Temperature-dependent Competition Research Articles

Chirality-Dependent Dynamic Evolution for Trions in Monolayer WS2.

Monolayer transition metal dichalcogenides exhibit valley-dependent excitonic characters with a large binding energy, acting as the building block for future optoelectronic functionalities. Herein, combined with pump-probe ultrafast transient transmission spectroscopy and theoretical simulations, we reveal the chirality-dependent trion dynamics in h-BN encapsulated monolayer tungsten disulfide. By resonantly pumping trions in a single valley and monitoring their temporal evolution, we identify the temperature-dependent competition between two relaxation channels driven by chirality-dependent scattering processes. At room temperature, the phonon-assisted upconversion process predominates, converting excited trions to excitons within the same valley on a sub-picosecond (ps) time scale. As temperature decreases, this process becomes less efficient, while alternative channels, notably valley depolarization process for trions, assume importance, leading to an increase of trion density in the unpumped valley within a ps time scale. Our time-resolved valley-contrast results provide a comprehensive insight into trion dynamics in 2D materials, thereby advancing the development of novel valleytronic devices.

Optical Bistability in Nanosilicon with Record Low Q-Factor.

We demonstrated optical bistability in an amorphous silicon Mie resonator with a size of ∼100 nm and Q-factor as low as ∼4 by utilizing photothermal and thermo-optical effects. We not only experimentally confirmed the steep intensity transition and the hysteresis in the scattering response from silicon nanocuboids but also established a physical model to numerically explain the underlying mechanism based on temperature-dependent competition between photothermal heating and heat dissipation. The transition between the bistable states offered particularly steep superlinearity of scattering intensity, reaching an effective nonlinearity order of ∼100th power over excitation intensity, leading to the potential of advanced optical switching devices and super-resolution microscopy.

The symmetry aspect of magnetocaloric effect in LaxBi0.3-xCa0.7MnO3 manganites

Manganites exhibit many novel properties, such as giant magnetoresistance and a large magnetocaloric effect (MCE). In this work, we report the magnetic properties and magnetocaloric effects of polycrystalline Bi0.3-xLaxCa0.7MnO3 (x = 0, 0.1, 0.2) ceramics from perspective of symmetry, where Raman active modes and corresponding phonons are used as probes to detect charge disorder and local structure distortion. Further, the large MCE is elucidated by the La doping induced Jahn-Teller distortion and valence perturbations. The orthorhombic and rhombohedral structural transition in doping manganites has shown a link between the complex magnetic phase transition and magnetic entropy. We attribute these intriguing behaviors to the temperature-dependent competition between the orbital order and charge order. Our results provide an approach to improve the MCE of manganite systems for future magnetic refrigeration applications.

Deciphering the Enigma of a Temperature-Dependent Best-Performance Field in Single-Molecule Magnets

Magnetic interactions give birth to complicated systems with novel magnetic properties, yet the complexity arising from complicated interactions makes the origination of properties cryptical. Here, we recreate an enigmatic phenomenon, namely, the temperature-dependent variation of the best-performance field, in a trinuclear 3d–4f single-molecule magnet {DyNi2} with a simple yet requisite magnetic structure. A concise model is proposed here to illustrate the origin of this anomalous phenomenon and reveal how the energy structure of the system impacts the field and temperature dependence of the relaxation time. According to the model, the temperature-dependent competition between the relaxation processes with smaller tunneling splitting occurring in ground states and the relaxation process with larger tunneling splitting yet related to the excited states, which encounter each other under an applied magnetic field, leads to the enigmatic phenomena.

OH(2Π) + C2H4 Reaction: A Combined Crossed Molecular Beam and Theoretical Study.

The reaction between the ground-state hydroxyl radical, OH(2Π), and ethylene, C2H4, has been investigated under single-collision conditions by the crossed molecular beam scattering technique with mass-spectrometric detection and time-of-flight analysis at the collision energy of 50.4 kJ/mol. Electronic structure calculations of the underlying potential energy surface (PES) and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of product branching fractions on the derived PES for the addition pathway have been performed. The theoretical results indicate a temperature-dependent competition between the anti-/syn-CH2CHOH (vinyl alcohol) + H, CH3CHO (acetaldehyde) + H, and H2CO (formaldehyde) + CH3 product channels. The yield of the H-abstraction channel could not be quantified with the employed methods. The RRKM results predict that under our experimental conditions, the anti- and syn-CH2CHOH + H product channels account for 38% (in similar amounts) of the addition mechanism yield, the H2CO + CH3 channel for ∼58%, while the CH3CHO + H channel is formed in negligible amount (<4%). The implications for combustion and astrochemical environments are discussed.

