Research Group for Condensed Matter Theory
Group Leader: Michiyasu Mori
Members: Hiroaki Onishi, Katsunori Kubo, Hiroto Adachi, Jun'ichi Ieda, Bo Gu,Mamoru Matsuo, Takanori Sugimoto, Zhuo Xu, Yuta Yamane, Yuichi Ohnuma

We study new functional materials emerging from theelectrons internal degrees of freedom (spin, charge and orbital)
and the electrons correlation. New principle and new function ofdevices are also our important targets. In this fiscal year, we
proposed a new method to generate a spinmotive force by atime-dependent electric-field [1]. This makes it possible tocontrol the spinmotive force by an electric field free frommagnetic properties such as ferromagnetic resonance frequency.
Furthermore, we found a new principle to generate a spincurrent in a non-magnetic metal by surface acoustic waves [2].
By this method, conventional metals such as Al and Cu can be
useful material for spintronics. The latter one is summarized inResearch Highlights. Below, progress on dynamical property ofa frustrated quantum spin system is reported [3].
Magnetic excitations in a frustrated two-legged spin-ladder
Spinons are elementary excitations characteristic of strongly
correlated electron systems in low dimensions, and observableby angle-resolved photoemission spectroscopy and neutron
scattering. There are a manifestation of electron
fractionalization called spin-charge separation, which instead of
forming a single quasiparticle, the electronic excitation splits
into spin (spinon) and charge (holon) degrees of freedom. In thefrustrated spin systems with exotic ground states such as spinliquid and valence-bond-solid states, the spinon is associatedwith a topological excitation similar to the magnetic domain
wall. In contrast to the collective excitation in a magnetic longranged ordered state, i.e., magnon, the spinon carries a fractional
quantum number, spin 1/2, accompanied by a spin string.
Because of this topological nature, two spinons make in someinstance a bound state called a triplon.
We theoretically studied the magnetic excitations in afrustrated
two-legspin-laddersystem,inwhichantiferromagnetic exchange interactions act on the nearestneighbor (J1) and next-nearest-neighbor (J2) bonds in the leg
direction, and on the nearest-neighbor (Jp) bonds in the rungdirection. A dynamical spin correlation function at zerotemperature is calculated using the dynamical density-matrix
renormalization-group method for possible magnetic phases, i.e.,columnar-dimer (CD) and rung-singlet (RS) phases. The CDphase is characterized by multi-spinon excitations with spin gap,
whereas the RS phase is dominated by the triplet excitation in
the rung direction. One major difference found between thesetwo phases appears in the spectral weight of magnetic
excitations, in particular, the bonding (qy=0) and anti-bonding(qy=) modes in the rung direction (See Fig. 1). Therefore, we
can distinguish one phase from the other by the difference of thespectral weight. Furthermore, we examine the effect offrustration on both modes in the rung-singlet phase with aperturbation theory from the strong coupling limit. The antibonding mode is shown to be stable against frustration, and awavenumber with minimum excitation energy is shifted frombeing commensurate to being incommensurate. In contrast, thebonding mode is merged into the continuum of multiple triplet
excitations by increasing frustration. By comparing our resultswith inelastic neutron scattering experiments for BiCu2PO6, themagnitude of the magnetic exchange interactions and the groundstate will be determined by analyzing some experimental dataobtained in J-PARC.
References[1] Y. Yamane et al., Phys. Rev. B 88, 014430 (2013).
[2] M. Matsuo et al., Phys. Rev. B 87, 180402(R) (2013).
[3] T. Sugimoto et al., Phys. Rev. B 87, 155143 (2013).

Fig. 1 The spectral weight of magnetic excitation in a 32-rung frustrated ladder for the three phases: (a), (b) theincommensurate columnar dimer (ICD) phase (Jp/J1=0.2, J2/J1=0.6), (c), (d) the incommensurate rungsinglet (IRS) phase (Jp/J1=1.0, J2/J1=0.6), and (e), (f) the commensurate rung singlet (CRC) phase((Jp/J1=1.0, J2/J1=0.1). (a), (c), and (e) [(b), (d), and (f)] represent the excitations with qy=0 [qy=], whichcorresponds to the bonding (anti-bonding) mode. We set the DMRG truncation number to be 150 and thebroadening factor as for 0.1.