Two-gap superconductivity in R2Fe3Si5 (R=Lu, Sc) and Sc5Ir4Si10

Abstract

R₂Fe₃Si₅ (R= Sc, Y, Lu) contains nonmagnetic iron and has a relatively high superconducting transition temperature T_c among iron-containing superconductors. An anomalous temperature dependence of specific heat C(T) has been reported for polycrystalline samples down to 1 K. We have grown R₂Fe₃Si₅ single crystals, confirmed the anomalous C(T) dependence, and found a second drop in specific heat below 1 K. In Lu₂Fe₃Si₅, we can reproduce C(T) below T_c, assuming two distinct energy gaps 2Δ ₁/k_BT_c = 4.4 and 2Δ ₂/k_BT_c = 1.1, with nearly equal weights, indicating that Lu₂Fe₃Si₅ is a two-gap superconductor similar to MgB₂. Hall coefficient measurements and band structure calculation also support the multiband contributions to the normal-state properties. The specific heat in the Sc₂Fe₃Si₅ single crystals also shows the two-gap feature. R₅Ir₄Si₁₀ (R = Sc, rare earth) is also a superconductor where competition between superconductivity and the charge-density wave is known for rare earths but not for Sc. We have performed detailed specific heat measurements on Sc₅Ir₄Si₁₀ single crystals and found that C(T) deviates slightly from the behavior expected for weak-coupling superconductors. C(T) for these superconductors can also be reproduced well by assuming two superconducting gaps.

Keywords: Lu2Fe3Si5; Sc2Fe3Si5; Sc5Ir4Si10; two-gap superconductivity.

Figure 1

Crystal structures of (a) R₂Fe₃Si₅ (R = Sc, Lu) and (b) Sc₅Ir₄Si₁₀. Large green circles are Lu or Sc atoms, medium-sized brown circles are Fe or Ir atoms, and small blue circles are Si atoms.

Figure 2

Effect of annealing at 1250 °C on temperature dependence of resistivity in Lu₂Fe₃Si₅. Inset shows the magnetization at H = 50 kOe for different annealing times.

Figure 3

Temperature dependence of Hall coefficient for (a) I∥c, H∥ab and (b) I∥ab, H∥c in Lu₂Fe₃Si₅ [10].

Figure 4

Fermi surfaces of Lu₂Fe₃Si₅ calculated by the FLAPW method [10]. Hole-like Fermi surfaces from the (a) 155th and (b) 156th bands, and (c) electron-like Fermi surface from the 157th band are shown.

Figure 5

Temperature dependence of specific heat for Lu₂Fe₃Si₅ [10]. C_e(T) based on the BCS model is shown by dashed line. The solid line shows the best fit using the two-gap model with D₁/k_BT_B=44 and D₂/k_BT_B=1.1. Inset shows an Arrhenius plot of C_e(T) and suggests the presence of the smaller gap.

Figure 6

Temperature dependence of resistivity along ab-axis for Sc₂Fe₃Si₅. Inset shows a magnification close to the superconducting transition temperature.

Figure 7

Temperature dependence of upper critical fields, H_c2^c and H_c2^ab, estimated from the magnetization-temperature curves for Sc₂Fe₃Si₅.

Figure 8

Temperature dependence of specific heat for Sc₂Fe₃Si₅. C_e(T) based on the BCS model is shown by dashed line. The solid line shows the best fit using the two-gap model with D₁/k_BT_B=3.53 (36%) and D₂/k_BT_B=1.70 (64%).

Figure 9

Temperature dependence of the upper critical fields, H_c2^c and H_c2^ab, estimated from the M–T (circles) and M–H (squares) curves for Sc₅Ir₄Si₁₀.

Figure 10

Temperature dependence of specific heat for Sc₅Ir₄Si₁₀. C_e(T) based on the BCS model is shown by dashed line. The solid line shows the best fit using the two-gap model. Inset shows Arrhenius plot of C_e(T). The solid line is a guide for the eye suggesting the presence of smaller gap.