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Register a new userAnomalous magneto-elastic coupling in Au-doped BaFe2As2
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We used polarization-resolved Raman scattering to study magneto-elastic coupling in Ba(Fe$_{1-x}$Au$_{x}$)$_2$As$_2$ crystals as a function of light Au-doping, materials for which temperatures of the structural transition ($T_S$) and of the magnetic ordering transition ($T_N$) split. We study the appearance of the $A_g$(As)phonon intensity in the $XY$ scattering geometry that is very weak just below $T_S$, but for which the intensity is significantly enhanced below $T_N$. In addition, the $A_g$(As) phonon shows an asymmetric line shape below $T_N$ and an anomalous linewidth broadening upon Au-doping in the magnetic phase. We demonstrate that the anomalous behavior of the $A_g$(As) phonon mode in the $XY$ scattering geometry can be consistently described by a Fano model involving the $A_g$(As) phonon mode interacting with the $B_{2g}$ symmetry-like magnetic continuum in which the magneto-elastic coupling constant is proportional to the magnetic order parameter.
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Within the BaFe2As2 crystal lattice, we partially substitute thallium for barium and report the effects of interlayer coupling in Ba1-xTlxFe2As2 crystals. We demonstrate the unusual effects of magneto-elastic coupling and charge doping in this iron-arsenide material, whereby Neel temperature rises with small x, and then falls with additional x. Specifically, we find that Neel and structural transitions in BaFe2As2 (TN =Ts= 133 K) increase for x=0.05 (TN = 138 K, Ts = 140 K) from magnetization, heat capacity, resistivity, and neutron diffraction measurements. Evidence from single crystal X-ray diffraction and first principles calculations attributes the stronger magnetism in x=0.05 to magneto-elastic coupling related to the shorter intraplanar Fe-Fe bond distance. With further thallium substitution, the transition temperatures decrease for x = 0.09 (TN = Ts = 131 K), and this is due to charge doping. We illustrate that small changes related to 3d transition-metal state can have profound effects on magnetism.
We present a comprehensive study of the low-temperature heat capacity and thermal expansion of single crystals of the hole-doped Ba1-xKxFe2As2 series (0<x<1) and the end-members RbFe2As2 and CsFe2As2. A large increase of the Sommerfeld coefficient is observed with both decreasing band filling and isovalent substitution (K, Rb, Cs) revealing a strong enhancement of electron correlations and the possible proximity of these materials to a Mott insulator. This trend is well reproduced theoretically by our Density-Functional Theory + Slave-Spin (DFT+SS) calculations, confirming that 122-iron pnictides are effectively Hund metals, in which sizable Hunds coupling and orbital selectivity are the key ingredients for tuning correlations. We also find direct evidence for the existence of a coherence-incoherence crossover between a low-temperature heavy Fermi liquid and a highly incoherent high-temperature regime similar to heavy fermion systems. In the superconducting state, clear signatures of multiband superconductivity are observed with no evidence for nodes in the energy gaps, ruling out the existence of a doping-induced change of symmetry (from s to d-wave). We argue that the disappearance of the electron band in the range 0.4<x<1.0 is accompanied by a strong-to-weak coupling crossover and that this shallow band remains involved in the superconducting pairing, although its contribution to the normal state fades away. Differences between hole- and electron-doped BaFe2As2 series are emphasized and discussed in terms of strong pair breaking by potential scatterers beyond the Born limit.
57Fe Mossbauer spectroscopy measurements are presented in the underdoped Ba(Fe{1-x}Cox)2As2 series for x=0.014 (T_c < 1.4K) and x=0.03 and 0.045 (T_c ~ 2 and 12K respectively). The spectral shapes in the so-called spin-density wave (SDW) phase are interpreted in terms of incommensurate modulation of the magnetic structure, and allow the shape of the modulation to be determined. In undoped BaFe2As2, the magnetic structure is commensurate, and we find that incommensurability is present at the lowest doping level (x=0.014). As Co doping increases, the low temperature modulation progressively loses its squaredness and tends to a sine-wave. The same trend occurs for a given doping level, as temperature increases. We find that a magnetic hyperfine component persists far above the SDW transition, its intensity being progressively tranferred to a paramagnetic component on heating.
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.
We report an ultrasonic investigation of the elastic moduli on a single crystal of hexagonal YMnO_3 as a function of temperature. Stiffening anomalies in the antiferromagnetic Neel state below T_N = 72.4 K are observed on all the four elastic moduli C_{ii}. The anomalies are the most important on C_{11} and C_{66} for in-plane elastic deformations; this is consistent with a strong coupling of the lattice with the in-plane exchange interactions. We use a Landau free energy model to account for these elastic anomalies. We derive an expression which relates the temperature profile of the anomaly to the order parameter; the critical exponent associated to this parameter $beta$ = 0.42 is not consistent with a chiral XY or 3D Heisenberg universality class, but more in agreement with a conventional antiferromagnetic long range order. A tiny softening anomaly on C_{11} for which hysteresis effects are observed could be indicative of an interaction between ferroelectric and magnetic domains at T_N. Moreover, magnetic fluctuations effects both above and below T_N are identified through abnormal temperature and magnetic field effects.
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