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.

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 .

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The magnetic and superconducting properties of a series of underdoped Ba 1−x Na x Fe 2 As 2 (BNFA) single crystals with 0.19 x 0.34 have been investigated with the complementary muon-spin-rotation (μSR) and infrared spectroscopy techniques. The focus has been on the different antiferromagnetic states in the underdoped regime and their competition with superconductivity, especially for the ones with a tetragonal crystal structure and a so-called double-Q magnetic order. Besides the collinear state with a spatially inhomogeneous spincharge-density wave (i-SCDW) order at x = 0.24 and 0.26, that was previously identified in BNFA, we obtained evidence for an orthomagnetic state with a "hedgehog"-type spin vortex crystal (SVC) structure at x = 0.32 and 0.34. Whereas in the former i-SCDW state the infrared spectra show no sign of a superconducting response down to the lowest measured temperature of about 10 K, in the SVC state there is a strong superconducting response similar to the one at optimum doping. The magnetic order is strongly suppressed here in the superconducting state and at x = 0.34 there is even a partial reentrance into a paramagnetic state at T T c .

The role of the crystal lattice for the electronic properties of cuprates and other high-temperature superconductors remains controversial despite decades of theoretical and experimental efforts. While the paradigm of strong electronic correlations suggests a purely electronic mechanism behind the insulator-to-metal transition, recently the mutual enhancement of the electron-electron and the electron-phonon interaction and its relevance to the formation of the ordered phases have also been emphasized.Here, we combine polarization-resolved ultrafast optical spectroscopy and state-ofthe-art dynamical mean-field theory to show the importance of the crystal lattice in the breakdown of the correlated insulating state in an archetypal undoped cuprate.We identify signatures of electron-phonon coupling to specific fully-symmetric optical modes during the build-up of a three-dimensional metallic state that follows charge photodoping. Calculations for coherently displaced crystal structures along the relevant phonon coordinates indicate that the insulating state is remarkably unstable toward metallization despite the seemingly large charge-transfer energy scale. This hitherto unobserved insulator-to-metal transition mediated by fully-symmetric lattice modes can find extensive application in a plethora of correlated solids. RESULTS Crystal Structure and Equilibrium Optical PropertiesAs a model material system we study La 2 CuO 4 (LCO), one of the simplest insulating cuprates exhibiting metallicity upon hole doping. In this solid, the two-dimensional network of corner-sharing CuO 4 units is accompanied by two apical O atoms below and above each CuO 4 plaquette. As a result, the main building blocks of LCO are CuO 6 octahedra ( Fig. 1 A) that are elongated along the c-axis due to the Jahn-Teller distortion. The unit cell of LCO is tetragonal above and orthorhombic below 560 K. A simplified scheme of the electronic density of states is shown in Fig. 1 B (left panel). An energy gap (∆ CT ∼2 eV) opens between the filled O-2p band and the unoccupied Cu-3d upper Hubbard band (UHB), thus being of the CT type. In contrast, the occupied Cu-3d lower Hubbard band (LHB) lies at lower energy. First, we present the optical properties of LCO in equilibrium. Figure 1 C shows the absorptive part of the optical conductivity (σ 1 ), measured via ellipsometry. The in-plane response (σ 1a , solid violet curve) is dominated by the optical CT gap at 2.20 eV [17, 18].This transition is a non-local resonant exciton that extends at least over two CuO 4 units.The strong coupling to the lattice degrees of freedom causes its broadened shape [18][19][20]. As such, this optical feature can be modeled as involving the formation of an electron-polaron and a hole-polaron, coupled to each other by a short-range interaction [18,21]. At higher energy (2.50−3.50 eV), the in-plane spectrum results from charge excitations that couple the O-2p states to both the Cu-3d states in the UHB and the La-5d /4f states. In contrast, the out-of-plane optical conductivity (σ ...