We report a Cu K- and L$_3$-edge resonant inelastic x-ray scattering study of charge and spin excitations of bulk Nd$_{2-x}$Ce$_x$CuO$_4$, with focus on post-growth annealing effects. For the parent compound Nd$_2$CuO$_4$ ($x = 0$), a clear charge-transfer gap is observed in the as-grown state, whereas the charge excitation spectra indicate that electrons are doped in the annealed state. This is consistent with the observation that annealed thin-film and polycrystalline samples of RE$_2$CuO$_4$ (RE = rare earth) can become metallic and superconducting at sufficiently high electron concentrations without Ce doping. For $x = 0.16$, a Ce concentration for which it is known that oxygen reduction destroys long-range antiferromagnetic order and induces superconductivity, we find that the high-energy spin excitations of non-superconducting as-grown and superconducting annealed crystals are nearly identical. This finding is in stark contrast to the significant changes in the low-energy spin excitations previously observed via neutron scattering.
We performed neutron powder diffraction measurements on the as-sintered (AS) and oxygen-reduced (OR) La$_{1.8}$Eu$_{0.2}$CuO$_{4+α-δ}$ (LECO). The structural parameters for oxygens in AS and OR samples refined by the Rietveld analysis are almost identical to those of the reference system, Pr$_{2}$CuO$_{4+α-δ}$. Thus, the two systems are comparable in terms of the structural changes in oxygens due to the annealing, although superconductivity appears only in LECO.
We performed Cu {\it K}-edge X-ray absorption fine structure measurements on T'-type La$_{1.8}$Eu$_{0.2}$CuO$_4$ (LECO) and Nd$_2$CuO$_4$ (NCO) to investigate the variation in the electronic state associated with the emergence of superconductivity due to annealing. The X-ray absorption near-edge structure spectra of as-sintered (AS) LECO are quite similar to those of AS NCO, indicating that the ground state of AS T'-type LECO is a Mott insulator. We found a significant variation of the electronic state at the Cu sites in LECO due to annealing. The electron density after annealing ($n_{\rm AN}$…
Electron-doped and hole-doped superconducting cuprates exhibit a symmetric phase diagram as a function of doping. This symmetry is however only approximate. Indeed, electron-doped cuprates become superconductors only after a specific annealing process: This annealing affects the oxygen content by only a tiny amount, but has a dramatic impact on the electronic properties of the sample. Here we report the occurrence of superconductivity in oxygen-deficient Nd$_{2-x}$Ce$_x$CuO$_4$ thin films grown in an oxygen-free environment, after annealing in pure argon flow. As verified by x-ray diffraction,…
The evolution of the electronic state through the reduction annealing has been investigated in electron-doped Pr_1.3-x_La_0.7_Ce_x_CuO_4+delta_ (x=0.10) single crystals with the so-called T' structure. From the ab plane and c axis electrical resistivity measurements in magnetic fields, it has been found that, through the reduction annealing, the strongly localized state of carriers accompanied by the antiferromagnetic (AF) pseudogap in the as-grown crystal changes to a metallic state bringing about the Kondo effect without AF pseudogap and to a superconducting state. These results are able to …
In order to realize superconductivity in cuprates with the T'-type structure, not only chemical substitution (Ce doping) but also post-growth reduction annealing is necessary. In the case of thin films, however, well-designed reduction annealing alone without Ce doping can induce superconductivity in the T'-type cuprates. In order to unveil the origin of superconductivity in the Ce-undoped T'-type cuprates, we have performed bulk-sensitive hard x-ray photoemission and soft x-ray absorption spectroscopies on superconducting and non-superconducting Nd$_{2-x}$Ce$_x$CuO$_4$ ($x=$ 0, 0.15, and 0.19…
We investigated Ce-substitution and reduction annealing effects on the electronic states at copper sites by Cu ${K}$-edge x-ray absorption near-edge structure measurements in Pr$_{2-x}$Ce$_x$CuO$_{4+α-δ}$ (PCCO) with varying $x$ and $δ$ (the amount of oxygen loss during annealing) values. Absorption near-edge spectra were modified by Ce-substitution and reduction annealing in a similar manner with increasing $x$ and $δ$. Considering electron doping by Ce-substitution, this similarity indicates an increase of electron number at the copper sites due to annealing $n_{\rm AN}$. Thus, the total num…
Oxygen $K$-edge X-ray absorption spectroscopy measurements were conducted on T*-type La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$CuO$_4$ (LESCO) to estimate the hole density ($n_{\rm h}$) and investigate the oxidation annealing effect on $n_{\rm h}$. A drastic increase in $n_{\rm h}$ due to annealing was found. The increase in $n_{\rm h}$ cannot be explained solely by the oxygen gain due to annealing, suggesting that delocalized holes are introduced into the CuO$_2$ plane. A phase diagram of LESCO was redrawn against $n_{\rm h}$.
