Weak magnetism and the Mott state of vanadium in superconducting Sr2VO3FeAs

Abstract

The symmetry requirement and the origin of magnetic orders coexisting with superconductivity have been strongly debated issues of iron-based superconductors (FeSCs). Observation of C 4 -symmetric antiferromagnetism in violation of the inter-band nesting condition of spin-density waves in superconducting ground state will require significant change in our understanding of the mechanism of FeSC. The superconducting material Sr 2 VO 3 FeAs, a bulk version of monolayer FeSC in contact with a perovskite layer with its magnetism (T N ~ 50 K) and superconductivity (T c ~ 37 K) coexisting at parent state, has no reported structural orthorhombic distortion and thus makes a perfect system to look for theoretically expected C 4 magnetisms 1-3 . Based on variable temperature spin-polarized scanning tunneling microscopy (SPSTM) with newly discovered imaging mechanism that removes the static surface reconstruction (SR) pattern by fluctuating it rapidly with spin-polarized tunneling current, we could visualize underlying C 4 symmetric (2 2) magnetic domains and its phase domain walls. We find that this magnetic order is perfectly consistent with the plaquette antiferromagnetic order in tetragonal Fe spin lattice expected from theories based on the Heisenberg exchange interaction of local Fe moments and the quantum order by disorder 4 . The inconsistency of its modulation Q vectors from the nesting condition also implies that the nesting-based C 2 symmetric magnetism is not a unique prerequisite of high-T c FeSC. Furthermore, the plaquette antiferromagnetic domain wall dynamics under the influence of small spin torque effect of spin-polarized tunneling current are shown to be consistent with theoretical simulation based on the extended Landau-Lifshitz-Gilbert equation.

Pressure dependence of the superconducting and magnetic transition temperatures in Sr2VO3FeAs

“…Sr 2 VO 3 FeAs is composed of alternating conducting FeAs and Sr 2 VO 3 buffer layers [26]. Upon lowering the temperature, Sr 2 VO 3 FeAs undergoes an antiferromagnetic transition presumably of the V 3d moments before it becomes superconducting with the FeAs layer being nonmagnetic [28,29]. Since the V 3d state is less localized than, e.g., the Gd 4f state in RuSr 2 GdCu 2 O 8 [5,6] and hybridizes with the Fe 3d state [30,31], Sr 2 VO 3 FeAs has the potential to exhibit significant coupling between the superconducting and the magnetic order.…”

Frustration-driven C 4 symmetric order in a naturally-heterostructured superconductor Sr2VO3FeAs

“…There is in fact a second-order transition observed at T 0 ∼ 155 K with a sizable entropy loss of ∼ 0.2Rln2 (R is the gas constant) [19][20][21]. With no evidence of a static magnetic order or another apparent symmetry breaking, the hidden nature of this phase transition, simi-lar to the famous "hidden-order" in underdoped cuprates or a heavy fermion system URu 2 Si 2 , remains elusive and controversial [19][20][21][22][23][24][25], posing a challenge to our understanding of the physics of FeSCs in proximity of a Mott insulator.…”

Order on Command

“…Each Cr tip is confirmed on Cr(001) steps for spin contrasts ) shows small randomly oriented domains of quasi-C 2 -symmetric atomic corrugations. These show no preference for any particular fourfold lattice direction over large scales, consistent with their identity as surface reconstructions (SRs) in the absence of bulk orthorhombicity [30].…”

“…At optimal doping, the FeAs layer usually prefers C 2 magnetism harboring superconductivity, while the VO 2 layer prefers C 4 magnetism [1][2][3]21]. Previous experimental studies of Sr 2 VO 3 FeAs [22][23][24][25][26][27][28], however, have reported inconsistent results about magnetic order; recent nuclear magnetic resonance (NMR) measurements on single crystals [29] and neutron diffraction [30] experiments show that there is no long-range magnetic order in the V lattice at any temperature, while in the Fe lattice a magnetic order with a small moment of ∼0.05μ B , possibly due to frustration, is developed below 50 K. Indeed, a theoretical generalized gradient approximation (GGA) calculation has suggested that there can be a number of competing metastable magnetic states composed of different symmetries in V and Fe layers [21]. This is a reasonable theoretical prediction considering the coupling and frustration between V and Fe layers (Supplemental Material Sec.…”