High pressure study of BaFe2As2--the role of hydrostaticity and uniaxial stress

Structural and Electronic Phase Transitions in Three Stable Tin-Sulfur Metallic Chalcogenides under High Pressure

As a representative homologous series, tin-bearing metallic chalcogenides (SnxSy) have sparked considerable attention because of their stoichiometric compositions and structural diversities. In this work, three stable compounds, SnS, Sn2S3, and SnS2, were screened from SnxSy and a comprehensive investigation on their structural and electrical transport properties was performed up to 60.1 GPa using a diamond anvil cell (DAC) under different hydrostatic environments. Upon nonhydrostatic compression, SnS underwent the Pnma-to-Cmcm transition accompanied by metallization at 7.6 GPa, followed by the Cmcm-to-Pm3̅m transformation at 17.8 GPa. For SnS2, the pressure-induced metallization and isostructural phase transition (IPT) occurred successively at 31.2 and 46.6 GPa, respectively. As an intermediate composition, Sn2S3 first experienced an IPT at 10.8 GPa, and then, the Pnma-to-Cmcm transition concomitantly with metallization occurred at 16.9 GPa, analogous to the high-pressure structural transformation routes of SnS and SnS2. The 0.6-5.4 GPa pressure hysteresis was detectable for the phase transitions of SnxSy under quasi-hydrostatic and hydrostatic conditions owing to the influence of deviatoric stress. In comprehensive consideration of our high-pressure Raman scattering and electrical conductivity results, the systematic construction of a pressure-phase state diagram on SnxSy not only unveils its composition-structure-property relation but also advances the in-depth exploration for other IVA-VIA metallic chalcogenides.

Pressure induced superconductivity in a CeRhSi3single crystal-the high pressure study

Pressure induced superconductivity in non-centrosymmetric CeRhSi₃and CeIrSi₃compounds has attracted significant attention of the scientific community since its discovery 15 years ago. Up-to-date, all reported experimental results were obtained employing the hybrid-cylinder piston pressure cells with a maximum reachable pressure of 3 GPa. Present study focuses on the superconducting state at higher, so far unreported, pressures using the Bridgman anvil cell and a CeRhSi₃single crystal synthesized by the Sn-true-flux method. The initial increase of superconducting critical temperature from 0.4 K at 1.1 GPa to 1.1 K at 2.4 GPa is followed by a gradual suppression of superconducting state upon increasing the pressure above 3.0 GPa, forming a typical dome. The pressure induced superconductivity is expected to be completely suppressed in the pressure region between 4.5 and 5.0 GPa. Temperature dependence of electrical resistivity in constant magnetic fields and high pressures, as well as the magnetoresistance measurements, reveal a large critical field, exceeding 19 T at 0.6 K and 2.4 GPa, sharply decreasing receding the superconductivity dome. The previously reportedT-pandH-Tphase diagrams are completed by our high-pressure data and discussed in the frame of previous results.

Imaging Anomalous Nematic Order and Strain in Optimally Doped BaFe_{2}(As,P)_{2}

We present the strain and temperature dependence of an anomalous nematic phase in optimally doped BaFe_{2}(As,P)_{2}. Polarized ultrafast optical measurements reveal broken fourfold rotational symmetry in a temperature range above T_{c} in which bulk probes do not detect a phase transition. Using ultrafast microscopy, we find that the magnitude and sign of this nematicity vary on a 50-100  μm length scale, and the temperature at which it onsets ranges from 40 K near a domain boundary to 60 K deep within a domain. Scanning Laue microdiffraction maps of local strain at room temperature indicate that the nematic order appears most strongly in regions of weak, isotropic strain. These results indicate that nematic order arises in a genuine phase transition rather than by enhancement of local anisotropy by a strong nematic susceptibility. We interpret our results in the context of a proposed surface nematic phase.

