Alla Chikina

Nature Materials, Published online: 09 March 2020; doi:10.1038/s41563-020-0632-9

A monolayer topological insulator is magnetized by proximity to a layered antiferromagnetic insulator.

One‐dimensional defect line patterns are formed in monolayer VSe₂ upon high‐temperature annealing. X‐ray magnetic circular dichroism and magnetic force microscopy confirm that such Se‐deficient reconstructed defects cause the onset of ferromagnetism in pristine VSe₂. This study provides a way to engineer the magnetism via controlling the atomic structures of surface defects in 2D crystals.

Abstract

There have been several recent conflicting reports on the ferromagnetism of clean monolayer VSe₂. Herein, the controllable formation of 1D defect line patterns in vanadium diselenide (VSe₂) monolayers initiated by thermal annealing is presented. Using scanning tunneling microscopy and q‐plus atomic force microscopy techniques, the 1D line features are determined to be 8‐member‐ring arrays, formed via a Se deficient reconstruction process. The reconstructed VSe₂ monolayer with Se‐deficient line defects displays room‐temperature ferromagnetism under X‐ray magnetic circular dichroism and magnetic force microscopy, consistent with the density functional theory calculations. This study possibly resolves the controversy on whether ferromagnetism is intrinsic in monolayer VSe₂, and highlights the importance of controlling and understanding the atomic structures of surface defects in 2D crystals, which could play key roles in the material properties and hence potential device applications.

A high‐quality single crystalline freestanding Fe₃O₄ thin film with strong magnetism has been synthesized by pulsed laser deposition using water‐dissolvable Sr₃Al₂O₆ sacrificial layer, and the resulting freestanding film is highly flexible. When transferred to the polydimethylsiloxane support layer, the Fe₃O₄ film can be bent with large deformation without affecting its magnetization, demonstrating its robust magnetism.

Abstract

Magnetic materials and devices that can be folded and twisted without sacrificing their functional properties are highly desirable for flexible electronic applications in wearable products and implantable systems. In this work, a high‐quality single crystalline freestanding Fe₃O₄ thin film with strong magnetism has been synthesized by pulsed laser deposition using a water‐dissolvable Sr₃Al₂O₆ sacrificial layer, and the resulting freestanding film, with magnetism confirmed at multiple length scales, is highly flexible with a bending radius as small as 7.18 µm and twist angle as large as 122°, in sharp contrast with bulk magnetite that is quite brittle. When transferred to a polydimethylsiloxane support layer, the Fe₃O₄ film can be bent with large deformation without affecting its magnetization, demonstrating its robust magnetism. The work thus offers a viable solution for flexible magnetic materials that can be utilized in a range of applications.

We report on muon spin rotation experiments probing the magnetic penetration depth (T) in the layered superconductors in 2H-NbSe₂ and 4H-NbSe₂. The current results, along with our earlier findings on 1T'-MoTe₂ (Guguchia et al.), demonstrate that the superfluid density scales linearly with T_c in the three transition metal dichalcogenide superconductors. Upon increasing pressure, we observe a substantial increase of the superfluid density in 2H-NbSe₂, which we find to correlate with T_c. The correlation deviates from the abovementioned linear trend. A similar deviation from the Uemura line was also observed in previous pressure studies of optimally doped cuprates. This correlation between the superfluid density and T_c is considered a hallmark feature of unconventional superconductivity. Here, we show that this correlation is an intrinsic property of the superconductivity in transition metal dichalcogenides, whereas the ratio T_c/T_F is approximately a factor of 20 lower than the ratio observed in hole-doped cuprates. We, furthermore, find that the values of the superconducting gaps are insensitive to the suppression of the charge density wave state.

Nature Materials, Published online: 04 November 2019; doi:10.1038/s41563-019-0518-x

A remarkably low critical current is found to reorient the magnetic order in a magnetically intercalated transition metal dichalcogenide, suggesting this class of materials could form a basis for antiferromagnetic spintronics.

Charged domain walls in non‐ferroelectric materials open a much wider choice of materials for domain wall‐based device engineering. By scanning transmission electron microcopy and electron energy loss spectroscopy, tail‐to‐tail charged domain walls, are revealed in LaAlO₃/SrTiO₃//NdGaO₃ heterostructures. The strong correlation between the domain wall and interface interdiffusion, plus oxygen octahedral rotation determine the complicated SrTiO₃ thickness dependent transport properties.

