When a parameter quench is performed in an isolated quantum system with a complete set of constants of motion, its out of equilibrium dynamics is considered to be well captured by the Generalized Gibbs Ensemble (GGE), characterized by a set {λα} of coefficients related to the constants of motion. We determine the most elementary GGE deviation from the equilibrium distribution that leads to detectable effects. By quenching a suitable local attractive potential in a one-dimensional electron system, the resulting GGE differs from equilibrium by only one single λα, corresponding to the emergence of an only partially occupied bound state lying below a fully occupied continuum of states. The effect is shown to induce optical gain, i.e., a negative peak in the absorption spectrum, indicating the stimulated emission of radiation, enabling one to identify GGE signatures in fermionic systems through optical measurements. We discuss the implementation in realistic setups.
In the topological phase of spin-orbit coupled nanowires Majorana bound states are known to localize at the nanowire edges and to exhibit a spin density orthogonal to both the magnetic field and the spin-orbit field. By investigating a nanowire exposed to a uniform magnetic field with an interface between regions with different spin-orbit couplings, we find that the orthogonal spin density is pinned at the interface even when both interface sides are in the topologically trivial phase, and even when no bound state is present at all. A bound state may additionally appear at the interface, especially if the spin-orbit coupling takes opposite signs across the interface. However, it can be destroyed by a smoothening of the spin-orbit profile or by a magnetic field component parallel to the spin-orbit field. In contrast, the orthogonal spin density persists in various and realistic parameter ranges. We also show that, while the measurement of bulk equilibrium spin currents has been elusive so far, such robust orthogonal spin density peak may provide a way to detect spin current variations across interfaces.
Semiconductor nanowires with strong Rashba spin-orbit coupling are currently on the spotlight of several research fields such as spintronics, topological materials and quantum computation. While most theoretical models assume an infinitely long nanowire, in actual experimental setups the nanowire has a finite length, is contacted to metallic electrodes and is partly covered by gates. By taking these effects into account through an inhomogeneous spin-orbit coupling profile, we show that in general two types of bound states arise in the nanowire, namely confinement bound states and interface bound states. The appearance of confinement bound states, related to the finite length of the nanowire, is favoured by a mismatch of the bulk band bottoms characterizing the lead and the nanowire, and occurs even in the absence of magnetic field. In contrast, an interface bound states may only appear if a magnetic field applied perpendicularly to the spin-orbit field direction overcomes a critical value, and is favoured by an alignment of the band bottoms of the two regions across the interface. We describe in details the emergence of these two types of bound states, pointing out their differences. Furthermore, we show that when a nanowire portion is covered by a gate the application of a magnetic field can change the nature of the electronic ground state from a confinement to an interface bound state, determining a redistribution of the electron charge.
At the interface between two massless Dirac models with opposite helicity a paradoxical situation arises: A transversally impinging electron can seemingly neither be transmitted nor reflected, due to the locking between spin and momentum. Here we investigate this paradox in one spatial dimension where, differently from higher dimensional realizations, electrons cannot leak along the interface. We show that models involving only massless Dirac modes lead to either no solutions or to trivial solutions to the paradox, depending on how the helicity change across the interface is modeled. However, non trivial scattering solutions to the paradox are shown to exist when additional massive Dirac modes are taken into account. Although these modes carry no current for energies within their gap, their interface coupling with the massless modes can induce a finite and tunable transmission. Finally, we show that such massless+massive Dirac model can be realized in suitably gated spin-orbit coupled nanowires exposed to an external Zeeman field, where the transmission coefficient can be controlled electrically.
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