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All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Electron–Hole Asymmetry of Surface States in Topological Insulator Sb 2 Te 3 Thin Films Revealed by Magneto-Infrared Spectroscopy

When surface states (SSs) form in topological insulators (TIs), they inherit the properties of bulk bands, including the electron–hole (e–h) asymmetry but with much more profound impacts. Here via combining magneto-infrared spectroscopy with theoretical analysis, we show that e–h asymmetry significantly modifies the SS electronic structures when interplaying with the quantum confinement effect. Compared with the case without e–h asymmetry, the SSs now bear not only a band asymmetry, such as that in the bulk, but also a shift of the Dirac point relative to the bulk bands and a reduction of the hybridization gap of up to 70%. Here, our results signify the importance of e–h asymmetry in the band engineering of TIs in the thin-film limit.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Extreme In-Plane Thermal Conductivity Anisotropy in Titanium Trisulfide Caused by Heat-Carrying Optical Phonons

High in-plane anisotropies arise in layered materials with large structural difference along different in-plane directions. Here, we report an extreme case in layered TiS 3 , which features tightly bonded atomic chains along the b-axis direction, held together by weaker, interchain bonding along the a-axis direction. Experiments show thermal conductivity along the chain twice as high as between the chain, an in-plane anisotropy higher than any other layered materials measured to date. We found that in contrast to most other materials, optical phonons in TiS 3 conduct an unusually high portion of heat (up to 66% along the b-axis direction). The large dispersiveness of optical phonons along the chains, contrasted to many fewer dispersive optical phonons perpendicular to the chains, is the primary reason for the observed high anisotropy in thermal conductivity. The finding discovers materials with unusual thermal conduction mechanism, as well as provides new material platforms for potential heat-routing or heat-managing devices.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Proximity-induced anisotropic magnetoresistance in magnetized topological insulators.

Topological insulators (TIs) host spin-momentum locked surface states that are inherently susceptible to magnetic proximity modulations, making them promising for nano-electronic, spintronic, and quantum computing applications. While much effort has been devoted to studying (quantum) anomalous Hall effects in magnetic magnetically doped TIs, the inherent magnetoresistance (MR) properties in magnetic proximity-coupled surface states remain largely unexplored. Here, we directly exfoliate Bi 2 Se 3 TI flakes onto a magnetic insulator, yttrium iron garnet, and measure the MR at various temperatures. We experimentally observe an anisotropic magnetoresistance that is consistent with a magnetized surface state. Our results indicate that the TI has magnetic anisotropy out of the sample plane, which opens an energy gap between the surface states. Additionally, by applying a magnetic field along any in-plane orientation, the magnetization of the TI rotates toward the plane and the gap closes. Consequently, we observe a large (similar to 6.5%) MR signal that is attributed to an interplay between coherent rotation of magnetization within a topological insulator and abrupt switching of magnetization in the underlying magnetic insulator.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Low-damage electron beam lithography for nanostructures on Bi 2 Te 3 -class topological insulator thin films

Nanostructured topological insulators (TIs) have the potential to impact a wide array of condensed matter physics topics, ranging from Majorana physics to spintronics. However, the most common TI materials, the Bi 2 Se 3 family, are easily damaged during nanofabrication of devices. In this paper, we show that electron beam lithography performed with a 30 or 50 kV accelerating voltage—common for nanopatterning in academic facilities—damages both nonmagnetic TIs and their magnetically doped counterparts at unacceptable levels. We additionally demonstrate that electron beam lithography with a 10 kV accelerating voltage produces minimal damage detectable through low-temperature electronic transport. In conclusion, although reduced accelerating voltages present challenges in creating fine features, we show that with careful choice of processing parameters, particularly the resist, 100 nm features are reliably achievable.

