Showing papers on "Relaxation (NMR) published in 2021"

Absence of local moments in the kagome metal KV3Sb5 as determined by muon spin spectroscopy

Abstract: We have carried out muon spin relaxation and rotation measurements on the newly discovered kagome metal KV3Sb5, and find a local field dominated by weak magnetic disorder which we associate with the nuclear moments present, and a modest temperature dependence which tracks the bulk magnetic susceptibility. We find no evidence for the existence of V4+local moments, suggesting that the physics underlying the recently reported giant unconventional anomalous Hall effect in this material warrants further studies.

Single-spin resonance in a van der Waals embedded paramagnetic defect.

Abstract: A plethora of single-photon emitters have been identified in the atomic layers of two-dimensional van der Waals materials1–8. Here, we report on a set of isolated optical emitters embedded in hexagonal boron nitride that exhibit optically detected magnetic resonance. The defect spins show an isotropic ge-factor of ~2 and zero-field splitting below 10 MHz. The photokinetics of one type of defect is compatible with ground-state electron-spin paramagnetism. The narrow and inhomogeneously broadened magnetic resonance spectrum differs significantly from the known spectra of in-plane defects. We determined a hyperfine coupling of ~10 MHz. Its angular dependence indicates an unpaired, out-of-plane delocalized π-orbital electron, probably originating from substitutional impurity atoms. We extracted spin–lattice relaxation times T1 of 13–17 μs with estimated spin coherence times T2 of less than 1 μs. Our results provide further insight into the structure, composition and dynamics of single optically active spin defects in hexagonal boron nitride. The optically detected magnetic resonance of a single defect in hexagonal boron nitride is reported.

Opening Magnetic Hysteresis by Axial Ferromagnetic Coupling: From Mono-Decker to Double-Decker Metallacrown

Abstract: Combining Ising-type magnetic anisotropy with collinear magnetic interactions in single-molecule magnets (SMMs) is a significant synthetic challenge. Herein we report a Dy[15-MCCu -5] (1-Dy) SMM, where a DyIII ion is held in a central pseudo-D5h pocket of a rigid and planar Cu5 metallacrown (MC). Linking two Dy[15-MCCu -5] units with a single hydroxide bridge yields the double-decker {Dy[15-MCCu -5]}2 (2-Dy) SMM where the anisotropy axes of the two DyIII ions are nearly collinear, resulting in magnetic relaxation times for 2-Dy that are approximately 200 000 times slower at 2 K than for 1-Dy in zero external field. Whereas 1-Dy and the YIII -diluted Dy@2-Y analogue do not show remanence in magnetic hysteresis experiments, the hysteresis data for 2-Dy remain open up to 6 K without a sudden drop at zero field. In conjunction with theoretical calculations, these results demonstrate that the axial ferromagnetic Dy-Dy coupling suppresses fast quantum tunneling of magnetization (QTM). The relaxation profiles of both complexes curiously exhibit three distinct exponential regimes, and hold the largest effective energy barriers for any reported d-f SMMs up to 625 cm-1 .

Importance of Broad Temperature Windows and Multiple Rheological Approaches for Probing Viscoelasticity and Entropic Elasticity in Vitrimers

Abstract: Model polydimethylsiloxane telechelic vitrimers with dynamic boronic ester bonds were synthesized to investigate the viscoelastic properties of dynamic networks with extremely low Tg via multiple rheological approaches. Frequency sweeps and stress relaxation tests, conducted at more than 120 °C above Tg, show the anticipated Arrhenius behavior of relaxation time with inverse temperature and give the same activation energy for a fixed molecular weight, obtained using a variety of analysis methods. Time–temperature superposition demonstrates that the flow regime is thermorheologically simple, while the modulus of the plateau regime increases with increasing temperature, consistent with a conserved network topology and associative bond exchange. As relaxation times decrease, the rubbery plateau modulus increases, indicating a decoupling of terminal dynamics from mechanics. Below 40 °C, a second Arrhenius regime with lower activation energy emerges, which is attributed to a transition from relaxation dominated by reaction exchange kinetics to relaxation dictated by local polymer dynamics. Our work points to the importance of assessing a broad temperature window and using multiple approaches in probing vitrimers and dynamic networks.

