טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentAharonovich Gil
SubjectA Study of Injected Carriers in Bilayers of Y Ba2Cu3Ox
and SrRuO3 in the Pseudogap Regime
DepartmentDepartment of Physics
Supervisor Professor Emeritus Gad Koren
Full Thesis textFull thesis text - English Version


Abstract

In the present study we looked for the signature of “pre-formed” Cooper pairs in the pseudogap regime of the high Tc superconductor (HTSC) YBa2Cu3Ox (YBCO). This was done by investigating the proximity effects in bilayers of YBCO and the itinerant ferromagnet SrRuO3 (SRO). The preformed pairs scenario is one of the hypothesis proposed to explain the pseudogap phase by assuming the existence of uncorrelated preformed pairs of charge carriers in this regime of the HTSC. In our study, we monitor the influence of a few nano meter thick YBCO layer on Tp, the ferromagnetic ordering temperature of SRO, in YBCO/SRO bilayers. Here, Tp is the temperature of maximum dM/dT or dR/dT where M and R are the magnetization and resistance of SRO, respectively. Tp shifts of the bilayers as compared to the reference Tp in pure SRO, were used as our probe to investigate the effects of injected carriers between YBCO and SRO. Large Tp shifts were observed when the YBCO layer was in its pseudogap regime. In several control experiments, we compared our results with those obtained in bilayers with SRO where instead of YBCO in the pseudogap regime, YBCO and La1.88Sr0.12CuO4−y out of the pseudogap regime, and Au and Cr were used. We found that in bilayers made of underdoped 10 nm YBCO on 5 nm SRO, the Tp values were shifted to lower temperatures by up to 6-8 K as compared to Tp ≈ 140 K of the 5 nm thick reference SRO film. In contrast, in the other type of bilayers, which are not in the pseudogap regime near the Tp of pure SRO, only smaller shifts of up to ±2 K were observed. These differences are discussed in this work in terms of proximity effects, where carriers from the YBCO layer are injected into the SRO layer and vice versa. Our results suggest that only correlated electrons in the pseudogap regime of YBCO are responsible for the observed large Tp shifts. Consequently, we conclude that the presumed pre-formed pairs are the likely candidates to explain this effect.