Record-High Superconductivity in Transition Metal Dichalcogenides Emerged in Compressed 2H-TaS2

Prediction of superconductivity in a series of tetragonal transition metal dichalcogenides

Transition metal dichalcogenides (TMDCs) represent a well-known material family with diverse structural phases and rich electronic properties; they are thus an ideal platform for studying the emergence and exotic phenomenon of superconductivity (SC). Herein, we propose the existence of tetragonal TMDCs with a distorted Lieb (dLieb) lattice structure and the stabilized transition metal disulfides (MS2), including dLieb-ZrS2, dLieb-NbS2, dLieb-MnS2, dLieb-FeS2, dLieb-ReS2, and dLieb-OsS2. Except for semiconducting dLieb-ZrS2 and magnetic dLieb-MnS2, the rest of metallic dLieb-MS2 was found to exhibit intrinsic SC with the transition temperature (TC) ranging from ∼5.4 to ∼13.0 K. The TC of dLieb-ReS2 and dLieb-OsS2 exceeded 10 K and was higher than that of the intrinsic SC in the known metallic TMDCs, which is attributed to the significant phonon-softening enhanced electron-phonon coupling strength. Different from the Ising spin-orbit coupling (SOC) effect in existing non-centrosymmetric TMDCs, the non-magnetic dLieb-MS2 monolayers exhibit the Dresselhaus SOC effect, which is featured by in-plane spin orientations and will give rise to the topological SC under proper conditions. In addition to enriching the structural phases of TMDCs, our work predicts a series of SC candidates with high intrinsic TC and topological non-triviality used for fault-tolerant quantum computation.

Double Superconducting Dome of Quasi Two-Dimensional TaS2 in Non-Centrosymmetric van der Waals Heterostructure

Two-dimensional van der Waals heterostructures (2D-vdWHs) based on transition metal dichalcogenides (TMDs) provide unparalleled control over electronic properties. However, the interlayer coupling is challenged by the interfacial misalignment and defects, which hinders a comprehensive understanding of the intertwined electronic orders, especially superconductivity and charge density wave (CDW). Here, by using pressure to regulate the interlayer coupling of non-centrosymmetric 6R-TaS2 vdWHs, we observe an unprecedented phase diagram in TMDs. This phase diagram encompasses successive suppression of the original CDW states from alternating H-layer and T-layer configurations, the emergence and disappearance of a new CDW-like state, and a double superconducting dome induced by different interlayer coupling effects. These results not only illuminate the crucial role of interlayer coupling in shaping the complex phase diagram of TMD systems but also pave a new avenue for the creation of a novel family of bulk heterostructures with customized 2D properties.

Synthesis and Characterization of 2D Ternary Compound TMD Materials Ta3VSe8

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are garnering considerable scientific interest, prompting discussion regarding their prospective applications in the fields of nanoelectronics and spintronics while also fueling groundbreaking discoveries in phenomena such as the fractional quantum anomalous Hall effect (FQAHE) and exciton dynamics. The abundance of binary compound TMDs, such as MX2 (M = Mo, W; X = S, Se, Te), has unlocked myriad avenues of exploration. However, the exploration of ternary compound TMDs remains relatively limited, with notable examples being Ta2NiS5 and Ta2NiSe5. In this study, we report the synthesis of a new 2D ternary compound TMD materials, Ta3VSe8, employing the chemical vapor transport (CVT) method. The as-grown bulk crystal is shiny and can be easily exfoliated. The crystal quality and structure are verified by X-ray diffraction (XRD), while the surface morphology, stoichiometric ratio, and uniformity are determined by scanning electron microscopy (SEM). Although the phonon property is found stable at different temperatures, magneto-resistivity evolves. These findings provide a possible approach for the realization and exploration of ternary compound TMDs.

Fingerprint of Dipole Moment Orientation of Water Molecules in Cu2+ Aqueous Solution Probed by X-ray Photoelectron Spectroscopy

