Phase transitions in condensed matter physics generally have been identified with an order parameter that spontaneously breaks the symmetry of the system and develops long range order, such as for the magnetization that develops in a ferromagnet below the Curie temperature or the staggered magnetization below the Néel temperature in an antiferromagnet. With the discovery of the Integer Quantum Hall Effect, and now for Topological Insulators (TI), the traditional classification of quantum phases of matter has expanded. These latter systems lack the concept of spontaneous symmetry breaking, rather exhibiting topological order associated with the presence of nontrivial topological components that lead to gapless boundary modes with chirality. One of the essential properties of TI's is a strong spin-orbit interaction, which interaction becomes larger the heavier the element. For the well-studied *3d* systems the spin-order interaction typically does not play an essential role, which is one of the reasons attention in the condensed matter community has turned to *4d*, *5d*, and *5f* systems.

Further expansion of these new and fascinating systems now includes Topological Semimetals. In the Weyl semimetal CeAlGe, Joe Checkelsky's group at M.I.T. has discovered the particularly interesting phenomenon of Singular Angular MagnetoResistance (SAMR). For a general explanation please see the news item at https://news.mit.edu/2019/approaching-magnetic-singularity-0620, while the publication can be found at *Science* **365**, 377 (2019); (perspective by Hassinger and Meng)*.*

**Half-Heusler Topological Insulators and Related**

In the so-called Z_{2} two-dimensional and three-dimensional systems, these topologically protected metallic states are of interest not only from a fundamental point of view, but also because they have the technological potential to transform spintronics and quantum computation applications. Even more interesting are systems that exhibit both symmetry breaking order parameters and topologically nontrivial states, which can give rise to exotic collective modes and states. One such family is represented by the cubic half-Heusler materials on which we have been collaborating. The antiferromagnetism breaks time reversal and translational symmetries but preserves the combination, leading to a new type of system, the antiferromagnetic topological insulator.

**Topological RPdBi half-Heusler semimetals:**** ****a new family of non-centrosymmetric magnetic superconductors**, Y. Nakajima, R. Hu, K. Kirshenbaum, A. Hughes, P. Syers, X. Wang, K. Wang, R. Wang, S. Saha, D. Pratt, J.W. Lynn, and J. Paglione, *Science Advances* **1**, e1500242 (2015).

**Large Anomalous Hall Effect in a Half Heusler Antiferromagnet**, T. Suzuki, R. Chisnell, A. Devarakonda, Y.-T. Liu, J. W. Lynn, and J. G. Checkelsky, *Nature Physics* **12**, 1119 (2016). *Press Release*.

**Carrier Density Control of Magnetism and Berry Phases in Doped EuTiO**_{3}, Kaveh Ahadi, Zhigang Gui, Zach Porter, Jeffrey W. Lynn, Zhijun Xu, Stephen D. Wilson, Anderson Janotti, and Susanne Stemmer,, *APL Materials* **6**, 056105 (2018).

**Amplitude Mode in a Classical, Two-dimensional Triangular Antiferromagnet, ** Rebecca L. Dally, Yang Zhao, Zhijun Xu, Robin Chisnell, Matthew Stone, Jeffrey W. Lynn, Leon Balents, and Stephen D. Wilson, *Nature Communications* **9**, 2188 (2018).

**Singular Angular Magnetoresistance and Spontaneous Symmetry Breaking in a Magnetic Nodal Semimeta**l, T. Suzuki, L. Savary, J.-P. Liu, J. W. Lynn, L. Balents, and J. G. Checkelsky, *Science * (2019)*. Press Release. *

**Correlating Magnetic Structure and Magnetotransport in Semimetal Thin Films of Eu _{1-x}Sm_{x}TiO_{3}**, Zach Porter, Ryan F. Need, Kaveh Ahadi, Yang Zhao, Zhijun Xu, Brian J. Kirby, Jeffrey W. Lynn, Susanne Stemmer, and Stephen D. Wilson,

**Magnetic Phase Transitions and Spin Density Distribution in the Molecular Multiferroic GaV _{4}S_{8}**, System, Jeffrey W. Lynn, William D. Ratcliff II, Rebecca L. Dally, Markus Bleuel, Lunyong Zhang, Heung-Sik Kim, Kristjan Haule, David Vanderbilt, Sang-wook Cheong (submitted).

**5d**** sytems; Iridates**

**Topological Singularity-Induced Kohn Anomaly in a Weyl Semimetal**, Thanh Nguyen, Fei Han, Nina Andrejevic, Ricardo Pablo-Pedro, Anuj Apte, Yoichiro Tsurimaki, Zhiwei Ding, Kunyan Zhang, Ahmet Alatas, Ercan Alp, Songxue Chi, Jaime Fernandez-Baca, Masaaki Matsuda, David A. Tennant, Yang Zhao, Zhijun Xu, Jeffrey W. Lynn, Shengxi Huang, and Mingda Li, *Phys. Rev. Lett.* **124**, 236401 (2020).

