Anh Pham

Weak d0 magnetism in C and N doped ZnO

We present an ab initio study of carbon and nitrogen substituting oxygen in zinc oxide structure. Detailed spin-polarized total-energy calculations of the various defect and dopant at different charge states and geometries indicate a non-zero spin magnetic moment only found from the CO-2 while NO shows no sign of localized magnetic moment. It is also revealed that CO has a tendency towards forming C2 complexes inside the ZnO structure with very weak antiferromagnetic spin arrangement. Furthermore, it was found that oxygen vacancy and hydrogen interstitial could not induce ferromagnetism in C doped ZnO.

Ferromagnetism in ZnO:Co originating from a hydrogenated Co–O–Co complex

The effects of hydrogen interstitials and oxygen vacancies on the overall ferromagnetic behaviour of Co doped ZnO (ZnO:Co) have been closely examined using different density functional calculations. The results demonstrate the importance of correcting the bandgap problem of the ZnO host as well as the lack of correlation in Cos 3d states which can severely affect the coupling of H and Cos impurity bands. Our results show that in hydrogenated ZnO:Co, hydrogen interstitial can also stabilize the ferromagnetic interaction at low Co concentrations, but this requires the formation of the in-plane O-H-Co-O-Co complex. In this structure, the hydrogen interstitial forms an anionic complex with the neighbouring oxygen, which polarizes the surrounding oxygen to mediate the ferromagnetism through the superexchange mechanism. An oxygen vacancy by itself would not cause ferromagnetism in ZnO:Co. On the other hand, in the presence of hydrogen interstitials, oxygen vacancies can significantly enhance the magnetic coupling between H and Co-O-Co as a shallow donor if it is far away from the in-plane O-H-Co-O-Co complex. However, the total energy results show that this is much more favoured when the oxygen vacancy is near the in-plane O-H-Co-O-Co complex, which can inhibit the ferromagnetic interaction between Co ions.

Optical bistability in mesoporous silicon microcavity resonators

We report on the observation of significant optical bistability in the transmission and reflection properties of mesoporous silicon microcavities when illuminated with a 150 ns pulsed laser at 532 nm. The observed optical hysteresis is shown to be transient in nature and the properties are strongly dependent on the porosity of the cavity layer. The onset and damage threshold intensity are also shown to be porosity dependent. Our modeling suggests that the observed effects are due to changes in the nonlinear refractive index where the transient lifetime increases with increasing porosity. We investigate the role of surface states on influencing the bistable process by passivating the internal porous surface with hydrosilylation chemistry.

Subtle interplay between localized magnetic moments and itinerant electrons in LaAlO3/SrTiO3 heterostructures

Clarification of the role of magnetic ordering and scattering in two-dimensional electron gas has become increasingly important to understand the transport and magnetic behavior in the LaAlO3 (LAO)/SrTiO3 (STO) heterostructures. In this work, we report the sheet resistance of the LAO/STO heterostructures as functions of temperature, magnetic field, and field orientation. An unexpected resistance minimum was discovered at ∼10 K under a sufficiently high in-plane magnetic field. An anisotropic magnetoresistance (MR) is clearly identified, indicating the presence of magnetic scattering which may be related to the interaction between itinerant electrons and localized magnetic moments in the LaAlO3/SrTiO3 heterostructures. It is believed that the high concentration of oxygen vacancies induced by the ultralow oxygen partial pressure during the deposition process plays a predominant role in the occurrence of the anisotropic MR.

Tuning conductivity and magnetism in isopolar oxide superlattices via compressive and tensile strain: A case study of SrVO3/SrMnO3 and SrCrO3/SrMnO3 heterostructure

We demonstrate the role of compressive and tensile strain to effectively control the conductivity and magnetism in isopolar materials utilizing density functional theory. Using the examples of superlattices containing transition metals with electronegativity differences such as SrVO3/SrMnO3 and SrCrO3/SrMnO3, our results show that the lattice strain can alter the apical oxygen shift at the interface of the transition metal layers, thus affecting the internal charge transfer process between d electrons. In addition, lattice compression and tensile strain can also modify the orbital occupancies of the manganite layers. As a result, various exotic effects can be realized in the SrMnO3 layer such as Mott insulator, quasi-two-dimensional conductivity, and long-range magnetism.

