First-Principles Investigation of Electronic Structure Features of TbOFeAs Compound

We report a theoretical investigation on the electronic and magnetic properties of rare-earth pnictide parent compound, such as TbOFeAs. Employing first-principles method supplemented by the local spin density approximation (LSDA), we discuss the electronic structure with the incorporation of the role of Coulomb on-site repulsion (U) of Tb 4f states as well as the spin-orbit (SO) coupling on the magnetic and nonmagnetic phases. For ferromagnetic (FM) and antiferromagnetic (AFM) phases, we have determined the spin and orbital magnetic moments of Tb ions and confer the significance of the spin-orbit interaction of Tb 4f states in this parent compound. In the FM state, the reduction of Fe moment is about a factor of 3.5 with respect to AFM configuration. The most energetically favorable state is AFM configuration. Our theoretical findings surmise that the magnetic moments on Fe sites carry an AFM order. Based on LSDA + U + SO approximation, we infer that the Tb magnetic moments also carry an AFM order, albeit the spin Tb sites in TbO layer possess the same orientation as the Fe spins in FeAs layer. With the incorporation of on-site Coulomb repulsion and spin-orbit interaction in AFM state, the Fe 3d states are large near the Fermi level and this phase is illustrating a metallic behavior. Moreover, the Fermi surface topology and nesting features are presented.     

Transport Measurements of the Heavy Fermion Superconductor CeCu2Si2 Under Pressure

Superconducting and normal state properties of a single crystal of CeCu₂Si₂ have been investigated by resistivity measurements under hydrostatic pressure and magnetic field. The low temperature resistivity behaviour ( = ₀ + AT ², which is observed up to a temperature T _A) is considered within the self-consistent renormalized (SCR) spin fluctuation model. Contrary to the model predictions, the product AT ² _A increases on approaching the quantum critical point (QCP). The temperature dependence of the upper critical field is analyzed assuming strong coupling and an intermediate regime between the clean and dirty limits. Both the low temperature resistivity behaviour at high pressure and the parameters deduced from the fits of H _c2 point to a pressure induced decrease of the quasi-particle effective mass m*. The anisotropy of the initial slope of H _c2(T) and therefore that of the effective mass was found to change under pressure.     

Magnetic Susceptibility of Heavy Fermion 3He-Bilayers

A ³He bilayer film adsorbed on graphite preplated with a bilayer of solid ⁴He has recently been shown to be a new heavy fermion system that can be tuned towards a quantum critical point (QCP) by varying the ³He coverage. We report new measurements of the temperature dependence of the magnetic susceptibility of this system over the temperature range 0.3 to 300 mK. The magnetisation of the ³He film is measured using pulsed NMR at a frequency of 60 kHz, on a SQUID detector setup with an untuned input circuit. The signal sensitivity attainable with this setup enables us to collect high-precision data with reasonable acquisition times, despite the low surface area of the sample (2 m²) and the small liquid susceptibility. The detailed NMR response provides an essential diagnostic tool to precisely adjust the ⁴He preplating. On cooling, the magnetisation evolves from Curie-law, through a distinct Kondo-like maximum at a temperature T ^*, to a weakly rising magnetisation far below the Curie law. This behaviour signifies the formation of the heavy fermion state of the coupled ³He layers. As the ³He coverage is increased towards the putative QCP, T ^* is tuned to lower temperature. These preliminary observations demonstrate the power of the technique, confirm previous results obtained in a different sample cell by a different method, and pave the way for a detailed study close to the QCP.     

A First-Principles Investigation on Electronic Structure,Optical and Thermoelectric Properties of Janus In2SeTe Monolayer

Journal of Superconductivity and Novel Magnetism - The Janus two-dimensional (2D) materials have recently demonstrated excellent physical and chemical properties. The electronic, thermoelectric,...     

First-Principles Study on the Electronic and Magnetic Properties of Zigzag AlN-SiC Nanoribbons

The electronic and magnetic properties of zigzag AlN-SiC nanoribbons are investigated by using the first-principles calculations. The band structures reveal that all the investigated AlN-SiC systems are the magnetic semiconductors, the band gaps of which decrease with the increasing width of the ribbon. The majority spin density is mostly contributed by the edge C atoms with dangling bonds. The total magnetic moments increase with the increasing width of the ribbon and decrease with the increase of the strain. These studies are helpful to the potential applications of the AlN-SiC ribbon in spintronics.     

