S. W. Tozer

Magnetic enhancement of superconductivity from electron spin domains.

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 CeCoIn5 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.

Fermi surface of the heavy-fermion superconductor CeCoIn5: The de Haas–van Alphen effect in the normal state

Measurements of the de Haas - van Alphen effect in the normal state of the heavy Fermion superconductor CeCoIn5 have been carried out using a torque cantilever at temperatures ranging from 20 to 500 mK and in fields up to 18 tesla. Angular dependent measurements of the extremal Fermi surface areas reveal a more extreme two dimensional sheet than is found in either CeRhIn5 or CeIrIn5. The effective masses of the measured frequencies range from 9 to 20 m*/m0.

Anisotropy of the Upper Critical Field in a Co-Doped BaFe2As2 Single Crystal

The temperature dependence of the upper critical magnetic field (Hc2) in a BaFe1.84Co0.16As2 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 3He temperatures permits us to construct an H-T phase diagram from quantitative Hc2(0) values and determine its behavior in low temperatures. Hc2(0) with H//ab (Hc2//(0)) and H^ab (Hc2^(0)) are ~ 55 T and 50 T respectively. These values are ~ 1.2 - 1.4 times larger than the weak-coupling Pauli paramagnetic limit (Hp = 1.84 Tc), indicating that enhanced paramagnetic limiting is essential and this superconductor is unconventional. While Hc2//ab is saturated at low temperature, Hc2 with H^ab (Hc2^) exhibits almost linear temperature dependence towards T = 0 K which results in reduced anisotropy of Hc2 in low temperature. The anisotropy of Hc2 was ~ 3.4 near Tc, and decreases rapidly with lower temperatures reaching ~ 1.1 at T = 0.7 K.

Electronic structure of CeRhIn 5 : de Haas–van Alphen and energy band calculations

The de Haas - van Alphen effect and energy band calculations are used to study angular dependent extremal areas and effective masses of the Fermi surface of the highly correlated antiferromagnetic material CeRhIn

Quantum Transport of Two-dimensional Dirac Fermions in SrMnBi(2)

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Anomalous superconductivity and field-induced magnetism in CeCoIn 5

. The agreement between experiment and theory is reasonable for the areas measured with the field applied along the (100) axis of the tetragonal structure, but disagree in size for the areas observed for the field applied along the (001) axis where the antiferromagnetic spin alignment is occurring. Detailed comparisons between experiment and theory are given.

Two-dimensional Dirac Fermions and Quantum Magnetoresistance in CaMnBi2

We report two-dimensional quantum transport in SrMnBi

Versatile and compact capacitive dilatometer

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Nonmetallic gasket and miniature plastic turnbuckle diamond anvil cell for pulsed magnetic field studies at cryogenic temperatures

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

Magnetic-field-induced lattice anomaly inside the superconducting state of CeCoIn5: anisotropic evidence of the possible Fulde-Ferrell-Larkin-Ovchinnikov state.

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