arXiv subjects

Christof Niedermayer

Publications and source records attributed to Christof Niedermayer.

21 records · Page 2Linked to original sources

arXiv2014

Magnetic nano-fluctuations in a frustrated magnet

Frustrated systems exhibit remarkable properties due to the high degeneracy of their ground states. Stabilised by competing interactions, a rich diversity of typically nanometre-sized phase structures appear in polymer and colloidal systems, while the surface of ice pre-melts due to geometrically frustrated interactions. Atomic spin systems where magnetic interactions are frustrated by lattice geometry provide a fruitful source of emergent phenomena, such as fractionalised excitations analogous to magnetic monopoles. The degeneracy inherent in frustrated systems may prevail all the way down to absolute zero temperature, or it may be lifted by small perturbations or entropic effects. In the geometrically frustrated Ising--like magnet Ca3Co2O6, we follow the temporal and spatial evolution of nanoscale magnetic fluctuations firmly embedded inside the spin--density--wave magnetic structure. These fluctuations are a signature of a competing ferrimagnetic phase with an incommensurability that is different from, but determined by the host. As the temperature is lowered, the fluctuations slow down into a super-paramagnetic regime of stable spatiotemporal nano-structures.

cond-mat.str-el↗

arXiv2014

Spin-charge interplay in antiferromagnetic La$_{2-x}$Sr$_{x}$CuO$_{4}$ studied by the muons, neutrons, and ARPES techniques

Exploring whether a spin density wave (SDW) is responsible for the charge excitations gap in the high-temperature superconducting cuprates is difficult, since the region of the phase diagram where the magnetic properties are clearly exposed is different from the region where the band dispersion is visible. On the one hand, long range magnetic order disappears as doping approaches 2% from below, hindering our ability to perform elastic neutron scattering (ENS). On the other hand, cuprates become insulating at low temperature when the doping approaches 2% from above, thus restricting angle-resolved photoemission spectroscopy (ARPES). In fact, ARPES data for samples with doping lower than 3% are rare and missing the quasiparticle peaks in the energy distribution curves (EDCs). The main problem is the high resistivity of extremely underdoped samples, which is detrimental to ARPES due to charging effects. Nevertheless, the resistivity of La$_{2-x}$Sr$_{x}$CuO$_{4}$ as a function of temperature, at 2% doping, has a broad minimum around 100K. This minimum opens a window for both experiments. By preparing a series of LSCO single crystals with $\sim $0.2-0.3% doping steps around 2%, we managed to find one to which both techniques apply. This allows us to explore the cross talk between the magnetic and electronic properties of the material.

cond-mat.supr-con↗

arXiv2012

New Perspectives for Cuprate Research: CLBLCO Single Crystals

We report the successful growth of a large (Ca$_{x}$La$_{1-x}$)(Ba$_{1.75-x}$La$_{0.25+x}$)Cu$_{3}$O$_{y}$ (CLBLCO) single crystal. In this material, $x$ controls the maximum of Tc ($T_{c}^{\max}$), with minimal structural changes. Therefore, it allows a search for correlations between material properties and $T_{c}^{\max}$. We demonstrate that the crystals are good enough for neutron scattering and cleave well enough for Raman scattering. These results open new possibilities for cuprate research.

cond-mat.supr-con↗