Effect of elastic strain and stress on the superconducting transition temperature of quench-condensed films

The volume dependence of the superconducting transition temperatureT _c of amorphous and highly disordered films is investigated by bending the film substrate. The volume coefficientd InT _c /d InV of annealed Sn and Pb films agrees with the volume coefficient known from hydrostatic pressure experiments. This bending technique is applied to superconducting modifications which can be produced only by quench condensation onto a substrate at liquid helium temperature. Amorphous films such as Bi, Ga, Sn₉₀Cu₁₀ and Pb_xBi_1–x, as well as highly disordered Sn and Pb films, are investigated. The relations found between the volume coefficient, the electron-phonon coupling constant and the Grüneisen parameter _G are in reasonable agreement with the volume dependence ofT _c for non-transition metals, obtained on the basis of the McMillan equation. In particular, the results represent an expansion of this kind of investigation to very strong-coupling superconductors.Dedicated to Prof. Dr. W. Buckel on the occasion of his 60th birthdayPaper based in part on a Habilitationsschrift submitted to the Fakultät für Physik, Universität Karlsruhe (TH)     

Superconductivity of vapour quenched beryllium and beryllium-based alloys

The properties of Be films, quench-condensed upon a³He cooled substrate, have been investigated by resistance and tunneling measurements. The superconducting transition temperature,T _c , of Be films increased with thickness and a thick film limit of 9.95 K could be estimated. Alloying with Al or Pb decreasedT _c. The ratios between energy gaps andT _c 's indicated that Be is a weak coupling superconductor, and no phonon induced structure could be traced in tunneling curves neither in pure Be nor in the Be based alloys. Resistance change during annealing as well as superconducting data indicated that the vapour quenched Be films were amorphous as deposited.     

Superconductivity and normal electrical conductivity of amorphous lead films nucleated with molybdenum

Lead films vapor quenched onto nucleating monolayers of Mo or W exhibit strong lattice disorder and can be considered to be amorphous. The amorphous-to-crystalline transformation temperatureT _tr is indicated by a sharp drop of the electrical resistivity in the course of annealing.T _tr is found to be proportional tod ^–2 for Pb thicknessd smaller than 30 nm. The superconducting transition temperatureT _c is by 0.6 to 1 K smaller in the amorphous state than after crystallization. In both states,T _c is proportional tod ^–1. Prenucleation with about half a monolayer of Mo leads to quite the sameT _c depression as observed earlier by Strongin et al. on Pb films vapor quenched onto predeposited films of SiO, Ge or Al₂O₃. For comparison, experiments have been carried out with 2.5 nm Ge predeposits. As with Mo prenucleation, a well defined transformation temperatureT _tr of about the same value has been observed.T _c of bulk amorphous Pb can be extrapolated to be about 6.6 K.     

Superconductivity of quench-condensed beryllium and beryllium-germanium films

Tunneling experiments on Be films condensed onto helium cooled substrates show that these films are homogeneously disordered with a uniform transition temperatureT _c when Ge is codeposited, whereas thick films of pure Be do not grow homogeneously. For films of Be+10 at.% Ge a ratio 2Δ/k _B T _c =3.7 is found. Phonon induced structure in the tunneling density of states is not observed. The metastable phase obtained by quench-condensation is considered to be a disordered high-temperature phase of Be which transforms to the room temperature phase at about 60 K.     

Quantum theory of radiation-pressure induced electrical currents: I. Free electrons

The maximum superconducting transition temperatures,T _c , of noncrystalline (Ti, Zr and Hf)-(3d metal) alloys are reported. The alloys have been produced by low temperature implantation of the 3d metals. The systematic behavior ofT _c as a function of 3d element content is discussed in comparison with metallic glasses and vapor-quenched films. A correlation between the recently observedd-band splitting in metallic glasses and superconducting behavior is pointed out.     