The electronic structure and geometrical organization of aqueous Cu2+ have been investigated by using X-ray photoelectron spectroscopy (XPS) at the Cu L-edge combined with state-of-the-art ab initio molecular dynamics and a quantum molecular approach designed to simulate the Cu 2p X-ray photoelectron spectrum. The calculations offer a comprehensive insight into the origin of the main peak and satellite features. It is illustrated how the energy drop of the Cu 3d levels (≈7 eV) following the creation of the Cu 2p core hole switches the nature of the highest singly occupied molecular orbitals (MOs) from the dominant metal to the dominant MO nature of water. It is particularly revealed how the repositioning of the Cu 3d levels induces the formation of new bonding (B) and antibonding (AB) orbitals, from which shakeup mechanisms toward the relaxed H-SOMO operate. As highlighted in this study, the appearance of the shoulder near the main peak corresponds to the characteristic signature of shakeup intraligand (1a1 → H-SOMO(1b1)) excitations in water, providing insights into the average dipole moment distribution (≈36°) of the first-shell water molecules surrounding the metal ion and its direct impact on the broadening of the satellite. It is also revealed that the main satellite at 8 eV from the main peak corresponds to (metal/1b2 → H-SOMO(1b1) of water) excitations due to a bonding/antibonding (B/AB) interaction of Cu 3d levels with the deepest valence O2p/H1s 1b2 orbitals of water. This finding underscores the sensitivity of XPS to the electronic structure and orientation of the nearest water molecules around the central ion.

Tuning the Flat Band in Bi2O2Se by Pressure to Induce Superconductivity

The discovery of superconductivity in twisted bilayer graphene has reignited enthusiasm in the field of flat-band superconductivity. However, important challenges remain, such as constructing a flat-band structure and inducing a superconducting state in materials. Here, we successfully achieved superconductivity in Bi2O2Se by pressure-tuning the flat-band electronic structure. Experimental measurements combined with theoretical calculations reveal that the occurrence of pressure-induced superconductivity at 30 GPa is associated with a flat-band electronic structure near the Fermi level. Moreover, in Bi2O2Se, a van Hove singularity is observed at the Fermi level alongside pronounced Fermi surface nesting. These remarkable features play a crucial role in promoting strong electron-phonon interactions, thus potentially enhancing the superconducting properties of the material. These findings demonstrate that pressure offers a potential experimental strategy for precisely tuning the flat band and achieving superconductivity.

Observation of Emergent Superconductivity in the Topological Insulator Ta2Pd3Te5 via Pressure Manipulation

Topological insulators offer significant potential to revolutionize diverse fields driven by nontrivial manifestations of their topological electronic band structures. However, the realization of superior integration between exotic topological states and superconductivity for practical applications remains a challenge, necessitating a profound understanding of intricate mechanisms. Here, we report experimental observations for a novel superconducting phase in the pressurized second-order topological insulator candidate Ta2Pd3Te5, and the high-pressure phase maintains its original ambient pressure lattice symmetry up to 45 GPa. Our in situ high-pressure synchrotron X-ray diffraction, electrical transport, infrared reflectance, and Raman spectroscopy measurements, in combination with rigorous theoretical calculations, provide compelling evidence for the association between the superconducting behavior and the densified phase. The electronic state change around 20 GPa was found to modify the topology of the Fermi surface directly, which synergistically fosters the emergence of robust superconductivity. In-depth comprehension of the fascinating properties exhibited by the compressed Ta2Pd3Te5 phase is achieved, highlighting the extraordinary potential of topological insulators for exploring and investigating high-performance electronic advanced devices under extreme conditions.

Tuneable Schottky contact of MoSi2N4/TaS2 van der Waals heterostructure

The two-dimensional M o S i 2 N 4 monolayer is an emerging semiconductor material that offers considerable promise due to its ultra-thin profile, tuneable mechanical properties, excellent optoelectronic properties and exceptional environmental stability. The van der Waals (vdW) heterostructure formed by stacking such two-dimensional monolayers has demonstrated superior performance across various domains. In this study, a vdW heterostructure combining the two-dimensional M o S i 2 N 4 and T a S 2 monolayers is examined using first-principles density functional theory. In its ground state, this van der Waals heterostructure establishes an ohmic contact with an exceptionally low potential barrier height. By modulating the vdW heterostructure with an applied electric field of -0.1 V/Å and under vertical stress, we discovered that M o S i 2 N 4 and T a S 2 can transition from an ohmic contact to a p-type Schottky with an ultra-low Schottky barrier height (SBH). Our observations may give valuable insights for designing reconfigurable, tuneable Schottky nano-devices with enhanced electronic and optical properties based on M o S i 2 N 4 / T a S 2 .

