Kouji Taniguchi

Electric-field-induced superconductivity at 9.4 K in a layered transition metal disulphide MoS2

Electro-static carrier doping was attempted in a layered transition metal disulphide MoS2 by constructing an electric double-layer transistor with an ionic liquid. With the application of gate voltage VG higher than 3 V, a metallic behavior was observed in the MoS2 channel. We found an onset of electric field-induced superconductivity in the field induced metallic phase. A maximum TC ∼ 9.4 K was observed, which could be higher than those in chemically doped bulk materials.

Ferroelectric Polarization Reversal by a Magnetic Field in Multiferroic Y-type Hexaferrite Ba2Mg2Fe12O22

Coexistence of the ferroelectric polarization and spontaneous magnetization has been found in Y-type hexaferrite Ba2Mg2Fe12O22. The reversal of magnetization by a small magnetic field below ~0.02 T accompanies an electric polarization reversal through the clamping of ferrimagnetic and ferroelectric domain walls. This behavior can be potentially used as a magnetically rewritable ferroelectric memory and an electrically rewritable magnetic memory.

Rechargeable magnesium-ion battery based on a TiSe2-cathode with d-p orbital hybridized electronic structure.

Rechargeable ion-batteries, in which ions such as Li+ carry charges between electrodes, have been contributing to the improvement of power-source performance in a wide variety of mobile electronic devices. Among them, Mg-ion batteries are recently attracting attention due to possible low cost and safety, which are realized by abundant natural resources and stability of Mg in the atmosphere. However, only a few materials have been known to work as rechargeable cathodes for Mg-ion batteries, owing to strong electrostatic interaction between Mg2+ and the host lattice. Here we demonstrate rechargeable performance of Mg-ion batteries at ambient temperature by selecting TiSe2 as a model cathode by focusing on electronic structure. Charge delocalization of electrons in a metal-ligand unit through d-p orbital hybridization is suggested as a possible key factor to realize reversible intercalation of Mg2+ into TiSe2. The viewpoint from the electronic structure proposed in this study might pave a new way to design electrode materials for multivalent-ion batteries.

Charge dynamics and possibility of ferromagnetism in (formula presented) (formula presented) and Sr)

Ferromagnetism has been reported recently in La-doped alkaline-earth hexaborides, A1-xLaxB6 (A=Ca, Sr, and Ba). We have performed the reflectivity, Hall resistivity, and magnetization measurements of A1-xLaxB6. The results indicate that A1-xLaxB6 can be regarded as a simple doped semimetal, with no signature of an excitonic state as suggested by several theories. It is also found that the surface of as-grown samples (10 micrometer in thickness) has a different electronic structure from a bulk one, and a fairly large number of paramagnetic moments are confined in this region. After eliminating these paramagnetic moments at the surface, we could not find any evidence of an intrinsic ferromagnetic moment in our samples, implying the possibility that the ferromagnetism of A1-xLaxB6 reported so far is neither intrinsic.

Spatial Redistribution of Oxygen Ions in Oxide Resistance Switching Device after Forming Process

The change in the spatial distribution of oxygen ions after an initial voltage application called the forming process was investigated for oxide resistance switching devices by secondary ion mass spectrometry mapping. To track the motion of oxygen ions, tracer 18O ions were implanted in a planar Pt/CuO/Pt device. We found clear evidence for the oxygen reduction in the conductive bridge structure formed between two electrodes. In addition, the oxygen ions in the bridge structure drift to the anode, implying the oxygen diffusion (migration) induced by high electric field and/or current density. We discuss those results in terms of a filament model.

