応用物理学輪講 I
5月28日
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発表の10日前までに宛てに「氏名」「指導 教員」「発表題目(英語)」「要旨(英語)」「発表言語(英語または日本語)」を送付して下さい。
発表日
2021年5月11日(火)14:55〜16:55

Aグループ

座長
山野 新一郎
指導
教員名
小芦 雅斗 教授
発表者名 越智 友崇
指導教員名 長田 俊人 教授
発表題目(英語) Search for nonlinear anomalous thermoelectric effect in 2D material WTe2
要旨(英語) The thermoelectric effect, in which the electromotive force is extracted from a temperature gradient, is attracting attention as an environmentally friendly method of power generation. The Nernst effect, in which a transverse voltage is obtained from a temperature gradient in a magnetic field, can be understood as an analogy of the electric Hall effect. Recently, the nonlinear anomalous Hall effect (NAHE), in which neither spontaneous magnetization nor external magnetic field is required and the transverse voltage is proportional to the square of the external electric field, has been observed in two-dimensional transition metal dichalcogenide WTe2 thin films [1]. From the same discussion, the nonlinear anomalous Nernst effect (NANE) is understood and its experimental result is expected. The concept of Berry curvature dipole (BCD) is behind these phenomena [2]. In this presentation, based on the idea of BCD, we finally describe these Hall and Nernst effects by a single equation and discuss the relationship between them. We will also discuss the results of test experiments to construct a measurement environment for the low temperature thermoelectric effect.
[1] Q. Ma et al., Nat.565,337-342 (2019)
[2] I. Sodemann and L. Fu, Phys. Rev. Lett.115, 216806(2015)
発表言語 日本語
発表者名 大西 由吾
指導教員名 永長 直人 教授
発表題目(英語) Theory of thermoelectric effect in insulators
要旨(英語) The thermoelectric effects are usually discussed for metals and semiconductors. There, it is assumed that electrons and/or holes carry heat and charge and contribute to thermoelectric responses. Similarly for insulators, thermally activated carriers are considered to be carriers of both heat and charge, which leads to the idea that insulators cannot exhibit large thermoelectric effects at low temperatures.
 Among thermoelectric effects, the Seebeck effect is special in some sense. In the Seebeck effect, the voltage drop is induced by the temperature difference without an electric current flowing. The Seebeck coefficient, which is defined as the ratio of the voltage drop and the temperature difference, is also special because it is not directly related to the carrier density. From these observations, an important question arises; whether the Seebeck effect can be defined for insulators without assuming the thermally activated carriers.
 In this presentation, I will talk about our new theory about the thermoelectric effect in insulators, especially the Seebeck effect [1]. We have revealed that the Seebeck effect can be defined for insulators and that the Seebeck coefficient even diverges as the temperature goes to 0. We also show that electric polarization plays an important role in the Seebeck effect in insulators and propose a new thermoelectric effect where the temperature gradient induces the electric polarization. Its dual effect is the ac heat current induced by the time-dependent electric field.
[1] Yugo Onishi, Naoto Nagaosa, “Theory of thermoelectric effect in insulators”, arXiv:2105.08228. 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 英語
発表者名 鎌谷 拓実
指導教員名 森本 高裕 准教授
発表題目(英語) Optical responses of Higgs and Leggett modes in multiband superconductors
要旨(英語) Collective excitations in superconductors are attracting much attention recently. One typical example is the Higgs mode, which is an amplitude mode of superconducting gap function. Higgs modes in superconductors are experimentally observed by Raman spectroscopy and optical measurements of third harmonic generation [1]. In multiband superconductors, the gap function contains multiple band degrees of freedom and becomes a matrix generally. Consequently, there appears another collective mode that corresponds to the oscillation of the relative phase between different bands, which is called Leggett modes. So far, Leggett modes are observed in Raman experiments using MgB2[2], while there is no optical measurement of Leggett modes.
 Motivated by these, we study optical responses of Higgs and Leggett modes in multi-band superconductors. We use a diagrammatic method to compute linear and nonlinear optical conductivities that arise from these collective excitations. We will discuss how these collective modes can be detected through (nonlinear) optical probes in the future experiments.
