We present two complementary methods to calculate the Andreev bound state energies of a single-level quantum dot connected to superconducting leads described by the superconducting impurity Anderson model. The first method, which is based on a mapping to a low-energy model, can be utilized to extract the Andreev bound state energies from finite-temperature, imaginary-time quantum Monte Carlo data without the necessity of any analytic continuation technique. The second method maps the full model on an exactly solvable superconducting atomic limit with renormalized parameters. As such, it represents a fast and reliable method for a quick scan of the parameter space. We demonstrate that after adding a simple band correction this method can provide predictions for measurable quantities, including the Josephson current, that are in a solid quantitative agreement with precise results obtained by the numerical renormalization group and quantum Monte Carlo.

We argue theoretically that, in doped $AB$ bilayer graphene, electron-electron coupling can give rise to the spontaneous formation of fractional metal phases. These states, being generalizations of a more common half-metal, have a Fermi surface that is perfectly polarized not only in terms of a spin-related quantum number, but also in terms of the valley index. The proposed mechanism assumes that the ground state of undoped bilayer graphene is a spin-density-wave insulator, with a finite gap in the single-electron spectrum. Upon doping, the insulator is destroyed and replaced by a fractional metal phase. As doping increases, transitions between various types of fractional metal (half-metal, quarter-metal, etc.) are triggered. Our findings are consistent with recent experiments on doped $AB$ bilayer graphene, in which a cascade of phase transitions between different isospin states was observed.

For electron optics in graphene, the propagation effect has so far been the only physical mechanism available. The resulting optics-inspired electronic components are large in size and operate at low temperatures to avoid violating the ballistic transport limits. Here, electron metasurfaces, i.e., electronic counterparts of optical metasurfaces, are introduced for graphene electronics. We theoretically implement various angles of electron beam bending, as well as beam splitting at corresponding angles in the same metasurface with the functionalities switched freely by controlling the applied gate biases. The wavefront of electron beams is shaped within a distance far below the ballistic transport distance at room temperature, allowing for the realization of optics-inspired electronic devices that can operate under ambient conditions. The concept of metasurface electron optics, based on elaborate design of more complex spatial phase patterns, might also open up a promising avenue for achieving more appealing applications such as electron metalenses, metasurface holography, as well as metasurface-based digital coding technology in graphene.

The scattering picture of electron transport in mesoscopic conductors shows that fluctuations of the current reveal additional information on the scattering mechanism not available through the conductance alone. The electronic fluctuations are coupled to the electromagnetic field and a junction at finite bias or temperature will emit radiation. The nonsymmetrized current-current correlators characterize the emission and absorption spectrum. Recent interest is focused on the so-called delta-$T$ noise, which is the nonequilibrium noise caused by a temperature difference between the terminals. Here, we generalize the notion of delta-$T$ noise to the nonsymmetrized current-current correlator at finite frequencies. We investigate the spectral density for energy-independent scattering and for a resonant level as an example of energy-dependent scattering. We find that a temperature difference $\mathrm{\Delta }T$ leads to a partial reduction of the noise for certain frequency ranges. This is a consequence of temperature broadening in combination with a frequency shift of the involved Fermi distributions. In the case of energy-independent scattering, the lowest order is a quadratic $\propto {\left(\mathrm{\Delta }T\right)}^2$ correction of the thermallike noise spectrum. For the resonance, an additional contribution to the delta-$T$ noise spectrum arises that is $\propto \mathrm{\Delta }T$ to the lowest order.

Angle-resolved photoemission spectroscopy experiments reveal a surprisingly richer surface electronic structure in $1T\text{−}{\mathrm{VSe}}_2$ than previously predicted or probed. Earlier claims supporting a charge density wave phase in this material are reexamined in terms of these findings and are found to be untenable. The Fermi surface is found to be gapless, while band warping effects, currently attributed to three-dimensional lattice distortion, result from the simultaneous dispersion of the closely lying multiple bands. Based on these findings, a charge density wave scenario in $1T\text{−}{\mathrm{VSe}}_2$ is unlikely. On the other side, the presence of multiple states crossing the Fermi level should constitute relevant constraints for any viable microscopic model of the structural phase transition of ${\mathrm{VSe}}_2$.

