Introduction

One big breaking through in the studies of strong spin orbital coupling (SOC) system is the discovery of A2B3 topological insulators (TIs)1. TIs are characterized by insulating bulk energy gap like in ordinary insulators but gapless edge or surface states protected by time-reversal symmetry and therefore trigger wide interests in physical sciences1,2,3,4,5,6,7,8. As one of the extraordinary states of topological quantum phases, topological superconductor with Majorana bound state at edge or surface, shows a potential application as well as provides theoretical foundation in topological quantum computations9. Superconductivity in TIs was firstly realized in Cu-intercalated Bi2Se310. In this system, the copper atoms reside in the van der Waals gaps between two Bi2Se3 layers resulting in the superconductivity with Cooper pairing existing up to 3.8 K. Further efforts on introducing superconductivity into topological insulator and exploring clean topological superconductors without doping were resorted to high pressure11,12,13. Comparing with chemical substitution, applying pressure is an effective tool of inducing novel properties in topological insulators by directly modifying the electronic structure without introducing defects or impurities. For instance, pressure can effectively tune the crystal field splitting and hybridization between Sb and Se14, resulting the emergence of topological insulator state. The A2B3-type TIs (A = Sb, Bi; B = Se, Te) except for Sb2Se3 exhibit rich and novel phenomena under high pressure, such as structure phase transitions11,12,13,15,16,17,18,19,20,21, insulator-to-metal transitions13,17 and even superconductivity11,12,13,18,19,20,21,22. The bulk superconducting states of Bi2Te3 and Sb2Te3 were observed experimentally as the increase of pressure and were proposed to be likely topologically related11,12. Additionally, the high pressure tuned superconductivities were realized in Bi2Se313,22 and CuxBi2Te319 system. All these TIs provided possible pathway to search for Majorana fermions through high pressure technique. Sb2Se3 compound, as a member of A2B3-type strong SOC semiconductors (A = Sb, Bi; B = S, Se, Te), has attracted great amount of attention in the past few years, due to its switching effects23, high thermoelectric power24 and good photovoltaic properties25. Unlike Bi2Te3, Bi2Se3 and Sb2Te3, Sb2Se3 was thought to be topological trivial1. However, it was recently theoretically26,27 predicted that Sb2Se3 could transform from band insulator to topological insulator under high pressure and thereafter the prediction of topology was experimentally elucidated14. Here, we report the effect of pressure on the transporting properties of Sb2Se3 in combination with structural studies through Raman spectra and in situ angle-dispersive synchrotron x-ray diffraction (AD-XRD). Our experiments indicated that Sb2Se3 transformed from insulator into metal like state and further to superconductive state as increase of pressure, during which there was no crystal structure phase transition occurred in our studied pressure range. Two new Raman active modes were observed at high pressure, respectively, where correspondingly the superconductivity emerges and the dTC/dp slop changes.

Results

Electronic transport properties of Sb2Se3 single crystal

Sb2Se3 is a band insulator with energy gap of ~1 eV28 and with resistance at the level of 109 Ω at ambient condition. Sb2Se3 compound undergoes an insulator to metal like transition upon compression, as shown in Fig. 1 (a). As increasing pressure, the resistance decreases over the entire measured temperature range, showing four orders of magnitude drop at room temperature with pressure up to 3.4 GPa, which could be related to the topological quantum transition27. The transport must contain conductance contribution from surface but the contribution cannot be differentiated at this moment. From 3.4 to 8.1 GPa, the temperature dependence of resistance is similar to that for topological insulator Bi2Se3 with an upturn of resistance at low temperature17. However, the upturn in resistance is barely visible, due to the high carrier density screening effect29. In Sb2Se3, superconductivity becomes more pronounced with pressure higher than 10 GPa, as shown in Fig. 1 (b). The signature of superconductivity is the resistance drop at around 2.2 K with decreasing temperature under pressure of 10 GPa. Resistance drops more obviously with further increasing pressure and zero-resistance state was fully realized above 15 GPa. Determination of superconducting transition temperature (TC) was based on the differential of resistance over temperature, as elucidated in Ref. 11. To validate the superconductive character, the evolutions of TC at 15.4 GPa as a function of magnetic field are performed, as shown in the inset of Fig. 1 (c). The suppression of TC with increasing magnetic field confirms the superconductivity. Using the Werthdamer-Helfand-Hohenberg formula30, , the upper critical field HC2 (0) is extrapolated to be 3.9 T with magnetic field H paralleling to c axis of Sb2Se3 single crystal at 15.4 GPa. Figure 2 (a) shows resistance evolution of Sb2Se3 compound as the function of pressures at various temperatures. The evolutions of TC from different experimental runs as functions of pressures are plotted in Fig. 2 (b). TC increases rapidly to 6 K at 20 GPa with a rate of 0.40 K/GPa. Subsequently, the increase rate of TC is 0.19 K/GPa and thereafter, TC increases very slowly to around 8 K at 40 GPa. We further conducted Hall effect measurement with a magnetic field H perpendicular to a–b plane of the Sb2Se3 single crystal and sweep the H at fixed temperatures (2 K, 30 K and 297 K, respectively) for each given pressure. The Hall resistance as a function of applied pressure at 30 K in Fig. 1 (d), shows a linear behavior with a positive slope, indicating that Sb2Se3 is of hole type carrier over the entire pressure range measured. Carrier densities increase almost two orders of magnitude as pressure increases from 10 GPa to 18 GPa (Fig. 2 (c)). For instance, the carrier density changes from ~1019 cm−3 to 1021 cm−3 with increasing rate of ~6.2 × 1019 cm−3/GPa from 10 GPa to 18 GPa at both 2 K and 30 K, suggesting significant change in the electronic band structure with pressure22. Apparently, the rapid increase of TC of the superconductive phase from 10 GPa to 18 GPa is intimately related to the increase of carrier density, as observed in Bi2Te321, Bi2Se313,22 or Sb2Te312 systems. The difference is that all these phenomena of Sb2Se3 occur within the initial ambient crystal structure31 (see Supplementary Fig. S1 online), other than pressure induced crystal structure changes in Bi2Te3, Bi2Se3 or Sb2Te3 compounds. It should be noticed that further increasing pressure will continuously increase the carrier density in the measured pressure range. The upright dotted lines in Fig. 2 represent the pressures, at which insulator to metal like and further to superconducting transitions occur.

