Low Temperature Regime Research Articles

Prediction of superionic state in LiH2 at conditions enroute to nuclear fusion

Hydrogen and lithium, along with their compounds, are crucial materials for nuclear fusion research. High-pressure studies have revealed intricate structural transitions in all these materials. However, research on lithium hydrides beyond LiH has mostly focused on the low-temperature regime. Here, we use density functional theory and ab initio molecular dynamics simulations to investigate the behavior of LiH2, a hydrogen-rich compound, near its melting point. Our study is particularly relevant to the low-pressure region of the compression pathway of lithium hydrides toward fusion. We discovered a premelting superionic phase transition in LiH2 that has significant implications for its mass transportation, elastic properties, and sound velocity. The theoretical boundary for the superionic transition and melting temperature was then determined. In contrast, we also found that the primary compound of lithium hydrides, LiH, does not exhibit a superionic transition. These findings have important implications for optimizing the compression path to achieve the ignition condition in inertial confinement fusion research, especially when lithium tritium-deuteride (LiTD) is used as the fuel.

Higher order Morita approximation and its validity for random copolymer adsorption onto homogeneous and periodic heterogeneous surfaces

Adsorption of a single AB random copolymer (RC) chain onto homogeneous and inhomogeneous ab surfaces with a regular periodic pattern is studied theoretically. For the averaging over disorder in the RC sequence, the constrained annealed approximation, known as the Morita approximation is employed. A general scheme for constructing the Morita approximation of an arbitrary order m is proposed; it is based on representation of the RC monomer sequence as a Markov chain of overlapping –1)-ads. The problem is solved within the framework of the generating functions approach for the two-dimensional partially directed walk model of the polymer on a square lattice. Temperature dependences of various adsorption characteristics are obtained. The accuracy of the Morita approximation is assessed by comparison with the numerical results for RC with quenched sequences obtained by the transfer matrix approach. It is shown that for RC adsorption on a homogeneous surface for AB-copolymers with adsorbing A and neutral B blocks, it is sufficient to use the Morita approximation of second or third order. If the non-adsorbing B block is repelled from the surface, then a higher order of the approximation (5th–6th) is required. An important indicator of validity of the Morita approximation is the entropy: if the order of the approximation is not high enough, the entropy becomes negative in the low-temperature regime which means that the Morita approximation is wrong in this order. In the case when the surface is periodically inhomogeneous, the Morita approximation works worse and very high orders are required, which can be beyond the computational capabilities. Moreover, the Morita approximations become degenerate: approximations of two or more consecutive orders give the same result.

Quantum Dynamical Investigation of Dihydrogen-Hydride Exchange in a Transition-Metal Polyhydride Complex.

The ongoing shift toward clean, sustainable energy is a primary driving force behind hydrogen fuel research. Safe and effective storage of hydrogen is a major challenge (particularly for mobile applications) and requires a detailed understanding of the atomic level interactions of hydrogen with its host materials. The light mass of hydrogen, however, implies that quantum effects are important, so a quantum dynamical treatment is required to properly account for these effects in computational simulations. As one such example, we describe herein the hydrogen exchange dynamics between a hydride and a dihydrogen ligand in the [FeH(H2)(PH3)4]+ model complex. A global three-dimensional (3D) potential energy surface (PES) was constructed by fitting to and interpolating from a discrete set of grid points computed using density functional theory; exact quantum dynamical calculations were then carried out on the 3D PES using discrete variable representation basis sets. Energy levels and their quantum tunneling splittings were computed up to 3000 cm-1 above the ground state. Within that energy range, all three fundamentals have been identified using wave function plots, as well as the first three overtones of the exchange (reaction coordinate) motion and several of its combination bands. From the tunneling splittings, the Boltzmann-averaged tunneling rates were computed. The Arrhenius plot of the total exchange rate shows a clear transition around 150 K, below which the activation energy is essentially zero and above which it is less than half of the electronic structure barrier. This indicates that exchange rates are governed by quantum tunneling throughout the relevant temperature range with the low-temperature regime dominated by a single quantum (ground) state. This work is the first-ever fully quantum dynamical study to investigate the hydrogen exchange dynamics between hydride and dihydrogen ligands coordinated to a transition-metal complex.

