Motivated by the recent observation of evidence of ferromagnetism in correlated insulating states in systems with moiré superlattices, we study a two-orbital quantum antiferromagnetic model on the triangular lattice, where the two orbitals physically correspond to the two valleys of the original graphene sheet. For simplicity this model has a $\mathrm{SU}{\left(2\right)}^s\otimes \mathrm{SU}{\left(2\right)}^v$ symmetry, where the two $\mathrm{SU}\left(2\right)$ symmetries correspond to the rotation within the spin and valley spaces, respectively. Through analytical argument, Schwinger boson analysis, and also density matrix renormalization group simulation, we find that even though all the couplings in the Hamiltonian are antiferromagnetic, there is still a region in the phase diagram with fully polarized ferromagnetic order. We argue that a Zeeman field can drive a metal-insulator transition in our picture, as was observed experimentally. We also construct spin liquids and topological ordered phases at various limits of this model. Then after doping this model with extra charge carriers, the system most likely becomes a spin-triplet/valley-singlet $d+id$ topological superconductor, as was predicted previously.

Magnetism in palladium has been the subject of much work and speculation. Bulk crystalline palladium is paramagnetic with a high magnetic susceptibility. Palladium under pressure and palladium nanoclusters have generated interest to scrutinize its magnetic properties. Here we report another possibility: palladium may become an itinerant ferromagnet in the amorphous bulk phase at atmospheric pressure. Atomic palladium is a $d^{10}$ element, whereas bulk crystalline Pd is a $d^{10-x}{\left(sp\right)}^x$ material; this, together with the possible presence of “unsaturated bonds” in amorphous materials, may explain the remnant magnetism reported herein. This work presents and discusses magnetic effects in bulk amorphous palladium.

Strong vortex pinning in FeSe could be useful for technological applications and could provide clues about the coexistence of superconductivity and nematicity. To characterize the pinning of individual, isolated vortices, we simultaneously apply a local magnetic field and image the vortex motion with scanning superconducting quantum interference devices susceptibility. We find that the pinning is highly anisotropic: the vortices move easily along directions that are parallel to the orientations of twin domain walls and pin strongly in a perpendicular direction. These results are consistent with a scenario in which the anisotropy arises from vortex pinning on twin domain walls and quantify the dynamics of individual vortex pinning in FeSe.

The electronic structure of a Möbius Kondo insulator candidate of CeRhSb has been investigated by employing angle-resolved photoemission spectroscopy, density functional theory (DFT) band calculations, and dynamical mean-field theory (DMFT) band calculations. Fermi surfaces (FSs) and band structures are successfully measured for three orthogonal crystallographic directions. A sharp Ce $4f$ peak is observed at the Fermi level ($E_{\text{F}}$), and its temperature ($T$) evolution agrees with that of the Ce $4f$ Kondo resonance. The metallic FSs are obtained for all three different (100), (010), and (001) planes. The Ce $4f$ FSs are described properly by the unfolded DFT calculations considering the reduced Ce-only unit cell. The $T$ dependence of Ce $4f$ states as well as the dispersive coherent Ce $4f$ bands are described well by the DMFT calculations and reveal the anisotropic $c\text{−}f$ hybridization. The photon energy dependence of the Fermi-edge states in CeRhSb reveals the three-dimensional character, consistent with the bulk states dispersing to $E_{\text{F}}$ over a larger energy scale rather than the predicted Möbius topological surface states.

We investigate the possibility of reducing the phase space of magnetic fluctuations at the weakly first-order Néel transition in Cr by neutron diffraction to test whether a second-order phase transition can be induced under the application of uniaxial pressure. Using an improved setup to reduce stress inhomogeneities we succeed in increasing significantly the pressure range until irreversible broadening due to plastic deformation occurs compared with previous studies. Despite the observed tripling of the intensity of the magnetic Bragg peak $(0,0,1-\delta )$ at $p_{\left[110\right]}\ge 450$ bar, indicating a full population of a single-${\mathbf{Q}}_{\pm }$-domain state, no hints of a tricritical point are observed, i.e., the phase transition remains weakly first order. Therefore, the reduction in the phase space of the magnetic fluctuations by uniaxial pressure does not lead to the predicted second-order phase transition.

