Layer-Number-Independent Two-Dimensional Ferromagnetism in Cr3Te4.

In a conventional magnetic material, a long-range magnetic order develops in three dimensions, and reducing a layer number weakens its magnetism. Here we demonstrate anomalous layer-number-independent ferromagnetism down to the two-dimensional (2D) limit in a metastable phase of Cr3Te4. We fabricated Cr3Te4 thin films by molecular-beam epitaxy and found that Cr3Te4 could host two distinct ferromagnetic phases characterized with different Curie temperatures (TC). One is the bulk-like "high-TC phase" showing room-temperature ferromagnetism, which is consistent with previous studies. The other is the metastable "low-TC phase" with TC ≈ 160 K, which exhibits a layer-number-independent TC down to the 2D limit in marked contrast with the conventional high-TC phase, demonstrating a purely 2D nature of its ferromagnetism. Such significant differences between two distinct phases could be attributed to a small variation in the doping level, making this material attractive for future ultracompact spintronics applications with potential gate-tunable room-temperature 2D ferromagnetism.

Pressure-induced topological phase transition in noncentrosymmetric elemental tellurium.

Recent progress in understanding the electronic band topology and emergent topological properties encourage us to reconsider the band structure of well-known materials including elemental substances. Controlling such a band topology by external field is of particular interest from both fundamental and technological viewpoints. Here we report possible signatures of the pressure-induced topological phase transition from a semiconductor to a Weyl semimetal in elemental tellurium probed by transport measurements. Pressure variation of the periods of Shubnikov-de Haas oscillations, as well as oscillation phases, shows an anomaly around the pressure theoretically predicted for topological phase transition. This behavior is consistent with the pressure-induced band deformation and resultant band-crossing effect. Moreover, effective cyclotron mass is reduced toward the critical pressure, potentially reflecting the emergence of massless linear dispersion. The present result paves the way for studying the electronic band topology in well-known compounds and topological phase transition by the external field.

Spin-Orbit-Induced Ising Ferromagnetism at a van der Waals Interface.

Magnetocrystalline anisotropy, a key ingredient for establishing long-range order in a magnetic material down to the two-dimensional (2D) limit, is generally associated with spin-orbit interaction (SOI) involving a finite orbital angular momentum. Here we report strong out-of-plane magnetic anisotropy without orbital angular momentum, emerging at the interface between two different van der Waals (vdW) materials, an archetypal metallic vdW material NbSe2 possessing Zeeman-type SOI and an isotropic vdW ferromagnet V5Se8. We found that the Zeeman SOI in NbSe2 induces robust out-of-plane magnetic anisotropy in V5Se8 down to the 2D limit with a more than 2-fold enhancement of the transition temperature. We propose a simple model that takes into account the energy gain in NbSe2 in contact with a ferromagnet, which naturally explains our observations. Our results demonstrate a conceptually new magnetic proximity effect at the vdW interface, expanding the horizons of emergent phenomena achievable in vdW heterostructures.

Probing the Chiral Domains and Excitonic States in Individual WS2 Tubes by Second-Harmonic Generation.

Distinct from carbon nanotubes, transition-metal dichalcogenide (TMD) nanotubes are noncentrosymmetric and polar and can exhibit some intriguing phenomena such as nonreciprocal superconductivity, chiral shift current, bulk photovoltaic effect, and exciton-polaritons. However, basic characterizations of individual TMD nanotubes are still quite limited, and much remains unclear about their structural chirality and electronic properties. Here we report an optical second-harmonic generation (SHG) study on multiwalled WS2 nanotubes on a single-tube level. As it is highly sensitive to the crystallographic symmetry, SHG microscopy unveiled multiple structural domains within a single WS2 nanotube, which are otherwise hidden under conventional white-light optical microscopy. Moreover, the polarization-resolved SHG anisotropy patterns revealed that different domains on the same tube can be of different chirality. In addition, we observed the excitonic states of individual WS2 nanotubes via SHG excitation spectroscopy, which were otherwise difficult to acquire due to the indirect band gap of the material.

Antiferromagnet-Semiconductor Van Der Waals Heterostructures: Interlayer Interplay of Exciton with Magnetic Ordering.