Temperature-Dependent Contact of Weakly Interacting Single-Component Fermi Gases and Loss Rate of Degenerate Polar Molecules.

Motivated by the experimental realization of single-component degenerate Fermi gases of polar ground state KRb molecules with intrinsic two-body losses [L. De Marco et al., A degenerate Fermi gas of polar molecules, Science 363, 853 (2019).SCIEAS0036-807510.1126/science.aau7230], this work studies the finite-temperature loss rate of single-component Fermi gases with weak interactions. First, we establish a relationship between the two-body loss rate and the p-wave contact. Second, we evaluate the contact of the homogeneous system in the low-temperature regime using p-wave Fermi liquid theory and in the high-temperature regime using the second-order virial expansion. Third, conjecturing that there are no phase transitions between the two temperature regimes, we smoothly interpolate the results to intermediate temperatures. It is found that the contact is constant at temperatures close to zero and increases first quadratically with increasing temperature and finally-in agreement with the Bethe-Wigner threshold law-linearly at high temperatures. Fourth, applying the local-density approximation, we obtain the loss-rate coefficient for the harmonically trapped system, reproducing the experimental KRb loss measurements within a unified theoretical framework over a wide temperature regime without fitting parameters. Our results for the contact are not only applicable to molecular p-wave gases but also to atomic single-component Fermi gases, such as ^{40}K and ^{6}Li.

Mean-field theory is exact for Ising spin glass models with Kac potential in non-additive limit on Nishimori line

Recently, Mori (2011 Phys. Rev. E 84 031128) has conjectured that the free energy of Ising spin glass models with the Kac potential in the non-additive limit, such as the power-law potential in the non-additive regime, is exactly equal to that of the Sherrington–Kirkpatrick model in the thermodynamic limit. In this study, we prove that his conjecture is true on the Nishimori line at any temperature in any dimension. One of the key ingredients of the proof is the use of the Gibbs–Bogoliubov inequality on the Nishimori line. We also consider the case in which the probability distribution of the interaction is symmetric, where his conjecture is true at any temperature in one dimension but is an open problem in the low-temperature regime in two or more dimensions.

Dependence of simulated radiation damage on crystal structure and atomic misfit in metals

This study investigates the evolution of radiation damage in three metals in the low temperature and high radiant flux regime using molecular statics and a Frenkel pair accumulation method to simulate up to 2.0 displacements per atom. The metals considered include Fe, equiatomic CrCoNi, and a fictitious metal with similar bulk properties to the CrCoNi composed of a single atom type referred to as an A-atom. CrCoNi is found to sustain higher concentrations of dislocations than either the Fe or A-atom systems and more stacking faults than the A-atom system. The results suggest that the difference between the concentrations of vacancies and interstitials is substantially smaller for CrCoNi than the A-atom system, perhaps reflecting that the sink capture radius is smaller in CrCoNi due to the roughened potential energy landscape. A model that partitions the major contributions from defects to the stored energy is described, and serves to highlight a general need for higher fidelity approaches to point defect identification.

Chilling and freezing stress tolerance in Moringa oleifera Lam.

Moringa oleifera Lam. is a tree species and popularly known for its multiple uses. The high nutritional value of moringa leaves make it a valuable product for human and animal nutrition. Native of India, moringa grows well in tropical and subtropical areas, being spread worldwide. However, the cold tolerance for this species remains unclear. This study aimed to evaluate the chilling tolerance of moringa under controlled low-temperature regimes and duration and assess whether this species has freeze-tolerance and cold acclimation ability. To evaluate chilling stress, we used three temperature regimes: 20/15, 15/10, and 10/5 °C, of which 10/5 °C was chosen to further investigate the effects of duration and recovery. Morphological (seedling height, number of leaves, number of leaflets, fresh weight, dry weight, and leaf area) and physiological indicators such as a chlorophyll fluorescence parameter (Fv/Fm) and electrolyte leakage (%) were used. All physiological parameters were compromised at progressively cooler temperature regimes. Chilling injury to photosynthetic apparatus and cellular membranes at 10/5 °C for 4 days was completely reversed during the recovery period. In contrast, 8 days of stress caused more severe injury which was only partially recovereable, and 12 days of stress inflicted severest and irreversible injury. The freezing tolerance and cold acclimation ability were evaluated using a laboratory freeze-thaw protocol. The tissues were 50% injured (LT50) at -2.8 °C. In addition, our data indicated the moringa did not show cold acclimation ability after 4 days at 15/10 °C or 4 days at 10/5 °C. The cold tolerance in moringa appears to be a complex system regulated by intensity and duration, and this study provided the baseline information to understand the physiological mechanism involved in these processes.

