The 8th QLC young colloquium
Date & Time : Monday, September 26, 2022. 13:30～15:10
TOKIMOTO, Jun Tokyo (Tokyo University of Science)
KITOU, Shunsuke (RIKEN)
WATANABE, Mori (Osaka University)
HU, Yajian (Kyoto University)
HAYASHIDA, Takeshi (University of Tokyo)
Place：online using “Zoom”
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Speaker：TOKIMOTO, Jun (Tokyo University of Science)
Title：Analysis of Photo-Excited States of the 2D Mott Insulator Using Randomized Singular Value Decomposition
The optical absorption spectrum of a two-dimensional Mott insulator differs significantly from that of a one-dimensional Mott insulator due to the lack of spin-charge separation, but the details of the origin of this difference are still unclear.
Therefore, we decompose the solution of the time-dependent Schrödinger equation into eigenmodes by using the Randomized singular value decomposition , and calculate the spin and charge correlations in each mode. This Randomized Singular Value Decomposition is a method that can decompose even matrices of a size that cannot be handled by conventional singular value decomposition methods by means of dimensionality compression, and can also automatically extract the important modes.
In this seminar, we will mainly present results for N=26 sites, V=0 electron-electron Coulomb interaction, and 0.1 < t/U < 0.01 (t = hopping, U = on-site Coulomb interaction). In this case, the dimension of the Hamiltonian is 1.7× ; however, we find that the optical absorption spectrum can be reproduced almost exactly by incorporating the contributions of about 3000 modes. Furthermore, by calculating spin and charge correlations, we find that the exciton-like states at the lower end of the band originate from spins, and that there are modes near the center of the band that retain the antiferromagnetic order to some extent. In this seminar, we will discuss these details.
Speaker：KITOU, Shunsuke (RIKEN）
Title：Observation of electron orbitals in strongly correlated electron systems using synchrotron radiation X-ray diffraction
As known as Neumann’s principle, the symmetry of the physical property is governed by the symmetry of the crystal. Therefore, the crystal structure is basic and essential information for understanding the physical properties. Recently, however, there are many substances whose unique quantum properties cannot be understood only from the symmetry of the crystal. To understand such systems, we have focused on the degrees of freedom of electron orbitals. The orbital degree of freedom, which corresponds to the spatial distribution of electrons, greatly affects physical properties such as transport and magnetic properties . Since the electron orbital is the smallest unit of “shape”, the orbital state in material can be understand by observing the spatial distribution of valence electrons. We have succeeded in direct observation of valence electron density distribution in crystalline materials by electron density analysis based on synchrotron radiation X-ray diffraction and core differential Fourier synthesis .
In this presentation, we will introduce the results of the electron density analysis in strongly correlated electron systems such as perovskite , spinel , and pyrochlore-lattice materials , as examples of 3d, 4d, and 4f orbital electron observations. Information such as crystal electric field, orbital hybridization, and relativistic spin-orbit interaction can be obtained from the valence electron density distribution.
 S. Kitou et al., Phys. Rev. Lett. 119, 065701 (2017).
 S. Kitou et al., Phys. Rev. Res. 2, 033503 (2020).
 T. Manjo, S. Kitou et al., Mater. Adv. 3, 3192 (2022).
 S. Kitou et al., submitted.
Speaker：WATANABE, Mori (Osaka University)
Title：Unique magnetoresistance and Hall effects in classical triangular antiferromagnet Ag2CrO2 thin films
Ag2CrO2, a triangular lattice antiferromagnet (TAFM) with S = 3/2 localized at the Cr site , has been gaining attraction in recent years. Below its magnetic transition temperature at TN = 24 K, some of the spin sites show a peculiar spin state, known as a partially disordered (PD) state, and act as effective free spins even under its magnetic transition temperature, making Ag2CrO2 an ideal stage to study spin liquid crystals. Moreover, it is one of the few TAFMs with high electrical conductivity. Although the bulk material is polycrystalline, recent research showed its crystallinity can be dramatically increased through the mechanical exfoliation technique similar to other van-der-Waals materials [2,3], making electrical transport measurements of high quality Ag2CrO2 possible.
