Lattice Relaxation Research Articles

Rate constants for saturation-recovery EPR and ELDOR of 14N-Spin labels

Saturation-recovery (SR)-EPR can determine electron spin–lattice relaxation rates in liquids over a wide range of effective viscosity, making it especially useful for biophysical and biomedical applications. Here, I develop exact solutions for the SR-EPR and SR-ELDOR rate constants of 14N-nitroxyl spin labels as a function of rotational correlation time and spectrometer operating frequency. Explicit mechanisms for electron spin–lattice relaxation are: rotational modulation of the N-hyperfine and electron-Zeeman anisotropies (specifically including cross terms), spin-rotation interaction, and residual frequency-independent vibrational contributions from Raman processes and local modes. Cross relaxation from mutual electron and nuclear spin flips, and direct nitrogen nuclear spin–lattice relaxation, also must be included. Both the latter are further contributions from rotational modulation of the electron-nuclear dipolar interaction (END). All the conventional liquid-state mechanisms are defined fully by the spin-Hamiltonian parameters; only the vibrational contributions contain fitting parameters. This analysis gives a firm basis for interpreting SR (and inversion recovery) results in terms of additional, less standard mechanisms.

Solid-state 13C NMR spectroscopic study of supercritical CO2 catalyzation treated polyethylene terephthalate textiles for platinum metallization

In this study, polyethylene terephthalate (PET) textiles were metallized with Pt via supercritical carbon dioxide (sc-CO2) catalyzation, using palladium(II) hexafluoroacetylacetonate (Pd(hfac)2) as the Pd catalyst source. The effects of the treatments on the structural changes and molecular mobility of PET were elucidated by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. The carbonyl and nonprotonated aromatic carbons and the methylene and protonated aromatic carbons were analyzed from the NMR spectra using a three-component model (crystalline, rigid amorphous, and mobile amorphous components), which inferred that the PET polymer textile comprised one crystalline and two amorphous regions. Additionally, when Pd catalyzation was performed on the textile under sc-CO2 conditions, the crystalline peak of the carbonyl carbon exhibited a chemical shift into a higher magnetic field, and the spin–lattice relaxation time increased. These results implied that Pd could exist in the vicinity of the CO carbon. The X-ray diffraction studies also showed crystallinity of PET increased by sc-CO2 catalyzation, while sc-CO2 annealing without Pd(hfac)2 did not increase it. Therefore, Pt films were successfully coated on the PET via catalytic plating using sc-CO2. The catalyzation reduced the molecular mobility of the PET polymer, indicating the incorporation of the catalytic metal into the polymer chain. The carbonyl carbon peak simultaneously shifted to a higher field, implying that the catalytic Pd could approach the carbonyl groups. The detailed analysis of the structure and molecular mobility provides valuable information that can contribute to the development of novel conductive materials.

Inverse Laplace Transform Approaches to βNMR Relaxation

Spin lattice relaxation is the simplest type of βNMR measurement. The usual approach is to implant a pulse of hyperpolarized nuclei and monitor the time-resolved β-decay asymmetry, yielding the ensemble average spin-lattice relaxation. In the simplest case, the asymmetry decays exponentially with a characteristic time constant T 1, but this ideal is rarely obtained in practice. In most data, the relaxation is more complicated. This can be the result of multiple crystallographic sites for the implanted probe each having a distinct T 1. The sample may also be inhomogeneous due to: impurities or defects (including interfaces that are particularly important for thin films), intrinsic phase separation, or, if it is a glass. There may also be a background signal from probe ions that stop outside the sample. The general approach to this problem has been the ad hoc development of an appropriate relaxation model that avoids overparametrization.Given the prevalence of more complicated relaxation, it is crucial to develop a systematic approach to relaxation modelling. The decomposition of a relaxing signal into exponentials is, however, a mathematically ill-posed problem[1]. This feature is intrinsic and unavoidable, but there are a number of methods to accommodate it for noisy real-world data, including nuclear spin relaxation[2, 3, 4].Here we demonstrate one of the best and most commonly used methods, Tikhonov regularization for the inverse Laplace transform, implemented for the particular features of βNMR relaxation data, most importantly the strong time dependence of the statistical uncertainty stemming from the radioactive lifetime of the probe.

