The recently discovered Dirac and Weyl semimetals are new members of topological materials. Starting from them, topological superconductivity may be achieved, e.g., by carrier doping or applying pressure. Here we report high-pressure resistance and X-ray diffraction study of the three-dimensional topological Dirac semimetal Cd3As2. Superconductivity with Tc≈2.0 K is observed at 8.5 GPa. The Tc keeps increasing to about 4.0 K at 21.3 GPa, then shows a nearly constant pressure dependence up to the highest pressure 50.9 GPa. The X-ray diffraction measurements reveal a structure phase transition around 3.5 GPa. Our observation of superconductivity in pressurised topological Dirac semimetal Cd3As2 provides a new candidate for topological superconductor, as argued in a recent point contact study and a theoretical work.
In recent few years, the search for topological superconductors (TSCs) has been a hot topic in condensed matter physics.1,2 The TSCs have a full pairing gap in the bulk and gapless surface states consisting of Majorana fermions.1 This is in close analogy to the topological insulators (TIs), which have a full insulating gap in the bulk and gapless edge or surface states.1 The TSC is of great importance, as it is not only a new kind of exotic superconductor but also one source of Majorana ferimons for future applications in quantum computations.1,2
Experimentally, the simplest way to get a candidate for TSC is to convert a TI into superconductor, by tuning the parameters such as doping or pressure. For example, by doping, CuxBi2Se3 and Cux(PbSe)5(Bi2Se3)6 are considered to be candidates for TSCs,3,
More recently, a new kind of topological material, the three-dimensional (3D) Dirac semimetal was discovered, with examples of SrMnBi2, Na3Bi and Cd3As2.18,
Here we present the resistance measurements on Cd3As2 single crystals under pressure up to 50.9 GPa. After an initial increase with pressure, the low-temperature resistance starts to decreases with pressure above 6.4 GPa. Superconductivity appears at 8.5 GPa with Tc≈2.0 K, and the Tc increases to about 4.0 K at 21.3 GPa, then persists to the highest pressure 50.9 GPa. A structure phase transition around 3.5 GPa is also observed by X-ray diffraction (XRD) measurements. These results suggest that Cd3As2 may be a new topological superconductor under high pressure.
Figure 1a shows the crystal structure of Cd3As2.30 The cubic Cd lattice with two vacancies resides in a face-centred cubic As lattice. Figure 1b plots a typical resistivity curve of Cd3As2 single crystal at 0 GPa. It is metallic and non-superconducting down to 1.5 K.
In Figure 2, the resistance curves for Cd3As2 single crystal under various pressures are plotted. From Figure 2a, the temperature dependence of resistance already changes to insulating behaviour (dR/dT<0) at 1.1 GPa. With increasing pressure, it becomes more and more insulating until 6.4 GPa. However, upon further increasing pressure, the resistance at low temperature decreases with pressure. In Figure 2b, it becomes more and more metallic up to 32.7 GPa. Figure 2c,d) show the low-temperature part of the resistance curves above 8.5 GPa. A drop of resistance is observed below 2.0 K at 8.5 GPa, which is like a superconducting transition. At 11.7 GPa, the resistance drops to zero, and the transition temperature Tc=3.3 K is defined at the cross of the two straight lines. The Tc increases to about 4.0 K at 21.3 GPa, then persists to the highest pressure 50.9 GPa.
To make sure the resistance drop in Figure 2 is a superconducting transition, we measure the low-temperature resistance under 13.5 GPa in magnetic fields applied perpendicular to the (112) plane, as shown in Figure 3a. The resistance drop is gradually suppressed to lower temperature with increasing field, which demonstrates that it is indeed a superconducting transition. Note that such a superconductivity we observed here is very unlikely due to contamination of pure As, as its highest Tc under pressures is much lower, and the pressure dependence of Tc is quite different.31
Figure 3b plots the temperature dependence of Hc2. Although limited by the temperature range we measured, one can see an apparently linear temperature dependence of Hc2. With a linear fit to the data, Hc2(0)≈4.29 T is roughly estimated. This value is higher than the orbital limiting field , according to Werthamer–Helfand–Hohenberg formula.32 It is much lower than the Pauli limiting field HP(0)=1.84Tc=7.89 T,33,34 suggesting an absence of Pauli pair breaking. The linear temperature dependence of Hc2 in Figure 3b is actually very interesting. It may come from a two-band Fermi surface topology as in MgB2,35,
We notice that no superconductivity was observed up to 13.43 GPa in an earlier pressure study of Cd3As2 single crystal.42 The reason may be that their sample is slightly different from ours, and pressure higher than 13.43 GPa is needed to induce superconductivity. Interestingly, we also notice two recent point contact studies on Cd3As2 polycrystal and single crystal, respectively.43,44 In both studies, indication of superconductivity was found around the point contact region on the surface, with Tc comparable to ours. In particular, no superconductivity is observed by the ‘soft’ point contact technique, therefore it was suggested that the superconductivity observed around the point contact region under the ‘hard’ tip might be induced by the local pressure.44 In this sense, our bulk resistance measurements under hydrostatic pressure confirm pressure-induced superconductivity in Cd3As2, although the local pressure under the ‘hard’ tip is more like uniaxial stress.
