Zeeman Quantum Metric Dipole

• Zeeman quantum metric dipole is a mixed position–spin geometric response that underpins the intrinsic spin magnetononlinear Hall effect with antisymmetric bilinear conductivity.
• It is constructed from interband position and spin matrix elements, distinguishing it from conventional quantum metric dipoles that involve only orbital responses.
• Its duality with the Zeeman Berry curvature delineates intrinsic (τ⁰) and extrinsic (τ-linear) spin responses, broadening its applicability across centrosymmetric and noncentrosymmetric materials.

Searching arXiv for papers on Zeeman quantum geometry and related quantum metric dipole responses. The Zeeman quantum metric dipole is a mixed spin–orbital geometric dipole that appears in the intrinsic bilinear electromagnetic response of Bloch electrons, specifically in the current $j_a=\sigma_{ab,c}E_bB_c$ generated by combined electric and magnetic fields. In the classification of electromagnetic responses by quantum geometry, it is defined from interband position and spin matrix elements through

$$\mathcal Q^{ac}_{nm}=\mathrm{Re}\!\left[r^a_{nm}\sigma^c_{mn}\right],$$

and enters the conductivity through the dipole structure $\mathcal Q^{ac}_{nm}v_n^b$. Its defining transport role is as the intrinsic, $\tau^0$, antisymmetric-in-$a,b$ geometric coefficient responsible for the spin magnetononlinear Hall effect (spin MNHE), completing the spin-sector counterpart of the familiar quantum-metric and Berry-curvature classifications of nonlinear electric response (Xiang et al., 3 Oct 2025).

1. Definition and geometric construction

The Zeeman quantum metric dipole is built from the interband spin matrix elements

$\sigma^c_{nl}=\langle n|\hat{\sigma}^c|l\rangle$

and interband position matrix elements

$r^b_{nl}=\langle n|i\partial_b|l\rangle.$

From these, the Zeeman quantum metric is defined as

$$\mathcal Q^{bc}_{nl}\equiv \mathrm{Re}\!\left[r^b_{nl}\sigma^c_{ln}\right].$$

The corresponding dipole structure relevant to transport is $\mathcal Q^{ac}_{nm}v_n^b$, or equivalently the antisymmetrized conductivity form built from $\mathcal Q^{ac}_{nm}v_n^b$ and $$\mathcal Q^{ac}_{nm}=\mathrm{Re}\!\left[r^a_{nm}\sigma^c_{mn}\right],$$0 (Xiang et al., 3 Oct 2025).

This object differs fundamentally from the conventional quantum metric dipole. The ordinary quantum metric is constructed from two position/Berry-connection matrix elements,

$$\mathcal Q^{ac}_{nm}=\mathrm{Re}\!\left[r^a_{nm}\sigma^c_{mn}\right],$$1

whereas the Zeeman quantum metric replaces one of those two geometric “legs” by a spin operator matrix element. It is therefore a mixed position–spin geometry, not a purely orbital one. In that sense, the Zeeman quantum metric dipole is not an alternative notation for the ordinary quantum metric dipole, but a distinct mixed spin–orbital geometric multipole (Xiang et al., 3 Oct 2025).

A broader geometric formulation casts this structure as part of Zeeman quantum geometry, where momentum-space translations are combined with spin rotations. In that setting, the relevant mixed tensor is the Zeeman geometric tensor $$\mathcal Q^{ac}_{nm}=\mathrm{Re}\!\left[r^a_{nm}\sigma^c_{mn}\right],$$2, whose real part is the Zeeman quantum metric,

$$\mathcal Q^{ac}_{nm}=\mathrm{Re}\!\left[r^a_{nm}\sigma^c_{mn}\right],$$3

This generalization extends ordinary quantum geometry to situations in which spin degrees of freedom participate directly in the geometric response (Ezawa, 5 Dec 2025).

2. Role in bilinear electromagnetic transport

The Zeeman quantum metric dipole arises in the bilinear conductivity tensor $$\mathcal Q^{ac}_{nm}=\mathrm{Re}\!\left[r^a_{nm}\sigma^c_{mn}\right],$$4 of the electromagnetic response

$$\mathcal Q^{ac}_{nm}=\mathrm{Re}\!\left[r^a_{nm}\sigma^c_{mn}\right],$$5

Within the density-matrix derivation based on the quantum Liouville equation solved to second order in the fields, the bilinear response separates into a term linear in the relaxation time $$\mathcal Q^{ac}_{nm}=\mathrm{Re}\!\left[r^a_{nm}\sigma^c_{mn}\right],$$6 and a $$\mathcal Q^{ac}_{nm}=\mathrm{Re}\!\left[r^a_{nm}\sigma^c_{mn}\right],$$7-independent intrinsic term. The intrinsic spin bilinear conductivity is built from the Zeeman quantum metric dipole and is explicitly antisymmetric under $$\mathcal Q^{ac}_{nm}=\mathrm{Re}\!\left[r^a_{nm}\sigma^c_{mn}\right],$$8. Because of that antisymmetry, it generates a Hall-like transverse current identified as the spin magnetononlinear Hall effect (Xiang et al., 3 Oct 2025).

