Superconductivity and topological states in the IV-VI system: (Sn,In)Te and (Pb,Sn)Se.
By introducing In into SnTe, we succeeded in making a superconducting SnInTe by molecular beam epitaxy and revealed the presence of superconducting fluctuations in the normal state, manifesting in quantum coherent corrections of the conductivity. This material – SnInTe – is a candidate topological superconductor that is free of radioactive and highly toxic elements and can be grown in wafer scale. It is thus potentially interesting for the search quasiparticles for topological quantum computing. Nanolett. 22 792 (2022)
Here‘s a broad audience summary of this work.
(Pb,Sn)Se and (Pb,Sn)Te also host gapless topological surface state protected by crystalline and time-reversal symmetry. The gapping of these states upon breaking of these symmetries results in new quantized anomalous Hall effects. However, it is unclear how large the proximity induced gap is. In a recent work, we have taken up the challenge of measuring this energy gap. arxiv2207.07685
The exceptionally high mobility that we have achieved ( Phys. Rev B 102 155307 (2020)) in the IV-VI system has allowed us to determine the g-factor of their surface charge carriers, through magnetooptical experiments. (submitted to Phys. Rev. B).
Supported by NSF-DMR-1905277
Magnetism in the Bi2Te3 system: MnBi2Te4
The Bi2Te3-Sb2Te3 system hosts the quantum anomalous Hall effect. Introducing Mn in Bi2Te3 can yields self-assembled MnBi2Te4. This superlattice is predicted to host a quantum anomalous Hall effect at elevated temperatures and a very rich magnetic phase diagram. We are currently working on the MBE synthesis of such superlattices to study their potential to achieve new anomalous Hall effects. Our most recent finding has been published here: npj Quantum Mater. 7 46 (2022)
Supported by NSF-DMR-1905277 and Notre Dame
Semiconducting Magnets: MnTe and EuIn2As2
While magnetic semiconductors have been extensively studied in the past two decades, the properties semiconducting magnets – materials that achieve magnetic order and have a semiconducting band structure – have remained elusive. We are currently investigating the origin of an unexpected anomalous Hall effect in hexagonal MnTe, thought to be due to the unusual symmetry of this material, by carefully tuning its charge density. MnTe has been predicted to be an “altermagnet” as it was argued toan exchange splitting of alternating sign at momentum points related by symmetry. Its optical and Hall properties remain poorly understood under this perspective.
Other semiconducting magnets of interest to us, and that we are currently developing include EuIn2As2 thin films. EuIn2As2 is a narrow gap semiconductor with an antiferromagnetic ground state predicted to host an inverted band structure.
Magnetic III-V materials
The magnetism in III-Mn-V materials is mediated by mobile charge carriers that result in a finite exchange interaction between Mn atoms. The origin of anomalous Hall effect in these materials is however still debated. We are examining this specific question in the context of recent theories linking strong anomalous Hall effects to the Berry curvature of the band structure. The ability to engineer band crossings in III-Vs using broken gap heterostructures (InAs/GaSb) and strain (GaMnAsP) opens up a new horizon in our understanding of how magnetic and band effects couple to yield novel spintronic phenomena. Phys. Rev. B. 105 125301 (2022)
Supported by NSF-DMR-1905277
MBE growth of novel thin films and heterostructures
We are also working on the MBE synthesis of new materials that host unexpected magnetic, topological and superconducting behavior or have functional properties relevant for data processing and sensing. For example, we have recently synthesized Sr-Bi2Se3 [J. of Applied. Phys. 129 085107 (2021)] by MBE and PbSe on GaAs[J. of Crys. Growth 570 126235 (2021)] and EuTe on MnBi2Te4.
Our method consists in a hybrid approach that combines material synthesis using MBE and CVD with electrical and optical characterization techniques. It allows us to produce, measure and identify materials with specific electronic and optical properties that can be integrated into functional devices. A list of the synthesis and characterization techniques used in our lab is given below:
- Molecular beam epitaxy (with Prof. Xinyu Liu)
- Chemical vapor deposition
- Infrared magnetooptical spectroscopy (on-site, at the NHMFL and at EMFL-Grenoble)
- Strong magnetic fields