Effective Exchange Interaction Research Articles

Stable and recyclable polyporous polyurethane foam highly loaded with UIO-66-NH2 nanoparticles for removal of Cr(Ⅵ) in wastewater

Efficient and recoverable adsorption technology plays a key role in removing the heavy metal Cr(Ⅵ) from wastewater. The metal-organic frameworks (MOFs) are very unstable in aqueous solution and their structure is easy to be destroyed. Of all MOFs, UIO-66-NH 2 (Zr-MOF) is widely used to adsorb heavy metal ions in wastewater. But UIO-66-NH 2 has disadvantages of small size and easy aggregation. When it is employed as the absorbent of industrial wastewater, it is impossible to be recycled after absorption. Therefore, in this study taking advantage of the excellent properties of polyurethane foam (PUF) such as porous network, light weight and abundant binding sites, UIO-66-NH 2 were loaded on PUF by in-situ growth method to obtain a composite UIO-66-NH 2 @PUF. BET and MIR results confirmed the co-existence of macropore, micropore and mesopore. The absorption kinetics and isotherms indicated that the adsorption capacity of UIO-66-NH 2 @PUF to Cr(Ⅵ) was 92 mg/g and Langmuir model could better describe adsorption process, as well as the process belongs to monolayer adsorption. After six cycles, the removal rate of UIO-66-NH 2 @PUF still reached to 79%. XRD, EDS and determination result of Zr(Ⅳ) concentration showed that the exfoliation of Zr(Ⅳ) was negligible, which proved UIO-66-NH 2 @PUF had excellent chemical stability after repeated use. The adsorption mechanism was the comprehensive effect of ligand exchange and Lewis acid-base interaction between Zr(Ⅳ) and Cr(Ⅵ) which was concluded by XPS. Owing to these attributes, loading MOFs powder on porous carrier makes it possible to reuse MOFs and avoid secondary pollution.

Hydrogeochemistry mechanism contrasts between low and high sulfate in limestone aquifers at the massive coalfield in Anhui province, China

Abstract High sulfate mine groundwater at the Huaibei coalfield has exceeded the permissible limit (250 mg/l) of World Health Organization, posing serious threat to nearby water resource. A total of 28 samples were collected from Taiyuan formation aquifer (TA) and Ordovician aquifer (QA). In view of the mean concentration in two aquifers, the TA and OA were identified as high sulfate group and low sulfate group. The contrast on hydrochemical characterization and water-rock interactions were analyzed to reveal the mechanism of sulfate enrichment in mine groundwater. Compared with low sulfate groundwater, the high sulfate groundwater possess a higher content of TDS, Na+ and Cl−, which elevated by 3.83, 4.0 and 3.29 times, respectively. The low sulfate groundwater is controlled by Ca-Na-HCO3 type (82%), whereas the high sulfate groundwater is dominated by Na-SO4-HCO3 type (88%). The geochemical process of low sulfate groundwater is influenced by dissolution of carbonate minerals and weak ion exchange. The mechanism of sulfate enrichment in mine groundwater are predominately controlled by both silicate and carbonate minerals weathering, strong ion exchange interaction and competitive effect. The outcomes enhance understanding of the mechanism of high sulfate mine water and provide theoretical support for mine wastewater treatment. HIGHLIGHTS Hydrogeochemistry mechanism contrasts between low and high sulfate in limestone aquifers were studied. Water environment for high sulfate mine groundwater was revealed from comparative analysis with low sulfate mine groundwater. Corresponding minerals dissolution dominated the hydro-chemistry process together with ion exchange and competitive effect.

Compensation and transition order temperature behavior of mixed spin-1 and spin-1/2 ising model on a centered honeycomb-hexagonal structure: two points of compensation

The magnetic properties and phase diagrams of the mixed spin-1/2 and spin-1 Ising model on a honeycomb inside a hexagonal structure have been studied using the Monte Carlo simulations based on the Metropolis update protocol. The effect of different exchange interactions (intralayer and interlayer Js') and single-ion anisotropy D on the magnetic properties, transition and compensation temperatures, and magnetic susceptibility have been investigated. The phase diagrams, T-J and T-D, for different exchange interactions and crystal field values have been examined. Compensation behavior exists for critical values of interlayer's exchange interactions. Our calculations reveal the existence of two points of compensation for a narrow range of negative D.Article highlightsA mixed spin Ising model on a centered honeycomb-hexagonal structure was proposedThe effects of crystal field and exchange coupling were studiedTwo points of compensation temperature were exist.

