We investigate black phosphorus by time- and angle-resolved photoelectron spectroscopy. The electrons excited by 1.57 eV photons relax down to a conduction band minimum within 1 ps. Despite the low band gap value, no relevant amount of carrier multiplication could be detected at an excitation density 3–6 × 1019 cm–3. In the thermalized state, the band gap renormalization is negligible up to a photoexcitation density that fills the conduction band by 150 meV. Astonishingly, a Stark broadening of the valence band takes place at an early delay time. We argue that electrons and holes with a high excess energy lead to inhomogeneous screening of near surface fields. As a consequence, the chemical potential is no longer pinned in a narrow impurity band.

Using x-ray emission spectroscopy, we find appreciable local magnetic moments until 30-40 GPa in the high-pressure phase of iron, however no magnetic order is detected with neutron powder diffraction down to 1.8 K contrary to previous predictions. Our first-principles calculations reveal a "spinsmectic" state lower in energy than previous results. This state forms antiferromagnetic bilayers separated by null spin bilayers, which allows a complete relaxation of the inherent frustration of antiferromagnetism on a hexagonal close-packed lattice. The magnetic bilayers are likely orientationally disordered, owing to the soft interlayer excitations and the neardegeneracy with other smectic phases. This possible lack of long-range correlation agrees with the null results from neutron powder diffraction. An orientationally-disordered, spinsmectic state resolves previously perceived contradictions in high pressure iron and could be integral to explaining its puzzling superconductivity.Iron is well-known since antiquity for its unique magnetic properties and continues to captivate scientists to this day. The study of iron and its alloys has many applications, including steel production and geophysics. Regarding the latter, the application of hydrostatic pressure induces a phase transition from the body-centered cubic (bcc) structure of α-iron to the hexagonal close-packed (hcp) structure of ε-iron (Fig. 1). Iron is being studied at increasingly high pressures and temperatures, since it and its alloys compose the majority of the Earth's core (1). Nonetheless, the relatively low-pressure, low-temperature region of ε-iron has remained a mystery for decades. The ferromagnetism (fm) found in α-iron disappears during the α-ε transition (2-4), however the magnetic state of 1 B.W.L and T.G contributed equally to this work 2 Corresponding author -matteo.dastuto@neel.cnrs.fr -Institut Néel CNRS -25, av des Martyrs -38042 Grenoble cedex 9; tel: (+33)(0)4 76 88 12 84 arXiv:1903.04792v2 [cond-mat.mtrl-sci]

In a self-driven mode, a graphene/InSe/MoS2 photodetector exhibits high photoresponsivity, fast photoresponse and high operational stability under ambient conditions.

We monitor the dynamics of hot carriers in InSe by means of two photons photoelectron spectroscopy (2PPE). The electrons excited by photons of 3.12 eV experience a manifold relaxation. First, they thermalize to the electronic states degenerate with theM valley. Subsequently, the electronic cooling is dictated by Fröhlich coupling with phonons of small momentum transfer. Ab-initio calculations predict cooling rates that are in good agreement with the observed dynamics. We argue that electrons accumulating in states degenerate with theM valley could travel through a multilayer flake of InSe with lateral size of 1 micrometer. The hot carriers pave a viable route to the realization of below-bandgap photodiodes and Gunn oscillators. Our results indicate that these technologies may find a natural implementation in future devices based on layered chalcogenides. PACS numbers:Van der Waals chalcogenides display a variety of different specifics that depend on their composition and number of layers. The weak mechanical binding of the atoms along the stacking direction facilitates the realization of heterostructures with different functionalities. Some recent achievements have been: the fine tuning of the band gap 1-3 , the control of valley polarization 4 and the realization of devices with high mobility 5-7 . In this context, InSe is one of the building blocks with the highest potentials.The bulk crystals of InSe can be thinned down to a few layers and encapsulated in hBN 7 . By these means, Bandurin and collaborators have fabricated transitors whose quality is high enough to observe Shubnikovde Haas oscillations and quantum Hall effect 7 . These results point out the two aspects making InSe particularly appealing. On one hand, the mobility of charge carriers rivals with the one measured in graphene 8 . On the other hand, the bulk band gap of 1.26 eV, is ideally suited for optoelectronic devices. Indeed, several groups have recently reported that InSe 10,11 and InSegraphene heterostructures 12 have excellent photoresponsivity in the visible spectral region. Such photodetectors could be patterned on a large scale over flexible supports 13 . Eventually, the application of larger bias can drive the photodetector in the avalanche regime 14 .The conception of devices based on InSe would greatly profit from a precise knowledge of the transient state following photoexcitation. Some pioneering experiments have investigated the coherent propagation and dephasing of the exciton-polariton 15,16 . These optical meth-ods revealed a beating polarization induced by resonant pulses, but provided no insights on the energy relaxation of hot carriers. Here, we address this issue by mapping the dynamics of excited electrons in reciprocal space 17,18 . Our two photon photoemission (2PPE) data show that photoexcitation above theM valley results in a high density of electrons with excess energy of ∼ = 0.6 eV and cooling time of 2 ps. First principle calculations of the electron-phonon coupling discriminate between the distinct scattering mechani...

