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Register a new userStructural transition and superconductivity in hydrothermally synthesized FeX (X = S, Se)
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Tetragonal beta-FeSe obtained by hydrothermal reaction is not superconducting and transforms to a triclinic structure at 60 K unlike superconducting FeSe from solid state synthesis, which becomes orthorhombic at 90 K. In contrast, tetragonal iron sulphide FeS from hydrothermal synthesis is superconducting at 4.8 K but undergoes no structural transition. Our results suggest that the absence of superconductivity in hydrothermally synthesized FeSe may be associated to the low-temperature structure with zigzag chains of iron atoms, which is different from the known orthorhombic Cmme structure of superconducting FeSe.
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Superconducting [(Li(1-x)Fex)OH](Fe(1-y)Liy)Se (x ~ 0.2, y ~ 0.08) was synthesized by hydrothermal methods and structurally characterized by single crystal X-ray diffraction. The crystal structure contains anti-PbO type (Fe(1-y)Liy)Se layers separated by layers of (Li(1-x)Fex)OH. Electrical resistivity and magnetic susceptibility measurements reveal superconductivity at 43 K. An anomaly in the diamagnetic shielding indicates ferromagnetic ordering near 10 K while superconductivity is retained. The ferromagnetism emerges from the iron atoms in the (Li(1-x)Fex)OH layer. Isothermal magnetization measurements confirm the superposition of ferromagnetic with superconducting hysteresis. The internal ferromagnetic field is larger than the lower, but smaller than the upper critical field of the superconductor, which gives evidence for a spontaneous vortex phase where both orders coexist. 57Fe-Mossbauer spectra, 7Li-NMR spectra, and muSR experiments consistently support this rare situation, especially in a bulk material where magnetism emerges from a 3d-element.
After our first discovery of superconductivity (SC) with $T_C$=3.7 K in TlNi$_2$Se$_2$, we grew successfully a series of TlNi$_2$Se$_{2-x}$S$_x$ (0.0 $leq$ x $leq$2.0) single crystals. The measurements of resistivity, susceptibility and specific heat were carried out. We found that SC with $T_C$=2.3 K also emerges in TlNi$_2$S$_2$ crystal, which appears to involve heavy electrons with an effective mass $m^*$=13$sim$25 $m_b$, as inferred from the normal state electronic specific heat and the upper critical field, $H_{C2}(T)$. It was found that the $T_C$ and superconducting volume fraction in TlNi$_2$Se$_{2-x}$S$_x$ crystals changes with the disorder degree induced by the partial substitution of S for Se, which is characterized by the residual resistivity ratio (textit{RRR}). The effect of the disorder on SC may provide some information for understanding the mechanism of SC in this new Ni-chalcogenide system.
High temperature superconductivity is often found in the vicinity of antiferromagnetism. This is also true in LaFeAsO$_{1-x}$F$_{x}$ ($x leq$ 0.2) and many other iron-based superconductors, which leads to proposals that superconductivity is mediated by fluctuations associated with the nearby magnetism. Here we report the discovery of a new superconductivity dome without low-energy magnetic fluctuations in LaFeAsO$_{1-x}$F$_{x}$ with 0.25$leq x leq$0.75, where the maximal critical temperature $T_c$ at $x_{opt}$ = 0.5$sim$0.55 is even higher than that at $x leq$ 0.2. By nuclear magnetic resonance and Transmission Electron Microscopy, we show that a C4 rotation symmetry-breaking structural transition takes place for $x>$ 0.5 above $T_c$. Our results point to a new paradigm of high temperature superconductivity.
Charge density wave (CDW), the periodic modulation of the electronic charge density, will open a gap on the Fermi surface that commonly leads to decreased or vanishing conductivity. On the other hand superconductivity, a commonly believed competing order, features a Fermi surface gap that results in infinite conductivity. Here we report that superconductivity emerges upon Se doping in CDW conductor ZrTe$_{3}$ when the long range CDW order is gradually suppressed. Superconducting critical temperature $T_c(x)$ in ZrTe$_{3-x}$Se$_x$ (${0leq}xleq0.1$) increases up to 4 K plateau for $0.04$$leq$$x$$leq$$0.07$. Further increase in Se content results in diminishing $T_{c}$ and filametary superconductivity. The CDW modes from Raman spectra are observed in $x$ = 0.04 and 0.1 crystals, where signature of ZrTe$_{3}$ CDW order in resistivity vanishes. The electronic-scattering for high $T_{c}$ crystals is dominated by local CDW fluctuations at high temperures, the resistivity is linear up to highest measured $T=300K$ and contributes to substantial in-plane anisotropy.
The discovery of intrinsic magnetism in atomically thin two-dimensional transition-metal trichalcogenides has attracted intense research interest due to the exotic properties of magnetism and potential applications in devices. Pressure has proven to be an effective tool to manipulate the crystal and electronic structures of the materials. Here, we report investigations on ferromagnetic van der Waals Cr2Si2Te6 via high-pressure synchrotron x-ray diffraction, electrical resistance, Hall resistance, and magnetoresistance measurements. Under compression, Cr2Si2Te6 simultaneously undergoes a structural transition, emergence of superconductivity at 3 K, sign change of the magnetoresistance, and dramatic change of the Hall coefficient at ~8 GPa. The superconductivity persists up to the highest measured pressure of 47.1 GPa with a maximum Tc = 4.5 K at ~30 GPa. The discovery of superconductivity in the two-dimensional van der Waals ferromagnetic Cr-based Cr2Si2Te6 provides new perspectives to explore superconductivity and the interplay between superconductivity and magnetism.
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