Costate Research Articles

An iterative method to solve the time delay optimal control problem with bounded control variable

This paper presents an iterative approach for solving the optimal control problem (OCP) with time-delayed terms and bounded control variables. The projection function is used to construct the iterative method to approximate the control law. Employing the approximation of control law the approximation of state and the co-state variables are obtained. To the best of our knowledge, this paper is the first attempt to solve bounded and delayed OCPs with a projection function. The convergence theorem for the proposed method is also proven. Several numerical examples are added, showing the developed method's effectiveness to solve the time-delayed OCPs with bounded control variables.

Performance improvement of oxygen electrode in reversible protonic ceramic electrochemical cell by co-doping of La and F

Reversible protonic ceramic electrochemical cell (R-PCEC) has demonstrated high potential as an energy storage and conversion device. Efforts have primarily concentrated on the development of oxygen electrodes being to increase their performance and reduce performance degradation. In this work, three perovskite materials, namely Ba0.9Co0.2Fe0.7Nb0.1O3-δ (BCFN), Ba0.8La0.1Co0.2Fe0.7Nb0.1O3-δ (BLCFN), and Ba0.8La0.1Co0.2Fe0.7Nb0.1F0.1O2.9-δ (BLCFNF) are investigated. Both doping with La and co-doping with La and F lead to a notable reduction in the average valence states of Fe, Co, and Nb elements, further resulting in an increase in oxygen vacancy concentration, thereby enhancing the oxygen migration ability of the material. The H2O partial pressure does not have a significant impact on the polarization resistance (Rp) of the oxygen electrode. Conversely, the rise in Rp is highly correlated with the decrease in oxygen partial pressure, suggesting that the primary factor influencing the reaction rate of the oxygen electrode process is the exchange of oxygen ions on the surface of the electrode. BLCFNF exhibits the best oxygen migration and surface exchange ability, resulting in the lowest Rp. When BLCFNF was used as the oxygen electrode in a R-PCEC with the humidity air (10%H2O) and humidified H2 (3%H2O) as oxidant and fuel, respectively, it exhibits the highest maximum power density of 472 mW cm−2, and a current density of −682.6 mA cm−2 (1.3 V) at 650 °C. In addition, BLCFNF also shows good durability in both discharge and electrolysis process. The results indicate that La and F co-doped BLCFNF material is a promising oxygen electrode material for R-PCECs.

Redox reactivity of LMCT and MLCT excited states of Earth‐abundant metal complexes

AbstractPrecious metal complexes, which act as excellent photoredox catalysts, have now being replaced by earth‐abundant metal complexes. This review focused on redox reactivity of ligand‐to‐metal charge transfer (LMCT) and metal‐to‐ligand charge transfer (MLCT) excited states of earth‐abundant metal complexes. Iron complexes with strongly σ‐donating NHC‐ligands (NHC = N‐heterocyclic carbene) has emerged, featuring long lived LMCT excited states due to a significantly increased barrier for deactivation via metal centered states. A Fe(III)‐NHC complex acts as an effective photoredox catalyst for various photocatalytic redox reactions. Although manganese(IV)‐oxo complexes have no long‐lived excited states (τ < < 1 ns). Once acids such as Sc(OTf)3 and HOTf are bound to the oxo moiety of Mn(IV)‐oxo complexes, the photoexcitation of acid‐bound Mn(IV)‐oxo complexes resulted in formation of the excited states with microseconds lifetimes, which are capable of oxidation of substrates including benzene to phenol. Photoexcited states of Mn(III), Mn(II) and Mn(I) complexes act as photoreductants to reduce substrates including O2. On the other hand, photoexcited states of Co(IV) and Co(III) complexes act as photooxidants, whereas those of Co(II) and Co(I) complexes act as photoreductants. With regard to the excited state lifetime, [Cr(tpe)2]3+ (tpe = 1,1,1‐tris(pyrid‐2‐yl)ethane) exhibited the longest luminescence lifetime (τ = 4500 μs), acting as an effective photoredox catalyst for photocatalytic redox reactions. The LMCT state of a Cr(0) complex acts as a super photoreductant. Thus, LMCT and MLCT excited states of earth‐abundant metal complexes are utilized as strong photooxidants and photoreductants, respectively.

