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Register a new userEvidence for the presence of the Fulde-Ferrell-Larkin-Ovchinnikov state in CeCu$_2$Si$_2$ revealed using $^{63}$Cu NMR
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Nuclear magnetic resonance measurements were performed on CeCu$_{2}$Si$_{2}$ in the presence of a magnetic field close to the upper critical field $mu_{0} H_{rm c2}$ in order to investigate its superconducting (SC) properties near pair-breaking fields. In lower fields, the Knight shift and nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ abruptly decreased below the SC transition temperature $T_{rm c}(H)$, a phenomenon understood within the framework of conventional spin-singlet superconductivity. In contrast, $1/T_1T$ was enhanced just below $T_{rm c}(H)$ and exhibited a broad maximum when magnetic fields close to $mu_0H_{rm c2}(0)$ were applied parallel or perpendicular to the $c$ axis, although the Knight shift decreased just below $T_{rm c}(H)$. This enhancement of $1/T_1T$, which was recently observed in the organic superconductor $kappa$-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$, suggests the presence of high-density Andreev bound states in the inhomogeneous SC region, a hallmark of the Fulde-Ferrell-Larkin-Ovchinnikov phase.
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The specific heat of the layered organic superconductor $kappa$-% (BEDT-TTF)$_2$Cu(NCS)$_2$, where BEDT-TTF is bisethylenedithio-% tetrathiafulvalene, has been studied in magnetic fields up to 28 T applied perpendicular and parallel to the superconducting layers. In parallel fields above 21 T, the superconducting transition becomes first order, which signals that the Pauli-limiting field is reached. Instead of saturating at this field value, the upper critical field increases sharply and a second first-order transition line appears within the superconducting phase. Our results give strong evidence that the phase, which separates the homogeneous superconducting state from the normal state is a realization of a Fulde-Ferrell-Larkin-Ovchinnikov state.
We show that in the presence of magnetic field, two superconducting phases with the center-of-mass momentum of Cooper pair parallel to the magnetic field are induced in spin-orbit-coupled superconductor Li$_2$Pd$_3$B. Specifically, at small magnetic field, the center-of-mass momentum is induced due to the energy-spectrum distortion and no unpairing region with vanishing singlet correlation appears. We refer to this superconducting state as the drift-BCS state. By further increasing the magnetic field, the superconducting state falls into the Fulde-Ferrell-Larkin-Ovchinnikov state with the emergence of the unpairing regions. The observed abrupt enhancement of the center-of-mass momenta and suppression on the order parameters during the crossover indicate the first-order phase transition. Enhanced Pauli limit and hence enlarged magnetic-field regime of the Fulde-Ferrell-Larkin-Ovchinnikov state, due to the spin-flip terms of the spin-orbit coupling, are revealed. We also address the triplet correlations induced by the spin-orbit coupling, and show that the Cooper-pair spin polarizations, generated by the magnetic field and center-of-mass momentum with the triplet correlations, exhibit totally different magnetic-field dependences between the drift-BCS and Fulde-Ferrell-Larkin-Ovchinnikov states.
We develop the Ginzburg-Landau theory of the vortex lattice in clean isotropic three-dimensional superconductors at large Maki parameter, when inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov state is favored. We show that diamagnetic superfluid currents mainly come from paramagnetic interaction of electron spins with local magnetic field, and not from kinetic energy response to the external field as usual. We find that the stable vortex lattice keeps its triangular structure as in usual Abrikosov mixed state, while the internal magnetic field acquires components perpendicular to applied magnetic field. Experimental possibilities related to this prediction are discussed.
We study attractively interacting spin-1/2 fermions on the square lattice subject to a spin population imbalance. Using unbiased diagrammatic Monte Carlo simulations we find an extended region in the parameter space where the Fermi liquid is unstable towards formation of Cooper pairs with non-zero center-of-mass momentum, known as the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. In contrast to earlier mean-field and quasi-classical studies we provide quantitative and well-controlled predictions on the existence and location of the relevant Fermi-liquid instabilities. The highest temperature where the FFLO instability can be observed is about half of the superfluid transition temperature in the unpolarized system.
The Higgs mode associated with amplitude fluctuations of the superconducting gap in uniform superconductors usually is heavy, which makes its excitation and detection difficult. We report on the existence of a gapless Higgs mode in the Fulde-Ferrell-Larkin-Ovchinnikov states. This feature is originated from the Goldstone mode associated with the translation symmetry breaking. The existence of the gapless Higgs mode is demonstrated by using both a phenomenological model and microscopic Bardeen-Cooper-Schrieffer (BCS) theory. The gapless Higgs mode can avoid the decay into other low energy excitations, which renders it stable and detectable.
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