Browsing by Author "Burset, Pablo"

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Composite two-particle sources
(2020-02-11) Moskalets, Michael; Kotilahti, Janne; Burset, Pablo; Flindt, Christian
A2 Katsausartikkeli tieteellisessä aikakauslehdessä
Multi-particle sources constitute an interesting new paradigm following the recent development of on-demand single-electron sources. Versatile devices can be designed using several single-electron sources, possibly of different types, coupled to the same quantum circuit. However, if combined non-locally to avoid cross-talk, the resulting architecture becomes very sensitive to electronic decoherence. To circumvent this problem, we here analyse two-particle sources that operate with several single-electron (or hole) emitters attached in series to the same electronic waveguide. Using Floquet scattering theory we demonstrate how such a device can emit exactly two electrons without exciting unwanted electron-hole pairs if the driving is adiabatic. Going beyond the adiabatic regime, perfect two-electron emission can be achieved by driving two quantum dot levels across the Fermi level of the external reservoir. If a single-electron source is combined with a source of holes, the emitted particles can annihilate each other in a process which is governed by the overlap of their wave functions. Importantly, the degree of annihilation can be controlled by tuning the emission times, and the overlap can be determined by measuring the shot noise after a beam splitter. In contrast to a Hong-Ou-Mandel experiment, the wave functions overlap close to the emitters and not after propagating to the beam splitter, making the shot noise reduction less susceptible to electronic decoherence.
Creation of Spin-Triplet Cooper Pairs in the Absence of Magnetic Ordering
(2018-01-19) Breunig, Daniel; Burset, Pablo; Trauzettel, Björn
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
In superconducting spintronics, it is essential to generate spin-Triplet Cooper pairs on demand. Up to now, proposals to do so concentrate on hybrid structures in which a superconductor (SC) is combined with a magnetically ordered material (or an external magnetic field). We, instead, identify a novel way to create and isolate spin-Triplet Cooper pairs in the absence of any magnetic ordering. This achievement is only possible because we drive a system with strong spin-orbit interaction-the Dirac surface states of a strong topological insulator (TI)-out of equilibrium. In particular, we consider a bipolar TI-SC-TI junction, where the electrochemical potentials in the outer leads differ in their overall sign. As a result, we find that nonlocal singlet pairing across the junction is completely suppressed for any excitation energy. Hence, this junction acts as a perfect spin-Triplet filter across the SC, generating equal-spin Cooper pairs via crossed Andreev reflection.
Dirac point formation revealed by Andreev tunneling in superlattice-graphene/superconductor junctions
(2019-11-27) Gomez Paez, Shirley; Martinez, Camilo; Herrera, William J.; Levy Yeyati, Alfredo; Burset, Pablo
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
A graphene superlattice is formed by a one-dimensional periodic potential and is characterized by the emergence of new Dirac points in the electronic structure. The group velocity of graphene's massless Dirac fermions at the new points is drastically reduced, resulting in a measurable effect in the conductance spectroscopy. We show here that tunnel spectroscopy using a superconducting hybrid junction is more sensitive to the formation of Dirac points in the spectrum of graphene superlattices due to the additional contribution of Andreev processes. We examine the transport properties of a graphene-based superlattice-superconductor hybrid junction and demonstrate that a superlattice potential can coexist with proximity-induced superconducting correlations. Both effects contribute to change graphene's spectrum for subgap energies, and as a result, the normalized tunneling conductance features sharp changes for voltages proportional to the energy separation between the original and newly generated Dirac points. Consequently, the superconducting differential conductance provides an excellent tool to reveal how the new Dirac points emerge from the original band. This result is robust against asymmetries and finite-size effects in the superlattice potential and is improved by an effective doping comparable to the superconducting gap.
Electron waiting times in hybrid junctions with topological superconductors
(2018-12-01) Mi, Shuo; Burset, Pablo; Flindt, Christian
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We investigate the waiting time distributions (WTDs) of superconducting hybrid junctions, considering both conventional and topologically nontrivial superconductors hosting Majorana bound states at their edges. To this end, we employ a scattering matrix formalism that allows us to evaluate the waiting times between the transmissions and reflections of electrons or holes. Specifically, we analyze normal-metal–superconductor (NIS) junctions and NISIN junctions, where Cooper pairs are spatially split into different leads. The distribution of waiting times is sensitive to the simultaneous reflection of electrons and holes, which is enhanced by the zero-energy state in topological superconductors. For the NISIN junctions, the WTDs of trivial superconductors feature a sharp dependence on the applied voltage, while for topological ones they are mostly independent of it. This particular voltage dependence is again connected to the presence of topological edge states, showing that WTDs are a promising tool for identifying topological superconductivity.
