arXiv subjects

Y. Yamasaki

Publications and source records attributed to Y. Yamasaki.

28 records · Page 2Linked to original sources

arXiv2012

Pseudogap-related charge dynamics in layered-nickelate R2-xSrxNiO4 (x sim 1)

Charge dynamics and its critical behavior are investigated near the metal-insulator transition of layered-nickelate R2-xSrxNiO4 (R=Nd, Eu). The polarized x-ray absorption spectroscopy experiment clearly shows the multi-orbital nature which enables the x2-y2-orbital-based checkerboard-type charge ordering or correlation to persist up to the critical doping region (x sim 1). In the barely metallic region proximate to the charge-ordered insulating phase, the nominal carrier density estimated from the Hall coefficient markedly decreases in accord with development of the pseudogap structure in the optical conductivity spectrum, while the effective mass is least enhanced. The present findings combined with the results of recent angle-resolved photoemission spectroscopy show that the pseudogap in the metal-insulator critical state evolves due to the checkerboard-type charge correlation to extinguish the coherent-motion carriers in a characteristic momentum (k)-dependent manner with lowering temperature.

cond-mat.str-el↗

arXiv2011

Effect of cation size variance on spin and orbital order in Eu$_{1-x}$(La$_{0.254}$Y$_{0.746}$)$_{x}$VO$_3$

We have investigated the $R$-ion ($R$ = rare earth or Y) size variance effect on spin/orbital order in Eu$_{1-x}$(La$_{0.254}$Y$_{0.746}$)$_{x}$VO$_3$. The size variance disturbs one-dimensional orbital correlation in $C$-type spin/$G$-type orbital ordered states and suppresses this spin/orbital order. In contrast, it stabilizes the other spin/orbital order. The results of neutron and resonant X-ray scattering denote that in the other ordered phase, the spin/orbital patterns are $G$-type/$C$-type, respectively.

cond-mat.str-el↗

arXiv2011

Origin of the large polarization in multiferroic YMnO$_3$ thin films revealed by soft and hard x-ray diffraction

We investigated the magnetic structure of an orthorhombic YMnO3 thin film by resonant soft x-ray and hard x-ray diffraction. We observed a temperature-dependent incommensurate magnetic reflection below 45 K and a commensurate lattice-distortion reflection below 35 K. These results demonstrate that the ground state is composed of coexisting E-type and cycloidal states. Their different ordering temperatures clarify the origin of the large polarization to be caused by the E-type antiferromagnetic states in the orthorhombic YMnO3 thin film.

cond-mat.str-el↗

arXiv2009

Terahertz time-domain spectroscopy of electromagnons in multiferroic perovskite manganites

Recent spectroscopic studies at terahertz frequencies for a variety of multiferroics endowed with both ferroelectric and magnetic orders have revealed the possible emergence of a new collective excitation, frequently referred to as electromagnon. It is magnetic origin, but becomes active in response to the electric field component of light. Here we give an overview on our recent advance in the terahertz time-domain spectroscopy of electromagnons or electric-dipole active magnetic resonances, focused on perovskite manganites--$R$MnO$_3$ ($R$ denotes rare-earth ions). The respective electric and magnetic contributions to the observed magnetic resonance are firmly identified by the measurements of the light-polarization dependence using a complete set of the crystal orientations. We extract general optical features in a variety of the spin ordered phases, including the $A$-type antiferromagnetic, collinear spin ordered, and ferroelectric $bc$ and $ab$ spiral spin ordered phases, which are realized by tuning the chemical composition of $R$, temperature, and external magnetic field. In addition to the antiferromagnetic resonances of Mn ions driven by the magnetic field component of light, we clarify that the electromagnon appears only for light polarized along the a-axis even in the collinear spin ordered phase and grows in intensity with evolution of the spiral spin order, but independent of the direction of the spiral spin plane ($bc$ or $ab$) or equivalently the direction of the ferroelectric polarization $P_{\rm s}$ ($P_{\rm s}\| c$ or $P_{\rm s}\| a$). A possible origin of the observed magnetic resonances at terahertz frequencies is discussed by comparing the systematic experimental data presented here with theoretical considerations based on Heisenberg model.

cond-mat.str-el↗

arXiv2008

Electric-dipole active two-magnon excitation in {\textit{ab}} spiral spin phase of a ferroelectric magnet Gd$_{\textbf{0.7}}$Tb$_{\textbf{0.3}}$MnO$_{\textbf 3}$

