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The development of defect analysis for inorganic semiconductors in the past century paved the way for the success story of today’s electronics. By analogy, defect analysis plays a critical role in ...

We present an experimental and theoretical study which compares the phonon anomalies and the electronic gap features in the infrared response of the weakly coupled two-leg-ladders in Sr 14 − x Ca x Cu24O41 (SCCO) with those of the underdoped high-T c superconductor YBa2Cu3O 6 + x (YBCO) and thereby reveals some surprising analogies. Specifically, we present a phenomenological model that describes the anomalous doping- and temperature-dependence of some of the phonon features in the a-axis response (field along the rungs of the ladders) of SCCO. It assumes that the phonons are coupled to charge oscillations within the ladders. Their changes with decreasing temperature reveal the formation of a crystal (density wave) of hole pairs that are oriented along the rungs. We also discuss the analogy to a similar model that was previously used to explain the phonon anomalies and an electronic plasma mode in the c-axis response (field perpendicular to the CuO2 planes) of YBCO. We further confirm that an insulator-like pseudogap develops in the a-axis conductivity of SCCO which closely resembles that in the c-axis conductivity of YBCO. Most surprisingly, we find that the c-axis conductivity (field along the legs of the ladders) of SCCO is strikingly similar to the in-plane one (field parallel to the CuO2 planes) of YBCO. Notably, in both cases a dip feature develops in the normal state spectra that is connected with a spectral weight shift toward low frequencies and can thus be associated with precursor superconducting pairing correlations that are lacking macroscopic phase coherence. This SCCO–YBCO analogy indicates that collective degrees of freedom contribute to the low-energy response of underdoped high T c cuprates and it even suggests that the charges in the CuO2 planes tend to segregate forming quasi-one-dimensional structures similar to the two-leg ladders, as predicted for the stripe-scenario or certain intertwinned states.

We have grown La$_{0.3}$Sr$_{0.7}$CoO$_{3-δ}$ thin films by pulsed laser deposition on (LaAlO$_3$)$_{0.3}$(Sr$_2$TaAlO$_3$)$_{0.7}$ substrates with and without a protective LaAlO$_3$ capping layer and investigated their structural and magnetic properties. We have observed that, in the uncapped films, the Curie temperature strongly decreased after annealing in helium atmosphere, and it significantly decreased even in samples stored for several weeks at room temperature. The decrease of the Currie temperature is caused by an increase of the concentration of oxygen vacancies, δ. However, we show that already a 3 nm thin LaAlO$_3$ capping layer can essentially conserve δ at room temperature, and it considerably slows down the formation of oxygen vacancies at elevated temperatures.

We proposed and demonstrated a preparation method of (001) preferentially oriented stress-free AlN piezoelectric thin films. The AlN thin films were deposited by a reactive sputtering technique at substrate temperatures up to 330 °C using a dual Kaufman ion-beam source setup. We deposited the AlN on Si (100), Si (111), amorphous SiO2, and a (001) preferentially oriented Ti thin film and compared their crystallographic, optical, and piezoelectric properties. The AlN thin films deposited on the (001) preferentially oriented Ti thin films have the highest crystallographic quality. The stress-free AlN reached a high value of the piezoelectric coefficient d33 = (7.33 ± 0.08) pC·N−1. The properties of the AlN thin film prepared at such low temperatures are suitable for numerous microelectromechanical systems, piezoelectric sensors, and actuators monolithically integrated with complementary metal-oxide-semiconductor signal-processing circuits.