42 ENGINEERING↗
DOE OSTI2022

Copper migration and surface oxidation of CuxBi2Se3 in ambient pressure environments

Abstract Chemical modifications such as intercalation can be used to modify surface properties or to further functionalize the surface states of topological insulators (TIs). Using ambient pressure x-ray photoelectron spectroscopy, we report copper migration in C u x B i 2 S e 3 , which occurs on a timescale of hours to days after initial surface cleaving. The increase in near-surface copper proceeds along with the oxidation of the sample surface and large changes in the selenium content. These complex changes are further modeled with core-level spectroscopy simulations, which suggest a composition gradient near the surface which develops with oxygen exposure. Our results shed light on a new phenomenon that must be considered for intercalated TIs—and intercalated materials in general—that surface chemical composition can change when specimens are exposed to ambient conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Intrinsic axion insulating behavior in antiferromagnetic MnBi 6 Te 10

A striking feature of time-reversal symmetry (TRS) protected topological insulators (TIs) is that they are characterized by a half integer quantum Hall effect on the boundary when the surface states are gapped by time-reversal breaking perturbations. While TRS-protected TIs have become increasingly under control, magnetic analogs are still a largely unexplored territory with novel rich structures. In particular, magnetic topological insulators can also host a quantized axion term in the presence of lattice symmetries. Since these symmetries are naturally broken on the boundary, the surface states can develop a gap without external manipulation. In this work, we combine theoretical analysis, density-functional calculations and experimental evidence to reveal intrinsic axion insulating behavior in MnBi 6 Te 10 . Additionally, the quantized axion term arises from the simplest possible mechanism in the antiferromagnetic regime where it is protected by inversion symmetry and the product of a fractional translation and TRS. The anticipated gapping of the Dirac surface state at the edge is subsequently experimentally established using angle resolved photoemission spectroscopy (ARPES). As a result, this system provides the magnetic analog of the simplest TRS-protected TI with a single, gapped Dirac cone at the surface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Observation of nonlinear planar Hall effect in magnetic-insulator–topological-insulator heterostructures

Interfacing topological insulators (TIs) with magnetic insulators (MIs) have been widely used to study the interaction between topological surface states and magnetism. Previous transport studies typically interpret the suppression of weak antilocalization or appearance of the anomalous Hall effect as signatures of the magnetic proximity effect (MPE) imposed to TIs. Here, in this work, we report the observation of the nonlinear planar Hall effect (NPHE) in Bi 2 ⁢Se 3 films grown on MI thulium and yttrium-iron-garnet (TmIG and YIG) substrates, which is an order of magnitude larger than that in Bi 2 ⁢Se 3 grown on nonmagnetic gadolinium-gallium-garnet (GGG) substrate. The nonlinear Hall resistance in TmIG/Bi 2 ⁢Se 3 depends linearly on the external magnetic field, while that in YIG/Bi 2 ⁢Se 3 exhibits an extra hysteresis loop around zero field. The magnitude of the NPHE is found to scale inversely with carrier density. We speculate that the observed NPHE is related to the MPE-induced exchange gap opening and out-of-plane spin textures in the TI surface states, which may be used as an alternative transport signature of the MPE in MI/TI heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Large bilinear magnetoresistance from Rashba spin-splitting on the surface of a topological insulator

In addition to the topologically protected linear dispersion, a band-bending-confined two-dimensional electron gas with tunable Rashba spin-splitting (RSS) was found to coexist with the topological surface states on the surface of topological insulators (TIs). Here, in this paper, we report the observation of large bilinear magnetoresistance (BMR) in Bi 2 ⁢Se 3 films decorated with transition-metal atoms. The magnitude of the BMR sensitively depends on the type and amount of atoms deposited, with a maximum achieved value close to those of strong Rashba semiconductors. Our first-principles calculations reproduce the quantum well states and reveal sizable RSS in all Bi 2 ⁢Se 3 heterostructures with broken inversion symmetry. Our results show that charge-spin interconversion through RSS states in TIs can be fine tuned through surface atom deposition and easily detected via BMR for potential spintronic applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Coexistence of ferromagnetism and antiferromagnetic dimers in topological insulators

