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Register a new userContribution of defects to the spin relaxation in copper nanowires
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The contributions to the spin relaxation in copper (Cu) nanowires are quantified by carefully analyzing measurements of both charge and spin transport in lateral spin valves as a function of temperature and thickness. The temperature dependence of the spin-flip scattering solely arises from the scattering with phonons, as in bulk Cu, whereas we identify grain boundaries as the main temperature-independent contribution of the defects in the nanowires. A puzzling maximum in the spin diffusion length of Cu at low temperatures is found, which can be explained by the presence of magnetic impurities. The results presented here suggest routes for improving spin transport in metallic nanostructures, otherwise limited by confinement effects.
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Spin-flip mechanism in Ag nanowires with MgO surface protection layers has been investigated by means of nonlocal spin valve measurements using Permalloy/Ag lateral spin valves. The spin flip events mediated by surface scattering are effectively suppressed by the MgO capping layer. The spin relaxation process was found to be well described in the framework of Elliott-Yafet mechanism and then the probabilities of spin-filp scattering for phonon or impurity mediated momentum scattering is precisely determined in the nanowires. The temperature dependent spin-lattice relaxation follows the Bloch-Gruneisen theory and falls on to a universal curve for the monovalent metals as in the Monod and Beuneu scaling determined from the conduction electron spin resonance data for bulk.
The monochromatic and geometrically anisotropic acoustic field generated by 400 nm and 120 nm diameter copper nanowires simply dropped on a 10 $mu$m silicon membrane is investigated in transmission using three-dimensional time-resolved femtosecond pump-probe experiments. Two pump-probe time-resolved experiments are carried out at the same time on both side of the silicon substrate. In reflection, the first radial breathing mode of the nanowire is excited and detected. In transmission, the longitudinal and shear waves are observed. The longitudinal signal is followed by a monochromatic component associated with the relaxation of the nanowires first radial breathing mode. Finite Difference Time Domain (FDTD) simulations are performed and accurately reproduce the diffracted field. A shape anisotropy resulting from the large aspect ratio of the nanowire is detected in the acoustic field. The orientation of the underlying nanowires is thus acoustically deduced.
We extend our previous shell effect observation in gold nanowires at room temperature under ultra high vacuum to the other two noble metals: silver and copper. Similar to gold, silver nanowires present two series of exceptionally stable diameters related to electronic and atomic shell filling. This observation is in concordance to what was previously found for alkali metal nanowires. Copper however presents only electronic shell filling. Remarkably we find that shell structure survives under ambient conditions for gold and silver.
We have measured the electronic heat capacity of thin film nanowires of copper and silver at temperatures 0.1 - 0.3 K; the films were deposited by standard electron-beam evaporation. The specific heat of the Ag films of sub-100 nm thickness agrees with the bulk value and the free-electron estimate, whereas that of similar Cu films exceeds the corresponding reference values by one order of magnitude. The origin of the anomalously high heat capacity of copper films remains unknown for the moment. Based on the low heat capacity and the possibility to devise a tunnel probe thermometer on it, the Ag films form a promising absorber material, e.g., for micro-wave photon calorimetry.
Spin-relaxation is conventionally discussed using two different approaches for materials with and without inversion symmetry. The former is known as the Elliott-Yafet (EY) theory and for the latter the Dyakonov-Perel (DP) theory applies, respectively. We discuss herein a simple and intuitive approach to demonstrate that the two seemingly disparate mechanisms are closely related. A compelling analogy between the respective Hamiltonian is presented and that the usual derivation of spin-relaxation times, in the respective frameworks of the two theories, can be performed. The result also allows to obtain the less canonical spin-relaxation regimes; the generalization of the EY when the material has a large quasiparticle broadening and the DP mechanism in ultrapure semiconductors. The method also allows a practical and intuitive numerical implementation of the spin-relaxation calculation, which is demonstrated for MgB$_2$ that has anomalous spin-relaxation properties.
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