In-situ crystallization of CoCr2O4 in tellurite glass with enhanced optical nonlinear limiting, photoluminescence, and magnetic properties: Influence of Co content

This article reports the in-situ growth of spinel CoCr2O4 nanocrystals (NC) in tellurite glass through tailoring the Co content by melt-quenching method. The microstructure, optical, and magnetic properties of NC/ glass systems were systematically studied using various techniques. Well-defined 30 nm crystallization of CoCr2O4 (space group of Fd3m) homogeneously distributed in the glassy matrix and the concurrent formation of cubic BiCoO3 (space group of P4mm) when the Co content exceeds 3%. The coexistence of multivalence and coordination states of Co and Cr significantly modified the glass structure with conversion of TeO4→TeO3, BO4→BO3, and CoO4→CoO6 units. Exceptional performance of the glass containing 5% Co-tuned CoCr2O4 was obtained, showcasing the decrease of Eg (2.09 eV), strong ferro-magnetization of 7.04 emu/g, large α3 (4.88×10−10 m/W) and good optical limiting property (threshhold of 1.31×1012 W/m2). Furthermore, the same glass showed excellent red emission between 650 and 800 nm under the excitation of 460 nm, with an increased lifetime (2.648 ms) and good stability of 90.7% within 25–300 °C. The emission mechanism was discussed. EPR analysis confirmed the active role and spin-states of magnetic Cr and Co ions, contributing synergistically to the improved optical and magnetic properties of the glasses.

Co/C Nanocomposites with Tunable Condensed States Induced by Conformation-Mediated Strategy for Electromagnetic Wave Absorption.

The strategic regulation of condensed state structures in multicomponent nanomaterials has emerged as an effective approach for achieving controllable electromagnetic (EM) properties. Herein, a novel conformation-mediated strategy is proposed to manipulate the condensed states of Co and C, as well as their interaction. The conformation of polyvinylpyrrolidone molecules is adjusted using a gradient methanol/water ratio, whereby the coordination dynamic equilibrium effectively governs the deposition of metal-organic framework precursors. This process ultimately influences the combined impact of derived Co and C in the resulting Co/C nanocomposites post-pyrolysis. The experimental results show that the condensed state structure of Co/C nanocomposites transitions from agglomerate state → to biphasic compact state → to loose packing state. Benefiting from the tunable collaboration between interfacial polarization and defects polarization, and the appropriate electrical conductivity, the diphasic compact state of Co/C nanocomposites achieves an effective absorbing bandwidth of 7.12GHz (2.1mm) and minimum reflection loss of -32.8dB. This study highlights the significance of condensed state manipulation in comprehensively regulating the EM wave absorption characteristics of carbon-based magnetic metal nanocomposites, encompassing factors such as conductivity loss, magnetic loss, defect polarization, and interface polarization.

Strontium doping-tailored inverse spinel phase in cobalt chromite with enhanced photo-degradation of ofloxacin

Spinel materials are attractive to photocatalytic degradation of contaminants. In this study, strontium (Sr) doped spinel cobalt chromite (CoCr2O4) was synthesized by solution combustion method and characterized using techniques such as X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and electrochemical impedance spectroscopy (EIS). 30 -50 nm-cubic CoCr2O4 nanocrystals were obtained, and phase transitions of CoO6→CoO4 and CrO4→CrO6 occurred as the Sr doping content increased until 0.6 % Sr doped sample displayed an inverse spinel phase. The modifications in composition and structure induced the co-existence of multivalent states for Co and Cr, which in turn changed the optical, complex impedance, and magnetic spin state of CoCr2O4. The 0.6 % Sr-doped sample showed the highest photocatalytic efficiency of 93 % due to enhanced optical absorption, reduced energy band gap, oxygen vacancies formation, and a high spin state of Co. The XRD structure and photocatalytic activity remained stable even after five cycles without any leakage of metal elements. The degradation pathway and Mott-Schottky curves were employed to elucidate the mechanism behind this enhancement. The strategy employed in this study proved effective, and the synthesized materials show promise for the photocatalytic degradation of contaminants.

