Superexchange Term Research Articles

Microscopic Study on Superexchange Dynamics of Composite Spin-1 Bosons.

We report on an experimental simulation of the spin-1 Heisenberg model with composite bosons in a one-dimensional chain based on the two-component Bose-Hubbard model. Exploiting our site- and spin-resolved quantum gas microscope, we observed faster superexchange dynamics of the spin-1 system compared to its spin-1/2 counterpart, which is attributed to the enhancement effect of multi-bosons. We further probed the nonequilibrium spin dynamics driven by the superexchange and single-ion anisotropy terms, unveiling the linear expansion of the spin-spin correlations, which is limited by the Lieb-Robinson bound. Based on the superexchange process, we prepared and verified the entangled qutrits pairs with these composite spin-1 bosons, potentially being applied in qutrit-based quantum information processing.

Quantum Magnetism in Wannier-Obstructed Mott Insulators

We develop a strong coupling approach towards quantum magnetism in Mott insulators for Wannier-obstructed bands. Despite the lack of Wannier orbitals, electrons can still singly occupy a set of exponentially localized but nonorthogonal orbitals to minimize the repulsive interaction energy. We develop a systematic method to establish an effective spin model from the electron Hamiltonian using a diagrammatic approach. The nonorthogonality of the Mott basis gives rise to multiple new channels of spin-exchange (or permutation) interactions beyond Hartree–Fock and superexchange terms. We apply this approach to a Kagome lattice model of interacting electrons in Wannier-obstructed bands (including both Chern bands and fragile topological bands). Due to the orbital nonorthogonality, as parameterized by the nearest-neighbor orbital overlap g, this model exhibits stable ferromagnetism up to a finite bandwidth W∼Ug, where U is the interaction strength. This provides an explanation for the experimentally observed robust ferromagnetism in Wannier-obstructed bands. The effective spin model constructed through our approach also opens up the possibility for frustrated quantum magnetism around the ferromagnet-antiferromagnet crossover in Wannier-obstructed bands.

Pressure-dependent X-ray diffraction of the multiferroics RMn2O5.

The room-temperature structural properties of the RMn2O5 multiferroics have been investigated under pressure, using powder X-ray scattering and density functional theory (DFT) calculations. It was possible to determine the lattice parameters and the main atomic positions as a function of pressure. Good agreement was observed between the X-ray and DFT results for most of the determined crystallographic data. From the DFT calculations, it was possible to infer the pressure evolution of the exchange interactions, and this analysis led to the conclusion that the onset of the q = (½, 0, ½) magnetic structure under pressure is related to the increase in the J1 super-exchange terms (due to the reduction in the Mn-O distances) compared with the Mn-R exchange interactions. In addition, the 1D antiferromagnetic character of the compounds should be reinforced under pressure.

Anisotropic spin-singlet pairings inCuxBi2Se3andBi2Te3

Possible anisotropic spin singlet pairings in Bi$_2$X$_3$ (X is Se or Te) are studied. Among six pairings compatible with the crystal symmetry, two novel pairings show nontrivial surface Andreev bound states, which form flat bands and could produce zero bias conductance peak in measurements like point contact spectroscopy. By considering purely repulsive short range Coulomb interaction as the pairing mechanism, the dominant superexchange terms are all antiferromagnetic, which would usually favor spin singlet pairing in Bi$_2$X$_3$. Mean field analyses show that the interorbital pairing interaction favors a mixed spatial-parity anisotropic pairing state, and one pairing channel with zero energy surface states has a sizable component. The results provide important new information for future experiments.

Magnetic Coupling and Single-Ion Anisotropy in Surface-Supported Mn-Based Metal–Organic Networks

The electronic and magnetic properties of Mn coordinated to 1,2,4,5-tetracyanobenzene (TCNB) in the Mn-TCNB 2D metal-ligand networks have been investigated by combining scanning tunneling microscopy and X-ray magnetic circular dichroism (XMCD) performed at low temperature (3 K). When formed on Au(111) and Ag(111) substrates the Mn-TCNB networks display similar geometric structures. Magnetization curves reveal ferromagnetic (FM) coupling of the Mn sites with similar single-ion anisotropy energies, but different coupling constants. Low-temperature XMCD spectra show that the local environment of the Mn centers differs appreciably for the two substrates. Multiplet structure calculations were used to derive the corresponding ligand field parameters confirming an in-plane uniaxial anisotropy. The observed interatomic coupling is discussed in terms of superexchange as well as substrate-mediated magnetic interactions.

