Correlated Oxide Dirac Semimetal in the Extreme Quantum Limit
Jong Mok OkNarayan MohantaJie ZhangSangmoon YoonSatoshi OkamotoEun Sang ChoiHua ZhouMegan BriggemanPatrick IrvinAndrew R. LupiniYun‐Yi PaiElizabeth SkoropataChanghee SohnHaoxiang LiH. MiaoBenjamin J. LawrieWoo Seok ChoiGyula EresJeremy LevyHo Nyung Lee
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Quantum materials (QMs) with strong correlation and non-trivial topology are indispensable to next-generation information and computing technologies. Exploitation of topological band structure is an ideal starting point to realize correlated topological QMs. Herein, we report that strain-induced symmetry modification in correlated oxide SrNbO3 thin films creates an emerging topological band structure. Dirac electrons in strained SrNbO3 films reveal ultra-high mobility (100,000 cm2/Vs), exceptionally small effective mass (0.04me), and non-zero Berry phase. More importantly, strained SrNbO3 films reach the extreme quantum limit, exhibiting a sign of fractional occupation of Landau levels and giant mass enhancement. Our results suggest that symmetry-modified SrNbO3 is a rare example of a correlated topological QM, in which strong correlation of Dirac electrons leads to the realization of fractional occupation of Landau levels.Keywords:
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The topological insulator and strong electronic correlation effect are two important subjects in the frontier studies of modern condensed matter physics. A topological insulator exhibits a unique pair of surface conduction bands with the Dirac dispersion albeit the bulk insulating behaviour. These surface states are protected by the topological order, and thus the spin and momentum of these surface electrons are locked together demonstrating the feature of time reversal invariance. On the other hand, the electronic correlation effect becomes the very base of many novel electronic states, such as high temperature superconductivity, giant magnetoresistance etc. Here we report the discovery of merging the two important components: Dirac electrons and the correlation effect in heterostructured Bi2Te3/Fe1+dTe. By measuring the scanning tunneling spectroscopy on Bi2Te3 thin films (a typical topological insulator) thicker than 6 quintuple layers on top of the Fe1+dTe single crystal (a parent phase of the iron based superconductors FeSe1-xTex), we observed the quantum oscillation of Landau levels of the Dirac electrons and the gapped feature at the Fermi energy due to the correlation effect of Fe1+dTe. Our observation challenges the ordinary understandings and must demonstrate some unexplored territory concerning the combination of topological insulator and strong correlation effect.
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A partially filled Landau level (LL) hosts a variety of correlated states of matter with unique properties. The ability to control these phases requires tuning the effective electron interactions within a LL, which has been difficult to achieve in GaAs-based structures. Here we consider a class of Dirac materials in which the chiral band structure, along with the mass term, gives rise to a wide tunability of the effective interactions by the magnetic field. This tunability is such that different phases can occur in a single LL, and phase transitions between them can be driven in situ. The incompressible, Abelian and non-Abelian, liquids are stabilized in interaction regimes different from GaAs. Our study points to a realistic method of controlling the correlated phases and studying the phase transitions between them in materials such as graphene, bilayer graphene, and topological insulators.
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Small potential variations in 3D semimetal Na 3 Bi enable close approach to the Dirac point, allowing exploration of new physics.
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The cubic ThTaN 3 compound has long been known as a semiconductor with a band gap of approximately 1 eV, but its electronic properties remain largely unexplored. By using density functional theory, we find that the band gap of ThTaN 3 is very sensitive to the hydrostatic pressure/strain. A Dirac cone can emerge around the Γ point with an ultrahigh Fermi velocity at a compressive strain of 8%. Interestingly, the effect of spin–orbital coupling (SOC) is significant, leading to a band gap reduction of 0.26 eV in the ThTaN 3 compound. Moreover, the strong SOC can turn ThTaN 3 into a topological insulator with a large inverted gap up to 0.25 eV, which can be primarily attributed to the inversion between the d-orbital of the heavy element Ta and the p-orbital of N. Our results highlight a new 3D topological insulator with strain-mediated topological transition for potential applications in future spintronics.
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We report experimental evidence for the existence of topologically protected charge carriers in the pure and robust 3D topological Dirac semimetal $\alpha$-Sn(100) grown on InSb(100) substrate by employing a simple macroscopic four-point resistance measurement. We analyzed and compared electrical characteristics of the constituting components of the sample, and proposed a k-band drift velocity model accordingly. In consequence, a topological band, with low carrier density and high mobility, was identified as the origin of the observed Shubnikov de Haas oscillations. The analysis of these quantum oscillations resulted in a non-trivial value of the phase shift $\gamma =0$, which is characteristic for massless Dirac fermions. This behavior was detected in the grown $\alpha$-Sn(100) films for both in plane and out of plane field orientation, suggesting 3D Topological Dirac semimetal behaviour of this material.