[1] R. Matsunaga et al., Science *345*, 6201 (2014)
[2] G. Blumber et al., Physics Review Letters *99*, 227002 (2007)
発表言語 英語

Bグループ

座長
?井 真央
指導
教員名
森本 高裕 准教授
座長
吉持 遥人
指導
教員名
関 真一郎 准教授
発表者名 荻原 琢磨
指導教員名 求 幸年 教授
発表題目(英語) Efficient real-frequency solver for dynamical mean-field theory
要旨(英語) Ab initio electronic structure calculations based on density functional theory (DFT) have been important theoretical and numerical tools in modern materials science. However, despite of their great success, the DFT-based methods have often failed to predict properties of correlated electron materials such as transition metal oxides. For example, standard DFT-based methods, local density approximation (LDA) and generalized gradient approximation (GGA), fail to reproduce experimentally observed crystal structures of strongly correlated electron materials. As a typical example, LDA and GGA have been known to underestimate the volume of FeO. To improve the accuracy of the DFT-based methods, a combination of DFT and dynamical mean-field theory (DMFT) [1] called DFT+DMFT has been proposed [2]. Indeed, the DFT+DMFT reproduces the crystal structure of FeO [3, 4].
 The DMFT maps a correlated lattice model onto a quantum impurity model in a self-consistent fashion so that the local one-particle Green functions of the lattice and impurity models become identical. The impurity model consisting of a correlated impurity site and non-interacting bath sites is much easier to solve than the original lattice model. Since the introduction of the DMFT, there has been an enormous development on how to solve the quantum impurity models.
 In our previous presentation, we reviewed an important previous work [5]. In this work, an exact diagonalization (ED) technique was used to solve the impurity model, and the DMFT loop was implemented by using real frequency to avoid analytical continuation. Na?ve implementation of the ED method has a problem that the Hilbert space dimension grows exponentially as the number of the bath sites increases. In Ref. [5], the Hilbert space dimension is effectively reduced by selecting basis functions with natural orbitals. Then they succeeded in solving a single impurity model with three hundred bath sites. It is highly desirable to extend the previous method to multiorbital systems and to realize an efficient impurity solver for the DFT+DMFT. Towards this goal, first, we have been working to reproduce the result of the previous work. In the presentation, we will report our progress.
[1] A. Georges, G. Kotliar, W. Krauth, and M. J. Rozenberg, Rev. Mod. Phys. 68, 13 (1996).
[2] G. Kotliar et al., Rev. Mod. Phys. 78, 865 (2006).
[3] K. Haule and T. Birol, Phys. Rev. Lett.115, 256402 (2015).
[4] A. Paul and T. Birol, Annu. Rev. Mater. Res. 49, 31 (2019).
[5] Y. Lu, M. Hoppner, O. Gunnarsson, and M. W. Haverkort, Phys. Rev. B 90, 085102 (2014). 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 加藤 啓輔
指導教員名 中村 泰信 教授
発表題目(英語) Magnon based quantum tranducer
要旨(英語) Recently, quantum computation has been attracting much attention by virtue of its potential to dramatically improve computational power for certain tasks in both academic and industrial areas. Amongst physical platforms for quantum computation, the superconducting qubit(quantum bit) is the most widely researched system around the world. The superconducting qubit is basically electric circuits made of superconducting material. It behaves as a quantum two-level system whose transition frequency is in the microwave
regime and its properties like the transition frequency and the magnitude of interaction with the external environment are tunable by changing circuit parameters. Although its tunability and some other points give an advantage over other physical systems that behave as a qubit, its microwave energy scale forces it to work in a dilution refrigerator because quantum states can easily decay in room temperature.
 Now I shall cast an eye on the quantum network. Quantum network is the network that connects many quantum devices in a quantum manner and allows more complicated computation, distribution of quantum entanglement, and so on. Each device is to be connected using optical fibre because of its low transmission loss. However, since a superconducting qubit works in the microwave regime, we need to convert quantum states from superconducting qubits to optical photons in the reversible manner. The physical system to do so is called quantum transducer.
