Unusual Temperature Dependence Research Articles

Discrete states and carrier-phonon scattering in quantum dot population dynamics.

The influence of the growth conditions of multilayer CdTe/ZnTe quantum dots (QDs) on Si substrate upon their carrier dynamics is studied using intensity integration and broadening photoluminescence. The unusual temperature dependence of the line broadening is explained using a model for interband transitions that involves a lowest discrete electronic state (1Se) with different discrete hole states (1S3/2 and 2S3/2) and a 1P transition. These transitions are expected to play a critical role in both the thermally activated energy and the line broadening of the QDs. We also demonstrate that a thermally activated transition between two different states occurs with band low-temperature quenching, with values separated by 5.8–16 meV. The main nonradiative process is thermal escape assisted by carrier scattering via emission of longitudinal phonons through the hole states at high temperature, with an average energy of 19.3–20.2 meV.

A novel N-terminal extension in mitochondrial TRAP1 serves as a thermal regulator of chaperone activity.

Hsp90 is a conserved chaperone that facilitates protein homeostasis. Our crystal structure of the mitochondrial Hsp90, TRAP1, revealed an extension of the N-terminal β-strand previously shown to cross between protomers in the closed state. In this study, we address the regulatory function of this extension or 'strap' and demonstrate its responsibility for an unusual temperature dependence in ATPase rates. This dependence is a consequence of a thermally sensitive kinetic barrier between the apo 'open' and ATP-bound 'closed' conformations. The strap stabilizes the closed state through trans-protomer interactions. Displacement of cis-protomer contacts from the apo state is rate-limiting for closure and ATP hydrolysis. Strap release is coupled to rotation of the N-terminal domain and dynamics of the nucleotide binding pocket lid. The strap is conserved in higher eukaryotes but absent from yeast and prokaryotes suggesting its role as a thermal and kinetic regulator, adapting Hsp90s to the demands of unique cellular and organismal environments.

Topological defects in systems with two competing order parameters: Application to superconductors with charge- and spin-density waves

On the basis of coupled Ginzburg-Landau equations we study nonhomogeneous states in systems with two order parameters (OPs). Superconductors with a superconducting OP $\ensuremath{\Delta}$ and a charge- or spin-density wave with amplitude $W$ are examples of such systems. When one OP, say $\ensuremath{\Delta}$, has a form of a topological defect, like, e.g., a vortex or domain wall between the domains with the phases 0 and $\ensuremath{\pi}$, the other OP $W$ is determined by the Gross-Pitaevskii equation and is localized at the center of the defect. We consider in detail the domain-wall defect for $\ensuremath{\Delta}$ and show that the shape of the associated solution for $W$ depends on temperature and doping (or on the curvature of the Fermi surface) $\ensuremath{\mu}$. It turns out that, provided the temperature or doping level is close to some discrete values ${T}_{n}$ and ${\ensuremath{\mu}}_{n}$, the spatial dependence of the function $W(x)$ is determined by the form of the eigenfunctions of the linearized Gross-Pitaevskii equation. The spatial dependence of ${W}_{0}$ corresponding to the ground state has the form of a soliton, while other possible solutions ${W}_{n}(x)$ have nodes. The inverse situation when $W(x)$ has the form of a topological defect and $\ensuremath{\Delta}(x)$ is localized at the center of this defect is also possible. In particular, we predict a surface or interfacial superconductivity in a system where a superconductor is in contact with a material that suppresses $W$. This superconductivity should have rather unusual temperature dependence existing only in certain intervals of temperature. Possible experimental realizations of such nonhomogeneous states of OPs are discussed.

Experimental and theoretical kinetics for the H2O+ + H2/D2 → H3O+/H2DO+ + H/D reactions: observation of the rotational effect in the temperature dependence.

Thermal rate coefficients for the title reactions computed using a quasi-classical trajectory method on an accurate global potential energy surface fitted to ∼81,000 high-level ab initio points are compared with experimental values measured between 100 and 600 K using a variable temperature selected ion flow tube instrument. Excellent agreement is found across the entire temperature range, showing a subtle, but unusual temperature dependence of the rate coefficients. For both reactions the temperature dependence has a maximum around 350 K, which is a result of H2O(+) rotations increasing the reactivity, while kinetic energy is decreasing the reactivity. A strong isotope effect is found, although the calculations slightly overestimate the kinetic isotope effect. The good experiment-theory agreement not only validates the accuracy of the potential energy surface but also provides more accurate kinetic data over a large temperature range.

