Echo Decay Research Articles

Comparison of Effects of Partial Discharge Echo in Various High-Voltage Insulation Systems

In this article, an extension of a conventional partial discharge (PD) approach called partial discharge echo (PDE), which is applied to different classes of electrical insulation systems of power devices, is presented. Currently, high-voltage (HV) electrical insulation is attributed not only to transmission and distribution grids but also to the industrial environment and emerging segments such as transportation electrification, i.e., electric vehicles, more-electric aircraft, and propulsion in maritime vehicles. This novel PDE methodology extends the conventional and established PD-based assessment, which is perceived to be one of the crucial indicators of HV electrical insulation integrity. PD echo may provide additional insight into the surface conditions and charge transport phenomena in a non-invasive way. It offers new diagnostic attributes that expand the evaluation of insulation conditions that are not possible by conventional PD measurements. The effects of partial discharge echo in various segments of insulation systems (such as cross-linked polyethylene [XLPE] power cable sections that contain defects and a twisted-pair helical coil that represents motor-winding insulation) are shown in this paper. The aim is to demonstrate the echo response on representative electrical insulating materials; for example, polyethylene, insulating paper, and Nomex. Comparisons of the PD echo decay times among various insulation systems are depicted, reflecting dielectric surface phenomena. The presented approach offers extended quantitative assessments of the conditions of HV electrical insulation, including its detection, measurement methodology, and interpretation.

Decoherence induced by a sparse bath of two-level fluctuators: Peculiar features of 1/f noise in high-quality qubits

Progress in fabrication of semiconductor and superconductor qubits has greatly diminished the number of decohering defects, thus decreasing the devastating low-frequency 1/f noise and extending the qubits' coherence times (dephasing time T2* and the echo decay time T2). However, large qubit-to-qubit variation of the coherence properties remains a problem, making it difficult to produce a large-scale register where all qubits have a uniformly high quality. In this work, we show that large variability is a characteristic feature of a qubit dephased by a sparse bath made of many (n≫1) decohering defects, coupled to the qubit with similar strength. We model the defects as two-level fluctuators (TLFs) whose transition rates γ are sampled from a log-uniform distribution over an interval [γm,γM], which is a standard model for 1/f noise. We investigate decoherence by such a bath in the limit of high-quality qubit, i.e., when the TLF density d is small (the limit of sparse bath, with d=n/w≪1, where n is the number of TLFs and w=ln[γM/γm] is the log-width of the distribution). We show that different realizations of the bath produce very similar noise power spectra S(f)∼1/f, but lead to drastically different coherence times T2* and T2. Thus the spectral density S(f) does not determine coherence of a qubit coupled to a sparse TLF bath, as opposed to a dense bath; instead, decoherence is controlled by only a few exceptional fluctuators, determined by their value of γ. We show that removing only two of these TLFs greatly increases T2 and T2* times. Our findings help theoretical understanding and further improvements in the coherence properties of semiconductor and superconductor qubits, battling the 1/f noise in these platforms. Published by the American Physical Society 2024

Quantitative determination of the carbonyl value in frying oils based on LF-NMR combined with chemometrics

Low-field nuclear magnetic resonance (LF-NMR) combined with chemometrics was used for the rapidly and nondestructively quantitative determination of carbonyl value (CV) of frying oils during frying process. The quality indices of frying oils during frying process were analyzed as well as the changes of its LF-NMR signal, followed by exploring the correlation between the quality indices of frying oils and the relaxation-characteristic indices and investigating the influences of modeling data and preprocessing methods on the modeling performance. Based on the echo decay curves data of LF-NMR and the relaxation-characteristic indices data, the prediction model of CV was established by partial least square (PLS), principal component regression (PCR), multiple linear regression (MLR), and support vector machine regression (SVR). Results showed that there was a significant correlation between CV and relaxation-characteristic indices (p < 0.01) with both modeling data and preprocessing methods affecting the model performance. The prediction performance of non-linear SVR model was significantly superior to that of linear modeling methods (PLS, PCR, and MLR). The best prediction model of CV was built using the SVR method based on echo decay curves data. The root mean square error of prediction and coefficient of determination were 2.599 mmol/kg and 0.9873, respectively.

