Electron Spin-spin Interaction Research Articles

Microscopic Origin of the Effective Spin-Spin Interaction in a Semiconductor Quantum Dot Ensemble

We present a microscopic model for a singly charged quantum dot (QD) ensemble to reveal the origin of the long-range effective interaction between the electron spins in the QDs. Wilson's numerical renormalization group (NRG) is used to calculate the magnitude and the spatial dependency of the effective spin-spin interaction mediated by the growth-induced wetting layer. Surprisingly, we found an antiferromagnetic Heisenberg coupling for very short inter-QD distances that is caused by the significant particle-hole asymmetry of the wetting layer band at very low filling. Using the NRG results obtained from realistic parameters as input for a semiclassical simulation for a large QD ensemble, we demonstrate that the experimentally reported phase shifts in the coherent spin dynamics between single- and two-color laser pumping can be reproduced by our model, solving a long-standing open problem of the microscopic origin of the inter-QD electron spin-spin interaction.

Monte Carlo study of the spin-spin interactions between radicals used for dynamic nuclear polarization

The spectrum of the electron spin-spin interactions largely determines which mechanism is responsible for the growth of the nuclear spin polarization in dynamic nuclear polarization (DNP). When electron spin-spin interactions are weak and their spectrum is narrow, the solid effect (SE) dominates the process. When they are stronger, the cross effect (CE) and thermal mixing (TM) come into play. Then a narrow spectrum favours the CE—that is an exchange of electron Zeeman energy with the nuclear spins—and a broad spectrum also TM—that is an exchange of electron spin-spin interaction energy with the nuclear spins. Moreover, the spectrum of the electron spin-spin interactions critically determines the rate of spectral diffusion of electron spin polarization across the electron spin resonance (ESR) line, and the associated conversion of electron Zeeman energy into electron spin-spin interaction energy. This way electron spin-spin interactions indirectly influence the DNP process. The present work describes Monte Carlo simulations of the spectrum of these interactions for approximately spherical radicals in glasses and analytical approximations of the simulation results. As an example application expressions for the relative strengths of the energy flows due to the CE and TM are derived.

Influence of applied electric and magnetic fields on a thermally-induced reentrance of a coupled spin-electron model on a decorated square lattice

The combination of an exact and Corner Transfer Matrix Renormalization Group (CTMRG) methods is used to study an influence of external electric and magnetic fields on existence of intriguing reentrant magnetic transitions in a coupled spin-electron model on a decorated square lattice. The two-dimensional (2D) decorated square lattice with localized nodal spins and delocalized electrons is taken into account. It was found that the competition among all involved interactions (the electron hopping, spin-spin and spin-electron interaction, external electric and magnetic fields) in combination with thermal fluctuations can produce new type of reentrant magnetic transitions. Depending on the model parameters the non-zero fields can stabilize or destabilize magnetic reentrance. In addition, an alternative and more effective way, for modulating the magnetic reentrance is found. An origin of intriguing low-temperature round maximum in the specific heat was explained as a consequence of rapid changes in the sublattice magnetizations, which is induced through a competition of all presented interactions.

Electronic spin-spin decoherence contribution in molecular qubits by quantum unitary spin dynamics

The realisation of quantum computers based on molecular electronic spins requires the design of qubits with very long coherence times, T2. Dephasing can proceed over several different microscopic pathways, active at the same time and in different regimes. This makes the rationalisation of the dephasing process not straightforward. Here we present a computational methodology able to address spin decoherence processes for a general ensemble of spins. The method consists in the propagation of the unitary quantum spin dynamics on a reduced Hilbert space. Then we study the dependence of spin dephasing over the magnetic dilution for a crystal of Vanadyl-based molecular qubits. Our results show the importance of long-range electronic spin-spin interactions and their effect on the shape of the spin-echo signal.

