Anatase TiO2 is among the most studied materials for light-energy conversion applications, but the nature of its fundamental charge excitations is still unknown. Yet it is crucial to establish whether light absorption creates uncorrelated electron–hole pairs or bound excitons and, in the latter case, to determine their character. Here, by combining steady-state angle-resolved photoemission spectroscopy and spectroscopic ellipsometry with state-of-the-art ab initio calculations, we demonstrate that the direct optical gap of single crystals is dominated by a strongly bound exciton rising over the continuum of indirect interband transitions. This exciton possesses an intermediate character between the Wannier–Mott and Frenkel regimes and displays a peculiar two-dimensional wavefunction in the three-dimensional lattice. The nature of the higher-energy excitations is also identified. The universal validity of our results is confirmed up to room temperature by observing the same elementary excitations in defect-rich samples (doped single crystals and nanoparticles) via ultrafast two-dimensional deep-ultraviolet spectroscopy.

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 Ba 1−x K x Fe 2 As 2 . In agreement with the predicted transition from a single-Q to a double-Q AF structure, we found that a distinct spin density wave (SDW) develops in the t-AF phase. The pair breaking peak of this SDW 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. Similar to the cuprate high-T c superconductors (HTS), the phase diagram of the iron arsenides is characterized by the interaction of superconductivity (SC) with electronic correlations that give rise to competing orders and may also be involved in the SC pairing [1][2][3][4][5][6][7]. The iron arsenides are considered to be archetypal HTS since they exhibit well resolved structural and magnetic phase transitions with long-range ordered states that, unlike the cuprates, are not obscured by strong fluctuation effects. However, the interpretation is still complicated by their multi-band and multi-orbital structure and a sizeable coupling between the charge, spin, orbital and lattice degrees of freedom [1,2]. A prominent example is the combined structural and magnetic transition in undoped and lightly doped samples for which it is still debated whether it is driven by the spin or orbital sectors [6,8,9]. A related, controversial issue is whether the antiferromagnetic (AF) order is better described by an itinerant picture, in terms of a spin density wave (SDW), or in terms of local moments

We studied the doping and temperature (T ) dependence of the infrared (IR) response of Ba(Fe 1-x Co x ) 2 As 2 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.

We present a combined study with time-domain terahertz and conventional far-infrared ellipsometry of the temperature dependent optical response of SrTiO3 thin films (82 and 8.5 nm) that are grown by pulsed-laser deposition on LSAT substrates. We demonstrate that terahertz ellipsometry is very sensitive to the optical response of these thin films, in particular, to the soft mode of SrTiO3. We show that for the 82 nm film the eigenfrequency of the soft mode is strongly reduced by annealing at 1200 • C, whereas for the 8.5 nm film it is hardly affected. For the latter, after annealing the mode remains at 125 cm −1 at 300 K and exhibits only a weak softening to about 90 cm −1 at 10 K. This suggests that this ultrathin film undergoes hardly any relaxation of the compressive strain due to the LSAT substrate.

Using muon spin rotation and infrared spectroscopy, we study the relation between magnetism and superconductivity in Ba 1−x K x Fe 2 As 2 single crystals from the underdoped to the slightly overdoped regime. We find that the Fe magnetic moment is only moderately suppressed in most of the underdoped region where it decreases more slowly than the Néel temperature T N . This applies for both the total Fe moment obtained from muon spin rotation and for the itinerant component that is deduced from the spectral weight of the spin-density-wave pair-breaking peak in the infrared response. In the moderately underdoped region, superconducting and static magnetic orders coexist on the nanoscale and compete for the same electronic states. The static magnetic moment disappears rather sharply near optimal doping, however, in the slightly overdoped region there is still an enhancement or slowing down of spin fluctuations in the superconducting state. Similar to the gap magnitude reported from specific-heat measurements, the superconducting condensate density is nearly constant in the optimally and slightly overdoped region, but exhibits a rather pronounced decrease on the underdoped side. Several of these observations are similar to the phenomenology in the electron-doped counterpart Ba(Fe 1−y Co y ) 2 As 2 .