Ultrafast exciton fluid flow in an atomically thin MoS semiconductor.

Excitons (coupled electron-hole pairs) in semiconductors can form collective states that sometimes exhibit spectacular nonlinear properties. Here, we show experimental evidence of a collective state of short-lived excitons in a direct-bandgap, atomically thin MoS semiconductor whose propagation resembles that of a classical liquid as suggested by the nearly uniform photoluminescence through the MoS monolayer regardless of crystallographic defects and geometric constraints. The exciton fluid flows over ultralong distances (at least 60 μm) at a speed of ~1.

Transition from Diffusive to Superdiffusive Transport in Carbon Nanotube Networks via Nematic Order Control.

The one-dimensional confinement of quasiparticles in individual carbon nanotubes (CNTs) leads to extremely anisotropic electronic and optical properties. In a macroscopic ensemble of randomly oriented CNTs, this anisotropy disappears together with other properties that make them attractive for certain device applications. The question however remains if not only anisotropy but also other types of behaviors are suppressed by disorder.

Zero-field quantum beats and spin decoherence mechanisms in CsPbBr perovskite nanocrystals.

Coherent optical manipulation of exciton states provides a fascinating approach for quantum gating and ultrafast switching. However, their coherence time for incumbent semiconductors is highly susceptible to thermal decoherence and inhomogeneous broadening effects. Here, we uncover zero-field exciton quantum beating and anomalous temperature dependence of the exciton spin lifetimes in CsPbBr perovskite nanocrystals (NCs) ensembles.

From rogue wave solution to solitons.

Using a generalized nonlinear Schrödinger equation, we investigate the transformation of a fundamental rogue wave solution to a collection of solitons. Taking the third-order dispersion, self-steepening, and Raman-induced self-frequency shift as the generalizing effects, we systematically observe how a fundamental rogue wave has an impact on its surrounding continuous wave background and reshapes its own characteristics while a group of solitons are created. Applying a local inverse scattering technique based on the periodization of an isolated structure, we show that the third-order dispersion and Raman-induced self-frequency shift generates a group of solitons in the neighborhood where the rogue wave solution emerges.

Ultrafast Thermalization Pathways of Excited Bulk and Surface States in the Ferroelectric Rashba Semiconductor GeTe.

A large Rashba effect is essential for future applications in spintronics. Particularly attractive is understanding and controlling nonequilibrium properties of ferroelectric Rashba semiconductors. Here, time- and angle-resolved photoemission is utilized to access the ultrafast dynamics of bulk and surface transient Rashba states after femtosecond optical excitation of GeTe.

Electro-Ionic Control of Surface Plasmons in Graphene-Layered Heterostructures.

Precise control of light is indispensable to modern optical communication devices especially as the size of such devices approaches the subwavelength scale. Plasmonic devices are suitable for the development of these optical devices due to the extreme field confinement and its ability to be controlled by tuning the carrier density at the metal/dielectric interface. Here, an electro-ionic controlled plasmonic device consisting of Au/graphene/ion-gel is demonstrated as an optical switch, where an external electric field modulates the real part of the electrical conductivity.

Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication.

Excitons play major roles in optical processes in modern semiconductors, such as single-wall carbon nanotubes (CNTs), transition metal dichalcogenides, and 2D perovskite quantum wells. They possess extremely large binding energies (>100 meV), dominating absorption and emission spectra even at high temperatures. The large binding energies imply that they are stable, that is, hard to ionize, rendering them seemingly unsuited for optoelectronic devices that require mobile charge carriers, especially terahertz emitters and solar cells.

Effect of Strong Electric Fields on Material Responses: The Bloch Oscillation Resonance in High Field Conductivities.

In this paper, we investigated the effect of strong electric fields on material responses and the Bloch oscillation resonance in high field conductivities. For this purpose, a high-order accurate explicit modal discontinuous Galerkin (DG) solver is employed for solving the quantum Boltzmann transport equation (BTE) in the context of electron transport at nanoscales under strongly out-of-equilibrium conditions. Here, we study the transient behavior and the convergence of a steady-state response to an external oscillating electric field switched on at time zero.

Modification of spintronic terahertz emitter performance through defect engineering.

Spintronic ferromagnetic/non-magnetic heterostructures are novel sources for the generation of THz radiation based on spin-to-charge conversion in the layers. The key technological and scientific challenge of THz spintronic emitters is to increase their intensity and frequency bandwidth. Our work reveals the factors to engineer spintronic Terahertz generation by introducing the scattering lifetime and the interface transmission for spin polarized, non-equilibrium electrons.

Ultrafast Spin-to-Charge Conversion at the Surface of Topological Insulator Thin Films.

Strong spin-orbit coupling, resulting in the formation of spin-momentum-locked surface states, endows topological insulators with superior spin-to-charge conversion characteristics, though the dynamics that govern it have remained elusive. Here, an all-optical method is presented, which enables unprecedented tracking of the ultrafast dynamics of spin-to-charge conversion in a prototypical topological insulator Bi Se /ferromagnetic Co heterostructure, down to the sub-picosecond timescale. Compared to pure Bi Se or Co, a giant terahertz emission is observed in the heterostructure that originates from spin-to-charge conversion, in which the topological surface states play a crucial role.

Distinctive Picosecond Spin Polarization Dynamics in Bulk Half Metals.

Femtosecond laser excitations in half-metal (HM) compounds are theoretically predicted to induce an exotic picosecond spin dynamics. In particular, conversely to what is observed in conventional metals and semiconductors, the thermalization process in HMs leads to a long living partially thermalized configuration characterized by three Fermi-Dirac distributions for the minority, majority conduction, and majority valence electrons, respectively. Remarkably, these distributions have the same temperature but different chemical potentials.