Electron Spin Resonance of P Donors in Isotopically PurifiedSiDetected by Contactless Photoconductivity

Coherence times of electron spins bound to phosphorus donors have been measured, using a standard Hahn echo technique, to be up to 20 ms in isotopically pure silicon with [P]$ = 10^{14}$ cm$^{-3}$ and at temperatures $\leq 4 $K. Although such times are exceptionally long for electron spins in the solid state, they are nevertheless limited by donor electron spin-spin interactions. Suppressing such interactions requires even lower donor concentrations, which lie below the detection limit for typical electron spin resonance (ESR) spectrometers. Here we describe an alternative method for phosphorus donor ESR detection, exploiting the spin-to-charge conversion provided by the optical donor bound exciton transition. We characterise the method and its dependence on laser power and use it to measure a coherence time of $T_2 = 130 $ms for one of the purest silicon samples grown to-date ([P]$ = 5\times 10^{11} $cm$^{-3}$). We then benchmark this result using an alternative application of the donor bound exciton transition: optically polarising the donor spins before using conventional ESR detection at 1.7~K for a sample with [P]$ = 4\times10^{12} $cm$^{-3}$, and measuring in this case a $T_2$ of 350 ms.

Effect of electron spin-spin interaction on level crossings and spin flips in a spin-triplet system

We study level crossings and spin flips in a system consisting of a spin-1 (an electron spin triplet) coupled to a nuclear spin of arbitrary size $K$, in the presence of a uniform magnetic field and the electron spin-spin interaction within the triplet. Through an analytical diagonalization based on the $\mathrm{SU}(3)$ Lie algebra, we find that the electron spin-spin interaction not only removes the curious degeneracy which appears in the absence of the interaction, but also produces some level anticrossings (LACs) for strong interactions. The real-time dynamics of the system shows that periodic spin flips occur at the LACs for arbitrary $K$, which might provide an option for nuclear or electron spin polarization.

Determination of Radical-Radical Distances in Light-Active Proteins and Their Implication for Biological Magnetoreception.

Light-generated short-lived radial pairs have been suggested to play pivotal roles in cryptochromes and photolyases. Cryptochromes are very probably involved in magnetic compass sensing in migratory birds and the magnetic-field-dependent behavior of insects. We examined photo-generated transient states in the cryptochrome of Drosophila melanogaster and in the structurally related DNA-repair enzyme Escherichia coli DNA photolyase. Using pulsed EPR spectroscopy, the exchange and dipolar contributions to the electron spin-spin interaction were determined in a straightforward and direct way. With these parameters, radical-pair partners may be identified and the magnetoreceptor efficiency of cryptochromes can be evaluated. We present compelling evidence for an extended electron-transfer cascade in the Drosophila cryptochrome, and identified W394 as a key residue for flavin photoreduction and formation of a spin-correlated radical pair with a sufficient lifetime for high-sensitivity magnetic-field sensing.

(Invited) Time-Resolved EPR Study on Charge Dynamics of Electron-Hole Pairs in Lead Iodide Perovskite Thin Film