Hydrostatic pressure effects on the static magnetism in Eu(Fe0.925Co0.075)2As2

EuFe₂As₂-based iron pnictides are quite interesting compounds, due to the two magnetic sublattices in them and the tunability to superconductors by chemical doping or application of external pressure. The effects of hydrostatic pressure on the static magnetism in Eu(Fe_0.925Co_0.075)₂As₂ are investigated by complementary electrical resistivity, ac magnetic susceptibility and single-crystal neutron diffraction measurements. A specific pressure-temperature (P-T) phase diagram of Eu(Fe_0.925Co_0.075)₂As₂ is established. The structural phase transition, as well as the spin-density-wave order of Fe sublattice, is suppressed gradually with increasing pressure and disappears completely above 2.0 GPa. In contrast, the magnetic order of Eu sublattice persists over the whole investigated pressure range up to 14 GPa, yet displaying a non-monotonic variation with pressure. With the increase of the hydrostatic pressure, the magnetic state of Eu evolves from the canted antiferromagnetic structure in the ground state, via a pure ferromagnetic structure under the intermediate pressure, finally to an "unconfirmed" antiferromagnetic structure under the high pressure. The strong ferromagnetism of Eu coexists with the pressure-induced superconductivity around 2 GPa. Comparisons between the P-T phase diagrams of Eu(Fe_0.925Co_0.075)₂As₂ and the parent compound EuFe₂As₂ were also made.

Strain induced superconductivity in the parent compound BaFe2As2

The discovery of superconductivity with a transition temperature, Tc, up to 65 K in single-layer FeSe (bulk Tc=8 K) films grown on SrTiO3 substrates has attracted special attention to Fe-based thin films. The high Tc is a consequence of the combined effect of electron transfer from the oxygen-vacant substrate to the FeSe thin film and lattice tensile strain. Here we demonstrate the realization of superconductivity in the parent compound BaFe2As2 (no bulk Tc) just by tensile lattice strain without charge doping. We investigate the interplay between strain and superconductivity in epitaxial BaFe2As2 thin films on Fe-buffered MgAl2O4 single crystalline substrates. The strong interfacial bonding between Fe and the FeAs sublattice increases the Fe-Fe distance due to the lattice misfit, which leads to a suppression of the antiferromagnetic spin density wave and induces superconductivity with bulk Tc≈10 K. These results highlight the role of structural changes in controlling the phase diagram of Fe-based superconductors.

Magnetic and structural transitions of SrFe2As2 at high pressure and low temperature

One of key issues in studying iron based superconductors is to understand how the magnetic phase of the parent compounds evolves. Here we report the systematic investigation of paramagnetic to antiferromagnetic and tetragonal to orthorhombic structural transitions of “122” SrFe₂As₂ parent compound using combined high resolution synchrotron Mössbauer spectroscopy and x-ray diffraction techniques in a cryogenically cooled high pressure diamond anvil cell. It is found that although the two transitions are coupled at 205 K at ambient pressure, they are concurrently suppressed to much lower temperatures near a quantum critical pressure of approximately 4.8 GPa where the antiferromagnetic state transforms into bulk superconducting state. Our results indicate that the lattice distortions and magnetism jointly play a critical role in inducing superconductivity in iron based compounds.

Pressure-decoupled magnetic and structural transitions of the parent compound of iron-based 122 superconductors BaFe2As2

The recent discovery of iron ferropnictide superconductors has received intensive concern in connection with magnetically involved superconductors. Prominent features of ferropnictide superconductors are becoming apparent: the parent compounds exhibit an antiferromagnetic ordered spin density wave (SDW) state, the magnetic-phase transition is always accompanied by a crystal structural transition, and superconductivity can be induced by suppressing the SDW phase via either chemical doping or applied external pressure to the parent state. These features generated considerable interest in the interplay between magnetism and structure in chemically doped samples, showing crystal structure transitions always precede or coincide with magnetic transition. Pressure-tuned transition, on the other hand, would be more straightforward to superconducting mechanism studies because there are no disorder effects caused by chemical doping; however, remarkably little is known about the interplay in the parent compounds under controlled pressure due to the experimental challenge of in situ measuring both of magnetic and crystal structure evolution at high pressure and low temperatures. Here we show from combined synchrotron Mössbauer and X-ray diffraction at high pressures that the magnetic ordering surprisingly precedes the structural transition at high pressures in the parent compound BaFe2As2, in sharp contrast to the chemical-doping case. The results can be well understood in terms of the spin fluctuations in the emerging nematic phase before the long-range magnetic order that sheds light on understanding how the parent compound evolves from a SDW state to a superconducting phase, a key scientific inquiry of iron-based superconductors.

