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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Topological Insulators versus Topological Dirac Semimetals in Honeycomb Compounds

Intriguing physical properties of materials stem from their chemical constituents, whereas the connection between them is often not clear. Here, we uncover a general chemical classification for the two quantum phases in the honeycomb ABX structure—topological insulator (TI) and topological Dirac semimetal (TDSM). First, we find among the 816 (existing as well as hypothetical) calculated compounds, 160 TIs (none were noted before), 96 TDSMs, 282 normal insulators (NIs), and 278 metals. Second, based on this classification, we have distilled a simple chemical regularity based on compound formulas for the selectivity between TI and TDSM: the ABX compounds that are TDSM have B atoms (part of the BX honeycomb layers) that come from the periodic table columns XI (Cu, Ag, Au) or XII (Zn, Cd, Hg), or Mg (group II), whereas the ABX compounds whose B at-oms come from columns I (Li, Na, K, Rb, Cs) or II (Ca, Sr, Ba) are TIs. Third, focusing on the ABX bismide compounds that are thermodynamically stable, we find a structural motif that delivers topological insulation and stability at the same time. Furthermore, this study opens the way to simultaneously design new topological materials based on the compositional rules indicated here.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Synthesis, Magnetic Properties, and Electronic Structure of Magnetic Topological Insulator MnBi 2 Se 4

The intrinsic magnetic topological insulators MnBi 2 Te 4 and MnBi 2 Se 4 support novel topological states related to symmetry breaking by magnetic order. Unlike MnBi 2 Te 4 , the study of MnBi 2 Se 4 has been inhibited by the lack of bulk crystals, as the van der Waals (vdW) crystal is not the thermodynamic equilibrium phase. In this study, we report the layer-by-layer synthesis of vdW MnBi 2 Se 4 crystals using nonequilibrium molecular beam epitaxy. Atomic-resolution scanning transmission electron microscopy and scanning tunneling microscopy identify a well-ordered vdW crystal with septuple-layer base units. The magnetic properties agree with the predicted layered antiferromagnetic ordering but disagree with its predicted out-of-plane orientation. Instead, our samples exhibit an easy-plane anisotropy, which is explained by including dipole–dipole interactions. Angle-resolved photoemission spectroscopy reveals the gapless Dirac-like surface state, which demonstrates that MnBi 2 Se 4 is a topological insulator above the magnetic-ordering temperature. These studies show that MnBi 2 Se 4 is a promising candidate for exploring rich topological phases of layered antiferromagnetic topological insulators.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Resistivity of Surface Steps in Bulk-Insulating Topological Insulators

Electron transport in topological insulators usually involves both topologically protected surface states and trivial electronic states in the bulk material. The surface transport is particularly interesting; however, it is also susceptible to atomic defects on the surfaces, such as vacancies, impurities, and step edges. Experimental determination of scattering effects of these surface defects requires both nanoscale spatial resolution and the ability to decipher surface transport from bulk transport. Here we directly measure the resistivity of individual surface steps in the surface dominating transport process of topological insulator Bi 2 Te 2 Se. A variable probe-spacing transport spectroscopy with a multiprobe scanning tunneling microscope is used to differentiate the surface conductance from bulk conductance, allowing the identification of a surface dominant transport regime. The technique also reveals a deviation from ideal 2D transport at atomic steps. Then, a multi-probe scanning tunneling potentiometry is employed to visualize the electrochemical potentials across individual step edges. A quantitative analysis of the potential distributions enables us to acquire a resistivity of 0.530 mΩ · cm for the one quintuple-layer atomic step. The result indicates that atomic defects, despite preserving the time-reversal symmetry, can still significantly affect the transport in topological insulators.

