Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session R39: Exotic Magnetic Orders and ExcitationsFocus Live

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Sponsoring Units: GMAG DMP Chair: Oleg Starykh, University of Utah 
Thursday, March 18, 2021 8:00AM  8:12AM Live 
R39.00001: Presaturation phase in the frustrated ferroantiferromagnet Pb_{2}VO(PO_{4})_{2} Florian Landolt, Simon L. Bettler, Zewu Yan, Severian Gvasaliya, Andrey Zheludev, Sanu Mishra, Ilya Sheikin, Steffen Kraemer, Mladen Horvatić, Alsu Gazizulina, Oleksandr Prokhnenko The compound Pb_{2}VO(PO_{4})_{2} belongs to the family of vanadyl phosphates and hosts magnetically almost isolated layers of V^{4+} ions which carry a spin S=½. The spins Hamiltonian approximates a square lattice with ferromagnetic nearest and anti ferromagnetic next nearest couplings. A previous neutron study has found that the antiferromagnetic ground state is less frustrated than thermodynamic measurements had initially suggested [1]. It is therefore rather surprisingly that for fields along the magnetic anisotropy axis, a new magnetic state emerges through a discontinuous transition and persists in a narrow field range just below saturation. In this talk the full HT phase diagram, determined by means of magnetization, torque, NMR and high field neutron diffraction experiments, are presented and properties of the presaturation phase are discussed [2]. 
Thursday, March 18, 2021 8:12AM  8:24AM Live 
R39.00002: Quantum phases diagram of the the ShastrySutherland compound SrCu_{2}(BO_{3})_{2} at high magnetic field and high pressure Zhenzhong Shi, Sachith Dissanayake, David E Graf, Philippe Corboz, Daniel Marc Silevitch, Hanna Dabkowska, Thomas F Rosenbaum, Frederic Mila, Sara Haravifard In the ShastrySutherland model, the ground state is expected to change from a spin dimer singlet state to a novel, intermediate 4spin plaquette singlet state, then to an antiferromagnet state, when the relative strength of the nearest neighbor and nextnearest neighbor interactions (J/J’) is tuned. SrCu_{2}(BO_{3})_{2} realizes the ShastrySutherland model at ambient pressure and can be tuned to other phases by application of high pressure, thus attracting considerable interests. Recent experimental advances have revealed the presence of an intermediate ground state with signatures of a 4spin plaquette singlet state at high pressure. However, this intermediate phase also feature signatures that clearly deviate from the prediction of the ideal ShastrySutherland model. Therefore, the nature of this plaquette phase and how it evolves into other phases remain unclear. Here, we report a comprehensive study of the quantum phase diagram of the plaquette state by tuning the pressure and magnetic field, using a highly sensitive magnetization measurement technique. Our results reveal the emergence of a sequence of ground states once the spin dimer singlet and the plaquette singlet phase are suppressed by the field, which are further corroborated by our theoretical simulations. 
Thursday, March 18, 2021 8:24AM  8:36AM Live 
R39.00003: Decay and renormalization in S=1 antiferromagnet Ba_{2}FeSi_{2}O_{7 } Hao Zhang, Seunghwan Do, Travis Williams, Tao Hong, Vasile Garlea, TaeHwan Jang, JaeHoon Park, Andy Christianson, Cristian Batista The order parameter of S=1 easyplane antiferromagnets can be characterized by a complex number. The phase fluctuations of the order parameter correspond to transverse modes (magnons), whereas amplitude fluctuations produce longitudinal modes. Due to the gapless nature of the transverse mode, the longitudinal mode is kinematically allowed to decay into pairs of transverse modes. Inelastic neutron scattering measurements on an effective S=1 tetragonal lattice compound Ba_{2}FeSi_{2}O_{7 }reveal this decay and the consequent renormalization of the longitudinal and transverse mode dispersions. By applying a generalized linear spin wave theory with oneloop corrections, we reproduce the observed decay and the renormalization of the different modes. Moreover, by including corrections beyond oneloop, we obtain a very accurate estimate of the critical singleion easyplane anisotropy required to induce a quantum phase transition into a quantum paramagnetic state. 
