We consider one-dimensional Hamiltonian systems whose ground states display symmetry-protected topological order. We show that ground states within the topological phase cannot be connected with each other through local operations and classical communication between a bipartition of the system. Our claim is demonstrated by analyzing the entanglement spectrum and Rényi entropies of different physical systems that provide examples for symmetry-protected topological phases. Specifically, we consider the spin-$1/2$ cluster-Ising model and a class of spin-1 models undergoing quantum phase transitions to the Haldane phase. Our results provide a probe for symmetry-protected topological order. Since the picture holds even at the system's local scale, our analysis can serve as a local experimental test for topological order.

A number of recent articles have reported the existence of topologically nontrivial states and associated end states in one-dimensional incommensurate lattice models that would usually only be expected in higher dimensions. Using an explicit construction, we here argue that the end states have precisely the same origin as their counterparts in commensurate models and that incommensurability does not in fact provide a meaningful connection to the topological classification of systems in higher dimensions. In particular, we show that it is possible to smoothly interpolate between states with commensurate and incommensurate modulation parameters without closing the band gap and without states crossing the band gap.

We have observed a massive reconstruction of the Fermi surface of single crystal chromium as a function of high pressure and high magnetic fields caused by the spin-flip transition, with multiple new orbits appearing above 0.93 GPa. In addition, some orbits have field-induced effective masses of ∼0.06–0.07 $m_e$, seen only at high magnetic fields. Based on the temperature insensitivity displayed by the oscillation amplitudes at these frequencies, we attribute the orbits to quantum interference rather than to Landau quantization.

The surface electronic structure of GaN(0001) surfaces is characterized by photoelectron spectroscopy. Depending on the surface preparation conditions, such as cooldown in nitrogen plasma after growth and additional vacuum annealing or Ga deposition, different surface states are observed at the valence-band edge and inside the band gap of GaN, while the surface Fermi-level position $E_F$ was found to be independent at 2.9–3.0 eV, indicative of unoccupied surface states that pin $E_F$. The experimental results are combined with band structures of different $2\times 2$ reconstructed surfaces calculated by density-functional theory. Comparing the experimental results with the theoretical density of surface states allows an identification of the microscopic origin of these states and an assignment of the related surface structure. The presence of a $2\times 2$ nitrogen adatom structure after growth is found that can be identified by its fingerprint surface states $\approx$0.9 eV above and $\approx$0.6 eV below the valence-band maximum (VBM). For Ga vacancy and adatom structures a similar agreement is found, revealing surface states $\approx$0.3 and $\approx$1.4 or $\approx$1.6 eV above the VBM, respectively.

We report the results of a study of ${}^4$He${}_N$ clusters, for $2\le N\le 40$, adsorbed on one and both sides of a graphene sheet. The ground-state properties are determined using variational and diffusion Monte Carlo calculations at zero temperature, and in addition path integral Monte Carlo simulations at finite temperature are performed for some selected cluster sizes. For the interaction of helium atoms with graphene, we compare several models: a smooth He-graphene potential that depends only on the distance to the graphene sheet and potentials constructed as a sum of individual He-C interactions where two possibilities for this He-C interaction are tested. In this way, we assess the effect of corrugation on the binding properties of helium clusters. Furthermore, we study the influence that the graphene-mediated McLachlan dispersion energy has on the He-He interaction. The McLachlan interaction weakens the attraction between helium atoms, which turns out to have a significant effect on the binding energy and the shape of adsorbed ${}^4$He clusters. We find that clusters adsorbed on opposite sides of graphene are bound, but according to the He-He pair distribution function across the graphene sheet, pair correlations are very weak. For a large enough number of particles, solidlike $\sqrt{3}\times \sqrt{3}$ structures start to become energetically preferred for the model of anisotropic corrugation without the McLachlan interaction. For the other models, the ground state of the studied clusters is clearly liquidlike.

