Session T40: Focus Session: Iron Based Superconductors: Doping and Magnetism

Doping dependence of chemical potential in Iron Pnictides by Photoemission

Recently superconductivity has been discovered in many iron pnictides when they are properly doped with charge carriers. It is important to study the chemical potential change as a function of charge carriers. We have performed a systematic photoemission study of core level shift and valence band as a function of doping. We will report our experimental results and comparisons to the first principal band calculations.   

Andreev Reflection Spectroscopy on BaFe$_{1.84}$Co$_{0.16}$As$_{2}$ Epitaxial Thin Films

We have performed point-contact Andreev reflection spectroscopy on high quality epitaxial thin films of BaFe$_{1.84}$Co$_{0.16}$As$_{2}$. In the Andreev reflection spectra, we observe the existence of multiple features which can be attributed to multiple order parameters in the material. Most of the features evolve with temperature and become less prominent close to the critical temperature. However, certain features survive up to a temperature considerably larger than the critical temperature. We discuss this observation in the light of other possible energy scales in this material.   

Effect of Excess Fe on the Conductance Spectra of Iron Chalcogenides

We present point contact spectroscopy measurements on single crystal $Fe_{1+y} Te_{1-x} Se_x$ using a nanometer scale gold tip, as a function of applied magnetic field, temperature, doping and contact resistance. The superconducting samples exhibit a zero bias peak that persists up to 5K above $T_c$ while the $x=0$ compound shows a dip at zero bias, unaffected by applied field of up to 9T. For superconducting compounds, features observed above $T_c$ are also unaffected while superconducting features are diminished by the applied field. The sample surface is characterized by atomic force microscopy while the sample bulk is examined by x-ray diffraction, energy dispersive x-ray spectroscopy and transmission electron microscopy. Point-to-point reproducibility is limited by inhomogeneities in the crystal structure and composition.   

Impurity Scattering Effect and Pairing Symmetry Revealed by Low Temperature Specific Heat in Iron Pnictide Superconductors

Low-temperature specific heat C$_{p}$ was measured on Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ single crystals in wide doping region. A sizeable residual specific heat coefficient $\gamma _{0}$ was observed in the low temperature limit of all samples. The specific heat jump near T$_{c}$, i.e. $\Delta $C$_{p}$/T$\vert _{Tc}$ was also determined. It is found that -$\gamma _{0}$,$_{ }\Delta $C$_{p}$/T$\vert _{Tc}$ and T$_{c}$ all share a similar evolution with doping. These can be well understood within the model of S$^{\pm }$ pairing manner when accounting the cobalt-dopants as unitary scattering centers in the FeAs planes. Our results revealed a non-trivial impact of impurity scattering in FeAs-based superconductors. Angle resolved low temperature specific heat was measured in FeSe$_{1-x}$Te$_{x}$ single crystals. Oscillations with periodicity of $\pi $/2 was observed at 9 T. This constrains the pairing symmetry with the two possibilities: either d-wave or S$^{\pm }$.   

Ce f-electron Kondo effect in the P- and F-doped CeFeAsO

In addition to high temperature superconductivity, the iron pnictides in the CeFeAsO family also provide an opportunity to study the interplay between the f-electron Kondo effect and the d-electron antiferromagnetic or superconducting order. Both the P/As [1,2,3] and F/O substitutions suppress the long-ranged d-electron antiferromagnetic order, and F/O doping also induces superconductivity. With this d-electron phase diagram in mind, we study the Kondo effect in a tunable d-electron antiferromagnetic [4] or superconducting [5] order. We show that the d-electron orders promote frustrating J1-J2-J3 interactions among the f-moments, leading to a co-existing f-electron antiferromagnetic order. The suppression of the d-electron antiferromagnetic and superconducting orders revives the f-electron Kondo effect. The experimentally observed f-electron Kondo physics in the P-doped CeFeAsO and in the pressurized CeFeAsO1-xFx is discussed in the context of our theory. [1] J. Dai, Q. Si, J.-X. Zhu, and E. Abrahams, PNAS 106, 4418 (2009). [2] C. de la Cruz et al, arXiv:0907.2853 (2009). [3] Y. Luo et al, arXiv:0907.2961 (2009). [4] J. Dai, J.-X. Zhu, and Q. Si, PRB 80, 020505(R) (2009). [5] J. Dai, J.-X. Zhu, and Q. Si, to be published (2009).   

Arsenic nuclear magnetic resonance in CaFe2As2

We present 75As nuclear magnetic resonance measurements in the paramagnetic and commensurate antiferromagnetic states of CaFe2As2. Single crystals were produced using a Sn flux method and characterized via powder X-Ray diffraction, susceptibility, and specific heat measurements. The NMR data show that the internal hyperfine field and electric field gradient change discontinuously at T0 = 169K. The observed hyperfine field is consistent with stripelike antiferromagnetic ordering of the Fe spins in the ab plane. Spin lattice relaxation data show metallic T1-1 $\sim $ T for T($<\sim )$ T0/3. However, T1-1 shows a small peak at 10K attributed to slow spin fluctuations that could indicate the emergence of antiferromagnetic domain wall motion.   

