Robust semi-Dirac points and unconventional topological phase transitions in doped superconducting Sr2IrO4 tunnel coupled to t2g electron systems

Mats Horsdal,Timo Hyart

doi:10.21468/scipostphys.3.6.041

Mats Horsdal, Timo Hyart

Open Access

https://doi.org/10.21468/scipostphys.3.6.041

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Journal: SciPost Physics	Publication Date: Dec 22, 2017
Citations: 4	License type: CC BY 4.0

Affiliation: University of Oslo, University of Jyväskylä

Abstract

Semi-Dirac fermions are known to exist at the critical points of topological phase transitions requiring fine-tuning of the parameters. We show that robust semi-Dirac points can appear in a heterostructure consisting of superconducting Sr2IrO4 and a t2g electron system (t2g-ES) without fine-tuning. They are topologically stable in the presence of the symmetries of the model, metallic t2g-ES and a single active band in Sr2IrO4. If the t2g metal is coupled to two different layers of Sr2IrO4 (effectively a multiband superconductor) in a three-layer-structure the semi-Dirac points can split into two stable Dirac points with opposite chiralities. A similar transition can be achieved if the t2g-ES supports intrinsic triplet superconductivity. By considering Sr2RuO4 as an example of a t2g-ES we predict a rich topological phase diagram as a function of various parameters.

Highlights

The idea to utilize strong intraionic spin-orbit coupling in a middle layer to convert singlet Cooper pairs into triplet ones in a three layer heterostructure was proposed [17], and the growth technology for the realization of such kind of heterostructures is under development [18]. The advantage of this idea is that the conversion from singlet to triplet Cooper pairs can take place in a single atomic layer, so that the induced superconducting order parameter is determined by microscopic energy scales given by tunneling amplitudes between the layers and the superconducting gap in the singlet superconductor. In this manuscript we explore the possibility and consequences of triplet pairing correlations in a heterostructure where doped superconducting Sr2IrO4 with strong intraionic spin orbit coupling is tunnel coupled to a t2g electron system (t2g -ES)
We show that the tunneling of the pseudospin singlet Cooper pairs from Sr2IrO4 into the t2g -ES naturally leads to an apperance of both triplet and singlet Cooper pairs in the t2g -ES, so that the triplet and singlet components of the induced order parameter are of similar magnitude
We find that the semi-Dirac points in the superconductor-metal heterostructures are topologically stable in the presence of the symmetries of the model, metallic t2g -ES and a single active band in the superconductor

Summary

Introduction

Native triplet superconductivity is typically fragile and appears only at very low temperatures [1,2,3,4]. The use of the bulk tight-binding parameters for Sr2RuO4 is a simplification because the Fermi surfaces have a weak dependence on the out-of-plane momentum [84] and in thin layers the γ band can be closer to a Lifshitz transition than in the bulk system [83] These effects and the number of atomic layers in the t2g -ES are not important for our qualitative conclusions, because the semi-Dirac points are robust as long as the symmetries are present and the tunneling matrices are topologically nontrivial as discussed in the previous section. This tunneling Hamiltonian (51) is consistent with the symmetries of the system.

Topological properties of the superconducting Sr2IrO4

Topological phase diagram for the three layer heterostructure

Summary and discussion