Bulk crystals of electron-doped cuprates with the T'-type structure require both Ce substitutions and reduction annealing for the emergence of superconductivity while the reduction annealing alone can induce superconductivity in thin films of the T'-type cuprates. In order to reveal low-energy electronic states which are responsible for the superconductivity, we have conducted angle-resolved photoemission spectroscopy measurements on thin films of the superconducting Ce-free T'-type cuprate Pr$_2$CuO$_4$. The results indicate that the overall band structure and the Fermi surface area of the su…
For electron-doped cuprates, the strong suppression of antiferromagnetic spin correlation by efficient reduction annealing by the "protect-annealing" method leads to superconductivity not only with lower Ce concentrations but also with higher transition temperatures. To reveal the nature of this superconducting state, we have performed angle-resolved photoemission spectroscopy measurements of protect-annealed electron-doped superconductors Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_{4}$ and directly investigated the superconducting gap. The gap was found to be consistent with $d$-wave symmetry, sugges…
Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$ (0$\leq x \leq$1.00) single crystals with high-quality have been grown successfully using the travelling-solvent floating-zone technique. The patterns of X-ray diffraction suggest high crystalline quality of the samples. After post-annealing in flowing oxygen at 600 $^o$C, the crystals show sharp superconducting transitions revealed by AC susceptibility. The hole concentration $p$ is deduced from superconducting transition temperature ($T_c$), which exhibits a linear relation with La doping level $x$. It ranges from the heavily overdoped regime ($p \approx$ 0.…
We investigated the magnetism and superconductivity in as-sintered (AS) and oxidation annealed (OA) T*-type La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$CuO$_4$ (LESCO) with 0.14 $\leq x \leq$ 0.28 by the first comprehensive muon spin rotation/relaxation ($μ$SR), magnetic susceptibility, and electrical resistivity measurements. In OA superconducting samples, no evidence of magnetic order was observed, whereas AS semiconducting samples exhibited evidence of a disordered magnetic state in the measured temperature range between $\sim$4 K and $\sim$8 K. Therefore, the ground state in LESCO drastically varies wit…
High quality electron-doped HTSC single crystals of $\rm Pr_{2-x}Ce_{x}CuO_{4+δ}$ and $\rm Nd_{2-x}Ce_{x}CuO_{4+δ}$ have been successfully grown by the container-free traveling solvent floating zone technique. The optimally doped $\rm Pr_{2-x}Ce_{x}CuO_{4+δ}$ and $\rm Nd_{2-x}Ce_{x}CuO_{4+δ}$ crystals have transition temperatures $T_{\rm c}$ of $25$\,K and $23.5$\,K, respectively, with a transition width of less than $1$\,K. We found a strong dependence of the optimal growth parameters on the Ce content $x$. We discuss the optimization of the post-growth annealing treatment of the samples, the…
We have performed Compton scattering measurements on as-grown and reduced single crystals of the electron-doped T'-cuprate Pr1.3-xLa0.7CexCuO4 (x = 0.10) to investigate the effect of reduction annealing on the electronic state. The obtained results have revealed that the numbers of electrons in the O2p orbital in the Zhang-Rice singlet band, the Cu3dx2-y2 orbital, and the Cu3d3z2-r2 orbital are increased by reduction annealing. The increase in the number of Cu3d3z2-r2 electrons suggests the existence of Cu3d3z2-r2 holes in the as-grown single crystal. This is spectroscopic evidence of local ho…
The superconducting nominal "non-doped" $La_{1.85}Y_{0.15}CuO_4$ (LYCO) thin films are successfully prepared by dc magnetron-sputtering and in situ post-annealing in vacuum. The best $T_{C0}$ more than 13K is achieved in the optimal LYCO films with highly pure c-axis oriented T'-type structure. In the normal state, the quasi-quadratic temperature dependence of resistivity, the negative Hall coefficient and effect of oxygen content in the films are quite similar to the typical Ce-doped T'-214 cuprates, suggesting that T'-LYCO shows the electron-doping nature like known n-type cuprates, and is n…
Samples of YBa2-xSrxCu3O7-d (x = 0.0, 0.50, 1.0) series were synthesized by solid-state reaction and first annealed in flowing O2 gas. All samples crystallized in orthorhombic structure, where orthorhombic distortion was found to decrease with an increase in x. Four probe resistivity measurements showed that superconducting transition temperature (Tc) decreases monotonically with x at a rate of dTc/dx = 1.3 K/at%. The sample with x=0 had a Tc = 91 K. The samples were further annealed in flowing N2 gas at 450 0C. The Tc of all samples decreased by nitrogen annealing. Interestingly, the decrease…
