The photoluminescence (PL) spectra of monolayer (1L) semiconductors feature peaks ascribed to different charge-carrier complexes. We perform diffusion quantum Monte Carlo simulations of the binding energies of these complexes and examine their response to electric and magnetic fields. We focus on quintons (charged biexcitons), since they are the largest free charge-carrier complexes in undoped and low doping transition-metal dichalcogenides (TMDs). We examine the accuracy of the Rytova-Keldysh interaction potential between charges by comparing the binding energies (BEs) of charge-carrier complexes in 1L-TMDs with results obtained using ab initio interaction potentials. Magnetic fields <8T change BEs by ∼0.2meVT−1, in agreement with experiments, with BE variations of different complexes being very similar. Our results will help identify charge complexes in the PL spectra of 1L semiconductors.

Bilayer graphene exhibits a rich phase diagram in the quantum Hall regime, arising from a multitude of internal degrees of freedom, including spin, valley, and orbital indices. The variety of fractional quantum Hall states between filling factors 1<ν≤2 suggests, among other things, a quantum phase transition between valley-unpolarized and polarized states at a perpendicular electric-field D*. We find that the behavior of D* with ν changes markedly as B is reduced. At ν=2, D* may even vanish when B is sufficiently small. We present a theoretical model for lattice-scale interactions, which explains these observations; surprisingly, both repulsive and attractive components in the interactions are required. Within this model, we analyze the nature of the ν=2 state as a function of the magnetic and electric fields and predict that valley coherence may emerge for D∼D* in the high-B regime. This suggests the system supports Kekulé bond ordering, which could, in principle, be verified via scanning tunneling measurements.

At low temperature and zero applied magnetic field, besides the equilibrium helical state, monoaxial chiral helimagnets have a continuum of helical states differing by the wave number of the modulation. The wave number of these states in units of the equilibrium state wave number is denoted here by p, and accordingly the corresponding states are called the p states. In this work we study in detail the metastability of the p states. The application of an external magnetic field in the direction of the chiral axis has a double effect: On the one hand, it introduces a conical deformation of the p states, and, on the other hand, it destabilizes some of them, shrinking the range of p in which the p states are metastable. If a polarized current is applied along the chiral axis, then the p states reach a steady moving state with a constant velocity proportional to the current intensity. Besides this dynamical effect, the polarized current also induces a conical deformation and reduces the range of stability of the p states. The stability diagram in the plane applied field–applied current intensity has interesting features that, among other things, permits the manipulation of p states by a combination of applied fields and currents. These features can be exploited to devise processes to switch between p states. In particular there are p states with negative p, opening the possibility to helicity switching. The theoretical feasibility of such processes, crucial from the point of view of applications, is shown by micromagnetic simulations. Analogous p states exists in cubic chiral helimagnets and therefore similar effects are expected in those systems.

Recent advances in material synthesis made it possible to realize two-dimensional monolayers of candidate materials for a quantum spin liquid (QSL) such as α−RuCl3, 1T-TaSe2, and 1T-TaS2. In this work, we propose an experimental setup that exploits nonlocal electrical probes to gain information on the transport properties of a gapless QSL. The proposed setup is a spinon-mediated drag experiment: a current is injected in one of the two layers and a voltage is measured on the second metallic film. The overall momentum transfer mechanism is a two-step process mediated by Kondo interaction between the local moments in the quantum spin liquid and the spins of the electrons. In the limit of negligible momentum relaxed within the QSL layer, we calculate the drag relaxation rate for Kitaev, Z2, and U(1) QSLs using Aslamazov-Larkin diagrams. We find, however, that the case of dominant momentum relaxation within the QSL layer is far more relevant and thus develop a model based on the Boltzmann kinetic equation to describe the proposed setup. Within this framework we calculate the low-temperature scaling behavior of the drag resistivity, both for U(1) and Z2 QSLs with Fermi surfaces. In some regimes we find a crossover in the temperature scaling that is different between the Z2 and U(1) QSL because of the non-Fermi-liquid nature of the latter, which reflects itself both in altered kinematic constraints for the momentum transfer as well as in the qualitative aspects of momentum relaxation within the QSL layer. Our findings suggest that parameters of the system can be tuned to make the spinon-mediated drag a significant fraction of the total transresistance.

