Recently, time-reversal symmetric but inversion broken systems with nontrivial Berry curvature in the presence of a built-in electric field have been proposed to exhibit a new type of linear Hall effect in ballistic regime, namely, the Magnus Hall effect. The transverse current here is caused by the Magnus velocity that is proportional to the built-in electric field enabling us to examine the Magnus responses, in particular, Magnus Hall conductivity and Magnus Nernst conductivity, with chemical potential. Starting with two-dimensional (2D) topological systems, we find that warping induced asymmetry in both the Fermi surface and Berry curvature can in general enhance the Magnus response for monolayer graphene and surface states of topological insulator. The strain alone is only responsible for Magnus valley responses in monolayer graphene, while warping leads to finite Magnus response there. Interestingly, on the other hand, strain can change the Fermi surface character substantially that further results in distinct behavior of Magnus transport coefficients as we observe in bilayer graphene. These responses there remain almost insensitive to warping unlike the case of monolayer graphene. Going beyond 2D systems, we also investigate the Magnus responses in three-dimensional multi-Weyl semimetals (mWSMs) to probe the effect of tilt and anisotropic nonlinear energy dispersion. Remarkably, Magnus responses can only survive for the WSMs with chiral tilt. In particular, our study indicates that the chiral (achiral) tilt engenders Magnus (Magnus valley) responses. Therefore, Magnus responses can be used as a tool to distinguish between the untilted and tilted WSMs in experiments. In addition, we find that the Magnus Hall responses get suppressed with increasing the nonlinearity associated with the band touching around the multi-Weyl node.

Starting from the rigorous quantum-field-theory formalism, we derive a formula for the screened conductivity designed to study the coupling of light with elementary electron excitations and the ensuing electromagnetic modes in two-dimensional (2D) semiconductors. The latter physical quantity consists of three fully separable parts, namely, intraband, interband, and ladder conductivities, and is calculated beyond the random phase approximation as well as from first principles. By using this methodology, we study the optical absorption spectra in 2D black phosphorous, so-called phosphorene, as a function of the concentration of electrons injected into the conduction band. The mechanisms of phosphorene exciton quenching versus doping are studied in detail. It is demonstrated that already small doping levels ($n\sim {10}^{12}{\text{cm}}^{-2}$) lead to a radical drop in the exciton binding energy, i.e., from 600 meV to 128 meV. The screened conductivity is applied to study the collective electromagnetic modes in doped phosphorene. It is shown that the phosphorene transversal exciton hybridizes with free photons to form an exciton-polariton. This phenomenon is experimentally observed only for the case of confined electromagnetic microcavity modes. Finally, we demonstrate that the energy and intensity of anisotropic 2D plasmon-polaritons can be tuned by varying the concentration of injected electrons.

We study the nonequilibrium dynamics and transport of a PT-symmetric Luttinger liquid (LL) after an interaction quench. The system is prepared in domain wall initial state. After a quantum quench to spatially homogeneous, PT-symmetric LL, the domain wall develops into a flat central region that spreads out ballistically faster than the conventional Lieb-Robinson maximal speed. By evaluating the current inside the regular light cone, we find a universal conductance $e^2/h$, insensitive to the strength of the PT-symmetric interaction. On the other hand, by repeating the very same time evolution with a Hermitian LL Hamiltonian, the conductance is heavily renormalized by the Hermitian interaction as $e^2/hK$ with $K$ the LL parameter. Our analytical results are tested numerically, confirming the universality of the conductance in the non-Hermitian realm.

The fermionic self-energy on the surface of a topological insulator proximity coupled to ferro- and antiferromagnetic insulators is studied. An enhanced electron-magnon coupling is achieved by allowing the electrons on the surface of the topological insulator to have a different exchange coupling to the two sublattices of the antiferromagnet. Such a system is therefore seen as superior to a ferromagnetic interface for the realization of magnon-mediated superconductivity. The increased electron-magnon-coupling simultaneously increases the self-energy effects. In this paper we show how the inverse quasiparticle lifetime and energy renormalization on the surface of the topological insulator can be kept low close to the Fermi level by using a magnetic insulator with a sufficient easy-axis anisotropy. We find that the antiferromagnetic case is most interesting from both a theoretical and an experimental standpoint due to the increased electron-magnon coupling, combined with a reduced need for easy-axis anisotropy compared to the ferromagnetic case. We also consider a set of material and instrumental parameters where these self-energies should be measurable in angle-resolved photoemission spectroscopy experiments, paving the way for a measurement of the interfacial exchange coupling strength.