Structure, magnetic morphology and magnetization correlations in pulsed laser deposited CoFe[formula omitted]O[formula omitted] ([formula omitted]) thin films

The present paper reports a complete study on the correlation between structure, morphology, and magnetic properties of (111)-oriented cobalt ferrite (CoFe2O4) thin films with varying film thickness. The CoFe2O4 (CFO) thin films were deposited on Pt-coated Si substrate by pulsed laser deposition (PLD) at 550 °C. The x-ray diffraction (XRD) data confirms the (111)-oriented growth of the cobalt ferrite films. The in-plane morphology of the films in the field emission scanning electron micrographs ensure the Stranski–Krastanov growth mechanism, and the atomic force micrographs confirms the effect of lattice relaxation on the morphology of the films with varying thickness. The possible cation distributions for the samples were determined from the Raman spectroscopy, which revealed the crystal structure-magnetic property correlations in cobalt ferrite films. The magnetic hysteresis (M−H) loops show a significant spin reorientation by showing the variation between the in-plane (IP) and out-of-plane (OP) magnetization. The presence of a kink on the OP M−H loops and its variation with film thickness clearly establishes the existence of competing magnetic anisotropies in the films. The high coercivity (HC) values observed for OP magnetization of cobalt ferrite films with thicknesses 115 nm and 125 nm may be explored for possible room-temperature (RT) device applications.

The Site and High Field βNMR Properties of 8Li+ Implanted in α-Al2O3

We present high (> 2 T) magnetic field βNMR measurements of 8Li+ implanted in single crystals of sapphire, a commonly used backing material for other samples. From the well-resolved quadrupolar splitting, we extract the electric field gradient (EFG) at the implanted 8Li+ site. Comparison with supercell density functional theory calculations of the EFG allows us to identify the octahedral interstitial site as the most likely candidate. In contrast to the zero field βNQR spectra, where multiple singals are detected, only a single site is evident at high field. We discuss possible explanations for this discrepancy. The spin lattice relaxation is extremely slow (1/T 1 < 0.02 s−1 ) over a broad temperature range from 4 to 300 K, demonstrating that cross relaxation with the 27Al nuclear spins is quenched in such high magnetic fields.

Production of Asiatic Hybrid Lily ‘Beni no Mai’ with More than Two Flowers within One Year from Bulbils Influenced by Development Stage, Flower Bud Removal, and Temperature Manipulation

Background and objective: The objective of this study was to produce Asiatic hybrid lily with more than two flowers in one year starting from bulbils. This study was to force 'Beni no Mai' bulbils influenced by harvesting date, flower bud removal, and temperature manipulation and to evaluate dormancy and maturity by nuclear magnetic resonance imaging (NMRI) and carbohydrates analysis.Methods: In experiment 1, bulbils were harvested at 0-30 days after anthesis (DAA), and were divided into 7 stages. And, soluble monosaccharides and monosaccharide constituents of bulbils were analyzed. In experiment 2, flower buds of plants that flowered on 21 April were not removed or removed on 1, 13, and 25 April. Bulbils harvested were divided into four groups and planted after cold treatment. In experiment 3, bulbils received a sequential temperature (SEQ CD) treatments for three weeks at 2.5, 5.0, and 7.5°C, 1 week at 10, 12.5, and 15°C, and 3 weeks at 2.5, 5.0, and 7.5°C. In experiment 4, bulbils were harvested 0-70 DAA, and were subjected to spin-spin lattice relaxation time by NMRI.Results: The bulbils harvested at 30 DAA produced two flowers within 282 days, and were better in growth. The bulbils harvested at 30 DAA showed higher concentration of soluble glucose and lower concentration of non-cellulosic neutral glucose. Growth of bulbils removed flower buds before flowering were better than those of bulbils that flower bud was removed at anthesis and 13 Apr. However, the number of flowers of bulbils harvested from plants removed flower buds on 1 Apr. were less than 2. The SEQ CD treatment of 7.5°C/3W-15°C/1W-5°C/3W in bulbils could produce the plants with 2.2 flowers within 300 days after planting. When bulbils were harvested at 0-28 DAA, yellow color corresponding to T2 relaxation time of 15-20 ms was dominant, and bulbils harvested at 56-70 DAA showed T2 relaxation time of 35-50 ms and 50-100 ms.Conclusion: Considering the results of soluble carbohydrate content and T2 relaxation time of bulbils, it was found that the bulbils had a shallower dormancy and were more mature at 30-42 DAA than before 28 DAA. Also, when these bulbils were planted after SEQ CD treatment at 7.5°C/3W-15°C/1W-5°C/3W, plants with two or more flowers could be produced within one year.