Colloidal gel systems exhibit increasingly slow relaxation and ultra-long-ranged spatial correlations of the dynamics similar to other jammed materials. These cooperative dynamics point to the presence of long-ranged stress correlation in these systems, which remain largely uninvestigated in the literature. In this work, we systematically investigate the nature of stress correlations in soft colloidal gel materials in the limit of moderate to high packing fractions and strong attraction. In this regime, centrosymmetric potential description for particle interaction fails as strong attraction can lead to frictional contacts, as shown explicitly in previous experiments. Accordingly, we model the system similarly to the cohesive granular media with Langevin dynamics to incorporate the effects of rolling and sliding resistant contacts and thermal fluctuations. We show that the spatial stress correlations are long ranged with very slow spatial decay close to the gel point. Similarly to previous studies on the frictional granular matter, the full stress autocorrelation matrix is dictated by the pressure and torque autocorrelations due to mechanical balance and material isotropy constraints. Surprisingly, it is observed that the gel materials do not behave as a normal elastic solid close to the gel point as assumed loosely in the literature because the real-space pressure fluctuations decay slower than normal. Furthermore, we link the abnormal pressure fluctuations to the non-hyperuniform behavior of the system (granular matter and gel) with respect to the local packing fraction fluctuations, thus relating the deviations from the normal elastic behavior across various jammed systems under a common framework.

Phase separation accompanied by domain growth and coarsening is a phenomenon common to a broad variety of dynamical systems. In this context, controlling such processes represents a relevant interdisciplinary problem. Through numerical modeling, we demonstrate two complementary approaches of coarsening control in bistable systems based on the example of a spatially extended model describing an optically addressed spatial light modulator with two-color illumination subject to optical feedback. The first method implies varying system parameters such that the system evolves as the pitchfork or saddle-node normal forms. The second method leverages noise, whose intensity serves as an additional system control parameter. Both deterministic and stochastic schemes allow us to control the direction and speed of the fronts separating spatial domains. The considered stochastic control represents a particular case of noise-sustained front propagation in bistable systems and involves the properties of the optical system under study. In contrast, the proposed deterministic control technique can generally be applied to bistable systems of different natures.

Chemical composition and pressure dependencies of the electronic structures of Bi2−xSbxTe3−ySey (y∼1.2) have been studied by the high-resolution x-ray absorption spectroscopy. We find a shift of the Bi-L3 absorption edge due to Sb substitution, suggesting a change in the Bi charge state. In the pressure dependence, the electronic structures of Bi2Te2Se and Bi1.5Sb0.5Te2Se start to change below the pressure of the first structural phase transition where the emergence of the superconductivity was observed and then show a large change just around that pressure. This is in contrast to the behavior observed in Bi2Se3-based compounds where the structural phase transition was necessary for the onset of superconductivity. We performed density functional theory calculations using the experimentally determined structures for Bi2Te2Se and Bi1.5Sb0.5Te2Se. We show that both compounds become metallic within the rhombohedral phase below the pressure of the first structural transition and we thus corroborate the experimental observation. The experimental and calculated results show that closing the gap and increasing the density of states in the rhombohedral phase are the triggers to induce superconductivity.

