We study the classification for a large class of interacting fermionic and bosonic symmetry-protected topological (SPT) states, focusing on the cases where interaction reduces the classification of free-fermion SPT states. We define a SPT state as whether it is separated from the trivial state through a bulk phase transition, which is a general definition applicable to SPT states with or without spatial symmetries. We show that in all dimensions short-range interactions can reduce the classification of free-fermion SPT states, and we demonstrate these results by making a connection between the fermionic and bosonic SPT states. We first demonstrate that our formalism gives the correct classification for several known SPT states, with or without interaction, and then we generalize our method to SPT states that involve the spatial inversion symmetry.

We study the zero-temperature quantum phase transition between liquid and hcp solid ${}^4\mathrm{He}$. We use the variational method with a simple yet exchange-symmetric and fully explicit wave function. It is found that the optimized wave function undergoes spontaneous symmetry breaking and describes the quantum solidification of helium at 22 atm. The explicit form of the wave function allows us to consider various contributions to the phase transition. We find that the employed wave function is an excellent candidate for describing both a first-order quantum phase transition and the ground state of a Bose solid.

We report a detailed investigation on the superconductivity in eight 1–4 unit-cell (UC) FeSe films on $\mathrm{SrTi}{\mathrm{O}}_3$ substrates by measuring their magnetization and sheet resistance in a field between $5\times {10}^{-2}$ and $7\times {10}^4\mathrm{Oe}$ over the last one and a half years as a function of temperature and frequency, from 2 to 300 K and from 0 to 1.5 kHz, respectively. The results show that samples of 1–4-UC FeSe films all display a complex superconducting structure, i.e., a Meissner state but populated with weak links below 20 K, and an unusual superconducting mesostructure up to 45 K, strongly suggesting that superconductivity may be of an interfacial nature. A model is proposed to account for the magnetizations in such a superconducting mesoscopic structure, similar to the Andreev reflection between the normal and superconducting carriers. Above 45 K, collective glasslike excitations are evident, although their nature is yet to be determined. The complex superconducting structure observed is consistent with the challenges in synthesizing ultrathin FeSe films with a superconducting temperature much higher than that of a bulk FeSe.

We introduce a disordered system, the super-Potts model, which is a more frustrated version of the Potts glass. Its elementary degrees of freedom are variables that can take $M$ values and are coupled via pairwise interactions. Its exact solution on a completely connected lattice demonstrates that, for large enough $M$, it belongs to the class of mean-field systems solved by a one-step replica symmetry breaking ansatz. Numerical simulations by the parallel tempering technique show that in three dimensions it displays a phenomenological behavior similar to the one of glass-forming liquids. The super-Potts glass is therefore a disordered model allowing one to perform extensive and detailed studies of the random first-order transition in finite dimensions. We also discuss its behavior for small values of $M$, which is similar to the one of spin glasses in a field.

We present an algorithm for Monte Carlo simulations of a nearest-neighbor spin-ice model based on its cluster representation. To assess its performance, we estimate a relaxation time, and find that, in contrast to the Metropolis algorithm, our algorithm does not develop spin freezing. Also, to demonstrate the efficiency, we calculate the spin and charge structure factors, and observe pinch points in a high-resolution color map. We then find that Debye screening works among defects and brings about short-range correlations, and that the deconfinement transition triggered by a fugacity of defects $z$ is dictated by a singular part of the free-energy density $f_{\mathrm{s}}\propto z$.