Van der Waals (vdW) heterostructures have attracted great interest because of their rich material combinations. The discovery of two-dimensional magnets has provided a new platform for magnetic vdW heterointerfaces; however, research on magnetic vdW heterointerfaces has been limited to those with ferromagnetic surfaces. Here, we report a magnetic vdW heterointerface using layered intralayer-antiferromagnetic MPSe3 (M = Mn, Fe) and monolayer transition-metal dichalcogenides (TMDs). We found an anomalous upshift of the excitonic peak in monolayer TMDs below the antiferromagnetic transition temperature in the MPSe3, capturing a signature of the interlayer exciton-magnon coupling. This is a concept extended from single materials to heterointerfaces. Moreover, this coupling strongly depends on the in-plane magnetic structure and stacking direction, showing its sensitivity to their magnetic interfaces. Our finding offers an opportunity to investigate interactions between elementary excitations in different materials across interfaces and to search for new functions of magnetic vdW heterointerfaces.

Superconducting 3R-Ta1+xSe2 with Giant In-Plane Upper Critical Fields.

Molecular-beam epitaxy (MBE) enables the stabilization of a nonequilibrium material phase, providing a powerful approach to the exploration of emergent phenomena in condensed-matter research. Here we demonstrate that one of the metallic two-dimensional (2D) materials, TaSe2, grown by MBE crystallizes into the pure 3R phase with the self-intercalated Ta atoms, 3R-Ta1+xSe2, which is thermodynamically metastable and does not exist in nature as a pure material phase. Interestingly, the thick-enough 3R-Ta1+xSe2 film exhibits a superconducting (SC) critical temperature (Tc) of 3.0 K, which is the highest among all of the polymorphs in TaSe2. Thickness-dependence measurements reveal that Tc decreases with decreasing thickness, accompanied by the development of the charge-density wave phase. The 3R-Ta1+xSe2 films exhibit large in-plane upper critical fields (Hc2) in their SC states even in the thick-enough regime, most likely due to the suppression of the interlayer hopping associated with the unique 3R stacking. Moreover, the temperature dependence of the in-plane Hc2 evolves from linear to square-root behavior with decreasing thickness, indicating crossover behavior from anisotropic three-dimensional superconductivity to 2D superconductivity. Our results unveil intriguing SC properties of metastable 3R-Ta1+xSe2 distinct from those of thermodynamically stable 2H-TaSe2, demonstrating the essential importance of the MBE-based approach to the exploration of novel quantum phenomena in 2D materials research.

Intrinsic 2D Ferromagnetism in V5Se8 Epitaxial Thin Films.

The discoveries of intrinsic ferromagnetism in atomically thin van der Waals crystals have opened a new research field enabling fundamental studies on magnetism at two-dimensional (2D) limit as well as development of magnetic van der Waals heterostructures. Currently, a variety of 2D ferromagnetism has been explored mainly by mechanically exfoliating "originally ferromagnetic (FM)" van der Waals crystals, while a bottom-up approach by thin-film growth technique has demonstrated emergent 2D ferromagnetism in a variety of "originally non-FM" van der Waals materials. Here we demonstrate that V5Se8 epitaxial thin films grown by molecular-beam epitaxy exhibit emergent 2D ferromagnetism with intrinsic spin polarization of the V 3d electrons despite that the bulk counterpart is "originally antiferromagnetic". Moreover, thickness-dependence measurements reveal that this newly developed 2D ferromagnet could be classified as an itinerant 2D Heisenberg ferromagnet with weak magnetic anisotropy, broadening a lineup of 2D magnets to those potentially beneficial for future spintronics applications.

Photoinduced Rashba Spin-to-Charge Conversion via an Interfacial Unoccupied State.

At interfaces with inversion symmetry breaking, the Rashba effect couples the motion of the electrons to their spin; as a result, a spin charge interconversion mechanism can occur. These interconversion mechanisms commonly exploit Rashba spin splitting at the Fermi level by spin pumping or spin torque ferromagnetic resonance. Here, we report evidence of significant photoinduced spin-to-charge conversion via Rashba spin splitting in an unoccupied state above the Fermi level at the Cu(111)/α-Bi_{2}O_{3} interface. We predict an average Rashba coefficient of 1.72×10^{-10} eV m at 1.98 eV above the Fermi level, by a fully relativistic first principles analysis of the interfacial electronic structure with spin orbit interaction. We find agreement with our observation of helicity dependent photoinduced spin-to-charge conversion excited at 1.96 eV at room temperature, with a spin current generation of J_{s}=10^{6} A/m^{2}. The present Letter shows evidence of efficient spin charge conversion exploiting Rashba spin splitting at excited states, harvesting light energy without magnetic materials or external magnetic fields.

Enhanced thermopower in ZnO two-dimensional electron gas.

