Leibniz Institute for Crystal Growth

MgO single crystals with diameters between 3.5 and 5 mm and lengths up to 40 mm were grown by the optical floating zone technique (OFZ). Despite challenging material properties such as the high melting point of 2825°C, very high evaporation rate and perfect {100} cleaving characteristics, crack-free crystals were grown at high growth rates exceeding 40 mm h-1 and at high thermal gradients. Chemical investigations revealed that the OFZ technique is suitable for the preparation of substrate crystals with a purity of 5N to facilitate the development of novel demonstrator devices based on epitaxially grown thin films. The achieved purity level is improved by more than one order of magnitude if compared to commercial MgO substrate single crystals graded as high purity.

The polar oxide Lithium Niobate Tantalate is probed using time-resolved luminescence spectroscopy with the goal of revealing an initial structural insight into the solid solution by analyzing the spectral properties and dynamics of radiatively decaying self-localization phenomena. A blue-green luminescence band can be induced by ultraviolet nanosecond laser pulses with a temperature-dependent intensity and spectral width, pointing to the radiative decay of optically generated self-trapped excitons as its origin, i.e., electron–hole pairs with strong coupling to either the NbO6- or TaO6-octahedra. The luminescence decay takes place in the microsecond time range and deviates significantly from a single exponential behavior, so the determined lifetime constants of up to ≈70 μs and stretching factors (1/3–1/5) are validated in more detail using alternative evaluation methods. We discuss our findings, considering the interplay of radiative and non-radiative decay channels, the transition from self-trapped to free excitons, and the presence of a structural disorder of the oxygen octahedra in the solid solutions. Overall, our results suggest self-trapped excitons as local probes for an initial structural elucidation and provide essential information about further experimental and theoretical studies on the atomic structure of Lithium Niobate Tantalate, but also for improving the crystal quality in the framework of applications in photonics and quantum optics.

Complementary time-resolved spectroscopies have been applied to study dynamics of molecular-type magnesium-related donors. Large interstate energy gaps of these donors prevent nonradiative decays through a first-order, one-phonon-assisted scattering – the main relaxation mechanism in shallow substitutional donors in low-doped silicon. Analysis reveals very short decay times of the deepest excited states of molecular donors: dephasing within less than 10 ps and relaxation rates above 30 ns −1 . These decays are several times shorter than those observed in single-electron hydrogen-like substitutional donors, but longer than those in helium-like interstitial atomic magnesium centers in silicon. Spectral correlations of temporal dependences of particular transients to the lattice phonon overtones suggest that phonon-assisted electronic scattering contributes also to decoherence of states in these double donors. Such efficient second-order phonon-assisted processes were underestimated for dynamics of deep impurities in semiconductors.

The numerical modelling of the melt flow in Si crystal growth plays an important role for improving the resistivity distribution of crystals grown in industrial processes. However, recent series of experiments have shown that the existing numerical model—a finite volume solver with incompressible laminar approximation of the melt flow—is not always accurate enough to describe the experimental results for 4″ crystals. To improve the simulation results, material properties have been revised. For some of them, such as the Marangoni or thermal expansion coefficients, the literature suggests different values varying by more than a factor of two. Therefore, simulations using different combinations of parameters were run to perform parameter calibration. The study demonstrated that the description of induced heat on the open melting front needs to be modified to obtain the shape of phase boundaries that provides the best agreement to the experiment. It was concluded that new values should be assigned to several material properties in the model, most importantly the Marangoni coefficient M=−1.2·10−4Nm·K, and that an appropriate turbulence model may help to describe the dopant transport more precisely.

Bulk single-crystal ZnGa2O4 samples with varying free-carrier concentrations are investigated using spectroscopic ellipsometry and Raman spectroscopy to explore their vibrational and excitonic properties. The fundamental band gap of inverse spinel ZnGa2O4 is determined to be 4.4±0.05eV, while the normal spinel phase exhibits a band gap of 5.4±0.02eV. For comparison, analysis of an insulating MgGa2O4 sample in the ultraviolet region yields a band gap of 5.64±0.03eV. The high-frequency dielectric constant is measured as ɛ∞=3.67±0.04 for ZnGa2O4 and ɛ∞=3.46±0.08 for MgGa2O4. The analysis of the infrared plasma frequency provides an effective electron mass of m*=(0.20±0.02)me for low carrier concentration. An increase in the effective mass to m*=(0.30±0.03)me at n=2.3×1019cm−3 is observed, reflecting the nonparabolicity of the conduction band. Optical mobility values are found to be consistent with Hall-effect measurements. Infrared phonon observations are in agreement with previous reports, while Raman spectra confirm five Raman-active phonon modes in their expected polarization symmetries. An additional Raman mode at higher frequencies suggests inversion of the spinel structure in all samples. Furthermore, Raman-inactive modes with Fano line shapes are observed at infrared-active frequencies and are attributed to off-stoichiometry in the samples.

