The random Zn${}_{1\text{−}x}$Be${}_x$Se zincblende alloy is known to exhibit a peculiar three-mode [$1\times$(Zn-Se),$2\times$(Be-Se)] vibration pattern near the Brillouin zone (BZ) center, of the so-called percolation type, apparent in its Raman spectra. This is due to an unusually large contrast between the physical properties (length, ionicity) of the constituting bonds. In the present work, the inelastic neutron scattering is applied to study the dispersion of modes away from the BZ center, with special attention to the $\vec{q}$ dependence of the BeSe-like transverse optic doublet. The discussion is supported by calculations of lattice dynamics done both ab initio (using the siesta code) and within the shell model. The BeSe-like doublet is found to survive nearly unchanged throughout the BZ up to the zone edge, indicating that its origin is at the ultimate bond scale. The microscopic mechanism of splitting is clarified by ab initio calculations. Namely, the local lattice relaxation needed to accommodate the contrast in physical properties of the Zn-Se and Be-Se bonds splits the stretching and bending modes of connected, i.e., percolativelike, (Be-Se) bonds.

Bismuth donors ion-implanted in ${}^{28}\mathrm{Si}$ and ${}^{\text{nat}}\mathrm{Si}$ are studied using magnetic resonance spectroscopy based on spin-dependent recombination. The hyperfine clock transition, at which the linewidth is significantly narrowed, is observed for the bismuth donors. The experimental results are modeled quantitatively by molecular orbital theory for a coupled pair consisting of a bismuth donor and a spin-dependent recombination readout center, including the effect of hyperfine and Zeeman interactions.

We find that interactions between intermolecular excited states can cause a line-shape narrowing in magnetophotoluminescence in an organic composite containing N,N-dimethylaniline and pyrene in the liquid state. The line-shape narrowing indicates that interactions between intermolecular excited states can decrease the spin-exchange interaction within intermolecular excited states. Our analysis shows that interactions between intermolecular excited states can occur through long-range Coulomb interaction, midrange spin-orbital interaction, and short-range spin interaction, with the consequence of line-shape modification in the development of magnetic field effects. Our experimental results reveal a parameter, the interactions between intermolecular excited states, involved in the development of magnetic field effects in organic semiconducting materials.

We report on resonant light scattering experiments in which a single InAs quantum dot is exposed to two independently tunable continuous-wave lasers while the scattered light is analyzed background free, with high spectral and temporal resolution. In contrast to the well-known monochromatic case exhibiting Rabi oscillations, in this bichromatic case the additional field introduces oscillations at half the difference of the laser frequencies and harmonics thereof, persisting beyond the natural lifetime. The familiar “dressed-states” ladder now contains an infinite number of levels per manifold. With suitable averaging, the rich resulting spectra and second-order correlations can be reproduced accurately using the Bloch equations and the quantum regression theorem.

The theoretical laser-flash method is used to calculate the thermal conductivity of the ${\text{Si}}_{200}{X}_{32}$ ($X=\text{H}$, D, or OH) and ${\text{Si}}_{296}{X}_{112}$ ($X=\text{H}$ or D) nanowires. The main emphasis is on the role of defects, which are described using first-principles methods. The defects considered are the surface of the nanowire, random distributions of substitutional C or Ge impurities, and monoatomic $\delta$ layers of C or Ge. The localized vibrational modes of these defects are explicitly included in the calculations and no empirical defect-related parameter is introduced. We find that the surface Si–H wag modes couple resonantly to each other much faster than they decay into bulk modes, which leads to distinct surface and bulk contributions to the thermal conductivity. The spatially-localized vibrational modes associated with the Ge or C impurities as well as the $\delta$ layers trap thermal phonons thus reducing the thermal conductivity.

We investigate the interplay of ferroelectricity and quantum electron transport at the nanoscale in the regime of Coulomb blockade. Ferroelectric polarization in this case is no longer the external parameter but should be self-consistently calculated along with electron hopping probabilities leading to physical transport phenomena studied in this paper. These phenomena appear mostly due to effective screening of a grain electric field by ferroelectric environment rather than due to polarization dependent tunneling probabilities. At small bias voltages polarization can be switched by a single excess electron in the grain. In this case transport properties of a single electron transistor exhibit the instability (memory effect).

Dynamical phase transitions (DPTs) occur after quenching some global parameters in quantum systems, and are signalled by the nonanalytical time evolution of the dynamical free energy, which is calculated from the Loschmidt overlap between the initial and time evolved states. In a recent Letter [M. Heyl et al., Phys. Rev. Lett. 110, 135704 (2013)], it was suggested that DPTs are closely related to equilibrium phase transitions (EPTs) for the transverse field Ising model. By studying a minimal model, the XY chain in a transverse magnetic field, we show analytically that this connection does not hold generally. We present examples where DPT occurs without crossing any equilibrium critical lines by the quench, and a nontrivial example with no DPT but crossing a critical line by the quench. Although the nonanalyticities of the dynamical free energy on the real time axis do not indicate the presence or absence of an EPT, the structure of Fisher lines for complex times reveals a qualitative difference.

