B10.

Project title:Supercurrent Control in Josephson Junctions

Recently there is a strong research interest in Josephson junctions (JJ) with the simple insulating tunneling barrier between two superconductors (SC) replaced by various nano-structures or materials. Ferromagnetic (FM) materials are considered to exploit the exchange spin splitting effects. Nanostructures such as nanowires, carbon nanotubes, Aharonov-Bohm rings and quantum dots are connected to superconducting leads to investigate how the supercurrent in the junction can be modulated by the nanostructures. One of the motivations for these studies is the realization of superconducting transistors with low power requirement, which control the supercurrent using a gate voltage or some other parameters. Another motivation is the realization of π junctions for applications in superconducting electronics or quantum computing. A more fundamental motivation for these studies is to understand the interplay of ferromagnetism and superconductivity and its relation with triplet superconductivity.

 

Despite the wide interest in these new JJs and the strong interest in semiconductor spintronics, JJs with spin-orbit interaction (SOI) in the coupling layer as a controlling parameter do not receive comparable attention. One explanation is: when some idealized structures such as one-dimensional structures are considered, the supercurrent is insensitive to the SOI strength.  Another possible reason is the intuitive expectation that SOI does not have strong effect on the supercurrent as it does not break the time-reversal symmetry as magnetic field does. However in some recent papers, Dell’Anna et al [Phys Rev B 75 085305 2007] and Dimitrova et al [J Exp Theor Phys 102 652 2006] pointed out that in the presence of a supercurrent, time reversal symmetry is effectively broken. Dell’Anna et al [Phys Rev B 75 085305 2007] show that the supercurrent in a JJ can be modulated significantly by the SOI strength of a multi-level dot connected to the superconducting leads. Dimitrova et al [J Exp Theor Phys 102 652 2006] show that SOI does affect the Andreev levels and their supercurrent. These results show that it is possible to control the supercurrent in a JJ with SOI with appropriate nanostructures. Apart from the fundamental interest, this topic is of immense practical importance, because SOI control of the supercurrent in a JJ has the following advantages over the existing methods of JJ current control:

 

(1) Full electrical control of the supercurrent is made possible by replacing the external magnetic field with the pseudo-magnetic field in SOI systems. The advantages of electrical control are: more compact in size, faster modulation.

(2) Replacing FM layers by SOI layers allow highly flexible control of supercurrent, which is not possible with FM layers. For example, the FM control is achieved by changing the thickness of the FM layer, which is very inconvenient for device applications.

(3) The exploitation of spin degree of freedom in supercurrent control can enable the realization of low dissipation transistors and quantum interference superconducting devices.

(4) The use of SOI in these devices allows possible integration with other spintronic devices.

 

Apart from these reasons, the study of SOI/SC hybrid structures allows us to study the triplet superconductivity from another perspective provided by the SOI systems. Therefore results obtained in this project will have significant impact in the field of superconducting devices and spintronics.

 

References:

F Dolcini and F Giazotto  Phys Rev B 75 140511 2007

J A van Dam, Y V Nazarov, E P A M Bakkers, S D Franceschi and L P Kouwenhoven Nature 442 667 2006

P Jarillo-Herrero, J A van Dam and L P Kouwenhoven  Nature 439 953 2006.

Y Doh, J A van Dam, A L Roest, E P A M Bakkers, L P Kouwenhoven, S D Franceschi   Science 309 272 2005.

M Choi, M Lee, K Kang and W Belzig Phys Rev B 70 20502 2004.

E Vecino, A Martin-Rodero and A L Yeyati  Phys Rev B 68 35105 2003.

F S Bergeret, A F Volkov, and K B Efetov  Phys Rev Lett 86 3140 2001.

 

 

Supervisor: Prof K S Chan (apkschan@cityu.edu.hk)

Webpage: http://www.ap.cityu.edu.hk/personal-website/Chan-KS.htm

Nature of Project: Theory and Computer Modelling

Suitable for: M.Phil. or Ph. D

Prerequisite: a Bachelor Degree in Physics with strong theory background; strong interest in superconductivity theory, many-body transport theory and Green’s function techniques (both equilibrium and non-equilibrium).