Adiabatic spin dynamics and effective exchange interactions from constrained tight-binding electronic structure theory: Beyond the Heisenberg regime

We consider an implementation of the adiabatic spin dynamics approach in a tight-binding description of the electronic structure. The adiabatic approximation for spin-degrees of freedom assumes that the faster electronic degrees of freedom are always in a quasi-equilibrium state, which significantly reduces the numerical complexity in comparison to the full electron dynamics. Non-collinear magnetic configurations are stabilized by a constraining field, which allows to directly obtain the effective magnetic field from the negative of the constraining field. While the dynamics are shown to conserve energy, we demonstrate that adiabatic spin dynamics does not conserve the total spin angular momentum when the lengths of the magnetic moments are allowed to change, which is confirmed by numerical simulations. Furthermore, we develop a method to extract an effective two-spin exchange interaction from the energy curvature tensor of non-collinear states, which we calculate at each time step of the numerical simulations. We demonstrate the effect of non-collinearity on this effective exchange and limitations due to multi-spin interactions in strongly non-collinear configurations beyond the regime where the Heisenberg model is valid. The relevance of the results are discussed with respect to experimental pump-probe experiments that follow the ultra-fast dynamics of magnetism.

The contribution of intermolecular spin interactions to the London dispersion forces between chiral molecules.

Dispersion interactions are one of the components of van der Waals (vdW) forces that play a key role in the understanding of intermolecular interactions in many physical, chemical, and biological processes. The theory of dispersion forces was developed by London in the early years of quantum mechanics. However, it was only in the 1960s that it was recognized that for molecules lacking an inversion center, such as chiral and helical molecules, there are chirality-sensitive corrections to the dispersion forces proportional to the rotatory power known from the theory of circular dichroism and with the same distance scaling law R-6 as the London energy. The discovery of the chirality-induced spin selectivity effect in recent years has led to an additional twist in the study of chiral molecular systems, showing a close relation between spin and molecular geometry. Motivated by it, we propose in this investigation to describe the mutual induction of charge and spin-density fluctuations in a pair A-B of chiral molecules by a simple physical model. The model assumes that the same fluctuating electric fields responsible for vdW forces can induce a magnetic response via a Rashba-like term so that a spin-orbit field acting on molecule B is generated by the electric field arising from charge density fluctuations in molecule A (and vice versa). Within a second-order perturbative approach, these contributions manifest as an effective intermolecular exchange interaction. Although expected to be weaker than the standard London forces, these interactions display the same R-6 distance scaling.

Dynamic dielectric properties of an antiferroelectric/ferroelectric mixed-spin (5/2, 3/2) Ising system under a time-dependent oscillating electric field

A mixed-spin (5/2, 3/2) Ising model was proposed to investigate the antiferroelectric/ferroelectric (AFE/FE) bilayer under a time-dependent oscillating electric field. Based on Monte Carlo simulation, the effects of exchange interaction, anisotropy, and external electric field on the electric properties of the system, including dynamic order parameter, electrical susceptibility, internal energy were studied in detail. We found the phenomenon of multi-saturation values of the dynamic order parameter. We plotted the phase diagrams of the critical temperature of the system under the influence of different parameters as well. Moreover, the hysteresis behavior of the system was studied and the rich multi-loop electric hysteresis behavior was found.

Valley-resolved quantum anomalous Hall effect in ferromagnetically proximitized monolayer MoTe2

We theoretically investigate the effect of Rashba field and magnetic exchange interaction on the topological properties of monolayer transition-metal dichalcogenides (TMDs) based on a tight-binding model. The tight-binding Hamiltonian incorporates magnetic exchange interaction and the Rashba field which can be induced by proximity to a ferromagnetic (FM) substrate, such as europium oxide (EuO) or similar materials. Through tuning the Rashba field and magnetic exchange interaction, we find a quantum state of matter---the valley-resolved quantum anomalous Hall state---in monolayer ${\mathrm{MoTe}}_{2}$. The most amazing property of this topological quantum state is that the edge states of this phase are all located only at valley $\ensuremath{-}K$ (or $+K$), i.e., they are completely valley resolved. Additionally, the calculated Chern number $\mathcal{C}=\ensuremath{-}2$ indicates that there are two pairs of valley-resolved chiral edge modes at the boundaries of the monolayer ${\mathrm{MoTe}}_{2}$ nanoribbon. The electronic transmission demonstrates the robustness of the edge states of the valley-resolved quantum anomalous Hall effect against defects. This newfound valley-resolved quantum anomalous Hall effect provides an idea for designing dissipationless valleytronics utilizing TMD/FM heterojunctions.