Study of the collagen tissue nanostructure by analyzing the echo decay obtained using the MRI technique.

The multicomponent relaxation observed in nuclear magnetic resonance experiments in biological tissues makes it difficult to establish a correlation between specific relaxation times and tissue structural parameters. The analysis of a nanostructure (the characteristic size of 10-1000 nm) is usually based on formulas for relaxation times which depend on structural parameters at the atomic or molecular levels in the size range of 0.1-5 nm. We have recently developed an analysis method in which relaxation times' anisotropy in a sample is explicitly related to its structure of nanocavities containing a liquid or gas. However, the method is based on the analysis of experimental data on the anisotropy of relaxation times obtained by using the standard NMR technique and rotating the sample relative to a magnetic field and requires a series of experiments. In the present study, to address this challenge, we develop a new method of analysis of a multi-exponential magnetic resonance signal that does not require determining relaxation times and eliminates the sample rotation and the necessity of a series of experiments. Using the magnetic resonance imaging (MRI) technique, the total signal from the whole sample was obtained as a sum of the signals (echo decays) from all voxels. In contrast to previous research, the volumes of nanocavities and their angular distribution can be obtained by analyzing a single total signal for the entire cartilage. In addition, within the framework of this approach, it is possible to identify the reason for the multicomponent nature of relaxation. The proposed method for analyzing a single multi-exponential signal (transverse relaxation) was implemented on cartilage. Using the signal, three anatomical zones of cartilage were studied, revealing significant structural differences between them. The proposed method not only avoids the need for sample rotation but also enables repeated application of layer-by-layer magnetic resonance imaging with micron resolution. The study results allow us to suggest that water molecules contributing to the echo decay are more likely located in nanocavities formed by the fibrillar structure rather than inside the fibrils.

Exploring tunneling ESEEM beyond methyl groups in nitroxides at low temperatures.

Tunneling of methyl rotors coupled to an electron spin causes magnetic field independent electron spin echo envelope modulation (ESEEM) at low temperatures. For nitroxides containing alkyl substituents, we observe this effect as a contribution at the beginning of the Hahn echo decay signal occurring on a faster time scale than the matrix-induced decoherence. The tunneling ESEEM contribution includes information on the local environment of the methyl rotors, which manifests as a distribution of rotation barriers P(V3) when measuring the paramagnetic species in a glassy matrix. Here, we investigate the differences in tunneling behaviour of geminal methyl and ethyl group rotors in nitroxides while exploring different levels of theory in our previously introduced methyl quantum rotor (MQR) model. Moreover, we extend the MQR model to analyze the tunneling ESEEM originating from two different rotor types coupled to the same electron spin. We find that ethyl groups in nitroxides give rise to stronger tunneling ESEEM contributions than methyl groups because the difference between hyperfine couplings of their methyl protons better matches the tunneling frequency. The methyl rotors of both ethyl and propyl groups exhibit distributions at lower rotation barriers compared to geminal methyl groups. This is in good agreement with density functional theory (DFT) calculations of their rotation barriers and showcases that conformational flexibility impacts the hindrance of rotation. Using Monte-Carlo based fitting in combination with an identifiability analysis of the MQR model parameter space, we extract rotation barrier distributions for the individual rotor types in mixed-rotor nitroxides as well as identify which rotors dominate the observed tunneling contribution in the Hahn echo decay signal.

Coherences of Photoinduced Electron Spin Qubit Pair States in Photosystem I.

