Detection of the Unusual Magnetic Orders in the Pseudogap Region of a High-Temperature SuperconductingYBa2Cu3O6.6Crystal by Muon-Spin Relaxation

Abstract

We present muon spin relaxation (µSR) measurements on a large YBa2Cu3O6.6 single crystal in which two kinds of unusual magnetic order have been detected in the pseudogap region by neutron scattering. A comparison is made to measurements on smaller, higher quality YBa2Cu3Oy single crystals. One type of magnetic order is observed in all samples, but does not evolve significantly with hole doping. A second type of unusual magnetic order is observed only in the YBa2Cu3O6.6 single crystal. This magnetism has an ordered magnetic moment that is quantitatively consistent with the neutron experiments, but is confined to just a small volume of the sample (∼ 3 %). Our findings do not support theories that ascribe the pseudogap to a state characterized by loop-current order, but instead indicate that dilute impurity phases are the source of the unusual magnetic orders in YBa2Cu3Oy.PACS numbers: 74.72. Bk, 74.25.Ha, 76.75.+i It is widely believed that the mysterious pseudogap region of high-transition temperature (high-T c ) copper oxide superconductors is caused by a "hidden order". Varma [1,2] has proposed that the pseudogap is caused by a circulating-current (CC) state that breaks timereversal symmetry and is characterized by a unique longrange pattern of loop currents in the CuO 2 planes that breaks rotational symmetry, but preserves the translational symmetry of the lattice (TSL). Alternatively, Chakravarty et al. [3] has attributed the pseudogap to a competing d-density wave (DDW) order. The DDW phase also breaks time reversal and rotational symmetries, but has an orbital current pattern that breaks the TSL. In both models the loop-current order is predicted to weaken with increased doping, and to vanish at a quantum critical point somewhat above optimal doping. Seemingly direct evidence for DDW order comes from neutron scattering experiments on underdoped YBa 2 Cu 3 O y (YBCO) with y = 6.6 [4, 5] and y = 6.45 [6], which reveal a weak antiferromagnetic (AF) ordered magnetic moment predominantly directed perpendicular to the CuO 2 planes. However, no such static magnetic order was observed by Stock et al. [7] in a neutron study of YBa 2 Cu 3 O 6.5 . A second unusual magnetic order recently detected in the pseudogap region of YBCO and HgBa 2 CuO 4+δ (Hg1201) by polarized neutron scattering [8-10] does not break the TSL, and hence is instead qualitatively consistent with the CC phase. However, the ordered moment in this case is not perpendicular to the CuO 2 planes as expected for the CC phase, but rather has a large in-plane component. Spin-orbit coupling [11] or orbital currents involving the apical oxygens [12] have been offered as possible reasons for why the magnetic moments are severely canted. The first scenario has also been invoked [13] to explain the onset of an accompanying weak ferromagnetism near T * , which has been detected in YBCO by high-resolution polar Kerr effect (PKE) experiments [14].What is most surprising is the lack of evidence from previous zero-field (ZF) µSR experiments for the existence of ...

Evidence for Skyrmions in the High-Temperature Superconductors

“…This relation shows that an extended object (assume, as an example, that R ∼ L/4, such that for L ∼ 3.0 nm, R ∼ 0.75 nm) can have a large magnetic moment, µ ∼ 0.01 µ B , attached to a very weak field, h 3 ∼ 0.02 mT. This magnetic field and magnetic moment is in the threshold limit of local fields from dynamical currents measured by NMR/NQR 21,22 and by µSR 23,24 measurements. Because of the elaborate circulating currents, the skyrmion lattice manifests itself as a periodic charge arrangement.…”

“…Indeed it is fundamental to estimate the value of h since NMR/NQR 21,22 and µSR 23,24 experiments put several bounds on its maximum value. An estimate of the maximum modules of h 3 can be obtained by calculating its value at the center of the skyrmion.…”

Numerical exploration of spontaneous broken symmetries in multiorbital Hubbard models

“…In addition, the Θ II phase (a q = 0 Fermi surface instability) faces the challenge of explaining the excitation gap in the pseudogap regime. Intense effort has been devoted to searching indirectly for orbital loop currents, using techniques such as NMR [47][48][49] and muon spin rotation, [50][51][52] but their existence in the cuprates remains inconclusive.…”

Spin rotational symmetry breaking by orbital current patterns in two-leg ladders

“…On the experimental front specific signatures compatible with uniform currents where observed [12][13][14][15] even if the issue is still controversial. 16,17 On the theoretical side significant efforts were made to probe the possible existence of such a phase, either using numerical approaches [18][19][20] or starting from Mott insulating side. 21 As a way of investigating the physics of these states in a more controlled way and also in view of possible connections with experimental systems, [22][23][24][25][26] phases exhibiting orbital current patterns ͑OCP͒ were also considered in the context of two-leg ladders.…”