Noncollinear magnetic structures with broken inversion symmetry have recently revolutionized the field of spintronics. Among them, transition metal monosilices and germanides are potential candidates due to their nontrivial spin textures. Here we report the growth and magnetic and electronic characterization of high-quality (111)-oriented thin films of MnSi by magnetron sputtering. While thicker films order magnetically similar to their bulk counterpart and according to previous reports in literature, x-ray magnetic circular dichroism measurements indicate that 30-nm-thick films do not develop long-range magnetic order, presumably due to magnetic frustration introduced by domains of different chiralities and spin disorder. X-ray absorption spectroscopy shows a stabilization of ${\mathrm{Mn}}^{+}$ oxidation state in epitaxial thin films evidencing different electronic structure as compared with bulk MnSi. Field- and angular-dependent magnetoresistance support a magnetically disordered scenario, while Hall resistivity develops features of zero-field topological order formerly ascribed to skyrmions. Our results highlight the importance of geometrical size effects and spin disorder in noncentrosymmetric magnets and suggest that the topological Hall effect in thin films cannot be taken as the sole criterium for the assignment of nontrivial magnetic structures.

We studied the magnetic phase diagram of the rare-earth manganites series ${\mathrm{Gd}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ (GCMO) over the full concentration range based on density functional theory. GCMO has been shown to form solid solutions. We take into account this disordered character by adapting special quasi-random structures at different concentration steps. The magnetic phase diagram is mainly described by means of the magnetic exchange interactions between the Mn sites, and Monte Carlo simulations were performed to estimate the corresponding transition temperatures. They agree very well with recent experiments. The hole-doped region $x<0.5$ shows a strong ferromagnetic ground state, which competes with A-type antiferromagnetism at higher Ca concentrations $x>0.6$.

A Josephson junction embedded in a dissipative circuit can be externally driven to induce nonlinear dynamics of its phase. Classically, under sufficiently strong driving and weak damping, dynamic multistability emerges, associated with dynamical bifurcations so that the often-used modeling as a Duffing oscillator, which can exhibit bistability at most, is insufficient. The present work analyzes in this regime corresponding quantum properties by mapping the problem onto a highly nonlinear quasienergy operator in a rotating frame. This allows us to identify in detail parameter regions where simplifications such as the Duffing approximation are valid, to explore classical-quantum correspondences, and to study how quantum fluctuations impact the effective junction parameters as well as the dynamics around higher-amplitude classical fixed points.

Weyl nodes in three-dimensional Weyl semimetals break the Liouville equation, leading to the Liouville anomaly. Here we present an approach to derive the semiclassical action and equations of motion for Weyl fermions in the presence of electromagnetic fields and torsions from the quantum field theory: combining the Wigner transformation with band projection operation. It has been shown that the Liouville anomaly, including a pure torsion anomaly term, a mixing term between the electromagnetic fields and torsions, and the conventional chiral anomaly, entirely differs from the counterpart of axial gauge fields. We find various torsional responses and reproduce the chiral vortical effect and the torsional chiral magnetic effect. A torsion-modified anomalous Hall effect due to the mixing term in the Liouville anomaly is predicted, and its implementation is also discussed. Therefore, our work not only provides insights into the torsional responses for Weyl fermions but also acts as a starting point to investigate their topological responses.

We investigate the universal information contained in the Rényi entanglement entropies for a free massless Dirac fermion in three spatial dimensions. Using numerical calculations on the lattice, we examine the case where the entangling boundary contains trihedral corners. The entropy contribution arising from these corners grows logarithmically in the entangled subsystem's size with a universal coefficient. Our numerical results provide evidence that this logarithmic coefficient has a simple structure determined by two universal functions characterizing the underlying critical theory and the geometry of the corner. This form is similar to that of the analogous coefficient appearing for smooth entangling surfaces. Furthermore, our results support the idea that one of the geometric factors in the corner coefficient is topological in nature and related to the Euler characteristic of the boundary, in direct analogy to the case of the smooth surface. We discuss implications, including the possibility that one can use trihedral corner contributions to the Rényi entropy to determine both of the universal central charges of the underlying critical theory.

We consider an interacting version of the Hofstadter model, which in the absence of interactions has a spectrum given by a Cantor set, provided that the adimensional parameter $\alpha$ is an irrational number. In the anisotropic situation where the hopping $t_2$ is smaller then $t_1$, we rigorously prove that the $n\mathrm{th}$ gap persists in the presence of interaction, even for interactions much stronger than the gap. We assume a Diophantine property for $\alpha$ and that $t_2/t_1,U/t_1$ are positive and smaller than some constant, weakly depending on $n$. The proof relies on a subtle interplay of renormalization group arguments combined with number-theoretic properties.

