Measurements of effective quasiparticle recombination times and of densities of electronic states at the Fermi level in superconducting Al- and Pb-films

Temperature-dependent quasiparticle recombination lifetimes _exp(T) and densitiesN ₀ of electronic states at the Fermi level have been measured from time decay experiments of excess quasiparticle concentrations in evaporated, superconducting Al- and Pb-tunnel junctions. Current pulses were used to inject excess, nonthermal quasiparticles in a single junction acting simultaneously as generator and detector. The experimental lifetimes in unperturbed Al show satisfactory agreement with calculations based on the 2-phonon trapping lifetime model. _exp decreases with increasing perturbations of the Al film structure by oxygen background evaporation. In Pb the measured times indicate 2-phonon volume losses. The densitiesN ₀ in Pb-films and unperturbed as well as oxygen-perturbed Al-films differ by less than 5% from the corresponding bulk material data. Therefore, in trying to explain the enhancement of the transition temperature from 1.23 K to 1.85 K in perturbed, granular Al-films a change ofN ₀ can be ruled out.     

Magnetism and electronic structure of the intermetallic compound Ce5CuBi3

We have studied the electronic structure as well as magnetic, electronic transport and thermodynamic properties of the intermetallic compound Ce₅CuBi₃. It was found that Ce₅CuBi₃ undergoes three successive phase transitions at 25?K, 13.7?K and 3.5?K. We attribute the multiple magnetic phase transition to be associated with the two non-equivalent magnetic sublattices of the magnetic Ce ions. The investigated compound is characterized by an enhanced ratio C_p /T at 2?K, which may be interpreted as being due to the nearness of the 4f-level to the Fermi level and some contribution of magnon excitation. The core-level photoemission spectra indicate that Ce ions in Ce₅CuBi₃ are very close to trivalent which is consistent with the magnetic susceptibility data. The calculated band structures using the scalar-relativistic linear muffin-tin orbital method in the atomic sphere approximation and the all-electron full potential linear augmented plane wave plus local orbitals method have been performed for the non-magnetic ground state and as well as for collinear ferromagnetic and ferrimagnetic spin alignments. The largest stabilization energy is found in the case of a ferromagnetic structure. The calculated moments on the two sites of the Ce atoms are in agreement with the experimental value (0.93?μ _B /Ce). The calculations predict that the studied compound has a pseudogap in the DOS curve. Analysis of the partial DOS suggests some differences in hybridization strengths between the Ce-Bi and Ce-Cu orbitals.     

The electronic structure of Ag x Pt1−x at the Fermi level

Results of magnetic susceptibility and low-temperature specific heat measurements on metastable single phase samples of the peritectic alloy system Ag _x Pt_1–x are presented, which cover the whole range of concentration. The experimental data are discussed on the base of recent Korringa-Kohn-Rostoker-Coherent Potential (KKR-CPA) band structure calculations.     

Unraveling the symmetry of the hole states near the Fermi level in the MgB2 superconductor

We use x-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS) to study the fine structure at the K edge of boron in MgB(2). We observe in XAS a peak of width 0.7 eV at the edge threshold, signaling a narrow energy region with empty boron p states near the Fermi level. The changes in the near edge structure observed in EELS with direction of the momentum transfer imply that these states have p(x)p(y) symmetry. Our observations are consistent with electronic structure calculations indicating a narrow energy window of empty p(x)p(y) states that falls to zero at 0.8 eV above the Fermi level. The disappearance of the p(x)p(y) feature in EELS at grain boundaries suggests that this signature may become powerful in probing superconductivity at nanoscale.     

Effect of magnetic doping on the electronic states of Ni

Angle-resolved photoemission is used to determine the change in the electronic states of Ni induced by doping with Fe and Cr. Well-defined spin and k states are selected using high energy and k resolution combined with single crystal alloys. Iron suppresses the mean free path of minority spins only, while chromium suppresses both spins and decreases the magnetic splitting. The strong variation of these effects from one impurity to the other supports the concept of magnetic doping.     

Evolution with composition of the d-band density of states at the Fermi level in highly spin polarized Co1-xFexS2

Highly spin polarized (SP) and half-metallic ferromagnetic systems are of considerable current interest and of potential importance for spintronic applications. Recent work has demonstrated that Co1-xFexS2 is a highly polarized ferromagnet (FM) where the spin polarization can be tuned by alloy composition. Using 59Co FM-NMR as a probe, we have measured the low-temperature spin relaxation in this system in magnetic fields from 0 to 1.0 T for 0相似文献   

Gap at the Fermi level in the intermetallic vacancy system RBiSn(R=Ti,Zr,Hf)

The new class of intermetallic compounds RNiSn(R=Ti,Zr,Hf) may be characterised by the presence of an ordered sublattice of Ni atom vacancies in comparison with normal metals RNi₂Sn with no Ni vacancies. We report unusual transport and optical properties of the RNiSn system. The electrical resistivity of RNiSn is very high (3<p<100) mOhm*cm; the temperature coefficient of resistivity (TCR) is negative and strongly dependent on the annealing conditions. For some samples ZrNiSn and for a single crystal of TiNiSn the resistivity can be described by the Mott's law at temperatures 0.1<T<20 K. A phase transition nearT=100 K without change of crystal structure was deduced from Hall effect data and the temperature dependence of the lattice constant. Preliminary data on transport phenomena in RPtSn and RPdSn(R=Ti,Zr,Hf) compounds are also reported. The unusual properties of RNiSn system might be related to a gap of the electron spectrum near the Fermi energy.     

