arXiv subjects

Naoki Ogawa

Publications and source records attributed to Naoki Ogawa.

At least 19 recordsLinked to original sources

arXiv2026

Switchable Magnetoelectric Transport in Graphene via a Van der Waals Multiferroic

Electric and magnetic control of transport properties at atomic interfaces is central to the development of next generation electronics and spintronics. Van der Waals multiferroics materials that simultaneously host dielectric and magnetic orders down to the monolayer limit offer a promising platform for such interfacial control, yet the realization of electronic functionalities that exploit the unique attributes of van der Waals multiferroics has largely remained elusive. Here, we realize a van der Waals heterostructure comprising graphene and the multiferroic CuCrP2S6, enabling gate-switchable magnetoelectric transport in graphene, mediated by the multiferroic layer. The charge-neutrality resistance peak of graphene exhibits pronounced hysteresis arising from polarization flip in the multiferroic state. Application of an in-plane magnetic field shifts this peak in a polarization-dependent manner, revealing magnetic-field-induced polarization modulation a direct signature of the magnetoelectric effect. Furthermore, cooling the device under an applied electric field enables domain control of the multiferroic order, allowing reversible switching of the interfacial magnetoelectric transport. These results provide the first demonstration of interfacial magnetoelectric transport in a vdW heterostructure, and establish a pathway for engineering two-dimensional van der Waals interfaces for functional device applications.

cond-mat.mes-hall↗

arXiv2026

Bridging dS/CFT and Celestial Holography via Ward-Takahashi Identities

In 2507.17558, we provide a map from a scalar theory on $(D+2)$-dimensional Minkowski spacetime to a scalar theory with a continuous mass spectrum on $(D+1)$-dimensional de Sitter spacetime, and propose a link between celestial amplitudes and cosmological correlators (the cosmological-celestial dictionary). We extend the construction to fields with spin 1 and 2, and find that massless spin fields map to spin fields with continuous mass spectra. In this construction, we identify the de Sitter counterparts of the Nambu-Goldstone modes associated with the asymptotic symmetries in Minkowski spacetime. For $U(1)$ gauge theories, the counterpart is restricted to the massless sector within the continuous Proca spectrum, while for linearized gravity supertranslations are encoded in the partially massless sector and superrotations in the strictly massless sector. Using the identification, we reveal that the associated Ward-Takahashi identities of the cosmological correlators reproduce the conformally soft photon and graviton theorems via the cosmological-celestial dictionary. In particular, the celestial stress tensor is derived from the asymptotic limit of gravitons in de Sitter spacetime.

hep-th↗

arXiv2025

Celestial Holography meets dS/CFT

We provide a concrete link between celestial amplitudes and cosmological correlators. We first construct a map from quantum field theories (QFTs) in $(D+2)$-dimensional Euclidean space to theories on the $(D+1)$-dimensional sphere, through a Weyl rescaling and a Fourier transformation. An analytic continuation extends this map to a relation between QFTs in Minkowski spacetime $\text{M}_{D+2}$ and in de Sitter spacetime $\text{dS}_{D+1}$ with the Bunch-Davies vacuum. Combining this relation with celestial holography, we show that the extrapolated operators in de Sitter space can be represented by operators on the celestial sphere $S^{D}$. Our framework offers a systematic route to transfer computational techniques and physical insights between celestial holography and the dS/CFT correspondence.

hep-th↗

arXiv2025

Harmonic and Subharmonic Magnon Generation in a Surface Acoustic Wave Resonator

We experimentally observe the generation of magnon harmonics and subharmonics in an on-chip surface acoustic wave resonator incorporating a thin Co$_{20}$Fe$_{60}$B$_{20}$ film, using micro-focused Brillouin light scattering. In our devices, rotating the in-plane magnetic field allows continuous tuning of the magnon-phonon coupling from weak to strong within the same resonator. In the weak coupling regime, we only observe fundamental magnetoelastic wave signal at $f_{1}$. Conversely, in the strong coupling regime, in addition to the fundamental magnetoelastic wave, we observe subharmonic and harmonic signals at $3/2f_{1}$, $2f_{1}$, and $3f_{1}$, which are well reproduced by our analytical model. Our results establish phonons as a means to generate and control nonlinear magnons in the strong coupling regime, providing a new route for magnonic signal processing.

