Small G-factor Research Articles

Reduced spin damping in inverse spinel Mn2TiO4 by ordered occupancy of magnetic ions

We investigated the ferromagnetic resonance of inverse spinel Mn2TiO4, and found the distortion of oxygen-octahedron caused by ordered occupancy of magnetic ions can effectively reduce magnetic damping of magnetic insulator materials. High-quality cubic and tetragonal Mn2TiO4 single crystals with disordered and ordered occupancy of Mn2+ and Ti4+ at B sites were grown in pure Argon and the mixture of Hydrogen and Argon, respectively. Cubic and tetragonal Mn2TiO4 exhibit the same magnetic behaviors, but the ferromagnetic resonance linewidth of tetragonal Mn2TiO4 is much smaller than that of cubic Mn2TiO4. Moreover, the weaker spin–orbit coupling is, the smaller g-factor and then the narrower ferromagnetic resonance linewidth are in Mn2TiO4 systems, suggesting the decrease of spin damping in tetragonal Mn2TiO4 originates from the diminution of orbital magnetic moment after distortion of oxygen-octahedron.

Engineering of inorganic nanostructures with hierarchy of chiral geometries at multiple scales

Engineering of inorganic nanostructures with hierarchy of chiral geometries at multiple scales

Spin-orbit-coupling induced electron-spin polarization in magnetically and electrically confined semiconductor microstructure

We theoretically investigate spin polarization for electrons in magnetically and electrically confined semiconductor microstructure, which is constructed by patterning a ferromagnetic stripe and a Schottky-metal stripe on top and bottom of GaAs/AlxGa1-xAs heterostructure, respectively. Both Zeeman interaction and spin–orbit coupling are taken into account; however, electron-spin polarization comes mainly from the latter due to a small effective g-factor for GaAs. Besides, both magnitude and sign of spin polarization can be manipulated by changing interfacial confining electric field or strain engineering, resulting in a tunable electron-spin filter for spintronics device applications.

Electrically controlled spin polarization in suspended GaAs quantum point contacts

We report on the observation of the lateral electric spin polarization effect in a suspended GaAs-based quantum point contact (QPC) separated from the substrate. The effect manifests itself in the experiment as the appearance of an additional half-integer plateaus at 0.5×2e2/h when the asymmetric voltage is applied to the side gates in zero magnetic field. The appearance of the plateaus has been attributed to the spin degeneracy lifting caused by the spin-orbit coupling associated with the lateral electric field in the asymmetrically biased QPC. We have experimentally demonstrated that, despite the relatively small g-factor in GaAs, the observation of the spin polarization in the GaAs-based QPCs becomes possible after the suspension due to the enhancement of the electron-electron interaction and the effect of the electric field guiding. These features are caused by a partial confinement of the electric field lines within a suspended semiconductor layer with a high dielectric constant.

Spin-polarized Goos-Hänchen displacement in a hybrid magnetic-electric-barrier nanostructure modulated by spin-orbit couplings

Spin-polarized Goos-Hänchen (GH) displacement is calculated for electrons in a hybrid magnetic-electric-barrier (MEB) nanostructure modulated by spin-orbit couplings (SOCs), which can be realized experimentally by the deposition of a ferromagnetic (FM) stripe and a Schottky-metal (SM) stripe in parallel configuration on the top of the GaAs/AlxGa1-xAs heterostructure. Both Zeeman interaction (ZI) and SOC are taken into account. It is shown that ZI has less contribution to spin-polarized GH displacement than SOC due to a small g-factor for GaAs. It is also shown that spin-polarized GH displacement can be controlled by Rashba or Dresselhause SOC--interfacial confining electric field or strain engineering, which results hence in a tunable spatial spin splitter for spintronics device applications.