Layered $5d$ transition metal dichalcogenide (TMD) $\mathrm{IrT}{\mathrm{e}}_2$ is distinguished from the traditional TMDs (such as $\mathrm{NbS}{\mathrm{e}}_2)$ by the existence of multiple charge-density wave (CDW)-like stripe phases and superconductivity at low temperatures. Despite intensive studies, there is still no consensus on the physical origin of the stripe phases or even the ground state modulation for this $5d$ material. Here we present atomic-scale evidence from scanning tunneling microscopy and spectroscopy (STM/STS) that the ground state of $\mathrm{IrT}{\mathrm{e}}_2$ is a $q=1/6$ stripe phase, identical to that of the Se-doped compound. Furthermore, our data suggest that the multiple transitions and stripe phases are driven by the intralayer Ir-Ir dimerization that competes against the interlayer Te-Te bonding. The competition results in a unified phase diagram with a series of hierarchical modulated stripe phases, strikingly similar to the renowned “devil's staircase” phenomena.

The configuration interaction (CI) method can be regarded as a generalization of the Hartree-Fock theory. The CI method includes multiple determinantal states to approximate the full many-body ground state, as opposed to just one in Hartree-Fock theory. In this work, we introduce an efficient and stable algorithm to obtain the optimal single-particle basis by formulating orbital searching in terms of a multivariable minimization problem. The algorithm is iterative and is based on imaginary-time dynamics. It is numerically efficient as the Hessian (second derivative of the energy with respect to the orbital variables) is not required; it is also stable as a reasonable initial orbital choice is not necessary. The algorithm can deal with Hamiltonians with complex coefficients. We demonstrate the power of the proposed algorithm by applying it to impurity problems, with the most complicated one including multiple correlated orbitals and spin-orbit coupling. Energies and spectral functions are computed to demonstrate the convergence of the CI method.

We evaluate the Landé $g$ factor of electrons in quantum dots (QDs) fabricated from GaAs quantum well (QW) structures of different well width. We first determine the Landé electron $g$ factor of the QWs through resistive detection of electron spin resonance and compare it to the enhanced electron $g$ factor determined from analysis of the magnetotransport. Next, we form laterally defined quantum dots using these quantum wells and extract the electron $g$ factor from analysis of the cotunneling and Kondo effect within the quantum dots. We conclude that the Landé electron $g$ factor of the quantum dot is primarily governed by the electron $g$ factor of the quantum well suggesting that well width is an ideal design parameter for $g$-factor engineering QDs.

We study the electromagnetic properties of a metamaterial consisting of polarizable (nano)particles and a single graphene sheet placed at the interface between two dielectrics. We show that the particle's polarizability is renormalized because of the electromagnetic coupling to surface plasmons supported by graphene, which results in a dispersive behavior, different for the polarizability components corresponding to the induced dipole moment, parallel and perpendicular to the graphene sheet. In particular, this effect is predicted to take place for a metallic particle whose bare polarizability in the terahertz (THz) region is practically equal to the cube of its radius (times $4\pi {\varepsilon }_0$). This opens the possibility to excite surface plasmons in graphene and enhance its absorption in the THz range by simply using a monolayer of metallic particles randomly deposited on top of it, as we show by explicit calculations.

We numerically investigate the encoding and retrieval processes for quantum memory realized in a semiconductor quantum dot by focusing on the effect of inhomogeneously polarized nuclear spins whose polarization depends on the local hyperfine coupling strength. We find that the performance of quantum memory is significantly improved by inhomogeneous nuclear polarization, as compared with homogeneous nuclear polarization. Moreover, the narrower the nuclear polarization distribution is, the better is the performance of the quantum memory. We ascribe the improvement in performance to the full harnessing of the highly polarized and strongly coupled nuclear spins by carefully studying the entropy change of individual nuclear spins during the encoding process. Our results shed light on the implementation of quantum memory in a quantum dot.

We investigate local control of the excitation of surface plasmon polaritons by a magnetic scatterer placed in the vicinity of a metallic surface. We show that under those conditions a change of about 27% in the surface plasmon intensity can be achieved by flipping the external magnetic field. The magnitude of this phenomenon is given by the magneto-optical Kerr effect (MOKE). A qualitative analysis of our numerical results, based on a perturbative approach, lead to simple analytical expressions for the longitudinal MOKE when the scatter is in close proximity to the surface. These results provide physical insight into the problem and may lead to the design of useful devices.

