When a light beam propagates in a crystal that is asymmetric with respect to both space inversion and time reversal, absorption coefficients between two-counter propagating light beams can be different. This nonreciprocal optical phenomenon is called directional dichroism, which has been intensively studied on the so-called multiferroic materials with broken space-inversion and time-reversal symmetries, in broad wavelength regions ranging from microwave, terahertz, visible, to x-ray. The directional dichroism can appear not only in ferromagnets but also in antiferromagnets when the symmetry requirement is fulfilled. Therefore, the directional dichroism can be employed as a unique working principle of magneto-optical devices based on antiferromagnets and as a useful probe of antiferromagnets. However, the magnitude of previously reported directional dichroism in near-infrared-to-visible (NIR-VIS) region is small, typically about 1% or less in the nonreciprocal to reciprocal components, except for some specific materials. Few reports on antiferromagnets make use of directional dichroism.

In this study, the authors investigated the directional dichroism of a multiferroic material Pb(TiO)Cu_{4}(PO_{4})_{4}
in high magnetic fields up to 49 tesla via magneto-optical spectroscopy in the NIR-VIS range combined with a pulse magnet technique. Measuring the optical absorption coefficient for counter-propagating light beams, directional dichroism signals were successfully observed in a magnetic-field-induced phase (16 to 45 tesla at a temperature of 2 K). The relative magnitude of the observed signals is significantly large, exceeding 13% at a photon energy of approximately 1.4 eV. Moreover, the magnetic-field dependence of the directional dichroism signals resembles that of a theoretically calculated antiferromagnetic order parameter of the field-induced phase. This strongly suggests that the nonreciprocal directional dichroism (NDD) in the field-induced phase originates from the antiferromagnetic order parameter.

In general, probing an antiferromagnetic order parameter is considerably difficult compared to probing a ferromagnetic order parameter, namely, macroscopic magnetization. There are few experimental probes of an antiferromagnetic order parameter in a high magnetic field regime which is hard to access by typical superconducting magnets. Therefore, the present work not only demonstrates a large NDD but also suggests that the measurement of NDD with a pulse magnet technique provides a unique way of investigating an antiferromagnetic order parameter in a high-field regime.

(Witten by K. Kimura on behalf of all authors)

It has been approximately a decade since Geim and Novoselov were honored with a Nobel prize for discovering graphene: a one-layer atomic sheet of graphite. Recently, a twisted stack of graphene (hereafter referred as twisted graphene) has been attracting an extensive attention. In its system, interesting phenomena are continuously being discovered, such as superconductivity and correlated insulating states. Twisted graphene has a characteristic feature: by stacking atomic layer graphene, a new ‘material’ can be produced without forming new chemical bonds. This differs from conventional materials.

A significant issue in designing the electronic band structure of a twisted graphene system and other stacked two-dimensional materials is the dielectric screening of an electric field applied via a gate voltage. Generally, when an electric field is applied to a conductor, electrons travel in the conductor so that no electric field is present inside it; as a result, the surface of the conductor become charged. This is known as electrostatic screening. However, what happens if the conductor is composed of a few layers of atomically thin graphene? Will only the surface layer be charged, or will all layers be charged uniformly? Discussions on this issue can be traced back to historic studies on graphite and its intercalation compounds. Indeed, this type of screening is important when designing or analyzing the electronic band structure at the device in operation.

The study explores this issue by conducting a low temperature transport experiment using twisted double bilayer graphene, which is a stack of bilayer graphene sheets whose relative crystal angle is adjusted by approximately 5°; this is considerably larger than the magic angle. The bilayer graphene was prepared by using the Scotch-tape-method.

Measurements at low temperatures and in a magnetic field exhibited quantum oscillations, which reflect the carrier density of the electronic band. Two bands with different carrier densities were revealed, which reflect the screened electric field of the gate voltage. The screening length was slightly longer than that of AB-stacked four-layer graphene, a material that has the same number of layers, but a stacking structure is the same as a natural graphite crystal. The results may correspond to the difference in the features of quantum mechanical wave functions perpendicular to the plane.

The present knowledge and methods could be used to tailor the valley structure of the band structure of the twisted graphene systems. We expect that solar cells or novel transistor devices based on new principles, will be realized in the near future. Diamonds are the queen of jewelry. Carbon fibers are a key item in sports. Therefore, will carbon also revolutionize electronics?

(Written by R. Yagi on behalf of all authors)

]]>Topological phases of matter, such as topological insulators, semimetals, and superconductors, have garnered much attention in condensed matter physics. These are strange materials characterized by the presence of conduction states on the surface while the bulk remains insulating. Moreover, the surface states are protected by certain symmetries against physical deformations or defects.