For developments of the light-energy conversion systems, it is essential to achieve efficient long-range charge-separations (CS). Light-induced electrons and holes generated by the electron-transfer (ET) reactions are thus required to escape from the electrostatic binding at the initial stage. In this respect, the lead iodide perovskite solar cells have attracted great attentions since they exhibit quite efficient energy conversion performances. This solar cell adopts the hybrid metal iodide perovskites as a photoactive layer with the general formulation of ABX3(CH3NH3PbI3), where A is the methylammonium (CH3NH3 +) cation, B is Pb2+ and M is I-. In the devise structure, this photoactive layer is sandwiched by the compact titanium oxide (TiO2) layer and by the hole-transporting layer to efficiently collect the photocarriers generated by the light irradiations of the perovskite, giving rise to ~20% power conversion efficiencies (PCE). Recent studies have clearly shown that the carrier mobilities of the electrons and the holes are significantly high in the CH3NH3PbI3 layer, resulting in micrometer ranges of the charge-diffusion lengths.[1] These efficient charge diffusions play a crucial role on the prevention of the unwanted geminate charge-recombination in the electron-hole pairs. However, little is known concerning the electronic interactions of the electron-hole pairs initially generated in the CH3NH3PbI3 layer. At lower temperature (T < 160 K), it is reported that the electron and the hole generated by the light irradiation exhibit a Wannier exciton character with a biding energy of ~30 meV.[2] Thus, low temperature investigations of the electronic characters in the electron-hole pairs are very important to understand the mechanisms of the efficient charge-conductions initiated by the primary electron-hole pairs in the CH3NH3PbI3 layer. Another importance is to understand electronic characters of the trapped-charges generated by the light irradiations of the photoactive layers. It is recently reported that the charge careers are trapped at the surface area of the crystalline CH3NH3PbI3 layer.[3] However, electronic orbital characteristics of the trapped charges are not well known in the devices. Time-resolved electron paramagnetic resonance (TREPR) and pulsed EPR methods have been powerful to obtain the electron spin-spin interactions including the spin-spin exchange coupling of the photoinduced CS states and thus have been utilized to characterize the interspin distances and the electronic couplings of the transient CS states.[4] Additionally, TREPR can detect the electron spin polarization (ESP) as an effect of the initial photochemical events. This phenomenon is referred to as the chemically induced dynamic electron polarization (CIDEP). According to the radical pair mechanism (RPM) CIDEP, the ESP is generated as a result of the coherent singlet-triplet interconversion in the presence of the exchange coupling (J) of the primary radical-pair states, producing the enhanced microwave absorption and the microwave emission in the separated radicals during the diffusive separations of the individual radicals in the radical-pair.[5] Since the ESP detected as the microwave absorption (A) or the emission (E) is thus sensitive to J, one is able to access the electronic interactions in the primary radical-pairs.[6] To unveil the above fundamental electronic characteristics for the light-induced charge conductions, we have observed low temperature TREPR spectra for the three layer thin films (Spiro-OMeTAD/CH3NH3PbI3/TiO2 on a glass substrate) composed of the organic hole transport layer (Spiro-OMeTAD), the CH3NH3PbI3 layer and the mesoporous TiO2 fabricated by spin coating method on the mesoporous TiO2 films. From the EPR spectra, we have characterized the spin density distributions of the trapped holes of the CH3NH3PbI3 layer at 110 K from the anisotropic hyperfine structures originating from the iodine atoms (I). Furthermore, from the ESP generated in the trapped hole, we estimated the J coupling in the primary electron-hole pairs which will give us an insight on the electronic coupling of the Wannier excitons. 1.﷒ Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Science 2013, 342, 344. 2.﷒ Yamada, Y.; Nakamura, T.; Endo, M.; Wakamiya, A.; Kanemitsu, Y. J. Am. Chem. Soc. 2014, 136, 11610-11613. 3.﷒ Tachikawa, T.; Karimata, I.; Kobori, Y. J. Phys. Chem. Lett. 2015, 6, 3195-3201. 4. Kobori, Y.; Miura, T. J. Phys. Chem. Lett. 2015, 6, 113-123. 5.﷒ Kobori, Y.; Yago, T.; Akiyama, K.; Tero-Kubota, S.; Sato, H.; Hirata, F.; Norris, J. R. J. Phys. Chem. B 2004, 108, 10226-10240. 6. Kobori, Y.; Yamauchi, S.; Akiyama, K.; Tero-Kubota, S.; Imahori, H.; Fukuzumi, S.; Norris, J. R. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10017-10022.

