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Register a new userQuantum oscillations in a bilayer with broken mirror symmetry: a minimal model for YBa$_2$Cu$_3$O$_{6 + delta}$
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Using an exact numerical solution and semiclassical analysis, we investigate quantum oscillations (QOs) in a model of a bilayer system with an anisotropic (elliptical) electron pocket in each plane. Key features of QO experiments in the high temperature superconducting cuprate YBCO can be reproduced by such a model, in particular the pattern of oscillation frequencies (which reflect magnetic breakdown between the two pockets) and the polar and azimuthal angular dependence of the oscillation amplitudes. However, the requisite magnetic breakdown is possible only under the assumption that the horizontal mirror plane symmetry is spontaneously broken and that the bilayer tunneling, $t_perp$, is substantially renormalized from its `bare value. Under the assumption that $t_perp= tilde{Z}t_perp^{(0)}$, where $tilde{Z}$ is a measure of the quasiparticle weight, this suggests that $tilde{Z} lesssim 1/20$. Detailed comparisons with new YBa$_2$Cu$_3$O$_{6.58}$ QO data, taken over a very broad range of magnetic field, confirm specific predictions made by the breakdown scenario.
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The phase diagram of cuprate high-temperature superconductors features an enigmatic pseudogap region that is characterized by a partial suppression of low energy electronic excitations. Polarized neutron diffraction, Nernst effect, THz polarimetery and ultrasound measurements on YBa$_2$Cu$_3$O$_y$ suggest that the pseudogap onset below a temperature T* coincides with a bona fide thermodynamic phase transition that breaks time-reversal, four-fold rotation and mirror symmetries respectively. However, the full point group above and below T* has not been resolved and the fate of this transition as T* approaches the superconducting critical temperature T$_c$ is poorly understood. Here we reveal the point group of YBa$_2$Cu$_3$O$_y$ inside its pseudogap and neighboring regions using high sensitivity linear and second harmonic optical anisotropy measurements. We show that spatial inversion and two-fold rotational symmetries are broken below T* while mirror symmetries perpendicular to the Cu-O plane are absent at all temperatures. This transition occurs over a wide doping range and persists inside the superconducting dome, with no detectable coupling to either charge ordering or superconductivity. These results suggest that the pseudogap region coincides with an odd-parity order that does not arise from a competing Fermi surface instability and exhibits a quantum phase transition inside the superconducting dome.
Polarized and unpolarized neutron diffraction has been used to search for magnetic order in YBa$_2$Cu$_3$O$_{6+x}$ superconductors. Most of the measurements were made on a high quality crystal of YBa$_2$Cu$_3$O$_{6.6}$. It is shown that this crystal has highly ordered ortho-II chain order, and a sharp superconducting transition. Inelastic scattering measurements display a very clean spin-gap and pseudogap with any intensity at 10 meV being 50 times smaller than the resonance intensity. The crystal shows a complicated magnetic order that appears to have three components. A magnetic phase is found at high temperatures that seems to stem from an impurity with a moment that is in the $a$-$b$ plane, but disordered on the crystal lattice. A second ordering occurs near the pseudogap temperature that has a shorter correlation length than the high temperature phase and a moment direction that is at least partly along the c-axis of the crystal. Its moment direction, temperature dependence, and Bragg intensities suggest that it may stem from orbital ordering of the $d$-density wave (DDW) type. An additional intensity increase occurs below the superconducting transition. The magnetic intensity in these phases does not change noticeably in a 7 Tesla magnetic field aligned approximately along the c-axis. Searches for magnetic order in YBa$_2$Cu$_3$O$_{7}$ show no signal while a small magnetic intensity is found in YBa$_2$Cu$_3$O$_{6.45}$ that is consistent with c-axis directed magnetic order. The results are contrasted with other recent neutron measurements.
Each unit cell in YBa$_2$Cu$_3$O$_{6+x}$ contains a pair of two-dimensional CuO$_2$ layers. While the crystal structure is globally inversion symmetric, the individual layers are not. This leads, necessarily, to a nonvanishing Rashba spin-orbit coupling (SOC) in the CuO$_2$ layers, with opposite signs of the coupling constant in each layer. These so-called Rashba bilayers generate hidden spin textures, with a vansishing net spin at each $k$-point in the Brillouin zone, but nonvanishing spin textures in each layer separately. Here, we trace the microscopic origin of the Rashba splitting through the orbital structure of the CuO$_2$ conduction bands, obtain a generic three-orbital model Hamiltonian, and show that the magnitude of the spin-splitting predicted by density functional theory is $sim 10$~meV.
We report a spectral ellipsometry study of multilayers composed of superconducting YBa$_2$Cu$_3$O$_{6+delta}$ (YBCO) and ferromagnetic La$_{0.7}$Ca$_{0.3}$MnO$_3$ in the spectral range of 0.7 - 6.5 eV. With increasing YBCO sublayer thickness, the optical spectral weight is enhanced at photon energies of 1.5 - 3.5 eV. The spectral weight enhancement is proportional to the number of interfaces of each multilayer sample, indicating its association with the interfacial electronic structure. Based on calculations in the framework of a multilayer model, we find that the shape of the interface-induced spectral weight is consistent with transfer of hole-carriers from YBCO to LCMO. Our results imply that the holes that are transferred across the interfaces accumulate in the LCMO layers, rather than being pinned by interfacial defects or annihilated by electron donors such as oxygen vacancies. Optical spectroscopy can thus serve as a non-destructive probe of charge transfer across buried interfaces in metal-oxide heterostructures.
We present local optical measurements of thermal diffusivity in the $ab$ plane of underdoped YBCO crystals. We find that the diffusivity anisotropy is comparable to reported values of the electrical resistivity anisotropy, suggesting that the anisotropies have the same origin. The anisotropy drops sharply below the charge order transition. We interpret our results through a strong electron-phonon scattering picture and find that both electronic and phononic contributions to the diffusivity saturate a proposed bound. Our results suggest that neither well-defined electron nor phonon quasiparticles are present in this material.
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