Superconductivity, Magnetoresistance, Magnetic Anomaly and Crystal Structure of New Phases of Topological Insulators Bi2Se3 and Sb2Te3

We synthesized a new metastable phase of Bi2Se3 topological insulator by a rapid quenching after a high-pressure-high-temperature treatment at P≈7.7 GPa; 673<T<1400 K. The structure of metastable phase is monoclinic P21/m type. We observed the zero-field magnetic susceptibility cusp and linear positive magnetoresistance indicating the topological insulator state. The annealing at 673 K during 2 hours resulted in complete reversible transformation into the initial crystalline rhombohedral structure. Also we synthesized bulk polycrystalline samples of metastable at ambient conditions monoclinic (C2/m) phase of Sb2Te3 by rapid quenching after a high-pressure–high-temperature treatment at P=3.7–7.7 GPa; T=873 K and found superconductivity with Tc=2 K. A zero-field magnetic susceptibility cusp and linear positive magnetoresistance indicate a topological insulator state.


Introduction
Experimental studies of bismuth and antimony chalcogenides substantially intensified last years due to discovery of their topological insulator (TI) properties [1][2][3][4][5][6][7][8][9][10][11][12][13] predicted earlier theoretically. Among other unusual properties of these materials superconductivity attracts much attention as far as it may possess the unconventional p-wave type. The unconventional superconductivity features have been indeed observed in Cu doped Bi2Se3 [7,8] and specially sintered Sb2Te3 [13]. Mostly the superconductivity in TIs was observed "in situ" under high-pressure [8,9]. In particular, in Bi2Se3 superconductivity appeared under pressure of 11 GPa with the critical temperature rising from 0.5 to 7 K under pressure up to 30 GPa, and the upper critical magnetic field of transition increased from 0.3 up to 4 T in the pressure range of 13-30 GPa [8]. The reduced critical field h * (T)=(Hc2(T)/Tc)/(dHc2(T)/dT)|Tc was calculated and compared to the models for orbitally limited swave and spin-triplet p-wave superconductors and found that the experimentally defined value of 2 1234567890 ''"" about h * (0) = 0.9 exceeded the upper limits of 0.7 for s-wave and 0.8 for the spin-triplet p-wave case. Thus the p-wave superconductivity type was concluded. By applying high-pressure the superconducting phases have been also found in other TIs like tellurides of Bi [9], Sb [10], In [12] and selenides Sb2Se3 [11]. However, the transitions to superconducting phases were reversible as well as in Bi2Se3. After the pressure releases they transformed back to non-superconducting phases. Besides superconductivity the peculiar properties of TIs like linear positive magnetoresistance effect and the zero-field paramagnetic cusp were observed [13,14].
Here we present synthesis of new Bi2Se3 metastable phases with monoclinic crystal structures by quenching after high-pressure-high-temperature treatment at high pressure and different temperatures and metastable superconducting phase Sb2Te3, linear positive magnetoresistance effect at low temperatures and the zero-field paramagnetic cusp at T=100 K and 300 K indicating topological insulator state.

Experimental
We used the commercially available high-purity (99.999%) Bi2Se3 and Sb2Te3 alloys with rhombohedral structure [15]. We have synthesized metastable high-pressure phases by rapid quenching after the electrical current thermoresistive heating in the "anvil with cavity"-type high-pressure apparatus [14]. The high-pressure-high-temperature experiments were carried out up to 7.7 GPa pressure with the heating up to 1673 K. The sample cooling rate was ≈ 60 o /min, and its pressure reduction rate in the reaction cell ≈ 1 GPa/min. The output samples were 2.5 mm thick and 4.5 mm in diameter. Metastable phases m-Sb2Te3 has monoclinic C2/m structure [16].
For the analysis of the crystal structure and physical properties of the samples we employed the powder X-ray diffraction, electrical, magnetic, and heat capacity measurements. PANalytical diffractometer with CuKα radiation source was used. We studied the temperature dependencies of the electrical resistivity of the samples via a conventional 4-probe method down to 1.8 K, current-voltage characteristics, effect of magnetic field up to 9 T and the heat capacity by differential scanning calorimetry in the temperature range of 1.8 -400 K using the Quantum Design® physical properties measurement system (PPMS) 3.

