Laser-coupled scanning tunneling microscopy and spectroscopy of quantum materials and qubit defects

Capturing the dynamics of quantum materials simultaneously at their intrinsic timescales and lengthscales has remained a long-sought goal in condensed matter physics. Laser-coupled scanning tunneling microscopy has provided a possible means to achieve this goal. However, to date, laser-coupled STM on quantum materials has been carried out with techniques utilizing transient bias voltages requiring significant modeling for interpretation of energy-resolved measurements. In this thesis, I describe the design and construction of a new laser-coupled scanning tunneling microscope which can simultaneously achieve sub-ps time resolution, sub-Å spatial resolution, and sub-meV energy resolution, along with measurements of a wide variety of quantum materials using our new instrument. Ch. 3 describes our measurements of nitrogen-vacancy (NV) centers in diamond, perhaps the most promising solid-state defects for qubit applications. By developing and applying a novel technique of covering the insulating diamond sample with a monolayer of graphene, we were able to map out the atomic-scale wavefunctions of individual NV centers and to manipulate the charge state of single NVs without disturbing nearest-neighbor defects even within just 15-20 nm of the defect of interest. Ch. 4 focuses on a femtosecond time-resolved STM study of the charge density wave (CDW) insulator (TaSe4)2I. In this sample, we show the first reported example of massive-to-massless parametric amplification of a collective sliding CDW phason mode. In this mechanism, the massive 0.22 THz phason yields two massless phasons at half frequency and with equal and opposite wavevectors, conserving energy and momentum. This parametric amplification is analogous to the decay of the massive Higgs boson into two massless photons. In addition, we find that the 0.11 THz half-frequency phasons compete with a CDW amplitudon collective mode at proximate frequency. We show this by a combination of time-resolved tunneling, time-resolved point-contact, and optical pump-probe reflectivity measurements all made within our new set-up. This comprises the first direct experimental evidence of competing collective modes in a CDW material. Due to the instability of iodine atoms under illumination however, (TaSe4)2I does not provide the optimal platform for combining atomic-scale mapping capabilities with our sub-ps time-resolved STM measurements. Ch. 5 thus showcases our next advance. In this chapter I describe our measurements on the topological crystalline insulator Pb0.7Sn0.3Se in which we show the existence of an enhanced transient current up to 1.2 ps at the topological step edges of the material, with the maximum edge-to-terrace contrast at the Dirac point close to EFermi − 80meV . This is contrasted with the trivial counterpart Pb0.9Sn0.1Se which shows no enhancement at the edges. We explain the transient tunneling current in this material using a bulk-to-surface diffusion scenario of bulk holes initially excited upon photoexcitation diffusing into topological surface states. I conclude by briefly describing our efforts in expanding this technique further by exploring various quantum materials from RbV3Sb4.97Sn0.03 to Bi2Se3, including the challenges addressed and an outlook for the future. This thesis therefore presents the journey starting from construction of our new laser-coupled STM and shows how step-by-step, from static photoluminescence, to sub-ps time resolution, to simultaneous ultrafast time resolution, atomic-scale spatial resolution, and meV-scale energy resolution, we extract important insights on a diverse range of quantum materials.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Raghavan, Arjun

Contributors dc:contributor

• Madhavan, Vidya
• Abbamonte, Peter
• Cahill, David G
• Fradkin, Eduardo H

• Physics
• Scanning
• Tunneling
• Microscopy
• Condensed
• Matter
• Laser
• Ultrafast

Statement dc:rights

• Copyright 2025 Arjun Raghavan

Language dc:language

en

Handle dc:identifier

https://hdl.handle.net/2142/132727

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/132727