The Zeeman quantum metric dipole is therefore the spin analog of the geometric mechanism that drives the intrinsic nonlinear Hall effect in purely electric response. In the $$\mathcal Q^{ac}_{nm}=\mathrm{Re}\!\left[r^a_{nm}\sigma^c_{mn}\right],$$9 problem, the ordinary quantum metric dipole governs intrinsic nonlinear Hall-type transport. In the $\mathcal Q^{ac}_{nm}v_n^b$0 problem, the mixed position–spin Zeeman quantum metric dipole plays the corresponding role for the intrinsic spin channel (Xiang et al., 3 Oct 2025).

This classification places the Zeeman quantum metric dipole within a larger response taxonomy:

Geometric object	Response channel	Sector
Quantum metric quadrupole	Orbital MNHE	Intrinsic
Zeeman quantum metric dipole	Spin MNHE	Intrinsic
Berry curvature quadrupole	Orbital PHE	Extrinsic
Zeeman Berry curvature dipole	Spin PHE / bilinear magnetoresistance	Extrinsic

A notable additional result is that the ordinary Hall effect due to the Lorentz force can also include an interband contribution from the quantum metric quadrupole, indicating that the bilinear $\mathcal Q^{ac}_{nm}v_n^b$1 response cannot be reduced to purely semiclassical orbital deflection (Xiang et al., 3 Oct 2025).

3. Duality with Zeeman Berry curvature and response symmetry

The natural partner of the Zeeman quantum metric is the Zeeman Berry curvature

$\mathcal Q^{ac}_{nm}v_n^b$2

The two form a real/imaginary dual pair, directly analogous to the standard metric/Berry-curvature duality of ordinary quantum geometry. In the bilinear electromagnetic response, this duality is operational: the Zeeman quantum metric dipole controls the intrinsic spin bilinear current, while the Zeeman Berry curvature dipole controls the extrinsic spin bilinear current (Xiang et al., 3 Oct 2025).

The $\mathcal Q^{ac}_{nm}v_n^b$3-linear extrinsic spin contribution is symmetric under $\mathcal Q^{ac}_{nm}v_n^b$4, and for that reason it is associated not with spin MNHE but with the spin planar Hall effect (spin PHE), or more generally bilinear magnetoresistance under coplanar fields. By contrast, the intrinsic Zeeman-quantum-metric channel is antisymmetric and therefore Hall-like. The resulting “quantum geometric duality” is:

• Zeeman quantum metric dipole $\mathcal Q^{ac}_{nm}v_n^b$5 intrinsic spin MNHE
• Zeeman Berry curvature dipole $\mathcal Q^{ac}_{nm}v_n^b$6 extrinsic spin PHE / bilinear magnetoresistance

This duality is central because it distinguishes not only two geometric tensors, but also two transport regimes: $\mathcal Q^{ac}_{nm}v_n^b$7 intrinsic Hall-type response versus $\mathcal Q^{ac}_{nm}v_n^b$8-linear extrinsic planar-Hall-type response (Xiang et al., 3 Oct 2025).

A broader theoretical extension further shows that the underlying Zeeman quantum geometric tensor

$\mathcal Q^{ac}_{nm}v_n^b$9

is generically non-Hermitian. It admits a decomposition into normal and anomalous metric-like and curvature-like sectors rather than the conventional Hermitian split into a single metric and a single Berry curvature. In that framework, the normal sector reduces to conventional Hermitian geometry, whereas the anomalous sector contains an imaginary symmetric metric-like tensor and a real antisymmetric curvature-like tensor with no counterpart in the standard quantum geometric tensor (Cui et al., 9 Apr 2026). This does not replace the bilinear-response definition of the Zeeman quantum metric dipole, but situates it in a wider non-Hermitian Zeeman-geometry setting.

4. Symmetry properties and material eligibility

The bilinear response tensor is treated as a rank-3 pseudotensor constrained by spatial symmetries and time reversal, with the transformation rule

$\tau^0$0

Within this symmetry framework, the Zeeman quantum metric dipole is $\tau^0$1-even, $\tau^0$2-even, and even under $\tau^0$3 (Xiang et al., 3 Oct 2025).

These parity assignments have direct consequences for material selection. Because the Zeeman quantum metric dipole is $\tau^0$4-even, the spin MNHE can appear in both centrosymmetric and noncentrosymmetric materials. Because it is $\tau^0$5-even, it is allowed in time-reversal-symmetric as well as magnetic settings. This sharply contrasts with the Zeeman Berry curvature dipole, which is odd under $\tau^0$6 and $\tau^0$7, and therefore characterizes a symmetry-distinct extrinsic spin PHE channel (Xiang et al., 3 Oct 2025).

