Emerging Superconductivity and Topological States in Bismuth Chalcogenides

Abstract

In this chapter, we review the recent experimental work in emerging superconductors, i.e., bismuth chalcogenides, including the newly discovered BiS(e)2-based layered superconductors and some topological superconductor candidates. Their crystal structure and various physical properties are reviewed in detail, with the correlation between structure and superconductivity as the main clue throughout this chapter. Bi2OS2 is the simplest structure in Bi─O─S compounds and probably the parent compound of this series. Superconductivity emerges when carriers are introduced by intercalation or chemical substitution. The superconducting layer is extended to BiSe2 layer in LaO1−xFxBiSe2, which has an improved superconductivity. Moreover, the topological insulator Bi2Se3 can be turned into superconductors by intercalating metal atoms into van der Waals space, e.g., SrxBi2Se3, a potential topological superconductor, whose quantum oscillations reveal a possible topological surface state. The intermediate external pressure can efficiently suppress superconductivity, which reemerges when pressure is further increased, while Tc is nearly invariant in high-pressure region, indicating an unconventional pairing state.

1. Introduction

Superconductivity was first discovered in the resistivity measurement of mercury by Kamerlingh Onnes in 1911. Its resistance abruptly vanishes at 4.1 K. Zero resistance means no energy loss in electric transport, which could greatly solve the energy crisis in the future. Since then, superconductivity has been a long-lasting hot topic in condensed matter physics. Exploring room temperature superconductors is one of the ultimate dreams.

However, so far, only two kinds of unconventional superconducting systems have exceeded the Macmillan limit at ambient pressure, i.e., the cuprate and iron-based superconductors. In general, the correlation of structure and typical properties is always a useful guideline for effectively searching for special functional materials. In fact, the structure of both cuprate and iron-based superconductors can be characterized as a sandwiched “hamburger” model. It consists of superconducting layers (CuO₂ plane, Fe₂M₂ (M = As, P, S, Se, and Te) layer) and spacer layers, which stack alternatively along the c-axis [1, 2]. Superconductivity occurs when the charged carriers are generated by the defects or substitution in superconducting layers or more commonly provided by the space layers; namely, a new superconducting layer probably means a new superconducting system. The spacer layer can be easily tuned by doping, substitution, intercalation, and pressure, which could affect superconductivity [3]. Therefore, materials with layered structure have been regarded as the most promising playground for exploring new high-T_c superconductors.

In 2010, superconductivity arising from the topological insulator Bi₂Se₃ by Cu intercalation was first reported [4]. It has drawn much attention since Cu_xBi₂Se₃ is proposed as a topological superconductor candidate, as evidenced by the zero-bias conductance peak and quantum oscillation experiment [5, 6]. Very recently, superconductivity with topological states was also reported in its isostructural compounds, Sr_xBi₂Se₃ and Nb_xBi₂Se₃ [7, 8]. In 2012, an exotic superconductivity was discovered in a new layered structure Bi₄O₄S₃ with zero-resistance superconducting temperature at about 4.5 K [9]. Soon, another new BiS₂-based superconductor LaO_0.5F_0.5BiS₂ was reported, whose structure is more definite and the zero-resistance superconducting temperature is about 8 K for the samples annealed under high pressure [10]. As its structure is very similar to the iron-based superconductor LaOFeAs, this system has been intensively researched, and lots of isostructural superconductors have been synthesized, including ReO_1−xF_xBiS₂ (Re: Ce, Pr, Nd, Yb), Sr_1−xRe_xFBiS₂ (Re: La, Ce), EuBiS₂F, and Eu₃Bi₃S₄F₄ [11, 12, 13, 14, 15]. These researches are focused on tuning the spacer layers. The attempts to explore new superconducting layers only succeed in LaO_xF_xBiSe₂ and Sr_0.5La_0.5FBiSe₂ [16, 17, 18]. So far, the superconducting layer of this system has been extended to BiCh₂ (Ch: S, Se). In this chapter, the crystal structure and superconducting properties of Bi─O─S superconductors, LaO_1−xF_xBiSe₂ single crystals, and Sr_xBi₂Se₃ single crystals are briefly reviewed.

