We report on measurements on Y1-xPrxBa2Cu3O7-delta single crystals, with x varying from 0 to 2.4%. The upper and the lower critical fields, Hc2 and Hc1, the Ginzburg-Landau parameter and the critical current density, Jc(B), were determined from magnetization measurements and the effective media approach scaling method. We present the influence of Pr substitution on the pinning force density as well as on the trapped field profiles analyzed by Hall probe scanning.
The effect of Zn substitution on the superconducting transition temperature, Tc, was investigated for the sintered Y1-xCaxBa2(Cu1-yZny)3O7-delta compounds over a wide range of hole concentration per CuO2 plane, p. Ca substitution enabled us to study the deeply overdoped region. p was changed by changing the oxygen deficiency (delta). A strongly p-dependent rate of suppression of Tc with Zn (dTc/dy) was found. From the analysis of the dTc(p)/dy and Tc(p,y) data, we found that the optimum hole content, popt, shifts to higher values with increasing Zn and superconductivity is at its strongest whe…
Density functional calculations are used to investigate the effects of partial substitutional alloying of the B site in MgB2 with C and Be alone and combined with alloying of the Mg site with Cu. The effect of such substitutions on the electronic structure, electron phonon coupling and superconductivity are discussed. We find that Be substitution for B is unfavorable for superconductivity as it leads to a softer lattice and weaker electron-phonon couplings. Replacement of Mg by Cu leads to an increase in the stiffness and doping level at the same time, while the carrier concentration can be co…
The effects of planar hole content, p, on the static magnetic susceptibility, chi(T), of Y1-xCaxBa2Cu3O7-delta polycrystalline samples were investigated over a wide range of Ca (x) and oxygen contents. Non-magnetic Ca2+, in the 3p6 state, induces a Curie-like contribution to chi(T) that increases systematically and non-linearly with x but is almost independent of p. We argue that this arises from statistical clusters containing two or more nearest neighbor Ca atoms. We have again found that the pseudogap in the quasi-particle spectral weight appears abruptly below a planar hole content p = 0.1…
We examined Lead (Pb) Substitution effect on a single crystal of a layered superconductor LaO0.5F0.5BiS2. Pb concentration dependence of the lattice constant showed slight anomaly at about 8% and 9% substitution of Pb for Bi. These samples showed the enhancement of the superconducting transition temperature and the superconducting volume fraction. Furthermore, these samples showed the anomaly in the temperature dependence of the resistivity at about 150K. These results were not observed in Pb substituted NdO0.7F0.3BiS2. Therefore, the enhancement of the superconducting properties by Pb substit…
The influence of Ca substitution on the superconducting properties of polycrystalline R1-xCaxBa2Cu3Oz (R = Eu, Gd, Er; x=0; 0.2; 0.25 and 0.3) samples has been studied by X-ray powder diffraction, ac susceptibility and dc magnetization measurements. The superconducting parameters such as critical temperature, inter- and intra-granular critical current and flux pinning are found to be strongly dependent both on Ca content and type of R element. The best combination of these parameters is found for the system Gd1-xCaxBa2Cu3Oz forming R1+yBa2-yCu3Oz clusters. The level of overdoping and the type …
In this work we present a systematic experimental and theoretical study of the structural, transport and superconducting properties of Sm(Fe1-xRux)As(O0.85F0.15) polycrystalline samples as a function of Ru content (x) ranging from 0 to 1. The choice of Ru as isoelectronic substitution at Fe site of F-doped compounds allows to better clarify the role of structural disorder in modifying the normal and superconducting properties of these newly discovered multiband superconductors. Two different regions are identified: the Fe-rich phase (x<0.5) where superconducting and normal state properties are…
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…
It is proposed that (i) the temperature dependence of the superconducting gap Delta(T) in high-Tc cuprates can be predicted just from the knowledge of Delta(0) and the critical temperature Tc; and, in particular, (ii) Delta(0)/Tc > 4 implies that Delta(Tc) is not equal to zero, while Delta(0)/Tc < 4 corresponds to Delta(Tc) = 0. A number of tunneling experiments appear to support the above proposition, and, furthermore, show reasonable quantitative agreement with a model (cond-mat/0308428), which is based on the two-dimensional stripe hypothesis.