The PD sites lead to unique electrical transport phenomena, such as large butterfly shaped hysteresis which only appears in the vicinity of TN . The result indicates that fluctuations of the PD spin state have a significant effect on the electrical transport properties of Ag2CrO2. In this presentation, we will report on electrical transport measurements of Ag2CrO2 thin films up to 8 T, in the temperature range from 5 to 44 K. The behavior of the magnetoresistance changes dramatically throughout this temperature range, and noticeably large magnetoresistance up to 80 % was observed owing to the high conductivity. We will also discuss Hall measurements where anomalous terms were present, possibly related to the higher order fluctuations of the spins .
 M. Watanabe et al., Appl. Phys. Lett. 117, 072403 (2020).
 M. Watanabe et al., AIP Adv. 11, 015005 (2021).
 H. Taniguchi et al., Sci. Rep. 10, 2525 (2020).
 J. Kondo, Prog. Theor. Phys. 27, 772 (1962).
Speaker：HU, Yajian (Kyoto University)
Title：Time-reversal symmetry breaking in charge density wave of CsV3Sb5 detected by polar Kerr effect
The Kagome lattice exhibits rich quantum phenomena owing to its unique geometric properties. Appealing realizations are the recently discovered Kagome metals AV3Sb5 (A = K, Rb, Cs), where unconventional charge density wave (CDW) is intertwined with superconductivity and non-trivial band topology. The CDW gap is strongly momentum-dependent, indicate that the CDW transition is triggered by Fermi surface nesting dominated by van Hove singularities . Moreover, some experiments, such as anomalous Hall effect and µSR, suggest the CDW is time-reversal symmetry-breaking (TRSB) [2,3]. Theories predict the CDW to be a rare occurrence of chiral CDW characterized by orbital loop current . However, key evidences of loop current, spontaneous TRSB and the coupling of its order parameter with the magnetic field remain elusive and contradictory.
Here, we investigate the CDW in CsV3Sb5 by magneto-optic polar Kerr effect with sub-microradian resolution. Under magnetic field, we observed a jump of the Kerr angle at the CDW transition. This jump is magnetic-field switchable and scales with field, indicating magneto–chirality coupling related to non-trivial band topology. At zero field, we found non-zero and field-trainable Kerr angle below TCDW, signaling spontaneous TRSB . Our results provide a crucial step to unveil quantum phenomena in correlated Kagome materials.
 Mielke et al. Nature 602, 245–250 (2022).
 Khasanov et al. Phys. Rev. Research 4, 023244 (2022).
 Denner et al. Phys. Rev. Lett. 127, 217601 (2021).
 Yajian Hu et al. arXiv:2208.08036 (2022).
Speaker：HAYASHIDA, Takeshi (University of Tokyo)
Title：Visualization of antiferromagnetic domains in Cr2O3 via nonreciprocal optical effects
The representative magnetoelectric antiferromagnet Cr2O3 exhibits two nonreciprocal optical effects, the electric-field-induced Faraday (E–induced Faraday) effect and spontaneous non-reciprocal reflection (NRR). E–induced Faraday is a rotation of a polarization plane of transmitted light induced in proportion to an applied electric field while NRR is a spontaneous rotation of a polarization plane of reflected light. Experimental observations of these effects were reported decades ago[1,2]. Because the rotation direction of the polarization plane is opposite between two distinct antiferromagnetic domains, spatially resolved measurements of these effects will visualize the antiferromagnetic domains. However, such a domain visualization has not been realized yet, most likely because these effects are small. A typical magnitude of the E–induced Faraday effect is the order of 10-5 [deg/V] while that of spontaneous NRR is the order of 10-2 [deg].
Here, we report the visualization of antiferromagnetic domains through spatial distribution measurements of the E–induced Faraday effect and spontaneous NRR, achieved by electric-field-modulation and polarization-modulation imaging techniques, respectively. In these methods, a large number of difference images are averaged, which makes it possible to obtain spatial distributions of small signals. The domain structures obtained via the two effects are well matched to each other, which complementarily supports their effectiveness for the domain observation.
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 Y. Uemura et al., Phys. Rev. Applied 11, 014046 (2019).
 T. Ishibashi et al., J. Appl. Phys. 100, 093903 (2006).
Committee Chair：Hiroshi WATANABE（Ritsumeikan University）,Hiroki WADATI（University of Hyogo）