Terukazu Nishizaki

Relaxor ferroelectricity induced by electron correlations in a molecular dimer Mott insulator

We have investigated the dielectric response in an antiferromagnetic dimer-Mott insulator beta-(BEDT-TTF)2ICl2 with square lattice, compared to a spin liquid candidate kappa-(BEDT-TTF)2Cu2(CN)3. Temperature dependence of the dielectric constant shows a peak structure obeying Curie-Weiss law with strong frequency dependence. We found an anisotropic ferroelectricity by pyrocurrent measurements, which suggests the charge disproportionation in a dimer. The ferroelectric actual charge freezing temperature is related to the antiferromagnetic interaction, which is expected to the charge-spin coupled degrees of freedom in the system.

Electrostatic and electrochemical tuning of superconductivity in two-dimensional NbSe2 crystals

We report modulation of the superconducting critical temperature (Tc) of ultrathin niobium diselenide (NbSe2) single crystals by gating an electric double-layer transistor. We realized reversible and irreversible changes of the Tc by adjusting the operating range of the voltage. The reversible and irreversible responses correspond to the electrostatic carrier doping and the electrochemical etching of the crystal, respectively. The results suggest that electric double-layer gating provides opportunities to control and functionalize collective electronic phenomena in two-dimensional crystals.

Extended Polymorphism of Two-Dimensional Material

When controlling electronic properties of bulk materials, we usually assume that the basic crystal structure is fixed. However, in two-dimensional (2D) materials, atomic structure or polymorph is attracting growing interest as a controlling parameter to functionalize their properties. Various polymorphs can exist in transition metal dichalcogenides (TMDCs) from which 2D materials are generated, and polymorphism has drastic impacts on the electronic states. Here we report the discovery of an unprecedented polymorph of a TMDC 2D material. By mechanical exfoliation, we made thin flakes from a single crystal of 2Ha-type tantalum disulfide (TaS2), a metallic TMDC with a charge-density-wave (CDW) phase. Microbeam X-ray diffraction measurements and electrical transport measurements indicate that thin flakes possess a polymorph different from any one known in TaS2 bulk crystals. Moreover, the flakes with the unique polymorph displayed the dramatically enhanced CDW ordering temperature. The present results suggest the potential existence of diverse structural and electronic phases accessible only in 2D materials.

Mesoscopic 2D Charge Transport in Commonplace PEDOT:PSS Films

The correlation between the transport properties and structural degrees of freedom of conducting polymers is a central concern in both practical applications and scientific research. In this study, we demonstrated the existence of mesoscopic two-dimensional (2D) coherent charge transport in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film by performing structural investigations and high-field magnetoconductance (MC) measurements in magnetic fields of up to 15 T. We succeeded in observing marked positive MCs reflecting 2D electronic states in a conventional drop-cast film. This low-dimensional feature is surprising, since PEDOT:PSS-a mixture of two different polymers-seems to be significantly different from crystalline 2D materials in the viewpoint of the structural inhomogeneity, especially in popular drop-cast thick films. The results of the structural experiments suggest that such 2D transport originates from the nanometer-scale self-assembled laminated structure, which is composed of PEDOT nanocrystals wrapped by insulating sheets consisting of amorphous PSSs. These results indicate that charge transport in the PEDOT:PSS film can be divided into two regimes: mesoscopic 2D coherent tunneling and macroscopic three-dimensional hopping among 2D states. Our findings elucidate the hieratical nature of charge transport in the PEDOT:PSS film, which could provide new insight into a recent engineering concern, i.e., the anisotropic conductance.

Doping dependencies of onset temperatures for the pseudogap and superconductive fluctuation in Bi2Sr2CaCu2O8+δ, studied from both in-plane and out-of-plane magnetoresistance measurements

To investigate the relationship between the pseudogap and superconductivity, we measured both the in-plane (ρab) and out-of-plane (ρc) resistivity for oxygen-controlled Bi2Sr2CaCu2O8+δ single crystals subject to magnetic fields (parallel to the c-axis) of up to 17.5 T. The onset temperature for the superconductive fluctuation, Tscf, is determined by the large positive in-plane magnetoresistance (MR) and negative out-of-plane MR observed near Tc, whereas the pseudogap opening temperature T* is determined by the semiconductive upturn of the zero-field ρc. Tscf was found to scale roughly as Tc, with a decreasing temperature interval between them upon doping. On the other hand, T* starts out much higher than Tscf but decreases monotonically upon doping; finally, at a heavily overdoped state, it is not observed above Tscf. These results imply that the pseudogap is not a simple precursor of superconductivity, but that further study is needed to determine whether or not T* exists below Tscf in the heavily overdo...

Unscaling Superconducting Parameters with Tc for Bi-2212 and Bi-2223: A Magnetotransport Study in the Superconductive Fluctuation Regime

To investigate the origin of the enhanced Tc (≈ 110 K) of the trilayer cuprate superconductor Bi2Sr2Ca2Cu3O10+δ (Bi-2223), we have performed systematic magnetoresistance (MR) measurements on this superconductor, as well as on the bilayer superconductor, Bi2Sr2CaCu2O8+δ (Bi-2212). The in-plane coherence length, ξab, and the specific-heat jump, ΔC, have been estimated using the theory of renormalized superconductive fluctuations, and the doping dependence of these parameters has been qualitatively explained using the Fermi arc approach. A detailed comparison of the superconducting parameters with Tc for these compounds suggests that an additional superconducting condensation energy exists, due to an increase in the number of stacking CuO2 planes in a unit cell.

Impurity effect on the inter-chain dielectric response of the CDW condensate in K0.3MoO3

We study the effect of impurity on the charge-density-wave (CDW) state via low-frequency dielectric measurements, especially for the inter-chain direction. The extremely high dielectric constant seen in non-doped K0.3MoO3 is strongly reduced by W- or V-doping, demonstrating that the three-dimensional (3D) phase rigidity is an innate characteristic of CDW condensate. In the impurity-doped K0.3MoO3, the 3D rigidity has been lost and the CDW state exhibits a different nature in which the dielectric response of CDW cannot be explained in terms of the screened phason model.

Single-crystal growth and the dependence on hole concentration and magnetic field of the magnetic ground state in the edge-sharingCuO2chain systemCa2+xY2−xCu5O10

We have succeeded in growing large-size single-crystals of Ca

Single-crystal growth and dependences on the hole concentration and magnetic field of the magnetic ground state in the edge-sharing CuO

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