The large in-plane anisotropy of the resistivity is a hallmark of the nematic state of the iron pnictides. Solving the Boltzmann transport equation, we show that the prominent doping dependence as well as the large values of the anisotropy can be well explained by momentum-dependent spin-fluctuation scattering without assuming anisotropic impurity states. Due to the forward-scattering corrections, the hot spots contribute to the resistive anisotropy even in the case of strong spin fluctuations, which makes large values of the anisotropy possible. The ellipticity of the electron pockets plays an important role in explaining the dominance of positive values of the anisotropy, i.e., larger resistivity in the direction with weaker spin fluctuations, throughout the doping range.

Knowledge of the electron sampling depth is important for quantitative analysis of x-ray absorption spectroscopy data, yet for oxides with the perovskite structure no quantitative values are so far available. Here, we study absorption saturation in films of two of the most-studied perovskites, ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ (LCMO) and ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ (YBCO), at the $L_{2,3}$ edges of Mn and Cu, respectively. By measuring the electron-yield intensity as a function of photon incidence angle and film thickness, the sampling depth $d$, photon attenuation length λ, and ratio $\lambda /d$ have been independently determined between 50 and 300 K. The extracted sampling depth $d_{\mathrm{LCMO}}\approx 3$ nm for LCMO films at high temperatures in the polaronic insulator state (150–300 K) is near the values reported for some transition metals ($d_{\mathrm{X}}=1.7–2.5$ nm, where X = Fe, Co, Ni) at room temperature, and it is much smaller than $d_{\mathrm{YBCO}}\approx 5$ nm measured for YBCO films and the value previously reported for ${\mathrm{Fe}}_3{\mathrm{O}}_4$ $(d_{\mathrm{Fe}3\mathrm{O}4}=4.5\text{nm})$. The measured $d_{\mathrm{LCMO}}$ increases to 4.5 nm when LCMO is in the metallic state at low temperatures. These results indicate that the sampling depth in oxides is strongly material dependent and can be measurably influenced by electronic phase transitions deriving from strong correlations.

We report ${}^{209}\mathrm{Bi}$ nuclear magnetic resonance spectra in nanowires and single crystals of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Crystals were powdered to simulate the random orientation of the nanowires, and the spectra are compared with theoretical expectations derived via an orientational study of a single crystal. The nanowire spectra are consistent with the randomly oriented powders; however, we find an unusual suppression of signal intensity as a function of field orientation that may be associated with screening by surface currents.

We present a theoretical study on the high-field charge transport on the surface of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and reproduce all the main features of the recent experimental results, i.e., the incomplete current saturation and the finite residual conductance in the high applied field regime [Costache et al., Phys. Rev. Lett. 112, 086601 (2014)]. Due to the hot-electron effect, the conductance decreases and the current shows a tendency of saturation with the increase of the applied electric field. Moreover, the electric field can excite carriers within the surface bands through interband precession and leads to a higher conductance. As a joint effect of the hot-electron transport and the carrier excitation, the conductance approaches a finite residual value in the high-field regime and the current saturation becomes incomplete. We thus demonstrate that, contrary to the conjecture in the literature, the observed transport phenomena can be understood qualitatively in the framework of surface transport alone. Furthermore, if a constant bulk conductance which is insensitive to the field is introduced, one can obtain a good quantitative agreement between the theoretical results and the experimental data.

We present a theoretical study of quantum resonances in the ballistic transport of graphene-based $n-p-n$ junctions subject to an externally applied electromagnetic field (EF). By making use of the Floquet analysis and the quasiclassical approach we analyze the dynamics of electrons in the presence of time and coordinate dependent potential $U(z,t)$. In the absence of the EF the resonant tunneling results in a set of sharp resonances in the dependence of dc conductance $\sigma$ on the gate voltage $V_g$. In irradiated $n-p-n$ junctions we obtain Fano-type resonances in the dependence of $\sigma \left(V_g\right)$. This coherent quantum-mechanical phenomenon is due to the interplay of two effects: (1) the resonant tunneling through quasibound states and (2) the quantum-interference effect in the region between the “resonant points,” where the resonant absorption (emission) of photons occurs, and junction interfaces.

