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Superconductivity in a Layered Ta4Pd3Te16 with PdTe2 Chains

Superconductivity in low-dimensional compounds has long attracted much interest. Here we report superconductivity in a low-dimensional ternary telluride Ta4Pd3Te16 in which the repeating layers contain edge-sharing octahedrally-coordinated PdTe2 chains along the crystallographic b axis. Measurements of electrical resistivity, magnetic susceptibility and specific heat on the Ta4Pd3Te16 crystals, grown via a self-flux method, consistently demonstrate bulk superconductivity at 4.6 K. Further analyses of the data indicate significant electron-electron interaction, which allows electronic Cooper pairing in the present system.

Recent decades witness increasing numbers of brand-new superconductors including the two classes of high-temperature superconductors ever discovered 1,2 in low-dimensional systems. Generally speaking, low dimensionality leads to special electronic structures and allows relatively strong fluctuations, which may enhance superconductivity, although charge-density wave (CDW) sometime competes, especially in the quasi-onedimensional case. 3 Thus it is rational to explore new superconductors in materials with low-dimensional crystal structures. From a crystal-chemistry viewpoint, low-dimensional structures easily appear in an inorganic metal compound whose constituent anions have relatively low electronegativity and high polarizability. For example, metal chalcogenides, as compared with metal oxides, tend to crystallize in layered structures, 4 which give rise to rich interesting physical phenomena including CDW and superconductivity. 3,5 Among chalcogenides, tellurides often differ from sulfides and selenides in crystal structures, electronic structures and physical properties, primarily because of the diffuse nature of the tellurium valence orbitals.
While binary chalcogenides such as NbSe 2 6 and NbSe 3 7 were intensively studied because of their CDW and superconductivity, ternary and multi-element chalcogenides have not been paid much attention since their births in the 1980s and 1990s. 4,8-12 Very recently, superconductivity was discovered in Pd-based ternary chalcogenides M 2 Pd x Q 5 (M=Nb and Ta, Q=S and Se) [13][14][15][16][17] and Nb 3 Pd x Se 7 18 . The crystal structure consists of corrugated M-Pd-Q layers in which the M-Q chains are interconnected abreast by square-planarly coordinated Pd atoms. 8 The most remarkable property of the new superconductors is an extremely high upper critical field along the chain direction. Furthermore, the material was notably found to have potential industrial applications by the successful fabrication of flexible Nb 2 Pd x S 5-δ superconducting fibers. 17 Nevertheless, none of their analogous tellurides has been synthesized because of the distinct feature of tellurium mentioned above.
Here we report superconductivity in a ternary Pd-based telluride Ta 4 Pd 3 Te 16 which was first synthesized and characterized by Mar and Ibers in 1991. 9 The compound crystallizes in space group I2/m with a monoclinic unit cell of a=17.687(4) Å, b=3.735(1) Å, c=19.510(4) Å, and β=110.42° at 111 K. Compared to the aforementioned M 2 Pd x Q 5 phase, the crystal structure has relatively flat Ta-Pd-Te layers, as illustrated in Figure 1a. The most remarkable difference, however, is that the Pd atoms are octahedrally coordinated, forming edge-sharing PdTe 2 chains along the b axis. This difference gives rise to totally different crystal field for Pd atoms, which possibly brings about a disparate electronic state. The PdTe 2 chains are separated by TaTe 3 chains (with face-sharing bicapped trigonal prisms) and Ta 2 Te 4 double chains (with edge-sharing distorted octahedra). Therefore, Ta 4 Pd 3 Te 16 is a layered compound with quasi-one-dimensional characteristics.
