ABSTRACT
Superconductivity discovered in boron-doped diamonds has been investigated by means of boron (11B)-nuclear magnetic resonance (NMR). 11B-NMR spectra for both (111) and (100) films are identified to arise from the substitutional B site as single occupation [B(1) site] and lower symmetric B site [B(2) site], probably substituted as boron pair or boron-hydrogen (B-H) complex, respectively. Clear evidence is presented that the effective carriers introduced by B(1) substitution are responsible for the superconductivity, whereas the charge neutral B(2) sites do not offer the carriers effectively. Related to this fact, the increase of the density of states is observed in the samples with high effective carrier density through the measurement of nuclear spin-lattice relaxation rate (1/T1). These results indicate that the evolution of superconductivity is driven by the effective carrier introduced by substitution at B(1) site. We will discuss the relation between the superconductivity and
the local structure of doped boron and propose how to increase the Tc in the diamond superconductor in this chapter. 10.1 IntroductionA beautiful jewel “diamond” has very attractive physical properties derived from a strong sp3 covalent bonding, i.e., the hardest material, the highest thermal conductivity, and very high Debye temperature (∼2200 K). Although the diamond is known as an insulator with a wide bandgap of 5.5 eV, boron (B) doping in diamond results in a p-type semiconductor with a hole binding energy (an acceptor level) of 0.37 eV. In 2004, Ekimov and coworkers discovered the superconductivity (SC) in heavily boron-doped diamond (BDD) synthesized by high-pressure and high-temperature technique [1]. The doping levels, where superconductivity has been observed, are in carrier concentration 4∼5 × 1021 cm−3, which was well above the metal-to-insulator transition (MIT) at 3 × 1020 cm−3 [2]. Many theoretical studies have stressed the similarity to MgB2 with a high-Tc value of 40 K, where strong coupling of the holes at the top of the valence band with optical phonons plays an important role for realizing a BCS-type superconductivity [3-6]. Soon after the first discovery of BDD, the superconductivity was also reported on the BDD film synthesized by microwave plasma-assisted chemical vapor deposition (MPCVD) method [7,8], which has provided with opportunities to demonstrate a variety of advanced spectroscopic studies on BDD [9-16]. For example, angle-resolved photoemission spectroscopy (ARPES) [9] and inelastic X-ray scattering [10] revealed a rigid shift of the valence band and phonon softening of the optical modes at the Γ point, respectively. Soft X-ray emission and absorption Spectroscopy (XES and XAS) unraveled that the impurity state of superconducting BDD is merged with the valence band [14]. These results suggest that holes in the merged state play an important role for the occurrence of the superconductivity in BDD. In the SC state, a typical s-wave superconducting gap with 2∆0/kBTc = 3.48 was observed by scanning tunneling spectroscopy (STS) [11], suggesting that the BDD is a phonon-mediated conventional BCS-type superconductor in weak coupling regime. However, in
association with the disorder induced by the doped boron, the SC state is very inhomogeneous, which was demonstrated by the STS [11,12] and ultrahigh resolution laser-excited photoemission spectroscopy [13], for example. One of the advantages of the study on BDD film is that B
concentration can be artificially tuned over a wide range, which enables us to elucidate how a superconducting transition temperature (Tc) depends on a doping level of boron in the BDDseBustarret, Umezawa. In particular, Umezawa et al. revealed that Tc for the (111) films is by more than two times higher than for the (100) films despite of an equivalent B concentration for both films, as shown in Fig. 10.1 [18]. This remarkable difference of Tc between (111) and (100) films might contain some hints for unravelling an origin for the onset of superconductivity in the BDD, and gives a further insight for searching higher-Tc superconductivity in the other related materials.
