Spin Exchange Mechanism Research Articles

Effect of magnetic scattering on the superfluid transition of He3 in nematic aerogel

We present the results of high magnetic field experiments in pure $^{3}\mathrm{He}$ (in the absence of $^{4}\mathrm{He}$ coverage) in nematic aerogel. In this case the aerogel strands are covered with few atomic layers of solid paramagnetic $^{3}\mathrm{He}$, which enables the spin-exchange mechanism for $^{3}\mathrm{He}$ quasiparticles scattering. Our earlier NMR experiments showed that in low fields, instead of the polar phase, the $A$ phase is expected to emerge in nematic aerogel. We use a vibrating wire resonator with the sample of aerogel attached to it and measure temperature dependencies of resonance properties of the resonator at different magnetic fields. A superfluid transition temperature of $^{3}\mathrm{He}$ in aerogel, obtained from the experiments, increases nonlinearly in applied magnetic field. And this increase is suppressed compared with that for the bulk ${A}_{1}$ phase, which we attribute to an influence of the magnetic scattering channel, previously considered theoretically for the case of $^{3}\mathrm{He}$ confined in isotropic silica aerogel. However, we observe the essential quantitative mismatch with theoretical expectations.

Atomic Intercalation Induced Spin-Flip Transition in Bilayer CrI3.

The recent discovery of 2D magnets has induced various intriguing phenomena due to the modulated spin polarization by other degrees of freedoms such as phonons, interlayer stacking, and doping. The mechanism of the modulated spin-polarization, however, is not clear. In this work, we demonstrate theoretically and computationally that interlayer magnetic coupling of the CrI3 bilayer can be well controlled by intercalation and carrier doping. Interlayer atomic intercalation and carrier doping have been proven to induce an antiferromagnetic (AFM) to ferromagnetic (FM) phase transition in the spin-polarization of the CrI3 bilayer. Our results revealed that the AFM to FM transition induced by atom intercalation was a result of enhanced superexchange interaction between Cr atoms of neighboring layers. FM coupling induced by O intercalation mainly originates from the improved superexchange interaction mediated by Cr 3d-O 2p coupling. FM coupling induced by Li intercalation was found to be much stronger than that by O intercalation, which was attributed to the much stronger superexchange by electron doping than by hole doping. This comprehensive spin exchange mechanism was further confirmed by our results of the carrier doping effect on the interlayer magnetic coupling. Our work provides a deep understanding of the underlying spin exchange mechanism in 2D magnetic materials.

Spin-polarization dependence of the Rb-Xe spin-exchange optical pumping process

We experimentally study the Rb-Xe spin-exchange optical pumping process. The dependence of the spin-exchange rate on the intensity of the pumping light is measured at different temperatures under low magnetic fields. We demonstrate that the spin-exchange rate will decrease as the Rb spin polarization increases, which agrees well with the theoretical prediction. In our measurement, three spin exchange and spin relaxation mechanisms, namely, the spin exchange rates between Rb and Xe atoms caused by van der Waals molecule (${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{vdW}}$) and binary collision (${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{bin}}$) processes, and the spin relaxation rate due to the wall collision of Xe atoms (${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{w}}$), have comparable magnitudes. These rates are extracted separately from the measured data with different Rb spin polarization at various temperatures. Our work provides a comprehensive confirmation of the physical picture of the spin-exchange optical pumping process.

Polyradical PROXYL/TEMPO Conjugates Connected by Ester/Amide Bridges: Synthesis, Physicochemical Studies, and DFT Calculations.

A series of di-/trinitroxide esters and amides featuring PROXYL and/or TEMPO radicals connected with alicyclic bridges were prepared in 61-92 % yields and their properties were analysed by using multiple experimental techniques. The examination of EPR spectra of radicals in organic solvents augmented with DFT calculations brought valuable information on the conformational dynamics and spin exchange mechanisms. Cyclic voltammetry investigations revealed (quasi)reversible electrochemical behaviour of studied nitroxides with their half-wave potentials ranging from -51 to -17 mV. SQUID measurements of selected radicals revealed that the magnetism of di- and trinitroxides is significantly different, since antiferromagnetic coupling in biradicals is notably larger than in triradicals. The single-crystal X-ray analysis of selected biradicals revealed the existence of 3D supramolecular networks of molecules linked through hydrogen-bonding interactions. These polynitroxide radicals can serve as promising bridging or chelating ligands in the synthesis of transition-metal-based molecular magnets.