Light-Driven Topological and Magnetic Phase Transitions in Thin Layer Antiferromagnets.

We theoretically study the effect of low-frequency light pulses in resonance with phonons in the topological and magnetically ordered two-septuple layer (2-SL) MnBi2Te4 (MBT) and MnSb2Te4 (MST). These materials share symmetry properties and an antiferromagnetic ground state in pristine form but present different magnetic exchange interactions. In both materials, shear and breathing Raman phonons can be excited via nonlinear interactions with photoexcited infrared phonons using intense laser pulses that can be attained in the current experimental setups. The light-induced transient lattice distortions lead to a change in the sign of the effective interlayer exchange interaction and magnetic order accompanied by a topological band transition. Furthermore, we show that moderate antisite disorder, typically present in MBT and MST samples, can facilitate such an effect. Therefore, our work establishes 2-SL MBT and MST as candidate platforms for achieving non-equilibrium magneto-topological phase transitions.

Microstructural evolutions, phase transformations and hard magnetic properties in polycrystalline Ce–Co–Fe–Cu alloys

This work focuses on systematic studies of Ce–Co based 1:5 permanent magnet alloys of CeCo4.4-xFexCu0.6 and CeCo3.9-xFexCu1.2 (x = 0, 0.3, 0.6, 0.9, 1.2, 1.8) by varying Co:Fe. The overarching aim of this manuscript is to elucidate the hard-magnetic properties through a better understanding of phase formation by the structural, microstructural, and magnetic properties in these materials. Improved mutual solubility of Fe in the 1:5 phase has been observed with an extended homogeneity range by Cu substitution. For both composition series, Fe contents of x ≤ 0.6 show a homogeneous microstructure with a single 1:5 phase and good magnetic properties. The composition region 0.6 < x ≤ 0.9 appears to be near the boundary of solubility and evolution of other phases. At x = 1.8, it is found that the homogeneous 1:5 phase and magnetic hardness deteriorated due to the evolution of secondary phases such as 2:17, 2:7, and Fe–Co. The addition of Fe improved both the magnetization and Curie temperature via increased effective exchange interactions, while an increase in Cu content enhanced coercivity.

Laser Control of Electronic Exchange Interaction within a Molecule.

Electronic interactions play a fundamental role in atoms, molecular structure and reactivity. We introduce a general concept to control the effective electronic exchange interaction with intense laser fields via coupling to excited states. As an experimental proof of principle, we study the SF_{6} molecule using a combination of soft x-ray and infrared (IR) laser pulses. Increasing the IR intensity increases the effective exchange energy of the core hole with the excited electron by 50%, as observed by a characteristic spin-orbit branching ratio change. This work demonstrates altering electronic interactions by targeting many-particle quantum properties.

Substrate orientation dependent characteristics of half-metallic and metallic superlattices [La0.7Sr0.3MnO3/LaNiO3]10

We report a detailed study on the orientation dependent growth characteristics, electronic structure, transport, magnetic, and vibrational excitations in atomically flat interfaces of [La0.7Sr0.3MnO3/LaNiO3]10 superlattices (SLs) coherently grown on (001/011/111)-SrTiO3 substrates by the pulsed laser deposition technique. X-ray reflectometry confirms the periodic superlattice stacks from the Kiessig interference fringes and well-defined even interfaces between the nickelate and manganite layers. A complex local atomic environment across the interfaces was noticed, yet trivalent La, divalent Sr, and mixed valent Ni2+/3+ and Mn3+/4+ electronic states prevail at the core level with enhanced relative intensity ratio of the Mn ions in the superlattices grown on (111) oriented SrTiO3 substrates as compared to those grown on (001) and (011) oriented SrTiO3. The temperature (5≤T≤300K) dependence of electrical resistivity ρ(T) analysis reveals 3D variable range hopping model [ρ(T)=ρ0exp⁡(T0/T)(1/4)] with large magnitude of hopping energies (≥40 meV) for the SL-111 system associated with the high energy gap developed by the accumulation of disorderness in the individual constituents of polar layers. Moreover, all SL systems exhibit reduced ferromagnetic ordering temperatures (67≤TC≤110K) with a low-temperature anomaly (11.4≤T∗≤22K) and a substantial enhancement in the effective exchange interaction (Jeff∼3.52meV) having altered ground state-spin configuration S∼1/2 different from S=3/2 of La0.75Sr0.25MnO3. Nevertheless, the SL-011 system exhibits large anisotropy field HK∼18kOe and cubic anisotropy constant K1∼9.3×103J/m3 in comparison to the other two orientations. The second order two-phonon interaction driven by the local polaronic distortion causes significant changes in the vibrational excitations of the investigated system. Nonetheless, most of the Raman modes follow the substrate-induced, highly oriented epitaxial growth pattern except for two modes ν4 (326cm−1) and ν8 (728cm−1), which slightly differ in the case of SL-111 superlattices.

