Unusual Temperature Dependence Research Articles

Temperature-dependent 57Fe Mössbauer spectroscopy and local structure of the mullite-type Bi2(FexGa1−x)4O9 (0.1≤x≤1) solid solution

The Bi2(FexGa1−x)4O9 oxide solid solution possessing a mullite-type structure has been investigated by 57Fe Mössbauer spectroscopy in dependence of composition (0.1≤x≤1) and temperature (293≤T/K≤1073). The spectra have been fitted with two doublets for tetrahedrally and octahedrally coordinated high-spin Fe3+ ions, respectively. The experimental areas of the subspectra were used to determine the distribution of iron on the two inequivalent structural sites. The fraction of iron cations occupying the octahedral site is found to increase with decreasing Fe content and the cation distribution is almost independent of temperature. The unusual temperature dependence of the quadrupolar splitting, QS, observed for the octahedral site with dQS/dT>0 is discussed in connexion with structural data for Bi2Fe4O9. The temperature dependence of Mössbauer isomer shifts and signal intensities is examined in the context of local vibrational properties of iron on the two inequivalent sites of the mullite-type lattice structure.

Saturation recovery electron spin–lattice relaxation studies on benzene anion radical and its derivatives: application of Kivelson–Orbach mechanism of electron spin relaxation

The role of low-lying excited states on the spin–lattice relaxation times (T1) of organic radicals has been investigated. To test the applicability of Kivelson's electric field fluctuation model (D. Kivelson, J. Chem. Phys. 45, 1324 (1966)), based on the Orbach mechanism of spin relaxation, the T1s of the anion radicals of benzene, benzene-1-d, toluene, ethyl benzene, isopropyl benzene, t-butyl benzene, p-xylene, 1,2,4-trimethyl benzene and 1,3,5-trimethyl benzene in liquid solutions, with potassium cation as the counter ion, have been measured by the pulse saturation recovery technique. The energy gap between the ground and the first excited electronic states changed with the substitutions to different extent. The spin–lattice relaxation rates showed correlation with this energy gap. Anion radicals of benzene and benzene-1-d showed the shortest T1 among the radicals studied here. A small but measurable energy splitting due to the deuterium substitution in benzene-1-d radical was obtained from the temperature dependence of T1. Spin–lattice relaxation times of benzene anion measured here decreased monotonically in the range of −60 to −125 °C, in contrast to some reported claims of very unusual temperature dependence, based on the continuous wave microwave power saturation studies. Our results also showed that the ion pairing between benzene anion and potassium cation did not significantly influence the spin–lattice relaxation times.

Excitons in nitride heterostructures: From zero- to one-dimensional behavior

We report an unusual temperature dependence of exciton lifetimes in arrays of GaN nanostructures grown on semipolar (11-22) oriented Al${}_{0.5}$Ga${}_{0.5}$N alloy by molecular beam epitaxy. Atomic force microscopy measurements revealed: (i) a one-dimensional ordering tendency along the [1-100] crystallographic direction together with (ii) an in-plane anisotropy of the nanostructure lateral shape with respect to [1-100] and [11-23] crystallographic axes. As a consequence, a morphological transition from dot-shaped islands forming an array of nanochains to wire-shaped objects elongated along the [1-100] direction was evidenced with the increase of the GaN deposited amount. Nanostructures of different dimensionality were fabricated including quantum dots (QDs), quantum wires (QWRs), and quantum wells (QWs), and the excitonic behavior was investigated as a function of the nanostructure shape. The measured temperature dependencies of the exciton radiative decay revealed its direct correlation with a spatial confinement, resulting in a temperature-independent exciton lifetime in the case of QDs, a square root dependence in the case of QWRs, and a linear dependence for QWs. These results, as well as absolute values of measured lifetimes, are in agreement with theoretical predictions.

Unusual Temperature Dependence of Heterogeneous Nucleation of Water Vapor on Ag Particles

Copyright 2013 American Association for Aerosol Research

Dynamic equilibrium explanation for nanobubbles' unusual temperature and saturation dependence

The dynamic equilibrium model suggests that surface nanobubbles can be stable due to an influx of gas in the vicinity of the bubble contact line, driven by substrate hydrophobicity, that balances the outflux of gas from the bubble apex. Here, we develop an alternate formulation of this mechanism that predicts rich behavior in agreement with recent experimental measurements. Namely, we find that stable nanobubbles exist in narrow temperature and dissolved gas concentration ranges, that there is a maximum and minimum possible bubble size, and that nanobubble radii decrease with temperature.

