In this work we present the result of the first experimental investigation of the role of oxygen content on the properties of the HoBaCo2O5+delta layered cobaltite. We have measured the variation of oxygen content as a function of temperature and oxygen partial pressure by means of thermogravimetry coupled to chemical titration analysis. On selected samples of accurately known oxygen content we have undertaken a systematic investigation of their structural, thermal and magnetic properties by means of x-ray diffraction, differential scanning calorimetry and magnetometry. The overall results gained by this study confirm the central role of oxygen content on the properties of these materials suggesting that, for the Ho composition, even very slight variation of the order of delta=0.01 has a dramatic influence on the magnetic and transport properties of the samples. In addition, we have presented results showing the strategy to check the quality of samples prepared at selected oxygen contents by annealing procedures.
In this paper we have studied, by means of high-resolution neutron powder diffraction and magnetic susceptibility, the structural and magnetic features of selected samples of the HoBaCo2O5+delta layered cobaltite in the low oxygen content range (0.02<delta<0.22). The results shows a strong antiferromagnetic contribution at room temperature coupled to an intermediate spin state of the Co3+ ions.
The ability to manipulate oxygen anion defects rather than metal cations in complex oxides can facilitate creating new functionalities critical for emerging energy and device technologies. However, the difficulty in activating oxygen at reduced temperatures hinders the deliberate control of important defects, oxygen vacancies. Here, strontium cobaltite (SrCoOx) is used to demonstrate that epitaxial strain is a powerful tool for manipulating the oxygen vacancy concentration even under highly oxidizing environments and at annealing temperatures as low as 300 C. By applying a small biaxial tensil…
Oxygen vacancies in transition metal oxides facilitate catalysis critical for energy storage and generation. However, it has proven elusive to promote vacancies at the lower temperatures required for operation in devices such as metal-air batteries and portable fuel cells. Here, we use thin films of the perovskite-based strontium cobaltite (SrCoOx) to show that epitaxial strain is a powerful tool towards manipulating the oxygen content under conditions consistent with the oxygen evolution reaction, yielding increasingly oxygen deficient states in an environment where the cobaltite would normal…
Using muon spin spectroscopy we have found that, for both Na$_x$CoO$_2$ (0.6 $\leq x \leq$ 0.9) and 3- and 4-layer cobaltites, a common low temperature magnetic state (which in some cases is manifest as an incommensurate spin density wave) forms in the CoO$_2$ planes. Here we summarize those results and report a dome-shaped relation between the transition temperature into the low-$T$ magnetic state and the composition $x$ for Na$_x$CoO$_2$ and/or the high-temperature asymptotic limit of thermopower in the more complex 3- and 4-layer cobaltites. This behavior is explained using the Hubbard mode…
Orientation control of oxygen vacancy channel (OVC) is a highly desirable for tailoring oxygen diffusion as it serves fast transport channel in ion conductors, which is widespread exploited in solid-state fuel cells, catalysts, and ion-batteries. Direct observation of oxygen-ions hopping towards preferential vacant sites is a key to clarifying migration pathways. Here we report the anisotropic oxygen-ion migration mediated by strain in ultrathin cobaltites via in-situ thermal activation in an atomic-resolved transmission electron microscopy. Oxygen migration pathways are constructed on the bas…
Rare earth perovskite cobaltites are increasingly recognized as materials of importance due to rich physics and chemistry in their ordered-disordered structure for the same composition. Apart from colossal magnetoresistance effect, like manganites, the different forms of cobaltites exhibit interesting phenomena including spin, charge and orbital ordering, electronic phase separation, insulator-metal transition, large thermoelectric power at low temperature. Moreover, the cobaltites which display colossal magnetoresistance effect could be used as read heads in magnetic data storage and also in …
In this article we present a neutron diffraction in-situ study of the thermal evolution and high-temperature structure of layered cobaltites Y(Ba, Sr)Co2 O5+δ. Neutron thermodiffractograms and magnetic susceptibility measurements are reported in the temperature range 20 K <= T <= 570 K, as well as high resolution neutron diffraction experiments at selected temperatures. Starting from the as-synthesized samples with δ ~ 0.5, we show that the room temperature phases remain stable up to 550 K, where they start loosing oxygen and transform to a vacancy-disordered "112" structure with tetragonal sy…
