Bose Sangita, Puthucode Anantha, Banerjee Rajarshi, Ayyub Pushan
Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai 400005, India.
J Phys Condens Matter. 2009 Jul 15;21(28):285305. doi: 10.1088/0953-8984/21/28/285305. Epub 2009 Jun 19.
The formation of amorphous phases in immiscible alloys with a large positive enthalpy of mixing is thermodynamically unfavorable. Co-sputter deposited Cu-Nb films exhibit a nanoscale phase separation into Cu-rich and Nb-rich amorphous regions. They show relatively high room temperature resistivity, a negative temperature coefficient of resistance (TCR), and an incomplete superconducting transition with onset at 3.7 K. Annealing the nanophase-separated amorphous films at 200 °C results in the nucleation of fcc Cu-rich nanocrystals within an Nb-rich amorphous matrix. This film exhibits multiple resistance steps, eventually showing a sharp drop with (T(C))(onset) = 3.7 K. Annealing at 350 °C leads to complete devitrification via the formation of large bcc Nb-rich grains encapsulating the existing fcc Cu nanocrystals. These films show low room temperature resistivity, positive TCR, and a sharp superconducting transition with onset at 5.2 K. The electrical transport and superconducting behavior appear to be consistent with a two-stage crystallization process.
在具有较大正混合焓的不混溶合金中形成非晶相在热力学上是不利的。共溅射沉积的Cu-Nb薄膜呈现出纳米级相分离，形成富Cu和富Nb的非晶区域。它们具有相对较高的室温电阻率、负电阻温度系数（TCR）以及在3.7 K开始的不完全超导转变。在200°C下对纳米相分离的非晶薄膜进行退火，会在富Nb非晶基体中形成富fcc Cu纳米晶体的形核。这种薄膜呈现出多个电阻台阶，最终在（T(C)）(起始) = 3.7 K时出现急剧下降。在350°C下退火会通过形成包裹现有fcc Cu纳米晶体的大的富bcc Nb晶粒导致完全失玻化。这些薄膜显示出低室温电阻率、正TCR以及在5.2 K开始的尖锐超导转变。电输运和超导行为似乎与两阶段结晶过程一致。
