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Register a new userReconsidering the interpretation of quantum oscillation experiments on underdoped YBa(2)Cu(3)O(6+x)
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On the basis of negative transport coefficients, it has been argued that the quantum oscillations observed in underdoped YBa(2)Cu(3)O(6+x) in high magnetic fields must be due to antinodal electron pockets. We point out a counter example in which electron-like transport in a hole-doped cuprate is associated with Fermi-arc states. We also present evidence that the antinodal gap in YBa(2)Cu(3)O(6+x) is robust to modest applied magnetic fields. We suggest that these observations should be taken into account when interpreting the results of the quantum oscillation experiments.
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Using neutron scattering, we investigate the effect of a magnetic field on the static and dynamic spin response in heavily underdoped superconducting YBa$_{2}$Cu$_{3}$O$_{6+x}$ (YBCO$_{6+x}$) with x=0.33 (T$_{c}$=8 K) and 0.35 (T$_{c}$=18 K). In contrast to the heavily doped and superconducting monolayer cuprates, the elastic central peak characterizing static spin correlations does not respond observably to a magnetic field which suppresses superconductivity. Instead, we find a magnetic field induced resonant enhancement of the spin fluctuations. The energy scale of the enhanced fluctuations matches the Zeeman energy within both the normal and vortex phases while the momentum dependence is the same as the zero field bilayer response. The magnitude of the enhancement is very similar in both phases with a fractional intensity change of $(I/I_{0}-1) sim 0.1$. We suggest that the enhancement is not directly correlated with superconductivity but is the result of almost free spins located near hole rich regions.
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.
The magnetization of three high-quality single crystals of YBa$_{2}$Cu$_{3}$O$_{6+x}$, from slightly overdoped to heavily underdoped,has been measured using torque magnetometry. Striking effects in the angular dependence of the torque for the two underdoped crystals, a few degrees above the superconducting transition temperature ($T_c$) are described well by the theory of Gaussian superconducting fluctuations using a single adjustable parameter. The data at higher temperatures ($T$) are consistent with a strong cut-off in the fluctuations for $Tgtrsim1.1T_c$. Numerical estimates suggest that inelastic scattering could be responsible for this cut-off.
We study the temperature dependence of the low energy phonons in the $(H, 0, L)$ reciprocal plane of the highly ordered ortho-II YBa$_2$Cu$_3$O$_{6.55}$ cuprate high temperature superconductor by means of high-resolution inelastic x-ray scattering. Anomalies associated with the emergence of long-range charge density wave (CDW) fluctuations are observed, and are qualitatively similar to those previously observed in the $(0, K, L)$ plane. This confirms the unconventional nature of this bi-dimensional CDW, which is not soft-phonon driven. With the support of first principles calculations, the symmetry of the anomalous phonon is identified and is found to match that of the charge modulation. This suggests in turn that these anomalies originate from a direct coupling between the phonons and the collective CDW excitations.
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.
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