Journal of Physics: Condensed Matter最新文献

An exotic quantum mechanical ground state, i.e. the non-magneticJ_eff= 0 state, has been predicted for higher transition metalt2g4systems, due to the influence of strong spin-orbit coupling (SOC) or in other words, due to unquenched orbital moment contribution. However, previous attempts to experimentally realize such a state in 5d⁴systems had mostly been clouded by solid-state effects or the reduced strength of the renormalized SOC that might allow significant triplon condensation. Interestingly, a recent study on vacancy ordered double perovskite compound K₂RuCl₆by Takahashiet al(2021Phys. Rev. Lett.127227201) concluded that even withinLScoupling regime the Ru⁴⁺4d⁴ions, within isolated RuCl₆octahedra, strongly accommodateJmultiplets havingJ_eff= 0 as the ground state with weakly interactingJ_eff= 1 excitation, due to large unquenced Ru orbital angular momentum in the system. In the present report, we show results from the double perovskite La₂ZnRuO₆, where Ru⁴⁺ions form isolated RuO₆octahedra but unlike K₂RuCl₆, they remain chemically connected via corner-sharing with nonmagnetic ZnO₆octahedra. Next, we move on to separate out the RuO₆octahedra further by doping the Ru-site with Ti⁴⁺, in order to probe the character of the Ru⁴⁺ions within a different structural background. We find that the system stabilizes inP21/nspace group with tilted octahedra without distortion as has been confirmed by the x-ray powder diffraction and x-ray absorption spectroscopic studies. Interestingly, the x-ray photoelectron spectroscopic valance band spectra indicated certain inhomogeneity around the half-doping region, while confirming insulating ground state for all. Moreover, unlike the vacancy ordered double perovskite cases, it is observed that here the Ru orbital angular momentum gets substantially quenched and only the Ru spin magnetic moments are realized.

Driven by the miniaturization of microelectronic devices and their multifunctionalities, the development of new quadruple-perovskite oxides with high dielectric constants and high Curie temperature are highly required. Herein, we report on the structural, dielectric and magnetic properties of Sb/Cr-doped CaCu₃Ti₄O₁₂(CCTO) quadruple perovskite oxides, CaCu₃Ti_3.9Sb_0.1O₁₂(CCTSO) and CaCu₃Ti₃CrO₁₂(CCTCO). Structural Rietveld refinements demonstrated that the CCTO, CCTSO, and CCTCO ceramics adopted a cubic crystal structure (Im3¯ space group). They exhibited spherical shapes with nearly uniform particle sizes. XPS spectra clarified Cu³⁺ions in the CCTSO ceramics, while Cu²⁺and Cu⁺ions, and Cr³⁺ions in the CCTCO ceramics. All the ceramic samples displayed nearly frequency independent dielectric behaviors at low temperatures but a relaxor dielectric behavior at high temperatures. Such relaxor dielectric behavior in the CCTO and CCTSO ceramics was ascribed to the movement of doubly ionized oxygen vacancies but in the CCTCO ceramics to the movement of singly ionized oxygen vacancies. Impedance and modulus spectra reveal the significance contributions of grains and grain boundaries with non-Debye behavior. At room temperature (RT) the dielectric constant (ϵ_r) and dielectric loss (tanδ) of CCTO ceramics at 1 kHz were 15 922 and 0.126, respectively. An order reduction of tanδwas achieved in the CCTSO ceramics. In the CCTCO ceramics, theϵ_rand tanδat RT and 1 kHz were 975 and 0.453, respectively. The CCTCO powders exhibited antiferroelectric behavior at 2 K with a saturation magnetization (M_S) of 1.42μ_B/f.u., while theM_Sfor the CCTSO powders was only 0.027μ_B/f.u. All ceramic samples exhibited semiconductor characteristics owing to their continuous decreases of resistivity from 2 K to 800 K. Our present work demonstrates an effective route to tuning the dielectric and magnetic properties of CCTO oxides via B-site non-magnetic/magnetic ion-doping.