cond-mat.mes-hall↗

arXiv2025

Flat Space Holography via AdS/BCFT

In this paper, we study a new class of AdS/BCFT setups, where the world-volumes of end-of-the-world branes (EOW branes) are given by flat spaces, to explore flat space holography from an AdS bulk. We show that they provide gravity duals of CFTs in the presence of null boundaries. Our holographic calculations lead to many new predictions on entanglement entropy, correlation functions and partition functions for CFTs with null boundaries. By considering a bulk region between two EOW branes, we present an AdS/BCFT explanation that the flat space gravity is dual to a Carrollian CFT (CCFT), including the swing surface calculation of entanglement entropy.

hep-th↗

arXiv2025

Quantum geometry in low-energy linear and nonlinear optical responses of magnetic Rashba semiconductor (Ge,Mn)Te

Quantum geometry appears as a key factor in understanding the optical properties of quantum materials, with the anticipation on diverging or quantized responses near the Dirac and Weyl points. Here we investigate linear and nonlinear optical responses -- optical conductivity and injection current -- in a magnetic Rashba semiconductor in the mid-infrared region, with varying the Fermi energy across the Dirac point. We reveal that the linear optical conductivity reflects quantum metric, which remains finite irrespective of the diminishing joint density-of-states at lower photon energy. It is also confirmed that the magnetic injection current enhances depending on the energy of the Fermi level relative to the Dirac point. These optical spectra are nicely reproduced by our theoretical calculations with geometrical effects taken into account.

cond-mat.mtrl-sci↗

arXiv2025

All-optical control of antiferromagnetic domains via an inverse optical magnetoelectric effect

Antiferromagnets are a promising platform for next-generation spintronics due to their ultrafast spin dynamics and robustness to external fields. All-optical control of antiferromagnetic order is essential to fully exploit their potential in energy-efficient and high-speed spintronic and memory applications. However, optical writing of antiferromagnetic domains remains a fundamental challenge, as conventional magneto-optical techniques rely on net magnetization, which is absent in antiferromagnets. In certain multiferroic antiferromagnets, the magnetic toroidal moment provides an additional degree of freedom through its inherent magnetoelectric coupling. This coupling at higher frequencies results in the optical magnetoelectric effect (OME), which manifests as a directional asymmetry in light propagation and enables optical probing of antiferromagnetic states. Here, we demonstrate all-optical writing of antiferromagnetic domains using the inverse optical magnetoelectric effect (IOME) in ferrotoroidic LiNiPO4. The writing process is nonvolatile, non-thermal, and deterministic, driven solely by reversing the light propagation direction. This directional control arises from a strong coupling between the photon linear momentum and the magnetic toroidal moment, enabling the repeatable switching between time-reversed domains with arbitrary light polarization. Our findings establish IOME as a distinct mechanism for manipulating antiferromagnetic order, opening a new paradigm in opto-magnetism driven by photon momentum.

cond-mat.mtrl-sci↗

arXiv2025

Probing de Sitter Space Using CFT States

In this paper we construct CFT states describing a putative holographic dual to local excitations in the three-dimensional de Sitter space (dS), called the bulk local states. We find that the conjugation operation in dS$_3/$CFT$_2$ is notably different from that in AdS$_3/$CFT$_2$. This requires us to combine two bulk local states constructed out of different primary states in a CPT-invariant way. This analysis explains why Green's functions in the dS Euclidean vacuum cannot simply be obtained from the Wick rotation of those in AdS. We also argue that this characteristic feature explains the emergence of a time coordinate from the dual Euclidean CFT. We show that the information metric for the quantum estimation of bulk coordinate values replicates the de Sitter space metric.

hep-th↗

arXiv2024

Emergence of ferroelectric topological insulator as verified by quantum Hall effect of surface states in (Sn,Pb,In)Te films

Emergent phenomena arising from nontrivial band structures based on topology and symmetry have been attracting keen interest in contemporary condensed-matter physics. Materials such as SnTe and PbTe are one such example, which demonstrate a topological phase transition while showing ferroelectric instability derived from their rock-salt structure. The ferroelectricity can lift the valley degeneracy, enabling the emergence of the Z2 topological insulator phase, although its observation in transport phenomena remains elusive. Here, we report magnetotransport properties of ferroelectric (Sn,Pb)Te thin films with finely-controlled Fermi levels via In doping. We identified the ferroelectric topological insulator phase from the observations of the quantum Hall states with filling factors of $ν$ = 1, 2 and 3 with both spin- and valley-degeneracy lifting. The electronic states are two-dimensional, indicating the ferroelectricity-induced topological surface states with a single Dirac cone. The finding of the new topological state with ferroelectricity will further expand the field of topological physics and advance the development of functional properties, such as topological nonlinear photonics and nonreciprocal transport with memory effect.

