Giant Switchable Rashba Effect in Oxide Heterostructures

One of the most fundamental phenomena and a reminder of the electron's relativistic nature is the Rashba spin splitting for broken inversion symmetries in two‐dimensional condensed matter systems. Typically, this splitting is a tiny relativistic correction. By coupling ferroelectric BaTiO₃ and a 5d (or 4d) transition metal oxide with a large spin‐orbit coupling, Ba(Os,Ir,Ru)O₃, it is shown that giant Rashba spin splittings are indeed possible and even controllable by an external electric field. Based on density functional theory and a microscopic tight binding understanding, it is concluded that the electric field is amplified and stored as a ferroelectric Ti‐O distortion which, through the network of oxygen octahedra, induces a large (Os,Ir,Ru)‐O distortion. The BaTiO₃/Ba(Os,Ru,Ir)O₃ heterostructure is hence the ideal test station for switching and studying the Rashba effect and allows applications at room temperature.     

Spatial Inhomogeneities in Single-Crystal HgBa2CuO4+δ from 63Cu NMR Spin and Quadrupole Shifts

⁶³Cu Nuclear Magnetic Resonance (NMR) measurements on a single crystal of the single-layer high-temperature superconductor HgBa₂CuO_4+δ are reported. From the analysis of the quadrupolar satellites and their anisotropic splitting a largely temperature-independent symmetric quadrupole tensor is deduced, despite substantial variation in the electrical field gradients at the Cu site. The strongly temperature-dependent magnetic shifts and linewidths of the ⁶³Cu central line for different field orientations also reveal substantial spin shift variations in the material. Linear dependences on the doping of both, the quadruple splitting as well as the spin shifts, explain over a large temperature range all the widths with a local doping variation corresponding to 2δ≈0.073.     

Structure, Phonon Vibration, and Spin Correlation of LaBa2Cu3−x Co x O y

Polycrystalline samples LaBa₂Cu_3?xCo _x O _y (0 ≤ x ≤ 1.0) were synthesized by solid state reaction method. The structure, phonon vibration, conduction, and spin correlation were investigated by means of X-ray diffraction, infrared spectra, resistivity, and electron spin resonance. It is found that there are orthorhombic–tetragonal and tetragonal–orthorhombic structural transitions with Co doping, and the conduction behavior changes from metallic to semiconducting. With the increase of Co content, the Cu(1)—O(1) phonon mode around 531 cm^?1 softens, the Cu(2)—O(2) phonon mode around 657 cm^?1 hardens, and the Cu(1)—O(4) mode around 583 cm^?1 is nearly unchanged. The Cu²⁺ spins tend to localize with Co doping. The changes in structure, phonon vibration, and spin correlation with Co doping are analyzed and discussed.     

Structural studies of a mixed-valence state in the incommensurate composite crystal Sr1.261CoO3

Abstract

The incommensurate modulated crystal structure of the hexagonal cobalt oxide Sr_1.261CoO₃ has been studied using a four-dimensional (4D) superspace profile analysis of neutron powder diffraction data. Sr_1.261CoO₃ is a composite crystal that consists of the [CoO₃] and [2Sr] subsystems. The [CoO₃] subsystem forms 1D chains that run parallel to the c-axis and consist of face-sharing CoO₆ polyhedra with octahedral (Oh) and trigonal prismatic (TP) coordinations. The structure analysis reveals that the [CoO₃] chains contain 73.9% Oh and 26.1% TP sites, and that the TP sites have longer Co–O bonds than the Oh sites: d_av. =2.039(4) Å (TP) and 1.895(3) Å (Oh). The averaged Co bond valences are Co^3.56(3)+ in the Oh sites and Co^2.45(3)+ in the TP sites, suggesting that a considerable amount of Co³⁺ ions are mixed with Co⁴⁺ions in the Oh sites and with Co²⁺ ions in the TP sites. The observed magnetic susceptibility can be well explained assuming that the compound has the Co mixed-valence state with the spin configurations of S=0 low-spin state for Co³⁺(dε⁶), S=1/2 low-spin state for Co⁴⁺(dε⁵) and S=3/2 high-spin state for Co²⁺(dε⁵dγ²). The Weiss temperature, approximately 0.8 K, implies that Sr_1.261CoO₃ naturally assumes a Curie paramagnetic state, probably owing to the obstruction of the intrachain magnetic interaction by the nonmagnetic Co³⁺ ions. These results suggest that the nonmagnetic Co³⁺ ions play an essential role in the magnetism of Sr_2γCoO₃ systems.     

