Structures and Properties of High-Concentration Doped Th:CaF₂ Single Crystals for Solid-State Nuclear Clock Materials

Area of Science:

• Materials Science
• Solid-State Physics
• Nuclear Physics

Background:

• Solid-state nuclear clocks require materials with excellent optical properties in the vacuum ultraviolet (VUV) range.
• Thorium-doped crystals are candidates for nuclear clocks, but high doping concentrations can affect VUV transparency.

Purpose of the Study:

• To investigate the structural and optical properties of high-concentration thorium-doped calcium fluoride (Th:CaF2) single crystals.
• To understand defect configurations and charge compensation mechanisms in Th:CaF2.
• To evaluate the VUV transmittance performance of Th:CaF2 for potential nuclear clock applications.

Main Methods:

• Theoretical calculations and experimental investigations were employed.
• Analysis of crystal structures, defects, and VUV transmittance.
• Study of defect configurations and charge compensation mechanisms (e.g., Ca vacancies, interstitial F atoms).
• Determination of defect formation energies under varying chemical potentials.

Main Results:

• High-concentration Th:CaF2 crystals were successfully cultivated.
• Significant VUV transmittance was maintained at high doping levels (e.g., ~62% at 150 nm for 1.91 × 10^20 cm^-3 doping).
• VUV transmittance showed a weak negative dependence on doping concentration.
• Defect configurations and charge compensation mechanisms were identified.

Conclusions:

• Th:CaF2 crystals exhibit exceptional comprehensive performance for VUV transparency and high doping concentrations.
• Controlling system factors impacting VUV transmittance is vital for realizing solid-state nuclear optical clocks.
• These findings advance the development of Th:CaF2 and other thorium-doped crystals for nuclear clock technology.