Model for coupled 4f−3d magnetic spectra: A neutron scattering study of the Yb-Fe hybridization in Yb3Fe5O12

In this work, we explore experimentally and theoretically the spectrum of magnetic excitations of the Fe$^{3+}$ and Yb$^{3+}$ ions in ytterbium iron garnet (Yb$_3$Fe$_5$O$_{12}$). We present a complete description of the crystal-field splitting of the $4f$ states of Yb$^{3+}$, including the effect of the exchange field generated by the magnetically ordered Fe subsystem. We also consider a further effect of the Fe-Yb exchange interaction, which is to hybridise the Yb crystal field excitations with the Fe spin-wave modes at positions in the Brillouin zone where the two types of excitations cross. We present detailed measurements of these hybridised excitations, and propose a framework which can be used in the quantitative analysis of the coupled spectra in terms of the anisotropic $4f-3d$ exchange interaction.

Low-field magnetic anisotropy of Sr2IrO4

Magnetic anisotropy in strontium iridate (Sr2IrO4) is essential because of its strong spin–orbit coupling and crystal field effect. In this paper, we present a detailed mapping of the out-of-plane (OOP) magnetic anisotropy in Sr2IrO4 for different sample orientations using torque magnetometry measurements in the low-magnetic-field region before the isospins are completely ordered. Dominant in-plane anisotropy was identified at low fields, confirming the b axis as an easy magnetization axis. Based on the fitting analysis of the strong uniaxial magnetic anisotropy, we observed that the main anisotropic effect arises from a spin–orbit-coupled magnetic exchange interaction affecting the OOP interaction. The effect of interlayer exchange interaction results in additional anisotropic terms owing to the tilting of the isospins. The results are relevant for understanding OOP magnetic anisotropy and provide a new way to analyze the effects of spin–orbit-coupling and interlayer magnetic exchange interactions. This study provides insight into the understanding of bulk magnetic, magnetotransport, and spintronic behavior on Sr2IrO4 for future studies.

The effect of spin exchange interaction on protein structural stability.

Partially charged chiral molecules act as spin filters, with preference for electron transport toward one type of spin ("up" or "down"), depending on their handedness. This effect is named the chiral induced spin selectivity (CISS) effect. A consequence of this phenomenon is spin polarization concomitant with electric polarization in chiral molecules. These findings were shown by adsorbing chiral molecules on magnetic surfaces and investigating the spin-exchange interaction between the surface and the chiral molecule. This field of study was developed using artificial chiral molecules. Here we used such magnetic surfaces to explore the importance of the intrinsic chiral properties of proteins in determining their stability. First, proteins were adsorbed on paramagnetic and ferromagnetic nanoparticles in a solution, and subsequently urea was gradually added to induce unfolding. The structural stability of proteins was assessed using two methods: bioluminescence measurements used to monitor the activity of the Luciferase enzyme, and fast spectroscopy detecting the distance between two chromophores implanted at the termini of a Barnase core. We found that interactions with magnetic materials altered the structural and functional resilience of the natively folded proteins, affecting their behavior under varying mild denaturing conditions. Minor structural disturbances at low urea concentrations were impeded in association with paramagnetic nanoparticles, whereas at higher urea concentrations, major structural deformation was hindered in association with ferromagnetic nanoparticles. These effects were attributed to spin exchange interactions due to differences in the magnetic imprinting properties of each type of nanoparticle. Additional measurements of proteins on macroscopic magnetic surfaces support this conclusion. The results imply a link between internal spin exchange interactions in a folded protein and its structural and functional integrity on magnetic surfaces. Together with the accumulating knowledge on CISS, our findings suggest that chirality and spin exchange interactions should be considered as additional factors governing protein structures.

