Single holes confined in semiconductor quantum dots are a promising platform for spin-qubit technology, due to the electrical tunability of the $g$ factor of holes. However, the underlying mechanisms that enable electric spin control remain unclear due to the complexity of hole-spin states. Here, we study the underlying hole-spin physics of the first hole in a silicon planar metal-oxide-semiconductor (MOS) quantum dot. We show that nonuniform electrode-induced strain produces nanometer-scale variations in the heavy-hole–light-hole (HH-LH) splitting. Importantly, we find that this nonuniform strain causes the HH-LH splitting to vary by up to 50% across the active region of the quantum dot. We show that local electric fields can be used to displace the hole relative to the nonuniform strain profile, allowing a mechanism for electric modulation of the hole $g$ tensor. Using this mechanism, we demonstrate tuning of the hole $g$ factor by up to 500%. In addition, we observe a potential sweet spot where $d{g}_{\left(1\overline{1}0\right)}/dV=0$, offering a configuration to suppress spin decoherence caused by electrical noise. These results open a path towards a technology involving engineering of nonuniform strains to optimize spin-based devices.

The exceptionally strong Coulomb interaction in semiconducting transition-metal dichalcogenides (TMDs) gives rise to a rich exciton landscape consisting of bright and dark exciton states. At elevated densities, excitons can interact through exciton-exciton annihilation (EEA), an Auger-like recombination process limiting the efficiency of optoelectronic applications. Although EEA is a well-known and particularly important process in atomically thin semiconductors determining exciton lifetimes and affecting transport at elevated densities, its microscopic origin has remained elusive. In this joint theory-experiment study combining microscopic and material-specific theory with time- and temperature-resolved photoluminescence measurements, we demonstrate the key role of dark intervalley states that are found to dominate the EEA rate in monolayer ${\mathrm{WSe}}_2$. We reveal an intriguing, characteristic temperature dependence of Auger scattering in this class of materials with an excellent agreement between theory and experiment. Our study provides microscopic insights into the efficiency of technologically relevant Auger scattering channels within the remarkable exciton landscape of atomically thin semiconductors.

Interactions between electrons can strongly affect the shape and functionality of multielectron quantum dots. The resulting charge distributions can be localized, as in the case of Wigner molecules, with consequences for the energy spectrum and tunneling to states outside the dot. The situation is even more complicated for silicon dots, due to the interplay between valley, orbital, and interaction energy scales. Here, we study two-electron wave functions in electrostatically confined quantum dots formed in a SiGe/Si/SiGe quantum well at zero magnetic field, using a combination of tight-binding and full configuration interaction methods, and taking into account atomic-scale disorder at the quantum well interface. We model dots based on recent qubit experiments, which straddle the boundary between strongly interacting and weakly interacting systems, and display a rich and diverse range of behaviors. Our calculations show that strong electron-electron interactions, induced by weak confinement, can significantly suppress the low-lying, singlet-triplet (ST) excitation energy. However, when the valley-orbit interactions caused by interfacial disorder are weak, the ST splitting can approach its noninteracting value, even when the electron-electron interactions are strong and Wigner-molecule behavior is observed. These results have important implications for the rational design and fabrication of quantum dot qubits with predictable properties.

The impact of Coulomb interaction on the electronic properties of a quantum spin Hall insulator is studied using quantum cluster methods, disentangling local from nonlocal effects. We identify different regimes, according to the value of the bare mass term, characterized by drastically different self-energy contributions. For small mass, nonlocal correlations start to be important and eventually dominate over local ones when getting close enough to the zero-mass semimetallic line. For intermediate and large mass, local correlation effects outweigh nonlocal corrections, leading to a first-order topological phase transition, in agreement with previous predictions.

Many advances in reflective metasurfaces have been made during the last few years, implementing efficient manipulations of wave fronts, especially for plane waves. Despite numerous solutions that have been developed throughout the years, a practical method to obtain subwavelength focusing without the generation of additional undesired scattering is a challenge to this day. In this paper, we introduce and discuss lossless reflectors for focusing incident waves into a point. The solution is based on the so-called power-flow conformal surfaces that allow theoretically arbitrary shaping of reflected waves. The metamirror shape is adapted to the power flow of the sum of the incident and reflected waves, allowing a local description of the reflector surface based on the surface impedance. In particular, we present a study of two scenarios. First, we study the scenario when the field is emitted by a point source and focused at an image point (in the considered example, with the $\lambda /20$ resolution). Second, we analyze a metasurface capable of focusing the power of an illuminating plane wave. This work provides a feasible strategy for various applications, including detecting biological signals near the skin, sensitive power focusing for cancer therapy, and point-to-point power transfer.

