Oxygen-enabled control of Dzyaloshinskii-Moriya Interaction in ultra-thin magnetic films

The search for chiral magnetic textures in systems lacking spatial inversion symmetry has attracted a massive amount of interest in the recent years with the real space observation of novel exotic magnetic phases such as skyrmions lattices, but also domain walls and spin spirals with a defined chirality. The electrical control of these textures offers thrilling perspectives in terms of fast and robust ultrahigh density data manipulation. A powerful ingredient commonly used to stabilize chiral magnetic states is the so-called Dzyaloshinskii-Moriya interaction (DMI) arising from spin-orbit coupling in inversion asymmetric magnets. Such a large antisymmetric exchange has been obtained at interfaces between heavy metals and transition metal ferromagnets, resulting in spin spirals and nanoskyrmion lattices. Here, using relativistic first-principles calculations, we demonstrate that the magnitude and sign of DMI can be entirely controlled by tuning the oxygen coverage of the magnetic film, therefore enabling the smart design of chiral magnetism in ultra-thin films. We anticipate that these results extend to other electronegative ions and suggest the possibility of electrical tuning of exotic magnetic phases

Indirect chiral magnetic exchange through Dzyaloshinskii–Moriya-enhanced RKKY interactions in manganese oxide chains on Ir(100)

Localized electron spins can couple magnetically via the Ruderman–Kittel–Kasuya–Yosida interaction even if their wave functions lack direct overlap. Theory predicts that spin–orbit scattering leads to a Dzyaloshinskii–Moriya type enhancement of this indirect exchange interaction, giving rise to chiral exchange terms. Here we present a combined spin-polarized scanning tunneling microscopy, angle-resolved photoemission, and density functional theory study of MnO_2 chains on Ir(100). Whereas we find antiferromagnetic Mn–Mn coupling along the chain, the inter-chain coupling across the non-magnetic Ir substrate turns out to be chiral with a 120° rotation between adjacent MnO_2 chains. Calculations reveal that the Dzyaloshinskii–Moriya interaction results in spin spirals with a periodicity in agreement with experiment. Our findings confirm the existence of indirect chiral magnetic exchange, potentially giving rise to exotic phenomena, such as chiral spin-liquid states in spin ice systems or the emergence of new quasiparticles

Direct bandgap quantum wells in hexagonal Silicon Germanium

Silicon is indisputably the most advanced material for scalable electronics, but it is a poor choice as a light source for photonic applications, due to its indirect band gap. The recently developed hexagonal Si1−xGex semiconductor features a direct bandgap at least for x &gt; 0.65, and the realization of quantum heterostructures would unlock new opportunities for advanced optoelectronic devices based on the SiGe system. Here, we demonstrate the synthesis and characterization of direct bandgap quantum wells realized in the hexagonal Si1−xGex system. Photoluminescence experiments on hex-Ge/Si0.2Ge0.8 quantum wells demonstrate quantum confinement in the hex-Ge segment with type-I band alignment, showing light emission up to room temperature. Moreover, the tuning range of the quantum well emission energy can be extended using hexagonal Si1−xGex/Si1−yGey quantum wells with additional Si in the well. These experimental findings are supported with ab initio bandstructure calculations. A direct bandgap with type-I band alignment is pivotal for the development of novel low-dimensional light emitting devices based on hexagonal Si1−xGex alloys, which have been out of reach for this material system until now.</p

Indirect chiral magnetic exchange through Dzyaloshinskii–Moriya-enhanced RKKY interactions in manganese oxide chains on Ir(100)

Localized electron spins can couple magnetically via the Ruderman–Kittel–Kasuya–Yosida interaction even if their wave functions lack direct overlap. Theory predicts that spin–orbit scattering leads to a Dzyaloshinskii–Moriya type enhancement of this indirect exchange interaction, giving rise to chiral exchange terms. Here we present a combined spin-polarized scanning tunneling microscopy, angle-resolved photoemission, and density functional theory study of MnO_2 chains on Ir(100). Whereas we find antiferromagnetic Mn–Mn coupling along the chain, the inter-chain coupling across the non-magnetic Ir substrate turns out to be chiral with a 120° rotation between adjacent MnO_2 chains. Calculations reveal that the Dzyaloshinskii–Moriya interaction results in spin spirals with a periodicity in agreement with experiment. Our findings confirm the existence of indirect chiral magnetic exchange, potentially giving rise to exotic phenomena, such as chiral spin-liquid states in spin ice systems or the emergence of new quasiparticles

Band lineup at hexagonal Six_xGe1−x_{1-x}/Siy_yGe1−y_{1-y} alloy interfaces

The natural and true band profiles at heterojunctions formed by hexagonal Si$_x$Ge$_{1-x}$ alloys are investigated by a variety of methods: density functional theory for atomic geometries, approximate quasiparticle treatments for electronic structures, different band edge alignment procedures, and construction of various hexagonal unit cells to model alloys and heterojunctions. We demonstrate that the natural band offsets are rather unaffected by the choice to align the vacuum level or the branch point energy, as well as by the use of a hybrid or the Tran-Blaha functional. At interfaces between Ge-rich alloys we observe a type-I heterocharacter with direct band gaps, while Si-rich junctions are type-I but with an indirect band gap. The true band lineups at pseudomorphically grown heterostructures are strongly influenced by the generated biaxial strain of opposite sign in the two adjacent alloys. Our calculations show that the type-I character of the interface is reduced by strain. To prepare alloy heterojunctions suitable for active optoelectronic applications, we discuss how to decrease the compressive biaxial strain at Ge-rich alloys.Comment: 23 pages, 8 figure

