Magic-angle helical trilayer graphene relaxes into commensurate moiré domains, whose topological and well-isolated sets of narrow bands possess ideal characteristics for realizing robust correlated topological phases, compared with other graphene-based moiré heterostructures. Combining strong-coupling analysis and Hartree-Fock calculations, we investigate the ground states at integer fillings $\nu$, and uncover a rich phase diagram of correlated insulators tuned by an external displacement field $D$. For small $D$, the system realizes several competing families of symmetry-broken generalized flavor ferromagnets, which exhibit various anomalous Hall signatures and Chern numbers as high as $\left|C\right|=6$. The interaction-induced dispersion renormalization is weak, so that the band flatness and the validity of strong-coupling theory are maintained at all integer fillings. For experimentally accessible displacement fields, the strong-coupling insulators at all $\nu$ undergo topological phase transitions, which appear continuous or weakly first order. For larger $D$, we also find translation symmetry-broken phases such as Kekulé spiral order. Our results demonstrate the robust capability of helical trilayer graphene to host gate-tunable topological and symmetry-broken correlated phases, and lay the groundwork for future theoretical studies on other aspects such as fractional topological states.

Superconductors possessing diverse symmetry-enforced topological states have been a subject of intense interest as they are arguably one of the most feasible candidates to realize so-called topological superconductivity, a source of Majorana fermions that hold great promise for topological quantum computing. Here we study the low-lying quasiparticle excitations in the superconducting intermetallic alloy ${\mathrm{V}}_2{\mathrm{Ga}}_5$ by low-temperature heat capacity and ultra-low-temperature thermal conductivity measurements. It is found that its electronic specific heat can be fitted by either a $d$-wave gap or two $s$-wave gaps. However, the low-$T$ thermal conductivity clearly points to the multiple nodeless energy gaps in its low-lying excitation spectrum. On the other hand, first-principles calculations reveal a multitude of topological fermions near the Fermi level, involving discrete Dirac nodes as well as Dirac nodal lines. These topological carriers, when condensed into Cooper pairs, provide a natural platform for achieving topologically nontrivial phases and possible Majorana fermions.

Experimental investigations of spin-charge interconversion in magnetic bilayers comprising a ferromagnet adjacent to a topological insulator have reported scattered results on the spin-charge and charge-spin conversion efficiency. Attempting to reconcile these contradicting experimental results, we develop a phenomenological theory of spin-charge interconversion accounting for both interfacial interconversion through the spin galvanic effect, also called the Rashba-Edelstein effect, as well as bulk interconversion via the spin Hall effect. We find that the spin current leakage into the nonmagnetic metal plays a central role during the spin-to-charge and charge-to-spin conversion, leading to dissymmetric interconversion processes. In particular, spin-to-charge conversion is much less sensitive to the spin current absorption in the nonmagnetic metal than charge-to-spin conversion. This suggests that spin pumping is a more trustable technique to extract the interfacial Rashba parameter than spin-orbit torque.

Amperean superconductivity is an exotic phenomenon stemming from attractive effective electron-electron interactions (EEEIs) mediated by a transverse gauge field. Originally introduced in the context of quantum spin liquids and high-$T_{\mathrm{c}}$ superconductors, Amperean superconductivity has been recently proposed to occur at temperatures on the order of $1–20\mathrm{K}$ in two-dimensional, parabolic-band, electron gases embedded inside deep subwavelength optical cavities. In this work, we first generalize the microscopic theory of cavity-induced Amperean superconductivity to the case of graphene and then argue that this superconducting state cannot be achieved in the deep subwavelength regime. In the latter regime, indeed, a cavity induces only EEEIs between density fluctuations rather than the current-current interactions which are responsible for Amperean pairing.

The interplay of frustrated interactions and lattice geometry can lead to a variety of exotic quantum phases. Here we unearth a particularly rich phase diagram of the Kitaev-Heisenberg model on the star lattice, a triangle decorated honeycomb lattice breaking sublattice symmetry. In the antiferromagnetic regime, the interplay of Heisenberg coupling and geometric frustration leads to the formation of valence bond solid (VBS) phases—a singlet VBS and a bond selective triplet VBS stabilized by the Kitaev exchange. We show that the ratio of the Kitaev versus Heisenberg exchange tunes between these VBS phases and chiral quantum spin-liquid regimes. Remarkably, the VBS phases host a whole variety of chiral triplon excitations with high Chern numbers in the presence of a weak magnetic field. We discuss our results in light of a recently synthesized star lattice material and other decorated lattice systems.

