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We present a direct measurement of the displacement noise spectrum of a macroscopic silicon flexure at room temperature. A cantilever attached to the 100 μm thick flexure holds a mirror which forms part of an optical cavity to enhance the displacement sensitivity to thermal noise. We predict the displacement noise spectrum using a simple model that assumes the dominant source of frequency-dependent loss is thermo-elastic damping and find good agreement with the experimental data. The measurement is consistent with a frequency-independent loss of [Formula: see text] combined with frequency-dependent thermo-elastic damping as the dominant losses. A crossover between the two that occurs well above the flexure resonant frequency allows a broadband measurement of the thermal noise of the silicon flexure. The flexure material, geometry, and measurement band are similar to those of planned future gravitational wave detectors.

Applied physics letters 118(8), 089901 (2021). doi:10.1063/5.0045052, Published by American Inst. of Physics, Melville, NY

We report on the observation of high-fidelity, high-Q, phononic combs in AT-cut 100-MHz shear-mode quartz resonators. The comb generation was observed on the first spurious mode above the fundamental mode resonance when driven a few kHz off resonance at power levels between +11 and +18 dBm. These combs exhibit the expected phase coherence between the teeth with no measurable line broadening from the nonlinear generation. Interestingly, the combs can exhibit a complex structure at particular temperatures. These changes in the comb structure over temperature are thought to be driven by the relative frequency shifts between the interacting modes. Teeth with both much larger and smaller frequency sensitivities to the drive frequency have been observed. These characteristics may be useful for enhancing the sensitivity of resonant-mode MEMS sensors or improving the phase stability of oscillators. In addition, if cryogenically cooled, these highly coherent combs may provide a component for phononic quantum processing studies.

C-axis-aligned one-dimensional artificial pinning centers (1D-APCs) provide strong correlated pinning of magnetic vortices and enhance critical current density Jc in superconducting YBa2Cu3O7-x (YBCO) films and coated conductors. A fundamental question arises on what determines the pinning efficiency of different 1D-APCs. To shed light on this question, this work investigates the correlation between the 1D-APC/YBCO interface and the pinning efficiency of 1D-APCs of BaHfO3 (BHO) and BaZrO3 (BZO) of comparable diameters of 5–6 nm. Intriguingly, a highly coherent BHO 1D-APC/YBCO interface is revealed even at a high BHO doping level of 6 vol. %, in contrast to a semi-coherent BZO 1D-APC/YBCO interface with a large number of dislocations. This leads to a profound effect on the pinning efficiency of these 1D-APCs. Specifically, a record high pinning force density peak Fp,max ∼ 182.0 GN m−3 at Hmax > 9.0 T and 65 K is obtained in the former, which is 2.5 times of the best reported in the latter. Moreover, a ratio of Hmax to accommodation field H* determined from the 1D-APC areal concentration is found up to 3.5 in the former in contrast to 0.7 in the latter, demonstrating the critical impact of the APC/YBCO interface on the pinning efficiency of 1D-APCs.

We use femtosecond laser pulses to generate coherent longitudinal acoustic phonons at frequencies of 1–1.4 THz and study their propagation in GaN-based structures at room temperature. Two InGaN-GaN multiple-quantum-well (MQW) structures separated by a 2.3 μm-thick GaN spacer are used to simultaneously generate phonon wave packets with a central frequency determined by the period of the MQW and detect them after passing through the spacer. The measurements provide lower bounds for phonon lifetimes in GaN, which are still significantly lower than those from first principles predictions. The material Q-factor at 1 THz is found to be at least as high as 900. The measurements also demonstrate a partial specular reflection from the free surface of GaN at 1.4 THz. This work shows the potential of laser-based methods for THz range phonon spectroscopy and the promise for extending the viable frequency range of GaN-based acousto-electronic devices.

The generation of spin currents by thermal gradients applied to a magnetic film is known as the spin Seebeck effect (SSE). The SSE is usually detected by an electric voltage generated in a metallic layer in contact with the magnetic film produced by the spin to charge current conversion through the inverse spin Hall effect (ISHE). The SSE has been widely studied in bilayers made of the insulating ferrimagnet yttrium iron garnet (YIG) and metals with large spin orbit coupling, such as platinum. Recently, the SSE has been observed in bilayers made of the antiferromagnets MnF2 and Cr2O3 with Pt at low temperatures and high magnetic fields. Here, we report measurements of the SSE at room temperature and low magnetic fields in bilayers made of well textured films of antiferromagnetic NiO with several metals. The detection of the spin current generated by the thermal gradient in the NiO layer is made by means of the ISHE in the nonmagnetic metals Pt and Ta, in the AF metal IrMn, and in the ferromagnetic metal N...

