We report the Se substitution effects on the crystal structure, superconducting properties, and valence states of self-doped BiCh2-based compound CeOBiS2-xSex. Polycrystalline CeOBiS2-xSex samples with x = 0-1.0 were synthesized. For x = 0.4 and 0.6, bulk superconducting transitions with a large shielding volume fraction were observed in magnetic susceptibility measurements; the highest transition temperature (Tc) was 3.0 K for x = 0.6. A superconductivity phase diagram of CeOBiS2-xSex was established based on Tc estimated from the electrical resistivity and magnetization measurements. The emergence of superconductivity in CeOBiS2-xSex was explained with two essential parameters of in-plane chemical pressure and carrier concentration, which systematically changed with increasing Se concentration.
We have investigated the thermoelectric properties of the novel layered bismuth chalcogenides LaOBiS2-xSex. The partial substitution of S by Se produced the enhancement of electrical conductivity (metallic characteristics) in LaOBiS2-xSex. The power factor largely increased with increasing Se concentration. The highest power factor was 4.5 uW/cmK2 at around 470 deg. C for LaOBiS1.2Se0.8. The obtained dimensionless figure-of-merit (ZT) was 0.17 at around 470 deg. C in LaOBiS1.2Se0.8.
La2O2Bi2Pb2S6 is a layered Bi-based oxychalcogenide with a thick four-layer-type conducting layer. Although La2O2Bi2Pb2S6 is a structural analogue of La2O2Bi3AgS6, which is a superconductor, insulating behavior has been observed in La2O2Bi2Pb2S6 at low temperatures, and no superconductivity has been reported. Herein, we demonstrate superconductivity in La2O2Bi2Pb2S6-xSex via partial substitution of Se in the S sites. Owing to the Se doping, the normal state electrical resistivity of La2O2Bi2Pb2S6-xSex at low temperatures was dramatically suppressed, and superconductivity was observed at a tran…
Superconducting single crystals of pure FeTe1 xSex and FeTe0.65Se0.35 doped with Co, Ni, Cu, Mn, Zn, Mo, Cd, In, Pb, Hg, V, Ga, Mg, Al, Ti, Cr, Sr or Nd into Fe ions site have been grown applying Bridgman's method. It has been found that the sharpness of transition to the superconducting state in FeTe1 xSex is evidently inversely correlated with crystallographic quality of the crystals. Among all of the studied dopants only Co, Ni and Cu substitute Fe ions in FeTe0.65Se0.35 crystals. The remaining examined ions do not incorporate into the crystal structure. Nevertheless, they form inclusions t…
We report the effect of Se substitution on the crystal structure and superconductivity of BiCh2-based (Ch: S, Se) layered compounds Eu0.5Ce0.5FBiS2-xSex (x = 0-1). Crystal structure analysis showed that both lattice constants, a and c, increased with increasing x, which is different from the related La-doped system, such as Eu0.5La0.5FBiS2-xSex. This is due to Se substitution for both in-plane and out-of-plane Ch sites in the present Ce-doped system. Zero resistivity was observed for x = 0.2-1 above 2 K. Superconducting properties of Eu0.5Ce0.5FBiS2-xSex was investigated by magnetic susceptibi…
Layered transition-metal dichalcogenides 1T-TaS2-xSex (0<=x<=2) single crystals have been successfully fabricated by using a chemical vapor transport technique in which Ta locates in octahedral coordination with S and Se atoms. This is the first superconducting example by the substitution of S site, which violates an initial rule based on the fact that superconductivity merely emerges in 1T-TaS2 by applying the high pressure or substitution of Ta site. We demonstrate the appearance of a series of electronic states in 1T-TaS2-xSex with Se content. Namely, the Mott phase melts into a nearly comm…
We report a comparative study of the series Fe1.1Te1-xSex and the stoichiometric FeTe1-xSex to bring out the difference in their magnetic, superconducting and electronic properties. The Fe1.1Te1-xSex series is found to be magnetic and its microscopic properties are elucidated through Moessbauer spectroscopy. The magnetic phase diagram of Fe1.1Te1-xSex is traced out and it shows the emergence of spin-glass state when the antiferromagnetic state is destabilized by the Se substitution. The isomer shift and quadrupolar splitting obtained from the Moessbauer spectroscopy clearly brings out the elec…
We investigated the effects of Se substitution on the lattice constants and superconducting properties of CeO0.5F0.5Bi(S1-xSex)2. With increasing Se concentration, the a lattice constant increased, while the c lattice constant did not show any significant increase between x = 0.1 and x = 0.5. Bulk superconductivity was observed in samples with x = 0.2-0.4, and the superconducting transition temperature was the highest at x = 0.3. The obtained superconductivity phase diagram was compared to those of LaO0.5F0.5Bi(S1-xSex)2 and NdO0.5F0.5Bi(S1-xSex)2.