We study heat transport in a gas of one-dimensional fermions in the presence of a small temperature gradient. At temperatures well below the Fermi energy there are two types of relaxation processes in this system, with dramatically different relaxation rates. As a result, in addition to the usual thermal conductivity, one can introduce the thermal conductivity of the gas of elementary excitations, which quantifies the dissipation in the system in the broad range of frequencies between the two relaxation rates. We develop a microscopic theory of these transport coefficients in the limit of weak interactions between the fermions.

In this paper we present first-principles calculations, based on both density functional theory and maximally localized Wannier functions, to study the electronic properties and interlayer coupling of twisted ${\mathrm{MoS}}_2/{\mathrm{NbSe}}_2$ heterobilayers. We accurately investigate different stacking configurations and commensurate twist angles by including an in-depth analysis of the interlayer van der Waals interaction. The metallic character of the investigated heterostructures is dominated, at the Fermi energy, by the ${\mathrm{NbSe}}_2$ atomic orbitals and shows a strong dependence on the twist angle. Notably, at the smallest considered twist angle, band structure flattening at the Fermi energy shows up, which should result in a lower conductivity of the metallic heterobilayer. The electrostatic potential analysis reveals no significant modification of the potential pattern with respect to the potentials of the isolated layers, with the exception of the interface region. A moderate electronic charge redistribution, compatible with electronically weakly coupled layers, is set up following the formation of the interface. The dependence of the electronic structure on the twist angle acts as a new degree of freedom for tuning properties relevant in electronic device applications.

Inversion asymmetry in two-dimensional materials grants them fascinating properties such as spin-coupled valley degrees of freedom and piezoelectricity, but at the cost of inversion domain boundaries if the epitaxy of the grown two-dimensional (2D) layer, on a polar substrate, cannot adequately distinguish what are often near-degenerate $0^{\circ }$ and ${180}^{\circ }$ orientations. We employ first-principles calculations to identify a method to lift this near degeneracy: the energetic distinction between eclipsed and staggered configurations during nucleation at a point defect in the substrate. For monolayer ${\mathrm{MoS}}_2$ grown on hexagonal boron nitride, the predicted defect complex can be more stable than common ${\mathrm{MoS}}_2$ point defects because it is both a donor-acceptor pair and a Frenkel pair shared between adjacent layers of a 2D heterostack. Orientation control is verified in experiments that achieve $\sim 90\%$ consistency in the orientation of as-grown triangular ${\mathrm{MoS}}_2$ flakes on hBN, as confirmed by aberration-corrected scanning/transmission electron microscopy. This defect-enhanced orientational epitaxy could provide a general mechanism to break the near-degeneracy of $0/{180}^{\circ }$ orientations of polar 2D materials on polar substrates, overcoming a long-standing impediment to scalable synthesis of single-crystal 2D semiconductors.

We systematically investigate the finite-size effects in non-Hermitian one-dimensional Su-Schrieffer-Heeger (SSH) and two-dimensional (2D) Chern insulator models. In the Hermitian SSH system, the finite-size energy gap is always real and shows a monotonic-exponential decay as the chain length grows. In contrast, for the non-Hermitian SSH model, the non-Hermitian intracell hoppings can modify the localization lengths of bulk and end states, giving rise to a complex finite-size energy gap that exhibits an oscillating exponential decay as the chain length grows. However, the imaginary staggered onsite potentials in the SSH model only change the end-state energy, leaving the localization lengths of the system unchanged. In this case, the finite-size energy gap can undergo a transition from real values to imaginary values. We observed similar phenomena for the finite-size effect in 2D non-Hermitian Chern insulator systems.

We perform a numerical study of a spin-1/2 model with ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ symmetry in one dimension which demonstrates an interesting similarity to the physics of two-dimensional deconfined quantum critical points (DQCP). Specifically, we investigate the quantum phase transition between Ising ferromagnetic and valence bond solid (VBS) symmetry-breaking phases. Working directly in the thermodynamic limit using uniform matrix product states, we find evidence for a direct continuous phase transition that lies outside of the Landau-Ginzburg-Wilson paradigm. In our model, the continuous transition is found everywhere on the phase boundary. We find that the magnetic and VBS correlations show very close power-law exponents, which is expected from the self-duality of the parton description of this DQCP. Critical exponents vary continuously along the phase boundary in a manner consistent with the predictions of the field theory for this transition. We also find a regime where the phase boundary splits, as suggested by the theory, introducing an intermediate phase of coexisting ferromagnetic and VBS order parameters. Interestingly, we discover a transition involving this coexistence phase which is similar to the DQCP, being also disallowed by the Landau-Ginzburg-Wilson symmetry-breaking theory.

