Search

Nuclear quadrupole resonance is a solid-state spectroscopy technique used for detecting compounds containing quadrupolar nuclei. Its advantages, such as high specificity and non-invasive scanning, have enabled its application in pharmaceutical analysis and prohibited substances detection. However, it has the disadvantages of very low signal-to-noise ratio and temperature dependence of the response signal, and multiple signal processing techniques have been developed to compensate them. The purpose of this article is to provide a comprehensive review of the signal processing and analysis techniques developed for improving detection in nuclear quadrupole resonance spectroscopy, in order to serve as a starting point for new researchers. An introduction in this field is given and the specific signal processing challenges are presented. A new classification of existing solutions is proposed and a comparative analysis is performed in different noise and temperature conditions on multiple statistical and machine learning algorithms. The classification solutions are shown to outperform the statistical ones and the use of machine learning techniques is proposed for future improvement. This study is also instructive for research regarding the selection of a signal processing and analysis algorithm for nuclear quadrupole resonance detection and can be used as a reference for future works.

The deviation of the electron density around the nuclei from spherical symmetry determines the electric field gradient (EFG), which can be measured by various types of spectroscopy. Nuclear Quadrupole Resonance (NQR) is particularly sensitive to the EFG. The EFGs, and by implication NQR frequencies, vary dramatically across materials. Consequently, searching for NQR spectral lines in previously uninvestigated materials represents a major challenge. Calculated EFGs can significantly aid at the search’s inception. To facilitate this task, we have applied high-throughput density functional theory calculations to predict EFGs for 15187 materials in the JARVIS-DFT database. This database, which will include EFG as a standard entry, is continuously increasing. Given the large scope of the database, it is impractical to verify each calculation. However, we assess accuracy by singling out cases for which reliable experimental information is readily available and compare them to the calculations. We further present a statistical analysis of the results. The database and tools associated with our work are made publicly available by JARVIS-DFT (https://www.ctcms.nist.gov/~knc6/JVASP.html) and NIST-JARVIS API (http://jarvis.nist.gov/)., Measurement(s) electric field gradient Technology Type(s) computational modeling technique Factor Type(s) material studied Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.13027220

In this study, the 14N NQR parameters of the possible isomers of borazyne were reported in the singlet and triplet states at the M062X/6-311G(d,p) level of theory. Electric field gradient tensors (qxx, qyy, qzz), nuclear quadrupole coupling constants (ηzz, ηyy, ηxx), asymmetry parameter (η), and nuclear quadrupole resonance frequencies (ν+, ν–, ν0) were estimated. Relative energy, frontier orbital energy, and HOMO–LUMO gap values of these isomers were calculated. From the energetic aspect, ortho and meta(2) isomers were the most stable isomers in the singlet and triplet states, respectively. The singlet state was a more stable spin isomer compared to the triplet state between all isomers, except for the meta(2) isomer. Aromaticity analysis of the studied molecules on the basis of nucleus-independent chemical shift values showed that the aromaticity of the triplet state was stronger than that of the singlet state for ortho and para isomers. For the meta(2) isomer, the aromaticity of the singlet state was stronger than that of the triplet state. To calculate the atomic charge of nitrogen atoms, natural bonding orbital method analysis was used.

An 14N nuclear quadrupole resonance (NQR) spectroscopy method has been developed using the singular spectrum analysis (SSA) based on the principal components concept. For the first time, it was demonstrated that this method can be used as a low cost and non-destructive quantitative method for analyses of small amounts (

The results of earlier studies, using the NQR method 63.65Cu of the compound of the transparent conductive oxide CuAlO2, were used to study the topology of the distribution of the electron density in this compound. It was shown that for Al – O and Cu – Cu pairs, the formation of a bond most likely proceeds according to the type of closed shells, and in the “copper” plane there can be a noticeable number of free electrons. As for the copper – oxygen bond, we can assume the formation of O-Cu-O chains with a covalent bond. The noticeable difference in the character of the electron density distribution in mutually perpendicular planes suggests a certain anisotropy of conductivity in a given compound.

The temperature dependence of the 1H nuclear magnetic resonance spin–lattice relaxation time T1 of [(CH3)2NH2]2KCo(CN)6 and the partially deuterated analogue [(CD3)2NH2]2KCo(CN)6 has been reported. A change in the molecular motion of organic cations through the first-order phase transition at Tc = 246 K has been discussed. Although this first-order transition has been reported as an isosymmetric transition, in which the space group of the crystal does not change, the number of the 14N nuclear quadrupole resonance line changed from three in the high-temperature phase to twelve in the low-temperature phase indicating the lowering of crystal symmetry.

Spatial inversion symmetry in crystal structures is closely related to the superconducting (SC) and magnetic properties of materials. Recently, several theoretical proposals that predict various interesting phenomena caused by the breaking of the local inversion symmetry have been presented. However, experimental validation has not yet progressed owing to the lack of model materials. Here we present evidence for antiferromagnetic (AFM) order in CeRh₂As₂ (SC transition temperature T[SC]∼0.37 K), wherein the Ce site breaks the local inversion symmetry. The evidence is based on the observation of different extents of broadening of the nuclear quadrupole resonance spectrum at two crystallographically inequivalent As sites. This AFM ordering breaks the inversion symmetry of this system, resulting in the activation of an odd-parity magnetic multipole. Moreover, the onset of antiferromagnetism T[N] within an SC phase, with T[N], 縺（もつ）れ結晶に起因した「超伝導反強磁性」状態を発見. 京都大学プレスリリース. 2022-02-07.

The recently reported AV3Sb5 (A = K, Rb, Cs) family of kagome metals are candidates for unconventional superconductivity and chiral charge density wave (CDW) order; both potentially arise from nested saddle points in their band structures close to the Fermi energy. Here, we use chemical substitution to introduce holes into CsV3Sb5 and unveil an unconventional coupling of the CDW and superconducting states. Specifically, we generate a phase diagram for CsV3Sb5−xSnx that illustrates the impact of hole doping the system and lifting the nearest van Hove singularity toward and above EF. Superconductivity exhibits a nonmonotonic evolution with the introduction of holes, resulting in two “domes” peaked at 3.6 and 4.1 K and the rapid suppression of three-dimensional CDW order. The evolution of CDW and superconducting order is compared with the evolution of the electronic band structure of CsV3Sb5−xSnx, where the complete suppression of superconductivity seemingly coincides with an electronlike band comprised of Sb pz orbitals pushed above EF.

The kagome Heisenberg antiferromagnet formed by frustrated spins arranged in a lattice of corner-sharing triangles is a prime candidate for hosting a quantum spin liquid (QSL) ground state consisting of entangled spin singlets. But the existence of various competing states makes a convincing theoretical prediction of the QSL ground state difficult, calling for experimental clues from model materials. The kagome lattice materials Zn-barlowite ZnCu$_{3}$(OD)$_{6}$FBr and herbertsmithite ZnCu$_{3}$(OD)$_{6}$Cl$_2$ do not exhibit long range order, and they are considered the best realizations of the kagome Heisenberg antiferromagnet known to date. Here we use $^{63}$Cu nuclear quadrupole resonance combined with the inverse Laplace transform (ILT) to probe locally the inhomogeneity of delicate quantum ground states affected by disorder. We present direct evidence for the gradual emergence of spin singlets with spatially varying excitation gaps, but even at temperatures far below the super-exchange energy scale their fraction is limited to approximately 60\% of the total spins. Theoretical models need to incorporate the role of disorder to account for the observed inhomogeneously gapped behaviour., Comment: 4 Figs