Subhro Bhattacharjee

Tunneling conductance of graphene NIS junctions.

We show that, in contrast with conventional normal metal-insulator-superconductor (NIS) junctions, the tunneling conductance of a NIS junction in graphene is an oscillatory function of the effective barrier strength of the insulating region, in the limit of a thin barrier. The amplitude of these oscillations is maximum for aligned Fermi surfaces of the normal and superconducting regions and vanishes for a large Fermi surface mismatch. The zero-bias tunneling conductance, in sharp contrast to its counterpart in conventional NIS junctions, becomes maximum for a finite barrier strength. We also suggest experiments to test these predictions.

Theory of tunneling conductance of graphene normal metal-insulator-superconductor junctions

We calculate the tunneling conductance of a graphene normal metal-insulator-superconductor (NIS) junction with a barrier of thickness d and with an arbitrary voltage V0 applied across the barrier region. We demonstrate that the tunneling conductance of such a NIS junction is an oscillatory function of both d and V0. We also show that the periodicity and amplitude of such oscillations deviate from their universal values in the thin barrier limit as obtained in earlier work [Phys. Rev. Lett. 97, 217001 (2006)] and become a function of the applied voltage V0. Our results reproduces the earlier results on tunneling conductance of such junctions in the thin [Phys. Rev. Lett. 97, 217001 (2006)] and zero [Phys. Rev. Lett. 97, 067007 (2006)] barrier limits as special limiting cases. We discuss experimental relevance of our results.

Quantum Phase Transition in Heisenberg-Kitaev Model

We explore the nature of the quantum phase transition between a magnetically ordered state with collinear spin pattern and a gapless

Spin?orbital locking, emergent pseudo-spin and magnetic order in honeycomb lattice iridates

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Ab initio holography

spin liquid in the Heisenberg-Kitaev model. We construct a slave particle mean field theory for the Heisenberg-Kitaev model in terms of complex fermionic spinons. It is shown that this theory, formulated in the appropriate basis, is capable of describing the Kitaev spin liquid as well as the transition between the gapless

Quantum spin liquids in the absence of spin-rotation symmetry: Application to herbertsmithite

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Z 2 topological liquid of hard-core bosons on a kagome lattice at 1 / 3 filling

spin liquid and the so-called stripy antiferromagnet. In particular, within a mean field theory, we have a discontinuous transition from the

Magnetic excitation spectra in pyrochlore iridates

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Spin-orbital liquids in non-Kramers magnets on the kagome lattice

spin liquid to the stripy antiferromagnet. We argue, however, that subtle spinon confinement effects, associated with the instability of gapped U(1) spin liquid in two spatial dimensions, are playing an important role at the transition. The possibility of an exotic continuous transition is briefly addressed.

Fractionalized topological insulators from frustrated spin models in three dimensions

The nature of the effective spin Hamiltonian and magnetic order in the honeycomb iridates is explored by considering a trigonal crystal field effect and spin?orbit (SO) coupling. Starting from a Hubbard model, an effective spin Hamiltonian is derived in terms of an emergent pseudo-spin-1/2 moment in the limit of large trigonal distortions and SO coupling. The present pseudo-spins arise from a spin?orbital locking and are different from the jeff?=?1/2 moments that are obtained when the SO coupling dominates and trigonal distortions are neglected. The resulting spin Hamiltonian is anisotropic and frustrated by further neighbour interactions. Mean-field theory suggests a ground state with four-sublattice zigzag magnetic order in a parameter regime that can be relevant to the honeycomb iridate compound Na2IrO3, where a similar magnetic ground state has recently been observed. Various properties of the phase, the spin-wave spectrum and experimental consequences are discussed. The present approach contrasts with the recent proposals to understand iridate compounds starting from the strong SO coupling limit and neglecting non-cubic lattice distortions.