Temperature Dependence Of Frequency Research Articles

Methanol decomposition on Ni(111) and O/Ni(111).

Methanol decomposition on Ni(111) surfaces has been studied in the presence and absence of oxygen using temperature-programmed desorption and temperature-dependent sum frequency generation spectroscopy. Under both conditions the C-H and O-H bonds break, forming carbon monoxide and atomic hydrogen on the surface. No C-O bond scission was observed, limiting the number of reaction pathways. The O-H bonds break first (>150K), forming surface methoxy, followed by C-H bond breakage (>250K). All atomic hydrogen desorbs from the surface as H2 through H+H recombinative desorption. H2 desorbs at a higher temperature in the presence of oxygen (>300K) than the absence of oxygen (>250K) as the oxygen on the surface stabilizes the H atoms, forming surface hydroxide (OH). The surface oxygen also appears to stabilize the O-H and C-H bonds, leading to slightly higher dissociation temperatures. The CO molecules occupy both the bridge sites and the top sites of the Ni atoms as surface H appears to force the CO molecules to the top sites. There is a slight blueshift in the C-O bond vibration for both the O covered and O free surfaces due to CO being more mobile. On the O free surface, the C-O peak width broadens as low-frequency modes are activated. Finally, CO desorbs between 350 and 400K.

Single-particle and collective excitations of polar water molecules confined in nano-pores within a cordierite crystal lattice.

Recently, the low-temperature phase of water molecules confined within nanocages formed by the crystalline lattice of water-containing cordierite crystals has been reported to comprise domains with ferroelectrically ordered dipoles within the a, b-planes which are antiferroelectrically alternating along the c-axis. In the present work, comprehensive broad-band dielectric spectroscopy is combined with specific heat studies and molecular dynamics and Monte Carlo simulations in order to investigate in more detail the collective modes and single-particle excitations of nanoconfined water molecules. From DFT-MD simulations we reconstruct the potential-energy landscape experienced by the H2O molecules. A rich set of anisotropic temperature-dependent excitations is observed in the terahertz frequency range. Their origin is associated with the complex rotational/translational vibrations of confined H2O molecules. A strongly temperature dependent relaxational excitation, observed at radio-microwave frequencies for the electric field parallel to the crystallographic a-axis, E||a is analyzed in detail. The temperature dependences of loss-peak frequency and dielectric strength of the excitation together with specific heat data confirm a ferroelectric order-disorder phase transition at T0 ≈ 3 K in the network of H2O dipoles. Additional dielectric data are also provided for polarization E||b, too. Overall, these combined experimental investigations enable detailed conclusions concerning the dynamics of the confined water molecules that develop within their microscopic energy landscapes.

Sol-Gel synthesis and investigation of dielectric behaviour in barium stannate

Barium stannate (BaSnO3) ceramics were synthesized via the sol–gel process and their dielectric study as a function of sintering temperatures was revealed for the first time. The structural, compositional, and microstructure properties also have been investigated. The crystallinity of the BaSnO3 was analysed by XRD. The morphology of synthesised samples was characterized by FESEM microscopy. Chemical composition and surface composition of prepared samples were identified from XPS and EDAX measurements. The room temperature frequency dependence of relative permittivity (dielectric constant) of barium stannate pellets has been determined over a range of frequencies from 4 Hz to 5 MHz. A high dielectric value ∼ 2476 was obtained at 1 KHz for samples sintered at 900 °C. The dielectric constant (εr) of about 889 was obtained for barium stannate pellets sintered at 1000 °C. The obtained results exhibited an anomalous behaviour in the dielectric response of BaSnO3 ceramics.

Radiation-stimulated phase transition to superionic state of the crystal TlSe

The temperature-frequency dependences of: electrical conductivity (sigma(μ,T)), the real and imaginary parts of the complex permittivity (ε'(ν,T)) and (ε''(ν,T)), the complex impedance of the TlSe compound when exposed to various doses of gamma radiation. The process of the occurrence of concentration polarization at the boundary of the ion conductor blocking contact is investigated. The temperature-frequency-dose interval of the occurrence of ionic conductivity and the Warburg impedance has been established and a radiationally stimulated phase transition to the superionic state of the TlSe crystal has been detected. Keywords: complex impedance, blocking contact, of ionic conductivity.

