Superhigh Uniform Magnetic Cr Substitution in a 2D Mo2C Superconductor for a Macroscopic-Scale Kondo Effect

Substitutional doping provides an effective strategy to tailor the properties of 2D materials, but it remains an open challenge to achieve tunable uniform doping, especially at high doping level. Here, uniform lattice substitution of a 2D Mo₂C superconductor by magnetic Cr atoms with controlled concentration up to ≈ 46.9 at% by chemical vapor deposition and a specifically designed Cu/Cr/Mo trilayer growth substrate is reported. The concentration of Cr atoms can be easily tuned by simply changing the thickness of the Cr layer, and the samples retain the original structure of 2D Mo₂C even at a very high Cr concentration. The controlled uniform Cr doping enables the tuning of the competition of the 2D superconductor and the Kondo effect across the whole sample. Transport measurements show that with increasing Cr concentration, the superconductivity of the 2D Cr-doped Mo₂C crystals disappears along with the emergence of the Kondo effect, and the Kondo temperature increases monotonously. Using scanning tunneling microscopy/spectroscopy, the mechanism of the doping level effect on the interplay and evolution between superconductivity and the Kondo effect is revealed. This work paves a new way for the synthesis of 2D materials with widely tunable doping levels, and provides new understandings on the interplay between superconductivity and magnetism in the 2D limit.     

Magnetic Properties of a Superconductor with no Inversion Symmetry

We study the magnetic properties of a superconductor in a crystal without symmetry, in particular how the lack of this symmetry exhibits itself. We show that, though the penetration depth itself shows no such effect, for suitable orientation of magnetic field, there is a magnetic field discontinuity at the interface which shows this absence of symmetry. The magnetic field profile of a vortex in the x − y plane is shown to be identical to that of an ordinary anisotropic superconductor to second order in a small parameter . For a vortex along z, there is an induced magnetization along the radial direction.     

Cooperative Time-Reversal Symmetry Breaking Induced by a Magnetic Field in a Quasi-2D d-Wave Superconductor

A model of quasi-two-dimensional d-wave superconductor, with strong nesting properties of the Fermi surface is considered. The orbital effect of a moderate magnetic field applied perpendicularly to the conducting planes is studied in the mean field approximation. It is shown that the field can induce a time-reversal symmetry-breaking spin density wave order coexisting with the superconducting order and can open a gap over the whole Fermi surface. The anomalies recently observed in the heat conductivity, penetration depth, and zero-bias conduction in cuprates might be ascribed to this effect.     

Superconductor stability, 1983: a review

Superconductor stability, as a characteristic of steady states, is reviewed. Reference is made to internally cooled superconductors, metastable magnets and superfluid helium.     

Effects of Magnetic Impurities on Two-Band Superconductor

The calculation was made of the effect of magnetic impurities on the normal and superconductive properties of a multiband s-wave superconductor by direct solution of the two-band Eliashberg equations. In this way, the critical temperature was determined, as well as the values of the superconductive gaps, and the shape of the superconductive density of states and other physical quantities that depend on the concentration of magnetic impurities. It was found that the gaps and the penetration lengths display an unusual behaviour as a function of temperature. The possibility is examined that the presence of a negative induced gap raises the critical temperature.     

Organic Superconductors as Stages for Exploration of Superconductivity in High Magnetic Fields

The low dimensionality of organic superconductors leads to pronounced anisotropy in the upper critical field. In the vicinity of the field direction parallel to the superconducting plane, the critical field shoots out due to the suppression of the orbital pair-breaking effect. Organic superconductors are suitable for the study of the high-field state related to the spin effect under an aligned field, since they are of a high crystalline quality. The reported experimental results covering the behavior at low temperatures are reviewed first, and the breakthrough of the BCS Pauli paramagnetic limit is discussed. The potential of the low-dimensional organic superconductor for study of the effect of electronic spectrum quantization is argued.     

Layered Superconductors in an External Magnetic Field

Presence of localized magnetic moments and inhomogeneites leads to an unconventional behavior of the upper critical field for layered superconductors. In addition, one can observe a large diamagnetic moment at H H _c2 and in the normal resistive state.     

Dielectric Resonators for the Measurement of Superconductor Thin Films Surface Impedance in Magnetic Fields at High Microwave Frequencies

We present the design and realization of cylindrical dielectric resonators operating in the 40–60 GHz frequency range, designed for the measurement of the surface resistance and of the surface reactance shift in High-T _c Superconductors (HTS) thin films in a dc magnetic field. The resonators are single tone, based on the TE ₀₁₁ mode, and multiple tone, the latter allowing in principle to exploit the simultaneous determination of the surface impedance at different frequencies. As an application example, we report the temperature and field dependencies of the effective surface impedance of some cuprate superconductors thin films. The results are compared with those obtained through the use of a standard metal cavity with a similar Q-factor and operating in the same frequency range. The comparison highlights a superior stability and a higher sensitivity, resulting in an increase of about two orders of magnitude in the resolution of the surface impedance measurement. By contrast, the dissipative part of the superconducting transition (above T/T _c = 0.97) is better studied with the metal cavity. We also present measurements in the vortex state in YBaCa₂Cu₃O_7−δ and Tl₂Ba₂CaCu₂O_8+x that show significantly different physics. PACS: 74.25 Nf.     