The recent discovery of superconductivity in ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$ has drawn significant attention in the field. A key open question regards the evolution of the electronic structure with respect to hole doping. Here we exploit x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) to probe the doping-dependent electronic structure of ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$. Upon doping, a high-energy feature in Ni $L_3$-edge XAS develops in addition to the main absorption peak, while XAS at the O $K$-, Nd $M_3$- and Nd $M_5$-edge exhibits a much weaker response. This implies that doped holes are mainly introduced into Ni $3d$ states. By comparing our data to atomic multiplet calculations including $D_{4h}$ crystal field, the doping-induced feature in Ni $L_3$-edge XAS is consistent with a $d^8$ spin-singlet state in which doped holes reside in the $3d_{x^2-y^2}$ orbitals. This is further supported by the softening of RIXS orbital excitations due to doping, corroborating with the Fermi level shift associated with increasing holes in the Ni $3d_{x^2-y^2}$ orbital.

In this paper we present an algorithm which allows single-stage direct Langevin dynamics simulations of the escape rate over arbitrarily high energy barriers employing the concept of the energy-dependent temperature (EDT). In our algorithm, simulation time required for the computation of the escape rate does not increase with the energy barrier. This is achieved by using in simulations an effective temperature which depends on the system energy: around the energy minima this temperature is high, whereas it tends towards room temperature when the energy approaches the saddle point value. Switching times computed via our EDT algorithm show an excellent agreement with results obtained with the established forward flux sampling (FFS) method. As the simulation time required by our method does not increase with the energy barrier, we achieve a very large speedup when compared even to our highly optimized FFS version with interfaces placed equidistantly in the energy space. In addition, our method does not suffer from stability problems occurring in multistage algorithms (like FFS and “energy bounce” methods) due to the multiplication of a large number of transition probabilities between the interfaces.

Luttinger's theorem connects a basic microscopic property of a given metallic crystalline material, the number of electrons per unit cell, to the volume, enclosed by its Fermi surface, which defines its low-energy observable properties. Such statements are valuable since, in general, deducing a low-energy description from microscopics, which may perhaps be regarded as the main problem of condensed matter theory, is far from easy. In this paper, we present a unified framework which allows one to discuss Luttinger theorems for ordinary metals as well as closely analogous exact statements for topological (semi)metals, whose low-energy description contains either discrete points or continuous line nodes. This framework is based on the 't Hooft anomaly of the emergent charge conservation symmetry at each point on the Fermi surface, a concept recently proposed by Else et al. [Phys. Rev. X 11, 021005 (2021)]. We find that the Fermi surface codimension $p$ plays a crucial role for the emergent anomaly. For odd $p$, such as ordinary metals ($p=1$) and magnetic Weyl semimetals ($p=3$), the emergent symmetry has a generalized chiral anomaly. For even $p$, such as graphene and nodal line semimetals (both with $p=2$), the emergent symmetry has a generalized parity anomaly. When restricted to microscopic symmetries, such as $\text{U}\left(1\right)$ and lattice symmetries, the emergent anomalies imply (generalized) Luttinger theorems, relating Fermi surface volume to various topological responses. The corresponding topological responses are the charge density for $p=1$, Hall conductivity for $p=3$, and polarization for $p=2$. As a by-product of our results, we clarify exactly what is anomalous about the surface states of nodal line semimetals.

