Low temperature formation of benzene from acetylene on a Pd(111) surface

Experiments with UPS, metastable noble gas deexcitation spectroscopy (MDS) and thermal desorption demonstrated that C₂H₂ adsorbed on Pd(111) at 140 K undergoes cyclotrimerisation to C₆H₆ after higher (? 100 L) exposures. If the surface is intermediately warmed up to 300 K, the low temperature state of adsorbed acetylene transforms irreversibly into another species which is unreactive. The surface species formed by reaction was identified by comparison with the electron spectroscopic data of C₆H₆ adsorbed from the gas phase as well as with those of free C₆H₆. The molecules are only weaky held on the surface and start to desorb already at about 150 K.     

Magnetic properties of (Co Pd ) superstructures on Pd(100) and Pd(111)

The magnetic properties of (Co_nPd_m)_r superstructures on Pd(100) and Pd(111) are evaluated using the fully-relativistic spin-polarized screened Korringa-Kohn-Rostoker method. It is found that only in the case of a Pd(111) substrate such superstructures exhibit perpendicular magnetism, while on a Pd(100) substrate the magnetization is oriented in-plane. Also investigated is the effect of interdiffusion in repeated superstructures. By using the inhomogeneous coherent potential approximation (CPA) for layered systems the effect of ordering into (repeated) superstructures can be described in an ab-initio-like manner. It is found that already small amounts of interdiffusion can be decisive for the actual value of the magnetic anisotropy energy. Received 3 November 1999 and Received in final form 18 January 2000     

Low temperature properties of (La,Nd)Sn3 alloys

Dilute (La_1–x Nd _x )Sn₃ alloys with 0.01<x<0.15 are characterized by determination of the superconducting transition temperatureT _c as well as low temperature measurements of the specific heat and inelastic neutron scattering. As an important result of these experiments we found that the Nd impurities in LaSn₃ exist as stable trivalent ions, in contrast to all other light rare earths. In addition, specific heat and neutron scattering results revealed the scheme of crystal field (CF) levels of Nd³⁺.Using this level scheme, the concentration dependence of the superconducting transition temperature,T _c (x), could be quantitatively fitted up tox9 at % by the theory of Keller and Fulde [J. Low. Temp. Phys.4, 289 (1971)]. From the relatively high initial slope ofT _c (x) we inferred that — besides isotropic spin exchange — other pair-breaking processes are important.In contrast toT _c (x), the reduced specific heat jumpsC/C ₀ as a function ofT _c /T _c0 (whereT _c0 andC ₀ refer to LaSn₃) were found to lie considerably below the theoretical curve for isolated Nd³⁺ ions. This is explained by Nd—Nd interactions resulting in a mean Zeeman splitting (<k _B T _c ) of the CF ground state, which could be directly observed in the form of broadened Schottky humps in the normal state specific heats.     

Angle resolved photoemission study of the Ce/Pd(1 1 1) interface

X-ray and UV excitation angle resolved photoemission spectroscopy of ultra-thin films of cerium deposited on a Pd(1 1 1) single-crystal surface has been carried out. Both core level and valence band spectra show a formation of a surface alloy exhibiting d- and f-electron orbital hybridization. An azimuth and polar angle mapping of X-ray excited photoemission intensities results in a surface-geometrical structure information. Mapping of ultra-violet photoelectron intensities as a function of emission angles is used for a band mapping and an electronic structure determination. The d-, f-hybridization occurs even for 0.7 ML of Ce deposited at room temperature. The surface annealing at 260 °C enhances this behavior. A shift of Pd 4d-derived states to higher binding energy in the Ce-Pd systems is observed.     

Low temperature magnetic and magnetostrictive properties in Pr(Fe_(1-x)Co_x)_(1.9) cubic Laves alloys

The structures,spin reorientations,magnetic,and magnetostrictive properties of the polycrystalline Pr(Fe_(1-x)Co_x)_(1.9)(x=0–1.0)cubic laves phase alloys between 5 K and 300 K are investigated.Large low-field magnetostrictions are observed at 5 K in the alloys with x=0.2 and 0.4 due to the low magnetic anisotropies of these two alloys.A large negative magnetostriction of about-1130 ppm is found in PrCo_(1.9) alloy at 5 K.The magnetizations of the alloys with 0≤x≤0.6decrease abnormally at the spin reorientation temperature T_(sr),and an abnormity is detected in the alloy with x=1.0 at its Curie temperature T_c(45 K).The substitution of Fe by Co increases the value of T_(sr) in the alloy with x value increasing from 0.0 to 0.4,and then reduces the value of Tsr with x value further increasing to 0.6.     

