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Photon energy and polarization dependence. a–d Comparison of various energy dependent scattering intensities across the Cu L3 edge. The data in a–c were measured on a 50 nm YBCO thin film grown on SrTiO3 (no chain order), whereas the data in d were collected from a single crystal of ortho-II ordered YB2Cu3O6.55. e Scattering geometry used to study the 3D-CO reflection at q3D. f Polarization dependence of the 3D-CO reflection as measured in e, and with photon energy tuned to the Cu L3 edge
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Incommensurate charge order (CO) has been identified as the leading competitor of high-temperature superconductivity in all major families of layered copper oxides, but the perplexing variety of CO states in different cuprates has confounded investigations of its impact on the transport and thermodynamic properties. The three-dimensional (3D) CO ob...
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... The epitaxial strain induced by the substrate has been used to control the atomic positions and therefore the strengths of the interactions. More in general, strain allows one to control the critical temperature [22], as well as the magnetic-exchange interaction [23], the nematicity [16,24], and the charge density waves [25][26][27][28][29]. ...
In underdoped cuprate high-Tc superconductors, various local orders and symmetry-breaking states, in addition to superconductivity, reside in the CuO2 planes. The confinement of the CuO2 planes can therefore play a fundamental role in modifying the hierarchy between the various orders and their intertwining with superconductivity. Here we present the growth of a-axis oriented YBa2Cu3O7−δ films, spanning the whole underdoped side of the phase diagram. In these samples, the CuO2 planes are confined by the film thickness, effectively forming unit-cell-thick nanoribbons. The unidirectional confinement at the nanoscale enhances the in-plane anisotropy of the films. By x-ray diffraction and resistance vs temperature measurements, we have discovered the suppression of the orthorhombic-to-tetragonal transition at low dopings, and a very high anisotropy of the normal state resistance in the b−c plane, the latter being connected to a weak coupling between adjacent CuO2 nanoribbons. These findings show that the samples we have grown represent a novel system, different from the bulk, where future experiments can possibly shed light on the rich and mysterious physics occurring within the CuO2 planes.
... These results call for examining the effect of VIS-UV light on other phenomena that emerge in the underdoped region of the cuprate's phase diagram. For example, charge density wave (CDW) order [45][46][47][48] whose competition with superconductivity can be tipped by disorder and scattering [49][50][51] as it has been shown in the case of external perturbations such as large magnetic fields [52] or strain [53,54]. These effects could be studied using techniques such as resonant inelastic X-ray Scattering [55,56] after illumination. ...
The normal-state conductivity and superconducting critical temperature of oxygen-deficient YBa2Cu3O7−δ can be persistently enhanced by illumination. Strongly debated for years, the origin of those effects—termed persistent photoconductivity and photosuperconductivity (PPS)—has remained an unsolved critical problem, whose comprehension may provide key insights to harness the origin of high-temperature superconductivity itself. Here, we make essential steps toward understanding PPS. While the models proposed so far assume that it is caused by a carrier-density increase (photodoping) observed concomitantly, our experiments contradict such conventional belief: we demonstrate that it is instead linked to a photo-induced decrease of the electronic scattering rate. Furthermore, we find that the latter effect and photodoping are completely disconnected and originate from different microscopic mechanisms, since they present different wavelength and oxygen-content dependences as well as strikingly different relaxation dynamics. Besides helping disentangle photodoping, persistent photoconductivity, and PPS, our results provide new evidence for the intimate relation between critical temperature and scattering rate, a key ingredient in modern theories on high-temperature superconductivity.
... Interestingly, these studies also reveal a peak at integer L values. Additionally, it is noteworthy that epitaxially grown YBa 2 Cu 3 O 6þx thin films exhibit integer L CO up to room temperature, which may be favored by enhanced Fermi surface nesting conditions [42]. The similar L-dependent behaviors of CO induced by external controls and observed in the metallic region suggest common interlayer correlations and suggest that CO is a prevalent feature in the normal state. ...
The extremely overdoped cuprates are generally considered to be Fermi liquid metals without exotic orders, whereas the underdoped cuprates harbor intertwined states. Contrary to this conventional wisdom, using Cu L3-edge and O K-edge resonant x-ray scattering, we reveal a charge order (CO) correlation in overdoped La2−xSrxCuO4 (0.35≤x≤0.6) beyond the superconducting dome. This CO has a periodicity of ∼6 lattice units with correlation lengths of ∼20 lattice units. It shows similar in-plane momentum and polarization dependence and dispersive excitations as the CO of underdoped cuprates, but its maximum intensity differs along the c direction and persists up to 300 K. This CO correlation cannot be explained by the Fermi surface instability and its origin remains to be understood. Our results suggest that CO is prevailing in the overdoped metallic regime and requires a reassessment of the picture of overdoped cuprates as weakly correlated Fermi liquids.
... It has since been observed that the application of certain perturbationshigh magnetic fields 4,23-25 , epitaxial strain in thin films 26 , or uniaxial strain 27,28can induce a CO phase with threedimensional (3D) coherence. Upon the application of these external influences, a second CO peak emerges, this time centered at integer L-values, evidencing an out-of-plane coupling that locks the phase of adjacent CuO 2 planes. ...
