Browsing by Author "Burset Atienza, Pablo"

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  • Cooling by Cooper pair splitting
    (2018-12-28) Sanchez, Rafael; Burset Atienza, Pablo; Levy Yeyati, Alfredo
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The electrons forming a Cooper pair in a superconductor can be spatially separated preserving their spin entanglement by means of quantum dots coupled to both the superconductor and independent normal leads. We investigate the thermoelectric properties of such a Cooper pair splitter and demonstrate that cooling of a reservoir is an indication of nonlocal correlations induced by the entangled electron pairs. Moreover, we show that the device can be operated as a nonlocal thermoelectric heat engine. Both as a refrigerator and as a heat engine, the Cooper pair splitter reaches efficiencies close to the thermodynamic bounds. As such, our work introduces an experimentally accessible heat engine and a refrigerator driven by entangled electron pairs in which the role of quantum correlations can be tested.
  • Current fluctuations in unconventional superconductor junctions with impurity scattering
    (2017-06-05) Burset Atienza, Pablo; Lu, Bo; Tamura, Shun; Tanaka, Yukio
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The order parameter of bulk two-dimensional superconductors is classified as nodal if it vanishes for a direction in momentum space, or gapful if it does not. Each class can be topologically nontrivial if Andreev bound states are formed at the edges of the superconductor. Nonmagnetic impurities in the superconductor affect the formation of Andreev bound states and can drastically change the tunneling spectra for small voltages. Here, we investigate the mean current and its fluctuations for two-dimensional tunnel junctions between normal-metal and unconventional superconductors by solving the quasiclassical Eilenberger equation self-consistently, including the presence of nonmagnetic impurities in the superconductor. As the impurity strength increases, we find that superconductivity is suppressed for almost all order parameters since (i) at zero applied bias, the effective transferred charge calculated from the noise-current ratio tends to the electron charge e, and (ii) for finite bias, the current-voltage characteristics follows that of a normal-state junction. There are notable exceptions to this trend. First, gapful nontrivial (chiral) superconductors are very robust against impurity scattering due to the linear dispersion relation of their surface Andreev bound states. Second, for nodal nontrivial superconductors, only p(x)-wave pairing is almost immune to the presence of impurities due to the emergence of odd-frequency s-wave Cooper pairs near the interface. Due to their anisotropic dependence on the wave vector, impurity scattering is an effective pair-breaking mechanism for the remaining nodal superconductors. All these behaviors are neatly captured by the noise-current ratio, providing a useful guide to find experimental signatures for unconventional superconductivity.
  • Majorana STM as a perfect detector of odd-frequency superconductivity
    (2017-05-24) Kashuba, Oleksiy; Sothmann, Bjoern; Burset Atienza, Pablo; Trauzettel, Bjoern
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We propose a novel scanning tunneling microscope (STM) device in which the tunneling tip is formed by a Majorana bound state (MBS). This peculiar bound state emerges at the boundary of a one-dimensional topological superconductor. Since the MBS has to be effectively spinless and local, we argue that it is the smallest unit that shows itself the properties of odd-frequency superconducting pairing. Odd-frequency superconductivity is characterized by an anomalous Green's function, which is an odd function of the time arguments of the two electrons building the Cooper pair. Interestingly, our Majorana STM can be used as the perfect detector of odd-frequency superconductivity. The reason is that a supercurrent between the Majorana STM and any other superconductor can only flow if the latter system exhibits itself odd-frequency pairing. To illustrate our general idea, we consider the tunneling problem of the Majorana STM coupled to a quantum dot placed on a surface of a conventional superconductor.
  • Odd-frequency superconductivity revealed by thermopower
    (2018-10-24) Hwang, Sun-Yong; Burset Atienza, Pablo; Sothmann, Björn
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Superconductivity is characterized by a nonvanishing superconducting pair amplitude. It has a definite symmetry in spin, momentum, and frequency (time). While the spin and momentum symmetry have been probed experimentally for different classes of superconductivity, the odd-frequency nature of certain superconducting correlations has not been demonstrated yet in a direct way. Here, we propose the thermopower as an unambiguous way to assess odd-frequency superconductivity. This is possible since the thermoelectric coefficient given by Andreev-like processes is only finite in the presence of odd-frequency superconductivity. We illustrate our general findings with a simple example of a superconductor-quantum-dot-ferromagnet hybrid.
  • Quantum interference in dynamically driven mesoscopic conductors
    (2021-08-24) Kotilahti, Janne
     Perustieteiden korkeakoulu | Master's thesis
    Mesoscopic conductors have become an important platform for experiments on the physics of individual electrons. They have enabled the development of electron quantum optics, where single-electron excitations are manipulated analogously to photons in quantum optics. This has led to the creation of electronic counterparts of various photonic interferometers and the possibility of using electrons as flying qubits. These advances pave the way towards future quantum technologies where single charges are controlled in nanoscale, quantum-coherent circuits to perform quantum computations. In this thesis, we study a system consisting of an electronic Mach-Zehnder interferometer and a metallic contact to which time-dependent voltage is applied. The voltage pulses excite single-electron quasiparticles called levitons, which then travel along edge states through the interferometer. By measuring the electrons ending up in different paths after the interferometer, one can find how quantum interference manifests itself in these systems. Our goal in this thesis is to improve the understanding of such experiments. To achieve this, we perform calculations based on Floquet scattering theory to analyze how quantum interference affects quantities such as the time-dependent current, transferred charge, and charge visibility. We also examine the first order correlation functions of the electrons to gain deeper analytical insights. We find that the transferred charge and current consist of cleanly separated classical and quantum contributions. The quantum parts of these quantities feature oscillations originating from quantum interference effects and can be accessed using measurements at different temperatures. Our analysis reveals that the number of these oscillations is proportional to the number of elementary charges excited by the voltage. We also show how the zeros of the correlation function encode all the relevant information about the interference of the excited particles. The results of this thesis form a theoretical guide for future experiments on interferometry of single- or few-electron excitations.
  • Transport spectroscopy of induced superconductivity in the three-dimensional topological insulator HgTe
    (2017) Wiedenmann, Jonas; Liebhaber, Eva; Kübert, Johannes; Bocquillon, Erwann; Burset Atienza, Pablo; Ames, Christopher; Buhmann, Hartmut; Klapwijk, Teun M.; Molenkamp, Laurens W.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The proximity-induced superconducting state in the three-dimensional topological insulator HgTe has been studied using electronic transport of a normal metal-superconducting point contact as a spectroscopic tool (Andreev point-contact spectroscopy). By analyzing the conductance as a function of voltage for various temperatures, magnetic fields, and gate voltages, we find evidence, in equilibrium, for an induced order parameter in HgTe of 70 µeV and a niobium order parameter of 1.1 meV. To understand the full conductance curve as a function of applied voltage we suggest a non-equilibrium-driven transformation of the quantum transport process where the relevant scattering region and equilibrium reservoirs change with voltage. This change implies that the spectroscopy probes the superconducting correlations at different positions in the sample, depending on the bias voltage.