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)2I3 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)2I3 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 2r-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 AsxSe1-x glasses and x = 0.20 for GexSe1-x glasses, the glasses show excellent glass forming ability separated between rigid (r > 2.4) and floppy (r < 2.4) glasses. Further, calorimetric and Raman scattering experiments revealed that the transition occurred within a certain composition range of the intermediate phase (IP) of the unstressed rigid phase.
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 AsxSe1-x glasses have an IP range of x = 0.29-0.37, where a rapid decrease in the number of wrong As-As bonds is observed. However, other anomalies found in Ge-Se glasses were not clearly observed, such as a rapid decrease in the pre-shoulder positions in the Se-Se partial structure factor, SSeSe(Q), a rapid decrease in the number of edge-sharing connections, and an exclusion tendency of the connections between the As (Ge) atoms sharing two Se atoms. These differences may be related to the anisotropic pyramidal AsSe3 units in the As-Se glasses, in contrast to the isotropic tetrahedral GeSe4 units around the Ge atoms in the Ge-Se glasses. This study was supported by a JSPS Grant-in-Aid for Transformative Research Areas (A) Hyper-Ordered Structures Science.
(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 kBT ln 2, where kB 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)
]]>In condensed matter physics, various types of phase transitions have been observed and their nature has been clarified by intensive experimental and theoretical studies. However, the order parameter of the phase below critical temperature T0 = 17.5 K in URu2Si2 has still been uncovered since its discovery in 1985 despite many efforts by several researchers. With the latest and high-resolution experiments, such as nuclear magnetic resonance, muon spin relaxation, and resonant X-ray scattering measurements, the symmetry of local environment of ions has been narrowed down. This enigmatic ordered phase is called the hidden order. Meanwhile, the candidate order parameter compatible with known experimental facts has recently been clarified from the symmetry aspect by Kambe et al. [Phys. Rev. B 97, 235142 (2018)], although its microscopic object and experimental identification have still been unclear.
In the present study, we theoretically propose that a staggered alignment of chiral charge, that appears in association with electron motion in crystals, corresponds to the hidden order parameter in URu2Si2, satisfying all the symmetry conditions accumulated by many experiments. The chiral charge represents composite degrees of freedom consisting of a spin and a hybridization between orbitals with different orbital angular momenta that can be regarded as a pseudoscalar monopole distinct from conventional electric and magnetic monopoles. Because it only locally breaks the mirror and inversion symmetries, the observation using microscopic probes is difficult; therefore, the order parameter is still hidden. By considering the minimal effective d-f hybridized model on the itinerant picture, we propose the methods to observe the hidden order parameter in experiments. One is an electric-field-induced static rotational deformation in bulk. The other is the observation of states at the surface or domain boundary that can be detected by state-of-the-art experimental methods such as nuclear quadrupole resonance and scanning transmission electron microscopy combined with convergent-beam electron diffraction under an electric field and circularly polarized second harmonic generation microscopy. Our result may provide the last information to resolve the hidden order parameter in URu2Si2.
(Written by Satoru Hayami on behalf of all authors.)
Thermoelectric (TE) materials have recently garnered significant attention toward realizing a low-carbon society. The maximum power of a TE material can be characterized by the power factor (PF), which is determined as PF = σS2, where σ and S denote the conductivity and Seebeck coefficient, respectively. Thus, both a large |S| and high σ are needed for the development of high-PF materials. However, it is well known that there is a tradeoff between |S| and σ; |S| is large but σ is small in semiconductors, and vice versa in metals.
Recently, Shimizu et al. reported that a high-quality ultrathin FeSe under a perpendicular electric field exhibits a high PF of 500 mW/(m·K2) at 100 K1). The film exhibits metallic character, with σ~4 × 106 S/m at 100 K and becomes a superconductor below 50 K. In contrast, |S| exhibits a large value of 350 μV/K, similar to that of a semiconductor. Such a high PF with both a large |S| and high σ cannot be explained in terms of a conventional one-band model. Thus, we proposed a two-band model with a chemical potential between upper and lower band bottoms as a simple theoretical model and elucidated the high PF. Here, we assumed that the two-band structure with finite splitting is realized by field in the ultrathin FeSe.
The present two-band model provides a guideline for designing high TE materials. For instance, nanowires and nanotubes with nanosized diameters are also promising materials with large |S| and high σ values because of sub-band structures. In contrast to FeSe thin films, tunes the chemical potential without affecting the sub-band gap suited for optimization of TE properties.
(Written by M. Matsubara on behalf of all the authors.)
Water dripping from faucets is a common occurrence in daily life. People have noticed that water does not drip at regular intervals, even if the flow rate remains the same. It may drip at short or long intervals with no predictable patterns. This chaotic behavior is caused by tiny droplet oscillations, which cause complex dynamics in classical fluids. Although there is considerable interest in understanding the differences between superfluid and dissipative classical flows, very little is known about their essential differences, particularly in highly nonlinear regimes. This study shows that the dripping behavior of superfluid 4He droplets is very different from that of classical droplets. The dripping periods of the superfluid droplets were quantized at discrete values, even when the input flow rates gradually varied. This is due to the high-amplitude and undamped oscillation of the droplets, which is a direct consequence of superfluidity.
To observe the dripping of the superfluid, spontaneous flow from a cup with an open top, the so-called superfluid film flow, was utilized. This has been used as a popular demonstration to demonstrate the peculiarity of a superfluid that can flow without dissipation through a nanometer-thick film formed on the cup surface. A high-speed video camera was used to capture the dripping of superfluid 4He droplets from the bottom of the cup via superfluid film flow.
When a superfluid pendant droplet is suspended from the bottom, it oscillates with a high amplitude and negligible damping. The amplitude is sufficiently high, causing it to consistently pinch off during the downward phase of the oscillation. The dripping periods of the droplets were discretized at the values specified by the number of oscillations when suspended from the bottom. This is in contrast to a water droplet, which oscillates at a small amplitude and only causes a delay or hast in the pinch-off, resulting in the chaotic behavior of classical fluids. The high-amplitude oscillation of the superfluid droplet removed the small difference and suppressed the occurrence of chaos in the dripping dynamics. The stably discretized periods indicate the spontaneous breaking of time translation symmetry, suggesting a new kind of time crystal realized in superfluid dripping; however, further investigation is needed for confirmation.
(Written by Nomura on behalf of all the authors )