Spin-Orbital von Neumann Entropy

• Spin-orbital von Neumann entropy is a metric that quantifies the entanglement between spin and orbital states by evaluating the entropy of the reduced density matrix.
• It distinguishes classical product phases from entangled quantum states by displaying discontinuous jumps or continuous peak structures across quantum phase transitions.
• Spectral generalizations of the metric reveal universal scaling laws in excitations, linking theoretical models with experimental observables such as magnetic moments and RIXS signatures.

Spin-orbital von Neumann entropy (vNE) rigorously quantifies the entanglement between spin and orbital degrees of freedom in quantum systems where each elementary constituent carries both spin and orbital states. It is universally defined as the vNE of the reduced spin (or orbital) density matrix after tracing out the complementary subsystem from the pure state density operator. This metric, and its spectral generalizations, have revealed essential features of quantum phase transitions, elementary excitation structures, and the interplay of spin-orbit interactions in both theoretical models and real materials. Spin-orbital entanglement entropy underpins phase diagrams, dynamical signatures accessible to experiment (e.g., RIXS), and offers an invariant probe of genuine strong correlation distinct from spin-coupling effects.

1. Formal Definition and Construction

Consider a quantum system described by a pure state $\vert\Psi\rangle$ in a composite Hilbert space $$\mathcal{H} = \mathcal{H}_\mathrm{spin} \otimes \mathcal{H}_\mathrm{orbital}$$. The total density matrix is $\rho_\mathrm{tot} = |\Psi\rangle\langle\Psi|$. The reduced spin density matrix is obtained by tracing out the orbital degrees,

$\rho_S = \mathrm{Tr}_O[\rho_\mathrm{tot}]\,,$

and analogously, $\rho_O = \mathrm{Tr}_S[\rho_\mathrm{tot}]$.

The spin-orbital von Neumann entropy is

$$S^0_\mathrm{vN} = -\mathrm{Tr}_S\!\big[\rho_S \log_2 \rho_S\big] = -\mathrm{Tr}_O\!\big[\rho_O \log_2 \rho_O\big]\,,$$

which vanishes if and only if $|\Psi\rangle$ factorizes. Nonzero $S^0_\mathrm{vN}$ directly quantifies the degree of non-separability between spin and orbital sectors (You et al., 2015, You et al., 2015).

For excited states $|\Psi_n\rangle$, generalization yields the spectral entropy function

$$\mathcal{S}_\mathrm{vN}(\omega) = -\sum_n \mathrm{Tr}_S \big[\rho_S^{(n)} \log_2 \rho_S^{(n)}\big]\ \delta(\omega - \omega_n)\,,$$

where $\omega_n = E_n - E_0$ (You et al., 2015, You et al., 2012).

2. Entropy in Spin-Orbital Models: Ground States and Phases

In one-dimensional SU(2)$\otimes$XXZ models with negative exchange coupling, the spin-orbital vNE differentiates classical product phases (I–IV), which have $S^0_\mathrm{vN}=0$, from entangled quantum phases. Notably:

• Majumdar-Ghosh-like dimer phase (V): At $\Delta=0$, $y<0$, ground states manifest maximal spin-orbital entanglement ($S^0_\mathrm{vN}=1$), invariant with chain length. The transition into this phase is marked by a discontinuous jump in $S^0_\mathrm{vN}$ at critical $x$ values (You et al., 2015, You et al., 2015).
• Antiferromagnetic-spin/alternating-orbital phase (III): Finite vNE emerges when orbital fluctuations are present ($\Delta>0$), showing area-law scaling. The III–V transition changes from first-order to continuous with increasing orbital exchange anisotropy; $S^0_\mathrm{vN}$ transitions smoothly, and $\partial_x S^0_\mathrm{vN}$ peaks rather than jumps.
• Orbital-dimer phase (VI): The $\Delta>0$ counterpart of the spin dimer phase, attaining a plateau $S^0_\mathrm{vN}=1$ (You et al., 2015).

In the SU(2)$\otimes$SU(2) limit ($\Delta=1$), the entanglement entropy at the I–III transition line scales extensively with system size: $S^0_\mathrm{vN} \sim \alpha L + \mathrm{const}$, indicating breakdown of mean-field factorization.