Out-of-plane spin-to-charge conversion at low temperatures in graphene/MoTe2 heterostructures

Multi-directional spin-to-charge conversion—in which spin polarizations with different orientations can be converted into a charge current in the same direction—has been demonstrated in low-symmetry materials and interfaces. This is possible because, in these systems, spin-to-charge conversion can occur in unconventional configurations in which charge current, spin current, and polarization do not need to be mutually orthogonal. Here, we explore, in the low temperature regime, the spin-to-charge conversion in heterostructures of graphene with the low-symmetry 1T' phase of MoTe2. First, we observe the emergence of charge conversion for out-of-plane spins at temperatures below 100 K. This unconventional component is allowed by the symmetries of both MoTe2 and graphene and likely arises from spin Hall effect in the spin–orbit proximitized graphene. Moreover, we examine the low-temperature evolution of non-local voltage signals arising from the charge conversion of the two in-plane spin polarizations, which have been previously observed at higher temperature. As a result, we report omni-directional spin-to-charge conversion—for all spin polarization orientations—in graphene/MoTe2 heterostructures at low temperatures.

Monte Carlo simulation of a magnetic nanoparticles system under an external rotating magnetic field

The magnetic response of an identical magnetic nanoparticles (MNP) system to a rotating external field (RMF) is studied via Monte Carlo simulations. The field of amplitude H0 and frequency ω, was applied in the y−z plane rotating clockwise. The energy was modeled by the Stoner–Wohlfarth scheme for fixed or random orientations of the anisotropy, and is in contact with a thermal bath at a temperature T. Interparticle dipolar interactions were also considered.In the non-interacting system and for low temperature, hysteresis is observed in the z magnetization component Mz for both orientations of the anisotropy axis and only in the y component (My) for the random case. Furthermore, the loop areas were estimated, and increased with ω for all orientations and (My,Mz) components. At higher temperatures the superparamagnetic state is observed, so both the blocking temperatures TB and loop areas were estimated. The values of TB were close from the room temperature TR=300K for all components, and the areas decreased with T but they are practically not zero at TB.When dipolar interactions are included a new scenario is revealed. In the low temperature regime, the blocked state is present for both My and Mz for all anisotropy orientations, and extends beyond the interval of amplitudes H0 estimated theoretically for the model without interactions and fixed anisotropy. The loops are displaced with respect to the origin of the magnetization-external field plane. When the temperature is raised, the blocked state extends for a larger range than the model without interactions, and the loop displacement decreases with T. These behaviors could be explained by observing that the average dipolar field per particle produces an effective field – the sum of both dipolar and external field – that is asymmetric with respect to the zero field line at low temperatures, becomes half-wave symmetric at higher temperatures, so the the centered character of the loops is restored . In addition, the loop areas show a peak for all orientations of the anisotropy axes in an intermediate range of temperatures. This result can be associated with a dominance of the anisotropy induced by the dipolar field.Finally, by comparing the areas of the loops of the models with and without interactions, it was found that the non-interacting model have larger areas at low temperatures that vanish near the room temperature, unlike the areas of the model with interactions due to the extension of the blocked state.

