Spin-spin Exchange Interaction Research Articles

Enhancing Photogenerated Radical Pair Properties in Donor-Chromophore-Acceptor Systems for Quantum Information Applications.

We report on new donor-chromophore-acceptor triads BDX-ANI-NDI and BDX-ANI-xy-NDI where the BDX donor is 2,2,6,6-tetramethylbenzo[1,2-d;4,5-d]bis[1,3]dioxole, the ANI chromophore is 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, the NDI acceptor is naphthalene-1,8:4,5-bis(dicarboximide), and xy is a 2,5-xylyl spacer. The results on these compounds are compared to the analogous derivatives having a p-methoxyaniline (MeOAn) as the donor. BDX•+ has no nitrogen atoms and only a single hydrogen atom coupled to its unpaired electron spin, and therefore has significantly decreased hyperfine interactions compared to MeOAn•+. We use femtosecond transient absorption (fsTA) and nanosecond TA (nsTA) spectroscopies, the latter with an applied static magnetic field, to study the charge transfer dynamics and determine the spin-spin exchange interaction (J) for BDX•+-ANI-NDI•- and BDX•+-ANI-xy-NDI•- at both ambient and cryogenic temperatures. Time-resolved electron paramagnetic resonance (EPR) and pulse-EPR measurements on these spin-correlated radical pairs (SCRPs) were used to probe their spin dynamics. We demonstrate that BDX•+-ANI-xy-NDI•- has an unusually long lifetime of ∼550 μs in glassy butyronitrile (PrCN) at 85 K, which makes it useful for pulse-EPR studies that target quantum information science (QIS) applications. We also show that rotation of the BDX group about the single bond linking it to the neighboring phenyl group has a significant impact on the spin dynamics, and in particular the magnitude of J. By comparing the results on these compounds to the analogous MeOAn series, insights into design principles for creating improved spin-correlated radical pair systems for QIS studies are obtained.

Control of magnetism on the topological SnTe(001) surface by doping, strain, and gap opening

The spin–spin exchange interaction is an important observable that helps in understanding the magnetism of materials. Topological materials are of particular interest due to their strong spin–orbit coupling, which makes them promising for applications in valleytronics and spintronics. In this study, we have theoretically demonstrated the ability to control the switching of ferromagnetic (FM) and antiferromagnetic (AFM) couplings between two magnetic impurities on the (001) surface of the topological crystalline insulator SnTe. By considering experimental parameters such as doping, strain, and gap opening at surface Dirac cones, we provide a generic way to build the FM-AFM and clockwise–counterclockwise phase diagrams of SnTe(001), which are still missing in the literature, especially for the spintronics community.

Monte Carlo study of Potts versus Kosterlitz-Thouless transitions in a 2D generalized XY model

The generalized XY model has seen renewed interest recently. This model contains both the spin-spin exchange interaction and q-fold nematic interaction. Due to competition between these two kinds of interaction, the model exhibits a rich phase diagram with various phases and unusual phase transitions. Here, we perform an extensive Monte Carlo simulation for this model, focusing on the case of q=3, which corresponds to a three-fold nematic interaction. We use the correlation length to locate the phase transition lines, including the Kosterlitz-Thouless (KT) line, the 1/3 KT line, and the Potts line, which meet at the tricritical point. Further, the magnetic correlation length gives more insights into the tricritical region. Finally, it signifies an intermediate region starting from the tricritical point, where the transition line is neither of the same physics as the Pott transition below nor the KT transition far above the tricritical point.

Manifestation of spin nematic ordering in the spin-1 chain system

Spin-1 chain material is considered. Recently, it was predicted that the spin nematic ordered phase can be realized due to the spin-elastic coupling. In this study we show that the appearance of the spin nematic ordering and phase transitions to such ordered states can be studied using magneto-acoustic experiments. One can observe such phases and phase transitions considering relative changes of the velocity of sound (related to the changes of the elastic modulus of the crystal) as a function of temperature and the external magnetic field. It is shown that those relative changes are proportional to the spin quadrupole susceptibility of the system. The results are obtained using the exact Bethe ansatz solution and Quantum Monte Carlo simulations for several typical values of the biquadratic spin-spin exchange interaction and for various values of the spin-elastic coupling and elastic modules.

