Low energy phases of bilayer Bi predicted by structure search in two dimensions

We employ an ab-initio structure search algorithm to explore the configurational space of Bi in quasi two dimensions. A confinement potential restricts the movement of atoms within a pre-defined thickness during structure search calculations within the minima hopping method to find the stable and metastable forms of bilayer Bi. In addition to recovering the two known low-energy structures (puckered monoclinic and buckled hexagonal), our calculations predict three new structures of bilayer Bi. We call these structures the $\alpha$, $\beta$, and $\gamma$ phases of bilayer Bi, which are, respectively, 63, 72, and 83 meV/atom higher in energy than that of the monoclinic ground state, and thus potentially synthesizable using appropriate substrates. We also compare the structural, electronic, and vibrational properties of the different phases. The puckered monoclinic, buckled hexagonal, and $\beta$ phases exhibit a semiconducting energy gap, whereas $\alpha$ and $\gamma$ phases are metallic. We notice an unusual Mexican-hat type band dispersion leading to a van Hove singularity in the buckled hexagonal bilayer Bi. Notably, we find symmetry-protected topological Dirac points in the electronic spectrum of the $\gamma$ phase. The new structures suggest that bilayer Bi provides a novel playground to study distortion-mediated metal-insulator phase transitions

Radiative impacts of the Australian bushfires 2019–2020 – Part 2: Large-scale and in-vortex radiative heating

Record-breaking wildfires ravaged south-eastern Australia during the fire season 2019–2020. The intensity of the fires reached its paroxysmal phase at the turn of the year 2019–2020, when large pyro-cumulonimbi developed. Pyro-convective activity injected biomass burning aerosols and gases in the upper-troposphere–lower-stratosphere (UTLS), producing a long-lasting perturbation to the atmospheric composition and the stratospheric aerosol layer. The large absorptivity of the biomass burning plume produced self-lofting of the plume and thus modified its vertical dynamics and horizontal dispersion. Another effect of the in-plume absorption was the generation of compact smoke-charged anticyclonic vortices which ascended up to 35 km altitude due to diabatic heating. We use observational and modelling description of this event to isolate the main vortex from the dominant Southern Hemispheric biomass burning aerosol plume. Entering this information into an offline radiative transfer model, and with hypotheses on the absorptivity and the angular scattering properties of the aerosol layer, we estimate the radiative heating rates (HRs) in the plume and the vortex. We found that the hemispheric-scale plume produced a HR of 0.08±0.05 K d−1 (from 0.01 to 0.15 K d−1, depending on the assumption on the aerosol optical properties), as a monthly average value for February 2020, which is strongly dependent on the assumptions on the aerosol optical properties and therefore on the plume ageing. We also found in-vortex HRs as large as 15–20 K d−1 in the denser sections of the main vortex (8.4±6.1 K d−1 on average in the vortex). Our results suggest that radiatively heated ascending isolated vortices are likely dominated by small-sized strongly absorbing black carbon particles. The hemispheric-scale and in-vortex HR estimates are consistent with the observed ensemble self-lofting (a few kilometres in 4 months) and the main isolated vortex rise (∼ 20 km in 2 months). Our results also show evidence of the importance of longwave emission in the net HR of biomass burning plumes.</p

Local electronic structure of Cr in the II-VI diluted ferromagnetic semiconductor Zn1−x_{1-x}Crx_xTe

The electronic structure of the Cr ions in the diluted ferromagnetic semiconductor Zn$_{1-x}$Cr$_x$Te ($x=0.03$ and 0.15) thin films has been investigated using x-ray magnetic circular dichroism (XMCD) and photoemission spectroscopy (PES). Magnetic-field ($H$) and temperature ($T$) dependences of the Cr $2p$ XMCD spectra well correspond to the magnetization measured by a SQUID magnetometer. The line shape of the Cr $2p$ XMCD spectra is independent of $H$, $T$, and $x$, indicating that the ferromagnetism is originated from the same electronic states of the Cr ion. Cluster-model analysis indicates that although there are two or more kinds of Cr ions in the Zn$_{1-x}$Cr$_x$Te samples, the ferromagnetic XMCD signal is originated from Cr ions substituted for the Zn site. The Cr 3d partial density of states extracted using Cr $2p \to 3d$ resonant PES shows a broad feature near the top of the valence band, suggesting strong $s$,$p$-$d$ hybridization. No density of states is detected at the Fermi level, consistent with their insulating behavior. Based on these findings, we conclude that double exchange mechanism cannot explain the ferromagnetism in Zn$_{1-x}$Cr$_{x}$Te.Comment: Accepted for New Journal of Physics. Single column, 21 pages, 7 figure

Trends of exchange interactions in dilute magnetic semiconductors

We discuss the importance of different exchange mechanisms like double exchange, p-d exchange and anti-ferromagnetic as well as ferromagnetic superexchange in dilute magnetic semiconductors (DMSs). Based on the coherent potential approximation for the electronic structure of the DMSs we show that the different mechanisms exhibit different dependences on the concentration of the magnetic impurities, on the hybridization with the wavefunctions of neighbouring impurities and on the position of the Fermi level in the band gap. However, common to all mechanisms is that, as long as half-metallicity is preserved, they are determined by the hybridization with the orbitals of neighbouring impurities and of the resulting energy gain due to the formation of bonding and anti-bonding hybrids. By calculating the exchange coupling constants J(ij) (E-F) as a function of the position of the Fermi level we obtain a universal trend for the exchange interactions with band filling

Influence of misfit dislocations on the magnetoresistance of MgO-based epitaxial magnetic tunnel junctions