We studied the infrared response of polycrystalline samples of the iron arsenide superconductor $(\mathrm{Rb},\mathrm{Cs}){\mathrm{Ca}}_{2}{\mathrm{Fe}}_{4}{\mathrm{As}}_{4}{\mathrm{F}}_{2}$ (Rb,Cs-12442), which has a bilayer structure similar to the high-${T}_{c}$ cuprates ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ (YBCO) and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$. The $c$-axis reflectivity spectra ${R}_{c}$ have been derived from the reflectivity spectra of the polycrystalline samples ${R}_{\mathrm{poly}}$ and the in-plane spectrum of a corresponding Cs-12442 crystal ${R}_{ab}$ using a geometrical averaging approach with ${R}_{c}=3{R}_{\mathrm{poly}}\ensuremath{-}2{R}_{ab}$. In analogy to the $c$-axis response of the cuprates, we observe a superconductivity-induced transverse plasma mode and a phonon anomaly that are both signatures of local electric field effects that arise from a large difference between the local conductivities in the intra- and interbilayer regions. Using a multilayer model developed for the cuprates, we obtain a good description of the $c$-axis response and derive the local conductivities at $T\ensuremath{\simeq}{T}_{c}$ of ${\ensuremath{\sigma}}_{1}^{\mathrm{bl}}(\ensuremath{\omega}\ensuremath{\rightarrow}0)\ensuremath{\simeq}1000{\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}{\mathrm{cm}}^{\ensuremath{-}1}$ and ${\ensuremath{\sigma}}_{1}^{\mathrm{int}}(\ensuremath{\omega}\ensuremath{\rightarrow}0)\ensuremath{\simeq}15{\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}{\mathrm{cm}}^{\ensuremath{-}1}$, respectively, that are similar to the ones previously found in underdoped YBCO. Different from the cuprates, we find no evidence of a normal-state pseudogap in terms of a partial suppression of the low-energy electronic states that sets in already well above ${T}_{c}$. There is also no clear sign of an onset of precursor superconducting pairing correlations well above ${T}_{c}\ensuremath{\simeq}$ 30 K. This highlights that the pseudogap and the precursor superconducting pairing well above ${T}_{c}$ are unique features of the cuprates with their strong electronic correlations and, for example, not just the result of a strongly anisotropic electronic response due to the layered crystal structure.

We present the design and performance of a vacuum far-infrared (similar to 50-680 cm(-1)) ellipsometer witha rotating analyser. The system is based on a Fourier transform spectrometer, an in-house built ellipsometer chamber and a closed-cycle bolometer. The ellipsometer chamber is equipped with a computer controlled theta-2 theta goniometer for automated measurements at various angles of incidence. We compare our measurements on SrTiO3 crystal with the results acquired above 300 cm(-1) with a commercially available ellipsometer system. After the calibration of the angle of incidence and after taking into account the finite reflectivity of mirrors in the detector part we obtain a very good agreement between the data from the two instruments. The system can be supplemented with a closed-cycle He cryostat for measurements between 5 and 400 K. (C) 2016 Published by Elsevier B.V.

We report on results of our analysis of the c-axis infrared conductivity,sigma(c )(omega), of bilayer LnBa(2)Cu(3)O(7-delta) (Ln=La, Nd, Y) and trilayer Bi2Sr2Ca2Cu3O10+delta high-T-c superconductors. The analysis employs the multilayer model involving the conductivity of the bilayer or trilayer unit, sigma(bl) (omega), and that of the spacing layers separating the latter units, sigma(int) (omega). For the YBa2Cu3O7-delta sample with concentration of holes p = 0.09, our fitting of the data strongly suggests that at low temperatures, the conductivity sigma(bl)(omega) possesses a pronounced and narrow Drude peak. For samples with p >= 0.115 however, the fitting indicates that sigma(bl)(omega) 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 sigma(bl)(omega) calculated using a microscopic gauge-invariant theory of sigma(c)(omega) 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-T-c cuprates the bilayer (or trilayer) splitting is already developed. The observed doping dependence is consistent with results from angular resolved photoemission spectroscopy.

We present results of our broadband ellipsometry measurements of the optical response of ferromagnetic La0.7Sr0.3CoO3. Our data show that the ferromagnetic transition is accompanied by a transfer of optical spectral weight from an absorption band centered at 1.5 eV to a narrow component of the Drude-like peak. The associated reduction of the intraband kinetic energy is significantly larger than k(B)T(c), confirming that the double exchange plays a major role in the ferromagnetism of doped cobaltites. In conjunction with results of recent theoretical studies, the temperature dependence of the Drude-like peak suggests that the double exchange is mediated by t(2g) orbitals.

