Session L23: Focus Session: Fe-based Superconductors - First Principles, Models, and Spectroscopy

Fe Based Superconductors from first Principles

Density functional theory (DFT) for superconductors (SCs) is, in principle, exact and contains no adjustable parameters.\footnote{\emph{Phys. Rev. Lett.}, 1988, 60, 2430} However, for real calculations an approximation to the pairing potential $\Delta_{\rm xc}$ is needed. Many-body perturbation theory can be used to derive approximations to $\Delta_{\rm xc}$. The perturbation series is developed in orders of the screened Coulomb and phonon interactions and truncated at the first order. Unfortunately, this approach fails to predict superconductivity in unconventional SCs, like Fe based compounds.\footnote{\emph{Phys. Rev. Lett.}, 2008, 101, 026403} Hence, an alternative pairing mechanism has to be present in these materials, the leading contender for which is spin fluctuations.\footnote{\emph{Phys. Rev. Lett.}, 2008, 101, 057003} We present a scheme to extend the current approximation $\Delta_{\rm xc}$ to include spin fluctuations, which involves two steps: (1) the fluctuations are calculated with time dependent DFT,\footnote{\emph{Phys. Rev. Lett.}, 2009, 102, 247206} (2) an effective electron interaction mediated by the spin fluctuations is constructed.\footnote{\emph{Phys. Rev. B, 1985, }32, 2156} We present results for FeSe.   

First-principles study on electronic structure of Ca-Fe-Pt-As-type iron-based superconductors

Since the discovery of the iron-based superconductor, LaFeAsO$_{1-x}$F$_x$ whose Tc reached 26K, various types of iron-based superconductors have been fabricated to attain higher Tc. Recently, new Ca-Fe-Pt-As system of iron-based superconductors was discovered, and the highest Tc in this class becomes 38K. In this system, the crystal structure is different from those of well-known iron-based superconductors. In particular, the blocking layers of some materials in this system are expected to be metallic unlike the other types of iron-based superconductors. We perform first-principles calculations for Ca-Fe-Pt-As system and evaluate their electronic structure.   

First-principles study of cobalt pnictides: Example compound SrCo$_{2}$N$_{2}$

Although it has been known for several decades that many materials with chemical form AT$_{2}$X$_{2}$ (A = alkaline earth element or lanthanide, T = transition metal, X = an element of groups IIIB-VIB) crystallize with the same structure, it was only recently that interest was renewed in these materials with the discovery that BaFe$_{2}$As$_{2}$ can be made to superconductor at temperatures as high as 38 K. Although the precise mechanism of superconductivity has yet to be determined, there is a growing interest in the study of materials that have similar electronic properties to BaFe$_{2}$As$_{2}$. In our work, we employ density functional theory in the local density approximation in order to study the isochemically substituted variant, SrCo$_{2}$N$_{2}$. In this work, we determine both the stability of SrCo$_{2}$N$_{2}$ as well as how the substitution changes the electronic properties in comparison to BaFe$_{2}$As$_{2}$.   

Density functional study on d-orbital characters of the Fe magnetic moment in BaFe$_{2}$As$_{2}$

There have been many published papers related on the orbital characters of band structures in the iron-based superconductors. However, the orbital characters of the Fe magnetic moment still remain unrevealed. By performing first-principles calculations of the electronic and magnetic properties with constraint on the real space shape of Fe magnetic moments, we study the d-orbital characters of the Fe magnetic moment in BaFe$_{2}$As$_{2}$. We compare obtained band structures with published angle-resolved photoemission spectroscopy (ARPES) result, and propose that the Fe magnetic moment in BaFe$_{2}$As$_{2}$ has in-plane d$_{xy}$ character. This work was supported by the NRF of Korea (Grant Nos. 2009-0081204 and 2011-0018306). Computational resources have been provided by KISTI Supercomputing Center (Project No. KSC-2011-C3-05)   

