Scaling behavior of the thermopower of the archetypal heavy-fermion metal YbRh2Si2

We reveal and explain the scaling behavior of the thermopower S/T exhibited by the archetypal heavy-fermion (HF) metal YbRh₂Si₂ under the application of magnetic field B at temperature T. We show that the same scaling is demonstrated by different HF compounds such as β-YbAlB₄ and the strongly correlated layered cobalt oxide [BiBa_0.66K_0.36O₂]CoO₂. Using YbRh₂Si₂ as an example, we demonstrate that the scaling behavior of S/T is violated at the antiferromagnetic phase transition, while both the residual resistivity ρ₀ and the density of states, N, experience jumps at the phase transition, causing the thermopower to make two jumps and change its sign. Our elucidation is based on flattening of the single-particle spectrum that profoundly affects ρ₀ and N. To depict the main features of the S/T behavior, we construct a T –B schematic phase diagram of YbRh₂Si₂. Our calculated S/T for the HF compounds are in good agreement with experimental facts and support our observations.     

Neutron diffraction study of magnetic ordering in ErMn2Si2, ErMn2Ge2 and ErFe2Si2

Neutron diffraction study of polycrystalline compounds ErMn₂Si₂, ErMn₂Ge₂ and ErFe₂Si₂ was performed in the temperature range between 1.8 and 293 K. All compounds have tetragonal, ThCr₂Si₂-type crystal structure. The antiferromagnetic collinear structure of ErMn₂Si₂ and ErMn₂Ge₂ at both RT and LNT, consists of a sequence + - + - of ferromagnetic layers of Mn atoms. The magnetic moment of an Mn atom (≈2μ_B) is parallel to the c-axist. At low temperatures (LHT and lower), the ferromagnetic ordering within the Er sublattice is observed. The magnetic moment (μ_Er ≈ 9μ_B) is perpendicular to the c-axis. From the temperature dependence of the intensities of the magnetic peaks, the following values for the Curie temperatures were obtained: (10±5) K for ErMn₂Si₂ and (8.5±3) K for ErMn₂Ge₂. For ErFe₂Si₂ a collinear antiferromagnetic structure of the + - - + type was found, the magnetic unit cell consisting of the chemical one, doubled along the c-axis.     

Magnetic field dependence of spin-flip cross section for polarized neutron scattering of CeCu2Si2

The magnetic field dependence of the spin-flip cross section for quasielastic polarized neutron scattering is investigated for the Ce Kondo system at zero temperature. An exact calculation is done for the differential cross section with a small energy transfer and also for the total cross section. In particular the latter case is discussed in detail in order to investigate the recent experiment done for the polycrystalline CeCu₂Si₂. With the use of the obtained results, two possibilities are discussed about the wavefunction for the crystal-field ground doublet of the f-electron in CeCu₂Si₂.     

Physical properties of quaternary compounds Gd2CoAl4T2 (T= Si,Ge) single crystals

We have synthesized and investigated physical properties of two new quaternary compounds Gd₂CoAl₄T₂ (T= Si, Ge) single crystals, which are isostructural to Tb₂NiAl₄Ge₂ and Er₂CoAl₄Ge₂. The most important structural feature of these materials is the anti-CaF₂-type CoAl₄T₂ slabs. These materials show metallic behavior below 300 K and there is a long-range antiferromagnetic (AFM) transition appearing at 20 and 27 K for Gd₂CoAl₄Ge₂ and Gd₂CoAl₄Si₂, respectively. Resistivity and heat capacity measurements also confirm these bulk AFM transitions. Further analysis indicates that this long-range antiferromagnetism should result from the magnetic interaction between local moments of Gd³⁺ ions.     

A Comparative Study of the Electronic and Magnetic Properties of Gd5Ge4 and Gd5Si4 Compounds

We present a comparative study of electronic structure and magnetic properties of Gd₅Si₄ and Gd₅Ge₄ compounds using first principles full potential linearized augmented plane wave (FP-LAPW) method based on density functional theory (DFT) using the WIEN2k code. The local-spin density approximation with correlation energy (LSDA+U) method has been used as the exchange-correlation potential. The optimized lattice constants are in good agreement with the experimental data. The total and partial density of states (DOS) of Gd₅Si₄ and Gd₅Ge₄ show the difference in Si 3p-Gd 5d and Ge 4p-Gd 5d hybridization, which have an effective role in indirect exchange interaction. In addition, the magnetic moments of Gd, Si, and Ge atoms and the compounds are calculated to clarify the differences in the magnetic properties of these compounds.     