Multi-particle interference in an electronic Mach-Zehnder interferometer
(2021-06) Kotilahti, Janne; Burset, Pablo; Moskalets, Michael; Flindt, Christian
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The development of dynamic single-electron sources has made it possible to observe and manipulate the quantum properties of individual charge carriers in mesoscopic circuits. Here, we investigate multi-particle effects in an electronic Mach-Zehnder interferometer driven by a series of voltage pulses. To this end, we employ a Floquet scattering formalism to evaluate the interference current and the visibility in the outputs of the interferometer. An injected multi-particle state can be described by its first-order correlation function, which we decompose into a sum of elementary correlation functions that each represent a single particle. Each particle in the pulse contributes independently to the interference current, while the visibility (given by the maximal interference current) exhibits a Fraunhofer-like diffraction pattern caused by the multi-particle interference between different particles in the pulse. For a sequence of multi-particle pulses, the visibility resembles the diffraction pattern from a grid, with the role of the grid and the spacing between the slits being played by the pulses and the time delay between them. Our findings may be observed in future experiments by injecting multi-particle pulses into a Mach-Zehnder interferometer.
On-demand thermoelectric generation of equal-spin Cooper pairs
(2020-04-28) Keidel, Felix; Hwang, Sun-Yong; Trauzettel, Bjoern; Sothmann, Bjoern; Burset, Pablo
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Superconducting spintronics is based on the creation of spin-triplet Cooper pairs in ferromagnet-superconductor (F-S) hybrid junctions. Previous proposals to manipulate spin-polarized supercurrents on demand typically require the ability to carefully control magnetic materials. We, instead, propose a quantum heat engine that generates equal-spin Cooper pairs and drives supercurrents on demand without manipulating magnetic components. We consider a S-F-S junction, connecting two leads at different temperatures, on top of the helical edge of a two-dimensional topological insulator. Heat and charge currents generated by the thermal bias are caused by different transport processes, where electron cotunneling is responsible for the heat flow to the cold lead and, strikingly, only crossed Andreev reflections contribute to the charge current. Such a purely nonlocal Andreev thermoelectric effect injects spin-polarized Cooper pairs at the superconductors, generating a supercurrent that can be switched on-off by tuning their relative phase. We further demonstrate that signatures of spin-triplet pairing are facilitated by rather low fluctuations of the thermoelectric current for temperature gradients smaller than the superconducting gap.
Spin-polarized multiple Andreev reflections in spin-split superconductors
(2020-01-08) Lu, Bo; Burset, Pablo; Tanaka, Yukio
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We study the transport properties of a voltage-biased contact between two spin-split superconductors separated by an insulating barrier of arbitrary transparency. At low transparency, the contribution of multiple Andreev reflections leads to a subharmonic gap structure that crucially depends on the amplitude and relative angle of the spin-splitting fields of each superconductor. For noncollinear fields, we find an interesting even-odd effect on the bound states within the gap, where the odd order multiple Andreev reflections split, but the even order ones remain at their expected positions. By computing the current-voltage characteristics, we determine the transparency required for the emergence of a subharmonic gap structure and show that the splitting of the odd bound states is associated with different threshold energies of spin-polarized Andreev processes. Our findings provide a tool to experimentally determine the amplitude and alignment of Zeeman fields in spin-split superconductors.
