Anisotropy of infrared-active phonon modes in the monodomain state of tetragonal SrTiO3{\mathrm{SrTiO}}_{3} (110)

With infrared (IR) and terahertz (THz) ellipsometry we investigated the anisotropy of the IR-active phonon modes in ${\mathrm{SrTiO}}_{3}$ (110) single crystals in the tetragonal state below the so-called antiferrodistortive transition at T∗=105 K. In particular, we show that the anisotropy of the oscillator strength of the so-called R mode, which becomes weakly IR active below T∗, is a valuable indicator of the orientation of the structural domains. Our results reveal that a monodomain state with the tetragonal axis (c axis) parallel to the [001] direction can be created by applying a moderate uniaxial stress of about 2.3 MPa along the [1-10] direction (with a simple mechanical clamp). The resulting splitting of the IR-active phonon modes is reported

Infrared study of the spin reorientation transition and its reversal in the superconducting state in underdoped Ba1−xKxFe2As2{\mathrm{Ba}}_{1-x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}

With infrared spectroscopy we investigated the spin-reorientation transition from an orthorhombic antiferromagnetic (o-AF) to a tetragonal AF (t-AF) phase and the reentrance of the o-AF phase in the superconducting state of underdoped Ba1−xKxFe2As2. In agreement with the predicted transition from a single-Q to a double-Q AF structure, we found that a distinct spin density wave develops in the t-AF phase. The pair breaking peak of this spin density wave acquires much more low-energy spectral weight than the one in the o-AF state which indicates that it competes more strongly with superconductivity. We also observed additional phonon modes in the t-AF phase which likely arise from a Brillouin-zone folding that is induced by the double-Q magnetic structure with two Fe sublattices exhibiting different magnitudes of the magnetic moment

Infrared ellipsometry study of the confined electrons in a high-mobility <i>γ</i>-Al₂O₃/SrTiO₃ heterostructure

With infrared ellipsometry we studied the response of the confined electrons in γ-Al₂O₃/SrTiO₃ (GAO/STO) heterostructures in which they originate predominantly from oxygen vacancies. From the analysis of a so-called Berreman mode, that develops near the highest longitudinal optical phonon mode of SrTiO₃, we derive the sheet carrier density, N s , the mobility, μ, and the depth profile of the carrier concentration. Notably, we find that N s and the shape of the depth profile are similar as in LaAlO₃/SrTiO₃ (LAO/STO) heterostructures for which the itinerant carriers are believed to arise from a polar discontinuity. Despite an order of magnitude higher mobility in GAO/STO, as obtained from transport measurements, the derived mobility in the infrared range exhibits only a twofold increase. We interpret this finding in terms of the polaronic nature of the confined charge carriers in GAO/STO and LAO/STO which leads to a strong, frequency-dependent interaction with the STO phonons

Signatures of the bonding-antibonding splitting in the cc-axis infrared response of moderately underdoped bilayer and trilayer cuprate superconductors

We report on results of our analysis of the c-axis infrared conductivity, σc(ω), of bilayer LnBa2Cu3O7−δ (Ln=La, Nd, Y) and trilayer Bi2Sr2Ca2Cu3O10+δ high-Tc superconductors. The analysis employs the multilayer model involving the conductivity of the bilayer or trilayer unit, σbl(ω), and that of the spacing layers separating the latter units, σint(ω). For the YBa2Cu3O7−δ sample with concentration of holes p=0.09, our fitting of the data strongly suggests that at low temperatures, the conductivity σbl(ω) possesses a pronounced and narrow Drude peak. For samples with p≥0.115 however, the fitting indicates that σbl(ω) is, at low temperatures, dominated by a mode at a finite energy in the range from 30 to 60 meV. The properties of this resonance are in accord with those of a collective mode that appears in the spectra of σbl(ω) calculated using a microscopic gauge-invariant theory of σc(ω) by J. Chaloupka and coworkers [Phys. Rev. B 79, 184513 (2009)]. The frequency and spectral weight of the latter mode are determined by the magnitude of the splitting between the bonding and the antibonding band of the bilayer or trilayer unit. Our results, in conjunction with the microscopic theory, thus demonstrate that in moderately underdoped bilayer and trilayer high-Tc cuprates the bilayer (or trilayer) splitting is already developed. The observed doping dependence is consistent with results from angular resolved photoemission spectroscopy

