arXiv subjects

Ryota Ono

Publications and source records attributed to Ryota Ono.

7 recordsLinked to original sources

arXiv2026

A microscopic design rule for spin supersolids in triangular-lattice magnets

Spin supersolids emerge as a central topic in frustrated magnetism, motivating the search for realization in quantum materials. To this end, we study the origin of exchange anisotropy, $\Delta$, in triangular-lattice cobaltate families $X_2$$Y$Co(PO$_4$)$_2$ and $X_2$Co(SeO$_3$)$_2$ ($X$ = Na, K, Rb, Cs; $Y$ = Mg, Ca, Sr, Ba) by tailoring realistic spin models. We show that $\Delta$ is determined by the ratio of trigonal crystal field to spin-orbit coupling strength. This framework explains contrasting anisotropies in these families, predicts systematic trends in $\Delta$ across $X/Y$-substitutions, and identifies candidate materials for spin supersolids. Our results establish trigonal field engineering as a microscopic route toward the design of spin supersolids.

cond-mat.str-el↗

arXiv2025

Finite temperature magnetic interactions from first principles

Density functional theory has demonstrated remarkable predictive power in calculating magnetic properties at zero temperature. At finite temperatures, thermally excited phonons may affect magnetism. Efficient ab-initio methods to calculate the temperature dependence of magnetic exchange interactions are still lacking despite the importance of room temperature magnetism for applications. Exchange is controlled by an interplay between metal-ligand hybridization, Hubbard repulsion, band gap, interatomic distances and bond angles, all of which change with temperature. Here we present a method to calculate the exchange interactions at finite temperatures from first principles using only two supercell calculations and quantify these mechanisms. Changes in bond angles and the band gap are identified as a primary factors. In NiO with 180-degree bonds exchange decreases with temperature, while in Cr$_2$O$_3$ with the bond angles away from 180 degrees the exchange increases by 10% at room temperature.

cond-mat.str-el↗

arXiv2024

Entangled orbital, spin, and ferroelectric orders in $p$-electron magnet CsO$_2$

Alkali superoxides differ from conventional transition metal magnets, exhibit magnetism from partially occupied oxygen molecular $π^*$-orbitals. Among them, CsO$_2$ stands out for its potential to exhibit novel quantum collective phenomena, such as an orbital order induced Tomonaga-Luttinger liquid state. Using ab-initio Hubbard models, superexchange theory, and experimental spin wave measurements, we propose that CsO$_2$ exhibits unconventional magnetoelectric characteristics at low temperature. Our analysis confirms a canted antiferromagnetic ground state and a spin-flop transition, with ferroelectricity is induced by breaking inversion and time-reversal symmetry in the spin-flop phase. Consequently, our analysis reveals a strong interplay not only between exchange interactions but also among magnetically-induced polarization and orbital order. The magnetic structure, stabilized by orbital order, induces magnetically-induced polarization through an antisymmetric mechanism. Overall, our results reveal the coexistence of three highly entangled orders in CsO$_2$, namely, orbital, spin and ferroelectricity.

cond-mat.str-el↗

arXiv2024

Multiferroic kinks and spin-flop transition in Ni$_{2}$InSbO$_6$ from first principles

Magnetoelectric multiferroics are key materials for next-generation spintronic devices due to their entangled magnetic and ferroelectric properties. Spiral multiferroics possess ferroelectric polarization and are particularly promising for electric control of magnetism and magnetic control of ferroelectricity. In this work, we uncover long-period incommensurate states characterized by unique multiferroic kinks in corundum nickelate Ni$_{2}$InSbO$_6$, a member of a promising family of polar magnets. Utilizing a 2-orbital $S=1$ model, we derive formulas for Heisenberg and anisotropic magnetic exchanges and magnetically-induced polarization, enabling their calculations from first principles. We use these parameters in Monte Carlo and Landau theory-based calculations to reproduce experimentally observed magnetic structures and polarization dependence on the magnetic field. We predict magnetic phase transitions between flat spiral, conical spiral, canted antiferromagnetic and ferromagnetic states under increasing magnetic fields. Kinks in the spiral phases repel each other through a Yukawa-like potential arising from exchange of massive magnons. We also find that suitably directed electric fields can be used to stabilize the ferromagnetic and spiral states. The findings open a new pathway to predictive first-principles modelling of multiferroics and will inspire experiments and technological applications based on multiferroic kinks.

