We present a systematic study on the influence of Rashba spin-orbit coupling, interlayer potential difference, and exchange field on the topological properties of bilayer graphene. In the presence of only Rashba spin-orbit coupling and interlayer potential difference, the band gap opening due to broken out-of-plane inversion symmetry offers new possibilities of realizing tunable topological phase transitions by varying an external gate voltage. We find a two-dimensional $Z_2$ topological insulator phase and a quantum valley Hall phase in AB-stacked bilayer graphene and obtain their effective low-energy Hamiltonians near the Dirac points. For AA stacking, we do not find any topological insulator phase in the presence of large Rashba spin-orbit coupling. When the exchange field is also turned on, the bilayer system exhibits a rich variety of topological phases including a quantum anomalous Hall phase, and we obtain the phase diagram as a function of the Rashba spin-orbit coupling, interlayer potential difference, and exchange field.

We calculate the Andreev conductance of a superconducting ring interrupted by a flux-biased Josephson junction, searching for electrical signatures of circulating edge states. Two-dimensional pair potentials of spin-singlet $d$-wave and spin-triplet $p$-wave symmetry support, respectively, (chiral) Dirac modes and (chiral or helical) Majorana modes. These produce $h/e$-periodic magnetoconductance oscillations of amplitude $\simeq (e^2/h)N^{-1/2}$, measured via an $N$-mode point contact at the inner or outer perimeter of the grounded ring. For Dirac modes the oscillations in the two contacts are independent, while for an unpaired Majorana mode they are phase locked by a topological phase transition at the Josephson junction.

We study molecular beam epitaxy growth, morphology, crystal structure, and magnetic properties of Co nanoislands on CaF${}_2$/Si(111) surface. In order to have a full appreciation of complex growth kinetics at different stages, a comprehensive study of Co growth on CaF${}_2$ is carried out by atomic force, scanning electron, and transmission electron microscopies in the direct space, as well as by x-ray and electron diffraction in the reciprocal space. These experimental data are complemented by theoretical modeling. Magnetic properties are characterized by magneto-optical Kerr effect and superconducting quantum interference device magnetometries. Key effects influencing the Co growth on fluorite are addressed, including the sticking probability, the preferential nucleation sites, the size and shape time evolution, the dependence of Co morphology on temperature and Co exposure, and the coalescence mechanism. The two-stage deposition technique is developed, whereby the low-temperature seeding stage is used to facilitate Co nucleation, and the follow-up high-temperature deposition yields Co particles with high crystalline quality. Our results enable precise control over the resulting morphology, spatial ordering, and crystal structure affecting the magnetic properties. In particular, it is demonstrated that the transformation from dense to isolated Co nanoparticles leads to the change of the in-plane and out-of-plane magnetic anisotropy and also the sign of polar and longitudinal magneto-optical Kerr effects.

We consider the quantum evolution and relaxation of an electron or hole which is coupled to a set of bath modes. In most applications the bath modes would be the vibronic coordinates but the model considered applies to any type of dynamic boson environment. The method is developed specifically for the problem of dynamic polaron formation in small nonperiodic systems. It can describe a broad group of experimental situations, including in particular electron localization in organics and polymeric materials and devices. The immediate bath is allowed to dissipate energy to a secondary bath. The bath obeys classical dynamics which puts some restriction on the range of validity of this approach. Using the density matrix formalism on a tight binding model consisting of a linear chain coupled to vibronic coordinates, we demonstrate in real time how the interaction with a dissipative bath makes the initial quantum distribution reach a steady-state population. This calculation is based on the Ehrenfest dynamics approximation. As an example we consider coupling at a single impurity site and find that for given parameters (bath coupling, site energy, and relaxation rate), the particle becomes dynamically localized in space on a particular time scale. This localized particle can be called a polaron. We define a population formation time in the same way as done in the experimental measurement. This formation time is studied as a function of the coupling strength, bandwidth, and energy dissipation rate. Energy dissipation plays a crucial role in the spatial localization process. The formation time shortens as the electron-vibration coupling increases, and as the intersite tunneling increases, but lengthens with impurity trap depth. Polaron formation is suppressed for sufficiently wide electronic bands.

