Effective Exchange Interaction Research Articles

Two-band model for magnetism and superconductivity in nickelates

The recently discovered superconductivity in Nd$_{1-x}$Sr$_x$NiO$_2$ provides a new opportunity for studying strongly correlated unconventional superconductivity. The single-hole Ni$^+$ ($3d^9$) configuration in the parent compound NdNiO$_2$ is similar to that of Cu$^{2+}$ in cuprates. We suggest that after doping, the intra-orbital spin-singlet and inter-orbital spin-triplet double-hole (doublon) configurations of Ni$^{2+}$ are competing, and we construct a two-band Hubbard model by including both the $3d_{x^2-y^2}$ and $3d_{xy}$-orbitals. The effective spin-orbital super-exchange model in the undoped case is a variant of the $SU(4)$ Kugel-Khomskii model augmented by symmetry breaking terms. Upon doping, the effective exchange interactions between spin-$\frac{1}{2}$ single-holes, spin-1 (triplet) doublons, and singlet doublons are derived. Possible superconducting pairing symmetries are classified in accordance to the $D_{4h}$ crystalline symmetry, and their connections to the superexchange interactions are analyzed.

A Projective Method for the Calculation of Excited-State Electronic Coupling: Isolating Charge Transfer/Recombination Processes in Organic Photovoltaics.

Electronic coupling between excited states is a vital parameter required to describe ultrafast energy and charge transfer processes that occur in photoresponsive organic materials. In such systems, short-range Coulombic, exchange, overlap, and configuration interaction effects must all be accounted for. Although a number of methods are available, the evaluation of coupling between arbitrary excited states remains challenging. In this contribution, a flexible and scalable method for the calculation of short-range electronic coupling between excited states is developed. Excitation- or charge-localized states are projected onto the adiabatic states of a dimeric molecular system using an efficient wave function overlap algorithm. In addition to correctly treating Coulombic, exchange, and overlap contributions, the inclusion of multistate interactions is inherent in the procedure. The method is then used to disentangle excitation energy transfer, charge transfer, and charge recombination processes in donor/acceptor systems relevant to organic photovoltaics, with a view toward the development of material design principles. Calculations were performed within single-excitation frameworks, but the scheme has the potential to be extended to multireference/higher-order excitation quantum-chemical methods.

Effective exchange interactions for bad metals and implications for iron-based superconductors

The experimentally observed bad metal behavior in parent iron pnictides and chalcogenides suggests that these systems contain strong electronic correlations and are on the verge of a metal-to-insulator transition. The magnetic excitations in this bad-metal regime mainly derive from the incoherent part of the electronic spectrum away from the Fermi energy. We present a microscopic study of the exchange interactions in such a regime within a slave rotor approach. We find that the exchange interaction is maximized near the Mott transition. Generalizations to the multi-orbital case are discussed, as are the implications for the strength of superconducting pairing amplitude in the iron-based superconductors.

Temperature dependence of the spin relaxation time of donor-bound electrons immersed in a CdTe quantum well

The behavior of the spin relaxation time of electrons bound to donors immersed in the middle of a CdTe quantum well was measured in the range of temperatures 10--80 K, by using picosecond pump-probe Kerr rotation. Different doping concentrations spanning from isolated donors up to a concentration beyond the metal-insulator transition were considered at a fixed magnetic field; at very low temperature, in insulating regime all electrons are bound to donors but for one high concentration we had to consider that a fraction of electrons is in conduction states. By increasing the temperature, the number of conduction electrons increases. The experimental temperature dependences were explained by invoking spin exchange between electron spins localized on donors and the spin of electrons promoted to conduction states. A good agreement between experiment and theory was found and allowed us to conclude that, while the spin of localized electrons undergoes the effect of both hyperfine and anisotropic exchange interactions, the D'yakonov-Perel' mechanism governs the spin relaxation of the conduction electrons for the whole range of the studied doping concentrations. Moreover, we identified the scattering mechanisms possibly undergone by the conduction electrons at low temperature.