We have investigated the optical properties of thin films of topological insulators Bi2Te3, Bi2Se3, and their alloys Bi-2(Te1-x Se-x)(3) on BaF2 substrates by a combination of infrared ellipsometry and reflectivity in the energy range from 0.06 to 6.5 eV. For the onset of interband absorption in Bi2Se3, after the correction for the Burstein-Moss effect, we find the value of the direct band gap of 215 +/- 10 meV at 10 K. Our data support the picture that Bi2Se3 has a direct band gap located at the Gamma point in the Brillouin zone and that the valence band reaches up to the Dirac point and has the shape of a downward-oriented paraboloid, i.e., without a camel-back structure. In Bi2Te3, the onset of strong direct interband absorption at 10 K is at a similar energy of about 200 meV, with a weaker additional feature at about 170 meV. Our data support the recent GW band-structure calculations suggesting that the direct interband transition does not occur at the Gamma point but near the Z-F line of the Brillouin zone. In the Bi-2(Te1-x Se-x)(3) alloy, the energy of the onset of direct interband transitions exhibits a maximum near x = 0.3 (i.e., the composition of Bi2Te2Se), suggesting that the crossover of the direct interband transitions between the two points in the Brillouin zone occurs close to this composition.

The work at the University of Fribourg was supported by the Schweizerische Nationalfonds (SNF) through Grant No. 200020-153660. B.P.P.M. wishes to acknowledge support from the Marsden Fund of New Zealand. The work at MUNI was financially supported by the Ministry of education youth and sports of the Czech Republic, under the project CEITEC 2020 (LQ1601). M.S., F.S., and G.H. acknowledge the support by the Spanish Government through Project No. MAT2014-56063-C2-1-R, the Severo Ochoa Grant No. SEV-2015-0496, and the Generalitat de Catalunya (Project No. 2014SGR 734). J. Mannhart is acknowledged for providing the LAO/STO (001) sample and J. Foncuberta for scientific discussion.

With infrared (IR) ellipsometry and DC resistance measurements we investigated the photo-doping at the (001) and (110) surfaces of SrTiO$_3$ (STO) single crystals and at the corresponding interfaces of LaAlO$_3$/SrTiO$_3$ (LAO/STO) heterostructures. In the bare STO crystals we find that the photo-generated 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 photo-generated charge carriers persist at low temperature at the STO (001) surface even after the UV 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* = 105 K. This is most evident from a corresponding photo-doping 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., Comment: 12 pages, 6 figures, 4 tables

We present broad-band infrared ellipsometry measurements of the c -axis dielectric response of underdoped YBa 2 Cu 3 O 7− d single crystals. Our data provide a clear spectroscopic distinction between the normal-state pseudogap and the superconducting gap. In particular, they establish that different energy scales are underlying the respective redistributions of spectral weight. Furthermore, our data are suggestive of a mutual competition between the two gaps and thus of an extrinsic nature of the pseudogap with respect to superconductivity.

Electronic devices that use the spin degree of freedom hold unique prospects for future technology. The performance of these 'spintronic' devices relies heavily on the efficient transfer of spin polarization across different layers and interfaces. This complex transfer process depends on individual material properties and also, most importantly, on the structural and electronic properties of the interfaces between the different materials and defects that are common to real devices. Knowledge of these factors is especially important for the relatively new field of organic spintronics, where there is a severe lack of suitable experimental techniques that can yield depth-resolved information about the spin polarization of charge carriers within buried layers of real devices. Here, we present a new depth-resolved technique for measuring the spin polarization of current-injected electrons in an organic spin valve and find the temperature dependence of the measured spin diffusion length is correlated with the device magnetoresistance.

We performed calculations of the in-plane infrared response of underdoped cuprate superconductors to clarify the origin of a characteristic dip feature which occurs in the published experimental spectra of the real part of the in-plane conductivity below an onset temperature ${{T}^{{\rm ons}}}$ considerably higher than ${{T}_{{\rm c}}}$. We provide several arguments, based on a detailed comparison of our results with the published experimental data, confirming that the dip feature and the related features of the memory function $M(\omega )={{M}_{1}}(\omega )+{\rm i}{{M}_{2}}(\omega )$ (a peak in M1 and a kink in M2) are due to superconducting pairing correlations that develop below ${{T}^{{\rm ons}}}$. In particular, we show that (i) the dip feature, the peak and the kink of the low-temperature experimental data can be almost quantitatively reproduced by calculations based on a model of a d-wave superconductor. The formation of the dip feature in the experimental data below ${{T}^{{\rm ons}}}$ is shown to be analogous to the one occurring in the model spetra below ${{T}_{{\rm c}}}$. (ii) Calculations based on simple models, for which the dip in the temperature range from ${{T}_{{\rm c}}}$ to ${{T}^{{\rm ons}}}$ is unrelated to superconducting pairing, predict a shift of the onset of the dip at the high-energy side upon entering the superconducting state, that is not observed in the experimental data; (iii) the conductivity data in conjunction with the recent photoemission data (Reber et al 2012 Nat. Phys. 8 606, Reber et al 2013 Phys. Rev. B 87 060506) imply the persistence of the coherence factor characteristic of superconducting pairing correlations in a range of temperatures above ${{T}_{{\rm c}}}$.

