non-SC Samples Research Articles

Activation conductivity and superconducting state in solid solutions (PbzSn1-z)0.8In0.2Te.

We have studied temperature (1.5 K < T < 300 K) and magnetic field H < 5 T dependences of the electrical resistivity in bulk polycrystalline samples of semiconductor solid solutions (PbzSn1-z)0.8In0.2Te with various Pb content 0.1 ≤ z ≤ 0.9. The transition to the superconducting (SC) state was observed in samples with z ≤ 0.5 at helium temperatures with the critical transition temperature increasing up to Tc = 4.1 K with an increase of the Pb content up to z = 0.5. The bulk superconductivity in solid solutions with z ≥ 0.6 was not detected at temperatures T > 1.5 K. An exponential increase of the resistivity was observed in samples (PbzSn1-z)0.8In0.2Te with z ≥ 0.5 in the temperature range 40 K < T < 120 K. Experimental data was interpreted considering a shift of the energy position of the indium impurity band EIn in the complex valence band spectrum of (PbzSn1-z)0.8In0.2Te with changing lead content z. The exponential increase of the resistivity with decreasing temperature in studied solid solutions is observed when an energy barrier Ea appears between valence band states and quasilocal impurity states of In. The energy barrier increases with an increase of the Pb content in compounds with 0.5 ≤ z ≤ 0.8 and reaches maximum value Ea = 9.7 meV in the solid solution with z = 0.8. The SC state at helium temperatures was observed in (PbzSn1-z)0.8In0.2Te at z ≤ 0.4 when there is no energy barrier between zone and impurity states (i.e. the indium impurity band is located within the valence Σ-band), or at z = 0.5 when the activation energy Еа = 0.7 meV is less than the superconducting gap ΔS. Possible reasons for a decrease in the resistivity at T < 4.2 K in non-SC samples (PbzSn1-z)0.8In0.2Te with z ≥ 0.6 are also discussed.

Electronic Phase Separation in Iron Selenide (Li,Fe)OHFeSe Superconductor System**Supported by the National Key Research and Development Program of China under Grant Nos 2017YFA0303003, 2016YFA0300300 and 2015CB921000, the National Natural Science Foundation of China under Grant Nos 11574370, 11474338, 11674374 and 61501220, the Strategic Priority Research Program and Key Research Program of Frontier Sciences of the Chinese Academy of Sciences under

The phenomenon of phase separation into antiferromagnetic (AFM) and superconducting (SC) or normal-state regions has great implication for the origin of high-temperature (high-Tc) superconductivity. However, the occurrence of an intrinsic antiferromagnetism above the Tc of (Li,Fe)OHFeSe superconductor is questioned. Here we report a systematic study on a series of (Li,Fe)OHFeSe single crystal samples with Tc up to ∼41 K. We observe an evident drop in the static magnetization at Tafm ∼ 125 K, in some of the SC (Tc ≲ 38 K, cell parameter c ≲ 9.27 Å) and non-SC samples. We verify that this AFM signal is intrinsic to (Li,Fe)OHFeSe. Thus, our observations indicate mesoscopic-to-macroscopic coexistence of an AFM state with the normal (below Tafm) or SC (below Tc) state in (Li,Fe)OHFeSe. We explain such coexistence by electronic phase separation, similar to that in high-Tc cuprates and iron arsenides. However, such an AFM signal can be absent in some other samples of (Li,Fe)OHFeSe, particularly it is never observed in the SC samples of Tc ≳ 38 K, owing to a spatial scale of the phase separation too small for the macroscopic magnetic probe. For this case, we propose a microscopic electronic phase separation. The occurrence of two-dimensional AFM spin fluctuations below nearly the same temperature as Tafm, reported previously for a (Li,Fe)OHFeSe (Tc ∼ 42 K) single crystal, suggests that the microscopic static phase separation reaches vanishing point in high-Tc (Li,Fe)OHFeSe. A complete phase diagram is thus established. Our study provides key information of the underlying physics for high-Tc superconductivity.

Phase diagram of (Li(1-x)Fe(x))OHFeSe: a bridge between iron selenide and arsenide superconductors.

Previous experimental results have shown important differences between iron selenide and arsenide superconductors which seem to suggest that the high-temperature superconductivity in these two subgroups of iron-based families may arise from different electronic ground states. Here we report the complete phase diagram of a newly synthesized superconducting (SC) system, (Li1-xFex)OHFeSe, with a structure similar to that of FeAs-based superconductors. In the non-SC samples, an antiferromagnetic (AFM) spin-density-wave (SDW) transition occurs at ∼127 K. This is the first example to demonstrate such an SDW phase in an FeSe-based superconductor system. Transmission electron microscopy shows that a well-known √5×√5 iron vacancy ordered state, resulting in an AFM order at ∼500 K in AyFe2-xSe2 (A = metal ions) superconductor systems, is absent in both non-SC and SC samples, but a unique superstructure with a modulation wave vector q = (1)/2(1,1,0), identical to that seen in the SC phase of KyFe2-xSe2, is dominant in the optimal SC sample (with an SC transition temperature Tc = 40 K). Hence, we conclude that the high-Tc superconductivity in (Li1-xFex)OHFeSe stems from the similarly weak AFM fluctuations as FeAs-based superconductors, suggesting a universal physical picture for both iron selenide and arsenide superconductors.