Non-Heisenberg magnetism in a quaternary spin-gapless semiconductor

Understanding of spin-gapless semiconductors with fully spin-polarized charge carriers is critically important because of their promise for spintronic applications. Here, we report non-collinear spin structures, magnetic ground state, and effective exchange interactions of the spin-gapless semiconductor CoFeCrAl investigated with noncollinear density functional calculations. The ground state of CoFeCrAl is ferrimagnetic and has a spin configuration with ↓ Fe, ↑ Co and ↑ Cr spins. In our constrained calculations, the magnetizations of the Fe, Co, and Cr sublattices are rotated by various angles θ, which give rise to three sets of noncollinear spin structures. For all three elements, the magnetic energy increases with the angle, which reconfirms the ferrimagnetic spin structure. During rotation, the magnitudes of the Co and Cr spins remain almost unchanged, whereas that of Fe strongly decreases as a function of the angle θ. This indicates that the finite-temperature behavior of CoFeCrAl is characterized by a pronounced non-Heisenberg behavior of the ↓ Fe moments, whereas the ↑ Co and ↑ Cr moments are Heisenberg-like. We discuss how this feature affects the finite-temperature behavior of the alloy beyond the commonly considered intersublattice Heisenberg exchange.

Characterization of localized effective spins in gapped quantum spin chains

We study properties of localized effective spins induced in gapped quantum spin chains by local inhomogeneities of the lattice. As a prototype, we study effective spins induced in impunity sites doped AKLT model by constructing the exact ground state in a matrix product state (MPS) form. We characterize their responses to external fields by studying an extended Zeeman interaction. We also study the antiferromagnetic bond-alternating Heisenberg chain with defect structures. For this model, an MPS representation similar to that for the AKLT model, "a uniform MPS with windows", is constructed, and it gives a good approximation of the ground state. We discuss the trade-off relation between the window length and the precision of the MPS ansatz. The effective exchange interaction between the induced spins is also investigated by using this representation.

Effect of intergrain exchange interaction on magnetic viscosity of nanocrystalline isotropic NdFeB magnets

The coefficient of magnetic viscosity, S, of NdFeB alloys with varied intergrain exchange interaction constant, KIEI, was measured. The observed dependences of fluctuation field, Hf, on the applied field, H, are strongly correlated with KIEI value. Cooperative magnetization reversal of single domain interacting particles leads to improve the temperature stability of permanent magnets and magnetic systems based on them.

Dependence of Energy Product to the Shell Thickness in FePt/Fe3O4 Core/Shell Nanoparticles

A new model is presented to calculate energy product variation for FePt/Fe3O4 core/shell in the terms of Fe3O4 shell thickness. The spins at the core/shell interface and some parts of the core volume have a random orientation due to the presence of chemical disorder created by the experimental procedures. The mentioned random effects based on the chemical disorder and intermixing of core and shell produces random interactions, which according to the presented model can behave similar to the spin glass system. Due to the existence of a competition between exchange and anisotropy energies, the coercivity field contain two parts: In the first part, the effects of exchange interactions will dominate the effects of anisotropy. As a result, this section will include low anisotropy and frustrated spins, which is coupled to neighbor particles due to the strong exchange interactions. In the second part, which is the volume part of matter, the anisotropy is higher; in this case, the spins are frozen and remain fixed so that the neighbor particles do not have much effect on them. Since the nanoscale particles have a significant value of surface to volume ratio, there will be a competition between these two parts. Energy product variations for FePt/Fe3O4 core/shell increases when coherent rotation is dominant in the core/shell system. By increasing shell thickness, coherent rotation of spins disappears and the magnetization rotation will not be uniform, which leads to a decrease in energy product. To increase the maximum product energy, we need to increase the value of coercivity which is achieved by increasing the core thickness. If we can increase the magnetocrystalline anisotropy constant of the core, then the coercivity and thus the maximum energy product will increase.

Voltage-Controlled Magnonic Spin Tunneling Junction

We theoretically investigate the effective exchange interaction, $J_\mathrm{eff}$, mediated by conductive electrons within a nonmagnetic metal spacer, in the presence of a bias voltage, sandwiched by two ferromagnetic insulators. On the basis of the tight-binding model, we show the voltage and spacer thickness dependences of $J_\mathrm{eff}$, and its contorollability is demonstrated. We also propose a new magnonic device with the functions of both field effect transistor and non-volatile memory.

