Spin-to-Charge Conversion (SCC) Overview

• Spin-to-charge conversion (SCC) is the process by which spin information is mapped onto charge observables using mechanisms such as the inverse spin Hall and Rashba–Edelstein effects.
• Experimental methods like ferromagnetic resonance, nonlocal spin transport, and ultrafast optical excitation reveal how geometry, symmetry, and interface properties determine conversion efficiency.
• Applications of SCC span spintronic devices, THz emitters, and quantum spin readout, with distinct figures of merit guiding performance and technological integration.

Spin-to-charge conversion (SCC) denotes a family of processes that map spin information onto charge observables. In spin transport, SCC usually means conversion of a pure spin current or spin accumulation into a charge current, voltage, or terahertz emission through spin–orbit-coupled bulk or interfacial states; in spin readout, it denotes mapping a spin state onto a charge-state distribution or a spatially separated charge distribution that can be detected electrically or optically. The term therefore spans bulk inverse spin Hall conversion, interfacial inverse Rashba–Edelstein conversion, mesoscopic spin-current detection, and charge-state-based readout of single spins in semiconductors and defects (Moharana et al., 2024, Shields et al., 2014, Pawłowski et al., 2018).

1. Core mechanisms and theoretical descriptions

In bulk-like spintronic implementations, the canonical SCC mechanism is the inverse spin Hall effect (ISHE). For a spin current density $\mathbf{J}_s$ with spin polarization $\boldsymbol{\sigma}$, the induced charge current density is written as

$$\mathbf{J}_c = \theta_\text{SH}\,\frac{2e}{\hbar}\,\mathbf{J}_s \times \boldsymbol{\sigma},$$

with $\theta_\text{SH}$ the spin Hall angle. This description underlies YIG/Au/chiral-molecule heterostructures and also serves as the reference against which chirality-induced asymmetries are identified (Moharana et al., 2024). In interfacial or strictly two-dimensional systems, SCC is instead frequently cast in inverse Rashba–Edelstein form,

$$j_c = \frac{2e}{\hbar}\,\lambda_{\text{IREE}}\,J_s,$$

where $j_c$ is a two-dimensional charge-current density and $\lambda_{\text{IREE}}$ is the inverse Rashba–Edelstein length (Cunha et al., 2024).

A common spin-current source is ferromagnetic-resonance spin pumping. In that framework the pumped spin current density is

$$\mathbf{J}_s = \frac{\hbar}{4\pi} g_{\uparrow\downarrow}\,\mathbf{m}\times\dot{\mathbf{m}},$$

with $g_{\uparrow\downarrow}$ the interfacial spin-mixing conductance and $\mathbf{m}(t)$ the unit magnetization vector. This formalism is used across YIG/Au, Py/graphene/WS$\boldsymbol{\sigma}$0, and related heterostructures to generate pure spin currents without a concomitant charge current (Moharana et al., 2024, Cunha et al., 2024).

At the theoretical level, SCC has also been formulated as a scattering problem. In a three-terminal ballistic quantum dot with strong spin–orbit coupling, a pure spin current in a single-channel probe produces an odd-in-$\boldsymbol{\sigma}$1 charge response, with

$\boldsymbol{\sigma}$2

providing a quantitative mesoscopic SCC protocol (Stano et al., 2010). In chaotic ballistic dots, a dimensionless SCC coefficient can be defined as $\boldsymbol{\sigma}$3; random-matrix analysis predicts zero ensemble average but universal mesoscopic fluctuations, thereby connecting SCC in 2DEG and graphene devices to symmetry class and channel configuration rather than to a single bulk material constant (Ramos et al., 2017).

The same conceptual structure extends to laterally pumped Rashba and topological-insulator interfaces. Floquet–NEGF calculations for microwave-driven ferromagnets predict $\boldsymbol{\sigma}$4–$\boldsymbol{\sigma}$5 for metallic Rashba interfaces, $\boldsymbol{\sigma}$6–$\boldsymbol{\sigma}$7 for lateral F/TI interfaces with perfect spin-momentum locking, and $\boldsymbol{\sigma}$8 for vertical F/TI pumping (Mahfouzi et al., 2011). These results make explicit that SCC efficiency depends not only on SOC strength, but also on geometry, dimensionality, and the spin texture of the conducting states.