Hysteretic electronic phase transitions in correlated charge density wave state of 1T−TaS2

The layered transition metal dichalcogenide $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ has evoked great interest owing to its particularly rich electronic phase diagram including different charge density wave (CDW) phases. However, few studies have focused on its hysteretic electronic phase transitions based on the in-depth discussion of the delicate interplay among temperature-dependent electronic interactions. Here, we report a sequence of spatial electronic phase transitions in the hysteresis temperature range (160--230 K) of $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ via variable-temperature scanning tunneling microscopy. Several emergent electronic states are investigated at multiscale during the commensurate CDW--triclinic CDW (CCDW-TCDW) phase transitions: a spotty-CDW state above $\ensuremath{\sim}160\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, a network-CDW (NCDW) state above $\ensuremath{\sim}180\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ during the warmup process, a belt-TCDW state below $\ensuremath{\sim}230\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, a NCDW state below $\ensuremath{\sim}200\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, and finally a mosaic-CDW state below $\ensuremath{\sim}160\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ during cooldown from the TCDW phase. These emergent electronic states are associated with the delicate temperature-dependent competition and/or cooperation of stacking-dependent interlayer interactions, intralayer electron-electron correlations, and electron-phonon ($e\text{\ensuremath{-}}ph$) coupling of $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$. Our results not only provide insight to understand the hysteretic electronic phase transitions in the correlated CDW state, but also pave a way to realize more exotic quantum states by accurately and effectively tuning various interior interactions in correlated materials.

Tempering of Au nanoclusters: capturing the temperature-dependent competition among structural motifs.

A computational approach to determine the equilibrium structures of nanoclusters in the whole temperature range from 0 K to melting is developed. Our approach relies on Parallel Tempering Molecular Dynamics (PTMD) simulations complemented by Harmonic Superposition Approximation (HSA) calculations and global optimization searches, thus combining the accuracy of global optimization and HSA in describing the low-energy part of configuration space, together with the PTMD thorough sampling of high-energy configurations. This combined methodology is shown to be instrumental towards revealing the temperature-dependent structural motifs in Au nanoclusters of sizes 90, 147, and 201 atoms. The reported phenomenology is particularly rich, displaying a size- and temperature-dependent competition between the global energy minimum and other structural motifs. In the case of Au90 and Au147, the global minimum is also the dominant structure at finite temperatures. In contrast, the Au201 cluster undergoes a solid-solid transformation at low temperature (<200 K). Results indicate that PTMD and HSA very well agree at intermediate temperatures, between 300 and 400 K. For higher temperatures, PTMD gives an accurate description of equilibrium, while HSA fails in describing the melting range. On the other hand, HSA is more efficient in catching low-temperature structural transitions. Finally, we describe the elusive structures close to the melting region which can present complex and defective geometries, that are otherwise difficult to characterize through experimental imaging.

The importance of species interactions in eco-evolutionary community dynamics under climate change

Eco-evolutionary dynamics are essential in shaping the biological response of communities to ongoing climate change. Here we develop a spatially explicit eco-evolutionary framework which features more detailed species interactions, integrating evolution and dispersal. We include species interactions within and between trophic levels, and additionally, we incorporate the feature that species’ interspecific competition might change due to increasing temperatures and affect the impact of climate change on ecological communities. Our modeling framework captures previously reported ecological responses to climate change, and also reveals two key results. First, interactions between trophic levels as well as temperature-dependent competition within a trophic level mitigate the negative impact of climate change on biodiversity, emphasizing the importance of understanding biotic interactions in shaping climate change impact. Second, our trait-based perspective reveals a strong positive relationship between the within-community variation in preferred temperatures and the capacity to respond to climate change. Temperature-dependent competition consistently results both in higher trait variation and more responsive communities to altered climatic conditions. Our study demonstrates the importance of species interactions in an eco-evolutionary setting, further expanding our knowledge of the interplay between ecological and evolutionary processes.

Analysis of Cultivable Microbial Community during Kimchi Fermentation Using MALDI-TOF MS.

