Electronic structure and magnetic properties in T2AlB2T_2\text{AlB}_2 (TT = Fe, Mn, Cr, Co, and Ni) and their alloys

The electronic structure and intrinsic magnetic properties of $\text{Fe}_2\text{AlB}_2$-related compounds and their alloys have been investigated using density functional theory. For $\text{Fe}_2\text{AlB}_2$, the crystallographic $a$ axis is the easiest axis, which agrees with experiments. The magnetic ground state of $\text{Mn}_2\text{AlB}_2$ is found to be ferromagnetic in the basal $ab$ plane, but antiferromagnetic along the $c$ axis. All $3d$ dopings considered decrease the magnetization and Curie temperature in $\text{Fe}_2\text{AlB}_2$. Electron doping with Co or Ni has a stronger effect on the decreasing of Curie temperature in $\text{Fe}_2\text{AlB}_2$ than hole doping with Mn or Cr. However, a larger amount of Mn doping on $\text{Fe}_2\text{AlB}_2$ promotes the ferromagnetic to antiferromagnetic transition. A very anisotropic magnetoelastic effect is found in $\text{Fe}_2\text{AlB}_2$: the magnetization has a much stronger dependence on the lattice parameter $c$ than on $a$ or $b$, which is explained by electronic-structure features near the Fermi level. Dopings of other elements on B and Al sites are also discussed.Comment: 10 pages, 9 figures, accepted by Phys. Rev.

Manipulation of double-dot spin qubit by continuous noisy measurement

We consider evolution of a double quantum dot (DQD) two-electron spin qubit which is continuously weakly measured with a linear charge detector (quantum point contact). Since the interaction between the spins of two electrons depends on their charge state, the charge measurement affects the state of two spins, and induces non-trivial spin dynamics. We consider the regimes of strong and weak coupling to the detector, and investigate the measurement-induced spin dynamics both analytically and numerically. We observe emergence of the negative-result evolution and the system stabilization due to an analog of quantum Zeno effect. Moreover, unitary evolution between the triplet and a singlet state is induced by the negative-result measurement. We demonstrate that these effects exist for both strong and weak coupling between the detector and the DQD system.Comment: 12 pages, 5 eps figure

Calculation of the optical spectra of β’-NiAl and CoAl

Band structures of β’-NiAl and CoAl have been calculated to interpret the experimental optical spectra. The optical transitions of both compounds are calculated as direct interband transitions including electric-dipole matrix elements between the eigenstates of the ground state of the system. All of the structures found in the optical spectra of both compounds involve states with some Ni or Co d character in both the initial and the final states. The overall agreement is good between the calculated spectrum and the optical data for β’-NiAl. For β’-CoAl there is qualitative agreement but some discrepancy for the energy positions and the intensities of the structures. A self-energy correction for the excitation spectrum has been used for β’-CoAl to improve the agreement

Optical properties and electronic structures of B2 and B19′ phases of equiatomic Ni-Ti alloys

The dielectric functions of equiatomic Ni-Ti alloys were measured by spectroscopic ellipsometry in the energy range of 1.5–5.4 eV at ∼423 and at ∼25 K. The peak at ∼2.26 eV in the B19′ (monoclinic structure) optical conductivity spectrum has a slightly larger magnitude than in the B2 (cubic CsCl structure) phase, while the shoulder at ∼3.5 eVbecomes weaker and almost indiscernible upon martensitic transformation. A new structure develops at ∼2.85 eV in the B19′ phase; however, it is also very weak. The band structures and the optical conductivity were calculated in both phases using the linearized-augmented-plane-wave method within the local-density approximation. k points near the Γ−X−M plane in the B2 phase and the corresponding k-points in B19′ phase contribute significantly to all three structures. The difference between the two spectra is due to the transitions between the folded-back bands from the B2phase because of the larger unit cell of the B19′ phase and the change in the electronic energy spectrum near the Fermi level. The overall optical properties of Ni-Ti alloys in the measured energy range are rather insensitive to the martensitic transformation because the states far from the Fermi level are mainly involved in the interband transitions

