Photoemission from buried interfaces in SrTiO3/LaTiO3 superlattices

We have measured photoemission spectra of SrTiO3/LaTiO3 superlattices with a topmost SrTiO3 layer of variable thickness. A finite coherent spectral weight with a clear Fermi cutoff was observed at chemically abrupt SrTiO3/LaTiO3 interfaces, indicating that an "electronic reconstruction" occurs at the interface between the Mott insulator LaTiO3 and the band insulator SrTiO3. For SrTiO3/LaTiO3 interfaces annealed at high temperatures (approximately 1000 degrees C), which leads to Sr/La atomic interdiffusion and hence to the formation of La(1-x)Sr(x)TiO3-like material, the intensity of the incoherent part was found to be dramatically reduced whereas the coherent part with a sharp Fermi cutoff was enhanced due to the spread of charge. These important experimental features are well reproduced by layer dynamical-mean-field-theory calculation.     

钛酸镧薄膜的制备及工艺优化

为了获得制备钛酸镧（LaTiO3）薄膜的最优工艺条件,采用电子束热蒸发技术在K9基底上制备了单层LaTiO3激光薄膜。研究了不同工艺条件对LaTiO3薄膜激光损伤特性的影响。研究结果表明,对LaTiO3薄膜激光损伤阈值（laser-induced damage threshold, LIDT）影响最大的工艺条件是沉积温度,其次是工作真空度,最后是蒸发束流。获得了制备单层LaTiO3激光薄膜的最优工艺条件:沉积温度175 ℃、工作真空度2.010-2 Pa、蒸发束流120 mA（8 keV）;证明了最优工艺下制备的LaTiO3薄膜具有良好的激光损伤特性、稳定性以及重复性,所制备LaTiO3薄膜的激光损伤阈值为16.9 J/cm2（1 064 nm,10 ns）。     

Presence of 3d quadrupole moment in LaTiO3 studied by 47,49Ti NMR

47,49Ti NMR spectra of LaTiO3 are reexamined and the orbital state of this compound is discussed. The NMR spectra of LaTiO3 taken at 1.5 K under zero external field indicate a large nuclear quadrupole splitting. This splitting is ascribed to the presence of the rather large quadrupole moment of 3d electrons at Ti sites, suggesting that the orbital liquid model proposed for LaTiO3 is inappropriate. The NMR spectra are well explained by the orbital ordering model expressed approximately as 1/square root of 3(d(xy)+d(yz)+d(zx)) originating from a crystal field effect. It is also shown that most of the orbital moment is quenched.     

Orbital liquid in three-dimensional mott insulator: LaTiO3

We present a theory of spin and orbital states in Mott insulator LaTiO3. The spin-orbital superexchange interaction between d(1)(t(2g)) ions in cubic crystal suffers from a pathological degeneracy of orbital states at the classical level. Quantum effects remove this degeneracy and result in the formation of the coherent ground state, in which the orbital moment of t(2g) level is fully quenched. We find a finite gap for orbital excitations. Such a disordered state of local degrees of freedom on unfrustrated, simple cubic lattice is highly unusual. Orbital liquid state naturally explains observed anomalies of LaTiO3.     

Optical study of the free-carrier response of LaTiO3/SrTiO3 superlattices

We used infrared spectroscopic ellipsometry to investigate the electronic properties of LaTiO_{3}/SrTiO_{3} superlattices (SLs). Our results indicated that, independent of the SL periodicity and individual layer thickness, the SLs exhibited a Drude metallic response with sheet carrier density per interface approximately 3x10;{14} cm;{-2}. This is probably due to the leakage of d electrons at interfaces from the Mott insulator LaTiO3 to the band insulator SrTiO3. We observed a carrier relaxation time approximately 35 fs and mobility approximately 35 cm;{2} V-1 s;{-1} at 10 K, and an unusual temperature dependence of carrier density that was attributed to the dielectric screening of quantum paraelectric SrTiO3.     

Spin dynamics and orbital state in LaTiO3

A neutron scattering study of the Mott-Hubbard insulator LaTiO3 ( T(N) = 132 K) reveals a spin wave spectrum that is well described by a nearest-neighbor superexchange constant J = 15.5 meV and a small Dzyaloshinskii-Moriya interaction ( D = 1.1 meV). The nearly isotropic spin wave spectrum is surprising in view of the absence of a static Jahn-Teller distortion that could quench the orbital angular momentum, and it may indicate strong orbital fluctuations. A resonant x-ray scattering study has uncovered no evidence of orbital order in LaTiO3.     

