Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userFrustrated Magnetic Interactions, Giant Magneto-Elastic Coupling, and Magnetic Phonons in Iron-Pnictides
144
0
0.0
(
0
)
Authors
Taner Yildirim
Ask ChatGPT about the research
No Arabic abstract
We present a detailed first principles study of Fe-pnictides with particular emphasis on competing magnetic interactions, structural phase transition, giant magneto-elastic coupling and its effect on phonons. The exchange interactions $J_{i,j}(R)$ are calculated up to $approx 12 $AA $. We find that $J_{i,j}(R)$ has an oscillatory character with an envelop decaying as $1/R^3$ along the stripe-direction while it is very short range along the diagonal direction and antiferromagnetic. A brief discussion of the neutron scattering determination of these exchange constants from a single crystal sample with orthorhombic twinning is given. The lattice parameter dependence of the exchange constants, $dJ_{i,j}/da$ are calculated for a simple spin-Peierls like model to explain the fine details of the tetragonal-orthorhombic phase transition. We then discuss giant magneto-elastic effects in these systems. We show that when the Fe-spin is turned off the optimized c-values are shorter than experimetnal values by 1.4 AA $ $ for CaFe$_2$As$_2$, by 0.4 AA $ $ for BaFe$_2$As$_2$, and by 0.13 AA $ $ for LaOFeAs. Finally, we show that Fe-spin is also required to obtain the right phonon energies, in particular As c-polarized and Fe-Fe in-plane modes. Since treating iron as magnetic ion always gives much better results than non-magnetic ones and since there is no large c-axis reduction during the normal to superconducting phase transition, the iron magnetic moment should be present in Fe-pnictides at all times. We discuss the implications of our results on the mechanism of superconductivity in these fascinating Fe-pnictide systems.
rate research
Read More
Many of the iron pnictides have strongly anisotropic normal-state characteristics, important for the exotic magnetic and superconducting behavior these materials exhibit. Yet, the origin of the observed anisotropy is unclear. Electronically driven nematicity has been suggested, but distinguishing this as an independent degree of freedom from magnetic and structural orders is difficult, as these couple together to break the same tetragonal symmetry. Here we use time-resolved polarimetry to reveal critical nematic fluctuations in unstrained Ba(Fe_(1-x)Co_x)_2As_2. The femtosecond anisotropic response, which arises from the two-fold in-plane anisotropy of the complex refractive index, displays a characteristic two-step recovery absent in the isotropic response. The fast recovery appears only in the magnetically ordered state, whereas the slow one persists in the paramagnetic phase with a critical divergence approaching the structural transition temperature. The dynamics also reveal a gigantic magnetoelastic coupling that far exceeds electron-spin and electron-phonon couplings, opposite to conventional magnetic metals.
The wave-vector q and doping (x,y) dependences of the magnetic energy, iron moment, and effective exchange interactions in LaFeAsO{1-x}F{x} and Ba{1-2y}K{2y}Fe2As2 are studied by self-consistent LSDA calculations for co-planar spin spirals. For the undoped compounds (x=0, y=0), the minimum of the calculated total energy, E(q), is for q corresponding to stripe antiferromagnetic order. Already at low levels of electron doping (x), this minimum becomes flat in LaFeAsO{1-x}F{x} and for x>=5, it shifts to an incommensurate q. In Ba{1-2y}K{2y}Fe2As2, stripe order remains stable for hole doping up to y=0.3. These results are explained in terms of the band structure. The magnetic interactions cannot be accurately described by a simple classical Heisenberg model and the effective exchange interactions fitted to E(q) depend strongly on doping. The doping dependence of the E(q) curves is compared with that of the noninteracting magnetic susceptibility for which similar trends are found.
We investigate the structural, electronic, and magnetic properties of the hypothetical compound BaFePn2 (Pn = As and Sb), which is isostructural to the parent compound of the high temperature superconductor LaFeAsO1-xFx. Using density functional theory, we show that the Fermi surface, electronic structure and the spin density wave instability of BaFePn2 are very similar to the Fe based superconductors. Additionally, there are very dispersive metallic bands of a spacer Pn layer, which are almost decoupled from FePn layer. Our results show that experimental study of BaFePn2 can test the role of charge and polarization fluctuation, importance of two dimensionality in mechanism of superconductivity.
Recent measurements of Fermi surface with de Haas-van Alphen oscillations in LaFePO showed a shrinking of the Fermi pockets with respect to first-principle LDA calculations, suggesting an energy shift of the hole and electrons bands with respect to LDA. We show that these shifts are a natural consequence of the strong particle-hole asymmetry of electronic bands in pnictides, and that they provide an indirect experimental evidence of a dominant interband scattering in these systems.
When either electron or hole doped at concentrations $xsim 0.1$, the LaOFeAs family displays remarkably high temperature superconductivity with T$_c$ up to 55 K. In the most energetically stable $vec Q_M = (pi,pi)$ antiferromagnetic (AFM) phase comprised of tetragonal-symmetry breaking alternating chains of aligned spins, there is a deep pseudogap in the Fe 3d states centered at the Fermi energy, and very strong magnetophonon coupling is uncovered. Doping (of either sign) beyond $x sim 0.1$ results in Fe 3d heavy mass carriers ($m^*sim 4-8$) with a large Fermi surface. Calculated Fe-Fe transverse exchange couplings $J_{ij}(R)$ reveal that exchange coupling is strongly dependent on the AFM symmetry and Fe-As distance.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