Increased Temperature and Discharge Influence Overwinter Growth and Survival of Juvenile Salmonids in a Hydropeaking River: Simulating Effects of Climate Change Using Individual-Based Modelling

Climate change causes warming of rivers and may increase discharge, particularly during winter. Downstream of hydropower plants, fluctuating water temperature and flow create dynamic overwintering conditions for juvenile salmonids. We used inSTREAM 7.2-SD to simulate the effects of increased temperature (+2 °C) and discharge (+10%) on the overwinter growth and mortality of one-summer- and two-summer-old Atlantic salmon and brown trout in a river with a hydropeaking flow regime in a 2 × 2 design with replicated simulations. Water temperature had a major positive relationship with growth for both species and year classes, whereas increased flow alone had no major general effect on overwinter growth. For one-summer-old trout experiencing the high temperature regime, however, increased flow resulted in reduced growth. There were no major effects from temperature and flow on the survival rate of the two-summer-old fishes. On the other hand, there were significant interaction effects for the one-summer-olds, indicating that the effect of flow depended on temperature. For one-summer-old salmon, high flow resulted in increased survival in the low temperature regime, whereas it resulted in reduced survival in high temperature. In contrast, for one-summer-old trout, high flow resulted in reduced survival in the low temperature regime and increased survival in the high temperature. Different hydropower operation alternatives may interact with warming, affecting the relative competitive abilities of stream salmonids. Ecological models that predict the effects of different environmental conditions, such as temperature and flow regimes, may offer insight into such effects when in situ experiments are not feasible.

Effect of Sm3+ and Yb3+ doping on the structural and magnetic properties of La2−xRExNiMnO6 (x = 0.1–0.7) double perovskites

The doping effect of samarium (Sm) and ytterbium (Yb) at the La sites of La2NiMnO6 (LMNO) double perovskite on the structural and magnetic properties was studied. The rare-earth (RE = Sm, Yb) ion-doped La2-x RExNiMnO6 (x = 0.1–0.7) double perovskites were synthesized using sol-gel auto combustion route. The X-ray diffraction patterns of the Sm- and Yb-doped samples indicated single-phase monoclinic symmetry (P21/n space group). The Raman spectra provided complementary information of structural phase stability. The antisymmetric (∼528 cm−1) and symmetric (∼670 cm−1) stretching modes of vibration and a shoulder at wavenumber ∼250 cm−1 of the Raman spectra were consistent with a monoclinic phase. The X-ray photoelectron spectroscopy revealed the presence of La3+, Sm3+, Yb3+, Ni3+, Ni2+, Mn3+, and Mn4+ ions and improvement of the ordering of Ni2+ and Mn4+ ions in monoclinic structure of the Sm- and Yb-doped double perovskite structure. The temperature and magnetic field dependent magnetization studies revealed ferromagnetic nature with Curie temperature (TC) in the range of 205–265 K. The increase of Sm and Yb doping at the La sites of LMNO decreased TC and the presence of an additional weak magnetic transition at low-temperature range (∼100–150 K). The downturn feature of magnetization curves for a low-temperature regime (≤50 K) indicated a decrease of ferromagnetic exchange interactions and was a signature of spin freezing behavior in Sm- and Yb-doped LNMO samples.

Skeletal kinetic mechanism for predicting formation of non-fuel hydrocarbons and soot in ethylene flames – A CFD approach

Since existing reduced mechanisms of ethylene oxidation are either too large or lack of sufficient chemical reactions to describe aromatics formation, the study aims at developing a skeletal mechanism with extended predictive capability and lowered computational cost in computational fluid dynamics modeling of ethylene combustion. The newly assembled and refined kinetic mechanism composed of 664 species and 3582 reactions is minimized to 79 species and 538 reactions without empirically adjusting kinetic parameters. In the 1-D simulation for a premixed flame of ethylene, the ethylene-PAH mechanism generally well predicts experimental profiles of five hydrocarbons and nine aromatics. Integrated into a 2-D axisymmetric laminar finite-rate model, the ethylene-PAH mechanism reproduces experimental data of seven C2–C5 hydrocarbons, five aromatic hydrocarbons and soot in a diffusion flame of ethylene. In comparison with the 158-species (Narayanaswamy) mechanism, the newly proposed ethylene mechanism leads to a speedup of approximately 5.5 times in the 2-D laminar diffusion flame simulation and improved accuracy for predicting species formation in both premixed and nonpremixed flames. Moreover, we unveil the correlation between reaction pathways and intermediate formation from ethylene oxidation at the low temperature regime.

