We use a new technique that allows in situ variation of uniaxial stress to probe the polarization dependence of the optical reflectivity of the representative underdoped iron-arsenide $\text{Ba(}{\text{Fe}}_{1-x}{\text{Co}}_x{\text{)}}_2{\text{As}}_2$ through the tetragonal-to-orthorhombic structural transition and with respect to their electronic nematic phase. These measurements reveal a hysteretic behavior of the anisotropic optical response to uniaxial stress in the orthorhombic state associated with twin boundary motion, whereas the pressure-induced anisotropy of the optical response is reversible in the tetragonal state.

We investigate atomic ordering in fcc Ni-rich Ni-Cr alloys using first-principles techniques and statistical mechanics simulations based on the Ising Hamiltonian with effective cluster interactions computed by the screened generalized perturbation method (SGPM) and projector augmented wave (PAW) method. We demonstrate that effective chemical interactions in this system are quite sensitive to alloy composition and in fact to the specific configurational state. The chemical interactions for the high-temperature random state produce the atomic short-range order (SRO) with intensity maximum close to the $\left(\frac{2}{3}\frac{2}{3}0\right)$ point of the reciprocal space in agreement with the previous first-principles investigation. A consistent with diffuse neutron scattering data maximum at the $\left(1\frac{1}{2}0\right)$ position is obtained only when we take into consideration relatively small strain-induced interactions, which solves a long-standing inconsistency between theory and experiment in this system. The calculated transition temperature of order-disorder transition of Ni${}_2$Cr alloy, 880 K, is in good agreement with the experimental value of 863 K.

Using first-principles density functional calculations, electronic and optical properties of ferromagnetic semiconductor EuO are investigated. In particular, we have developed a way to obtain the spin-dependent optical response of the magnetic materials, which is helpful to verify the spin-dependent band structure of EuO. Significantly different optical responses from spin-up and spin-down channels are obtained in both linear and nonlinear cases, making it possible to distinguish contributions from different spin channels in the optical absorption spectra if the spin-flip process can be neglected. In addition, the red-shift of the absorption edge from paramagnetic to ferromagnetic ordering is explained by exchange interactions. Using such a method, we have also compared the optical properties of multiferroic EuO which is induced by strong epitaxial strain. Our results show that from tensile to compressive strain, the blue-shift of the leading absorption peaks in the optical spectra, the red-shift of the optical band gap in the spin-up state can be observed, consistent to the energy difference between spin-splitting orbits. The spin-dependent nonlinear optical properties reveal that in the infrared and visible light regions, the contributions to second-harmonic generation (SHG) susceptibilities are mainly from spin-majority channels. In addition, the strain effect is also discussed. With the increase of epitaxial strain, the larger energy shift of the leading absorption peaks and the more remarkable nonlinear optical response can be obtained.

The Kondo cloud, represented by the correlations between the magnetic moment and the spin density in the leads of a Kondo setup, has until now eluded observation. We exploit the unique coupling of spin and direction of motion of the excitations in the recently discovered helical liquids in a setup with two leads to establish a proportionality between the Kondo cloud and the time-resolved current cross correlations. This relation holds around a specific choice of model parameters. Thereby, we propose a direct way to detect the Kondo cloud in a least invasive manner since the current cross correlations are measurable far away from the magnetic moment. We furthermore discuss how our predictions are modified if the model parameters are varied away from the specific choice.

The excitonic spectrum of radially polarized semiconductor rings has been analyzed theoretically, in the presence of an in-plane electric field and a perpendicular magnetic field. Based on the numerically exact solution, a regime has been found where the exciton behaves as a single carrier or quasiparticle, with an effective and tunable electric charge determined by the ring geometry. A protocol is proposed for the storage of excitons without destroying them, consisting in converting them from “bright” to “dark,” by performing a sequence of well-defined steps. Accurate analytical approximations are provided for each of the exciton regimens found: quasifree, locked, and broken.

The ${\text{LiNbO}}_3$(2$\overline{1}\overline{1}0$) surface, commonly referred to as X-cut, is investigated by means of atomic force microscopy and first-principles calculations. Atomically resolved atomic force microscopy images show geometrical patterns not compatible with truncated bulk terminations. Fast Fourier transformation of the real-space images shows an oblique surface unit cell with lattice parameters of $a=0.75\pm 0.02$ nm, $b=0.54\pm 0.02$ nm, and $\alpha =94.8^{\circ }$. Comparing these experimental results with the theoretical models of stable surface terminations provides clear evidence for the formation of a -${\text{Li}}_{12}$ termination. The LN X-cut is, thus, characterized by a nonstoichiometric and Li-enriched composition. An analysis of the surface electronic charge indicates that the atomic force microscopy contrast is governed by the charge accumulation at the oxygen ions.