International audienc

Spin-Polarized Electron Tunneling in bcc FeCo/MgO/FeCo(001) Magnetic Tunnel Junctions

International audienceIn combining spin-and symmetry-resolved photoemission, magnetotransport measurements and ab initio calculations we detangled the electronic states involved in the electronic transport in Fe 1Àx Co x ð001Þ=MgO=Fe 1Àx Co x ð001Þ magnetic tunnel junctions. Contrary to previous theoretical predictions , we observe a large reduction in TMR (from 530 to 200% at 20 K) for Co content above 25 atomic % as well as anomalies in the conductance curves. We demonstrate that these unexpected behaviors originate from a minority spin state with Á 1 symmetry that exists below the Fermi level for high Co concentration. Using angle-resolved photoemission, this state is shown to be a two-dimensional state that occurs at both Fe 1Àx Co x ð001Þ free surface, and more importantly at the interface with MgO. The combination of this interface state with the peculiar density of empty states due to chemical disorder allows us to describe in details the complex conduction behavior in this system. Since the discovery of giant magnetoresistance (GMR) in spin valves in 1988 [1], a new branch of physics referred to as spintronics has considerably developed. The discovery of the large tunnel magnetoresistance (TMR) in 1995 [2], the prediction of the spin-transfer mechanism in 1996 [3,4], and the demonstration of spin-dependent coherent tunnel-ing in MgO-based epitaxial MTJs in 2001-2004 [5–10], have largely contributed to developments in this field. Currently, a number of new areas are being explored, such as rf oscillators, devices and memories based on the spin-transfer-torque effect, electric field assisted switching , magnonics, or spincaloritronics [11]. In addition, industrial-scale devices such as magnetic recording heads already use the exceptional electrical properties of GMR and TMR. The technology transfer from research to industry continues today, with MRAM demonstrators based on MgO-based MTJs [12] and rf oscillators using spintronics devices. While commercialization as well as broad utilization into various areas of research has been rapid for spin-tronic devices, in many cases a full understanding of the underlying physics is lacking. MgO-based MTJs with FeCo or FeCoB electrodes are a striking example of this situation. FeCoðBÞ=MgO=FeCoðBÞð001Þ multilayers, fabricated by molecular beam epitaxy (MBE) or sputtering deposition are widely utilized for their high spin current injection efficiency and exceptional electrical sensitivity to any change in the magnetic configuration of the electrodes. Because of the huge TMR predicted by ab initio calculation for the equimolar and B2 ordered Fe 0:5 Co 0:5 alloy and for pure bcc Co (1000%–6000% at 0 K [13]), bcc FeCo(001) electrodes are now extensively used in MTJ fabrication. However, the situation is not so clear regarding the reported results. First, large TMR were actually obtained on MBE grown Fe=bcc Co=MgO=Co=Feð001Þ [14]. However, a heating of the whole stacking up to 250 C during 30 minutes suggest a possible alloying between Fe and Co. On the other hand, contrary to expectations, epitaxial Fe 0:5 Co 0:5 =MgO=Feð001Þ and Fe=MgO=Feð001Þ MTJs exhibit the same TMR [15]. It should be noted that the B2 order assumed in Ref. [13] is not observed. Finally, reported TMR of sputtered FeCo=MgO=FeCoð001Þ MTJs present a nonmonotonic dependence as a function of the Co concentration with a maximum around 25% of Co [16]. The detailed effect of Co alloying into Fe on the spin-dependent tunneling remains therefore obscure. In this Letter, we explain quantitatively the unexpected transport properties observed in FeCo=MgO=FeCoð001Þ MTJs. We demonstrate that transport measurements alone are not sufficient to complete the current understanding of this system, and that spin-, symmetry-, and angle-resolved photoemission, together with DFT calculations taking into account the chemical disorder, offer a unique path to probing directly the tunneling electrons. We use a specific photoemission experiment to untangle the different Bloch waves responsible for the conduction along (001) as a function of their symmetry (Á 1 or Á 5) and spin state (majority " or minority #), in contrast to standard transport measurements where all these contributions are mixed. bcc MgO-based MTJs with Fe 1Àx Co x ð001Þ electrodes were grown by coevaporation using MBE. The epitaxial relationship, growth mode, and surface flatness were controlled using reflection high energy electron diffraction (RHEED). In addition, the evaporation rates of the Co and Fe sources, and consequently the alloys stoichiometry, were accurately controlled by recording the intensit

The unexpected radiative impact of the Hunga Tonga eruption of 15th January 2022

International audienceThe underwater Hunga Tonga-Hunga Ha-apai volcano erupted in the early hours of 15th January 2022, and injected volcanic gases and aerosols to over 50 km altitude. Here we synthesise satellite, ground-based, in situ and radiosonde observations of the eruption to investigate the strength of the stratospheric aerosol and water vapour perturbations in the initial weeks after the eruption and we quantify the net radiative impact across the two species using offline radiative transfer modelling. We find that the Hunga Tonga-Hunga Ha-apai eruption produced the largest global perturbation of stratospheric aerosols since the Pinatubo eruption in 1991 and the largest perturbation of stratospheric water vapour observed in the satellite era. Immediately after the eruption, water vapour radiative cooling dominated the local stratospheric heating/cooling rates, while at the top-of-the-atmosphere and surface, volcanic aerosol cooling dominated the radiative forcing. However, after two weeks, due to dispersion/dilution, water vapour heating started to dominate the top-of-the-atmosphere radiative forcing, leading to a net warming of the climate system