We report synthesis of under doped non-superconducting parent compound of iron chalcogenide, i.e FeTe1-xSex (x=0.0, 0.01, 0.03, 0.05) single crystals by flux free method. These FeTe1-xSex (x=0.0, 0.01, 0.03, 0.05) single crystals are crystallized in the tetragonal structure with space group P4/nmm at room temperature. Detailed scanning electron microscopy (SEM) results showed that the crystals are formed in slab-like morphology structure. Powder X-Ray diffraction (XRD) results show that (0 0 l) peaks are shifted towards higher angel with increasing Se concentration. Coupled magnetic and struct…
We have systematically investigated the crystal structure, magnetic susceptibility, and electrical resistivity of the BiS2-based superconductor LaO0.5F0.5Bi(S1-xSex)2 (x = 0 - 0.7). With expanding lattice volume by Se substitution, bulk superconductivity was induced for x > 0.2, and the highest Tc of 3.8 K was observed in x = 0.5 (LaO0.5F0.5BiSSe). Metallic conductivity was observed for x > 0.3 in the resistivity measurement, whereas semiconducting-like behavior was observed for x < 0.2. The induction of bulk superconductivity by the partial substitution of S by Se in the LaO0.5F0.5BiS2 superc…
Ordering of Interstitial Iron Atoms and Local Structural Distortion Induced by Iron Polycomplex in Fe1+yTe1-xSex as Seen via Transmission Electron Microscopy
Eu0.5La0.5FBiS2-xSex is a new BiS2-based superconductor system. In Eu0.5La0.5FBiS2-xSex, electron carriers are doped to the BiS2 layer by the substitution of Eu by La. Bulk superconductivity in this system is induced by increasing the in-plane chemical pressure, which is controlled by the Se concentration (x). In this study, we have analysed the crystal structure of Eu0.5La0.5FBiS2-xSex using synchrotron powder diffraction and the Rietveld refinement. The precise determination of the structural parameters and thermal factors suggest that the emergence of bulk superconductivity in Eu0.5La0.5FBi…
We have investigated Se substitution effect to superconductivity of an optimally-doped BiS2-based superconductor Eu0.5La0.5FBiS2. Eu0.5La0.5FBiS2-xSex samples with x = 0-1 were synthesized. With increasing x, in-plane chemical pressure is enhanced. For x = 0.6, 0.8, and 1, superconducting transitions with a large shielding volume fraction are observed in magnetic susceptibility measurements, and the highest Tc is 3.8 K for x = 0.8. From low-temperature electrical resistivity measurements, a zero-resistivity state is observed for all the samples, and the highest Tc is observed for x = 0.8. With…
We examined the crystal structure of the new thermoelectric material LaOBiS2-xSex, whose thermoelectric performance is enhanced by Se substitution, by using powder synchrotron X-ray diffraction and Rietveld refinement. The emergence of metallic conductivity and enhancement of the thermoelectric power factor of LaOBiS2-xSex can be explained with the higher in-plane chemical pressure caused by the increase of Se concentration at the in-plane Ch1 site (Ch = S, Se). High-temperature X-ray diffraction measurements for optimally substituted LaOBiSSe revealed anomalously large atomic displacement par…
The superconducting properties and structural parameters, including the Grüneisen parameter (γG), of the BiCh2-based (Ch: S, Se) layered superconductor LaO0.5F0.5BiS2-xSex were investigated. The superconducting transition temperature (Tc) increased with increasing Se concentration (x), and bulk superconductivity was induced by Se substitution. γG exhibited a larger value when x greater than 0.4, where the superconducting properties were improved. These results suggest that the increase of lattice anharmonicity is preferable for superconductivity in LaO0.5F0.5BiS2-xSex. The discussion includes …
From our powder x ray diffraction pattern, electrical transport and magnetic studies we report the effect of isovalent Se substitution at S sites in the newly discovered systems EuSr2Bi2S4F4 and Eu2SrBi2S4F4. We have synthesized two new variants of 3244 type superconductor with Eu replaced by Sr which is reported elsewhere [Z. Haque et. al.]. We observe superconductivity at Tc 2.9 K (resistivity) and 2.3 K (susceptibility) in EuSr2Bi2S4-xSexF4 series for x = 2. In the other series Eu2SrBi2S4-xSexF4, two materials (x= 1.5; Tc = 2.6 K and x = 2; Tc = 2.75 K) exhibit superconductivity.