This publication presents the first comprehensive experimental study of electron spin coherences in photosynthetic reaction center proteins, specifically focusing on photosystem I (PSI). The ultrafast electron transfer in PSI generates spin-correlated radical pairs (SCRPs), which are entangled spin pairs formed in well-defined spin states (Bell states). Since their discovery in our group in the 1980s, SCRPs have been extensively used to enhance our understanding of structure-function relationships in photosynthetic proteins. More recently, SCRPs have been utilized as tools for quantum sensing. Electron spin decoherence poses a significant challenge in realizing practical applications of electron spin qubits, particularly the creation of quantum entanglement between multiple electron spins. This work is focused on the systematic characterization of decoherence in SCRPs of PSI. These decoherence times were measured as electron spin echo decay times, termed phase memory times (TM), at various temperatures. Decoherence was recorded on both transient SCRP states P700+A1- and thermalized states. Our study reveals that TM exhibits minimal dependence on the biological species, biochemical treatment, and paramagnetic species. The analysis indicates that nuclear spin diffusion and instantaneous diffusion mechanisms alone cannot explain the observed decoherence. As a plausible explanation we discuss the assumption that the low-temperature dynamics of methyl groups in the protein surrounding the unpaired electron spin centers is the main factor governing the loss of the spin coherence in PSI.

Long-Lived Exciton Coherence in Mixed-Halide Perovskite Crystals.

Compositional engineering of the optical properties of hybrid organic-inorganic lead halide perovskites is crucial for the realization of efficient solar cells and light-emitting devices. We study the effect of band gap fluctuations on coherent exciton dynamics in a mixed FA0.9Cs0.1PbI2.8Br0.2 perovskite crystal by using photon echo spectroscopy. We reveal a narrow homogeneous exciton line width of 16 μeV at a temperature of 1.5 K. The corresponding exciton coherence time T2 = 83 ps is exceptionally long due to the localization of excitons at the scale of tens to hundreds of nanometers. From spectral and temperature dependences of the two- and three-pulse photon echo decay, we conclude that for low-energy excitons pure decoherence associated with elastic scattering on phonons is comparable with the exciton lifetime, while for excitons with higher energies, inelastic scattering to lower energy states via phonon emission dominates.

Structural organization of phase-separated bioactive glasses and the clustering of Si, P, B, Na and F atoms investigated by solid-state NMR and Monte Carlo simulations

In this paper we studied the effect of NaBO2 addition to a phase-separated alkali-free bioactive glass with a composition of 38.49 SiO2 • 36.07 CaO • 19.24 MgO • 5.61 P2O5 • 0.59 CaF2. Microscopy reveals binodal phase separation involving two Si-containing microphases with a droplet size of ∼200 μm, driven by the thermodynamic LLPS mechanism. The local environments and spatial distribution of silicate, phosphate, and fluoride ions in this phase-separated system were studied, using 29Si, 31P, 11B, 19F, 25Mg, and 23Na nuclear magnetic resonance (NMR) and infrared spectroscopy. The silicate units are dominantly of the metasilicate (Si2) type. The phosphate units exist mostly as orthophosphate (P0) while the borate is present in the form of pyroborate (B1). Multinuclear dipolar re-coupling experiments indicate that the minority components F, P, B and Na all occur within a common phase. Thus, atomic distribution scenarios involving the separation of these components into separate phases can be excluded. The 31P spin echo decay (SED) method was used along with Monte Carlo simulations to characterize the spatial distribution of the phosphate component. Based on the analysis, the phosphate component forms clusters of sizes 1-4 nm, which are embedded in an environment more dilute in phosphate, having a random distribution. While 19F SED results indicate that the fluoride ions do not form clusters and are close to randomly distributed, dipolar recoupling of 31P suggests a local environment resembling that of fluorapatite.

Structural impact of niobium oxide on lithium silicate glasses: Results from advanced interaction-selective solid-state nuclear magnetic resonance and Raman spectroscopy

