Large Relaxation Rate Research Articles

Increasing the accuracy of exchange parameters reporting on slow dynamics by performing CEST experiments with ‘high’ B1 fields

Over the last decade chemical exchange saturation transfer (CEST) NMR methods have emerged as powerful tools to characterize biomolecular conformational dynamics occurring between a visible major state and ‘invisible’ minor states. The ability of the CEST experiment to detect these minor states, and provide precise exchange parameters, hinges on using appropriate B1 field strengths during the saturation period. Typically, a pair of B1 fields with ω1 (=2πB1) values around the exchange rate kex are chosen. Here we show that the transverse relaxation rate of the minor state resonance (R2,B) also plays a crucial role in determining the B1 fields that lead to the most informative datasets. Using K=kexkex+R2,B12 ≥ kex, to guide the choice of B1, instead of kex, leads to data wherefrom substantially more accurate exchange parameters can be derived. The need for higher B1 fields, guided by K, is demonstrated by studying the conformational exchange in two mutants of the 71 residue FF domain with kex ∼ 11 s−1 and ∼ 72 s−1, respectively. In both cases analysis of CEST datasets recorded using B1 field values guided by kex lead to imprecise exchange parameters, whereas using B1 values guided by K resulted in precise site-specific exchange parameters. The conclusions presented here will be valuable while using CEST to study slow processes at sites with large intrinsic relaxation rates, including carbonyl sites in small to medium sized proteins, amide 15N sites in large proteins and when the minor state dips are broadened due to exchange among the minor states.

Near infrared emissions from both high efficient quantum cutting (173%) and nearly-pure-color upconversion in NaY(WO4)2:Er3+/Yb3+ with thermal management capability for silicon-based solar cells

Raising photoelectric conversion efficiency and enhancing heat management are two critical concerns for silicon-based solar cells. In this work, efficient Yb3+ infrared emissions from both quantum cutting and upconversion were demonstrated by adjusting Er3+ and Yb3+ concentrations, and thermo-manage-applicable temperature sensing based on the luminescence intensity ratio of two super-low thermal quenching levels was discovered in an Er3+/Yb3+ co-doped tungstate system. The quantum cutting mechanism was clearly decrypted as a two-step energy transfer process from Er3+ to Yb3+. The two-step energy transfer efficiencies, the radiative and nonradiative transition rates of all interested 4 f levels of Er3+ in NaY(WO4)2 were confirmed in the framework of Föster-Dexter theory, Judd-Ofelt theory, and energy gap law, and based on these obtained efficiencies and rates the quantum cutting efficiency was furthermore determined to be as high as 173% in NaY(WO4)2: 5 mol% Er3+/50 mol% Yb3+ sample. Strong and nearly pure infrared upconversion emission of Yb3+ under 1550 nm excitation was achieved in Er3+/Yb3+ co-doped NaY(WO4)2 by adjusting Yb3+ doping concentrations. The Yb3+ induced infrared upconversion emission enhancement was attributed to the efficient energy transfer 4I11/2 (Er3+) + 2F7/2 (Yb3+) → 4I15/2 (Er3+) + 2F5/2 (Yb3+) and large nonradiative relaxation rate of 4I9/2. Analysis on the temperature sensing indicated that the NaY(WO4)2:Er3+/Yb3+ serves well the solar cells as thermos-managing material. Moreover, it was confirmed that the fluorescence thermal quenching of 2H11/2/4S3/2 was caused by the nonradiative relaxation of 4S3/2. All the obtained results suggest that NaY(WO4)2:Er3+/Yb3+ is an excellent material for silicon-based solar cells to improve photoelectric conversion efficiency and thermal management.