We propose a theoretical model of a master equation for cathodoluminescence (CL). The master equation describes simultaneous excitation of multiple emitters by an incoming electron and radiative decay of individual emitters. We investigate the normalized second-order correlation function, $g^{\left(2\right)}\left(\tau \right)$, of this model. We derive the exact formula for the zero-time delay correlation, $g^{\left(2\right)}\left(0\right)$, and show that the model successfully describes giant bunching (superbunching) in the CL. We also derive an approximate form of $g^{\left(2\right)}\left(\tau \right)$, which is valid for small excitation rate. Furthermore, we discuss the state of the radiation field of the CL. We reveal that the superbunching results from a mixture of an excited photon state and the vacuum state and that this type of state is realized in the CL.

Realizing ferromagnetic (FM) semiconductors with Curie temperatures ($T_c$) above 300 K is highly desirable for spintronics but remains a big challenge. Up to now, the oxide double perovskite ${\mathrm{La}}_2{\mathrm{NiMnO}}_6$ is regarded as one of the most promising FM semiconductors, where the near-${180}^{\circ }$ strong superexchange interaction brings its $T_c$ up to 280 K, but it is still below room temperature. Here, we propose that, by using ${\mathrm{H}}^{-}$ instead of ${\mathrm{O}}^{2-}$ as magnetic mediator, the resulted perfect ${180}^{\circ }$ bond angle and short interaction distance between magnetic ions can notably raise the $T_c$ beyond 300 K. The idea is verified by theoretically designing a class of thermodynamically stable hydride double perovskites ${\mathrm{A}}_2{\mathrm{NiVH}}_6 (\mathrm{A}=\mathrm{Na},\mathrm{K},\mathrm{Rb},\mathrm{Cs})$ with ferromagnetic $T_c$ up to 789 K. Moreover, the ${\mathrm{A}}_2{\mathrm{NiVH}}_6$ are identified as intriguing bipolar magnetic semiconductors with the carrier's spin orientation readily reversible by electrical gating. In addition, the possible Ni/V disorder further changes ${\mathrm{A}}_2{\mathrm{NiVH}}_6$ to be compensated ferrimagnetic semiconductors with vanishing magnetization, which may have unique advantages in antiferromagnetic spintronics.

Three-dimensional Hopf insulators are a class of topological phases beyond the tenfold-way classification. At the critical point of the transition between two distinct Hopf insulators with rotational symmetry, the band-touching points are point Berry dipoles with two opposite Berry charges overlapping in a mirror-symmetric way, and carry unique Berry curvature structures leading to a special quantization of Berry flux. Close to such Berry-dipole transitions, we find that the extrinsic and intrinsic nonlinear Hall conductivity tensors in the weakly doped regime are characterized by two universal functions of the ratio between doping level and bulk energy gap, and are directly proportional to the change in Hopf invariant across the transition. Our work suggests that the nonlinear Hall effects display a generalized-sense quantized behavior across Berry-dipole transitions, establishing a correspondence between nonlinear Hall effects and Hopf invariants.

The theory of the linear photogalvanic effect is developed for direct optical transitions between surface states of three-dimensional topological insulators. The photocurrent governed by the orientation of the polarization plane of light and caused by the warping of the energy dispersion of two-dimensional carriers is calculated. It is shown that both the shift contribution caused by coordinate shifts of the particle wave packets during the optical transitions and the ballistic contribution caused by interference of the optical absorption and scattering by disorder have generally the same order of magnitude. The ballistic contribution is present owing to electron-hole asymmetry of topological surface states and has a frequency dependence in contrast to the shift photocurrent. In the nonlinear-in-the-light-intensity regime appearing due to saturation of the direct optical transitions, the ballistic contribution dominates. The nontrivial dependence of the photocurrent on the polarization plane orientation in the nonlinear regime appears solely due to the ballistic contribution. Our findings allow separating the ballistic and shift contributions to the Linear photogalvanic current in experiments.

