A disorder-enhanced quasi-one-dimensional superconductor

Exploring disorder correlations in superconducting systems: spectroscopic insights and matrix element effects

Understanding the intricate interplay between disorder and superconductivity has become a key area of research in condensed matter physics, with profound implications for materials science. Recent studies have shown that spatial correlations of disorder potential can improve superconductivity, prompting a re-evaluation of some theoretical models. This paper explores the influence of disorder correlations on the fundamental properties of superconducting systems, going beyond the traditional assumption of spatially uncorrelated disorder. In particular, we investigate the influence of disorder correlations on key spectroscopic superconductor properties, including the density of states, as well as on the matrix elements of the superconducting coupling constant and their impact on the localization length. Our findings offer valuable insights into the role of disorder correlations in shaping the behavior of superconducting materials.

Pressure-induced disorder and nanosizing inhibits superconductivity in In2Te3

The generation of disorder often gives rise to profound and irreversible physical phenomena. Here, we explore the influence of disorder on the superconducting properties of In2Te3through comprehensive high-pressure investigations. Building upon previous findings, we investigated the progressive suppression of superconductivity in In2Te3during the depressurization process: the increased disorder that ultimately leads to the complete disappearance of the superconducting state. Simultaneously, our high-pressure x-ray diffraction analysis reveals an irreversible structural phase transition. Furthermore, microstructure analysis using transmission electron microscopy clearly demonstrates both grain refinement and a substantial enhancement of disorder. These findings not only provide valuable insights into the mechanism by which disorder suppresses superconductivity, but also offer guidance for future advancements in the fabrication of atmospheric-pressure superconductors.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Vapor-Phase Indium Intercalation in van der Waals Nanofibers of Atomically Thin W6Te6 Wires

One-dimensional (1D) conducting materials are of great interest as potential building blocks for integrated nanocircuits. Ternary 1D transition-metal chalcogenides, consisting of M₆X₆ wires with intercalated A atoms (M = Mo or W; X = S, Se, or Te; A = alkali or rare metals, etc.), have attracted much attention due to their 1D metallic behavior, superconductivity, and mechanical flexibility. However, the conventional solid-state reaction usually produces micrometer-scale bulk crystals, limiting their potential use as nanoscale conductors. Here we demonstrate a versatile method to fabricate indium (In)-intercalated W₆Te₆ (In-W₆Te₆) bundles with a nanoscale thickness. We first prepared micrometer-long, crystalline bundles of van der Waals W₆Te₆ wires using chemical vapor deposition and intercalated In into the crystal via a vapor-phase reaction. Atomic-resolution electron microscopy revealed that In atoms were surrounded by three adjacent W₆Te₆ wires. First-principles calculations suggested that their wire-by-wire stacking can transform through postgrowth intercalation. Individual In-W₆Te₆ bundles exhibited metallic behavior, as theoretically predicted. We further identified the vibrational modes by combining polarized Raman spectroscopy and nonresonant Raman calculations.

Superconductivity in Crystallographically Disordered LaHg6.4

The influence of structural disorder on superconductivity is not yet fully understood. A concurrent examination of crystallographic and physical properties of LaHg_6.4 reveals that this material enters a superconducting state below T_c = 2.4 K while showing crystallographic disorder in one dimension. Lanthanum mercuride, which crystallizes in a new structure type (space group Cmcm, a = 9.779(2) Å, b = 28.891(4) Å, c = 5.0012(8) Å, Z = 8), has remained out of reach for nearly 50 years. In this crystal structure, strong disorder is present in the channels that propagate along the [001] direction. By implementing a combination of cutting-edge synthesis and characterization techniques, we were able to circumvent the complexity associated with the low formation temperature and chemical reactivity of this substance and study the superconductivity of LaHg_6.4 in detail.

Long-Range Ordered Amorphous Atomic Chains as Building Blocks of a Superconducting Quasi-One-Dimensional Crystal

Crystalline and amorphous structures are two of the most common solid-state phases. Crystals having orientational and periodic translation symmetries are usually both short-range and long-range ordered, while amorphous materials have no long-range order. Short-range ordered but long-range disordered materials are generally categorized into amorphous phases. In contrast to the extensively studied crystalline and amorphous phases, the combination of short-range disordered and long-range ordered structures at the atomic level is extremely rare and so far has only been reported for solvated fullerenes under compression. Here, a report on the creation and investigation of a superconducting quasi-1D material with long-range ordered amorphous building blocks is presented. Using a diamond anvil cell, monocrystalline (TaSe₄ )₂ I is compressed and a system is created where the TaSe₄ atomic chains are in amorphous state without breaking the orientational and periodic translation symmetries of the chain lattice. Strikingly, along with the amorphization of the atomic chains, the insulating (TaSe₄ )₂ I becomes a superconductor. The data provide critical insight into a new phase of solid-state materials. The findings demonstrate a first ever case where superconductivity is hosted by a lattice with periodic but amorphous constituent atomic chains.

