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Yokoyama T.,Tokyo Institute of Technology | Balatsky A.V.,Los Alamos National Laboratory | Nagaosa N.,University of Tokyo | Nagaosa N.,Cross Correlated Materials Research Group CMRG
Physical Review Letters | Year: 2010

We investigate the formation of one-dimensional channels on the topological surface under the gate electrode. The energy dispersion of these channels is almost linear in momentum, and its velocity and sign are sensitively dependent on the strength of the gate voltage. Consequently, the local density of states near the gated region has an asymmetric structure with respect to zero energy. In the presence of the electron-electron interaction, the correlation effect can be tuned by the gate voltage. We also suggest a tunneling experiment to verify the presence of these bound states. © 2010 The American Physical Society. Source

Yokoyama T.,University of Tokyo | Tanaka Y.,Nagoya University | Nagaosa N.,University of Tokyo | Nagaosa N.,Cross Correlated Materials Research Group CMRG
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We investigate charge transport in two-dimensional ferromagnet/ferromagnet junction on a topological insulator. The conductance across the interface depends sensitively on the directions of the magnetizations of the two ferromagnets, showing anomalous behaviors compared with the conventional spin valve. This stems from the way how the wave functions connect between both sides. It is found that the conductance depends strongly on the in-plane direction of the magnetization. Moreover, in sharp contrast to the conventional magnetoresistance effect, the conductance at the parallel configuration can be much smaller than that at the antiparallel configuration. © 2010 The American Physical Society. Source

Kohara Y.,University of Tokyo | Yamasaki Y.,University of Tokyo | Yamasaki Y.,High Energy Accelerator Research Organization | Onose Y.,University of Tokyo | And 4 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Magnetoelectric (ME) properties in yttrium iron garnet (YIG: Y3 Fe5 O12), including both the first-order and second-order effects, have long been under dispute. In particular, the conflict between observations of the first-order ME effect and the centrosymmetric lattice structure has remained as a puzzling issue. As a key to solve the problem, we found that YIG shows quantum ME relaxation; the dielectric relaxation process is correlated closely with the magnetic one and has characteristic features of quantum tunneling. An application of magnetic field enhances the dielectric relaxation strength (by 300% at 10 K with 0.5 T), which gives rise to the large second-order ME (magnetocapacitance) effect critically dependent on the magnetization direction. The temperature and magnetic-field dependence of dielectric relaxation strength is well described by the noninteracting transverse-field Ising model for the excess-electron or Fe2+ center with the quantum tunneling and spin-orbit coupling effects. We could also spectroscopically identify such a ME Fe2+ center in terms of linear dichroism under a magnetic field along the specific direction. On this basis, the fictitious first-order ME effect-the magnetic-field induced electric polarization without the presence of external electric field-as observed for the electric-field cooled sample is ascribed to the combined effect of the above large second-order ME effect and the poling induced charge accumulation. The correlation between the ME effect and the thermally stimulated depolarization current indicates that hopping electrons freeze below around 125 K and the frozen-in dipoles generate an internal electric field (i.e., an electret-like effect). Investigation of electron-compensating doping effect on dielectric relaxation phenomena gives compelling evidences that excess electrons forming Fe2+ ions play a critical role in the charge accumulation as well as in the ME effect in YIG. © 2010 The American Physical Society. Source

Ishii I.,University of Tokyo | Tokunaga Y.,Japan Science and Technology Agency | Fujioka J.,Japan Science and Technology Agency | Onose Y.,University of Tokyo | And 4 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Bilayered-structure manganites Pr (Sr1-y Cay) 2 Mn2 O7 show the competing spin-charge-orbital orders; the ground state is the spin CE-type charge-orbital ordered state for y>0.4 and the spin-A-type ferroic x2 - y2 orbital ordered state for y<0.4. For the two representative crystals of y=0.9 and y=0, we have investigated the optical conductivity and Raman spectra to probe the charge, spin, and lattice dynamics of the respective phases. For y=0.9, the charge-orbital ordered states are characterized by strongly activated Raman phonon bands corresponding to the modulated breathing and Jahn-Teller lattice distortions, as observed commonly for other charge-orbital ordered manganites. As the unique features of this bilayered manganite, however, we found that the infrared-active phonon mode can also show up in the Raman spectra in the lower-temperature charge-orbital ordered state, confirming the breaking of centrosymmetry to produce the spontaneous electric polarization. The other feature to be noted is the appearance of the two-magnon excitation band of the CE-type spin order in the Raman spectra which estimates the energy scale of spin exchange energies. For y=0, on the other hand, we have observed a charge gap in the optical conductivity spectrum which becomes narrowed with the evolution of the A-type (in-plane ferromagnetic) spin order but remains finite (0.15 eV) at the ground state. The persistence of the charge gap is ascribed to the vertical charge ordering or charge-density wave (CDW) that was previously verified by diffuse x-ray scattering. Such charge-order or CDW formation also activates additional infrared phonon modes at 26 and 35 meV, whose spectral intensity appears to be enhanced by resonance with the underlying electronic continuum. © 2010 The American Physical Society. Source

Lee J.S.,Japan Science and Technology Agency | Lee J.S.,University of Tokyo | Nakamura M.,Cross Correlated Materials Research Group CMRG | Okuyama D.,Cross Correlated Materials Research Group CMRG | And 7 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We have explored the competing phases in the anisotropically strained manganese oxide with use of a composition-spread thin film where two contrastive ground states, i.e., ferromagnetic metal and charge/orbital-ordered insulator, are designed to compete with a continuous variation in the chemical composition. Taking advantage of a systematic inspection of the phase evolutions, we have confirmed that competing electronic orders could be realized in thin-film form. In particular, we found a possibly one-dimensional orbital-ordered insulating state near the metal-insulator phase boundary which can be disturbed by a relatively low magnetic field. © 2010 The American Physical Society. Source

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