Temperature dependent electrical and magnetic properties of topological insulator
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... The mixture of high-purity Bi, Se, and Tr elements was sealed in evacuated quartz ampoules. The ampoule was heated up to 850 o C for 12 hs and was kept at that temperature for 1 h. Then, it was slowly cooled to 620 o C for 46 hs and was quenched in cold water. The obtained crystals were easily cleaved along the plane with shiny flat surface. X-ray diffraction (XRD) measurements were carried out by using Bruker D8 diffractometer with Cu K a radiation. The samples were characterized by using electron probe micro- analyzer (EPMA) and inductively coupled plasma (ICP) spectrometer. The transport properties were measured by using Quantum Design physical property measurement system (PPMS). The superconducting quantum interference device-vibrating sample magnetometer (SQUID-VSM) from Quantum Design was used to measure the magnetic properties. Figure 1 shows the XRD data for all single crystals. It is clear that the samples are single phased with the rhombo- hedral structure of Bi 2 Se 3 (R3m space group). 11 Most of XRD peaks correspond to (0 0 L) reflections, indicating the cleaved surface is the ab plane. No significant difference between intercalated and substituted samples (Tr x Bi 2 Se 3 and Bi 2-x Tr x Se 3 , respectively) is observed, in contrast to the previous report on Cu-intercalated Bi 2 Se 3 where the c-axis lat- tice is increased. 8 A splitting in the peaks around 60 and 70 degree occurs due to the difference in wavelengths in the x-ray source of Cu K a 1 and Cu K a 2 radiations used to measure the diffraction patterns. Furthermore, an additional peak around 40 degree for Cr-substituted and Fe-intercalated Bi 2 Se 3 might be detected because the single crystals are mounted with a slight tilt angle or a slight misalignment of the crystallinity. The stoichiometric ratio was nominally checked by the EPMA and ICP techniques. We could check the stoichiometric composition of roughly Bi: Se 2:3 and the corresponding doping level. A small deviation from the stoichiometric ratio might stem from the composition gradi- ent inside the quartz ampoule. 9 We measured the temperature dependence of electrical resistivity, which shows typical metallic behavior for all single crystals of Tr x Bi 2 Se 3 and Bi 2-x Tr x Se 3 (Tr 1⁄4 Cr, Fe, Cu) with x 1⁄4 0.15. In Fig. 2, the typical resistivity curve for the Cu-substituted Bi 2 Se 3 is displayed. It is known that the as- grown crystals of Bi 2 Se 3 display metallic behavior because the Fermi energy lies in the conduction band due to the gen- eration of electrons donated by Se vacancies. 12,13 By chemical doping, we can drive the Fermi level inside the energy gap, leading to nonmetallic behavior. For example, nonmetallic resistivity profile was observed in Bi 2-x Ca x Se 3 with a narrow doping window 0.002 < x < 0.0025, 12 and p-type conducting profile was observed with higher doping level. 13 However, all resistivity curves of Tr x Bi 2 Se 3 and Bi 2-x Tr x Se 3 (Tr 1⁄4 Cr, Fe, Cu) for x 1⁄4 0.15 show metallic behavior, which is expected for Se vacancies. This result implies that there is no significant change of the Fermi energy by doping for Cr, Fe, and Cu. The charge carrier type was found by the Hall measurement. The sign of linear slope of Hall voltage versus magnetic field is negative, giving rise to the n-type carriers for all the crystals. The estimated carrier density of the as-grown Bi 2 Se 3 is approximately 5.8 Â 10 19 /cm 3 , independent of temperature. For the Cu doped cases, the Cu-substituted BiSe 3 shows a temperature independence of carrier density $ 2.7 Â 10 18 /cm 3 between 50 and 300 K, while the Cu-intercalated Bi 2 Se 3 shows a strong temperature dependence of carrier density $ 1.7 Â 10 18 /cm 3 at 300 K and 2.2 Â 10 20 /cm 3 at 50 K. The increment of carrier density at low temperature in the Cu-intercalated Bi 2 Se 3 could be responsible for the superconducting transition with T C 1⁄4 3 K. In the inset of Fig. 2, a diamagnetic signal below T C is verified for the superconductivity. The origin of nonzero resistivity below T C is the small amount of superconducting volume fraction, as described in Ref. 8. Note that at 300 K the carrier density for the Cu-intercalated Bi Se is lower than that for the Cu-substituted Bi 2 Se 3 . This tendency is common for the Cr and Fe doped cases, where the carrier density is independent of temperature. For example, the carrier density of the Cr- intercalated Bi 2 Se 3 is 1 order of magnitude lower than that of the Cr-substituted Bi 2 Se 3 . On the other hand, the mobility found in the intercalated samples is much higher