Abstract

Charged domain walls provide possibilities in effectively manipulating electrons at nanoscales for developing next‐generation electronic devices. Here, using the atom‐resolved imaging and spectroscopy on LaAlO₃/SrTiO₃//NdGaO₃ heterostructures, the evolution of correlated lattice instability and charged domain walls is visualized crossing the conducting LaAlO₃/SrTiO₃ heterointerface. When increasing the SrTiO₃ layer thickness to 20 unit cells and above, both LaAlO₃ and SrTiO₃ layers begin to exhibit measurable polar displacements to form a tail‐to‐tail charged domain wall at the LaAlO₃/SrTiO₃ interface, resulting in the charged redistribution within the 2‐nm‐thick SrTiO₃ layer close to the LaAlO₃/SrTiO₃ interface. The mobile charges in different heterostructures can be estimated by summing up Ti³⁺ concentrations in the conducting channel, which is sandwiched by SrTiO₃ layers with interdiffusion and/or oxygen octahedral rotations. Those estimated mobile charges are quantitatively consistent with results from Hall measurements. The results not only shed light on complex oxide heterointerfaces, but also pave a new path to nanoscale charge engineering.

Author(s): Steven S. -L. Zhang, Anton A. Burkov, Ivar Martin, and Olle G. Heinonen

Weyl semimetals (WSM) are a newly discovered class of quantum materials which can host a number of exotic bulk transport properties, such as the chiral magnetic effect, negative magnetoresistance, and the anomalous Hall effect. In this work, we investigate theoretically the spin-to-charge conversion...

[Phys. Rev. Lett.] Published Wed Oct 02, 2019

Symmetry prohibits pyroelectricity from being present in the majority of crystallographic material classes, including centrosymmetric SrTiO₃. Nonetheless, this study reports the emergence of pyroelectricity at the surface of SrTiO₃ where the lattice symmetry is broken. The discovery paves the way for observing pyroelectricity and piezoelectricity in any material, independent of its bulk lattice symmetry.

Abstract

Symmetry‐imposed restrictions on the number of available pyroelectric and piezoelectric materials remain a major limitation as 22 out of 32 crystallographic material classes exhibit neither pyroelectricity nor piezoelectricity. Yet, by breaking the lattice symmetry it is possible to circumvent this limitation. Here, using a unique technique for measuring transient currents upon rapid heating, direct experimental evidence is provided that despite the fact that bulk SrTiO₃ is not pyroelectric, the (100) surface of TiO₂‐terminated SrTiO₃ is intrinsically pyroelectric at room temperature. The pyroelectric layer is found to be ≈1 nm thick and, surprisingly, its polarization is comparable with that of strongly polar materials such as BaTiO₃. The pyroelectric effect can be tuned ON/OFF by the formation or removal of a nanometric SiO₂ layer. Using density functional theory, the pyroelectricity is found to be a result of polar surface relaxation, which can be suppressed by varying the lattice symmetry breaking using a SiO₂ capping layer. The observation of pyroelectricity emerging at the SrTiO₃ surface also implies that it is intrinsically piezoelectric. These findings may pave the way for observing and tailoring piezo‐ and pyroelectricity in any material through appropriate breaking of symmetry at surfaces and artificial nanostructures such as heterointerfaces and superlattices.

Consecutively tailoring few‐layer transition metal dichalcogenides from the 2H to T_d phase may realize the long‐sought topological superconductivity by incorporating the quantum spin Hall effect and superconductivity. This study demonstrates that the transitions from T_d to 1T' to 2H phase can be realized in Se‐substituted MoTe₂ thin films. More importantly, the observed superconductivity enhancement can be interpreted as two‐band superconductivity.

Abstract

Consecutively tailoring few‐layer transition metal dichalcogenides MX₂ from 2H to T _d phase may realize the long‐sought topological superconductivity in a single material system by incorporating superconductivity and the quantum spin Hall effect together. Here, this study demonstrates that a consecutive structural phase transition from T _d to 1T′ to 2H polytype can be realized by increasing the Se concentration in Se‐substituted MoTe₂ thin films. More importantly, the Se‐substitution is found to dramatically enhance the superconductivity of the MoTe₂ thin film, which is interpreted as the introduction of two‐band superconductivity. The chemical‐constituent‐induced phase transition offers a new strategy to study the s⁺⁻ superconductivity and the possible topological superconductivity, as well as to develop phase‐sensitive devices based on MX₂ materials.

