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Insulator-to-superconductor transition in quasi-one-dimensional HfS3 under pressure

Various transition metal trichalcogenides (TMTC) show the charge-density-wave and superconductivity, which provide an ideal platform to study the correlation between these two orderings and the mechanism of superconductivity. Currently, almost all metallic TMTC compounds can show superconductivity either at ambient pressure or at high pressure. However, most TMTC compounds are semiconductors and even insulators. Does the superconductivity exist in any non-metal TMTC compound? In this work, we managed to manipulate the electronic behavior of highly insulating HfS3 in term of pressure. HfS3 underwent an insulator-semiconductor transition near 17 GPa with a band gap reduce of ~1 eV. The optical absorption and Raman measurement provide the consistent results, suggesting the structural origin of the electronic transition. Upon further compression, HfS3 becomes a superconductor. The superconducting transition was initialized as early as 50.6 GPa and the zero-resistance is reached above 91.2 GPa. The superconducting behavior is further confirmed by both the magnetic field effect and current effect. This work sheds the light that all TMTC may be superconductors, and opens a new avenue to explore the abundant emergence phenomena in TMTC material family.


Introduction
Transition metal chalcogenides (TMCs) are a sort of layer-structured semiconductors or metals with great potential as next generation electronics and optoelectronics (1-3). There are two important branches in TMC material family, transition metal dichalcogenides (TMDCs) and transition metal trichalcogenides (TMTCs).
Different from the TMDC in form of typical two-dimension structure, TMTCs show the characters of quasione-dimension properties. The chemical formula of TMTCs is MX3, where M is transition metal atom belong to either group IVB (Ti, Zr, Hf) or VB (Nb, Ta) and X is chalcogen atom from group VIA (S, Se, Te). As shown in Table I, most of them crystalize in monoclinic structure (P21/m) while NbSe3 and TaSe3 can also stabilize in triclinic and orthorhombic structure, respectively (3,4). Physical properties of MX3 vary from superconducting (SC) to semiconducting (even highly insulating) according to the distinctive stacking sequences of the one-dimensional (1D) chain variants, which allows their application in various areas. Charge density waves (CDW) transitions have shown in NbS3 (5,6), TaS3 (7-9), NbSe3 (10)(11)(12) while coexistence with SC has also been observed in both ZrTe3 (13,14) and HfTe3 (15). TaSe3 is a semimetal which was proved to be a topological insulator recently (16,17) and has a superconducting transition at ~2.2 K (18) as well.
Trisulfides with M from group IVB are all semiconductors with band gap from ~1 to ~ 3 eV (3,(19)(20)(21)(22)(23)(24)(25)(26)(27). With the introduction of pressure, which can manipulate the layer distance and interaction in a large range, some TMC materials show various emergence phenomena. In ZrTe3, TCDW increases to ~105 K at 1.1 GPa, while the filamentary SC disappears. TCDW shows a dome-like behavior and disappear at ~ 5 GPa when a bulk SC emerges (28)(29)(30). Suppression of the CDW and emerging of SC has also observed in metallic NbSe3 (10)(11)(12) o-TaS3 (31) under pressure, which is also observed in some TMDC materials, such as TaS2 (32). It is noted that all these TMTC materials with SC transition are metallic at ambient pressure. Theoretical calculation shows semiconducting TiS3 could be a superconductor at 80 GPa (33). However, it is still an open question that whether any semiconducting or insulating MX3 could become a superconductor upon compression by experiment.
Among all MX3 compounds, the band gap of HfS3 is almost the largest (1.7~3.1 eV) which makes it highly insulating (23)(24)(25)(26)(27). Here we choose this material as a prototype to study the pressure effects on its electronic properties, with the aim to find out whether SC state in such an insulating MX3 can be induced by pressure or not. In this work, we found that the band gap of HfS3 is sensitive to external pressure. HfS3 underwent an insulator-semiconductor transition near 17 GPa with sudden reduce of band gap. Upon further compression, signature of SC transition starts to show above 50.6 GPa and zero resistance is observed above 91.2 GPa. Tc increases with further compression and reaches 8.1 K at 121 GPa. This work has demonstrated that the electronic properties of layered and insulating TMTC HfS3 can be well manipulated by pressure, while the pressure plays a role similar with electron doping, and the SC transition is firstly achieved in such an insulating TMTC compound, which suggests other semiconducting TMTC are also promising superconductors under high pressure.