In the context of inhomogeneous one-dimensional finite systems, recent numerical advances [Phys. Rev. B 103, 125155 (2021)] allow us to compute the exact coupling-constant dependent exchange-correlation kernel $f_{\text{xc}}^{\lambda }(x,x^{\prime },\omega )$ within linear response time-dependent density-functional theory. This permits an improved understanding of ground-state total energies derived from the adiabatic-connection fluctuation-dissipation theorem (ACFDT). We consider both one-shot and self-consistent ACFDT calculations, and demonstrate that chemical accuracy is reliably preserved when the frequency dependence in the exact functional $f_{\text{xc}}\left[n\right](\omega =0)$ is neglected. This performance is understood on the grounds that the exact $f_{\text{xc}}\left[n\right]$ varies slowly over the most relevant $\omega$ range (but not in general), and hence the spatial structure in $f_{\text{xc}}\left[n\right](\omega =0)$ is able to largely remedy the principal issue in the present context: self-interaction (examined from the perspective of the exchange-correlation hole). Moreover, we find that the implicit orbitals contained within a self-consistent ACFDT calculation utilizing the adiabatic exact kernel $f_{\text{xc}}\left[n\right](\omega =0)$ are remarkably similar to the exact Kohn-Sham orbitals, thus further establishing that the majority of the physics required to capture the ground-state total energy resides in the spatial dependence of $f_{\text{xc}}\left[n\right]$ at $\omega =0$.

Superconductivity in topological quantum materials is much sought after, as it represents the key avenue to searching for topological superconductors, which host a full pairing gap in the bulk but Majorana bound states at the surface. To date, however, the simultaneous realization of nontrivial band topology and superconductivity in the same material under ambient conditions remains rare. In this paper, we study both superconducting and topological properties of the binary compound ${\mathrm{PtPb}}_4$ $(T_c\sim 2.7 \mathrm{K})$ that was recently reported to exhibit large Rashba splitting, inherent to the heavy $5d$ Pt and $6p$ Pb. We show that in ${\mathrm{PtPb}}_4$, the specific heat jump at $T_c$ reaches $\mathrm{\Delta }C/\gamma T_c\sim 1.70\pm 0.04$, larger than the 1.43 expected for weak-coupling BCS superconductors. Moreover, the measurement of quantum oscillation suggests the possibility of a topological band structure, which is further studied by density functional theory calculations. Our study may stimulate future experimental and theoretical investigations in this intriguing material.

Defects in transition-metal dichalcogenides (TMDs) monolayers are ubiquitous and have great effects on the electronic and optical properties. As a consequence, an in-depth understanding of how defects influence the performance of materials is crucial for the design and manipulation of electronic and optoelectronic devices. In this work, we investigate the effect of four common point defects, i.e., $V_{\text{S}}$, $V_{\text{Se}}$, $V_{{\text{MoS}}_3}$, and $V_{{\text{MoSe}}_3}$ on the electronic and excitonic properties in the Janus-MoSSe monolayer, by employing the first-principles method combined with the $GW$-Bethe-Salpeter equation. Results show that the presence of defects indeed alters the electronic and optical properties of the Janus-MoSSe monolayer, but in different ways. For monochalcogen vacancies, the first bright exciton ($X_1$) is more localized and has a smaller exciton radius as compared with that in the pristine Janus-MoSSe monolayer. In addition, the exciton binding energies become much larger, with the value up to 1.58 eV. As a result, excitons are easy to combine with defects, becoming centers of luminescence. While for the systems with ${\mathrm{MoS}}_3$ and ${\mathrm{MoSe}}_3$ vacancy clusters, the distribution of $X_1$ has changed significantly even though the exciton binding energies are comparable with that of the pristine system. Further analysis indicates that this can be ascribed to the competition mechanism between the existence of defects and Coulomb screening effect. Based on these findings, we obtained a thorough understanding of how point defects influence the nature of the Janus-TMD monolayer, which provides useful theoretical guidance for further experimental research.