We study the acoustic-induced interactions between a pair of identical elastic plates patterned with periodical structures. Remarkable mutual forces, both repulsions and attractions, have been observed in the subwavelength regime. The dramatic effect stems from the resonant enhancement of the local field sandwiched between the double plates. The parameter sensitivity of the magnitude and the sign of the interaction (i.e., repulsion or attraction) depend directly on the vibration morphology of the resonant mode. In practical applications, the sign of the interaction can be switched by controlling the external frequency. Both the adjustable magnitude and the switchable sign of the contactless interaction endow this simple and compact double-plate structure with great potential in ultrasonic applications.

The magnetic structure of the ${\mathrm{Eu}}^{2+}$ moments in the superconducting ${\mathrm{EuFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$ sample with $x=0.19$ has been determined using neutron scattering. We conclude that the ${\mathrm{Eu}}^{2+}$ moments are aligned along the c direction below $T_{\mathrm{C}}=19.0\left(1\right)$ K with an ordered moment of 6.6(2) ${\mu }_B$ in the superconducting state. An impurity phase similar to the underdoped phase exists within the bulk sample which orders antiferromagnetically below $T_{\mathrm{N}}=17.0\left(2\right)$ K. We found no indication of iron magnetic order, nor any incommensurate magnetic order of the ${\mathrm{Eu}}^{2+}$ moments in the sample.

Multiferroics offer exciting opportunities for electric-field control of magnetism. Single-phase multiferroics suitable for such applications at room temperature need much more study. Here, we propose the concept of an alternative type of multiferroics, namely, the “asymmetric multiferroic.” In asymmetric multiferroics, two locally stable ferroelectric states are not symmetrically equivalent, leading to different magnetic properties between these two states. Furthermore, we predict from first principles that a Fe-Cr-Mo superlattice with the ${\mathrm{LiNbO}}_3$-type structure is such an asymmetric multiferroic. The strong ferrimagnetism, high ferroelectric polarization, and significant dependence of the magnetic transition temperature on polarization make this asymmetric multiferroic an ideal candidate for realizing electric-field control of magnetism at room temperature. Our study suggests that the asymmetric multiferroic may provide an alternative playground for voltage control of magnetism and find its applications in spintronics and quantum computing.

Superconducting transition temperatures of ${\mathrm{Sr}}_2{\mathrm{VO}}_{3-\delta }\mathrm{FeAs}$ ($\delta =0.1,0.5$) can be tuned by adjusting the dopant concentration of oxygen vacancy. The association between microstructure and superconductivity is not clear and urgently needs to be clarified. Directly via the in situ transmission electron microscopy (TEM) cooling experiment, we demonstrate novel atomic-level microstructural features, which are induced by oxygen deficiency and play key roles in determining the superconducting property. The poor superconducting sample ${\mathrm{Sr}}_2{\mathrm{VO}}_{2.5}\mathrm{FeAs}$ exhibits high density of stacking faults distributed along the [001] orientation with periodic weak image contrast and extra streaking diffraction spots, due to the periodic extraction of partial SrO atom stripes from the ${\mathrm{Sr}}_2{\mathrm{VO}}_{2.5}$ blocking unit. The superconducting sample ${\mathrm{Sr}}_2{\mathrm{VO}}_{2.9}\mathrm{FeAs}$ shows a well-formed structure with only a limited amount of point vacancies. When $\delta$ is changed from 0.1 to 0.5, the valence state of the Fe ion inside the FeAs blocking unit is slightly reduced from +2.0 to +($2-\delta$). Surprisingly, when the sample ${\mathrm{Sr}}_2{\mathrm{VO}}_{2.5}\mathrm{FeAs}$ is cooled down to the superconducting state, a structural reconstruction process occurs as is inferred from the in situ TEM cooling experiments. The evolution of superconductivity on oxygen stoichiometry can further be supported by the dependence of the Hall coefficient and resistivity. Our findings might shed new light on understanding the superconductivity of the ${\mathrm{Sr}}_2{\mathrm{VO}}_{3-\delta }\mathrm{FeAs}$ system.

