Kai-Cheng Zhang

Origin of ferromagnetism in Cu-doped SnO2: A first-principles study

We investigate the magnetic properties of Cu-doped SnO2, which was reported recently in experiments to possess ferromagnetism at room temperature, by first-principles calculations. Our results show that the doped system prefers the antiferromagnetic state for only Cu substitution. The oxygen vacancies are more inclined to form on the sites nearest to Cu defects. However, only those oxygen vacancies away from Cu defects contribute to the ferromagnetism significantly. Our results reveal that Sn vacancies are much easier to form than oxygen vacancies for their much lower formation energy. Moreover, the Cu-doped system always prefers the ferromagnetic state in the presence of Sn vacancies. Therefore, the origin of ferromagnetism in Cu-doped SnO2 can be attributed to the magnetic mediation of Sn vacancies.

Thermoelectric effect in an Aharonov-Bohm ring with an embedded quantum dot

Thermoelectric effect is studied in an Aharonov-Bohm interferometer with an embedded quantum dot (QD) in the Coulomb blockade regime. The electrical conductance, electron thermal conductance, thermopower, and thermoelectric figure-of-merit are calculated by using the Keldysh Greens function method. It is found that the figure-of-merit ZT of the QD ring may be quite high due to the Fano effect originated from the quantum interference effect. Moreover, the thermoelectric efficiency is sensitive to the magnitude of the dot-lead and inter-lead coupling strengthes. The effect of intradot Coulomb repulsion on ZT is significant in the weak-coupling regime, and then large ZT values can be obtained at rather high temperature.

Ferromagnetism of Cd doped SnO2: A first-principles study

The magnetic properties of Cd-doped SnO2 are studied by first-principles calculations. Our results reveal that the doped system favors a ferromagnetic state and high Curie-temperature can be expected in it. The doped Cd atoms do not tend to form clusters in tin oxide. The systems can be favorably synthesized in oxygen-rich ambient. The origin of ferromagnetism can be attributed to the hole-mediated p-d hybridization between Cd and O atoms.

Density-functional study on the ferromagnetism of Mn-doped SnO2

So far, both room-temperature ferromagnetism and paramagnetism have been reported in Mn-doped SnO2 films and their magnetic properties remain still puzzling. We have systematically investigated the magnetic properties of Mn-doped SnO2 by first-principles calculations. Our results reveal that the magnetic coupling is too weak and consequently paramagnetism is only found in the doped system when Mn substitutes Sn atoms. The formation energy of Mn impurity is much higher without the presence of vacancy, which prevents their substitutions significantly. In the presence of vacancies, the formation energy of Mn is greatly reduced and they can substitute Sn heavily. Compared to Sn vacancies, oxygen vacancies mediate the ferromagnetism much more effectively. The origin of ferromagnetism in Mn-doped SnO2 can be attributed to the oxygen-vacancy mediated RKKY interaction.

Magnetism induced by boron impurities in amorphous silicon

With first-principles calculations, magnetism is found in amorphous silicondoped with B impurities. The maximum magnetic moment per impurity atom is predicted to be ∼1.0 μ B which originates mostly from unsaturated bond around three-fold coordinated Si atoms. Stoner criterion is employed to account for the magnetism induced by p-type impurities. The obtained spin polarized energies are around 50 meV, indicating that the magnetism found in amorphous silicon is able to survive even at room temperature.

Interface spacing, stability, band offsets, and electronic properties on a (001) SrHfO3/GaAs interface: First-principles calculations

SrHfO3 is a potential dielectric material for metal–oxide–semiconductor devices. The SrHfO3/GaAs interface has attracted attention because of its unique properties. In this paper, the interface properties of (001) SrHfO3/GaAs are investigated by first-principles calculations based on density functional theory. First, the adsorption behavior of Sr, Hf, and O atoms on the GaAs surface is investigated. An O atom has a lower adsorption energy on a Ga surface than on an As surface. Then, some possible (001) SrHfO3/GaAs configurations are considered to analyze the interface spacing, stability, band offsets, and charge transfer. The SrO/Ga(1) and HfO2/Ga(2) configurations are lower in binding energy than other interface configurations, indicating that they are more stable. Finally, we study the electronic properties of the SrO/Ga(1) and HfO2/Ga(2) configurations. The electronic density of states suggests that these systems exhibit metallic behavior. The band offset and charge transfer are related to the interfac...

Half-metallic interface with stronger ferromagnetism in (110)-oriented LaxSr1−xMnO3/SrTiO3 ultrathin superlattices

Using the density functional calculations, we elucidate the nature of (110)- and (001)-oriented LaxSr1−xMnO3/SrTiO3 ultrathin superlattices with different La concentrations. In comparison to the (001)-oriented superlattice, the (110)-oriented one keeps ferromagnetic and half-metallic in a wider x range and has much greater exchange energy at the same La concentration, verifying previous experimental results. The significant advantages of the (110) superlattice is the result of the uniform distribution of the eg states in the 110 superlattice. We also find that the exchange energy is not sensitive to the strain between the layers.