While niobium-containing oxide glasses are used in many technological applications, especially optical glasses, the exact structural role of Nb and its impact on the structure of covalent/ionic oxide glasses remain incompletely understood. In the present study, this issue is addressed for Nb2O5-containing lithium silicate glasses using 7Li, 29Si, and 93Nb magic-angle-spinning (MAS) and advanced NMR methods sensitive to 7Li–7Li and 7Li–93Nb magnetic dipole-dipole interactions, accompanied by molecular dynamics (MD) and Monte-Carlo (MC) simulations. In glasses along the compositional line (1–y)Li2Si2O5–yNb2O5, 29Si MAS-NMR spectra reveal a gradual silica network repolymerization with increasing y, indicating that the accommodation of the niobium oxide component into the glassy network requires on average about 0.7 equivalents of lithium oxide. Significant 7Li–93Nb interactions were also detected and quantified by Rotational Echo Saturation Pulse Double Resonance (RESPDOR) experiments. Consistent with this finding, profound changes in the local environments of the lithium ions and their spatial distributions are observed in 7Li satellite transition (SATRAS) spectra and 7Li dipolar spin echo decays, whereas 93Nb NMR and Raman spectra suggest the formation of six-coordinated NbO6 polyhedra with both, corner- and edge-sharing connectivity. Results obtained on glasses along a second composition line, SiO2–[(Li2O)1.4(Nb2O5)], confirm the above-described structural concepts. The average homo- and heteronuclear dipolar interaction strengths as expressed by the second moments M2(Li-Li) and M2(Li-Nb) are consistent with random spatial distributions of the lithium and niobium species, with a tendency towards Nb clustering at higher concentrations. All of these results suggest that niobium oxide adopts a network former role in these glasses. The current study defines a comprehensive general strategy for elucidating the structural impact of NMR active intermediate oxides incorporated into silicate glasses.

A comparison of pulse and CW EPR [formula omitted]-relaxation measurements of an inhomogeneously broadened nitroxide spin probe undergoing Heisenberg spin exchange

Nitroxide spin probes are inhomogeneously broadened (IHB) by intramolecular hyperfine interactions with protons (deuterons) producing lines of Voigt shape. Thus, to study T2 relaxation by continuous wave (CW) EPR, the Voigt must be deconvoluted to find the Lorentzian component. For homogeneously broadened lines, T2 is obtained directly from the Lorentzian line widths ΔHppL; however, for IHB lines finding T2 from ΔHppL is more complicated. It has been known for many years that values of ΔHppL of high precision may be obtained from IHB lines; however, direct, accurate comparison of spin exchange frequencies obtained from electron spin echo decay and CW EPR data has been lacking. It is demonstrated here that despite complications in the interpretation of experiments, these two techniques yield the same spin exchange rate constant for spin probes that are the most difficult to treat.

&lt;sup&gt;7&lt;/sup&gt;Li Diffusion in Thin Disks of Single-Crystal Garnet LLZO-Ta Studied by PFG-NMR Spectroscopy

Pulsed field-gradient (PFG) NMR spectroscopy was applied to thin disk samples (0.5, 1 and 2 mm thick, 4 mm diameter) of LLZO-Ta (Li&lt;sub&gt;6&lt;/sub&gt;La&lt;sub&gt;3&lt;/sub&gt;Zr&lt;sub&gt;1.5&lt;/sub&gt;Ta&lt;sub&gt;0.5&lt;/sub&gt;O&lt;sub&gt;12&lt;/sub&gt;) single crystal. We have measured the diffusion of &lt;sup&gt;7&lt;/sup&gt;Li in thin pellets (0.5 to 3 mm thick) of NASICON-type LAGP powder and found that the Li&lt;sup&gt;+&lt;/sup&gt; diffuses preferentially in the thicker direction. In this study, we placed 1 mm thick, 4 mm diameter disk-shaped single crystals vertically, i.e., parallel to the PFG direction. We found that &lt;sup&gt;7&lt;/sup&gt;Li ions diffuse similarly as in rod-shaped single crystals. The &lt;sup&gt;7&lt;/sup&gt;Li diffusion is slow (diffusion coefficient; ~2 × 10&lt;sup&gt;-13&lt;/sup&gt; m&lt;sup&gt;2&lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;) at Δ ≥ 100 milliseconds and gradually becomes faster to ~1 × 10&lt;sup&gt;-11&lt;/sup&gt; m&lt;sup&gt;2&lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt; at Δ ≤ 10 milliseconds at 28°C. A diffraction pattern was observed in the echo decay plots, which was not observed in well-prepared single crystal rod samples. The diffraction patterns are often observed in powder inorganic electrolytes (LLZOs, LLTO, LAGP, and (Li&lt;sub&gt;2&lt;/sub&gt;S)&lt;sub&gt;x&lt;/sub&gt;(P&lt;sub&gt;2&lt;/sub&gt;S&lt;sub&gt;5&lt;/sub&gt;)&lt;sub&gt;y&lt;/sub&gt;). In this study, edge effects are assumed for the diffraction pattern of the vertically placed thin disk of the single crystal. For horizontally placed disks, at long observation times (Δ ≥ 100 milliseconds), the diffusion coefficients of &lt;sup&gt;7&lt;/sup&gt;Li agreed with those of the rod-shaped samples. However, as Δ became shorter, contrary to the vertically placed disk, the diffusion of &lt;sup&gt;7&lt;/sup&gt;Li decreased and the plots became less linear. Surprisingly, as Δ became shorter (Δ ≤ 15 milliseconds), sinusoidal patterns were observed in the real and imaginary elements of the echo attenuation plots,. The pattern is reproducible and the first example of sinusoidal real and imaginary components of the echo attenuation plot was observed, but an adequate explanation has not been obtained.

