Low-energy spectrum and finite temperature properties of quantum rings

Recently it was demonstrated that the rotational and vibrational spectra of quantum rings containing few electrons can be described quantitatively by an effective spin-Hamiltonian combined with rigid center-of-mass rotation and internal vibrations of localized electrons. We use this model Hamiltonian to study the quantum rings at finite temperatures and in presence of a nonzero magnetic field. Total spin, angular momentum and pair correlation show similar phase diagram which can be understood with help of the rotational spectrum of the ring. Received 18 January 2002 Published online 13 August 2002     

Enhanced dielectric properties of lead barium zirconate thin films by manganese doping

(Pb_0.5Ba_0.5)ZrO₃ (PBZ) and 1 mol% Mn-doped (Pb_0.5Ba_0.5)ZrO₃ (Mn-PBZ) sol were successfully fabricated, and corresponding thin films were deposited on Pt(1 1 1)/TiO₂/SiO₂/Si(1 0 0) substrates by spin-coating method. Effects of Mn doping on the microstructure and electrical properties of PBZ thin films were investigated systemically. X-ray diffraction patterns showed that both films had a polycrystalline perovskite structure, and that the degree of (1 1 1) orientation were increased by Mn doping. Dielectric measurements illustrated that Mn-doped PBZ thin films not only had a larger dielectric constant, but also possessed a smaller dielectric loss. Accordingly, the tunability and the figure of merit of PBZ films were improved by Mn doping.     

Origin of the 330 nm absorption band and effect of doping Yb in LiYF4 crystals

The electronic structures of LiYF₄ (YLF) crystals containing F color center (YLF-F) and Yb doped YLF crystals (Yb³⁺:YLF, Yb²⁺:YLF) are systematically studied within the framework of the density functional theory. The calculated results indicate that the 330 nm absorption band originates from the F center in YLF crystals. Thus the doping of Yb³⁺ can weaken the 330 nm absorption band by competing with F vacancies in capturing free electrons arising after γ-irradiation and change to Yb². By analyzing the lattice relaxation and the electronic structure of YLF containing Yb²⁺, we can reasonably believe that once Yb²⁺ is formed in YLF crystal, its compensating hole will turn out to be shared by two F⁻ nearest to Yb²⁺ forming a diatomic fluoride molecular ion () perturbed by Yb²⁺, or to say V_F color center. According to the molecular-orbital linear combination of atomic orbital (MO-LCAOs) theory, compared to the alkali halides, e.g. LiF, the in V_F center in LiYF₄ peaks at about 340 nm, which is in agreement with the experimental results.     

Electronic structure of graphene/Y2C heterostructure and related doping effect

Until now, many attempts have been made to dope graphene in various ways, but each method turned out to have pros and cons. In this study, to overcome the limitations of doping methods, yttrium hypocarbide (Y₂C) is investigated as one prospective material to dope graphene, using density functional theory calculations. In monolayer Y₂C, the anionic electrons localized away from Y atomic layers are confirmed to contribute to occupied states near the Fermi level. Next, we investigate the electronic structure of graphene in heterojunction with Y₂C. Anionic electrons of Y₂C occupy the empty states of graphene in graphene/Y₂C heterostructure, which makes the Dirac cone of graphene located at about 1.7 eV below the Fermi level. Such charge transfer of anionic electrons to graphene and the flatness of electric cloud of anionic electrons leads to evenly n-doped graphene in graphene/Y₂C heterostructure. This suggests that Y₂C is a good candidate to dope graphene.     

Ultrafast transverse thermoelectric response in c‐axis inclined epitaxial La0.5Sr0.5CoO3 thin films

Ultrafast transverse thermoelectric voltage response has been observed in c‐axis inclined epitaxial La_0.5Sr_0.5CoO₃thin films. Voltage signals with the rise time of 7 ns have been detected under the irradiation of pulse laser with duration of 28 ns. A concept, named response rate ratio, has been proposed to evaluate the intrinsic response rate, and this ratio in La_0.5Sr_0.5CoO₃is smaller than that in other reported materials. The low resistivity is thought to be responsible for the ultrafast response, as low resistivity induces small optical penetration depth, and response time has a monotonous increasing relationship with this depth.