Ordering and criticality in an underscreened Kondo chain

Motivated by the nickel valence controversy in the perovskite nickelate RNiO_3, we consider a one-dimensional underscreened Kondo chain consisting of alternating spin-1 ("nickel") and electron ("oxygen") sites, which in addition to the usual electron hopping and spin-spin interaction between the S=1 spin and the electron also contains a spin mediated electron hopping term. Using the density-matrix renormalization group (DMRG), we obtained the zero temperature phase diagram of the model, as well as various correlation functions in each phase. Importantly, for a certain range of parameters the model exhibits a quasi-long-range spiral (QS) order. To understand the DMRG results, we construct a mean-field theory based on a Schwinger fermion decomposition of the S=1 spins, from which we argue that the QS phase corresponds to a phase in proximity to the spin Bose metal state proposed by D. N. Sheng, O. I. Motrunich, and M. P. A. Fisher [Phys. Rev. B 79, 205112 (2009)]. Notably, we find no evidence for a phase with the symmetry of "nickel"-centered charge order, which has been argued to arise due to site-selective Kondo screening of half the S=1 spins, and suggest that order of this type occurs only due to an additional energy gain from spontaneous lattice distortions, not present in this model.

Spin-spin and spin-orbit interactions in nanographene fragments: A quantum chemistry approach

The relativistic behavior of graphene structures, starting from the fundamental building blocks--the poly-aromatic hydrocarbons (PAHs) along with other PAH nanographenes--is studied to quantify any associated intrinsic magnetism in the triplet (T) state and subsequently in the ground singlet (S) state with account of possible S-T mixture induced by spin-orbit coupling (SOC). We employ a first principle quantum chemical-based approach and density functional theory (DFT) for a systematic treatment of the spin-Hamiltonian by considering both the spin-orbit and spin-spin interactions as dependent on different numbers of benzene rings. We assess these relativistic spin-coupling phenomena in terms of splitting parameters which cause magnetic anisotropy in absence of external perturbations. Possible routes for changes in the couplings in terms of doping and defects are also simulated and discussed. Accounting for the artificial character of the broken-symmetry solutions for strong spin polarization of the so-called "singlet open-shell" ground state in zigzag graphene nanoribbons predicted by spin-unrestricted DFT approaches, we interpolate results from more sophisticated methods for the S-T gaps and spin-orbit coupling (SOC) integrals and find that these spin interactions become weak as function of size and increasing decoupling of electrons at the edges. This leads to reduced electron spin-spin interaction and hence almost negligible intrinsic magnetism in the carbon-based PAHs and carbon nanographene fragments. Our results are in agreement with the fact that direct experimental evidence of edge magnetism in pristine graphene has been reported so far. We support the notion that magnetism in graphene only can be ascribed to structural defects or impurities.

Size-dependent magnetic ordering and spin dynamics in DyPO4and GdPO4nanoparticles

Low-temperature magnetic susceptibility and heat-capacity measurements on nanoparticles (d? 2.6 nm) of the antiferromagnetic compounds DyPO4 (TN=3.4 K) and GdPO4 (TN=0.77 K) provide clear demonstrations of finite-size effects, which limit the divergence of the magnetic correlation lengths, thereby suppressing the bulk long-range magnetic ordering transitions. Instead, the incomplete antiferromagnetic order inside the particles leads to the formation of net magnetic moments on the particles. For the nanoparticles of Ising-type DyPO4 superparamagnetic blocking is found in the ac susceptibility at ?1 K, those of the XY-type GdPO4 analog show a dipolar spin-glass transition at ?0.2 K. Monte Carlo simulations for the magnetic heat capacities of both bulk and nanoparticle samples are in agreement with the experimental data. Strong size effects are also apparent in the Dy3+ and Gd3+ spin dynamics, which were studied by zero-field muon spin rotation (?SR) and high-field 31P-nuclear magnetic resonance (31P-NMR) nuclear relaxation measurements. The freezing transitions observed in the ac susceptibility of the nanoparticles also appear as peaks in the temperature dependence of the zero-field ?SR rates, but at slightly higher temperatures, as to be expected from the higher frequency of the muon probe. For both bulk and nanoparticles of GdPO4, the muon and 31P-NMR rates are for T?5 K dominated by exchange-narrowed hyperfine broadening arising from the electron spin-spin interactions inside the particles. The dipolar hyperfine interactions acting on the muons and the 31P are, however, much reduced in the nanoparticles. For the DyPO4 analogs the high-temperature rates appear to be fully determined by electron spin-lattice relaxation processes.