Dissecting the Nature of Exciton Interactions in Ethyne-Linked Tetraarylporphyrin Arrays

We investigate how electronic energy transfer in a series of three ethyne-linked zinc- and free base tetraarylporphyrin dimers is tuned by the type of linker and by substitution on the porphyrin rings. We use time-dependent density functional theory (TD-DFT) combined with a recently developed fully polarizable QM/MM/PCM method. This allows us to dissect the bridge-mediated contributions to energy transfer in terms of superexchange (through-bond) interactions and Coulomb (through-space) terms mediated by the polarizability of the bridge. We explore the effects of the substituents and of the bridge-chromophore mutual orientation on these contributions. We find that bridge-mediated superexchange contributions largely boost energy transfer between the porphyrin units. When the effect of the solvent is also considered through the polarizable continuum model (PCM), we find good agreement with the through-bond versus through-space contributions determined experimentally, thus indicating the need to properly incl...

Singlet Excitation Energy Transfer Mediated by Local Exciton Bridges

Singlet excitation energy transfer (EET) generally occurs via a Förster mechanism and originates from direct electronic coupling between the donor (D) and acceptor (A). However, indirect EET (the so-called superexchange mechanism) can be crucial in some situations. In this study, we have explored the contributions of various linker types, linker lengths, and tunneling energies in D–linker–A molecular systems using quantum chemical methods [Kawatsu et al. J. Phys. Chem. A2011, 115, 10814], obtaining conditions in which the superexchange term emerges. To analyze the results in terms of physical parameters, a model Hamiltonian was developed and found to yield results qualitatively consistent with those from the quantum chemical calculations. In a model compound, the superexchange term can be up to 4 times larger than the direct (Förster) term when the energy of the excited state at a linker unit is close to the tunneling energy and when the interactions between these excited states are strong. The ratio of the indirect term to the direct term is greatest at a certain D–A distance because the number of multistep terms increases and the sizes of multistep indirect terms decay exponentially. Single-step indirect terms, which exhibit a polynomial decay, dominate the indirect term for larger D–A distances. The decay behavior with the distance is very different from that of the McConnell type donor–bridge–acceptor superexchange mechanism proposed for the charge transfer (CT). The result of pathway analysis showed that the exciton states mediate EET. The coupling between the local excited states is driven by the pseudo-Coulombic interaction, even in the superexchange terms.

Effective single-band models for strongly interacting fermions in an optical lattice

To test effective Hamiltonians for strongly interacting fermions in an optical lattice, we numerically find the energy spectrum for two fermions interacting across a Feshbach resonance in a double well potential. From the spectrum, we determine the range of detunings for which the system can be described by an effective lattice model, and how the model parameters are related to the experimental parameters. We find that for a range of strong interactions the system is well described by an effective $t-J$ model, and the effective superexchange term, $J$, can be smoothly tuned through zero on either side of unitarity. Right at and around unitarity, an effective one-band general Hubbard model is appropriate, with a finite and small on-site energy, due to a lattice-induced anharmonic coupling between atoms at the scattering threshold and a weakly bound Feshbach molecule in an excited center of mass state.

Orbitally induced string formation in the spin-orbital polarons

We study the spectral function of a single hole doped into the (a,b) plane of the Mott insulator LaVO$_3$, with antiferromagnetic (AF) spin order of S=1 spins accompanied by alternating orbital (AO) order of active $\{d_{yz},d_{zx}\}$ orbitals. Starting from the respective t-J model, with spin-orbital superexchange and effective three-site hopping terms, we derive the polaron Hamiltonian and show that a hole couples simultaneously to the collective excitations of the AF/AO phase, magnons and orbitons. Next, we solve this polaron problem using the self-consistent Born approximation and find a stable quasiparticle solution -- a spin-orbital polaron. We show that the spin-orbital polaron resembles the orbital polaron found in $e_g$ systems, as e.g. in K$_2$CuF$_4$ or (to some extent) in LaMnO$_3$, and that the hole may be seen as confined in a string-like potential. However, the spins also play a crucial role in the formation of this polaron -- we explain how the orbital degrees of freedom: (i) confine the spin dynamics acting on the hole as the classical Ising spins, and (ii) generate the string potential which is of the joint spin-orbital character. Finally, we discuss the impact of the results presented here on the understanding of the phase diagrams of the lightly doped cubic vanadates.