Experiments:
The standard four-probe electrical resistance measurement under high pressure was measured in a commercial cryostat from 1.7 K to 300 K by a Keithley 6221 current source and a 2182A nanovoltmeter. A BeCu alloy diamond anvil cell (DAC) with two opposing anvils was used to generate high pressure. For Run #1, 300 µm culets were used and 100 µm culets were used for Run #2. In these experiments, a thin single crystal sample was loaded into the sample chamber in a rhenium gasket with c-BN insulating layer. A ruby ball is loaded to serve as in-ternal pressure standard for Run #1, while the Raman signal of diamond culet was used to calibrate the pressure for Run #2. The high-pressure Raman spectra were collected using a Renishaw Micro-Raman spectroscopy system equipped with a second-harmonic Nd:YAG laser (operating at 532 nm). A 532 nm laser was used, with a spot size of 1-2 µm. A pair of low fluorescent diamonds were used for Raman measurement.
The laser power was maintained at relatively low power level to avoid overheating during measurements. KBr was used as pressure medium. In-situ high-pressure UV-VIS absorption spectroscopy was performed on a home-designed spectroscopy system (Ideaoptics, Shanghai, China) with a 405 nm laser. The high-pressure infrared experiments were performed at room temperature on a Bruker VERTEX 70v infrared spectroscopy system with HYPERION 2000 microscope. A thin HfS3 single crystal was loaded and KBr was used as pressure medium. The spectra were collected in transmission mode in the range of 600-8000 cm -1 with a resolution of 4 cm -1 through a ∼50×50 µm 2 aperture.

