My thesis presents a resonant inelastic and elastic X-ray scattering (RIXS/REXS) study on epitaxial YBa2Cu3O7/Nd1-x(Ca1-ySry)xMnO3 heterostructures (NYN). It shows that the Cu-charge density wave (CDW) can be strongly manipulated by controlling the hole doping and tolerance factor of Nd1-x(Ca1-ySry)xMnO3, i.e. by changing x and y.
At x=0.35 we observe a quasi-2D Cu-CDW order with dx2-y2 orbital character that resembles the one that is commonly found in underdoped bulk YBCO. The strength of the corresponding Bragg peak at Q||0.3 r.l.u. gets strongly enhanced as the tolerance factor of the manganite layers1 is decreased and its CE-type antiferromagnetic and charge/orbital ordered (COO) is reinforced. Eventually, it becomes even stronger than in bulk YBCO.
For NYN multilayers with x=0.5 we discovered a new kind of Cu-based charge order with a strong Bragg peak at Q||=0.1 r.l.u. and an unusually large correlation length of about 40 nm 3. This Cu-based charge order seems to be of a dz2 type, instead of the usual dx2-y2 type, and persists well above 170K. Its origin is presently not understood, but seems to be rooted in the particular properties of this cuprate/manganite interface. Additional, X-ray absorption spectroscopy (XAS) data reveal indeed that a significant charge transfer and orbital reconstruction occurs at this interface that may exhibit a modulation along the lateral direction.
Finally, we studied how the Cu-CDW order at x=0.35 is affected by a large magnetic field up to 6.9 Tesla. Unlike the COO of the manganite layers, which is strongly suppressed by such a large magnetic field, we find that the Cu-CDW exhibits an anomalous enhancement above 6T 2. Additional magneto-transport measurements indicate that this enhancement of the Cu-CDW is accompanied by a crossover in the dynamical properties of the domain boundaries of the Mn-COO, which become more mobile and flexible above 6 Tesla. Our findings thus point towards an indirect coupling mechanism between the Cu-CDW and Mn-COO that involves the domain boundaries of the Mn-COO2.
While further studies are required to fully understand this interfacial coupling mechanism(s), the possibility of tuning the Cu-CDW holds great prospects for studying its relationship with high temperature superconductivity and hopefully, for future quantum devices.
References
[1] E. Perret et al., Coupled Cu and Mn charge and orbital orders in YBa2Cu3O7/Nd0.65(Ca1-ySry)0.35MnO3 multilayers, Communications Physics, 1, Article number: 45 (2018); DOI: 10.1038/s42005-018-0046-z.
[2] R. Gaina et al., Magnetic-field dependence of the copper charge density wave order in a YBa2Cu3O7/ Nd0.65(Ca0.7Sr0.3)0.35MnO3 superlattice, manuscript under submission
[3] R. Gaina et al., Long-ranged Cu-based order with dz2 orbital character at a YBa2Cu3O7/ manganite interface. npj Quantum Materials 6, 12 (2021); https://doi.org/10.1038/s41535-021-00311-y