Session S25: Superconductivity: Less Common Materials II

Instability of three-band Luttinger liquids: renormalization group analysis and possible application to K$_{\mathrm{2}}$Cr$_{\mathrm{3}}$As$_{\mathrm{3}}$

Motivated by recently discovered quasi-one-dimensional superconductor K$_{\mathrm{2}}$Cr$_{\mathrm{3}}$As$_{\mathrm{3}}$ with $D_{3h}_{\mathrm{\thinspace }}$lattice symmetry, we study one-dimensional three-orbital Hubbard models with generic electron repulsive interaction described by intra-orbital repulsion $U$, inter-orbital repulsion$U'$, and Hund's coupling $J$. As extracted from density functional theory calculation, two of the three atomic orbitals are degenerate and the third one is non-degenerate, and the system is presumed to be at incommensurate filling. With the help of bosonization, we have usual three-band Luttinger liquids in the normal state. Possible charge density wave (CDW), spin density wave (SDW) and superconducting instabilities are analyzed by one-loop renormalization group. The ground state depends on the ratio $J/U$. For the physical relevant parameter region, $\mbox{0}1/2$.   

Evaluations of MgB$_{\mathrm{2}}$ Coatings on 2'' Copper Discs for Superconducting Radio Frequency Applications.

We propose that coating the inner walls of copper RF cavities with superconducting MgB$_{\mathrm{2}}$ ($T_{c} \quad =$ 39 K) can result in a viable alternative to the already established niobium-based SRF technology. This approach improves the thermal conductivity, allows for operation at higher temperatures, and reduces the need for large helium refrigeration, thereby resulting in lower operational costs. For our studies, we grew MgB$_{\mathrm{2}}$ films via hybrid physical chemical vapor deposition (HPCVD) on 2'' Cu substrates. Since Mg and Cu readily form an alloy at higher temperatures, the HPCVD setup was modified in order to achieve lower deposition temperatures, minimize alloy formation, and provide high quality MgB$_{\mathrm{2}}$ films. This method yielded MgB$_{\mathrm{2}}$ coatings on 2'' Cu discs with transition temperatures around 38 K. The samples were characterized with regards to their RF attributes and showed similar performance in comparison to Nb reference samples. The presented results show that MgB$_{\mathrm{2}}$ coated copper can be a suitable alternative for use in SRF cavities.   

Electronic Pair-Binding and Hund's Rule Violations in Doped C60

We calculate the electronic properties of the t-J model on a C60 molecule using the density-matrix renormalization group and show that Hund's first rule is violated and that for an average of three added electron per molecule, an effective attraction (pair-binding) arises for intermediate values of t$=$J. Specifically, it is energetically favorable to put four electrons on one C60 and two on a second rather than putting three on each. Our results show that a dominantly electronic mechanism of superconductivity is possible in doped C60.   

Camelback-shaped band reconciles heavy-electron behavior with weak electronic Coulomb correlations in superconducting TlNi$_{2}$Se$_{2}$

Combining photoemission spectroscopy, Raman spectroscopy, and first-principles calculations, we characterize superconducting TlNi$_{2}$Se$_{2}$ as a material with weak electronic Coulomb correlations leading to a bandwidth renormalization of 1.4. We identify a camelback-shaped band, whose energetic position strongly depends on the selenium height. While this feature is universal in transition metal pnictides, in TlNi$_2$Se$_2$ it lies in the immediate vicinity of the Fermi level, giving rise to a pronounced van Hove singularity (VHS). The resulting heavy band mass resolves the apparent puzzle of a large normal-state Sommerfeld coefficient in this weakly correlated compound. The correlation effect evolution in pnictides upon d-shell filling in the presence of significant Hund's exchange coupling will also be discussed.   

The happy marriage between electron-phonon superconductivity and Mott physics in Cs$_3$C$_{60}$: A first-principle phase diagram

The phase diagram of doped fullerides like Cs$_3$C$_{60}$ as a function of the spacing between fullerene molecules is characterized by a first-order transition between a Mott insulator and an s-wave superconductor with a dome-shaped behavior of the critical temperature. By means of an ab-initio modeling of the bandstructure, the electron-phonon interaction and the interaction parameter and a Dynamical Mean-Field Theory solution, we reproduce the phase diagram and demonstrate that phonon superconductivity benefits from strong correlations [1] confirming earlier model predictions [2]. The role of correlations is manifest also in infrared measurements carried out by L. Baldassarre [3]. The superconducting phase shares many similarities with "exotic" superconductors with electronic pairing, suggesting that the anomalies in the "normal" state, rather than the pairing glue, can be the real common element unifying a wide family of strongly correlated superconductors including cuprates and iron superconductors. [1] Y. Nomura, S. Sakai, K. Nakamura, M. Capone, and R. Arita, Science Advances 1, e1500568 (2015) [2] M. Capone et al., Science 296, 2364 (2002); M. Capone et al., Rev. Mod. Phys. 81, 943 (2009) [3] L. Baldassarre et al. Sci. Rep. 5, 15240 (2015   

