We consider the role of potential scatterers in the nematic phase of Fe-based superconductors above the transition temperature to the (pi,0) magnetic state but below the orthorhombic structural transition. The anisotropic spin fluctuations in this re
gion can be frozen by disorder, to create elongated magnetic droplets whose anisotropy grows as the magnetic transition is approached. Such states act as strong anisotropic defect potentials which scatter with much higher probability perpendicular to their length than parallel, although the actual crystal symmetry breaking is tiny. We calculate the scattering potentials, relaxation rates, and conductivity in this region, and show that such emergent defect states are essential for the transport anisotropy observed in experiments.
A novel feature of the iron arsenides is the magnetoelastic coupling between the long wavelength in-plane strains of the lattice and the collective spin fluctuations of the electrons near the magnetic ordering wavevectors. Here, we study its microsco
pic origin from an electronic model with nested Fermi pockets and a nominal interaction. We find the couplings diverge with a power-law as the system is tuned to perfect nesting. Furthermore, the theory reveals how nematicity is boosted by nesting. These results are relevant for other systems with nesting driven density wave transitions.
Using electronic Raman spectroscopy, we report direct measurements of charge nematic fluctuations in the tetragonal phase of strain-free Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ single crystals. The strong enhancement of the Raman response at low temperatu
res unveils an underlying charge nematic state that extends to superconducting compositions and which has hitherto remained unnoticed. Comparison between the extracted charge nematic susceptibility and the elastic modulus allows us to disentangle the charge contribution to the nematic instability, and to show that charge nematic fluctuations are weakly coupled to the lattice.
Single band theories of quantum criticality successfully describe a single-particle lifetime with non-Fermi liquid temperature dependence. But, they fail to obtain a charge transport rate with the same dependence unless the interaction is assumed to
be momentum independent. Here we demonstrate that a quantum critical material, with a long range mode that transmutes electrons between light and heavy bands, exhibits a quasi-linear temperature dependence for {it both} the single-particle and the charge transport lifetimes, despite the strong momentum dependence of the interaction.
We examine the relevance of magneto-elastic coupling to describe the complex magnetic and structural behaviour of the different classes of the iron superconductors. We model the system as a two-dimensional metal whose magnetic excitations interact wi
th the distortions of the underlying square lattice. Going beyond mean field we find that quantum fluctuation effects can explain two unusual features of these materials that have attracted considerable attention. First, why iron telluride orders magnetically at a non-nesting wave-vector $(pi/2, pi/2)$ and not at the nesting wave-vector $(pi, 0)$ as in the iron arsenides, even though the nominal band structures of both these systems are similar. And second, why the $(pi, 0)$ magnetic transition in the iron arsenides is often preceded by an orthorhombic structural transition. These are robust properties of the model, independent of microscopic details, and they emphasize the importance of the magneto-elastic interaction.
Iron telluride doped lightly with selenium is known to undergo a first order magneto-structural transition before turning superconducting at higher doping. We study the effects of magneto-elastic couplings on this transition using symmetry considerat
ions. We find that the magnetic order parameters are coupled to the uniform monoclinic strain of the unit cell with one iron per cell, as well as to the phonons at high symmetry points of the Brillouin zone. In the magnetic phase the former gives rise to monoclinic distortion while the latter induces dimerization of the ferromagnetic iron chains due to alternate lengthening and shortening of the nearest-neighbour iron-iron bonds. We compare this system with the iron arsenides and propose a microscopic magneto-elastic Hamiltonian which is relevant for all the iron based superconductors. We argue that this describes electron-lattice coupling in a system where electron-electron interaction is crucial.
We study the finite-frequency inter-band transition peak in the optical conductivity of a heavy fermion system close to a Kondo breakdown quantum critical point, where the lattice Kondo temperature vanishes. As the system approaches the phase transit
ion from the heavy Fermi liquid side, we find a new cross-over regime where the peak position is related to, but is not directly proportional to, the lattice Kondo scale. In particular, the position of the peak moves to lower energies, but remains finite at the critical point. On the other hand, the peak value changes non-monotonically and eventually the peak disappears at the quantum critical point, indicating the decoupling of the narrow band of f-electrons from the conduction band. We argue that these are unique signatures of a Kondo breakdown transition, and therefore can be useful to distinguish it experimentally from a spin density wave instability.
We compute the tunneling conductance of graphene as measured by a scanning tunneling microscope (STM) with a normal/superconducting tip. We demonstrate that for undoped graphene with zero Fermi energy, the first derivative of the tunneling conductanc
e with respect to the applied voltage is proportional to the density of states of the STM tip. We also show that the shape of the STM spectra for graphene doped with impurities depends qualitatively on the position of the impurity atom in the graphene matrix and relate this unconventional phenomenon to the pseudopsin symmetry of the Dirac quasiparticles in graphene. We suggest experiments to test our theory.