Effect of different carboxylic acids on the aromatic hydroxylation with H2O2 in the presence of an iron aminopyridine complex

In this contribution, the effect of the structure of the catalytic additive – carboxylic acid – on the catalytic performance of the iron based catalyst [(PDP)Fe(OTf)2], 2 (PDP = N,N′-bis(2-pyridylmethyl)-(S,S)-2,2′-bipyrrolidine) in the selective aromatic oxidation of alkylbenzenes with H2O2 is presented. Eight (linear and branched) carboxylic acids have been tested; in the presence of 2-ethylhexanoic acid, the system [(PDP)Fe(OTf)2]/RCOOH/H2O2 has demonstrated the highest substrate conversion and the highest selectivity for oxygen incorporation into the aromatic ring (up to > 99%) at the same time. Low-temperature EPR spectroscopic study of the system [(PDP)Fe(OTf)2]/2-ethylhexanoic acid/H2O2 witness the presence of the low-spin perferryl intermediate 2aEHA with small g-factor anisotropy (g1 = 2.069, g2 = 2.007, g3 = 1.963), which directly reacts with benzene at −80 °C with the rate constant k2 = 0.6 M−1s−1, and with toluene with k2 > 1 M−1s−1, thus giving evidence for its key role in the selective oxygenation of aromatic substrates.

Lateral-electric-field-induced spin polarization in a suspended GaAs quantum point contact

The conductance of a GaAs-based suspended quantum point contact (QPC) equipped with lateral side gates has been experimentally studied in the absence of the external magnetic field. The half-integer conductance plateau (0.5×2e2/h) has been observed when an asymmetric voltage between the side gates is applied. The appearance of this plateau has been attributed to the spin degeneracy lifting caused by the spin-orbit coupling associated with the lateral electric field in the asymmetrically biased QPC. We have experimentally demonstrated that, despite the relatively small g-factor in GaAs, the observation of the spin polarization in the GaAs-based QPC became possible after the suspension due to the enhancement of the electron-electron interaction and the effect of the electric field guiding. These features are caused by a partial confinement of the electric field lines within a suspended semiconductor layer with a high dielectric constant.

EPR-Determined Anisotropy of the g-Factor and Magnetostriction of a Cu2MnBO5 Single Crystal with a Ludwigite Structure

Electron paramagnetic resonance (EPR) and magnetostriction of the Cu2MnBO5 single crystal have been studied. The EPR spectrum consists of a single Lorentzian line due to the exchange-coupled system of spins of Cu2+ and Mn3+ ions. It has been established experimentally that the g-factor in the paramagnetic region is strongly anisotropic and anomalously small, which is not typical of the exchange-coupled system of spins of Cu2+ and Mn3+ ions. At a temperature of 150 K, the g-factors along the crystallographic a, b, and c axes are 2.04, 1.96, and 1.87, respectively. Such small effective g-factor values can be due to the effect of the anisotropic Dzyaloshinskii–Moriya exchange interaction between the spins of Cu2+ and Mn3+ ions directed along the a axis. The presence of two Cu2+ and Mn3+ Jahn–Teller ions occupying four nonequivalent positions in the crystal is responsible for the absence of the inversion center. It is found that the behavior of the magnetostriction of Cu2MnBO5 is not typical of transition-metal crystals but is closer to the behavior of crystals containing rare-earth ions.

Circular Dichroism of CdSe Nanocrystals Bound by Chiral Carboxylic Acids.

Chiral semiconductor nanocrystals, or quantum dots (QDs), are promising materials for applications in biological sensing, photonics, and spin-polarized devices. Many of these applications rely on large dissymmetry, or g-factors, the difference in absorbance between left- and right-handed circularly polarized light compared to the unpolarized absorbance. The majority of chiral QDs, specifically CdSe, reported to date have used thiolated amino acid ligands to introduce chirality onto the nanoparticles, but these systems have ultimately reported small g-factors of ∼2 × 10-4. In an effort to realize chiral CdSe QDs with higher g-factors and to expand the set of designer chiral semiconductor nanocrystals, we have employed chiral carboxylic acids as a distinct class of ligands for chiral CdSe nanoparticles. Through this family of chiral carboxylic acid ligands, we performed a direct comparison between carboxylate-bound and thiolate-bound chiral CdSe QDs. Spectral analysis revealed that the resulting circular dichroism shifts originate from the splitting of the exciton by the ligand-nanocrystal interaction. Subsequent examination of a series of chiral carboxylic acid ligands revealed a 30-fold range in g-factor through relatively small changes in the structure of the ligand. Finally, we showed that increasing the number of stereocenters on the ligand can further enhance the dissymmetry factors. This versatile and tunable combination of nanocrystals and ligands will inform future designs of chiral nanomaterials and their applications.

