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Register a new userInherent spin density wave instability by vortices in superconductors with strong Pauli effects
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A novel spin density wave (SDW) instability mechanism enhanced by vortices under fields is proposed to explain the high field and low temperature (HL) phase in CeCoIn$_5$. In the vortex state the strong Pauli effect and the nodal gap conspire to enhance the momentum resolved spectral weight exclusively along the nodal direction over the normal value, providing a favorable nesting condition for SDW with ${bf Q}=(2k_F, 2k_F, 0.5)$ only under high field ($H$). Observed mysteries of the field-induced SDW confined within $H_{c2}$ are understood consistently, such facts that ${bf Q}$ is directed to the nodal direction independent of $H$, SDW diminishes under tilting field from the $ab$ plane, and the SDW transition line in $(H,T)$ has a positive slope.
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The magnetic field distribution around the vortices in TmNi2B2C in the paramagnetic phase was studied experimentally as well as theoretically. The vortex form factor, measured by small-angle neutron scattering, is found to be field independent up to 0.6 Hc2 followed by a sharp decrease at higher fields. The data are fitted well by solutions to the Eilenberger equations when paramagnetic effects due to the exchange interaction with the localized 4f Tm moments are included. The induced paramagnetic moments around the vortex cores act to maintain the field contrast probed by the form factor.
Very recently a new family of layered materials, containing BiS2 planes was discovered to be superconducting at temperatures up to Tc=10 K, raising questions about the mechanism of superconductivity in these systems. Here, we present state-of-the-art first principles calculations that directly address this question and reveal several surprising findings. The parent compound LaOBiS2 possesses anharmonic ferroelectric soft phonons at the zone center with a rather large polarization of $approx 10 mu C/cm^2$, which is comparable to the well-known ferroelectric BiFeO3. Upon electron doping, new unstable phonon branches appear along the entire line Q=(q,q,0), causing Bi/S atoms to order in a one-dimensional charge density wave (CDW). We find that BiS2 is a strong electron-phonon coupled superconductor in the vicinity of competing ferroelectric and CDW phases. Our results suggest new directions to tune the balance between these phases and increase Tc in this new class of materials.
We predict a novel buckling instability in the critical state of thin type-II superconductors with strong pinning. This elastic instability appears in high perpendicular magnetic fields and may cause an almost periodic series of flux jumps visible in the magnetization curve. As an illustration we apply the obtained criteria to a long rectangular strip.
In the present work we investigate the behavior of a vortex in a long superconducting cylinder near to a columnar defect at the center. The derivations of the local magnetic field distribution and the Gibbs free energy will be carried out for a cylinder and a cavity of arbitrary sizes. From the general expressions, it considered two particular limits: one in which the radius of the cavity is very small but the radius of the superconducting cylinder is kept finite; and one in which the radius of the superconducting cylinder is taken very large (infinite) but the radius of the cavity is kept finite. In both cases the maximum number of vortices which are allowed in the cavity is determined. In addition, the surface barrier field for flux entrance into the cavity is calculated.
Neutron diffraction studies of Ba(Fe[1-x]Co[x])2As2 reveal that commensurate antiferromagnetic order gives way to incommensurate magnetic order for Co compositions between 0.056 < x < 0.06. The incommensurability has the form of a small transverse splitting (0, +-e, 0) from the nominal commensurate antiferromagnetic propagation vector Q[AFM] = (1, 0, 1) (in orthorhombic notation) where e = 0.02-0.03 and is composition dependent. The results are consistent with the formation of a spin-density wave driven by Fermi surface nesting of electron and hole pockets and confirm the itinerant nature of magnetism in the iron arsenide superconductors.
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