Frustration and the Kondo Effect in Heavy Fermion Materials

The observation of a separation between the antiferromagnetic phase boundary and the small-large Fermi surface transition in recent experiments has led to the proposal that frustration is an important additional tuning parameter in the Kondo lattice model of heavy fermion materials. The introduction of a Kondo (K) and a frustration (Q) axis into the phase diagram permits us to discuss the physics of heavy fermion materials in a broader perspective. The current experimental situation is analyzed in the context of this combined “QK” phase diagram. We discuss various theoretical models for the frustrated Kondo lattice, using general arguments to characterize the nature of the f-electron localization transition that occurs between the spin liquid and heavy Fermi liquid ground-states. We concentrate in particular on the Shastry–Sutherland Kondo lattice model, for which we establish the qualitative phase diagram using strong coupling arguments and the large-N expansion. The paper closes with some brief remarks on promising future theoretical directions.     

High Pressure Resistivity of the Heavy Fermion Compound CeCu6

High pressure resistivity measurements up to 8.5 GPa performed on the archetypal heavy fermion compound CeCu₆ are reported down to temperatures of 30 mK. The pressure variation of the different characteristic temperatures and resistivity exponents are analyzed in the framework of recent theories aiming to understand quantum phase transitions.     

Thermopower of Yb Heavy Fermion Compounds at High Pressure

The absolute thermopower S(T) of magnetic and non-magnetic Yb-based heavy fermions compounds was investigated at ambient pressure. High pressure experiments were performed for YbSiT _N =1.6 K) and YbCu ₂ Si ₂ . For the latter, which is not magnetically ordered at P=0, the chemical pressure effect was studied within the solid-solution Yb(Ni _x Cu _1–x ) ₂ Si ₂ (0x1). All S(T)-curves obtained at ambient pressure are characterised by a giant negative peak at a temperature T _{min, S} , and by an additional positive maximum at low temperature for the magnetic compounds. With increasing pressure, S(T _min,S ) and T _min,S were reduced, reflecting a reduction of the Kondo effect. At low temperature a magnetic transition is observed at P>8 GPa in the case of YbCu ₂ Si ₂ without a sign change in contrast to the alloys Yb(Ni _x Cu _1–x ) ₂ Si ₂.     

Al-Zn-Mg合金的相图计算及电子结构与力学性能的第一性原理计算

通过热力学计算得到了Al-Zn-Mg合金室温平衡相图,计算结果表明,Al-xZn-yMg(x=5.0％ ～6.5％;y=1.5％ ～3.0％,均为质量分数)合金室温下稳定存在的第二相为MgZn2、Al2Mg3Zn3、Mg32(Al,Zn)49.采用基于密度泛函理论的第一性原理赝势平面波方法,计算研究了三种相的生成焓和结合能.计算结果表明,MgZn2具有较弱的合金化能力及相结构稳定性.根据合金中第二相的含量,筛选出每种第二相含量最多的三个合金成分为Al-6.0Zn-2.9Mg、Al-6.5Zn-1.7Mg和Al-6.5Zn-2.5Mg,构建了Al37 Zn1 Mg2、Al49 Zn2 Mg1和Al34 Zn1 Mg1三个固溶体超胞,计算研究了三者的弹性常数、能带结构和电子态密度等本征特性.计算结果表明:Al49 Zn2 Mg1的强度和塑性最好;Al37 Zn1 Mg2的抗剪切能力和刚度最好;Al49 Zn2 Mg1呈韧性,而Al34 Zn1 Mg1和Al37 Zn1 Mg2均呈脆性.能带结构和电子态密度计算结果表明三者均具有金属键,存在金属导电性,Al37 Zn1 Mg2的共价性强于Al49 Zn2 Mg1和Al34 Zn1 Mg1,Al37 Zn1 Mg2结构最稳定.     

Electronic Structure and Thermodynamic Properties of the Cubic Antiperovskite Compound InNCe_{3} via First-Principles Calculations

Elastic, thermodynamic, electronic, and magnetic properties in the cubic antiperovskite InNCe $_{3}$ 3 compound are derived from the full-potential linear muffin-tin orbital method. From the computed elastic constants, theoretical values of Young’s modulus, the shear modulus, Poisson’s ratio, Lamé’s coefficients, sound velocities, and the Debye temperature are evaluated. Analysis of the ratio between the bulk modulus and the shear modulus shows that InNCe $_{3}$ 3 is brittle in nature. The variations of elastic constants with pressure indicate that this compound possesses higher mechanical stability in the pressure range from 0 to 40 GPa. The electronic and magnetic properties of this compound are calculated by adding the Coulomb interaction $U$ U to improve the results.     