Topological charge-density and spin-density waves in InAs/GaSb quantum wells under an in-plane magnetic field

We study interaction effect of quantum spin Hall state in InAs/GaSb quantum wells under an in-plane magnetic field by using the self-consistent mean field theory. We construct a phase diagram as a function of intra-layer and inter-layer interactions, and identify two novel phases, a charge/spin density wave phase and an exciton condensate phase. The charge/spin density wave phase is topologically non-trivial with helical edge transport at the boundary, while the exciton condensate phase is topologically trivial. The Zeeman effect is strongly renormalized due to interaction in certain parameter regimes of the system, leading to a much smaller g-factor, which may stabilize the helical edge transport.

Dramatic Effect of Carboxylic Acid on the Electronic Structure of the Active Species in Fe(PDP)-Catalyzed Asymmetric Epoxidation

The electronic structure of the iron–oxygen intermediates responsible for catalytic transformations in the biomimetic catalyst systems [((S,S)-PDP)FeII(OTf)2]/H2O2/RCOOH has been found to be strongly dependent on the structure of the carboxylic acid RCOOH. For carboxylic acids with primary and secondary α-carbon atoms (acetic acid, butyric acid, caproic acid), the active species exhibit electron paramagnetic resonance (EPR) spectra with large g-factor anisotropy (g1 = 2.7, g2 = 2.4, g3 = 1.7), whereas for those with tertiary α-carbon atoms (2-ethylhexanoic acid, valproic acid, 2-ethylbutyric acid), the active species display EPR spectra with small g-factor anisotropy (g1 = 2.07, g2 = 2.01, g3 = 1.96). The EPR spectra of the latter intermediates are very similar to those of the intermediates previously assigned to oxoiron(V) species. The systems featuring intermediates of the second type ensure higher enantioselection in the epoxidation of electron-deficient olefins.

A 32-Channel Head Coil Array with Circularly Symmetric Geometry for Accelerated Human Brain Imaging

The goal of this study is to optimize a 32-channel head coil array for accelerated 3T human brain proton MRI using either a Cartesian or a radial k-space trajectory. Coils had curved trapezoidal shapes and were arranged in a circular symmetry (CS) geometry. Coils were optimally overlapped to reduce mutual inductance. Low-noise pre-amplifiers were used to further decouple between coils. The SNR and noise amplification in accelerated imaging were compared to results from a head coil array with a soccer-ball (SB) geometry. The maximal SNR in the CS array was about 120% (1070 vs. 892) and 62% (303 vs. 488) of the SB array at the periphery and the center of the FOV on a transverse plane, respectively. In one-dimensional 4-fold acceleration, the CS array has higher averaged SNR than the SB array across the whole FOV. Compared to the SB array, the CS array has a smaller g-factor at head periphery in all accelerated acquisitions. Reconstructed images using a radial k-space trajectory show that the CS array has a smaller error than the SB array in 2- to 5-fold accelerations.

Nodal bilayer-splitting controlled by spin-orbit interactions in underdoped high-Tc cuprates.

The highest superconducting transition temperatures in the cuprates are achieved in bilayer and trilayer systems, highlighting the importance of interlayer interactions for high Tc. It has been argued that interlayer hybridization vanishes along the nodal directions by way of a specific pattern of orbital overlap. Recent quantum oscillation measurements in bilayer cuprates have provided evidence for a residual bilayer-splitting at the nodes that is sufficiently small to enable magnetic breakdown tunneling at the nodes. Here we show that several key features of the experimental data can be understood in terms of weak spin-orbit interactions naturally present in bilayer systems, whose primary effect is to cause the magnetic breakdown to be accompanied by a spin flip. These features can now be understood to include the equidistant set of three quantum oscillation frequencies, the asymmetry of the quantum oscillation amplitudes in c-axis transport compared to ab-plane transport, and the anomalous magnetic field angle dependence of the amplitude of the side frequencies suggestive of small effective g-factors. We suggest that spin-orbit interactions in bilayer systems can further affect the structure of the nodal quasiparticle spectrum in the superconducting phase. PACS numbers: 71.45.Lr, 71.20.Ps, 71.18.+y