We present a combined experimental and theoretical study of the di-exchange spin waves in layered structures constituted by permalloy(30 nm)/Cu(10 nm)/permalloy ($d$)/Cu(10 nm)/permalloy (30 nm), which differ by the variable thickness $d$ of the middle permalloy film (with $d=15$, 30, 60 nm). Both the plane layered films and two sets of stripes having widths of 150 and 400 nm were studied. The spin-wave dispersions (frequency vs wave vector) were measured by Brillouin light scattering and the experimental data interpreted using a microscopic (Hamiltonian-based) theory that takes into account the magnetic dipole-dipole and exchange interactions within each stripe and the dipole-dipole coupling between stripes of the stack. Overall, good agreement is found, showing that modes of the layered films constituted by surface dipolar and exchange-dominated waves give rise to complicated spectra in the case of layered stripes because of the lateral confinement. The role of the middle-layer thickness $d$ in the determination of the perpendicular profile of the spin modes is analyzed in detail.

The upper critical field $H_{c2}$ of optimally doped, iron-based superconductor SrFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ ($x$ $=$ 0.35, $T_c$ $=$ 25 K) was measured as a function of temperature down to 1.6 K for two principal directions of magnetic field $H\parallel c$ and $H\parallel a$. Measurements were performed in pulsed magnetic fields up to 65 T using a tunnel-diode resonator technique on as-grown and heavy-ion-irradiated single crystals, with columnar defect density corresponding to a matching field $B_{\varphi }$ $=$ 25 T. The $H_{c2,c}\left(T\right)$ is close to $T$ linear, while clear saturation is observed for $H_{c2,a}\left(T\right)$, leading to a strongly temperature-dependent anisotropy parameter $\gamma$. The linear shape of $H_{c2,c}\left(T\right)$ is very similar to that observed in nodal KFe${}_2$As${}_2$, but very different from full-gap LiFeAs. Irradiation does not introduce any additional features on the $H_{c2}\left(T\right)$ line corresponding to the matching field. Instead, it suppresses uniformly both $T_c$ and $H_{c2}$, keeping their ratio constant.

We reconsider the effect of disorder on the properties of a superconductor characterized by a sign-changing order parameter appropriate for Fe-based materials. Within a simple two-band model, we calculate simultaneously $T_c$, the change in residual resistivity $\Delta {\rho }_0$, and the zero-energy density of states, and show how these results change for various types of gap structure and assumptions regarding the impurity scattering. The rate of $T_c$ suppression is shown to vary dramatically according to details of the impurity model considered. We search therefore for a practical, experimentally oriented signature of a gap of the $s_{\pm }$ type, and propose that observation of a particular evolution of the penetration depth, nuclear magnetic resonance relaxation rate, or thermal conductivity temperature dependence with disorder would suffice.

We explore the stability of structures exhibiting hybridization gaps across a broad range of binary and ternary intermetallic compositions by means of band structure and total energy calculations. This search reveals previously unknown metal-based insulators, some with large gaps exceeding 1 eV, such as Al${}_2$Fe and Al${}_4$IrRe. We confirm large gaps using a hybrid density functional including exact exchange, and predict a gap of 2.2 eV for AlMnSi in the Pearson type tP6 structure, which is a chemically ordered ternary variant of the prototype MoSi${}_2$ (Pearson type tI6) structure.

The prediction that the piezoelectric tensor is diverging at the critical point is verified for BaTiO${}_3$ (BTO), which is the oldest perovskite structured ferroelectric material with an extremely long and eventful research history. Here we investigate experimentally by dielectric and calorimetric measurements the existence and the position of the critical point in the electric-field-temperature ($E$-$T$) phase diagram of BTO in the vicinity of the paraelectric to ferroelectric phase transition. Measurements of the piezoelectric coefficient $d_{31}$ as a function of the temperature and the electric field applied along the [001] direction show a critical enhancement of the piezoelectric response in the vicinity of the critical point, in agreement with recent calculations by Porta et al. [J. Phys.: Condens. Matter 22, 345902 (2010)]. The electrocaloric responsivity is found to be enhanced due to the latent heat on the paraelectric to ferroelectric transition locus below the critical point.

