We report the complex dielectric function of the quasi-one-dimensional chalcogenide Ta2NiSe5, which undergoes a structural phase transition presumably associated with exciton condensation below T-c = 326 K [Y. Wakisaka et al., Phys. Rev. Lett. 103, 026402 (2009); Y. F. Lu et al., Nat. Commun. 8, 14408 (2017)], and of the isostructural Ta2NiSe5, which does not exhibit such a transition. Using spectroscopic ellipsometry, we have detected exciton doublets with pronounced Fano line shapes in both the compounds. The exciton Fano resonances in Ta2NiSe5 display an order-of-magnitude higher intensity than those in Ta2NiSe5. In conjunction with prior theoretical work [E. Rashba, Sov. Phys. Semicond. 8, 807 (1975)], we attribute this observation to the giant oscillator strength of spatially extended exciton-phonon bound states in Ta2NiSe5. The formation of exciton-phonon complexes in Ta2NiSe5 and Ta2NiSe5 is confirmed by the pronounced temperature dependence of sharp interband transitions in the optical spectra, the peak energies and widths of which scale with the thermal population of optical phonon modes. The description of the optically excited states in terms of strongly overlapping exciton complexes is in good agreement with the hypothesis of an exciton insulator ground state.

We report on the growth of stoichiometric Sr_2IrO_4 single crystals, which allow us to unveil their intrinsic magnetic properties. The effect of different growth conditions has been investigated for crystals grown by the flux method. We find that the magnetic response depends very sensitively on the details of the growth conditions. We assess the defect concentration based on magnetization, X-ray diffraction, Raman scattering, and optical conductivity measurements. We find that samples with a low concentration of electronically active defects show much reduced in-gap spectral weight in the optical conductivity and a pronounced two-magnon peak in the Raman scattering spectrum. A prolonged exposure at high temperature during the growth leads to higher defect concentration likely due to creation of oxygen vacancies. We further demonstrate a systematic intergrowth of Sr_2IrO_4 and Sr_3Ir_2O_7 phases by varying the growth temperature. Our results thus emphasize that revealing the intrinsic magnetic properties of Sr_2IrO_4 and related materials requires a scrupulous control of the crystal growth process.

The almost multiferroic perovskite EuTiO3 (ETO) has been prepared as films on substrates of SrTiO3. For all prepared film thicknesses highly transparent insulating films with atomically flat surfaces and excellent orientation have been grown. They were characterized by X-ray diffraction, magnetic susceptibility and birefringence measurements and found to exhibit bulk properties, namely an antiferromagnetic transition at T-N = 5.1 K and a structural transition at T-S = 282 K. The latter could only be identified due to the high transparency of the samples since the optical band gap is of the order of 4.5eV and larger than observed before for any bulk and thin film samples.

Optical excitation of apical oxygen vibrations in YBa2Cu3O6+delta has been shown to enhance its c axis superconducting-phase rigidity, as evidenced by a transient blueshift of the equilibrium interbilayer Josephson plasma resonance. Surprisingly, a transient c axis plasma mode could also be induced above T-c by the same apical oxygen excitation, suggesting light activated superfluid tunneling throughout the pseudogap phase of YBa2Cu3O6+delta. However, despite the similarities between the transient plasma mode above Tc and the equilibrium Josephson plasmon, alternative explanations involving high-mobility quasiparticle transport should be considered. Here, we report an extensive study of the relaxation of the light induced plasmon into the equilibrium incoherent phase. These new experiments allow for a critical assessment of the nature of this mode. We determine that the transient plasma relaxes through a collapse of its coherence length rather than its carrier (or superfluid) density. These observations are not easily reconciled with quasiparticle interlayer transport and rather support transient superfluid tunneling as the origin of the light induced interlayer coupling in YBa2Cu3O6+delta.

