Preamble

Possible High-Tc Superconductivity at 45 K in the Ge-Doped Cluster Mott Insulator GaNb4Se8

Ji-Hai Yuan¹,²#, Ya-Dong Gu¹,²#, Yun-Qing Shi¹,², Hao-Yu He¹,², Qing-Song Liu¹,², Jun-Kun Yi¹,², Le-Wei Chen¹,², Zheng-Xin Lin¹,², Jia-Sheng Liu¹,², Meng Wang¹,², Zhi-An Ren¹,²*

¹ Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, China
² School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China

These authors contributed equally to this work.

• Corresponding author. E-mail: renzhian@iphy.ac.cn

Abstract

Ge-doped GaNb4Se8 polycrystalline samples were synthesized using a solid-state reaction method. Zero-resistance transitions were observed in one batch of samples, with the highest onset superconducting critical temperature (Tc) reaching 45 K. This discovery may represent a new class of Nb-based high-Tc superconductors emerging from doped Mott insulators.

Keywords: GaNb4Se8, doping, Mott insulator, superconductivity

Introduction

In 1986, Bednorz and Müller discovered superconductivity in Ba-doped La2CuO4 with a critical temperature (Tc) of 30 K, ushering in a new era of high-Tc superconductivity research [1]. La2CuO4 exemplifies a broad family of copper oxides hosting strongly correlated electrons that form an antiferromagnetic Mott insulating state. Carrier doping into their CuO2 planes disrupts electronic correlations and gives rise to rich phase diagrams that include the emergence of high-Tc superconductivity. Following this paradigm, the pioneering work was rapidly extended, with Tc reaching 93 K in YBa2Cu3O7 and a record of 135 K in HgBa2Ca2Cu3O8 that remains unbroken today [2-3]. The discovery of iron-based and nickel-based superconductors in this century has further demonstrated that doping strongly correlated electron systems is a pivotal route for discovering high-Tc superconductors [4-8].

GaNb4Se8 is a well-known cubic lacunar spinel with the general formula AM4X8 (A = Al, Ga, Ge; M = V, Nb, Mo, Ta; X = chalcogen). First synthesized in 1984 by Benyaich et al. as a cluster compound [9], it features Nb4 tetramers arranged in a pyrochlore-type network as shown in Fig. 1 [FIGURE:1]. In GaNb4Se8, an unpaired electron occupies the molecular t2 orbital, forming a nonmagnetic Mott insulating ground state. The interplay of electron correlations, spin-orbit coupling, and Jahn-Teller effects creates complex physics in this material, which undergoes a cubic-to-cubic structural transition and quadrupolar ordering of molecular orbitals on Nb4 clusters at TQ = 50 K, followed by structural distortions along different directions into an orthorhombic lattice at TM = 31 K [10-15]. The realization of a valence bond solid state at low temperatures in GaNb4Se8 has also been proposed [15]. Under high pressure, GaNb4Se8 transforms from a Mott insulator to a metallic and superconducting state with Tc = 2.9 K at 13 GPa [16]. However, no successful carrier doping study has been reported for GaNb4Se8 to date.

In recent years, our research has focused on the synthesis and fundamental physical properties of Mo/Nb-based transition metal compounds, leading to the discovery of a series of superconducting materials. These include the quasi-one-dimensional K2Mo3As3 [17], the ternary boride Mo5GeB2 [18], the P/As-doped silicide Mo5Si3 [19-20], the gallium cluster compound Mo4Ga20As [21], the Chevrel-phase KxMo6Se8 [22], the intercalated dichalcogenide Ge-NbSe2 [23], and the binary NbS [24]. Here we report the possible discovery of high-Tc superconductivity in the cluster Mott insulator GaNb4Se8 induced by Ge doping.

Figure 1. Schematic crystal structure of Ge-doped GaNb4Se8, the Nb4 tetrahedron cluster, and the molecular orbital scheme of the Nb4 tetramer.