Simultaneous tuning of magnetocrystalline anisotropy and spin reorientation transition via Cu substitution in Mn-Ni-Ga magnets for nanoscale biskyrmion formation

Skyrmions with multiple helicity or topology in centrosymmetric crystals are intriguing magnetic-domain objects due to their diverse dynamics under external stimuli. Here we illustrate how the two key gradients of magnetocrystalline anisotropy (MCA) and spin reorientation transition (SRT) affect the skyrmion formation and topology by Cu substitution in the biskyrmion-host MnNiGa alloy. The MCA and SRT are simultaneously tuned in a large scope, while the original high Curie temperature (${T}_{C}$) is retained. Detailed neutron-scattering studies revealed the construction of a noncollinear canted magnetic structure below the SRT temperature (${T}_{\mathrm{SR}}$), which effectively correlates the SRT with the evolution of the MCA, as well as the exchange interaction. The Cu substitution raises the ${T}_{\mathrm{SR}}$ to merge with the ${T}_{\mathrm{C}}$, and meanwhile, reduces the $c$-axis anisotropy. Lorentz transmission electron microscopy revealed the formation of stacked biskyrmions from above room temperature to lower temperatures in $\mathrm{MnN}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{u}}_{x}\mathrm{Ga}\phantom{\rule{0.28em}{0ex}}(x=0--0.3)$ in the presence of proper MCA. Micromagnetic simulations further confirmed the great effect of uniaxial anisotropy on the stabilization of biskyrmions. Our work has helped clarify the evolution of magnetic structures and their correlation to the SRT, providing an account of the effect of MCA and exchange interaction on the biskyrmion formation.

Nonvolatile Control of the Electronic Properties of In2-xCrxO3 Semiconductor Films by Ferroelectric Polarization Charge.

A series of Cr-doped In2-xCrxO3 (ICO) semiconductor thin films were epitaxially grown on (111)-oriented 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-0.29PT) single-crystal substrates by the pulsed laser deposition. Upon the application of an electric field to the PMN-0.29PT substrate along the thickness direction, we realized in situ, reversible, and nonvolatile control of the electronic properties and Fermi level of the films, which are manifested by abundant physical phenomena such as the n-type to p-type transformation, metal-semiconductor transition, metal-insulator transition, crossover of the magnetoresistance (MR) from negative to positive, and a large nonvolatile on-and-off ratio of 5.5 × 104% at room temperature. We also strictly disclose that both the sign and the magnitude of MR are determined by the electron carrier density of ICO films, which could modify the s-d exchange interaction and weak localization effect. Our results demonstrate that the ferroelectric gating approach using PMN-PT can be utilized to gain deeper insight into the carrier-density-related electronic properties of In2O3-based semiconductors and provide a simple and energy efficient way to construct multifunctional devices which can utilize the unique properties of composite materials.

Electron Paramagnetic Resonance in HoxLu1−xB12 Dodecaborides

The high-frequency (60 GHz) electron paramagnetic resonance is studied in HoxLu1−xB12 single crystals at low temperatures (2–80 K) in an applied magnetic field up to 7 T within a wide doping range, 0.01 ≤ x ≤ 1. These compounds are characterized by the presence of a low-temperature cage glass phase with random displacements of magnetic holmium ions from the centrosymmetric position in B12 boron cuboctahedra. For the samples with x ≥ 0.1, it is demonstrated that the electron paramagnetic resonance in the form of a single broad line with a g-factor of about 5 appears in the cage glass phase at T 0.3, the pronounced broadening of the resonance line and a steep decrease in the g-factor related to antiferromagnetic correlations is observed on cooling at T < 30 K. The performed simulation of the electron paramagnetic resonance spectra suggests the dominant role of the exchange interaction effects and positional disorder in determining the characteristic features of spin dynamics in HoxLu1−xB12.