Anomalously small magnetic relaxation rate of Ca10(Pt3As8)(Fe2−x Pt x As2)5 superconductor

Large critical current density and low magnetic relaxation are the main challenges for practical applications of high-temperature superconductors (HTSs). Magnetic relaxation refers to the decay of superconducting current density caused by thermal activation and/or quantum fluctuations. This process involves the motion of vortices, particularly vortex creep, which leads to energy dissipation. HTSs typically exhibit strong superconducting fluctuations and magnetic relaxation due to their short coherence length and large anisotropy. These characteristics are usually manifested by the large Ginzburg number Gi and high magnetic relaxation rate S. The correlation between the relaxation rate S and Gi is informative to understand the interplay between vortex dynamics and relevant parameters. Recently, Eley et al (2017 Nat. Mater. 16 409) proposed that almost all the HTSs follow a universal lower limit S∼ Gi 1/2(T/T c), providing new clues to design HTSs with small relaxation rate and evaluate their application potential. Here, we systematically studied the vortex dynamics in the Ca10(Pt3As8)(Fe2−x Pt x As2)5 superconductor with a large Gi∼ 0.16. Strikingly, this material exhibits a small relaxation rate (S ∼0.02 at T c/4 under 1 T), approaching the proposed lower limit of S∼ Gi 1/2(T/T c). We propose that such a small value of S in Ca10(Pt3As8)(Fe2−x Pt x As2)5 may originate from its unique structure with metallic skutterudite blocking layers. Our results suggest a promising new avenue for the search and design of HTSs with low magnetic relaxation.

Characterisation of magnetic relaxation on extremely long timescales.

The use of magnetisation decay measurements to characterise very slow relaxation of the magnetisation in single-molecule magnets is becoming increasingly prevalent as relaxation times move to longer timescales outside of the AC susceptibility range. However, experimental limitations and a poor understanding of the distribution underlying the stretched exponential function, commonly used to model the data, may be leading to misinterpretation of the results. Herein we develop guidelines on the experimental design, data fitting, and analysis required to accurately interpret magnetisation decay measurements. Various measures of the magnetic relaxation rate extracted from magnetisation decay measurements of [Dy(Dtp)2][Al{OC(CF3)3}4] previously characterised by Evans et al., fitted using combinations of fixing or freely fitting different parameters, are compared to those obtained using the innovative square-wave "waveform" technique of Hilgar et al. The waveform technique is comparable to AC susceptometry for measurement of relaxation rates on long timescales. The most reliable measure of the relaxation time for magnetisation decays is found to be the average logarithmic relaxation time, e〈ln[τ]〉, obtained via a fit of the decay trace using a stretched exponential function, where the initial and equilibrium magnetisation are fixed to first measured point and target values respectively. This new definition causes the largest differences to traditional approaches in the presence of large distributions or relaxation rates, with differences up to 50% with β = 0.45, and hence could have a significant impact on the chemical interpretation of magnetic relaxation rates. A necessary step in progressing towards chemical control of magnetic relaxation is the accurate determination of relaxation times, and such large variations in experimental measures stress the need for consistency in fitting and interpretation of magnetisation decays.

Rabi oscillation of spin-polarized rubidium in the spin-exchange relaxation-free regime.

The transient dynamics of atomic spins under oscillating and static magnetic fields have been studied in the spin-exchange relaxation-free (SERF) regime with a dual-beam configuration. The spin-relaxation rate can be accurately measured by detecting the transient response signal of the free induction decay (FID) process within several milliseconds. Leveraging this convenient method for measuring a large relaxation rate in a small cell volume, the dependence of the spin-relaxation rate on the probe intensity and ambient magnetic field was studied in the limit of low spin polarization. Moreover, by theoretical analysis of the dynamic evolution of the Rabi oscillation generated by a consecutive oscillating field and a small static magnetic field, we experimentally demonstrate that the amplitude of the Rabi oscillation is affected by the amplitude of the oscillating field in the SERF regime. According to the retrieved frequency of the FID signal and amplitude of relevant Rabi oscillation, the coil constants were 75.55 ± 0.78~nT/mA, 151.5 ± 0.9~nT/mA, and 116.6 ± 0.3~nT/mA along the x-, y-, and z-axes, respectively.

