Spin-exchange Energy Research Articles

External magnetic fields prompting blended ferron magnetization and demagnetization in antiferromagnetic clusters

The paper studies the magnetization and demagnetization of a stable, spin-polarized impurity interacting with phonons and magnons, dubbed blended-ferron in an antiferromagnetic cluster and subjected to an external magnetic field. The weak magnetic field is observed enhance local magnetic reordering and blended-ferron formation. Strong magnetic field enhances ferron demagnetization (with excess spin-down population 0ph,0mag,↓ ) imitating optical polaron characteristics and enhanced ferron magnetization for excess spin-up population 0ph,0mag,↑. For the critical magnetic field, the blended ferron achieves infinite effective mass and exhibits ferromagnetism leading to a spin flop phenomenon. The spin exchange energy is observed to be modified by the phonons which together with the magnons are responsible for the blended-ferron characteristics.

Controlling the Spin Exchange Energy through Charge Transfer for Triplet State Management in Organic Semiconductors

Controlling the Spin Exchange Energy through Charge Transfer for Triplet State Management in Organic Semiconductors

Effect of the magnetic order on the magneto-photocurrent of organo-metal halide perovskites

The optical properties of perovskite materials are spin dependent. Therefore, tuning the spin-exchange energy in perovskite materials can provide a way towards improving their photovoltaic and light-emission performances. In this study, the effect of an externally applied magnetic field on the magneto-photocurrent of hybrid-perovskite-based devices was investigated. We show that a magnetic field can be used to tune the short-range exchange energy of excited states, which results in a negative magneto-photocurrent. Besides, doping perovskite with manganese provides an additional way to tune the long-range spin-exchange energy between the excited states through the formation of polarons. This can lead to an increase of the performance of perovskite-based optoelectronic devices.

Higher-order spin-hole correlations around a localized charge impurity

Analysis of higher-order correlation functions has become a powerful tool for investigating interacting many-body systems in quantum simulators, such as quantum gas microscopes. Experimental measurements of mixed spin-charge correlation functions in the 2D Hubbard have been used to study equilibrium properties of magnetic polarons and to identify coherent and incoherent regimes of their dynamics. In this paper we consider theoretically an extension of this technique to systems which use a pinning potential to reduce the mobility of a single dopant in the Mott insulating regime of the 2D Hubbard model. We find that localization of the dopant has a dramatic effect on its magnetic dressing. The connected third-order spin correlations are weakened in the case of a mobile hole but strengthened near an immobile hole. In the case of the fifth-order correlation function, we find that its bare value has opposite signs in cases of the mobile and of fully pinned dopant, whereas the connected part is similar for both cases. We study suppression of higher-order correlators by thermal fluctuations and demonstrate that they can be observed up to temperatures comparable to the spin-exchange energy $J$. We discuss implications of our results for understanding the interplay of spin and charge in doped Mott insulators.

Theoretical Investigation in Coherent Manipulation throughout the Calculation of the Local Density of States in FM-DQD-FM Device

In this work, theoretical investigation in coherent manipulation throughout local density of states calculation for serially coupled double quantum dots embedded between ferromagnetic leads (FM-QD1-QD2-FM) by using the non-equilibrium Green's function approach. Since the local density of states are formulated incorporating the spin polarization and the type of spin configuration on the leads. Our model incorporates the inter-dot hopping, the intra-dot Coulomb correlation, the spin exchange energy and the coupling interactions between the quantum dots and leads. The results concerned to the parallel configuration at strong inter-dot coupling regime shows that the spin down electrons in the quantum dots may be more coupled coherently if the regime is tuned. The local density of states of the two dots for spin up electrons shows a broad hump with small splitting i.e. the case is decoherent for spin up electrons. In the case of weak interdot coupling it is obvious that the spin dependent density of states on the quantum dots show that the resonances are not well splitted. For the antiparallel configuration in the strong coupling regime, the spin dependent density of states of the double quantum dots show four peaks but with broaden and overlapping. In the case of weak coupling regime, the total spin dependent density of states, which have two peaks with certain board, one can conclude that the states are not coupled coherently. The case of the antiferromagnetic nature of the spin exchange interaction, our calculations for the parallel and antiparallel configurations (in strong and weak regimes) show a decoherence state.

