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We theoretically investigate the properties of holes in a Si$_{x}$Ge$_{1-x}$/Ge/ Si$_{x}$Ge$_{1-x}$ quantum well in a perpendicular magnetic field that make them advantageous as qubits, including a large ($>$100~meV) intrinsic splitting between the light and heavy hole bands, a very light ($\sim$0.05$\, m_0$) in-plane effective mass, consistent with higher mobilities and tunnel rates, and larger dot sizes that could ameliorate constraints on device fabrication. Compared to electrons in quantum dots, hole qubits do not suffer from the presence of nearby quantum levels (e.g., valley states) that can compete with spins as qubits. The strong spin-orbit coupling in Ge quantum wells may be harnessed to implement electric-dipole spin resonance, leading to gate times of several nanoseconds for single-qubit rotations. The microscopic mechanism of this spin-orbit coupling is discussed, along with its implications for quantum gates based on electric-dipole spin resonance, stressing the importance of coupling terms that arise from the underlying cubic crystal field. Our results provide a theoretical foundation for recent experimental advances in Ge hole-spin qubits., 1 pages

Using a combination of theoretical (first-principles total-energy and electronic structure calculations) as well as experimental (structural, thermodynamics) techniques, we systematically investigated the influence of B incorporation on the structural, electronic and thermodynamic properties of a series of technologically-important B-containing AlNi matrix. Special attention was paid to calculating the energy cost of placing B at various sites within the cubic unit cell. The most energetically favorable defects were identified to be, depending on initial stoichiometry, substitutional B at Al site (B Al ), Ni vacancy ( V Ni ), or Ni antisite (Ni Al ). We show that the induced variation in the lattice parameters can be correlated with the type and concentration of the involved defects: e.g. the surge of V Ni defects leads to a stronger lattice-parameter reduction, that of Ni Al ones to a relatively weaker reduction while that of B Al defects to a much weaker influence. Both electronic band structure calculations as well as thermodynamics measurements indicate that the 3 d bands of Ni are fully occupied and magnetically unpolarized and that the resulting N ( E F ) is very small: all studied compounds are normal conductors with no trace of superconductivity or magnetic polarization.

Phosphorous-doped single wall carbon nanotubes were synthesized by an aerosol-assisted chemical vapor deposition by adding triphenilphosphine in the ethanol-ferrocene precursor solution. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were carried out along with density functional theory (DFT) calculations in order to understand the nature of phosphorous chemical doping. Binding energies of P 2p electrons, experimentally measured by XPS, are 129.6 eV and 134.3 eV. DFT calculations allow for the assignment of these levels to substitutional phosphorous and phosphorous-oxygen groups, respectively. The nature of phosphorous doping is elucidated through band structure and density of states calculations.

We have synthesized boron-doped single wall carbon nanotubes in a high vacuum chemical vapor deposition (CVD) system using a new boron precursor. Transmission electron microscopy was used in order to confirm the presence of single wall carbon nanotubes and field emission scanning electron microscopy to allow a qualitative characterization of the produced tubes. To estimate the doping level, we compared the Raman spectra with pure single wall carbon nanotubes and we found an upshifted G band as an evidence of doping. X-ray photoelectron spectroscopy analysis and ab initio electronic structure calculations reveals the presence of substitutional boron atoms incorporated on the tubes. We have also developed a simple method to determine quantitatively in which temperature range the carbon nanotubes are produced more efficiently by high vacuum CVD.

Here we calculate the electric field gradient (EFG) at the nucleus of the substitutional Ta impurity site in Zr 2 T and Hf 2 T (T=Cu, Ag, Au, and Pd) C11 b inter-metallic compounds. We use the ab initio FP-LAPW method as embodied in the Wien97 code in a super-cell approach and include lattice relaxations around the impurity. Our results are compared with EFG values inferred from measurements of the quadrupole coupling constants at the 111 Ta probe in these compounds performed with the time differential perturbed angular correlation (TDPAC) technique. We also performed EFG calculations for the pure inter-metallic compounds. Through the comparison of theoretical and experimental EFGs in these cases, we elucidate the role played by the Ta probe in the TDPAC measurements of Hf and Zr compounds. Our results show that, although the EFGs at the Hf site are very similar to the EFGs at the Ta impurity, there is no direct correlation between the Zr and Ta EFGs.