Investigation into electrochemical catalytic properties and electronic structure of Mn doped SrCoO3 perovskite catalysts

High activity, long lifetime and low-cost cathode catalyst plays an irreplaceable role in oxygen reduction reactions process. In the present work, SrCoO3 and SrCo0.875Mn0.125O3 perovskite are prepared by sol–gel method and first principles calculation to study the role of Mn doping. The characterizations of phase, microstructure and valence state of elements demonstrate that Mn doping stabilizes the cubic structure of SrCo0.875Mn0.125O3 perovskite and forms a porous structure with large specific surface area. The high-valence and multivalent states of Co and Mn are produced, which provide abundant active sites for oxygen reduction reaction. The charge transfer resistance (1.820 Ω·cm2), Tafel slope (88 mV dec−1), discharge capacity (4.173 mWh/cm2), and positive shift of onset potential and half-wave potentials reveal that the SrCo0.875Mn0.125O3 perovskite is a promising cathode catalyst. Mn doping helps the O p-band center move up relative to the Fermi level, and adsorption energy of O2 and formation energy of oxygen vacancies increase. The improvement of these parameters has a positive effect on predicting the catalytic activity of SrCo0.875Mn0.125O3 perovskite. The possible oxygen reduction reaction mechanism of SrCo0.875Mn0.125O3 perovskite is discussed. Mn doping accelerates the transport process of SrCo0.875Mn0.125O3 perovskite.

J_{eff}=1/2 Hyperoctagon Lattice in Cobalt Oxalate Metal-Organic Framework.

We report the magnetic properties of a cobalt oxalate metal-organic framework featuring the hyperoctagon lattice. Our thermodynamic measurements reveal the J_{eff}=1/2 state of the high-spin Co^{2+} (3d^{7}) ion and the two successive magnetic transitions at zero field with two-stage entropy release. ^{13}C-NMR measurements reveal the absence of an internal magnetic field in the intermediate temperature phase. Multiple field-induced phases are observed before full saturation at around 40T. We argue the unique cobalt oxalate network gives rise to the Kitaev interaction and/or a bond frustration effect, providing an unconventional platform for frustrated magnetism on the hyperoctagon lattice.

Synthesis of nano-crystalline K2NiF4-type phases La1+xSr1-xCoO4 (x = 0.0, 0.2, and 0.5): Effect of mixed valence state of Co on structural, magnetic and photocatalytic properties

A lot of work has already been reported on bulk K2NiF4-type layered perovskite oxides but hardly is known about their nano-crystalline state as they are generally prepared at high temperatures. In this connection, we have synthesized nanocrystalline La1+xSr1-xCoO4 layered perovskites oxides via sol-gel Pechini method. Efforts were also done to synthesize phases with x > 0.5 but remained unsuccessful. The Rietveld refinement investigation has confirmed the successful formation of single-phase compounds that crystallize in the I4/mmm space group with tetragonal symmetry. The X-ray photoelectron spectroscopy (XPS) analysis revealed that Co in La1+xSr1-xCoO4 has mixed valence states of +2 and + 3. Dominant anti-ferromagnetic (AFM) super-exchange Co2+―O―Co2+ and Co3+―O―Co3+ interactions were observed in all the synthesized nanophases and the existence of weak ferromagnetic (FM) interactions due to the generation of double-exchange Co2+―O―Co3+ interaction because of mixed valent state of Co. The increase in Néel temperature (TN) values with La doping has been attributed by the increased prevalence of anti-ferromagnetic (AFM) super-exchange Co2+―O―Co2+ and Co3+―O―Co3+ interactions. The band gap energy values (Eg) for all the prepared nano samples vary from 1.57 to 1.64 eV, rendering them highly competent catalysts for degradation of dyes. Additionally, the photocatalytic performance of the prepared nano photocatalysts was evaluated using cationic (MB & RhB) and anionic (MO) dye contaminants as well as their ternary mixture. LaSrCoO4 photocatalyst has been found to efficiently and swiftly degrade all three dye pollutants separately as well as in mixtures, with outstanding recyclability and durability. It has been observed that the x = 0.0 photocatalyst showed best degradation efficacy for RhB dye, i.e., ∼99 % in 100 min, compared to ∼91 % and ∼84 % for the x = 0.2 and 0.5 photocatalysts in the same time interval, respectively. These results suggest that LaSrCoO4 nano powder can be considered a promising photocatalyst in the dyes degradation.

