Negative magnetoresistance in boron-doped nanocrystalline diamond films

Abstract

We report on the observation of a negative magnetoresistance ͑NMR͒ regime in boron-doped nanocrystalline diamond films at low temperatures. A comparative analysis of our experimental results and those reported for systems composed of superconducting granules embedded in an insulating matrix ͑also referred as granular films͒ suggest the presence of superconducting regions inside the insulating films as causing the NMR. By considering the latter scenario, the experimental observations are explained by modeling the systems as consisting of a distribution of superconducting granules whose global properties are tuned by the intergrain distance.

Superconductivity in doped semiconductors

“…In the range between TC and about 40K, the conductivity extrapolates to a finite temperature according to the weak localization expression 25,109,126 : (T) = 0 + AeeT 1/2 + BepT, where Aee and B ep refer respectively to interelectronic and electron-phonon interactions. As described below, the resistivity peak at low T is significantly enhanced in heavily doped nanocrystalline (nc-C:B) diamond films [127][128][129] while the magnetoresistance changed sign 128,130 with the field, as expected for a granular system. The weak localization observed in the granular disordered metal nc-C:B has been proposed to herald an unconventional superconductivity in that material through a spin-flip pairing mechanism 127,131 .…”

“…This has been performed in boron doped nanocrystalline diamond : for grain sizes smaller than the thickness and comparable to the coherence length 152 , or comparable to the thickness 81,153 , the granularity of the system takes over and percolation effects are expected. The role of the grain boundaries and intragranular metallic transport parameters, quite obvious in the normal state 130,154,155 takes another twist below TC, with an influence of the intergranular Josephson coupling, as well as that of the heterogeneity of the local superconductivity among the grains and even within the same grain, which has been assessed by scanning tunnel microscopy 77,78 . For micrometer-sized grains, the enhanced weak localization effects may be hidden at low temperatures by a sharp and pronounced "bosonic" resistivity peak occuring at a temperature close to the onset of superconductivity 129 .…”

Large magnetoresistance of insulating silicon films with superconducting nanoprecipitates

“…thin amorphous or granular metallic films on insulators, [23][24][25][26] fractal Pb films on silicon, 27 Pb-Si hetorojunctions, 28 amorphous metal-metalloid films, [29][30][31][32][33] cuprate superconductors [34][35][36][37][38] and heavily doped semiconductors. [39][40][41][42][43] There has been an academic debate about the nature of this phase transition and the origin of the large negative MR. 12,[44][45][46] Some theories consider a global quantum phase transition 21,33,47,48 or quantum corrections to the classical magnetotransport. 12,24,49,50 Here we report on large negative and positive MR in Ga-rich, p-type Si films and demonstrate that a simple phenomenological model based on local superconductivity and hopping transport can describe the complex temperature and field dependence of the resistance.…”

Magnetic field-driven superconductor–insulator transition in boron-doped nanocrystalline chemical vapor deposition diamond

“…Chemical vapor deposition ͑CVD͒, a method widely used in the semiconductor industry, is being employed to produce high-purity BDDs, offering opportunities for their practical uses 5 and providing stages for fundamental research on superconductivity. 4,[6][7][8][9][10][11][12][13] Although important knowledge has been accumulated concerning the properties of boron-doped CVD diamond thin films, 4,[6][7][8][9]11,12 only a few research reports have been devoted to the granular nature of this superconductivity. The influence of the inevitable granularity of CVD diamond thin films on their transport properties was only lately brought to attention.…”