Determination of the nearest-neighbor interaction in VO2_2 via fractal dimension analysis

The Ising model is one of the simplest and most well-established concepts to simulate phase transformations in complex materials. However, its most central constant, the interaction strength J between two nearest neighbors, is hard to obtain. Here we show how this basic constant can be determined with a fractal dimension analysis of measured domain structures. We apply this approach to vanadium dioxide, a strongly correlated material with a first-order insulator-to-metal phase-transition with enigmatic properties. We obtain a nearest-neighbor interaction of 13.8 meV, a value close to the thermal energy at room temperature. Consequently, even far below the transition temperature, there are spontaneous local phase-flips from the insulating into the metallic phase. These fluctuations explain several measured anomalies in VO$_2$, in particular the low thermal carrier activation energy and the finite conductivity of the insulating phase. As a method, our fractal dimension analysis links the Ising model to macroscopic material constants for almost any first-order phase transition.Comment: {\dag}These authors contributed equally to this wor

Liquid-diffusion-limited growth of vanadium dioxide single crystals

Vanadium dioxide is a strongly correlated material with an ultrafast first-order phase transition between monoclinic/insulator and rutile/metallic close to room temperature. The unusual and complex properties of this transition make VO_{2} one of the most heavily investigated materials in modern condensed matter physics. Consequently, high-quality single crystals are in large demand. Here we report the growth of mm-sized VO_{2} crystals by thermal decomposition of liquid V_{2}O_{5} at ∼1000^{∘}C. Time-resolved zirconia sensor measurements of the oxygen release reveal that the crystal growth rate is limited by liquid-phase diffusion; the properties of the gaseous environment, which were previously assumed to be decisive, are almost insignificant. Consequently, large and stoichiometric single crystals of VO_{2} can be obtained at lower temperatures and gas purities than usually applied. These results signify the role of gas-liquid diffusion in crystal growth and will simplify future research on VO_{2} and related materials for applications in ultrafast electronics and thermal energy management

Laser-induced magnetization precession in the magnetite Fe₃O₄ in the vicinity of a spin-reorientation transition

Using time-resolved magneto-optical pump-probe technique we demonstrate excitation of magnetization precession in a single crystalline bulk magnetite Fe3O4 below and in the vicinity of the Verwey and spin-reorientation (SR) phase transitions. Pronounced temperature dependence of the precession amplitude is observed suggesting that the excitation occurs via laser-driven spin-reorientation transition. Similarity observed between the characteristic features of the laser-induced ultrafast SR and Verwey transitions suggests that they both rely on the same microscopic processes.publishe

Polarized phonons carry angular momentum in ultrafast demagnetization

Magnetic phenomena are ubiquitous in nature and indispensable for modern science and technology, but it is notoriously difficult to change the magnetic order of a material in a rapid way. However, if a thin nickel film is subjected to ultrashort laser pulses, it loses its magnetic order almost completely within femtosecond timescales1. This phenomenon is widespread2-7 and offers opportunities for rapid information processing8-11 or ultrafast spintronics at frequencies approaching those of light8,9,12. Consequently, the physics of ultrafast demagnetization is central to modern materials research1-7,13-28, but a crucial question has remained elusive: if a material loses its magnetization within mere femtoseconds, where is the missing angular momentum in such a short time? Here we use ultrafast electron diffraction to reveal in nickel an almost instantaneous, long-lasting, non-equilibrium population of anisotropic high-frequency phonons that appear within 150-750 fs. The anisotropy plane is perpendicular to the direction of the initial magnetization and the atomic oscillation amplitude is 2 pm. We explain these observations by means of circularly polarized phonons that quickly absorb the angular momentum of the spin system before macroscopic sample rotation. The time that is needed for demagnetization is related to the time it takes to accelerate the atoms. These results provide an atomistic picture of the Einstein-de Haas effect and signify the general importance of polarized phonons for non-equilibrium dynamics and phase transitions.publishe