We present a rigorous quasinormal mode approach to describe the strong coupling behavior between a monolayer of ${\mathrm{MoSe}}_2$ and a single gold nanoparticle. The onset of strong coupling, described through a classical spectral mode splitting (analog of vacuum Rabi splitting) is quantified by computing the full three-dimensional hybrid quasinormal modes of the combined structure, allowing one to accurately model light-matter interactions without invoking the usual phenomenological theories of strong coupling. We explore the hybrid quasinormal modes as a function of gap size and temperature, and find spectral splittings in the range of around 80–110 meV, with no fitting parameters for the material models. We also show how the hybrid modes exhibit Fano-like resonances and quantify the complex poles of the hybrid modes as well as the Purcell factor resonances from embedded dipole emitters. The Rabi splitting is found to be larger at elevated temperatures for very small gap separations between the metal nanoparticle and the monolayer, but smaller at elevated temperatures for larger gaps. We also show how these spectral splittings can differ qualitatively from the actual complex poles of the hybrid quasinormal modes.

Electron dynamics in external electric fields governs the behavior of solid-state electronic devices. First-principles calculations enable precise predictions of charge transport in low electric fields. However, studies of high-field electron dynamics remain elusive due to a lack of accurate and broadly applicable methods. Here, we develop an efficient approach to solve the real-time Boltzmann transport equation with both the electric field term and ab initio electron-phonon collisions. These simulations provide field-dependent electronic distributions in the time domain, allowing us to investigate both transient and steady-state transport in electric fields ranging from low to high ($>10$ kV/cm). The broad capabilities of our approach are shown by computing nonequilibrium electron occupations and velocity-field curves in Si, GaAs, and graphene, obtaining results in quantitative agreement with experiment. Our approach sheds light on microscopic details of transport in high electric fields, including the dominant scattering mechanisms and valley occupation dynamics. Our results demonstrate quantitatively accurate calculations of electron dynamics in low to high electric fields, with broad application to power and micro-electronics, optoelectronics, and sensing.

We use tunable dipolar interactions between the spins of nitrogen-vacancy (NV) centers in diamond to rotate a diamond crystal. Specifically, we employ cross-relaxation between the electronic spin of pairs of NV centers in a trapped diamond to enhance the anisotropic NV paramagnetism and thus to increase the associated spin torque. Our observations open a path towards the use of mechanical oscillators to detect paramagnetic defects that lack optical transitions, to the investigation of angular momentum conservation in spin relaxation processes, and to different means of cooling the motion of mechanical oscillators.

Chiral bobber (ChB) is an exotic three-dimensional topological texture in chiral magnets. It is composed of a Bloch-point singularity and a skyrmion tube; the incommensurate characteristic length scales of these two components would thus lead to novel dynamics of a ChB. Here, we report the current-induced motion and the accompanying Hall effect of a ChB. Surprisingly, we find that the Hall angle of a ChB can be tuned in a wide range from −90 ° to positive angles. A further study of spin-wave excitations identified high-frequency resonant modes, which are related to the velocity of ChB and lattice constants. The excited frequency can be two orders of magnitude higher than that of skyrmions. The broad ranges of Hall angles and frequency spectra could boost the development of spintronic devices using topological textures.

We have developed a sensitive technique to probe the magnetic skyrmion phases and dynamics by employing the interfacial coupling effect in a magnetoelectric composite configuration. The study on a MnSi single crystal sample using this technique provides clear evidence for the skyrmion lattice phase and coexistence of skyrmion and conical phase. Above the Curie temperature $T_{\mathrm{C}}$, a region with strong spin fluctuation is revealed as well. By tuning the density of skyrmion or disorder, a transition from the skyrmion lattice to skyrmion-conical coexisting phase is observed. The observation is in good agreement with a theoretical model which predicts the dissipation behavior in the coexistence phase.

