Anomalous Temperature Dependence Research Articles

Effectiveness of AlGaAs barrier layers as a redistribution channel of photoexcited carriers on anomalous temperature dependence of photoluminescence properties of GaAs quantum dots

Photoluminescence (PL) measurements at a wide temperature range, up to room temperature, for high-quality and high-density GaAs/AlGaAs quantum dots (QDs) fabricated by droplet epitaxy were carried out to investigate the anomalous temperature dependence of the PL peak energy from the QD ensemble. In addition to a reported redshift that deviated from the so-called Varshni's curve of the PL peak energy in the low temperature region, a new blueshift was observed above 200 K. We analyzed the experimental results using a steady-state rate equation model and observed a good agreement. The distribution of the QD sizes and the presence of the AlGaAs barrier layer as a carrier coupling channel were considered in this model. This means that the wetting layer proposed thus far is not a necessary condition for explaining the anomalous temperature behavior of the PL properties. In addition, it was found that the anomalous temperature behavior was smeared out by the insertion of a GaAs height adjustment layer in order to homogenize the apparent QD size. We found that sufficient control of the QD size is a necessary factor for high temperature stability of QD devices.

Anomalous Temperature Dependence of the Hall Coefficient of Heavily Al-Doped 4H-SiC Epilayers in the Band Conduction Region

We investigate the temperature dependence of the resistivity and Hall coefficient for heavily Al-doped p-type 4H-SiC epilayers with Al concentrations (C_Al) of > 2E19 cm^−3, which are substrates for the collectors of insulated-gate bipolar transistors. The signs of the measured Hall co- efficients (R_H) changed from positive to negative at low temperatures. For epilayers with C_Al values of < 3E19 cm^−3, a negative R_H was observed in the hopping conduction region. In contrast, for epilayers with C_Al values of > 3E19 cm^−3, a negative R_H was observed in not only the hopping conduction region but also the band conduction region, which is a striking feature because the movement of free holes in the valence band should make R_H positive. For an epilayer with C_Al of 1.8E20 cm^−3, the sign of R_H clearly changed three times in the band conduction region. Moreover, the activation energies of the temperature-dependent R_H values were similar to those of the temperature-dependent resistivity in the corresponding temperature ranges, irrespective of the conduction mechanisms (band and hopping conduction).

Reinvestigation of the intrinsic magnetic properties of [formula omitted] alloys and crystallization behavior of ribbons

New determination of the magnetic anisotropy from single crystals of (Fe1-xCox)2B alloys are presented. The anomalous temperature dependence of the anisotropy constant is discussed using the standard Callen-Callen theory, which is shown to be insufficient to explain the experimental results. A more material specific study using first-principles calculations with disordered moments approach gives a much more consistent interpretation of the experimental data. Since the intrinsic properties of the alloys with x=0.3-0.35 are promising for permanent magnets applications, initial investigation of the extrinsic properties are described, in particular the crystallization of melt spun ribbons with Cu, Al, and Ti additions. Previous attempts at developing a significant hysteresis have been unsuccessful in this system. Our melt-spinning experiment indicates that this system shows rapid crystallization.

Correlations between the structural, magnetic, and ferroelectric properties of BaMO3: M = Ti1-x(Mn/Fe)x compounds: A Raman study

Multiferroics possess two or more switchable states such as polarization, magnetization, etc. Phonon excitations in multiferroic phase are strongly modified by magnetoelectric coupling, spin-phonon coupling, and anharmonic phonon-phonon interactions. Here, we have investigated the correlation between phonons and multiferroic order parameters in hexagonal BaMO3: M = Ti1-x(Mn/Fe)x systems using powder x-ray diffraction (PXRD), Raman spectroscopic, and magnetic measurements. The structural transformation from a polar tetragonal to a non-polar 6H-type hexagonal phase is observed as a function of doping (Mn/Fe). Magnetic measurements reveal that the BaTi1-xMnxO3 is paramagnetic while BaTi1-xFexO3 exhibits composition-dependent ferromagnetic order. Importantly, Anomalous temperature-dependence is observed for two phonons (E1g at ∼ 152 cm−1 and A1g at ∼ 636 cm−1) in both the systems exhibiting similar trend with the doping (Mn/Fe) irrespective of the differences in their magnetic ground state. Hence, we attribute the phonon anomalies in both the (Fe/Mn doped) systems to strong anharmonic phonon-phonon interactions arising from large atomic displacements involved in the vibrations. In addition, we have also observed signatures of correlation of phonons with ferroelectric phase as well as magnetically ordered state suggesting the presence of a strain-induced magnetoelectric coupling in the doped compounds.