Effect of fluoride (CoF2) based electrode material for high energy and power density asymmetric flexible supercapacitors

Herein, an asymmetric supercapacitor (ASC) device was assembled by employing the pristine CoF2 as active positive electrode material and activated carbon (AC) as negative electrode material. The CoF2 electrode exhibited an outstanding electrochemical behavior with enhanced specific capacitance (Cs) of 872 F/g at 1 A/g current density in three electrode configurations. The reversible Faradaic redox reactions in CoF2 instigate the formation of cobalt-oxyfluorides and generating the mixed valence state of Co (Co2+ and Co3+) which improves its electrical conductivity and electrochemical behaviors. Furthermore, the room temperature paramagnetic nature of CoF2 increases the gradient force of ions which could possibly increase the redox reactions at the electrolyte/electrode interface. The constructed asymmetric device structure of CoF2//AC revealed a quasi-rectangular shaped cyclic voltametric curve and symmetrical semi-triangular shaped galvanostatic charge-discharge profiles, respectively, which affirms the combined electric double layer capacitance and pseudocapacitive nature by its hybrid assembly. Hence, the constructed CoF2//AC ASC demonstrated a remarkable specific capacitance of 141 F/g, energy density of 44.06 Wh.Kg−1, and power density of 422.97 W.Kg−1 at 1 A/g with the high cell voltage of 1.5 V. The CoF2//AC ASC device retained c.a. 92 % of its initial capacitance and attained over 96 % of Coulombic efficiency after 2000 cycles of charge-discharge profiles at 1 A/g, which suggests the high electrochemical stability. This admirable electrochemical performance and the origin towards the magneto-capacitance of CoF2 could pave a new way to explore next generation high-performance supercapacitors.

Construction of high performance fuel electrode in solid oxide electrolysis cells by tuning the surface oxygen defect to promote electrochemical reduction of CO2

B-site Co–Fe-based perovskite materials have shown potential as ceramic cathodes in solid oxide electrolysis cells (SOECs) for direct CO2 electrolysis. Nevertheless, their operational utilization is constrained by insufficient catalytic activity and durability in pure CO2. In this work, F− doping and A-site deficiency strategies are proposed and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF), F-doped La0.6Sr0.4Co0.2Fe0.8F0.1O2.9 (LSCFF) and A-site deficiency combined with F-doped (La0.6Sr0.4)0.95Co0.2Fe0.8F0.1O2.9 (LSCFF95) are prepared as cathode materials for CO2 electrolysis. After doping with F−, the valence states of both Co and Fe elements decrease, while generating more oxygen vacancies. With the further introduction of A-site deficiency, the valence states further decrease, resulting in more oxygen vacancies. The enhancement of oxygen vacancies leads to an improvement in the oxygen surface exchange coefficient (kchem) and the bulk diffusion coefficient (Dchem) of LSCFF and LSCFF95 materials. At 800 °C, kchem of LSCFF and LSCFF95 reach 24.21 × 10−4 and 33.54 × 10−4 cm s−1, respectively, which are 51% and 110% higher than the value of 16.03 × 10−4 cm s−1 for LSCF. Meanwhile, the Dchem values of LSCFF and LSCFF95 are measured to be 37.52 × 10−5 and 47.81 × 10−5 cm2 s−1, respectively. These values are 49% and 90% higher than the value of 25.22 × 10−5 cm2 s−1 for LSCF. Consequently, F− doping and A-site deficiency significantly increase the electrochemical performance. For example, F− doping decreases the polarization resistance (Rp) value of the SOEC with LSCF cathode from 0.19 Ω cm2 to 0.13 Ω cm2 at 800 °C. Furthermore, when the A-site deficiency is introduced, the Rp value is further reduced to 0.11 Ω cm2. Therefore, the SOEC with the LSCFF95 cathode exhibits excellent performance. Specifically, it has demonstrated an impressively high current density of 2.85 A cm−2 at 800 °C and 1.5 V, and has exhibited exceptional durability over prolonged operation.

