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Institute
Surface systems attract great scientific attention due to novel and exotic properties. The atomically structured surfaces lead to a reduced dimensionality which alters electronic correlations, vibrational properties, and their impact on each other. The emerging physical phenomena are not observed for related bulk materials. In this thesis, ordered (sub)monolayers of metal atoms (Au and Sn) on semiconductor substrates (Si(111) and Ge(111)) and ultrathin intermetallic films (CePt5 and LaPt5) on metal substrate (Pt(111)) are investigated by polarized in situ surface Raman spectroscopy. The surface Raman spectra exhibit features of specific elementary excitations like surface phonons and electronic excitations, which are suitable to gain fundamental insights into the surface systems.
The Au-induced surface reconstructions (5x2) and (r3xr3) constitute quasi-one- and two-dimensional Au structures on the Si(111) substrate, respectively. The new reconstruction-related Raman peaks are analyzed with respect to their polarization and temperature behavior. The Raman results are combined with firstprinciples calculations to decide between different proposed structural models. The Au-(5x2)/Si(111) reconstruction is best described by the model of Kwon and Kang, while for Au-(r3xr3)/Si(111) the conjugate honeycomb-chained-trimer model is favored. The Sn-induced reconstructions with 1/3 monolayer on Ge(111) and Si(111) are investigated to reveal their extraordinary temperature behavior. Specific surface phonon modes are identified that are predicted within the dynamical fluctuation model. Contrary to Sn/Si(111), the corresponding vibrational mode of Sn/Ge(111) exhibits a nearly harmonic character. The reversible structural phase transition of Sn/Ge(111) from (r3xr3) to (3x3) is observed, while no phase transition is apparent for Sn/Si(111). Moreover, Raman spectra of the closely related systems Sn-(2r3x2r3)/Si(111) and thin films of a-Sn as well as the clean semiconductor surfaces Si(111)-(7x7) and Ge(111)-c(2x8) are evaluated and compared.
The CePt5/Pt(111) system hosts 4f electrons whose energy levels are modified by the crystal field and are relevant for a description of the observed Kondo physics. In contrast, isostructural LaPt5/Pt(111) has no 4f electrons. For CePt5/Pt(111), distinct Raman features due to electronic Raman scattering can be unambiguously related to transitions between the crystal-field states which are depth-dependent. This assignment is supported by comparison to LaPt5/Pt(111) and group theoretical considerations. Furthermore, the vibrational properties of CePt5 and LaPt5 reveal interesting similarities but also striking differences like an unusual temperature shift of a vibration mode of CePt5, which is related to the influence of 4f electrons.
Perovskite oxides are a very versatile material class with a large variety of outstanding physical properties.
A subgroup of these compounds particularly tempting to investigate are oxides involving high-\(Z\) elements, where spin-orbit coupling is expected to give rise to new intriguing phases and potential application-relevant functionalities. This thesis deals with the preparation and characterization of two representatives of high-\(Z\) oxide sample systems based on KTaO\(_3\) and BaBiO\(_3\).
KTaO\(_3\) is a band insulator with an electronic valence configuration of Ta 5\(d\)\(^0\) . It is shown that by pulsed laser deposition of a disordered LaAlO\(_3\) film on the KTaO\(_3\)(001) surface, through the creation of oxygen vacancies, a Ta 5\(d\)\(^{0+\(\delta\)}\) state is obtained in the upmost crystal layers of the substrate. In consequence a quasi two dimensional electron system (q2DES) with large spin-orbit coupling emerges at the heterointerface. Measurements of the Hall effect establish sheet carrier densities in the range of 0.1-1.2 10\(^{14}\) cm\(^2\), which can be controlled by the applied oxygen background pressure during deposition and the LaAlO\(_3\) film thickness. When compared to the prototypical oxide q2DESs based on SrTiO\(_3\) crystals, the investigated system exhibits exceptionally large carrier mobilities of up to 30 cm\(^2\)/Vs (7000 cm\(^2\)/Vs) at room temperature (below 10 K). Through a depth profiling by photoemission spectra of the Ta 4\(f\) core level it is shown that the majority of the Ta 5\(d\)\(^0\) charge carriers, consisting of mobile and localized electrons, is situated within 4 nm from the interface at low temperatures. Furthermore, the momentum-resolved electronic structure of the q2DES \(buried\) underneath the LaAlO\(_3\) film is probed by means of hard X-ray angle-resolved photoelectron spectroscopy. It is inferred that, due to a strong confinement potential of the electrons, the band structure of the system is altered compared to \(n\)-doped bulk KTO. Despite the constraint of the electron movement along one direction, the Fermi surface exhibits a clear three dimensional momentum dependence, which is related to a depth extension of the conduction channels of at least 1 nm.