The superconducting properties of $\mathrm{Ca}T{\mathrm{Si}}_3$ (where T = Pt and Ir) have been investigated using muon spectroscopy. Our muon-spin-relaxation results suggest that in both these superconductors time-reversal symmetry is preserved, while muon-spin-rotation data show that the temperature dependence of the superfluid density is consistent with an isotropic $s$-wave gap. The magnetic penetration depths determined from our transverse-field muon-spin-rotation spectra are found to be 448(6) and 150(7) nm for ${\mathrm{CaPtSi}}_3$ and ${\mathrm{CaIrSi}}_3$, respectively.

String-net models allow us to systematically construct and classify (2+1)-dimensional [(2+1)D] topologically ordered states which can have gapped boundaries. We can use a simple ideal string-net wave function, which is described by a set of F-matrices [or more precisely, a unitary fusion category (UFC)], to study all the universal properties of such a topological order. In this paper, we describe a finite computational method, Q-algebra approach, that allows us to compute the non-Abelian statistics of the topological excitations [or more precisely, the unitary modular tensor category (UMTC)], from the string-net wave function (or the UFC). We discuss several examples, including the topological phases described by twisted gauge theory [i.e., twisted quantum double $D^{\alpha }\left(G\right)$]. Our result can also be viewed from an angle of holographic bulk-boundary relation. The (1+1)-dimensional [(1+1)D] anomalous topological orders, that can appear as edges of (2+1)D topological states, are classified by UFCs which describe the fusion of quasiparticles in (1+1)D. The (1+1)D anomalous edge topological order uniquely determines the (2+1)D bulk topological order (which are classified by UMTC). Our method allows us to compute this bulk topological order (i.e., the UMTC) from the anomalous edge topological order (i.e., the UFC).

The importance of ceria (${\mathrm{CeO}}_2$) in many applications originates from the ease of oxygen vacancy formation and healing. The ordering of vacancies and the whereabouts of the excess charge in bulk ${\mathrm{CeO}}_2$ are of no less significance than at ceria surfaces, but they have not received the same attention. In this work, the formation of neutral oxygen vacancies in bulk ${\mathrm{CeO}}_2$ is investigated using density-functional theory (DFT) in the $\mathrm{DFT}+U$ ($U$ is an effective onsite Coulomb interaction parameter) approach for a broad range of vacancy concentrations $\Theta$ $(\frac{1}{64}\le \Theta \le \frac{1}{4})$. We find that the excess charge prefers to be localized in cation sites such that the mean ${\mathrm{Ce}}^{3+}$ coordination number is maximized, and if nearest-neighbor cation sites are reduced, they rather be nonuniformly distributed. Furthermore, we show that a vacancy repels other vacancies from its nearest-neighbor shell and that the [110] and [111] directions are possible directions for clustering of second- and third-neighbor vacancies, respectively. Vacancies prefer not to share cations. The results are discussed in a simple physical picture which enables the separation of the different contributions to the averaged vacancy formation energy. We also consider cells with fluorite structure and same stoichiometries as in existing bulk phases, i.e., ${\mathrm{Ce}}_{11}{\mathrm{O}}_{20}$ ($\Theta =\frac{1}{11}$), ${\mathrm{Ce}}_7{\mathrm{O}}_{12}$ ($\Theta =\frac{1}{7}$), and ${\mathrm{Ce}}_2{\mathrm{O}}_3$ ($\Theta =\frac{1}{4}$), as well as the corresponding real structures. We find that the vacancy ordering and the location of the excess electrons are consistent with the results for single-phase reduced ${\mathrm{CeO}}_2$, but the ${\mathrm{Ce}}_{11}{\mathrm{O}}_{20}$, ${\mathrm{Ce}}_7{\mathrm{O}}_{12}$, and ${\mathrm{Ce}}_2{\mathrm{O}}_3$ structures are substantially more stable. The stability of these phases as a function of pressure and temperature is discussed. Vacancy-induced lattice relaxation effects are crucial for the interpretation of the results.

