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Spectroscopic properties and martensitic phase transition of Y4Al2O9:Ce single crystals under high pressure
摘要： High pressure studies of monoclinic yttrium aluminum oxide single crystals (Y4Al2O9 -YAM) doped with Ce3+ ions grown by micro-pulling-down method are reported. The results of absorption measurements in the mid-infrared prove the existence of four different sites in which Ce3+ ions substitute Y ions, in accordance with the crystallographic structure of YAM. The lowest 5d level of Ce3+ is located very close to the bottom of the conduction band for two of four Ce3+ related centers at ambient pressure, which results in strong temperature quenching of their luminescence. Two other Ce3+ centers do not emit at ambient pressure since their 5d levels are resonant with the conduction band. Application of high pressure above 11 GPa restores their luminescence. Consequences of such energy structure of various Ce3+ sites in YAM for the Dorenbos theory are discussed. The bandgap energy of YAM is found to be 5.93 eV, which is the smallest out of the three yttrium aluminum oxides. High-pressure synchrotron angle dispersive x-ray diffraction and Raman scattering measurements identify a phase transition occurring at pressures between 8 and 11 GPa, which has martensitic character. The second phase transition to another structure (likely with hexagonal symmetry) occurs at a pressure around 16 GPa.
关键词： high pressure,phase transition,X-ray diffraction,luminescence,single crystal
更新于2025-09-10 09:29:36
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Optical properties of CH3NH3PbI3 crystal grown using inverse temperature crystallization
摘要： Perovskite CH3NH3PbI3 (MAPbI3) single crystal was grown using inverse temperature crystallization method. Crystallinity of the perovskite was con?rmed by X-ray di?raction. Photoluminescence (PL) spectra revealed abnormal behavior due to a temperature-induced orthorhombic to the tetragonal phase transition. Four PL emission peaks, A, B, C, and D, were observed in the low temperature regime. Peaks A and B were observed at 756 and 776 nm at 12 K, and were blue-shifted and disappeared at 130 and 70 K, respectively. Peaks C and D were observed at 789 and 807 nm at 40 K and were also blue-shifted to 780 and 794 nm at 100 K. On the other hand, the peak C red-shifted to 799 nm from 100 to 140 K because of an orthorhombic to the tetragonal phase change and was also blue-shifted above 140 K. From the excitation intensity- and temperature-dependent PL results, peaks A and B were assigned to the free-exciton and bound-exciton of the orthorhombic phase crystal, respectively. In addition, peaks C and D were associated with the free-exciton and bound-exciton of the tetragonal phase crystal, respectively. The activation energy of peak C was calculated to be 98 meV from temperature dependence of the PL intensity.
关键词： Phase transition,MAPbI3,Perovskite,Photoluminescence,Exciton
更新于2025-09-10 09:29:36
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Scratching of silicon surfaces
摘要： Nanoscale scratching of silicon surfaces is the elementary abrasive event for various machining techniques including fixed abrasive wiresaw slicing, grinding, elliptical ultrasonic cutting and single-point diamond turning. The understanding of this process is essential for improving the surface quality and reducing sub-surface damage. Nanoscratching experiments are performed using a well characterized diamond tip geometry. The finite element method is employed in order to simulate the scratching process with a continuum constitutive model developed for phase transformation in silicon (Budnitzki, M., Kuna, M., 2016. Stress induced phase transitions in silicon. JMPS 95, 64–91). The required material parameters were determined from indentation experiments in a (111) single crystal Si wafer. The simulation results agree very well with data from scratch experiments without requiring additional calibration.
关键词： silicon,scratching,phase transition,grinding,wiresawing,simulation
更新于2025-09-10 09:29:36
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Hydrogen-bonded and supramolecular ferroelectricity in a new hybrid (C<sub>12</sub>H<sub>25</sub>NH<sub>3</sub>)<sub>2</sub>CoCl<sub>4</sub>
摘要： A new organic-inorganic layered halide perovskite ((C12H25NH3)2CoCl4) has been synthesized and characterized by: chemical analysis, X-ray powder diffraction, differential scanning calorimetry, and differential thermal analysis thermographs. The dielectric and ferroelectric properties have been investigated. Analysis of the ac conductivity σac (ω, T) measured in the temperature interval from 110K to 370K and at selected frequencies in the range 1 kHz< f<100 kHz and the results show that the Jonscher universal power law is accurately obeyed. The existence of ferroelectricity has been demonstrated by the hysteresis P-E loop. The collective data evidenced that the hybrid behaves as a proper ferroelectric rather than a relaxor one and undergoes an order-disorder ferroelectric phase transition at the Curie temperature Tc (Tc?360K) accompanied with a structural distortion from an intercalated to non- intercalated state as the other Co and/or Zn hybrids. Since the room temperature space group of the hybrid is centrosymmetric (P21/c), the ferroelectricity should be due to the supramolecular interaction which results from the induced dipoles. The hydrogen bonding system of the type N-H....Cl plays the key role in the mechanism of supramolecular ferroelectricity.
关键词： ac induced conductivity and complex dielectric permittivity,phase transition,Supramolecular ferroelectricity,polarization
更新于2025-09-10 09:29:36
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Ferroelectric behavior in nanocrystalline KNbO <sub/>3</sub> synthesized by a modified polymerized complex method
摘要： Effect of nanocrystalline size of the particles on ferroelectric properties of KNbO3 is studied. Properties like hysteresis parameters, phase transition temperatures and domain morphology in nanocrystallites were studied by using P-E hysteresis, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) techniques, respectively. Nanocrystalline KNbO3 compound was synthesized by a modified polymerized complex method and room-temperature structural phase was characterized by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. Field emission gun-scanning electron microscopy (FEG-SEM) was used to reveal the size and morphology of the particles. It was found that the present nanoparticles of sizes ranging from 50 to 250 nm bears polarization values <1 lC/cm2, and undergo cubic-tetragonal and tetragonal-orthorhombic phase transitions at 312 and 87 (cid:1)C respectively. Although the value of polarization is small as compared to the bulk reported in the literature, the nature of P-E loops along with phase transitions and presence of 60, 90 and 180(cid:1) domain structures indicates retention of ferroelectric nature in KNbO3 even at the nanolevel.