Enhanced Thermoelectric Performance by Resonant Doping and Embedded Magnetic Impurity

The thermoelectric energy-conversion efficiency is significantly enhanced by introducing magnetism. Doping $\mathrm{Cr}$ atoms in a ${\mathrm{Cu}}_{2}{\mathrm{Sn}\mathrm{S}}_{3}(\mathrm{CTS})$ matrix modifies the electron density of state (e-DOS) and causes a special electron-transport mechanism by localized impurity spin moments. The localized spin moments cause a spin-spin exchange interaction with the spin of itinerant electrons of the CTS matrix. This is clearly verified in M-T measurement, which deviated from Curie's law. The figure of merit (zT) of the magnetic nanocomposite was 8 times higher than that of pristine CTS by the synergetic effect of modifying e-DOS near the Fermi level and electron transport by localized spin moments.

Zoo of silicon-based quantum bits

Zoo of silicon-based quantum bits

Ultrafast kinetics of the antiferromagnetic-ferromagnetic phase transition in FeRh

Understanding how fast short-range interactions build up long-range order is one of the most intriguing topics in condensed matter physics. FeRh is a test specimen for studying this problem in magnetism, where the microscopic spin-spin exchange interaction is ultimately responsible for either ferro- or antiferromagnetic macroscopic order. Femtosecond laser excitation can induce ferromagnetism in antiferromagnetic FeRh, but the mechanism and dynamics of this transition are topics of intense debates. Employing double-pump THz emission spectroscopy has enabled us to dramatically increase the temporal detection window of THz emission probes of transient states without sacrificing any loss of resolution or sensitivity. It allows us to study the kinetics of emergent ferromagnetism from the femtosecond up to the nanosecond timescales in FeRh/Pt bilayers. Our results strongly suggest a latency period between the initial pump-excitation and the emission of THz radiation by ferromagnetic nuclei.

Radical-Enhanced Intersystem Crossing in Perylene-Oxoverdazyl Radical Dyads.

Attaching stable radicals to organic chromophores is an effective method to enhance the intersystem crossing (ISC) of the chromophores. Herein we prepared perylene-oxoverdazyl dyads either by directly connecting the two units or using an intervening phenyl spacer. We investigated the effect of the radical on the photophysical properties of perylene and observed strong fluorescence quenching due to radical enhanced ISC (REISC). Compared with a previously reported perylene-fused nitroxide radical compound (triplet lifetime, τT =0.1 μs), these new adducts show a longer-lived triplet excited state (τT =9.5 μs). Based on the singlet oxygen quantum yield (ΦΔ =7 %) and study of the triplet state, we propose that the radical enhanced internal conversion also plays a role in the relaxation of the excited state. Femtosecond fluorescence up-conversion indicates a fast decay of the excited state (<1.0 ps), suggesting a strong spin-spin exchange interaction between the two units. Femtosecond transient absorption (fs-TA) spectra confirmed direct triplet state population (within 0.5 ps). Interestingly, by fs-TA spectra, we observed the interconversion of the two states (D1 ↔Q1 ) at ∼80 ps time scale. Time-resolved electron paramagnetic resonance (TREPR) spectral study confirmed the formation of the quartet sate. We observed triplet and quartet states simultaneously with weights of 0.7 and 0.3, respectively. This is attributed to two different conformations of the molecule at excited state. DFT computations showed that the interaction between the radical and the chromophore is ferromagnetic (J>0, 0.05∼0.10 eV).

Photogenerated Spin-Correlated Radical Pairs: From Photosynthetic Energy Transduction to Quantum Information Science.

More than a half century ago, the NMR spectra of diamagnetic products resulting from radical pair reactions were observed to have strongly enhanced absorptive and emissive resonances. At the same time, photogenerated radical pairs were discovered to exhibit unusual electron paramagnetic resonance spectra that also had such resonances. These non-Boltzmann, spin-polarized spectra were observed in both chemical systems as well as in photosynthetic reaction center proteins following photodriven charge separation. Subsequent studies of these phenomena led to a variety of chemical electron donor-acceptor model systems that provided a broad understanding of the spin dynamics responsible for these spectra. When the distance between the two radicals is restricted, these observations result from the formation of spin-correlated radical pairs (SCRPs) in which the spin-spin exchange and dipolar interactions between the two unpaired spins play an important role in the spin dynamics. Early on, it was recognized that SCRPs photogenerated by ultrafast electron transfer are entangled spin pairs created in a well-defined spin state. These SCRPs can serve as spin qubit pairs (SQPs), whose spin dynamics can be manipulated to study a wide variety of quantum phenomena intrinsic to the field of quantum information science. This Perspective highlights the role of SCRPs as SQPs, gives examples of possible quantum manipulations using SQPs, and provides some thoughts on future directions.

