Quantum spin helices more stable than the ground state: onset of helical protection

Abstract

Topological magnetic structures are promising candidates for resilient information storage. An elementary example are spin helices in one-dimensional easy-plane quantum magnets. To quantify their stability, we numerically implement the stochastic Schrödinger equation and time-dependent perturbation theory for spin chains with fluctuating local magnetic fields. We find two classes of quantum spin helices that can reach and even exceed ground-state stability: Spin-current-maximizing helices and, for fine-tuned boundary conditions, the recently discovered "phantom helices". Beyond that, we show that the helicity itself (left- or right-rotating) is even more stable. We explain these findings by separated helical sectors and connect them to topological sectors in continuous spin systems. The resulting helical protection mechanism is a promising phenomenon towards stabilizing helical quantum structures, e.g., in ultracold atoms and solid state systems. We also identify an - up to our knowledge - previously unknown new type of phantom helices.

6+4 pages, 3 figures; version 2: minor updates, additional references

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• Exact NESS of XXZ circuits boundary driven with arbitrary resets or fields
• Generalized Josephson effect with arbitrary periodicity in quantum magnets
• Universal scaling of higher-order cumulants in quantum isotropic spin chains