The electronic structure of the Cr(001) surface with its sharp resonance at the Fermi level is a subject of controversial debate of many experimental and theoretical works. To date, it is unclear whether the origin of this resonance is an orbital Kondo or an electron-phonon coupling effect. We have combined ab initio density functional calculations with dynamical mean-field simulations to calculate the orbitally resolved spectral function of the Cr(001) surface. The calculated orbital character and shape of the spectrum is in agreement with data from (inverse) photoemission experiments. We find that dynamic electron correlations crucially influence the surface electronic structure and lead to a low energy resonance in the $d_{z^2}$ and $d_{xz/yz}$ orbitals. Our results help to reconvene controversial experimental results from (I)PES and STM measurements.

Infrared optical investigations of $\alpha \text{−}\left(\text{BEDT-TTF}\right){}_2{\mathrm{I}}_3$ have been performed in the spectral range from 80 to $8000 {\mathrm{cm}}^{-1}$ down to temperatures as low as 10 K by applying hydrostatic pressure. In the metallic state, $T>135$ K, we observe a 50% increase in the Drude contribution as well as the mid-infrared band due to the growing intermolecular orbital overlap with pressure up to 11 kbar. In the ordered state, $T<T_{\mathrm{CO}}$, we extract how the electronic charge per molecule varies with temperature and pressure: Transport and optical studies demonstrate that charge order and metal-insulator transition coincide and consistently yield a linear decrease of the transition temperature $T_{\mathrm{CO}}$ by $8\text{–}9$ K/kbar. The charge disproportionation $\mathrm{\Delta }\rho$ diminishes by $0.017e$/kbar and the optical gap $\mathrm{\Delta }$ between the bands decreases with pressure by $-47 {\mathrm{cm}}^{-1}$/kbar. In our high-pressure and low-temperature experiments, we do observe contributions from the massive charge carriers as well as from massless electrons to the low-frequency optical conductivity, however, without being able to disentangle them unambiguously.

Employing the $\mathbf{k}·\mathbf{p}$ expansion for a family of tight-binding models for ${\mathrm{SmB}}_6$, we analytically compute topological surface states on a generic $\left(lmn\right)$ surface. We show how the Dirac-cone spin structure depends on model ingredients and on the angle $\theta$ between the surface normal and the main crystal axes. We apply the general theory to (001), (110), (111), and (210) surfaces, for which we provide concrete predictions for the spin pattern of surface states which we also compare with tight-binding results. As shown in previous work, the spin pattern on a $\left(001\right)$ surface can be related to the value of mirror Chern numbers, and we explore the possibility of topological phase transitions between states with different mirror Chern numbers and the associated change of the spin structure of surface states. Such transitions may be accessed by varying either the hybridization between conduction and $f$ electrons or the crystal-field splitting of the low-energy $f$ multiplets, and we compute corresponding phase diagrams. Experimentally, chemical doping is a promising route to realize such transitions.

We introduce a two-ladder lattice model with interacting Majorana fermions that could be realized on the surfaces of a topological insulator film. We study this model with a combination of analytical and numerical techniques, and we find a phase diagram that features both gapless and gapped phases as well as interesting phase transitions including a quantum critical point in the tricritical Ising (TCI) universality class. The latter occurs at an intermediate-coupling strength at a meeting point of a first-order transition line and an Ising critical line, and it is known to be described by a superconformal field theory with central charge $c=\frac{7}{10}$. We discuss the experimental feasibility of constructing the model and tuning parameters to the vicinity of the TCI point where signatures of the elusive supersymmetry can be observed.

We report transverse and longitudinal magnetotransport properties of ${\mathrm{NbAs}}_2$ single crystals. Attributing to the electron-hole compensation, nonsaturating large transverse magnetoresistance reaches up to 8000 at 9 T at 1.8 K with mobility around 1 to $2{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{S}}^{-1}$. We present a thorough study of angular-dependent Shubnikov–de Haas (SdH) quantum oscillations of ${\mathrm{NbAs}}_2$. Three distinct oscillation frequencies are identified. First-principles calculations reveal four types of Fermi-surface pockets: electron $\alpha$ pocket, hole $\beta$ pocket, hole $\gamma$ pocket, and small electron $\delta$ pocket. Although the angular dependence of $\alpha ,\beta$, and $\delta$ agree well with the SdH data, $\gamma$ pocket is missing in SdH. Negative longitudinal magnetoresistance is observed which may be linked to novel topological states in this material, although systematic study is necessary to ascertain its origin.

