Topological phases in iridium oxide superlattices: Quantized anomalous charge or valley Hall insulators

Chen, Yige

doi:10.1103/physrevb.90.195145

Cited by 31 publications

(35 citation statements)

References 51 publications

(65 reference statements)

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“…A tiny trace of unreacted Y 2 O 3 (∼1.06%) is observed. The analysis of the structural data suggests the monoclinic P 2 1 /n space group with lattice parameters: a = 5.7870(7)Å, b = 5.7912(6)Å, c = 8.1805(1)Å, β = 90.214 (6) • , and R f = 2.11, R Bragg = 2.01, and reduced χ 2 = 2.32. The result of the Rietveld refinement of the synchrotron XRD data using the cubic structural model discovered by single crystal XRD is depicted in Figure S3.…”

Section: Resultsmentioning

confidence: 97%

Iridium double perovskite Sr2YIrO6 : A combined structural and specific heat study

et al. 2017

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Recently, the iridate double perovskite Sr 2 YIrO 6 has attracted considerable attention due to the report of unexpected magnetism in this Ir 5+ (5d 4 ) material, in which according to the J eff model, a nonmagnetic ground state is expected. However, in recent works on polycrystalline samples of the series Ba 2−x Sr x YIrO 6 no indication of magnetic transitions have been found. We present a structural, magnetic, and thermodynamic characterization of Sr 2 YIrO 6 single crystals, with emphasis on the temperature and magnetic field dependence of the specific heat. As determined by x-ray diffraction, the Sr 2 YIrO 6 single crystals have a cubic structure, with space group F m3m. In agreement with the expected nonmagnetic ground state of Ir 5+ (5d 4 ) in Sr 2 YIrO 6 , no magnetic transition is observed down to 430 mK. Moreover, our results suggest that the low-temperature anomaly observed in the specific heat is not related to the onset of long-range magnetic order. Instead, it is identified as a Schottky anomaly caused by paramagnetic impurities present in the sample, of the order of n ∼ 0.5(2)%. These impurities lead to non-negligible spin correlations, which nonetheless, are not associated with long-range magnetic ordering.

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Section: Resultsmentioning

confidence: 97%

Iridium double perovskite Sr2YIrO6 : A combined structural and specific heat study

et al. 2017

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“…We will then introduce a simple model system that allows us to characterize the allowed Dirac phases in 2D, and conclude with a brief discussion of the possible material venues for these phases, including the layered iridium oxide superlattices recently proposed and studied in Ref. [18].It has long been known that nonsymmorphic symmetries lead to extra degeneracies in electronic band structures that cause bands to "stick together" due to the existence of higher-dimensional projective representations of the little groups of certain values of k [19]. This fact can be understood as a simple consequence of fractional translation symmetries.…”

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confidence: 99%

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confidence: 99%

Dirac Semimetals in Two Dimensions

2015

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Graphene is famous for being a host of 2D Dirac fermions. However, spin-orbit coupling introduces a small gap, so that graphene is formally a quantum spin Hall insulator. Here we present symmetry-protected 2D Dirac semimetals, which feature Dirac cones at high-symmetry points that are not gapped by spin-orbit interactions and exhibit behavior distinct from both graphene and 3D Dirac semimetals. Using a two-site tight-binding model, we construct representatives of three possible distinct Dirac semimetal phases, and show that single symmetry-protected Dirac points are impossible in two dimensions. An essential role is played by the presence of nonsymmorphic space group symmetries. We argue that these symmetries tune the system to the boundary between a 2D topological and trivial insulator. By breaking the symmetries we are able to access trivial and topological insulators as well as Weyl semimetal phases.Over the past decade, graphene has attracted intense interest as a material with Dirac cones at the Fermi energy and, as a consequence, a number of unique electronic properties [1,2]. The Dirac points in graphene, as in similar materials [3][4][5][6], are protected by symmetry, but only in the absence of spin-orbit coupling. Spin-orbit coupling opens a gap at the Dirac point, leading to a topological insulating phase [7,8]. The discovery of topological insulators (TIs) heightened interest in three-dimensional Dirac semimetals, which host 3D Dirac points when spinorbit coupling is included [9,10]. The concept of Dirac and Weyl superconductors has also been recently introduced [11]. In this Letter we introduce a system that has symmetry-protected 2D Dirac points in the presence of spin-orbit coupling and provide a classification of such systems in general. These are of interest because they are symmetry tuned to the boundary between topological and trivial insulating phases.Three-dimensional Dirac semimetals fall into two distinct classes.In Ref.[10] we introduced a Dirac semimetal with Dirac points at high-symmetry points on the surface of the Brillouin zone (BZ). Candidate materials include β-cristobalite BiO 2 [10], as well as distorted spinel materials such as BiZnSiO 4 [12]. In these materials the semimetallic state is at the boundary between strong and weak topological insulating phases, and an essential role is played by the nonsymmorphic symmetry of the crystal space group. A distinct class of Dirac semimetals was introduced in Refs. [13,14], and has been observed in Cd 3 As 2 and Na 3 Bi [15][16][17]. Here, the Dirac points arise due to a band inversion and occur at a generic point on a C 3 symmetry axis in the interior of the BZ. Opening a gap by lowering the symmetry in these materials necessarily leads to a topological insulator -the trivial insulator is not adjacent. The 2D Dirac semimetals we introduce here are analogous to the former class: they arise due to a nonsymmorphic symmetry that requires the conduction and valence bands to touch and exist in the presence of significant spin-orbit coupling. We...

