arXiv subjects

Y. Moritomo

Publications and source records attributed to Y. Moritomo.

At least 19 recordsLinked to original sources

arXiv2016

Ultrafast Polaron Dynamics in Layered and Perovskite Manganites: 2D and 3D Polarons

We have studied the sub-picosecond quasiparticle dynamics in the perovskite manganite La0.7Ca0.3MnO3 and the layered manganite La1.4Sr1.6Mn2O7 using ultrafast optical spectroscopy. We found that for T > TC, initial relaxation proceeds on the time scale of several hundred femtoseconds and corresponds to the redressing of a photoexcited electron to its polaronic ground state. The temperature and dimensionality dependence of this polaron redressing time provides insight into the relationship between polaronic motion and spin dynamics on a sub-picosecond time scale. We also observe a crossover to a more conventional electron-phonon relaxation in the ferromagnetic metallic phase below Tc.

cond-mat.str-el↗

arXiv2014

Temperature and Field Dependence of Magnetic Domains in La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$

Colossal magnetoresistance and field-induced ferromagnetism are well documented in manganite compounds. Since domain wall resistance contributes to magnetoresistance, data on the temperature and magnetic field dependence of the ferromagnetic domain structure are required for a full understanding of the magnetoresistive effect. Here we show, using cryogenic Magnetic Force Microscopy, domain structures for the layered manganite La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ as a function of temperature and magnetic field. Domain walls are suppressed close to the Curie temperature T$_C$, and appear either via the application of a c-axis magnetic field, or by decreasing the temperature further. At temperatures well below T$_C$, new domain walls, stable at zero field, can be formed by the application of a c-axis field. Magnetic structures are seen also at temperatures above T$_C$: these features are attributed to inclusions of additional Ruddleston-Popper manganite phases. Low-temperature domain walls are nucleated by these ferromagnetic inclusions.

cond-mat.str-el↗

arXiv2012

Large thermal Hall coefficient in bismuth

We present a systematical study of thermal Hall effect on a bismuth single crystal by measuring resistivity, Hall coefficient, and thermal conductivity under magnetic field, which shows a large thermal Hall coefficient comparable to the largest one in a semiconductor HgSe. We discuss that this is mainly due to a large mobility and a low thermal conductivity comparing theoretical calculations, which will give a route for controlling heat current in electronic devices.

cond-mat.mtrl-sci↗

arXiv2011

Thermal rectification in bulk materials with asymmetric shape

We investigate thermal rectification in a bulk material with a pyramid shape to elucidate shape dependence of the thermal rectification, and find that rectifying coefficient R is 1.35 for this shape, which is smaller than R=1.43 for a rectangular shape. This result is fully duplicated by our numerical calculation based on Fourier's law. We also apply this calculation to a given shape, and show a possible way to increase R depending on the shape.

cond-mat.mtrl-sci↗

arXiv2006

Enhanced Optical Gap in Bi-layered Manganites La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ near $x=0.4$

We have systematically investigated the optical conductivity spectra of La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ ($0.3 \leqslant x \leqslant 0.5$). We find that just above the magnetic ordering temperatures, the optical gap shows an enhancement up to $\sim 0.3$ eV near $x=0.4$. Based on a $x$-dependent comparison of the nesting vector of the hypothetical Fermi surface and the superlattice wave-vector, we suggest that the peculiar $x$-dependence of the optical gap can be understood in terms of charge and lattice correlation enhanced by the charge density wave instability in nested Fermi surface.

cond-mat.str-el↗

arXiv2005

Nano-magnetic droplets and implications to orbital ordering in La1-xSrxCoO3

Inelastic cold neutron scattering on LaCoO3 provided evidence for a distinct low energy excitation at 0.6 meV coincident with the thermally induced magnetic transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic (AFM) correlations that are dynamic follow the activation to the excited state, identified as the intermediate S=1 spin triplet. This is indicative of dynamical orbital ordering favoring the observed magnetic interactions. With hole doping as in La1-xSrxCoO3, the FM correlations between Co spins become static and isotropically distributed due to the formation of FM droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism.

cond-mat.str-el↗

arXiv2005

Pressure/temperature/substitution-induced melting of A-site charge disproportionation in Bi_(1-x)La_(x)NiO_3 (0 =< x =< 0.5)

Metal-insulator transitions strongly coupled with lattice were found in Bi1-xLaxNiO3. Synchrotron X-ray powder diffraction revealed that pressure (P ~ 3 GPa, T = 300 K), temperature (T ~ 340 K, x = 0.05), and La-substitution (x ~ 0.075, T = 300 K) caused the similar structural change from a triclinic (insulating) to an orthorhombic (metallic) symmetry, suggesting melting of the A-site charge disproportionation. Comparing crystal structure and physical properties with the other ANiO3 series, an electronic state of the metallic phase can be described as [A3+Ld, Ni2+L1-d], where a ligand-hole L contributes to a conductivity. We depicted a schematic P-T phase diagram of BiNiO3 including a critical point (3 GPa, 300 K) and an inhomogeneous region, which implies universality of ligand-hole dynamics in ANiO3 (A = Bi, Pr, Nd,...).

