Anomalous thermal expansion of high-temperature superconductors at low temperatures

The origins of the anomalous (negative) thermal expansion of high-temperature superconductors (HTSCs) and of the anomalously strong effect of a magnetic field on the thermal expansion coefficient at low temperatures are discussed. A physical model is proposed based on the stabilizing influence of a charge density wave (CDW) arising in the oxygen sublattice in addition to the antiferromagnetic and spin-Peierls ordering in the copper ion sublattice of an HTSC compound. The effects of temperature, magnetic field, and doping on the interaction of such a CDW with the ionic subsystem of an HTSC compound are studied. The conclusions derived from the model are in agreement with experimental data.     

μ−SR studies of high-T c oxide-superconductor systems

Application of negative muons to studies of electronic states at oxygen sites of high-T _c copper-oxide superconductors as well as 3d transition metal oxides are described. A characteristic feature of the muonic oxygen probes in oxides is discussed. Muonic oxygen can be considered as a nitrogen impurity in the oxides. Around muonic oxygen one hole seems to be introduced and the behavior of the hole depends on the properties of the host oxides. A new impurity problem in strongly correlated electron systems is formulated.     

Measurement of the Barkas effect in hydrogen

The stopping power of gaseous hydrogen for positive and negative muons at energies ranging from 3 to 100 keV has been measured by time-of-flight. A pronounced Barkas effect was observed: the energy loss in hydrogen for negative muons was found to be significantly smaller than that for positive muons. Received: 23 April 1998 / Accepted: 7 May 1998     

Low-temperature anomalies in thermal expansion of HTSCs: System Bi2Sr2−x LaxCuO6

The data on thermal expansion at low temperatures have been obtained for HTSC single crystals of the system Bi₂Sr_2?x La_xCuO₆ with different doping levels. Anomalous (negative) thermal expansion is observed in the temperature range from 5 to 20 K. It is shown that the anomaly vanishes in an overdoped sample. An anomalously strong effect of magnetic fields of 2–4 T on the negative thermal expansion domain is observed. The effect of field screening, frozen field, doping level, defects and oxygen vacancies on the region of thermal expansion anomalies is investigated. The origin of the observed anomalies in the properties of the system Bi₂Sr_2?x La_xCuO₆, as well as other HTSC systems in which similar anomalies have been observed, is discussed.     

Study of the charge asymmetry in electromagnetic interactions of cosmic ray muons

The Durgapur cosmic ray spectrograph has been utilised to study the electromagnetic interaction of cosmic ray muons in iron in the momentum range 5–100 GeV/c. The dependence of the interaction cross section on the charge of the muons for the production of a single electron secondary due to the knock-on process and for two and more two electrons due to both knock-on and pair production has been investigated.The ratio of the interaction cross section for positive muons to that for negative muons in the magnetic-iron has been found to be 0.952 ± 0.045 for the production of one secondary particle. For production of two and more than two secondaries the ratios are 1.05 ± 0.09 and 0.91 ± 0.08 respectively. The present results do not indicate any charge asymmetry of cosmic ray muons in respect of electromagnetic interactions in iron.     

高温高压合成顶角氧掺杂型高温超导系列

 简要介绍了(Sr,Ca)_n+1Cu_nO_2n(Cl,O)₂,即三元数法的Cl-“02n”（或四元数Cl-02(n-1)n）超导系列,简称Cl-系超导体。和通常的高温超导体不同,Cl-系的电荷库由卤化物构成,而非通常的氧化物。即Cl-系的导电区和电荷库由顶角氯连接,根据这一结构特点用“顶角氧”掺杂,高压合成了T_c≈80 K的(Sr,Ca)₃Cu₂O₄(Cl,O)₂超导体。该化合物具有和全氧化物高温超导体可比的超导性能,表明高温高压合成全新的卤氧超导新材料的潜力,以及开发卤氧化物高温超导体群的研究前景。     

Frictional accumulation of negative muons and its application

A novel method is proposed for the efficient conversion of intermediate energy negative muons into a low-energy muon beam. It is based on using an electric field to eject muons from a moderator consisting of a large number of thin carbon foils placed perpendicularly to the axis of a high-field solenoid. High-energy muons are made to slow down within the moderator to an energy where further slowing down is inhibited by the electric field acceleration between the foils. The muons accumulate at low energy within the moderator hopping from one foil to the next until they come out as a low-energy muon beam. The resulting phase compression factor exceeds 1000. Efficient initial injection of the muons into the moderator is obtained either by letting the muons enter it in a direction opposite to the acceleration force or by producing the muons within a magnetic trap containing the moderator. A practical configuration based on the second scheme is presented. By implementing the method into the most intense muon production configurations a new pathway is opened that may ultimately compete with other schemes in the selection of the optimal source for high-energy muon colliders.     

Feasibility studies of the geochemical solar neutrino experiment205Tl(v,e−)205Pb

New investigations on the signal-to-background ratio of the geochemical solar neutrino experiment²⁰⁵Tl(v,e^?)²⁰⁵Pb using thallium minerals from Allchar (Yugoslavia) are presented. The contributions of several background processes producing²⁰⁵Pb induced by natural radioacitivities, stopped negative muons and fast muons are estimated. The production rate of²⁰⁵Pb by cosmic ray muons, derived from experimental investigations with 120, 200 and 280 GeV muons at the high energy muon channel of CERN, is significantly higher than expected by former estimations. The feasibility of the geochemical²⁰⁵T1 solar neutrino experiment is discussed.     