重费米子超导与竞争序

与其他非常规超导系列相比, 重费米子超导体往往具有丰富多样的竞争序, 超导与各种竞争序相伴而生, 电子配对与反铁磁涨落、铁磁涨落、价态涨落、电四极矩涨落等量子临界涨落密切相关, 扩充了非常规超导的研究内容. 重费米子材料中的f电子往往同时参与超导与各种竞争序的形成, 表现出局域与巡游的二重性. 重费米子二流体理论为理解重费米子超导与竞争序的关系提供了新的思路.     

Magnetic structures of R3Cu4Si4 (R=Dy, Ho and Er)

Polycrystalline samples of R₃Cu₄Si₄ (R=Dy, Ho, Er) intermetallics were studied with neutron diffraction methods. All of them crystallize in the orthorhombic structure of Gd₃Cu₄Ge₄-type and order antiferromagnetically at low temperatures. Magnetic moments localized at the rare earth atoms, that occupy two non-equivalent 2d and 4e sublattices, order simultaneously in Dy₃Cu₄Si₄. The order is described by the propagation vector accompanied by , δ=0.025(2). In Ho₃Cu₄Si₄ two propagation vectors are needed to model the magnetic order. These are: for the 4e sublattice, which disorders as the first when the temperature increases, and for the 2d sublattice. A similar situation is observed for Er₃Cu₄Si₄ where the propagation vectors are: k=(0,1−δ,0), δ=0.097(2) for the 4e sublattice, which disorders as the first with increasing temperature, and , δ=0.0015(6) for the 2d sublattice.     

Impact of Al addition on the formation of Ni germanosilicide layers under different temperature annealing

Solid reactions between Ni and relaxed Si_0.7Ge_0.3 substrate were systematically investigated with different Al interlayer thicknesses. The morphology, composition, and micro-structure of the Ni germanosilicide layers were analyzed with different annealing temperatures in the appearance of Al. The germanosilicide layers were characterized by Rutherford backscattering spectrometry, cross-section transmission electron microscopy, scan transmission electron microscopy, and secondary ion mass spectroscopy. It was shown that the incorporation of Al improved the surface and interface morphology of the germanosilicide layers, enhanced the thermal stabilities, and retarded the Ni-rich germanosilicide phase to mono germanosilicide phase. With increasing annealing temperature, Al atoms distributed from the Ni/Si_0.7Ge_0.3 interface to the total layer of Ni₂Si_0.7Ge_0.3, and finally accumulated at the surface of NiSi_0.7Ge_0.3. We found that under the assistance of Al atoms, the best quality Ni germanosilicide layer was achieved by annealing at 700 ℃ in the case of 3 nm Al.     

Disorder effects on transport near AFM quantum phase transitions

We discuss three different scenarios recently proposed to account for the non-Fermi liquid behavior near antiferromagnetic (AFM) quantum critical points in heavy-Fermion systems: (i) scattering of Fermi liquid quasiparticles by strong spin fluctuations near the spin-density-wave instability, (ii) the breakdown of the Kondo effect due to the competition with the RKKY interaction, and (iii) the formation of magnetic regions due to rare configurations of the disorder. Here we focus on the first scenario and show that it explains in some detail the anomalous temperature dependence of the resistivity observed, e.g. in CePd₂Si₂, CeNi₂Ge₂ or CeIn₃. The interplay of strongly anisotropic scattering due to critical spin-fluctuations and weak isotropic impurity scattering leads to a regime with a resistivity

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for sufficiently large T and small ρ₀.     