Infrared ellipsometry study of photogenerated charge carriers at the (001) and (110) surfaces of SrTiO3\mathrm{SrTi}{\mathrm{O}}_{3} crystals and at the interface of the corresponding LaAlO3/SrTiO3\mathrm{LaAl}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3} heterostructures

With infrared (IR) ellipsometry and dc resistance measurements, we investigated the photodoping at the (001) and (110) surfaces of SrTiO3 (STO) single crystals and at the corresponding interfaces of LaAlO3/SrTiO3 (LAO/STO) heterostructures. In the bare STO crystals, we find that the photogenerated charge carriers, which accumulate near the (001) surface, have a similar depth profile and sheet carrier concentration as the confined electrons that were previously observed in LAO/STO (001) heterostructures. A large fraction of these photogenerated charge carriers persist at low temperature at the STO (001) surface even after the ultraviolet light has been switched off again. These persistent charge carriers seem to originate from oxygen vacancies that are trapped at the structural domain boundaries, which develop below the so-called antiferrodistortive transition at T∗=105K. This is most evident from a corresponding photodoping study of the dc transport in STO (110) crystals for which the concentration of these domain boundaries can be modified by applying a weak uniaxial stress. The oxygen vacancies and their trapping by defects are also the source of the electrons that are confined to the interface of LAO/STO (110) heterostructures, which likely do not have a polar discontinuity as in LAO/STO (001). In the former, the trapping and clustering of the oxygen vacancies also has a strong influence on the anisotropy of the charge carrier mobility. We show that this anisotropy can be readily varied and even inverted by various means, such as a gentle thermal treatment, UV irradiation, or even a weak uniaxial stress. Our experiments suggest that extended defects, which develop over long time periods (of weeks to months), can strongly influence the response of the confined charge carriers at the LAO/STO (110) interface

X-ray absorption study of the ferromagnetic Cu moment at the YBa2Cu3O7/La2/3Ca1/3MnO3{\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}/{\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_{3} interface and variation of its exchange interaction with the Mn moment

With x-ray absorption spectroscopy and polarized neutron reflectometry we studied how the magnetic proximity effect at the interface between the cuprate high-TC superconductor YBa2Cu3O7 (YBCO) and the ferromagnet La2/3Ca1/3MnO3 (LCMO) is related to the electronic and magnetic properties of the LCMO layers. In particular, we explored how the magnitude of the ferromagnetic Cu moment on the YBCO side depends on the strength of the antiferromagnetic (AF) exchange coupling with the Mn moment on the LCMO side. We found that the Cu moment remains sizable if the AF coupling with the Mn moments is strongly reduced or even entirely suppressed. The ferromagnetic order of the Cu moments thus seems to be intrinsic to the interfacial CuO2 planes and related to a weakly ferromagnetic intraplanar exchange interaction. The latter is discussed in terms of the partial occupation of the Cu 3d3z2−r2 orbitals, which occurs in the context of the so-called orbital reconstruction of the interfacial Cu ions

Příprava dopovaných SnO2 tenkých vrstev ze SnCl2 a SnCl4

Thin layer electrodes based on SnO2 were prepared by two methods (sputtering and spray deposition ) and their electrical and electrochemical properties compared

Electric-Field-Induced Polar Order and Localization of the Confined Electrons in LaAlO₃/SrTiO₃ Heterostructures

With ellipsometry, x-ray diffraction, and resistance measurements we investigated the electric-field effect on the confined electrons at the LaAlO₃/SrTiO₃ interface. We obtained evidence that the localization of the electrons at negative gate voltage is induced, or at least enhanced, by a polar phase transition in SrTiO₃ which strongly reduces the lattice polarizability and the subsequent screening. In particular, we show that the charge localization and the polar order of SrTiO₃ both develop below ∼50  K and exhibit similar, unipolar hysteresis loops as a function of the gate voltage