Unravelling the Anisotropic Behavior of Nickel—Wires Prepared through External Magnetic Field Assisted Hydrazine Reduction Method

Ni wires, prepared through a hydrazine reduction, were exposed to external magnetic fields of different geometrical shape and configuration during the synthesis denoted as Ni-Non-Magnetic, Ni-Double, Ni-Single, Ni-Ring. Their effect on the wire morphology, magnetization and magnetic anisotropy was then investigated via various characterization techniques viz. X-ray diffraction (XRD), high-resolution field emission scanning electron microscopy (HR-FESEM), and vibrating sample magnetometer (VSM). The polycrystalline single phase of the Ni-wires with face centered cubic symmetry was confirmed through the analysis of XRD patterns. Analysis of HR-FESEM images revealed that the Ni-particles were aligned in form of wire-like morphology. The Ni-single sample formed the wires with minimum diameter compared to the parent sample. The magnetization measurements performed at 300 K and 50 K demonstrated the ferromagnetic behavior of all the samples. The room temperature saturation magnetization (MS) and anisotropy constant (K) of the Ni-wires were reduced upon providing the external field during synthesis. However, the low temperature (50 K) magnetization behavior was rather opposing, indicating enhanced values of MS and K. Among all, Ni-ring sample showed maximum anisotropy with a value of 3.84 × 104 erg/cm3 at 50 K. The ambiguous nature of the anisotropic constant and saturation magnetization ascribed partly to the variation in diameters of Ni-wires and to the intermittent spin-spin exchange interactions of unaligned/partially aligned particles during the synthesis. Briefly, in the present study, it was established that the morphology and magnetic anisotropy of the Ni-wires could be tailored through the external magnetic field assisted synthesis method.

Spectroscopic Investigation of a Metal-Metal-Bonded Fe6 Single-Molecule Magnet with an Isolated S = 19/2 Giant-Spin Ground State.

The metal-metal-bonded molecule [Bu4N][(HL)2Fe6(dmf)2] (Fe6) was previously shown to possess a thermally isolated spin S = 19/2 ground state and found to exhibit slow magnetization relaxation below a blocking temperature of ∼5 K [J. Am. Chem. Soc. 2015, 137, 13949-13956]. Here, we present a comprehensive spectroscopic investigation of this unique single-molecule magnet (SMM), combining ultrawideband field-swept high-field electron paramagnetic resonance (EPR) with frequency-domain Fourier-transform terahertz EPR to accurately quantify the spin Hamiltonian parameters of Fe6. Of particular importance is the near absence of a 4th-order axial zero-field splitting term, which is known to arise because of quantum mechanical mixing of spin states on account of the relatively weak spin-spin (superexchange) interactions in traditional polynuclear SMMs such as the celebrated Mn12-acetate. The combined high-resolution measurements on both powder samples and an oriented single crystal provide a quantitative measure of the isolated nature of the spin ground state in the Fe6 molecule, as well as additional microscopic insights into factors that govern the quantum tunneling of its magnetization. This work suggests strategies for improving the performance of polynuclear SMMs featuring direct metal-metal bonds and strong ferromagnetic spin-spin (exchange) interactions.

Charge Transfer and Spin Dynamics in a Zinc Porphyrin Donor Covalently Linked to One or Two Naphthalenediimide Acceptors.

Quantum coherence effects on charge transfer and spin dynamics in a system having two degenerate electron acceptors are studied using a zinc 5,10,15-tri(n-pentyl)-20-phenylporphyrin (ZnP) electron donor covalently linked to either one or two naphthalene-1,8:4,5-bis(dicarboximide) (NDI) electron acceptors using an anthracene (An) spacer, ZnP-An-NDI (1) and ZnP-An-NDI2 (2), respectively. Following photoexcitation of 1 and 2 in toluene at 295 K, femtosecond transient absorption spectroscopy shows that the electron transfer (ET) rate constant for 2 is about three times larger than that of 1, which can be accounted for by the statistical nature of incoherent ET as well as the electron couplings for the charge separation reactions. In contrast, the rate constant for charge recombination (CR) of 1 is about 25% faster than that of 2. Using femtosecond transient infrared spectroscopy and theoretical analysis, we find that the electron on NDI2•- in 2 localizes onto one of the two NDIs prior to CR, thus precluding electronically coherent CR from NDI2•-. Conversely, CR in both 1 and 2 is spin coherent as indicated by the observation of a resonance in the 3*ZnP yield following CR as a function of applied magnetic field, giving spin-spin exchange interaction energies of 2J = 210 and 236 mT, respectively, where the line width of the resonance for 2 is greater than 1. These data show that while CR is a spin-coherent process, incoherent hopping of the electron between the two NDIs in 2, consistent with the lack of delocalization noted above, results in greater spin decoherence in 2 relative to 1.

