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Eiji Saitoh

Publications and source records attributed to Eiji Saitoh.

At least 37 records · Page 2Linked to original sources
arXiv2023

Cryogenic spin Peltier effect detected by a RuO$_2$-AlO$_x$ on-chip microthermometer

We report electric detection of the spin Peltier effect (SPE) in a bilayer consisting of a Pt film and a Y$_{3}$Fe$_5$O$_{12}$ (YIG) single crystal at the cryogenic temperature $T$ as low as 2 K based on a RuO$_2$$-$AlO$_x$ on-chip thermometer film. By means of a reactive co-sputtering technique, we successfully fabricated RuO$_2$$-$AlO$_x$ films having a large temperature coefficient of resistance (TCR) of $\sim 100\% ~\textrm{K}^{-1}$ at around $2~\textrm{K}$. By using the RuO$_2$$-$AlO$_x$ film as an on-chip temperature sensor for a Pt/YIG device, we observe a SPE-induced temperature change on the order of sub-$μ\textrm{K}$, the sign of which is reversed with respect to the external magnetic field $B$ direction. We found that the SPE signal gradually decreases and converges to zero by increasing $B$ up to $10~\textrm{T}$. The result is attributed to the suppression of magnon excitations due to the Zeeman-gap opening in the magnon dispersion of YIG, whose energy much exceeds the thermal energy at 2 K.

cond-mat.mes-hall↗
arXiv2023

Holographic Dual of Quantum Spin Chain as Chern-Simons-Scalar Theory

We construct a holographic dual theory of one-dimensional anisotropic Heisenberg spin chain, which includes two Chern-Simons gauge fields and a charged scalar field. Thermodynamic quantities of the spin chain at low temperatures, which are exactly calculated from the integrability, are completely reproduced by the dual theory on three-dimensional black hole backgrounds and the exact matching of the parameters between the dual theory and the spin chain is obtained. The holographic dual theory provides a new theoretical framework to analyze the quantum spin chain and one-dimensional quantum many-body systems.

hep-th↗
arXiv2023

Spin and spin current -- From fundamentals to recent progress

Along with the progress of spin science and spintronics research, the flow of electron spins, (i.e. spin current), has attracted interest. New phenomena and electronic states were explained in succession using the concept of spin current. Moreover, as many of the conventionally known spintronics phenomena became well organized based on spin current, it has rapidly been recognized as an essential concept in a wide range of condensed matter physics. In this article, we focus on recent developments in the physics of spin, spin current, and their related phenomena, where the conversion between spin angular momentum and different forms of angular momentum plays an essential role. Starting with an introduction to spin current, we first discuss the recent progress in spintronic phenomena driven by spin-exchange coupling: spin pumping, topological Hall torque, and emergent inductor. We, then, extend our discussion to the interaction/interconversion of spins with heat, lattice vibrations, and charge current and address recent progress and perspectives on the spin Seebeck and Peltier effects. Next, we review the interaction between mechanical motion and electron/nuclear spins and argue the difference between the Barnett field and rotational Doppler effect. We show that the Barnett effect reveals the angular momentum compensation temperature, at which the net angular momentum is quenched in ferrimagnets.

cond-mat.mtrl-sci↗
arXiv2023

Quantum Circuit Distillation and Compression

Quantum coherence in a qubit is vulnerable to environmental noise. When long quantum calculation is run on a quantum processor without error correction, the noise often causes fatal errors and messes up the calculation. Here, we propose quantum-circuit distillation to generate quantum circuits that are short but have enough functions to produce an output almost identical to that of the original circuits. The distilled circuits are less sensitive to the noise and can complete calculation before the quantum coherence is broken in the qubits. We created a quantum-circuit distillator by building a reinforcement learning model, and applied it to the inverse quantum Fourier transform (IQFT) and Shor's quantum prime factorization. The obtained distilled circuit allows correct calculation on IBM-Quantum processors. By working with the quantum-circuit distillator, we also found a general rule to generate quantum circuits approximating the general $n$-qubit IQFTs. The quantum-circuit distillator offers a new approach to improve performance of noisy quantum processors.