We have investigated the effect of Zn substitution on the hole content, p, and the oxygen deficiency, d, for a series of high-quality crystalline c-axis oriented thin films and polycrystalline Y1-xCaxBa2(Cu1-yZny)3O7-delta compounds over a wide range of composition (x, y, and delta). p was determined from the room-temperature thermopower, S[290K]. When annealed at a given temperature and oxygen partial pressure, a small but systematic variation in S[290K] was observed with Zn, for all the samples. From the analysis of the temperature derivative of resistivity, drho/dT, at 300K, which varies al…
Static and dynamic transport measurements on stacks of intrinsic Josephson junctions on Bi2Sr2CaCu2O8+delta single crystals, near to optimal doping, are presented. The zero bias minimum in dynamic conductance is consistent with a normal state pseudo-gap. At Tc, a critical current develops simultaneously with new features consistent with a gap in the superconducting density states at an already well-established energy. The lack of thermal broadening and the small scale of the features force us to conclude a high degree of coherent interlayer tunneling in both the superconducting and normal stat…
Thin films of Fe(1+delta)Se(1-x)Te(x) (delta ~ 0.18 & x ~ 0.5) have been successfully grown on (100) oriented single crystalline SrTiO3 and LaAlO3 substrates by pulsed laser deposition. The crystal structure was characterized by x-ray diffraction, and the superconducting properties by electrical resistivity measurements. X-ray diffraction analysis establishes the epitaxial growth of the films with c-axis orientation. Atomic force microscopy showed a smooth surface morphology for the films grown on SrTiO3 substrates. All the films are observed to be superconducting with a Tc of ~ 8-14 K dependi…
We have measured the effect of oxygen isotope substitution on the ab-plane reflectance of underdoped YBCO. The frequency shift of the transverse optic phonons due to the substitution of O-16 by O-18 yields an isotope effect of the expected magnitude for copper-oxygen stretching modes with alpha=0.5 +- 0.1. The reflectance shoulder at 400 - 500 cm^-1 shows a much smaller exponent of alpha=0.1 +- 0.1 in the normal state and alpha=0.23+- 0.1 in the superconducting state. These observations suggest that the shoulder is of electronic origin and not due to a phonon mode as has been suggested recentl…
The influence of a partial substitution with S, Te, Co, Ni and Cu atoms on the electronic structure of the FeSe superconductor has been investigated within the density functional theory. The results of the supercell calculations reveal distinct changes of electronic structures of the substituted FeSe systems, which can be responsible for their superconducting properties. The replacement of Se atoms by Te or S ones yields imperfect nesting between the holelike and electronlike Fermi surface (FS) sheets, which enhances magnetic fluctuations responsible for superconducting pairing, thus leading t…
We elucidate the magnetic phases and superconducting transition temperatures (Tc) in Sr2VFeAsO3-delta (21113V), an iron-based superconductor with a thick-blocking layer fabricated from a perovskite-related transition metal oxide. At low temperatures (T < 37.1 K), 21113V exhibited a superconducting phase in the range 0.031 =< delta =< 0.145 and an antiferromagnetic (AFM) iron sublattice in the range 0.267 =< delta =< 0.664. Mixed-valent vanadium exhibited a dominant AFM phase in 0.031 =< delta =< 0.088, and a partial ferrimagnetic (Ferri.) phase in the range 0.124 =< delta =< 0.664. The Ferri. …
We have performed transverse- and zero-field muon spin rotation/relaxation experiments, as well as magnetometry measurements, on samples of Fe1-xVxSe and their Li+NH3 intercalates Li0.6(NH2)0.2(NH3)0.8 Fe1-x Vx Se. We examine the low vanadium substitution regime: x = 0.005, 0.01, and 0.02. The intercalation reaction significantly increases the critical temperature (Tc) and the superfluid stiffness for all x. The nonintercalated samples all exhibit Tc = 8.5 K while the intercalated samples all show an enhanced Tc > 40 K. Vanadium substitution has a negligible effect on Tc, but seems to suppress…
The non-equilibrium state of the high-Tc superconductor Bi2Sr2CaCu2O8+delta and its ultrafast dynamics have been investigated by femtosecond time- and angle-resolved photoemission spectroscopy well below the critical temperature. We probe optically excited quasiparticles at different electron momenta along the Fermi surface and detect metastable quasiparticles near the antinode. Their decay through e-e scattering is blocked by a phase space restricted to the nodal region. The lack of momentum dependence in the decay rates is in agreement with relaxation dominated by Cooper pair recombination i…
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.