We have studied structures of the chiral amino acid alanine adsorbed on Cu(110) via low-energy electron diffraction (LEED) as well as scattering of fast light atoms and molecules. The adsorption process was controlled in situ by the intensity of specularly reflected 2-keV He atoms. For projectile energies less than 1 keV, we applied the method of fast atom diffraction for studies on the structure of adsorbed alanine molecules on an atomically flat Cu(110) surface with focus on a $p$$(3\times 2)$ adsorbate phase. The results are consistent with LEED and explain distortions in LEED patterns via an elongated surface unit cell with incommensurate $c$$(3.16\times 2)$ symmetry of parts of the adsorbate. From triangulation using fast atoms via the azimuthal rotation of the target surface, the positions of protruding methyl groups are derived.

The dynamics of excited charge carriers at the graphene/Ni(111) interface has been investigated by means of time-resolved two-photon photoemission spectroscopy, employing femtosecond-XUV pulses with an energy of 39.2 eV produced by high-order-harmonic generation. Due to the interplay of substrate and adsorbate band structures, the dependence of the lifetimes on the energy $(E-E_F)$ of the excited carriers was found to be similar to that of Ni $3d$ electrons measured for clean Ni in the energy range $(E-E_F)<1$ eV, while it resembled that of graphite from 1 eV above $E_F$ onwards. This result is suggested to be the effect of the peculiar electronic structure of the interface, which still possesses features belonging to the pristine graphene layer, such as a residual saddle point.

We study the critical properties of the quantum anomalous Hall (QAH) plateau transition in magnetic topological insulators. We introduce a microscopic model for the plateau transition in the QAH effect at the coercive field and then map it to the network model of quantum percolation in the integer quantum Hall effect plateau transition. Generally, an intermediate plateau with zero Hall conductance could occur at the coercive field. ${\sigma }_{xx}$ would have double peaks at the coercivity while ${\rho }_{xx}$ only has single peak. Remarkably, this theoretical prediction is already borne out in experiment. Universal scaling of the transport coefficients ${\rho }_{xy}$ and ${\rho }_{xx}$ are predicted.

We find a characteristic low-energy peak structure located in the range 100–300 cm${}^{-1}$ in the optical conductivity spectra of a quasi-two-dimensional organic compound with a triangular lattice, $\theta$-${\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{CsZn}\left(\mathrm{SCN}\right)}_4$, in which two different types of short-range charge orderings (COs) coexist. Upon lowering the temperature, the low-energy peak becomes significant and shifts to much lower frequencies only for the polarization of $\mathit{E}\parallel \mathit{a}$, in contrast to the other broad electronic bands in the midinfrared region. On introducing disorder, the low-energy peak is strongly suppressed in comparison with the broad electronic bands. This result indicates that the low-energy peak is attributed to a collective excitation that originates from the short-range CO with a relatively long-period $3\times 3$ pattern. The present results shed light on the understanding of the low-energy excitation in the glassy electronic state, where the charge degrees of freedom remain at low temperatures.

The realization of a high spin state, which is related to ferromagnetism and molecular magnetism, has been an attractive research topic. Here, using the nonequilibrium condition, we realize a high population of the quadruplet states (total spin $S$ = 3/2) in a double quantum dot containing three electrons. Owing to Pauli exclusion, the quadruplet states are forbidden to transit to states with electron double occupancy. Thus, it is located at the end of the dead-end path in a charge transfer cycle, and the three-electron state is stacked at the quadruplet state once it is accessed. As a result of the high population in the three-electron quadruplet spin states, current suppression is observed even under a high bias, which was also reproduced by a simple theoretical model.

We consider magnetic breakdown in twisted bilayer graphene where electrons may hop between semiclassical $k$-space trajectories in different layers. These trajectories within a doubled Brillouin zone (BZ) constitute a network in which an $S$ matrix at each saddle point (SP) is used to model interlayer tunneling. Although semiclassical orbits at a given energy all have the same shape, we find that the doubled BZ supports two distinct quantization conditions above the SP, versus only one below it, yielding a more intricate spectrum. Possible experimental signatures are discussed.

I comment on two recent papers on the Kerr effect as evidence of gyrotropic order in cuprates, and I suggest that the arguments may not be sound. The difficulty is that, in practically all cases, the wave vector $k_z$ perpendicular to the copper-oxygen plane is not a good quantum number. This appears to be problematic for Orenstein and Moore [Phys. Rev. B 87, 165110 (2013)], whereas, in Hosur et al. [Phys. Rev. B 87, 115116 (2013)], the symmetry arguments may turn out to be robust, but the microscopic picture is wanting. Thus, the Kerr effect in cuprates remains a puzzle as there is little doubt that the arguments presented against time-reversal symmetry breaking appear to be rather strong on experimental grounds in both of these papers.

In response to the Comment of Chakravarty, we explain that interplane incoherence is unlikely to invalidate the Berry phase mechanism of gyrotropy in metals, although it will certainly modify the observed magnitude of the effect.

