Metallic Spin Chains: the Effects of Electronic Correlations*

Much of what is known about one-dimensional systems comes from theories and experiments that are carried out on insulating spin chain systems where strong correlations localize electron moments, or alternatively on one-dimensional metals where correlations are very weak, and the electron states are fully delocalized. There is considerable evidence in three-dimensional materials that varying the strength of electron correlations can lead to electronic localization, accompanied by T=0 quantum fluctuations between two Fermi surfaces with different sizes, one where electron states are included and one where they are not. We seek experimental evidence that there is an analog of this sort of electronic localization in one-dimensional systems. We focus our search on metallic systems, where the degree of hybridization between moment-bearing electrons and conduction electrons can have different strengths. The first system is Yb₂Pt₂Pb, where the 1-D chains of Yb moments are only weakly coupled to itinerant states. Fractionalized spinon excitations are observed in neutron scattering measurements, evidence for the one dimensionality of the magnetic subsystem. We present as well measurements on Ti₄MnBi₂, whose crystal structure features well separated chains of Mn ions. The Curie –Weiss susceptibility reveals that the Mn moments are effective spin S=1/2, while spin-polarized DFT calculations suggest that magnetic order is present, associated with the significantly delocalized d_x2-y2 and d_xy Mn orbitals. Specific heat and susceptibility measurements find a very broad transition to antiferromagnetism at 1.9 K, accompanied by significant fluctuations that may originate with electronic localization, or with frustration of the magnetic order by low dimensionality.