Spin Energy Gap Research Articles

Density-dependent spin polarization in ultra-low-disorder quantum wires.

There is controversy as to whether a one-dimensional (1D) electron gas can spin polarize in the absence of a magnetic field. Together with a simple model, we present conductance measurements on ultra-low-disorder quantum wires supportive of a spin polarization at B=0. A spin energy gap is indicated by the presence of a feature in the range (0.5-0.7)x2e(2)/h in conductance data. Importantly, it appears that the spin gap is not constant but a function of the electron density. Data obtained using a bias spectroscopy technique are consistent with the spin gap widening further as the Fermi level is increased.

Instability of a system and its estimation in terms of the hybrid density functional theory method: a magnetic effective density functional (MEDF) approach

Potential curves and high and low spin energy gaps for radical clusters were calculated by spin polarized molecular orbital methods. Through-space effective exchange integrals (J ab) and relative energies of spin projected low spin states by post-Hartree-Fock (HF) calculation were reproduced by the hybrid density functional theory (DFT) method. The hybrid parameters that could reproduce post-HF values such as UCCSD(T)'s for each model had close relations with the instabilities of those systems. Information entropy and related chemical indices were used to estimate the magnitude of the instabilities. A magnetic effective density functional (MEDF) scheme for spin clusters was proposed for practical computation of J ab values in molecular magnetic materials.

Spin-crossover Phenomena of Iron Complexes in the NaTSM Intercalation Compounds

The spin-crossover phenomena of iron complexes accompany with molecular deformations such as symmetric breathing and trigonal torsion. Thus they can serve as a sensitive probe to molecular surroundings. A confinement effect into interlayer spaces of a clay host compound, NaTSM (sodium fluorotetrasilicic mica), was studied for iron complexes by magnetic susceptibility and 57 Fe Mössbauer spectroscopy. Guest complexes examined are [Fe II -(amp) 3 ] 2+ (amp=2-aminomethylpyridine), [Fe III (acpa) 2 ] + (Hacpa= N -(1-acetyl-2-propylidene)-2-pyridylmethylamine), and [Fe III - (sal 2 trien)] + (H 2 sal 2 trien= N , N ′-disalicylidenetriethylenetetramine). A remarkable reduction of the high spin-low spin energy gap in the interlayer space was observed.

Spin-crossover Phenomena of Iron Complexes in the NaTSM Intercalation Compounds

The spin-crossover phenomena of iron complexes accompany with molecular deformations such as symmetric breathing and trigonal torsion. Thus they can serve as a sensitive probe to molecular surroundings. A confinement effect into interlayer spaces of a clay host compound, NaTSM (sodium fluorotetrasilicic mica), was studied for iron complexes by magnetic susceptibility and 57 Fe Mössbauer spectroscopy. Guest complexes examined are [Fe II -(amp) 3 ] 2+ (amp=2-aminomethylpyridine), [Fe III (acpa) 2 ] + (Hacpa= N -(1-acetyl-2-propylidene)-2-pyridylmethylamine), and [Fe III - (sal 2 trien)] + (H 2 sal 2 trien= N , N ′-disalicylidenetriethylenetetramine). A remarkable reduction of the high spin-low spin energy gap in the interlayer space was observed.

High Magnetic Field Studies of AlGaN/GaN Heterostructures Grown on Bulk GaN, SiC, and Sapphire Substrates

ABSTRACTWe present the results of the high magnetic field studies of properties of two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures grown over high-pressure bulk GaN, sapphire, and insulating SiC substrates. The experimental results include the low field Hall measurements, cyclotron resonance measurements, and cryogenic temperature Quantum Hall Effect studies as well as room-temperature characteristics of High Electron Mobility Transistors fabricated on all these substrates. The room temperature high field measurements allow us to clearly separate the contributions of a parasitic parallel conduction from 2DEG conduction in all investigated heterostructures.The magnetotransport measurements are performed in the magnetic fields up to 30 Tesla for temperatures between 50mK-300K. This high magnetic field in combination with very high mobilities (over 60.000 cm2/Vs) in the sample on the bulk GaN substrates allow us to observe features related both to cyclotron resonance and spin splitting. The temperature dependence of this splitting determines the spin and cyclotron resonance energy gaps and, in combination with cyclotron resonance and tilted field experiments, allows us to determine the complete energy structure of 2DEG conduction band. We also present the first experimental results showing so called “the exchange enhancement” of the energy gaps between spin Landau levels.

