Concentration Of Paramagnetic Impurities Research Articles

Thermal conductivity of type-Ib HPHT synthetic diamond irradiated with electrons

The effect of irradiation with 3 MeV electrons and subsequent high-temperature annealing on the thermal conductivity κT of synthetic HPHT diamond containing impurity nitrogen at a concentration of about 100 ppm was studied at temperatures from 5 to 410 K. At each stage of the experiment, the optical absorption and photoluminescence spectra, magnetization and thermal conductivity were measured on two plates from the same diamond single crystal. Color centers and their concentration in different locations in the samples were determined from the optical spectra. The concentration of paramagnetic impurities in the samples was determined from the temperature and field dependencies of the paramagnetic magnetization. Thermal conductivity is suppressed by two orders of magnitude at temperatures from 35 to 50 K after irradiation to a fluence of 5×1018 cm−2. Annealing at a temperature of 1800° C restored the thermal conductivity of diamond at moderate and high temperatures, but not at temperatures below 120 K. The measured data on thermal conductivity were analyzed within the framework of the phenomenological Callaway model in order to understand the role of various phonon scattering processes and how it changes during irradiation and annealing.

Determination of nitrogen spin concentration in diamond using double electron-electron resonance

Diamond has been extensively investigated recently due to a wide range of potential applications of nitrogen-vacancy (NV) defect centers existing in a diamond lattice. The applications include magnetometry and quantum information technologies, and long decoherence time $({T}_{2})$ of NV centers is critical for those applications. Although it has been known that ${T}_{2}$ highly depends on the concentration of paramagnetic impurities in diamond, precise measurement of the impurity concentration remains challenging. In the present work we show a method to determine a wide range of the nitrogen concentration $(n)$ in diamond using a wide-band high-frequency electron spin resonance and double electron-electron resonance spectrometer. Moreover, we investigate ${T}_{2}$ of the nitrogen impurities and show the relationship between ${T}_{2}$ and $n$. The method developed here is applicable for various spin systems in solid and implementable in nanoscale magnetic resonance spectroscopy with NV centers to characterize the concentration of the paramagnetic spins within a microscopic volume.

On the Paramagnetic Impurity Concentration of Silicate Glasses from Low-Temperature Physics

The concentration of paramagnetic trace impurities in glasses can be determined via precise SQUID measurements of the sample’s magnetization in a magnetic field. However, the existence of quasi-ordered structural inhomogeneities in the disordered solid causes correlated tunneling currents that can contribute to the magnetization, surprisingly, also at the higher temperatures. We show that taking into account such tunneling systems gives rise to a good agreement between the concentrations extracted from SQUID magnetization and those extracted from low-temperature heat capacity measurements. Without suitable inclusion of such magnetization contribution from the tunneling currents, we find that the concentration of paramagnetic impurities gets considerably over-estimated. This analysis represents a further positive test for the structural inhomogeneity theory of the magnetic effects in the cold glasses.

Time-dependent equation for the magnetic order parameter near the quantum critical point in multiband superconductors with a spin-density wave

Using a simple two-band model for Fe-based pnictides and the generalized Eilenberger equation, we present a microscopic derivation of a time-dependent equation for the amplitude of the spin density wave near the quantum critical point where it turns to zero. This equation describes the dynamics of the magnetic---$m$, as well as the superconducting order parameter---$\Delta$. It is valid at low temperatures $T$ and small $m$ (${T, m \ll \Delta}$) in a region of coexistence of both order parameters, $m$ and $\Delta$. The boundary of this region is found in the space of the nesting parameter $\{\mu_{0},\mu_{\phi}\}$ where $\mu_{0}$ describes the relative position of the electron and the hole pockets on the energy scale, and $\mu_{\phi}$ accounts for the ellipticity of the electron pocket. At low $T$ the number of quasiparticles is small due to the presence of the energy gap $\Delta$, and therefore the quasiparticles do not play a role in the relaxation of $m$. This circumstance allows one to derive the time-dependent equation for $m$ in contrast to the case of conventional superconductors for which the time-dependent Ginzburg--Landau equation can be derived near $T_{\text{c}}$ only in some special cases (high concentration of paramagnetic impurities. In the stationary case the derived equation is valid at arbitrary temperatures. We find a solution of the stationary equation which describes a domain wall in the magnetic structure. In the center of the domain wall the superconducting order parameter has a maximum, which means a local enhancement of superconductivity. Using the derived time-dependent equation for $m$, we investgate also the stability of a uniform commensurate SDW and obtain the values of $\{\mu_{0}, \mu_{\phi}\}$ at which the first order transition into the state with ${m = 0}$ takes place or the transition to the state with an inhomogeneous SDW occurs.

