Polar Kerr-Effect Measurements of the High-TemperatureYBa2Cu3O6+xSuperconductor: Evidence for Broken Symmetry near the Pseudogap Temperature

Abstract

The polar Kerr effect in the high-T c superconductor YBa 2 Cu 3 O 6x was measured at zero magnetic field with high precision using a cyogenic Sagnac fiber interferometer. We observed nonzero Kerr rotations of order 1 rad appearing near the pseudogap temperature T and marking what appears to be a true phase transition. Anomalous magnetic behavior in magnetic-field training of the effect suggests that time reversal symmetry is already broken above room temperature. , large compared to the superconducting (SC) transition temperature, T c . Two major classes of theories have been introduced in an attempt to describe the pseudogap state: One in which the pseudogap temperature T represents a crossover into a state with preformed pairs with a d wave gap symmetry [6,7], and another in which T marks a true transition into a phase with broken symmetry that ends at a quantum critical point, typically inside the superconducting dome. While at low doping this phase may compete with superconductvity, it might provide fluctuations that are responsible for the enhanced transition temperature near its quantum critical point (e.g., as in Ref. [8] In this Letter, we report high resolution optical Kerreffect measurements on YBa 2 Cu 3 O 6x crystals with various hole concentrations p. (p is, in turn, a monotonic function of the oxygen concentration x, and it also depends on oxygen ordering in the chains [12].) We identify a sharp phase transition at a temperature T s p, below which there is a nonzero Kerr angle, indicating the existence of a phase with broken time reversal symmetry (TRS). Both the magnitude and hole concentration dependence of T s are in close correspondence with those of the pseudogap crossover temperature, T , which has been identified in other physical quantities. In particular, as shown in Fig. 1, T s is substantially larger than the superconducting T c in underdoped materials, but drops rapidly with increasing hole concentration, so that it is smaller than T c in a near optimally doped crystal and extrapolates to zero at a putative quantum critical point under the superconducting dome. The magnitude of the Kerr rotation in YBa 2 Cu 3 O 6x (YBCO) is smaller by 4 orders of magnitude than that observed in other itinerant ferromagnetic oxides [13,14], and the temperature dependence is ''superlinear'' near T c , FIG. 1 (color online). The onset of the Kerr-effect signal, T s (circles), and T c (red squares) for the YBa 2 Cu 3 O 6x samples reported in this Letter. Also shown are T c p (from [12]) and T N p (from [22]).

Quantum oscillations in the high- T c cuprates

“…Experimental evidence has been lacking thus far for a divergent susceptibility characteristic of a proposed QCP under the superconducting dome [88][89][90][91][92][93][94], in part due to the robust superconductivity that obscures any such signatures [95]. While an array of thermodynamic signatures from experiments such as heat capacity, muon spin resonance, neutron scattering and others show a discontinuity above the superconducting dome [96][97][98][99], only a few hints of such a second-order phase transition under the dome have been indicated by the polar Kerr effect in YBa 2 Cu 3 O 6+x in the vicinity of optimal doping [100]. A possible scenario is a secondary symmetry-breaking transition located at the MI QCP that destroys the Fermi surface [20,34], subsequent to a symmetrybreaking transition at optimal doping that reconstructs the Fermi surface.…”

Planar metamaterial with transmission and reflection that depend on the direction of incidence

“…13 In conclusion, we have demonstrated strong resonant directional and polarization asymmetry of normal incidence reflection and transmission of circularly polarized light from and through a planar chiral metamaterial. Nanoscaled versions of the metamaterial structure will depend on the resonant plasmonic response at optical frequencies.…”

Coexistence of Charge-Density-Wave and Pair-Density-Wave Orders in Underdoped Cuprates

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]) The two most frequently discussed candidates for electronic order are incommensurate charge density-wave (CDW) order (Refs. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]) and incommensurate pair-density-wave order (PDW), which is a SC order with a finite Cooper pair momentum Q (Refs.…”