We study the properties of localized vibrational modes associated with structural defects in a sheet of graphene. For the examples of the Stone-Wales defects, one- and two-atom vacancies, many-atom linear vacancies, and adatoms in a honeycomb lattice, we demonstrate that the local defect modes are characterized by stable oscillations with the frequencies lying outside the linear frequency bands of an ideal graphene. In the frequency spectral density of thermal oscillations, such localized defect modes lead to the additional peaks from the right side of the frequency band of the ideal sheet of graphene, which indicate the presence of defects in the graphene flakes.

Molecular nanomagnets are zero-dimensional spin systems that exhibit quantum-mechanical behavior at low temperatures. Exploiting quantum-information theoretic measures, we quantify here the size of linear superpositions that can be generated within the ground multiplet of high- and low-spin nanomagnets. Amongst the former class of systems, we mainly focus on Mn${}_{12}$ and Fe${}_8$. General criteria for maximizing such measures are also outlined, and illustrated with reference to spin clusters of different geometries. The actual character (micro or macroscopic) of linear superpositions in low-spin systems is inherently ill-defined. Here, this issue is addressed with specific reference to the Cr${}_7$Ni and V${}_{15}$ molecules, characterized by an $S=1/2$ ground state. In both cases, the measures we obtain are larger than those of a single $s=1/2$ spin but not proportionate to the number and value of the constituent spins.

Low-energy excitations in the rare earth kagome spin system Pr${}_3$Ga${}_5$SiO${}_{14}$ (PGS) have been investigated using high-field electron spin resonance (ESR) techniques. Previous work has shown that PGS does not exhibit long-range magnetic order at temperatures down to 30 mK. The present low-temperature (1.3–20 K) field-scan measurements, made on a single crystal sample in applied fields up to 30 T and for several microwave frequencies, give a series of temperature-dependent absorption peaks that are very different to conventional ESR spectra. The observed response is interpreted in terms of discrete spin-wave excitations in antiferromagnetically correlated spin clusters. We present a theoretical model which, in the limit of small magnetic frustration effects, qualitatively describes the microwave-induced transitions.

We show that the acoustic response of perforated and elastically filled rigid screens can give rise to a broad landscape of tunable devices. We begin presenting deep-subwavelength transmission properties of a structured plate and demonstrate the immediate relationship to truly bound surface modes. We extend our theoretical model to analyze structured metal-fluid-metal wave guides for the confinement of sound and present exact expressions for the dispersion relations which describe the hybridization of resonances. We discuss the validity of our analytical model by direct comparison to full-wave simulations and use this technique in the search for broadband response in composite structures where the effective mass density and bulk modulus are simultaneously negative and exhibiting weak influences by viscous losses.

The optimal Co concentration in Fe${}_{1-x}$Co${}_x$/MgO magnetic tunnel junctions (MTJs) that maximizes tunneling magnetoresistance (TMR) is still under investigation. We perform a first-principles transport study on MTJs using disordered electrodes modeled using the virtual crystal approximation (VCA) and ordered alloys with various MgO barrier thicknesses. We find that 10–20$\%$ Co concentration maximizes TMR using VCA to represent disorder in the electrodes. This TMR peak arises due to a minority $d$-type interfacial resonance state (IRS) that becomes filled with small Co doping, leading to a decrease in antiparallel conductance. Calculations with ordered Fe${}_{1-x}$Co${}_x$ electrodes confirm the filling of this minority $d$-type IRS for small Co concentrations. In addition, we construct a $10\times 10$ supercell without VCA to explicitly represent disorder at the Fe${}_{1-x}$Co${}_x$/MgO interface, which demonstrates a quenching of the minority-$d$ IRS and significant reduction in available states at the Fermi level that agrees with VCA calculations. These results explain recent experimental findings and provide implications for the impact of IRS on conductance and TMR in Fe${}_{1-x}$Co${}_x$/MgO tunnel junctions.