(Bi1-x Agx)2(Te1-ySey)3粉体的机械合金化制备及其放电等离子烧结体的热电输运特性

采用机械合金化制备了n型(Bi1-x Agx)2(Te1-y Sey)3合金粉体,对其进行XRD分析表明Bi,Te,Ag,Se单质粉末,经2 h球磨后实现了合金化;SEM分析表明随着机械合金化时间延长粉体颗粒变得均匀、细小,颗粒尺寸在微米至哑微米数量级.采用放电等离子烧结制备了块体样品,研究了合金成分和球磨时间对热电件能的影响.结果表明材料的热电性能与掺杂元素有密切关系.Ag有利于提高功率因子和降低晶格热导率,球磨10 h的(Bi0.99Ag0.01)2(Te0.96Se0.04)3合金粉末的烧结块体具有最大的功率因子和最低的晶格热导率,并在323 K取得最高ZT值0.52.     

Magnetic properties of multiferroics-semiconductors Eu(1-x)Ce(x)Mn2O5

Studies of magnetization, magnetoresistance, and magnetic oscillations in semiconductor-multiferroics Eu(1-x)Ce(x)Mn2O5 (x = 0.2-0.25) (ECMO) at temperatures ranging from 5 to 350 K in magnetic fields up to 6 T are presented. It is shown that phase separation and charge carrier self-organization in the crystals give rise to a layered superstructure perpendicular to the c axis. An effect of magnetic field cycling on the superstructure, magnetization, and magnetoresistance is demonstrated. X-ray diffraction studies of ECMO demonstrating the effect of magnetic field on the superstructure are presented. The de Haas-van Alphen magnetization oscillations in high magnetic fields and the temperature-induced magnetic oscillations in a fixed magnetic field are observed at low temperatures. Below 10 K the quantum corrections to magnetization due to the weak charge carrier localization in 2D superlattice layers occur. It is shown that at all the temperatures the Eu(1-x)Ce(x)Mn2O5 magnetic state is dictated by superparamagnetism of isolated ferromagnetic domains.     

Low temperature thermodynamical properties of the organic chain conductor (TMTSF)AsF

We report on specific heat measurements of the quasi-one-dimensional organic salt (TMTSF)₂AsF₆ in its spin density wave state between 75 mK and 7 K. Similarly to (TMTSF)₂PF₆, we find discontinuities in the lattice contribution at 1.9 K an d 3.5 K ascribed to sub-spin density wave phases. Time-dependent effects due to dynamics of low-energy excitations in metastable states occur only below 0.2 K which yields an activation energy for the equilibrium energy relaxation process of 0.34 K, 4-5 times smaller than found for (TMTSF)₂PF₆. Finally the reduction of the low-energy excitations contribution to the specific heat in comparison to PF₆ reveals an intermediate cubic-like regime between 0.25 and 0.5 K that we tentatively describe as the phason contribution of the incommensurate spin density wave modulation. Received: 17 March 1998 / Revised: 27 July 1998 / Accepted: 22 September 1998     

邮票上的物理学史(71)--低温物理学(续)

再看超流．液氦的超流动性是苏联物理学家卡皮察于1938年发现的(图13,俄国2000年,俄国20世纪科学．上面的俄文是“卡皮察在1938年发现液氦的超流动性”)．他在用毛细管测量液氦的黏性时发现,液氦的黏性在低于2．2K时趋于消失,可以完全畅通无阻地流过极细的管道或狭缝而不损耗其动能;同时热导率异常高．     

Low temperature magnetostriction and magnetization of RE x Y1−x Pd3-alloys

Low temperature magnetostriction and magnetization measurements are reported for the series of RE _x Y_1–x Pd₃ alloys and for some REPd₃ compounds. The magnetostriction per Rare Earth ion is nearly independent of concentration and conserves volume at low fields. In both the alloys and compounds with the heavy Rare Earths and with Nd the magnetostriction tends to saturate at high fields together with the magnetization. The saturated quadrupolar magnetostriction as extracted from these data via the Callen projection, is of order a few times 10^–3 per Rare Earth ion. Sign and magnitude of this quadrupole do not correlate with the Stevens factor. The results are discussed in terms of aspherical screening of the nuclear charge from the outer valence electrons by the intervening 4f charge distribution.Supported by Sonderforschungsbereich 125, Deutsche Forschungsgemeinschaft, FRG     

