Tunable spin-phonon polarons in a chiral molecular qubit framework

Chiral structures that produce asymmetric spin-phonon coupling can theoretically generate spin-phonon polarons – quasiparticles exhibiting non-degenerate spin states with phonon displacements. These quasiparticles are speculated to be the origin of chirality-induced spin selectivity and presumably can display exotic dynamic behaviors. However, direct experimental evidence of spin-phonon polarons has been lacking. Using a chiral molecular qubit framework embedding stable semiquinone-like radicals, we report spin dynamic signatures that indicate the formation of spin-phonon polarons for the first time. Our non-adiabatic model reveals that these quasiparticles introduce an active spin relaxation channel when polaron reorganization energy approaches Zeeman splitting. This new channel manifests itself as anomalous, temperature-independent spin relaxation, which can be suppressed by high magnetic fields or pore-filling solvents (e.g. CH2Cl2, CS2). Such field- and guest-tunable relaxation is unattainable in conventional spin systems. Harnessing this mechanism could boost repetition rates in spin-based quantum information technologies without compromising coherence or quantum sensing performance.

💡 Research Summary

The authors report the first experimental evidence for spin‑phonon polarons—quasiparticles in which opposite spin states are coupled to distinct lattice displacements—in a chiral molecular‑qubit framework. The material, Zn₃(HOTP), is a three‑dimensional metal‑organic framework built from Zn²⁺ ions and 2,3,6,7,10,11‑hexaoxytriphenylene (HOTP) ligands. Spontaneous oxidation of a small fraction of the HOTP ligands creates semiquinone‑like radicals that act as stable organic spin qubits. Structural analysis (continuous‑rotation electron diffraction, EXAFS, second‑harmonic generation) confirms a non‑centrosymmetric P6₃ space group, a 1.6 nm cylindrical pore system, and an incommensurate modulation along the c‑axis. Raman and FT‑IR spectroscopy reveal low‑frequency phonons (<200 cm⁻¹) and hydrogen‑bond vibrations that are likely to couple strongly to the radical spins.

Continuous‑wave EPR detects two spin species: a Curie‑like component (g = 2.00341) whose susceptibility follows a conventional temperature dependence, and a temperature‑independent paramagnetic (TIP) component (g = 2.00237) attributed to delocalized electrons moving along the π‑stacked HOTP columns. Electrical measurements (σ ≈ 0.46 S cm⁻¹) support the presence of itinerant charge carriers. Quantitative EPR indicates that only ~1 % of the ligands form small polarons (localized radicals) that retain quantum coherence, whereas the majority form large Fröhlich‑type polarons that decohere too rapidly to be observed in pulsed experiments.

Pulsed EPR on the coherent small polarons shows an unusually short spin‑lattice relaxation time (T₁) that is almost temperature‑independent: 31.9 µs at 13 K, decreasing to ~4 µs at 295 K. Raising the static magnetic field dramatically prolongs T₁ (to 3.39 ms at 1.22 T and 1.59 ms at 3.34 T) and introduces a clear temperature dependence, indicating the coexistence of a field‑dependent direct process and field‑independent two‑phonon (Raman and local‑mode) processes. This behavior deviates markedly from that of previously studied stable organic radical qubits.

To rationalize the anomalous relaxation, the authors develop a non‑adiabatic microscopic model based on a polaron transformation and Fermi’s golden rule, analogous to Marcus theory for electron transfer. The model predicts that when the polaron reorganization energy (λ) approaches the Zeeman splitting (ΔE_Z) of the two spin states, a new relaxation channel opens: a spin‑phonon polaron state in which each spin is associated with a distinct phonon displacement. This channel yields temperature‑independent relaxation because the energy gap between the spin‑polarized polaron states matches the phonon bath, allowing efficient energy exchange irrespective of thermal occupation.

Experimentally, the new channel can be switched off by either (i) increasing the magnetic field, which detunes λ from ΔE_Z, or (ii) filling the framework pores with non‑polar organic solvents such as dichloromethane, tetrahydrofuran, or carbon disulfide. Guest molecules modify the low‑frequency phonon spectrum and reduce the effective spin‑phonon coupling, thereby suppressing the polaron‑mediated relaxation and restoring longer T₁ values.

The work establishes three key advances: (1) direct experimental validation that chiral crystals can generate asymmetric spin‑phonon coupling leading to spin‑phonon polarons; (2) demonstration that the polaron reorganization energy can be engineered to match Zeeman splitting, providing a tunable “on/off” switch for spin relaxation; and (3) a practical route to accelerate spin‑based quantum information protocols—by deliberately invoking the fast relaxation channel to increase repetition rates—while preserving coherence when the channel is disabled. The authors suggest that further exploration of other chiral frameworks, systematic guest‑induced phonon engineering, and integration with quantum devices could unlock new functionalities for spintronics, quantum sensing, and chiral‑induced spin selectivity phenomena.