Study of spin-phonon coupling in multiferroic BiFeO3 through Raman spectroscopy

Spin-phonon coupling in multiferroic BiFeO3 single crystal is studied through temperature dependant polarized Raman spectroscopy. The phonon hardening appears at 200 K and becomes stronger below ~135 K. Clear anomalous temperature dependence of the phonon hardening near the magnetic transitions evidence the presence of a spin-phonon coupling in BiFeO3. Different phonon modes show very distinct spin-phonon coupling behaviors. Possible interpretations of the anomalous of the phonon hardening are discussed. We suggest that the phonon hardening would be correlated to spin reorientation and partially influenced by strains of Bi vacancies.

Topological insulators based on HgTe

The most interesting experimental results obtained in studies of 2D and 3D topological insulators (TIs) based on HgTe quantum wells and films are reviewed. In the case of 2D TIs, these include the observation of nonlocal ballistic and diffusion transport, the magnetic breakdown of 2D TIs, and an anomalous temperature dependence of edge-channel resistance. In 3D TIs, a record-setting high mobility (up to 5 × 105 cm2 V−1 s−1) of surface two-dimensional Dirac fermions (DFs) has been attained. This enabled determining all the main TI parameters (the bulk gap and the density of DFs on both of its surfaces) and provided information on the phase of the Shubnikov–de Haas oscillations of DFs, which indicates the rigid topological coupling between the fermion spin and momentum. Prospects for further research are discussed in the conclusion.

Unconventional spin excitations in the S=32 triangular antiferromagnet RbAg2Cr[VO4]2

We present muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) measurements of the $S=3/2$ undistorted triangular lattice established in ${\mathrm{RbAg}}_{2}\mathrm{Cr}{[{\mathrm{VO}}_{4}]}_{2}$. The zero- (ZF) and longitudinal-field $\ensuremath{\mu}\mathrm{SR}$ spectra evidence the absence of spin freezing and long-range magnetic ordering down to $T=25$ mK, supporting the formation of a dynamic ground state. Noticeably, we observe an anomalous temperature dependence of the ZF muon spin relaxation rate ${\ensuremath{\lambda}}_{Z}F(T)$, featuring a decrease below $T=20$ K. This suggests the alteration of the dominant relaxation mechanism by the development of short-range magnetic correlations. A subsequent leveling off of ${\ensuremath{\lambda}}_{Z}F(T)$ below $T=2$ K indicates persistent spin dynamics and reveals the presence of exotic magnetic excitations. The field dependence of the muon spin relaxation rate at $T=25$ mK is well described by a diffusive spin transport model with algebraic spin-spin correlations. The suppressed long-range order and the peculiar temperature-dependent behavior of ${\ensuremath{\lambda}}_{Z}F(T)$ will be discussed in terms of the exchange interaction between ${\mathrm{Cr}}^{3+}$ moments via nonmagnetic ${[{\mathrm{VO}}_{4}]}^{3\ensuremath{-}}$ entities. In the title compound, the degeneracy of the ${t}_{2g}$-orbital set is not lifted by a space group symmetry reduction or subject to significant anisotropy resulting from spin-orbit coupling.

A Review for Consistent Analysis of Hydrogen Permeability through Dense Metallic Membranes.

The hydrogen permeation coefficient (ϕ) is generally used as a measure to show hydrogen permeation ability through dense metallic membranes, which is the product of the Fick’s diffusion coefficient (D) and the Sieverts’ solubility constant (K). However, the hydrogen permeability of metal membranes cannot be analyzed consistently with this conventional description. In this paper, various methods for consistent analysis of hydrogen permeability are reviewed. The derivations of the descriptions are explained in detail and four applications of the consistent descriptions of hydrogen permeability are introduced: (1) prediction of hydrogen flux under given conditions, (2) comparability of hydrogen permeability, (3) understanding of the anomalous temperature dependence of hydrogen permeability of Pd-Ag alloy membrane, and (4) design of alloy composition of non-Pd-based alloy membranes to satisfy both high hydrogen permeability together with strong resistance to hydrogen embrittlement.

Anomalous Temperature Dependence of Quantum Correction to the Conductivity of Magnetic Topological Insulators.