A resonant photoemission study of the Ce and Ce-oxide/Pd(1 1 1) interfaces

We have investigated the Ce 4f electronic states in the Ce/Pd(1 1 1) and Ce-oxide/Pd(1 1 1) systems, using resonant photoemission (Ce 4d → 4f transitions), and XPS to understand Pd-Ce interactions in ultra thin layers of cerium and ceria deposited on Pd(1 1 1). Cerium deposited on Pd(1 1 1) at room temperature forms surface Ce-Pd alloys with Ce rich character, while a Pd rich Ce-Pd alloy is formed by heating to 700 °C. A modification of the chemical state of Ce can also be seen after oxygen exposure. RPES provides evidence that Ce-oxide layers deposited on Pd(1 1 1) have a CeO₂ (Ce⁴⁺) character, however a net contribution of the Ce³⁺ states is also revealed. The Ce³⁺ states have surface character and are accompanied by oxygen vacancies. Heating to 600 °C causes Ce-oxide reduction. A significant shift of Pd 4d-derived states, induced by Pd 4d and Ce 4f hybridization, was observed. The resonant features in the valence band corresponding to Ce⁴⁺, Ce³⁺ and Ce⁰ states have been investigated for various Pd−Ce(CeO_x) coverages.     

Low temperature properties of magnetic films

The effect of surface oxides on the low temperature magnetic properties of permalloy films is reviewed. Emphasis is placed on the work of Hagedorn and Mitchell and coworkers. It is shown that the unidirectional anisotropy in permalloy (typically exhibited at temperatures below 40 °K) can be correlated with the presence of -Fe₂O₃ as detected in electron diffraction studies. A possible mechanism for the phenomena is discussed which hinges on the depression of the Morin transition in -Fe₂O₃ as a consequence of a change in thed spacing of the -Fe₂O₃.The author would like to thank Professors L. D. Roberts and C. S. Smith of the University North Carolina for their helpful discussions during the course of this work.     

Low temperature specific heat of (TTF) (TCNQ)

Low temperature specific heat measurements on (TTF) (TCNQ) are reported. The Debye temperature is 89.5 K. No term linear in temperature is observed, and comparison with other data indicates the low temperature phase is that of a small band gap non-magnetic semiconductor.     

Low temperature specific heats of very dilute alloys of Ce in Al

Measurements of the specific heat of two alloys of Ce (0.00275 and 0.0121 at % Ce) in Al have been made in the region 0.3<T<3.8 K. Within the limits of detection (<1 ppm), the alloys contained no metallic impurities other than Ce and their homogeneity was checked by neutron activation analysis. For the alloys in their normal electronic states, the change/c was found to be 6±3 and 3.5±1.5mJ/at% K². The behavior of the alloys in their superconducting states (e.g.: the shift ofT _c with the concentration of Ce; the correlation betweenT _C/T_C0 andC/C ₀) indicates the existence of short-lived local moments associated with the Ce impurities.     

Structure and electrical and mechanical properties of (Pd87.3W12.7)1−xNix

The effect of composition and temperature on the properties of the alloy PdV-20 doped with nickel was investigated. It was established that the microhardness, electroresistivity, and the normal elasticity modulus depend nonmonotonically on the content of nickel, and also that these quantities are affected significantly by the past thermal history of the samples. The results' are discussed, taking into account the nature of their intrinsic short-range order structure.Translated from Izvestiya Vysshikh. Uchebnykh Zavedenii, Fizika, No. 5, pp. 42–46, May, 1991.     

Low temperature properties of Pr amorphous alloys

Starting from the Fert and Campbell Hamiltonian and assuming a suitable η-distribution, the magnetization and the specific heat for Pr amorphous alloys are evaluated in the molecular field approximation with ferromagnetic interaction.Recent low-temperature experimental data on the specific heat of Pr amorphous alloys are reasonably explained.