Circular dichroism (CD) is an intriguing chiroptical phenomenon associated with the interaction of chiral structures with circularly polarized lights. Although the CD effect has been extensively studied in optics, it has not yet been demonstrated in acoustic systems. Here, we demonstrate the acoustic CD effect in a three-dimensional chiral metamaterial supporting circularly polarized transverse sound. We find that the effect is negligible in the lossy metamaterial possessing $C_4$ rotational symmetry but can be strongly enhanced in the $C_2$-symmetric system with inhomogeneous loss. The phenomena can be understood based on the properties of the metamaterial's complex band structure and the quality factors of its eigenmodes. We show that the enhanced CD in the $C_2$-symmetric system is attributed to the polarization band gaps and the non-Hermitian exceptional points appearing near the Brillouin-zone center and boundaries. The results contribute to the understanding of chiral sound-matter interactions and can find applications in acoustic sensing of chiral structures and sound manipulations based on vector properties.

We present a rigorous theoretical framework underpinning the technique of spin-echo modulated small-angle neutron scattering (SEMSANS), and show how the technique can be extended in order to generate spin-textured neutron beams with orbital angular momentum (OAM) via birefringent neutron spin-polarization devices known as magnetic Wollaston prisms. Neutron OAM beams are mathematically characterized by a “cork-screw” phase singularity $e^{i\ell \varphi }$ about the propagation axis where $\ell$ is the OAM quantum number. To understand the precise relationship between the emergent OAM state and the variety of spin textures realized by various setups, we have developed a path-integral approach that in the interferometric limit makes a judicious use of magnetic Snell's law. We show that our proposed technique produces a complex two-dimensional pattern of spin-OAM entangled states which may be useful as a probe of quantum magnetic materials. We compare our path-integral approach to the well-known single-path Larmor precession model and present a pedagogical derivation of magnetic Snell's law of refraction for both massive and massless particles based on Maupertuis's action principle.

We investigate the nematic and loop-current-type orders that may arise as vestigial precursor phases in a model with an underlying pair-density wave (PDW) instability. We discuss how such a vestigial phase gives rise to a highly anisotropic stiffness for a coexisting single-component superconductor with low intrinsic stiffness, as is the case for the underdoped cuprate superconductors. Next, focusing on a regime with a mean-field PDW ground state with loop-current and nematic $xy$ (${\mathrm{B}}_{2g}$) order, we find a preemptive transition into low- and high-temperature vestigial phases with loop-current and nematic order corresponding to $xy$ (${\mathrm{B}}_{2g}$) and $x^2-y^2$ (${\mathrm{B}}_{1g}$) symmetry, respectively. Near the transition between the two phases, a state of soft nematic order emerges for which we expect that the nematic director is readily pinned away from the high-symmetry directions in the presence of an external field. Results are discussed in relation to findings in the cuprates, especially to the recently inferred highly anisotropic superconducting fluctuations [Wårdh et al., arXiv:2203.06769], giving additional evidence for an underlying ubiquitous PDW instability in these materials.

We analyze results of a recent experiment [D. Razmadze, E. C. T. O'Farrell, P. Krogstrup, and C. M. Marcus, Phys. Rev. Lett. 125, 116803 (2020)] on transport through a quantum dot between two full-shell nanowires and show that the observed effects are caused by the Kondo effect enhancement due to a nontrivial geometry (magnetic flux in a full-shell nanowire) rather than the presence of Majorana bound states. Moreover, we propose that such a setup presents a unique and convenient system to study the competition between superconductivity and the Kondo effect and has significant advantages in comparison to other known approaches, as the important parameter is controlled by the magnetic flux through the full-shell nanowire, which can be significantly varied with small changes of magnetic field, and does not require additional gates. This competition is of fundamental interest as it results in a quantum phase transition between an unscreened doublet and a many-body Kondo singlet ground state of the system.