We study, using quantum Monte Carlo (QMC) simulations, the ground state properties of spin-1 bosons trapped in a square optical lattice. The phase diagram is characterized by the mobility of the particles (Mott insulating or superfluid phase) and by their magnetic properties. For ferromagnetic on-site interactions, the whole phase diagram is ferromagnetic and the Mott insulators-superfluid phase transitions are second order. For antiferromagnetic on-site interactions, spin nematic order is found in the odd Mott lobes and in the superfluid phase. Furthermore, the superfluid-insulator phase transition is first or second order depending on whether the density in the Mott is even or odd. Inside the even Mott lobes we observe a singlet-to-nematic transition for certain values of the interactions. This transition appears to be first order.

The experimental advances in cold atomic and molecular gases stimulate the investigation of lattice correlated systems beyond the conventional on-site Hubbard approximation, by possibly including multiparticle processes. We study fermionic extended Hubbard models in a one-dimensional lattice with different types of particle couplings, including also three- and four-body interactions up to nearest neighboring sites. By using the bosonization technique, we investigate the low-energy regime and determine the conditions for the appearance of ordered phases, for arbitrary particle filling. We find that three- and four-body couplings may significantly modify the phase diagram. In particular, diagonal three-body terms that directly couple the local particle densities have qualitatively different effects from off-diagonal three-body couplings originating from correlated hopping, and favor the appearance of a Luther-Emery phase even when two-body terms are repulsive. Furthermore, the four-body coupling gives rise to a rich phase diagram and may lead to the realization of the Haldane insulator phase at half-filling.

We study the problem of disorder-free metals in the vicinity of a quantum critical point in $d=3+1$ dimensions. We begin with perturbation theory in the Yukawa coupling between the electrons and undamped bosons (order-parameter fluctuations) and show that the perturbation expansion breaks down below energy scales where the bosons get substantially Landau damped. Above this scale, however, we find a regime in which low-energy fermions obtain an imaginary self-energy that varies linearly with frequency, realizing the “marginal Fermi liquid” phenomenology [Varma et al., Phys. Rev. Lett. 63, 1996 (1989)]. We discuss a large-$N$ theory in which the marginal Fermi liquid behavior is enhanced while the role of Landau damping is suppressed, and show that quasiparticles obtain a decay rate parametrically larger than their energy.

We study the static structure factor of the fractional Chern insulator Laughlin-like state and provide analytical forms for this quantity in the long-distance limit. In the course of this we identify the averaged over the Brillouin zone Fubini-Study metric as the relevant metric in the long-distance limit. We discuss under which conditions the static structure factor will assume the usual behavior of a Laughlin-like fractional quantum Hall system, i.e., the scenario of S. M. Girvin, A. H. MacDonald, and P. M. Platzman [Phys. Rev. B 33, 2481 (1986)]. We study the influence of the departure of the averaged over the Brillouin zone Fubini-Study metric from its fractional quantum Hall value which appears in the long-distance analysis as an effective change of the filling factor. According to our exact-diagonalization results on the Haldane model and analytical considerations we find persistence of the fractional Chern insulator state even in this region of the parameter space.

Anisotropic and inhomogeneous metamaterial shells are studied in order to exploit all their resonant mode richness. These multilayer structures are based on a cylindrical distribution of radially dependent constitutive parameters including an inner void cavity. Shell, cavity, and whispering gallery modes are characterized, and special attention is paid to the latter ones. The whispering gallery modes are created at the boundary layers of the shell with the background. Energy localization is produced with highly radiative characteristics when the localization takes place at the external layer. These low-$Q$ resonant states have frequencies that are independent of the shell thickness. However, their quality factors can be controlled by the number of layers forming the shell, which allows confining electromagnetic waves at the interface layers (internal or external), and make them suitable for the harvesting of electromagnetic energy.

Homogenization theory is used to derive the effective properties of gratings with complex subwavelength structures. Going beyond the effect of the filling fraction, geometrical effects are analyzed using a two-step homogenization process. An explicit expression for the transmission spectrum is derived, able to predict the Fabry-Perot resonances and the Brewster angle realizing broadband extraordinary transmission. With the same filling fraction, one expects from this analytical expression that gratings with different geometries may display very different transmission properties. This sensitivity to the microstructure geometry is exemplified in the case of gratings made of hard material and made of dielectric material. The analytical results are shown to be within a few percentage points as compared to full-wave numerical simulations, paving the way for transmission properties tuned by structural and geometrical manipulations.