Temperature and frequency dependence of dielectric relaxation in a metal–organic perovskite-like framework

This study dielectrically analyzed the three-dimensional metal–organic framework complex of imidazolium magnesium formate [(C3N2H5)(Mg(HCOO)3)] (HImMg) to investigate the temperature–frequency dependence of the perovskite-like structure’s dielectric relaxation processes. HImMg was synthesized via solution diffusion at room temperature. The high-temperature phase transition of the HImMg was confirmed at around 178 °C by differential scanning calorimetry and temperature-dependent X-ray powder diffraction. Single-crystal X-ray diffraction revealed a monoclinic structure in the space group P21/n, indicating a centrosymmetric system, with the HIm+ dipole in the cages of the Mg(HCOO)3. The dielectric values were recorded between −50 °C and 200 °C and at radio and microwave frequency ranges between 1 kHz and 1 GHz. Dielectric relaxation was calculated using the Arrhenius and Cole–Cole relationships. Low-temperature activation energies of 0.14, 0.35, and 0.25 eV were connected to the gradual freezing of the imidazolium cations’ reorientation movements. This behavior may demonstrate that the dielectric relaxations of HImMg are caused by the orientational motion of dipolar guest molecules.

Studies of structural, dielectric and impedance spectroscopy of Sr/ Zr modified BiFeO3 ceramics

Bismuth ferrite (BiFeO3) is the most commonly used multiferroics for multifunctional applications. The polycrystalline samples of co-doped (Sr/Zr) bismuth ferrite (BiFeO3) of a general chemical composition, Bi1-xSrx)(Fe1-xZrx)O3, x = 0.05, 0.1, 0.15, 0.20, were prepared using a standard solid-state reaction method. The basic structural data of the above synthesized electro-ceramics were obtained by X-ray diffraction data and pattern. The surface morphology of the samples was recorded at room temperature by a scanning electron microscope. The scanning electron microscope images show the uniform distribution of grains throughout the surface of the samples. Studies of dielectric and impedance properties of the samples at varying frequency (1 kHz–1000 kHz) and temperature (250C–300 °C) have provided many important information about the materials. The existence of ferroelectricity in the materials was studied by the electric field dependent polarization. The contributions of both grains and grain boundaries towards the total resistance and capacitive behavior of the compounds have been studied by analyzing the frequency and temperature dependence of impedance data.. Study of frequency-temperature dependence of ac-conductivity has provided the information on the type of conduction mechanism. The J–E characteristics of the samples provide the Ohmic behavior with slope closer to 1.

Structural, Electronic Band Transition and Optoelectronic Properties of p-Type Transparent Conductive CuCr1–xNixO2 Semiconductor Films

The transparent conductive CuCr1–xNixO2 (0 ≤ x ≤ 8%) films were prepared by sol–gel method with two-step annealing process. The structure, morphology, phonon modes, chemical composition, optical bandgap, and electrical properties were systematically studied. It was found that the crystal quality improves first and then decreases with increasing the Ni content, and the inflection point occurs when the doping content is 2% and all samples have the (00l) preferred orientation. In the visible region, the transmittances of these samples range from 75% to 55% as the Ni dopant increases. On the basis of the theoretical calculation, we are inclined to the view that there are two ways of coexisting electronic transitions. The direct band gap decreases from 3.13 to 3.02 eV, and the indirect band gap decreases from 2.78 to 2.49 eV with increasing Ni content. Transmittance spectra of the CuCrO2 film at the temperatures from 323 to 453 K present the fact that the absorption edge shows a slight redshift, which is due to the lattice thermal expansion and the electron–phonon interaction. Raman spectroscopy shows that Ni doping can substitute the Cr sites and further affect Cu–O bonds, which is caused by oxygen deficiency or a relatively smaller crystallite size. Moreover, it suggested that the nonlinear temperature dependence of phonon frequency and line width is a sign of nonharmonic lattice dynamics. The inherent anharmonic force from the crystal results in energy exchange between harmonic phonon normal modes, thereby promoting thermal balance, and in the lattice. The Cu cations have been confirmed to exist in the valence state of Cu1+, and all doped Ni cations exist in a positive divalent state by XPS. Hall effect measurement shows that the conductivities and carrier concentrations of the doped CuCr1–xNixO2 films can be improved by several orders of magnitude larger than that of undoped CuCrO2. The present work suggests that these well crystallized films without a secondary phase can be applied in future p-type optoelectronic devices.

