Electronic phenomena studied in the nordic countries

Unexpected suppression of magnetic stripes by field in underdoped LSCO

Not scheduled

15m

Max IV Lund

Speaker

Prof. Kim Lefmann (University of Copenhagen)

Description

The origin of high temperature superconductivity (HTSC) is still under debate despite thirty years of intensive research. An increasing amount of evidence points to the interplay between the magnetism and superconductivity as the key to solve this puzzle. We have chosen to study one of the cuprate compounds with a relatively simple crystal structure, namely La$_{2-x}$Sr$_x$CuO$_4$. The phase diagram of this copper–oxide is already well defined starting from a Mott insulator parent compound that evolves into a spin-glass phase for Sr doping $0.015 < x < 0.05$ and becomes superconducting at $x = 0.055$ reaching a maximum of $T_c \approx 38$ K at $x = 0.15$. What is interesting to notice is that the properties of this superconductor vary greatly with doping and behave differently upon application of an external magnetic field. In optimally doped compounds $(0.14 < x < 0.18)$ a spin gap was observed which decreases upon application of an external magnetic field and eventually vanishes at the critical field required to induce long-range magnetic order. The underdoped region $(0.055 < x < 0.13)$ on the other hand, is explained by a stripe model, a static spin and charge modulation in the form of stripes of antiferromagnetically arranged copper spins separated by holes. Previous studies reported an enhancement of the magnetic stripe order by an external magnetic field, a phenomenon which is believed to be related to weakening of superconductivity by the field. We chose to employ neutron scattering measurements on a low doped sample with $x = 0.08$ and superconducting transition temperature $T_c = 19$ K, as this region of the phase diagram was sparsely studied until now. Surprisingly our latest results show a suppression of the low-energy (0.6 meV energy transfer) spin excitations with increased field contrary to expectations. This is, nonetheless, in agreement with other experiments performed by our group on oxygen doped samples LCO+O. Another starling observation is the absence of a static spin order below $T_c$ which does not develop upon application of a magnetic field either. There are still many pieces of the puzzle to be gathered, but we believe that a precise characterization of the region close to the quantum phase transition between insulation and superconducting phases has the potential to reveal important insights into the mechanism of HTSC.