K. L. Chen

Off-axis pulsed laser deposited YBa2Cu3O7−δ thin films for device applications

Abstract YBa2Cu3O7−δ (YBCO) thin films deposited by off-axis pulsed laser deposition could be used for fabricating multilayer devices. Growing YBCO films with high critical current density and high Tc (Tc is around 90 K and Jc is above 107 A/cm2 at 77 K), there was a tendency that the surface morphology of YBCO films is rough. This causes problems in fabricating multilayer devices that require a smooth surface. In this work, we have developed two-step procedures of growing YBCO thin films with high critical current and a smooth surface for multilayer device applications. In the two-step procedure, we first grew YBCO film with higher Tc (Tc is around 90 K, but usually the surface is with defects like holes), next we planarized the film by growing the film with lower Tc (Tc is around 83 K and the surface is smooth). YBCO films deposited at low temperatures have critical current densities (Jc) around 106 A/cm2 at 77 K in zero magnetic field. These films are almost free from laser droplets and outgrowth-free surface. In two-step procedures, we could grow films with a smooth surface and high critical current density. Methods for growing smooth surfaces in two-step procedures will be presented.

Photoreflectance characterization of an InAlAs/InGaAs heterostructure bipolar transistor

We have measured the photoreflectance spectrum at 300 K from a lattice‐matched InAlAs/InGaAs heterostructure bipolar transistor grown by molecular beam epitaxy. The energy features of photoreflectance spectra have been identified and the built‐in dc electric fields and associated doping profiles have been evaluated in the n‐InAlAs emitter from the observed Franz–Keldysh oscillations. The undoped InGaAs spacer between emitter and base was added on to change the built‐in electric field. The results showed that the energy features above the InGaAs band gap are the transitions from the valence band to the quantized state of the conduction band. The quantum well of the conduction band is in the interface of the InAlAs and InGaAs heterojunction. The interface charge densities in the spacer channel are determined to be 3.54×1011 cm−2 and 4.22×1011 cm−2, corresponding to the samples with spacer thicknesses of 300 and 500 A, respectively. A triangular potential profile model was used to calculate the microstructur...

Influence of bicrystal microstructural defects on high-transition-temperature direct-current superconducting quantum interference device

Using atomic force microscopy and scanning electron microscopy (SEM), we investigate the correlations between the microstructural defects and the electrical characteristics of the bicrystal grain-boundary Josephson junctions and dc superconducting quantum inference devices (SQUIDs). The structural defects are shown to correlate qualitatively with the characteristics of grain-boundary Josephson junctions patterned on the YBa2Cu3O7−x film. SEM images show that these defects grown on the grain boundary were a few submicron depth of the groove. The low flux noise characteristics were observed when the groove depth was smaller than 18nm in the junctions of the SQUID. The existence of these defects is expected to affect the supercurrent and the motion of the magnetic flux in the films, which dominate the excess noise in the SQUID with bicrystal junctions.

Using Bio-Functionalized Magnetic Nanoparticles and Dynamic Nuclear Magnetic Resonance to Characterize the Time-Dependent Spin-Spin Relaxation Time for Sensitive Bio-Detection

In this work, we report the use of bio-functionalized magnetic nanoparticles (BMNs) and dynamic magnetic resonance (DMR) to characterize the time-dependent spin-spin relaxation time for sensitive bio-detection. The biomarkers are the human C-reactive protein (CRP) while the BMNs are the anti-CRP bound onto dextran-coated Fe3O4 particles labeled as Fe3O4-antiCRP. It was found the time-dependent spin-spin relaxation time, T2, of protons decreases as time evolves. Additionally, the ΔT2 of of protons in BMNs increases as the concentration of CRP increases. We attribute these to the formation of the magnetic clusters that deteriorate the field homogeneity of nearby protons. A sensitivity better than 0.1 μg/mL for assaying CRP is achieved, which is much higher than that required by the clinical criteria (0.5 mg/dL). The present MR-detection platform shows promise for further use in detecting tumors, viruses, and proteins.

Temperature and concentration-dependent relaxation of ferrofluids characterized with a high-Tc SQUID-based nuclear magnetic resonance spectrometer

We investigated the relaxation of protons in magnetic fluids using a high-Tc SQUID magnetometer. It was found that the longitudinal relaxation rate, 1/T1, is slower than the transverse relaxation rate, 1/T2, for ferrofluids in the same field. This is due to the fact that the 1/T1 process involves returning the magnetization to the z-direction, which automatically involves the loss of magnetization in the x-y plane governed by the 1/T2 process. Additionally, 1/T1 and 1/T2 at high temperatures are slower than the corresponding relaxation rates at low temperatures, which is due to the enhanced Brownian motion of nanoparticles at high temperatures.