Starting from a model Hamiltonian for the normal state of the topological insulator Bi${}_2$Se${}_3$, we construct a pseudospin basis for the single-particle wave functions. Considering weak superconducting pairing near the Fermi surface, we express the recently proposed superconducting order parameters for Cu-doped Bi${}_2$Se${}_3$ in this basis. For the odd-parity states, the $\vec{d}\left(\vec{k}\right)$ vectors specifying the order parameter can have unusual momentum $\vec{k}$ dependence for certain parameter regimes. Some peculiar results in the literature for surface states are discussed in light of the forms of these $\vec{d}\left(\vec{k}\right)$'s. Properties of the even-parity states are also illuminated using this pseudospin basis. Results from this single-band description are compared with those from the full two-band model.

Using polarized neutron reflectometry (PNR) we have investigated a [${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$(10 nm)/${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_3$(9 nm)]${}_{10}$ (YBCO/LCMO) superlattice grown by pulsed laser deposition on a ${\mathrm{La}}_{0.3}{\mathrm{Sr}}_{0.7}{\mathrm{Al}}_{0.65}{\mathrm{Ta}}_{0.35}{\mathrm{O}}_3$ (LSAT) substrate. Due to the high structural quality of the superlattice and the substrate, the specular reflectivity signal extends with a high signal-to-background ratio beyond the fourth-order superlattice Bragg peak. This allows us to obtain more detailed and reliable information about the magnetic depth profile than in previous PNR studies on similar superlattices that were partially impeded by problems related to the low-temperature structural transitions of the SrTiO${}_3$ substrates. In agreement with the previous reports, our PNR data reveal a strong magnetic proximity effect showing that the depth profile of the magnetic potential differs significantly from the one of the nuclear potential that is given by the YBCO and LCMO layer thickness. We present fits of the PNR data using different simple blocklike models for which either a large ferromagnetic moment is induced on the YBCO side of the interfaces or the ferromagnetic order is suppressed on the LCMO side. We show that a good agreement with the PNR data and with the average magnetization as obtained from dc magnetization data can only be obtained with the latter model where a so-called depleted layer with a strongly suppressed ferromagnetic moment develops on the LCMO side of the interfaces. We also show that the PNR data are still compatible with the presence of a small, ferromagnetic Cu moment of $0.25{\mu }_{\mathrm{B}}$ on the YBCO side that was previously identified with x-ray magnetic circular dichroism and x-ray resonant magnetic reflectometry measurements on the same superlattice [D. K. Satapathy et al., Phys. Rev. Lett. 108, 197201 (2012)]. We discuss that the depleted layer thus should not be mistaken with a “dead” layer that is entirely nonmagnetic but rather may contain a canted antiferromagnetic or an oscillatory type of ordering of the Mn moments that is not detected with the PNR technique.

We study the nature of $\left(001\right)$ surface states in Pb${}_{0.73}$Sn${}_{0.27}$Se in the newly discovered topological-crystalline-insulator (TCI) phase as well as the corresponding topologically trivial state above the band-gap-inversion temperature. Our calculations predict not only metallic surface states with a nontrivial chiral spin structure for the TCI case, but also nonmetallic (gapped) surface states with nonzero spin polarization when the system is a normal insulator. For both phases, angle- and spin-resolved photoelectron spectroscopy measurements provide conclusive evidence for the formation of these $\left(001\right)$ surface states in Pb${}_{0.73}$Sn${}_{0.27}$Se, as well as for their chiral spin structure.

Valence instability is a key ingredient of the unusual properties of $f$ electron materials, yet a clear understanding is lacking as it involves a complex interplay between $f$ electrons and conduction states. Here we propose a unified picture of pressure-induced valence transition in Sm and Yb monochalcogenides, considered as a model system for mixed valent $4f$-electron materials. Using high-resolution x-ray-absorption spectroscopy, we show that the valence transition is driven by the promotion of a $4f$ electron specifically into the lowest unoccupied (LU) $5d$ $t_{2g}$ band. We demonstrate with a promotional model that the nature of the transition at low pressures is intimately related to the density of states of the LU band, while at high pressures it is governed by the hybridization strength.