Reentrant Superconductivity in Quasi-Low-Dimensional Systems: Magnetic Field Effects

The possibilities of survival of superconductivity and of reentrant behavior at high magnetic fields, i.e., for fields beyond the Pauli paramagnetic (Clogston–Chandrasehkar) limit, are discussed. These possibilities are a consequence of a magnetic field induced dimensional crossover (MFIDC) and of pairing in the extreme quantum limit. In particular, when there is no Josephson coupling between layers in weak fields, the magnetic field induces a magnetic field dependent Josephson coupling at large magnetic fields. Furthermore, the cases of singlet and triplet layered superconductors are considered for systems that behave as quasi-one-dimensional metals in their normal state. A possible phase transition between singlet and triplet superconductivity is pointed out. A special emphasis is given to organic quasi-one-dimensional superconductors like (TMTSF)₂PF₆ and (TMTSF)₂ClO₄ in the light of new experimental results. In addition, the cases of quasi-two-dimensional superconductors like strontium ruthenate and copper oxides are briefly discussed.     

Bi（2223）超导体的磁屏蔽效应

研究了Ｂｉ（２２２３）超导体磁屏蔽筒的磁屏蔽效应，从而得出磁屏蔽效果与超本的屏蔽空间大小有关，以及将０．３ｍｍ厚的软铁筒放于两超导体屏蔽筒之间，可使磁屏蔽效果提高为单超导体蔽筒的２１倍的结论。     

Superconductor Thermal Stability Under the Effect of a Two-Dimensional Dual-Phase-Lag Heat Conduction Model

The thermal stability of superconducting thick wire has been numerically investigated under the effect of a two-dimensional dual-phase-lag heat conduction model. Two types of superconductors are considered, Types I and II. It is found that the dual-phase-lag model predicts a wider stable region as compared to the predictions of the parabolic and the hyperbolic heat conduction models. Also, the superconductor thermal stability under the effect of different design, geometrical, and operating conditions is studied.     

Magnetic Flux Penetration in a Mesoscopic Superconductor with a Slit

In this work, the vortex nucleation process in a mesoscopic squared superconductor with a slit is numerically investigated in terms of time dependent Ginzburg–Landau theory. We have calculated, simultaneously, the Gibbs free energy and the vortex configuration in function of time, it allows to identify the correspondence between special points in the Gibbs free energy and the configuration of the vortex system.     

On the Applicability of the Magnetic Susceptibility Models for the Magnetic Superconductor Ru-1212

The applicability of theoretical models for the ac susceptibility measurements of polycrystalline RuSr₂GdCu₂O₈ superconductor has been examined within the temperature range between 8–50 K, ac magnetic field 0.5–25 G, and frequency 20–12500 Hz. In general, a reasonable qualitative agreement between theory and experiment was attained. An evident and detectable asymmetry was observed within the Cole–Cole polar plots with a peak enhancement for both theoretical and experimental data. The modified critical state models are found to generate much better explanation of the ac susceptibility measured data than Bean’s model. For fields above 20 G, the results are agreed roughly with the Bean critical state model, while below 20 G, the Kim–Anderson model is more suitable to account of the magnetic performance. The temperature and field amplitude dependencies of the flux-creep exponent, n, were extracted from the real part of susceptibility, χ′, dependence on frequency. The flux-creep exponent was found to decrease with both temperature and ac field amplitude in accordance to a power-law of the form: n(T,H)=n ₀(H)T ^−s(H). Such dependence might be an indication of a crossover to flux-creep bundles regime.     

Emergence of a Non-Van der Waals Magnetic Phase in a Van der Waals Ferromagnet

Manipulation of long-range order in 2D van der Waals (vdW) magnetic materials (e.g., CrI₃, CrSiTe₃ ,etc.), exfoliated in few-atomic layer, can be achieved via application of electric field, mechanical-constraint, interface engineering, or even by chemical substitution/doping. Usually, active surface oxidation due to the exposure in the ambient condition and hydrolysis in the presence of water/moisture causes degradation in magnetic nanosheets that, in turn, affects the nanoelectronic /spintronic device performance. Counterintuitively, the current study reveals that exposure to the air at ambient atmosphere results in advent of a stable nonlayered secondary ferromagnetic phase in the form of Cr₂Te₃ (T_C2 ≈160 K) in the parent vdW magnetic semiconductor Cr₂Ge₂Te₆ (T_C1 ≈69 K). The coexistence of the two ferromagnetic phases in the time elapsed bulk crystal is confirmed through systematic investigation of crystal structure along with detailed dc/ac magnetic susceptibility, specific heat, and magneto-transport measurement. To capture the concurrence of the two ferromagnetic phases in a single material, Ginzburg-Landau theory with two independent order parameters (as magnetization) with a coupling term can be introduced. In contrast to the rather common poor environmental stability of the vdW magnets, the results open possibilities of finding air-stable novel materials having multiple magnetic phases.     

