FeSe$_{1-x}$Te$_{x}$ superconductors manifest some intriguing electronic properties depending on the value of $x$. In FeSe single crystal, the nematic phase and Dirac band structure have been observed, while topological surface superconductivity with the Majorana bound state was found in the crystal of $x \sim 0.55$. Therefore, the electronic properties of single crystals with $0 < x \leq 0.5$ are crucial for probing the evolution of those intriguing properties as well as their relations. However, this study is still left blank due to the lack of single crystals because of phase separation. Here, we report the synthesis, magnetization, electronic transport properties, and hydrostatic pressure effect of FeSe$_{0.67}$Te$_{0.33}$ single crystals free of phase separation. A structural (nematic) transition is visible at $T_{s} = 39$ K, below which the resistivity exhibits a Fermi-liquid behavior. Analysis of upper critical fields suggests that spin-paramagnetic effect should be taken into account for both $H \parallel c$ axis and $H \parallel ab$ plane. A crossover from the low-$H$ quadratic to the high-$H$ quasi-linear behavior is observed in the magnetoresistance, signifying the possible existence of Dirac-cone state. Besides, the strong temperature dependence of Hall coefficient, violation of (modified) Kohler's rule, and two-band model analysis indicate the multiband effects in FeSe$_{0.67}$Te$_{0.33}$ single crystals. Hydrostatic pressure measurements reveal that $T_{s}$ is quickly suppressed with pressure while $T_{c}$ is monotonically increased up to 2.31 GPa, indicating the competition between nematicity and superconductivity. No signature of magnetic order that has been detected in FeSe$_{1-x}$S$_{x}$ is observed. Our findings fill up the blank of the knowledge on the basic properties of FeSe$_{1-x}$Te$_{x}$ system with low-Te concentrations.
Superconductivity in FeSe was investigated under high pressure through the measurements of DC magnetization by using a diamond anvil cell. We successfully observed that the disappearance of the superconductivity as a results of the appearance of the non-superconducting ortho I\hspace{-1pt}I phase above $\sim$7 GPa ($\sim$5 GPa), when Ar (glycerin) is used as the pressure transmitting media. Contrary to this, it has been found that the superconductivity even survives under pressure above 7 GPa, when the thickness ($t$) of a platelet-single crystal specimen is reduced. The survival of the superc…
Suppression of phase separation and giant enhancement of superconducting transition temperature in FeSe$_{1-x}$Te$_{x}$ thin films
我们证明了成功的制造与$0 \le x \le 1$的FeSe $_{1-x}$ Te $_{x}$薄膜的CaF $_2$衬底上，包括区域的$0.1 \le x \le 0.4$，这是众所周知的“相分离区”，通过脉冲激光沉积，这是一种非平衡的方法。在由此产生的薄膜中，我们观察到一个巨大的增强的超导转变温度，$T_\mathrm{c}$，在该区域的$0.1 \le x \le 0.4$：最大值达到23 K，这是大约1.5倍大的散装样品的FeSe $_{1-x}$ Te $_{x}$报告的值。我们提出了一个完整的相图FeSe $_{1-x}$ Te $_{x}$薄膜。令人惊讶的是，在$0.1<x<0.2$处观察到$T_\mathrm{c}$的突然抑制，而对于$0.2 \le x < 1$，$T_\mathrm{c}$随着$x$的降低而增加。也就是说，在$x=0-0.1$和$x \ge 0.2$中实现的超导性之间存在明显的差异。为了获得具有高$T_\mathrm{c}$的FeSe $_{1-x}$ Te $_{x}$薄膜，发现Te含量$x$和面内晶格应变的控制是关键因素。
Our study of FeSe$ _ {1-x}$Te$ _ {x}$ crystals with x $<$ 0.15 shows that the phase separation in these compositions occurs into phases with a different stoichiometry of iron. This phase separation may indicate structural instability of the iron plane in the studied range of compositions. To explain it, we discuss the bond polarity and the peculiarity of the direct $d$ exchange in the iron plane in the framework of the basic phenomenological description such as the Bethe-Slater curve. With this approach, when the distance between iron atoms is close to the value at which the sign of the magnet…
