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Understanding Charge Density Wave Phase Transition in 1T-TaS2 via Machine Learning Force Field

2024-12-31T15:00:00Z

Title: Understanding Charge Density Wave Phase Transition in 1T-TaS2 via Machine Learning Force Field Author(s): Koh, Kahyeon Abstract: Recently, the charge density wave (CDW) phases and their properties in various condensed matter systems have been widely studied. In condensed matter, the formation of CDWs and periodic lattice distortions leads to changes in the electronic transport properties. Among these materials, 1T-type tantalum disulfide (1T-TaS2) is a layered material, and its CDW phases indicate a two-step phase transition depending on thickness and temperature. Experimental studies have shown that its physical properties depend on the stacking order of its layers, which affects the CDW phase transitions. However, due to its strong correlation between electronic structure and complex atomic geometry, understanding the CDW phase transition in 1T-TaS2 is challenging from a theoretical perspective. In this study, we investigated the tendency for structural change from molecular dynamics (MD) with the machine learning force field (MLFF), to simulate multiscale dynamics. We extracted a MLFF for a 1T-TaS2 monolayer based on Ab Initio Molecular Dynamics (AIMD) simulation training data on an Angstrom scale. Using this MLFF in MD simulations, we performed large-scale (∼< 100 nm^2) and long-time (∼< ns) simulations of temperature-dependent dynamics. Through these simulations, we discovered CDW phase transitions and domain wall formations at various temperatures and times, and analyzed their atomic composition. Our results not only theoretically predicted the temperature-dependent bulk CDW phase transition of 1T-TaS2, but also observed microscopic dynamics. This suggests that it will provide explanations consistent with previous experimental results. In future studies, our MLFF approach methodology could be applied to investigate bulk 1T-TaS2. Moreover, our findings may contribute to future studies that analyze light-induced hidden phases in 1T-TaS2 in detail, and could be extended to understand CDW phase transitions in other materials.

Traversable wormhole and quantum teleportation

2022-12-31T15:00:00Z

Title: Traversable wormhole and quantum teleportation Author(s): Yeong Han Park Abstract: The quantum informative aspect of holographic principle incorporated with the ER$=$EPR conjecture suggests the dual geometry for an entangled thermofield double state is a non-traversable wormhole and recent studies have shown its traversability is governed by a non-local coupling of two boundaries. In the paper, we elaborate on the principle and the method on the wormhole opening and the signal teleportation for the supplementation of the duality including the formulation of double trace deformations and boundary conditions in the context of AdS/CFT correspondence. We also give proposals for further generalizing and resolving the dual configurations regarding several issues and subtleties.

The quantum paraelectricity of perovskite oxides from first-principle calculation

2023-12-31T15:00:00Z

Title: The quantum paraelectricity of perovskite oxides from first-principle calculation Author(s): Lym, Yongsik Abstract: Perovskites have been widely utilized in various applications due to their ability to transition into different phases such as ferroelectricity, ferromagnetism, and superconductivity, depending on external conditions. A new class of phase known as quantum paraelectricity (QPE) has been discovered, which do not undergo phase transitions even at low temperatures away from conventional ferroelectric materials. The underlying cause of this behavior is attributed to quantum fluctuations occurring at low temperature, and extensive research is being conducted to microscopically elucidate the properties of QPE in terms of quantum fluctuation. A representative study is proceeded in the SrTiO3 by highlighting the interaction between lattice elongation and ferroelectric soft modes based on the first principle calculations. In our research, we investigated QPE behavior in the specific perovskite oxides, such as KTaO3 and EuTiO3, and aimed to unravel the nature of QPE behavior. For this, we first studied the methodology of the aforementioned study on SrTiO3 to verify our numerical configurations, and then applied the methodology to our chosen QPE candidates (KTaO3 and EuTiO3). The specific methodology involved the following steps: We used four functionals (LDA, PBE, PBEsol, HSE06) to find the one that best describes the ferroelectric soft mode. For this, we performed DFPT (Density Functional Perturbation Theory) and then confirmed whether these methods achieved an unstable equilibrium at the equilibrium point. Subsequently, to incorporate quantum fluctuations of the material, we introduced the lattice–Schr¨odinger equation, calculated the mode frequencies from this, and used these frequencies as a measure of QPE characteristics. We can confirm QPE for the above two materials from PBE functionals. In the case of KTaO3, unlike SrTiO3, the influence of lattice elongation was negligible, and due to the cubic symmetry of the structure, there were no modes preferred in specific axial directions. Therefore, the mode could be described as a linear combination of modes in each axial direction as 3D quantum harmonic oscillator model. In the case of EuTiO3, the interaction between the spins on Eu and the lattice affects the QPE iproperties. Furthermore, by analyzing the cause of the variation in mode frequencies depending on the degree of spin interaction, we identified that this variation was related to the electron occupancy of the 3d-orbitals of the transition metal. In conclusion, we verified that both materials show QPE based on the first-principle calculation.

Terahertz Radiation Generated by an Asymmetric Current in a Laser-Produced Plasma in a Gaseous Medium

2022-12-31T15:00:00Z

Title: Terahertz Radiation Generated by an Asymmetric Current in a Laser-Produced Plasma in a Gaseous Medium Author(s): Rajaram Shrestha Abstract: When atoms are exposed to an intense laser field, they can be tunnel-ionized. The liberated electrons are accelerated in the laser field and gain a transverse velocity. If the liberated electrons are driven asymmetrically, the net current would not be canceled out, and the THz radiation can be emitted. The THz radiation has been extensively used in various applications, including pump-probe experiments, medical image processing, non-destructive testing, security, ultrafast technologies, and nonlinear THz spectroscopy. Thus, it is important to understand the generation mechanism and the optimal conditions for the generation of intense THz radiation. One way to induce the asymmetric current is to use an ultrashort laser pulse. We theoretically studied the generation of THz radiation using ultrashort laser pulses. The dependence of the THz generation on the laser pulse duration is analyzed in single-cycle and few-cycle regimes. It is shown that the single-cycle or few-cycle laser pulses can be used to generate intense THz emissions by the asymmetric current. Also, the intensity of the THz radiation strongly depends on the carrier-envelope phase (CEP) of the driving laser pulse. It is shown that the intensity of the THz radiation is pronounced at the CEP of 90-degree, and it decreases with increasing the laser pulse duration. Therefore, the use of the ultrashort laser pulse with the right CEP is critical for efficient THz radiation. The other approach to induce the asymmetric current is to use two-color laser fields. The two color laser fields were obtained by superimposing the fundamental laser field with its second harmonic. We analyzed the THz radiation generated using the two-color laser fields theoretically by using the semi-classical theory and by solving the time-dependent Schrodinger equation. These analyses provide detailed electron dynamics in the intense two-color laser field. We also performed THz generation experiments using the two-color laser fields in a gaseous medium. Our experimental results exhibit a strong dependence on the relative phase and the intensity of the two-color laser field, showing that the asymmetric current can be controlled using these laser parameters. Our analysis provides a detailed description of the asymmetric plasma current in a gaseous medium, which is critical to understand the electron dynamics in the laser-produced plasma. In addition, we also found the optimal conditions for the generation of intense THz radiation. These findings will be highly useful in both the fundamental research and the THz applications.