Abstract (WS-A, poster)

Ishizuka, Hiroaki

1. Affiliation: University of Tokyo
2. Country: JAPAN
3. Participation: from 11/7 to 11/11
4. Keywords:
5. Title: Monte Carlo study of a spin-ice type Kondo lattice model on a pyrochlore lattice
   Abstract:
   Itinerant electron systems on a geometrically frustrated lattices have gained much interest as they bring about novel magnetic states and transport phenomena. The metallic pyrochlore oxides are prototypical materials of this kind, which are regarded to be treated by a Kondo lattice model on a pyrochlore lattice. In fact, some Mo and Ir pyrochlore oxides were known to show peculiar conduction properties, such as the unconventional anomalous Hall effect and resistivity minimum. As the origin of such phenomena, strong local correlations intrinsic to these highly frustrated systems have attracted attention. However, comprehensive understanding of possible orderings and related transport phenomena has not been reached so far. To clarify thermodynamic properties brought about by the interaction between localized spins and itinerant electrons, we investigate a spin-ice type Kondo lattice model on a pyrochlore lattice. By applying the Monte Carlo simulation using polynomial expansion technique, we systematically study the magnetic phase diagram of this model. We show that the system exhibits keen competition between various magnetic phases depending on the spin-charge coupling and electron density. Peculiar fluctuations of two-in two-out type or all-in/all-out type local spin configurations appear in the paramagnetic state above the critical temperatures. The mechanism of the orderings and fluctuations will be discussed.

Kaneko, Ryui

1. Affiliation: Department of Applied Physics, University of Tokyo
2. Country: Japan
3. Participation: from 11/7 to 11/11
4. Keywords: kagome lattice, Heisenberg model, volborthite, magnetization step
5. Title: Magnetization Process of Antiferromagnetic Heisenberg Model on Spatially Anisotropic Kagome Lattice
   Abstract:
   Motivated by recent experiments for volborthite, a typical spin 1/2 antiferromagnet with kagome lattice structure, we study magnetization process of the classical and quantum Heisenberg model on the kagome lattice with the spatial anisotropy as well as the next-nearest-neighbor interaction in applied magnetic fields. Recent studies on volborthite have revealed the existence of three steps in the magnetization curve. These steps are not anticipated in the magnetization process of the isotropic model. First, in the classical model with the spatial anisotropy as well as the additional ferromagnetic next-nearest-neighbor interaction with reasonable values for volborthite, by using the Monte Carlo method, we find that two first-order transitions appear around zero and one third of the saturation field. Then, to elucidate the nature of the transitions, by using the mean-field theory as well as the Monte Carlo method, we calculate the magnetization process in the absence of the ferromagnetic next-nearest-neighbor interaction. We show that the spatial anisotropy in the exchange interaction with amplitudes indeed expected in volbothite induces the lowest first-order transition under magnetic fields. We also show that the ferromagnetic next-nearest-neighbor interaction shifts the lowest transition field to nonzero field at T=0. The transition line drops sharply in the h-T phase diagram, just as in volborthite. Finally, by using the linear-spin-wave approximation as well as the exact diagonalization method, we show that the first-order transition or sharp crossover exist even for the quantum model. Our results successfully reproduce essential aspects of the experimental results observed in volborthite.

Okubo, Tsuyoshi

1. Affiliation: Osaka University
2. Country: JAPAN
3. Participation: from 11/7 to 11/18
4. Keywords: Frustration, topological excitation
5. Title: Multiple-Q states and skyrmion lattice of the triangular-lattice Heisenberg model under magnetic fields with an incommensurate helical structure
   Abstract:
   We study the ordering of the classical antiferromagnetic Heisenberg model on the triangular lattice with the next-nearest-neighbor (or the third-neighbor) interaction under magnetic fields by means of a mean-field analysis and a Monte Carlo simulation. When further-neighbor interactions become dominant, the ground state in zero magnetic field takes an incommensurate helical spin structure, which has three-fold degeneracy associated with three equivalent directions of the wavevector on the triangular lattice. We show that not only a simple single-Q state but also a variety of multiple-Q states, including the so-called "skyrmion-lattice" state, are stabilized under applied magnetic fields.