It turns out that such systems can be classified by a “topological invariant” or a topological number such that two systems with the same topological number are equivalent. The topological invariant is classified by the symmetry and dimensionality of the system and is well-defined for gapped Hamiltonian systems. A well-known classification scheme is based on onsite symmetries, such as time-reversal symmetry, particle-hole symmetry, and chiral symmetry. However, recent schemes involve both crystal symmetry and onsite symmetry.

Interestingly, topological superconductors often have gapless points called “superconducting nodes” that are characterized by topological invariants, even though topology for the wavefunction can only be defined for gapped Hamiltonians. While there exist some classification theories for the superconducting order parameter predicting such nodes, they are sometimes incompatible with the actual superconducting gap structure and do not account for space group symmetry and higher-spin states.

In this paper, we present a new classification theory for superconducting nodes to resolve these two problems. We constructed this theory based on arguments from group theory and topology. An earlier topological classification theory focused on the *d*-dimensional Brillouin zone (BZ). However, since a band gap cannot exist in a whole *d*-dimensional BZ in nodal superconductors, we instead considered a *p*-dimensional (*p* < *d*) sphere surrounding nodes in the BZ.

We fixed a specific ** k **point in the BZ and then developed the classification theory on the high-symmetry points using the Wigner criteria and the orthogonality test. Our results predicted two new types of excitation structures, i.e., unconventional superconducting nodes protected by crystal symmetry and topology.

By performing classification on high-symmetry planes, we predicted nontrivial gap structures appearing from nonsymmorphic space group symmetry, which can be decomposed into a product of non-primitive lattice translation and a point group operation. Meanwhile, performing classification on high-symmetry lines helped predict nontrivial gap structures dependent on the angular momentum of the normal Bloch state.

Because these classification structures are determined only by symmetry, they can be universally applied to many candidate superconductors, potentially leading to the discovery of superconductors hosting unconventional nodes.

]]>Photoinduced phase transition is the cooperative change of the electronic states and/or the crystal structures of a material induced by ultrashort laser pulses. Its mechanism is closely related to the dynamics of elementary excitations which modulate the quantum-mechanical states of strongly correlated electron systems and/or strongly coupled electron-phonon systems in a coherent regime. As slight fluctuations trigger the macroscopic change of the system in phase transitions, the transient dynamics of light-induced fluctuations play an important role in controlling the photoinduced phase transitions. It has been theoretically revealed that the generation dynamics of photoinduced quantum entanglement causes the aforementioned fluctuations.

A model of remote electron-phonon systems which is not directly interacting with each other was selected. The manner in which the entanglement between the phonons of these systems appears as photoirradiation proceeds was calculated. When two systems are sufficiently separated in space, there is no direct quantum-mechanical correlation between them, and the quantum entanglement is generated between them when “quantized light” is irradiated. The problem that requires the quantum nature of light has not been raised frequently although the relationship between the nonequilibrium state and the entanglement is considered a fundamental problem of physics. The results of this study elucidate the role of light quantization on the transient dynamics of condensed matter.

The results of this study will be extended to the study of photoinduced fluctuations leading to macroscopic change of the physical properties or phase transitions. Simultaneously, it is expected to be closely related to the research in the field of quantum information as the proposed theoretical framework is similar to the one for the fundamental problem of quantum information. Specifically, the photoinduced entanglement generation between the remote qubits can be used as a quantum memory. The results of this study show that a large degree of freedom of phonon states will enable a larger quantum entanglement between phonons than qubits, and that an effective quantum entanglement storage/memory is expected. The quantum many-body problems regarding the electrons and bosons (phonons and photons) have a long history, and it is expected that further detailed studies will continue in the future.

(Written by K. Ishida on behalf of all the authors)

]]>Investigating the effect of electric current on magnetic materials is crucial in spintronics. In ferromagnets, electric currents are known to drive domain-wall motion and cause a Doppler shift in the spin-wave spectrum. These phenomena, known as the spin-transfer effect, can be understood as the exchange of spin angular momentum between magnetization and conduction electrons. In antiferromagnets, neither the conduction electrons nor antiferromagnetic spins carry macroscopic spin angular momentum, and the spin-transfer effect is not intuitively understood, calling for a microscopic analysis.

It was long supposed that there is only one (reactive) spin-transfer torque in antiferromagnets, as in ferromagnets. In this study, starting from a microscopic Hamiltonian with conduction electrons, we show that antiferromagnets have two different types of spin-transfer torques: one arising through the coupling to the uniform spin density ( ) and the other through the staggered spin density ( ). The two spin-transfer torques make equal contributions of to the spin-wave Doppler shift, while only one ( ) acts on domain walls [1]. This feature is in stark contrast to ferromagnets, in which a single spin-transfer torque leads to both Doppler shift and domain-wall motion. The Doppler shift depends on chirality of antiferromagnetic magnons, thus an electric current can be used to differentiate the two modes via the shift in wavelength or frequency.

