FeSe基超导单晶与薄膜研究新进展:自旋向列序、电子相分离及高临界参数

FeSe基超导体的超导临界温度可大范围调控,物理现象丰富,是非常规超导机理研究的热点.由于较高的超导临界参数及易于加工等特点,FeSe基超导体在超导应用开发方面也日益受到重视.大尺寸高质量的单晶和薄膜形态的FeSe基超导材料,对于相关基础科学研究和应用开发都极为重要.作者近年来先后开发和发明了水热离子交换（ion-exchange）、离子脱插（ion-deintercalation）、基底辅助水热外延生长方法,成功解决了二元FeSe和插层（Li,Fe）OHFeSe超导体高质量单晶和薄膜的生长和物性调控难题.进而在相关物理问题的研究中取得新进展,包括发现二元FeSe中自旋向列序与超导电性密切相关,观测到（Li,Fe）OHFeSe中的电子相分离现象.此外,（Li,Fe）OHFeSe超导薄膜呈现很高的超导临界电流密度和上临界磁场,其应用前景值得关注.     

On the use of STM superconducting tips at very low temperatures

We report on high quality local tunnel spectroscopy measurements in superconductors using in-situ fabricated superconducting tips as counterelectrode. The experiments were made at very low temperatures using a dilution refrigerator and a ³He cryostat. Spectra obtained with superconducting tip and sample of Al show that the spectroscopic resolution of our set-up is of 15 eV. Following the observation of Josephson current in tunnelling regime (with tips of Pb and of Al), we discuss the feasibility of Scanning Josephson Spectroscopy with atomic size resolution. Experiments showing new applications of these superconducting tips under applied external magnetic fields are also reported.Received: 12 May 2004, Published online: 7 September 2004PACS: 73.63.Rt Nanoscale contacts - 74.25.Fy Transport properties (electric and thermal conductivity, thermoelectric effects, etc.) - 74.25.Ha Magnetic properties - 74.50. + r Tunnelling phenomena; point contacts, weak links, Josephson effects - 74.78.Na Mesoscopic and nanoscale systems - 74.80.Fp Point contacts; SN and SNS junctions     

Electron transport in a mesoscopic superconducting / ferromagnetic hybrid conductor

We present electrical transport experiments performed on submicron hybrid devices made of a ferromagnetic conductor (Co) and a superconducting (Al) electrode. The sample was patterned in order to separate the contributions of the Co conductor and of the Co-Al interface. We observed a strong influence of the Al electrode superconductivity on the resistance of the Co conductor. This effect is large only when the interface is highly transparent. We characterized the dependence of the observed resistance decrease on temperature, bias current and magnetic field. As the differential resistance of the ferromagnet exhibits a non-trivial asymmetry, we claim that the magnetic domain structure plays an important role in the electron transport properties of superconducting / ferromagnetic conductors. Received 9 July 2002 / Received in final form 22 October 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: herve.courtois@grenoble.cnrs.fr RID="b" ID="b"associated to Université Joseph Fourier     

铁硒基超导研究新进展:高质量(Li,Fe)OHFeSe单晶薄膜

单晶薄膜形态的高温超导材料对于相关基础科学研究和应用开发都极为重要.多带的铁基高温超导体往往呈现丰富的物理现象,并具有较高的超导临界参数.特别是近年发现的插层（Li,Fe）OHFeSe超导体,无论对高温超导机理还是应用研究而言,都日益受到重视,已成为铁基家族中重要的典型材料.但是,该化合物含有OH键,加热易分解.因此,现有的常规高温成膜技术均不适用于生长该薄膜材料.为解决这一生长难题,我们最近发明了基体辅助水热外延生长法,实现了超导薄膜制备技术上的突破.本文简要介绍用此软化学成膜技术首次成功制备出（Li,Fe）OHFeSe单晶薄膜.该薄膜材料具有优良的结晶质量和较高的超导临界参数,特别是其高的临界电流密度和上临界场对应用开发有实际价值.因此,（Li,Fe）OHFeSe超导单晶薄膜的成功合成,为机理研究和应用开发分别提供了重要的实验载体和备选材料.另外,该薄膜技术也有望应用于其他功能材料的探索与合成,尤其是对常规手段难以获取的材料更具重大价值.     

Quantum diffusion of the positive muon in the superconducting state of Al

We report low temperature studies of muon diffusion and trapping in Al doped with Li impurities. The trapping rate at Li and the deduced muon diffusion rate increase more rapidly with decreasing temperature in the superconducting state compared with the normal state. The temperature dependence of the quantum diffusion rate in the superconducting state is close to that predicted by the Kondo theory.     

