Electron-phonon coupling and superconductivity-induced distortion of the phonon lineshape in V3Si

Abstract

Phonon measurements in the A15-type superconductors were complicated in the past because of the unavailability of large single crystals for inelastic neutron scattering, e.g. in the case of Nb3Sn, or unfavorable neutron scattering properties in the case of V3Si. Hence, only few studies of the lattice dynamical properties with momentum resolved methods were published, in particular below the superconducting transition temperature . Here, we overcome these problems by employing inelastic x-ray scattering and report a combined experimental and theoretical investigation of lattice dynamics in V3Si with the focus on the temperature-dependent properties of low-energy acoustic phonon modes in several high-symmetry directions. We paid particular attention to the evolution of the soft phonon mode of the structural phase transition observed in our sample at = ., i.e., just above the measured superconducting phase transition at = .. Theoretically, we predict lattice dynamics including electron-phonon coupling based on density-functional-perturbation theory and discuss the relevance of the soft phonon mode with regard to the value of . Furthermore, we explain superconductivity-induced anomalies in the lineshape of several acoustic phonon modes using a model proposed by Allen et al. [Physical Review B 56, 5552 (1997)].

Peierls-type structural phase transition in the low-dimensional superconductor Sc3CoC4

“…Among the best studied representatives are the charge-density-wave (CDW) compounds [34][35][36][37][38][39] and the binary A15 compounds [105,106] displaying low-dimensional and three-dimensional structural motifs, respectively. Similar to our results for Sc 3 CoC 4 above, many of these superconducting materials have been found to undergo phonon softening processes towards their structural phase transition temperatures [34,62,90,91,96,97,[105][106][107][108][109][110][111][112][113][114][115]. Furthermore, the structural and superconducting phase transitions in Sc 3 CoC 4 , CDWs and A15 compounds are susceptible to a manipulation by means of applied strain or hydrostatic pressure [34-36, 40-44, 51, 52, 57, 105, 111, 116-118].…”

“…In line with the condensation of a zone-center phonon mode [114,115], the symmetry reduction in V 3 Si and Nb 3 Sn from space group P m3n to space group P 4 2 /mmc involves only a translationengleiche (t3) step and no doubling of the unit cell volume [106,[124][125][126] like in the HT-LT transition of Sc 3 CoC 4 . The structural changes due to the tetragonal distortion of the A15 compounds therefore do not allow an alternative description by a commensurate or incommensurate displacive modulation of the cubic parent phase.…”

Superconductivity in the cobalt-doped V3Si A15 intermetallic compound

“…We speculate that a structural phase transition may have occurred at T * in the high-content cobalt-doped samples (x ⩾ 0.15). Previous studies have shown a martensitic cubic to tetragonal phase transition at a temperature of about 18.9 K in the V 3 Si compound [16][17][18][19]. Besides, the hydrostatic pressure will gradually suppress this martensitic phase transition [36,37].…”

“…These include the anisotropy of the upper critical field and specific heat, De Hass-Van Alphen effect, and large ratio of T c with Fermi temperature (T F ). Strong evidence exists that a martensitic cubic to tetragonal phase transition occurs at about 18.9 K, slightly above the T c [16][17][18][19]. Unusual softening of the acoustic phonons in V 3 Si has been reported [20,21].…”

“…Unusual softening of the acoustic phonons in V 3 Si has been reported [20,21]. The inelastic x-ray scattering investigation of the lattice dynamics of V 3 Si indicates only a small impact of the soft phonon mode of the martensitic transition on the superconducting properties [17]. The theoretical analysis and experimental results recently present strong support for an s ++ pairing with two barely coupled gaps and weak interband coupling in the V 3 Si superconductor [22], which is very similar to that of the MgB 2 superconductor [23].…”