arXiv subjects

Atsushi Miyake

Publications and source records attributed to Atsushi Miyake.

At least 55 records · Page 3Linked to original sources

arXiv2019

Rich magnetoelectric phase diagrams of multiferroic single-crystal alpha-NaFeO2

The magnetic and dielectric properties of the multiferroic triangular lattice magnet compound alpha-NaFeO2 were studied by magnetization, specific heat, dielectric permittivity, and pyroelectric current measurements and by neutron diffraction experiments using single crystals grown by a hydrothermal synthesis method. This work produced magnetic field (in the monoclinic ab-plane, B_ab, and along the c*-axis, B_c) versus temperature magnetic phase diagrams, including five and six magnetically ordered phases in B_ab and along B_c, respectively. Comparing the polarization direction to the magnetic structures in the different ferroelectric phases, we conclude that the extended inverse Dzyaloshinskii-Moriya mechanism expressed by the orthogonal components p1 ~ rij x (Si x Sj ) and p2 ~ Si x Sj can explain the polarization directions. Based on calculations incorporating exchange interactions up to fourth-nearest-neighbor (NN) couplings, we infer that competition among antiferromagnetic second NN interactions in the triangular lattice plane, as well as weak interplane antiferromagnetic interactions, are responsible for the rich phase diagrams of alpha-NaFeO2.

cond-mat.str-el↗

arXiv2018

Magnetic field effect on the chiral magnetism of noncentrosymmetric UPtGe: experiment and theory

The effect of differently oriented magnetic field on chiral incommensurate helimagnet UPtGe is studied both experimentally and theoretically. The magnetization measurements up to the field above the saturation have revealed an isotropic magnetic response below 20 T and a remarkable nonmonotonic anisotropy in high fields. Moreover, the two principally different phase transitions from the noncollinear incommensurate to the field-induced ferromagnetic state have been observed. These properties are successfully explained by density-functional theory calculations taking into account the noncollinearity of the magnetic structures, arbitrary directed magnetic field, and relativistic effects. We also estimate the strength of different competing magnetic interactions and discuss possible scenarios of the field-induced phase transformations.

cond-mat.str-el↗

arXiv2018

Magnetoelectric behavior from cluster multipoles in square cupolas: Study of Sr(TiO)Cu$_4$(PO$_4$)$_4$ in comparison with Ba and Pb isostructurals

We report our combined experimental and theoretical study of magnetoelectric properties of an antiferromagnet Sr(TiO)Cu$_4$(PO$_4$)$_4$, in comparison with the isostructurals Ba(TiO)Cu$_4$(PO$_4$)$_4$ and Pb(TiO)Cu$_4$(PO$_4$)$_4$. The family of compounds commonly possesses a low-symmetric magnetic unit called the square cupola, which is a source of magnetoelectric responses associated with the magnetic multipoles activated under simultaneous breaking of spatial inversion and time reversal symmetries. Measuring the full magnetization curves and the magnetic-field profiles of dielectric constant for Sr(TiO)Cu$_4$(PO$_4$)$_4$ and comparing them with the theoretical analyses by the cluster mean-field theory, we find that the effective $S=1/2$ spin model, which was used for the previous studies for Ba(TiO)Cu$_4$(PO$_4$)$_4$ and Pb(TiO)Cu$_4$(PO$_4$)$_4$, well explains the experimental results by tuning the model parameters. Furthermore, elaborating the phase diagram of the model, we find that the square cupolas could host a variety of magnetic multipoles, i.e., monopole, toroidal moment, and quadrupole tensor, depending on the parameters that could be modulated by deformations of the magnetic square cupolas. Our results not only provide a microscopic understanding of the series of the square cupola compounds, but also stimulate further exploration of the magnetoelectric behavior arising from cluster multipoles harboring in low-symmetric magnetic units.