Tungsten(VI) oxide as a versatile support for Pd nanocatalysts for alkaline direct alcohol fuel cells

The instability of carbon support materials has motivated the development of metal oxides supports which are stable under the fuel cell environment. In this study, tungsten (VI) oxide (WO3) is utilized as a secondary support and cocatalyst for the electrooxidation of methanol and ethanol. Functionalized carbon nanodots employed as primary supports were blended with WO3 nanoparticles to form a composite support onto which Pd nanoparticles were deposited by a borohydride reduction method. The synthesized Pd/fCNDs-WO3 electrocatalysts were characterized by Transmission Electron microscopy (TEM), X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy. XRD results proved that incorporating WO3 into Pd/fCNDs electrocatalyst shifts the Pd diffraction peaks to lower 2Ɵ value due to lattice relaxation. XPS results revealed that W exist in oxidised form and confirmed the strong interaction between the support material and the catalyst. The Pd/fCNDs-WO3 electrocatalysts exhibited a remarkable catalytic activity towards methanol and ethanol oxidation. High current densities of 87.24 mA cm−2 and 44.23 mA cm−2 were obtained for ethanol and methanol oxidation, respectively, using a catalyst with 2.5% Pd loading. EIS, CA and stability tests revealed that the presence of WO3 in Pd/fCNDs electrocatalyst improves the kinetics, tolerance to poisoning and long-term durability in alkaline conditions. This superior performance is attributed to the electronic coupling between Pd and WO3 nanoparticles.

Giant Density of States Enhancement Driven by a Zero-Mode Landau Level in Semimetallic Black Phosphorus under Pressure.

Dirac fermion systems form a unique Landau level at the Fermi level-the so-called zero mode-whose observation itself will provide strong evidence of the presence of Dirac dispersions. Here, we report the study of semimetallic black phosphorus under pressure by ^{31}P-nuclear magnetic resonance measurements in a wide range of magnetic field up to 24.0T. We have found a field-induced giant enhancement of 1/T_{1}T, where 1/T_{1} is the nuclear spin lattice relaxation rate: 1/T_{1}T at 24.0T reaches more than 20 times larger than that at 2.0T. The increase in 1/T_{1}T above 6.5T is approximately proportional to the squared field, implying a linear relationship between the density of states and the field. We also found that, while 1/T_{1}T at a constant field is independent of temperature in the low-temperature region, it steeply increases with temperature above 100K. All these phenomena are well explained by considering the effect of Landau quantization on three-dimensional Dirac fermions. The present study demonstrates that 1/T_{1} is an excellent quantity to probe the zero-mode Landau level and to identify the dimensionality of the Dirac fermion system.

Pseudomagnetic fields, particle-hole asymmetry, and microscopic effective continuum Hamiltonians of twisted bilayer graphene

Using the method developed in the companion paper, we construct the effective continuum theories for two different microscopic tight binding models of the twisted bilayer graphene at the twist angle of $1.05^\circ$, one Slater-Koster based and the other ab-initio Wannier based. The energy spectra obtained from the continuum theory -- either for rigid twist or including lattice relaxation -- are found to be in nearly perfect agreement with the spectra from the tight binding models when the gradient expansion is carried out to second order, demonstrating the validity of the method. We also analyze the properties of the Bloch states of the resulting narrow bands, finding non-negligible particle-hole symmetry breaking near the $\Gamma$ point in our continuum theory constructed for the ab-initio based microscopic model due to a term in the continuum theory that was previously overlooked. This reveals the difference with all existing continuum models where the particle-hole symmetry of the narrow band Hilbert space is nearly perfect.