关键词： domain structure,Nanocrystalline material,phase transition,ferroelectrics
更新于2025-09-10 09:29:36
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Spectroscopic signature of trimer Mott insulator and charge disproportionation in
摘要： We have measured the re?ectivity spectra of the barium iridate 9R BaIrO3, the crystal structure of which consists of characteristic Ir3O12 trimers. In the high-temperature phase above the transition temperature Tc (cid:2) 180 K, we ?nd that the optical conductivity involves two temperature-dependent optical transitions with an ill-de?ned Drude response. These features are reminiscent of the optical spectra in the organic dimer Mott insulators, implying a possible emergence of an unusual electronic state named trimer Mott insulator in BaIrO3, where the carrier is localized on the trimer owing to the strong Coulomb repulsion. Along with a pronounced splitting of the phonon peak observed below Tc, which is a hallmark of charge disproportionation, we discuss a possible phase transition from the trimer Mott insulator to a charge-ordered insulating phase in BaIrO3.
关键词： phase transition,charge disproportionation,BaIrO3,optical conductivity,trimer Mott insulator
更新于2025-09-10 09:29:36
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Quantum Zeno effect and the many-body entanglement transition
摘要： We introduce and explore a one-dimensional “hybrid” quantum circuit model consisting of both unitary gates and projective measurements. While the unitary gates are drawn from a random distribution and act uniformly in the circuit, the measurements are made at random positions and times throughout the system. By varying the measurement rate we can tune between the volume law entangled phase for the random unitary circuit model (no measurements) and a “quantum Zeno phase” where strong measurements suppress the entanglement growth to saturate in an area law. Extensive numerical simulations of the quantum trajectories of the many-particle wave functions (exploiting Clifford circuitry to access systems up to 512 qubits) provide evidence for a stable “weak measurement phase” that exhibits volume-law entanglement entropy, with a coef?cient decreasing with increasing measurement rate. We also present evidence for a continuous quantum dynamical phase transition between the “weak measurement phase” and the “quantum Zeno phase,” driven by a competition between the entangling tendencies of unitary evolution and the disentangling tendencies of projective measurements. Detailed steady-state and dynamic critical properties of this quantum entanglement transition are accessed.
关键词： quantum circuit model,volume law entangled phase,projective measurements,weak measurement phase,quantum Zeno effect,many-body entanglement transition,area law,quantum dynamical phase transition
更新于2025-09-09 09:28:46
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A Scanning Tunneling Microscopy Study of Monolayer and Bilayer Transition-Metal Dichalcogenides Grown by Molecular-Beam Epitaxy
摘要： This review presents an account of some recent scanning tunneling microscopy and spectroscopy (STM/S) studies of monolayer and bilayer transition-metal dichalcogenide (TMD) films grown by molecular-beam epitaxy (MBE). In addition to some intrinsic properties revealed by STM/S, defects such as inversion domain boundaries and point defects, their properties and induced effects, are presented. More specifically, the quantum confinement and moiré potential effects, charge state transition, quasi-particle interference and structural phase transition as revealed by STM/S are described.
关键词： transition-metal dichalcogenides,quantum confinement,structural phase transition,molecular-beam epitaxy,moiré potential,quasi-particle interference,scanning tunneling microscopy,charge state transition
更新于2025-09-09 09:28:46
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Infrared Investigations of the Neutral-Ionic Phase Transition in TTF-CA and Its Dynamics
摘要： The neutral-ionic phase transition in TTF-CA was investigated by steady-state and time-resolved infrared spectroscopy. We describe the growth of high-quality single crystals and their characterization. Extended theoretical calculations were performed in order to obtain the band structure, the molecular vibrational modes and the optical spectra along all crystallographic axes. The theoretical results are compared to polarization-dependent infrared reflection experiments. The temperature-dependent optical conductivity is discussed in detail. We study the photo-induced phase transition in the vicinity of thermally-induced neutral-ionic transition. The observed temporal dynamics of the photo-induced states is attributed to the random-walk of neutral-ionic domain walls. We simulate the random-walk annihilation process of domain walls on a one-dimensional chain.
关键词： domain-wall motion,neutral-ionic phase transition,steady-state and transient infrared spectroscopy,TTF-CA,charge-transfer salts,characterization,random-walk simulations,light-induced phase transition,vibrational spectroscopy,crystal growth
更新于2025-09-09 09:28:46
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Effective Three-Body Interactions in Jaynes-Cummings-Hubbard Systems
摘要： A generalisation of the Jaynes-Cummings-Hubbard model for coupled-cavity arrays is introduced, where the embedded two-level system in each cavity is replaced by a Ξ-type three-level system. We demonstrate that the resulting effective polariton-polariton interactions at each site are both two-body and three-body. By tuning the ratio of the two transition dipole matrix elements, we show that the strength and sign of the two-body interaction can be controlled whilst maintaining a three-body repulsion. We then proceed to demonstrate how different two-body and three-body interactions alter the mean field superfluid-Mott insulator phase diagram, with the possible emergence of a pair superfluid phase in the two-body attractive regime.
关键词： Jaynes-Cummings-Hubbard model,pair superfluid phase,three-level system,superfluid-Mott insulator phase transition,polariton-polariton interactions
更新于2025-09-09 09:28:46