Direct observation of ultrafast hydrogen bond strengthening in liquid water

Abstract: Water is one of the most important, yet least understood, liquids in nature. Many anomalous properties of liquid water originate from its well-connected hydrogen bond network1, including unusually efficient vibrational energy redistribution and relaxation2. An accurate description of the ultrafast vibrational motion of water molecules is essential for understanding the nature of hydrogen bonds and many solution-phase chemical reactions. Most existing knowledge of vibrational relaxation in water is built upon ultrafast spectroscopy experiments2–7. However, these experiments cannot directly resolve the motion of the atomic positions and require difficult translation of spectral dynamics into hydrogen bond dynamics. Here, we measure the ultrafast structural response to the excitation of the OH stretching vibration in liquid water with femtosecond temporal and atomic spatial resolution using liquid ultrafast electron scattering. We observed a transient hydrogen bond contraction of roughly 0.04 A on a timescale of 80 femtoseconds, followed by a thermalization on a timescale of approximately 1 picosecond. Molecular dynamics simulations reveal the need to treat the distribution of the shared proton in the hydrogen bond quantum mechanically to capture the structural dynamics on femtosecond timescales. Our experiment and simulations unveil the intermolecular character of the water vibration preceding the relaxation of the OH stretch. Liquid ultrafast electron scattering measures structural responses in liquid water with femtosecond temporal and atomic spatial resolution to reveal a transient hydrogen bond contraction then thermalization preceding relaxation of the OH stretch.

Collective Vibrational Strong Coupling Effects on Molecular Vibrational Relaxation and Energy Transfer: Numerical Insights via Cavity Molecular Dynamics Simulations*.

Abstract: For a small fraction of hot CO2 molecules immersed in a liquid-phase CO2 thermal bath, classical cavity molecular dynamics simulations show that forming collective vibrational strong coupling (VSC) between the C=O asymmetric stretch of CO2 molecules and a cavity mode accelerates hot-molecule relaxation. This acceleration stems from the fact that polaritons can be transiently excited during the nonequilibrium process, which facilitates intermolecular vibrational energy transfer. The VSC effects on these rates 1) resonantly depend on the cavity mode detuning, 2) cooperatively depend on Rabi splitting, and 3) collectively scale with the number of hot molecules. For larger cavity volumes, the average VSC effect per molecule can remain meaningful for up to N≈104 molecules forming VSC. Moreover, the transiently excited lower polariton prefers to relax by transferring its energy to the tail of the molecular energy distribution rather than distributing it equally to all thermal molecules. As far as the parameter dependence is concerned, the vibrational relaxation data presented here appear analogous to VSC catalysis in Fabry-Perot microcavities.

Effect of rotation and magnetic field on a numerical-refined heat conduction in a semiconductor medium during photo-excitation processes

Abstract: A novel model in photo-thermoelasticity theory is investigated in the paper understudy. The model is obtained theoretically for a semiconductor elastic medium, which is in a rotation case. The interaction between main physical quantities during photothermal transport process is expressed in the governing equations. In addition, the numerical-refined multi-phase-lags relaxation times (thermal memories) are studied in the context of the heat equation when the medium is exposed to an external magnetic field. Moreover, the harmonic wave method in two-dimensional (2D) is introduced during the coupling processes between multi-waves. As such, the complete exact solutions of the main physical fields of semi-infinite semiconductor medium are obtained. Some plasma, mechanical and thermal forces are applied at the outer surface of the elastic medium to determine the unknown parameters. Many comparisons are displayed graphically when the physical constants of silicon (Si) material are used. Theoretical results are discussed under the impact of magnetic field and rotation field.

An unusual mechanism of building up of a high magnetization blocking barrier in an octahedral alkoxide Dy3+-based single-molecule magnet

Abstract: We report a new octahedral [Dy(OCPh3)2(THF)4][BPh4] luminescent Single-Molecule Magnet (SMM) exhibiting massive crystal-field splitting and an anisotropic barrier of 1385 cm−1. Magnetic measurements combined with ab initio analysis reveal a novel mechanism behind the high blocking barrier based on the quenching of one-phonon transitions between the three low-lying crystal-field multiplets due to large energy gaps between them exceeding the available phonon energies and forcing the activated relaxation to proceed through the fourth doublet. The observed nonetheless short relaxation time is due to appreciable non-axial anisotropy, which opens a tunnelling relaxation path via interaction with the nuclear spins. Reducing the equatorial crystal-field quenches drastically the quantum tunnelling of magnetization, allowing for full exploitation of the high blocking barrier of the complex as in the best known SMMs.

