We present results of a Raman scattering study of the Kagome ferromagnet Co_3Sn_2S_2, with a focus on electronic and phononic excitations and their interplay. We provide a theoretical analysis of the electronic band structure, enabling a semi-quantitative explanation of the spectra. A prominent feature in the electronic spectra is a redistribution of spectral weight from low to high energies in all polarization configurations starting at the Curie temperature T_C. In the symmetry-resolved spectra, the suppression of the A_1g continuum in the ferromagnetic state arises from the redistribution of electronic states below T_C, while a strong enhancement of the A_2g continuum is linked to the dynamics of fermions near the Fermi level E_ F being characterized by spin-momentum locking near Weyl points. The A_1g phonon modulates the position of these Weyl points and couples strongly to the related fermions close to E_ F. These results allow a comprehensive understanding of the bulk band structure evolution as a function of temperature in Co_3Sn_2S_2, offering key insights for further studies of the driving force behind the long-range magnetic order and novel topological states in this compound.

We conducted a comparative study of the rare-earth infinite-layer nickelates films, RNiO2 (R = La, Pr, and Nd) using resonant inelastic X-ray scattering (RIXS). We found that the gross features of the orbital configurations are essentially the same, with minor variations in the detailed hybridization. For low-energy excitations, we unambiguously confirm the presence of damped magnetic excitations in all three compounds. By fitting to a linear spin-wave theory, comparable spin exchange coupling strengths and damping coefficients are extracted, indicating a universal magnetic structure in the infinite-layer nickelates. Interestingly, while signatures of a charge order are observed in LaNiO2 in the quasi-elastic region of the RIXS spectrum, it is absent in NdNiO2 and PrNiO2. This prompts further investigation into the universality and the origins of charge order within the infinite-layer inickelates.

Polarons-fermionic charge carriers bearing a strong companion lattice deformation-exhibit a natural tendency for self-localization due to the recursive interaction between electrons and the lattice. While polarons are ubiquitous in insulators, how they evolve in transitions to metallic and superconducting states in quantum materials remains an open question. Here, we use resonant inelastic x-ray scattering to track the electron-lattice coupling in the colossal magneto-resistive bi-layer manganite La_{1.2}Sr_{1.8}Mn_{2}O_{7} across its metal-to-insulator transition. The response in the insulating high-temperature state features harmonic emissions of a dispersionless oxygen phonon at small energy transfer. Upon cooling into the metallic state, we observe a drastic redistribution of spectral weight from the region of these harmonic emissions to a broad high energy continuum. In concert with theoretical calculations, we show that this evolution implies a shift in electron-lattice coupling from static to dynamic lattice distortions that leads to a distinct polaronic ground state in the low temperature metallic phase-a dynamic polaron liquid.

Tip-enhanced Raman spectroscopy (TERS) combines inelastic light scattering well below the diffraction limit down to the nanometer range and scanning probe microscopy and, possibly, spectroscopy. In this way, topographic and spectroscopic as well as single- and two-particle information may simultaneously be collected. While single molecules can now be studied successfully, bulk solids are still not meaningfully accessible. It is the purpose of the work presented here to outline approaches toward this objective. We describe a home-built, liquid helium cooled, ultrahigh vacuum TERS. The setup is based on a scanning tunneling microscope and, as an innovation, an off-axis parabolic mirror having a high numerical aperture of ∼0.85 and a large working distance. The system is equipped with a fast load-lock chamber, a chamber for the in situ preparation of tips, substrates, and samples, and a TERS chamber. Base pressure and temperature in the TERS chamber were ∼3 × 10-11 mbar and 15 K, respectively. Polarization dependent tip-enhanced Raman spectra of the vibration modes of carbon nanotubes were successfully acquired at cryogenic temperature. The new features described here including very low pressure and temperature and the external access to the light polarizations, thus the selection rules, may pave the way toward the investigation of bulk and surface materials.

We studied the electronic Raman spectra of (Li_{1-x}Fe_{x})OHFeSe as a function of light polarization and temperature. In the B_{1g} spectra alone we observe the redistribution of spectral weight expected for a superconductor and two well-resolved peaks below T_{c}. The nearly resolution-limited peak at 110  cm^{-1} (13.6 meV) is identified as a collective mode. The peak at 190  cm^{-1} (23.6 meV) is presumably another collective mode since the line is symmetric and its energy is significantly below the gap energy observed by single-particle spectroscopies. Given the experimental band structure of (Li_{1-x}Fe_{x})OHFeSe, the most plausible explanations include conventional spin-fluctuation pairing between the electron bands and the incipient hole band and pairing between the hybridized electron bands. The absence of gap features in A_{1g} and B_{2g} symmetry favors the second case. Thus, in spite of various differences between the pnictides and chalcogenides, this Letter demonstrates the proximity of pairing states and the importance of band structure effects in the Fe-based compounds.

Theoretical analysis typically involves imaginary-time correlation functions. Inferring real-time dynamical response functions from this information is notoriously difficult. However, as we articulate here, it is straightforward to compute imaginary-time correlators from the measured frequency dependence of (real-time) response functions. In addition to facilitating comparison between theory and experiment, the proposed approach can be useful in extracting certain aspects of the (long-time relaxational) dynamics from a complex data set. We illustrate this with an analysis of the nematic response inferred from Raman scattering spectroscopy on the iron-based superconductor Ba(Fe1-xCox)(2)As-2, which includes a new method for identifying a putative quantum-critical contribution to that response.