Derivation of lifetime in the new got equation based on the temperature dependent frequency factor and the analysis of trap and recombination center distribution of Al2O3:C

A new lifetime equation in the latest general order based on the temperature-dependent frequency factor has been derived, and the lifetime of the trap of Al2O3:C analyzed. The analysis of Al2O3:C in the new general order equation based on the temperature-dependent frequency factor shows that the activation energy of Al2O3:C is 1.153 ± 0.003 eV. The coefficient of the temperature-dependent frequency factor is in the range of (3.49–4.39) x106(s−1 oK−2). The most remarkable feature of the Al2O3:C is that it has nearly (1−3)% of the empty traps lying in the vicinity of the filled traps, i.e., the filled traps are surrounded mainly by recombination centers, signified by the value of the parameter η. The lifetime of the 450°K trap is 18–19 months for a 5 mGy dose, which by first-order kinetics is 3 months, and 6 months by the general order lifetime equation. Thus, the lifetime equations of the first order and general order kinetics are not valid for evaluating the lifetime of a trap.

Integrated reference cavity with dual-mode optical thermometry for frequency correction

Photonic integrated resonators have advantages over traditional benchtop cavities in terms of size, weight, and cost with the potential to enable applications that require spectrally pure light. However, integrated resonators suffer from temperature-dependent frequency variations and are sensitive to external environmental perturbations, which hinders their usage in precision frequency applications. One solution is to use interrogation of the cavity temperature through dual-mode optical thermometry (DMOT) by measuring the shift of the resonance frequency difference between two polarization or optical frequency modes. Yet this approach has only been demonstrated in bulk-optic whispering gallery mode and fiber resonators. In this paper, we implement dual-mode optical thermometry in an ultra-high Q integrated silicon nitride resonator. A dual-mode resonance frequency difference temperature sensitivity of 188 ± 15 M H z / K is measured. We demonstrate feedforward DMOT frequency correction that, under an applied external temperature ramp, is able to reduce the optical frequency change to 0.31 kHz/s as compared to an uncorrected 10.03 kHz/s, a factor of 30 × reduction. These results show promise for on-chip frequency correction solutions for quantum, metrology, atomic, and coherent optical communications applications.

Developing machine learning potential for classical molecular dynamics simulation with superior phonon properties

Classical molecular dynamics is one of the most important methods for exploring material properties and uncovering physical mechanism, but the predicted results strongly depend on the used potentials. Using ab initio molecular dynamics simulations to obtain atomic conformations and the associated energy and force as training set and testing set, we developed a Gaussian approximation potential model for single-layer MoS2based on the machine learning method. The phonon dispersion relations calculated from the developed potential are compared well to that of density functional theory,which confirms the accuracy of the developed potential in the harmonic interaction. We also calculated the temperature-dependent Raman-active phonon frequency and linewidth of the single-layer MoS2 using classical molecular dynamics. The obtained temperature-dependent phonon frequency and linewidth are compared with the corresponding experimental results, indicating that the developed potential still has high accuracy in the anharmonic interactions. The detailed process and methods to obtain the MoS2potential in this work can be extended and applied to the development and investigation of the high-precision potential of emerging new materials.

