Transient absorption and photoluminescence are experimentally investigated in the polaronic reference system lithium niobate, LiNbO 3 (LN), with the aim to refine the microscopic model of small polaron dynamics in materials with strong electron-phonon coupling. As a unique feature, our study is performed by using two different spectroscopic methods, in crystals with dopants enhancing photorefraction or damage resistance, and over a broad temperature range from 15-400 K. Although being self-consistent for particular experimental conditions, the hitherto used microscopic polaronic models reveal inconsistencies when applied to this larger data set. We show that comprehensive modeling is unlocked by the inclusion of an additional type of polaronic state with the following characteristics: (i) strongly temperature-and dopantdependent relaxation times, (ii) an absorption feature in the blue-green spectral range, and (iii) a Kohlrausch-Williams-Watts decay shape with a temperature-dependent stretching factor β (T) showing a behavior contrary to that of small, strong-coupling polarons. The hypothesis of self-trapped excitons (STEs, i.e. bound electron-hole pairs strongly coupled to Nb 5+ and O 2− within a niobium-oxygen octahedron) and their pinning on defects as the microscopic origin of these characteristics is supported by a spectroscopic linkage of photoluminescence at low (15 K) and elevated (300 K) temperatures and explains the long-lifetime components in transient absorption as due to pinned STEs.

Magnetite (Fe3O4) is an eligible candidate for magnetic tunnel junctions (MTJs) since it shows a high spin polarization at the Fermi level as well as a high Curie temperature of 585°C. In this study, Fe3O4/MgO/Co-Fe-B MTJs were manufactured. A sign change in the TMR is observed after annealing the MTJs at temperatures between 200°C and 280°C. Our findings suggest an Mg interdiffusion from the MgO barrier into the Fe3O4 as the reason for the change of the TMR. Additionally, different treatments of the magnetite interface (argon bombardment, annealing at 200°C in oxygen atmosphere) during the preparation of the MTJs have been studied regarding their effect on the performance of the MTJs. A maximum TMR of up to -12% could be observed using both argon bombardment and annealing in oxygen atmosphere, despite exposing the magnetite surface to atmospheric conditions before the deposition of the MgO barrier.

Different aspects of ferroelectric LiTaO3 (LT) such as polaronic defects, optical response and electrical conductivity are investigated by the most recent theoretical and experimental approaches. Comparing the results with the state-of-the-art knowledge of the widely studied LiNbO3 (LN), we evaluate the general assumption that there is little difference between the aforementioned properties of LT and LN. First-principles calculations reveal the existence of point defects in LT qualitatively compatible with the polaronic picture established in LN. Though, peculiar differences with respect to the individual binding energies and polaronic deformation can be revealed. Accordingly, (sub-)picosecond transient absorption measurements show pronounced differences in the kinetics in the sub-ps time domain of small polaron formation and, even more pronounced, in the long-term evolution identified with small polaron hopping. In contrast, (sub-)ps transient luminescence, attributed to the relaxation of self-trapped excitons in LN, shows very similar kinetics. Electrical conductivity measurements are performed in air as function of temperature. Up to about 600 °C they demonstrate similar temperature dependence for the two materials, from which rather comparable activation energies can be extracted. However, in the high-temperature range from about 600 °C to 920 °C both materials show noticeable differences. The results suggest that the fundamental microscopic understanding of LN can be in part transferred to LT. However, due to differences in structure, energetic landscape and temperature behavior, discrepancies between the two materials bear a striking potential for novel applications, even at high temperatures.

Iron oxide films were reactively grown on iron buffer films, which were deposited before on MgO(001) substrates to analyze the influence of the initial iron buffer layers on the magnetic properties of the magnetite films. X-ray photoelectron spectroscopy and low energy electron diffraction showed that magnetite films of high crystalline quality in the surface near region were formed by this two-step deposition procedure. The underlying iron film, however, was completely oxidized as proved by x-ray reflectometry and diffraction. The structural bulk quality of the iron oxide film is poor compared to magnetite films directly grown on MgO(001). Although the iron film was completely oxidized, we found drastically modified magnetic properties for these films using the magnetooptic Kerr effect. The magnetite films had strongly increased coercive fields, and their magnetic in-plane anisotropy is in-plane rotated by 458 compared to magnetite films formed directly by one step reactive growth on MgO(001). V C 2015 AIP Publishing LLC.

Femtosecond-pulse-induced (E pump = 2.5 eV) picosecond infrared absorption is studied in the spectral region between 0.30 eV and 1.05 eV in LiNbO 3 :Mg. We find a noninstantaneous mid-infrared absorption peak in the time domain up to 1 ps and a broad-band, long-lived absorption (maximum at 0.85 eV, width ≈ 0.5 eV), for t > 1 ps. The modelling succeeds by considering small Nb 4+ Nb electron polaron formation along the sequence: (i) twophoton injection of hot electron-hole pairs at Nb-O-octahedra, (ii) dissociation and electron cooling by electron-phonon-scattering, and (iii) electron self-localization by strong electronphonon-coupling.

