Tiege Zhou

Nonlinear Inductance of Intrinsic Josephson Junction Arrays and Its Application to Tunable Filters

Nonlinear inductance of intrinsic Josephson junction arrays in Tl-2212 thin films has been investigated. Misaligned Tl-2212 thin films were fabricated on 20deg vicinal substrates and then patterned into long bridges by standard photolithography and wet chemical etching. Each bridge forms a large array of intrinsic Josephson junctions. We have measured the Josephson inductance of the bridges in the zero-DC-voltage state at 77 K and show that its dependence on the critical current is in a good agreement with theory. Also, we have numerically studied the nonlinearity of the Josephson inductance and present dependence of the distortion factor on the bias and AC signal currents. Based on the obtained results, we have designed a 10-pole narrow-band lumped-element filter with the tunable center frequency in the range from 1900 MHz to 1960 MHz. Simulations show that the filter has good selectivity.

Carrier-dependent magnetic anisotropy of Gd-adsorbed graphene

Using first-principles calculation based on density functional theory, we study the magnetic anisotropy of Gd-adsorbed graphene and its dependence on carrier accumulation. We show that carrier accumulation not only impacts the magnitude of magnetic anisotropy but also switches its sign. Hole accumulation enhances the perpendicular anisotropy up to ∼16 meV per Gd atom, while electron accumulation switches the anisotropy from perpendicular to in-plane direction. Moreover, we find that the first order perturbation of spin-orbit coupling interaction induces a pseudo-gap at Γ for the perpendicular magnetization, which leads to the the anomalous magnetic anisotropy for the neutral composite. Our findings pave the way for magneto-electric materials based on rare-earth-decorated graphene for voltage-controlled spintronics.

Magnetic anisotropy in the boron nitride monolayer doped by 3d transitional metal substitutes at boron-site

Based on density functional theory with the generalized gradient approximation, a computational study is carried out to reveal the magnetism of the hexagonal boron nitride monolayers (h-BN) doped by 3d transitional metal atoms at boron-site. The magnetic moments and magnetic anisotropy energies (MAEs) of the most stable structures are given. Among the systems investigated, the MAE of Fe doped h-BN is 1.19 meV with an easy axis in the h-BN plane, while that of Mn is 0.63 meV with an easy axis perpendicular to the plane. For Sc and Co, the doped systems are nonmagnetic. For Ti, V, Cr, Ni, Cu, and Zn, the MAEs are relatively small. Theoretical analyses by perturbation theory and molecular orbital clarify that the MAE of Fe originates from the second order spin orbit coupling, whereas the MAE of Mn is contributed by the unquenched orbital momentum.

Spatiotemporal chaos in capacitively coupled intrinsic Josephson junction arrays and control by a shunted LCR series resonance circuit

The dynamics of capacitively coupled intrinsic Josephson junction arrays with a McCumber parameter βc<1 driven by a dc current are investigated by using numerical simulations. For the first time, spatiotemporal chaotic behaviors are found. Analysis of the dynamics of individual junctions shows that the chaos is caused by the strong diffusive coupling between the junctions in the array. The chaotic regions of different parameters are also presented, from which the conditions for chaos are deduced. The properties of the array shunted by an LCR series resonance circuit are also investigated. The results show that the spatiotemporal chaos can be effectively controlled by the shunted LCR circuit. This presents a new spatiotemporal chaos control method.

A computer-aided method for postproduction tuning of HTS filters

The empirical tuning of high temperature superconducting (HTS) filters is usually time-consuming and expensive. A computer-aided tuning method is presented in this paper. By means of optimization, the method diagnoses detuning of individual resonators quantitatively in the tuning process, and then by using neural network models the method is able to give the operator a decision on how much the corresponding tuning screws should be tuned. So, compared with the empirical tuning method, this method can be used to tune the HTS filters accurately with less time and cost.

Study on chaotic behaviors of RCLSJ model Josephson junctions

Chaotic behaviors of the dc-biased resistively-capacitively-inductively shunted Josephson junctions are studied numerically. The existence of the chaos is proved by the spectrum and strange attractor. We also find out the route to chaos is intermittence. The parameter space in which chaos exits is obtained, and different features of the chaos in different parameter range are also given.

HTS filter subsystem for future mobile communication system

A high-temperature superconducting (HTS) filter subsystem consisting of a 10-pole HTS filter with group delay self-equalization, a cryogenic low noise amplifier (LNA), and a cooling device is presented in this paper. The HTS filter has a 0.51% fractional bandwidth at 1955 MHz, simulated in Sonnet, and is fabricated using double-sided Tl2Ba2CaCu2O8 films on 0.5 mm thick LaAlO3 substrate. Measured responses show that the minimum insertion loss of the filter is 0.16 dB, and the out-of-band rejection is better than 75 dB. The slope is 27 dB MHz−1 at the low band edge and 22 dB MHz−1 at the upper band edge. The HTS filter subsystem has a gain of 19.3 dB and a noise figure (NF) of 0.8 dB. Furthermore, the phase distortion of HTS filters is considered, and an efficient design approach for self-equalized HTS filters is proposed.

An improved technique for the growth of (Tl0.95Bi0.2)(Sr0.8Ba0.2)2Ca2Cu3Ox superconducting thin films in oxygen and?argon

We fabricated (Tl0.95Bi0.2)(Sr0.8Ba0.2)2Ca2Cu3Ox (Tl, Bi-1223) superconducting thin films on a LaAlO3(001) substrate by a three-step process including magnetron sputtering and two post-annealing processes. In the post-annealing process we found that the (Tl0.95Bi0.2)(Sr0.8Ba0.2)2Ca1Cu2Ox (Tl, Bi-1212) phase formation played a very important role in obtaining the Tl, Bi-1223 superconducting phase. So, the precursors were annealed at lower temperature to form the Tl, Bi-1212 phase entirely, and then almost pure Tl, Bi-1223 phase superconducting thin films could be obtained by annealing the Tl, Bi-1212 film at higher temperature. The superconducting critical temperature Tc of the Tl, Bi-1223 film annealed in oxygen and in argon was 111 K and 107 K, respectively. The critical current density Jc (77 K, 0 T) of the Tl, Bi-1223 film annealed in oxygen was as high as 3.7 MA cm−2.

Preparation and characterization of Tl2Ba2CaCu2O8 superconducting films on biaxially textured Ni substrates

The preparation and properties of Tl-2212 (Tl2Ba2CaCu2O8) thin films on biaxially textured Ni substrates, buffered with CeO2/YSZ/CeO2 trilayers, have been studied. Tl-2212 thin films were grown using a two-step fabrication process that involved magnetron sputtering and low temperature post-annealing. XRD θ–2θ scans, rocking curves and -scans proved that the Tl-2212 thin films were epitaxially grown on the buffered RABiTS. The critical temperature Tc and critical current Jc (77 K, 0 T) were 103 K and 2.6 MA cm−2, respectively.

THz radiation sensing and emission: The phase-locking properties of Josphson junction arrays coupled with THz frequency Fabry-Perot resonators

A new simulation model of the Josephson junction array coupled with a Fabry-Perot resonator is presented in this paper, which considered the excitation of the resonance mode by the array and the influence of the resonance mode on the array. Taking advantage of the new model, we investigate the phase-locking properties of the junctions. The results include the influence of the Q value of the resonator, the strength of the coupling between the junctions and the resonator as well as the number of junctions in the array on the phase locking properties. It is useful in the detection of THz signal and fabrication of THz sources using Josephson junction arrays.