Resonances and Lander Modes Observed by InSight on Mars (1–9 Hz)

ABSTRACT The National Aeronautics and Space Administration’s (NASAs) Interior exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) lander successfully touched down on Mars in November 2018, and, for the first time, a seismometer was deployed on the surface of the planet. The seismic recordings reveal diurnal and seasonal changes of the broadband noise level that are consistent with variations of the local atmospheric conditions. The seismic data include a variety of spectral peaks, which are interpreted as wind-excited, mechanical resonances of the lander, resonances of the subsurface, or artifacts produced in the measurement system. Understanding the origin of these signals is critical for the detection and characterization of marsquakes as well as for studies investigating the ambient noise. We identify the major spectral peaks up to 9 Hz, corresponding to the frequency range the most relevant to observed marsquakes. We track the variations in frequency, amplitude, and polarization of these peaks over the duration of the mission so far. The majority of these peaks can readily be classified as measurement artifacts or lander resonances (lander modes), of which the latter have a temperature-dependent peak frequency and a wind-sensitive amplitude. Of particular interest is a prominent resonance at 2.4 Hz, which is used to discriminate between seismic events and local noise and is possibly produced by a subsurface structure. In contrast to the lander modes, the 2.4 Hz resonance has distinctly different features: (1) a broad and stable spectral shape, slightly shifted on each component; (2) predominantly vertical energy; (3) temperature-independent peak frequency; (4) comparatively weak amplification by local winds, though there is a slow change in the diurnal and seasonal amplitude; and (5) excitation during all seismic events that excite this frequency band. Based on these observations, we suggest that the 2.4 Hz resonance is the only mode below 9 Hz that could be related to a local ground structure.

Anomalous temperature dependence of the effective mass in p -type Bi2Te3

Infrared reflectance measurements have been carried on highly $p$-type ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ crystals over the broad temperature range from 10 to 650 K and their response function investigated. An anomalous nonmonotonous temperature dependence of the plasma edge frequency is observed---with a pronounced redshift and blueshift that appear above and below room temperature, respectively. Such behavior is explained in terms of the temperature evolution of the free carrier optical weight $N/{m}^{*}$ given by particular temperature dependence of the total carrier concentration $N$ (determined by transport measurements) and characteristic (optical) mass ${m}^{*}$. The effective mass strongly varies from ${m}^{*}=0.1{m}_{0}$ (${m}_{0}$ electron rest mass) at 10 K to ${m}^{*}=0.5{m}_{0}$ respective ${m}^{*}=0.9{m}_{0}$ at 650 K depending on the evaluating procedure. The interpretation is corroborated by complementary magnetotransport and magnetoreflectivity experiments.

Sharp-Wave Ripple Frequency and Interictal Epileptic Discharges Increase in Tandem During Thermal Induction of Seizures in a Mouse Model of Genetic Epilepsy.

Dravet Syndrome (DS) is a genetic, infantile-onset epilepsy with refractory seizures and severe cognitive impairment. While network level pathophysiology is poorly understood, work in genetic mouse models of DS reveals selective reduction of inhibitory interneuron excitability, a likely mechanism of seizures and comorbidities. Consistent with the critical role of interneurons in timing and recruitment of network activity, hippocampal sharp wave ripples (SPW-R)—interneuron dependent compound brain rhythms essential for spatial learning and memory—are less frequent and ripple frequency is slower in DS mice, both likely to impair cognitive performance. Febrile seizures are characteristic of DS, reflecting a temperature-dependent shift in excitation–inhibition balance. DS interneurons are sensitive to depolarization block and may fall silent with increased excitation precipitating epileptic transformation of ripples. To determine the temperature dependence of SWP-R features and relationship of SPW-R to hippocampal interictal activity, we recorded hippocampal local field potentials in a DS mouse model and wildtype littermate controls while increasing core body temperature. In both genotypes, temperature elevation speeds ripple frequency, although DS ripples remain consistently slower. The rate of SPW-R also increases in both genotypes but subsequently falls in DS mice as interictal epileptic activity simultaneously increases preceding a thermally-evoked seizure. Epileptic events occur intermixed with SPW-R, some during SPW-R burst complexes, and transiently suppress SPW-R occurrence suggesting shared network elements. Together these data demonstrate a temperature dependence of SPW-R rate and ripple frequency and suggest a pathophysiologic mechanism by which elevated temperature transforms a normal brain rhythm into epileptic event.

