Spin-rotation coupling, which refers to the coupling between mechanical rotations and electron spins, leads to various phenomena, such as the Einstein-de Haas effect and the Barnett effect, in which the mechanical rotation of a crystal acts on the electron spins as an effective magnetic field. Recently, surface acoustic waves and chiral phonons involving the rotational motion of atoms have attracted considerable attention in spintronics. Surface acoustic waves refer to classical surface waves in continuous elastic media, whereas chiral phonons represent the microscopic local rotation of atoms with a phonon angular momentum.

Previous studies have indicated a phenomenon involving the conversion of phonon angular momentum into electron spins and charges in nonmagnetic materials and shown that chiral phonons behave as an effective magnetic field. Chiral phonons accompanying atomic rotations modulate electron hopping and spin-orbital coupling because the distance between atoms periodically changes with time. In the case of magnets, spin-spin interactions, such as exchange interactions and Dzyaloshinskii-Moriya interaction, always exist. These spin-spin interactions govern the collective propagation of precessions of electron spins, which are known as magnons. However, the interplay between atomic rotation and magnons in magnets is not fully understood.

In this study, we investigate a new conversion of chiral phonons into magnons in both ferromagnets and antiferromagnets and show that chiral phonons modulate spin-spin interactions. Because the masses of atoms are much larger than those of electrons, the behavior of magnon dynamics in an adiabatic response to atomic rotations leads to a geometric effect. We demonstrate that this effect requires breaking the spin-rotation symmetry. Consequently, the geometric effect due to chiral phonons results in a nontrivial change in magnon excitations. Specifically, chiral phonons with clockwise and counterclockwise rotational modes induce a change in magnon numbers with opposite signs, which corresponds to increasing and decreasing spin magnetization due to the chiral nature of atomic rotations.

The proposed effect is universal for a wide range of magnets. Measurement in an antiferromagnet is easier than in a ferromagnet because the net spin magnetization becomes nonzero owing to the geometric effect induced by chiral phonons. Our theory can generally be applied to real materials and is expected to be experimentally realized in some candidate materials, such as antiferromagnets XPS3 (X =Fe, Mn, and Ni), with a strong magnetic anisotropy.

(Written by D. Yao and S. Murakami)

]]>In the muon spin relaxation (µSR) measurements, the distribution (described by linewidth D) of internal magnetic field *H*(*t*) and its temporal fluctuations (with mean fluctuation rate *n*) can be observed by implanting spin-polarized muons into a material. However, distinguishing whether the fluctuations are caused by the diffusive motion of the muon itself or the motion of the ions around it, is difficult. In this study, by reviewing the strong collision model, which is an assumption used to describe spin relaxation, we observed that the difference in the cause of the fluctuation appeared as a difference in the spin relaxation function. The new model reproduces well the spin relaxation owing to the local rotational motion of cation molecules observed in hybrid perovskites; this opens the way to distinguish the cause of fluctuations solely from the µSR data.

When a muon exhibits jump-diffusion in a nonmagnetic material, the configuration of the nuclear magnetic moments around the muon changes simultaneously before and after the site change. The fluctuation of *H*(*t*) owing to these jumps is well approximated by the strong collision model [where the autocorrelation of *H*(*t*) is given by equation *H*(*t*)*H*(0)~D^{2}exp(-*n**t*)], and the time evolution of the muon spin polarization, *G _{z}*(

Therefore, we performed Monte Carlo simulations for *G _{z}*(

Written by T. U. Ito and R. Kadono

]]>Spin current, the flow of spin angular momentum, is a central element in spintronics for future technological applications. Thus, elucidating various mechanisms to generate spin currents is an important topic. Since the discovery of the gyromagnetic effect more than 100 years ago by Einstein, de Haas, and Barnett, spin angular momentum has been known to be mutually converted with the mechanical angular momentum associated with rotational motion of materials. This suggests that spin currents can be generated mechanically. Present-day experiments have shown that spin currents are generated by shear flows in liquid metals and by surface acoustic waves in solids.

The electron spin also interacts with its orbital motion through relativistic effects, that is, the spin-orbit interaction (SOI). The SOI is responsible for various spin-current generation methods because it bends the electron orbits in spin-dependent directions. In particular, the Rashba SOI appears in systems with broken spatial inversion symmetries, such as at the surfaces and interfaces of materials. When generating spin currents using surface acoustic waves, the effects of Rashba SOI may be utilized.

