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تسجيل مستخدم جديدAnomalous Thermal Diffusivity in Underdoped YBa$_2$Cu$_3$O$_{6+x}$
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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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The application of large magnetic fields ($B sim B_{c2}$) to layered cuprates suppresses their high temperature superconducting behaviour and reveals competing ground states. In the widely-studied material YBa$_2$Cu$_3$O$_{6+x}$ (YBCO), underdoped ($
p sim 1/8$) samples show signatures of field-induced electronic and structural changes at low temperatures. However, the microscopic nature of the field-induced reconstruction and the high-field state are unclear. Here we report an x-ray study of the high-field charge density wave (CDW) in YBCO, for doping, $0.1 lesssim p lesssim 0.13$. For $p sim 0.123$, we find that a field ($B sim 10$~T) induces new CDW correlations along the CuO chain ($b$) direction only, leading to a 3-D ordered state along this direction at $B sim 15$~T. The CDW signal along the $a$-direction is also enhanced by field, but does not develop a new pattern of correlations. We find that field modifies the coupling between the CuO$_2$ bilayers in the YBCO structure, and causes the sudden appearance of 3D CDW order. The mirror symmetry of individual bilayers is broken by the CDW at low and high fields, allowing recently suggested Fermi surface reconstruction.
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
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The possibility of enhancing desirable functional properties of complex materials by optical driving is motivating a series of studies of their nonlinear terahertz response. In high-Tc cuprates, large amplitude excitation of certain infrared-active l
attice vibrations has been shown to induce transient features in the reflectivity suggestive of non-equilibrium superconductivity. Yet, a microscopic mechanism for these observations is still lacking. Here, we report measurements of time- and scattering-angle-dependent second-harmonic generation in YBa$_2$Cu$_3$O$_{6+x}$, taken under the same excitation conditions that result in superconductor-like terahertz reflectivity. We discover a three-order-of-magnitude amplification of a 2.5-terahertz electronic mode, which is unique because of its symmetry, momentum, and temperature dependence. A theory for parametric three-wave amplification of Josephson plasmons, which are assumed to be well-formed below T$_c$ but overdamped throughout the pseudogap phase, explains all these observations and provides a mechanism for non-equilibrium superconductivity. More broadly, our work underscores the role of parametric mode mixing to stabilize fluctuating orders in quantum materials.
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1+}$O$_x$ chains at any $x$. Ordering produces one-dimensional metallic bands, while configurations with non-aligned oxygens are insulating. The Cu$^{2+}$O$_2$ planes remain insulating and antiferromagnetic up to a threshold between $x$=0.25 and 0.5, above which a paramagnetic normal-metal state prevails. The in-plane antiferro-paramagnetic competition depends on $x$, but only weakly on the ordering state of the chains.
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