Structure, Phonon Vibration, and Spin Correlation of LaBa2Cu3?xCoxOy

Polycrystalline samples LaBa₂Cu_3–xCo _x O _y (0 x 1.0) were synthesized by solid state reaction method. The structure, phonon vibration, conduction, and spin correlation were investigated by means of X-ray diffraction, infrared spectra, resistivity, and electron spin resonance. It is found that there are orthorhombic–tetragonal and tetragonal–orthorhombic structural transitions with Co doping, and the conduction behavior changes from metallic to semiconducting. With the increase of Co content, the Cu(1)—O(1) phonon mode around 531 cm^–1 softens, the Cu(2)—O(2) phonon mode around 657 cm^–1 hardens, and the Cu(1)—O(4) mode around 583 cm^–1 is nearly unchanged. The Cu²⁺ spins tend to localize with Co doping. The changes in structure, phonon vibration, and spin correlation with Co doping are analyzed and discussed.     

The search for novel superhard and incompressible materials on the basis of higher borides of <Emphasis Type="Italic">s</Emphasis>, <Emphasis Type="Italic">p</Emphasis>, <Emphasis Type="Italic">d</Emphasis> metals

The state of the art in the search for novel superhard and (or) incompressible materials on the basis of higher borides of s, p, d metals has been briefly reviewed. The information has been considered about experimental and theoretical studies of the following groups of borides: diborides of 4d, 5d heavy metals (Tc, Ru, Rh, Re, Os, and Ir), hexagonal tetraborides with the WB₄-type structure, and AMB₁₄ borides (where A, M are s, p metals) as well as of a number of related systems.     

Anomalous Jahn-Teller distortions in La0.75Ca0.25MnO3 system: An X-ray absorption study

MnK-edge X-ray absorption near edge structure (XANES) measurements have been made to study the temperature-dependent lattice effects in the La_0.75Ca_0.25MnO₃ system. The high-resolution XANES spectra recorded with high signal-to-noise ratio have allowed us to study the temperature dependence of the Jahn-Teller splitting indicated directly by the dipole forbidden pre-peaks corresponding to transition in Mn 3d states. The results show splitting of thee _g state in its two orbitals (d x² -_y ² andd _3z ²-_r ². The energy splitting shows anomalous temperature dependence across the giant magnetoresistance transition temperature.     

Vingt nouveaux germaniures ternaires TR5T4Ge10 de metaux tr des terres rares et T = Co,Rh, Ir. Supraconductivite de Lu5Rh4Ge10 et Lu5Ir4Ge10

Twenty ternary germanides, which crystallize in the Sc₅Co₄Si₁₀ type, have been prepared in the RE-T-Ge systems where RE is a rare earth and T = Co, Rh, Ir: with Co, RE = Tm-Lu; with Rh, RE = Y, Gd-Lu; with Ir, RE = Gd-Lu. Lu₅Rh₄Ge₁₀ and Lu₅Ir₄Ge₁₀ show superconducting transitions at 2.2?1.6 K and 2.01?1.94 K respectively.     

Magnetic properties of RRh3B2(R=La,Ce, Nd,Gd) compounds

The¹¹Bnmr results on RRh₃B₂(R=La, Ce, Nd and Gd) are reported. For CeRh₃B₂, specific heat and electrical resistivity are reported. From a comparative study of the¹¹Bnmr Knight shifts and the spin lattice relaxation times of these compounds it is shown that in CeRh₃B₂, there is strong hybridization of 4f states with the conduction electrons. A local moment on Ce with admixture of 4f and 5d–6s orbitals is suggested.     