We explore the topological properties of a modified Haldane model (MHM) in which the strength of the nearest-neighbor and next-nearest-neighbor hopping terms is made unequal and the threefold rotational symmetry ${\mathcal{C}}_3$ is broken by introducing a dimerization term ($|t_{1w\left(2w\right)}|<t_{1s\left(2s\right)}$) in the Hamiltonian. Using the parameter $\eta =t_{1w}/t_{1s}=t_{2w}/t_{2s}$, we show that this MHM supports a transition from the quantum anomalous Hall insulator to a higher-order topological insulator (HOTI) phase at $\eta =\pm 0.5$. It also hosts a zero-energy corner mode on a nanodisk that can transition to a trivial insulator without gap closing when the inversion symmetry is broken. The gap-closing critical states are found to be magnetic semimetals with a single Dirac node which, unlike the classic Haldane model, can move along the high-symmetry lines in the Brillouin zone. Our MHM offers a rich tapestry of HOTIs and other topological and nontopological phases.

Non-Hermtian (NH) Hamiltonians effectively describing the physics of dissipative systems have become an important tool with applications ranging from classical metamaterials to quantum many-body systems. Exceptional points, the NH counterpart of spectral degeneracies, are among the paramount phenomena unique to the NH realm. While realizations of second-order exceptional points have been reported in a variety of microscopic models, higher-order ones have largely remained elusive in the many-body context, as they in general require fine tuning in high-dimensional parameter spaces. Here, we propose a microscopic model of correlated fermions in three spatial dimensions and demonstrate the occurrence of interaction-induced fourth-order exceptional points that are protected by chiral symmetry. We demonstrate their stability against symmetry-breaking perturbations and investigate their characteristic analytical and topological properties.

The quasi-one-dimensional layered compound ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ has been proposed to undergo a transition to an excitonic insulator at $T_{\mathrm{c}}=326$ K. We found a clear anomaly at $T_{\mathrm{c}}$ in the in-plane thermal conductivities both parallel ($\parallel a$) and perpendicular ($\parallel c$) to the one-dimensional chains, ${\kappa }_{\mathrm{a}}$ and ${\kappa }_{\mathrm{c}}$. While ${\kappa }_{\mathrm{a}}$ shows a rapid decrease below $T_{\mathrm{c}}$, ${\kappa }_{\mathrm{c}}$ shows a pronounced V-shaped suppression centered at $T_{\mathrm{c}}$. We argue that the decrease of ${\kappa }_{\mathrm{a}}$ represents the suppression of the quasiparticle contribution below $T_{\mathrm{c}}$ due to the excitonic transition. On the other hand, the V-shaped suppression of ${\kappa }_{\mathrm{c}}$ comes from the enhanced phonon scattering by soft phonons associated with the monoclinic transition with momentum $\mathbf{q}\parallel c$. The continued suppression of ${\kappa }_{\mathrm{c}}$ up to an extremely high temperature above $T_{\mathrm{c}}$ suggests the persistence of phonon softening likely coupled to electronic, presumably excitonic, fluctuations.

We show theoretically that the magnetic ions, randomly distributed in a two-dimensional (2D) semiconductor system, can generate a ferromagnetic long-range order via the RKKY interaction. The main physical reason is the discrete (rather than continuous) symmetry of the 2D Ising model of the spin-spin interaction mediated by the spin-orbit coupling of 2D free carriers, which precludes the validity of the Mermin-Wagner theorem. Further, the analysis clearly illustrates the crucial role of the molecular field fluctuations as opposed to the mean field. The developed theoretical model describes the desired magnetization and phase-transition temperature $T_c$ in terms of a single parameter, namely, the chemical potential $\mu$. Our results highlight a pathway to reach the highest possible $T_c$ in a given material as well as an opportunity to control the magnetic properties externally (e.g., via a gate bias). Numerical estimations show that magnetic impurities such as ${\mathrm{Mn}}^{2+}$ with spins $S=5/2$ can realize ferromagnetism with $T_c$ close to room temperature.

