Session V23: Superconductivity: Mostly Transport

Thermal Conductivity Data Near T$_{c}$ as a Probe of Unconventional Superconductivity

In classical phonon-mediated superconductors, the thermal conductivity, kappa, just below T$_{c}$ decreases since electrons in the superfluid state carry no heat. A decrease in kappa occurs just below T$_{c}$ even for BCS superconductors with relatively high transition temperatures such as Nb$_{3}$Sn and MgB$_{2}$. By contrast, in all of the unconventional superconductors, of which the author is aware, the thermal conductivity always increases just below T$_{c}$. It is hypothesized in these unconventional superconductors that magnetic or some other type of fluctuation that scatters phonons above T$_{c}$ are frozen out below T$_{c}$, resulting in a larger kappa. This suggests that a relatively simple thermal conductivity measurement may be a way of identifying superconductors with an unusual pairing mechanism. Although this phenomenology does not help identify new superconductors, it may help determine which materials have the potential for a much higher T$_{c}$. Several related examples of the effects of magnetic fluctuations or excitations on thermal transport also will be presented. This research was supported by the Materials Sciences and Engineering Division, Office of Science, U. S. Department of Energy.   

Critical currents in thin-film superconductors via two-coil mutual inductance measurements

Following Claassen et al.(RSI 1991), we determine the critical current density, $J_c(T)$, in a superconducting thin film by measuring the inductive coupling between two coils located on opposite sides of the film as a function of temperature. For several values of the AC drive magnetic field, we record the temperature at which inductive coupling between the coils jumps dramatically. The peak current density in the film at such temperature is computed by numerical simulation. For thin niobium films, we obtain $J_c(0)$ values greater than 10 percent of the Ginzburg-Landau (GL) prediction with the correct temperature dependence. This suggests the possibility of practical measurement of the GL coherence length in thin films without need for the strong magnetic fields used to measure $B_{c2}$. Application of this technique to cuprates and pnictides will be discussed.   

Anomalous dependence of interface superconductivity on carrier density in metal-insulator bilayer

The interface superconductivity is of particular interest for its higher superconducting temperature (Tc) and close connection to the HTS study. However, despite of the experimental and theoretical progress, the answer to the following key question still remains ambiguous: can the underlying mechanism of the interface superconductivity be interpreted solely in terms of charge transfer and Sr interdiffusion? One experimental approach to answer this question is to study the dependence of Tc on the carrier density of La$_{2-x}$Sr$_{x}$CuO$_{4}$/La$_{2}$CuO$_{4}$ (LSCO/LCO) bilayer by changing the doping level in the metallic LSCO layer. The charge transfer and cation interdiffusion at the interface is proportional to the carrier density in LSCO layer; therefore, Tc of the interface is expected to show a significant and non-monotonic dependence on the carrier density. To exam this prediction, we synthesized a series of La$_{2-x}$Sr$_{x}$CuO$_{4}$/La$_{2}$CuO$_{4}$ bilayers with x ranging in 0.27 to 0.47 by atomic layer-by-layer molecular beam epitaxy (ALL-MBE). The uniqueness of these samples is a 4{\%} Sr doping gradient of the central doping level across the 10mm width of the substrate. These combinatorial samples realize an extremely fine tuning of doping and enable us to construct a phase diagram that is 30 times denser than a normal method can achieve.   

Magnetotransport of La$_{(2-x)}$Sr$_{x}$CuO$_{4}$ with Nearly Continuous Doping

Hall resistivity and longitudinal magnetoresistance were measured in fields up to 35 T. The samples were grown using Combinatorial Molecular Beam Epitaxy (COMBE) where the strontium doping changes continuously across the sample. Thirty simultaneous transport measurements were taken on a sample from a single growth, which allows for unprecedented resolution in doping ($\Delta p \approx 0.0002 $). A series of these measurements were performed with the goal of examining the phase diagram of the hole-doped cuprates, particularly an increase in the Hall number around optimum doping.   