We study thermalization and spectral properties of extended systems connected, through their boundaries, to a thermalizing Markovian bath. Specifically, we consider periodically driven systems modeled by brickwork quantum circuits where a finite section (block) of the circuit is constituted by arbitrary local unitary gates while its complement, which plays the role of the bath, is dual unitary. We show that the evolution of local observables and the spectral form factor are determined by the same quantum channel, which we use to characterize the system's dynamics and spectral properties. In particular, we identify a family of fine-tuned quantum circuits—which we call strongly nonergodic—that fails to thermalize even in this controlled setting, and, accordingly, their spectral form factor does not follow the random matrix theory prediction. We provide a set of necessary conditions on the local quantum gates that lead to strong nonergodicity, and in the case of qubits, we provide a complete classification of strongly nonergodic circuits. We also study the opposite extreme case of circuits that are almost dual unitary, i.e., where thermalization occurs with the fastest possible rate. We show that, in these systems, local observables and spectral form factor approach, respectively, thermal values and random matrix theory prediction exponentially fast. We provide a perturbative characterization of the dynamics and, in particular, of the timescale for thermalization.

We present ab initio calculations of photoionization thresholds and cross sections of the negatively charged nitrogen-vacancy (NV) center in diamond. We consider photoionization from the ground ${}^{3}A_2$ and the excited ${}^{3}E$ states. After the ionization from the ${}^{3}E$ level, we show that the NV center transitions into the metastable ${}^{4}A_2$ electronic state of the neutral defect. We reveal how spin polarization of ${\mathrm{NV}}^{-}$ gives rise to spin polarization of the ${}^{4}A_2$ state, explaining electron spin resonance experiments. We obtain smooth curves of photoionization cross sections as a function of energy by employing dense $k$-point meshes for the Brillouin-zone integration together with the band unfolding technique to rectify the distortions of the band structure induced by the artificial periodicity of the supercell approach. Our calculations provide a comprehensive picture of photoionization mechanisms of ${\mathrm{NV}}^{-}$. They will be useful in interpreting and designing experiments on charge-state dynamics at NV centers. In particular, we offer a consistent explanation of recent results of spin-to-charge conversion of NV centers.

It has been suggested that the enlarged spin susceptibility in topological insulators, described by interband Van Vleck's formalism, accounts for the ferromagnetism of bismuth-antimony topological chalcogenides doped with transition metal impurities. In contrast, earlier studies of HgTe and related topological systems pointed out that the interband analog of the Ruderman-Kittel-Kasuya-Yosida interaction (the Bloembergen-Rowland mechanism) leads to antiferromagnetic coupling between pairs of localized spins. Here, we critically revisit these two approaches, show their shortcomings, and elucidate why the magnitude of the interband contribution is small even in topological systems. From the proposed theoretical approach and our computational studies of magnetism in Mn-doped HgTe and CdTe, we conclude that in the absence of band carriers, the superexchange dominates and its sign depends on the coordination and charge state of magnetic impurities rather than on the topological class of the host material.

Small-twist-angle transition metal dichalcogenide (TMD) heterobilayers develop isolated flat moiré bands that are approximately described by triangular lattice generalized Hubbard models [F. Wu, T. Lovorn, E. Tutuc, and A. H. MacDonald, Phys. Rev. Lett. 121, 026402 (2018)]. In this paper we explore the metallic and insulating states that appear under different control conditions at a density of one electron per moiré period and the transitions between them. By combining fully self-consistent Hartree-Fock theory calculations with strong-coupling expansions around the atomic limit, we identify four different magnetic states and one nonmagnetic state near the model phase diagram's metal-insulator phase transition line. Ferromagnetic insulating states, stabilized by nonlocal direct exchange interactions, are surprisingly prominent.

Dirac semimetals lack a simple bulk-boundary correspondence. Recently, Dirac materials with fourfold rotation symmetry have been shown to exhibit a higher-order bulk-hinge correspondence: they display “higher-order Fermi arcs,” which are localized on hinges where two surfaces meet and connect the projections of the bulk Dirac points. In this paper, we classify higher-order Fermi arcs for Dirac semimetals protected by a rotation symmetry and the product of time-reversal and inversion. Such Dirac points can be either linear in all directions or linear along the rotation axis and quadratic in other directions. By computing the filling anomaly for momentum-space planes on either side of the Dirac point, we find that all linear Dirac points exhibit higher-order Fermi arcs terminating at the projection of the Dirac point, while the Dirac points that are quadratic in two directions lack such higher-order Fermi arcs. When higher-order Fermi arcs do exist, they obey either a ${\mathbb{Z}}_2$ (fourfold rotation axis) or ${\mathbb{Z}}_3$ (three- or sixfold rotation axis) group structure. Finally, we build two models with sixfold symmetry to illustrate the cases with and without higher-order Fermi arcs. We predict higher-order Fermi arcs in ${\mathrm{Na}}_3\mathrm{Bi}$.