In an extended superconductor-topological insulator-superconductor (S-TI-S) Josephson junction in a magnetic field, localized Majorana bound states (MBS) are predicted to exist at the cores of Josephson vortices where the local phase difference across the junction is an odd-multiple of $\pi$. These states contribute a supercurrent with a $4\pi$-periodic current-phase relation (CPR) that adds to the conventional $2\pi$-periodic sinusoidal CPR. In this work, we present a comprehensive experimental study of the critical current vs. applied magnetic field diffraction patterns of lateral Nb-${\mathrm{Bi}}_2{\mathrm{Se}}_3$-Nb Josephson junctions. We compare our observations to a model of the Josephson dynamics in the S-TI-S junction system to explore what features of MBS are, or are not, exhibited in these junctions. Consistent with the model, we find several distinct deviations from a Fraunhofer diffraction pattern that is expected for a uniform $\sin \left(\varphi \right)$ CPR. In particular, we observe abrupt changes in the diffraction pattern at applied magnetic fields in which the current-carrying localized MBS are expected to enter the junction, and a lifting of the odd-numbered nodes consistent with a $4\pi$-periodic $\sin (\varphi /2)$ component in the CPR. We also see that although the even-numbered nodes often remain fully formed, we sometimes see deviations that are consistent with quasiparticle-induced fluctuations in the parity of the MBS pairs that encode quantum information.

We report anomalous circularly polarized Raman spectra of phonons in right- and left-handed quartzes. The phonon branches splitting from the $E$-mode at a finite wave number were found to be chiral with mutually opposite angular momenta. Our analysis reveals the Raman selection rules for chiral phonons. We also find that the conservation of angular momentum should be satisfied between two photons and one chiral phonon in the Raman process. Furthermore, we experimentally proved that the helicity of phonons with a certain frequency and wave vector should be opposite in the other enantiomer.

We consider nonequilibrium time evolution in quantum spin chains after a global quench. Usually a nonequilibrium quantum many-body system locally relaxes to a (generalized) Gibbs ensemble built from conserved operators with quasilocal densities. Here we exhibit explicit examples of local Hamiltonians that possess conservation laws with densities that are not quasilocal but act as such in the symmetry-restricted space where time evolution occurs. Because of them, the stationary state emerging at infinite time can exhibit exceptional features. We focus on a specific example with a spin-flip symmetry, which is the commonest global symmetry encountered in spin-$1/2$ chains. Among the exceptional properties, we find that, at late times, the excess of entropy of a spin block triggered by a local perturbation in the initial state grows logarithmically with the subsystem's length. We establish a connection with symmetry-protected topological order in equilibrium at zero temperature and study the melting of the order induced either by a (symmetry-breaking) rotation of the initial state or by an increase of the temperature.

From a systematic study of thermal and charge transport in various single crystals of compensated topological insulators we identify the evolution of a large low-temperature thermal Hall effect as a characteristic common feature. In order to separate phononic and electronic contributions in the measured longitudinal and transverse thermal conductivity, the electronic contributions are estimated from corresponding electrical resistivity and Hall effect measurements on the same samples by using the Wiedemann-Franz law. As may be expected for charge-compensated topological insulators the longitudinal thermal conductivity is phonon dominated in all samples. However, we also find a pronounced field-linear thermal Hall effect that becomes most pronounced in the low-temperature range, where all samples are good electrical insulators. This indicates an underlying phononic mechanism of the thermal Hall effect and in this respect the topological insulators resemble other, mainly ionic, insulators, which have been reported to show a phonon-induced thermal Hall effect, but its underlying phononic mechanism remains to be identified. Our observation of a comparable thermal Hall ratio in topological insulators supports a theoretical scenario that explains a thermal Hall effect through skew scattering on charged impurities.