A broad continuum-like spin excitation (1--10 meV) with a peak structure around 2.4 meV has been observed in the ferroelectric $ab$ spiral spin phase of Gd$_{0.7}$Tb$_{0.3}$MnO$_3$ by using terahertz (THz) time-domain spectroscopy. Based on a complete set of light-polarization measurements, we identify the spin excitation active for the light $E$ vector only along the a-axis, which grows in intensity with lowering temperature even from above the magnetic ordering temperature but disappears upon the transition to the $A$-type antiferromagnetic phase. Such an electric-dipole active spin excitation as observed at THz frequencies can be ascribed to the two-magnon excitation in terms of the unique polarization selection rule in a variety of the magnetically ordered phases.

cond-mat.str-el↗

arXiv2008

Electrically driven spin excitation in a ferroelectric magnet DyMnO_3

Temperature (5--250 K) and magnetic field (0--70 kOe) variations of the low-energy (1--10 meV) electrodynamics of spin excitations have been investigated for a complete set of light-polarization configurations for a ferroelectric magnet DyMnO$_3$ by using terahertz time-domain spectroscopy. We identify the pronounced absorption continuum (1--8 meV) with a peak feature around 2 meV, which is electric-dipole active only for the light $E$-vector along the a-axis. This absorption band grows in intensity with lowering temperature from the spin-collinear paraelectric phase above the ferroelectric transition, but is independent of the orientation of spiral spin plane ($bc$ or $ab$), as shown on the original $P_{\rm s}$ (ferroelectric polarization) $\parallel c$ phase as well as the magnetic field induced $P_{\rm s}\parallel a$ phase. The possible origin of this electric-dipole active band is argued in terms of the large fluctuations of spins and spin-current.

cond-mat.mtrl-sci↗

arXiv2008

Correlation between spin helicity and electric polarization vector in quantum-spin chain magnet LiCu$_2$O$_2$

Measurements of polarized neutron scattering were performed on a $S=1/2$ chain multiferroic LiCu$_2$O$_2$. In the ferroelectric ground state with the spontaneous polarization along the c-axis, the existence of transverse spiral spin component in the $bc$-plane was confirmed. When the direction of electric polarization is reversed, the vector spin chirality as defined by ${\bf C}_{ij} = {\bf S}_i \times {\bf S}_j$ ($i$ and $j$ being the neighboring spin sites) is observed to be reversed, indicating that the spin-current model or the inverse Dzyaloshinskii-Moriya mechanism is applicable even to this $e_{\mathrm{g}}$-electron quantum-spin system. Differential scattering intensity of polarized neutrons shows a large discrepancy from that expected for the classical-spin $bc$-cycloidal structure, implying the effect of large quantum fluctuation.

cond-mat.str-el↗

arXiv2007

Electric Control of Spin Helicity in a Magnetic Ferroelectric

Magnetic ferroelectrics or multiferroics, which are currently extensively explored, may provide a good arena to realize a novel magnetoelectric function. Here we demonstrate the genuine electric control of the spiral magnetic structure in one of such magnetic ferroelectrics, TbMnO3. A spin-polarized neutron scattering experiment clearly shows that the spin helicity, clockwise or counter-clockwise, is controlled by the direction of spontaneous polarization and hence by the polarity of the small cooling electric field.

cond-mat.str-el↗

arXiv2006

Impurity-doping induced ferroelectricity in frustrated antiferromagnet CuFeO2

Dielectric responses have been investigated on the triangular-lattice antiferromagnet CuFeO2 and its site-diluted analogs CuFe1-xAlxO2 (x=0.01 and 0.02) with and without application of magnetic field. We have found a ferroelectric behavior at zero magnetic field for x=0.02. At any doping level, the onset field of the ferroelectricity always coincides with that of the noncollinear magnetic structure while the transition field dramatically decreases to zero field with Al doping. The results imply the further possibility of producing the ferroelectricity by modifying the frustrated spin structure in terms of site-doping and external magnetic field.

cond-mat.str-el↗

arXiv2005

Change in lattice modulation upon gigantic magnetoelectric transition in GdMnO3 and TbMnO3

Single-crystal synchrotron x-ray diffraction measurements in strong magnetic fields have been performed for magnetoelectric compounds GdMnO3 and TbMnO3. It has been found that the P || a ferroelectric phase induced by the application of a magnetic field at low temperatures is characterized by commensurate lattice modulation along the orthorhombic b axis with q = 1/2 and q = 1/4. The lattice modulation is ascribed to antiferromagnetic spin alignment with a modulation vector of (0 1/4 1). The change of the spin structure is directly correlated with the magnetic-field-induced electric phase transition, because any commensurate spin modulation with (0 1/4 1) should break glide planes normal to the a axis of the distorted perovskite with the Pbnm space group.

cond-mat.str-el↗