First Order Phase Transition and Superconductivity in BaNiAs Single Crystals
We report the synthesis and physical properties of single crystals of stoichiometric BaNiAs that crystalizes in the ThCrSi structure with lattice parameters = 4.112(4) Å and = 11.54(2) Å. Resistivity and heat capacity show a first order phase transition at = 130 K with a thermal hysteresis of 7 K. The Hall coefficient is weakly temperature dependent from room temperature to 2 K where it has a value of -4x10 -cm/Oe. Resistivity, ac-susceptibility, and heat capacity find evidence for bulk superconductivity at = 0.7 K. The Sommerfeld coefficient at is 11.6 0.9 mJ/molK. The upper critical field is anisotropic with initial slopes of d/d = -0.19 T/K and d/d = -0.40 T/K, as determined by resistivity.
The large superconducting transition temperatures found in the oxypnictide system has stimulated a great deal of research activity world-wide. Perhaps, even more remarkable than the large transition temperatures (up to 55 K for SmFeAs(O,F)ZARen2008a (2008)) is the large tunability these systems possess. Superconductivity has been found in RTPn(O,F) (ZrCuSiAs structure type) at ambient pressure with Rare-earth R = La, Ce, Pr, Nd, Sm, or Gd; Transition metal T = Fe or Ni, and Pnictide Pn = P or As ZARen2008a (2008); Kamihara (2008); XHChenNature2008 (2008, 2008, 2008, 2008, 2008, 2007, 2008, 2008, 2008, 2006). Some compounds require chemical substitution, while for others the parent compounds also superconduct. Furthermore, superconductivity has also been discovered in the related ThCrSi structure type, where it has been found by doping AFeAs on the A site (with A = Ba, Sr, Ca, Eu) Rotter (2008); Chen (2008); Sasmal (2008); Wu2008Ca (2008); Jeevan (2008), under pressure in AFeAsPark2008CaFe2As2 (2008, 2008, 2008), and at ambient pressure in the stoichiometric compounds BaNiPMine2008BaNi2P2 (2008), LaRuPJeitschko1987LaRu2P2 (2008), CsFeAs, and KFeAsSasmal (2008).
The common structural element between the ZrCuSiAs and ThCrSi structure types are TPn layers which are alternately stacked with RO or A layers in the RTPnO and ATPn families, respectively. The fact that the highest transition temperatures in both families occur in compounds containing FeAs layers suggests that the TPn layers are the active layers while the RO or A layers act as a spacer that can fine tune the electronic structure of the TPn layer and act as a charge reservoir layer, but do not control the physics. Since superconductivity has been found in LaNiAsOWatanabe2008LaNiAsO (2008), one might expect that superconductivity would also be found in the ThCrSi structure type with an active NiAs layer.
Following this reasoning, we have synthesized single crystals of BaNiAs. We find a first order phase transition at = 130 K (cooling) with 7 K thermal hysteresis. By analogy with AFeAs(A = Ba, Sr, Ca)Rotter (2008); Wu2008Ca (2008, 2008, 2008, 2008) we identify this transition as a magnetic spin density wave (SDW) transition concomitant with a structural transition. Here we show that BaNiAs is also a bulk superconductor at = 0.7 K, well below the first order phase transition .
Single crystals of BaNiAs were grown in Pb flux in the ratio Ba:Ni:As:Pb=1:2:2:20. The starting elements were placed in an alumina crucible and sealed under vacuum in a quartz ampoule. The ampoule was placed in a furnace and heated to 600 C at 100 C hr, and held at that temperature for 4 hours. This sequence was repeated at 900C and at a maximum temperature of 1075 C, with hold times of 4 hr, each. The sample was then cooled slowly (C hr) to 650 C, at which point the excess Pb flux was removed with the aid of a centrifuge. The resulting plate-like crystals of typical dimensions 1 x 1 x 0.1 mm are micaceous and air sensitive and are oriented with the -axis normal to the plate. BaNiAs crystallizes in the ThCrSi tetragonal structure (space group no. 139). Single crystal refinement [R(I2) = 5.37%] at room temperature gives lattice parameters = 4.112(4) Å and = 11.54(2) Å and fully occupied atomic positions Ba 2a(0,0,0), Ni 4d(0.5,0,0.5) and As 4e(0,0,z) with z = 0.3476(3) consistent with previous reportsPfisterer1980 (1980, 1983). Powder X-ray diffraction data was consistent with the single crystal diffraction data.
Specific heat measurements were carried out using an adiabatic relaxation method in a commercial cryostat from 2 K to 300 K, and in a dilution refrigerator down to 150 mK. Electrical transport measurements were performed using a LR-700 resistance bridge with an excitation current of 0.2 mA, on samples for which platinum leads were spot welded. X-ray data were collected at room temperature on a Bruker APEXII diffractometer, with charge-coupled-device detector, and graphite monchromated MoK ( = 0.71073 Å) radiation. The data were corrected for absorption and Lorentz-polarization effects..