Generalised Ramachandran pairing interaction in helium-4 and helium-3 superfluids

We make use of the ionisation energy theory, Ramachandran interaction theory and the energy-level spacing renormalisation group technique to extend the Drude Hamiltonian to derive the Ramachandran pairing mechanism. This particular mechanism is exploited to explain the notorious discontinuous jumps in the specific heat data at critical points in both He-4 and He-3 superfluids. The well-known spin states (due to Balian–Werthamer and Anderson–Brinkman–Morel theories) and the Leggett’s spontaneously broken spin-orbit symmetry are shown to enhance Ramachandran attraction between two He-3 atoms without the need to invoke the spin-induced pairing or the phonon-mediated Cooper pairing mechanism in superconductors. In addition, we shall show physically that the spin-exchange mechanism can neither cause pairing between electrons nor between He-3 atoms.

Probing of mechanical behaviour, quantum mechanism of spin exchange and magnetism of SnV2O4 and SnCr2O4 spinel oxides by DFT

ABSTRACT The spinel oxides SnV2O4 and SnCr2O4 are analysed to reveal the mechanical, electronic and magnetic properties using Wien2k software. The negative formation energy and positive elastic constants shows the studied materials are thermodynamically and mechanically favorable. In ferromagnetic (FM) state more energy release than antiferromagnetic (AFM) state. The above room temperature magnetism is elaborated by spin polarisation, exchange mechanism, direct Δx(d) and indirect Δx(pd) exchange energy and crystal field energy Δx(CF). The shifting of magnetic moment form V/Cr sites to Sn/V sites illustrate the ferromagnetism is caused by pd hybridisation that affects the net orientation of electrons spin and hence, it increases the importance of the materials for spin-dependent electronic applications.

Spin density encodes intramolecular singlet exciton fission in pentacene dimers

The formation of two triplet excitons at the cost of one photon via singlet exciton fission in organic semiconductors can potentially enhance the photocurrent in photovoltaic devices. However, the role of spin density distribution in driving this photophysical process has been unclear until now. Here we present the significance of electronic spin density distribution in facilitating efficient intramolecular singlet exciton fission (iSEF) in π-bridged pentacene dimers. We synthetically modulate the spin density distribution in a series of pentacene dimers using phenyl-, thienyl- and selenyl- flanked diketopyrrolopyrrole (DPP) derivatives as π-bridges. Using femtosecond transient absorption spectroscopy, we find that efficient iSEF is only observed for the phenyl-derivative in ~2.4 ps while absent in the other two dimers. Electronic structure calculations reveal that phenyl-DPP bridge localizes α- and β-spin densities on distinct terminal pentacenes. Upon photoexcitation, a spin exchange mechanism enables iSEF from a singlet state which has an innate triplet pair character.

Polyradical PROXYL/TEMPO-Derived Amides: Synthesis, Physicochemical Studies, DFT Calculations, and Antimicrobial Activity.

A series of polynitroxide amides possessing 2,2,5,5-tetramethyl-1-pyrrolidinyloxy (PROXYL) and/or 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) units connected through various bridges were synthesized and their properties were analyzed. EPR spectroscopy provided detailed insight into their paramagnetic character and related properties. A thorough examination of the EPR spectra of dinitroxides in organic solvents provided valuable information on the intramolecular motions, thermodynamics, and spin-exchange mechanisms. Analysis of low-temperature X- and Q-band EPR spectra of the dissolved dinitroxides provided spin-spin distances that were comparable with the theoretical values obtained by DFT. Cyclic voltammetry investigations revealed (quasi)reversible electrochemical behavior for PROXYL-derived biradicals, whereas significant loss of the reversibility was found for TEMPO-containing bi- and polyradicals. The inhibitory activities of the nitroxides against model bacteria, yeasts, and filamentous fungi were assessed.

Interatomic Spin Coupling in Manganese Clusters Registered on Graphene.

Different interatomic spin interactions in graphene-regulated Mn atomic clusters are investigated by low-temperature scanning tunneling microscopy and magnetic-field-dependent inelastic spin excitation spectroscopy. All dimers observed exhibit an antiferromagnetic (AFM) singlet ground state and spin transition from the singlet to triplet states, but their AFM coupling strength shows a unique dependence on their site registration on the graphene. Intriguing spin coupling can be found in the graphene-mediated Mn trimers, which manifest multilevel spin excitations. In combination with Heisenberg spin modeling and first-principles numerical simulation, an exclusive noncollinear spin configuration of the Mn trimer regulated by the graphene template can be determined, and our observed experimental exchange energies cannot be understood by a direct spin exchange mechanism, but suggest a nonlocal Ruderman-Kittel-Kasuya-Yosida indirect spin exchange mechanism through substrate modulation, which has not yet been achieved in graphene so far.