The Ta 4 Pd 3 Te 16 crystals were grown by a self-flux method, similar to previous report, 9 but the growth parameters were varied. Powders of the elements Ta (99.97%), Pd (99.995%) and Te (99.99%), all from Alfa Aesar, in an atomic ratio of Ta:Pd:Te = 2:3:15 were thoroughly mixed together, then loaded, and sealed into a quartz ampoule evacuated. The ampoule was slowly heated up to 1223 K, holding for 24 h. After that, it was allowed to cool to 923 K at a rate of 5 K/h, followed by cooling down to room temperature. Shinning gray-black soft crystals in flattened needle shape were harvested with a typical dimension of 2.5×0.25×0.1 mm 3 , as shown in the left panel of Figure 1b. The scanning-electron-microscope (SEM) 19 image shown in the chain direction. The chemical composition was determined by energy dispersive X-ray spectroscopy (EDXS), 19 which gives the atomic ratio Ta:Pd:Te=4:3:16 within the measurement precision (±3-5% depending on the elements measured). This result indicates that there is no significant deficiency for Pd, contrasting with the cases of the M 2 Pd x Q 5 and Nb 3 Pd x Se 7 phases mentioned above. Shown in the left panel of Figure 1c is the Xray diffraction (XRD) 19 pattern at 298 K by a conventional θ-2θ scan for the crystals lying on a sample holder. Only multiple reflections of (-103) planes can be seen, consistent with the layered crystal structure depicted in Figure 1a. The inter-plane spacing is determined to be 6.530 Å, close to the calculated value of 6.502 Å using the reported cell parameters at 111 K, 9 pointing to the Ta 4 Pd 3 Te 16 phase. The full width at half maximum (FWHM) is only 0.04°, e.g., for the (-309) peak, indicating high quality of the crystals. We also employed highresolution transmission electron microscopy (HRTEM) 19 to confirm the structure further. As shown in the right panel of Figure 1c, the lattice parameter b measured in the [-103] zone image is ∼3.76(5) Å, close to the reported value as well. The HRTEM image is found to be completely consistent with the crystal structure of Ta 4 Pd 3 Te 16 , as illustrated in the upper right inset. The room-temperature ρ b is 6.1×10 -7 Ω⋅m, comparable to that of a normal metal. With decreasing T, ρ b (T) decreases almost linearly down to ∼50 K without any sharp anomaly due to a possible CDW transition. The ρ b (T) data from 6 K to 35 K obeys ρ b =ρ 0 +AT α with α=2.00 (Figure 2c), which is one of the hallmarks of Fermi liquid, suggesting dominant electronelectron scattering at low temperatures. The residual resistivity ρ 0 by the extrapolation turns out to be 2.34×10 -8 Ω⋅m. Consequently, the residual resistivity ratio (RRR, defined as the ratio of the resistivity at room temperature and at zero temperature) is 26. It is much larger than those (∼2) of the M 2 Pd x Q 5 compounds, 14,15,17 in which the Pd deficiency may cause impurity scatterings. The much less impurity scattering in our Ta 4 Pd 3 Te 16 crystals meets with the exact stoichiometry without Pd deficiency evidenced from the above EDXS result.
A superconducting transition takes place below 5 K, as clearly shown in Figure 2b. The onset, midpoint, and zeroresistance temperatures are 4.62 K, 4.46 K and 4.11 K, respectively. Superconductivity is confirmed by dc magnetic susceptibility measurement 20 (Figure 2d), showing strong diamagnetism below 4.6 K. Since the zero-field-cooling (ZFC) signal is a measure of magnetic shielding and, the sample was a crystal, the superconducting volume fraction can be estimated to be 90%±4% (the error mainly comes from the weighing accuracy). The field-cooling (FC) diamagnetic signal is obviously weaker due to flux pinning effect. Nevertheless, it is still much stronger than that in M 2 Pd x Q 5 system, 14,15,17 indicating comparatively weak flux pinning in Ta 4 Pd 3 Te 16 . Other superconducting properties such as critical magnetic fields will be reported elsewhere. 21 Low-T specific heat, C(T), may supply important information to a superconductor. At temperatures much below Debye temperature Θ D , the specific heat contributed from phonons can be expressed by Debye T 3 law: C ph =βT 3 . Thus the total specific heat measured 20 can be expressed by C=γT+βT 3 , and the plot of C/T vs. T 2 allows a linear fitting of normal-state C(T) of the Ta 4 Pd 3 Te 16 crystals, as shown in Figure 3a. The fitting yields a Sommerfeld parameter γ=42.8±1.6 mJ⋅K -2 ⋅mol -1 and a phononic coefficient β=11.11±0.05 mJ⋅K -4 ⋅mol -1 . The Θ D value is then determined to be 159.1±0.3 K by the formula Θ D =[(12/5)NRπ 4 /β] 1/3 , where N counts the number of atoms per formula unit (fu), and R is the gas constant. The density of states (DOS) at Fermi level, N(E F ), is also estimated to be 18.1±0.7 eV -1 ⋅fu -1 by the relation N(E F )=3γ/(k B 2 π 2 ) for the case of non-interacting electron systems.
