IRIS Pol. Torinohttps://iris.polito.itIl sistema di repository digitale IRIS acquisisce, archivia, indicizza, conserva e rende accessibili prodotti digitali della ricerca.Thu, 09 Apr 2020 14:34:39 GMT2020-04-09T14:34:39Z10631Josephson current in a four-terminal superconductor/exciton-condensate/superconductor systemhttp://hdl.handle.net/11583/2462590Titolo: Josephson current in a four-terminal superconductor/exciton-condensate/superconductor system
Abstract: We investigate the transport properties of a bilayer exciton condensate that is contacted by four superconducting leads. We focus on the equilibrium regime and investigate how the Josephson currents induced in the bilayer by phase biases applied to the superconducting electrodes are affected by the presence of an exciton condensate in the bulk of the system. As long as the distance between the superconducting electrodes is much larger than the exciton coherence length, the Josephson current depends only on the difference between the phase biases in the two layers. This result holds true in both short- and long-junction limits. We relate it to a new correlated four-particle Andreev process, which occurs at the superconductor/exciton-condensate interface. The system we investigate provides an implementation of the supercurrent mirror proposed by Kitaev as a viable way to realize topologically protected qubits.
Sat, 01 Jan 2011 00:00:00 GMThttp://hdl.handle.net/11583/24625902011-01-01T00:00:00ZMicroscopic treatment of energy dissipation and decoherence via many-body Lindblad superoperatorshttp://hdl.handle.net/11583/2620520Titolo: Microscopic treatment of energy dissipation and decoherence via many-body Lindblad superoperators
Abstract: Starting from a recent reformulation of the Markov limit, we apply the mean-field approximation to the resulting Lindblad-type many-electron dynamics, and derive a closed equation of motion for the electronic single-particle density matrix in the presence of one- and two-body scattering mechanisms. The proposed formulation preserves the positive-definite character of the single-particle density matrix. This result is in striking contrast with conventional Markov approaches, where the single-particle mean-field equations can lead to positivity violations and therefore to unphysical results.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/11583/26205202015-01-01T00:00:00ZTopological Josephson phi0-junctionshttp://hdl.handle.net/11583/2615514Titolo: Topological Josephson phi0-junctions
Abstract: We study the effect of a magnetic field on the current-phase relation of a topological Josephson junction formed by connecting two superconductors through the helical edge states of a quantum spin-Hall insulator. We predict that the Zeeman effect along the spin quantization axis of the helical edges results in an anomalous Josephson relation that allows for a supercurrent to flow in the absence of superconducting phase bias. We relate the associated field-tunable phase shift phi0 in the Josephson relation of such a phi0-junction to the existence of a so-called helical superconductivity, which may result from the interplay of the Zeeman effect and spin-orbit coupling. We analyze the dependence of the magneto-supercurrent on the junction length and discuss its observability in suitably designed hybrid structures subject to an in-plane magnetic field.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/11583/26155142015-01-01T00:00:00ZDispersionless propagation of electron wavepackets in single-walled carbon nanotubeshttp://hdl.handle.net/11583/2614484Titolo: Dispersionless propagation of electron wavepackets in single-walled carbon nanotubes
Abstract: We investigate the propagation of electron wavepackets in single-walled carbon nanotubes via a Lindblad-based density-matrix approach that enables us to account for both dissipation and decoherence effects induced by various phonon modes. We show that, while in semiconducting nanotubes the wavepacket experiences the typical dispersion of conventional materials, in metallic nanotubes its shape remains essentially unaltered, even in the presence of the electron-phonon coupling, up to micron distances at room temperature.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/11583/26144842015-01-01T00:00:00ZElectron tunneling into a quantum wire in the Fabry-Perot regimehttp://hdl.handle.net/11583/2263406Titolo: Electron tunneling into a quantum wire in the Fabry-Perot regime
Abstract: We study a gated quantum wire contacted to source and drain electrodes in the Fabry-Pérot regime. The wire is also coupled to a third terminal (tip), and we allow for an asymmetry of the tip tunneling amplitudes of right-moving and left-moving electrons. We analyze configurations where the tip acts as an electron injector or as a voltage probe and show that the transport properties of this three-terminal setup exhibit very rich physical behavior. For a noninteracting wire we find that a tip in the voltage-probe configuration affects the source-drain transport in different ways, namely, by suppressing the conductance, by modulating the Fabry-Pérot oscillations, and by reducing their visibility. The combined effect of electron-electron interaction and finite length of the wire, accounted for by the inhomogeneous Luttinger liquid model, leads to significantly modified predictions as compared to models based on infinite wires. We show that when the tip injects electrons asymmetrically the charge fractionalization induced by interaction cannot be inferred from the asymmetry of the currents flowing in source and drain. Nevertheless interaction effects are visible as oscillations in the nonlinear tip-source and tip-drain conductances. Important differences with respect to a two-terminal setup emerge, suggesting new strategies for the experimental investigation of Luttinger liquid behavior.