A highly sensitive YBCO serial SQUID magnetometer with a flux focuser

We have designed and fabricated a low noise, high Tc superconducting quantum interference device (SQUID) magnetometer with an improved yield. In order to reduce the field noise level of the magnetometer, we increased the voltage modulation depth with a series SQUID array and enhanced the effective area with a flux focuser. Ten washer-type SQUIDs were connected in series, thereby increasing the voltage flux transfer function by a factor of 10. An enhancement in effective area by a factor of 5 was achieved by coupling the 10-SQUID array to a single-layered flux focuser. For the directly coupled SQUID array magnetometer, the yield was improved by selecting the best SQUIDs to have in series in order to reduce the flux noise. The magnetic field sensitivity of 40 fT Hz−1/2 in the white regime and 100 fT Hz−1/2 at 1 Hz is demonstrated by using a single layer of high Tc film. The proposed high yield magnetometers would be suitable for building low noise multi-channel magnetocardiographs.

High quality step-edge substrates for high-Tc superconducting devices

Despite the significant progress in fabrication methods of step edge, the lack of reproducibility still hinders their use in more complicated systems. To pursue the high reproducibility and quality of step edge for high-Tc superconducting devices, we have developed the technique to fabricate high quality step-edge substrates with arbitrary step angles. We used two steps to improve the step ramp quality substantially. The surface microscopy of step substrates shows high uniformity with respect to any step angle. There are no needles, waves, trenches, cascades, or other flaws on these surfaces. Serial Josephson junctions and superconducting quantum interference device arrays were fabricated onto step-edge substrates. The step-edge devices exhibit excellent results.

Magnetic Clustering Effect during the Association of Biofunctionalized Magnetic Nanoparticles with Biomarkers.

We report herein an investigation into dynamic magnetic clustering that occurs during immunoassays as biofunctionalized magnetic nanoparticles (BMNs) become associated with biotargets. We measure the dynamic effective relaxation time τ eff(t) and use scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to investigate the C-reactive protein (CRP) as it associates with the BMN Fe3O4-antiCRP to form the magnetic cluster Fe3O4-antiCRP-CRP. The results indicate that τ eff(t) increases with increasing association time. In addition, the ration Δτ eff/τ 0 as a function of CRP concentration follows a characteristic logistic function, which provides a basis for estimating the quantity of biomolecules with a detection sensitivity close to 0.1 ppm. After the association, SEM and TEM images show that CRP and Fe3O4-antiCRP conjugate to form Fe3O4-antiCRP-CRP clusters hundreds of nanometers in size. The SEM and TEM images provide direct evidence of the formation of magnetic clustering.

Spin-spin relaxation of protons in ferrofluids characterized with a high-Tc superconducting quantum interference device-detected magnetometer in microtesla fields

In this work, the spin-spin relaxation of protons in ferrofluids is characterized using a high-Tc SQUID-based detector in microtesla fields. We found that spin-spin relaxation rate is enhanced in the presence of superparamagnetic nanoparticles. The enhanced relaxation rates are attributed to the microscopic field gradients from magnetic nanoparticles that dephase protons’ spins nearby. The relaxation rates decrease when temperatures increase. Additionally, the alternating current magnetic susceptibility was inversely proportional to temperature. Those characteristics explained the enhanced Brownian motion of nanoparticles at high temperatures. Characterizing the relaxation will be helpful for assaying bio-molecules and magnetic resonance imaging in microtesla fields.

Photoreflectance characterization of graded InAlAs/InGaAs heterojunction bipolar transistor layers

The photoreflectance (PR) spectroscopic technique has been used to investigate the microstructures of the graded InAlAs/InGaAs heterojunction bipolar transistor at room temperature. The energy features of the PR spectrum were fitted and identified as band‐to‐band transitions in the graded layers which were grown using pulsed molecular beam epitaxy and InGaAs as well as InAlAs layers. A linear variation relationship of band gaps with Al composition z was observed and approximated by Eg=0.737+0.759z eV. From the observed Franz‐Keldysh oscillations, we have evaluated the built‐in dc electric fields in the i‐InGaAs collector and n‐InAlAs emitter regions. These electric fields are in good agreement with the continuity condition of electric displacements in the interfaces.