Session H2: Emergent Topological Phenomena in Pyrochlore Iridates II

Metal-insulator transitions of bulk and domain-wall states in pyrochlore iridates.

A family of pyrochlore iridates $R_{\mathrm{2}}$Ir$_{\mathrm{2}}$O$_{\mathrm{7}}$ offers an ideal platform to explore intriguing phases such as topological Mott insulator and Weyl semimetal [1]. Here we report transport and spectroscopic studies on the metal-insulator transition (MIT) induced by the modulations of effective electron correlation and magnetic structures, which is finely tuned by external pressure, chemical substitutions ($R \quad =$ Nd$_{\mathrm{1-}}_{x}$Pr$_{x}$ and Sm$_{y}$Nd$_{\mathrm{1-}}_{y})$, and magnetic field. A reentrant insulator-metal-insulator transition is observed near the paramagnetic insulator-metal phase boundary reminiscent of a first-order Mott transition for $R \quad =$ Sm$_{y}$Nd$_{\mathrm{1-}}_{y}$ compounds ($y$\textasciitilde 0.8). The metallic states on the magnetic domain walls (DWs), which are observed for $R =$ Nd in real space [2] as well as in transport properties [3], is simultaneously turned into the insulating one. These findings imply that the DW electronic state is intimately linked to the bulk states. For the mixed $R \quad =$ Nd$_{\mathrm{1-}}_{x}$Pr$_{x}$ compounds, the divergent behavior of resistivity with antiferromagnetic order is significantly suppressed by applying a magnetic field along [001] direction [4]. It is attributed to the phase transition from the antiferromagnetic insulating state to the novel Weyl (semi-)metal state accompanied by the change of magnetic structure. The present study combined with experiment and theory suggests that there are abundant exotic phases with physical parameters such as electron correlation and Ir-5$d$ magnetic order pattern. Work performed in collaboration with J. Fujioka, B.-J. Yang, C. Terakura, N. Nagaosa, Y. Tokura (University of Tokyo, RIKEN CEMS), J. Shiogai, A. Tsukazaki, S. Nakamura, S. Awaji (Tohoku University). $^{\mathrm{1}}$This work was supported by JSPS FIRST Program and Grant-in-Aid for Scientific Research (Grants No. 80609488 and No. 24224009). [1] W. Witczak-Krempa, G. Chen, Y. B. Kim, and L. Balents, Annu. Rev. Condens. Matter Phys. \textbf{5}, 57 (2014). [2] Eric Yue Ma, Yong-Tao Cui, Kentaro Ueda, Shujie Tang, Kai Chen, Nobumichi Tamura, Phillip M. Wu, Jun Fujioka, Yoshinori Tokura, and Zhi-Xun Shen, Science \textbf{350}, 538 (2015). [3] K. Ueda, J. Fujioka, Y. Takahashi, T. Suzuki, S. Ishiwata, Y. Taguchi, M. Kawasaki, and Y. Tokura, Phys. Rev. B \textbf{89}, 075127 (2014). [4] K. Ueda, J. Fujioka, B.-J. Yang, J. Shiogai, A. Tsukazaki, S. Nakamura, S. Awaji, N. Nagaosa, and Y. Tokura, Phys. Rev. Lett. \textbf{115}, 056402 (2015).   

Mobile metallic domain walls in an all-in-all-out magnetic insulator

Magnetic domain walls are boundaries between regions with different configurations of the same magnetic order. In a magnetic insulator where the magnetic order is tied to its bulk insulating property, it has been postulated that electrical properties are drastically different along the domain walls, where the order is inevitably disturbed. Here we report the discovery of highly conductive magnetic domain walls in a magnetic insulator Nd$_{2}$Ir$_{2}$O$_{7}$, which has an unusual all-in-all-out magnetic order, via transport and spatially resolved microwave impedance microscopy. The domain walls have a virtually temperature-independent sheet resistance (averaged over mesoscopic distances) of \textasciitilde 1 kilohm per square, show smooth morphology with no preferred orientation, are free from pinning by disorders, and have strong thermal and magnetic field responses that agree with expectations for all-in-all-out magnetic order.   

Topological Domain-Wall Metals in Pyrochlore Iridates

Emergent quantum phases [1-3] of pyrochlore iridium oxides $R_{\mathrm{2}}$Ir$_{\mathrm{2}}$O$_{\mathrm{7}}$ ($R$: rare-earth elements) have attracted a broad interest. Previous theoretical studies have predicted Weyl semimetals in non-collinear magnetic phases, called the all-in-all-out (AIAO) orders in Y$_{\mathrm{2}}$Ir$_{\mathrm{2}}$O$_{\mathrm{7}}$ [1]. The Weyl electrons are, however, easily annihilated in a pair [4]. Recently, we have predicted that magnetic domain walls in the AIAO phase of the pyrochlore iridium oxides host two-dimensional metallic states characterized by a zero-dimensional class A Chern number [5], even after the pair-annihilation of the Weyl electrons. By employing a symmetry adapted effective hamiltonian, we also predict a helical transport emerging from a spontaneous symmetry breaking at the magnetic domain wall as well as a subsequent metal-insulator transition [6]. [1] X. Wan, A. M. Turner, A. Vishwanath, and S. Y. Savrasov, Phys. Rev. B 83, 205101 (2011). [2] E.-G. Moon, C. Xu, Y. B. Kim, and L. Balents, Phys. Rev. Lett. 111, 206401 (2013). [3] I. F. Herbut and L. Janssen, Phys. Rev. Lett. 113, 106401 (2014). [4] K. Ueda, et al., Phys. Rev. Lett. 109, 136402 (2012). [5] Y. Yamaji and M. Imada, Phys. Rev. X 4, 021035 (2014). [6] Y. Yamaji and M. Imada, arXiv:1507.04153.   

Exotic topological states near a quantum metal-insulator transition in pyrochlore iridates

Pyrochlore iridates have attracted great interest as prime candidates that may host topologically nontrivial states, spin ice ordering and quantum spin liquid states, in particular through the interplay between different degrees of freedom, such as local moments and mobile electrons. Based on our extensive study using our high quality single crystals, we will discuss such examples, i.e. chiral spin liquid in a quadratic band touching state, Weyl semimetallic state and chiral domain wall transport nearby a quantum insulator-semimetal transition in pyrochlore iridates. References: [1] D. E. MacLaughlin et al Phys. Rev. B 92,054432 (2015) [2] Y. Machida et al, Nature 463 210 (2010) [3] T. Kondo et al, unpublished. [4] Z. Tian et al unpublished. [5] A. Sushkov et al. arXiv1507.01038 (2015) [6] Y. Yamaji et al, arXiv 1507.04153v1 (2015)   

Interplay of magnetic and electronic states in pyrochlore iridates

The pyrochlore iridates are a series of compounds undergoing antiferromagnetic ordering and metal-insulator transitions. They are of interest because they combine electron correlation effects and the potential for non-trivial band topology. We will discuss the theoretical picture of these materials, from electronic structure to magnetism and phase transitions, and how they may be controlled through applied fields and temperature. Comparison will be made between theory and recent experiments.