Decreasing Sample Temperature Research Articles

Deuterium plasma-driven permeation through vanadium, niobium and tantalum membranes

Deuterium (D) plasma-driven permeation (PDP) experiments for vanadium (V), niobium (Nb) and tantalum (Ta) membranes are performed between 573–873 K in the linear plasma facility PREFACE. The sample temperature, membrane thickness and ion incident energy effects on D permeation behavior through these three metals are studied. PDP fluxes of D through Ta and Nb decrease with decreasing sample temperature, except for the 0.05 mm thick Nb case. Moreover, a break point around 750 K can be observed on the flux-temperature curves for all V samples. The D PDP flux through Nb decreases as the thickness increases. For Ta and V samples, however, the permeation fluxes do not change significantly when their thicknesses are thinner than 0.1 mm. A drop in D permeation fluxes for the three materials is observed when increasing the ion incident energy. The surface morphology is checked by Scanning Electron Microscopy (SEM). Only the V membranes exposed to D plasma show surface exfoliation.

CF3Br plasma cryo etching of low-k porous dielectric

Process of plasma etching of CVD low-k dielectric was studied. We used CF3Br low pressure ICP plasma for etching at cryo temperatures (-20°C — -100°C), pressures (5-20 mTorr) and RF bias with effective DC voltage 80-140 V. Refractive index of film and its thickness were measured by spectral ellipsometry. Ellipsometric porosimetry was employed to compare pore size distribution before and after etching of films. Measurements show increasing of etch rate increase with decreasing sample temperature.

Three-Dimensional Measurement of Ice Crystals in Frozen Materials by Near-Infrared Imaging Spectroscopy

A novel technique was developed to recognize ice crystals in biological materials and to analyze their three-dimensional morphology using a Cryogenic Micro-Slicer Spectral Imaging System with a micro-slicer unit and a near-infrared spectral imaging unit. Consecutive cross-sections of a frozen sample were exposed by the multi-slicing operations with a minimum thickness of 1 µm, and their images were taken by the imaging unit. Spectroscopic analysis using a near-infrared spectrum meter showed an absorption peak at 1460 nm for pure water. Based on the observations of the absorption band of ice crystals in the wavelength range of 1450–1570 nm and its peak at 1495 nm, a commodity-type bandpass filter with a central wavelength of 1500 nm was adopted to identify ice crystals in near-infrared images. The absorption peak of water exhibited a tendency to move toward longer wavelengths with decreasing sample temperature from 25 °C to −15 °C. The filtered images of ice crystals in frozen samples were darker than the other components at the peak wavelength of ice crystals. The three-dimensional reconstructed morphology of ice crystals revealed that they were formed along the direction of heat transfer while freezing. The proposed method provides a novel tool to investigate the effects of freezing conditions on the size, morphology and distribution of ice crystals.

NMR at low and ultralow temperatures.