Resistivity and heat capacity shown in figures 1 and 2, respectively, provide clear evidence for a first order transition which occurs at 130 K upon cooling, and at 137 K upon warming, consistent with earlier magnetic susceptibility results on polycrystaline samplesPfisterer1983 (1983). The resistivity anomaly is very similar to that observed in CaFeAsWu2008Ca (2008, 2008, 2008) with a RRR (= (300 K)/(4 K)) of 5, while the absolute magnitude of the resistivity is more than an order of magnitude less than in the AFeAs compounds. The thermal hysteresis of 7 K is clearly observed in the resistivity data shown as an inset to figure 1. The Hall coefficient is negative over the entire temperature range, and indicates a weak anomaly at . The value at 2 K is = -4x10 -cm/Oe. The sharp anomaly at 137 K in the heat capacity data of figure 2 (taken upon warming) is also consistent with a first order phase transition. From 2 to 6 K the heat capacity data was fit to . This yields a Sommerfeld coefficient = 10.8 0.1 mJ/mol K. Assuming that the term is due solely to acoustic phonons, the coefficient = 1.10 0.01 mJ/mol K gives a Debye temperature = 206 K.
At low temperatures, ac-susceptibility, heat capacity, and resistivity provide evidence for bulk superconductivity. As shown in figure 3a the onset of diamagnetism starts at 0.7 K and is estimated to be 50% volume fraction, by comparing the signal to that of a piece of Pb with a comparable volume. The low temperature heat capacity data on a second sample shown in figure 3b reveals a sharp anomaly at 0.68 K with a jump = 11.15 mJ/mol K. Taking the value of the Sommerfeld coefficient at ( = 12.5 mJ/mol KlowTCpfootnote ()) gives the ratio = 1.31. The large ratio confirms the bulk nature of superconductivity, but further work is necessary to determine whether the heat capacity data can reveal any sign of unconventional superconducting behavior.
From the resistivity data at low temperatures shown in figure 4 we can extract additional information. The resistivity sample has trace amounts of Pb impurities which gives a partial transition at 7.2 K. At roughly 1.5 K there is an additional downturn in the resistivity data, which then goes to zero abruptly at 0.7 K. Since the bulk transition occurs sharply at 0.7 K in zero field, we attribute the downturn at 1.5 K to an unknown impurity phase which is also superconducting. Upon application of a magnetic field, we estimate the upper critical field for both the bulk superconductor and the impurity phase. We extract the upper critical field, for BaNiAs by taking the temperature at which = 0.5 -cm (the lower dashed line in figure 4a and 4b). This gives initial slopes of = -0.396 T/K and = -0.186 T/K with an anisotropy of 2.1. From these initial slopes we estimate the zero-temperature upper critical field (0) = -0.7 WHH1966 (1966) to be 0.19 T and 0.09 T for and , respectively, yielding a Ginzburg-Landau coherence length = 420 Å and = 610 Å, using the formula = (), where = 2.07 10 Oe cm is the flux quantum. Surprisingly, for the magnetic field in the -plane the resistive anomaly develops a shoulder. Consequently, the upper critical field of the impurity phase, for which we obtain a rough estimate by taking the midpoint of the resistive transition, has even greater anisotropy than the bulk BaNiAs superconductor.
Whether superconductivity can coexist with the low temperature orthorhombic structure and/or the spin-density wave (SDW) ground state is unclear. While coexistence of SDW and SC order is observed in the phase diagram of some doped compounds (e.g. ref. 2), whether or not this is microscopic coexistence remains to be determined. Of the stoichiometric compounds which superconduct in either the ZrCuSiAsWatanabe2007LaNiPO (2007, 2008, 2008) or ThCrSiPark2008CaFe2As2 (2008, 2008, 2008, 2008); Sasmal (2008) structure, to our knowledge, none have yet been shown to coexist with a magnetic ground state. Microscopic confirmation of a low temperature orthorhombic possessing a spin-density wave is still needed in BaNiAs. However, the similarity of the first order anomaly here to those found in the AFeAs systems Rotter (2008); Krellner2008Sr (2008, 2008, 2008, 2008) where an orthorhombic SDW state has been determined Huang2008BaNeutrons (2008, 2008) is suggestive that a similar situation occurs in BaNiAs. Thus, the clear observation of bulk superconductivity below the first order transition in BaNiAs, may constitute the first example of coexistence of these three order parameters in a system with active TPn layers.
The observation of bulk superconductivity at 0.7 K in BaNiAs completes a form of continuity with regards to the presence of superconductivity in going from the ZrCuSiAs structure type to the ThCrSi structure type, independent of whether the active layers are FeAs, NiP, or NiAs. When the active layers are FeAs, the stoichiometric materials possess SDW order, and require doping or pressure to produce superconductivity. In the cases of NiP and NiAs layers, the stoichiometric parent compounds possess superconductivity in both structure types. The biggest difference with these comparisons is that BaNiAshas a first order phase transition, while LaNiAsO does not.
In conclusion, we have synthesized single crystals of BaNiAs, which possesses both a first order transition at 130 K, which is likely a combined structural and magnetic transition, and superconductivity at 0.7 K. It will be interesting to study the dependence of doping, pressure, and isoelectronic substitution on these transitions to help elucidate the origin of superconductivity as well as the influence of competing orders.
Acknowledgements.We thank H. Lee for assistance with the measurements. Work at Los Alamos National Laboratory was performed under the auspices of the U.S. Department of Energy.
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