Session S52: Invited Session: Partial Order and Spin Liquids In Novel Frustrated Materials

Emergent Separation of Valence Bond Regimes in LiZn$_2$Mo$_3$O$_8$

LiZn$_2$Mo$_3$O$_8$ is a S = 1/2 triangular lattice antiferromagnet in which the basic magnetic building block is an Mo3O13 cluster rather than an individual ion. Rather than forming the 120$^{\circ}$ ordered magnetic state expected for a Heisenberg nearest neighbor triangular antiferromagnet, LiZn$_2$Mo$_3$O$_8$ instead favors a complex valence bond order across different energy and lengthscales: approximately two thirds of the spins form singlets at T $\sim$ 100 K, while the remainder form valence bonds at lower temperature, as indicated by inelastic neutron scattering. No static magnetic order is detected by uSR down to T = 0.05 K. What is the origin of this spontaneous separation into different regimes of magnetic fluctuations? Recent experimental data, including the impact of hole doping, will be compared to different theoretical models that have been proposed for this behavior, including an emergent honeycomb lattice by octahedral rotations and partial charge ordering driven by extra intracluster degrees of freedom.   

Low energy XY spin clusters in a pyrochlore antiferromagnet with weak disorder

The spin liquid state of the Heisenberg antiferromagnet (HAFM) on the pyrochlore lattice arises from an extensive degeneracy of correlated yet disordered ground states. How this spin liquid is modified in real materials with imperfect Heisenberg exchange is a rich field of study, with many possible outcomes depending on the relevant perturbations. We have studied a single crystal of a new pyrochlore antiferromagnet, NaCaCo$_2$F$_7$. High spin ($S=3/2$) Co$^{2+}$ forms a fully ordered pyrochlore sub lattice, while non-magnetic Na$^{+}$ and Ca$^{2+}$ are intermixed on the A-site. Despite isotropic magnetic properties and large antiferromagnetic coupling ($\Theta_{CW}$ = -140 K), a freezing transition is observed at temperature much lower than the exchange energy ($T_f \sim$ 3 K), thus revealing the relatively weak exchange disorder induced by the mixed ion A-site. Unexpectedly, our inelastic neutron scattering measurements reveal that the frozen state is of local $XY$ character and supports low energy $XY$ fluctuations. Yet the system can break free from the $XY$ states at energies above 2.5 meV $\sim$ 30 K; at these energy scales we observe the collinear Ising configurations expected for the weak-disorder HAFM model. The frozen state in NaCaCo$_2$F$_7$ provides a new outlook on the role of disorder in selecting spin configurations from the Heisenberg pyrochlore spin liquid state.   

Longitudinal and transverse heat transports of quantum spin liquids

Study of disordered states of quantum spins in two-dimensions, so-called quantum spin liquids (QSLs), has been attracting attention because 2D QSL can be a new state of matter characterized by unknown quasiparticles. Recent discoveries of materials possessing an ideal 2D triangular or a kagom\'{e} lattice have spurred a lot of experimental pursuit to identify the ground state. Especially, identifying the elementary excitation characterizing the ground state has been the central focus of attention. In this presentation, I will present our transport studies of organic insulators with triangular lattices [1] and inorganic kagom\'{e} material Volborthite Cu$_3$V$_2$O$_7$(OH)$_2$$\cdot$2H$_2$O. From NMR studies of the slightly distorted kagom\'{e} material, multiple ordered phases have been found in low temperatures and under high fields [2]. Upon entering one of the ordered phases under field, we have found an increase of the thermal conductivity, showing an additional thermal transport due to spin wave excitations. Above the ordering temperature, we've found a finite transverse heat transfer in this transparent insulator. We will discuss origins of the thermal-Hall conductivity in terms of the spinon thermal-Hall effect [3].\\[4pt] [1] K. Kanoda and R. Kato, Annu. Rev. Condens. Matter Phys. \textbf{2}, 167 (2011).\\[0pt] [2] M. Yoshida et al.: J. Phys. Soc. Jpn. \textbf{81} (2012) 024703.\\[0pt] [3] H. Katsura et al.: Phys. Rev. Lett. \textbf{104}, 066403 (2010).   

Doping and disorder in spin liquids

The question of what happens when an unconventional spin state is doped has been a central theme of the field of strongly correlated electrons since Anderson's proposal of doping the RVB liquid state to obtain a high-temperature superconductor. Recently, there has been much progress in constructing models which exhibit topological spin liquid phases, e.g. in Kitaev's models or in spin ice. In this talk, we address the properties of defects in such exotic spin states.   

Unconventional magnetic order stabilized by Kitaev interactions in the three-dimensional honeycomb polytypes of Li2IrO3

Materials that realize Kitaev spin models with bond-dependent anisotropic interactions have long been searched for, as the resulting frustration effects are predicted to stabilize novel forms of magnetic order or quantum spin liquids. Here we explore the magnetism of the recently-synthesized iridates $\beta$- and $\gamma$-Li$_2$IrO$_3$, which have the topology of three-dimensional Kitaev lattices of inter-connected Ir honeycombs. Using single-crystal resonant magnetic x-ray diffraction we find in both cases a surprisingly complex, yet highly symmetric, incommensurate magnetic structure with non-coplanar and counter-rotating Ir moments [1,2]. Our experimental results combined with a theoretical analysis [3] of candidate spin Hamiltonians provide strong evidence that both $\beta$ and $\gamma$-Li$_2$IrO$_3$ realize a spin Hamiltonian with dominant Kitaev interactions. [1] A. Biffin, R.D. Johnson, I. Kimchi, R. Morris, A. Bombardi, J.G. Analytis, A. Vishwanath, and R. Coldea, Phys. Rev. Lett. 113, 197201 (2014). [2] A. Biffin, R.D. Johnson, Sungkyun Choi, F. Freund, S. Manni, A. Bombardi, P. Manuel, P. Gegenwart, and R. Coldea, Phys. Rev. B. 90, 205116 (2014). [3] I. Kimchi, R. Coldea and A. Vishwanath, arXiv:1408.3640 (2014).