LATTICE INSTABILITIES OF BaTiO 3 /SrTiO 3 ARTIFICIAL SUPERLATTICE

KIM, LEEJUN,KIM, JUHO,WAGHMARE, UMESH,JUNG, DONGGEUN,LEE, JAICHAN Gordon and Breach Science Publishers 2005 Integrated ferroelectrics Vol.73 No.-

<P>We have investigated lattice instabilities of ultra-short period BaTiO 3 /SrTIO 3 (BTO/ STO) artificial superlattice (double-perovskite 10-atom unit cell) as a function of epitaxial strain using first-principles density functional theory (DFT) calculations based on pseudopotentials and a plane-wave basis. We find a structural transition from the tetragonal phase at compressive strain to the monoclinic phase at tensile strain. In the tetragonal phase, the Ti ions displacement patterns of the BTO/STO superlattice show ferrielectric phase behavior. Likewise, those of the monoclinic BTO/STO superlattice along [001] direction are very similar to the tetragonal BTO/STO superlattice. However, the displacement patterns in the xy -plane BTO/STO superlattice exhibit different behavior which both Ti ions displace parallely along [110] direction and against direction of oxygen displacements. It means that the monoclinic phase of BTO/STO superlattice is ferrielectric along [001] direction similar to tetragonal BTO/STO superlattice, but is ferroelectric in the xy- plane.</P>

양자점형 초격자의 다중양자우물 내 플라즈몬의 거동 비교

안형수,이상칠,김석환 한국물리학회 2022 새물리 Vol.72 No.3

The behavior of plasmons was studied in multiple quantum wells (MQWs) in four quantum dots-type superlattices, i.e., GaAs/AlxGa1−xAs, Ge/SiO2, InAs/InSn, and GaSb/AlxIn1−xSb, using the parabolic confining potential for the different dielectric interfaces. Using the random-phase approximation, the density–density correlation functions, based on the intraband and interband transitions in an MQW in a quantum dots-type superlattice system, were calculated as functions of the radius of the quantum dots in the MQW superlattice. The dispersion energies for the plasmons at the surface and in the bulk states in the MQW in the four quantum dots-type MQW superlattices, respectively, were calculated in the region between the zeroth and the first roots of the first type of first-order cylindrical Bessel function. The dispersion energies of the intraband and interband transitions for the bulk and the surface plasmons, respectively, in the four quantum dots-type MQW superlattices, were compared. The dispersion energy of the intraband and interband transitions decreased as the radius of the quantum dot-type superlattice increased due to the size-effect caused by quantum confinement.

Ballistic Diffusive Approximation에 의한 Quantum Dot Superlattice의 나노열전달 해석

김원갑(Won-Kap Kim),정재동(Jae Dong Chung) 대한기계학회 2004 대한기계학회 춘추학술대회 Vol.2004 No.4

Understanding the thermal conductivity and heat transfer processes in superlattice structures is critical for the development of thermoelectric materials and optoelectronic devices based on quantum structures. Chen(1) developed ballistic diffusive equation(BDE) for alternatives of the Boltzmann equation that can be applied to the complex geometrical situation. In this study, a simulation code based on BDE is developed and applied to the 1-dimensional transient heat conduction across a thin film and transient 2-dimensional heat conduction across the film with heater. The obtained results are compared to the results of the Chen(1) and Yang and Chen. Finally, steady 2-dimensional heat conduction in the quantum dot superlattice are solved to obtain the equivalent thermal conductivity of the lattice and also compared with the experimental data from Borca-Tasciuc.

초격자 다공질규소 표면에서 제 2고조파 발생

조창호,박종대,Cho, Chang-Ho,Park, Jong-Dae 배재대학교 자연과학연구소 2002 自然科學論文集 Vol.12 No.1

Second harmonic generation was measured in porous silicon superlattices surface which were made by changing and anodization current density and the anodization time periodically in the process of obtaining porous silicon. The technique of second harmonic generation is used as a probe to the surface of porous silicon superlattice by using Nd:YAG laser. We have investigated the surface structure and nonlinear-optics properties of porous silicon superlattice. 전류밀도와 에칭시간을 변화시키면서 제작된 다공질 규소 초격자의 표면에서 Nd:YAG 레이저의 제2 고조파를 발생시켰다. 이로부터 대칭구조가 바뀌고 유전율이 상승됨이 관측되었다.