Molybdenum bronze $\eta \text{−}{\mathrm{Mo}}_4{\mathrm{O}}_{11}$ with a monoclinic structure has received continued interest due to a variety of magnetotransport properties in its charge density wave (CDW) state, while the orthorhombic phase $\gamma \text{−}{\mathrm{Mo}}_4{\mathrm{O}}_{11}$ has been less reported. We successfully grew high-quality $\gamma \text{−}{\mathrm{Mo}}_4{\mathrm{O}}_{11}$ single crystals via a chemical vapor transport method. Specific heat, magnetic susceptibility, and various electric transport properties in DC and pulsed fields were carefully measured. Our main findings include (1) $\gamma \text{−}{\mathrm{Mo}}_4{\mathrm{O}}_{11}$ belongs to a quasi-two-dimensional (Q2D) system and undergoes a transition at $T_{\mathrm{CDW}}=\sim 96\mathrm{K}$ due to a CDW instability. Unlike the robust transition in $\eta \text{−}{\mathrm{Mo}}_4{\mathrm{O}}_{11}$, the CDW modulation in $\gamma \text{−}{\mathrm{Mo}}_4{\mathrm{O}}_{11}$ is rather broad and flexible, which results in negligible anomaly in specific heat. (2) $\gamma \text{−}{\mathrm{Mo}}_4{\mathrm{O}}_{11}$ shows a clear nonlinear current-voltage relation below $T_{\mathrm{CDW}}$ due to a sliding motion of the CDW electrons. This indicates that the CDW gap in $\gamma \text{−}{\mathrm{Mo}}_4{\mathrm{O}}_{11}$ is nearly fully opened, and the residual small number of electrons contribute to the CDW transport, different from the situation in $\eta \text{−}{\mathrm{Mo}}_4{\mathrm{O}}_{11}$. (3) The interlayer magnetoresistivity exhibits magnetic transition at ∼5 T and quantum oscillations at $B>15\mathrm{T}$ in applied magnetic fields up to 60 T. These field-induced oscillations featured with two main frequencies ($f_{\alpha }=30\mathrm{T}$ and $f_{\beta }=131\mathrm{T}$) are reminiscent of a Fermi surface reconstruction by magnetic breakdown in a strong field in $\eta \text{−}{\mathrm{Mo}}_4{\mathrm{O}}_{11}$. In this paper, we reveal that $\gamma \text{−}{\mathrm{Mo}}_4{\mathrm{O}}_{11}$ is distinct from traditional quasi-one-dimensional or Q2D CDW compounds and provide an opportunity to study the CDW electrons and nonlinear transport properties in such a Q2D system.

We report muon spin rotation (μSR) and neutron diffraction on the rare-earth-based magnets ${\left({\mathrm{Mo}}_{2/3}\mathrm{R}{\mathrm{E}}_{1/3}\right)}_2\mathrm{AlC}$, also predicted as parent materials for two-dimensional (2D) derivatives, where the rare earth (RE) = Nd, Gd (only μSR), Tb, Dy, Ho, and Er. By crossing information between the two techniques, we determine the magnetic moment ($m$), structure, and dynamic properties of all compounds. We find that only for RE = Nd and Gd the moments are frozen on a microsecond timescale. Out of these two, the most promising compound for a potential 2D high $m$ magnet is the Gd variant, since the parent crystals are pristine with $m=6.5\pm 0.5{\mu }_{\mathrm{B}}$, Néel temperature of $29\pm 1\mathrm{K}$, and the magnetic anisotropy between out-of- and in-plane coupling is smaller than ${10}^{-8}$. This result suggests that magnetic ordering in the Gd variant is dominated by in-plane magnetic interactions and should therefore remain stable when exfoliated into 2D sheets.

