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.     

Long-Range Spin-Triplet Superconductivity Induced by Magnetic Field in d Wave Superconductor/Ferromagnet Hybrid System

We theoretically study the long-range spin- triplet superconductivity in d wave superconductor/ ferromagnet/ferromagnet (S/F₁/F₂) trilayer junction, in which the magnetization of F₁ layer could be rotated in the y–z plane by an external magnetic field. The four-component Eilenberger equations were constructed to calculate the superconducting order parameters and density of states (DOS). Near the clean limit, the p wave equal-spin triplet component could be induced when the magnetization directions of F₁/F₂ layers are non-collinear, and the DOS exhibits a split zero-bias conductance peak. The various parameters such as ferromagnetic exchange energy, thickness of ferromagnetic layers, and angles between F₁/F₂ magnetization directions are studied for the effect on inducing triplet superconductivity. By magnetic field controlling the emergence of equal-spin triplet pairings or not, such a tunable S/F₁/F₂ trilayer junction based on long-range spin-triplet superconductivity could be used as a superconducting switch device, which would open up a new view of spintronics.     

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 Susceptibility and High Field Magneto-transport of Silver-Added Bi-2223 Superconductor: a Revisit

The effect of Ag addition on weak link behaviour of a Bi-2223 (Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O₁₀) polycrystalline sample has been investigated in terms of AC susceptibility, critical current density (J _c), electrical resistivity ρ(T)H and upper critical field (H _c2). A series of phase pure Bi-2223-Ag _x (x = 0.0–0.3) samples are prepared by the solid-state synthesis route. The phase purity, crystal structures and surface morphology are being studied using the X-ray diffraction and scanning electron microscopy (SEM) technique, respectively. The effect of Ag addition on inter- and intragranular coupling has been investigated by means of AC susceptibility and magneto-transport ρ(T)H measurements, and the results are compared with the pure Bi-2223 sample. Enhancement in granular coupling between the grains of the 20 wt% Ag-added Bi-2223 sample has been witnessed. Critical current density (J _c) has been estimated using the AC susceptibility technique, and the results are interpreted in terms of inter- and intragranular coupling of the investigated samples. The high field magneto-transport technique has been used to estimate the upper critical field (H _c2) and thermally activation flux flow (TAFF) activation energy. The ensuing results revealed that H _c2 increases for the 20 wt% Ag-added sample along with enhancement in grain alignment and intergrain connectivity.     

磁诱导下氧化亚铜晶体的制备及表征

在恒定磁场的诱导下，恒电流电沉积制备了氧化亚铜（Cu2O）晶体。X射线衍射（XRD）和X射线光电子能谱仪（XPS）的测定结果表明，电沉积制备的氧化亚铜（Cu2O）为纯净、立方晶系的Cu2O晶体；扫描电子显微镜（SEM）分析结果表明，有无磁场电沉积时，氧化亚铜均表现为多面体聚集，但电结晶行为表现不同，在磁诱导下Cu2O电结晶径向生长的速率明显优于轴向生长，并出现孔洞现象。     

Analytic Expressions for a 2D Permanent Magnetic Lattice with a 3D Bias Magnetic Field for Ultracold Atoms

In this paper, we discuss a permanent magnetic lattice for trapping and manipulating ultracold atoms and quantum degenerate gases. Considering three components of an external bias magnetic field, B_1x, B_1y and B_1z which are applied for controlling the magnetic lattice parameters, we obtain analytic expressions for the magnetic field minima locations as well as their absolute values, curvatures of the absolute value of the magnetic field, trap frequencies and barrier heights between minima in all directions. B_1z had previously been considered in numerical works, but not in analytic calculations, due to difficulty in obtaining the analytic expressions for the physical parameters and sufficiency of considering only B_1x and B_1y. Including the third component in this study provides further control over the magnetic lattice parameters. Also, in the presence of unavoidable bias magnetic fields with a z-component such as the Earth’s magnetic field, our analytic calculations could be useful. In numerical calculations, we have considered up to n _s  = 1001 square magnetic slabs in each direction of the 2D lattice.     