Gigantic Optical Magnetoelectric Effect in CuB2O4

Although it has been well known that materials in which both space inversion and time reversal symmetries are broken can host optical magneto-electric effect, i.e., change in optical constants with the reversal of propagating direction of light, the largest change in absorption ever reported on this effect was 0.2%. Here we show that optical absorption in noncentrosymmetric weak ferromagnetic material CuB 2 O 4 changes by more than 100% with reversal of a low magnetic field of 300 Oe. The gigantic optical magneto-electric effect is ascribed to the canted antiferromagnetic spin ordering of square-coordinated Cu 2+ sites, where the local inversion is slightly broken.

Rechargeable Mg battery cathode TiS3 with d–p orbital hybridized electronic structures

Rechargeable performance is realized in Mg batteries using a TiS3 cathode without the nanometer-scale downsizing of electrode particles. The specific capacity is about 80 mAh/g for the first 50 cycles at room temperature. This observed specific capacity is comparable to that of the prototype cathode for Mg batteries. First-principles calculation indicates that TiS3 is a semiconductor with d–p orbital hybridized electronic structures around the Fermi level. The reversible electrode performance is likely assisted by the delocalized electronic distribution over metal–ligand units through d–p orbital hybridization.

A new “zero-strain” material for electrochemical lithium insertion

A new “zero-strain” system, spinel Li1+xRh2O4, is found as an insertion material for lithium ions. A very small volume change of ca. 0.5% is observed in the two-phase reaction process. The “zero-strain” behavior is likely induced by itinerant t2g electrons with a suppressed Jahn–Teller effect.

Charge dynamics and possibility of ferromagnetism in A1-xLaxB6 (A=Ca and Sr)

Ferromagnetism has been reported recently in La-doped alkaline-earth hexaborides, A1-xLaxB6 (A=Ca, Sr, and Ba). We have performed the reflectivity, Hall resistivity, and magnetization measurements of A1-xLaxB6. The results indicate that A1-xLaxB6 can be regarded as a simple doped semimetal, with no signature of an excitonic state as suggested by several theories. It is also found that the surface of as-grown samples (10 micrometer in thickness) has a different electronic structure from a bulk one, and a fairly large number of paramagnetic moments are confined in this region. After eliminating these paramagnetic moments at the surface, we could not find any evidence of an intrinsic ferromagnetic moment in our samples, implying the possibility that the ferromagnetism of A1-xLaxB6 reported so far is neither intrinsic.

Frontispiece: Magnetic Switching by the In Situ Electrochemical Control of Quasi-Spin-Peierls Singlet States in a Three-Dimensional Spin Lattice Incorporating TTF-TCNQ Salts

Magnetic phase switching in a coordination polymer is reported, which is demonstrated by combining two processes: (A) the pre-organization of magnetic/redox-active molecules into a framework, and (B) a post-treatment through electrochemical tuning of the pre-organized molecules. A TTF.+ -TCNQ.- salt (TTF=tetrathiafulvalene; TCNQ=7,7,8,8-tetracyano-p-quinodimethane) was incorporated into a three-dimensional framework with paddlewheel-type dimetal(II, II) units ([M2II,II ]; M=Ru with S=1, 1; and Rh with S=0, 2), where the [M2II,II ] and TCNQ.- units form the coordinating framework, and TTF.+ is located in the pores of framework, forming an irregular π-stacking alternating column with the TCNQ.- in the framework. In 1, the spins of [Ru2II,II ] and TCNQ.- units make a magnetic correlation through the framework upon decreasing the temperature from 300 K, which is, however, suddenly suppressed below 137 K (=Td (1)) by the formation of a spin singlet in the TTF.+ -TCNQ.- columns, as seen in the spin-Peierls transition (Td (2)=200 K). This material was incorporated as a cathode in a Li-ion battery (LIB); a long-range ferrimagnetic correlation was formed through the three-dimensional [{Ru2II,II }2 TCNQ]- framework at Tc =78 K in the discharge process. The reversible magnetic phase switching between the non-volatile ferrimagnetic and paramagnetic states, resulting from the local spin tuning of quasi-spin-Peierls singlet, is demonstrated through the discharge/charge cycling of the LIB.