Effect of Te doping on superconductivity and charge-density wave in dichalcogenides 2H-NbSe 2- x Te x ( x = 0, 0.1, 0.2)

Electrical transport phenomena in two-dimensional metallic 2H-NbSe2: an experimental and theoretical study

Two-dimensional (2D) metallic transition metal dichalcogenides (TMDCs) have attracted extensive interest in various fields owing to their unique electronic properties. However, studies on their transport properties and the modulation of these properties based on their band structure are limited. Herein, we studied the transport phenomena in 2D metallic 2H-NbSe2 using experimental and theoretical approaches. The transport properties, including electrical conductivity (σ) and Seebeck coefficient (S), of mechanically exfoliated 2H-NbSe2 nanosheets were measured. We observed field effect-dependent variations in σ and S of the 2H-NbSe2 nanosheets. Theoretical calculations of the electronic band structures and estimations of the transport properties of 2D 2H-NbSe2 crystals were conducted to verify and explain the experimental results. The superconducting transition temperature of the exfoliated NbSe2 nanosheets validated the reliability of the sample preparation procedures and indicated the high quality of the samples. Our findings provide a basis for understanding the electrical properties of metallic TMDCs intended for various applications.

Organic self-assembled monolayers on superconducting NbSe2: interfacial electronic structure and energetics

While organic self-assembled monolayers (SAMs) have been widely used to modify the work function of metal and metal-oxide surfaces, their application to tune the critical temperature of a superconductor has only been considered recently when SAMs were deposited on NbSe₂monolayers (Calavalle et al 2021Nano Lett.21136-143). Here, we describe the results of density functional theory calculations performed on the experimentally reported organic/NbSe₂systems. Our objectives are: (i) to determine how the organic layers impact the NbSe₂work function and electronic density of states; (ii) to understand the possible correlation with the experimental variations in superconducting behavior upon SAM deposition. We find that, upon adsorption of the organic monolayers, the work-function modulation induced by the SAM and interface dipoles is consistent with the experimental results. However, there occurs no significant difference in the electronic density of states near the Fermi level, a consequence of the absence of any charge transfer across the organic/NbSe₂interfaces. Therefore, our results indicate that it is not a SAM-induced tuning of the NbSe₂density of states near the Fermi level that leads to the tuning of the superconducting critical temperature. This calls for further explorations, both experimentally and theoretically, of the mechanism underlying the superconducting critical temperature variation upon formation of SAM/NbSe₂interfaces.

Superconducting dome associated with the suppression and re-emergence of charge density wave states upon sulfur substitution in CuIr2Te4chalcogenides

We report the path from the charge density wave (CDW)-bearing superconductor CuIr₂Te₄to the metal insulator transition (MIT)-bearing compound CuIr₂S₄by chemical alloying with the gradual substitution of S for Te. The evolution of structural and physical properties of the CuIr₂Te_4-xS_x(0 ⩽x⩽ 4) polycrystalline system is systemically examined. The x-ray diffraction (XRD) results imply CuIr₂Te_4-xS_x(0 ⩽x⩽ 0.5) crystallizes in a NiAs defected trigonal structure, whereas it adapts to the cubic spinel structure for 3.6 ⩽x⩽ 4 and it is a mixed phase in the doping range of 0.5 <x< 3.6. Unexpectedly, the resistivity and magnetization measurements reveal that small-concentration S substitution for Te can suppress the CDW transition, but it reappears aroundx= 0.2, and the CDW transition temperature enhances clearly asxaugments for 0.2 ⩽x⩽ 0.5. Besides, the superconducting critical temperature (T_c) first increases with S doping content and then decreases after reaching a maximumT_c= 2.82 K for CuIr₂Te_3.85S_0.15. MIT order has been observed in the spinel region (3.6 ⩽x⩽ 4) associated withT_MIincreasing withxincreasing. Finally, the rich electronic phase diagram of temperature versusxfor this CuIr₂Te_4-xS_xsystem is assembled, where the superconducting dome is associated with the suppression and re-emergence of CDW as well as MIT states at the end upon sulfur substitution in the CuIr₂Te_4-xS_xchalcogenides.

Topical review: recent progress of charge density waves in 2D transition metal dichalcogenide-based heterojunctions and their applications

Charge density wave (CDW) is an intriguing physical phenomenon especially found in two-dimensional (2D) layered systems such as transition-metal dichalcogenides (TMDs). The study of CDW is vital for understanding lattice modification, strongly correlated electronic behaviors, and other related physical properties. This paper gives a review of the recent studies on CDW emerging in 2D TMDs. First, a brief introduction and the main mechanisms of CDW are given. Second, the interplay between CDW patterns and the related unique electronic phenomena (superconductivity, spin, and Mottness) is elucidated. Then various manipulation methods such as doping, applying strain, local voltage pulse to induce the CDW change are discussed. Finally, examples of the potential application of devices based on CDW materials are given. We also discuss the current challenge and opportunities at the frontier in this research field.