Inhomogeneous reduction-annealing effects on the electron-doped cuprate superconductor revealed by micro-focused angle-resolved photoemission spectroscopy
We use thermodynamic and neutron scattering measurements to study the effect of oxygen annealing on the superconductivity and magnetism in Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-δ}$. Although the transition temperature $T_c$ measured by susceptibility and superconducting coherence length increase smoothly with gradual oxygen removal from the annealing process, bulk superconductivity, marked by a specific heat anomaly at $T_c$ and the presence of a neutron magnetic resonance, only appears abruptly when $T_c$ is close to the largest value. These results suggest that the effect of oxygen annealing must …
Post-annealing effects on the crystal structure and superconductivity of the lithium- and hexamethylenediamine (HMDA)-intercalated superconductor Lix(C6H16N2)yFe2-zSe2 have been investigated. Through the post-annealing, a two-step reduction of the interlayer spacing between neighboring Fe layers, d, has been observed. It has been found that a new phase of Lix(C6H16N2)yFe2-zSe2 with d= 10.30(2) Å and Tc = 41 K different from the as-intercalated phase is stabilized owing to the possible stable inclination of HMDA intercalated between FeSe layers. This result supports the domic relation between T…
In order to clarify the origin of anomalous superconductivity in (Ca,RE)Fe2As2 system, Pr doped and Pr,Co co-doped CaFe2As2 single crystals were grown by the FeAs flux method. These samples showed two-step superconducting transition with Tc1 = 25~42 K, and Tc2 < 16 K, suggesting that (Ca,RE)Fe2As2 system has two superconducting components. Post-annealing performed for these crystals in evacuated quartz ampoules at various temperatures revealed that post-annealing at ~400°C increased the c-axis length for all samples. This indicates that as-grown crystals have a certain level of strain, which i…
We investigated the annealing effects on the physical properties of SrFe$_2$(As$_{1-x}$P$_x$)$_2$ ($x=0.35$). The superconducting transition temperature ($T_c$) increased from 26 K to 33 K by annealing. The X-ray diffraction measurement suggested that the annealed crystals have the shorter/longer $a/c$-axes and the larger pnictogen height $h_\mathrm{Pn}$. This must be linked to the $T_c$-enhancement by annealing. Moreover, it was found that the post-annealing decreased the electronic specific heat coefficient at $T$=0 K, $γ_r$, and changed the magnetic field ($H$) dependence from sub-linear $γ…
Here, we report the synthesis and characterization of sulphur-substituted iron telluride i.e. FeTe1-xSx; (x = 0-30 %) system and study the impact of low temperature oxygen (O2) annealing as well. Rietveld analysis of room temperature x-ray diffraction (XRD) patterns shows that all the compounds are crystallized in a tetragonal structure (space group P4/nmm) and no secondary phases are observed. Lattice constants are decreased with increasing S concentration. The parent compound of the system i.e. FeTe does not exhibit superconductivity but shows an anomaly in the resistivity measurement at aro…
New iron-based intercalation superconductors Ax(C2H8N2)yFe2-zSe2 (A = Li, Na) with Tc = 45 K have successfully been synthesized via intercalation of dissolved alkaline metal in ethylenediamine. The c-axis lengths of Ax(C2H8N2)yFe2-zSe2 (A = Li, Na) are 20.74(7) Å and 21.9(1) Å, respectively, and are about 50 % larger than that of KxFe2Se2, indicating that not only alkaline metal but also ethylenediamine is intercalated between the Se-Se layers of FeSe. It seems that the high-Tc of Ax(C2H8N2)yFe2-zSe2 (A = Li, Na) is caused by the possible two-dimensional electronic structure due to the large c…
MgB2 thin films were cold-grown on sapphire substrates by pulsed laser deposition (PLD), followed by post-annealing in mixed, reducing gas, Mg-rich, Zr gettered, environments. The films had Tcs in the range 29 K to 34 K, Jcs (20K, H=0) in the range 30 kA/cm2 to 300 kA/cm2, and irreversibility fields at 20 K of 4 T to 6.2 T. An inverse correlation was found between Tc and irreversibility field. The films had grain sizes of 0.1-1 micron and a strong biaxial alignment was observed in the 950C annealed film.
