[en] We discuss the synthesis, characterization, and comprehensive study of Ba-122 single crystals with various substitutions and various superconducting transition temperatures. We use five complementary techniques to obtain a self-consistent set of data on the superconducting properties of Ba-122. A major conclusion of our work is the coexistence of two superconducting condensates differing in the electron–boson coupling strength. The two gaps that develop in distinct Fermi surface sheets are nodeless in the k_xk_y plane and exhibit s-wave symmetry; the two-band model suffices for the description of the main parameters of the superconducting state. A moderate interband coupling and a considerable Coulomb repulsion in the description of the two-gap superconducting state of barium pnictides favor the s⁺⁺ model. (conferences and symposia)

[en] The reflectance spectra of the topological insulator Pb_1–xSn_xSe (x = 0.2, 0.34) films grown by molecular-beam epitaxy on a ZnTe/GaAs substrate are measured in the range of 12–2500 cm^–1 at room temperature. Using dispersion analysis, the frequencies of transverse optical phonons, plasma frequencies, highfrequency permittivities, and layer thicknesses are determined. In the quasi-steady-state approximation, the interface mode frequencies of the four-layer structure are calculated as a function of the overlap parameter χ₁ (0 ≤ χ₁ ≤ 1). The parameter describes the degree of overlap of two interface modes localized at planes bounding the layer to the right and left. The existence of interacting interface modes in the structure makes its spectrum different from the sum of component spectra. These differences manifest themselves in the experiment. The conditions of the interaction of interface modes with IR radiation are discussed.

[en] The infrared reflection spectra of PbTe/CdTe multilayer nanostructures grown by molecular-beam epitaxy are measured in the frequency range of 20–5000 cm"−"1 at room temperature. The thicknesses and high-frequency dielectric constants of the PbTe and CdTe layers and the frequencies of the transverse optical (TO) phonons in these structures are determined from dispersion analysis of the spectra. It is found that the samples under study are characterized by two TO phonon frequencies, equal to 28 and 47 cm"−"1. The first frequency is close to that of TO phonons in bulk PbTe, and the second is assigned to the optical mode in structurally distorted interface layers. The Raman-scattering spectra upon excitation with the radiation of an Ar"+ laser at 514.5 nm are measured at room and liquid-nitrogen temperatures. The weak line at 106 cm"−"1 observed in these spectra is attributed to longitudinal optical phonons in the interface layers

[en] Optical spectroscopy was used to study the electrodynamics and hidden transport properties of a BaFe_1.91Ni_0.09As₂ thin superconducting (SC) film. We analyzed the normal state data using a Drude–Lorentz model with two Drude components: one narrow (D₁) and another broad one (D₂). In the SC state, two gaps with $2{\mathrm{\Delta <!--\; ??\; -->}}_0^{\left(2\right)}/k_{\text{B}}{T}_{\text{c}}=1.9$–2.0 and $2{\mathrm{\Delta <!--\; ??\; -->}}_0^{\left(1\right)}/k_{\text{B}}{T}_{\text{c}}=4.0$–4.3 are formed from the narrow component D₁ while the broad component D₂ remains ungapped. The calculated total DC resistivity of the film and the low-temperature scattering rate for the narrow Drude component show a hidden Fermi-liquid behavior. The change of total electron–boson coupling (λ _tot) and representative energy (Ω₀) in the normal state with respect to the SC state is typical of other iron-based materials as well as high-temperature superconducting (HTSC) cuprates. (paper)

[en] The temperature dependences of the plasma frequency, superfluid density and London penetration depth were determined from the terahertz spectra of conductivity and the dielectric permittivity of BaFe_1.91Ni_0.09As₂ film with critical temperature T _c = 19.6 K. Part of the experimental data were analyzed within a simple three-band Eliashberg model where the mechanism of superconducting coupling is mediated by antiferromagnetic spin fluctuations, whose characteristic energy Ω₀ scales with T _c according to the empirical law ${\mathrm{\Omega <!--\; ??\; -->}}_0=4.65k_B{T}_c$, and with a total electron-boson coupling strength λ _tot = 2.17. (paper)

[en] In a superconductor that lacks inversion symmetry, the spatial part of the Cooper pair wave function has a reduced symmetry, allowing for the mixing of spin-singlet and spin-triplet Cooper pairing channels and thus providing a pathway to a non-trivial superconducting state. Materials with a non-centrosymmetric crystal structure and with strong spin–orbit coupling are a platform to realize these possibilities. Here, we report the synthesis and characterisation of high quality crystals of Sn₄As₃, with non-centrosymmetric unit cell (R3m). We have characterised the normal and superconducting states using a range of methods. Angle-resolved photoemission spectroscopy shows a multiband Fermi surface and the presence of two surface states, confirmed by density-functional theory calculations. Specific heat measurements reveal a superconducting critical temperature of T _c ∼ 1.14 K and an upper critical magnetic field of μ ₀ H _c ≳ 7 mT, which are both confirmed by ultra-low temperature scanning tunneling microscopy and spectroscopy. Scanning tunneling spectroscopy shows a fully formed superconducting gap, consistent with conventional s-wave superconductivity. (paper)

[en] Accurate low temperature charge transport measurements in combination with high-precision x-ray diffraction experiments have allowed detection of the symmetry lowering in the single domain Tm_0.19Yb_0.81B₁₂ crystals that belong to the family of dodecaborides with metal–insulator transition. Based on the fine structure analysis we discover the formation of dynamic charge stripes within the semiconducting matrix of Tm_0.19Yb_0.81B₁₂. The charge dynamics in these conducting nano-size channels is characterized by broad-band optical spectroscopy that allowed estimating the frequency (∼2.4 × 10¹¹ Hz) of quantum motion of the charge carriers. It is suggested that cooperative Jahn–Teller effect in the boron sublattice is a cause of the large-amplitude rattling modes of the Tm and Yb ions responsible for the ‘modulation’ of the conduction band along one of the directions through the variation of 5d-2p hybridization of electron states. (paper)