Nuclear magnetic relaxation in the quasi-two-dimensional (C2H5NH3)2Cd1−xMnxCl4 mixed crystal systems

We have studied the quasi-two-dimensional layered compound systems (C2H5NH3)2Cd1−xMnxCl4 (0≤x≤1) by means of 1H nuclear magnetic resonance (NMR) measurements. In contrast to the case of the unmixed crystal compounds (x=0, 1) following a single-exponential type of the NMR spin–lattice relaxation, a stretched-exponential type of recovery, M(t)=M0[1−exp(−t/T1S)n], was necessary for the stoichiometric composition systems in addition to it, the exponent n depending on the randomly distributed paramagnetic Mn impurity concentration x. The spin–lattice relaxation rate T1S−1 thus obtained showed a maximum at a percolation threshold xc≈0.3, in agreement with a percolation theory of the paramagnetic impurity relaxation. The stacking dimensions of the paramagnetic ions were derived from the exponent n as a function of x.

Scalable Fabrication of High Purity Diamond Nanocrystals with Long-Spin-Coherence Nitrogen Vacancy Centers

The combination of long spin coherence time and nanoscale size has made nitrogen vacancy (NV) centers in nanodiamonds the subject of much interest for quantum information and sensing applications. However, currently available high-pressure high-temperature (HPHT) nanodiamonds have a high concentration of paramagnetic impurities that limit their spin coherence time to the order of microseconds, less than 1% of that observed in bulk diamond. In this work, we use a porous metal mask and a reactive ion etching process to fabricate nanocrystals from high-purity chemical vapor deposition (CVD) diamond. We show that NV centers in these CVD nanodiamonds exhibit record-long spin coherence times in excess of 200 μs, enabling magnetic field sensitivities of 290 nT Hz(-1/2) with the spatial resolution characteristic of a 50 nm diameter probe.

On the interpretation of 1H 2-dimensional NMR relaxation exchange spectra in cements: Is there exchange between pores with two characteristic sizes or Fe 3+ concentrations?

We discuss the effect of paramagnetic impurity content (thought to be predominantly Fe 3+) in cement pastes on the interpretation of 1H NMR T 2– T 2 exchange spectra. Through measurements on synthesised C–S–H with greatly reduced paramagnetic impurity concentration, we show that the spectra cannot be explained by exchange between regions of different Fe 3+ concentration but rather are explained by exchange between regions of different pore size.

Ultralong spin coherence time in isotopically engineered diamond

As quantum mechanics ventures into the world of applications and engineering, materials science faces the necessity to design matter to quantum grade purity. For such materials, quantum effects define their physical behaviour and open completely new (quantum) perspectives for applications. Carbon-based materials are particularly good examples, highlighted by the fascinating quantum properties of, for example, nanotubes or graphene. Here, we demonstrate the synthesis and application of ultrapure isotopically controlled single-crystal chemical vapour deposition (CVD) diamond with a remarkably low concentration of paramagnetic impurities. The content of nuclear spins associated with the (13)C isotope was depleted to 0.3% and the concentration of other paramagnetic defects was measured to be <10(13) cm(-3). Being placed in such a spin-free lattice, single electron spins show the longest room-temperature spin dephasing times ever observed in solid-state systems (T2=1.8 ms). This benchmark will potentially allow observation of coherent coupling between spins separated by a few tens of nanometres, making it a versatile material for room-temperature quantum information processing devices. We also show that single electron spins in the same isotopically engineered CVD diamond can be used to detect external magnetic fields with a sensitivity reaching 4 nT Hz(-1/2) and subnanometre spatial resolution.

