We present a quantitative theory of the gate-voltage tuned superconductor-to-insulator transition (SIT) observed experimentally in the 2D electron system created in the (111) interface between crystalline ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ and ${\mathrm{La}\mathrm{Al}\mathrm{O}}_3$. Considering two fundamental opposing effects of Cooper-pair fluctuations—the critical conductivity enhancement, known as paraconductivity, and its suppression associated with the loss of unpaired electrons due to Cooper-pairs formation—we generalize the standard thermal fluctuations theory to include interaction between fluctuations within the self-consistent field approximation and quantum tunneling between mesoscopic superconducting puddles within a phenomenological approach. Relying on the quantitative agreement found between our theory and a large body of experimental sheet-resistance data, we conclude that spin-orbit scatterings, via significant enhancement of the interaction between fluctuations, strongly enhance the sheet resistance peak at high fields and reveal anomalous metallic behavior at low fields, due to mixing of relatively heavy electron bands with a light electron band near a Lifshitz point. The large enhancement of the interaction between fluctuations at high fields, where the sheet resistance is strongly amplified, is shown to result in localization of Cooper-pair fluctuations within mesoscopic puddles.

Transition metal (TM) defects in silicon carbide (SiC) are a promising platform in quantum technology, especially because some TM defects emit in one of the telecom bands. We develop a theory for the interaction of an active electron in the $D$ shell of a TM defect in SiC with the TM nuclear spin and derive the effective hyperfine tensor within the Kramers doublets formed by the spin-orbit coupling. Based on our theory we discuss the possibility to exchange the nuclear and electron states with potential applications for nuclear spin manipulation and long-lived nuclear-spin based-quantum memories.

We have studied rectified current in a geometrically symmetric nanoelectromechanical shuttle with periodic kicks and sinusoidal ac bias voltages. The rectified current is exactly zero under the geometrical symmetry generated by electrons transferring from source to drain electrodes via the movable shuttle. We investigate nonzero rectified currents through the regular motion of the shuttle in which the time-translational symmetry is broken. The motion of the shuttle, moreover, becomes chaotic with the same mechanism as a kicked rotor and generates a scattered current with increasing kick strength. We point out that the time-translational symmetry breaking of the instantaneous current has an important role in the manipulation of the rectified current.

We present a formula for thermal transport in the bulk of Bose systems based on the quantum Boltzmann equation (QBE). First, starting from the quantum kinetic equation and using the Born approximation for impurity scattering, we derive the QBE of Bose systems and provide a formula for thermal transport subjected to a temperature gradient. Next, we apply the formula to magnons. Assuming a relaxation time approximation and focusing on the linear response regime, we show that the longitudinal thermal conductivity of the QBE exhibits the different behavior from the conventional Boltzmann equation. The thermal conductivity of the QBE reduces to the Drude type in the limit of the quasiparticle approximation, while not in the absence of the approximation. Finally, applying the quasiparticle approximation to the QBE, we find that the correction to the conventional Boltzmann equation is integrated as the self-energy into the spectral function of the QBE, and this enhances the thermal conductivity. Thus, we shed light on the thermal transport property of the QBE beyond the conventional.

Exploring materials with topological magnetic structures has always been the subject of intense research due to fundamental and technological interests. Here we report the topological Hall effect in the frustrated B2-ordered ${\mathrm{Mn}}_{0.74}{\mathrm{Co}}_{0.57}{\mathrm{Al}}_{0.69}$ thin films. A large topological Hall resistivity of $58.3\mathrm{n}\mathrm{\Omega }$ cm is found at 300 K. It is shown that a spin reorientation transition occurs around 200 K through the temperature-dependent magnetization and angular-dependent ferromagnetic resonance measurements. Using the obtained magnetic anisotropy parameters, we perform the micromagnetic simulations to simulate the formation and evolution of topological magnetic structures with temperature, demonstrating the influence of the higher-order magnetic anisotropies. This work enriches the variety of materials exhibiting topological Hall effect and helps to clarify the mechanism of new emergent topological features in magnetic materials.

Two-dimensional (2D) valleytronic materials, exhibiting valley-contrasting physics, are both fundamentally intriguing and practically appealing to be used in nanoscale devices. In this work, through first-principles calculations and model analysis, we identify single-layer ${\mathrm{Cr}}_2{\mathrm{Se}}_3$ as a promising 2D valleytronic semiconductor. We reveal that single-layer ${\mathrm{Cr}}_2{\mathrm{Se}}_3$ is a ferromagnetic semiconductor and harbors valley features. In contrast to most previously reported similar systems suffering from a natural in-plane magnetization, it favors the robust out-of-plane magnetization. This, combined with the broken inversion symmetry and strong spin-orbit coupling strength, renders that a sizable valley polarization occurs spontaneously in single-layer ${\mathrm{Cr}}_2{\mathrm{Se}}_3$, thus facilitating the observation of intriguing anomalous valley Hall effect. More remarkably, by employing strain and ferroelectric substrate, the on-off switching of the valley polarization is realized in single-layer ${\mathrm{Cr}}_2{\mathrm{Se}}_3$. The underlying physics is discussed in detail. Our work provides a tantalizing candidate for realizing and manipulating the valley and spin physics.

