Quasi-two-dimensional metallic ground state ofCa3Ru2O7

Spin-wave excitation in the antiferromagnetic bilayer ruthenateCa3Ru2O7

“…Therefore, the above discussions provide evidences of the necessity of including a finite linewidth in the model to describe the broadening of the spectra, implying a finite lifetime of the magnetic excitation that is presumably due to the existence of a non-negligible number of itinerant charge carriers even below T MI . This is supported by various experimental observations: a) The in-plane resistivity of Ca 3 Ru 2 O 7 shows metallic behavior at low temperatures, although the resistivity along the c-axis enhances only by one order of magnitude or so below T MI [33]; b) ARPES experiments reveal a small Fermi surface pocket surviving at low temperatures [34]; c) Optical conductivity measurements show the existence of pseudogap opening below T MI which argue that the in-plane metallic behavior is triggered by hopping of the carriers due to the strong hybridization between different Ru t 2g orbital states [35]. Thus, it is reasonable to attribute the finite lifetime of the spin wave excitation observed in our experiment to the existence of itinerant charge carriers within the planes which induces the magnon broadening via the magnon-itinerant charge carrier scattering, a feature similar to the ferromagnetic metallic bilayer manganite [27].…”

Specific heat of (Ca1−xSrx)3Ru2O7 single crystals

“…Similarly, for the 0.36 mg sample, we measured the specific heat for a variety of chopping frequencies (with ∆T(ω) ~ 1/ω), again with negligible difference in results, as shown in the Figure 3, also indicating that the transition is not "sluggish". For all samples, the anomaly is very sharp, ∆T ~ 0.25 K, 8 as observed by others [14,17]. The measured transition entropy, ∆S ≡ ∫dT ∆c P /T, is somewhat sample dependent but reasonably well-defined, with an average value ∆S = (0.31 ± 0.08) R, very close to the value measured by McCall, et al [14].…”

“…oxygen-stoichiometric) crystals. Finally, while the T c transition, on the basis of the abruptness of the structural, magnetic, and resistance changes, appears to be first order, no thermal hysteresis has been observed in any measurement [11][12][13][14][15][16][17], so the free energy change between the two states is presumably very small.…”

“…There is a second, more unusual, phase transition at T c = 48 K; here all components of the susceptibility drop abruptly upon cooling through T c [11,14], which has been suggested to be due to formation of a charge-density-wave [15], while the lattice contracts along c by ~ 0.1 % [13,16]. Crystals grown by a flux method become nonmetallic below T C [11], but for those grown in a floating zone furnace only the c-axis resistivity becomes 3 activated, so the transition appears to correspond to a change from three-dimensional to two-dimensional transport [16,17]. It is not yet clear which type of growth yields more defect-free (e.g.…”

Colossal Magnetoresistance in a Mott Insulator via Magnetic Field-Driven Insulator-Metal Transition

“…The bilayer ruthentate, Ca3Ru2O7, undergoes an antiferromagnetic transition at Néel temperature TN ~ 56 K followed by a metal-insulator transition at TMIT ~ 48 K [15]. The metallicity of Ca3Ru2O7 reappears below 30 K [16], which originates from a very small (~0.36%), ungapped section of the Fermi surface surviving through the MIT [17]. The magnetic structures below and above TMIT are characterized as ferromagnetic bilayers stacked antiparallel along the c axis [18][19][20], with the spin direction along the a axis for TMIT < T < TN (denoted as AFM-a) and along the b axis for T < TMIT (AFM-b).…”