Model Parameterization for Coherent Manipulation in Spin Current through FM-QD1-QD2-FM

In this work, model parameterization for coherent manipulation, for serially coupled double quantum dots embedded between ferromagnetic leads (FM-QD1-QD2-FM), is presented. Theoretical model based to the non-equilibrium Green’s function approach is considered. Since the spin current and its spin channels are formulated incorporating the spin polarization and the type of spin configuration on the leads. Our model incorporates the inter-dot hopping, the intra-dot Coulomb correlation, the spin exchange energy and the coupling interactions between the quantum dots and leads. The results concerned to the parallel configuration at strong inter-dot coupling regime shows the preferable coherent state for spin down electrons only at relatively high polarization. While, for weak coupling regime and antiparallel configuration, the decoherent state is dominating. Interesting results have been concluded by calculating the molecular energy levels of the quantum dots. In all our calculations the same spin polarization on the leads is considered. The case of different spin polarization on the lead is also investigated but no unprecedented features are concluded as active region is symmetric.

Z_{2} Parton Phases in the Mixed-Dimensional t-J_{z} Model.

We study the interplay of spin and charge degrees of freedom in a doped Ising antiferromagnet, where the motion of charges is restricted to one dimension. The phase diagram of this mixed-dimensional t-J_{z} model can be understood in terms of spinless chargons coupled to a Z_{2} lattice gauge field. The antiferromagnetic couplings give rise to interactions between Z_{2} electric field lines which, in turn, lead to a robust stripe phase at low temperatures. At higher temperatures, a confined meson-gas phase is found for low doping whereas at higher doping values, a robust deconfined chargon-gas phase is seen, which features hidden antiferromagnetic order. We confirm these phases in quantum MonteCarlo simulations. Our model can be implemented and its phases detected with existing technology in ultracold atom experiments. The critical temperature for stripe formation with a sufficiently high hole concentration is around the spin-exchange energy J_{z}, i.e., well within reach of current experiments.

Ultrafast Enhancement of Ferromagnetic Spin Exchange Induced by Ligand-to-Metal Charge Transfer

We theoretically predict and experimentally demonstrate a nonthermal pathway to optically enhance superexchange interaction energies in a material based on exciting ligand-to-metal charge-transfer transitions, which introduces lower-order virtual hopping contributions that are absent in the ground state. We demonstrate this effect in the layered ferromagnetic insulator CrSiTe_{3} by exciting Te-to-Cr charge-transfer transitions using ultrashort laser pulses and detecting coherent phonon oscillations that are impulsively generated by superexchange enhancement via magneto-elastic coupling. This mechanism kicks in below the temperature scale where short-range in-plane spin correlations begin to develop and disappears when the excitation energy is tuned away from the charge-transfer resonance, consistent with our predictions.

Scale-invariant magnetic anisotropy in RuCl3 at high magnetic fields

In RuCl$_3$, inelastic neutron scattering and Raman spectroscopy reveal a continuum of non-spin-wave excitations that persists to high temperature, suggesting the presence of a spin liquid state on a honeycomb lattice. In the context of the Kitaev model, magnetic fields introduce finite interactions between the elementary excitations, and thus the effects of high magnetic fields - comparable to the spin exchange energy scale - must be explored. Here we report measurements of the magnetotropic coefficient - the second derivative of the free energy with respect to magnetic field orientation - over a wide range of magnetic fields and temperatures. We find that magnetic field and temperature compete to determine the magnetic response in a way that is independent of the large intrinsic exchange interaction energy. This emergent scale-invariant magnetic anisotropy provides evidence for a high degree of exchange frustration that favors the formation of a spin liquid state in RuCl$_3$.

