1. Full name: Nguyen Thuy Trang                                                2. Gender: Female

3. Date of birth: 27/08/1985                                                        4. Place of birth: Ha Noi

5. Admission decision number: 2048/QĐ-SĐH

6. Changes in academic process: Nothing

7. Official thesis title: Low dimensional complex oxide: Simulating and investigating some properties

8. Major: Solid state physics                                                      9. Code: 62 44 07 01

10. Supervisors: Assoc. Prof. Hoang Nam Nhat 

11. Summary of the new findings of the thesis:

- In case of heterostructure La1-xSrxMnO3/BaTiO3, on the base of first principles calculations, it was shown that:

 (1) The mechanism of magnetoelectric coupling is based on the electrostatic screening effect. The magnetoelectric coupling coefficient S was estimated to be 2.16×10-10 G.cm2/V in case of (001) orientation and doubled 4.28×10-10 G.cm2/V in case of (111) orientation.

(2) BO6 distortions caused by broken symmetry distribute the screening charge and hence affect the strength of the coupling.

(3) In case of (001) orientation, the reconstruction of magnetic order occurs when the polarization of BaTiO3 is switched if an appropriate strain is applied. However, such effect hardly occurs in (111) orientation case.

- In case of heterostructure LaAlO3/SrTiO3, on the base of first principles calculations, it was shown that:

1) An electron gas forms when the thickness of LaAlO3 layer is larger than 3 atomic layers.

2) The origin of this electron gas is, again, the electrostatic screening effect at the interface between nonpolar material SrTiO3 and polar material LaAlO3.

3) BO6 distortions cause a potential well in the interface region which confines the electron gas.

4) The bare surface of SrTiO3 similarly affects the electron gas. The sudden reducing of Madelung potential at the surface causes ferroelectric distortion and the electron polarization, which, in turn, creates a potential well in the surface region.

- In case of 1D antiferromagnet Ca2CuO3, there are some achievements as following

1) First principles electronic structure exhibits the covalence insulating nature which is caused by the one dimensionality of the system. The energy band gap of this system is smaller than other complex metal oxide because the strong pd hybridization significantly reduces the effective on-site repulsion.

2) First principle Raman spectrum and experimental Raman spectrum agree quite well with each other, demonstrating that the Raman activation of forbidden modes observed in experiments is related to the 1 dimensionality of the system.

12. Practical applicability, if any: The thesis provides information on key factor controlling properties of low dimensional complex oxide systems. It is helpful for practically designing materials with optimal properties.

13. Further research directions, if any:

- Studying the influences of strain, oxygen vacancies, cation intermixing on behaviors of LaAlO3/SrTiO3 interface.

- Studying the influences of strain, dopants on ME coupling at LAMO/BTO interface.

14. Thesis-related publications:

Nam Nhat Hoang, Thuy Trang Nguyen, Hong Van Bui and Duc Tho Nguyen, (2009), “Origin of the forbidden phonons in Raman scattering spectra of uranium-doped Ca2CuO3, a spin ½ chain system”, Journal of Raman Spectroscopy 40, pp. 170-175.

Nguyen Thuy Trang, Nguyen Van Chinh, Bach Thanh Cong and Nguyen Hoang Linh (2010), “Study on lattice and electronic structures at the surface of BaTiO3 thin films by DFT method”, Proceeding of National Conference of Theoretical Physics 35, pp. 209-220.

Hoang Nam Nhat and Nguyen Thuy Trang (2010), “Ground state of spin chain system by Density Functional Theory”, Computational Materials Science 49, pp. S348–S354.

Nguyen Thuy Trang, Tran Van Nam, Pham Ba Duy and Bach Huong Giang (2014), “Two-dimensional electron gas at the n-type interface of LaAlO3/SrTiO3 heterostructure: a first principles study”, Journal of Physics: Conference Series 537, pp. 012007.

Bach Huong Giang, Nguyen Thuy Trang, Tran Van Nam and Bach Thanh Cong (2015), “Magnetism in the interfaces of the sandwiched PbTiO3/LaAlO3/SrTiO3 heterostructure”, Communication in Physics 25, pp. 219-225.

Nguyen Thuy Trang, Kunihiko Yamauchi,  Tamio Oguchi and Hoang Nam Nhat (2016), “Influences of orientation on magnetoelectric coupling at La1-xSrxMnO3/BaTiO3 interface from ab initio calculations”, Journal of Electronic Materials, Journal of Electronic Materials 46, pp. 3808-3814s.

Nguyen Thuy Trang, Hoang Nam Nhat and Bach Thanh Cong (2017), “Strain effects on multiferroic heterointerface La0.5Sr0.5MnO3/BaTiO3 by DFT calculations”, Advances in Natural Sciences: Nanoscience and Nanotechnology (Submitted).