     

La1．6（MoO4）3∶Eu3＋0．4纳米晶合成及发光特性

采用燃烧法合成了La1．6（MoO4）3∶Eu3＋0．4纳米晶末,研究了其声子-掺杂-晶格相互作用和发光性质。 X射线粉末衍射（XRD）分析表明,在500～900℃退火后,La1．6（MoO4）3∶Eu3＋0．4样品为单一晶相。对样品进行了光致发光（PL）测量,激发Mo6＋-O2－电荷迁移带,观察到Eu3＋的系列发光,表明Mo6＋-O2－带和Eu3＋间存在能量传递,中心波长分别在λ1＝469 nm和λ2＝426 nm处的两个one-phonon边带,相应的声子能量分别为767和1202 cm－1,分别对应于Mo O和Mo—O—Mo伸缩振动。同时,计算了两个局域模电子-声子耦合强度的黄昆因子分别为S1＝0.055和S2＝0.037,为揭示其三价离子高传导特性及其负热膨胀物理特性提供了实验基础。     

First-principles investigation of the elastic and electronic properties of the binary intermetallics in the Al–La alloy system

The structural, elastic and electronic properties of Al₂La, AlLa₃ and Al₃La binary intermetallics in the Al–La alloy system were investigated using the first-principles method. The calculated lattice constants were consistent with the experimental values. Formation enthalpy and cohesive energy showed that the studied Al₂La, AlLa₃ and Al₃La all have a higher structural stability, and the alloying ability of Al₂La and Al₃La is stronger than that of AlLa₃. The single-crystal elastic constants (C_ij) as well as polycrystalline elastic parameters (bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio υ and anisotropy value A) were calculated by the Voigt–Reuss–Hill (V–R–H) approximations, and the relationship of these elastic parameters between Al₂La, AlLa₃ and Al₃La phases were discussed in detail. The results showed that Al₂La and Al₃La which are anisotropic materials are absolutely brittle, while the isotropic AlLa₃ is slightly ductile. Finally, the electronic density of states (DOS) was also calculated to reveal the underlying mechanism of structural stability.     

Electronic and magnetic behaviors of graphene with 5d series transition metal atom substitutions: A first-principles study

The electronic structures and magnetic behaviors of graphene with 5d series transition metal atom substitutions are investigated by performing first-principles calculations. All the impurities are tightly bonded to single vacancy in a graphene sheet. The substitutions of La and Ta lead to Fermi level shifting to valence and conduction band, respectively. Both the two substitutions result in metallic properties. Moreover, the Hf, Os and Pt-substituted systems exhibit semiconductor properties, while the Re and Ir-substituted ones exhibit robust half-metallic properties. Interestingly, W-substituted system shows dilute magnetic semiconductor property. On the other hand, the substitution of Ta, W, Re and Ir induce 0.86 μ_B, 2 μ_B, 1 μ_B and 0.99 μ_B magnetic moment, respectively. Our studies demonstrate that the 5d series transition metal substituted graphene have potential applications in nanoelectronics, spintronics and magnetic storage devices.     

Energy of exciton in quantum dot for a potential containing Coulomb and quadratic terms

We theoretically study the energy levels of an exciton in a quantum dot. We take in to account both quadratic and Coulomb terms. Next, we use the method of series to solve the Schrödinger equation exactly. Using this formalism, we have calculated the exciton energy in both ground and excited states. The results are comparable to those of variational exact diagonolization, full configuration interaction, Hartree-Fock and 1/N methods. Our approach could be fitted for any desired material.     

Impurity substitution effects in BiFeO3 thin films—From a viewpoint of FeRAM applications

Impurity substitution effects in BiFeO₃ thin films are reviewed from a viewpoint of FeRAM (ferroelectric random access memory) applications, in which such characteristics as a large remanent polarization, a low coercive voltage, and excellent fatigue endurance are most important. First, it is described that substitution experiments for Bi and Fe atoms in the films have already been conducted using almost all rare earth and transition metal elements. A list of the published paper is given in a form of the periodic table of elements. Then, two typical cases, La substitution for the Bi site and Mn substitution for the Fe site, are reviewed in detail. Particular attention is paid to the role of these impurity atoms by which the ferroelectric and insulating properties of BiFeO₃ films are significantly improved. Finally, impurity effects due to substitution or co-substitution of other elements are reviewed.