Metastable Superconductivity in Two-Dimensional IrTe2 Crystals

Chemical Doping and O-Functionalization of Carbon-Based Electrode to Improve Vanadium Redox Flow Batteries

The vanadium redox flow battery (VRFB) holds promise for large-scale energy storage applications, despite its lower energy and power densities compared to advanced secondary batteries available today. Carbon materials are considered suitable catalyst electrodes for improving many aspects of the VRFB. However, pristine graphite structures in carbon materials are catalytically inert and require modification to activate their catalytic activity. Among the various strategies developed so far, O-functionalization and chemical doping of carbon materials are considered some of the most promising pathways to regulate their electronic structures. Building on the catalytic mechanisms involved in the VRFB, this concise review discusses recent advancements in the O-functionalization and chemical doping of carbon materials. Furthermore, it explores how these materials can be tailored and highlights future directions for developing more promising VRFBs to guide future research.

Evolution of the surface phase transitions in IrTe2

The phase transitions in IrTe2 have been extensively studied but the symmetry at each phase is yet to be settled. Employing second harmonic generation (SHG) measurements over a temperature range of 4 -300 K, we probe the evolution of the symmetry of IrTe2. Our results indicate shifts in two distinct transition temperatures (Ts1 and Ts2 with Ts1 > Ts2) through thermal cycling, providing an explanation for the variations of reported values in literature. The SHG polarimetry measurements identify symmetries in different temperature ranges, confirming the trigonal symmetry above Ts1, the triclinic symmetry between Ts1 and Ts2, and the coexistence of multiple stripe phases below Ts2. The most striking feature is the reemergence of a trigonal phase as reflected by six-fold symmetry below ~ 10 K which is likely responsible for phenomena observed at low temperatures.

Enhancement of superconductivity in Zr2S2C arising from phonon softening on transition from bulk to monolayer

We report a new compound, Zr2S2C, belonging to the transition metal carbo-chalcogenide (TMCC) family. Through first-principles calculations, our analysis of phonon dispersion spectra indicates that the compound is dynamically stable in both bulk and monolayer forms. We systematically investigated the electronic structure, phonon dispersion, and electron-phonon coupling (EPC) driven superconducting properties in bulk and monolayer Zr2S2C. The results demonstrate the metallic character of bulk Zr2S2C, with a weak EPC strength (λ) of 0.41 and superconducting critical temperature (Tc) of ∼3 K. The monolayer Zr2S2C has an enhancedλof 0.62 andTcof ∼6.4 K. The increasedλvalue in the monolayer results from the softening of the acoustic phonon mode. We found that when biaxial strain is applied, the low energy acoustic phonon mode in monolayer becomes even softer. This softening leads to a transformation of the Zr2S2C monolayer from its initial weak coupling state (λ= 0.62) to a strongly coupled state, resulting in an increasedλvalue of 1.33. Consequently, the superconducting critical temperature experiences a twofold increase. These findings provide a theoretical framework for further exploration of the layered two-dimensional TMCC family, in addition to offering valuable insights.

Charge density waves in two-dimensional transition metal dichalcogenides

Charge density wave (CDW is one of the most ubiquitous electronic orders in quantum materials. While the essential ingredients of CDW order have been extensively studied, a comprehensive microscopic understanding is yet to be reached. Recent research efforts on the CDW phenomena in two-dimensional (2D) materials provide a new pathway toward a deeper understanding of its complexity. This review provides an overview of the CDW orders in 2D with atomically thin transition metal dichalcogenides (TMDCs) as the materials platform. We mainly focus on the electronic structure investigations on the epitaxially grown TMDC samples with angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy as complementary experimental tools. We discuss the possible origins of the 2D CDW, novel quantum states coexisting with them, and exotic types of charge orders that can only be realized in the 2D limit.

Structurally Constrained Evolutionary Algorithm for the Discovery and Design of Metastable Phases

Metastable materials are abundant in nature and technology, showcasing remarkable properties that inspire innovative materials design. However, traditional crystal structure prediction methods, which rely solely on energetic factors to determine a structure's fitness, are not suitable for predicting the vast number of potentially synthesizable phases that represent a local minimum corresponding to a state in thermodynamic equilibrium. Here, we present a new approach for the prediction of metastable phases with specific structural features and interface this method with the XtalOpt evolutionary algorithm. Our method relies on structural features that include the local crystalline order (e.g, the coordination number or chemical environment), and symmetry (e.g, Bravais lattice and space group) to filter the breeding pool of an evolutionary crystal structure search. The effectiveness of this approach is benchmarked on three known metastable systems: XeN8, with a two-dimensional polymeric nitrogen sublattice, brookite TiO2, and a high pressure BaH4 phase, which was recently characterized. Additionally, a newly predicted metastable melaminate salt, P1̅ WC3N6, was found to possess an energy that is lower than that of two phases proposed in a recent computational study. The method presented here could help in identifying the structures of compounds that have already been synthesized, and in developing new synthesis targets with desired properties.