With infrared ellipsometry we studied the response of the confined electrons in a $��$-Al$_2$O$_3$/SrTiO$_3$ 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 SrTiO$_3$, we derive the sheet carrier density, ${N_{s}}$, the mobility, $��$, and also the depth profile of the carrier concentration. Notably, we find that ${N_{s}}$ and the shape of the depth profile are similar as in LaAlO$_3$/SrTiO$_3$ heterostructures for which the itinerant carriers are believed to arise from a polar discontinuity. The main differences concern the higher mobility and a relatively stronger confinement of the electrons in $��$-Al$_2$O$_3$/SrTiO$_3$, 12 pages, 2 figures

We report on the infrared spectra of the antisymmetric stretching vibration of Oi in Si-rich Si1xGex alloys. The shift of its frequency is obtained as a function of the germanium content x below 0:5. We present a decomposition of the low-temperature spectra using GaussLorentz profiles. A low-lying broad band is found for x ź 0:08, and assigned tentatively to the vibrations of Si2O units with the nearest sites occupied by several Ge atoms. We compare our experimental findings with results of a recent ab initio study of the phonon modes.

We present an extension of the model proposed recently to account for dramatic changes below Tc (anomalies) of some c-axis polarized infrared-active phonons in bilayer cuprate superconductors, that applies to trilayer high-Tc compounds. We discuss several types of phonon anomalies that can occur in these systems and demonstrate that our model is capable of explaining the spectral changes occurring upon entering the superconducting state in the trilayer compound Tl2Ba2Ca2Cu3O10. The low-temperature spectra of this compound obtained by Zetterer and coworkers display an additional broad absorption band, similar to the one observed in underdoped YBa2Cu3O7−δ and Bi2Sr2CaCu2O8. In addition, three phonon modes are strongly anomalous. We attribute the absorption band to the transverse Josephson plasma resonance, similar to that of the bilayer compounds. The phonon anomalies are shown to result from a modification of the local fields induced by the formation of the resonance. The spectral changes in Tl2Ba2Ca2Cu3O10 are compared with those occurring in Bi2Sr2Ca2Cu3O10, reported recently by Boris and coworkers.

The infrared response of superlattices (SLs) composed of YBa 2 Cu 3 O 7 (YBCO) and La 0.67 Ca 0.33 MnO 3 (LCMO) on SrTiO 3 substrate has been studied by ellipsometry in the range from 400 to 5000 cm -1 . We use a model calculation of the optical response to distinguish the contribution of the SL and the substrate. The infrared data provide an evidence of a strong interaction between the YBCO and LCMO layers. The mid-infrared resonance located at approximately 1000-3000 cm -1 is discussed in terms of the polaronic absorption in LCMO. The resonance could indicate a charge transfer from the YBCO to the LCMO layers.

We studied the doping and temperature (T) dependence of the infrared (IR) response of Ba(Fe1_xCox)2As2 single crystals. We show that a weak band around 1000 cm_1, that was previously interpreted in terms of interaction of the charge carriers with magnetic excitations or of a pseudogap, is rather related to low energy interband transitions. Specifically, we show that this band exhibits a similar doping and T dependence as the hole pockets seen by angle resolved photoemission spectroscopy (ARPES). Notably, we find that it vanishes as a function of doping near the critical point where superconductivity is suppressed in the overdoped regime. Our IR data thus provide bulk specific information (complementary to the surface sensitive ARPES) for a Lifshitz transition. Our IR data also reveal a second low-energy band around 2300 cm_1 which further emphasizes the necessity to consider the multiband nature of these iron arsenides in the analysis of the optical response.

