Scholarly article on topic 'Superconductivity in anti-post-perovskite vanadium compounds'

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Academic research paper on topic "Superconductivity in anti-post-perovskite vanadium compounds"

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SUPERCONDUCTING PROPERTIES AND MATERIALS

METALS AND ALLOYS

ELECTRONIC PROPERTIES AND MATERIALS

STRUCTURAL GEOLOGY

Received 5 June 2013

Accepted 12 November 2013

Published 29 November 2013

Correspondence and requests for materials should be addressed to B.W. (bswang@issp. u-tokyo.ac.jp) or K.O. (ohgushi@issp.u-tokyo.

ac.jp)

Superconductivity in anti-post-perovskite vanadium compounds

Bosen Wang & Kenya Ohgushi

Institute for Solid State Physics, University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8581, Japan.

Superconductivity, which is a quantum state induced by spontaneous gauge symmetry breaking, frequently emerges in low-dimensional materials. Hence, low dimensionality has long been considered as necessary to achieve high superconducting transition temperatures (TC). The recently discovered post-perovskite (ppv) MgSiO3, which constitutes the Earth's lowermost mantle (D" layer), has attracted significant research interest due to its importance in geoscience. The ppv structure has a peculiar two-dimensional character and is expected to be a good platform for superconductivity. However, hereunto, no superconductivity has been observed in isostructural materials, despite extensive investigation. Here, we report the discovery of superconductivity with a maximum TC of 5.6 K in V3PnNx (Pn = P, As) phases with the anti-ppv structure, where the anion and cation positions are reversed with respect to the ppv structure. This discovery stimulates further explorations of new superconducting materials with ppv and anti-ppv structures.

ince 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 chalco-genide 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 superconductivity1,2,4-6. 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 ofthe 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 chalcogenides13-19. 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.

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 (p) ofV3PnNx (Pn = P, As) as a function of temperature (T). The room temperature resistivities p3ooK, are approximately 340 and 240 pfi cm for V3PN and V3AsN, respectively. p

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.

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).

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 p (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 j, as HC2 = W0/2pj2 (where W0 is the magnetic flux quantum). Using this formula, j = 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 — a(T/TC)2] (where a is the fitting parameter), and the lower critical fields obtained at the ground state are HC1(0) = 207 and

Figure 2 | Evidence for bulk superconductivity in V3PnN (Pn = P, As). (a) Temperature (T) dependent resistivity (p) 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-Tcurve 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 = y T + b T3 in the normal state and the function based on the BCS model in the superconducting state. Inset shows the CJy TC value, where Cel is the electron contribution of the specific heat. The solid curve is calculated from the BCS model with an isotropic gap.

Figure 3 | Characterization of the superconductivity in V3PnN (Pn = P, As). Magnetic field (H) dependence of (a-c) resistivity (p) 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).

134 Oe for V3PN and V3AsN, respectively (inset of Fig. 3g). The London penetration depths 1L, and the Ginzburg-Landau parameters k are estimated from the formula HC2/HC1 = 2k2/lnk with k = lL/j to be 1L = 157 and 187 nm and k = 16.2 and 17.2 for V3PN and V3AsN, respectively.

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 = y T + b T3 (Fig. 2e), where the former and latter terms represent the electron and phonon contributions, respectively. We estimated y = 19.5 and 22.0 mJ/mol K2, and b = 0.083 and 0.20 mJ/mol K4 for V3PN and V3AsN, respectively. The DC/yTC value (inset of Fig. 2e), where DC 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 6D = 489 and 364 K for V3PN and V3AsN, respectively, using the relationship b = 12k4NR/50d3 (where N is the number of atoms in a formula unit, 5, and R is the gas constant). It is worth noting that 0D for V3PN is larger than that for V3AsN, because materials with smaller mass exhibit harder phonons. From the McMillan formula, TC = (6D/1.45) exp {-1.04(1 1 Aph)/[Aph -m*(1 1 0.62Aph)]}26, together with the assumption of the Coulomb pseudopotential m* = 0.15, the electron-phonon coupling constants

can be estimated as 1ph = 0.55 and 0.54 for V3PN and V3AsN, respectively. These are typical values for phonon-mediated weakly

coupling BCS superconductors24,25,27.

On the other hand, the importance ofa strong electron correlation effect manifests itself from an analysis of the Wilson ratio Rw = n2k2Bxs/3m2By, where kB is the Boltzmann constant, xs is the spin susceptibility, and mB is the Bohr magneton28. The magnetic susceptibility (x = M/H) in the normal state exhibits weakly T-dependent behaviour and can be well fitted to the formula x = x0 1 CCW/(T — h) (Fig. 2b), where x0 is a temperature-independent term, CCW is the Curie-Weiss constant, and h is the Weiss temperature. The fitting results give x0 = 4.13 X 10—4 and 3.75 X 10—4 emu/mol, CCW = 1.09 X 10—2and8.08 X 10—3 emuK/mol,andh = —26.5and —13.0 Kfor V3PN and V3AsN, respectively. Using the obtained x0 values as xs, 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 p values in the normal state are in the order of 10—4 V cm (Figs. 2a, 2c), which is much larger than typical p 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

Figure 4 | Lattice parameters and superconducting transition temperature as a function of N-content (x). The values of a, b, c, Vand TC were determined from resistivity (p) and susceptibility (x) 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).

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 KforV3PN09, 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 ofsuperconductivity. This suggestion is reinforced by the higher TC for V3PN than for V3AsN. The former system has stronger intraplane 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 Ka 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.

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Acknowledgments

The authors acknowledge discussions with Prof. Y. Ueda, Dr. Y. Hirata, Dr. Y. Q. Zhang, and Dr. F. Du. We also thank M. Isobe and T. Yamauchi for their technical supports. This work was supported by the Grant Program of the Sumitomo Foundation, and the Grant Program of the Murata Science Foundation.

Author contributions

B.W. prepared the samples and carried out the experiments. The authors equally contributed to analysing the results. B.W. wrote the paper with assistance from K.O. K.O. directed the research.

Additional information

Supplementary information accompanies this paper at http://www.nature.com/ scientificreports

Competing financial interests: The authors declare no competing financial interests.

How to cite this article: Wang, B. & Ohgushi, K. Superconductivity in anti-post-perovskite vanadium compounds. Sri. Rep. 3, 3381; DOI:10.1038/srep03381 (2013).

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