Effect of electron spin-spin exchange interaction on spin precession in coupled quantum well

Electron spin-spin interaction in an asymmetric coupled quantum well (CQW) was investigated through electron spin-precession measurements. Precession (Larmor) frequencies from electrons localized in two CQWs were measured by means of polarization- and time-resolved photoluminescence measurements under a high magnetic field. At a low excitation power density, the Larmor frequency of the CQW was same as that of a single quantum well. The Larmor frequency of electron spin in one well was shifted to that in the other well as the excitation power density was increased. These experimental results are quantitatively explained by an exchange interaction between electrons localized in the two wells.

On the role of correlation energy produced by the spin-spin interaction in the formation of binding energy for two-electron atomic systems

An algorithm is proposed for computing the correlation energy produced by the spin-spin interaction VSS of electrons in He-like atomic systems. The algorithm takes into account the singular nature of VSS and the formation of a compact finite motion of electrons in the range of distances Ze2/mc2 < r12 < ħ/mc between the electrons under the action of the magnetic fields of spins for a singlet ground state. Good agreement with experimental values of the ionization potential is attained for a wide set of two-electron atomic systems without resorting to variational procedures, but only using hydrogen-like wavefunctions and correctly taking into account the singular nature of the spin-spin interaction of electrons.

Analysis of Electron Spin-Spin Interaction in Coupled Quantum Well

Electron spin-spin interaction in asymmetric coupled GaAs/AlGaAs quantum well (QW) is investigated through electron spin precession measurements. In the experiment, the precession (Larmor) frequency of electrons localized in thin and thick QWs are measured by means of polarization- and time-resolved photoluminescence measurements under magnetic field. The Larmor frequency observed in thin well was transformed to that of thick well with increasing excitation power density. This dependence can be reproduced by theoretical calculations, which take into account electron spin-spin interactions in GaAs wells.

Structure and dynamics of a DNA-based model system for the study of electron spin–spin interactions

We report on the structure and dynamics of a model system for measuring long-range distances in biological macromolecules by saturation-recovery EPR. Four DNA duplexes that incorporate a paramagnetic dysprosium ion (Dy(III)) and a nitroxide spin-label were examined by electron paramagnetic resonance (EPR), circular dichroism (CD), and ultra-violet absorbance (UV) spectroscopy. Dy(III) is chelated by the modified base deoxythymidine-EDTA, (dT-EDTA). Electron spin-spin interactions between the Dy(III) ion and the nitroxide radical are observed at distances as great as approximately 5.3 nm. A slight change in the conformation of those nucleotides lying between the EDTA(Dy(III)) complex and the nitroxide spin-label results in a "stiffening" of the DNA helix on the EPR time scale. Changes in conformation and helix dynamics are due to the binding of the EDTA(Dy(III)) complex to the phosphodiester backbone of the complementary strand. Molecular mechanics calculations indicate that binding occurs in the 5' direction on the complementary strand, at a position 3 or 4 phosphates distant from the dT-EDTA(Dy(III))*dA base pair.

Long-Range Electron Spin-Spin Interactions from Unparticle Exchange

Unparticles as suggested by Georgi are identities that are not constrained by dispersion relations but are governed by their scaling dimension d. Their coupling to particles can result in macroscopic interactions between matter that are generally an inverse nonintegral power of distance. This is totally different from known macroscopic forces. We use the precisely measured long-ranged spin-spin interaction of electrons to constrain unparticle couplings to the electron. For 1<d<1.5 the axial vector unparticle coupling is excluded, and for 1<d<1.3 the pseudoscalar and vector couplings are also ruled out. These bounds and the ones for other ranges of d exceed or are complementary to those obtained previously from exotic positronium decays.