Control of materials through custom design of ionic distributions represents a powerful new approach to develop future technologies ranging from spintronic logic and memory devices to energy storage. Perovskites have shown particular promise for ionic devices due to their high ion mobility and sensitivity to chemical stoichiometry. In this work, we demonstrate a solid-state approach to control of ionic distributions in (La,Sr)CoO$_{3}$ thin films. Depositing a Gd capping layer on the perovskite film, oxygen is controllably extracted from the structure, up-to 0.5 O/u.c. throughout the entire 36…
Manipulation of oxygen vacancies (V_O) in single oxide layers by varying the electric field can result in significant modulation of the ground state. However, in many oxide multilayers with strong application potentials, e.g. ferroelectric tunnel junctions and solid-oxide fuel cells, understanding V_O behaviour in various layers under an applied electric field remains a challenge, owing to complex V_O transport between different layers. By sweeping the external voltage, a reversible manipulation of V_O and a corresponding fixed magnetic phase transition sequence in cobaltite/manganite (SrCoO3-…
A positive muon spin rotation and relaxation ($μ^+$SR) experiment on [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$], ({\sl i.e.}, Ca$_3$Co$_4$O$_9$, a layered thermoelectric cobaltite) indicates the existence of two magnetic transitions at $\sim$ 100 K and 400 - 600 K; the former is a transition from a paramagnetic state to an incommensurate ({\sf IC}) spin density wave ({\sf SDW}) state. The anisotropic behavior of zero-field $μ^+$SR spectra at 5 K suggests that the {\sf IC-SDW} propagates in the $a$-$b$ plane, with oscillating moments directed along the c-axis; also the {\sf IC-SDW} is found to exist not…
We study experimentally and theoretically the effect of Eu doping and partial oxygen isotope substitution on the transport and magnetic characteristics and spin-state transitions in (Pr$_{1-y}$Eu$_y$)$_{0.7}$Ca$_{0.3}$CoO$_3$ cobaltites. The Eu doping level $y$ is chosen in the range of the phase diagram near the crossover between the ferromagnetic and spin-state transitions ($0.10 < y < 0.20$). We prepared a series of samples with different degrees of enrichment by the heavy oxygen isotope $^{18}$O, namely, containing 90%, 67%, 43%, 17%, and 0% of $^{18}$O. Based on the measurements of ac mag…
Magnetism of a misfit layered cobaltite [Ca$_2$Co$_{4/3}$Cu$_{2/3}$O$_4$]$_x^{\rm RS}$[CoO$_2$] ($x \sim$ 0.62, RS denotes a rocksalt-type block) was investigated by a positive muon spin rotation and relaxation ($μ^+$SR) experiment. A transition to an incommensurate ({\sf IC}) spin density wave ({\sf SDW}) state was found below 180 K (= $T_{\rm C}^{\rm on}$); and a clear oscillation due to a static internal magnetic field was observed below 140 K (= $T_{\rm C}$). Furthermore, an anisotropic behavior of the zero-field $μ^+$SR experiment indicated that the {\sf IC-SDW} propagates in the $a$-$b$ …
In this article we study the effects of a partial substitution of Ba with the smaller cation Ca in the layered cobaltites YBaCo_2O_{5+δ} for δ\approx 0.5. Neutron thermodiffractograms are reported for the compounds YBa_{0.95}Ca_{0.05}Co_2O_{5.5} (x_{Ca}=0.05) and YBa_{0.90}Ca_{0.10}Co_2O_{5.5} (x_{Ca}=0.10) in the temperature range 20 K \leq T \leq 300 K, as well as high resolution neutron diffraction experiments at selected temperatures for the samples x_{Ca}=0.05, x_{Ca}=0.10 and the parent compound x_{Ca}=0. We have found the magnetic properties to be strongly affected by the cationic subst…
We have investigated the systematic transport properties of the layered 112-type cobaltite LaBaCo2O5.5 by means of electrical resistivity, magnetoresistance, electroresistance and thermoelectric measurements in various conditions. In order to understand the complex conduction mechanism of LaBaCo2O5.5, the transport data have been analyzed using different theoretical models. The system shows semiconductor-semiconductor like transition (TSC) around 326K, corresponding to ferromagnetic transition and in the low temperature region resistivity data follows the Motts variable range hopping model. In…
The oxygen-deficient perovskite cobaltite SrCo1-xNbxO3-d was synthesized by direct solid-state reaction and its magnetotransport properties were investigated. This cobaltite exhibits an unusual ferromagnetic behavior with a transition temperature Tm = 130-150 K and a spin glass like behavior below Tm. Importantly, this phase reaches a large magnetoresistance (MR) value, MR = -(rH - r0) / r0 = 30% at 5 K in 7 T. The large MR effect is believed to be related to the disordered magnetic state induced by the Nb-for-Co substitution.
