No abstract

Complex and correlated quantum systems with promise for new functionality often involve entwined electronic degrees of freedom. In such materials, highly unusual properties emerge and could be the result of electron localization. Here, a cubic heavy fermion metal governed by spins and orbitals is chosen as a model system for this physics. Its properties are found to originate from surprisingly simple low-energy behavior, with 2 distinct localization transitions driven by a single degree of freedom at a time. This result is unexpected, but we are able to understand it by advancing the notion of sequential destruction of an SU(4) spin–orbital-coupled Kondo entanglement. Our results implicate electron localization as a unified framework for strongly correlated materials and suggest ways to exploit multiple degrees of freedom for quantum engineering.

57 Fe Mössbauer spectra of Fe 3 O 2 BO 3 reveal a combined effect of charge ordering and electron delocalization between 112 and 450 K. On the basis of the temperature dependence of the isomer shifts and quadrupole interactions, together with the information from previously obtained transport data, we are able to discuss the arrangement of Fe 2+ and Fe 3+ in the structure and the dynamics of the electronic configurations. We found a charge-delocalization transition around 300 K. Below this temperature, formation of pairs of Fe ions with mixed valence takes place in part of the crystalline structure.

The phase diagram of the heavy fermion compound YbFe 2 Ge 2 under high pressures P ഛ 18.2 GPa was obtained by electrical resistivity measurements. Pressure drives the system from a paramagnetic Fermi liquid state to a magnetically ordered state, with a quantum critical point at P C Ϸ 9.4 GPa. In the vicinity of P C a non-Fermi-liquid behavior ascribed to two-dimensional antiferromagnetic fluctuations is observed. In the magnetic side, the resistivity shows the existence of spin-wave excitations characteristic of an antiferromagnet.

Temperature dependent magnetization, muon spin rotation and 57 Fe Mössbauer spectroscopy experiments performed on crystals of intermetallic 0.14, 0.17, 0.22, 0.27, 0.29, 0.32) are reported. Whereas at y = 0.11 even a sensitive magnetic microprobe such as µSR does not detect magnetism, all other samples display weak ferromagnetism with a magnetic moment of up to 0.22 µB per Fe atom. As a function of doping and of temperature a crossover from short range to long range magnetic order is observed, characterized by a broadly distributed spontaneous internal field. However, the y = 0.14 and y = 0.17 remain in the short range ordered state down to the lowest investigated temperature. The transition from short range to long range order appears to be accompanied by a change of the character of the spin fluctuations, which exhibit spin wave excitations signature in the LRO part of the phase diagram. Mössbauer spectroscopy for y = 0.27 and 0.32 indicates that the internal field lies in the plane perpendicular to the crystallographic c axis. The field distribution and its evolution with doping suggest that the details of the Fe magnetic moment formation and the consequent magnetic state are determined not only by the dopant concentration but also by the way the replacement of the Ga atoms surrounding the Fe is accomplished.