Superconductivity in Li OHFe S single crystals with a shrunk c-axis lattice constant

By using a hydrothermal ion-exchange method, we have successfully grown superconducting crystals of Li OHFe S with Tcof about 2.8 K. Being different from the sister sample(Li1-xFex)OHFe Se, the energy dispersion spectrum analysis on Li OHFe S shows that the Fe/S ratio is very close to 1:1, suggesting an almost charge neutrality and less electron doping in the Fe S planes of the system. Comparing with the non superconducting Li OHFe S crystal, each peak of the X-ray diffraction pattern of the superconducting crystal splits into two, and the diffraction peaks locating at lower reflection angles are consistent with that of non-superconducting ones. The rest set of diffraction peaks with higher reflection angles is corresponding to the superconducting phase, suggesting that the superconducting phase may has a shrunk c-axis lattice constant. Magnetization measurements indicate that the magnetic shielding due to superconductivity can be quite high under a weak magnetic field. The resistivity measurements under various magnetic fields show that the upper critical field is quite low, which is similar to the tetragonal Fe S superconductor.     

非铁磁性管套法硼化镁带材的基本物理特性

采用粉末管套法,在不锈钢、铜和铝非铁磁性金属管套中制备了MgB2超导带材。电阻温度特性测量表明,以不锈钢管为包套制备的MgB2超导带材具备较好的电输运性能:临界转变温度最高(Tc=33.7K)、转变宽度最窄(零电阻转变ΔTc=1.2K,完全抗磁转变Tm=1.9K)。XRD测量显示,三种管套都能够使MgB2良好成相,MgB2和MgO相的相含量比例R(SS、Cu、A l)分别为7.15、2.45、3.77,样品中MgO相应该是Mg和管套中残余的氧反应的结果,铜管和铝管中还分别检测到了Cu2Mg和A l3Mg2相。对不锈钢管套MgB2带材的热导率测量显示,在超导转变温度以下热输运能力单调下降,满足多晶导体内热运动随温度变化的理论结果。     

点缺陷对L1_2-Al_3Li金属间化合物热力学性能的影响

运用基于第一性原理的平面波赝势法研究了L12-Al3Li金属间化合物中Li原子空位和Al原子反位缺陷对Al3Li热力学性能的影响,结果表明:Al反位缺陷易与周围原子形成局域共价键,使晶体体积增大,而Li空位缺陷却减小了晶体体积.Li空位缺陷使L12-Al3Li的硬度增加,延展性降低,德拜温度值升高.Al反位缺陷降低了晶体的硬度,增加了延展性,降低了德拜温度值.在德拜温度以下,Li空位缺陷减小了L12-Al3Li的热容,而Al反位缺陷使晶体热容增大.晶格畸变对L12-Al3Li晶体的热力学性能有重要影响.     

点缺陷对 L12-Al3Li金属间化合物热力学性能的影响

运用基于第一性原理的平面波赝势法研究了L12-Al3Li金属间化合物中Li原子空位和Al原子反位缺陷对Al3Li热力学性能的影响,结果表明：Al反位缺陷易与周围原子形成局域共价键,使晶体体积增大,而Li空位缺陷却减小了晶体体积。Li空位缺陷使L12-Al3Li的硬度增加,延展性降低,德拜温度值升高。Al反位缺陷降低了晶体的硬度,增加了延展性,降低了德拜温度值。在德拜温度以下,Li空位缺陷减小了L12-Al3Li的热容, 而Al反位缺陷使晶体热容增大。晶格畸变对L12-Al3Li晶体的热力学性能有重要影响     

Al(111)/Al_3Li(111)的界面性质

运用基于第一性原理的平面波贋势法,计算研究了Al (111)/Al_3Li (111)的界面性质.结果表明:Al (111)/Al_3Li (111)的界面具有三种原子配位关系结构,其中界面处仍保持与基体Al一致的三明治堆垛构型的界面稳定性最好.计算表明,该结构界面最薄弱层,位于Al_3Li (111)内,其分离功最小(约1.53 J/m~2),强度最弱,而基体Al和Al_3Li内部的强度随着到界面距离的增大而逐渐增强.     

Persistence of Li induced Kondo moments in the superconducting state of cuprates

Using 7Li NMR shift data, the anomalous local moment induced by spinless Li impurities persists below T(c) in YBa 2Cu 3O6+y. In the underdoped regime, the moments retain their Curie law below Tc. In contrast, near optimal doping, the large Kondo screening observed above Tc (TK = 135 K) is strongly reduced below Tc as expected theoretically when the superconducting gap develops. The limited spatial extent of the induced moment (on first near neighbor Cu) is not drastically modified below Tc, which allows a comparison with STM determination of the local density of states. Our results constrain theoretical models of the impurity electronic properties.     

Superconductivity in ordered LiBe alloy under high pressure: A first-principles study

The electronic, vibrational, and superconducting properties of LiBe alloy in the P2₁/m structure under pressure have been investigated using first-principles calculations. The calculated electron–phonon coupling (EPC) of LiBe with both linear response theory and the rigid muffin-tin approximation suggested that pairing electrons are mainly mediated by the Li low-lying phonon vibrations, and the increase of the Li EPC matrix element $〈I_i^2〉$ with pressure is responsible for the increased EPC parameter $\lambda$. The application of the Allen–Dynes modified McMillan equation reveals high superconducting critical temperatures of 15.2 K at 80 GPa and 18.4 K at 100 GPa for P2₁/m LiBe.     