Conformal field theory has recently been applied to derive few-body Hamiltonians whose ground states are lattice versions of fractional quantum Hall states. The exact lattice models involve interactions over long distances, which is difficult to realize in experiments. It seems, however, that such long-range interactions should not be necessary, as the correlations decay exponentially in the bulk. This poses the question, whether the Hamiltonians can be truncated to contain only local interactions without changing the physics of the ground state. Previous studies have, in a couple of cases with particularly much symmetry, obtained such local Hamiltonians by keeping only a few local terms and numerically optimizing the coefficients. Here, we investigate a different strategy to construct truncated Hamiltonians, which does not rely on optimization, and which can be applied independently of the choice of lattice. We test the approach on two models with bosonic Laughlin-like ground states with filling factor $1/2$ and $1/4$, respectively. We first investigate how the coupling strengths in the exact Hamiltonians depend on distance, and then we study the truncated models. For the case of $1/2$ filling, we find that the truncated model with truncation radius $\sqrt{2}$ lattice constants on the square lattice and 1 lattice constant on the triangular lattice has an approximate twofold ground-state degeneracy on the torus, and the overlap per site between these states and the states constructed from conformal field theory is higher than 0.99 for the lattices considered. For the model at $1/4$ filling, our results give some hints that a truncation radius of $\sqrt{5}$ on the square lattice and $\sqrt{7}$ on the triangular lattice might be enough, but the finite-size effects are too large to judge whether the topology is, indeed, present in the thermodynamic limit. The states with high overlap also have the expected topological entanglement entropies.

We measured basic parameters such as the lifetime, mobility, and diffusion constant of trapped paraexcitons in ${\mathrm{Cu}}_2\mathrm{O}$ at very low temperatures (below 1 K) using a dilution refrigerator. To obtain these parameters, we observed the space- and time-resolved luminescence spectrum of paraexcitons in a strain-induced trap potential. We extracted the lifetime of 410 ns from the measurements of the decay of the luminescence intensity. By comparing the experimental results and numerical calculations, we found that the mobility and the diffusion constant increase as the temperature of the paraexcitons decreases below 1 K. In particular, we obtained a mobility of $5.1\times {10}^7{\mathrm{cm}}^2$/V s at the corresponding temperature of 280 mK. To the best of our knowledge, this value is the highest exciton mobility that has been measured. These results show that the mean-free path of the paraexcitons reaches a size ($\sim 300 \mu \mathrm{m}$) comparable to that of the cloud of trapped paraexcitons ($\sim 100\mu \mathrm{m}$). From our analyses, we found that the spatial distribution of the paraexcitons can reach a distribution that is defined by the statistical distribution function and the shape of the three-dimensional trap potential at ultralow temperatures (well below 1 K). Our survey shows that the ultralow temperature ensures that the Bose-Einstein condensation transition in a trap potential can be investigated by examining the spatial distribution of the density of $1s$ paraexcitons.

We study the Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling between localized magnetic impurities in the ${\text{MoS}}_2$ nanoribbons. Our calculations are based on a three-orbital tight-binding model for a ${\text{MoS}}_2$ finite nanoribbon system with periodic boundary conditions. We consider impurities hybridized to different d orbitals of the host transition metal. The large intrinsic spin-orbit coupling leads to the spatial anisotropy in the RKKY interaction amplitude and appearance of collinear and noncollinear terms. We find that the RKKY interaction is sensitive to the Fermi energy values and changes dramatically in doped systems. We analyze the exponent decay behavior of the RKKY interaction envelope for doping levels inside the band gap of the infinite layer.

Coupling a normal-metal wire to a superconductor induces an excitation gap ${\mathrm{\Delta }}_{\mathrm{ind}}$ in the normal metal. In the absence of disorder, the induced excitation gap is strongly suppressed by finite-size effects if the thickness $D_{\mathrm{S}}$ of the superconductor is much smaller than the thickness $D_{\mathrm{N}}$ of the normal metal and the superconducting coherence length $\xi$. We show that the presence of disorder, either in the bulk or at the exposed surface of the superconductor, significantly enhances the magnitude of ${\mathrm{\Delta }}_{\mathrm{ind}}$, such that ${\mathrm{\Delta }}_{\mathrm{ind}}$ approaches the superconducting gap $\mathrm{\Delta }$ in the limit of strong disorder. We also discuss the shift of energy bands inside the normal-metal wire as a result of the coupling to the superconducting shell.

We show that the conductivity of a two-dimensional electron gas can be intrinsically anisotropic despite isotropic Fermi surface, energy dispersion, and disorder configuration. In the model we study, the anisotropy stems from the interplay between Dirac and Schrödinger features combined in a special two-band Hamiltonian describing the quasiparticles similar to the low-energy excitations in phosphorene. As a result, even scalar isotropic disorder scattering alters the nature of the carriers and results in anisotropic transport. Solving the Boltzmann equation exactly for such carriers with pointlike random impurities, we find a hidden knob to control the anisotropy just by tuning either the Fermi energy or temperature. Our results are expected to be generally applicable beyond the model studied here, and should stimulate further search for the alternative ways to control electron transport in advanced materials.

