Zhengming Ji

Suppression of superconductivity in epitaxial NbN ultrathin films

This paper studies the suppression of superconducting transition temperature (T(c)) of ultrathin NbN film. We fabricated epitaxial NbN superconducting thin films of thicknesses ranging from 2.5 to 100 nm on single crystal MgO (100) substrates by dc magnetron sputtering. We performed structure analyses and measured their electric and far infrared properties. The experimental results were compared with several mechanisms of the suppression of superconductivity proposed in the literature, including the weak localization effect, the proximity effect, and quantum size effect (electron wave leakage model). We found that the electron wave leakage model matches best to the experimental data

Low-loss terahertz metamaterial from superconducting niobium nitride films

This paper reports a type of low Ohmic loss terahertz (THz) metamaterials made from low-temperature superconducting niobium nitride (NbN) films. Its resonance properties are studied by THz time domain spectroscopy. Our experiments show that its unloaded quality factor reaches as high as 178 at 8 K with the resonance frequency at around 0.58 THz, which is about 24 times that of gold metamaterial at the same temperature. The unloaded quality factor keeps at a high level, above 90, even when the resonance frequency increases to 1.02 THz, which is close to the gap frequency of NbN film. All these experimental observations fit well into the framework of Bardeen-Copper-Schrieffer theory and equivalent circuit model. These new metamaterials offer an efficient way to the design and implementation of high performance THz electronic devices.

Thermal escape from a metastable state in periodically driven Josephson junctions.

Resonant activation and noise-enhanced stability were observed in an underdamped real physical system, i.e., Josephson tunnel junctions. With a weak sinusoidal driving force applied, the thermal activated escape from a potential well underwent resonancelike behaviors as a function of the driving frequency. The resonances also crucially depended on the initial condition of the system. Numerical simulations showed good agreement with the experimental results.

Phase locking a 3 mm waveband Gunn oscillator with a high Tc superconducting Josephson junction at 77 K

Using a high‐temperature superconducting YBa2Cu3O7−δ Josephson‐junction harmonic mixer operated at 77 K and an analog phase‐locking loop, we have locked a 3 mm waveband Gunn oscillator to a synthesizer for harmonic numbers ranging from 10 to 52. The spectrum of the phase‐locked intermediate frequency reveals good phase‐locked quality even for the 48th harmonic mixing between the signal at 104.3 GHz and the local oscillator at about 2 GHz. Our experimental results show that a high Tc superconducting harmonic mixer can work efficiently with high harmonic number for phase locking in the millimeter waveband.

The Josephson Effect in a Ceramic Bridge at Liquid Nitrogen Temperature

A bridge junction can be made by cutting a bulk ceramic superconductor into the right sizes. Using this sort of bridge, both dc and ac Josephson effects are confirmed in liquid nitrogen. We have observed microwave-induced steps at either nhf/2e or 2nhf/2e, where n is an integral, as well as the modulation of a critical supercurrent with the increase of microwave power or the externally applied magnetic field.

Controlling the Intrinsic Josephson Junction Number in a Bi2Sr2CaCu2O8+δ Mesa

In fabricating Bi2Sr2CaCu2O8+δ intrinsic Josephson junctions in 4-terminal mesa structures, we modify the conventional fabrication process by markedly reducing the etching rates of argon ion milling. As a result, the junction number in a stack can be controlled quite satisfactorily as long as we carefully adjust those factors such as the etching time and the thickness of the evaporated layers. The error in the junction number is within ±1. By additional ion etching if necessary, we can controllably decrease the junction number to a rather small value, and even a single intrinsic Josephson junction can be produced.

Fabrication and characterization of NbN, AlN and NbN/AlN/NbN on MgO substrates

At ambient substrate temperatures, NbN and AlN thin films as well as NbN/AlN/NbN sandwiches are prepared on single crystal MgO substrates, using direct current (dc) or radio frequency (RF) magnetron sputtering techniques. Excellent single crystal orientations of these structures are revealed by x-ray diffraction and transmission electron microscopy (TEM), while x-ray photoelectron spectroscope (XPS) shows that the stoichiometric composition of the NbN films is 1: 0.9 (Nb:N). Furthermore, AFM (atomic force microscope) scans indicate a root mean square (rms) roughness of 0.755 nm for AlN/NbN, and 0.83 nm for AlN, both over an area of 5 µm × 5 µm.

Time-resolved measurement of capacitance in a Josephson tunnel junction

The capacitance of a Josephson tunnel junction is an important parameter, which determines its plasma frequency ωp and thus the energy level spacing in the quantum regime. When a microwave signal at the frequency of ω is applied to a current-biased junction in its zero-voltage state, the escape rate from this state is somehow enhanced. The enhancement is expected to go through a pronounced maximum when ωp=ω. Making use of such microwave generated resonant activation in the thermal regime, we have developed a time-resolved measurement scheme for determining the junction capacitance with an uncertainty of less than 2%.

Fabrication of AlN thin films on different substrates at ambient temperature

Aluminium nitride (AlN) is very useful as a barrier in superconductor?insulator?superconductor (SIS) device or as an insulating layer in many other applications. At ambient temperature, we deposit AlN thin films onto different substrates (such as MgO, LaAlO3 and Si) by using radio-frequency magnetron sputtering and pure Al target. X-ray diffraction (XRD) and ?-scan patterns show that the films grown on MgO substrates are excellent epitaxial films with (101) orientation of a hexagonal lattice. A possible structure of the interface between the film and the substrate is suggested and discussed.

Harmonic frequency mixing using highT c superconductor Josephson junction mounted on pulse tube cryocooler

A frequency mixing system including microwave coupling and intermediate frequency (IF) measurement arrangements is designed. In lieu of liquid nitrogen, a pulse tube cryocooler is used to cool the whole system. With YBa2Cu3O7/Yttrium stabilized zirconia (YBCO/YSZ) bicrystal Josephson junction as the mixing element, 36th harmonic frequency mixing at the 8 mm waveband is obtained.