No abstract available

[en] The tunneling conductance in a topological insulator (TI) ferromagnet/d_x²−y²+id_xy mixed wave superconductor (FM/d_x²−y²+id_xy S) junction is studied based on the Blonder–Tinkham–Klapwijk (BTK) theory. It is demonstrated that the conductance strongly depends on the magnetic gap, the superconducting pair symmetry orientation, and the magnitude of the ratio of Δ₂/Δ₁. Here Δ₁(Δ₂) is the absolute pair potential of d_x²−y²(d_xy) component. For a large magnetic gap, the tunneling spectrum is characterized by a dip structure. When α=0.25π, the conductance dip appears at eV=±Δ₂. Thus, the d_x²−y²+id_xy pairing symmetry as well as the magnitude of the gap can be determined from the experiment of scanning tunneling spectroscopy. - Highlights: • Using BTK theory studied tunneling conductance in TI-based FM/d_x²+y²+id_xy S junctions. • A clear difference in tunneling spectra between d_x²−y² and d_x²−y²+id_xy-wave. • In FM/d_x²−y²+id_xy S junction, a dip structure appears at eV=±Δ₂ for α=0.25π. • The conductance depends on the magnetic gap, angle α, and the ratio of Δ₂/Δ₁. • The result can be used to determine the magnitude of gaps and BTRS states pairing

No abstract available

[en] We theoretically investigate resonant tunneling through S- and U-shaped nanostructured graphene nanoribbons. A rich structure of resonant tunneling peaks is found emanating from different quasi-bound states in the middle region. The tunneling current can be turned on and off by varying the Fermi energy. Tunability of resonant tunneling is realized by changing the width of the left and/or right leads and without the use of any external gates.

No abstract available

[en] We study the properties of the Schroedinger equation in d dimensions for a class of potentials, exhibiting a geometrical hierarchical structure. The main feature of such models is that for low energy the particle can move to infinity only by tunneling through a sequence of barriers of increasing length. The qualitative properties of these models may be similar to those arising in periodic potentials perturbed over different scales. The main result which holds for the whole class of potentials is that quantum evolution is very slow and can be characterized by: r²(t) <= C (ln t)sub(β) where r(t) is the distance traveled by a wave packet of sufficiently low energy initially localized near the origin. By imposing symmetries compatible with the hierarchical structure we obtain the remarkable result that r²(t) >= C' (ln t)sub(β') at least for a sequence of increasing times, i.e. the motion is actually characterized by a logarithmic growth. For these symmetric cases the spectral properties of the Hamiltonian are studied in detail in the low energy region and we show that the spectrum is not discrete but of zero Lebesgue measure. Finally we add an arbitrarily weak random perturbation and we show that in all cases r²(t) <= const with probability one

[en] Superconducting quantum circuits hold great potential in providing revolutionizing practical applications such as quantum sensing or computing. However, in many cases noise limits the operation and the fidelity of these circuits. Here we introduce a concept that exploits noise instead of trying to reduce it. Our concept uses photon-assisted single-electron tunneling as a controlled source for dissipation in superconducting qubits. We show how the recently developed quantum-circuit refrigerator $[$1$]$, QCR, is suitable to control the dynamics of superconducting qubits. In our experiments, the QCR works as a voltage-controlled environmental bath for the qubit. The qubit-bath coupling strength can be tuned over several orders of magnitude on a nanosecond timescale. Such a tunable environment is promising for fast qubit reset and studies of dissipative open quantum circuits. Our highly integrable circuit architecture may prove useful in the initialization of qubit arrays and in dissipation-assisted quantum annealing.

[en] In this paper we try to give an answer to the question: how a particle tunneling through a classically forbidden region behaves? Working out the quantum mechanical analysis in a particular case first proposed by Stevens, we find that the group velocity plays an important role, so confirming the results previously obtained by some authors. (orig.)

[en] Short communication

[en] Electron tunnelling in double junctions with ferromagnetic barriers and nonmagnetic electrodes is analysed in the sequential and coherent limit of electron tunnelling. The free-electron-like one band model is used. The tunnelling current and its spin polarisation, as well as tunnel magnetoresistance (TMR) are determined. The spin accumulation in the central electrode, leading to a nonvanishing TMR effect, is taken into account and analysed in the sequential limit. In the coherent limit the influence of resonant states on the results obtained is analysed. The conditions leading to an enhancement of TMR and negative differential resistance are discussed. The influence of the parameters of the junction on the results obtained is also investigated