A correlated ferromagnetic polar metal by design.

Polar metals have recently garnered increasing interest because of their promising functionalities. Here we report the experimental realization of an intrinsic coexisting ferromagnetism, polar distortion and metallicity in quasi-two-dimensional Ca3Co3O8. This material crystallizes with alternating stacking of oxygen tetrahedral CoO4 monolayers and octahedral CoO6 bilayers. The ferromagnetic metallic state is confined within the quasi-two-dimensional CoO6 layers, and the broken inversion symmetry arises simultaneously from the Co displacements. The breaking of both spatial-inversion and time-reversal symmetries, along with their strong coupling, gives rise to an intrinsic magnetochiral anisotropy with exotic magnetic field-free non-reciprocal electrical resistivity. An extraordinarily robust topological Hall effect persists over a broad temperature-magnetic field phase space, arising from dipole-induced Rashba spin-orbit coupling. Our work not only provides a rich platform to explore the coupling between polarity and magnetism in a metallic system, with extensive potential applications, but also defines a novel design strategy to access exotic correlated electronic states.

Theory of bulk photovoltaic effect in Anderson insulator.

The localization of wavefunction by disorder makes a conductive material an insulator with vanishing conductivity at zero temperature. A similar outcome is expected for the photocurrent in semiconductor p-n junctions because the photoexcited carriers cannot drift through the device. In contrast, we here show numerically that the bulk photovoltaic effect-the photovoltaic effect in noncentrosymmetric bulk materials-occurs in a noncentrosymmetric, disordered, one-dimensional insulator where all eigenstates are localized. We find this photocurrent remains, even when the energy scale of random potential is larger than the bandwidth. On the other hand, the photocurrent decays exponentially when the excitation is local, i.e., when only a part of the device is illuminated. The photocurrent also vanishes if the relaxation occurs only by contact with the electrodes. Our result implies that the ratio of the photovoltaic current and the direct current by the variable-range hopping increases with decreasing temperature. These results suggest a route to design high-efficiency solar cells and photodetectors.

Large Photogalvanic Spin Current by Magnetic Resonance in Bilayer Cr Trihalides.

Spin current is a key to realizing various phenomena and functionalities related to spintronics. Recently, the possibility of generating spin current through a photogalvanic effect of magnons was pointed out theoretically. However, neither a candidate material nor a general formula for calculating the photogalvanic spin current in materials is known so far. In this Letter, we develop a general formula for the photogalvanic spin current through a magnetic resonance process. This mechanism involves a one-magnon excitation process in contrast to the two-particle processes studied in earlier works. Using the formula, we show that GHz and THz waves create a large photogalvanic spin current in the antiferromagnetic phase of bilayer CrI_{3} and CrBr_{3}. The large spin current arises from an optical process involving two magnon bands, which is a contribution unknown to date. This spin current appears only in the antiferromagnetic ordered phase and is reversible by controlling the order parameter. These results open a route to material design for the photogalvanic effect of magnetic excitations.

Purcell-like Enhancement of Electron-Phonon Interactions in Long-Period Superlattices: Linear-Temperature Resistivity and Cooling Power.

In the Purcell effect, the efficiency of optical emitters is enhanced by reducing the optical mode volume. Here we predict an analogous enhancement for electron-phonon (el-ph) scattering, achieved by compressing the electronic Wannier orbitals. Reshaped Wannier orbitals are a prominent attribute of graphene moiré superlattices, where the orbital size is tunable by the twist angle. A reduction in the orbital size leads to an enhancement in the el-ph interaction strength, yielding the values considerably larger than those in pristine monolayer graphene. The enhanced coupling boosts the el-ph scattering rates, pushing them above the values expected for the flat-band-enhanced density of electronic states. The enhanced phonon emission and scattering rates are manifested through the observables such as the electron-lattice cooling and the linear-temperature (T) resistivity, both of which are directly tunable by the moiré twist angle.

Rectification of Spin Current in Inversion-Asymmetric Magnets with Linearly Polarized Electromagnetic Waves.

