Introduction

Since the discovery of high-TC superconductivity in cuprates with the layered-perovskite (pv) structure1, extensive effort has been devoted to finding other superconducting materials. After a quarter century of investigation, many layered superconducting families have been discovered, such as a ruthenate Sr2RuO42, boride MgB23, hafnium nitride chloride4, cobaltate NaxCoO2·yH2O5, an intercalated graphite C6Ca6, a chalcogenide CuxTiSe27 and iron-based pnictides and chalcogenides8,9. The layered characteristics of the host crystal structures are widely believed to be essential in producing superconductivity due to the anisotropic electronic structures. This provides an important reference for the design of superconducting materials and the exploration of new mechanisms for superconductivity1,2,4,5,6. Recently, the ppv transition of MgSiO3 was discovered using a laser-heated diamond anvil cell10,11. Consequently, this phase has received more attention because it is considered to be the main constitute of the Earth's lowermost mantle (D" layer, ca. 2700–2900 km deep) (Fig. 1a). The ppv crystal structure is comprised of alternately-stacked SiO6 octahedra and Mg atoms along the b axis and it has typical two-dimensional characteristics. This has motivated research into the physical phenomena of this phase, including superconductivity. However, ppv-MgSiO3 is stable only under extreme conditions (120 GPa and 2200°C)12 and is unquenchable to ambient pressure, which has restricted further research into chemical substitution and carrier doping of the structure. Therefore, there have been attempts to establish analogue materials that are stable under ambient conditions. Almost 20 ppv-type compounds have been identified to date, including the MgGeO3, NaIrO3 and CaBO3 (B = Ru, Rh, Sn, Ir and Pt) oxides, Na(Mg,Zn)F3 fluorides and (U,Th)MnSe3 chalcogenides13,14,15,16,17,18,19. As with perovskite-type materials, many interesting physical phenomena have been observed in ppv-type materials, such as metal-insulator phase transition13 and low-dimensional magnetism17. However, no superconductivity has been reported so far for the ppv family, because most compounds with ppv structure are Mott insulators owing to the strong electron correlation effect.

Figure 1
figure 1

Schematic illustration of Earth's interior and crystal structures of ppv-MgSiO3/anti-ppv-V3PnN.

(a) The outermost solid shell is the crust composed of a variety of rocks. The mantle below the crust has layer structures corresponding to the structural transition induced by pressure in Mg-Si-O system. The lowermost layer is called the D” layer, where the main constitute is the ppv-type silicate. The outer and inner cores below the mantle consist of liquid and solid iron alloys, respectively. (b) Crystal structure of ppv-MgSiO3 and anti-ppv-V3PnN (Pn = P, As) with the Cmcm (#63) space group. Solid lines represent a unit cell. The anti-ppv structure has layer structures composed of NV6 octahedra and PnV8 polyhedra.

Full size image

In this letter, we report the observation of superconductivity in V3PnNx (Pn = P, As). These compounds crystallize in the filled Re3B structure with the orthorhombic Cmcm (#63) space group, as depicted in Fig. 1b20. The positions occupied by the anions and cations are opposite to those of the ppv structure. Considering also the nomenclature of the anti-pv structure21,22, we call this structure the anti-ppv structure (see crystal structure details in Section I of the Supplementary Information). The anti-ppv-type V3PnNx is composed of alternately-stacked NV6 octahedral layers and PnV8 bicapped trigonal prisms layers along the b axis, which gives rise to quasi-two-dimensional electronic states. Within the ac-plane, NV6 octahedra are connected by edge sharing along the a axis and corner sharing along the c axis20. Here, we also notice that V-V metallic bonds play the key role in stabilizing this structure20.

