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Joint research project

Tuning correlated transport and localization in low-dimensional electron systems with spin-orbit interaction

Project leaders
Stefano Roddaro, Alexander Shashkin
Agreement
RUSSIA - RFBR-suspended - Russian Foundation for Basic Research
Call
CNR/RFBR 2015-2017
Department
Physical sciences and technologies of matter
Thematic area
Physical sciences and technologies of matter
Status of the project
New

Research proposal

Spin-orbit (SO) interaction couples the real-space motion of charge carriers to their spin configuration. As such, it can have an important impact on research and applications in semiconductors, in particular when combined with the engineering of the electron quantum states in low-dimensional systems. As far as applications are concerned, SO interaction opens the way to the electrical control of the spin of carriers and is thus relevant to spintronics [1]. In addition, SO impact on localization could provide new ways to control electrical conductivity that go beyond conventional field-effect transistors, whose performance is bound to the sub-threshold slope limits [2]. From the pure science point of view, SO interaction is a remarkable example of relativistic effects in the solid state and is expected to have an important impact on the fundamental properties of a quantum-confined electron system, including metal-insulator transition in 2D [3].
The project will focus on the impact of SO interaction on correlated transport phenomena. In particular, we will investigate the interplay between SO interaction, Coulomb correlations and localization in two and one spatial dimensions [3]. The behavior of single-particle wave functions in a random static potential is governed by a genuine quantum interference effect (conventional Anderson's localization scenario) and is very sensitive to extra interactions mediated by SO or Coulomb forces. Therefore, understanding the physics of localization is one of the key ingredients for control of quantum states in novel materials. A set of promising strong-SO material systems will be investigated:

In(x)Ga(1-x)As high-mobility two-dimensional heterostructures;
Few-layer graphene, in its pristine and weakly hydrogenated form;
Self-assembled InAs and InSb nanowires.

All these electron systems allow a field-effect control of the carrier density and mobility and can display a strong Rashba-type SO coupling leading to a very large g-factor. These properties make them ideal for the investigation of the impact of SO interaction on correlated transport phenomena in low dimensions.
In a weakly disordered 2D case, interference can lead to weak localization (WL) or weak anti-localization (WAL) corrections, depending on the strength of the SO interaction. Extrapolating to stronger disorder one expects a transition from a metallic to an insulating phase to emerge as a function of SO strength. This simplified hypothetical picture is further modified by Coulomb correlations, dominating at small carrier densities [3]. We choose InGaAs-based quantum wells and hydrogenated graphene [4,5] as two representative 2D materials with strong SO interactions. Higher mobility of the former system as well as a linear energy spectrum together with a tunability of the SO coupling of the latter will allow to investigate the corresponding phase diagram in detail. Transport and thermodynamic studies will be complemented by local probing of conductance and density of states by means of SGM/STM with the emphasis on their mesoscopic fluctuations [6].
In contrast to higher dimensions, a quasi-1D electronic system in semiconductor NWs offers a wide range of control on disorder and carrier densities and thus represents an ideal testbed for the investigation of Anderson's localization in presence of the SO interaction. In addition, at low temperatures, a few micron long NWs are perfectly suited for studies of non-equilibrium current fluctuations. Our main goal is to investigate WL/WAL using the statistics of current fluctuations. In particular, we expect deviations of different signs in shot noise Fano factor in respect to a universal value F=1/3 depending on the sign of the interference correction to the conductance. Apart from the ensemble averaged Fano factor value, its mesoscopic fluctuations will also be studied using gate voltage or magnetic field as a parameter to reveal the impact of quantum interference on the shot noise. Beyond few theoretical studies (e.g. De Jong and Beenakker Phys. Rev. B 46, 13400, 1992), this intriguing research program has not been realized experimentally so far.
The success of the research program is warranted by the past and long-standing fruitful collaborations between CNR and ISSP [7-9] as well as by the joint expertise of the involved teams, in particular:

CNR: MBE growth of high-mobility III-V heterostructures; CBE growth of self-assembled nanowires; design, fabrication and testing of low-dimensional nanodevices [10]; low-temperature scanning-probe microscopy; graphene hydrogenation.
ISSP: design, fabrication and testing of low-dimensional nanodevices; low-temperature magneto-transport [11]; low-temperature current noise measurement.

The project will be the occasion to strengthen existing synergies between the teams and will provide new competitive edges to the scientific activities in both CNR and ISSP. The research plan will also be a very good chance for the professional training of young researchers, postdocs and students involved in the activities.
References:

J.Balakrishnan et al., Nature Phys. 8, 284 (2013).
D.M.Newns et al., APL 73, 780 (1998).
V.F. Gantmakher and V.T. Dolgopolov, Physics-Uspekhi 51, 1 (2008).
S.Goler, et al. , Jour. Phys. C 117, 11506 (2013).
A.Iagallo et al., PRB 88, 235406 (2013).
I.S.Brumistrov et al., PRB 89, 035430 (2014).
A.Kononov et al. PRB 89, 075312 (2014).
M. G. Prokudina et al., PRL 112, 216402 (2014).
A. Mokashi et al. PRL 109, 096405 (2012).
F.Rossella et al., in press on Nature Nanotech (2014).
A. A. Shashkin et al., PRL 112, 186402 (2014).

Research goals


Investigation of the metal-insulator transition (MIT) phenomenology in high-mobility and strong-SO InGaAs two-dimensional systems. A particular attention will be devoted to the control of the transition by changing electron concentration and the magnetic field. Samples will be grown at CNR and used by ISSP to fabricate test devices and to investigate SO effects in this material system.
Demonstration of strong and controllable SO effects in graphene by hydrogenation. Samples will be hydrogenated at CNR, who will also be responsible for the key material characterization by scanning-probe microscopy and for the fabrication of the devices. The investigation of SO effect will be carried on by both CNR and ISSP.
Investigation of localization effects in semiconductor NWs. Particular attention will be devoted to the impact of SO interaction and carrier density. Samples will be grown at CNR, who will also take care of the sample nanofabrication. Shot noise measurements will be performed at ISSP.

In addition, depending on the success of research activities we foresee few exploratory targets. In particular we plan to investigate thermopower in strong SO materials in the metal-insulator transition in 2D, thanks to the highly homogeneous 2D system which we plan to obtain from previous steps of the project. In the 1D case we will implement novel local temperature measurement techniques and investigate the thermoelectric performance of strongly interacting NWs.

Last update: 07/07/2025