Joint measurements of two-Pauli observables are powerful tools for both the control and protection of quantum information. By following a simple recipe for measurement choices, single- and two- qubit rotations using two-Pauli parity and single qubit measurements are guaranteed to be unitary while requiring only a single ancilla qubit. This language for measurement based quantum computing is shown to be directly applicable to encoded double quantum dot singlet-triplet spin qubits, by measuring spin-parity between dots from neighboring qubits. Along with exchange interaction, a complete, leakage free, measurement based gate set can be shown, up to a known Pauli correction. Both theoretically exact spin-parity measurements and experimentally demonstrated asymmetric spin-parity measurements are shown to be viable for achieving the proposed measurement based scheme, provided some extra leakage mitigating measurement steps. This method of spin qubit control offers a leakage suppressed, low resource overhead implementation of a measurement-based control that is viable on current spin qubit processor devices.

The magnetic, thermal, and transport properties of the noncentrosymmetric compound Eu2Pd2Sn are reexamined with the inclusion of detailed measurements. In its paramagnetic phase, the outstanding feature of this compound is the formation of Eu2+ dimers, which allows us to understand the deviation of the magnetic susceptibility χ(T) from the Curie-Weiss law below about 70 K, the field-dependent magnetization M(B) below ≈80K, and the reduced entropy at the ordering temperature S(TN)=0.64Rln(8). A significant change in the exchange interactions occurs between T≈70K (where θP=18K) and TN=13.3K (to θP=−4.5K). The strong electronic overlap, arising from the reduced Eu-Eu spacing (compared with that in pure Eu2+) is expected to favor dimer quasiparticles formation, leading to a significant change in the magnetic structure. From the analysis of the derivatives of the magnetic parameters, ∂χ/∂T and ∂M/∂B, as well as the field dependence of the specific heat and mangnetoresistance, a comprehensive magnetic phase diagram is obtained. Two critical points are recognized and a tentative description of the magnetic structures is proposed. The possible formation of a skyrmion lattice, arising from the presence of magnetically frustrated pockets in the phase diagram, is suggested by theoretical studies on hexagonal structures that exhibit similar interaction patterns.

Odd-frequency superconductivity is an exotic superconducting state in which the symmetry of the gap function is odd in frequency. Here, we show that an inherent odd-frequency mode emerges dynamically under application of a Lorentz transformation of the anomalous Green function with the general frequency-dependent gap function. To see this, we consider a Dirac model with quartic potential and perform a mean-field analysis to obtain a relativistic Bogoliubov–de Gennes system. Solving the resulting Gor'kov equations yields expressions for relativistic normal and anomalous Green functions. The form of the relativistically invariant pairing term is chosen such that it reduces to BCS form in the nonrelativistic limit. We choose an ansatz for the gap function in a particular frame which is even frequency and analyze the effects on the anomalous Green function under a boost into a relativistic frame. The odd-frequency pairing emerges dynamically as a result of the boost. In the boosted frame, the order parameter contains terms which are both even and odd in frequency. The relativistic correction to the anomalous Green function to first order in the boost parameter is completely odd in frequency. In this paper, we provide evidence that odd-frequency pairing may form intrinsically within relativistic superconductors.

We develop a model which incorporates both intra- and intervalley scatterings to master equation, to explore exciton valley coherence in monolayer WS2 subjected to magnetic field. For linearly polarized (LP) excitation accompanied with an initial coherence, our determined valley dynamics manifests the coherence decay being faster than the exciton population relaxation and agrees with experimental data by Hao et al. [Nat. Phys. 12, 677 (2016)]. Further, we reveal that magnetic field may quench the electron-hole (e-h) exchange-induced pure dephasing—a crucial decoherence source—as a result of lifting of valley degeneracy, allowing one to magnetically regulate valley coherence. In particular, at low temperatures for which the exciton-phonon (ex-ph) interaction is weak, we find that the coherence time is expected to attain τC∼1 ps, facilitating full control of qubits based on the valley pseudospin. For dark excitons, we demonstrate an emerging coherence even in the absence of initial coherent state, which has a long coherence time (∼15 ps) at low temperature. Our work provides an insight into tunable valley coherence and coherent valley control based on dark excitons.