quant-ph↗
arXiv2023

Electrical detection of parallel parametric amplification and attenuation in $\mathrm{Y}_3\mathrm{Fe}_5\mathrm{O}_{12}$/$\mathrm{Pt}$ bilayer disk

We report a systematic quantitative evaluation of parametric amplification gain of magnetization dynamics in ytirrium iron garnet ($\mathrm{Y}_3\mathrm{Fe}_5\mathrm{O}_{12}$) thin disk via a.c. spin pumping and inverse spin Hall effect. We demonstrate its signature phase-dependence where amplification and attenuation occur every $\fracπ{2}$ phase shift of the input signal. The results also show the pump-power dependence of the gain that is explained well by our theoretical model. Finally, the optimal conditions for the amplification is investigated by measuring the magnetic field dependence, where we find the highest gain of 11.4 dB.

physics.app-ph↗
arXiv2023

Detection of temporal fluctuation in superconducting qubits for quantum error mitigation

We have investigated instability of a superconducting quantum computer by continuously monitoring the qubit output. We found that qubits exhibit a step-like change in the error rates. This change is repeatedly observed, and each step persists for several minutes. By analyzing the correlation between the increased errors and anomalous variance of the output, we demonstrate quantum error mitigation based on post-selection. Numerical analysis on the proposed method was also conducted.

quant-ph↗
arXiv2023

Switching of magnon parametric oscillation by magnetic field direction

Parametric oscillation occurs when the resonance frequency of an oscillator is periodically modulated. Owing to time-reversal symmetry breaking in magnets, nonreciprocal magnons can be parametrically excited when spatial-inversion symmetry breaking is provided. This means that magnons with opposite propagation directions have different amplitudes. Here we demonstrate switching on and off the magnon parametric oscillation by reversing the external field direction applied to a Y$_3$Fe$_5$O$_{12}$ micro-structured film. The result originates from the nonreciprocity of surface mode magnons, leading to field-direction dependence of the magnon accumulation under a nonuniform microwave pumping. Our numerical calculation well reproduces the experimental result.

cond-mat.mes-hall↗
arXiv2023

Ginzburg-Landau action and polarization current in an excitonic insulator model of electronic ferroelectricity

In comparison to transport of spin polarization in ferromagnets, transport of electric polarization in ferroelectrics remains less explored. Taking an excitonic insulator model of electronic ferroelectricity as a prototypical example, we theoretically investigate the low-energy dynamics and transport of electric polarization by microscopically constructing the Ginzburg-Landau action. We show that, because of the scalar nature of the excitonic order parameter, only the longitudinal fluctuations are relevant to the transport of electric polarization. We also formulate the electric polarization diffusion equation, in which the electric-polarization current is defined purely electronically without recourse to the lattice degrees of freedom.

cond-mat.str-el↗
arXiv2023

Mechanism of electrical switching of ultra-thin CoO/Pt bilayers

We study current-induced switching of the Néel vector in CoO/Pt bilayers to understand the underlaying antiferromagnetic switching mechanism. Surprisingly, we find that for ultra-thin CoO/Pt bilayers electrical pulses along the same path can lead to an increase or decrease of the spin Hall magnetoresistance signal, depending on the current density of the pulse. By comparing the results of these electrical measurements to XMLD-PEEM imaging of the antiferromagnetic domain structure before and after the application of current pulses, we reveal the reorientation of the Néel vector in ultra-thin CoO(4 nm). This allows us to determine that even opposite resistance changes can result from a thermomagnetoelastic switching mechanism. Importantly, our spatially resolved imaging shows that regions where the current pulses are applied and regions further away exhibit different switched spin structures, which can be explained by a spin-orbit torque based switching mechanism that can dominate in very thin films.

cond-mat.mtrl-sci↗
arXiv2023

Laser-induced Creation of antiferromagnetic 180-degree domains in NiO/Pt bilayers

