A. M. Kadigrobov

Influence of the Rashba effect on the Josephson current through a superconductor/Luttinger liquid/superconductor tunnel junction

The Josephson current through a 1D quantum wire with Rashba spin-orbit and electron-electron interactions is calculated. We show that the interplay of Rashba and Zeeman interactions gives rise to a supercurrent through the 1D conductor that is anomalous in the sense that it persists in the absence of any phase difference between the two superconducting leads to which it is attached. The electron dispersion asymmetry induced by the Rashba interaction in a Luttinger-liquid wire plays a significant role for poorly transmitting junctions. It is shown that for a weak or moderate electron-electron interaction the spectrum of plasmonic modes confined to the normal part of the junction becomes quasi-random in the presence of dispersion asymmetry.

Hot electrons in magnetic point contacts as a photon source

We propose to use a point contact between a ferromagnetic and a normal metal in the presence of a magnetic field for creating a large inverted spin population of hot electrons in the contact core. The key point of the proposal is that when these hot electrons relax by flipping their spin, microwave photons are emitted, with a frequency tunable by the applied magnetic field. While point contacts are an established technology, their use as a photon source is a new and potentially very useful application. We show that this photon emission process can be detected by means of transport spectroscopy and demonstrate stimulated emission of radiation in the 10-100GHz range for a model point contact system using a minority-spin ferromagnetic injector. These results can potentially lead to new types of lasers based on spin injection in metals.

Stimulated emission and absorption of photons in magnetic point contacts

Point contacts between high anisotropy ferromagnetic SmCo5 and normal metal Cu are used to achieve a strong spin-population inversion in the contact core. Subjected to microwave irradiation in resonance with the Zeeman splitting in Cu, the inverted spin population relaxes through stimulated spin-flip photon emission, detected as peaks in the point-contact resistance. Resonant spin-flip photon absorption is detected as resistance minima, corresponding to sourcing the photon field energy into the electrical circuit. These results demonstrate fundamental mechanisms that are potentially useful in designing metallic spin-based lasers.

Magnetotransport along a barrier: Multiple quantum interference of edge states

Transport in a two-dimensional (2D) electron gas subject to an external magnetic field is analyzed in the presence of a longitudinal barrier. We show that quantum interference of the edge states bound by the longitudinal barrier results in a drastic change of the electron motion: the degenerate discrete Landau levels are transformed into an alternating sequence of energy bands and energy gaps. These features of the electron spectrum should result in a high sensitivity of thermodynamic and transport properties of the 2D electron gas to external fields. In particular, we predict giant oscillations of the ballistic conductance and discuss nonlinear current-voltage characteristics, coherent Bloch oscillations, and effects of impurities.

Self-excited oscillations of charge-spin accumulation due to single-electron tunneling

We theoretically study electronic transport through a layer of quantum dots connecting two metallic leads. By the inclusion of an inductor in series with the junction, we show that steady electronic transport in such a system may be unstable with respect to temporal oscillations caused by an interplay between the Coulomb blockade of tunneling and spin accumulation in the dots. When this instability occurs, a new stable regime is reached, where the average spin and charge in the dots oscillate periodically in time. The frequency of these oscillations is typically on the order of 1 GHz for realistic values of the junction parameters.

Joule heating and current-induced instabilities in magnetic nanocontacts

We consider the electrical current through a magnetic point contact in the limit of a strong inelastic scattering of electrons. In this limit local Joule heating of the contact region plays a decisive role in determining the transport properties of the point contact. We show that if an applied constant bias voltage exceeds a critical value, the stationary state of the system is unstable, and that periodic, nonharmonic oscillations in time of both the electrical current through the contact and the local temperature in the contact region develop spontaneously. Our estimations show that the necessary experimental conditions for observing such oscillations with characteristic frequencies in the range 10(8)-10(9) Hz can easily be met. We also show a possibility to manipulate upon the magnetization direction of a magnetic grain coupled through a point contact to a bulk ferromagnet by exciting the above-mentioned thermal-electric oscillations.

Thermal-magnetic-electric oscillator based on spin-valve effect

A. M. Kadigrobov, 2 S. Andersson, Hee Chul Park, 4 D. Radić, 5 R. I. Shekhter, M. Jonson, 6, 7 and V. Korenivski Department of Physics, University of Gothenburg, SE-412 96 Göteborg, Sweden Theoretische Physik III, Ruhr-Universität Bochum, D-44801 Bochum, Germany Nanostructure Physics, Royal Institute of Technology, SE-106 91 Stockholm, Sweden Department of Physics, Chungnam National University, Daejeon 305-764, Republic of Korea Department of Physics, Faculty of Science, University of Zagreb, 1001 Zagreb, Croatia School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, Scotland, UK 7 Division of Quantum Phases and Devices, School of Physics, Konkuk University, Seoul 143-701, Republic of KoreaA thermal-magnetic-electric valve with the free layer of exchange-spring type and inverse magnetoresistance is investigated. The structure has S-shaped current-voltage characteristics and can exhibit spontaneous oscillations when integrated with a conventional capacitor within a resonator circuit. The frequency of the oscillations can be controlled from essentially dc to the GHz range by the circuit capacitance.

Microwave-induced spin-flip scattering of electrons in point contacts

We investigate resonant interaction of conduction electrons with an electromagnetic field that irradiates a point contact between a ferromagnetic and a normal metal in the presence of a strong magnetic field of order 1 T. We show that electron spin-flips caused by resonant absorption and stimulated emission of photons result in a sharp peak in the magnetic-field dependence of the point-contact resistance. The height of the peak is shown to be directly proportional to the net rate of energy transfer to the electromagnetic field in the point contact due to absorption and stimulated emission of photons. Estimations indicate that our theory can serve as a basis for the explanation of recent experiments [A.M. Kadigrobov et al., New J. Phys. 13, 023007 (2011)].

Stimulated emission of radiation using spin-population inversion in metals

The discovery of masers and lasers has led to major breakthroughs in science and technology. Later important developments include compact semiconductor lasers for visible to near-infrared and quantum-cascade lasers for far-infrared to THz radiation. In conclusion, we have fabricated ballistic-range magnetic point contact arrays integrated in IR-THz range metal-oxide resonators and observed threshold-type I-V excitations of giant magnitude, correlated with optical emission from the devices, consistent with the expected spin-flip laser effect in the system.

Spin laser based on magnetic nano-contact array

Arrays of 10 nm diameter point contacts of exchange-coupled spin-majority/spin-minority ferromagnetic metals, integrated into infrared-terahertz range photon resonators, are fabricated and measured electrically and optically. Giant, threshold-type electronic excitations under high-current pumping of the devices are observed as abrupt but reversible steps in device resistance, in many cases in excess of 100%, which correlate with optical emission from the devices. The results are interpreted as due to stimulated spin-flip electron-photon relaxation in the system.The discovery of masers and lasers has led to major breakthroughs in science and technology. Later important developments include compact semiconductor lasers for visible to near-infrared and quantum-cascade lasers for far-infrared to THz radiation. In conclusion, we have fabricated ballistic-range magnetic point contact arrays integrated in IR-THz range metal-oxide resonators and observed threshold-type I-V excitations of giant magnitude, correlated with optical emission from the devices, consistent with the expected spin-flip laser effect in the system.