Field-induced quantum spin disordered state in spin-1/2 honeycomb magnet Na2Co2TeO6

Spin-orbit coupled honeycomb magnets with the Kitaev interaction have received a lot of attention due to their potential of hosting exotic quantum states including quantum spin liquids. Thus far, the most studied Kitaev systems are 4d/5d-based honeycomb magnets. Recent theoretical studies predicted that 3d-based honeycomb magnets, including Na2Co2TeO6 (NCTO), could also be a potential Kitaev system. Here, we have used a combination of heat capacity, magnetization, electron spin resonance measurements alongside inelastic neutron scattering (INS) to study NCTO’s quantum magnetism, and we have found a field-induced spin disordered state in an applied magnetic field range of 7.5 T < B (⊥ b-axis) < 10.5 T. The INS spectra were also simulated to tentatively extract the exchange interactions. As a 3d-magnet with a field-induced disordered state on an effective spin-1/2 honeycomb lattice, NCTO expands the Kitaev model to 3d compounds, promoting further interests on the spin-orbital effect in quantum magnets.

Realization of quantum nanomagnets in metal-free porphyrins

Quantum nanomagnets exhibit collective quantum behaviors beyond the usual long range ordered states due to the interplay of low dimension, competing interactions and strong quantum fluctuations. Despite numerous theoretical works treating quantum magnetism, the experimental study of individual quantum nanomagnets remains very challenge, greatly hindering the development of this cutting-edge field. Here, we demonstrate an effective strategy to realize individual quantum nanomagnets in metal-free porphyrins by using combined on-surface synthesis and atom manipulation approaches, with the ultimate ability to arrange coupled spins one by one as envisioned by Richard Feynman 60 years ago. A series of metal-free porphyrin nanomagnets have been constructed on Au(111) and their collective magnetic properties have been thoroughly characterized on the atomic scale by scanning probe microscopy together with theoretical calculations. Our results reveal that the constructed S=1/2 antiferromagnets host a gapped excitation in consistent with isotropic Heisenberg antiferromagnets S=1/2 model, while the S=1 antiferromagnets with odd-number units exhibit two zero-mode end states due to quantum fluctuations. Our achieved strategy not only provides a unique testing bed to study the strongly correlated effects of quantum magnetism in purely organic materials, but expands the functionalities of porphyrins with implications for quantum technological applications.

Spin Dynamics in Quantum Sine-Gordon Spin Chains: High-Field ESR Studies

A spin-1/2 Heisenberg antiferromagnetic chain is one of the most important paradigmatic models in quantum magnetism. Its ground state is a spin singlet, while the excitation spectrum is formed by gapless fractional excitations, spinons. The presence of alternating g-tensors and/or the staggered Dzyaloshinskii-Moriya interaction results in opening the energy gap $$\varDelta \propto H^{2/3}$$ Δ ∝ H 2 / 3 , once the magnetic field H is applied. A fairly good understanding of this phenomenon was achieved in the framework of the sine-Gordon quantum-field theory, taking into account the effective transverse staggered field induced by the applied uniform field. The theory predicts solitons and antisolitons as elementary excitations, as well as their bound states, breathers. Here, I review recent high-field electron spin resonance spectroscopy studies of such systems, focusing on peculiarities of their spin dynamics in the sine-Gordon regime and beyond.

Van Hove singularity in the magnon spectrum of the antiferromagnetic quantum honeycomb lattice

In quantum magnets, magnetic moments fluctuate heavily and are strongly entangled with each other, a fundamental distinction from classical magnetism. Here, with inelastic neutron scattering measurements, we probe the spin correlations of the honeycomb lattice quantum magnet YbCl3. A linear spin wave theory with a single Heisenberg interaction on the honeycomb lattice, including both transverse and longitudinal channels of the neutron response, reproduces all of the key features in the spectrum. In particular, we identify a Van Hove singularity, a clearly observable sharp feature within a continuum response. The demonstration of such a Van Hove singularity in a two-magnon continuum is important as a confirmation of broadly held notions of continua in quantum magnetism and additionally because analogous features in two-spinon continua could be used to distinguish quantum spin liquids from merely disordered systems. These results establish YbCl3 as a benchmark material for quantum magnetism on the honeycomb lattice.