Abstract
Unconventional superconductivity typically occurs in materials in which a small change of a parameter such as bandwidth or doping leads to antiferromagnetic or Mott insulating phases. As such competing phases are approached, the properties of the superconductor often become increasingly exotic. For example, in organic superconductors and underdoped high-Tc cuprate superconductors a fluctuating superconducting state persists to temperatures significantly above Tc. By studying alloys of quasi-two-dimensional organic molecular metals in the κ-(BEDT-TTF)2X family, we reveal how the Nernst effect, a sensitive probe of superconducting phase fluctuations, evolves in the regime of extreme Mott criticality. We find strong evidence that, as the phase diagram is traversed through superconductivity towards the Mott state, the temperature scale for superconducting fluctuations increases dramatically, eventually approaching the temperature at which quasiparticles become identifiable at all.
Highlights
Superconducting fluctuations in organic molecular metals enhanced by Mott criticality Moon-Sun Nam[1], Cecile Meziere[2], Patrick Batail[2], Leokadiya Zorina[3], Sergey Simonov3 & Arzhang Ardavan[1]
In the classic case of the Q2D k-(BEDT-TTF)2X family of organic molecular metals, t/U can be adjusted by varying the lattice spacing, either continuously by applying hydrostatic pressure to a Mott-insulating parent compound such as X 5 Cu[N(CN)2]Cl, or sampling discretely by varying the anion volume. [in k-(BEDT-TTF)2X, there is a degree of frustration determined by a second hopping energy scale, t9, with important consequences for proximity to and stability of the Mott state[4,5,6]
The superconducting Br1.0, Br0.8, and Br0.73 compounds each show metallic conductivity below 64 K, 62 K and 54 K respectively. It is a general characteristic of superconducting members of the k-(BEDT-TTF)2X family that they exhibit a crossover in this temperature range from a regime of diffusive conductivity characterised by negative dR/dT at high temperatures to a regime in which the electronic transport is coherent and metallic at lower temperatures
Summary
Superconducting fluctuations in organic molecular metals enhanced by Mott criticality Moon-Sun Nam[1], Cecile Meziere[2], Patrick Batail[2], Leokadiya Zorina[3], Sergey Simonov3 & Arzhang Ardavan[1]. This motivates a picture of an alternative kind of transition out of the superconducting state, into a state which does not exhibit the signatures of long range phase coherence such as zero resistance, but which does preserve certain features such as a gapped or pseudo-gapped spectrum, a degree of quasiparticle pairing, and remnants of the vortex physics Experimental support for this picture was offered by measurements of magnetothermoelectric effects in under-doped high-Tc superconductors, which revealed a Nernst signal (a voltage transverse to both an applied temperature gradient www.nature.com/scientificreports and magnetic field) characteristic of mobile superconducting vortices persisting to temperatures well above the zero magnetic field resistive transition temperature[12]. This model offers a good quantitative account of the Nernst effect in a range of systems including amorphous superconducting thin films[16,17] and optimally- and overdoped high-Tc superconductors[18,19], though it is less successful for under-doped high-Tc superconductors, perhaps owing to the proximity of the Mott state[18]
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