Valley relaxation of resident electrons and holes in a monolayer semiconductor: Dependence on carrier density and the role of substrate-induced disorder

Abstract: Analogous to the keen interest in electron, hole, and exciton spin relaxation during the early days of semiconductor spintronics, measurements of valley relaxation in monolayer transition-metal dichalcogenide (TMD) semiconductors such as WSe2 are currently a focus of attention for potential applications in valleytronics. For many notional valleytronic devices, the important parameter is the intrinsic valley relaxation time of the resident electrons and holes that exist in n-type and p-type TMD monolayers. Using optical methods, the authors determine these timescales as a systematic function of carrier density, and study the (important) role of the underlying substrate. Microsecond-long valley relaxation of carriers is revealed at low densities.

Experimental test of a predicted dynamics–structure–thermodynamics connection in molecularly complex glass-forming liquids

Abstract: Understanding in a unified manner the generic and chemically specific aspects of activated dynamics in diverse glass-forming liquids over 14 or more decades in time is a grand challenge in condensed matter physics, physical chemistry, and materials science and engineering. Large families of conceptually distinct models have postulated a causal connection with qualitatively different "order parameters" including various measures of structure, free volume, thermodynamic properties, short or intermediate time dynamics, and mechanical properties. Construction of a predictive theory that covers both the noncooperative and cooperative activated relaxation regimes remains elusive. Here, we test using solely experimental data a recent microscopic dynamical theory prediction that although activated relaxation is a spatially coupled local-nonlocal event with barriers quantified by local pair structure, it can also be understood based on the dimensionless compressibility via an equilibrium statistical mechanics connection between thermodynamics and structure. This prediction is found to be consistent with observations on diverse fragile molecular liquids under isobaric and isochoric conditions and provides a different conceptual view of the global relaxation map. As a corollary, a theoretical basis is established for the structural relaxation time scale growing exponentially with inverse temperature to a high power, consistent with experiments in the deeply supercooled regime. A criterion for the irrelevance of collective elasticity effects is deduced and shown to be consistent with viscous flow in low-fragility inorganic network-forming melts. Finally, implications for relaxation in the equilibrated deep glass state are briefly considered.

Fundamental limits of electron and nuclear spin qubit lifetimes in an isolated self-assembled quantum dot

Abstract: Combining external control with long spin lifetime and coherence is a key challenge for solid state spin qubits. Tunnel coupling with electron Fermi reservoir provides robust charge state control in semiconductor quantum dots, but results in undesired relaxation of electron and nuclear spins through mechanisms that lack complete understanding. Here, we unravel the contributions of tunnelling-assisted and phonon-assisted spin relaxation mechanisms by systematically adjusting the tunnelling coupling in a wide range, including the limit of an isolated quantum dot. These experiments reveal fundamental limits and trade-offs of quantum dot spin dynamics: while reduced tunnelling can be used to achieve electron spin qubit lifetimes exceeding 1 s, the optical spin initialisation fidelity is reduced below 80%, limited by Auger recombination. Comprehensive understanding of electron-nuclear spin relaxation attained here provides a roadmap for design of the optimal operating conditions in quantum dot spin qubits.

Nutation in antiferromagnetic resonance

Abstract: The effect of inertial spin dynamics is compared between ferromagnetic, antiferromagnetic, and ferrimagnetic systems. The linear response to an oscillating external magnetic field is calculated within the framework of the inertial Landau-Lifshitz-Gilbert equation using analytical theory and computer simulations. Precession and nutation resonance peaks are identified, and it is demonstrated that the precession frequencies are reduced by the spin inertia, while the lifetime of the excitations is enhanced. The interplay between precession and nutation is found to be the most prominent in antiferromagnets, where the timescale of the exchange-driven sublattice dynamics is comparable to inertial relaxation times. Consequently, antiferromagnetic resonance techniques should be better suited for the search for intrinsic inertial spin dynamics on ultrafast timescales than ferromagnetic resonance.