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 similar to 50 K and exhibit similar, unipolar hysteresis loops as a function of the gate voltage.

We have measured the temperature dependence of the direct band gap, $E_g$, in SrTi$^{16}$O$_3$ and BaTiO$_3$ and related materials with quantum-paraelectric and ferroelectric properties using optical spectroscopy. We show that $E_g$ exhibits an anomalous temperature dependence with pronounced changes in the vicinity of the ferroelectric transition that can be accounted for in terms of the Fr\"ohlich electron-phonon interaction with an optical phonon mode, the so-called soft mode. We demonstrate that these characteristic changes of $E_g$ can be readily detected even in very thin films of SrTi$^{16}$O$_3$ with a strain-induced ferroelectric order. Optical spectroscopy thus can be used as a relatively simple but sensitive probe of ferroelectric order in very thin films of these titanates and probably also in subsequent multilayers and devices., Comment: 6 pages, 3 figures, and supplementary material; accepted for publication in Phys. Rev. B

Far-infrared spectra of ferrimagnetic Dy3Fe5O12 single crystals have been studied in the spectral range between 12 and 700 cm(-1) and in a wide temperature range between 5 K and 300 K using transmission spectroscopy and rotating analyzer ellipsometry. In the temperature range below T-C = 16 K for the magnetic ordering of Dy3+ spins, a number of ligand field and Kaplan-Kittel (KK) exchange resonance modes have been found. Temperature dependencies of their frequencies allowed us to estimate the ratio between the Fe-Dy and Dy-Dy exchange constants. We found that variation of the oscillator strength of the KK modes correlates with the magnetic-field-induced changes of magnetic permeability. The low-temperature peak in the static value of the dielectric function at T-C = 16 K may be related to the observed temperature-induced variation of the oscillator strength of the optical phonons.

Physical review letters 110, 136805 (2013). doi:10.1103/PhysRevLett.110.136805, Published by APS, College Park, Md.

Using muon spin rotation (\muSR) and infrared spectroscopy we investigated the recently discovered superconductor K0.73Fe1.67Se2 with Tc = 32 K. We show that the combined data can be consistently described in terms of a macroscopically phase segregated state with a matrix of ~88% volume fraction that is insulating and strongly magnetic and inclusions with a ~12% volume fraction which are metallic, superconducting and non-magnetic. The electronic properties of the latter, in terms of the normal state plasma frequency and the superconducting condensate density, appear to be similar as in other iron selenide or arsenide superconductors., Comment: 22 pages, 8 figures. (citation list correction.)

With infrared ellipsometry, x-ray diffraction, and electric transport 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 low temperature and negative gate voltage is induced, or at least strongly enhanced, by a pyroelectric phase transition in SrTiO3 which strongly reduces the lattice polarizability and the subsequent Coulomb screening. In particular, we show that the charge localisation and the polar order of SrTiO3 both develop below about 50 K and exhibit similar, unipolar hysteresis loops as a function of the gate voltage. Our findings suggest that the pyroelectric order also plays an important role in the quantum phase transition at very low temperatures where superconductivity is suppressed by an electric field., Comment: 5 pages, 4 figures, supplementary material

Far-infrared spectra of magnetodielectric Dy3Fe5O12 garnet were studied using a combination of transmittance, reflectivity, and rotating analyzer ellipsometry. In addition to purely dielectric and magnetic modes, we observed several hybrid modes with a mixed magnetic and electric dipole activity. Using 4 x 4 matrix formalism for materials with mu(omega) 1, we modeled the experimental optical spectra and determined the far-infrared dielectric and magnetic permeability functions. The matching condition mu(omega_h)Se = epsilon(omega_h)Sm for the oscillator strengths Se/m explains the observed vanishing of certain hybrid modes at omega_h in reflectivity.

We show that a miltilayer analysis of the infrared c-axis response of RBa2Cu3O7-delta (R=Y,Gd,Eu) provides important new information about the anomalous normal-state properties of underdoped curprate high temperature superconductors. In addition to competing correlations which give rise to a pseudogap that depletes the low-energy electronic states below T*>>Tc, it enables us to identify the onset of a precursor superconducting state below Tons>Tc. We map out the doping phase diagram of Tons which reaches a maximum of 180K at strong underdoping and present magnetic field dependent data which confirm our conclusions.

