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Atomic ordering, magnetic properties, and electronic structure of Mn<sub>2</sub>CoGa Heusler alloy
摘要： The magnetic properties and atomic arrangement of Mn2CoGa Heusler alloy were investigated experimentally and by theoretical calculations. The magnetic moment derived from spontaneous magnetization at 5 K was 2.06 μB/f.u. and was close to the integer number of the expected value from theoretical calculation and the Slater–Pauling rule predicted by Galanakis et al. The Curie temperature and L21-B2 order-disorder phase transition temperature were 741 and 1047 K, respectively. Powder neutron diffraction experiment results suggested that the atomic arrangement prefers an L21b-type structure rather than that of Hg2CuTi, being consistent with our previous results of high-angle annular dark-field-scanning transmission electron microscopic observations. The magnetic moments obtained were in good agreement with the theoretical values in the model of the L21b-type structure. The density of states obtained by the first-principles calculation combined with the coherent potential approximation in Mn2CoGa with the L21b-type crystal structure maintained the half-metallic character, even though disordering by Mn and Co atoms was introduced.
关键词： powder neutron diffraction,magnetic moment,Half-metal-type ferromagnets,order-disorder phase transition,atomic ordering,Curie temperature
更新于2025-09-09 09:28:46
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Engineering SrSnO <sub/>3</sub> Phases and Electron Mobility at Room Temperature Using Epitaxial Strain
摘要： High-speed electronics require epitaxial films with exceptionally high carrier mobility at room temperature. Alkaline-earth stannates with high room-temperature mobility show outstanding prospects for oxide electronics operating at ambient temperatures. However, despite significant progress over the last few years, mobility in stannate films has been limited by dislocations due to the inability to grow fully coherent films. Here, we demonstrate the growth of coherent, strain-engineered phases of epitaxial SrSnO3 (SSO) films using a radical-based molecular beam epitaxy approach. Compressive strain stabilized the high-symmetry tetragonal phase of SSO at room temperature (RT), which, in bulk, exists only at temperatures between 1062 K and 1295 K. We achieved a mobility enhancement of over 300% in doped films compared with the low temperature orthorhombic polymorph. Using comprehensive temperature-dependent synchrotron-based X-ray measurements, electronic transport and first principles calculations, crystal and electronic structures of SSO films were investigated as a function of strain. We argue that strain-engineered films of stannate will enable high mobility oxide electronics operating at RT with the added advantage of being optically transparent.
关键词： phase transition,Hybrid molecular beam epitaxy,half-order diffraction,strain engineering,density functional theory,high mobility,Octahedral rotations
更新于2025-09-09 09:28:46
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Distributions of steady states in a network of degenerate optical parametric oscillators in solving combinatorial optimization problems
摘要： We investigate a network of degenerate optical parametric oscillators (DOPOs) as a model of the coherent Ising machine, an architecture for solving Ising problems. The network represents the interaction in the Ising model, which is a generalization of a previously proposed one for the two-DOPO case. Dynamics of the DOPOs is described by the Fokker-Planck equation in the positive P representation. We obtain approximate distribution of steady states for arbitrary Ising problems under some ansatz. Using the method of statistical mechanics, we analytically demonstrate that the most probable states in a particular range of the parameters correspond to the true optimal states for two rather simple problems, i.e., fully connected ferromagnetic coupling without or with binary random fields. In particular, for the random-field problem, the distribution correctly detects the phase transition that occurs in the target Ising model with varying the magnitude of the fields.
关键词： statistical mechanics,coherent Ising machine,degenerate optical parametric oscillators,phase transition,Ising problems,Fokker-Planck equation
更新于2025-09-09 09:28:46
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Nodal points of Weyl semimetals survive the presence of moderate disorder
摘要： In this work we address the physics of individual three-dimensional Weyl nodes subject to a moderate concentration of disorder. Previous analysis indicates the presence of a quantum phase transition below which disorder becomes irrelevant and the integrity of sharp nodal points of vanishing spectral density is preserved in this system. This statement appears to be at variance with the inevitable presence of statistically rare fluctuations which cannot be considered as weak and must have strong influence on the system’s spectrum, no matter how small the average concentration. We here reconcile the two pictures by demonstrating that rare fluctuation potentials in the Weyl system generate a peculiar type of resonances which carry spectral density in any neighborhood of zero energy, but never at zero. In this way, the vanishing of the DoS for weak disorder survives the inclusion of rare events. We demonstrate this feature by considering three different models of disorder, each emphasizing specific aspects of the problem: a simplistic box potential model, a model with Gaussian distributed disorder, and one with a finite number of s-wave scatterers. Our analysis also explains why the protection of the nodal DoS may be difficult to see in simulations of finite size lattices.