Unveiling the Roles of Precursor Structure and Controlled Sintering on Ni-Phyllosilicate-Derived Catalysts for Low-Temperature Methane Decomposition

Catalytic methane decomposition (CMD) is a promising technology for large-scale production of COx-free H2 from natural gas that can also produce valuable carbon byproducts. Although equilibrium conversions and reaction rates of CMD generally increase with temperature, operation in a low-temperature regime with simultaneous H2 recovery could potentially lead to operating cost and energy savings. Here, we report that well-dispersed Ni–SiO2, derived from high-temperature reduction of nickel phyllosilicates, is active for CMD at temperatures below 500 °C, with initial H2 production rates of up to 5.3 mol H2/gcat·h at 25% CH4 conversion. This ability to achieve rates comparable to other well-established catalysts is contrary to expectations that small (<ca. 10 nm) Ni nanoparticles are inactive for CMD because of rapid deactivation and attributed here to an unusual mobility of nickel–silica interfaces in the presence of CH4 that leads to controlled sintering of the originally well-dispersed Ni nanoparticles. We further show that the ratio of 1:1 and 2:1 nickel phyllosilicates in the precursor, which governs catalyst reducibility and can be tuned by adding NH4F to the synthesis mixture, is a key descriptor of catalytic performance. Our findings provide valuable insight into catalyst and process design for low-temperature CMD.

Triangular lattice Hubbard model physics at intermediate temperatures

Moire systems offer an exciting playground to study many-body effects of strongly correlated electrons in regimes that are not easily accessible in conventional material settings. Motivated by a recent experiment on $\text{WSe}_2/\text{WS}_2$ moire bilayers [Y. Tang et al., Nature 579, 353-358 (2020)], which realizes a triangular superlattice with a small hopping (of approximately 10 Kelvin), with tunable density of holes, we explore the Hubbard model on the triangular lattice for intermediate temperatures $t \lesssim T < U$. Employing finite temperature Lanczos calculations, and closely following the fitting protocols used in the experiment, we recover the observed trends in the reported Curie-Weiss temperature $\Theta$ with filling, using the reported interaction strength $U/t=20$. We focus on the large increase of $|\Theta|$ on decreasing the density below half filling and the sign change of $\Theta$ at higher fillings, which signals the onset of ferromagnetism. The increase in $|\Theta|$ is also seen in the $t$-$J$ model (the low energy limit of the Hubbard model) in the intermediate temperature range, which we clarify is opposite to the trend in its high temperature limit. Differences between the low, intermediate and high temperature regimes are discussed. Our numerical calculations also capture the crossover between short-range antiferromagnetic to ferromagnetic order in the intermediate temperature regime, a result broadly consistent with the experimental findings. We find that this behavior is a finite-temperature remnant of the underlying zero temperature phase transition, which we explore with ground state density matrix renormalization group calculations. We provide evidence of ferromagnetism characterized by weak (but robust) correlations that explain the small $\Theta$ seen in the experiment.

How Temperature Affects the Selectivity of the Electrochemical CO2 Reduction on Copper.

Copper is a unique catalyst for the electrochemical CO2 reduction reaction (CO2RR) as it can produce multi-carbon products, such as ethylene and propanol. As practical electrolyzers will likely operate at elevated temperatures, the effect of reaction temperature on the product distribution and activity of CO2RR on copper is important to elucidate. In this study, we have performed electrolysis experiments at different reaction temperatures and potentials. We show that there are two distinct temperature regimes. From 18 up to ∼48 °C, C2+ products are produced with higher Faradaic efficiency, while methane and formic acid selectivity decreases and hydrogen selectivity stays approximately constant. From 48 to 70 °C, it was found that HER dominates and the activity of CO2RR decreases. Moreover, the CO2RR products produced in this higher temperature range are mainly the C1 products, namely, CO and HCOOH. We argue that CO surface coverage, local pH, and kinetics play an important role in the lower-temperature regime, while the second regime appears most likely to be related to structural changes in the copper surface.