All-inorganic perovskite quantum dots films have received significant research interest for photovoltaic applications because of their better mechanical durability than bulk film and the various tunable properties that perovskite quantum dots exhibit. Here we develop a simple, almost analytic approach to calculating the electronic states in self-organized perovskite quantum dot systems. We present an extension to the tight-binding method, where the role of atoms is now played by quantum dots. This generalized tight-binding approach is applied to assess the feasibility of achieving miniband formation through phenyl-C60-butyric acid methyl ester (PCBM)/CsPbI3 quantum dots films. Type-II band alignment in these hybrid heterostructures led to the development of a nearly-free-electron model combined with a Hartree variational approach for describing conduction-band states. By implementing our approach, we reveal that PCBM/CsPbI3 quantum dot films are indirect-band-gap systems. A 41-meV bandwidth for the hole ground miniband is found due to the coupling between dots. Energy-gap values consistent with the experiment are obtained. This approach opens the way for calculating relevant optical properties in a broad class of perovskite quantum dot systems, such as the absorption coefficient.

Pursuit of superconductivity in light-element systems at ambient pressure is of great experimental and theoretical interest. In this work, we combine a machine learning (ML) method with first-principles calculations to efficiently search for the energetically favorable ternary Ca-B-C compounds. Three layered borocarbides (stable CaBC5 and metastable Ca2BC11 and CaB3C3) are predicted to be phonon-mediated superconductors at ambient pressure. The stable CaBC5 and the low-energy metastable Ca2BC11 (with formation energy only 9.5 meV/atom above the convex hull) have a superconducting Tc of 5.2 and 8.9 K, respectively. While the hexagonal CaB3C3 possesses a Tc of 26.1 K, it is metastable with formation energy of 153 meV/atom above the convex hull. The ML-guided approach opens up a way for greatly accelerating the discovery of new high-Tc superconductors.

Superconductivity has been experimentally observed in monolayer WTe2, in which the suggested in-plane field measurements are of a spin-triplet nature. Furthermore, it has been proposed that with a p-wave pairing, the material is a second-order topological superconductor with a pair of Majorana zero modes at the corners of a finite sample. We show that for a repulsive on-site interaction and sizable Fermi surfaces, the desired p-wave state arises naturally due to the Kohn-Luttinger mechanism, and indeed a finite superconducting sample hosts corner Majorana zero modes. We study the behavior of the critical temperature in response to external in-plane magnetic fields. We find an enhancement to the critical temperature that depends on the direction of the magnetic field, which can be directly verified experimentally.

We construct a set of lattice models of noninteracting topological insulators with chiral symmetry in three dimensions. We build a model of the topological insulators in the class AIII by coupling lower dimensional models of Z classes. By coupling the two AIII models related by time-reversal symmetry we construct other chiral symmetric topological insulators that may also possess additional symmetries (the time-reversal and/or particle-hole). There are two different chiral symmetry operators for the coupled model that correspond to two distinct ways of defining the sublattices. The integer topological invariant (the winding number) in case of weak coupling can be either the sum or difference of indices of the basic building blocks, dependent on the preserved chiral symmetry operator. The value of the topological index in case of weak coupling is determined by the chiral symmetry only and does not depend on the presence of other symmetries. For Z topological classes AIII, DIII, and CI with chiral symmetry are topologically equivalent, it implies that a smooth transition between the classes can be achieved if it connects the topological sectors with the same winding number. We demonstrate this explicitly by proving that the gapless surface states remain robust in Z classes as long as the chiral symmetry is preserved, and the coupling does not close the gap in the bulk. By studying the surface states in Z2 topological classes, we show that class CII and AII are distinct, and cannot be adiabatically connected.

