arXiv subjects

T. Kimura

Publications and source records attributed to T. Kimura.

At least 37 records · Page 2Linked to original sources

arXiv2016

Magnetodielectric detection of magnetic quadrupole order in Ba(TiO)Cu$_4$(PO$_4$)$_4$ with Cu$_4$O$_{12}$ square cupolas

In vortex-like spin arrangements, multiple spins can combine into emergent multipole moments. Such multipole moments have broken space-inversion and time-reversal symmetries, and can therefore exhibit linear magnetoelectric (ME) activity. Three types of such multipole moments are known: toroidal, monopole, and quadrupole moments. So far, however, the ME-activity of these multipole moments has only been established experimentally for the toroidal moment. Here, we propose a magnetic square cupola cluster, in which four corner-sharing square-coordinated metal-ligand fragments form a noncoplanar buckled structure, as a promising structural unit that carries an ME-active multipole moment. We substantiate this idea by observing clear magnetodielectric signals associated with an antiferroic ME-active magnetic quadrupole order in the real material Ba(TiO)Cu$_4$(PO$_4$)$_4$. The present result serves as a useful guide for exploring and designing new ME-active materials based on vortex-like spin arrangements.

cond-mat.str-el↗

arXiv2016

Stress-induced magnetic domain selection reveals a conical ground state for the multiferroic phase of Mn2GeO4

At ambient pressure (P) and below 5.5 K, olivine-type Mn2GeO4 hosts a multiferroic (MF) phase where a multicomponent, i.e., multi-k magnetic order generates spontaneous ferromagnetism and ferroelectricity (FE) along the c axis. Under high P the FE disappears above 6 GPa, yet the P evolution of the magnetic structure remained unclear based on available data. Here we report high P single crystal neutron diffraction experiments in theMF phase at T = 4.5 K.We observe clearly that the incommensurate spiral component of the magnetic order responsible for FE varies little with P up to 5.1 GPa. With support from high P synchrotron x-ray diffraction measurements at room temperature (T), the P driven suppression of FE is proposed to occur as a consequence of a crystal structure transition away from the olivine structure. In addition, in the low T neutron scattering experiments an emergent nonhydrostatic P component, i.e., a uniaxial stress, leads to the selection of certain multi-k domains. We use this observation to deduce a double-k conical magnetic structure for the ambient P ground state, this being a key ingredient for a model description of the MF phase.

cond-mat.mtrl-sci↗

arXiv2014

Experimental Realization of a Quantum Breathing Pyrochlore Antiferromagnet

The synthesis and characterization of a new Yb-based material Ba3Yb2Zn5O11 are reported. This material is identified as the first model system of a pseudospin-1/2 quantum antiferromagnet on "breathing" pyrochlore lattice characterized by an alternating array of small and large Yb tetrahedra. Despite dominant antiferromagnetic interactions J ~ 7 K, a large amount of magnetic entropy (25%) remains at 0.38 K, indicating that each small Yb tetrahedron forms a unique doubly degenerate singlet state. These results are well described by the Heisenberg pseudospin-1/2 single tetrahedron model.

cond-mat.str-el↗

arXiv2014

Tuning order-by-disorder multiferroicity in CuO by doping

The high Curie temperature multiferroic compound, CuO, has a quasidegenerate magnetic ground state that makes it prone to manipulation by the so called ``order-by-disorder'' mechanism. First principle computations supplemented with Monte Carlo simulations and experiments show that isovalent doping allows to stabilize the multiferroic phase in non-ferroelectric regions of the pristine material phase-diagram with experiments reaching a 250% widening of the ferroelectric temperature window with 5% of Zn doping. Our results allow to validate the importance of a quasidegenerate ground state on promoting multiferroicity on CuO at high temperatures and open a path to the material engineering of new multiferroic materials.

cond-mat.mtrl-sci↗

arXiv2013

Spatially Resolved Magnetization in the Bose-Einstein Condensed State of BaCuSi2O6: Evidence for Imperfect Frustration

In order to understand the nature of the two-dimensional Bose-Einstein condensed (BEC) phase in BaCuSi2O6, we performed detailed 63Cu and 29Si NMR above the critical magnetic field, Hc1= 23.4 T. The two different alternating layers present in the system have very different local magnetizations close to Hc1; one is very weak, and its size and field dependence are highly sensitive to the nature of inter-layer coupling. Its precise value could only be determined by "on-site" 63Cu NMR, and the data are fully reproduced by a model of interacting hard-core bosons in which the perfect frustration associated to tetragonal symmetry is slightly lifted, leading to the conclusion that the population of the less populated layers is not fully incoherent but must be partially condensed.

cond-mat.str-el↗

arXiv2012

Two types of adjacent dimer layers in the low temperature phase of BaCuSi2O6

The low-temperature crystal structure of BaCuSi2O6 has been investigated with high-resolution synchrotron x-ray and neutron powder diffraction techniques and has been found to be on average (ignoring the incommensurate modulation) orthorhombic, with the most probable space group Ibam. The Cu-Cu dimers in this material are forming two types of layers with distinctly different interatomic distances. Subtle changes also modify the partially frustrated interlayer Cu-Cu exchange paths. The present results corroborate the interpretation of low-temperature nuclear magnetic resonance and inelastic neutron scattering data in terms of distinct dimer layers.

