The 212 species of structural phase transitions which break macroscopic symmetry are analyzed with respect to the occurrence of time-reversal-invariant vector and bidirector order parameters. The possibility of discerning the orientational domain states of the low-symmetry phase by these “vectorlike” physical properties has been derived using a computer algorithm exploiting the concept of polar, axial, chiral, and neutral dipoles. It is argued that the presence of a pseudo-Lifshitz invariant of chiral bidirector symmetry in a Ginzburg-Landau functional of uniaxial ferroelectrics can induce electric Bloch skyrmions in the same way as the Dzyaloshinskii-Moriya interaction induces bulk magnetic Bloch skyrmions in chiral magnets. It is found that this is possible for three types of fully ferroelectric phase transitions with a chiral paraelectric phase.

We demonstrate the existence of generalized Aubry-André self-duality in a class of non-Hermitian quasiperiodic lattices with complex potentials. From the self-duality relations, the analytical expression of mobility edges is derived. Compared to Hermitian systems, mobility edges in non-Hermitian ones not only separate localized from extended states but also indicate the coexistence of complex and real eigenenergies, making possible a topological characterization of mobility edges. An experimental scheme, based on optical pulse propagation in synthetic photonic mesh lattices, is suggested to implement a non-Hermitian quasicrystal displaying mobility edges.

Using first-principles calculations, we demonstrate that hexagonal monolayers of chromium pnictides $\mathrm{Cr}X (X\text{=P},\mathrm{As},\mathrm{Sb})$ are ferromagnetically ordered systems with the Curie temperature well above 300 K. All three systems are found to be dynamically stable, having no anomalies in the phonon spectrum. Electronic structure calculations show that $\mathrm{Cr}X$ are half-metals in a wide energy range, which make them perfect spin polarizers and candidates for observing anomalous Hall effect. Our predictions are robust with respect to external dielectric screening induced, for example, by substrates or encapsulation.

We analyze the properties of the edge states of the one-dimensional Kitaev model with long-range anisotropic pairing and tunneling. Tunneling and pairing are assumed to decay algebraically with exponents $\alpha$ and $\beta$, respectively, and $\alpha ,\beta >1$. We determine analytically the decay of the edge modes. We show that the decay is exponential for $\alpha =\beta$ and when the coefficients scaling tunneling and pairing terms are equal. Otherwise, the decay is exponential at sufficiently short distances and then algebraic at the asymptotics. We show that the exponent of the algebraic tail is determined by the smallest exponent between $\alpha$ and $\beta$. Our predictions are in agreement with numerical results found by exact diagonalization and reported in the literature.

We study the topological properties of Bose-Mott insulators in one-dimensional non-Hermitian superlattices, which may serve as effective Hamiltonians for cold atomic optical systems with either two-body loss or one-body loss. We find that in the strongly repulsive limit, the Mott insulator states of the Bose-Hubbard model with a finite two-body loss under integer fillings are topological insulators characterized by a finite charge gap, nonzero integer Chern numbers, and nontrivial edge modes in a low-energy excitation spectrum under an open boundary condition. The two-body loss suppressed by the strong repulsion results in a stable topological Bose-Mott insulator which has features similar to the Hermitian case. However, for the non-Hermitian model related to the one-body loss, we find the non-Hermitian topological Mott insulators are unstable with a finite imaginary excitation gap. Finally, we also discuss the stability of the Mott phase in the presence of two-body loss by solving the Lindblad master equation.

Recent experiments on twisted bilayer graphene show the urgent need for establishing a low-energy lattice model for the system. We use the constrained random phase approximation to study the interaction parameters of such models, taking into account screening from the moiré bands left outside the model space. Based on an atomic-scale tight-binding model, we numerically compute the polarization function and study its behavior for different twist angles. We discuss an approximation scheme which allows us to compute the screened interaction, in spite of the very large number of atoms in the unit cell. We find that the polarization has three different momentum regimes. For small momenta, the polarization is quadratic, leading to a linear dielectric function expected for a two-dimensional dielectric material. For large momenta, the polarization becomes independent of the twist angle and approaches that of uncoupled graphene layers. In the intermediate-momentum regime, the dependence on the twist angle is strong. Close to the largest magic angle the dielectric function peaks at a momentum of $1/\left(4\text{nm}\right)$, attaining values of 18–25, depending on the exact model, meaning very strong screening at intermediate distances. We also calculate the effective screened Coulomb interaction in real space and give estimates for the on-site and extended interaction terms for the recently developed hexagonal-lattice model. For freestanding twisted bilayer graphene, the effective interaction decays slower than $1/r$ at intermediate distances $r$, while it remains essentially unscreened at large enough $r$.

