Spin Density Wave Excitations in the Specific Heat of La2CuO4.11 Single Crystals

We measured the specific heat vs. temperature in single crystal samples of superconducting La₂CuO_4.11, in magnetic fields up to 15 T. After subtraction of the phonon contribution to the specific heat, obtained measuring a nonsuperconducting crystal, we found a broad anomaly centered at 50 K. This excess specific heat is attributed to fluctuations of the Cu²⁺ spins possibly enhanced by an interplay with the charge degrees of freedom.     

Quantum Theory of Spin Wave Gap in Ultrathin Magnetic Films

A simple quantum model is presented for the spin wave energy gap in single-layer and thin magnetic films that include both the magnetic out-of-plane and in-plane anisotropies. The films are assumed to be under the influence of the out-of-plane direction of the applied magnetic field at zero temperature. The calculated equations present a nonzero spin wave gap at zero magnetic field which is strongly affected by anisotropies. The effects of the film thickness and the role of the applied field are also examined. We discuss the results in connection with experimental data reported for nanocrystalline amorphous CoFeB films with growth-induced anisotropy.     

Electronic and Phonon Structures of BaFe2As2 Superconductor by Ab-initio Density Functional Theory

Electronic and phonon structures of the BaFe₂As₂ superconductor in the magnetic–orthorhombic phase have been investigated by the ab-initio density functional theory using the pseudopotential Quantum Espresso code. Density of state and electronic band structure for this phase has been studied, but phonon dispersion has been obtained only for the nonmagnetic–orthorhombic phase. Electronic band structure and density of states are in good agreement with other calculations in the literature. The electronic state near the Fermi energy are essentially made from Fe3d and As4p orbital that indicate in-plane conductivity in FeAs layers in this system. Comparing calculated phonon dispersions with experimental data justify magnetic dependence of some phonon modes.     

Superconductivity, Spin Density Wave and Field Induced Spin Density Wave versus anion ordering in the (TMTSF)2ClO4 salt

An effect of electron correlation is studied in an angular dependence of magnetoconductivity of quasi one-dimensional organic conductors. We investigate the effect of both quasi-particle’s lifetime and velocity due to the electron correlation near SDW in the magnetoconductivity. We found that the momentum dependence of lifetime strongly suppresses the one-dimensional axis magnetoconductivity at a magic angle θ=45°. On the other hand, the change of the momentum dependence of the velocity originating from the electron correlation enhances the interchain magnetoconductivity at θ=0°. These effects are explained from the momentum dependence of lifetime and velocity over the electron’s commensurate orbital on Fermi surface at the magic angles.     

Selective Surface Enhanced Raman Scattering for Quantitative Detection of Lung Cancer Biomarkers in Superparticle@MOF Structure

Surface enhanced Raman scattering (SERS) is a trace detection technique that extends even to single molecule detection. Its potential application to the noninvasive recognition of lung malignancies by detecting volatile organic compounds (VOCs) that serve as biomarkers would be a breakthrough in early cancer diagnostics. This application, however, is currently limited by two main factors: (1) most VOC biomarkers exhibit only weak Raman scattering; and (2) the high mobility of gaseous molecules results in a low adsorptivity on solid substrates. To enhance the adsorption of gaseous molecules, a ZIF‐8 layer is coated onto a self‐assembly of gold superparticles (GSPs) in order to slow the flow rate of gaseous biomarkers and depress the exponential decay of the electromagnetic field around the GSP surfaces. Gaseous aldehydes that are released as a result of tumor‐specific tissue composition and metabolism, thereby acting as indicators of lung cancer, are guided onto SERS‐active GSPs substrates through a ZIF‐8 channel. Through a Schiff base reaction with 4‐aminothiophenol pregrafted onto gold GSPs, gaseous aldehydes are captured with a 10 ppb limit of detection, demonstrating tremendous prospects for in vitro diagnoses of early stage lung cancer.     

Magnetic Studies of Spin Wave in Fe/Ag Multilayer Films

Magnetic properties of Fe/Ag multilayer films are investigated and examined versus Fe layer thickness t _Fe. As a result, spontaneous magnetization M(T) temperature dependence has been revealed to be well described by a T ^3/2 law in all multilayer films (7 ?≤t _Fe≤60 ?). Spin-wave theory based on anisotropic ferromagnetic system has been also used to explain magnetization temperature dependence. For Fe layer thickness, approximate values for J ₀ bulk exchange interaction and J _s surface exchange interaction have been estimated. First principle calculations based on density functional theory (DFT) and Korringa–Kohn–Rostoker (KKR)—coherent potential approximation (CPA) method—combined with Local Spin Density Approximation (LSDA), are performed as well. Magnetic moment, in fcc Ag_1−x Fe _x and bcc Fe_1−x Ag _x systems, versus x is presented and discussed in terms of Fe content on magnetic coupling. Reasonable agreement between experimental data and theoretical calculations is highlighted.     

