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arXiv:2510.25537 · Physical Review Letters

Chirality-Induced Spin Currents in a Fermi Gas

Camen A. Royse, John E. Thomas

Published Feb 26, 2026• 0 citations
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Abstract

We observe and model spin currents arising from chirality and effective spin-exchange interactions in a weakly interacting ^{6}Li Fermi gas. Chirality is introduced by a static displacement between the center of the trapped atoms and the center of an applied magnetic bowl, which produces left- or right-handed spatially varying spin rotation. Spin current is directly observed via oscillations in the centers of mass of the spin-up and spin-down components, which appear to bounce off of or pass through one another, depending on the degree of handedness and s-wave scattering length. We show that this behavior obeys a driven oscillator equation with an effective spin-dependent driving force. Our measurements demonstrate chirality-induced spin selectivity via the direction of the current flow, extending CISS phenomena to Fermi gases.

DOI
10.1103/73th-7hwt

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We observe and model spin currents arising from chirality and effective spin-exchange interactions in a weakly interacting ^{6}Li Fermi gas. Chirality is introduced by a static displacement between the center of the trapped atoms and the center of an applied magnetic bowl, which produces left- or right-handed spatially varying spin rotation. Spin current is directly observed via oscillations in the centers of mass of the spin-up and spin-down components, which appear to bounce off of or pass through one another, depending on the degree of handedness and s-wave scattering length. We show that this behavior obeys a driven oscillator equation with an effective spin-dependent driving force. Our measurements demonstrate chirality-induced spin selectivity via the direction of the current flow, extending CISS phenomena to Fermi gases.
The chiral magnetic and chiral separation effects-quantum-anomaly-induced electric current and chiral current along an external magnetic field in parity-odd quark-gluon plasma-have received intense studies in the community of heavy-ion collision physics. We show that analogous effects occur in rotating trapped Fermi gases with Weyl-Zeeman spin-orbit coupling where the rotation plays the role of an external magnetic field. These effects can induce a mass quadrupole in the atomic cloud along the rotation axis which may be tested in future experiments. Our results suggest that the spin-orbit coupled atomic gases are potential simulators of the chiral magnetic and separation effects.
We develop a quantized screw-mode mechanism for chirality-induced spin selectivity (CISS). The corresponding quantum, termed a heliciton, is a screw-symmetric environmental excitation with phase φ−qzφ-qzφ−qz, longitudinal momentum ℏq\hbar qℏq, and energy ℏΩq\hbarΩ_qℏΩq​. Its absorption and emission convert the static local chiral vertex developed in our preceding work into an inelastic resonant scattering process. In first Born approximation, absorption maps ↑,k\uparrow,k↑,k to the ↓,k+q\downarrow,k+q↓,k+q sideband, whereas emission maps ↓,k\downarrow,k↓,k to the ↑,k−q\uparrow,k-q↑,k−q sideband. The two outputs share the sampled-current overlap Jχ(k)\mathcal J_χ(k)Jχ​(k) but differ in ladder factors, final momenta, and detunings. With spectral factors S+\mathcal S_+S+​ and S−\mathcal S_-S−​, Psb=[S−−O(T)S+]/[S−+O(T)S+]P_{\rm sb}=[\mathcal S_--\mathcal O(T)\mathcal S_+]/ [\mathcal S_-+\mathcal O(T)\mathcal S_+]Psb​=[S−​−O(T)S+​]/[S−​+O(T)S+​], where O(T)=exp⁡[−ℏΩq/(kBT)]\mathcal O(T)=\exp[-\hbarΩ_q/(k_BT)]O(T)=exp[−ℏΩq​/(kB​T)]. An isolated emission or absorption resonance yields Psb≃+1P_{\rm sb}\simeq+1Psb​≃+1 or −1-1−1, respectively, in the resolved sideband sector. Reversing the screw handedness interchanges the spin identities of the two sidebands while leaving their spectral and occupation weights unchanged, and therefore reverses PsbP_{\rm sb}Psb​ at every temperature. At high temperature, absorption and emission have nearly equal occupation weights; at low temperature, absorption is exponentially suppressed while spontaneous emission remains. Liquid-nitrogen temperature can already produce a pronounced asymmetry for higher-ΩqΩ_qΩq​ modes. Thus a spatially resolved Dirac wave with spin-dependent helical conserved current couples locally to a heliciton and produces thermally weighted spin- and momentum-resolved sidebands without an ad hoc spin-dependent potential.
