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 two dimensional Dirac fermions and quantum magnetoresistance in single crystals of CaMnBi2. The non-zero Berry's phase, small cyclotron resonant mass and first-principle band structure suggest the existence of the Dirac fermions in the Bi square nets. The in-plane transverse magnetoresistance exhibits a crossover at a critical field B * from semiclassical weak-field B 2 dependence to the high-field unsaturated linear magnetoresistance (∼ 120% in 9 T at 2 K) due to the quantum limit of the Dirac fermions. The temperature dependence of B * satisfies quadratic behavior, which is attributed to the splitting of linear energy dispersion in high field. Our results demonstrate the existence of two dimensional Dirac fermions in CaMnBi2 with Bi square nets.

The temperature dependence of the upper critical magnetic field (H c2 ) in a BaFe 1.84 Co 0.16 As 2 single crystal was determined via resistivity, for the inter-plane (H⊥ab) and in-plane (H//ab) directions in pulsed and static magnetic fields of up to 60 T. Suppressing superconductivity in a pulsed magnetic field at 3 He temperatures permits us to construct an H-T phase diagram from quantitative H c2 (0) values and determine its behavior in low temperatures. Hc 2 (0) with H//ab (H c2// (0)) and H⊥ab (H c2⊥ (0)) are ~ 55 T and 50 T respectively. These values are ~ 1.2 -1.4 times larger than the weak-coupling Pauli paramagnetic limit (H p = 1.84 T c ), indicating that enhanced paramagnetic limiting is essential and this superconductor is unconventional. While H c2// ab is saturated at low temperature, H c2 with H⊥ab (H c2⊥ ) exhibits almost linear temperature dependence towards T = 0 K which results in reduced anisotropy of H c2 in low temperature. The anisotropy of H c2 was ~ 3.4 near T c , and decreases rapidly with lower temperatures reaching ~ 1.1 at T = 0.7 K.

We report two-dimensional quantum transport in SrMnBi2 single crystals. The linear energy dispersion leads to the unusual nonsaturated linear magnetoresistance since all Dirac fermions occupy the lowest Landau level in the quantum limit. The transverse magnetoresistance exhibits a crossover at a critical field B * from semiclassical weak-field B 2 dependence to the high-field linear-field dependence. With increase in the temperature, the critical field B * increases and the temperature dependence of B * satisfies quadratic behavior which is attributed to the Landau level splitting of the linear energy dispersion. The effective magnetoresistant mobility µMR ∼ 3400 cm 2 /Vs is derived. Angular dependent magnetoresistance and quantum oscillations suggest dominant two-dimensional (2D) Fermi surfaces. Our results illustrate the dominant 2D Dirac fermion states in SrMnBi2 and imply that bulk crystals with Bi square nets can be used to study low dimensional electronic transport commonly found in 2D materials like graphene.PACS numbers: 72.20. My,75.47.Np Dirac fermions have raised great interest in condensed matter physics, as seen on the example of materials such as graphene 1 and topological insulators (TIs).2 The linear dispersion between momentum and energy of Dirac fermions brings forth some spectacular properties, such as zero effective mass and large transport mobility. 1,2In addition to the surface/interface states in TIs and graphene, Dirac states in bulk materials were discussed in organic conductors 3 and iron-based superconductors such as BaFe 2 As 2 .4,5 Recently, highly anisotropic Dirac states were observed in SrMnBi 2 , 6,7 where linear energy dispersion originates from the crossing of two Bi 6p x,y bands in the double-sized Bi square nets. SrMnBi 2 has a crystal structure similar to that of the superconducting Fe pnictides and is a bad metal. 7,8The Fermi velocity along Γ − M symmetry line is ν F ≈ 1.51 × 10 6 m/s, whereas the Fermi velocity in the orthogonal direction experiences nearly one order of magnitude decrease. 7,8One of the interesting properties of Dirac materials is the quantum transport phenomena.9,10 Unlike the conventional electron gas with parabolic energy dispersion, where Landau levels (LLs) are equidistant, 11 the distance between the lowest and 1 st LLs of Dirac fermions in magnetic field is very large and the quantum limit where all of the carriers occupy only the lowest LL is easily realized under moderate fields.12,13 Consequently some quantum transport phenomena such as quantum Hall effect and large linear magnetoresistance (MR) could be observed by conventional experimental methods in Dirac fermion system. 14-17Here we show two-dimensional (2D) quantum transport in bulk SrMnBi 2 single crystals. The linear energy dispersion leads to the unusual nonsaturated linear MR since all Dirac fermions occupy the lowest LL in the quantum limit. The transverse MR exhibits a crossover at a critical field B * from semiclassical weak-field MR ∼ B 2 to the high-field MR ∼ B dependence. The ...