L. M. Wang

Bandgap, mid-gap states, and gating effects in MoS2.

The discovery of graphene has put the spotlight on other layered materials including transition metal dichalcogenites (TMD) as building blocks for novel heterostructures assembled from stacked atomic layers. Molybdenum disulfide, MoS2, a semiconductor in the TMD family, with its remarkable thermal and chemical stability and high mobility, has emerged as a promising candidate for postsilicon applications such as switching, photonics, and flexible electronics. Because these rely on controlling the position of the Fermi energy (EF), it is crucial to understand its dependence on doping and gating. To elucidate these questions we carried out gated scanning tunneling microscopy (STM) and spectroscopy (STS) measurements and compared them with transport measurements in a field effect transistor (FET) device configuration. This made it possible to measure the bandgap and the position of EF in MoS2 and to track its evolution with gate voltage. For bulk samples, the measured bandgap (∼ 1.3 eV) is comparable to the value obtained by photoluminescence, and the position of EF (∼ 0.35 eV) below the conduction band, is consistent with N-doping reported in this material. We show that the N-doping in bulk samples can be attributed to S vacancies. In contrast, the significantly higher N-doping observed in thin MoS2 films deposited on SiO2 is dominated by charge traps at the sample-substrate interface.The discovery of graphene has put the spotlight on other layered materials including transition metal dichalcogenites (TMD) as building blocks for novel heterostructures assembled from stacked atomic layers. Molybdenum disulfide, MoS2, a semiconductor in the TMD family, with its remarkable thermal and chemical stability and high mobility, has emerged as a promising candidate for post-silicon applications such as switching, photonics, and flexible electronics. Since these rely on controlling the position of the Fermi energy (EF), it is crucial to understand its dependence on doping and gating. Here we employed scanning tunneling microscopy (STM) and spectroscopy (STS) with gating capabilities to measure the bandgap and the position of EF in MoS2, and to track its evolution with gate voltage. For bulk samples, the measured bandgap (~1.3eV) is comparable to the value obtained by photoluminescence, and the position of EF (~0.35eV) below the conduction band, is consistent with n-doping reported in this material. Using topography together with spectroscopy we traced the source of the n-doping in bulk MoS2 samples to point defects, which we attribute to S vacancies. In contrast, for thin films deposited on SiO2, we found significantly higher levels of n-doping that cannot be attributed to S vacancies. By combining gated STS with transport measurements in a field effect transistor (FET) configuration, we demonstrate that the higher levels of n-doping in thin film samples is due to charge traps at the sample-substrate interface.

Optimum sputtering conditions on the in-situ growth of superconducting YBa2Cu3Oy films with an off-axis RF sputtering configuration

Abstract The effects of sputtering conditions on the transport properties of YBa 2 Cu 3 O y (YBCO) thin films were investigated in this work using an off-axis RF sputtering configuration. These conditions included the substrate temperature, the distance from the substrate to the target, the total pressure of the sputtering gas and the gas composition. The substrates were MgO (001) and SrTiO 3 (001) and the sputtering gas was a mixture of Ar and O 2 . The YBCO films were characterized by resistivity measurements, X-ray diffraction and scanning force microscopy (SFM). Additionally, the optimum conditions of growing high-quality YBCO films were obtained. Those conditions were found to be: (1) the substrate is 3.5 cm horizontal and 3.0 cm vertical away from center of target, (2) the substrate temperature is 680°C for MgO and it is 720°C for SrTiO 3 , and (3) the pressure of the sputtering gas of Ar : O 2 is (3 : 7) at 300 mTorr. The high-quality YBCO films revealed a transition temperature T c ( R = 0) at 87 ∼ 90 K, a normal-state resistance ratio R 300K / R 100K ∼ 3 and a normal-state resistivity at 300 K smaller than 200 μΩ cm.

Quarter-Wavelength Stepped-Impedance YBCO Resonators for Miniaturized Dual-Band High-Tc Superconducting Filters

A dual-band high-temperature superconducting (HTS) bandpass filter (BPF) is proposed for wireless local area network applications. Using quarter-wavelength stepped-impedance resonators, the designed filter can be miniaturized and shows a dual-band response. The simulation results show the dual-band feature of two passbands at 2.4 and 5.2 GHz, each with a minimum in-band insertion loss of about 0.02 dB. The filter was made out of patterned double-sided deposited YBa2Cu3Oy (YBCO) films integrated with a gold-coated housing. The realized HTS BPF shows two passbands at 2.41 and 5.24 GHz with minimum insertion losses of 0.86 and 1.97 dB, respectively. The measured results show a good HTS BPF performance. Moreover, the temperature-dependent center frequencies can be well described by the modified two-fluid model based formulas, indicating that the frequency shift in HTS BPF is dominated by the temperature dependence of the magnetic penetration depth.

