AVS 69 Session 2D-WeA: 2D-Materials: Electronic/Magnetic/Optical Properties

Recently we have discovered a non-reciprocal Josephson diode effect in several Josephson junctions, both lateral and vertical, formed from conventional superconducting electrodes (Nb, NbSe₂) separated by several non-superconducting metals including the 2D van der Waals metals, NiTe₂ [1] and WTe₂[2], as well as sputtered layers of platinum that are magnetically proximitized via a magnetic insulator (YIG) [3]. Each of these materials becoming superconducting by proximity to the conventional superconducting electrodes. The superconductivity can be sustained over long distances of, in some cases, up to ~1 micron. The critical supercurrent densities for current flowing in opposite directions within the junction are distinct and can vary by up to 40% or more. For the van der Waals metals the non-reciprocity is only observed in the presence of a small magnetic field oriented perpendicular to the supercurrent, whereas for the Pt based junctions the diode effect is observed in zero field. For vertical Josephson junctions formed from WTe₂ we show that the non-reciprocity depends on the orientation of the magnetic field with respect to the crystal structure of the WTe₂, proving thereby the intrinsic origin of the Josephson diode effect. We also show how the magnitude of the asymmetry increases with the thickness of the WTe₂ barrier[2]. Finally, we discuss our recent work on spin-triplet supercurrent spin valves using 2D chiral Kagome antiferromagnets [4].

[1]B. Pal et al., "Josephson diode effect from Cooper pair momentum in a topological semimetal," Nat. Phys., vol. 18, pp. 1228-1233, 2022.

[2]J.-K. Kim et al., "Intrinsic supercurrent non-reciprocity coupled to the crystal structure of a van der Waals Josephson barrier," arXiv:2303.13049, 2023.

[3]K.-R. Jeon et al., "Zero-field polarity-reversible Josephson supercurrent diodes enabled by a proximity-magnetized Pt barrier," Nat. Mater., vol. 21, pp. 1008-1013, 2022.

[4]K.-R. Jeon et al., "Chiral antiferromagnetic Josephson junctions as spin-triplet supercurrent spin valves and d.c. SQUIDs," Nat. Nanotechnol., 2023.

Transition metal dichalcogenide bilayers exhibit improved stability and higher carrier mobility compared to their monolayer counterparts, and may be attractive for a variety of applications. Both 2H and 3R bilayers are energetically stable and are expected to exhibit semiconducting behavior. However, 2H has received the bulk of attention due to its ready availability in the form of mechanically exfoliated flakes. Here, we detail the energies and temperature dependent behaviors of the ground and the first excited excitonic states in both 2H and 3R WSe₂ bilayers. Samples are obtained through chemical vapor deposition, encapsulated with hBN, and reflectance contrast (RC) is measured to identify 1s and 2s excitonic states. At cryogenic temperatures, a splitting of approximately 17 meV is experimentally observed in both the 1s and 2s states of 3R bilayers. This splitting is consistent with our DFT calculations and is due to lack of inversion symmetry, with the two peaks corresponding to distinct excitonic transitions in the upper and lower layers of the 3R WSe₂. As temperatures increases, excitonic states broaden and RC intensity decreases, preventing detection of 2s states above 100 K. The 1s state is evident at all temperatures between 4 K and 300 K, and splitting of the 1s state in 3R samples is detectable to approximately 250 K. This work provides much needed insight into bilayer systems and demonstrates that interlayer interactions are strong enough to significantly modify the optical properties in WSe₂ samples.

Angle-resolved photoemission is a premiere technique for measuring the electronic structure of materials, as represented by momentum-resolved electronic bands. Furthermore, with modern high-resolution instrumentation it is possible to access the single-particle spectral function A(k,ω), which gives important information on the renormalization of excited state energies and lifetimes due to many-body interactions and defect scattering.