Efficient low-temperature simulations for fermionic reservoirs with the hierarchical equations of motion method: Application to the Anderson impurity model

The hierarchical equations of motion (HEOM) approach is an accurate method to simulate open system quantum dynamics, which allows for systematic convergence to numerically exact results. To represent effects of the bath, the reservoir correlation functions are usually decomposed into summation of multiple exponential terms in the HEOM method. Since the reservoir correlation functions become highly non-Markovian at low temperatures or when the bath has complex band structures, a present challenge is to obtain accurate exponential decompositions that allow efficient simulation with the HEOM. In this work, we employ the barycentric representation to approximate the Fermi function and hybridization functions in the frequency domain. This method, by approximating these functions with optimized rational decomposition, greatly reduces the number of basis functions in decomposing the reservoir correlation functions, which further allows the HEOM method to be applied to ultralow temperature and general band structures. We demonstrate the efficiency, accuracy, and long-time stability of this decomposition scheme by applying it to the Anderson impurity model in the low-temperature regime with the Lorentzian and tight-binding hybridization functions.

Copper, Uranium and REE Mineralisation in an Exhumed Oil Reservoir, Southwest Orkney, Scotland

Copper, uranium, and rare earth element (REE) mineralisation occurs in hydrocarbon-bearing Devonian continental sandstones in southwest Orkney, Scotland. The aeolian Yesnaby Sandstone Formation and fluvial Harra Ebb Sandstone Formation were mineralised following oil emplacement. The REE-bearing APS mineral florencite is particularly associated with bituminous nodules, many of which contain brannerite. Subsequently hydrothermal copper and other sulphides, and barite, further mineralised the oil reservoir at a temperature of ~190 °C. Oil was mobilised through mineralised fractures at this stage. Biodegradation of the oil occurred later, following the Carboniferous-Permian uplift. The occurrence confirms that Cu-APS mineralisation is possible in relatively low-temperature regimes in sedimentary basins.

Thermodynamics of a quantum ring modified by Lorentz violation

In this work, we investigate the consequences of Lorentz-violating terms in the thermodynamic properties of a 1-dimensional quantum ring. In particular, we use the ensemble theory to obtain our results of interest. The thermodynamic functions as well as the spin currents are calculated as a function of the temperature. We observe that parameter ξ, which triggers the Lorentz symmetry breaking, plays a major role in low temperature regime. Finally, depending on the configuration of the system, electrons can rotate in two different directions: clockwise and counterclockwise.

Improvement of the manufacturing method of semi-finished fruit and vegetables of a high degree of readiness

The method of production of concentrated fruit and vegeTable semi-finished product based on apples, Jerusalem artichoke and quince has been improved. Selected components of plant raw materials are chosen due to the presence of such useful substances as pectins, vitamins, minerals, etc. The obtained semi-finished products of a high degree of readiness are semi-finished products with a natural nutrient composition. A feature of the proposed method is the use of low-temperature modes of concentration and drying of raw materials. For the developed method, the hardware set of the line with the use of advanced thermal equipment was selected. Chopped raw materials are blanched in a universal multifunctional device: apple and Jerusalem artichoke in a steam environment (103 °C and 108 °C) for 4 and 8 minutes. Quince is blanched under conditions of pre-exposure in a 0.1 % solution of citric acid in an aqueous medium at a temperature of 75...80 °C (4...8 min.).&#x0D; Concentration to a pasty state takes place in the improved design of the rotary-film apparatus with a reduced processing time of 0.75...1.25 s to a DM content of 28...30 %. To obtain a powdery state, a cylindrical IR dryer was used for drying to the final moisture content at the level of 4...6 % DM. It is also possible to dry the extracts formed after wiping the raw materials to a final moisture content of 10...13 % DM in an IR dryer. The use of low-temperature regimes in the range of 45...60 oC helps to obtain high-quality semi-finished products with maximum preservation of the initial properties of raw materials. By means of organoleptic evaluation, the best experimental sample with the content of blend components was found: apple 40 %; Jerusalem artichoke 35 %; quince 25 %. Developed semi-finished products are additives or fillers in various food products to improve their nutritional value

Holographic thermal propagator for arbitrary scale dimensions

Using the AdS/CFT correspondence we model the behaviour of the two-point correlator of an operator with arbitrary scale dimension ∆ in arbitrary spacetime dimension d for small but non-zero temperature. The obtained propagator coincides in the low temperature regime with the known result for d = 4 for large ∆ at the order Td as well as with the Td and T2d terms of the exact all order result for d = 2. Furthermore, for arbitrary d we explicitly write down the expression for the order Td of the propagator for arbitrary ∆, and present a conjecture for the order T2d in the large ∆ limit.