We analyze a zero-dimensional correlated system with special emphasis on the non-Hermitian point-gap topology protected by chiral symmetry. Our analysis elucidates that correlations destroy an exceptional point on a topological transition point, which separates two topological phases in the noninteracting case; one of them is characterized by the zeroth Chern number $N_{0\mathrm{Ch}}=0$, and the other is characterized by $N_{0\mathrm{Ch}}=2$. This fact implies that correlations allow to continuously connect the two distinct topological phases in the noninteracting case without closing the point gap, which is analogous to the reduction of topological classifications by correlations in Hermitian systems. Furthermore, we also discover a Mott exceptional point, an exceptional point where only spin degrees of freedom are involved.

We investigate the properties of heavily C-doped GaN grown by hydride vapor phase epitaxy using both optical experiments and hybrid density functional theory calculations. Previous work has established that carbon acceptors $\left({\mathrm{C}}_{\mathrm{N}}\right)$ give rise to a yellow luminescence band near 2.2 eV along with a blue luminescence band near 2.9 eV. Photoluminescence measurements show the yellow band shifting as a function of carbon concentration, suggesting a change in the behavior of carbon species as carbon content increases. With hybrid density functional theory we calculate the electrical and optical behavior of carbon centers containing multiple carbon impurities, which may arise in heavily doped material. We compare the behavior of these complexes to the isolated centers, and find that the dicarbon donor-acceptor $\left({\mathrm{C}}_{\mathrm{Ga}}\text{−}{\mathrm{C}}_{\mathrm{N}}\right)$ complex is a candidate to explain the shift in the yellow luminescence peak. Tricarbon complexes have high formation energies and modest binding energies, and also give rise to optical transitions that are inconsistent with the observed spectra. We also identify the split dicarbon interstitial on the gallium site as a low-energy species with a large binding energy that may act to compensate carbon acceptors. Local vibrational modes are calculated for carbon impurity centers, and we compare these results to recent experiments. Dicarbon and tricarbon complexes involving ${\mathrm{C}}_{\mathrm{Ga}}$ and ${\mathrm{C}}_{\mathrm{N}}$ exhibit modes that are only slightly higher than the isolated species, while carbon interstitials and related complexes give rise to vibrational modes much higher than ${\mathrm{C}}_{\mathrm{Ga}}$ and ${\mathrm{C}}_{\mathrm{N}}$.

We present a comprehensive study on heralded spin preparation employing excited state resonances of droplet-etched GaAs quantum dots. This achievement will facilitate future investigations of spin qubit based quantum memories using the GaAs quantum dot material platform. By observation of excitation spectra for a range of fundamental excitonic transitions, the properties of different quantum dot energy levels, i.e., shells, are revealed. The innovative use of polarization-resolved excitation and detection in the context of quasiresonant excitation spectroscopy of quantum dots greatly simplifies the determination of the spin preparation fidelities—irrespective of the relative orientations of laboratory and quantum dot polarization eigenbases. By employing this method, spin preparation fidelities of quantum dot ground states of up to 85% are found. Additionally, the characteristic nonradiative decay time is investigated as a function of ground state, excitation resonance, and excitation power level, yielding decay times as low as 29 ps for $s-p$ shell exited state transitions. Finally, by time-resolved correlation spectroscopy it is demonstrated that the employed excitation scheme has a significant impact on the electronic environment of quantum dot transitions and their apparent brightness.

We present the numerical, theoretical, and experimental study of a terahertz metasurface supporting a pseudo-anapole. Pseudo-anapole effect arises when electric and toroidal dipole moments both tend to a minimum, instead of destructive interference between electric and toroidal dipole moments in conventional anapole mode. Such overlap allows resonance suppression of electric type radiation. Thus it becomes possible to study the multipoles of other families and higher order excitations. We estimate multipole contribution to the metasurface response via the multipole expansion method. The series is extended with such terms as mean-square radii and multipole interference. We also study the metasurface geometrical tunability. Via scaling, we demonstrate that it is possible to control the metasurface toroidal and electric responses independently. This in turn proves the fact that these multipoles have different physical origin. Moreover, we demonstrate that the proposed metasurface allows excitation of coherent magnetic dipole and electric quadrupole modes, which is crucial for planar cavities and lasing spasers in nanophotonics.