2. Geometries and measurement strategies

Experimentally, SCC is accessed through a small set of recurring geometries. One class uses spin pumping in ferromagnetic resonance. In YIG/Au/chiral-molecule devices, a grounded coplanar waveguide excites YIG at $\boldsymbol{\sigma}$9 GHz, pure spin current is pumped into a $$\mathbf{J}_c = \theta_\text{SH}\,\frac{2e}{\hbar}\,\mathbf{J}_s \times \boldsymbol{\sigma},$$0 nm Au layer, and the resulting ISHE voltage is measured along a $$\mathbf{J}_c = \theta_\text{SH}\,\frac{2e}{\hbar}\,\mathbf{J}_s \times \boldsymbol{\sigma},$$1 wide, $$\mathbf{J}_c = \theta_\text{SH}\,\frac{2e}{\hbar}\,\mathbf{J}_s \times \boldsymbol{\sigma},$$2 long bar (Moharana et al., 2024). In Py/graphene/WS$$\mathbf{J}_c = \theta_\text{SH}\,\frac{2e}{\hbar}\,\mathbf{J}_s \times \boldsymbol{\sigma},$$3, a $$\mathbf{J}_c = \theta_\text{SH}\,\frac{2e}{\hbar}\,\mathbf{J}_s \times \boldsymbol{\sigma},$$4 nm Py layer is driven in an X-band cavity at $$\mathbf{J}_c = \theta_\text{SH}\,\frac{2e}{\hbar}\,\mathbf{J}_s \times \boldsymbol{\sigma},$$5 GHz and the SCC signal is detected between Ti/Au electrodes separated by $$\mathbf{J}_c = \theta_\text{SH}\,\frac{2e}{\hbar}\,\mathbf{J}_s \times \boldsymbol{\sigma},$$6 mm (Cunha et al., 2024). In TI/ferromagnet structures, the same spin-pumping logic is implemented with Co as injector and the charge response is fitted by symmetric and antisymmetric Lorentzians to separate SCC from rectification backgrounds (Longo et al., 2023).

A second class uses nonlocal spin transport. In graphene/NbSe$$\mathbf{J}_c = \theta_\text{SH}\,\frac{2e}{\hbar}\,\mathbf{J}_s \times \boldsymbol{\sigma},$$7, spins are injected nonlocally from Co contacts into graphene, diffuse into the graphene/NbSe$$\mathbf{J}_c = \theta_\text{SH}\,\frac{2e}{\hbar}\,\mathbf{J}_s \times \boldsymbol{\sigma},$$8 region, and are converted into a voltage on NbSe$$\mathbf{J}_c = \theta_\text{SH}\,\frac{2e}{\hbar}\,\mathbf{J}_s \times \boldsymbol{\sigma},$$9 contacts. Field rotation and Hanle precession allow decomposition of the signal into $\theta_\text{SH}$0-, $\theta_\text{SH}$1-, and $\theta_\text{SH}$2-polarized SCC components, thereby demonstrating omnidirectional conversion in a single device (Ingla-Aynés et al., 2022).

A third class is ultrafast. In SnBi$\theta_\text{SH}$3Te$\theta_\text{SH}$4/Co, femtosecond optical excitation of Co generates a sub-picosecond spin current into the topological insulator, and the SCC-induced current transient radiates a THz pulse. The measured THz field obeys

$\theta_\text{SH}$5

so THz emission spectroscopy becomes a probe of ultrafast SCC and of whether the relevant channels are interfacial or bulk (Rongione et al., 2022).

Mesoscopic SCC can also be read out electrically without ferromagnetic resonance. In the three-terminal quantum-dot proposal, a single-channel QPC is tuned near half transmission and acts as a spin-sensitive probe whose odd-in-$\theta_\text{SH}$6 current directly encodes the spin current (Stano et al., 2010). In quantum-dot and defect readout, the measurement geometry changes again: the “charge” variable is read by a charge sensor, a QPC, or charge-selective fluorescence after the spin state has been mapped to a charge distribution (Pawłowski et al., 2018, Shields et al., 2014).

3. Representative material platforms

The contemporary SCC literature spans bulk metals, Rashba interfaces, proximitized graphene, topological insulators, semimetal surfaces, oxides, and molecule-functionalized metals. Representative platforms and reported figures are summarized below.

