Probing emergent behaviour in epitaxial nanocomposites using ultrafast optical spectroscopy

• Main Author: Saritha Krishnankutty Nair
• Other Authors: Chia Ee Min, Elbert
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Science::Physics::Optics and light; DRNTU::Science::Physics::Atomic physics::Solid state physics
• Online Access: https://hdl.handle.net/10356/53524
• Institution: Nanyang Technological University

In this thesis, after explaining the experimental setup and data analysis procedure, the main experimental results are presented. Optical pump-optical probe technique has been used to study the photoinduced quasiparticle dynamics of Bi2Sr2CaCu2O8+ (Bi-2212) single crystals of different hole-doping levels, uranium intermetallic compounds UX3 (X = Al; Ga; Sn), and finally BiFeO3=YBa2Cu3O7 ?(BFO/YBCO) bilayer thin film heterostructure grown on STO substrate. The temperature dependence of the quasiparticle dynamics is closely studied in all of the above materials. Though Bi-2212 and UX3 samples are not thin films, the results of pump-probe studies on these samples supported the applicability of this technique in studying the relaxation dynamics in epitaxial thin lm heterostructures. Photoexcited quasiparticle relaxation dynamics in overdoped Bi-2212 (Tc = 65 K, hole doping p = 0:22) single crystal is investigated as a function of temperature. We provide evidence of a 22 meV pseudogap (T = 100 K) at this doping level. Moreover, this pseudogap vanishes at T . Our data support the scenario where both the superconducting gap and pseudogap coexist in the superconducting state. Our results also suggest an increased scattering rate between electrons and spin fluctuation as the sample enters the pseudogap phase. The first investigation of probe polarization dependence of pump-probe response of overdoped, optimally doped and underdoped Bi-2212 crystals is also reported. We observed a polarization dependence similar to that reported in orthorhombic YBCO. The temperature dependence of relaxation dynamics of UX3 (X = Al; Ga; Sn) compounds is studied using the time resolved pump-probe technique in reflectance geometry. UGa3 is an itinerant antiferromagnet, while UAl3 and USn3 are spin fluctuation systems. For UGa3, our data are consistent with the formation of a spin density wave gap as evidenced from the quasidivergence of the relaxation time near the N eel temperature TN. For UAl3 and USn3, the relaxation dynamics shows a change from single-exponential to two-exponential behavior below a particular temperature, suggestive of coherence formation of the 5f electrons withthe conduction band electrons. This particular temperature can be attributed to the spin fluctuation temperature Tsf , a measure of the strength of Kondo coherence. Our Tsf is consistent with other data such as resistivity and susceptibility measurements. The temperature dependence of the relaxation amplitude and time of UAl3 and USn3 were also tted by the Rothwarf-Taylor model. Our results show ultrafast optical spectroscopy is sensitive to the c-f Kondo hybridization in the f -electron systems. The temperature dependence of relaxation dynamics in bilayer thin film heterostructure composed of multiferroic BFO and superconducting YBCO grown on (001) SrTiO3 substrate is studied by pump probe technique. The relaxation dynamics is compared with YBCO and BFO thin lms of same growth conditions. The superconductivity of YBCO is found to be retained in the heterostructure. We observe the speeding up of YBCO dynamics in the superconducting state in the heterostructure. Also, BFO dynamics is seen to be a ected by the YBCO underlayer in the superconducting state in the heterostructure. We performed terahertz time-domain spectroscopy measurements on YBCO films grown on MgO and LSAT substrates. Our data agrees with literature on optical conductivity measurements of YBCO in the THz regime. This will be used as a starting point to detailed study on terahertz dynamics of heterostructures such as BFO/YBCO bilayer.