Session G35: Focus Session: Search for New Superconductors I

Superconductivity and New Compounds in the Bi-O-S System

Recent reports of superconductivity in Bi$_4$O$_4$S$_3$ and LaO$_{1-x}$F$_x$BiS$_2$ have stimulated interest in a potentially new family of layered superconductors based on BiS$_2$ units. The most interesting structural feature of the reported crystal structure of Bi$_4$O$_4$S$_3$ is that it contains both reduced sulfides (S$^{2-}$) and oxidized sulfates (S$^{6+}$) within the same compound. However, the scattering factors of oxygen and sulfur relative to bismuth make the precise structure of Bi$_4$O$_4$S$_3$ difficult to determine by x-ray diffraction, and limit the ability to compare with theoretical predictions of electronically driven structural distortions. Here we report the results of studies on the structure and physical properties of compounds in the Bi-O-S system through electron diffraction, high resolution transmission electron microscopy, synchrotron x-ray diffraction, and IR spectroscopy, including the discovery of two new ternary Bi-O-S phases.   

Phonon-mediated superconductivity in electrostatically and chemically doped single-layer MoS2

MoS$_2$ is a semiconductor with a layered structure that can be exfoliated to make few-layer and single-layer samples. Superconductivity has recently been reported in electrostatically doped few-layer MoS$_2$ samples, with a transition temperature above 9 K, which is higher than the maximum T$_c$ found in intercalated bulk MoS$_2$. Here we report a density functional theory study of electron-phonon coupling in doped single-layer MoS$_2$. With electrostatic n-type doping at levels comparable to those achieved in MoS$_2$ field-effect transistors, the electron-phonon coupling constant is calculated to be consistent with a superconducting T$_c$ of 5-10 K. While deposition of alkali atoms on the surface also introduces carriers into the conduction band, we find that in some cases, it creates significant changes in the electronic structure, leading to a weaker interaction between electrons and phonons. The dependence of the electron-phonon interaction on carrier concentration, for both n-type and p-type doping, will be discussed.   

Superconductivity in Nb$_3$Pd$_{0.75}$Se$_7$

Here, we report the discovery of superconductivity in the transition metal chalcogenide Nb$_3$Pd$_{0.75}$S$_7$ with a transition temperature $T_c = 1.9 $ K. In extremely thin, needle like single crystals we observe upper critical fields $H_{c2}^b(T \rightarrow 0$ K) $\simeq 14 $ T for fields directed along the needle axis, or the crystallographic \emph{b}-axis. This value is 4 times larger than the expected weak coupling Pauli limiting field. For fields applied along two directions perpendicular to the \emph{b}-axis, we observe considerably smaller but anisotropic upper critical fields. For fields along and perpendicular to the $b$-axis we observe a temperature-dependent anisotropy $\gamma = H_{c2}^b/H_{c2}^{\perp b}$ as large as 6 (as $T \rightarrow T_c$). This behavior suggests that this compound is a multi-band superconductor. The low symmetry of its crystallographic structure implying low electronic dimensionality, coupled to multi-band behavior and very high upper critical fields, suggests an unconventional superconducting ground-state.   

The Search for Higher Temperature Superconductors: Two Case Studies

The recent confluence of optimism in the prospects for higher temperature superconductivity and the documented need for new higher temperature superconductors (if electric power applications above liquid nitrogen temperatures are to be possible) has simulated several focused programs in the search new and improved high-Tc superconductors. In this talk, we review these motivating factors and present the results of two case studies. The first is the study of the high Tc bismuthate superconductors to understand the mechanism of their superconductivity and the factors governing Tc. We find that the bismuthates are moderately correlated materials with a dynamically enhanced electron-phonon interaction that exhibit dimorphism and a sensitivity of Tc to disorder. The second is the study of Cu/CuO interfaces (for which evidence of trace high temperature superconductivity has been reported) at which we find a new proximity effect in which antiferromagnetism is induced into a metal (Cu) by proximity to a charge transfer insulator (CuO).   

Electronic properties of $\alpha $-FeSi$_{2}$ -- single crystal study

The discovery of high temperature superconductivity (HTS) in Fe pnictides has simulated a lot of work in field of Fe-based materials. We focus on the tetragonal (high-temperature) form of the iron disilicide, which crystal structure resembles one of the HTS, LiFeAs ($T_{sc}=$18 K). Single crystals of $\alpha $-FeSi$_{2}$ with Fe$_{0.83}$Si$_{2}$ composition were grown and magnetic, transport and heat capacity studies were performed in consistent way. Magnetic susceptibility of $\alpha $-FeSi$_{2}$ increases in a linear fashion with increasing temperature, as was commonly observed among Fe HTS. In a contrast to superconducting pnictides, where $\chi (T)$ $\sim$ $T$ is associated to antiferromagnetic fluctuations, in $\alpha $-FeSi$_{2}$ this behavior is rather related to the electronic structure of this metal. In Fe-based HTS proximity of the SDW instability seems to be crucial for the emergence of superconducting state -- in $\alpha $-FeSi$_{2}$ the experimental data do not find evidence for any strong electronic correlations. Our LDA and DMFT calculations results find low density of states, supporting weakness of correlations and suggest electronic configuration of Fe close to d$^{6}$.   