We report on the magnetic, resistive, and structural studies of perovskite La$_{1/3}$Sr$_{2/3}$CoO$_{3-δ}$. By using the relation of synthesis temperature and oxygen partial pressure to oxygen stoichiometry obtained from thermogravimetric analysis, we have synthesized a series of samples with precisely controlled $δ=0.00-0.49$. These samples show three structural phases at $δ=0.00-0.15$, $\approx0.25$, $\approx0.5$, and two-phase behavior for other oxygen contents. The stoichiometric material with $δ=0.00$ is a cubic ferromagnetic metal with the Curie temperature $T_{\rm C}=274$ K. The increas…
A new nanoscale ordered perovskite cobaltite, which consists of 90 degree ordered domains of the layered-112 LaBaCo2O6 has been evidenced by high resolution- transmission electron microscopy. This new form, like the disordered La0.5Ba0.5CoO3 and ordered LaBaCo2O6, exhibits a ferromagnetic transition at TC around 179 K. However, it differs from the two previous forms by its strong magnetic anisotropy, and correlatively by its high value of coercivity (0.42 Tesla) at low temperature. We suggest that this behaviour originates from the locking of magnetic spins in the 90 degree oriented nano-domai…
We have investigated cobaltite relatives of the layered perovskite cuprates and nickelates, Pr$_{2-x}$Ca$_x$CoO$_4$ ($0.39 \leq x \leq 0.73$) and La$_{2-x}$Sr$_x$CoO$_4$ ($x = 0.61$), using elastic neutron scattering. We have discovered doping-dependent incommensurate short-range ordering of charges and magnetic moments, which in cobaltites occur in the range of heavy doping, $ 0.5 \lesssim x \lesssim 0.75$. The charge order exists already at room temperature and shows no change on cooling. The incommensurability of its propagation vector, ${\bf Q}_c = (ε_c,0,l)$, roughly scales with the conce…
The magnetic and magnetotransport properties of the oxygen deficient perovskites, SrCo1-xMxO3-d with M = Nb and Ru, were investigated. Both Nb- and Ru-substituted cobaltites are weak ferromagnets, with transition temperatures Tm of 130-150 K and 130-180 K, respectively, and both exhibit a spin glass behavior at temperatures below Tf = 80-90 K. It is demonstrated that there exists a strong competition between ferromagnetism and spin glass state, where Co4+ induces ferromagnetism, whereas Nb or Ru substitution at the cobalt sites induces magnetic disorder, and this particular magnetic behavior i…
Sodium cobaltate with high Na content is a promising thermoelectric material. It has recently been reported that oxygen vacancies can alter the material properties, reducing its figure of merit. However, experimental data concerning the oxygen stoichiometry are contradictory. We therefore studied the formation of oxygen vacancies in Na_xCoO_2 with first principles calculations, focusing on x = 0.75. We show that a very low oxygen vacancy concentration is expected at the temperatures and partial pressures relevant for applications.
The layered perovskite PrBaCo2O5.5+x demonstrates a strong negative thermal expansion (NTE) which holds potential for being fabricated into composites with zero thermal expansion. The NTE was found to be intimately associated with the spontaneous magnetic ordering, known as magnetovolume effect (MVE). Here we report with compelling evidences that the continuous-like MVE in PrBaCo2O5.5+x is intrinsically of discontinuous character, originating from an magnetoelectric transition from an antiferromagnetic insulating large-volume (AFILV) phase to a ferromagnetic metallic small-volume (FMSV) phase.…
Oxygen octahedral distortions including tilts/rotations, deformations and off-centering in (layered) perovskites play the key role in their numerous functional properties. At the polar-centrosymmetric phase boundary in bi-layered perovskite Ca3-xSrxTi2O7 with x~1, we found the presence of abundant topological Z8 vortex-antivortex pairs, associated with four oxygen octahedral tilts at domains and another four different oxygen octahedral tilts at domain boundaries. Our discovery opens a new revenue to unveil topological defects associated with various types of oxygen octahedral distortions.