Signatures of nontrivial Rashba metal states in a transition metal dichalcogenides Josephson junction

Nontrivial Rashba metal states in conventional semiconductor materials generated by both Rashba spin-orbit coupling and ferromagnetic exchange coupling coexisting were recently predicted and exploited. Single layered transition metal dichalcogenides (TMDC) featuring those states and their potential applications have been less focused. We find that, in the materials with Rashba spin-orbit coupling only, nontrivial Rashba metallic states can be manipulated by an external gate voltage. Based on extensive numerical simulations, the relationships between the supercurrent and nontrivial Rashba metallic states in the TMDC Josephson junction have been investigated. It is shown that, in the absence of the Rashba spin-orbit coupling, the critical supercurrent exhibits a stark difference between normal Rashba metal state and anomalous Rashba metal state in the finite junction as compared to the case of the short junction. While in the case of the finite Rashba spin-orbit coupling, the critical supercurrent demonstrates a reentrant behavior when Fermi level sweeps from anomalous Rashba metal state to Rashba ring metal state. Intriguingly, not only at the conversion of the nontrivial Rashba metallic states but also in the Rashba ring metal state the reentrant behavior exhibits again, which could be well explained by the mixing of spin-triplet Cooper pairs with spin-singlet Cooper pairs in Ising superconductor. Such a reentrant effect offers a new way to detect Ising superconductivity based on the TMDC systems. Meanwhile our study also clarified that the nontrivial Rashba metallic state plays an important role in controlling the supercurrent in the TMDC Josephson junction, which is useful for designing future superconducting devices.

NbSeTe-a new layered transition metal dichalcogenide superconductor

Transition metal dichalcogenides (TMDCs) usually exhibit layered polytypic structures due to the weak interlayer coupling. 2H-NbSe₂ is one of the most widely studied in the pristine TMDC family due to its high superconducting transition temperature (T _c  =  7.3 K) and the occurrence of a charge-density wave (CDW) order below 33 K. The coexistence of CDW with superconductivity poses an intriguing open question about the relationship between Fermi surface nesting and Cooper pairing. Past studies of this issue have mostly been focused on doping 2H-NbSe₂ by 3d transition metals without significantly changing its crystal structure. Here we replaced the Se by Te in 2H-NbSe₂ in order to design a new 1T polytype layered TMDC NbSeTe, which adopts a trigonal structure with space group P [Formula: see text] m1. We successfully grew large size and high-quality single crystals of 1T-NbSeTe via the vapor transport method using I ₂ as the transport agent. Temperature-dependent resistivity and specific heat data revealed a bulk T _c at 1.3 K, which is the first observation of superconductivity in pure 1T-NbSeTe phase. This compound enlarged the family of superconducting TMDCs and provides an opportunity to study the interplay between CDW and superconductivity in the trigonal structure.

Enhancement of superconducting critical current density by Fe impurity substitution in NbSe2 single crystals and the vortex pinning mechanism

Magnetization measurements have been used to determine the effect of magnetic impurities (Fe) on the Larkin-Ovchinnikov (LO) 3D collective pinning model in NbSe₂ single crystals. Upon increasing the concentration of Fe impurities, the superconducting critical current density enhances appreciably compared to pure NbSe₂ reflecting the fact that the addition of magnetic impurities assists in improving the practical applicability of NbSe₂. The random pinning potential that is introduced by the Fe impurities also shows a considerable change in the interaction between the vortices and the core region, resulting in a competitive nature of single vortex, small bundle and large bundle pinning regimes in the H-T phase diagram. The intrinsic disorder in pure NbSe₂ single crystals shows δT_c flux pinning; however, the extrinsic disorder created by Fe atoms in NbSe₂ shows δl flux pinning. Furthermore, the field dependence of the pinning force on both NbSe₂ and Fe-incorporated NbSe₂ represents the existence of point pinning and the surface pinning mechanism with a broadening of the f_p curves in the Fe-incorporated single crystals.

Coexistence of superconductivity and a charge density wave in LaPt2(Si1-x Ge x )2 (0 ⩽ x ⩽ 0.5)

Interplay between a charge density wave (CDW) and superconductivity in LaPt2(Si1-x Ge x )2 has been studied by electrical transport and magnetic measurements. LaPt2Si2 crystallizes in CaBe2Ge2 type structure which shows a first order structural phase transition from tetragonal to orthorhombic accompanied by a CDW transition at 112 K and superconducting transition at around 1.22 K as confirmed by temperature dependence of resistivity and magnetic measurements. For 2[Formula: see text] doping of germanium, while the CDW temperature T CDW decreases, the superconducting transition temperature T C shows an increase. T CDW increases for 5[Formula: see text] doping of germanium and the superconducting transition decreases. These findings demonstrate the competing nature of a CDW and superconductivity.

Emergence of coherence in the charge-density wave state of 2H-NbSe2

A charge-density wave (CDW) state has a broken symmetry described by a complex order parameter with an amplitude and a phase. The conventional view, based on clean, weak-coupling systems, is that a finite amplitude and long-range phase coherence set in simultaneously at the CDW transition temperature T_cdw. Here we investigate, using photoemission, X-ray scattering and scanning tunnelling microscopy, the canonical CDW compound 2H-NbSe₂ intercalated with Mn and Co, and show that the conventional view is untenable. We find that, either at high temperature or at large intercalation, CDW order becomes short-ranged with a well-defined amplitude, which has impacts on the electronic dispersion, giving rise to an energy gap. The phase transition at T_cdw marks the onset of long-range order with global phase coherence, leading to sharp electronic excitations. Our observations emphasize the importance of phase fluctuations in strongly coupled CDW systems and provide insights into the significance of phase incoherence in ‘pseudogap’ states.

Charge density waves are described by a complex order parameter whose amplitude is expected to vanish at the transition temperature. This study shows that the transition in 2H-NbSe₂ is driven by fluctuations of the phase of the order parameter, with a finite amplitude surviving in the disordered state.