Observation of magnetic anisotropy increment with temperature in composition-graded FeCoZr thin films

Through a systematic investigation, we demonstrate that FeCoZr thin films deposited by gradient composition sputtering technique possess a unique magnetic thermal behavior, namely, the increase in magnetic anisotropy with temperature. Moreover, this gradient composition sputtering technique also offers a viable method to tailor the high-frequency magnetic properties as well as their thermal stability by changing deposition angle and deposition power. The unusual temperature dependence of magnetic anisotropy in such thin films can be interpreted in terms of stress-induced magnetic anisotropy arising from the composition gradient in the films.

Spin-state crossover model for the magnetism of iron pnictides.

We propose a minimal model describing magnetic behavior of Fe-based superconductors. The key ingredient of the model is a dynamical mixing of quasidegenerate spin states of Fe(2+) ion by intersite electron hoppings, resulting in an effective local spin S(eff). The moments S(eff) tend to form singlet pairs and may condense into a spin nematic phase due to the emergent biquadratic exchange couplings. The long-range ordered part m of S(eff) varies widely, 0 ≤ m ≤ S(eff), but magnon spectra are universal and scale with S(eff), resolving the puzzle of large but fluctuating Fe moments. Unusual temperature dependences of a local moment and spin susceptibility are also explained.

Optical absorption and photoreflectance spectroscopy of the single-crystalline chalcopyrite semiconductor AgGaSe2

Optical absorption and photoreflectance (PR) spectra have been measured on the single-crystalline chalcopyrite semiconductor AgGaSe2 for light polarization perpendicular (E ⊥ c) and parallel to the c-axis (E ‖ c) at T = 15–300 K. Optical absorption measurements suggest that AgGaSe2 is a direct-gap semiconductor having an optical band gap of E0 ∼ 1.8 eV at T = 15–300 K. The temperature-dependent PR spectra are obtained at T = 20–300 K in the 1.8–2.5 eV spectral ranges. The lowest band-gap energy E0 of AgGaSe2 shows unusual temperature dependence at T ≤ 80 K. The resultant temperature coefficients dE0/dT are positive at T ≤ 70 K and negative above 70 K, and are explained by considering the effects of thermal expansion and electron-phonon interaction. The spin-orbit and crystal-field splitting parameters are also determined to be Δso = 327 meV and Δcr = −288 meV at T = 20 K, respectively.

Theory of Topological Quantum Phase Transitions in 3D Noncentrosymmetric Systems

We construct a general theory describing the topological quantum phase transitions in 3D systems with broken inversion symmetry. While the consideration of the system's codimension generally predicts the appearance of a stable metallic phase between the normal and topological insulators, it is shown that a direct topological phase transition between two insulators is also possible when an accidental band crossing occurs along directions with high crystalline symmetry. At the quantum critical point, the energy dispersion becomes quadratic along one direction while the dispersions along the other two orthogonal directions are linear, which manifests the zero chirality of the band touching point. Because of the anisotropic dispersion at quantum critical point, various thermodynamic and transport properties show unusual temperature dependence and anisotropic behaviors.

Role of Quantum Effects in the Glass Transition

It is shown that quantum effects lead to a significant decrease of the glass transition temperature T(g) with respect to the melting temperature T(m), so that the ratio T(g)/T(m) can be much smaller than the typical value of 2/3 in materials where T(g) is near or below ~60 K. Furthermore, it is demonstrated that the viscosity or structural relaxation time in such low temperature glass formers should exhibit highly unusual temperature dependence, namely a decrease of the apparent activation energy upon approaching T(g) (instead of traditional increase).

