BME

 

Quantum transport Laboratory

 

Budapest University of Technology and Economics

Faculty of Natural Sciences

Institute of Physics

Quantum transport

Experimental method and facilities

Fabrication of nanocircuits

 

For the creation of pre-designed nanostructures the electron-beam lithography (EBL) is the most effective method. We are operating an electron-beam lithography system (a) in collaboration with MFA (Research Institute for Technical Physics and Matherial Science), which is based on a Jeol 848 scanning electron microscope. The principle of EBL (b): a thin polymer layer is scanned by the electron beam. The irradiated regions can be removed by chemical processing, forming a mask for evaporation. Metal is evaporated on the mask, and the remainder of the PMMA is removed. With these steps complex metallic nanostructures can be produced down to range of 40-100 nm. We use the EBL system to fabricated electric circuits from different nanoscaled objects (e.g. semiconducting nanowires, graphene, carbon nanotubes). These objects are contacted with different type of electrodes (normal, superconducting, ferromagnetic), local gates are fabricated in their close vicinity, with the aim to define hybrid nanodevices demonstrating novel quantum behaviors.

 

a) E-beam lithography system based on a Jeol 848 Scanning Electron Microscope (SEM) b) Principle of electron beam lithography c)Design of a hybrid nanocircuit and SEM picture of its realization e) Graphene flake contacted in a Hall-geometry, f) Typical resolution of our E-beam lithography system: ~40nm holes in a 400nm thick PMMA, g) Fabricated nanocircuits are bounded in chip carriers and placed into sample holders in order to study their transport behavior at low temperature.

 

 

Low Temperature measurement facilities

 

In order to preserve quantum coherence the thermal fluctuation has to be suppressed, which requires low temperature environment. Our laboratory is equipped with several cryogenic equipments to allow transport measurements in the temperature range of 300K-18mK. Three liquid helium cryostats, various insets (VTI, He3 sorption pump, He3-He4 dilution refrigerator), several sample holders, two superconducting magnets are available, the required liquid helium is produced by an own liquefier.

 

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a) One of the helium cryostats of our laboratory. They are equipped with 3 insets for various purposes: b)(from left to right) Standard Variable temperature inset which allows measurements in the 1.5K-350K range, top loading 3He sorption pumped fridge (Tmin = 300 mK), and 3He-4He dilution fridge, which reaches Tmin = 18 mK. c) Two of our He-cryostat contains superconducting magnets of maximum field B = 14 T and B=8T.

The helium liquefier operated by us covers the large helium consumption of the low-temperature experiments.

We also regularly supply helium for ESR and NMR spectrometers in 4 other laboratories at the Budapest University of Technology and Ecomnomics and at the Eötvös Loránt University.

 

 

 

Some recent results

·         Electrons forming a Cooper pair are entangled in a spin-singlet state. Thus, a superconductor connected to two normal conductors could serve as a natural source of spatially separated entangled electrons. However, 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 be realized by incorporating quantum dots between the superconductor and the two normal metal drains. We have demonstrated the first experimental realization of such a Cooper pair splitter device based on InAs nanowires [1].

a) Concept of a Cooper pair splitter device based two quantum dots coupled to a superconducting electrode (S). b) Experimental realization, where the quantum dots are defined in an InAs nanowire

 

 

·        We studied the ferromagnetic proximity effect in InAs nanowire based quantum dots coupled to a ferromagnetic (F) and a superconducting (S) lead. The influence of the F lead is detected through the splitting of the spin-1/2 Kondo resonance. We show that the F lead induces a local exchange field on the quantum dot, which has varying amplitude and sign depending on the charge states. The interplay of the F and S correlations generates an exchange field related subgap feature [2].

a) SEM image of an InAs nanowire contacted by a ferromagnetic (F) and a superconducting (S) electrode. b) This structure realizes a quantum dot coupled to two electron reservoirs with competing spin correlations (S & F).

 

·        We studied 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 ½ -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 we have demonstrated electric gate tunability of the g-factor as well, which could provide a fast and selective manipulation of quantum dot based spin qubit [3].

Conductance of an InAs nanowire based quantum dot as a function of two gate electrodes. The numbers show the g-factor values determined by Kondo splitting. The g-factor shows a strong fluctuation for neighboring charge states and it can be even tuned for the same charge state by gate electrodes (see dash line).

 

Recent publications

[1] L. Hofstetter, A. Geresdi, M. Aagesen, J. Nygård, C. Schönenberger, and S. Csonka, Ferromagnetic Proximity Effect in a Ferromagnet–Quantum-Dot–Superconductor Device, Phys. Rev. Lett. 104, 246804 (2010)

[2] L. Hofstetter, S. Csonka, J. Nygard, C. Schönenberger, Cooper pair splitter realized in a two-quantum-dot Y-junctions Nanowire Quantum Dots, Nature, 461, 960 (2009).

[3] S. Csonka, L. Hofstetter, F. Freitag, S. Oberholzer, T. S. Jespersen, M. Aagesen, J. Nygard, C. Schonenberger, Giant fluctuations and gate control of the g-factor in InAs Nanowire Quantum Dots, Nano Letters, 8, 3932 (2008).

 

Collaboration

- group of L. P. Biro and group of J. Volk,  MFA, Research Institute for Technical Physics and Matherial Science, Hungarian Academy of Science, Budapest, HU

- Nanoelectronics group of C. Schönenberger, University of Basel, CH

- group of J. Nygard, Niels Bohr Institute, University of Copenhagen, DK

- Quantronics group, SPEC, CEA-Saclay, FR