Solid state nuclear magnetic resonance (NMR) measurements at low temperatures have been common in physical sciences for many years and are becoming increasingly important in studies of biomolecular systems. This Account reviews a diverse set of projects from my laboratory, dating back to the early 1990s, that illustrate the motivations for low-temperature solid state NMR, the types of information that are available from the measurements, and likely directions for future research. These projects include NMR studies of both physical and biological systems, performed at low (cooled with nitrogen, down to 77 K) and ultralow (cooled with helium, below 77 K) temperatures, and performed with and without magic-angle spinning (MAS). NMR studies of physical systems often focus on phenomena that occur only at low temperatures. Two examples from my laboratory are studies of molecular rotation and orientational ordering in solid C60 at low temperatures and studies of unusual electronic states, called skyrmions, in two-dimensionally confined electron systems within semiconductor quantum wells. To study quantum wells, we used optical pumping of nuclear spin polarizations to enhance their NMR signals. The optical pumping phenomenon exists only at ultralow temperatures. In studies of biomolecular systems, low-temperature NMR has several motivations. In some cases, low temperatures suppress molecular tumbling, thereby permitting solid state NMR measurements on soluble proteins. Studies of AIDS-related peptide/antibody complexes illustrate this effect. In other cases, low temperatures suppress conformational exchange, thereby permitting quantitation of conformational distributions. Studies of chemically denatured states of the model protein HP35 illustrate this effect. Low temperatures and rapid freeze-quenching can also be used to trap transient intermediate states in a non-equilibrium kinetic process, as shown in studies of a transient intermediate in the rapid folding pathway of HP35. NMR sensitivity generally increases with decreasing sample temperature. Therefore, it can be useful to carry out experiments at the lowest possible temperatures, particularly in studies of biomolecular systems in frozen solutions. However, solid state NMR studies of biomolecular systems generally require rapid MAS. A novel MAS NMR probe design that uses nitrogen gas for sample spinning and cold helium only for sample cooling allows a wide variety of solid state NMR measurements to be performed on biomolecular systems at 20-25 K, where signals are enhanced by factors of 12-15 relative to measurements at room temperature. MAS NMR at ultralow temperatures also facilitates dynamic nuclear polarization (DNP), allowing sizeable additional signal enhancements and large absolute NMR signal amplitudes with relatively low microwave powers. Current research in my laboratory seeks to develop and exploit DNP-enhanced MAS NMR at ultralow temperatures, for example, in studies of transient intermediates in protein folding and aggregation processes and studies of peptide/protein complexes that can be prepared only at low concentrations.

Twin structures in copper after shock and Shockless high-rate loading

The structure of copper formed after high-rate loading up to pressures of 20–80 GPa with a strain rate of 105–109 sec−1 is considered. In situations with pressures above 20 GPa and strain rates above 106 sec−1, the deformation twins are grouped into packets, which are seen in an optical microscope as parallel bands of localized strains inside individual grains. The number of bands in the structure increases with increasing grain size and strain rate, with decreasing sample temperature, and with increasing period of sample loading. The characteristic time of formation of twin bands in copper is estimated as 0.3 µsec.

Viscoelastic properties of liver measured by oscillatory rheometry and multifrequency magnetic resonance elastography

The mechanical properties of liver can sensitively indicate the progression of hepatic fibrosis. Mechanical tissue characterization involves the analysis of the complex shear modulus measured either by oscillatory rheometry or by in vivo elastography. In this study, bovine liver specimens were investigated by oscillatory rheometry and multifrequency magnetic resonance elastography (MRE) in a common frequency range between 25.0 and 62.5 Hz. The results were compared with in vivo MRE of human liver. Storage and loss moduli were quantified, and the data were also analyzed employing a springpot model, yielding a stiffness-related parameter of 2.96+/-0.53 kPa in bovine liver by rheometry and of 2.20+/-0.45 kPa in human liver by in vivo MRE. Furthermore, MRE of excised bovine liver showed that stiffness tended to increase with decreasing sample temperature. In conclusion, mechanical tissue characterization by multifrequency MRE agrees well with oscillatory rheometry, which validates MRE as a method for investigating the rheology of liver tissue.

Investigation of Carrier Recombination Processes in GaAs/AlAs Multiple Quantum Wells Using Piezoelectric Photothermal and Surface Photovoltage Techniques

We investigate the confinement, radiative, and nonradiative recombinations of photoexcited carriers in each subband of GaAs/AlAs multiple quantum wells (MQWs) using two nondestructive techniques, namely, the surface photovoltage (SPV) and the piezoelectric photothermal (PPT) techniques. In the temperature range from 160 to 300 K, there is no difference between the temperature dependences of carrier confinement in 1HH, 2HH, and 3HH subband transitions. As temperature decreased, increases in the value of the nonradiative recombination component in 2HH and 3HH were observed. These are attributed to the phonon-emitting intersubband relaxation. It was found that the number of emitted phonons by this relaxation increased with increasing order of the subband and with decreasing sample temperature. We conclude that this phonon-emitting intersubband relaxation is the most important process for electron transition in the higher-order subband in MQWs and for PPT signal generation. This is the first time that phonon-emitting intersubband relaxation was directly detected from the nonradiative recombination viewpoint.

