Majorana modes can arise as zero-energy bound states in a variety of solid state systems. A two-dimensional phase supporting these quasiparticles, for instance, emerges on the surface of a topological superconductor with the zero modes localized at the cores of vortices. At low energies, such a setup can be modeled by Majorana modes that interact with each other on the Abrikosov lattice. In experiments, the lattice is usually triangular. Motivated by the practical relevance, we explore the phase diagram of this Hubbard-like Majorana model using a combination of mean-field theory and numerical simulation of thin torus geometries through the density matrix renormalization group algorithm. Our analysis indicates that attractive interactions between Majoranas can drive a phase transition in an otherwise gapped topological state.

We predict a mechanism for achieving complete population inversion in a continuously driven InAs/GaAs semiconductor quantum dot featuring V-type transitions. This highly nonequilibrium steady state is enabled by the interplay between V-type interband transitions and a non-Markovian decoherence mechanism, introduced by acoustic phonons. The population trapping mechanism is generalized to a chain of coupled emitters. Exploiting the population inversion, we predict unidirectional excitation transport from one end of the chain to the other without external bias, independent of the unitary interdot coupling mechanism.

We investigate the electrodynamics of a disk-shaped two-dimensional electron system in the proximity of a screening gate. The two-dimensional plasmons in the system under study exhibit the linear dispersion characteristic of acoustic plasma waves. We experimentally achieve the limit when the velocity of acoustic waves approaches the speed of light. We show that retardation effects lead to strong coupling between acoustic plasmons and the light, which is manifested in the renormalization of the electron effective mass. We develop a theory to substantiate the observed phenomena.

Color centers in hexagonal boron nitride (h-BN) are among the brightest emission centers known, yet the origins of these emission centers are not well understood. Here, using first-principles calculations in combination with the generating function method, we systematically elucidate the coupling of specific defects to the vibrational degrees of freedom. We show that the line shape of many defects exhibits strong coupling to high frequency phonon modes and that ${\mathrm{C}}_{\text{N}}$, ${\mathrm{C}}_{\text{B}}$, ${\mathrm{C}}_{\text{B}}-{\mathrm{C}}_{\text{N}}$ dimer, and ${\mathrm{V}}_{\text{B}}$ can be associated with experimental line shapes. Our detailed theoretical study serves as a guide to identify optically active defects in h-BN that can suit specific applications in photonic-based quantum technologies, such as single photon emitters, hybrid spin-photon interfaces, or spin-mechanics interfaces.

In many situations, excitons—bound particle-hole pairs above an insulating ground state—carry an electric dipole moment, allowing them to be manipulated via coupling to an electric field. For two-dimensional systems, we demonstrate that this property of an exciton is uniquely determined by the quantum geometry of its eigenstates, and demonstrate its intimate connection with a quantity which we call the quantum geometric dipole. We demonstrate that this quantity arises naturally in the semiclassical equations of motion of an exciton in an electric field, adding a term additional to the anomalous velocity coming from Berry's curvature. In a uniform electric field, this contributes a drift velocity to the exciton akin to that expected for excitons in crossed electric and magnetic fields, even in the absence of a real magnetic field. We compute the quantities relevant to semiclassical exciton dynamics for several interesting examples of bilayer systems with weak interlayer tunneling and Fermi energy in a gap, where the exciton may be sensibly described as a two-body problem. These quantities include the exciton dispersion, its quantum geometric dipole, and its Berry's curvature. For a simple example of two gapped-graphene layers in a vanishing magnetic field, we demonstrate that there is a nonvanishing quantum geometric dipole when the layers are different, e.g., have different gaps, but vanishes when the layers are identical. We further analyze examples in the presence of magnetic fields, allowing us to examine cases involving graphene, in which a gap is opened by Landau level splitting. Heterostructures involving transition metal dichalcogenides materials are also considered. In each case, the quantum geometric dipole and Berry's curvatures play out in different ways. In some cases, the lowest energy exciton state is found to reside at finite momentum, with interesting possible consequences for Bose condensation in these systems. Additionally, we find situations in which the quantum geometric dipole increases monotonically with exciton momentum, suggesting that the quantum geometry can be exploited to produce photocurrents from initially bound excitons with electric fields, without the need to overcome an effective barrier via tunneling or thermal excitation. We speculate on further possible effects of the semiclassical dynamics in geometries where the constituent layers are subject to the same or different electric fields.