cond-mat.str-el↗

arXiv2012

Coupling of magnetic and ferroelectric hysteresis by a multi-component magnetic structure in Mn2GeO4

The olivine compound Mn2GeO4 is shown to feature both a ferroelectric polarization and a ferromagnetic magnetization that are directly coupled and point along the same direction. We show that a spin spiral generates ferroelectricity (FE), and a canted commensurate order leads to weak ferromagnetism (FM). Symmetry suggests that the direct coupling between the FM and FE is mediated by Dzyaloshinskii-Moriya interactions that exist only in the ferroelectric phase, controlling both the sense of the spiral rotation and the canting of the commensurate structure. Our study demonstrates how multi-component magnetic structures found in magnetically-frustrated materials like Mn2GeO4 provide a new route towards functional materials that exhibit coupled FM and FE.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic Phase Diagram of CuO

High resolution ultrasonic velocity measurements have been used to determine the temperature -- magnetic-field phase diagram of the monoclinic multiferroic CuO. A new transition at TN3 = 230 K, corresponding to an intermediate state between the antiferromagnetic non-collinear spiral phase observed below TN2 = 229.3 K and the paramagnetic phase, is revealed. Anomalies associated with a first order transition to the commensurate collinear phase are also observed at TN1 = 213 K. For fields with B along the b axis, a spin-flop transition is detected between 11 T - 13 T at lower temperatures. Moreover, our analysis using a Landau-type free energy clearly reveals the necessity for an incommensurate collinear phase between the spiral and the paramagnetic phase. This model is also relevant to the phase diagrams of other monoclinic multiferroic systems.

cond-mat.mtrl-sci↗

arXiv2012

Estimation of the spin polarization for Heusler-compound thin films by means of nonlocal spin-valve measurements: Comparison of Co$_{2}$FeSi and Fe$_{3}$Si

We study room-temperature generation and detection of pure spin currents using lateral spin-valve devices with Heusler-compound electrodes, Co$_{2}$FeSi (CFS) or Fe$_{3}$Si (FS). The magnitude of the nonlocal spin-valve (NLSV) signals is seriously affected by the dispersion of the resistivity peculiarly observed in the low-temperature grown Heusler compounds with ordered structures. From the analysis based on the one-dimensional spin diffusion model, we find that the spin polarization monotonically increases with decreasing the resistivity, which depends on the structural ordering, for both CFS and FS electrodes, and verify that CFS has relatively large spin polarization compared with FS.

cond-mat.mes-hall↗

arXiv2012

Soft X-ray harmonic comb from relativistic electron spikes

We demonstrate a new high-order harmonic generation mechanism reaching the `water window' spectral region in experiments with multi-terawatt femtosecond lasers irradiating gas jets. A few hundred harmonic orders are resolved, giving uJ/sr pulses. Harmonics are collectively emitted by an oscillating electron spike formed at the joint of the boundaries of a cavity and bow wave created by a relativistically self-focusing laser in underdense plasma. The spike sharpness and stability are explained by catastrophe theory. The mechanism is corroborated by particle-in-cell simulations.

physics.plasm-ph↗

arXiv2011

Raman scattering study of delafossite magnetoelectric multiferroic compounds: CuFeO2 and CuCrO2

Ultrasonic velocity measurements on the magnetoelectric multiferroic compound CuFeO2 reveal that the antiferromagnetic transition observed at TN1 = 14 K might be induced by an R-3m -> C2/m pseudoproper ferroelastic transition (G. Quirion, M. J. Tagore, M. L. Plumer, O. A. Petrenko, Phys. Rev. B 77, 094111 (2008)). In that case, the group theory states that the order parameter associated with the structural transition must belong to a two dimensional irreducible representation Eg (x^2 - y^2, xy). Since this type of transition can be driven by a Raman Eg mode, we performed Raman scattering measurements on CuFeO2 between 5 K and 290 K. Considering that the isostructural multiferroic compound CuCrO2 might show similar structural deformations at the antiferromagnetic transition TN1 = 24.3 K, Raman measurements have also been performed for comparison. At ambient temperature, the Raman modes in CuFeO2 are observed at omega_Eg = 352 cm^-1 and omega_Ag = 692 cm^-1, while these modes are detected at omega_E_g = 457 cm^-1 and omega_Ag = 709 cm^-1 in CuCrO2. The analysis of the temperature dependence of modes shows that the frequency of all modes increases down to 5 K. This typical behavior can be attributed to anharmonic phonon-phonon interactions. These results clearly indicate that none of the Raman active modes observed in CuFeO2 and CuCrO2 drive the pseudoproper ferroelastic transition observed at the Néel temperature TN1. Finally, a broad band at about 550 cm^-1 observed in the magnetoelectric phase of CuCrO2 below TN2 could be attributed to a magnon mode.

cond-mat.mtrl-sci↗

arXiv2011

Room-temperature generation of giant pure spin currents using Co$_2$FeSi spin injectors