The shape of 3d-orbitals often governs the electronic and magnetic properties of correlated transition metal oxides. In the superconducting cuprates, the planar confinement of the dx2−y2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${d}_{{x}^{2}-{y}^{2}}$$\end{document} orbital dictates the two-dimensional nature of the unconventional superconductivity and a competing charge order. Achieving orbital-specific control of the electronic structure to allow coupling pathways across adjacent planes would enable direct assessment of the role of dimensionality in the intertwined orders. Using Cu L3 and Pr M5 resonant x-ray scattering and first-principles calculations, we report a highly correlated three-dimensional charge order in Pr-substituted YBa2Cu3O7, where the Pr f-electrons create a direct orbital bridge between CuO2 planes. With this we demonstrate that interplanar orbital engineering can be used to surgically control electronic phases in correlated oxides and other layered materials.
... The epitaxial strain induced by the substrate has been used to control the atomic positions and therefore the strengths of the interactions. Strain allows to control the critical temperature [28], as well as the magneticexchange interaction [29], the nematicity [23,30] and the charge density waves [31][32][33][34]. ...
In underdoped cuprate high $T_{\mathrm{c}}$ superconductors, various local orders and symmetry breaking states, in addition to superconductivity, resides in the CuO$_2$ planes. The confinement of the CuO$_2$ planes can therefore play a fundamental role in modifying the hierarchy between the various orders and their intertwining with superconductivity. Here we present the growth of $a$-axis oriented YBa$_2$Cu$_3$O$_{7-\delta}$ films, spanning the whole underdoped side of the phase diagram. In these samples, the CuO$_2$ planes are confined by the film thickness, which make them in form of unit cell nanoribbons. The unidirectional confinement at the nanoscale enhances the anisotropy of the films. By X-ray diffraction and resistance vs temperature measurements, we have discovered the suppression of the orthorhombic-to-tetragonal transition at low dopings, and a very high anisotropy of the normal resistance in the $b$-$c$ plane, the latter being connected to a weak coupling between adjacent CuO$_2$ nanoribbons. These findings show that the samples we have grown represent a novel system, different from the bulk, where future experiments can possibly shed light on the rich and mysterious physics occurring within the CuO$_2$ planes.
... Yet, the electronic energy gain involved in these distortions goes well beyond the simple perturbation of the levels close to the Fermi level. A last challenging example is provided by the now common observations of 2D or 3D, static or fluctuating, CDW orders in Cu-based high-T c superconductors [21][22][23][24][25]. ...
Within the standard perturbative approach of Peierls, a charge-density wave is usually assumed to have a cosine shape of weak amplitude. In nonlinear physics, we know that waves can be deformed. What are the effects of the nonlinearities of the electron-lattice models in the physical properties of Peierls systems? A nonlinear discrete model, introduced by Brazovskii, Dzyaloshinskii and Krichever, allows to address this issue beyond perturbation theory. For some special parameters of the model, at integrable points, it has exact solutions: the wave is cnoidal rather than simply harmonic but its envelope function remains continuous. As a consequence, the charge-density wave may slide over the lattice potential at no energy cost, in agreement with the Fr\"ohlich's argument. When the integrability is broken, however, the envelope function may be discontinuous, electrons tend to form some stronger distinct chemical bonds, i.e. local dimers or oligomers. The wave is intrinsically pinned by the lattice. We argue that an Aubry transition from the sliding phase to the insulating pinned phase occurs when the nonlinearities become strong.
... It has since been observed that the application of certain perturbations -high magnetic fields [4,23,24], epitax- ial strain [25], or uniaxial stress [26,27] -can induce a CO phase with three-dimensional (3D) coherence. Upon the application of these external influences, a second CO peak emerges, this time centered at integer L-values, evidencing an out-of-plane coupling that locks the phase of adjacent CuO 2 planes. ...
... Upon the application of these external influences, a second CO peak emerges, this time centered at integer L-values, evidencing an out-of-plane coupling that locks the phase of adjacent CuO 2 planes. The 3D CO peaks have significantly increased out-of-plane correlation lengths, achieving up to 55Å [24], 61Å [25], and 94Å [26], respectively. All of these 3D CO correlation lengths are still considerably shorter than the typical crystalline c-axis correlation lengths found in this compound. ...
... The XAS reveals two resonances that correspond to the Pr M 5 (∼930 eV) and the Cu L 3 (∼933 eV) edges; the 3D CO scattered intensity also exhibits features corresponding to these two resonances. Unlike in YBCO films with 3D CO [25], we do not observe a significant shift in spectral weight to higher energy that would indicate CO coupling through the CuO chains. Notably, the CO peak resonates more strongly at the Pr edge than at the Cu edge; while it is not straightforward to draw quantitative conclusions from this result, the intense scattering signal nonetheless confirms involvement by the Pr ions in the 3D CO mechanism. ...
