We consider a ballistic Josephson junction with a quantum point contact in a two-dimensional electron gas with Rashba spin-orbit coupling. The point contact acts as a spin filter when embedded in a circuit with normal electrodes. We show that with an in-plane external magnetic field an anomalous supercurrent appears even for zero phase difference between the superconducting electrodes. In addition, the external field induces large critical current asymmetries between the two flow directions, leading to supercurrent rectifying effects. PACS numbers: 74.45.+c, 71.70.Ej, 72.25.Dc, 74.50.+r Josephson junctions (JJ) are the basic building blocks for superconducting electronics with applications that range from SQUID magnetometers to possible quantum computing devices. In superconductor-normal metal-superconductor (S-N-S) junctions the supercurrent flow is due to the Andreev states-a coherent superposition of electron and holes states. These states depend on the electronic structure of the normal material and on the properties of the S-N interface [1,2,3,4]. Modern technologies based on two dimensional electron gases (2DEGs) [5,6] or nanowires [7] allow for a precise control of such electronic properties, and thus of the JJ characteristics. Moreover, spin-orbit (SO) effects offer new alternatives to control the spin and charge transport [8,9].Superconducting rectifiers are among the new devices proposed and studied during the last few years. Most of these proposals are based on the dynamics of vortices [10,11]. Here we show that in systems with SO-coupling rectifying properties can be obtained by controlling the spin of the Andreev states. To this end we consider a ballistic JJ with a quantum point contact (QPC) in a 2DEG with SO interaction. The QPC can be tuned to control the number of transmitting channels and thus the critical current of the junction [3,4,5,12,13]. On the other hand, the QPC with SO coupling may act as a spin filter producing spin-polarized currents when embedded in a circuit with normal leads [14,15,16]. The normal current also generates an in-plane magnetization-perpendicular to the current -as well as out-of-plane spin-Hall textures [17]. Both effects are maximized at the core of the QPC [18]. As the SO-coupling preserves time-reversal symmetry (TRS), we expect that these peculiarities of the transmitting channels do not harm the Josephson effect when the leads become superconducting. However, the Josephson current itself breaks the TRS and, as we show below, it reveals striking effects of the SO-coupling. For example, the supercurrent generates spin polarization in the 2DEG [19] and the QPC in a similar way normal current does [17,18]. This is due to the distinctive spin texture of each Andreev state that contributes to the local magnetization in a supercurrent-carrying state.More striking effects take place if an external in-plane magnetic field is applied. Its effect on the supercurrent characteristics depends on the nature of the junction. In the absence of SO-coupling, the Zeeman field ...

We study Josephson junctions (JJs) in which the region between the two superconductors is a multichannel system with Rashba spin-orbit coupling (SOC) where a barrier or a quantum point contact (QPC) is present. These systems might present unconventional Josephson effects such as Josephson currents for zero phase difference or critical currents that depend on the current direction. Here, we discuss how the spin polarizing properties of the system in the normal state affect the spin characteristics of the Andreev bound states inside the junction. This results in a strong correlation between the spin of the Andreev states and the direction in which they transport Cooper pairs. While the current-phase relation for the JJ at zero magnetic field is qualitatively unchanged by SOC, in the presence of a weak magnetic field, a strongly anisotropic behavior and the mentioned anomalous Josephson effects follow. We show that the situation is not restricted to barriers based on constrictions such as QPCs and should generically arise if in the normal system the direction of the carrier's spin is linked to its direction of motion.

Quantum wires subject to the combined action of spin-orbit and Zeeman coupling in the presence of \emph{s}-wave pairing potentials (superconducting proximity effect in semiconductors or superfluidity in cold atoms) are one of the most promising systems for the developing of topological phases hosting Majorana fermions. The breaking of time-reversal symmetry is essential for the appearance of unpaired Majorana fermions. By implementing a \emph{time-dependent} spin rotation, we show that the standard magnetostatic model maps into a \emph{non-magnetic} one where the breaking of time-reversal symmetry is guaranteed by a periodical change of the spin-orbit coupling axis as a function of time. This suggests the possibility of developing the topological superconducting state of matter driven by external forces in the absence of magnetic fields and magnetic elements. From a practical viewpoint, the scheme avoids the disadvantages of conjugating magnetism and superconductivity, even though the need of a high-frequency driving of spin-orbit coupling may represent a technological challenge. We describe the basic properties of this Floquet system by showing that finite samples host unpaired Majorana fermions at their edges despite the fact that the bulk Floquet quasienergies are gapless and that the Hamiltonian at each instant of time preserves time-reversal symmetry. Remarkably, we identify the mean energy of the Floquet states as a topological indicator. We additionally show that the localized Floquet Majorana fermions are robust under local perturbations. Our results are supported by complementary numerical Floquet simulations.Comment: 18 pages, 6 figure

Investigation of the decay processes leading to the phonon lifetime of optically excited vibrational modes of few-layer MoSe2 membranes.

We study the edge states in a two dimensional electron gas with a transverse magnetic field and Rashba spin-orbit coupling. In the bulk, the interplay between the external field perpendicular to the gas plane and the spin-orbit coupling leads to two branches of states that, within the same energy window, have different cyclotron radii. For the edge states, surface reflection generates hybrid states with the two cyclotron radii. We analyze the spectrum and spin structure of these states and present a semiclassical picture of them.

We study the spin polarization induced by a current flow in clean two dimensional electron gases with Rashba spin-orbit coupling. This geometric effect originates from special properties of the electron's scattering at the edges of the sample. In wide samples, the spin polarization has it largest value at low energies (close to the bottom of the band) and goes to zero at higher energies. In this case, the spin polarization is dominated by the presence of evanescent modes which have an explicit spin component outside the plane. In quantum wires, on the other hand, the spin polarization is dominated by interference effects induced by multiple scattering at the edges. Here, the spin polarization is quite sensitive to the value of the Fermi energy, especially close to the point where a new channel opens up. We analyzed different geometries and found that the spin polarization can be strongly enhanced.

Efficient generation of phonons is an important ingredient for a prospective electrically-driven phonon laser. Hybrid quantum systems combining cavity quantum electrodynamics and optomechanics constitute a novel platform with potential for operation at the extremely high frequency range (30–300 GHz). We report on laser-like phonon emission in a hybrid system that optomechanically couples polariton Bose-Einstein condensates (BECs) with phonons in a semiconductor microcavity. The studied system comprises GaAs/AlAs quantum wells coupled to cavity-confined optical and vibrational modes. The non-resonant continuous wave laser excitation of a polariton BEC in an individual trap of a trap array, induces coherent mechanical self-oscillation, leading to the formation of spectral sidebands displaced by harmonics of the fundamental 20 GHz mode vibration frequency. This phonon “lasing” enhances the phonon occupation five orders of magnitude above the thermal value when tunable neighbor traps are red-shifted with respect to the pumped trap BEC emission at even harmonics of the vibration mode. These experiments, supported by a theoretical model, constitute the first demonstration of coherent cavity optomechanical phenomena with exciton polaritons, paving the way for new hybrid designs for quantum technologies, phonon lasers, and phonon-photon bidirectional translators.

It has been predicted recently that an electron beam can be polarized when it flows adiabatically through a quantum point contact in a system with spin-orbit interaction. Here, we show that a simple transverse electron focusing setup can be used to detect such polarized current. It uses the amplitude's asymmetry of the spin-split transverse electron focusing peak to extract information about the electron's spin polarization. On the other hand, and depending on the quantum point contact geometry, including this one-body effect can be important when using the focusing setup to study many-body effects in quantum point contacts.