In this paper, we study the static polarization in ABC-stacked multilayer graphene. Since the density of states diverges for these systems if the number of layers exceeds three, screening effects are expected to be important. In the random phase approximation, screening can be included through the polarization. We derive an analytical integral expression for the polarization in both the full-band model and an effective two-band model. Numerical evaluation of these integrals is very time consuming in the full-band model. Hence, for ABC-stacked trilayer graphene, we use the two-band model to calculate the low momentum part of the polarization. The results for the two-band model are universal, i.e., independent of doping. The high momentum part is linear and is determined by calculating two points, such that we can determine the slope. For ABC stacked trilayer graphene, the slope is given by three times the monolayer value. We compare our results to previous ones in the literature and discuss the similarities and discrepancies. Our results can be used to include screening in ABC-stacked multilayer systems in a way that all the characteristics of the polarization function are included. The numerical results for the polarization of trilayer graphene are used to sketch the screened potential.

Low-temperature scanning tunneling microscopy has been employed to analyze the formation of quantum well states (QWS) in two-dimensional gold islands, containing between 50 and 200 atoms, on MgO thin films. The energy position and symmetry of the eigenstates are revealed from conductance spectroscopy and imaging. The majority of the QWS originates from overlapping Au 6$p$ orbitals in the individual atoms and is unoccupied. Their characteristic is already reproduced with simple particle-in-a-box models that account for the symmetry of the islands (rectangular, triangular, or linear). However, better agreement is achieved when considering the true atomic structure of the aggregates via a density functional tight-binding approach. Based on a statistically relevant number of single-island data, we have established a correlation between the island geometry and the gap between the highest-occupied and the lowest-unoccupied molecular orbital in the finite-sized islands. The linear eccentricity is identified as a suitable descriptor for this relationship, as it combines information on both island size and island shape. Finally, the depth of the confinement potential is determined from the spatial extension of QWS beyond the physical boundaries of the Au islands. Our paper demonstrates how electron quantization effects can be analyzed in detail in metal nanostructures. The results may help elucidating the interplay between electronic and chemical properties of oxide-supported clusters as used in heterogeneous catalysis.

We use micro-Raman and photoluminescence (PL) spectroscopy at 300 K to investigate the influence of uniaxial tensile strain on the vibrational and optoelectronic properties of monolayer and bilayer MoS${}_2$ on a flexible substrate. The initially degenerate $E^{\prime }$ monolayer Raman mode is split into a doublet as a direct consequence of the strain applied to MoS${}_2$ through Van der Waals coupling at the sample-substrate interface. We observe a strong shift of the direct band gap of 48 meV/(% of strain) for the monolayer and 46 meV/% for the bilayer, whose indirect gap shifts by 86 meV/%. We find a strong decrease of the PL polarization linked to optical valley initialization for both monolayer and bilayer samples, indicating that scattering to the spin-degenerate $\Gamma$ valley plays a key role.

Magnetotransport experiments on ABC-stacked suspended trilayer graphene reveal a complete splitting of the 12-fold degenerated lowest Landau level, and, in particular, the opening of an exchange-driven gap at the charge neutrality point. A quantitative analysis of distinctness of the quantum Hall plateaus as a function of field yields a hierarchy of the filling factors: $\nu =6$, 4, and 0 are the most pronounced, followed by $\nu =3$, and finally $\nu =1$, 2, and 5. Apart from the appearance of a $\nu =4$ state, which is probably caused by a layer asymmetry, this sequence is in agreement with Hund's rules for ABC-stacked trilayer graphene.

Using time-dependent Ginzburg-Landau theory, we study the role of amplitude and phase fluctuations in the recovery of charge-stripe and spin-stripe phases in response to a pump pulse that melts the orders. For parameters relevant to the case where charge order precedes spin order thermodynamically, amplitude recovery governs the initial time scales, while phase recovery controls behavior at longer times. In addition to these intrinsic effects, there is a longer spin reorientation time scale related to the scattering geometry that dominates the recovery of the spin phase. Coupling between the charge and spin orders locks the amplitude and similarly the phase recovery, reducing the number of distinct time scales. Our results well reproduce the major experimental features of pump-probe x-ray diffraction measurements on the striped nickelate La${}_{1.75}$Sr${}_{0.25}$NiO${}_4$. They highlight the main idea of this work, which is the use of time-dependent Ginzburg-Landau theory to study systems with multiple coexisting order parameters.