29Si cross-polarization magic-angle spinning NMR spectroscopy––an efficient tool for quantification of thaumasite in cement-based materials

Abstract 29Si{1H} cross-polarization (CP) magic-angle spinning (MAS) NMR spectroscopy is a powerful and reliable tool for the quantification of thaumasite in cement-based materials. The most efficient method for quantifying thaumasite from 29Si{1H} CP/MAS NMR spectra is described and it is shown that the method allows detection of thaumasite contents below approximately 10 wt.% with a relative precision of 15% and contents above 10 wt.% with a relative precision of 10%. The applicability of 29Si{1H} CP/MAS NMR for quantification of thaumasite is demonstrated for different Portland cement pastes and shows that thaumasite contents as low as 0.2–0.5 wt.% can be detected in cementitious systems with low concentrations of paramagnetic impurities. For a Portland cement containing various amounts of limestone dust and stored at 5 °C in a MgSO4 solution, large quantities of thaumasite have been detected. Furthermore, the quantity of thaumasite is found to be less sensitive to the amount of added limestone dust. For samples of a Portland cement with a fixed content of limestone dust but different quantities of added gypsum, the increased contents of gypsum are observed to result in larger quantities of thaumasite after prolonged hydration.

Electron Dephasing in Metallic Narrow Wires at Low Temperatures

The low temperature behavior of the electron phase coherence time in metals, τ φ , is currently a subject of controversy. While theory predicts that τ φ should increase on cooling as an inverse power law of temperature, many samples exhibit a saturation of τ φ below a temperature between 0.1 and 1 K. In this paper we present evidence that the saturation of τ φ often observed in weakly-disordered polycrystalline metals (Au, Ag, and Cu) is most likely due to a dilute concentration of paramagnetic impurities, either on the surface or in the bulk.

Spin‐lattice relaxation of spin‐½ nuclei in solids containing diluted paramagnetic impurity centers. III. Dynamic polarization of nuclear spin system by nuclear orientation via electron spin locking

AbstractDynamic nuclear polarization of nuclei by means of paramagnetic electron spin locking (Hartmann‐Hahn cross‐polarization between paramagnetic electrons and nuclei, or NOVEL) is discussed. The theory is demonstrated by experiments executed at 2.4 and 9.6 GHz on a natural type Ib diamond. It is shown that the 13C polarization rate is independent of the microwave frequency, in agreement with theory. NOVEL polarization takes place only while the spin‐locking pulse is on. The rate at which the nuclei are polarized is proportional to the electron polarization in the rotating frame. Therefore, the length of the spin‐locking pulse is limited by the value of T1ρ(e), and because T1ρ(e) ≪ T1(e) for diamond the effective NOVEL polarization rate of 13C nuclei is usually relatively low. A comparison between the relative effectiveness of 13C polarization rates between NOVEL and the solid‐state effect is made for high and low paramagnetic impurity concentrations. The dependence of the 13C polarization rate on the paramagnetic impurity concentration has been determined for a suite of natural diamonds. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson 19A: 44–49, 2003.

Spin‐lattice relaxation of spin‐½ nuclei in solids containing diluted paramagnetic impurity centers. II. Dynamic polarization of nuclear spin system by thermal mixing and solid‐state effect

AbstractDynamic nuclear polarization of nuclear spins via the solid‐state and thermal mixing effects is discussed. Continuous‐wave S‐ and X‐band microwave radiation have been employed to measure 13C signal enhancements and polarization times for 13C nuclei in a natural type Ib diamond as a function of magnetic field. It was found that thermal mixing plays an important role in the 13C signal enhancement because the central electron spin resonance (ESR) line width HL ≃ H0γC/γe, resulting in flip‐flip and flip‐flop forbidden transitions taking place simultaneously. On the other hand, the 13C spin‐lattice relaxation rate is determined to a large extent by the solid‐state effect (forbidden transitions). 13C polarization rates have also been measured for a suite of natural diamonds. It is shown that the polarization rate is proportional to the paramagnetic impurity concentration, in agreement with the theory. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson 19A: 36–43, 2003.

Model-independent measurements of hydrogen diffusivity in the yttrium dihydrides

The hydrogen diffusivities D in yttrium dihydrides YH x have been measured by means of pulsed-field-gradient nuclear magnetic resonance (PFG-NMR) up to 920 K for x=1.91 and up to 620 K for x=1.95 and 2.03. The observed D values are generally significantly greater than those in the dihydrides of titanium and zirconium. The activation enthalpy H a, obtained by fitting an Arrhenius expression to the diffusivities, decreases substantially with increasing x from H a=0.53 eV ( x=1.91) to H a=0.38 eV ( x=2.03). Measurements of the proton spin-lattice relaxation Γ 1 have been performed on the same samples. The temperature and concentration dependences of the dipolar relaxation Γ 1d are consistent with the PFG results. Besides Γ 1d and the electronic relaxation Γ 1e, the Γ 1 data reveal contributions due to very small concentrations of paramagnetic impurities Γ 1p, which dominate the measured rates below about 500 K, even at impurity levels as low as 20 ppm or less.

