Electrophysical studies of nanoporous carbon materials prepared of silicon carbide powders

Measurements of the electrophysical properties of nanoporous carbon (NPC) samples (conductivity and Hall effect), as well as studies of the same samples using electron spin resonance (ESR), are summarized. Free holes are shown to play the major part in charge transport in such materials, thus permitting identification of the ESR signal with free holes. An analysis of the shape of the resonance lines, as well as of their dependence on temperature and the technology employed in sample preparation, established the ESR signals to consist of two superimposed resonance lines associated with carriers, free or localized to various extents, whose magnetic susceptibility obeys Pauli and Curie-Weiss laws, respectively. The temperature dependences of the ESR signal parameters were studied, and the experimental relations were compared with theory to determine the model-parameters. An analysis of the temperature behavior of these parameters suggests the conclusion that NPC samples are heterophase porous systems whose properties are dominated by structural characteristics.     

Features of magnetic resonance in nanoporous carbon with pores filled with transition-metal atoms

Properties of nanoporous carbon (NPC) free of metal atoms and NPC containing atoms of Ni, Co and Pd in their pores are studied by electron spin resonance (ESR). The asymmetrical ESR line with the so-called Dyson line shape points out that charge carriers are responsible for the resonance spectrum in metal-free NPC. Although the amount of Ni, Co, and Pd introduced into nanopores is small, the NPC properties change significantly. A bulk ferromagnetism is observed in the case of NPC with Co and Pd, but not in NPC:Ni. Co atoms in pores of NPC cause the formation of a new material, namely, a disordered ferromagnetic medium with some features in the Co atom distribution. Magnetic properties are strongly temperature-dependent. The temperature dependences of the conductivity and ESR integral intensity in NPC:Ni reveal an exponential growth with the same activation energy. The magnetic resonance spectrum of NPC:Pd consists of four signals for NPC which is produced from SiC. There are one ferromagnetic and three paramagnetic signals belonging to the carbon dangle sp³- and sp²-bonds kind and to the paramagnetic clusters of Pd atoms.     

Combined k-space q-space pulsed ESR imaging: mapping of restricted diffusion in (FA)(2)PF(6)

A (FA)(2)PF(6) crystal from the family of the quasi-one-dimensional organic conductors was selectively damaged by a beam of Helium ions with a slitted mask placed in the beam's trajectory. Pulsed ESR density weighted imaging of the damaged crystal revealed the appearance of regions where the ESR signal was absent. The one-dimensional motion of the charge carriers was thus restricted to the undamaged sections. The local charge carrier spin dynamics in these restricted areas was probed by combined k-space q-space pulsed ESR imaging. The local expected appearance of the restricted pulsed gradient spin echo (PGSE) "diffusive diffraction" effect is shown. The position of the diffraction minima is compatible with the density imaging results.     

Spatially separated spin carriers in three-ports graphene nanoribbons

Exporting the different spin signals to different ports is of practical importance to graphene-based spintronic devices. In this work, we have designed a three-ports graphene nanoribbon (GNR) device by inserting square-shaped carbon tetragon (CT) into GNR symmetrically, and calculated the magnetic moment distribution and transmission spectrum by using first-principles calculation and quantum transport simulation. Our results show that CT can bring non-equivalent path for two spin transport channels resulting in one spin is easier to transport than the other in each output port. Overall whole model, the spin states would be separated in real space but degenerated in energy. After correcting the device with asymmetric edge hydrogenation, we can achieve spatially separated spin carriers in real space and stable spin transporting. Our results suggest this model can serve as the most basic logic device for applying in future spintronics.     

储锂硅材料的ESR谱研究

采用了电化学方法制备储锂硅材料,并用电子自旋共振(ESR)方法进行研究. 实验结果表明, 储锂前的硅ESR行为符合居里自旋的ESR特征, ESR信号主要来源于硅材料中的晶格缺陷、 表面悬空键等局域化自旋中心. 储锂后硅材料中产生了泡利自旋,居里自旋的强度比储锂前增大2～3倍. 此外,对硅和储锂硅ESR谱线的g因子和ΔH_pp随温度的变化情况也进行了分析. 硅材料电化学储锂时,与锂离子中和的电子主要参与形成Li-Si共价键,对ESR信号贡献很小.     

以VO2+为自旋探针的配位共聚催化剂分子筛载体的ESR研究

以VO²⁺为自旋探针,观察了VO²⁺负载的分子筛载体的微环境,在计算机上模拟了复杂的多中心的实验ESR谱,发现VO²⁺在载体上的分散状况不仅取决于制备条件,也和分子筛的种类有关。利用这种方法,可得探针在载体上所处微环境的信息,进而可用于指导配位聚合催化剂载体的选择。     

ESR study of calix[4]arene by spin-trapping method

Electron spin resonance (ESR) measurements are introduced to investigate the interaction between 5,11,17,23-tetrabromo-25,26,27,28-tetramethoxycalix[4]arene (II ₄) and 2-methyl-2-nitrosopropane (MNP) or nitrosodurene (ND) which are used as spin-trapping reagents under ultraviolet irradiation. In most cases, the complicated ESR spectra were observed because of the overlap of ESR signals attributed to several spin adducts in different conformations or in an inclusion state. However, after the optimum amount of ND was added into theII ₄ solution, the ESR spectrum showed that each nitrogen hyperfine line was further split to 1:2:1 triplet lines, and this is a typical spectrum ascribed to the spin adduct of ND at one para position ofII ₄.     