Density functional theory investigation of half-metallic ferromagnetism in V-doped GaP alloys

The spin polarized structural, electronic, and magnetic properties of zincblende Ga1−xVxP (x = 0, 0.25, 0.50, 0.75, and 1) alloys are studied by using the full-potential linearized augmented plane waves with local orbital (FP-L/APW + lo) method within the spin-polarized density functional theory (spin-DFT). The exchange and correlation (XC) energy is defined in all this approach by the generalized gradient approximation (GGA-PBE) of Perdew-Burke-Ernzerhof, whereas the GGA + U (U is the Hubbard term of the Coulomb repulsion correlation) functional is employed specially to treat and improve the electronic and magnetic properties of these compounds. The structural properties show the stability of Ga1−xVxP alloys in ferromagnetic phase, where their equilibrium structural parameters (lattice constant (a0), bulk modulus (B0), and its first-pressure derivative (B’) are evaluated. The elastic constants for the cubic system (C11, C12, and C44) and anisotropy are computed in the goal to prove the mechanical stability of these compounds. The electronic results reveal that both Ga0.25V0.75P and VP alloys have a perfect half-metallic nature, while the Ga0.75V0.25P alloy is a semiconductor. From band structures, we observe that the unfilled 3d-V orbitals increase the rise of spin-exchange splitting energy Δx(d) and Δx(pd); consequently, we found that the minority-spin case presents an attractive effective potential comparing to the majority-spin case. The total magnetic moment of Ga0.75V0.25P, Ga0.25V0.75P and VP alloys are found in integer value (2μB), affirming their complete half-metallic behavior. Moreover, we estimate the s-d exchange constant N0α (conduction band) and the p-d exchange constant N0β (valence band) to analyze contributions of conduction and valence bands in the exchange splitting process; in the other hand, the observed p-d hybridization reduces the atomic magnetic moment of V element from its free space charge value and produces weak local magnetic moments on the nonmagnetic sites of Ga and P atoms.

Spin-incoherent Luttinger liquid of one-dimensional SU( κ ) fermions

We theoretically investigate one-dimensional (1D) SU($\kappa$) fermions in the regime of spin-incoherent Luttinger liquid. We specifically focus on the Tonks-Girardeau gas limit where its density is sufficiently low that effective repulsions between atoms become infinite. In such case, spin exchange energy of 1D SU($\kappa$) fermions vanishes and all spin configurations are degenerate, which automatically puts them into spin-incoherent regime. In this limit, we are able to express the single-particle density matrices in terms of those of anyons. This allows us to numerically simulate the number of particles up to $N=32$. We numerically calculate single-particle density matrices in two cases: (1) equal populations for each spin components (balanced) and (2) all $S_z$ manifolds included. In contrast to noninteracting multi-component fermions, the momentum distributions are broadened due to strong interactions. As $\kappa$ increases, the momentum distributions are less broadened for fixed $N$, while they are more broadened for fixed number of particle per spin component. We then compare numerically calculated high momentum tails with analytical predictions which are proportional to $1/p^4$, in good agreement. Thus, our theoretical study provides a comparison with the experiments of repulsive multicomponent alkaline-earth fermions with a tunable SU($\kappa$) spin-symmetry in the spin-incoherent regime.

Non-Fermi-liquid to Fermi-liquid transports in iron-pnictide Ba(Fe1−xCox)2As2 and the electronic correlation strength in superconductors newly probed by the normal-state Hall angle

Electrical transports in iron-pnictide Ba(Fe1−xCox)2As2 (BFCA) single crystals are heavily debated in terms of the hidden Fermi-liquid (HFL) and holographic theories. Both HFL and holographic theories provide consistent physic pictures and propose a universal expression of resistivity to describe the crossover of transports from the non-Fermi-liquid (FL) to FL behavior in these so-called ‘strange metal’ systems. The deduced spin exchange energy J and model-dependent energy scale W in BFCA are almost the same, or are of the same order of several hundred Kelvin for over-doped BFCA, which is in agreement with the HFL theory. Moreover, a drawn line of W/3.5 for BFCA in the higher-doping region up to the right demonstrates the crossover from non-FL-like behavior to FL-like behavior at high doping, and shows a new phase diagram of BFCA. The electronic correlation strength in superconductors has been newly probed by the normal-state Hall angle, which found that, for the first time, correlation strength can be characterized by the ratios of Tc to the Fermi temperature TF, J/TF, and the transverse mass to longitudinal mass.

