Iron-based magnetic superconductors AEuFe4As4 (A= Rb, Cs): Natural superconductor-ferromagnet hybrids

Superconductivity (SC) and ferromagnetism (FM) are normally antagonistic, and their coexistence in a single crystalline material appears to be very rare. Over a decade ago, the iron-based pnictides of doped EuFe2As2 were found to render such a coexistence primarily because of the Fe-3d multiorbitals which simultaneously satisfy the superconducting pairing of Fe-3d electrons and the ferromagnetic exchange interaction among Eu local spins. In 2016, the discovery of the iron-based superconductors AEuFe4As4 (A= Rb, Cs) provided an additional and complementary material basis for the study of the coexistence and interplay between SC and FM. The two sibling compounds, which can be viewed as the intergrowth or hybrid between AFe2As2 and EuFe2As2, show SC in the FeAs bilayers at T c = 35 – 37 K, followed by a magnetic ordering at T m ∼ 15 K in the sandwiched Eu2+-ion sheets. Below T m, the Eu2+ spins align ferromagnetically within each Eu plane, making the system as a natural atomic-thick superconductor-ferromagnet superlattice. This paper reviews the main research progress in the emerging topic during the past five years, and an outlook for the future research opportunities is also presented.

Vacancy tuned thermoelectric properties and high spin filtering performance in graphene/silicene heterostructures

The main contribution of this paper is to study the spin caloritronic effects in defected graphene/silicene nanoribbon (GSNR) junctions. Each step-like GSNR is subjected to the ferromagnetic exchange and local external electric fields, and their responses are determined using the nonequilibrium Green’s function (NEGF) approach. To further study the thermoelectric (TE) properties of the GSNRs, three defect arrangements of divacancies (DVs) are also considered for a larger system, and their responses are re-evaluated. The results demonstrate that the defected GSNRs with the DVs can provide an almost perfect thermal spin filtering effect (SFE), and spin switching. A negative differential thermoelectric resistance (NDTR) effect and high spin polarization efficiency (SPE) larger than 99.99% are obtained. The system with the DV defects can show a large spin-dependent Seebeck coefficient, equal to Ss ⁓ 1.2 mV/K, which is relatively large and acceptable. Appropriate thermal and electronic properties of the GSNRs can also be obtained by tuning up the DV orientation in the device region. Accordingly, the step-like GSNRs can be employed to produce high efficiency spin caloritronic devices with various features in practical applications.

EPR study of paramagnetic centers in SiO2:C: Zn nanocomposites obtained by infiltration of fumed silica with luminescent Zn(acac)2 solution

Silica-carbon with zinc (SiO2:C:Zn) nanocomposites obtained via infiltration with aged luminescent zinc acetylacetonate (Zn(acac)2) ethanol solution of two concentrations (1 or 4%) into the fumed silica (SiO2) matrix have been studied using EPR within the temperature range 6…296 K before and after thermal annealing. The EPR spectrum of SiO2:C:Zn nanocomposites consists of three signals with the Lorentzian lineshape corresponding to paramagnetic centers with S = 1/2, which are related to carbon dangling bonds (CDB) (g = 2.0029(3)), silicon dangling bonds (g = 2.0062(3)) and oxygen-centered carbon-related radicals (CRR) (g = 2.0042(3)). A small EPR linewidth (<1 mT) observed for CDB and oxygen-centered CRR allows us to conclude that they are in the sp3-hybridized state. It was found that the temperature dependence of the EPR signal integrated intensity of the CDB and oxygen-centered CRR follows the Curie–Weiss law with a small positive value of the Curie–Weiss constant, which indicates that the weak ferromagnetic exchange interaction takes place in the spin system of CDB and oxygen-centered CRR. It was supposed that the carbon-related centers are clustered in SiO2:C:Zn nanocomposites. We assume that the oxygen-centered CRR in the sp3-hybridized state are associated with luminescent centers in previously reported aged Zn(acac)2/C2H5OH solution.

Magnetic and f-electron effects in LaNiO2 and NdNiO2 nickelates with cuprate-like $$3{d}_{{x}^{2}-{y}^{2}}$$ band