Nature of tunneling electron spin resonance of an isolated surface spin

A phenomenological model has been proposed for tunneling electron spin resonance (ESR) of an isolated surface spin situated in a scanning tunneling microscope (STM), which explains the dependence of features (local maxima) of the tunneling current on the radio-frequency (RF) electric field and on the position of the tip with respect to the spin. A crossover of the line shape of the resonance signals, whose nature in weak and strong pumping fields corresponds to Lorentzian and Fano resonances, respectively, has been interpreted. New ESR–STM effects that are linear and nonlinear in the RF field and are promising for developing the methods of controlling spin qubits have been predicted.     

Low-Temperature Phase Transition Detected by ESR in ����-(BEDT-TTF)2IBr2 Single Crystals

Hysteresis of the electron spin resonance (ESR) spectra of localized charge carriers has been observed in ????-(BEDT-TTF)₂IBr₂ single crystals in the temperature range of 15?C30?K. The first-order phase transition has been detected by ESR components. Two lines observed in the ESR spectrum correspond to low-temperature and high-temperature phases.     

Simultaneous ESR and optical absorption measurements of K in solid Ar

An apparatus was developed for simulataneous electron spin resonance (ESR) and optical absorption experiments on atoms imbedded in rare gas matrices. In a first application the temperature dependence of ESR signals and of optical absorption spectra of K atoms in Ar matrices were studied. One of the numerous ESR lines could be unambiguously assigned to the so-called red triplet absorption due to their identical annealing behaviour at 12–14 K. The correspondence of other ESR lines to optical absorptions is discussed.     

连续波电子自旋共振的二维相关谱

在数学相关分析的基础上,建立了获得连续波电子自旋共振（continuous-wave electronspin resonance, CW-ESR）的二维相关谱的方法. 对光照产生的2-甲基-2-亚硝基-丙烷(2-methyl-2-nitrosopropane, MNP)的自捕捉自由基的一系列CW-ESR谱进行相关分析处理,首次得到了同步和异步的二维电子自旋共振（2D ESR）相关谱. 这种利用数学相关分析方法所得到的2D ESR相关谱将为扩大CW ESR的应用领域及获取更多的有用信息开拓新的途径.     

The determination of pair distance distributions by pulsed ESR using Tikhonov regularization

Pulsed ESR techniques with the aid of site-directed spin labeling have proven useful in providing unique structural information about proteins. The determination of distance distributions in electron spin pairs directly from the dipolar time evolution of the pulsed ESR signals by means of the Tikhonov regularization method is reported. The difficulties connected with numerically inverting this ill-posed mathematical problem are clearly illustrated. The Tikhonov regularization with the regularization parameter determined by the L-curve criterion is then described and tested to confirm its accuracy and reliability. The method is applied to recent experimental results on doubly labeled proteins that have been studied using two pulsed ESR techniques, double quantum coherence (DQC) ESR and double electron-electron resonance (DEER). The extracted distance distributions are able to provide valuable information about the conformational constraints in various partially folded states of proteins. This study supplies a mathematically reliable method for extracting pair distributions from pulsed ESR experimental data and has extended the use of pulsed ESR to provide results of greater value for structural biology.     

Spin-polarized transport and submillimetric microwave spectroscopy of solids

The problem of spin transport (spin transfer and localization in space by charge carriers) is considered from the standpoint of implementing this phenomenon in microelectronic devices based on novel physical principles. Experimental data are presented to confirm the possibility of creating extremely-high-frequency solid state microelectronic devices, operating in the millimetric and submillimetric wavelength range, which can be used as the main elements for spin informatics. These devices can be based on ferromagnetic semiconductor-nonmagnetic semiconductor junctions, the output parameters of which are controlled both by the transport current and by an external magnetic field.     