 It is not an easy task to convert quantum states from superconducting qubits to optical photons because energy magnitude of each systems are quite different. There are several candidates for quantum transducer. In the midst of them, I shall explain one using magnon, the quanta of ferromagnetic resonance. First, I confirmed that the NMR spectral shift (Knight shift), which measures the magnitude of the local magnetic field created by the electron spins, is scaled to the spin susceptibility and found that the spectrum is widened with decreasing temperature, suggesting an increasing inhomogeneity of the local spin susceptibility upon cooling.
 Second, the nuclear spin-lattice relaxation rate 1/T1, which measures the intensity of electron spin fluctuations, showed a temperature dependence similar to in the spin-liquid compound κ-(ET)2Cu2(CN)3. A previous study found both materials to behave similar in spin susceptibility; thus, κ-(ET)4Hg2.89Br8 is regarded as a doped spin liquid hosting both mobile carriers and a quantum spin liquid.
 Furthermore, a decreases in the Knight shift and a cubic temperature dependence of 1/T1 below 10 K suggest that superconductivity emerging in this material has spin-singlet d-wave symmetry with highly enhanced fluctuations above Tc.
発表言語 日本語
発表者名 上島 卓也
指導教員名 沙川 貴大 教授
発表題目(英語) Power-efficiency tradeoff of heat engines in the nonlinear regime
要旨(英語) According to thermodynamics, the efficiency of heat engines generally cannot surpass the universal efficiency, called Carnot efficiency. In the reversible limit, the Carnot efficiency can be achieved, where, however, the heat engine cannot produce nonzero power due to its long-time operation. When it comes to operating heat engines in a finite time, power and efficiency cannot be optimized simultaneously in general. This trade-off relation is well-established in the linear response regime, where the temperatures of the reservoirs are slightly different, and the tradeoff relation is fully characterized only by two parameters. However, it is known that those parameters can no longer characterize the tradeoff relation beyond the linear response regime, thus it is a difficult task to explore the trade-off relation there.
 Pareto front and efficiency bound are two promising approaches to investigate the tradeoff relation in the nonlinear regime. Focusing on small time-independent systems like quantum dots, we calculated the Pareto front numerically and confirm that both power and efficiency are enhanced by the nonlinearity. Furthermore, we developed a new efficiency bound taking into account higher-order fluctuations of power. According to our calculation in the quantum dots, our new bound turned out to be tighter than the conventional efficiency bound. We plan further investigate larger and complicated systems and extract universal properties of the tradeoff relation from them.
 In this presentation, I will show the aforementioned results on power and efficiency and discuss the trade-off relation in the nonlinear regime.
発表言語 日本語
発表者名 川﨑 彬斗
指導教員名 古澤 明 教授
発表題目(英語) Temporal mode function engineering of Non-Gaussian state
要旨(英語) In continuous variable quantum information processing with light, highly non-classical states called non-Gaussian states are essential to realize fault-tolerant and universal quantum computation. The standard method for generating non-Gaussian states is called "heralding state preparation". In this method, non-Gaussian state can be generated on one side of the two entangled modes by projective measurement of the other side.
On the other hand, for quantum computation in real experimental systems, it is important to properly define the temporal mode function that describes the shape of the wave packet of light that contains the quantum state. For example, a wave packet defined in finite time with no DC component has been proposed to be useful for large-scale high-speed quantum computation [1].There have been previous studies [2, 3] on the generation of non-Gaussian states defined by temporal mode function of simple shapes in heralding scheme, but there has been no method to freely shape an arbitrary temporal mode function. We propose that arbitrary temporal mode functions can be engineered by combining a broadband quantum entanglement source and appropriate frequency filtering in measurement system of the heralding scheme.
[1] J. Yoshikawa et al., APL Photonics 1, 060801 (2016).
[2] H. Ogawa et al., Phys. Rev. Lett. 116. 233602 (2016).
[3] S. Takeda et al., Phys. Rev. A 87, 043803 (2013).
発表言語 英語