Doped Two-dimensional Semiconductor Superlattice: Photo-stimulated Quantum Thermo-magnetoelectric Effects under the Influence of a Confined Phonon

Bau Nguyen Quang,Quynh Nguyen Thi Lam,Ba Cao Thi Vi,Hung Le Thai 한국물리학회 2020 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.77 No.12

Photo-stimulated quantum thermo-magnetoelectric effects in doped two-dimensional semiconductor superlattices, including the photo-stimulated quantum Ettingshausen effect and the photo-stimulated quantum Peltier effect, have been theoretically studied by using the quantum kinetic equation method. In this work, we assume that the electron-confined acoustic phonon scattering is essential. Moreover, the presence of the laser radiation (LR) is also taken into account to determine the influence of confined phonons on the aforementioned effects. We have defined the analytical expressions for the kinetic tensors and the Ettingshausen and the Peltier coefficients, presented the numerically calculated the theoretical results for the GaAs:Si/GaAs:Be doped semiconductor superlattice and compared them with these for the case of an unconfined acoustic phonon. The results obtained indicated that the formulas for the kinetic tensors, the Ettingshausen coefficient (EC) and the Peltier coefficient (PC) contain the quantum number m specifying the confinement of a phonon and approach the results for an unconfined phonon as m goes to zero. We found that the kinetic tensors, the EC and the PC oscillate with changing magnetic field and that the confinement of a phonon causes a shift of the peaks in these oscillations to lower energy. The dependences of both EC and PC on the temperature were found to be nonlinear. Moreover, all the coefficients level off when the temperature was less than 4.5 K or greater than 5.5 K. The EC also depended on the doping concentration in a nonlinear way and reaches a positive constant value when the semiconductor superlattice was doped with a high concentration. Most of the numerical results showed that the magnitude of the tensors, the EC as well as the PC, within a confined acoustic phonon varie significantly in comparison with the unconfined phonon case. This means that the confinement of the phonon affects the thermo-magnetoelectric effect quantitatively and qualitatively. These results contribute to completing the theory of the thermo-magnetoelectric effects in the low-dimensional semiconductor systems.

반도체초격자에서 와니어-슈타르크-자기소리양자 공명

이상칠,안형수,김석환 한국물리학회 2016 새물리 Vol.66 No.11

반도체초격자 축을 따라 평행하게 걸린 자기장과 전기장에 의한 란다우 준위와 와니어-슈타르크 준위가 형성될 때 양자수송 이론에 바탕을 둔 전기전도도와 완화함수를 온도, 초격자 주기, 미니밴드 너비, 전자밀도, 자기장, 전기장의 함수로 구하여 와니어-슈타르크 자기소리양자 공명 조건을 얻었다. 자기장, 전기장 및 소리 양자에너지 함수인 전기전도도와 완화함수의 와니어-슈타르크 자기소리양자 공명 조건으로부터 얻어진 선모양에 대한 거동을 수치적으로 조사하였다. 란다우 준위와 와니어-슈타르크 준위 사이의 전자 전이에 기인한 공명 봉우리의 수는 전기장이 커질수록 감소하고, 공명 봉우리에서의 자기장의 세기는 반도체초격자의 주기가 커질수록 증가한다. We obtained the conditions for the Wannier-Stark magnetophonon resonance from the electric conductivity and a relaxation function, which are functions of the temperature, the period of the superlattice, the miniband width, the electron density, and the magnetic and the electric fields, based on both Wannier-Stark quantum transport theory and the Landau levels obtained when using magnetic and steady strong electric fields parallel to the axis of the semiconductor superlattice, respectively. The behaviors of the line shapes obtained from the Wannier-Stark magnetophonon resonance of the electric conductivity and the relaxation function as functions of the magnetic field, the electric field and the phonon energy were investigated numerically. The number of resonance peaks due to electron transitions between the Landau and the Wannier-Stark levels decreased with increasing electric field, and the magnetic-field strengths at the resonance peaks increased with increasing period in the semiconductor superlattice.