System	Dominant SCC interpretation	Reported figure
YIG/Au + L-/D-AHPA	ISHE in Au modulated by chirality; chiral-induced unidirectional SCC (Moharana et al., 2024)	$\theta_\text{SH}$7 reaches up to nearly $\theta_\text{SH}$8; $\theta_\text{SH}$9 nV for L-AHPA
SLG/WS$$j_c = \frac{2e}{\hbar}\,\lambda_{\text{IREE}}\,J_s,$$0/Py	IREE at the graphene/WS$$j_c = \frac{2e}{\hbar}\,\lambda_{\text{IREE}}\,J_s,$$1 interface (Cunha et al., 2024)	$$j_c = \frac{2e}{\hbar}\,\lambda_{\text{IREE}}\,J_s,$$2 nm
Graphene/NbSe$$j_c = \frac{2e}{\hbar}\,\lambda_{\text{IREE}}\,J_s,$$3	Omnidirectional SCC of spins oriented in $$j_c = \frac{2e}{\hbar}\,\lambda_{\text{IREE}}\,J_s,$$4, $$j_c = \frac{2e}{\hbar}\,\lambda_{\text{IREE}}\,J_s,$$5, and $$j_c = \frac{2e}{\hbar}\,\lambda_{\text{IREE}}\,J_s,$$6 (Ingla-Aynés et al., 2022)	At $$j_c = \frac{2e}{\hbar}\,\lambda_{\text{IREE}}\,J_s,$$7 K, $$j_c = \frac{2e}{\hbar}\,\lambda_{\text{IREE}}\,J_s,$$8, $$j_c = \frac{2e}{\hbar}\,\lambda_{\text{IREE}}\,J_s,$$9, $j_c$0
Sb$j_c$1Te$j_c$2/Bi$j_c$3Te$j_c$4/Au/Co/Au	Surface-state IEE in a TI heterostructure (Longo et al., 2023)	$j_c$5 nm
KTaO$j_c$6(111) 2DEG	IREE at a Rashba interface (Al-Tawhid et al., 20 Feb 2025)	$j_c$7 nm at $j_c$8 K
Graphene/WSe$j_c$9	Gate-tunable SHE with no REE observed (Herling et al., 2020)	SCC length larger than $\lambda_{\text{IREE}}$0 nm

The same theme appears in semimetallic and topological surface systems. In Co/Sb/Py trilayers, spin-pumping voltages at Co and Py resonance have the same sign for a given field direction, and the SCC signal is almost independent of Sb thickness for $\lambda_{\text{IREE}}$1 nm; the text indicates that the effect persists up to $\lambda_{\text{IREE}}$2 nm. That behavior was interpreted as surface-state-mediated and IREE-like rather than as bulk ISHE (Gomes et al., 2023). In SnBi$\lambda_{\text{IREE}}$3Te$\lambda_{\text{IREE}}$4/Co, the magnetic THz amplitude is ~$\lambda_{\text{IREE}}$5, ~$\lambda_{\text{IREE}}$6, and ~$\lambda_{\text{IREE}}$7 of Co/Pt for $\lambda_{\text{IREE}}$8, $\lambda_{\text{IREE}}$9, and $$\mathbf{J}_s = \frac{\hbar}{4\pi} g_{\uparrow\downarrow}\,\mathbf{m}\times\dot{\mathbf{m}},$$0 septuple layers after renormalization, and the weak thickness dependence was used to argue that the dominant SCC is interfacial and TSS-mediated (Rongione et al., 2022).

These platforms make clear that SCC is not tied to a single microscopic origin. Bulk spin Hall conversion, interfacial Edelstein conversion, surface-state-mediated conversion, and chirality-modulated conversion can all dominate, depending on whether the active states are bulk diffusive states, Rashba-split interface bands, topological surface states, or spin-selective molecular channels.

4. Symmetry, chirality, and directionality

A defining feature of SCC is its sensitivity to symmetry. In lateral Rashba or topological-insulator interfaces, the conversion direction is fixed by spin–momentum locking, so rotating the injected spin polarization or changing the precession axis can switch SCC on or off. The Floquet–NEGF results for F/TI and F/Rashba interfaces make this explicit: lateral geometries are efficient because the pumped spin polarization can be mapped directly onto momentum imbalance along the interface, whereas vertical geometries are far less efficient even when SCC remains symmetry-allowed (Mahfouzi et al., 2011).

