. We show that spin-active scattering is an essential ingredient in the description of quantum point contacts. This holds also for quantum dots in an even charge state, whereas it is strongly suppressed in a typical Kondo situation. We explain this feature by the strong asymmetry of the hybridizations with the quantum dot and show how Kondo peak splitting in a magnetic field can be used for spin filtering. For the quantum dot in the even state, spin-active scattering allows for an explanation of the experimentally observed mini-gap feature.

Comparison of gate geometries for tunable, local barriers in InAs nanowires

We report measurements and analysis of gate-induced electrostatic barriers for electron transport in InAs nanowires. Three types of local gates are analyzed; narrow gates (50−100 nm) located on top of or below the nanowire, and wide gates overlapping the interfaces between nanowire and source and drain electrodes. We find that applying negative potentials to the local gate electrodes induces tunable barriers of up to 0.25 eV and that transport through the wire can be blocked at neutral and slightly positive potentials on the nanowire-contact gates, indicating that built-in barriers can exist at the nanowire-contact interface. The contact gates can be biased to remove the unwanted interface barriers occasionally formed during processing. From the temperature dependence of the conductance, the barrier height is extracted and mapped as a function of gate voltage. Top and bottom gates are similar to each other in terms of electrostatic couplings (lever arms ∼0.1−0.2 eV/ V) and threshold voltages for barrier i...