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Register a new userIntra-Unit-Cell magnetic correlations near optimal doping in $rm YBa_2Cu_3O_{6.85}$
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Understanding high-temperature superconductivity requires a prior knowledge of the nature of the enigmatic pseudogap metallic state, out of which the superconducting state condenses. In addition to the electronic orders involving charge degrees of freedoms recently reported inside the pseudogap state, a magnetic intra-unit-cell (IUC) order was discovered in various cuprates to set in just at the pseudogap temperature, T*. In nearly optimally doped YBa$_2$Cu$_3$O$_{6.85}$, polarized neutron scattering measurements, carried out on two different spectrometers, reveal new features. The order is made of finite size planar domains, hardly correlated along the c-axis. At high temperature, only the out-of-plane magnetic components correlate, revealing a strong Ising anistropy, as originally predicted in the loop current model. Below T*, a correlated in-plane response develops, giving rise the apparent tilt of the magnetic moment at low temperature. The discovery of these two regimes put stringent constraints on the intrinsict nature of IUC order, tightly bound to the pseudogap physics.
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Within the complex phase diagram of the hole-doped cuprates, seizing the nature of the mysterious pseudo-gap phase is essential to unravel the microscopic origin of high-temperature superconductivity. Below the pseudo-gap temperature $rm T^{star}$, evidences for intra-unit-cell orders breaking the 4-fold rotation symmetry have been provided by neutron diffraction and scanning tunneling spectroscopy. Using polarized neutron diffraction on a detwinned $rm YBa_2Cu_3O_{6.6}$ sample, we here report a distinct a-b anisotropy of the intra-unit-cell magnetic structure factor below $rm T^{star}$, highlighting that intra-unit-cell order in this material breaks the mirror symmetry of the CuO$_2$ bilayers. This is likely to originate from a crisscrossed arrangement of loop currents within the $rm CuO_2$ bilayer, resulting in a bilayer mean toroidal axis along the $rm {bf b}$ direction.
We have used Resonant Inelastic X-ray Scattering (RIXS) and dynamical susceptibility calculations to study the magnetic excitations in NaFe$_{1-x}$Co$_x$As (x = 0, 0.03, and 0.08). Despite a relatively low ordered magnetic moment, collective magnetic modes are observed in parent compounds (x = 0) and persist in optimally (x = 0.03) and overdoped (x = 0.08) samples. Their magnetic bandwidths are unaffected by doping within the range investigated. High energy magnetic excitations in iron pnictides are robust against doping, and present irrespectively of the ordered magnetic moment. Nevertheless, Co doping slightly reduces the overall magnetic spectral weight, differently from previous studies on hole-doped BaFe$_{2}$As$_{2}$, where it was observed constant. Finally, we demonstrate that the doping evolution of magnetic modes is different for the dopants being inside or outside the Fe-As layer.
Polarized beam neutron scattering measurements on a highly perfect crystal of ${rm YBa_2Cu_3O_{6.6}}$ show a distinct magnetic transition with an onset at about 235K, the temperature expected for the pseudogap transition. The moment is found to be about 0.1 $mu_B$ for each sublattice and have a correlation length of at least 75 AA. We found the critical exponent for the magnetic neutron intensity to be 2$beta$ =0.37$pm$ 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.
We report low temperature muon spin relaxation (muSR) measurements of the high-transition-temperature (Tc) cuprate superconductors Bi{2+x}Sr{2-x}CaCu2O{8+delta} and YBa2Cu3O6.57, aimed at detecting the mysterious intra-unit cell (IUC) magnetic order that has been observed by spin polarized neutron scattering in the pseudogap phase of four different cuprate families. A lack of confirmation by local magnetic probe methods has raised the possibility that the magnetic order fluctuates slowly enough to appear static on the time scale of neutron scattering, but too fast to affect $mu$SR or nuclear magnetic resonance (NMR) signals. The IUC magnetic order has been linked to a theoretical model for the cuprates, which predicts a long-range ordered phase of electron-current loop order that terminates at a quantum crictical point (QCP). Our study suggests that lowering the temperature to T ~ 25 mK and moving far below the purported QCP does not cause enough of a slowing down of fluctuations for the IUC magnetic order to become detectable on the time scale of muSR. Our measurements place narrow limits on the fluctuation rate of this unidentified magnetic order.
We report a polarized neutron scattering study of the orbital-like magnetic order in strongly underdoped ${rm YBa_2Cu_3O_{6.45}}$ and ${rm YBa_2(Cu_{0.98}Zn_{0.02})_3O_{6.6}}$. Their hole doping levels are located on both sides of the critical doping $p_{MI}$ of a metal-insulator transition inferred from transport measurements. Our study reveals a drop down of the orbital-like order slightly below $p_{MI}$ with a steep decrease of both the ordering temperature $T_{mag}$ and the ordered moment. Above $p_{MI}$, substitution of quantum impurities does not change $T_{mag}$, whereas it lowers significantly the bulk ordered moment. The modifications of the orbital-like magnetic order are interpreted in terms of a competition with electronic liquid crystal phases around $p_{MI}$. This competition gives rise to a mixed magnetic state in ${rm YBa_2Cu_3O_{6.45}}$ and a phase separation in ${rm YBa_2(Cu_{0.98}Zn_{0.02})_3O_{6.6}}$.
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