N. Shitsevalova — Page 2

arXiv2005

Superconductivity mediated by a soft phonon mode: specific heat, resistivity, thermal expansion and magnetization of YB6

The superconductor YB6 has the second highest critical temperature Tc among the boride family MBn. We report measurements of the specific heat, resistivity, magnetic susceptibility and thermal expansion from 2 to 300 K, using a single crystal with Tc = 7.2 K. The superconducting gap is characteristic of medium-strong coupling. The specific heat, resistivity and expansivity curves are deconvolved to yield approximations of the phonon density of states, the spectral electron-phonon scattering function and the phonon density of states weighted by the frequency-dependent Grueneisen parameter respectively. Lattice vibrations extend to high frequencies >100 meV, but a dominant Einstein-like mode at ~8 meV, associated with the vibrations of yttrium ions in oversized boron cages, appears to provide most of the superconducting coupling and gives rise to an unusual temperature behavior of several observable quantities. A surface critical field Hc3 is also observed.

cond-mat.supr-con↗

arXiv2003

Characterization of the electronic properties of YB$_4$ and YB$_6$ using $^{11}$B NMR and first-principles calculations

Two compounds, tetragonal YB$_4$ and cubic YB$_6$, have been investigated by electric-field gradient (EFG) and Knight shift measurements at the boron sites using the $^{11}$B nuclear magnetic resonance (NMR) technique and by performing first-principles calculations. In YB$_6$ $^{11}$B ($I=3/2$) NMR spectra reveal patterns typical for an axially symmetric field gradient with a quadrupole coupling frequency of $ν_Q=600\pm15$ kHz. In the second boride (YB$_4$) three different EFGs were observed corresponding to the three inequivalent crystallographic sites for the boron atoms ($4h$, $4e$, and $8j$). They correspond to: $ν_Q(4h)=700\pm30$ kHz with an asymmetry parameter $η=0.02\pm0.02$, $ν_Q(4e)=515\pm30$ kHz, $η=0.00+0.02/-0.00$, and $ν_Q(8j)=515\pm40$ kHz, $η=0.46\pm0.08$. The Knight shifts measured by Magic-Angle Spinning (MAS) NMR at room temperature are very small being $0.6\pm8$ ppm and $-1\pm8$ ppm for YB$_4$ and YB$_6$, respectively. For the theoretical calculations structure optimizations were performed as a first step. For the obtained structural parameters the EFGs were computed within the local-density approximation. Very satisfactory agreement between experimental and theoretical results is obtained both for the structural parameters and the B EFGs thus confirming the underlying structural models. In addition to the EFGs, band structures, densities of states, and valence-electron densities are presented and the bonding situation in the two yttrium borides is discussed. The band-structure results are compatible with the very low values for the Knight shifts mentioned above.

cond-mat↗