Observation of an Incommensurate Charge Density Wave in Monolayer TiSe_{2}/CuSe/Cu(111) Heterostructure.

TiSe_{2} is a layered material exhibiting a commensurate (2×2×2) charge density wave (CDW) with a transition temperature of ∼200 K. Recently, incommensurate CDW in bulk TiSe_{2} draws great interest due to its close relationship with the emergence of superconductivity. Here, we report an incommensurate superstructure in monolayer TiSe_{2}/CuSe/Cu(111) heterostructure. Characterizations by low-energy electron diffraction and scanning tunneling microscopy show that the main wave vector of the superstructure is ∼0.41a^{*} or ∼0.59a^{*} (here a^{*} is in-plane reciprocal lattice constant of TiSe_{2}). After ruling out the possibility of moiré superlattices, according to the correlation of the wave vectors of the superstructure and the large indirect band gap below the Fermi level, we propose that the incommensurate superstructure is associated with an incommensurate charge density wave (I-CDW). It is noteworthy that the I-CDW is robust with a transition temperature over 600 K, much higher than that of commensurate CDW in pristine TiSe_{2}. Based on our data and analysis, we present that interface effect may play a key role in the formation of the I-CDW state.

Understanding petroleum vapor fate and transport through high resolution analysis of two distinct vapor plumes.

No field study has provided a detailed characterization of the molecular composition and spatial distribution of a vadose zone plume of petroleum volatile organic compounds (VOCs), which is critical to improve the current understanding of petroleum VOC transport and fate. This is study reports a high-resolution analysis of two distinct vapor plumes emanating from two different light non-aqueous phase liquid (LNAPL) sources (an aliphatic-rich LNAPL for Zone #1vs an aromatic-rich LNAPL for Zone #2) at a large petrochemical site. Although deep soil vapor signatures were similar to the source zone LNAPL signatures, the composition of the shallow soil vapors reflected preferential attenuation of certain hydrocarbons over others during upward transport in the vadose zone. Between deeper and shallower soil gas samples, attenuation of aromatics was observed under all conditions, but important differences were observed in attenuation to aliphatic compound classes. Attenuation of all aliphatic compounds was observed under aerobic conditions but little attenuation of any aliphatics was observed under anoxic conditions without methane. In contrast, under methanogenic conditions, paraffins attenuated more than isoparaffins and naphthenes. These results suggest that isoparafins and naphthenes may present more of a vapor intrusion risk than benzene or other aromatic hydrocarbons commonly considered to be petroleum vapor intrusion risk drivers. While the overall vapor composition changed significantly within the vadose zone, diagnostic ratios of relatively recalcitrant alkylcyclopentanes were preserved in shallow soil vapor samples. These alkylcyclopentanes may be useful for distinguishing between petroleum vapor intrusion and other sources of petroleum VOCs detected in indoor air.

Time-resolved ARPES with tunable 12-21.6 eV XUV at 400 kHz repetition rate.

Time-resolved and angle-resolved photoemission spectroscopy (trARPES) is a powerful method to detect the non-equilibrium electronic structure in solid systems. In this study, we report a trARPES apparatus with tunable photon energy selectively among 12, 16.8, and 21.6 eV at a repetition rate of 400 kHz. The energy and temporal resolutions of the three harmonics are determined as 104/111/157 meV and 276/190/154 fs, respectively. The photon flux on the sample is estimated to be 1010-1011 photons/s by using a photodiode. Finally, the performance of this setup is verified by both equilibrium and non-equilibrium ARPES measurements on topological materials Zr2Te2P and Bi2Se3. Meanwhile, the importance of the tunability of the extreme ultraviolet (XUV) source is highlighted by comparing experimental results measured with the three different photon energies.

Charge instability of topological Fermi arcs in chiral crystal CoSi.

Topological boundary states emerged at the spatial boundary between topological non-trivial and trivial phases, are usually gapless, or commonly referred as metallic states. For example, the surface state of a topological insulator is a gapless Dirac state. These metallic topological boundary states are typically well described by non-interacting fermions. However, the behavior of topological boundary states with significant electron-electron interactions, which could turn the gapless boundary states into gapped ordered states, e.g., density wave states or superconducting states, is of great interest theoretically, but is still lacking evidence experimentally. Here, we report the observation of incommensurable charge density wave (CDW) formed on the topological boundary states driven by the electron-electron interactions on the (001) surface of CoSi. The wavevector of CDW varies as the temperature changes, which coincides with the evolution of topological surface Fermi arcs with temperature. The orientation of the CDW phase is determined by the chirality of the Fermi arcs, which indicates a direct association between CDW and Fermi arcs. Our finding will stimulate the search of more interactions-driven ordered states, such as superconductivity and magnetism, on the boundaries of topological materials.

A robust and tunable Luttinger liquid in correlated edge of transition-metal second-order topological insulator Ta2Pd3Te5.

The interplay between topology and interaction always plays an important role in condensed matter physics and induces many exotic quantum phases, while rare transition metal layered material (TMLM) has been proved to possess both. Here we report a TMLM Ta2Pd3Te5 has the two-dimensional second-order topology (also a quadrupole topological insulator) with correlated edge states - Luttinger liquid. It is ascribed to the unconventional nature of the mismatch between charge- and atomic- centers induced by a remarkable double-band inversion. This one-dimensional protected edge state preserves the Luttinger liquid behavior with robustness and universality in scale from micro- to macro- size, leading to a significant anisotropic electrical transport through two-dimensional sides of bulk materials. Moreover, the bulk gap can be modulated by the thickness, resulting in an extensive-range phase diagram for Luttinger liquid. These provide an attractive model to study the interaction and quantum phases in correlated topological systems.

Intrinsic surface p-wave superconductivity in layered AuSn4.

The search for topological superconductivity (TSC) is currently an exciting pursuit, since non-trivial topological superconducting phases could host exotic Majorana modes. However, the difficulty in fabricating proximity-induced TSC heterostructures, the sensitivity to disorder and stringent topological restrictions of intrinsic TSC place serious limitations and formidable challenges on the materials and related applications. Here, we report a new type of intrinsic TSC, namely intrinsic surface topological superconductivity (IS-TSC) and demonstrate it in layered AuSn4 with Tc of 2.4 K. Different in-plane and out-of-plane upper critical fields reflect a two-dimensional (2D) character of superconductivity. The two-fold symmetric angular dependences of both magneto-transport and the zero-bias conductance peak (ZBCP) in point-contact spectroscopy (PCS) in the superconducting regime indicate an unconventional pairing symmetry of AuSn4. The superconducting gap and surface multi-bands with Rashba splitting at the Fermi level (EF), in conjunction with first-principle calculations, strongly suggest that 2D unconventional SC in AuSn4 originates from the mixture of p-wave surface and s-wave bulk contributions, which leads to a two-fold symmetric superconductivity. Our results provide an exciting paradigm to realize TSC via Rashba effect on surface superconducting bands in layered materials.