Supercapacitors have recently gained popularity as possible energy storage systems due to their high cycling ability and increased power density. However, one of the major drawbacks of supercapacitors is that they have a low energy density, which makes them less effective than batteries. Herein, we explore different methods of increasing the supercapacitor performance of the perovskite SrCoO3. We carry out first-principles calculations to systematically study how SrCoO3/graphene interface, oxygen vacancies, and doping improve the performance of strontium cobaltite as an anion-intercalation-typ…
Debye Institute for Nanomaterials Science, Utrecht University（乌得勒支大学德拜纳米材料科学研究所）；Department of Chemical Engineering, University College London（伦敦大学学院化学工程系）
Solar water splitting has attracted much attention as a clean and renewable route to produce hydrogen fuel. Since the oxygen evolution half-reaction (OER) requires high overpotentials, much research has focused on finding catalyst materials that minimize this energy loss. Oxynitrides with a layered perovskite structure have the potential to combine the superior photocatalytic properties of layered perovskite oxides with enhanced visible-light absorption caused by the band gap narrowing due to less electronegative nitrogen ions. In this paper, we study the OER on the (001) and (100) surfaces of…
Defects and their interactions in crystalline solids often underpin material properties and functionality as they are decisive for stability, result in enhanced diffusion, and act as a reservoir of vacancies. Recently, lithium-rich layered oxides have emerged among the leading candidates for the next-generation energy storage cathode material, delivering 50 % excess capacity over commercially used compounds. Oxygen-redox reactions are believed to be responsible for the excess capacity, however, voltage fading has prevented commercialization of these new materials. Despite extensive research th…
The magnetic and magnetotransport properties of the Sr3Fe2-xCoxO7-d system (0.2 <= x <= 1.0) were systematically investigated. This oxide system exhibits a giant magnetoresistance (GMR) effect at low temperatures, reaching up to 80% in 7 T at 5 K. Ac-susceptibility measurements show that there exists a strong competition between ferromagnetic (F) and spin glass states, and the balance between these two magnetic states can be controlled by varying cobalt (x) and/or oxygen contents (d). Importantly, the MR effect is closely related to the magnetic property: the development of magnetic disorderin…
A general spin-state model and a qualitative physical picture have been proposed for a class of lately synthesized layered cobalt oxides (LCOs) by means of density functional calculations. As the plane corrugation of the cobalt-oxygen layer decreases, the LCOs evolve from a high-spin (HS) superexchange-coupled antiferromagnetic (AFM) insulator to an almost-HS AFM/ferromagnetic (FM) competing system where the FM coupling is mediated via the p-d exchange by an increasing amount of delocalized pdσholes having mainly the planar O 2p character. It is tentatively suggested that the delocalized holes…
Nickel-iron layered double hydroxide (NiFe LDH) platelets with high morphological regularity and sub-micrometre lateral dimensions were synthesized using a homogeneous precipitation technique for highly efficient catalysis of the oxygen evolution reaction (OER). Considering edge sites are the point of activity, efforts were made to control platelet size within the synthesized dispersions. The goal is to controllably isolate and characterize size-reduced NiFe LDH particles. Synthetic approaches for size control of NiFe LDH platelets have not been transferable based on published work with other …
The structural, electronic, and optical properties of twelve multicomponent oxides with layered structure, RAMO$_4$, where R$^{3+}$=In or Sc; A$^{3+}$=Al or Ga; and M$^{2+}$=Ca, Cd, Mg, or Zn, are investigated using first-principles density functional approach. The compositional complexity of RAMO$_4$ leads to a wide range of band gap values varying from 2.45 eV for InGaCdO$_4$ to 6.29 eV for ScAlMgO$_4$. Strikingly, despite the different band gaps in the oxide constituents, namely, 2-4 eV in CdO, In$_2$O$_3$, or ZnO; 5-6 for Ga$_2$O$_3$ or Sc$_2$O$_3$; and 7-9 eV in CaO, MgO, or Al$_2$O$_3$, …