Magnetic glass state and magnetoresistance in SrLaFeCoO6 double perovskite

Unusual features in magnetization resembling the kinetic arrest of a magnetic glass state are observed in the La-doped double perovskite, SrLaFeCoO6. Neutron powder diffraction experiments confirm the presence of antisite disorder as well as a lack of long-range magnetic order down to 4 K in this double perovskite which displays spin glass-like features in dc and ac susceptibilities. Magnetic relaxation observed through cooling and heating under unequal fields (CHUF) point towards unusual domain dynamics which is supported by a broad memory effect. Among the two anomalies that are observed at 75 K and at 250 K in the magnetic measurements, the former is associated with a spin-freezing temperature below which the magnetic glass state is experimentally verified. The magnetometric experiments detailed in the paper bring out the non-equilibrium metastable magnetic states in this disordered magnetic system. The magnetic glass state described above manifests in the electrical resistivity through the formation of a ‘hard gap’ because of the spin-exchange energy following the formation of magnetic glass. It is observed that the combination of disorder and magnetic glass state leads to a large, negative magnetoresistance (MR) of ≈47 at 5 K in 8 T.

Surface ferromagnetism inHfO2induced by excess oxygen

Abstract First-principles numerical simulations based on density functional theory are performed to examine surface electronic and magnetic properties of cubic, tetragonal, and monoclinic HfO 2 with low index terminations. Our systematic calculations reveal that i) stoichiometric surfaces and Hf rich non-stoichiometric surfaces are non magnetic, and ii) O rich non-stoichiometric surfaces are ferromagnetic and metallic. The ferromagnetism found here is attributed to O surface electronic states with large O 2 p spin exchange energy. This finding provides a novel pathway to d0 ferromagnetism for simple oxides with no magnetic ions involved. We also calculate the surface energy and discuss a recent controversial issue of ferromagnetism observed experimentally in HfO 2 .

Spin-incoherent one-dimensional spin-1 Bose Luttinger liquid

We investigate spin-incoherent Luttinger liquid of a one-dimensional spin-1 Bose gas in a harmonic trap. In this regime highly degenerate spin configurations emerge since the spin exchange energy is much less than the thermal energy of the system, while the temperature is low enough that the lowest energetic orbitals are occupied. As an example we numerically study the momentum distribution of a one-dimensional spin-1 Bose gas in Tonks- Girardeau gas limit and in the sector of zero magnetization.We find that the momentum distributions broaden as the number of atoms increase due to the averaging of spin function overlaps. Large momentum ($p$) asymptotic is analytically derived, showing the universal $1/p^4$ dependence. We demonstrate that the spin-incoherent Luttinger liquid has a momentum distribution also distinct from spinless bosons at finite temperature.

Effective Hamiltonians for correlated narrow energy band systems and magnetic insulators: Role of spin-orbit interactions in metal-insulator transitions and magnetic phase transitions.

Using a second-quantized many-electron Hamiltonian, we obtain (a) an effective Hamiltonian suitable for materials whose electronic properties are governed by a set of strongly correlated bands in a narrow energy range and (b) an effective spin-only Hamiltonian for magnetic materials. The present Hamiltonians faithfully include phonon and spin-related interactions as well as the external fields to study the electromagnetic response properties of complex materials and they, in appropriate limits, reduce to the model Hamiltonians due to Hubbard and Heisenberg. With the Hamiltonian for narrow-band strongly correlated materials, we show that the spin-orbit interaction provides a mechanism for metal-insulator transition, which is distinct from the Mott-Hubbard (driven by the electron correlation) and the Anderson mechanism (driven by the disorder). Next, with the spin-only Hamiltonian, we demonstrate the spin-orbit interaction to be a reason for the existence of antiferromagnetic phase in materials which are characterized by a positive isotropic spin-exchange energy. This is distinct from the Néel-VanVleck-Anderson paradigm which posits a negative spin-exchange for the existence of antiferromagnetism. We also find that the Néel temperature increases as the absolute value of the spin-orbit coupling increases.