Superconductivity in self-flux-synthesized single crystalline R_2Pt_3Ge_5(R=La,Ce,Pr)

In order to study the basic superconductivity properties of R_2Pt_3Ge_5, we synthesized the single crystalline samples by the Pt–Ge self-flux method. R_2Pt_3Ge_5(R = La, Ce) were also grown for a systematic study. Zero-resistivity was observed in both the La-and Pr-based samples below the reported superconducting transition temperatures. However, magnetic susceptibility measurements showed low superconductivity volume fractions in both La_2Pt_3Ge_5 and R_2Pt_3Ge_5(less than2%). Ce_2Pt_3Ge_5 did not show any signature of superconductivity. From the specific heat measurements, we did not observe a superconducting transition peak in R_2Pt_3Ge_5, suggesting that it is not a bulk superconductor. The magnetic susceptibility and heat capacity measurements revealed two antiferromagnetic(AFM) orders in R_2Pt_3Ge_5 at T_(N1)= 4.2 K and T_(N2)= 3.5K, as well as a single AFM transition at TN= 3.8 K in Ce_2Pt_3Ge_5.     

700℃退火下铝调制镍硅锗薄膜的外延生长机理

文章研究了在700℃退火下,铝插入层调制镍和硅锗合金反应形成单相镍硅锗化物的生长机理.透射电镜测试结果表明,镍硅锗薄膜和硅锗衬底基本达到赝晶生长;二次质谱仪和卢瑟福沟道背散射测试结果表明,在镍硅锗薄膜形成的过程中,铝原子大部分移动到镍硅锗薄膜的表面.研究结果表明,铝原子的存在延迟了镍和硅锗合金的反应,镍硅锗薄膜的热稳定性和均匀性都得到了提高.最后,基于上述实验结果给出了铝原子调制形成外延镍硅锗薄膜的生长机理.     

Phase transition in Pb4.9A0.1Ge2.7Si0.3O11 (A=Ca,Sr,Ba) ferroelectrics

Polycrystalline samples of modified lead germanate Pb_4.9A_0.1Ge_2.7Si_0.3O₁₁ (A=Ca,Sr,Ba) have been prepared by solid-state reaction technique. X-ray studies have been carried out to check the formation of single-phase composition and to obtain preliminary structural data. Measurements of dielectric permittivity () and loss (tan δ) have been studied both as a function of frequency (10²–10⁴ Hz) and temperature (room temperature to 180°C). Phase transition was observed in all three cases but with different T_c value. From the plots of conductivity vs. temperature using dielectric data, collected at 10 kHz activation energy was calculated to investigate the conduction mechanism in them.     

Neutron scattering studies on heavy-fermion superconductors

We review recent neutron scattering studies on uranium-based heavy-fermion superconductors. The coupling between magnetic and superconducting order parameters was observed in UPt₃, UPd₂Al₃, URu₂Si₂, and UNi₂Al₃. In UPd₂Al₃, the superconducting gap appears in the spin excitation spectra. These results are indicative of the strong interplay between magnetism and superconductivity. We also report the unusual behaviors of the weak antiferromagnetic ordering, the long-range magnetic correlation in UPt₃ at ultra-low temperatures, and the pressure-induced magnetic transition from the weak (0.02μ_B/U) to a high moment state (0.4μ_B/U) at 1.5 GPa in URu₂Si₂.     

Note on the crystal structure and the magnetic properties of the compound CeNi9In2

The crystal structure of the compound CeNi₉In₂ has been studied by time-of-flight neutron diffraction at room temperature. A strong site preference of the s,p element was observed. It is shown that In occupies almost exclusively the 4g site. From magnetic measurements and resistivity measurements it is derived that CeNi₉In₂ is a IV compound in which Ce is close to tetravalent.     

Na_2Ge_2Se_5电子结构和光学性质的第一性原理研究

Na_2Ge_2Se_5是一种优异的红外非线性晶体材料.采用基于第一性原理的密度泛函理论赝势平面波方法对Na_2Ge_2Se_5进行结构优化,并以此为基础计算研究了Na_2Ge_2Se_5的电子结构和光学性质.结果表明:Na2Ge2Se5是宽禁带间接带隙半导体,价带至导带的电子跃迁主要来自于Ge和Se的48,4p态;Na对光学性质的贡献较小,Ge和Se之间的相互耦合作用决定了Na_2Ge_2Se_5的光学性质;该晶体在紫外区有强烈的反射和吸收,静态折射率为2.133,双折射率值适中,为0.145.理论计算结果表明,Na_2Ge_2Se_5是一种性能优良的红外非线性光学晶体材料.     

