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Register a new userCarrier flow and nonequilibrium superconductivity in superconductor-based light-emitting diode
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Superconductor-based light-emitting diode (superconductor-based LED) in strong light-confinement regime are characterized as a superconductor-based three-terminal device, and its transport properties are quantitatively investigated. In the gate-controlled region, we confirm the realization of new-type Josephson field effect transistor (JoFET) performance, where the channel cross-sectional area of the junction is directly modulated by the gate voltage. In the current-injected region, the superconducting critical current of $mu$A order in the Josephson junction is found to be modulated by the steady current injection of pA order. This ultrahigh monitoring sensitivity of the radiative recombination process can be explained by taking into account the fact that the energy relaxation of the absorbed photons causes the conversion of superconducting pairs to quasiparticles in the active layer. Using quasiparticle density and superconducting pair density, we discuss the carrier flows together with the non-equilibrium superconductovity in the active layer and the superconducting electrodes, which take place for compensating the conversion.
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Recent experiments, in which Terahertz (THz) light has been used to induce nonequilibrium superconducting states, have raised a number of intriguing fundamental questions. Theoretically, these experiments are most often described within the Floquet formalism, which suffers a number of well-known limitations (e.g. Floquet heating). Alternative approaches rely on heavy numerical methods. In this Article we develop an analytical theory of nonequilibrium superconductivity that combines path integrals on the Kostantinov-Perel time contour with adiabatic perturbation theory [G. Rigolin, G. Ortiz, and V.H. Ponce, Phys. Rev. A~{bf 78}, 052508 (2008)]. We consider a general system of electrons and Raman phonons coupled by the Frohlich interaction, in the presence of a time-dependent external field which acts on the phonon subsystem. The latter is supposed to model the THz light-induced excitation of nonlinear interactions between infrared and Raman phonons. Assuming that the external field has a slow dependence on time, we derive equations for the dynamical superconducting gap, calculating the leading adiabatic term and the first non-adiabatic correction. Our nonequilibrium formulas can be solved numerically with a minimal increase of computational complexity with respect to that needed to calculate the superconducting gap at equilibrium.
We demonstrate Cooper-pairs drastic enhancement effect on band-to-band radiative recombination in a semiconductor. Electron Cooper pairs injected from a superconducting electrode into an active layer by the proximity effect recombine with holes injected from a p-type electrode and dramatically accelerate the photon generation rates of a light emitting diode in the optical-fiber communication band. Cooper pairs are the condensation of electrons at a spin-singlet quantum state and this condensation leads to the observed enhancement of the electric-dipole transitions. Our results indicate the possibility to open up new interdisciplinary fields between superconductivity and optoelectronics.
Planar light-emitting diodes (LEDs) fabricated within a single high-mobility quantum well are demonstrated. Our approach leads to a dramatic reduction of radiative lifetime and junction area with respect to conventional vertical LEDs, promising very high-frequency device operation. Devices were fabricated by UV lithography and wet chemical etching starting from p-type modulation-doped AlGaAs/GaAs heterostructures grown by molecular beam epitaxy. Electrical and optical measurements from room temperature down to 1.8 K show high spectral purity and high external efficiency. Time-resolved measurements yielded extremely short recombination times of the order of 50 ps, demonstrating the relevance of the present scheme for high-frequency device applications in the GHz range.
When both inversion and time-reversal symmetries are broken, the critical current of a superconductor can be nonreciprocal. In this work we show that in certain classes of two-dimensional superconductors with antisymmetric spin-orbit coupling, Cooper pairs acquire a finite momentum upon the application of an in-plane magnetic field, and as a result, critical currents in the direction parallel and antiparallel to the Cooper pair momentum become unequal. This supercurrent diode effect is also manifested in the polarity-dependence of in-plane critical fields induced by a supercurrent. These nonreciprocal effects may be found in polar SrTiO$_3$ film, few-layer MoTe$_2$ in the $T_d$ phase, and twisted bilayer graphene in which the valley degrees of freedom plays the role analogous to spin.
Layered superconductors have provided some interesting fields in condensed matter physics owing to the low dimensionality of their electronic states. For example, the high-Tc (high transition temperature) cuprates and the Fe-based superconductors possess a layered crystal structure composed of a stacking of spacer (blocking) layers and conduction (superconducting) layers, CuO2 planes or Fe-Anion layers. The spacer layers provide carriers to the conduction layers and induce exotic superconductivity. Recently, we have reported superconductivity in the novel BiS2-based layered compound Bi4O4S3. It was found that superconductivity of Bi4O4S3 originates from the BiS2 layers. The crystal structure is composed of a stacking of BiS2 superconducting layers and the spacer layers, which resembles those of high-Tc cuprate and the Fe-based superconductors. Here we report a discovery of a new type of BiS2-based layered superconductor LaO1-xFxBiS2, with a Tc as high as 10.6 K.
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