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Register a new userRecent high-magnetic-field studies of unusual groundstates in quasi-two-dimensional crystalline organic metals and superconductors
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After a brief introduction to crystalline organic superconductors and metals, we shall describe two recently-observed exotic phases that occur only in high magnetic fields. The first involves measurements of the non-linear electrical resistance of single crystals of the charge-density-wave (CDW) system (Per)$_2$Au(mnt)$_2$ in static magnetic fields of up to 45 T and temperatures as low as 25 mK. The presence of a fully gapped CDW state with typical CDW electrodynamics at fields higher that the Pauli paramagnetic limit of 34 T suggests the existence of a modulated CDW phase analogous to the Fulde-Ferrell-Larkin-Ovchinnikov state. Secondly, measurements of the Hall potential of single crystals of $alpha$-(BEDT-TTF)$_2$KHg(SCN)$_4$, made using a variant of the Corbino geometry in quasistatic magnetic fields, show persistent current effects that are similar to those observed in conventional superconductors. The longevity of the currents, large Hall angle, flux quantization and confinement of the reactive component of the Hall potential to the edge of the sample are all consistent with the realization of a new state of matter in CDW systems with significant orbital quantization effects in strong magnetic fields.
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Although quasi-two-dimensional organic superconductors such as $kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$ seem to be very clean systems, with apparent quasiparticle mean-free paths of several thousand AA, the superconducting transition is intrinsically broad (e.g $sim 1$ K wide for $T_c approx 10$ K). We propose that this is due to the extreme anisotropy of these materials, which greatly exacerbates the statistical effects of spatial variations in the potential experienced by the quasiparticles. Using a statistical model, we are able to account for the experimental observations. A parameter $bar{x}$, which characterises the spatial potential variations, may be derived from Shubnikov-de Haas oscillation experiments. Using this value, we are able to predict a transition width which is in good agreement with that observed in MHz penetration-depth measurements on the same sample.
We report the results of high-field, low-temperature MuSR measurements of the quasi two-dimensional organic superconductors k{appa}-(ET)2Cu(NCS)2 and k{appa}-(ET)2Cu[N(CN)2]Br. The MuSR lineshapes for these compounds indicate the existence of partially-ordered vortex lattice phases in the high magnetic field regime, up to 2.5 T for the former compound and 4 T for the latter compound. The observed sharp loss of order is found to be consistent with a vortex-lattice melting transition that is predicted by numerical simulations of weakly coupled layers of pancake vortices. It is argued that the robustness of the partially-ordered vortex lattice phases could be due to strong flux-line pinning by a dilute ensemble of defects.
We report comprehensive Raman and infrared investigations of charge-order (CO) fluctuations in the organic metal $beta^{primeprime}$-(BEDT-TTF)$_2$SF$_5$CHFSO$_3$ and superconductor $beta^{primeprime}$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$. The charge-sensitive vibrational bands have been analyzed through an extension of the well-known Kubo model for the spectral signatures of an equilibrium between two states. At room temperature, both salts exhibit charge fluctuations between two differently charged molecular states with an exchange frequency of about $6times10^{11} {rm s}^{-1}$. The exchange rate of the metallic salt remains roughly constant down to 10 K, while in the superconductor the exchange velocity starts to decrease below 200 K, and a frozen charge-ordered state emerges, and coexists with the charge-order fluctuation state down to the superconducting temperature. These findings are confronted with other existing spectroscopic experiments, and a tentative phase diagram is proposed for the $beta^{primeprime}$ BEDT-TTF quarter-filled salts.
Muon spin relaxation ($mu$SR) measurements in high transverse magnetic fields ($parallel hat c$) revealed strong field-induced quasi-static magnetism in the underdoped and Eu doped (La,Sr)$_{2}$CuO$_{4}$ and La$_{1.875}$Ba$_{0.125}$CuO$_{4}$, existing well above $T_{c}$ and $T_{N}$. The susceptibility-counterpart of Cu spin polarization, derived from the muon spin relaxation rate, exhibits a divergent behavior towards $T sim 25$ K. No field-induced magnetism was detected in overdoped La$_{1.81}$Sr$_{0.19}$CuO$_{4}$, optimally doped Bi2212, and Zn-doped YBa$_{2}$Cu$_{3}$O$_{7}$.
We classify discrete-rotation symmetric topological crystalline superconductors (TCS) in two dimensions and provide the criteria for a zero energy Majorana bound state (MBS) to be present at composite defects made from magnetic flux, dislocations, and disclinations. In addition to the Chern number that encodes chirality, discrete rotation symmetry further divides TCS into distinct stable topological classes according to the rotation eigenspectrum of Bogoliubov-de Gennes quasi-particles. Conical crystalline defects are shown to be able to accommodate robust MBS when a certain combination of these bulk topological invariants is non-trivial as dictated by the index theorems proved within. The number parity of MBS is counted by a $mathbb{Z}_2$-valued index that solely depends on the disclination and the topological class of the TCS. We also discuss the implications for corner-bound Majorana modes on the boundary of topological crystalline superconductors.
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