Control of dimensionality has proven to be an effective way to manipulate the electronic properties of materials, thereby enabling exotic quantum phenomena, such as superconductivity, quantum Hall effects, and valleytronic effects. Another example is thermoelectricity, which has been theoretically proposed to be favorably controllable by reducing the dimensionality. Here, we verify this proposal by performing a systematic study on a gate-tuned 2D electron gas (2DEG) system formed at the surface of ZnO. Combining state-of-the-art electric-double-layer transistor experiments and realistic tight-binding calculations, we show that, for a wide range of carrier densities, the 2DEG channel comprises a single subband, and its effective thickness can be reduced to [Formula: see text] 1 nm at sufficiently high gate biases. We also demonstrate that the thermoelectric performance of the 2DEG region is significantly higher than that of bulk ZnO. Our approach opens up a route to exploit the peculiar behavior of 2DEG electronic states and realize thermoelectric devices with advanced functionalities.

Metastable Superconductivity in Two-Dimensional IrTe2 Crystals.

Two-dimensional (2D) materials exhibit unusual physical and chemical properties that are attributed to the thinning-induced modification of their electronic band structure. Recently, reduced thickness was found to dramatically impact not only the static electronic structure, but also the dynamic ordering kinetics. The ordering kinetics of first-order phase transitions becomes significantly slowed with decreasing thickness, and metastable supercooled states can be realized by thinning alone. We therefore focus on layered iridium ditelluride (IrTe2), a charge-ordering system that is transformed into a superconductor by suppressing its first-order transition. Here, we discovered a persistent superconducting zero-resistance state in mechanically exfoliated IrTe2 thin flakes. The maximum superconducting critical temperature ( Tc) was identical to that which is chemically optimized, and the emergent superconductivity was revealed to have a metastable nature. The discovered robust metastable superconductivity suggests that 2D material is a new platform to induce, control, and functionalize metastable electronic states that are inaccessible in bulk crystals.

Diameter-Dependent Superconductivity in Individual WS2 Nanotubes.

Transition metal dichalcogenide nanotubes are fascinating platforms for the research of superconductivity due to their unique dimensionalities and geometries. Here we report the diameter dependence of superconductivity in individual WS2 nanotubes. The superconductivity is realized by electrochemical doping via the ionic gating technique in which the diameter of the nanotube is estimated from the periodic oscillating magnetoresistance, known as the Little-Parks effect. The critical temperature of superconductivity displays an unexpected linear behavior as a function of the inverse diameter, that is, the curvature of the nanotube. The present results are an important step in understanding the microscopic mechanism of superconductivity in a nanotube, opening up a new way of superconductivity in crystalline nanostructures.

Exciton Hall effect in monolayer MoS2.

The spontaneous Hall effect driven by the quantum Berry phase (which serves as an internal magnetic flux in momentum space) manifests the topological nature of quasiparticles and can be used to control the information flow, such as spin and valley. We report a Hall effect of excitons (fundamental composite particles of electrons and holes that dominate optical responses in semiconductors). By polarization-resolved photoluminescence mapping, we directly observed the Hall effect of excitons in monolayer MoS2 and valley-selective spatial transport of excitons on a micrometre scale. The Hall angle of excitons is found to be much larger than that of single electrons in monolayer MoS2 (ref. ), implying that the quantum transport of the composite particles is significantly affected by their internal structures. The present result not only poses a fundamental problem of the Hall effect in composite particles, but also offers a route to explore exciton-based valleytronics in two-dimensional materials.

Layer-by-Layer Epitaxial Growth of Scalable WSe2 on Sapphire by Molecular Beam Epitaxy.

Molecular beam epitaxy (MBE) provides a simple but powerful way to synthesize large-area high-quality thin films and heterostructures of a wide variety of materials including accomplished group III-V and II-VI semiconductors as well as newly developing oxides and chalcogenides, leading to major discoveries in condensed-matter physics. For two-dimensional (2D) materials, however, main fabrication routes have been mechanical exfoliation and chemical vapor deposition by making good use of weak van der Waals bonding nature between neighboring layers, and MBE growth of 2D materials, in particular on insulating substrates for transport measurements, has been limited despite its fundamental importance for future advanced research. Here, we report layer-by-layer epitaxial growth of scalable transition-metal dichalocogenide (TMDC) thin films on insulating substrates by MBE and demonstrate ambipolar transistor operation. The proposed growth protocol is broadly applicable to other TMDCs, providing a key milestone toward fabrication of van der Waals heterostructures with various 2D materials for novel properties and functionalities.

Extended Polymorphism of Two-Dimensional Material.