Shinaoka, Hiroshi

1. Affiliation: Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology
2. Country: Japan
3. Participation: from 11/7 to 11/11
4. Keywords: Geometrical frustration, randomness, strongly correlated electronic systems, classical Monte Carlo simulation, first-principles calculation
5. Title: First-principles study on the magnetism and metal-insulator transition in pyrochlore oxide Cd2Os2O7
   Abstract:
   In the past few decades, geometrically frustrated magnetic materials have been attracting increasing attention. Pyrochlore oxide Cd2Os2O7 is a typical example of such frustrated magnets, which exhibits a metal-insulator transition concurrently with the emergence of a Neel order at about 225 K . Despite extensive studies since its discovery in 1974, however, the nature and origin of the transition have not yet been fully clarified. In this talk, we present a fully relativistic spin-polarized LSDA+U study on Cd2Os2O7. We discuss the stability of several magnetic ordering patterns suggested by recent experiments, as well as the origin of the metal-insulator transition.

Tonegawa, Takashi

1. Affiliation: Kobe Unicersity
2. Country: Japan
3. Participation: from 11/7 to 11/11
4. Keywords: quantum spin chain, frustration, quantum phase transition
5. Title: Ground-State Phase Diagram of an Anisotropic $S=2$ Antiferromagnetic Chain with Quartic, Uniaxial, On-Site Anisotropy
   Abstract:
   (Authors) Takashi Tonegawa, Kiyomi Okamoto, Hiroki Nakano, Toru Sakai, Kiyohide Nomura, and Makoto Kaburagi (Abstract) Recently, we [1-3] have numerically determined the ground-state phase diagram of an anisotropic $S=2$ antiferromagnetic chain described by the Hamiltonian,
   ${\cal H} = \sum_j (S_j^x S_{j+1}^x + S_j^y S_{j+1}^y + \Delta S_j^z S_{j+1}^z) + D \sum_j (S_j^z)^2,$
   where ${\vec S}_j=(S_j^x, S_j^y, S_j^z)$ is the $S=2$ operator at the $j$-th site, $\Delta\geq0$, and $D\geq0$. Each phase boundary line is obtained by the level spectroscopy or the phenomenological renormalization analysis of numerical results of exact-diagonalization calculations. Then, we have found that, in a certain region of $\Delta>1$ and $D>0$, where the Ising-type nearest-neighbor interactions and the easy-plane-type on-site anisotropy are competing with each other, the intermediate-$D$ (ID) state appears as the ground state. In spite of the fact that the ID state was predicted by Oshikawa [4] in 1992, it had been considered for about twenty years that this phase does not exist in the phase diagram.
   The region for the ID state is fairly narrow in the obtained phase diagram. In order to clarify the nature of the ID state in more details, it is desirable to investigate $S=2$ chains having a wider ID region in their ground-state phase diagrams. In this sense, it would be interesting to treat the case where the quartic term $D_4 \sum_j (S_j^z)^4$ [4] is included in the on-site anisotropy. As a first step, we investigate in this study the case where the on-site anisotropy term $D \sum_j (S_j^z)^2$ in the above ${\cal H}$ is replaced by $D' \sum_j \{(S_j^z)^4\!-\!(S_j^z)^2\}$, keeping in mind the fact that the ID state in the $S=2$ chain corresponds essentially to the Haldane state in the $S=1$ chain. In this form of the on-site anisotropy, the $S_j^z=0$ state and the $S_j^z=1$ state are equally realized, while the $S_j^z=2$ state is suppressed when $D'$ is positive and sufficiently large. In fact, we find in this case that when $D'>0.4$, the ID state appears as the ground state in a wide region; for example, for $D'=0.5$ and $0.6$, it appears when $1.80<\Delta<2.39$ and $1.63<\Delta<2.73$, respectively. We also find that, due to the existence of the $(S_j^z)^4$ term, the ferrimagnetic state characterized by the magnetization per spin taking finite but unsaturated values which continuously change with $D'$ and $\Delta$ appears as the ground state in the region where $D'>0$ and $\Delta<-1$.
   [1] T. Tonegawa, K. Okamoto, H. Nakano, T. Sakai, K. Nomura, and M. Kaburagi: J. Phys. Soc. Jpn. 80 (2011) 043001.
   [2] K. Okamoto, T. Tonegawa, H. Nakano, T. Sakai, K. Nomura, and M. Kaburagi: J. Phys.; Conf. Series, in press; arXiv:1101.2799.
   [3] K. Okamoto, T. Tonegawa, H. Nakano, T. Sakai, K. Nomura, and M. Kaburagi: J. Phys.; Conf. Series, in press.
   [4] M. Oshikawa: J. Phys.: Cond. Matter 4 (1992) 7469.