Recently, new layered superconductors having a BiS2-type conduction layer have been discovered. NdO1-xFxBiS2 is a typical BiS2-based superconductor with a maximum Tc of 5.4 K. In this study, the effect of element substitution within the superconducting layer of BiS2-based NdO0.5F0.5BiS2 was investigated. We systematically synthesized two kinds of polycrystalline samples of NdO0.5F0.5Bi(S1-xSex)2 and NdO0.5F0.5Bi1-ySbyS2 by a two-step solid-state reaction method. The phase purity and the changes in lattice constants were investigated by x-ray diffraction. The superconducting properties were inv…
We have carried out detailed magnetic and transport studies of the new Sr0.5Ce0.5FBiS2-xSex (x = 0.5, 1) superconductors derived by doping Se in Sr0.5Ce0.5FBiS2. Se-doping produces several effects: it suppresses semiconducting like behavior observed in the undoped Sr0.5Ce0.5FBiS2, ferromagnetic ordering temperature, TFM, decreases considerably from 7.5 K (in Sr0.5Ce0.5FBiS2) to 3.5 K and superconducting transition temperature, Tc, gets enhanced slightly to 2.9 - 3.3 K. Thus in these Se-doped materials, TFM is just marginally higher than Tc. Magnetization studies provide an evidence of bulk sup…
In order to understand the mechanisms behind the emergence of superconductivity by the chemical pressure effect in REO0.5F0.5BiS2 (RE = La, Ce, Pr, and Nd), where bulk superconductivity is induced by the substitutions with a smaller-radius RE, we performed synchrotron powder X-ray diffraction, and analyzed the crystal structure and anisotropic displacement parameters. With the decrease of the RE3+ ionic radius, the in-plane disorder of the S1 sites significantly decreased, very similar to the trend observed in the Se-substituted systems: LaO0.5F0.5BiS2-xSex and Eu0.5La0.5FBiS2-xSex. Therefore,…
Structures for realizing hole-doped MgB$_2$ without appealing to chemical substitutions are proposed. These structures which consist of alternating MgB$_2$ and graphene layers have small excess energy compared to bulk graphite and MgB$_2$. Density functional theory based first-principles electronic structure calculations show significant charge transfer from the MgB$_2$ layer to graphene, resulting in effectively hole-doped MgB$_2$. Substantial enhancement in the density of states at the Fermi level of the proposed structure is predicted.
Chemical substitution, combined with strain, allows the charge-transfer energy in hole-doped cuprates to be broadly tuned. We theoretically characterize the structural and electronic properties of the family of compounds $R_2$CuO$_2$S$_2$, constructed by sulfur replacement of the apical oxygens and rare earth substitutions in the parent cuprate La$_2$CuO$_4$. Additionally, the enthalpies of formation for possible synthesis pathways are determined.
We report on the effect of substitution for Cu on Tc of electron-doped infinite-layer superconductors Sr0.9La0.1Cu1-xRxO2, R = Zn and Ni. We found that Tc was nearly constant until x = 0.03 for R = Zn, while superconductivity was nearly suppressed for x = 0.02 with dTc/dx = 20 K/% for R = Ni. This behavior is very similar to that of conventional superconductors. These findings are discussed in terms of the superconducting gap symmetry in the cuprate superconductors including another electron-doped superconductor, (Nd,Ce)2CuO4-y.