Emergence of Nontrivial Low-Energy Dirac Fermions in Antiferromagnetic EuCd2As2

Parity-time symmetry plays an essential role for the formation of Dirac states in Dirac semimetals. So far, all of the experimentally identified topologically nontrivial Dirac semimetals (DSMs) possess both parity and time reversal symmetry. The realization of magnetic topological DSMs remains a major issue in topological material research. Here, combining angle-resolved photoemission spectroscopy with density functional theory calculations, it is ascertained that band inversion induces a topologically nontrivial ground state in EuCd₂As₂. As a result, ideal magnetic Dirac fermions with simplest double cone structure near the Fermi level emerge in the antiferromagnetic (AFM) phase. The magnetic order breaks time reversal symmetry, but preserves inversion symmetry. The double degeneracy of the Dirac bands is protected by a combination of inversion, time-reversal, and an additional translation operation. Moreover, the calculations show that a deviation of the magnetic moments from the c-axis leads to the breaking of C3 rotation symmetry, and thus, a small bandgap opens at the Dirac point in the bulk. In this case, the system hosts a novel state containing three different types of topological insulator: axion insulator, AFM topological crystalline insulator (TCI), and higher order topological insulator. The results provide an enlarged platform for the quest of topological Dirac fermions in a magnetic system.     

Interplay Between Spin and Charge Excitations in the Coupled Spin-Pseudospin Systems

A coupled spin-pseudospin Hamiltonian in one-dimension (1D) that models the charge-ordering instability of the anisotropic Hubbard ladder at quarter filling is studied by the quantum Monte Carlo and density-matrix renormalization group methods. We show that there is a parameter and temperature region where the spin degrees of freedom are separated from the charge degrees of freedom and behave like a 1D antiferromagnetic Heisenberg model. Anomalous behaviors observed in the disorder phase of -NaV₂O₅ may be due to the spin-charge coupling in the present model.     

Electron-Conduction Mechanism and Specific Heat Above Transition Temperature in LaFeAsO and BaFe2As2 Superconductors

The ionization energy theory is used to calculate the evolution of the resistivity and specific heat curves with respect to different doping elements in the recently discovered superconducting pnictide materials. Electron-conduction mechanism in the pnictides above the structural transition temperature is explained unambiguously, which is also consistent with other strongly correlated materials, such as cuprates, manganites, titanates and magnetic semiconductors.     

Magnetic and Photocatalytic Activity of Pure BaFe12O19 and BaFe12O19-TiO2 Hexagonal Ferrite Nanocomposites

Journal of Superconductivity and Novel Magnetism - Hard hexagonal barium ferrite BaFe12O19 (BaM), as well as core–shell structure BaFe12O19-TiO2 composite nanoparticles, was successfully...     

A Structural and Electronic Properties DFT Study of BaFe2As2 and Two Derived Substitution Compounds

In this work, we calculated the structural and electronic properties of a pnictide iron based superconductor in both orthorhombic (Fmmm) and tetragonal (4I/mmm) phases using density functional theory in the generalized gradient approximation (GGA). We used pseudopotential and all-electron DFT variants. We found that the calculated structures are in good agreement with the experimental and other theoretical calculations. Two substitution compounds issued from the original compound were studied by the same methods.     

Ultrafast transient generation of spin-density-wave order in the normal state of BaFe2As2 driven by coherent lattice vibrations

The interplay among charge, spin and lattice degrees of freedom in solids gives rise to intriguing macroscopic quantum phenomena such as colossal magnetoresistance, multiferroicity and high-temperature superconductivity. Strong coupling or competition between various orders in these systems presents the key to manipulate their functional properties by means of external perturbations such as electric and magnetic fields or pressure. Ultrashort and intense optical pulses have emerged as an interesting tool to investigate elementary dynamics and control material properties by melting an existing order. Here, we employ few-cycle multi-terahertz pulses to resonantly probe the evolution of the spin-density-wave (SDW) gap of the pnictide compound BaFe(2)As(2) following excitation with a femtosecond optical pulse. When starting in the low-temperature ground state, optical excitation results in a melting of the SDW order, followed by ultrafast recovery. In contrast, the SDW gap is induced when we excite the normal state above the transition temperature. Very surprisingly, the transient ordering quasi-adiabatically follows a coherent lattice oscillation at a frequency as high as 5.5 THz. Our results attest to a pronounced spin-phonon coupling in pnictides that supports rapid development of a macroscopic order on small vibrational displacement even without breaking the symmetry of the crystal.     