The electronic specific heat in the superconducting state can be obtained by a simple subtraction: C el =C−βT 3 . Figure 3b plots C el /T as a function of temperature. A clear specific-heat jump, ∆C el , shows up below 4.6 K, which further confirms bulk superconductivity. The ∆C el /T c value is estimated to be 54±3 mJ⋅K -2 ⋅mol -1 and, by an entropy conserving construction, the midpoint temperature of the thermodynamic transition is determined to be 4.1 K, coincident with the zero-resistance temperature. The C el (T) data below 1 K tend to saturate, suggesting that fully gapped superconductivity is likely. However, with the limited data of ultra-low temperatures, one cannot rule out the possibility of nodal and/or multiple superconducting gaps owing to the complexity of the Fermi surfaces. 22 In the case of full-gap scenario, the residual electronic specific-heat coefficient is 7(1) mJ⋅K -2 ⋅mol -1 at T→0 K, indicating presence of non-superconducting metallic portion in the sample. Assuming that the non-superconducting phase has the same γ-value, the superconducting molar fraction is then estimated to be 84%±6%, basically consistent with the above magnetic measurement result.  where the Coulomb repulsion parameter µ* is set to be 0.13 empirically. This resultant λ ph is 0.77, suggesting that Ta 4 Pd 3 Te 16 belongs to an intermediately coupled superconductor. In addition, the e-ph coupling strength can also be evaluated by comparing the DOS from specific-heat measurement and the " bare " DOS from band-structure calculations, N bs (E F ). Using the calculation result of N bs (E F )=5.5 eV -1 ⋅fu -1 , 22 and applying the relation N(E F )/N bs (E F )=1+λ, one obtains an unusually large coupling constant of λ=2.3. This points to electron-nonphonon couplings, λ nph =λ−λ ph =1.53, which is about twice of λ ph itself, in accordance with the dominant electron-electron scattering revealed by the T 2dependence in low-T resistivity.
The significant electron-electron interactions are also manifested by the criteria of Kadowaki-Woods ratio, 24 defined as A/γ 2 . Note that there are three Pd atoms in one chemical formula of Ta 4 Pd 3 Te 16 , hence the Sommerfeld parameter should be reasonably considered as γ Pd =14.3±0.5 mJ⋅K -2 ⋅mol-Pd -1 , which is close to those of M 2 Pd x Q 5 13,15,16 and cobalt oxyhydrate 25 superconductors. In this circumstance, the A/γ Pd 2 value is calculated to be 19 µΩ⋅cm⋅mol 2 ⋅K 2 ⋅J -2 , placing this material into the group of strongly correlated electron systems including heavy fermion compounds. 24 Superconductivity in the quasi-one-dimensional Ta 4 Pd 3 Te 16 reminds us of possible excitonic superconducting mechanism, first proposed by Little in 1964. 26 Such an unconventional electronic Cooper pairing by exchanging excitons was hotly discussed recently, 27 and the stabilization of excitons was probed 28 in Ta 2 NiSe 5 which has a similar quasi-one-dimensional crystal structure. 10 In Ta 4 Pd 3 Te 16 , charge transfer from the Te 2to Pd 4+ /Ta 5+ was evident according to the band-structure calculations. 22 Taken the remarkable electron-electron interaction into consideration together, the unconventional electronic Cooper pairing is then possible, which deserves further investigations.
To our knowledge, only few multi-element telluride (except for doped or intercalation binary tellurides) has been discovered to be a superconductor so far. 29 On the other hand, dozens of layered multi-element tellurides have been synthesized, 4,11-12 but they have not been well studied either by chemical doping/intercalation or by physical property measurements. We hope that the discovery of superconductivity in Ta 4 Pd 3 Te 16 will stimulate exploration of superconductivity for the future in numerous multi-element layered tellurides.