Fe–Co magnetic nanoclusters by density functional theory calculations

ABSTRACT

We present density functional theory results referring to the structural, electronic and magnetic properties of 13, 55, 147 and 309 Fe–Co (magnetic–magnetic) icosahedral nanoclusters (ICO) comparing with our previous results on Fe–Cu (magnetic–nonmagnetic). It came out that the Fe atoms always favour the edge surface sites exhibiting higher average magnetic moment (MM) for Fe and FeCo ICOs than FeCu while the local Fe MM is greater for FeCu₁₂ and Fe₆Cu₄₉ ones. This is due to the strong hybridisation of Fe 3d–Co 3d states, while in Fe–Cu the Fe spin down states are restricted close to fermi without been affected by the corresponding Cu states. These results could be used for the design of environmental sustainable smart magnetic alloys.

This is part of a thematic issue on Nanoscale Materials Characterisation and Modeling by Advances Microscopy Methods - EUROMAT.     

Quaternary chalcogenides of formula MII2MV2−2yM′2yS6 and Se6

Compositions of the title formula where M^II = Ba and Sr, M^V = Ta and Nb, M′ = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Rh and Ir, and y = 0.5 and 0.33 were prepared by two methods: reaction of the elements in evacuated silica tubes and reaction of M^IICO₃, M^V₂O₅, and M′ oxides with H₂S. When M^II = Ba, all members have the BaNiO₃ structure. When M′ = Mn and Cu, structural distortions are present. Some compositions have Ta or S vacancies. When M^II = Sr, compositions were prepared only with Ta, for y = 0.5 and when M′ = Cr and Fe. Electrical measurements on selected compositions showed semiconductivity.     

Anisotropic Transferred Hyperfine Interaction in a Heavy-Fermion Compound: Paramagnetic NMR Line Shift in a CeCu6 Single Crystal

We report on single crystal nuclear magnetic resonance spectroscopy for ⁶³Cu at the Cu(5) sites in the heavy-fermion paramagnet CeCu₆ for constant external magnetic field strength (7 T) applied along the “hardest” magnetic direction b and varied temperature (5 − 100 K). The transferred field coupling constant α _b (5) = + 3.40(31) kOe/μ _B is derived from the van Vleck like paramagnetic line shift. These results establish the predominance of anisotropic spin and orbital polarization acting on the Cu sites neighbouring the Ce 4f local moment. Because magnetically induced electric quadrupolar interaction at this Cu site is not unusually enhanced, we conclude that the exchange polarization mechanism at the Ce site is the source of the observed anisotropy. Impact of the substitution of Cu(2) by Au on the Ce³⁺ single-ion ground state properties and magnetic characteristics of CeCu_6-x Au _x is discussed.      

Sorption of 131I? and 131IO3? from Aqueous Solutions on Porous Inorganic Sorbents Modified with d-Element Ammoniates at 25°C

Sorption of ¹³¹I⁻ and ¹³¹IO ₃ ⁻ from aqueous solutions on porous inorganic sorbents modified with d elements is studied at 25°C. The sorbents modified with Ag, Zn, Ni, Cu, and Co ammoniates are characterized by low distribution coefficients of ¹³¹I⁻ and ¹³¹IO ₃ ⁻ (K _d <150 ml g⁻¹ at V/m = 1000). Calcination of the sorbents modified with Ag, Cu, and Zn ammoniates at a temperature above 250°C affects the chemical state of the metals in the sorbent matrix, which is accompanied by slight increase in K _d of ¹³¹I⁻ and ¹³¹IO ₃ ⁻ as compared to as-prepared samples. With the γ-Al₂O₃-based sorbents modified with Ni or Cu, K _d of ¹³¹I⁻ and ¹³¹IO ₃ ⁻ is well comparable with that for the Ag-modified sorbents based on MSKG silica gel.__________Translated from Radiokhimiya, Vol. 47, No. 3, 2005, pp. 265–268.Original Russian Text Copyright © 2005 by Kulyukhin, Mizina, Krasavina, Rumer, Konovalova, Tanashchuk, Bogachev.     