Itinerant f-electron systems of cerium and uranium compounds

The f-electron systems of the cerium and uranium compounds have attracted attention in the context of heavy fermion and anisotropic superconductivity. We clarified an itinerant f-electron nature for UPt₃ by the de Haas–van Alphen experiments, detecting heavy conduction electrons with 100m₀. The dHvA oscillation was also observed clearly in both the normal and superconducting mixed states in CeRu₂,URu₂Si₂ and UPd₂Al₃. An anisotropic energy gap with a line node for URu₂Si₂ was discussed from the angular dependence of the dHvA amplitude in the mixed state.     

退火处理对W:Bi4Ge3O12和Bi12GeO20晶体发光性能的影响

通过提拉法制备了W:Bi4 Ge3 O12和Bi12GeO20晶体,测试了晶体的吸收光谱、光致发光谱和发光衰减时间等.W:Bi4 Ge3 O12的可见光发光强度比纯Bi4 Ge3 O12有所增强,而且N2中退火处理对W:Bi4Ge3O12发光有进一步增强作用.Bi12GeO20在N2中退火处理后在745 nm附近有发光...     

The vacancy defects and oxygen atoms occupation effects on mechanical and electronic properties of Mo_5Si_3 silicides

Improving brittle behavior and mechanical properties is still a big challenge for high-temperature structural materials.By means of first-principles calculations,in this paper,we systematically investigate the effect of vacancy and oxygen occupation on the elastic properties and brittle-orductile behavior on Mo_5Si_3.Four vacancies (Si_(–Va1),Si_(–Va2),Mo_(–Va1),Mo_(–Va2)) and oxygen occupation models (O_(–Mo1),O_(–Mo2),O_(–Si1),O_(–Si2)) are selected for research.It is found that Mo_(–Va2)vacancy has the stronger structural stability in the ground state in comparison with other vacancies.Besides,the deformation resistance and hardness of the parent Mo_5Si_3are weakened due to the introduction of different vacancy defects and oxygen occupation.The ratio of B/G indicates that oxygen atoms occupation and vacancy defects result in brittle-to-ductile transition for Mo_5Si_3.These vacancies and the oxygen atoms occupation change the localized hybridization between Mo–Si and Mo–Mo atoms.The weaker O–Mo bond is a contributing factor for the excellent ductile behavior in the O_(-Si2)model for Mo_5Si_3.     

Appearance of inverse giant magnetoresistance in noncollinear magnetic systems

We show for a simple d-band TB Hamiltonian that noncollinear magnetic configurations can contribute to large inverse giant magnetoresistance (IGMR) ratios. We make a systematic study as a function of band filling, magnetic moment and canting angle for some simple model examples and use the outcome of this study to interpret the experimentally observed IGMR ratios on LaMn₂Ge₂.     

Structural stability of ultra-high temperature refractory material MoSi_2 and Mo_5Si_3 under high pressure

In-situ angle dispersive x-ray diffraction(ADXRD) with synchrotron radiation source is performed on an ultra-high temperature refractory of MoSi_2 and Mo_5Si_3 by using a diamond anvil cell(DAC) at room temperature. While the pressureinduced volume reduction is almost constant, the value of the bulk modulus increases with the decrease of molybdenum content in the system. According to the Brich–Murnaghan equation, the bulk modulus 222.1(2.1) GPa with its pressure derivative 4 of MoSi_2, and the bulk modulus 308.4(7.6) GPa with its pressure derivative 0.7(0.1) of Mo_5Si_3 are obtained.The experimental data show that MoSi_2 has distinct anisotropic behavior, Mo_5Si_3 is less anisotropic than MoSi_2. The result shows that MoSi_2 and Mo_5Si_3 have the structural stabilities under high pressure. When the pressure reaches up to 41.1 GPa, they can still maintain their body-cantered tetragonal structures.