The edge states in the quantum spin Hall effect are expected to be protected by time reversal symmetry. The experimental observation of the quantized conductance was reported in the InAs/GaSb quantum well (Du et al., arXiv:1306.1925), up to a large magnetic field, which raises a question on the robustness of the edge states in the quantum spin Hall effect under time reversal symmetry breaking. Here we present a theoretical calculation on topological invariants for the Benevig-Hughes-Zhang model in an external magnetic field, and find that the quantum spin Hall effect remains robust up to a large magnetic field. The critical value of the magnetic field breaking the quantum spin Hall effect is dominantly determined by the band gap at the $\Gamma$ point instead of the indirect band gap between the conduction and valence bands. This illustrates that the quantum spin Hall effect could persist even under time reversal symmetry breaking.

We study a Luttinger liquid (LL) coupled to a generic environment consisting of bosonic modes with arbitrary density-density and current-current interactions. The LL can be either in the conducting phase and perturbed by a weak scatterer or in the insulating phase and perturbed by a weak link. The environment modes can also be scattered by the imperfection in the system with arbitrary transmission and reflection amplitudes. We present a general method of calculating correlation functions under the presence of the environment and prove the duality of exponents describing the scaling of the weak scatterer and of the weak link. This duality holds true for a broad class of models and is sensitive to neither interaction nor environmental modes details, thus it shows up as the universal property. It ensures that the environment cannot generate new stable fixed points of the renormalization group flow. Thus, the LL always flows toward either conducting or insulating phase. Phases are separated by a sharp boundary which is shifted by the influence of the environment. Our results are relevant, for example, for low-energy transport in (i) an interacting quantum wire or a carbon nanotube where the electrons are coupled to the acoustic phonons scattered by the lattice defect; (ii) a mixture of interacting fermionic and bosonic cold atoms where the bosonic modes are scattered due to an abrupt local change of the interaction; (iii) mesoscopic electric circuits.

We have observed a magnetic vortex lattice (VL) in ${\mathrm{BaFe}}_2\left({\mathrm{As}}_{0.67}{\mathrm{P}}_{0.33}\right){}_2$ (BFAP) single crystals by small-angle neutron scattering. With the field along the $c$ axis, a nearly isotropic hexagonal VL was formed in the field range from 1 to 16 T, and no symmetry changes in the VL were observed. The temperature dependence of the VL signal was measured and confirms the presence of (non-$d$-wave) nodes in the superconducting gap structure for measurements at 5 T and below. The nodal effects were suppressed at high fields. At low fields, a VL reorientation transition was observed between 1 and 3 T, with the VL orientation changing by ${45}^{\circ }$. Below 1 T, the VL structure was strongly affected by pinning and the diffraction pattern had a fourfold symmetry. We suggest that this (and possibly also the VL reorientation) is due to pinning to defects aligned with the crystal structure, rather than being intrinsic. The temperature dependence of the scaled intensity suggests that BFAP possesses at least one full gap and one nodal gap with circular symmetry. Judging from the symmetry, the node structure should take the form of an “accidental” circular line node, which is consistent with recent angle-resolved photoemission spectroscopy results [Y. Zhang, Z. R. Ye, Q. Q. Ge, F. Chen, J. Jiang, M. Xu, B. P. Xie, and D. L. Feng, Nature Physics 8, 371 (2012).].

Lossless propagation of longitudinal magnetic dipole waves along chains of high-index subwavelength particles is predicted for a narrow frequency range around the magnetic Mie resonance of the individual particles. Mathematical analogies between dipole and magnetoinductive waves are used to reduce back-reflections thus improving the power transfer efficiency of respective particle waveguides. The proposed technique can be used to optimize the propagation of even more complex particle-based configurations.