Experimental results
3.1. Crystal structure of metastable phase of m-Bi2Se3. , 3m R a = 4.143 Å, c = 28.636 Å, the cell parameters are the same as that of ICDD database PDF-2 [17]. The peaks in the diffraction patterns of samples, obtained at temperatures below 1173 K (Fig 1, No 2-4) were broad, indicating disorder or amorphization of the structure. After treatment at higher temperatures 1373 -1473 K (Fig 1, No 6-7) the crystal structure became more ordered, these peaks are split and narrowed.
Since the diffraction peaks of the metastable phase of m-Bi2Se3 did not match to the high-pressure phase peaks obtained in the high-pressure diamond-anvil cell [5] we carried out a crystal-chemical analysis of the structures of all known sesquioxide chalcogenides with large size cations. Almost all sulphides and some selenides of rare-earth elements (RE) crystallize in the orthorhombic structure with the space group Pnma. Metastable phases with the orthorhombic structure Pnma were found after high pressures and high temperatures treatment of RE sulphides like Tm2S with initial monoclinic P21/m structure [18]. The Pnma structure was previously detected in the metastable phase of Bi2Se3 obtained after the treatment at lower pressures and temperatures [19] and in the present work at P = 4 GPa and T = 673 K. Here it should be mentioned, that after annealing at the temperature of 120 0 C we observed transformation of the metastable high-pressure monoclinic phase m-Bi2Se3i into an orthorhombic phase with a Pnma structure. Thus we assumed that the structure of the new metastable The parameters of the unit cell and the coordinates of the atoms in the structure of the new phase (Fig.1, No. 3) with the space group P21/m were obtained by the method of full-profile analysis by the use of FULLlPROF program [21]. Here are its cell sizes: space group P21/m, a=12.25(6)Å, b = 4.106(8) Å, c = 11.49(7) Å, β = 115.088º, Z = 4. The coordinates of 10 independent atoms are also defined. Thus, by quenching after HPHT treatment we obtained new metastable structures of m-Bi2Se3 (P21/m). The structure remains layered, but the interlayer distance decreased. The main difference is the decrease in the distance between metal atoms Bi-Bi 3.5-3.7Å.  octahedral (BiSe6) and mono-and double-capped trigonal prisms (BiSe7 and BiSe8) with the common ribs along the short axis "b". The interatomic distances of Bi-Bi in the P21/m structure are much smaller than in the Pnma structure. After the annealing of metastable phase at T = 300 ° C during 1 hour structure of the sample returned back into initial rhombohedral structure.

Magnetization
Magnetization versus magnetic field dependence of m-Bi2Se3 sample (Fig. 1a, No.7) shows general diamagnetic behavior with paramagnetic contribution (Fig. 2a). Fig. 2b displays the magnetic susceptibility with Langevin approximation. The approximation definitely shows the zero-field paramagnetic anomaly in the field less of about 2 kOe. Such an anomaly, a zero-field paramagnetic cusp, is typical for topological insulators [22].
The same cusp was observed in m-Sb2Te3 samples, as shown in Fig. 3a. In m-Sb2Te3 we observed superconductivity at T=2K which was suppressed by low magnetic field as shown in Fig. 3b. While electrical and magnetic measurements clearly indicate transition into superconducting state, the DSC study did not show any peak in heat capacity at critical temperature. That may evidence twodimensional character of superconductivity.   Applying strong magnetic field at fixed temperatures T=1.8 and T=10 K we observed linear positive transverse magnetoresistance in metastable phase m-Bi2Se3 in strong magnetic field (Fig. 4a). In m-Sb2Te3 transverse magnetoresistance is positive and increases linearly with the magnetic field at H>25 kOe as shown in Fig. 4b.

Discussion
We synthesized new metastable phases m-Bi2Se3 and m-Sb2Te3, and determined the crystal structure of m-Bi2Se3 phase. Earlier superconductivity in high-pressure phases was observed "in situ" [4,5,8,9], but after the pressure release the structures transformed back into non-superconductive. The quenched metastable phase m-Sb2Te3 possess superconductivity transition at normal pressure. It is totally superconductive below Tc = 1.75 K.
It should be noted, that the R(T) dependence in our case display not simply metallic behavior, it looks more likely as degenerated semiconductor. The linear magnetoresistance (LMR) effect in high fields (see Fig. 4) and the zero-field paramagnetic cusp (see Fig. 2b and 3a) are typical in the topological insulators. Both m-Bi2Se3 and m-Sb2Te3 have positive linear magnetoresistance at H>25 kOe. It should be mentioned, that the disorder in narrow-gap semiconductors also causes linear magnetoresistance [23], but a relatively narrow temperature range of superconductivity transition can indicate good degree of crystallinity in our case. Thus we believe that this effect as well as the zerofield paramagnetic cusp manifests a topological insulator state.

Conclusion
We synthesized bulk polycrystalline samples of new metastable phase of Bi2Se3 topological insulator using a high-pressure and high-temperature treatment at P = 7.7 GPa; T = 673-1473 K with the subsequent quenching and investigated their electrical and magnetic properties. The monoclinic phase has space group P21/m, and the unit cell parameters a=12.25(6)Å, b = 4.106 (8) Å, c = 11.49(7) Å, β = 115.088º, Z = 4. We observed the linear positive transverse magnetoresistance effect and the zerofield paramagnetic cusp which are proper to topological insulators. Also we synthesized bulk polycrystalline samples of metastable at ambient conditions monoclinic (C2/m) phase of Sb2Te3. We observed superconductivity at T=2K in m-Sb2Te3 The critical current value of about 3 mA and an absence of the heat capacity peak at superconductivity transition indicate low-dimensional character of superconductivity in the case of m-Sb2Te3 phase.