We study the $n=2$ Rényi entanglement entropy of the triangular quantum dimer model via Monte Carlo sampling of Rokhsar-Kivelson- (RK-) like ground-state wave functions. Using the construction proposed by Kitaev and Preskill [Phys. Rev. Lett. 96, 110404 (2006)] and an adaptation of the Monte Carlo algorithm described by Hastings et al. [Phys. Rev. Lett. 104, 157201 (2010)], we compute the topological entanglement entropy (TEE) at the RK point $\gamma =(1.001\pm 0.003)\ln 2$, confirming earlier results. Additionally, we compute the TEE of the ground state of a generalized RK-like Hamiltonian and demonstrate the universality of TEE over a wide range of parameter values within a topologically ordered phase approaching a quantum phase transition. For system sizes that are accessible numerically, we find that the quantization of TEE depends sensitively on correlations. We characterize corner contributions to the entanglement entropy and show that these are well described by shifts proportional to the number and types of corners in the bipartition.

The many-body Coulomb repulsive energy of strictly correlated electrons provides direct information on the exact Hohenberg-Kohn exchange-correlation functional in the strong interaction limit. Until now the treatment of strictly correlated electrons has been based on the calculation of comotion functions with the help of semianalytic formulations. This procedure is system-specific and has been limited to spherically symmetric atoms and strictly one-dimensional systems. We develop a nested optimization method which solves the Kantorovich dual problem directly, and thus facilitates a general treatment of strictly correlated electrons for systems including atoms and small molecules.

A new mechanism for the generation of efficient zero phase delay of electromagnetic wave propagation based on wavefront modulation is investigated in this paper. Both numerical simulations and experiments have demonstrated the zero phase delay behaviors of wave propagation in a two-dimensional triangular photonic crystal. The zero phase delay propagation, independent of the propagating distance, is attributed to an invariable wavefront modulation in a photonic crystal in the direction of wave propagation where the phase velocity is perpendicular to the group velocity with parallel wavefronts (or phasefronts) extending along the direction of the energy flow. This effect can be extended to the three-dimensional cases or other artificially engineered materials and may open a new route for obtaining perfect zero phase delay propagation for an electromagnetic wave instead of using zero-index or zero-averaged-index materials and may have significant potential in many applications.

We investigate experimentally and theoretically the highly direction-selective emission with small angular divergence in a metal-dielectric-metal structure with a subwavelength metal aperture layer. The thicknesses of the dielectric layer and top metal layer play important roles in controlling the emission direction and angular divergence, respectively. Dispersion curve calculations based on the transfer matrix method indicate that the directional emission is mediated by radiative waveguide modes. We show that the directional emission in a metal-dielectric-metal structure is independent of the polarization of the incident light in contrast to plasmonic beaming structures, such as a subwavelength aperture surrounded by surface corrugations with a strong polarization dependence.

We study fermions in two dimensions interacting via a long-ranged $1/r$ potential for small particle separations and a short-ranged $1/r^3$ potential for larger separations in comparison to a length scale $\xi$. We compute the energy of the Wigner crystal and of the homogeneous Fermi liquid phases using a variational approach, and determined the phase diagram as a function of density and $\xi$ at zero temperature. We discuss the collective modes in the Fermi liquid phase, finite temperature effects on the phase diagram, and possible experimental realizations of this model.

In the present work, we studied the electronic and optical properties of ZnO quantum dots (QDs) subjected to externally applied hydrostatic pressure. Our single-particle calculations are based on the empirical pseudopotential method and the excitonic effects are considered by employing the configuration interaction approach. The optical band gap, Stokes shift, and optical emission polarization have been investigated as a function of the applied pressure. It is found that the applied pressure causes a linear increase in the optical band gap. The pressure coefficient appears to be highly size dependent, exhibiting a monotonic increase with increasing QD size. In contrast to this monotonic behavior, the applied pressure induces a nonmonotonic Stokes shift which presents a minimum value at a critical pressure. For pressures larger than this critical value, the optical emission polarization exhibits a sharp transition from in-plane to out-of-plane polarization. Finally, it is found that the critical pressure at which the crossing takes place strongly depends on the QD size, showing larger values for larger QD sizes. Beyond this crossing point, the lowest optically bright exciton state mainly originates from one Slater determinant, where both the single-particle electron and hole states have an $S$-type envelope function and the hole state originates mainly from the bulk Bloch C band.