Session X9: Superconductivity: Electronic Transport Measurements of Copper Oxide Superconductors

Quantum superconductor-to-insulator transition in 2D $Y_{1-x}Ca_xBa_2Cu_3O_{7-\delta}$ films

This study investigates the relationship between the critical temperature, $T_C$, and the zero temperature superfluid density, $n_s(0)$, close to the quantum superconductor-to- insulator transition in 2D cuprate films. We use a two-coil technique, at frequencies up to 50 kHz, to measure the temperature dependence of superfluid density in severely underdoped $Y_{1-x}Ca_xBa_2Cu_3O_{7-\delta}$ films, as thin as 1-2 unit cells and with transition temperatures as low as 3 K. Superconducting films are grown by pulsed laser deposition on $SrTiO_3$ substrates, with thin insulating $PrBa_2Cu_3O_{7- \delta}$ protecting the film above and below. The zero temperature superfluid density in these films is comparable to values measured in thicker $Y_{1-x}Ca_xBa_2Cu_3O_{7-\delta}$ films, which suggests continuous superconducting layers. In 1-2 unit cell thick films, we find $T_C \propto n_s(0)$, which is expected in 2D. This result is different from the approximate $T_c \propto n_s(0)^{1/2}$ dependence previously reported in $YBa_2Cu_3O_{7-\delta}$ films and crystals and confirmed by our measurements on thicker $Y_{1-x}Ca_xBa_2Cu_3O_ {7-\delta}$ films.   

Exploring the critical dynamic exponent using YBCO films and untwinned crystal in zero and non-zero magnetic field

The phase transition in high Tc superconductors in zero and non-zero magnetic field has been intensely studied. However, there are debates on the critical dynamic exponent z from both simulations and experiments. We will report our result on z from our dc transport measurement in both YBCO films and untwinned crystals. Our results show that the finite size effects, which can cause misinterpretation in the film data, are absent in the crystal data, and the data on both the films and crystals give a very similar result for the critical dynamic exponent. (This work was supported by NSF grant number DMR-0302596)   

Upper critical field in electron-doped Pr$_{2-x}$Ce$_{x}$CuO$_{4-\delta }$ in parallel magnetic fields

We report a comprehensive study of the resistive superconducting transition in the electron-doped Pr$_{2-x}$Ce$_{x}$CuO$_{4-\delta }$ films down to 1.5K for magnetic field up to 58T applied parallel to the conducting ab-planes. We find that the parallel critical field (H$_{c2//ab})$ exceeds 58T for underdoped and optimally doped films. For the overdoped films, 58T is sufficient to suppress the superconductivity. An H$_{c2//ab}$ -T phase diagram is established. A comparison between our experimental results and theories for orbital and spin pairbreaking effects will be presented.   

$T_c$ suppression and resistivity in cuprates with out-of-plane defects

The suppression of the critical temperature due to isotropic impurity scattering in a $d$-wave superconductor is expected to be described by the Abrikosov-Gor'kov formula. However recent experiments on cuprate superconductors with out-of-plane defects show a nearly linear decrease of the critical temperature as a function of the residual resistivity in contradiction to the Abrikosov-Gor'kov result. Other experiments suggest further that the $T_c$ suppression is stronger for out-of-plane than for in-plane impurities. Both results can be explained assuming that elastic forward scattering is the dominant scattering process for out-of-plane disorder. We present a simple model of forward scattering allowing an analytical solution that already includes the key features of $T_c$ suppression for out-of-plane defects. We also verify the intuitive result of this simple model by comparing it to numerical calculations assuming randomly distributed out- of-plane defects and a realistic band structure appropriate for YBCO.   

c-axis magnetotransport and noise in underdoped La$_{2-x}$Sr$_x$CuO$_4$

We report a study of c-axis magnetotransport and noise on high quality single crystals of La$_{2-x}$Sr$_x$CuO$_4$ ($x=0.03$). The measurements were performed at temperatures 0.110$\leq T$(K) $\leq 50$ and fields $0\leq B$(T)$\leq 18$ parallel and perpendicular to the c axis. Our experiments have revealed for the first time a number of glassy features in the charge response at very low $T$, such as memory effects and history dependence. In the same $T$ range, we have observed positive magnetoresistance (MR), which exhibits hysteretic behavior. The hysteretic effects decrease with increasing $T$ and vanish at $\sim1.5$~K. The crossover from positive to negative MR takes place at higher $T$ and $B$. We have also observed switching fluctuations in the time-dependent resistance at the lowest $T$, with switching times varying from several minutes to several hours. The possible origins of the observed glassiness will be discussed.   