Stability and electronic properties of Cd0.75Mn0.25S and Cd0.75Mn0.25Se in B3 phase

We studied the structural, elastic, spin-polarized electronic band structures and magnetic properties of the diluted magnetic semiconductor Cd1−x Mn x S and Cd1−x Mn x Se in zinc blende phase (B3) for x = 0.25 using ab initio method. The calculations were performed by using density functional theory as implemented in the Spanish Initiative for Electronic Simulations with Thousands of Atoms code using local density approximation. Calculated electronic band structures and magnetic properties of Cd1−x Mn x S are discussed in terms of contribution of Mn 3d 5 4s 2, Cd 4d 10 5s 2, S 3s 2 3p 4 orbitals. The total magnetic moment is found to be 5.00 µb for Cd1−x Mn x S and Cd1−x Mn x Se at x = 0.25. This value indicates that Mn atom adds no hole carrier to the perfect CdS crystal. We determine the spin-exchange splitting energies produced by Mn 3d states, s-d exchange constant N 0 α, and p-d exchange constant N 0 β. We found that Mn-doped systems are ferromagnetic. Calculated results are in good agreement with previous studies.

Weakly-correlated nodeless superconductivity in single crystals of Ca3Ir4Sn13 and Sr3Ir4Sn13 revealed by critical fields, Hall effect, and magnetoresistance measurements

We report a study of single crystal Ca3Ir4Sn13 (CIS) and Sr3Ir4Sn13 (SIS) by measuring the longitudinal and Hall resistivities, upper and lower critical fields and magnetoresistance, as well as the magnetization. The sign change in the Hall coefficient observed on both the CIS and SIS provides direct evidence for the Fermi surface reconstructing during the superlattice phase transition. Both materials are of current interest due to indications of superconductivity associated with charge-density-wave (CDW) ordering. Observations of the diamagnetic feature and the lower critical field Hc1(T) in both CIS and SIS can be realized by means of the nodeless single-gap BCS theory. In addition, a weak electronic correlation in both systems has been revealed by the small values of the spin exchange energy, upper critical field and Δ(0)/kBTc ratio, derived respectively from the normal-state Hall effect, resistive transition and temperature-dependent Hc1. It is noticeable that the magnetoresistance of SIS shows a rapid increase below T′ ∼ 40 K, following Kohler’s scaling rule. The results of the magnetic susceptibility and Hall coefficient also exhibit anomalous features near T′. With respect to these observations, this suggests that the existence of an additional phonon mode with energy of about 4.0 meV in SIS is responsible for the presence of lattice instability toward a phase transition.

Spin exchange energy for a pair of valence band holes in artificial molecules

We study the exchange energy for two interacting holes in vertically stacked InGaAs quantum dots using the configuration interaction method and an axially symmetric 4-band Kohn-Luttinger Hamiltonian that introduces heavy-hole and light-hole subbands mixing due to the spin–orbit interaction. We demonstrate that, as a consequence of the band mixing, a singlet–triplet degeneracy appears for a single specific value of the interdot barrier, for which the bonding and antibonding heavy hole have identical energy. We explain this degeneracy using a simple model matrix Hamiltonian as due to the interaction effects for a degenerate ground state of the hole. We demonstrate that, in conditions of the singlet–triplet degeneracy due to hybridization of the bonding and antibonding energy levels, the spin exchange energy becomes insensitive to external electric fields.

Optically tunable spin-exchange energy at donor:acceptor interfaces in organic solar cells

Spin-exchange energy is a critical parameter in controlling spin-dependent optic, electronic, and magnetic properties in organic materials. This article reports optically tunable spin-exchange energy by studying the line-shape characteristics in magnetic field effect of photocurrent developed from intermolecular charge-transfer states based on donor:acceptor (P3HT:PCBM) system. Specifically, we divide magnetic field effect of photocurrent into hyperfine (at low field < 10 mT) and spin-exchange (at high field > 10 mT) regimes. We observe that increasing photoexcitation intensity can lead to a significant line-shape narrowing in magnetic field effect of photocurrent occurring at the spin-exchange regime. We analyze that the line-shape characteristics is essentially determined by the changing rate of magnetic field-dependent singlet/triplet ratio when a magnetic field perturbs the singlet-triplet transition through spin mixing. Based on our analysis, the line-shape narrowing results indicate that the spin-exchange energy at D:A interfaces can be optically changed by changing photoexcitation intensity through the interactions between intermolecular charge-transfer states. Therefore, our experimental results demonstrate an optical approach to change the spin-exchange energy through the interactions between intermolecular charge-transfer states at donor:acceptor interface in organic materials.