Since the discovery of superconductivity, there has been a drive to understand the mechanisms by which it occurs. The BCS (Bardeen-Cooper-Schrieffer) model successfully treats the electrons in conventional superconductors as pairs coupled by phonons (vibrational modes of oscillation) moving through the material, but there is as yet no accepted model for high-transition-temperature, organic or 'heavy fermion' superconductivity. Experiments that reveal unusual properties of those superconductors could therefore point the way to a deeper understanding of the underlying physics. In particular, the response of a material to a magnetic field can be revealing, because this usually reduces or quenches superconductivity. Here we report measurements of the heat capacity and magnetization that show that, for particular orientations of an external magnetic field, superconductivity in the heavy-fermion material CeCoIn(5) is enhanced through the magnetic moments (spins) of individual electrons. This enhancement occurs by fundamentally altering how the superconducting state forms, resulting in regions of superconductivity alternating with walls of spin-polarized unpaired electrons; this configuration lowers the free energy and allows superconductivity to remain stable. The large magnetic susceptibility of this material leads to an unusually strong coupling of the field to the electron spins, which dominates over the coupling to the electron orbits.

We report magnetocaloric and magnetic-torque evidence that in Cs2CuBr4--a geometrically frustrated Heisenberg S=1/2 triangular-lattice antiferromagnet--quantum fluctuations stabilize a series of spin states at simple increasing fractions of the saturation magnetization Ms. Only the first of these states--at M=1/3Ms--has been theoretically predicted. We discuss how the higher fraction quantum states might arise and propose model spin arrangements. We argue that the first-order nature of the transitions into those states is due to strong lowering of the energies by quantum fluctuations, with implications for the general character of quantum phase transitions in geometrically frustrated systems.

We report the first magneto-caloric and calorimetric observations of a magnetic-field-induced phase transition within a superconducting state to the long-sought exotic "FFLO" superconducting state first predicted over 50 years ago. Through the combination of bulk thermodynamic calorimetric and magnetocaloric measurements in the organic superconductor κ -(BEDT-TTF)2Cu(NCS)2, as a function of temperature, magnetic field strength, and magnetic field orientation, we establish for the first time that this field-induced first-order phase transition at the paramagnetic limit Hp for traditional superconductivity is to a higher entropy superconducting phase uniquely characteristic of the FFLO state. We also establish that this high-field superconducting state displays the bulk paramagnetic ordering of spin domains required of the FFLO state. These results rule out the alternate possibility of spin-density wave (SDW) ordering in the high field superconducting phase. The phase diagram determined from our measurements -including the observation of a phase transition into the FFLO phase at Hp -is in good agreement with recent NMR results and our own earlier tunnel-diode magnetic penetration depth experiments, but is in disagreement with the only previous calorimetric report. 74.25.Dw, 74.70.Kn, 74.25.Ha Magnetic fields destroy superconductivity. In most cases, this occurs due to the formation of magnetic vortices -non-superconducting regions containing a magnetic field flux line shielded by circulating electrons -which increase in density as the magnetic field strength increases, ultimately displacing the superconducting phase. In the absence of magnetic vortices, the paramagnetic spin susceptibility of the electrons making up the superconducting "Cooper pairs" places another upper limit on superconductivity in magnetic fields. Because the electrons in these pairs have oppositely aligned magnetic moments (spins), the reduction in magnetic energy due to flipping the spin of an individual electron will exceed the reduction in electronic energy available from the formation of the Cooper pairs above a critical magnetic field H P known as the Clogston-Chandrasakar paramagnetic limit [1,2]. A phase transition from the superconducting to the normal metallic state is therefore expected at H P . Some 50 years ago, however, Fulde and Ferrell [3] and Larkin and Ovchinnikov [4] predicted that there might instead be a phase transition at H P to a different superconducting phase in which paramagnetic spin domains coexist with a spatially inhomogeneous superconducting phase. This "FFLO state" is expected to exist at fields above H P in electronically clean, anisotropic superconducting materials [5][6][7].The search for inhomogeneous superconductivity has spanned many years, and began in low dimensional single layers of superconducting materials [8]. The first clear calorimetric observations of a bulk field induced phase transition between two superconducting phases were observed in the heavy fermion compound CeCoIn 5 [9,10]. This tran...

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