Decoupling of superconducting planes of La$_{1.905}$Ba$_{0.095}$CuO$_4$ in a transverse magnetic field

We have measured the resistivity parallel and perpendicular to the CuO$_2$ planes in single crystals of La$_{1.905}$Ba$_{0.095}$CuO$_4$ for magnetic fields up to 35~T applied along the $c$-axis. Below the zero-field superconducting transition temperature of 32 K, we observe that, above a threshold field, the $c$-axis resistivity grows with field, eventually reaching a maximum and then decreasing. At the resistivity maximum, interlayer pair tunneling becomes insignificant. Under the same field and temperature conditions, the in-plane resistivity remains quite low, reflecting robust superconductivity. We identify a regime in which the superconducting planes are effectively decoupled. At 20~K, a field much greater than 35~T would be required to destroy the in-plane pairing, despite the fact that the field also induces both charge and spin stripe order (J.S. Wen {\it et al.}, arXiv:1009.0031).   

Spatial Inhomogeneity in Oxygen Modulated Potassium Tungsten Oxide Thin Films: implications for superconductivity and metal-insulator transitions

High quality potassium tungsten oxide (K0.33 WOy) films were synthesized by pulsed laser deposition followed by annealing in vacuum. Oxygen concentration modulated anomalous correlation of metal-insulator and superconductivity transitions were studied; a similar scenario was recently suggested in the literature [1] for polycrystalline rubidium tungsten oxide samples. Detailed studies of the transport properties below and above the superconducting transition temperature exhibit a diversity of unexpected behavior. Some of our results can be interpreted as a signature of reduced dimensionality in the ab-plane in oxygen-rich insulating samples, implying a formation of spatially inhomogeneous electronic structure. We compare such phenomenology to the behavior of other materials with strong electron-phonon interactions, and discuss its implication for the possible high temperature superconducting anomaly in sodium tungsten oxides reported in the literature [2]. [1] D. C. Ling et. al., J. Phys. Conf. Ser. 150, 052141 (2009). [2] S. Reich, and Y. Tsabba, Eur. Phys. J. B 9, 1 (1999).   

Thermal Hall effect in YBCO: Probing Fermi-surface reconstruction inside the superconducting state

The thermal Hall (Righi-Leduc) effect was measured in the cuprate superconductor YBCO at a doping $p$ = 0.11, as a function of magnetic field $H$ up to 29 T. At temperatures well below the zero-field superconducting $T_{c}$, the thermal Hall conductivity \textit{$\kappa $}$_{xy}$ is positive at low field and then turns over to become negative at fields above 15 T. The negative \textit{$\kappa $}$_{xy}$ is consistent with the negative Hall and Seebeck coefficients observed in the normal state above 25 T [1,2]. This further supports our interpretation: the Fermi surface of YBCO contains a small electron-like pocket [3] in that region of the phase diagram, the result of a Fermi-surface reconstruction attributed to stripe order [4]. In the $T $= 0 limit at $H$ = 29 T, we find reasonable agreement with the Wiedemann-Franz law, \textit{$\kappa $}$_{xy}$/$T=L_{0}$\textit{$\sigma $}$_{xy}$. The fact that \textit{$\kappa $}$_{xy}$ changes sign at $H \quad \approx $ 15 T is consistent with a scenario of phase competition whereby stripe order emerges only at finite field, in agreement with recent NMR studies that detect the onset of charge-stripe order above 15 T [5]. \\[4pt] [1] LeBoeuf \textit{et al}., \textit{PRB} \textbf{83}, 054056 (2011); [2] Lalibert\'{e} \textit{et al}., \textit{Nat. Commun.} \textbf{2}, 432 (2011); [3] LeBoeuf \textit{et al}., \textit{Nature} \textbf{450}, 533 (2007); [4] Chang \textit{et al}., \textit{PRL} \textbf{104}, 057005 (2010); [5] Wu \textit{et al., Nature }\textbf{477}$, $191 (2011).   