2. Crystal structure and superconducting properties

2.1. Bi─O─S superconductors

The element composition of Bi₄O₄S₃ is the same as Bi₄O₄(SO₄)_xBi₂S₄ (x = 0.5), and its parent Bi₆O₈S₅ is an oxide insulator composed of alternatively stacked BiS₂ and Bi₂O₂ + SO₄ + Bi₂O₂ layers along the c-axis. It has a tetragonal structure with I4/mmm space group and its schematic crystal structure is shown in Figure 1(c). Band calculations demonstrate that the half vacancy of SO₄ layer generates electron carriers into BiS₂ layer. The normal state of Bi₄O₄S₃ is metallic and the superconductivity mainly originates from the Bi 6p_x and 6p_y orbitals in BiS₂ layers. Therefore, the BiS₂ layer is called the superconducting layer in this family.

Figure 1.

Crystal structures of (a) Bi₂OS₂, (b) Bi₃O₂S₃, and (c) Bi₄O₄S₃ [21].

However, the chemical composition studies show that it probably contains two new Bi─O─S phases, i.e., Bi₂OS₂ and Bi₃O₂S₃. Their schematic structures can be seen in Figure 1(a) and (b). Bi₂OS₂ is an insulating phase and its content is less than 10%. Bi₃O₂S₃ is the main phase and likely accounts for the 4.5 K superconductivity in Bi₄O₄S₃. And the superconductivity can be suppressed by the amount of Bi₂OS₂-like stacking faults [19]. Once the quality of Bi₃O₂S₃ sample is improved, the superconducting volume fraction will be enhanced with its zero-resistance superconducting temperature increased up to 4.9 K [20].

The crystal structure of Bi₃O₂S₃ is similar to Bi₄O₄S₃ with the same I4/mmm space group, a = 3.9674 Å and b = 41.2825 Å. The electron carriers are believed to be generated from S₂²⁻ layers replacing the vacancy of SO₄²⁻ layers in Bi₄O₄S₃. The chemical composition of Bi₂OS₂ can also be expressed as BiOBiS₂. Then we can see it is isostructural with LaOBiS₂ with P4/nmm space group, a = b = 3.9744 Å and c = 13.7497 Å. BiOBiS₂ has the simplest structure and composition, then it is probably the parent compound of this BiS₂-based family. Besides, superconductivity is likely to be induced by introducing carriers into spacer layer. In fact, F-doped Bi₂OS₂ has been reported to exhibit bulk superconductivity below 5 K [21, 22].

Figure 2 shows the powder XRD patterns of Bi₃O₂S₃, BiO_1−xF_xBiS₂, and Bi₂OS₂ samples. We can see that samples of Bi─O─S compounds tend to contain impurities such as Bi₂O₃, Bi, and Bi₂S₃, because their synthesis temperature is relatively low (520°C for Bi₄O₄S₃ and Bi₃O₂S₃, and 400°C for Bi₂(O,F)S₂) [9, 19, 20, 21]. Besides, these samples can only be synthesized in a narrow temperature region. Another difficulty in detecting their actual composition and structure is that several strong diffraction peaks in the powder XRD patterns are very close to each other. Hence, bulk superconductivity is very important in this system. Up to now, high-quality samples, especially single crystals, are still needed to investigate the relationship of structure and properties, in view of the multiple competing low-energy crystal structures in this system.

Figure 2.

Powder XRD patterns of Bi₃O₂S₃, Bi₂OS₂, and Bi₂O_1−xF_xS₂ polycrystalline samples. The special characters (*, #) represent the impurity phases.

The physical properties of Bi─O─S superconductors are introduced, taking Bi₃O₂S₃ and F-doped Bi₂OS₂ for instance [20, 21]. Figure 3(a) shows the temperature dependence of resistivity and magnetoresistivity under different applied magnetic fields for Bi₃O₂S₃. Its normal state is metallic-like and a sharp drop in resistivity appears at 5.8 K and quickly down to zero at 4.9 K. The upper critical field is estimated from resistivity versus temperature curves under different applied magnetic fields perpendicular to the sample surface, as seen in the insets of Figure 3(a). According to the Werthamer-Helfand-Hohenberg (WHH) formula, the upper critical field μ₀H_c2(0) is evaluated to be about 4.84 T.

Figure 3.

(a) Temperature dependence of resistivity for Bi₃O₂S₃. The lower inset shows the curves of resistivity versus temperature under different applied magnetic fields and the upper inset shows the field dependence of T_c^onset and T_c^zero. (b) Temperature dependence of magnetic susceptibility for Bi₃O₂S₃ and the insets show the magnetic field dependence of magnetic susceptibility at 2 K. (c) Hall resistivity versus magnetic field at different temperatures. (d) Curves of C/T versus T² in superconducting state (0 T) and normal state (9 T). The upper inset shows the data of normal state at low temperature region. The lower inset shows the temperature dependence of calculated electron specific heat in superconducting state [20].