The addition of magnetic impurities in topological insulators (TIs) can drive ferromagnetic order that leads to quantum anomalous Hall transport well below the Curie temperature. The fragility of the quantized regime has been ascribed to the random nature of the magnetic moment distribution. Here, we refine this hypothesis by using inelastic neutron scattering and density-functional theory calculations to show that two antagonistic components define the magnetism in Mn-substituted SnTe, thereby limiting the effectiveness of dilute magnetic TIs. One component is strongly bound antiferromagnetic dimers that compete with ferromagnetic order. In conclusion, the other component consists of undimerized moments where ferromagnetic order develops via long-range interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Millikelvin-resolved ambient thermography

Thermography detects surface temperature and subsurface thermal activity of an object based on the Stefan-Boltzmann law. Impacts of the technology would be more far-reaching with finer thermal sensitivity, called noise-equivalent differential temperature (NEDT). Existing efforts to advance NEDT are all focused on improving registration of radiation signals with better cameras, driving the number close to the end of the roadmap at 20 to 40 mK. In this work, we take a distinct approach of sensitizing surface radiation against minute temperature variation of the object. The emissivity of the thermal imaging sensitizer (TIS) rises abruptly at a preprogrammed temperature, driven by a metal-insulator transition in cooperation with photonic resonance in the structure. The NEDT is refined by over 15 times with the TIS to achieve single-digit millikelvin resolution near room temperature, empowering ambient thermography for a broad range of applications such as in operando electronics analysis and early cancer screening.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Growth and Development of Quantum Materials

Topological insulators (TIs) display a new state of quantum matter, which is broadly known as quantum materials. The TIs have a unique property of being an insulator as a bulk property and having conducting surface states, which are symmetry-protected Dirac Fermions and well isolated from the bulk valence and conduction bands. Ideally, these topological surface states and bulk electronic states should act independently. The degree to which they intermix depends on the crystalline quality, composition and defects present in the material. These materials thus demand very high-quality crystals to observe the desired quantum properties. We proposed growth of high-quality single crystals and thin films to enhance the ability to fabricate devices with unique performance attributes. Two separate approaches will be made to make the thin films, one is exfoliation of 2D monolayer from the grown single crystals and the second is thin films grown by pulsed laser deposition. Both techniques will subsequently allow for fabrication of meso structures using a Focused Ion Beam technique.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Phase-Transition Interlayer Enables High-Performance Solid-State Sodium Batteries with Sulfide Solid Electrolyte

All-solid-state sodium (Na) batteries (ASSSBs) using sulfide-based solid electrolytes (SEs) have attracted intensive attention due to their superior safety, high energy density and low cost. However, interfacial issue is one of the biggest challenges to achieve high-performance sulfide-based ASSSBs due to the serious reactions between active Na metal and sulfide SEs at the interface. To address the interfacial challenges, we propose in this paper a simple and efficient approach by introducing a polymer electrolyte as an interlayer to stabilize the interface. Na 3 SbS 4 as a model sulfide SE is used to demonstrate the interlayer strategy to stabilize the interface by preventing the detrimental reactions and inhibiting Na dendrites. As a result, stable Na plating/stripping was observed in Na symmetric cells under the current density of 0.1 mA cm –2 . Moreover, ASSSBs with Na metal and TiS 2 electrode delivered long-term stability over 300 cycles remaining a specific capacity above 100 mAh g –1 , and FeS 2 ||Na cells exhibited an impressive specific capacity of up to 200 mAh g –1 after the 20th cycles. This work demonstrates an efficient strategy to address interfacial challenges between sulfide SEs and Na metal, which contributes to the development of ASSSBs in next-generation energy storage systems.