Newly-synthesized structure T (sT) hydrate show promising practical applications in hydrogen storage and transport, yet the properties remain poorly understood. Here, we develop a machine learning potential (MLP) of sT hydrogen hydrate derived from quantum-mechanical molecular dynamics simulations. Using this MLP forcefield, the structural, hydrogen diffusion, mechanical and thermal properties of sT hydrogen hydrate are extensively explored. It is revealed that the translational and rotational mobilities of hydrogen molecule in sT hydrate are limited due to unique shape and finite dimensional cavities, and tiny windows of neighboring cavities. sT hydrogen hydrate exhibits unique uniaxial tension stress-strain response, with first nonlinear increase to GPa-level but followed by deep drop in the stretching stress, indicating brittle failure, similar to that by Density Functional Theory and empirical forcefields. Moreover, temperature-dependent thermal conductivity in sT hydrogen hydrate is mainly contributed by hydrogen-bonded network formed by host water molecules, while hydrogen guest molecules play an insignificant role in the thermal transport.

The design of solid-state materials requests a thorough understanding of the structural preferences among plausible structure models. Since the bond energy contributes to the formation energy of a given structure model, it also is decisive to determine the nature of chemical bonding for a given material. In this context, we were motivated to explore the correlation between chemical bonding and structural distortions within the low-dimensional tellurium fragments in TbCu_0.33Te₂. The ternary telluride was obtained from high-temperature solid-state reactions, while structure determinations based on x-ray diffraction experiments did not point to the presence of any structural distortion above 100 K. However, the results of first-principles-based computations indicate that a potential structural distortion within the low-dimensional tellurium fragments also correlates to an optimization of overall bonding.

In the present work, we reinvestigate the atomic ordering of a Pb-free morphotropic phase boundary (MPB) compositionviz.,K0.5Na0.5NbO3(KNN50) and its vicinity at various length scales using high-resolution x-ray diffraction and pair distribution function data. We have observed a monoclinic phase (Space Group: Pm) at long/short ranges differing from a very recent report by Sahaet al2024J. Phys.: Condens. Matter36425703. Moreover, the ferroelectric (polarization) dominance of short-range ordering (SRO) over long-range ordering (LRO) has been observed and quantified for the very first time using the amplitude of the ferroelectric frozen phonon mode (Γ4-) (corresponding to the high symmetry cubic phase), thereby structure is linked with ferroelectric (or polarization) property for a widely studied MPB systemviz.,KxNa(1-x)NbO3(KNNxforx= 0.40, 0.50, and 0.60). Two uniquely identified monoclinic phases has been observed for SRO (MSRO) and LRO (MLRO) for all the compositions. The amplitude of ferroelectric frozen phonon mode (Γ4-) corresponding toMSROis significantly higher (≈150%-180%) thanMLRO. A peak is observed in the amplitude ofΓ4-and intensity of prominent Raman peaks (ν₁andν₅) forx= 0.50, which is held responsible for high physical propertiesviz.,dielectric permittivity, piezoelectric coefficient, remnant polarization, electromechanical coupling coefficient, and many more widely reported in literature for KNN50.

In nonmetallic crystals, heat is transported by phonons of different frequencies, each contributing differently to the overall heat flux spectrum. In this study, we demonstrate a significant redistribution of heat flux among phonon frequencies when phonons transmit across the interface between dissimilar solids. This redistribution arises from the natural tendency of phononic heat to re-establish the bulk distribution characteristic of the material through which it propagates. Remarkably, while the heat flux spectra of dissimilar solids are typically distinct in their bulk forms, they can become nearly identical in superlattices or sandwich structures where the layer thicknesses are smaller than the phonon mean free paths. This phenomenon reflects that the redistribution of heat among phonon frequencies to the bulk distribution does not occur instantaneously at the interface, rather it develops over a distance on the order of phonon mean-free-paths.