cond-mat.str-el↗

arXiv2018

Quantitative evaluation of Dirac physics in PbTe

The magnetic field dependence of electronic transport, magnetic, and elastic properties in single crystals of PbTe were investigated in high magnetic fields up to 55 T. The magnetoresistance, magnetization, ultrasonic attenuation, and sound velocity showed clear quantum oscillations with pronounced Zeeman-splitting, which causes a large second harmonic in the frequency spectra. The ratio of the Zeeman to the cyclotron energy, which is regarded as an index of `Diracness' [H. Hayasaka and Y. Fuseya, J. Phys.: Condens. Matter \textbf{28,} 31LT01 (2016).], was determined to be 0.52 and 0.57 in samples with different carrier densities. We also pointed out that the effect of Zeeman-splitting seriously affects the Landau-level fan diagram analysis, which is widely used to extract the nontrivial Berry's phase from the quantum oscillations.

cond-mat.str-el↗

arXiv2018

Magnetic Structural Unit with Convex Geometry: a Building Block Hosting an Exchange-striction-driven Magnetoelectric Coupling

We perform a combined experimental and theoretical study of a magnetic-field ($B$) induced evolution of magnetic and ferroelectric properties in an antiferromagnetic material Pb(TiO)Cu$_4$(PO$_4$)$_4$, whose structure is characterized by a staggered array of Cu$_4$O$_{12}$ magnetic units with convex geometry known as square cupola. Our experiments show a $B$-induced phase transition from a previously reported low-$B$ linear magnetoelectric phase to a new high-$B$ magnetoelectric phase, which accompanies a 90$^\circ$ flop of electric polarization and gigantic magnetodielectric effect. Moreover, we observe a $B$-induced sign reversal of ferroelectric polarization in the high-$B$ phase. Our model and first-principles calculations reveal that the observed complex magnetoelectric behavior is well explained in terms of a $B$-dependent electric polarization generated in each Cu$_4$O$_{12}$ unit by the so-called exchange striction mechanism. The present study demonstrates that the materials design based on the magnetic structural unit with convex geometry deserves to be explored for developing strong magnetoelectric couplings.

cond-mat.mtrl-sci↗

arXiv2018

Magnetic field induced phenomena in UIrGe in fields applied along b axis

The metamagnetic transition between the antiferromagnetic and paramagnetic state in UIrGe has been studied at various temperatures by magnetization, heat capacity and magnetocaloric-effect measurements on a single crystal in static and pulsed magnetic fields applied along the orthorhombic b-axis. A first-order transition is observed at temperatures below 13 K and a second-order one at higher temperatures up to the Néel temperature (TN = 16.5 K). The first-order transition is accompanied by a dramatic increase of the Sommerfeld coefficient. Magnetization measurements extended to the paramagnetic range revealed an anomalous S-shape (inflection point at a magnetic field Hm) in magnetization isotherms at temperatures above 13 K and a temperature dependence of susceptibility with a maximum at Tmax well above TN. The lines representing the temperature-induced evolution of Hm and field-induced evolution of Tmax, respectively, are bound for the point in the magnetic phase diagram at which the order of metamagnetic transition changes. A tentative scenario explaining these anomalies by antiferromagnetic correlations or short-range order in the paramagnetic state is discussed.

cond-mat.str-el↗

arXiv2018

Observation of Poiseuille Flow of Phonons in Black Phosphorus

The travel of heat in insulators is commonly pictured as a flow of phonons scattered along their individual trajectory. In rare circumstances, momentum-conserving collision events dominate, and thermal transport becomes hydrodynamic. One of these cases, dubbed the Poiseuille flow of phonons, can occur in a temperature window just below the peak temperature of thermal conductivity. We report on a study of heat flow in bulk black phosphorus between 0.1 and 80 K. We find a thermal conductivity showing a faster than cubic temperature dependence between 5 and 12 K. Consequently, the effective phonon mean free path shows a nonmonotonic temperature dependence at the onset of the ballistic regime, with a size-dependent Knudsen minimum. These are hallmarks of Poiseuille flow previously observed in a handful of solids. Comparing the phonon dispersion in black phosphorus and silicon, we showthat the phase space for normal scattering events in black phosphorus is much larger. Our results imply that the most important requirement for the emergence of Poiseuille flowis the facility ofmomentum exchange between acoustic phonon branches. Proximity to a structural transition can be beneficial for the emergence of this behavior in clean systems, even when they do not exceed silicon in purity.