Insulating systems are characterized by their insensitivity to twisted boundary conditions as quantified by the charge stiffness and charge localization length. The latter quantity was shown to be related to the expectation value of the many-body position operator and serves as a universal criterion to distinguish between metals and insulators. In this work we extend these concepts to a new class of quantum systems having conserved charge and dipole moments. We refine the concept of a charge insulator by introducing notions of multipolar insulators, e.g., a charge insulator could be a dipole insulator or dipole metal. We develop a universal criterion to distinguish between these phases by extending the concept of charge stiffness and localization to analogous versions for multipole moments but with our focus on dipoles. We are able to relate the dipole localization scale to the expectation value of a recently introduced many-body quadrupole operator. This refined structure allows for the identification of phase transitions where charge remains localized but, e.g., dipoles delocalize. We illustrate the proposed criterion using several exactly solvable models that exemplify these concepts and discuss a possible realization in cold-atom systems.

The effective interaction of downfolded low-energy models for electrons in solids can be obtained by integrating out the high energy bands away from the target band near the Fermi level. Here we apply the constrained random-phase approximation (cRPA) and constrained functional renormalization group (cfRG), which can go beyond cRPA by including all one-loop diagrams, to calculate and compare the effective interactions of the three-band Emery model, which is often used to investigate cuprate high-temperature superconductors. At half-filling we find that the effective interaction increases as the charge transfer energy (${\mathrm{\Delta }}_{dp}$) increases and similar behavior is obtained as a function of the interatomic $2p\text{−}3d$ interaction ($U_{dp}$). However, the effective interaction is more sensitive to ${\mathrm{\Delta }}_{dp}$ than $U_{dp}$. For most of the parameter sets, the effective static interaction is overscreened in cRPA compared to cfRG. The low-energy models at half-filling are solved within dynamical mean-field theory (DMFT). The results show that despite the different static interactions, the systems with cRPA and cfRG interaction exhibit a Mott transition at similar values of ${\mathrm{\Delta }}_{dp}$. We also investigate the effective interaction as a function of doping. The cfRG effective interaction decreases as the electron number increases and displays a trend opposite to that of cRPA. Antiscreening is observed for the hole-doped case. For all the cases studied, the near cancellation of the direct particle-hole channel is observed. This indicates that at least for the downfolding of the on-site interaction terms, methods beyond cRPA may be required.

Weyl semimetals are three-dimensional, topologically protected, gapless phases which show exotic phenomena such as Fermi arc surface states or negative magnetoresistance. It is an open question whether interparticle interactions can turn the topological semimetal into a topologically nontrivial Mott-insulating phase. We investigate an experimentally motivated model for Weyl physics of cold atoms in optical lattices, with the main focus on interaction effects and topological properties, by means of dynamical mean-field theory. We characterize topological phases by numerically evaluating the Chern number via the Ishikawa-Matsuyama formula for interacting phases. Within our studies, we find that the Chern numbers become trivial when interactions lead to insulating behavior. For a deeper understanding of the Weyl-semimetal-to-Mott-insulator topological phase transition, we evaluate the topological properties of quasiparticle bands as well as so-called blind bands. Finally, we consider a system with an open boundary along one spatial direction in order to study correlation effects of surface states. In a narrow regime close to the topological phase transition, we find a correlation-induced state in which the surface becomes metallic while the bulk is semimetallic.

Nonperturbative constraints on many-body physics, such as the famous Lieb-Schultz-Mattis theorem, are valuable tools for studying strongly correlated systems. To this end, we present a number of nonperturbative results that constrain the low-energy physics of systems having conserved dipole moments. We find that for these systems, a unique translationally invariant gapped ground state is only possible if the polarization of the system is integer. Furthermore, if a lattice system also has $\text{U}\left(1\right)$ subsystem charge conservation symmetry, a unique gapped ground state is only possible if the particle filling along these subsystems is integer. We also apply these methods to spin systems, and determine criteria for the existence of a different type of magnetic response plateau in the presence of a nonuniform magnetic field. Finally, we formulate a version of Luttinger's theorem for one-dimensional systems consisting of dipoles.

