Evidence for spin droplets (ferrons) formation in the heavy fermion metal CeB$_6$ with dynamic charge stripes

TL;DR

This study provides evidence for the formation of spin droplets, or ferrons, in CeB$_6$ due to dynamic charge stripes, revealed through low-temperature transport, thermal, and diffraction experiments, highlighting a nanoscale phase separation.

Contribution

The paper introduces a phenomenological model linking spin droplets to charge stripes and Jahn-Teller modes in CeB$_6$, demonstrating their role in magnetic and electronic properties.

Findings

Exponential field dependence of resistivity, thermal conductivity, and specific heat in CeB$_6$.

Effective magnetic moments in the AFQ phase range from 1.4 to 1.9 μB.

Detection of electron phase separation and dynamic charge stripes via X-ray diffraction.

Abstract

The presented studies of resistivity (R), thermal conductivity (k) and specific heat (C) at low temperature 1.8-7 K in magnetic field up to 90 kOe made it possible to detect for the first time the exponential field dependences R(H), 1/k(H), $C(H) \sim \exp(-{\mu}_{\rm eff}H/kBT)$ of the charge transport and thermal characteristics in the so-called antiferroquadrupole (AFQ) phase of the archetypal heavy-fermion CeB$_6$ hexaboride. From magnetoresistance measurements it is shown that in the AFQ state the effective magnetic moment varies in the range ${\mu}_{\rm eff}(T) = 1.4$-1.9$\mu$B, and its value is very close to ${\mu}_{\rm eff}(\tau)(T) = 2{\mu}$B, derived from the field dependence of the relaxation time $|tau(H)$ observed in the heat capacity and thermal conductivity experiments. The phenomenological model proposed here allowed us to attribute the magnetic moments to spin droplets (ferrons), that appear in the bulk AFQ phase of CeB$_6$ crystals. The relevant electron phase separation at the nanoscale, manifested by dynamic charge stripes, that leads to the formation of ferrons, was revealed from the analysis of low-temperature X-ray diffraction experiments using the maximum entropy method. We argue that the Jahn-Teller collective mode of B$_6$ clusters is responsible for the charge stripe formation, which subsequently induces transverse quasi-local vibrations of Ce ions in the form of pairs and triples. These lead to 4f-5d spin fluctuations providing spin-polarons (ferrons) in the CeB$_6$ matrix.