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Heterostructures of strongly correlated oxides demonstrate various intriguing and potentially useful interfacial phenomena. LaMnO3/SrMnO3 superlattices are presented showcasing a new high‐temperature ferromagnetic phase with Curie temperature, T C ≈360 K, caused by electron transfer from the surface of the LaMnO3 donor layer into the neighboring SrMnO3 acceptor layer. As a result, the SrMnO3 (top)/LaMnO3 (bottom) interface shows an enhancement of the magnetization as depth‐profiled by polarized neutron reflectometry. The length scale of charge transfer, λTF ≈2 unit cells, is obtained from in situ growth monitoring by optical ellipsometry, supported by optical simulations, and further confirmed by high resolution electron microscopy and spectroscopy. A model of the inhomogeneous distribution of electron density in LaMnO3/SrMnO3 layers along the growth direction is concluded to account for a complex interplay between ferromagnetic and antiferromagnetic layers in superlattices.

With terahertz and infrared spectroscopy we studied the in-plane response of an underdoped, twinned YBa2Cu3O6.6 single crystal with Tc=58(1)K in high magnetic fields up to B=30 Tesla (T) applied along the c axis. Our goal was to investigate the field-induced suppression of superconductivity and to observe the signatures of the three-dimensional (3D) incommensurate copper charge density wave (Cu-CDW), which was previously shown to develop at such high magnetic fields. Our study confirms that a B field in excess of 20 T gives rise to a full suppression of the macroscopic response of the superconducting condensate. However, it reveals surprisingly weak signatures of the 3D Cu-CDW at high magnetic fields. At 30 T there is only a weak reduction of the spectral weight of the Drude-response (by about 3%), which is accompanied by an enhancement of the so-called mid-infrared (MIR) band as well as a narrow electronic mode around 240cm−1 (and, possibly, another one around 90cm−1), which is interpreted in terms of a pinned phase mode of the CDW. The pinned phase mode and the MIR band are strong features already without magnetic field, which suggests that prominent but short-ranged and slowly fluctuating (compared to the picosecond infrared timescale) CDW correlations exist all along, i.e., even at zero magnetic field.

We studied the infrared response of polycrystalline samples of the iron arsenide superconductor (Rb,Cs)Ca2Fe4As4F2 (Rb,Cs-12442), which has a bilayer structure similar to the high-Tc cuprates YBa2Cu3O7 (YBCO) and Bi2Sr2CaCu2O8. The c-axis reflectivity spectra Rc have been derived from the reflectivity spectra of the polycrystalline samples Rpoly and the in-plane spectrum of a corresponding Cs-12442 crystal Rab using a geometrical averaging approach with Rc=3Rpoly−2Rab. 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≃Tc of σbl1(ω→0)≃1000Ω−1cm−1 and σint1(ω→0)≃15Ω−1cm−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 Tc. There is also no clear sign of an onset of precursor superconducting pairing correlations well above Tc≃ 30 K. This highlights that the pseudogap and the precursor superconducting pairing well above Tc 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.

Ultrasonics have been an incisive probe of internal interfaces in a wide variety of systems ranging from stars to solids. For thin-film structures, however, ultrasound is largely ineffective because the signal is dominated by the substrate. Using confocal Raman spectromicroscopy, we show that multiple reflection of sound waves at internal interfaces of a metal-oxide superlattice generates standing waves that are insensitive to the substrate. Such modes had previously been observed only in high-quality superlattices of elemental semiconductors, and their observation in complex metal- oxide heterostructures is testimony to recent progress in this field. We use the high spatial resolution of the Raman microscope to demonstrate the high sensitivity of the mode frequency to atomic-scale thickness variations of the superlattice. Spectroscopy of acoustic standing waves can hence serve as a powerful characterization tool of thin-film structures. In analogy to ultrasound spectroscopy of bulk solids, lineshape analysis of these modes has the potential to yield detailed information about the internal structure of the interfaces as well as the coupling of sound waves to the low- frequency spin, charge, and orbital dynamics in metal-oxide superlattices.