Motivated by the earlier experimental results and \textit{ab initio} studies on the electronic structure of layered ruthenates (Sr$_2$RuO$_4$ and Ca$_2$RuO$_4$) we introduce and investigate the multiband $d-p$ charge transfer model describing a single RuO$_4$ layer, similar to the charge transfer model for a single CuO$_2$ plane including apical oxygen orbitals in high $T_c$ cuprates. The present model takes into account nearest-neighbor anisotropic ruthenium-oxygen $d-p$ and oxygen-oxygen $p-p$ hopping elements, crystal-field splittings and spin-orbit coupling. The intraorbital Coulomb repuls…
Highly active, durable and cost-effective electrocatalysts for water oxidation to evolve oxygen gas hold a key to a range of renewable energy solutions including water splitting and rechargeable metal-air batteries. Here, we report the synthesis of ultrathin nickel iron layered double hydroxide nanoplates on mildly oxidized multi-walled carbon nanotubes. Incorporation of Fe into the nickel hydroxide induced the formation of NiFe-layered double hydroxide. The nanoplates were covalently attached to a network of nanotubes, affording excellent electrical wiring to the nanoplates. The ultra-thin Ni…
Institute of Solid State and Semiconductor Physics（固体态与半导体物理研究所）；CNRS, Université de Bordeaux, Bordeaux INP, ICMB UMR 5026（法国国家科学研究中心，波尔多大学，波尔多理工学院，ICMB UMR 5026）；Institut Laue-Langevin（朗之万研究所）；Joint Institute for Nuclear Research（联合核研究所）；Kant Baltic Federal University（康特波罗的海联邦大学）；Karlsruhe Institute of Technology（卡尔斯鲁厄理工学院）；State Key Laboratory of Solidification Processing（凝固技术国家重点实验室）
Layered LiMnO$_2$ and Li$_2$MnO$_3$ are of great interest for lithium-ion battery cathodes because of their high theoretical capacities. The practical application of these materials is, however, limited due to poor electrochemical performance. We herein report a comprehensive first-principles study of defect physics in LiMnO$_2$ and Li$_2$MnO$_3$ using hybrid-density functional calculations. We find that manganese antisites have low formation energies in LiMnO$_2$ and may act as nucleation sites for the formation of impurity phases. The antisites can also occur with high concentrations in Li$_…
Single crystals of the metallic Ruddlesden-Popper trilayer nickelates R$_4$Ni$_3$O$_{10}$ (R=La, Pr) were successfully grown using an optical-image floating zone furnace under oxygen pressure (pO$_2$) of 20 bar for La$_4$Ni$_3$O$_{10}$ and 140 bar for Pr$_4$Ni$_3$O$_{10}$. A combination of synchrotron and laboratory x-ray single crystal diffraction, high-resolution synchrotron x-ray powder diffraction and measurements of physical properties revealed that R$_4$Ni$_3$O$_{10}$ (R=La, Pr) crystallizes in the monoclinic $P$2$_1$/$a$ (Z=2) space group at room temperature, and that a metastable ortho…
We present a first-principles investigation of ferroelectricity in layered perovskite oxide La2Ti2O7 (LTO), one of the compounds with highest Curie temperature known (1770 K). Our calculations reveal that LTO's ferroelectric transition results from the condensation of two soft modes that have the same symmetry and are strongly coupled anharmonically. Further, the leading instability mode essentially consists of rotations of the oxygen octahedra that are the basic building block of the perovskite structure; remarkably, because of the particular topology of the lattice, such O6 rotations give ra…
We find that layered materials composed of various oxides of cations with $s^2$ electronic configuration, $XY_2$O$_4$, $X$=In or Sc, $Y$=Ga, Zn, Al, Cd and/or Mg, exhibit isotropic electron effective mass which can be obtained via averaging over those of the corresponding single-cation oxide constituents. This effect is due to a hybrid nature of the conduction band formed from the s-states of {\it all} cations and the oxygen p-states. Moreover, the observed insensitivity of the electron effective mass to the oxygen coordination and to the distortions in the cation-oxygen chains suggests that s…
In the ferromagnetic metallic state of the colossal magnetoresistive (CMR) manganite La1.2Sr1.8Mn2O7, the spin density distribution is essentially in agreement with the standard picture in which the unpaired electrons occupy the three t2g orbitals, dxy, dyz, and dxz. However we find a small spin density ~4% of the total Mn spin) on the apical O(2) oxygen atom at both 100 K and 220 K and we suggest that this is due to covalency effects. Surprisingly we find no evidence of spin on the other apical oxygen O(1) suggesting that the Mn eg electron distribution along the c-axis is highly anisotropic.
