Transport Properties Of Topological Dirac Materials Bi_0.05Sb_1.95Te₃ And ZrTe₅

Generally, topological Dirac materials have time reversal symmetry (TRS) protected band structures, which have a capability of anti-backscattering and are regarded as a potential candidate for energy-saving electronic devices. Due to those properties, topological Dirac materials have been widely studied recently. Topologically, insulator includes topological trivial insulator and nontrivial insulator (topological insulator). While for metal, it can also be divided into topological trivial metal (normal metal) and nontrivial metal. Based on telluride class Dirac materials Bi0.05Sb1.95Te3 and ZrTes, this thesis presents the low temperature transport properties in three-dimensional (3D) topological insulator (TI) and 3D Dirac semimetal. Details can be concluded as follows:一. Magnetic proximity effect between topological insulator Bi0.05Sb1.95Te3 and ferrimagnetic insulator BaFe12O19.We have grown the high quality topological insulator Bi0.05Sb1.95Te3 films on different substrates via Van der Waals epitaxial techniques. And based on this techniques, we have successfully grown Bi0.05Sb1.9sTe3 films on ferrimagnetic magnetic insulator BaFe12O19(BaM) with large lattice mismatch. By using Nano-fabrication skills, we have fabricated four-probe devices and studied the transport properties in our TI/BaM heterostructures. The transport properties of our TI/BaM heterostructures can be concluded as follow:1. We have clearly observed the weak localization effect in our TI/BaM heterostructures, which is manifested by a negative magnetoresistance in low temperature region, revealing that the time reversal symmetry of the topological surface state has been broken by ferrimagnetic insulator substrate via magnetic exchange interactions.2. Analyzing on the weak localization effect, a surface state gap of about 10 meV is revealed in our TI/BaM heterostructures.Contrast to magnetic doping, extra magnetic impurities might not be introduced in our TI/BaM heterostructures, and also a relative high Curie temperature can be reached. Our study reveals that the topological insulator-ferrimagnetic insulator heterostructures provide an idea platform to study the transport properties on a TRS-broken surface state.二. Transport evidence for Dirac semimetal phase in layered material ZrTe5.The ZrTes nanoribbons with different thickness are exfoliated from the bulk compound, and the nano-devices with different thickness are fabricated by using nano-fabrication techniques in our lab. After a detailed transport study on these nano-devices with different thickness, we get the following experiment results:1. Chiral magnetic effect is revealed in different directions. When the current is applied along a-axis of ZrTe5 nanoribbons, a negative magnetoresistance is revealed when the magnetic field B is parallel to the current direction. While both the current and magnetic field B are applied along c-axis, the negative magnetoresistance can also be identified in experiments. After a detailed analysis, we find that the observed positive magnetoconductance (negative magnetoresistance) exhibits a quadratic field dependence, and also the observed negative magnetoresistance is highly sensitive to the intersection angle between magnetic field B and the current, indicating the chiral magnetic effect in ZrTe5.2. By tracking the quantum oscillation in ZrTe5, a nontrivial Berry phase is revealed in layered material ZrTe5 nanoribbons. Analyzing on the linear dependence of the Landau index n on 1/B, an intercept of about 1/8 is revealed, which is consistence with the nontrivial Berry phase anticipated for three dimensional Dirac semimetal.3. An anisotropic three dimensional (3D) Fermi surface is demonstrated after analyzing the quantum oscillation in different directions.Our experiments reveal that a 3D Dirac semimetal phase exists in layered material ZrTe5. And in this layered material, a magnetic field of about S T can drive the system into the quantum limit, demonstrating that ZrTe5 is an idea layered material to study the quantum transport properites of 3D Dirac Fermions. 三. Zeeman effect in Dirac semimetal ZrTesFurthermore, we have studied the Zeeman effect in ZrTe5 bulk compound. Different from ZrTe5 nanoribbons, the ZrTe5 single crystal exhibits an obvious Zeeman splitting in high magnetic field. A sharp drop of magnetoresistance in high field region is revealed while the Fermi level is driven into the first Landau level. After a detailed analysis on the angle dependent magnetoresistance, we find: 1. A large effective Lande g factor of about 32 is estimated for a-c plane carriers in our ZrTe5 single crystal, after a detailed analysis on the quantum oscillation. While for the carrriers in other planes, the effective g factor is much smaller than that in a-c plane, indicating a large and anisotropic g factor.2. Due to the weak interlayer coupling in ZrTe5, a small band width along interlayer direction of about 10 meV is revealed via quantum oscillations. While B>8 T (applied along interlayer direction), Zeeman splitting might exceed the band width along b-axis due to large Lande g factor and quenches the dispersion in this direction. Correspondingly, the 3D Dirac semimetal is transformed to a 2D massive Dirac metal in high field region.3. While the system becomes a 2D massive Dirac metal in high field region, a sharp drop of magnetoresistance in high field region is induced due to orbital effect. Correspondingly, the angle dependent magnetoresistance exhibit a splitting in high field region, indicating the orbital splitting for the 2D massive Dirac metal phase in high field region.Our experiments reveal a fierce competition between the relative weak interlayer coupling (a small band width) and large Zeeman splitting, which induces a transformation from 3D Weyl semimetal in low field region to a 2D massive Dirac metal in high field region Also, an obvious orbital splitting is demonstrated while the system becomes a 2D massive Dirac metal.In brief, according to our study on telluride class topological insulator, a surface gap can be opened on the topological insulator surface state via magnetic proximity, which paves the way for further study on the TRS-broken topological surface state. The study on layered material ZrTe5 reveals that both the chiral magnetic effect and nontrivial Berry phase exist in this layered material, indicating that ZrTe5 is a 3D Dirac semimetal. Besides, in this layered material, a strong Zeeman effect is revealed in single crystal ZrTe5, which induces a transformation of the Fermi surface from 3D to 2D in high field region (above 8 T). An orbital splitting is also revealed while the system becomes a 2D system. These quantum transport properties demonstrate that ZrTe5 is an idea platform to study the 3D topological Dirac Fermions.