Exchange Splitting Energy Research Articles

Model of photoluminescence from ion-synthesized silicon nanocrystal arrays embedded in a silicon dioxide matrix

A four-level model of photoluminescence from Si nanocrystal arrays embedded in a SiO2 matrix is suggested. The model allows for thermally activated transitions between singlet and triplet levels in the exchange-split energy state of an exciton in an excited silicon nanocrystal. An expression is derived for the temperature dependence of the intensity of photoluminescence monochromatic components. A correlation is found between the amount of splitting and the emitted photon energy by comparing model data with our experimental data for ion-synthesized Si nanocrystals in a SiO2 matrix. The model explains the finiteness of the photoluminescence intensity at temperatures close to 0 K and the nonmonotonicity of the temperature run of the intensity.

Spin coherence generation in negatively charged self-assembled (In,Ga)As quantum dots by pumping excited trion states

Spin coherence generation in an ensemble of negatively charged (In,Ga)As/GaAs quantum dots was investigated by picosecond time-resolved pump-probe spectroscopy measuring ellipticity. Robust coherence of the ground-state electron spins is generated by pumping excited charged exciton (trion) states. The phase of the coherent state, as evidenced by the spin ensemble precession about an external magnetic field, varies relative to spin coherence generation resonant with the ground state. The phase variation depends on the pump photon energy. It is determined by (a) pumping dominantly either singlet or triplet excited states, leading to a phase inversion, and (b) the subsequent carrier relaxation into the ground states. From the dependence of the precession phase and the measured $g$ factors, information about the quantum dot shell splitting and the exchange energy splitting between triplet and singlet states can be extracted in the ensemble.

Structural stability, band structure and magnetic properties of ZnS and Zn0.75Cr0.25S under pressure

Due to the significant luminescent properties of ZnS and the half metallic features of Cr doped ZnS (Zn0.75Cr0.25S), many researches have been done to make possible the use of these materials in optical and spintronic devices. The environmental conditions, such as, high pressure and high temperature, are crucial for the device service, however, it is difficult to exploit this issue from the experimental aspect. Therefore, spin-polarized first-principle calculations were performed to study the structural stability, band structure and magnetic properties of zincblende ZnS and Zn0.75Cr0.25S under pressure. It is shown that ZnS can exist stably under pressure up to 10GPa, while Zn0.75Cr0.25S becomes unstable under a pressure of 5GPa or so. The bulk modulus (BH), Poisson ratio (v) and anisotropic parameter (A) of ZnS and Zn0.75Cr0.25S increase with pressure, but the shear modulus (GH) decreases. BH and GH of Zn0.75Cr0.25S are smaller than those of ZnS, while v and A of Zn0.75Cr0.25S are larger than those of ZnS. Both ZnS and Zn0.75Cr0.25S behave in a ductile manner, while Zn0.75Cr0.25S is more anisotropic and is easier to induce deformation than ZnS. As for ZnS, the direct band structure is kept but the band gap increases gradually with the pressure. As for Zn0.75Cr0.25S, the half metallic features are maintained up to a pressure of 10GPa, but both the band gap for minority spin and the half metallic band gap have the largest values under 6Pa. The spin exchange splitting energy Δx(d) and Δx(pd) decrease with the increase of pressure. The negative values of Δx(pd) under pressure imply that the effective potential (Eep) of the minority spin is more attractive than that of majority spin.

Spin thermopower and thermoconductance in a ferromagnetic graphene nanoribbon

The spin thermoelectric properties of a zigzag edged ferromagnetic (FM) graphene nanoribbon are studied theoretically by using the non-equilibrium Green’s function method combined with the Landauer–Büttiker formula. By applying a temperature gradient along the ribbon, under closed boundary conditions, there is a spin voltage ΔVs inside the terminal as the response to the temperature difference ΔT between two terminals. Meanwhile, the heat current ΔQ is accompanied from the ‘hot’ terminal to the ‘cold’ terminal. The spin thermopower S = ΔVs/ΔT and thermoconductance κ = ΔQ/ΔT are obtained. When there is no magnetic field, S versus ER curves show peaks and valleys as a result of band selective transmission and Klein tunneling with ER being the on-site energy of the right terminal. The results are in agreement with the semi-classical Mott relation. When |ER| < M (M is the FM exchange split energy), κ is infinitesimal because tunneling is prohibited by the band selective rule. While |ER| > M, the quantized value of appears. In the quantum Hall regime, because Klein tunneling is suppressed, S peaks are eliminated and the quantized value of κ is much clearer. We also investigate how the thermoelectric properties are affected by temperature, FM exchange split energy and Anderson disorder. The results indicate that S and κ are sensitive to disorder. S is suppressed for even small disorder strengths. For small disorder strengths, κ is enhanced and for moderate disorder strengths, κ shows quantized values.