Role of exchange coupling between the hard and soft magnetic phases on the absorption of microwaves by SrFe10Al2O19/Co0.6Zn0.4Fe2O4 nanocomposite

(1-x)SrFe10Al2O19/(x)Co0.6Zn0.4Fe2O4-(SFAO/CZFO) hard/soft nanocomposite ferrite materials were synthesized by ‘one-pot’ self-propagating combustion route. The co-existence of the two magnetic phases were confirmed by XRD, FESEM, EDS and VSM. The prepared nanocomposite samples were also characterized by TGA/DSC, Raman spectroscopy and VNA. Exchange coupling between the hard and the soft magnetic grains was observed by determining the switching field distribution (SFD) curve. As a result of the competing effects of exchange interaction and dipolar interaction, magnetic parameters were observed to be sensitive to the incorporation of soft magnetic phase into the nanocomposite. Results showed that with the inclusion of soft magnetic phase, exchange coupling behaviour between the hard and the soft ferrite phases had significant influence on the microwave absorption capacity of the samples. Related electromagnetic parameters and impedance matching ratio of the nanocomposite system were discussed. A minimum reflection loss of −42.9 dB with an absorber thickness of 2.5 mm was attained by the nanocomposite (90 wt%)SrFe10Al2O19/(10 wt %)Co0.6Zn0.4Fe2O4 at a matching frequency of 11.45 GHz. This assured the candidacy of SrFe10Al2O19/Co0.6Zn0.4Fe2O4 nanocomposite as a promising microwave absorption material in the X-band (8–12 GHz).

Electronic and magnetic properties of iridium ilmenites AIrO3 (A=Mg, Zn, and Mn)

We theoretically investigate the electronic band structures and magnetic properties of ilmenites with edge-sharing IrO$_6$ honeycomb layers, $A$IrO$_3$ with $A=$ Mg, Zn, and Mn, in comparison with a collinear antiferromagnet MnTiO$_3$. The compounds with $A=$ Mg and Zn were recently reported in Y.~Haraguchi {\it et al.}, Phys. Rev. Materials {\bf 2}, 054411 (2018), while MnIrO$_3$ has not been synthesized yet but the honeycomb stacking structure was elaborated in a superlattice with MnTiO$_3$ in K.~Miura {\it et al.}, Commun. Mater. {\bf 1}, 55 (2020). We find that, in contrast to MnTiO$_3$, where an energy gap opens in the Ti $3d$ bands by antiferromagnetic ordering of the high-spin $S=5/2$ moments, MgIrO$_3$ and ZnIrO$_3$ have a gap in the Ir $5d$ bands under the influence of both spin-orbit coupling and electron correlation. Their electronic structures are similar to those in the spin-orbit coupled Mott insulators with the $j_{\rm eff}=1/2$ pseudospin degree of freedom, as found in monoclinic $A_2$IrO$_3$ with $A=$ Na and Li which have been studied as candidates for the Kitaev spin liquid. Indeed, we find that the effective exchange interactions between the $j_{\rm eff}=1/2$ pseudospins are dominated by the Kitaev-type bond-dependent interaction and the symmetric off-diagonal interactions. On the other hand, for MnIrO$_3$, we show that the local lattice structure is largely deformed, and both Mn $3d$ and Ir $5d$ bands appear near the Fermi level in a complicated manner, which makes the electronic and magnetic properties qualitatively different from MgIrO$_3$ and ZnIrO$_3$. Our results indicate that the IrO$_6$ honeycomb network in the ilmenites $A$IrO$_3$ with $A=$ Mg and Zn would offer a good platform for exotic magnetism by the spin-orbital entangled moments like the Kitaev spin liquid.

