4 results
Search Results
Now showing 1 - 4 of 4
- ItemStrong out-of-plane magnetic anisotropy in ion irradiated anatase TiO2 thin films(New York, NY : American Inst. of Physics, 2016) Stiller, M.; Barzola-Quiquia, J.; Esquinazi, P.; Spemann, D.; Meijer, J.; Lorenz, M.; Grundmann, M.The temperature and field dependence of the magnetization of epitaxial, undoped anatase TiO2 thin films on SrTiO3 substrates was investigated. Low-energy ion irradiation was used to modify the surface of the films within a few nanometers, yet with high enough energy to produce oxygen and titanium vacancies. The as-prepared thin film shows ferromagnetism which increases after irradiation with low-energy ions. An optimal and clear magnetic anisotropy was observed after the first irradiation, opposite to the expected form anisotropy. Taking into account the experimental parameters, titanium vacancies as di-Frenkel pairs appear to be responsible for the enhanced ferromagnetism and the strong anisotropy observed in our films. The magnetic impurities concentrations was measured by particle-induced X-ray emission with ppm resolution. They are ruled out as a source of the observed ferromagnetism before and after irradiation.
- ItemOrigin of Terahertz Soft-Mode Nonlinearities in Ferroelectric Perovskites(College Park, Md. : APS, 2021) Pal, Shovon; Strkalj, Nives; Yang, Chia-Jung; Weber, Mads C.; Trassin, Morgan; Woerner, Michael; Fiebig, ManfredSoft modes are intimately linked to structural instabilities and are key for the understanding of phase transitions. The soft modes in ferroelectrics, for example, map directly the polar order parameter of a crystal lattice. Driving these modes into the nonlinear, frequency-changing regime with intense terahertz (THz) light fields is an efficient way to alter the lattice and, with it, the physical properties. However, recent studies show that the THz electric-field amplitudes triggering a nonlinear soft-mode response are surprisingly low, which raises the question on the microscopic picture behind the origin of this nonlinear response. Here, we use linear and two-dimensional terahertz (2D THz) spectroscopy to unravel the origin of the soft-mode nonlinearities in a strain-engineered epitaxial ferroelectric SrTiO3 thin film. We find that the linear dielectric function of this mode is quantitatively incompatible with pure ionic or pure electronic motions. Instead, 2D THz spectroscopy reveals a pronounced coupling of electronic and ionic-displacement dipoles. Hence, the soft mode is a hybrid mode of lattice (ionic) motions and electronic interband transitions. We confirm this conclusion with model calculations based on a simplified pseudopotential concept of the electronic band structure. It reveals that the entire THz nonlinearity is caused by the off-resonant nonlinear response of the electronic interband transitions of the lattice-electronic hybrid mode. With this work, we provide fundamental insights into the microscopic processes that govern the softness that any material assumes near a ferroic phase transition. This knowledge will allow us to gain an efficient all-optical control over the associated large nonlinear effects.
- ItemSuppression of nematicity by tensile strain in multilayer FeSe/SrTiO3 films(College Park, MD : APS, 2023) Lou, Rui; Suvorov, Oleksandr; Grafe, Hans-Joachim; Kuibarov, Andrii; Krivenkov, Maxim; Rader, Oliver; Büchner, Bernd; Borisenko, Sergey; Fedorov, AlexanderThe nematicity in multilayer FeSe/SrTiO3 films has been previously suggested to be enhanced with decreasing film thickness. Motivated by this, there have been many discussions about the competing relation between nematicity and superconductivity. However, the criterion for determining the nematicity strength in FeSe remains highly debated. The understanding of nematicity as well as its relation to superconductivity in FeSe films is therefore still controversial. Here, we fabricate multilayer FeSe/SrTiO3 films using molecular beam epitaxy and study the nematic properties by combining angle-resolved photoemission spectroscopy, Se77 nuclear magnetic resonance, and scanning tunneling microscopy experiments. We unambiguously demonstrate that, near the interface, the nematic order is suppressed by the SrTiO3-induced tensile strain; in the bulk region further away from the interface, the strength of nematicity recovers to the bulk value. Our results not only solve the recent controversy about the nematicity in multilayer FeSe films, but also offer valuable insights into the relationship between nematicity and superconductivity.
- ItemUltrafast structural changes in SrTiO3 due to a superconducting phase transition in a YBa2Cu3O7 top layer(College Park, MD : Institute of Physics Publishing, 2010) Lübcke, A.; Zamponi, F.; Loetzsch, R.; Kämpfer, T.; Uschmann, I.; Große, V.; Schmidl, F.; Köttig, T.; Thürk, M.; Schwoerer, H.; Förster, E.; Seidel, P.; Sauerbrey, R.We investigate the structural response of SrTiO3 when Cooper pairs are broken in an epitaxially grown YBa2Cu3O 7 top layer due to both heating and optical excitation. The crystal structure is investigated by static, temperaturedependent and time-resolved x-ray diffraction. In the static case, a large strain field in SrTiO3 is formed in the proximity of the onset of the superconducting phase in the top layer, suggesting a relationship between both effects. For the time-dependent studies, we likewise find a large fraction of the probed volume of the SrTiO3 substrate strained if the top layer is superconducting. Upon optical breaking of Cooper pairs, the observed width of the rocking curve is reduced and its position is slightly shifted towards smaller angles. The dynamical theory of x-ray diffraction is used to model the measured rocking curves. We find that the thickness of the strained layer is reduced by about 200 nm on a sub-ps to ps timescale, but the strain value at the interface between SrTiO3 and YBa2Cu3O7 remains unaffected. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.