In this study, we investigated spin-current generation from dynamic lattice distortions in systems with Rashba SOI. Unlike prior theoretical studies, we started from a multiorbital tight-binding model to derive a Rashba model perturbed by lattice distortions. This method enabled us to treat the lattice distortion effects microscopically through the modulation of hopping integrals and local rotation of the crystal axes. By calculating the linear response to the effective perturbations, we observed that surface acoustic waves can generate a variety of spin currents through the Rashba SOI, including unconventional spin currents, such as the quadrupolar spin current, perpendicular spin current, and helicity current.

Written by Y. Ogawa on behalf of all authors.

]]>The spin Seebeck effect (SSE) is an established method for generating a DC spin current using a temperature gradient. In the standard experimental setup, a temperature gradient is applied to a bilayer junction of a target magnet and a metal; the gradient direction is perpendicular to their interface. The generated spin current runs along the gradient in the magnet and is injected into the metal, where it is converted into an electric voltage via the inverse-spin Hall effect. Because the spin current is essentially non-conserved and almost impossible to directly detect, attachment of a metal is necessary to (indirectly) observe the spin current as an electric signal.

Since its discovery in 2008, the SSE has been explored in various magnetic materials including antiferromagnets, which are one of the most fundamental magnetic materials and have garnered attention as a new platform for ultrafast spintronics because their magnetic excitations (magnons) reside in a higher-energy THz regime than magnons in ferromagnets. Thus, it is important to understand the nonequilibrium phenomena in antiferromagnets, including the SSE. Most theoretical research on SSE in antiferromagnets relies on phenomenological arguments; thus, development of microscopic understanding is desirable.

Based on spin-wave theory and the non-equilibrium Green’s function approach, we analyze the DC spin current flowing from the antiferromagnet to the attached metal. We determine the relationship between the spin current and the inelastic neutron scattering spectrum. The tunnel spin current is expressed with dynamical spin structure factors that are proportional to the density of states of spin-current carriers and can be measured by inelastic neutron scattering. Our analysis shows that the sign of the spin current is determined by the spin polarization direction of the dominant carrier and that the sign reverses at the spin-flop transition point. This result is in agreement with a recent SSE experiment on Cr2O3. Furthermore, our theory predicts that in a low-temperature regime, the spin current in the canted phase exhibits a non-monotonic field dependence, which could be potentially observed in future experiments.

In addition to these predictions, our microscopic theory makes it possible to quantitatively compare the SSE of antiferromagnets with those of other magnets. Such a quantitative comparison is difficult within the phenomenology, which is an advantage of our microscopic theory.

Development of microscopic theories generally deepens the understanding of observed phenomena and helps predict new related phenomena. Our findings are expected to contribute to understanding the SSE in antiferromagnets and unexplored non-equilibrium dynamics in future spintronics and non-equilibrium physics.

Written by K. Masuda and M. Sato.

Quantum contact interactions in low energy regime are one of the most fundamental type of interactions in nature. In these interactions, the wavelengths of particles are longer than the characteristic range of the interactions, and particles are no longer able to resolve their microscopic details. Consequently, short-range interactions can be described as contact or point-like interactions.

In this context, recent studies have shown that three-body contact interactions of non-identical particles in one dimension are particularly unique. They can be topologically nontrivial, and so, can be classified by unitary irreducible representations of the pure twin group. The pure twin group is a set of trajectories of particles in spacetime, allowing only two particles to interact simultaneously. However, the description of these interactions within the framework of quantum mechanics, especially in the Hamiltonian formalism, remains unknown.

Addressing this gap, a new study published in *Progress of Theoretical and Experimental*
*Physics* explores the Hamiltonian descriptions of the topologically nontrivial three-body contact interactions by using the path-integral formalism. The study revealed that these interactions can be described by fictitious, infinitely thin magnetic fluxes in the many-body configuration space. When the trajectories of the particles wind around these fluxes, they acquire the Aharonov–Bohm phases, which can solely be determined by topology and group theory. Additionally, the study also introduces a new special parameter to describe the contact interactions of more than three non-identical particles.

In summary, this topological description of three-body contact interactions expands our understanding of quantum contact interactions. Moreover, owing to its similarity to particles called anyons in two dimensions, these findings hold great potential for quantum computing.

]]>Conservation laws that hold in classical theory can be violated in quantum mechanics, a phenomenon known as the “chiral anomaly.” This intriguing violation of the conservation laws has been the focal point of research in particle physics and astrophysics for many years. However, recently, physical phenomena stemming from chiral anomalies have been observed in topological materials, revealing that chiral anomalies are a universal phenomenon that also manifest in solid-state systems.