Results

Figure 2a presents the electrical resistivity (ρ) of V3PnNx (Pn = P, As) as a function of temperature (T). The room temperature resistivities ρ300K, are approximately 340 and 240 μΩ cm for V3PN and V3AsN, respectively. ρ decreases gradually with cooling (metallic behaviour) and drops sharply to zero at TC = 4.2 and 2.6 K for V3PN and V3AsN (Fig. 2c), respectively, which indicates the appearance of superconductivity. The width of the transition temperature is narrow (ca. 0.3 K), which implies good sample quality. Figure 2d presents the magnetic susceptibility (M/H) under zero-field cooling (ZFC) and field cooling (FC) conditions at H = 10 Oe. The superconducting volume fraction estimated from ZFC data at 1.8 K are approximately 190 and 167% for V3PN and V3AsN, respectively. The volume fractions exceeding 100% is attributable to the polycrystalline nature of samples. The specific heat shown in Fig. 2e has a sudden increase around TC. These results provide unambiguous evidence for the bulk superconductivity in V3PnN (Pn = P, As).

Figure 2
figure 2

Evidence for bulk superconductivity in V3PnN (Pn = P, As).

(a) Temperature (T) dependent resistivity (ρ) at zero magnetic field (H). (b) DC susceptibility (M/H) curve under the ZFC condition at H = 1 kOe. Solid lines indicate fitting results (see text). (c) Enlargement of low-temperature resistivity data. Arrows indicate the superconducting transition temperature (TC). (d) Low-temperature M/H-T curve under ZFC and FC conditions at H = 10 Oe. (e) Temperature dependence of specific heat (C/T) at zero magnetic field. Solid lines indicate fitting with C = γ T + β T3 in the normal state and the function based on the BCS model in the superconducting state. Inset shows the Cel/γTC value, where Cel is the electron contribution of the specific heat. The solid curve is calculated from the BCS model with an isotropic gap.

Full size image

Detailed measurements of the field-dependent resistivity and magnetization presented in Figs. 3a–f allow for better characterization of the superconducting state. The H-dependence of ρ (Figs. 3a–c) gives the upper critical field HC2, as shown in Fig. 3g. There is another weak transition above HC2 that originates from a small amount of the VNx impurity phase, as shown in Fig. S1(a). Close to TC, HC2 is linearly dependent on T in accordance with the Werthamer-Helfand-Hohenberg theory23. Using the formula HC2(0) = −0.693TCdHC2/dT provided HC2 at the ground state; HC2(0) = 34.9 and 27.9 kOe was obtained for V3PN and V3AsN, respectively. According to the Bardeen-Copper-Schrieffer (BCS) theory24, the HC2 value is related to the coherent length ξ, as HC2 = Φ0/2πξ2 (where Φ0 is the magnetic flux quantum). Using this formula, ξ = 9.7 and 10.9 nm are obtained for V3PN and V3AsN, respectively. The magnetization isotherms at 1.8 K exhibit typical type-II superconductor behaviour (insets of Figs. 3d, 3f). The lower critical field HC1 was determined from the magnetic field, where the magnetization departs from the linear H-dependence (indicated by arrows in Figs. 3d, 3f). The temperature dependence of HC1 is well fitted with the empirical function HC1(T) = HC1(0)[1 − α(T/TC)2] (where α is the fitting parameter) and the lower critical fields obtained at the ground state are HC1(0) = 207 and 134 Oe for V3PN and V3AsN, respectively (inset of Fig. 3g). The London penetration depths λL and the Ginzburg-Landau parameters κ are estimated from the formula HC2/HC1 = 2κ2/lnκ with κ = λL/ξ to be λL = 157 and 187 nm and κ = 16.2 and 17.2 for V3PN and V3AsN, respectively.

Figure 3
figure 3

Characterization of the superconductivity in V3PnN (Pn = P, As).

Magnetic field (H) dependence of (a–c) resistivity (ρ) and (d–f) magnetization (M) at fixed temperatures. Arrows in (a–c) indicate the upper critical field (HC2) at 1.8 K. There is another weak transition above HC2 that originates from a small amount of the VNx impurity phase, as shown in Fig. S1(a). Arrows in Figs. (d–f) indicate the lower critical field (HC1), where M departs from the linear H-dependence. Insets in (d–f) are magnetization isotherms at 1.8 K over a wider H range. (g) Temperature (T) dependence of the upper and lower critical fields (HC2 and HC1). Solid lines indicate fitting results (see text).