**Influence of Electron-doping on the Ground State of (Sr**_{1-x}**La**_{x})_{2}**IrO**_{4}, Xiang Chen, Tom Hogan, D. Walkup, Wenwen Zhou, M. Pokharel, Mengliang Yao, Wei Tian, Thomas Z. Ward, Y. Zhao, D. Parshall, C. Opeil, J. W. Lynn, Vidya Madhavan, and Stephen D. Wilson,* Phys. Rev. * B* ** 92, 017125 (2015)*.

**Carrier Localization and Electronic Phase Separation in a doped Spin-Orbit Driven Mott Phase in Sr _{3}(Ir_{1-x}Ru_{x})_{2}O_{7},** Chetan Dhital, Tom Hogan, Wenwen Zhou, Xiang Chen, Zhensong Ren, Mani Pokharel, Yoshinori Okada, M. Heine, Wei Tian, Z. Yamani, C. Opeil, J. S. Helton, J. W. Lynn, Ziqiang Wang, Vidya Madhavan, and Stephen D. Wilson, Nature Communications 5, 3377 (2014).

**Cd-doping effects in Ce _{2}MIn_{8} (M=Rh and Ir) Heavy Fermion Compounds**, C. Adriano, C. Giles, E. M. Bitter, L. N. Coelho, F. de Bergevin, C. Mazzoll, L. Paolasini, W. Ratcliff, R. Bindel, J. W. Lynn, Z. Fisk, and P. G. Pagliuso,

**Novel Coexistence of Superconductivity with Two Distinct Magnetic Orders in Heavy Fermion Ce(Rh,Ir)In _{5}, **A. D. Christianson, A. Llobet, W. Bao, J. S. Gardner, I. P. Swainson, J. W. Lynn, J.-M. Mignot, K. Prokes, P. G. Pagliuso, N. O. Moreno, J. L. Sarrao, and A. H. Lacerda

**Covalency Effects in the Magnetic Form Factor of Ir in K**_{2}**IrCl**_{6}, J. W. Lynn, G. Shirane, and M. Blume, * Phys. Rev. Lett. 37, 154 (1976)*.

**4d**** materials; Ruthenates**

**Phase Diagram of ** **a****-RuCl**_{3}** in an in-plane Magnetic Field**, J. A. Sears, Y. Zhao, Z. Xu, J. W. Lynn, and Young-June Kim, Phys. Rev. B **95**, 180411(R) (2017).

**Spin Dynamics and Two-dimensional Correlations in an FCC Antiferromagnetic Sr**_{2}**YRuO _{6},** S. M. Disseler, J. W. Lynn, R. F. Jardim, M. S. Torikachvili, and E. Granado, Phys. Rev. B

**Two-Dimensional Magnetic Correlations and Partial Long-Range Order in Geometrically Frustrated Sr _{2}YRuO_{6}**, E. Granado, J. W. Lynn, R. F. Jardim, and M. Torikachvili,

**Spin-valve Effect and Magnetoresistivity in Single Crystalline Ca _{3}Ru_{2}O_{7}**, W. Bao, Z. Q. Mao, Z. Qu, and J. W. Lynn,

**Unusual heavy-mass nearly ferromagnetic state with a surprisingly large Wilson ratio in double layered ruthenates (Sr _{1-x}Ca_{x})_{3}Ru_{2}O_{7}**, Z. Qu, L. Spinu, H. Q. Yuan, V. Dobrosavljeviuc, W. Bao, J. W. Lynn, M. Nicklas, J. Peng, T. J. Liu, D. Fobes, E. Flesch, and Z. Q. Mao,

**Structure and Magnetism of Single Crystal Sr _{4}Ru_{3}O_{10}: A Ferromagnetic Triple-Layer Ruthenate**

**Synthesis and crystal structure of La _{3}RuO_{7}**, P. Khalifah, Q. Huang, J. W. Lynn, R. W. Erwin, and R. J. Cava, Materials Research Bulletin 35, 1 (2000).

**Oxygen Displacements and Search for Magnetic Order in Sr _{3}Ru_{2}O_{7}**, Q. Huang, J. W. Lynn, R. W. Erwin, J. Jarupatrakorn, and R. J. Cava,

**5f**** materials; Hidden order in URu**_{2}**Si**_{2}

**Orbital-selective Kondo-lattice and Enigmatic f-electrons emerging from inside the Antiferromagnetic Phase of a Heavy Fermion**, Ioannis Giannakis, Justin Leshen, Mariam Kavai, Sheng Ran, Shanta R. Saha, Y. Zhao, Z. Xu, J. W. Lynn, Nicholas P. Butch, and Pegor Aynajian,

**High temperature singlet-based magnetism from Hund's rule correlations**, Lin Miao,^{ }Rourav Basak,* *Sheng Ran, Yishuai Xu, Haowei He, Jonathan D. Denlinger, Yi-De Chuang, Y. Zhao, Z. Xu, J. W. Lynn, J. R. Jeffries, S. R. Saha, Ioannis Giannakis, Pegor Aynajian, Chang-Jong Kang, Yilin Wang, Gabriel Kotliar, Nicholas P. Butch, L. Andrew Wray, *Nature Communications*** 10, **644 (2019).