Thursday, March 18, 2021 8:36AM  9:12AM Live 
R39.00004: Spectral signatures of magnon interactions in antiferromagnets in high fields Invited Speaker: Anna Keselman In this talk I will discuss signatures of interactions between magnons that show up in the dynamical spin correlations of antiferromagnets in presence of a magnetic field, focusing on the high magnetization regime. I will first present our results for the antiferromagnetic spin1/2 chain, where we uncover the appearance of twomagnon bound states in the transverse dynamical susceptibility. This bound state feature generalizes the one arising from string states in the Bethe ansatz solution of the integrable case. I will then address higherdimensional systems and argue that these observations are not unique to 1D. 
Thursday, March 18, 2021 9:12AM  9:24AM Live 
R39.00005: Quantum antiferromagnets in the XY limit SANTANU PAL, Prakash Sharma, Hitesh J. Changlani, Sumiran Pujari Exactly solvable Hamiltonians form important cornerstones in our understanding of quantum magnetism. Recently a class of ground state solvable (GSS) frustrated quantum magnets on any lattice composed of triangular motifs has been reported^{1}. The exact solutions (ES) require consistent threecoloring of the lattice sites. We apply the general principle of exact coloring GSS to longrange magnetic order on the triangular and extend it to unfrustrated bipartite lattices where the ES corresponds to a twocoloring ground state (GS). For several magnetization sectors, we show how the presence of ES in the deep XY regime of the XXZ model accounts for the corresponding antiferromagnetically ordered GS. We numerically demonstrate that for square and triangular lattice, the ordered physics of the solvable points respectively at J_{z} /J_{xy} (=Δ)=1 and Δ=0.5 adiabatically extend close to the Heisenberg regime Δ∼1. These findings are contrasted with an anisotropic generalization of the MajumdarGhosh model, which is proven to have exact dimer GS and threecoloring GS at Δ=0.5, but where only the former survives as the GS for Δ>0.5. 
Thursday, March 18, 2021 9:24AM  9:36AM Live 
R39.00006: A series of magnon crystals appearing under ultrahigh magnetic fields in a kagomé antiferromagnet Ryutaro Okuma, Daisuke Nakamura, Tsuyoshi Okubo, Atsushi Miyake, Akira Matsuo, Koichi Kindo, Masashi Tokunaga, Naoki Kawashima, Shojiro Takeyama takeyama@issp.utokyo.ac.jp, Zenji Hiroi Geometrical frustration and a high magnetic field are two key factors for realizing unconventional quantum states in magnetic materials. Specifically, conventional magnetic order can potentially be destroyed by competing interactions and may be replaced by an exotic state that is characterized in terms of quasiparticles called magnons, the density and chemical potential of which are controlled by the magnetic field. Here we show that a synthetic copper mineral, Cdkapellasite, which comprises a kagomé lattice consisting of cornersharing triangles of spin1/2 Cu^{2+} ions, exhibits an unprecedented series of fractional magnetization plateaus in ultrahigh magnetic fields of up to 160 T. We propose that these quantum states can be interpreted as crystallizations of emergent magnons localized on the hexagon of the kagomé lattice. [1] Okuma, R., et al. Nature communications 10.1 (2019): 17. 
Thursday, March 18, 2021 9:36AM  9:48AM Live 
R39.00007: Frustrationdriven marginal phase transition at finite temperature in an Ising model in one dimension Weiguo Yin The Ising model, with simple shortrange interactions between constituents (e.g., spins), is a basic mathematical model in statistical mechanics for describing phase transitions in various manybody systems. It was rigorously proven that in one dimension, phase transitions do not exist in this model at any nonzero temperature because the free energy is analytic. Here I show that a family of strongly frustrated onedimensional Ising model can exhibit a firstorderlike phase transition at finite temperature via a virtual level crossing in the free energy, resulting in a large latent heat and an ultranarrow peak in the specific heat [1]. The critical temperature goes to zero as the transition width goes to zero; therefore, this ultranarrow phase transition does not violate the existing theorems and is an extension of the zerotemperature phase transition to the finite temperature regime, thus named marginal phase transition. These exact results expose a mathematical structure and unconventional order parameters that have not appeared before in phase transition problems, shedding new light not only on our understanding of phase transitions and the dynamic actions of frustration but also on onedimensional device applications. [1] W. Yin, arXiv:2006.08921; arXiv:2006.15087 (2020). 
