arXiv subjects

T. Kanda

Publications and source records attributed to T. Kanda.

7 recordsLinked to original sources

arXiv2025

High-field NMR study of field-induced states in Pb(TiO)Cu$_4$(PO$_4$)$_4$

The square cupola antiferromagnet Pb(TiO)Cu$_4$(PO$_4$)$_4$ exhibits the intriguing magnetoelectric responses arising from the consecutive change in the magnetic quadrupolar-type configuration of magnetic moments under external magnetic fields higher than 15 T. To clarify the high-field magnetic structures in Pb(TiO)Cu$_4$(PO$_4$)$_4$, an NMR measurement was performed in pulsed fields up to 32.2 T significantly extending the field range accessible by superconducting magnets. The double-peak structure of NMR spectra emerging above 29 T applied along the [001] direction evidences the successive magnetic transitions. The field dependence of NMR spectra was analyzed on the basis of cluster mean-field theory, which allows us to propose possible magnetic structures for the high-field magnetic states.

cond-mat.str-el↗

arXiv2025

Interface-induced collective phase transition in VO2-based bilayers studied by layer selective spectroscopy

We investigated the origin of collective electronic phase transitions induced at the heterointerface between monoclinic insulating VO2 and rutile metallic electron-doped VO2 layers using in situ soft x-ray photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS) on nanoscale VO2/V0.99W0.01O2 (001)R bilayers. Thanks to the surface sensitivity of PES and XAS, we determined the changes in the electronic structure and V-V dimerization in each constituent layer separately. The layer selective observation of the electronic and crystal structures in the upper VO2 layer of the bilayer indicates that the monoclinic insulating phase VO2 layer undergoes a transition to the rutile metallic phase by forming the heterointerface. Detailed temperature-dependent measurements reveal that the rutile metallic phase VO2 undergoes a transition to the monoclinic insulating phase with a decrease in temperature, as in the case of a VO2 single-layer film. Furthermore, during the temperature-induced phase transition in the VO2 layer, the spectra are well described by an in-plane phase separation of the rutile metallic and monoclinic insulating phases. These results suggest that the interface-induced transition from the monoclinic insulating to the rutile metallic phase in the VO2 layer of bilayers occurs as a collective phase transition derived from the static energy balance between the interfacial energy and the bulk free energies of the constituent layers.

cond-mat.str-el↗

arXiv2022

Electronic phase diagram of Cr-doped VO2 epitaxial films studied by in situ photoemission spectroscopy

Through in situ photoemission spectroscopy (PES), we investigated the changes in the electronic structure of Cr-doped VO2 films coherently grown on TiO2 (001) substrates. The electronic phase diagram of CrxV1-xO2 is drawn by a combination of electric and spectroscopic measurements. The phase diagram is similar to that of bulk CrxV1-xO2, while the temperature of metal-insulator transition (TMIT) is significantly suppressed by the epitaxial strain effect. In the range of x = 0-0.04, where TMIT remains unchanged as a function of x, the PES spectra show dramatic change across TMIT, demonstrating the characteristic spectral changes associated with the Peierls phenomenon. In contrast, for x > 0.04, the TMIT linearly increases, and the metal-insulator transition (MIT) may disappear at x = 0.08-0.12. The PES spectra at x = 0.08 exhibit pseudogap behavior near the Fermi level, whereas the characteristic temperature-induced change remains almost intact, suggesting the existence of local V-V dimerization. The suppression of V-V dimerization with increasing x was confirmed by polarization-dependent x-ray absorption spectroscopy. These spectroscopic investigations reveal that the energy gap and V 3d states are essentially unchanged with 0 $\le$ x $\le$ 0.08 despite the suppression of V-V dimerization. The invariance of the energy gap with respect to x suggests that the MIT in CrxV1-xO2 arises primarily from the strong electron correlations, namely the Peierls-assisted Mott transition. Meanwhile, the pseudogap at x = 0.08 eventually evolves to a full gap (Mott gap) at x = 0.12, which is consistent with the disappearance of the temperature-dependent MIT in the electronic phase diagram. These results demonstrate that a Mott insulating phase without V-V dimerization is stabilized at x > 0.08 as a result of the superiority of Mott instability over the Peierls one.