Results and discussions:
The electrical transport measurements on single crystal HfS3 were conducted as a function of temperature up to 121 GPa. Two runs of experiments have been conducted and the resistance-temperature (R-T) curves are shown in Fig. 1(a) and 1(b), respectively. At ambient condition, the resistance of the HfS3 sample is too large to be measured, the multimeter shows readable resistance value ~31 kΩ from 17.7 GPa at 300 K and ~8.8 MΩ at 2 K. As pressure further increases, the temperature dependent resistance always shows semiconductor feature while the resistance decreases monotonously with pressure, as shown in Fig. 1(a). However, staring from 50.6 GPa, the resistance below 5 K shows some anomaly as seen in inset of Fig. 1(a), and the phenomenon is much clearer at 65.1 GPa with a sudden drop of the resistance below ~5 K, as shown in Fig.   1(b). Such a sharp transition suggests a possible superconducting transition. This is further confirmed by the resistance measurement at much higher pressure. The sample reaches zero resistance at ~1.8 K at 91.2 GPa and further compression can move both the Tc onset and Tc zero to higher temperature (the Tc onset is determined from the cross point of two straight lines above and below the transition, while Tc zero is defined as zeroresistance temperature), as seen in the inset of Fig. 1 To further verify the pressure-induced superconductivity in HfS3, electric transport measurements under external magnetic field and various currents have both been conducted. Fig. 2 (a) displays the R-T curve at 121 GPa under various magnetic fields up to 2 T. It can be seen clearly that Tc is suppressed to lower temperature with increasing magnetic field, which is typical for a bulk superconducting transition. The upper critical field μ0Hc2 as a function of critical temperature Tc onset is plotted in Fig. 2  The resistance of HfS3 crystal becomes measurable only from above 17.7 GPa, which suggests that the electronic transition might take place near this pressure point. To better understanding the electronic structure evolution, optical absorbance of HfS3 as a function of pressure via both UV-VIS and IR spectroscopy has been carried out (Fig. 3 (a-c)). At ambient pressure, HfS3 shows a band gap of 2.45 eV, consistence with previous results (24)(25)(26). As shown in Fig. 3 (d), the band gap decreases gradually with pressure and then has a sudden drop above 15.3 GPa, indicating a phase transition which is consistence with the electric transport results.
Above 17.9 GPa, the band gap is out of the detecting range of UV-VIS spectroscopy. The color of the sample also changes from transparence to black (Fig. 3e), which makes the resistance of HfS3 to be small enough to be detected by a multimeter. To get information under higher pressure, IR absorption measurements has been conducted, as displayed in Fig. 3(b-c) and results show that the absorbance is almost 0 up to 12.9 GPa, following with an increasing trend with pressure. Above 15.6 GPa, the absorbance drops a little bit due to the phase transition and then increases again. A band gap of 0.82 eV is obtained at 19 GPa, which is much smaller as compared with that of the ambient phase. Under further compression, it decreases slowly and reaches 0.36 eV at 53.7 GPa. The transmission spectra also agree with this. The transmittance decreases with pressure and was strongly suppressed at the highest pressure of 53.7 GPa. Meanwhile, the optical interference oscillation amplitude of the transmittance signal is also much smaller at 53.7 GPa. This suggested that the band gap HfS3 is gradually closing and may started to transform to a semimetal state. The optical analysis confirms that there is an electronic transition near 17 GPa, the band gap is reduced by ~1 eV near this transition, which is consistent with the electric transport behavior in HfS3. When pressure was released, the band gap was recovered with a small hysteresis. A band gap of 2.34 eV is obtained when the pressure was released to 0.4 GPa, close to that at ambient condition.
To understand the mechanism of the electronic transition observed by transport measurement and optical absorption experiment, we collected the Raman spectra under high pressure, as presented in Fig. 4(a). At ambient temperature and pressure, the crystal structure of HfS3 belongs to the C2h point group and shows typical quasi-one-dimension feature, as seen in Fig. 4 (b-c). The Raman-active phonon modes can be represented by the irreducible representations as  = 8Ag + 4Bg (54). Ag phonon modes vibrate perpendicular to the quasi-one-dimension chain and are prominent while Bg modes involving atomic displacement along the chain are weak. The Raman modes observed in this work can be divided into four groups. Group I have three Ag lines at 74, 113 and 132 cm -1 and one Bg mode at 141 cm -1 as shown in Fig. 4 (a) and 4 (d). It stems from rigid-chain modes and is associated with the out-of-plane vibration of 1D-like chains extending along the baxis. The intensity of 74 cm -1 peak is quite weak compared with others and cannot be detected above 7 GPa.
All these three Ag modes share the similar pressure coefficients within the range of 1.5 ~ 2 cm -1 /GPa. The Bg mode at 141 cm -1 shows a relatively higher d/dp value and starts to decay above 7 GPa. At around 16~18 GPa, all peaks in this group disappear while a new mode at 159 cm -1 appears, indicating that a phase transition should occur. Group II corresponds to internal deformation of the chains and includes 3 Bg modes (219, 246 and 260 cm -1 ) and 2 Ag modes (267 and 274 cm -1 ). The first two Bg modes show different pressure coefficients and merge together above ~7 GPa. It finally disappears above 13.6 GPa while another mode at 238 cm -1 starts to show up. The Ag mode at 274 cm -1 shows a quite high value of pressure coefficient and becomes undetectable above 7 GPa. For 260 and 267 cm -1 modes, a reversal of relative intensity can be observed at 3.1 GPa. Both of them last up to 19.7 GPa and then are replaced by a new mode at 321.4 cm -1 . Similar with group II, group III also corresponds to internal deformation of the chains. In this group, the Ag line at 322 cm -1 shows a moderate d/dp coefficient as compared with that of other modes. The other mode in this group, the Ag line at 353 cm -1 , decays a lot from 3.1 GPa and then raise up from 13.6 GPa. The strengthening of this mode should be corresponding to the intermediate phase since it disappears above 19.7 GPa together with the 322 cm -1 , and then the 385 cm -1 mode comes out, as shown in Fig. 4 (a) and 4 (e). Group IV has one strong Ag line at 524 cm -1 , which is predominantly made of in-plane out of phase motion of SII-SIII pair (Fig. 4c). It shows a negative pressure coefficient for the original low-pressure phase and then a positive trend with the highest d/dp value than the other new modes (Table II) after the phase transition above 19.7 GPa. Based on the Raman results, we found that the electronic transition near 17 GPa is due to a structural phase transition, which started from ~13.6 GPa and completed at ~19.7 GPa. The strongly suppression of Raman signal above 40 GPa also suggests that HfS3 could start to show some feature of a metal in which Raman signal is generally absent or very weak.
According to previous study on TiS3, there is an isostructural phase transition near 22 GPa (35), which may also apply to the phase transition near 17 GPa observed in HfS3. Further study is still required to reveal the accurate structural information under high pressure. Grüneisen parameters of all modes of the ambient phase are calculated by using the relationship i=(B0/i) (di/dp), where B0 is the bulk modulus of HfS3. All results are listed in Table II. The quite large value of  of the rigid Ag mode 74 cm -1 and Bg mode 141 cm -1 implies more contribution to the thermal expansion coefficient, similar as the situation in TiS3 (37). Another phenomenon that needs to be noted is that the peak intensity of all high-pressure modes is much weaker than that of the ambient phase. It further decreases with pressure and mode Ihp becomes undetectable at the highest pressure (40.5 GPa) reached in this experiment.
Based on spectroscopy and electric transport measurements, the pressure-temperature (P-T) phase diagram of HfS3 has been plotted and is shown in Fig.5. Within the highest-pressure limit (121 GPa) in this work, Tc zero increases with pressure, which differs from the decreasing trend of theoretically predicted Tc in TiS3 (33), but resembles that of ZrTe3 (39).

Conclusion
In conclusion, HfS3 undergoes an insulator-to-superconductor transition under high pressure up to 121 GPa.