Studies of Structural, Electrical, and Magnetic Characteristics of Double Perovskite Ceramic: La2FeMnO6

Herein, detailed studies of structural, electrical, and magnetic characteristics of a double perovskite lanthanum iron manganate (La2FeMnO6) are reported. From the Rietveld analysis of X‐ray diffraction data and pattern, the orthorhombic structure of the compound is determined. The least‐squares refined lattice parameters of the compound are: a = 5.5370 Å, b = 7.7988 Å, and c = 5.5069 Å. The frequency–temperature dependence of the dielectric parameters is explained by the Maxwell–Wagner model to provide the polarization mechanism. Analysis of temperature dependence of impedance and modulus data of different frequency has shown the contributions of grains and grain boundaries in the electrical characteristics of the material. This analysis also shows the semiconductor characteristics, through a negative temperature coefficient of resistance nature. Based on both impedance and modulus spectroscopy, the dielectric relaxation process is found to be of a non‐Debye type. The study of scaling behavior of impedance and modulus data suggests that relaxation phenomena are nearly independent of temperature. Detailed analysis of the frequency dependence of conductivity data of different temperatures using the Jonscher's power law has shown the existence of different types of conduction mechanisms in the material. The nature of the current–voltage (I–V) curves suggests that the material behaves as a semiconductor diode type to be used for the rectifying purpose. The conduction mechanism follows the space charge‐limited conduction (SCLC) process which is obtained from leakage current analysis. The study of magnetic hysteresis of the material suggests that the material exhibits soft ferromagnetic ordering.

Temperature Dependence of Spin-Phonon Coupling in [VO(acac)2]: A Computational and Spectroscopic Study.

Molecular electronic spins are good candidates as qubits since they are characterized by a large tunability of their electronic and magnetic properties through a rational chemical design. Coordination compounds of light transition metals are promising systems for spin-based quantum information technologies, thanks to their long spin coherence times up to room temperature. Our work aims at presenting an in-depth study on how the spin–phonon coupling in vanadyl-acetylacetonate, [VO(acac)2], can change as a function of temperature using terahertz time-domain spectroscopy and density functional theory (DFT) calculations. Powder THz spectra were recorded between 10 and 300 K. The temperature dependence of vibrational frequencies was then accounted for in the periodic DFT calculations using unit-cell parameters measured at two different temperatures and the optimized ones, as usually reported in the literature. In this way, it was possible to calculate the observed THz anharmonic frequency shift with high accuracy. The overall differences in the spin–phonon coupling magnitudes as a function of temperature were also highlighted showing that the computed trends have to be ascribed to the anisotropic variation of cell parameters.

Strong negative electrothermal feedback in thermal kinetic inductance detectors

We demonstrate strong negative electrothermal feedback accelerating and linearizing the response of a thermal kinetic inductance detector (TKID). TKIDs are a proposed highly multiplexable replacement to transition-edge sensors and measure power through the temperature-dependent resonant frequency of a superconducting microresonator bolometer. At high readout probe power and probe frequency detuned from the TKID resonant frequency, we observe electrothermal feedback loop gain up to L≈16 through measuring the reduction of settling time. We also show that the detector response has no detectable non-linearity over a 38% range of incident power and that the noise-equivalent power is below the design photon noise.

Josephson Plasmon Resonance in Tl2Ba2CaCu2O8 High‐Temperature Superconductor Tunable Terahertz Metamaterials

AbstractThe Josephson plasmon resonance (JPR) offers a valuable probe to investigate the superconductivity in layered cuprate superconductors. However, the coupling between free space radiation and JPR in high‐temperature superconductor (HTS) film remains challenging because the excitation of JPR demands the c‐axis oriented electric field. The subwavelength resonators in metamaterials can enhance the localized electric field, which can be utilized to resolve this difficulty. Here, a tunable terahertz (THz) metamaterial made from Tl2Ba2CaCu2O8 (Tl‐2212) HTS film is developed. The spectral response of Tl‐2212 metamaterial has a tunable property at temperatures up to 90 K. The resonant excitation of Josephson plasmon in the metamaterial is observed. Simulation results indicate that the scattering of subwavelength resonators can provide the component of the z‐axis electric field for the resonant excitation. The coupling between JPR and resonance modes of metamaterials is observed and explained using coupled mode theory. The temperature dependence of JPR frequency shows accordance with the experimental results of the pure film. This work provides an avenue to excite the JPR and probe superconducting condensate in the layered superconductor. The development of Josephson plasmonic metamaterials may contribute to tunable and nonlinear THz devices.