We investigate quasiparticle relaxation dynamics in ${\mathrm{URu}}_{2-x}{\mathrm{Fe}}_x{\mathrm{Si}}_2$ single crystals using ultrafast optical-pump optical-probe (OPOP) spectroscopy as a function of temperature and Fe substitution ($x$), crossing from the hidden-order (HO) phase ($x=0$) to the large-moment antiferromagnet (LMAFM) phase ($x=0$.12). At low temperature, the dynamics for $x=0$ and $x=0$.12 are consistent with the low-energy electronic structure of the HO and LMAFM phases that emerge from the high-temperature paramagnetic (PM) phase. In contrast, near the bicritical point separating HO and LMAFM ($x=0$.1), two transitions occur over a narrow temperature range (from 15.5–17.5 K). A PM to HO transition occurs at an intermediate temperature followed by a transition to the LMAFM phase at lower temperature. While the data at low temperatures are consistent with the expected coexistence of LMAFM and HO, the data in the intermediate temperature phase are not, and instead suggest the possibility of an unexpected coexistence of HO and PM. Additionally, the dynamics in the PM phase reflect the presence of a hybridization gap as well as strongly interacting spin and charge degrees of freedom. OPOP yields insights into meV-scale electrodynamics with sub-Kelvin temperature resolution, providing a complementary approach to study low-energy electronic structure in quantum materials.

We consider the effect of disorder on the spectrum of quasiparticles in the point-node and nodal-line superconductors. Due to the anisotropic dispersion of quasiparticles disorder scattering may render the Hamiltonian describing these excitations non-Hermitian. Depending on the dimensionality of the system, we show that the nodes in the spectrum are replaced by Fermi arcs or Fermi areas bounded by exceptional points or exceptional lines, respectively. These features are illustrated by first considering a model of a proximity-induced superconductor in an anisotropic two-dimensional (2D) Dirac semimetal, where a Fermi arc in the gap bounded by exceptional points can be realized. We next show that the interplay between disorder and supercurrents can give rise to a 2D Fermi surface bounded by exceptional lines in three-dimensional (3D) nodal superconductors.

We study the positive longitudinal magnetoconductivity (LMC) and planar Hall effect in Weyl semimetals, following a recent-developed theory by integrating the Landau quantization with Boltzmann equation. It is found that, in the weak magnetic field regime, the LMC and planar Hall conductivity (PHC) obey ${cos}^6\theta$ and ${cos}^5\theta sin\theta$ dependence on the angle $\theta$ between the magnetic and electric fields. For higher magnetic fields, the LMC and PHC cross over to ${cos}^2\theta$ and $cos\theta sin\theta$ dependence, respectively. Interestingly, the PHC could exhibit quantum oscillations with varying $\theta$, due to the periodic-in-$1/B$ oscillations of the chiral chemical potential. When the magnetic and electric fields are noncollinear, the LMC and PHC will deviate from the classical $B$-quadratic dependence, even in the weak magnetic field regime.

Broad-beam low-energy ion bombardment can lead to the spontaneous formation of nanoscale surface structures, but the dominant mechanisms driving evolution remain controversial. Using coherent x-ray scattering to examine the classic case of ion-beam rippling of ${\mathrm{SiO}}_2$ surfaces, we study the relationship between the average kinetics of ripple formation and the underlying fluctuation dynamics. The early stage growth of fluctuations is well fit with a linear theory formalism employing a viscous relaxation term with full wave-number dependence. In this regime, the x-ray photon correlation spectroscopy two-time correlation function shows distinctive behavior with memory stretching back to the beginning of the bombardment. For a given length scale, correlation times do not grow significantly beyond the characteristic time associated with the early-stage ripple growth. In the late stages of patterning, when the average surface structure on a given length scale is no longer evolving, dynamical processes continue on the surface. Nonlinear processes dominate at long length scales, leading to compressed exponential decay of the speckle correlation functions, while at short length scales the dynamics appears to approach a linear behavior consistent with viscous flow relaxation. This behavior is found to be consistent with simulations of a recent nonlinear growth model. In addition, it is shown that the surface ripple velocity, an important parameter of the ion-driven surface evolution, can be measured with coherent x-ray scattering in conjunction with use of an inhomogeneous ion beam. The change in viewpoint exemplified by this study, from a focus on only average surface kinetics to one incorporating the underlying nanoscale dynamics, is rapidly becoming more widely applicable as new and upgraded x-ray sources with higher coherent flux come online.