Spin waves carry their propagation direction in the handedness of their circularly polarized stray field, a property inaccessible to scalar magnetometers. In the Damon-Eshbach surface wave of an in-plane magnetized film, the dipolar field above the film rotates with a handedness fixed by the direction of travel. The nitrogen-vacancy (NV) centre in diamond is an intrinsic polarization analyser: a right-handed field about the NV axis drives one spin transition, and a left-handed field drives the other, in both the ground and the excited state. Here, we exploit the two excited-state transitions, which lie inside the spin-wave band of yttrium iron garnet (YIG), as a pair of chirality channels and image Damon-Eshbach waves in both directions. Because reflection from an etched boundary reverses the handedness, the incident and reflected waves appear as two lines in one spectrum and are imaged separately. In the ground state, the same rule applies, and the selected field drives Rabi oscillations, a route to chirality-selective control of the NV spin by magnons. Reading the two channels gives the direction of a spin wave at one point, without a phase reference. This advances chirality-resolved quantum sensing and provides insight into chiral spin-wave dynamics.
We use the Bogoliubov--de Gennes formalism and study the ground-state phases of trapped spin-orbit-coupled Fermi gases in two dimensions. Our main finding is that the presence of a symmetric (Rashba-type) spin-orbit coupling spontaneously induces counterflowing mass currents in the vicinity of the trap edge, i.e., $\ensuremath{\uparrow}$ and $\ensuremath{\downarrow}$ particles circulate in opposite directions with equal speed. These currents flow even in noninteracting systems, but their strength decreases toward the molecular Bose-Einstein-condensate limit, which can be achieved by increasing either the spin-orbit coupling or the interaction strength. These currents are also quite robust against the effects of asymmetric spin-orbit couplings in the xxx and yyy directions, gradually reducing to zero as the spin-orbit coupling becomes one dimensional. We compare our results with those of chiral ppp-wave superfluids and superconductors.
The chirality-induced spin selectivity (CISS) effect has been invoked to explain recent reports of differences in the time-resolved EPR signals between chiral and achiral molecules. However, the microscopic origin of these differences and their connection to CISS remains contested, particularly since these systems lack a metal interface. Here we introduce an intramolecular spinterface-like mechanism that naturally arises within donor-chiral bridge-acceptor (D--χχχB--A) complexes and quantitatively reproduces experimentally reported observed spin polarization in time-resolved EPR studies. In our two-electron Lindblad model, the photoexcited charge-transfer electron traversing the chiral bridge exchanges with the residual donor electron, which acts as a localized magnetic moment analogous to an induced magnetic moment on an electrode surface. The resulting through-bridge charge current produces an effective solenoidal field at the donor--bridge interface, breaking spin degeneracy and directional symmetry, thus enabling spin-selective transport without invoking intrinsic spin-orbit coupling on the bridge. We show that the interplay between this current-induced field, donor thermalization (which breaks time-reversal symmetry), and bridge spin mixing yields tens-of-percent polarization over realistic experimental conditions and charge-transfer time scales, matching reported CISS signatures in triads and DNA hairpins. By explicitly resolving the dependence on solenoidal coupling strength, temperature, and spin-mixing rates, the model identifies the regime in which internal spinterfaces can generate robust CISS-like spin filtering. These findings demonstrate that CISS-like signals in isolated D--χχχB--A complexes are fully compatible with a spinterface mechanism, providing a unified conceptual framework for interpreting both device-based and molecule-internal CISS platforms.
We show that an analogue to the classical Einstein-de Haas effect can appear in ultracold dipolar Fermi gases. The anisotropic nature of dipole-dipole interactions can lead to a transfer of magnetization into orbital angular momentum. Remarkably, distinct from a Bose-Einstein condensate, this transfer is accompanied by twisting motion, where individual spin components rotate in opposite directions with larger orbital angular momenta than the full system, possibly leading to easier experimental observation of the effect. This feature is induced by the deformation of the Fermi surface and the direction of the twisting motion can be controlled by an s-wave scattering length or external magnetic field, possibly providing a method of measuring scattering lengths of strongly dipolar atomic species.