Weak localization in systems with chiral spin textures and skyrmion crystals

Theory of interference-induced quantum corrections to conductivity is developed for two dimensional systems with chiral spin textures including skyrmions. The effect of exchange interaction between electrons and spin textures on weak localization of electronic waves is studied. The spin dephasing rates are calculated as functions of the spin texture size. The anomalous magnetoresistance is shown to be governed by the size and magnetization spatial distribution of the spin textures. The effect of average magnetization-induced spin splitting on weak localization is analyzed. The sign-alternating weak-antilocalization magnetoresistance is demonstrated for skyrmion crystals. We argue that analysis of the low-field magnetoresistance serves as an independent tool for an experimental detection of chiral spin textures and, in particular, skyrmions.

Devlin-like approach to a spin-1 transverse XY model with biquadratic exchange and single-ion anisotropy

The effect of the biquadratic exchange interaction on the phase diagram of a d-dimensional spin-1 transverse XY model with easy-axis single-ion anisotropy is studied by employing the Devlin-like two-time Green functions framework. The chain of equations of motion is closed adopting the random phase approximation for the exchange higher order Green functions and treating exactly the crystal-field anisotropy terms. For short-range interactions and d > 2, analytical estimates and numerical calculations predict a reentrant behavior of the critical lines close to the magnetic-field-induced quantum critical point for appropriate values of the single-ion anisotropy parameter and suitable combinations of the bilinear and biquadratic exchange couplings. Remarkably, increasing the biquadratic exchange reduces or destroies the reentrant character of the quantum critical lines, in qualitative agreement with the findings of the Anderson-Callen-like strategy. In our formalism, the easy-plane anisotropy case can be studied similarly but the phase diagram and the quantum critical scenario do not present any reentrant phenomena.

Voltage control of superconducting exchange interaction and anomalous Josephson effect

Exerting control of the magnetic exchange interaction in heterostructures is of both basic interest and has potential for use in spin-based applications relying on quantum effects. We here show that the sign of the exchange interaction in a spin-valve, determining whether the ferro- or antiferromagnetic configuration is favored, can be controlled via an electric voltage. This occurs due to an interplay between a nonequilibrium quasiparticle distribution and the presence of spin-polarized Cooper pairs. Additionally, we show that a voltage-induced distribution controls the anomalous supercurrent that occurs in magnetic Josephson junctions, obviating the challenging task to manipulate the magnetic texture of the system. This demonstrates that two key phenomena in superconducting spintronics, the magnetic exchange interaction and the phase shift generating the anomalous Josephson effect, can be controlled electrically. Our findings are of relevance for spin-based superconducting devices which in practice most likely have to be operated precisely by nonequilibrium effects.

High magnetic field phase diagram and failure of the magnetic Grüneisen scaling in LiFePO4

We report the magnetic phase diagram of single-crystalline LiFePO$_4$ in magnetic fields up to 58~T and present a detailed study of magneto-elastic coupling by means of high-resolution capacitance dilatometry. Large anomalies at \tn\ in the thermal expansion coefficient $\alpha$ imply pronounced magneto-elastic coupling. Quantitative analysis yields the magnetic Gr\"uneisen parameter $\gamma_{\rm mag}=6.7(5)\cdot 10^{-7}$~mol/J. The positive hydrostatic pressure dependence $dT_{\rm N}/dp = 1.46(11)$~K/GPa is dominated by uniaxial effects along the $a$-axis. Failure of Gr\"uneisen scaling below $\approx 40$~K, i.e., below the peak temperature in the magneto-electric coupling coefficient [\onlinecite{toft2015anomalous}], implies several competing degrees of freedom and indicates relevance of recently observed hybrid excitations~[\onlinecite{yiu2017hybrid}]. A broad and strongly magnetic-field-dependent anomaly in $\alpha$ in this temperature regime highlight the relevance of structure changes. Upon application of magnetic fields $B||b$-axis, a pronounced jump in the magnetisation implies spin-reorientation at $B_{\rm SF} = 32$~T as well as a precursing phase at 29~T and $T=1.5$~K. In a two-sublattice mean-field model, the saturation field $B_{\rm sat,b} = 64(2)$~T enables the determination of the effective antiferromagnetic exchange interaction $J_{\rm af} = 2.68(5)$~meV as well as the anisotropies $D_{\rm b} = -0.53(4)$~meV and $D_{\rm c} = 0.44(8)$~meV.