Features of Self-Diffusion of Tridecane Molecules in a Porous Medium of Kaolinite Used as a Model of a Chemically Inert Membrane

The present work focused on the experimental study of the specific features of self-diffusion of tridecane molecules in macroporous kaolinite, which is used as a raw material for the production of chemically inert membranes. The measurements of self-diffusion coefficients by pulsed magnetic field gradient nuclear magnetic resonance (PMFG NMR) revealed an increased translational mobility of tridecane molecules in kaolinite with incomplete filling of the pore space. This effect was accompanied by a sharp change in the slope of the Arrhenius plot of the self-diffusion coefficients of tridecane molecules in kaolinite. An analysis of the diffusion spin echo decay in the tridecane-kaolinite system revealed a discrepancy between the experimental data and the theoretical predictions, considering the effect of the geometry of porous space on molecular mobility. It was shown that the experimental results could be interpreted in terms of a model of two phases of tridecane molecules in the pores of kaolinite, in the gaseous and adsorbed state, coexisting under the fast-exchange conditions. Within the framework of the model, the activation energies of self-diffusion were calculated, which agreed satisfactorily with the experimental data. Additionally, the effects of the internal magnetic field gradients arising in a porous medium loaded with a gas or liquid on the data of the PFG NMR measurements were calculated. It was shown that the effect of magnetic field inhomogeneities on the measured self-diffusion coefficients of tridecane in kaolinite is small and could be neglected.

Nuclear pair electron spin echo envelope modulation

The interaction of electron spins with homonuclear spin pairs in their vicinity is one of the dominating mechanisms of electron spin echo decay at low temperature and low concentration. Here, we study this mechanism using established concepts of electron spin echo envelope modulation (ESEEM). We obtain analytical expressions for the Hahn echo, the refocused echo, the stimulated echo, and Carr–Purcell pulse trains with small numbers of π pulses. Hahn echo decay is well approximated by the product of nuclear pair ESEEM functions. The same approximation can explain dependence of stimulated echo decay on the first interpulse delay and provides reasonable time scale estimates for decay of Carr–Purcell echos after an odd number of π pulses. Carr–Purcell echoes after an even number of π pulses are rather sensitive to correlations within larger nuclear spin clusters. Approximations improve for both odd and even numbers of π pulses by factorising the nuclear spin bath into disjoint clusters, provided that modulation due to pairs of spins belonging to different clusters is considered in addition to cluster modulation. The analytical ESEEM expressions for the Hahn echo and the Carr–Purcell echo after two π pulses have the same mathematical form as the filter functions of these sequences of spin noise spectroscopy. This coincidence provides a computationally very efficient way of predicting Hahn echo decay induced by homonuclear spin pairs. The analytical pair product approximation predicts the previously observed (Bahrenberg et al., 2021) increase of the refocused echo amplitude when one refocusing time is incremented and other one is fixed but longer. In contrast, the spin-noise concept fails to predict this effect.

Distance measurements between 5 nanometer diamonds - single particle magnetic resonance or optical super-resolution imaging?

