arXiv subjects

Bailey R. Nebgen

Publications and source records attributed to Bailey R. Nebgen.

4 recordsLinked to original sources

arXiv2026

A table-top few-femtosecond broadband extreme-ultraviolet absorption spectrometer with cryogenic cooling

We present a table-top cryogenic ultrafast broadband XUV absorption spectroscopy (c-UBXAS) beamline designed for temperature-dependent and time-resolved investigations of quantum materials. The instrument combines a broadband high-harmonic generation source spanning 22-73 eV with automated image registration and cryogenic sample control down to 20 K, enabling element-specific measurements under both equilibrium and nonequilibrium conditions. The beamline provides sub-50-meV energy resolution and a sub-10-fs instrument response function, while maintaining long-term stability suitable for extended, ultrasensitive measurements. Benchmark experiments on NiI$_2$, a van der Waals multiferroic, reveal temperature-dependent spectral evolution across its magnetic phase transitions, demonstrating the ability to identify element-specific contributions to the ground state and its transient response in the femtosecond regime. This instrument introduces the opportunity to unravel the very first response of a solid-state material during light-matter interaction, paving the way for understanding the complex phase space of nonequilibrium dynamics and emergent states in strongly correlated systems where the few-femtosecond electronic response has eluded most other types of time-resolved techniques.

physics.ins-det↗

arXiv2025

Isostructural electronic transition in MoS$_2$ probed by solid-state high harmonic generation spectroscopy

Studying materials under extreme pressure in diamond anvil cells (DACs) is key to discovering new states of matter, yet no method currently allows the direct measurement of the electronic structure in this environment. Solid-state high harmonic generation (sHHG) offers a new all-optical window into the electronic structure of materials. We demonstrate sHHG spectroscopy inside a DAC by probing $2H$-MoS$_2$, up to 30 GPa, revealing a pressure-induced crossover of the lowest direct bandgap from the $\textbf{K}$-point to the $Γ$-point. This transition manifests as a sharp minimum in harmonic intensity and a 30° rotation of the sHHG polarization anisotropy, despite the absence of a structural phase change. First-principles simulations attribute these features to interference between competing excitation pathways at distinct points in the Brillouin zone. Our results establish sHHG as a sensitive probe of electronic transitions at high pressure, enabling access to quantum phenomena that evade detection by conventional techniques.

cond-mat.mtrl-sci↗

arXiv2024

Core-level signature of long-range density-wave order and short-range excitonic correlations probed by attosecond broadband spectroscopy

Advances in attosecond core-level spectroscopies have successfully unlocked the fastest dynamics involving high-energy electrons. Yet, these techniques are not conventionally regarded as an appropriate probe for low-energy quasiparticle interactions that govern the ground state of quantum materials, nor for studying long-range order because of their limited sensitivity to local charge environments. Here, by employing a unique cryogenic attosecond beamline, we identified clear core-level signatures of long-range charge-density-wave (CDW) formation in a quasi-2D excitonic insulator candidate, even though equilibrium photoemission and absorption measurements of the same core levels showed no spectroscopic singularity at the phase transition. Leveraging the high time resolution and intrinsic sensitivity to short-range charge excitations in attosecond core-level absorption, we observed compelling time-domain evidence for excitonic correlations in the normal-state of the material, whose presence has been subjected to a long-standing debate in equilibrium experiments because of interfering phonon fluctuations in a similar part of the phase space. Our findings support the scenario that short-range excitonic fluctuations prelude long-range order formation in the ground state, providing important insights in the mechanism of exciton condensation in a quasi-low-dimensional system. These results further demonstrate the importance of a simultaneous access to long- and short-range order with underlying dynamical processes spanning a multitude of time- and energy-scales, making attosecond spectroscopy an indispensable tool for both understanding the equilibrium phase diagram and for discovering novel, nonequilibrium states in strongly correlated materials.

cond-mat.str-el↗

arXiv2023

A solid-state high harmonic generation spectrometer with cryogenic cooling

Solid-state high harmonic generation spectroscopy (sHHG) is a promising technique for studying electronic structure, symmetry, and dynamics in condensed matter systems. Here, we report on the implementation of an advanced sHHG spectrometer based on a vacuum chamber and closed-cycle helium cryostat. Using an in situ temperature probe, it is demonstrated that the sample interaction region retains cryogenic temperature during the application of high-intensity femtosecond laser pulses that generate high harmonics. The presented implementation opens the door for temperature-dependent sHHG measurements down to few Kelvin, which makes sHHG spectroscopy a new tool for studying phases of matter that emerge at low temperatures, which is particularly interesting for highly correlated materials.

physics.app-ph↗