Internal Josephson Oscillations Research Articles

Spin Mixing Dynamics in a Spin–Orbit Coupled Bose–Einstein Condensate

We investigate the spin mixing dynamics of a spin–orbit coupled Bose–Einstein condensate by using a variational approach. It is shown that the spin mixing dynamics can be well described by an internal Josephson equation. In the nonzero momentum phase, the spin mixing dynamics leads to a spin polarization, i.e., most atoms are suppressed in the one of the two spin states. In the zero momentum phase, the internal Josephson oscillation between the two spin states takes place. Thus, the coupling effects of spin–orbit coupling, Raman coupling and atomic interaction can result in the transition between the spin polarization and the internal Josephson oscillation of the two spin states, which provide a theoretical evidence for elaborating the spin mixing dynamics. Moreover, at the phase transition point, the oscillation period of the spin mixing dynamics tends to infinity. This can be used to explore the phase transition experimentally. The accurate manipulation of the spin mixing dynamics can provide a theoretical evidence for designing the atomic device.

Dynamics of a Spinor Exciton–Polariton System in Laterally Strained GaAs Microcavities under Resonant Photoexcitation

The evolution of the spatial coherence and the polarization has been studied in a freely decaying polariton condensate that is resonantly excited by linearly polarized picosecond laser pulses at the lower and upper sublevels of the lower polariton branch in a high-Q GaAs-based microcavity with a reduced lateral symmetry without excitation of the exciton reservoir. It is found that the condensate inherits the coherence of the exciting laser pulse at both sublevels in a wide range of excitation densities and retains it for several dozen picoseconds. The linear polarization of the photoexcited condensate is retained only in the condensate at the lower sublevel. The linearly polarized condensate excited at the upper sublevel loses its stability at the excitation densities higher a threshold value: it enters a regime of internal Josephson oscillations with strongly oscillating circular and diagonal linear degrees of polarization. The polariton–polariton interaction leads to the nonlinear Josephson effects at high condensate densities. All the effects are well described in terms of the spinor Gross–Pitaevskii equations. The cause of the polarization instability of the condensate is shown to be the spin anisotropy of the polariton–polariton interaction.

Solitons and Josephson-type oscillations in Bose-Einstein condensates with spin-orbit coupling and time-varying Raman frequency

The existence and dynamics of solitons in quasi-one-dimensional Bose-Einstein condensates (BEC) with spin-orbit coupling (SOC) and attractive two-body interactions are described for two coupled atomic pseudo-spin components with slowly and rapidly varying time-dependent Raman frequency. By varying the Raman frequency linearly in time, it was shown that ordinary nonlinear Schr\"odinger-type bright solitons can be converted to striped bright solitons and vice versa. The internal Josephson oscillations between atom-number of the coupled soliton components, and the corresponding center-of-mass motion, are studied for different parameter configurations. In this case, a mechanism to control the soliton parameters is proposed by considering parametric resonances, which can emerge when using time-varying Raman frequencies. Full numerical simulations confirm variational analysis predictions when applied to the region where regular solitons are expected. In the limit of high frequencies, the system is described by a time-averaged Gross-Pitaevskii formalism with renormalized nonlinear and SOC parameters and modified phase-dependent nonlinearities. By comparing full-numerical simulations with averaged results, we have also studied the lower limits for the frequency of Raman oscillations in order to obtain stable soliton solutions.

Динамика спинорной экситон-поляритонной системы в латерально сжатых GaAs микрорезонаторах при резонансном фотовозбуждении

AbstractThe evolution of the spatial coherence and the polarization has been studied in a freely decaying polariton condensate that is resonantly excited by linearly polarized picosecond laser pulses at the lower and upper sublevels of the lower polariton branch in a high-Q GaAs-based microcavity with a reduced lateral symmetry without excitation of the exciton reservoir. It is found that the condensate inherits the coherence of the exciting laser pulse at both sublevels in a wide range of excitation densities and retains it for several dozen picoseconds. The linear polarization of the photoexcited condensate is retained only in the condensate at the lower sublevel. The linearly polarized condensate excited at the upper sublevel loses its stability at the excitation densities higher a threshold value: it enters a regime of internal Josephson oscillations with strongly oscillating circular and diagonal linear degrees of polarization. The polariton–polariton interaction leads to the nonlinear Josephson effects at high condensate densities. All the effects are well described in terms of the spinor Gross–Pitaevskii equations. The cause of the polarization instability of the condensate is shown to be the spin anisotropy of the polariton–polariton interaction.

Internal Josephson oscillations for distinct momenta Bose-Einstein condensates

The internal Josephson oscillations between an atomic Bose-Einstein condensate (BEC) and a molecular one are studied for atoms in a square optical lattice subjected to a staggered gauge field. The system is described by a Bose-Hubbard model with complex and anisotropic hopping parameters that are different for each species, i.e., atoms and molecules. When the flux per plaquette for each species is small, the system oscillates between two conventional zero-momentum condensates. However, there is a regime of parameters in which Josephson oscillations between a vortex-carrying atomic condensate (finite momentum BEC) and a conventional zero-momentum molecular condensate may be realized. The experimental observation of the oscillations between these qualitatively distinct BEC's is possible with state-of-the-art Ramsey interference techniques.