Pressure engineering of the Dirac fermions in quasi-one-dimensional Tl2Mo6Se6

Topological band dispersions other than the standard Dirac or Weyl fermions have garnered the increasing interest in materials science. Among them, the cubic Dirac fermions were recently proposed in the family of quasi-one-dimensional (q-1D) conductors A₂Mo₆X₆ (A  =  Na, K, In, Tl; X  =  S, Se, Te), where the band crossing is characterized by a linear dispersion in one k-space direction but the cubic dispersion in the plane perpendicular to it. It is not yet clear, however, how the external perturbations can alter these nontrivial carriers and ultimately induce a new distinct quantum phase. Here we study the evolution of Dirac fermions, in particular the cubic Dirac crossing, under external pressure in the representative q-1D Tl₂Mo₆Se₆ via the first-principles calculations. Specifically, it is found that the topological properties, including the bulk Dirac crossings and the topological surface states, change progressively under pressure up to 50 GPa where it undergoes a structural transition from the hexagonal phase to body-centered tetragonal phase. Above 50 GPa, the system is more likely to be topologically trivial. Further, we also investigate its phonon spectra, which reveals a gradual depletion of the negative phonon modes with pressure, consistent with the more three-dimensional Fermi surface in the high-pressure phase. Our work may provide a useful guideline for further experimental search and the band engineering of the topologically nontrivial fermions in this intriguing state of matter.

Anisotropic Collective Charge Excitations in Quasimetallic 2D Transition-Metal Dichalcogenides

The quasimetallic 1T' phase 2D transition-metal dichalcogenides (TMDs) consist of 1D zigzag metal chains stacked periodically along a single axis. This gives rise to its prominent physical properties which promises the onset of novel physical phenomena and applications. Here, the in-plane electronic correlations are explored, and new mid-infrared plasmon excitations in 1T' phase monolayer WSe2 and MoS2 are observed using optical spectroscopies. Based on an extensive first-principles study which analyzes the charge dynamics across multiple axes of the atomic-layered systems, the collective charge excitations are found to disperse only along the direction perpendicular to the chains. Further analysis reveals that the interchain long-range coupling is responsible for the coherent 1D charge dynamics and the spin-orbit coupling affects the plasmon frequency. Detailed investigation of these charge collective modes in 2D-chained systems offers opportunities for novel device applications and has implications for the underlying mechanism that governs superconductivity in 2D TMD systems.

Substrate mediated nitridation of niobium into superconducting Nb2N thin films for phase slip study

Here we report a novel nitridation technique for transforming niobium into hexagonal Nb₂N which appears to be superconducting below 1K. The nitridation is achieved by high temperature annealing of Nb films grown on Si₃N₄/Si (100) substrate under high vacuum. The structural characterization directs the formation of a majority Nb₂N phase while the morphology shows granular nature of the films. The temperature dependent resistance measurements reveal a wide metal-to-superconductor transition featuring two distinct transition regions. The region close to the normal state varies strongly with the film thickness, whereas, the second region in the vicinity of the superconducting state remains almost unaltered but exhibiting resistive tailing. The current-voltage characteristics also display wide transition embedded with intermediate resistive states originated by phase slip lines. The transition width in current and the number of resistive steps depend on film thickness and they both increase with decrease in thickness. The broadening in transition width is explained by progressive establishment of superconductivity through proximity coupled superconducting nano-grains while finite size effects and quantum fluctuation may lead to the resistive tailing. Finally, by comparing with Nb control samples, we emphasize that Nb₂N offers unconventional superconductivity with promises in the field of phase slip based device applications.

Na-doping effects on structural evolution and superconductivity in (K1-x Na x )2Cr3As3 (x = 0-1)

In this report, we studied the effects of isovalent Na-doping on the recently discovered quasi-one-dimensional Cr-based unconventional superconductor K₂Cr₃As₃. A series of polycrystalline samples with nominal component (K_1-x Na _x )₂Cr₃As₃ (x  =  0-1) were synthesized by the solid state reaction method. From crystal structure and chemical phase characterizations, we found two distinct chemical phases with the same hexagonal structure but distinguished by different site occupancy of Na⁺ ions at the two kinds of K-site in the K₂Cr₃As₃ lattice structure. When x  ⩽  0.4, the doped samples form a continuous sosoloid phase of (K_1-x Na _x )₂Cr₃As₃ with the Na⁺ ions randomly doping at the K-sites (denoted as α-phase); when x  ⩾  0.5, a novel individual phase of (K_0.25Na_0.75)₂Cr₃As₃ emerges, in which the Na⁺ ions selectively occupy all the '3k' sites and the K⁺ ions occupy the '1c' sites (denoted as β-phase). No chemical phase of Na₂Cr₃As₃ was detected. Superconductivity in these samples was studied by electrical transport and magnetic susceptibility measurements, and it evolves in a much sophisticated manner. In the α-phase, the superconducting T _c decreases quickly upon Na-doping. All these α-phase samples have surprisingly low superconducting volume fraction and relatively low T _c compared with the undoped K₂Cr₃As₃. However, the β-phase has a clearly enhanced T _c up to 7.6 K which locates between the values of K₂Cr₃As₃ and Na₂Cr₃As₃, and exhibits a full superconducting shielding signal.

