arXiv subjects
Publications and source records attributed to Inti Sodemann.
The first-order interaction correction to the irreducible polarization function of pristine graphene is studied at arbitrary relation between momentum and frequency. The results are used to calculate the dielectric function and the dynamical conductivity of graphene beyond the standard random-phase approximation. The computed static dielectric constant compares favorably with recent experiments.
We estimate the strength of interaction-enhanced coherence between two graphene or topological insulator surface-state layers by solving imaginary-axis gap equations in the random phase approximation. Using a self-consistent treatment of dynamic screening of Coulomb interactions in the gapped phase, we show that the excitonic gap can reach values on the order of the Fermi energy at strong interactions. The gap is discontinuous as a function of interlayer separation and effective fine structure constant, revealing a first order phase transition between effectively incoherent and interlayer coherent phases. To achieve the regime of strong coherence the interlayer separation must be smaller than the Fermi wavelength, and the extrinsic screening of the medium embedding the Dirac layers must be negligible. In the case of a graphene double-layer we comment on the supportive role of the remote $π$-bands neglected in the two-band Dirac model.
We study the influence of population imbalance on the pairing, spin, and density instabilities of a two component ideal Fermi gas after a sudden quench of interactions near a Feshbach resonance. Over a large region of parameters the pairing instability is dominated by finite momentum pairing, suggesting the possibility of observing FFLO-like states in the unstable initial dynamics. Long-wavelength density instabilities are found on the BCS side of the resonance, and are interpreted as a precursor of the phase separation expected at equilibrium. On the BEC side of the resonance, the pairing instability is present for scattering lengths that are larger than a critical value that is only weakly dependent on population imbalance and always smaller than the scattering length at which the Stoner-like spin instability occurs.
Switching diagrams of nanoscale ferromagnets driven by a spin-transfer torque are studied in the macrospin approximation. We consider a disk-shaped free layer with in-plane easy axis and external magnetic field directed in-plane at 90 degrees to that axis. It is shown that this configuration is sensitive to the angular dependence of the spin-transfer efficiency factor and can be used to experimentally distinguish between different forms of $g(θ)$, in particular between the original Slonczewski form and the constant $g$ approximation. The difference in switching diagrams is especially pronounced at large spin polarizations, with the Slonczewski case exhibiting an anomalous region.
We derive an invariant form of the current-induced switching condition in spin-transfer devices and show that for energy minima and maxima the "switching ability" of the current is determined by the spin torque divergence. In contrast, energy saddle points are normally stabilized by current-induced merging with other equilibria. Our approach provides new predictions for several experimental setups and shows the limitations of some frequently used approximations.