Observation of Negative and Positive Trions in the Electrochemically Carrier-Doped Single-Walled Carbon Nanotubes

Understanding of electronic and optical features of single-walled carbon nanotubes (SWNTs) has been a central issue in science and nanotechnology of carbon nanotubes. We describe the detection of both the positive trion (positively charged exciton) and negative trion (negatively charged exciton) as a three-particle bound state in the SWNTs at room temperature by an in situ photoluminescence spectroelectrochemistry method for an isolated SWNT film cast on an ITO electrode. The electrochemical hole and electron dopings enable us to detect such trions on the SWNTs. The large energy difference between the singlet bright exciton and the negative and positive trions showing a tube diameter dependence is determined by both the exchange splitting energy and the trion binding energy. In contrast to conventional compound semiconductors, on the SWNTs, the negative trion has almost the same binding energy to the positive trion, which is attributed to nearly identical effective masses of the holes and electrons.

Temperature dependent optical properties of PbS nanocrystals

A comprehensive study of the optical properties of PbS nanocrystals (NCs) is reported that includes the temperature dependent absorption, photoluminescence (PL) and PL lifetime in the range of 3–300 K. The absorption and PL are found to display different temperature dependent behaviour though both redshift as temperature is reduced. This results in a temperature dependent Stokes shift which increases from ∼75 meV at 300 K with reducing temperature until saturating at ∼130 meV below ∼150 K prior to a small reduction to 125 meV upon cooling from 25 to 3 K. The PL lifetime is found to be single exponential at 3 K with a lifetime of τ1 = 6.5 μs. Above 3 K biexponential behaviour is observed with the lifetime for each process displaying a different temperature dependence. The Stokes shift is modelled using a three-level rate equation model incorporating temperature dependent parameter values obtained via fitting phenomenological relationships to the observed absorption and PL behaviour. This results in a predicted energy difference between the two emitting states of ∼6 meV which is close to the excitonic exchange energy splitting predicted theoretically for these systems.

Spin-Waves in Stoner Ferromagnetic Phase of a Two-Dimensional Electron System

We have studied the collective excitations (spin-wave energies) in the ground state of a two-dimensional (2D) electron system using the random phase approximation (RPA). To do so, we have calculated the magnetic transverse susceptibility in 2D electron system by Hartree-Fock (HF) approximation of Hubbard model at T=0 which is generally referred to as the Stoner model. The spin-waves dispersion of three-dimensional (3D) electron system due to its quadratic behavior starts from zero at q0 while our numerical results show that the spin-waves dispersion of 2D electron system starts from the value Δ, known as exchange splitting energy, even at q=0.

First-principles study of oxygen adsorption on Fe(1 1 0) surface

We investigate three superstructures c(2×2), c(3×1) and c(1×1) of oxygen atom adsorption on Fe(110) surface by using first-principles density functional theory. The former two superstructures have been found by experiments at low oxygen exposures. The oxygen adsorption results in some electron transfer from surface Fe atom to O atom due to the strong electronegativity of oxygen atom, which also causes a positive work function change. For all three superstructures, the calculated surface spin density at Fermi level is positive, unlike that on a clean Fe(110) surface. The adsorbate–substrate hybridization is mainly from the interaction between O 2p and Fe 3d orbitals, and O 2p state exhibits obvious exchange spin–split induced by the Fe–O magnetic interaction. The adsorbate O 2p bands of c(2×2) and c(3×1) superstructures exhibit a weaker dispersion, whereas those bands of c(1×1) structure show larger dispersion due to the O–O interaction. Through the band structure calculation, we also calculate the exchange splitting energy of O 2px orbital for c(3×1) structure in k space and compare them with experimental results.

First principle electronic structure calculations of ternary alloys Hg1−xMnxTe in zinc-blende structure

We have investigated the structural, electronic, magnetic and optical properties of Hg1−xMnxTe in the zinc-blende phase for 0≤x≤1. The calculations were performed by using the full potential linearized augmented plane wave plus local orbitals method within the framework of the density functional theory. The lattice constants of Hg1−xMnxTe at different Mn concentrations exhibit Vegard's law perfectly. For spin-up channel the Mn 3d bands are occupied and mixed with the Te 5p bands whereas for spin-down channel the Mn 3d bands are unoccupied. The values of the p–d exchange splitting energy, ∆x(pd) as produced by the Mn 3d states are given. The contribution of the valence band and the conduction band in the process of exchange and splitting is described by the exchange coupling constants N0α and N0β. Due to p–d hybridization the magnetic moment of the Mn atom reduces, which results in small local magnetic moments on the non-magnetic Hg and Te sites. The potential applications of Hg1−xMnxTe in infrared device have been discussed on the basis of its optical properties.

