Cooper Pair Recombination Research Articles

Anisotropic purity of entangled photons from Cooper pairs in heterostructures

It was theoretically proposed that a forward-biased p-n junction with a superconducting layer (P-N-S), would produce pure, polarization entangled photons due to inherent spin-singlet pairing of electrons. However, any heterostructure interface generically induces Rashba spin-orbit coupling, which in turn generates a mixed singlet-triplet superconducting order parameter. Here we study the effect of triplet pairing on the purity of photons produced through Cooper pair recombination. A unique directional dependence of the state purity is found for a triplet superconductor with fixed $\vec{d}$: pure, entangled photons are produced when the photon polarization axis is parallel to $\vec{d}$. Induced triplet pairing in a singlet superconductor is shown to degrade the state purity, while induced singlet pairing in a triplet superconductor is shown to enhance the production of entangled pairs. These considerations may aid the design of functional devices to produce entangled photons.

Time-resolved measurements of Cooper-pair radiative recombination in InAs quantum dots

We studied InAs quantum dots (QDs) where electron Cooper pairs penetrate from an adjacent niobium (Nb) superconductor with the proximity effect. With time-resolved luminescence measurements at the wavelength around 1550 nm, we observed luminescence enhancement and reduction of luminescence decay time constants at temperature below the superconducting critical temperature (TC) of Nb. On the basis of these measurements, we propose a method to determine the contribution of Cooper-pair recombination in InAs QDs. We show that the luminescence enhancement measured below TC is well explained with our theory including Cooper-pair recombination.

Cooper-Pair Radiative Recombination in Semiconductor Heterostructures: Impact on Quantum Optics and Optoelectronics

The injection of Cooper pairs into a normal medium such as a semiconductor is known as the proximity effect at the superconductor/normal interface. We confirm this injection as well as the contribution of Cooper pairs to a drastic enhancement of inter-band optical transitions in semiconductor heterostructures. In this paper we investigate and clarify the relation of internal quantum efficiencies and radiative lifetimes in Cooper-pair light emitting diodes (CP-LEDs). A quantitative description of the dynamic photon generation processes is given, and the contribution of the Cooper-pair recombination relative to normal-electron recombination in CP-LEDs is discussed in detail.

Momentum-Resolved Ultrafast Electron Dynamics in SuperconductingBi2Sr2CaCu2O8+δ

The nonequilibrium state of the high-T(c) superconductor Bi(2)Sr(2)CaCu(2)O(8+δ) and its ultrafast dynamics have been investigated by femtosecond time- and angle-resolved photoemission spectroscopy well below the critical temperature. We probe optically excited quasiparticles at different electron momenta along the Fermi surface and detect metastable quasiparticles near the antinode, since their decay toward the nodal region through e-e scattering is blocked by phase space restrictions. The observed lack of momentum dependence in the decay rates is in agreement with relaxation dynamics dominated by Cooper pair recombination in a boson bottleneck limit.

Neutrino emission due to Cooper-pair recombination in neutron stars reexamined

Neutrino emission in processes of breaking and formation of neutron and proton Cooper pairs is calculated within the Larkin-Migdal-Leggett approach for a superfluid Fermi liquid. We demonstrate explicitly that the Fermi-liquid renormalization respects the Ward identity and ensures the weak vector-current conservation. The systematic expansion of the emissivities for small temperatures and nucleon Fermi velocity ${v}_{\mathrm{F},i},i=n,p,$ is performed. Both neutron and proton processes are mainly controlled by the axial-vector current contributions, which are not strongly changed in the superfluid matter. Thus, compared to earlier calculations, the total emissivity of processes on neutrons paired in the $1{S}_{0}$ state is suppressed by a factor of $\ensuremath{\simeq}(0.9\text{\ensuremath{-}}1.2){v}_{\mathrm{F},n}^{2}$. A similar suppression factor ($~{v}_{\mathrm{F},p}^{2}$) arises for processes on protons.

Ultrafast carrier dynamics in optically excited YBa/sub 2/Cu/sub 3/O/sub 7-δ/ thin films diagnosed by a pump-and-probe terahertz experiment

A pump-and-probe terahertz emission technique is reported to investigate the nonequilibrium process of ultrafast generation and relaxation of hot quasiparticles and their recombination to form Cooper pairs in superconducting YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films following femtosecond laser excitation. Subpicosecond electromagnetic pulses emitted by an YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// microbridge change after an excitation of the material due to an additional laser pulse, the so-called pump pulse. The modulation of the emitted terahertz pulse amplitude is used to measure the time resolved evaluation of the hot carrier excitation and relaxation after a time delay to the pump laser pulse. The results are compared to terahertz pump-and-probe experiments on GaAs photoswitches and can be explained by avalanche Cooper pair breaking, supercurrent modulation followed by carrier acceleration and Cooper pair recombination accompanied with deceleration of the supercurrent carriers.

Pump-and-probe terahertz method to investigate YBa 2Cu 3O 7− δ thin films

Ultrafast optical modulation of the super carrier density inside of a YBa 2Cu 3O 7− δ dipole antenna structure can be used to generate single subpicosecond electromagnetic pulses, i.e. terahertz (THz) radiation. We investigate the dependence of the emitted THz radiation on an addition excitation of the material by means of a common pump-and-probe technique. After femtosecond laser excitation the pump induced changes of the signal are connected to the nonequilibrium quasiparticle dynamics. This process contains both the ultrafast generation and relaxation of hot quasiparticles and their recombination to Cooper pairs in superconducting YBa 2Cu 3O 7− δ thin films. The modulation of the emitted THz pulse amplitude is used to measure the time resolved evaluation of the hot carrier excitation and relaxation after a time delay to the pump laser pulse. The results can be explained by the avalanche Cooper pair breaking, by the supercurrent modulation followed by carrier acceleration and by the Cooper pair recombination which cause the deceleration of the supercurrent carriers.

Divergence of the quasiparticle lifetime with doping and evidence for pre-formed pairs below T* in YBa 2Cu 3O 7−δ: Direct measurements by femtosecond time-resolved spectroscopy

We report new time-resolved data of quasiparticle relaxation and Cooper pair recombination dynamics in YBa 2Cu 3O 7−δ, measured as a function of temperature and doping δ using femtosecond optical spectroscopy. The data show the existence of a normal state pseudo-gap for in-plane charge excitations below T* and an unusual divergence of the quasiparticle relaxation time in the superconducting state τ s with δ towards optimum doping. In the underdoped state, no change in the amplitude of the induced transmission (which is proportional to the DOS at E F), or relaxation time τ s is observed at T c. From the sum rule, both observations signify that no gap opens at T c in underdoped YBCO for δ>0.15. T c in this case only signifies the onset of phase coherence. The presented data thus suggest pair formation with an associated redistribution of the DOS starting at T* and the establishment of phase coherence at T c consistent with Bose–Einstein condensation. In the optimally doped material, δ≈0.1, on the other hand, both a divergence in lifetime and a change of the DOS occur at T c, signifying the opening of a gap and the occurrence of pairing takes place simultaneously.