Kimchi, a traditional Korean fermented vegetable, has received considerable attention for its health-promoting effects. This study analyzes the cultivable microbial community in kimchi fermented at different temperatures using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) to comprehensively understand the factors affecting the quality of kimchi. Of the 5204 strains isolated from kimchi, aligned with the in-house database, 4467 (85.8%) were correctly identified at the species level. The fermentation temperature affected the microbial community by varying the pH and acidity, which was mainly caused by temperature-dependent competition between the different lactic acid bacteria (LAB) species in kimchi. LAB, such as Levilactobacillus (Lb.) brevis and Lactiplantibacillus (Lpb.) plantarum associated with rancidity and tissue softening, proliferated faster at higher temperatures than at low temperature. In addition, LAB, such as Latilactobacillus (Lat.) sakei and Leuconostoc (Leu.) mesenteroides, which produce beneficial substances and flavor, were mainly distributed in kimchi fermented at 4 °C. This study shows as a novelty that MALDI-TOF MS is a robust and economically affordable method for investigating viable microbial communities in kimchi.

Assessing temperature‐dependent competition between two invasive mosquito species

Invasive mosquitoes are expanding their ranges into new geographic areas and interacting with resident mosquito species. Understanding how novel interactions can affect mosquito population dynamics is necessary to predict transmission risk at invasion fronts. Mosquito life-history traits are extremely sensitive to temperature, and this can lead to temperature-dependent competition between competing invasive mosquito species. We explored temperature-dependent competition between Aedes aegypti and Anopheles stephensi, two invasive mosquito species whose distributions overlap in India, the Middle East, and North Africa, where An.stephensi is currently expanding into the endemic range of Ae.aegypti. We followed mosquito cohorts raised at different intraspecific and interspecific densities across five temperatures (16-32°C) to measure traits relevant for population growth and to estimate species' per capita growth rates. We then used these growth rates to derive each species' competitive ability at each temperature. We find strong evidence for asymmetric competition at all temperatures, with Ae.aegypti emerging as the dominant competitor. This was primarily because of differences in larval survival and development times across all temperatures that resulted in a higher estimated intrinsic growth rate and competitive tolerance estimate for Ae.aegypti compared to An.stephensi. The spread of An.stephensi into the African continent could lead to urban transmission of malaria, an otherwise rural disease, increasing the human population at risk and complicating malaria elimination efforts. Competition has resulted in habitat segregation of other invasive mosquito species, and our results suggest that it may play a role in determining the distribution of An.stephensi across its invasive range.

Imaging current distribution in a topological insulator Bi2Se3 in the presence of competing surface and bulk contributions to conductivity

Two-dimensional (2D) topological surface states in a three-dimensional topological insulator (TI) should produce uniform 2D surface current distribution. However, our transport current imaging studies on Bi2Se3 thin film reveal non-uniform current sheet flow at 15 K with strong edge current flow. This is consistent with other imaging studies on thin films of Bi2Se3. In contrast to strong edge current flow in thin films, in single crystal of Bi2Se3 at 15 K our current imaging studies show the presence of 3.6 nm thick uniform 2D sheet current flow. Above 70 K, this uniform 2D sheet current sheet begins to disintegrate into a spatially non-uniform flow. The flow becomes patchy with regions having high and low current density. The area fraction of the patches with high current density rapidly decreases at temperatures above 70 K, with a temperature dependence of the form 1/left| {T - 70} right|^{0.35}. The temperature scale of 70 K coincides with the onset of bulk conductivity in the crystal due to electron doping by selenium vacancy clusters in Bi2Se3. Thus our results show a temperature dependent competition between surface and bulk conductivity produces a temperature dependent variation in uniformity of current flow in the topological insulator.

Thermal Traits of MNPs under High-Frequency Magnetic Fields: Disentangling the Effect of Size and Coating.

We investigated the heating abilities of magnetic nanoparticles (MNPs) in a high-frequency magnetic field (MF) as a function of surface coating and size. The cobalt ferrite MNPs were obtained by a hydrothermal method in a water–oleic acid–ethanol system, yielding MNPs with mean diameter of about 5 nm, functionalized with the oleic acid. By applying another cycle of hydrothermal synthesis, we obtained MNPs with about one nm larger diameter. In the next step, the oleic acid was exchanged for 11-maleimidoundecanoic acid or 11-(furfurylureido)undecanoic acid. For the heating experiments, all samples were dispersed in the same solvent (dichloroethane) in the same concentration and the heating performance was studied in a broad interval of MF frequencies (346–782 kHz). The obtained results enabled us to disentangle the impact of the hydrodynamic, structural, and magnetic parameters on the overall heating capabilities. We also demonstrated that the specific power absorption does not show a monotonous trend within the series in the investigated interval of temperatures, pointing to temperature-dependent competition of the Brownian and Néel contributions in heat release.