Pulsed laser deposition growth and transport studies of superconducting La$_{1.85}$Y$_{0.15}$CuO$_{4}$ thin films

The recent MBE growth of the ``nominally undoped" cuprate superconductor (La,RE)$_{2}$CuO$_{4}$ has led to the speculation that this system is a band metal and not a ``doped Mott insulator" as found in all other superconducting cuprates $^1$. Here we report the first pulsed laser deposition (PLD) growth of insulating and superconducting La$_{1.85}$Y$_{0.15} $CuO$_{4}$ thin films, which are prepared under different oxygen conditions. We also report resistivity, Hall, Nernst, and magnetoresistance measurements, which show that La$_{1.85}$Y$_{0.15}$CuO$_{4}$ is an electron-doped, Mott-Hubbard system, where the carriers originate from oxygen nonstoichiometry produced by the oxygen reduction. This work is supported by NSF (DMR 0352735).\\ $^1$A. Tsukada et al., Sol. Stat. Commu. 133, 427 (2005).   

Superconducting Phase Diagram and Vortex-glass Scaling of the Electron-Doped Superconductor Sm$_{2-x}$Ce$_x$CuO$_{4-y}$

We report the growth and characterization of thin films of the electron-doped superconductor Sm$_{2-x}$Ce$_x$CuO$_{4-y}$ over a wide doping range 0.13 $\leq$ x $\leq$ 0.19. The shape of the superconducting dome is similar to other electron-doped compounds, such as Nd$_{2-x}$Ce$_x$CuO$_{4-y}$ and Pr$_{2-x}$Ce$_x$CuO$_{4-y}$, and shows a peak in the superconducting transition temperature T$_c$ at a doping level x $\approx$ 0.15. Magnetoresistance data $\rho$(H, T) in fields up to 17 T reveal a metal to insulator transition in the underdoped region. Analysis of Hall effect measurements from the underdoped to overdoped regime is presented along with vortex-glass (VG) scaling analysis of the transport measurements. VG scaling shows no change in the dynamical vortex behavior from the underdoped to overdoped regions. This research was sponsored by the DOE under Grant No. DE-FG02-04ER46105 and the CULAR program no. 9985-001. A portion of this work was performed at the National High Magnetic Field Laboratory (NHMFL), which is supported by NSF Cooperative Agreement No. DMR-0084173, by the State of Florida, and by the DOE.   

Metal-insulator transition in YBa$_2$Cu$_3$O$_{\delta}$

The non-superconducting ground state of the underdoped cuprates YBa$_2$Cu$_2$O$_{\delta}$ and YBa$_2$Cu$_4$O$_8$ was examined by measuring the electrical resistivity of high-quality single crystals in magnetic fields up to 60 T. A metal-insulator-like crossover is observed near a critical doping $p$~=~0.1, i.e., far in the underdoped region. This shows that the pseudogap phase present below $T^{\star}$ is a metal, characterized by a resistivity of the Fermi-liquid form, $\rho_0 + AT^2$, where $\rho_0$ is small, just as in the strongly overdoped regime. In the metallic phase, the transverse magneto-resistance undergoes a qualitative change between $p$~=~0.14, where it saturates at high field, and $p$~=~0.11, where it does not saturate. This would suggest that a change in the nature of the ground state occurs in the vicinity of $p$~=~1/8.   

The Effect Of hBN addition on properties of Bi-2223 superconductors.

The effect of the addition of hBN (x = 0.005, 0.015, 0.025, 0.035 and 0.05 g.) in the \textit{Bi-2223} superconducting samples have been investigated in terms of magnetoresistivity, T$_{c}$, $J_{c}^{trans}$, and ac susceptibility. The samples were prepared by the conventional solid-state reaction method. We estimated the transition temperature, the activation energy values of the samples from the resistivity vs. temperature measurements in dc magnetic fields up to 0.7 T. We observed that transition temperature, $T_{c}$, and transport critical current density, $J_{c}^{trans}$, depend on the hBN addition. They both decrease with increasing x up to 0.025 g. With further increasing x up to 0.035 g. Activation energy, $U_{0},$ is calculated from resistivity versus temperature at various magnetic fields. It is observed that $U_{0}$ depend on both the hBN content of samples and the applied magnetic field. The ac susceptibility measurements were carried out at different values of the ac magnetic field amplitudes up to 555 A/m. The imaginary part of ac susceptibility measurements is used to calculate J$_{c}$ using the Bean's Model. The intergranular critical current density is also seen to vary with increasing hBN content. We extracted the peak temperature, $T_{p}$, and the pinning force density from our previous ac susceptibility measurements. The pinning force density varied with increasing the hBN content of samples.   