Temperature-sensitive junction transformations for mid-wavelength HgCdTe photovoltaic infrared detector arrays by laser beam induced current microscope

In this paper, we report on the disappearance of the photosensitive area extension effect and the unusual temperature dependence of junction transformation for mid-wavelength, n-on-p HgCdTe photovoltaic infrared detector arrays. The n-type region is formed by B+ ion implantation on Hg-vacancy-doped p-type HgCdTe. Junction transformations under different temperatures are visually captured by a laser beam induced current microscope. A physical model of temperature dependence on junction transformation is proposed and demonstrated by using numerical simulations. It is shown that Hg-interstitial diffusion and temperature activated defects jointly lead to the p-n junction transformation dependence on temperature, and the weaker mixed conduction compared with long-wavelength HgCdTe photodiode contributes to the disappearance of the photosensitive area extension effect in mid-wavelength HgCdTe infrared detector arrays.

Substrate Specificity of Human Protein Arginine Methyltransferase 7 (PRMT7): THE IMPORTANCE OF ACIDIC RESIDUES IN THE DOUBLE E LOOP

Protein arginine methyltransferase 7 (PRMT7) methylates arginine residues on various protein substrates and is involved in DNA transcription, RNA splicing, DNA repair, cell differentiation, and metastasis. The substrate sequences it recognizes in vivo and the enzymatic mechanism behind it, however, remain to be explored. Here we characterize methylation catalyzed by a bacterially expressed GST-tagged human PRMT7 fusion protein with a broad range of peptide and protein substrates. After confirming its type III activity generating only ω-N(G)-monomethylarginine and its distinct substrate specificity for RXR motifs surrounded by basic residues, we performed site-directed mutagenesis studies on this enzyme, revealing that two acidic residues within the double E loop, Asp-147 and Glu-149, modulate the substrate preference. Furthermore, altering a single acidic residue, Glu-478, on the C-terminal domain to glutamine nearly abolished the activity of the enzyme. Additionally, we demonstrate that PRMT7 has unusual temperature dependence and salt tolerance. These results provide a biochemical foundation to understanding the broad biological functions of PRMT7 in health and disease.

Unusual temperature dependence of quadrupole interaction at 111Cd in UMn2⋅

We report the temperature dependence of electric quadrupole interaction frequency of 111Cd probe in UMn2 measured by time differential perturbed angular correlation spectroscopy. The measurements carried out in the temperature range do not show any indication of magnetic ordering. Interestingly the quadropole frequency νQ decrease at low temperatures, quite in contrast with the usual phonon associated increase observed in metals and alloys. We believe, this unusual feature may be related to dynamic lattice distortion driven by spin fluctuation.

Multiple lattice instabilities resolved by magnetic-field and disorder sensitivities inMgV2O4

Ultrasound velocity measurements of the orbital-degenerate frustrated spinel MgV$_2$O$_4$ are performed in the high-purity single crystal which exhibits successive structural and antiferromagnetic phase transitions, and in the disorder-introduced single crystal which exhibits spin-glass-like behavior. The measurements reveal that two-types of unusual temperature dependence of the elastic moduli coexist in the cubic paramagnetic phase, which are resolved by magnetic-field and disorder sensitivities: huge Curie-type softening with decreasing temperature, and concave temperature dependence with a characteristic minimum. These elastic anomalies suggest the coupling of lattice to coexisting orbital fluctuations and orbital-spin-coupled excitations.

Elastic properties of stishovite and the CaCl2-type silica at the mantle temperature and pressure: An ab initio investigation

The elastic constant tensors of stishovite and the CaCl2-type silica at the Earth's mantle temperature and pressure were determined using first-principles calculations with local density approximation. The elastic properties of stishovite show not only strong pressure dependence but also temperature dependence. By increasing temperature, the shear instability of stishovite is shifted to an elevated pressure with a slope of ∼5.4±1.4 MPa/K. The softening of the shear modulus and the positive Clapeyron slope result in crossing of the sound velocities at different temperatures, which leads to the unusual positive temperature dependence of the sound velocities around the phase boundary. The transition from stishovite to the CaCl2-type silica at the lower mantle's temperature occurs at a depth far deeper than 1200 km and is accompanied by a velocity jump of ∼0.98±0.08 km/s in S wave velocity (VS) and ∼0.45±0.15 km/s in P wave velocity (VP). This transition is likely related to the seismic discontinuity at the depth of ∼1670 km in the vicinity of Mariana Island. The unusual positive temperature dependence of VS of stishovite and strong anisotropy of stishovite and the CaCl2-type silica around the phase boundary provide potential ways to identify the origin of the seismic discontinuity.