Transition of vortex pinning behaviour induced by an artificial microstructure design in Ba(Fe0.94Co0.06)2As2 pnictide superconductor

Among various systems of iron-based superconductors, the doped Ba122 polycrystals have shown the largest application value because of their brilliant properties and chemical stability. Since there have been few reports on the vortex dynamics in Ba122 polycrystal, we firstly conduct a comparative investigation on the differences of vortex dynamics in Ba(Fe0.94Co0.06)2As2 single crystal, grain oriented polycrystal and random oriented powder in this paper. Herein, the polycrystal, which has c-axis grain texture, is prepared by the compaction of designed single crystal powder. The second magnetization peak (SMP) effects are observed in the hysteresis loops of single crystal and totally disappear in those of polycrystal and powder. Conventional magnetization relaxation and Maley's method are applied to analyse the vortex dynamics. In single crystal, the SMP effect is primarily related to the crossover of elastic-plastic vortex creep. The weak pinning in single vortex regime results in large magnetization relaxation rates (S) in low magnetic field regime. In polycrystal for H//AD (axial direction), the introduction of strong pins is effective to single vortex pinning, which brings about much lower S values in low magnetic field regime. Furthermore, the results of single crystal for H//ab, polycrystal for H//RD (radial direction) and random oriented powder are analyzed. Synergistic effects of strong pins and microscopic anisotropy on the vortex dynamics are discussed. These results will provide new knowledge for the design of high-performance Ba122 superconductors.

Cavity-mediated level attraction and repulsion between magnons

We characterize some of the distinctive hallmarks of magnon-magnon interaction mediated by the intracavity field of a microwave cavity, along with their testable ramifications. In general, we foreground two widely dissimilar parameter domains that bring forth the contrasting possibilities of level splitting and level crossing. The former is observed in the regime of strong magnon-photon couplings, particularly when the three modes bear comparable relaxation rates. This character is marked by the appearance of three distinguishable and nonconverging polariton branches in the spectral response to a cavity drive. However, when the bare modes are resonant and the couplings are perfectly symmetrical, one of the spectral peaks gets wiped out. This anomalous extinction of polaritonic response can be traced down to the existence of a conspicuous dark mode alongside two frequency-shifted bright modes. In an alternate parameter regime, where the magnon modes are weakly coupled to the cavity, features of level attraction unfold, subject to a large relaxation rate for the cavity mode. Concurrently, for antisymmetric detunings to the magnon modes, a transmission window springs into existence, exhibiting transparency in the limit of negligible dissipation from the magnons. The emergence of level attraction can be reconciled with a theoretical model that embodies the dynamics of the magnon-magnon subsystem when the cavity field decays rapidly into its steady state. In this limit, we identify a purely dissipative coupling between the magnon modes.

Exciton Delocalization Counteracts the Energy Gap: A New Pathway toward NIR-Emissive Dyes.

Exciton coupling between the transition dipole moments of ordered dyes in supramolecular assemblies, so-called J/H-aggregates, leads to shifted electronic transitions. This can lower the excited state energy, allowing for emission well into the near-infrared regime. However, as we show here, it is not only the excited state energy modifications that J-aggregates can provide. A bay-alkylated quaterrylene was synthesized, which was found to form J-aggregates in 1,1,2,2-tetrachloroethane. A combination of superradiance and a decreased nonradiative relaxation rate made the J-aggregate four times more emissive than the monomeric counterpart. A reduced nonradiative relaxation rate is a nonintuitive consequence following the 180 nm (3300 cm–1) red-shift of the J-aggregate in comparison to the monomeric absorption. However, the energy gap law, which is commonly invoked to rationalize increased nonradiative relaxation rates with increasing emission wavelength, also contains a reorganization energy term. The reorganization energy is highly suppressed in J-aggregates due to exciton delocalization, and the framework of the energy gap law could therefore reproduce our experimental observations. J-Aggregates can thus circumvent the common belief that lowering the excited state energies results in large nonradiative relaxation rates and are thus a pathway toward highly emissive organic dyes in the NIR regime.

Resonant transmission of fermionic carriers: Comparison between solid-state physics and quantum optics approaches

We revisit the phenomenon of the resonant transmission of fermionic carriers through a quantum device connected to two contacts with different chemical potentials. We show that, besides the traditional Landauer-B\"uttiker approach in solid-state physics, this phenomenon can also be described by the non-Markovian master equation for the reduced density matrix of the fermions in the quantum device. We identify validity regions for both approaches in the system parameter space and argue that for large relaxation rates the accuracy of the latter approach greatly exceeds the accuracy of the former.

