Field-Free Superconducting Diodes

• Field-free superconducting diodes are defined as Josephson junction devices that display nonreciprocal, dissipationless current transport achieved by breaking inversion and time-reversal symmetries.
• Key design approaches include intrinsic symmetry breaking in layered heterostructures, spontaneous ordering in magic-angle twisted bilayer graphene, and engineered vortex effects to achieve high diode efficiencies.
• These diodes promise energy-efficient applications in superconducting logic, memory, and quantum circuits, though challenges remain in device uniformity, scalability, and integration.

Field-free superconducting diodes are Josephson junction-based or bulk superconducting devices that exhibit nonreciprocal supercurrent transport—dissipationless transport (zero resistance) in one current direction, while the opposite direction yields a resistive (normal) state—even in the absence of any externally applied magnetic field. These devices critically depend on the engineered breaking of inversion symmetry, time-reversal symmetry, or both, by intrinsic material properties, heterostructure design, circuit architecture, or extrinsic mechanisms such as strain and thermal gradients. Field-free superconducting diodes provide a platform for energy-efficient, directional superconducting electronics that circumvent the need for magnetic field biasing, thus enabling scalable logic, memory, and quantum technologies.

1. Core Physical Mechanisms of Field-Free Superconducting Diodes

The essential criterion for nonreciprocal supercurrent (diode behavior) is breaking both time-reversal symmetry (TRS) and inversion symmetry (IS). This can be realized via several orthogonal mechanisms:

• Intrinsic inversion symmetry breaking in layered or van der Waals heterostructures: For example, NbSe$_2$/Nb$_3$Br$_8$/NbSe$_2$ junctions break IS via the use of an odd-layer obstructed atomic insulator (Nb$_3$Br$_8$), leading to asymmetric Josephson tunneling and a robust diode effect without any extrinsic fields (Wu et al., 2021). The polarization due to displaced Wannier charge centers in the barrier facilitates preferential Cooper pair transmission.
• Spontaneous electronic ordering: Devices in magic-angle twisted bilayer graphene leverage a gate-tunable correlated insulator that develops valley polarization and orbital magnetization, spontaneously breaking TRS and creating a finite effective internal field, thus yielding a programmable, field-free superconducting diode (Diez-Merida et al., 2021).
• Artificial symmetry breaking via magnetic or structural design: Planar Josephson junctions can achieve nonreciprocity by combining self-field effects from non-uniform current bias with stray fields from a trapped Abrikosov vortex, creating a built-in phase bias at zero applied field. The diode polarity and state are switchable by vortex injection/removal and current path reconfiguration, enabling diode-memory devices (Golod et al., 2022).
• Ferromagnet/noncentrosymmetric superconductor multilayers: [Nb/V/Co/V/Ta]$_{20}$ superlattices exploit interface-driven Rashba spin–orbit coupling and the exchange field of engineered ferromagnetic (Co) layers to generate a nonreciprocal superconducting state without external field, with the diode polarity settable by minor hysteresis loops of the layer magnetization (Narita et al., 2022).
• Strain, geometric asymmetry, and thermoelectric effects: In single-layer FeSe and FeTe$_{0.55}$Se$_{0.45}$, field-free SDE arises because large thermoelectric response (Seebeck effect) and intentionally asymmetric (triangular) device geometry cause in-plane temperature gradients under current, leading to nonreciprocal critical currents as the thermoelectric current adds or subtracts from the injected bias (Nagata et al., 3 Sep 2024, Dong et al., 1 Oct 2025). In strained NbSe$_2$, uniaxial stress introduces real-space polarity and symmetry breaking, directly producing diode action (Li et al., 28 Sep 2025).
• Synthetic symmetry breaking by engineered magnetic textures: Nanostructuring superconducting films with arrays of magnetic dots (zero net flux) locally modifies Meissner screening and facilitates vortex–antivortex pair nucleation dependent on current polarity, enabling fully fluxonic, field-free SDEs (Jiang et al., 2022).
• Back-action mechanisms and circuit-level engineering: Gate-tunable critical current (transconductance) in a metallic/semiconducting weak link becomes a function of applied current via the voltage drop across a control resistor. This feedback (back-action) alters the current–phase relation, resulting in nonreciprocal supercurrent transport highly tunable by device and circuit parameters (Margineda et al., 2023, Shi et al., 23 May 2025).