Suppression of spin-exchange relaxation in tilted magnetic fields within the geophysical range

We present a detailed experimental and theoretical study on the relaxation of spin coherence due to the spin-exchange mechanism arising in the electronic ground states of alkali-metal vapor atoms. As opposed to the well-explored formation of a stretched state in a longitudinal geometry (magnetic field parallel to the laser propagation direction) we employ adapted hyperfine-selective optical pumping in order to suppress spin-exchange relaxation. By comparing measurements of the intrinsic relaxation rate of the spin coherence in the ground state of cesium atoms with detailed density-matrix simulations we show that the relaxation due to spin-exchange collisions can be reduced substantially even in a tilted magnetic field of geomagnetic strength, the major application case of scalar magnetic surveying. This explains the observed striking improvement in sensitivity and further deepens the understanding of the light-narrowed ${M}_{x}$ magnetometer, which was presented recently. Additionally, new avenues for investigating the dynamics in alkali-metal atoms governed by the spin-exchange interaction and interacting with arbitrary external fields open up.

Correlation between charge transfer and exchange coupling in carbon-based magnetic materials

Several forms of carbon-based magnetic materials, i.e. single radicals, radical dimers, and alternating stacks of radicals and diamagnetic molecules, have been investigated using density-functional theory with dispersion correction and full geometry optimization. Our calculated results demonstrate that the C31H15 (R4) radical has a spin of ½. However, in its [R4]2 dimer structure, the net spin becomes zero due to antiferromagnetic spin-exchange between radicals. To avoid antiferromagnetic spin-exchange of identical face-to-face radicals, eight alternating stacks, R4/D2m/R4 (with m = 3-10), were designed. Our calculated results show that charge transfer (Δn) between R4 radicals and the diamagnetic molecule D2m occurs with a mechanism of spin exchange (J) in stacks. The more electrons that transfer from R4 to D2m, the stronger the ferromagnetic spin-exchange in stacks. In addition, our calculated results show that Δn can be tailored by adjusting the electron affinity (Ea) of D2m. The correlation between Δn, Ea, m, and J is discussed. These results give some hints for the design of new ferromagnetic carbon-based materials.

Doping-induced spin-manipulation in complex trimer system Ca3Cu3(PO4)4: A density functional investigation

Doping induced spin-manipulation with magnetic (Ni) and non-magnetic (Mg) dopants constitutes the experimental attempts to obtain a singlet ground state system from the linear chain Heisenberg antiferromagnetic Cu-based d9 spin-1/2 trimer compound Ca3Cu3(PO4)4 with doublet ground state. The present study is a density-functional investigation of the effects of such doping on the spin-exchange mechanism and electronic structure of the parent compound. Site-selective doping with zero-spin dopants like Mg is proved to be more efficient than an integral spin dopant Ni in obtaining a spin-gap system with singlet ground state, as also observed in the experimental studies. Doping induced dimerized state is found to be the lowest in ground-state energy. Calculated spin exchange couplings along various possible pathways are observed to attain good agreement with earlier experimental results with suitable optimization of Coulomb repulsion (U) and exchange (J) parameters.

Antioxidative properties of nitroxyl radicals and hydroxyamines in reactions with triplet and deaminated kynurenine

The reactions of triplet kynurenine and 4-(2-aminophenyl)-4-oxocrotonic acid, formed upon the thermal decomposition of kynurenine, with nitroxyl radicals and cyclic N-hydroxylamines were studied. Nitroxyl radicals were found to quench efficiently the triplet state of kynurenine (rate constant 3–6·.108 L mol-1 s-1). The quenching proceeds via the spin-exchange mechanism and affords no new products. Neither nitroxyl radicals, nor hydroxyl-lamines react with 4-(2-aminophenyl)-4-oxocrotonic acid under conditions similar to physiological.

Coexistence of spin polarization and pairing correlations in metallic grains

We investigate the competition between magnetic depairing interactions, due to spin-exchange mechanism and∕or to spin-dependent asymmetric bandwidths, and pairing coupling in metallic grains. We present a detailed analysis of the quantum ground state in different regimes arising from the interplay between ferromagnetic and pairing correlations for different fillings. We find out that the occurrence of a ground state with coexisting spin-polarization and pairing correlations is enhanced when the asymmetric spin-dependent distribution of the single-particle energies is considered. The mechanisms leading to such a stable quantum state are finally clarified.

Analysis of the magneto-structural correlations in the meso-tetraphenylporphyrinatomanganese(iii) tetracyanoethenide family of molecule-based magnets

The dependence of the nearest neighbor magnetic interaction, JAB, as a function of geometry for [Mn(porphyrin)][TCNE] materials has been evaluated by the results of CASSCF, CASPT2 and B3LYP calculations. The B3LYP results were validated for seven representative [Mn(porphyrin)][TCNE] molecule-based magnets and were found to be in good agreement with the experimental data for observed values of JAB/kB less than −150 K, and the CASPT2 method for JAB/kB greater than −150 K. Then, the JAB was systematically evaluated as a function of the intermolecular coordinates that define the position and orientation of [TCNE]˙− relative to [Mn(porphyrin)]+. The calculated JAB/kB (Ĥ = −2ΣJABŜAŜB) are antiferromagnetic in all regions, with values in the −112 to −457 K range. The most negative JAB values occur for the smallest plane angles between the [Mn(porphyrin)]+ and [TCNE]˙− planes. The B3LYP results were then compared to those obtained using the three spin exchange mechanisms (i.e. orthogonality, spin coupling, and CI mechanisms), and none of these mechanisms give trends in accord with the B3LYP results. Therefore, first principles methods, such as DFT or CASPT2, are needed for more detailed calculations.