关键词： Weyl semimetals,rare fluctuations,disorder,spectral density,quantum phase transition
更新于2025-09-09 09:28:46
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across the quantum phase transition from topological to trivial insulator
摘要： Using spin- and angle-resolved photoemission spectroscopy and relativistic many-body calculations, we investigate the evolution of the electronic structure of (Bi1?xInx )2Se3 bulk single crystals around the critical point of the trivial to topological insulator quantum-phase transition. By increasing x, we observe how a surface gap opens at the Dirac point of the initially gapless topological surface state of Bi2Se3, leading to the existence of massive fermions. The surface gap monotonically increases for a wide range of x values across the topological and trivial sides of the quantum-phase transition. By means of photon-energy-dependent measurements, we demonstrate that the gapped surface state survives the inversion of the bulk bands which occurs at a critical point near x = 0.055. The surface state exhibits a nonzero in-plane spin polarization which decays exponentially with increasing x, and which persists in both the topological and trivial insulator phases. Our calculations reveal qualitative agreement with the experimental results all across the quantum-phase transition upon the systematic variation of the spin-orbit coupling strength. A non-time-reversal symmetry-breaking mechanism of bulk-mediated scattering processes that increase with decreasing spin-orbit coupling strength is proposed as explanation.
关键词： quantum-phase transition,topological insulator,electronic structure,photoemission spectroscopy,spin-orbit coupling
更新于2025-09-09 09:28:46
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Temperature-dependent Raman spectroscopic study of ferroelastic K <sub/>2</sub> Sr(MoO <sub/>4</sub> )
摘要： Raman scattering measurements of K2Sr(MoO4)2 were performed in the temperature range of 25–750 ?C. The Raman spectrum of the low-temperature phase α-K2Sr(MoO4)2 that was obtained by first-principle calculations indicated that the Raman bands in the wavenumber region of 250–500 cm?1 are related to Mo–O bending vibrations in MoO4 tetrahedra, while the Raman bands in the wavenumber region of 650–950 cm?1 are attributed to stretching vibrations of Mo–O bonds. The temperature-dependent Raman spectra reveal that K2Sr(MoO4)2 exhibits two sets of modifications in the Raman spectra at ～ 150 ?C and ～ 475 ?C, attributed to structural phase transitions. The large change of the Raman spectra in the temperature range of 150 ?C to 475 ?C suggests structural instability of the medium-temperature phase β -K2Sr(MoO4)2.
关键词： ferroelastics,K2Sr(MoO4)2 crystal,phase transition,Raman spectroscopy
更新于2025-09-09 09:28:46
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Tungsten doped M-phase VO2 mesoporous nanocrystals with enhanced comprehensive thermochromic properties for smart windows
摘要： The phase transition temperature (~ 68 oC) of M-VO2 film can be lowered significantly by tungsten (W) doping into the crystal lattice of VO2 due to the reduction of the strength of V-V pair interaction. However, W doping was always coupled with a serious weakening of luminous transmittance and solar modulation efficiency because W dopants can increase the electron concentration of VO2 film. Herein, the simultaneous introduction of W dopants and mesopores into M-VO2 nanocrystals was employed to prepare VO2 film. Interestingly, the obtained 0.4 at% W-doped mesoporous VO2 nanocrystals based composite films exhibited enhanced comprehensive thermochromic performance with excellent solar modulation efficiency (ΔTsol = 11.4%), suitable luminous transmittance (Tlum = 61.6%) and low phase transition temperature around 43 oC, much lower than 65.3 oC of undoped VO2. It was demonstrated that the lower phase transition temperature of VO2 can be primarily attributed to abundant lattice distortion after W doping, whereas the mesoporous structure can facilitate the uniform distribution of W dopants in VO2 nanocrystals, enhance the luminous transmittance and guarantee enough VO2 nanocrystals in the composite film to keep relatively high solar modulation efficiency. Therefore, this work can provide a new way to balance the three important parameters for the thermochromic performance of VO2 film (ΔTsol, Tlum and Tc) and probably promote the application of VO2 nanocrystals in the energy efficient windows.