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/11583/22634062009-01-01T00:00:00ZFractional charge in the noise of Luttinger liquid systemshttp://hdl.handle.net/11583/2372983Titolo: Fractional charge in the noise of Luttinger liquid systems
Sat, 01 Jan 2005 00:00:00 GMThttp://hdl.handle.net/11583/23729832005-01-01T00:00:00ZRole of the equilibrium size of Kadanoff blocks in the loop-expansion techniquehttp://hdl.handle.net/11583/2263403Titolo: Role of the equilibrium size of Kadanoff blocks in the loop-expansion technique
Abstract: A method developed by the present authors in a previous paper [Phys. Rev. E 57, 2594 (1998)] leads to the introduction of the equilibrium size of the Kadanoff blocks as a useful tool to approach the critical properties of the φ4 model. The present paper aims to elucidate the role of the equilibrium size of the Kadanoff blocks in the loop-expansion technique currently used in the field-theoretic renormalization. While the standard results are readily obtained, aspects emerge that help clarify the true nature of the smallness parameter in the loop-expansion technique.
Thu, 01 Jan 1998 00:00:00 GMThttp://hdl.handle.net/11583/22634031998-01-01T00:00:00ZPhotoexcitation in two-dimensional topological insulators: Generating and controlling electron wavepackets in Quantum Spin Hall systemshttp://hdl.handle.net/11583/2725379Titolo: Photoexcitation in two-dimensional topological insulators: Generating and controlling electron wavepackets in Quantum Spin Hall systems
Abstract: One of the most fascinating challenges in Physics is the realization of an electron-based counterpart of quantum optics, which requires the capability to generate and control single electron wave packets. The edge states of quantum spin Hall (QSH) systems, i.e., two-dimensional (2D) topological insulators realized in HgTe/CdTe and InAs/GaSb quantum wells, may turn the tide in the field, as they do not require the magnetic field that limits the implementations based on quantum Hall effect. However, the band structure of these topological states, described by a massless Dirac fermion Hamiltonian, prevents electron photoexcitation via the customary vertical electric dipole transitions of conventional optoelectronics. So far, proposals to overcome this problem are based on magnetic dipole transitions induced via Zeeman coupling by circularly polarised radiation, and are limited by the g-factor. Alternatively, optical transitions can be induced from the edge states to the bulk states, which are not topologically protected though.
Here we show that an electric pulse, localized in space and/or time and applied at a QSH edge, can photoexcite electron wavepackets by intra-branch electrical transitions, without invoking the bulk states or the Zeeman coupling. Such wavepackets are spin-polarised and propagate in opposite directions, with a density profile that is independent of the initial equilibrium temperature and that does not exhibit dispersion, as a result of the linearity of the spectrum and of the chiral anomaly characterising massless Dirac electrons. We also investigate the photoexcited energy distribution and show how, under appropriate circumstances, minimal excitations (Levitons) are generated. Furthermore, we show that the presence of a Rashba spin–orbit coupling can be exploited to tailor the shape of photoexcited wavepackets. Possible experimental realizations are also discussed.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/11583/27253792018-01-01T00:00:00ZTuning excess noise by Aharonov–Bohm interferometryhttp://hdl.handle.net/11583/2374819Titolo: Tuning excess noise by Aharonov–Bohm interferometry
Abstract: A voltage bias applied to a conductor induces a change of the current noise with respect to the equilibrium noise known as excess noise. We analyze the excess noise of the electronic current flowing through a mesoscopic Aharonov–Bohm ring threaded by a magnetic flux, coupled to a side gate, and contacted by two metallic electrodes. It is shown that the excess noise can be controlled both magnetically and electrostatically, demonstrating the full tunability of the system. At zero frequency, the ratio of the noise strength to the current (Fano factor) can thereby be minimized. Remarkably, at finite frequency, regions of negative excess noise emerge.
Fri, 01 Jan 2010 00:00:00 GMThttp://hdl.handle.net/11583/23748192010-01-01T00:00:00Zdc Josephson Effect in Metallic Single-Walled Carbon Nanotubeshttp://hdl.handle.net/11583/2263400Titolo: dc Josephson Effect in Metallic Single-Walled Carbon Nanotubes
Abstract: The dc Josephson effect is investigated in a single-walled metallic carbon nanotube connected to two superconducting leads. In particular, by using the Luttinger liquid theory, we analyze the effects of the electron–electron interaction on the supercurrent. We find that in the long junction limit the strong electronic correlations of the nanotube, together with its peculiar band structure, induce oscillations in the critical current as a function of the junction length and/or the nanotube electron filling. These oscillations represent a signature of the Luttinger liquid physics of the nanotube, for they are absent if the interaction is vanishing. We show that this effect can be exploited to reverse the sign of the supercurrent, realizing a tunable π-junction.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/11583/22634002007-01-01T00:00:00Z