Temperature Electron Paramagnetic Resonance Research Articles

Electron spin resonance studies of hydrogen adsorption on single-walled carbon nanotubes

Temperature and pressure dependent electron spin resonance (ESR) studies were conducted on two oxidatively purified single-walled carbon nanotube samples. For both samples, the ESR signal is composed of two components. The narrower component is ascribed to localized spins due to defects, the broader one to conduction electrons. Hydrogen predominantly interacts with defects. Below the condensation point, addition of hydrogen is suggested to cause diamagnetic dilution of the conducting sample, which leads to increased signal intensity.

Magnetic phase diagram of Nd0.5Ca0.5−ySryMnO3

The phase diagram obtained for Nd0.5Ca0.5−ySryMnO3 from the temperature dependent magnetization and electron spin resonance measurements are presented in this paper. It is found from magnetization measurements that y=0.25 is a critical point where a magnetic transformation occurs, accompanied by a nonmonotonic variation in the c lattice parameter. Below a certain transition temperature, compounds with y<0.25 exhibit charge ordering and those with y≥0.25 show ferromagnetic ordering. An antiferromagnetic transition occurs for y=0 at still lower temperatures whereas a phase separation into antiferromagnetically and ferromagnetically ordered states occurs for 0<y≤0.5. Electron spin resonance analysis shows that the phase separation occurs at temperatures above the ferromagnetic transition temperature in all the compounds with y>0.

Magnetic properties of frustrated Y-doped [formula omitted

We report temperature dependent magnetic susceptibility and electron spin resonance (ESR) measurements on single crystals of the frustrated antiferromagnet Gd 1 - x Y x InCu 4 ( x = 0 , 0.01 , 0.05 and 0.1). The magnetic susceptibility data revealed, as expected, a decreasing in the antiferromagnetic ordering temperature as function of the Y-concentration, while the high- T Curie–Weiss temperatures remain nearly unaffect by the doping. For T ≳ 100 K , a single Dysonian Gd 3 + ESR line with a nearly temperature independent g ∼ 1.989 ( 8 ) is observed, and its linewidth follows a Korringa-like ( Δ H / Δ T ∼ 0.75 ( 5 ) Oe / K ) behavior as a function of temperature. Below T ≈ 100 K both the ESR g-value and linewidth are affected by the presence of strong short-range magnetic correlation in the paramagnetic phase well above T N , consistently with the frustrated character of this compound. The role of chemical disorder and dilution effects induced by Y-doping in the properties of this highly frustrated material is discussed.

Pseudorotation-driven dynamical structure of the tropyl radical

Despite intensive studies of the neutral tropyl radical, none of its structure, energetics, and vibrational modes are still clear. This system has puzzled scientists for over a decade since one vibrational mode frequency sharply varies from imaginary number 3000i cm-1 to the real number 6000 cm-1, depending on the calculation methods employed. We find that the origin of this peculiar mode is due to the pseudorotation (omegairot) involved in the interconversion of two nearly isoenergetic Jahn-Teller configurations (elongated structure 2B1 and compressed structure 2A2 with C2v symmetry). Here, we first report that this interconversion is not via D7h or C2v symmetry configuration but via Cs symmetry (i.e., by changing the C2v axis). This interconversion barrier is found negligibly small. Thus, the two conformers are considered to be not two different structures but a dynamically identical structure with partial quantum statistical distributions on the potential energy surface. Owing to the nearly barrierless pseudorotation, the overall structure in a short time scale (less than femtosecond) would be Cs-like between 2A2 and 2B1 configurations with small fluctuation of bond distances. However, the dynamical transitions between the 2B1 and 2A2 configurations via 14 different pseudorotation pathways would make the tropyl radical have the effective D7h structure in either a nonshort time scale (greater than femtosecond) or at nonlow temperatures, which explains the high temperature electron spin resonance experiments.