When controlling electronic properties of bulk materials, we usually assume that the basic crystal structure is fixed. However, in two-dimensional (2D) materials, atomic structure or polymorph is attracting growing interest as a controlling parameter to functionalize their properties. Various polymorphs can exist in transition metal dichalcogenides (TMDCs) from which 2D materials are generated, and polymorphism has drastic impacts on the electronic states. Here we report the discovery of an unprecedented polymorph of a TMDC 2D material. By mechanical exfoliation, we made thin flakes from a single crystal of 2Ha-type tantalum disulfide (TaS2), a metallic TMDC with a charge-density-wave (CDW) phase. Microbeam X-ray diffraction measurements and electrical transport measurements indicate that thin flakes possess a polymorph different from any one known in TaS2 bulk crystals. Moreover, the flakes with the unique polymorph displayed the dramatically enhanced CDW ordering temperature. The present results suggest the potential existence of diverse structural and electronic phases accessible only in 2D materials.

Quantifying van der Waals Interactions in Layered Transition Metal Dichalcogenides from Pressure-Enhanced Valence Band Splitting.

van der Waals (vdW) forces, despite being relatively weak, hold the layers together in transition metal dichalcogenides (TMDs) and play a key role in their band structure evolution, hence profoundly affecting their physical properties. In this work, we experimentally probe the vdW interactions in MoS2 and other TMDs by measuring the valence band maximum (VBM) splitting (Δ) at K point as a function of pressure in a diamond anvil cell. As high pressure increases interlayer wave function coupling, the VBM splitting is enhanced in 2H-stacked MoS2 multilayers but, due to its specific geometry, not in 3R-stacked multilayers, hence allowing the interlayer contribution to be separated out of the total VBM splitting, as well as predicting a negative pressure (2.4 GPa) where the interlayer contribution vanishes. This negative pressure represents the threshold vdW interaction beyond which neighboring layers are electronically decoupled. This approach is compared to first-principles calculations and found to be widely applicable to other group-VI TMDs.

Exceptionally High Electric Double Layer Capacitances of Oligomeric Ionic Liquids.

Electric double layer (EDL) capacitors are promising as next-generation energy accumulators if their capacitances and operation voltages are both high. However, only few electrolytes can simultaneously fulfill these two requisites. Here we report that an oligomeric ionic liquid such as IL4TFSI with four imidazolium ion units in its structure provides a wide electrochemical window of ∼5.0 V, similar to monomeric ionic liquids. Furthermore, electrochemical impedance measurements using Au working electrodes demonstrated that IL4TFSI exhibits an exceptionally high EDL capacitance of ∼66 µF/cm2, which is ∼6 times as high as those of monomeric ionic liquids so far reported. We also found that an EDL-based field effect transistor (FET) using IL4TFSI as a gate dielectric material and SrTiO3 as a channel material displays a very sharp transfer curve with an enhanced carrier accumulation capability of ∼64 µF/cm2, as determined by Hall-effect measurements.

Gate-Tuned Thermoelectric Power in Black Phosphorus.

The electric field effect is a useful means of elucidating intrinsic material properties as well as for designing functional devices. The electric-double-layer transistor (EDLT) enables the control of carrier density in a wide range, which is recently proved to be an effective tool for the investigation of thermoelectric properties. Here, we report the gate-tuning of thermoelectric power in a black phosphorus (BP) single crystal flake with the thickness of 40 nm. Using an EDLT configuration, we successfully control the thermoelectric power (S) and find that the S of ion-gated BP reached +510 µV/K at 210 K in the hole depleted state, which is much higher than the reported bulk single crystal value of +340 µV/K at 300 K. We compared this experimental data with the first-principles-based calculation and found that this enhancement is qualitatively explained by the effective thinning of the conduction channel of the BP flake and nonuniformity of the channel owing to the gate operation in a depletion mode. Our results provide new opportunities for further engineering BP as a thermoelectric material in nanoscale.

Gate-Optimized Thermoelectric Power Factor in Ultrathin WSe2 Single Crystals.

We report an electric field tuning of the thermopower in ultrathin WSe2 single crystals over a wide range of carrier concentration by using electric double-layer (EDL) technique. We succeeded in the optimization of power factor not only in the hole but also in the electron side, which has never been chemically accessed. The maximized values of power factor are one-order larger than that obtained by changing chemical composition, reflecting the clean nature of electrostatic doping.

Thermoelectric Detection of Multi-Subband Density of States in Semiconducting and Metallic Single-Walled Carbon Nanotubes.

Thermoelectric detection of a multi-subband density of states in semiconducting and metallic single-walled carbon nanotubes is demonstrated by scanning the Fermi energy from electron-doped to hole-doped regions. The Fermi energy is systematically controlled by utilizing the strong electric field induced in electric-double-layer transistor configurations, resulting in the optimization of the thermoelectric power factor.