Anisotropy in frustrated ferromagnetic spin-1/2 chain $\beta$-TeVO$_4$: dynamical spin stripes instead of spin nematicity

Frustrated spin-1/2 chains with competing nearest- and next-nearest-neighbor interactions have attracted a lot of attention due to their rich magnetic phase diagram. The vector-chiral (VC) ground state with chiral spin-spin correlations is in the applied magnetic field succeeded by a spin-density-wave (SDW) state and finally, just before the saturation, the spin-nematic state is predicted [1]. According to the theory, the amplitude modulated correlations in the SDW phase at high magnetic field give way to bound magnon pairs, forming a long-range order of magnetic multipoles [2]. Adding the magnetic anisotropy can lead to novel magnetic phases [3], making the phase diagram even richer.
Here we focus on $\beta$-TeVO$_4$ [4,5], which has been recently recognized as a very good realization of a spin-1/2 frustrated ferromagnetic chain [6-9]. The compound undergoes a long-range magnetic ordering below $T_{N1}$ = 4.6 K and the experimentally derived phase diagram almost exactly matches the theoretically predicted diagram [6]. Yet, the VC ground state and the SDW phase are separated by an unconventional spin-stripe phase that exists between 2 and 4.5 T [6]. At $\sim$18.7 T, the SDW phase is followed by the high-field (HF) phase, with an unknown type of incommensurate magnetic order [10]. Finally, saturation is achieved by a first-order transition, occurring between 21.72 and 21.78 T, where the HF and the fully-saturated phases coexist [11].
I will present neutron-diffraction, nuclear-magnetic-resonance and muons-spin-relaxation results that provide an insight into the magnetic ordering in the HF state and, most importantly, reveal a new type of low-frequency bound-phason excitations in the spin-stripe phase [12]. In particular, we find that the HF order is of an SDW type with different alignments of magnetic moments on the neighboring chains, which is most likely driven by magnetic anisotropy. The latter is also responsible for the establishment of the spin-stipe phase, which appears static on the neutron-scattering timescale, i.e., at $\nu$ > 10 GHz, yet our results show that is, in fact, dynamical at MHz frequencies. Such a display of spin-only manifestation of the fluctuating-stripe physics [12] is unique and has, so far, been studied exclusively in the context of nematic phases in high-temperature superconductors. Our discovery, hence, draws attention to other frustrated spin-1/2 chain compounds with complicated and unresolved magnetic phase diagrams, where similar effects may occur.

[1] T. Hikihara et al., Phys. Rev. B 78, 144404 (2008).
[2] N. Shannon et al.. Phys. Rev. Lett. 96, 027213 (2006)
[3] E. Cemal et al., Phys. Rev. Lett. 120, 067203 (2018)
[4] G. Meunier et al., J. Solid. State. Chem. 6, 67 (1973).
[5] Yu Savina et al., Phys. Rev. B 84, 104447 (2011).
[6] M. Pregelj et al., Nat. Commun. 6, 7255 (2015).
[7] Yu Savina et al., Low Temp. Phys. 41, 659 (2015).
[8] F. Weickert et al., Phys. Rev. B 94, 064403 (2016).
[9] M. Pregelj et al., Phys. Rev. B 98, 094405 (2018).
[10] M. Pregelj et al., Phys. Rev. B 100, 094433 (2019).
[11] M. Pregelj et al., Phys. Rev. B 102, 081104(R) (2020).
[12] M. Pregelj et al., npj Quantum Mater. 4, 22 (2019).