Static Theory of a Self-Driven Photo-Thermoelectrical High-Temperature Superconductor Bolometer

At the present time, radiation detection in the far infrared with millisecond response time requires use of complicated technology to obtain mK-degree operating temperatures and very high quality, critical temperature materials (e.g., W, Al). It is also customary to use films of nanometer thickness in the manufacture of such devices. In this paper, we present theory and computations for the performance a composite high-temperature superconductor (HTSC)–thermolectric materials bolometer, operated at 77 K and with films of thickness around 1 . A practical device with performance comparable to available photoelectrical bolometers is investigated. The simplicity of the technology involved makes it an attractive proposition. A semimetal film leg (SML; e.g., BiSb) is connected in parallel with a HTSC leg (HTSCL; e.g., YBCO). The bolometer is operated in a photovoltaic mode with no external source of bias power. The thermoelectric voltage developed across the SML leg due to radiation absorbed on the sensitive area of the bolometer remains shorted till there is a sufficient temperature rise of the HTSCL so that its resistance becomes comparable to that of the SML. The voltage thus developed across the bolometer would drive an external electronic readout circuit.     

5 mm微肋管内R404A沸腾换热模型的适配性

本文针对5 mm微肋管内R404A流动沸腾换热进行实验研究,并将研究结果与筛选出的一批换热模型进行适配性验证。实验工况:热流密度5～25 kW/m²、饱和温度0℃、质量流率200～500 kg/(m²·s)、干度为0.1～0.9。结果表明:Zhang Xiaoyan等的模型由于工质热物性差异较大,过高的预测了部分数据; Liu Zhongliang等的模型低估了热流密度对传热系数的影响,过低的预测了实验数据; S. M. Kim等的模型不能体现高干度区域传热系数的衰减,整体预测精度不高; K. E. Gungor等的模型能够很好的解释管内传热的过程,同时预测精度较高,平均绝对偏差仅27.46%。乘以修正系数1.372后的模型平均绝对偏差仅为8. 95%,落在30%偏差带上的数据多达98.18%。     

A Possible 2D H x WO3 Superconductor with a T c of 120 K

Magnetic susceptibility measurements of WO₃ crystals hydrogen doped on the surface suggest 2D local non-percolated superconductivity with an onset temperature of 120 K. Observed zero field cooled vs. field cooled magnetization response is characteristic of type II superconductivity. The diamagnetic response at the critical temperature is field dependent, and is suppressed by a magnetic field of ∼1700 Oe.     

Magnetic flowmeters with controlled boundary conditions

Magnetic flowmeters for liquid metal with improved properties are considered for which distribution of the electric potential at a channel surface is controlled by means of an extraneous current fed to the measurement electrodes. __________ Translated from Izmeritel’naya Tekhnika, No. 5, pp. 69–72, May, 2007.     

Magnetic Structure Determinations at NBS/NIST

Magnetic neutron scattering plays a central role in determining and understanding the microscopic properties of a vast variety of magnetic systems, from the fundamental nature, symmetry, and dynamics of magnetically ordered materials to elucidating the magnetic characteristics essential in technological applications. From the early days of neutron scattering measurements at NBS/NIST, magnetic diffraction studies have been a central theme involving many universities, industrial and government labs from around the United States and worldwide. Such measurements have been used to determine the spatial arrangement and directions of the atomic magnetic moments, the atomic magnetization density of the individual atoms in the material, and the value of the ordered moments as a function of thermodynamic parameters such as temperature, pressure, and applied magnetic field. These types of measurements have been carried out on single crystals, powders, thin films, and artificially grown multilayers, and often the information collected can be obtained by no other experimental technique. This article presents, in an historical perspective, a few examples of work carried out at the NIST Center for Neutron Research (NCNR), and discusses the key role that the Center can expect to play in future magnetism research.     

磁通,磁场自动测量系统的自校准方法

利用计算机及数模、模数转换技术,在原有的永磁材料自动控制测量系统基础上,将磁通、磁场两个物理量溯源到电压Ｖ、时间ｔ、长度ｍ三个基本物理量,实现测量系统高精度的自我校准。