Zero-field and transverse-field muon spin rotation/relaxation ($μ$SR) experiments were undertaken in order to elucidate microscopic properties of a strongly-coupled superconductor Mo$_{8}$Ga$_{41}$ with $T_{\text{c}}=9.8$ K. The upper critical field extracted from the transverse-field $μ$SR data exhibits significant reduction with respect to the data from thermodynamic measurements indicating the coexistence of two independent length scales in the superconducting state. Accordingly, the temperature-dependent magnetic penetration depth of Mo$_{8}$Ga$_{41}$ is described using the model, in which…
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 Ba substitution by Sr has been studied in two Hg-2212 series: Hg2(Ba(1-y)Sr(y))2YCu2O8-d and Hg2(Ba(1-y)Sr(y))2(Y0.80Ca0.20)Cu2O8-d. In both series a Tc enhancement of about 40 K is observed when Sr substitutes Ba from y = 0 to y = 1.0. The y = 0 compound of the first series is the non superconducting Hg2Ba2YCu2O8-d prototype. In the second series, this y = 0 compound is already superconducting at 21 K. Indeed the members of this series present a higher charge carrier density in their CuO2 superconducting planes than their homologues of the first series due to the doping introduced by the …
We present the effect of yttrium substitution on superconductivity in the La$_{1-\textit{x}}$Y$_{\textit{x}}$O$_{0.5}$F$_{0.5}$BiS$_{2}$ system. Polycrystalline samples with nominal Y concentrations up to 40% were synthesized and characterized via electrical resistivity, magnetic susceptibility, and specific heat measurements. Y substitution reduces the lattice parameter \textit{a} and unit cell volume \textit{V}, and a correlation between the lattice parameter \textit{c}, the La-O-La bond angle, and the superconducting critical temperature $T_c$ is observed. The chemical pressure induced by Y…
Taking cue from the increase in the superconducting transition temperature (Tc) of Fe1+xSe via nominal (2 wt%) substitution of Cr instead of excess Fe, we have now extended our study with nominal substitution (<=5 wt%) with other transition metals (Ni, Co, Fe, Mn, Cr, V and Ti) in place of excess iron. The Tc is found to increase (maximum ~11 K) or get suppressed depending on the substituted transition metal. Our studies indicate that the superconducting transition temperature depends on various parameters like the ionic size of the transition metal, its magnetic moment as well as the amount o…
We report effects of La substitution on superconducting state of heavy fermion superconductor CeCoIn5, as seen in transport and magnetization measurements. As opposed to the case of conventional superconductors, pair breaking by nonmagnetic La results in depression of Tc and indicates strong gap anisotropy. Upper critical field Hc2 values decrease with increased La concentration, but the critical field anisotropy, gamma=Hc2(a)/Hc2(c), does not change in the Ce_{1-x}La_xCoIn5 (x=0-0.15). The electronic system is in the clean limit for all values of x.
