Coexistence of ferromagnetism and superconductivity in Ni/Bi bilayers

Magnetic and Electronic Properties of Sr Doped Infinite-Layer NdNiO2 Supercell: A Screened Hybrid Density Functional Study

To understand the influence of doping Sr atoms on the structural, magnetic, and electronic properties of the infinite-layer NdSrNiO₂, we carried out the screened hybrid density functional study on the Nd_9-nSr_nNi₉O₁₈ (n = 0-2) unit cells. Geometries, substitution energies, magnetic moments, spin densities, atom- and lm-projected partial density of states (PDOS), spin-polarized band structures, and the average Bader charges were studied. It showed that the total magnetic moments of the Nd₉Ni₉O₁₈ and Nd₈SrNi₉O₁₈ unit cells are 37.4 and 24.9 emu g^-1, respectively. They are decreased to 12.6 and 4.2 emu g^-1 for the Nd₇Sr₂Ni₉O₁₈-Dia and Nd₇Sr₂Ni₉O₁₈-Par unit cells. The spin density distributions demonstrated that magnetic disordering of the Ni atoms results in the magnetism decrease. The spin-polarized band structures indicated that the symmetry of the spin-up and spin-down energy bands around the Fermi levels also influence the total magnetic moments. Atom- and lm-projected PDOS as well as the band structures revealed that Ni(dx2-y2) is the main orbital intersecting the Fermi level. As a whole, electrons of Sr atoms tend to locate locally and hybridize weakly with the O atoms. They primarily help to build the infinite-layer structures, and influence the electronic structure near the Fermi level indirectly.

Phase evolution in thermally annealed Ni/Bi multilayers studied by X-ray absorption spectroscopy

The thin films of Ni and Bi are known to form NiBi₃ and NiBi compounds spontaneously at the interface, which become superconducting below 4.2 K and show ferromagnetism either intrinsically or due to Ni impurities. Formation of NiBi₃ and NiBi is a slow diffusion reaction, which means the local environment around Ni and Bi atoms may vary with time and temperature. In this report, we assess the feasibility of using X-ray Absorption Spectroscopy (XAS) as a tool to track the changes in local bonding environment in NiBi₃ and NiBi. Thermal annealing at temperatures up to 500 °C was used to induce changes in the local environment in NiBi₃ system. Consequent decomposition of NiBi₃ into NiO and Bi has been tracked through changes in structural and magnetization behavior, which matched well with the findings of XAS. In addition, the magnetic hysteresis measurements indicated that NiO should be the dominant phase when NiBi₃ is annealed at 500 °C. This was corroborated from XAS and was found to be >90%. The shift in K-edge of Ni in annealed samples was attributed to increasing charge state on Ni atom, which was ascertained by Bader charge analysis using Density Functional Theory (DFT). This study correlating macroscopic properties of NiBi₃ with local bonding environment of the system indicates that XAS can be a very reliable tool for studying dynamics of diffusion in the NiBi₃ system.

Non-conventional superconductivity in magnetic In and Sn nanoparticles

We report on experimental evidence of non-conversional pairing in In and Sn nanoparticle assemblies. Spontaneous magnetizations are observed, through extremely weak-field magnetization and neutron-diffraction measurements, to develop when the nanoparticles enter the superconducting state. The superconducting transition temperature TC shifts to a noticeably higher temperature when an external magnetic field or magnetic Ni nanoparticles are introduced into the vicinity of the superconducting In or Sn nanoparticles. There is a critical magnetic field and a critical Ni composition that must be reached before the magnetic environment will suppress the superconductivity. The observations may be understood when assuming development of spin-parallel superconducting pairs on the surfaces and spin-antiparallel superconducting pairs in the core of the nanoparticles.