The perovskite ruthenates ($A\mathrm{Ru}{\mathrm{O}}_3, A=\mathrm{Ca}$, Ba, or Sr) exhibit unique properties owing to a subtle interplay of crystal structure and electronic-spin degrees of freedom. Here, we demonstrate an intriguing continuous tuning of crystal symmetry from orthorhombic to tetragonal (no octahedral rotations) phases in epitaxial $\mathrm{SrRu}{\mathrm{O}}_3$ achieved via Ba substitution (${\mathrm{Sr}}_{1\text{−}x}{\mathrm{Ba}}_x\mathrm{Ru}{\mathrm{O}}_3$ with $0\le x\le 0.7$). An initial Ba substitution to $\mathrm{SrRu}{\mathrm{O}}_3$ not only changes the ferromagnetic properties, but also tunes the perpendicular magnetic anisotropy via flattening the Ru–O–Ru bond angle (to ${180}^{\circ }$), resulting in the maximum Curie temperature and an extinction of $\mathrm{Ru}{\mathrm{O}}_6$ rotational distortions at $x\approx 0.20$. For $x\le 0.2$, the reduction of $\mathrm{Ru}{\mathrm{O}}_6$ octahedral rotational distortion dominantly enhances the ferromagnetism in the system, though competing with the effect of the $\mathrm{Ru}{\mathrm{O}}_6$ tetragonal distortion. Further increasing Ba substitution $\left(x>0.2\right)$ gradually enhances the tetragonal-type distortion, resulting in the tuning of $\mathrm{Ru}\text{−}4d$ orbital occupancy and suppression of ferromagnetism. Our results demonstrate that isovalent substitution of the $A\text{−}\mathrm{site}$ cations significantly and controllably impacts both electronic and magnetic properties of perovskite oxides.

In this study, we report the magnetic energy landscape of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ employing the generalized Bloch approach within density functional theory. We identify previously suggested magnetic fluctuations and ferromagnetic and spin-density-wave instabilities together with other predominant instabilities. We show that epitaxial strain can change the overall magnetic tendency of the system and tune the relative weight of the various magnetic instabilities in the system. Especially, the balance between spin-density-wave and ferromagnetic instabilities can be controlled by the strain and, eventually, can lead to new magnetic phases as well as superconducting phases with possibly altered pairing channels. Our findings are compared with previous theoretical models and experimental reports for the various magnetic features of the system and offers a first-principles explanation for them.

Spintronic THz emitters (STE) are efficient THz sources constructed using thin heavy-metal (HM) and ferromagnetic-metal (FM) layers. To improve the performance of the STE, different structuring methods (trilayers, stacked bilayers) have been experimentally applied. A theoretical description of the overall THz emission process is necessary to optimize the efficiency of STE. In particular, geometry, composition, pump laser frequency, and spin diffusion will be significant in understanding the pathways for further research developments. This work will apply a generalized model based on a modified transfer matrix method. We will consider the spin generation and diffusion in the FM and HM layers and explain the spintronic THz emission process. This model is suitable for calculating emitted THz signal as a function of FM and HM thicknesses for different geometrical configurations. We will investigate a bilayer geometry as a test case, but the extension to a multi-layer configuration is straightforward. We will show how the different configurations of the sample will influence the THz emission amplitude.

By means of exact diagonalizations, the Bernevig-Hughes-Zhang model at quarter-filling in the limit of strong Hubbard on-site repulsion is investigated. We find that the noninteracting metallic state will be turned into a Chern insulator with saturated magnetization under strong correlations. That is, at such a metal-insulator transition, both the topological and the magnetic properties of the system are changed due to spontaneous breaking of time-reversal symmetry in the ground states. According to our findings, this topological phase transition seems to be of first order. Our results illustrate the interesting physics in topological Mott transitions and provide guidance to the search of more interaction-induced topological phases in similar systems.

The quantum spin Hall state can be understood in terms of spontaneous O(3) symmetry breaking. Topological skyrmion configurations of the O(3) order parameter vector carry a charge 2e, and as shown previously, when they condense, a superconducting state is generated. We show that this topological route to superconductivity survives easy-plane anisotropy. Upon reducing the O(3) symmetry to $\mathrm{O}\left(2\right)\times {\mathrm{Z}}_2$, skyrmions give way to merons that carry a unit charge. On the basis of large-scale auxiliary field quantum Monte Carlo simulations, we show that at the particle-hole symmetric point, we can trigger a continuous and direct transition between the quantum spin Hall state and $s$-wave superconductor by condensing pairs of merons. This statement is valid in both strong and weak anisotropy limits. Our results can be interpreted in terms of an easy-plane deconfined quantum critical point. However, in contrast to the previous studies in quantum spin models, our realization of this quantum critical point conserves $\text{U}\left(1\right)$ charge such that skyrmions are conserved.