C-axis resistivity, pseudogap, superconductivity, and Widom line in doped Mott insulators

Layered doped Mott insulators, such as the cuprates, show unusual temperature dependence of the resistivity. Intriguingly, the resistivity perpendicular to the CuO$_2$ planes, $\rho_c(T)$, shows both metallic ($d\rho_c/dT > 0$) and semi-conducting ($d\rho_c/dT<0$) behavior. We shed light on this puzzle by calculating $\rho_c$ for the two-dimensional Hubbard model within plaquette cellular dynamical mean-field theory and strong-coupling continuous-time quantum Monte Carlo as the impurity solver. The temperature, $T$, and doping, $\delta$, dependencies of $\rho_c$ are controlled by the first-order transition between pseudogap and correlated metal phases from which superconductivity can emerge. On the large doping side of the transition $\rho_c(T)$ is metallic, while on the low-doping side $\rho_c(T)$ changes from metallic to semi-conducting behavior with decreasing $T$. As a function of doping, the jump in $\rho_c$ across the first-order transition evolves into a sharp crossover at higher temperatures. This crossover coincides with the pseudogap temperature $T^*$ in the single-particle density of states, the spin susceptibility and other observables. Such coincidence in crossovers is expected along the continuation of the first-order transition into the super-critical regime, called the Widom line. This implies that not only the dynamic and the thermodynamic properties but also the DC transport in the normal state are governed by the hidden first-order transition. $\rho_c(T)$ has a high-temperature quasi-linear regime where it can exceed the Mott-Ioffe-Regel limit and when it has a minimum it is nearly parallel to the Widom line.

Anomalous surface lattice dynamics in the low-temperature phase of Ba(Fe 1− x Co x ) 2 As 2

In complex materials, how correlation between charge, spin, and lattice affects the emergent phenomena remains unclear. The newly discovered iron-based high-temperature superconductors and related compounds present to the community a prototype family of materials, where interplay between charge, spin, and lattice degrees of freedom can be explored. With the occurrence of structural, magnetic, and superconducting transitions in the bulk of these materials, creating a surface will change the delicate balance between these phases, resulting in new behavior. A surface lattice dynamics study on (001) Ba(Fe(1-x)Co(x))(2)As(2), through electron energy loss spectroscopy measurements, reveals unusual temperature dependence of both the phonon frequency and line width in the low-temperature orthorhombic phase. The rate of change of phonon frequency with temperature is gigantic, two orders of magnitude larger than in the bulk. This behavior cannot be explained using conventional models of anharmonicity or electron-phonon coupling; instead, it requires that a large surface-spin-charge-lattice coupling be included. Furthermore, the higher surface-phase-transition temperature driven by surface stabilization of the low-temperature orthorhombic phase seems to turn the first-order transition (bulk) into the second-order type, equivalent to what is observed in the bulk by applying a uniaxial pressure. Such equivalence indicates that the surface mirrors the bulk under extreme conditions.

Unusual temperature dependence of smectic layer structure associated with the nematic–smectic C phase transition in a hockey-stick-shaped four-ring compound

In a newly designed four-ring asymmetrical bent-core compound, we observed smectic-C-type diffuse layer reflection over the entire nematic temperature range. At the nematic–smectic C phase transition, a sharp layer reflection emerges in addition to the diffuse reflection with different layer tilt angles.

Phenylalanine‐based polyarylacetylenes as enantiomer‐differentiating alignment media

Lyotropic liquid crystalline phases of a phenylalanine-based polyacetylene are introduced as new enantiodifferentiating alignment media. Based on the unusual temperature dependence of the quadrupolar splitting of the (2)H-signal of the solvent (CDCl(3)), three distinct states of the phase with different orientational properties can be identified. This offers the opportunity to measure multiple alignment data sets without changing the sample. Unexpectedly, the largest difference in the orientation of the enantiomers of isopinocampheol was found in the high temperature domain of the phase. This is even more astonishing because the helical structure of the polymer backbone seems to break down at temperatures above approximately 10 °C, leaving the stereogenic centers of the amino acid moieties in the repeating units as the only reason for the enantiodifferentiation.

Unusual temperature dependence of the spectral weight near the Fermi level of NdNiO3thin films

We investigate the behavior of the spectral weight near the Fermi level of NdNiO3 thin films as a function of temperature across the metal-to-insulator transition (MIT) by means of ultraviolet photoelectron spectroscopy. The spectral weight was found to exhibit thermal hysteresis, similar to that of the dc conductivity. A detailed analysis of the temperature dependence reveals two distinct regimes of spectral loss close to the Fermi level. The temperature evolution of one regime is found to be independent from the MIT.