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“…A number of model [6][7][8][9] and materialspecific studies [10][11][12][13][14][15][16] have probed the possibilities that such systems may offer. The advent of (111)-oriented growth of oxides [17][18][19][20][21][22][23][24][25] provides a new platform for design of new materials, and especially Chern insulators.…”

Section: Introductionmentioning

confidence: 99%

“…, there are competing indications whether the bilayer perovskite platform will be both ferromagnetic (FM) and insulating as required for a Chern insulator, or rather will assume some less desirable configuration. 10,17,22 It is highly encouraging that (111)-grown SrIrO 3 and (Ca,Sr)IrO 3 have been synthesized, with the former found to be magnetic and insulating. 18,22 We provide below detailed predictions that, once all effects are accounted for, 2LRuO and 2LOsO buckled honeycomb lattices will be Chern insulators with substantial gaps.…”

Section: Introductionmentioning

confidence: 99%

Wide gap Chern Mott insulating phases achieved by design

et al. 2017

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Quantum anomalous Hall insulators, which display robust boundary charge and spin currents categorized in terms of a bulk topological invariant known as the Chern number (Thouless et al Phys. Rev. Lett. 49, 405-408 (1982)), provide the quantum Hall anomalous effect without an applied magnetic field. Chern insulators are attracting interest both as a novel electronic phase and for their novel and potentially useful boundary charge and spin currents. Honeycomb lattice systems such as we discuss here, occupied by heavy transition-metal ions, have been proposed as Chern insulators, but finding a concrete example has been challenging due to an assortment of broken symmetry phases that thwart the topological character. Building on accumulated knowledge of the behavior of the 3d series, we tune spin-orbit and interaction strength together with strain to design two Chern insulator systems with bandgaps up to 130 meV and Chern numbers C = −1 and C = 2. We find, in this class, that a trade-off between larger spin-orbit coupling and strong interactions leads to a larger gap, whereas the stronger spin-orbit coupling correlates with the larger magnitude of the Hall conductivity. Symmetry lowering in the course of structural relaxation hampers obtaining quantum anomalous Hall character, as pointed out previously; there is only mild structural symmetry breaking of the bilayer in these robust Chern phases. Recent growth of insulating, magnetic phases in closely related materials with this orientation supports the likelihood that synthesis and exploitation will follow. npj Quantum Materials (2017) 2:4 ; doi:10.1038/s41535-016-0007-2 INTRODUCTIONThe honeycomb lattice, 1,2 particularly in conjunction with its Dirac points and two-valley nature in graphene, 3 has provided the basic platform for a great number of explorations into new phases of matter and new phenomena. Yet the single band, uncorrelated case of graphene is only the simplest level of what can be realized on honeycomb lattices. The recognition that a perovskite (111) bilayer of LaXO 3 encased in LaAlO 3 (we use the notation 2LXO for an X bilayer, X=transition metal [TM]) provides a honeycomb lattice that led to a call for engineering ( i.e., design) of a Chern insulator in such systems. 4,5 The origin of the buckled honeycomb lattice is depicted in Fig. 1. A number of model 6-9 and materialspecific studies [10][11][12][13][14][15][16] have probed the possibilities that such systems may offer. The advent of (111)-oriented growth of oxides 17-25 provides a new platform for design of new materials, and especially Chern insulators.The degree of generalization from graphene is huge. Graphene has a hopping amplitude, or equivalently a velocity, that sets the energy scale, and a two-valley degree of freedom. 2LXO, on the other hand, presents a multi-orbital system with a number of additional degrees of freedom: intraatomic Coulomb repulsion U and Hund's rule spin interaction J H , cubic crystal field splitting Δ cf , trigonal crystal field splitting δ cf , spin-orbit coupling...

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Topological phases in iridium oxide superlattices: Quantized anomalous charge or valley Hall insulators

Cited by 31 publications

References 51 publications

Iridium double perovskite Sr2YIrO6 : A combined structural and specific heat study

Iridium double perovskite Sr2YIrO6 : A combined structural and specific heat study

Dirac Semimetals in Two Dimensions

Wide gap Chern Mott insulating phases achieved by design

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