Compositionally tunable vanadium oxyhydrides Sr2VO4-xHx (x = 0 - 1) without considerable anion vacancy were synthesized by high-pressure solid state reaction. The crystal structures and their properties were characterized by powder neutron diffraction, synchrotron X-ray diffraction, thermal desorption spectroscopy, and first-principles density functional theory (DFT) calculations. The hydrogen anions selectively replaced equatorial oxygen sites in the VO6 layers via statistical substitution of hydrogen in the low x region (x < 0.2). A new orthorhombic phase (Immm) with an almost entirely hydro…
By means of first-principle FLAPW-GGA calculations, we have investigated the electronic properties of the newly discovered layered quaternary systems SrFeAsF and CaFeAsF as parent phases for a new group of oxygen-free FeAs superconductors. The electronic bands, density of states, Fermi surfaces, atomic charges, together with Sommerfeld coefficients and molar Pauli paramagnetic susceptibility have been evaluated and discussed in comparison with oxyarsenide LaFeAsO - a parent phase for a new class of high-temperature (Tc about 26-56K) oxygen-containing FeAs superconductors. Similarity of our dat…
We have investigated the crystal, electronic, and magnetic structure of Mn5O8 by means of state of-the-art density functional theory calculations and neutron powder diffraction (NPD) measurements. This compound stabilizes in the monoclinic structure with space group C2/m where the Mn ions are in the distorted octahedral and trigonal prismatic coordination with oxygen atoms. The calculated structural parameters based on total energy calculations are found to be in excellent agreement with low temperature NPD measurements when we accounted correct magnetic structure and Coulomb correlation effec…
Doping is considered to be the main method for improving the thermoelectric performance of layered sodium cobaltate (Na$_{1-x}$CoO$_2$). However, in the vast majority of past reports, the equilibrium location of the dopant in the Na$_{1-x}$CoO$_2$'s complex layered lattice has not been confidently identified. Consequently, a universal strategy for choosing a suitable dopant for enhancing Na$_{1-x}$CoO$_2$'s figure of merit is yet to be established. Here, by examining the formation energy of Gd and Yb dopants in Na$_{0.75}$CoO$_2$ and Na$_{0.50}$CoO$_2$, we demonstrate that in an oxygen poor en…
MXenes are a class of 2D/layered materials which are highly conductive, hydrophilic, have a large electrochemical surface area and are easily processible into electrodes for energy applications. Since the discovery of MXenes over ten years ago, these materials have been mainly used in the preparation of electrodes for batteries and supercapacitors. However, due to their aforementioned properties, MXenes could potentially be utilised as a component in the catalyst layer for the Oxygen Evolution Reaction (OER). This opinion piece will discuss some of the recent literature in the area of hybrid c…
The effect of Ru substitution on the local structure of layered SmFe$_{1-x}$Ru$_x$AsO$_{0.85}$F$_{0.15}$ superconductor has been studied by As $K$- and Sm $L_3$ - edges x-ray-absorption spectroscopy. The extended x-ray-absorption fine-structure measurements reveal distinct Fe-As and Ru-As bondlengths in the Ru substituted samples with the latter being $\sim$0.03 Å longer. Local disorder induced by the Ru substitution is mainly confined to the FeAs layer while the SmO spacer layer sustains a relative order, consistent with the x-ray-absorption near-edge structure spectra. The results suggest th…
Dielectric and magnetic phenomena in spin and charge frustrated system RFe2O4 (R is a rare-earth metal ion) are studied. An electronic model for charge, spin and orbital degrees in a pair of triangular-lattice planes is derived. We analyze this model by utilizing the mean-field approximation and the Monte-Carlo simulation in a finite size cluster. A three fold-type charge ordered structure with charge imbalance between the planes is stabilized in finite temperatures. This polar charge order is reinforced by spin ordering of Fe ions. This novel magneto-dielectric phenomenon is caused by spin fr…