We present a quantitative microscopic theory of the disorder- and phonon-induced coupling between surface and bulk states in doped topological insulator films. We find a simple mathematical structure for the surface-to-bulk scattering matrix elements and confirm the importance of bulk-surface coupling in transport and photoemission experiments, assessing its dependence on temperature, carrier density, film thickness, and particle-hole asymmetry.

We theoretically investigate the efficiency of an entanglement swapping procedure based on the use of quantum dots as sources of entangled photon pairs. The four-photon interference that affects such efficiency is potentially limited by the fine-structure splitting and by the time correlation between cascaded photons, which provide which-path information. The effect of spectral inhomogeneity is also considered, and a possible quantum eraser experiment is discussed for the case of identical dots.

Using ab initio methods based on the density functional theory, we study the magnetic properties of different point defects in graphene. We consider separately, atomic hydrogen, atomic fluorine, and single vacancies. The three defects have completely different magnetic properties. A local spin one-half magnetic moment is well defined at a hydrogen impurity, while single fluorine adatoms do not induce a well-defined magnetic moment unless there is a fluorine concentration of at least of a 0.5%. In this case, the induced magnetic moment is of the order of 0.45 ${\mu }_B$ per defect. This behavior is interpreted as being due to the charge transfer between fluorine and graphene. The case of magnetic moments localized at $\pi$ electrons near vacancies is different from both previous cases; the size of the induced magnetic moment decreases with the dilution of defects and it is compatible with zero in the isolated vacancy. The effect of hole doping on these magnetic behaviors is studied and compared with the available experimental data. In the three cases, hole doping inhibits the formation of $\pi$ states magnetic moments.

Understanding the influence of dipolar interactions in magnetic hyperthermia experiments is of crucial importance for fine optimization of nanoparticle (NP) heating power. In this study we use a kinetic Monte Carlo algorithm to calculate hysteresis loops that correctly account for both time and temperature. This algorithm is shown to correctly reproduce the high-frequency hysteresis loop of both superparamagnetic and ferromagnetic NPs without any ad hoc or artificial parameters. The algorithm is easily parallelizable with a good speed-up behavior, which considerably decreases the calculation time on several processors and enables the study of assemblies of several thousands of NPs. The specific absorption rate (SAR) of magnetic NPs dispersed inside spherical lysosomes is studied as a function of several key parameters: volume concentration, applied magnetic field, lysosome size, NP diameter, and anisotropy. The influence of these parameters is illustrated and comprehensively explained. In summary, magnetic interactions increase the coercive field, saturation field, and hysteresis area of major loops. However, for small amplitude magnetic fields such as those used in magnetic hyperthermia, the heating power as a function of concentration can increase, decrease, or display a bell shape, depending on the relationship between the applied magnetic field and the coercive/saturation fields of the NPs. The hysteresis area is found to be well correlated with the parallel or antiparallel nature of the dipolar field acting on each particle. The heating power of a given NP is strongly influenced by a local concentration involving approximately 20 neighbors. Because this local concentration strongly decreases upon approaching the surface, the heating power increases or decreases in the vicinity of the lysosome membrane. The amplitude of variation reaches more than one order of magnitude in certain conditions. This transition occurs on a thickness corresponding to approximately 1.3 times the mean distance between two neighbors. The amplitude and sign of this variation is explained. Finally, implications of these various findings are discussed in the framework of magnetic hyperthermia optimization. It is concluded that feedback on two specific points from biology experiments is required for further advancement of the optimization of magnetic NPs for magnetic hyperthermia. The present simulations will be an advantageous tool to optimize magnetic NPs heating power and interpret experimental results.

