応用物理学輪講 I
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廉 東奇
長谷川 幸雄 教授
渡辺 柊
中村 泰信 教授
発表者名 熊崎 紘介
指導教員名 沙川 貴大 教授
発表題目(英語) Implementing noncommutative time-dependent Hamiltonians by a quantum clock
要旨(英語) When considering manipulating a quantum system with external control, there are two theoretical frameworks to treat the time dependence of the Hamiltonian. One is a nonautonomous framework in which the classical external field is considered and the Hamiltonian of the system is time-dependent, and the other is an autonomous framework in which the external quantum system interacting with the system is explicitly considered and the Hamiltonian of the whole system is time-independent. In this study, we investigate the relationship between these frameworks.
 We focus on the fact that the time-dependent Hamiltonian can be implemented approximately to the system by using an ancilla system called a clock [1]. In particular, we investigate how the error between the state evolution with a finite-dimensional clock as an ancilla system and the evolution of the time-dependent Hamiltonian scales with respect to the dimension of the clock. In a previous study, the scaling of the exponential decay at the time after one cycle of the clock has been theoretically shown, but the analysis of the implementation error of the state evolution was restricted to the case where the time-dependent Hamiltonian commutes with itself at different times [2]. In our study, we perform numerical calculations for the case where the Hamiltonian does not commute with itself at different times and observe the scaling of the exponential decay of the implementation error at the time after one cycle as in the commutative case. The scaling of the error is also analyzed for times other than after one cycle, and in contrast, a power-law decay is observed. Furthermore, we have theoretically derived such a power-law decay. This result suggests that high accuracy implementation with the clock can only be achieved at specific times.
[1] A. S. L. Malabarba et al., New J. Phys. *17*, 045027 (2015).
[2] M. P. Woods, R. Silva and J. Oppenheim, Ann. Henri Poincare *20, *125-218(2019). Recently, a new criterion of the second law of thermodynamics is proposed, which is called concavity [1]. An energy eigenstate that meets the criterion indeed obeys the principle of maximum work for adiabatic processes, which is an expression of the second law. The concavity also can be used to judge whether a mixed state obeys the second law or not and would be useful to study quantum heat engines.
 In quantum heat engines [2], there are several examples that show higher efficiency than the Carnot efficiency [3]. However, the reason why these quantum engines show high efficiency has not been clarified yet.
In this presentation, I will introduce the concept of concavity for a single energy eigenstate and a mixed state, and show a numerical estimate of the efficiency of a single qubit heat engine.
[1] C. Itoi and M. Amano, J. Phys. Soc. Jpn. 89, 104001 (2020)
[2] J. Goold et al., J. Phys. A: Math. Theor. 49 143001 (2016)
[3] J. Roßnagel et al., PRL 112, 030602 (2014)
発表言語 日本語
発表者名 古賀 淳平
指導教員名 石坂 香子 教授
発表題目(英語) Ultrafast phase transition dynamics in photoinduced TaTe2 and construction of ultrafast electron diffraction
要旨(英語) Optical manipulating the structure and function of materials remains one of the ultimate challenges of science. MTe2(M=V, Nb, Ta) is known to form M-M bonding, leading to the formation of a double zigzag chain in room temperature. It is also reported that in VTe2, this V-V bonding is related to topological surface states. However, the ultrafast response of MTe2 to optical driving remains unknown.
 Ultrafast electron diffraction (UED) is time resolved measurement for time dependence of electron diffraction images after photoexcitation by pump-probe method with short pulse laser. This method reveals ultrafast optical response and lattice dynamics. In this study, we have used UED for elucidating photo-induced dynamics of TaTe2. We observed ultrafast intensity increase in the Bragg intensity (< 0.5 ps), that implies structural phase transition.
 In this presentation, I will discuss the origin of photoinduced structural phase transition from the analysis of the electron diffraction intensity and introduce reconstruction of UED equipment to improve time resolution. Here, we focus on pyrochlore oxides (A2B2O7) which are theoretically predicted to be another candidate of the host of flat band structure. Among them, Sn2Nb2O7 and Sn2Ta2O7 are specifically expected to be promising compounds according to some first principle calculations. We aim to synthesize high-quality single crystalline thin films of them by pulsed laser deposition method in order to materialize the flat band structure.
 In this presentation, I will explain why ferromagnetism emerges in the flat band system, and why pyrochlore oxides can be candidates of it.
 Besides, I will introduce my experimental results of synthesizing and evaluating thin films of these pyrochlore oxides and discuss difficulties in inducing ferromagnetism in them.
発表言語 日本語
発表者名 小林 拓豊
指導教員名 吉岡 孝高 准教授
発表題目(英語) A Laser Cooling Experiment of Positronium
要旨(英語) Positronium (Ps) is a hydrogen-like atom consisting of an electron and its anti-particle (a positron). Ps is an important research target for precise verification of fundamental physics because it is a simple system consisting of two leptons. If Ps can be cooled down to below 10 K, it is expected to lead to important applications such as the verification of the Standard Model of elementary particles by precise measurement of its atomic properties, the search for matter-antimatter asymmetry, and the realization of Bose-Einstein condensation for the first time in a system containing antimatter. We are working on the realization of laser cooling of Ps.
 In order to realize effective cooling of Ps, a special light source is required to adapt to the characteristics of Ps, which has the short lifetime as 142 ns and the wide Doppler width around 500 GHz at room temperature. We have developed a prototype laser based on our original design. The laser was transported and installed at an accelerator facility where Ps can be produced. In addition, we have constructed a laser system to evaluate the velocity distribution of Ps by measuring the Doppler spectrum of the 1S-2P transition.