Scattering from a dielectric nanowire coated by helical graphene strips (nanoribbons) is investigated for a dual-polarized wave at oblique incidence. In the long-wavelength approximation, the helical strips are treated as a homogeneous layer with averaged tensor conductivity. It is shown that the performance of well-known surface cloaks in the form of graphene monolayer, axial, and azimuthal graphene strips can be deteriorated in a wide range of incidence angles. To overcome this problem, helical graphene strips are proposed as an advanced metasurface for dual-polarized all-angle cloaking of the dielectric nanowire in the terahertz band. It is found that such metasurface suppresses scattering from the nanowire more efficiently as compared to graphene monolayer, regardless of the angle of wave incidence. Moreover, dual-polarized all-angle cloaking of the dielectric nanowire can be broadly tuned in frequency with parameters of helical graphene strips.

Recent studies of van der Waals heterostructures involving transition metal dichalcogenide (TMD) overlayers have revealed the formation of highly ordered mirror twin boundaries (MTBs) dividing domains in the single 2H phase. Here, using a multiscale modeling approach, we identify that the MTB network formation results from a delicate interplay between strain accumulation in the heterostructure and single-crystal preference of the growing overlayer. We determine the energy costs for the creation of the MTB by first-principles calculations, from which we show that even the presence of a perceived-to-be negligible strain is able to induce the formation of the MTB networks as an effective strain-relief mechanism of the growing TMD monolayers, as observed experimentally. This counterintuitive finding demonstrates the importance of collective effects in weakly interacting systems, i.e., in van der Waals epitaxy.

A gear effect is demonstrated at parallel and cross junctions between boron nitride nanotubes (BNNTs) via atomistic simulations. The atoms of neighboring BNNTs are meshed together at the junctions like gear teeth through long-range noncovalent interactions, which are shown to be able to transmit motion and power. The sliding motion of a BNNT can be spontaneously translated to rotating motion of an adjoining one or viceversa at a well-defined speed ratio. The transmittable motion and force strongly depend on the helical lattice structure of BNNTs represented by a chiral angle. The motion transmission efficiency of the parallel junctions increases up to a maximum for certain BNNTs depending on the displacement rate. It then decreases with increasing chiral angles. For cross junctions, the angular motion transmission ratio increases with decreasing chiral angles of the driven BNNTs, while the translational one exhibits the opposite trend.

Lattice vibrations of point defects are essential for understanding nonradiative electron and hole capture in semiconductors as they govern properties including persistent photoconductivity and the Shockley-Read-Hall recombination rate. Although the harmonic approximation is sufficient to describe a defect with small lattice relaxation, for cases of large lattice relaxation it is likely to break down. We describe a first-principles procedure to account for anharmonic carrier capture and apply it to the important case of the DX center in GaAs. This is a system where the harmonic approximation grossly fails. Our treatment of the anharmonic Morse-like potentials accurately describes the observed electron capture barrier, predicting the absence of quantum tunneling at low temperature, and a high hole capture rate that is independent of temperature. The model also explains the origin of the composition-invariant electron emission barrier. These results highlight an important shortcoming of the standard approach for describing point defect ionization that is accompanied by large lattice relaxation, where charge transfer occurs far from the equilibrium configuration.

Nanoscale optical isolators are crucial for nanophotonic, quantum-optical, and optoelectronic applications and have attracted considerable attention recently. The application of external magnetic fields is the standard way for creating nonreciprocity as the basis for optical isolation. However, the magneto-optical response is usually small, thus it should be enhanced to create strong enough nonreciprocity for high-contrast isolation. We demonstrate that a graphene waveguide ring resonator allows a nanoscale platform for a high-contrast optical isolator. Graphene provides strong subwavelength confinement and low loss, therefore large nonreciprocity can be realized for ultracompact high-contrast isolators by plasmon resonance enhancement combined with resonator resonance enhancement. The resonance frequency is widely tunable by controlling the gate voltages of the rolling graphene sheet of a graphene resonator. Manipulation of the waveguide-resonator coupling can be achieved by controlling the gate voltage of the planar graphene sheet. The concept of magnetically biased ultracompact graphene waveguide ring resonators for one-way plasmon flow allows one to tune the operation band over a one-octave-spanning broad frequency range, keeping the contrast higher than 99% and a moderate insertion loss lower than 50%. High performance is achievable by only adjusting both gate voltages of the planar and rolling graphene sheets.