**Distinct magnetic spectra in the hidden order and antiferromagnetic phases in URu**_{2-x}**Fe**_{x}**Si**_{2}, Nicholas P. Butch, Sheng Ran, Inho Jeon, Noravee Kanchanavatee, Kevin Huang, Alexander Breindel, M. Brian Maple, Ryan L. Stillwell, Yang Zhao, Leland Harriger, and Jeffrey W. Lynn, Phys. Rev. B 94, 201102 (2016).

**Chemical Pressure Tuning of URu**_{2}**Si**_{2}** via Isoelectronic Substitution of Ru with Fe**, Pinaki Das, N. Kanchanavatee, J. S. Helton, K. Huang, R. E. Baumbach, E. D. Bauer, B. D. White, V. W. Burnett, M. B. Maple, J.W. Lynn, and M. Janoschek, * Phys. Rev. * B **91**, 085122 (2015).

**Symmetry and Correlations Underlying Hidden Order in URu**_{2}**Si**_{2}, Nicholas P. Butch, Michael E. Manley, Jason R. Jeffries, Marc Janoschek, Kevin Huang, M. Brian Maple, Ayman H. Said, Bogdan M. Leu, and Jeffrey W. Lynn, * Phys. Rev. * B **91**, 035128 (2015).

**Absence of a static in-plane magnetic moment in the "hidden-order" phase of URu**_{2}**Si**_{2}, P. Das, R. E. Baumbach, E. D. Bauer, K. Huang, M. B. Maple, Y. Zhao, J. Helton, J. W. Lynn, and M. Janoschek, *New J. Phys.* **15**, 053031(2013).

**Antiferromagnetic Critical Pressure in URu _{2}Si_{2} under Hydrostatic Conditions**, N. P. Butch, J. R. Jeffries, S. X. Chi, J. B. Leão, and J. W. Lynn, and M. B. Maple,

**Non-Fermi Liquid Behavior and Quantum Criticality in Sc _{1-x}U_{x}Pd_{3} and URu_{2-x}Re_{x}Si_{2}**, M. B. Maple, N. P. Butch, E. D. Bauer, V. S. Zapf, P.-C. Ho, S. D. Wilson , P. Dai, D. T. Adroja, S.-H. Lee, J.-H. Chung, J. W. Lynn,

**Spin Ice, Magnetic Monopoles, and Spin Liquids**

Magnetic Monopoles are spin excitaitons in spin-ice, and as there is no long range order these are also topoloigcal excitations of a kind:

**Observation of Magnetic Monopoles in Spin Ice**, Hiroaki Kadowaki, Naohiro Doi, Yuji Aoki, Yoshikazu Tabata, Taku J. Sato, J. W. Lynn, K. Matsuhira, and Z. Hiroi, J. Phys. Soc. Japan 78, 103706 (2009).

**Quantum Spin Fluctuations in the Spin Liquid State of Tb**_{2}**Ti**_{2}**O**_{7}, Hiroshi Takatsu, Hiroaki Kadowaki, Taku J. Sato, Jeffrey W. Lynn, Yoshikazu Tabata, Terno Yamakaki, and Kazuyuki Matsuhira, *J. Phys. Cond. Matr. * **24**, 052201 (2012).

**Quadrupole Order in the Frustrated Pyrochlore Tb**_{2+x}**Ti**_{2−}_{x}**O**_{7+y}, H. Takatsu, S. Onoda, S. Kittaka, A. Kasahara, Y. Kono, T. Sakakibara, Y. Kato, B. Fåk, J. Ollivier, J. W. Lynn, T. Taniguchi, M. Wakita, and H. Kadowaki, * Phys. Rev. Lett. 116, 217201 (2016).*

**Spin correlations of quantum-spin-liquid and quadrupole-ordered states of Tb**_{2+x}**Ti**_{2-x}O_{7+y}, Hiroaki Kadowaki, Mika Wakita, Björn Fåk, Jacques Ollivier, Seiko Ohira-Kawamura, Kenji M Kofu-Nakajima, and Jeffrey W. Lynn, Phys. Rev. B **99**, 014406 (2019)**.**

**Absence of Magnetic Order in the Spin Liquid Candidate Ca _{3}Cu_{2}GeV_{2}O_{12}**,

**Materializing Rival Ground States in the Barlowite Family of Kagome Magnets: Quantum Spin Liquid, Spin Ordered, and Valence Bond Crystal States****,** Rebecca W. Smaha, Wei He, Jack Mingde Jiang, Jiajia Wen, Yi-Fan Jiang, John P. Sheckelton, Charles J. Titus, Suyin Grass Wang, Yu-Sheng Chen, Simon J. Teat, Adam A. Aczel, Yang Zhao, Guangyong Xu, Jeffrey W. Lynn, Hong-Chen Jiang and Young S. Lee, *Nature Partner Journals: Quantum Materials ***5**, 23 (2020).

See Monopoles and Related Systems

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Created January 21, 2020, Updated June 15, 2020