Low temperature electron spin resonance investigation of ultrananocrystalline diamond films as a function of nitrogen content

We have investigated the room and low temperature ESR signals in a series of ultrananocrystalline diamond films grown with different N 2 enrichments in the gas phase. We have found that exchange interaction is playing the major role in spin–spin interaction and determines the linewidths. The relatively high values of spin–lattice relaxation and other features of the ESR signals show that the ESR active centers are sitting in the grain boundaries' regions.

Low temperature electron spin resonance investigation on SWNTs after hydrogen treatment

We report a study of hydrogen adsorption experiment on SWNT samples deposited by chemical vapor deposition (CVD) and investigated with Electron Spin Resonance spectroscopy (ESR) before and after hydrogen treatment. The ESR investigation was performed at room and low temperature (4–30 K). We found that ESR technique is very useful to investigate paramagnetic centers and their interaction with hydrogen. The morphology and structure of the samples were characterized by Micro-Raman spectroscopy. Subsequently the thermal stability was investigated by thermogravimetric analysis (TGA) testing. Brunauer Emmett Teller (BET) analysis was used to calculate the specific surface area and porosity of the as grown samples. All characterizations showed a small presence of residual Fe impurity (3.3 wt.%).

Low temperature electron spin resonance theory for systems with multichannel Kondoimpurities

Recent experiments observed an electronic spin resonance (ESR) signal from localizedytterbium ions in a Yb-based compound, manifesting non-Fermi-liquid properties.Motivated by these experiments we develop a theory for the low temperature ESR inmetallic systems with multichannel magnetic (Kondo) impurities.

Analysis of Different Vanadium Charge States in Vanadium Doped 6H-SiC by Low Temperature Optical Absorption and Electron Paramagnetic Resonance

Analysis of Different Vanadium Charge States in Vanadium Doped 6H-SiC by Low Temperature Optical Absorption and Electron Paramagnetic Resonance

Spin defects and transport in hydrogenated nanocrystalline silicon carbide films produced by photo-CVD technique

Hydrogenated nanocrystalline silicon-carbide (nc-SiC:H) films with a varied carbon content have been prepared by a mercury-sensitized photo-assisted chemical vapor deposition (photo-CVD) technique. The structural analysis by Raman spectroscopy and X-ray diffraction indicated that the deposited material is composed of nanocrystalline silicon grains embedded into an amorphous matrix. The transport behavior and spin defect densities ( N S) of the films have been investigated by temperature dependent conductivity and electron spin resonance (ESR). It was found that during nc-SiC:H film growth the increase in carbon content (C-content) hinders the growth of a crystalline fraction. The decrease in film crystallinity caused by C-content increase (from 5 to 12 vol%) is accompanied by the decrease in spin densities from N S≈10 19 to N S=3×10 18 cm −3 and diminishes the room temperature dark conductivity by two orders of magnitude.

Hemin: A Possible Cause of Oxidative Stress in Blood Circulation of β-Thalassemia/Hemoglobin E Disease

A correlation between endogenous hemin and pro-oxidant activity was revealed in serum of g -thalassemia/hemoglobin E disease ( g -thal/Hb E), which is the most common prevalent type of thalassemia in Thailand. The technique of low temperature electron spin resonance spectroscopy was used for characterization and quantification of high spin ferric heme, which had been identified as hemin (iron (III)-protoporphyrin IX). Hemin was present at levels ranging from 50 to 280 w M in serum of g -thal/Hb E but not detectable in serum of non-thalassemia. Pro-oxidant activity in serum of g -thal/Hb E was demonstrated by luminol-mediated chemiluminescence, a sensitive method for screening of free radical generation in vitro. In the presence of H2O2, the chemiluminescence intensity (CL) was about 20 fold enhanced in serum of g -thal/Hb E, indicating its extensive pro-oxidant activity. The CL showed a good correlation with serum hemin, r =0.778 (p <0.001), while the correlations with total serum iron and serum ferritin were 0.260 (p =0.259) and 0.519 (p =0.004), respectively. Our finding suggested that serum hemin readily catalyzed free radical reactions and it may contribute a major pro-oxidant in blood circulation of g -thal/Hb E.