Using both an effective three-band model and ab initio calculations, we have investigated various topological features in the cubic ferromagnetic $5d^{1,2}$ systems showing large spin-orbit coupling (SOC): ${\mathrm{Ba}}_2{\mathrm{NaOsO}}_6, {\mathrm{Sr}}_2{\mathrm{SrOsO}}_6$, and ${\mathrm{Ba}}_{2}B{\mathrm{ReO}}_6$ ($B$=Mg, Zn). In the presence of time-reversal symmetry ($\mathcal{T}$), spinless Dirac nodal loops linked to each other at the $W$ points appear in the mirror planes. Remarkably, breaking $\mathcal{T}$ leads to spinful magnetic Weyl nodal loops (MWNLs) that are robust even at large SOC and correlation strength $U$ variation due to the combination of mirror symmetry and broken $\mathcal{T}$. Additionally, there are two types of magnetic Weyl points with chiral charges $\left|\chi \right|=1,2$ along the $C_{4v}$ symmetry line, and another type-II MWNL encircling the zone center, that are dependent on $U$. Furthermore, the ferromagnetic ${\mathrm{Ba}}_2{\mathrm{ZnReO}}_6$ is an ideal half semimetal with MWNLs and magnetic Weyl nodes at the Fermi level without the interference of topologically trivial bulk states. These systems give rise to a remarkably large anomalous Hall conductivity ${\sigma }_{xy}$ of up to 1160 (${\mathrm{\Omega }\mathrm{cm})}^{-1}$. Our findings may apply widely for $t_{2g}$ systems with cubic (or slightly distorted) fcc-like structures.

Using density functional theory and phenomenological Landau-Khalatnikov theory, we investigate the effect of spontaneous polarization switching in ${\mathrm{BaTiO}}_3$ on the oxygen vacancy-induced electron gas at the adjacent ${\mathrm{SrTiO}}_3/EuO$ interface in epitaxial ${\mathrm{BaTiO}}_3/{\mathrm{SrTiO}}_3/\mathrm{EuO}$ heterostructures. The ferroelectric field effect, induced by ${\mathrm{BaTiO}}_3$ (BTO) in oxygen-deficient ${\mathrm{SrTiO}}_3$ (STO) in the presence of a ferromagnetic semiconductor EuO, results in a spin-polarized carrier gas at the interface due to the magnetoelectric proximity effect.

Energy transfer is an important photophysical process that plays a significant role in determining the performance of many optoelectronic and light-harvesting devices. Combining carrier dynamics measurements with Monte Carlo simulations, we study the influence of local microscopic structures on energy transfer efficiencies in quantum-dot films. We find that in thin films, the formation of local domains leads to reduced energy transfer efficiencies, even though macroscopically the energy transfer rate remains intact. Compared to packing density, the vertical interlayer energy transfer has a small impact on the overall energy transfer efficiencies in our structures. In thick three-dimensional films, energy transfer outpaces biexciton recombination, suggesting the possibility to harvest multiexcitons in quantum-dot films for device applications.

The low-temperature spin-orbit density phase of Pb quantum wires on Si(557) was investigated by scanning tunneling microscopy. High resolution images of this phase allow the proposal of an atomistic model for the densely-packed Pb coverages on the (223) facets. A metal-insulator phase transition was confirmed by local spectroscopy and the BCS-like gap feature of $2\mathrm{\Delta }\approx 30\text{meV}$ at 10 K correlates with the transition temperature of $T_c=78\text{K}$ seen in surface transport experiments. Moreover, Pb excess coverage adds an extra row of Pb atoms on the mini-(111) terraces. These atoms induce a metallic state located at the step edge at low temperature and quench the charge and spin correlations of the spin-orbit density phase.

