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Register a new userQuasiparticle Spin Resonance and Coherence in Superconducting Aluminium
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Spin/magnetisation relaxation and coherence times, respectively T_1 and T_2, initially defined in the context of nuclear magnetic resonance (NMR), are general concepts applicable to a wide range of systems, including quantum bits [1-4]. At first glance, these ideas might seem to be irrelevant to conventional Bardeen-Cooper-Schrieffer (BCS) superconductors, as the BCS superconducting ground state is a condensate of Cooper pairs of electrons with opposite spins (in a singlet state) [5]. It has recently been demonstrated, however, that a non-equilibrium magnetisation can appear in the quasiparticle (i.e. excitation) population of a conventional superconductor, with relaxation times on the order of several nanoseconds [6-10]. This raises the question of the spin coherence time of quasiparticles in superconductors and whether this can be measured through resonance experiments analogous to NMR and electron spin resonance (ESR). We have performed such measurements in aluminium and find a quasiparticle spin coherence time of 95+/-20ps.
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We report on nonlocal spin transport in mesoscopic superconducting aluminum wires in contact with the ferromagnetic insulator europium sulfide. We find spin injection and long-range spin transport in the regime of the exchange splitting induced by europium sulfide. Our results demonstrate that spin transport in superconductors can be manipulated by ferromagnetic insulators, and opens a new path to control spin currents in superconductors.
We present a comprehensive theoretical framework for the Andreev bound state population dynamics in superconducting weak links. Contrary to previous works, our approach takes into account the generated nonequilibrium distribution of the continuum quasiparticle states in a self-consistent way. As application of our theory, we show that the coupling of the superconducting contact to environmental phase fluctuations induces a charge imbalance of the continuum quasiparticle population. This imbalance is due to the breaking of the left-right symmetry in the rates connecting continuum quasiparticles and the Andreev bound state system, and causes a quasiparticle current on top of the Josephson current in a ring geometry. We evaluate the phase dependence of the quasiparticle current for realistic choices of the model parameters. Our theory also allows one to analyze the quantum coherent evolution of the system from an arbitrary initial state.
In this paper we analyze, using scanning tunneling spectroscopy, the density of electronic states in nearly optimally doped BSCCO in zero field. Focusing on the superconducting gap, we find patches of what appear to be two different phases in a background of some average gap, one with a relatively small gap and sharp large coherence peaks and one characterized by a large gap with broad weak coherence peaks. We compare these spectra with calculations of the local density of states for a simple phenomenological model in which a 2 xi_0 * 2 xi_0 patch with an enhanced or supressed d-wave gap amplitude is embedded in a region with a uniform average d-wave gap.
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