In this paper, we present a new noninvasive and nondestructive approach to recover scattering and absorption coefficients from reflectance measurements of highly absorbing and optically inhomogeneous media. Our approach is based on the Yang and Miklavcic theoretical model of light propagation through turbid media, which is a generalization of the Kubelka-Munk theory, extended to accommodate optically thick samples. We show its applications to paper, a material primarily composed of a web of fibers of cellulose, whose optical properties are strongly governed by light scattering effects. Samples studied were ancient and industrial paper sheets, aged in different conditions and highly absorbing in the ultraviolet region. The recovered experimental absorptions of cellulose fibers have been compared to theoretical ab initio quantum-mechanical computational simulations carried out within time-dependent density functional theory. In this way, for each sample, we evaluate the absolute concentration of different kinds of oxidized groups formed upon aging and acting as chromophores causing paper discoloration. We found that the relative concentration of different chromophores in cellulose fibers depends on the aging temperature endured by samples. This clearly indicates that the oxidation of cellulose follows temperature-dependent reaction pathways. Our approach has a wide range of applications for cellulose-based materials, like paper, textiles, and other manufactured products of great industrial and cultural interest, and can potentially be extended to other strongly absorbing inhomogeneous materials.

Quantum transport in a three-dimensional Weyl (massless Dirac) electron system with long-range Gaussian impurities is studied theoretically using a self-consistent Born approximation (SCBA). We find that the conductivity significantly changes its behavior at a certain critical disorder strength which separates the weak and strong disorder regimes. In the weak disorder regime, the SCBA conductivity mostly agrees with the Boltzmann conductivity, except for the Weyl point (the band touching point) at which the SCBA conductivity exhibits a sharp dip. In the strong disorder regime, the Boltzmann theory fails in all the energy regions and the conductivity becomes larger the disorder potential increases, contrary to the usual metallic behavior. At the Weyl point, the conductivity and the density of states are exponentially small in the weak disorder regime, and they abruptly rise at the critical disorder strength. The qualitative behavior near zero energy is well described by an approximate analytic solution of the SCBA equation. The theory applies to three-dimensional gapless band structures including Weyl semimetals.

Cylindrical BaTiO${}_3$ nanorods embedded in $\langle 100\rangle$-oriented SrTiO${}_3$ epitaxial film in a brushlike configuration are investigated in the framework of the Ginzburg-Landau-Devonshire model. It is shown that strain compatibility at BaTiO${}_3$/SrTiO${}_3$ interfaces keeps BaTiO${}_3$ nanorods in the rhombohedral phase even at room temperature. Depolarization field at the BaTiO${}_3$/SrTiO${}_3$ interfaces is reduced by an emission of the 109${}^{\circ }$ or 71${}^{\circ }$ domain boundaries. In case of 10–80-nm diameter nanorods, the ferroelectric domains are found to form a quadruplet with a robust flux-closure arrangement of the in-plane components of the spontaneous polarization. The out-of-plane components of the polarization are either balanced or oriented up or down along the nanorod axis. Switching of the out-of-plane polarization with coercive field of about $5\times {10}^6$ V/m occurs as a collapse of a 71${}^{\circ }$ cylindrical domain boundary formed at the curved circumference surface of the nanorod. The remnant domain quadruplet configuration is chiral, with the $C_4$ macroscopic symmetry. More complex stable domain configurations with coexisting clockwise and anticlockwise quadruplets contain interesting arrangement of strongly curved 71${}^{\circ }$ boundaries.

We report on spherical neutron polarimetry and unpolarized neutron diffraction in zero magnetic field as well as flipping ratio and static magnetization measurements in high magnetic fields on the multiferroic square lattice antiferromagnet ${\text{Ba}}_2{\text{CoGe}}_2{\text{O}}_7$. We found that in zero magnetic field the magnetic space group is $C{m}^{\prime }m{2}^{\prime }$ with sublattice magnetization parallel to the [100] axis of this orthorhombic setting. The spin canting has been found to be smaller than $0.2^{\circ }$ in the ground state. This assignment is in agreement with the field-induced changes of the magnetic domain structure below 40 mT as resolved by spherical neutron polarimetry. The magnitude of the ordered moment has been precisely determined. Above the magnetic ordering temperature short-range magnetic fluctuations are observed. Based on the high-field magnetization data, we refined the parameters of the recently proposed microscopic spin model describing the multiferroic phase of ${\text{Ba}}_2{\text{CoGe}}_2{\text{O}}_7$.

We study theoretically the electrical current and low-frequency noise for a linear Josephson junction structure on a topological insulator, in which the superconductor forms a closed ring and currents are injected from normal regions inside and outside the ring. We find that this geometry offers a signature for the presence of gapless one-dimensional Majorana fermion modes that are predicted in the channel when the phase difference $\phi$, controlled by the magnetic flux through the ring, is $\pi$. We show that for low temperature the linear conductance jumps when $\phi$ passes through $\pi$, accompanied by nonlocal correlations between the currents from the inside and outside of the ring. We compute the dependence of these features on temperature, voltage, and linear dimensions, and discuss the implications for experiments.