We consider the “transport” of the state of a spin across a Heisenberg-coupled spin chain via the use of repeated SWAP gates, starting with one of two states—one in which the leftmost spin is down and the others up, and one in which the leftmost two spins are in a singlet state (i.e., they are entangled), and the others are again all up. More specifically, we transport the state of the leftmost spin in the first case and the next-to-leftmost spin in the second to the other end of the chain, and then back again. We accomplish our SWAP operations here by increasing the exchange coupling between the two spins that we operate on from a base value $J$ to a larger value $J_{\text{SWAP}}$ for a time $t=\pi \hslash /4J_{\text{SWAP}}$. We determine the fidelity of this sequence of operations in a number of situations—one in which only nearest-neighbor coupling exists between spins and there is no magnetic dipole-dipole coupling or noise (the most ideal case), one in which we introduce next-nearest-neighbor coupling, but none of the other effects, and one in which all of these effects are present. In the last case, the noise is assumed to be quasistatic, i.e., the exchange couplings are each drawn from a Gaussian distribution, truncated to only nonnegative values. We plot the fidelity as a function of $J_{\text{SWAP}}$ to illustrate various effects, namely crosstalk due to coupling to other spins, as well as noise, that are detrimental to our ability to perform a SWAP operation. Our theory should be useful to the ongoing experimental efforts in building semiconductor-based spin quantum computer architectures.

The second-order nonlinear optical (NLO) processes, such as photogalvanic effect and second-order harmonic generation (SHG), play crucial roles in probing and controlling light-matter interactions for energy and device applications. Combining quantum perturbation theory and first-principles simulations, we predict a giant injection-current photogalvanic effect and SHG in a family of emerging axion insulators, the even septuple layers of $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ (MBT) materials. Their amplitudes of photocurrent and SHG are about 1–2 orders of magnitude larger than those of proper ferroelectrics and antiferromagnetic $\mathrm{Cr}{\mathrm{I}}_3$. Moreover, unlike the widely studied injection current observed under circularly polarized light, the injection photocurrent of MBT only emerges under linearly polarized light, making it suitable for device applications. These unique characters are from a combination effect of parity-time symmetry and significant spin-orbit coupling. Our predicted enhanced NLOs are valuable for characterizing subtle magnetic orders and shed light on infrared photodetecting and photovoltaic applications based on magnetic topological materials.

The clear evidence for the nuclear spin conversion of matrix-isolated ${\mathrm{H}}_2$ is presented. We trapped ${\mathrm{H}}_2$ in a ${\mathrm{CO}}_2$ film at 5.4 K and detected the induced infrared-absorption band including both the components of ortho and para ${\mathrm{H}}_2$. By analyzing its time evolution, the conversion rate was determined to be $9.4\times {10}^{-4}{\mathrm{s}}^{-1}$. We confirmed that this fast conversion was induced just by the ${\mathrm{CO}}_2$ film with the uniquely unstable structure. We also identified ${\mathrm{H}}_2$ at another kind of trapping sites, accompanied by higher anisotropy of the confining potential, and estimated its conversion rate to be $6\times {10}^{-3}{\mathrm{s}}^{-1}$.

We study the neutral excitations of fractional quantum Hall states in electronic systems of finite width where an external anisotropy is introduced by tilting the magnetic field. As in the isotropic case, the neutral collective excitation can be worked out through the conserving method of composite fermions in the Hamiltonian theory because the interaction potential has a natural cutoff due to the quantum well width. We show how such a computation can be carried out perturbatively for an anisotropic interaction. We find that unlike the charge gap, the neutral collective gap is much more sensitive to the tilt and can thus, for certain fractional quantum Hall states, be easily destroyed by the parallel component of the magnetic field. We also discuss the convergence of the collective spectrum to the activation gap in the large-momentum limit.