Mobility estimation in microsized bismuth wire arrays

Wire-diameter dependencies were investigated using microwire sample arrays with diameters from 2 to 25 μm and polycrystalline bulk bismuth. Seebeck coefficient and resistivity were measured from 50 to 300 K. Although the temperature dependence of the Seebeck coefficients of all samples were similar to that of the bulk, the resistivity dependence of the 2-μm-diameter microwire array sample had a small positive temperature coefficient, and was quite different from the other samples. Mobility was estimated using a simple model to identify the dependences on temperature and wire diameter. The ratio of electron mobility divided by hole mobility gradually decreased with decreasing temperature for samples with wire diameters below 6 μm, and electron mobility was suppressed at low temperatures, explaining the promotion of the boundary scattering. Therefore, small positive temperature coefficients of resistivity were obtained for very small diameter microwire array samples. To compare with the temperature dependence of the nanowire array samples that typically have negative temperature coefficient reported, a model for mobility was developed to represent the temperature dependence of resistivity.

Analysis of polarization‐dependent photoreflectance studies for c ‐plane GaN films grown on a ‐plane sapphire

AbstractThe optical properties of c ‐plane GaN layers grown by molecular beam epitaxy on a ‐plane sapphire substrate are investigated. Polarization‐dependent photoreflectance spectroscopy was applied with the electric field vector of the probe beam being either parallel or perpendicular to the [10$ \bar 1 $0] direction of the GaN films. A pronounced in‐plane optical anisotropy under normal incidence of light is observed which is attributed to anisotropic in‐plane strain. It originates from the difference of the thermal expansion coefficients of the substrate parallel and perpendicular to its c‐axis. Temperature‐dependent studies and a comparison to photoluminescence and reflectance anisotropy studies indicate that the strain (anisotropy) increases with decreasing sample temperature. The dependence of transition energies and oscillator strengths of GaN as a function of strain values is calculated by k·p perturbation theory for comparison. The results emphasize the experimental data. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Energy spike effects in ion-bombarded GaN

We study structural disorder in GaN bombarded at room temperature with 1.3 keV amu−1 PFn (n = 0, 2 and 4) cluster ions. Results are compared with our previous studies of irradiation with atomic ions of different masses. An algorithm for cascade density calculations that take into account the formation of subcascades is presented. Quantitative analysis of both new and previous data shows that an increase in the cascade density above a certain critical value results in a rapid increase in the rate of planar amorphization and the rate of damage buildup in the crystal bulk. Both such rates increase with decreasing sample temperature. This threshold-like behaviour suggests an important role of nonlinear energy spikes in the formation of stable implantation disorder in GaN. We also discuss the striking difference between cascade density effects in damage buildup in different semiconductors, including GaN, ZnO and Si.

XANES measurements of the rate of radiation damage to selenomethionine side chains.

The radiation-induced disordering of selenomethionine (SeMet) side chains represents a significant impediment to protein structure solution. Not only does the increased B-factor of these sites result in a serious drop in phasing power, but some sites decay much faster than others in the same unit cell. These radio-labile SeMet side chains decay faster than high-order diffraction spots with dose, making it difficult to detect this kind of damage by inspection of the diffraction pattern. The selenium X-ray absorbance near-edge spectrum (XANES) from samples containing SeMet was found to change significantly after application of X-ray doses of 10-100 MGy. Most notably, the sharp ;white line' feature near the canonical Se edge disappears. The change was attributed to breakage of the Cgamma-Se bond in SeMet. This spectral change was used as a probe to measure the decay rate of SeMet with X-ray dose in cryo-cooled samples. Two protein crystal types and 15 solutions containing free SeMet amino acid were examined. The damage rate was influenced by the chemical and physical condition of the sample, and the half-decaying dose for the selenium XANES signal ranged from 5 to 43 MGy. These decay rates were 34- to 3.8-fold higher than the rate at which the Se atoms interacted directly with X-ray photons, so the damage mechanism must be a secondary effect. Samples that cooled to a more crystalline state generally decayed faster than samples that cooled to an amorphous solid. The single exception was a protein crystal where a nanocrystalline cryoprotectant had a protective effect. Lowering the pH, especially with ascorbic or nitric acids, had a protective effect, and SeMet lifetime increased monotonically with decreasing sample temperature (down to 93 K). The SeMet lifetime in one protein crystal was the same as that of the free amino acid, and the longest SeMet lifetime measured was found in the other protein crystal type. This protection was found to arise from the folded structure of the protein molecule. A mechanism to explain observed decay rates involving the damaging species following the electric field lines around protein molecules is proposed.

