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A weird electronic state accompanied with an anomalous superconductingprecursor and/or exotic orders, called the pseudogap state, arises prior to asuperconducting condensate in underdoped cuprates that are situated near Motttransition. Another way to make the system approach the Mott transition is thevariation of bandwidth or correlation strength, which gives a new dimension toexploring this exotic state. Here we report nuclear magnetic resonance (NMR)studies on layered organic superconductors with half-filled bands whose widthsare pressure-tuned near the Mott transition. The system situated on the vergeof the Mott transition shows a pseudogap-like anomalous suppression of spinexcitations on cooling from well above the superconducting critical temperature$T_{\mathrm{c}}$. The pressure variation of the NMR relaxation rate shows thatthe pseudogap-like behavior is rapidly suppressed by applying pressure. The NMRexperiments under various magnetic fields varied up to 18 T proves the absenceof symmetry breaking orders that compete with superconductivity, such as chargeorders, in the metallic phase. Remarkably, the pseudogap-like behavior above$T_{\mathrm{c}}$ and the superconducting condensate fade out in parallel underascending magnetic fields with similar field-orientation dependence, indicatinga superconducting precursor is the predominant origin of the pseudogap. Ourfurther investigation of different materials, which take different "distances"from the Mott transition by chemical pressure, confirms that thesuperconducting precursor is not the conventional amplitude fluctuationsarising from low dimensionality but unconventional preformation of Cooper pairsenhanced near the Mott transition. These findings conclude that preformedCooper pairs persist up to twice as high as $T_{\mathrm{c}}$ on the verge ofthe bandwidth-controlled Mott transition.

Microscopic Evidence for Preformed Cooper Pairs in Pressure-Tuned Organic Superconductors near Mott Transition