Evidence of High-Temperature Superconductivity at 18 K in Nanosized Rhombohedral Bi Enhanced by Ni-Doping

Superconductivity in bulk rhombohedral Bi has recently been detected to appear below 0.53 mK and 5.2 μT. Here, we unambiguously demonstrate that superconductivity in rhombohedral Bi can be greatly enhanced by incorporating Ni ions onto the Bi sites and reducing the size to the nanometer scale. The superconducting transition temperature T C of 12 nm rhombohedral Bi nanoparticles (NPs) reaches 4 K at ambient pressure. T C is significantly enhanced to reach 7, 12, and 18 K in 6, 8, and 10% Ni-doped Bi NPs, respectively, where superconductivity is found to coexist with ferromagnetism. Ni-doping causes a significant amount of electronic charges to shift toward the interconnecting regions between neighboring Bi ions. First-principles calculations reveal that the Ni ions serve as charge and spin suppliers for the developments of superconductivity and ferromagnetism.

Nodeless Bulk Superconductivity in the Time-Reversal Symmetry Breaking Bi/Ni Bilayer System

Epitaxial bilayer films of Bi(110) and Ni host a time-reversal symmetry breaking superconducting order with an unexpectedly high transition temperature T_{c}=4.1  K. Using time-domain THz spectroscopy, we measure the low energy electrodynamic response of a Bi/Ni bilayer thin film from 0.2 to 2 THz as a function of temperature and magnetic field. We analyze the data in the context of a Bardeen-Cooper-Schrieffer-like superconductor with a finite normal-state scattering rate. In a zero magnetic field, all states in the film become fully gapped, providing important constraints into possible pairing symmetries. Our data appear to rule out the odd-frequency pairing that is natural for many ferromagnetic-superconductor interfaces. By analyzing the magnetic field-dependent response in terms of a pair-breaking parameter, we determine that superconductivity develops over the entire bilayer sample which may point to the p-wave like nature of unconventional superconductivity.

Time-reversal symmetry-breaking superconductivity in epitaxial bismuth/nickel bilayers

Superconductivity that spontaneously breaks time-reversal symmetry (TRS) has been found, so far, only in a handful of three-dimensional (3D) crystals with bulk inversion symmetry. We report an observation of spontaneous TRS breaking in a 2D superconducting system without inversion symmetry: the epitaxial bilayer films of bismuth and nickel. The evidence comes from the onset of the polar Kerr effect at the superconducting transition in the absence of an external magnetic field, detected by the ultrasensitive loop-less fiber-optic Sagnac interferometer. Because of strong spin-orbit interaction and lack of inversion symmetry in a Bi/Ni bilayer, superconducting pairing cannot be classified as singlet or triplet. We propose a theoretical model where magnetic fluctuations in Ni induce the superconducting pairing of the [Formula: see text] orbital symmetry between the electrons in Bi. In this model, the order parameter spontaneously breaks the TRS and has a nonzero phase winding number around the Fermi surface, thus making it a rare example of a 2D topological superconductor.

Influence of spin-polarized current on superconductivity and the realization of large magnetoresistance

The superconducting state can be influenced by injecting spin-polarized current in a controlled manner by properly tailoring the interfacial transmittivity between a ferromagnet (F) and a superconductor (S), resulting in a large magnetoresistance of over 1100% for a F/I/S/I/F multilayer system (I insulator). Because of the competition between ferromagnetism and superconductivity, the superconducting transition temperature (T(C)) in the spin-parallel configuration is shifted below that in the spin antiparallel configuration. The T(C) shift is attributed to ferromagnet-induced nonequilibrium spin carriers in the superconductors.

Observation of superconductivity in granular bi nanowires fabricated by electrodeposition

Bulk rhombohedral Bi at ambient pressure is a well-known semimetal, and its transition to a superconductor has not been observed, at least down to 50 mK. We report that, unlike bulk rhombohedral Bi, granular Bi nanowires with well-defined rhombohedral grains of approximately 10 nm diameter, fabricated by electrochemically depositing Bi into porous polycarbonate membranes at ambient pressure, are superconducting with two transition temperatures, Tc, of 7.2 and 8.3 K. These Tc values coincide with Tc values of the high-pressure phases Bi-III and Bi-V, respectively. Analysis of our structural and transport data indicates that the superconductivity in granular Bi nanowires probably arises from grain boundary areas where there are structural reconstructions between the grains showing a preferred orientation within a small angular distribution.