Quantum ‘spin Liquid’ Shows Unexpected Behaviour Defying Current Simulations

Scientists are increasingly focused on understanding dynamical correlations in quantum materials, yet simulating these behaviours remains a significant challenge. A. Scheie, J. Willsher, and E. A. Ghioldi, alongside Kevin Wang from the University of California, Berkeley, and et al., present a real-space analysis of time-dependent van Hove spin correlations in the 2D triangular antiferromagnet KYbSe, utilising high-resolution neutron spectroscopy. Their work reveals non-linear, sub-ballistic low-temperature transport within the material, a phenomenon currently uncaptured by existing theoretical simulations. This observation suggests the emergence of collective hydrodynamic behaviour, potentially linked to a spin liquid phase, and importantly establishes KYbSe as a crucial benchmark system for advancing the capabilities of quantum simulation techniques.

Emergent Hydrodynamics and Quantum Spin Liquid Behaviour in KYbSe2 are observed

Scientists have uncovered a surprising dynamic in the quantum magnet KYbSe2, revealing non-linear sub-ballistic low-temperature transport that defies explanation by current theoretical models. This research, detailed in a recent publication, focuses on analysing real-space time-dependent van Hove spin correlations within KYbSe2 using high-resolution Fourier-transformed neutron spectroscopy.
The team meticulously compared these experimental findings with results from five distinct theoretical simulations based on the material’s established spin Hamiltonian. Their analysis demonstrates a behaviour suggesting emergent collective hydrodynamics, potentially linked to the quantum critical phase of a quantum spin liquid, and establishes a crucial benchmark for advancing quantum simulations.

The study centres on KYbSe2, a two-dimensional triangular antiferromagnet with nearly isotropic Heisenberg spin interactions and a J2/J1 ratio of 0.044(5). Despite exhibiting weak magnetic order below 290 mK, the material’s inelastic neutron spectrum is dominated by a diffuse signal with a defined lower bound, indicating a departure from conventional magnon physics.

Researchers extracted the real-space magnetic Van Hove correlation function G(r, t) from inelastic neutron scattering data collected at 0.3 K. This function was then compared against G(r, t) data generated from five theoretical approaches: linear spin wave theory, Schwinger boson theory, classical Landau-Lifshitz dynamics, random phase approximation simulations, and Matrix Product Simulations.

The core discovery lies in the observation of a non-linear light cone in the experimental data at 0.3 K, a feature absent in all five theoretical simulations. This light cone, analogous to those found in special relativity, delineates the boundary between static and dynamic regions, revealing the fastest-moving quasiparticles within the material.

Notably, the onset of this light cone is non-uniform, with correlations evolving at the third neighbour position before the second neighbour along the b-axis. The observed behaviour suggests a collective dynamic that challenges existing theoretical frameworks and presents a compelling target for validation through future quantum simulations.

Real-space mapping of dynamical spin correlations in KYbSe2 using neutron spectroscopy reveals complex magnetic excitations

High-resolution Fourier-transformed neutron spectroscopy served as the primary experimental technique for this work, enabling the analysis of real-space time-dependent van Hove spin correlations in the 2D triangular antiferromagnet KYbSe2. The inelastic neutron scattering data, S(q, ω), were transformed into the real-space correlation function G(r, t) to facilitate comparison with theoretical simulations.

This transformation allowed the researchers to move from momentum and frequency space, typical for analysing dynamical correlations, to the real-space, real-time settings more natural for quantum simulators. Specifically, the study focused on KYbSe2, a material possessing a 2D triangular lattice of magnetic Yb3+ ions with nearly isotropic Heisenberg exchange interactions and a J2/J1 ratio of 0.044(5).

Measurements were conducted at a low temperature of 0.3 K to observe the material’s behaviour in a regime where collective effects are prominent. The resulting G(r, t) data were then compared against five distinct theoretical simulations, each based on the established spin Hamiltonian for KYbSe2. These simulations included linear spin wave theory (LSWT) calculated using SpinW software, Schwinger boson theory, classical Landau-Lifshitz (LL) dynamics modelled with SU(N)NY software, random phase approximation (RPA) simulations of a U(1) quantum spin liquid, and Matrix Product Simulations (MPS) performed on a 6-site circumference cylinder.

Each simulation employed different assumptions regarding the ground state and dynamics of the system, providing a comprehensive benchmark for the experimental results. The comparison revealed a nonlinear light cone in the experimental G(r, t) at 0.3 K, a feature absent in all five theoretical models, suggesting emergent collective hydrodynamics.

Real-space spin correlations in KYbSe2 reveal non-uniform light cone dynamics arising from strong electron correlations

Researchers obtained the real-space magnetic Van Hove correlation function G(r, t) from KYbSe₂ inelastic neutron scattering at T = 0.3 K, alongside calculations from five theoretical descriptions including linear spin wave theory, Schwinger boson theory, classical Landau-Lifshitz dynamics, random phase approximation simulations, and Matrix Product Simulations. At 0.3 K, the KYbSe₂ correlation function G(r, t) exhibited a clear “light cone” in both real and imaginary components, separating static and dynamic regions.

Interestingly, the onset of this light cone was non-uniform, with the third neighbor along b showing evolving correlations before the second neighbor along b. The study revealed a temporary inversion of r = 0 spin correlations in the real part at 0.3 K, with modulation of the spin correlation pattern at finite distances.

At elevated temperatures of 1 K and 2 K, the spin pattern fully inverted above the light cone, resulting in a pattern opposite the initial state, reminiscent of a “Higgs-mode” observed in cold-atom simulations of quantum criticality. Additionally, the Quantum Fisher Information Matrix was computed for KYbSe₂, describing quantum correlations as a function of distance and providing a benchmark for quantum computers.

Analysis of the 2D G(r, t) revealed a nonlinear light cone, a feature absent in all theoretical simulations. The imaginary component of G(r, t) for KYbSe₂ and the five theoretical simulations were plotted, identifying the onset of the light cone as the midpoint of the rise in Im[G(r, t)]. KYbSe₂ exhibited a marked upward bend around six lattice units, both along the near neighbor and next-near-neighbor directions, quantified by a dynamical exponent z = 1.4(2). This superdiffusive exponent deviates from the expected z = 1 for ballistic transport and z = 2 for diffusive transport.

Van Hove correlations reveal limitations in modelling sub-ballistic magnetic transport due to energy-dependent scattering rates

Researchers analysing the 2D triangular antiferromagnet KYbSe₂ have identified non-linear, sub-ballistic low-temperature transport, a phenomenon not currently reproduced by existing theoretical simulations. This analysis focused on real-space, time-dependent van Hove spin correlations derived from high-resolution neutron spectroscopy, offering a different perspective compared to traditional momentum and frequency-based approaches.

The observed behaviour suggests the emergence of collective hydrodynamics, potentially linked to the critical phase of a spin liquid, and establishes a valuable benchmark for validating future computational models. The key finding of sub-ballistic magnetic transport indicates a limitation in current theoretical frameworks used to describe the behaviour of quantum systems on a lattice.

The authors acknowledge that state-of-the-art high-performance computing methods struggle to accurately capture this observed behaviour. However, they propose that the signal presents an opportunity to advance and validate quantum computing technologies, particularly in the simulation of spin dynamics, alongside addressing fundamental questions in many-body physics regarding the propagation of quantum correlations. Future research could focus on developing theoretical models capable of explaining the observed non-linear light cone and exploring the potential of quantum or high-performance computing simulations to replicate these findings.