X. W. Zhu

Oxygen-Assisted Chemical Vapor Deposition Growth of Large Single-Crystal and High-Quality Monolayer MoS2

Monolayer molybdenum disulfide (MoS2) has attracted great interest due to its potential applications in electronics and optoelectronics. Ideally, single-crystal growth over a large area is necessary to preserve its intrinsic figure of merit but is very challenging to achieve. Here, we report an oxygen-assisted chemical vapor deposition method for growth of single-crystal monolayer MoS2. We found that the growth of MoS2 domains can be greatly improved by introducing a small amount of oxygen into the growth environment. Triangular monolayer MoS2 domains can be achieved with sizes up to ∼350 μm and a room-temperature mobility up to ∼90 cm(2)/(V·s) on SiO2. The role of oxygen is not only to effectively prevent the poisoning of precursors but also to eliminate defects during the growth. Our work provides an advanced method for high-quality single-crystal monolayer MoS2 growth.

Electron-Phonon Coupling on the Surface of the Topological Insulator Bi2Se3 Determined from Surface-Phonon Dispersion Measurements

In this Letter, we report measurements of the coupling between Dirac fermion quasiparticles (DFQs) and phonons on the (001) surface of the strong topological insulator Bi2Se3. While most contemporary investigations of this coupling have involved examining the temperature dependence of the DFQ self-energy via angle-resolved photoemission spectroscopy measurements, we employ inelastic helium-atom scattering to explore, for the first time, this coupling from the phonon perspective. Using a Hilbert transform, we are able to obtain the imaginary part of the phonon self-energy associated with a dispersive surface-phonon branch identified in our previous work [Phys. Rev. Lett. 107, 186102 (2011)] as having strong interactions with the DFQs. From this imaginary part of the self-energy we obtain a branch-specific electron-phonon coupling constant of 0.43, which is stronger than the values reported from the angle-resolved photoemission spectroscopy measurements.

Classification of charge density waves based on their nature

Significance Charge density waves (CDWs) are observed in many solids, especially in low-dimensional systems. Their existence was first predicted in the 1930s by Rudolf Peierls, who prophesied that CDWs would exist in an ideal one-dimensional (1D) chain of atoms, lowering the energy of the system and driving a reconstruction of the lattice. In 1959, Walter Kohn pointed out that this nesting results in what is now known as a “Kohn anomaly,” a simultaneous softening of coherent lattice vibrations, i.e., phonon softening. This simple textbook picture of the origin of CDWs does not seem to be correct in many materials and in this report we propose a previously unidentified classification of CDWs based upon their nature. The concept of a charge density wave (CDW) permeates much of condensed matter physics and chemistry. CDWs have their origin rooted in the instability of a one-dimensional system described by Peierls. The extension of this concept to reduced dimensional systems has led to the concept of Fermi surface nesting (FSN), which dictates the wave vector (q→CDW) of the CDW and the corresponding lattice distortion. The idea is that segments of the Fermi contours are connected by q→CDW, resulting in the effective screening of phonons inducing Kohn anomalies in their dispersion at q→CDW, driving a lattice restructuring at low temperatures. There is growing theoretical and experimental evidence that this picture fails in many real systems and in fact it is the momentum dependence of the electron–phonon coupling (EPC) matrix element that determines the characteristic of the CDW phase. Based on the published results for the prototypical CDW system 2H-NbSe2, we show how well the q→-dependent EPC matrix element, but not the FSN, can describe the origin of the CDW. We further demonstrate a procedure of combing electronic band and phonon measurements to extract the EPC matrix element, allowing the electronic states involved in the EPC to be identified. Thus, we show that a large EPC does not necessarily induce the CDW phase, with Bi2Sr2CaCu2O8+δ as the example, and the charge-ordered phenomena observed in various cuprates are not driven by FSN or EPC. To experimentally resolve the microscopic picture of EPC will lead to a fundamental change in the way we think about, write about, and classify charge density waves.

Interaction of phonons and dirac fermions on the surface of Bi2Se3: a strong Kohn anomaly.

We report the first measurements of phonon dispersion curves on the (001) surface of the strong three-dimensional topological insulator Bi2Se3. The surface phonon measurements were carried out with the aid of coherent helium beam surface scattering techniques. The results reveal a prominent signature of the exotic metallic Dirac fermion quasiparticles, including a strong Kohn anomaly. The signature is manifest in a low energy isotropic convex dispersive surface phonon branch with a frequency maximum of 1.8 THz and having a V-shaped minimum at approximately 2kF that defines the Kohn anomaly. Theoretical analysis attributes this dispersive profile to the renormalization of the surface phonon excitations by the surface Dirac fermions. The contribution of the Dirac fermions to this renormalization is derived in terms of a Coulomb-type perturbation model.

Role of SrTiO 3 phonon penetrating into thin FeSe films in the enhancement of superconductivity

The significant role of interfacial coupling in the enhancement of superconductivity in FeSe films on

High resolution electron energy loss spectroscopy with two-dimensional energy and momentum mapping

{\mathrm{SrTiO}}_{3}

Misconceptions associated with the origin of charge density waves

has been widely recognized, but the explicit origin of this coupling is yet to be identified. Here, by surface phonon measurements using high-resolution electron energy loss spectroscopy, we found the electric field generated by Fuchs-Kliewer (F-K) phonon modes of

δ-Doping of oxygen vacancies dictated by thermodynamics in epitaxial SrTiO3 films

{\mathrm{SrTiO}}_{3}

Lattice dynamics of ultrathin FeSe films on SrTiO 3

can penetrate into FeSe films and strongly interact with the electrons therein. The mode-specific electron-phonon coupling constant for the

Superconducting transition of FeSe / SrTiO3 induced by adsorption of semiconducting organic molecules

\ensuremath{\sim}92