Measuring radiofrequency fields in NMR spectroscopy using offset-dependent nutation profiles

The application of NMR spectroscopy for studying molecular and reaction dynamics relies crucially on the measurement of the magnitude of radiofrequency (RF) fields that are used to nutate or lock the nuclear magnetization. Here, we report a method for measuring RF field amplitudes that leverages the intrinsic modulations observed in offset-dependent NMR nutation profiles of small molecules. Such nutation profiles are exquisitely sensitive to the magnitude of the RF field, and B1 values ranging from 1 to 2000 Hz, as well the inhomogeneity in B1 distributions, can be determined with high accuracy and precision using this approach. In order to measure B1 fields associated with NMR experiments carried out on protein or nucleic acids, where these modulations are obscured by the large transverse relaxation rate constants of the analyte, our approach can be used in conjunction with a suitable external small molecule standard, expanding the scope of the method for large biomolecules.

Relaxivity of Gd‐Based MRI Contrast Agents in Crosslinked Hyaluronic Acid as a Model for Tissues

The efficiency of MRI contrast agents depends on the relaxation rate enhancement that they can induce at imaging fields. It is well known that, at these fields, large relaxation rates are obtained by binding of gadolinium(III) ions to large molecules. By the same token, the interaction of the gadolinium(III) complexes with macromolecules that are found in biological tissues can be responsible for an increase of the relaxation rate with respect to the value observed in liquids. We investigate here the relaxation enhancement of gadoteridol (Gd-HP-DO3A) in crosslinked hyaluronic acid, taken as model tissue, using fast field-cycling relaxometry. The analysis of the relaxation profiles as a function of the magnetic fields indicates that a sizable increase in the relaxation rates is due to a modest interaction of the contrast agent with the hydrogel and to the slower mobility of the water molecules outside the first-coordination sphere of the gadolinium(III) ion.

In vivo measurements of irreversible and reversible transverse relaxation rates in human basal ganglia at 7 T: making inferences about the microscopic and mesoscopic structure of iron and calcification deposits

The goal of this study was to measure irreversible and reversible transverse relaxation rates in the globus pallidus and putamen at 7T, and to use these rates to make inferences about the sub-voxel structure of iron and calcification deposits. Gradient Echo Sampling of a Spin Echo (GESSE) data were acquired at 7T on eighteen volunteers spanning a large range of ages (23-85years), with calcifications in the globus pallidus incidentally observed in one volunteer. Maps of transverse relaxation rates were derived from the GESSE data, and the mean value of these rates in globus pallidus and putamen was estimated for each volunteer. Both irreversible and reversible transverse relaxation rates increased with the expected age-dependent iron content in these structures, except for the individual with calcifications for whom extremely large reversible relaxation rates but normal irreversible relaxation rates were found in the globus pallidus. Given the sensitivity of irreversible and reversible transverse relaxation rates to microscopic and mesoscopic field variations, respectively, our findings suggest that joint consideration of these rates may yield information not only about the amount of iron and calcification deposited in the brain, but also about the sub-voxel structure of these deposits, perhaps revealing certain aspects of their geometry and cellular distribution.

Dynamics of the intrinsically disordered protein NUPR1 in isolation and in its fuzzy complexes with DNA and prothymosin α

Intrinsically disordered proteins (IDPs) explore diverse conformations in their free states and, a few of them, also in their molecular complexes. This functional plasticity is essential for the function of IDPs, although their dynamics in both free and bound states is poorly understood. NUPR1 is a protumoral multifunctional IDP, activated during the acute phases of pancreatitis. It interacts with DNA and other IDPs, such as prothymosin α (ProTα), with dissociation constants of ~0.5 μM, and a 1:1 stoichiometry. We studied the structure and picosecond-to-nanosecond (ps-ns) dynamics by using both NMR and SAXS in: (i) isolated NUPR1; (ii) the NUPR1/ProTα complex; and (iii) the NUPR1/double stranded (ds) GGGCGCGCCC complex. Our SAXS findings show that NUPR1 remained disordered when bound to either partner, adopting a worm-like conformation; the fuzziness of bound NUPR1 was also pinpointed by NMR. Residues with the largest values of the relaxation rates (R1, R1ρ, R2 and ηxy), in the free and bound species, were mainly clustered around the 30s region of the sequence, which agree with one of the protein hot-spots already identified by site-directed mutagenesis. Not only residues in this region had larger relaxation rates, but they also moved slower than the rest of the molecule, as indicated by the reduced spectral density approach (RSDA). Upon binding, the energy landscape of NUPR1 was not funneled down to a specific, well-folded conformation, but rather its backbone flexibility was kept, with distinct motions occurring at the hot-spot region.