We theoretically propose a method of rectifying spin current with a linearly polarized electromagnetic wave in inversion-asymmetric magnetic insulators. To demonstrate the proposal, we consider quantum spin chains as a simple example; these models are mapped to fermion (spinon) models via Jordan-Wigner transformation. Using a nonlinear response theory, we find that a dc spin current is generated by the linearly polarized waves. The spin current shows rich anisotropic behavior depending on the direction of the electromagnetic wave. This is a manifestation of the rich interplay between spins and the waves; inverse Dzyaloshinskii-Moriya, Zeeman, and magnetostriction couplings lead to different behaviors of the spin current. The resultant spin current is insensitive to the relaxation time of spinons, a property of which potentially benefits a long-distance propagation of the spin current. An estimate of the required electromagnetic wave is given.

Berry Phase of Phonons and Thermal Hall Effect in Nonmagnetic Insulators.

A mechanism for the phonon Hall effect (PHE) in nonmagnetic insulators under an external magnetic field is theoretically studied. PHE is known in (para)magnetic compounds, where the magnetic moments and spin-orbit interaction play an essential role. In sharp contrast, we here discuss that the PHE also occurs in nonmagnetic band insulators subject to the magnetic field. We find that a correction to the Born-Oppenheimer approximation gives rise to a Raman-type interaction between the magnetic field and the phonons; this interaction gives rise to the Berry curvature of a phonon band. This Berry curvature results in the finite thermal Hall conductivity κ_{H} in nonmagnetic band insulators. The value of κ_{H} is calculated for square and honeycomb lattices. The order of the magnitude estimation for κ_{H} is given for Si at room temperature.

Bulk Topological Proximity Effect.

Existing proximity effects stem from systems with a local order parameter, such as a local magnetic moment or a local superconducting pairing amplitude. Here, we demonstrate that despite lacking a local order parameter, topological phases also may give rise to a proximity effect of a distinctively inverted nature. We focus on a general construction in which a topological phase is extensively coupled to a second system, and we argue that, in many cases, the inverse topological order will be induced on the second system. To support our arguments, we rigorously establish this "bulk topological proximity effect" for all gapped free-fermion topological phases and representative integrable models of interacting topological phases. We present a terrace construction which illustrates the phenomenological consequences of this proximity effect. Finally, we discuss generalizations beyond our framework, including how intrinsic topological order may also exhibit this effect.

Emergent Electromagnetic Induction and Adiabatic Charge Pumping in Noncentrosymmetric Weyl Semimetals.

The photovoltaic effect due to the adiabatic quantum phase in noncentrosymmetric Weyl semimetals is studied. We particularly focus on the case in which an external ac electric field is applied. By considering a generalized Weyl Hamiltonian with nonlinear terms, we show that the photocurrent is induced by circularly, rather than linearly, polarized light. This photovoltaic current can be understood as an emergent electromagnetic induction in momentum space; the Weyl node is a magnetic monopole in momentum space, the circular motion of which induces the electric field. This result is distinct from conventional photovoltaic effects, and the estimated photocurrent is ∼10^{-1}-10^{1} nA, which can be detected experimentally.

Thermal Hall effects due to topological spin fluctuations in YMnO3.

The thermal Hall effect in magnetic insulators has been considered a powerful method for examining the topological nature of charge-neutral quasiparticles such as magnons. Yet, unlike the kagome system, the triangular lattice has received less attention for studying the thermal Hall effect because the scalar spin chirality cancels out between adjacent triangles. However, such cancellation cannot be perfect if the triangular lattice is distorted. Here, we report that the trimerized triangular lattice of multiferroic hexagonal manganite YMnO3 produces a highly unusual thermal Hall effect under an applied magnetic field. Our theoretical calculations demonstrate that the thermal Hall conductivity is related to the splitting of the otherwise degenerate two chiralities of its 120˚ magnetic structure. Our result is one of the most unusual cases of topological physics due to this broken Z2 symmetry of the chirality in the supposedly paramagnetic state of YMnO3, due to strong topological spin fluctuations with the additional intricacy of a Dzyaloshinskii-Moriya interaction.

Ultrafast Umklapp-assisted electron-phonon cooling in magic-angle twisted bilayer graphene.

Understanding electron-phonon interactions is fundamentally important and has crucial implications for device applications. However, in twisted bilayer graphene near the magic angle, this understanding is currently lacking. Here, we study electron-phonon coupling using time- and frequency-resolved photovoltage measurements as direct and complementary probes of phonon-mediated hot-electron cooling. We find a remarkable speedup in cooling of twisted bilayer graphene near the magic angle: The cooling time is a few picoseconds from room temperature down to 5 kelvin, whereas in pristine bilayer graphene, cooling to phonons becomes much slower for lower temperatures. Our experimental and theoretical analysis indicates that this ultrafast cooling is a combined effect of superlattice formation with low-energy moiré phonons, spatially compressed electronic Wannier orbitals, and a reduced superlattice Brillouin zone. This enables efficient electron-phonon Umklapp scattering that overcomes electron-phonon momentum mismatch. These results establish twist angle as an effective way to control energy relaxation and electronic heat flow.