Abnormal Metal-Semiconductor-Like Transition and Exceptional Enhanced Superconducting State in Pressurized Restacked TaS2

Interlayer coupling and stacking order play essential roles in shaping the exotic electronic properties of two-dimensional materials. Here, we employ restacked TaS₂─a novel transition metal dichalcogenide (TMD) with weak vdW bonding and twisted angles─to investigate the strain effects of interlayer modulation on the electronic properties. Under pressure, an unexpected transition from metallic to semiconducting-like states occurs. Superconductivity coexists with the semiconducting-like state over a wide pressure range, which has never before been observed in TMDs. Upon further compression, a new superconducting SC-II state emerges without structural evolution and gradually replaces the initial SC-I state. The emerging SC-II state exhibits robust zero-resistance superconductivity and an ultrahigh upper critical field. The abundant electronic state changes in RS-TaS₂ are strongly related to band-structure engineering resulting from pressure-induced interlayer stacking angle modulation. Our results reveal the remarkable effect of interlayer rearrangement on electronic properties and provide a special way to explore the unique properties of 2D materials.

Modulating Charge-Density Wave Order and Superconductivity from Two Alternative Stacked Monolayers in a Bulk 4Hb-TaSe2 Heterostructure via Pressure

Two-dimensional (2D) van der Waals heterostructures (VDWHs) containing a charge-density wave (CDW) and superconductivity (SC) have revealed rich tunability in their properties, which provide a new route for optimizing their novel exotic states. The interaction between SC and CDW is critical to its properties; however, understanding this interaction within VDWHs is very limited. A comprehensive in situ study and theoretical calculation on bulk 4Hb-TaSe₂ VDWHs consisting of alternately stacking 1T-TaSe₂ and 1H-TaSe₂ monolayers are investigated under high pressure. Surprisingly, the superconductivity competes with the intralayer and adjacent-layer CDW order in 4Hb-TaSe₂, which results in substantially and continually boosted superconductivity under compression. Upon total suppression of the CDW, the superconductivity in the individual layers responds differently to the charge transfer. Our results provide an excellent method to efficiently tune the interplay between SC and CDW in VDWHs and a new avenue for designing materials with tailored properties.

Self-Assembled TaOX/2H-TaS2 as a van der Waals Platform of a Multilevel Memristor Circuit Integrated with a β-Ga2O3 Transistor

Two-dimensional (2D)-layered material tantalum disulfide (2H-TaS₂) is known to be a van der Waals conductor at room temperature. Here, 2D-layered TaS₂ has been partially oxidized by utraviolet-ozone (UV-O₃) annealing to form a 12-nm-thin TaO_X on conducting TaS₂, so that the TaO_X/2H-TaS₂ structure might be self-assembled. Utilizing the TaO_X/2H-TaS₂ structure as a platform, each device of a β-Ga₂O₃ channel MOSFET and a TaO_X memristor has been successfully fabricated. An insulator structure of Pt/TaO_X/2H-TaS₂ shows good a dielectric constant (k ∼ 21) and strength (∼3 MV/cm) of achieved TaO_X, which is enough to support a β-Ga₂O₃ transistor channel. Based on the quality of TaO_X and low trap density of the TaO_X/β-Ga₂O₃ interface, which is achieved via another UV-O₃ annealing, excellent device properties such as little hysteresis (<∼0.04 V), band-like transport, and a steep subthreshold swing of ∼85 mV/dec are achieved. With a Cu electrode on top of the TaO_X/2H-TaS₂ structure, the TaO_X acts as a memristor operating around ∼2 V for nonvolatile bipolar and unipolar mode memories. The functionalities of the TaO_X/2H-TaS₂ platform become more distinguished finally when the Cu/TaO_X/2H-TaS₂ memristor and β-Ga₂O₃ MOSFET are integrated to form a resistive memory switching circuit. The circuit nicely demonstrates the multilevel memory functions.

Interpretation of Shakeup Mechanisms in Copper L-Shell Photoelectron Spectra

We report on an original full ab initio quantum molecular approach designed to simulate Cu 2p X-ray photoelectron spectra. The description includes electronic relaxation/correlation and spin-orbit coupling effects and is implemented within nonorthogonal sets of molecular orbitals for the initial and final states. The underlying mechanism structuring the Cu 2p photoelectron spectra is clarified thanks to a correlation diagram applied to the CuO₄C₆H₆ paradigm. This diagram illustrates how the energy drop of the Cu 3d levels following the creation of the Cu 2p core hole switches the nature of the highest singly occupied molecular orbital (H-SOMO) from dominant metal to dominant ligand character. It also reveals how the repositioning of the Cu 3d levels induces the formation of new bonding and antibonding orbitals from which shakeup mechanisms toward the relaxed H-SOMO operate. The specific nature, ligand → ligand and metal → ligand, of these excitations building the satellite lines is exposed. Our approach finally applied to the real Cu(acac)₂ system clearly demonstrates how a definite interpretation of the XPS spectra can be obtained when a correct evaluation of binding energies, intensities, and relative widths of the spectral lines is achieved.