Predicting the compositional phase stability of strongly correlated electron materials is an outstanding challenge in condensed matter physics, requiring precise computations of total energies. In this work, we employ the density functional theory plus dynamical mean-field theory ($\mathrm{DFT}\text{+}\mathrm{DMFT}$) formalism to address local correlations due to transition metal $d$ electrons on compositional phase stability in the prototype rechargeable battery cathode material ${\mathrm{Li}}_x{\mathrm{CoO}}_2$, and detailed comparisons are made with the simpler $\mathrm{DFT}\text{+}U$ approach (i.e., the Hartree-Fock solution of the DMFT impurity problem). Local interactions are found to strongly impact the energetics of the band insulator ${\mathrm{LiCoO}}_2$, most significantly via the $E_g$ orbitals, which are partially occupied via hybridization with O $p$ states. We find ${\mathrm{CoO}}_2$ and ${\mathrm{Li}}_{1/2}{\mathrm{CoO}}_2$ to be moderately correlated Fermi liquids with quasiparticle weights of 0.6–0.8 for the $T_{2g}$ states, which are most impacted by the interactions. As compared to $\mathrm{DFT}\text{+}U, \mathrm{DFT}\text{+}\mathrm{DMFT}$ considerably dampens the increase in total energy as $U$ is increased, which indicates that dynamical correlations are important to describe this class of materials despite the relatively modest quasiparticle weights. Unlike $\mathrm{DFT}\text{+}U$, which can incorrectly drive ${\mathrm{Li}}_x{\mathrm{CoO}}_2$ toward spurious phase separating or charge-ordered states, $\mathrm{DFT}\text{+}\mathrm{DMFT}$ correctly captures the system's phase stability and does not exhibit a strong charge-ordering tendency. Most importantly, the error within $\mathrm{DFT}\text{+}U$ varies strongly as the composition changes, challenging the common practice of artificially tuning $U$ within $\mathrm{DFT}\text{+}U$ to compensate the errors of Hartree-Fock. $\mathrm{DFT}\text{+}\mathrm{DMFT}$ predicts the average intercalation voltage decreases relative to DFT, opposite to the result of $\mathrm{DFT}\text{+}U$, which would yield favorable agreement with experiment in conjunction with the overprediction of the voltage by the strongly constrained and appropriately normed DFT functional.

Non-Fermi liquids in $d=2$ spatial dimensions can arise from coupling a Fermi surface to a gapless boson. At finite temperature, however, the perturbative quantum field theory description breaks down due to infrared divergences. These are caused by virtual static bosonic modes and affect both fermionic and bosonic correlators. We show how these divergences are resolved by self-consistent boson and fermion self-energies that resum an infinite class of diagrams and correct the standard Eliashberg equations. Extending a previous approach in $d=3-\epsilon$ dimensions, we find a new “thermal non-Fermi liquid” regime that violates the scaling laws of the zero-temperature fixed point and dominates over a wide range of scales. We conclude that basic properties of quantum phase transitions and quantum-classical crossovers at finite temperature are modified in crucial ways in systems with soft bosonic fluctuations, and we begin a study of some of the phenomenological consequences.

We reveal that the interplay between Rashba spin-orbit coupling and proximity-induced magnetization in a two-dimensional electron gas leads to peculiar transport properties and large anisotropy of magnetoresistance. While the related tunneling anisotropic magnetoresistance (TAMR) has been extensively studied before, we predict an effect with a different origin arising from the evolution of a resonant condition with the in-plane rotation of magnetization and having a much larger magnitude. The resonances in the tunneling emerge from a spin-parity-time symmetry of the scattering states. However, such a symmetry is generally absent from the system itself and only appears for certain parameter values. Without resonant behavior in the topological surface states of a proximitized three-dimensional topological insulator (TI), TAMR measurements can readily distinguish them from often misinterpreted trivial Rashba-like states inherent to many TIs.

Following previous work on graphene on Ni(111) and Co(0001) surfaces [Phys. Rev. B 97, 195405 (2018)], the spin polarization of graphene on a ${\mathrm{Co}}_2{\mathrm{FeGe}}_{0.5}{\mathrm{Ga}}_{0.5}$ (CFGG) (001) surface was examined using spin-polarized surface positronium spectroscopy. The graphene was found to be magnetically insulated from the CFGG(001) even after heat treatment, which increased the spin polarizations of graphene on Ni(111) and Co(0001). First-principles calculations indicated that graphene is weakly bound to the CFGG(001) surface mainly through the van der Waals interaction, and consequently the interlayer distance between graphene and the CFGG(001) surface is greater ($\sim 3$ Å) than that between graphene and Ni(111) or Co(0001) ($\sim 2$ Å). The weak hybridization between graphene and CFGG(001) sufficiently reduces the spin polarization in graphene, which enables the magnetic insulation of graphene from CFGG.

We use low-depth quantum circuits, a specific type of tensor networks, to classify two-dimensional symmetry-protected topological many-body localized phases. For (anti)unitary onsite symmetries we show that the (generalized) third cohomology class of the symmetry group is a topological invariant; however, our approach leaves room for the existence of additional topological indices. We argue that our classification applies to quasiperiodic systems in two dimensions and systems with true random disorder within times which scale superexponentially with the inverse interaction strength. Our technique might be adapted to supply arguments suggesting the same classification for two-dimensional symmetry-protected topological ground states with a rigorous proof.