We present a numerical method for the study of correlated quantum impurity problems out of equilibrium, which is particularly suited to address steady-state properties within dynamical mean field theory. The approach, recently introduced by Arrigoni et al. [Phys. Rev. Lett. 110, 086403 (2013)], is based upon a mapping of the original impurity problem onto an auxiliary open quantum system, consisting of the interacting impurity coupled to bath sites as well as to a Markovian environment. The dynamics of the auxiliary system is governed by a Lindblad master equation whose parameters are used to optimize the mapping. The accuracy of the results can be readily estimated and systematically improved by increasing the number of auxiliary bath sites, or by introducing a linear correction. Here, we focus on a detailed discussion of the proposed approach including technical remarks. To solve for the Green's functions of the auxiliary impurity problem, a non-Hermitian Lanczos diagonalization is applied. As a benchmark, results for the steady-state current-voltage characteristics of the single-impurity Anderson model are presented. Furthermore, the bias dependence of the single-particle spectral function and the splitting of the Kondo resonance are discussed. In its present form, the method is fast, efficient, and features a controlled accuracy.

Universal features in the scalings of Shannon-Rényi entropies of many-body ground states are studied for interacting spin-$\frac{1}{2}$ systems across (2+1)-dimensional $O\left(3\right)$ critical points, using quantum Monte Carlo simulations on dimerized and plaquettized Heisenberg models on the square lattice. Considering both full systems and line-shaped subsystems, SU(2) symmetry breaking on the Néel ordered side of the transition is characterized by the presence of a logarithmic term in the scaling of Shannon-Rényi entropies, which is absent in the disordered gapped phase. Such a difference in the scalings allows to capture the quantum critical point using Shannon-Rényi entropies for line-shaped subsystems of length $L$ embedded in $L\times L$ tori, as the smaller subsystem entropies are numerically accessible to much higher precision than for the full system. Most interestingly, at the quantum phase transition an additive subleading constant $b_{\infty }^{*\mathrm{line}}=0.41\left(1\right)$ emerges in the critical scaling of the line Shannon-Rényi entropy $S_{\infty }^{\mathrm{line}}$. This number appears to be universal for 3d $O\left(3\right)$ criticality, as confirmed for the finite-temperature transition in the 3d antiferromagnetic spin-$\frac{1}{2}$ Heisenberg model. Additionally, the phases and phase transition can be detected in several features of the participation spectrum, consisting of the diagonal elements of the reduced density matrix of the line subsystem. In particular, the Néel ordering transition can be simply understood in the $\left\{S^z\right\}$ basis by a confinement mechanism of ferromagnetic domain walls.

We present measurements of the resistivity ${\rho }_{x,x}$ of ${\mathrm{URu}}_2{\mathrm{Si}}_2$ high-quality single crystals in pulsed high magnetic fields up to 81 T at a temperature of 1.4 K and up to 60 T at temperatures down to 100 mK. For a field H applied along the magnetic easy axis c, a strong sample dependence of the low-temperature resistivity in the hidden-order phase is attributed to a high carrier mobility. The interplay between the magnetic and orbital properties is emphasized by the angle dependence of the phase diagram, where magnetic transition fields and crossover fields related to the Fermi surface properties follow a 1/$cos\theta$ law, $\theta$ being the angle between H and c. For $\mathbf{H}\parallel \mathbf{c}$, a crossover defined at a kink of ${\rho }_{x,x}$, as initially reported in [Shishido, Phys. Rev. Lett. 102, 156403 (2009)], is found to be strongly sample dependent: its characteristic field ${\mu }_0{H}^{*}$ varies from $\simeq$20 T in our best sample with a residual resistivity ratio $\mathrm{RRR}=$ ${\rho }_{x,x}\left(300\mathrm{K}\right)/$ ${\rho }_{x,x}\left(2\mathrm{K}\right)$ of 225 to $\simeq$25 T in a sample with a RRR of 90. A second crossover is defined at the maximum of ${\rho }_{x,x}$ at the sample-independent low-temperature (LT) characteristic field ${\mu }_0{H}_{\rho ,\max }^{\mathrm{LT}}\simeq 30$ T. Fourier analyses of Shubnikov-de Haas oscillations show that $H_{\rho ,\max }^{\mathrm{LT}}$ coincides with a sudden modification of the Fermi surface, while $H^{*}$ lies in a regime where the Fermi surface is smoothly modified. For $\mathbf{H}\parallel \mathbf{a}$, (i) no phase transition is observed at low temperature and the system remains in the hidden-order phase up to 81 T, (ii) quantum oscillations surviving up to 7 K are related to a new orbit observed at the frequency $F_{\lambda }\simeq 1350$ T and associated with a low effective mass $m_{\lambda }^{*}=(1\pm 0.5)m_0$, where $m_0$ is the free electron mass, and (iii) no Fermi surface modification occurs up to 81 T.