The recently discovered compounds Ca10(Fe1-xPtxAs)10(Pt3+yAs8) exhibit superconductivity up to 38 K, and contain iron arsenide (FeAs) and platinum arsenide (Pt3+yAs8) layers separated by layers of Ca atoms. We show that high Tc's above 15 K only emerge if the iron-arsenide layers are at most free of platinum-substitution (x \rightarrow 0) in contrast to recent reports. In fact Pt-substitution is detrimental to higher Tc, which increases up to 38 K only by charge doping of pure FeAs layers. We point out, that two different negatively charged layers [(FeAs)10]n- and (Pt3+yAs8)m- compete for the …
By doping MgB_2 superconductor with SiC nano-particles, we have successfully introduced pinning sites directly into the crystal lattice of MgB_2 grains (intra-grain pinning). It became possible due to the combination of counter-balanced Si and C co-substitution for B, leading to a large number of intra-granular dislocations and the dispersed nano-size impurities induced by the substitution. The magnetic field dependence of the critical current density was significantly improved in a wide temperature range, whereas the transition temperature in the sample MgB_2(SiC)_x having x = 0.34, the highe…
We study the effects of Cu substitution in Fe1.1Te, the non-superconducting parent compound of the iron-based superconductor, Fe1+yTe1-xSex, utilizing neutron scattering techniques. It is found that the structural and magnetic transitions, which occur at \sim 60 K without Cu, are monotonically depressed with increasing Cu content. By 10% Cu for Fe, the structural transition is hardly detectable, and the system becomes a spin glass below 22 K, with a slightly incommensurate ordering wave vector of (0.5-d, 0, 0.5) with d being the incommensurability of 0.02, and correlation length of 12 angstrom…
In this study, Si and C were incorporated into polycrystalline MgB2 via in situ reaction of Mg and B with either SiC or with separate Si and C (Si+C). The electrical transport and magnetic properties of the two series of samples were compared. The corrected resistivity at 40K, Rho A(40K), is higher for the SiC reacted samples regardless of carbon (C) substitution level, indicating larger intragrain scattering because of the simultaneous reaction between Mg and SiC and carbon substitution during the formation of MgB2. In addition, because of the cleaner reaction route for the SiC reacted sample…
We report the effect of adipic acid (C6H10O4) doping on lattice parameters, microstructure, critical temperature (Tc), current density (Jc), and irreversibility field (Hirr) for MgB2 superconductor. Actual carbon (C) substitution level for boron (B) is estimated to be from 0.40 percent to 2.95 percent for different doping levels. A reduction in Tc from 38.43 to 34.93 K and in lattice parameter a from 3.084(3) A to 3.075(6) Ais observed for the10 wt percent C6H10O4 doped sample in comparison to pristine MgB2. This is an indication of C substitution at boron sites, with the C coming from the dec…
$^{75}$As Nuclear Magnetic (NMR) and Quadrupolar (NQR) Resonance were used, together with Mössbauer spectroscopy, to investigate the magnetic state induced by Mn for Fe substitutions in F-doped LaFe$_{1-x}$Mn$_{x}$AsO superconductors. The results show that $0.5$% of Mn doping is enough to suppress the superconducting transition temperature $T_c$ from 27 K to zero and to recover the magnetic structure observed in the parent undoped LaFeAsO. Also the tetragonal to orthorhombic transition of the parent compound is recovered by introducing Mn, as evidenced by a sharp drop of the NQR frequency. The…
We investigate the thermodynamic and transport properties of molybdenum-doped BaFe2As2 (122) crystals, the first report of hole doping using a 4d element. The chemical substitution of Mo in place of Fe is possible up to ~ 7%. For Ba(Fe1-xMox)2As2, the suppression rate of the magnetic transition temperature with x is the same as in 3d Cr-doped 122 and is independent of the unit cell changes. This illustrates that temperature-composition phase diagram for hole-doped 122 can be simply parameterized by x, similar to the electron-doped 122 systems found in literature. Compared to 122 with a coupled…