The shielding volume fraction is about 100%, revealing bulk superconductivity, as seen in Figure 3(b). The divergence in temperature dependence of magnetic susceptibility and the M-H curves characterize Bi₃O₂S₃ as a type-II superconductor. The Hall effect shows a remarkable nonlinear magnetic field dependence of transverse resistivity, which means it is likely a multiband superconductor [23]. However, the Hall resistivity at different temperatures is all negative, indicating that the dominant charge carriers are electron-type. The evaluated charge carrier density is about 1.5 × 10¹⁹ cm⁻³. It is much lower than those of cuprate and iron-based superconductors, implying a low superfluid density. Chemical substitution effects seem to increase the charge carrier density, but ultimately inhibit the superconductivity [24, 25, 26].

A clear specific heat anomaly appears around the superconducting transition temperature, as seen in Figure 3(d), confirming the bulk superconductivity in Bi₃O₂S₃. The electronic specific heat coefficient γ and phonon specific heat coefficient β for the normal state under 9 T are obtained as 1.65 mJ/(mol K²) and 2.6 mJ/(mol K⁴), respectively, using linear fitting of C/T versus T². As the phononic contribution to the heat capacity is generally independent of the external magnetic field, the electronic specific heat of superconducting state can be expressed by the equation

$${\mathrm{C}}_{\mathrm{e}}\left(\mathrm{T}\right)=\mathrm{C}\left(\mathrm{T},\mathrm{H}=0\right)-\mathrm{C}\left(\mathrm{T},\mathrm{H}=9\mathrm{T}\right)+\mathrm{\gamma }\text{T}.$$E1

The estimated value of ΔC_e/γT_c is comparable to the BCS weak-coupling limit 1.43.

Undoped Bi₂OS₂ was predicted to be an insulating oxide by the band structure calculations. However, we can see it is almost metallic from 300 K to 30 K, and a weak semiconductor behavior emerges below 30 K, which may be originating from the impurities. The F-doping can significantly decrease the normal state resistivity and increase the shielding volume fraction, as shown in Figure 4. The best doping ratio is about 0.24. From the temperature dependence of magnetic susceptibility, the best doped sample has a bulk type-II-like superconductivity. When doping content exceeds 0.27, superconductivity disappears and the resistivity increases quickly. Besides, the quality of samples (x > 0.27) synthesized by conventional solid state reaction method begins to deteriorate with increasing doping content [21]. In fact, the Bi₂(O,F)S₂ samples synthesized by topotactic fluorination using XeF₂ also contain bismuth impurity [22]. It is difficult to get pure samples because the optimal synthesis temperature is only around 400°C.

Figure 4.

(a) Temperature dependence of resistivity for BiO_1−xF_xBiS₂. The inset shows the variation of T_c with different F-doping content. (b) Temperature dependence of magnetic susceptibility for BiO_1−xF_xBiS₂ under ZFC process. The inset presents the FC and ZFC data for x = 0.24 sample [21].

2.2. Re(O,F)BiCh₂ (Ch: S, Se) superconductors

Re(O,F)BiS₂ (Re: La, Ce, Pr, Nd, Yb) superconductors have been intensively studied since the report of LaO_0.5F_0.5BiS₂. Their structure is more definite and similar to “1111” phase of iron-based superconductors. Single crystals of this structure have been successfully synthesized [27]. Structure tuning is mainly concentrated on the spacer layers rather than the superconducting layer. And only the electron-doping into the insulating parent can induce superconductivity [28]. Here, we introduce the crystal structure and various physical properties of LaO_1−xF_xBiSe₂ single crystals, which also firstly extend the superconducting layer to BiSe₂ layer.

The powder XRD pattern and crystal structure of LaO_0.59F_0.41BiSe₂ superconducting single crystal are presented in Figure 5. No impurity phase is found and each peak is indexed. It has a P4/nmm tetragonal lattice with the refined lattice constants a = b = 4.1377 Å and c = 14.1566 Å, which are larger than those of LaO_0.5F_0.5BiS₂ for the larger ionic radius of Se²⁻. Figure 6 shows a comparison of the temperature dependence of resistivity for La(O,F)BiS₂ and La(O,F)BiSe₂ samples. LaOBiS₂ can be described as an insulator while LaOBiSe₂ is metallic. For LaO_0.5F_0.5BiS₂, it exhibits a semiconducting behavior before the superconducting transition begins. The transport property of LaO_0.5F_0.5BiSe₂ is similar to Bi₃O₂S₃ but with a lower residual resistivity. Other isostructural compounds such as LaO_0.5F_0.5BiTe₂ and LaO_0.5F_0.5SbS₂ are also reported, but no superconductivity can be observed down to 1.7 K [16].