Experimental Methods

Ge-doped Ga1-xGexNb4Se8 polycrystalline samples were synthesized by solid-state reaction. High-purity (99.99%) Ga, Nb, Se, and Ge powders were weighed and sealed in an evacuated quartz ampule. The ampule was heated at 900 °C for 72 hours. The resulting mixture was reground thoroughly, pressed into pellets, sealed again in an evacuated quartz ampule, and heated at 1000 °C for another 72 hours. All preparation procedures were carried out in a glove box filled with high-purity Ar gas (O2 and H2O content less than 0.1 ppm) to avoid contamination. The obtained samples are black and appear stable in air. We note that GeSe crystals often accumulate at the end of the quartz ampule due to their volatility at lower temperatures, making Ge doping at the Ga site extremely difficult and uncontrollable. Powder X-ray diffraction (XRD) data were collected using a PANalytical X-ray diffractometer with Cu-Kα radiation. Electrical resistance measurements were performed on a Quantum Design physical property measurement system (PPMS) using the standard four-probe method. DC magnetization was measured with a Quantum Design magnetic property measurement system.

Results and Discussion

Figure 2 [FIGURE:2]. (a) Temperature dependence of electrical resistance for two Ge-doped GaNb4Se8 samples. (b) Comparison of zero-resistance transitions with three other superconductors: W5Si3-xPx, KxMo6Se8, and Ba(FeCo)2As2.

Among hundreds of samples synthesized, only one batch within the same ampule exhibited zero-resistance transitions, though the electrical properties were inhomogeneous from piece to piece. The nominal composition of this batch was Ga0.9Ge0.2Nb4Se8, with excess Ge added to compensate for volatilization losses. Figure 2a shows the temperature dependence of resistance for sample #1 and sample #2 measured with an applied current of 1 mA. We note that these samples had irregular shapes, as they were cut without polishing, with dimensions of approximately 5 mm in length and cross-sectional areas of 1–2 mm². Sample #1 and sample #2 exhibit superconducting transitions with onset Tc values of 45 K and 40 K, respectively, achieving zero resistance at lower temperatures. Both samples show resistivity anomalies around 60–80 K, indicating additional phase transitions.

The zero-resistance transitions for sample #1 and sample #2 are magnified in Figure 2b and compared with three other superconductors—W5Si3-xPx, KxMo6Se8, and Ba(FeCo)2As2—demonstrating good zero-resistance superconductivity in both samples. Notably, just below the zero-resistance transition at 34 K in sample #1, the resistance shows a small segment of negative values. This phenomenon arises from local reverse superconducting percolation paths between the voltage probes during measurement and has been occasionally observed in our superconducting research when samples are not single-phase. The W5Si3-xPx superconductor exhibits the same negative resistance phenomenon at 6 K. This negative resistance segment further confirms the occurrence of genuine superconductivity in sample #1.

We note that for this batch of samples, all superconducting signals disappeared after storage in a glove box for several days, and the samples became insulating. The zero-resistance transitions represent all the data we have measured to demonstrate superconductivity. One possible explanation for the loss of superconductivity is that the Ge-doped samples are unstable and gradually decompose into the parent compound GaNb4Se8 over time.

After the superconducting signals vanished, the chemical phases of these samples were examined by powder XRD at ambient temperature. Figure 3a [FIGURE:3] shows the XRD pattern of the Ga0.9Ge0.2Nb4Se8 sample together with that of pure undoped GaNb4Se8. NbSe2 phase and other minor impurities are observed in the Ga0.9Ge0.2Nb4Se8 sample, with the main phase being GaNb4Se8 that is almost identical to the undoped compound in lattice constant. Figure 3b shows the temperature dependence of magnetization for the Ga0.9Ge0.2Nb4Se8 sample after the loss of zero resistance, measured under a field of 100 Oe. The behavior is similar to that of the undoped sample reported in Ref. [12]. Figure 3c displays the temperature dependence of resistance for undoped GaNb4Se8, showing typical Mott insulating behavior.

Summary

In summary, zero resistance was observed in Ge-doped cluster Mott insulator GaNb4Se8 with an onset Tc of 45 K. Since no other physical phenomenon has produced zero resistance at such temperatures to date, our discovery may represent a new class of Nb-based high-Tc superconductors.

Figure 3. (a) Powder XRD patterns of the Ga0.9Ge0.2Nb4Se8 sample and undoped GaNb4Se8. (b) Temperature dependence of magnetization for the Ga0.9Ge0.2Nb4Se8 sample after the loss of zero resistance. (c) Temperature dependence of electrical resistance for undoped GaNb4Se8.

Acknowledgments

This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB25000000), the CAS Superconducting Research Project (Grant No. SCZX-0103), and the National Key Research and Development Program of China (Grant No. 2021YFA1401800).

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