Figure 1
figure 1

Electronic transport properties of Sb2Se3 single crystal.

(a) Temperature dependence of a–b plane resistance below 8.1 GPa. (b) The a–b plane resistance as a function of temperature at various pressures showing a superconducting transition around 2.2 K at 10 GPa. (c) The superconducting transition of Sb2Se3 with applied magnetic field H perpendicular to the a–b plane of the Sb2Se3 single crystal at 15.4 GPa. The inset shows TC evolution as a function of magnetic field H. (d) The Hall resistance of Sb2Se3 as a function of applied pressure at 30 K, showing a linear behavior with a positive slope.

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Figure 2
figure 2

Pressure induced transitions in Sb2Se3.

(a) Resistance evolution of Sb2Se3 single crystal as a function of pressure at fixed temperatures (280 K, 200 K, 2 K), in which insulator, metal like and superconductor are denoted. (b) Pressure dependence of superconductive transition temperature TC of Sb2Se3. Solid lines are guides to the eye. Increasing rate of TC decreases upon compression. (c) Pressure induced changes of carrier density at temperature of 2 K, 30 K and 297 K, respectively.

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Figure 3 gives the ac magnetic susceptibility measurement of Sb2Se3 single crystal as functions of pressure and temperature. The sudden drop of the real part of ac susceptibility appears at pressure above 15 GPa in the measured temperature range (2 K to 300 K), which gets more pronounced with further increasing pressure, strongly indicating the bulk nature of the superconductivity. The TC increases upon compression and non-monotonically increased to 39 GPa, which shows consistent trend compared with the TC obtained from resistance measurement.

Figure 3
figure 3

The real part of the ac susceptibility of Sb2Se3 single crystal as a function of temperature at various pressures.

The inset is the TC as a function of pressure.

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High pressure structural evolution of Sb2Se3 crystal

To understand the pressure induced novel physical properties in terms of structural mechanism, we conducted in situ Raman spectra and angle resolved x-ray diffraction (AD-XRD) measurements at high pressures. The Sb2Se3 crystallizes in space group Pbnm28,31,32 at ambient condition and sketch of the coordination environment around the Sb1 and Sb2 cations is shown in Supplementary Fig. S2 (a) online. The AD-XRD results (see Supplementary Fig. S1 online) reveal that Sb2Se3 maintains ambient-pressure phase within the pressure range in our experiment, consistent with the recent observation31. At this point, the ambient structure of Sb2Se3 stabilizes within entire pressure range we studied, indicating that insulator to metal like to superconducting transitions as increase of pressure are type of electronic phase changes related to local structure evolutions. Raman spectroscopy is sensitive to local bond vibrations and symmetric broken33,34,35,36, therefore it can provide evidence for the rich property evolution of Sb2Se3 under high pressure. As shown in Fig. 4, there are two new vibration modes appeared at 10 and 20 GPa, respectively, which agrees well with the reported results14,31. Vibration modes of Raman spectra are fitted by Lorentzian peak configuration with pressure up to ~20 GPa, as plotted in Fig. 4. At ambient pressure, seven modes denoted as M1 (105 cm−1), M2 (122.6 cm−1), M3 (131 cm−1), M4 (155.2 cm−1), M5 (181 cm−1), M6 (189.6 cm−1) and M7 (213.3 cm−1) are clearly seen in Fig. 4 (a). The vibration modes as the function of pressure are fitted to the linear equation (dashed line) (Fig. 5 (d), except for M5 and M8 modes). M2 and M5 modes get softened around 2.5 GPa, while M6 and M7 modes almost keep constant. At ~10 GPa, M5 disappears accompanied by a new mode M8 showing up, where superconductivity occurs. Further increasing pressure to around 20 GPa, another new mode M9 is observed, which results in the change of increase rate of TC.

