We present a detailed study of 75 As nuclear magnetic resonance Knight shift and spin-lattice relaxation rate in the normal state of stoichiometric polycrystalline LiFeAs. Our analysis of the Korringa relation suggests that LiFeAs exhibits strong antiferromagnetic fluctuations, if transferred hyperfine coupling is a dominant interaction between 75 As nuclei and Fe electronic spins, whereas for an on-site hyperfine coupling scenario, these are weaker, but still present to account for our experimental observations. Density-functional calculations of electric field gradient correctly reproduce the experimental values for both 75 As and 7 Li sites.

Mixed spin-singlet and spin-triplet pairing can occur in noncentrosymmetric superconductors. In this respect, a comprehensive characterization of the noncentrosymmetric superconductor BeAu was carried out. It was established that BeAu undergoes a structural phase transition from a low-temperature noncentrosymmetric FeSi structure type to a high-temperature centrosymmetric structure in the CsCl type at T-s = 860 K. The low-temperature modification exhibits a superconducting transition below T-c = 3.3 K. The values of lower (H-c1 = 32 Oe) and upper (H-c2 = 335 Oe) critical fields are rather small, confirming that this type-II (kappa(G-L) = 2.3) weakly coupled (lambda(e-p) = 0.5, Delta C-e/gamma T-n(c) approximate to 1.26) superconductor can be well understood within the Bardeen-Cooper-Schrieffer theory. The muon spin relaxation analysis indicates that the time-reversal symmetry is preserved when the superconducting state is entered, supporting conventional superconductivity in BeAu. From the density functional band structure calculations, a considerable contribution of the Be electrons to the superconducting state was established. On average, a rather small mass renormalization was found, consistent with the experimental data

We explored the role of valence electron concentration in bond formation and superconductivity of mixed silicon−aluminum networks by using high-pressure synthesis to obtain the BaAl 4 -type structural pattern in solid solution samples SrAl 4−x Si x where 0 ≤ x ≤ 2. Local ordering of aluminum and silicon in SrAl 4−x Si x was evidenced by nuclear magnetic resonance experiments. Subsequent bonding analysis by quantum chemical techniques in real space demonstrated that the strong deviation of the lattice parameters in SrAl 4−x Si x from Vegard's law can be attributed to the strengthening of interatomic Al−Al and Al−Si bonds within the layers (perpendicular to [001]) for 0 ≤ x ≤ 1.5, followed by the breaking of the interlayer bonds (parallel to [001]) for 1.5 < x ≤ 2 and leading to the structural transition from the BaAl 4 structure type with three-dimensional anionic framework at lower x values to the two-dimensional anion of the BaZn 2 P 2 structure type with increasing x values. Low-temperature measurements of the resistivity and heat capacity reveal that SrAl 2.5 Si 1.5 and SrAl 2 Si 2 prepared at high pressures exhibit superconductivity with critical temperatures of 2.1 and 2.6 K, respectively.

In search of the origin of superconductivity (SC) in diluted rhenium superconductors and their significantly enhanced T c compared to pure Be (0.026 K), we investigated the intermetallic ReBe 22 compound, mostly by means of muon-spin rotation/relaxation (μSR). At a macroscopic level, its bulk SC (with T c =9.4 K) was studied via electrical resistivity, magnetization, and heat-capacity measurements. The superfluid density, as determined from transverse-field μSR and electronic specific-heat measurements, suggest that ReBe 22 is a fully-gapped superconductor with some multigap features. The larger gap value, D = 1.78 l 0 T k B c , with a weight of almost 90%, is slightly higher than that expected from the BCS theory in the weak-coupling case. The multigap feature, rather unusual for an almost elemental superconductor, is further supported by the field-dependent specific-heat coefficient, the temperature dependence of the upper critical field, as well as by electronic bandstructure calculations. The absence of spontaneous magnetic fields below T c , as determined from zero-field μSR measurements, indicates a preserved time-reversal symmetry in the superconducting state of ReBe 22 . In general, we find that a dramatic increase in the density of states at the Fermi level and an increase in the electron-phonon coupling strength, both contribute to the highly enhanced T c value of ReBe 22 .