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.…
We present detailed heat capacity measurements for Ba(Fe(1-x)Cox)2As2 and Ba(Fe(1-x)Nix)2As2 single crystals in the vicinity of the superconducting transitions. The specific heat jump at the superconducting transition temperature (Tc), Delta Cp/Tc, changes by a factor ~ 10 across these series. The Delta Cp/T$ vs. Tc data of this work (together with the literature data for Ba(Fe0.939Co0.061)2As2, (Ba0.55K0.45)Fe2As2, and (Ba0.6K0.4)Fe2As2) scale well to a single log-log plot over two orders of magnitude in Delta Cp/Tc and over about an order of magnitude in Tc, giving Delta Cp/Tc ~ Tc^2.
We report the path from the charge density wave (CDW)-bearing superconductor CuIr2Te4 to the metal insulator transition (MIT)-bearing compound CuIr2S4 by chemical alloying with the gradual substitution of S for Te. The evolution of structural and physical properties of the CuIr2Te4-xSx polycrystalline system with the doping range from 0 to 4 is systemically examined. The X-ray diffraction (XRD) results imply CuIr2Te4-xSx in the range between 0 and 0.5 crystallizes in a NiAs defected trigonal structure, whereas it adapts to the cubic spinel structure in the doping range from 3.6 to 4 and it is …
We study the effects of C substitutions in MgB_2 within the two-band model in the Eliashberg formulation. We use as input the B-B stretching-mode frequency and the partial densities of states N_{sigma}(EF) and N_{pi}(EF), recently calculated for Mg(B_{1-x}C_{x})_2 at various x values from first-principles density functional methods. We then take the prefactor in the Coulomb pseudopotential matrix, mu, and the interband scattering parameter, Gamma^{sigma pi}, as the only adjustable parameters. The dependence on the C content of Tc and of the gaps (Delta_{sigma} and Delta_{pi}) recently measured…
Re-entrant unconventional superconductivity induced by rare-earth substitution in Nd1-xEuxNiO2 thin films
Yale University（耶鲁大学）；Max Planck Institute for the Structure and Dynamics of Matter（马克斯·普朗克物质结构动力学研究所）；National High Magnetic Field Laboratory（国家高磁场实验室）；Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）；Florida State University（佛罗里达州立大学）；University of Oxford（牛津大学）
Wiring systems powered by high-efficient superconductors have long been a dream of scientists, but researchers have faced practical challenges such as finding flexible materials. Here we report superconductivity in Nb2PdxS5-delta fibers with transition temperature up to 7.43 K, which have typical diameters of 0.3-3 micrometer. Superconductivity occurs in a wide range of Pd and S contents, suggesting that the superconductivity in this system is very robust. Long fibers with suitable size provide a new route to high-power transmission cables and electronic devices.
We report electronic Raman scattering measurements on optimally doped YBa2Cu3O7 where Zn or Ni impurities have been substituted by Cu. Using Raman selection rules, we have probed the superconducting gap in the nodal and antinodal regions. We show that under impurity substitutions, the energy of the antinodal peak detected in the superconducting state is not related to the critical temperature Tc and that signatures of superconductivity disappear in the nodal regions. Our experimental findings advocate in favor of gapless arcs around the nodes. The breakdown of the relationship between the anti…
Superconducting properties of hypothetical simple hexagonal CaB2 are studied using the fully anisotropic Eliashberg formalism based on electronic and phononic structures and electron-phonon interactions which are obtained from ab initio pseudopotential density functional calculations. The superconducting transition temperature Tc, the superconducting energy gap Delta(k) on the Fermi surface, and the specific heat are obtained and compared with corresponding properties of MgB2. Our results suggest that CaB2 will have a higher Tc and a stronger two-gap nature, with a larger Delta(k) in the sigma…
Photoemission spectra of Bi_2Sr_2CaCu_2O_{8+delta} below T_c show two features near the (pi,0) point of the zone: a sharp peak at low energy and a higher binding energy hump. We find that the sharp peak persists at low energy even as one moves towards (0,0), while the broad hump shows significant dispersion which correlates well with the normal state dispersion. We argue that these features are naturally explained by the mixing of electrons with a bosonic mode which appears only below T_c, and speculate that the latter may be related to the resonance seen in recent neutron data.