cond-mat.mtrl-sci↗

arXiv2018

Magnetic and Structural Properties of A-Site Ordered Chromium Spinel Sulfides: Alternating Antiferromagnetic and Ferromagnetic Interactions in the Breathing Pyrochlore Lattice

We report a comprehensive study on the magnetic and structural properties of the spinel sulfides LiInCr4S8, LiGaCr4S8, and CuInCr4S8, where Li+/Cu+ and Ga3+/In3+ ions form a zinc-blende-type order. On the basis of synchrotron X-ray diffraction and magnetization data obtained using polycrystalline samples, these three sulfides are suggested to be breathing pyrochlore magnets with alternating antiferromagnetic and ferromagnetic interactions on the small and large tetrahedra, respectively. The measured magnetization processes of the three sulfides up to 72 T are significantly different. The magnetization curves of LiInCr4S8 and CuInCr4S8 have large hysteresis loops with different shapes, while there is no hysteresis in that of LiGaCr4S8. Geometrical frustration of the small tetrahedron is likely to give rise to a wide variety of ground states, indicating the rich physics in these antiferromagnetic-ferromagnetic breathing pyrochlore magnets.

cond-mat.mtrl-sci↗

arXiv2017

Quantum Hall states observed in thin films of Dirac semimetal Cd3As2

A well known semiconductor Cd3As2 has reentered the spotlight due to its unique electronic structure and quantum transport phenomena as a topological Dirac semimetal. For elucidating and controlling its topological quantum state, high-quality Cd3As2 thin films have been highly desired. Here we report the development of an elaborate growth technique of high-crystallinity and high-mobility Cd3As2 films with controlled thicknesses and the observation of quantum Hall effect dependent on the film thickness. With decreasing the film thickness to 10 nm, the quantum Hall states exhibit variations such as a change in the spin degeneracy reflecting the Dirac dispersion with a large Fermi velocity. Details of the electronic structure including subband splitting and gap opening are identified from the quantum transport depending on the confinement thickness, suggesting the presence of a two-dimensional topological insulating phase. The demonstration of quantum Hall states in our high-quality Cd3As2 films paves a road to study quantum transport and device application in topological Dirac semimetal and its derivative phases.

cond-mat.mtrl-sci↗

arXiv2017

Consecutive magnetic phase diagram of UCoGe-URhGe-UIrGe system

We prepared single crystals in UCo1-xRhxGe and UIr1-xRhxGe systems to establish a complex dU-U-T (dU-U is the shortest interatomic uranium distance and T is temperature) magnetic phase diagram. This recognized a characteristic maximum in magnetic susceptibility at temperature Tmax along the b axis as an important parameter. Three magnetically ordered regions can be distinguished within this scope; first a ferromagnetic region with Curie temperature < Tmax, second a ferromagnetic region with Curie temperature = Tmax and finally an antiferromagnetic region existing on the UIrGe side with Néel temperature < Tmax.

