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Register a new userAnalog of Magnetoelectric Effect in High-Tc Granular Superconductors
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We propose the existence of an electric-field induced nonlinear magnetization in a weakly coupled granular superconductor due to time-parity violation. As the field increases the induced magnetization passes from para- to dia-magnetic behavior. We discuss conditions under which this effect could be experimentally measured in high-temperature superconductors.
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We measure the local harmonic generation from superconducting thin films at microwave frequencies to investigate the intrinsic nonlinear Meissner effect near Tc in zero magnetic field. Both second and third harmonic generation are measured to identify time-reversal symmetry breaking (TRSB) and time-reversal symmetric (TRS) nonlinearities. We perform a systematic doping-dependent study of the nonlinear response and find that the TRS characteristic nonlinearity current density scale follows the doping dependence of the de-pairing critical current density. We also extract a spontaneous TRSB characteristic current density scale that onsets at Tc, grows with decreasing temperature, and systematically decreases in magnitude (at fixed T/Tc) with under-doping. The origin of this current scale could be Josephson circulating currents or the spontaneous magnetization associated with a TRSB order parameter.
A quarter of a century after their discovery the mechanism that pairs carriers in the cuprate high-Tc superconductors (HTS) still remains uncertain. Despite this the general consensus is that it is probably magnetic in origin [1] so that the energy scale for the pairing boson is governed by J, the antiferromagnetic exchange interaction. Recent studies using resonant inelastic X-ray scattering strongly support these ideas [2]. Here as a further test we vary J (as measured by two-magnon Raman scattering) by more than 60% by changing ion sizes in the model HTS system LnA2Cu3O7-{delta} where A=(Ba,Sr) and Ln=(La, Nd, Sm, Eu, Gd, Dy, Yb, Lu). Such changes are often referred to as internal pressure. Surprisingly, we find Tcmax anticorrelates with J where internal pressure is the implicit variable. This is the opposite to the effect of external pressure and suggests that J is not the dominant energy scale governing Tcmax.
The recent observation of quantum oscillations in underdoped high-Tc superconductors, combined with their negative Hall coefficient at low temperature, reveals that the Fermi surface of hole-doped cuprates includes a small electron pocket. This strongly suggests that the large hole Fermi surface characteristic of the overdoped regime undergoes a reconstruction caused by the onset of some order which breaks translational symmetry. Here we consider the possibility that this order is stripe order, a form of charge / spin modulation observed most clearly in materials like Eu-doped and Nd-doped LSCO. In these materials, the onset of stripe order is indeed the cause of Fermi-surface reconstruction. We identify the critical doping where this reconstruction occurs and show that the temperature dependence of transport coefficients at that doping is typical of metals at a quantum critical point. We discuss how the pseudogap phase may be a fluctuating precursor of the stripe-ordered phase.
Local antiferromagnetism coexists with superconductivity in the cuprates. Charge segregation provides a way to reconcile these properties. Direct evidence for modulated spin and charge densities has been found in neutron and X-ray scattering studies of Nd-doped La(2-x)Sr(x)CuO(4). Here we discuss the nature of the modulation, and present some new results for a Zn-doped sample. Some of the open questions concerning the connections between segregation and superconductivity are described.
We report experimental evidence for the phase diagram of doped cuprate superconductors as a function of the micro-strain (e) of the Cu-O bond length, measured by Cu K-edge EXAFS, and hole doping (d). This phase diagram shows a QCP at P(e*,d*) where for the micro-strain e larger than the critical value e* charge-orbital-spin stripes and free carriers co-exist. The superconducting phase occurs in the region of critical fluctuations around this QCP. The critical temperature is function of two variables and Tc shows its maximum at the strain driven QCP. The critical fluctuations near this strain QCP give the self-organization of a metallic superlattice of quantum wires superstripes that favors the amplification of the critical temperature.
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