Emergent superconductivity at 16.3 K in an altermagnetic candidate Na$_{2-x}$V$_2$Se$_2$O with broken inversion symmetry

Abstract: Altermagnets (AMs), characterized by zero net magnetization and momentum-dependent spin splitting, are anticipated to hold significant potential for generating multiple exotic and uncommon superconducting states. However, superconductivity has not yet been realized in AMs to date. Recently, two-dimensional (2D) V$2$Ch$_2$O (Ch = Se, Te) monolayers, as well as AV$_2$Ch$_2$O (A = K, Rb, Cs) crystals containing [V$_2$Ch$_2$O]$^{δ-}$ building layers, have been predicted and/or demonstrated to be promising altermagnetic materials. Our preliminary attempts to explore superconductivity in these materials by applying pressure or chemical doping were unsuccessful. Here we report the discovery of superconductivity at a relatively high transition temperature of ~ 16.3 K in a newly synthesized layered compound, Na${2-x}$V$_2$Se$_2$O, a variant of AV$_2$Ch$_2$O. In this structure, the [V$_2$Ch$_2$O]$^{δ-}$ layers are interspersed with double layers of Na$^+$ instead of a single layer of A$^+$, with sodium sites being only half-filled. This new family of layered vanadium oxychalcogenides, lacking inversion symmetry, represents an intriguing platform for exploring altermagnetic superconductors, and holds the potential to reveal novel phenomena, such as topological states, van Hove singularities, and finite-momentum superconductivity. Furthermore, this material acts as a "bridge" between the cuprate/nickelate and iron-pnictide high temperature superconductors, providing new hope and opportunity to expand the category of layered superconductors with higher critical temperatures (T$_c$) and enhancing our understanding of the underlying mechanisms in these systems.

• The paper demonstrates emergent superconductivity at 16.3 K in Na2-xV2Se2O, coexisting with altermagnetic order and a spin-density-wave transition.
• It employs DFT calculations and transport measurements to reveal a quasi-2D Fermi surface with Dirac points and tunable electronic properties via Na vacancies.
• The study highlights the role of non-centrosymmetric structure in inducing parity-mixed pairing, opening avenues for unconventional superconducting states.

Emergent Superconductivity in Non-Centrosymmetric Altermagnetic Na$_{2-x}$V$_2$Se$_2$O

Introduction and Context

The discovery of superconductivity in the layered vanadium oxychalcogenide Na$_{2-x}$V$_2$Se$_2$O, which also manifests features of altermagnetism and pronounced lack of inversion symmetry, represents a significant development for the study of quantum materials. This work offers compelling evidence for the coexistence of superconductivity with a spin-density-wave (SDW)-like transition in a system previously characterized as an altermagnetic (AM) candidate. The confluence of layered structure, tunable carrier concentration through self-doping (Na-vacancies), and non-centrosymmetric crystallography highlights this material as a highly controllable platform for investigating the interplay between superconductivity and exotic magnetic orders.

Crystal Structure, Defect Chemistry, and Electronic Landscape

Unlike the established AV$_2$Ch$_2$O (A = K, Rb, Cs; Ch = Se, Te) altermagnet family, Na$_{2-x}$V$_2$Se$_2$0O crystallizes in an orthorhombic (Ammm, No. 65) structure at room temperature, with alternating stacking of double Na$_2$1-deficient layers and [V$_2$2Se$_2$3O]$_2$4 conducting slabs. The Na-site occupancy is approximately 50%, directly impacting the vanadium oxidation state and resulting carrier concentration. This deviation from full occupancy not only suggests the possibility of ordered sodium vacancy superstructures but also allows controlled modulation of electronic properties via native self-doping.

First-principles DFT calculations demonstrate three bands crossing the Fermi level, a quasi-two-dimensional (2D) Fermi surface with van Hove singularities near $_2$5, and the presence of Dirac points, collectively indicating strong electronic correlations, enhanced two-dimensionality, and the potential for unconventional superconductivity. The dominant V-3$_2$6 character at $_2$7, and the band topology, are only weakly modified by SOC, as expected for the constituent light elements.

Magnetism: Altermagnetism and SDW Phenomena

Magnetic measurements show a SDW-like anomaly at $_2$8 K, coupled with strong magnetic anisotropy and a likely canted AFM ground state. The simultaneous lack of inversion symmetry and the occurrence of non-collinear magnetism points to a substantial Dzyaloshinskii-Moriya interaction (DMI), which can stabilize intricate spin textures. The magnetic response, particularly the temperature dependence of susceptibility and the metamagnetic features, reflect a ground-state complexity influenced by both the inherent AM order (momentum-dependent spin splitting with zero net moment) and structural distortion.