On the magnetic tricritical point in BaFe$_{2(1-x)}$Co$_{2x}$As$_{2}$

We present here high resolution magnetization measurements on high-quality BaFe$_{2(1-x)}$Co$_{2x}$As$_{2}$, 0$\leq$x$\leq$0.046 as-grown single crystals. The results confirm the existence of a magnetic tricritical point in the ($x$,$T$) plane at x$^{m}_{tr}$$\approx$0.022 [1,2]. We show that the extrapolated T$_{c}$ onset doping could be close to the magnetic tricritical point x$^{m}_{tr}$. It has been speculated that the magnetic critical point is relevant to the superconductivity in this series [3]. Finally, we comment on the universality of the tricritical point in the superconducting 122s.\\[4pt] [1] M. G. Kim \emph{et al.}, Phys. Rev. B {\bf 83}, 134522 (2011).\\[0pt] [2] C. R. Rotundu and R. J. Birgeneau, Phys. Rev. B {\bf 84}, 092501 (2011).\\[0pt] [3] G. Giovannetti \emph{et al.}, Nature Communications {\bf 2}, article number 398 (2011).   

$Rb_xFe_2Se_2$: A study of superconductivity under high pressure

Superconductivity was reported in $Rb_xFe_2Se_2$ in early 2011, after the discovery of similar Fe-based chalcogenides, $K_xFe_2Se_2$ and $Cs_xFe_2Se_2$ in December 2010. These Fe-based chalcogenides have similar structures to the iron pnictides, with a superconducting transition of approximately 30 K. Here we report the results of the physical characterization and the subsequent high-pressure study on two samples with slightly different compositions of $Rb_xFe_2Se_2$. From resistivity measurements, $Rb_{0.93(2)}Fe_{1.70(2)}Se_2$ (sample A) was found to be superconducting and $Rb_{0.90(1)}Fe_{1.78(1)}Se_2$ (sample B) was found to be semiconducting. Further sample characterization was conducted through magnetic and thermoelectric power measurements, which support the initial resistivity findings. High pressure resistivity measurements were conducted with a BeCu clamp cell up to approximately 1.8 GPa. Initially, the Tc of sample A increases slightly until p reaches $\sim$ 1GPa. However, at further increasing pressure, $T_c$ starts to decrease and superconductivity is completely suppressed at about 6 GPa. Sample B was not found to be superconducting up to 1.8 GPa.   

Unusual superconducting state in Pr-doped CaFe$_{2}$As$_{2}$

We report the detection of unusual superconductivity up to 49 K in Pr-doped single crystalline CaFe$_{2}$As$_{2}$ as evidenced from the resistive, magnetic and thermoelectric measurements. This superconducting transition observed suggests the possible existence of two phases: a field as low as 500 Oe can totally suppress the observed diamagnetic susceptibility above 21K. The 49 K part has a low critical field $<$ 4 Oe, and the other at 21K, with a high upper critical field $>$ 5T. Our observations are in strong contrast to previous reports of doping or pressurizing layered compounds AeFe$_{2}$As$_{2}$ (Ae122), where Ae = Ca, Sr, or Ba. In Ae 122, hole-doping has been previously observed to generate superconductivity with a transition temperature (Tc) only up to 38 K and pressurization has been reported to produce superconductivity with a Tc up to 30 K. The experiment results of resistivity and inductance measurements of Pr-doped Ca122 under pressure up to 2.0 GPa will also be discussed.   

Fermi-liquid behavior of quasiparticle scattering in the normal state of BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$

We present studies of the galvanomagnetic effects of compensated BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ (x=0.32$\sim $0.6) superconductors. The magnetoresistance follows the relaxed Kohler's scaling for all doping levels. Using a two-band model, we quantitatively extracted the scattering parameter m*/$\tau $ and the carrier density of the electron and hole bands. The temperature dependence of the carrier concentration reveals the semimetal properties of BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$. The Fermi-liquid behavior, m*/$\tau \sim $T$^{2}$, is observed from optimal doped x=0.32 to over-doped x=0.6 crystals, suggesting that the proximity of the SDW state does not play an important role in transport. Our analysis suggests that the normal state transport properties of BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ can be well understood in the framework of a compensated two-band Fermi-liquid semimetal.   