Barrier-controlled electron transport in Sn-doped ZnO polycrystalline thin films

We present a study of the temperature-dependent electrical conductivity in Sn-doped ZnO polycrystalline thin films with different Sn contents (0, 1, 3, 5, and 7at.%). The electrical conductivities show unusual temperature dependence in a temperature range of 110–400K, where electronic spatial potential fluctuations at the grain boundaries are observed. By applying a model supposing potential fluctuations at the grain boundaries we find that the potential barrier height decreases from 0.269eV down to 0.112eV and the homogeneity coefficient, which describes the homogeneity of films, decreases from 4.41 to 2.91 with increases in Sn content from 1at.% up to 7at.%. From these results, we see that the electron transport is essentially controlled by potential fluctuations at the grain boundaries for Sn-doped ZnO polycrystalline thin films. Our findings could be important for the design of high efficiency solar cell based on ZnO thin films.

Site-selective quantum correlations revealed by magnetic anisotropy in the tetramer system SeCuO3

We present the investigation of a monoclinic compound SeCuO${}_{3}$ using x-ray powder diffraction, magnetization, torque, and electron-spin-resonance. Structurally based analysis suggests that SeCuO${}_{3}$ can be considered as a three-dimensional network of tetramers. The values of intratetramer exchange interactions are extracted from the temperature dependence of the susceptibility and amount to $\ensuremath{\sim}\phantom{\rule{-0.16em}{0ex}}200$ K. The intertetramer coupling leads to the development of long-range antiferromagnetic order at ${T}_{N}=8$ K. An unusual temperature dependence of the effective $g$ tensors is observed, accompanied with a rotation of macroscopic magnetic axes. We explain this unique observation as due to site-selective quantum correlations.

Strong magnetic instability in correlated metallic Bi2Ir2O7

We report an experimental/theoretical study of single-crystal Bi2Ir2O7 that possesses a metallic state with strongly exchange-enhanced paramagnetism. The ground state of Bi2Ir2O7 is characterized by the following features: (1) a divergent low-temperature magnetic susceptibility that indicates no long-range order down to 50 mK; (2) strongly field-dependent coefficients of the low-temperature T and T3 terms of the specific heat; (3) a conspicuously large Wilson ratio RW ≈ 53.5; and (4) unusual temperature and field dependences of the Hall resistivity that abruptly change below 80 K, without any clear correlation with the magnetic behavior. All these unconventional properties suggest the existence of an exotic ground state in Bi2Ir2O7.

Crystal Structure, Magnetic and Transport Properties of CeRu1–xNixAl (x = 0.5)

AbstractSingle crystals of CeRu0.5Ni0.5Al can be obtained from a Ce‐Ni melt using elements as starting materials. The presented compound crystallizes in the orthorhombic space group Pnma (no. 62) with a = 700.38(7), b = 416.92(4) and c = 1562.84(16) pm in the LaNiAl structure type. The compound shows an unusual temperature dependence of the magnetic susceptibility for T &lt; 20 K although the cerium cations exhibit no local moment behavior. This feature is also visible in the electrical resistivity.

Elastic constants of langasite and alpha quartz at high temperatures measured by antenna transmission acoustic resonance

A method for measuring elastic constants of piezoelectric materials at high temperature up to 1224 K is proposed. It determines all independent elastic constants by measuring resonance frequencies of a rectangular parallelepiped piezoelectric specimen contactlessly using its own piezoelectricity with an antenna. Without using conventional contacting piezoelectric transducers, vibrational sources are excited directly in the specimen by the oscillating electric field. Capability of the method is demonstrated by measuring the elastic constants of langasite at high temperature up to 1224 K, and temperature coefficients of the elastic constants are determined. In addition, elastic constants of alpha quartz are measured at high temperature up to just below the alpha-beta phase transition temperature. Considering the local deformation with temperature increment, an interpretation based on the strain energy reduction is proposed for the unusual temperature dependence of C(66). Furthermore, the internal-friction tensor is measured, and the relationship between the observed anisotropy in internal friction and the structural evolution with temperature increment is discussed.