A Comparison of the Magnetic Structures of KMn4(PO4)3 and KCo4(PO4)3 Based on the Connectivity of the Coordination Polyhedra

The crystal structures of KMn(4)(PO(4))(3) and KCo(4)(PO(4))(3) have been determined by neutron diffraction at room temperature. Both compounds are orthorhombic with similar cell parameters, but they crystallize in different space groups, Pnam for the Mn phosphate and Pnnm for the Co analogue. On the basis of the metal cation polyhedra and their connectivity, the crystal structures have been rationalised, which allow interpretation of the main magnetic interactions between them. Magnetic measurements show ferromagnetic behaviour for the Co compound, whereas in the Mn derivative antiferromagnetism is observed. Both magnetic structures are described and qualitatively analysed in terms of superexchange and super-superexchange interactions.

Role of the Dzyaloshinskii-Moriya interaction in multiferroic perovskites

With the perovskite multiferroic RMnO3 (R = Gd, Tb, Dy) as guidance, we argue that the Dzyaloshinskii-Moriya interaction (DMI) provides the microscopic mechanism for the coexistence and strong coupling between ferroelectricity and incommensurate magnetism. We use Monte-Carlo simulations and zero temperature exact calculations to study a model incorporating the double-exchange, superexchange, Jahn-Teller and DMI terms. The phase diagram contains a multiferroic phase between A and E antiferromagnetic phases, in excellent agreement with experiments.

Microscopic theory of magnetic interactions in KCuF3 and LaMnO3 – The role of charge transfer

AbstractWe analyze the spin exchange interactions Jab and Jc in the A ‐type antiferromagnetic phase of KCuF3 and LaMnO3. One finds that the quasi one‐dimensional spin interactions in KCuF3 are consistent with the charge‐transfer type of insulating state, while the modifications of Jab and Jc by the charge‐transfer superexchange terms are moderate in the Mott insulator LaMnO3. We point out on the example of LaMnO3 that a reliable determination of the experimental microscopic parameters of spin‐orbital models is possible only when results of magnetic and optical experiments can be combined. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin and mixed spin‐and‐orbital excitations in KCuF3

AbstractWe investigate the effective spin‐orbital superexchange model for KCuF3 including the superexchange term due to charge‐transfer excitations. We investigate the spin wave and spin‐and‐orbital wave excitations using the bosonic representation of spin and orbital operators for the antiferromagnetic ground state with the ferro‐type of orbital order of occupied eg orbitals. It is shown that antiferromagnetic order is stabilized either by the charge‐transfer superexchange or by the crystal field splitting of the eg energy levels. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Molecular electronics: Some views on transport junctions and beyond

The field of molecular electronics comprises a fundamental set of issues concerning the electronic response of molecules as parts of a mesoscopic structure and a technology-facing area of science. We will overview some important aspects of these subfields. The most advanced ideas in the field involve the use of molecules as individual logic or memory units and are broadly based on using the quantum state space of the molecule. Current work in molecular electronics usually addresses molecular junction transport, where the molecule acts as a barrier for incoming electrons: This is the fundamental Landauer idea of "conduction as scattering" generalized to molecular junction structures. Another point of view in terms of superexchange as a guiding mechanism for coherent electron transfer through the molecular bridge is discussed. Molecules generally exhibit relatively strong vibronic coupling. The last section of this overview focuses on vibronic effects, including inelastic electron tunneling spectroscopy, hysteresis in junction charge transport, and negative differential resistance in molecular transport junctions.