We microscopically calculated the spin-transfer torques due to electrons on a two-dimensional square lattice by considering not only the nearest-neighbor (inter-sublattice) hopping but also the next-nearest-neighbor (intra-sublattice) hopping; the former induces “antiferromagnetic transport” in the sense that the electrons feel the alternating magnetization, whereas the latter induces “ferromagnetic transport,” rendering the electrons feel a uniform magnetization. One can interpolate the two transport regimes (ferromagnetic and antiferromagnetic) by varying the hopping parameters. In the limit of ferromagnetic transport, the two spin-transfer torques reduce to the well-known spin-transfer torque in a ferromagnet. In the antiferromagnetic transport regime, the two torques collaborate or compete, and the overall Doppler shift depends on microscopic parameters (such as band filling); it is negative (same sign as for ferromagnets) at small band filling and changes sign as the lower band becomes filled towards the antiferromagnetic band gap.

[1] J. J. Nakane and H. Kohno, Phys. Rev. B **103**, L180405 (2021).

(Written by J. Nakane on behalf of all authors)

]]>Electrons in an atom possess spins with two states, one facing upwards and the other downwards. The magnetic moments and degrees of freedom associated with the spins can be manipulated to carry information and transfer energy. Spin caloritronics is a new and upcoming field of research that looks for new ways to drive and control thermal transport and thermoelectric conversion mediated by the spin of electrons.

Most of the fundamental research in this field is focused on heat-to-charge and heat-to-spin conversion phenomena shown by hybrid structures and magnetic materials, and not much is known about the spin-caloritronic properties that give rise to heat currents.

To unravel properties of heat conversion, generation, and transport mediated by spin, a team of researchers from Japan proposed a new concept called “spintronic thermal management.” The concept provides a window to the demonstration of unique heat control functionalities such as local temperature modulation, spintronic thermal switching, active control of thermoelectric conversion, and unidirectional remote heating.

The team also classified the basic behaviors of spintronic thermal management into magneto-thermoelectric effects, thermomagnetic effects, and thermospin effects based on an extensive overview of the conversion phenomenon between spin, charge, and heat currents associated with spin caloritronics.

Ultimately, the study provides a comprehensive understanding of a basic physical phenomenon that opens up avenues for new material development and device engineering for spintronic thermal management. These findings could also come in handy while designing advanced thermal management technologies for high-functioning and reliable electronic devices with better heat distribution and cooling systems.

]]>The decay of quantum systems through a potential barrier is ubiquitous in many fields of physics and chemistry, occurring in electron conduction within devices and quantum dots as well as chemical reactions, and nuclear fission. The usual framework for describing the barrier crossing is the "transition-state" theory, which was invented long ago to investigate chemical reactions and nuclear fission. The main assumption of this theory is that the decay rate is entirely determined at the highest point of the barrier and does not depend on what occurs afterward. This remarkable theory has been part of reaction theory since the 1930s and has been used in disciplines as widely distinct as chemistry and nuclear physics. However, this theory has not been validated for the quantum mechanics of many interacting particles.

In this study, a model was proposed to study reactions in systems with many interacting particles. The model allowed us to test whether the assumptions of the transition state theory could be satisfied in a full quantum theory. The model contains two reservoirs of internal states connected to each other by additional barrier-top states, and the reservoir states were considered using random matrix theory.

For the first time, we demonstrated that a main assumption of the transition-state theory can be easily satisfied by a more detailed theory, that is, the overall decay rate going through the far-side reservoir is largely determined by the barrier region and internal properties of the reservoirs, and is insensitive to the individual decay rates of the states out of the far-side reservoir.

In a subsequent paper (K. Hagino and G.F. Bertsch, Phys. Rev. E104, L052104(2021)), we applied the same model to discuss the variation of the decay rates between states. There is a well-established theory known as the Porter-Thomas distribution that has been validated for many systems, including atomic spectra, chemical reaction spectra, and nuclei. Surprisingly, our transition-state model revealed that this distribution differed depending on how large the individual rates were. The new distribution followed a different random matrix model, namely the Gaussian Unitary Ensemble, proposed by Dyson in 1960.

(Written by K. Hagino on behalf of all authors)

In quantum field theory (or QFT), perturbation theory is a mathematical approximation used to describe a complex quantum system with a simpler one. However, this approach is not valid for strongly coupled systems or phase transitions, for which the perturbative expansions are not convergent.

A technique called “resurgence theory” could, however, allow us to understand non-perturbative effects from information concealed within a perturbative series. This approach has been used to describe a wide range of systems in quantum mechanics, hydrodynamics, and string theory. But so far, there has been little focus on describing systems with phase transitions.

Against this backdrop, physicists from Japan studied the resurgence structure of a three-dimensional supersymmetric quantum electrodynamics (or SQED) model with a second-order quantum phase transition to explore the relations between resurgence and phase transitions.

