Peculiar Temperature Dependence Research Articles

Dynamics in polymers with phase separated dynamic bonds: the case of a peculiar temperature dependence.

The topic of polymers with dynamic bonds (stickers) appears as an exciting and promising area of materials science, thanks to their attractive self-healable, recyclable, extremely tough, and super extensible properties. Polymers with phase separated dynamic bonds revealed several unique properties, but mechanisms controlling their viscoelastic properties remain poorly understood. In this work, we present a dynamic analysis of a model polymer system with phase separated hydrogen bonding functionalities. The results confirm that terminal relaxation in these systems is independent of polymer segmental dynamics and is instead controlled by structural relaxations in clusters of stickers. Detailed analysis revealed a surprising result: terminal relaxation time of these systems has weaker temperature dependence than that of structural relaxation in clusters, although the former is slower than the latter. Borrowing ideas from the field of block copolymers, we ascribed this unusual result to an LCST-like behavior for the miscibility of the stickers in the polymer matrix. The presented results and ideas deepen the understanding of the viscoelasticity for polymers with dynamic bonds, enabling intelligent design of functional materials with desired macroscopic properties.

Luminescence mechanism analysis of a TADF molecule showing peculiar thermal behavior.

In recent years, much attention has been paid to the development of thermally activated delayed fluorescence (TADF) materials with short delayed-fluorescence lifetimes to improve the device performances of OLEDs. In principle, by reducing the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) overlap, while the energy difference between S1-T1 (ΔEST) and activation energy (Ea) can be reduced, and the reverse intersystem crossing rate constant (kRISC) can be accelerated, a decrease in the radiative rate constant happens, necessitating an advanced molecular design. Furthermore, a molecule based on heptazine as a parent skeleton has recently been found to have a peculiar temperature dependence of luminescence decay, suggesting a negative gap (NG) material. In this report, we show that 9-[1,4]benzoxaborino[2,3,4-kl]phenoxaborine-7-yl-1,3,6,8-tetramethyl-9H-carbazole (TMCz-BO), a donor-acceptor linked TADF molecule with a very short delay lifetime of 750 ns, exhibits a peculiar thermal behavior similar to that of NG materials based on the temperature dependence of its luminescence decay in solution.

Scaling similarities and quasinormal modes of D0 black hole solutions

We study the gravity solution dual to the D0 brane quantum mechanics, or BFSS matrix model, in the ’t Hooft limit.The classical physics described by this gravity solution is invariant under a scaling transformation, which changes the action with a specific critical exponent, sometimes called the hyperscaling violating exponent. We present an argument for this critical exponent from the matrix model side, which leads to an explanation for the peculiar temperature dependence of the entropy in this theory, S ∝ T9/5. We also present a similar argument for all other Dp-brane geometries.We then compute the black hole quasinormal modes. This involves perturbing the finite temperature geometry. These perturbations can be easily obtained by a mathematical trick where we view the solution as the dimensional reduction of an AdS2+9/5× S8 geometry.

Transparent Colloids of Detonation Nanodiamond: Physical, Chemical and Biological Properties.

Aqueous suspensions (colloids) containing detonation nano-diamond (DND) feature in most applications of DND and are an indispensable stage of its production; therefore, the interaction of DND with water is actively studied. However, insufficient attention has been paid to the unique physico-chemical and biological properties of transparent colloids with low DND content (≤0.1%), which are the subject of this review. Thus, such colloids possess giant dielectric permittivity which shows peculiar temperature dependence, as well as quasi-periodic fluctuations during slow evaporation or dilution. In these colloids, DND interacts with water and air to form cottonwool-like fibers comprising living micro-organisms (fungi and bacteria) and DND particles, with elevated nitrogen content due to fixation of atmospheric N2. Prolonged contact between these solutions and air lead to the formation of ammonium nitrate, sometimes forming macroscopic crystals. The latter was also formed during prolonged oxidation of fungi in aqueous DND colloids. The possible mechanism of N2 fixation is discussed, which can be attributable to the high reactivity of DND.

Anomalous Vapor and Ice Nucleation in Water at Negative Pressures: A Classical Density Functional Theory Study.