We theoretically investigate the quantum reflection of different atoms by two-dimensional (2D) materials of the graphene family (silicene, germanene, and stanene), subjected to an external electric field and circularly polarized light. By using Lifshitz theory to compute the Casimir-Polder potential, which ensures that our predictions apply to all regimes of atom-2D surface distances, we demonstrate that the quantum reflection probability exhibits distinctive, unambiguous signatures of topological phase transitions that occur in 2D materials. We also show that the quantum reflection probability can be highly tunable by these external agents, depending on the atom-surface combination, reaching a variation of 40% for rubidium in the presence of a stanene sheet. Our findings attest not only that dispersive forces play a crucial role in quantum reflection, but also that the topological phase transitions of the graphene family materials can be comprehensively and efficiently probed via atom-surface interactions at the nanoscale.

Antiferromagnetism in a prototypical quantum critical metal ${\mathrm{CeCoIn}}_5$ is known to be induced by slight substitutions of nonmagnetic Zn atoms for In. In nominally 7% Zn-substituted ${\mathrm{CeCoIn}}_5$, an antiferromagnetic (AFM) state coexists with heavy fermion superconductivity. Heterogeneity of the electronic states is investigated in Zn-doped ${\mathrm{CeCoIn}}_5$ by means of nuclear quadrupole and magnetic resonances (NQR and NMR). Site-dependent NQR relaxation rates $1/T_1$ indicate that the AFM state is locally nucleated around Zn substituents in the matrix of a heavy fermion state, and percolates through the bulk at the AFM transition temperature $T_{\mathrm{N}}$. At lower temperatures, an anisotropic superconducting (SC) gap below the SC transition temperature $T_{\mathrm{c}}$, and the SC state permeates through the AFM regions via a SC proximity effect. Applying an external magnetic field induces a spin-flop transition near 5 T, reducing the volume of the AFM regions. Consequently, a short-ranged inhomogeneous AFM state survives and coexists with a paramagnetic Fermi liquid state at high fields.

We investigate an interaction-driven transition between crystalline and liquid states of strongly correlated spinless fermions within topological flat bands at low density (with filling factors $\nu =\frac{1}{5}, \frac{1}{7}, \frac{1}{9}$). Using exact diagonalization for finite-size systems with periodic boundary conditions, we distinguish different phases, whose stability depends on the interaction range, controlled by the screening parameter of the Coulomb interaction. The crystalline phases are identified by a crystallization strength, calculated from the Fourier transforms of pair correlation density, while the fractional Chern insulator (FCI) phases are characterized using momentum counting rules, entanglement spectrum, and overlaps with corresponding fractional quantum Hall states. The type of the phase depends on a particular single-particle model and its topological properties. We show that for $\nu =\frac{1}{7}$ and $\frac{1}{5}$ it is possible to tune between the Wigner crystal and fractional Chern insulator phase in the kagome lattice model with the band carrying the Chern number $C=1$. In contrast, in the $C=2$ models, the Wigner crystallization was absent at $\nu =\frac{1}{5}$, and appeared at $\nu =\frac{1}{9}$, suggesting that $C=2$ FCIs are more stable against the formation of crystalline order.

We present gate voltage and temperature dependent transport measurements of InAs/GaSb/InAs triple quantum wells (TQWs) with a designed hybridization gap energy of 4 meV comparable to its traditional double quantum well counterpart. Gate voltage dependent measurements enable us to monitor two electron densities deep in the nonhybridized electron regime and further reveal a clear hybridization gap and a Van Hove singularity in the valence band as a result of the hybridized electron-hole band structure of the TQWs. The evolution of the charge carrier densities and types is studied in detail. Electron and hole densities coexist if the Fermi energy is within the gap and the bottom of the valence band at the Γ point. On the contrary, only single carrier types can be found far in the conduction and valence band. Thus, we are able to identify the topological band structure of these TQWs. Furthermore, the temperature evolution of the hybridized gap of the triple quantum well is studied. We find a rather temperature insensitive hybridization gap energy.

Magnetic-field-driven domain wall dynamics in a ferrimagnetic GdFeCo thin film with perpendicular magnetic anisotropy is studied using low-temperature magneto-optical Kerr microscopy. Measurements performed in a practically athermal condition allow for the direct experimental determination of the velocity ($\beta =0.30\pm 0.03$) and correlation length ($\nu =1.3\pm 0.3$) exponents of the depinning transition. The whole family of exponents characterizing the transition is deduced, providing evidence that the depinning of magnetic domain walls is better described by the quenched Edwards-Wilkinson universality class.