Bloch Oscillations in Fibonacci lattices: polaron formation

We investigated the dynamics of an electron subjected to a uniform electric field in the scope of a tight-binding electron-phonon interacting approach. We aimed at describing the transport in a one-dimensional lattice in which the on-site energies are distributed according to a Fibonacci sequence. Within this physical picture, we obtained a novel dynamical process with no counterpart in ordered lattices. Our findings showed that in low-disorder limit, the electron performs spatial Bloch oscillations, generating, in the turning points of its trajectory, composite quasi-particles-namely, polarons. When it comes to highly disordered systems, two strongly localized polarons are formed in the region where the oscillating charge is confined, thus propagating excitations that are present in the lattice.

Disorder Quenching of the Charge Density Wave in ZrTe_{3}

The charge density wave (CDW) in ZrTe_{3} is quenched in samples with a small amount of Te isoelectronically substituted by Se. Using angle-resolved photoemission spectroscopy we observe subtle changes in the electronic band dispersions and Fermi surfaces upon Se substitution. The scattering rates are substantially increased, in particular for the large three-dimensional Fermi surface sheet. The quasi-one-dimensional band is unaffected by the substitution and still shows a gap at low temperature, which starts to open from room temperature. Long-range order is, however, absent in the electronic states as in the periodic lattice distortion. The competition between superconductivity and the CDW is thus linked to the suppression of long-range order of the CDW.

Disorder Enhanced Superconductivity toward TaS2 Monolayer

Appearance of disorder is commonly known as detrimental to two-dimensional (2D) superconductivity, and typically results in decrement of the critical transition temperature ( T_c). Herein, an anomalous enhancement of superconductivity was observed in TaS₂ monolayer with function of disorder induced by structural defect. Owing to controlled pore density by acid concentration during chemical exfoliation, the disorder level in TaS₂ framework can be effectively regulated. Dome-shaped behavior was uncovered in disorder dependence of superconductivity toward the chemically functionalized TaS₂ monolayers, with T_c enhanced from 2.89 to 3.61 K when below critical disorder level. The disorder-engineered T_c enhancement, which distinctly differs from monotonic decrement in deposited 2D superconductors, can be ascribed to the increment of carrier density induced by Ta atom absence. The exotic superconducting enhancement would give help to deeply understand the correlation between superconductivity and disorder in 2D system.

Superconductivity at 10.4 K in a novel quasi-one-dimensional ternary molybdenum pnictide K2Mo3As3

Here we report the discovery of the first ternary molybdenum pnictide based superconductor K₂Mo₃As₃. Polycrystalline samples were synthesized by the conventional solid state reaction method. X-ray diffraction analysis reveals a quasi-one-dimensional hexagonal crystal structure with (Mo₃As₃)^2- linear chains separated by K⁺ ions, similar as previously reported K₂Cr₃As₃, with the space group of P-6m2 (No. 187) and the refined lattice parameters a = 10.145(5) Å and c = 4.453(8) Å. Electrical resistivity, magnetic susceptibility, and heat capacity measurements exhibit bulk superconductivity with the onset T_c at 10.4 K in K₂Mo₃As₃ which is higher than the isostructural Cr-based superconductors. Being the same group VIB transition elements and with similar structural motifs, these Cr and Mo based superconductors may share some common underlying origins for the occurrence of superconductivity and need more investigations to uncover the electron pairing within a quasi-one-dimensional chain structure.

Multiband One-Dimensional Electronic Structure and Spectroscopic Signature of Tomonaga-Luttinger Liquid Behavior in K_{2}Cr_{3}As_{3}

We present angle-resolved photoemission spectroscopy measurements of the quasi-one-dimensional superconductor K_{2}Cr_{3}As_{3}. We find that the Fermi surface contains two Fermi surface sheets, with linearly dispersing bands not displaying any significant band renormalizations. The one-dimensional band dispersions display a suppression of spectral intensity approaching the Fermi level according to a linear power law, over an energy range of ∼200  meV. This is interpreted as a signature of Tomonoga-Luttinger liquid physics, which provides a new perspective on the possibly unconventional superconductivity in this family of compounds.

Negative Magnetoresistance in Amorphous Indium Oxide Wires

We study magneto-transport properties of several amorphous Indium oxide nanowires of different widths. The wires show superconducting transition at zero magnetic field, but, there exist a finite resistance at the lowest temperature. The R(T) broadening was explained by available phase slip models. At low field, and far below the superconducting critical temperature, the wires with diameter equal to or less than 100 nm, show negative magnetoresistance (nMR). The magnitude of nMR and the crossover field are found to be dependent on both temperature and the cross-sectional area. We find that this intriguing behavior originates from the interplay between two field dependent contributions.