High-pressure behavior of superconducting boron-doped diamond

This work investigates the high-pressure structure of freestanding superconducting $(T_c=4.3$ K) boron-doped diamond (BDD) and how it affects the electronic and vibrational properties using Raman spectroscopy and x-ray diffraction in the 0–30 GPa range. High-pressure Raman scattering experiments revealed an abrupt change in the linear pressure coefficients, and the grain boundary components undergo an irreversible phase change at 14 GPa. We show that the blueshift in the pressure-dependent vibrational modes correlates with the negative pressure coefficient of $T_c$ in BDD. The analysis of x-ray diffraction data determines the equation of state of the BDD film, revealing a high bulk modulus of $B_0=510\pm 28$ GPa. The comparative analysis of high-pressure data clarified that the $s{p}^2$ carbons in the grain boundaries transform into hexagonal diamond.

Figure 1

(a) Resistivity vs temperature curve for the BDD film with a boron concentration $n_{\text{B}}=2.7\times {10}^{21}$ ${\mathrm{cm}}^{-3}$, as estimated using Raman spectroscopy [30, 31, 32]. Inset: Cross-sectional SEM image of the $60\mu \mathrm{m}$ thick BDD film. (b) and the inset of (c) present the temperature and field dependence down to 100 mK of transport measurements of $60\mu \mathrm{m}$ thick BDD film. (c) illustrates the upper critical field $H_{c2}$ extracted using the 90% ${\rho }_n$ criteria. Open symbols are taken from the resistivity vs magnetic field curves. (d) Temperature dependence of the total specific heat in zero magnetic field. The solid line is the specific heat fitting below 10 K using $C_p/T={\gamma }_n+\beta T^2$; here, the inset illustrates the temperature dependence of the magnetic susceptibility $\chi$, in an external field of 10 Oe. $\chi$ was deduced from the dc magnetization, measured by following the zero-field-cooled (ZFC) and field-cooled (FC) protocols.

Figure 2

(a) Raman spectra of BDD at ambient pressure (red) and the decompressed Raman spectrum at 0.2 GPa (black). (b) Raman spectra of freestanding BDD under hydrostatic pressure; dotted lines indicate the abrupt change in the pressure coefficient at 14 GPa. The intense signal at $\sim 1300{\mathrm{cm}}^{-1}$ is due to the diamond anvil. (c) Pressure dependence of various vibrational modes in the superconducting BDD; solid spheres represent the peak center obtained using a Gaussian fit on various peaks and the solid line represents a linear fit. Here, the hatched line indicates an abrupt change in the pressure coefficient.

Figure 3

(a) Representative XRD patterns of BDD under various pressures. The dotted lines indicate the shift of the diffraction peaks, a result of lattice contraction under compression. The extra peak at $7.{32}^{\circ }$ is due the Fe in the steel gasket. (b) Volume change vs the applied pressure behavior of BDD, where the solid spheres represent the volume at a given pressure and the continuous curve represents a fit to the Vinet's equation.

Figure 4

Temperature dependence of resistivity under various applied pressures. Superconductivity is suppressed with increasing pressure. Inset: Pressure-induced suppression of the $T_c$.