EPR spectra of deuterated methyl radicals trapped in low temperature matrices

EPR spectra of CHD2 and CD3 radicals are investigated in low-temperature matrices of H2, D2, and Ne at temperatures of 1.6–4.2 K. A method of condensation from the gas phase on a cold substrate is used. With decreasing sample temperature, a transformation of the shape of the CD3 spectrum in H2, D2, and Ne matrices and CHD2 spectrum in H2 and Ne is observed. This transformation was reversible in the above temperature range. The temperature effects are explained as reflecting a change in the populations of the lowest rotational states of the radicals. The temperature behavior of the EPR spectra for radicals trapped in various matrices are compared on the basis of the present data and known results for deuterated methyl radicals in Ar obtained in photolytic experiments. As a result the existence of a hindering barrier for the radical rotation is suggested.

Probing dynamics of a phase transition of two-dimensional nano-domains with STM imaging and manipulation

A reversible, temperature-driven structural surface phase transition of Pb/Si(1 1 1) nano-domains is studied with a variable-temperature scanning tunneling microscope (STM). Finite-size effects of the transition are clearly demonstrated. Most importantly, structural fluctuations in the low-temperature phase can be induced by the direct interaction between the tip atoms and the surface atoms. The structural changes reveal dynamics in the low-temperature phase. Amazingly, the largest size of the domains that can be manipulated decreases with decreasing sample temperature.

EPR spectra and rotation of CH3, CH2D, CHD2, and CD3 radicals in solid H2

The EPR spectra of CH3, CH2D, CHD2, and CD3 radicals in a H2 matrix have been observed in the temperature range 1.6–4.2 K. The radicals were obtained by condensation of two gas flows on a cold substrate: deuterium mixed with 2 mol % methane passed through a discharge and pure hydrogen bypassing the discharge. The CD3 and CHD2 spectra were found to be a superposition of two spectra: high- and low-temperature. A transformation of the shape of the CD3 and CHD2 spectra with decreasing sample temperature was observed. This was attributed to a change in the populations of the lowest rotational states of the radicals. Compared to the known results for deuterated methyl radicals in Ar, the present observations suggest the existence of a barrier hindering the rotation of radicals in solid H2.

Thermoelectric powerand band spectrum transformation in Y1-xCaxBa2-xLaxCu3Oy

The superconducting and normal-state transportproperties (resistivity ρ and thermoelectric power S) oftwo sets of ceramic samples with compositionY1-xCaxBa2-xLaxCu3Oy (x = 0-0.4) have been investigated.The co-doping effect was studied at different conditions of the oxygen subsystem, namely, at anear-stoichiometric oxygen content and at that decreased by annealing in vacuum atmosphere.The thermoelectric power changes only slightly with increasing doping level,while the S(T) dependence acquires additional features,exhibiting a linear increase with decreasing temperature forheavily-doped samples and a rise in the S(T) curve slope as x grows.The results obtained are analysed in terms of aphenomenological narrow-band model that makes it possible to determinethe band spectrum parameters in the normal state and to tracetheir changes with varying composition.In co-doped Y1-xCaxBa2-xLaxCu3Oy, impuritieswith valencies different from those of the native cations cancel out the influence of one another on thecharge balance in the lattice.All the results obtained indicate that the normal-state and superconducting properties of theinvestigated system are mainly determined by this compensation effect.Comparison of the variation of the critical temperatureTc with the changing conduction bandwidth WD shows thatthere is a correlation between these parameters, according towhich the superconducting properties of doped YBa2Cu3Oydepend strongly on the parameters of the band spectrum structurein the normal state.Analysis of how the conduction band is modified and S(T) transforms suggests that calcium isresponsible for additional states in the conduction band.