Figure 5.

(a) Powder XRD pattern (black circles) with the Rietveld refinement (red curve) and Miller indices for LaO_0.59F_0.41BiSe₂. The inset table summarizes the structural parameters. (b) Crystal structure of LaO_0.59F_0.41BiSe₂. The rectangle indicates the unit cell [17].

Figure 6.

A comparison of the temperature dependence of resistivity between (a) La(O,F)BiS₂ and (b) La(O,F)BiSe₂.

Fluorine doping effect on the superconductivity of LaO_1−xF_xBiSe₂ single crystals is shown in Figure 7(a) and (b). F-doping can significantly decrease the resistivity of normal state and increase the superconducting transition temperature and shielding volume fraction. Unfortunately, the flux method can only grow single crystals with the largest F content of about 0.5. For example, the sample with F-doping amount of 0.52 was grown by a nominal component of 0.9. The magnetic susceptibility measurement shows LaO_1−xF_xBiSe₂ has a bulk superconductivity and belongs to the type-II superconductors. Upper critical magnetic field can be evaluated from the resistivity versus temperature under various magnetic fields. As seen in Figure 7(c) and (d), the upper critical fields at zero temperature are estimated to be 29 T and 1 T for H∥ab and H⊥ab, respectively, which indicate large anisotropy.

Figure 7.

Superconducting properties of LaO_1−xF_xBiSe₂ single crystals with different F-doping contents. (a) Temperature dependence of resistivity and an enlarged view near the superconducting transition temperature for all samples. (b) ZFC and FC magnetic susceptibility versus temperature with magnetic field applied parallel to ab-plane for all samples. (c) and (d) Resistivity versus temperature with magnetic field applied perpendicular to and parallel to ab-plane, respectively, for the x = 0.52 sample [17].

The anisotropy parameter γ_s of the LaO_1−xF_xBiSe₂ superconducting single crystal is investigated by measuring the angular dependence of resistivity under various magnetic fields at 3 K (see Figure 8). Note that the angle θ describes the deviation of magnetic field with respect to the ab-plane of single crystal. Only the data with magnetic field below 1 T are selected for the reduced magnetic field, because the H_C2(0) for H⊥ab is about 1 T. The reduced magnetic field is calculated by the equation

$$H_{\mathit{red}}=H\sqrt{{\text{sin}}^2\theta +{\gamma }_s^{-2}{\text{cos}}^2\theta }.$$E2

Figure 8.

Anisotropy of LaO_1−xF_xBiSe₂ superconducting single crystal. (a) Angular dependence of resistivity taken under magnetic fields from 0.1 T to 6 T at 3 K for LaO_0.48F_0.52BiSe_1.93 single crystal. (b) Scaling of the resistivity vs. the reduced magnetic field H_red [17].

According to the Ginzburg-Landau theory [29], the curves of resistivity versus reduced magnetic field under different magnetic fields should merge into one. The resultant anisotropy parameter at 3 K is about 30 (see Figure 8(b)), which is close to the result of upper critical field within the ab-plane.

Considering that the T_c of LaO_0.5F_0.5BiS₂ is increased from 2.7 K to 10.6 K under a hydrostatic pressure of 1.68 GPa [30], the highest T_c among the BiS₂-based superconductors, higher T_c, above 10.6 K is expected for LaO_0.5F_0.5BiSe₂ under external pressure since its zero-resistance temperature is about 3.5 K. However, we find that its superconductivity and shielding volume fraction decrease unexpectedly with increasing pressure below 1 GPa hydrostatic pressure, as seen in Figure 9(a). Another experiment with higher pressure shows that a new superconducting phase emerges at about 1.2 GPa and T_c reaches about 6.5 K at 2.17 GPa [31]. Accompanied by this crossover, the normal state is switched from that with a low temperature resistivity upturning to a metallic one. Accordingly, the normal state resistivity also shows a nonmonotonic change with the external pressure. These facts suggest that the BiSe₂-based system is very different from the BiS₂-based system.

Figure 9.

