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Register a new userLight-induced new collective modes in La$_{1.905}$Ba$_{0.095}$CuO$_4$ superconductor
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We report near and mid-infrared pump c-axis terahertz probe measurement on a superconducting single crystal La$_{1.905}$Ba$_{0.095}$CuO$_4$ with T$_c$=32 K. The measurement reveals that the pump-induced change occurs predominantly at the Josephson plasma edge position below T$_c$. Upon excited by the strong near-infrared pulses, the superconducting state is severely disturbed and incoherent quasiparticle excitations develop in frequency regime above the static plasma edge. However, within very short time delay ($sim$1.5 ps) we observe the reappearance of a very sharp Josephson plasma edge at frequency lower than the static Josephson plasma edge and the emergence of a new light-induced Josephson mode at higher energy. The results imply that the light can induce new Josephson couplings with different coupling strengths. Similar but weaker effect is observed for the mid-infrared pump. No pump induced effect is detected above T$_c$.
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We present an experimental study of the anisotropic resistivity of superconducting La(2-x)Ba(x)CuO(4) with x=0.095 and transition temperature Tc=32 K. In a magnetic field perpendicular to the CuO(2) layers, H(perp), we observe that the resistivity perpendicular to the layers, rho(perp), becomes finite at a temperature consistent with previous studies on very similar materials; however, the onset of finite parallel resistivity, rho(par), occurs at a much higher temperature. This behavior contradicts conventional theory, which predicts that rho(perp) and rho(par) should become finite at the same temperature. Voltage vs. current measurements near the threshold of voltage detectability indicate linear behavior perpendicular to the layers, becoming nonlinear at higher currents, while the behavior is nonlinear from the onset parallel to the layers. These results, in the presence of moderate H(perp), appear consistent with superconducting order parallel to the layers with voltage fluctuations between the layers due to thermal noise. In search of uncommon effects that might help to explain this behavior, we have performed diffraction measurements that provide evidence for H(perp)-induced charge and spin stripe order. The field-induced decoupling of superconducting layers is similar to the decoupled phase observed previously in La(2-x)Ba(x)CuO(4) with x=1/8 in zero field.
We explore the evolution of superconductivity in La(2-x)Ba(x)CuO(4) with x=0.095 in magnetic fields of up to 35 T applied perpendicular to the CuO(2) planes. Previous work on this material has shown that perpendicular fields enhance both charge and spin stripe order within the planes. We present measurements of the resistivity parallel and perpendicular to the planes, as well as the Hall effect. Measurements of magnetic susceptibility for fields of up to 15 T applied both parallel and perpendicular to the planes provide complementary measures of the superconductivity. We show that fields sufficient to destroy pair tunneling between the planes do not disrupt the superconducting correlations within the planes. In fact, we observe an onset of large amplitude but phase disordered superconductivity within the planes at approximately 30 K that is remarkably insensitive to field. With further cooling, we observe a phase-transition-like drop in the in-plane resistivity to an apparent state of superconductivity, despite the lack of phase coherence between the layers. These observations raise interesting questions concerning the identification of the upper critical field, where pairing is destroyed, in underdoped cuprates.
Optical excitation of stripe-ordered La$_{2-x}$Ba$_x$CuO$_4$ has been shown to transiently enhance superconducting tunneling between the CuO$_2$ planes. This effect was revealed by a blue-shift, or by the appearance of a Josephson Plasma Resonance in the terahertz-frequency optical properties. Here, we show that this photo-induced state can be strengthened by the application of high external magnetic fields oriented along the c-axis. For a 7-Tesla field, we observe up to a ten-fold enhancement in the transient interlayer phase correlation length, accompanied by a two-fold increase in the relaxation time of the photo-induced state. These observations are highly surprising, since static magnetic fields suppress interlayer Josephson tunneling and stabilize stripe order at equilibrium. We interpret our data as an indication that optically-enhanced interlayer coupling in La$_{2-x}$Ba$_x$CuO$_4$ does not originate from a simple optical melting of stripes, as previously hypothesized. Rather, we speculate that the photo-induced state may emerge from activated tunneling between optically-excited stripes in adjacent planes.
The correlations between stripe order, superconductivity, and crystal structure in La(2-x)Ba(x)CuO(4) single crystals have been studied by means of x-ray and neutron diffraction as well as static magnetization measurements. The derived phase diagram shows that charge stripe order (CO) coexists with bulk superconductivity in a broad range of doping around x=1/8, although the CO order parameter falls off quickly for x<>1/8. Except for x=0.155, the onset of CO always coincides with the transition between the orthorhombic and the tetragonal low temperature structures. The CO transition evolves from a sharp drop at low x to a more gradual transition at higher x, eventually falling below the structural phase boundary for optimum doping. With respect to the interlayer CO correlations, we find no qualitative change of the stripe stacking order as a function of doping, and in-plane and out-of-plane correlations disappear simultaneously at the transition. Similarly to the CO, the spin stripe order (SO) is also most pronounced at x=1/8. Truly static SO sets in below the CO and coincides with the first appearance of in-plane superconducting correlations at temperatures significantly above the bulk transition to superconductivity (SC). Indications that bulk SC causes a reduction of the spin or charge stripe order could not be identified. We argue that CO is the dominant order that is compatible with SC pairing but competes with SC phase coherence. Comparing our results with data from the literature, we find good agreement if all results are plotted as a function of x instead of the nominal x, where x represents an estimate of the actual Ba content, extracted from the doping dependence of the structural transition between the orthorhombic phase and the tetragonal high-temperature phase.
Interlayer transport in high-$T_C$ cuprates is mediated by superconducting tunneling across the CuO$_2$ planes. For this reason, the terahertz frequency optical response is dominated by one or more Josephson plasma resonances and becomes highly nonlinear at fields for which the tunneling supercurrents approach their critical value, $I_C$. These large terahertz nonlinearities are in fact a hallmark of superconducting transport. Surprisingly, however, they have been documented in La$_{2-x}$Ba$_x$CuO$_4$ also above $T_C$ for doping values near $x=1/8$, and interpreted as an indication of superfluidity in the stripe phase. Here, Electric Field Induced Second Harmonic (EFISH) is used to study the dynamics of time-dependent interlayer voltages when La$_{2-x}$Ba$_x$CuO$_4$ is driven with large-amplitude terahertz pulses, in search of other characteristic signatures of Josephson tunnelling in the normal state. We show that this method is sensitive to the voltage anomalies associated with 2$pi$ Josephson phase slips, which near $x=1/8$ are observed both below and above $T_C$. These results document a new regime of nonlinear transport that shares features of sliding charge-density-waves and superconducting phase dynamics.
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