25 ENERGY STORAGE↗
DOE OSTI2021

Anisotropic Properties of Quasi‐1D In 4 Se 3 : Mechanical Exfoliation, Electronic Transport, and Polarization‐Dependent Photoresponse

Abstract Theoretical and experimental investigations of various exfoliated samples taken from layered In 4 Se 3 crystals are performed. In spite of the ionic character of interlayer interactions in In 4 Se 3 and hence much higher calculated cleavage energies compared to graphite, it is possible to produce few‐nanometer‐thick flakes of In 4 Se 3 by mechanical exfoliation of its bulk crystals. The In 4 Se 3 flakes exfoliated on Si/SiO 2 have anisotropic electronic properties and exhibit field‐effect electron mobilities of about 50 cm 2 V −1 s −1 at room temperature, which are comparable with other popular transition metal chalcogenide (TMC) electronic materials, such as MoS 2 and TiS 3 . In 4 Se 3 devices exhibit a visible range photoresponse on a timescale of less than 30 ms. The photoresponse depends on the polarization of the excitation light consistent with symmetry‐dependent band structure calculations for the most expected ac cleavage plane. These results demonstrate that mechanical exfoliation of layered ionic In 4 Se 3 crystals is possible, while the fast anisotropic photoresponse makes In 4 Se 3 a competitive electronic material, in the TMC family, for emerging optoelectronic device applications.

Vorobeva, Nataliia S.↗
DOE OSTI2024

Strain‐Driven Stabilization of a Room‐Temperature Chiral Multiferroic with Coupled Ferroaxial and Ferroelectric Order

Noncollinear ferroic materials are sought after as testbeds to explore the intimate connections between topology and symmetry, which result in electronic, optical, and magnetic functionalities not observed in collinear ferroic materials. For example, ferroaxial materials have rotational structural distortions that break mirror symmetry and induce chirality. When ferroaxial order is coupled with ferroelectricity arising from a broken inversion symmetry, it offers the prospect of electric-field-control of the ferroaxial distortions and opens up new tunable functionalities. However, chiral multiferroics, especially ones stable at room temperature, are rare. A strain-stabilized, room-temperature chiral multiferroic phase in single crystals of BaTiS 3 is reported here. Using first-principles calculations, the stabilization of this multiferroic phase having P6 3 space group for biaxial tensile strains exceeding 1.5% applied on the basal ab-plane of the room temperature P6 3 cm phase of BaTiS 3 is predicted. The chiral multiferroic phase is characterized by rotational distortions of TiS 6 octahedra around the long c-axis and polar displacement of Ti atoms along the c-axis. Furthermore, the ferroaxial and ferroelectric distortions and their domains in P6 3 -BaTiS 3 are directly resolved using atomic resolution scanning transmission electron microscopy. Landau-based phenomenological modeling predicts a strong coupling between the ferroelectric and the ferroaxial order making P6 3 -BaTiS 3 an attractive test bed for achieving electric-field-control of chirality.

Chalcogenide↗
DOE OSTI2020

Visualizing Tailored Spin Phenomena in a Reduced‐Dimensional Topological Superlattice

Abstract Emergent topological insulators (TIs) and their design are in high demand for manipulating and transmitting spin information toward ultralow‐power‐consumption spintronic applications. Here, distinct topological states with tailored spin properties can be achieved in a single reduced‐dimensional TI‐superlattice, (Bi 2 /Bi 2 Se 3 )‐(Bi 2 /Bi 2 Se 3 ) N or (□/Bi 2 Se 3 )‐(Bi 2 /Bi 2 Se 3 ) N ( N is the repeating unit, □ represents an empty layer) by controlling the termination via molecular beam epitaxy. The Bi 2 ‐terminated superlattice exhibits a single Dirac cone with a spin momentum splitting ≈0.5 Å −1 , producing a pronounced inverse Edelstein effect with a coherence length up to 1.26 nm. In contrast, the Bi 2 Se 3 ‐terminated superlattice is identified as a dual TI protected by coexisting time reversal and mirror symmetries, showing an unexpectedly long spin lifetime up to 1 ns. The work elucidates the key role of dimensionality and dual topological phases in selecting desired spin properties, suggesting a promise route for engineering topological superlattices for high‐performance TI‐spintronic devices.