We show that a multilayer analysis of the infrared c-axis response of RBa2Cu3O(7-δ) (R=Y, Gd, Eu) provides important new information about the anomalous normal-state properties of underdoped cuprate high temperature superconductors. In addition to competing correlations which give rise to a pseudogap that depletes the low-energy electronic states below T*≫T(c), it enables us to identify the onset of a precursor superconducting state below T(ons)T(c). We map out the doping phase diagram of T(ons) which reaches a maximum of 180 K at strong underdoping and present magnetic field dependent data which confirm our conclusions.

We measure the anisotropic midinfrared response of electrons and phonons in bulk YBa(2)Cu(3)O(7-δ) after femtosecond photoexcitation. A line shape analysis of specific lattice modes reveals their transient occupation and coupling to the superconducting condensate. The apex oxygen vibration is strongly excited within 150 fs, demonstrating that the lattice absorbs a major portion of the pump energy before the quasiparticles are thermalized. Our results attest to substantial electron-phonon scattering and introduce a powerful concept probing electron-lattice interactions in a variety of complex materials.

We report muon spin rotation (mu SR) and infrared spectroscopy experiments on underdoped BaFe1.89Co0.11As2 which show that bulk magnetism and superconductivity (SC) coexist and compete on the nanometer length scale. Our combined data reveal a bulk magnetic order, likely due to an incommensurate spin density wave (SDW), which develops below T-mag approximate to 32 K and becomes reduced in magnitude (but not in volume) below T-c = 21.7 K. A slowly fluctuating precursor of the SDW seems to develop already below the structural transition at T-s approximate to 50 K. The bulk nature of SC is established by the mu SR data which show a bulk SC vortex lattice and the IR data which reveal that the majority of low-energy states is gapped and participates in the condensate at T << T-c.

We performed combined infrared reflection and ellipsometry measurements of the in-plane optical response of single crystals of the pnictide high-temperature superconductor ${\text{BaFe}}_{1.87}{\text{Co}}_{0.13}{\text{As}}_{2}$ with ${T}_{c}=24.5\text{ }\text{K}$. Our experimental data provide evidence for multiple superconducting energy gaps and can be well described in terms of three isotropic gaps with $2\ensuremath{\Delta}/{k}_{B}{T}_{c}$ of 3.1, 4.7, and 9.2. The obtained low-temperature value of the in-plane magnetic penetration depth is 270 nm.

We measure the anisotropic midinfrared response of electrons and phonons in bulk YBa(2)Cu(3)O(7-δ) after femtosecond photoexcitation. A line shape analysis of specific lattice modes reveals their transient occupation and coupling to the superconducting condensate. The apex oxygen vibration is strongly excited within 150 fs, demonstrating that the lattice absorbs a major portion of the pump energy before the quasiparticles are thermalized. Our results attest to substantial electron-phonon scattering and introduce a powerful concept probing electron-lattice interactions in a variety of complex materials.

With infrared ellipsometry and transport measurements we investigated the electrons at the interface between ${\mathrm{LaAlO}}_{3}$ and ${\mathrm{SrTiO}}_{3}$. We obtained a sheet carrier concentration of ${N}_{\mathrm{s}}\ensuremath{\approx}5--9\ifmmode\times\else\texttimes\fi{}{10}^{13}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$, an effective mass of ${m}^{*}=3.2\ifmmode\pm\else\textpm\fi{}0.4{m}_{e}$, and a strongly frequency dependent mobility. The latter are similar as in bulk ${\mathrm{SrTi}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{x}{\mathrm{O}}_{3}$ and therefore suggestive of polaronic correlations. We also determined the vertical concentration profile which has a strongly asymmetric shape with a rapid initial decay over the first 2 nm and a pronounced tail that extends to about 11 nm.

We report muon spin rotation (μSR) and infrared spectroscopy experiments on underdoped BaFe1.89Co0.11As2 which show that bulk magnetism and superconductivity (SC) coexist and compete on the nanometer length scale. Our combined data reveal a bulk magnetic order, likely due to an incommensurate spin density wave (SDW), which develops below T(mag)≈32 K and becomes reduced in magnitude (but not in volume) below Tc=21.7 K. A slowly fluctuating precursor of the SDW seems to develop already below the structural transition at T(s)≈50 K. The bulk nature of SC is established by the μSR data which show a bulk SC vortex lattice and the IR data which reveal that the majority of low-energy states is gapped and participates in the condensate at T≪T(c).