Electrical Readout of the Local Nuclear Polarization in the Quantum Hall Effect: A Hyperfine Battery

It is demonstrated that the now well-established "flip-flop" mechanism of spin exchange between electrons and nuclei in the quantum Hall effect can be reversed. We use a sample geometry which utilizes separately contacted edge states to establish a local nuclear spin polarization--close to the maximum value achievable--by driving a current between electron states of different spin orientation. When the externally applied current is switched off, the sample exhibits an output voltage of up to a few tenths of a mV, which decays with a time constant typical for the nuclear spin relaxation. The surprising fact that a sample with a local nuclear spin polarization can act as a source of energy and that this energy is well above the nuclear Zeeman splitting is explained by a simple model which takes into account the effect of a local Overhauser shift on the edge state reconstruction.

Nuclear exchange coupling and electronic structure of low-dimensional semiconductors

We review the nuclear magnetic resonance (NMR) studies of the indirect nuclear exchange coupling and electronic structure of the chain and layered semiconductors Tl(I)M(III)X2 (M = Tl, Ga, In, X = Se, S, Te) and some other low-dimensional Tl-contained semiconducting compounds. Both univalent and trivalent Tl atoms in these compounds show essential chemical shielding anisotropy despite their formal spherically symmetric 5d106s2 and 5d10 electron configurations. Such a behavior results from the sp-hybridization of the Tl electron wave functions. Strong exchange coupling among the spins of Tl1+ and M3+ ions, which reside in neighboring chains or layers, is observed. Such coupling is realized due to the overlap of the Tl1+ and M3+ electron wave functions across the intervening chalcogen atom. This overlap is the important mechanism in the formation of the valence and conduction bands and determines the electronic structure and properties of the compounds. The long-range indirect nuclear exchange coupling via a chalcogen atom is an analog of the Kramers mechanism of electron spin exchange via a nonmagnetic bridge ion. Recent photoemission spectroscopy studies and band-structure calculations of several aforementioned compounds have confirmed the NMR results on the interchain and interlayer overlap.

Phase transformation in a lattice system in the presence of spin-exchange dynamics

A joint action of the Glauber single-spin-flip and the Kawasaki spin-exchange mechanisms upon the processes of phase transformation is examined in the framework of the one-dimensional kinetic Ising model. It is shown that the addition of the Kawasaki dynamics to that of Glauber accelerates the process of phase transformation in the initial stage, but slows it down in later stages. For the truncated form of Glauber dynamics, which excludes the processes of splitting and coagulation of clusters, the addition of the Kawasaki dynamics always accelerates the phase transformation process. Acting alone, the Kawasaki mechanism provides a cluster growth proportional to t(1/2) (where t is the time) in the initial stage and proportional to t(1/3) (Lifshitz-Slyozov-Wagner law) in the intermediate stage. In the final stage, a cluster size approaches exponentially its equilibrium value.

Optical and magneto-optical properties of Gd monochalcogenides

The electronic structure of Gd monochalcogenides are investigated theoretically using thefull potential linear muffin-tin orbital method. The on-site Coulomb interaction(U)is accounted for by the local spin-density approximation. These calculations corroboratethe fact that they are all metallic. Magnetism is driven by the spin exchangemechanism only. A deep minimum in the reflectivity spectrum occurs in GdS andGdSe. The largest Kerr rotation among the Gd monochalcogenides, of about−5°,is found at the shoulder followed by a rotation of about4° at the plasma resonance energy. Both the plasma resonance and the shoulder effectare equally responsible for the moderately large magneto-optical signals in Gdmonochalcogenides. A reasonably good agreement between theory and experiment isobtained.

Quantum computation and the production of entangled photons using coupled quantum dots

We review recent theoretical progress on the use of electron spins as qubits in coupled semiconductor quantum dots for quantum information processing. We discuss the spin exchange mechanism and its microscopic origin in both laterally and vertically tunnel-coupled quantum dots and explain how it can be used to implement the quantum XOR gate which, in combination with single spin rotations, allows to perform arbitrary quantum computations. In addition to their functionality as a quantum gate, coupled quantum dots can act as a source for photon pairs in entangled polarization states which are useful for quantum communication. We describe a mechanism for the production of such entangled photon pairs via a biexciton state in tunnel-coupled quantum dots.