Large-gap insulating dimer ground state in monolayer IrTe2

Monolayers of two-dimensional van der Waals materials exhibit novel electronic phases distinct from their bulk due to the symmetry breaking and reduced screening in the absence of the interlayer coupling. In this work, we combine angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy to demonstrate the emergence of a unique insulating 2 × 1 dimer ground state in monolayer 1T-IrTe₂ that has a large band gap in contrast to the metallic bilayer-to-bulk forms of this material. First-principles calculations reveal that phonon and charge instabilities as well as local bond formation collectively enhance and stabilize a charge-ordered ground state. Our findings provide important insights into the subtle balance of interactions having similar energy scales that occurs in the absence of strong interlayer coupling, which offers new opportunities to engineer the properties of 2D monolayers.

Spatially Separated Superconductivity and Enhanced Charge-Density-Wave Ordering in an IrTe2 Nanoflake

The interplay among collective electronic states like superconductivity (SC) and charge density wave (CDW) is of significance in transition metal dichalcogenides. To date, a consensus on the relationship between SC and CDW has not been established in IrTe₂. Here we use the Au-assisted exfoliation method to cleave IrTe₂ down to 10 nm. A striking feature is the concurrence of phase separation in a single piece of nanoflake, i.e., the superconducting (P3̅m1) and CDW (P3̅) phases. In the former area, the dimensional fluctuations suppress the CDW ordering and induce SC at 3.5 K. The CDW area at the phase boundary shows enhanced T_CDW at 605 K (T_CDW = 280 K in the bulk phase), which is accompanied by a unique wrinkle. Detailed analyses suggest that the strain-induced bond breaking of Te-Te dimers favors the CDW. Our works provide compelling evidence of competition between SC and CDW in IrTe₂.

Real-Space Observation of Emergent Complexity of Phase Evolution in Micrometer-Sized IrTe_{2} Crystals

We report complex behaviors in the phase evolution of transition-metal dichalcogenide IrTe_{2} thin flakes, captured with real-space observations using scanning Raman microscopy. The phase transition progresses via growth of a small number of domains, which is unlikely in statistical models that assume a macroscopic number of nucleation events. Consequently, the degree of phase evolution in the thin flakes is quite variable for the selected specimen and for a repeated measurement sequence, representing the emergence of complexity in the phase evolution. In the ∼20-μm^{3}-volume specimen, the complex phase evolution results in the emergent coexistence of a superconducting phase that originally requires chemical doping to become thermodynamically stable. These findings indicate that the complexity involved in phase evolution considerably affects the physical properties of a small-sized specimen.

Superconductivity emerging from a stripe charge order in IrTe2 nanoflakes

Superconductivity in the vicinity of a competing electronic order often manifests itself with a superconducting dome, centered at a presumed quantum critical point in the phase diagram. This common feature, found in many unconventional superconductors, has supported a prevalent scenario in which fluctuations or partial melting of a parent order are essential for inducing or enhancing superconductivity. Here we present a contrary example, found in IrTe₂ nanoflakes of which the superconducting dome is identified well inside the parent stripe charge ordering phase in the thickness-dependent phase diagram. The coexisting stripe charge order in IrTe₂ nanoflakes significantly increases the out-of-plane coherence length and the coupling strength of superconductivity, in contrast to the doped bulk IrTe₂. These findings clarify that the inherent instabilities of the parent stripe phase are sufficient to induce superconductivity in IrTe₂ without its complete or partial melting. Our study highlights the thickness control as an effective means to unveil intrinsic phase diagrams of correlated van der Waals materials.

Superconductivity often appears due to suppression of competing electronic orders. Here, the authors present a contrary example showing a superconducting dome inside the parent phase with a stripe charge order in IrTe₂ nanoflakes and identify their unusual superconducting properties.

Chemical Vapor Deposition of IrTe2 Thin Films

Two-dimensional (2D) IrTe2 has a profound charge ordering and superconducting state, which is related to its thickness and doping. Here, we report the chemical vapor deposition (CVD) of IrTe2 films using different Ir precursors on different substrates. The Ir(acac)3 precursor and hexagonal boron nitride (h-BN) substrate is found to yield a higher quality of polycrystalline IrTe2 films. Temperature-dependent Raman spectroscopic characterization has shown the q1/8 phase to HT phase at ~250 K in the as-grown IrTe2 films on h-BN. Electrical measurement has shown the HT phase to q1/5 phase at around 220 K.

Kinetic approach to superconductivity hidden behind a competing order

Exploration for superconductivity is one of the research frontiers in condensed matter physics. In strongly correlated electron systems, the emergence of superconductivity is often inhibited by the formation of a thermodynamically more stable magnetic/charge order. Thus, to develop the superconductivity as the thermodynamically most stable state, the free-energy balance between the superconductivity and the competing order has been controlled mainly by changing thermodynamic parameters, such as the physical/chemical pressure and carrier density. However, such a thermodynamic approach may not be the only way to materialize the superconductivity. We present a new kinetic approach to avoiding the competing order and thereby inducing persistent superconductivity. In the transition-metal dichalcogenide IrTe₂ as an example, by using current pulse-based rapid cooling of up to ~10⁷ K s^-1, we successfully kinetically avoid a first-order phase transition to a competing charge order and uncover metastable superconductivity hidden behind. Because the electronic states at low temperatures depend on the history of thermal quenching, electric pulse applications enable nonvolatile and reversible switching of the metastable superconductivity, a unique advantage of the kinetic approach. Thus, our findings provide a new approach to developing and manipulating superconductivity beyond the framework of thermodynamics.