AVS 69 Session MI+2D+TF-ThM: 2D Magnetism and Superconductivity

Tunneling spectroscopy is an indispensable experimental tool of modern condensed matter physics. Vertical planar tunneling, which uses a fixed-width tunnel barrier, offers advantages over other spectroscopic tools such as scanning tunneling microscopy (STM). One such advantage is the ability to tunnel in reorientable and very large (≥40 T) magnetic fields at dilution refrigerator temperatures (≤30 mK), a capability that has application in, for example, determining the order parameter symmetry of novel two-dimensional (2D) superconductors. We demonstrate a novel vertical planar tunneling architecture for van der Waals heterostructures based on via contacts, namely, metallic contacts embedded into through-holes in hexagonal boron nitride (hBN). This via-based architecture overcomes limitations of other planar tunneling designs and produces high-quality, ultra-clean tunneling structures from a variety of 2D materials. The physical area of our via-based tunnel contacts is limited only by nanofabrication techniques, and we demonstrate a crossover from diffusive to point contacts in the small-contact-area limit by studying the spectrum of a 2D superconductor, NbSe₂. We show that our tunneling technique may enable highly-sought measurements of newly-discovered 2D superconductors such as monolayer 1T'-WTe₂, rhombohedral trilayer graphene, twisted trilayer graphene, and twisted bilayer BSCCO.

Co₃Sn₂S₂ is a magnetic Weyl semimetal below its Curie temperature (T_c) of 177K. I will discuss spatial and temperature-dependent angle-resolved photoemission spectroscopy (ARPES) and x-ray photoelectron spectroscopy (XPS) studies in this system.Across T_c, we observe signatures of a topological phase transition, but also observe changes in bulk bands which are inconsistent with a simple lifting of exchange interactions, suggesting enhanced electronic correlations in the regime without long-range magnetic order.I will also discuss spatial-dependent ARPES and XPS data which quantify the characteristic differences between Sn- and S- terminated surfaces, with relevance for interpreting surface-dominated phenomena.

FeSe, a bulk superconductor with a T_C of 9 K has attracted a high level of attention since a skyrocketing boost in TC was reported for a single unit cell (UC) layer of FeSe grown on SrTiO3(001) by molecular beam epitaxy (MBE) to as high as 100 K. FeSe-SrTiO3 heterostructures have since been fabricated by many groups but the record TC proved difficult to reproduce and thus the mechanism behind it remains concealed. After extensive work in the past, the field appears to agree on certain key “ingredients” in the heterostructure sample preparation that are believed essential for the boost in TC. Those are; 1. an ultra-clean substrate surface of a double TiO₂₂ termination realized by a chemical and thermal ex-situ and/or thermal in-situ substrate preparation; 2. ultra-thin – one UC thickness – limit of FeSe; 3. a high number of Se vacancies in the FeSe film ensured through post-growth annealing steps in ultra-high vacuum (UHV) for several hours; 4. followed by a capping layer growth protecting FeSe against oxidation during ex-situ characterization.

Dynamic processes govern a multitude of phenomena in physical, chemical and material sciences. Time- and spatially resolved element-specific monitoring of such processes is crucial in the understanding of phenomena like magnetization dynamics, battery charging and discharging, and phase transitions of several kinds. Time-Resolved Scanning Transmission X-ray Microscopy (TR-STXM) [1] is a versatile tool fulfilling these demands on the mesoscopic scale, offering element-specific observations with sub 50 nm spatial resolution and picosecond time sampling. By introducing a phased-locked-loop excitation synchronization scheme, TR-STXM also allows to sample dynamics originating from continuous wave excitations. This presentation introduces the TR-STXM technique and its principle of operation, and the setup developed at the Advanced Light Source at Lawrence Berkeley National Laboratory. The presentation is complemented by examples of dynamic magnetic measurements, which allow for local monitoring of magnetization dynamics in fields such as spintronics, magnonics, biomedical and energy related applications. Here we demonstrate TR-STXM results on Py/Co microstructures [2], Py stripe ensembles [3] and magnetite nanoparticle chains inside magnetotactic bacteria Magnetospirillum Magnetotacticum [4,5], showcasing localized uniform and non-uniform resonant magnetic responses, supplemented by micromagnetic simulations in good agreement.

This work is funded by German Research Foundation projects OL513/1-1, 321560838, 405553726 TRR 270, and the Austrian Science Fund project: I 3050-N36. Lawrence Berkeley National Laboratory is acknowledged for funding through LDRD Award: Development of a Continuous Photon Counting Scheme for Time Resolved Studies. T.F. and R.F. acknowledge support from STROBE: A National Science Foundation S&T Center, under Grant No. DMR-1548924. This research used resources of the Advanced Light Source, a U.S. DOE Office of Science User Facility under contract no. DE-AC02- 05CH11231. The use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. We thank HZB for the allocation of synchrotron radiation beamtime.

[1] T. Feggeler, A. Levitan, et al. J. Electron Spectrosc. Relat. Phenom. 2023. 267: 147381.

[2] T. Feggeler, R. Meckenstock, et al. Sci. Rep.2022, 12:18724.

[3] S. Pile, T. Feggeler, et al. Appl. Phys. Lett. 2020, 116(7): 072401.

[4] T. Feggeler, R. Meckenstock, et al. Phys. Rev. Res. 2021, 3(3): 033036.

[5] T. Feggeler, J. Lill, et al. New J. Phys. 2023, 25(4): 043010.