Continuum of quantum fluctuations in a three-dimensional S = 1 Heisenberg magnet

Abstract

Conventional crystalline magnets are characterized by symmetry breaking and normal modes of excitation called magnons with quantized angular momentum . Neutron scattering correspondingly features extra magnetic Bragg diffraction at low temperatures and dispersive inelastic scattering associated with single magnon creation and annihilation. Exceptions are anticipated in socalled quantum spin liquids as exemplified by the one-dimensional spin-1/2 chain which has no magnetic order and where magnons accordingly fractionalize into spinons with angular momentum /2. This is spectacularly revealed by a continuum of inelastic neutron scattering associated with two-spinon processes and the absence of magnetic Bragg diffraction. Here, we report evidence for these same key features of a quantum spin liquid in the three-dimensional Heisenberg antiferromagnet NaCaNi 2 F 7 . Through specific heat and neutron scattering measurements, Monte Carlo simulations, and analytic approximations to the equal time correlations, we show that NaCaNi 2 F 7 is an almost ideal realization of the spin-1 antiferromagnetic Heisenberg model on a pyrochlore lattice with weak connectivity and frustrated interactions. Magnetic Bragg diffraction is absent and 90% of the spectral weight forms a continuum of magnetic scattering not dissimilar to that of the spin-1/2 chain but with low energy pinch points indicating NaCaNi 2 F 7 is in a Coulomb phase. The residual entropy and diffuse elastic scattering points to an exotic state of matter driven by frustration, quantum fluctuations and weak exchange disorder.

Thermodynamics of the pyrochlore-lattice quantum Heisenberg antiferromagnet

“…4c of Ref. [82]. To compare with measured data, we notice that the neutron momentum transfer denoted in Ref.…”

“…To compare with measured data, we notice that the neutron momentum transfer denoted in Ref. [82] as (h, k, l) corresponds to (q x /(2π), q y /(2π), q z /(2π)) and thus, e.g., the vector (0, 0, 2) of Ref. [82] is the vector (0, 0, 4π) in our notations.…”

“…[82] as (h, k, l) corresponds to (q x /(2π), q y /(2π), q z /(2π)) and thus, e.g., the vector (0, 0, 2) of Ref. [82] is the vector (0, 0, 4π) in our notations. The pinch points at, e.g., Q 0 = (4π, 4π, 0) and other symmetry related points such as (0, 0, 4π) indicate that each tetrahedron has vanishing total magnetization (ice rule).…”

Experimental signatures of a three-dimensional quantum spin liquid in effective spin-1/2 Ce2Zr2O7 pyrochlore

“…26 ) and the spin-1 antiferromagnet NaCaNi 2 F 7 (ref. 27 ). Nevertheless, there is no consensus on the experimental confirmation of a QSL with spin quantum number fractionalization in a 3D pyrochlore lattice spin-1/2 magnet.…”

Goldstone modes in the emergent gauge fields of a frustrated magnet

“…Many of them show spin freezing with a transition temperature much smaller than the dominant interaction scale (which is characterised by the Curie-Weiss constant) and this freezing is plausibly attributed to weak exchange disorder. A large magnetic heat capacity C M at low temperature is characteristic arXiv:1908.02271v1 [cond-mat.str-el] 6 Aug 2019 of these systems [23][24][25][26], suggesting soft, gapless modes. The Goldstone modes described by the theory we develop here give rise to a large value for C M because the excitation speed is small if exchange disorder is weak.…”