Theoretical insight on layered Mn-compounds under pressure

Since the unexpected discovery of Fe-based superconductors, the search for new materials with a higher superconducting $T_c$ has been reinvigorated. Mn based compounds in the ThCr$_2$Si$_2$ structure contain square lattice layers of the transition metal atom, like the cuprates and the iron arsenides, but the strong antiferromagnetic correlations typical of these compounds suggest that superconductivity in the manganites is impossible. However new experiments carried under pressure show that some of these layered Mn-compounds go through an electron delocalization transition . Here we study these compounds from a theoretical point of view. Looking at the magnetic properties and the Fermi surfaces close to the electron delocalization transition, we discuss the possibility of superconductivity for these compounds.   

Renormalization group study up to two-loop order of an effective two-band model for iron-based superconductors

We perform a renormalization group (RG) study up to two-loop order of an effective low-energy two-band model to describe some of the recently discovered iron-based superconductors. Our starting point is the itinerant electronic model proposed by Chubukov \emph{et al.} [Phys. Rev. B \textbf{78}, 134512 (2008)], which displays two small, almost nested Fermi pockets with one hole pocket centered at $(0,0)$ and one electron pocket centered at $\mathbf{Q} = (\pi,\pi)$ in the folded Brillouin zone. We then proceed to implement a two-loop RG calculation for this model of four-point vertex corrections, quasiparticle weight and several order-parameter susceptibilities in order to evaluate the robustness of one-loop RG results available in the literature with respect to including self-energy effects and higher-order quantum fluctuations.   

Microscopic derivation of GL theory for magnetism and superconductivity in multiband electronic systems

The superconductivity in pnictides arises due to interband scattering between Cooper pairs belonging to hole and particle Fermi surfaces. The amplitude of the scattering, while weak at bare level, is enhanced under RG flow, and competes with the SDW to become the leading instability when the hole and particle FS's are nearly, but not perfectly, nested. This motivates us to construct a GL theory with multiple order parameters from the microscopic action. It allows us to study the two competing orders and their interplay including a possible coexistence. The presence of both attractive and repulsive pairing terms requires a novel approach to the derivation with imaginary terms in the action as one consequence. The construction of the GL theory is a two-step process, with RG flow to an intermediate cut-off scale, determined by the deviations from the nesting, as the first step.   

Impurity Dimer Scattering as Reflection of Band Reconstruction in Iron Pnictides

While the impurity induced nanoscale electronic disorder has been extensively reported in the underdoped iron pnictides its microscopic origin remains a theoretical challenge. Recent STM measurements reveal a resonance in the impurity dimer scattering in cobalt-doped iron arsenides. These resonant dimers are randomly distributed but aligned with antiferromagnetic a-axis. We present a theoretical study of the impurity induced quasiparticle interference patterns in these materials, based on the five orbital model. The local density of states oscillates with the ``imperfect nesting'' wavevector encoded in the reconstructed Fermi pockets, provided one assumes the ordered pocket density wave (PDW) state along the b-axis. This anisotropic oscillation pattern breaks C$_{4}$ symmetry and is in agreement with the electronic dimmer resonance found in the STM experiments, hinting at the existence of~``hidden'' PDW order in iron-based superconductors [1]. [1] J. Kang and Z. Tesanovic, Phys. Rev. B 83, 020505(R) (2011).   

Jump in specific heat at the superconducting transition in iron pnictides

Experiments reveal that in iron-based superconductors the jump $\Delta C$ of the specific heat at the superconducting transition is not proportional to the transition temperature $T_c$, as expected in the BCS theory. Rather, the ratio $\Delta C/T_c$ varies with $T_c$, and has a peak near optimal doping and decreases at smaller and larger dopings. We show that this behavior can be naturally explained by the interplay between superconductivity and antiferromagnetism in the underdoped regime. We demonstrate that $\Delta C/T_c$ is indeed peaked at the doping where the coexistence phase with antiferromagnetism develops, and decreases at deviations from this doping in both directions.   

