Showing papers on "Nuclear quadrupole resonance published in 2021"

$s$-wave superconductivity in kagome metal CsV$_{3}$Sb$_{5}$ revealed by $^{121/123}$Sb NQR and $^{51}$V NMR measurements

Abstract: We report $^{121/123}$Sb nuclear quadrupole resonance (NQR) and $^{51}$V nuclear magnetic resonance (NMR) measurements on kagome metal CsV$_3$Sb$_5$ with $T_{\rm c}=2.5$ K. Both $^{51}$V NMR spectra and $^{121/123}$Sb NQR spectra split after a charge density wave (CDW) transition, which demonstrates a commensurate CDW state. The coexistence of the high temperature phase and the CDW phase between $91$ K and $94$ K manifests that it is a first order phase transition. At low temperature, electric-field-gradient fluctuations diminish and magnetic fluctuations become dominant. Superconductivity emerges in the charge order state. Knight shift decreases and $1/T_{1}T$ shows a Hebel--Slichter coherence peak just below $T_{\rm c}$, indicating that CsV$_3$Sb$_5$ is an s-wave superconductor.

S-Wave Superconductivity in Kagome Metal CsV$_{3}$Sb$_{5}$ Revealed by $^{121/123}$Sb NQR and $^{51}$V NMR Measurements

Abstract: We report $^{121/123}$Sb nuclear quadrupole resonance (NQR) and $^{51}$V nuclear magnetic resonance (NMR) measurements on kagome metal CsV$_3$Sb$_5$ with $T_{\rm c}=2.5$ K. Both $^{51}$V NMR spectra and $^{121/123}$Sb NQR spectra split after a charge density wave (CDW) transition, which demonstrates a commensurate CDW state. The coexistence of the high temperature phase and the CDW phase between $91$ K and $94$ K manifests that it is a first order phase transition. At low temperature, electric-field-gradient fluctuations diminish and magnetic fluctuations become dominant. Superconductivity emerges in the charge order state. Knight shift decreases and $1/T_{1}T$ shows a Hebel--Slichter coherence peak just below $T_{\rm c}$, indicating that CsV$_3$Sb$_5$ is an s-wave superconductor.

Recent advances in NMR crystallography and polymorphism

Abstract: This chapter covers some recent developments in NMR crystallography with a focus on its application towards the identification and characterization of crystal structures and their polymorphs. NMR crystallography is a discipline that uses insights or constraints obtained from solid-state NMR spectroscopy and first-principles calculations to predict, identify, validate, or refine crystal structures. In recent years, there have been plenty of developments in the area of NMR crystallography as it applies to study polymorphism in organic and inorganic systems, the understanding of noncovalent bonding networks directing crystal growth, and the study of dynamic processes present within solids. We discuss these topics through the lens of solid-state NMR and consider some of the more innovative or unusual strategies being applied to these problems, such as dynamic nuclear polarization (DNP) or nuclear quadrupole resonance (NQR) spectroscopy. The focus is on literature reports from 2016 to mid-2020.

Barnett field, rotational Doppler effect, and Berry phase studied by nuclear quadrupole resonance with rotation

Abstract: We report the observation of the Barnett field, rotational Doppler effect, and Berry phase using the rotating nuclear quadrupole resonance (NQR) method. We have developed coil-spinning techniques that enable us to systematically study the effects of rotation in setups involving rotation of the signal detector, rotation of the sample, and simultaneous rotation of both the signal detector and sample. Applying these setups to NQR measurements, we observe NQR line splittings in which the spectral structures are clearly distinct among the setups. By analyzing these structures, we clarify the origin of the NQR line splittings and discuss the relationship between the rotational Doppler effect, Barnett field effect, and Berry phase in terms of the rotational degrees of freedom, such as the relative rotation and the sample rotation itself, and the observation frame of reference. We also provide clear evidence of the difference between the rotational Doppler effect and the Barnett field and the equivalence of the Barnett field and the Berry phase.