Quantum transport in magnetic topological insulators reveals a strong interplay between magnetism and topology of electronic band structures. A recent experiment on magnetically doped topological insulator Bi_{2}Se_{3} thin films showed the anomalous temperature dependence of the magnetoconductivity while their field dependence presents a clear signature of weak antilocalization [Tkac etal., Phys. Rev. Lett. 123, 036406 (2019)PRLTAO0031-900710.1103/PhysRevLett.123.036406]. Here, we demonstrate that the tiny mass of the surface electrons induced by the bulk magnetization leads to a temperature-dependent correction to the π Berry phase and generates a decoherence mechanism to the phase coherence length of the surface electrons. As a consequence, the quantum correction to conductivity can exhibit nonmonotonic behavior by decreasing the temperature. This effect is attributed to the close relation of the Berry phase and quantum interference of the topological surface electrons in quantum topological materials.

Critical phenomena of magnetization, magnetocaloric effect, and superparamagnetism in nanoparticles of non-stoichiometric manganite

Magnetic nanoparticles of the La 0 . 6 Ag 0 . 2 Mn 1 . 2 O 3 manganite with superstoichiometric manganese have been obtained using nitrate pyrolysis method. Structure, phase composition, morphology, and size of the nanoparticles have been determined. Magnetic properties of the La 0 . 6 Ag 0 . 2 Mn 1 . 2 O 3 nanoparticles have been studied near the temperature of their magnetic ordering. It has been shown that magnetization near critical temperature is equilibrium and superparamagnetic. It has been found that magnetization and a change in entropy have critical behavior with a large value of the magnetocaloric effect. Interrelation between the critical indices of magnetization and the magnetocaloric effect has been established. In strong magnetic field, these properties of the nanoparticles have behaviour comparable with bulk samples. In weak magnetic field, the effect of superparamagnetism on the entropy has been found. The magnitude of the contribution from superparamagnetism to the magnetocaloric effect has anomalous temperature dependence. The obtained results of the influence of superparamagnetism on the thermomagnetic properties of the nanoparticles near critical temperature are important to understand and can be used for applied applications. • Magnetically homogeneous La 0 · 6 Ag 0 · 2 Mn 1 · 2 O 3 nanopowder has been prepared. • It has been found that the soft magnetic nanoparticles are in SPM state near T C . • Critical indices obtained experimentally are in good agreement with each other. • In small H , an entropy change Δ S M associated with SPM near T C has anomalous maximum. • High values of Δ S M in the nanopowder under small magnetic field have been obtained.

Strong Reduction in Amplitude of the Interfacial Segmental Dynamics in Polymer Nanocomposites

Despite the wide use of polymer nanocomposites (PNCs) in various applications, our understanding of the microscopic parameters controlling their macroscopic properties remains limited. In this study, we examine the dielectric strength of segmental dynamics, ΔeIL(T) in the interfacial polymer layer surrounding the nanoparticles in PNCs. The presented analysis reveals a significant drop in ΔeIL(T) and its anomalous temperature dependence in the polymer layer adsorbed to nanoparticles. The drop in ΔeIL(T) was observed in all samples regardless of whether segmental relaxation time in the interfacial layer was slower or faster than in the bulk polymer, excluding interpretation of the “dead” layer. We ascribe the observed decrease in the dielectric strength to the restricted amplitude of segmental relaxation in the interfacial/adsorbed layer. Our results provide a new perspective on discussion of dynamics in the interfacial layer in PNCs and thin polymer films, demonstrating that not only segmental relaxation time but also its amplitude can be strongly affected by the interface.

The negativity contour: a quasi-local measure of entanglement for mixed states

In this paper, we study the entanglement structure of mixed states in quantum many-body systems using the negativity contour, a local measure of entanglement that determines which real-space degrees of freedom in a subregion are contributing to the logarithmic negativity and with what magnitude. We construct an explicit contour function for Gaussian states using the fermionic partial-transpose. We generalize this contour function to generic many-body systems using a natural combination of derivatives of the logarithmic negativity. Though the latter negativity contour function is not strictly positive for all quantum systems, it is simple to compute and produces reasonable and interesting results. In particular, it rigorously satisfies the positivity condition for all holographic states and those obeying the quasi-particle picture. We apply this formalism to quantum field theories with a Fermi surface, contrasting the entanglement structure of Fermi liquids and holographic (hyperscale violating) non-Fermi liquids. The analysis of non-Fermi liquids show anomalous temperature dependence of the negativity depending on the dynamical critical exponent. We further compute the negativity contour following a quantum quench and discuss how this may clarify certain aspects of thermalization.