For ideal invisibility cloaks, electromagnetic radiations cannot penetrate through the cloaking shell. However, researchers have suggested that the originally hidden object with a cloaking shell is visible to the outside world when it is covered by an “anticloak.” In this paper, we give the first experimental verification of the “anticloak” for the Laplace equation, which requires negative conductivity profiles. The proposed device consists of two layers—the cloak and anticloak, which can work individually or collectively and give rise to different functions. Functionality switching among the invisibility cloak, anticloak, transparent cloak, superscatterer, and illusion is suggested and demonstrated. Based on the circuit theory, we fabricate a switching resistor network with both positive and negative resistors to emulate the cloaking and anticloaking interactions. The switchable cloaking, anticloaking, transparent, superscatterer, and illusion performances are confirmed by the experiments and/or numerical simulations.

The quantum phase diagram of disordered electron systems as a function of the concentration of magnetic impurities $n_m$ and the local exchange coupling $J$ is studied in the dilute limit. We take into account the Anderson localization of the electrons by a nonperturbative numerical treatment of the disorder potential. The competition between Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction $J_{\mathrm{RKKY}}$ and the Kondo effect, as governed by the temperature scale $T_K$, is known to give rise to a rich magnetic quantum phase diagram, the Doniach diagram. Our numerical calculations show that in a disordered system both the Kondo temperature $T_K$ and $J_{\mathrm{RKKY}}$ as well as their ratio $J_{\mathrm{RKKY}}/T_K$ is widely distributed. However, we find a sharp cutoff of that distribution, which allows us to define a critical density of magnetic impurities $n_c$ below which Kondo screening wins at all sites of the system above a critical coupling $J_c$, forming the Kondo phase [see Fig. 3(b)]. As disorder is increased, $J_c$ increases and a spin coupled phase is found to grow at the expense of the Kondo phase. From these distribution functions we derive the magnetic susceptibility which show anomalous power-law behavior. In the Kondo phase that power is determined by the wide distribution of the Kondo temperature, while in the spin coupled phase it is governed by the distribution of $J_{\mathrm{RKKY}}$. At low densities and small $J<J_c$ we identify a local-moment phase (LM). We also report results on a honeycomb lattice, graphene, where we find that the spin coupled phase is more stable against Kondo screening, but is more easily destroyed by disorder into a LM phase.

We show that fermion charge fractionalization can take place in a recently proposed chiral gauge model for graphene even in the absence of Kekulé distortion in the graphene honeycomb lattice. In this model, electrons couple in a chiral way to a pseudomagnetic field with a vortex profile in such a way that it can be used to describe the influences of topological defects, such as disclinations, on the electronic states. We also extend the model by adding the coupling of fermions to an external magnetic field and show that the fermion charge can be fractionalized by means of only gauge potentials. It is shown that the chiral fermion charge can also have fractional value. We also relate the fractionalization of the fermion charge to the parity anomaly in an extended quantum electrodynamics, which involves vector and axial-vector gauge fields.

We present a theory of the cavity quantum electrodynamics of the graphene cyclotron resonance. By employing a canonical transformation, we derive an effective Hamiltonian for the system comprising two neighboring Landau levels dressed by the cavity electromagnetic field (integer quantum Hall polaritons). This generalized Dicke Hamiltonian, which contains terms that are quadratic in the electromagnetic field and respects gauge invariance, is then used to calculate thermodynamic properties of the quantum Hall polariton system. Finally, we demonstrate that the generalized Dicke description fails when the graphene sheet is heavily doped, i.e., when the Landau level spectrum of two-dimensional massless Dirac fermions is approximately harmonic. In this case we “integrate out” the Landau levels in valence band and obtain an effective Hamiltonian for the entire stack of Landau levels in conduction band, as dressed by strong light-matter interactions.