The variation of TC in MgB2 has been systematically investigated for monovalent cation substitution in Mg1-xLixB2 and in Mg1-xCuxB2 and simultaneous cation and Carbon substitution in Mg0.80Li0.20B2-xCx and Mg0.95Cu0.05B2-xCx using dc resistivity and ac susceptibility techniques. TC seems to be uniquely determined by the electron count in the sample, being constant at the MgB2 value for electron counts lower than MgB2 but rapidly decreasing for larger electron counts. The Debye temperature extracted from fitting normal state resistivity to the Bloch-Gruneisen formula shows no systematics with T…
We have carried out an investigation of the structural, magnetic, transport and superconducting properties of Fe$_{1+z}$Te$_{1-x-y}$Se$_x$S$_y$ ceramic compounds, for $z=0$ and some specific Se (0$\leq$ x $\leq$ 0.5) and S (0 $\leq$ y $\leq$0.12) contents. The incorporation of Se and S to the FeTe structure produces a progressive reduction of the crystallographic parameters as well as different degrees of structural disorder associated with the differences of the ionic radius of the substituting cations. In the present study, we measure transport properties of this family of compounds and we s…
The model that explains many of the details of superconducting and magnetic phase diagrams of YBa2Cu3O{6+/delta} and La2-xSrxCuO4 is presented. The model is based on the assumption of rigid localization of doped charges in the close vicinity of doped ion. This localization results in local variation of electronic structure of the parent charge-transfer insulator that depends on the local mutual arrangement of the doped charges. It is shown that in such system the negative-U centers (NUCs) may form under certain conditions on the pairs of neighboring Cu cations in CuO2 plane. We consider the me…
We investigate the electronic background as well as the O2-O3 mode at 330 cm^-1 of highly doped YbBa2Cu3O7-delta in B1g symmetry. Above the critical temperature Tc the spectra consist of an almost constant electronic background and superimposed phononic excitations. Below Tc the superconducting gap opens and the electronic background redistributes exhibiting a 2Delta peak at 320 cm^-1. We use a model that allows us to separate the background from the phonon. In this model the phonon intensity is assigned to the coupling of the phonon to inter- and intraband electronic excitations. For excitati…
Using finite size scaling we analyze specific heat and London penetration depth data to estimate the spatial extent of the superconducting grains in so called high-quality Bi2Sr2CaCu2O8+delta single crystals. Contrary to the previously investigated type II superconductors, YBa2Cu3O7+delta, MgB2,2H-NbSe2 and Nb77Zr23, where the lower bound for the length scale of the grains ranges from 182 A to 814 A, our analysis uncovers nanoscale superconducting grains. This clarifies the relevance of the spatial variations in the electronic characteristics observed in underdoped Bi2Sr2CaCu2O8+delta with sca…
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…
``Sub-meV'' resolution photoemission spectroscopy was used to study carbon-substitution dependence on the multiple superconducting gap of Mg(B$_{1 - x}$C$_{x})_{2}$. Two features corresponding to $σ$ and $π$ gaps are clearly observed in the raw spectra up to carbon concentration x = 7.5 {%}. The observed x dependence of the two gaps shows a qualitatively different behavior: a marked change of the $σ$ gap proportional to the $T_{c}$ variation and a negligible one of the $π$ gap. This as well as the temperature dependence can be explained with the two-band mean-field theory. Implications from th…