The motion of light interstitials in metals: Recent experiments

Recent theoretical considerations suggest, that at low temperatures the transport properties of light interstitials in metals are dominated by the interaction with the conduction electrons. After briefly surveying the state of the theory, experimental examples for the low and high temperature cases are presented. We begin with the low T regime and display first neutron scattering experiments on H trapped at O-impurities in Nb where a coherent tunneling state is observed. Its dependence on temperature and its properties in normal and superconducting Nb are discussed. Thereafter muon diffusion results in Al are examined. In Al, muon diffusion is observed via diffusion limited trapping and local properties of the impurity as well as long range diffusion are playing a role. Using a large variety of impurities it was possible to evaluate the diffusion coefficient over a large T-range. Muon motion in Al at accessible temperatures falls entirely in the hopping regime and constitutes an example for the high T regime of the theory.     

Metastable phases in YBCO films created by short-term annealings

The effect of short-term low-temperature annealing in air and in vacuum on the properties of HTSC films of YBCO is studied. It is shown that, under certain conditions of preparation of initial samples, a transition from the HTSC phase with the superconducting transition temperature T_c=90 K to a phase with T_c=60 K occurs without a noticeable change in the oxygen content. It is found that, as a result of short-term annealings, a transition from the HTSC phase with T_c=60 K to the phase with T_c=90 K can occur only through the vacuum annealing stage, which converts the sample into the superconducting state. Short-term annealings lead to multiple reversible “switching” of the films from one phase to another. The obtained results are of practical interest, since the proposed method can be used to quickly obtain superconducting YBCO films in various phase states. It is shown, in addition, that the annealing procedure makes it possible not only to increase the oxygen concentration but also to produce a structural rearrangement of a YBCO film.     

用赝势方法计算简单金属的弹性常数

本文利用单参数Heine-Abarenkov模型势及Hubbard-Sham介电函数计算了七种简单金属(Li,Na,K,Rb,Cs,Al和Pb)的二阶与三阶弹性常数,两个可调参数用零温零压下晶体的弹性常数C₄₄与晶格常数的实验值来确定,计算结果与实验值或其他作者的理论计算值符合得比较好,尤其是Al的三阶弹性常数,本文的计算结果比其他作者的计算结果更接近于实验值。 关键词：     

Die Übergangstemperatur der Supraleitung verschiedener Metalle unter dem Einfluß sehr dünner Oxydschichten

The superconducting transition of thin films of Al, In, Tl, Pb, Sn, and Ga⁺ is investigated by resistance measurements before and after the low temperature oxidation of the surface. The films are condensed onto a crystalline quartz plate to a thickness of about 100 Å or less at a temperature of mostly 105 °K. The oxidation process below 40 °K was described in a previous paper. After a theory ofMott, it may be understood by the tunneling of electrons from the metal to some acceptor levels of the oxygen molecules at the outside of the oxide layer. This way, a strong electric field arises which, on the one hand, facilitates the motion of metal ions outwards to form oxide molecules and, on the other hand, is suspected to affect the transition temperature. The experimental results show a shift of the transition to higher temperatures with the metals Al, In, and Tl, however, with the metals Sn, Pb, and Ga⁺ to lower temperatures. These results agree qualitatively very well with direct measurements of the influence of surface charges on the superconducting transition of Sn and In byGlover andSherrill. TheT _c -shift depends clearly upon the film thickness and amounts to about 0.3 to 0.4 °K for the thinnest films used. Some experiments are undertaken in order to prove the existence of the electric field. Removing the residual oxygen molecules on top of the oxide layer destroys the acceptor levels and causes a reduction of the transition temperature to the value observed with the oxygen free film. As expected, the thickness of the oxide layer is not important for the magnitude of theT _c shift.     

The synthesis and characterization of LiFeAs and NaFeAs

The newest homologous series of superconducting Fe-pnictides, LiFeAs (Li111) and NaFeAs (Na111) have been synthesized and investigated. Both crystallize with the layered tetragonal anti-PbFCl-type structure in P4/nmm space group. Polycrystalline samples and single crystals of Li111 and Na111 display superconducting transitions at ～18 K and 12–25 K, respectively. No magnetic order has been found in either compound, although a weak magnetic background is clearly in evidence. The origin of the carriers and the stoichiometric compositions of Li111 and Na111 were explored.     

Superconductivity in the alkali metals

At ambient pressure there are 29 elemental superconductors in the periodic table, none of which is an alkali metal. The first alkali metal to become superconducting under high pressure is Cs followed years later by Li. Alkali metals are believed to be exemplary free-electron systems. The fact that an alkali metal becomes superconducting at all is surprising and is a result of the fact that under pressure it shows marked deviations from free-electron behaviour where, counterintuitively, bands narrow and gaps widen. For this reason the alkali metals are among the most interesting systems known to study in high-pressure experiments and superconductivity is one of their most fascinating properties.     

Electron excitation cross sections of Li,Na and K

The Vainshtein approximation with exchange via the Ochkur-Rudge method and Ochkur's modified first Born approximation are used to calculate electron excitation cross sections of Li, Na and K. The results are compared with experiments and the frozen core Glauber cross sections.