cond-mat.mtrl-sci↗

arXiv2024

Superconducting diode effect under time reversal symmetry

In noncentrosymmetric superconductors, superconducting and normal conductions can interchange based on the current flow direction. This effect is termed a superconducting diode effect (SDE), which is a focal point of recent research. The broken inversion and time reversal symmetry is believed to be the requirements of SDE but their intrinsic role has remained elusive. Here, we report strain-controlled SDEs in a layered trigonal superconductor, PbTaSe2. The SDE was found exclusively in a strained device with its absence in an unstrained device, despite that it is allowed in unstrained trigonal structure. Moreover, the zero-field or magnetic field-even (magnetic field-odd) SDE is observed when the strain and current are along armchair (zigzag) direction The results unambiguously demonstrate the intrinsic SDE under time-reversal symmetry and the critical role of strain-induced electric polarization.

cond-mat.supr-con↗

arXiv2024

Odd-parity multipole order in the spin-orbit coupled metallic pyrochlore Pb$_2$Re$_2$O$_{7-δ}$

The pyrochlore oxide Pb2Re2O7-δ (PRO) is a candidate spin-orbit-coupled metal (SOCM) that exhibits a structural phase transition with inversion symmetry breaking. In this study, we report the results of detailed X-ray diffraction (XRD) measurements on single crystals of PRO to clarify the crystal structure below the phase transition temperature at Ts = 300 K. In the XRD patterns, a clear peak splitting is observed below Ts, indicating a cubic to tetragonal transition. Based on the group-subgroup relationship and the observed reflection conditions, the space group of the low-temperature phase is proposed to be I4122, which agrees with optical second harmonic generation measurements. This space group is the same as that of the lowest temperature structure of the analogous SOCM Cd2Re2O7 (CRO), which is realized by the emergence of odd-parity multipole order. The comparison between PRO and CRO allows for advancing our understanding on the symmetry-lowering complex order exhibited by SOCMs.

cond-mat.str-el↗

arXiv2024

Celestial CFT from $H_3^+$-WZW Model

Recently, there has been a growing interest in celestial holography, which is holography in asymptotically flat spacetimes. This holographic duality exhibits numerous mysterious and fruitful features, particularly on the dual CFT side. In this paper, we present the candidate of dual CFT for Minkowski spacetime extracted from $SL(2,\mathbb{C})/SU(2)\cong H^+_3$ Wess-Zumino-Witten (WZW) model, the simplest non-compact CFT. We demonstrate that it reproduces the well-known principal series and correlation functions dual to the bulk scattering amplitudes.

hep-th↗

arXiv2024

Electric-field-enhanced second-harmonic domain contrast and nonreciprocity in a van der Waals antiferromagnet

Imaging antiferromagnetic 180° domains with actively controlled visibility is vital for both fundamental science and sophisticated applications. While optical second-harmonic generation (SHG) is a well-known technique for distinguishing such domains in non-centrosymmetric antiferromagnets, a general material-based strategy to control domain contrast remains elusive. Using van der Waals antiferromagnet MnPS$_3$ as a proof of concept, we demonstrate the tuning of nonreciprocity-induced domain contrast in SHG through applying an in-plane electric field that transforms the magnetic point group to its unitary subgroup. The interference among intrinsic electric-dipole, magnetic-dipole, and field-induced electric-dipole transitions, each carrying distinct characters under space-inversion ($\mathcal{P}$) and time-reversal ($\mathcal{T}$) operations, enables large tuning of domain contrast and nonreciprocity in a broad spectral range. This strategy, generically applicable to systems characterized by $\mathcal{PT}$-symmetric magnetic groups with a polar unitary subgroup, offers a path to fast electrical modulation of nonlinear nonreciprocal photonic behaviors using antiferromagnets.