Effect of Surface Roughness on the Magnetic Field Profile in the Meissner State of a Superconductor

It is well known that the London model (valid for a hard type II superconductor) predicts an externally applied magnetic field decays exponentially as a function of depth into the superconductor on a length scale λ. Direct measurements of the field profile using low energy μSR on high-Tc superconductors, such as YBa₂Cu₃ O _x measure deviations from a simple exponential decay. In particular, there is a short length scale δ close to the surface where the magnetic field does not decay. It has been proposed that this is due to surface roughness, which leads to a suppression of the order parameter near the surface. A model of surface roughness has been studied for the case of a sinusoidally modulated surface roughness on an isotropic superconductor showing that in some cases the profiles resulting from surface roughness may be qualitatively similar to the dead layer phenomena in that the field magnitude decay rate may be slowed near the surface relative to a flat interface but that for modest roughness, the quantitative value of the length over which the field decay is slowed is much smaller than the experiments measure. In this paper, we extend this work in two directions: firstly, by using atomic force microscopy data of YBa₂Cu₃ O _x crystals, we predict the expected field profiles within the context of the Isotropic London model of superconductivity given their actual surface geometry; and secondly, we consider how surface roughness could affect experimental values for λ and δ. The main finding is that roughness within an isotropic model does not produce the dead layers found in experiments on YBa₂Cu₃ O _x . However, we suggest that roughness in a highly anisotropic superconductor could account for the observed dead layer.     

Point-Contact Spectroscopy in Mn-Doped MgB2 Single Crystals: Effects of Magnetic Impurities in a Two-Band Superconductor

We performed point-contact spectroscopy (PCS) measurements in Mg_1−x Mn _x B₂ single crystals, with x≤0.015 and bulk T _c down to 13.3 K. The gaps Δ _σ and Δ _π were obtained by fitting the conductance curves of the point contacts with the two-band Blonder–Tinkham–Klapwijk (BTK) model. Both Δ _σ and Δ _π decrease with the critical temperature of the junctions T _c ^A , but remain clearly distinct down to the lowest critical temperature (T _c ^A ≅9 K). Once analyzed within the Eliashberg theory, the gap trends as a function of T _c ^A can be explained by a doping-induced increase in the pair-breaking scattering within the σ band, with smaller contributions from the π-π or the σ-π channels.     

Comprehensive Electrical Control of Metamagnetic Transition of a Quasi-2D Antiferromagnet by In Situ Anisotropic Strain

Effective nonmagnetic control of the spin structure is at the forefront of the study for functional quantum materials. This study demonstrates that, by applying an anisotropic strain up to only 0.05%, the metamagnetic transition field of spin–orbit-coupled Mott insulator Sr₂IrO₄ can be in situ modulated by almost 300%. Simultaneous measurements of resonant X-ray scattering and transport reveal that this drastic response originates from the complete strain-tuning of the transition between the spin-flop and spin-flip limits, and is always accompanied by large elastoconductance and magnetoconductance. This enables electrically controllable and electronically detectable metamagnetic switching, despite the antiferromagnetic insulating state. The obtained strain-magnetic field phase diagram reveals that C₄-symmetry-breaking anisotropy is introduced by strain via pseudospin-lattice coupling, directly demonstrating the pseudo-Jahn–Teller effect of spin–orbit-coupled complex oxides. The extracted coupling strength is much weaker than the superexchange interactions, yet crucial for the spontaneous symmetry-breaking, affording the remarkably efficient strain-control.     

Giant Enhancement in the Supercapacitance of NiFe–Graphene Nanocomposites Induced by a Magnetic Field

The rapid rise in energy demand in the past years has prompted a search for low‐cost alternatives for energy storage, supercapacitors being one of the most important devices. It is shown that a dramatic enhancement (≈1100%, from 155 to 1850 F g^?1) of the specific capacitance of a hybrid stimuli‐responsive FeNi₃–graphene electrode material can be achieved when the charge/discharge cycling is performed in the presence of an applied magnetic field of 4000 G. This result is related to an unprecedented magnetic‐field‐induced metal segregation of the FeNi₃ nanoparticles during the cycling, which results in the appearance of small Ni clusters (<5 nm) and, consequently, in an increase in pseudocapacitive sites. The results open the door to a systematic improvement of the capacitance values of hybrid supercapacitors, while moving the research in this area towards the development of magnetically addressable energy‐storage devices.     

Magnetic Pair Breaking in Superconducting SrPd2Ge2 Investigated by Scanning Tunnelling Spectroscopy

SrPd₂Ge₂ has been structurally identical with the 122-type iron pnictide compounds with iron completely replaced by Pd and pnictogen replaced by Ge yielding the superconducting transition below 3 K. Our previous works showed that in contrast to iron pnictides this system is electronically fully three-dimensional and revealed conventional superconductivity, even of type I. Here, by means of sub-Kelvin STM spectroscopy, we show that the dirty limit occurs in the sample driving it to type-II superconductivity. STM also proves that in magnetic field the sample undergoes the phase transition to the Abrikosov vortex-lattice state. The de Gennes–Maki theory of gapless superconductivity for dirty superconductors is successfully applied to model the tunnelling spectra of the superconducting density of states in magnetic field. We show that this theory is applicable in a wide range of magnetic fields starting from 60 % of the upper critical field H _c2.     