关键词： thermochromic performance,phase transition temperature,mesoporous structure,W-doped VO2
更新于2025-09-09 09:28:46
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The influence of external stress/strain on the phase transitions and dielectric properties of potassium niobate crystal
摘要： We investigated the effects of one-dimensional shock wave compression on the structural, dielectric, and ferroelectric properties of a potassium niobate ferroelectric crystal using Landau–Ginsburg–Devonshire phenomenological theory. Our results show that the cubic to tetragonal and tetragonal to orthorhombic phase transition temperatures increase with increasing shock compression and the derivative of the phase transition temperatures with respect to stain induced by shock compression was estimated to be 21.9 °C/10?3. Additionally, the orthorhombic to rhombohedral phase transition temperature decreased with increasing shock compression and the derivative of the phase transition temperatures with respect to stain induced by shock compression was estimated to be ?144.3 °C/10?3. This result represents a 15.3% increase in the remnant polarization of the tetragonal KNbO3 crystal when the stain induced by shock compression increased from 0 to 4×10?3 at 400 °C. The dielectric susceptibilities ε33 and ε22 were calculated to decrease by approximately 20% and 40%, respectively, when the stain induced by shock compression increased from 0 to 1.6×10?3 at 350 °C. The increase in the remnant polarization and the decrease in the dielectric properties induced by shock compression provide an effective way to improve the power output capacity of a ferroelectric-pulsed power supply.
关键词： phase transition,lead-free ferroelectric crystal,ferroelectric properties,dielectric properties,shock compression
更新于2025-09-09 09:28:46
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Temperature Dependence of the Luminescence Spectra of a 5CB Liquid Crystal and its Phase Transitions
摘要： The spectroluminescence properties of 4cyano4'pentylbiphenyl CH3(CH2)4(C6H4)2CN (5CB) were studied in the temperature range 4.2–297 K. A red shift of the fluorescence spectrum was noted with increasing temperature. The long-wavelength shifts in these spectra were also analyzed. Comparison of the temperature dependence curves for the emission at the band maxima λmax and their halfwidths Δλ/2 in the fluorescence spectra and the results of a differential scanning calorimetry (DSC) study showed that phase transitions occur in the 5CB liquid crystals at ~230 and ~260 K.
关键词： liquid crystal,phase transition,differential scanning calorimetry,luminescence,low temperature,conformation change
更新于2025-09-09 09:28:46
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Structural, Dielectric and Impedance Spectroscopic Studies on Fe Doped BaTiO <sub/>3</sub>
摘要： Compositions with x=0.03, 0.05 and 0.10 are synthesized in the system BaFexTi1–xO3 using conventional solid state reaction method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). XRD patterns reveal the coexistence of tetragonal and hexagonal phases which are strongly affected by the level of Fe doping. The hexagonal phase increases with increasing x, being maximum for x=0.10. SEM images show the presence of agglomerated grains which increase in size with amount of Fe doping. SEM for x=0.10 shows mostly rod-like or rectangular grains indicating predominantly hexagonal phase. Dielectric and impedance measurements have been carried out in the frequency range 20 Hz-1 MHz as a function of temperature (300-650 K). The samples with x=0.03 and 0.05 exhibit diffuse phase transition. Using complex impedance analysis equivalent circuit models have been developed which represent the data well and successfully separate out the contributions of tetragonal phase, hexagonal phase, grain boundaries and electrodes.
关键词： Impedance spectroscopy,Fe doped BaTiO3,Dielectric response,Diffuse phase transition
更新于2025-09-09 09:28:46