Recent discovery of superconductivity in the doped infinite-layer nickelates has renewed interest in understanding the nature of high-temperature superconductivity more generally. The low-energy electronic structure of the parent compound NdNiO2, the role of electronic correlations in driving superconductivity, and the possible relationship between the cuprates and the nickelates are still open questions. Here, by comparing LaNiO2 and NdNiO2 systematically within a parameter-free, all-electron first-principles density-functional theory framework, we reveal the role of Nd 4f electrons in shaping the ground state of pristine NdNiO2. Strong similarities are found between the electronic structures of LaNiO2 and NdNiO2, except for the effects of the 4f electrons. Hybridization between the Nd 4f and Ni 3d orbitals is shown to significantly modify the Fermi surfaces of various magnetic states. In contrast, the competition between the magnetically ordered phases depends mainly on the gaps in the Ni $$3{d}_{{x}^{2}-{y}^{2}}$$ 3 d x 2 − y 2 band. Our estimated value of the on-site Hubbard U in the nickelates is similar to that in the cuprates, but the value of the Hund’s coupling JH is found to be sensitive to the Nd magnetic moment. In contrast with the cuprates, NdNiO2 presents 3D magnetism with competing antiferromagnetic and (interlayer) ferromagnetic exchange, which may explain why the Tc is lower in the nickelates.

Pure thermal spin current and perfect spin-filtering with negative differential thermoelectric resistance induced by proximity effect in graphene/silicene junctions

The spin-dependent Seebeck effect (SDSE) and thermal spin-filtering effect (SFE) are now considered as the essential aspects of the spin caloritronics, which can efficiently explore the relationships between the spin and heat transport in the materials. However, there is still a challenge to get a thermally-induced spin current with no thermal electron current. This paper aims to numerically investigate the spin-dependent transport properties in hybrid graphene/silicene nanoribbons (GSNRs), using the nonequilibrium Green’s function method. The effects of temperature gradient between the left and right leads, the ferromagnetic exchange field, and the local external electric fields are also included. The results showed that the spin-up and spin-down currents are produced and flow in opposite directions with almost equal magnitudes. This evidently shows that the carrier transport is dominated by the thermal spin current, whereas the thermal electron current is almost disappeared. A pure thermal spin current with the finite threshold temperatures can be obtained by modulating the temperature, and a negative differential thermoelectric resistance is obtained for the thermal electron current. A nearly zero charge thermopower is also obtained, which further demonstrates the emergence of the SDSE. The response of the hybrid system is then varied by changing the magnitudes of the ferromagnetic exchange field and local external electric fields. Thus, a nearly perfect SFE can be observed at room temperature, whereas the spin polarization efficiency is reached up to 99%. It is believed that the results obtained from this study can be useful to well understand the inspiring thermospin phenomena, and to enhance the spin caloritronics material with lower energy consumption.

Auxiliary Atomic Relay Center Facilitates Enhanced Magnetic Couplings in Blatter’s Radical

<div>The recent accomplishments in obtaining the strong ferromagnetic exchange interactions in organic diradicals have made the field quite fascinating and even more promising towards its technological applications. In this context, herein we report a unique combination of remarkably strong ferromagnetic exchange interactions coupled with the molecular rigidity utilizing super-stable Blatter’s radical as a spin source. The planar analogues of the parent Blatter’s radical obtained by annulation with a chalcogen coupled to nitronyl nitroxide (NN) are investigated using density functional theory (DFT) along with the wave function based multi-configurational self-consistent field (MCSCF) methods e.g. CASSCF/NEVPT2. The calculations reveal phenomenal modulation in exchange couplings upon annulation such that remarkably strong ferromagnetic interactions are realized especially for a certain class of the Blatter - nitronyl nitroxide diradicals. The modulation of spin spin interactions is rationalised by variation in spin density distribution and molecular torsional angles. We demonstrate that annulation in OMMs opens an additional coupling pathway via auxiliary X-atom acting as atomic relay center which strongly manipulates the magnitude of exchange couplings.</div>

Auxiliary Atomic Relay Center Facilitates Enhanced Magnetic Couplings in Blatter’s Radical

<div>The recent accomplishments in obtaining the strong ferromagnetic exchange interactions in organic diradicals have made the field quite fascinating and even more promising towards its technological applications. In this context, herein we report a unique combination of remarkably strong ferromagnetic exchange interactions coupled with the molecular rigidity utilizing super-stable Blatter’s radical as a spin source. The planar analogues of the parent Blatter’s radical obtained by annulation with a chalcogen coupled to nitronyl nitroxide (NN) are investigated using density functional theory (DFT) along with the wave function based multi-configurational self-consistent field (MCSCF) methods e.g. CASSCF/NEVPT2. The calculations reveal phenomenal modulation in exchange couplings upon annulation such that remarkably strong ferromagnetic interactions are realized especially for a certain class of the Blatter - nitronyl nitroxide diradicals. The modulation of spin spin interactions is rationalised by variation in spin density distribution and molecular torsional angles. We demonstrate that annulation in OMMs opens an additional coupling pathway via auxiliary X-atom acting as atomic relay center which strongly manipulates the magnitude of exchange couplings.</div>