The 1111-type iron arsenide hydride CaFe1-xCoxAsH was synthesized by high-pressure solid-state reaction, and its electronic structure and superconducting properties were investigated. Bulk superconductivity was observed at x = 0.07-0.26. A maximum superconducting critical temperature Tcmax of 23 K was observed at x = 0.07. These values are in agreement with those of CaFe1-xCoxAsF. The calculated Fermi surface of CaFeAsH has a small three-dimensional 3D hole pocket around the Gamma point. This is a result of weak covalent bonding between the As 4p and H 1s orbitals. No such covalency exists in …
The superconducting transition temperature, T$_{C}$, the residual resistivity $ρ_{0}$ and the slope of resistivity curve at high temperature, d$ρ$/dT, have been measured in a series of MgB$_{2}$ samples that have been chemically substituted to varying degree with Li or Cu at the Mg-site and by Li or Cu at the Mg-site along with C substitution at the B-site. DC resistivity and ac susceptibility measurements were employed to extract the above parameters. T$_{C}$ versus the electron count (estimated from simple chemical valence count arguments) shows a universal behaviour, with T$_{C}$ being cons…
A detailed study of the effect caused by the partial substitution of Ru by Ir on the magnetic and superconducting properties of the ruthenocuprate Ru(1-x)Ir(x)Sr2GdCu2o8; 0 <= x <= 0.10; is presented. The combined experimental results of structural, electrical, and magnetic measurements indicate that Ir substitutes Ru for x <= 0.10 with no significant structural distortions. Ir-doping gradually suppresses both the magnetic and the superconducting states. However, all samples were observed to attain the zero-resistance state at temperatures >= 2 K up to the highest applied magnetic field of 18 …
We study the perpendicular transport characteristics of small superconductor/ferromagnetic insulator/superconductor (YBa$_2$Cu$_3$O$_{7-x}$/LaMnO$_{3+δ}$/YBa$_2$Cu$_3$O$_{7-x}$) tunnel junctions. At a large bias voltage $V\sim 1$ V we observe a step-like onset of excess current that occurs below the superconducting transition temperature $T<T_c$ and is easily suppressed by a magnetic field. The phenomenon is attributed to a novel type of the superconducting proximity effect of non-equilibrium electrons injected into the conduction band of the ferromagnetic insulator via a Fowler-Nordheim tunne…
We investigated carbon substitution effect on boron plane of superconducting MgB$_2$. MgB$_2$ and MgB$_{1.8}$C$_{0.2}$ samples are synthesized under high pressure furnace. MgB$_{1.8}$C$_{0.2}$ are characterized as AlB$_2$-type single phase with smaller B-B distance. During the superconducting transition, two distinct onset temperatures are observed in MgB$_{1.8}$C$_{0.2}$.
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 an experimental study of the effect of Mn impurities in the optimally doped LaFeAsO$_{0.89}$F$_{0.11}$ compound. The results show that a very tiny amount of Mn, of the order of 0.1\%, is enough to destroy superconductivity and to recover at low temperatures both the magnetic ground state and the orthorhombic structure of the pristine LaFeAsO parent compound. The results are discussed within a model where electron correlations enhance the Ruderman-Kittel-Kasuya-Yosida interaction among impurities.