13C Spin–Lattice Relaxation in Natural Diamond: Zeeman Relaxation at 4.7 T and 300 K Due to Fixed Paramagnetic Nitrogen Defects

13C spin–lattice relaxation times in the laboratory frame, ranging from 1.4 to 36 h, have been measured on a suite of five natural type Ia and Ib diamonds at 4.7 T and 300 K. Each of the diamonds contains two types of fixed paramagnetic centers with overlapping inhomogeneous electron paramagnetic resonance (EPR) lines. EPR techniques have been employed to identify these defects and to determine their concentrations and relaxation times at X-band. Spin–lattice relaxation behavior of 13C in diamonds containing paramagnetic P1, P2, N2, and N3 centers are discussed. Depending on the paramagnetic impurity types and concentrations present in each diamond, three different nuclear spin–lattice relaxation (SLR) paths exist, namely that due to electron SLR mechanisms and two types of three-spin processes (TSPs). The one three-spin process (TSP1) involves a simultaneous transition of two electron spins belonging to the same hyperfine EPR line and a flip of a 13C spin, while the other process (TSP2) involves two electron spins belonging to different hyperfine EPR lines and a 13C spin. It is shown that the thermal contact between the 13C nuclear Zeeman and electron dipole–dipole interaction reservoirs is field dependent, thus forming a bottleneck in the 13C relaxation path due to TSP1 at high magnetic fields.

Dynamic nuclear polarization of diamond. III. Paramagnetic electron relaxation times from enhanced C13 nuclear magnetic resonance signals

It is shown that by varying pulse lengths and delay times in electron spin resonance microwave pulse sequences, designed to enhance C13 nuclear magnetic resonance signals in diamond, and measuring the resulting C13 nuclear magnetic resonance signal, the paramagnetic impurity concentration, electron spin–spin and spin-lattice relaxation times in the laboratory and rotating frames, as well as the electron-nuclear interaction time, can be obtained. It is also shown that the maximum C13 polarization at a specific microwave frequency is dependent on the electron spin-lattice relaxation times in the laboratory and rotating frames.

Dynamic nuclear polarization of diamond. I. Solid state and thermal mixing effects

The dynamic nuclear polarization of 13C nuclei in a suite of seven natural type Ia and Ib diamonds, using continuous wave S- and X-band microwave radiation, is described. The 13C signal enhancement and polarization time have been measured for one of the type Ib diamonds as a function of magnetic field in the vicinity of the resonance field. The total paramagnetic impurity concentration (P1 and other centers) in this diamond is 2×1018 cm−3 (23 atomic parts per million), while the concentration of P1 centers is 9.3 ppm. Since the central electron spin resonance (ESR) linewidth HL is comparable with H0γC/γe, flip–flip and flip–flop forbidden transitions take place simultaneously. Consequently thermal mixing plays an important role in the 13C signal enhancement. However, the 13C spin-lattice relaxation rate is determined to a large extent by the solid state effect (forbidden transitions). The 13C polarization rates have been measured for the suite of diamonds by executing dynamic nuclear polarization (DNP) experiments on both hyperfine and central ESR lines. It is shown that the polarization rate is proportional to the paramagnetic impurity concentration of the sample, in agreement with the existing theory. It has been found that in type Ib diamonds with relatively low nitrogen impurity concentrations the dynamic nuclear polarization of a single hyperfine line yields an equilibrium 13C polarization that is one-quarter of that obtained in the case of dynamic nuclear polarization of the central line. In samples containing P1 and P2 centers (type Ia), or in type Ib samples with relatively high concentrations of P1 centers, the same equilibrium 13C polarization is obtained for the DNP of hyperfine and central transitions. This phenomenon is explained in terms of a model in which thermal contact is established between the electron Zeeman reservoir and the nuclear spin reservoirs via the spin–spin interaction reservoir if HL⩾H0γC/γe.

