Nianhua Peng

Irradiation enhanced paramagnetism on graphene nanoflakes

We have studied the magnetization of vertically aligned graphene nanoflakes irradiated with nitrogen ions of 100 KeV energy and doses in the range 1011–1017 ions/cm2. The non-irradiated graphene nanoflakes show a paramagnetic contribution, which is increased progressively by ion irradiation at low doses up to 1015/cm2. However, further increase on implantation dose reduces the magnetic moment which coincides with the onset of amorphization as verified by both Raman and x-ray photoelectron spectroscopic data. Overall, our results demonstrate the absence of ferromagnetism on either implanted or unimplanted samples from room temperature down to a temperature of 5 K.

Ion beam synthesis of superconducting MgB2 thin films

Superconducting MgB2 thin films have been fabricated by 80 keV 11B ion implantation into commercial Mg ribbon with 11B doses up to 1018 ions/cm2, followed by thermal annealing at 500 °C. Temperature dependent dc magnetization measurements confirmed superconducting phase transitions between 11 and 18 K for samples containing nanocrystalline MgB2 grains embedded in Mg substrate with a small amount of MgO inclusion.

Realization and properties of YBa2Cu3O7−δ Josephson junctions by metal masked ion damage technique

We have developed a simple process to fabricate high-TC Josephson junctions by a combination of focused ion beam milling and 100 keV H2+ ion implantation. The resistively shunted junction-like current–voltage characteristics were observed in the temperature range of 48 to 4.2 K. The devices showed clear dc and ac Josephson effects. This technique is very promising in terms of simplicity and flexibility of fabrication and has potential for high-density integration.

Realization and properties of MgB2 metal-masked ion damage junctions

Ion beam damage combined with nanoscale focused-ion-beam direct milling was used to create manufacturable superconductor–normal–superconductor type (SNS) Josephson junctions in 100-nm-thick MgB2 with TC of 38 K. The junctions show nonhysteretic current–voltage characteristics between 36 and 4.2 K. Experimental evidence for the dc and ac Josephson effects in MgB2 metal-masked ion damage junctions are presented. This technique is particularly useful for prototyping devices due to its simplicity and flexibility of fabrication and has a great potential for high-density integration.

Electrical properties of electron and ion beam irradiated YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//

An electron beam with sufficient energy can be used to create a high quality Josephson junction in a single layer of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//. The number of junctions thus produced is severely limited by the serial nature of the technique. An alternative method to create similar high quality Josephson junctions without such a serious throughput limitation is possibly the combination of high resolution masking and ion irradiation. For this reason we have studied the electrical properties of both electron and proton irradiated YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// in some detail. It was found that the resistivity of electron beam irradiated barriers of intermediate length (200 nm) are strongly influenced by a proximity effect when the irradiated material has a finite T/sub c/. At higher electron doses the superconducting properties are fully suppressed and the electrical behaviour is dominated by a Variable Range Hopping mechanism.

2D Monte Carlo simulation of proton implantation of superconducting YBa2Cu3O7−δ thin films through high aspect ratio Nb masks

Abstract Perturbation of proton beam damage profile due to sidewall interactions in very high aspect ratio implant masks has been studied using Monte Carlo simulations. The model structure is composed of amorphous Nb metal mask, crystalline high temperature superconducting YBa 2 Cu 3 O 7− δ (YBCO) thin film, and amorphous LaAlO 3 substrate. The simulation results reveal the existence of enhanced proton beam penetration in target materials due to sidewall interactions.

Monte Carlo simulations of masked ion beam irradiation damage profiles in YBa2Cu3O7−δ thin films

Abstract Monte Carlo simulations of irradiation damage profiles in high aspect ratio Nb masked YBa 2 Cu 3 O 7− δ thin film reveal an improved damage profile when O + beams are used instead of proton beams. The overall damage profile is insensitive to either beam divergence or beam misalignment but the damage accumulation levels are significantly affected by these effects.

Transfer function and noise properties of YBa2Cu3O7−δ direct-current superconducting-quantum-interference-device magnetometers with resistively shunted inductances

We have investigated resistively shunted direct-coupled high-Tc dc superconducting quantum interference device (SQUID) magnetometers with four different inductances (50, 100, 150, and 200 pH). The SQUIDs were based on 200 nm thick YBa2Cu3O7−δ films deposited on bicrystal substrates with a 24° misorientation angle, and the shunt resistors were fabricated by masked ion damage. At T=77 K, good quantitative agreement was observed between the measured maximum voltage modulation depth ΔV and calculated values based on the theoretical predictions by Enpuku et al., whereas the white magnetic flux noise SΦ at 10 kHz of all four devices was found to be a factor of 2.3 higher than predicted. The lowest white magnetic field noise of 153 fT/Hz was obtained for the magnetometer with a SQUID inductance of 100 pH with an outer dimension of the pickup loop of just 2 mm.

A potential integrated low temperature approach for superconducting MgB/sub 2/ thin film growth and electronics device fabrication by ion implantation

By implanting high fluence B into Mg targets, we demonstrate clearly that superconducting MgB/sub 2/ thin films can be formed at a low process temperature. The superconducting transition temperature T/sub c/ values observed are dependent on the growth condition. The ion beam synthesis experimental data are discussed and compared with simulation results of the B implantation process in Mg target. We propose that ion beam synthesis of MgB/sub 2/ is a potential approach for the mass production of superconducting electronics devices.

Irradiation damage technology for manufacturable Josephson junctions

Abstract We have shown in a series of studies that irradiation of YBa 2 Cu 3 O 7− δ (YBCO) with ions of energy in the range of 30–350 keV through a suitable mask can be used to create highly localized damage regions in the films. This technique has been successfully employed to create high quality Josephson junctions in YBCO with an ion beam implanter capable of in situ low temperature electrical measurement during implantation and focussed ion beam nanolithography. The fabricated devices show a clear dc and ac Josephson effects. This technique is very promising in terms of simplicity and flexibility of fabrication and has potential for high density integration.