36 MATERIALS SCIENCE↗
DOE OSTI2021

3D Artificial Solid-Electrolyte Interphase for Lithium Metal Anodes Enabled by Insulator–Metal–Insulator Layered Heterostructures

Despite considerable efforts to prevent lithium (Li) dendrite growth, stable cycling of Li metal anodes with various structures remains extremely difficult due to the direct contact of the liquid electrolyte with Li. Rational design of solid-electrolyte interphase (SEI) for 3D electrodes is a promising but still challenging strategy for preventing Li dendrite growth and avoiding lithium–electrolyte side reactions in Li-metal batteries. Here, a 3D architecture is constructed with g-C 3 N 4 /graphene/g-C 3 N 4 insulator–metal–insulator sandwiched nanosheets to guide uniform Li plating/stripping in the van der Waals gap between the graphene and the g-C 3 N 4 , and the function of which can be regarded as a 3D artificial SEI. Li deposition on the surface of g-C 3 N 4 is suppressed due to its insulating nature. However, its uniform lithiophilic sites and nanopore channels enable homogeneous lithium plating between the graphene and the g-C 3 N 4 , prohibiting the direct contact of the electrolyte with the Li metal. The use of the g-C 3 N 4 -layer-modified 3D anode enables long-term Li deposition with a high Coulombic efficiency and stable cycling of full cells under high cathode loading, limited Li excess, and lean electrolyte conditions. The concept of a 3D artificial SEI will shed light on developing safe and stable Li-metal anodes.

25 ENERGY STORAGE↗
DOE OSTI2018

Coupled Dirac Plasmons in Topological Insulators

Topological insulators are predicted to house spin-polarized two-dimensional Dirac plasmons. In topological insulator thin films, Dirac plasmons on the top and bottom surfaces will be coupled, giving rise to an unusual dispersion relationship. These plasmons are of interest both for fundamental science as well as applications in THz sensing and waveguiding. In this article, we present conclusive evidence for coupled Dirac plasmons excited in the surface states of topological insulator thin film stripe arrays by investigating how the plasmon frequency depends on both film thickness and stripe width. Bi 2 Se 3 films of thickness 50-200nm are patterned into periodic stripes of width 1-4μm to excite plasmons. The plasmon frequency dependence on both film thickness and stripe width is found to be in good agreement with theoretical models for couple 2D Dirac plasmons. Alternative explanations such as 2D massive plasmons are shown to be inconstant with data. Finally, effective mode indexes of up to 211 are reported, demonstrating incredible confinement of light in the THz frequency range.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Electronic response of a Mott insulator at a current-induced insulator-to-metal transition

The Mott insulator Ca 2 RuO 4 exhibits an insulator-to-metal transition induced by d.c. current. Despite the thorough examination of the structural changes associated with this transition, a comprehensive knowledge of the response of electronic degrees of freedom is still lacking. Here we demonstrate current-induced modifications of the electronic states of Ca 2 RuO 4 . Angle-resolved photoemission spectroscopy in conjunction with four-probe electrical transport (transport-ARPES) measurements reveal a clear reduction of the Mott gap and a modification in the dispersion of the Ru bands. Based on a free-energy analysis, we show that the current-induced phase is electronically distinct from the high-temperature zero-current metallic phase. Our results highlight strong interplay of lattice- and orbital-dependent electronic responses in the current-driven insulator-to-metal transition.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Molecular beam epitaxial growth of Bi{sub 2}Te{sub 3} and Sb{sub 2}Te{sub 3} topological insulators on GaAs (111) substrates: a potential route to fabricate topological insulator p-n junction

High quality Bi{sub 2}Te{sub 3} and Sb{sub 2}Te{sub 3} topological insulators films were epitaxially grown on GaAs (111) substrate using solid source molecular beam epitaxy. Their growth and behavior on both vicinal and non-vicinal GaAs (111) substrates were investigated by reflection high-energy electron diffraction, atomic force microscopy, X-ray diffraction, and high resolution transmission electron microscopy. It is found that non-vicinal GaAs (111) substrate is better than a vicinal substrate to provide high quality Bi{sub 2}Te{sub 3} and Sb{sub 2}Te{sub 3} films. Hall and magnetoresistance measurements indicate that p type Sb{sub 2}Te{sub 3} and n type Bi{sub 2}Te{sub 3} topological insulator films can be directly grown on a GaAs (111) substrate, which may pave a way to fabricate topological insulator p-n junction on the same substrate, compatible with the fabrication process of present semiconductor optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Improved interfacial and electrical properties of vanadyl-phthalocyanine metal-insulator-semiconductor devices with silicon nitride as gate insulator