Dirac half-metal in a triangular ferrimagnet.

An idea is proposed for realizing a fully spin-polarized Dirac semimetal in frustrated itinerant magnets. We show that itinerant electrons on a triangular lattice exhibit the Dirac cone dispersion with half-metallic behavior in the presence of a three-sublattice ferrimagnetic order. The Dirac nodes have the same structure as those of graphene. By variational calculation and Monte Carlo simulation, we demonstrate that the ferrimagnetic order with the Dirac node spontaneously emerges in a simple Kondo lattice model with Ising anisotropy. The realization will be beneficial for spintronics as a candidate for a spin-current generator.

Partial disorder in an Ising-spin Kondo lattice model on a triangular lattice.

A phase diagram of an Ising-spin Kondo lattice model on a triangular lattice near 1/3 filling is investigated by Monte Carlo simulation. We identify a partially disordered phase with the coexistence of magnetic order and paramagnetic moments, which was unstable in two-dimensional Ising models with localized spins only. The partial disorder emerges in the competing regime between a two-sublattice stripe phase and three-sublattice ferrimagnetic phase, at finite temperatures above an electronic phase separation. The peculiar magnetic structure accompanies a charge order and develops a gap in the electronic structure. The results manifest a crucial role of the nonperturbative interplay between spin and charge degrees of freedom in stabilizing the partial disorder.

Non-Kondo mechanism for resistivity minimum in spin ice conduction systems.

We present a mechanism of resistivity minimum in conduction electron systems coupled with localized moments, which is distinguished from the Kondo effect. Instead of the spin-flip process in the Kondo effect, electrons are elastically scattered by local spin correlations which evolve in a particular way under geometrical frustration as decreasing temperature. This is demonstrated by the cellular dynamical mean-field theory for a spin-ice-type Kondo lattice model on a pyrochlore lattice. Peculiar temperature dependences of the resistivity, specific heat, and magnetic susceptibility in the non-Kondo mechanism are compared with the experimental data in metallic Ir pyrochlore oxides.

Above-ordering-temperature large anomalous Hall effect in a triangular-lattice magnetic semiconductor.

While anomalous Hall effect (AHE) has been extensively studied in the past, efforts for realizing large Hall response have been mainly limited within intrinsic mechanism. Lately, however, a theory of extrinsic mechanism has predicted that magnetic scattering by spin cluster can induce large AHE even above magnetic ordering temperature, particularly in magnetic semiconductors with low carrier density, strong exchange coupling, and finite spin chirality. Here, we find out a new magnetic semiconductor EuAs, where Eu2+ ions with large magnetic moments form distorted triangular lattice. In addition to colossal magnetoresistance, EuAs exhibits large AHE with an anomalous Hall angle of 0.13 at temperatures far above antiferromagnetic ordering. As also demonstrated by model calculations, observed AHE can be explained by the spin cluster scattering in a hopping regime. Our findings shed light on magnetic semiconductors hosting topological spin textures, developing a field targeting diluted carriers strongly coupled to noncoplanar spin structures.

Giant anomalous Hall effect from spin-chirality scattering in a chiral magnet.

The electrical Hall effect can be significantly enhanced through the interplay of the conduction electrons with magnetism, which is known as the anomalous Hall effect (AHE). Whereas the mechanism related to band topology has been intensively studied towards energy efficient electronics, those related to electron scattering have received limited attention. Here we report the observation of giant AHE of electron-scattering origin in a chiral magnet MnGe thin film. The Hall conductivity and Hall angle, respectively, reach [Formula: see text] Ω-1 cm-1 and [Formula: see text]% in the ferromagnetic region, exceeding the conventional limits of AHE of intrinsic and extrinsic origins, respectively. A possible origin of the large AHE is attributed to a new type of skew-scattering via thermally excited spin-clusters with scalar spin chirality, which is corroborated by the temperature-magnetic-field profile of the AHE being sensitive to the film-thickness or magneto-crystalline anisotropy. Our results may open up a new platform to explore giant AHE responses in various systems, including frustrated magnets and thin-film heterostructures.