Growth Mechanism of Metal Clusters on a Graphene/Ru(0001) Template

Using first‐principles calculations combined with scanning tunneling microscopy experiments, we investigated the adsorption configurations, electronic structures and the corresponding growth mechanism of several transition metal (TM) atoms (Pt, Ru, Ir, Ti, Pd, Au, Ag, and Cu) on a graphene/Ru(0001) moiré template (G/Ru(0001)) at low coverage. We find that Pt, Ru, Ir, and Ti selectively adsorb on the fcc region of G/Ru(0001) and form ordered dispersed metal nanoclusters. This behavior is due to the unoccupied d orbital of the TM atoms and the strong sp³ hybridization of carbon atoms in the fcc region of G/Ru(0001). Pd, Au, Ag, and Cu form nonselective structures because of the fully occupied d orbital. This mechanism can be extended to metals on a graphene/Rh(111) template. By using Pt as an example, we provide a layer by layer growth path for Pt nanoclusters in the fcc region of the G/Ru(0001). The simulations of growth mechanism agree well with the experimental observations. Moreover, they also provide guidance for the selection of suitable metal atoms to form ordered dispersed metal nanoclusters on similar templates.     

Neutron structural investigations of Y1?xCaxBa2Cu3?yCoyO7±δ

The results of a systematic powder neutron study on Y_1–x CaxBa₂Cu_3–y Co _y O_7± for A (x=y=0), B (x=0;y=0.2), C (x= 0;y=0.4), D (x=y=0.2), and E (x=y=0.4) are investigated with a view to understanding the relation between the structural parameters and superconductivity. Rietveld refinements of the structures show that: (a) Co substitutes at the chain Cu(1) sites only, except for sample E, where the presence of a minor amount of Co at the planar Cu(2) site cannot be ruled out; (b) Co substitution reduced thec-parameter, which is reduced even further upon substitution of Ca at the Y-sites; (c) the occupancy factors of the chain O(1) site indicate an average coordination of 4.8 and 5.1 for Co for samples B and C, but only 4 for D and E; (d) Cu/Co(1) atoms for B and C display large thermal parameters, suggesting a displacement from their ideal centrosymmetric location; (e) the apical Cu(1)-O(4) bond lengthens upon substitution of Co in samples B and C but undergoes shortening upon substitution of Ca in the case of samples D and E. The apical Cu(2)-O(4) bond, on the other hand, shows just the opposite trend; (f) samples D and E show a reduction in the separation of CuO₂ layers and their oxygen content; and (e) the bond valence of the Cu(2) ion shows the lowest value of 2.127 for the nonsuperconducting sample C.     

Performance of an Array of Tin Dioxide-Based Ceramic Sensors for CO and SO2 Selective Detecting

Changes in electrical resistance of tin dioxide based ceramic pellets have been studied (as received and doped with Tl, Zn, Rh, Cu, Pt, Ir, Cd and Co) in air and at various concentrations of nitrogen mixed with CO and SO₂. In response to changes in the amount of adsorbed oxygen, the electric conductivity in direct current of the materials tested modifies accordingly. This behaviour is influenced by physical (diffusion in nitrogen) and chemical (CO and SO₂ oxidation) phenomena. It was showed that the electric answers of the sensors doped with Zn, Cu, Pt and Ir could be combined for the simultaneous determination of CO and SO₂.     

Iridium‐Based Multimetallic Porous Hollow Nanocrystals for Efficient Overall‐Water‐Splitting Catalysis

The development of active and durable bifunctional electrocatalysts for overall water splitting is mandatory for renewable energy conversion. This study reports a general method for controllable synthesis of a class of IrM (M = Co, Ni, CoNi) multimetallic porous hollow nanocrystals (PHNCs), through etching Ir‐based, multimetallic, solid nanocrystals using Fe³⁺ ions, as catalysts for boosting overall water splitting. The Ir‐based multimetallic PHNCs show transition‐metal‐dependent bifunctional electrocatalytic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in acidic electrolyte, with IrCo and IrCoNi PHNCs being the best for HER and OER, respectively. First‐principles calculations reveal a ligand effect, induced by alloying Ir with 3d transition metals, can weaken the adsorption energy of oxygen intermediates, which is the key to realizing much‐enhanced OER activity. The IrCoNi PHNCs are highly efficient in overall‐water‐splitting catalysis by showing a low cell voltage of only 1.56 V at a current density of 2 mA cm^?2, and only 8 mV of polarization‐curve shift after a 1000‐cycle durability test in 0.5 m H₂SO₄ solution. This work highlights a potentially powerful strategy toward the general synthesis of novel, multimetallic, PHNCs as highly active and durable bifunctional electrocatalysts for high‐performance electrochemical overall‐water‐splitting devices.     