Chiral anomaly effects in solids result in pronounced responses, including transport phenomena without scattering, such as negative magnetoresistance, planar Hall effects, and anomalous Hall effects. Consequently, topological materials that exhibit chiral anomalies have garnered significant interest for potential device applications.

Broken chiral symmetry is crucial for the occurrence of chiral anomalies, and the dimensionality of electron motion is another key factor. According to quantum field theory, chiral anomalies are present in three-dimensional (3D) systems but absent in two-dimensional (2D) systems. Typically, Dirac semimetals are categorized as either 2D or 3D, with no materials bridging these dimensions yet. To explore the physics behind chiral anomaly effects, candidate materials capable of transitioning between 2D and 3D dimensionalities have been searched.

We propose that organic conductor α-(BEDT-TTF)_{2}I_{3}
under high pressures emerges as a prime candidate material. This is the first bulk (multilayered) massless Dirac fermion system identified. Although various physical phenomena within this system have traditionally been interpreted through a 2D framework, the evidence of coherent interlayer tunneling at low temperatures reveals its intrinsic 3D nature. Theoretical insights indicate that this system functions as a 3D Dirac semimetal with chiral symmetry broken by electron correlation.

In the 3D Dirac semimetal with broken chiral symmetry, a “magnetic monopole”—stemming from the topology of the wave function—is formed at the two Dirac points, K and K', analogous to the N and S poles. The observation of negative magnetoresistance and planar Hall effects, driven by a virtual magnetic field (known as the Berry curvature) emanating from the Dirac points, confirms the realization of a 3D Dirac semimetal with chiral anomalies in α-(BEDT-TTF)_{2}I_{3} under high pressures at low temperatures. This system provides a pivotal platform for probing chiral anomalies.

(Written by N. Tajima on behalf of all authors)

]]>Rigidity percolation (mean-field) theory is a simple theory that can explain several anomalies in the thermodynamic properties of covalently bonded glasses such as chalcogenides. According to this theory, the atoms in covalent glasses are constrained by either bonds or bond angles. When the constituent atom has coordination number *r*, the bond constraint and bond angle constraint are *r*/2 and 2*r*-3 per atom, respectively. If the averaged sum of these constraints is equal to the degree of freedom of three in three dimensions, i.e., *r* = 2.4 at *x* = 0.40 for As* _{x}*Se

This article presents a series of structural studies using element-selective diffraction techniques with synchrotron X-rays and high-flux neutron sources to investigate the structural changes across the IP transition phase in chalcogenide glasses. Technical improvements were achieved by developing a new detection system with a curved graphite analyzer crystal and a 1-m-long detector arm at the beamline BM02 of the European Synchrotron Radiation Facility (ESRF). Experimental X-ray and neutron diffraction data and anomalous X-ray scattering (AXS) results close to the absorption edges of the constituent elements were analyzed using reverse Monte Carlo modeling to reveal changes in intermediate-range atomic configurations across the IP range from rigid to floppy. The As* _{x}*Se

(Written by S. Hosokawa on behalf of all the authors.)

]]>Understanding the thermodynamic properties of computation is not only physically interesting but also holds significant practical implications.

In 1961, Rolf Landauer from IBM introduced the Landauer principle, establishing a lower bound for the dissipation of energy required to reliably erasing one bit of information. The bound is expressed as *k*_{B}*T *ln 2, where *k*_{B} is the Boltzmann constant, and* T* is the temperature of a thermal reservoir. This value is approximately 3.0✕10^{-21} J at room temperature. Although extremely small, achieving this limit is feasible through the quasi-static erasure process of memory. However, practical implementation may result in increased energy dissipation. Beyond serving as mere memory systems, computers execute complex mathematical operations through logic circuits composed of numerous logic gates. Hence, discussing the thermodynamic properties of this system is interesting.

Recent advancements in nonequilibrium statistical mechanics have unveiled instances of dissipation surpassing the Landauer bound in practical applications. In addition to memory systems, the thermodynamic analysis of more complex computers, such as logic circuits, Brownian computers, and models proposed in computer science, has become possible. However, existing studies are limited to ideal models and settings. For physically implemented computers, only a few studies have analyzed the relationship between computational processes and their thermodynamic properties.

This study focuses on a specific logic gate and analyzes the thermodynamic properties in terms of the extended Landauer bound. NAND gates, comprising CMOS transistors operating in sub-threshold regions, exhibit additional dissipation due to dynamic changes in the logical states encoded in the output voltage. These findings have been quantitatively revealed.