We report the NMR Knight shift and spin-lattice relaxation of ${}^8\mathrm{Li}{}^{+}$ implanted $\sim 100$ nm into single crystals of semimetallic Sb, Bi, and topologically insulating ${\mathrm{Bi}}_{0.9}{\mathrm{Sb}}_{0.1}$. We find small negative shifts (of order 100 ppm) in all three. In the insulator, the shift is nearly temperature independent, while in Bi and Sb it becomes more negative at low temperature without following the bulk susceptibility, suggesting two distinct temperature dependent contributions, possibly from the orbital and spin response. However, a simple model is unable to account for the observed shift. The spin-lattice relaxation differs in both scale and temperature dependence in all three. It is Korringa-like in Bi and remarkably is fastest in the insulating alloy and slowest in Sb with the highest bulk carrier density. These surprising results call for detailed calculations, but phenomenologically demonstrate that $\beta$-detected NMR of implanted ${}^8\mathrm{Li}{}^{+}$ is sensitive to the magnetic response of low-density carriers. The prospects for depth-resolved studies of conventional and topological surface states at lower implantation energies are good.

We report the discovery of an antiferromagnetic coupling of the magnetic moment of chromium(II) tetraphenyl-porphyrin (CrTPP) molecules to the magnetization of the clean ferromagnetic Co(001) substrate. We assign this unusual molecule-substrate exchange coupling to the less than half-filled chromium $3d$ orbitals interacting with Co valence band electrons via porphyrin-ligand molecular orbitals. X-ray magnetic circular dichroism, x-ray photoelectron spectroscopy, and scanning tunneling microscopy are combined with $\mathrm{DFT}+U$ calculations and provide evidence for a surprising type of antiferromagnetic ${90}^{\circ }$ indirect magnetic exchange coupling.

Ab initio calculations of the electric polarization of correlation-driven insulating materials, namely, Mott insulators, are lacking. Using a combination of density-functional theory and dynamical mean-field theory we study the electric polarization of the Mott insulator ${\mathrm{Sr}}_{0.5}{\mathrm{Ba}}_{0.5}{\mathrm{MnO}}_3$. We predict a ferroelectric polarization of $\simeq 16.5\mu \text{C}/{\text{cm}}^2$ in the high-temperature paramagnetic phase and recover a measured value of $\simeq 13.3\mu \text{C}/{\text{cm}}^2$ in the low-temperature antiferromagnetic phase. Our calculations reveal that the driving force for the ferroelectricity, the hybridization between Mn $e_g$ and O p orbitals, is suppressed by correlations, in particular, by the Hund's coupling and by the onset of magnetic order. They also confirm that the half-filled Mn $t_{2g}$ orbitals give rise to the antiferromagnetic Mott phase. This magnetic ordering leads to changes in the ionic polar displacement and, in turn, to the electronic polarization. In addition, for a fixed ionic displacement, we find that there is a reduction in the electronic contribution due to partial magnetic polarization of the Mn ${\mathrm{e}}_g$ orbitals. The reduction of the polarization due to ionic displacement dominates over the additional electronic part, hence the net magnetoelectric coupling is negative.

We show that, after a transformation, the dynamics of linear perturbations (spin waves) around a singular Bloch point soliton is formally equivalent to a quantum system of an electron in a magnetic monopole field. The analytical solution to this problem is known and allows us to find the spectrum and the scattering of a wave in a Bloch point field.

A systematic study of the angular and temperature dependencies of the ferromagnetic resonance (FMR) of manganese germanide ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$ grown on Ge(111) by solid phase epitaxy is reported. The FMR measurements performed in the out-of-plane geometry show that an out-of-plane anisotropy contribution exists in this material, and it increases with decreasing temperatures. However, the perpendicular magnetization curves imply that the shape anisotropy determines the orientation of the magnetization at any temperature. It is also shown that the Callen-Callen law cannot be used to describe the temperature dependence of the perpendicular anisotropy in ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$. Such temperature dependence of the magnetocrystalline anisotropy shows the invalidity of the single-ion anisotropy model for ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$.