Gellan Gum-Montmorillonite Nanocomposites for Electrochromic Devices

The present paper shows the results of nanocomposite polymer electrolytes (SPEs) of montmorillonite (MMT; Na+SYN-1) and gellan gum obtained by the solution casting method. The membrane samples were characterized by ATR-FTIR, time-domain nuclear magnetic resonance (TD-NMR), and scanning electron microscopy (SEM). Then, two samples were chosen and applied in small electrochromic devices (ECDs). The ATR-FTIR revealed shifts in bands of acetate and glycosidic bonds at 1032 and 1611 cm−1, respectively, indicating an interaction between the gellan gum and Na+SYN-1. The spin–lattice relaxation time of the proton nuclei (T1) suggests the poor dispersion of MMT in the matrix, especially above 20 wt.%. However, SEM pictures pointed to a more homogeneous surface of the nanocomposite containing 40 wt.% Na+SYN-1 when compared to the sample without clay. NPEs with 10 and 40 wt.% Na+SYN-1 were applied in ECDs, and the voltammograms showed a decrease in anodic and cathodic peaks after 2519 chronocoulometric cycles for the sample with 10 wt.% Na+SYN-1 and after 420 cycles for the sample with 40 wt.% Na+SYN-1. There was also a decrease in the charge density in both ECDs and an oscillating difference in transmittance between the colored/discolored states during the chronocoulometric cycles of the ECD with GG-MMT10. Further studies may reveal more property improvements in gellan gum nanocomposites.

Pressure-induced core defects and photoluminescent quenching in carbon quantum dots

Carbon quantum dots (CDs) with favorable luminescent features for biphotonic applications have attracted much interest in modulating their photoluminescence (PL) efficiency. A surface state with various defects is believed to play a key role in the emissive intensity. Here, pressure-induced quenching of PL is observed in red emissive CDs (R-CDs) and is ascribed to defects in carbon cores upon compression. In the power-law fitting to the excitation power-dependent PL of R-CDs at high pressure, the coefficient k parameter related to the emissive mechanism decreases from 1 under ambient pressure to much less than 1 under the application of pressure, suggesting a transition from single exciton recombination to defect-related emission. With the k parameter decreasing to 0.69 at 1.6 GPa, the pressure-induced defects reduce the PL intensity by approximately one order of magnitude. Furthermore, the attenuation and broadening of the G band characterizing the sp2 hybrid structure of carbon cores in the Raman spectra for R-CDs at high pressure support that the pressure-induced lattice relaxation impairs the crystalline symmetry of the carbon core and results in the dramatic quenching of PL. Our results highlight the importance of the well-crystallized carbon core in designing CDs with high quantum yields.

Nuclear dipolar relaxation induced by interacting ground state electron spins

In samples used for dynamic nuclear polarization (DNP), spin–lattice relaxation times are usefully increased by going to high magnetic field and low temperature, typically several tesla and below 1 K. But the relaxation times for dipolar components of the nuclear spin energy remain stubbornly shorter than those for the Zeeman energy: dipolar order decays faster than the polarization itself by a huge factor—up to four orders of magnitude or more in the materials studied thus far. Such fast nuclear dipolar relaxation poses experimental challenges, for instance, when transferring polarization between different nuclear spin species via intermediate nuclear order: a proven technique to polarize rare nuclear spins. The origin of this fast nuclear dipolar relaxation remained a mystery for a long time—existing theories of nuclear spin–lattice relaxation could at best predict about one order of magnitude difference between the nuclear dipolar and nuclear Zeeman relaxation rates—until it was recently discovered to be due to conversion of nuclear dipolar energy into super-hyperfine energy induced by nuclear flip-flop transitions. A previous article showed that the inclusion of this relaxation path enables a quantitative explanation of nuclear dipolar relaxation induced by photo-excited triplet states. This article extends the theory to nuclear dipolar relaxation induced by ground state electron spins and demonstrates that this new mechanism enables a precise quantitative prediction from first principles of the nuclear dipolar relaxation rate for Ca(OH)2 doped with O2- centres—in which DNP is caused by the solid effect (SE)—and LiF doped with F-centres—in which DNP is caused by thermal mixing (TM)—both at 5.5 T and 0.4 K. It is noticed that the proposed mechanism extends to other spin systems which has implications for e.g. TM and spectral diffusion.

