Possible unconventional superconductivity in substituted BaFe2As2 revealed by magnetic pair-breaking studies

Scanning tunnelling spectroscopy as a probe of multi-Q magnetic states of itinerant magnets

The combination of electronic correlations and Fermi surfaces with multiple nesting vectors can lead to the appearance of complex multi-Q magnetic ground states, hosting unusual states such as chiral density waves and quantum Hall insulators. Distinguishing single-Q and multi-Q magnetic phases is however a notoriously difficult experimental problem. Here we propose theoretically that the local density of states (LDOS) near a magnetic impurity, whose orientation may be controlled by an external magnetic field, can be used to map out the detailed magnetic configuration of an itinerant system and distinguish unambiguously between single-Q and multi-Q phases. We demonstrate this concept by computing and contrasting the LDOS near a magnetic impurity embedded in three different magnetic ground states relevant to iron-based superconductors—one single-Q and two double-Q phases. Our results open a promising avenue to investigate the complex magnetic configurations in itinerant systems via standard scanning tunnelling spectroscopy, without requiring spin-resolved capability.

It remains difficult to distinguish single-Q and multi-Q magnetic states experimentally. Here, Gastiasoro et al. show that the magnetic configuration of an itinerant system can be mapped out to the local density of states near a magnetic impurity, distinguishing unambiguously between single-Q and multi-Q phases.

Combined external pressure and Cu-substitution studies on BaFe₂As₂ single crystals

We report a combined study of external pressure and Cu-substitution on BaFe2As2 single crystals grown by the in-flux technique. At ambient pressure, the Cu-substitution is known to suppress the spin density wave (SDW) phase in pure BaFe2As2(T(SDW) ≈ 140 K) and to induce a superconducting (SC) dome with a maximum transition temperature T(c)(max) ≃ 4.2 K. This T(c)(max) is much lower than the T(c) ∼ 15-28 K achieved in the case of Ru, Ni and Co substitutions. Such a lower T(c) is attributed to a Cu(2+) magnetic pair-breaking effect. The latter is strongly suppressed by applied pressure, as shown herein, Tc can be significantly enhanced by applying high pressures. In this work, we investigated the pressure effects on Cu(2+) magnetic pair-breaking in the BaFe(2-x)Cu(x)As2 series. Around the optimal concentration (x(opd) = 0.11), all samples showed a substantial increase of T(c) as a function of pressure. Yet for those samples with a slightly higher doping level (over-doped regime), T(c) presented a dome-like shape with maximum T(c) ≃ 8 K. Remarkably interesting, the under-doped samples, e.g. x = 0.02 display a maximum pressure induced T(c) ≃ 30 K which is comparable to the maximum T(c)'s found for the pure compound under external pressures. Furthermore, the magnetoresistance effect as a function of pressure in the normal state of the x = 0.02 sample also presented an evolution consistent with the screening of the Cu(2+) local moments. These findings demonstrate that the Cu(2+) magnetic pair-breaking effect is completely suppressed by applying pressure in the low concentration regime of Cu(2+) substituted BaFe2As2.