In this article, we review our exploration of unconventional electronic states near the metal-insulator boundary in black phosphorus and lead telluride, which have long been known as narrow-gap semiconductors with simple crystal structure. In such low carrier systems with high mobility, the energy band structure and physical property can be drastically affected by application of high magnetic field and high pressure. High pressure is a powerful tool to continuously tune the band structure from semiconductor to semimetal without degradation of the mobility.
It is becoming evident that Dirac fermions are present in several materials other than graphene. The organic conductor α-(BEDT-TTF)2I3 is known to be a first bulk crystal which realizes a massless Dirac fermion system under high pressure. By taking advantage of its bulk nature, we experimentally study the thermodynamic and thermoelectric properties of Dirac fermions by using this system. Here, we will show the experimental technique and experimental results of the specific heat and thermopower of α-(BEDT-TTF)2I3 under high pressure.
In this article, the effects of pressure on topological quantum materials, a polar semiconductor BiTeBr and a type-II Dirac semimetal PdTe2, are reviewed. The first topic is the pressure-induced trivial-nontrivial electronic transition in BiTeBr. I explain that the topological transition was clearly revealed by the Wilson-loop analysis used for evaluating topological Z2 invariants. The second topic is the introduction of superconducting properties of PdTe2 under multi-extreme conditions. I report an unusual superconducting phase diagram in addition to experimental results of electrical resistivity and ac-susceptibility in PdTe2.
We present a review of the superconducting properties of Cu, Sr and Nb doped Bi2Se3 compounds with a focus on high pressure effects. The parent compound Bi2Se3 is a topological insulator at ambient pressure and exhibits pressure-induced crystallographic phase transitions and superconductivity above 10 GPa. The doped compounds are all bulk superconductors with Tc～3 K and have been investigated intensively in the past decade because of their candidature for topological superconductivity (TSC). A key role as regards topological superconductivity is played by spontaneous rotational symmetry breaking (RSB), which is observed, for instance, as an anisotropy in the upper critical field Bc2. We discuss the pressure variation of Tc and the concomitant effect on the upper critical field. An analysis of the basal-plane anisotropy of Bc2 is presented in the context of an odd-parity unconventional superconducting state.
In this article, recent topics in the high-pressure synthesis and physical properties under high pressure circumstance on Topological Kondo Insulators SmB6 and YbB12 were reviewed. As for high pressure synthesis of substitution by Ca for YbB12, high-pressure equipment using the Walker module is introduced. This equipment is used for material development of novel RB12. Furthermore some physical properties of SmB6 and YbB12 under high pressure condition are also introduced.