Neural Time-Reversed Generalized Riccati Equation

Optimal control deals with optimization problems in which variables steer a dynamical system, and its outcome contributes to the objective function. Two classical approaches to solving these problems are Dynamic Programming and the Pontryagin Maximum Principle. In both approaches, Hamiltonian equations offer an interpretation of optimality through auxiliary variables known as costates. However, Hamiltonian equations are rarely used due to their reliance on forward-backward algorithms across the entire temporal domain. This paper introduces a novel neural-based approach to optimal control. Neural networks are employed not only for implementing state dynamics but also for estimating costate variables. The parameters of the latter network are determined at each time step using a newly introduced local policy referred to as the time-reversed generalized Riccati equation. This policy is inspired by a result discussed in the Linear Quadratic (LQ) problem, which we conjecture stabilizes state dynamics. We support this conjecture by discussing experimental results from a range of optimal control case studies.

Experimental Proposal on Non-Hermitian Skin Effect by Two-dimensional Quantum Walk with a Single Trapped Ion

Non-Hermitian Hamiltonians are widely used in describing open systems with gain and loss, among which a key phenomenon is the non-Hermitian skin effect. Here we report an experimental scheme to realize a two-dimensional (2D) discrete-time quantum walk with non-Hermitian skin effect in a single trapped ion. It is shown that the coin and 2D walker states can be labeled in the spin of the ion and the coherent-state lattice of the ion motion, respectively. We numerically observe a directional bulk flow, whose orientations are controlled by dissipative parameters, showing the emergence of the non-Hermitian skin effect. We then discuss an experimental implementation of our scheme in a laser-controlled trapped Ca+ ion. Our experimental proposal may be applicable to research of dissipative quantum walk systems and may be able to generalize to other platforms, such as superconducting circuits and atoms in cavity.

Orbital Occupancy Modulation to Optimize Intermediate Absorption for Efficient Electrocatalysts in Water Electrolysis and Zinc-Ethanol-Air Battery.

Spin engineering is a promising way to modulate the interaction between the metal d-orbital and the intermediates and thus enhance the catalytic kinetics. Herein, an innovative strategy is reported to modulate the spin state of Co by regulating its coordinating environment. o-c-CoSe2 -Ni is prepared as pre-catalyst, then in situ electrochemical impedance spectroscopy (EIS)and in situ Raman spectroscopy are employed to prove phase transition, and CoOOH/Co3 O4 is formed on the surface as active sites. In hybrid water electrolysis, the voltage has a negative shift, and in zinc-ethanol-air battery, the charging voltage is lowered and the cycling stability is greatly increased. Coordinated atom substitution and crystalline symmetry change are combined to regulate the absorption ability of reaction intermediates with balanced optimal adsorption. Coordinated atom substitution weakens the adsorption while the crystalline symmetry change strengthens the adsorption. Importantly, the tetrahedral sites are introduced by Ni doping which enables the co-existence of four-coordinated sites and six-coordination sites in o-c-CoSe2 -Ni. The dz2 + dx2 -y2 orbital occupancy decreases after the atomic substitution, while increases after replacing the CoSe6 -Oh field with CoSe6 -Oh /CoSe4 -Td . This work explores a new direction for the preparation of efficient catalysts for water electrolysis and innovative zinc-ethanol-air battery.