In situ ESR studies over wide temperature range for conducting polymers

In this work, in situ variable temperature electron spin resonance (ESR) measurements were realized for the first time to verify the spin nature of conducting polymers at controlled potentials. By using mixed solvents consisting of water and propanol in 1:1 volume ratio the temperature window was widened to 100 °C (−40 to +60 °C) for the study of polyaniline. According to the temperature dependence of ESR intensities obtained with this technique, it can be clearly seen that polyaniline in HCl solutions took on the Curie spin behavior at low doping level (0.0 V vs AgCl/Ag), and turned to the Pauli spin behavior at high doping level (0.4 V where conductivity reached about its maximum).

Thiourea protects against copper-induced oxidative damage by formation of a redox-inactive thiourea-copper complex

Although thiourea has been used widely to study the role of hydroxyl radicals in metal-mediated biological damage, it is not a specific hydroxyl radical scavenger and may also exert antioxidant effects unrelated to hydroxyl radical scavenging. Thus, we investigated the effects of thiourea on copper-induced oxidative damage to bovine serum albumin (1 mg/ml) in three different copper-containing systems: Cu(II)/ascorbate, Cu(II)/H 2O 2, and Cu(II)/H 2O 2/ascorbate [Cu(II), 0.1 mM; ascorbate, 1 mM; H 2O 2, 1 mM]. Oxidative damage to albumin was measured as protein carbonyl formation. Thiourea (0.1–10 mM) provided marked and dose-dependent protection against protein oxidation in all three copper-containing systems. In contrast, only minor protection was observed with dimethyl sulfoxide and mannitol, even at concentrations as high as 100 mM. Strong protection was also observed with dimethylthiourea, but not with urea or dimethylurea. Thiourea also significantly inhibited copper-catalyzed oxidation of ascorbate, and competed effectively with histidine and 1,10-phenanthroline for binding of cuprous, but not cupric, copper, as demonstrated by both UV-visible and low temperature electron spin resonance measurements. We conclude that the protection by thiourea against copper-mediated protein oxidation is not through scavenging of hydroxyl radicals, but rather through the chelation of cuprous copper and the formation of a redox-inactive thiourea-copper complex.

EPR and magnetic susceptibility measurements onCeB6

We report measurements of temperature and magnetic field dependent magnetic susceptibility and electron paramagnetic resonance in the paramagnetic phase of ${\mathrm{CeB}}_{6}.$ From calculations of the possible ground state level structure of the interacting $4f$ electrons it is found that the fourfold degenerate ${\ensuremath{\Gamma}}_{8}$ is split into two doublets with equal g factors and an energy separation of the order of 30 K.

Spin dynamics in perovskites, pyrochlores, and layered manganites

High temperature electron spin resonance (ESR) and magnetic susceptibility (χ) are analyzed for manganites related with colossal magnetoresistance (CMR). The properties of compounds with different crystalline structures: three-dimensional (3D) perovskites, pyrochlore, and La1.2Sr1.8Mn2O7, a two-dimensional layer, are compared. In the paramagnetic regime, and outside the critical regions associated with phase transitions, the temperature dependence of the ESR linewidth presents a universal behavior dominated by the variations of χ(T), ΔHpp(T)=[C/Tχ(T)]ΔHpp(∞). The high temperature limit of the linewidth, ΔHpp(∞), is related to the parameters of the Hamiltonian describing the interactions of the spin system. The role played by magnetic anisotropy, isotropic superexchange, and double exchange is revealed and discussed in the analysis of the experimental data. In CMR and non-CMR pyrochlores, ΔHpp(∞)∝ωp2/J where J is proportional to the Curie–Weiss temperature, including the hybridization mechanism producing CMR. Instead, ΔHpp(∞) of CMR perovskites seems not to be affected by the double-exchange interaction. In contrast with the 3D perovskites, the ESR linewidth and resonance field of La1.2Sr1.8Mn2O7, a bilayer compound, although isotropic at high temperatures, becomes anisotropic for Tc=125 K&amp;lt;T&amp;lt;Tp≈450 K.