Ti3SnC2电子结构的第一性原理研究

采用密度泛函数理论框架下的第一性原理研究了Ti3SnC2的电子结构,利用GGA-PW91基组对Ti3SnC2晶体结构进行了优化,并计算了Ti3SnC2的能带结构、总态密度和各原子的分态密度.对能带和总态密度的计算结果表明,Ti3SnC2在费米能级处电子态密度较高,材料表现出较强的金属性,同时材料的导电性为各向异性.Ti3SnC2各原子的分态密度图的计算结果表明,其导电性主要由Ti2的3d电子决定,Ti1的3d态电子、Sn的5p态电子和C的2p态电子也有少量贡献.决定材料电学性质的主要是Ti的3d、Sn的5p和C的2p态电子的p-d电子轨道杂化,而p-d电子轨道杂化成键则使材料具有比较稳定的结构.     

First-Principles Study on Half-Metallic Full-Heusler Compound Mn2ZnSi

By first-principle calculation, the electronic structure and magnetic properties of the Mn₂ZnSi full-Heusler alloy with CuHg₂Ti-type structure are investigated. Calculations show that Mn₂ZnSi compound presents half-metallic ferrimagnetic properties under the equilibrium lattice constant. The influence of spin-orbit interaction for the magnetic moments is investigated. The result shows spin-orbit interaction has little influence on magnetic moment. The bulk modulus of Mn₂ZnSi obtained by a fit of the Murnaghan equation of state is 134.3 GPa, which is more compressible than some other Heusler alloys. At the pressure range of 0 to 17.7 GPa, Mn₂ZnSi presents half-metallic character. Mn₂ZnSi would be a promising material for future spintronic applications.     

Unconventional Superconductivity and Heavy Fermion Behavior in PrOs4Sb12

Research carried out during the last several years on the heavy fermion superconductor PrOs₄Sb₁₂ is reviewed. The compound PrOs₄Sb₁₂ exhibits unconventional strong coupling superconductivity below a superconducting critical temperature T _c = 1.85 K. The superconducting state breaks time reversal symmetry, apparently consists of several distinct superconducting phases, some of which may have point nodes in the energy gap, and may involve triplet spin pairing of electrons. The determination of the Pr³⁺ energy level scheme in the crystalline electric field, as well as the discovery and characterization of the high field-ordered phase (HFOP), which has been identified with antiferroquadrupolar order, are described. Studies of the substitutional systems Pr(Os_1−x Ru _x )₄Sb₁₂, Pr_1−x La _x Os₄Sb₁₂, and Pr_1−x Nd _x Os₄Sb₁₂ are summarized and compared. PACS 71.27.+a; 74.62.Dh; 74.70.Tx.     

K掺杂立方相Ca2Si电子结构及光学性能的研究

基于第一性原理的密度泛函理论赝势平面波方法,采用广义梯度近似（GGA）对掺K的立方相Ca2Si的电子结构和光学性能,包括能带结构、态密度、介电函数、折射率、反射率、吸收系数、光电导率及能量损失函数进行理论计算,结果表明,掺K后立方相Ca2Si的能带向高能方向发生了偏移,形成直接带隙的P型半导体,禁带宽度为0.6230eV,光学带隙变宽,价带主要是Si的3p、Ca的4s、3d以及K的3p、4s态的贡献;静态介电函数ε1（0）=14.4;折射率n0=3.8;吸收系数最大峰值为3.47×105cm-1。通过掺杂调制材料电子结构和光电性能,为Ca2Si材料光电性能的开发与应用提供了理论依据。     

First-Principles Study on the Stability and Electronic Properties of Bi-Doped Sr3Ti2O7

The electronic and crystal structural properties of Bi-doped Sr_3Ti_2O_7are studied using the first principles density functional theory(DFT)based on pseudopotentials basis and plane-wave method.Our results show that the formation energy of Bi doping in Site-1 and Site-2 of Sr_3Ti_2O_7increases with increasing doping concentration.And at the same doping concentration,the formation energy of Bi doping in Site-2 is lower than that in Site-1.The undoped Sr_3Ti_2O_7is found to be an insulator and its Fermi level stays at the top of the valence band.While the Fermi level of the Bi-doped Sr_3Ti_2O_7moves into the bottom of conduction band,the system undergoes an insulator-to-metal phase transition.Furthermore,our calculation results demonstrated that the Fermi level of the Bi-doped Sr_3Ti_2O_7goes deeper into the bottom of conduction band with increasing doping concentration.     

Effect of Pressure on Transport Properties of Heavy Fermion CePtSi<Subscript>2</Subscript>

We measured the resistivity of heavy fermion CePtSi₂ under pressure. At ambient pressure, CePtSi₂ shows an antiferromagnetic (AF) transition at 2 K and a Fermi liquid like T ² dependence in resistivity below 1.5 K. With increasing pressure, the AF phase and T ² dependence are suppressed. Above 1.4 GPa, a T linear dependence and pressure-induced superconductivity were found with the maximum T _c=0.14 K at 1.7 GPa. Above 2 GPa, the T ² dependence recovers just above T _c.