BOSE CONDENSATION AND ELECTRON PAIRING IN ONE-DIMENSIONAL HUBBARD MODEL

The ground-state properties in one-dimensional Hubbard model with on-site attraction and repulsion of electrons in the presence of magnetic field h are calculated by means of the exact Bethe-ansatz formalism and the generalized self-consistent field (GSCF) approach for general electron concentrations n and arbitrary interaction strength. The ground-state properties, including the energy, the average spin (magnetization) and the kinetic energy are compared over a wide range of parameter space. The GSCF theory is in qualitative and in some cases in good quantitative agreement with the exact results. The GSCF theory at U≤0 (or U≥0) differentiates the spin (or charge) energy gap from the BCS (or antiferromagnetic) order parameter and suggests a smooth crossover from the phase with the itinerant BCS-like behavior to the Bose condensation regime of the local pairs.

Spin dynamics and energy gap of a Fe dimer from susceptibility and 1H nuclear magnetic resonance

The iron(III) S=5/2 dimer [Fe(OMe)(dbm)2]2 (in short Fe2) has a nonmagnetic S=0 ground state. The separation between the singlet ground state and the first excited (triplet) state is determined from susceptibility measurements to be about 22 K; for a dimer this value is equal to the antiferromagnetic exchange constant J. Proton nuclear magnetic resonance measurements were performed on Fe2. The nuclear spin-lattice relaxation rate (NSLR) was studied as a function of temperature at 31 and 67 MHz and as a function of the resonance frequency (10–67 MHz) at T=295 K. At room temperature the 1H NSLR is independent of frequency contrary to the strong dependence found in planar ring compounds like Fe6 and ferric wheel (Fe10). The temperature dependence of the proton NSLR shows an exponential decrease on lowering the temperature from which we estimate a gap value about double the value obtained from the uniform susceptibility, a result which is unexpected if the NSLR were simply proportional to the concentration of dimers in the triplet excited state.

Magneto-optics of strongly correlated two-dimensional electrons in single heterojunctions

Abstract Investigations of two-dimensional (2D) electron systems in semiconductors subjected to a strong perpendicular magnetic field with the use of photoluminescence are reviewed. The foundation of the optical spectroscopy method using the radiative recombination of 2D electrons with photoexcited holes bound to acceptors in a δ-doped monolayer in GaAs/Al x Ga1-x As single heterojunctions is presented. Optical spectroscopy studies of the energy spectra of 2D electrons imposed on transverse magnetic fields in the regimes of the integer and fractional quantum Hall effects are discussed. The relationship between the mean energy of the 2D electron gas and the first moment of their radiative recombination is analysed. It is shown that the magnetic field dependence of the first moment provides a method to measure the cyclotron, enhanced spin and quasiparticle energy gaps at the same time. Therefore it is shown how magneto-optics ‘see’ the ground state of interacting 2D electrons in the extreme quantum limit an...

High-field magnetization of Tb single crystals

The magnetization of Tb single crystals was measured in magnetic fields to 34T along the hard direction at temperature of 1.8, 4.2, 65.5 and 77K, and along with easy direction at 4.2 and 77K. The data are compared with the results of a self-consistent spin wave calculation using a phenomenological Hamiltonian including isotropic exchange interactions, effective single-ion anisotropy and magnetoelastic contributions. The parameters of this Hamiltonian were determined by fitting the theoretical results for the spin wave dispersion and energy gap as a function of temperature and magnetic field to existing data on Tb. The conduction-electron polarization at zero field and temperature is (0.33+or-0.05) mu B/ion, and the susceptibility is greater than the Pauli susceptibility calculated from the band-structure.