More Like this

1. Multiphase magnetism in Yb 2 Ti 2 O 7

   https://doi.org/10.1073/pnas.2008791117  

   Scheie, Allen ; Kindervater, Jonas ; Zhang, Shu ; Changlani, Hitesh J. ; Sala, Gabriele ; Ehlers, Georg ; Heinemann, Andre ; Tucker, Gregory S. ; Koohpayeh, Seyed M. ; Broholm, Collin ( October 2020 , Proceedings of the National Academy of Sciences)

   We use neutron scattering to show that ferromagnetism and antiferromagnetism coexist in the low T state of the pyrochlore quantum magnet${\text{Yb}}_2{\text{Ti}}_2{\mathrm{O}}_7$. While magnetic Bragg peaks evidence long-range static ferromagnetic order, inelastic scattering shows that short-range correlated antiferromagnetism is also present. Small-angle neutron scattering provides direct evidence for mesoscale magnetic structure that we associate with metastable antiferromagnetism. Classical Monte Carlo simulations based on exchange interactions inferred from$⟨111⟩$-oriented high-field spin wave measurements confirm that antiferromagnetism is metastable within the otherwise ferromagnetic ground state. The apparent lack of coherent spin wave excitations and strong sensitivity to quenched disorder characterizing${\text{Yb}}_2{\text{Ti}}_2{\mathrm{O}}_7$is a consequence of this multiphase magnetism.

    

   more » « less
2. Survival of itinerant excitations and quantum spin state transitions in YbMgGaO4 with chemical disorder

   https://doi.org/10.1038/s41467-021-25247-6  

   Rao, X. ; Hussain, G. ; Huang, Q. ; Chu, W. J. ; Li, N. ; Zhao, X. ; Dun, Z. ; Choi, E. S. ; Asaba, T. ; Chen, L. ; et al ( December 2021 , Nature Communications)

   Abstract A recent focus of quantum spin liquid (QSL) studies is how disorder/randomness in a QSL candidate affects its true magnetic ground state. The ultimate question is whether the QSL survives disorder or the disorder leads to a “spin-liquid-like” state, such as the proposed random-singlet (RS) state. Since disorder is a standard feature of most QSL candidates, this question represents a major challenge for QSL candidates. YbMgGaO 4 , a triangular lattice antiferromagnet with effective spin-1/2 Yb 3+ ions, is an ideal system to address this question, since it shows no long-range magnetic ordering with Mg/Ga site disorder. Despite the intensive study, it remains unresolved as to whether YbMgGaO 4 is a QSL or in the RS state. Here, through ultralow-temperature thermal conductivity and magnetic torque measurements, plus specific heat and DC magnetization data, we observed a residual κ 0 / T term and series of quantum spin state transitions in the zero temperature limit for YbMgGaO 4 . These observations strongly suggest that a QSL state with itinerant excitations and quantum spin fluctuations survives disorder in YbMgGaO 4 . 

   more » « less
3. Imaging antiferromagnetic domain fluctuations and the effect of atomic scale disorder in a doped spin-orbit Mott insulator

   https://doi.org/10.1126/sciadv.abi6468  

   Zhao, He ; Porter, Zach ; Chen, Xiang ; Wilson, Stephen D. ; Wang, Ziqiang ; Zeljkovic, Ilija ( November 2021 , Science Advances)

   Correlated oxides can exhibit complex magnetic patterns. Understanding how magnetic domains form in the presence of disorder and their robustness to temperature variations has been of particular interest, but atomic scale insight has been limited. We use spin-polarized scanning tunneling microscopy to image the evolution of spin-resolved modulations originating from antiferromagnetic (AF) ordering in a spin-orbit Mott insulator perovskite iridate Sr 3 Ir 2 O 7 as a function of chemical composition and temperature. We find that replacing only several percent of lanthanum for strontium leaves behind nanometer-scale AF puddles clustering away from lanthanum substitutions preferentially located in the middle strontium oxide layer. Thermal erasure and reentry into the low-temperature ground state leads to a spatial reorganization of the AF puddles, which nevertheless maintain scale-invariant fractal geometry in each configuration. Our experiments reveal multiple stable AF configurations at low temperature and shed light onto spatial fluctuations of the AF order around atomic scale disorder in electron-doped Sr 3 Ir 2 O 7 . 

   more » « less
4. Competing stripe and magnetic phases in the cuprates from first principles

   https://doi.org/10.1073/pnas.1910411116  

   Zhang, Yubo ; Lane, Christopher ; Furness, James W. ; Barbiellini, Bernardo ; Perdew, John P. ; Markiewicz, Robert S. ; Bansil, Arun ; Sun, Jianwei ( December 2019 , Proceedings of the National Academy of Sciences)

   Realistic description of competing phases in complex quantum materials has proven extremely challenging. For example, much of the existing density-functional-theory-based first-principles framework fails in the cuprate superconductors. Various many-body approaches involve generic model Hamiltonians and do not account for the interplay between the spin, charge, and lattice degrees of freedom. Here, by deploying the recently constructed strongly constrained and appropriately normed (SCAN) density functional, we show how the landscape of competing stripe and magnetic phases can be addressed on a first-principles basis both in the parent insulator YBa₂Cu₃O₆and the near-optimally doped YBa₂Cu₃O₇as archetype cuprate compounds. In YBa₂Cu₃O₇, we find many stripe phases that are nearly degenerate with the ground state and may give rise to the pseudogap state from which the high-temperature superconducting state emerges. We invoke no free parameters such as the HubbardU, which has been the basis of much of the existing cuprate literature. Lattice degrees of freedom are found to be crucially important in stabilizing the various phases.

    

   more » « less
5. Superdislocations and point defects in pyrochlore Yb2Ti2O7 single crystals and implication on magnetic ground states

   https://doi.org/10.1038/s41598-018-35283-w  

   Shafieizadeh, Zahra ; Xin, Yan ; Koohpayeh, Seyed M. ; Huang, Qing ; Zhou, Haidong ( November 2018 , Scientific Reports)

   This study reports atomic-scale characterization of structural defects in Yb₂Ti₂O_7,a pyrochlore oxide whose subtle magnetic interactions is prone to small perturbations. Due to discrepancies in the reported magnetic ground states, it has become a pressing issue to determine the nature of defects in this system. In the present study, we use atomic resolution scanning transmission electron microscopy techniques to identify the type of defects in the ytterbium titanate single crystals grown by the conventional optical floating zone (FZ) method. In addition to the known point defects of substitution Yb on Ti B-sites, extended defects such as dissociated superdislocations and anti-phase boundaries were discovered for the first time in this material. Such defects were prevalently observed in the FZ grown single crystals (of a darker color), in contrast to the stoichiometric white polycrystalline powders or high quality colorless single crystals grown by the traveling solvent floating zone technique. The lattice strains from these extended defects result in distortions of Yb-tetrahedron. A change of Ti valance was not detected at the defects. Our findings provide new insights into understanding the nature of defects that are of great importance for the physical property studies of geometrically frustrated compounds. Furthermore, this work sheds light on the complicated core structure of superdislocations that have large Burgers vectors in oxides with complex unit cells.

    

   more » « less