High-pressure effect on the superconductivity of LaO_0.5F_0.5BiSe₂ single crystal. (a) High-pressure effect on the temperature dependence of magnetic susceptibility. (b) and (c) High-pressure effect on the transport properties of two single crystal samples of LaO_0.5F_0.5BiSe₂ [31].

2.3. M_xBi₂Ch₂ (Ch: Se, Te) superconductors

Topological insulator has linearly dispersive band structures and its topological surface state exhibits metallic properties while the bulk state is insulating. If its spin-momentum locking effect combines with superconductivity, Majorana fermion may exist, which is useful for quantum computing. At first, the topological superconductors were mostly focused on the proximity-induced superconductivity. The discovery of Cu_xBi₂Se₃ superconductor opens a new gate to topological superconductors, i.e., superconductors induced by doping into topological insulators, which are expected to be the candidate of three-dimensional topological superconductors. Recently, a series of superconductors based on the topological insulators have been reported, such as Cu_x(PbSe)₅(Bi₂Se₃)₆ [32], Sr_xBi₂Se₃ [7], Nb_xBi₂Se₃ [8], and Tl_xBi₂Te₃ [33]. Here, we put emphasis on the crystal structure and physical properties of Sr_xBi₂Se₃ single crystals.

The structure of Sr_xBi₂Se₃ is similar to that of Cu_xBi₂Se₃ and isomorphic to the parent Bi₂Se₃. Sr atoms may act as a bipolar dopant that can be embedded in the van der Waals space or randomly substitute for Bi. The actual Sr doping content of Sr_xBi₂Se₃ is very little so that it is hard to define its precise position. Nevertheless, the lattice constants of Sr_xBi₂Se₃ are a little larger than those of Bi₂Se₃, while the lattice constants of Bi_2−xSr_xSe₃ are smaller. The c-axis lattice constant of Bi_2−xSr_xSe₃ decreases slightly with increasing doping content (see Figure 10(b)). In addition, all samples grown in Bi_2−xSr_xSe₃ ratio show no signs of superconductivity at 1.8 K, as seen in Figure 11(a). Therefore, we could use Figure 10(a) as the schematic structure diagram.

Figure 10.

Crystal structure of Sr_xBi₂Se₃ superconductors. (a) Schematic diagram of Sr_xBi₂Se₃ crystal structure. (b) Powder XRD patterns of Sr_xBi₂Se₃, Bi₂Se₃, and Bi_2−xSr_xSe₃ [7].

Figure 11.

Superconducting properties of Sr_xBi₂Se₃. (a) Temperature dependence of resistivity for Sr_xBi₂Se₃ and Bi_2−xSr_xSe₃. (b) Hall resistivity versus magnetic field curves measured at different temperatures. (c) Temperature dependence of estimated Hall coefficient and charge carrier density. (d) Temperature dependence of susceptibility for samples with different Sr contents. (e) Plot of T_c^onset, T_c^zero, and shielding volume fraction as a function of Sr content [7].

The linear curves of Hall resistivity versus magnetic field indicate that Sr_xBi₂Se₃ has only one electron-like bulk carrier. The carrier density increases slightly with decreasing temperature. Its average is around 2.3 × 10¹⁹ cm⁻³, about 1–2 orders of magnitude lower than Cu_xBi₂Se₃. Figure 11(d) and (e) shows that the T_c of superconducting samples changes little with different Sr contents, but the shielding volume fraction is very different. Only those samples with Sr content above 0.06 have a large shielding volume fraction. Moreover, the superconductivity is very stable in air, as evidenced by the almost unchanged shielding volume fraction for the sample placed in air even for a month. This provides great convenience for experimental research.

The topological surface state of Sr_xBi₂Se₃ single crystal has been investigated through Shubnikov-de Haas oscillation measurements. Clear oscillations in resistivity and Hall resistivity can be observed under high magnetic field at different temperatures, as shown in Figure 12(a) and (c). The oscillation amplitudes become more pronounced for higher magnetic field and lower temperature. However, the oscillatory periods measured at different temperatures remain constant, so only the data at 0.35 K with the most noticeable oscillations are selected to deduce the Landau level indices. In fact, the measured resistivity and Hall resistivity actually contain contributions from both the surface and bulk conductance when a large parallel bulk conduction channel is present. Therefore, the least confusing method is to convert resistivity into conductance to determine the Landau index because its components are additive [34]. The following equations are used to calculate conductance

$$G_{\mathit{xx}}=\frac{R_{\mathit{xx}}}{R_{\mathit{xx}}^2+R_{\mathit{xy}}^2},G_{\mathit{xy}}=\frac{R_{\mathit{xy}}}{R_{\mathit{xx}}^2+R_{\mathit{xy}}^2}.$$E3

Figure 12.