应力作用下GdTiO_3磁序相变的第一性原理研究

采用基于密度泛函理论的第一性原理方法,系统研究了GdTiO_3薄膜在压缩应力和拉伸应力作用下的磁序相变.计算结果表明:1)在LaAlO_3压缩衬底的作用下,GdTiO_3薄膜从铁磁基态转变为G型反铁磁基态.该结果不同于YTiO_3和LaTiO_3在LaAlO_3压缩衬底作用时都呈现A型反铁磁基态的情况.若进一步加大压缩应力,例如在(001)平面施加YTiO_3衬底,此时GdTiO_3薄膜基态才为A型反铁磁态.2)在LaScO_3和BaZrO_3拉伸衬底的作用下,GdTiO_3薄膜的基态仍是铁磁态,但是随着拉伸应力的增大,A型反铁磁态的能量和铁磁态的能量差逐渐缩小,即GdTiO_3薄膜的基态有转变为A型反铁磁态的趋势.3)在外加应力的作用下,GdTiO_3薄膜基态的磁序发生了相变,但是其绝缘性并没有变,说明GdTiO_3薄膜仍为Mott型绝缘体.     

Correlation-driven charge order at the interface between a Mott and a band insulator

To study digital Mott insulator LaTiO3 and band insulator SrTiO3 interfaces, we apply correlated band theory within the local density approximation including a Hubbard U to (n, m) multilayers, 1相似文献   

Raman scattering in the Mott insulators LaTiO3 and YTiO3: evidence for orbital excitations

Raman scattering is used to observe pronounced electronic excitations around 230 meV--well above the two-phonon range--in the Mott insulators LaTiO3 and YTiO3. Based on the temperature, polarization, and photon energy dependence, the modes are identified as orbital excitations. The observed profiles bear a striking resemblance to magnetic Raman modes in the insulating parent compounds of the superconducting cuprates, indicating an unanticipated universality of the electronic excitations in transition metal oxides.     

Transition from orbital liquid to Jahn-Teller insulator in orthorhombic perovskites RTiO3

Following the same strategy used for RVO3, thermal conductivity measurements have been made on a series of single-crystal perovskites RTiO3 (R=La,Nd,...,Yb). Results reveal explicitly a transition from an orbital liquid to an orbitally ordered phase at a magnetic transition temperature, which is common for both the antiferromagnetic and ferromagnetic phases in the phase diagram of RTiO3. This spin/orbital transition is consistent with the mode softening at T_{N} in antiferromagnetic LaTiO3 and is supported by an anomalous critical behavior at T_{c} in ferromagnetic YTiO3.     

Evidence for Jahn-Teller distortions at the antiferromagnetic transition in LaTiO3

LaTiO3 is known as a Mott insulator which orders antiferromagnetically at T(N)=146 K. We report on results of thermal expansion and temperature dependent x-ray diffraction together with measurements of the heat capacity, electrical transport measurements, and optical spectroscopy in untwinned single crystals. At T(N) significant structural changes appear, which are volume conserving. Concomitant anomalies are also observed in the dc resistivity, in bulk modulus, and optical reflectivity spectra. We interpret these experimental observations as evidence of orbital order.     

Wedge-shaped potential and Airy-function electron localization in oxide superlattices

Oxide superlattices and microstructures hold the promise for creating a new class of devices with unprecedented functionalities. Density-functional studies of the recently fabricated, lattice-matched perovskite titanates (SrTiO3)n/(LaTiO3)m reveal a classic wedge-shaped potential well for the monolayer (m = 1) structure, originating from the Coulomb potential of a two-dimensional charged La sheet. The potential in turn confines the electrons in the Airy-function-localized states. Magnetism is suppressed for the monolayer structure, while in structures with a thicker LaTiO3 part, bulk antiferromagnetism is recovered, with a narrow transition region separating the magnetic LaTiO3 and the nonmagnetic SrTiO3.     