Quantum logic gates from time-dependent global magnetic field in a system with constant exchange

We propose a method that implements a universal set of one- and two-quantum-bit gates for quantum computation in a system of coupled electron pairs with constant non-diagonal exchange interaction. In our proposal, suppression of the exchange interaction is performed by the continual repetition of single-spin rotations. A small g-factor difference between the electrons allows for addressing qubits and avoiding strong magnetic field pulses. Numerical experiments were performed to show that, to implement the one- and two-qubit operations, it is sufficient to change the strength of the magnetic field by a few Gauss. This introduces one and then the other electron in a resonance. To determine the evolution of the two-qubit system, we use the algorithms of optimal control theory.

Control of Majorana edge modes by a g-factor engineered nanowire spin transistor

We propose the manipulation of Majorana edge states via hybridization and spin currents in a nanowire spin transistor. The spin transistor is based on a heterostructure nanowire comprising of semiconductors with large and small g-factors that form the topological and non-topological regions respectively. The hybridization of bound edge states results in spin currents and 4π-periodic torques, as a function of the relative magnetic field angle – an effect which is dual to the fractional Josephson effect. We establish relation between torques and spin-currents in the non-topological region where the magnetic field is almost zero and spin is conserved along the spin–orbit field direction. The angular momentum transfer could be detected by sensitive magnetic resonance force microscopy techniques.

NIR to visible up-conversion, infrared luminescence, thermoluminescence and defect centres in Y2O3 : Er phosphor

Er3+ doped Y2O3 phosphor was prepared by the solution combustion method and characterized using powder x-ray diffraction and energy-dispersive analysis of x-ray mapping studies. Room temperature near infrared (NIR) to green up-conversion (UC) emissions in the region 520–580 nm {(2H11/2, 4S3/2) →4I15/2} and red UC emissions in the region 650–700 nm (4F9/2 →4I15/2) of Er3+ ions have been observed upon direct excitation to the 4I11/2 level using ∼972 nm laser radiation of nanosecond pulses. The possible mechanisms for the UC processes have been discussed on the basis of the energy level scheme, the pump power dependence as well as based on the temporal evolution. The excited state absorption is observed to be the dominant mechanism for the UC process. Y2O3 : Er exhibits one thermally stimulated luminescence (TSL) peak around 367 °C. Electron spin resonance (ESR) studies were carried out to study the defect centres induced in the phosphor by gamma irradiation and also to identify the centres responsible for the TSL peak. Room temperature ESR spectrum of irradiated phosphor appears to be a superposition of at least three distinct centres. One of them (centre I) with principal g-values g|| = 2.0415 and g⊥ = 2.0056 is identified as centre while centre II with an isotropic g-factor 2.0096 is assigned to an F+-centre (singly ionized oxygen vacancy). Centre III is also assigned to an F+-centre with a small g-factor anisotropy (g|| = 1.974 and g⊥ = 1.967). Additional defect centres are observed during thermal annealing experiments and one of them appearing around 330 °C grows with the annealing temperature. This centre (assigned to an F+-centre) seems to originate from an F-centre (oxygen vacancy with two electrons) and the F-centre appears to correlate with the observed TSL peak in Y2O3 : Er phosphor. The trap depth for this peak has been determined to be 0.97 eV from TSL data.