SdH oscillations under high magnetic field for Sr_xBi₂Se₃ single crystal. (a) and (c) Magnetic field dependence of resistivity and Hall resistivity at different temperatures. (b) and (d) Magnetic field dependence of the fitted longitudinal and Hall conductivity at 0.35 K [7].

After removing the nonoscillatory background, the oscillatory components are obtained and plotted as a function of 1/B. The frequencies are 146 T for longitudinal conductance and 144.8 T for Hall conductance, which are comparable to those of Bi₂Se₃ but smaller than Cu_xBi₂Se₃. The integer Landau index n corresponds to the valleys in ΔG_xx, while the valleys in ΔG_xy are assigned to n + 1/4 [see Figure 13(a) and (c)]. The 1/4 shift arises to match the valleys in dΔG_xy/dB with the valleys in ΔG_xx [34]. The obtained intercepts of the linear fittings for n versus 1/B are both close to the value for an ideal Dirac system, i.e., −0.5 rather than 0 or 1 (see Figure 13(b) and (d)). Thus, it provides transport evidence for the existence of Dirac fermions in Sr_xBi₂Se₃ superconductor.

Figure 13.

(a) and (c) Oscillatory component of the longitudinal and Hall conductivity at 0.35 K plotted against 1/B. (b) The Landau index n versus 1/B, where n and n + 1/2 correspond to the valleys and peaks of ΔG_xx. (d) n versus 1/B derived from (c), where n + 1/4 corresponds to the valleys of ΔG_xy [7].

The superconductivity of Sr_xBi₂Se₃ is very sensitive to external pressure below 1 GPa, as seen in Figure 14(a) and (b). With the increasing applied pressure, the T_c and shielding volume fraction decrease but the normal state resistivity increases. This depression of superconductivity can be attributed to the reduction of charge carrier density, which is apparent from the normal state resistivity. However, if the pressure continues to increase, the normal state resistivity begins to decrease and a sign of superconducting transition occurs at 6 GPa. Then, the T_c^onset and the charge carrier density estimated from the normal state resistivity gradually increase with the increasing pressure, and T_c^onset reaches around 8 K when P > 14 GPa. But unfortunately, the T_c^onset remains almost constant for the pressure up to 40 GPa, although the normal state resistivity keeps decreasing. The reemerging superconductivity is very robust and the T_c^onset still changes little under 80 GPa [35]. In fact, the whole process contains three structural phases, i.e., R-3 m, C2/m, and I4/mmm, as seen in Figure 14(d). The structural transitions and pressure-invariant T_c are very similar to the parent compound Bi₂Se₃, which needs further investigations.

Figure 14.

(a) Temperature dependence of magnetic susceptibility under different pressures. (b) and (c) Temperature dependence of resistance under high pressure. (d) The structural phase diagram on pressure for Sr_xBi₂Se₃ [35].

3. Conclusions

The discovery of superconductivity in layered compound Bi₄O₄S₃ brings in a new BiS₂-based superconducting family, including the Bi─O─S compounds, Re(O,F)BiS₂, and MFBiS₂ superconductors. The superconducting layer is extended to BiSe₂ layer in LaO_1−xF_xBiSe₂ and Sr_1−xLa_xFBiSe₂. The crystal structure and various superconducting properties are reviewed for selective systems. Hall effect and specific heat suggest that they are probably multiband superconductors and can be described by BCS weak-coupling theory. Moreover, bismuth chalcogenide topological insulators can be turned into superconductors by doping, which are potential candidates for 3D topological superconductors. For example, the topological surface state of Sr_xBi₂Se₃ is well supported by SdH oscillations under high magnetic field. The intermediate external pressure can efficiently suppress the superconductivity, which reemerges when pressure is further increased, while Tc is nearly invariant in high-pressure region, indicating an unconventional pairing state.

Acknowledgments

We acknowledge the support from the National Natural Science Foundation of China (Grant nos. 51603207, U1532267, and 11574288).

J.S. would like to dedicate his best love to his dear wife, Lv Youyou, who always encourages and supports his research and life. Best wishes for their love and coming baby. W.Z. would like to express his special thanks to his dearest daughters, Lily and Amy.

Conflict of interest

The authors declare no competing financial interests.