New mechanism of plasmons specific for spin-polarized nanoparticles

Here it is experimentally shown that Co nanoparticles with a single-domain crystal structure support a plasmon resonance at approximately 280 nm with better quality than gold nanoparticle resonance in the visible. Magnetic nature of the nanoparticles suggests a new type of these plasmons. The exchange interaction of electrons splits the energy bands between spin-up electrons and spin-down electrons. It makes it possible for coexistence of two independent channels of conductivity as well as two independent plasmons in the same nanoparticle with very different electron relaxation. Indeed, the density of empty states in a partially populated d-band is high, resulting in a large relaxation rate of the spin-down conduction electrons and consequently in low quality of the plasmon resonance. In contrast, the majority electrons with a completely filled d-band do not provide final states for the scattering processes of the conduction spin-up electrons, therefore supporting a high quality plasmon resonance. The scattering without spin flip is required to keep these two plasmons independent.

Experimental study on stress relaxation properties of structural cables

Cable stress relaxation causes tension loss at high stress in high-strength structural cables, thereby influencing the stiffness and strength of cable-supported or tension structures. Previous studies have mainly focused on steel wires (SWs) and spiral strands; however, several new types of structural cable, such as semi-parallel wire strands (SPWS), Galfan spiral strands (GSSs), and steel tie rods, have emerged and have been widely applied in recent decades. The axial force during normal service state occasionally reaches 55% of the ultimate tensile strength. Relaxation data on structural cables are rare and are urgently required for structural design and lifetime safety evaluation. Therefore, experimental studies on five types of structural cable were conducted, in which the initial stress levels were 70%, 55%, and 40% of the ultimate tensile strength. Three different working ambient temperatures of 15 ± 2 °C, 20 ± 2 °C, and 25 ± 2 °C were considered. Special onsite anchoring methods and anchorage devices were established and proven effective. Results showed that logarithmic expression could well express the relationship between relaxation rate and time duration. The relaxation rates of SPWSs were larger than those of SWs. GSSs had considerably larger relaxation rates than other cable types due to their material properties and special twist characteristic. The relaxation rates of all types of cable increased with the initial stress and temperature. The recommended values for the 50-year relaxation rates for different structural cables were predicted and calculated.

An innovative approach for numerical simulation of stress relaxation of structural cables

Relaxation test on cable with large diameter is usually difficult and money consuming due to its extremely large ultimate tensile strength. A simplified finite element modeling method for stress relaxation simulation of steel wire, semi-parallel wire strands and spiral strands was proposed. Simplified relationship between stress relaxation and creep of steel wires was derived and a simplified modeling method on spiral wire composition was proposed. The whole numerical approach incorporated the steel wire creep model and the simplified cable compositions to realize the simulation of stress relaxation behavior of structural cables. The effectiveness of proposed creep calibration and cable stress relaxation simulations were verified through relaxation test data of single-wires and 19-wire semi-parallel wire strands. The 19-wire, 37-wire, 61-wire, 91-wire and 127-wire semi-parallel wire strands and spiral strands at different initial stress levels were all simulated. Due to the spiral wire composition and corresponding gradient wire stress distribution under axial tension, relaxation rate of semi-parallel wire strands and spiral strands were all larger than that of single steel wire. Semi-parallel wire strands presented a decreasing relaxation rate with the increase of cable dimension. Spiral strands had larger relaxation rate than similar-sized semi-parallel wire strands, and present a slight relaxation increase trend with cable dimension. The proposed finite element modeling method can realize the stress relaxation analysis of large diameter structural cables, with only stress relaxation test data of single wires or small diameter cables.