Engineering spin polarization in a driven multistranded magnetic quantum network

Possible engineering of spin polarization through a multistranded magnetic quantum network in the presence of light irradiation is reported. Each site of the network is associated with a net magnetic moment which provides a spin-dependent scattering via spin-spin exchange interaction, yielding a finite spin polarization across the network. The quantum system is described within a tight-binding framework where the irradiation effect is incorporated through Floquet-Bloch prescription. The spin-polarization coefficient, measured by determining spin-dependent transmission probabilities following the Green's function formalism, shows several atypical features with irradiation parameters. Apart from getting a high degree of spin polarization, a complete phase reversal can be achieved. These two issues are extremely important in designing efficient spin-based electronic devices. A detailed mathematical description is given to decouple the Hamiltonian of a generalized multistranded magnetic ladder into distinct one-dimensional lattices in separate spin subspaces for any arbitrary orientation of local spin, which clearly illustrates the allowed energy spin subbands. Effects of system size and uncorrelated disorder on spin polarization are critically examined. Finally, we discuss the possible experimental realization of our magnetic quantum systemto regulate the degree and phase of spin polarization by irradiating a magnetic sample, and thus we believe that the present analysis may provide a route of engineering spin-dependent electron transfer through a spin-polarized device.

Magnetic Properties of π-Conjugated Hybrid Phenoxyl-Nitroxide Radicals with Extended π-Spin Delocalization.

A series of stable and genuinely organic open-shell systems, π-conjugated phenoxyl-nitroxide free radicals (hybrid phenoxyl-nitroxide radicals), have been synthesized and their magnetic properties in the crystalline state investigated, revealing their usefulness as new building blocks for molecular magnetic materials. The salient electronic structure of the hybrid phenoxyl-nitroxide radicals is extended π-spin delocalization from the nitroxide moiety, mediating the localization effect intrinsic to nitroxide radicals. Five representative hybrid radicals containing an aliphatic, aromatic, and heteroaromatic substituent in the side part of the compact hybrid radical centers were synthesized, and their molecular/crystal structures in the crystalline state were determined by X-ray diffraction analyses. CW X-band ESR, 1H-ENDOR spectroscopy, and DFT calculations for the hybrid radicals confirmed that an unpaired spin delocalizes over the whole molecular frame including the nonconjugated fragments, suggesting the possibility of tuning their electronic properties through substituent effects in the crystalline state. Significant influence of the phenoxyl moiety on the electronic structure was analyzed in terms of the g-tensor calculations. The SQUID magnetization measurements revealed that the nitroxides bearing alkyl or aromatic substituents behave as 3D Curie-Weiss paramagnets with weak antiferromagnetic (AFM) (Θ = -1 to -2.6 K) or ferromagnetic (FM) (Θ = +0.33 K) spin-spin exchange interactions. On the other hand, heteroaromatically substituted hybrid phenoxyl-nitroxide showed significant AFM interactions with J/kB = -25.6 K. The analysis of the bulk magnetic properties based on the crystallographic data and DFT calculations revealed competition between the intermolecular AFM and FM interactions which originate from the C-O(phenoxyl)···Me(nitroxide) or (N)O-C(arom) infinite 1D head-to-tail chains and the C(arom)-C(arom) head-over-tail dimers forming 3D networks in their crystal lattices.

Manipulation of the RKKY exchange by voltages

In the last years, electric fields have been used to control the magnetic exchange interactions and anisotropies in nanometric devices. In this paper, we study the spin-spin exchange interaction between two magnetic impurities embedded in a three-dimensional nonrelativistic electron gas, namely the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. The gas is confined in an insulating structure, and an applied voltage produces local changes in the electron density, which modulates the Fermi level of the system. Using a simple model, we demonstrate that this voltage modifies the strength and wavelength of the coupling between the impurities. Depending on the voltage, the effective RKKY exchange can change from a ferro- to an antiferromagnetic coupling, and vice versa. The spin-spin coupling can also be switched on and off by the voltage.

Low working temperature near liquid helium boiling point of RNiAl2 (R = Tm, Tb and Gd) compounds with large magnetocaloric effect