Optimal High-$T_C$ Superconductivity in $\bf{Cs_3C_{60}}$

The highest superconducting transition temperatures in the $(A_{1-x}B_x)_3$C$_{60}$ superconducting family are seen in the A15 and FCC structural phases of Cs$_3$C$_{60}$ (optimized under hydrostatic pressure), exhibiting measured values for near-stoichiometric samples of $T_{C0}^{meas.}$ = 37.8 K and 35.7 K, respectively. It is argued these two Cs-intercalated C$_{60}$ compounds represent the optimal materials of their respective structures, with superconductivity originating from Coulombic $e$-$h$ interactions between the C$_{60}$ molecules, which host the $n$-type superconductivity, and mediating holes associated with the Cs cations. A variation of the interlayer Coulombic pairing model [Harshman and Fiory, J. Supercond. Nov. Magn. $\underline{28}$, 2967 (2015), and references therein] is introduced in which $T_{C0}^{calc.} \propto 1/\ell\zeta$, where $\ell$ relates to the mean spacing between interacting charges on surfaces of the C$_{60}$ molecules, and $\zeta$ is the average radial distance between the surface of the C$_{60}$ molecules and the neighboring Cs cations. For stoichiometric Cs$_3$C$_{60}$, $T_{C0}^{calc.}$ = 38.08 K and 35.67 K for the A15 and FCC macrostructures, respectively; the dichotomy is attributable to differences in $\zeta$.   

Electrical transport properties of C$_{60}$ superconductors in the vicinity of Mott metal-insulator transition

It was revealed in 2008 that Cs$_{3}$C$_{60}$ showed superconductivity up to 38K under high pressure [1]. As the lattice constant is increased, the superconducting critical temperature ($T_{C})$ decreases after experiencing the maximum $T_{C}$ and finally Cs$_{3}$C$_{60}$ becomes a Mott insulator [2]. This result suggests that C$_{60}$ superconductors cannot be simply explained by the BCS theory [1-4]. There are strong electron correlations in the vicinity of Mott metal-insulator transition. Electrical transport properties are required because they provide us importantly intrinsic information on the electronic states, but the systematical electrical transport measurements have not been made due to the experimental difficulties. It will be reported that we have successfully obtained the electrical transport properties of expanded C$_{60}$ superconductors in pellet form by using a specially designed pressure cell. [1] A. Y. Ganin \textit{et al.}, \textit{Nat. Mater}. \textbf{7}, 367 (2008). [2] A. Y. Ganin \textit{et al.}, \textit{Nature} \textbf{466}, 221 (2010). [3] Y. Takabayashi \textit{et al.}, \textit{Science}, \textbf{323}, 1585 (2009). [4] R. H. Zadik \textit{et al.}, \textit{Sci. Adv.} \textbf{1}, e1500059 (2015).   

Isotope Effect on Electron-Phonon Coupling in Multiband Superconductor MgB2

We systematically investigate the isotope effect of electron-phonon coupling in multi-band superconductor MgB$_2$ by laser based Angle Resolved Photoemission Spectroscopy. The kink structure around 70 meV on two $\sigma$ bands, which is caused by electron coupling to $E_{2g}$ phonon mode, is shifted to higher binding energy in Mg$^{10}$B$_2$ than that in Mg$^{11}$B$_2$. The measured shifting energy of 3.5 meV is consistent with theoretical calculation based on harmonic phonon in MgB$_2$. Our temperature dependent measurement also indicates the isotope effect of kink structure is not dependent on superconducting transition.   

Momentum-resolved electronic structure of the superconductor parent compound BaBiO$_3$

We use \emph{in situ} angle-resolved photoemission to study thin films of BaBiO$_3$, a parent compound of bismuthate superconductors with $T_c$ up to 30 K. By simple electron counting, BaBiO$_3$ should be metallic. However, in analogy with many unconventional and high-$T_c$ superconductor families, it is instead insulating, and superconductivity emerges with doping. Our experiments reveal a folded band structure consistent with known BiO$_6$ breathing distortions. However, charge ordering often thought to accompany the distortions is virtually nonexistent. The data combined with DFT calculations indicate that states near $E_F$ are primarily oxygen-derived. Hence BaBiO$_3$ appears to be characterized by negative charge transfer energy. This can account for the seeming discrepancy between the atomic structure and "missing" charge order. It should also be relevant for understanding the doping evolution and superconductivity in bismuthates.   

