Session Q22: Focus Session: Fe-based Superconductivity - Doped and Undoped BaFe2As2

Nature of phase transitions in the parent and lightly electron doped BaFe$_{2}$As$_{2}$ compounds

We present a combined high-resolution x-ray diffraction and x-ray resonant magnetic scattering study of as-grown BaFe$_{2}$As$_{2}$. The structural and magnetic transitions must be described as a two-step process. The undoped BaFe$_{2}$As$_{2}$ parent compound manifests a second-order structural transition from the high-temperature paramagnetic tetragonal structure to a paramagnetic orthorhombic phase and a first-order antiferromagnetic transition from the paramagnetic orthorhombic phase to an antiferromagnetic orthorhombic phase at slightly lower temperature. As electrons are introduced by Co or Rh, the phase transitions evolve toward second-order transitions. Using these results, we provide an estimate of the position of a tricritical point in the phase diagram of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$.   

Neutron powder diffraction studies on Ba$_{1-x}$A$_{x}$Fe$_{2}$As$_{2}$ (A=Na, K)

In the so-called 122 iron pnictide superconductors, the SDW is suppressed and superconductivity can be induced by various means including charge doping, pressure, and isovalent substitution. Using neutron powder diffraction and SQUID magnetization measurements, we have investigated the effects of both potassium and sodium substitution on superconductivity, structural transformation and magnetic ordering in Ba$_{1-x}$A$_{x}$Fe$_{2}$As$_{2 }$(A=K,Na) producing detailed phase diagrams of both systems. We present a comparison of the similarities and differences in the various internal atomic and magnetic structural parameters as a function of temperature and composition of these two hole-doped systems.   

Neutron diffraction study of the development of incommensurate magnetic order in Ba(Fe$_{1-x}M_{x}$)As$_{2}$ ($M=$ Co, Ni, Cu)

We employ neutron diffraction and electronic structure calculations using both virtual crystal and coherent potential approximations to investigate the conditions for incommensurate magnetism in several electron-doped compounds Ba(Fe$_{1-x}M_{x}$)As$_{2}$ with $M=$Co, Ni, Cu. Incommensurate order is observed for $M=$ Co and Ni with $x \approx 0.06$ and 0.03, respectively. Assuming each Co (Ni) ion donates one (two) additional electrons, the appearance of incommensurate magnetism in the Co and Ni doped systems occurs at similar electron concentrations. For $M=$ Cu, Cu should donate three electrons and one expects that incommensurate magnetism will appear at $x \approx 0.02$. However, we find that the magnetism remains commensurate until it disappears at $x \approx 0.05$. Thus, Cu doping behaves differently than either Co or Ni, as can be explained from the electronic structure.   

Magnetic moment in single crystalline BaFe$_{2-x}$Zn$_{x}$As$_{2}$

Nature of the magnetism for iron-based superconductors (FeSCs) has been actively studied since the discovery of this new family of compounds in 2008, largely owing to its significance for interpreting the paring mechanism. The approach through impurity substitution to shed light into this issue is always one of major ways. The substitution shows distinct responses to species of impurities, where partially replacement of Fe in parent FeSCs with a variety of $d$-metals like Co, Ni Ru, Rh, Pd, Ir, and Pt generally results in superconductivity, while recent progress in Zn doped FeSCs gives rather contrary result, where Zn severely degenerates the $T_{C}$. Herein we show the magnetic and electrical studies on BaFe$_{2-x}$Zn$_{x}$As$_{2}$ single crystals. Nonmagnetic Zn doping progressively suppresses the SDW without resulting in superconductivity, while it alternatively develops the spin-glass state, possibly suggestive of local magnetic moment around the Fe sites induced by Zn. The characterizations by X-ray diffraction, magnetic and electrical transport properties, specific heat capacity, and Hall coefficient have been done and the results will be discussed in detail.   