Polymorphism and Conformational Equilibrium of Nitro-Acetophenone in Solid State and under Matrix Conditions.

Abstract: Conformational and polymorphic states in the nitro-derivative of o-hydroxy acetophenone have been studied by experimental and theoretical methods. The potential energy curves for the rotation of the nitro group and isomerization of the hydroxyl group have been calculated by density functional theory (DFT) to estimate the barriers of the conformational changes. Two polymorphic forms of the studied compound were obtained by the slow and fast evaporation of polar and non-polar solutions, respectively. Both of the polymorphs were investigated by Infrared-Red (IR) and Raman spectroscopy, Incoherent Inelastic Neutron Scattering (IINS), X-ray diffraction, nuclear quadrupole resonance spectroscopy (NQR), differential scanning calorimetry (DSC) and density functional theory (DFT) methods. In one of the polymorphs, the existence of a phase transition was shown. The position of the nitro group and its impact on the crystal cell of the studied compound were analyzed. The conformational equilibrium determined by the reorientation of the hydroxyl group was observed under argon matrix isolation. An analysis of vibrational spectra was achieved for the interpretation of conformational equilibrium. The infrared spectra were measured in a wide temperature range to reveal the spectral bands that were the most sensitive to the phase transition and conformational equilibrium. The results showed the interrelations between intramolecular processes and macroscopic phenomena in the studied compound.

Equatorial Electronic Structure in the Uranyl Ion: Cs2UO2Cl4 and Cs2UO2Br4.

Abstract: Electric field gradient (EFG) tensors in the equatorial plane of the linear UO22+ ion have been measured by nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) experiments and computed by relativistic Kohn-Sham methods with and without environment embedding for Cs2UO2Cl4 and Cs2UO2Br4. This approach expands the possibilities for probing the electronic structure in uranyl complexes beyond the strongly covalent U-O bonds. The combined analyses find that one of the two largest principal EFG tensor components at the halogen sites points along the U-X bond (X = Cl, Br), and the second is parallel to the UO22+ ion; in Cs2UO2Cl4, the components are nearly equal in magnitude, whereas in Cs2UO2Br4, due to short-range bromide-cesium interactions, the equatorial component is dominant for one pair of Br sites and the axial component is larger for the second pair. The directions and relative magnitudes of the field gradient principal axes are found to be sensitive to the σ and π electron donation by the ligands and the model of the environment. Chlorine-35 NQR spectra of 235U-depleted and 235U-enriched Cs2UO2Cl4 exhibited no uranium-isotope-dependent shift, but the resonance of the depleted sample displayed a 58% broader line width.

Development of a Low-Cost, Portable NQR Spectrometer for RDX Explosives Detection

Abstract: Nuclear quadrupole resonance (NQR) technology is a promising approach to detect so-called “minimum metal” landmines, as it can look directly for their explosive content. Conventional commercially available NQR devices, however, are large and expensive, and they require a transmitter power amplifier with a power generator, which is not suitable for outdoor use and mass production. Here we present a small and inexpensive portable device developed for detecting landmines using NQR. The device uses a field-programmable gate array (FPGA) and low-impedance transmission and reception circuits that include a dual supply class-D power amplifier powered by conventional batteries to ensure sufficient magnetic field excitation for mine detection. The pulse width modulation signals that are fed into the power amplifier have been designed to protect the low impedance transmit-receive switch circuit from the high voltage. The system has been tested successfully in the laboratory with 100g of explosive RDX at a distance of 10cm from the antenna, corresponding to a plausible anti-personal (AP) mine scenario. Detection was achieved with a signal to noise ratio (SNR) ~2 in 2/3 of the time that a previous prototype required (120 sec). Moreover, the new device can detect RDX explosives with a measurement time and SNR comparable to mine detectors built with commercial-off-the-shelf (COTS) components, but at lower cost and smaller form factor.

Implementation of the Configuration Structure of an Integrated Computational Core of a Pulsed NQR Sensor Based on FPGA.

