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Peter Grünberg Institute

# Ballistic Transport

In nanowires composed of very clean materials, ballistic transport can be observed.

In contrast to diffusive transport, the carriers in the nanowire are not scattered while propagating through the material. An image of the corresponding sample is shown below.

Scanning electron beam micrograph of a semiconductor nanowire with top gate electrodes

By a applying a voltage to one of the gate fingers a constriction is defined in the nanowire. The electrons are propagating ballistically through this constriction. The effective width can be adjusted by the gate voltage. The smaller the gate voltage is the smaller the width of the constriction is and the lower the conductance is. In case that the transport is effectively one-dimensional and the states are quantized, characteristic steps in the conductance are observed (see figure below).

InAs nanowire: quantized conductance

Measurements of quantized conductance can be utilized to obtain fundamental parameters, i.e. the energy splitting for spin up and down electrons in a magnetic field. Furthermore, in InAs quantum point contacts the appearence of a helical gap was observed, which is relevant for research in the field of topological quantum computation [3].

### References

[1] Heedt, S.; Prost, W.; Schubert, J.; Grützmacher, D. and Schäpers, T., Ballistic Transport and Exchange Interaction in InAs Nanowire Quantum Point Contacts, Nano Letters, 16, 3116-3123 (2016) (http://dx.doi.org/10.1021/acs.nanolett.6b00414).

[2] Heedt, S.; Manolescu, A.; Nemnes, G. A.; Prost, W.; Schubert, J.; Grützmacher, D. and Schäpers, T., Adiabatic Edge Channel Transport in a Nanowire Quantum Point Contact Register, Nano Letters, 16, 4569-4575 (2016) (http://dx.doi.org/10.1021/acs.nanolett.6b01840).

[3] Heedt, S.; Ziani, N. T.; Crepin, F.; Prost, W.; Trellenkamp, S.; Schubert, J.; Grützmacher, D.; Trauzettel, and Schäpers, T., Signatures of interaction-induced helical gaps in nanowire quantum point contacts Nature Physics, (2017) (http://dx.doi.org/10.1038/nphys4070)