75As-NMR studies on Ba(Fe1-xNix)2As2 single crystals

75As nuclear magnetic resonance (NMR) were measured for Ba(Fe1-xNix)2As2 single crystals with x = 0.05 and x = 0.1 under 0 GPa and 1.5 GPa, respectively. For the optimal doped sample with x = 0.05, the superconducting transition temperature Tc is strongly suppressed from 18 K to 5 K, while for the over-doped sample with x = 0.1, it is turned from the superconducting ground state to a disordered paramagnetic state under 1.5 GPa. Our experimental results show that the antiferromagnetic spin fuctuations are suppressed as well as Tc. The experimental results can be explained with the two-band model. As a result, the electronic band is downward shifted with increase of pressure and the electrons become the dominant carriers in the system.   

Do Transition Metal Substitutions Dope Carriers in Iron Based Superconductors?

We investigate the currently debated issue concerning whether transition metal substitutions dope carriers in iron based superconductors. From first-principles calculations of the configuration-averaged spectral function [1,2] of BaFe$_2$As$_2$ with disordered Co/Zn substitutions of Fe, important doping effects are found beyond merely changing the carrier density. While the chemical potential shifts suggest doping of a large amount of carriers, a reduction of the coherent carrier density was found due to the loss of spectral weight. Therefore, none of the change in the Fermi surface, density of states, or charge distribution can be solely used for counting doped coherent carriers, let alone presenting the full effects of the disordered substitutions. Our study highlights the necessity of including disorder effects in the studies of doped materials in general. [1] W. Ku, T. Berlijn. and C.-C. Lee, Phys. Rev. Lett. 104, 216401 (2010) [2] T. Berlijn, D. Volja and W. Ku, Phys. Rev. Lett. 106, 077005 (2011)   

X-ray absorption spectroscopy study in the BaFe2As2 family

One of the representative Fe-based superconductor families, BaFe2As2 (Tc =38K) is a semimetal with the same number of hole and electron carriers, and is in a spin density wave state below 139K. It has been reported that various types of ``doped'' BaFe2As2 systems can obtained by substitution of Ba, Fe, and As atoms. However, an important issue has been recently raised regarding whether each type of substitution indeed induces effective charge doping or not. It is essential to clarify whether each type of substitution indeed induce an effective doping in BaFe2As2 system. To clarify the carrier doping issue, we performed high resolution X-ray absorption spectroscopy experiment on Ba(Fe,Co)2As2, Ba(Fe,Ru)2As2, BaFe2(As,P)2 which are representative ``doped'' BaFe2As2 systems.   

Competing Magnetic Phases in Ba(Fe$_{0.925}$Mn$_{0.075}$)$_2$As$_2$

Inelastic neutron scattering measurements on Ba(Fe$_{0.925}$Mn$_{0.075}$)$_2$As$_2$ show broad, diffusive spin fluctuations at two different propagation vectors corresponding to stripe magnetic order and conventional N\'eel antiferromagnetic order. Below {\it T} = 80\,K long-range stripe magnetic ordering occurs and sharp spin wave excitations appear at the stripe propagation vector while diffusive spin fluctuations remain at the N\'eel propagation vector. These results suggest that low concentrations of Mn dopants introduce a competing magnetic phase that may prevent the development of superconductivity in Ba(Fe$_{1-x}$Mn$_{x}$)$_2$As$_2$.   

Infrared pseudogap in P- and Co-doped BaFe$_{2}$As$_{2}$ superconductors

We investigated the in-plane electronic response of P- and Co-doped BaFe$_{2}$As$_{2}$ compounds using infrared spectroscopy. We found hallmarks of the normal-state pseudogap in the optical spectra of the BaFe$_{2}$As$_{2}$ system, which are very similar with those of the cuprates. Based on the evolution of the electronic response with doping and across the superconducting transition, we suggest that the antiferromagnetic fluctuations can be a possible origin of the infrared pseudogap in the iron pnictides. We will also discuss implications of our results for the origin of the pseudogap in the cuprates.   

