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Superconductor-metal-insulator transitions
Vortex states at low temperature in disordered thin and thick films of a-Mo$_x$Si$_{1-x}$
S. Okuma, M. Morita Research Center for Very Low Temperature System, Tokyo Institute of Technology, 2-2-1, Ohokayama, Meguro-ku, Tokyo 152-8551, Japan
Abstract:
We have measured the ac complex resistivity in the
linear regime, as well as dc resistivity, for thick ($100$, $300$ nm)
amorphous (a-)Mo$_x$Si$_{1-x}$ films at low temperatures
($T > 0.04$ K) in constant fields $B$. The critical behavior
associated with the second-order transition has been observed
for both dc and ac resistivities, which is similar to that observed
for granular indium films. This is the first convincing evidence
for the vortex glass transition (VGT) in the homogeneously
disordered low-$T_C$ superconductors containing microscopic
pinning centers. We have found that the VGT persists down to
$T\sim0.1T_{C0}$ up to $B\sim0.9B_{C2}(0)$, where $T_{C0}$ and $B_{C2}(0)$ are the
mean-field transition temperature and upper critical field at $T=0$, respectively. At $T\to0$ the VGT line $B_g(T)$ extrapolates to a field below $B_{C2}(0)$, indicative of the presence of a $T=0$ quantum-vortex-liquid phase in the region $B_g(0)<B<B_{C2}(0)$.
For thin ($4$ nm) films the ($T=0$) field-driven superconductor-insulator transition takes place at $B_C$. We have not obtained evidence of the metallic quantum liquid phase below $B_C$, while in $B>B_C$ an anomalous negative magnetoresistance (MR) suggesting the presence of the localized Cooper pairs has
been observed. The negative MR is commonly observed for thin films; however, for thick films the MR is always positive. This means that the two-dimensionality plays an important role in
the appearance of the negative MR (or localized Cooper pairs).
The negative MR is no longer visible as the field is applied
parallel to the film surface, consistent with the view that mobile
vortices, as well as localized Cooper pairs, are present in $B>B_C$.
Citation:
S. Okuma, M. Morita, “Vortex states at low temperature in disordered thin and thick films of a-Mo$_x$Si$_{1-x}$”, UFN, 171, supplement № 10 (2001), 108–113; Phys. Usp., 44:10 suppl. (2001), s108–s113
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https://www.mathnet.ru/eng/ufn5643 https://www.mathnet.ru/eng/ufn/v171/i13/p108
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