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The formation of quasiparticle zero-energy bound states at surfaces and interfaces of high-Tc superconductors is a direct consequence of the d-wave symmetry of the order parameter. In many cases it is possible to understand the novel physics appearing at surfaces/interfaces of d-wave superconductors by a simple consideration of the specific properties of the zero-energy states. Here we discuss how the large zero-energy density of states makes breaking of time-reversal symmetry energetically favorable, both at surfaces and at Josephson junctions. [Copyright &y& Elsevier]

We discuss how lifetime broadening of quasiparticle states influences single- and two-particle current transport through zero-energy states at normal metal/d-wave superconductor junctions. We distinguish between intrinsic broadening (imaginary part η of the energy), which couples the bound states with the superconducting reservoir, and broadening due to leakage through the junction barrier, which couples the bound states with the normal metal reservoir. We show that shot noise is highly sensitive to the mechanism of broadening, while the conductance is not. In the limit of small but finite intrinsic broadening, compared to the junction transparency D, η/Δ0≪D, the low-voltage shot noise at zero frequency and zero temperature becomes proportional to the magnitude η of intrinsic broadening (Δ0 is the maximum d-wave gap). [Copyright &y& Elsevier]

We present SuperConga, an open-source framework for simulating equilibrium properties of unconventional and ballistic singlet superconductors, confined to two-dimensional (2D) mesoscopic grains in a perpendicular external magnetic field, at arbitrary low temperatures. It aims at being both fast and easy to use, enabling research without access to a computer cluster, and visualization in real-time with OpenGL. The core is written in C++ and CUDA, exploiting the embarrassingly parallel nature of the quasiclassical theory of superconductivity by utilizing the parallel computational power of modern graphics processing units. The framework self-consistently computes both the superconducting order-parameter and the induced vector potential and finds the current density, free energy, induced flux density, local density of states (LDOS), and the magnetic moment. A user-friendly Python frontend is provided, enabling simulation parameters to be defined via intuitive configuration files, or via the command-line interface, without requiring a deep understanding of implementation details. For example, complicated geometries can be created with relative ease. The framework ships with simple tools for analyzing and visualizing the results, including an interactive plotter for spectroscopy. An overview of the theory is presented, as well as examples showcasing the framework's capabilities and ease of use. The framework is free to download from https://gitlab.com/superconga/superconga, which also links to the extensive user manual, containing even more examples, tutorials, and guides. To demonstrate and benchmark SuperConga, we study the magnetostatics, thermodynamics, and spectroscopy of various phenomena. In particular, we study flux quantization in solenoids, vortex physics, surface Andreev bound-states, and a "phase crystal." We compare our numeric results with analytics and present experimental observables, e.g., the magnetic moment and LDOS, measurable with, for example, scanning probes, STM, and magnetometry. [ABSTRACT FROM AUTHOR]

We report a theoretical study of time-dependent transport in a ballistic graphene field effect transistor. We develop a model based on Floquet theory describing Dirac electron transmission through a harmonically driven potential barrier. Photon-assisted tunneling results in excitation of quasibound states at the barrier. Under resonance conditions, the excitation of the quasibound states leads to promotion of higher-order sidebands and, in particular, an enhanced second harmonic of the source-drain conductance. The resonances in the main transmission channel are of the Fano form, while they are of the Breit-Wigner form for sidebands. For weak ac drive strength Z1, the dynamic Stark shift scales as Z41, while the resonance broadens as Z21 . We discuss the possibility of utilizing the resonances in prospective ballistic high-frequency devices, in particular frequency doublers operating at high frequencies and low temperatures. [ABSTRACT FROM AUTHOR]

We study time-dependent electron transport and quantum noise in a ballistic graphene field effect transistor driven by an ac gate potential. The nonlinear response to the ac signal is computed through Floquet theory for scattering states and Landauer-Büttiker theory for charge current and its fluctuations. Photon-assisted excitation of a quasibound state in the top-gate barrier leads to resonances in transmission that strongly influence the noise properties. For strong doping of graphene under source and drain contacts, when electrons are transmitted through the channel via evanescent waves, the resonance leads to a substantial suppression of noise. The Fano factor is then reduced well below the pseudodiffusive value, F < 1/3, also for strong ac drive. The good signal-to-noise ratio (small Fano factor) on resonance suggests that the device is a good candidate for high-frequency (THz) radiation detection. We show analytically that Klein tunneling (total suppression of back-reflection) persists for perpendicular incidence also when the barrier is driven harmonically. Although the transmission is inelastic and distributed among sideband energies, a sum rule leads to total suppression of shot noise. [ABSTRACT FROM AUTHOR]

We present results for time-dependent electron transport in a ballistic graphene field-effect transistor with an ac-driven gate. Nonlinear response to the ac drive is derived utilizing Floquet theory for scattering states in combination with Landauer-Büttiker theory for transport. We identify two regimes that can be useful for applications: (i) low and (ii) high doping of graphene under source and drain contacts, relative to the doping level in the graphene channel, which in an experiment can be varied by a back gate. In both regimes, inelastic scattering induced by the ac drive can excite quasibound states in the channel that leads to resonance promotion of higher-order sidebands. Already for weak to intermediate ac drive strength, this leads to a substantial change in the direct current between source and drain. For strong ac drive with frequency Ω, we compute the higher harmonics of frequencies nΩ (n integer) in the source-drain conductance. In regime (ii), we show that particular harmonics (for instance, n=6) can be selectively enhanced by tuning the doping level in the channel or by tuning the drive strength. We propose that the device operated in the weak-drive regime can be used to detect THz radiation, while in the strong-drive regime, it can be used as a frequency multiplier. [ABSTRACT FROM AUTHOR]

We present atomistic calculations of quantum coherent electron transport through fulleropyrrolidine terminated molecules bridging a graphene nanogap. We predict that three difficult problems in molecular electronics with single molecules can be solved by utilizing graphene contacts: (1) a back gate modulating the Fermi level in the graphene leads facilitates control of the device conductance in a transistor effect with high on-off current ratio; (2) the size mismatch between leads and molecule is avoided, in contrast to the traditional metal contacts; (3) as a consequence, distinct features in charge flow patterns throughout the device are directly detectable by scanning techniques. We show that moderate graphene edge disorder is unimportant for the transistor function. [ABSTRACT FROM AUTHOR]

We have investigated the transport properties of optimally doped Pr2-xCexCuO4-δ (PCCO) nanobridges with width down to 100 nm. The critical current density of the nanobridges approaches the Ginzburg-Landau theoretical limit, which demonstrates nanostructures with properties close to the as-grown films. The current voltage characteristics are hysteretic with a sharp voltage switch, of the order of a few millivolts, that we interpret with the occurrence of a hot spot formation. The values of the retrapping current and the voltage switch obtained by modeling the heat transport in the nanobridges are very close to the experimental ones. This feature, together with the extremely short recombination times, make PCCO nanostructures attractive candidates for ultrafast single photon detectors. [ABSTRACT FROM AUTHOR]