Superconducting transition temperatures of amorphous binary alloy systems based on Bi,Pb, Ga and Be

Quench condensed binary alloy films are produced by evaporation from two separated furnaces. The films contain the whole composition range of the respective alloy system in well defined arrangement.T _c is measured as a function of concentration. Eight predominantly amorphous alloy systems are studied: Bi—Ga, Pb—Ga, Pb—Bi, Be—Bi, Be—Pb, Be—Ga, Be—Al, Be—Li. In Bi—Ga and Pb—GaT _c is a linear function of concentration in the amorphous composition range. In Pb—BiT _c has a maximum. All Be-alloys show lower transition temperatures than pure quench condensed Be. Except for Be—Li all systems have aT _c minimum. The experiments are compared to aT _c calculation using tunelling spectroscopy data. Except for the Be-alloys the agreement is satisfying.     

Effects of dipolar forces on critical sound attenuation in uniaxial systems

Critical sound attenuation in uniaxial dipolar systems is investigated above and below T_c by using the dynamic renormalization-group analysis for an elastically coupled Ising-type model at d = 4. We predict a logarithmic crossover of the divergence of sound damping as $\text{t =}\text{|T ? T}{}_{\mathrm{c}}\text{|}\text{T}{}_{\mathrm{c}}\text{→ 0}$ and derive a universal, crossover-independent ratio R = 12.984|lnt| between the relaxation and the fluctuation damping below T_c. The effects of elastic anisotropy on the phase transition are discussed.     

The upper critical magnetic field Bc2(T) of amorphous molybdenum films

Molybdenum films are obtained by quenched condensation of the metal vapour onto a He-cooled substrate. During annealing the electrical resistance decreases over a small temperature range; this is the typical behaviour of amorphous metals. The films, about 300 Å thick and with a high resistivity, have a positive Hall effect. The transition temperature T_c = 8.0 K is strongly enhanced compared to crystalline Mo. The upper critical field strongly enhanced compared to crystalline Mo. The upper critical field B_c2(T) has been measured. In the examined field range up to 80 kG, B_c2 is a linear function of temperature; the slope has the large value dB_c2/dT = -45 kG/K.     

The elastic behaviour of ferroelastic Sb5O7I polytype 2MC studied by ultrasonic experiments and Brillouin scattering

The temperature dependence of the elastic functions of the improper ferroelastic polytype 2MC-Sb₅O₇I has been investigated in the temperature range from room temperature to well above the structural phase transition atT _c=481 K. The stiffnessesc(c^*c^*),c(a,a),c ₂₂ andc(c^*a) show a considerable softening up to 20% aroundT _c whereasc ₄₄ remains unaffected by the phase transition. The experimental results are discussed considering cubic and quartic anharmonic coupling between two components of the zone boundary order parameter and elastic waves.Carried out in Laboratories RCA Ltd, Zürich, SwitzerlandCarried out in Fakultät für Physik der Universität, Regensburg, Federal Republic of Germany     

非过渡金属非晶态超导电性的研究

本文中分析了非过渡金属非晶态超导体的超导参量、声子谱参量与霍耳系数之间的经验关系。研究了非晶态超导体的T_c,并得出,声子谱的软化所导致的T_c的提高幅度与电-声子耦合常数λ的提高幅度成线性关系;声子谱的高频截止频率愈高,其T_c也愈高。讨论了利用声子谱的软化虽然能大幅度地提高T_c值,但要获得包括金属Be在内的非过渡金属的高T_c非晶态超导体的希望是渺茫的。还讨论了非晶态超导体的上临界场H_c2和能隙2Δ₀所表现出的强耦合效应等问题。 关键词：     

Direct angle resolved photoemission spectroscopy and superconductivity of strained high-T c films