We have investigated the interfacial and electrical properties of vanadyl-phthalocyanine (VOPc) metal-insulator-semiconductor devices by the measurement of capacitance and conductance. The devices have been fabricated on ordered para-sexiphenyl (p-6P) layer with silicon nitride (SiN{sub x}) as gate insulator. The VOPc/p-6P/SiN{sub x} devices have shown a negligible hysteresis, low series resistance, and high operated frequency. Bulk traps have been distinguished from interface traps by two loss peaks in conductance measurement. Trap densities and distribution of trap energy level have been obtained. The improved properties indicate that VOPc/ p-6P devices with SiN{sub x} insulator hold a great promise of application in flexible displays.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Spin transport in normal metal/insulator/topological insulator coupled to ferromagnetic insulator structures

In this study, we investigate the spin transport in normal metal (NM)/insulator (I)/topological insulator (TI) coupled to ferromagnetic insulator (FI) structures. In particular, we focus on the barrier thickness dependence of the spin transport inside the bulk gap of the TI with FI. The TI with FI is described by two-dimensional (2D) Dirac Hamiltonian. The energy profile of the insulator is assumed to be a square with barrier height V and thickness d along the transport-direction. This structure behaves as a tunnel device for 2D Dirac electrons. The calculation is performed for the spin conductance with changing the barrier thickness and the components of magnetization of FI layer. It is found that the spin conductance decreases with increasing the barrier thickness. Also, the spin conductance is strongly dependent on the polar angle θ, which is defined as the angle between the axis normal to the FI and the magnetization of FI layer. These results indicate that the structures are promising candidates for novel tunneling magnetoresistance devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Enhancing metal-insulator-insulator-metal tunnel diodes via defect enhanced direct tunneling

Metal-insulator-insulator-metal tunnel diodes with dissimilar work function electrodes and nanolaminate Al{sub 2}O{sub 3}-Ta{sub 2}O{sub 5} bilayer tunnel barriers deposited by atomic layer deposition are investigated. This combination of high and low electron affinity insulators, each with different dominant conduction mechanisms (tunneling and Frenkel-Poole emission), results in improved low voltage asymmetry and non-linearity of current versus voltage behavior. These improvements are due to defect enhanced direct tunneling in which electrons transport across the Ta{sub 2}O{sub 5} via defect based conduction before tunneling directly through the Al{sub 2}O{sub 3}, effectively narrowing the tunnel barrier. Conduction through the device is dominated by tunneling, and operation is relatively insensitive to temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Opposite current-induced spin polarizations in bulk-metallic Bi 2 Se 3 and bulk-insulating Bi 2 Te 2 Se topological insulator thin flakes

One of the most fundamental and exotic properties of three-dimensional (3D) topological insulators (TIs) is spin-momentum locking (SML) of their topological surface states (TSSs), promising for potential applications in future spintronics. However, other possible conduction channels, such as a trivial two-dimensional electron gas (2DEG) with strong Rashba-type spin-orbit interaction (SOI) and bulk-conducting states that may possess a spin Hall effect (SHE), can coexist in 3D TIs, making determining the origin of the current-induced spin polarization (CISP) difficult. Here, we directly compared the CISP between bulk-insulating Bi 2 Te 2 Se (BTS221) and bulk-metallic Bi2Se3 thin flakes using spin potentiometry. In the bulk-insulating BTS221, the observed CISP has a sign consistent with the expected helicity of the SML of the TSS, but an opposite sign to its calculated bulk spin Hall conductivity. However, compared to BTS221, an opposite CISP is observed in the bulk-metallic Bi2Se3, consistent with both the expectations of its Rashba-Edelstein effect of the band-bending induced 2DEG and bulk intrinsic spin Hall Effect (SHE). If one assumes a representative occupation of the Rashba band of 3×10 13 cm –2 in Bi 2 Se 3 with a relevant relaxation time of 100 fs, the contribution to the CISP could be more dominated by the bulk intrinsic SHE. Our results provide an electrical way to distinguish the TSS from other possible conducting channels in spin transport measurements on 3D TIs, and open ways for the potential applications in charge-spin conversion devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Insulating phases of vanadium dioxide are Mott-Hubbard insulators