The transition‐metal diphosphide WP₂ is a type‐II Weyl semimetal candidate. The anisotropy of its ac‐plane resistivity, which mainly arises from the scattering rate anisotropy, increases sharply at temperature T ≤ 100 K without phase transitions and can be tuned by magnetic fields. The broken inversion symmetry in WP₂ is identified by combining linearly polarized Raman spectroscopy and first‐principle calculations.

Abstract

A transition metal diphosphide, WP₂, is a candidate for type‐II Weyl semimetals (WSMs) in which spatial inversion symmetry is broken and Lorentz invariance is violated. As one of the prerequisites for the presence of the WSM state in WP₂, spatial inversion symmetry breaking in this compound has rarely been investigated. Furthermore, the anisotropy of the WP₂ electrical properties and whether its electrical anisotropy can be tuned remain elusive. Angle‐resolved polarized Raman spectroscopy, electrical transport, optical spectroscopy, and first‐principle studies of WP₂ are reported. The energies of the observed Raman‐active phonons and the angle dependences of the detected phonon intensities are consistent with results obtained by first‐principle calculations and analysis of the proposed crystal symmetry without spatial inversion, showing that spatial inversion symmetry is broken in WP₂. Moreover, the measured ratio (R_c /R_a ) between the crystalline c‐axis and a‐axis electrical resistivities exhibits a weak dependence on temperature (T) in the temperature range from 100 to 250 K, but increases abruptly at T ≤ 100 K, and then reaches the value of ≈8.0 at T = 10 K, which is by far the strongest in‐plane electrical resistivity anisotropy among the reported type‐II WSM candidates with comparable carrier concentrations. Optical spectroscopy study, together with the first‐principle calculations on the electronic band structure, reveals that the abrupt enhancement of the electrical resistivity anisotropy at T ≤ 100 K mainly arises from a sharp increase in the scattering rate anisotropy at low temperatures. More interestingly, the R_c /R_a of WP₂ at T = 10 K can be tuned from 8.0 to 10.6 as the magnetic field increases from 0 to 9 T. The so‐far‐strongest and magnetic‐field‐tunable electrical resistivity anisotropy found in WP₂ can serve as a degree of freedom for tuning the electrical properties of type‐II WSMs, which paves the way for the development of novel electronic applications based on type‐II WSMs.

A strong interfacial coupling between the trions in 2D MoS₂ and soft phonons emerging below the antiferrodistortive phase transition in SrTiO₃ gives rise to a new quasiparticle, the “Polaronic Trion.” The polaronic trion has a very high binding energy that is electric field tunable, significant for trion‐based low‐loss optoelectronic devices.

Abstract

The reduced electrical screening in 2D materials provides an ideal platform for realization of exotic quasiparticles, that are robust and whose functionalities can be exploited for future electronic, optoelectronic, and valleytronic applications. Recent examples include an interlayer exciton, where an electron from one layer binds with a hole from another, and a Holstein polaron, formed by an electron dressed by a sea of phonons. Here, a new quasiparticle is reported, “polaronic trion” in a heterostructure of MoS₂/SrTiO₃ (STO). This emerges as the Fröhlich bound state of the trion in the atomically thin monolayer of MoS₂ and the very unique low energy soft phonon mode (≤7 meV, which is temperature and field tunable) in the quantum paraelectric substrate STO, arising below its structural antiferrodistortive (AFD) phase transition temperature. This dressing of the trion with soft phonons manifests in an anomalous temperature dependence of photoluminescence emission leading to a huge enhancement of the trion binding energy (≈70 meV). The soft phonons in STO are sensitive to electric field, which enables field control of the interfacial trion–phonon coupling and resultant polaronic trion binding energy. Polaronic trions could provide a platform to realize quasiparticle‐based tunable optoelectronic applications driven by many body effects.

Selective tuning of ambipolar WSe₂ monolayer to p‐ and n‐type semiconductors by chemical doping is demonstrated. The chemical doping not only allows to control over the main charge carriers, but also increases the carrier mobility of the WSe₂ significantly. Furthermore, a complementary metal‐oxide‐semiconductor inverter and an in‐plane p–n junction with superior performance are successfully fabricated by integrating the chemically doped WSe₂.

