Session Q2: CM.2 Phase Transitions: Superconductivity

Fe moments in the pressure-induced collapsed tetragonal phase of (Ca$_{0.67}$Sr$_{0.33})$Fe$_{2}$As$_{2}$

The tetragonal AEFe$_{2}$As$_{2}$ (AE$=$alkaline earth element) family of iron-based superconductors exhibits magnetic order at ambient pressure and low temperature. Under pressure, the magnetic order is suppressed, and an isostructural volume collapse is induced due to increased As-As bonding across the mirror plane of the structure. This collapsed tetragonal phase has been shown to support superconductivity under some conditions, and theoretical calculations suggest an unconventional origin. Theoretical calculations also reveal that enhanced As-As bonding and the magnitude of the Fe moments are correlated, suggesting that the Fe moments can be quenched in the collapsed tetragonal phase. Whether the Fe moments persist in the collapsed tetragonal phase has implications for the pairing mechanism of the observed, pressure-induced superconductivity in these compounds. We will present pressure- dependent x-ray emission spectroscopy (XES) measurements that probe the Fe moments through the volume collapse transition of (Ca$_{0.67}$Sr$_{0.33})$Fe$_{2}$As$_{2}$. These measurements will be compared with previously reported phase diagrams that include superconductivity. Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the US Department of Energy (DOE), National Nuclear Security Administration under Contract No. DE-AC52-07NA27344.   

Enhancement of Superconductivity of Beryllium at High Pressure

Among elements shows superconductivity at high pressure, some elements show the large enhancement of the transition temperature (Tc) at higher pressures. In the case of lithium, the Tc at ambient pressure is 0.4 mK which is the lowest observed value in whole elements, however, is enhanced by pressure up to near 20 K [1]. And calcium, which is on the same group II and not superconductive at ambient pressure, shows the highest Tc of elements at 29 K under pressure [2]. Then we focused on beryllium which is near to them on the periodic table. At ambient pressure, Tc of beryllium is 24 mK. We measured the electrical resistance at high pressure (P\textless 50 GPa) and low temperature (T\textgreater 100 mK) and found that the Tc rose up to few Kelvin at pressure above 20 GPa and reached up to 3.7 K at 30 GPa. In this pressure range the hcp crystal structure is stable at room temperature. We performed a powder X-ray diffraction measurement at room temperature and low temperature in BL10XU at SPring-8 and found a discontinuous change in c/a ratio at around 25 GPa.\\[4pt] [1] K. Shimizu, H. Ishikawa, D. Takao, T. Yagi and K. Amaya, Nature 419, 597-599 (2002).\\[0pt] [2] M. Sakata, Y. Nakamoto, and K. Shimizu, Phys. Rev. B 83, 220512 (2011).\\[0pt] [3] K. Nakano, Y. Akahama and H. Kawamura, J. Phys.: Condens. Matter 14, 10569--10573 (2002).   

Insulator-metal transitions and superconductivity in solids at high pressures

Under high pressure, simple molecular solids transform into non-molecular (extended) solids as compression energies approach the energies of strong covalent bonds in constituent chemical species. As a result, it is common to observe the transformation of molecular solids into more compact extended structures with more itinerant electrons, which softens repulsive interatomic interactions at high density. This allows exotic properties to be tuned, such as mechanical strength, nonlinear second harmonic optical properties, electric and optical conductivites, and magnetic properties of condensed-matter systems. Carbon dioxide, for example, exhibits a richness of high-pressure polymorphs with a great diversity in intermolecular interaction, chemical bonding, and crystal structures. Thus, group IV sulfides, in comparison with their chemical analog CO$_{\mathrm{2}}$, provide opportunities to exploit the relationship between the structural phase transition, electronic metallization, and superconductivity. We present integrated spectral, structural, resistance, and theoretical evidence for several systems of simple molecular group IV sulfides that undergo pressure-induced electronic phase transitions to novel metallic, magnetically ordered, and/or superconducting states.   

High-pressure studies for hydrogen-doped LaFeAsO$_{\mathrm{1-x}}$H$_{\mathrm{x}}$ and SmFeAsO$_{\mathrm{1-x}}$H$_{\mathrm{x}}$

Iron-based superconductor LaFeAsO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$ shows the conventional superconducting dome in an $x$-$T$ phase diagram with a maximum $T_{\mathrm{c}}$ of 26 K at $x=$0.1. However, the over-doped region has not been investigated, because of the poor solubility of fluorine above $x=$0.2. Recently, hydrogen was doped for LaFeAsO$_{\mathrm{1-x}}$H$_{\mathrm{x}}$ above $x=$0.5. It is interesting that LaFeAsO$_{\mathrm{1-x}}$H$_{\mathrm{x}}$ exhibits the second superconducting dome in the over-doped region (0.2 \textless $x$ \textless 0.5) with a maximum $T_{\mathrm{c}}$ of 36 K, in addition to the conventional dome. Since large enhancement of $T_{\mathrm{c}}$ under high pressure was reported for LaFeAsO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$, it is intriguing to study the superconducting properties in LaFeAsO$_{\mathrm{1-x}}$H$_{\mathrm{x\thinspace }}$ (x \textgreater 0.2) under high pressure. Marvelous results that $T_{\mathrm{c}}$ of $x=$ 0.2, which corresponds to the ravine between two domes, is enhanced largely from 18 K to 52 K with pressure of 6 GPa are obtained from resistivity measurements. These results are compared with the superconducting properties under high pressure of SmFeAsO$_{\mathrm{1-x}}$H$_{\mathrm{x}}$.   

Uniaxial pressure effect of Metal-Insulator Transition (T$_{\mathrm{MI}}$) in oriented Sm$_{0.55}$(Sr$_{0.5}$Ca$_{0.5}$)$_{0.45}$MnO$_3$

Perovskite type manganites $R_{1-x}A_{x}$\textit{MnO}$_{3}$ ($R$: rare earth ions, $A$: alkaline earth ions) exhibit various fundamental phenomena like colossal magnetoresistance (CMR), phase separation, and first-order ferromagnetic (FM) to paramagnetic (PM) phase transition etc. Similar to CMR, piezoresistance (PR), the change in electrical resistance in response to external pressure, can also be important parameter for various technological applications. Several studies shows that the order of phase transition can be changed either by applying external perturbations like magnetic field, pressure ($P)$, etc. or internally like doping etc. \textit{SSCMO} single crystal was grown using floating zone technique and the quality was carefully checked and aligned along the c axis as well as \textit{ab-}plane. We have investigated the effect of uniaxial pressure ($P)$ on electrical resistivity along the \textit{ab-}plane and $c-$axis in a single crystal of \textit{SSCMO}. A huge \textit{PR}$\sim $\textit{10}$^{7}$\textit{{\%}} at $P =$\textit{ 90 MPa} and a remarkable increase ($\sim $\textit{79 K/GPa}) of $T_{MI}$ have been observed with the application of $P$ \textbar \textbar $c-$axis, while $T_{MI}$ decreases at the rate of $\sim $\textit{77 K/GPa} for $P \quad \bot $c axis. These values of \textit{PR} and \textit{dT}$_{MI}$ \textit{/dP} are much larger than those observed in other perovskite and bilayer manganites. Hence, these materials may be used for various technological applications.