Effect of exchange interaction on electronic instabilities in the honeycomb lattice: A functional renormalization group study

The impact of local and nonlocal density-density interactions on the electronic instabilities in the honeycomb lattice is widely investigated. Some early studies proposed the emergence of interaction-induced topologically nontrivial phases, but recently, it was denied in several works including renormalization group calculations with refined momentum resolution. We use the truncated unity functional renormalization group to study the many-body instabilities of electrons on the half-filled honeycomb lattice, focusing on the effect of the exchange interaction. We show that varying the next-nearest-neighbor repulsion and nearest-neighbor exchange integral can lead to diverse ordered phases, namely, the quantum spin Hall, the spin-Kekul\'e, and some spin- and charge-density-wave phases. The quantum spin Hall phase can be induced by a combination of the ferromagnetic exchange and pair hopping interactions. Another exotic phase, the spin-Kekul\'e phase, develops in a very small region of the parameter space considered. We encounter the three-sublattice charge-density-wave phase in a large part of the parameter space. It is replaced by the incommensurate charge density wave when increasing the exchange integral. In order to reduce the computational effort, we derive the explicit symmetry relations for the bosonic propagators of the effective interaction and propose a linear-response-based approach for identifying the form factor of order parameter. Their efficiencies are confirmed by numerical calculations in our work.

Coulomb and exchange interaction effects on the exact two-electron dynamics in the Hong-Ou-Mandel interferometer based on Hall edge states

The electronic Hong-Ou-Mandel interferometer in the integer quantum Hall regime is an ideal system to probe the building up of quantum correlations between charge carriers and it has been proposed as a viable platform for quantum computing gates. Using a parallel implementation of the split-step Fourier method, we simulated the antibunching of two interacting fermionic wave packets impinging on a quantum point contact. Numerical results of the exact approach are compared with a simplified theoretical model based on one-dimensional scattering formalism. We show that, for a realistic geometry, the Coulomb repulsion dominates over the exchange energy, this effect being strongly dependent on the energy broadening of the particles. We define analytically the spatial entanglement between the two regions of the quantum point contact, and obtain quantitatively its entanglement-generation capabilities.

Impact of off-diagonal exchange interactions on the Kitaev spin-liquid state of α−RuCl3

Motivated by the recent experimental observation of the half-quantized thermal Hall conductivity for the candidate material of the Kitaev spin liquid, $\alpha$-RuCl$_3$[ Y. Kasahara et al., Nature 559, 227 (2018)], we investigate effects of non-Kitaev exchange interactions, which exist in the real material, on the gapped chiral spin liquid state realized in the case with an applied magnetic field. It is found that off-diagonal exchange interactions enhance significantly the mass gap of Majorana fermions. This result provides a possible explanation for robust quantization of the thermal Hall conductivity observed in the above-mentioned experiment. Furthermore, we demonstrate that if the Kitaev spin liquid state and the zigzag antiferromagnetic order coexist around the border of these two phases, off-diagonal exchange interactions can induce Fermi surfaces of Majorana fermions, leading to a state similar to the U(1) spin liquid.

Fermion exchange interaction between magnetic Skyrmions

We propose a new mechanism of exchange interaction between the solitons related with the presence of localized fermions. Since the Atiyah-Patodi-Singer index theorem implies that such modes always exist for any topological soliton, their appearance may significantly alter the usual pattern of interaction. We elaborate this possibility considering, as a particular example, the chiral magnetic skyrmions coupled to spin-isospin fermions. It is shown that there are sequences of fermionic modes localized on the skyrmions. We investigate the additional effect of exchange interaction between the soltions with localized fermionic modes and demonstrate the existence of a stable system of magnetic skyrmions bounded by the strong attractive interaction mediated by the chargeless fermions.