Ab-initio investigations of structural, electronic, magnetic and optical properties of ferromagnetic Cd 1– xMn xTe

We report ab-initio calculations of the structural, electronic, magnetic and optical properties of the alloy Cd 1– x Mn x Te as a function of the Mn concentration ‘ x’. Ab-initio calculations are based on the density functional theory (DFT) within the generalized gradient approximation (GGA). The calculated lattice constants of the Cd 1– x Mn x Te alloys exhibit Vegard's law downward bowing parameter. For the minority spin channel the Fermi level shifts toward higher energy with the value of ‘ x’ in Cd 1– x Mn x Te. The spin-exchange splitting energy Δ x (d) increases with increasing ‘ x’ in Cd 1– x Mn x Te and the values of p–d exchange splitting energy Δ x (pd) of Cd 1– x Mn x Te show that the effective potential for the minority spin is more attractive than that for the majority spin. The values of exchange constants N 0 α and N 0 β obtained for Cd 1– x Mn x Te are in agreement with the reported data. The magnetic moment per Mn atom reduces from its free space charge value of 5μ B to around 4μ B due to p–d hybridization and this results into an appearance of small local magnetic moments on the non-magnetic Cd and Te sites. The absorption threshold shifts toward higher energy and the static refractive index decreases with the increasing value of ‘ x’ in Cd 1– x Mn x Te.

Magnetoelectric effect in ferromagnetic-ferroelectric tunneling junctions

The interfacial magnetoelectric (ME) effect in ferromagnetic-ferroelectric (FM-FE) tunneling junctions was investigated in this work. We found that the tunneling magnetoresistance (TMR) changes with the reversing of the electric polarization of the FE barrier. The theoretical results also indicated that TMR is strongly dependent on the electric polarization, the exchange splitting energy, the screening lengths in the electrodes, and the dielectric constant of the FE barrier layer. These results may provide some insights into switching magnetization electrically for spintronics and presenting independent tunneling states in a single junction for multi-value storage memory applications.

Triplet excitons in a ladder-type conjugated polymer: Application in solar cells

Inclusion of trace quantities of heavy metal atoms (Pd) in a ladder-type polymer (PhLPPP) backbone allows transitions between the singlet and the triplet manifolds. Temperature-dependent photoinduced absorption studies of PhLPPP show the triplet–triplet absorption peak ( T 1 → T N) at 1.3 eV to blue-shift with increasing temperatures, suggesting a localization of the excitons on smaller chain segments with large exchange splitting energy. Furthermore, solar cells were fabricated using PhLPPP as the donor material; on an average the power conversion efficiencies of PhLPPP-based solar cells were 3–10 times greater compared to the ladder-type polymer with no incorporation of Pd atoms.

Numerical evaluations on the asymmetric bias dependence of magnetoresistance in double spin filter tunnel junctions

In double spin filter tunnel junctions, the spin information is generated and analyzed purely from the spin filtering effect with nonmagnetic electrodes. In this article we numerically evaluate the bias dependence of magnetoresistance in such tunnel junctions (nonmagnetic metal/spin filter/nonmagnetic insulator/spin filter/nonmagnetic metal), particularly in cases when different spin filter materials are utilized. A large magnetoresistance with nonmonotonic and asymmetric bias dependence is expected within the framework of Wentzel–Kramers–Brillouin approximation. We illustrate the systematic influence of tunnel barrier height, tunnel barrier thickness, and exchange energy splitting on magnetoresistance, particularly focusing on the asymmetric behavior of the magnetoresistance bias dependence.

Large Magnetoresistance Based on Double Spin Filter Tunnel Barriers

We propose and theoretically analyse a double magnetic tunnel device that takes advantages of the spin filter effect. Two magnetic tunnel barriers are formed by different spin filters which have different barrier heights. The magnetoresistance of the device is low (high) when the magnetic moments of the two spin filters are parallel (antiparallel). We present a theoretical calculation of the magnetoresistance based on electric tunnel effect. In addition, the effect of the difference barrier heights and exchange splitting energies between the two spin filters are also analysed in detail. The numerical results show that the spin filter in this configuration gives a magnetoresistance larger than that with standard magnetic tunnel junctions.