Unraveling Site Selective Magnetic Properties of Cobalt Sites in Critical Elements Lean RE(TM)5 Magnet Materials

We report here our discovery of crystallographic and interstitial sites and onsite electron correlation propelled intrinsic and derived permanent magnetic properties of critical elements lean RE(TM)5 (RE = La, Ce and TM = Fe, Co) magnet materials. A full potential linearized augmented plane wave (FP-LAPW) method within the local density approximation (LDA) is used to investigate and analyze the electronic structure and magnetism of these RE(TM)5 type structures. To better correlate the experimental results, the effective Coulomb (U) and exchange (J) interactions (Hubbard parameters) are crucial at the transition metal sites. Results show that the main propeller of magnetic anisotropy in these compounds is the cobalt atoms at the 2c sites not the 3g sites. This site-specific property and site preference energetics are used to replace 3g sites with non-critical elements such as iron that exhibits a larger magnetic moment. Based on this strategic replacement, we predict two new compounds that have a larger hardness parameter with a relatively large energy product due to the atomic dilution caused by interstitial addition of nitrogen in the compounds: CeCo2Fe3N2 and LaCo2Fe3N2.

Effective exchange interaction for terahertz spin waves in iron layers

The exchange stiffness is a central material parameter of all ferromagnetic materials. Its value controls the Curie temperature as well as the dynamic properties of spin waves to a large extent. Using ultrashort spin current pulses we excite perpendicular standing spin waves (PSSW) in ultrathin epitaxial iron layers at frequencies of up to 2.4 THz. Our analysis shows that for the PSSWs the observed exchange stiffness of iron is about 20% smaller compared to the established iron bulk value. In addition, we find an interface-related reduction of the effective exchange stiffness for layers with the thickness below 10 nm. To understand and discuss the possible mechanisms of the exchange stiffness reduction we develop an analytical one-dimensional model. In doing so we find that the interface induced reduction of the exchange stiffness is mode dependent.

One-dimensional ferromagnetic semiconductor CrSbSe3 with high Curie temperature and large magnetic anisotropy

In recent years, low-dimensional ferromagnetic semiconductors have attracted tremendous interest due to their great significance for both fundamental physics and device applications. In this work, a one dimensional (1D) ferromagnetic semiconductor $\mathrm{CrSb}{\mathrm{Se}}_{3}$ with high Curie temperature $({T}_{\mathrm{C}})$ and sizable magnetic anisotropy is proposed based on the first-principle calculations and theoretical model. Quasi-1D chain structure with a moderate cleavage energy of $0.49\phantom{\rule{0.16em}{0ex}}\mathrm{J}/{\mathrm{m}}^{2}$ suggests the possibility to exfoliate a 1D $\mathrm{CrSb}{\mathrm{Se}}_{3}$ ladder (a ladder structure formed by two rows of 1D monoatomic Cr chain) from the bulk. Monte Carlo simulation based on anisotropic Heisenberg model predicts ${T}_{\mathrm{C}}$ to be up to 170 K, which is much higher than that of the bulk. The high ${T}_{\mathrm{C}}$ of 1D $\mathrm{CrSb}{\mathrm{Se}}_{3}$ ladder originates from the combined effect of the strong intraladder ferromagnetic exchange interaction and large magnetic anisotropy, while the lower ${T}_{\mathrm{C}}$ of the bulk results from the weak interladder indirect exchange interaction. In addition, we find that the splitting of top valence bands depends on the direction of magnetization due to the reduced structural symmetry and spin-orbit coupling, indicating a strong magnetoband structure effect. Our theoretical prediction not only provides a material platform for understanding the fundamental physics of 1D magnetism, but also provides a strategy for the search for the 1D ferromagnetic semiconductor with high Curie temperature.

Collective spin and charge excitations in the t-J-U model of high-[formula omitted] cuprates

The t-J-U model of high-Tc copper-oxide superconductors incorporates both the on-site Coulomb repulsion and kinetic exchange interaction and yields a semi-quantitative description of the static properties of those materials. We extend this analysis to dynamic quantities and address collective spin- and charge excitations in the correlated metallic state of the t-J-U model. We employ VWF+1/Nf approach that combines the variational wave function (VWF) approach with the expansion in the inverse number of fermionic flavors (1/Nf). It is shown that the resonant (paramagnon) contribution to the dynamic magnetic susceptibility remains robust as one interpolates between the Hubbard- and t-J-model limits, whereas the incoherent continuum undergoes substantial renormalization. Energy of the collective charge mode diminishes as the strong-coupling limit is approached. We also introduce the concept of effective kinetic exchange interaction that allows for a unified interpretation of magnetic dynamics in the Hubbard, t-J, and t-J-U models. The results are discussed in the context of recent resonant inelastic x-ray scattering experiments for the high-Tc cuprates.