A real-time energy management strategy for fuel cell vehicle based on Pontryagin’s minimum principle

This paper proposes a Pontryagin's minimum principle (PMP) energy management strategy (EMS) based on driving cycle recognition for fuel cell vehicle powertrains, aiming to minimize hydrogen consumption and fuel cell degradation. Firstly, the neural network-based driving cycle recognizer is optimized using the tuna swarm optimization (TSO) algorithm and trained under four typical driving cycles. Then, the optimal co-state variables for the four driving cycles are obtained by iteration. Finally, the co-state variables are dynamically updated based on real-time driving cycle recognition results. Comparative analysis demonstrates that the PMP-DCR effectively improves fuel cell lifetime and vehicle economy under short-distance driving cycles. Based on the combined driving cycle, the proposed PMP-DCR EMS exhibits similar economy performance to optimal dynamic programming (DP) EMS, reducing equivalent hydrogen consumption by 13.8% and 9.2%, and decreasing fuel cell degradation rates by 93% and 8.7% in comparison to the conventional power-following and PMP EMS, respectively.

High-spin Co3+ in cobalt oxyhydroxide for efficient water oxidation

Cobalt oxyhydroxide (CoOOH) is a promising catalytic material for oxygen evolution reaction (OER). In the traditional CoOOH structure, Co3+ exhibits a low-spin state configuration (t2g6eg0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${t}_{2{{{{{\\rm{g}}}}}}}^{6}{e}_{{{{{{\\rm{g}}}}}}}^{0}$$\\end{document}), with electron transfer occurring in face-to-face t2g*\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${t}_{2{{{{{\\rm{g}}}}}}}^{*}$$\\end{document} orbitals. In this work, we report the successful synthesis of high-spin state Co3+ CoOOH structure, by introducing coordinatively unsaturated Co atoms. As compared to the low-spin state CoOOH, electron transfer in the high-spin state CoOOH occurs in apex-to-apex eg*\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${e}_{{{{{{\\rm{g}}}}}}}^{*}$$\\end{document} orbitals, which exhibits faster electron transfer ability. As a result, the high-spin state CoOOH performs superior OER activity with an overpotential of 226 mV at 10 mA cm−2, which is 148 mV lower than that of the low-spin state CoOOH. This work emphasizes the effect of the spin state of Co3+ on OER activity of CoOOH based electrocatalysts for water splitting, and thus provides a new strategy for designing highly efficient electrocatalysts.

Discrete-Time Quantum Walks Community Detection in Multi-Domain Networks

Abstract The problem of detecting communities in real-world networks has been extensively studied in the past, but most of the existing approaches work on single-domain networks, i.e. they consider only one type of relationship between nodes. Single-domain networks may contain noisy edges and they may lack some important information. Thus, some authors have proposed to consider the multiple relationships that connect the nodes of a network, thus obtaining multi-domain networks. However, most community detection approaches are limited to multi-layer networks, i.e. networks generated from the superposition of several single-domain networks (called layers) that are regarded as independent of each other. In addition to being computationally expensive, multi-layer approaches might yield inaccurate results because they ignore potential dependencies between layers. This paper proposes a multi-domain discrete-time quantum walks (MDQW) model for multi-domain networks. First, the walking space of network nodes in multi-domain network is constructed. Second, the quantum permutation circuit of the coin state is designed based on the coded particle state. Then, using different coin states, the shift operator performs several quantum walks on the particles. Finally, the corresponding update rule is selected to move the node according to the measurement result of the quantum state. With continuous update iteration, the shift operator automatically optimizes the discovered community structure. We experimentally compared our MDQW method with four state-of-the-art competitors on five real datasets. We used the normalized mutual information (NMI) to compare clustering quality, and we report an increase in NMI of up to 3.51 of our MDQW method in comparison with the second-best performing competitor. The MDQW method is much faster than its competitors, allowing us to conclude that MDQW is a useful tool in the analysis of large real-life multi-domain networks. Finally, we illustrate the usefulness of our approach on two real-world case studies.