The dependences of ESR line widths and spin - spin relaxation times of single nitrogen defects on the concentration of nitrogen defects in diamond

Line widths and spin - spin relaxation times of P1 centres in synthetic Ib and natural Ia diamonds with concentrations of P1 and P2 centres covering the range 0.03 - 400 atomic parts per million have been measured. At concentrations higher than about ten atomic parts per million the line width is linearly dependent on the concentration. At lower concentrations the electron - dipolar contribution to the line width dominates and the width of the line remains constant. Since the pulse sequence employed for measurements eliminates the effects of inhomogeneous line broadening, of the line remains linearly dependent on the total paramagnetic impurity concentration, even at very low paramagnetic impurity concentrations.

Ultraviolet Interstellar Linear Polarization. III. Features

Astro-2 has revealed a broad, weak spectral feature in the ultraviolet interstellar linear polarization for two lines of sight, confirming the original detection toward HD 197770 and adding HD 147933-4. These are the only two polarization features found in some 30 lines of sight now observed. Both features are centered close to 2175 A, the position of the ubiquitous ultraviolet extinction bump. Two possibilities are considered for the source of the polarization feature: changes in the mass distribution of the aligned silicate grains responsible for the continuum polarization, and alignment of the small graphite grains responsible for the extinction bump. While the former approach meets with some success for the HD 197770 feature, it is not possible to produce a feature as clearly peaked as in HD 147933-4. The central wavenumbers and widths of both polarization features correspond closely to those of the corresponding extinction bump. Taken together, it seems the graphite grains are a more likely source of the polarization features. Both polarization features have amplitudes that are very small compared to the amount of excess extinction present in the 2175 A bump, implying poor polarization efficiency. Many other lines of sight have been observed with sufficient signal-to-noise ratios such that features should have been clearly detected if the same polarization efficiency applied, and so real variations in the alignment or shape of the grains responsible seem to occur from one line of sight to another. The weak alignment might be caused by unusually low concentrations of paramagnetic impurities in rapidly spinning small grains.

Nuclear spin-lattice relaxation mechanisms in kaolinite confirmed by magic-angle spinning

Spin-lattice relaxation mechanisms in kaolinite have been reinvestigated by magic-angle spinning (MAS) of the sample. MAS is useful to distinguish between relaxation mechanisms: the direct relaxation rate caused by the dipole-dipole interaction with electron spins is not affected by spinning while the spin diffusion-assisted relaxation rate is. Spin diffusion plays a dominant role in 1H relaxation. MAS causes only a slight change in the relaxation behavior, because the dipolar coupling between 1H spins is strong. 29Si relaxes directly through the dipole-dipole interaction with electron spins under spinning conditions higher than 2 kHz. A spin diffusion effect has been clearly observed in the 29Si relaxation of relatively pure samples under static and slow-spinning conditions. 27Al relaxes through three mechanisms: phonon-coupled quadrupole interaction, spin diffusion and dipole-dipole interaction with electron spins. The first mechanism is dominant, while the last is negligibly small. Spin diffusion between 27Al spins is suppressed completely at a spinning rate of 2.5 kHz. We have analyzed the relaxation behavior theoretically and discussed quantitatively. Concentrations of paramagnetic impurities, electron spin-lattice relaxation times and spin diffusion rates have been estimated.

Ultra-stable sapphire resonator-oscillator

Preliminary measurements of the properties of cryogenic sapphire resonators with various concentrations of paramagnetic impurities are reported. These resonators are mounted in both copper and niobium shields. It is found that one can achieve extrema in the frequency-temperature function in the temperature range 5-18 K due to the compensating effects of the dielectric constant temperature dependence and the Curie law dependence on the susceptibility of the paramagnetic impurities. In samples where dominant paramagnetic impurity has a high ESR (electron spin resonance) unloaded resonator Q factors as high as 4*10/sup 9/ are observed. The increased Q factors coupled with reduced shield interactions should result in an improvement in frequency stability for an oscillator based around this resonator as compared to the best results achieved to date as the authors' laboratory (an Allan variance of 9*10/sup -15/). The preliminary measurement of the tilt sensitivity of the oscillator is reported to be a fractional frequency shift of 6.5*10/sup -12/ per degree. >