Abstract (WS-B, poster)

PS-B-1. Akagi, Yutaka

1. Affiliation: Department of Applied Physics, University of Tokyo
2. Country: Japan
3. Participation: from 11/14 to 11/18
4. Keywords: Hall effect, spin chirality, frustration
5. Title: Spin-chirality ordering and kinetic-driven effective interactions in geometrically-frustrated ferromagnetic Kondo-lattice systems
   Abstract:
   Recently, noncoplanar spin configurations with spin scalar chirality have drawn considerable attention as an origin of the anomalous Hall effect in geometrically frustrated systems. In this mechanism, itinerant electrons acquire an internal magnetic field according to the solid angle spanning three spins through the so-called Berry phase, which can result in the anomalous Hall effect. The idea was first explored in the ferromagnetic Kondo lattice model on a kagome lattice [1], and extended to other lattice systems, such as a face-centered-cubic lattice [2] and a triangular lattice [3]. In particular, it was pointed out that in the triangular lattice system the perfect nesting of the Fermi surface at 3/4 electron filling might lead to a noncoplanar four-sublattice ordering and the anomalous Hall effect [3]. While these studies have successfully revealed the nontrivial relation between the Berry phase and anomalous Hall effect, a crucial question has been left unclear so far, i.e., when and how such noncoplanar spin order emerges and what is the role of coupling between charge and spin degrees of freedom in energetically stabilizing such ordering. To clarify the parameter range and the stabilization mechanism of the noncoplanar ordering, we study a ferromagnetic Kondo lattice model on a triangular lattice, and obtain the groundstate phase diagram in the parameter space of electron density, Hund's-rule coupling and antiferromagnetic superexchange interaction between localized spins [4]. In order to determine the ground state for each parameter set, we evaluate and compare the energies of various spin-ordered states up to four-sublattice orders. As a result, we find that a noncoplanar four-sublattice spin ordering with finite spin scalar chirality emerges in the region near 1/4 filling, in addition to the 3/4 filling indicated in the previous study [3]. This new phase is stabilized in a wider parameter region, covering both metallic and insulating phases, compared to the 3/4 filling phase. The anomalous Hall effect takes place in these chiral-ordered phases, and in particular, the Hall conductivity is quantized according to the Chern number in the insulating regions. We also reveal significance of kinetic-driven multiple-spin interactions hidden in geometrically-frustrated Kondo lattice models. Carefully examining the perturbation in terms of the spin-charge coupling up to the fourth order, we find that a positive biqaudratic interaction is critically enhanced and plays a crucial role on stabilizing a spin scalar chiral ordering near 1/4 filling in a triangular lattice case. The origin of large positive biquadratic interaction is ascribed to the Fermi surface connection by the ordering wave vectors of four sublattice order, which we call the generalized Kohn anomaly [5]. The mechanism is potentially common to frustrated spin-charge coupled systems, leading to emergence of unusual magnetic orders. We also show the results on other frustrated lattices such as face-centered-cubic, checkerboard, and pyrochlore lattices.
   [1] K. Ohgushi, S. Murakami, and N. Nagaosa, Phys. Rev. B 62 (2000) R6065.
   [2] R. Shindou and N. Nagaosa, Phys. Rev. Lett. 87 (2001) 116801.
   [3] I. Martin and C.D. Batista, Phys. Rev. Lett. 101 (2008) 156402.
   [4] Y. Akagi and Y. Motome, J. Phys. Soc. Jpn. 79 (2010) 083711.
   [5] Y. Akagi, M. Udagawa, and Y. Motome, in preparation.

PS-B-2. Hayami, Satoru

1. Affiliation: Department of Applied Physics, University of Tokyo
2. Country: Japan
3. Participation: from 11/14 to 11/18
4. Keywords: partial disorder, charge disproportionation, quantum critical point, geometrical frustration, periodic Anderson model
5. Title: Partial disorder in a frustrated periodic Anderson model
   Abstract:
   Partial disorder is one of the peculiar orderings to geometrically-frustrated systems. It is stabilized by relieving the frustration with self-organizing the system into coexistence of magnetically-ordered sites and nonmagnetic sites. In fact, several rare-earth systems exhibit intriguing partially-disordered (PD) states, in which magnetically-ordered sites coexist with Kondo spin singlets. Such coexistence of magnetic moments and spin singlets is expected to cause interesting magnetotransport phenomena, but the comprehensive understanding is not reached yet. In this contribution, we investigate the nature and origin of the peculiar PD state related with the Kondo singlet formation. We clarify the ground state phase diagram of a periodic Anderson model on a triangular lattice by the Hartree-Fock approximation with three-site unit cell. As a result, we find a collection of PD states at several commensurate fillings such as at half-filling and 1/6 filling. The PD state is stabilized by releasing the frustration with forming a collinear antiferromagnetic order on an unfrustrated honeycomb subnetwork and leaving the remaining sites non-magnetic. We will discuss the electronic and magnetic properties of the PD states in detail.