Generation, manipulation, and detection of a pure spin current, i.e., the flow of spin angular momentum without a charge current, are prospective approaches for realizing next-generation spintronic devices with ultra low electric power consumptions. Conventional ferromagnetic electrodes such as Co and NiFe have so far been utilized as a spin injector for generating the pure spin currents in nonmagnetic channels. However, the generation efficiency of the pure spin currents is extremely low at room temperature, giving rise to a serious obstacle for device applications. Here, we demonstrate the generation of giant pure spin currents at room temperature in lateral spin valve devices with a highly ordered Heusler-compound Co$_2$FeSi spin injector. The generation efficiency of the pure spin currents for the Co$_2$FeSi spin injectors reaches approximately one hundred times as large as that for NiFe ones, indicating that Heusler-compound spin injectors enable us to materialize a high-performance lateral spin device. The present study is a technological jump in spintronics and indicates the great potential of ferromagnetic Heusler compounds with half metallicity for generating pure spin currents.

cond-mat.mes-hall↗

arXiv2011

Indication of intrinsic spin Hall effect in 4d and 5d transition metals

We have investigated spin Hall effects in 4$d$ and 5$d$ transition metals, Nb, Ta, Mo, Pd and Pt, by incorporating the spin absorption method in the lateral spin valve structure; where large spin current preferably relaxes into the transition metals, exhibiting strong spin-orbit interactions. Thereby nonlocal spin valve measurements enable us to evaluate their spin Hall conductivities. The sign of the spin Hall conductivity changes systematically depending on the number of $d$ electrons. This tendency is in good agreement with the recent theoretical calculation based on the intrinsic spin Hall effect.

cond-mat.mes-hall↗

arXiv2011

Generation of Dicke states using adiabatic passage

Entangled states of two ions are realized by using an adiabatic process. Based on the proposal by Linington and Vitanov, we have generated Dicke states in optical qubits of two $^{40}$Ca$^+$ ions by applying frequency-chirped optical pulses with time-dependent envelopes to perform rapid adiabatic passage on sideband transitions. One of the biggest advantages of adiabatic approaches is their robustness against variations in experimental parameters, which is verified by performing experiments for different pulse widths or peak Rabi frequencies. Fidelities exceeding 0.5, which is the threshold for inseparable states, are obtained over wide ranges of parameter values.

quant-ph↗

arXiv2011

Laser-Shock Compression and Hugoniot Measurements of Liquid Hydrogen to 55 GPa

The principal Hugoniot for liquid hydrogen was obtained up to 55 GPa under laser-driven shock loading. Pressure and density of compressed hydrogen were determined by impedance-matching to a quartz standard. The shock temperature was independently measured from the brightness of the shock front. Hugoniot data of hydrogen provide a good benchmark to modern theories of condensed matter. The initial number density of liquid hydrogen is lower than that for liquid deuterium, and this results in shock compressed hydrogen having a higher compression and higher temperature than deuterium at the same shock pressure.

cond-mat.mtrl-sci↗

arXiv2010

Identifying the multiferroic phase of doped CuFeO_2 using inelastic neutron scattering

We report inelastic neutron scattering measurements that provide a distinct dynamical "fingerprint" for the multiferroic ground state of 3.5% Ga-doped CuFeO_2. The complex ground state is stabilized by the displacement of the oxygen atoms, which are also responsible for the multiferroic coupling predicted by Arima. By comparing the observed and calculated spectrum of spin excitations, we conclude that the magnetic ground state is a distorted screw-type spin configuration that requires a mechanism for magnetoelectric coupling different from the generally accepted spin-current model.

cond-mat.str-el↗

arXiv2009

Infrared optical absorption spectra of CuO single crystals: Fermion-spinon band and dimensional crossover of the antiferromagnetic order

We have obtained mid-infrared optical absorption spectra of the S=1/2 quasi one-dimensional CuO using polarized transmission measurement and interpreted the spectra in terms of phonon assisted magnetic excitations. When the electric field is parallel to the main antiferromagnetic direction a Delta shaped peak is observed with the maximum at 0.23eV which is attributed to spinons along Cu-O chains. At low temperatures in the antiferromagnetic phase another peak appears at 0.16eV which is attributed to two-magnon absorption but the spinon peak remains. This behavior is interpreted as due to a dimensional crossover where the low temperature three-dimensional magnetic phase keeps short range characteristics of a one-dimensional magnet.

cond-mat.str-el↗

arXiv2009

Magnetically tunable dielectric materials

The coupling between localized spins and phonons can lead to shifts in the dielectric constant of insulating materials at magnetic ordering transitions. Studies on isostructural SeCuO3 (ferromagnetic) and TeCuO3 (antiferromagnetic) illustrate how the q-dependent spin-spin correlation function couples to phonon frequencies leading to a shift in the dielectric constant. A model is discussed for this spin-phonon coupling. The magnetodielectric coupling in multiferroic materials can be very large at a ferroelectric transition temperature. This coupling is investigated in the recently identified multiferroic Ni3V2O8.

cond-mat.mtrl-sci↗