Subgap states in two-dimensional spectroscopy of graphene-based superconducting hybrid junctions
(2019-04-03) Casas, Oscar E.; Gómez Páez, Shirley; Levy Yeyati, Alfredo; Burset, Pablo; Herrera, William J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Several recent works have predicted that unconventional and topological superconductivity can arise in graphene, either intrinsically or by proximity effect. Then, the analysis of the spectroscopic and transport properties in graphene would be a valuable source of information in the study of the emergent superconducting order parameter. Using Green's functions techniques, we study the transport properties of a finite size ballistic graphene layer placed between a normal state electrode and a graphene lead with proximity-induced unconventional superconductivity. Our microscopic description of such a junction allows us to consider the effect of edge states in the graphene layer and the imperfect coupling to the electrodes. The tunnel conductance through the junction and the spectral density of states feature a rich interplay between graphene's edge states, interface bound states formed at the graphene-superconductor junction, Fabry-Pérot resonances originated from the finite size of the graphene layer, and the characteristic Andreev surface states of unconventional superconductors. Within our analytical formalism, we identify the separate contribution from each of these subgap states to the conductance and density of states. Our results provide an advisable tool to determine experimentally the pairing symmetry of unconventional superconductivity that can arise in graphene.
Thermoelectric detection of Andreev states in unconventional superconductors
(2020-12-18) Savander, Tony; Tamura, Shun; Flindt, Christian; Tanaka, Yukio; Burset, Pablo
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We theoretically describe a thermoelectric effect that is entirely due to Andreev processes involving the formation of Cooper pairs through the coupling of electrons and holes. The Andreev thermoelectric effect can occur in ballistic ferromagnet-superconductor junctions with a dominant superconducting proximity effect on the ferromagnet, and it is very sensitive to surface states emerging in unconventional superconductors. We consider hybrid junctions in two and three dimensions to demonstrate that the thermoelectric current is always reversed in the presence of low-energy Andreev bound states at the superconductor surface. A microscopic analysis of the proximity-induced pairing reveals that the thermoelectric effect only arises if even- and odd-frequency Cooper pairs coexist in mixed singlet and triplet states. Our results are an example of the richness of emergent phenomena in systems that combine magnetism and superconductivity, and they open a pathway for exploring exotic surface states in unconventional superconductors.
Thermoelectric effects in hybrid superconducting junctions
(2020-03-17) Savander, Tony
Perustieteiden korkeakoulu | Master's thesis
In this work, we study thermoelectric effects in ferromagnet-superconductor hybrid junctions, focusing on the thermoelectric current created by a temperature gradient. The main objective is to verify the necessary conditions in order to have a thermoelectric effect. Another main objective is to determine the effect of the symmetry of the superconducting pairing potential on the thermoelectric current.
Time-Domain Spectroscopy of Mesoscopic Conductors Using Voltage Pulses
(2019-02-27) Burset, Pablo; Kotilahti, Janne; Moskalets, Michael; Flindt, Christian
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The development of single-electron sources is paving the way for a novel type of experiment in which individual electrons are emitted into a quantum-coherent circuit. However, to facilitate further progress toward fully coherent on-chip experiments with electrons, a detailed understanding of the quantum circuits is needed. Here, it is proposed to perform time-domain spectroscopy of mesoscopic conductors by applying Lorentzian-shaped voltage pulses to an input contact. Specifically, it is shown how characteristic timescales of a quantum-coherent conductor can be extracted from the distribution of waiting times between charge pulses propagating through the circuit. To illustrate the idea, Floquet scattering theory is employed to evaluate the electron waiting times for an electronic Fabry–Pérot cavity and a Mach–Zehnder interferometer. The perspectives for an experimental realization of the proposal are discussed and possible avenues for further developments are identified.
Tunable hybridization of Majorana bound states at the quantum spin Hall edge
(2018-02-07) Keidel, Felix; Burset, Pablo; Trauzettel, Björn
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Confinement at the helical edge of a topological insulator is possible in the presence of proximity-induced magnetic (F) or superconducting (S) order. The interplay of both phenomena leads to the formation of localized Majorana bound states (MBS) or likewise (under certain resonance conditions) the formation of ordinary Andreev bound states (ABS). We investigate the properties of bound states in junctions composed of alternating regions of F or S barriers. Interestingly, the direction of magnetization in F regions and the relative superconducting phase between S regions can be exploited to hybridize MBS or ABS at will. We show that the local properties of MBS translate into a particular nonlocal superconducting pairing amplitude. Remarkably, the symmetry of the pairing amplitude contains information about the nature of the bound state that it stems from. Hence this symmetry can in principle be used to distinguish MBS from ABS, owing to the strong connection between local density of states and nonlocal pairing in our setup.