Transient absorption is studied in Fe-doped lithium niobate single crystals with the goal to control and probe a blue absorption feature related with excitonic states bound to Fe Li defect centers. The exciton absorption is deduced from the comparison of ns-pump, supercontinuumprobe spectra obtained in crystals with different Fe-concentration and Fe 2+/3+ Li-ratio, at different pulse peak and photon energies as well as by signal separation taking well-known small polaron absorption bands into account. As a result, a broad-band absorption feature is deduced being characterized by an absorption cross-section of up to σ max (2.85 eV) = (4 ± 2) • 10 −22 m 2. The band peaks at about 2.85 eV and can be reconstructed by the sum of two Gaussians centered at 2.2 eV (width ≈ 0.5 eV) and 2.9 eV (width ≈ 0.4 eV), respectively. The appropriate build-up and decay properties strongly depend on the crystals' composition as well as the incident pulse parameters. All findings are comprehensively analyzed and discussed within the model of Fe 2+ Li − O − − V Li excitonic states.

Based on a model of coupled processes with differently time-dependent decay kinetics we present a critical review on photoluminescence (PL) and transient absorption (TA) experiments in undoped and Mg or Fe-doped LiNbO 3 , together with a comprehensive interpretation of visible radiative and parallel non-radiative decay processes on timescales ranging from 50 ns up to minutes. Analogies and peculiarities of the kinetics of mobile self-trapped and pinned excitons are investigated and compared with those of hopping polarons in the same system. Exciton hopping with an activation energy of ≈0.18 eV is shown to govern the lifetime and quenching of the short PL component above 100 K. Strong interaction between excitons and dipolar pinning defects explains the exorbitant lifetimes and large depinning energies characterizing delayed TA components in doped LiNbO 3 , while restricted hopping of the pinned excitons is proposed to play a role in strongly delayed PL in LiNbO 3 :Mg exhibiting a narrowed emission band due to locally reduced electron-phonon coupling. Atomistic models of pinned excitons are proposed corresponding to charge-compensated dipolar defects predicted by theories of dopant incorporation in LiNbO 3 and are systematically assigned to absorption bands observed near the UV edge. Excitation in these bands is shown to lead directly to pinned exciton states confirming also the previously proposed two-step exciton-decay scenario in LiNbO 3 :Fe. Weak intrinsic sub-80 ns luminescence in congruent LiNbO 3 is explained as an opposite effect of enhanced electron-phonon coupling for excitons pinned on Nb Li antisite defects. The comparison of the different observed stretching behaviors in the paradigmatic system LiNbO 3 provides an intuitive picture of the underlying physical processes. The findings are relevant not only for holographic and non-linear optical applications of LiNbO 3 but are of general interest also for the treatment of stretched exponential or other time-dependent kinetics in complex condensed systems ranging from nanocrystals and polymers to liquids and biophysical systems.

Various manifestations of small polarons strongly affect the linear and nonlinear optical properties of the oxide crystal lithium niobate (LiNbO 3 , Ln). While related transient absorption phenomena in Ln have been extensively studied in recent decades, a sound microscopic picture describing the blue-green (photo)luminescence of lithium niobate single crystals is still missing. in particular, almost nothing is known about: (i) the luminescence build-up and (ii) its room temperature decay. We present here the results of our systematic experimental study using nominally undoped and Mg-doped LN crystals with different Mg concentration. Picosecond luminescence was detected by means of femtosecond fluorescence upconversion spectroscopy (FLUPS) extended to the inspection of oxide crystals in reflection geometry. Two distinct luminescence decay components on the picosecond time scale are revealed. While a short exponential decay is present in each sample, a longer non-exponential decay clearly depends on the crystal composition. Since transient absorption spectroscopy excludes geminate small polaron annihilation as microscopic cause of the luminescence, both decay components are discussed in the context of self-trapped exciton (Ste) transport and decay. ABO 3 perovskite-like ferroelectrics exhibiting unique electronic properties 1 are important materials for optical frequency converters 2 , THz pulse generation 3 or promise advances in the fields of photovoltaics 4 , oxide electronics 5 and electroluminescent devices 6. In these materials, strong charge carrier-phonon coupling often lead to pronounced photo-induced polaronic effects manifesting in, e.g., laser-induced bulk damage 7,8 , and broad photoluminescence bands emitted from BO 6 octahedra 9-11. Lithium niobate is a frequently studied example of ferroelectric perovskite-like oxides, known for its rich defect structure leading to a conglomeration of intrinsic and extrinsic small polarons 7. Whereas light-induced phenomena in LN, such as transient absorption, have been extensively studied in recent decades relating them with formation and transport of small polarons 12-23 , a sound microscopic picture describing the blue-green (photo)luminescence of lithium niobate single crystals is still missing. While recently geminate small polaron annihilation has been proposed to lead to a two component luminescence decay in Mg-doped lithium niobate at low temperatures 24 , Messerschmidt et al. showed that low temperature luminescence and absorption decay on very different time scales 25. They revived the idea of radiatively decaying self-trapped excitons located at niobium-oxygen octahedra, which has been introduced over thirty years ago in the pioneering work of Blasse et al. 9,26-29. They concluded that an effect of STE formation on the light-matter interaction at elevated temperatures could per se no longer be neglected. In fact, no absorption feature of intrinsic self-trapped excitons has been reported so far, while Messerschmidt et al. related long-lived STEs pinned on ...