Fe-based Superconductors

Competiting/coexisting phases in Ba(Fe,Co)2As2

The phase diagram of Fe-based superconductors shown in fig. 1 presents many similarities with that of cuprates. The undoped parent compound host antiferromagnetic order and doping suppresses the magnetic order and superconducting dome pops up.[1] In addition, there is also a structural phase transition which strongly influences the magnetic order. It has been found that sizable fluctuation of lattice and magnetic orders exists above the long range ordering temperature.[2] Interestingly, the magnetic order even coexists with superconductivity in the underdoped region. Therefore, the magnetic phase has attracted a great attention as a common player for the high temperature superconductivity in Fe-based and cuprates superconductors.

Figure 1. Phase diagram of Ba(Fe1-xCox)2As2 superconductors. TSDW is the transition temperature to the antiferromagnetic (AFM) spin density wave (SDW) state, TS the structural phase transition temperature, Tc the transition temperature to the superconducting (SC) state, and T* the onset temperature of the structural and magnetic phase fluctuations.[1,2] 

We examine the phase diagram in the ultrafast time scale. Details of temperature and doping dependence of the inequilibrium behaviors should allow us to trace the evolutions of magnetic and superconducting orders. Most of all, the pump-probe investigations of underdoped samples where both orders survive are expected to reveal how these two order parameters compete or cooperate in the ultrafast time scale. Finally, the comprehensive understanding the phase competition behaviors in Fe-based superconductors should shed light on a hidden key to understand the mechanism of the HTSC. 

Figure 2. Spectral features of superconducting gap and spin density wave gap in Ba(Fe,Co)2As2. In case of the superconducting transition, the suppressed spectral weight is transferred to the delta function at zero frequency in contrast to the spin density wave transition where the suppressed weight is added to the region above the gap edge.[3,4,5] 

In the doping depenent study, we benefit from that the magnetic order of spin density waves is accompanied by a distinct spectroscopic feature. Although optical investigation is insensitive to the magnetic order, huge spectral weight redistribution as shown in fig. 2 allows us to trace the magnetic order of the system. It is also important that this spectroscopic feature can be well distinguished from the superconducting gap response because of their different energy scales and the different spectral weight redistribution. Although the spin density wave order gets weaker as doping goes on, we expect that highly sensitive pump-probe measurement over the broadband extending from terahertz to mid-infrared regions should allow us to follow the ultrafast recoveries of those two gap features, mapping their phase diagram in the ultrafast time domain.

Ultrafast dynamics of spin density waves in BaFe2As2

Recently, we have investigated the ultrafast dynamics of spin density waves in BaFe2As2. In this study, we have not only obtained characteristic features, such as recovery time constant, of the spin density waves but also found a very strong coupling between the spin order and lattice.[3] The intense pump pulses can excite a coherent A1g phonon oscillations of As along c-axis. Interestingly, we observed that the coherent lattice motion transiently modulate the feature induced by the spin density waves even at temperature higher than the macroscopic spin density waves ordering temperature. This suggests that the magnetic ordering is very strongly coupled to the lattice in Fe-based superconductors.

Figure 3. Ultrafast recovery dynamics of the spin density waves in BaFe2As2. (a) Two-dimensional map of the photo-induced change of the optical conductivity at TL ~134 K and .(a) its averages in spectral regions A, B and C.(c)Comparison of the dispersion of the oscillatory signal with the one of the SDW-induced changes in the equilibrium state. [3] 

Ultrafast lattice dynamics in Ba(Fe,Co)2As2 

In addition to the study with broadband terahertz-infared pulses, we perform optical pump-X-ray probe experiment using the free electron hard X-ray laser facility at LCLS (Linar coherent light source), SLAC (Stanford linear accelerator center) in collaboration with sicentists at SLAC. The ultrafast X-ray diffraction experiments on BaFe2As2 has revealed that strong pump pulses forces the As move away from the Fe square lattice. The modified band structure facilitates another channel of nesting take places, which is supposed to induce the transient generation of spin density waves in BaFe2As2 according to the As vibration as observed in the previous infrared work. [6] 

Figure 4. Ultrafast X-ray diffraction on BaFe2As2. The pump pulses make As move away from the Fe square lattice, which modulates the Fe-As-Fe bond angle. [6]

Related articles 

[1] “Anomalous Suppression of the Orthorhombic Lattice Distortion in Superconducting Ba(Fe1-xCox)2As2 Single Crystals”, S. Nandi et al., Phys. Rev. Lett. 104, 057006 (2010).

[2] “Electronic nematicity above the structural and superconducting transition in BaFe2(As1-xPx)2”, S. Kasahara et al. Nature 486, 382 (2012).

[3] “Ultrafast transient generation of spin density wave order in the normal state of BaFe2As2 driven by coherent lattice vibrations”, K. W. Kim et al., Nature Materials 11, 497 (2012).

[4]  “Evidence for multiple superconducting gaps in optimally doped BaFe1.87Co0.13As2 from infrared spectroscopy”, K. W. Kim et al., Phys. Rev. B 81, 214508 (2010).

[5] “Origin of the Spin DensityWave Instability in AFe2As2 (A = Ba, Sr) as Revealed by Optical Spectroscopy”, W. Z. Hu et al., Phys. Rev. Lett. 101, 257005 (2008).

[6] “Direct characterization of photoinduced lattice dynamics in BaFe2As2”, S. Gerber, K. W. Kim et al., Nature Communications 6, 7377(2015).