Quantum coherence in sub-10 nm metal wires
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Quantum coherence in sub-10 nm metal wires Douglas Natelson1 , Robert L. Willett, Kenneth W. West, and Loren N. Pfeiffer Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974 1 Department of Physics and Astronomy, MS61, Rice University, Houston, TX 77005 ABSTRACT We report weak localization studies of quantum coherence in metal nanowires with widths as small as 5 nm, demonstrating that structures fabricated at sub-50 nm length scales can reveal coherence phenomena not accessible in larger devices. Through selective etching of cleaved molecular-beam epitaxy (MBE)-grown substrates, we produce precise nanoscale surface relief then used as a stencil for metal deposition. This nonlithographic method of lateral definition allows the fabrication of metal (AuPd) nanowires greater than one micron in length with widths below 5 nm, a previously unexplored size regime in studies of quantum corrections to the conductance of disordered metals. Analyzing magnetoresistance data, we find that the coherence time, τφ , shows a low temperature T dependence close to quasi1D theoretical expectations (τφ ∼ T −2/3 ) in 5 nm wide wires, while exhibiting a relative saturation as T → 0 for wide samples of the same material. Since an externally controlled parameter, the sample geometry, can cause a single material to exhibit both suppression and divergence of τφ , this finding provides a new constraint on models of dephasing phenomena. INTRODUCTION The ability to create devices on steadily decreasing size scales together with increased access to extremely low temperatures has resulted in the observation of many novel phenomena. These include quantum corrections to electrical conductance such as weak localization (WL) [1], universal conductance fluctuations as a function of external magnetic field (MFUCF) [2], and similar fluctuations as a function of time (TDUCF) [3]. At the heart of these phenomena is the interplay between quantum coherence, electron-electron interactions, and disorder. These effects may be analyzed quantitatively to infer the quantum coherence properties of interacting electrons. One typically defines [4] a phenomenological coherence time scale, τφ , for the electrons. For t τφ , interactions between a particular electron |ψ and its environment (other electrons, phonons, etc.) are sufficiently weak that nonnegligible quantum interference effects remain. Conversely, for t τφ the electron and environmental degrees of freedom have coupled and evolved enough to suppress the contribution of quantum interference terms to expectation values of electron observables. Since τφ is essentially the lifetime of a particular state of electronic excitation, the validity of the quasiparticle picture of metals implies [4] the expectation that /τφ kb T ; that is, quasiparticles must be distinct from the Fermi sea. Indiffusive systems with diffusion constant D, one can also speak of a coherence length Lφ ≡ Dτφ .
D10.3.1
Of particular interest is the temperature dependence of τφ , which provides information about the phase-breaking process
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