Pressure effect on the electronic transport properties of Fe$_{1+y}$Te$_{1-x}$Se$_{x}$

We present a systematic study of electronic transport as function of pressure up to 25 kbar of Fe$_{+y}$Te$_{1-x}$Se$_{x}$ single crystalline samples (with $y=0.02$, 0.05, and $x=0$, 0.2, and 0.3). Pressure is demonstrated to be a clean control parameter to drive the system with high Fe-excess through the metal-insulator (MIT) transition, in analogy with increasing the Se-doping or reducing the Fe-excess. The scaling of resistivity $\rho (T, p)$ below 50 K identified a critical pressure of $p_{c}=8$ kbar which separates non-metallic and metallic temperature dependences. At the $p_{c}$ the low-temperature sheet resistance is in the 6.5 k$\Omega$/square range. The Seebeck coefficient ($S$) at $p_{c}$ changes sign from negative to positive indicating a change in the electronic structure and in the balance between the electron and hole carriers. The $S$ at the highest pressure exhibits low positive values similar to the metallic, superconducting cuprates. The critical MIT behavior, related to a quantum phase transition, indicates a universality of the Fe- and Cu-based high-$T_{c}$ superconductors.   

DC transport properties of Fe(Se,Te) crystals: Effects of Se substitution into Te site

FeTe is an antiferromagnetic semimetal and superconductivity shows up when Te is substituted with Se, but effects of Se substitution are still unclear. To elucidate this issue, we study the behavior of Hall resistance in detail using single crystalline FeSe$_x$Te$_{1-x}$ with $x = 0$ to 0.4. Single crystals are grown by Bridgman method, and Hall resistance is analyzed within the standard 2-carrier model. We find that the electron density $n_e$ is larger than the hole density $n_h$ for $x \leq 0.3$ samples, but $n_h$ becomes greater than $n_e$ for $x = 0.4$ crystal. We also find that electron mobility enhances especially at low temperatures with Se substitution.   

Ferroelectricity and magnetoelectric coupling in underdoped La$_{2}$CuO$_{4+x}$

La$_{2}$CuO$_{4}$ is an archetypal antiferromagnetic Mott insulator. Upon oxygen doping, a wide range groundstates may be accessed, including glassy magnetic phases and high-T$_{c}$ superconductivity. However, determining the nature of the charge correlations coexistent with magnetic order has remained elusive, particularly in the highly underdoped limit. In this study, we show that the first holes added to La$_{2}$CuO$_{4}$ drive the formation of a ferroelectric phase below 4.5K, with slow charge fluctuations developing below 40K. We invoke the formation of polar nanoregions -- which are a natural consequence of non-stoichiometric oxygen doping -- to explain the emergent ferroelectricity. An anisotropic magnetoelectric coupling is observed and attributed to the Dzyaloshinskii-Moriya interaction. Although this interaction is not responsible for the electronic ordering (unlike in other multiferroic perovskites), the presence of weak magnetoelectricity allows us to confirm charge carrier doping as the cause of ferroelectricity in La$_{2}$CuO$_{4+x}$.   

Surface pinning effects in Ag doped superconducting polycrystalline Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-x }$

The current-voltage measurements ($I $--$V$ curves) with different current sweep rates (d$I$/d$t)$ were carried out to investigate the effects of silver on the flux dynamics in Ag doped superconducting polycrystalline Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-x }$sample (YBCO/Ag). Standard and reverse procedures were used in the measurements of$ I $-- $V$ curves. In the standard procedure, the dc driving current is cycled up and down; whereas, in the reverse procedure, the current is first cycled down and, then, cycled up. The reverse procedure enables us to investigate the flux motion evolving from the outer surface of the sample to its interior. Upon cycling transport current, the $I $-- $V $curves of the YBCO/Ag sample exhibit hysteresis effects for both procedures and are sensitive to the variation of d$I$/d$t$. The experimental data reveal that the irreversibilities in the $I $-- $V $curves of undoped YBCO are more prominent than those of YBCO/Ag. One of the main observations in the $I $-- $V$ curves of YBCO/Ag is the peculiar voltage jumps and drops. It was found that these instabilities depend strongly on the magnitude of external parameters d$I$/d$t$ and $H$. The instabilities and short and long lived plateau regions observed in $I$ -- $V$ curves were explained in terms of plastic flow of flux lines along easy motion channels which are considered mainly as metallic silver paths in the YBCO/Ag sample. In order to understand better the flux dynamics evolving in YBCO/Ag, the results of present measurements were compared to our previous studies on polycrystalline samples of undoped YBCO and MgB$_{2}$.