Recent theoretical calculations and experimental results suggest that the strongly correlated material SmB${}_6$ may be a realization of a topological Kondo insulator. We have performed an angle-resolved photoemission spectroscopy study on SmB${}_6$ in order to elucidate elements of the electronic structure relevant to the possible occurrence of a topological Kondo insulator state. The obtained electronic structure in the whole three-dimensional momentum space reveals one electron-like $5d$ bulk band centered at the $X$ point of the bulk Brillouin zone that is hybridized with strongly correlated $f$ electrons, as well as the opening of a Kondo band gap ($\Delta$${}_B$ $\sim$ 20 meV) at low temperature. In addition, we observe electron-like bands forming three Fermi surfaces at the center $\overline{\Gamma }$ point and boundary $\overline{X}$ point of the surface Brillouin zone. These bands are not expected from calculations of the bulk electronic structure, and their observed dispersion characteristics are consistent with surface states. Our results suggest that the unusual low-temperature transport behavior of SmB${}_6$ is likely to be related to the pronounced surface states sitting inside the band hybridization gap and/or the presence of a topological Kondo insulating state.

We study the problem of disorder-free metals near a continuous quantum critical point. We depart from the standard paradigm [Hertz, Phys. Rev. B 14, 1165 (1976) and Millis, Phys. Rev. B 48, 7183 (1993)] and treat both fermions and bosons (i.e., order parameter fields) on equal footing. We construct a Wilsonian effective field theory that integrates out only high-energy boson and fermion modes. Below the upper critical dimension of the theory ($d=3$ spatial dimensions), we find new fixed points in which the bosons are described by the Wilson-Fisher fixed point and are coupled to a non-Fermi-liquid metal. We describe subtleties with the renormalization group flow of four-Fermi interactions, which can be surmounted in a controlled large-$N$ limit. In this limit, we find that the theory has no superconducting instability.

It was recently discovered that a transparent $n$-type (Ba,La)SnO${}_3$ system has electrical mobility as high as 320 cm${}^2$ V${}^{-1}$ s${}^{-1}$ at room temperature and superior thermal stability up to ∼500 °C. To understand comparatively the carrier-scattering mechanism in the doped BaSnO${}_3$, we investigate the physical properties of the single crystals of BaSn${}_{1-x}$Sb${}_x$O${}_3$ ($x$ $=$ 0.03, 0.05, and 0.10), which also show the $n$-type characters via the Sn site doping by Sb. Transmittance of the grown single crystals in the visible spectral region turn out to be similar to that of the (Ba,La)SnO${}_3$ system, maintaining optical transparency. Temperature-dependent Hall effect measurements reveal that the electrical mobility at room temperature reaches as high as 79.4 cm${}^2$ V${}^{-1}$ s${}^{-1}$ at a carrier density of 1.02×10${}^{20}$ cm${}^{-3}$, and upon increasing carrier density further, it systematically decreases nearly proportional to the inverse of the carrier density. The overall reduced mobility of the Ba(Sn,Sb)O${}_3$ system as compared to the (Ba,La)SnO${}_3$ system is attributed to the enhanced scattering caused by the Sb ions located in the direct conduction path. Based on the inverse proportionality between the carrier density and the electrical mobility, we suggest that the neutral impurity scattering becomes particularly strong in the Ba(Sn,Sb)O${}_3$.

Band gaps and band-edge energy levels are computed using the many-body perturbation theory within the $GW$ approximation, with four common plasmon pole models (PPMs) and numerical integration employed to evaluate the dynamic screening matrix. Although the Hybertsen-Louie PPM is often adopted in $GW$ calculations because it predicts band gaps best matching experimental data, we show that it is the Godby-Needs construction that agrees consistently with numerical integration on dynamic screening for materials with distinct characteristics. The variation in predicted band gaps due to different PPMs used can be as large as 1 eV in strongly localized electronic systems, and the band-edge energy levels are more sensitive to the choice of PPM than band gap even in simple semiconductors.