Evidence of Strong Orbital-Selective Spin-Orbital-Phonon Coupling in CrVO_{4}.

Coupling of orbital degree of freedom with a spin exchange, i.e., Kugel-Khomskii-type interaction (KK), governs a host of material properties, including colossal magnetoresistance, enhanced magnetoelectric response, and photoinduced high-temperature magnetism. In general, KK-type interactions lead to deviation in experimental observables of coupled Hamiltonian near or below the magnetic transition. Using diffraction and spectroscopy experiments, here we report anomalous changes in lattice parameters, electronic states, spin dynamics, and phonons at four times the Néel transition temperature (T_{N}) in CrVO_{4}. The temperature is significantly higher than other d-orbital compounds such as manganites and vanadates, where effects are limited to near or below T_{N}. The experimental observations are rationalized using first-principles and Green's function-based phonon and spin simulations that show unprecedentedly strong KK-type interactions via a superexchange process and an orbital-selective spin-phonon coupling coefficient at least double the magnitude previously reported for strongly coupled spin-phonon systems. Our results present an opportunity to explore the effect of KK-type interactions and spin-phonon coupling well above T_{N} and possibly bring various properties closer to application, for example, strong room-temperature magnetoelectric coupling.

Quasi-One-Dimensional Fermi Surface Nesting and Hidden Nesting Enable Multiple Kohn Anomalies in α-Uranium.

The topology of the Fermi surface controls the electronic response of a metal, including charge density wave (CDW) formation. A topology conducive for Fermi surface nesting (FSN) allows the electronic susceptibility χ_{0} to diverge and induce a CDW at wave vector q_{CDW}. Kohn extended the implications of FSN to show that the imaginary part of the lattice dynamical susceptibility χ_{L}^{''} also responds anomalously for all phonon branches at q_{CDW}-a phenomenon referred to as the Kohn anomaly. However, materials exhibiting multiple Kohn anomalies remain rare. Using first-principles simulations of χ_{0} and χ_{L}^{''}, and previous scattering measurements [Crummett et al., Phys. Rev. B 19, 6028 234 (1979)PRBMDO0163-1829], we show that α-uranium harbors multiple Kohn anomalies enabled by the combined effect of FSN and "hidden" nesting, i.e., nesting of electronic states above and below the Fermi surface. FSN and hidden nesting lead to a ridgelike feature in the real part of χ_{0}, allowing interatomic forces to modulate strongly and multiple Kohn anomalies to emerge. These results emphasize the importance of hidden nesting in controlling χ_{0} and χ_{L}^{''} to exploit electronic and lattice states and enable engineering of advanced materials, including topological Weyl semimetals and superconductors.

High-Temperature Thermal Cycling Effect on the Irreversible Responses of Lattice Structure, Magnetic Properties, and Electrical Conductivity in Co2.75Fe0.25O4+δ Spinel Oxide.

We report high-temperature synchrotron X-ray diffraction (SXRD), magnetization, and current-voltage (I-V) characteristics for the samples of Co2.75Fe0.25O4 ferrite. The material was prepared by chemical reaction of the Fe and Co nitrate solutions at pH ∼ 11 and subsequent thermal annealing. Physical properties of the samples were measured by cycling the temperature from 300 K to high temperature (warming mode) and returning back to 300 K (cooling mode). The lattice structure showed sensitivity to high measurement temperatures. Magnetization curves showed a defect-induced ferromagnetic phase at higher temperatures and superparamagnetic blocking of the ferrimagnetic particles near to 300 K or below. Electrical conductivity exhibited a thermal hysteresis loop at higher measurement temperatures. The samples exhibited new form (not studied so far) of surface magnetism in Co rich spinel oxides and irreversibility phenomena in the lattice structure, magnetization, and conductivity on cycling the measurement temperatures.