cond-mat.str-el↗

arXiv2005

Unusual electron-doping effects in Sr$_{2-x}$La$_x$FeMoO$_6$ observed by photoemission spectroscopy

We have investigated the electronic structure of electron-doped Sr$_{2-x}$La$_x$FeMoO$_6$ ($x$=0.0 and 0.2) by photoemission spectroscopy and band-structure calculations within the local-density approximation+$U$ (LDA+$U$) scheme. A characteristic double-peak feature near the Fermi level ($E_{\rm F}$) has been observed in the valence-band photoemission spectra of both $x$=0.0 and 0.2 samples. A photon-energy dependence of the spectra in the Mo 4$d$ Cooper minimum region compared with the band-structure calculations has shown that the first peak crossing $E_{\rm F}$ consists of the (Fe+Mo) $t_{2g\downarrow}$ states (feature A) and the second peak well below $E_{\rm F}$ is dominated by the Fe $e_{g\uparrow}$ states (feature B). Upon La substitution, the feature A moves away from $E_{\rm F}$ by $\sim$50 meV which is smaller than the prediction of our band theory, 112 meV. In addition, an intensity enhancement of $both$ A and B has been observed, although B is not crossing $E_{\rm F}$. Those two facts are apparently incompatible with the simple rigid-band shift due to electron doping. We point out that such phenomena can be understood in terms of the strong Hund's rule energy stabilization in the 3$d^5$ configuration at the Fe sites in this compound. From an observed band-narrowing, we have also deduced a mass enhancement of $\sim$2.5 with respect to the band theory, in good agreement with a specific heat measurement.

cond-mat.str-el↗

arXiv2004

Universal features of charge and spin order in a half-doped layered perovskite

We have investigated the peculiar structure of charge and spin ordering in the half-doped layered perovskite oxide La$_{1.5}$Sr$_{0.5}$CoO$_4$ by elastic neutron scattering. Two samples with different preparation histories were studied. We find that the generic features of the ordered states, such as their short-range, glassy nature and the spin incommensurability, are sample-independent. At the same time, some subtle features of the ordered phases, such as the correlation lengths, differ.

cond-mat.str-el↗

arXiv2004

Resonant inelastic x-ray scattering study of hole-doped manganites La1-xSrxMnO3 (x=0.2 and 0.4)

Electronic excitations near the Fermi energy in the hole doped manganese oxides (La1-xSrxMnO3, x=0.2 and 0.4) have been elucidated by using the resonant inelastic x-ray scattering (RIXS) method. A doping effect in the strongly correlated electron systems has been observed for the first time. The scattering spectra show that a salient peak appears in low energies indicating the persistence of the Mott gap. At the same time, the energy gap is partly filled by doping holes and the energy of the spectral weight shifts toward lower energies. The excitation spectra show little change in the momentum space as is in undoped LaMnO3, but the scattering intensities in the low energy excitations of x=0.2 are anisotropic as well as temperature dependent, which indicates a reminiscence of the orbital nature.

cond-mat.str-el↗

arXiv2002

Spin/Orbital Pattern-Dependent Polaron Absorption in Nd(1-x)Sr(x)MnO3

We investigated optical properties of Nd(1-x)Sr(x)MnO3 (x= 0.40, 0.50, 0.55, and 0.65) single crystals. In the spin/orbital disordered state, their conductivity spectra look quite similar, and the strength of the mid-infrared absorption peak is proportional to x(1-x) consistent with the polaron picture. As temperature lowers, the Nd(1-x)Sr(x)MnO3 samples enter into various spin/orbital ordered states, whose optical responses are quite different. These optical responses can be explained by the spin/orbital ordering pattern-dependent polaron hopping.

cond-mat.str-el↗

arXiv2001

Highly Degenerate Canted Spin Structure in Bilayer Manganite La$_{1.1}$Sr$_{1.9}$Mn$_2$O$_7$

We have examined magnetic ordering of the two-dimensional (2D) bilayer manganite La$_{1.1}$Sr$_{1.9}$Mn$_2$O$_7$ with neutron diffraction technique. Due to the two-dimensionality, there appears intense 2D ridge scattering along the tetragonal $(00l)$ direction, and the analyses of such scattering leads to a conclusion that the low temperature spin structure is a canted antiferromagnetic ordering.

