Departmental Oral Examination (Thesis Title: "Temperature-driven spectral weight transfer in doped magnetic insulators")

Event Date and Time: 
Fri, 2016-08-26 10:00 - 13:00
Henn 309
Intended Audience: 

In this thesis we study the effects of finite temperature (T) on the single-electron spectral function of doped magnetic insulators.

First, we derive the low-temperature correction to the self-energy of a charge carrier injected with parallel spin into a ferromagnetic background which is modeled with both Heisenberg and Ising Hamiltonians so that differences due to gapless versus gapped magnons can be understood. Beside the expected thermal broadening of the T=0 quasiparticle peak which becomes a resonance inside a continuum, we find that spectral weight is transferred to regions lying outside this continuum, because the carrier and a thermal magnon can bind into a spin-polaron.  This work is valid in dimensions d > 1, because it does not include the role of magnetic domains which are important in 1d.

We then consider the role of these magnetic domains in 1d systems, for models where spin-polaron formation is impossible. We present Monte Carlo simulations for the spectral function of three related models of a charge carrier that is injected into an Ising chain. Both ferromagnetic and antiferromagnetic coupling between the Ising spins are considered. The interaction between the carrier and the Ising spins is also of Ising type. In two of the models the charge carrier is hosted by a different band, while in the third model it is hosted by the same band as the Ising spins. We find that the carrier's spectral function exhibits a distinctive fine structure due to its temporary entrapping inside small magnetic domains, and use these results to construct an accurate (quasi)analytic approximation for low and medium T. While at T=0, for ferromagnetic order all three models have identical, low-energy quasiparticles, at finite T the low-energy behavior of the first two models remains equivalent, but that of the third model is controlled by rare events due to thermal fluctuations, which transfer spectral weight below the T=0 quasiparticle peak, generating a pseudogaplike phenomnology. Taken together, our results show that the temperature evolution of the spectral weight of weakly doped magnetic insulators can be very complex.

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