Commensurate and incommensurate double moiré interference in twisted trilayer graphene

Twisted graphene multi-layers have been recently demonstrated to share several correlation-driven behaviours with twisted bilayer graphene. In general, the van Hove singularities (VHSs) can be used as a proxy of the tendency for correlated behaviours. In this paper, we adopt an atomistic method by combining tight-binding method with the semi-classical molecular dynamics to investigate the electronic structures of twisted trilayer graphene (TTG) with two independent twist angles. The two independent twist angles can lead to the interference of the moir\'e patterns forming a variety of commensurate/incommensurate complex supermoir\'e patterns. In particular, the lattice relaxation, twist angle and angle disorder effects on the VHS are discussed. We find that the lattice relaxation significantly influence the position and magnitude of the VHSs. In the supermoir\'e TTG, the moir\'e interference provides constructive or destructive effects depending on the relative twist angle. By modulating the two independent twist angles, novel superstructures, for instance, the Kagome-like lattice, could constructed via the moir\'e pattern. Moreover, we demonstrate that a slight change in twist angles (angle disorder) provides a significant suppression of the peak of the VHSs. Apart from the moir\'e length, the evolution of the VHSs and the LDOS mapping in real space could be used to identify the twist angles in the complicated TTG. In practice, our work could provide a guide for exploring the flat band behaviours in the supermoir\'e TTG experimentally.

Phonon-assisted upconversion in twisted two-dimensional semiconductors

Phonon-assisted photon upconversion (UPC) is an anti-Stokes process in which incident photons achieve higher energy emission by absorbing phonons. This letter studies phonon-assisted UPC in twisted 2D semiconductors, in which an inverted contrast between UPC and conventional photoluminescence (PL) of WSe2 twisted bilayer is emergent. A 4-fold UPC enhancement is achieved in 5.5° twisted bilayer while PL weakens by half. Reduced interlayer exciton conversion efficiency driven by lattice relaxation, along with enhanced pump efficiency resulting from spectral redshift, lead to the rotation-angle-dependent UPC enhancement. The counterintuitive phenomenon provides a novel insight into a unique way that twisted angle affects UPC and light-matter interactions in 2D semiconductors. Furthermore, the UPC enhancement platform with various superimposable means offers an effective method for lighting bilayers and expanding the application prospect of 2D stacked van der Waals devices.

Lattice relaxation effects on the collective resonance spectra of a finite dipole array.

Dielectric/plasmonic lattice relaxation spectroscopy is theoretically discussed in this work. A lattice relaxation effect generally occurs in nanocrystals, which means that from the bulk phase to the crystal surface, lattice parameters show a gradual shift. Here, lattice relaxation is introduced into finite polarizable point arrays or rod arrays as an adjusting tool, and its effect on lattice resonance extinction spectrum peaks is calculated. DDA (discrete dipole approximation) and FDTD (finite difference time domain) methods are applied. Different from an ideal infinite array, a finite array exhibits a broad, rippled extinction spectral peak. The application of an expanded/contracted lattice relaxation to the finite array can compress the ripple on one shoulder of the peak, as a cost, and the other shoulder of the peak gets more rippled, showing a "ripple transfer" effect. The strategy introduced in this work can contribute to the micro/nano optical measurement, on-chip adjustable optical cavity for OPOs (optical parameter oscillators)/lasers and controlling of fluorescence or hot-electron chemistry.

A theoretical framework for the design of molecular crystal engines.

Photomechanical molecular crystals have garnered attention for their ability to transform light into mechanical work, but difficulties in characterizing the structural changes and mechanical responses experimentally have hindered the development of practical organic crystal engines. This study proposes a new computational framework for predicting the solid-state crystal-to-crystal photochemical transformations entirely from first principles, and it establishes a photomechanical engine cycle that quantifies the anisotropic mechanical performance resulting from the transformation. The approach relies on crystal structure prediction, solid-state topochemical principles, and high-quality electronic structure methods. After validating the framework on the well-studied [4 + 4] cycloadditions in 9-methyl anthracene and 9-tert-butyl anthracene ester, the experimentally-unknown solid-state transformation of 9-carboxylic acid anthracene is predicted for the first time. The results illustrate how the mechanical work is done by relaxation of the crystal lattice to accommodate the photoproduct, rather than by the photochemistry itself. The large ∼107 J m-3 work densities computed for all three systems highlight the promise of photomechanical crystal engines. This study demonstrates the importance of crystal packing in determining molecular crystal engine performance and provides tools and insights to design improved materials in silico.