Intrinsic phonon anharmonicity in heavily doped graphene probed by Raman spectroscopy

The temperature-dependent (T-dependent) linewidth (ΓG) and frequency shift (ΔωG) of the G mode provide valuable information on the phonon anharmonicity of graphene-based materials. In contrast to the negligible contribution from electron-phonon coupling (EPC) to the linewidth of a Raman mode in semiconductors, ΓG in pristine graphene is dominated by EPC contribution at room temperature due to its semimetallic characteristics. This leads to difficulty in resolving intrinsic contribution from phonon anharmonicity to ΓG. Here, we probed the intrinsic phonon anharmonicity of heavily-doped graphene by T-dependent Raman spectra based on FeCl3-based stage-1 graphite intercalation compound (GIC), in which the EPC contribution is negligible due to the large Fermi level (EF) shift. The ΔωG and ΓG exhibit a nonlinear decrease and noticeable broadening with increasing temperature, respectively, which are both dominated by phonon anharmonicity processes. The contribution of phonon anharmonicity to ΓG of heavily-doped graphene decreases as the EF approaches to the Dirac point. However, the T dependence of ΔωG is almost independent on EF and qualitatively agrees with the theoretical result of pristine graphene. These results provide a deeper understanding of the role of phonon anharmonicity on the Raman spectra of heavily doped graphene.

Dielectric anomalous peaks accenting ferroelectricity prospects of Li and Mg co-doped ZnO ceramics

This paper presents effects of lithium (Li) and magnesium (Mg) dopants on the dielectric properties of ZnO ceramics with emphasis on ferroelectricity prospects of co-doped samples with Zn0.7Li0.28Mg0.02O compositions. Ceramics with average relative density ∼97% were produced by solid-state-reaction of mechanically activated powder precursors using 1000 °C for 2 h sintering conditions. Room temperature A.C. conductivity σa.c(ω) and temperature-frequency (T-ω) dependence of dielectric permittivity (ε′) and loss (ε″) of the samples are measured within temperature range from −100 to 150 °C in air and in a vacuum cryostat with liquid nitrogen coolant. Undoped ZnO σa.c(ω) data exhibits a nearly insensitive dependence to the ω while that of doped samples show significant ω dependence and higher σa.c(ω) values than undoped ZnO ceramics. Anomalous ε′ and ε″ peaks were observed in Zn0.7Li0.28Mg0.02O specimens around TC of −25 °C during cooling and −10 °C during heating measurements in air. However, dielectric properties of the same Zn0.7Li0.28Mg0.02O ceramics obtained in vacuum cryostat and from undoped ZnO ceramics in air and in vacuum cryostat did not reveal any ε′ and ε″ transition peaks at low temperature.

Degradation mechanisms of InGaN/GaN UVA LEDs under swift heavy ion irradiation: role of defects

This work investigates the intrinsic degradation mechanism of InGaN/GaN UVA (320–400 nm) light emitting diodes (LEDs) after swift heavy ion irradiation from the defect aspect using temperature-dependent low frequency noise measurement. The UVA LEDs were exposed to 1423 MeV Ta ions with a fluence of 108–1010 ions cm−2. The optical power decreases and the leakage current increases evidently after irradiation. The defect behaviors were characterized to analyze the degradation mechanism. It is indicated that the ∼0.2 eV energy level defect decreases, and the defect with an energy level of about 0.65 eV increases with the increasing fluence, resulting in device degradation. Combining the defect energy level and the decreased yellow luminescence in the photoluminescence spectrum, the possible defect transform process is the newly created N interstitial atoms fill in the intrinsic VGa and form NGa after irradiation.