Heterostructures of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ on transition metal dichalcogenides (TMDCs) offer a new materials platform for studying novel quantum states by exploiting the interplay among topological orders, charge orders, and magnetic orders. The diverse interface attributes, such as the material combination, charge rearrangement, defects, and strain, can be utilized to manipulate the quantum properties of this class of materials. Recent experiments with ${\mathrm{Bi}}_2{\mathrm{Se}}_3/{\mathrm{NbSe}}_2$ heterostructures show signatures of strong Rashba band splitting due to the presence of a BiSe buffer layer, but the atomic level mechanism is not fully understood. We conduct first-principles studies of the ${\mathrm{Bi}}_2{\mathrm{Se}}_3/\text{BiSe}/\mathrm{TMDC}$ heterostructures with five different TMDC substrates ($1T$ phase ${\mathrm{VSe}}_2, {\mathrm{MoSe}}_2$, and ${\mathrm{TiSe}}_2$ and $2H$ phase ${\mathrm{NbSe}}_2$ and ${\mathrm{MoSe}}_2$). We find significant charge transfer at both BiSe/TMDC and ${\mathrm{Bi}}_2{\mathrm{Se}}_3/\text{BiSe}$ interfaces driven by the work function difference, which stabilizes the BiSe layer as an electron donor. The electric field of the ${\mathrm{Bi}}_2{\mathrm{Se}}_3/\text{BiSe}$ interface dipole breaks the inversion symmetry in the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ layer, leading to the giant Rashba band splitting in two quintuple layers and the recovery of the Dirac point in three quintuple layers, with the latter otherwise occurring only in thicker samples with at least six ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ quintuple layers. In addition, we find that strain can significantly affect the charge transfer at the interfaces. Our study presents a promising avenue for tuning topological properties in heterostructures of two-dimensional materials, with potential applications in quantum devices.

In the last few decades, ultrafast demagnetization elicited by ultrashort laser pulses has been the subject of a large body of work that aims to better understand and control this phenomenon. Although specific magnetic materials’ properties play a key role in defining ultrafast demagnetization dynamics, features of the driving laser pulse such as its duration and photon energy might also contribute. Here, we report ultrafast demagnetization of a cobalt/platinum multilayer in a broad spectral range spanning from the near-infrared (near-IR) to the mid-infrared (mid-IR), with wavelengths between 0.8 and 8.7 µm. The ultrafast dynamics of the macroscopic magnetization is tracked via the time-resolved magneto-optical Kerr effect. We show that the ultrafast demagnetization of the sample can be efficiently induced over that entire excitation spectrum with minimal dependence on the excitation wavelength. Instead, we confirm that the temporal profile of the pump excitation pulse is an important factor influencing ultrafast demagnetization dynamics.

Ferrotoroidic order has recently been established as a new form of spontaneous long-range order. Its inherent magnetoelectric properties have the potential to be utilized for spintronics based on magnetization-free systems. While spintronics applications have been mostly discussed for conducting systems, the ferroic properties of ferrotoroidic order have been mainly studied in insulating materials thus far. Here we report on the observation of ferrotoroidic order in a rare-earth tetraboride, $\mathrm{Nd}{\mathrm{B}}_4$, that shows metallic conductivity and no spontaneous magnetization. Using optical second harmonic generation (SHG), we spatially resolve the formation, distribution, and behavior of the ferrotoroidic domains across the ordering temperature. Due to the interference between the domains with the opposite ferrotoroidic polarity, SHG signals cancel out at the domain boundaries, which enables us to image the ferrotoroidic domain pattern. This study opens the way to explore unique functionalities of ferrotoroidic domains in metals, such as an electric-current-induced domain switching.

We performed angle-resolved photoemission spectroscopy (ARPES) on hexagonal MnTe, a candidate for an altermagnet with a high critical temperature $(T_{\mathrm{N}}=307\text{K})$. By utilizing photon-energy-tunable ARPES in combination with first-principles calculations, we found that the band structure in the antiferromagnetic phase exhibits a strongly anisotropic band-splitting associated with the time-reversal-symmetry breaking, providing direct experimental evidence for the altermagnetic band splitting. The magnitude of the splitting reaches 0.8 eV at non-high-symmetry momentum points, which is much larger than the spin-orbit gap of $\sim 0.3$ eV along the $\mathrm{\Gamma }K$ high-symmetry cut. The present result paves the way toward realizing exotic physical properties associated with the altermagnetic spin splitting.

