Electron mass enhancement and magnetic phase separation near the Mott transition in double-layer ruthenates

Abstract

We present a detailed investigation of the specific heat in Ca3(Ru1-xMx)2O7 (M = Ti, Fe, Mn) single crystals. With different dopants and doping levels, three distinct regions are present, including a quasi-2D metallic state with an antiferromagnetic (AFM) order formed by ferromagnetic bilayers (AFM-b), a Mott insulating state with G-type AFM order (G-AFM) and a localized state with a mixed AFM-b and G-AFM phase. Our specific heat data provide deep insights into the Mott transitions induced by Ti and Mn dopings. We observed not only an anomalous large mass enhancement but also an additional term in the specific heat i.e. 2 C T in the localized region. The 2 C T  term is most likely due to the long-wavelength excitations with both FM and AFM components. A decrease of Debye temperature is observed in the G-type AFM region, indicating a lattice softening associated with the Mott transition. n  . However, non-zero value of  in insulating materials were reported before. P. W. Anderson proposed that the linear specific heat component is a general feature on the glassy state, including spin glass, due to statistical distribution of localized "tunneling levels[28]. Interestingly, various studies of manganites reveal large γ in insulating crystalline compositions. For example, in Nd0.67Sr0.33MnO3, a value of 2 25 / mJ mol K   was observed and in the electron-doped system La2.3Ca0.7Mn2O3, 2 41 / mJ mol K   .[29,30]. In the hole-doped LaMnO3+δ,  reaches as high as 2 23/ mJ mol K [31]. In La0.2Sr0.8MnO3, 2 5.6 / mJ mol K   [32] .Obviously that the finite linear specific heat coefficient not only appear in spin glass phase.In undoped Ca3Ru2O7, the small value of sommerfeld coefficient (  ~ 1.7 mJ/ Ru mol T 2 ) arising from the non-nesting fermi surface pockets survived. In Ru-site doped Ca3Ru2O7, the itinerant Ru t2g electrons may be localized due to the potential fluctuations arising from cation substitution and spin-dependent fluctuations due to local deviations from AFM-b magnetic order. If the doping concentration is fairly low, the localization length may be fairly large. Charge carriers can thus hop through a number of Ru ions, defines limited length of bilayer FM clusters. On the other hand, the electron levels although localized, are not largely spaced in energy, allowing for thermal excitations that contribute with a linear term to specific heat. From Fig. 4, we noticed that enhanced  appears simultaneously with the T 2 term. As state above, the T 2 term is due to the longwavelength excitations with both FM and AFM components in the magnetic phase separation region.The proposed scenario above is consistent with this picture.Above scenario explain the non-zero sommerfeld coefficient in insulating state. Why this coefficient is one order larger than pristine compound. Actually, enhanced  near Mott transition is ubiquitous, especially the ones with antiferromagnetism. For example, the heavy fermion compounds, the high-Tc cuprates, the Mott-Hubbard systems V2O3 and Ni(Se1-xSx)2 [8,33,34]. In V2O3 system, met...