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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 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.

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.

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.

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 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.

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 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 ${\ensuremath{\lambda}}_{ab}(0{)}^{\ensuremath{-}2}$.

The far-infrared (FIR) c axis conductivity of a Bi2223 crystal has been measured by ellipsometry. Below T(c) a strong absorption band develops near 500 cm(-1), corresponding to a transverse Josephson plasmon. The related increase in FIR spectral weight leads to a giant violation of the Ferrell-Glover-Tinkham sum rule. The gain in c axis kinetic energy accounts for a sizable part of the condensation energy. We also observe phonon anomalies which suggest that the Josephson currents lead to a drastic variation of the local electric field within the block of closely spaced CuO2 planes.