2. Device Architectures and Key Experimental Signatures

Field-free superconducting diodes have been realized in a diverse range of architectures, summarized below.

Device Platform	Symmetry-Breaking Mechanism	Max Diode Efficiency
NbSe$_2$/Nb$_3$Br$_8$/NbSe$_2$ JJ	Broken IS (polar barrier)	$\sim 10^4$ RR
MATBG Josephson junction	Intrinsic TRS breaking (valley)	Programmable/persistent
Nb-based planar JJs	Vortex + bias asymmetry	$>70\%$
[Nb/V/Co/V/Ta] multilayers	IS + exchange + Rashba SOC	Magnetically tunable
FeSe, FeTe$_{x}$Se$_{1-x}$ flakes	Geometry + thermoelectric	$\sim$ few \%
Graphene triode, circuit back-action	Phase bias/circuit shift, self-action	$>90\%$, $>60\%$

Key experimental signatures include:

• Critical current asymmetry: $A/c = |I_c^-| - I_c^+ > 0$, i.e. the critical current for negative bias greatly exceeds that for positive bias (or vice versa), quantifiable via the rectification ratio.
• Half-wave rectification: Robust rectified response under AC or pulsed currents between $I_c^+$ and $|I_c^-|$, with negligible dissipation in the forward direction and a large voltage in the reverse.
• Field invariance: Symmetric diode response with respect to small applied fields, confirming that nonreciprocity is not magnetochiral in origin (distinct from field-induced SDEs).
• Switchability and memory operation: Vortex trapping or gating allows switching diode polarity and storing persistent nonreciprocal states (Golod et al., 2022, Margineda et al., 2023).
• Fraunhofer interference: Genuine Josephson coupling established via single-slit Fraunhofer $I_c(B)$ patterns, with central peak shifted as expected in the presence of internal magnetization or asymmetry.

3. Microscopic Theoretical Formulation

The superconducting diode effect in these systems is understood as a deviation from the conventional Josephson current–phase relation, enforced by IS and TRS. For standard tunnel JJs,

$I_s = I_{cs} \sin \phi$

ensures $I(\phi) = -I(-\phi)$ and $I_c^+ = |I_c^-|$.

In the presence of symmetry breaking,

• For $\varphi_0$-junctions ($\sin(\phi + \varphi_0)$), or more generally in higher harmonics or phase-biased states,
• Or when the Josephson coupling energy $E_J$ becomes current-dependent (back-action), i.e. $I_c = I_c^0 + \alpha I$ :

$$I(\phi) = \frac{I_c^0 \sin \phi}{1 - \alpha \sin \phi}$$

Here $I_c^+ \neq |I_c^-|$ and the diode efficiency can approach $\eta \rightarrow 1$ as $|\alpha| \rightarrow 1$ (Margineda et al., 2023).

For barrier-induced polarization (NbSe$_2$/Nb$_3$Br$_8$/NbSe$_2$),

• The barrier is an obstructed atomic insulator with Wannier centers shifted relative to the lattice, creating a built-in out-of-plane polarization. This manifests as an increased tunneling probability for Cooper pairs in one direction, breaking inversion symmetry at a microscopic level (Wu et al., 2021).

For current and temperature-induced SDE in materials with a large Seebeck coefficient $S$:

$i_{th} = S \Delta T$

$j_{net} = j_{app} \pm i_{th}$

and critical current asymmetry emerges when $j_{net}$ reaches the depairing limit on only one polarity (Nagata et al., 3 Sep 2024).