We report the influence of 6 MeV Zr$^{4+}$ irradiation and post-irradiation annealing (200 °C) in the in-field dependences of the critical current densities J$_c$ of 1.3 $μ$m thick GdBa$_2$Cu$_3$O$_{7-d}$ coated conductors grown by co-evaporation. Samples were irradiated with 6 MeV Zr$^{4+}$ and fluences between 2.3x10$^{11}$ cm$^{-2}$ and 3x10$^{12}$ cm$^{-2}$. The correlation between the superconducting critical temperature T$_c$ and in-field dependences of critical current densities J$_c$ has been analyzed. In addition, random disorder introduced by irradiation was reduced by thermal anneal…
We report the effect of annealing on the superconductivity of MgB2 thin films as functions of the postannealing temperature in the range from 700 C to 950 C and of the postannealing time in the range from 30 min to 120 min. On annealing at 900 C for 30 min, we obtained the best-quality MgB2 films with a transition temperature of 39 K and a critical current density of ~ 10^7 A/cm^2. Using the scanning electron microscopy, we also investigated the film growth mechanism. The samples annealed at higher temperatures showed the larger grain sizes, well-aligned crystal structures with preferential or…
Since the discovery of superconductivity at 26 K in oxy-pnictide LaFeAsO1-xFx, enormous interests have been stimulated in the field of condensed matter physics and material sciences. Among the many kind of structures in the iron pnictide superconductors, FeSe with the PbO structure has received special attention since there is not poisonous pnictogen element in chemical composition and its structure is the simplest one. However, the superconducting transition temperature (Tc) in iron chalcogenide compounds is not enhanced as high as other iron pnictide superconductors under ambient pressure un…
We have measured the effect of a c-axis aligned magnetic field on the long-range magnetic order of insulating Nd2CuO4, as-grown nonsuperconducting and superconducting Nd1.85Ce0.15CuO4. On cooling from room temperature, Nd2CuO4 goes through a series of antiferromagnetic (AF) phase transitions with different noncollinear spin structures. In all phases of Nd2CuO4, we find that the applied c-axis field induces a canting of the AF order but does not alter the basic zero-field noncollinear spin structures. Similar behavior is also found in as-grown nonsuperconducting Nd1.85Ce0.15CuO4. These results …
We present comprehensive neutron scattering studies of nonsuperconducting and superconducting electron-doped Pr0.88LaCe0.12CuO4(PLCCO). At zero field, the transition from antiferromagnetic (AF) as-grown PLCCO to superconductivity without static antiferromagnetism can be achieved by annealing the sample in pure Ar at different temperatures, which also induces an epitaxial (Pr,La,Ce)2O3 phase as an impurity. When the superconductivity first appears in PLCCO, a quasi-two-dimensional (2D) spin-density-wave (SDW) order is also induced, and both coexist with the residual three-dimensional (3D) AF st…
Nb3Sn and V3Si thin films are promising candidates as thin films for the next generation of superconducting radio-frequency (SRF) cavities. However, sputtered films often suffer from stoichiometry and strain issues during deposition and post annealing. In this study, we explore the structural and chemical effects of thermal annealing, both in-situ and post-sputtering, on DC-sputtered Nb3Sn and V3Si films of varying thickness on Nb or Cu substrates, extending from our initial studies [1]. Through annealing at 950 °C, we successfully enabled recrystallization of 100 nm thin Nb3Sn films on Nb sub…