Charge-doping-induced variation of the superconducting BaFe2As2 electronic structure and the emerging physical effects: a DFT+DMFT study

We performed a first-principles study in the framework of density functional theory combined with dynamical mean-field theory (DFT+DMFT) on the relationship between the charge doping and the correlations in BaFe₂As₂ and investigated how the relationship can affect its spectral function. Structural positions were systematically optimized for a wide range of electron and hole dopings in BaFe_2?xCo_xAs₂ (0?≤?x?≤?0.7) and Ba_1?xK_xFe₂As₂ (0?≤?x?≤?1), respectively, and the found physical characteristics are well compatible with the existing experimental results. Our results could clarify both the importance of structural parameters in iron-based superconductors and the capability of DFT+DMFT in predicting them. The calculated mass enhancements showed that the strength of the electronic correlations varies systematically from weak to strong when moving from the heavily electron-doped regime to the heavily hole-doped one. Since the BaFe₂As₂ compound has a multi-orbital nature, its correlations are orbital-dependent and increase as hole-doping increases. The Fe-3d_xy (xy) orbital is much more correlated than the others because it has reached its half-filled situation and has a narrower energy range around the Fermi level. Our findings can be consistently understood as the tendency of the heavily hole-doped BaFe₂As₂ compound to the orbital-selective Mott phase. Furthermore, the fact that the superconducting state of the heavily hole-doped BaFe₂As₂ is an extreme case of such a selective Mottness constrains the non-trivial role of the electronic correlations in iron-pnictide superconductors. The results are consistent with the previous theoretical and experimental literature. In particular, the calculated spectral function is compatible with the existing experimental results on every charge-doped BaFe₂As₂ compound.

     

Multiband Calculations on the Magnetic Properties of Superconductor KFe2As2

Based on two-band Ginzburg–Landau theory, we study the temperature dependence of upper critical fields for superconducting crystal KFe₂As₂. The results reproduce the experimental data in a broad temperature range and directly underlie the multigap superconductivity in this crystal. Our calculations also indicate that the specific heat jump at the superconducting critical temperature is about 0.77, in accordance with the experimental data.     

Neutron and Raman Scattering Studies of Hgba2CuO4+δ

The HgBa₂CuO_4+δ sample was characterized by Neutron diffraction and magnetic measurements. Both of the measurements indicate a high purity of the sample. Raman measurement was performed on a HgBa₂CuO_4+δ compound of T_c = 96 K. The apical oxygen vibration at 592 cm^?1 was found to show (a) an above T_c anomaly, and (b) frequency hardening and linewidth broadening below the superconducting phase transition. The latter is attributed to the coupling of the phonon to the electronic excitation and related to the opening of the superconducting gap below the phonon frequency.     

Magnetic Field Dependence of the Energy Gap in Nanotubes

The dependence of the energy gap in the band structure of a carbon nanotube on the flux of an external uniform magnetic field is studied in the framework of the zero-range potential method. Taking into account a renormalization procedure for the coupling constant of the zero-range potential, we obtain an estimate for the maximal gap between the conductivity band and the valence one; this estimate is closer to experimental data when compared to the value given by the tight-binding approximation. It is shown numerically that decreasing the diameter of nanotubes breaks down the flux periodicity of the gap structure.     

Berry Phase and Topological Spin Transport in the Chiral d-Density Wave State

In this paper we demonstrate the possibility of dissipationless spin transport in the chiral d-density wave state, by the sole application of a uniform Zeeman field gradient. The occurrence of these spontaneous spin currents is attributed to the parity (℘) and time-reversal ( ) violation induced by the density wave order parameter. We calculate the spin Hall conductance and reveal its intimate relation to the Berry phase which is generated when the Zeeman field is applied adiabatically. Finally, we demonstrate that in the zero temperature and doping case, the spin Hall conductance is quantized as it becomes a topological invariant.      

磁性固相萃取剂Fe3 O4/PDA/PAMAM的制备及其在Pb2+和Cd2+检测中的应用

重金属离子污染严重影响着人类的生活环境和身体健康,因此准确检测重金属离子的含量已成为当今环境保护及检测领域亟待解决的问题.针对重金属离子痕量分析困难及传统磁性固相萃取技术存在的萃取容量小、对重金属离子选择萃取性差等缺点,以溶剂热法制备的Fe3 O4磁性纳米颗粒为磁核,并配合多巴胺改性及具有丰富端基的聚酰胺-胺树状大分子...     

Prospects of the Emerging Raman Scattering Tools for Surface and Nanoanalysis

Demands for label free chemical characterisation in ambient conditions are ever increasing. Only a handful of ambient techniques can provide chemical and structural information with sub micrometre spatial resolution. Raman scattering is a promising method for surface and nanoanalysis for micro- to nanometre length scale characterisation. This article gives an overview of the emerging Raman scattering tools, current state of the art, applications and standardisation requirement for the techniques.