Abstract

Monolayers of transition metal dichalcogenides (TMDCs) have attracted a great interest for post‐silicon electronics and photonics due to their high carrier mobility, tunable bandgap, and atom‐thick 2D structure. With the analogy to conventional silicon electronics, establishing a method to convert TMDC to p‐ and n‐type semiconductors is essential for various device applications, such as complementary metal‐oxide‐semiconductor (CMOS) circuits and photovoltaics. Here, a successful control of the electrical polarity of monolayer WSe₂ is demonstrated by chemical doping. Two different molecules, 4‐nitrobenzenediazonium tetrafluoroborate and diethylenetriamine, are utilized to convert ambipolar WSe₂ field‐effect transistors (FETs) to p‐ and n‐type, respectively. Moreover, the chemically doped WSe₂ show increased effective carrier mobilities of 82 and 25 cm² V⁻¹s⁻¹ for holes and electrons, respectively, which are much higher than those of the pristine WSe₂. The doping effects are studied by photoluminescence, Raman, X‐ray photoelectron spectroscopy, and density functional theory. Chemically tuned WSe₂ FETs are integrated into CMOS inverters, exhibiting extremely low power consumption (≈0.17 nW). Furthermore, a p‐n junction within single WSe₂ grain is realized via spatially controlled chemical doping. The chemical doping method for controlling the transport properties of WSe₂ will contribute to the development of TMDC‐based advanced electronics.

In article number https://doi.org/10.1002/adma.2019010771901077, Lih‐Juann Chen, Yi‐Hsien Lee, and co‐workers demonstrate ultraclean transfer of synthesized monolayers for artificially stacked monolayers with tunable twisting and a heterointerface. Diverse Moiré electronic superlattices are directly visualized by scanning tunneling microscopy, which opens a new avenue toward correlated properties in artificial 2D lattices.

Topological surface states (TSSs) in a topological insulator are expected to be able to produce a spin-orbit torque that can switch a neighboring ferromagnet. This effect may be absent if the ferromagnet is conductive because it can completely suppress the TSSs, but it should be present if the ferromagnet is insulating. This study reports TSS-induced switching in a bilayer consisting of a topological insulator Bi₂Se₃ and an insulating ferromagnet BaFe₁₂O₁₉. A charge current in Bi₂Se₃ can switch the magnetization in BaFe₁₂O₁₉ up and down. When the magnetization is switched by a field, a current in Bi₂Se₃ can reduce the switching field by ~4000 Oe. The switching efficiency at 3 K is 300 times higher than at room temperature; it is ~30 times higher than in Pt/BaFe₁₂O₁₉. These strong effects originate from the presence of more pronounced TSSs at low temperatures due to enhanced surface conductivity and reduced bulk conductivity.

Controlling charge density in two-dimensional (2D) materials is a powerful approach for engineering new electronic phases and properties. This control is traditionally realized by electrostatic gating. Here, we report an optical approach for generation of high carrier densities using transition metal dichalcogenide heterobilayers, WSe₂/MoSe₂, with type II band alignment. By tuning the optical excitation density above the Mott threshold, we realize the phase transition from interlayer excitons to charge-separated electron/hole plasmas, where photoexcited electrons and holes are localized to individual layers. High carrier densities up to 4 x 10¹⁴ cm^–2 can be sustained under both pulsed and continuous wave excitation conditions. These findings open the door to optical control of electronic phases in 2D heterobilayers.

In article number 1802093, Wenjing Zhang, Andrivo Rusydi, Andrew T. S. Wee, and co‐workers derive the general trends for the high‐yield phase‐transition‐process of two‐dimensional transition metal dichalcogenides (2D‐TMDs) on metals (Au, Ag, Cu). While each 2D‐TMD possesses different intrinsic 1H‐1T' energy barriers, the use of a metallic substrate with higher chemical reactivity plays a more pivotal role in increasing the 1H‐1T' phase‐transition yield which is brought about via the enhancement of the interfacial hybridizations.

In article number https://doi.org/10.1002/advs.2019004461900446, Shi Jie Wang, Wenjing Zhang, Andrivo Rusydi, Andrew T. S. Wee, and co‐workers observe high‐energy excitons that are generated by a new mechanism in monolayer‐MoS₂ on SrTiO₃, which are attributed to the change in many‐body interactions that couples with interfacial orbital‐hybridization. The interfacial interactions lead to a fermi‐surface feature at the interface. The results provide an understanding of 2D‐transition metal dichalcogenides heterointerfaces and show the crucial role that many‐body interactions play at the atomic level.