Is the structural relaxation of glasses controlled by equilibrium shear viscosity

Abstract: The knowledge of relaxation processes is fundamental in glass science and technology because relaxation is intrinsically related to vitrification, tempering, and annealing and several applications of glasses. However, there are conflicting reports on whether the structural relaxation time of glass can be calculated using the Maxwell equation, which relates the relaxation time with the shear viscosity and shear modulus. Hence, the objective of this work was to test whether these two relaxation times are comparable. We studied the kinetics of structural relaxation of a lead metasilicate glass by measurements of the refractive index variation over time at temperatures between 5 and 25 K below the fictive temperature, which was initially set 5 K below the glass transition temperature. We also measured the equilibrium shear viscosity above and below the glass transition range, expanding the current knowledge by one order of magnitude. We found that the Kohlrausch equation describes very well the experimental structural relaxation kinetics in the whole temperature range and the Kohlrausch exponent increases with temperature, in agreement with studies for other glasses. The experimental average structural relaxation times are much longer than the values computed from isostructural viscosity, as expected. Still, they are less than one order of magnitude higher than the average relaxation time computed by the Maxwell equation. Thus, these results demonstrate that the structural relaxation process is not controlled by isostructural viscosity and that the equilibrium shear viscosity only gives a lower boundary for structural relaxation kinetics.

Structural, dielectric, electrical and modulus spectroscopic characteristics of CoFeCuO4 spinel ferrite nanoparticles

Abstract: In this work, our focus is upon the synthesis of material having useful and less expensive high frequency applications. For this purpose, we used the sol–gel method to elaborate the Cu-substituted CoFe2O4. The XRD analysis confirmed the single-phase cubic spinel structure with the Fd 3 - m space group. The grain size distribution was traced through different methods. It confirms the nano-metric size of the compound. To elaborate the electrical and dielectric properties of the prepared sample, impedance spectroscopy was invested. It demonstrated the relaxing nature of our material and indicated that the substitution of Fe by Cu on CoFe2O4 increases the ac conductivity and decreases the dielectric loss especially at higher frequencies; the low loss values indicate the potential of these ferrites for high frequency applications at temperatures close to the room temperature. The activation energy value was estimated based on the relaxation time and the conductance curves analysis, the found value is around 0.33 eV which confirms the semiconductor character of our compound and its capacity to achieve a significant gain in productivity. Moreover, this obtained value indicates the relationship between the electrical conductivity mechanism and the relaxation one.

Translational and reorientational dynamics in deep eutectic solvents.

Abstract: We performed rheological measurements of the typical deep eutectic solvents (DESs) glyceline, ethaline, and reline in a very broad temperature and dynamic range, extending from the low-viscosity to the high-viscosity supercooled-liquid regime. We find that the mechanical compliance spectra can be well described by the random free-energy barrier hopping model, while the dielectric spectra on the same materials involve significant contributions arising from reorientational dynamics. The temperature-dependent viscosity and structural relaxation time, revealing non-Arrhenius behavior typical for glassy freezing, are compared to the ionic dc conductivity and relaxation times determined by broadband dielectric spectroscopy. For glyceline and ethaline, we find essentially identical temperature dependences for all dynamic quantities. These findings point to a close coupling of the ionic and molecular translational and reorientational motions in these systems. However, for reline, the ionic charge transport appears decoupled from the structural and reorientational dynamics, following a fractional Walden rule. In particular, at low temperatures, the ionic conductivity in this DES is enhanced by about one decade compared to expectations based on the temperature dependence of the viscosity. The results for all three DESs can be understood without invoking a revolving-door mechanism previously considered as a possible charge-transport mechanism in DESs.