cond-mat.mtrl-sci↗

arXiv2021

Magnetically induced polarization in centrosymmetric bonds

We reveal the microscopic origin of electric polarization $\vec{P}$ induced by noncollinear magnetic order. We show that in Mott insulators, such $\vec{P}$ is given by all possible combinations of position operators $\hat{\vec{r}}_{ij} = (\vec{r}_{ij}^{\, 0},\vec{\boldsymbol{r}}_{ij}^{\phantom{0}})$ and transfer integrals $\hat{t}_{ij} = (t_{ij}^{0},\boldsymbol{t}_{ij}^{\phantom{0}})$ in the bonds, where $\vec{r}_{ij}^{\, 0}$ and $t_{ij}^{0}$ are spin-independent contributions in the basis of Kramers doublet states, while $\vec{\boldsymbol{r}}_{ij}^{\phantom{0}}$ and $\boldsymbol{t}_{ij}^{\phantom{0}}$ stem solely from the spin-orbit interaction. Among them, the combination $t_{ij}^{0} \vec{\boldsymbol{r}}_{ij}^{\phantom{0}}$, which couples to the spin current, remains finite in the centrosymmetric bonds, thus yielding finite $\vec{P}$ in the case of noncollinear arrangement of spins. The form of the magnetoelectric coupling, which is controlled by $\vec{\boldsymbol{r}}_{ij}^{\phantom{0}}$, appears to be rich and is not limited to the phenomenological law $\vec{P} \sim \boldsymbolε_{ij} \times [\boldsymbol{e}_{i} \times \boldsymbol{e}_{j}]$ with $\boldsymbolε_{ij}$ being the bond vector connecting the spins $\boldsymbol{e}_{i}$ and $\boldsymbol{e}_{j}$. Using density-functional theory, we illustrate how the proposed mechanism work in the spiral magnets CuCl$_2$, CuBr$_2$, CuO, and $α$-Li$_2$IrO$_3$, providing consistent explanation to available experimental data.

cond-mat.str-el↗

arXiv2021

Surface band characters of Weyl semimetal candidate material MoTe$_2$ revealed by one-step ARPES theory

The layered 2D-material MoTe$_2$ in the T$_d$ crystal phase is a semimetal which has theoretically been predicted to possess topologically non-trivial bands corresponding to Weyl fermions. Clear experimental evidence by angle-resolved photoemission spectroscopy (ARPES) is, however, lacking, which calls for a careful examination of the relation between ground state band structure calculations and ARPES intensity plots. Here we report a study of the near Fermi-energy band structure of MoTe$_2$(T$_d$) by means of ARPES measurements, density functional theory, and one-step-model ARPES calculations. Good agreement between theory and experiment is obtained. We analyze the orbital character of the surface bands and its relation to the ARPES polarization dependence. We find that light polarization has a major efect on which bands can be observed by ARPES. For s-polarized light, the ARPES intensity is dominated by subsurface Mo d orbitals, while p-polarized light reveals the bands composed mainly derived from Te p orbitals. Suitable light polarization for observing either electron or hole pocket are determined

cond-mat.mtrl-sci↗

arXiv2020

Fingerprints of spin-current physics on magnetoelectric response in the spin-$1/2$ magnet Ba$_2$CuGe$_2$O$_7$

The single-site anisotropy vanishes for the spin-1/2 as a consequence of Kramers degeneracy. We argue that similar property holds for the magnetically induced electric polarization P, which should depend only on the relative orientation of spins in the bonds but not on the direction of each individual spin. Thus, for insulating multiferroic compounds, P can be decomposed in terms of pairwise isotropic, antisymmetric, and anisotropic contributions, which can be rigorously derived in the framework of the superexchange (SE) theory, in an analogy with the spin Hamiltonian. The SE theory also allows us to identify the microscopic mechanism, which stands behind each contribution. The most controversial and intriguing one is antisymmetric or spin-current mechanism. In this work, we propose that the disputed magnetoelectric (ME) properties of Ba2CuGe2O7 can be explained solely by the spin-current mechanism, while other contributions are either small or forbidden by symmetry. First we explicitly show how the cycloidal spin order induces the experimentally observed P in the direction perpendicular to the xy plane, which can be naturally explained by the spin-current mechanism operating in the out-of-plane bonds. Then, we unveil previously overlooked ME effect, where the application of the magnetic field perpendicular to the plane not only causes the incommensurate-commensurate transition, but also flips P into the plane due to the spin-current mechanism operating in the neighboring bonds within this plane. In both cases, the magnitude and direction of P can be controlled by rotating the spin pattern in the xy plane. Our analysis is based on a realistic spin model, which was rigorously derived from the first-principles calculations and supplemented with the new algorithm for the construction of localized Wannier functions obeying the crystallographic symmetry of Ba2CuGe2O7.

cond-mat.str-el↗