Effects of anharmonic lattice distortion on orbital and magnetic orderings in KCuF_3

Lattice, magnetic and orbital structures in KCuF3 are self-consistently determined by our cluster self-consistent field approach based on a spin-orbital-lattice Hamiltonian. Two stable structures are obtained and found to be degenerate, which confirms the presence of the coexistent phases observed experimentally. We clearly show that due to the inherent frustration, the ground state of the system only with the superexchange interaction is degenerate; while the Jahn-Teller distortion, especially the anharmonic effect, stabilizes the orbital ordered phase at about 23% in the x2-y2 orbit and at 77% in the 3z2-r2 orbit. Meanwhile the magnetic moment of Cu is considerably reduced to 0.56μB, and magnetic coupling strengths are highly anisotropic, Jx/Jxy ≈ 18. These results are in good agreement with the experiments, implying that the anharmonic Jahn-Teller effect plays an essential role in stabilising the orbital ordered ground state of KCuF3.

Effects of Rattling Behavior of K and Cd Atoms along Different Directions in Anisotropic KCdAs on Lattice Thermal Transport and Thermoelectric Properties

We employ advanced first principles methodology,merging self-consistent phonon theory and the Boltzmann transport equation,to comprehensively explore the thermal transport and thermoelectric properties of KCdAs.Notably,the study accounts for the impact of quartic anharmonicity on phonon group velocities in the pursuit of lattice thermal conductivity and investigates 3ph and 4ph scattering processes on phonon lifetimes.Through various methodologies,including examining atomic vibrational modes and analyzing 3ph and 4ph scattering processes,the article unveils microphysical mechanisms contributing to the lowκL within KCdAs.Key features include significant anisotropy in Cd atoms,pronounced anharmonicity in K atoms,and relative vibrations in non-equivalent As atomic layers.Cd atoms,situated between As layers,exhibit rattling modes and strong lattice anharmonicity,contributing to the observed lowκL.Remarkably flat bands near the valence band maximum translate into high PF,aligning with ultralowκL for exceptional thermoelectric performance.Under optimal temperature and carrier concentration doping,outstanding ZT values are achieved:4.25(a(b)-axis,p-type,3×10^(19)cm^(−3),500 K),0.90(c-axis,p-type,5×10^(20)cm^(−3),700 K),1.61(a(b)-axis,n-type,2×10^(18)cm^(−3),700 K),and 3.06(c-axis,n-type,9×10^(17)cm^(−3),700 K).

Band modification towards high thermoelectric performance of SnSb_(2)Te_(4) with strong anharmonicity driven by cation disorder

As a typical (IV–VI)_(x)(V_(2)VI_(3))_(y) compound, the tetradymite-like layered SnSb_(2)Te_(4) -based compounds have attracted increasing attention in the thermoelectric community owing to the intrinsically low lattice thermal conductivity. Nevertheless, the effect of cations disorder on the inherent physical characteristics remains puzzling, and its inferior Seebeck coefficient is the bottleneck to achieving high thermoelectric performance. In this work, the thermoelectric properties of polycrystalline In_(x)Sn_(1−x)Sb_(2)(Te_(1−y)Se_(y))_(4) (0≤x≤0.1,0≤y≤0.15) samples are comprehensively investigated. In conjunction with the calculated band structure and experimental results, the Seebeck coefficient and power factor are markedly improved after the introduction of indium and selenium, which originates from the combined effects of the emergent resonant states and converged valence bands along with optimal carrier concentration. Additionally, compared with the ordered lattice structure, the disordered cations occupancy in SnSb_(2)Te_(4) further strengthens lattice anharmonicity and reduces phonon group velocity verified by first-principles calculations, securing intrinsically low lattice thermal conductivity. Finally, a record zT value of ∼0.6 at 670 K and an average zT of ∼0.4 between 320 and 720 K are obtained in the In0.1 Sn0.9 Sb2 Te3.4 Se0.6 sample, being one of the highest zT values among SnSb2 Te4 -based materials. This work not only demonstrates that SnSb2 Te4 -based compounds are promising thermoelectric candidates, but also provides guidance for the promotion of thermoelectric performance in a broad temperature range.

Ultrasonic generation via direct interaction between photons and phonons in anharmonic lattice of ionic crystals

A theory of ultrasonic generation via direct interaction of transverse optic (TO) phonons with photons in anharmonic lattice of ionic crystals is presented. There are two methods of supplying light energy for the excitation of TO lattice wave as a high frequency ultrasound: (A) incident light comes from the source outside the cavity? fulfilled with ionic crystal medium, (B) photon mode of the cavity possesses the gain of amplification by stimulated radiation of active atoms doping in the medium. More attention is drawn to the case (B). The working system of case (B), as a mixture of lasing action and ultrasonic generation, has the threshold phenomena like usual laser. And the linear stability analysis shows that the nonlineax phonon-photon coupling and the interaction among phonons themselves, both of which reflect the anharmonicity of lattice vibration, are necessary to the stable ultrasonic output. So this laser-ultrasonic generation mixture would be also a measure to investigate the lattice-dynamic nonlinearity and correlated electromagnetic properties of ionic crystals.