In contrast to the abundance of work on the anomalous behavior of water, the relationship between the water's thermodynamic anomalies and kinetics of phase transition from metastable water is relatively unexplored. In this work, we have employed classical density functional theory to provide a unified and coherent picture of nucleation (both vapor and ice) from metastable water at negative pressure conditions. Our results suggest a peculiar nonmonotonic temperature dependence of vapor-liquid surface tension at temperatures where vapor-liquid coexistence is metastable with respect to the ice phase. The vapor nucleation barrier on isochoric cooling also shows a nonmonotonic temperature dependence. We further report that, for low density isochores, the temperature of the minimum vapor nucleation barrier does not coincide with the temperature of maximum density (TMD) where metastability is maximum. The difference between the and the TMD, however, decreases with increasing the density of the isochore. The vapor nucleation barrier along isobars shows an interesting crossover behavior in the vicinity of the Widom line on lowering the temperature. Our results on the ice nucleation suggest an anomalous retracing behavior of the nucleation barrier along isotherms at negative pressures and theoretically validate the recent findings that the reentrant ice(Ih)-liquid coexistence line can induce a drastic change in the kinetics of ice nucleation. Thus, this study establishes a direct connection between the metastable water's thermodynamic anomalies and the (vapor and ice) nucleation kinetics. In addition, this study provides deeper insights into the origin of the isothermal compressibility maximum on isochoric cooling.

Peculiar temperature dependence of magneto-optic Kerr rotation associated with antiferromagnetic–paramagnetic transition

We investigate the magneto-optic Kerr effect in perpendicularly magnetized Pt/Co/Ir/Cr2O3/Pt thin films, associated with the antiferromagnetic–paramagnetic transition of the Cr2O3 layer. The magneto-optic Kerr rotation angle (θ K) shows oscillatory behavior as a function of the photon energy of incident light owing to interference in the Cr2O3 layer. The temperature dependence of θ K at 2.67 eV (λ = 465 nm), at which the largest θ K is obtained, shows a sharp dip at 287.0 K. The dip temperature is similar to the reported Néel temperature for Cr2O3 thin films. Although the θ K spectra measured at several temperatures are generally explained by the classical interference model, θ K is enhanced at 2.36–2.79 eV (λ = 525–445 nm) close to the dip temperature. This peculiar enhancement in θ K is discussed on the basis of the anomaly in the optical parameters of the Cr2O3 layer associated with the antiferromagnetic–paramagnetic transition.

Anomalous Temperature Dependence of High-Harmonic Generation in Mott Insulators.

We reveal the crucial effect of strong spin-charge coupling on high-harmonic generation (HHG) in Mott insulators. In a system with antiferromagnetic correlations, the HHG signal is drastically enhanced with decreasing temperature, even though the gap increases and the production of charge carriers is suppressed. This anomalous behavior, which has also been observed in recent HHG experiments on Ca_{2}RuO_{4}, originates from a cooperative effect between the spin-charge coupling and the thermal ensemble, as well as the strongly temperature-dependent coherence between charge carriers. We argue that the peculiar temperature dependence of HHG is a generic feature of Mott insulators, which can be controlled via the Coulomb interaction and dimensionality of the system. Our results demonstrate that correlations between different degrees of freedom, which are a characteristic feature of strongly correlated solids, have significant and nontrivial effects on nonlinear optical responses.

Walker breakdown of Brownian domain wall dynamics

The Brownian motion of domain walls in uniaxial and biaxial ferromagnetic nanowires is studied, comparing spin dynamics simulations with analytical calculations within the framework of a collective coordinate approach. Our results demonstrate that the interplay between spatial and angular dynamics gives rise to a complex time dependence of the MSD in biaxial nanowires and to a drastically reduced diffusion coefficient in uniaxial nanowires, analogous to magnetic skyrmions. This diffusion suppression is also responsible for the peculiar temperature dependence of the diffusion coefficient in biaxial systems: while it is found to scale linearly with temperature up to a certain threshold, the emergence of a Walker breakdown of Brownian motion is responsible for a reduction of the diffusion coefficient with increasing temperature above this threshold.

Two distinctive temperature dependences of the London penetration depth in high-Tc cuprate superconductors as support for the theory of Bose-liquid superconductivity