We characterize the incorporation of Ba adatoms into the Ge(001) surface, resulting in the formation of one-dimensional structures with an internal $2\times 3$ periodicity, after the deposition of Ba atoms at 970 K or at room temperature followed by a 770 K anneal. Scanning tunneling microscopy (STM) data were compared with theoretically simulated STM images generated by density functional theory electronic structure calculations. Excellent agreement between experiment and simulation was found when using an adopted structural model that assumes partial removal of the surface Ge dimers in the [1–10] surface direction and subsequent addition of a single Ba atom to the substrate second layer. Structural assignments for a number of defects observed within regions of the $2\times 3$ reconstruction were also obtained.

We study quantum point contacts in two-dimensional topological insulators by means of quantum transport simulations for InAs/GaSb heterostructures and HgTe/(Hg,Cd)Te quantum wells. In InAs/GaSb, the density of edge states shows an oscillatory decay as a function of the distance to the edge. This is in contrast to the behavior of the edge states in HgTe quantum wells, which decay into the bulk in a simple exponential manner. The difference between the two materials is brought about by spatial separation of electrons and holes in InAs/GaSb, which affects the magnitudes of the parameters describing the particle-hole asymmetry and the strength of intersubband coupling within the Bernevig-Hughes-Zhang model. We show that the character of the wave-function decay impacts directly the dependence of the point contact conductance on the constriction width and the Fermi energy, which can be verified experimentally and serves to accurately determine the values of the relevant parameters. In the case of InAs/GaSb heterostructures, the conductance magnitude oscillates as a function of the constriction width following the oscillations of the edge state penetration, whereas in HgTe/(Hg,Cd)Te quantum wells a single switching from transmitting to reflecting contact is predicted.

The temperature dependence of the optical response in the far-infrared (FIR) region of metallic and semiconducting rich single-walled carbon nanotubes (SWCNTs) was investigated by micro-Fourier transform infrared spectrometry with a focused beam of synchrotron radiation. The temperature dependence of the FIR spectra of both types of SWCNT showed negligibly small variations within a wide temperature range from 4 to 300 K. Upon comparison with a theoretical model for the diffusive region, it is speculated that these results might have been caused by a strong suppression of phonon scattering in relatively short CNTs with lengths of less than 1 μm.

Magnetic coupling is generally much weaker than electric Coulomb interaction. This also applies to the well-known magnetic “meta-atoms,” or split-ring resonators (SRRs) as originally proposed by Pendry et al. [IEEE Trans. Microwave Theory Tech. 47, 2075 (1999)], in which the associated electric dipole moments usually dictate their interaction. As a result, stereometamaterials, a stack of identical SRRs, were found with electric coupling so strong that the dispersion from merely magnetic coupling was overturned. Recently, other workers have proposed a new concept of magnetic localized surface plasmons, supported on metallic spiral structures (MSSs) at a deep-subwavelength scale. Here, we experimentally demonstrate that a stack of these magnetic “meta-atoms” can have dominant magnetic coupling in both of its two configurations. This allows magnetic-coupling-dominant energy transport along a one-dimensional stack of MSSs, as demonstrated with near-field transmission measurement. Our work not only applies this type of magnetic “meta-atom” into metamaterial construction, but also provides possibilities of magnetic metamaterial design in which the electric interaction no longer takes precedence.

We report a precise experimental study on the shot noise of a quantum point contact (QPC) fabricated in a GaAs/AlGaAs based high-mobility two-dimensional electron gas (2DEG). The combination of unprecedented cleanliness and very high measurement accuracy has enabled us to discuss the Fano factor to characterize the shot noise with a precision of 0.01. We observed that the shot noise at zero magnetic field exhibits a slight enhancement exceeding the single particle theoretical prediction, and that it gradually decreases as a perpendicular magnetic field is applied. We also confirmed that this additional noise completely vanishes in the quantum Hall regime. These phenomena can be explained by the electron heating effect near the QPC, which is suppressed with increasing magnetic field.

Highly integrated single-photon sources are key components in future quantum-optical circuits. Whereas the probabilistic generation of single photons can routinely be done by now, their triggered generation is a much greater challenge. Here, we describe the triggered generation of single photons in a hybrid plasmonic device. It consists of a lambda-type quantum emitter coupled to a multimode optical nanoantenna. For moderate interaction strengths between the subsystems, the description of the quantum optical evolution can be simplified by an adiabatic elimination of the electromagnetic fields of the nanoantenna modes. This leads to an insightful analysis of the emitter's dynamics, entails the opportunity to understand the physics of the device, and to identify parameter regimes for a desired operation. Even though the approach presented in this work is general, we consider a simple exemplary design of a plasmonic nanoantenna, made of two silver nanorods, suitable for triggered generation of single photons. The investigated device realizes single photons, triggered, potentially at high rates, and using low device volumes.