We measure the surface of CH3NH3PbI3 single crystals by making use of two photon photoemission spectroscopy. Our method monitors the electronic distribution of photoexcited electrons, explicitly discriminating the initial thermalization from slower dynamical processes. The reported results disclose the fast dissipation channels of hot carriers (0.25 ps), set a upper bound to the surface induced recombination velocity (< 4000 cm/s) and reveal the dramatic effect of shallow traps on the electrons dynamics. The picosecond localization of excited electrons in degraded CH3NH3PbI3 samples is consistent with the progressive reduction of photoconversion efficiency in operating devices. Minimizing the density of shallow traps and solving the aging problem may boost the macroscopic efficiency of solar cells to the theoretical limit.

Manipulation of intrinsic electronic structures by electron or hole doping in a controlled manner in van der Waals layered materials is the key to control their electrical and optical properties. Two-dimensional indium selenide (InSe) semiconductor has attracted attention due to its direct band gap and ultrahigh mobility as a promising material for optoelectronic devices. In this work, we manipulate the electronic structure of InSe by in situ surface electron doping and obtain a significant band gap renormalization of ∼120 meV directly observed by high-resolution angle resolved photoemission spectroscopy. This moderate doping level (carrier concentration of 8.1 × 1012 cm–2) can be achieved by electrical gating in field effect transistors, demonstrating the potential to design of broad spectral response devices.

Electrostatic gating or alkali metal evaporation can be successfully employed to tune the interface of layered black phosphorus (BP) from a semiconductor to a 2D Dirac semimetal. Although Angle Resolved Photoelectron Spectroscopy (ARPES) experiments have captured the collapse of the band gap in the inversion layer, a quantitative estimation of band structure evolution has been hindered by the short escape depth and matrix elements of the probed photoelectrons. Here, we precisely monitor the evolution of electronic states using time-resolved ARPES at a photon energy of 6.3 eV. The probing depth of laser -based ARPES is long enough to observe the buried electronic states originating from the valence band maximum. Our data shows that the band gap has a maximal value of 0.32 eV in the pristine sample, and that it shrinks down monotonically by increasing the carrier concentration in the topmost layer. Most interestingly, the band velocity of the valence band increases by a factor of two along the armchair direction, surpassing the value reported in graphene on silicon carbide (SiC). This control of band structure via external gating will be of interest with regard to the design of optoelectronic devices.

New data on Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) reveal interesting aspects of photoexcited superconductors. The electrons dynamics show that inelastic scattering by nodal quasiparticles decreases when the temperature is lowered below the critical value of the superconducting phase transition. This drop of electronic dissipation is astonishingly robust and survives to photoexcitation densities much larger than the value sustained by long-range superconductivity. The unconventional behavior of quasiparticle scattering is ascribed to superconducting correlations extending on a length scale comparable to the inelastic mean-free path. Our measurements indicate that strongly driven superconductors enter in a regime without phase coherence but finite pairing amplitude.