PS-B-3. Kariya, Natsuki

1. Affiliation: Department of Physics,Graduate School of Science,The University of Tokyo
2. Country: Japan
3. Participation: from 11/14 to 11/18 from 12/5 to 12/9
4. Keywords: nonequilibrium phenomena in strongly correlated system
5. Title: Nonequilibrium dynamics of magnon BEC in the spin dimer system
   Abstract:
   The spin dimer system, which consists of "dimer", a pair of spin 1/2 bound anti ferromagnetically, is very fascinating because quantum phase transition occurs in the system by adding the sufficient external magnetic field to the system, and this phase transition can be regarded as BEC of magnon. We are interested in the dynamical problem in the system. Specifically, we prepare the initial state as magnon BEC state by adding sufficiently strong magnetic field, and at a given time, we suddenly turn the field around the critical point. After this quench, the BEC will become unstable. We study the dynamics of destabilaized magnon BEC by using truncated Wigner approximation(TWA), which is the way to describe the dynamics of the observable using phase space representation. In this poster, we would like to present the result we calculated

PS-B-4. Suzuki, Takafumi

1. Affiliation: University of Hyogo
2. Country: Japan
3. Participation: from 11/7 to 11/25
4. Keywords: topological insulators, critical phenomena, quantum spin systems
5. Title: Edge states and their stability in two-dimensional quantum antiferromagnets
   Abstract:
   Recently, topological insulators (TI) [1] have been much attracted from both experimental and theoretical view points. They can be characterized not by conventional local order parameters, but by topological quantities of the bulk or gapless surface states [2,3]. The TI phases and the surface states are quite stable against any perturbations with time reversal symmetry. On the other hand, it is known that there exists another kind of topological gapped state, which is called Haldane-gap state [4], in one-dimensional quantum spin systems. Similarly to TIs, this gapped state has no local order and is characterized by the non-local (string) order parameter or free spins at the edges. Due to the recent theoretical studies, the Haldane-gap state has been more deeply understood based on symmetry arguments and artificial quantities such as entanglement entropy [5, 6]. 　In this study, motivated by the recent development of theories for topological phases and surface states, we consider properties of edge states in two-dimensional quantum spin systems by applying the quantum Monte Carlo method. Particularly, we focus on the following three points; (1) which spin systems can have gapless edge states, (2) the stability of the gapless edge states, and (3) the difference between the edge modes of TIs and spin systems.
   [1] See, for example, M. Z. Hasan and C. L. Kane, RMP82, 3045 (2010); X.-L. Qi and S. C. Zhang, arXiv:1008.2026.
   [2] A. P. Schnyder, S. Ryu, A. Furusaki, and A. W. W. Ludwig, PRB 78, 195125 (2008).
   [3] A. Kitaev, AIP Conf. Proc. 1134, 22 (2009).
   [4] F.D.M. Haldane, Phys. Lett. 93A, 464 (1983); PRL50, 1153 (1983).
   [5] F. Pollmann, E. Berg, A. M. Turner, and M. Oshikawa, arXiv:0909.4059; F. Pollmann, A. M. Turner, E. Berg, and M. Oshikawa, PRB81, 064439 (2010).
   [6] Z.-C. Gu and X.-G. Wen, PRB80, 155131 (2009); X. Chen, Z.-C. Gu, and X.-G. Wen, PRB83, 035107 (2011).

PS-B-5. Takayoshi, Shintaro

1. Affiliation: Institute for Solid State Physics, University of Tokyo
2. Country: Japan
3. Participation: from 11/14 to 11/18
4. Keywords: Triangular lattice, Heisenberg antiferromagnets
5. Title: New phases induced by four-spin interaction in stacked triangular-lattice antiferromagnets in magnetic field
   Abstract:
   Classical antiferromagnetic Heisenberg model on triangular lattice (CAFHT) is one of the representative models with geometrical frustration. It is well-known that the Y-shape, up-up-down (uud) and V-shape phases appear as we apply magnetic field. Since these phases originate from order-by-disorder mechanism, they are expected to be weak against small perturbations. However, the stability of these three phases has not been investigated well so far. In this study, as such perturbations, we consider four-spin interactions (ring-exchange and biquadratic term). It is expected that an even small four-spin interaction can induce new phases or violate the three (Y, uud, and V) phases. We determine the global phase diagram of the ferromagnetically-stacked CAFHT with the four-spin terms, by using Monte Carlo simulation with reasonable accuracy. It is found that a small ring-exchange term induces a scalar chiral ordered phase, up-up-up-down phase and a four-sublattice planar spiral phase with uud axial order, while a small biquadratic term does not provoke any new phase and just stabilizes the uud phase. We also show that a sufficiently strong biquadratic term induces a spin-nematic order. In the conference, we will discuss the detail of the phase structure and related magnetic compounds.