Electropolymerisation dynamics of a highly conducting metallopolymer: poly-[Os(4 ′-(5-(2,2 ′-bithienyl))-2,2 ′:6 ′,2 ″-terpyridine) 2] 2+

The potential, time, and concentration dependence of the potentiostatic electropolymerisation of the thienyl-substituted transition metal complex [Os(bttpy) 2 2+] onto platinum disk microelectrodes is reported, bttpy is 4 ′-(5-(2,2 ′-bithienyl))-2,2 ′:6 ′,2 ″-terpyridine. Oxidative electropolymerisation of the thienyl bridges is highly efficient with an electropolymerisation efficiency of 36 ± 3% being observed across a wide range of potentials and monomer concentrations. The osmium centres are oxidised when polymerisation proceeds and the deposited polymer is highly conducting allowing high surface coverage films, up to 6 × 10 −7 mol cm −2, to be deposited within 60 s. SEM imaging reveals that smooth films can be produced using moderate overpotentials for electrodeposition. Significantly, diffusional mass transport controls the rate of film deposition allowing the radial diffusion field found at microelectrodes to be exploited to favour film growth co-planar with the electrode surface. The electropolymerisation rate increases approximately linearly with increasing monomer concentration from approximately 8 to 400 μM and this first-order dependence likely arises from mass transport limitations. The rate of homogeneous charge transport through potentiostatically deposited films, characterised by D 1/2 C, where D is the apparent charge transport diffusion coefficient and C is the concentration of osmium centres, is 3.5 ± 0.5 × 10 −7 mol cm −2 s −1/2 and is largely insensitive to the deposition potential, and is very similar to that previously observed for potentiodynamically deposited films. This charge transport parameter is approximately two orders of magnitude larger than those found for non-conjugated bridges and is interpreted in terms of resonant superexchange across the quaterthienyl bridge.

Strong magnetic linear dichroism in Fe L23 and O K electron energy-loss near-edge spectra of antiferromagnetic hematite ?-Fe2O3

We report strong magnetic linear dichroism (MLD) at the Fe L 23 and O K edges of the antiferromagnetic compound hematite α-Fe2 O3 in high-resolution orientation- and temperature-dependent electron energy-loss spectroscopy (EELS). Large intensity differences of corresponding spectral features are observed when the Fe L 23 and O K edges are measured with momentum transfer either parallel or perpendicular to the magnetization. The resultant difference spectra for the Fe L 23 edges is consistent with the MLD observed in X-ray absorption spectroscopy. For the first time we have observed MLD at the O K edge, where the magnetic origin of this dichroism is demonstrated by temperature-dependent investigations across the Morin transition temperature T M = 263 K, at which the Fe electron spins, i.e. the magnetic moments, rotate by 90°. The O K edge MLD is interpreted in terms of superexchange between the spins of the Fe 3d and O 2p electrons through overlapping Fe 3d and O 2p orbitals. The experiments were performed in a transmission electron microscope (TEM), yielding information about the anisotropic electronic structure at nanoscale spatial resolution when operated with a focused electron probe. The effects of MLD at the Fe L 23 edges on the determination of Fe3+/ΣFe in hematite at submicrometre scale using different independent quantification methods are discussed.

Exchange interactions in the perovskitesCa1−xSrxMnO3andRMnO3(R=La,Pr,Sm)

Measurement of the variation of the N\'eel temperature ${T}_{N}$ with pressure P and the compressibility \ensuremath{\kappa} of the orbitally ordered $\mathrm{Mn}(\mathrm{III}){\mathrm{O}}_{3}$ array of the $R{\mathrm{MnO}}_{3}$ family $(R=\mathrm{L}\mathrm{a},\mathrm{P}\mathrm{r},\mathrm{S}\mathrm{m})$ and of the $\mathrm{Mn}(\mathrm{IV}){\mathrm{O}}_{3}$ array of the ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ system have enabled the determination for each of a systematic increase in \ensuremath{\kappa}, the magnitude of the Bloch parameter ${\ensuremath{\alpha}}_{B}\ensuremath{\equiv}d\mathrm{ln}{T}_{N}/d\mathrm{ln}V,$ and ${T}_{N}$ with decreasing bending of the (180\ifmmode^\circ\else\textdegree\fi{}-\ensuremath{\varphi}) Mn-O-Mn bond angles. The data demonstrate that the anomalously high value of $\ensuremath{-}{\ensuremath{\alpha}}_{B}=5.5$ reported previously for ${\mathrm{LaMnO}}_{3}$ is not due to orbital order and its attendant anisotropic magnetic order. A higher $|{\ensuremath{\alpha}}_{B}|$ than 3.3 and an increasing compressibility characterize a system on the approach from the localized-electron side to a first-order transition from localized to itinerant electronic behavior. Moreover, the puzzling empirical relationship ${T}_{N}\ensuremath{\sim}〈{\mathrm{cos}}^{2}\ensuremath{\varphi}〉$ for the \ensuremath{\pi}-bonding $\mathrm{Mn}(\mathrm{IV}){\mathrm{O}}_{3}$ array is rationalized by dominance of a semicovalent exchange term over the superexchange term in the perturbation theory for the spin-spin interatomic interactions. The data also provide indirect evidence for a transition from in-phase to out-of-phase stacking along the c axis of the (001)-plane orbital ordering that has been predicted by Mizokawa, Khomskii, and Sawatzky.