Measuring distances in proteins by saturation-recovery EPR

We describe a protocol for detecting electron spin-spin interactions between a radical and a metal ion in a protein or protein complex by saturation-recovery electron paramagnetic resonance (EPR). This protocol can be used with a protein containing an endogenous metal center and either an endogenous or synthetic radical species. We suggest a two-step approach whereby dipole-dipole or exchange interactions are first detected by continuous-wave EPR experiments and then quantified by saturation-recovery EPR. The latter measurements make it possible to measure long distances to within a few Angstroms. The protocol for making distance measurements by saturation-recovery EPR will take approximately 6 days to complete.

Two modes of ligand binding in maltose-binding protein of Escherichia coli. Electron paramagnetic resonance study of ligand-induced global conformational changes by site-directed spin labeling.

Binding of ligands to the maltose-binding protein (MBP) of Escherichia coli often causes a global conformational change involving the closure of its two lobes. We have introduced a cysteine residue onto each of these lobes by site-directed mutagenesis and modified these residues with spin labels. Using EPR spectroscopy, we examined the changes, caused by the ligand binding, in distance between the two spin labels, hence between the two lobes. The binding of both maltose and maltotetraose induced a considerable closure of the N- and C-terminal lobes of MBP. Little closure occurred upon the binding of maltotetraitol or beta-cyclodextrin. Previous study by fluorescence and UV differential absorbance spectroscopy (Hall, J. A., Gehring, K., and Nikaido, H. (1997) J. Biol. Chem. 272, 17605-17609) showed that maltose and a large portion of maltotetraose bound to MBP via one mode (R mode or "end-on" mode), which is physiologically active and leads to the subsequent transport of the ligands across the cytoplasmic membrane. In contrast, maltotetraitol and beta-cyclodextrin bound to MBP via a different mode (B mode or "middle" mode), which is physiologically inactive. The present work suggests that the B mode is nonproductive because ligands binding in this manner prevent the closure of the two domains of MBP, and, as a result, the resulting ligand-MBP complex is incapable of interacting properly with the inner membrane-associated transporter complex.

Time-resolved infrared diode laser spectroscopy of the ν1 band of the iron carbonyl radical (FeCO) produced by the ultraviolet photolysis of Fe(CO)5

The infrared spectrum of the iron carbonyl radical FeCO generated by the 193 nm excimer laser photolysis of iron pentacarbonyl Fe(CO)5 was observed by time-resolved diode laser spectroscopy. The 85 lines, mostly observed as triplets split by the electron spin-spin interaction, were assigned to the ν1 (CO stretch) band of FeCO. The electronic ground state of FeCO was confirmed experimentally to have 3Σ− symmetry. Molecular constants in the ground and ν1 vibrational states were derived from an analysis of the infrared spectrum combined with pure rotational lines in the lowest spin component Ω=0 observed by Fourier transform microwave (FTMW) spectroscopy. The rotational and centrifugal distortion constants in the ground state were determined as B0=4364.266(55) MHz and D0=1.378(25) kHz, where the figures in parentheses are standard errors to be attached to the last digit. The spin-spin and spin-rotation coupling constants are λ0=663.1(40) GHz and γ0=−974(27) MHz, respectively. The origin of the ν1 vibrational band determined is 1946.470 60(12) cm−1, which is consistent with the value derived from photodetachment spectroscopy of the FeCO− anion. The collisional quenching rate of FeCO by Fe(CO)5 was measured to be 1.13(4)×10−10 cm3 molec −1 s−1 by a kinetic study.

Magnetooptical method of measuring the characteristics of the reservoir of spin-spin interactions in paramagnets

Based on the phenomenon of coherent spin Raman scattering a magnetooptical method of measuring the characteristics of the reservoir of electronic spin-spin interactions in paramagnets was implemented. A formula relating the intensity of the inelastically scattered light to the temperature of the spin-spin reservoirs under conditions of saturation of the EPR of paramagnetic ions has been derived. For the Ce3+ ions in the (CexLa1−x)2Mg3(NO3)12·24H2O crystal values of the time of spin-lattice relaxation of the spin-spin reservoir and the local magnetic field have been measured, and a relation between the spin-spin reservoir temperature and the external magnetic field strength under conditions of stationary saturation of the EPR of the Ce3+ ions has been obtained.