Geometric frustrated kagome systems can show complex and exotic magnetic properties. We theoretically predict ways in which these can be manipulated in two-dimensional (2D) multiferroic materials from first-principles density functional theory calculations. We propose that ${\mathrm{Ti}}_3{X}_8$ ($X=\mathrm{Br}$ or I) compounds are shown to form 2D intrinsic semiconductors with breathing kagome lattices containing coexisting ferroelectric (FE) and ferromagnetic ordering. Inside the lattice, Ti atoms distort from high-symmetry locations to produce trimers with shorter interatomic distances that form the basis of local cluster magnets. Lattice breathing interchanges trimer patterns, switching the direction of out-of-plane FE polarization while simultaneously rearranging the interactions between the cluster magnets. FE switching of the monolayer ${\mathrm{Ti}}_3{X}_8$, which is concomitant with the direction reversal of the vector of the Dzyaloshinskii-Moriya interaction, is feasible to be manipulated by the application of out-of-plane electric fields. Through the interlayer interaction, the coupling of FE and magnetism is achieved in bilayer ${\mathrm{Ti}}_3{\mathrm{I}}_8$. The magnetic configurations are transformed between interlayer ferromagnetism and antiferromagnetism by switching the FE polarization directions of bilayer ${\mathrm{Ti}}_3{\mathrm{I}}_8$. Our findings expand the arena for realizing 2D multiferroics and magnetoelectric effect.

We present benchmark calculations of the Anderson lattice model based on the recently developed “ghost Gutzwiller approximation”. Our analysis shows that, in some parameters regimes, the predictions of the standard Gutzwiller approximation can be incorrect by orders of magnitude for this model. We show that this is caused by the inability of this method to describe simultaneously the Mott physics and the hybridization between correlated and itinerant degrees of freedom—whose interplay often governs the metal-insulator transition in real materials. Finally, we show that the ghost Gutzwiller approximation solves this problem, providing us with results in remarkable agreement with dynamical mean-field theory throughout the entire phase diagram, while being much less computationally demanding. We provide an analytical explanation of these findings and discuss their implications within the context of ab-initio computation of strongly correlated matter.

Flat bands have band crossing points with other dispersive bands in many systems including the canonical flat-band models in the Lieb and kagome lattices. Here we show that some of such band degeneracy points of nondegenerate flat bands are unavoidable because of the symmetry representation (SR) of flat bands under unitary symmetry. We refer to such a band degeneracy point of flat bands as a SR-enforced band crossing. SR-enforced band crossing is distinct from the conventional band degeneracy protected by symmetry eigenvalues or topological charges in that its protection requires both specific symmetry representation and band flatness of the flat band, simultaneously. Even $n$-fold rotation $C_n$ ($n=2,3,4,6$) symmetry, which cannot protect band degeneracy without additional symmetries due to its abelian nature, can protect SR-enforced band crossings in flat-band systems. In two-dimensional flat-band systems with $C_n$ symmetry, when the degeneracy of a SR-enforced band crossing is lifted by a $C_n$ symmetry-preserving perturbation, we obtain a nearly flat Chern band. Our theory not only explains the origin of the band crossing points of FBs existing in various models, but also gives a strict no-go theorem for isolated FBs in a given lattice arising from the SR.

Conventional nanoscale logic gates involve several critical challenges, such as the appearance of leakage currents, and so new approaches to logic calculation devices have been exploited. However, although polarization switching in ferroelectrics has certain advantages in this regard, including nonvolatility, these materials are not leading candidates for logic gates because they disappear at the nanoscale and do not typically allow the two-inputs–one-output operation which is necessary for logic gates. Here, we demonstrate the possibility that ultrasmall ferroelectric nanoscale logic gates can be generated from mixed dislocations in ${\mathrm{SrTiO}}_3$. Phase-field simulations show that the unique strain field associated with a mixed dislocation induces a few nanometer polarization spiral, the chirality of which can be switched by electric fields that are both horizontal and vertical to the spiral. Due to this polarization structure, or, and, and not operations can be performed depending on the strength of the electric fields. These results suggest a different means of fabricating ultrathin logic gates, and could potentially lead to energy-efficient ultrahigh logic density devices.

The observed tendency in tetrahedral solids for the pressure-induced softening of zone-edge TA phonons to correlate with the threshold pressure $P_T$ of the fourfold to sixfold phase change is examined for 25 materials. A strong correlation is found for an effective pressure shift obtained by using a simple tight-binding model to exclude the frequency-stiffening non-central-force contribution to the zone-edge TA modes. The correlation is further improved by accounting for the material's shear rigidity. A parameter $P_T^{\mathrm{correl}}$ is defined that correlates linearly to $P_T$ with near-unity slope and high quality factor up to the transition of BP at ∼1.6 Mbar. The pressure-softening of the zone-edge TA modes is driven by the negative tension produced in the bonds by compression. This tension originates from increased electron-overlap repulsion, which modifies the metallic and ionic character of the bonding, leading eventually to the phase change. This is the underlying mechanism that links the pressure-softening of the zone-edge TA phonons to the phase transition.