强相互作用极化费米气体中涡旋束缚态的研究

涡旋态的研究对理解冷原子体系超流特性有很重要的价值. 文章在简要回顾涡旋态研究历史的基础上,介绍了文章作者近期在超冷极化费米气体中涡旋态的工作.通过应用平均场的方法,从理论上研究了强相互作用极化费米气中单个涡旋态的结构.发现在涡旋态核中,Andreev 束缚态的填充可引起一种量子相变.这就提供了一种新颖的探测Andreev 束缚态的方法,在冷原子物理中,可通过吸收成像方法来完成.进一步文章作者对涡旋态的核尺寸进行了研究,发现涡旋态的核尺寸随着体系极化程度的增加而变大.     

强相互作用极化费米气体中涡旋束缚态的研究

涡旋态的研究对理解冷原子体系超流特性有很重要的价值.文章在简要回顾涡旋态研究历史的基础上,介绍了文章作者近期在超冷极化费米气体中涡旋态的工作.通过应用平均场的方法,从理论上研究了强相互作用极化费米气中单个涡旋态的结构.发现在涡旋态核中,Andreev束缚态的填充可引起一种量子相变.这就提供了一种新颖的探测Andreev束缚态的方法,在冷原子物理中,可通过吸收成像方法来完成.进一步文章作者对涡旋态的核尺寸进行了研究,发现涡旋态的核尺寸随着体系极化程度的增加而变大.     

La,Ce,Nd掺杂对单层MoS_2电子结构的影响

为了研究稀土掺杂对单层MoS2电子结构的影响,文章基于密度泛函理论框架下的第一性原理,采用平面波赝势方法分别计算了本征及La,Ce,Nd掺杂单层MoS2的晶格参数、能带结构、态密度和差分电荷密度.计算发现,稀土掺杂所引起的晶格畸变与杂质原子的共价半径大小有关,La杂质附近的键长变化最大,Nd杂质附近的键长变化最小.能带结构分析表明,La掺杂可以在MoS2的禁带中引入3个能级,Ce掺杂可以形成6个新能级,Nd掺杂可以形成4个能级,并对杂质能级属性进行了初步分析.差分电荷密度分布显示,稀土掺杂可以使单层MoS2中的电子分布发生改变,尤其是f电子的存在会使差分电荷密度呈现出反差极大的物理图象.     

Study of the spin polarization of unoccupied states near the Fermi level by spin-dependent near-edge absorption spectroscopy

Spin-dependent near-edge absorption spectroscopy is presented as a new method for studying the spin density of states near the Fermi level at the absorbing atom site. Using circularly polarized synchrotron radiation the spin-dependent inner-shell absorption coefficient is measured as a function of the photoelectron energy. The spin-dependent absorption profile is expected to reflect directly the spin-density distribution of the states populated in the absorption process. The spin densities of 3d-and 4f-elements in pure systems, ferromagnetic alloy and compounds, and 5d-impurities in Fe have been investigated. The results are compared with spin-resolved band-structure calculations for the ferromagnetic ground state.     

Indication of the presence of acceptor states around the Fermi level of amorphous silicon

A junction was grown by sputtering of silicon on a single crystal of high resistivity n-type silicon. The reverese bias V-I characteristics followed Fowler-Nerdheim model with a sudden change in the slope at 0.4 volt indicating an increase in the barrier height. This is attributed to tunneling of extra electrons from the bottom of the conduction band of the substrate to the acceptor traps that face each ether at this bias.     

掺Ga对ZnO电子态密度和光学性质的影响

采用密度泛函理论下的第一性原理平面波赝势方法,研究了掺Ga对纤锌矿ZnO电子态密度和光学性质的影响.从晶体配位场理论分析了掺Ga前后ZnO的成键情况及态密度的变化.计算得到掺Ga后电子浓度为2.42×10²¹ cm^-3,ZnO的载流子浓度提高了10⁴倍.比较分析掺Ga前后ZnO的介电函数、复折射率、吸收光谱和反射光谱可得,ZnO光吸收边向高能端移动,光学带隙增大.在可见光区,ZnO光吸收系数与反射率减小,光透过率显著提高,使ZnO:Ga成为 关键词： 密度泛函理论 态密度 光学性质 ZnO:Ga     

Electronic quasiparticles and evolution of Fermi level spin states in thin magnetic layers

Here we report on high-resolution photoemission of iron layers grown on a W(1 1 0) substrate. The evolution of the substrate states upon sub-monolayer adsorption of Fe atoms leads to a shift in surface state binding energy. For thicker (1 1 0) films, sharp metallic surface states are obtained. Their dispersion displays the signature of quasiparticle renormalization due to dressing with excitations. The energy scale is characteristic for the spin wave spectrum in iron, thereby giving evidence of electron-magnon coupling. Furthermore, it is found that quantum well states occur as a function of layer thickness. These modify the spin density of states at the Fermi level in the ferromagnetic film.