Pseudodirect to direct compositional crossover in wurtzite GaP/InxGa1-xP core-shell nanowires

Thanks to their uniqueness, nanowires allow the realization of novel semiconductor crystal structures with yet unexplored properties, which can be key to overcome current technological limits. Here we develop the growth of wurtzite GaP/In(x)G(x)G(1-x) core shell nanowires with tunable indium concentration and optical emission in the visible region from 590 nm (2.1 eV) to 760 nm (1.6 eV). We demonstrate pseudodirect (Gamma(8c)-Gamma(9v)) to direct ((Gamma 7c-Gamma 9v)) transition crossover through experimental and theoretical approach. Time resolved and temperature dependent photoluminescence measurements were used, which led to the observation of a steep change in carrier lifetime and temperature dependence by respectively one and 3 orders of magnitude in the range 0.28 +/- 0.04 &lt;= 0.41 +/- 0.04. Our work reveals the electronic properties of wurtzite In(x)G(1-x)P

Cu, Sm co-doping effect on the CO oxidation activity of CeO₂. A combined experimental and density functional study

The co-doping effect of a rare earth (RE) metal and a transition metal (TM) on ceria oxidation catalysis through the evaluation of samarium-copper co-doped catalysts with Ce-Sm-xCu-O (x: 0–20 at.%, Ce/Sm = 1) nominal compositions, is discussed. The CO oxidation reaction was used as a prototype reaction due to its pivotal role in the fuel cell technology. Ce-Sm-20Cu-O catalyst presented a 64% increase in the CO oxidation activity compared to that of pristine ceria. Diffraction and Raman studies proved that the Cu, Sm co-doping induces many defects related to the dopants (Sm, Cu) and the oxygen vacant sites, while the presence of hybrid CuO/Ce-Sm(Cu)-O fluorite/SmO8 (cubic metastable) phases is the most representative scenario of this oxide microstructure. A size polydispersity of CuO phases was achieved by introducing air cooling during the microwave heating. Cu, Sm atoms were uniformly doped in CeO2 structure according to the HAADF-STEM studies. These results are in agreement with EDS analysis, where Cu, Sm and Ce are located in all the analyzed areas without any preferential distribution. The XPS studies demonstrated the co-presence of Cu2+/Cu1+ and Ce4+/Ce3+ redox couples in agreement with the Bader charge analysis from the ab initio calculations, the latter influencing greatly the oxidation activity of the catalysts. Density functional theory (DFT) calculations shed light on the oxide surface and the underlying mechanism governing the oxidation catalysis taking place. In particular, Cu2+ and Sm3+ dopants were found to be located in the nearest neighbor (NN) sites of oxygen vacancies. Different oxygen vacancies configurations were studied (single vs. double, surface vs. subsurface), where the single vacancies are more stable on the surface, whereas the double vacancies configurations are more stable on the subsurface. Regarding the Ce3+ location, in the presence of single and double oxygen vacancy, the Ce3+ ions prefer to be located in the 1st NN/2nd NN and 2nd NN of the first Ce layer, relative to the oxygen vacancy, respectively. The total Density of States (DOS) analysis of the co-doped systems revealed that the dopants induced new surface states inside the ceria band gap, which can accommodate the unpaired electrons of the vacant oxygen sites. These electronic modifications justify the much lower energy of oxygen vacancy formation (Evf) in both cases, the Sm-doped, and Cu, Sm -doped CeO2 (1 1 1) geometries. Specifically, the Evf lowering upon doping was found to be almost two times larger for the Cu adjacent oxygen vacancies (Cu2+-□) compared to the Sm ones (Sm3+-□), consistent with the CO adsorption trend as the Cu-Sm-CeO2 (1 1 1) system is energetically more favorable than the Sm-CeO2 (1 1 1) and pure CeO2 (1 1 1) surfaces

Indirect chiral magnetic exchange through Dzyaloshinskii–Moriya-enhanced RKKY interactions in manganese oxide chains on Ir(100)

Localized electron spins can couple magnetically via the Ruderman–Kittel–Kasuya–Yosida interaction even if their wave functions lack direct overlap. Theory predicts that spin–orbit scattering leads to a Dzyaloshinskii–Moriya type enhancement of this indirect exchange interaction, giving rise to chiral exchange terms. Here we present a combined spin-polarized scanning tunneling microscopy, angle-resolved photoemission, and density functional theory study of MnO2 chains on Ir(100). Whereas we find antiferromagnetic Mn–Mn coupling along the chain, the inter-chain coupling across the non-magnetic Ir substrate turns out to be chiral with a 120° rotation between adjacent MnO2 chains. Calculations reveal that the Dzyaloshinskii–Moriya interaction results in spin spirals with a periodicity in agreement with experiment. Our findings confirm the existence of indirect chiral magnetic exchange, potentially giving rise to exotic phenomena, such as chiral spin-liquid states in spin ice systems or the emergence of new quasiparticles