Effects of disordered isovalent substitution in Fe-based superconductor

Using a recently developed first-principles method for disordered materials [1-2], we investigate the effect of isovalent substitution in Fe-based superconductors, BaFe$_2$(As$_{1-x}$P$_x$)$_2$, FeTe$_{1-x}$Se$_x$, and Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$. For anion substitutions (the first two cases) effects of impurity scattering are found mostly in the anion bands. By contrast, the Ru substitution introduces much stronger scattering in the Fe bands. Surprisingly, in all the cases, the pockets near the chemical potential are the least affected, due to the low density of states near the chemical potential. Together, our results suggest an interesting scenario of enhancing superconductivity.\\[4pt] [1] T. Berlijn, D. Volja, W. Ku, Phys. Rev. Lett. 106, 077005 (2011).\\[0pt] [2] W.Ku, T. Berlijn, CC. Lee, Phys. Rev. Lett. 104, 216401 (2010).   

Electron Injection and ``Oxygen Effect'' in BaFe$_{2}$As$_{2}$: M\"{o}ssbauer Studies

It is widely believed that a few percent substitution of Co (with d7) or Ni (with d8) for Fe (with d6) in BaFe$_{2}$As$_{2}$ results in donation~of electrons, killing of magnetism, and thereby induction of superconductivity. We are exploring the possibility of injecting electrons by physico-chemical techniques instead of substitutions.~ We have also discovered that traces of chemisorbed oxygen can change drastically the magnetic behavior in polycrystalline BaFe$_{2}$As$_{2}$ at low temperatures. BaFe$_{2}$As$_{2}$ exhibits considerable hysteresis~behavior as well that of thermal history.   

Effects of annealing on Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Ba(Fe$_{1-x-y}$Co$_x$TM$_y$)$_2$As$_2$ (TM=Mn,Cr) single crystals

Single crystals of Ba(Fe$_{1-x-y}$Co$_x$TM$_y$)$_2$As$_2$ (TM=Cr, Mn) have been grown and characterized by structural, magnetic and transport measurements, both in the as-grown state (quenched from $\sim 1000^{\circ}$~C) as well as after post-growth annealing. This phase space has many parameters and is rich and complex, with superconducting transition temperatures depending upon x and y, as well as annealing temperature and time. In this talk, we will present T-x and T-y, as well as T-time and T-T (for annealing) phase diagrams and discuss the implications for future research into these complex materials.   

Infrared conductivity of BaFe$_2$As$_2$ superconductors: Effects of in-plane and out-of-plane doping

We measured the \emph{in-plane} optical conductivity of a nearly optimally doped ($T_{c}$ = 39.1 K) single crystal of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$. Upon entering the superconducting state the optical conductivity vanishes below $\sim$ 20 meV, indicating a fully gapped system. A model with two different isotropic gaps is required to describe the optical response of this material. The temperature dependence of the gaps indicate a strong interband interaction, but no impurity scattering induced pair breaking is present. This contrasts to the large residual conductivity observed in optimally doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ and strongly supports an $s_\pm$ gap symmetry for both compounds.   

Ultrafast Probing of Antiferromagnetism and Dynamic Critical Behaviors in Iron Pnictide

Iron pnictide superconductors and their parent compounds exhibit superconductivity, antiferromagnetism, poor conductivity in undoped phase, structural instabilities, and additional peculiarities. Much of the complexity originates from strong interactions among the spin, charge, lattice degrees of freedom, where correlated excitations and self-organization among them occur and cause exotic cooperative behaviors. Time-resolved magneto-optical spectroscopy has been developed as a power tool to dynamically disentangle various degrees of freedoms by exploring their responses to fs photoexcitation. Using femtosecond optical and magneto-optical technique, we were able to measure and decoupling dynamics of AFM, structure and electronic excitations in parent BaFe$_{2}$As$_{2}$ and also underdoped sample (Co=4.7{\%}). The technique demonstrated provides an extremely powerful tool to reveal fs dynamics and critical behaviors of various order parameters in the iron pnictide system.