SENSE imaging with a quadrature half‐volume transverse electromagnetic (TEM) coil at 4T

To demonstrate the feasibility of high-field SENSE imaging of large objects, such as the human head, using a semicircular (half-volume) coil for both transmission and multi-channel reception. As a proof of concept, we present experimental data obtained using a seven-element half-volume (180 degrees of arc) transmit/receive quadrature transverse electromagnetic (TEM) coil. SENSE images of the human brain were acquired with a reduction factor of R=2, using two degenerate linear modes of the same coil as independent receive channels at 4T. Since the need for additional hardware (i.e., a separate set of receive coils) is eliminated, the design can be substantially simplified. The experimental data demonstrate that linear modes of the half-volume TEM coil have essentially no noise correlation, and their sensitivity profiles satisfy the requirement for small g-factors. Also, this type of coil provides efficient transmission with a relatively large uniform region and a reception profile that is more uniform than that of the surface coils. We demonstrate the feasibility of SENSE imaging using a half-volume coil. Half-volume coils allow reduced total power deposition compared to full-volume coils, and may replace the latter in body imaging applications in which the target region of interest (ROI) is smaller than the entire torso.

Cyclotron masses andg-factors of hybridized electron-hole states inInAs∕GaSbquantum wells

Using the eight-band k center dot p model and the Burt-Foreman envelope function theory to perform self-consistent calculations, we have studied the effect of electron-hole hybridization on the cyclotron masses m(*) and the effective g-factors g(*) of two-dimensional quasiparticles in InAs/GaSb quantum wells under a magnetic field applied perpendicular to the interfaces. We can modify the degree of hybridization by changing the InAs and/or GaSb layer width, or by inserting a thin AlSb barrier. While electron-light-hole hybridization dominates at both low and high fields, due to a sequence of anticrossings between electronlike and heavy-holelike levels, there is also an important contribution from heavy-hole states to the strong hybridization in the intermediate field range. The field-dependence of the hybridized energy eigenstates is manifested in the variations of m(*) and g(*). Characteristic discontinuous changes of both m(*) and g(*) appear at each anticrossing, resulting in a magnetic-field-driven oscillating behavior of these quantities for electronlike states of a given Landau level index. The electron g-factor can change sign when two eigenstates anticross. Hybridization of electron and hole states enhances the electron effective mass, and we have found a complicated dependence of this effect on the interaction strength. Without inserting an AlSb barrier, the strong interaction between the electronlike and the light-holelike states at low magnetic fields produces a large level repulsion, and hence relatively small effective masses and g-factors associated with these states. Intermediate interaction leads to weaker level repulsion and therefore very heavy electron cyclotron masses as well as large g-factors associated with the lowest Landau levels. A weak interaction only enhances the cyclotron masses of the electronlike states slightly. The hole effective masses change with both the magnetic field and the sample structure in a more complicated fashion. (Less)

Material dependence of spin transport modulated by magnetic barriers in semiconductor nano-wires

We present the properties of ballistic spin transport through magnetic barrier structures in semiconductor nano-wires. The Landauer's approach is adopted to calculation of the transmission probability and the conductance for various host material nano-wires which are different remarkably from each other in effective g-factors. A host material having small effective g-factor is quantized in the conductance and the spin-dependence is disappeared in it. Nevertheless this kind of behavior is broken for the host material having large effective g-factor and the spin-dependent splitting is shown.

Spin lifetime from Hanle-effect measurements in samples with InAs quantum dots embedded in different AlxGa1−xAs matrices

Excitation of electron hole pairs by circularly polarized light yields an electron spin polarization in InAs self-assembled quantum dots embedded in AlxGa1−xAs. This spin polarization causes circular polarization of the photoluminescence light, its degree decreasing in a magnetic field perpendicular to the initial spin vector of the electrons (Hanle effect). With increasing aluminium content x the width of the Hanle curve is reduced from about 100 mT for x = 0 to 17.5 mT for x = 1. The corresponding electron spin lifetime increases with increasing x. For samples with QDs in the indirect gap matrix material AlAs, the spin lifetimes exceed 1.5 ns. A finite photoluminescence polarization evidences that the exciton spin lifetime inside a quantum dot cannot be much smaller than its recombination time. A single Lorentzian shape of the obtained Hanle curves and the observed field-independent part of the photoluminescence polarization could be explained by a small (zero) exciton g-factor in the dots at moderate magnetic fields, which could stem from the zero hole g-factor due to the electron–hole exchange interaction. The spin lifetime deduced from the Hanle curves could in this case be attributed to the thermalized electron capture time in the quantum dots.