Superconductivity in Scandium Borocarbide with orbital hybridization

Exploration of superconductivity in light element compounds has drawn considerable attention because those materials can easily realize the high $T_{c}$ superconductivity, such as ${\mathrm{LnNi}}_{2}{\mathrm{B}_{2}}{\mathrm{C}}$ ($T_{c}$ =17 K), ${\mathrm{Mg}}{\mathrm{B}}_{2}$ ($T_{c}$ =39 K), and very recently super-hydrides under pressure ($T_{c}$ =250 K). Here we report the discovery of bulk superconductivity at 7.8 K in scandium borocarbide ${\mathrm{Sc}}_{20}{\mathrm{B}}{\mathrm{C}}_{27}$ with a tetragonal lattice which structure changes based on the compound of ${\mathrm{Sc}}_{3}{\mathrm{C}}_{4}$ with very little B doping. Magnetization and specific heat measurements show bulk superconductivity. An upper critical field of Hc2(0) ~ 8 T is determined. Low temperature specific-heat shows that this system is a BCS fully gapped s-wave superconductor. Electronic structure calculations demonstrate that compared with ${\mathrm{Sc}}_{3}{\mathrm{C}}_{4}$ there are more orbital overlap and hybridization between Sc 3d electrons and 2p electrons of C-C(B)-C fragment in ${\mathrm{Sc}}_{20}{\mathrm{B}}{\mathrm{C}}_{27}$, which form a new electric conduction path of Sc-C(B)-Sc. Those changes influence the band structure at the Fermi level and may be the reason of superconductivity in ${\mathrm{Sc}}_{20}{\mathrm{B}}{\mathrm{C}}_{27}$.


I. INTRODUCTION
High T C superconductor may be anticipated for hydrides, carbides, and borides due to their light masses and strong covalent bonding, which yields high vibrational or both lead to high Tc values, such as for LnNi 2 B 2 C (Tc = 17 K) [2][3][4], MgB 2 (T C = 39 K) [5], Y 2 C 3 (Tc = 18 K) [6], and the recently synthesized hydrides at mega-bar pressures, in which nearly room-temperature superconductivity has been realized in H 3 S (Tc = 203 K) and LaH 10-x (Tc > 260 K) [7][8]. Metal carbides are good candidates to explore high Tc superconductors and they also provide a bridge which links the organic and inorganic materials [9]. There are many carbide compounds containing C2 fragments that are superconductors, such as Y 2 C 3 (18 K) [6], YC 2 (3.9 K) [10], Y 2 C 2 I 2 (10 K) [11], whose structures contain molecular anionic fragments, (C-C) or (B-C-B), that interact with a metal framework. Sc 3 C 4 is the first model compound containing C 3 4fragments and some other examples are Mg 2 C 3 [12] and Ca 3 Cl 2 C 3 [13]. Sc 3 C 4 crystallizes in a tetragonal structure (Z = 10, P4/mnc, a = 748.73(5) pm, c = 1502.6(2) pm) as shown in Fig. 1(a). The unit cell contains eight C3 fragments, two C2 fragments and twelve isolated C atoms, with the S C30 (C 3 ) 8 (C 2 ) 2 (C) 12 composition [14][15][16][17]. Sc 3 C 4 is a metallic conductor and a Pauli paramagnet, but is not a superconductor. Oyama analyzed the electronic band structures of Sc 3 C 4 and found that it is Sc 3d orbitals instead of the p orbitals of C2 and C3 that provide large contribution at E F [18]. There is no sufficient overlap between C p orbitals and Sc 3d orbitals around E F and the calculated spectra of the molecular fragments do not meet the energy requirements as found in LnC 2 , Ln 2 C 3 , and Ln 2 C 2 X 2 superconductors [18]. It is worth trying to change the electronic structure of Sc 3 C 4 via doping elements or applying pressure to induce superconductivity.
Here we report the crystal structure and basic superconducting properties of the new superconductor Sc 20 BC 27 , whose structure has an adjustment on the base of We synthesized and confirmed the superconductivity on the base of doping little B in Sc 3 C 4 in 2018 [19]. Recently, Ninomiya and coworkers have independently obtained similar results on the occurrence of superconductivity through resistivity and susceptibility measurements and confirmed the final crystal structure of this superconductor [20]. Now we found our result of structure is the same with that of Ninomiya et al., indicating that they are the same compound.

II. EXPERIMENTAL AND THEORETICAL METHODS
The starting materials for the synthesis of polycrystalline Sc 3 C 4 were graphite (C 99.99%), amorphous boron (B 99.95) and piece of scandium (Sc 99.99%). The Sc and C, B chunks were weighed out in a 3:4:0.1 ratio. Then C and B powder was pressed into a pellet and put together with piece of Sc, arc-melted to have a metal chunk for subsequent meltings, and the samples were arc-melted four times in Ar atmosphere of 500 mbar. In between each melting the arc-melted buttons were flipped to ensure homogeneous samples.
The resulting crystals were characterized by X-ray diffraction (XRD) with Cu K α1 radiation at room temperature. The XRD pattern of Sc 20 BC 27 was analyzed with the GSAS program with a user interface EXPGUI [21]. We carried out the specific heat,

DC susceptibility and electrical resistivity measurements in a Quantum Design Physical
Property Measurement System (PPMS-9), respectively.
We conducted first-principles electronic structure calculations on Sc 20 BC 27 and Sc 3 C 4 using the projector augmented wave (PAW) method [22] as implemented in the VASP package [23][24]. The generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) type was adopted for the exchange-correlation functional [25].       To further confirm the bulk superconductivity, we resorted to DC magnetic susceptibility measurements in Sc 20 BC 27 . Figure 4 shows zero field cooling (ZFC) and

III. RESULTS AND ANALYSIS
field cooling (FC) processes of the susceptibility  in the low temperature under the applied field of H = 10 Oe. Diamagnetic signal is probed below T c due to superconducting transition and tends to saturate at low temperatures. The shielding fraction estimated from ZFC data at 2 K is about 80%.  states is plotted as in the inset of Fig. 5(a), the normalized specific heat jump at T c is found to be C /( - 0 )T c = 1.35, which is close to the BCS prediction for weak-coupling superconductivity of 1.43 [27]. . This distortion also occurs in the carbide LaNi 2 B 2 C where B-C-Bπ-nonbonding orbital is tuned by the position of the La dx 2 -y 2 orbital, which leads to second order Jahn-Teller instabilities [19]. But in other systems with (C-C(B)-C) fragment, the angle has little change, for instance, it is 174.6°in La 5 B 2 C 60 [28] and 174.3°in Y 15 B 4 C 14 [29].

IV. CONCLUSION
We report the experimental results for a polycrystalline sample of ternary borocarbide Sc 20 BC 27 . Sc 20 BC 27 were successfully synthesized using arcing method.
The Rietveld refinements demonstrate that Sc 20 BC 27 has a tetragonal lattice structure without C-C fragment compared with Sc 3 C 4 . Bulk superconductivity with T c ~ 7.8 K is observed from the resistivity, susceptibility, and specific heat measurements. Low temperature specific heat data shows the superconductivity is s-wave pairing symmetry.
The specific heat shows linear relation with magnetic field, suggesting the existence of Further experimental studies are needed to increase the superconducting transition temperature via doping other elements or applying high pressure.