To explore material dependence of layered cuprate superconductors, we examine effective two-particle interactions for Hg1201 and Tl1201, where Tl1201 having a nearly half value of Tc of Hg1201 even at the optimal oxygen concentration. Although the 3dx_2-y_2 band, the Fermi surface, and its Wannier-orbitals are similar for these superconductors, there is an apparent difference in the unoccupied levels above EF. Based on a multi-reference density-functional-theory formulation, effective two-particle exchange interactions are estimated to derive enhancement in intra-layer exchange interactions fo…
Soft-x-ray photoemission and absorption spectroscopies are employed to investigate the electronic structures of Sr_{1-x}Rh_{2}O_{4}. Similarly to the layered cobaltates such as Na_{1-x}CoO_{2}, a valence-band satellite feature (VBS) occurs at higher binding energy to the O 2p band. We find that the VBS resonates at the O 1s edge. Additionally, core absorption shows clear x dependence in the O 1s edge rather than in the Rh 3p edge. These results indicate that the holes in the initial state mainly have O 2p character presumably due to d-p rehybridizations affected by Sr^{2+} vacancy potentials. …
In order to understand the material dependence of $T_c$ within the single-layered cuprates, we study a two-orbital model that considers both $d_{x^2-y^2}$ and $d_{z^2}$ orbitals. We reveal that a hybridization of $d_{z^2}$ on the Fermi surface substantially affects $T_c$ in the cuprates, where the energy difference $ΔE$ between the $d_{x^2-y2}$ and $d_{z^2}$ orbitals is identified to be the key parameter that governs both the hybridization and the shape of the Fermi surface. A smaller $ΔE$ tends to suppress $T_c$ through a larger hybridization, whose effect supersedes the effect of diamond-sha…
The electronic properties of single- and multi-cation transparent conducting oxides (TCOs) are investigated using first-principles density functional approach. A detailed comparison of the electronic band structure of stoichiometric and oxygen deficient In$_2$O$_3$, $α$- and $β$-Ga$_2$O$_3$, rock salt and wurtzite ZnO, and layered InGaZnO$_4$ reveals the role of the following factors which govern the transport and optical properties of these TCO materials: (i) the crystal symmetry of the oxides, including both the oxygen coordination and the long-range structural anisotropy; (ii) the electroni…
The subtle balance of electronic correlations, crystal field splitting and spin--orbit coupling in layered Ir$^{4+}$ oxides can give rise to novel electronic and magnetic phases. Experimental progress in this field relies on the synthesis of epitaxial films of these oxides. However, the growth of layered iridates with excellent structural quality is a great experimental challenge. Here we selectively grow high quality single--phase films of Sr$_2$IrO$_4$, Sr$_3$Ir$_2$O$_7$, and SrIrO$_3$ on various substrates from a single Sr$_3$Ir$_2$O$_7$ target by tuning background oxygen pressure and epita…
Sr substituted perovskites La1.8Sr0.2MMnO6 (M = Ni, Co) were synthesized using solid state reaction technique to present a systematic study on their morphological, structural and magnetic properties. The average grain size of the as-prepared La1.8Sr0.2NiMnO6 samples are in the range of 0.2-0.7 μm and those for La1.8Sr0.2CoMnO6 manganites are 0.1-2.8 μm, which is significantly less than that of unsubstituted La2NiMnO6 (LNMO) and La2CoMnO6 (LCMO) manganites. The XPS analysis enlightened about phase purity, binding energy and oxygen vacancy of La1.8Sr0.2MMnO6 manganites. The Sr substituted LNMO h…
We have successfully synthesized a new layered iron oxychalcogenide BaFe2OSe2 single crystal. This compound is built up of Ba and Fe-Se(O) layers alternatively stacked along the c-axis. The Fe-Se(O) layers contain double chains of edge-shared Fe-Se(O) tetrahedra that propagate along the b-axis and are bridged by oxygen along the a-axis. Physical property measurements indicate that BaFe2OSe2 is a semiconductor without the Curie-Weiss behavior up to 350 K. There is a possible long range antiferromagnetic (AFM) transition at 240 K, corresponding to the peak in specific heat measurement and two gl…