Interpretation of the optical properties of Nb

The interband optical conductivity of Nb was calculated from a self-consistent relativistic band structure. For the optical conductivity, the major relativistic effect is the lowering of the s-like bands relative to the d-like bands, not spin-orbit splitting. k-space searches identified the regions of the Brillouin zone contributing to the principal structures of the optical conductivity. The regions were found to be large volumes of the zone that did not include symmetry points or lines. Along with previous results for Mo, the good agreement between theory and experiment suggests that the effects of the omitted self-energy are small for these two bcc metals

Optical Properties of Mo

The interband optical conductivity of Mo was calculated using a self-consistent relativistic band structure. Including electric dipole matrix elements, the results are in excellent quantitative agreement with experiment in the 1–6-eV region. The use of nonrelativistic bands and the neglect of the dipole matrix elements each lead to poorer agreement with experiment. The major relativistic effect is the lowering of the s-like bands, not the spin-orbit splitting. k-space searches identified the regions of the Brillouin zone contributing to the three principal structures, which were found to be large volumes of the zone consisting of general points, away from symmetry points or lines

Optical properties and electronic structures of equiatomic XTi (X=Fe,Co,andNi) alloys

The dielectric functions of equiatomic XTi (X=Fe,Co,andNi) alloys in the B2 phase (cubic CsCl structure) were measured by spectroscopic ellipsometry in the energy range of 1.5-5.4 eV. The optical conductivity spectra show close resemblance to each other with a peak at 1.9-2.3 eV and another at 3.1-3.4 eV. Fine structures observed in previously reported measurements were not seen. The band structures and the optical conductivity spectra were calculated in the B2 phase using the linearized-augmented-plane-wave method with the local density approximation. The agreement between the measured and calculated spectra is markedly improved by the inclusion of a quasiparticle self-energy correction. Strong optical transitions are identified and the similarities and differences among the optical conductivity spectra of the three compounds are explained

Optical and magneto-optical properties of RFe2 (R=Gd,Tb,Ho,Lu) and GdCo2

The conductivity tensors of single crystals and polycrystals of RFe2 (R=Gd,Tb,Ho,Lu) and GdCo2 were determined in the visible and near UV ranges. The magneto-optical Kerr effect (MOKE) was studied at different temperatures and magnetic fields. The single-crystal data show more features and larger magnitudes in the MOKE spectrum than the polycrystalline data under the same experimental conditions. The theoretical optical conductivity tensors for these compounds were calculated using the tight-binding linear-muffin-tin orbital (TB-LMTO) method in the local spin-density approximation. The agreement between theory and experiment was poor except for LuFe2, in which the 4f shell is completely closed

Ellipsometric and Kerr-effect studies of Pt3−X (X=Mn,Co)

The conductivity tensor of polycrystalline Pt3X (X=Mn,Co) was determined between 1.6 and 5.2 eV. Samples were arc melted, mechanically polished, and annealed at 500°C for 1 h in Ar. The complex dielectric function was measured from 1.3 to 5.2 eV at room temperature with a rotating analyzer ellipsometer. The magneto-optic Kerr effect was studied between 10 and 293 K in magnetic fields up to 3 T. We used the tight-binding linear-muffin-tin-orbital method in the local spin-density approximation to determine the band structure, density of states, and optical conductivity. Including an empirical quasiparticle self-energy and a lifetime broadening yields good agreement of experimental and calculated spectra

Optical properties and electronic structure of single crystals of LuAl2 and YbAl2

The optical conductivities of single crystals of LuAl2 and YbAl2 were measured by spectroscopic ellipsometry in the energy range of 1.4–5.5eV for LuAl2 and 1.4–5.2eV for YbAl2. The optical conductivity spectra of LuAl2 and YbAl2 show similar features except for a difference in magnitude. Both have peaks near 1.8–2.1eV and broad shoulders between 3.0 and 4.0eV. The shoulder is weaker in YbAl2. The band structure, density of states, and optical conductivity were calculated with the tight-binding linear muffin-tin orbital method in the atomic sphere approximation. The calculated optical conductivity with the inclusion of energy-dependent broadening agrees well with the experimental data. Oxidation effects on the surface of the sample were modeled using a three-phase model. The calculated optical conductivity of the clean surface is enhanced over that of the oxidized surface