We present a systematic study of strain effects on the electronic band structure of the group-III-nitrides (AlN, GaN and InN) in the wurtzite phase. The calculations are based on density functional theory with band-gap-corrected approaches including the Heyd-Scuseria-Ernzerhof hybrid functional (HSE) and quasiparticle $G_0{W}_0$ methods. We study strain effects under realistic strain conditions, hydrostatic pressure, and biaxial stress. The strain-induced modification of the band structures is found to be nonlinear; transition energies and crystal-field splittings show a strong nonlinear behavior under biaxial stress. For the linear regime around the experimental lattice parameters, we present a complete set of deformation potentials ($a_{\mathrm{cz}}$, $a_{\mathrm{ct}}$, $D_1$, $D_2$, $D_3$, $D_4$, $D_5$, $D_6$) that allows us to predict the band positions of group-III nitrides and their alloys (InGaN and AlGaN) under realistic strain conditions. The benchmarking $G_0{W}_0$ results for GaN agree well with the HSE data and indicate that HSE provides an appropriate description for the band structures of nitrides. We present a systematic study of strain effects on the electronic band structure of the group-III nitrides (AlN, GaN, and InN). We quantify the nonlinearity of strain effects by introducing a set of bowing parameters. We apply the calculated deformation potentials to the prediction of strain effects on transition energies and valence-band structures of InGaN alloys and quantum wells (QWs) grown on GaN, in various orientations (including $c$-plane, $m$-plane, and semipolar). The calculated band gap bowing parameters, including the strain effect for $c$-plane InGaN, agree well with the results obtained by hybrid functional alloy calculations. For semipolar InGaN QWs grown in $\left(20\overline{2}1\right)$, $\left(30\overline{3}1\right)$, and $\left(30\overline{3}\overline{1}\right)$ orientations, our calculated deformation potentials have provided results for polarization ratios in good agreement with the experimental observations, providing further confidence in the accuracy of our values.

We use the renormalization group method to examine the effect of phonon-mediated interaction on $d$-wave superconductivity, as driven by spin fluctuations in a quasi-one-dimensional electron system. The influence of a tight-binding electron-phonon interaction on the spin-density-wave and $d$-wave superconducting instability lines is calculated for arbitrary temperature, phonon frequency, and antinesting of the Fermi surface. The domain of electron-phonon coupling strength where spin-density-wave order becomes unstable against the formation of a bond-order wave or Peierls state is determined at weak antinesting. We show the existence of a positive isotope effect for spin-density-wave and $d$-wave superconducting critical temperatures which scales with the antinesting distance from quantum critical point where the two instabilities merge. We single out a low phonon frequency zone where the bond-order-wave ordering gives rise to triplet $f$-wave superconductivity under nesting alteration, with both orderings displaying a negative isotope effect. We also study the electron-phonon strengthening of spin fluctuations at the origin of extended quantum criticality in the metallic phase above superconductivity. The impact of our results on quasi-one-dimensional organic conductors like the Bechgaard salts where a Peierls distortion is absent and superconductivity emerges near a spin-density-wave state under pressure is emphasized.

We provide a simple set of rules for predicting interference effects in off-resonant transport through single molecule junctions. These effects fall into two classes, showing, respectively, an odd or an even number of nodes in the linear conductance within a given molecular charge state, and we demonstrate how to decide the interference class directly from the contacting geometry. For neutral alternant hydrocarbons, we employ the Coulson-Rushbrooke-McLachlan pairing theorem to show that the interference class is decided simply by tunneling on and off the molecule from same or different sublattices. More generally, we investigate a range of smaller molecules by means of exact diagonalization combined with a perturbative treatment of the molecule-lead tunnel coupling. While these results generally agree well with $GW$ calculations, they are shown to be at odds with simpler mean-field treatments. For molecules with spin-degenerate ground states, we show that for most junctions interference causes no transmission nodes, but we argue that it may lead to a nonstandard gate dependence of the zero-bias Kondo resonance.