The valley-contrasting orbital magnetic moment of Bloch electrons allows the lifting of valley degeneracy by an out-of-plane magnetic field. We demonstrate that this leads to negative magnetoresistance, utilizing a gapped two-dimensional multivalley model as an example. An intuitive physical picture in terms of the increased carrier density from a magnetic gating effect is proposed for this negative magnetoresistance. In particular, giant negative magnetoresistance is achieved after one of the two valleys is depleted by the magnetic field. This mechanism of negative magnetoresistance is argued to be relevant in ionic-liquid gated gapped graphene with a small effective mass.

Even a small amplitude charge-density wave (CDW) can reconstruct a Fermi surface, giving rise to new quantum oscillation frequencies. Here, we investigate quantum oscillations when the CDW has a finite correlation length $\xi$—a case relevant to the hole-doped cuprates. By considering the Berry phase induced by a spatially varying CDW phase, we derive an effective Dingle factor that depends exponentially on the ratio of the cyclotron orbit radius, $R_c$, to $\xi$. In the context of $\mathrm{YBa}{}_2\mathrm{Cu}{}_3\mathrm{O}{}_y$ (YBCO), we conclude that the values of $\xi$ reported to date for bidirectional CDW order are, prima facie, too short to account for the observed Fermi surface reconstruction; on the other hand, the values of $\xi$ for the unidirectional CDW are just long enough.

The classification of phase transitions is a central and challenging task in condensed matter physics. Typically, it relies on the identification of order parameters and the analysis of singularities in the free energy and its derivatives. Here, we propose an alternative framework to identify quantum phase transitions, employing both unsupervised and supervised machine learning techniques. Using the axial next-nearest-neighbor Ising (ANNNI) model as a benchmark, we show how unsupervised learning can detect three phases (ferromagnetic, paramagnetic, and a cluster of the antiphase with the floating phase) as well as two distinct regions within the paramagnetic phase. Employing supervised learning we show that transfer learning becomes possible: a machine trained only with nearest-neighbor interactions can learn to identify a new type of phase occurring when next-nearest-neighbor interactions are introduced. All our results rely on few- and low-dimensional input data (up to twelve lattice sites), thus providing a computational friendly and general framework for the study of phase transitions in many-body systems.

We study the electron-magnon scattering in bulk Fe, Co, and Ni within the framework of many-body perturbation theory implemented in the full-potential linearized augmented-plane-wave method. To this end, a $\mathbf{k}$-dependent self-energy ($GT$ self-energy) describing the scattering of electrons and magnons is constructed from the solution of a Bethe-Salpeter equation for the two-particle (electron-hole) Green function, in which single-particle Stoner and collective spin-wave excitations (magnons) are treated on the same footing. Partial self-consistency is achieved by the alignment of the chemical potentials. The resulting renormalized electronic band structures exhibit strong spin-dependent lifetime effects close to the Fermi energy, which are strongest in Fe. The renormalization can give rise to a loss of quasiparticle character close to the Fermi energy, which we attribute to electron scattering with spatially extended spin waves. This scattering is also responsible for dispersion anomalies in conduction bands of iron and for the formation of satellite bands in nickel. Furthermore, we find a band anomaly at a binding energy of 1.5 eV in iron, which results from a coupling of the quasihole with single-particle excitations that form a peak in the Stoner continuum. This band anomaly was recently observed in photoemission experiments. On the theory side, we show that the contribution of the Goldstone mode to the $GT$ self-energy is expected to (nearly) vanish in the long-wavelength limit. We also present an in-depth discussion about the possible violation of causality when an incomplete subset of self-energy diagrams is chosen.

We perform a systematic study of the macroscopic dielectric function and electron energy loss (EEL) spectra for graphite. We obtain the dispersion behavior for the $\pi$ plasmon, as a function of the momentum transfer $q$ for two nonequivalent paths that traverse the first four Brillouin zones. We carry out these calculations within both time-dependent density functional theory (with two exchange-correlation functionals) and the Bethe-Salpeter equation. Additionally, we explore the effects of using the complete excitonic Hamiltonian (with all electron-hole pairs and antipairs), and within the Tamm-Dancoff approximation (neglecting antipairs). By analyzing the behavior of the macroscopic dielectric function, we are able to determine which peaks are predominantly from plasmonic behavior or only interband transitions. We compare the calculated spectra against several experiments that span almost five decades; our results present clear trends that follow the physical origins of the observed peaks. We carry out this study over a large range of momentum transfer in order to better evaluate the different theoretical methods compared to experiment, and predict the plasmonic behavior beyond available experimental data. Our results indicate that including the complete Hamiltonian with the exciton coupling included is essential for accurately describing the observed EEL spectra and plasmon dispersion of graphite, particularly for low values of momentum transfer. However, the solution of the Bethe-Salpeter equation is computationally intensive, so time-dependent density functional theory methods used in conjunction with the complete Hamiltonian may be an attractive alternative.