Simulation of thermoluminescence dose response in cluster systems with deep traps

The TL dose response was simulated within the framework of a kinetic model with cluster defects which contain electron TL-active traps and deep traps. It was shown that superlinearity of the TL dose dependence arises due to the temperature-dependent competition between the intracluster capture of electrons by deep traps and their thermal release into the conduction band. The regularities of the influence of the main parameters of the model (heating rate, activation energies of the ground and excited levels of TL-active traps) on the TL yield and nonlinearity of the dose dependence were found.

Effects of the grain size on domain structure and thermal stability of sintered Nd-Fe-B magnets

Thermal stability of coercivity and domain structure in sintered Nd–Fe–B magnets with various grain sizes has been studied. Microstructure analysis shows that the improved thermal stability arises from the grain morphology beneficial to a small effective demagnetization factor Neff. In-situ magnetic domain evolution during thermal demagnetization indicates that lower surface stray field in fine-grained magnets suppresses the reversion of domain in high temperature condition, which results from temperature-dependent competition between the local demagnetizing field and effective anisotropy field of surface grains.

The electron trap parameter extraction-based investigation of the relationship between charge trapping and activation energy in IGZO TFTs under positive bias temperature stress

Experimental extraction of the electron trap parameters which are associated with charge trapping into gate insulators under the positive bias temperature stress (PBTS) is proposed and demonstrated for the first time in amorphous indium-gallium-zinc-oxide thin-film transistors. This was done by combining the PBTS/recovery time-evolution of the experimentally decomposed threshold voltage shift (ΔVT) and the technology computer-aided design (TCAD)-based charge trapping simulation. The extracted parameters were the trap density (NOT) = 2.6×1018 cm−3, the trap energy level (ΔET) = 0.6 eV, and the capture cross section (σ0) = 3 × 10−19 cm2.Furthermore, based on the established TCAD framework, the relationship between the electron trap parameters and the activation energy (Ea) is comprehensively investigated. It is found that Ea increases with an increase in σ0, whereas Ea is independent of NOT. In addition, as ΔET increases, Ea decreases in the electron trapping-dominant regime (low ΔET) and increases again in the Poole–Frenkel (PF) emission/hopping-dominant regime (high ΔET). Moreover, our results suggest that the cross-over ΔET point originates from the complicated temperature-dependent competition between the capture rate and the emission rate. The PBTS bias dependence of the relationship between Ea and ΔET suggests that the electric field dependence of the PF emission-based electron hopping is stronger than that of the thermionic field emission-based electron trapping.

Temperature dependent competition between different recombination channels in organic heterojunction solar cells

A modification of the Shockley–Queisser theory for organic heterojunctions is presented with a special focus on constellations, where a linear extrapolation of the temperature dependence of the open circuit voltage results in the optical gap of the absorber rather than in the intermolecular charge transfer (CT) gap. We demonstrate that, depending on the electronic coupling strength between donor and acceptor molecules, either singlet or CT recombination is dominant in different temperature regimes. The different regimes are separated by a transition temperature that is usually well above room temperature (RT). However, in the case of small energy level offset and weak electronic coupling, it can be around 300 K or even below. We point out that a linear extrapolation of the open circuit voltage Voc towards 0 K for measured temperatures larger than the transition temperature results in a photovoltaic gap that is close to the optical gap, whereas for values below the transition temperature the CT gap will be extracted. We show that for α-sexithiophene (6T)/diindenoperylene (DIP) solar cells heating the substrate during 6T deposition leads to a molecular configuration at the interface where the coupling between donor and acceptor molecules is strongly reduced. This leads to a transition temperature well below RT which is confirmed by temperature dependent electroluminescence measurements. By comparing the temperature dependent spectra of high temperature and RT grown 6T/DIP solar cells to the spectra of the individual materials, the different contributions from the CT gap and the optical gap are separated.

