Cu nuclear magnetic resonance study of charge and spin stripe order in La1.875_{1.875}Ba0.125_{0.125}CuO4_4

We present a Cu nuclear magnetic/quadrupole resonance study of the charge stripe ordered phase of LBCO, with detection of previously unobserved ('wiped-out') signal. We show that spin-spin and spin-lattice relaxation rates are strongly enhanced in the charge ordered phase, explaining the apparent signal decrease in earlier investigations. The enhancement is caused by magnetic, rather than charge fluctuations, conclusively confirming the long-suspected assumption that spin fluctuations are responsible for the wipeout effect. Observation of the full Cu signal enables insight into the spin and charge dynamics of the stripe-ordered phase, and measurements in external magnetic fields provide information on the nature and suppression of spin fluctuations associated with charge order. We find glassy spin dynamics, in agreement with previous work, and incommensurate static charge order with charge modulation amplitude similar to other cuprate compounds, suggesting that the amplitude of charge stripes is universal in the cuprates.Comment: 7 pages, 5 figure

Probing optically silent superfluid stripes in cuprates

Unconventional superconductivity in the cuprates emerges from, or coexists with, other types of electronic order. However, these orders are sometimes invisible because of their symmetry. For example, the possible existence of superfluid charge stripes in the normal state of single layer cuprates cannot be validated with infrared optics, because interlayer tunneling fluctuations vanish on average. Similarly, it is not easy to establish if charge orders are responsible for dynamical decoupling of the superconducting layers over broad ranges of doping and temperatures. Here, we show that TeraHertz pulses can excite nonlinear tunneling currents between linearly de-coupled charge-ordered planes. A giant TeraHertz third harmonic signal is observed in La1.885Ba0.115CuO4 far above Tc=13 K and up to the charge ordering temperature TCO = 55 K. We model these results by considering large order-parameter-phase oscillations in a pair density wave condensate, and show how nonlinear mixing of optically silent tunneling modes can drive large dipole-carrying super-current oscillations. Our results provide compelling experimental support for the presence of hidden superfluid order in the normal state of cuprates. These experiments also underscore the power of nonlinear TeraHertz optics as a sensitive probe of frustrated excitations in quantum solids.Comment: 9 pages main text, 5 figures, 12 page supplementar

Measurement of the dynamic charge response of materials using low-energy, momentum-resolved electron energy-loss spectroscopy (M-EELS)

One of the most fundamental properties of an interacting electron system is its frequency- and wave-vector-dependent density response function, $\chi({\bf q},\omega)$. The imaginary part, $\chi''({\bf q},\omega)$, defines the fundamental bosonic charge excitations of the system, exhibiting peaks wherever collective modes are present. $\chi$ quantifies the electronic compressibility of a material, its response to external fields, its ability to screen charge, and its tendency to form charge density waves. Unfortunately, there has never been a fully momentum-resolved means to measure $\chi({\bf q},\omega)$ at the meV energy scale relevant to modern elecronic materials. Here, we demonstrate a way to measure $\chi$ with quantitative momentum resolution by applying alignment techniques from x-ray and neutron scattering to surface high-resolution electron energy-loss spectroscopy (HR-EELS). This approach, which we refer to here as "M-EELS," allows direct measurement of $\chi''({\bf q},\omega)$ with meV resolution while controlling the momentum with an accuracy better than a percent of a typical Brillouin zone. We apply this technique to finite-q excitations in the optimally-doped high temperature superconductor, Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (Bi2212), which exhibits several phonons potentially relevant to dispersion anomalies observed in ARPES and STM experiments. Our study defines a path to studying the long-sought collective charge modes in quantum materials at the meV scale and with full momentum control.Comment: 26 pages, 10 sections, 7 figures, and an appendi

Ultrafast Melting of Superconductivity in an Iron-Based Superconductor

Intense debate has recently arisen regarding the photoinduced changes to the iron-chalcogenide superconductors, including the enhancement of superconductivity and a metastable state. Here, by employing high energy resolution, we directly observe the melting of superconductivity on ultrafast timescales. We demonstrate a distinctly nonequilibrium response on short timescales, where the gap fills in prior to the destruction of the superconducting peak, followed by a metastable response. We propose that the former is due to pair phase decoherence and speculate that the latter is due to the increase in double stripe correlations that are known to compete with superconductivity. Our results add to exciting new developments on the iron-based superconductors, indicating that the photoinduced metastable state possibly competes with superconductivity

Magnetic order tuned by Cu substitution in Fe1.1-zCuzTe

We study the effects of Cu substitution in Fe1.1Te, the non-superconducting parent compound of the iron-based superconductor, Fe1+yTe1-xSex, utilizing neutron scattering techniques. It is found that the structural and magnetic transitions, which occur at \sim 60 K without Cu, are monotonically depressed with increasing Cu content. By 10% Cu for Fe, the structural transition is hardly detectable, and the system becomes a spin glass below 22 K, with a slightly incommensurate ordering wave vector of (0.5-d, 0, 0.5) with d being the incommensurability of 0.02, and correlation length of 12 angstrom along the a axis and 9 angstrom along the c axis. With 4% Cu, both transition temperatures are at 41 K, though short-range incommensurate order at (0.42, 0, 0.5) is present at 60 K. With further cooling, the incommensurability decreases linearly with temperature down to 37 K, below which there is a first order transition to a long-range almost-commensurate antiferromagnetic structure. A spin anisotropy gap of 4.5 meV is also observed in this compound. Our results show that the weakly magnetic Cu has large effects on the magnetic correlations; it is suggested that this is caused by the frustration of the exchange interactions between the coupled Fe spins.Comment: 7 pages, 7 figures, version as appeared on PR

Enhanced low-energy magnetic excitations via suppression of the itinerancy in Fe0.98-zCuzTe0.5Se0.5

We have performed resistivity and inelastic neutron scattering measurements on three samples of Fe0.98-zCuzTe0.5Se0.5 with z = 0, 0.02, and 0.1. It is found that with increasing Cu doping the sample's resistivity deviates progressively from that of a metal. However, in contrast to expectations that replacing Fe with Cu would suppress the magnetic correlations, the low-energy (no larger than 12 meV) magnetic scattering is enhanced in strength, with greater spectral weight and longer dynamical spin-spin correlation lengths. Such enhancements can be a consequence of either enlarged local moments or a slowing down of the spin fluctuations. In either case, the localization of the conduction states induced by the Cu doping should play a critical role. Our results are not applicable to models that treat 3d transition metal dopants simply as effective electron donors.Comment: 5 pages, 5 figures. To appear in PR