Largely Enhanced Mobility in Trilayered LaAlO3/SrTiO3/LaAlO3 Heterostructures

LaAlO3 (LAO)/SrTiO3 (STO)/LaAlO3 (LAO) heterostructures were epitaxially deposited on TiO2-terminated (100) SrTiO3 single-crystal substrates by laser molecular beam epitaxy. The electron Hall mobility of 1.2 × 104 cm2/V s at 2 K was obtained in our trilayered heterostructures grown under 1 × 10-5 Torr, which was significantly higher than that in single-layer 5 unit cells LAO (∼4 × 103 cm2/V s) epitaxially grown on (100) STO substrates under the same conditions. It is believed that the enhancement of dielectric permittivity in the polar insulating trilayer can screen the electric field, thus reducing the carrier effective mass of the two-dimensional electron gas formed at the TiO2 interfacial layer in the substrate, resulting in a largely enhanced mobility, as suggested by the first-principle calculation. Our results will pave the way for designing high-mobility oxide nanoelectronic devices based on LAO/STO heterostructures.

Engineering the Electronic and Magnetic Properties of Sc 2 CF 2 MXene Material through Vacancy Doping and Lattice Straining

MXenes is a new group of two-dimensional materials via etching of the ‘A’ element from MAX phases. Depending on the functional group, MXenes can be semiconductors or metals. In this paper, first-principles calculations have been performed to investigate the effects of single vacancy defects on a semiconducting MXene, Sc2CF2 monolayer. The theoretical results show that V-Sc can induce magnetism in the host monolayer, while V-C and V-F result in n-type conductivity. For V-Sc doped Sc2CF2, tensile strains enhance the total magnetic moment which remains constant with applied compressive strains. As a result, by manipulating the fabrication parameters, the magnetic and conductive properties of Sc2CF2 can be tuned without the need of chemical doping.

Defect induced charge trapping in C-doped α-Al2O3

The charged defect states of C-doped α-Al2O3 are investigated systematically with the density functional theory to study their thermodynamic stability and possible effects on the crystal structure and electrical conductivity. Our results reveal that under reducing (Al-rich) synthesis conditions, the most stable defect configuration is CO−2 with a deep (-1|-2) thermodynamic transition level. As a result, C defects are expected to act as double acceptors by introducing appreciable deep electron traps in the host band gap of α-Al2O3. These results are consistent with the proposal that a large number of F+-centers are formed as charge compensators to CO−2 ions in α-Al2O3.

Phononic Structure Engineering: the Realization of Einstein Rattling in Calcium Cobaltate for the Suppression of Thermal Conductivity.

Phonons in condensed matter materials transmit energy through atomic lattices as coherent vibrational waves. Like electronic and photonic properties, an improved understanding of phononic properties is essential for the development of functional materials, including thermoelectric materials. Recently, an Einstein rattling mode was found in thermoelectric material Na0.8CoO2, due to the large displacement of Na between the [CoO2] layers. In this work, we have realized a different type of rattler in another thermoelectric material Ca3Co4O9 by chemical doping, which possesses the same [CoO2] layer as Na0.8CoO2. It remarkably suppressed the thermal conductivity while enhancing its electrical conductivity. This new type of rattler was investigated by inelastic neutron scattering experiments in conjunction with ab-initio molecular dynamics simulations. We found that the large mass of dopant rather than the large displacement is responsible for such rattling in present study, which is fundamentally different from skutterudites, clathrates as well as Na analogue. We have also tentatively studied the phonon band structure of this material by DFT lattice dynamics simulation, showing the relative contribution to phonons in the distinct layers of Ca3Co4O9.

Optical hysteresis in mesoporous silicon microcavities

We report on the observation of transient optical bistability in mesoporous silicon microcavity resonators in transmission and reflection when illuminated by a 532nm nanosecond pulsed laser. The properties of the bistablility such as the hysteresis area, the onset and damaged threshold intensities are shown to be porosity dependent. Our model suggests that the bistability is due to the induced nonlinear refractive index changes and the transient lifetime increases with increasing porosity. The role of surface states in the bistability process was also investigated by passivating the internal porous surface using the hydrosilylation chemistry.