Abstract: This paper presents a method for implementing the configuration structure of an integrated computational core of a pulsed nuclear quadrupole resonance (NQR) sensor based on a field-programmable gate array (FPGA), which comprises the following modules: a three-channel direct digital synthesizer (DDS), a pulse sequence shaper and a software-defined radio. Experimental studies carried out using the in-circuit analyzer SignalTap Logic Analyzer have confirmed the reliability of the correct and stable operation of the functional modules of the configuration structure at all stages of signal transformations, starting from the formation of the envelope of the excitation pulses and ending with the obtainment of low-frequency quadrature signals at the outlet of the compensating filters. The time and frequency dependences of the amplitude of the output signals generated using the DDS based on a 48 bit phase accumulator are investigated. This development can be used when creating pulsed coherent NQR sensors in the frequency range of 1 MHz–50 MHz.

A Novel SSA-NLLSF Approach for Denoising NQR Signals

Abstract: Nuclear quadrupole resonance (NQR) spectroscopy is used to identify narcotics and explosive materials. Detection of NQR signal generated by $${14}^{{N}}$$ isotope in an open environment is a challenging task due to the presence of external random noise and RF interference. Unlike the existing wavelet-based and other frequency-domain approaches that use averaged data, the present work exploits raw data by saving the acquisition time. In this context, a novel singular spectral analysis (SSA) and non-linear least square fit (NLLSF) algorithms are proposed to denoise the NQR signal and obtain the required parameters for detection of the NQR signal. Considering signal to noise ratio (SNR), segmental SNR (SSNR), and Pink noise as the performance parameters, the proposed algorithms are tested under various noise conditions by passing synthesized NQR signal and observed 35.7 dB gain SSNR. Furthermore, tests are carried out on NQR spectroscopy data acquired from the $$\hbox {NaNO}_{2}$$ sample, and an improvement in terms of signal quality and acquisition time are noticed.

Two-Frequency Planar Gradiometer for Distant NQR Detection of Explosives

Abstract: The experimental realization of the two-frequency 14N nuclear quadrupole resonance (NQR) technique with use of the gradiometer radiofrequency (RF) probe for applications in the remote detection of explosives and other substances has been proposed. The planar two-frequency gradiometer has been designed, modeled and manufactured. The regions, where the RF magnetic fields are nearly orthogonal to each other, have been determined by finite-element modeling studies. The two-frequency experiments have been carried out for the detection of hexahydro-1,3,5-trinitro-s-triazine C3H6N6O6 (RDX). The possibility of the combined use of the two-frequency 14N NQR technique and the gradiometer RF probe in practice for detection of explosives hidden under clothes or underground has been also discussed.

Application of reinforcement learning for NQR excitation sequence optimization

Abstract: Nuclear quadrupole resonance is a spectroscopy technique well-known for its high specificity and its downsides, like very low signal-to-noise ratio, susceptibility to radio frequency interference and temperature dependence of the resonance lines. Multiple optimization techniques have been applied in nuclear resonance spectroscopy to improve the excitation sequence and Bayesian optimization has been shown to give good results. This paper proposes a different approach to finding the optimized excitation sequence by applying reinforcement learning algorithms. Six solutions are proposed, validated and tested in both simulated and real environments. The solutions are shown to be inappropriate for this application and to provide no information gain, similar to the random algorithm. Bayesian optimization is considered better suited for nuclear quadrupole resonance sequence optimization.