Magnetic domain tuning and the emergence of bubble domains in the bilayer manganiteLa2−2xSr1+2xMn2O7(x=0.32)

We report a magnetic force microscopy study of the magnetic domain evolution in the layered manganite La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ (with $x=0.32$). This strongly correlated electron compound is known to exhibit a wide range of magnetic phases, including a recently uncovered biskyrmion phase. We observe a continuous transition from dendritic to stripe-like domains, followed by the formation of magnetic bubbles due to a field- and temperature dependent competition between in-plane and out-of-plane spin alignments. The magnetic bubble phase appears at comparable field- and temperature ranges as the biskyrmion phase, suggesting a close relation between both phases. Based on our real-space images we construct a temperature-field phase diagram for this composition.

Ac susceptibility of Ho (Fe,CO)11Ti compounds and their hydrides

Abstract The structural characterization and ac susceptibility behavior of polycrystalline samples of HoFe 11− x Co x TiH y ( x =1, 2, 4, 6, 7, 9, 11 and y =0, 1) intermetallics are investigated. All compounds crystallize in tetragonal ThMn 12 -type structure. The replacement of Fe by Co causes the lattice constants to decrease, while hydrogen insertion leads to cell volume expansion. A spin-reorientation (SR) transition from a uniaxial to cone/planar structure is observed for the HoFe 11− x Co x Ti compounds of higher Co content ( x >7). By hydrogenation, SR transition appears in all samples due to relative reinforcement of Ho contribution to anisotropy. T SR values increase by Co-content because of the preferential substitution of Fe by Co atoms and consequently, concentration-dependence of contribution of transition metal sublattice to anisotropy, as well as that of Ho. The concentration-dependence of magnetization direction and hydrogen-induced SR in the studied samples are discussed in the framework of two-magnetic sublattice model by bearing in mind the temperature-dependent competition of their anisotropies and dependence of magnetic anisotropy on crystal field interactions.

Continuous regime of chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU) reactivity of CuO oxygen carriers

Chemical-looping with CuO and CH4 presents an interesting reaction system, in which CuO participates in heterogeneous reduction reactions with the gaseous fuel, as well as oxygen uncoupling and catalytic reactions. Therefore, there exists a temperature dependent competition between the processes typically termed as chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU). The objective of this work is to carry out an assessment of the rates of all the relevant reactions involved at various temperatures and thus to show a continuous regime, where the CLC reactions domination proceeds to CuO oxygen uncoupling dominance, as temperature increases. The possible reactions occurring in CuO-based chemical-looping include heterogeneous reactions with Cu oxides, CuO oxygen uncoupling and Cu-catalyzed reactions. The significance of each individual reaction and the corresponding kinetic parameters are experimentally examined in a bench-scale fixed-bed, alternating-flow chemical-looping reactor at conditions relevant to both CLC and CLOU. In situ XRD characterization is used to determine the phases in which Cu exists during reduction and oxidation and possible interactions with the SiO2 oxygen carrier support. A kinetic scheme is proposed, applicable to the entire regime of CLC and CLOU, which is validated with in-house experiments and experimental data from the literature, at conditions where heterogeneous reduction, oxygen uncoupling, Cu-catalyzed, and gas phase oxidation reactions present significant contributions.

Local Symmetry Breaking in the High Temperature Regime of SnTe

The term emphanisis [1] has been coined to define the appearance of local off-centering displacements of ions from a high-symmetry ground state on warming, as recently discovered in PbTe [2]. Such a phenomenon is unusual because, in the canonical view of structural transformations, a low-symmetry ground state evolves into a higher symmetry state on warming. Although it is not uncommon for remnants of a low-symmetry phase to appear as spatial fluctuations at high temperature, the emergence of a locally broken symmetry state from a high symmetry ground state is quite rare. Emphanisis may be behind some long-known, but poorly understood anomalies seen in the lead chalcogenides. However, the origin and nature of emphanisis are still the subject of controversy. Several explanations for emphanisis have been suggested, including a simple response to an underlying anharmonic potential [3], a dynamic ferroelectric-like off-centering [2], and a temperature-dependent competition between ionicity and covalency [1], but an understanding remains elusive. In this talk I will report on atomic pair distribution function (PDF) measurements of the lead-free compound SnTe, which is isostructural to PbTe at high T but with a ferroelectric phase below Tc ~ 100K. Our data show that SnTe also exhibits an emphanitic response, but with an onset temperature well above Tc and a symmetry that is distinct from that of the ferroelectric phase. Taken together these results suggests that the emphanitic and ferroelectric responses are quite distinct.