The symmetry content is significant because quantum metric dipoles in other contexts are often strongly constrained by inversion and time-reversal. For example, in nonmagnetic Cd$\tau^0$8As$\tau^0$9, the ordinary quantum metric dipole vanishes unless inversion and time-reversal constraints are lifted; there, realistic strain breaks inversion and the magnetic field supplies time-reversal breaking, enabling a finite ordinary QMD and a time-reversal-odd nonlinear Hall response (Zhao et al., 10 Aug 2025). The Zeeman quantum metric dipole follows a different symmetry logic: it is already even under both $a,b$0 and $a,b$1, and its allowed material space is correspondingly broader (Xiang et al., 3 Oct 2025).

5. Model realization and experimental signatures

A concrete model realization appears on the surface Dirac cone of a three-dimensional topological insulator, described by the tilted surface Hamiltonian

$a,b$2

For this system, an in-plane magnetic field induces a regime in which the orbital planar Hall effect is suppressed, leaving the spin PHE as the dominant bilinear response. The allowed conductivity takes the form

$a,b$3

and this response is governed by the Zeeman Berry curvature dipole rather than the Zeeman quantum metric dipole (Xiang et al., 3 Oct 2025).

Although this topological-insulator example illustrates the extrinsic spin channel, it is important within the same classification structure because the Zeeman quantum metric dipole defines the intrinsic spin MNHE partner. The authors further report that the resulting spin PHE can be “quantized” in the sense of being independent of the chemical potential above and below the charge-neutral point, and that it can produce a sizable measurable Hall voltage. The example therefore provides an experimentally accessible fingerprint of Zeeman quantum geometry more broadly, even though the observed channel is the Zeeman-Berry-curvature partner rather than the intrinsic Zeeman-quantum-metric channel itself (Xiang et al., 3 Oct 2025).

A plausible implication is that experimental identification of the intrinsic Zeeman-quantum-metric channel will require disentangling antisymmetric spin MNHE contributions from symmetric spin PHE contributions. That separation is naturally suggested by the tensor symmetry under $a,b$4, which distinguishes the intrinsic and extrinsic spin sectors at the level of response form (Xiang et al., 3 Oct 2025).

6. Relation to neighboring quantum-geometric responses

The Zeeman quantum metric dipole belongs to a broader family of quantum-geometric multipoles, but it is not interchangeable with them. Several nearby concepts are frequently conflated.

First, it is distinct from the ordinary quantum metric dipole that governs intrinsic nonlinear Hall effects in purely electric $a,b$5 response. Light-controlled Berry-dipole semimetals provide an example of that ordinary channel: circularly polarized light can induce an asymmetric off-diagonal quantum metric, generate a tunable ordinary QMD, and reverse the sign of the nonlinear Hall response beyond a threshold amplitude around $a,b$6 (Chowdhury et al., 5 Jun 2026). That mechanism is conceptually analogous in the sense that an external control parameter produces a momentum-space asymmetry in a metric tensor, but it is not a Zeeman quantum metric dipole.

Second, it is distinct from magnetic-field-induced ordinary QMD in nonmagnetic Dirac semimetals. In Cd$a,b$7As$a,b$8, Zeeman splitting and orbital field coupling reshape the ordinary quantum metric distribution and generate a finite QMD, which then drives a time-reversal-odd nonlinear Hall response. That work concerns magnetic-field tuning of the conventional metric dipole rather than the mixed position–spin Zeeman quantum metric dipole (Zhao et al., 10 Aug 2025).

Third, it is distinct from the quantum metric quadrupole. In few-layer WTe$a,b$9, the observed third-order longitudinal nonlinear response is attributed to the quantum metric quadrupole, with cubic current scaling $\sigma^c_{nl}=\langle n|\hat{\sigma}^c|l\rangle$0 and persistence up to room temperature. That mechanism concerns higher spatial moments of the ordinary quantum metric and does not support a Zeeman quantum metric dipole interpretation (Liu et al., 22 Jan 2025).

Fourth, it should not be confused with ordinary quantum-metric transport in purely electric settings. The longitudinal nonreciprocal DC current driven by a quantum-metric dipole in shifted quasiequilibrium (Kitamura et al., 2 Jul 2026), the orbital magneto-electric effect generated by a quantum-metric-controlled nonequilibrium dipole (Cullen et al., 5 May 2025), the linear displacement current solely determined by the quantum metric (Xiang et al., 2023), and nonreciprocal directional dichroism induced by the quantum metric dipole (Gao et al., 2018) all concern conventional metric geometry or orbital dipole mechanisms rather than the mixed spin–orbital Zeeman object.

In this comparative landscape, the Zeeman quantum metric dipole is specifically the mixed tensorial moment

$\sigma^c_{nl}=\langle n|\hat{\sigma}^c|l\rangle$1

appearing in the intrinsic part of the bilinear $\sigma^c_{nl}=\langle n|\hat{\sigma}^c|l\rangle$2 conductivity and responsible for the spin magnetononlinear Hall effect (Xiang et al., 3 Oct 2025). Its significance lies in extending quantum-geometric response classification from purely electric nonlinear transport to electromagnetic cross transport with explicit spin Zeeman coupling.