The shape of 3$d$-orbitals often governs the electronic and magnetic properties of correlated transition metal oxides. In the superconducting cuprates, the planar confinement of the $d_{x^2-y^2}$ orbital dictates the two-dimensional nature of the unconventional superconductivity and a competing charge order. Achieving orbital-specific control of the electronic structure to allow coupling pathways across adjacent planes would enable direct assessment of the role of dimensionality in the intertwined orders. Using Cu-$L_3$ and Pr-$M_5$ resonant x-ray scattering and first-principles calculations, we report a highly correlated three-dimensional charge order in Pr-substituted YBa$_2$Cu$_3$O$_{7}$, where the Pr $f$-electrons create a direct orbital bridge between CuO$_2$ planes. With this, we demonstrate that interplanar orbital engineering can be used to surgically control electronic phases in correlated oxides and other layered materials.
... Finally, Fig. 4f shows the evolution of the amplitude of the elastic Bragg peak in the RIXS data at x = 0.5 as a function of the incident photon energy (see Supplementary Note 5 and Supplementary Fig. 6 for more details and raw spectra). It exhibits a clear resonance with a maximum slightly below the resonance of the TEY-XAS signal (see Fig. 4d) and significantly below one of the resonance for the Bragg peak at Q || ≈ 0.3 at x = 0.35 22 and in plain YBCO 31 . This trend is consistent with our interpretation that this Cu-d z 2 order originates predominantly from the interfacial CuO 2 planes. ...
The interplay of nearly degenerate orders in quantum materials can lead to a myriad of emergent phases. A prominent case is that of the high- T c cuprates for which the relationship between superconductivity and a short-ranged, incommensurate charge density wave in the CuO 2 planes involving the $$d_{x^2 - y^2}$$ d x 2 − y 2 orbitals (Cu-CDW) is a subject of great current interest. Strong modifications of the strength and coherence of this Cu-CDW have been achieved by applying large magnetic fields, uniaxial pressure, or via the interfacial coupling in cuprate/manganite multilayers. However, such modifications do not alter the dominant orbital character. Here we investigate cuprate/manganite multilayers with resonant inelastic X-ray scattering (RIXS) and show that a new kind of Cu-based density wave order can be induced that has not been previously observed in the cuprates. This order has an unusually small in-plane wave vector in the range of Q || < 0.1 reciprocal lattice units (r.l.u.), a large correlation length of about 40 nm, and a predominant $$d_{z^2}$$ d z 2 orbital character, instead of the typical $$d_{x^2 - y^2}$$ d x 2 − y 2 one. Its appearance is determined by the hole doping of the manganite which is a key parameter controlling the interfacial charge transfer and orbital reconstruction. We anticipate that the observation of a previously unknown type of density wave order at the YBCO interface will allow for fresh perspectives on the enigmatic relation between superconductivity and charge order (CO) in the cuprates.
... An enhancement of the static Cu-CDW order was also observed in thin films of underdoped YBCO, for which a three-dimensional CDW order can develop in the absence of a large magnetic field or external strain [24]. It has been speculated that the interaction with the STO substrate helps to stabilize the Cu-CDW order in these thin films. ...
For a YBa2Cu3O7/Nd0.65(Ca0.7Sr0.3)0.35MnO3 (YBCO/NCSMO) superlattice, we studied with resonant elastic x-ray scattering (REXS) at the Cu L3 edge how the copper sublattice charge density wave (Cu-CDW) order in YBCO is affected by a large magnetic field up to 6.9 T that weakens the CE-type antiferromagnetic (AF) and the charge/orbital (Mn-CO) orders of the manganite in favor of a ferromagnetic state. While a field of only 2 T induces a strong ferromagnetic moment in the manganite, we find that the Bragg peak of the Cu-CDW hardly changes up to 6 T. Moreover, as the magnetic field is further increased to 6.9 T, the Cu-CDW Bragg peak gets suddenly enhanced and broadened, whereas the ferromagnetic moment of the manganite is already saturated. The observed uncorrelated magnetic field dependence of the charge orders in the cuprate and manganite layers suggests that these orders are not directly coupled across the interface. We rather interpret our data in terms of an indirect coupling via the domain boundaries of the Mn-CO and the related disorder and lattice strain. This interpretation is supported by additional studies of the magnetoelectric response, which provide evidence for a crossover in the dynamics of the Mn-CO in the range between 6 and 7 T, from a low-field state with pinned domains to a high-field state with more mobile and flexible domain boundaries. We attribute the concomitant enhancement and broadening of the Cu-CDW Bragg peak to this crossover.
... 23,39) Particularly, even at zero magnetic field, it was found that a long-range three dimensional CDW is induced by uniaxial pressure parallel to the CuO chain, rather than perpendicular to it, suggesting a strong correlation between the CuO chain and the CDWs in YBCO. 50,51) Such a correlation with lattice distortion is also true for the stripe order in the La-based cuprate. 52) By contrast, Bi2201 is a single-CuO 2 -layer compound without a CuO chain, and exhibits no structural transitions. ...