Electron spin resonance of carbon nanotubes prepared under two kinds of inert gas ambient

The nanotubes were prepared by the dc arc-discharge method in helium and argon gas ambient under controlled pressure, and subsequently purified by oxidation in air. The room temperature electron spin resonance (ESR) spectra of purified carbon nanotubes prepared under different inert gases and their pressure were measured. The dependence of the ESR line shape, linewidth, g-value of the nanotubes on the inert gases and their pressures is found and discussed.

Aromatization of isophorone to 3,5-xylenol over Cr 2O 3/SiO 2 catalysts

SiO 2 supported Cr 2O 3 catalysts with varying Cr content have been prepared and are characterized by nitrogen adsorption, low temperature oxygen chemisorption (LTOC), X-ray diffraction (XRD) and electron spin resonance (ESR) techniques. Aromatization of isophorone is carried out on these catalysts. A good correlation obtained between the oxygen uptake and aromatization yield helped to identify the nature of the active species.

Low-Temperature Electron Spin Resonance Theory for Half-Integer Spin Antiferromagnetic Chains

A theory of low temperature (T) electron spin resonance (ESR) in half-integer spin antiferromagnetic chains is developed using field theory methods and avoiding previous approximations. It is compared to experiments on Cu benzoate. Power laws are predicted for the line-width broadening due to various types of anisotropy. At T -> 0, zero width absorption peaks occur in some cases. The second ESR peak in Cu benzoate, observed at T<.76K, is argued not to indicate Neel order as previously claimed, but to correspond to a sine-Gordon "breather" excitation.

Gd-doping effects on properties of amorphous silicon films prepared by electron beam evaporations

Gd-doped amorphous silicon films have been prepared by the electron beam evaporation technique, employing the experimental methods of dc conductivity temperature properties, ESR (electron spin resonance) spectra, and optical band gap Eopt measurements. We have investigated the optical and electrical properties of the films. The results show that at 290 K &lt; T &lt; 330 K, hopping conduction in Gd impurity states near Fermi level is predominant, and at 330 K &lt; T &lt; 500 K extended state conduction dominates due to electrons exited from the impurity states. At a Gd concentration of about 1.0 at.% spin density Ns, peak-peak width ΔBpp and line-shape factor l of ESR spectra change their dependence on Gd contents. The optical gap of the films narrows with increasing Gd contents in the films from 1.68 eV to 0.42 eV. The results were explained on the basis of the partial compensation of Gd atoms for dangling bonds .

Room Temperature Electron Spin Resonance of the Purified Carbon Nanotubes Produced in Different Helium Pressures

The electron spin resonance (ESR) of purified carbon nanotubes prepared under different helium pressures from 20.0 to 80.0 kPa in arc discharge has been measured. The dependence of the ESR spin density, linewidth and g value of the purified nanotubes on the helium pressure is found. The electronic properties of purified nanotubes varying with He pressure are discussed.

Esr of Purified Carbon Nanotubes Produced Under Different Helium Pressures

ABSTRACTCarbon nanotubes were prepared by the dc arc-discharge method under a controlled helium pressure ranging from 10 to 80 kPa and subsequently purified by oxidation in air. The purified carbon nanotubes were observed by transmission electron microscopy. The room temperature electron spin resonance (ESR) spectra of the purified nanotubes were measured. The variations in the ESR line shape, g-value, linewidth and relative spin density of the purified nanotubes on helium pressure were studied and discussed.