In the framework of the theory of Bose-liquid superconductivity, we show that in high-Tc cuprate superconductors the magnetic field penetration depth (known as the London penetration depth λL(T)) decreases first exponentially below the superconducting transition temperature Tc with decreasing the temperature T and then λL(T) decreases like a certain power of T at lower temperatures due to the vanishing of the energy gap Δg(T) in the excitation spectrum of a superfluid Bose-liquid at a characteristic temperature Tc⁎ lower than Tc. Below Tc, the London penetration depth λL(T) has an exponential temperature dependence down to Tc⁎, which is different from a BCS- like exponential temperature dependence, and then λL(T) will follow a power law temperature dependence down to T=0, which is also different from power-law temperature dependences predicted by some BCS-like (d-wave) pairing theories. In unconventional (bosonic) cuprate superconductors characterized by the strong interboson coupling constant (γB≳1), λL(T) exhibits the distinctly different temperature dependences in different temperature intervals 0≤T≤Tc⁎<<Tc and Tc⁎<T<Tc when the energy gap Δg(T) vanishes at a temperature Tc⁎ well below Tc. In the weak interboson coupling (γB<<1) the energy gap Δg(T) vanishes at a temperature Tc⁎ close to Tc and the reduced London penetration depth λL(T)/λL(0) has an unusual Tn power-law temperature dependence in a wide temperature range 0<T<Tc⁎ below Tc, i.e. the expression λL(T)/λL(0)≃[1−aL(T)(T/Tc)n]−1/2 (where n≳4 and aL(T)≲1) is valid only below Tc⁎ and is similar to the well-known empirical Gorter-Casimir law. Our theoretical predictions of the distinctive power-law and exponential temperature dependences of λL(T)/λL(0) are in good agreement with the experimental results reported for GdBa2Cu3O7−δ ceramics and YBa2Cu3O7−δ films. The peculiar temperature dependences of λL(T)/λL(0) observed in these high-Tc materials at low and high temperatures do not follow the predictions of the s- and d-wave BCS-like theories of Fermi-liquid superconductivity and give evidence for the predictions of the theory of Bose-liquid superconductivity.

Dynamics of phase transitions in Ba1−xSrxV13O18

We studied the slow dynamics of phase transitions in ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{V}}_{13}{\mathrm{O}}_{18}$, which is composed of quasitriangular lattices of V ions and exhibits a phase transition into a V orbital ordered state with V trimerization. We found a clear time dependence of magnetic susceptibility and strain on a phase transition, and the transformation time estimated from the time dependence exhibits a peculiar temperature dependence, consistent with nucleation-growth behavior. Detailed analysis indicates that there is a distribution of the transformation time in ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{V}}_{13}{\mathrm{O}}_{18}$, and the formation of twin structures associated with the phase transition may affect the nucleation-growth dynamics and causes such a distribution of the transformation time.

Dynamics of Arabic gum aqueous solutions as revealed by NMR relaxometry.

The purpose of this article is to study molecular dynamics through nuclear magnetic relaxation (NMR) dispersion of Arabic gum aqueous solutions analysed in terms of two-fraction exchange model. The experiments revealed that relaxation of water molecules was non-monoexponential, which was interpreted in terms of a model describing the magnetization transfer due to exchange of water and polysaccharide protons. The analysis showed that water dynamics decreased slightly with gum content. Polymer-chain dynamics was assigned to regime II of the tube/reptation model. Peculiar temperature dependence of exchange rate was observed in the whole concentration range of Arabic gum solutions. NMR relaxation probed in a broad frequency and temperature range allows probing of the molecular dynamics of complex food systems. © 2022 Society of Chemical Industry.

Hopping conduction in FeSi. I. The Hall, Seebeck, and Nernst effects due to hopping conduction in the top and bottom impurity Hubbard bands

FeSi is known as a narrow-gap semiconductor showing peculiar temperature dependence of transport properties, which evoked debate for over 50 years. In this study, it is shown that the peculiar temperature dependence of the electrical conductivity σ, the Hall coefficient RH, the Hall mobility μH, the Seebeck coefficient S, and the Nernst coefficient Q of FeSi can be well explained in a model that includes the conduction and the valence band with parabolic dispersions together with the top and bottom impurity Hubbard bands. In particular, the coincidence of the hump of σ(T), the maximum of S(T), the minimum of μH(T), and the maximum of Q(T) can be attributed to the contribution from hopping conduction in the top impurity Hubbard band.

Dark matter from an even lighter QCD axion: trapped misalignment

We show that dark matter can be accounted for by an axion that solves the strong CP problem, but is much lighter than usual due to a Z \U0001d4a9 symmetry.The whole mass range from the canonical QCD axion down to the ultra-light regime is allowed, with 3 ≤ \U0001d4a9 ≲ 65. This includes the first proposal of a "fuzzy dark matter" QCD axion with m a ∼ 10-22 eV.A novel misalignment mechanism occurs — trapped misalignment — due to the peculiar temperature dependence of the Z \U0001d4a9 axion potential. The dark matter relic density is enhanced because the axion field undergoes two stages of oscillations: it is first trapped in the wrong minimum, which effectively delays the onset of true oscillations. Trapped misalignment is more general than the setup discussed here, and may hold whenever an extra source of Peccei-Quinn breaking appears at high temperatures. Furthermore, it will be shown that trapped misalignment can dynamically source the recently proposed kinetic misalignment mechanism. All the parameter space is withintantalizing reach of the experimental projects for the next decades. For instance, even Phase I ofCASPEr-Electric could discover this axion.