Using high resolution tender x-ray resonant inelastic scattering and hard x-ray nonresonant inelastic scattering beyond the dipole limit we were able to detect electronic excitations in intermetallic UGa2 that are highly atomic in nature. Analysis of the spectral lineshape reveals that the local 5f2 configuration characterizes the correlated nature of this ferromagnet. The orientation and directional dependence of the spectra indicate that the ground state is made of the Γ1 singlet and/or Γ6 doublet symmetry. With the ordered moment in the ab plane, we infer that the magnetism originates from the higher lying Γ6 doublet being mixed with the Γ1 singlet due to intersite exchange, qualifying UGa2 to be a true quantum magnet. The ability to observe atomic excitations is crucial to resolve the ongoing debate about the degree of localization versus itineracy in U intermetallics.

We investigate the dispersive paramagnetic excitons on the honeycomb lattice that originate from the crystalline electric field split localized f-electron states in the paramagnetic state due to intersite exchange. We start with a symmetry analysis of possible Ising-type singlet-singlet and xy-type singlet-doublet models. The former supports only symmetric intersite exchange while the latter additionally allows for antisymmetric Dzyaloshinski-Moriya exchange interactions. We calculate the closed expressions for magnetic exciton dispersion using both response function formalism and bosonic Bogoliubov approach. We do this for the most general model that shows inversion-symmetry breaking on the honeycomb lattice but also discuss interesting special cases. By calculating Berry curvatures and Chern numbers of paramagnetic excitons we show that the xy model supports nontrivial topological states in a wide range of parameters. This leads to the existence of excitonic topological edge states with Dirac dispersion lying in the zone boundary gap without the presence of magnetic order.

A large class of type-I fracton models, including the X-cube model, have been found to be fixed points of the foliated renormalization group (RG). The system size of such foliated models can be changed by adding or removing decoupled layers of 2D topological states and continuous deformation of the Hamiltonian. In this paper, we study a closely related model—the Ising cage-net model—and find that this model is not foliated in the same sense. In fact, we point out certain unnatural restrictions in the foliated RG, and find that removing these restrictions leads to a generalized foliated RG under which the Ising cage-net model is a fixed point, and which includes the original foliated RG as a special case. The Ising cage-net model thus gives a prototypical example of the generalized foliated RG, and its system size can be changed either by condensing/uncondensing bosonic planon excitations near a 2D plane or through a linear-depth quantum circuit in the same plane. We show that these two apparently different RG procedures are closely related, as they lead to the same gapped boundary when implemented in part of a plane. Finally, we briefly discuss the implications for foliated fracton phases, whose universal properties will need to be reexamined in light of the generalized foliated RG.

Magnetic topological materials are a realization of topologically nontrivial electronic band structure with magnetic correlation effects; they offer novel opportunities in manipulating charge/spin transport as well as spin texture. In the search for emergent phenomena that are specific in this class of materials, here we report on a type of quantum oscillation of a polar magnetic Weyl semimetal (WSM) NdAlSi in both the temperature dependent electrical resistivity and specific heat at a constant magnetic field. It is revealed that they arise from the destructive interference between field dependent quantum oscillations of the spin-split Fermi surfaces due to the strong Weyl fermion–4f electron exchange interaction combined with Rashba-Dresselhaus and Zeeman effects. Our results demonstrate that the f electrons bearing magnetic WSMs possess even richer responses to external stimuli compared to known d-electron magnetic WSMs and call for further in-depth investigations.

We argue that the formation of Yu-Shiba-Rusinov excitations in proximitized thin films is largely mediated by a type of Andreev-bound state named after de Gennes and Saint-James. This is shown by studying an experimentally motivated model and computing the overlap of the wave functions of these two subgap states. We find the overlap stays close to unity even as the system moves away from weak coupling across the parity-changing quantum phase transition. Based on this observation, we introduce a single-site model of the bound state coupled to a quantum spin. The adequacy of this description is assessed by reintroducing the coupling to the continuum as a weak perturbation and studying its scaling flow using Anderson's poor man's scaling.