Research topics

Electrostatically confined quantum devices on TMDCs

Electrostatically tunable QPCs, a first step towards the realization of electrostatically controlled devices. QPCs are one-dimensional ballistic channels connected to large source and drain regions. The most common and controllable way of fabricating a QPC is by electron confinement using gate voltages to result in quantization.  This will result in having a discrete number of channels each contributing 2e^2/h (spin degenerate) for conduction, as a result, the conductance is quantized in the units of the same. The quantized conductance plateaus can be controlled by the changing the width of the channel using gate-voltages. 

Only a handful of works have been performed on QPCs on TMDCs.  Difficulty in getting Ohmic contacts to the system is a major reason. It is possible to overcome this issue by techniques like contacting with graphene, using low work-function metals or using heavily doped TMDCs.  Presence of high SOC and spin valley effect in TMDC brings further interesting physics and applications for QPCs on them. It would be beneficial for creating spin channels and gain degenerate conductance was recently demonstrated in WSe2. 

Further constriction to zero dimension would lead to formation of quantum dots where electrons are confined to a few to ~100 nm in space. These allows controlling and manipulating individual electrons.  Some TMDCs are also potential hosts for QDs where electron confinement is easier due to the semiconducting nature. It has been proposed that the electron spin or valley degree of freedom alone or spin-valley coupled states can form robust basis states for quantum information processing. MoS2 is the most prominent material in the TMDC class in which we already acquired processing and measurement experience. The Kramer’s pair given by the opposite spins in the K and K’ valleys, |K,up> and |K’,down>  could be used the basis states. The outstanding features of such a qubit in a TMDC-material are the large band gap, large SOC, the tunability of the spin and valley degrees of freedoms, and its intrinsic optical activity and very small hyperfine interaction. In contrast to the conventional spin-qubits, the spin-orbit interaction offers electronic control of these qubits. The optical activity of the valley states also offers a convenient strategy to couple the qubit states to optical cavities for long-distance information transfer and entanglement. We also note that the valley Hall effect and topologically coupled valley currents have been demonstrated in MoS2.

Our work on electrostatic confinement of electrons in MoS2/hBN system was one the first works in this direction. We also work on fabrication and measurements on 2DEG based QPCs. 

One bachelor's thesis supervised on this topic:

BA Nanoscience, UHH: 'Fabrication and quantum transport measurements in nanostructures on MoS2/hBN heterostructures', Jan Stelzner (2022),- co-supervisor 

2D superconductivity

Unlike its 3D counterpart, superconductivity shows some interesting effects in 2D. While there are theories like Mermin-Wagner theorem predicting that long-range order is not possible in dimensions less than or equal to two while superconductivity can still be observed in 2D. They are stabilized by the vortex dynamics in the material syatem that brings out many interesting physics. 

The Berezinskii-Kosterlitz-Thouless (BKT) phase transition and Bose metal phase, driven by vortices and their dynamics are the hallmark features of a clean two-dimensional superconductor. Materials with a minimal structural disorder and high conductivity are essential for the observation of these features. Among the van der Waals materials, transition metal dichalcogenides, such as NbSe2, MoS2 etc. are interesting candidates. Apart from the high degree of electrical tunability, what makes MoS2 fascinating is the presence of 2D polymorphs displaying a wide spectrum of electrical properties; 2H and 3R are large bandgap semiconductors, 1T’ and 1T” are narrow bandgap semiconductors and 1T is metallic. 2D superconductivity is limited to systems having sheet-resistance less than the quantum resistance; the metallic nature and high carrier concentration make 1T-MoS2 a natural choice. 

Previously, we reported the observation of 2D superconductivity, BKT phase transition and the emergence of Bose metallic state in an ~8 nm thick (~12 layers) 1T-MoS2 sample. The electrical characterization reveals a transition temperature Tc ~920 mK. BKT transition and anisotropy in the magneto-transport confirm the dimensionality of the superconductivity. Magneto-transport measurements reveal a parallel critical field manifold of the Pauli limit, possibly due to the presence of high spin orbit coupling in our system, making it a potential candidate for the realization of superconducting circuit elements operational at high magnetic fields. The sample also exhibits a transition to Bose metallic state, a signature of a clean 2D superconducting system. The observation of 2D superconductivity in 1T-MoS2 and the capability to scalably engineer this phase on the semiconducting 2H-MoS2 phase opens up a new route for the realization and study of monolithic hybrid quantum circuits [Communications Physics 1, 90 (2018)].