4. Materials Systems and Efficiency Benchmarks

The realization of field-free superconducting diodes spans diverse material platforms:

• Van der Waals heterostructures: NbSe$_2$/Nb$_3$Br$_8$/NbSe$_2$ (Wu et al., 2021)—record rectification ratio ($\sim 10^4$), low switching current density ($2.2 \times 10^2$ A/cm$^2$), robust over $10^4$ cycles.
• Correlated moiré systems: Magic-angle TBG (Diez-Merida et al., 2021)—gate-tunable, persistent, and electrically switchable diode effect.
• Intrinsic high-T$_c$ materials: BSCCO flakes (Qi et al., 5 Jan 2025)—field-free SDE at up to $72$ K, $22\%$ efficiency at $53$ K.
• Iron-based superconductors: FeSe, FeTe$_x$Se$_{1-x}$ (Nagata et al., 3 Sep 2024, Li et al., 16 Oct 2024, Dong et al., 1 Oct 2025)—geometry and strain-induced SDE; rectification up to several percent.
• Superconductor/ferromagnet superlattices: [Nb/V/Co/V/Ta]$_{20}$ (Narita et al., 2022)—polarity controllable, field-free operation, integrated with lithographic processing.
• Planar and fluxonic designs: Nanostructured Nb with Abrikosov vortex manipulation (Golod et al., 2022); magnetic dot arrays (Jiang et al., 2022).

The highest diode efficiencies ($>90\%$) have been reported in graphene Josephson triodes with phase biasing (Chiles et al., 2022), and in back-action engineered devices where the critical current is a circuit-controlled function (Margineda et al., 2023, Shi et al., 23 May 2025). In topological platforms—d-wave altermagnet-induced Shiba chains and 2D Shiba lattices—diode efficiencies above $40\%$ have been theoretically demonstrated, with field-free operation and strong potential for integration in quantum devices (Samanta et al., 29 Jul 2025, Bhowmik et al., 14 Aug 2025).

5. Design Principles, Symmetry Considerations, and Tunability

Symmetry analysis is central to understanding and engineering field-free SDE:

• Device and material symmetry: Joint breaking of TRS and IS is necessary. Internal (bulk, barrier, or interface) polarization, chiral magnetic or pairing textures (e.g. $d+id'$ or $d+is$ pairing in cuprate or heavy-fermion SCs), and engineered Rashba effect or magnetic textures all serve this purpose (Vakili et al., 17 Jun 2024, Roig et al., 2023, Cheng et al., 4 Aug 2024).
• Tunable polarity and strength: Devices based on multilayers or trapped vortices can switch diode polarity via magnetization direction or vortex injection (Golod et al., 2022, Narita et al., 2022). Circuit-level configurations allow electrical switchability and continuous tuning of diode response (Shi et al., 23 May 2025, Margineda et al., 2023).
• Control of efficiency: Orientational tuning (e.g., relative angle between altermagnetic leads in AMSC/NM/AMSC junctions), chemical potential gating, strain engineering, and choice of experimental geometry (triodes, planar JJs, or asymmetric films) afford further tunability and optimization of the nonreciprocal response.

6. Applications, Challenges, and Prospects

Field-free superconducting diodes provide pathways toward:

• Superconducting digital logic and memory: Non-dissipative, directionally rectifying elements for RSFQ and adiabatic logic; memory states via vortex pinning or magnetization configuration (Golod et al., 2022, Narita et al., 2022).
• Energy-efficient superconducting electronics: High rectification ratios and low power dissipation support next-generation computing architectures (Wu et al., 2021, Li et al., 21 Jun 2025).
• Quantum technologies: Topological SDE platforms (FFLO states in Shiba chains or lattices with Majorana zero modes) enable scalable, dissipationless current biasing and robust quantum logic elements (Samanta et al., 29 Jul 2025, Bhowmik et al., 14 Aug 2025).
• Configurable cryogenic circuits: Circuit-level diode architectures permit integration and switchability in multi-component cryo-electronic systems (Shi et al., 23 May 2025).
• Programmable, scalable platforms: Junction geometries and nonreciprocity settable via gate tuning, stacking angle, or external stress, compatible with planar fabrication and CMOS processes (Li et al., 21 Jun 2025).

Remaining technical challenges include controlling domain formation (in spontaneous TRS-breaking materials), managing device-to-device variability (in barrier and interface quality), optimizing integration with other components, and extending efficiency and robustness to higher (above liquid nitrogen) temperatures and practical current densities.

In conclusion, field-free superconducting diodes harness broken symmetries, polarization effects, circuit engineering, and advanced quantum material properties to realize unidirectional, dissipationless supercurrent transport. These systems are poised to underlie future ultralow-power superconducting electronics, programmable logic, and robust quantum information platforms, with continual advances expected in architecture, scalability, and operational metrics.