NanoARPES machines capable of spatially-resolving the ARPES spectrum on the mesoscopic scale are coming online at synchrotrons around the world, with spatial resolution in the range 100-1000nm. Combining these new small probes with arbitrary stacking orders of diverse materials assembled micromechanically, nanoARPES can obtain information on new materials far faster than in the past, when probe sizes limited us to wafer-scale, epitaxially grown heterostructures. Using small x-ray probes, nanoARPES can also study naturally or spontaneously-formed heterogenous materials. Most if not all nanoARPES machines are accessible through proposals to international user facilities, and the community specializing in these experiments is growing rapidly.

Most excitedly, nanoARPES offers the possibility to measure 2D materials and heterostructures at true device scales, and can thus enable for the characterization of materials under in operando conditions. In this talk I will review recent developments and pioneering experiments at the MAESTRO beamline at the Advanced Light Source in Berkeley, CA. These capabilities include the application of external fields (electrical, magnetic, and optical) and current. Prospects for the application of magnetic fields to create a new "MagnetoARPES" technique will be presented.

Two-dimensional (2D) materials provide unprecedented opportunity to engineer their physical properties by modification to the electronic structure utilizing external perturbations- strain, gating, adsorbates, defects, twist-angle, and interface engineering. This is expected to cause changes to the Hamiltonian describing the system and has resulted in exotic phenomena such as superconductivity, bound quasiparticles, topological states as well as magnetic phases, with implications for novel electronics and spin-device applications. In this talk, I will present our work on directly visualizing (without any assumption) the electronic structure of atomically thin systems utilizing in-operando angle-resolved photoemission spectroscopy with nanoscale spatial resolution (nanoARPES) on 2D heterostructures and their fully functional devices [1,2,3,4,5]. I will present the experiments which demonstrate on-demand tuning of the electronic band structure in atomically thin systems, such as transition metal dichalcogenides (TMDCs) and graphene, by varying the twist-angle between the atomic layers and external dopants. Specifically, I will discuss the electric field tuning of the electronic interactions that result in van Hove singularity and flat bands in twisted bilayer graphene and twisted double bilayer graphene heterostructures. In addition, I will show our recent results where we observe the formation of quasiparticle polarons due to many-body interactions in graphene/TMDC heterostructures.

References:

[1] Katoch et. al., Nature Physics 14, 355-359 (2018).

[2] Ulstrup [https://arxiv.org/search/?searchtype=author&query=Ulstrup%2C+S], et. al.,Science Advances, Vol. 6, no. 14, eaay6104, (2020).

[3] Jones, et. al., 2D Mater., 9 015032 (2022).

[4] Muzzio, et. al., Physical review B Rapid Communications 101, 201409(R) (2020).

[5] Jones, et. al., Adv. Mater., 32, 2001656 (2020).

Two-dimensional (2D) materials such as transition metal dichalcogenides are excellent candidates for creating novel nano-electronic and photonic devices. Previous research indicates that the edge states of MoS₂ could strongly influence its conductivity, and the 2D honeycomb structure enables different electronic performance along the zigzag and armchair edges. Understanding and controlling the conductivity is essential in devices like field effect transistors that use MoS₂ as the channel. To date, transport along edge states of MoSe₂ nanoribbons, which have substantially reduced dimensionality relative to 2D crystals, has not been explored. In this project, we used chemical vapor deposition (CVD) to synthesize MoSe₂ nanoribbons through a particle-seeded approach. This approach yields directed growth of monolayer MoSe₂ to form high aspect ratio (>7) nanoribbons. Tip-enhanced photoluminescence (TEPL) is used to probe the optical properties of the edge and surface of the MoSe₂ nanoribbons. To perform electronic transport measurements, we used e-beam lithography to pattern contacts on the nanoribbons in a Hall bar configuration with the side contacts at the edges and tips of the nanoribbons. The influence of edge states on the electrical performance of MoSe₂ nanoribbons was investigated by conductivity and Hall transport measurements. Current flow in the transverse and longitudinal directions of the nanoribbon was compared to analyze the importance of edge states on MoSe₂ nanoribbon conductivity.