Terahertz ellipsometry study of the soft mode behavior in ultrathin SrTiO films

We present a combined study with time-domain terahertz and conventional far-infrared ellipsometry of the temperature dependent optical response of SrTiO₃thin films (82 and 8.5 nm) that are grown by pulsed-laser deposition on (La0.3Sr0.7) (Al0.65Ta0.35)O₃ (LSAT) substrates. We demonstrate that terahertz ellipsometry is very sensitive to the optical response of these thin films, in particular, to the soft mode of SrTiO₃. We show that for the 82 nm film the eigenfrequency of the soft mode is strongly reduced by annealing at 1200 °C, whereas for the 8.5 nm film it is hardly affected. For the latter, after annealing the mode remains at 125 cm⁻¹ at 300 K and exhibits only a weak softening to about 90 cm⁻¹ at 10 K. This suggests that this ultrathin film undergoes hardly any relaxation of the compressive strain due to the LSAT substrate

Granular superconductivity and charge/orbital order in YBa 2 Cu 3 O 7 /manganite trilayers

We studied how the electronic, superconducting, and magnetic properties of YBa2Cu3O7/Nd1−x(Ca1−ySry )xMnO3 multilayers depend on the tolerance factor and the hole doping of the manganite. In particular, we investigated the granular superconducting state and the related magnetic-field-driven insulator-to- superconductor transition that was previously discovered in corresponding multilayers with Pr0.5La0.2Ca0.3MnO3 [B. P. P. Mallett et al., Phys. Rev. B 94, 180503(R) (2016)]. We found that this granular uperconducting state occurs only when the manganite layer is in a charge/orbital ordered and CE-type antiferromagnetic state (Mn-CO/OO). The coupling mechanism underlying this intriguing proximity effect seems to involve the domain boundaries of the Mn-CO/OO and/or the charge disordered regions of the manganite layer that become more numerous as the hole doping is reduced below x = 0.5

Coupled Cu and Mn charge and orbital orders in YBa₂Cu₃O₇/Nd 0.65 (Ca 1-y Sr y ) 0.35 MnO₃ multilayers

The observation of a charge density wave in the underdoped cuprate high Tc superconductors (Cu-CDW) raised a debate about its relationship with superconductivity. In bulk YBa2Cu3O7−δ the Cu-CDW is incipient and mainly pinned by defects. Nevertheless, a large magnetic field can induce a true long-range Cu- CDW order as it suppresses superconductivity. An enhanced Cu-CDW order was also observed in YBa2Cu3O7/La2/3Ca1/3MnO3 multilayers. Here, we show that the magnitude of the Cu-CDW in YBa2Cu3O7−δ / Nd0.65(Ca1-ySry)0.35MnO3 multilayers can be varied by adjusting the strength of the manganite charge and orbital order via the Sr content (tolerance factor). Furthermore, we resolve the reconstruction of the crystal field levels of the interfacial Cu ions that are also affected by the manganite charge and orbital order. This tuneable interfacial coupling and Cu- CDW in YBa2Cu3O7−δ can be used for studying the relationship between the Cu- CDW and superconductivity and, possibly, for inducing new intertwined quantum states

Backfolded acoustic phonons as ultrasonic probes in metal-oxide superlattices

Ultrasonics have been an incisive probe of internal interfaces in a wide variety of systems ranging from stars to solids. For thin-film structures, however, ultrasound is largely ineffective because the signal is dominated by the substrate. Using confocal Raman spectromicroscopy, we show that multiple reflection of sound waves at internal interfaces of a metal-oxide superlattice generates standing waves that are insensitive to the substrate. Such modes had previously been observed only in high-quality superlattices of elemental semiconductors, and their observation in complex metal- oxide heterostructures is testimony to recent progress in this field. We use the high spatial resolution of the Raman microscope to demonstrate the high sensitivity of the mode frequency to atomic-scale thickness variations of the superlattice. Spectroscopy of acoustic standing waves can hence serve as a powerful characterization tool of thin-film structures. In analogy to ultrasound spectroscopy of bulk solids, lineshape analysis of these modes has the potential to yield detailed information about the internal structure of the interfaces as well as the coupling of sound waves to the low- frequency spin, charge, and orbital dynamics in metal-oxide superlattices

Long-ranged Cu-based order with dz2d_{z^2} orbital character at a YBa2Cu3O7/ manganite interface

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-Tc cuprates for which the relationship between superconductivity and a short-ranged, incommensurate charge density wave in the CuO2 planes involving the dx2−y2 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 dz2 orbital character, instead of the typical dx2−y2 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