Spin-Polarized Currents Flow Only in Odd-Layered Materials

Researchers are increasingly focused on the relationship between superconductivity and unconventional magnetism to create novel quantum phenomena. Chuang Li, Jin-Xing Hou from Hefei National Laboratory, and Shuai-Ling Zhu, Hao Zheng, and Yang Liu working with colleagues at the Center for Correlated Matter and School of Physics, Zhejiang University, have investigated Josephson junctions fabricated from CsV₂Te₂O materials, which exhibit a unique form of magnetism known as altermagnetism. This collaborative work, also including contributions from Yu Song, Song-Bo Zhang from Hefei National Laboratory, and Lun-Hui Hu, reveals a striking ‘altermagnetic even-odd effect’ where the number of layers dictates the flow of spin-polarized supercurrents, effectively acting as a switch. The discovery of this layer-dependent behaviour not only demonstrates a robust spin-selective Josephson effect but also suggests a broader, general principle applicable to understanding magnetic and transport properties in hidden altermagnets.

Scientists are uncovering increasingly subtle ways to control the flow of electricity with zero resistance. A surprising discovery concerning layered materials reveals that the very number of atomic layers can dictate the direction of supercurrents. This precise control could prove vital for building future quantum technologies with unprecedented functionality.

Researchers have uncovered a novel phenomenon in layered superconducting materials that could revolutionize the design of spintronic devices. The work, centred on compounds from the CsV2Te2O family, demonstrates a controllable spin-polarized supercurrent, an electrical current carrying both charge and spin, dependent on the number of atomic layers within the material.

This discovery hinges on the material’s unique ‘hidden altermagnetism’, a subtle form of magnetism where spin polarization exists at the layer level without a net magnetic moment. By meticulously constructing Josephson junctions, devices allowing supercurrent to flow between two superconductors, from these materials, scientists have revealed an ‘altermagnetic even-odd effect’.

Specifically, the team found that spin-polarized supercurrents appear only in junctions composed of an odd number of layers, while those with an even number exhibit complete cancellation of this spin polarization. This layer parity acts as an effective switch, controlling the flow of spin information. Investigations into vertical junctions, where layers are stacked directly on top of each other, further refined this understanding.

Odd-layer barriers enhance the transport of electrons with aligned spins, while even layers promote opposite spin configurations, resulting in a predictable oscillation in the total supercurrent as the number of layers increases. This layer-dependent behaviour represents a general characteristic of hidden altermagnets, extending beyond the specific CsV2Te2O system and potentially applicable to a wider range of magnetic and transport phenomena.

The ability to manipulate spin currents without relying on conventional magnetism opens exciting possibilities for creating energy-efficient spintronic devices, electronics that utilise the spin of electrons in addition to their charge, and could pave the way for entirely new paradigms in quantum information processing and low-power electronics. The research establishes a unique platform for gate-tunable, spin-polarized supercurrents, offering a significant step towards advanced superconducting spintronics.

Modelling supercurrent flow through an altermagnetic Josephson junction

A tight-binding Hamiltonian, HJJ(x, ky), underpinned the theoretical investigation of Josephson junctions constructed from CsV₂Te₂O, a material exhibiting hidden altermagnetism. This model explicitly describes the superconducting leads, the altermagnetic barrier, and the coupling between them, allowing for calculations across a finite number of layers with open boundaries in the x-direction and translational invariance along y.

The Hamiltonian comprises terms representing the left and right superconducting leads (HSC1 and HSC2), the central altermagnet (HAM), and the spin-independent hopping (Hcoup) facilitating electron transfer between these regions. Current conservation principles were then applied to directly calculate the total Josephson current, Ix tot(φJ), across the altermagnetic region, utilising the phase difference (φJ) and the inverse temperature (β).