Here, we present comprehensive broadband optical spectroscopy data on two insulating phases of vanadium dioxide (VO 2 ): monoclinic M 2 and triclinic. The main result of our work is that the energy gap and the electronic structure are essentially unaltered by the first-order structural phase transition between the M 2 and triclinic phases. Moreover, the optical interband features in the M 2 and triclinic phases are remarkably similar to those observed in the well-studied monoclinic M 1 insulating phase of VO 2 . As the energy gap is insensitive to the different lattice structures of the three insulating phases, we rule out vanadium-vanadium pairing (the Peierls component) as the dominant contributor to the opening of the gap. Rather, the energy gap arises primarily from intra-atomic Coulomb correlations.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Two-dimensional ferroelectric topological insulators in functionalized atomically thin bismuth layers

We introduce a class of two-dimensional (2D) materials that possess coexisting ferroelectric and topologically insulating orders. Such ferroelectric topological insulators (FETIs) occur in noncentrosymmetric atomic layer structures with strong spin-orbit coupling (SOC). Here, we showcase a prototype 2D FETI in an atomically thin bismuth layer functionalized by CH 2 OH, which exhibits a large ferroelectric polarization that is switchable by a ligand molecule rotation mechanism and a strong SOC that drives a band inversion leading to the topologically insulating state. An external electric field that switches the ferroelectric polarization also tunes the spin texture in the underlying atomic lattice. Moreover, the functionalized bismuth layer exhibits an additional quantum order driven by the valley splitting at the K and K' points in the Brillouin zone stemming from the symmetry breaking and strong SOC in the system, resulting in a remarkable state of matter with the simultaneous presence of the quantum spin Hall and quantum valley Hall effect. These phenomena are predicted to exist in other similarly constructed 2D FETIs, thereby offering a unique quantum material platform for discovering novel physics and exploring innovative applications.

2-dimensional systems↗
DOE OSTI2018

Two-stage proximity-induced gap opening in topological-insulator–insulating-ferromagnet ( Bi x Sb 1 - x ) 2 Te 3 – EuS bilayers

To further investigate the interplay between ferromagnetism and topological insulators, thin films of the low-carrier topological insulator (Bi x Sb 1 - x ) 2 Te 3 were deposited on the insulating ferromagnet EuS (100) in situ. AC susceptibility indicates magnetic anomalies between T ≈ 30 K and T ≈ 60 K , well above the Curie temperature T C ≈ 15 K of EuS. When the Fermi level is close to the Dirac point and the surface state dominates the electric conduction, sharp increases in resistance with decreasing temperatures were observed concurrently with the magnetic anomalies. Positive-negative magnetoresistance crossovers were observed at the Curie temperature, which seem only to appear when the sheet resistance exceeds the Mott-Ioffe-Regel limit h/e 2 . Finally, a two-stage gap-opening process due to magnetic proximity is proposed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Skyrme insulators: insulators at the brink of superconductivity

Current theories of superfluidity are based on the idea of a coherent quantum state with topologically protected, quantized circulation. When this topological protection is absent, as in the case of 3He-A, the coherent quantum state no longer supports persistent superflow. In this paper, we argue that the loss of topological protection in a superconductor gives rise to an insulating ground state. Specifically, we introduce the concept of a Skyrme insulator to describe the coherent dielectric state that results from the topological failure of superflow carried by a complex vector order parameter. Here, we apply this idea to the case of SmB6, arguing that the observation of a diamagnetic Fermi surface within an insulating bulk can be understood as a realization of this state. Our theory enables us to understand the linear specific heat of SmB6 in terms of a neutral Majorana Fermi sea and leads us to predict that in low fields of order a Gauss, SmB6 will develop a Meissner effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