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Quantum limit is quite easy to achieve once the band crossing exists exactly at the Fermi level ($E_F$) in topological semimetals. In multilayered Dirac fermion system, the density of Dirac fermions on the zeroth Landau levels (LLs) increases in proportion to the magnetic field, resulting in intriguing angle- and field-dependent interlayer tunneling conductivity near the quantum limit. BaGa$_2$ is an example of multilayered Dirac semimetal with anisotropic Dirac cone close to $E_F$, providing a good platform to study its interlayer transport properties. In this paper, we report the negative interlayer magnetoresistance (NIMR, I//c and B//c) induced by the tunneling of Dirac fermions on the zeroth LLs of neighbouring Ga layers in BaGa$_2$. When the field deviates from the c-axis, the interlayer resistivity $\rho_{zz}(\theta)$ increases and finally results in a peak with the field perpendicular to the c-axis. These unusual interlayer transport properties (NIMR and resistivity peak with B$\perp$c) are observed together for the first time in Dirac semimetal under ambient pressure and are well explained by the model of tunneling between Dirac fermions in the quantum limit.
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We demonstrate theoretically the coexistence of Dirac semimetal and topological insulator phases in InSb/$\alpha$-Sn conventional semiconductor superlattices, based on advanced first-principles calculations combined with low-energy $k\cdot p$ theory. By proper interfaces designing, a large interface polarization emerges when the growth direction is chosen along {[}111{]}. Such an intrinsic polarized electrostatic field reduces band gap largely and invert the band structure finally, leading to emerge of the topological Dirac semimetal phase with a pair of Dirac nodes appearing along the (111) crystallographic direction near the $\Gamma$ point. The surface states and Fermi arc are clearly observed in (100) projected surface. In addition, we also find a two-dimensional topological insulator phase with large nontrivial band gap approaching 70 meV, which make it possible to observe the quantum spin Hall effect at room temperature. Our proposal paves a way to realize topological nontrivial phases coexisted in conventional semiconductor superlattices by proper interface designing.
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A well known semiconductor Cd3As2 has reentered the spotlight due to its unique electronic structure and quantum transport phenomena as a topological Dirac semimetal. For elucidating and controlling its topological quantum state, high-quality Cd3As2 thin films have been highly desired. Here we report the development of an elaborate growth technique of high-crystallinity and high-mobility Cd3As2 films with controlled thicknesses and the observation of quantum Hall effect dependent on the film thickness. With decreasing the film thickness to 10 nm, the quantum Hall states exhibit variations such as a change in the spin degeneracy reflecting the Dirac dispersion with a large Fermi velocity. Details of the electronic structure including subband splitting and gap opening are identified from the quantum transport depending on the confinement thickness, suggesting the presence of a two-dimensional topological insulating phase. The demonstration of quantum Hall states in our high-quality Cd3As2 films paves a road to study quantum transport and device application in topological Dirac semimetal and its derivative phases.
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Field-induced multiple metal-insulator crossovers of correlated Dirac electrons of perovskite CaIrO3
Abstract The interplay between electron correlation and topology of relativistic electrons may lead to a fascinating stage of the research on quantum materials and emergent functions. The emergence of various collective electronic orderings/liquids, which are tunable by external stimuli, is a remarkable feature of correlated electron systems, but has rarely been realized in the topological semimetals with high-mobility relativistic electrons. Here, we report that the correlated Dirac electrons in perovskite CaIrO 3 show unconventional field-induced successive metal–insulator–metal crossovers in the quantum limit accompanying a giant magnetoresistance (MR) with MR ratio of 3500 % (18 T and 1.4 K). In conjunction with the numerical calculation, we propose that the insulating state originates from the collective electronic ordering such as charge/spin density wave promoted by electron correlation, whereas it turns into the quasi-one-dimensional metal at higher fields due to the field-induced reduction of chemical potential, highlighting the highly field-tunable character of correlated Dirac electrons.
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The prospect of a Dirac half metal, a material which is characterized by a bandstructure with a gap in one spin channel but a Dirac cone in the other, is of both fundamental interest and a natural candidate for use in spin-polarized current applications. However, while the possibility of such a material has been reported based on model calculations[H. Ishizuka and Y. Motome, Phys. Rev. Lett. 109, 237207 (2012)], it remains unclear what material system might realize such an exotic state. Using first-principles calculations, we show that the experimentally accessible Mn intercalated epitaxial graphene on SiC(0001) transits to a Dirac half metal when the coverage is > 1/3 monolayer. This transition results from an orbital-selective breaking of quasi-2D inversion symmetry, leading to symmetry breaking in a single spin channel which is robust against randomness in the distribution of Mn intercalates. Furthermore, the inclusion of spin-orbit interaction naturally drives the system into the quantum anomalous Hall (QAH) state. Our results thus not only demonstrate the practicality of realizing the Dirac half metal beyond a toy model but also open up a new avenue to the realization of the QAH effect.
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