Focusing on the particular case of the discrete symmetry group ${\mathbb{Z}}_N\times {\mathbb{Z}}_N$, we establish a mapping between symmetry-protected topological phases and symmetry-broken phases for one-dimensional spin systems. It is realized in terms of a nonlocal unitary transformation which preserves the locality of the Hamiltonian. We derive the image of the mapping for various phases involved, including those with a mixture of symmetry breaking and topological protection. Our analysis also applies to topological phases in spin systems with arbitrary continuous symmetries of unitary, orthogonal, and symplectic type. This is achieved by identifying suitable subgroups ${\mathbb{Z}}_N\times {\mathbb{Z}}_N$ in all these groups, together with a bijection between the individual classes of projective representations.

Renormalization of quasiparticles and excitons in carbon nanotubes (CNTs) near a metallic surface has been studied within a many-body formalism using an embedding approach newly implemented in the GW and Bethe-Salpeter methods. The quasiparticle band-gap renormalization in semiconducting CNTs is found to scale as $-1/\left(2h_a\right)$, with $h_a$ the apparent nanotube height, and it can exceed half an eV. Also, the binding energy of excitons is reduced dramatically—by as much as $75\%$—near the surface. Compensation between quasiparticle and excitonic effects results in small changes in the optical gap. The important role played by the nanotube screening response in establishing these effects is emphasized and a simple electrostatic model with no adjustable parameters explains the results of state-of-the-art calculations and generalizes them to a large variety of CNTs.

Electronic excited states at a bcc(110) lithium surface, both bare and covered by ionic ultrathin (1–2 monolayers) LiF epitaxial films, are investigated via many-body perturbation theory calculations achieving an atomistic level of detail. The full self-consistent solution of the GW equations is used to account for correlation effects and to properly describe the screened potential in the vacuum. In addition to the correct prediction of image-potential states, we find that the mixing between resonances and image states and the charge compression due to the dielectric ultrathin overlayer give rise to excitations with a hybrid localized but low-lying character whose accurate description cannot intrinsically be achieved via simple models or low-level calculations, but which are expected to play a crucial role in determining the electronic response and transport properties of these systems.

Topological crystalline insulators represent a novel topological phase of matter in which the surface states are protected by discrete point group symmetries of the underlying lattice. Rock-salt lead-tin-selenide alloy is one possible realization of this phase, which undergoes a topological phase transition upon changing the lead content. We used scanning tunneling microscopy (STM) and angle resolved photoemission spectroscopy (ARPES) to probe the surface states on (001) Pb${}_{1-x}$Sn${}_x$Se in the topologically nontrivial ($x=0.23$) and topologically trivial ($x=0$) phases. We observed quasiparticle interference with STM on the surface of the topological crystalline insulator and demonstrated that the measured interference can be understood from ARPES studies and a simple band structure model. Furthermore, our findings support the fact that Pb${}_{0.77}$Sn${}_{0.23}$Se and PbSe have different topological nature.

We investigate the transport of electrons in disordered and pristine graphene devices. Fano shot noise, a standard metric to assess the mechanism for electronic transport in mesoscopic devices, has been shown to produce almost the same magnitude ($\approx 1/3$) in ballistic and diffusive graphene devices and is therefore of limited applicability. We consider a two-terminal geometry where the graphene flake is contacted by narrow metallic leads. We propose that the dependence of the conductance on the position of one of the leads, a conductance profile, can give us insight into the charge flow, which can in turn be used to analyze the transport mechanism. Moreover, we simulate scanning probe microscopy (SPM) measurements for the same devices, which can visualize the flow of charge inside the device, thus complementing the transport calculations. From our simulations, we find that both the conductance profile and SPM measurements are excellent tools to assess the transport mechanism differentiating ballistic and diffusive graphene systems.