Synchronization in stacked array of the Josephson junctions in Bi 2Sr 2CaCu 2O 8+δ

Intrinsic stacked array of Josephson junctions will find potential application as voltage standard or HTSC Josephson spectral detector if internal Josephson oscillations in different layers can be synchronized under electromagnetic radiation. In our experiments we studied synchronization is stacked array of Josephson oscillations contained up to 2500 junctions in Bi 2Sr 2CaCu 2O 8+δ. Phase locking of the internal Josephson oscillation in array to externally applied microwave radiation was manifested by observation of the collective Shapiro steps. Synchronization of all junctions in array was possible in the vortex glass state in high magnetic fields and vortex liquid state on the B– T c-axis vortex phase diagram in frequency range 80–140 GHz. In those regions damping is significantly enhanced and hysteresis on I– V characteristics collapsed. Observations deny major opinion that intrinsic Josephson junctions in Bi 2Sr 2CaCu 2O 8+δ are always underdamped. We interpret our results as a reflecting the pancake vortex disorder and fluctuations in CuO 2 layers.

Applications of Superconductivity for Implementation of Phase Conjugation in the Microwave Region

This paper extends a preliminary proposal and description of a superconductivity-based phase conjugating antenna for operation in the microwave range. The complementary properties of high-temperature superconductors and other oxide functional materials are emphasised. The potential applications of such an antenna are very wide. We consider the requirements for the Josephson junction which is at the heart of the phase conjugation operation, especially issues of the use of the internal Josephson oscillation or of an external signal to provide the local oscillator requirement. Other design requirements are also explored and the necessary substrate properties are set out which enable suitable junction properties, as well as phase shifter and interconnection functions.

Shapiro step response in Bi 2Sr 2CaCu 2O 8+ δ in parallel and tilted magnetic field

We report the first observation of Shapiro step response in the Josephson flux-flow state of Bi 2Sr 2CaCu 2O 8+ δ in strong magnetic field ( H ∥>1 T) in ab plane. Phase of external microwave radiation and internal Josephson oscillation were locked in whole stack ( N=67) of intrinsic Josephson junctions. Steps were observed at voltages V st= pNhν/2 e, with p integer, ν=45–142 GHz. As a matter of fact V st remains constant and demonstrates that the Josephson vortices move at the same speed over the whole stack of junctions keeping the high special regularity. We also observed Shapiro steps at fields tilted with respect to the c-axis. The height of Shapiro steps and Josephson critical current increase sharply with increasing tilted angle and then both decrease at larger angle. We attribute this anomaly to the interaction between Josephson and pancake vortices.

Resistively-shunted Bi 2Sr 2CaCu 2O 8+ x intrinsic Josephson junctions

Observation of Shapiro steps is probably the most convincing demonstration of the Josephson effects in superconducting weak links. To observe Shapiro steps in Bi 2Sr 2CaCu 2O 8+ x intrinsic Josephson junctions (IJJs), we used side-wall of gold to resistively-shunt these junctions in a conventional mesa structure. In the current–voltage ( I– V) characteristics of these shunted IJJs, the voltage jumps and the hysteresis were greatly reduced. In particular, individual Shapiro steps were clearly observed with irradiation up to 100 GHz, thus providing direct evidence for the ac Josephson effect in intrinsic high- T c stack junctions. Also discussed will be the conditions for achieving stable phase locking between the external irradiation and internal Josephson oscillation. We believe that resistive shunting of the junction stacks makes it possible for them to be used in on-chip detection or coherent oscillation up to terahertz region.

Design and fabrication of a voltage divider utilizing high-T/sub c/ ramp-edge Josephson junctions with a ground plane

Circuit operation of a voltage divider utilizing YBa/sub 2/Cu/sub 3/O/sub x//Co-doped PrBa/sub 2/Cu/sub 3/O/sub y//YBa/sub 2/Cu/sub 3/O/sub x/ ramp-edge Josephson junctions with a ground plane was demonstrated for the voltage region of 0-400 /spl mu/V at 12.5 K, where the maximum voltage corresponds to /spl sim/200 GHz internal Josephson oscillation. This voltage was around the critical current-normal resistance (I/sub c/R/sub n/) product of 420 /spl mu/V. In the design of the voltage divider, a transmission line model was adopted for an inductance loop because of the large dielectric constant of the insulation layer interposed between the ground plane and the wiring serving as a loop inductance. In the fabrication of the circuit, a SrTiO/sub 3//Co-doped PrBa/sub 2/Cu/sub 3/O/sub y//SrTiO/sub 3/ multilayer insulation and four-directional ramp-edge junction process using a photoresist reflow technique were adopted.

Low noise parametric upconversion with a self-pumped Josephson junction

Experimental results are reported on a Josephson parametric upconverting amplifier which is self-pumped via the internal Josephson oscillations derived from a dc bias voltage. A niobium point-contact SQUID is simultaneously coupled to a 115-MHz tank circuit and a 9-GHz cavity. Josephson oscillations at 9 GHz produce parametric upconversion of power in the 115-MHz circuit to sidebands at 9±0.115 GHz with measured power gain of 25. The amplifier noise temperature, referred to the input at 115 MHz, was established to be <15 K by varying the temperature of an input termination from 4.2 to 300 K. This result establishes the behavior of self-pumped SQUID upconverters at microwave frequencies and demonstrates that the achievable noise performance is not greatly in excess of the thermal limit.

Self-Pumped Josephson Parametric Amplification

Parametric gain has been demonstrated at 30 MHz using the nonlinear inductance of Josephson point-contact devices. SQUID-type configurations were used to achieve degenerate-mode negative-resistance amplification with self-pumping achieved via the internal Josephson oscillations. Gain as large as 11 dB was measured. The small-signal analysis predicts parametric amplification and delineates the importance of the critical current, circuit loss, and circuit coupling efficiency.