Superconductivity in ternary borides MReB (M = Mo, W) with the CuAl2-type structure

Abstract We report the structural and superconducting properties of ternary transition metal borides MoReB and WReB. Both compounds crystallize in the tetragonal CuAl2-type structure, in which the Re and Mo/W atoms share the same crystallographic site and form one-dimensional zigzag chains along the c-axis. The results of resistivity, magnetic susceptibility, and specific heat measurements indicate that these borides are weakly coupled, fully gapped, type-II superconductors. The superconducting transition temperature T c , upper critical field, and Ginzburg-Laudau parameter are determined to be 5.25 K, 2.1 T, 15.6 for MoReB, and 5.30 K, 2.0 T, 15.7 for WReB. Despite their nearly identical T c values, the Sommerfeld coefficient of WReB is more than 20% smaller than that of MoReB, suggesting that the electronic density of states at the Fermi level is not the dominant factor in determining T c . In addition, the upper critical fields of both MoReB and WReB exhibit an anomalous linear temperature dependence and possible origins are discussed.

Anomalous temperature dependence of the electrical conductivity in metal/β-Ga2O3/n-Si structures

Electrical characteristics of Metal/β-Ga2O3/n-Si structures with pure β-Ga2O3, Ti – and Cr – doped β-Ga2O3 films have been investigated. For all samples, regardless of the dopant type, a current decrease with the temperature increase is observed both at forward and reverse biases. The anomalous temperature dependencies are explained by the presence of traps located in a certain energy range near the bottom of the conduction band Ec. When the positive potential is applied to the electrode on the side of gallium oxide, the current-voltage characteristics are described as space charge limited current in the presence of energy distributed traps. In the case of a negative potential applied to the electrode on the side of gallium oxide, the reverse currents are due to the hopping conduction to the nearest neighboring state unoccupied by the electron.

Retrieving CeB6 's lost magnetic entropy

The reported temperature variations of CeB6 s magnetic entropy are inconsistent with the fourfold degeneracy of the crystal field ground state. This old question is here addressed through new specific heat measurements and an improved description, in the cage context, of both the phonons and crystal field contributions to the specific heat. The antiferromagnetic transition is characterized as first-order and its latent heat determined. From the phonons dispersion for a cage compound, the lattice specific heat contribution is derived from the LaB6 data. Once corrected for the first-order transition and lattice contributions, the magnetic entropy displays the characteristic plateau of the quadruplet crystal field ground state, but at temperatures in excess of 30 K. Below 30 K, as the ordering temperature is approached, the magnetic entropy is substantially reduced. This anomalous temperature dependence is consistent with a crystal field ground state split by the rare-earth movement, a phenomenon specific to rare-earth cage compounds.

Simultaneous Enhancement and Modulation of Upconversion by Thermal Stimulation in Sc2Mo3O12 Crystals.

Rational control of photoluminescence against a change in temperature is important for fundamental research and technological applications. Herein, we report an anomalous temperature dependence of upconversion luminescence in Yb/Ho co-doped Sc2Mo3O12 crystals. By leveraging negative thermal expansion of the crystal lattice, energy transfer between the lanthanide dopants is promoted as the temperature is increased from 303 to 573 K, resulting in an ∼5-fold enhancement of the emission. Meanwhile, the emission profile is also substantially altered due to the concurrent thermal quenching of selective energy states, corresponding to a clear shift in color from green to red. Via correlation of the red-to-green emission intensity ratio of Ho3+ dopant ions with temperature, a ratiometric luminescence thermometer is constructed with a maximum sensitivity of 2.75% K-1 at 543 K. As the Sc2Mo3O12 crystals are thermally stable and nonhygroscopic, our findings highlight a general approach for highly reversible control of upconversion by temperature in ambient air.