Chiral molecular interfaces add a different symmetry breaking. In YIG/Au functionalized with L- or D-$$\mathbf{J}_s = \frac{\hbar}{4\pi} g_{\uparrow\downarrow}\,\mathbf{m}\times\dot{\mathbf{m}},$$1-helix polyalanine, the absolute ISHE voltage differs for $$\mathbf{J}_s = \frac{\hbar}{4\pi} g_{\uparrow\downarrow}\,\mathbf{m}\times\dot{\mathbf{m}},$$2 and $$\mathbf{J}_s = \frac{\hbar}{4\pi} g_{\uparrow\downarrow}\,\mathbf{m}\times\dot{\mathbf{m}},$$3, the sign of the asymmetry flips between L and D, and a racemic monolayer restores the symmetric bare-Au behavior. The corresponding spin selectivity,

$$\mathbf{J}_s = \frac{\hbar}{4\pi} g_{\uparrow\downarrow}\,\mathbf{m}\times\dot{\mathbf{m}},$$4

reaches up to nearly $$\mathbf{J}_s = \frac{\hbar}{4\pi} g_{\uparrow\downarrow}\,\mathbf{m}\times\dot{\mathbf{m}},$$5, peaks near $$\mathbf{J}_s = \frac{\hbar}{4\pi} g_{\uparrow\downarrow}\,\mathbf{m}\times\dot{\mathbf{m}},$$6, and remains near zero for the racemic layer. Angle dependence shows that spin selectivity is maximal when the spin angular momentum is aligned with the molecular axis, so the SCC is not only chiral but vectorial (Moharana et al., 2024).

Broken interfacial symmetries can also extend the set of allowed SCC tensor components. In twisted graphene/NbSe$$\mathbf{J}_s = \frac{\hbar}{4\pi} g_{\uparrow\downarrow}\,\mathbf{m}\times\dot{\mathbf{m}},$$7, the data are consistent with $$\mathbf{J}_s = \frac{\hbar}{4\pi} g_{\uparrow\downarrow}\,\mathbf{m}\times\dot{\mathbf{m}},$$8-SCC dominated by ISHE in proximitized graphene, $$\mathbf{J}_s = \frac{\hbar}{4\pi} g_{\uparrow\downarrow}\,\mathbf{m}\times\dot{\mathbf{m}},$$9-SCC dominated by conventional IEE, and $g_{\uparrow\downarrow}$0-SCC attributed to an unconventional Edelstein component enabled by broken mirror symmetries at the twisted interface. The result is omnidirectional SCC: spins polarized along all three spatial directions are converted into a charge current flowing along a fixed in-plane direction (Ingla-Aynés et al., 2022).

The angular dependence itself can become a spectroscopic probe. At KTaO$g_{\uparrow\downarrow}$1(111), the in-plane-field dependence of the SCC voltage deviates from simple $g_{\uparrow\downarrow}$2 behavior and is fitted by

$g_{\uparrow\downarrow}$3

with $g_{\uparrow\downarrow}$4 at $g_{\uparrow\downarrow}$5 K and $g_{\uparrow\downarrow}$6 at $g_{\uparrow\downarrow}$7 K. That nontrivial behavior was interpreted as a fingerprint of the $g_{\uparrow\downarrow}$8 symmetry of the Fermi states, anisotropic spin texture, and out-of-plane spin canting (Al-Tawhid et al., 20 Feb 2025). A plausible implication is that angle-resolved SCC can function as an electrical probe of spin texture in systems where direct spectroscopic access is difficult.

5. Spin-state readout implementations

Outside spin-current transport, SCC has become a readout primitive for single spins. In an InSb nanowire quantum dot, a time-dependent Rashba interaction

$g_{\uparrow\downarrow}$9

drives opposite displacements of the $\mathbf{m}(t)$0 and $\mathbf{m}(t)$1 components of a single-electron wavefunction. A subsequent barrier pulse freezes the spatially separated charge packets in the left and right halves of the dot, so that the final charges satisfy $\mathbf{m}(t)$2 and $\mathbf{m}(t)$3. For the simulated device the maximal error is $\mathbf{m}(t)$4, giving a fidelity of $\mathbf{m}(t)$5–$\mathbf{m}(t)$6 (Pawłowski et al., 2018). Here SCC means a coherent spin-to-position and hence spin-to-charge mapping rather than a spin-current transduction.

In NV centers in diamond, SCC maps the electronic spin state onto the long-lived charge states NV$\mathbf{m}(t)$7 and NV$\mathbf{m}(t)$8. A room-temperature protocol based on spin-dependent shelving and photoionization achieved a minimum spin readout noise

$\mathbf{m}(t)$9

within a factor of three of the spin projection noise level, and charge readout fidelity $\boldsymbol{\sigma}$00 in $\boldsymbol{\sigma}$01s (Shields et al., 2014). For shallow NV centers implanted $\boldsymbol{\sigma}$02 nm below the surface, SCC remained viable for nanoscale sensing, with $\boldsymbol{\sigma}$03–$\boldsymbol{\sigma}$04, and glycerol coating improved both charge initialization and stability (Giri et al., 2022).