We have performed a systematic study of the evolution of the superconducting and normal state properties of neutron irradiated MgB$_2$ wire segments as a function of fluence and post exposure annealing temperature and time. All fluences used suppressed the transition temperature, Tc, below 5 K and expanded the unit cell. For each annealing temperature Tc recovers with annealing time and the upper critical field, Hc2(T=0), approximately scales with Tc. By judicious choice of fluence, annealing temperature and time, the Tc of damaged MgB2 can be tuned to virtually any value between 5 and 39 K. F…
Large crystals of La$_{2-x}$Ca$_{1+x}$Cu$_2$O$_{6}$ (La-Ca-2126) with $x=0.10$ and 0.15 have been grown and converted to bulk superconductors by high-pressure oxygen annealing. The superconducting transition temperature, $T_c$, is as high as 55~K; this can be raised to 60~K by post-annealing in air. Here we present structural and magnetic characterizations of these crystals using neutron scattering and muon spin rotation techniques. While the as-grown, non-superconducting crystals are single phase, we find that the superconducting crystals contain 3 phases forming coherent domains stacked alon…
The effect of thermal annealing on the properties of Al-AlOx-Al single electron tunneling transistors is reported. After treatment of the devices by annealing processes in forming gas atmosphere at different temperatures and for different times, distinct and reproducible changes of their resistance and capacitance values were found. According to the temperature regime, we observed different behaviors as regards the resistance changes, namely the tendency to decrease the resistance by annealing at T = 200 degree C, but to increase the resistance by annealing at T = 400 degree C. We attribute th…
We have studied the evolution of superconducting and normal state properties of neutron irradiated Mg(B$_{.962}$C$_{.038}$)$_2$ wire segments as a function of post exposure annealing time and temperature. The initial fluence fully suppressed superconductivity and resulted in an anisotropic expansion of the unit cell. Superconductivity was restored by post-exposure annealing. The upper critical field, H$_{c2}$(T=0), approximately scales with T$_c$ starting with an undamaged T$_c$ near 37 K and H$_{c2}$(T=0) near 32 T. Up to an annealing temperature of 400 $^ o$C the recovery of T$_c$ tends to c…
As superconducting quantum circuits scale to larger sizes, the problem of frequency crowding proves a formidable task. Here we present a solution for this problem in fixed-frequency qubit architectures. By systematically adjusting qubit frequencies post-fabrication, we show a nearly ten-fold improvement in the precision of setting qubit frequencies. To assess scalability, we identify the types of 'frequency collisions' that will impair a transmon qubit and cross-resonance gate architecture. Using statistical modeling, we compute the probability of evading all such conditions, as a function of …
Superconducting MgB2 thin films were prepared on Al2O3(0001) and MgO(100) substrates. Boron thin films were deposited by the electron-beam evaporation followed by post-annealing process with magnesium. Proper post annealing conditions were investigated to grow good superconducting MgB2 thin films. The X-ray diffraction patterns showed randomly orientated growth of MgB2 phase in our thin films. The surface morphology was examined by scanning electron microscope (SEM) and atomic force microscope (AFM). Critical current density (Jc) measured by transport method was about 10^7 A/cm^2 at 15 K, and …
Critical temperature Tc of the Li2(Pd1-xPtx)3B was reported to be 7-8K for x=0 and 2.2-2.8K for x=1. In this article we present our preliminary results on behavior of magnetization-temperature curves with starting composition of Pd-B precursor, y-Li concentration in LiyPd3B and post-annealing of the Pd-end compound. Results suggest that to maximize Tc ratio Pd:B should be close to 3:1, while y-Li has to be optimum. The lowest Tc for LiyPd3B was 4.4-4.6K, while post-annealings at 560 deg. C allowed enhancement of Tc up to 8.2-8.4K. Compositions Li2Z3B with Z=Ni, Ru, Rh, Re, Ag are not supercond…