The second material, BaBiO\(_3\), is a charge-ordered insulator, which has recently been predicted to emerge as a large-gap topological insulator upon \(n\)-doping. This study reports on the thin film growth of pristine BaBiO\(_3\) on Nb:SrTiO\(_3\)(001) substrates by means of pulsed laser deposition. The mechanism is identified that facilitates the development of epitaxial order in the heterostructure despite the presence of an extraordinary large lattice mismatch of 12 %. At the heterointerface, a structurally modified layer of about 1.7 nm thickness is formed that gradually relieves the in-plane strain and serves as the foundation of a relaxed BBO film. The thereupon formed lattice orders laterally in registry with the substrate with the orientation BaBiO\(_3\)(001)||SrTiO\(_3\)(001) by so-called domain matching, where 8 to 9 BaBiO\(_3\) unit cells align with 9 to 10 unit cells of the substrate. Through the optimization of the deposition conditions in regard to the cation stoichiometry and the structural lattice quality, BaBiO\(_3\) thin films with bulk-like electronic properties are obtained, as is inferred from a comparison of valence band spectra with density functional theory calculations. Finally, a spectroscopic survey of BaBiO\(_3\) samples of various thicknesses resolves that a recently discovered film thickness-controlled phase transition in BaBiO\(_3\) thin films can be traced back to the structural and concurrent stoichiometric modifications occuring in the initially formed lattice on top of the SrTiO\(_3\) substrate rather than being purely driven by the smaller spatial extent of the BBO lattice.
Lattice dynamics and spin-phonon coupling in the multiferroic oxides Eu(1-x)Ho(x)MnO3 and ACrO2
(2019)
The focus of this thesis is the investigation of the lattice dynamics and the coupling of magnetism and phonons in two different multiferroic model systems. The first system, which constitutes the main part in this work is the system of multiferroic manganites RMnO$_{3}$, in particular Eu$_{1-x}$Ho$_{x}$MnO$_{3}$ with $0 \le x \le 0.5$. Its cycloidal spin arrangement leads to the emergence of the ferroelectric polarization via the inverse Dzyaloshinskii-Moriya interaction. This system is special among RMnO$_{3}$ as with increasing Ho content $x$, Eu$_{1-x}$Ho$_{x}$MnO$_{3}$ does not only become multiferroic, but due to the exchange interaction with the magnetic Ho-ion, the spin cycloid (and with it the electric polarization) is also flipped for higher Ho contents. This makes it one of the first compounds, where the cycloidal reorientation happens spontaneously, rather than with the application of external fields.
On the other hand, there is the delafossite ACrO$_{2}$ system. Here, due to symmetry reasons, the spin-spiral pattern can not induce the polarization according to the inverse Dzyaloshinskii-Moriya interaction mechanism. Instead, it is thought that another way of magnetoelectric coupling is involved, which affects the charge distribution in the $d-p$ hybridized orbitals of the bonds.
The lattice vibrations as well as the quasi-particle of the multiferroic phase, the electromagnon, are studied by Raman spectroscopy. Lattice vibrations like the B$_{3g}$(1) mode, which involves vibrations of the Mn-O-Mn bonds modulate the exchange interaction and serve as a powerful tool for the investigation of magnetic correlations effects with high frequency accuracy. Raman spectroscopy acts as a local probe as even local magnetic correlations directly affect the phonon vibration frequency, revealing coupling effects onto the lattice dynamics even in the absence of global magnetic order. By varying the temperature, the coupling is investigated and unveils a renormalization of the phonon frequency as the magnetic order develops. For Eu$_{1-x}$Ho$_{x}$MnO$_{3}$, the analysis of this spin-induced phonon frequency renormalization enables the quantitative determination of the in-plane spin-phonon coupling strengths. This formalism, introduced by Granado et al., is extended here to evaluate the out-of-plane coupling strengths, which is enabled by the identification of a previously elusive feature as a vibrational mode. The complete picture is obtained by studying the lattice- and electromagnon dynamics in the magnetic field.
Further emphasis is put towards the development of the cycloidal spin structure and correlations with temperature. A new model of describing the temperature-dependent behavior of said spin correlations is proposed and can consistently explain ordering phenomena which were until now unaddressed. The results are underscored with Monte Carlo based simulations of the spin dynamics with varying temperature.
Furthermore, a novel effect of a tentative violation of the Raman selection rules in Eu$_{1-x}$Ho$_{x}$MnO$_{3}$ was discovered. While the phonon modes can be separated and identified by their symmetry by choosing appropriate polarization configurations, in a very narrow temperature range, Eu$_{1-x}$Ho$_{x}$MnO$_{3}$ shows an increase of phonon intensities in polarization configurations where they should be forbidden. This is interpreted as a sign of local disorder, caused by 90° domain walls and could be explained within the model framework.
This course of action is followed with the material system of delafossites ACrO$_{2}$. Being a relatively new class of multiferroic materials, the investigations on ACrO$_{2}$ are also of characterizing nature. For this, shell model calculations are performed as a reference to compare the vibrational frequencies obtained by the Raman experiments to. A renormalization of the vibrational frequencies is observed in this system as well and systematically analyzed across the sample series of \textit{A}=Cu, Pd and Ag. Eventually, the effect of applying an external magnetic field is studied. A particularly interesting feature specific for CuCrO$_{2}$ is a satellite peak which appears at lower temperatures. It is presumably related to a deformation of the lattice and therefore going to be discussed in further detail.