In Fig. 3.21, we observed shoulders in the RT curve of (SnS)(TaS2)2 that is possibly due to the incomplete suppression of CDW transition in 1T-TaS2. Comparing with the complete suppression of CDW in (SnS)(TaS2) (see Fig. 3.18), these shoulders imply that CDW is suppressed by decreasing the ratio of TaS2 in the misfit compound. We also note that the critical temperature Tc in (SnS)(TaS2) (2.9K) is higher than (SnS)(TaS2)2
(2.2K). Similar suppression of superconductivity in (SnS)(TaS2)2 is found in the current density and magnetic field dependence of Tc. Figure 4.9 is the critical current Jcvs. the normalized temperature Tc/Tc0 (open circles), where Tc0is the critical temperature mea-sured with the lowest current (100µA). It can be seen that Tc decreased about 14% with current density J=5.6 A/cm2 in (SnS)(TaS2)2, while the decrease of Tc in (SnS)(TaS2) is less than 1%. Figure 4.10(a) and (b) are the critical field Hc vs. the normalized Tc
(open circles) for (SnS)(TaS2) and (SnS)(TaS2)2, respectively, where Tc0 is the critical temperature measured without external field. The red curves are the fittings to the empir-ical formula Hc(T ) ≈ Hc(0)[1− (Tc(H)/Tc(0))2]. The obtained critical field Hc(0) of (SnS)(TaS2) (0.45T) is about twice the value of (SnS)(TaS2)2(0.21T). These comparisons suggest that superconductivity is more fragile in (SnS)(TaS2)2 than in (SnS)(TaS2). This is consistent with the usually adopted picture that superconductivity and CDW are com-peting electronic states at low temperatures [56, 38]. Similar suppression of CDW is also observed in the misfit superconductor (PbSe)(TiSe2)2. No sign of CDW transition can be seen in the RT curve (Fig. 3.24). Figure 4.11 is Hc vs. the normalized Tc(open circles) for (PbSe)(TiSe2)2. The red curve is the fitting to the empirical formula mentioned above, and the obtained Hcis about 0.31T.
We also observed dramatic increase in the carrier concentrations of these misfit super-conductors (see Sec. 3.3), which is usually explained by the rigid band model we intro-duced in Sec. 1.3. This is consistent with the previous observations [38, 57] that charge doping can suppress CDW and induce superconductivity at low temperatures. This may suggests that charge transfer in these misfit compounds are closely related to the compe-tition between CDW and superconductivity. However, the quantitative study on this issue is beyond the scope of this work.
)b* )c*
Figure 4.9: Jc plots against the normalized Tc (Tc0 is the critical temperature measured with the lowest current) (open circles) for (a)(SnS)(TaS2) and (b)(SnS)(TaS2)2.
)b* )c*
Figure 4.10: Hc plot against the normalized Tc (Tc0 is the critical temperature without external field) (open circles) for (a)(SnS)(TaS2) and (b)(SnS)(TaS2)2. The red curves are the fittings to the empirical formula (see main text). The comparison of the fitted Hc(0) indicate that the superconductivity is destroyed at a higher magnetic field in (SnS)(TaS2).
0.0 0.2 0.4 0.6 0.8 1.0 0.0
0.1 0.2 0.3 0.4
H c
(0) ~ 0.31T
(PbSe)(TiSe 2
) 2
0
H C
(T)
(T C
/T C0
) 2
RS1142-B1
Figure 4.11: Hcplot against the normalized Tc(open circles) for (PbSe)(TiSe2)2. The red curve is the fitting to the empirical formula.
Conclusion
The topological insulators are insulators with conducting gapless surface states. In this work, we tried to identify the surface states through transport measurements. We observed prominent quantum oscillations in the MR of bulk Bi2Se3 crystals. The angular depen-dence of the oscillation frequency corresponds to a 3D Fermi surface. This indicates that the Fermi level is in the bulk conduction band and the signals from surface states are too small to be identified. We tried to tune the Fermi level in the exfoliated micro-sized Bi2Se3 crystals by applying top gate and bottom gate voltage. However, the observed field effect is not large enough to bring Fermi level into bulk band-gap, and the quantum oscillations are suppressed in the micro-sized Bi2Se3 crystals. This is possibly due to disorders and defects induced by the exfoliation process. We also tried to tune Fermi level by calcium doping and adding excess selenium. However, the results indicate that they may not be effective methods for tuning Fermi level. In this thesis, we described strategies for iden-tifying the surface states through transport measurements. Our experimental setup can be used to identify the surface states of new topological insulators in the future.
CDW is a special phase of periodic modulation of the electron density and the lattice constants. We studied misfit compounds derived from CDW materials, where supercon-ductivity was observed at temperatures below 5K. We observed evidences showing that CDW and superconductivity are competing electronic orderings in these compounds.
In-69
creasing the ratio of CDW material in misfit compound leads to the lowering of Tcand Hc. The measured carrier concentration in these compounds indicate that interlayer charge transfer is present, which is possibly related to the suppression of CDW. However, the quantitative study on this issue is beyond the scope of this work.
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