Full size image

The appearance of superconductivity in both Pn = P and As compounds, as well as N-deficient systems (Fig. 4), indicate that the V-3d electrons are predominantly responsible for the emergence of superconductivity. The specific heat was closely examined to reveal bosons that act as the glue for Cooper pairs. The specific heat at the normal state can be well fitted using the function C = γ T + β T3 (Fig. 2e), where the former and latter terms represent the electron and phonon contributions, respectively. We estimated γ = 19.5 and 22.0 mJ/mol K2 and β = 0.083 and 0.20 mJ/mol K4 for V3PN and V3AsN, respectively. The ΔC/γTC value (inset of Fig. 2e), where ΔC is the specific heat jump at TC, is approximately 0.86 and 1.22 for V3PN and V3AsN, respectively. These values are slightly smaller than 1.43 expected for a typical BCS superconductor with a weak-coupling limit, which implies that the electron-phonon coupling is the glue for the Cooper pairs24,25. The Debye temperature was estimated to be θD = 489 and 364 K for V3PN and V3AsN, respectively, using the relationship β = 12π4NR/5θD3 (where N is the number of atoms in a formula unit, 5 and R is the gas constant). It is worth noting that θD for V3PN is larger than that for V3AsN, because materials with smaller mass exhibit harder phonons. From the McMillan formula, TC = (θD/1.45) exp {−1.04(1 + λph)/[λph − μ*(1 + 0.62λph)]}26, together with the assumption of the Coulomb pseudopotential μ* = 0.15, the electron-phonon coupling constants can be estimated as λph = 0.55 and 0.54 for V3PN and V3AsN, respectively. These are typical values for phonon-mediated weakly coupling BCS superconductors24,25,27.

Figure 4
figure 4

Lattice parameters and superconducting transition temperature as a function of N-content (x).

The values of a, b, c, V and TC were determined from resistivity (ρ) and susceptibility (χ) data for (a–c) V3PNx and (d–f) V3AsNx. The b/[ac]1/2 values, which quantify the two dimensionality of the systems, are also plotted in (c) and (f).

Full size image

On the other hand, the importance of a strong electron correlation effect manifests itself from an analysis of the Wilson ratio Rw = π2k2Bχs/3μ2Bγ, where kB is the Boltzmann constant, χs is the spin susceptibility and μB is the Bohr magneton28. The magnetic susceptibility (χ = M/H) in the normal state exhibits weakly T-dependent behaviour and can be well fitted to the formula χ = χ0 + CCW/(T − θ) (Fig. 2b), where χ0 is a temperature-independent term, CCW is the Curie-Weiss constant and θ is the Weiss temperature. The fitting results give χ0 = 4.13 × 10−4 and 3.75 × 10−4 emu/mol, CCW = 1.09 × 10−2 and 8.08 × 10−3 emu K/mol and θ = −26.5 and −13.0 K for V3PN and V3AsN, respectively. Using the obtained χ0 values as χs, we acquire Rw = 1.42 and 1.45 for V3PN and V3AsN, respectively. The enhancement from the Fermi liquid value of 1 indicates a moderate electron correlation effect in the present compounds. Moreover, the ρ values in the normal state are in the order of 10−4 Ω cm (Figs. 2a, 2c), which is much larger than typical ρ values for conventional intermetallic compounds and indicates strong electron-electron interaction. Therefore, magnetic fluctuations originating from the strong electron correlation effect could not be excluded as the pairing glue for Cooper pairs.