Lattice relaxation, mirror symmetry and magnetic field effects on ultraflat bands in twisted trilayer graphene

Abstract: Twisted graphene multilayers exhibit strongly correlated insulating states and superconductivity due to the presence of ultraflat bands near the charge neutral point. In this paper, the response of ultraflat bands to lattice relaxation and a magnetic field in twisted trilayer graphene (tTLG) with different stacking arrangements is investigated by using a full tight-binding model. We show that lattice relaxations are indispensable for understanding the electronic properties of tTLG, in particular, of tTLG in the presence of mirror symmetry. Lattice relaxations renormalize the quasiparticle spectrum near the Fermi energy and change the localization of higher energy flat bands. Furthermore, different from the twisted bilayer graphene, the Hofstadter butterfly spectrum can be realized at laboratory accessible strengths of magnetic field. Our work verifies tTLG as a more tunable platform than the twisted bilayer graphene in strongly correlated phenomena.

Fitting Side-Chain NMR Relaxation Data Using Molecular Simulations.

Abstract: Proteins display a wealth of dynamical motions that can be probed using both experiments and simulations. We present an approach to integrate side-chain NMR relaxation measurements with molecular dynamics simulations to study the structure and dynamics of these motions. The approach, which we term ABSURDer (average block selection using relaxation data with entropy restraints), can be used to find a set of trajectories that are in agreement with relaxation measurements. We apply the method to deuterium relaxation measurements in T4 lysozyme and show how it can be used to integrate the accuracy of the NMR measurements with the molecular models of protein dynamics afforded by the simulations. We show how fitting of dynamic quantities leads to improved agreement with static properties and highlight areas needed for further improvements of the approach.

Electrokinetic oscillatory flow and energy conversion of viscoelastic fluids in microchannels: a linear analysis

Abstract: We study the electrokinetic flow of viscoelastic fluids subjected to an oscillatory pressure gradient, and particularly focus on the resonance behaviors in the flow. The governing equations are restricted to linear regime so that the velocity and streaming potential fields can be solved analytically. Based on the interaction of viscoelastic shear waves, we explain the mechanism of resonance, and derive a critical Deborah number Dec = 1/4 which dictates the occurrence of resonance. Using the Maxwell fluid model, we show that the resonance enhances electrokinetic effects and results in a dramatic increase of electrokinetic energy conversion efficiency. However, by applying the Oldroyd-B fluid model it reveals that the amplification of efficiency is suppressed even for a very small Newtonian solvent contribution. This may be one of the reasons that experimental verification regarding the high efficiency predicted by Bandopadhyay & Chakraborty (Appl. Phys. Lett., vol. 101, 2012, 043905) is unavailable in the literature. Furthermore, the damping effect of solvent viscosity is more significant for higher-order resonances. Introducing the factor of multiple relaxation times, we show that the occurrence of resonances for the streaming potential field and the flow rate are still dominated by Dec. For the efficiency in the multi-mode case, the occurrence of resonance is dominated by the Deborah number De and the mode number N, and the resonance disappears for small De or large N. In addition, a new type of scaling relation between the streaming potential field and EDL thickness can be identified at large De.

Significantly enhanced dielectric properties and chain segmental dynamics of PEO/SnO2 nanocomposites

Abstract: Poly(ethylene oxide) (PEO) matrix- and tin oxide (SnO2) nanofiller-based polymer nanocomposite films were prepared by solution cast followed by melt press. The effect of SnO2 concentration (i.e. 1, 3, and 5 wt%) on the structure, dielectric permittivity, and chain segmental motion of PEO was investigated with X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and dielectric relaxation spectroscopy (DRS) techniques. Results reveal that the dispersion of SnO2 nanoparticles has enormously altered the contents of different crystallite phases of the PEO and some amount of the degree of crystallinity. The DRS study in the range of 20 Hz–1 MHz at 30 °C confirmed that the complex dielectric permittivity and electrical conductivity at lower frequencies increased largely on the dispersion of SnO2 nanoparticles in the PEO matrix which was attributed to the contribution of interfacial polarization effect, moreover, a significant increase was also noted in the molecular polarization at higher frequencies. The relaxation processes corresponding to Maxwell–Wagner–Sillars mechanism and PEO chain segmental motion were analysed from the electric modulus spectra. The chain segmental dynamics and dc electrical conductivity have nonlinearly enhanced with the increase in SnO2 content in the polymer structure. The temperature-dependent dielectric and relaxation behaviour of PEO-3 wt% SnO2 film was also reported. It was observed that the relaxation time and electrical conductivity of the film exhibited Arrhenius behaviour of low activation energies.