We demonstrate how the antiferromagnetic order in heterostructures of NiO/Pt thin films can be modified by optical pulses. We irradiate our samples with laser light and identify an optically induced creation of antiferromagnetic domains by imaging the created domain structure utilizing the X-ray magnetic linear dichroism effect. We study the effect of different laser polarizations on the domain formation and identify a polarization-independent creation of 180° domain walls and domains with 180° different Néel vector orientation. By varying the irradiation parameters, we determine the switching mechanism to be thermally induced and demonstrate the reversibility. We thus demonstrate experimentally the possibility to optically create antiferromagnetic domains, an important step towards future functionalization of all optical switching mechanisms in antiferromagnets.

cond-mat.mtrl-sci↗
arXiv2023

Spin Seebeck Effect: Sensitive Probe for Elementary Excitation, Spin Correlation, Transport, Magnetic Order, and Domains in Solids

The spin Seebeck effect (SSE) refers to the generation of a spin current as a result of a temperature gradient in a magnetic material, which can be detected electrically via the inverse spin Hall effect in a metallic contact. Since the discovery of SSE in 2008, intensive studies on SSE have been conducted to elucidate its origin. SSEs appear in a wide range of magnetic materials including ferro-, ferri-, and antiferro-magnets and also paramagnets with classical or quantum spin fluctuation. SSE voltage reflects fundamental properties of a magnet, such as elementary excitation, static magnetic order, spin correlation, and spin transport. In this article, we review recent progress on SSEs in various systems, with particular emphasis on its emerging role as a probe of these magnetic properties in solids. We also briefly discuss the recently-discovered nuclear SSE.

cond-mat.mtrl-sci↗
arXiv2023

Mechanism of paramagnetic spin Seebeck effect

We have theoretically investigated the spin Seebeck effect (SSE) in a normal metal (NM)/paramagnetic insulator (PI) bilayer system. Through a linear response approach, we calculated the thermal spin pumping from PI to NM and backflow spin current from NM to PI, where the spin-flip scattering via the interfacial exchange coupling between conduction-electron spin in NM and localized spin in PI is taken into account. We found a finite spin current appears at the interface under the difference in the effective temperatures between spins in NM and PI, and its intensity increases by increasing the density of the localized spin $S$. Our model well reproduces the magnetic-field-induced reduction of the paramagnetic SSE in Pt/Gd$_3$Ga$_5$O$_{12}$ experimentally observed when the Zeeman energy is comparable to the thermal energy, which can be interpreted as the suppression of the interfacial spin-flip scattering. The present finding provides an insight into the mechanism of paramagnetic SSEs and the thermally induced spin-current generation in magnetic materials.

cond-mat.mes-hall↗
arXiv2022

Composition-tunable magnon-polaron anomalies in spin Seebeck effects in epitaxial Bi$_x$Y$_{3-x}$Fe$_{5}$O$_{12}$ films

We have investigated hybridized magnon-phonon excitation (magnon polarons) in spin Seebeck effects (SSEs) in Bi$_x$Y$_{3-x}$Fe$_{5}$O$_{12}$ (Bi$_x$Y$_{3-x}$IG; $x=0$, $0.5$, and $0.9$) films with Pt contact. We observed sharp peak structures in the magnetic field $H$ dependence of the longitudinal SSE (LSSE) voltages, which appear when the phonon dispersions are tangential to the magnon dispersion curve in Bi$_x$Y$_{3-x}$IG. By increasing the Bi amount $x$, the peak fields in the LSSE shift toward lower $H$ values due to the reduction of the sound velocities in Bi$_x$Y$_{3-x}$IG. We also measured the SSE in a nonlocal configuration and found that magnon-polaron anomalies appear with different signs and intensities. Our result shows composition-tunability of magnon-polaron anomalies and provides a clue to further unravel the physics of magnon-polaron SSEs.