cond-mat.str-el↗

arXiv2023

Celestial two-point functions and rectified dictionary

A naive celestial dictionary causes massless two-point functions to take the delta-function forms in the celestial conformal field theory (CCFT). We rectify the dictionary, involving the shadow transformation so that the two-point functions follow the standard power-law. In this new definition, we can smoothly take the massless limit of the massive dictionary. We also compute a three-point function using the new dictionary and discuss the OPE in CCFT.

hep-th↗

arXiv2023

Emergent zero-field anomalous Hall effect in a reconstructedrutileantiferromagnetic metal

Anomalous Hall effect (AHE) emerged in antiferromagnetic metals shows intriguing physics and application potential. In contrast to certain noncollinear antiferromagnets, rutile RuO$_2$ has been proposed recently to exhibit a crystal-assisted AHE with collinear antiferromagnetism. However, in RuO$_2$, the on-site magnetic moment accompanying itinerant 4d electrons is quite small, and more importantly, the AHE at zero external field is prohibited by symmetry because of the high-symmetry [001] direction of the Néel vector. Here, we show the AHE at zero field in the collinear antiferromagnet, Cr-doped RuO$_2$. The appropriate doping of Cr at Ru sites results in a rotation of the Néel vector from [001] to [110] and enhancement of the on-site magnetic moment by one order of magnitude while maintaining a metallic state with the collinear antiferromagnetism. The AHE with vanishing net moment in the Ru$_{0.8}$Cr$_{0.2}$O$_2$ exhibits an orientation dependence consistent with the [110]-oriented Néel vector. These results open a new avenue to manipulate AHE in antiferromagnetic metals.

cond-mat.mtrl-sci↗

arXiv2023

Exciton-magnon splitting in van der Waals antiferromagnet MnPS$_3$ unveiled by second-harmonic generation

Ultrafast and coherent generation of magnons is of great significance for high-speed antiferromagnetic spintronics. One possible route is by exciton-magnon pairwise optical excitation. To date, such exciton-magnon transitions have been studied mostly by linear optical means in a limited number of conventional three-dimensional antiferromagnets. Here we investigate this correlated transition in van der Waals antiferromagnet MnPS$_3$ by using resonant second-harmonic generation spectroscopy, a nonlinear optical probe sensitive to the symmetry of electronic and magnetic excitations. Two exciton-magnon peaks are observed, in line with the exciton-induced splitting of magnon density of states predicted by a Koster-Slater type theory, indicating significant exciton-magnon interactions. In addition, a large linear magnetoelectric effect of excitons is observed. These findings provide renewed understandings on the correlation effects in two-dimensional magnets with enhanced quasiparticle scattering cross-sections, and point to potential coherent control among charge, spin and orbital degrees of freedom in two-dimensional magnets by optical means.

cond-mat.str-el↗

arXiv2023

Soft hair, dressed coordinates and information loss paradox

Understanding the dynamics of soft hair might shine some light on the information loss paradox. In this paper, we introduce a new coordinate system, dressed coordinates, in order to analyze the quantum states of Hawking radiation as a first step toward understanding the connection between soft hair and the information paradox. Dressed coordinates can be introduced by an operator-dependent coordinate transformation that makes the soft hair degrees of freedom apparently disappear from the metric. We show that some results of previous studies, such as the angle-dependent Hawking temperature and the soft graviton theorem, can be easily reproduced using these dressed coordinates. Finally, we discuss future possible applications of dressed coordinates toward understanding the information loss paradox.

hep-th↗

arXiv2022

Wedge Holography in Flat Space and Celestial Holography

In this paper, we study codimension two holography in flat spacetimes, based on the idea of the wedge holography. We propose that a region in a $d+1$ dimensional flat spacetime surrounded by two end of the world-branes, which are given by $d$ dimensional hyperbolic spaces, is dual to a conformal field theory (CFT) on a $d-1$ dimensional sphere. Similarly, we also propose that a $d+1$ dimensional region in the flat spacetime bounded by two $d$ dimensional de Sitter spaces is holographically dual to a CFT on a $d-1$ dimensional sphere. Our calculations of the partition function, holographic entanglement entropy and two point functions, support these duality relations and imply that such CFTs are non-unitary. Finally, we glue these two dualities along null surfaces to realize a codimension two holography for a full Minkowski spacetime and discuss a possible connection to the celestial holography.

hep-th↗