We consider a problem of nonlinear response to an external electromagnetic radiation in conventional disordered superconductors which contain a small amount of weak magnetic impurities. We focus on the diffusive limit and use Usadel equation to analyze the excitation energy and dispersion relation of the collective modes. We determine the resonant frequency and dispersion of both amplitude (Schmidt-Higgs) and phase (Carlson-Goldman) modes for moderate strength of magnetic scattering. We find that the minimum energy required for the excitation of the both of these modes decreases with an increase in spin-flip scattering. Surprisingly we also find that as a result the Carlson-Goldman mode becomes gapless and as a consequence can only be excited at some finite value of the threshold momentum. We thus discover yet another physical realization of a state with gapped momentum dispersion of one of its collective modes. The value of the threshold momentum is determined by the distance between the two consecutive spin-flip scattering events which, in turn, is proportional to the scattering time between two consecutive scattering events. The amplitude mode is diffusive and becomes strongly suppressed with the increase in spin-flip scattering. Possible ways to experimentally verify our results are also discussed.

The magnetic properties of solids are typically analyzed in terms of Heisenberg models where the electronic structure is approximated by interacting localized spins. However, even in such models the evaluation of thermodynamic properties constitutes a major challenge and is usually handled by a mean field decoupling scheme. The random phase approximation (RPA) comprises a common approach and is often applied to evaluate critical temperatures although it is well known that the method is only accurate wellbelowthe critical temperature. In the present work we compare the performance of the RPA with a different decoupling scheme proposed by Callen as well as the mean field decoupling of interacting Holstein-Primakoff (HP) magnons. We consider three-dimensional (3D) as well as two-dimensional (2D) model systems where the Curie temperature is governed by anisotropy. In 3D, the Callen method is the most accurate in the classical limit, and we show that the Callen decoupling (CD) produces the best agreement with experiments for bcc Fe, fcc Ni and fcc Co with exchange interactions obtained from first principles. In contrast, for low spin systems where a quantum mechanical treatment is pertinent, the HP and RPA methods are superior to the CD. In 2D systems with magnetic order driven by single-ion anisotropy, it is shown that HP fails rather dramatically and both RPA and Callen approaches severely overestimates Curie temperatures. The most accurate approach is then constructed by combining RPA with the CD of single-ion anisotropy, which yields the correct lack of order forS=1/2. We exemplify this by the case of monolayer CrI₃using exchange constant extracted from experiments.

Based on the recent discovery of intrinsic magnetism in monolayer films VSe₂, we have constructed a two-dimensional (2D) Heisenberg model incorporating the 1Tand 2Hstructures. These configurations consist of three layers: the upper and lower surface layers and a middle layer. Using the retarded Green's function method, we investigate the spin-wave energy spectrum, spin-wave density of states, and transition temperature of the system. It is found that in the 2Hstructure, the spin-wave energy spectrum of the system exhibits three direct energy gaps, with one branch being independent of the wave vector. Further analysis shows that at this constant energy, a particular surface state emerges in the 2Hstructure. In contrast, the spin-wave energy spectrum in the 1Tstructure features only two energy gaps-one direct energy gap1 and one indirect energy gap3-without forming a unique surface state. Single-ion anisotropy and interlayer interactions between the upper and lower surface layers influence the energy gaps in the spin-wave energy spectrum and the system's transition temperature. This theoretical work sheds light on forming particular surface states in monolayer 2Hstructure magnetic materials. It provides crucial theoretical support for designing and fabricating next-generation low-dimensional magnetic random-access memory.

In the current era of nanotechnology, the isolation of graphene has acted as a catalyst for the study and creation of many innovative two-dimensional (2D) materials with distinctive functions. The recent synthesis of biphenylene (BPN), a porous 2D carbon allotrope, has ignited significant research interest due to its unique and tunable properties, making it a promising candidate for diverse applications in hydrogen storage, batteries, sensing, electrocatalysis, and beyond. Although a considerable amount of research has been carried out on BPN, there is hardly any review article on this fascinating material. Therefore, this comprehensive review article focuses on the fascinating properties of the advanced graphene family and 2D BPN. Additionally, there has been an in-depth discussion on 2D BPN, covering its synthesis process, recent advancements, and its applications in various fields. Moreover, this review will be useful to professionals and researchers in materials science, physics, chemistry, chemical engineering, and related subjects since it provides detailed information on the characteristics, synthesis, and applications of 2D BPN.