Engineering physics of superconducting hot-electron bolometer mixers

Superconducting hot-electron bolometers are presently the best performing mixing devices for the frequency range beyond 1.2 THz, where good quality superconductor-insulator-superconductor (SIS) devices do not exist. Their physical appearance is very simple: an antenna consisting of a normal metal, sometimes a normal metal-superconductor bilayer, connected to a thin film of a narrow, short superconductor with a high resistivity in the normal state. The device is brought into an optimal operating regime by applying a dc current and a certain amount of local- oscillator power. Despite this technological simplicity its operation has been found to be controlled by many different aspects of superconductivity, all occurring simultaneously. A core ingredient is the understanding that there are two sources of resistance in a superconductor: a charge conversion resistance occurring at an normal-metal-superconductor interface and a resistance due to time- dependent changes of the superconducting phase. The latter is responsible for the actual mixing process in a non-uniform superconducting environment set up by the bias-conditions and the geometry. The present understanding indicates that further improvement needs to be found in the use of other materials with a faster energy-relaxation rate. Meanwhile several empirical parameters have become physically meaningful indicators of the devices, which will facilitate the technological developments.Comment: This is an author-processed copy of an Invited contribution to the Special Issue of the IEEE Transactions on Terahertz Science and Technology dedicated to the 28th IEEE International Symposium on Space Terahertz Technology (ISSTT2017

Influence of spin filtering and spin mixing on the subgap structure of I-V characteristics in superconducting quantum point contact

The effect of spin filtering and spin mixing on the dc electric current for voltage biased magnetic quantum point contact with superconducting leads is theoretically studied. The I-V characteristics are calculated for the whole range of spin filtering and spin mixing parameters. It is found that with increasing of spin filtering the subharmonic step structure of the dc electric current, typical for low-transparency junction and junction without considerable spin filtering qualitatively changes. In the lower voltage region and for small enough spin mixing the peak structure arises. When spin mixing increases the peak subgap structure evolves to the step structure. The voltages where subharmonic gap features are located are found to be sensitive to the value of spin filtering. The positions of peaks and steps are calculated analytically and the evolution of the subgap structure from well-known tunnel limit to the large spin filtering case is explained in terms of multiple Andreev reflection (MAR) processes. In particular, it is found that for large spin filtering the subgap feature at $eV_k$ arises from $2k^{\rm th}$ and $(2k\pm 1)^{\rm th}$ order MAR processes, while in the tunnel limit the step at $eV_n$ is known to result from $n^{\rm th}$ order MAR process.Comment: 9 pages, 3 figure

Subharmonic Gap Structure in Superconductor/Ferromagnet/Superconductor Junctions

The behavior of dc subgap current in magnetic quantum point contact is discussed for the case of low-transparency junction with different tunnel probabilities for spin-up ($D_\uparrow$) and spin-down ($D_\downarrow$) electrons. Due to the presence of Andreev bound states $\pm \epsilon_0$ in the system the positions of subgap electric current steps $eV_n = (\Delta \pm \epsilon_0)/n$ are split at temperature $T \neq 0$ with respect to the nonmagnetic result $eV_n=2\Delta/n$. It is found that under the condition $D_\uparrow \neq D_\downarrow$ the spin current also manifests subgap structure, but only for odd values of $n$. The split steps corresponding to $n=1,2$ in subgap electric and spin currents are analytically calculated and the following steps are described qualitatively.Comment: 4 pages, 1 figure, minor stylistic changes, journal-ref adde

Nonequilibrium Transport in Superconductor/Ferromagnet/Superconductor Diffusive Junctions: Interplay between Proximity Effect and Ferromagnetism

The theory of the I-V characteristics in diffusive superconductor/weak ferromagnet/superconductor (SFS) junction is developed. We show that the exchange field $h$ of the ferromagnet manifests itself as an additional conductance peak at $eV \sim \Delta+h$ in the phase-coherent regime, when the Thouless energy is of the order of superconducting order parameter. The excess current exhibits non-monotonous dependence on the exchange field and non-trivial temperature behavior, which is strongly influenced by the temperature dependence of the exchange field.Comment: 5 pages, 2 figure

