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Register a new userHigh Temperature Superconductivity in a Lightly Doped Quantum Spin Liquid
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We have performed density-matrix renormalization group studies of a square lattice $t$-$J$ model with small hole doping, $deltall 1$, on long 4 and 6 leg cylinders. We include frustration in the form of a second-neighbor exchange coupling, $J_2 = J_1/2$, such that the undoped ($delta=0$) parent state is a quantum spin liquid. In contrast to the relatively short range superconducting (SC) correlations that have been observed in recent studies of the 6-leg cylinder in the absence of frustration, we find power law SC correlations with a Luttinger exponent, $K_{sc} approx 1$, consistent with a strongly diverging SC susceptibility, $chi sim T^{-(2-K_{sc})}$ as the temperature $Tto 0$. The spin-spin correlations - as in the undoped state - fall exponentially suggesting that the SC pairing correlations evolve smoothly from the insulating parent state.
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Broad interest in quantum spin liquid (QSL) phases was triggered by the notion that they can be viewed as insulating phases with preexisting electron-pairs, such that upon light doping they might automatically yield superconductivity. Yet despite intense efforts, definitive evidence is lacking. We address the problem of a lightly doped QSL through a large-scale density-matrix renormalization group study of the $t$-$J$ model on the triangular lattice with a small but non-zero concentration of doped holes. The ground state is consistent with a Luther-Emery liquid with power-law superconducting and charge-density-wave correlations associated with partially-filled charge stripes. In particular, the superconducting correlations are dominant on both four-leg and six-leg cylinders at all hole doping concentrations. Our results provide direct evidences that doping a QSL can naturally lead to robust superconductivity.
Inelastic neutron scattering (INS), electron spin (ESR) and nuclear magnetic resonance (NMR) measurements were employed to establish the origin of the strong magnetic signal in lightly hole-doped La_{1-x}Sr_xCoO_3, x=0.002. Both, INS and ESR low temperature spectra show intense excitations with large effective g-factors ~10-18. NMR data indicate the creation of extended magnetic clusters. From the Q-dependence of the INS magnetic intensity we conclude that the observed anomalies are caused by the formation of octahedrally shaped spin-state polarons comprising seven Co ions.
We show that, at weak to intermediate coupling, antiferromagnetic fluctuations enhance d-wave pairing correlations until, as one moves closer to half-filling, the antiferromagnetically-induced pseudogap begins to suppress the tendency to superconductivity. The accuracy of our approach is gauged by detailed comparisons with Quantum Monte Carlo simulations. The negative pressure dependence of Tc and the existence of photoemission hot spots in electron-doped cuprate superconductors find their natural explanation within this approach.
The formation of domains comprising alternating hole rich and hole poor ladders recently observed by Scanning Tunneling Microscopy by Kohsaka et al., on lightly hole doped cuprates, is interpreted in terms of an attractive mechanism which favors the presence of doped holes on Cu sites located each on one side of an oxygen atom. This mechanism leads to a geometrical pattern of alternating hole-rich and hole-poor ladders with a periodicity equal to 4 times the lattice spacing in the CuO plane, as observed experimentally. To cite this article: G. Deutscher, P.-G. de Gennes, C. R. Physique 8 (2007).
Unrevealing local magnetic and electronic correlations in the vicinity of charge carriers is crucial in order to understand rich physical properties in correlated electron systems. Here, using high-energy optical conductivity (up to 35 eV) as a function of temperature and polarization, we observe a surprisingly strong spin polarization of the local spin singlet with enhanced ferromagnetic correlations between Cu spins near the doped holes in lightly hole-doped La$_{1.95}$Sr$_{0.05}$Cu$_{0.95}$Zn$_{0.05}$O$_{4}$. The changes of the local spin polarization manifest strongly in the temperature-dependent optical conductivity at ~7.2 eV, with an anomaly at the magnetic stripe phase (~25 K), accompanied by anomalous spectral-weight transfer in a broad energy range. Supported by theoretical calculations, we also assign high-energy optical transitions and their corresponding temperature dependence, particularly at ~2.5 ~8.7, ~9.7, ~11.3 and ~21.8 eV. Our result shows the importance of a strong mixture of spin singlet and triplet states in hole-doped cuprates and demonstrates a new strategy to probe local magnetic correlations using high- energy optical conductivity in correlated electron systems.
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