The electronic excitations of bilayer graphene (BLG) under a magnetic field are investigated with the use of the Peierls tight-binding model in conjunction with random-phase approximation (RPA). The interlayer atomic interactions, interlayer Coulomb interactions, and magnetic field effects are simultaneously included in the dielectric-function matrix. That enables us to derive the magneto-Coulomb-excitation spectrum of different stacking structures. The two typical arrangements of BLGs, AB and AA, are considered in this article. AB-BLG exhibits many discrete energy-loss peaks, which correspond to the quantization of electron energies. On the other hand, the AA-BLG spectra possess a unique and pronounced peak at low frequency. This peak represents the collective excitation of the entire low-frequency Landau states. The dependence of the energy-loss peaks on the momentum transfer and the magnetic field strength is presented. Accordingly, two kinds of plasmon modes produced by the layer stacking are clearly distinguished.

It is known that while group VII 4$d$ Tc and 5$d$ Re have hexagonally close-packed (hcp) ground states, 3$d$ Mn adopts a complex $\chi$-phase ground state, exhibiting complex noncollinear magnetic ordering. Density functional theory (DFT) calculations have shown that without magnetism, the $\chi$ phase is still the ground state of Mn implying that magnetism and the resultant atomic-size difference between large- and small-moment atoms are not the critical factors, as is commonly believed, in driving the anomalous stability of the $\chi$ phase over hcp. Using a canonical tight-binding (TB) model, it is found that for a more than half-filled $d$ band, while harder potentials stabilize close-packed hcp, a softer potential stabilizes the more open $\chi$ phase. By analogy with the structural trend from open to close-packed phases down the group IV elements, the anomalous stability of the $\chi$ phase in Mn is shown to be due to 3$d$ valent Mn lacking $d$ states in the core which leads to an effectively softer atomic repulsion between the atoms than in 4$d$ Tc and 5$d$ Re. Subsequently, an analytic bond-order potential (BOP) is developed to investigate the structural and magnetic properties of elemental Mn at 0 K. It is derived within BOP theory directly from a new short-ranged orthogonal $d$-valent TB model of Mn, the parameters of which are fitted to reproduce the DFT binding energy curves of the four experimentally observed phases of Mn, namely, $\alpha$, $\beta$, $\gamma$, $\delta$, and $\epsilon$-Mn. Not only does the BOP reproduce qualitatively the DFT binding energy curves of the five different structure types, it also predicts the complex collinear antiferromagnetic (AFM) ordering in $\alpha$-Mn, the ferrimagnetic ordering in $\beta$-Mn, and the AFM ordering in $\gamma$-, $\delta$-, and $\epsilon$-Mn that are found by DFT. A BOP expansion including 14 moments is sufficiently converged to reproduce most of the properties of the TB model with the exception of the elastic shear constants, which require further moments. The current TB model, however, predicts values of the shear moduli and the vacancy formation energies that are approximately a factor of 2 too small, so that a future more realistic model for MD simulations will require these properties to be included from the outset in the fitting database.

We present a study on the existence of Dirac cones in two-dimensional phononic crystals. Specifically, we start from the phononic crystals made of rotatable regular hexagonal or triangular rods arranged in a hexagonal lattice. The additional freedom of the rotation enables the crystals with various symmetries. The numerical and experimental band structures manifest consistently that part of the symmetries can support Dirac cones stably, yet the others cannot. The phenomena are explained successfully by the k $·$ p perturbation theory combined with symmetry analysis. Based on this theoretical framework, a systemic exploration on symmetry is further performed, which yields a complete summary on the existence of Dirac cones. The conclusion also remains in the other types of artificial crystals for classical waves, either scalar or vectorial ones.

A superconducting-to-magnetic transition is reported for $\mathrm{La}\mathrm{Fe}\mathrm{As}{\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}$ where a per-thousand amount of Mn impurities is dispersed. By employing local spectroscopic techniques like muon spin rotation ($\mu$SR) and nuclear quadrupole resonance (NQR) on compounds with Mn contents ranging from $x=0.025$% to $x=0.75\%$, we find that the electronic properties are extremely sensitive to the Mn impurities. In fact, a small amount of Mn as low as 0.2% suppresses superconductivity completely. Static magnetism, involving the FeAs planes, is observed to arise for $x>0.1$% and becomes further enhanced upon increasing Mn substitution. Also a progressive increase of low-energy spin fluctuations, leading to an enhancement of the NQR spin-lattice relaxation rate $T_1^{-1}$, is observed upon Mn substitution. The analysis of $T_1^{-1}$ for the sample closest to the crossover between superconductivity and magnetism ($x=0.2$%) points toward the presence of an antiferromagnetic quantum critical point around that doping level.

We report on the possibility of controlling the maximum demagnetization induced by a femtosecond laser pulse in a CoFeB-based magnetic tunnel junction, by tuning the external bias applied to the junction. We explain this effect in terms of laser-induced spin-polarized currents which can be controlled by tuning the conductivity of the junction. This demonstrates the contribution of spin-polarized currents for laser-induced demagnetization and may pave the way to increase the speed of spintronic devices, using ultrashort laser pulses.