Magnetic Excitations in LaMnPO

We performed inelastic neutron scattering experiments on LaMnPO at the BT-7 triple-axis spectrometer at NIST Center for Neutron Research. LaMnPO is an insulating pnictide compound and is antiferromagnetically ordered below T$_{N}$ = 375 K. Constant energy scans were performed above T$_{N}$, and revealed spin-spin correlations in the paramagnetic state with characteristic wavevector Q = 1.6 \AA$^{-1}$, near the antiferromagnetic ordering wavevector Q$_{AFM}$ = 1.55 \AA$^{-1}$. We performed constant wavevector scans above and below T$_{N}$ and these show there is a q-dependent and temperature-dependent energy gap in the magnetic excitations that vanishes at T$_{N}$ = 375 K. Constant energy scans below T$_{N}$ show the peak in the magnetic excitations does not change up to a measured energy transfer of 15 meV, suggesting exchange interactions are quite strong. The magnetic excitations in LaMnPO are similar to those observed in the parent compounds of the iron pnictide superconductors. Research supported by a National Security Science and Engineering Faculty Fellowship by the AFOSR   

Pressurized LaMnPO: antiferromagnetic insulator to magnetic metal

It is felt that high temperature superconductivity stems from proximity to an electron delocalization transition, such as the metal-insulator transitions exhibited by the cuprates or the antiferromagnetic transitions of the iron pnictides. We subjected the manganese pnictide LaMnPO to hydrostatic pressures up to 43 GPa, measured x-ray diffraction patterns, and solved the crystal structures at various pressures. We then performed LSDA electronic structure calculations using the observed lattice constants and atomic parameters to obtain the magnitudes of the insulating gap and the ordered state magnetic moment. While the calculations found the gap to close near 10 GPa, the magnetic moment persisted until a structural collapse at 31 GPa. These results imply that the metal-insulator transition and the antiferromagnetic transition are separated within the pressure phase diagram of LaMnPO. We discuss these results in light of the inherent differences between Mott-like and Hund's-like systems.   

Complex magnetic properties in multilayer rare earth oxypnictides

Intensive research interest on layered transition metal pnictide materials was stimulated by the discovery of high temperature superconductivity in Fe-pnictides a few years ago. To study the relationship between superconductivity, crystal structure and magnetism, and to search for novel superconductors of better application potential, more transition metal pnictides are worth investigating. In this talk, I will discuss physical properties of members of a particular class of layered oxypnictides, with four transition metal pnictogen layers per unit cell. While varying the rare earth ion, we find that one compound is a low temperature superconductor (Tc $\sim$ 2.1 K), and others show diverse magnetic properties, including ferromagnetic or antiferromagnetic order, or spin glass behavior. I will show our observation from measurements of DC and AC magnetization, specific heat and resistivity. The understanding of the physical properties of these isostructual compounds may serve as a guide in the search for superconductivity in these systems.   

Angle Resolved Photoemission Spectroscopy Study on Layered Oxy-pnictide BaTi2As2O

Recently, superconductivity has been discovered in Ba$_{1-x}$Na$_x$Ti$_2$Sb$_2$O, a titanium-based oxy-pnictide with an anti-CuO$_2$ type Ti$_2$O plane and a CDW/SDW anomaly at 54K. The isostructured BaTi$_2$As$_2$O, where signs of CDW/SDW at 200K has been observed, could be viewed as one of the parent compounds of this new family of superconductors. Here we report the Angle Resolved Photoemission Spectroscopy Study on BaTi$_2$As$_2$O. Parallel sections were found in Fermi surface structure, indicating possible nesting condition. The orbital character of bands supports the important role of Ti-Ti direct interaction. No abrupt change was observed at the critical temperature; however, spectral weight change takes place at wide energy scale both above and below the critical temperature, revealing the strong electron-lattice coupling effect in this system.   