Anisotropy of microstructure and tensile properties of Ti–48Al–2Cr–2Nb fabricated by electron beam melting

The anisotropy of Ti–48Al–2Cr–2Nb (TiAl-4822) fabricated by electron beam melting (EBM) with 3 different microstructures, namely hot isostatic pressing (HIP), duplex (DP) and fully lamellar (FL) microstructure was investigated. Results show that the HIP material exhibits microstructure with alternating layers of coarse and fine grains. The α2 phase is more concentrated in the layer of fine grains and exhibits a strong texture with (0001) nearly perpendicular to the building direction, which further leads to the preferential orientation of lamellae. Non-uniform distribution of Al results in the selective formation of lamellar colonies in DP microstructure. The variation of critical resolved shear stress (CRSS) for slip activation is believed to cause the anomalous temperature dependence of yield strength at intermediate temperature. For all 3 microstructures, the ductility in Z direction is lower compared with XY direction, which is attributed to the difference in crack initiation mode and preferential orientation of lamellae.

Magnetic behavior of a ferroferrimagnetic ternary alloy AB_{ρ}C_{1-ρ} with a selective site disorder: Case study of a mixed-spin Ising model on a honeycomb lattice.

Phase transitions, compensation phenomenon, and magnetization of a ferroferrimagnetic ternary alloy AB_{ρ}C_{1-ρ} composed of three different kinds of magnetic ions A, B, and C with the spin magnitudes 1/2, 1, and 3/2 are examined within the framework of a mixed-spin Ising model on a honeycomb lattice with a selective annealed site disorder on one of its two sublattices. It is supposed that the first sublattice of a bipartite honeycomb lattice is formed by the spin-1/2 magnetic ions, while the sites of the second sublattice are randomly occupied either by the spin-1 magnetic ions with a probability ρ or the spin-3/2 magnetic ions with a probability 1-ρ, both being subject to a uniaxial single-ion anisotropy. The model under investigation can be exactly mapped into an effective spin-1/2 Ising model on a triangular lattice through the generalized star-triangle transformation. For a specific concentration of the spin-1 (spin-3/2) magnetic ions, it is shown that the ferroferrimagnetic version of the studied model may display a compensation temperature at which the total magnetization vanishes below a critical temperature. The critical temperature strikingly may also become independent of the concentration of the randomly mixed spin-1 and spin-3/2 magnetic ions for a specific value of a uniaxial single-ion anisotropy. The spontaneous magnetic order may be notably restored at finite temperatures through the order-by-disorder mechanism above a disordered ground state, which results in an anomalous temperature dependence of the total magnetization with double reentrant phase transitions.

Synthesis and Characterization of Two New Second Harmonic Generation Active Iodates: K3Sc(IO3)6 and KSc(IO3)3Cl.

Transparent single crystals of two new iodates K3Sc(IO3)6 and KSc(IO3)3Cl have been synthesized hydrothermally. Single-crystal X-ray diffraction was used to determine their crystal structures. Both compounds crystallize in non-centrosymmetric space groups. The compound K3Sc(IO3)6 crystallizes in the orthorhombic space group Fdd2. The crystal structure is made up of [ScO6] octahedra, [IO3] trigonal pyramids, and [KO8] distorted cubes. The compound KSc(IO3)3Cl crystallizes in the trigonal space group R3. The building blocks are [ScO6] octahedra, [KO12] polyhedra, and [IO3] trigonal pyramids. The Cl– ions act as counter ions and reside in tunnels in the crystal structure. The second harmonic generation (SHG) measurements at room temperature, using 1064 nm radiation, on polycrystalline samples show that the SHG intensities of K3Sc(IO3)6 and KSc(IO3)3Cl are around 2.8 and 2.5 times that of KH2PO4 (KDP), respectively. In addition, K3Sc(IO3)6 and KSc(IO3)3Cl are phase-matchable at the fundamental wavelength of 1064 nm. The large anharmonicity in the optical response of both compounds is further supported by an anomalous temperature dependence of optical phonon frequencies as well as their enlarged intensities in Raman scattering. The latter corresponds to a very large electronic polarizability.

Sign switching of dimer correlations in SrCu2(BO3)2 under hydrostatic pressure

Magnetic and vibrational excitations in SrCu$_2$(BO$_3$)$_2$ are studied using Raman spectroscopy at hydrostatic pressures up to 34 kbar and temperatures down to 2.6 K. The frequency of a particular optical phonon, the so-called pantograph mode, shows a very strong anomalous temperature dependence below about 40 K. We link the magnitude of the effect to the magnetic exchange energy on the dimer bonds in the Sutherland-Shastry spin lattice in this material. The corresponding dimer spin correlations are quantitatively estimated and found to be strongly pressure dependent. At around P$_2\sim$22 kbar they switch from antiferromagnetic to being predominantly ferromagnetic.