We present a study concerning the partial substitution of yttrium at the lanthanum site of the undoped LaFeAsO and superconducting LaFeAsO0.85F0.15 compounds. We prepared samples with a nominal yttrium content up to 70% producing simultaneous shrinkage of both the a- and c-lattice parameters by 1.8% and 1.7%, respectively. The chemical pressure provided by the partial substitution with this smaller ion size causes a lowering of the spin density wave temperature in the undoped compounds, as well as an increase of the superconducting transition temperatures in the doped ones. The 15% fluorine-do…
Rh17S15 has recently been shown to be a strongly correlated superconductor with a transition temperature of 5.4 K. In order to understand the nature of strong correlations we study the effect of substitution of some of the Rh and S atoms by other elements such as Fe, Pd, Ir and Ni on the Rh side and Se on the S side in this work. We find that while substitution of Ir and Se lower the transition temperature considerably, that of Fe, Pd and Ni destroy superconductivity down to 1.5 K. The resistivity data in these doped samples show a minimum which is presumably disorder induced. A reduction of T…
The in-plane anisotropy of resistivity has been investigated for Ba(Fe$_{1-x}$TM$_{x}$)$_2$As$_2$ (TM-Ba122, TM$=$Cr, Mn, and Co) where the substitution sites are the same but the doped carriers are different for different TM elements. The Hall coefficient measurements indicated that hole carriers are effectively doped by Cr substitution but not by Mn substitution. It has been found that the resistivity difference $Δρ=ρ_{\rm b}-ρ_{\rm a}$ in the antiferromagnetic-orthorhombic (AFO) phase of Cr-Ba122 is initially positive but it turns to negative with increasing Cr content, whereas the positive…
We have performed an angle-resolved photoemission study of the electron-doped high-Tc superconductors (HTSCs) Sm_{1.85}Ce_{0.15}CuO_{4} (SCCO) and Eu_{1.85}Ce_{0.15}CuO_{4} (ECCO). Around the nodal point (pi/2, pi/2), we observed an energy gap at the Fermi level (EF) for both samples, resulting from the strong effects of antiferromagnetism. The magnitude of this gap in ECCO is larger than that in SCCO, implying that the effects of antiferromagnetism in ECCO are stronger than that in SCCO. Also, we quantitatively confirmed the effects of antiferromagnetism by the band dispersions to tight-bindi…
We studied the effects of isoelectronic Ru substitution at the Fe site on the energy gaps of optimally F-doped SmFeAsO by means of point-contact Andreev reflection spectroscopy. The results show that the SmFe_{1-x}Ru_{x}AsO_{0.85}F_{0.15} system keeps a multigap character at least up to x=0.50, and that the gap amplitudes Delta_1 and Delta_2 scale almost linearly with the local critical temperature TcA. The gap ratios 2Delta_i/(kB Tc) remain approximately constant only as long as Tc > 30 K, but increase dramatically when Tc decreases further. This trend seems to be common to many Fe-based supe…
We report on structural, susceptibility, conductivity, and heat-capacity studies of FeSe0.5Te0.5 single crystals with 2% substitution of Mn for Fe. Mn-doped samples show a higher onset temperature, a narrower width of the superconducting transition, and a higher magnitude of the jump in the specific heat at Tc in compared to undoped samples. From the resistivity data in magnetic fields parallel to the c axis we derived an upper critical field Hc2 of ~580 kOe for doped samples compared to 490 kOe for pure samples. Using a single-band BCS model we can describe the electronic specific heat in the…
We have investigated the superconductivity discovered in boron (B)-doped diamonds by means of 11B-NMR on heteroepitaxially grown (111) and (100) films. 11B-NMR spectra for all of the films are identified to arise from the substitutional B(1) site as single occupation and lower symmetric B(2) site substituted as boron+hydrogen(B+H) complex, respectively. A clear evidence is presented that the effective carriers introduced by B(1) substitution are responsible for the superconductivity, whereas the charge neutral B(2) sites does not offer the carriers effectively. The result is also corroborated …