We consider topological phases in periodically driven (Floquet) systems exhibiting many-body localization, protected by a symmetry $G$. We argue for a general correspondence between such phases and topological phases of undriven systems protected by symmetry $\mathbb{Z}⋊G$ where the additional $\mathbb{Z}$ accounts for the discrete time-translation symmetry. Thus, for example, the bosonic phases in $d$ spatial dimensions without intrinsic topological order [symmetry-protected topological (SPT) phases] are classified by the cohomology group $H^{d+1}[\mathbb{Z}⋊G,\mathrm{U}\left(1\right)]$. For unitary symmetries, we interpret the additional resulting Floquet phases in terms of the lower-dimensional SPT phases that are pumped to the boundary during one time step. These results also imply the existence of novel symmetry-enriched topological (SET) orders protected solely by the periodicity of the drive.

We demonstrate that light creates a highly magnetized region in a magnetic semiconductor far above its critical temperature. A near-gap photon generates a quasiparticle of nonzero magnetic moment, named magnetic polaron, which is constituted by the photoexcited electron and about 1000 spin-polarized lattice atoms surrounding the photoexcited electron. The photoinduced magnetization follows a Langevin function, whose shape uniquely determines the magnetic moment of an individual polaron. In EuTe at 5 K the magnetic moment reaches a giant value of over 500 Bohr magnetons, thus the photoinduced magnetization saturates with a magnetic field of only 50 mT, which characterizes the magnetic polaron system as superparamagnetic. The polaron has an average lifetime of $15\mu \mathrm{s}$. When temperature is increased its magnetic moment decreases, but at 150 K it still has a large value of about 80 Bohr magnetons. The paramagnet of polarons is fully controlled by light. Because the magnetic polaron affects only spin orientation, but not the charge distribution, in the superparamagnetic state the ideal optical quality of the host semiconductor is preserved.

$\mathrm{N}{\mathrm{d}}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Mn}{\mathrm{O}}_3$ is particularly representative of mixed-valent manganites since their three characteristic macroscopic phases (charge-ordered insulator, ferromagnetic-metallic, and paramagnetic insulator) appear at different temperatures. We here report a complete x-ray emission and absorption spectroscopy (XES-XAS) study of $\mathrm{N}{\mathrm{d}}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Mn}{\mathrm{O}}_3$ as a function of temperature to investigate the electronic and local structure changes of the Mn atom in these three phases. Compared with the differences in the XES-XAS spectra between $\mathrm{N}{\mathrm{d}}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Mn}{\mathrm{O}}_3$ and the single-valent reference compounds $\mathrm{NdMn}{\mathrm{O}}_3$ $\left(\mathrm{M}{\mathrm{n}}^{3+}\right)$ and Sr/$\mathrm{CaMn}{\mathrm{O}}_3$ $\left(\mathrm{M}{\mathrm{n}}^{4+}\right)$, only modest changes have been obtained across the magnetoelectrical transitions. The XES spectra, including both the Mn Kα and Kβ emission lines, have mainly shown a subtle decrease in the local spin density accompanying the passage to the ferromagnetic-metallic phase. For the same phase, the small intensity variations in the pre-edge region of the high-resolution XAS spectra reflect an increase of the $p\text{−}d$ mixing. The analysis of these XAS spectra imply a charge segregation between the two different Mn sites far from one electron, being in intermediate valences $\mathrm{M}{\mathrm{n}}^{+3.5\pm \delta /2}(\delta <0.2e-)$ for all the phases. Our results indicate that the spin, charge, and geometrical structure of the Mn atom hardly change among the three macroscopic phases demonstrating the strong competition between the ferromagnetic conductor and the charge-ordered insulator behaviors in the manganites.

Three-dimensional (3D) Dirac and Weyl semimetals are novel states of quantum matter. We classify stable 3D Dirac and Weyl semimetals with reflection and rotational symmetry in the presence of time reversal symmetry and spin-orbit coupling, which belong to seventeen different point groups. They have two classes of reflection symmetry, with the mirror plane parallel and perpendicular to rotation axis. In both cases two types of Dirac points, existing through accidental band crossing (ABC) or at a time reversal invariant momentum (TBC), are determined by four different reflection symmetries. We classify those two types of Dirac points with a combination of different reflection and rotational symmetries. We further classify Dirac and Weyl line nodes to show in which types of mirror plane they can exist. Finally we discuss that Weyl line nodes and Dirac points can exist at the same time taking ${\mathrm{C}}_{4\mathrm{v}}$ symmetry as an example.