Pressure-induced crystal structure phase transition
Before discussing whether the pressure-induced superconductivity is topological or not, it is important to know whether it is accompanied by a structure phase transition, as observed in pressurised TIs.9,
The unusual Tc–p phase diagram
In Figure 5, we plot the temperature versus pressure phase diagram for Cd3As2. As the resistance was only measured down to 1.8 K, we cannot judge whether the superconductivity emerges at the same time as the structural transition near 3.5 GPa, or inside the high-pressure phase. Nevertheless, after increasing from 1.8 to about 4.0 K, there is apparently a region of constant Tc from 21.3 to 50.9 GPa. Such a phase diagram is very similar to that of 3D TI Bi2Se3, which also shows a nearly constant Tc from 30 to 50 GPa after an initial increase of Tc starting from 12 GPa.11 A constant Tc over such a large pressure range is highly anomalous, as Kirshenbaum et al.11 already pointed out. For Bi2Se3, two mechanisms with contrasting pressure-dependant Tc may be balanced to produce a pressure-invariant Tc over a wide range of pressure.11 It was argued that the unique pressure evolution of Tc and the anomalous linear temperature dependence of Hc2 are two evidences for unconventional superconductivity in Bi2Se3.11 The similarity between Cd3As2 and Bi2Se3 under pressure is worthy of further investigation.
Now we discuss whether the superconducting state of Cd3As2 under high pressure is topological or not. In ref. 44, the observation of zero bias conductance peak and double conductance peaks under ‘hard’ tip reveal p-wave like unconventional superconductivity in Cd3As2. Considering its special topological property, they suggested that Cd3As2 under high pressure is a candidate of the TSC.44 Furthermore, a recent theoretical work also argued that Cd3As2 likely realises a TSC with bulk point nodes and a surface Majorana fermion quartet.45 Under high pressure, the symmetry-lowering effect may stabilise the TSC phase by increasing the condensation energy, as the point nodes in the TSC phase are gapped when C4 reduces to C2 (the structure phase transition from tetragonal to monoclinic).45 These two works suggest that the superconductivity we observe under hydrostatic pressure is topological, although detailed band structure calculation for the high-pressure phase of Cd3As2 is needed to give more information about this possible TSC phase.
In summary, we have done resistance measurements on the 3D Dirac semimetal Cd3As2 single crystals under pressures up to 50.9 GPa. It is found that superconductivity with Tc≈2.0 K emerges at 8.5 GPa. The Tc increases to 4.0 K at 21.3 GPa, then it shows an anomalous constant pressure dependence up to the highest pressure measured. High-pressure powder XRD measurements reveal a structure phase transition around 3.5 GPa. Our observation of superconductivity in Cd3As2 under high pressure provides an interesting candidate for topological superconductor.
Materials and Methods
High-quality Cd3As2 single crystals were grown from Cd flux.28 The largest natural surface was determined as (112) plane by XRD. The resistivity in vacuum (0 GPa) was measured on a large sample with dimension of 1.50×0.40 mm2 in the (112) plane and 0.15 mm in thickness. The resistance measurement under pressure between 1.1 and 50.9 GPa was performed using diamond anvil cell with solid transmitting medium hexagonal boron nitride.9,13,14 The sample size is about 80×80 μm2 in the (112) plane, with the thickness of ∼10 μm. The pressure was determined by ruby fluorescence method at room temperature before and after each cooling down. The high-pressure powder XRD measurements with synchrotron radiation were performed at the HPCAT of Advanced Photon Source of Argonne National Lab (Lemont, IL, USA) using a symmetric Mao Bell diamond anvil cell at room temperature. The X-ray wavelength is 0.434 Å.
We thank X Dai and Z Fang for helpful discussions. This work is supported by the Ministry of Science and Technology of China (No. 2012CB821402, No. 2015CB921401 and No. 2016YFA0300503), the Natural Science Foundation of China, Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning and STCSM of China (No. 15XD1500200).
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