Coherent back-scattering near the two-dimensional metal-insulator transition

We have studied corrections to conductivity due to the coherent backscattering in low-disordered two-dimensional electron systems in silicon for a range of electron densities including the vicinity of the metal-insulator transition, where the dramatic increase of the spin susceptibility has been observed earlier. We show that the corrections, which exist deeper in the metallic phase, weaken upon approaching to the transition and practically vanish at the critical density, thus suggesting that the localization is suppressed near and at the transition even in zero field.Comment: to appear in PR

Noise and Full Counting Statistics of Incoherent Multiple Andreev Reflection

We present a general theory for the full counting statistics of multiple Andreev reflections in incoherent superconducting-normal-superconducting contacts. The theory, based on a stochastic path integral approach, is applied to a superconductor-double barrier system. It is found that all cumulants of the current show a pronounced subharmonic gap structure at voltages $V=2\Delta/en$. For low voltages $V\ll\Delta/e$, the counting statistics results from diffusion of multiple charges in energy space, giving the $p$th cumulant $\propto V^{2-p}$, diverging for $p\geq 3$. We show that this low-voltage result holds for a large class of incoherent superconducting-normal-superconducting contacts.Comment: 4 pages, 4 figure

Spin-independent origin of the strongly enhanced effective mass in a dilute 2D electron system

We have accurately measured the effective mass in a dilute two-dimensional electron system in silicon by analyzing temperature dependence of the Shubnikov-de Haas oscillations in the low-temperature limit. A sharp increase of the effective mass with decreasing electron density has been observed. Using tilted magnetic fields, we have found that the enhanced effective mass is independent of the degree of spin polarization, which points to a spin-independent origin of the mass enhancement and is in contradiction with existing theories

Reentrant behavior in the superconducting phase-dependent resistance of a disordered 2-dimensional electron gas

We have investigated the bias-voltage dependence of the phase-dependent differential resistance of a disordered T-shaped 2-dimensional electron gas coupled to two superconducting terminals. The resistance oscillations first increase upon lowering the energy. For bias voltages below the Thouless energy, the resistance oscillations are suppressed and disappear almost completely at zero bias voltage. We find a qualitative agreement with the calculated reentrant behavior of the resistance and discuss quantitative deviations.Comment: 4 pages, 5 figures, to be published in Phys. Rev.

Theory of Nonequilibrium Spin Transport and Spin Transfer Torque in Superconducting-Ferromagnetic Nanostructures

Spin transport currents and the spin-transfer torques in voltage-biased superconducting-ferromagnetic nanopillars (SFNFS point contacts) are computed. We develop and implement an algorithm based on the Ricatti formulation of the quasiclassical theory of superconductivity to solve the time-dependent boundary conditions for the nonequilibrium Green's functions for spin transport through the ferromagnetic interfaces. A signature of the nonequilibrium torque is a component perpendicular to the plane spanned by the two ferromagnetic moments. The perpendicular component is absent in normal-metal-ferromagnetic nanopillars (NFNFN) contacts, but is shown to have the same order of magnitude as the in-plane torque for non-equilibrium SFNFS contacts. The out-of-plane torque is due to the rotation of quasiparticle spin by the exchange fields of the ferromagnetic layers. In the ballistic limit the equilibrium torque is related to the spectrum of spin-polarized Andreev bound states, while the {\sl ac} component, for small bias voltages, is determined by the nearly adiabatic dynamics of the Andreev bound states. The nonlinear voltage dependence of the non-equilibrium torque, including the subharmonic gap structure and the high-voltage asymptotics, is attributed to the interplay between multiple Andreev reflections, spin filtering and spin mixing. These properties of spin angular momentum transport may be exploited to control the state of nanomagnets.Comment: 15 pages, 14 figure