Quantum melting of charge ice and non-Fermi-liquid behavior: an exact solution for the extended Falicov-Kimball model in the ice-rule limit.

An exact solution is obtained for a model of itinerant electrons coupled to ice-rule variables on the tetrahedron Husimi cactus, an analogue of the Bethe lattice of corner-sharing tetrahedra. It reveals a quantum critical point with the emergence of non-Fermi-liquid behavior in melting of the "charge ice" insulator. The electronic structure is compared with the numerical results for the pyrochlore-lattice model to elucidate the physics of electron systems interacting with the tetrahedron ice rule.

Anomalous electrical magnetochiral effect by chiral spin-cluster scattering.

The non-collinear spin configurations give rise to many nontrivial phenomena related to the Berry phase. They are often related to the vector and scalar spin chiralities. The scalar spin chirality leads to the topological Hall effect in metals, while the vector spin chirality to the ferroelectricity of spin origin, i.e., multiferroics in insulators. However, the role of the vector spin chirality in conducting systems has not yet been elucidated. Here we show theoretically that the spin correlation with vector spin chirality in chiral magnets scatters electrons asymmetrically, resulting in nonreciprocal transport phenomena, i.e., electrical magnetochiral effect (eMCE). This asymmetric scattering appears in the leading-order scattering term, implying a large nonreciprocity in the charge and spin currents. We find that the temperature and magnetic field dependence of the eMCE reproduces that observed in MnSi. Our results reveal the microscopic mechanism of eMCE and its potential in producing a large nonreciprocal response.

Author Correction: Butterfly-shaped magnetoresistance in triangular-lattice antiferromagnet Ag2CrO2.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Butterfly-shaped magnetoresistance in triangular-lattice antiferromagnet Ag2CrO2.

Spintronic devices using antiferromagnets (AFMs) are promising candidates for future applications. Recently, many interesting physical properties have been reported with AFM-based devices. Here we report a butterfly-shaped magnetoresistance (MR) in a micrometer-sized triangular-lattice antiferromagnet Ag2CrO2. The material consists of two-dimensional triangular-lattice CrO2 layers with antiferromagnetically coupled S = 3/2 spins and Ag2 layers with high electrical conductivity. The butterfly-shaped MR appears only when the magnetic field is applied perpendicularly to the CrO2 plane with the maximum MR ratio (≈15%) at the magnetic ordering temperature. These features are distinct from those observed in conventional magnetic materials. We propose a theoretical model where fluctuations of partially disordered spins with the Ising anisotropy play an essential role in the butterfly-shaped MR in Ag2CrO2.

Minimal motif peptide structure of metzincin clan zinc peptidases in micelles.

It is well known that the functions of metalloproteins generally originate from their metal-binding motifs. However, the intrinsic nature of individual motifs remains unknown, particularly the details about metal-binding effects on the folding of motifs; the converse is also unknown, although there is no doubt that the motif is the core of the reactivity for each metalloprotein. In this study, we focused our attention on the zinc-binding motif of the metzincin clan family, HEXXHXXGXXH; this family contains the general zinc-binding sequence His-Glu-Xaa-Xaa-His (HEXXH) and the extended GXXH region. We adopted the motif sequence of stromelysin-1 and investigated the folding properties of the Trp-labeled peptides WAHEIAHSLGLFHA (STR-W1), AWHEIAHSLGLFHA (STR-W2), AHEIAHSLGWFHA (STR-W11), and AHEIAHSLGLFHWA (STR-W14) in the presence and absence of zinc ions in hydrophobic micellar environments by circular dichroism (CD) measurements. We accessed successful incorporation of these zinc peptides into micelles using quenching of Trp fluorescence. Results of CD studies indicated that two of the Trp-incorporated peptides, STR-W1 and STR-W14, exhibited helical folding in the hydrophobic region of cetyltrimethylammonium chloride micelle. The NMR structural analysis of the apo STR-W14 revealed that the conformation in the C-terminus GXXH region significantly differred between the apo state in the micelle and the reported Zn-bound state of stromelysin-1 in crystal structures. The structural analyses of the qualitative Zn-binding properties of this motif peptide provide an interesting Zn-binding mechanism: the minimum consensus motif in the metzincin clan, a basic zinc-binding motif with an extended GXXH region, has the potential to serve as a preorganized Zn binding scaffold in a hydrophobic environment.