자기장하의 GaAs/Al$_x$Ga$_{1-x}$As 반도체초격자내 다중양자우물내 플라스몬속성의 이론적 조사

이상칠,안형수,김석환 한국물리학회 2017 New Physics: Sae Mulli Vol.67 No.2

Extending a previous theoretical framework for the multiple-quantum-well (MQW) system in the GaAs/Al$_x$Ga$_{1-x}$As semiconductor superlattice, we obtained, on the basis of a simple model of a half-parabolic confining potential varying with the composition of the Al$_x$Ga$_{1-x}$As barrier layer, the density-density correlation function, including intrasubband and intersubband transitions, by using the random-phase approximationin under a magnetic field perpendicular to the axis of the superlattice. The dispersion energies for the surface and the bulk states were obtained as a function of compositional ratio of $x$ in the Al$_x$Ga$_{1-x}$As barrier layer and the magnetic field strength, respectively. The Raman intensities due to the bulk and the surface were also obtained for incoming light energy as a function of the compositional ratio $x$ and the magnetic field strength. GaAs/Al$_x$Ga$_{1-x}$As 반도체초격자내 다중양자우물계에 대한 이전의 이론적 토대를 확장하여 초격자축에 수직한 자기장하의 막 위상 근사방법을 사용하고 부 밴드 내와 부 밴드 사이의 전이가 포함된 밀도-밀도 상관 함수가 반-포물선형 구속 퍼텐셜을 기초로 얻어 졌다. 표면과 벌크 상태들에 대한 분산에너지를 Al$_x$Ga$_{1-x}$As 장벽층의 조성비와 자기장세기의 함수로 각각 얻었다. 또한 표면과 벌크 상태들에 대한 라만 세기를 조성비와 자기장세기의 함수로 얻는다.

Epitaxially strained ultrathin LaNiO3/LaAlO3 and LaNiO3/SrTiO3 superlattices: A density functional theory + U study

Kim Heung-Sik,Park Sang Hyeon,Han Myung Joon 한국물리학회 2023 Current Applied Physics Vol.50 No.-

By employing first-principles electronic structure calculations we investigate nickelate superlattices [LaNiO3]1/ [LaAlO3]1 and [LaNiO3]1/[SrTiO3]1 with (001) orientation under epitaxial tensile strain. Within density functional theory augmented by mean-field treatment of on-site electronic correlations, the ground states show remarkable dependence on the correlation strength and the strain. In the weakly and intermediately correlated regimes with small epitaxial strain, the charge-disproportionated insulating states with antiferromagneitc order is favored over the other orbital and spin ordered phases. On the other hand, in the strongly correlated regime or under the large tensile strain, ferromagnetic spin states with Jahn-Teller orbital order become most stable. The effect from polar interfaces in LaNiO3]1/[SrTiO3]1 is found to be noticeable in our single-layered geometry. Detailed discussion is presented in comparison with previous experimental and theoretical studies.

분자동역학법을 이용한 초격자 내부의 경계면 열저항의 해석

최순호(Soon-Ho Choi),이정혜(Jung-Hye Lee),최현규(Hyun-Kue Choi),윤석훈(Seok-Hun Yoon),오철(Cheol Oh),김명환(Myoung-Hwan Kim) 대한기계학회 2004 대한기계학회 춘추학술대회 Vol.2004 No.4

From the viewpoint of a macro state, there is no thermal boundary resistance (TBR) at an interlace if both surfaces at an interface arc perfectly contacted. However, recent molecular dynamics (MD) studies reported that there still exists the TBR at the interface in all ideal epitaxial superlttice. Our previous studies suggested thc model to predict the TBR not only quantitatively also qualitatively in superlattices. The suggested model was based on the classical theory of a wave rellection, and provided highly satisfactory results for an engineering purpose. However. it was not the complete model because our previous model was derived by considering only the effects from a mass ratio and a potential ratio of two species. The interaction of two species presented by the Lennard-Jones (L-J) potential is governed by the mutual ratio of the masses, the potential well depths. and the diameters. In this study. we performed the preliminary simulations to investigate the effect resulting from the diameter ratio of two species for the completion of our model and confirmed that it was also a ruling factor to the TBR at an interface in superlattices.

Fe3O4/CoFe2O4 superlattices: MBE growth and magnetic properties

Quang Van Nguyen,Yooleemi Shin,Duong Anh Tuan,Nguyen Thi Minh Hai,Sunglae Cho,Christian Meny 한국진공학회 2016 한국진공학회 학술발표회초록집 Vol.2016 No.2