Electrostatic control has been proposed as a further enhancement route. An ambient-condition NV study using surface electrodes modeled SCC through electrode-induced shifts of the NV levels relative to the conduction band. That scheme predicted $\boldsymbol{\sigma}$05 optical spin contrast for a single SCC cycle, $\boldsymbol{\sigma}$06 for a three-pulse SCC sequence, and, in an idealized $\boldsymbol{\sigma}$07 limit, $\boldsymbol{\sigma}$08 for a single SCC cycle and $\boldsymbol{\sigma}$09 for three cycles (Hanlon et al., 2022). In this branch of the field, the essential metrics are not $\boldsymbol{\sigma}$10 or $\boldsymbol{\sigma}$11, but charge-state contrast, readout noise, and mapping fidelity.

6. Metrics, applications, and unresolved questions

SCC is quantified by several non-equivalent figures of merit. Bulk and interfacial spin-current devices use $\boldsymbol{\sigma}$12, $\boldsymbol{\sigma}$13, $\boldsymbol{\sigma}$14, or the SCC length $\boldsymbol{\sigma}$15; chiral interfaces additionally use the effective spin selectivity $\boldsymbol{\sigma}$16; mesoscopic detectors use $\boldsymbol{\sigma}$17; single-spin readout uses $\boldsymbol{\sigma}$18, charge-readout fidelity, or direct mapping error (Cunha et al., 2024, Moharana et al., 2024, Ramos et al., 2017, Shields et al., 2014, Pawłowski et al., 2018). The diversity of metrics reflects a genuine diversity of physical realizations rather than a notational inconsistency.

Applications follow the same branching. Chirality-induced SCC has been discussed as a spin diode or spin rectifier and as a route toward “three-dimensional functionalization” of molecule–metal spintronic interfaces (Moharana et al., 2024). Omnidirectional SCC in graphene/NbSe$\boldsymbol{\sigma}$19 suggests spin–orbit torques capable of switching or reading magnetic states pointing in any direction (Ingla-Aynés et al., 2022). TI and semimetal surface systems are being developed as SCC elements for room-temperature spin pumping and THz spintronic emitters (Longo et al., 2023, Rongione et al., 2022). In quantum technologies, NV-based SCC improves room-temperature magnetic sensing, while all-electric semiconductor implementations offer ultrafast spin readout without reservoirs or static magnetic fields (Shields et al., 2014, Pawłowski et al., 2018).

Several issues remain unsettled. One is microscopic decomposition: in many heterostructures, the measured signal can contain competing contributions from bulk ISHE, interfacial IREE, changes in spin-mixing conductance, and spin-memory-loss channels. This is explicit in chiral Au interfaces, where the open question is how much of the asymmetry comes from $\boldsymbol{\sigma}$20, effective $\boldsymbol{\sigma}$21, or Rashba-like interfacial conversion, and in graphene/NbSe$\boldsymbol{\sigma}$22, where proximitized graphene, NbSe$\boldsymbol{\sigma}$23 bulk, and NbSe$\boldsymbol{\sigma}$24 surface remain difficult to separate experimentally (Moharana et al., 2024, Ingla-Aynés et al., 2022). Another is the role of orbital structure. Graphene/WS$\boldsymbol{\sigma}$25, KTaO$\boldsymbol{\sigma}$26(111), and related systems point to strong coupling between SCC, charge transfer, orbital hybridization, and spin texture, suggesting that “spin-to-charge” can in practice involve a substantial orbital intermediary (Cunha et al., 2024, Al-Tawhid et al., 20 Feb 2025). In defect-based SCC, surface charge stability, ionization pathways, and photonic collection efficiency remain decisive constraints for shallow centers and nanoscale devices (Giri et al., 2022).

Taken together, these developments establish SCC as a unifying concept across spintronics and quantum readout. Its modern significance lies less in any single mechanism than in the fact that spin information can be transduced by bulk spin Hall physics, interfacial Edelstein physics, topological or chiral surface states, coherent mesoscopic scattering, or engineered charge-state dynamics, with the dominant channel selected by symmetry, dimensionality, and materials design.