Regensburg 2025 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 41: Quantum-Critical Phenomena (joint session TT/DY)

TT 41.2: Talk

Thursday, March 20, 2025, 09:45–10:00, H31

Terahertz Crystal Electric Field Transitions in a Kondo-Lattice Antiferromagnet — •Payel Shee¹, Chia-Jung Yang², Shishir Kumar Pandey³, Ashis Kumar Nandy¹, Ruta Kulkarni⁴, Arumugam Thamizhavel⁴, Manfred Fiebig², and Shovon Pal¹ — ¹NISER, HBNI, Jatni, India. — ²ETH Zurich, Switzerland. — ³Artificial Intelligence for Science Institute, Beijing, China. — ⁴Tata Institute of Fundamental Research, Mumbai, India.

The interplay between the Kondo effect and Ruderman-Kittel-Kasuya-Yosida (RKKY) leads to the emergence of many intriguing phenomena in strongly correlated systems. Metallic materials doped with magnetic impurities are ideal for such studies. These impurities interact with the crystal electric field (CEF) produced by neighboring ions, lifting the degeneracy of their energy levels and creating CEF states. Given that CEF excitations occur in the millielectronvolt (meV) range, the terahertz (THz) frequency range is particularly suited for these investigations. Using time-domain THz reflection spectroscopy, we show the first direct evidence of two low-energy CEF transitions at 0.6 THz (2.5 meV) and 2.1 THz (8.7 meV) in CeAg₂Ge₂, a prototype Kondo-lattice antiferromagnet. In addition, we also observe that the lower CEF transition peak undergoes a blue-shift once the sample enters into the antiferromagnetic phase. The temporal spectral weights obtained directly from the THz time traces corroborate the corresponding CEF energy scales of the compound [2].

[1] S. Pal et al., Phys. Rev. Lett. 122, 096401 (2019);

[2] P. Shee et al., Phys. Rev. B 109, 075133 (2024).

Keywords: Strongly-correlated materials; Heavy-fermions; Terahertz (THz) spectroscopy; Quantum phase transition