超高速碰撞产生瞬态磁场的时间尺度特征

在超高速碰撞的早期阶段会产生瞬态等离子体云,等离子体云能以某种机理产生电流和磁场。在靶板表面的等离子体云中产生的非线性电子温度和电子密度梯度将产生磁场,场的持续时间从10-6s到约60s,依赖于弹丸的碰撞能量。本文利用超高速碰撞产生等离子体诱生磁场的一维理论模型,理论推导了喷出物诱生磁场的峰值,得到了碰撞喷出物膨胀过程中磁场增强、磁场衰减的时间尺度特征及磁感应强度峰值。结合超高速正碰撞实验,给出了碰撞喷出物膨胀等离子体云中瞬态磁场的时间尺度,并与理论时间尺度进行了比较,结果基本一致。     

Efficient Energy Funneling in Quasi-2D Perovskites: From Light Emission to Lasing

Quasi-2D Ruddlesden–Popper halide perovskites with a large exciton binding energy, self-assembled quantum wells, and high quantum yield draw attention for optoelectronic device applications. Thin films of these quasi-2D perovskites consist of a mixture of domains having different dimensionality, allowing energy funneling from lower-dimensional nanosheets (high-bandgap domains) to 3D nanocrystals (low-bandgap domains). High-quality quasi-2D perovskite (PEA)₂(FA)₃Pb₄Br₁₃ films are fabricated by solution engineering. Grazing-incidence wide-angle X-ray scattering measurements are conducted to study the crystal orientation, and transient absorption spectroscopy measurements are conducted to study the charge-carrier dynamics. These data show that highly oriented 2D crystal films have a faster energy transfer from the high-bandgap domains to the low-bandgap domains (<0.5 ps) compared to the randomly oriented films. High-performance light-emitting diodes can be realized with these highly oriented 2D films. Finally, amplified spontaneous emission with a low threshold 4.16 µJ cm⁻² is achieved and distributed feedback lasers are also demonstrated. These results show that it is important to control the morphology of the quasi-2D films to achieve efficient energy transfer, which is a critical requirement for light-emitting devices.     

Nucleate Boiling on a Single Site: Contact Angle Analysis for a Quasi-2D Growing Vapour Bubble

Heat transfer prediction under boiling condition is still unresolved. In this paper, a basic study on bubble growth is carried out. Recent works show that contact line region plays an important role for heat and mass transfer in nucleate boiling regime. Three dimension experimental set-up lead to a mirage effect which disturbs measurements. To overcome this problem, a new quasi two dimensional experimental set-up is designed. This Hele–Shaw like configuration allows measuring the contact angle and contact line displacement during the bubble growth. A noticeable behavior of the contact angle is observed, and the influence of the sub-cooling level on the bubble growth rate and the contact angle value is studied.     

Alignment under Magnetic Field of Mixed Fe2O3/SiO2 Colloidal Mesoporous Particles Induced by Shape Anisotropy

When using the bottom‐up approach with anisotropic building‐blocks, an important goal is to find simple methods to elaborate nanocomposite materials with a truly macroscopic anisotropy. Here, micrometer size colloidal mesoporous particles with a highly anisotropic rod‐like shape (aspect ratio ≈ 10) have been fabricated from silica (SiO₂) and iron oxide (Fe₂O₃). When dispersed in a solvent, these particles can be easily oriented using a magnetic field (≈200 mT). A macroscopic orientation of the particles is achieved, with their long axis parallel to the field, due to the shape anisotropy of the magnetic component of the particles. The iron oxide nanocrystals are confined inside the porosity and they form columns in the nanochannels. Two different polymorphs of Fe₂O₃ iron oxide have been stabilized, the superparamagnetic γ‐phase and the rarest multiferroic ε‐phase. The phase transformation between these two polymorphs occurs around 900 °C. Because growth occurs under confinement, a preferred crystallographic orientation of iron oxide is obtained, and structural relationships between the two polymorphs are revealed. These findings open completely new possibilities for the design of macroscopically oriented mesoporous nanocomposites, using such strongly anisotropic Fe₂O₃/silica particles. Moreover, in the case of the ε‐phase, nanocomposites with original anisotropic magnetic properties are in view.