Influence of Spin and Charge Fluctuations on Magnetic-Field-Induced Insulator-to-Metal Transitions

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In certain condensed-matter materials, the complex interplay of electronic, magnetic and struc- tural degrees of freedom causes unexpected and spectacular new physical phenomena. Among these are high-temperature superconductivity in cuprates and the so-called colossal magnetoresistance (CMR) in manganites and magnetic semiconductors. A universal characteristic of these complex materials apparently is the occurrence of intrinsic (i.e. non-chemical) spatial phase separation at the nanoscale, often leading to percolative electronic and magnetic phase transitions. Interestingly, many aspects of such inhomogeneous states, in particular their dynamic properties, until now have not been investigated in much detail and, therefore, are not well understood. It turns out that electronic phase separation and colossal magnetoresistance also occur in less-complex rare-earth compounds. Our project aims to study the fundamental properties and the dynamics of magnetic and electronic phase separation on the basis of a comparatively simple model system, namely the cubic and magnetically-isotropic EuB6. In systematic investigations of the substitution/doping series (Eu,Ca)B6 and (Eu,Sm)B6 we aim to follow the development of magnetic polarons and understand the strong enhancement of the CMR effect upon isoelectronic substitution and the effect of adding itinerant carriers and enhancing a local antiferromagnetic coupling. To that end, we aim to employ a unique combination of macroscopic, microscopic and time-resolved measurement techniques, which ideally fits to the questions we aim to answer in this proposal. The methods are, in particular, a combination of time-resolved electronic transport (resistance noise spectroscopy) and magnetic-flux-noise spectroscopy allowing to study both the charge and spin channels of slow percolation dynamics, as well as atomically resolved low-temperature scanning tunneling microscopy and spectroscopy, with the option of employing spin-polarized tips, to visualize and ultimately verify the existence of magnetic clusters and their percolation upon entering the magnetic state or in increasing magnetic fields. The general applicability of the polaron concept as derived from the above investigations is going to be scrutinized by studying selected other CMR materials, such as HgCr2Se4 and Eu5In2Sb6. The chosen approach is set out to reveal direct evidence for or against certain theoretically predicted types of behavior and to delineate important system parameters that help to better understand the influence of the state of magnetization on the electronic properties. The expected results on an exemplary model system promise insights for a better understanding and further development of more complex materials. Moreover, in view of the very recent proposal of a non-trivial topology in EuB6 (which is discussed for SmB6 for some time) our measurements on the (Eu,Sm)B6 substitution series will most certainly contribute to this discussion.