Surface triggered stabilization of metastable charge-ordered phase in SrTiO3

Charge ordering (CO), characterized by a periodic modulation of electron density and lattice distortion, has been a fundamental topic in condensed matter physics, serving as a potential platform for inducing novel functional properties. The charge-ordered phase is known to occur in a doped system with high d-electron occupancy, rather than low occupancy. Here, we report the realization of the charge-ordered phase in electron-doped (100) SrTiO3 epitaxial thin films that have the lowest d-electron occupancy i.e., d1-d0. Theoretical calculation predicts the presence of a metastable CO state in the bulk state of electron-doped SrTiO3. Atomic scale analysis reveals that (100) surface distortion favors electron-lattice coupling for the charge-ordered state, and triggering the stabilization of the CO phase from a correlated metal state. This stabilization extends up to six unit cells from the top surface to the interior. Our approach offers an insight into the means of stabilizing a new phase of matter, extending CO phase to the lowest electron occupancy and encompassing a wide range of 3d transition metal oxides.

Low field-cooled induced large exchange bias effect and DFT calculations in ferromagnetic Sm2CoMnO6

In the present report, we study the large exchange bias effect in Sm2CoMnO6 (SCMO) polycrystalline samples synthesized with the presence of two crystallographic phases: ordered-phase (monoclinic; P21/n) and disordered-phase (orthorhombic; Pnma). X-ray photoelectron spectroscopy study revealed the presence of mixed valence states for Co (2+ & 3+) and Mn (4+ & 3+). M(T) data exhibits an inhomogeneous magnetic state with the presence of ferromagnetic ordering at TC ∼128 K due to the super-exchange interactions of Co2+-O2--Mn4+ and antiferromagnetic-like spin correlations for T < 50 K, attributed to Co3+-O2--Co3+, and Mn3+-O2--Mn3+ interactions. M(H) loop shift with a conventional exchange bias (EB) effect of 10 kOe for a field-cooled (HFC) of 10 kOe at 2 K was observed. Such a large value of the EB effect for low HFC in SCMO is comparable to that of large EB compounds, such as La1·5Sr0·5CoMnO6 and NiFe2O4/CoO nanocomposites. The zero-field cooled asymmetry in M(H) loop termed as spontaneous exchange bias effect (SEB) observed for T < 20 K. The systematic study of EB effects like HEB and MEB with T(K) and HFC was explained qualitatively by the presence of unidirectional anisotropy formed at the interface of inhomogeneous magnetic phases. Further, density functional theory (DFT) calculations validate the ferromagnetic ground state of SCMO with Co and Mn networks. Moreover, the semiconductor characteristics of SCMO are established with a band gap of 1.3 eV.

Spin glass state in Co doped NdMnO3

The temperature-dependent ac susceptibility studies of in-phase component (χ′ac) and out of phase component (χac″) was done on a single phase polycrystalline NdMn0.6Co0.4O3 sample. The measurements were conducted in 4 Oe ac field and 0 Oe dc field at frequencies of 111, 311, 1011, and 1311 Hz. As the frequency increases, the magnitude of χ′ac decreases, and its peak position, corresponding to Tf, moves towards higher temperatures. This strongly suggests the presence of a spin glass (SG) state and the absence of long-range ferromagnetic ordering. The value of relative shift in freezing temperature (δTf) is found to be δTf = 0.0089, which suggests the existence of canonical SG state in NdMn0.6Co0.4O3 sample. To confirm it the ac susceptibility data was further analyzed using Arrhenius, Vogel Fulcher, and Power–law.