In cuprate superconductors, due to strong electronic correlations, there are multiple intertwined orders which either coexist or compete with superconductivity. Among them the antiferromagnetic (AF) order is the most prominent one. In the region where superconductivity sets in, the long-range AF order is destroyed. Yet the residual short-range AF fluctuations are present up to a much higher doping and their role in the emergence of the superconducting phase is still highly debated. Here, by using a spin polarized scanning tunneling microscope, for the first time, we directly visualize an emerg…
2H-NbSe2 is one of the most widely researched transition metal dichalcogenide (TMD) superconductors, which undergoes charge-density wave (CDW) transition at TCDW about 33 K and superconducting transition at Tc of 7.3 K. To explore the relation between its superconductivity and Fermi surface nesting, we combined S substitution with Cu intercalation in 2H-NbSe2 to make CuxNbSe2-ySy. Upon systematic substitution of S and intercalation of Cu ions into 2H-NbSe2, we found that when the Cu and S contents increases, the Tc decreases in CuxNbSe2-ySy. While at higher x and y values, Tc keeps a constant …
The effect of Sr chemical pressure on superconductivity was investigated in Hg2(Ba1-ySry)2YCu2O8-d. The samples were synthesized at high pressure-high temperature from y = 0.0 to full substitution, y = 1.0. These Sr-substituted compounds are superconducting, without Ca doping on the Y site, and show an increasing Tc with Sr, reaching 42 K for y = 1.0. A detailed neutron powder diffraction study compares the structural changes induced by this chemical Sr/Ba substitution and the mechanical pressure effects in the Hg-2212 system. It shows the strong decrease of the three Ba/Sr-O distances and con…
High-resolution angle-resolved photoemission measurements were taken on FeSe$_{1-x}$S$_x$ (x=0, 0.04, and 0.08) superconductors. With an ultrahigh energy resolution of 0.4 meV, unusual two hole bands near the Brillouin-zone center, which was possibly a result of additional symmetry breaking, were identified in all the sulfur-substituted samples. In addition, in both of the hole bands highly anisotropic superconducting gaps with resolution limited nodes were evidenced. We find that the larger superconducting gap on the outer hole band is reduced linearly to the nematic transition temperature wh…
We have investigated the Pb-substitution effect on the superconductivity of NaCl-type In1-xPbxTe samples obtained using high-pressure synthesis. Polycrystalline samples with x = 0-0.8 were obtained, and the lattice parameter systematically increased by Pb substitution. The superconducting transition temperature (Tc ~3 K for x = 0) increased to 5.3 K (for x = 0.4) by Pb substitutions. According to the comparison of the electronic states between NaCl-type InTe and PbTe, we consider that the Pb substitution increases electron carriers. Possible mechanisms of the enhancement of Tc in this system h…
We report on the c-axis resistivity rho_c(H) in Bi_2Sr_2CaCu_2O_{8+δ} that peaks in quasi-static magnetic fields up to 60 T. By suppressing the Josephson part of the two-channel (Cooper pair/quasiparticle) conductivity σ_c (H), we find that the negative slope of ρ_c(H) above the peak is due to quasiparticle tunneling conductivity σ_q(H) across the CuO_2 layers below H_{c2}. At high fields (a) σ_q(H) grows linearly with H, and (b) ρ_c(T) tends to saturate (sigma_c \neq 0) as T->0, consistent with the scattering at the nodes of the d-gap. A superlinear sigma_q(H) marks the normal state above T_c…
We present low-temperature thermal conductivity measurements on the cuprate Tl_2Ba_2CuO_{6+delta} throughout the overdoped regime. In the T -> 0 limit, the thermal conductivity due to d-wave nodal quasiparticles provides a bulk measurement of the superconducting gap, Delta. We find Delta to decrease with increasing doping, with a magnitude consistent with spectroscopic measurements (photoemission and tunneling). This argues for a pure and simple d-wave superconducting state in the overdoped region of the phase diagram, which appears to extend into the underdoped regime down to a hole concentra…
A complete knowledge of its excitation spectrum could greatly benefit efforts to understand the unusual form of superconductivity occurring in the lightly hole-doped copper-oxides. Here we use tunnelling spectroscopy to measure the T\to 0 spectrum of electronic excitations N(E) over a wide range of hole-density p in superconducting Bi_{2}Sr_{2}CaCu_{2}O_{8+/delta}. We introduce a parameterization for N(E) based upon an anisotropic energy-gap /Delta (\vec k)=/Delta_{1}(Cos(k_{x})-Cos(k_{y}))/2 plus an effective scattering rate which varies linearly with energy /Gamma_{2}(E) . We demonstrate tha…
