Masashi Tokunaga

Detection of Berry’s Phase in a Bulk Rashba Semiconductor

Spin Berrys Phase When a quantum mechanical system performs an adiabatic cyclic path in the space of the parameters that affect its state (such as, for example, the magnetic field) its wave function may acquire an additional phase rather than go back to its original value. This quantity, called the Berrys phase, is associated with the topological properties of the parameter space and has been observed in materials such as graphene and bismuth. Murakawa et al. (p. 1490) observe a Berrys phase equal to π in the material BiTeI in which the phenomenon is predicted to be a consequence of a very strong coupling of spin and orbital degrees of freedom realized through the so-called Rashba effect. Transport measurements indicate a nontrivial spin texture stemming from strong spin-orbit coupling in the material BiTeI. The motion of electrons in a solid has a profound effect on its topological properties and may result in a nonzero Berry’s phase, a geometric quantum phase encoded in the system’s electronic wave function. Despite its ubiquity, there are few experimental observations of Berry’s phase of bulk states. Here, we report detection of a nontrivial π Berry’s phase in the bulk Rashba semiconductor BiTeI via analysis of the Shubnikov–de Haas (SdH) effect. The extremely large Rashba splitting in this material enables the separation of SdH oscillations, stemming from the spin-split inner and outer Fermi surfaces. For both Fermi surfaces, we observe a systematic π-phase shift in SdH oscillations, consistent with the theoretically predicted nontrivial π Berry’s phase in Rashba systems.

Dzyaloshinskii-Moriya interaction and spin reorientation transition in the frustrated kagome lattice antiferromagnet

Magnetization, specific heat, and neutron scattering measurements were performed to study a magnetic transition in jarosite, a spin-52 kagome lattice antiferromagnet. When a magnetic field is applied perpendicular to the kagome plane, magnetizations in the ordered state show a sudden increase at a critical field H c , indicative of the transition from antiferromagnetic to ferromagnetic states. This sudden increase arises as the spins on alternate kagome planes rotate 180 ° to ferromagnetically align the canted moments along the field direction. The canted moment on a single kagome plane is a result of the Dzyaloshinskii-Moriya interaction. For H H c , the Zeeman energy overcomes the interlayer coupling causing the spins on the alternate layers to rotate, aligning the canted moments along the field direction. Neutron scattering measurements provide the first direct evidence of this 180 ° spin rotation at the transition.

High-Field Study of Strong Magnetoelectric Coupling in Single-Domain Crystals of BiFeO3

Magnetic and dielectric properties of single-domain crystals of BiFeO 3 were studied in pulsed magnetic fields up to 55 T. At low temperatures, metamagnetic transitions accompanied with sharp changes in electric polarization ( P ) were observed at around 18 T. The angular dependence of the transition field coincides with that of the transition from the cycloidal state to the canted antiferromagnetic spin state studied in the framework of the Landau–Ginzburg theory incorporated with the Lifshitz-like invariant. The parasitic P component caused by the cycloidal spin structure amounts to 210±30 µC/m 2 in terms of the projected component on the pseudocubic principal axis, which can be controlled by applying magnetic fields at least up to 500 K. This result indicates that the microscopic magnetoelectric coupling in BiFeO 3 is not weak: In fact, it is rather strong as compared to that in the canonical multiferroic material TbMnO 3 .

Magnetization ''Steps'' on a Kagome Lattice in Volborthite

Magnetic properties of the spin-1/2 kagome-like compound volborthite are studied using a high-quality polycrystalline sample. It is evidenced from magnetization and specific heat measurements that the spins on the kagome lattice still fluctuate at low temperature, down to T =60 mK that corresponds to 1/1500 of the nearest-neighbor antiferromagnetic interaction, exhibiting neither a conventional long-range order nor a spin gap. In contrast, 51 V NMR experiments revealed a sharp peak at 1 K in relaxation rate, which indicates that a certain exotic order occurs. Surprisingly, we have observed three “steps” in magnetization as a function of magnetic field, suggesting that at least four liquid-like or other quantum states exist under magnetic fields.

Versatile helimagnetic phases under magnetic fields in cubic perovskite SrFeO 3

A helical spin texture is of great current interest for a host of novel spin-dependent transport phenomena. We report a rich variety of nontrivial helimagnetic phases in the simple cubic perovskite

Magnetic control of transverse electric polarization in BiFeO 3

{\mathrm{SrFeO}}_{3}

Current oscillation and low-field colossal magnetoresistance effect in phase-separated manganites.

under magnetic fields up to 42 T. Magnetic and resistivity measurements revealed that the proper-screw spin phase proposed for

Disordered ground state and magnetic field-induced long-range order in an S=3/2 antiferromagnetic honeycomb lattice compound Bi3Mn4O12(NO3).

{\mathrm{SrFeO}}_{3}

Quantum Hall effect in a bulk antiferromagnet EuMnBi2 with magnetically confined two-dimensional Dirac fermions.

can be subdivided into at least five kinds of ordered phases. Near the multicritical point, an unconventional anomalous Hall effect was found to show up and was interpreted as due to a possible long-period noncoplanar spin texture with scalar spin chirality.

Role of f electrons in the Fermi surface of the heavy fermion superconductor beta-YbAlB4.

Numerous attempts have been made to realize crossed coupling between ferroelectricity and magnetism in multiferroic materials at room temperature. BiFeO3 is the most extensively studied multiferroic material that shows multiferroicity at temperatures significantly above room temperature. Here we present high-field experiments on high-quality mono-domain BiFeO3 crystals reveal substantial electric polarization orthogonal to the widely recognized one along the trigonal c axis. This novel polarization appears to couple with the domains of the cycloidal spin order and, hence, can be controlled using magnetic fields. The transverse polarization shows the non-volatile memory effect at least up to 300 K.