Investigation of the normal and superconducting states of Ba$_x$Na$_{1-x}$Ti$_2$Sb$_2$O (0 $\geq$ x $\geq$ 0.33) : a pnictide oxide compound with hole doped titanium-oxygen layers

The interest in layered transition metal oxides/pnictides was re-ignited by the discovery of the iron pnictides; 2 such as examples are, Na$_2$Ti$_2$Pn$_2$O and BaTi$_2$As$_2$O. Both compounds are comprised of a layered structure and exhibit a SDW/CDW, similar to the iron pnictide parent compounds. It is well established that by suppressing the SDW, superconductivity emerges in pnictides; therefore, can a similar approach be used for these titanium based pnictide oxides? To date, the lowering of the critical temperature for the SDW/CDW has been reported, but no superconductivity was seen for BaTi$_2$As$_2$O. We report the effects of hole doping on BaTi$_2$Sb$_2$O and its influence on the SDW and superconducting states. Initial findings from neutron scattering will also be discussed. Our parent compound, which is similar to the BaTi$_2$As$_2$O in structure, shows a SDW/CDW at 57 K. A systematic lowering of the critical temperature is seen for the SDW/CDW with increased doping. In addition, the superconducting temperature increases up to 6 K. The phase diagram as a function of doping is derived from the normal and superconducting states of the system. The lowering of the critical temperature of the SDW/CDW seems to be the key for the emergence of superconductivity.   

Crystal structure and superconducvitiy in BaPbO$_3$/BaBiO$_3$ thin films

Thin bilayers of BaPbO$_3$ and BaBiO$_3$ were grown on SrTiO$_3$ by e-beam evaporation in the hope of testing the proximity effect route to high T$_{\mathrm{c}}$ superconductivity suggested by Kivelson et al [Phys.Rev.B 78, 094509]. X-ray diffraction measurements show that the bilayers are single-phase, but fully relaxed. Depth-profiling by XPS showed that for a deposition temperature of 500 $^{\circ}$C there is a gradual intermixing of Pb and Bi in the top BaPbO$_3$ layer. This could result in a superconducting Ba(Pb,Bi)O$_3$ film, but XRD points to well-resolved layers. Superconductivity in these films is BCS-like, with $\xi_{GL}(0)$ $\sim$ 10 nm comparable to bulk values. However, the superconductivity was not primarily correlated with the Bi content as determined from surface XPS scans, but by the crystal structure. The superconducting films consistently have a larger unit cell volume, mostly due to larger in-plane lattice constants. This increase coincides with a higher Ba/Pb elemental ratios, which in literature has been linked to the occurrence of the tetragonal form of Ba(Pb,Bi)O$_3$ [Sol.State.Comm. 60, 897-900]. This larger unit cell may result in a lower tilt angle of the oxygen octahedra, which has a positive influence on the superconductivity [Phys.Rev.B 83, 174512].   

Multigap Superconductivity at 5.4 K in $\beta$-Bi$_2$Pd

We report the superconducting properties of new multigap superconductor Bi$_2$Pd($\beta$-Bi$_2$Pd; space group: $I4/mmm$)[1]. $\beta$-Bi$_2$Pd single crystals were grown via a melt-growth method. The temperature dependences of the electrical resistivity and the magnetic susceptibility reveal that the superconducting transition occurs at 5.4 K in the $\beta$-Bi$_2$Pd single crystal. This value is greater than the value of 4.25 K reported in the previous paper [2]. Here, it is interesting to note that the $T_{\mathrm{c}}$ of $\beta$-Bi$_2$Pd reported here is almost the same as that of Pd-intercalated Bi$_2$Te$_3$ with a very small superconducting volume fraction ($<1 \%$) in ref. [3], where the possibility that the topological insulator Bi$_2$Te$_3$ can be made into an SC by Pd intercalation between the Bi$_2$Te$_3$ layers is argued. In addition, the temperature dependences of the upper critical magnetic field and the specific heat suggest that $\beta$-Bi$_2$Pd is a multiple-band/multiple-gap superconductor. \\[4pt] [1] Y. Imai \textit{et al.}, J. Phys. Soc. Jpn. 81 (2012) 113708. (arXiv: 1207.5905.)\\[0pt] [2] N.~E. Alekseevski \textit{et al.}, Zh. Eksp. Teor. Fiz. 27 (1954) 125.\\[0pt] [3] Y.~S. Hor \textit{et al.}, J. Phys. Chem. Sol. 72 (2011) 572.   

Different doping from apical and planar oxygen vacancies in Ba$_{2}$CuO$_{4-\delta}$ and La$_{2}$CuO$_{4-\delta}$

First principles band-structure calculations for large supercells of Ba$_{2}$CuO$_{4-\delta}$ and La$_{2}$CuO$_{4-\delta}$ with different distributions and concentrations of oxygen vacancies show that the effective doping on copper sites strongly depends on where the vacancy is located. A vacancy within the Cu layer produces a weak electron doping effect while a vacancy located at an apical oxygen site acts as a stronger electron dopant on the copper layers and gradually brings the electronic structure close to that of La$_{2-x}$Sr$_x$CuO$_{4}$. These effects are very robust and only depend marginally on lattice distortions. Our results show that deoxygenation can reduce the effect of traditional La/Sr or La/Nd substitutions. Our study clearly identifies location of the dopant in the crystal structure as an important factor in doping of the cuprate planes.