Paramagnetic Spin Correlations in CaFe2As2 Single Crystals

Magnetic correlations in the high temperature paramagnetic phase of CaFe$_{2}$As$_{2}$ ($T_N=172$ K) have been examined by means of inelastic neutron scattering from $180$ K up to 300 K. Despite the first-order nature of the magnetic ordering, strong but short-ranged anti-ferromagnetic (AFM) correlations are clearly observed. These correlations, which consist of quasielastic scattering centered at the wavevector $\mathbf{Q}_{AFM}$ of the low-temperature AFM structure, are observed up to the highest measured temperature of 300 K and at high energy transfer (above 60 meV). The weak $L$ dependence of the scattering implies rather weak interlayer coupling corresponding to nearly two-dimensional fluctuations above $T_N$. The spin correlation lengths within the Fe layer are found to be anisotropic, consistent with underlying fluctuations of the AFM stripe structure. These experimental features can be adequately reproduced by a scattering model $S(\mathbf{Q},\hbar\omega$) that describes short-ranged aniostropic spin correlations with overdamped spin dynamics.   

Are the iron pnictides quantum critical?

Unconventional superconductivity is often found in the vicinity of a doping- or pressure-induced magnetic instability, or quantum phase transition (QCP). For a pressure-induced QCP, the Gr\"{u}neisen parameter is expected to diverge as temperature approaches zero and is thus a very sensitive parameter for detecting the presence of a QCP. Experimentally, the Gr\"{u}neisen parameter can ``simply'' be determined by the ratio of the thermal expansivity to the specific heat. Here we report on high-resolution thermal expansion and specific heat measurements of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ single crystals as a function of doping. The question of quantum criticality will be addressed by a detailed analysis of the measured thermodynamic data.   

Magnetic degeneracy and hidden metallicity of the spin density wave state in ferropnictides

We analyze spin density wave (SDW) order in iron-based superconductors and electronic structure in the SDW phase. We consider an itinerant model for Fe-pnictides with two hole bands centered at $(0,0)$ and two electron bands centered at $(0,\pi)$ and $(\pi,0)$ in the unfolded BZ. A SDW order in such a model is generally a combination of two components with momenta $(0,\pi)$ and $(\pi,0)$, both yield $(\pi,\pi)$ order in the folded zone. Neutron experiments, however, indicate that only one component is present. We show that $(0,\pi)$ or $(\pi,0)$ order is selected if we assume that only one hole band is involved in the SDW mixing with electron bands. A SDW order in such 3-band model is highly degenerate for a perfect nesting and hole-electron interaction only, but we show that ellipticity of electron pockets and interactions between electron bands break the degeneracy and favor the desired $(0,\pi)$ or $(\pi,0)$ order. We further show that stripe-ordered system remains a metal for arbitrary coupling. We analyze electronic structure for parameters relevant to the pnictides and argue that the resulting electronic structure is in good agreement with ARPES experiments. We discuss the differences between our model and $J_1-J_2$ model of localized spins.   

Competition and coexistence between the magnetic and the superconducting state of the iron arsenides

The new iron arsenide superconductors present a very rich phase diagram, displaying superconducting, antiferromagnetic and structural order. Here, we develop a microscopic theory to study the interplay between electronic and magnetic degrees of freedom. In this model, electrons sharing the same bands are responsible for both superconductivity and itinerant magnetism, causing these two states to compete. Then, two distinct outcomes are possible: either these two states are mutually exclusive and phase separate or they can coexist microscopically. Using a mean-field approach, we analyze the relation between these two scenarios and the symmetry of the Cooper-pair wave function. In particular, we show that while the so-called $s^{++}$ state is generally incompatible to the antiferromagnetic phase, the unconventional $s^{+-}$ state can coexist with magnetism depending on the Fermi surface topology. Thus, valuable information about the nature of the superconducting phase can be extracted directly from the phase diagrams of the iron arsenides.   

Properties of Spin-Waves in Iron Pnictides

The spectrum of spin density-wave (SDW) fluctuation at zero temperature is studied within a multiband Hubbard-like Model. It is assumed that the formation of the SDW is driven by a short range interaction in a particle-hole channel. The mean-field solution reveals that the ground state is an itinerant, antiferromagnetically ordered phase, with staggered magnetic moment modulated at a wave vector M and a partially gapped Fermi surface, consisting of several disconnected pockets. A familiar Hubbard-Stratonovich transformation is used to obtain the effective action for this SDW state and the quantum fluctuations around the mean-field SDW solution are analyzed for realistic band structure of iron pnictides. In particular, we find that the damping is greatly enhanced by the inter-pocket decay of a spin-wave when its momentum reaches a critical value. We compare our theoretical results with the neutron scattering experiments and discuss interplay of SDW and superconductivity.   

Commensurate and incommensurate magnetic order in CaFe$_4$As$_3$

Magnetic order in newly discovered orthorhombic CaFe$_4$As$_3$ was examined through neutron diffraction on powder and single crystalline samples. The structure can be described as interleaving FeAs strips extending along the $b$ axis. Longitudinally polarized magnetic order with an incommensurate propagation vector $0.37b^{\ast} [Preview Abstract]