Superconductivity-driven ferromagnetism and spin manipulation using vortices in the magnetic superconductor EuRbFe4As4

In Situ Local Imaging of Ferromagnetism and Superconductivity in RbEuFe4As4

The coexistence of superconductivity and ferromagnetism is an intrinsically interesting research focus in condensed matter physics, but the study is limited by low superconducting (Tc) and magnetic (Tm) transition temperatures in related materials. Here, we used a scanning superconducting quantum interference device to image the in situ diamagnetic and ferromagnetic responses of RbEuFe4As4 with high Tc and Tm. We observed significant suppression of the superfluid density in the vicinity of the magnetic phase transition, signifying fluctuation-enhanced magnetic scatterings between Eu spins and Fe 3d conduction electrons. Intriguingly, we observed multiple ferromagnetic domains that should be absent in an ideal magnetic helical phase. The formation of these domains demonstrates a weak c-axis ferromagnetic component probably arising from the Eu spin-canting effect, indicative of possible superconductivity-driven domain Meissner and domain vortex-antivortex phases, as revealed in EuFe2(As0.79P0.21)2. Our observations highlight that RbEuFe4As4 is a unique system that includes multiple interplay channels between superconductivity and ferromagnetism.

Smectic pair-density-wave order in EuRbFe4As4

The pair density wave (PDW) is a superconducting state in which Cooper pairs carry centre-of-mass momentum in equilibrium, leading to the breaking of translational symmetry^1-4. Experimental evidence for such a state exists in high magnetic field^5-8 and in some materials that feature density-wave orders that explicitly break translational symmetry^9-13. However, evidence for a zero-field PDW state that exists independent of other spatially ordered states has so far been elusive. Here we show that such a state exists in the iron pnictide superconductor EuRbFe₄As₄, a material that features co-existing superconductivity (superconducting transition temperature (T_c) ≈ 37 kelvin) and magnetism (magnetic transition temperature (T_m) ≈ 15 kelvin)^14,15. Using spectroscopic imaging scanning tunnelling microscopy (SI-STM) measurements, we show that the superconducting gap at low temperature has long-range, unidirectional spatial modulations with an incommensurate period of about eight unit cells. Upon increasing the temperature above T_m, the modulated superconductor disappears, but a uniform superconducting gap survives to T_c. When an external magnetic field is applied, gap modulations disappear inside the vortex halo. The SI-STM and bulk measurements show the absence of other density-wave orders, indicating that the PDW state is a primary, zero-field superconducting state in this compound. Both four-fold rotational symmetry and translation symmetry are recovered above T_m, indicating that the PDW is a smectic order.

Effective Exchange Energy in a Thin, Spatially Inhomogeneous CuNi Layer Proximized by Nb

Thin films of diluted magnetic alloys are widely used in superconducting spintronics devices. Most studies rely on transport measurements and assume homogeneous magnetic layers. Here we examine on a local scale the electronic properties of the well-known two-layer superconductor/ferromagnet structure Nb/CuNi. Scanning tunneling spectroscopy experiments demonstrated significant spatial variations of the tunneling conductance on nanoscale, with characteristic gapped, nongapped, and strongly zero-bias peaked spectra. The microscopic theory successfully reproduced the observed spectra and relied them to spatial variations of CuNi film thickness and composition, leading to strong variations of the effective exchange energy. The observed inhomogeneities put constraints on the use of diluted magnetic alloys in nanoscale devices.

Pressured-induced superconducting phase with large upper critical field and concomitant enhancement of antiferromagnetic transition in EuTe2

We report an unusual pressure-induced superconducting state that coexists with an antiferromagnetic ordering of Eu²⁺ moments and shows a large upper critical field comparable to the Pauli paramagnetic limit in EuTe₂. In concomitant with the emergence of superconductivity with T_c ≈ 3–5 K above P_c ≈ 6 GPa, the antiferromagnetic transition temperature T_N(P) experiences a quicker rise with the slope increased dramatically from dT_N/dP = 0.85(14) K/GPa for P ≤ P_c to 3.7(2) K/GPa for P ≥ P_c. Moreover, the superconducting state can survive in the spin-flop state with a net ferromagnetic component of the Eu²⁺ sublattice under moderate magnetic fields μ₀H ≥ 2 T. Our findings establish the pressurized EuTe₂ as a rare magnetic superconductor possessing an intimated interplay between magnetism and superconductivity.

Here, the authors report pressure-induced superconductivity with concomitant enhancement of antiferromagnetic transition in layered EuTe₂. The superconductivity is distinctly characterized by the high upper critical fields exceeding the Pauli limit among binary tellurides, a prerequisite of the coexistence of ferromagnetism with superconductivity.