cond-mat.str-el↗

arXiv2017

Different metamagnetism between paramagnetic Ce and Yb isomorphs

To solve the puzzle of metamagnetic phenomena in heavy fermion systems, we have compared paramagnetic isostructural Ce and Yb systems, CeNi$_2$Ge$_2$ and YbNi$_2$Ge$_2$, both of which are located near a magnetic instability. The most intriguing result is discovery of a metamagnetic-like transition in both systems from magnetization measurements in a pulsed magnetic fields. This is the first observation of the metamagnetism for isomorphic Ce and Yb paramagnetic systems. Similar to other metamagnets, the metamagnetic transition fields for both compounds are well scaled by the temperature $T_χ^{\rm max}$, at which the magnetic susceptibility shows a maximum. In addition, for CeNi$_2$Ge$_2$ a peak of nonlinear susceptibility $χ_3$ appears at approximately $T_χ^{\rm max}/2$, as for other heavy-fermion metamagnets. In contrast, YbNi$_2$Ge$_2$ shows only a sign change for $χ_3$ at $T_χ^{\rm max}$, as observed in itinerant metamagnets located near the ferromagnetic critical point. The metamagnetism of CeNi$_2$Ge$_2$ corresponds to a typical Kondo lattice system, whereas that of YbNi$_2$Ge$_2$ is similar to the nearly ferromagnetic itinerant systems. Other possibilities for the metamagnetic behavior of YbNi$_2$Ge$_2$ are also discussed.

cond-mat.str-el↗

arXiv2017

Magnetic Transitions under Ultrahigh Magnetic Fields of up to 130 T in the Breathing Pyrochlore Antiferromagnet LiInCr4O8

The magnetization processes of the spin-3/2 antiferromagnet LiInCr4O8 comprising a "breathing" pyrochlore lattice, which is an alternating array of small and large tetrahedra, are studied under ultrahigh magnetic fields of up to 130 T using state-of-the-art pulsed magnets. A half magnetization plateau is observed above 90 T to 130 T, suggesting that LiInCr4O8 has a strong spin-lattice coupling, similar to conventional chromium spinel oxides. The magnetization of LiGa0.125In0.875Cr4O8, in which the structural and magnetic transitions at low temperatures have been completely suppressed, shows a sudden increase above 13 T, indicating that a spin gap of 2.2 meV exists between a tetramer singlet ground state and an excited state with total spin 1, with the latter being stabilized by the application of a magnetic field. The breathing pyrochlore antiferromagnet is found to be a unique frustrated system with strong spin-lattice coupling and bond alternation.

cond-mat.str-el↗

arXiv2017

Two-carrier analyses of the transport properties of black phosphorus under pressure

We report on the electronic transport properties of black phosphorus and analyze them using a two-carrier model in a wide range of pressure up to 2.5 GPa. In semiconducting state at 0.29 GPa, the remarkable non-linear behavior in the Hall resistance is reasonably reproduced by assuming the coexistence of two kinds of hole with different densities and mobilities. On the other hand, two-carrier analyses of the magnetotransport properties above 1.01 GPa suggest the coexistence of high mobility electron and hole carriers that have almost the same densities, i.e., nearly compensated semimetallic nature of black phosphorus. In the semimetallic state, analyses of both the two-carrier model and quantum oscillations indicate a systematic increase in the carrier densities as pressure increases. An observed sign inversion of Hall resistivity at low magnetic fields suggests the existence of high mobility electrons (\sim105 cm2 V-1 s-1) that is roughly ten times larger than that of holes, in the semimetallic black phosphorus. We conclude that the extremely large positive magnetoresistance that has been observed in semimetallic state cannot be reproduced by a conventional two-carrier model.

cond-mat.str-el↗

arXiv2017

Successive field-induced transitions in BiFeO$_{3}$ around room temperature

The effects of high magnetic fields applied perpendicular to the spontaneous ferroelectric polarization on single crystals of BiFeO$_3$ were investigated through magnetization, magnetostriction, and neutron diffraction measurements. The magnetostriction measurements revealed lattice distortion of $2\times 10^{-5}$, during the reorientation process of the cycloidal spin order by applied magnetic fields. Furthermore, anomalous changes in magnetostriction and electric polarization at a larger field demonstrate an intermediate phase between cycloidal and canted antiferromagnetic states, where a large magnetoelectric effect was observed. Neutron diffraction measurements clarified that incommensurate spin modulation along [110] direction in the cycloidal phase becomes commensurate in the intermediate phase. Theoretical calculations based on the standard spin Hamiltonian of this material suggest an antiferromagnetic cone-type spin order in the intermediate phase.

