# Calculated Changes in the Elastic Properties of MgCNi3 at the Superconducting Transition

Authors: R. Abd-Shukor
Journal: Advances in Materials Science and Engineering
 (2013)

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## Abstract

We calculated the elastic properties of MgCNi3 at the superconducting transition (Tc) using various thermodynamic and acoustic data. From the calculations, a step discontinuity of 8 ppm in the bulk modulus, 7 ppm in the Young’s modulus, and 3 ppm in the longitudinal sound velocity (vl) is expected at Tc. The step discontinuities at the transition temperature indicated the importance of lattice changes to the superconducting mechanism of MgCNi3. The Debye temperature was calculated to be 460 K. The electron-phonon coupling constants calculated in the weak and strong coupling limits of the BCS theory and the van Hove scenario showed that MgCNi3 is a moderately strong coupled superconductor.


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## Body

## 1. Introduction

Superconductivity in MgCNi3 with transition temperature (Tc) near 8 K is of interest especially from the pairing mechanism point of view. The electron-phonon coupling constant of MgCNi3 has been determined using specific heat measurements [1]. Evidence of strong electron-phonon coupling and the van Hove type singularity in the electronic density of states near the Fermi energy has also been suggested [2].The nature of superconductivity in this material is still controversial. A step discontinuity in the longitudinal sound velocity indicating lattice softening has been observed in the conventional superconductors. Thus, information on the lattice properties atTc is very useful in understanding the MgCNi3 superconductor. The acoustic method is a sensitive probe of phonon states and electron-phonon coupling in materials. The elastic properties of MgCNi3 have also been reported [3, 4].The objectives of this work were to estimate the electron-phonon coupling constant, changes in the elastic properties, and sound velocity at the transition temperature of MgCNi3. The elastic properties of MgCNi3 including the change in the bulk and Young’s moduli and discontinuity in the longitudinal sound velocity at Tc were calculated using various sound velocity and thermodynamic data. The acoustic Debye temperature was also calculated and compared to reported values determined by other methods. The electron-phonon coupling constant in various scenarios is also reported.


## 2. Basic Formulation

The standard isotropic elastic medium approximation can be applied to the polycrystalline samples. In this case, two independent elastic moduli, namely, the shear modulusG can be written as G=ρνs2, and the bulk modulus B can be written as B=ρνl2-(4/3)ρνs2, where ρ is the mass density, νs is the shear velocity, and νl is the longitudinal velocity [5]. The longitudinal modulus can be written as L=ρνl2, and the Young’s modulus is Y=G(3L-4G)/(L-G).For the superconducting transition in zero fields, the thermodynamics of second-order phase transition [6, 7] gives the following anomalies: (1a)ΔBB=-ΔCpTcB(∂Tc∂P)2,(1b)ΔYY=-ΔCpTcY(∂Tc∂σi)2,where σi is the ith component of the stress, ΔCp is the discontinuity in specific heat at Tc, and P is the pressure. B is expected to show a step discontinuity, and G is supposed to show a change in the slope at Tc from the mean-field transition [6]. The change in B can be written as
(2)ΔBB≈νl2(2Δνl/νl)-(4/3)νs2(2Δνs/νs)νl2-(4/3)νs2.


## 3. Results and Discussion

Several papers have reported the elastic moduli of MgCNi3 [3, 4, 8] with a considerable spread in values. Because this material is usually prepared in the polycrystalline form, in this paper, we have used values for polycrystalline form as reported by Shein et al. [3]. Using bulk modulus B=171.05 GPa, shear modulus of G=61.46, and Young’s modulus Y=168.16 GPa as published previously [3], the longitudinal sound velocity was calculated to be νl=6270 m/s and shear sound velocity νs=3100 m/s.The sound velocities can be used to calculate the acoustic Debye temperature by using the standard formulaθD=(h/kB)(3N/4πV)1/3νm, where h is the Planck’s constant, kB is the Boltzmann’s constant, N is the number of mass-point, V is the atomic volume, and νm is the mean velocity given by 3/νm3=1/νl3+2/νs3. Using this expression, the acoustic Debye temperature of MgCNi3 is calculated to be 460 K. This is near to the θD values calculated theoretically [4, 8, 9] but much higher than the value determined from specific heat measurement [10, 11].The BCS theory for weak coupling givesTc=1.13θDe-1/λ, where λ is the electron-phonon coupling. The electron-phonon coupling constant for the weak coupling case calculated using the previous acoustic Debye temperature is λ=0.25.The relevance of van Hove density of states has been suggested for MgCNi3 [2]. The singularity in the density of states at the Fermi level was taken into account in the van Hove scenario of a two-dimensional system. In this scenario, Tc is given as Tc=2.72TFe(-1/λ), where TF=EF/k≈10θD (EF is the Fermi energy) [12]. The electron-phonon coupling constant calculated using this formula is λ=0.018.The McMillan expression for the strong coupling BCS case is given as [13] Tc=(θD/1.45)exp[-1.04(1+λ)/(λ-μ*(1+0.62λ))], where μ* (with conventional values of 0.05-0.1) is the screened Coulomb repulsion between electrons in a Cooper pair. The electron-phonon coupling constant derived from this expression ranged from 0.49 to 0.59. This is consistent with most measured values for MgCNi3 [1, 10]. Thus, this material is a moderately strong coupled superconductor.The following is a discussion on the elastic properties of MgCNi3 at Tc. Using (1a), ΔCp/Tc=66 mJ/mol K2 [10], and ∂Tc/∂P=0.15 K/GPa [14], the calculated step discontinuity in the bulk modulus is 8 ppm. Using (2), the discontinuity in the longitudinal velocity is 3 ppm. By assuming that ∂Tc/∂σi≈∂Tc/∂P, the expected discontinuity in the Young’s modulus calculated using (1b) is 7 ppm. These discontinuities are similar to Pb [15] and LiFeAsOF [16] superconductors but are much smaller than the calculated [17] and observed [18] values in the cuprate-based high Tc superconductors and MgB2 [19]. Table 1 shows the sound velocity and changes in bulk and Young’s modulus at Tc of MgCNi3 and other superconductors.Table 1
Sample
T
c (K)
v
l (m s−1)
v
s (m s−1)
θ
D (K)
Δ
B
/
B (ppm)
Δ
v
l
/
v
l (ppm)
Δ
Y
/
Y (ppm)
MgCNi3 (this work)
8
7980
4800
460
8
3
7
LaFeAsO0.89F0.11 [16]
26
5090
2900
367
16
5
—
MgB2 [19]
39
9650
5070
880
66
21
76
LaOFeP [20]
5.8
7980
4800
491
59
15
101
LiFeAs [20]
18
8250
4820
509
25
7
38The second-order phase transition does not allow step discontinuity in the shear velocity atTc in zero fields; hence, the shear velocity was assumed continuous at Tc. In these calculations, the thermodynamic data used are representative of the various reported values, which are of the same order of magnitude. The expected step discontinuity in the longitudinal velocity in MgCNi3 is similar to the measured value of Pb which is a BCS-type superconductors and are within the resolution range of most ultrasonic apparatus. Our calculations showed that MgCNi3 is a moderately strong coupled BCS-type superconductor.



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*Source: 247393-2013-01-16.xml*