5 nanometer sized detonation nanodiamonds (DNDs) are studied as potential single-particle labels for distance measurements in biomolecules. Nitrogen-vacancy (NV) defects in the crystal lattice can be addressed through their fluorescence and optically-detected magnetic resonance (ODMR) of a single particle can be recorded. To achieve single-particle distance measurements, we propose two complementary approaches based on spin-spin coupling or optical super-resolution imaging. As a first approach, we try to measure the mutual magnetic dipole-dipole coupling between two NV centers in close DNDs using a pulse ODMR sequence (DEER). The electron spin coherence time, a key parameter to reach long distance DEER measurements, was prolonged using dynamical decoupling reaching T 2,DD ≈ 20 μs, extending the Hahn echo decay time T 2 by one order of magnitude. Nevertheless, an inter-particle NV-NV dipole coupling could not be measured. As a second approach, we successfully localize the NV centers in DNDs using STORM super-resolution imaging, achieving a localization precision of down to 15 nm, enabling optical nanometer-scale single-particle distance measurements.

Echo modulations under homonuclear decoupling

A simple Hahn echo in a single-spin system with a static Hamiltonian can lead to echo modulations if the static Hamiltonian contains a component along the direction of the echo pulse. These modulations manifest as side bands in the Fourier transform of the echo decay. Experimentally, echo modulations that can be explained by such a model have been observed under homonuclear decoupling in solids where pulse imperfections can lead to residual effective fields in the interaction frame that have arbitrary orientations in space. We show analytically that such echo modulations are significantly reduced in intensity using double-echo sequences in agreement with experimental observations. Using pulse shapes for homonuclear decoupling that mimic and amplify the pulse distortions expected from pulse transients, we show that these effective fields can be one explanation for the observed reduction in echo modulations going from a single to a double Hahn-echo sequence.

Optically induced nuclear spin–spin couplings in GaAs manifested by spin echo decays under optical pumping

Optically induced nuclear spin–spin coupling in semiconductors is expected to serve for gate operations in nuclear-magnetic-resonance (NMR) quantum computations. However, not much is known about its characteristics. In this study, we investigate optically induced couplings in GaAs via double-resonance spin-echo decay experiments under optical pumping. The squared echo decay rate left( {1/T_2} right)^2 of 71Ga measured under various light powers revealed that optically induced couplings increase with increasing light power, and the sign of heteronuclear couplings between Ga–As pairs is opposite to that of dipolar couplings, which cancel each other out. Under higher light power, the optically induced “controllable” coupling is expected to eventually surpass the “uncontrollable” nuclear dipolar coupling, becoming dominant, which is preferable for the gate operation in NMR quantum computation.

Microsized Pore Structure Determination in EPDM Rubbers Using High-Pressure 129Xe NMR Techniques.

Microsized pore parameters, such as pore size and distance between pores in a series of model EPDM rubbers, were determined in situ under the pressure of 500 psi using 129Xe nuclear magnetic resonance (NMR) techniques: spin-lattice (T1) and spin-spin (T2) relaxation measurements, pulsed-field gradient (PFG) NMR, and two-dimensional exchange spectroscopy (2D EXSY). The T1/T2 (≫1) ratio for the xenon confined in the pores is larger than that for nonconfined free xenon. This suggests that almost the entire pore surface interacts with xenon atoms like a closed pore. While these pores still connect each other through very narrow diffusion/exchange channels, it is possible to observe the echo decay in PFG-NMR and cross-peaks in 2D EXSY. The results show that both diffusion (Dpore ≈ 2.1 × 10-10 m2/s) and exchange (exchange rate, τexch = a few tens of milliseconds) of xenon between a pore within the material and outer surface are prolonged. The exchange distances (l), which correspond to the xenon gas penetration depth, were estimated to be 70-100 μm based on the measured diffusion coefficients and exchange rate (1/τexch). NMR diffraction analysis reveals that pore size (a) and pore distance (b) are on the order of magnitude of micrometers and tens of micrometers, while the diffusion coefficients of xenon gas in the diffusion channels (Deff) are about 10-8 m2/s. Overall, this study suggests that the pores with a few micrometers connected through very narrow flowing channels with the length of several tens of micrometers are developed 70 to 100 μm below the rubber surface. Furthermore, the overall steady-state diffusion of xenon is slower, approximately 2 orders of magnitudes, than the diffusion in the channel between the pores. The pore and exchange distances correlated with the composition of rubbers showed that the properties of EPDM rubber as a high-pressure gas barrier could be improved by reducing the size of cracks and the depth of gas penetration by the addition of both carbon black and silica fillers.