Since 1997 we systematically perform direct angle resolved photoemission spectroscopy (ARPES) on in-situ grown thin (<30 nm) cuprate films. Specifically, we probe low-energy electronic structure and properties of high-T _c superconductors (HTSC) under different degrees of epitaxial (compressive vs. tensile) strain. In overdoped and underdoped in-plane compressed (the strain is induced by the choice of substrate) ≈15 nm thin La_{2 − x} Sr _x CuO₄ (LSCO) films we almost double T _c to 40 K, from 20 K and 24 K, respectively. Yet the Fermi surface (FS) remains essentially two-dimensional. In contrast, ARPES data under tensile strain exhibit the dispersion that is three-dimensional, yet T _c drastically decreases. It seems that the in-plane compressive strain tends to push the apical oxygen far away from the CuO₂ plane, enhances the two-dimensional character of the dispersion and increases T _c, while the tensile strain acts in the opposite direction and the resulting dispersion is three-dimensional. We have established the shape of the FS for both cases, and all our data are consistent with other ongoing studies, like EXAFS. As the actual lattice of cuprates is like a ‘Napoleon-cake’, i.e. rigid CuO₂ planes alternating with softer ‘reservoir’, that distort differently under strain, our data rule out all oversimplified two-dimensional (rigid lattice) mean field models. The work is still in progress on optimized La-doped Bi-2201 films with enhanced T _c.      

Superconducting transition temperatures of vapour quenched beryllium

T_c for vapour quenched Be is depressed in thin films. In the limit of infinite thickness we measure T_c = 9.95 ± 0.06 K.     

A mode coupling analysis of the central peak at order disorder phase transitions

The influence of local and translation invariant memory effects on the critical dynamics of a model undergoing a continous structural phase transition from a disordered (T>T _c ) to an ordered distorted phase (T>T _c ) is studied by mode coupling theory above the critical temperatureT _c . It is shown that besides the existence of critical slowing-down modes there also exists a central peak as a consequence of correlations of the critical modes, increasing with the critical exponent when approachingT _c . The dependence of the central peak on the wavevector , temperatureT and on the spatial dimensiond is investigated. Ford=3 a scenario withlocal long time memory correlations is compared with respect to its temperature and momentum dependence with a model using translation invariant long time memory correlations.     

Size and interface effects on Curie temperature of perovskite ferroelectric nanosolids

A simple and unified model, without any adjustable parameter, is established for size effect on Curie temperature of low-dimensional ferroelectrics (thin films, nanowires and nanoparticles), T _c(D), where D denotes size of low-dimensional ferroelectrics. T _c(D) function is based on consideration on the size dependence of spontaneous polarization of low-dimensional ferroelectrics P _s(D), which is determined by the misfit strain at the ferroelectrics/substrate interface. It is shown that P _s(D) and T _c(D) functions decrease or increase when the misfit strain is tensile or compressive. The numerically predicted results are in agreement with the available experimental results of BaTiO₃ and PbTiO₃ nanoparticles and thin films.     

Theory for superconductivity in amorphous transition metals

A theory is presented for superconductivity in amorphous transition metals. It is shown that in contrast to simple metals for transition metals the changes in the phonon spectrum, in the electronic density of states and in the electronic matrix elements which result from strong lattice disorder can enhance as well as decreaseT _c. The numerical results for the superconducting transition temperatureT _c of amorphous 4d-and 5d-transition metals agree well with the experimental results.     

Anomalous temperature dependence of magnetic relaxation in YBa<Subscript>2</Subscript>Cu<Subscript>2</Subscript>O<Subscript>7 − δ</Subscript> oxygen-deficient single crystals

The dependence of the magnetization relaxation rate S = ?d lnM/dlnt on temperature T is measured in YBa₂Cu₃O_{7 ? δ} samples with various oxygen concentrations. It is found that the S(T) curve changes qualitatively when oxygen deficit δ exceeds the threshold value δ_th = 0.37. For δ < δ_th (T _c > 60 K, where T _c is the superconducting transition temperature), function S(T) has the well-known peak at T/T _c = 0.4. For δ > δ_th (at T _c < 51 K), this peak transforms into a plateau and a new sharp peak appears at T/T _c = 0.1. The threshold value δ_th of the oxygen deficit corresponds to the transition of the sample from the disordered state into the ordered state of oxygen vacancies. We consider the change in the shape of the S(T) curve as a macroscopic manifestation of this transition.     

