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Register a new userRapid Suppression of the Charge Density Wave in YBa$_2$Cu$_3$O$_{6.6}$ under Hydrostatic Pressure
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Added by
Sofia Michaela Souliou
Publication date
2017
fields
Physics
and research's language is
English
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We report on the effects of hydrostatic pressure (HP) on the charge density wave observed in underdoped cuprates. We studied YBa$_2$Cu$_3$O$_{6.6}$ ($T_c$=61 K) using high-resolution inelastic x-ray scattering (IXS), and reveal an extreme sensitivity of the phonon anomalies related to the charge density wave (CDW) order to HP. The amplitudes of the normal state broadening and superconductivity induced phonon softening at Q$_{CDW}$ rapidly decrease as HP is applied, resulting in the complete suppression of signatures of the CDW below $sim$1 GPa. Additional IXS measurements on YBa$_2$Cu$_3$O$_{6.75}$ demonstrate that this very rapid effect cannot be explained by pressure-induced modification of the doping level and highlight the different role of external pressure and doping in tuning the phase diagram of the cuprates. Our results provide new insights into the mechanisms underlying the CDW formation and its interplay with superconductivity.
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Hole-doped cuprate superconductors show a ubiquitous tendency towards charge order. Although onset of superconductivity is known to suppress charge order, there has not so far been a decisive demonstration of the reverse process, namely, the effect of charge order on superconductivity. To gain such information, we report here the dependence of the critical temperature $T_{mathrm{c}}$ of YBa$_2$Cu$_3$O$_{6.67}$ on in-plane uniaxial stress up to 2 GPa. At a compression of about 1 GPa along the $a$ axis, 3D-correlated charge density wave (3D CDW) order appears. We find that $T_{mathrm{c}}$ decreases steeply as the applied stress crosses 1 GPa, showing that the appearance of 3D CDW order strongly suppresses superconductivity. Through the elastocaloric effect we resolve the heat capacity anomaly at $T_{mathrm{c}}$, and find that it does not change drastically as the 3D CDW onsets, which shows that the condensation energy of the 3D CDW is considerably less than that of the superconductivity.
The charge density wave in the high-temperature superconductor YBa$_2$Cu$_3$O$_{7-x}$ (YBCO) is now known to have two different ordering tendencies differentiated by their $c$-axis correlations. These correspond to ferro- (F-CDW) and antiferro- (AF-CDW) couplings between CDW in neighbouring CuO$_2$ bilayers. This discovery has prompted a number of fundamental questions. For example, how does superconductivity adjust to two competing orders and are either of these orders responsible for the electronic reconstruction? Here we use high-energy x-ray diffraction to study YBa$_2$Cu$_3$O$_{6.67}$ as a function of magnetic field and temperature. We show that regions of the sample with F-CDW correlations suppress superconductivity more strongly than those with AF-CDW correlations. This implies that an inhomogeneous superconducting state exists, in which some regions show a weak or fragile form of superconductivity. By comparison of F-CDW and AF-CDW correlation lengths, it is furthermore concluded that F-CDW ordering is sufficiently long-range to modify the electronic structure. Our study thus suggests that F-CDW correlations have an important impact on superconducting and normal state properties of underdoped YBCO.
Neutron Scattering measurements for YBa$_2$Cu$_3$O$_{6.6}$ have identified small magnetic moments that increase in strength as the temperature is reduced below $T^ast$ and further increase below $T_c$. An analysis of the data shows the moments are antiferromagnetic between the Cu-O planes with a correlation length of longer than 195 AA in the $a$-$b$ plane and about 35 AA along the c-axis. The origin of the moments is unknown, and their properties are discusssed both in terms of Cu spin magnetism and orbital bond currents.
The competition between superconductivity and charge density wave (CDW) order in underdoped cuprates has now been widely reported, but the role of disorder in this competition has yet to be fully resolved. A central question is whether disorder sets the length scale of the CDW order, for instance by pinning charge density fluctuations or disrupting an otherwise long range order. Using resonant soft x-ray scattering, we investigate the sensitivity of CDW order in YBa$_2$Cu$_3$O$_{6+x}$ (YBCO) to varying levels of oxygen disorder. We find that quench cooling YBCO$_{6.67}$ (YBCO$_{6.75}$) crystals to destroy their o-V and o-VIII (o-III) chains decreases the intensity of the CDW superlattice peak by a factor of 1.9 (1.3), but has little effect on the CDW correlation length, incommensurability, and temperature dependence. This reveals that while quenched oxygen disorder influences the CDW order parameter, the spatial extent of the CDW order is insensitive to the level of quenched oxygen disorder and may instead be a consequence of competition with superconductivity.
We report the results a comprehensive study of charge density wave (CDW) correlations in untwinned YBCO6+x single crystals with 0.4<x<0.99 using Cu-L3 edge resonant x-ray scattering (RXS). Evidence of CDW formation is found for 0.45<x<0.93, but not for samples with x<0.44 that exhibit incommensurate spin-density-wave order, and in slightly overdoped samples with x=0.99. This suggests the presence of two proximate zero-temperature CDW critical points at doping pc1~0.08 and pc2~0.18. The CDW reflections are observed at incommensurate in-plane wave vectors (d_a, 0) and (0, d_b). Both decrease linearly with increasing doping, in agreement with recent reports on Bi-based high-Tc superconductors, but in sharp contrast to the behavior of the 214 family. The CDW intensity and correlation length exhibit maxima at p~0.12, coincident with a plateau in the superconducting transition temperature Tc. The onset temperature of the CDW reflections depends non-monotonically on p, with a maximum of~160 K for p~0.12. The RXS reflections exhibit a uniaxial intensity anisotropy. We further observe a depression of CDW correlations upon cooling below Tc, and (for samples with p> 0.09) an enhancement of the signal when an external magnetic field up to 6 T is applied in the superconducting state. For samples with p~0.08, where prior work has revealed a field-enhancement of incommensurate magnetic order, the RXS signal is field-independent. This supports a previously suggested scenario in which incommensurate charge and spin orders compete against each other, in addition to individually competing against. We discuss the relationship of these results to stripe order 214, the pseudogap phenomenon, superconducting fluctuations, and quantum oscillations.
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