Dielectric and magnetic birefringence in low‐chlorine‐doped n ‐type Zn1–xMnxSe

AbstractGeneralized spectroscopic ellipsometry over the spectral range from 1.24 to 3.34 eV is used to investigate a set of molecular beam epitaxy grown lowchlorine‐doped n ‐type Zn1–x Mnx Se films on (001)‐oriented GaAs for x = 0.0, 0.02, 0.14 and 0.28. We present evidence for intrinsic optical anisotropy in dependence of the Mn concentration caused by wurtzitestructure domain formation. We employ a previously established dielectric function model that accounts for band‐gap transition energy splitting in cubic semiconductors [Phys. Rev. B 60, 16618 (1999)]. Room temperature magneto‐optic generalized ellipsometry in the Kerrconfiguration reveals the sp‐d exchange energy splitting parameters upon expanding our anisotropy model by inclusion of chiral spin‐polarized band‐to‐band transition contributions. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Energy dependence of the electron‐hole in‐plane anisotropy in InAs/GaAs quantum dots

AbstractDue to a regrettable technical error some Greek and special letters in the paper were not reproduced. Please, find here the fully corrected article.In pump‐probe experiments, we measured the anisotropic exchange energy splitting of the low‐lying electron‐hole pair states in an ensemble of as‐grown InAs/GaAs self‐assembled quantum dots. This splitting shows a monotonic decrease with the energy of interband emission. We discuss this result considering different parameters: shape anisotropy, wavefunction confinement, size of the quantum dots, and piezoelectric effect. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Energy dependence of the electron‐hole in‐plane anisotropy in InAs/GaAs quantum dots

AbstractIn pump‐probe experiments, we measured the anisotropic exchange energy splitting of the low‐lying electron‐hole pair states in an ensemble of as‐grown InAs/GaAs self‐assembled quantum dots. This splitting shows a monotonic decrease with the energy of interband emission. We discuss this result considering different parameters: shape anisotropy, wavefunction confinement, size of the quantum dots, and piezoelectric effect. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Density functional perturbational orbital theory of spin polarization in electronic systems. I. Formalism

A perturbational approach is presented for the general analysis of spin-polarization effect on electronic structures and energies within spin-density functional formalism. Explicit expressions for the changes in Kohn-Sham [Phys. Rev. 140, 1133 (1965)] orbital energies and coefficients as well as for the change in total electronic energy are derived upon using the local spin density and self-interaction-corrected exchange-correlation functionals. The application of the method for atoms provides analytical expressions for the exchange splitting energy and spin-polarization energy. The atomic exchange parameters are obtained from the expressions for the elements with Z=1-92 and they match well with Stoner exchange parameters for 3d metal elements.

Size and shape effects on excitons and biexcitons in single InAs∕InP quantum dots

Single InAs quantum dots grown on an InP vicinal substrate are studied using polarized microphotoluminescence. The study of electron-hole exchange energy splitting reveals that the energy splitting is influenced by quantum confinement. The biexciton binding energies found for single dots grown on the vicinal substrate (around 2.4meV) are high as compared to those of dots grown on an InP(001) substrate and this is attributed to a difference in spatial confinement. Moreover, the quantum dot size is shown to influence directly the biexciton binding energy.

Effect of Ni concentration on quantum-well states of the alloy systemAg∕Fe1−xNix: A spin- and angle-resolved photoemission study

We investigate the quantum-well states (QWSs) of Ag on an $\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Ni}$ alloy film grown on a W(110) substrate, using spin- and angle-resolved photoemission spectroscopy (SPARPES). As the Ni content is increased, the $\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Ni}$ alloy undergoes a transition from a bcc (110) structure to a fcc (111) structure at a Ni concentration of 30%. In the bcc (110) region, we found that as the Ni concentration was increased, the binding energy shifted linearly toward higher binding energy in the QWSs of a thin Ag film, with a maximum shift of $0.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for a Ni concentration of 30%. Our SPARPES data and phase accumulation model calculation indicate that this shift is due to electron doping resulting from inserting Ni, which has two more electrons than Fe. In the fcc (111) region, by contrast, the binding energy shift in the QWSs is barely noticeable, indicating that the potential shift caused by electron doping is insufficiently large in this region. Moreover, as the Ni concentration is increased, a decrease in the polarization of QWSs is observed, which can be attributed to the reduced exchange splitting energy.