Investigation of some physical properties of gold diffusion-doped Bi$_{1.8}$Pb$_{0.35}$Sr$_{1.9}$Ca$_{2.1}$Cu$_{3}$O$_{y}$ superconductors

We have investigated the effect of the gold-diffusion doped Bi-2223 superconducting samples on the crystal structure and superconducting properties using XRD, SEM, T$_{c}$, J$_{c}$, and room temperature resistivity measurements. Doping of Bi-2223 was carried out by means of gold diffusion from an evaporated onto pellets at the sintering. XRD patterns and SEM micrographs are used to obtain information about Bi-2223 phase ratio, lattice parameters calculations and grain size, respectively. The gold diffusion in the Bi$_{1.8}$Pb$_{0.35}$Sr$_{1.9}$Ca$_{2.1}$Cu$_{3}$O$_{y}$ has been also studied over the temperature range 500-800 $^{^{\circ}}$C$_{ }$using the technique of successive removal of thin layers and measurement of the sample resistivity at room temperature. The temperature dependence of the Au diffusion coefficient in the range 500-800 $^{^{\circ}}$C was described by the relation D=4x10$^{-4}$exp(- 1.08 eV/k$_{B}$T). For the Au-doped sample, the critical transition temperature and J$_{c}$ were increased from 100$\pm $1 K to 104$\pm $1 K and from 40 and 125 A cm$^{-2}$, respectively, in comparison with those of undoped Bi-2223. The possible reasons for the observed increases in critical temperature and critical current density due to Au diffusion were discussed.   

The effect of \textit{Sm} substitution on properties of Bi$_{1.6}$Pb$_{0.4}$Sr$_{2}$Ca$_{2-x}$Sm$_{x}$Cu$_{3}$O$_{y}$ superconductors.

The effect of the partial substitution of \textit{Ca} by \textit{Sm} in the \textit{Bi-2223} superconducting samples have been investigated in terms of XRD, magnetoresistivity, critical temperature, transport critical current density, and ac susceptibility. The samples were prepared by the conventional solid-state reaction method. XRD patterns are used to calculate lattice parameters and obtain about Bi-2223 phase ratio. The volume fraction was determined from the intensities of Bi-2223 and Bi-2212 peaks. The room temperature XRD patterns of the samples showed the presence of Bi-2223 phase decreases with increasing the \textit{Sm} content. We estimated the transition temperature, the activation energy values of the samples from the resistivity vs. temperature measurements in dc magnetic fields up to 0.6 T. We observed that transition temperature, $T_{c}$, and transport critical current density, $J_{c}$\textit{trans}, depend on the Sm substitution. They both decrease with increasing the \textit{Sm} substitution. Activation energy, $U_{0},$ is calculated from resistivity versus temperature at various magnetic fields in both high temperature region and low temperature region. It is observed that $U_{0}$ depend on both the \textit{Sm} content of samples and the applied magnetic field.   

Orientation and Grain Boundaries in Bulk Samples of Bi-2223 Phase.

In polycrystalline high-T$_{c}$ compounds the critical current densities are mostly limited by the grain boundaries. Their characterization is of significant importance to understand and improve the superconducting properties. In our work, superconducting Bi$_{1.8}$Pb$_{0.35}$Sr$_{1.9}$Ca$_{2.1}$Cu$_{3}$O$_{y}$ samples were prepared by the standard solid-state reaction method. 20 samples were pressed in pellets and optimum annealing temperatures were found as 835$^{ o}$C for the first annealing and 830$^{ o}$C for the second annealing respectively. The optimum Between the first and second annealing procedure, the pellets are pressed piled up. After the second annealing, six bar samples were cut, three of them were perpendicular and the others parallel to the pressing surface. The transport T$_{c}$ and J$_{c}$ were measured using standard four-probe method. Micro structural examination of the samples was done using SEM. Degrading effect of the magnetic field on J$_{c}$ is much more significant for the perpendicular sample. This is in agreement with literature and ensures that our samples had preferred orientation. We have proved that preferred orientation in bulk pellet samples of (Bi, Pb)-2223 phase can be achieved by the method described in this work. Orientation of the magnetic field with respect to grain boundaries may be an important issue for transport critical current values in applied magnetic fields.