Masato Matsuura

Electric Control of Spin Helicity in Multiferroic Triangular Lattice Antiferromagnet CuCrO2 with Proper-Screw Order

We have carried out a spin-polarized-neutron study on multiferroic CuCrO 2 to clarify the origin of the ferroelectricity. The neutron results demonstrate that an incommensurate proper-screw magnetic structure of CuCrO 2 induces electric polarization. Not only the magnetic structure but also the oxygen location contributes to the ferroelectricity of CuCrO 2 . The electric polarization of CuCrO 2 can be explained not by a conventional spin-current model but by a theoretical prediction proposed by Arima. The spin helicities of CuCrO 2 can be reversed by the reversal of the electric field E in the multiferroic phase.

Polar rotor scattering as atomic-level origin of low mobility and thermal conductivity of perovskite CH 3 NH 3 PbI 3

Perovskite CH3NH3PbI3 exhibits outstanding photovoltaic performances, but the understanding of the atomic motions remains inadequate even though they take a fundamental role in transport properties. Here, we present a complete atomic dynamic picture consisting of molecular jumping rotational modes and phonons, which is established by carrying out high-resolution time-of-flight quasi-elastic and inelastic neutron scattering measurements in a wide energy window ranging from 0.0036 to 54 meV on a large single crystal sample, respectively. The ultrafast orientational disorder of molecular dipoles, activated at ∼165 K, acts as an additional scattering source for optical phonons as well as for charge carriers. It is revealed that acoustic phonons dominate the thermal transport, rather than optical phonons due to sub-picosecond lifetimes. These microscopic insights provide a solid standing point, on which perovskite solar cells can be understood more accurately and their performances are perhaps further optimized.

Magnetic Phase Diagram and Metal-Insulator Transition of NiS2-xSex.

Magnetic phase diagram of NiS 2- x Se x has been reexamined by systematic studies of electrical resistivity, uniform magnetic susceptibility and neutron diffraction using single crystals grown by a chemical transport method. The electrical resistivity and the uniform magnetic susceptibility exhibit the same feature of temperature dependence over a wide Se concentration. A distinct first order metal-insulator (M-I) transition accompanied by a volume change was observed only in the antiferromagnetic ordered phase for 0.50≤ x ≤0.59. In this region, the M-I transition makes substantial effects to the thermal evolution of staggered moments. In the paramagnetic phase, the M-I transition becomes broad; both the electrical resistivity and the uniform magnetic susceptibility exhibit a broad maximum around the temperatures on the M-I transition-line extrapolated to the paramagnetic phase.

The Performance of TOF near Backscattering Spectrometer DNA in MLF, J-PARC

The time-of-flight (TOF) type near-backscattering spectrometer (n-BSS), DNA, with Si crystal analyzers was built and started operation in 2012 at the Materials and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC). DNA is the first n-BSS with pulse shaping chopper installed at a spallation pulsed neutron source. It offers currently the highest energy-resolution of about 2.4 micro eV by operating a pulse shaping double-disk chopper at 225 Hz whose phase is optimized to the narrowest slit of 10 mm width. Energy resolution can be flexibly compromised with intensity during experiment by using two type slits with different widths and changing the copper frequency. An example of measurement with high energy-resolution under the condition that the pulse shaping chopper was operated is shown, where the limited measurable energy range was widely expanded by multi incident energy band technique. The experimental data demonstrate extremely high signal-to-noise ratio (~10 5 ) of this spectrometer.

Superparamagnetism Induced by Polar Nanoregions in Relaxor Ferroelectric (1 x)BiFeO3-xBaTiO3

A new class of superparamagnetism was found in relaxor ferroelectric 2/3BiFeO 3 –1/3BaTiO 3 . The size of the magnetic particle, estimated from the superparamagnetic magnetization curve, coincides ...

Diffuse scattering anisotropy and inhomogeneous lattice deformations in the lead magnoniobate relaxor PMN above the Burns temperature

The results of diffuse neutron scattering experiment on PbMg1/3Nb2/3O3 single crystal above the Burns temperature are reported. It is shown that the high temperature elastic diffuse component is highly anisotropic in low-symmetry Brillouin zones and this anisotropy can be described using Huang scattering formalism assuming that the scattering originates from mesoscopic lattice deformations due to elastic defects. The qualitative agreement between this model and the experimental data is achieved with simple isotropic defects. It is demonstrated that weak satellite maxima near the Bragg reflections can be interpreted as the finite resolution effect.