Correlation of the temperature coefficient of resistivity for doped manganites to the transition temperature, polaron binding energy, and magnetic order

Based on the phase-coexistence transport model, an expression for the temperature coefficient of resistivity (TCR) behavior in doped manganites is proposed. The derived maximum TCR value (TCRmax), being related to polaron binding energy, transition temperature, and disorder in doped manganites, describes the observed “universal law” that the TCRmax is decreased, when TC increases. The calculated results are strongly supported by experiments and are analyzed within the framework of a microscopic transport mechanism. Correlation of the proposed TCR behavior to the transport parameters creates an opportunity to improve the TCRmax value in doped manganites, for uncooled bolometric applications.

Magneto-optical properties of La0.7Sr0.3MnO3 thin films with perpendicular magnetic anisotropy

We report the magneto-optical (MO) properties of compressively strained La0.7Sr0.3MnO3 (LSMO) thin films epitaxially grown on a LaAlO3 substrate. The magnetic force microscope images show the stripe magnetic domains, characteristic of films with the perpendicular magnetic anisotropy (PMA). The optical reflectance and transmittance of the samples were measured over a broad energy range from the far infrared through the ultraviolet. To extract the optical constants of the films, we analyzed all of the layers of this thin-film structure using a Drude-Lorentz model. From the parameters obtained, we compute the optical constants, such as frequency-dependent optical conductivity and the diagonal components of the dielectric tensor. Moreover, the MO polar Kerr spectra of the samples were measured in an applied magnetic field of 1.5T between 0.74 and 5.8eV. The off-diagonal components of the dielectric tensor were then calculated by analyzing Kerr rotation, ellipticity, and the determined diagonal elements of the...

Time-dependent phase lag of biofunctionalized magnetic nanoparticles conjugated with biotargets studied with alternating current magnetic susceptometor for liquid phase immunoassays

In this work, the time-dependent phase lag θ of biofunctionalized magnetic nanoparticles (BMNs) conjugated with biotargets is studied with a home-made alternating current (ac) susceptometor for liquid phase immunoassays. The sensing unit of the ac susceptometor composed of excitation, pick-up, and compensation coils are balanced to 0.03 ppm. The BMNs are anti-goat C-reactive protein coated onto dextran-coated magnetic nanoparticles composed of Fe3O4, labeled as Fe3O4-antiCRP. The bio-targets are human CRP. As the human CRP is conjugated with reagents Fe3O4-antiCRP, the magnetic clusters of Fe3O4-antiCRP-CRP are formulated. Due to the clustering effect, the Brownian relaxation of BMNs will be depressed, which in turn enhances the effective relaxation time. By monitoring the dynamic phase lag, we demonstrate a sensitive platform of assaying human CRP. The detection platform is robust, easy to use and can be applied for assaying a wide variety of biotargets including viruses, proteins, tumor markers, chemica...

Characteristics of Ultra-Wideband Bandpass YBCO Filter With Impendence Stub

A compact ultra-wideband (UWB) bandpass filter (BPF) is presented for applications to short-range and high-speed wireless communication. Superconducting YBa2Cu3Oy (YBCO) stepped impedance resonators and coupled-line sections as inverter circuits are designed to form the basic filter structure. In the filter design, connected high-low stepped impedance microstrip lines construct the resonators, and open-stub lines are utilized to add return-loss poles in the pass-band and create transmission zeros in the lower/upper stop-band region. Simulation results show that the passband from 3.0 GHz to 8.6 GHz has a 3-dB fractional bandwidth of 99 percent, computed insertion losses better than 0.03 dB, and return losses greater than 15 dB. Rejection levels in the upper/lower stop-bands are better than 20 dB. For fabrication, high-Tc superconducting (HTS) YBCO films were deposited on double-side-polished 0.5-mm-thick MgO (100) substrates by a radio-frequency sputtering system. The filter was made out of patterned double-sided deposited YBCO films integrated with a gold-coated housing. The realized HTS UWB BPF shows a wide passband within 2.9-8.3 GHz with a maximum insertion loss of 0.88 dB. The measured results show good HTS UWB BPF performance. Moreover, the temperature-dependent frequency responses and the insertion loss can be described by the modified two-fluid-model-based formulas, indicating that the frequency shift and the increase in insertion loss for HTS BPF are both dominated by the temperature dependence of the magnetic penetration depth.

Thickness-dependent optical properties of La0.7Sr0.3MnO3 thin films

We report on a systematic study of the thickness dependence of the optical properties of La0.7Sr0.3MnO3 thin films epitaxially grown on a LaAlO3 substrate. The x-ray powder-diffraction data indicate that the c-axis lattice constant is enhanced with decreasing the film thickness due to the compressive strain in the film plane produced by lattice mismatch. Magnetization curves show a decrease of the Curie temperature (TC) for decreasing thickness of films. Optical reflectance and transmittance measurements provide evidence that the position of Mn-O stretching mode shifts toward low frequency and the energy of the charge-transfer transition between O 2p and Mn 3d states increases with the decrease of film thickness. Most importantly, an analysis of the small-polaron absorption in the mid-infrared region shows that the polaron binding energy increases with decreasing the film thickness, suggesting that the strain dependence of TC mainly results from the strain-induced electron–phonon coupling.

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.