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Register a new userEmergent charge order near the doping-induced Mott-insulating quantum phase transition in Sr$_{3}$Ru$_{2}$O$_{7}$
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We investigate the double layered Sr$_{3}$(Ru$_{1-x}$Mn$_{x}$)$_{2}$O$_{7}$ and its doping-induced quantum phase transition (QPT) from a metal to an antiferromagnetic (AFM) Mott insulator. Using spectroscopic imaging with the scanning tunneling microscope (STM), we visualize the evolution of the electronic states in real- and momentum-space. We find a partial-gap in the tunneling density of states at the Fermi energy (E$_{F}$) that develops with doping to form a weak Mott insulating ({Delta} ~ 100meV) state. Near the QPT, we discover a spatial electronic reorganization into a commensurate checkerboard charge order. These findings share some resemblance to the well-established universal charge order in the pseudogap phase of cuprates. Our experiments therefore demonstrate the ubiquity of the incipient charge order that emanates from doped Mott insulators.
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Tailoring transport properties of strongly correlated electron systems in a controlled fashion counts among the dreams of materials scientists. In copper oxides, varying the carrier concentration is a tool to obtain high-temperature superconducting phases. In manganites, doping results in exotic physics such as insulator-metal transitions (IMT), colossal magnetoresistance (CMR), orbital- or charge-ordered (CO) or charge-disproportionate (CD) states. In most oxides, antiferromagnetic order and charge-disproportionation are asssociated with insulating behavior. Here we report the realization of a unique physical state that can be induced by Mo doping in LaFeO$_3$: the resulting metallic state is a site-selective Mott insulator where itinerant electrons evolving in low-energy Mo states coexist with localized carriers on the Fe sites. In addition, a local breathing-type lattice distortion induces charge disproportionation on the latter, without destroying the antiferromagnetic order. A state, combining antiferromangetism, metallicity and CD phenomena is rather rare in oxides and may be of utmost significance for future antiferromagnetic memory devices.
The electrical, magnetic, and structural properties of Sr$_3$(Ru$_{1-x}$Mn$_x$)$_2$O$_7$ (0 $leq x leq$ 0.2) are investigated. The parent compound Sr$_3$Ru$_2$O$_7$ is a paramagnetic metal, critically close to magnetic order. We have found that, with a Ru-site doping by only a few percent of Mn, the ground state is switched from a paramagnetic metal to an antiferromagnetic insulator. Optical conductivity measurements show the opening of a gap as large as 0.1 eV, indicating that the metal-to-insulator transition is driven by the electron correlation. The complex low-temperature antiferromagnetic spin arrangement, reminiscent of those observed in some nickelates and manganites, suggests a long range orbital order.
Triple-layered ruthenate Sr$_4$Ru$_3$O$_{10}$ shows a first-order itinerant metamagnetic transition for in-plane magnetic fields. Our experiments revealed rather surprising behavior in the low-temperature transport properties near this transition. The in-plane magnetoresistivity $rho$$_{ab}$(H) exhibits ultrasharp steps as the magnetic field sweeps down through the transition. Temperature sweeps of $rho$$_{ab}$ for fields within the transition regime show non-metallic behavior in the up-sweep cycle of magnetic field, but show a significant drop in the down-sweep cycle. These observations indicate that the transition occurs via a new electronic phase separation process; a lowly polarized state is mixed with a ferromagnetic state within the transition regime.
We have investigated the effect of pressure on the electronic, magnetic, and structural properties on a single crystal of conducting, ferromagnet (T$_{C}$=157K) La$_{0.82}$Sr$_{0.18}$CoO$_{3}$ located near the boundary of the metal-insulator transition. Contrary to the results reported on related systems, we find a transition from the conducting state to an insulating state and a decrease of T$_{C}$ with increasing pressure while the lattice structure remains unchanged. We show that this unusual behavior is driven by a gradual change of the spin state of Co$^{3+}$ ions from magnetic intermediate-spin (t$_{2g}^5$e$_{g}^{1}$; S=1) to a nonmagnetic low-spin (t$_{2g}^6$e$_{g}^{0}$; S=0) state.
We present a detailed NMR study of the insulator to metal transition induced by an applied pressure $p$ in the A15 phase of Cs$_{3}$C$_{60}$. We evidence that the insulating antiferromagnetic (AF) and superconducting (SC) phases only coexist in a narrow $p$ range. At fixed $p$, in the metallic state above the SC transition $T_c$, the $^{133}$Cs and $^{13}$C NMR spin lattice relaxation data are seemingly governed by a pseudogap like feature. We prove that this feature, also seen in the $^{133}$Cs NMR shift data is rather a signature of the Mott transition, which broadens and smears out progressively for increasing $(p,T)$. The analysis of the variation of the quadrupole splitting $ u _{Q}$ of the $^{133}$Cs NMR spectrum precludes any cell symmetry change at the Mott transition and only monitors a weak variation of lattice parameter. These results open an opportunity to consider theoretically the Mott transition in a multiorbital three dimensional system well beyond its critical point.
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