The team used two approaches in their study: one involved a “Lefschetz thimble analysis,” in which the path integral representations of physical observables were decomposed in terms of Lefschetz thimbles. The thimble decomposition, in turn, could either change discontinuously to give rise to “Stokes phenomena” or remain unchanged but switch dominant saddle points to give “anti-Stokes phenomena.” The other was “Borel resummation technique,” which could decode these phenomena from a purely perturbative expansion.

The team interpreted the second-order phase transition as a simultaneous Stokes and anti-Stokes phenomena and showed that the order of phase transition was governed by the number of saddles colliding and by their collision angle at the critical point. In addition, supersymmetry led to an infinite number of Stokes phenomena. Finally, they showed that Borel resummation can be used to understand phase transitions.

These findings could open up potential applications of resurgence in QFT along with opportunities to explore problems such as hadron dynamics.

]]>Cuprate superconductors, discovered in 1986, still confront researchers of condensed matter physics with unresolved challenges. On top of this, superconductivity research is now booming again—thanks to recent studies that demonstrate how phenomena like charge order and fluctuations can coexist and interact with high-temperature superconductivity.

To help researchers approach the daunting amount of literature on this subject, the Special Topics issue of the Journal of the Physical Society of Japan includes 12 papers on recent experimental and theoretical research on remarkable new phenomena in high-temperature superconductors.

First, Uchida provides an overview of the field and future prospects, while Tranquada and colleagues pursue the relationship between charge order and superconductivity using scattering and transport techniques in cuprate families.

Fujita and colleagues provide an atomic-scale visualization of Cooper-pair density waves using scanning tunneling microscopy techniques. Meanwhile, Lee summarizes recent findings on the charge density wave in superconducting cuprates using X-ray scattering.

Also employing X-ray scattering techniques, Arpaia and Ghiringhelli explore high temperature and high energy charge fluctuations, whereas Le Tacon and colleagues investigate charge order and phonon anomalies under uniaxial stress.

Abbamonte and colleagues search for a new ordered state using X-ray diffraction, and Kawasaki and colleagues employ nuclear magnetic resonance to probe charge order and fluctuations.

On the theoretical side, Imada explores the relationships between charge order and superconductivity and how to measure them using spectroscopic methods.

Devereaux and colleagues analyze charge-spin fluctuations using large-scale numerical calculation of the Hubbard model, while Yamase delves into the theory of bond charge order, collective charge fluctuations, and nematic order. Finally, Kontani and colleagues explore the theory of various liquid crystal orders in cuprate superconductors and related materials.

The sheer amount of knowledge that has been accumulated on high-temperature superconductivity makes it hard for new researchers to approach the subject, but this issue will hopefully be a useful source of information so that anyone can approach and grasp the hottest spots of the field.

]]>Research on photoinduced phase transitions has progressed recently accelerated because of the rapid development of laser technology. Irradiation by circularly polarized light was theoretically proven to induce a photoinduced topological phase transition to the Chern insulator phase in a tight-binding model on the honeycomb lattice via a special kind of band structure resembling those predicted by Haldane. According to this prediction, the possible emergence of the photoinduced topological phase in graphene has been explored, and an observation of photoinduced Hall currents in graphene was argued in this context.

Since these pioneering studies, photoinduced topological phase transitions have undergone extensive theoretical investigations. However, the further development of this growing research field requires proposals of novel target materials and theoretical predictions of interesting physical phenomena. In this study, we theoretically predicted the occurrence of photoinduced topological phase transitions and the emergence of the topologically nontrivial Chern insulator phase as a nonequilibrium steady state in the organic salt α-(BEDT-TTF)_{2}I_{3} under irradiation with elliptically polarized light.

We constructed rich nonequilibrium phase diagrams in the plane of the x-axis and y-axis components of the amplitude of elliptically polarized light by calculating the band structures, Chern numbers, and Hall conductivity in a photodriven α-(BEDT-TTF)_{2}I_{3}
system using the Floquet theory. These include the Chern insulator phases, non-topological insulator phases, and semimetal phases. In addition, calculations of the Hall conductivity using the Floquet–Keldysh scheme predicted that the quantization of Hall conductivity can be observed in this nonequilibrium Chern insulator phase at low temperatures, just as it is observed in equilibrium Chern insulators. Furthermore, we revealed that the present photoinduced Chern insulator phase possesses another feature of the equilibrium Chern insulators, namely, the gapless state localized at the edges. The predicted quantized Hall conductivity and edge current owing to the predicted edge states in the photoinduced Chern insulator phase are expected to be observed in future experiments for α-(BEDT-TTF)_{2}I_{3}. Our results expand a range of target materials and contribute to the research on the optical manipulation of electronic states in matter.

(Written by M. Mochizuki on behalf of all authors)

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