We report muon spin rotation (muSR) and infrared (IR) spectroscopy experiments on underdoped BaFe1.89Co0.11As2 which show that bulk magnetism and superconductivity (SC) coexist and compete on the nanometer length scale. Our combined data reveal a bulk magnetic order, likely due to an incommensurate spin density wave (SDW), which develops below Tmag \approx 32 K and becomes reduced in magnitude (but not in volume) below Tc = 21.7 K. A slowly fluctuating precursor of the SDW seems to develop alrady below the structural transition at Ts \approx 50 K. The bulk nature of SC is established by the muSR data which show a bulk SC vortex lattice and the IR data which reveal that the majority of low-energy states is gapped and participates in the condensate at T << Tc.

We measure the anisotropic midinfrared response of electrons and phonons in bulk YBa2Cu3O7-�� after femtosecond photoexcitation. A line shape analysis of specific lattice modes reveals their transient occupation and coupling to the superconducting condensate. The apex oxygen vibration is strongly excited within 150 fs, demonstrating that the lattice absorbs a major portion of the pump energy before the quasiparticles are thermalized. Our results attest to substantial electron-phonon scattering and introduce a powerful concept probing electron-lattice interactions in a variety of complex materials.

We measure the anisotropic mid-infrared response of electrons and phonons in bulk YBa2Cu3O7 after femtosecond photoexcitation. A line shape analysis of specific lattice modes reveals their transient occupation and coupling to the superconducting condensate. The apex oxygen vibration is strongly excited within 150 fs demonstrating that the lattice absorbs a major portion of the pump energy before the quasiparticles are thermalized. Our results attest to substantial electron-phonon scattering and introduce a powerful concept probing electron-lattice interactions in a variety of complex materials., Comment: 4 pages, 4 figures + supplementary material

We show that a multilayer analysis of the infrared c-axis response of RBa2Cu3O7-d (R=Y, Gd, Eu) provides important new information about the anomalous normal state properties of underdoped cuprate high temperature superconductors. Besides competing correlations which give rise to a pseudogap that depletes the low-energy electronic states below T*>>Tc, it enables us to identify the onset of a precursor superconducting state below Tons>Tc. We map out the doping phase diagram of Tons which reaches a maximum of ~180 K at strong underdoping and present magnetic field dependent data which confirm our conclusions., 5 pages, 3 figures

With infrared ellipsometry and transport measurements we investigated the electrons at the interface between LaAlO3 and SrTiO3. We obtained a sheet carrier concentration of N(s) approximately = 5-9x10(13) cm(-2), an effective mass of m*=3.2+/-0.4m(e), and a strongly frequency dependent mobility. The latter are similar as in bulk SrTi(1-x)Nb(x)O3 and therefore suggestive of polaronic correlations. We also determined the vertical concentration profile which has a strongly asymmetric shape with a rapid initial decay over the first 2 nm and a pronounced tail that extends to about 11 nm.

We performed combined infrared reflection and ellipsometry measurements of the in-plane optical reponse of single crystals of the pnictide high temperature superconductor BaFe$_{1.87}$Co$_{0.13}$As$_{2}$ with $T_{c}$ = 24.5 K. We observed characteristic superconductivity-induced changes which provide evidence for at least three different energy gaps. We show that a BCS-model of isotropic gaps with 2$\Delta/k_{B}T_{c}$ of 3.1, 4.7, and 9.2 reproduces the experimental data rather well. We also determine the low-temperature value of the in-plane magnetic penetration depth of 270 nm.