In the present work, we investigate the electronic and elastic properties in equilibrium and under strain of the type-II Dirac semimetal ${\mathrm{NiTe}}_2$ using density functional theory. Our results demonstrate the tunability of Dirac nodes' energy and momentum with strain and that it is possible to bring them closer to the Fermi level, while other metallic bands are suppressed. We also derive a minimal 4-band effective model for the Dirac cones, which accounts for the aforementioned strain effects by means of lattice regularization, providing an inexpensive way for further theoretical investigations and easy comparison with experiments. On an equal footing, we propose the static control of the electronic structure by intercalating alkali species into the van der Waals gap, resulting in the same effects obtained by strain engineering and removing the requirement of in situ strain. Finally, evaluating the wave-function's symmetry evolution as the lattice is deformed, we discuss possible consequences, such as Liftshitz transitions and the coexistence of type-I and type-II Dirac cones, thus motivating future investigations.

We theoretically investigate the properties of holes in a ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}/\mathrm{Ge}/{\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}$ quantum well in a perpendicular magnetic field that make them advantageous as qubits, including a large ($>100$ meV) intrinsic splitting between the light and heavy hole bands, a very light ($\sim 0.05m_0$) in-plane effective mass, consistent with higher mobilities and tunnel rates, and larger dot sizes that could ameliorate constraints on device fabrication. Compared to electrons in quantum dots, hole qubits do not suffer from the presence of nearby quantum levels (e.g., valley states) that can compete with spins as qubits. The strong spin-orbit coupling in Ge quantum wells may be harnessed to implement electric-dipole spin resonance, leading to gate times of several nanoseconds for single-qubit rotations. The microscopic mechanism of this spin-orbit coupling is discussed, along with its implications for quantum gates based on electric-dipole spin resonance, stressing the importance of coupling terms that arise from the underlying cubic crystal field. Our results provide a theoretical foundation for recent experimental advances in Ge hole-spin qubits.

Two-dimensional (2D) ferroelectric materials (FEMs) and their application in ferroelectric tunnel junctions (FTJs) have attracted a great deal of attention during the past several years due to their great potential in nonvolatile memory devices. Particularly, the all-2D FTJs, which have only atomic-layer thickness, have been demonstrated to show very high tunnel electroresistance (TER) ratio. Nevertheless, to better integrate with the present semiconductor technology, it is necessary to consider metal contacts in the construction of FTJs with 2D FEMs. However, due to the unknown interaction between traditional metals and 2D FEMs, it is not clear whether ferroelectricity still persists when the 2D FEMS are in contact with metals and whether the corresponding FTJs exhibit high TER effect as demanded for memory devices. To probe this, we construct FTJs with top contact between Au(010) and ${\mathrm{In}}_2{\mathrm{Se}}_3$, a 2D FEM with out-of-plane ferroelectric polarization. By density functional calculations combined with a nonequilibrium Green function technique, we find that not only the ferroelectricity still persists in the metal/FEM contact, but also a giant TER ratio as high as ${10}^4\%$ is achieved. The giant TER arises from the change of the metal/FEM contact from a Schottky type to an Ohmic type accompanying with the ferroelectric polarization reversal. In the meantime, the tunnel barrier height between Au(010) and ${\mathrm{In}}_2{\mathrm{Se}}_3$ is zero, which means good ability of electron injection from metal to semiconductor and low contact resistance. Our study suggests that, by properly selecting the metal materials, giant TER ratio and high performance can be achieved in FTJs constructed with 2D FEMs and metal contacts.

The low-energy (intraband) range of the third harmonic generation of graphene in the terahertz regime is governed by the damping terms induced by the interactions. A controlled many-body description of the scattering processes is thus a compelling and desirable requirement. In this paper, using a Kadanoff-Baym approach, we systematically investigate the impact of many-body interaction on the third harmonic generation of graphene, taking elastic impurity scattering as a benchmark example. We predict the onset in the mixed inter- and intraband regime of incoherent features driven by the interaction at four- and five-photon transition frequencies in the third harmonic optical conductivity with a spectral weight proportional to the scattering rate. We also show that in spite of the complex many-body physics, the purely intraband term governing the limit $\omega \to 0$ resembles the constraints of the phenomenological model. We ascribe this agreement to the fulfilling of the conservation laws enforced by the conserving approach. However, the overlap with incoherent features and the impact of many-body-driven multiphoton vertex couplings severely limit the validity of phenomenological description.

