Showing papers by "Tomas Löfwander published in 2016"

Hot spot formation in electron-doped PCCO nanobridges

Abstract: 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.

Nonlinear response of a ballistic graphene transistor with an ac-driven gate: High harmonic generation and terahertz detection

Abstract: 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-Buttiker 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 Omega, we compute the higher harmonics of frequencies n Omega (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.

Resonant second-harmonic generation in a ballistic graphene transistor with an ac-driven gate

Abstract: 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 Z(1), the dynamic Stark shift scales as Z(1)(4), while the resonance broadens as Z(1)(2). 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.

Basic Theory of Electron Transport Through Molecular Contacts

Abstract: In this thesis, we focus on different aspects of electron transport in nanostructured graphene (such as graphene nanoribbons). We develop and implement numerical methods to study quantum coherent electron transport on an atomistic level, complemented by analytical calculations based on the Dirac approximation valid close to the points K and K ′ in the graphene Brillouin zone. By simulating a graphene nanogap bridged with 1,4-phenylenediamine molecules anchored via C60 molecules, we show that a transistor effect can be achieved by back-gating the system. By simulating STMmeasurements on nanoribbons with single impurities, we investigate the interplay between size quantization and the local scatterers, and show analytically how the features of the Fourier transformed local density of states can be explained by electrons scattering between different transverse modes of the ribbons. We extend the analys to also include analytical transport calculations, and explain the origin of characteristic dips found in the transmission and their relations to quasi-bound states formed around the ribbon impurities. We construct and simulate graphene ribbons with transverse grain boundaries, and illustrate how such grain boundaries form metallic states bridging the two edges of the ribbon together. This is a plausible candidate to explain the attenuation (or even destruction) of the quantum Hall effect often seen in quantum Hall bar measurements, especially with graphene grown on metals (such as copper) where grain boundaries are common. The introductory chapters also present a basic introduction to the field of graphene and graphene ribbons, and we thoroughly present the tight-binding techniques used for simulation.

Spin-polarized currents and noise in normal-metal/superconductor junctions with Yu-Shiba-Rusinov impurities

Abstract: Conventional superconductors disordered by magnetic impurities demonstrate physical properties that are drastically different from their pristine counterparts. In our previous work [D. Persson et al., Phys. Rev. B 92, 245430 (2015)], we explored the spectral and thermodynamic properties of such systems for two extreme cases: completely random and ferromagnetically aligned impurity magnetic moments. Here we consider the transport properties of these systems and show that they have a potential to be used in superconducting spintronic devices. Each magnetic impurity contributes a Yu-Shiba-Rusinov (YSR) bound state to the spectrum, residing at subgap energies. Provided the YSR states form metallic bands, we demonstrate that the tunneling current carried by these states can be highly spin polarized when the impurities are ferromagnetically ordered. The spin polarization can be switched by tuning the bias voltage. Moreover, even when the impurity spins are completely uncorrelated, one can still achieve almost 100% spin polarization of the current, if the tunnel interface is spin active. We compute electric current and noise, varying parameters of the interface between tunneling and fully transparent regimes, and analyze the relative role of single-particle and Andreev reflection processes.