Sun, Rui↗
DOE OSTI2024

Salt‐Assisted Vapor–Liquid–Solid Growth of 1D van der Waals Materials

The method of salt-assisted vapor–liquid–solid (VLS) growth is introduced to synthesize 1D nanostructures of trichalcogenide van der Waals (vdW) materials, exemplified by niobium trisulfide (NbS 3 ). The method uses a unique catalyst consisting of an alloy of Au and an alkali metal halide (NaCl) to enable rapid and directional growth. High yields of two types of NbS 3 1D nanostructures, nanowires and nanoribbons, each with sub-ten nanometer diameter, tens of micrometers length, and distinct 1D morphology and growth orientation are demonstrated. Strategies to control the location, size, and morphology of growth, and extend the growth method to synthesize other transition metal trichalcogenides, NbSe 3 and TiS 3 , as nanowires are demonstrated. Finally, the role of the Au–NaCl alloy catalyst in guiding VLS synthesis is described and the growth mechanism based on the relationships measured between structure (growth orientation, morphology, and dimensions) and growth conditions (catalyst volume and growth time) is discussed. These results introduce opportunities to expand the library of emerging 1D vdW materials to make use of their unique properties through controlled growth at nanoscale dimensions.

1D van der Waals material↗
DOE OSTI2024

Giant Modulation of Refractive Index from Picoscale Atomic Displacements

Abstract It is shown that structural disorder—in the form of anisotropic, picoscale atomic displacements—modulates the refractive index tensor and results in the giant optical anisotropy observed in BaTiS 3 , a quasi‐1D hexagonal chalcogenide. Single‐crystal X‐ray diffraction studies reveal the presence of antipolar displacements of Ti atoms within adjacent TiS 6 chains along the c ‐axis, and threefold degenerate Ti displacements in the a – b plane. 47/49 Ti solid‐state NMR provides additional evidence for those Ti displacements in the form of a three‐horned NMR lineshape resulting from a low symmetry local environment around Ti atoms. Scanning transmission electron microscopy is used to directly observe the globally disordered Ti a–b plane displacements and find them to be ordered locally over a few unit cells. First‐principles calculations show that the Ti a – b plane displacements selectively reduce the refractive index along the ab ‐plane, while having minimal impact on the refractive index along the chain direction, thus resulting in a giant enhancement in the optical anisotropy. By showing a strong connection between structural disorder with picoscale displacements and the optical response in BaTiS 3 , this study opens a pathway for designing optical materials with high refractive index and functionalities such as large optical anisotropy and nonlinearity.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Highly Efficient Room‐Temperature Spin‐Orbit‐Torque Switching in a Van der Waals Heterostructure of Topological Insulator and Ferromagnet

Abstract All‐Van der Waals (vdW)‐material‐based heterostructures with atomically sharp interfaces offer a versatile platform for high‐performing spintronic functionalities at room temperature. One of the key components is vdW topological insulators (TIs), which can produce a strong spin‐orbit‐torque (SOT) through the spin‐momentum locking of their topological surface state (TSS). However, the relatively low conductance of the TSS introduces a current leakage problem through the bulk states of the TI or the adjacent ferromagnetic metal layers, reducing the interfacial charge‐to‐spin conversion efficiency ( q ICS ). Here, a vdW heterostructure is used consisting of atomically‐thin layers of a bulk‐insulating TI Sn‐doped Bi 1.1 Sb 0.9 Te 2 S 1 and a room‐temperature ferromagnet Fe 3 GaTe 2, to enhance the relative current ratio on the TSS up to ≈20%. The resulting q ICS reaches ≈1.65 nm −1 and the critical current density J c ≈0.9 × 10 6 Acm −2 at 300 K, surpassing the performance of TI‐based and heavy‐metal‐based SOT devices. These findings demonstrate that an all‐vdW heterostructure with thickness optimization offers a promising platform for efficient current‐controlled magnetization switching at room temperature.

2D ferromagnet↗