Wide temperature AC-calorimetry equipped in a constant loading cubic-anvil-type pressure apparatus.

AC-calorimetry was developed for a cubic-anvil-type pressure apparatus, which can explore the electromagnetic properties of matter over temperatures of 2-300 K and pressures of 0-15 GPa. This method was designed to observe the specific heat of fragile crystals that are difficult to mold into desired forms, such as ß-Na0.33V2O5 and BaFe2S3. The calorimeter has two main components: a thermometer and a heater. We employed an AuFe (0.07 mol. %)-Chromel thermocouple and NiCr alloy foil/wire as the thermometer and heater, respectively. Using this calorimetry, we successfully observed the pressure dependencies of several transition temperatures in ß-Na0.33V2O5 and the jump in the specific heat (ΔCac/T) at the superconducting transition in Pb-metal when under pressure. Meanwhile, the pressure dependencies of the observed ΔCac/T do not coincide with the literature, which may be attributed to the pressure dependence of the thermoelectric power for the AuFe-Chromel thermocouple at around 5 K.

Contrasting Pressure-Induced Metallization Processes in Layered Perovskites, α-Sr_{2}MO_{4} (M=V, Cr).

Electric resistivity, magnetic susceptibility, and x-ray diffraction measurements under high pressure are performed in both α-Sr_{2}VO_{4} and α-Sr_{2}CrO_{4}, which are carefully prepared with regard to their stoichiometry. These measurements reveal contrasting and peculiar metallization processes of these compounds with increasing pressure. In contrast to a previously reported one in a V compound, we find two kinds of pressure-induced metallic states at low- (T<50 K) and high-temperature (T>100 K) regions. The high-temperature one seems to emerge beyond the pressure-induced Mott transition. The low-temperature one might imply a topological nature of the V compound, which is expected in the spin-orbit coupled 3d^{1} state that arises from their degenerated d_{zx} and d_{yz} orbits.

Erratum: Pressure-Induced Mott Transition Followed by a 24-K Superconducting Phase in BaFe_{2}S_{3} [Phys. Rev. Lett. 115, 246402 (2015)].

This corrects the article DOI: 10.1103/PhysRevLett.115.246402.

Pressure-Induced Mott Transition Followed by a 24-K Superconducting Phase in BaFe_{2}S_{3}.

We performed high-pressure study for a Mott insulator BaFe_{2}S_{3}, by measuring dc resistivity and ac susceptibility up to 15 GPa. We found that the antiferromagnetic insulating state at the ambient pressure is transformed into a metallic state at the critical pressure, P_{c}=10 GPa, and the superconductivity with the optimum T_{c}=24 K emerges above P_{c}. Furthermore, we found that the metal-insulator transition (Mott transition) boundary terminates at a critical point around 10 GPa and 75 K. The obtained pressure-temperature (P-T) phase diagram is similar to those of the organic and fullerene compounds; namely, BaFe_{2}S_{3} is the first inorganic superconductor in the vicinity of bandwidth control type Mott transition.

Pressure-induced superconductivity in the iron-based ladder material BaFe2S3.

All the iron-based superconductors identified so far share a square lattice composed of Fe atoms as a common feature, despite having different crystal structures. In copper-based materials, the superconducting phase emerges not only in square-lattice structures but also in ladder structures. Yet iron-based superconductors without a square-lattice motif have not been found, despite being actively sought out. Here, we report the discovery of pressure-induced superconductivity in the iron-based spin-ladder material BaFe2S3, a Mott insulator with striped-type magnetic ordering below ∼120 K. On the application of pressure this compound exhibits a metal-insulator transition at about 11 GPa, followed by the appearance of superconductivity below Tc = 14 K, right after the onset of the metallic phase. Our findings indicate that iron-based ladder compounds represent promising material platforms, in particular for studying the fundamentals of iron-based superconductivity.

Discovery of ferromagnetic-half-metal-to-insulator transition in K2Cr8O16.

The hollandite chromium oxide K2Cr8O16 has been synthesized in both powder and single-crystal form under high pressure. Combining electrical resistivity, magnetic susceptibility, and x-ray diffraction, we found that K2Cr8O16 is a ferromagnetic metal (or half-metal) with T(C)=180 K and shows a transition to an insulator at 95 K without any apparent structural change but retaining ferromagnetism. K2Cr8O16 is quite unique in three aspects: It has a rare mixed valence of Cr3+ and Cr4+; it has a metal (or half-metal)-to-insulator transition in a ferromagnetic state; and the resulting low-temperature phase is a rare case of a ferromagnetic insulator. This discovery could open a new frontier on the relation of magnetism and conducting properties in strongly correlated electron systems.