Fate of soliton matter upon symmetry-breaking ferroelectric order

Abstract: In a one-dimensional (1D) system with degenerate ground states, their domain boundaries, dubbed solitons, emerge as topological excitations often carrying unconventional charges and spins; however, the soliton excitations are vital in only the nonordered regime. Then a question arises: How do the solitons conform to a three-dimensional (3D) ordered state? Here, using a quasi-1D organic ferroelectric, tetrathiafulvalene-$p$-chloranil (TTF-CA), with degenerate polar dimers, we pursue the fate of spin-soliton charge-soliton composite matter in a 1D polar-dimer liquid upon its transition to a 3D ferroelectric order by resistivity, nuclear magnetic resonance (NMR), and nuclear quadrupole resonance (NQR) measurements. We demonstrate that the soliton matter undergoes neutral spin-spin soliton pairing and spin-charge soliton pairing to form polarons, coping with the 3D order. Below the ferroelectric transition, the former contributes to the magnetism through triplet excitations, which rapidly fade out on cooling, whereas the latter carries electrical current with paramagnetic spins that more moderately decrease with temperature. The nearly perfect scaling between NMR and NQR relaxation rates in the ferroelectric phase evidences that spin carriers diffuse with lattice distortion, namely, in the form of polarons. From the combined analyses of conductivity and NMR relaxation rate, we derive the excitation energies of polaron excitations and diffusion. Our results reveal the whole picture of soliton matter that condenses into the 3D ordered state.

Continuous random network avoidance and microscopic relaxation time in As2Se3 glass

Abstract: Recent nuclear magnetic resonance (NMR) studies on SiO2 structural glass has shown a positive correlation exists between Si-O bond lengths and Si-O-Si bond angles. This indicates that when the glass solidified it avoided forming as a continuous random network (CRN). A microscopic tight binding (TB) model was used some years ago to predict the nuclear quadrupole resonance (NQR) distribution of powdered As2Se3 glass. Similar bond length- bond angle correlations were found. Here we extend this model and show that these correlations are directly attributable to a competition between valence electronic energy and emergent frustration forces as the glass solidifies into a locally preferred structure (LPS). Recent studies have shown that fast atomic motion occurs in metallic glasses at temperatures far below the glass temperature. Using this model we calculate a microscopic relaxation time and show it is a product of CRN avoidance.

Magnetically induced local lattice anomalies and low-frequency fluctuations in the Mott insulator La 2 O 3 Fe 2 Se 2

Abstract: We report a study of the Mott insulator ${\mathrm{La}}_{2}{\mathrm{O}}_{3}{\mathrm{Fe}}_{2}{\mathrm{Se}}_{2}$ by means of $^{139}\mathrm{La}$ nuclear quadrupole resonance (NQR). The NQR spectra evidence a single La site in the paramagnetic phase and two inequivalent La sites, La1 and La2, in the antiferromagnetic phase. These two sites are characterized by different quadrupole couplings, indicative of distinct lattice configurations segregated in domains. The dependence of the quadrupole coupling for La2 on temperature suggests that the structural distortion is driven by the magnetic order parameter. The nuclear transverse relaxation rate $1/{T}_{2}$ evidences fluctuations in the paramagnetic phase with characteristic frequencies well below the Heisenberg exchange frequency and likely associated with nematic fluctuations.

Star-of-David pattern charge density wave with additional modulation in the kagome superconductor CsV$_3$Sb$_5$ revealed by $^{51}$V-NMR and $^{121/123}$Sb-NQR.

Abstract: $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) is a novel kagome superconductor coexisting with the charge density wave (CDW) order. Identifying the structure of the CDW order is crucial for understanding the exotic normal state and superconductivity in this system. Here, we report $^{51}$V nuclear magnetic resonance (NMR) and $^{121/123}$Sb nuclear quadrupole resonance (NQR) studies on kagome-metal CsV$_3$Sb$_5$. Below the CDW transition temperature $T_\textrm{CDW} \sim$ 98 K, an abrupt change of spectra was observed, indicating that the transition is of the first order. By further analysing the spectra, we find that the CDW order is commensurate. And most remarkably, we obtain the first experimental evidence that the charge modulation of the CDW order is of star-of-David pattern and accompanied by an additional charge modulation in bulk below $T^* \sim$ 40 K. Our results revealing the unconventional CDW order provide new insights into $A$V$_3$Sb$_5$.