Full Access to Nanoscale Bismuth–Palladium Intermetallics by Low-Temperature Syntheses

The microwave-assisted polyol process was applied and modified to synthesize phase-pure micro- or nanocrystalline samples of all intermetallic phases in the bismuth–palladium system. Reaction temperatures range between 170 and 240 °C, whereas conventional syntheses from melt necessitate 500 to 1000 °C. Reaction times of few minutes up to 1 h are sufficient. Although not stable at the temperature of synthesis, high-temperature phases are accessible as well. Differences in the redox potentials of the two metals have effectively been compensated by adding auxiliaries such as oleylamine, oleic acid, and potassium hydroxide. The samples were characterized by X-ray powder diffraction, scanning electron microscopy, and energy-dispersive electron spectroscopy. Magnetic properties and electrical conductivity of the nanocrystalline samples were measured. The high temperature compound γ-BiPd showed superconductivity with Tc = 3.2 K. Nanocrystalline nc-Bi2Pd5 revealed an unusual temperature dependence of the electrical conductivity indicating an electronic phase transition at about 230 K. The electronic band structures of γ-BiPd, Bi2Pd5, Bi12Pd31, and BiPd3 were calculated including spin–orbit coupling.

Structure sensitivity in the ruthenium nanoparticle catalyzed aqueous-phase Fischer–Tropsch reaction

Low-temperature Fischer–Tropsch reaction data are reported for Ru nanoparticles suspended in the water phase. Their activity and selectivity strongly depends on particle size, when varied between 1 to 5 nm. Small particles display high oxygenates selectivity. The Anderson–Schulz–Flory (ASF) chain-growth probability for oxygenates is significantly lower than that observed for hydrocarbons. The chain growth parameter for hydrocarbon formation is independent of particle size. For oxygenates it is constant only for particles larger than 3 nm. Oxygenate and hydrocarbon formation occur on different sites. The ASF chain-growth probability for oxygenate formation increases with temperature. For very small 1.2 nm particles it shows a maximum as a function of temperature. This unusual temperature dependence is due to relatively slow CO dissociation compared to the rate of C–C bond formation.

Physico-chemical concept of drag reduction nature in dilute polymer solutions (the Toms effect)

The physicochemical concept of turbulent drag reduction (the Toms effect) integrates physicochemical characteristics of polymer solutions with hydrodynamic and rheological flow parameters into a generalized equation, where the increment in volumetric flow rate QP is a function of the external shear stress τw, temperature, volume of macromolecular coils with immobilized solvent Vc and a function of their volume fraction Ψ=C·[η]/(1+C·[η]). The QP depends on the coil intrinsic elasticity [G]=kT/Vc as well. This model allows one: (1) to describe the Toms effect in terms of useful elastic work spent by macromolecular coils with immobilized solvent to overcome the frictional forces (i.e. the forces of intermolecular interactions), (2) to forecast the initial conditions of the Toms effect (τ*≈(RT)/(M·[η])) and (3) to explain the unusual temperature dependence of the polymer solutions flow.

Self-consistent theory of ferromagnetism on the surface of a topological insulator

The Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between magnetic impurities, mediated by Dirac surface states on the surface of a topological insulator, leads to impurities' ferromagnetic ordering. We present a self-consistent theory of the ordering, which takes into account a gap in the surface spectrum induced by the exchange field of magnetic impurities. We show that the gap does not change the general structure of RKKY interaction but considerably influences its strength. This feedback can be both positive and negative, depending on the ratio between the chemical potential and the gap, and it qualitatively modifies the temperature dependence of the spin polarization of magnetic impurities. The resulting unusual temperature dependence can be directly measured in angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) experiments.

Positive temperature variation of the bandgap energy in the single-crystalline chalcopyrite semiconductor AgInS2

Optical absorption spectra have been measured on the single-crystalline chalcopyrite semiconductor AgInS2 using polarized light at T = 10–300 K. The bandgap energy Eg of AgInS2 shows unusual temperature dependence at low temperatures. The resultant temperature coefficient ∂Eg/∂T is found to be positive at T < 130 K and negative above 130 K. This result has been successfully explained by considering the effects of thermal expansion and electron–phonon interaction. The free-exciton emission of photoluminescence spectra also indicates positive temperature dependence of the peak energies at low temperatures. The exciton binding energy of AgInS2 is determined to be 26.4 meV.