Learning heterogenous reaction rates from stochastic simulations.

Reaction rate equations are ordinary differential equations that are frequently used to describe deterministic chemical kinetics at the macroscopic scale. At the microscopic scale, the chemical kinetics is stochastic and can be captured by complex dynamical systems reproducing spatial movements of molecules and their collisions. Such molecular dynamics systems may implicitly capture intricate phenomena that affect reaction rates but are not accounted for in the macroscopic models. In this work we present a data assimilation procedure for learning nonhomogeneous kinetic parameters from molecular simulations with many simultaneously reacting species. The learned parameters can then be plugged into the deterministic reaction rate equations to predict long time evolution of the macroscopic system. In this way, our procedure discovers an effective differential equation for reaction kinetics. To demonstrate the procedure, we upscale the kinetics of a molecular system that forms a complex covalently bonded network severely interfering with the reaction rates. Incidentally, we report that the kinetic parameters of this system feature peculiar time and temperature dependences, whereas the probability of a network strand to close a cycle follows a universal distribution.

Correlation between crystal structure and magnetism in PLD grown epitaxial films of ε-Fe2O3 on GaN

ABSTRACT In the present paper we discuss correlations between crystal structure and magnetic properties of epitaxial ε-Fe2O3 films grown on GaN. The large magnetocrystalline anisotropy and room temperature multiferroic properties of this exotic iron oxide polymorph, make it a perspective material for the development of low power consumption magnetic media storage devices. Extending our recent progress in PLD growth of ε-Fe2O3 on the surface of technologically important nitride semiconductors, we apply reciprocal space tomography by electron and x-ray diffraction to investigate the break of crystallographic symmetry occurring at the oxide-nitride interface resulting in the appearance of anisotropic crystallographic disorder in the sub-100 nm ε-Fe2O3 films. The orthorhombic-on-hexagonal nucleation scenario is shown responsible for the development of a peculiar columnar structure observed in ε-Fe2O3 by means of HRTEM and AFM. The complementary information on the direct and reciprocal space structure of the columnar ε-Fe2O3 films is obtained by various techniques and correlated to their magnetic properties. The peculiar temperature dependence of magnetization studied by the small-field magnetization derivative method and by neutron diffraction reveals the existence of a magnetic softening below 150 K, similar to the one observed earlier solely in nanoparticles. The magnetization reversal in ε-Fe2O3 films probed by X-ray magnetic circular dichroism is found different from the behavior of the bulk averaged magnetization measured by conventional magnetometry. The presented results fill the gap between the numerous studies performed on randomly oriented ε-Fe2O3 nanoparticles and much less frequent investigations of epitaxial epsilon ferrite films with lattice orientation fixed by the substrate.

Elucidating the origin of planar defects that enhance critical current density in CaKFe4As4 single crystals

CaKFe4As4 is a new member of the 1144-type iron-based superconductors (IBSs) which is expected to show remarkable superconducting properties because its crystal structure is similar to that of 122-type IBSs. Recently, substantial anisotropy between the in-plane and out-of-plane critical current densities (J C), with peculiar temperature dependence of in-plane J C at high fields, has been reported in CaKFe4As4 single crystals. The anisotropy of J C was attributed to the presence of planar defects, which were shown to be confined to the ab plane. However, the origin of these defects remains unclear. To elucidate the origin of these planar defects, we herein present our results on atomic-scale microstructure analysis of high-quality CaKFe4As4 single crystals. Using high-resolution scanning transmission electron microscopy (STEM), along with electron energy loss spectroscopy, we have demonstrated that these planar defects consist of one- or two-layer KFe2As2 step network, which is spread over the periodically ordered KFe2As2 and CaFe2As2 monolayers. Further, we report that the dark contrast regions observed via conventional STEM are local strain distributions, which originate from the substitution of Ca by K in a unit cell.

Isomeric effects in structure formation and dielectric dynamics of different octanols.