Calculations for substitutional Fe in Al in the absence of lattice relaxation yield a large value for the magnetic moment at the impurity site. The lack of magnetic response observed experimentally has been attributed to the presence of spin fluctuations. However, more recent calculations show that the moment at the Fe impurity could vanish in the presence of lattice relaxation around the impurity, thus rendering unnecessary a description in terms of spin fluctuations. We have performed electronic-structure calculations to investigate the magnetic behavior of substitutional Fe impurities in Zn, Cd, and Al hosts as a function of lattice relaxation. In the case of Fe in Al, the theoretical results are sensitive to the lattice constant used for fcc Al. If the experimental value is used, the Fe moment decreases drastically but does not vanish at the experimentally observed relaxation values. In the case of Zn and Cd hosts, the calculated moment remains quite large for all reasonable values of relaxation. Our results indicate that a description in terms of spin fluctuations and high values of the Kondo temperature should be used for Fe in Zn and Cd and may also be appropriate to describe Fe impurities in Al.

We have used first-principles linear muffin-tin orbital (LMTO) electronic structure calculations in the atomic sphere approximation (ASA), implemented directly in real space (RS) to investigate theoretically the behavior of the isomer shift of substitutional Fe impurities in metallic hosts. We find that the isomer shift in these systems is dominated by the 4$s$ contribution at the Fe site. Since both the volume occupied by the impurity and the valence of the metallic host vary widely in the systems studied here, the influence of these factors on the isomer shift could be investigated. When RS-LMTO-ASA basis functions are used, a simple picture emerges, which allows us to understand the observed trends of the isomer shift in terms of two quantities: the number of 4$s$ electrons at the impurity site and the probability of finding one of these electrons close to the nucleus. We find that probability follows a nearly universal curve as a function of the volume occupied by the impurity in the host, which leads to more negative isomer shift values as the Fe is compressed into smaller volumes. The number of 4$s$ electrons at the Fe site depends both on the volume occupied by the impurity in the host and on the host valence. Our approach can partially deconvolute these dependences, calling attention to the interesting processes which regulate the isomer shift trends.

By using the time-differential perturbed angular correlation technique, the electric field gradients (EFG) at (181)Hf/(181)Ta and (111)In/(111)Cd probe sites in the MoSi(2)-type compound Ti(2)Ag have been measured as a function of temperature in the range from 24 to 1073 K. Ab initio EFG calculations have been performed within the framework of density functional theory using the full-potential augmented plane wave+local orbitals method as implemented in the WIEN2k package. These calculations allowed assignments of the probe lattice sites. For Ta, a single well-defined EFG with very weak temperature dependence was established and attributed to the [4(e)4mm] Ti site. For (111)Cd probes, two of the three measured EFGs are well defined and correlated with substitutional lattice sites, i.e. both the [4(e)4mm] Ti site and the [2(a)4/mmm] Ag site.

The quadrupolar hyperfine interactions of in-diffused (111)In --(111)Cd probes in polycrystalline isostructural Zr(4)Al(3) and Hf(4)Al(3) samples containing small admixtures of the phases (Zr/Hf)(3)Al(2) were investigated. A strong preference of (111)In solutes for the contaminant (Zr/Hf)(3)Al(2) minority phases was observed. Detailed calculations of the electric field gradient (EFG) at the Cd nucleus using the full-potential augmented plane wave + local orbital formalism allowed us to assign the observed EFG fractions to the various lattice sites in the (Zr/Hf)(3)Al(2) compounds and to understand the preferential site occupation of the minority phases by the (111)In atoms. The effects of the size of the supercell and relaxation around the oversized In and Cd probe atoms were investigated in detail.