The recently realized bilayer graphene system with a twist angle of ${30}^{\circ }$ offers a new type of quasicrystal which unites the dodecagonal quasicrystalline nature and graphene's relativistic properties. Here, we introduce a concise theoretical framework that fully respects both the dodecagonal rotational symmetry and the massless Dirac nature, to describe the electronic states of the system. We find that the electronic spectrum consists of resonant states labeled by 12-fold quantized angular momentum, together with the extended relativistic states. The resulting quasiband structure is composed of the nearly flat bands with spiky peaks in the density of states, where the wave functions exhibit characteristic patterns which fit to the fractal inflations of the quasicrystal tiling. We also demonstrate that the 12-fold resonant states appear as spatially localized states in a finite-size geometry, which is another hallmark of quasicrystal. The theoretical method introduced here is applicable to a broad class of “extrinsic quasicrystals” composed of a pair of two-dimensional crystals overlaid on top of the other with incommensurate configurations.

We present the spontaneous Brillouin light scattering (BLS) under simultaneous one-dimensional confinement of sound and light and show that the photon-phonon coupling results from nontrivial interplay of the photoelastic and moving-interface effects. We reveal two types of BLS self-canceling: governed by mode symmetry and driven by destructive interference of the two effects. We show that the latter can be adjusted by the light polarization and phonon wave number. Furthermore, we present a measurement of the shear-horizontal waves in thin membranes.

Strain and strain adaptation mechanisms in modern functional materials are of crucial importance for their performance. Understanding these mechanisms will advance innovative approaches for material properties engineering. Here we study the strain adaptation mechanism in a thin film model system as a function of epitaxial strain. Chemically disordered FeRh thin films are deposited on W-V buffer layers, which allow for large variation of the preset lattice constants, e.g., epitaxial boundary condition. It is shown by means of high-resolution x-ray reciprocal space maps and transmission electron microscopy that the system reacts with a tilting mechanism of the structural units in order to adapt to the lattice constants of the buffer layer. This response is explained by density functional theory calculations, which evidence an energetic minimum for structures with a distortion of $c/a\approx 0.87$. The experimentally observed tilting mechanism is induced by this energy gain and allows the system to remain in the most favorable structure. In general, it is shown that the use of epitaxial model heterostructures consisting of alloy buffer layers of fully miscible elements and the functional material of interest allows to study strain adaptation behaviors in great detail. This approach makes even small secondary effects observable, such as the directional tilting of the structural domains identified in the present case study.

In spin-ice research, small variations in structure or interactions drive a multitude of different behaviors, yet the collection of known materials relies heavily on the “227” pyrochlore structure. Here, we present thermodynamic, structural, and inelastic-neutron-scattering data on a spin-ice material, ${\mathrm{MgEr}}_2{\mathrm{Se}}_4$, which contributes to the relatively underexplored family of rare-earth spinel chalcogenides. X-ray and neutron diffraction confirm a normal spinel structure and place ${\mathrm{Er}}^{3+}$ moments on an ideal pyrochlore sublattice. Measurement of crystal electric field excitations with inelastic neutron scattering confirms that the moments have perfect Ising character, and further identifies the ground-state Kramers doublet as having dipolar-octupolar form with a significant multipolar character. Heat capacity and magnetic neutron diffuse scattering have icelike features, but are inconsistent with Monte Carlo simulations of the nearest-neighbor and next-nearest-neighbor dipolar spin-ice (DSI) models. A significant remnant entropy is observed as $T\to 0$ K, but again falls short of the full Pauling expectation for DSI, unless significant disorder is added. We show that these observations are fully in line with what has been recently reported for ${\mathrm{CdEr}}_2{\mathrm{Se}}_4$, and point to the importance of quantum fluctuations in these materials.

Magnetic structure of the substituted multiferroics-ferroborates ${\mathrm{Nd}}_{0.9}{\mathrm{Tb}}_{0.1}{\mathrm{Fe}}_3{\left({\mathrm{BO}}_3\right)}_4$ and ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Fe}}_3{\left({\mathrm{BO}}_3\right)}_4$ were determined in the framework of a self-consistent refinement of the single crystal neutron diffraction data. The small substitution of Nd for Tb leads to the reorientation of the main antiferromagnetic vector L from the basal plane towards the hexagonal axis. The reorientation takes place via an angular structure for which L does not coincide with the principal crystallographic directions and evolves with temperature due to competing magnetic anisotropies of Fe, Nd, and Tb subsystems. Our refinement at 2 K reveals the existence of distortions in the collinear antiferromagnetic Fe spin arrangement suggested before in other ferroborates. Therefore, besides the main antiferromagnetic vector L, the magnetic structure involves additional fine symmetrized combinations of spin components allowed by symmetry. They coexist with certain L components and could originate from the antisymmetric Dzyaloshinsky-Moriya Fe-Fe exchange interactions. At higher temperatures, the magnetic structure is described by the simple collinear model, where the L vector is deviated from the hexagonal plane.