Mott transition and suppression of orbital fluctuations in orthorhombic 3d1 perovskites

Using t(2g) Wannier functions, a low-energy Hamiltonian is derived for orthorhombic 3d(1) transition-metal oxides. Electronic correlations are treated with a new implementation of dynamical mean-field theory for noncubic systems. Good agreement with photoemission data is obtained. The interplay of correlation effects and cation covalency (GdFeO3-type distortions) is found to suppress orbital fluctuations in LaTiO3 and even more in YTiO3, and to favor the transition to the insulating state.     

Surface gap and surface electronic states in CuGeO3 single crystal

The surface and bulk electronic excitations of CuGeO₃ are investigated by means of electron energy loss and polarized X-ray absorption spectroscopy. CuGeO₃ shows a surface charge transfer gap of about 3.0±0.3 eV. The unoccupied oxygen derived density of states, as probed by X-ray absorption at the O 1s edge, is in good agreement with recent many-body calculations.     

Determination of the orbital moment and crystal-field splitting in LaTiO3

Utilizing a sum rule in a spin-resolved photoelectron spectroscopic experiment with circularly polarized light, we show that the orbital moment in LaTiO3 is strongly reduced from its ionic value, both below and above the Ne el temperature. Using Ti L2,3 x-ray absorption spectroscopy as a local probe, we found that the crystal-field splitting in the t2g subshell is about 0.12-0.30 eV. This large splitting does not facilitate the formation of an orbital liquid.     

Universality of Short-Range Nucleon-Nucleon Correlations in Nuclei

We investigate the short-range correlations in light nuclei. The highly correlated many-body states are obtained with an explicitly correlated basis which enables us to get a precise solution of a many-body Schrödinger equation for a realistic interaction. We show two-body density distributions for the different spin-isospin channels calculated from three- and four-body states to investigate the short-range correlations between nucleon pairs. At distances below 1 fm a universal behavior is found which does not depend on the many-body states. The universality is also seen in high momentum components of the two-body momentum distributions.     

Hartree-Fock基矢下第一性原理的多体微扰理论计算

从现实核力出发（手征有效场论N3LO）,应用多体微扰理论对一些双幻核进行计算。借助相似重整化群理论对手征有效场论核力进行"软化"处理。在Hartree-Fock基矢下对有效哈密顿量进行多体微扰理论计算,对能量的修正计算到第三阶,对波函数微扰修正到第二阶。利用反对称化的Goldstone图对波函数进行展开,进而对单体密度进行修正,从单体密度出发对原子核半径进行计算。与实验数据进行对比,给出了很好的计算结果。Starting from chiral N3LO, we have applied many-body perturbation theory (MBPT) to the structure of spherical, doubly closed-shell nuclei. The two-body N3LO interaction is softened by a similarity renormalization group transformation. The MBPT calculations are performed within the Hartree-Fock (HF) bases. Higher-order corrections in the HF basis are small relative to the leading-order perturbative result. Corrections up to the third order in energy and up to the second order in wave function are evaluated. Using the anti-symmetrized Goldstone diagram expansions of the wave function, we directly correct the one-body density for the calculation of the radius. Our results are in very good agreement with experimental data.     

Si_3分子基态(X~1A_1)的结构与分析势能函数

应用群论及原子分子反应静力学方法推导Si分子的电子态及其离解极限,在B3P86/CC-PVTZ水平上,对Si3分子基态进行优化计算,得出Si3基态的单重态能量最低,其稳定构性为的C2V构型,平衡核间距Re=0.2176nm、∠213=79.7°,能量为-869.2057a.u..同时计算出基态的简正振动频率:对称伸缩振动频率ν(B2)=547.6446cm-1,弯曲振动频率ν(A1)=185.6100cm-1和反对称伸缩振动频率ν(A1)=559.6090cm-1.在此基础上,使用多体项展式理论方法,导出了基态Si3分子的全空间解析势能函数,该势能函数准确再现了Si3(C2V)平衡结构.     

Single-particle relaxation time of the two-dimensional electron gas in Si/SiGe: Many-body effects

The single-particle relaxation time of the two-dimensional electron gas in SiGe/Si/SiGe quantum wells is calculated. Many-body effects beyond the random-phase approximation become important at low electron density. For charged impurity scattering (remote doped), the importance of these many-body effects as functions of the electron density and spacer width is analyzed. Induced by many-body effects, a strong reduction of the single-particle relaxation time at low electron densities is predicted. The relation with the transport scattering time is described, multiple-scattering effects are commented, and the determination of many-body effects in existing samples is discussed.