Quasiparticle self-consistent GW calculations are presented for the band structure of β-Ga2O3, including a lattice polarization correction of the screened Coulomb interaction W. It is found that this correction is of the order of 0.5 eV. When an estimated zero-point motion correction is also included, the direct gap is found to be 4.8 ± 0.1 eV in good agreement with experiment. The indirect gap is found to be 0.1 eV smaller. The origin of the anisotropy of the absorption edge is interpreted in terms of selection rules and the symmetry labeling of the bands at Γ.

The amphiphilic nature of graphene oxide (GO) is exploited as a seed layer to facilitate the ultrathin and conformal high-κ metal oxide (MOX) deposition on defect-free epitaxial graphene (EG) by atomic layer deposition (ALD). Three different high-κ metal oxides (Al2O3, HfO2 and TiO2) with various thicknesses (4–20 nm) were grown on ultrathin (1.5 nm) GO seed layers on EG. The quality of such dielectrics was examined by fabricating various metal-insulator-graphene (MIG) type devices. For MIG tunnel devices, on-off ratios of 104 and 103 were obtained for 4 nm Al2O3 and HfO2 dielectric layers, respectively. Additionally, no defect/trap assisted conduction behavior was observed. Graphene field effect transistors (GFETs) with bi-layer metal oxide stack (6 nm TiO2/14 nm HfO2) demonstrated a peak on-state current of 0.16 A/mm, an on-resistance of 6.8 Ω mm, an Ion/Ioff ratio of ∼4, and a gate leakage current below 10 pA/mm at Vds = 1 V and Vgs = 4 V. Capacitance-voltage measurement of the same GFETs exhibited a l...

Mn ions were implanted in p-type cubic GaN at doses from 0.6 to 2.4×1016cm−2 at 200 keV energy. A 200-nm-thick epitaxial layer, grown by molecular beam epitaxy on GaAs(001) substrate, is used for the Mn implantation. The Mn implanted samples were subjected to an annealing at 950 °C for 1–5 min. The structural quality of the samples was investigated by high resolution x-ray diffraction and Raman spectroscopy. The annealing procedure leads to a significant increasing of the crystalline quality of the samples. Hysteresis loops were observed for all cubic GaMnN annealed samples and ferromagnetism was detected up to room temperature.

We have observed field emission with an energy distribution of 68meV full width at half maximum (FWHM) from a thermally cleaned Mo⟨110⟩ single tip. The emission spectra are taken with the emitting surface at 295K, using a double-pass cylindrical mirror analyzer in ultrahigh vacuum. This narrow energy distribution is attributed to a nanotip spontaneously formed on the ∼75nm radius Mo tip from in situ buildup by field-induced surface diffusion. Auger electron spectroscopy showed that residual surface Ca segregated from the bulk during thermal cleaning and is likely the source of mobile atoms that formed the nanotip. The emission spectra show a discernible doublet that is attributed to a variation in the localized density of states of the nanotip. Sub-Langmuir oxygen exposure of the Mo tip immediately increased the energy distribution to a FWHM of >1eV.

In YBa2Cu3O7−δ (YBCO) thick films, the superconducting critical current density (Jc) decreases with increasing film thickness (t). The mechanisms responsible for this Jc–t behavior remain unclear. To probe the correlation between the film microstructure and Jc-thickness behavior, we have deposited YBCO thick films up to 3.0μm in thickness on flat and surface-miscut (100) SrTiO3 substrates with 5°, 10°, and 15° vicinal angles. The microstructures of the YBCO films were found to evolve differently on flat and miscut substrates, resulting in different Jc–t behaviors. Surprisingly the small miscut angles of 5°–10° were favorable to obtain higher Jc and smaller Jc reduction at larger film thickness.

Cross-sectional transmission electron microscopy and atomic force microscopy have been used to study the microstructure of a thin heteroeptiaxial GaN film grown on (0001) 4H-SiC mesa surfaces with and without atomic scale steps. Analysis of a mesa that was completely free of atomic-scale surface steps prior to III–N film deposition showed that these GaN layers had a wide variation in island height (1–3μm) and included the presence of pit-like defects on the film surface. This sample had a low dislocation density (5×108∕cm2) as compared to conventionally grown samples on unpatterned (0001) on-axis 4H-SiC (2×109∕cm2), coupled with a 3–5 times increase in grain size. A comparison of a GaN film on the step-free 4H-SiC mesa region with a GaN film on a stepped 4H-SiC mesa region on the same substrate showed that the presence of surface steps reduced the overall grain size of the film from 7–10μm to a grain size of about 2–3μm. Since the GaN films grow via a Volmer–Weber mechanism, a decrease in the number of he...