Emergence of superconductivity and magnetic ordering tuned by Fe-vacancy in alkali-metal Fe chalcogenides RbxFe2-ySe2

Samples of RbxFe2-ySe2 exhibiting superconductivity [superconducting (SC) samples] undergo a phase-separation into two phases, a Fe-vacancy ordered phase with antiferromagnetic (AFM) transition at TN1~500 K (AFM1 phase) and a phase with little Fe- vacancy and SC transition at Tc~30 K (SC phase). The samples of RbxFe2-ySe2 exhibiting no SC behaviour (non-SC samples) are phase-separated into three phases, the AFM1 phase, another AFM phase with TN2 ~150 K (AFM2 phase), and a paramagnetic phase with no SC transitions (paramagnetic non-SC phase). In this paper, we present the experimental results of magnetic susceptibility, electrical resistivity, and NMR measurements on single crystals of RbxFe2-ySe2 to reveal physical properties of these co-existing phases in the SC and non-SC samples. The 87Rb and 77Se NMR spectra show that the Fe vacancy concentration is very small in the Fe planes of the SC phase, whereas the AFM2 and paramagnetic non-SC phases in non-SC samples have larger amount of Fe vacancies. The randomness induced by the Fe vacancy in the non-SC samples makes the AFM2 and paramagnetic non-SC phases insulating/semiconducting and magnetically active, resulting in the absence of the superconductivity in RbxFe2-ySe2.

Lower symmetrical behaviour of electronic state in the FeAs plane of Ba(Fe1-xCox)2As2 -75As NMR study-

To investigate the anisotropy within the FeAs plane in the tetragonal phase of Ba(Fe1-xCox)2As2, 75As NMR measurements on the electric field gradient (EFG) at the As site have been carried out for a Ba(Fe1-xCox)2As2 single crystal of x~0.08 with the superconducting (SC) transition temperature of Tc~23 K. We present a method how to analyse the 75As NMR spectra and deduce the anisotropic parameter of the EFG, η, that shows the electric inplane anisotropy at the As site. The EFG of the As site with no Co atoms at the nearest and next nearest Fe sites has the η value of 0.08-0.10 similar to that in the non-SC samples of x~0.02 in the tetragonal phase. The in-plane anisotropy in the x~0.08 sample remains even near Tc. We discuss the relationship between the in-plane anisotropy and local physical properties.

Electronic structure and phase separation of superconducting and nonsuperconductingKxFe2−ySe2revealed by x-ray photoemission spectroscopy

We have investigated the electronic structure of superconducting (SC) and nonsuperconducting (non-SC) K${}_{x}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ using x-ray photoemission spectroscopy (XPS). The spectral shape of the Fe 2$p$ XPS is found to depend on the amount of Fe vacancies. The Fe 2${p}_{3/2}$ peak of the SC and non-SC Fe-rich samples is accompanied by a shoulder structure on the lower binding energy side, which can be attributed to the metallic phase embedded in the Fe${}^{2+}$ insulating phase. The absence of the shoulder structure in the non-SC Fe-poor sample allows us to analyze the Fe 2$p$ spectra using a FeSe${}_{4}$ cluster model. The Fe 3$d$-Se 4$p$ charge-transfer energy of the Fe${}^{2+}$ insulating phase is found to be $\ensuremath{\sim}$2.3 eV which is smaller than the Fe 3$d$-Fe 3$d$ Coulomb interaction of $\ensuremath{\sim}$3.5 eV. This indicates that the Fe${}^{2+}$ insulating state is the charge-transfer type in the Zaanen-Sawatzky-Allen scheme. We also find a substantial change in the valence-band XPS as a function of Fe content and temperature. The metallic state at the Fermi level is seen in the SC and non-SC Fe-rich samples and tends to be enhanced with cooling in the SC sample.

Electronic structure of heavily electron-doped BaFe1.7Co0.3As2 studied by angle-resolved photoemission

We have performed high-resolution angle-resolved photoemission spectroscopy on heavily electron-doped non-superconducting (SC) BaFe1.7Co0.3As2. We find that the two hole Fermi surface pockets at the Brillouin zone center observed in the hole-doped superconducting Ba0.6K0.4Fe2As2 are absent or very small in this compound, while the two electron pockets at the zone corner significantly expand due to electron doping by the Co substitution. Comparison of the Fermi surface between non-SC and SC samples indicates that the coexistence of hole and electron pockets connected via the antiferromagnetic wave vector is essential in realizing the mechanism of superconductivity in the iron-based superconductors.

