Enhanced superconductivity, Kondo behavior, and negative-curvature resistivity of oxygen-irradiated thin films of aluminium

We followed the evolution of the normal and superconducting properties of Al thin films after each session of various successive oxygen irradiations at ambient temperature. Such irradiated films, similar to the granular ones, exhibit enhanced superconductivity, Kondo behavior, and negative-curvature resistivity. Two distinct roles of oxygen are identified: as a damage-causing projectile and as an implanted oxidizing agent. The former gives rise to the processes involved in the conventional recovery stages. The latter, considered within the context of the Cabrera-Mott model, gives rise to a multistep process which involves charges transfer and creation of stabilized vacancies and charged defects. Based on the outcome of this multistep process, we consider (i) the negative-curvature resistivity as a manifestation of a thermally assisted liberation of trapped electric charges, (ii) the Kondo contribution as a spin-flip scattering from paramagnetic, color-center-type defects, and (iii) the enhancement of $T_{\text{c}}$ as being due to a lattice softening facilitated by the stabilized defects and vacancies. The similarity in the phase diagrams of granular and irradiated films as well as the aging effects are discussed along the same line of reasoning.

Figure 1

(a) A multistep kinetic scheme for oxidation of aluminum as described in the Cabrera-Mott model (adapted from Ref. [12]). Directly after ionization [Eq. (1a)], electrons pass freely through the oxide interface until reaching incorporated oxygen which are then ionized to ${\mathrm{O}}_2^{-}$ [Eq. (1b)]. This charge transfer together with the left-behind positive ${\mathrm{Al}}^{3+}$ induce a local electric field which drives the slow migration of ${\mathrm{Al}}^{3+}$ across the oxide interface, leaving behind a vacancy ${\mathrm{V}}_{\text{Al}}$ [Eq. (1c)]. Equation (1d) indicates a typical formation of ${\mathrm{Al}}_2{\mathrm{O}}_3$ by the combination of migrated ${\mathrm{Al}}^{3+}$ and ionized ${\mathrm{O}}_2^{-}$. Panels (b) and (c): Illustration of the reaction at metal-oxide interface (represented by the solid green line) of a directly exposed film [panel (b) is before an event of Eq. (1a) while panel (c) is after the event of Eq. (1c); adapted from Ref. 13]. (d) We consider that, during the codeposition process of a granular film, the incorporated oxygen does not enter as an idle and neutral entity, rather it does react with Al matrix, just as in the normal oxidation process of Cabrera-Mott, leading to an interaction similar to that described in panels (a)–(c). Ultimately this accumulates into an extended nanosized grain. (e) Similar reaction occurring at the boundary of an isolated oxide grain of an irradiated film. In all cases, incorporated and ionized oxygen are represented, with no loss of generality, by ${\mathrm{O}}_2$ and ${\mathrm{O}}_2^{-}$.

Figure 2

$\rho (t,300\mathrm{K},6\text{th})$ measured with a current density $j={10}^7\mathrm{A}/{\mathrm{m}}^2$ during two consecutive O-irradiation sessions without breaking a vacuum of $1.0\times {10}^{-7}$ Torr. The irradiation intervals are shown as vertical gray areas. The inset highlights the presence of relaxation processes: In their absence, the resistivity rise should have followed the dotted line. During the steady state, the resistivity rises linearly with a rate given by the dot-dashed line in the inset. Directly after beam stoppage, there are two contributions: a fast-decaying one governed by one class of relaxations (depicted as green crosses with values given by the right-hand $y$ axis) and a slow contribution governed by a second class of relaxations; the latter is evident as a weak decay in the almost horizontal, dashed lines. The fast-decaying ${\rho }_d(t>106000\mathrm{s}$, 300 K, $6\text{th})$ is well fitted to Eq. (5) of the first of Refs. [14] (solid red line).