Temperature dependence of magnetoresistance in neutron-irradiated and unirradiated high resistivity p-type silicon

The temperature dependence of the transverse magnetoresistance in irradiated and unirradiated p-type Si is studied in the range from 120 to 290 K. The magnetoresistance coefficients for the unirradiated [001] and [110] samples increases with decreasing sample temperature in the range from 160 to 290 K, however, this behavior is reversed below 160 K. It is proposed that this reversal is due to the double injection effect. The magnetoresistance coefficient for the irradiated [100] sample increases with decreasing sample temperature in the range of 120 to 290 K and is greater than that for the unirradiated [001] sample. This result can be explained by increased scattering due to the increased number of defects produced by irradiation. On the other hand, the magnetoresistance coefficient for the unirradiated [110] sample is found to be greater than that of the unirradiated [001] sample.

Relationship between the Structural Transitions and the Low Temperature Electronic State of (La, Nd)2-xMxCuO4(M=Ba, Sr;x∼1/8)

Relationship between successive structural transitions and transport anomalies of La 2- x - y Nd y Ba x CuO 4 has been studied, where electrical resistivities ρ, Hall coefficients R H , thermoelectric powers S and ultrasonic sound velocities V s were measured. The transport properties exhibit no anomalous behavior at T d2 , the critical temperature of the transition from the low temperature orthorhombic (LTO1) to the low temperature tetragonal (LTT(Ba)) phase, indicating that the conduction electrons are not affected by this transition. Instead, a sudden decrease in S is observed with decreasing temperature for samples with x ∼ 0.125 at T ′ d3 ∼ 70 K. Although T ′ d3 seemed to coincide with the temperature T d3 of the tetragonal → orthorhombic transition reported by Hara et al. , results of the present X-ray diffraction study do not indicate any evidence for the existence of such a transition.

Isotopically selective photodissociation of gas phase OCS at low termperatures

Two-step laser photodissociation of gas phase OCS over the temperature range 296–150 K was used to enrich oxygen and sulfur isotopes. The 0–2 v 2 vibrational transition of OCS was pumped with a line-tuned CO 2 TEA laser, followed by UV photodissociation with a KrF laser at 249 nm. The degree of isotope enrichment achieved in the photodissociation products was found to increase significantly with decreasing sample temperature, reaching a maximum enrichment factor 4.84±0.5 at 150 K for enrichment of 18O. The observed temperature dependence was fit to a simple theory based on the effect of the thermal population of excited vibrational levels on the photodissociation cross-section. The dependence of isotope enrichment on the densities of OCS, xenon diluent gas and C 2D 4 scavenger, was also investigated.

Extended Helmholtz resonator in low-temperature photoacoustic spectroscopy

The acoustic transfer function of resonant PA cells is investigated experimentally and theoretically in the 10-300-K temperature range. The underlying cell design consists of two cavities that are interconnected by a cylindrical tube. The acoustic properties are treated in the scope of a generalized Helmholtz resonator model, which includes elements of acoustic transmission lines. For the low-temperature measurements the cell construction was modified to fit into a commercial optical cryostat. Photoacoustic signals are recorded from carbon and graphon samples as a function of the chopping frequency and of the temperature. With decreasing sample temperature the photoacoustic signal amplitude increases proportional to T(-2.75), as the phase angle and the resonance frequencies are simultaneously reduced.