Thursday, March 18, 2021 9:48AM  10:00AM Live 
R39.00008: Twophoton magnetic resonance as detector of magnon pairs and spin nematic orders Masahiro Sato, Yoshitaka Morisaku Spinnematic (quadrupolar) order is a typical lessvisible order in magnetic systems. Proposing an efficient experimental way of detecting spinnematic (SN) order is important to deepen the understanding of spin nematicity and frustrated magnetism. Several theoretical works have shown that in forced fully polarized states of SN ordered magnets, magnon pairs (molecule of two magnons) generally appear in addition to usual magnons. Moreover, a SN order is usually shown to occur as the result of the BoseEinstein condensation of magnon pairs in a broad class of SN magnets. Therefore, the detection of magnon pairs can provide an indirect but strong evidence for the emergence of SN order. Since magnon pairs have angular momentum 2hbar, they can be excited through twophoton absorption with strong electromagnetic (EM) waves. Based on these backgrounds, we theoretically consider the magnonpair resonance driven by strong EM wave in a fully polarized state of SN magnets. Using the quantum master equation for a nano spin model, we predict that magnonpair resonance peak can be detected with currently available terahertz laser. I explain the essential aspects of our proposal. 
Thursday, March 18, 2021 10:00AM  10:12AM Live 
R39.00009: Dynamics of a fractal set of 1st order phase transitions and comparison to XPCS data in Lu2CoMnO6 Adra Carr, John Bowlan, Claudio Mazzoli, Andi Barbour, Wen Hu, Stuart B Wilkins, Colby S Walker, Xiaxin Ding, Jonghyuk Kim, Nara Lee, Youngjai Choi, ShiZeng Lin, Richard Sandberg, Vivien Zapf The Axial Next Nearest Neighbor Ising model predicts a fractal (infinite) set of phases with incommensurate wave vectors that are separated by 1st order phase boundaries. This behavior emerges from a simple frustration condition between nearest and nextnearest neighbors along a chain of Ising spins. Here, we probe the dynamics of this scenario using Monte Carlo simulations and compare to Xray photon correlation spectroscopy (XPCS) measurements in Lu2CoMnO6. We resonantly probe the dynamics of speckle within Mn and Co L3edge antiferromagnetic Bragg peaks. Bragg peaks of incommensurate order slide towards commensurate `up up down down' spin order with decreasing temperature and increasing time. Analysis of the speckle dynamics supports a counterintuitive dependence of the dynamics on temperature that is consistent with our simulations. 
Thursday, March 18, 2021 10:12AM  10:48AM Live 
R39.00010: A zoo of presaturation phases in novel quantum ferroantiferromagnets. Invited Speaker: Andrey Zheludev The standard paradigm of magneticsaturationbyBECofmagnons breaks down close to a classical ferroantiferromagnetic transition [1]. Upon decreasing the external magnetic field the fully polarized state collapses before the single magnon instability is reached. New quantum presaturation phases with unusual long range orders may emerge, including spin density waves, nematics and chiral states. 
Thursday, March 18, 2021 10:48AM  11:00AM Live 
R39.00011: Observation of additional magnetic modulation vectors in the element Neodymium Hasitha Suriya Arachchige, Lisa DeBeerSchmitt, Andrew May, David Parker, Markus Bleuel, Ganesh Pokharel, Binod K Rai, Joseph Paddison, Wei Tian, Masaaki Matsuda, Yaohua Liu, David George Mandrus, Cristian Batista, Andrew Christianson The rare earth element neodymium exhibits a series of complex magnetic phases as a function of temperature and applied magnetic fields. Neodymium crystallizes in a double hexagonal closed packed structure consisting of triangular nets of Nd stacked along the caxis. Here we report the observation of a new set of magnetic modulation vectors through small angle neutron scattering (SANS) and singlecrystal neutron diffraction measurements. While we find distinct modulations occur in different phases, this talk focuses on the modulations occurring for 5.9(1) ≤ T ≤ 7.6(1) K and µ_{0}H ≤ 1.0(1) T for fields applied along the caxis. Within this phase, the modulation vectors are found to occur at 0.06 a* implying a characteristic length scale of ~52 Å. We discuss the possibility that these additional modulation vectors indicate the presence of frustration driven spin texture in the element Nd. 
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