B111, B112, B113, and B114: The most stable core-shell borospherenes with an icosahedral B12 core at the center exhibiting superatomic behaviors

Boron allotropes are known to be predominately constructed by icosahedral B₁₂ cages, while icosahedral-B₁₂ stuffing proves to effectively improve the stability of fullerene-like boron nanoclusters in the size range between B₉₈–B₁₀₂. However, the thermodynamically most stable core-shell borospherenes with a B₁₂ icosahedron at the center still remains unknown. Based on the structural motif of D_5h C₇₀ and extensive first-principles theory calculations, we predict herein the high-symmetry C_5v B₁₁₁⁺ (3) which satisfies the Wade’s n+1 and n+2 skeletal electron counting rules exactly and the approximately electron sufficient C_s B₁₁₁ (4), C_s B₁₁₂ (5), C_s B₁₁₃ (6), and C_s B₁₁₄ (7) which are the most stable neutral core-shell borospherenes with a B₁₂ icosahedron at the center reported to date in the size range between B₆₈–B₁₃₀, with C_s B₁₁₂ (5) being the thermodynamically most favorite species in the series. Detailed orbital and bonding analyses indicate that these spherically aromatic species all contain a negatively charged icosahedral B₁₂²⁻ core at the center which exhibits typical superatomic behaviors in the electronic configuration of 1S²1P⁶1D¹⁰1F⁸, with its dangling valences saturated by twelve radial B-B 2c-2e σ bonds between the B₁₂ inner core and the B₇₀ outer shell. The infrared (IR) and Raman spectra of the concerned species are computationally simulated to facilitate their future characterizations.

     

Magnetic Moments in SmCo5 and SmCo5?x Cu x Films

SmCo₅ is an emerging perpendicular magnetic material for super-high density magnetic recording, due to its large magnetic anisotropy energy. In this paper, the magnetic moments of SmCo_5?x Cu _x have been studied using first principles calculation based on density-functional theory (DFT). Calculations are performed using the pseudopotential plane wave DFT code Vienna ab initio simulation package (VASP) with the projector augmented wave (PAW) method. The local density approximation LDA+U method is used for the calculation of the exchange correlation energy of Sm. The calculation results show that the average Co magnetic moment of SmCo_5?x Cu _x decreases with the increase of Cu doping concentrations, and the influence of the Cu doping on the spin state of Co is greater than that of Sm. The magnetic anisotropy energy of SmCo₅ is analyzed. The electronic density of states and the differential in spin densities of atoms show that the spatial distribution of 4f electron and the 4f?C3d coupling are the controlling factors of the magnetic anisotropy energy of SmCo₅.     

Multimetal Borides Nanochains as Efficient Electrocatalysts for Overall Water Splitting

The development of cost‐efficient, active, and stable electrode materials as bifunctional catalysts for electrochemical water splitting is crucial to high‐performance renewable energy storage and conversion devices. In this work, the synthesis of Co‐based multi‐metal borides nanochains with amorphous structure is reported for boosting the oxygen evolution (OER) and hydrogen evolution reactions (HER) by one‐pot NaBH₄ reduction of Co²⁺, Ni²⁺, and Fe²⁺ under ambient temperature. In all the investigated Co‐based metal borides, NiCoFeB nanochains show the excellent OER performance with a low overpotential of 284 mV at 10 mA cm^‐2 and Tafel slope of 46 mV dec^‐1, respectively, together with excellent catalytic stability, and robust HER performance with an overpotential of 345 mV at 10 mA cm^‐2. The density functional theory (DFT) calculations reveal that the excellent electrocatalytic performance is mainly attributed to optimal electronic structure by tuning the Co‐3d band activities by the incorporation of Ni and Fe for enhanced water splitting via the potentially existed Co⁰ state. Moreover, the electrolyzer using NiCoFeB nanochains as anode and cathode offers 10 mA cm^‐2 at a cell voltage of 1.81 V, comparable to commercial Pt/C // Ir/C, providing a simple method to design and explore highly efficient and cheap bifunctional electrocatalysts for overall water splitting.