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Resolving the microscopic pairing mechanism and its experimental identification in unconventional superconductors is among the most vexing problems of contemporary condensed matter physics. We show that Raman spectroscopy provides an avenue towards this aim by probing the structure of the pairing interaction at play in an unconventional superconductor. As we study the spectra of the prototypical Fe-based superconductor Ba 1−x K x Fe 2 As 2 for 0.22 ≤ x ≤ 0.70 in all symmetry channels, Raman spectroscopy allows us to distill the leading s-wave state. In addition, the spectra collected in the B 1g symmetry channel reveal the existence of two collective modes which are indicative of the presence of two competing, yet subdominant , pairing tendencies of d x 2 Ày 2 symmetry type. A comprehensive functional Renormalization Group and random-phase approximation study on this compound confirms the presence of the two sub-leading channels, and consistently matches the experimental doping dependence of the related modes. The consistency between the experimental observations and the theoretical modeling suggests that spin fluctuations play a significant role in superconducting pairing.

We report inelastic light scattering results on the stoichiometric and fully ordered superconductor CaKFe4As4 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 Tc and 7 of the 8 Raman active phonons. The main focus is placed on the analysis of the electronic excitations. Below Tc 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 A1g and B2g symmetry are in approximate agreement with the results from photoemission studies. In B1g symmetry the difference between normal and superconducting state is most pronounced, and the feature is shifted downwards with respect to those in A1g and B2g 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.

Quantification of bacteria attached to sand grains is usually not possible without destroying biofilms or the sand structure. In situ quantification of autofluorescing or of fluorescence-carrying bacteria might be an alternative if fluorescence intensity (FI) was strong enough and could be related to cell numbers. For calibration, a Hele-Shaw cell was filled with sterile quartz sand and sectioned for inoculation with increasing cell densities of Escherichia coli strain HB101 K12 pGLO, overexpressing green fluorescent protein (GFP). Digital photographs of the single sections were taken under ultraviolet light at wavelength l = 365 nm for fluorescence quantification by TotalLab Quant software. The FI was related to respective cell densities per quartz sand volume. Growth of GFP-labeled E. coli, suspended in Lysogeny broth (LB) medium (dissolved organic C = 6.8 g L −1 ), could thus be quantified noninvasively in capillary fringe (CF) laboratory systems simulating static or hydrodynamic conditions. Best growth conditions existed in the transition zone of the CF, where after 3 d the highest cell densities of 5.0 to 6.5 ´ 10 8 cells cm −3 were determined. The E. coli cells that grew attached as a biofilm in the different sections of the CF could also be quantified under flow-through conditions and after fluctuation of the water table. For reliable results, FI calibration must be performed in the same setup as used for experiments, and fluorescence characteristics of the bacterial strain should be well known. This noninvasive method using bright fluorescing bacteria can be advantageous for the determination of bacterial growth and biofilm formation in laboratory experiments with unsaturated soil under changing hydraulic conditions.

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In tests with Hele–Shaw cells containing silica sand, O2 profiles were measured after formation of a capillary fringe (CF) with a nutrient broth, that was inoculated with Pseudomonas putida. After 7 d, the CF became anaerobic up to 2 cm above the water level. Within the next 3 cm at decreasing water saturation, the O2 concentration approached its maximum value. After growth, most P. putida cells were found at the 4‐ to 5‐cm height of the CF, whereas cell densities decreased significantly toward the top of the CF due to water‐limited respiratory activity. This was shown in separate batch experiments by measuring respiration rates of P. putida for defined water saturations. In silica sand (rough surface) or glass beads (smooth surface), respiration of P. putida was significantly reduced at a water saturation of ≤7.5% but was always higher for growth on sand than on glass beads, presumably due to different water bioavailability at the surface. The highest O2 consumption rates of P. putida in both media were measured at 60 to 85% water saturation, which represents the saturated–unsaturated interface region in a CF. To find the minimum required water availability for respiration, batch assays with increasing agar concentrations in LB medium were conducted. The O2 consumption rates of P. putida decreased to zero above an agar concentration of 18% (w/v), equivalent to a water activity <0.978. The results indicate that limited bacterial respiration due to water potential stress would only occur in the top few millimeters of a CF in sand.

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 Eg modes and the As A1g mode. Each of the two Eg phonons in the tetragonal phase is observed to split into a B2g and a B3g 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 Eg mode, respectively. The detailed study of the fully symmetric As mode using parallel incident and outgoing photon polarizations along either the antiferromagnetic 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.

Laboratory‐scale flow‐through experiments in a sand‐filled chamber were performed to investigate the impact of aerobically growing Escherichia coli HB101‐K12 on oxygen transfer across the capillary fringe (CF) to oxygen‐depleted groundwater. In the experiments, both the effects of different nutrient concentrations and transient flow conditions were tested. The results of biotic experiments under oligotrophic and eutrophic conditions (6.8 g organic C L−1) were compared with those of an abiotic experiment. Moreover, in each experiment steady‐state and transient conditions were considered, the latter induced by changing the water‐table height. Growth of E. coli was quantified by cell counting in effluent samples and could be monitored due to intracellular production of the green fluorescent protein (GFP). Under eutrophic conditions, highest cell densities and strongest cell attachment were observed in the transition region of the CF. In this region, intensive bacterial respiration decreased oxygen concentrations relatively quickly, causing a steep oxygen gradient that resulted in a higher oxygen flux across the unsaturated–saturated interface. Under oligotrophic conditions, this effect was considerably reduced, but was still detectable. Due to oxygen supply from entrapped air, bacterial growth was slightly enhanced within the upper, newly formed CF region, directly after raising the water table. After lowering the water table to the initial height only a minor microbial impact on oxygen transfer was noticed, even under eutrophic conditions, because of the fast diffusion of oxygen in the (partially) air‐filled pore spaces. Bacteria that remained in the transition region of the CF grew to higher densities due to better oxygen supply.