Quantum Dot Spectral Tuning of Multijunction III-V Solar Cells

AbstractImproving the production of photocurrent in the middle junction of a InGaP/Ga(In)As/Ge triple-junction solar cells (TJSC) can improve the overall conversion efficiency of cell. One possible method to improve the middle junction photocurrent is inserting a quantum dot (QD) superlattice (SL) into the stack. It has been predicted that QD-SL enhanced TJSCs have an efficiency ceiling of 47% under a one-sun AM0 illumination spectrum. Additionally, QD array enhanced GaAs cells have the added benefit of possible intermediate band effects, anisotropic absorption and enhanced radiation tolerance. Embedding InAs quantum dots (QDs) in a single junction GaAs solar cell can increase sub-GaAs bandgap photocurrent generation. This method has been shown to improve the short circuit current density (Jsc) of single junction cells under simulated 1 sun air mass zero (AM0) illumination. However, the increase in strain due to the InAs QD self-assembly may cause defects that reduce the minority carrier lifetime resulting in losses in the cell open circuit voltage (Voc) on the order of 300-500 mV. The introduction of strain compensation (SC) layers into the superlattice (SL) structure of a QD solar cell has previously been shown to improve the device performance, including the partial recovery of Voc. Strain compensation can be used effectively to balance the residual strain, impede dislocation formation, and improve the solar cell characteristics. The effect of GaP strain compensation on the solar cell electrical and material properties was investigated. High resolution X-ray diffraction (HRXRD) scans along the symmetric (004) Bragg peak show weak intensity and wide FWHM at the zero order SL peak (SL0) for non-SC samples. Optimum SC thickness was theoretically determined using a zero in plane stress method and experimentally verified using HRXRD. Optimal strain compensation was then used to increase the QD SL stacking from 5x to 10x and 20x. Use of SC resulted in shifting of the SL0 peak toward the substrate peak as well as reduced FWHM and improved SL peak definition. Kinematical diffraction modeling of the QD structures using numerical simulation indicated this peak shift resulted from reduced overall strain in the SL stack up to 5ML of SC. The material quality improvement in the SC QD solar cells was manifested in an improved spectral response and Jsc. The optoelectronic results for GaAs solar cells with QD SL’s demonstrate a strong dependence on GaP SC layer thickness. In addition, comparison of multi junction (MJ) solar cells which incorporate the SC QD SL’s demonstrate the utility of additional sub-GaAs bandgap current contribution as a tool for additional current-matching optimization in MJ solar cells.

X-ray absorption spectroscopic studies of the T′-type Gd 1.85− xPr xCe 0.15CuO y system

X-ray absorption near edge structure (XANES) studies of the T′-type Gd 1.85− x Pr x Ce 0.15CuO y ; x = 0.00, 1.10, 1.35 and 1.85 are carried out. The PrL 3 − , Ce L 3 3− and Cu K-absorption edge regions in these compounds have been measured. Pr in the reduced superconducting (SC) samples is found to be in 3 + state, while in the oxygenated non-SC samples, signatures of both Pr 3+ and Pr 4+ are observed. Ce remains throughout in the mixed (3+, 4+) valent state. As the Pr concentration ( x) in the reduced samples is increased, the unoccupied DOS, as reflected by intensity of the white line at the Cu K-edge, decreases. This is attributed to an increase in the electron density due to the creation of oxygen vacancies.

Magnetic and structural characterization of superconducting sodium-doped C60 prepared with the thermal decomposition of sodium azide

Abstract We have prepared Na4C60 samples by the different ways of the thermal decomposition of sodium azide NaN3. The superconducting (SC) samples were obtained by the decomposition under the dynamic vacuum, while the non-SC samples by the decomposition in the sealed tube. The SC sample was composed of two fee phases with a lattice constant a=14.207(5)A (phase A) and a=14.339(4)A (phase B). On the contrary, the non-SC sample had single fee phase which can be assigned to phase A. This result strongly suggests that phase B is the SC phase.

Magnetic circular dichroism properties of Y-Ba-Cu-O transparent films

Magnetic circular dichroism (MCD) in the Yba 2Cu 3 O 7− x c-oriented epitaxial optically transparent films has been investigated. For superconducting (sc) and non-sc samples the MCD dependence on magnetic field is described well by the linear law up to 7 T. The dispersion of MCD is different in sc and non-sc films. The paramagnetic behavior of the temperature dependence of amplitude is peculiar to most of the MCD bands.

Effect of quenching on the magnetic properties of RBa 2Cu 3O 7−y (r=rare earth) compounds

The RBa 2Cu 3O 7−y (R = Nd, Sm, Eu, Dy, Ho and Er) compounds have been prepared in the orthorhombic superconducting (SC) form by slow-cooling the samples to room temperature in oxygen atmosphere and in the tetragonal non-superconducting (non-SC) form by fast quenching to liquid nitrogen temperatures. Magnetic susceptibility measurements reveal a slight increase in the susceptibility of the non-superconducting samples over that of the superconducting samples above 100K. This may be due to a small magnetic moment on copper in non-SC samples. Crystal field effects are evident in the susceptibility of some of the non-SC compounds.