Figure 4
figure 4

Pressure evolution of Raman spectra (λ = 532 nm, T = 300 K).

From low to high wavenumber, phonon modes are denoted as M1, M2, M3, M4, M5, M6 and M7 at 0.9 GPa. The black squares are experimental data points, the red solid lines are the total fitting curves from the sum of the individual Lorentzian fits (blue peaks) to the experimental data points. Arrows mark the appearances of new vibration modes.

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Figure 5
figure 5

The relation of electrical properties and structure as a function of pressure.

(a) TC from two individual resistance measurement experiments, (b) the angles of and , (c) a/b ratio and (d) Evolutions of Raman vibration modes as functions of pressures. The vertical shadowed boxes indicate the pressures for insulator-metal like-superconductors transitions and the change of TC slop as the function of high pressure and all the solid and dotted lines are guidelines for your eyes.

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Discussions

In Fig. 1(b), there appear to be two transitions for the two highest pressures. As in Bi2Se322 and Bi4Te335 compounds, they also demonstrated two transitions around BCC phase coexists with other phase. Considering the non-hydrostatic pressure environment, the high pressure BCC phase could be present with the Pbnm phase in the resistance measurement at the two highest pressures. However, the pressure gradient in the sample chamber could be the other possible reason of TC distribution. Figure 5 summarizes the insulator to metal like to superconductivity transitions of Sb2Se3 crystal as a function of pressure through the structure parameters and corresponding electronic property evolution. At ~2.5 GPa, insulator to metal like transition is directly monitored by sharp drop of resistance as shown in Fig. 1 (a). Similar to the three-dimensional time-reversal invariant topological insulators of Bi2Se333 or Sb2Te334compounds, all of which show electronic topological transition (ETT) in this low pressure range, our Raman results in Fig. 5 (d) show that the M2 and M5 vibration modes gradually soften with pressure up to 2.5 GPa, which agrees well with the phonon softening and the asymmetric peak configuration of the linewidth14. Specifically, M2 phonon mode shows a softening and then the sign of slope changes and FWHM of M2 mode is maximum near 2.5 GPa (see Supplementary Fig. S3 online), which may be related to ETT or Lifshitz transition. Our spectra results indicated that during Raman experiments higher laser power will heat and transform the specimen to a new phase as evidenced by vibration mode dramatic change (see Supplementary Fig. S4 online). There is no crystal structural phase transition in the pressure range we studied and the lattice parameters and volume of our data are consistent with the results in Ref. 31. However, the anomaly of the bond length in both experiments together with the Raman vibration mode change confirm that the insulator to metal like transition is local structural distortion related. To understand the structural related mechanism of transport properties, we plot the bond angles of to demonstrate their correlation with the insulator to metal like transition and high pressure tuned superconductivity. These two angles increase linearly with pressure up to ~2.5 GPa, then the value of turns to decrease linearly and the value of sharply drop to ~91° and increases linearly with pressure to ~10 GPa, as shown in Fig. 5 (b). The a/b ratio reaches to 1 at 2.5 GPa and followed by parabolic trend with pressure up to 10 GPa. As mentioned above, the crystal field splitting and hybridization must be modulated accordingly. More specifically, the variation of will mainly rotate the polyhedral of Sb(1)Se7 along the c axis while evolution of will tilt and distort the polyhedral of Sb(1)Se7 at high pressure, as the schematic of the crystal structure shown in Supplementary Fig. S2 (b) and (c) online. At ~10 GPa, the sign of increase rate of changes, the value of suddenly increases (as shown in Fig. 5 (b)) and a new Raman vibrational mode M8 at 200 cm−1 emerges (as shown in Fig. 5 (d)), which should correspond to the metal like state to a superconductive state transition in transport measurement. The values of angles of and in Fig. 5 (b) decrease in the pressure range of 10 to 20 GPa with a slope of −0.425°/GPa and −0.295°/GPa, respectively, then change to a smaller decreasing rate and turns to increase with pressure higher than 20 GPa. At 20 GPa, the change of increase rate of the TC should be due to anomalies of and and the appearance of another new Raman vibrational mode M9 at ~190 cm−1. The dependence of TC in Sb2Se3 on pressure is similar to that in Bi2Se3, in which pressure-induced unconventional superconducting phase has been reported13,22. From 10 GPa to ~20 GPa, the increased carrier density suggests an increased electronic density of states, which promotes an increase of TC. The a/b ratio stays close to 1 between 10 and 20 GPa followed by a decrease with higher pressure, as shown in Fig. 5 (c). Further increasing pressure to 30 GPa, TC keeps increasing. Like Bi2Se322, balanced electronic contribution, phonon contribution and some other parameters may lead to the very slow variation from 30 GPa to 40 GPa (see Fig. 2 (b)). However, the relationship between TC increase (>20 GPa) and carrier density need to be elaborated and established by further experiments.

