Thermal conductivity of anisotropic spin - 1/2 two leg ladder:Green's function approach

We study the thermal transport of a spin-1/2 two leg antiferromagnetic ladder in the direction of legs. The possible effect of spin-orbit coupling and crystalline electric field are investigated in terms of anisotropies in the Heisenberg interactions on both leg and rung couplings. The original spin ladder is mapped to a bosonic model via a bond-operator transformation where an infinite hard-core repulsion is imposed to constrain one boson occupation per site. The Green's function approach is applied to obtain the energy spectrum of quasi-particle excitations responsible for thermal transport. The thermal conductivity is found to be monotonically decreasing with temperature due to increased scattering among triplet excitations at higher temperatures. A tiny dependence of thermal transport on the anisotropy in the leg direction at low temperatures is observed in contrast to the strong one on the anisotropy along the rung direction, due to the direct effect of the triplet density. Our results reach asymptotically the ballistic regime of the spin - 1/2 Heisenberg chain and compare favorably well with exact diagonalization data

Dynamics of spin and orbital phase transitions in YVO3

YVO3 exhibits a well separated sequence of orbital and spin order transitions at 200 K and 116 K, followed by a combined spin-orbital reorientation at 77 K. It is shown that the spin order can be destroyed by a sufficiently strong optical pulse within less than 4 ps. In contrast, the orbital reordering transition from C-type to G-type orbital order is slower than 100 ps and goes via an intermediate nonthermal phase. We propose that the dynamics of phase transitions is subjected to symmetry relations between the associated phases.Comment: 5 pages, 3 figure

Probing ultrafast symmetry breaking in photo-stimulated matter

The nature of a phase transition is inherently connected to the changes in the crystalline symmtry, which is typically probed by elastic or inelastic scattering with neutrons, electrons or photons. When such a phase transition is stimulated by light or other sudden perturbations the solid evolves along a non-equilibrium pathway of which the underlying physics is poorly understood. Here we use picosecond Raman scattering to study the photo-induced ultrafast dynamics in Peierls distorted Antimony. We find evidence for an ultrafast non-thermal reversible structural phase transition. Most surprisingly, we find evidence that this transition evolves toward a lower symmetry, in contrast to the commonly accepted rhombohedral-to-simple cubic transition path. Our study demonstrates the feasibility of ultrafast Raman scattering symmetry analysis of photo-induced non-thermal transient phases

Photo-induced magnetization enhancement in two-dimensional weakly anisotropic Heisenberg magnets

By comparing the photo-induced magnetization dynamics in simple layered systems we show how light-induced modifications of the magnetic anisotropy directly enhance the magnetization. It is observed that the spin precession in (CH3NH3)2CuCl4, initiated by a light pulse, increases in amplitude at the critical temperature TC. The phenomenon is related to the dependence of the critical temperature on the axial magnetic anisotropy. The present results underline the possibility and the importance of the optical modifications of the anisotropy, opening new paths toward the control of the magnetization state for ultrafast memories.Comment: 5 pages, 3 figures, supplementary info as SIr.pd

Coherent amplitudon generation in K_0.3MoO_3 through ultrafast inter-band quasi particle decay

The charge density wave system K_0.3MoO_3 has been studied using variable energy pump-probe spectroscopy, ellipsometry, and inelastic light scattering. The observed transient reflectivity response exhibits quite a complex behavior, containing contributions due to quasi particle excitations, coherent amplitudons and phonons, and heating effects. The generation of coherent amplitudons is discussed in terms of relaxation of photo-excited quasi particles, and is found to be resonant with the interband plasmon frequency. Two additional coherent excitations observed in the transients are assigned to zone-folding modes of the charge density wave state

Hubbard exciton revealed by time-domain optical spectroscopy

We use broadband ultra-fast pump-probe spectroscopy in the visible range to study the lowest excitations across the Mott-Hubbard gap in the orbitally ordered insulator YVO3. Separating thermal and non-thermal contributions to the optical transients, we show that the total spectral weight of the two lowest peaks is conserved, demonstrating that both excitations correspond to the same multiplet. The pump-induced transfer of spectral weight between the two peaks reveals that the low-energy one is a Hubbard exciton, i.e. a resonance or bound state between a doublon and a holon. Finally, we speculate that the pump-driven spin-disorder can be used to quantify the kinetic energy gain of the excitons in the ferromagnetic phase.Comment: 5 pages and 6 figures, 9 pages and 12 figures with additional material

Magnetic interlayer coupling between ferromagnetic SrRuO3_3 layers through a SrIrO3_3 spacer

A key element to tailor the properties of magnetic multilayers is the coupling between the individual magnetic layers. In case of skyrmion hosting multilayers, coupling of skyrmions across the magnetic layers is highly desirable. Here the magnetic interlayer coupling was studied in epitaxial all-oxide heterostructures of ferromagnetic perovskite SrRuO$_3$ layers separated by spacers of the strong spin-orbit coupling oxide SrIrO$_3$. This combination of oxide layers is being discussed as a potential candidate system to host N\'{e}el skyrmions. First order reversal curve (FORC) measurements were performed in order to distinguish between magnetic switching processes of the individual layers and to disentangle the signal of soft magnetic impurities from the samples$'$ signal. Additionally, FORC investigations enabled to determine whether the coupling between the magnetic layers is ferromagnetic or antiferromagnetic. The observed interlayer coupling strength was weak for all the heterostructures, with SrIrO$_3$ spacers between 2 monolayers and 12 monolayers thick.Comment: 22 page