cond-mat.str-el↗

arXiv2001

Spin-entropy driven melting of the charge order in La1.5Sr0.5CoO4

We studied the melting of the charge order in the half-doped tetragonal perovskite La1.5Sr0.5CoO4 by elastic neutron scattering. We found that diffuse peaks, corresponding to the breathing-type modulation of oxygen positions in the CoO6 octahedra, disappear above Tc = 825(27) K. This melting of the diffuse superstructure is reversible (no change in the correlation lengths upon annealing is observed) and accompanied by a large nonlinear thermal expansion along the tetragonal c-axis, with a cusp at Tc. We conclude that quenched disorder of the doped Sr ions is at the origin of this anisotropic charge glass state. We also suggest that its melting is driven by the transition from the intermediate- to the high-spin state of Co{3+} ion, which is favored by the spin entropy at high T.

cond-mat.str-el↗

arXiv2001

Effects of transition metal doping in MgB$_2$ superconductor

Effects of chemical substitution of the divalent transition metals has been systematically investigated in Mg$_{1-x}M_x$B$_2$ (x=0.03; M=Mn, Fe, Co, Ni and Zn). Substitution of magnetic ions, i.e., Mn$^{2+}$, Fe$^{2+}$, Co$^{2+}$ and Ni$^{2+}$, for the Mg$^{2+}$ ion suppresses $T_{\rm c}$: d$T_{\rm c}$/d$x$ is the largest (= - 159 K) in the Mn-doped sample. We have found Zn-substitution increases $T_{\rm c}$ ($ΔT_{\rm c} \approx$ 0.2 K at x=0.03), perhaps due to the enhanced density of state near the Fermi level.

cond-mat.supr-con↗

arXiv2001

Spin dynamical properties and orbital states of the layered perovskite La_2-2x_Sr_1+2x_Mn_2_O_7 (0.3 <= x < 0.5)

Low-temperature spin dynamics of the double-layered perovskite La_2-2x_Sr_1+2x_Mn_2_O_7 (LSMO327) was systematically studied in a wide hole concentration range (0.3 <= x < 0.5). The spin-wave dispersion, which is almost perfectly 2D, has two branches due to a coupling between layers within a double-layer. Each branch exhibits a characteristic intensity oscillation along the out-of-plane direction. We found that the in-plane spin stiffness constant and the gap between the two branches strongly depend on x. By fitting to calculated dispersion relations and cross sections assuming Heisenberg models, we have obtained the in-plane (J_para), intra-bilayer (J_perp) and inter-bilayer (J') exchange interactions at each x. At x=0.30, J_para=-4meV and J_perp=-5meV, namely almost isotropic and ferromagnetic. Upon increasing x, J_perp rapidly approaches zero while |J_para| increases slightly, indicating an enhancement of the planar magnetic anisotropy. At x=0.48, J_para reaches -9meV, while J_perp turns to +1meV indicating an antiferromagnetic interaction. Such a drastic change of the exchange interactions can be ascribed to the change of the relative stability of the d_x^2-y^2 and d_3z^2-r^2 orbital states upon doping. However, a simple linear combination of the two states results in an orbital state with an orthorhombic symmetry, which is inconsistent with the tetragonal symmetry of the crystal structure. We thus propose that an ``orbital liquid'' state realizes in LSMO327, where the charge distribution symmetry is kept tetragonal around each Mn site.

cond-mat.str-el↗

arXiv2000

Magnon scattering processes and low temperature resistivity in CMR manganites

Low temperature resistivity of CMR manganites is investigated. At the ground state, conduction electrons are perfectly spin polarized, which is called half-metallic. From one-magnon scattering processes, it is discussed that the resistivity of a half metal as a function of temperature scales as rho(T) - rho(0) propto T^3. We take (Nd,Tb,Sr)MnO_3 as an example to compare theory and experiments. The result is in a good agreement.

cond-mat.str-el↗

arXiv2000

Observation of anomalous single-magnon scattering in half-metallic ferromagnets by chemical pressure control

Temperature variation of resistivity and specific heat have been measured for prototypical half-metallic ferromagnets, R_0.6Sr_0.4MnO_3, with controlling the one-electron bandwidth W. We have found variation of the temperature scalings in the resistivity from T^2 (R = La, and Nd) to T^3 (R = Sm), and have interpreted the $T^3-law in terms of the anomalous single-magnon scattering (AMS) process in the half-metallic system.

cond-mat.str-el↗

arXiv2000

Competition of charge, orbital, and ferromagnetic correlations in layered manganites

The competition of charge, orbital, and ferromagnetic interactions in layered manganites is investigated by magneto-Raman scattering spectroscopy. We find that the colossal magnetoresistance effect in the layered compounds results from the interplay of the orbital and ferromagnetic double-exchange correlations. Inelastic scattering by charge-order fluctuations dominates the quasiparticle dynamics in the ferromagnetic-metal state. The scattering is suppressed at low frequencies, consistent with the opening of a charge-density wave pseudogap.

cond-mat.str-el↗