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Register a new userAn optically stimulated superconducting-like phase in K3C60 far above equilibrium Tc
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The control of non-equilibrium phenomena in complex solids is an important research frontier, encompassing new effects like light induced superconductivity. Here, we show that coherent optical excitation of molecular vibrations in the organic conductor K3C60 can induce a non-equilibrium state with the optical properties of a superconductor. A transient gap in the real part of the optical conductivity and a low-frequency divergence of the imaginary part are measured for base temperatures far above equilibrium Tc=20 K. These findings underscore the role of coherent light fields in inducing emergent order.
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We report on a photo-induced transient state of YBa2Cu2O6+x in which transport perpendicular to the Cu-O planes becomes highly coherent. This effect is achieved by excitation with mid-infrared optical pulses, tuned to the resonant frequency of apical oxygen vibrations, which modulate both lattice and electronic properties. Below the superconducting transition temperature Tc, the equilibrium signatures of superconducting interlayer coupling are enhanced. Most strikingly, the optical excitation induces a new reflectivity edge at higher frequency than the equilibrium Josephson plasma resonance, with a concomitant enhancement of the low frequency imaginary conductivity. Above Tc, the incoherent equilibrium conductivity becomes highly coherent, with the appearance of a reflectivity edge and a positive imaginary conductivity that increases with decreasing frequency. These features are observed up to room temperature in YBa2Cu2O6.45 and YBa2Cu2O6.5. The data above Tc can be fitted by hypothesizing that the light re-establishes a transient superconducting state over only a fraction of the solid, with a lifetime of a few picoseconds. Non-superconducting transport could also explain these observations, although one would have to assume transient carrier mobilities near 10^4 cm^2/(V.sec) at 100 K, with a density of charge carriers similar to the below Tc superfluid density. Our results are indicative of highly unconventional non-equilibrium physics and open new prospects for optical control of complex solids.
In this brief report an attempt is made for a mise-a-point of the subject of the phase fluctuations of the superconducting order parameter above Tc in cuprates, particularly as they appear in underdoped compounds. Measurements of torque magnetometry, Nernst effect and isothermal diamagnetic magnetization curves published in the last years are taken into consideration. Although by different experimental approaches and in different magnetic field ranges it can be stated that vortex-antivortex excitations and phase fluctuations among islands of local non-zero order parameter lacking of long range coherence do occur in a relevant temperature range above Tc, particularly in underdoped compounds. The role of the diamagnetic magnetization curves on approaching Tc from above in opening the field with clear signature is remarked, while enlightening comparison with other approaches appear possible.
In contrast to a complex feature of antinodal state, suffering from competing order(s), the pure pairing gap of cuprates is obtained in the nodal region, which therefore holds the key to the superconducting mechanism. One of the biggest question is whether the point nodal state as a hallmark of d-wave pairing collapses at Tc like the BCS-type superconductors, or it instead survives above Tc turning into the preformed pair state. A difficulty in this issue comes from the small magnitude of the nodal gap, which has been preventing experimentalists from solving it. Here we use a laser ARPES capable of ultrahigh energy resolution, and detect the point nodes surviving far beyond Tc in Bi2212. By tracking the temperature evolution of spectra, we reveal that the superconductivity occurs when the pair breaking rate is suppressed smaller than the single particle scattering rate on cooling, which governs the value of Tc in cuprates.
The upper critical field $H_{c2}$ of purple bronze Li$_{0.9}$Mo$_6$O$_{17}$ is found to exhibit a large anisotropy, in quantitative agreement with that expected from the observed electrical resistivity anisotropy. With the field aligned along the most conducting axis, $H_{c2}$ increases monotonically with decreasing temperature to a value five times larger than the estimated paramagnetic pair-breaking field. Theories for the enhancement of $H_{c2}$ invoking spin-orbit scattering or strong-coupling superconductivity are shown to be inadequate in explaining the observed behavior, suggesting that the pairing state in Li$_{0.9}$Mo$_6$O$_{17}$ is unconventional and possibly spin-triplet.
Magnetic excitation spectra c(q,w) of YBa2Cu3Oy and La214 systems have been studied. For La1.88Sr0.12CuO4, c(q,w) have been measured up to ~30 meV and existing data have been analyzed up to the energy w~150 meV by using the phenomenological expression of the generalized magnetic susceptibility c(q,w)=c0(q,w)/{1+J(q)c0(q,w)}, where c0(q,w) is the susceptibility of the electrons without the exchange coupling J(q) among them. In the relatively low energy region up to slightly above the resonance energy Er, it has been reported by the authors group that the expression can explain characteristics of the q- and w-dependence of the spectra of YBa2Cu3Oy (YBCO or YBCOy). Here, it is also pointed out that the expression can reproduce the rotation of four incommensurate peaks of c(q,w) within the a*-b* plane about (p/a, p/a) {or so-called (p, p)} point by 45 degree, which occurs as w goes to the energy region far above Er from E below Er. For La2-xSrxCuO4 and La2-xBaxCuO4, agreements between the observed results and the calculations are less satisfactory than for YBCO, indicating that we have to take account of the existence of the stripes to consistently explain the observed c(q,w) of La214 system especially near x=1/8.
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