3. Behavior Across Quantum Phase Transitions

The evolution of $S^0_\mathrm{vN}$ and its spectral function across phase transitions provides a fingerprint of underlying entanglement changes:

• First-order transitions: $S^0_\mathrm{vN}$ jumps discontinuously, e.g., II$\leftrightarrow$V boundary at $\Delta=0$.
• Continuous transitions: $S^0_\mathrm{vN}$ demonstrates peak structure in its derivative, with restructuring of low-lying entangled excitations (You et al., 2015).
• In the spectral entropy $\mathcal{S}_\mathrm{vN}(\omega)$, entangled ground states correspond to level crossings, while disentangled regimes exhibit zero-entropy excitations (pure magnons, orbitons). In dimer phases, even elementary spin excitations manifest finite spin-orbital entanglement.

4. Entanglement of Excitations and Universal Scaling

Beyond ground states, the vNE of excited states reveals universal properties:

• Bound states (BS) and on-site spin–orbit excitons (SOEX) exhibit logarithmic system-size scaling: $S_\mathrm{vN}(L) \sim \log_2 L + c_0$ with $c_0$ dependent on the mode (e.g., $c_0=0$ for SOEX, $c_0<0$ for BS branches) (You et al., 2015, You et al., 2012).
• Continuum states possess vNE saturating to O(1), highlighting the critical distinction between localized bound entanglement and extended excitations.
• This log-scaling is robust under variation of orbital anisotropy and exchange parameters and is experimentally testable via spectroscopic probes such as RIXS (You et al., 2012).

5. Spin-Orbital von Neumann Entropy in Quantum Materials

In single-ion systems such as $5d^1$ compounds, the spin-orbital vNE is extracted from relativistic crystal field theory:

• The entropy is defined as

$$\Delta S_{\rm vN}^{\rm SO} = S(\rho) - S(\rho_\mathrm{spin}) - S(\rho_\mathrm{orbital})$$

where $S(\rho)$ is the total system vNE, and the difference isolates spin–orbital entanglement (García-Rojas et al., 22 Nov 2025).

• Empirical analysis shows that $\Delta S_{\rm vN}^{\rm SO}$ correlates directly with the measured effective magnetic moment $\mu_\mathrm{eff}$, not with the spin–orbit coupling strength $\xi$ or crystal field splitting $Dq$ alone.
• Table: Representative values in select perovskites and molecular systems:

Compound	$\xi_{5d}$ (eV)	$Dq$ (eV)	$p/q$	$\Delta S^{SO}(\Gamma_8^-)$	$\Delta S^{SO}(\Gamma_8^+)$	$\Delta S^{SO}(\Gamma_8^{cd})$
K$_2$TaCl$_6$	0.211	0.293	1.046	–0.0020	0.0205	0.0522
Ba$_2$MgReO$_6$	0.584	0.498	0.787	–0.0115	0.0882	0.2112
Ba$_2$NaOsO$_6$	0.614	0.538	0.778	–0.0114	0.0874	0.2094

A plausible implication is that spin-orbital entanglement, as rigorously quantified by $\Delta S_{\rm vN}^{\rm SO}$, is tied to observable magnetic properties even for single-ion systems, and cannot be inferred solely from SOC strength (García-Rojas et al., 22 Nov 2025).

6. Spin-Free Orbital Entropy: Invariance and Correlation Diagnostics

In quantum chemistry, particularly in density matrix renormalization group (DMRG) frameworks, the raw spin-orbital entropy can conflate static spin-coupling (multiplet) effects with genuine spatial multireference correlation. Spin-free orbital entropy, defined by collapsing spin-resolved microstates into aggregated occupation states, yields quantities invariant with respect to spin projection $M_s$. This enables:

• Extraction of a three-state local basis entropy per orbital (occupied, singly-occupied, doubly-occupied).
• $M_s$-independent mapping of spatial correlation, discriminating true strong correlation from mere spin-coupling.
• Mututal information and pair entropy measures that robustly identify entangled active spaces (Pittner, 7 Feb 2025).

7. Experimental Access and Broader Context

Spin-orbital vNE and its spectral analogs underpin experimental identification of entangled excitations. Resonant inelastic x-ray scattering protocols directly probe dynamical spin-orbital correlation functions, which couple uniquely to entangled bound states with large vNE (You et al., 2012). Material-specific studies leveraging optical and magnetic measurements combined with crystal-field theory extract vNE fingerprints in transition-metal systems (García-Rojas et al., 22 Nov 2025).

This metric, whether in condensed-matter chains, quantum chemistry active spaces, or single-ion environments, serves as a universal quantifier of spin-orbital entanglement, illuminating underlying quantum mechanisms and emergent physical observables.