This calculation involved tracing over the fermionic Matsubara frequencies and employing the electron and hole hopping matrices, Te/h(ky), alongside the anomalous Green’s functions, F(x, ky, ω) and F’(x, ky, ω), which encode the proximity-induced pairing correlations. These Green’s functions were decomposed into spin-singlet, equal-spin-triplet, opposite-spin-triplet, and orbital components, revealing that only the equal-spin-triplet channel develops a non-zero pairing within the altermagnet.

Consequently, the total Josephson current naturally separates into distinct contributions from singlet, triplet, and mixed spin states, enabling a detailed analysis of spin-selective transport. The study deliberately excluded spin-orbit coupling to ensure the vanishing of the mixed term, Ix mix, simplifying the analysis and isolating the fundamental altermagnetic effects. Supercurrents were calculated for both x and y-oriented junctions, revealing a pronounced spin polarization where the x-direction predominantly carried spin-up current and the y-direction carried spin-down current, a phenomenon termed the spin-selective Josephson effect.

Spin polarisation and layer-dependent supercurrent modulation in CsV2Te2O Josephson junctions

Planar Josephson junctions fabricated from monolayer CsV2Te2O exhibit a fully spin-polarized supercurrent, a consequence of the material’s unique electronic structure and hidden altermagnetism. Calculations reveal quasi-1D, nearly flat, spin-polarized bands arising from the altermagnet, which, when coupled to a d-wave superconductor, produce this directional anisotropy in the supercurrent.

The strength of the altermagnetic spin-splitting, M, is on the order of 1.0 eV, a value large enough to neglect spin-orbit coupling in this system. This substantial splitting directly contributes to the observed spin selectivity. Moving to multilayer structures, the research uncovers a distinct altermagnetic even-odd effect. Spin-polarized supercurrents are consistently observed in planar junctions comprising an odd number of layers, while these currents are completely suppressed in even-layer configurations.

This cancellation of the spin-polarized supercurrent between layers in even-layer structures demonstrates a clear switching mechanism governed by layer parity. The effective Hamiltonian used to model the monolayer system incorporates parameters derived from experimental observations, including an on-site energy splitting of 1.24 eV between the dxz and dyz orbitals.

Vertical junctions further illuminate this layer-parity dependence. Odd-layer barriers enhance equal-spin triplet transport, while even-layer barriers favour opposite-spin transport. This results in a robust period-two oscillation in the total supercurrent as the number of layers is increased, specifically, the supercurrent alternates between larger and smaller values with each added layer, superimposed on an overall decay with increasing barrier thickness.

This oscillation confirms the sensitivity of the supercurrent to the magnetic configuration within the multilayer stack. The band structure calculations, based on a monolayer model, highlight the dominance of the 3d orbitals of vanadium atoms near the Fermi level.

Layered superconductors reveal inherent spin control via the even-odd effect

Scientists are increasingly focused on manipulating spin within superconducting materials, and this work demonstrates a surprisingly direct route to achieving that goal. For years, controlling spin currents has demanded complex material designs and external fields, often hindering practical application. The discovery of robust, layer-dependent spin polarization in these altermagnetic Josephson junctions offers a potentially simpler paradigm.

The ‘even-odd effect’, where the number of atomic layers dictates whether a spin-polarized current flows, is particularly compelling, acting as an inherent switch without needing external intervention. This isn’t merely an incremental advance; it speaks to a fundamental shift in how we might engineer spintronic devices. Beyond the immediate implications for superconducting circuits, the principles at play could extend to other magnetic and transport phenomena, potentially influencing the development of novel magnetic sensors or memory technologies.

The ability to control spin flow with such precision opens doors to exploring more exotic states of matter. However, scaling these devices remains a significant hurdle. Maintaining the delicate balance required for the layer-dependent effects across larger areas and more complex architectures will be challenging. Furthermore, the materials themselves, while promising, are relatively new, and their long-term stability and compatibility with existing fabrication techniques need thorough investigation.

Future research will likely focus on exploring different altermagnetic materials and heterostructures, seeking to amplify the observed effects and integrate them into functional prototypes. The broader effort may well see this layer-parity control become a cornerstone of future spintronic designs.