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Register a new userQuantum phase transition in overdoped La$_{2-x}$Sr$_x$CuO$_4$ evinced by the superfluid density
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T. Schneider
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The superfluid density of overdoped La$_{2-x}$Sr$_x$CuO$_4$ thin films of high quality have been measured with Tc (defined by the onset of the Meissner effect) from 5.1 to 41.6 K by Bosovic et al. Given this Tc the superfluid density shows no clear evidence of critical fluctuations and no indication of vortex unbinding as T->Tc. Nevertheless, the superfluid density displays the expected behavior for for a quantum phase transition (QPT) in the (3+1)D-xy universality class, rho(T=0)~Tc^2. However, this relation is also a hallmark of dirty superconductors, treated in the mean-field approximation. Here we attempt to clear out the nature of the suppression of ro(T) as Tc->0. Noting that for any finite system the continuous transition will be rounded we perform a finite size scaling analysis. It uncovers that the ro(T) data data are consistent with a finite length limited 3D-xy transition. In some films it is their thickness and in others their inhomogeneity that determines the limiting length. Having established the precondition for the occurrence of a QPT mapping on the (3+1) D-xy model, we explore the consistency with the hallmarks of this transition. In particular with the relations rho(T)/rho(0)=1-ycT/Tc, rho((0)^Tc^2, yc=alpha*Tc/ro(0), where alpha is the coefficient in rho(T)=rho(0)-alphaT as T->0. The emerging agreement with these characteristics points clearly to a quantum fluctuations induced suppression, revealing the crossover from the thermal to the quantum critical regime as Tc ->0. In the classical-quantum mapping it corresponds to a 3D to (3 + 1)D crossover.
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Recently, several experiments on La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) challenged the Fermi liquid picture for overdoped cuprates, and stimulated intensive debates [1]. In this work, we study the magnetotransport phenomena in such systems based on the Fermi liquid assumption. The Hall coefficient $R_H$ and magnetoresistivity $rho_{xx}$ are investigated near the van Hove singularity $x_{tinytext{VHS}}approx0.2$ across which the Fermi surface topology changes from hole- to electron-like. Our main findings are: (1) $R_H$ depends on the magnetic field $B$ and drops from positive to negative values with increasing $B$ in the doping regime $x_{tinytext{VHS}}<xlesssim0.3$; (2) $rho_{xx}$ grows up as $B^2$ at small $B$ and saturates at large $B$, while in the transition regime a nearly linear behavior shows up. Our results can be further tested by future magnetotransport experiments in the overdoped LSCO.
The dynamical stripe structure relating to the 1/8 problem was investigated in La$_{2-x}$Sr$_x$CuO$_4$ utilizing the high frequency response of Raman scattering. The split of the two-magnon peak due to the formation of the stripe structure was observed at whole Sr concentration region from $x=0.035$ to 0.25 at low temperatures. Especially clear split was observed at low carrier concentration region $x=0.035 - 0.06$ and at $x sim 1/8$. The onset temperatures of these stripe structures are as high as 300-350 K, which are much higher than the temperatures measured by slow response probes.
We present results of inelastic light scattering experiments on single-crystalline La$_{2-x}$Sr$_{x}$CuO$_4$ in the doping range $0.00 le x=p le 0.30$ and Tl$_2$Ba$_2$CuO$_{6+delta}$ at $p=0.20$ and $p=0.24$. The main emphasis is placed on the response of electronic excitations in the antiferromagnetic phase, in the pseudogap range, in the superconducting state, and in the essentially normal metallic state at $x ge 0.26$, where no superconductivity could be observed. In most of the cases we compare B$_{1g}$ and B$_{2g}$ spectra which project out electronic properties close to $(pi,0)$ and $(pi/2, pi/2)$, respectively. In the channel of electron-hole excitations we find universal behavior in B$_{2g}$ symmetry as long as the material exhibits superconductivity at low temperature. In contrast, there is a strong doping dependence in B$_{1g}$ symmetry: (i) In the doping range $0.20 le p le 0.25$ we observe rapid changes of shape and temperature dependence of the spectra. (ii) In La$_{2-x}$Sr$_{x}$CuO$_4$ new structures appear for $x < 0.13$ which are superposed on the electron-hole continuum. The temperature dependence as well as model calculations support an interpretation in terms of charge-ordering fluctuations. For $x le 0.05$ the response from fluctuations disappears at B$_{1g}$ and appears at B$_{2g}$ symmetry in full agreement with the orientation change of stripes found by neutron scattering. While, with a grain of salt, the particle-hole continuum is universal for all cuprates the response from fluctuating charge order in the range $0.05 le p < 0.16$ is so far found only in La$_{2-x}$Sr$_{x}$CuO$_4$. We conclude that La$_{2-x}$Sr$_{x}$CuO$_4$ is close to static charge order and, for this reason, may have a suppressed $T_c$.
We investigate whether the spin or charge degrees of freedom are responsible for the nodal gap in underdoped cuprates by performing inelastic neutron scattering and x-ray diffraction measurements on La$_{2-x}$Sr$_x$CuO$_4$, which is on the edge of the antiferromagnetic phase. We found that fluctuating incommensurate spin-density-wave (SDW) with a the bottom part of an hourglass dispersion exists even in this magnetic sample. The strongest component of these fluctuations diminishes at the same temperature where the nodal gap opens. X-ray scattering measurements on the same crystal show no signature of charge-density-wave (CDW). Therefore, we suggest that the nodal gap in the electronic band of this cuprate opens due to fluctuating SDW with no contribution from CDW.
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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