Wet‐Etching Induced Abnormal Phase Transition in Highly Strained VO2/TiO2 (001) Epitaxial Film

The metal–insulator transition (MIT) behavior in vanadium dioxide (VO₂) epitaxial film is known to be dramatically affected by interfacial stress due to lattice mismatching. For the VO₂/TiO₂ (001) system, there exists a considerable strain in ultra‐thin VO₂ thin film, which shows a lower T_c value close to room temperature. As the VO₂ epitaxial film grows thicker layer‐by‐layer along the “bottom‐up” route, the strain will be gradually relaxed and T_c will increase as well, until the MIT behavior becomes the same as that of bulk material with a T_c of about 68 °C. Whereas, in this study, we find that the VO₂/TiO₂ (001) film thinned by “top‐down” wet‐etching shows an abnormal variation in MIT, which accompanies the potential relaxation of film strain with thinning. It is observed that even when the strained VO₂ film is etched up to several nanometers, the MIT persists, and T_c will increase up to that of bulk material, showing the trend to a stress‐free ultra‐thin VO₂ film. The current findings demonstrate a facial chemical‐etching way to change interfacial strain and modulate the phase transition behavior of ultrathinVO₂ films, which can also be applied to other strained oxide films.     

The magnetism of nickel monolayers

Nickel allows to study the largest variety of phenomena in the magnetism of UHV ultrathin films. The low critical temperature of 630 K for the bulk favors experiments from 0 K to aboveT _c and from one monolayer to infinite thick films. The thickness dependence ofT _c (d) for the (001) and the (111) orientation is compared. Susceptibility measurements in UHV are presented, and from _max the film geometry can be deduced. Ferromagnetic resonance measures the second- and fourth-order anisotropy constants. These give a clear understanding of when and how the reorientation transition from the in-plane to the perpendicular orientation occurs and its nature. Magnetic resonance and circular X-ray dichroism measure the spin and orbital parts of the magnetic moment µ, its anisotropy µ, and the 3d and 4sp contributions. Finally, we show how a 4 Monolayer (ML) Ni(001) film can be transformed into NiO by controlled oxygen dosage and thermal treatment.Paper presented at the annual Spring Meeting of the Deutsche Physikalische Gesellschaft for Condensed- Matter Division, Berlin, Germany. 20–24 March, 1995     

Phase transitions in SrTiO3-based solid solutions

Phase transitions in solid solutions of strontium titanate with titanates of the divalent metals Pb, Ba, and Ca are considered. It is shown that the critical concentration x _c in the well-known expression relating the transition temperature to the polar state in SrTiO₃ with the concentration x of divalent metals, T _c=A(x−x _c)^1/2, is practically the same for all impurities. For large values of x, the ferroelectric transition temperature T _c depends linearly on x. The volume and impurity contributions to the T _c(x) relation are determined for this concentration range. Fiz. Tverd. Tela (St. Petersburg) 39, 1645–1651 (September 1997)     

Nanocrystalline titanium carbide thin films deposited by reactive magnetron sputtering

We investigated the structure and composition of titanium carbide thin films deposited by the reactive magnetron sputter ion plating process as a function of deposition parameters. The films were sputtered onto unheated glass substrates by means of an unbalanced planar d.c. magnetron equipped with a titanium target using a mixture of argon and methane. The deposition parameters ranged from 0.05 Pa to 2 Pa for total working gas pressurep _T, from 10% to 60% (volume) for relative methane concentration in the working gas mixture, from 45 mm to 85 mm for the substrate-to-target distanced _s-t and from −50V to −800V for the substrate biasU _S. It was found that the crystallinity of the thin films strongly decreases with increasingp _T,d _s-t andU _S. The experiments described show the conditions necessary to obtain sputter-deposited nanocrystalline titanium carbide films. This work has been partially supported by the Grant Agency of the Czech Republic under Grant No. 106/96/K245 and by the Ministry of Education of the Czech Republic under Grant No. VS96 059.