arXiv subjects

Rudi Hackl

Publications and source records attributed to Rudi Hackl.

22 records · Page 2Linked to original sources

arXiv2015

Nematic fluctuations and the magneto-structural phase transition in ${\rm Ba(Fe_{1-x}Co_x)_2As_2}$

An inelastic light (Raman) scattering study of nematicity and critical fluctuations in ${\rm Ba(Fe_{1-x}Co_x)_2As_2}$ ($0\le x \le 0.051$) is presented. It is shown that the response from fluctuations appears only in $B_{1g}$ (${x^2-y^2}$) symmetry. The scattering amplitude increases towards the structural transition at $T_s$ but vanishes only below the magnetic ordering transition at $T_{\rm SDW} < T_s$, suggesting a magnetic origin of the fluctuations. The theoretical analysis explains the selection rules and the temperature dependence of the fluctuation response. These results make magnetism the favorite candidate for driving the series of transitions.%Below $T_{\rm SDW}$ the gap of the magnetically ordered phase opens up.

cond-mat.str-el↗

arXiv2015

Manifestation of nematic degrees of freedom in the Raman response function of iron pnictides

We establish a relation between the Raman response function in the $B_{1g}$ channel and the electronic contribution to the nematic susceptibility within the spin-driven approach to electron nematicity of the iron based superconductors. The spin-driven nematic phase, characterized by the broken $C_4$ symmetry, but unbroken $O(3$) spin-rotational symmetry, is generated by the presence of magnetic fluctuations associated with the striped phase. It occurs as a separate phase between $T_N$ and $T_s$ in systems where the structural and magnetic phase transitions are separated. Detecting the presence of nematic degrees of freedom in iron-based superconductors is a difficult task, since it involves measuring higher order spin correlation functions. We show that the nematic degrees of freedom manifest themselves in the experimentally measurable Raman response function. We calculate the Raman response function in tetragonal phase in the large $N$ limit by considering Aslamazov-Larkin type of diagrams that contain a series of inserted fermionic boxes that resemble the nematic coupling constant of the theory. These diagrams effectively account for collisions between spin fluctuations. By summing an infinite number of such higher order diagrams, we demonstrate that the electronic Raman response function shows a clear maximum at the structural phase transition in the $B_{1g}$ channel. Hence, the Raman response function can be used to probe nematic degrees of freedom.

cond-mat.str-el↗

arXiv1999

Superconducting Gap and Pseudogap in Bi-2212

We present results of Raman scattering experiments in differently doped Bi-2212 single crystals. Below Tc the spectra show pair-breaking features in the whole doping range. The low frequency power laws confirm the existence of a $d_{x^2-y^2}$-wave order parameter. In the normal state between Tc and T* = 200K we find evidence for a pseudogap in B2g symmetry. Upon doping its effect on the spectra decreases while its energy scale appears to be unchanged.

cond-mat.supr-con↗

arXiv1999

Pseudogap and Superconducting Gap in Y-123: A Raman Study

We present results of electronic Raman-scattering experiments in differently doped Y-123. In B2g symmetry, an analysis of the data in terms of a memory function approach is presented and dynamical relaxation rates $Γ(ω,T)$ and mass-enhancement factors $1+λ(ω,T)$ for the carriers are obtained. Starting from temperatures T > 180K, $Γ(ω,T)$ and $1+λ(ω,T)$ are extrapolated to lower temperatures and used to re-calculate Raman spectra. By comparison with our data, we find a loss of spectral weight between Tc < T < T* at all doping levels x. T* is comparable to the pseudogap temperature found in other experiments. Below Tc, the superconducting gap is observed. It depends on x and scales with Tc whereas the energy scale of the pseudogap remains the same.

cond-mat.supr-con↗