We describe a simple hybrid superconductor–ferromagnetic-insulator structure manifesting spin-resolved Andreev bound states in which dynamic magnetization is employed to probe spin related physics. We show that, at low bias and below $T_c$, the transfer of spin angular momentum pumped by an externally driven ferromagnetic insulator is greatly affected by the formation of spin-resolved Andreev bound states. Our results indicate that these bound states capture the essential physics of condensate-facilitated spin flow. For finite thicknesses of the superconducting layer, comparable to the coherence length, resonant Andreev bound states render highly transmitting subgap spin transport channels. We point out that the resonant enhancement of the subgap transport channels establishes a prototype Fabry-Pérot resonator for spin pumping.

Harmonic generation in atoms and molecules has reshaped our understanding of ultrafast phenomena beyond the traditional nonlinear optics and has launched attosecond physics. Harmonics from solids represent a new frontier, where both majority and minority spin channels contribute to the harmonics. This is true even in a ferromagnet whose electronic states are equally available to optical excitation. Here, we demonstrate that harmonics can be generated mostly from a single spin channel in half-metallic chromium dioxide. An energy gap in the minority channel greatly reduces the harmonic generation, so the harmonics predominantly emit from the majority channel, with a small contribution from the minority channel. However, this is only possible when the incident photon energy is well below the energy gap in the minority channel, so all the transitions in the minority channel are virtual. The onset of the photon energy is determined by the transition energy between the dipole-allowed transition between the $\mathrm{O}\text{−}2p$ and $\mathrm{Cr}\text{−}3d$ states. The harmonics mainly from a single spin channel can be detected, regardless of laser field strength, as long as the photon energy is below the minority band energy gap. This prediction should be tested experimentally.

We propose a method for controlling the exchange interactions of Mott insulators with strong spin-orbit coupling. We consider a multiorbital system with strong spin-orbit coupling and a circularly polarized light field and derive its effective Hamiltonian in the strong-interaction limit. Applying this theory to a minimal model of $\alpha \text{−}{\mathrm{RuCl}}_3$, we show that the magnitudes and signs of three exchange interactions, $J$, $K$, and $\mathrm{\Gamma }$, can be changed simultaneously. Then, considering another case in which one of the hopping integrals has a different value and the other parameters are the same as those for $\alpha \text{−}{\mathrm{RuCl}}_3$, we show that the Heisenberg interaction $J$ can be made much smaller than the anisotropic exchange interactions $K$ and $\mathrm{\Gamma }$.

Magnetic impurities inserted in an $s$-wave superconductor give rise to spin-polarized in-gap states called Shiba states. We study the back-action of these induced states on the dynamics of the classical moments. We show that the Shiba state pertains to both reactive and dissipative torques acting on the precessing classical spin that can be detected through ferromagnetic resonance measurements. Moreover, we highlight the influence of the bulk states as well as the effect of the finite linewidth of the Shiba state on the magnetization dynamics. Finally, we demonstrate that the torques are a direct measure of the even and odd frequency triplet pairings generated by the dynamics of the magnetic impurity. Our approach offers noninvasive alternative to the scanning tunneling microscopy techniques used to probe the Shiba states.

Collinear and noncollinear calculations based on density functional theory are carried out to elucidate the magnetic ordering of Mn trimers on a $\mathrm{GaN}\left(000\overline{1}\right)$ substrate. These trimers had previously been observed in $3a\times 3a$ surface reconstructions through Mn deposition onto the N-polar face of wurtzite $\mathrm{GaN}\left(000\overline{1}\right)$. We start off by studying the effect of spin orbit coupling for the case of monomers and dimers of Mn atoms on top of a GaN surface. Based on an effective spin Hamiltonian, we estimate the magnetic anisotropy energy (MAE) for those cases and found that it is four orders of magnitude weaker than the exchange magnetic coupling between Mn adatoms. In the Mn trimer case, the magnetic ground state has the Mn spins in-plane with the GaN surface in which the relative spin orientation within each trimer is noncollinear due to the competition between the two antiferromagnetic interactions that affect each Mn spin in the trimer, which leads to the found energy minimum with 120 degree angles between the spins. By exploring the nature and fundamental mechanisms for the magnetic interaction among the Mn trimers, we find that the surface states of the substrate play a key role, involving a Ruderman-Kittel-Kasuya-Yosida (RKKY)-type interaction. We report on an electron-mediated long-distance exchange coupling between localized magnetic moments on a $\mathrm{GaN}\left(000\overline{1}\right)$ surface.