Transport signatures, including moderate to high magnetoresistance (up to 100% at 2 K, 16 T) and a pronounced upturn in low-temperature resistivity, echo phenomena encountered in cuprate, iron-pnictide, and nickelate high-$_2$9 superconductor families. Hall effect data establish hole-type carriers as dominant, with vacancies in the Na layers acting as effective hole dopants.

Superconductivity: Discovery, Properties, and Interplay with Magnetism

The central result is the unambiguous detection of superconductivity with $_2$0 up to 16.3 K in Na$_2$1V$_2$2Se$_2$3O, with the superconducting state interwoven with the underlying SDW/AM background. Both single-crystal and polycrystalline samples exhibit sharp diamagnetic transitions and resistivity drops, although the superconducting volume fraction remains low ($_2$4), which is consistent with nascent superconductors or those with substantial intrinsic disorder.

The upper critical field $_2$5, while not exceeding the BCS paramagnetic limit, is moderately high and displays field anisotropy typical of 2D or weakly-coupled layer systems. The suppression of $_2$6, possibly arising from inter-band (spin-triplet or parity-mixed) pairing or pair-breaking by disorder, warrants further investigation via directional critical field measurements and advanced spectroscopies.

Superconductivity emerges under self-doping conditions but can be tuned by topochemical approaches; for example, sodium deintercalation or intercalation in related compounds can induce or suppress superconductivity, respectively. This is reminiscent of the role played by alkali/alkaline-earth layer engineering in cuprates, highlighting the importance of blocking layer chemistry.

Theoretical and Practical Implications

This study provides concrete evidence that superconductivity can emerge from a magnetic ground state characterized by altermagnetism. The lack of inversion symmetry introduces parity-mixing, enabling the stabilization of unconventional pairing, possibly with a sizable triplet component or even topological structures, given the Dirac-like features and Fermiology.

From a theoretical standpoint, these results support predictions that altermagnetic spin fluctuations—especially in non-centrosymmetric environments—can mediate nontrivial superconducting states [28,29,31-36]. The observed phenomena in Na$_2$7V$_2$8Se$_2$9O suggest new routes to higher-$_{2-x}$0 superconductivity in 3$_{2-x}$1 multi-valent vanadium oxides, and provide a bridge between cuprate, nickelate, and iron-pnictide systems, all unified by the presence of 2D conducting planes proximate to magnetic ordering.

Practically, the work expands the design space for layered superconductors: manipulation of the blocking layer and precise defect control are poised to yield further increases in $_{2-x}$2 and tuning of the superconducting order parameter symmetry. The sensitivity of the superconducting state to carrier concentration, disorder, and structural symmetry implies technological potential for quantum electronic and spintronic devices, especially those leveraging parity-mixed, topological superconductivity.

Outlook and Future Directions

Key outstanding issues include the stabilization of bulk superconductivity (currently hindered by disorder and phase inhomogeneity), definitive identification of the superconducting gap structure (nodal vs fully gapped, singlet vs triplet or mixed), and direct confirmation of topological states via ARPES or tunneling spectroscopy. Further exploration of blocking layer chemistry, Na-vacancy ordering phenomena, and controlled suppression of magnetism via doping or pressure will be instrumental in optimizing superconductivity and unraveling the mechanism for Cooper pairing in these altermagnetic platforms.

Given the theoretical prediction that AM superconductors may support a variety of exotic states—including finite-momentum pairing (FFLO), topological superconductivity, and strong diode effects [31-36]—Na$_{2-x}$3V$_{2-x}$4Se$_{2-x}$5O and its derivatives represent fertile ground for the pursuit of new quantum phases.

Conclusion

Na$_{2-x}$6V$_{2-x}$7Se$_{2-x}$8O establishes a new paradigm in the search for unconventional superconductivity by actualizing superconductivity at 16.3 K in a non-centrosymmetric, layered altermagnetic system. The combination of strongly-correlated electronic structure, complex magnetism, and tunable doping via alkali-deficient blocking layers opens unexplored regimes for understanding and manipulating quantum ordered states. The demonstration of superconductivity in this class of materials provides a foundation for probing the interplay between altermagnetism and superconductivity, with far-reaching implications for both fundamental physics and applied quantum technology.