Contrasting the experimental properties of hydrogen in SnO2, In2O3, and TiO2

IR spectroscopy has been used to investigate the properties of H and D in single crystals of the transparent conducting oxides, SnO2, and In2O3. H introduces several O-H stretching lines and also the broad absorption arising from free carriers. IR spectroscopy has been used to identify the sources of n-type conductivity, its thermal stability, and the reactions of H-containing defects. The properties of OH and OD centers in TiO2, while studied for decades, reveal new surprises and properties that are in sharp contrast to the shallow, H-related donors seen in SnO2 and In2O3. Recent theory and EPR experiments find that electrons in TiO2 become self-trapped at Ti sites to form small polarons. The OD center in TiO2 shows a multiline vibrational spectrum with an unusual temperature dependence that can be explained by a small polaron model with the donor electron self-trapped at different Ti sites near the OD oscillator.

Trap states in chemically derived graphene oxide revealed by anomalous temperature-dependent photoluminescence

We carry out a comprehensive temperature-dependent photoluminescence (PL) study on chemically derived graphene oxide (GO) sheets. According to the unusual temperature dependence, we introduce a trap state ∼114 meV beneath the LUMO, which implies an additional carrier decay process.

Changes of the crystal structure at the relaxor ferroelectric phase transition of strontium barium niobate (SBN53)

The average crystal structures of the ferroelectric and the intermediate paraelectric phases of SBN53 were refined at various temperatures up to 200 °C from single crystal X‐ray diffraction data. Shifts of the atoms reduce the microscopic spontaneous electric polarization which, however, does not entirely vanish on heating at the ferroelectric (FE) phase transition near 95 °C. Most atoms show an unusual temperature dependence of the atomic displacement parameters (adp). At the FE transition, a very large anisotropic increase of the U33 adp of part of the niobium atoms indicates enhanced structural disorder in the intermediate high temperature phase. A similarly high anisotropic U11 adp of the large Me2 site can be assigned to static disorder caused by incomplete occupation by Ba2+ and Sr2+ which is not affected by the FE phase transition.

Unusual temperature dependence of the splay elastic constant of a rodlike nematic liquid crystal doped with a highly kinked bent-core molecule

We report an unusual temperature dependence of the elastic constants of a rodlike nematic liquid crystal (RLC) mixed with a highly kinked bent-core liquid crystal (BLC). On cooling through the nematic phase, the splay elastic constant (K(11)) of the RLC-BLC mixture increased below the nematic-isotropic phase transition temperature, but started to decrease midway through the nematic phase. The decrease of K(11) was more prominent with a greater concentration of BLC. On the other hand, the bend elastic constant (K(33)) of the RLC-BLC mixture monotonically increased through the nematic phase with decreasing temperature.

The unusual temperature dependence of the switching behavior in a ferroelectric single crystal with dislocations

The unusual temperature-induced switching behavior in a ferroelectric single crystal with dislocation arrays is investigated by using phase field simulations. The results show that the influence of temperature on the hysteresis loop of a ferroelectric is dependent on the dislocation arrays. In the presence of dislocation arrays, the dependence of the coercive field on the temperature is different from that of a dislocation-free ferroelectric. The coercive field increases when the temperature increases from room temperature to a critical temperature, which is attributed to the pinning of domains by the dislocation arrays. Above the critical temperature, both the coercive field and the remnant polarization decrease with temperature. It is found that double hysteresis loops can be induced by dislocation arrays when the temperature is higher than the Curie temperature. This work exhibits the complex role of temperature and dislocations in the polarization switching of ferroelectric single crystal.

Phonon-Assisted Exciton Transfer into Silicon Using Nanoemitters: The Role of Phonons and Temperature Effects in Förster Resonance Energy Transfer

We study phonon-assisted Förster resonance energy transfer (FRET) into an indirect band-gap semiconductor using nanoemitters. The unusual temperature dependence of this energy transfer, which is measured using the donor nanoemitters of quantum dot (QD) layers integrated on the acceptor monocrystalline bulk silicon as a model system, is predicted by a phonon-assisted exciton transfer model proposed here. The model includes the phonon-mediated optical properties of silicon, while considering the contribution from the multimonolayer-equivalent QD film to the nonradiative energy transfer, which is derived with a d(-3) distance dependence. The FRET efficiencies are experimentally observed to decrease at cryogenic temperatures, which are well explained by the model considering the phonon depopulation in the indirect band-gap acceptor together with the changes in the quantum yield of the donor. These understandings will be crucial for designing FRET-enabled sensitization of silicon based high-efficiency excitonic systems using nanoemitters.