Thermal diffusivity measurement on Nb by Femtosecond optical pump-probe spectroscopy Md Mahmudur Rahman Ph.D. student Advisor Dr. Hani Elsayed-Ali Department of Electrical and Computer Engineering Old Dominion University Presented by Ganapati Myneni ISOHIM/ODU-AARC TTC2019 Vancouver February 6, 2019

Thermal diffusivity2K α2K ~ k/ρC = 2333 cm2 s-1 Temperature and magnetic field dependence of heat capacity of superconducting large grain Niobium S B Roy et al (unpublished) During cavity operation heat is deposited in the sc layer of ~60 nm τ (1.5 GHz) ~ 6.6×10-10 s Thermal diffusivity2K α2K ~ k/ρC = 2333 cm2 s-1 Jlab/JSA LLC Provisional patent January 2016

Examples of time scales in solids Process Time scale Electron –Electron interaction 10-16 to 10-13 sec Electrons interacting with atomic vibration (Phonons) 10-13 to 10-12 sec 10 August 2019

Pump-probe technique hνprobe Energy En e- hνpump EF e from below the Fermi level (EF) knocked out into an intermediate state with pump pulse. Probe pulse interrogate the system. The time scale of the e to decay back to equilibrium can be measured. hνprobe En Energy e- hνpump EF 10 August 2019

Pump-probe setup 10 August 2019

Spatial difference converts into time delay between pump and probe beam Liner stage Time delay, 𝜏= 2𝑑 𝑐 c = speed of light d = distance varied (1 mm ~ 6.66 ps ) (1 µm ~ 6.66 fs ) Pump Detector Probe d The beam path difference between pump and probe beam is varied 10 August 2019

Thermal conductivity measurement of metallic thin film Ingot Nb at room temperature K = 55 W/mK Heat equation 𝐶 𝜕𝑇 𝑥,𝑡 𝜕𝑡 =−𝐾 𝜕 2 𝑇 𝑥,𝑡 𝜕𝑥 2 +𝑆 C = Heat capacity (J/m3 . K) K = Thermal conductivity (W/m.K) S = Source term 10 August 2019

Low temperature measurements started After using 5pt FFT filer