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Register a new userUniversal size-dependent nonlinear charge transport in single crystals of the Mott insulator Ca2RuO4
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The surprisingly low current densities required for inducing the insulator to metal transition has made Ca$_2$RuO$_4$ an attractive candidate material for developing novel Mott-based electronics devices. The mechanism underlying the resistive switching, however, remains to be a controversial topic in the field of correlated electron systems. Here we report a four orders of magnitude increase in the current density required for driving Ca$_2$RuO$_4$ out of the insulating state upon decreasing the crystal size. We investigate this unprecedented effect by conducting an extensive size-dependent study of electrical transport in high-purity Ca$_2$RuO$_4$ single crystals. We establish that the size dependence is not a result of Joule heating, by integrating a microscopic platinum thermometer. Our detailed study demonstrates that the universally observed transport characteristics of Ca$_2$RuO$_4$ are a result of a strongly inhomogenous current distribution in the nominally homogeneous crystal.
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We have investigated the in-plane uniaxial pressure effect on the antiferromagnetic Mott insulator Ca2RuO4 from resistivity and magnetization measurements. We succeeded in inducing the ferromagnetic metallic phase at lower critical pressure than by hydrostatic pressure, indicating that the flattening distortion of the RuO6 octahedra is more easily released under in-plane uniaxial pressure. We also found a striking in-plane anisotropy in the pressure responses of various magnetic phases: Although the magnetization increases monotonically with pressure diagonal to the orthorhombic principal axes, the magnetization exhibits peculiar dependence on pressure along the in-plane orthorhombic principal axes. This peculiar dependence can be explained by a qualitative difference between the uniaxial pressure effects along the orthorhombic a and b axes, as well as by the presence of twin domain structures.
We study the origin of the temperature-induced Mott transition in Ca2RuO4. As a method we use the local-density approximation+dynamical mean-field theory. We show the following. (i) The Mott transition is driven by the change in structure from long to short c-axis layered perovskite (L-Pbca to S-Pbca); it occurs together with orbital order, which follows, rather than produces, the structural transition. (ii) In the metallic L-Pbca phase the orbital polarization is ~0. (iii) In the insulating S-Pbca phase the lower energy orbital, ~xy, is full. (iv) The spin-flip and pair-hopping Coulomb terms reduce the effective masses in the metallic phase. Our results indicate that a similar scenario applies to Ca_{2-x}Sr_xRuO_4 (x<0.2). In the metallic x< 0.5 structures electrons are progressively transferred to the xz/yz bands with increasing x, however we find no orbital-selective Mott transition down to ~300 K.
We have investigated the dispersion renormalization $Z_{disp}$ in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) over the wide doping range of $x=0.03-0.30$, for binding energies extending to several hundred meVs. Strong correlation effects conspire in such a way that the system exhibits an LDA-like dispersion which essentially `undresses ($Z_{disp}to 1$) as the Mott insulator is approached. Our finding that the Mott insulator contains `nascent or `preformed metallic states with a vanishing spectral weight offers a challenge to existing theoretical scenarios for cuprates.
Whether or not anomalies in the thermal conductivity from insulating cuprates can be attributed to antiferromagnetic order and magnons in a 2D Mott insulator remains an intriguing open question. To shed light on this issue, we investigate the thermal conductivity $kappa$ and specific heat $c_v$ of the half-filled 2D single-band Hubbard model using the numerically exact determinant quantum Monte Carlo algorithm and maximum entropy analytic continuation. Both $c_v$ and $kappa$ possess two peaks as a function of temperature, with scales related to the Hubbard interaction energy $U$ and spin superexchange energy $J$, respectively. At low temperatures where the charge degrees of freedom are gapped-out, our results for the contribution to both $c_v$ and the Drude weight associated with $kappa$ from the kinetic energy agree well with spin-wave theory for the spin-$frac{1}{2}$ antiferromagnetic Heisenberg model.
The Mott insulator is the quintessential strongly correlated electronic state. We obtain complete insight into the physics of the two-dimensional Mott insulator by extending the slave-fermion (holon-doublon) description to finite temperatures. We first benchmark its predictions against state-of-the-art quantum Monte Carlo simulations, demonstrating quantitative agreement. Qualitatively, the short-ranged spin fluctuations both induce holon-doublon bound states and renormalize the charge sector to form the Hubbard bands. The Mott gap is understood as the charge gap renormalized downwards by these spin fluctuations. As temperature increases, the Mott gap closes before the charge gap, causing a pseudogap regime to appear naturally during the melting of the Mott insulator.
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