We systematically compare predictions of various exchange correlation functionals for the structural and magnetic properties of perovskite ${\mathrm{Sr}}_{1-x}{\mathrm{Ba}}_x{\mathrm{MnO}}_3 \left(0\le x\le 1\right)$—a representative class of multiferroic oxides. The local spin density approximation (LSDA) and spin-dependent generalized gradient approximation with Perdew-Burke-Ernzerhof parametrization (sPBE) make substantial different predictions for ferroelectric atomic distortions, tetragonality, and ground state magnetic ordering. Neither approximation quantitatively reproduces all the measured structural and magnetic properties of perovskite ${\mathrm{Sr}}_{0.5}{\mathrm{Ba}}_{0.5}{\mathrm{MnO}}_3$. The spin-dependent generalized gradient approximation with Perdew-Burke-Ernzerhof revised for solids parametrization (sPBEsol) and the charge-only Perdew-Burke-Ernzerhof parametrized generalized gradient approximation with Hubbard $U$ and Hund's $J$ extensions both provide overall better agreement with measured structural and magnetic properties of ${\mathrm{Sr}}_{0.5}{\mathrm{Ba}}_{0.5}{\mathrm{MnO}}_3$, compared to LSDA and sPBE. Using these two methods, we find that different from previous predictions, perovskite ${\mathrm{BaMnO}}_3$ has large Mn off-center displacements and is close to a ferromagnetic-to-antiferromagnetic phase boundary, making it a promising candidate to induce effective giant magnetoelectric effects and to achieve cross-field control of polarization and magnetism.

We present the results of precise correlated-electron calculations on the monomer lattices of the organic charge-transfer solids $\kappa \text{−}(\text{BEDT-}{\mathrm{TTF})}_{2}X$ for 32 and 64 molecular sites. Our calculations are for band parameters corresponding to $X=\text{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Cl}$ and ${\mathrm{Cu}}_2({\mathrm{CN})}_3$, which are semiconducting antiferromagnetic and quantum spin liquid, respectively, at ambient pressure. We have performed our calculations for variable electron densities $\rho$ per BEDT-TTF molecule, with $\rho$ ranging from 1 to 2. We find that $d$-wave superconducting pair-pair correlations are enhanced by electron-electron interactions only for a narrow carrier concentration about $\rho =1.5$, which is precisely the carrier concentration where superconductivity in the charge-transfer solids occurs. Our results indicate that the enhancement in pair-pair correlations is not related to antiferromagnetic order, but to a proximate hidden spin-singlet state that manifests itself as a charge-ordered state in other charge-transfer solids. Long-range superconducting order does not appear to be present in the purely electronic model, suggesting that electron-phonon interactions also must play a role in a complete theory of superconductivity.

We challenge the hypothesis that the ground states of a physical system whose degeneracy depends on topology must necessarily realize topological quantum order and display nonlocal entanglement. To this end, we introduce and study a classical rendition of the Toric Code model embedded on Riemann surfaces of different genus numbers. We find that the minimal ground state degeneracy (and those of all levels) depends on the topology of the embedding surface alone. As the ground states of this classical system may be distinguished by local measurements, a characteristic of Landau orders, this example illustrates that topological degeneracy is not a sufficient condition for topological quantum order. This conclusion is generic and, as shown, it applies to many other models. We also demonstrate that certain lattice realizations of these models, and other theories, display a ground state entropy (and those of all levels) that is “holographic”, i.e., extensive in the system boundary. We find that clock and $U\left(1\right)$ gauge theories display topological (in addition to gauge) degeneracies.

We study the texture of helical currents in metallic planar strips in the presence of Rashba spin-orbit coupling (RSOC) on the lattice at zero temperature. In the noninteracting case and in the absence of external electromagnetic sources, we determine, by exact numerical diagonalization of the single-particle Hamiltonian, the distribution across the strip section of these Rashba helical currents (RHC) as well as their sign oscillation, as a function of the RSOC strength, strip width, electron filling, and strip boundary conditions. Then, we study the effects of charge currents introduced into the system by an Aharonov-Bohm flux for the case of rings or by a voltage bias in the case of open strips. The former setup is studied by variational Monte Carlo, and the latter by the time-dependent density-matrix renormalization-group technique. Particularly for strips formed by two, three, and four coupled chains, we show how these RHC vary in the presence of such induced charge current, and how their differences between spin-up and spin-down electron currents on each chain help to explain the distribution across the strip of charge currents, both of the spin-conserving and the spin-flipping types. We also predict the appearance of polarized charge currents on each chain. Finally, we show that these Rashba helical currents and their derived features remain in the presence of an on-site Hubbard repulsion as long as the system remains metallic, at quarter filling, and even at half filling where a Mott-Hubbard metal-insulator transition occurs for large Hubbard repulsion.