With optical spectroscopy we provide evidence that the insulator-metal transition in Sr2Ir1−xRhxO4 occurs close to a crossover from the Mott- to the Slater-type. The Mott gap at x=0 persists to high temperature and evolves without an anomaly across the Néel temperature, TN. Upon Rh doping, it collapses rather rapidly and vanishes around x=0.055. Notably, just as the Mott gap vanishes yet another gap appears that is of the Slater-type and develops right below TN. This Slater gap is only partial and is accompanied by a reduced scattering rate of the remaining free carriers, similar as in the parent compounds of the iron arsenide superconductors.

We studied how the electronic, superconducting, and magnetic properties of YBa2Cu3O7/Nd1−x(Ca1−ySry )xMnO3 multilayers depend on the tolerance factor and the hole doping of the manganite. In particular, we investigated the granular superconducting state and the related magnetic-field-driven insulator-to- superconductor transition that was previously discovered in corresponding multilayers with Pr0.5La0.2Ca0.3MnO3 [B. P. P. Mallett et al., Phys. Rev. B 94, 180503(R) (2016)]. We found that this granular uperconducting state occurs only when the manganite layer is in a charge/orbital ordered and CE-type antiferromagnetic state (Mn-CO/OO). The coupling mechanism underlying this intriguing proximity effect seems to involve the domain boundaries of the Mn-CO/OO and/or the charge disordered regions of the manganite layer that become more numerous as the hole doping is reduced below x = 0.5.

By means of infrared spectroscopy, we determine the temperature-doping phase diagram of the Fano effect for the in-plane Fe-As stretching mode in Ba1−xKxFe2As2. The Fano parameter 1=q2, which is a measure of the phonon coupling to the electronic particle-hole continuum, shows a remarkable sensitivity to the magnetic and structural orderings at low temperatures. Most strikingly, at elevated temperatures in the paramagnetic tetragonal state we observe a linear correlation between 1=q2 and the superconducting critical temperature Tc. Based on theoretical calculations and symmetry considerations, we identify the relevant interband transitions that are coupled to the Fe-As mode. In particular, we show that a sizable xy orbital component at the Fermi level is fundamental for the Fano effect and, thus, possibly also for the superconducting pairing.

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.

The observation of a charge density wave in the underdoped cuprate high Tc superconductors (Cu-CDW) raised a debate about its relationship with superconductivity. In bulk YBa2Cu3O7−δ the Cu-CDW is incipient and mainly pinned by defects. Nevertheless, a large magnetic field can induce a true long-range Cu- CDW order as it suppresses superconductivity. An enhanced Cu-CDW order was also observed in YBa2Cu3O7/La2/3Ca1/3MnO3 multilayers. Here, we show that the magnitude of the Cu-CDW in YBa2Cu3O7−δ / Nd0.65(Ca1-ySry)0.35MnO3 multilayers can be varied by adjusting the strength of the manganite charge and orbital order via the Sr content (tolerance factor). Furthermore, we resolve the reconstruction of the crystal field levels of the interfacial Cu ions that are also affected by the manganite charge and orbital order. This tuneable interfacial coupling and Cu- CDW in YBa2Cu3O7−δ can be used for studying the relationship between the Cu- CDW and superconductivity and, possibly, for inducing new intertwined quantum states.

We performed optical studies on CaFeAsF single crystals, a parent compound of the 1111-type iron-based superconductors that undergoes a structural phase transition from tetragonal to orthorhombic at Ts=121 K and a magnetic one to a spin density wave (SDW) state at TN=110 K. In the low-temperature optical conductivity spectrum, after the subtraction of a narrow Drude peak, we observe a pronounced singularity around 300cm−1 that separates two regions of quasilinear conductivity. We outline that these characteristic absorption features are signatures of Dirac fermions, similar to what was previously reported for the BaFe2As2 system [Z.-G. Chen et al., Phys. Rev. Lett. 119, 096401 (2017)]. In support of this interpretation, we show that for the latter system this singular feature disappears rapidly upon electron and hole doping, as expected if it arises from a van Hove singularity in between two Dirac cones. Finally, we show that one of the infrared-active phonon modes (the Fe-As mode at 250cm−1) develops a strongly asymmetric line shape in the SDW state and note that this behavior can be explained in terms of a strong coupling with the Dirac fermions.