The effects of nonmagnetic Zn and magnetic Ni substitution for Cu site on magnetism are studied by measurements of uniform magnetic susceptibility for lightly doped La_{2-x}Sr_xCu_{1-z}M_zO_4 (M=Zn or Ni) polycrystalline samples. For the parent x=0, Zn doping suppresses the Néel temperature T_N whereas Ni doping hardly changes T_N up to z=0.3. For the lightly doped samples with T_N~0, the Ni doping recovers T_N. For the superconducting samples, the Ni doping induces the superconductivity-to-antiferromagnetic transition (or crossover). All the heavily Ni doped samples indicate a spin glass beha…
Superconductivity is achieved in Nd1-xCexOBiS2 via electron doping using mixed-valence Ce ions. Single crystals with x = 0.2, 0.3, 0.4, and 0.5 are generated using a CsCl flux method. Plate-like single crystals with dimensions of 0.8 0.8 0.2 mm3 were obtained. The magnetic susceptibility chi(T) indicates large diamagnetism, and the electrical resistivity rho(T) indicates zero resistivity. The maximum value of Tc is observed at 4.7 K in Nd0.8Ce0.2OBiS2 from chi(T). From the rho(T) measurements taken in several magnetic fields, the upper critical field mu0Hc2(0) is estimated to be about 12 and 0…
Non-magnetic impurities in iron-based superconductors can provide an important tool to understand the pair symmetry and they can influence significantly the transport and the superconducting behaviour. Here, we present a study of the role of strong impurity potential in the Fe plane, induced by Cu substitution, on the electronic and superconducting properties of single crystals of FeSe. The addition of Cu quickly suppresses both the nematic and superconducting states, and increases the residual resistivity due to enhanced impurity scattering. Using magnetotransport data up to 35 T for a small …
We investigate the currently debated issue concerning whether transition metal substitutions dope carriers in iron based superconductors. From first-principles calculations of the configuration-averaged spectral function of BaFe$_2$As$_2$ with disordered Co/Zn substitutions of Fe, important doping effects are found beyond merely changing the carrier density. While the chemical potential shifts suggest doping of a large amount of carriers, a reduction of the coherent carrier density is found due to the loss of spectral weight. Therefore, none of the change in the Fermi surface, density of state…
We report the influence on the superconducting critical temperature $T_c$ in doped SrTiO$_3$ of the substitution of the natural $^{16}$O atoms by the heavier isotope $^{18}$O. We observe that for a wide range of doping this substitution causes a strong ($\sim 50 \%$) enhancement of $T_c$. Also the magnetic critical field $H_{c2}$ is increased by a factor $\sim 2$. Such a strong impact on $T_c$ and $H_{c2}$, with a sign opposite to conventional superconductors, is unprecedented. The observed effect could be the consequence of strong coupling of the doped electrons to lattice vibrations (phonons…
We have succeeded in synthesizing electron-doped polycrystalline bulk samples of T'-La1.8Eu0.2CuO4-yFy (y = 0 - 0.15) by the fluorination of undoped (Ce-free) T'-La1.8Eu0.2CuO4 using NH4F. The magnetic susceptibility measurements have revealed that the superconducting transition temperature, Tc, increases with increasing y, exhibits the maximum of 23 K at y = 0.025, and decreases. The dome-like dependence of Tc on the doped carrier concentration in the T'-type (La,Eu)-based cuprates is explained in terms of the pairing mediated by spin fluctuations based on the d-p model calculation [K. Yamaza…
The superconductor MgCNi3 has been chemically doped by partial substitution of Ru for Ni in the solid solution MgCNi3-xRux for 0<x<0.5. Magnetic and specific heat measurements show that the Sommerfeld parameter (gamma_exp) and TC decrease immediately on Ru substitution, but that a TC above 2K is maintained even for a relatively large decrease in gamma_exp. Ferromagnetism is not observed to develop through Ru substitution, and the normal state magnetic susceptibility is suppressed.
This report describes some transport and magnetic properties of doped Bi2212 and Bi2223 superconducting whiskers. These materials have advantages over polycrystalline sample as well as large bulk crystals in that they provide narrow transitions in resistance versus temperature as well as in magnetization versus temperature curves. In addition they are easy to grow and and have short oxygen annealing times. Here Data on transport and magnetic properties of these superconducting whiskers are presented.
