arXiv subjects

Takuto Yamada

Publications and source records attributed to Takuto Yamada.

5 recordsLinked to original sources

arXiv2023

Strain-induced Landau levels of Majorana fermions in an anisotropically interacting Kitaev model on a honeycomb lattice

The energy structure of an anisotropically interacting Kitaev model on a honeycomb lattice under triaxial strain is investigated. A numerical calculation shows that quantized states appear in the low-energy region, even when the anisotropy of the interaction is rather strong. Their energies are proportional to the square root of the quantum number and the quantized state at zero energy appears only on one sublattice. These findings indicate the emergence of the strain-induced Landau levels of Majorana fermions, which is also confirmed by an analytical calculation. These Landau levels are stable, when the direction of triaxial strain is slightly changed from the bond direction.

cond-mat.str-el↗

arXiv2021

Ground-state phase diagram of anisotropically interacting Heisenberg-$Γ$ models on a honeycomb lattice

In this paper, we investigate the ground-state phase diagram of the $S=1/2$ Heisenberg-$Γ$ model on a honeycomb lattice by dimer series expansion and exact diagonalization. We focus on the effects of the anisotropy of the interactions; by tuning the coupling constants, the system changes between the isolated dimer and the spin-chain models. We find that, in the spin-chain limit, there are three kinds of states: a Tomonaga-Luttinger liquid and two magnetically long-range-ordered states. All three states become two-dimensional long-range ordered states by the infinitesimal interchain interaction except for the case where the Heisenberg interaction is much weaker than the off-diagonal symmetric ($Γ$) interaction. Starting from the isolated dimer limit, a triplet dimer phase survives up to the isotropically interacting system in a large part of the phase diagram where the Heisenberg and $Γ$ interactions are ferromagnetic and antiferromagnetic, respectively. Otherwise, a phase transition to a magnetically ordered phase occurs before the interaction becomes isotropic. This indicates that the quantum spin liquid proposed in the $Γ$ model [A. Catuneanu et al., npj Quantum Mater. 3, 23 (2018)] is unstable against the anisotropy of the interactions.

cond-mat.str-el↗

arXiv2020

Ground-state properties of the $K-Γ$ model on a honeycomb lattice

We investigate the ground-state properies of the $K-Γ$ model on a honeycomb lattice using series expansions and numerical exact diagonalizations, where the model includes Kitaev ($K$) and symmetric off-diagonal ($Γ$) interactions. Starting from the weakly interacting dimers on the specific bond, we strengthen the interdimer interactions to the isotropically interacting system. We show that depending on $Γ$ and $K$, the dimer state survives up to the isotropically interacting system, where the phase transition occurs, or obeys a phase transition to a magnetically ordered state at an anisotropic interaction. The results are summarized in the phase diagram. We also show that the Kekulé dimerized state is unstable in the isotropic $K-Γ$ model.

cond-mat.str-el↗

arXiv2016

Clues and criteria for designing Kitaev spin liquid revealed by thermal and spin excitations of honeycomb iridates Na$_2$IrO$_3$

Contrary to the original expectation, Na$_2$IrO$_3$ is not a Kitaev's quantum spin liquid (QSL) but shows a zig-zag-type antiferromagnetic order in experiments. Here we propose experimental clues and criteria to measure how a material in hand is close to the Kitaev's QSL state. For this purpose, we systematically study thermal and spin excitations of a generalized Kitaev-Heisenberg model studied by Chaloupka $et$ $al$. in Phys. Rev. Lett. 110, 097204 (2013) and an effective ab initio Hamiltonian for Na$_2$IrO$_3$ proposed by Yamaji $et$ $al$. in Phys. Rev. Lett. 113, 107201 (2014), by employing a numerical diagonalization method. We reveal that closeness to the Kitaev's QSL is characterized by the following properties, besides trivial criteria such as reduction of magnetic ordered moments and Neel temperatures: (1) Two peaks in the temperature dependence of specific heat at $T_{\ell}$ and $T_h$ caused by the fractionalization of spin to two types of Majorana fermions. (2) In between the double peak, prominent plateau or shoulder pinned at $(R/2)\ln 2$ in the temperature dependence of entropy, where $R$ is the gas constant. (3) Failure of the linear spin wave approximation at the low-lying excitations of dynamical structure factors. (4) Small ratio $T_{\ell}/T_h$ close to or less than 0.03. According to the proposed criteria, Na$_2$IrO$_3$ is categorized to a compound close to the Kitaev's QSL, and is proven to be a promising candidate for the realization of the QSL if the relevant material parameters can further be tuned by making thin film of Na$_2$IrO$_3$ on various substrates or applying axial pressure perpendicular to the honeycomb networks of iridium ions. Applications of these characterization to (Na$_{1-x}$Li$_x$)$_2$IrO$_3$ and other related materials are also discussed.

cond-mat.str-el↗

arXiv2015

Dynamical properties of the honeycomb-lattice Iridates ${\rm Na_2IrO_3}$

We investigate the dynamical properties of ${\rm Na_2IrO_3}$. For five effective models proposed for ${\rm Na_2IrO_3}$, we numerically calculate dynamical structure factors (DSFs) with an exact diagonalization method. An effective model obtained from $ab$ $initio$ calculations explains inelastic neutron scattering experiments adequately. We further calculate excitation modes based on linearized spin-wave theory. The spin-wave excitation of the effective models obtained by $ab$ $initio$ calculations disagrees with the low-lying excitation of DSFs. We attribute this discrepancy to the location of ${\rm Na_2IrO_3}$ in a parameter space close to the phase boundary with the Kitaev spin-liquid phase.

cond-mat.str-el↗