The understanding of the microstructure of associated liquids promoted by hydrogen-bonding and constrained by steric hindrance is highly relevant in chemistry, physics, biology and for many aspects of daily life. In this study we use a combination of X-ray diffraction, dielectric spectroscopy and molecular dynamics simulations to reveal temperature induced changes in the microstructure of different octanol isomers, i.e., linear 1-octanol and branched 2-, 3- and 4-octanol. In all octanols, the hydroxyl groups form the basis of chain-, cyclic- or loop-like bonded structures that are separated by outwardly directed alkyl chains. This clustering is analyzed through the scattering pre-peaks observed from X-ray scattering and simulations. The charge ordering which pilots OH aggregation can be linked to the strength of the Debye process observed in dielectric spectroscopy. Interestingly, all methods used here converge to the same interpretation: as one moves from 1-octanol to the branched octanols, the cluster structure evolves from loose large aggregates to a larger number of smaller, tighter aggregates. All alcohols exhibit a peculiar temperature dependence of both the pre-peak and Debye process, which can be understood as a change in microstructure promoted by chain association with increased chain length possibly assisted by ring-opening effects. All these results tend to support the intuitive picture of the entropic constraint provided by branching through the alkyl tails and highlight its capital entropic role in supramolecular assembly.

Enhancement of the transverse thermoelectric conductivity originating from stationary points in nodal lines

Motivated by the recent discovery of a large anomalous Nernst effect in Co$_2$MnGa, Fe$_3X$ ($X$=Al, Ga) and Co$_3$Sn$_2$S$_2$, we performed a first-principles study to clarify the origin of the enhancement of the transverse thermoelectric conductivity ($\alpha_{ij}$) in these ferromagnets. The intrinsic contribution to $\alpha_{ij}$ can be understood in terms of the Berry curvature ($\Omega$) around the Fermi level, and $\Omega$ is singularly large along nodal lines (which are gapless in the absence of the spin-orbit coupling) in the Brillouin zone. We find that not only the Weyl points but also stationary points in the energy dispersion of the nodal lines play a crucial role. The stationary points make sharp peaks in the density of states projected onto the nodal line, clearly identifying the characteristic Fermi energies at which $\alpha_{ij}$ is most dramatically enhanced. We also find that $\alpha_{ij}/T$ breaks the Mott relation and show a peculiar temperature dependence at these energies. The present results suggest that the stationary points will give us a useful guiding principle to design magnets showing a large anomalous Nernst effect.

Dehydration behavior of a natural hydrated Ca-chabazite studied by in-situ high-temperature single-crystal X-ray diffraction

Abstract Dehydration behavior of a natural hydrated Ca-chabazite with composition (Ca1.57Na0.49)(Al3.39Si8.55)O24·12.47H2O has been studied by the structure analyses based on the in-situ high-temperature single-crystal X-ray diffraction data at 100, 200 and 250 °C. Among the five water sites (OW1–OW5) and four exchangeable-cation sites (Ca1–Ca4) found previously in the room-temperature (RT) structure, the present structure analyses revealed the presence of OW1, OW4, Ca3 sites at 100 °C and of OW3, Ca3, Ca4 sites at 200 and 250 °C. The presence of the new extraframework sites was also revealed: OW2’ site at 100 °C and Ca5 site at 200 and 250 °C. From the temperature dependence of the occupation for these extraframework sites, the dehydration and cation migration processes are summarized and rationalized as follows. With heating up to 100 °C, OW3 water molecules, having no bonds to exchangeable cations, are completely desorbed. Simultaneously, Ca1 and Ca2 cations lose the large amounts of water molecules as ligands (OW1 and OW5 for Ca1, OW4 for Ca2); this brings about the simultaneous migration of both cations to adjacent Ca3 site, surrounded by more ligands. With further heating up to 250 °C, OW2’ water molecules, having no bonds to exchangeable cations, are completely desorbed. Simultaneously, most of the Ca3 cations migrate to adjacent Ca4 and Ca5 sites by losing the large amounts of water molecules as ligands (OW1, OW4). A part of the water molecules desorbed in the temperature range of 100–250 °C migrate to OW3 site to be bonded to the remaining Ca3 cations. The peculiar temperature dependence of Si/Al−O1, −O2 and −O3 bond lengths in the framework structure can be explained in terms of their bond-valence variations due to such dehydration and cation migration processes. On the other hand, the variation of Si/Al−O4 bond length is dominated by thermal expansion effect rather than the bond valence effect.

Effects of artificial square nanorods array to square bulk superconducting plate

It is known that the superconducting properties are improved by adding nanorods to a superconductor. As increasing nanorods into superconductor, the pinning force becomes stronger. Using the molecular dynamics method, we investigate vortex motions in a superconductor with a nanorod array. We obtain trajectories of vortices and standard deviation of vortex positions. We find peculiar temperature dependence of vortex motion.