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Register a new userQuantized Transport in Two-Dimensional Spin-Ordered Structures
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We study in detail the transport properties of a model of conducting electrons in the presence of double-exchange between localized spins arranged on a 2D Kagome lattice, as introduced by Ohgushi, Murakami, and Nagaosa (2000). The relationship between the canting angle of the spin texture $theta$ and the Berry phase field flux per triangular plaquette $phi$ is derived explicitly and we emphasize the similarities between this model and Haldanes honeycomb lattice version of the quantum Hall effect (Haldane, 1988). The quantization of the transverse (Hall) conductivity $sigma_{xy}$ is derived explicitly from the Kubo formula and a direct calculation of the longitudinal conductivity $sigma_{xx}$ shows the existence of a metal-insulator transition as a function of the canting angle $theta$ (or flux density $phi$). This transition might be linked to that observable in the manganite compounds or in the pyrochlore ones, as the spin ordering changes from ferromagnetic to canted.
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Quantum spin liquids (QSLs) are fluid-like states of quantum spins where its long-range ordered state is destroyed by quantum fluctuations. The ground state of QSL and its exotic phenomena, which have been extensively discussed for decades, have yet to be identified. We employ thermal transport measurements on newly discovered QSL candidates, $kappa$-(BEDT-TTF)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2, and report that the two organic insulators possess different QSLs characterized by different elementary excitations. In $kappa$-(BEDT-TTF)2Cu2(CN)3, heat transport is thermally activated at low temperatures, suggesting presence of a spin gap in this QSL. In stark contrast, in EtMe3Sb[Pd(dmit)2]2, a sizable linear temperature dependence of thermal conductivity is clearly resolved in the zero-temperature limit, showing gapless excitation with a long mean free path (~1,000 lattice distances). Such a long mean free path demonstrates a novel feature of QSL as a quantum-condensed state with long-distance coherence.
The bulk electric polarization works as a nonlocal order parameter that characterizes topological quantum matters. Motivated by a recent paper [H. Watanabe et al., Phys. Rev. B 103, 134430 (2021)], we discuss magnetic analogs of the bulk polarization in one-dimensional quantum spin systems, that is, quantized magnetizations on the edges of one-dimensional quantum spin systems. The edge magnetization shares the topological origin with the fractional edge state of the topological odd-spin Haldane phases. Despite this topological origin, the edge magnetization can also appear in topologically trivial quantum phases. We develop straightforward field theoretical arguments that explain the characteristic properties of the edge magnetization. The field theory shows that a U(1) spin-rotation symmetry and a site-centered or bond-centered inversion symmetry protect the quantization of the edge magnetization. We proceed to discussions that quantum phases on nonzero magnetization plateaus can also have the quantized edge magnetization that deviates from the magnetization density in bulk. We demonstrate that the quantized edge magnetization distinguishes two quantum phases on a magnetization plateau separated by a quantum critical point. The edge magnetization exhibits an abrupt stepwise change from zero to $1/2$ at the quantum critical point because the quantum phase transition occurs in the presence of the symmetries protecting the quantization of the edge magnetization. We also show that the quantized edge magnetization can result from the spontaneous ferrimagnetic order.
Unconventional surface states protected by non-trivial bulk orders are sources of various exotic quantum transport in topological materials. One prominent example is the unique magnetic orbit, so-called Weyl orbit, in topological semimetals where two spatially separated surface Fermi-arcs are interconnected across the bulk. The recent observation of quantum Hall states in Dirac semimetal Cd3As2 bulks have drawn attention to the novel quantization phenomena possibly evolving from the Weyl orbit. Here we report surface quantum oscillation and its evolution into quantum Hall states in Cd3As2 thin film samples, where bulk dimensionality, Fermi energy, and band topology are systematically controlled. We reveal essential involvement of bulk states in the quantized surface transport and the resultant quantum Hall degeneracy depending on the bulk occupation. Our demonstration of surface transport controlled in film samples also paves a way for engineering Fermi-arc-mediated transport in topological semimetals.
We study origin of Rashba spin-orbit interaction at SrTiO$_3$ surfaces and LaAlO$_3$/SrTiO$_3$ interfaces by considering the interplay between atomic spin-orbit coupling and inversion asymmetry at the surface or interface. We show that, in a simple tight-binding model involving 3d $t_{2g}$ bands of Ti ions, the induced spin-orbit coupling in the $d_{xz}$ and $d_{yz}$ bands is cubic in momentum whereas the spin-orbit interaction in the $d_{xy}$ band has linear momentum dependence. We also find that the spin-orbit interaction in one-dimensional channels at LaAlO$_3$/SrTiO$_3$ interfaces is linear in momentum for all bands. We discuss implications of our results for transport experiments on SrTiO$_3$ surfaces and LaAlO$_3$/SrTiO$_3$ interfaces. In particular, we analyze the effect of a given spin-orbit interaction term on magnetotransport of LaAlO$_3$/SrTiO$_3$ by calculating weak anti-localization corrections to the conductance and to universal conductance fluctuations.
The two-electron doped rare earth mangnites Ca_1-x Ce_x MnO_3 (x = 0.1,0.2) are probed using resistivity, ac susceptibility and electron paramagnetic resonance (EPR) measurements across their respective charge ordering (CO) temperatures T_CO = 173 K and 250 K. The EPR g factor and intensity as well as the transport and magnetic behaviours of the two compositions are qualitatively similar and are as expected for CO systems. However, the EPR linewidth, reflective of the spin dynamics, for x = 0.1, shows a strongly anomalous temperature dependence, decreasing with decreasing temperature below T_CO in contrast with the sample with x = 0.2 and other CO systems. Keeping in view the evidence for magnetic frustration in the system, we propose that the anomalous temperature dependence of the linewidth is the signature of the occurrence of a disorder driven spin liquid phase, present along with charge ordering.
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