The Landauer bound stems from logical irreversibility and the inability to accurately infer the input from the output state after computation. This reduces the number of logical states (*M*) to be realized before and after the computation, thus increasing the corresponding entropy (*H),* up to ln 2 in the case of a 1-bit complete information erasure. In this study, alongside the dissipation associated with this logical irreversibility, an additional dissipation, contingent on the initial system distribution, was identified through an investigation of the Kullback-Leibler divergence. While no difference was observed in the former dissipation under varying input voltage conditions, the latter exhibited greater dissipation under certain conditions. We interpret this factor as a consequence of logic state flipping.

The relevance of thermodynamic properties for more complex physical computers has not been completely understood, and further research is required.

(Written by D. Yoshino on behalf of all authors)

]]>Accordingly, there have been many attempts to develop a consistent QFT of gravity. One potential candidate for this is quadratic gravity (QG). QG is a modification of Einstein’s field equations in general relativity by adding higher-order derivative terms that address the infinities arising during gravitational interactions at very high energies, making the theory renormalizable. However, this modification, similar to Lee–Wick’s quantum electrodynamics model, leads to the presence of “ghost particles” of negative norm, which endanger the unitarity of the theory.

Lee and Wick had noticed that the ghost acquires a complex mass by radiative corrections and had claimed that such complex ghosts would never be created during the collision of physical particles due to energy conservation, thus preserving the physical unitarity. If this claim is true, all theories, in principle, can be made renormalizable or even finite without violating unitarity by simply adding higher-order derivative terms. This would also make QG a viable theory.

In this study, we addressed the above unitarity problem based solely on QFT. To this end, we faithfully applied the operator formalism of indefinite metric QFT and calculated the amplitudes of the scattering process of physical particles.

Interestingly, our calculations showed that complex ghosts can indeed be created with a non-zero probability during the collisions of physical particles, thereby violating the physical unitarity contrary to Lee and Wick's claim. Even if one devises a clever method to modify scattering amplitudes to satisfy unitarity, it will no longer be a QFT. Notably, we identified an energy limit below which no complex ghosts are created and thus unitarity holds.

The complex delta function (a generalization of the Dirac delta function), which appears at each interaction vertex of the complex ghost, played a central role in our investigation. In particular, we found that the usual Feynman rule applied at each vertex with a complex energy conservation law assumed in advance is wrong and has no ground in QFT. To address this, we also gave the corrected starting expression.

In conclusion, this study addresses the unitarity problem in the context of QG and takes us a step closer to realizing a proper renormalizable QFT of gravity.

]]>Different external environments, such as temperature and pressure, can change physical properties, such as transport, magnetism, and light transparency. One phenomenon is the Pressure-induced Insulator-to-Metal Transition (PIMT), in which a transparent, nonconducting material (insulator) becomes opaque and begins to conduct electricity (metallization) when pressure is applied. Some materials exhibit the PIMT at pressures below 10,000 atm (approximately 1 GPa). One substance that exhibits PIMT at low pressure is samarium monosulfide (SmS), an insulator at ambient pressure (1 atm) with high electrical resistance and transmits infrared and terahertz regions. When SmS is damaged by a needle tip or applying pressure above approximately 7,000 atm (~0.7 GPa), it undergoes PIMT and transforms into a metal. This property change, as well as the color change of SmS from black to golden-yellow, has attracted the attention of many researchers and has been the subject of numerous studies. However, the origin of PIMT is yet to be identified, even more than 50 years after its discovery.

More recently, the behavior of SmS in external environments that differs from that of pressure has attracted attention. The first is the negative differential resistance (dV/dI) when a current higher than the normal measurement condition is applied to SmS at low temperatures. Because ordinary materials obey Ohm's law, dV/dI is constant regardless of the current magnitude. However, SmS exhibits negative dV/dI at low temperatures, where the voltage decreases when the current is increased.

Here, the origin of dV/dI was investigated using optical reflectivity spectra in the region from the terahertz to vacuum-ultraviolet and angle-integrated photoelectron spectra under the application of current. The obtained spectra suggest that the material exhibited a Current-induced Insulator-to-Metal Transition (CIMT). Detailed analysis of the electronic state change revealed that the CIMT was caused by the hybridization of the Sm 4f orbital and Sm 5d conduction band owing to the pushing out of the 4f electrons localized in the atom by the applied electric current. The metallic state of the CIMT has a lower conduction electron density than the metallic phase of the PIMT of SmS, suggesting that the same phase transition in the PIMT cannot occur by increasing only the 4f-5d hybridization strength. This result suggests that effects other than hybridization are required for PIMT.

(Written by S. Kimura on behalf of all the authors)

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