Besides elucidation of the microscopic mechanism, identification of the chemical factors responsible for the appearance of superconductivity is important to further increase TC in this new family of superconductors. Therefore, we have focused on the effects of N-defects on the superconductivity. The TC values determined from the low-temperature resistivity and magnetization of V3PNx (x = 0.6–1.3) and V3AsNx (x = 0.5–1.3) (x is the nominal composition and the actual composition is expected to be less than 1) have been summarized in Figs. 4c and 4f (raw data are shown in Fig. S3 of supplementary information). For both cases, TC is significantly enhanced with increasing x in the low x region, then decreases slightly after reaching a maximum at just below the stoichiometric composition (x = 1). The highest TC achieved was 5.6 K for V3PN0.9 and the corresponding electronic properties are presented in Figs. 2a–e, 3b, 3e and 3g. In addition, as x increases, the lattice expands along the b and c axes and contracts along the a axis, which results in a slight increase of the unit cell volume. Here, we note that the b/(ac)1/2 value, which quantifies the two-dimensionality of the system, is well correlated with the TC values, as shown in Figs. 4c and 4f.

Discussion

On the basis of these experimental results, we now discuss the mechanism of superconductivity in V3PnN. The strong correlation between TC and b/(ac)1/2 values suggests that the quasi two dimensionality of crystal structure plays an important role in the appearance of superconductivity. This suggestion is reinforced by the higher TC for V3PN than for V3AsN. The former system has stronger intra-plane coupling because the smaller atomic radius of P compared with that of As makes the in-plane V-V bond distances shorter, resulting in the enhanced hybridization of 3d orbitals in the plane; moreover, more ionic character of V-P bonds compared with V-As bonds due to the larger electronegativity of P than that of As also enhances the two-dimensionality of the system. Then, how this two dimensionality favor the superconductivity? If the electron-phonon interaction mediates Cooper pairs as suggested by electronic properties, the TC value is considered to be enhanced by the larger density of states at the Fermi energy in low dimensional systems. Another possibility is that the electron correlation effect pronounced in the low dimensional crystal structure due to the smaller kinetic energy of electrons stabilizes the superconducting states. Generally, TC is known to be very sensitive to various factors in reported superconductors. Therefore, further detailed studies are required to identify the mechanism of superconductivity, especially on hybridization of the V-3d and N-2p bands, the direct-overlapping of V-3d orbitals across two V atoms and the change in carrier density introduced by N-defects.

To summarize, we have discovered superconductivity with maximum TC at 5.6 and 2.6 K for V3PNx and V3AsNx with the anti ppv structure. Two-dimensionality is the key for the appearance of superconductivity; however, to elucidate the microscopic mechanism of superconductivity, further experimental and theoretical studies are required. These findings should stimulate future experimental and theoretical research on ppv-type materials to explore advanced functionalities.

Methods

An optimized synthesis method20 was employed. Powders of elemental vanadium (99.999%), vanadium nitride (99.9%) and phosphorus (99.99%) or arsenic (99.9%) were mixed in a stoichiometric ratio, pressed into pellets in a nitrogen-filled glove box and then sealed in a quartz tube under 0.3 atm of argon gas. The quartz tube was slowly heated to 673 K, held for 24 h to avoid rapid volatilization of the phosphorus or arsenic, then heated to 1273 K for 12 h and held for 120 h. After quenching the tubes to room temperature, the product was pulverized and pressed into pellets. The pellets were annealed inside a quartz tube at 1273 K for 48 h. To remove oxide impurities during this procedure, the V3PNx and V3AsNx pellets were wrapped with molybdenum and tantalum foil, respectively, inside the quartz tube. The as-synthesized samples were dark grey coloured with a metallic luster and were stable in air. The samples were characterized using powder X-ray diffraction (Rigaku, Smartlab) with Cu Kα radiation.

The detailed structural parameters were obtained by Rietveld refinement using Rietica software29. Details of the analysis are presented in section I of the Supplementary Information. Magnetic, electrical and heat capacity measurements were performed using a commercial apparatus (Quantum Design) from 1.8 to 300 K. DC resistivity measurements were performed using the four-probe method with gold paste as electrodes.