skip to main content


This content will become publicly available on December 1, 2024

Title: Diffusive excitonic bands from frustrated triangular sublattice in a singlet-ground-state system
Abstract Magnetic order in most materials occurs when magnetic ions with finite moments arrange in a particular pattern below the ordering temperature. Intriguingly, if the crystal electric field (CEF) effect results in a spin-singlet ground state, a magnetic order can still occur due to the exchange interactions between neighboring ions admixing the excited CEF levels. The magnetic excitations in such a state are spin excitons generally dispersionless in reciprocal space. Here we use neutron scattering to study stoichiometric Ni 2 Mo 3 O 8 , where Ni 2+ ions form a bipartite honeycomb lattice comprised of two triangular lattices, with ions subject to the tetrahedral and octahedral crystalline environment, respectively. We find that in both types of ions, the CEF excitations have nonmagnetic singlet ground states, yet the material has magnetic order. Furthermore, CEF spin excitons from the tetrahedral sites form a dispersive diffusive pattern around the Brillouin zone boundary, likely due to spin entanglement and geometric frustrations.  more » « less
Award ID(s):
1917511 2116515
NSF-PAR ID:
10417050
Author(s) / Creator(s):
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;
Date Published:
Journal Name:
Nature Communications
Volume:
14
Issue:
1
ISSN:
2041-1723
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    The status of nickelate superconductors in relation to cuprate high temperature superconductors is one of the concepts being discussed in high temperature superconductivity in correlated transition metal oxides. New additions to the class of infinite layer nickelates can provide essential input relating to connections or distinctions. A recently synthesized compound Ba2NiO2(AgSe)2, which contains isolated ‘infinite layer’ NiO2planes, may lead to new insights. Our investigations have discovered that, at density functional theory mean field level, the ground state consists of an unusualegsinglet on the Ni2+ion arising from large but separate Mott insulating gaps in bothegorbitals, but with different, anti-Hund’s, spin directions of their moments. This textured singlet incorporates at the least new physics, and potentially a new platform for nickelate superconductivity, which might be of an unconventional form for transition metal oxides due to the unconventional undoped state. We include in this paper a comparison of electronic structure parameters of Ba2NiO2(AgSe)2with a better characterized infinite layer nickelate LaNiO2. We provide more analysis of thed8anti-Hund’s singlet that emerges in this compound, and consider a minimally correlated wavefunction for this singlet in an itinerant background, and begin discussion of excitations—real or virtual—that may figure into new electronic phases.

     
    more » « less
  2. Abstract

    Haldane topological materials contain unique antiferromagnetic chains with symmetry-protected energy gaps. Such materials have potential applications in spintronics and future quantum computers. Haldane topological solids typically consist of spin-1 chains embedded in extended three-dimensional (3D) crystal structures. Here, we demonstrate that [Ni(μ−4,4′-bipyridine)(μ-oxalate)]n(NiBO) instead adopts a two-dimensional (2D) metal-organic framework (MOF) structure of Ni2+spin-1 chains weakly linked by 4,4′-bipyridine. NiBO exhibits Haldane topological properties with a gap between the singlet ground state and the triplet excited state. The latter is split by weak axial and rhombic anisotropies. Several experimental probes, including single-crystal X-ray diffraction, variable-temperature powder neutron diffraction (VT-PND), VT inelastic neutron scattering (VT-INS), DC susceptibility and specific heat measurements, high-field electron spin resonance, and unbiased quantum Monte Carlo simulations, provide a detailed, comprehensive characterization of NiBO. Vibrational (also known as phonon) properties of NiBO have been probed by INS and density-functional theory (DFT) calculations, indicating the absence of phonons near magnetic excitations in NiBO, suppressing spin-phonon coupling. The work here demonstrates that NiBO is indeed a rare 2D-MOF Haldane topological material.

     
    more » « less
  3. 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
  4. null (Ed.)
    Abstract The spin- $$\frac{1}{2}$$ 1 2 kagome antiferromagnet is considered an ideal host for a quantum spin liquid (QSL) ground state. We find that when the bonds of the kagome lattice are modulated with a periodic pattern, new quantum ground states emerge. Newly synthesized crystalline barlowite (Cu 4 (OH) 6 FBr) and Zn-substituted barlowite demonstrate the delicate interplay between singlet states and spin order on the spin- $$\frac{1}{2}$$ 1 2 kagome lattice. Comprehensive structural measurements demonstrate that our new variant of barlowite maintains hexagonal symmetry at low temperatures with an arrangement of distorted and undistorted kagome triangles, for which numerical simulations predict a pinwheel valence bond crystal (VBC) state instead of a QSL. The presence of interlayer spins eventually leads to an interesting pinwheel q  = 0 magnetic order. Partially Zn-substituted barlowite (Cu 3.44 Zn 0.56 (OH) 6 FBr) has an ideal kagome lattice and shows QSL behavior, indicating a surprising robustness of the QSL against interlayer impurities. The magnetic susceptibility is similar to that of herbertsmithite, even though the Cu 2+ impurities are above the percolation threshold for the interlayer lattice and they couple more strongly to the nearest kagome moment. This system is a unique playground displaying QSL, VBC, and spin order, furthering our understanding of these highly competitive quantum states. 
    more » « less
  5. Abstract

    The emergence of ferromagnetism in materials where the bulk phase does not show any magnetic order demonstrates that atomically precise films can stabilize distinct ground states and expands the phase space for the discovery of materials. Here, the emergence of long-range magnetic order is reported in ultrathin (111) LaNiO3(LNO) films, where bulk LNO is paramagnetic, and the origins of this phase are explained. Transport and structural studies of LNO(111) films indicate that NiO6octahedral distortions stabilize a magnetic insulating phase at the film/substrate interface and result in a thickness-dependent metal–insulator transition att = 8 unit cells. Away from this interface, distortions relax and bulk-like conduction is regained. Synchrotron x-ray diffraction and dynamical x-ray diffraction simulations confirm a corresponding out-of-plane unit-cell expansion at the interface of all films. X-ray absorption spectroscopy reveals that distortion stabilizes an increased concentration of Ni2+ions. Evidence of long-range magnetic order is found in anomalous Hall effect and magnetoresistance measurements, likely due to ferromagnetic superexchange interactions among Ni2+–Ni3+ions. Together, these results indicate that long-range magnetic ordering and metallicity in LNO(111) films emerges from a balance among the spin, charge, lattice, and orbital degrees of freedom.

     
    more » « less