Figure 3

(a) Representative $\rho (T,n\mathrm{th})$ curves. The solid lines are linear fit with a slope ${\left(\partial \rho /\partial T\right)}_{\text{100-220K}}$. Due to aging effects, a unique room-temperature resistivity for each warming-up measuring cycle is taken to be the extrapolated ${\rho }_{\text{300}\text{K}}^{\mathrm{ext}}$, solid circle on the high-$T$ linear extrapolation. (b) Evolution of ${\left(\partial \rho /\partial T\right)}_{\text{100-220K}}$ as a function of ${\rho }_{\text{300}\text{K}}^{\mathrm{ext}}$. (c) An expanded view of low-$T\rho (T,H,n\mathrm{th})$ curves of panel $a$. Open (solid) symbols denote zero-field (5kOe) curve. (d) Hall coefficients of as-prepared film and that of the same film measured 85 days after the $7\mathrm{th}$ irradiation. (e) $\mathrm{\Delta }\rho (H/T,n\mathrm{th})=\rho (H/T,n\mathrm{th})-\rho (0,T,n\mathrm{th})\mathrm{versus}$ ${\left(H/T\right)}^2$: showing the breakdown of the ${\left(H/T\right)}^2$ scaling. These curves are from $n\mathrm{th}=6$, 7 irradiation and are limited to the range of $H_{c2}<H<20$ kOe and $T_c<T<T_{\text{K}}^{mn}$ (small $H/T)$. (f) The same as panel $e$ but are scaled to $\sqrt{H/T}$. This better scaling is emphasized by the solid line fit $\mathrm{\Delta }\rho (H/T,6\mathrm{th})\propto {\left(H/T\right)}^{1/2}$.

Figure 4

Manifestation of the influence of aging on the three main feature of the phase diagrams. The $i\text{th}$ cycle represents the order and time (in days) of the cooling-warming measurement carried out after the 7th irradiation session. (a) The influence of aging on the position and height of the peak maximum of NCR. The thick black line is the cooling curve directly after the first, $i=1$, warming-up measurement while the red dashed line is the cooling and warming curves of $i=5$ cycle after 85 days. The solid thin straight lines are linear fits within the range $100\le T\le 220$ K. The solid circles represent each of $\rho (T_{\text{NCR}},0\mathrm{kOe},7\text{th},i\text{th})$ and ${\rho }_{\text{300}\text{K}}^{\mathrm{ext}}(0\mathrm{kOe},7\text{th},i\text{th})$. (b) Thermal evolution of $\rho (T,5\mathrm{kOe},7\text{th},i\text{th})-\rho (20\mathrm{K},0\mathrm{kOe},7\text{th},i\text{th})$ in a semilog plot. For $T<10\mathrm{K}$, this shows a log-in-T dependence which is not a quantum localization effect since our film does not manifest a 2-dimensional character. (c) The degradation of $T_{\text{c}}$ with aging. The arrows highlight the tendency of aging influence while the solid lines in panels (b) and (c) are guides to the eye. Just as for $T_{\text{NCR}}$ of panel (a), there is no appreciable change in $T_{\text{K}}^{min}$ and $T_{\text{c}}$ for $t>85$ days.

Figure 5

Normal-state and superconducting phase diagram of different Al thin films shown as a log-log plot of $T\text{vs}{\rho }_{300\text{K}}$. $▾,♠,♣,♦$: $T_{\text{c}}^{\mathrm{zero}},T_{\text{c}}^{\mathrm{onset}}(=T_{\text{K}}^{max}),T_{\text{K}}^{min},T_{\text{NCR}}$, resp., as obtained by this work.$★,♠,♣,♦$: $T_{\text{c}}^{\mathrm{zero}},T_{\text{c}}^{\mathrm{onset}}(=T_{\text{K}}^{max}),T_{\text{K}}^{min},T_{\text{NCR}}$, resp., of granular films deposited at 77 K (Refs. [3, 4]). The location of each thermal event is explicitly shown at the right-side inset. $▿,▾$: $T_c$ of granular films deposited at room-temperature (Refs. [5] and [17], resp.). $★$: $T_c$ after O implantation at liquid helium (Ref. [6]). Left-hand-side inset: A semilog plot of our irradiated film's $\rho (T,6\text{th},0\mathrm{kOe})$ and $\rho (T,6\text{th},5\mathrm{kOe})$ (open and close circles, resp.). Right-hand-side inset: A semilog plot of $\rho \left(T\right)$ curve of a granular sample having ${\rho }_{300K}=310\mu \mathrm{\Omega }\mathrm{cm}$ (taken from Refs. [3, 4]). The main graph is extrapolated down to $2.75\mu \mathrm{\Omega }\mathrm{cm}$ of bulk Al [18]. The lines are guides to the eye.