Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H2: Emergent Topological Phenomena in Pyrochlore Iridates IIInvited

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Sponsoring Units: DCMP DMP GMAG Chair: ZhiXun Shen, Stanford Univ Room: Ballroom II 
Tuesday, March 15, 2016 2:30PM  3:06PM 
H2.00001: Metalinsulator transitions of bulk and domainwall states in pyrochlore iridates. Invited Speaker: Kentaro Ueda 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 metalinsulator 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 insulatormetalinsulator transition is observed near the paramagnetic insulatormetal phase boundary reminiscent of a firstorder 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 Ir5$d$ 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 GrantinAid for Scientific Research (Grants No. 80609488 and No. 24224009). [1] W. WitczakKrempa, G. Chen, Y. B. Kim, and L. Balents, Annu. Rev. Condens. Matter Phys. \textbf{5}, 57 (2014). [2] Eric Yue Ma, YongTao Cui, Kentaro Ueda, Shujie Tang, Kai Chen, Nobumichi Tamura, Phillip M. Wu, Jun Fujioka, Yoshinori Tokura, and ZhiXun 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). [Preview Abstract] 
Tuesday, March 15, 2016 3:06PM  3:42PM 
H2.00002: Mobile metallic domain walls in an allinallout magnetic insulator Invited Speaker: Eric Yue Ma 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 allinallout magnetic order, via transport and spatially resolved microwave impedance microscopy. The domain walls have a virtually temperatureindependent 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 allinallout magnetic order. [Preview Abstract] 
Tuesday, March 15, 2016 3:42PM  4:18PM 
H2.00003: Topological DomainWall Metals in Pyrochlore Iridates Invited Speaker: Youhei Yamaji Emergent quantum phases [13] of pyrochlore iridium oxides $R_{\mathrm{2}}$Ir$_{\mathrm{2}}$O$_{\mathrm{7}}$ ($R$: rareearth elements) have attracted a broad interest. Previous theoretical studies have predicted Weyl semimetals in noncollinear magnetic phases, called the allinallout (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 twodimensional metallic states characterized by a zerodimensional class A Chern number [5], even after the pairannihilation 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 metalinsulator 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. [Preview Abstract] 
Tuesday, March 15, 2016 4:18PM  4:54PM 
H2.00004: Exotic topological states near a quantum metalinsulator transition in pyrochlore iridates Invited Speaker: Zhaoming Tian 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 insulatorsemimetal 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) [Preview Abstract] 
Tuesday, March 15, 2016 4:54PM  5:30PM 
H2.00005: Interplay of magnetic and electronic states in pyrochlore iridates Invited Speaker: Leon Balents The pyrochlore iridates are a series of compounds undergoing antiferromagnetic ordering and metalinsulator transitions. They are of interest because they combine electron correlation effects and the potential for nontrivial 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. [Preview Abstract] 
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