We study the crossover from the weak coupling Bardeen-Cooper-Schrieffer (BCS) state to Bose-Einstein condensation (BEC) at strong coupling in a two-band superconductor with orbitals of opposite parity coexisting at a common Fermi surface in the metallic state. In such systems hybridization can play a role similar to spin-orbit interaction in fermionic spinor gases, enhancing interband pairing and opening the possibility for driving the BCS-BEC crossover. Building on a mean-field analysis, we investigate the BCS-BEC crossover induced by a variation of the hybridization strength. We show that the relevant scale for the crossover depends strongly on the ratio of effective masses, with the crossover favored in systems with one dispersive and one flat band. Including the effect of thermal pair fluctuations in a one-loop approximation, we calculate the dependence of the critical temperature on the microscopic parameters in the strong coupling regime.

Recently, high-entropy alloys (HEAs) have emerged as a unique platform for discovering superconducting materials and offer avenues to explore exotic superconductivity. The highly disordered nature of HEAs suggests the regular phonon required for BCS superconductivity may be unlikely to occur. Therefore, understanding the microscopic properties of these superconducting HEAs is important. We report a detailed characterization of the superconducting properties of the noncentrosymmetric ($\alpha \text{−}\mathrm{Mn}$ structure) HEAs ${\left(\mathrm{HfNb}\right)}_{0.10}{\left(\mathrm{MoReRu}\right)}_{0.90}$ and ${\left(\mathrm{ZrNb}\right)}_{0.10}{\left(\mathrm{MoReRu}\right)}_{0.90}$ by using magnetization, specific heat, AC transport, and muon-spin relaxation/rotation ($\mu \mathrm{SR}$). Despite the disordered nature, low-temperature specific heat and transverse-field muon spin rotation measurements suggest a nodeless isotropic superconducting gap, and zero-field $\mu \mathrm{SR}$ measurements confirm that time reversal symmetry is preserved in the superconducting ground state.

We investigate theoretically the superconducting state of the undoped Fe-based superconductor ThFeAsN. Using input from ab initio calculations, we solve the Fermi-surface based, multichannel Eliashberg equations for Cooper-pair formation mediated by spin and charge fluctuations, and by the electron-phonon interaction (EPI). Our results reveal that spin fluctuations alone, when coupling only hole-like with electron-like energy bands, can account for a critical temperature $T_c$ up to $\sim 7.5\mathrm{K}$ with an $s_{\pm }$-wave superconducting gap symmetry, which is a comparatively low $T_c$ with respect to the experimental value $T_c^{\exp }=30\mathrm{K}$. Other combinations of interaction kernels (spin, charge, electron-phonon) lead to a suppression of $T_c$ due to phase frustration of the superconducting gap. We qualitatively argue that the missing ingredient to explain the gap magnitude and $T_c$ in this material might be the first-order correction to the EPI vertex. In the noninteracting state this correction adopts a form supporting the $s_{\pm }$ gap symmetry, in contrast to EPI within Migdal's approximation, i.e., EPI without vertex correction, and therefore it enhances tendencies arising from spin fluctuations.

We report muon spin rotation $\left(\mu \mathrm{S}\mathrm{R}\right)$ and magnetization measurements on superconducting ${\mathrm{Sr}}_2{\mathrm{VO}}_3\mathrm{Fe}\mathrm{As}$ under pressure. At ambient pressure, ${\mathrm{Sr}}_2{\mathrm{VO}}_3\mathrm{Fe}\mathrm{As}$ undergoes an antiferromagnetic transition of the V moments at $T_N$ and becomes superconducting at $T_c<T_N$. As a function of pressure, $T_N$ initially decreases while $T_c$ increases. Surprisingly, once $T_N\approx T_c$ at 0.6 GPa, $T_N$ reverses its trend and increases together with $T_c$, which might indicate a cooperative coupling of the superconducting and the magnetic order.