Magnetic and Superconducting Properties of CeRhGe2 and CePtSi2

We synthesized a new compound of CeRhGe 2 with the orthorhombic CeNiSi 2 -type structure. By measuring the electrical resistivity, magnetic susceptibility, magnetization, specific heat, and neutron scattering for single crystals, CeRhGe 2 was found to order antiferromagnetically below T N = 7.6 K with an ordered moment of 1.3 µ B /Ce oriented along the magnetic easy-axis of the c -axis. Similar measurements were also carried out for CePtSi 2 single crystals. From the experimental results of anisotropic magnetic susceptibility and magnetization, the crystalline electric field (CEF) scheme was obtained for CeRhGe 2 and CePtSi 2 , together with the similar compound CeNiGe 2 . Furthermore, we carried out pressure experiments by measuring the electrical resistivity of CePtSi 2 and CeRhGe 2 . In the quantum critical region where the Neel temperature becomes zero, we observed superconductivity in CePtSi 2 and most likely CeRhGe 2 , at 1.5 and 7.1 GPa, respectively. The upper critical field of CePtSi 2 was found ...

A New Growth Mechanism of Polar Nanoregions by Phonon-Relaxation Mode-Coupling in a Relaxor Ferroelectric

We have performed inelastic neutron scattering measurements to elucidate the interplay between soft phonons and polar nanoregions (PNRs) in a relaxor ferroelectric 0.7Pb(Mg 1/3 Nb 2/3 )O 3 –0.3PbTiO 3 . In the thermal evolution of transverse phonons, we observed successive damping for both optical (TO) and acoustic (TA) modes, which can be described by a mode-coupling between phonons and relaxation motion of polarization in PNRs. The TO phonon damping at higher temperatures is induced by PNRs which drag surrounding soft mode regions into relaxation mode ones: Pb ions shift along a polarization of each PNR from ideal cubic positions. On the other hand, the TA phonon damping at lower temperatures corresponds to the transverse relaxation motion of polarization within PNRs. Such new growth mechanism of PNRs not only explains the relaxor properties over a wide temperature range but also unifies different approaches based on order–disorder-type and displacive-type lattice dynamics.

Anomalous phase of MnP at very low field

Manganese phosphide MnP has been investigated for decades because of its rich magnetic phase diagram. It is well known that the MnP exhibits the ferromagnetic phase transition at TC = 292 K and the helical magnetic phase below TN = 47 K at zero field. Recently, a novel magnetic phase transition was observed at T* = 282 K when the magnetic field is lower than 5 Oe. However, the nature of the new phase has not been illuminated yet. In order to reveal it, we performed the AC and the DC magnetization measurements for a single crystal MnP at very low field. A divergent behavior of the real and the imaginary part of the AC susceptibility and a sharp increase of the DC magnetization was observed at T*, indicating the magnetic phase transition at T*. Furthermore a peculiar temperature hysteresis was observed: namely, the magnetization depends on whether cooling sample to the temperature lower than TN or not before the measurements. This hysteresis phenomenon suggests the complicated nature of the new phase and a strong relation between the magnetic state of the new phase and the helical structure.

Study of Slow Lattice Dynamics in Relaxor Ferroelectric PMN–30%PT by Neutron Spin Echo Technique

We have used the neutron spin echo technique to characterize the dynamics of polar nanoregions (PNRs) in the relaxor ferroelectric (1- x )Pb(Mg 1/3 Nb 2/3 )O 3 – x PbTiO 3 (PMN– x PT) with x =30% ( T C ∼400 K). We measured the normalized intermediate scattering function I ( Q , t )/ I ( Q ,0) for a butterfly-shaped diffuse scattering around the (100) Bragg point, which is associated with atomic shifts, i.e., polarizations in PNRs. For T ≥500 K, I ( Q , t )/ I ( Q ,0) shows a peak structure at 0.5 ns, which is ascribed to the coexistence of echo signals from a relaxation mode and those from a low-frequency vibrational mode with ω∼26 µeV. Such a low-frequency vibrational mode for directions indicate a flat free-energy surface between the rhombohedral and tetragonal phases. On cooling below 450 K, the peak structure in I ( Q , t )/ I ( Q ,0) disappears and diffuse scattering intensity increases, which indicates that the condensed or pinned low-frequency mode for directions is the origin of PNRs.