Two-Dimensional CW-ESR Correlation Spectroscopy

In the present paper, a new method called two-dimensional continuous-wave electron spin resonance (2-D CW-ESR) correlation spectroscopy is introduced. This method is based on mathematical analysis of ESR data obtained from common CW-ESR spectra. It is different from conventional 2-D ESR, which is built on multiple-pulse sequences. Synchronous and asynchronous 2-D CW-ESR correlation spectra defined by two independent magnetic field variable axes are generated by a correlation analysis of dynamic fluctuations of ESR signals induced by an external perturbation. The basic property of 2-D ESR correlation spectra is described. Several examples are investigated and discussed on applications of 2-D ESR correlation spectra in the spin self-trapping of 2-methyl-2-nitrosopropane (MNP), the mixture of spin adducts generated by self-trapping of MNP and 5,5-dimethyl-1-pyrroline N-oxide, and the coordination reaction of benzo-15-crown-5 with cupric ion in boron trifluoride-ether solution. The potential applications of this new 2-D CW-ESR method are discussed. Authors' address: Yong Li, Department of Chemistry, Tsinghua University, Beijing 100084, China     

Electron spin resonance determination of proton locations in a hydride

The low temperature electron spin resonance (ESR) spectrum of dilute Er in ScH_x powder contains distinct signals which are due to the presence of octahedral as well as tetrahedral protons in the vicinity of the Er ions. The occupation probability for the octahedral protons versus the bulk hydrogen-to-metal ratio is determined from the ESR signal intensities and compared with a lattice-gas calculation.     

Electron spin resonance study of a CuIr2S4 single crystal

A spinel sulphide CuIr₂S₄ single crystal, which exhibits an orbitally induced Peierls phase transition at ～230?K, is investigated by electron spin resonance (ESR) spectroscopy. The phase transition is clearly manifested on the ESR spectra. It is suggested that the ESR signals are produced by a few non-dimerized Ir⁴⁺ ions. Moreover, an extra ESR spectrum appears at low temperature in addition to the paramagnetic ESR signals of Ir⁴⁺ ions, which is suggested to be caused by the Jahn–Teller effect of the non-dimerized Ir⁴⁺ ions. From the ESR results, it is found that the Jahn–Teller splitting energy ΔE _JT is much smaller than the spin-dimerization gap.     

Electron spin resonance in Ge0.99Mn0.01 nanowires

Electron spin resonance (ESR) is measured in arrays of Ge_0.99Mn_0.01 nanowires (NWs). We show that the ESR spectrum comprises four lines corresponding to the different magnetic subsystems. Two lines are attributed to spin-wave excitations in the Mn⁺³ ion subsystem. The third component corresponds to the resonance in the Mn⁺² ions. The last component has an asymmetric Dyson form and characterizes the resonance in the mobile paramagnetic centers. We demonstrate that temperature dependences of ESR parameters are correlated in subsystems of the localized magnetic centers (Mn⁺³ and Mn⁺² ions) and delocalized band carriers. It implies the presence of exchange interactions between the localized Mn ions and spin-polarized carriers.     

The status quo and prospect of ESR imaging applications to study on catalysts

ESR imaging technique supplies us with the possibility of spatial imaging of spin species, and has been widely accepted as a useful tool in many fields. ESR imaging can be applied to observing the spatial distribution of spin species and determining the diffusion processes. These applications will certainly interest chemists in catalyst field. In this paper, the characteristics and limitations of ESR imaging technique, and the status quo and prospect, of ESR imaging applications to catalyst study are discussed. With a current ESR apparatus, the sensitivity and spatial resolution are good enough for the most applications in catalyst studies. Although few works in this field have been reported, its prospect will be interesting. We suggestin situ observation of spatial distribution of paramagnetic species (such as reactants, products and intermediates) in a catalyst system, determination of diffusion in a catalyst pellet and characterization of poisoning in micro fixed-bed reactors with ESR imaging method. Not only paramagnetic species could be studied directly, but also antimagnetic species, which quench spin signals, could be studied indirectly with this method.     

Electron spin resonance characterization of calcretes from Thar desert for dating applications

Results of a study to characterize electron spin resonance (ESR) signals from naturally occurring carbonates from Thar desert are reported. Three radiation induced signals at g=2.0071 (signal A, very weak), g=2.0012 (signal B) and g=2.0003 (signal C) with linewidths ranging from 0.5–2 G are seen. A broad signal (BL) with linewidth 7 G is also seen at 2.0038. Detailed ESR experiments involving (1) sensitivity of ESR signals to ionizing radiation, (2) intensity variation of ESR signals with microwave power at different temperatures, (3) analysis of acid insoluble residue to estimate the interference from ESR signals of silicate fraction, and (4) X-ray diffraction analysis (to study possible phase changes on heating), led to the choice of signal BL for dating. Experiments on acid insoluble residue revealed that signal C has a contribution from the E′₁ centre of quartz and signal B is the parallel component (g_∥) of the E′₁ centre. Paleodose estimates based on signal BL are stratigraphically consistent.     

Electron Spin Resonance Measurements of a Demagnetizing Field on the Surface of Metal Samples

By comparing the signals of electron spin resonance (ESR) from two crystals of a diamond (spin–labels) the demagnetizing field of the Co, Fe, and Ni samples in the shape of strongly elongated ellipsoids of revolution (disks) has been measured. The magnetic permeabilities of the metals in the external magnetic field corresponding to the ESR of the broken chemical bonds in a natural diamond irradiated with fast reactor neutrons have been determined.