Search

2D layered Fe3GeTe2 has attracted increasing attention due to its high magnetic ordering temperature and novel physical properties. Lattice dynamics is a fundamental property of Fe3GeTe2, and its relationships with the number of layers and interlayer spin ordering have not yet been explored in depth. Here, by first‐principles density functional theory calculations, the phonon vibrations and Raman intensities of Fe3GeTe2 are systematically studied from the bulk to monolayer structures. Furthermore, the spin‐phonon coupling effect is investigated by considering different interlayer magnetic orderings: ferromagnetic and antiferromagnetic. It is found that the frequencies of Raman modes in Fe3GeTe2 exhibit considerable dependence on the layer number and spin ordering. The results not only reveal the notable spin‐phonon interactions in Fe3GeTe2, but also demonstrate that Raman modes can be utilized for characterizing the sample thickness and interlayer spin ordering in this 2D magnet. [ABSTRACT FROM AUTHOR]

We investigate the current debate on the Mn valence in Ga1-xMnxN, a diluted magnetic semiconductor (DMS) with a potentially high Curie temperature. From a first-principles Wannier-function analysis, we unambiguously find the Mn valence to be close to 2+ (d5), but in a mixed spin configuration with average magnetic moments of 4µB. By integrating out high-energy degrees of freedom differently, we further derive for the first time from first-principles two low-energy pictures that reflect the intrinsic dual nature of the doped holes in the DMS: (1) an effective d4 picture ideal for local physics, and (2) an effective d5 picture suitable for extended properties. In the latter, our results further reveal a few novel physical effects, and pave the way for future realistic studies of magnetism. Our study not only resolves one of the outstanding key controversies of the field, but also exemplifies the general need for multiple effective descriptions to account for the rich low-energy physics in many-body systems in general. [ABSTRACT FROM AUTHOR]

The iron platinum arsenides Ca10(Fe1−xPtxAs)10(PtnAs8) are the first Fe-based superconductors with metallic spacer layers. Furthermore they display a large variation in their critical temperatures depending on the amount of Pt in their spacer layers: (n=3,4). To gain more insight into the role of the spacer layer the electronic structures of the iron platinum arsenides are represented in the momentum space of the underlying Fe sublattice using a first-principles unfolding method. We find that Ca10(FeAs)10(Pt4As8), contrary to Ca10(FeAs)10(Pt3As8), shows a net electron doping and a non-negligible interlayer coupling. Both effects could account for the difference in the critical temperatures. [ABSTRACT FROM AUTHOR]

We investigate the effect of disordered vacancies on the normal-state electronic structure of the newly discovered alkali-intercalated iron selenide superconductors. To this end, we use a recently developed Wannier function based method to calculate from first principles the configuration-averaged spectral function (A(k,ω)) of K0.8Fe1.6Se2 with disordered Fe and K vacancies. We find that the disorder can suppress the expected Fermi surface reconstruction without completely destroying the Fermi surface. More interestingly, the disorder effect raises the chemical potential significantly, giving enlarged electron pockets similar to highly doped KFe2Se2, without adding carriers to the system. [ABSTRACT FROM AUTHOR]

We investigate the physical effects of translational symmetry breaking in Fe-based high-temperature superconductors due to alternating anion positions. In the representative parent compounds, including the newly discovered Fe-vacancy-ordered K0.8Fe1.6Se2, an unusual change of orbital character is found across the one-Fe Brillouin zone upon unfolding the first-principles band structure and Fermi surfaces, suggesting that covering a larger one-Fe Brillouin zone is necessary in experiments. Most significantly, the electron pockets (critical to the magnetism and superconductivity) are found only created with broken symmetry, advocating strongly its full inclusion in future studies, particularly on the debated nodal structures of the superconducting order parameter. [ABSTRACT FROM AUTHOR]

We present neutron inelastic scattering measurements of the full three-dimensional spin-wave dispersion in the multiferroic material BiFeO3, for temperatures from 5 to 700 K. Despite the presence of strong electromagnetic coupling, the magnetic excitations behave as conventional magnons over all parts of the Brillouin zone. At low temperature the spin waves are well-defined coherent modes, described by a classical model for a G-type antiferromagnet. A softening of the spin-wave velocity and broadening in energy is already present at room temperature, which is well below the Neel temperature TN ~ 640 K, and increases on heating. In addition, a strong hybridization of the Fe 3d and 0 2p states is found to modify significantly the distribution of the spin-wave spectral weight, which implies that the spins are not restricted to the Fe atomic sites as previously believed. [ABSTRACT FROM AUTHOR]

Charge transfer and electron-phonon coupling (EPC) are proposed to be two important constituents associated with enhanced superconductivity in the single unit cell FeSe films on oxide surfaces. Using high-resolution electron energy loss spectroscopy combined with first-principles calculations, we have explored the lattice dynamics of ultrathin FeSe films grown on SrT103. We show that, despite the significant effect from the substrate on the electronic structure and superconductivity of the system, the FeSe phonons in the films are unaffected. The energy dispersion and linewidth associated with the Fe- and Se-derived vibrational modes are thickness and temperature independent. Theoretical calculations indicate the crucial role of antiferromagnetic correlation in FeSe to reproduce the experimental phonon dispersion. Importantly, the only detectable change due to the growth of FeSe films is the broadening of the Fuchs-Kliewer (F-K) phonons associated with the lattice vibrations of SrTiO3(001) substrate. If EPC plays any role in the enhancement of film superconductivity, it must be the interfacial coupling between the electrons in FeSe film and the F-K phonons from substrate rather than the phonons of FeSe. [ABSTRACT FROM AUTHOR]

We generalize the typical medium dynamical cluster approximation to disordered systems with multiple bands. Using our extended formalism, we perform a systematic study of the non-local correlation effects induced by disorder on the density of states and the mobility edge of the Anderson localized states. We apply our method to the three dimensional multiband Anderson model with both inter- and intra-band hopping and disorder potential and find fast convergence with increasing cluster size. Our results are consistent with the ones obtained by the transfer matrix and the kernel polynomial methods. Our findings show that the typical medium dynamical cluster approximation method can be used to study the Anderson localization in real materials. [ABSTRACT FROM AUTHOR]

Recent investigations have shown that Fe1+yTe1-xSex can be made superconducting by annealing it in Se and O vapors. The current lore is that these chalcogen vapors induce superconductivity by removing the magnetic excess Fe atoms. To investigate this phenomenon, we performed a combination of magnetic susceptibility, specific heat, and transport measurements together with scanning tunneling microscopy and spectroscopy and density functional theory calculations on Fe1+yTe1-xSex treated with Te vapor. We conclude that the main role of the Te vapor is to quench the magnetic moments of the excess Fe atoms by forming FeTem (m ⩾ 1) complexes. We show that the remaining FeTem complexes are still damaging to the superconductivity and therefore that their removal potentially could further improve superconductive properties in these compounds. [ABSTRACT FROM AUTHOR]