cond-mat.mtrl-sci↗

arXiv2016

Magnetoelectric Behavior from $S=1/2$ Asymmetric Square Cupolas

Magnetoelectric properties are studied by a combined experimental and theoretical study of a quasi-two-dimensional material composed of square cupolas, Ba(TiO)Cu$_4$(PO$_4$)$_4$. The magnetization is measured up to above the saturation field, and several anomalies are observed depending on the field directions. We propose a $S$=1/2 spin model with Dzyaloshinskii-Moriya interactions, which well reproduces the full magnetization curves. Elaborating the phase diagram of the model, we show that the anomalies are explained by magnetoelectric phase transitions. Our theory also accounts for the scaling of the dielectric anomaly observed in experiments. The results elucidate the crucial role of the in-plane component of Dzyaloshinskii-Moriya interactions, which is induced by the noncoplanar buckling of square cupola. We also predict a `hidden' phase and another magnetoelectric response both of which appear in nonzero magnetic field.

cond-mat.str-el↗

arXiv2016

Quasi-two-dimensional Bose-Einstein condensation of spin triplets in dimerized quantum magnet Ba$_2$CuSi$_2$O$_6$Cl$_2$

We synthesized single crystals of composition Ba$_2$CuSi$_2$O$_6$Cl$_2$ and investigated its quantum magnetic properties. The crystal structure is closely related to that of the quasi-two-dimensional (2D) dimerized magnet BaCuSi$_2$O$_6$ also known as Han purple. Ba$_2$CuSi$_2$O$_6$Cl$_2$ has a singlet ground state with an excitation gap of $Δ/k_{\rm B}\,{=}\,20.8$ K. The magnetization curves for two different field directions almost perfectly coincide when normalized by the $g$-factor except for a small jump anomaly for a magnetic field perpendicular to the $c$ axis. The magnetization curve with a nonlinear slope above the critical field is in excellent agreement with exact-diagonalization calculations based on a 2D coupled spin-dimer model. Individual exchange constants are also evaluated using density functional theory (DFT). The DFT results demonstrate a 2D exchange network and weak frustration between interdimer exchange interactions, supported by weak spin-lattice coupling implied from our magnetostriction data. The magnetic-field-induced spin ordering in Ba$_2$CuSi$_2$O$_6$Cl$_2$ is described as the quasi-2D Bose-Einstein condensation of triplets.

cond-mat.str-el↗

arXiv2015

Magnetic-Field-Induced Polar Phase in Chiral Magnet CsCuCl$_3$

Magnetoelectric effects in the chiral magnet CsCuCl$_3$ have been investigated through the magnetization and electric polarization measurements in pulsed high magnetic fields. If the magnetic field is applied normal to the spin chiral c-axis, a plateau and a jump in magnetization are observed. Between the plateau and the jump in magnetization, a small but significant electric polarization of about 0.25 μC/m$^2$ along the a-axis is observed with the field applied almost perpendicular to the ac-plane. In addition, the paramagnetoelectric effect expected from the point group symmetry is confirmed. The emergence of the electric polarization is explained by the cooperation of the local electric polarization on the chiral solitonic spin arrangement and the paramagnetoelectric effect. Hence, we interpret this novel multiferroic phase in CsCuCl$_3$ as a polar solitonic phase.

cond-mat.str-el↗

arXiv2015

Possible Excitonic Phase of Graphite in the Quantum Limit State

The in-plane resistivity, Hall resistivity and magnetization of graphite were investigated in pulsed magnetic fields applied along the \textit{c}-axis. The Hall resistivity approaches zero at around 53 T where the in-plane and out-of-plane resistivities steeply decrease. The differential magnetization also shows an anomaly at around this field with a similar amplitude compared to that of de Haas-van Alphen oscillations at lower fields. This transition field appears insensitive to disorder, but reduces with doping holes. These results suggest the realization of the quantum limit states above 53 T. As a plausible explanation for the observed gapped out-of-plane conduction above 53 T, the emergence of the excitonic BCS-like state in graphite is proposed.

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