Observation of magnetic order in a superconductingYBa2Cu3O6.6single crystal using polarized neutron scattering

Abstract

Polarized beam neutron-scattering measurements on a highly perfect crystal of YBa 2 Cu 3 O 6.6 show a distinct magnetic transition to a different phase with an onset at about 235 K, the temperature expected for the pseudogap transition. The moment is found to be about 0.1 B for each sublattice and has a correlation length of at least 75 Å. We found the critical exponent for the magnetic neutron intensity to be 2␤ = 0.37Ϯ 0.12. This is the proper range for the class of transition that has no specific-heat divergence possibly explaining why none is found at the pseudogap transition.Superconductivity produces a gap in the quasiparticle spectrum which for conventional materials disappears as the temperature is increased to T c , where the superconducting electron pairs are no longer bound together. However, for the underdoped cuprate materials a pseudogap measured by a number of techniques appears at a temperature T ‫ء‬ Ͼ T c , with T ‫ء‬ increasing as T c gets smaller. This pseudogap associated with T ‫ء‬ is one of the most puzzling and important attributes of the cuprate superconductors. 1,2 Indeed since superconductivity originates from the pseudogap state, it is this state that has to be understood to determine the mechanism for hightemperature superconductivity. One possible origin of the pseudogap is to postulate that phase incoherent pairs are established as the material is cooled through the pseudogap temperature T ‫ء‬ , with superconductivity developing at a lower temperature T c when phase coherence is established. 3 This picture has been studied by the Nernst effect, 4 which shows the presence of vortexlike excitations in the pseudogap phase. However, upon warming the Nernst effect 4 disappears well before T ‫ء‬ is reached, suggesting that well-defined preformed pairs may not be present at temperatures as high as T ‫ء‬ .Completely different approaches 5-7 to the problem postulated a state with broken symmetry that displayed a pattern of circulating currents ͑CC phase͒ in the a-b plane. As the sample is cooled, this state appears at T ‫ء‬ and for near optimal doping ends at a quantum critical point. The present study considers a magnetic signal found at certain ͑H , K , L͒ positions in the reciprocal lattice. These would correspond to the phases considered by Varma 5,6 which preserve the translational symmetry of the lattice. Since translational symmetry is not broken, the signal to observe the phases occurs at the Bragg positions of the unit cell which means they sit on top of the much more intense nuclear scattering. A very sensitive polarized beam experiment is thus needed to observe them. Fauqué et al. 8 have undertaken polarized neutron-scattering measurements to search for this phase at the expected positions for several underdoped YBa 2 Cu 3 O 6+x ͑YBCO͒ samples. They observed an increase in polarized neutron scattering upon cooling, which represents a magnetic signal, which could be associated with the pseudogap temperature T ‫ء‬ . We have undertaken similar polarized neutron measurements on a well ...

Spin Polaron-Bipolaron Scenario of Three Gap-Like Energy Scales in Superconducting Cuprates

“…These results are consistent with a variant of previously proposed current-loop order and of circulating currents which induce orbital moments. Also the polarized beam neutron-scattering measurements on a highly perfect crystal of YBa 2 Cu 3 O 6.6 have shown a distinct magnetic transition to a different phase with an onset at about 235 K, the temperature expected for the pseudogap transition [35]. These data support the scenario in which the superconducting and pseudogap state are two distinct and competing phenomena.…”

Experimental consequences of thes-wavecos(kx)cos(kyParish1, Hu2, Bernevig3 2008Phys. Rev. B11651062View full textAdd to dashboardCite

“…Moreover, the NNN coupling strength J 2 is stronger than the NN coupling strength J 1 . The J 1 − J 2 model produces half-filled magnetic physics consistent with experimental neutron data 15 . A nematic magnetic phase transition has been predicted in this model 16,17 , consistent with the experimental observation of a structural transition preceding the spin density wave (SDW) formation.…”

Absence of Static Loop-Current Magnetism at the Apical Oxygen Site inHgBa2CuO4+δfrom NMR

“…A more stringent test, if the orbital currents have the symmetry shown in Fig.1, is to examine the NMR linewidth for broadening that might reflect a small frequency splitting. Our measurements at all temperatures of the angular variation of the linewidth is less than 0.5 kHz which places a bound on orbital current fields of 0.4 G. According to Lederer and Kivelson,[19] orbital currents inferred from neutron scattering results [8,10,11] should lead to local fields of order H max ∼ 200 G oriented as indicated in Fig.1. Taken together, our measurements of the angular variations of the apical orbital shift and NMR linewidth, in-plane and out of-plane, place substantial constraints on predictions from the theory for a static orbital field to be less than 0.4 G.…”

“…Polarized neutron scattering in YBa 2 Cu 3 O 7−δ [8][9][10], HgBa 2 CuO 4+δ (Hg1201) [11,12], Bi 2 Sr 2 CaCu 2 O 8+δ [13], and La 2−x Sr x CuO 4 (short ranged) [14] show the appearance of broken time reversal symmetry that correlates with the onset of the pseudogap phase with a magnetic moment tilted away from the crystalline c-axis. A muon spin relaxation (µSR) experiment in zero field was unable to detect this magnetism [15] although screening effects may account for muon insensitivity.…”