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Register a new userQuasiclassical theory of $C_4$-symmetric magnetic order in disordered multiband metals
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Recent experimental studies performed in the normal state of iron-based superconductors have discovered the existence of the $C_4$-symmetric (tetragonal) itinerant magnetic state. This state can be described as a spin density wave with two distinct magnetic vectors ${vec Q}_1$ and ${vec Q}_2$. Given an itinerant nature of magnetism in iron-pnictides, we develop a quasiclassical theory of tetragonal magnetic order in disordered three-band metal with anisotropic band structure. Within our model we find that the $C_4$-symmetric magnetism competes with the $C_2$-symmetric state with a single ${vec Q}$ magnetic structure vector. Our main results is that disorder promotes tetragonal magnetic state which is in agreement with earlier theoretical studies.
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We consider a general problem of a Josephson contact between two multiband superconductors with coexisting superconducting and magnetic phases. As a particular example, we use the quasiclassical theory of superconductivity to study the properties of a Josephson contact between two disordered $s^{pm}$-wave superconductors allowing for the coexistence between superconductivity and spin-density-wave orders. The intra- and inter-band scattering effects of disorder are treated within the self-consistent Born approximation. We calculate the spatial profile of the corresponding order parameters on both sides of the interface assuming that the interface has finite reflection coefficient and use our results to evaluate the local density of states at the interface as well as critical supercurrent through the junction as a function of phase or applied voltage. Our methods are particularly well suited for describing spatially inhomogeneous states of iron-based superconductors where controlled structural disorder can be created by an electron irradiation. We reveal the connection between our theory and the circuit-theory of Andreev reflection and extend it to superconducting junctions of arbitrary nature. Lastly, we outline directions for further developments in the context of proximity circuits of correlated electron systems.
The crossover between a free magnetic moment phase and a Kondo phase in low dimensional disordered metals with dilute magnetic impurities is studied. We perform a finite size scaling analysis of the distribution of the Kondo temperature as obtained from a numerical renormalization group calculation of the local magnetic susceptibility and from the solution of the self-consistent Nagaoka-Suhl equation. We find a sizable fraction of free (unscreened) magnetic moments when the exchange coupling falls below a disorder-dependent critical value $J_{rm c}$. Our numerical results show that between the free moment phase due to Anderson localization and the Kondo screened phase there is a phase where free moments occur due to the appearance of random local pseudogaps at the Fermi energy whose width and power scale with the elastic scattering rate $1/tau$.
In comparison to 3d or 4f metals, magnetism in actinides remains poorly understood due to experimental complications and the exotic behavior of the 5f states. In particular, plutonium metal is most especially vexing. Over the last five decades theories proposed the presence of either ordered or disordered local moments at low temperatures. However, experiments such as magnetic susceptibility, electrical resistivity, nuclear magnetic resonance, specific heat, and elastic and inelastic neutron scattering show no evidence for ordered or disordered magnetic moments in any of the six phases of plutonium. Beyond plutonium, the magnetic structure of other actinides is an active area of research given that temperature, pressure, and chemistry can quickly alter the magnetic structure of the 5f states. For instance, curium metal has an exceedingly large spin polarization that results in a total moment of about 8 Bohr magneton/atom, which influences the phase stability of the metal. Insight in the actinide ground state can be obtained from core-level x-ray absorption spectroscopy (XAS) and electron energy-loss spectroscopy (EELS). A sum rule relates the branching ratio of the core-level spectra measured by XAS or EELS to the expectation value of the angular part of the spin-orbit interaction.
Motivated by recent experimental realizations of polar metals with broken inversion symmetry, we explore the emergence of strong correlations driven by criticality when the polar transition temperature is tuned to zero. Overcoming previously discussed challenges, we demonstrate a robust mechanism for coupling between the critical mode and electrons in multiband metals. We identify and characterize several novel interacting phases, including non-Fermi liquids, when band crossings are close to the Fermi level and present their experimental signatures for three generic types of band crossings.
Using powder neutron diffraction we have discovered an unusual magnetic order-order transition in the Ising spin chain compound Ca3Co2O6. On lowering the temperature an antiferromagnetic phase with propagation vector k=(0.5,-0.5,1) emerges from a higher temperature spin density wave structure with k=(0, 0, 1.01). This transition occurs over an unprecedented time-scale of several hours and is never complete.
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