Infrared Study of the Spin Reorientation Transition and Its Reversal in the Superconducting State in UnderdopedBa1−xKxFe

Abstract

With infrared spectroscopy we investigated the spin-reorientation transition from an orthorhombic antiferromagnetic (o-AF) to a tetragonal AF (t-AF) phase and the reentrance of the o-AF phase in the superconducting state of underdoped Ba 1−x K x Fe 2 As 2 . In agreement with the predicted transition from a single-Q to a double-Q AF structure, we found that a distinct spin density wave (SDW) develops in the t-AF phase. The pair breaking peak of this SDW acquires much more low-energy spectral weight than the one in the o-AF state which indicates that it competes more strongly with superconductivity. We also observed additional phonon modes in the t-AF phase which likely arise from a Brillouin-zone folding that is induced by the double-Q magnetic structure with two Fe sublattices exhibiting different magnitudes of the magnetic moment. Similar to the cuprate high-T c superconductors (HTS), the phase diagram of the iron arsenides is characterized by the interaction of superconductivity (SC) with electronic correlations that give rise to competing orders and may also be involved in the SC pairing [1][2][3][4][5][6][7]. The iron arsenides are considered to be archetypal HTS since they exhibit well resolved structural and magnetic phase transitions with long-range ordered states that, unlike the cuprates, are not obscured by strong fluctuation effects. However, the interpretation is still complicated by their multi-band and multi-orbital structure and a sizeable coupling between the charge, spin, orbital and lattice degrees of freedom [1,2]. A prominent example is the combined structural and magnetic transition in undoped and lightly doped samples for which it is still debated whether it is driven by the spin or orbital sectors [6,8,9]. A related, controversial issue is whether the antiferromagnetic (AF) order is better described by an itinerant picture, in terms of a spin density wave (SDW), or in terms of local moments

Muon spin rotation study of the magnetic structure in the tetragonal antiferromagnetic state of weakly underdoped Ba 1− x K x Fe 2 As 2

“…A state with very similar phenomenology was observed earlier in BKFA at elevated pressures [19]. In BKFA, even a reentrance of the o-AF state at lower temperature has been observed which seems to be induced (or at least supported) by superconductivity [17,20].…”

“…A state with very similar phenomenology was observed earlier in BKFA at elevated pressures [19]. In BKFA, even a reentrance of the o-AF state at lower temperature has been observed which seems to be induced (or at least supported) by superconductivity [17,20].Motivated by these observations, a range of so-called double-Q magnetic structures have been proposed for the t-AF state [21][22][23]. These correspond to different superpositions of the single-Q states with wave vectors (0, π) and (π, 0).…”

Anomalous Composition-Induced Crossover in the Magnetic Properties of the Itinerant-Electron AntiferromagnetCa1−xSrxCo2−yAs2Sangeetha1, Smetana2, Mudring3 et al. 2017Phys. Rev. Lett.213540View full textAdd to dashboardCite

“…A subsequent investigation by polarized and unpolarized neutron diffraction determined that the ordered moments in the new phase are aligned along the c axis instead of along the ab plane as in the stripe AFM structure [8]. The work on the Ba 1−x Na x Fe 2 As 2 system was followed by the observation of the same AFM C 4 phase in the Ba 1−x K x Fe 2 As 2 [9][10][11], Sr 1−x Na x Fe 2 As 2 [12,13], and Ca 1−x Na x Fe 2 As 2 [14] systems. These results are important to understanding the mechanism of superconductivity and other aspects of the hole-doped iron arsenides [15][16][17][18][19][20][21].…”

Emergent Magnetic Degeneracy in Iron Pnictides due to the Interplay between Spin-Orbit Coupling and Quantum Fluctuations

“…While early experiments reported the prevalence of a stripe spin-density wave (SSDW) in the phase diagrams of these systems [7,8], a series of recent experiments in several different compounds found a richer behavior [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. As optimal doping is approached, the SSDW transition temperature is suppressed to zero, signaling a putative magnetic quantum critical point (QCP).…”