Pressure-induced frustration in charge ordered spinel AlV₂O₄

AlV2O4 is the only spinel compound so far known that exists in the charge ordered state at room temperature. It is known to transform to a charge frustrated cubic spinel structure above 427 ° C. The presence of multivalent V ions in the pyrochlore lattice of the cubic spinel phase brings about the charge frustration that is relieved in the room temperature rhombohedral phase by the clustering of vanadium into a heptamer molecular unit along with a lone V atom. The present work is the first demonstration of pressure-induced frustration in the charge ordered state of AlV2O4. Synchrotron powder x-ray diffraction studies carried out at room temperature on AlV2O4 subjected to high pressure in a diamond anvil cell show that the charge ordered rhombohedral phase becomes unstable under the application of pressure and transforms to the frustrated cubic spinel structure. The frustration is found to be present even after pressure recovery. The possible role of pressure on vanadium t2g orbitals in understanding these observations is discussed.

Pressure-induced superconductivity in Ba0.5Sr0.5Fe2As2

High-pressure electrical resistance measurements have been performed on single crystal Ba(0.5)Sr(0.5)Fe(2)As(2) platelets to pressures of 16 GPa and temperatures down to 10 K using designer diamond anvils under quasi-hydrostatic conditions with an insulating steatite pressure medium. The resistance measurements show evidence of pressure-induced superconductivity with an onset transition temperature at ∼31 K and zero resistance at ∼22 K for a pressure of 3.3 GPa. The transition temperature decreases gradually with increasing pressure before completely disappearing for pressures above 12 GPa. The present results provide experimental evidence that a solid solution of two 122-type materials, i.e., Ba(1-x)Sr(x)Fe(2)As(2) (0 < x < 1), can also exhibit superconductivity under high pressure.

Co-substitution effects on the Fe valence in the BaFe2As2 superconducting compound: a study of hard x-ray absorption spectroscopy

The Fe K x-ray absorption near edge structure of BaFe(2-x)Co(x)As(2) superconductors was investigated. No appreciable alteration in shape or energy position of this edge was observed with Co substitution. This result provides experimental support to previous ab initio calculations in which the extra Co electron is concentrated at the substitute site and do not change the electronic occupation of the Fe ions. Superconductivity may emerge due to bonding modifications induced by the substitute atom that weakens the spin-density-wave ground state by reducing the Fe local moments and/or increasing the elastic energy penalty of the accompanying orthorhombic distortion.

Structural phase transitions in EuFe2As2 superconductor at low temperatures and high pressures

The crystal structure of EuFe(2)As(2) has been studied up to a pressure of 35 GPa and down to a temperature of 8 K using temperature dependent x-ray diffraction in a diamond anvil cell at a synchrotron source. At 4.3 GPa, we have detected a structural phase transition from a high temperature tetragonal phase with I4/mmm space group to a low temperature orthorhombic phase with Fmmm space group around 120 K. With the application of pressure at a low temperature of 10 K, the orthorhombic phase is suppressed and a phase change to a collapsed tetragonal phase with I4/mmm space group is observed at 11 GPa. This collapsed tetragonal phase is similar to the one observed at ambient temperature and pressure above 8.5 GPa. We have shown that the collapsed tetragonal phase of EuFe(2)As(2) has the same pressure-volume (P-V) equation of state at ambient temperature and at 10 K, implying that the high pressure phase of EuFe(2)As(2) has a negligible thermal expansion coefficient.

Phase transition and superconductivity of SrFe(2)As(2) under high pressure

High pressure x-ray diffraction and electrical resistance measurements have been carried out on SrFe(2)As(2) to a pressure of 23 GPa and temperature of 10 K using a synchrotron source and designer diamond anvils. At ambient temperature, a phase transition from the tetragonal phase to a collapsed tetragonal (CT) phase is observed at 10 GPa under non-hydrostatic conditions. The experimental relation that T-CT transition pressure for 122 Fe-based superconductors is dependent on ambient pressure volume is affirmed. The superconducting transition temperature is observed at 32 K at 1.3 GPa and decreases rapidly with a further increase of pressure in the region where the T-CT transition occurs. Our results suggest that T(C) falls below 10 K in the pressure range of 10-18 GPa where the CT phase is expected to be stable.

Patterned anvils for high pressure measurements at low temperature

Multiprobe high pressure measurements require electrical leads in the sample chamber. Compared to conventional wire-based techniques, metallic tracks patterned onto the anvil surface improve reliability and ease of use, and enable novel and more demanding measurements under high pressure. We have developed new anvil designs based on sputter-deposited tracks on alumina and moissanite anvils. These anvils allow convenient and reliable measurements of electrical transport properties or of the magnetic susceptibility under hydrostatic conditions, as demonstrated by test measurements on Pb and Ca(3)Ru(2)O(7).