 In this presentation, I will introduce the design of the laser cooling experiment and show the results so far.
発表言語 日本語
発表者名 阪本 天志
指導教員名 吉岡 孝高 准教授
発表題目(英語) Exploration of laser cooling method for carbon atoms
要旨(英語) Laser cooling and trapping of atoms at very low temperatures are techniques of great value in a wide range of applications, such as precision spectroscopy, observation of quantum degenerate states, and
chemical reaction studies.
 However, these techniques can be applied only to limited atomic species, such as alkali and alkali-earth metals which have convenient energy levels suited to current laser technology.
 In particular, many atomic species of chemical and biological interest, including carbon, are currently out of reach due to the lack of sufficiently powerful lasers in deep ultraviolet which are required for cooling.
 In this presentation, I will introduce our research plan and current progress of light source development toward the realization of laser cooling of carbon atoms with the background of a proposal for a new laser cooling method[1].
[1] A. M. Jayich et al. Physical Review X6, 041004 (2016)
発表言語 日本語


楊 暁達
香取 秀俊 教授
発表者名 小林 弘和
指導教員名 渡辺 悠樹 准教授
発表題目(英語) Modern theory of polarization and orbital magnetization
要旨(英語) In thermodynamics, Polarization and orbital magnetization can be interpreted as responses to energy changes about the external fields. Therefore two quantities are one of the most important and fundamental quantities of the system and studied for a long time. As is well known, classical electromagnetism gives their definitions which include the positions. However, under the periodic boundary condition(PBC), which is frequently used for the quantum many body systems, the position is ill-defined.
 To solve above problem, new definitions called modern theory were suggested [1,2]. These definitions are based on bloch wave functions, then well-defined under PBC. Although these definitions are studied carefully [3], the extension to higher-order terms, multipoles, has not yet been done.
 In this presentation, I would like to explain the detail of the above problems and the results of analysis using the tight binding model.
[1]R. Resta, Phys. Rev. Lett. 80, 1800 (1998).
[2]D. Vanderbilt and R. D. King-Smith, Phys. Rev. B 48, 4442 (1993).
[3]R. Bianco and R. Resta, Phys. Rev. B 93, 174417 (2016). We have constructed a mid-infrared spectroscopy system using a mid-infrared comb and an extremely-high-order dispersion element and a two-dimensional array type detector. Our laboratory has constructed highly sensitive, high-resolution and real-time mid-infrared spectroscopic system in the 3-5 um band. The 3-5 um band has sharp absorption lines of light molecules, and a high-resolution spectroscopic system is required. On the other hand, the 8-12 um band has wide and complex absorption lines of heavy molecules such as aromatic compounds. So wider spectral system is required in the 8-12 um band.
 In this presentation, I will introduce the setup of mid-infrared spectroscopy system in the 8-12 um band .
発表言語 日本語
発表者名 小山 奏汰
指導教員名 長谷川 達生 教授
発表題目(英語) Crystal structure prediction of layered organic semiconductors based on symmetry
要旨(英語) Although the greatest advantage of organic semiconductors is the flexibility of molecular design, the quality of their properties can only be determined by knowing the arrangement of molecules, or their crystal structure. For example, if a layered herringbone structure, which is said to be suitable for TFT structures, can be predicted at the molecular design stage, the development of new organic semiconductors can be made more efficient. However, it is difficult to predict the crystal structure of molecular materials because of the myriad of candidate structures in the vast parameter space, although many attempts have been made to search for energetically stable structures. Based on the empirical assumption that only structures with relatively high symmetry can be realized at room temperature among the innumerable candidate structures that are stable below absolute zero, we believe that a stepwise method to predict the crystal structure of polyacene semiconductors under the two assumptions of symmetry and stratification is effective. In this study, we report the results of similar attempts to apply the same approach to pentacene and anthracene, which are typical organic semiconductors with at least three polymorphs and a smaller number of fused rings, respectively. In the case of pentacene, we have succeeded in reproducing the actual crystal structure of pentacene by clarifying the mechanism of the structural polymorphism through a stepwise structure search. On the other hand, for anthracene, the molecular long axis is shorter than that of pentacene, and the parallelism of the long axis between neighboring molecules is expected to be broken, so that the prediction of the structure with the introduction of the torsion of the long axis can be extended to the system with slightly broken stratification.
発表言語 日本語
発表者名 財津 英明
指導教員名 岩佐 義宏 教授
発表題目(英語) Gate-induced superconductivity and its phase diagram, and some technical improvement
要旨(英語) In this study, we focus on a semiconductor, as a candidate material to reach the BCS-BEC crossover region. Because it has a simpler crystal and band structure than ZrNCl, if it is possible to reach the BCS-BEC crossover region with this material, it should be possible to obtain a hint of universal mechanism. Previous studies were limited to polycrystalline ones and no carrier density dependence was reported.
 Based on this background, we have designed a gate-induced device that, for the first time, induces superconductivity in a single crystal of the material and shows that, like LixZrNCl, the transition temperature increases in the low-carrier region. This result is non-trivial and suggests that this material has a phase diagram suitable for studying the BCS-BEC crossover.
 In addition, we have evaluated the anisotropy of superconductivity in this material, which is the first time to be achieved with single crystal, and its properties as an unconventional superconductor and its relation to the BCS-BEC crossover will also be of interest.
 Furthermore, I want to talk about some technical improvement on 2D material experiment.
発表言語 日本語