With infrared ellipsometry and transport measurements we investigated the electrons at the interface between LaAlO3 and SrTiO3. We obtained a sheet carrier density of Ns~5-9x 10E13 cm^-2, an effective mass of m*~3m_e, and a strongly frequency dependent mobility. The latter are similar as in bulk SrTi1-xNbxO3 and therefore suggestive of polaronic correlations of the confined carriers. We also determined the vertical density profile which has a strongly asymmetric shape with a rapid initial decay over the first 2 nm and a pronounced tail that extends to about 11 nm., Comment: 4 pages, 3 figures, 1 EPAPS file (3 figures)

We present far-infrared ellipsometric measurements of polycrystalline samples of the pnictide superconductor RFeAsO0.82F0.18 (R=Nd and Sm). We find evidence that the electronic properties are strongly anisotropic such that the optical spectra are dominated by the weakly conducting c-axis response similar to the cuprate high-temperature superconductors. We deduce an upper limit of the c-axis superconducting plasma frequency of omega pl,c(SC) or =6 microm and lambda c/lambda ab > or =30 as compared to lambda ab=185 nm from muon spin rotation [A. Drew, arXiv:0805.1042 [Phys. Rev. Lett. (to be published)]]. We also observe a gaplike suppression of the conductivity in the superconducting state with a shoulderlike feature at omegaSC* approximately 300 cm(-1) and spectral shape which is consistent with an unconventional order parameter with 2Delta approximately omegaSC* approximately 37 meV.

Muon spin rotation experiments were performed on the pnictide high temperature superconductor ${\mathrm{SmFeAsO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}$ with $x=0.18$ and 0.3. We observed an unusual enhancement of slow spin fluctuations in the vicinity of the superconducting transition which suggests that the spin fluctuations contribute to the formation of an unconventional superconducting state. An estimate of the in-plane penetration depth ${\ensuremath{\lambda}}_{ab}(0)=190(5)\text{ }\text{ }\mathrm{nm}$ was obtained, which confirms that the pnictide superconductors obey an Uemura-style relationship between ${T}_{c}$ and ${\ensuremath{\lambda}}_{ab}(0{)}^{\ensuremath{-}2}$.

We present far-infrared ellipsometric measurements of polycrystalline samples of the pnictide superconductor RFeAsO0.82F0.18 (R=Nd and Sm). We find evidence that the electronic properties are strongly anisotropic such that the optical spectra are dominated by the weakly conducting c-axis response similar to the cuprate high-temperature superconductors. We deduce an upper limit of the c-axis superconducting plasma frequency of omega pl,c(SC)or =260 cm(-1) corresponding to a lower limit of the c-axis magnetic penetration depth of lambda cor =6 microm and lambda c/lambda abor =30 as compared to lambda ab=185 nm from muon spin rotation [A. Drew, arXiv:0805.1042 [Phys. Rev. Lett. (to be published)]]. We also observe a gaplike suppression of the conductivity in the superconducting state with a shoulderlike feature at omegaSC* approximately 300 cm(-1) and spectral shape which is consistent with an unconventional order parameter with 2Delta approximately omegaSC* approximately 37 meV.

Muon spin rotation experiments were performed on the pnictide high temperature superconductor SmFeAsO1-xFx with x=0.18 and 0.3. We observed an unusual enhancement of slow spin fluctuations in the vicinity of the superconducting transition which suggests that the spin fluctuations contribute to the formation of an unconventional superconducting state. An estimate of the in-plane penetration depth lambda ab(0)=190(5) nm was obtained, which confirms that the pnictide superconductors obey an Uemura-style relationship between Tc and lambda ab(0);(-2).

We present ellipsometric measurements of the far-infrared dielectric response of polycrystalline samples of the new pnictide superconductor RO0.82F0.18FeAs (R=Nd and Sm). We find evidence that the electronic properties are strongly anisotropic such that the optical spectra are dominated by the weakly conducting c-axis response similar as in the cuprate high-temperature superconductors (HTSC). Accordingly, we obtain an upper limit of the c-axis superconducting plasma frequency of $\omega_{{\rm pl},c}^{\rm SC}\leq 260\cm$ which corresponds to a lower limit of the c-axis magnetic penetration depth of $\lambda_c\geq6\mum$ and an anisotropy of $\lambda_c/\lambda_{ab}\geq 30$ as compared to $\lambda_{ab}=185$ nm from muon spin rotation [A. Drew {\it et al.}, arXiv:0805.1042]. Also in analogy to the cuprate HTSC, our spectra exhibit the signatures of a gap-like suppression of the conductivity in the superconducting state with a large gap magnitude of $2\Delta\approx300\cm$ (37 meV) and a ratio of $2\Delta/k_{\rm B}\tc\approx8$ that is suggestive of strong coupling.