Exchange interactions and anisotropic spin waves in bilayer manganites

We present a theory of the magnetic interactions in the bilayer manganites, based upon the manganite t-J model, and show that the ferromagnetic phase is an anisotropic orbital liquiddisordered state. By including the correlations of the e g electrons we explain why the intralayer exchange coupling increases with hole doping x. The transition from the ferromagnetic to A-type antiferromagnetic phase in La222xSr112xMn2O7 at x.0.45 occurs when the occupancy of 3z 2 2r 2 orbitals drops and the superexchange term exceeds the double exchange in the interlayer coupling.

Superexchange Mediated Charge Hopping in DNA

We explore the relationship between the electronic-nuclear level structure, the electronic couplings, and the dynamics of hole hopping transport in DNA. We utilized the electronic coupling matrix elements for hole transfer between nearest-neighbor nucleobases in DNA [Voityuk, A. A.; Jortner, J.; Bixon, M.; Rösch, N. J. Chem. Phys. 2001, 114, 5614] to evaluate intrastrand and interstrand superexchange electronic couplings, which determine hole hopping rates within the framework of a semiempirical quantum mechanical-kinetic model. Calculations of the exponential distance (R) dependence of the superexchange mediated intrastrand electronic couplings |Vsuper|2 ∝ exp(−βR) between guanines (G) in “short” G+(T−A)nG (n ≲ 3) duplexes result in β = 0.8−0.9 Å-1. We interpret the experimental data on time-resolved hole transport in the presence of a site-specifically bound methyl transferase mutant in DNA [Wagenknecht, H.-A.; Rajski, S. R.; Pascally, M.; Stemp, E. D. A.; Barton, J. K. J. Am. Chem. Soc. 2001, 123, 4400] in terms of composite sequential, interstrand and intrastrand superexchange mediated, and direct interstrand hole hopping. This mechanism accounts for the rate determining step, for the weak duplex size dependence of the rate, and for the long-range charge transport induced by interstrand superexchange via short (T−A) bridges, containing a single mediating nucleobase. For hole transfer via longer (T−A)n (n ≳ 3) bridges, the superexchange mechanism is replaced by the parallel mechanism of thermally induced hole hopping (TIH) via long (A)n chains. A kinetic analysis of the experimental data for hole transport through seven GG pairs separated by (T−A)n (n = 2−5) bridges across the 3‘−5‘ strand of the DNA duplex [Sartor, V.; Boone, E.; Schuster, G. B. J. Phys. Chem. B, 2001, 105, 11057] reveals that the superexchange−TIH crossover occurs at n = nx = 3. The explorations of the range of applicability and the breakdown of the superexchange mechanism in DNA lay the foundations for the scrutiny of the universality and system specificity of this mechanism in large-scale chemical and biophysical systems.

Hydrothermal Synthesis, Structural, and Magnetic Studies of a New Fluoropyrophosphate: BaFeII2P2O7F2

BaFe2(P2O7)F2 has been synthesized hydrothermally (24 h, 700°C). Its structure, determined using room-temperature single-crystal X-ray diffraction data, is othorhombic (space group Pmmn, No. 59) with cell parameters a=6.878(1) Å, b=11.847(2) Å, c=4.588(1) Å, V=373.9(1) Å3, and Z=2. The reliability factors converge to R1=2.0% and ωR2=5.8% for 1267 unique reflections (I>2σ(I)). The structure, which is compared to that of the minerals Lawsonite and Henomartinite, consists of octahedra infinite chains [FeO3F]5n−n and P2O7 pyrophosphate linked to generate a three-dimensional framework. Mössbauer data are consistent with the presence of one Fe2+ site in the high spin state. Magnetic couplings are discussed in terms of superexchange and super-superexchange interactions.