Twisted bilayer graphene (TBLG) subject to a sequence of commensurate external periodic potentials reveals the formation of moiré fractals (MFs) that share striking similarities with the central place theory of economic geography, thus uncovering a remarkable connection between twistronics and the geometry of economic zones. MFs arise from the self-similarity of the emergent hierarchy of Brillouin zones (BZs), forming a nested subband structure within the bandwidth of the original moiré bands. We derive the fractal generators for TBLG under these external potentials and explore their impact on the hierarchy of the BZ edges and the wave functions at the Dirac point. By examining realistic supermoiré structures and demonstrating their equivalence to MFs with periodic perturbations under specific conditions, we establish MFs as a general description for such systems. Furthermore, we uncover parallels between the modification of the BZ hierarchy and magnetic BZ formation in Hofstadter's butterfly, allowing us to construct an incommensurability measure for MFs versus twist angle. The resulting band structure hierarchy bolsters correlation effects, pushing more bands within the same energy window for both commensurate and incommensurate TBLG.

The polarization discontinuity across interfaces in polar nitride-based heterostructures can lead to the formation of two-dimensional electron and hole gases. In the past, the observation of these electron and hole gases has been achieved through various experimental techniques, most often by electronic measurements but occasionally by optical means. However, the occurrence of a two-dimensional hole gas has never been demonstrated optically in nitride-based heterostructures. The objective of this article is to demonstrate, thanks to the combination of various optical spectroscopy techniques coupled to numerical simulations, the presence of a two-dimensional hole gas in a GaN/AlGaN/GaN heterostructure. This is made possible thanks to a GaN/AlGaN/GaN heterostructure displaying a micrometer-thick AlGaN layer and a GaN cap thicker than in conventional GaN-based HEMTs structures. The band structure across the whole heterostructure was established by solving self-consistently the Schrödinger and Poisson equations and by taking into account the experimentally determined strain state of each layer. The appearance of a two-dimensional hole gas in such structure is thus established first theoretically. Continuous and quasicontinuous photoluminescence, spanning six orders of magnitude excitation intensities, reveal the presence of a broad emission band at an energy around 50 meV below the exciton emission and whose energy blueshifts with increasing excitation power density, until it is completely quenched due to the complete screening of the internal electric field. Time-resolved measurements show that the emission arising from the two-dimensional hole gas can be assigned to the recombination of holes in the potential well with electrons located in the top GaN as well as electron from the bottom AlGaN, each of them displaying different decay times due to unequal electric fields. Besides the optical demonstration of a two-dimensional hole gas in a nitride-based heterostructure, our work highlights the rich optical recombination processes involved in the emission from such a hole gas.

Chiral coupling, which allows directional interactions between quantum dots (QDs) and photonic crystal waveguide modes, holds promise for enhancing the functionality of quantum photonic integrated circuits. Elliptical polarizations of QD transitions already offer a considerable enhancement in directionality. However, in epitaxial QD fabrication, the lack of precise control over lateral QD positions still poses a challenge in achieving efficient chiral interfaces. Here, we present a theoretical analysis in which we propose to optimize the polarization of a QD emitter against the spatially averaged directionality and demonstrate that the resulting emitter offers a considerable technological advantage in terms of the size and location of high-directionality areas of the waveguide as well as their overlap with the regions of large Purcell enhancement, thereby improving the scalability of the device. Moreover, using $\mathit{k}·\mathit{p}$ modeling, we demonstrate that the optimal elliptical polarization can be achieved for neutral exciton transitions in a realistic QD structure. Our results present a viable path for efficient chiral coupling in QD-based photonic integrated circuits, to a large extent overcoming the challenges and limitations of the present manufacturing technology.

The study of quantum criticality and entanglement in systems with long-range (LR) interactions is still in its early stages, with many open questions remaining to be explored. In this work, we investigate critical exponents and scaling of entanglement entropy (EE) in the LR bilayer Heisenberg model using large-scale quantum Monte Carlo simulations. By applying modified (standard) finite-size scaling above (below) the upper critical dimension and field theory analysis, we obtain precise critical exponents in three regimes: the LR Gaussian regime with a Gaussian fixed point, the short-range (SR) regime with Wilson-Fisher exponents, and a LR non-Gaussian regime where the critical exponents vary continuously from LR Gaussian to SR values. We compute the Rényi EE both along the critical line and in the Néel phase, and we observe that as the LR interaction is enhanced, the area-law contribution in EE gradually vanishes both at quantum critical points (QCPs) and in the Néel phase. The log-correction in EE arising from sharp corners at the QCPs also decays to zero as the LR interaction grows, whereas that for Néel states, caused by the interplay of Goldstone modes and restoration of the symmetry in a finite system, is enhanced. Relevant experimental settings to detect these nontrivial properties for quantum many-body systems with LR interactions are discussed.