Flux dynamics and vortex phase diagram of the new superconductor (Li1−xFex)OHFeSe single crystals

Critical current density, transport properties, and magnetic relaxation have been investigated in (Li1−xFex)OHFeSe single crystals at different temperatures and magnetic fields. A vortex phase diagram, which shows a large separation between the Hc2 and Hirr, has been predicated according to the obtained data. The small value of Hirr at high temperature as well as the upturn of Hirr with the cooling temperature indicates poor flux pinning ability and strong influence of thermal fluctuation. A crossover field Hcr, which is located below the irreversibility field and denotes the phase transition from vortex elastic creep to plastic creep, has been identified. Hcr is also strongly dependent on the temperature, confirming the influence of thermal activation of flux motion at finite temperature. The large relaxation rate and its strong dependence on magnetic field and temperature have been attributed to thermally activated vortex motion due to a very small flux pinning potential.

Triple quantum filtered spectroscopy of homonuclear three spin-1/2 systems employing isotropic mixing

We report the design and performance evaluation of novel pulse sequences for triple quantum filtered spectroscopy in homonuclear three spin-1/2 systems, employing isotropic mixing (IM) to excite triple quantum coherence (TQC). Our approach involves the generation of combination single quantum coherences (cSQC) from antisymmetric longitudinal or transverse magnetization components employing isotropic mixing (IM). cSQC’s are then converted to TQC by a selective 180° pulse on one of the spins. As IM ideally causes magnetization to evolve under the influence of the spin coupling Hamiltonian alone, TQC is generated at a faster rate compared to sequences involving free precession. This is expected to be significant when the spins have large relaxation rates. Our approach is demonstrated experimentally by TQC filtered 1D spectroscopy on a 1H AX2 system (propargyl bromide in the presence of a paramagnetic additive), as well as a 31P linear AMX system (ATP in agar gel). The performance of the IM-based sequences for TQC excitation are compared against the standard three pulse sequence (Ernst et al., 1987) and an AX2 spin pattern recognition sequence (Levitt and Ernst, 1983). The latter reaches the unitary bound on TQC preparation efficiency starting from thermal equilibrium in AX2 systems, not considering relaxation. It is shown that in systems where spins relax rapidly, the new IM-based sequences indeed perform significantly better than the above two known TQC excitation sequences, the sensitivity enhancement being especially pronounced in the case of the proton system investigated. An overview of the differences in relaxation behavior is presented for the different approaches. Applications are envisaged to Overhauser DNP experiments and to in vivo NMR.

Potential distribution around a test charge in a positive dust-electron plasma

The electrostatic potential caused by a test-charge particle in a positive dust-electron plasma is studied, accounting for the dust-charge fluctuations associated with ultraviolet photoelectron and thermionic emissions. For this purpose, the set of Vlasov–Poisson equations coupled with the dust charging equation is solved by using the space–time Fourier transform technique. As a consequence, a modified dielectric response function is obtained for dust-acoustic waves in a positive dust-electron plasma. By imposing certain conditions on the velocity of the test charge, the electrostatic potential is decomposed into the Debye–H¨uckel (DH), wake-field (WF), and far-field (FF) potentials that are significantly modified in the limit of a large dust-charge relaxation rate both analytically and numerically. The results can be helpful for understanding dust crystallization/coagulation in twocomponent plasmas, where positively charged dust grains are present.

Remarkable magnetic relaxation and metamagnetic transition in phase-separated La0.7Ca0.3Mn0.9Cu0.1O3 perovskite

The magnetic and electrical properties in phase-separated La0.7Ca0.3Mn0.9Cu0.1O3 perovskite were studied in detail. Experimental results show that the substitution of Cu at Mn-site suppresses the ferromagnetic (FM) phase of La0.7Ca0.3MnO3 matrix and induces a charge ordering transition at ∼146 K. At ∼256 K, a Griffiths-like behaviour is observed. At low temperatures, the occurrence of an irreversible metamagnetic transition associated with a large relaxation rate is analyzed, clearly indicating the dynamic phase coexistence and evolution of FM islands. A magnetic phase diagram is finally constructed based on the magnetic measurements, summarizing the dynamic evolution process of phase separation.