Phase diagram of electron and hole-doped SrFe2As2 single crystals is investigated using Co and Mn substitution at the Fe-sites. We found that the spin-density-wave state is suppressed by both dopants, but the superconducting phase appears only for Co (electron)-doping, not for Mn (hole)-doping. Absence of the superconductivity by Mn-doping is in sharp contrast to the hole-doped system with K-substitution at the Sr sites. Distinct structural change, in particular the increase of the Fe-As distance by Mn-doping is important to have a magnetic and semiconducting ground state as confirmed by first…
Different element substitution effects in transition metal oxypnictide Re(O$_{1-x}$F$_x$)TAs with Re=La, Ce, Nd, Eu, Gd, Tm, T=Fe, Ni, Ru, were studied. Similar to the La- or Ce-based systems, we found that the pure NdOFeAs shows a strong resistivity anomaly near 145 K, which was ascribed to the spin-density-wave instability. Electron doping by F increases T$_c$ to about 50 K. While in the case of Gd, the T$_c$ is reduced below 10 K. The tetragonal ZrCuSiAs-type structure could not be formed for Eu or Tm substitution in our preparing process. For Ni-based case, although both pure and F-doped L…
We report a breakdown of the scaling of the antiferromagnetic transition temperature T_N on chemical doping for Pt-doped CaFe2As2. The substitution of Pt is ineffective in the reduction of antiferromagnetic ordering as well as in inducing superconductivity in Ca(Fe1-xPtx)2As2. The Pt-doped CaFe2As2 that does not exhibit superconductivity will, however, provide us an ideal opportunity to elucidate the role of chemical doping in the occurrence of superconductivity in iron-based materials.
Superconductivity (SC) in the Ba-122 family of iron-based compounds can be controlled by aliovalent or isovalent substitutions, applied external pressure, and strain, the combined effects of which are sometimes studied within the same sample. Most often, the result is limited to a shift of the SC dome to different doping values. In a few cases, the maximum SC transition at optimal doping can also be enhanced. In this work, we study the combination of charge doping together with isovalent P substitution and strain by performing ionic gating experiments on BaFe$_2$(As$_{0.8}$P$_{0.2}$)$_2$ ultra…
La-substituted CaFeAsH,(Ca1-xLax)FeAsH, were synthesized by the solid state reaction at 1173 K under high pressure of 2.5 GPa. The r-T anomaly corresponding to tetragonal to orthorhombic structural transition was suppressed by La-substitution and then the superconductivity was observed at an electron doping content of Ne > 0.05. The maximum onset Tc of 47.4 K was reached for Ne = 0.18, which is significant increase compared to 23 K achieved by direct electron doping to the FeAs-layer via Co-substitution to the Fe-site. This value of Tc value fits well to the phenomenological relation between T…
The influence of low-level Pr substitution in Y1-xPrxBa2Cu3O(7-delta) single crystals on the anisotropy of the superconducting state parameters was investigated by torque magnetometry in magnetic fields of up to 9 T. An averaged anisotropy parameter, γ, of 7.4 was found and no influence of the Pr ion concentration on γwas observed up to a Pr content of 2.4%. A pronounced maximum at angles between 0.5 and 1 degree out of the ab-plane was observed in all crystals in the irreversible angular dependence of the torque. This maximum is attributed to intrinsic pinning associated with kinked vortices.…
The crystal structure of boron doped superconducting MgC_{1-x}{11}^B_{x}Ni_{3}, studied by powder neutron diffraction, is reported. The solubility limit of boron is determined to be approximately x=0.16. The unit cell expands from a = 3.81089(2) to 3.81966(2) Angstroms as x increases from x=0 to x=0.155. Boron ({11}^B) doping decreases Tc with increasing x: from 7.09K (x=0) to 6.44K (x=0.155).