Generalized GW+Boltzmann Approach for the Description of Ultrafast Electron Dynamics in Topological Insulators.

Quantum-phase transitions between trivial insulators and topological insulators differ from ordinary metal-insulator transitions in that they arise from the inversion of the bulk band structure due to strong spin-orbit coupling. Such topological phase transitions are unique in nature as they lead to the emergence of topological surface states which are characterized by a peculiar spin texture that is believed to play a central role in the generation and manipulation of dissipationless surface spin currents on ultrafast timescales. Here, we provide a generalized G W +Boltzmann approach for the description of ultrafast dynamics in topological insulators driven by electron-electron and electron-phonon scatterings.

Spin polarisation of ultrashort spin current pulses injected in semiconductors.

Ultrashort spin current pulses have great potential to become carriers of information in future ultrafast spintronics. They present the outstanding property of an extremely compressed time profile, which can allow for the building up of spintronics operating at the unprecedented THz frequencies. The ultrashort spin pulses, however, still lack other desirable features.

Ultrafast and Gigantic Spin Injection in Semiconductors.

The injection of spin currents in semiconductors is one of the big challenges of spintronics. Motivated by the ultrafast demagnetization and spin injection into metals, we propose an alternative femtosecond route based on the laser excitation of superdiffusive spin currents in a ferromagnet such as Ni. Our calculations show that even though only a fraction of the current crosses the Ni-Si interface, the laser-induced creation of strong transient electrical fields at a ferromagnet-semiconductor interface allows for the injection of chargeless spin currents with record spin polarizations of 80%.

Unified Picture for the Colossal Thermopower Compound FeSb2.

We identify the driving mechanism of the gigantic Seebeck coefficient in FeSb2 as the phonon-drag effect associated with an in-gap density of states that we demonstrate to derive from excess iron. We accurately model electronic and thermoelectric transport coefficients and explain the so far ill-understood correlation of maxima and inflection points in different response functions. Our scenario has far-reaching consequences for attempts to harvest the spectacular power factor of FeSb2.

Momentum-resolved spin dynamics of bulk and surface excited States in the topological insulator Bi(2)Se(3).

The prospect of optically inducing and controlling a spin-polarized current in spintronic devices has generated wide interest in the out-of-equilibrium electronic and spin structure of topological insulators. In this Letter we show that only measuring the spin intensity signal over several orders of magnitude by spin-, time-, and angle-resolved photoemission spectroscopy can provide a comprehensive description of the optically excited electronic states in Bi_{2}Se_{3}. Our experiments reveal the existence of a surface resonance state in the second bulk band gap that is benchmarked by fully relativistic ab initio spin-resolved photoemission calculations.

What about temperature? Haloperidol-induced hypotermia.

The use of treatment with antipsychotic drugs highlights the difficulty of finding the right balance between the benefit on psychotic symptoms and the risk of the occurrence of adverse reactions. There is a strong genetic and pharmacological evidence supporting the hypothesis that activation of D2 receptors could lead to hypothermia and that the treatment haloperidol is capable of inducing hypothermia apomorfino similarily in laboratory animals. It also seems that, haloperidol is not the only antipsychotic able to determine this type of reaction, as some evidence suggests that other drugs such as reserpine, chlorpromazine would be capable of inducing hypothermia.

Terahertz spin current pulses controlled by magnetic heterostructures.

In spin-based electronics, information is encoded by the spin state of electron bunches. Processing this information requires the controlled transport of spin angular momentum through a solid, preferably at frequencies reaching the so far unexplored terahertz regime. Here, we demonstrate, by experiment and theory, that the temporal shape of femtosecond spin current bursts can be manipulated by using specifically designed magnetic heterostructures.

Ultrafast spin transport as key to femtosecond demagnetization.

Irradiating a ferromagnet with a femtosecond laser pulse is known to induce an ultrafast demagnetization within a few hundred femtoseconds. Here we demonstrate that direct laser irradiation is in fact not essential for ultrafast demagnetization, and that electron cascades caused by hot electron currents accomplish it very efficiently. We optically excite a Au/Ni layered structure in which the 30 nm Au capping layer absorbs the incident laser pump pulse and subsequently use the X-ray magnetic circular dichroism technique to probe the femtosecond demagnetization of the adjacent 15 nm Ni layer.

Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current.

Uncovering the physical mechanisms that govern ultrafast charge and spin dynamics is crucial for understanding correlated matter as well as the fundamental limits of ultrafast spin-based electronics. Spin dynamics in magnetic materials can be driven by ultrashort light pulses, resulting in a transient drop in magnetization within a few hundred femtoseconds. However, a full understanding of femtosecond spin dynamics remains elusive.

Ab Initio investigation of the Elliott-Yafet electron-phonon mechanism in laser-induced ultrafast demagnetization.

The spin-flip (SF) Eliashberg function is calculated from first principles for ferromagnetic Ni to accurately establish the contribution of Elliott-Yafet electron-phonon SF scattering to Ni's femtosecond laser-driven demagnetization. This is used to compute the SF probability and demagnetization rate for laser-created thermalized as well as nonequilibrium electron distributions. Increased SF probabilities are found for thermalized electrons, but the induced demagnetization rate is extremely small.

Superdiffusive spin transport as a mechanism of ultrafast demagnetization.

We propose a semiclassical model for femtosecond laser-induced demagnetization due to spin-polarized excited electron diffusion in the superdiffusive regime. Our approach treats the finite elapsed time and transport in space between multiple electronic collisions exactly, as well as the presence of several metal films in the sample. Solving the derived transport equation numerically we show that this mechanism accounts for the experimentally observed demagnetization within 200 fs in Ni, without the need to invoke any angular momentum dissipation channel.