The structural aspects of the metastable states for K${}_{0.32}$Co[Fe(CN)${}_6$]${}_{0.76}·$3H${}_2$O have been investigated by synchrotron x-ray powder diffraction. The title compound exhibits nonequilibrium high spin (HS) states of Co${}^{\mathrm{II}}$($S=3/2$)-Fe${}^{\mathrm{III}}$($S=1/2$) configuration, induced by rapid cooling or photoexcitation from the low-temperature (LT) phase. By introducing a new local order parameter of tilting angle between cyanide-bridged Fe and Co-based octahedra, we discovered the existence of a precursor phenomenon triggering the collective instability during the thermal relaxation of the photoexcited (PX) state. Moreover, we introduced a methodology, based on the flow diagram studies, which allowed us to clearly distinguish the obtained metastable states through their strength of spin-lattice coupling, leading to various pathways in the phase space during the combined electroelastic relaxation process.

Dynamical Jahn-Teller (DJT) effect in a spin-orbital coupled system on a honeycomb lattice is examined, motivated from recently observed spin-liquid behavior in Ba${}_3$CuSb${}_2$O${}_9$. An effective vibronic Hamiltonian, where the superexchange interaction and the DJT effect are taken into account, is derived. We find that the DJT effect induces a spin-orbital resonant state where local spin-singlet states and parallel orbital configurations are entangled with each other. This spin-orbital resonant state is realized in between an orbital-ordered state, where spin-singlet pairs are localized, and an antiferromagnetic ordered state. Based on the theoretical results, a possible scenario for Ba${}_3$CuSb${}_2$O${}_9$ is proposed.

We numerically solve the coupled Landau-Ginzburg-Maxwell equations for a model of a spin triplet $p_x+i{p}_y$ superconductor in which whole or half-quanta of flux thread through a hole. We recover the pattern of stable and unstable regions for the half-flux quanta observed in a recent experiment. We also examine a scenario with bent, Abrikosov integer vortices and find it implausible for the thin, mesoscopic ring geometry considered.

The interpretation of Kerr rotation measurements in the superconducting phase of Sr${}_2$RuO${}_4$ is a controversial topic. Both intrinsic and extrinsic mechanisms have been proposed, and it has been argued that the intrinsic response vanishes by symmetry. We focus on the intrinsic contribution and clarify several conflicting results in the literature. On the basis of symmetry considerations and detailed calculations, we show that the intrinsic Kerr signal is not forbidden in a general multiband system but has a rich structure in the near-infrared regime. We distinguish different optical transitions determined by the superconducting gap (far infrared) and the interorbital coupling of the normal state (near infrared). We argue that the low-frequency transitions do not contribute to the Hall conductivity, while only the interorbital transitions in the near-infrared regime contribute. Finally, we discuss the difficulties to connect the calculations for the optical Hall conductivity to the experimental measurement of the Kerr angle. We will compare different approximations which might lead to conflicting results.

In some magnetic ferroelectrics the polarization P direction can be selected by a magnetic field H. Here we show that in a (Y${}_{1-x}$Sm${}_x$)MnO${}_3$ single crystal, both the direction and the magnitude of P can be controlled solely by H. We argue that this remarkable dual control arises from both the phase coexistence of $bc$-cycloidal regions within an $ab$-cycloidal matrix and the annihilation of multiferroic domain walls by $H$ cycling. We show that phase coexistence occurs even within the high-temperature spin-collinear phase and argue that this could be a general property arising from the strong frustration of magnetic interactions in these oxides.

We present a modified bond-valence model of PbTiO${}_3$ based on the principles of bond-valence and bond-valence vector conservation. The relationship between the bond-valence model and the bond-order potential is derived analytically in the framework of a tight-binding model. An energy term, bond-valence vector energy, is introduced into the atomistic model and the potential parameters are reoptimized. This model potential can be applied both to canonical-ensemble (NVT) and isobaric-isothermal ensemble (NPT) molecular dynamics (MD) simulations. This model reproduces the experimental phase transition in NVT MD simulations and also exhibits the experimental sequence of temperature-driven and pressure-driven phase transitions in NPT simulations. We expect that this improved bond-valence model can be applied to a broad range of inorganic materials.