A polycrystalline TmNiAl2 compound with transition temperature near the liquid helium boiling point was successfully synthesized. Magnetic measurements show that FM (ferromagnetic) to FM and FM to PM (paramagnetic) transitions take place at 2.4 K and 4.0 K, respectively. Magnetic entropy change (–ΔSM) is calculated, and its maximal value [(–ΔSM)max] reaches as high as 20.7 J/kg K with the field change of 0–5 T. The low transition temperature together with large (–ΔSM)max at the liquid helium temperature zone originates from the weak spin-spin exchange interaction between Tm atoms. For a clear comparison, the magnetocaloric effect (MCE) of polycrystalline RNiAl2 (R = Tb and Gd) samples was also prepared and investigated. For the TbNiAl2 compound, FIM (ferrimagnetic) to AFM (antiferromagnetic) and AFM to PM transitions occur at 11.0 K and 21.5 K, respectively. As for the GdNiAl2 compound, an FM to PM transition occurs at 30.5 K. The value of (–ΔSM)max under the field change of 0–5 T is calculated to be 11.8 J/kg K and 17.3 J/kg K for TbNiAl2 and GdNiAl2, respectively. Compared with other RNiAl2-series MCE materials, TmNiAl2 exhibits the lowest working temperature and relatively larger (–ΔSM)max. The large MCE at low temperature indicates that TmNiAl2 is competitive among the MCE materials working at a liquid helium temperature zone.

Tuning the orbital-lattice fluctuations in the mixed spin-dimer system Ba3−xSrxCr2O8

In $A_{3}$Cr$_{2}$O$_{8}$, where $A$ = Sr or Ba, the Cr$^{5+}$ ions surrounded by oxygen ions in a tetrahedral coordination are Jahn-Teller active. The Jahn-Teller distortion leads to a structural transition and a related emergence of three twinned monoclinic domains below the structural phase transition. This transition is highly dynamic over an extended temperature range for $A$ = Sr. We have investigated mixed compounds Ba$_{3-x}$Sr$_{x}$Cr$_{2}$O$_{8}$ with $x=2.9$ and $x=2.8$ by means of X-ray and neutron diffraction, Raman scattering and calorimetry. Based on the obtained evolution of the phonon frequencies, we find a distinct suppression of the orbital-lattice fluctuation regime with increasing Ba content. This stands in contrast to the linear behaviour exhibited by unit cell volumes, atomic positions and intradimer spin-spin exchange interactions.

Emergence of antiferromagnetic quantum domain walls

Magnetic domain walls (DWs) are widely regarded as classical objects in physics community, even though the concepts of electron spins and spin-spin exchange interaction are quantum mechanical in nature. One intriguing question is whether DWs can survive at the quantum level and acquire the quantum properties such as entanglement. Here we show that spins within a DW are highly entangled in their quantum description. The total magnetization of a magnetic DW is nonzero, which is a manifestation of the global entanglement of the collective spin state. These results significantly deepen our understanding of magnetic DWs and enable the application of DWs in quantum information science. The essential physics can be generalized to skyrmions so that they can also play a role in quantum information processing.

On the optical and magnetic studies of YCrO3 perovskites

In this paper we studied the Electron Paramagnetic Resonance (EPR) and Optical spectroscopies of mono disperse yttrium chromite (YCrO3) nanoparticles prepared by a modified polymeric precursor method. The particles crystallized in roughly spherical shape with size ranging from 100 to 200 nm. Diffuse reflectance spectrum exhibited very well defined bands related to the 4A2 → 4T2, 4A2 → 4T1, 4A2 → 2T1, 4A2 → 2E transitions, characteristic of Cr3+ (3d3, 2D5/2) ions in nearly octahedral symmetry. The crystalline field Dq, Racah parameters (B and C) and Dq/B, could be determined and the results indicated that Cr3+ ions are in a strong ligand field site. The EPR spectra of YCrO3 shows a single broad resonance in the whole range of temperature studied, 100–300 K. Combining optical and EPR results the effective g-values of the paramagnetic Cr3+ were estimated to be 1.97 and 1.98, respectively, typical of Cr3+ in orthorhombic crystal field environment. The onset of the antiferro-paramagnetic transition, TÑ140 K, attributed to Cr3+-Cr3+ exchange interaction (J = −41.5 cm−1) was determinate by means of Magnetization and EPR measurements. The results were discussed in terms of the magnetic short-rang spin-spin exchange interaction between pairs of Cr3+.

Continuous wave electron paramagnetic resonance of nitroxide biradicals in fluid solution.

Nitroxide biradicals have been prepared with electron-electron spin-spin exchange interaction, J, ranging from weak to very strong. EPR spectra of these biradicals in fluid solution depend on the ratio of J to the nitrogen hyperfine coupling, AN, and the rates of interconversion between conformations with different values of J. For relatively rigid biradicals EPR spectra can be simulated as the superposition of AB splitting patterns arising from different combinations of nitrogen nuclear spin states. For more flexible biradicals spectra can be simulated with a Liouville representation of the dynamics that interconvert conformations with different values of J on the EPR timescale. Analysis of spectra, factors that impact J, and examples of applications to chemical and biophysical problems are discussed.