Information and communication technologies will revolutionize the society by enhancing the information density of memories and speed of data transfer of hand-held equipment by two orders of magnitude. The next-generation data storage and logic circuits require smaller and faster devices that consume less power, operate with higher reliability and provide new functionalities like non-volatile updatability. Nanotechnology promises to fulfill these demands by designing sensors and devices based on novel physical phenomena. Our project aims to increase our fundamental understanding of transport in nano-scale devices.

Technological needs and scientific curiosity are going to drive the miniaturization of magnetic structures into the mesoscopic and nanoscopic regimes in which the basic physics has still to be explored. Theoretical understanding has progressed to the stage where materials-specific predictions can be made. More recently, the study of the dynamics of the magnetization vectors in the presence of charge and spin currents is increasing.

We have contributed to the fundamental understanding of spin transport and magnetization dynamics in nano-scale normal metals, ferromagnets, and semiconductors. In particular, we have considered how an electric current can initiate a motion of the magnetization direction in nano-scale ferromagnet. An improved understanding of this phenomenon is important for all use of nano-scale ferromagnets, for example in future magnetic RAM.

One of the surprising discoveries we have made is that the critical current for when the domain wall starts its motion is much less in III-V ferromagnetic semiconductors than in the conventional ferromagnets Co, Ni, and Fe. This could be of technological importance.

We have also contributed to the understanding of the interplay between spin-transport and Coulomb blockade effects, spin Hall effects, and entanglement in solid state systems. The spin Hall effect is the spin analogue of the Hall effect. The Hall effect denotes the phenomenom that when a system is biased by a longitudinal electric field, a magnetic field can set up a transverse potential. Similarly, the spin-orbit interaction can produce a transverse spin accumulation in response to a longitudinal electric current flow. We have made impact in the field by considering the spin Hall effect in two-dimensional electron gases that can be realized in semiconductor hetero-structures and in other novel materials.

We have also investigated, in detail, conductance spectra and tunnelling characteristics of both spin-and charge flow in hybrid structures of superconductors, metals, and ferromagnets. It has been shown in a number of different settings that dissipation-free charge, as well as spin current, can be manipulated by a magnetic field, unlike the situation in a conventional superconductor/metal structure.

We have investigated the details of induced spin triplet pairing in a conventional superconductor by the proximity to a ferromagnet, and how this induced spin-triplet pairing affects conductance spectra. This is very useful in determining the symmetry of the superconducting gaps in magnetic superconductors and in superconducting/magnetic systems. The symmetry of the superconducting gap is one of the basic issues to understanding novel superconductivity in a material. 

We have set up a general mean-field theory for superconductors exhibiting multiple other broken symmetries, such as ferromagnetism and lack of inversion symmetry. The mean field theory is complicated, but we have succeeded in deriving mean-field stationary self-consistent coupled equations for all the order parameters of the problem, and solved them in special cases. This includes a detailed study of thermodynamic quantities of a ferromagnetic superconductor that has hitherto not been carried out before.

We have further considered tunneling between a spin-singlet superconductor with d-wave symmetry, and graphene. Graphene and high-temperature superconductors share a superficially similar peculiar feature of their electronic structure, namely gapless fermionic excitations only at specific points in the Brillouin-zone. In high-temperature superconductors, this is a feature of their d-wave superconductivity and persists when the system is doped, while in graphene it is a consequence of the band-structure and is special to the half filled undoped case. We have found novel oscillations at sub-gap energies in the tunnelling spectra of hybrid structures of this system. These novel oscillations are due to the possibility of having undamped “relativistic” electrons inside a tunnel barrier, unlike non-relativistic particles. This possibility relates directly to the linear-in-momentum dispersion relation of these relativistic electrons.

The research results have been published in a number of papers, and some are in press or in preparation. It is expected that there will be a flow of papers stemming from the research at CAS in our group during 2006/2007 at least for the next year or two.