Freezing of the Lattice in the Kagome Lattice Heisenberg Antiferromagnet Zn-Barlowite ZnCu_{3}(OD)_{6}FBr.

We use ^{79}Br nuclear quadrupole resonance (NQR) to demonstrate that ultraslow lattice dynamics set in below the temperature scale set by the Cu-Cu superexchange interaction J (≃160 K) in the kagome lattice Heisenberg antiferromagnet Zn-barlowite. The lattice completely freezes below 50K, and ^{79}Br NQR line shapes become twice broader due to increased lattice distortions. Moreover, the frozen lattice exhibits an oscillatory component in the transverse spin echo decay, a typical signature of pairing of nuclear spins by indirect nuclear spin-spin interaction. This indicates that some Br sites form structural dimers via a pair of kagome Cu sites prior to the gradual emergence of spin singlets below ∼30 K. Our findings underscore the significant roles played by subtle structural distortions in determining the nature of the disordered magnetic ground state of the kagome lattice.

Influencing mechanics and correction method of nuclear magnetic resonance measurement in igneous rocks reservoir

Nuclear magnetic resonance (NMR) measurement provides lithology-independent porosity and pore structure for sedimentary reservoirs. Recently, oil and gas were discovered in igneous rock formations, however, NMR logging porosity is substantially lower than the true porosity, and the T2 distributions does not characterize the pore-size distributions. These observations limit the use of NMR logging in igneous rock formations and new research is needed to be developed. To address this gap in the literature, using numerical simulation of NMR behavior, we conduct a theoretical study to examine the impact of magnetic susceptibility, echo spacing, and fluid type on the T2 distributions in igneous rock. The simulation results and laboratory experiment show that, the paramagnetic minerals affect the magnetization of fluids in pores and rate of the echo decay, and NMR porosity is thus significantly reduced. To effectively correct the NMR measurement, a new approach is proposed to improve the underestimation of NMR porosity and T2 distribution shifting to shorter times. For a pore model, the corrected porosity and T2 distribution are both in good agreement with the model. In a case study from Chepaizi area of Junggar Basin, China, the corrected NMR porosity matches well with core measurement, and the corrected T2 distribution shifts to longer times more than before. Moreover, bound fluid volume and movable fluid volume are both corrected, and the NMR permeability also agrees well with core laboratory measurement, which proves that the correction method effective. Therefore, this technology enables NMR log for evaluating the igneous rock reservoir quality.

Effects of surface treatments on flux tunable transmon qubits

One of the main limitations in state-of-the art solid-state quantum processors is qubit decoherence and relaxation due to noise from adsorbates on surfaces, impurities at interfaces, and material defects. For the field to advance towards full fault-tolerant quantum computing, a better understanding of these microscopic noise sources is therefore needed. Here, we use an ultra-high vacuum package to study the impact of vacuum loading, UV-light exposure, and ion irradiation treatments on relaxation and coherence times, as well as slow parameter fluctuations of flux tunable superconducting transmon qubits. The treatments studied do not significantly impact the relaxation rate Γ1 and the echo decay rate {{{Gamma }}}_{2,{{{rm{SS}}}}}^{{{{rm{e}}}}} at the sweet spot, except for Ne ion bombardment which reduces Γ1. In contrast, flux noise parameters are improved by removing magnetic adsorbates from the chip surfaces with UV-light and NH3 treatments. Additionally, we demonstrate that SF6 ion bombardment can be used to adjust qubit frequencies in situ and post-fabrication without affecting qubit relaxation and coherence times at the sweet spot.