In transmission studies of two-dimensional AlGaAs-based photonic crystal waveguide structures, we demonstrate high coupling efficiencies from ridge waveguides to photonic crystal waveguides using photonic crystal waveguide tapers. Enhanced bending efficiencies and bend bandwidths are achieved by use of modified waveguide bends.

Ferromagnetism is observed at and above room temperature in pulsed laser deposited epitaxial films of Co-doped Ti-based oxide perovskite (La1-xSrxTiO3-d). The system has the characteristics of an intrinsic diluted magnetic semiconductor (metal) at low concentrations (

Current collapse is observed to be induced in AlGaN/GaN high-electron-mobility transistors as a result of short-term bias stress. This effect was seen in devices grown by both metalorganic chemical vapor deposition (MOCVD) and molecular-beam epitaxy (MBE). The induced collapse appears to be permanent and can be reversed by SiN passivation. The traps responsible for the collapse have been studied by photoionization spectroscopy. For the MOCVD-grown devices, the same traps cause the collapse in both unstressed and stressed devices. These effects are thought to result from hot-carrier damage during stress.

We investigate some properties of one-dimensional photonic band-gap materials that have the possibility of total internal reflection at one or more interfaces. The proposed structures have application for electro-optic modulation, sensing, and other optical devices. A specific example for electro-optic modulation is presented. Certain advantages of this arrangement may make it a desired geometry for specific applications like sensing.

Planar InAs/InP quantum-dot microcavities using multilayer SiO2/Ta2O5 Bragg reflectors have been studied in emission. For an eight-pair mirror design, the stop band extends from 700 to 900 meV and the cavity linewidth is ∼500 μeV for emission at 850 meV. Measurements as a function of incident power show quantum-dot saturation behavior, with the emission intensity going to the wetting layer outside the cavity stop band. Saturation behavior at fixed pump power is also observed as a function of decreasing temperature. Dispersion measurements as a function of emission angle show polarization splitting in reasonable agreement with theory.

The microstructure of sputter-deposited Cu films is shown to be unstable at room temperature (RT). The average grain size increases significantly during RT storage for a couple of hours after deposition. This RT grain growth is shown to be very dependent on the deposition parameters (substrate temperature and sputter gas pressure) and hence on the microstructure of the as-deposited film. The microstructure of sputter-deposited films is usually summarized in a structure-growth-zone model. It is found that significant RT grain growth only occurs for Cu layers with a zone-T-type, as-deposited microstructure.

We have measured the room-temperature electroreflectance (ER) of hexagonal gallium nitride. Our measurements were obtained at the fundamental optical gap and at the E1 critical point regions. The measurements were performed using front contact ER. In the fundamental region, the ER spectra were found to be of excitonic nature and in the low field regime. In the E1 region, the ER spectra were also in the low field regime, but were fitted with two-dimensional critical points.

We have applied a low-energy positron beam to study epitaxial Si-doped GaN layers, where the grain size varies from 0.2 to 2–5 μm. Negatively charged Ga vacancies are found in n-type samples. Their concentration is independent of the grain size, suggesting that Ga vacancies exist in the grain interior. Positrons are observed to get trapped also at other negatively charged centers. The positron trapping rate at these defects correlates with the grain-boundary density. We attribute the observed shallow positron traps, which do not contain open volume, to negatively charged edge-type dislocations which define the grain boundaries.

The nature of the electron states associated with hydrogen in InN has been inferred by studying the behavior of positive muons, which mimic protons when implanted into semiconductors. The muons capture electrons below 60 K, forming paramagnetic centers with a binding energy of about 12 meV. Together with an exceedingly small muon-electron hyperfine constant indicative of a highly delocalized electron wave function, the results confirm the recently predicted shallow-donor properties of hydrogen in InN.

We report on the growth of self-assembled ternary InAsP quantum dots embedded into GaAs. A comparison between average sizes and recombination energies for different dot species (InAs, InAsP, and InP) confirms the formation of the ternary alloy islands. By carefully changing the growth conditions, we determined a global growth map for InAs, InAsP, and InP dots, showing that it is possible to tailor the optical characteristics of the InAsP species, thus covering the energy range between InAs and InP quantum dot emission.