We present room-temperature spectroscopy of Tb 3+ :YLF crystals with different doping concentrations between 0.2% and 15% evaluating the potential for laser operation at 415 nm or 435 nm starting from the 5 D 3 level of Tb 3+ . Evaluation of the fluorescence decay curves by the Inokuti-Hirayama model yields a radiative lifetime of the 5 D 3 level of 2.53 ms. The shorter effective lifetime of a Tb 3+ (0.2%):YLF sample of 2.1 ms indicates quenching of the 5 D 3 level by cross relaxation even at such low Tb 3+ doping concentrations. The ratio of the 5 D 3 and 5 D 4 fluorescence yields a branching ratio of the 5 D 3 → 5 D 4 transition of 0.45 in Tb 3+ :YLF, which allows to calculate the stimulated emission cross sections for the potential UV and blue laser transitions 5 D 3 → 7 F J to be on the order of 10 -21 cm 2 for the first time. In pump-probe measurements, we observe gain around 415 nm and 435 nm for an Tb 3+ (0.8%):YLF sample, but no detrimental excited state absorption. However, room-temperature lasing is not achieved for up to 0.87 W of incident cw UV pump power. Our calculations find a Tb 3+ doping concentration of 1.7% to yield an optimized product of doping ion density and quantum efficiency, which maximizes the possible single-pass gain to 1.4% assuming typical laser parameters. This study thus paves the way for future UV to blue lasing of Tb 3+ starting from the 5 D 3 level with further optimized conditions. • Doping-concentration dependent spectroscopy of Tb:YLF was performed under UV excitation • Cross relaxation quenches the 5 D 3 level in Tb:YLF even at 0.2% doping • Radiative lifetime of the 5 D 3 level in Tb:YLF was determined by Inokuti-Hirayama as 2.5 ms • No detrimental excited state absorption around 415 nm and 435 nm results in net gain with emission cross sections around 10 -21 cm 2 • Tb:YLF is a promising material for direct violet-blue laser emission

Pattern formation in spin systems with continuous-rotational symmetry (CRS) provides a powerful platform to study emergent complex magnetic phases and topological defects in condensed-matter physics. However, its understanding and correlation with unconventional magnetic order along with high-resolution nanoscale imaging are challenging. Here, we employ scanning nitrogen vacancy (NV) magnetometry to unveil the morphogenesis of spin cycloids at both the local and global scales within a single ferroelectric domain of (111)-oriented BiFeO3, which is a noncollinear antiferromagnet, resulting in formation of a glassy labyrinthine pattern. We find that the domains of locally oriented cycloids are interconnected by an array of topological defects and exhibit isotropic energy landscape predicted by first-principles calculations. We propose that the CRS of spin-cycloid propagation directions within the (111) drives the formation of the labyrinthine pattern and the associated topological defects such as antiferromagnetic skyrmions. Unexpectedly, reversing the as-grown ferroelectric polarization from [[Formula: see text][Formula: see text][Formula: see text]] to [111] produces a noncycloidal NV image contrast which could be attributed to either the emergence of a uniformly magnetized state or a reversal of the cycloid polarity. These findings highlight that (111)-oriented BiFeO3 is not only important for studying the fascinating subject of pattern formation but could also be utilized as an ideal platform for integrating novel topological defects in the field of antiferromagnetic spintronics.

We present time-domain THz spectroscopy of thin films of the heavy-fermion superconductor ${\mathrm{CeCoIn}}_{5}$. Below the $\ensuremath{\approx}40$ K Kondo coherence temperature, a narrow Drude-like peak forms, as a result of the $f$-orbital--conduction-electron hybridization and the formation of the heavy-fermion state. The complex optical conductivity is analyzed through a Drude model and extended Drude model analysis. Via the extended Drude model analysis, we measure the frequency-dependent scattering rate ($1/\ensuremath{\tau}$) and effective mass (${m}^{*}/{m}_{b}$). This scattering rate shows a linear dependence on temperature, which matches the dependence of the resistivity as expected. Nevertheless, the width of the low-frequency Drude peak itself that is set by the renormalized quasiparticle scattering rate ($1/{\ensuremath{\tau}}^{*}={m}_{b}/{m}^{*}\ensuremath{\tau}$) shows a ${T}^{2}$ dependence. This is the scattering rate that characterizes the relaxation time of the renormalized quasiparticles. This gives evidence for a Fermi liquid state, which in conventional transport experiments is hidden by the strong temperature dependent mass.