Results of our study on the superconducting and normal state properties of the recently discovered superconductor MgCNi_3, the effect of Fe and Co substitution at the Ni site and the effect of pressure are reported. It is shown that a two band model provides a consistent interpretation of the temperature dependence of the normal state resistance and the Hall constant. Whereas band structure calculations suggest an increase in T_c upon partial substitution of Ni with Fe and Co, Co substitution quenches superconductivity and Fe substitution leads to an increase followed by a decrease in T_c. The…
The Hall effect and its associated anomalies offer an important means for describing superconducting property in cuprate compounds. Here, we investigate the electron- and hole-doping effects (implemented through Al- and Ag-doping, respectively) on the Hall effect and transport properties of Y123 superconducting materials (YBa2Cu3-xXxO7-d; X= Ag, Al). The Hall anomaly, in both normal and vortex states, generally depends on multiple factors, such as temperature, magnetic field, and amount/type of doping. Specifically, for our samples, we observed that hole doping first enhances the critical temp…
Thus far, there is no cuprate system where both n-type and p-type charge carriers can be doped without changing the crystallographic structure. For studying the electron-hole symmetry in an identical structure, we try to dope n-type carriers to YBa2Cu3Oy system by reducing oxygen content and substituting La3+ ions for Ba2+. Single crystals of La-doped YBa2Cu3Oy are grown by a flux method with Y2O3 crucibles and it is confirmed that La actually substitutes \~13% of Ba. The oxygen content y can be varied between 6.21 and 6.95 by annealing the crystals in an atmosphere with controlled oxygen part…
We study how Co- and Ni-substitution affect the electronic structure of the iron-based superconductor, LaFeAsO. We perform {\it ab initio} supercell calculations and unfold the first Brillouin zone (BZ) to calculate the spectral function in the BZ for the normal cell. The charge density distribution in real space shows that doped extra electrons are trapped around Co (Ni) atom. This seems to mean that Co(Ni)-substitution does not work as carrier doping. However, the present momentum-space analysis indicates that the Fermi-surface volume indeed expands by substitutions, which can be well descri…
We have systematically studied the substitution of nonmagnetic Zn and magnetic Ni at iron sites in Ce based oxypnictide. The parent compound (CeOFeAs) shows an anomaly in resistivity around 150 K due to structural transition from tetragonal (space group: P4/nmm) to orthorhombic structure (space group: Cmma). Substitution of Zn suppresses this anomaly to lower temperature (~130 K) but Ni substitution does not show any anomaly around this temperature and the compound behaves like a metal. Further, we find that non magnetic (Zn) doping leads to higher impurity scattering as compared to magnetic N…
Effects of the Zn- and Ni-substitution on the Cu-spin dynamics in the electron-doped Pr0.86LaCe0.14Cu1-y(Zn,Ni)yO4+a-d with y = 0, 0.01, 0.02, 0.05 and different values of the reduced oxygen content d have been studied using zero-field muon-spin-relaxation (muSR) measurements at temperatures down to 2 K. For the as-grown sample (d = 0, y = 0) and the sample with a very small d value (d < 0.01, y = 0), a muon-spin precession due to long-range antiferromagnetic order has been observed. On the other hand, no precession has been observed for moderately oxygen-reduced samples (0.01 < d < 0.09). It …
We have synthesized the single-phase polycrystalline samples of Sn1-xAgxTe, Ag-doped topological crystalline insulator SnTe, with a range of x = 0-0.5 using a high-pressure synthesis method. The crystal structure of Sn1-xAgxTe at room temperature is a cubic NaCl-type structure, which does not vary upon Ag substitution. Bulk superconductivity with a transition temperature of 2.4 K was observed for x = 0.15-0.25, and the optimal Ag content was x = 0.2. The Sn1-xAgxTe superconducting phase will be useful for understanding the superconductivity nature and mechanisms of the carrier-doped SnTe syste…
We develop a theory of superconducting pairing in low-density Strontium titanate due to quadratic coupling of electron density to soft transverse optical phonons. It leads to static attractive potential between electrons which decay length scales inversely with soft optical gap. For low electron densities attraction between electrons is local and transition temperature Tc was found. The Tc(n) dependence in agreement with experimental data for low doping was calculated. Next, we show that suppression of Tc by hydrostatic pressure and strong increase of Tc due to isotop substitution are explaine…
We report a systematic study on the effect of partial substitution of Sc$^{3+}$ by Ti$^{4+}$ in Sr$_{2}$ScFeAsO$_{3}$, Ca$_{2}$ScFeAsO$_{3}$ and Sr$_{3}$Sc$_{2}$Fe$_{2}$As$_{2}$O$_{5}$ on their electrical properties. High level of doping results in an increased carrier concentration and leads to the appearance of superconductivity with the onset of T$_{c}$ up to 45 K.