Structural and vibrational properties of wurtzite ZnO with oxygen-deficient defects: ab initio and potential-based calculations.

The structural and vibrational properties of the ZnO wurtzite phase with oxygen vacancies in different charged states are studied using first-principles and potential-based methods. The calculations based on density-functional theory are performed to determine the atomic configurations around defects. The DFT results are discussed and compared with those obtained using the static lattice method in the traditional shell model. Both computational approaches predict the same character of crystal lattice relaxation around oxygen vacancies. The phonon local symmetrized densities of states are calculated using the Green function method. The frequencies of localized vibrations of various symmetry types induced by oxygen vacancies in neutral and positively charged states are determined. The calculation results allow estimating the effect of oxygen vacancies on the formation of the intense Raman peak.

Twistronics of Janus transition metal dichalcogenide bilayers

Twisted multilayers of two-dimensional (2D) materials are an increasingly important platform for investigating quantum phases of matter, and in particular, strongly correlated electrons. The moir\'e pattern introduced by the relative twist between layers creates effective potentials of long wavelength, leading to electron localization. However, in contrast to the abundance of 2D materials, few twisted heterostructures have been studied until now. Here we develop a first-principles continuum theory to study the electronic bands introduced by moire patterns of twisted Janus transition metal dichalcogenide (TMD) homo- and heterobilayers. The model includes lattice relaxation, stacking-dependent effective mass, and Rashba spin-orbit coupling. We then perform a high-throughput generation and characterization of DFT-extracted continuum models for more than a hundred possible combinations of materials and stackings. Our model predicts that the moir\'e physics and emergent symmetries depend on chemical composition, vertical layer orientation, and twist angle, so that the miniband wave functions can form triangular, honeycomb, and kagome networks. Rashba spin-orbit effects, peculiar of these systems, can dominate the moir\'e bandwidth at small angles. Our work enables detailed investigation of Janus twisted heterostructures, allowing the discovery and control of novel electronic phenomena.

Moiré Potential, Lattice Relaxation, and Layer Polarization in Marginally Twisted MoS2 Bilayers.

Artificially twisted heterostructures of semiconducting transition-metal dichalcogenides (TMDs) offer unprecedented control over their electronic and optical properties via the spatial modulation of interlayer interactions and structural reconstruction. Here we study twisted MoS2 bilayers in a wide range of twist angles near 0° using scanning tunneling microscopy/spectroscopy. We investigate the twist angle dependence of the moiré pattern, which is dominated by lattice reconstruction for small angles (<2°), leading to large triangular domains with rhombohedral stacking. Local spectroscopy measurements reveal a large moiré-potential strength of 100-200 meV for angles <3°. In reconstructed regions, we see a bias-dependent asymmetry between neighboring triangular domains, which we relate to the vertical polarization that is intrinsic to rhombohedral stacked TMDs. This viewpoint is further supported by spectroscopy maps and ambient piezoresponse measurements. Our results provide a microscopic perspective of this new class of interfacial ferroelectrics and can offer clues for designing novel heterostructures that harness this effect.

Unraveling Water and Salt Transport in Polyamide with Nuclear Magnetic Resonance Spectroscopy

Quantifying water and salt transport properties in polyamide is of growing importance as the use of reverse osmosis membranes grows in many industries. Here, using solid-state nuclear magnetic resonance (NMR) spectroscopy, we measure the translational diffusion coefficients using pulsed-field gradient NMR, examine ion dynamics with NMR relaxometry, and determine the activation energy barriers of hydrogen and sodium ions in ion-exchanged polyamide using variable-temperature NMR. We identify two predominant diffusion components within the spectra associated with bound and unbound hydrogen and sodium ions. We show that the diffusion coefficient of the bound hydrogen ions decreases by ∼50% while the unbound hydrogen diffusion coefficient remains constant as the salinity of the mixture doubles. Conversely, the bound sodium diffusion coefficient remained constant while the unbound sodium diffusion coefficient decreased by ∼40% as the salinity of the mixture doubled. Through examining the spin–lattice relaxation time (T1) we also report the associated activation energy for sodium and hydrogen. We believe these molecular-scale measurements can aid in extending physics-based models of salt and water transport in high-pressure reverse osmosis applications.