A quantum many-body system with a conserved electric charge can have a DC resistivity that is either exactly zero (implying it supports dissipationless current) or nonzero. Exactly zero resistivity is related to conservation laws that prevent the current from degrading. In this paper, we carefully examine the situations in which such a circumstance can occur. We find that exactly zero resistivity requires either continuous translation symmetry or an internal symmetry that has a certain kind of “mixed anomaly” with the electric charge. (The symmetry could be a generalized global symmetry associated with the emergence of unbreakable loop or higher dimensional excitations). However, even if one of these is satisfied, we show that there is still a mechanism to get nonzero resistivity, through critical fluctuations that drive the susceptibility of the conserved quantity to infinity; we call this mechanism “critical drag.” Critical drag is thus a mechanism for resistivity that, unlike conventional mechanisms, is unrelated to broken symmetries. We furthermore argue that an emergent symmetry that has the appropriate mixed anomaly with electric charge is in fact an inevitable consequence of compressibility in systems with lattice translation symmetry. Critical drag, therefore, seems to be the only way (other than through irrelevant perturbations breaking the emergent symmetry that disappear at the renormalization group fixed point) to get nonzero resistivity in such systems. Finally, we present a very simple and concrete model—the “quantum Lifshitz model”—that illustrates the critical drag mechanism as well as the other considerations of the paper.

We have carried out a comprehensive investigation of the magnetic properties of the intercalated ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{O}}_7$ by the means of dc magnetization, polarized neutron scattering, and x-ray magnetic circular dichroism (XMCD) measurements. The results obtained by these techniques support a spin order in each of the Fe-O bilayers that is the same as observed in the nonintercalated $\mathrm{Lu}{\mathrm{Fe}}_2{\mathrm{O}}_4$. In the Fe-O single layers, a magnetic moment appears to be induced by the application of a magnetic field, paramagnetic-like in the first approximation. Furthermore, a spin-charge coupling was revealed by the XMCD measurements.

Recently, three different magnetic states were observed experimentally in trilayer ${\mathrm{CrI}}_3$ under pressure, including ferromagnetic (FM)-$\uparrow \uparrow \uparrow$, FM-$\downarrow \uparrow \downarrow$, and FM-$\uparrow \uparrow \downarrow$. To reveal the nature of the three observed magnetic states, we studied the magnetic properties of four possible stacking structures in trilayer ${\mathrm{CrI}}_3$: Structures I (rhombohedral), II (monoclinic), III (hexagonal), and IV (triclinic). We find that all four stacking structures possess the FM-$\uparrow \uparrow \uparrow$ ground state. After applying a few strains, FM-$\downarrow \uparrow \downarrow$ becomes the ground state in structures II and III, and FM-$\uparrow \uparrow \downarrow$ is preferred in structure IV, while FM-$\uparrow \uparrow \uparrow$ persists in structure I. Our results unveil that the three magnetic states observed in trilayer ${\mathrm{CrI}}_3$ may correspond to different stacking structures with small tensile strains, which can well interpret the experimentally obtained pressure-dependent interlayer coupling and Curie temperature. Our present study paves a way to design magnetic multilayers with required magnetic states by tuning stacking and strain.

We develop a theory for the dynamics of an Andreev bound state (ABS) hosted by a weak link of finite length for which charging effects are important. Assuming that the charging energy associated with an electron in the ABS is small compared to the superconducting order parameter in the leads, we derive the linear response of both the current through the link and charge accumulated in it with respect to the phase and gate voltage biases. The resulting matrix encapsulates the spectroscopic properties of a weak link embedded in a microwave resonator. In the low-frequency limit, we obtain the response functions analytically using an effective low-energy Hamiltonian, which we derive. This Hamiltonian minimally accounts for Coulomb interaction and is suitable for a phenomenological description of a weak link having a finite length.

We demonstrate that phase-coherent heat transport constitutes a powerful tool to probe Majorana physics in topological Josephson junctions. We predict that the thermal conductance transverse to the direction of the superconducting phase bias is universally quantized by half the thermal conductance quantum at phase difference $\varphi =\pi$. This is a direct consequence of the parity-protected counterpropagating Majorana modes which are hosted at the superconducting interfaces. Away from $\varphi =\pi$, we find a strong suppression of the thermal conductance due to the opening of a gap in the Andreev spectrum. This behavior is very robust with respect to the presence of magnetic fields. It is in direct contrast to the thermal conductance of a trivial Josephson junction which is suppressed at any phase difference $\varphi$. Thus, thermal transport can provide strong evidence for the existence of Majorana modes in topological Josephson junctions.