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Register a new userObservation and origin of the $Delta$-manifold in Si:P $delta$-layers
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By creating a sharp and dense dopant profile of phosphorus atoms buried within a silicon host, a two-dimensional electron gas is formed within the dopant region. Quantum confinement effects induced by reducing the thickness of the dopant layer, from $4.0$,nm to the single-layer limit, are explored using angle-resolved photoemission spectroscopy. The location of theoretically predicted, but experimentally hitherto unobserved, quantum well states known as the $Delta$-manifold is revealed. Moreover, the number of carriers hosted within the $Delta$-manifold is shown to be strongly affected by the confinement potential, opening the possibility to select carrier characteristics by tuning the dopant-layer thickness.
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Recently, a single atom transistor was deterministically fabricated using phosphorus in Si by H-desorption lithography with a scanning tunneling microscope (STM). This milestone in precision, achieved by operating the STM in the conventional tunneling mode, typically utilizes very slow ($sim!10^2~mathrm{nm^2/s}$) patterning speeds. By contrast, using the STM in a high voltage ($>10~mathrm{V}$) field emission mode, patterning speeds can be increased by orders of magnitude to $gtrsim!10^4~mathrm{nm^2/s}$. We show that the rapid patterning negligibly affects the functionality of relatively large micron-sized features, which act as contacting pads on these devices. For nanoscale structures, we show that the resulting transport is consistent with the donor incorporation chemistry enhancing the device definition to a scale of $10~mathrm{nm}$ even though the pattering spot size is $40~mathrm{nm}$.
Mn doping of group-IV semiconductors (Si/Ge) is achieved by embedding a thin Mn-film as a {delta}-doped layer in group-IV matrix. The Mn-layer consists of a dense layer of monoatomic Mn-wires, which are oriented perpendicular to the Si(001)-(2x1) dimer rows, or Mn-clusters. The nanostructures are covered with an amorphous Si or Ge capping layer, which conserves the identity of the {delta}-doped layer. The analysis of the bonding environment with STM is combined with the element-specific detection of the magnetic signature with X-ray magnetic circular dichroism. The largest moment (2.5 {mu}B/Mn) is measured for Mn-wires, which have ionic bonding character, with an a-Ge overlayer cap, a-Si capping leads to a slightly reduced moment which has its origin in subtle variation of bonding geometry. Our results directly confirm theoretical predictions on magnetism for Mn-adatoms on Si(001). The moment is quenched to 0.5{mu}B/Mn for {delta}-doped layers, which are dominated by clusters, and thus develop an antiferromagnetic component from Mn-Mn bonding.
Perpendicular magnetization is essential for high-density memory application using magnetic materials. High-spin polarization of conduction electrons is also required for realizing large electric signals from spin-dependent transport phenomena. Heusler alloy is a well-known material class showing the half-metallic electronic structure. However, its cubic lattice nature favors in-plane magnetization and thus minimizes the perpendicular magnetic anisotropy (PMA), in general. This study focuses on an inverse-type Heusler alloy, Mn$_{2-delta}$CoGa$_{1+delta}$ (MCG) with a small off-stoichiometry ($delta$) , which is expected to be a half-metallic material. We observed relatively large uniaxial magnetocrystalline anisotropy energy ($K_mathrm{u}$) of the order of 10$^5$ J/m$^3$ at room temperature in MCG films with a small tetragonal distortion of a few percent. A positive correlation was confirmed between the $c/a$ ratio of lattice constants and $K_mathrm{u}$. Imaging of magnetic domains using Kerr microscopy clearly demonstrated a change in the domain patterns along with $K_mathrm{u}$. X-ray magnetic circular dichroism (XMCD) was employed using synchrotron radiation soft x-ray beam to get insight into the origin for PMA. Negligible angular variation of orbital magnetic moment ($Delta m_mathrm{orb}$) evaluated using the XMCD spectra suggested a minor role of the so-called Brunos term to $K_mathrm{u}$. Our first principles calculation reasonably explained the small $Delta m_mathrm{orb}$ and the positive correlation between the $c/a$ ratio and $K_mathrm{u}$. The origin of the magnetocrystalline anisotropy was discussed based on the second-order perturbation theory in terms of the spin-orbit coupling, claiming that the mixing of the occupied $uparrow$- and the unoccupied $downarrow$-spin states is responsible for the PMA of the MCG films.
We have prepared a number of GaAs structures delta-doped by Sn using the well-known molecular beam epitaxy growth technique. The samples obtained for a wide range of Sn doping densities were characterised by magnetotransport experiments at low temperatures and in high magnetic fields up to 38 T. Hall-effect and Shubnikov-de Haas measurements show that the electron densities reached are higher than for other delta-dopants, like Si and Be. The maximum carrier density determined by the Hall effect equals 8.4x10^13 cm^-2. For all samples several Shubnikov-de Haas frequencies were observed, indicating the population of multiple subbands. The depopulation fields of the subbands were determined by measuring the magnetoresistance with the magnetic field in the plane of the delta-layer. The experimental results are in good agreement with selfconsistent bandstructure calculations. These calculation shows that in the sample with the highest electron density also the conduction band at the L point is populated.
We present the synthesis of D0$_{22}$ Mn$_{3 - delta}$Ga ($delta$ = 0, 1) Heusler alloys by Spark Plasma Sintering method. The single phase Mn$_3$Ga (T$_mathrm{c}$ $simeq$ 780 K) is synthesized, while Mn$_2$Ga (T$_mathrm{c}$ $simeq$ 710 K) is found to coexist with a near-stoichiometric room temperature paramagnetic Mn$_9$Ga$_5$~($approx$ 15 %) phase due to its lower formation energy, as confirmed from our density functional theory (DFT) calculations. The alloys show hard magnetic behavior with large room temperature spontaneous magnetization m$_s$(80 kOe) = 1.63 (0.83) $mu_mathrm{B}$/f.u. and coercivity H$_mathrm{c}$ = 4.28 (3.35) kOe for Mn$_3$Ga (Mn$_2$Ga). The magnetic properties are further investigated till T$_mathrm{c}$ and the H$_mathrm{c}$ (T) analysis by Stoner-Wohlfarth model shows the nucleation mechanism for the magnetization reversal. The experimental results are well supported by DFT calculations, which reveal that the ground state of D0$_{22}$ Mn$_2$Ga is achieved by the removal of Mn-atoms from full Heusler Mn$_3$Ga structure in accordance with half Heusler alloy picture.
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