Abstract
We have recently reported the first direct calorimetric observation of a magnetic-field-induced first-order phase transition into a high-field FFLO superconducting state at the Clogston-Chandrasekar ‘Pauli’ paramagnetic limitHp in a 2D superconductor κ − (BEDT-TTF)2Cu(NCS)2. The high-field state is both higher entropy and strongly paramagnetic, as thermodynamically required for the FFLO state. Here we compare our results with theoretical predictions for the field dependence of the high-field FFLO state in the 2D limit, revealing tentative evidence for transitions between FFLO states of differing order parameter. We also present calorimetric evidence for a 1st order phase transition into the FFLO state for a second 2D organic superconductor: β″ − (BEDT-TTF)2SF5(CH)2(CF)2(SO)3.
Highlights
What is the highest possible applied magnetic field in which superconductivity can exist for an electronically 2D superconductor? One possible answer derives from the paramagnetic spin susceptibility of the conduction electrons
Because the electrons in BCS superconducting pairs have oppositely aligned spins with momenta (k ↑, −k ↓), the reduction in Zeeman energy that arises from realignment of the electron spin in an applied magnetic field must eventually exceed the reduction in electronic energy available from the formation of superconducting Cooper pairs as the magnetic field increases in strength
Predictions for field-induced FFLO states in a 2D superconductor The additional features seen in the specific heat and magnetocaloric effect above Hp may indicate the existence of phase transitions within the FFLO region
Summary
What is the highest possible applied magnetic field in which superconductivity can exist for an electronically 2D superconductor? One possible answer derives from the paramagnetic spin susceptibility of the conduction electrons. For a 2D superconductor like κ − (ET)2Cu(NCS)2 with a zero field superconducting critical temperature Tc = 9.1 ± 0.2 K [4, 5] and a Pauli limit Hp = 20.7 ± 0.4 T [4, 8], theory predicts T ∗ = 5.1 ± 0.1 K and Hc22D(0) = 29.5 ± 0.7 T.
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