cond-mat.str-el↗

arXiv2021

Respiratory Rate Estimation Based on WiFi Frame Capture

This paper presents a method that estimates the respiratory rate based on the frame capturing of wireless local area networks. The method uses beamforming feedback matrices (BFMs) contained in the captured frames, which is a rotation matrix of channel state information (CSI). BFMs are transmitted unencrypted and easily obtained using frame capturing, requiring no specific firmware or WiFi chipsets, unlike the methods that use CSI. Such properties of BFMs allow us to apply frame capturing to various sensing tasks, e.g., vital sensing. In the proposed method, principal component analysis is applied to BFMs to isolate the effect of the chest movement of the subject, and then, discrete Fourier transform is performed to extract respiratory rates in a frequency domain. Experimental evaluation results confirm that the frame-capture-based respiratory rate estimation can achieve estimation error lower than 3.2 breaths/minute.

eess.SP↗

arXiv2021

ACK-Less Rate Adaptation for IEEE 802.11bc Enhanced Broadcast Services Using Sim-to-Real Deep Reinforcement Learning

In IEEE 802.11bc, the broadcast mode on wireless local area networks (WLANs), data rate control that is based on acknowledgement (ACK) mechanism similar to the one in the current IEEE 802.11 WLANs is not applicable because ACK mechanism is not implemented. This paper addresses this challenge by proposing ACK-less data rate adaptation methods by capturing non-broadcast uplink frames of STAs. In IEEE 802.11bc, an use case is assumed, where a part of STAs in the broadcast recipients is also associated with non-broadcast APs, and such STAs periodically transmit uplink frames including ACK frames. The proposed method is based on the idea that by overhearing such uplink frames, the broadcast AP surveys channel conditions at partial STAs, thereby setting appropriate data rates for the STAs. Furthermore, in order to avoid reception failures in a large portion of STAs, this paper proposes deep reinforcement learning (DRL)-based data rate adaptation framework that uses a sim-to-real approach. Therein, information of reception success/failure at broadcast recipient STAs, that could not be notified to the broadcast AP in real deployments, are made available by simulations beforehand, thereby forming data rate adaptation strategies. Numerical results show that utilizing overheard uplink frames of recipients makes it feasible to manage data rates in ACK-less broadcast WLANs, and using the sim-to-real DRL framework can decrease reception failures.

cs.NI↗

arXiv2020

Thickness dependence of electronic and crystal structures in VO$_2$ ultrathin films: suppression of the collaborative Mott-Peierls transition

Through ${in~situ}$ photoemission spectroscopy, we investigated the change in the electronic and crystal structures of dimensionality-controlled VO$_2$ films coherently grown on TiO$_2$(001) substrates. In the nanostructured films, the balance between the instabilities of a bandlike Peierls transition and a Mott transition is controlled as a function of thickness. The characteristic spectral change associated with temperature-driven metal-insulator transition in VO$_2$ thick films holds down to 1.5 nm (roughly corresponding to five V atoms along the [001] direction), whereas VO$_2$ films of less than 1.0 nm exhibit insulating nature without V-V dimerization. These results suggest that the delicate balance between a Mott instability and a bandlike Peierls instability is modulated at a scale of a few nanometers by the dimensional crossover effects and confinement effects, which consequently induce the complicated electronic phase diagram of ultrathin VO$_2$ films.

cond-mat.str-el↗

arXiv2016

KMS States of Weakly Coupled Anharmonic Crystals and the Resolvent CCR Algebra

We consider equilibrium states of weakly coupled anharmonic quantum oscillators on Z. We consider the Resolvent CCR Algebra introduced by D.Buchholtz and H.Grundling, and we show that the infinite volume limit of equilibrium states satisfies the KMS (Kubo-Martin-Schwinger) condition with good regularity(= locally Fock representation). Uniqueness of the KMS states is proven as well.

math-ph↗