Py/FeMn/Cu(001) and Py/FeMn/Ni/Cu(001) films were grown and studied as a function of both the FeMn and the Ni thicknesses using rotating magneto-optic Kerr effect and x-ray magnetic circular dichroism. For Py/FeMn/Cu(001), we find that the FeMn antiferromagnetic order switches the sign of the Py fourfold magnetic anisotropy but has little effect on the step-induced uniaxial magnetic anisotropy. For Py/FeMn/Ni/Cu(001), we find that out-of-plane Ni magnetization has little effect on the Py magnetic anisotropy, but in-plane Ni magnetization enhances the Py magnetic anisotropy in the region just above antiferromagnetic transition thickness. The underlying mechanism could be attributed to the FeMn 3Q spin structure.

We analyze a criterion which guarantees that the ground states of certain many-body systems are stable under perturbations. Specifically, we consider PEPS, which are believed to provide an efficient description, based on local tensors, for the low energy physics arising from local interactions. In order to assess stability in the framework of PEPS, one thus needs to understand how physically allowed perturbations of the local tensor affect the properties of the global state. In this paper, we show that a restricted version of the local topological quantum order (LTQO) condition [Michalakis and Pytel, Commun. Math. Phys. 322, 277 (2013)] provides a checkable criterion which allows us to assess the stability of local properties of PEPS under physical perturbations. We moreover show that LTQO itself is stable under perturbations which preserve the spectral gap, leading to nontrivial examples of PEPS which possess LTQO and are thus stable under arbitrary perturbations.

We compare the photoluminescence spectra of the low-temperature Mott insulator $\kappa$-(BEDT–TTF)${}_2$Cu[N(CN)${}_2$]Cl ($T_{\mathrm{MIT}}=40$ K) with spectra of metallic $\kappa$-(BEDT–TTF)${}_2$Cu[N(CN)${}_2$]Br, which is superconducting below $T_c=11.8$ K, in the temperature range between 300 and 20 K. In the Mott insulating state of $\kappa$-(BEDT–TTF)${}_2$Cu[N(CN)${}_2$]Cl we observe a luminescence band at 1.95 eV due to the recombination of an exciton created by a highest occupied molecular orbital–lowest unoccupied molecular orbital optical excitation. This luminescence is quenched both in the high-temperature bad metal state of $\kappa$-(BEDT–TTF)${}_2$Cu[N(CN)${}_2$]Cl and in metallic $\kappa$-(BEDT–TTF)${}_2$Cu[N(CN)${}_2$]Br. The observation of the luminescence of an exciton provides evidence for the local character of excitations in the Mott insulating state.

Ground-plane cloaks, which transform a curved mirror into a flat one, and recently reported at wavelengths ranging from the optical to the visible spectrum, bring the realm of optical illusion a step closer to reality. However, all carpet-cloaking experiments have thus far been carried out in the far field. Here, we demonstrate numerically and experimentally that a dielectric photonic crystal (PC) of an irregular shape made of a honeycomb array of air holes can scatter waves in the near field like a PC with a flat boundary at stop band frequencies. This mirage effect relies upon a specific arrangement of dielectric pillars placed at the nodes of a quasiconformal grid dressing the PC. Our carpet is experimentally shown to flatten the scattered wave fronts of a PC with a bump throughout the range of wavelengths from 1520 to 1580 nm within the stop band extending from 1280 to 1940 nm. The device has been fabricated using a single-mask advanced nanoelectronics technique on III-V semiconductors and the near field measurements have been carried out in order to image the wave fronts’ curvatures around the telecommunication wavelength 1550 nm. Interestingly, comparisons of our near-field experimental results with full-wave simulations suggest the relatively low aspect ratio of the fabricated carpet (pillars have 200 nm diameter and 2 $\mu$m height) makes it inherently three dimensional. Moreover, this carpet is constrained to normal incidence. We therefore propose an elaborated design of the carpet (with pillars of varying radii) which should work at different angles of incidence.