Doctoral thesis

Phononic and electronic excitations in complex oxides studied with advanced Infrared and Raman spectroscopy techniques


  • Fribourg, Switzerland : [éditeur non identifié], 2021

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Thèse: Université de Fribourg (Suisse), 2021

English Complex oxides with perovskite structure exhibit a rich spectrum of exotic and emergent phases, including superconductivity, spin and charge ordering, ferroelectricity and flexoelectricity. Interestingly, several of these orders can coexist within the same material. A certain phase can be favored over the others by exposing the material to external stimuli. Decreasing the spatial dimensions in these materials to the atomic level, e.g. in the form of thin films, provides an additional control parameter. In this thesis, several oxide-based materials, including the high-temperature superconductor YBa2Cu3O6+x (YBCO) in the form of bulk crystals and as thin films in heterostructures, and the dielectrics SrTiO3 and KTaO3, have been investigated. The measurements were performed as a function of various external stimuli, such as high magnetic or electric fields, temperature and UV-light illumination. Using advanced infrared and Raman spectroscopy techniques, novel experimental results have been obtained on the underlying physical mechanisms that lead to the interesting effects in these materials, which can be divided into three main topics:

I. Magnetic field effect on the optical response of an underdoped YBa2Cu3O6.6 single crystal
The cuprate high-temperature superconductors are among the most intensively studied oxidebased materials. Even though they were discovered almost 35 years ago, there is still no consensus about the underlying pairing mechanism. Therefore, it is necessary to further investigate the cuprates’ ground state orders and reveal their interplay. Special focus is drawn to the recently discovered three-dimensional (3D) copper charge density wave (Cu-CDW). Having its maximum in the underdoped region of the phase diagram, this 3D CDW was shown to develop at high magnetic fields above 15-20 Tesla, or when applying uniaxial pressure along the system’s a-axis. Here, the impact of a magnetic field of up to 30 Tesla on the infrared spectrum of an underdoped, twinned YBa2Cu3O6.6 single crystal with Tc = 58(1) Kelvin was studied. The magnetic field was applied along the c-axis while the in-plane reflectivity was detected from 40 cm-1 to 6000 cm-1. It was found that a magnetic field in excess of 20 Tesla suppresses the macroscopic response of the superconducting condensate. At the same time, surprisingly weak signatures of the 3D CDW have been observed in the form of a weak reduction of the Drude response which is compensated by an enhancement of the mid-infrared band and a narrow electronic mode around 240 cm-1 (and possibly a second one around 90 cm-1). The latter modes were interpreted as the phasons of the CDW along the b- and a-axis, respectively. These findings suggest that short-ranged and slowly fluctuating CDW correlations are an intrinsic property of the YBCO crystal and are prominent already at zero magnetic field.

II. Observation of backfolded acoustic phonons in metal-oxide heterostructures
The quality control of thin film structures is a fundamental necessity for exploring the phenomena in such systems. Therefore, it is important to improve existing technologies as well as to find new approaches for studying thin films. In this thesis, a promising ultrasonic probe was demonstrated in the form of backfolded acoustic phonons, which were observed for the first time in metal-oxide superlattices with Raman spectroscopy. These low-frequency modes originate from the reflection of sound waves at internal interfaces of the superlattice. They are independent of the substrate properties and their frequency is highly sensitive to atomic-scale thickness variations of the superstructure. These modes can hence serve as a powerful characterization tool for thin film structures. In addition, a lineshape analysis of these superlattice modes can potentially yield further information about the coupling of sound waves to the low-energy spin, charge and orbital excitations.

III. Electric Field effect on SrTiO3- and KTaO3-based heterostructures
Strontium titanate (SrTiO3) has been one of the most studied perovskite oxides for decades, and yet new exotic phenomena and functionalities are still being discovered today. Currently, heterostructures of AlOx/SrTiO3 or LaAlO3/SrTiO3, which feature the formation of a twodimensional electron gas at their interface, are heavily investigated. Especially interesting is the electric field effect, which can drive the system e.g. from an insulating to a metallic or even a superconducting state. As part of this dissertation, back-gate voltages of up to 8 kV/cm were applied to heterostructures based on SrTiO3 and KTaO3 while investigating them using a combination of infrared ellipsometry and confocal Raman spectroscopy. We have observed anomalous polarization effects that are highly asymmetric and non-collinear with respect to the applied field. These polarization effects show a high remanence and hysteresis, and are strongly surface-sensitive. We have shown that these observations originate from the interplay of the oxygen vacancies and the tetragonal domain boundaries of SrTiO3 below 110 Kelvin, which are absent in KTaO3. In addition, we demonstrated that radiating UV light onto the sample surface suppresses these anomalous polarization effects. With this, we found an optical switch that allows to restore the original polarization state of the system.

The here presented results have implication for a broad field of ongoing research in solid state physics. They contribute in understanding the macroscopic effects in complex oxides, which might play a crucial role in future quantum technology. Moreover, the state of the art capabilities of optical spectroscopy were presented as a suitable characterization method in materials science.
Faculté des sciences et de médecine
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  • Ressource en ligne consultée le 2022-01-11
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