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

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

Superconductivity and charge-carrier localization in ultrathin La1.85Sr0.15CuO4/La2CuO4{\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_{4}/{\mathrm{La}}_{2}{\mathrm{CuO}}_{4} bilayers

La1.85Sr0.15CuO4/La2CuO4 (LSCO15/LCO) bilayers with a precisely controlled thickness of N unit cells (UCs) of the former and M UCs of the latter ([LSCO15_N/LCO_M]) were grown on (001)-oriented SrLaAlO4 (SLAO) substrates with pulsed laser deposition (PLD). X-ray diffraction and reciprocal space map (RSM) studies confirmed the epitaxial growth of the bilayers and showed that a [LSCO15_2/LCO_2] bilayer is fully strained, whereas a [LSCO15_2/LCO_7] bilayer is already partially relaxed. The in situ monitoring of the growth with reflection high energy electron diffraction (RHEED) revealed that the gas environment during deposition has a surprisingly strong effect on the growth mode and thus on the amount of disorder in the first UC of LSCO15 (or the first two monolayers of LSCO15 containing one CuO2 plane each). For samples grown in pure N2O gas (growth type B), the first LSCO15 UC next to the SLAO substrate is strongly disordered. This disorder is strongly reduced if the growth is performed in a mixture of N2O and O2 gas (growth type A). Electric transport measurements confirmed that the first UC of LSCO15 next to the SLAO substrate is highly resistive and shows no sign of superconductivity for growth type B, whereas it is superconducting for growth type A. Furthermore, we found, rather surprisingly, that the conductivity of the LSCO15 UC next to the LCO capping layer strongly depends on the thickness of the latter. A LCO capping layer with 7 UCs leads to a strong localization of the charge carriers in the adjacent LSCO15 UC and suppresses superconductivity. The magnetotransport data suggest a similarity with the case of weakly hole doped LSCO single crystals that are in a so-called ‘cluster-spin-glass state.’ We discussed several mechanisms that could lead to such a localization of holes that are embedded in a short-range ordered antiferromagnetic background for the case of a thick LCO capping layer with M=7 but not for a thin one with M=2

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

Terahertz vortex beam as a spectroscopic probe of magnetic excitations

Circularly polarized light with spin angular momentum is one of the most valuable probes of magnetism. We demonstrate that light beams with orbital angular momentum (OAM), or vortex beams, can also couple to magnetism exhibiting dichroisms in a magnetized medium. Resonant optical absorption in a ferrimagnetic crystal depends strongly on both the handedness of the vortex and the direction of the beam propagation with respect to the sample magnetization. This effect exceeds the conventional dichroism for circularly polarized light. Our results demonstrate the high potential of the vortex beams with OAM as a new spectroscopic probe of magnetism in matter

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

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

Temperature-driven topological phase transition and intermediate Dirac semimetal phase in ZrTe5{\mathrm{ZrTe}}_{5}

We present an infrared spectroscopy study of ZrTe5, which confirms a recent theoretical proposal that this material exhibits a temperature-driven topological quantum phase transition from a weak to a strong topological insulating state with an intermediate Dirac semimetal state around Tp ≃ 138 K. Our study details the temperature evolution of the energy gap in the bulk electronic structure. We found that the energy gap closes around Tp, where the optical response exhibits characteristic signatures of a Dirac semimetal state, i.e., a linear frequency-dependent optical conductivity extrapolating to the origin (after subtracting a weak Drude response). This finding allows us to reconcile previous diverging reports about the topological nature of ZrTe5 in terms of a variation of Tp that depends on the crystal growth condition

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

Optimized calibration and measurement procedures in rotating analyzer and rotating polarizer ellipsometry

Accurate spectroellipsometric (SE) measurements in the rotating analyzer (RAE) or rotating polarizer (RPE) configurations require accurate values of the polarizer/analyzer(/retarder) azimuths. While the readings are usually fairly accurate, true values are influenced by possible offsets between the plane of incidence, physical axes of the elements, and the instrument scales. The offsets are often determined by specialized calibration procedures. We describe SE measurements designed to obtain the calibration parameters together with the target ellipsometric spectra. We use multiple settings of the polarizer (analyzer) azimuths in RAE (RPE), respectively, to optimize precision and accuracy of SE measurements, and to economize measurement time. The optimization concerns the choice of measurement parameters as well as the subsequent data analysis. We present in detail examples of visible-ultraviolet measurements