We compute the compressibility of the conduction electrons in both bulk orthorhombic FeSe and monolayer FeSe on SrTiO$_3$ substrate, including dynamical electronic correlations within slave-spin mean-field + density-functional theory. Results show a zone of enhancement of the electronic compressibility crossing the interaction-doping phase diagram of these compounds in accord with previous simulations on iron pnictides and in general with the phenomenology of Hund's metals. Interestingly at ambient pressure FeSe is found slightly away from the zone with enhanced compressibility but moved right…
Muon spin rotation/relaxation measurements were performed to investigate magnetic properties of FeSe$_{1-x}$S$_x$ thin films on LaAlO$_3$. A drastic decrease of the initial asymmetry was observed together with the peak structure in the temperature dependence of the relaxation rate of muon spins almost at the same temperature where kink anomalies were observed in the temperature dependent resistivity. With increasing S content, the anomaly temperature increased and the magnetic fluctuations at the lowest temperature were suppressed. These results show that the S substitution induces magnetism a…
The influence of spin-phonon coupling on antiferromagnetic spin fluctuations in FeSe under pressure: the First-principles calculations with van der Waals corrections
We report on the structural phase transitions in the S doped FeSe superconductor by powder synchrotron X ray diffraction at high pressures up to 18.5 GPa under compression and decompression modes. In order to create high quasi hydrostatic pressures, diamond anvil cells filled with helium as a pressure transmitting medium were used. It was found that at ambient pressure and room temperature, S doped FeSe has a tetragonal structure. Under compression, in the region of 10 GPa, a phase transition from the tetragonal into the orthorhombic structure is observed, which persists up to 18.5 GPa. Our re…
We report the evolution of the electronic nematic susceptibility in FeSe via Raman scattering as a function of hydrostatic pressure up to 5.8 GPa where the superconducting transition temperature $T_{c}$ reaches its maximum. The critical nematic fluctuations observed at low pressure vanish above 1.6 GPa, indicating they play a marginal role in the four-fold enhancement of $T_{c}$ at higher pressures. The collapse of nematic fluctuations appears to be linked to a suppression of low energy electronic excitations which manifests itself by optical phonon anomalies at around 2 GPa, in agreement with…
Dome of magnetic order inside the nematic phase of sulfur-substituted FeSe under pressure
研究了硫取代Fe（Se $_{1-x}$ S $_{x}$）的结构、磁性和超导转变以及超导上临界场的压力依赖性。电阻测量进行单晶体上的三个取代水平（$x$ =0.043，0.096，0.12）下的静水压力高达1.8 GPa和磁场高达9 T，并与纯FeSe的数据进行比较。我们的结果说明了化学和物理压力对Fe（Se $_{1-x}$ S $_{x}$）的影响。随着硫含量的增加，低压区的磁有序被强烈抑制到相图中的一个小的圆顶状区域。然而，$T_s$是少得多的抑制硫取代和$T_c$的Fe（Se $_{1-x}$ S $_{x}$）表现出类似的非单调压力依赖性与局部最大值和局部最小值存在于低压范围内的所有$x$。$T_c$中的局部最大值与$T_c$上方磁序的出现相一致。在这个压力下，上临界场的斜率突然减小。$T_c$的最小值与由$T_c$归一化的上临界场斜率的宽最大值相关。
Iron-based superconductors are well-known for their intriguing phase diagrams, which manifest a complex interplay of electronic, magnetic and structural degrees of freedom. Among the phase transitions observed are superconducting, magnetic, and several types of structural transitions, including a tetragonal-to-orthorhombic and a collapsed-tetragonal transition. In particular, the widely-observed tetragonal-to-orthorhombic transition is believed to be a result of an electronic order that is coupled to the crystalline lattice and is, thus, referred to as nematic transition. Nematicity is therefo…
