Spin-orbit coupling has been conjectured to play a key role in the low-energy electronic structure of Sr 2 RuO 4 . By using circularly polarized light combined with spin-and angle-resolved photoemission spectroscopy, we directly measure the value of the effective spin-orbit coupling to be 130 AE 30 meV. This is even larger than theoretically predicted and comparable to the energy splitting of the d xy and d xz;yz orbitals around the Fermi surface, resulting in a strongly momentum-dependent entanglement of spin and orbital character in the electronic wavefunction. As demonstrated by the spin expectation value h ⃗ s k · ⃗ s −k i calculated for a pair of electrons with zero total momentum, the classification of the Cooper pairs in terms of pure singlets or triplets fundamentally breaks down, necessitating a description of the unconventional superconducting state of Sr 2 RuO 4 in terms of these newly found spin-orbital entangled eigenstates. DOI: 10.1103/PhysRevLett.112.127002 PACS numbers: 74.25.Jb, 74.20.Rp, 74.70.Pq, 79.60.-i After a flurry of experimental activity [1-5], Sr 2 RuO 4 has become a hallmark candidate for spin-triplet chiral p-wave superconductivity, the electronic analogue of superfluid 3 He [6][7][8]. However, despite the apparent existence of such a pairing, some later experiments [9-11] do not fully support this conclusion, as they cannot be explained within a theoretical model using spin-triplet superconductivity alone [12]. A resolution might come from the inclusion of spin-orbit (SO) coupling, which has been conjectured to play a key role in the normal-state electronic structure [13] and may be important when describing superconductivity as well. By mixing the canonical spin eigenstates, the relativistic SO interaction might play a fundamental role beyond simply lifting the degeneracy of competing pairing states [13][14][15][16][17].Thus far, the experimental study of SO coupling's effects on the electronic structure of Sr 2 RuO 4 has been limited to the comparison of band calculations against angle-resolved photoemission spectroscopy (ARPES) [13,[18][19][20][21] -no success has been obtained in observing experimentally either the strength of SO coupling or its implications for the mixing between spin and orbital descriptions. Here we probe this directly by performing spin-resolved ARPES [22], with circularly polarized light: by using the angular momentum inherent in each photon-along with electricdipole selection rules [23]-to generate spin-polarized photoemission from the SO mixed states. Combined with a novel spin-and orbitally-resolved ab initio based tightbinding (TB) modeling of the electronic structure [24], these results demonstrate the presence of a nontrivial spinorbital entanglement over much of the Fermi surface, i.e., with no simple way of factoring the band states into the spatial and spin sectors. Most importantly, the analysis of the corresponding Cooper pair spin eigenstates establishes the need for a description of the unconventional superconductivity of Sr 2 RuO 4 beyond...

We report susceptibility, specific heat, and neutron diffraction measurements on NaCu2O2, a spin-1/2 chain compound isostructural to LiCu2O2, which has been extensively investigated. Below 12 K, we find a long-range ordered, incommensurate magnetic helix state with a propagation vector similar to that of LiCu2O2. In contrast to the Li analogue, substitutional disorder is negligible in NaCu2O2. We can thus rule out that the helix is induced by impurities, as was claimed on the basis of prior work on LiCu2O2. A spin Hamiltonian with frustrated longer-range exchange interactions provides a good description of both the ordered state and the paramagnetic susceptibility.PACS numbers: 75.10. Pq, 75.40.Cx, 75.25.+z Copper oxides are excellent model systems for lowdimensional spin-1/2 quantum antiferromagnets. In particular, copper oxides with magnetic backbones comprised of chains of CuO 4 squares have been shown to exhibit quasi-one-dimensional behavior. Two classes of copper oxide spin chain materials are known. Compounds in which adjacent squares share their corners are excellent realizations of the one-dimensional (1D) spin-1/2 Heisenberg Hamiltonian [1, 2, 3]. Linear Cu-O-Cu bonds along the spin chains give rise to a large antiferromagnetic nearest-neighbor exchange coupling. In compounds built up of edge-sharing squares, on the other hand, the Cu-O-Cu bond angle is nearly 90 • , so that the nearest-neighbor coupling is more than an order of magnitude smaller [4]. Because of the anomalously small nearest-neighbor coupling, longer-range frustrating exchange interactions have a pronounced influence on the physical properties of these materials. Edge-sharing copper oxides thus provide uniquely simple model systems to test current theories of spin correlations in frustrated quantum magnets.At low temperatures, the ground state of edge-sharing copper oxides is either a 3D-ordered antiferromagnet [5,6,7] or a spin-Peierls state [8], depending on whether interchain exchange interactions or spin-phonon interactions are dominant. In the former case, the magnetic order is almost always collinear. An interesting exception was recently discovered in LiCu 2 O 2 [9, 10, 11], which undergoes a transition to a magnetic helix state at low temperatures. While such a state is expected for classical spin models with frustrating interactions, quantum models predict a gapped spin liquid state in the range of exchange parameters that was claimed to describe the spin system in LiCu 2 O 2 . Since the ionic radii of Li + and Cu 2+ are similar, chemical disorder was identified as a possible solution to this puzzle. Indeed, a chemical analysis of the sample used in the neutron scattering study of Ref. [11] showed that about 16% of the Cu 2+ ions in the spin chains were replaced by nonmagnetic Li + impurities. Since even much lower concentrations of nonmagnetic impurities are found to induce magnetic long-range order in other quasi-1D spin-gap systems, the authors of Ref.[11] attributed the unexpected helix state to the Here we report magnet...

Microwave absorption measurements in magnetic fields from 0 up to 16 T were used to determine the temperature range of superconducting fluctuations above the superconducting critical temperature T c in YBa 2 Cu 3 O 7−δ . Measurements were performed on deeply underdoped, slightly underdoped, and overdoped single crystals. The temperature range of the superconducting fluctuations above T c is determined by an experimental method which is free from arbitrary assumptions about subtracting the nonsuperconducting contributions to the total measured signal and/or theoretical models to extract the unknown parameters. The superconducting fluctuations are detected in the ab plane, and c-axis conductivity, by identifying the onset temperature T . Within the sensitivity of the method, this fluctuation regime is found only within a fairly narrow region above T c . Its width increases from 7 K in the overdoped sample (T c = 89 K) to, at most, 23 K in the deeply underdoped sample (T c = 57 K), so that T falls well below the pseudogap temperature T * . Implications of these findings are discussed in the context of other experimental probes of superconducting fluctuations in the cuprates.

In strongly correlated systems the electronic properties at the Fermi energy (EF) are intertwined with those at high-energy scales. One of the pivotal challenges in the field of high-temperature superconductivity (HTSC) is to understand whether and how the high-energy scale physics associated with Mott-like excitations (|E−EF|>1 eV) is involved in the condensate formation. Here, we report the interplay between the many-body high-energy CuO2 excitations at 1.5 and 2 eV, and the onset of HTSC. This is revealed by a novel optical pump-supercontinuum-probe technique that provides access to the dynamics of the dielectric function in Bi2Sr2Ca0.92Y0.08Cu2O8+δ over an extended energy range, after the photoinduced suppression of the superconducting pairing. These results unveil an unconventional mechanism at the base of HTSC both below and above the optimal hole concentration required to attain the maximum critical temperature (Tc).