Superconductivity in ternary borides MReB (M = Mo, W) with the CuAl2-type structure

doi:10.1016/j.jallcom.2020.154855

Journal of Alloys and Compounds

Volume 832, 15 August 2020, 154855

https://doi.org/10.1016/j.jallcom.2020.154855 Get rights and content

Highlights

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In both MoReB and WReB, the Mo/W and Re atoms share the same crystallographic site.
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Both borides are found to be weakly coupled, fully gapped, type-II superconductors.
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Tc of these borides is not determined primarily by the density of states at the Fermi level.
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A linear temperature dependence of upper critical field is observed in both cases, ascribing to Fermi surface anisotropy or multiband effects.

Abstract

We report the structural and superconducting properties of ternary transition metal borides MoReB and WReB. Both compounds crystallize in the tetragonal CuAl₂-type structure, in which the Re and Mo/W atoms share the same crystallographic site and form one-dimensional zigzag chains along the c-axis. The results of resistivity, magnetic susceptibility, and specific heat measurements indicate that these borides are weakly coupled, fully gapped, type-II superconductors. The superconducting transition temperature $T_{c}$ , upper critical field, and Ginzburg-Laudau parameter are determined to be 5.25 K, 2.1 T, 15.6 for MoReB, and 5.30 K, 2.0 T, 15.7 for WReB. Despite their nearly identical $T_{c}$ values, the Sommerfeld coefficient of WReB is more than 20% smaller than that of MoReB, suggesting that the electronic density of states at the Fermi level is not the dominant factor in determining $T_{c}$ . In addition, the upper critical fields of both MoReB and WReB exhibit an anomalous linear temperature dependence and possible origins are discussed.

Introduction

Boride intermetallics have long been regarded as potential candidates for high temperature superconductors because of the light mass of boron, which could give rise to high frequency phonon modes and strong electron-phonon coupling [1]. Decades of efforts culminated in the discovery of superconductivity at 39 K in simple MgB₂ [2]. In comparison, binary borides of other main group metals, such as AlB₂ [3], BeB_2.75 [4], are either nonsuperconducting or superconducting with $T_{c}$ below 1 K. On the other hand, superconductivity is observed in a large number of transition-metal borides, with stoichiometries ranging from metal-rich to boron-rich. Examples include Re₃B ( $T_{c}$ = 4.7 K) [5], Mo₂B ( $T_{c}$ = 5.85 K) [6], W₂B ( $T_{c}$ = 3.22 K) [6], Re₇B₃ ( $T_{c}$ = 3.3 K) [7], WB_4.2 ( $T_{c}$ = 2.05 K) [8], ZrB₁₂ ( $T_{c}$ = 6.03 K) [9]. In recent years, W₇Re₁₃B ( $T_{c}$ = 7.1 K) [10], Mo₇Re₁₃B ( $T_{c}$ = 8.3 K) [11], Mo₂Re₃B ( $T_{c}$ = 8.5 K) [12], and Cr₂Re₃B ( $T_{c}$ = 4.8 K) [13] are found to be superconductors one after another. These ternary borides, all containing Group VIB elements and 5d transition metal Re, appear to have higher $T_{c}$ compared with binary ones (except MgB₂), which motivates us to study superconductivity in other (IVB)-Re-B systems.

In this respect, it is noted that Mo_2−xRe_xB and W_2−xRe_xB have been studied by Havinga $e t$ $a l$ . in early 1970s [14,15]. Nearly single phase samples were obtained by splat cooling up to x $\approx$ 1 for Mo_2−xRe_xB and up to x $\approx$ 1.3 for W_2−xRe_xB [14]. While these samples were found to have the same CuAl₂-type structure as Mo₂B and W₂B, no structural refinement was performed. For both cases, $T_{c}$ falls within the range of $\sim$ 3– $\sim$ 6 K and displays an oscillatory dependence on the valence electron concentration [15]. This is qualitatively explained in terms of Brillouin zone effects, considering both variations in the density of states at the Fermi level [ $N (E_{F})$ ] and effect of wave-function on the electron phonon coupling strength. However, the superconducting transitions are rather broad for some compositions, presumably due to the splat cooling [15]. Furthermore, the superconducting properties other than $T_{c}$ , which may provide valuable information about the superconducting mechanism, have not been explored to date.

In this paper, we present studies of crystal structure, resistivity, magnetic susceptiblity, and specific heat properties on high-quality polycrystalline MoReB and WReB (x = 1) prepared by arc-melting. Compared with the samples prepared by splat cooling [14,15], these arc-melted ones are of high crystallinity and exhibit sharp superconducting transitions. This allows us to determine the distribution of Mo/W and Re atoms in the crystal lattice and various superconducting parameters, such as normalized specific heat jump, electron-phonon coupling strength, lower/upper critical fields, as well as penetration and coherence lengths. The properties of MoReB and WReB are also compared and discussed.

Section snippets

Experimental details

Polycrystalline MReB (M = Mo, W) samples were prepared by melting stoichiometric high-purity Mo (99.9%), W (99.99%), Re (99.99%) and B (99.999%) elements in an arc furnace under high-purity argon atmosphere. To ensure homogeneity, the melts were turned over and remelted several times, following by rapid cooling on a water-chilled copper plate. The phase purity of as-cast samples was checked by powder x-ray diffraction (XRD) using a Bruker D8 Advance x-ray diffractometer with Cu Kα radiation at

Structural and chemical characterization

The schematic structure of M₂B is shown in Fig. 1(a) and (b). In this structure, the M and B atoms occupy two distinct crystallographic sites (0.1581, 0.6518, 0) and (0, 0, 0.25). As a consequence, the nearest M and B atoms form one-dimensional zigzag and linear chains along the c-axis, respectively, which bears similarity with the W₅Si₃-type superconductors [17]. The XRD patterns at room temperature for the MoReB and WReB samples, together with the structural refinement profiles, are shown in

Conclusions

In summary, we have studied in detail the structural and physical properties of MoReB and WReB. Both compounds are confirmed to crystallize in the tetragonal CuAl₂-type structure, with a random distribution of Mo/W and Re atoms on the same crystallographic site. We show that both MoReB and WReB are bulk type-II superconductors with $T_{c}$ values of 5.25 K and 5.30 K, respectively. Their specific heat jumps are well described by the weak-coupling BCS theory, which points to a fully gapped

CRediT authorship contribution statement

Yanwei Cui: Investigation, Formal analysis. Jifeng Wu: Investigation. Bin Liu: Investigation. Qinqing Zhu: Investigation. Guorui Xiao: Investigation. Siqi Wu: Investigation. Guanghan Cao: Resources. Zhi Ren: Supervision, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The work at Zhejiang University is supported by the National Key Research and Development Program of China (No.2017YFA0303002) and the Fundamental Research Funds for the Central Universities of China.

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View full text

Superconductivity in ternary borides MReB (M = Mo, W) with the CuAl2-type structure

Highlights

Abstract

Introduction

Section snippets

Experimental details

Structural and chemical characterization

Conclusions

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgments

Phyica C

Physica B

Cryogenics

J. Less Common. Met.

J. Less Common. Met.

J. Less Common. Met.

Phys. Rev.

Nature

Phys. Rev. B

Phys. Rev.

J. Phys. Soc. Jpn.

Supercond

Sci. Technol.

Science

J. Phys. Soc. Jpn.

J. Phys. Soc. Jpn.

Superconductivity in ternary borides MReB (M = Mo, W) with the CuAl₂-type structure