Comparison of Tunneling in Fe-based Superconductors with Multi-band MgB$_{\mathrm{2}}$

MgB$_{\mathrm{2}}$ is an s-wave, phonon coupled, multiband superconductor that exhibits novel tunneling spectra including a subtle dip feature due to quasiparticle transfer between bands. Since this feature mimics the above-gap spectral dip feature observed in Fe-based superconductors, typically attributed to a strong coupling boson, it is worthwhile to consider whether quasiparticle transfer is relevant. We first show that the dip in MgB$_{\mathrm{2}}$ appears in the $\pi $-band, DOS ($\Delta =$2.4 meV) and is due to quasiparticle transfer to the $\sigma $-band with $\Delta =$7.2 meV. Reviewing the spectral dip in Fe-based superconductors, including new data on FeSe crystals, there are inconsistencies with quasiparticle transfer as the origin. The conclusion is that the spectral dip is more likely due to a boson, the resonance spin excitation, as found in cuprate superconductors.   

Robust Resistive Critical Field in Noncentrosymmetric B20 AuBe

AuBe is a chiral-structured (B20 structure) superconductor. The B20 structure in magnetic systems was discovered to host a magnetic topological structure, the Skyrmion lattice, and our research focused on what behavior the same structure would effect in a superconducting system. Samples were arc-melted in an Ar atmosphere and characterized via powder XRD. Specific heat measurement revealed bulk superconductivity with an exponential form below Tc while magnetization showed Type I behavior near the Tc of 3.2 K and a crossover to Type II behavior at approximately 1.2 K. Resistance measurement revealed a critical field that deviates from that found in magnetization measurements at approximately 2.4 K linearly rising with decreasing T to approximately 3.5x Hc2 at T$=$0.3K. The resistive critical field was also found to be robust against a Cr film deposited on the surface of AuBe. We find similarity between this superconductivity crossover behavior and robust low temperature critical field with other noncentrosymmetric superconductors in literature. Additionally, we measured the de Haas-van Alphen effect in polycrystalline samples and derived an effective electron mass of 0.16mo for a small spherical piece of Fermi surface.   

Angle Resolved Thermal Conductivity of Superconducting CeCoIn$_{5}$ along the Nodal Direction

The thermal conductivity measurement in a rotating magnetic field is a powerful probe of the structure of the superconducting energy gap. The four-fold oscillation in thermal conductivity of CeCoIn$_{5}$, with the heat current in the anti-nodal direction, has revealed the d-wave nature of its order parameter. We have measured the thermal conductivity with the heat current along the [110] (nodal) direction and the magnetic field rotating in the \textit{ab}-plane. In contrast to the smooth oscillation found with the heat current along the anti-nodal direction, a sharp increase of thermal conductivity was observed when the magnetic field is also in the [110] direction, parallel to the heat current. This suggests that the scattering of the nodal quasiparticle is strongly suppressed along the magnetic field direction. In addition, a smaller increase of the thermal conductivity was observed when the magnetic field is approximately 30 degree away from the nodal direction, perhaps due to a Fermi surface anomaly.   

electric dipole superconductor in bilayer exciton system

Recently, it was reported that the bilayer exciton systems could exhibit many new phenomena, including the large bilayer counterflow conductivity, the Coulomb drag, etc. These phenomena imply the formation of exciton condensate superfluid state. On the other hand, it is now well known that the superconductor is the condensate superfluid state of the Cooper pairs, which can be viewed as electric monopoles. In other words, the superconductor state is the electric monopole condensate superfluid state. Thus, one may wonder whether there exists electric dipole superfluid state. In this talk, we point out that the exciton in a bilayer system can be considered as a charge neutral electric dipole. And we derive the London-type and Ginzburg-Landau-type equations of electric dipole superconductivity. From these equations, we discover the Meissner-type effect (against spatial variation of magnetic fields), and the dipole current Josephson effect. The frequency in the AC Josephson effect of the dipole current is equal to that in the normal (monopole) superconductor. These results can provide direct evidence for the formation of exciton superfluid state in the bilayer systems and pave new ways to obtain the electric dipole current.