The complex structure versus rich properties evolution of Sb2Se3 as a function of pressure, such as ETT and topological superconducting state, invites further studies from theoretical prediction (electronic structural calculation) to validate the possible topological character.

Methods

Sample synthesis

Sb2Se3 single crystal was grown by Bridgeman method. High purity Sb and Se elements are mixed at a molar ratio of 2:3, grounded thoroughly in an agate mortar to ensure homogeneity and then pressed into a cylinder. The mixture was sealed in an evacuated quartz tube under vacuum of 10−4 Pa and then heated to 850°C for 24 hours. The quartz tube and sample inside were slowly cooled down to 500°C with a controlled rate of 10°C/h, then followed by furnace cooling to room temperature. The crystal quality was assured by x ray diffraction & EDX measurements.

Measurement of high pressure properties

The resistance measurement of Sb2Se3 single crystal under pressure was using the standard four-probe method in a diamond anvil cell (DAC) made of CuBe alloy11,12,13,18. The diamond was 300 μm in diameter, center flat with 8 degree bevel out to 500 μm. A T301 stainless steel gasket was preindented from a thickness of 250 μm to 40 μm and a hole was drilled at center with diameter of 150 μm. Fine cubic boron nitride (cBN) powder was used to cover the gasket to keep the electrode leads being insulated from the metallic gasket. The cBN powder was pressed and further drilled into a center chamber with a diameter of 100 μm, in which a Sb2Se3 single crystal piece in dimension of 60 μm × 50 μm × 30 μm was loaded simultaneously with soft hexagonal boron nitride (hBN) fine powder surrounding it as pressure transmitting medium. Slim gold wires of 18 μm in diameter were used as electrodes. Pressure was determined by ruby fluorescence method36. The DAC was placed inside a MagLab system to perform the experiments with an automatic temperature control. A thermometer located near the diamond of the DAC is used for monitoring the sample temperature. The Hall coefficient at high pressure was measured using the van der Pauw method. The ac susceptibility was measured using inductance method. Two identical coils were wound with one is placed around diamond anvil and the compensating coil directly adjacent. Ac susceptibility measurements were carried out under high pressure at 3.8 Oe rms and 1000 Hz. CuBe gasket was preindented from 500 μm to 250 μm and drilled with a center hole of 350 μm in diameter. The Sb2Se3 single crystal in dimension of 150 μm × 150 μm × 100 μm was placed into the preindented CuBe gasket chamber with neon loaded as pressure transmitting medium. Ruby spheres placed at the top of sample served as a pressure marker.

Raman spectroscopy & angle-dispersive powder x-ray diffraction measurement under high pressures

The monochromatic ADXRD measurements at pressure were performed at the 16BMD beamline of the High Pressure Collaborative Access Team (HPCAT) at Advanced Photon Source (APS) of Argonne National Lab (ANL), using a symmetric DAC. The diamond culet was 300 μm in diameter. The gasket was rhenium which was preindented from 250 μm to 40 μm and drilled with a center hole of 150 μm in diameter. The hole was compressed to 120 μm with neon loaded as a nearly hydrostatic pressure transmitting medium. Ruby spheres were placed around sample to monitor pressure. The x-ray wavelength was 0.4246 Å. The XRD patterns were collected with a MAR 3450 image plate detector. The two-dimensional image plate patterns obtained were converted to one-dimensional 2θ versus intensity data using the Fit2D software package37. Refinements of the measured XRD patterns were performed using the GSAS + EXPGUI software packages38. The high-pressure Raman experiments were conducted on single crystal Sb2Se3 using Renishaw inVia Raman microscope with laser wavelength 532 nm and spectral resolution ~1 cm−1. The gasket was T301 stainless steel, which was preindented from 250 μm to 40 μm and drilled with a center hole of 120 μm in diameter. Ruby spheres were pressure monitors.