Unconventional superconductivity generally occurs in proximity to magnetic phases. Raman spectroscopy affords a window into most of the related excitations including pairing interactions. Thus, the superconducting ground state and putative sub-leading pairing tendencies can be probed along with magnetic excitations and fluctuations. In this thesis, Raman spectroscopy identifies the hierarchy of pairing channels in CaKFe4As4 and critical fluctuations in Ba(Fe1-xCox)2As2. The fluctuations are analysed using a novel method tailored for the low energy Raman response.

We report inelastic light scattering results on the stoichiometric and fully ordered superconductor CaKFe_4As_2 as a function of temperature and light polarization. In the energy range between 10 and 315 cm^-1 (1.24 and 39.1 meV) we observe the particle-hole continuum above and below the superconducting transition temperature T_c and 7 of the 8 Raman active phonons. The main focus is placed on the analysis of the electronic excitations. Below T_c all three symmetries projected with in-plane polarizations display a redistribution of spectral weight characteristic for superconductivity. The energies of the pair-breaking peaks in A_1g and B_2g symmetry are in approximate agreement with the results from photoemission studies. In B_1g symmetry the difference between normal and superconducting state is most pronounced, and the feature is shifted downwards with respect to those in A_1g and B_2g symmetry. The maximum peaking at 134 cm^-1 (16.6 meV) has a substructure on the high-energy side. We interpret the peak at 134 cm^-1 in terms of a collective Bardasis-Schrieffer (BS) mode and the substructure as a remainder of the pair-breaking feature on the electron bands. There is a very weak peak at 50 cm^-1 (6.2 meV) which is tentatively assigned to another BS mode.

We report results of Raman scattering experiments on twin-free BaFe2As2 with the main focus placed on understanding the influence of electronic and spin degrees of freedom on the lattice dynamics. In particular, we scrutinize the E-g modes and the As A(1g) mode. Each of the two E-g phonons in the tetragonal phase is observed to split into a B-2g and a B-3g mode upon entering the orthorhombic stripe-magnetic phase. The splitting amounts to approximately 10 cm(-1) and less than 5 cm(-1) for the low- and the high-energy E-g mode, respectively. The detailed study of the fully symmetric As mode using parallel incident and outgoing photon polarizations along either the anti-ferromagnetic or the ferromagnetic Fe-Fe direction reveals an anisotropic variation of the spectral weight with the energy of the exciting laser indicating a polarization-dependent resonance effect. Along with the experiments we present results from density functional theory calculations of the phonon eigenvectors, the dielectric function, and the Raman tensor elements. The comparison of theory and experiment indicates that (i) orbital-selective electronic correlations are crucial to understand the lattice dynamics and (ii) all phonon anomalies originate predominantly from the magnetic ordering and the corresponding reconstruction of the electronic bands at all energies.

We report the development of a detection optics for the integration of Raman scattering and scanning probe microscopy at low temperature based on a parabolic mirror. In our set-up, half of the paraboloid mirror covers a solid angle of π corresponding to a numerical aperture of N.A. ≈ 0.85. The optical system can be used for far- and near-field spectroscopy. In the far field, the polarizations can be maintained to within 80%–90%. In combination with a scanning microscope (AFM/STM), tunneling or near-field experiments are possible with less than 10% loss of aperture. Our set-up provides ideal conditions for the future development of tip-enhanced Raman spectroscopy at low temperature.

We study the interplay of fluctuations and superconductivity in BaFe2As2 (Ba‐122) compounds with Ba and Fe substituted by K (p doping) and Co (n doping), respectively. To this end, we measured electronic Raman spectra as a function of polarization and temperature. We observe gap excitations and fluctuations for all doping levels studied. The response from fluctuations is much stronger for Co substitution and, according to the selection rules and the temperature dependence, originates from the exchange of two critical spin fluctuations with characteristic wave vectors (±π,0) and (0,±π) . At 22% K doping ( p=0.22 ), we find the same selection rules and spectral shape for the fluctuations but the intensity is smaller by a factor of 5. Since there exists no nematic region above the orthorhombic spin‐density‐wave (SDW) phase, the identification of the fluctuations via the temperature dependence is not possible. The gap excitations in the superconducting state indicate strongly anisotropic near‐nodal gaps for Co substitution which make the observation of collective modes difficult. The variation with doping of the spectral weights of the A1g and B1g gap features does not support the influence of fluctuations on Cooper pairing. Therefore, the observation of Bardasis–Schrieffer modes inside the nearly clean gaps on the K‐doped side remains the only experimental evidence for the relevance of fluctuations for pairing.

Inelastic Raman scattering is used to probe the critical spin fluctuations in an iron pnictide superconductor, providing insights into the origin of nematic order in this system. Nematic fluctuations and order play a prominent role in material classes such as the cuprates1, some ruthenates2 or the iron-based compounds3,4,5,6 and may be interrelated with superconductivity7,8,9,10,11. In iron-based compounds12 signatures of nematicity have been observed in a variety of experiments. However, the fundamental question as to the relevance of the related spin13, charge9,14 or orbital8,15,16 fluctuations remains open. Here, we use inelastic light (Raman) scattering and study Ba(Fe1−xCox)2As2 (0 ≤ x ≤ 0.085) for getting direct access to nematicity and the underlying critical fluctuations with finite characteristic wavelengths17,18,19,20,21. We show that the response from fluctuations appears only in B1g (x2 − y2) symmetry (1 Fe unit cell). The scattering amplitude increases towards the structural transition at Ts but vanishes only below the magnetic ordering transition at TSDW < Ts, suggesting a magnetic origin of the fluctuations. The theoretical analysis explains the selection rules and the temperature dependence of the fluctuation response. These results make magnetism the favourite candidate for driving the series of transitions.