We report the complex dielectric function of high-quality Na3Ir3O8 single crystals determined by spectroscopic ellipsometry in the spectral range from 15 meV to 2 eV. The far-infrared phonon spectra exhibit highly asymmetric line shapes characteristic of Fano resonances. With decreasing temperature, we observe a sharp increase of the infrared intensity of the Fano-shaped phonon modes accompanied by concomitant changes in the low-energy electronic background, formed by electronic transitions between Ir 5d t(2g) bands of a mostly J(eff) = 1/2 character. The role of the complex hyperkagome lattice structure and strong spin-orbit coupling is considered.

We report the complex dielectric function of high-quality AFe(2)As(2), (A = Ca, Sr, Ba) single crystals with T-N approximate to 150 K, 200 K, and 138 K, respectively, determined by broadband spectroscopic ellipsometry at temperatures 10 <= T <= 300 K and wave numbers from 100 cm (1) to 52000 cm (1). In CaFe2As2 we identify the optical spin-density-wave gap 2 Delta(SDW) approximate to 1250 cm(-1). The 2 Delta(SDW)/(k(B)T(N))ratio, characterizing the strength of the electron-electron coupling in the spin-density-wave state, amounts to approximate to 12 in CaFe2As2, significantly larger than the corresponding values for the SrFe2As2 and BaFe2As2 compounds: 8.7 and 5.3, respectively. We further show that, similarly to the Ba-based compound, two characteristic SDW energy gaps can be identified in the infrared-conductivity spectra of both SrFe2As2 and CaFe2As2 and investigate their detailed temperature dependence in all three materials. This analysis reveals the existence of an anomaly in CaFe2As2 at a temperature T* approximate to 80 K, well below the Neeel temperature of this compound, which implies weak coupling between the two SDW subsystems. The coupling between the two subsystems evolves to intermediate in the Sr-based and strong in the Ba-based material. The temperature dependence of the infrared phonons reveals clear anomalies at the corresponding Neel temperatures of the investigated compounds. In CaFe2As2, the phonons exhibit signatures of SDW fluctuations above T-N and some evidence for anomalies at T*. Investigation of all three materials in the visible spectral range reveals a spin-density-wave-induced suppression of two absorption bands systematically enhanced with decreasing atomic number of the intercalant. A dispersion analysis of the data in the entire spectral range clearly shows that CaFe2As2 is significantly more metallic than the other two compounds. Our results single out CaFe2As2 in the class of ThCr2Si2-type iron-based materials by demonstrating the existence of two weakly coupled and extremely metallic electronic subsystems.

Antiferromagnetic semiconductors are new alternative materials for spintronic applications and spin valves. In this work, we report a detailed investigation of two antiferromagnetic semiconductors $A$MnAs ($A=\mathrm{Li}$, LaO), which are isostructural to the well-known LiFeAs and LaOFeAs superconductors. Here we present a comparison between the structural, magnetic, and electronic properties of LiMnAs, LaOMnAs, and related materials. Interestingly, both LiMnAs and LaOMnAs show a variation in resistivity with more than five orders of magnitude, making them particularly suitable for use in future electronic devices. Neutron and x-ray diffraction measurements on LiMnAs show a magnetic phase transition corresponding to the N\'eel temperature of 373.8 K, and a structural transition from the tetragonal to the cubic phase at 768 K. These experimental results are supported by density functional theory calculations.

We report the complex dielectric function of high-quality nearly-stoichiometric Rb2Fe4Se5 (RFS) single crystals with Tc=32 K determined by wide-band spectroscopic ellipsometry and time-domain transmission spectroscopy in the spectral range 1 meV<=$\hbar$\omega<=6.5 eV at temperatures 4 K<=T<=300 K. This compound simultaneously displays a superconducting and a semiconducting optical response. It reveals a direct band-gap of 0.45 eV determined by a set of spin-controlled interband transitions. Below 100 K we observe in the lowest THz spectral range a clear metallic response characterized by the negative dielectric permittivity \epsilon 1 and bare (unscreened) \omega pl=100 meV. At the superconducting transition this metallic response exhibits a signature of a superconducting gap below 8 meV. Our findings suggest a coexistence of superconductivity and magnetism in this compound as two separate phases.