We performed time- and polarization-resolved extreme ultraviolet momentum microscopy on topological Dirac semimetal candidate 1T-ZrTe$_2$. Excited states band mapping uncovers the previously inaccessible linear dispersion of the Dirac cone above the Fermi level. We study the orbital texture of bands using linear dichroism in photoelectron angular distributions. These observations provide hints on the topological character of 1T-ZrTe$_2$. Time-, energy- and momentum-resolved nonequilibrium carrier dynamics reveal that intra- and inter-band scattering processes play a capital role in the relaxation mechanism, leading to multivalley electron-hole accumulation near the Fermi level. We also show that electrons' inverse lifetime has a linear dependence on their binding energy. Our time- and polarization-resolved XUV photoemission results shed light on the excited state electronic structure of 1T-ZrTe$_2$ and provide valuable insights into the relatively unexplored field of quantum-state-resolved ultrafast dynamics in 3D topological Dirac semimetals.

The on-demand selective population transfer between states in multilevel quantum systems is a challenging problem with implications for a wide-range of physical platforms including photon and exciton-polariton Bose- Einstein condensates (BECs). Here, we introduce an universal strategy for this selective transfer based on a strong time-periodic energy modulation, which is experimentally demonstrated by using a GHz acoustic wave to control the gain and loss of confined modes of an exciton-polariton BEC in a microcavity. The harmonic acoustic field shifts the energy of the excitonic BEC component relative to the photonic ones, which generates a dynamic population transfer within a multimode BEC that can be controlled by the acoustic amplitude. In this way, the full BEC population can be selectively transferred to the ground state to yield a single-level emission consisting of a spectral frequency comb with GHz repetition rates as well as picosecond-scale correlations. A theoretical model reproduces the observed time evolution and reveals a dynamical interplay between bosonic stimulation and the adiabatic Landau-Zener-like population transfer. Our approach provides a new avenue for the Floquet engineering of light-matter systems and enables tunable single- or multi-wavelength ultrafast pulsed laser-like emission for novel information technologies.

We present a study on the magnetic behavior of dextran-coated magnetite nanoparticles (DM NPs) with sizes between 3 and 19 nm, synthesized by hydrothermal-assisted co-precipitation method. The decrease of saturation magnetization ($M_s$) with decreasing particle size has been modeled by assuming the existence of a spin-disordered layer at the particle surface, which is magnetically dead. Based on this core-shell model and taking into account the weight contribution of the non-magnetic coating layer (dextran) to the whole magnetization, the dead layer thickness ($t$) and saturation magnetization $M_s$ of the magnetic cores in our samples were estimated to be $t = 6.8~\mathrmÅ$ and $M_s = 98.8~\mathrm{emu/g}$, respectively. The data of $M_s$ were analyzed using a law of approach to saturation, indicating an increase in effective magnetic anisotropy ($K_{eff}$) with decreasing particle size as expected from the increased surface/volume ratio in small MNPs. The obtained $K_{eff}$ values were successfully modeled by including an extra contribution of dipolar interactions due to the formation of chain-like clusters of MNPs. The surface magnetic anisotropy ($K_s$) was estimated to be about $K_s = 1.04\times10^5~\mathrm{J/m^3}$. Our method provides a simple and accurate way to obtain the $M_s$ core values in surface-disordered MNPs, a relevant parameter required for magnetic modeling in many applications.