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Superconducting phases of (NH3)yMxFeSe1-zTez have been synthesized by the intercalation of metal atoms (M: Li, Na, Ca) into FeSe and FeSe0.5Te0.5 using a low-temperature liquid NH3 technique. The superconducting transition temperature (Tc) is 31.5 K for Na-doped FeSe, and for Li-, Na-, and Ca-doped FeSe0.5Te0.5 it is 26, 22, and 17 K, respectively. The 31.5 K superconducting is the superconducting phase in ammoniated Na-doped FeSe. The Tc is lower than that (onset Tc = 46 K) of the superconducting phase reported previously. The reason why the Tc of this phase is lower is discussed based on the structure. The pressure dependences of Tc in the (NH3)yNa0.5FeSe and (NH3)yNa0.4FeSe0.5Te0.5 samples have been measured and a negative pressure dependence is observed; i.e., a decrease in lattice constant c leads to a decrease in Tc, consistent with the behavior of (NH3)yCs0.4FeSe reported previously by our group. Furthermore, the magnetic behavior of (NH3)yNa0.4FeSe0.5Te0.5 has been fully investigated at different applied magnetic fields (H) to determine the critical magnetic field. This is a successful metal intercalation into FeSe1-zTez (z≠0) and an observation of superconductivity. [ABSTRACT FROM AUTHOR]

Alkali-metal-intercalated FeSe materials, (NH3)yCs0.4FeSe (M: K, Rb, and Cs), have been synthesized using the liquid NH3 technique. (NH3)yCs0.4FeSe shows a superconducting transition temperature (Tc) as high as 31.2 K, which is higher by 3.8 K than the Tc of nonammoniated Cs0.4FeSe. The Tcs of (NH3)yCs0.4FeSe and (NH3)yCs0.4FeSe are almost the same as those of nonammoniated K0.4FeSe and Rb0.4FeSe. The Tc of (NH3)yCs0.4FeSe shows a negative pressure dependence. A clear correlation between Tc and lattice constant c is found for ammoniated metal-intercalated FeSe materials, suggesting a correlation between Fermi-surface nesting and superconductivity. [ABSTRACT FROM AUTHOR]

We have studied new superconductors prepared by metal doping of two-dimensional (2D) layered materials, FeSe and FeSe0.5Te0.5, using ethylenediamine (EDA). The superconducting transition temperatures (Tcs) of metal-doped FeSe and metal-doped FeSe0.5Te0.5, i.e., (EDA)yMxFeSe and (EDA)yMxFeSe0.5Te0.5 (M: Li, Na, and K), were 31-45 K and 19-25 K, respectively. The stoichiometry of each sample was clarified by energy dispersive x-ray (EDX) spectroscopy, and the x-ray powder diffraction pattern indicated a large expansion of lattice constant c, indicating the cointercalation of metal atoms and EDA. The pressure dependence of superconductivity in (EDA)yNaxFeSe0.5Te0.5 has been investigated at a pressure of 0-0.8GPa, showing negative pressure dependence in the same manner as (NH3)yNaxFeSe0.5Te0.5 The Tc-c phase diagrams of MxFeSe and MxFeSe0.5Te0.5 were drawn afresh from the Tc and c of (EDA)yMxFeSe and (EDA)yMxFeSe0.5Te0.5, showing that the Tc increases with increasing c but that extreme expansion of c reverses the Tc trend. [ABSTRACT FROM AUTHOR]

Field-effect transistors (FETs) were fabricated using exfoliated single crystals of Mo(Se1-xTex)2 with an x range of 0 to 1, and the transistor properties fully investigated at 295 K in four-terminal measurement mode. The chemical composition and crystal structure of exfoliated single crystals were identified by energy-dispersive x-ray spectroscopy (EDX), single-crystal x-ray diffraction, and Raman scattering, suggesting the 2H- structure in all Mo(Se1-xTex)2. The lattice constants of a and c increase monotonically with increasing x, indicating the substitution of Se by Te. When x<0.4 in a FET with a thin single crystal of Mo(Se1-xTex)2, n-channel FET properties were observed, changing to p-channel or ambipolar operation for x>0.4. In contrast, the polarity of a thick single-crystal Mo(Se1-xTex)2 FET did not change despite an increase in x. The change of polarity in a thin single-crystal FET was well explained by the variation of electronic structure. The absence of such change in the thick single-crystal FET can be reasonably interpreted based on the large bulk conduction due to naturally accumulated electrons. The μ value in the thin single-crystal FET showed a parabolic variation, with a minimum μ at around x=0.4, which probably originates from the disorder of the single crystal caused by the partial replacement of Se by Te, i.e., a disorder that may be due to ionic size difference of Se and Te. [ABSTRACT FROM AUTHOR]

Na-intercalated FeSe0.5Te0.5 was prepared using the liquid NH3 technique, and a superconducting phase exhibiting a superconducting transition temperature (Tc) as high as 27 K was discovered. This can be called the high-Tc phase since a 21 K superconducting phase was previously obtained in (NH3)yNaxFeSe0.5Te0.5. The chemical composition of the high-Tc phase was determined to be (NH3)0.61(4)Na0.63(5)Fe0.85Se0.55(3)Te0.44(2). The x-ray diffraction patterns of both phases show that a larger lattice constant c (i.e., FeSe0.5Te0.5 plane spacing) produces a higher Tc. This behavior is the same as that of metal-doped FeSe, suggesting that improved Fermi-surface nesting produces the higher Tc. The high-Tc phase converted to the low-Tc phase within several days, indicating that it is a metastable phase. The temperature dependence of resistance for both phases was recorded at different magnetic fields, and the critical fields were determined for both phases. Finally, the Tc versus c phase diagram was prepared for the metal-doped FeSe0.5Te0.5, which is similar to that of metal-doped FeSe, although the Tc is lower. [ABSTRACT FROM AUTHOR]

The superconducting transition temperature Tc of ammoniated metal-doped FeSe (NH3)yMxFeSe (M: metal atom) has been scaled with the FeSe plane spacing, and it has been suggested that the FeSe plane spacing depends on the location of metal atoms in (NH3)yMxFeSe crystals. Although the crystal structure of (NH3)yLixFeSe exhibiting a high Tc (∼44 K) was determined from neutron diffraction, the structure of (NH3)yMxFeSe exhibiting a low Tc (∼32 K) has not been determined thus far. Here, we determined the crystal structure of (NH3)yCs0.4FeSe(Tc=33K) through the Rietveld refinement of the x-ray diffraction (XRD) pattern measured with synchrotron radiation at 30 K. The XRD pattern was analyzed based on two different models, on-center and off-center, under a space group of I4/mmm. In the on-center structure, the Cs occupies the 2a site and the N of NH3 may occupy either the 4c or 2b site, or both. In the off-center structure, the Cs may occupy either the 4c or 2b site, or both, while the N occupies the 2a site. Only an on-center structure model in which the Cs occupies the 2a and the N of NH3 occupies the 4c site provided reasonable results in the Rietveld analysis. Consequently, we concluded that (NH3)yCs0.4FeSe can be assigned to the on-center structure, which produces a smaller FeSe plane spacing leading to the lower Tc. [ABSTRACT FROM AUTHOR]