The standard approach to realize a spin-liquid state is through magnetically frustrated states, relying on ingredients such as the lattice geometry, dimensionality, and magnetic interaction type of the spins. While Heisenberg spins on a pyrochlore lattice with only antiferromagnetic nearest-neighbor interactions are theoretically proven disordered, spins in real systems generally include longer-range interactions. The spatial correlations at longer distances typically stabilize a long-range order rather than enhancing a spin-liquid state. Both states can, however, be destroyed by short-range static correlations introduced by chemical disorder. Here, using disorder-free specimens with a clear long-range antiferromagnetic order, we refine the spin structure of the Heisenberg spinel ${\mathrm{ZnFe}}_2{\mathrm{O}}_4$ through neutron magnetic diffraction. The unique wave vector $(1,0,\frac{1}{2})$ leads to a spin structure that can be viewed as alternatively stacked ferromagnetic and antiferromagnetic tetrahedra in a three-dimensional checkerboard form. Stable coexistence of these opposing types of clusters is enabled by the bipartite breathing pyrochlore crystal structure, leading to a second-order phase transition at 10 K. The diffraction intensity of ${\mathrm{ZnFe}}_2{\mathrm{O}}_4$ is an exact complement to the inelastic scattering intensity of several chromate spinel systems which are regarded as model classical spin liquids. Our results challenge this attribution, and suggest instead of the six-spin ring mode, spin excitations in chromate spinels are closely related to the $(1,0,\frac{1}{2})$ type of spin order and the four-spin ferromagnetic cluster locally at one tetrahedron.

The diversity of magnetic orders that appear in layered magnetic materials is of great interest from the fundamental point of view and for applications. In particular, the magnetic sublayers, introduced by intercalation into van der Waals gaps of the host transition-metal dichalcogenide (TMD), are known to produce various magnetic states, with some being tunable by pressure and doping. The magnetic sublayers and their magnetic ordering strongly modify the electronic coupling between layers of the host compound. Understanding the roots of this variability, starting from the underlying electronic structure, is a significant challenge. Here we employ the angle-resolved photoelectron spectroscopy at various photon energies, the ab initio electronic structure calculations, and modeling to address the particular case of Ni-intercalate, ${\mathrm{Ni}}_{1/3}{\mathrm{NbS}}_2$. We find that the bands around the Fermi level bear the signature of a strong yet unusual hybridization between ${\mathrm{NbS}}_2$ conduction band states and the Ni $3d$ orbitals. The hybridization between metallic ${\mathrm{NbS}}_2$ layers is almost entirely suppressed in the central part of the Brillouin zone, including the part of the Fermi surface around the $\mathrm{\Gamma }$ point. Simultaneously, it gets very pronounced towards the zone edges. It is shown that this behavior is the consequence of the rather exceptional, symmetry imposed, spatially strongly varying, zero total hybridization between relevant Ni magnetic orbitals and the neighboring Nb orbitals that constitute the metallic bands. We also report the presence of the so-called $\beta$ feature, discovered only recently in two other magnetic intercalates with very different magnetic orderings. In ${\mathrm{Ni}}_{1/3}{\mathrm{NbS}}_2$, the feature shows only at particular photon energies, indicating its bulk origin. Common to prior observations, it appears as a series of very shallow electron pockets at the Fermi level, positioned along the edge of the Brillouin zone. Unforeseen by ab initio electronic calculations, and its origin still unresolved, the feature appears to be a robust consequence of the intercalation of $2\mathrm{H}\text{−}{\mathrm{NbS}}_2$ with magnetic ions.

Several studies have explored how phononic switching of magnetization can be achieved in doped films of yttrium iron garnet (YIG) with an initially homogeneous magnetic state. In this paper, we investigate how this mechanism operates in high-purity YIG films with a nonuniform stripe domain structure. We demonstrate that a mid-infrared pulse causes the stripe domains to rotate and form a distinctive nonuniform pattern that ultimately follows the sample's easy axes. Irradiation with subsequent pulses progressively displaces the reoriented domains, thus extending the total affected area. Micromagnetic simulations, modeling the excitation as a strain pulse, provide a conceptual understanding of the observed magnetization pattern. Our observations thus extend the mechanism of phononic switching of magnetization—which manifests as stripe reorientation in the stripe domain structure—which was previously observed only in homogeneous magnetic textures.