Optical Characterization of Time Reversal Symmetry Breaking States in Heavy Fermion Superconductors PDF Download

Author: Elizabeth Regina Schemm
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Languages : en
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Book Description
Heavy fermion materials have been of interest for decades because of the numerous ordered phases they exhibit at low temperatures, often resulting in novel bulk properties including various forms of magnetic ordering and unconventional superconductivity. A full understanding of these phases and their associated order parameters requires knowledge of their corresponding symmetries. This thesis examines specifically the role of time-reversal symmetry (TRS), as probed by polar Kerr effect (PKE) measurements, in identifying the order parameter of the canonical heavy fermion superconductors UPt_3 and URu_2Si_2. In UPt_3, the onset of PKE is observed below a temperature T_Kerr that coincides with the low temperature "B phase" superconducting transition temperature T_c- ~ 480 mK. In contrast, no change in Kerr effect is observed through either the high temperature "A phase" superconducting transition at T_c+ ~ 550 mK or the small-moment antiferromagnetic (AF) transition at T_N ~ 5 K. These results indicate that TRS is broken only in the B phase, independently of the higher temperature AF order, thus placing strong restrictions on the theory of superconductivity in this system. The case of URu_2Si_2 is more complex. There is a finite PKE loosely associated with the so-called "hidden order" (HO) transition at T_HO ~ 17.5 K. However, the magnitude of the signal appears to depend on the strength of the magnetic field used to train it, and for high fields the signal clearly onsets at ~25 K -- well above T_HO. At lower temperatures, an additional Kerr signal appears below the superconducting transition T_c ~ 1.5 K, which is independent of impurity concentration and which can be trained independently of the HO signal, all of which is consistent with broken TRS intrinsic to superconductivity in \URS. At the same time, an anomaly in the PKE data at ~0.8-1 K is consistently observed whose origins remain a puzzle; there is still more to be learned about URu_2Si_2 within the superconducting state.