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Register a new userBi$_{2}$Se$_{3}$ thin films heteroepitaxially grown on $alpha$-RuCl$_{3}$
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Combining various two-dimensional materials into novel van der Waals (vdW) heterostructures has been shown to lead to new emergent quantum systems. A novel heterostructure composed of a vdW topological insulator (TI) such as Bi$_{2}$Se$_{3}$ with a quantum spin liquid (QSL) such as $alpha$-RuCl$_{3}$ is of great interest for the potential for the chiral Dirac electrons in the TI surface states to interact strongly with the fractionalized fermionic spin excitations in the QSL. We report the heteroepitaxial growth of Bi$_{2}$Se$_{3}$ thin films on $alpha$-RuCl$_{3}$ as well as the characterization of their structural and electrical properties. Bi$_{2}$Se$_{3}$ thin films with an atomically smooth and uniform surface are grown by molecular beam epitaxy. The heterostructure exhibits a preferential epitaxial relationship corresponding to $(5 times 5)-$Bi$_{2}$Se$_{3}/(2sqrt{3} times 2sqrt{3})R30deg-alpha$-RuCl$_{3}$ commensurate supercells with a periodicity of 1.2 nm. The formation of the superlattice despite a lattice mismatch as large as 60% is attributed to the van der Waals heteroepitaxy. Magnetotransport measurements as a function of temperature show Bi$_{2}$Se$_{3}$ films grown on $alpha$-RuCl$_{3}$ are heavily $n$-doped, $n_{e}$ ~10$^{14}$ cm$^{-2}$, with mobility $mu$ ~450 cm$^{2}$ V$^{-1}$ s$^{-1}$ at low temperatures.
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Bi$_{2}$Se$_{3}$, one of the most widely studied topological insulators (TIs), is naturally electron-doped due to n-type native defects. However, many years of efforts to achieve p-type Bi$_{2}$Se$_{3}$ thin films have failed so far. Here, we provide a solution to this long-standing problem, showing that the main culprit has been the high density of interfacial defects. By suppressing these defects through an interfacial engineering scheme, we have successfully implemented p-type Bi$_{2}$Se$_{3}$ thin films down to the thinnest topological regime. On this platform, we present the first tunable quantum Hall effect (QHE) study in Bi$_{2}$Se$_{3}$ thin films, and reveal not only significantly asymmetric QHE signatures across the Dirac point but also the presence of competing anomalous states near the zeroth Landau level. The availability of doping tunable Bi$_{2}$Se$_{3}$ thin films will now make it possible to implement various topological quantum devices, previously inaccessible.
Crystalline symmetries have played a central role in the identification of topological materials. The use of symmetry indicators and band representations have enabled a classification scheme for crystalline topological materials, leading to large scale topological materials discovery. In this work we address whether amorphous topological materials, which lie beyond this classification due to the lack of long-range structural order, exist in the solid state. We study amorphous Bi$_2$Se$_3$ thin films, which show a metallic behavior and an increased bulk resistance. The observed low field magnetoresistance due to weak antilocalization demonstrates a significant number of two dimensional surface conduction channels. Our angle-resolved photoemission spectroscopy data is consistent with a dispersive two-dimensional surface state that crosses the bulk gap. Spin resolved photoemission spectroscopy shows this state has an anti-symmetric spin texture resembling that of the surface state of crystalline Bi$_2$Se$_3$. These experimental results are consistent with theoretical photoemission spectra obtained with an amorphous tight-binding model that utilizes a realistic amorphous structure. This discovery of amorphous materials with topological properties uncovers an overlooked subset of topological matter outside the current classification scheme, enabling a new route to discover materials that can enhance the development of scalable topological devices.
The magnetic insulator $alpha$-RuCl$_{3}$ is a promising candidate to realize Kitaev interactions on a quasi-2D honeycomb lattice. We perform extensive susceptibility measurements on single crystals of $alpha$-RuCl$_{3}$, including angle-dependence of the in-plane longitudinal and transverse susceptibilities, which reveal a unidirectional anisotropy within the honeycomb plane. By comparing the experimental results to a high-temperature expansion of a Kitaev-Heisenberg-$Gamma$ spin Hamiltonian with bond anisotropy, we find excellent agreement with the observed phase shift and periodicity of the angle-resolved susceptibilities. Within this model, we show that the pronounced difference between in-plane and out-of-plane susceptibilities as well as the finite transverse susceptibility are rooted in strong symmetric off-diagonal $Gamma$ spin exchange. The $Gamma$ couplings and relationships between other terms in the model Hamiltonian are quantified by extracting relevant Curie-Weiss intercepts from the experimental data.
We study the chemical ordering in Bi$_2$Te$_{3-x}$Se$_x$ grown by molecular beam epitaxy on Si substrates. We produce films in the full composition range from x = 0 to 3, and determine their material properties using energy dispersive X-ray spectroscopy, X-ray diffraction and Raman spectroscopy. By fitting the parameters of a kinetic growth model to these results, we obtain a consistent description of growth at a microscopic level. Our main finding is that despite the incorporation of Se in the central layer being much more probable than that of Te, the formation of a fully ordered Te-Bi-Se-Bi-Te layer is prevented by kinetic of the growth process. Indeed, the Se concentration in the central layer of Bi$_2$Te$_2$Se$_1$ reaches a maximum of only $approx$ 75% even under ideal growth conditions. A second finding of our work is that the intensity ratio of the 0 0 12 and 0 0 6 X-ray reflections serves as an experimentally accessible quantitative measure of the degree of ordering in these films.
We report $beta$-detected nuclear magnetic resonance ($beta$-NMR) measurements in Bi$_{2}$Se$_{3}$:Ca (BSC) and Bi$_{2}$Te$_{3}$:Mn (BTM) single crystals using $^{8}$Li$^{+}$ implanted to depths on the order of 100 nm. Above $sim 200$ K, spin-lattice relaxation (SLR) reveals diffusion of $^{8}$Li$^{+}$, with activation energies of $sim 0.4$ eV ($sim 0.2$ eV) in BSC (BTM). At lower temperatures, the nuclear magnetic resonance (NMR) properties are those of a heavily doped semiconductor in the metallic limit, with Korringa relaxation and a small, negative, temperature-dependent Knight shift in BSC. From this, we make a detailed comparison with the isostructural tetradymite Bi$_{2}$Te$_{2}$Se (BTS) [McFadden et al., Phys Rev. B 99, 125201 (2019)]. In the magnetic BTM, the effects of the dilute Mn moments predominate, but remarkably the $^{8}$Li signal is not wiped out through the magnetic transition at 13 K, with a prominent critical peak in the SLR that is suppressed in a high applied field. This detailed characterization of the $^{8}$Li NMR response is an important step towards using depth-resolved $beta$-NMR to study the low-energy properties of the chiral topological surface state (TSS). With the bulk NMR response now established in several Bi$_{2}$Ch$_{3}$ tetradymite topological insulators (TIs), the prospect of directly probing their chiral TSS using the depth resolution afforded by $beta$-NMR remains strong.
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