Research Highlights

One of our research directions is to explore ways of engineering superconducting correlations in proximized systems with the aim to obtain exotic pairings.

As an example we believe spin-triplet pairing can be engineered in single atomic-layers of transition-metal dichalcogenides (TMDCs). These are very special materials with very rich physics ranging from Dirac like spectrum and strong spin-orbit interactions (SOI) to correlated phase like superconductivity (SC) and charge density wave. We have shown that when an external spin splitting field is present, a variety of superconducting order parameters can emerge from the interplay of SOI, magnetism and SC. Interestingly, we find that the entire spectrum of possibilities for the induced pairing amplitudes is covered; to be symmetric or antisymmetric with respect to the valley and spin degrees of freedom, as well as even or odd in frequency. Specifically, both spin-singlet and triplet pairings between electrons from the same and opposite valleys arise due to the combined effects of intrinsic spin-orbit coupling and a magnetic-substrate-induced exchange field. As a key finding, we reveal the existence of an exotic even-frequency triplet pairing between equal-spin electrons from different valleys, which arises whenever the spin orientations in the two valleys are noncollinear. All types of superconducting order turn out to be highly tunable via straightforward manipulation of the external exchange field.

In another previous investigation, we had revealed that p-doped semiconductors at the proximity of a conventional superconductor can host a combination of s-wave and d-wave correlations. Moreover since the effective low energy properties of this system is usually described by spin-3/2 quasiparticles and the so-called Luttinger Hamiltonian, the superconducting pairing function is not limited to singlet and triplet one and can have higher spin components of Cooper pairing. These pairings are called as septet or quintet and recently these type of pairing have been observed in in some half-Heusler compounds which turn to be topological semimetals and undergo a superconducting transition.

One of our research directions is to explore various features ranging from the band structures to quantum transport in topological insulating and superconducting phases.

In two dimensions, one of the acclaimed topological insulators is silicene, which is nothing but a single-layer of silicon. Despite its many similarities to graphene presence of spin-orbit interaction leads to topological insulating phase at low energies. Nevertheless the topological gap of silicene is tiny which puts strong limitations on exploring topological aspects of silicene. Using exhaustive first principle calculations based on density functional theory we predicted that presence of haloganted silicon substrate underneath the silicene could leads to larger gap while keeping the low energy effective properties especially the non-trivial topology of the system unaffected.

Moreover, employing non-equilibrium Green’s function techniques transport properties of a zigzag silicene ribbon in the presence of magnetic impurities has been investigated in our group couples of years ago. We find that although time-reversal symmetry is already broken with a negligible amount of magnetic impurities, however when their concentration is low, the edge currents are not significantly affected. In this case, when the exchange field lies in the x–y plane, the spin mixing around magnetic impurity is more profound rather than the case in which the exchange field is directed along the z-axis. In addition, when the exchange field of magnetic impurities is placed in the x–y plane, a spin-polarized conductance is observed which can be promising for spintronic applications of silicene doped with magnetic impurities.

Engineering superconducting states in two-dimensional systems

Phys Rev. B 95, 104515 (2017); Phys Rev. B 89, 184507 (2014); Physica C 548, 123 (2018)

Topological aspects of two-dimensional systems

Phys Rev. B 97, 125301 (2018); Superlattices Microstruct. 100, 214 (2016); J. Phys. D: Appl. Phys. 49, 355001 (2016)

Spintronics and spin transport with graphene

Phys. Rev. Lett. 105, 146803 (2010); J. Phys. D: Appl. Phys. 48, 295004 (2014); Phys Rev. B 91, 155407 (2015)

Collaborations:

Alireza Akbari

APCTP & POSTECH, Pohang, SOUTH KOREA

Saeed Abedinpour

IASBS, Zanjan, IRAN

Habib Rostami

Nordita, Stockholm, SWEDEN

Fariborz Parzgar

University of Uppsala, Uppsala, SWEDEN

Michele Governale & Uli Zuelicke

Victoria University of Wellington, NEW ZEALAND

Jürgen König

Duisburg-Essen Universität, GERMANY

Reza Asgari

IPM, Tehran, IRAN

Alireza Qaiumzadeh

NTNU, Trondheim, NORWAY