Beyond anharmonicity: signature of spin-phonon coupling at the surface of BaFe$_{2}$As$_{2}$

High Resolution Electron Energy Loss Spectroscopy (HREELS) has been used to investigate the temperature dependence of the lattice dynamics of cleaved single crystals of BaFe$_{2}$As$_{2}$, one of the parent compounds of Fe-based superconductors. Both the phonon frequency as well as phonon linewidth of the intense 32 meV out-of-plane Fe/As mode (A$_{2u})$ and the 24 meV out-of-plane As vibration mode (A$_{1g})$ show a dramatic temperature dependence and anomalous behavior below $\sim $150K. The anomalous behavior is associated with the coupled elasto-magnetic transition in the bulk but occurs appreciably higher at the surface than in the bulk ($\sim $138 K). The anharmonicity at the surface is considerably larger than that in the bulk for the orthorhombic phase, but is significantly less in the tetragonal phase. A detailed discussion is given in terms of the interplay between the spin and lattice in this novel system.   

Low magnetic field microwave absorption and ESR studies of coexisting superconductivity and ferromagnetism in Eu-based pnictides

We have studied the coexistence of FM ordering and SC in the doped pnictide EuFe$_2$(As$_{1-x}$P$_x$)$_2$ system, with doping levels of $x=0.3$ and 0.27, by combined ESR and non-resonant low field microwave absorption (LFMA) methods, and magnetization in SQUID. LFMA uses an external magnetic field to create vortices in the material, which are non-resonantly excited by MW radiation. The resulting spectrum's hysteresis shape and form changes with $T$, revealing transitions around 9 and 20 K, and showing the FM onset and SC $T_c$. Two types of coexisting LFMA signals have been observed: a) a superconducting hysteretic LFMA loop, appearing below $T_c$ and changing its shape, phase and intensity with $T$, and b) the non-hysteretic, reversed phase LFMA, above SC $T_c$, which is assigned to the magnetically ordered ferromagnetic phase, since it does not depend on $T$. Additionally, in the $x=0.27$ sample, a reemergence of hysteresis is seen near 18K, corresponding with the reentrant superconductivity observed in transport measurements. LFMA has also been used to determine the position of the irreversibility line and to estimate the critical current density for the two samples.   

ab Plane Point Contact Spectroscopy of Co-doped Iron Pnictide Superconductors

Point contact measurements on iron-based superconductors provide valuable information about these new superconductors, including gap sizes, number of gaps, and gap symmetries. We use a point contact apparatus capable of taking measurements in the ab{\-}plane using a tungsten wire lowered mechanically onto a superconducting sample. Using this method allows us to measure the variation of superconducting gap(s) with Z, a dimensionless barrier strength parameter. Previous point contact measurements have shown Co-doped iron pnictide superconductors to be a two gapped material. Our measurements confirm the presence of two gaps in single crystal Ba(Fe$_{0.926}$Co$_{0.074)2}$As$_{2}$ and the Z variation of the measurements imply the gaps are isotropic. Acknowledgments: NSF DMR-0804452, DOE contract {\#} DE-FG02-86ER45268   

Highly anisotropic Dirac fermions in a Bi square net of SrMnBi$_2$

We report the highly anisotropic Dirac fermions in a Bi square net of SrMnBi$_2$, based on a first principle calculation, angle resolved photoemission spectroscopy, and quantum oscillations for high-quality single crystals. We found that the Dirac dispersion is generally induced in the (SrBi)$^{+}$ layer containing a double-sized Bi square net. In contrast to the commonly observed isotropic Dirac cone, the Dirac cone in SrMnBi$_2$ is highly anisotropic with a large momentum-dependent disparity of Fermi velocities of $\sim 8$. These findings demonstrate that a Bi square net, a common building block of various layered pnictides, provide a new platform that hosts highly anisotropic Dirac fermions.