cond-mat.mtrl-sci↗
arXiv2022

Nonlinear magnon polaritons

We experimentally and theoretically demonstrate that nonlinear spin-wave interactions suppress the hybrid magnon-photon quasiparticle or "magnon polariton" in microwave spectra of an yttrium iron garnet film detected by an on-chip split-ring resonator. We observe a strong coupling between the Kittel and microwave cavity modes in terms of an avoided crossing as a function of magnetic fields at low microwave input powers, but a complete closing of the gap at high powers. The experimental results are well explained by a theoretical model including the three-magnon decay of the Kittel magnon into spin waves. The gap closure originates from the saturation of the ferromagnetic resonance above the Suhl instability threshold by a coherent back reaction from the spin waves.

cond-mat.mes-hall↗
arXiv2022

Identifiying the domain wall spin structure in current-induced switching of antiferromagnetic NiO/Pt

The understanding of antiferromagnetic domain walls, which are the interface between domains with different Néel order orientations, is a crucial aspect to enable the use of antiferromagnetic materials as active elements in future spintronic devices. In this work, we demonstrate that in antiferromagnetic NiO/Pt bilayers circular domain structures can be generated by switching driven by electrical current pulses. The generated domains are T-domains, separated from each other by a domain wall whose spins are pointing toward the average direction of the two T-domains rather than the common axis of the two planes. Interestingly, this direction is the same for the whole circular domain indicating the absence of strong Lifshitz invariants. The domain wall can be micromagnetically modeled by strain distributions in the NiO thin film induced by the MgO substrate, deviating from the bulk anisotropy. From our measurements we determine the domain wall width to have a full width at half maximum of $Δ= 98 \pm 10$ nm, demonstrating strong confinement.

cond-mat.mtrl-sci↗
arXiv2022

Strain-induced Shape Anisotropy in Antiferromagnetic Structures

We demonstrate how shape-induced strain can be used to control antiferromagnetic order in NiO/Pt thin films. For rectangular elements patterned along the easy and hard magnetocrystalline anisotropy axes of our film, we observe different domain structures and we identify magnetoelastic interactions that are distinct for different domain configurations. We reproduce the experimental observations by modeling the magnetoelastic interactions, considering spontaneous strain induced by the domain configuration, as well as elastic strain due to the substrate and the shape of the patterns. This allows us to demonstrate and explain how the variation of the aspect ratio of rectangular elements can be used to control the antiferromagnetic ground state domain configuration. Shape-dependent strain does not only need to be considered in the design of antiferromagnetic devices, but can potentially be used to tailor their properties, providing an additional handle to control antiferromagnets.

cond-mat.mtrl-sci↗
arXiv2022

Deciphering quantum fingerprints in electric conductance

When the electric conductance of a nano-sized metal is measured at low temperatures, it often exhibits complex but reproducible patterns as a function of external magnetic fields, called quantum fingerprints in electric conductance. Such complex patterns are due to quantum-mechanical interference of conduction electrons; when thermal disturbance is feeble and coherence of the electrons extends all over the sample, the quantum interference pattern reflects microscopic structures, such as crystalline defects and the shape of the sample, giving rise to complicated interference. Although the interference pattern carries such microscopic information, it looks so random that it has not been analysed. Here we show that machine learning allows us to decipher quantum fingerprints; fingerprint patterns in magneto-conductance are shown to be transcribed into spatial images of electron wave function intensities (WIs) in a sample by using generative machine learning. The output WIs reveal quantum interference states of conduction electrons, as well as sample shapes. The present result augments the human ability to identify quantum states, and it should allow microscopy of quantum nanostructures in materials by making use of quantum fingerprints.

cond-mat.mes-hall↗
arXiv2022

Indirect optical manipulation of the antiferromagnetic order of insulating NiO by ultrafast interfacial energy transfer

We report the ultrafast, (sub)picosecond reduction of the antiferromagnetic order of the insulating NiO thin film in a Pt/NiO bilayer. This reduction of the antiferromagnetic order is not present in pure NiO thin films after a strong optical excitation. This ultrafast phenomenon is attributed to an ultrafast and highly efficient energy transfer from the optically excited electron system of the Pt layer into the NiO spin system. We propose that this energy transfer is mediated by a stochastic exchange scattering of hot Pt electrons at the Pt/NiO interface.

cond-mat.mtrl-sci↗