We investigate the high-pressure phase of the iron-based superconductor FeSe$_{0.89}$S$_{0.11}$ using transport and tunnel diode oscillator studies. We construct detailed pressure-temperature phase diagrams that indicate that outside of the nematic phase, the superconducting critical temperature reaches a minimum before it is quickly enhanced towards 40 K above 4 GPa. The resistivity data reveal signatures of a fan-like structure of non-Fermi liquid behaviour which could indicate the existence of a putative quantum critical point buried underneath the superconducting dome around 4.3 GPa. Furth…
In the vicinity of a quantum critical point, quenched disorder can lead to a quantum Griffiths phase, accompanied by an exotic power-law scaling with a continuously varying dynamical exponent that diverges in the zero-temperature limit. Here, we investigate a nematic quantum critical point in the iron-based superconductor FeSe$_{0.89}$S$_{0.11}$ using applied hydrostatic pressure. We report an unusual crossing of the magnetoresistivity isotherms in the non-superconducting normal state which features a continuously varying dynamical exponent over a large temperature range. We interpret our resu…
A notable characteristic of PbTaSe$_2$, a prototypical noncentrosymmetric (NCS) superconductor, is that its superconductivity can be modulated through a structural transition under hydrostatic pressure [Phys. Rev. B 95, 224508 (2017)]. Here we report on simultaneous pressure-sensitive point-contact Andreev reflection (PCAR) spectroscopy and bulk resistance measurements on PbTaSe$_2$, to elucidate the nature of the surface and bulk superconductivity and their evolution with hydrostatic pressure. It is found that in high pressure region the superconducting gap opening temperature $T_c^A$ is sign…
Magnetic phase separation and superfluid density measurements in alkali metal-organic solvent intercalated iron selenide superconductor Lix(C5H5N)yFe2-zSe2
Separation of antiferromagnetism and high-temperature superconductivity in Ca$_{1-x}$La$_x$Fe$_2$As$_2$ under pressure
本文报道了外加压力对异价La掺杂铁磷属元素化物材料Ca $_{1-x}$ La $_{x}$ Fe $_{2}$ As $_{2}$单晶中磁序和超导序的影响。使用电输运，弹性中子散射和共振隧道二极管振荡器测量样品在两个准流体静力学和流体静力学压力条件下，我们报告了一系列的相图跨越范围的替代浓度为反铁磁和超导基态，包括压力调谐通过反铁磁（AFM）量子临界点。我们的研究结果表明，所观察到的超导相与$T_{c}$ =47 K的最大转变温度是固有的这些材料，只有出现在通过AFM临界点的压力调谐抑制磁序。与具有ThCr $_2$ Si $_2$结构的所有其他金属间铁-磷属元素化物超导体相比，该超导相似乎仅以与迄今报道的具有最高转变温度的氧-和氟基铁-磷属元素化物超导体类似的方式从反铁磁相存在。低- $T_{c}$系统与共存的超导性和磁性之间的不寻常的二分法和最高- $T_{c}$系统的趋势，以显示非共存提供了一个重要的洞察不同的转变温度限制在铁基超导体家族的不同成员。
Phase separation at the magnetic-superconducting transition in La$_{0.7}$Y$_{0.3}$FeAsO$_{1-x}$F$_{x}$
本文报道了La $_{0.7}$ Y $_{0.3}$ FeAsO $_{1-x}$ F $_{x}$类材料的$μ^{+}$ SR和{} $^{19}$ F-NMR研究。在这里，抗磁性La $_{1-y}$ Y $_{y}$取代增加了化学压力，因此大大提高了最佳超导转变温度。我们研究了保持Y含量恒定（$y = 0.3$）和在$0.025 \leq x \leq 0.15$范围内改变F含量的磁-超导相变。我们的研究结果表明如何磁性和超导性共存的$x = 0.065$。这种共存是由于在宏观区域中的两相的偏析，类似于在所施加的流体静压下在LaFeAsO $_{1-x}$ F $_{x}$材料中观察到的。这种情况是定性不同的纳米尺度共存时，观察到的两个序参数的La是完全取代磁性稀土离子，如Sm或Ce。
The nematic electronic state and its associated nematic critical fluctuations have emerged as potential candidates for superconducting pairing in various unconventional superconductors. However, in most materials their coexistence with other magnetically-ordered phases poses significant challenges in establishing their importance. Here, by combining chemical and hydrostatic physical pressure in FeSe$_{0.89}$S$_{0.11}$, we provide a unique access to a clean nematic quantum phase transition in the absence of a long-range magnetic order. We find that in the proximity of the nematic phase transiti…
Linear laws of volume elasticity in the properties and structural phase transitions: physical process of the parameter effect (TPH) in magnet-structures
