Yanwei Cao

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.

Polarity compensation in ultra-thin films of complex oxides: The case of a perovskite nickelate

We address the fundamental issue of growth of perovskite ultra-thin films under the condition of a strong polar mismatch at the heterointerface exemplified by the growth of a correlated metal LaNiO3 on the band insulator SrTiO3 along the pseudo cubic [111] direction. While in general the metallic LaNiO3 film can effectively screen this polarity mismatch, we establish that in the ultra-thin limit, films are insulating in nature and require additional chemical and structural reconstruction to compensate for such mismatch. A combination of in-situ reflection high-energy electron diffraction recorded during the growth, X-ray diffraction, and synchrotron based resonant X-ray spectroscopy reveal the formation of a chemical phase La2Ni2O5 (Ni2+) for a few unit-cell thick films. First-principles layer-resolved calculations of the potential energy across the nominal LaNiO3/SrTiO3 interface confirm that the oxygen vacancies can efficiently reduce the electric field at the interface.We address the fundamental issue of growth of perovskite ultra-thin films under the condition of a strong polar mismatch at the heterointerface exemplified by the growth of a correlated metal LaNiO

InP-based InAs/InGaAs quantum wells with type-I emission beyond 3 μm

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Anomalous orbital structure in a spinel–perovskite interface

on the band insulator SrTiO

Metallic Conductance at the Interface of Tri-color Titanate Superlattices

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Direct Determination of the Electron-Phonon Coupling Matrix Element in a Correlated System

along the pseudo cubic [111] direction. While in general the metallic LaNiO

Magnetic Interactions at the Nanoscale in Trilayer Titanates.

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ECR ion sources at the Institute of Modern Physics: From classical to fully superconducting device

film can effectively screen this polarity mismatch, we establish that in the ultra-thin limit, films are insulating in nature and require additional chemical and structural reconstruction to compensate for such mismatch. A combination of in-situ reflection high-energy electron diffraction recorded during the growth, X-ray diffraction, and synchrotron based resonant X-ray spectroscopy reveal the formation of a chemical phase La

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

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Measurements of bremsstrahlung spectra of Lanzhou ECR Ion Source No. 3 (LECR3)

Ni