The ISIS pulsed muon facility: Past,present and future

The pulsed muon facility at ISIS of the Rutherford Appleton Laboratory has been operational since March 1987. It is now fully scheduled for condensed matter research using polarised surface muons, atomic physics with sub-surface muons, and μCF experiments requiring negative cloud muons. The design and performance of the present beam are briefly discussed and recent improvements to the facility are described. Essential future upgrades have recently received international support and funding, which will lead to a complete facility comparable in extent to those of the continuous meson factories at PSI and TRIUMF, but with the unique advantages of the pulsed nature of the source. Such an upgraded facility will provide unprecedented opportunities for muon science at ISIS, unmatched by any other facility until the end of the decade.     

The properties of short-circuited HTSC coils

The properties of short-circuited multiturn superconducting coils have been studied; coils with nonsuperconducting contacts have been fabricated from a high-temperature superconducting (HTSC) tape made by Super Power Company. The magnetic flux captured by HTSC coils has been measured at different values of magnetic field of the magnetizing solenoid. the critical current in the coils have been experimentally determined based on the maximum values of the field they captured. It is ~50% of the nominal value for this HTSC tape. The range of external magnetic field, where HTSC coils keep the captured magnetic flux, has been experimentally found. The obtained results have demonstrated the possibility of designing magnet systems with levitating coils made of HTSC tape, in which levitation is controlled without using feedbacks.     

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.     

Possible nature of the pseudogap anomalies in HTSC

An HTSC model, in which the interaction of valence-band electrons with diatomic negative U centers is assumed to be responsible for the anomalous properties of HTSC compounds, is proposed and used to explain the nature of the pseudogap and pseudogap anomalies (including the giant Nernst effect, the anomalous diamagnetism above T _c, the “transfer” of the optical spectral weight). For YBa₂Cu₃O_{6 + δ}, the pseudogap opening temperature T* and T _c are calculated as functions of the degree of doping δ. The calculated dependences agree quantitatively with the experimental dependences without using scale fitting parameters. The good agreement between the calculated and experimental results can serve as an argument for the proposed HTSC model.     

Production of high-energy muons in gamma showers

We have calculated the number of high-energy muons in gamma showers generated by photoproduction and by muon pair creation. The prompt muons have flatter energy spectrum than the muons, which come from photoproduction and contribute significant fraction of the total muon rates for E_μ ? 1 TeV. The total rate of high-energy muons in gamma showers is, however, very low.     

Muon induced fission of 237Np, 239Pu and 242Pu

The absolute yields for prompt and delayed fission induced by negative muons in ²³⁷Np, ²³⁹Pu and ²⁴²Pu have been measured and compared with photofission data.     

Measurement of the spectral flux density of negative muons at very low energies

The spectral flux density of negative muons has been measured down to 5 eV.     

高温超导体场致发射的非平衡效应

高温超导体场致发射时 ,非平衡状态将使电场在超导发射体内增加穿透深度 ,这将使其场发射性质发生改变。本文对高温超导体场致发射的非平衡状态进行了研究。     

Atomic capture of muons and density

Including a density dependence, as well as a Z-dependence, an empirical expression was formulated for atomic capture ratios of negative muons in oxides, chlorides and fluorides, using a chi-squared minimization method. The overall chi-square per point produced by this expression is lower than the same obtained from any of the present atomic capture models.     

Model for superconducting current in HTSC: How to calculate the maximum intrinsic values

Summary In order to explain the HTSC electrical density values, the authors propose a new original model for these new superconductors, introducing the parameterSc, given by the ratio between the coherence distance ξ₀ and the lattice constant λ₁. The value ofSc is less than 1 in HTSC and higher than 1 in normal s.c. In HTSC the pair electron transport mechanism became different than in other s.c., in which a wave function linear coherence is established. A Josephson intrinsic electrical conduction mechanism is established. The HTSC behaves like a good diamagnetic material but a bad superconductor.     

Low energy muons as probes of thin films and surfaces

Polarized muons with kinetic energies of a few eV (epithermal μ⁺) can be generated by slowing down energetic muons in appropriate moderators consisting of a thin layer of a van der Waals gas frozen on a substrate. The availability of polarized muons with kinetic energies in the eV to several keV range opens the possibility to extend the μSR technique to the study of thin films and surfaces (low energy μSR, LE-μSR). We summarize the characteristics of the very slow polarized muons and of a low energy beam based on the moderation technique. We discuss the implantation of muons in thin film samples and the potential and limitations of LE-μSR. As an example first results obtained by implanting slow μ⁺ in a sample consisting of a Ni film deposited on Ag are presented.     

Transfer of negative muons from hydrogen to oxygen

Negative muons were trapped in gaseous mixtures of hydrogen and oxygen. The time distributions of the muonic oxygen X-rays showed a clear deviation from a single exponential structure. From these time distributions we evaluated the transfer rate from thermalized muonic hydrogen atoms to oxygen. In order to test the hypothesis of an energy-dependent transfer rate to oxygen we performed Monte Carlo simulations of the thermalization and the transfer process. We report on the present status of our investigations.