Scanning tunneling spectroscopy measurements conducted on epitaxially grown bilayers of half-metallic ferromagnetic La${}_{2/3}$Ca${}_{1/3}$MnO${}_3$ (LCMO) on superconducting (100)YBa${}_2$Cu${}_3$O${}_{7-\delta }$ reveal long-range penetration of superconducting order into the LCMO. This anomalous proximity effect manifests itself in the tunneling spectra measured on the LCMO layer as gaps and zero-bias conductance peaks. Remarkably, these proximity-induced spectral features were observed for bilayers with LCMO thickness of up to 30 nm, an order of magnitude larger than the expected ferromagnetic coherence length in LCMO. We argue that this long-range proximity effect can be accounted for by the formation of spin-triplet pairing at the LCMO side of the bilayer due to magnetic inhomogeneity at the interface or at domain walls. Possible symmetries of the induced order parameter are discussed.

Photoemission electron microscopy (PEEM) with linearly polarized x rays is used to determine the orientation of antiferromagnetic domains by monitoring the relative peak intensities at the $3d$ transition metal $L_2$ absorption edge. In such an analysis it is necessary to take into account the orientations of the x-ray polarization $\mathbf{Ê}$ and magnetization $\mathbf{Ĥ}$ with respect to the crystalline axes. We address this problem by presenting a general expression of the angular dependence for both x-ray absorption (XA) spectroscopy and x-ray magnetic linear dichroism (XMLD) for arbitrary directions of $\mathbf{Ê}$ and $\mathbf{Ĥ}$ in the (001) cubic plane. In cubic symmetry the angular-dependent XMLD is a linear combination of two spectra with different photon energy dependencies. As a result the intensity maxima of the XA and XMLD are no longer found for $\mathbf{Ê}$ parallel to $\mathbf{Ĥ}$ and their directions will vary with photon energy. When $\mathbf{Ê}$ or $\mathbf{Ĥ}$ is along a high-symmetry axis there is only one spectrum. The angular-dependent XMLD can be separated into an isotropic part, which is symmetric around $\mathbf{Ĥ}$, and an anisotropic part, which depends on the orientation of the crystal axes. The anisotropic part has maximal intensity when $\mathbf{Ê}$ and $\mathbf{Ĥ}$ have equal but opposite angles with respect to the [100] direction. The Ni${}^{2+}$ $L_2$ edge has the peculiarity that the isotropic part vanishes, which has the interesting consequence that the maximum in the XMLD intensity is obtained not only for $\mathbf{Ê}\parallel \mathbf{Ĥ}\parallel$ [100] but also for ($\mathbf{Ê}\parallel$ [110], $\mathbf{Ĥ}\parallel$ [1$\mathrm{1̄}$0]). We apply the angular-dependent theory to determine the spin orientation near the Co/NiO(100) interface. The PEEM images show that the ferromagnetic Co spins and antiferromagnetic NiO spins are aligned perpendicular to each other. By rotating the sample with respect to the linear x-ray polarization we furthermore find that the perpendicular coupling with the ferromagnetic Co layer at the interface causes a canting of the antiferromagnetic Ni moments. This shows that taking into account the angular dependence of the XMLD in the detailed analysis of PEEM images can lead to an accurate retrieval of the spin axes of the antiferromagnetic domains.

We study the topologically nontrivial semimetals by means of the six-band Kane model. Existence of surface states is explicitly demonstrated by calculating the local density of states (LDOS) on the material surface. In the strain-free condition, surface states are divided into two parts in the energy spectrum, one part is in the direct gap, the other part including the crossing point of surface state Dirac cone is submerged in the valence band. We also show how uniaxial strain induces an insulating band gap and raises the crossing point from the valence band into the band gap, making the system a true topological insulator. We predict the existence of helical edge states and spin Hall effect in the thin-film topological semimetals, which could be tested by future experiments. Disorder is found to significantly enhance the spin-Hall effect in the valence band of the thin films.

Recently, Shih et al. [Phys. Rev. Lett. 105, 146401 (2010)] published a theoretical band gap for wurtzite ZnO, calculated with the non-self-consistent $\mathit{GW}$ approximation, that agreed surprisingly well with experiment while deviating strongly from previous studies. They showed that a very large number of empty bands is necessary to converge the gap. We reexamine the $\mathit{GW}$ calculation with the full-potential linearized augmented-plane-wave method and find that even with 3000 bands the band gap is not completely converged. A hyperbolical fit is used to extrapolate to infinite bands. Furthermore, we eliminate the linearization error for high-lying states with local orbitals. In fact, our calculated band gap is considerably larger than in previous studies, but somewhat smaller than that of Shih et al.

A closed-form algorithm for computing subsurface Green functions—the blocks of a material’s Green function between the surface and the bulk—is presented, where we assume the system satisfies a common principal-layer approximation. By exploiting the block tridiagonal and nearly block Toeplitz structure of the Hamiltonian and overlap matrices, this method scales independently of the system size (constant scaling), allowing studies of large systems. As a proof-of-concept example, we investigate the decay of surface effects in an armchair graphene nanoribbon, demonstrating the persistence of surface effects hundreds of atomic layers ($~$0.5 $\mu$m) away from a surface. We finally compare the surface-to-bulk transitions of finite and semi-infinite systems, finding that finite systems exhibit amplified surface effects.

We calculate the probability distribution of the Andreev reflection eigenvalues $R_n$ at the Fermi level in the circular ensemble of random-matrix theory. Without spin-rotation symmetry, the statistics of the electrical conductance $G$ depends on the topological quantum number $Q$ of the superconductor. We show that this dependence is nonperturbative in the number $N$ of scattering channels by proving that the $p$-th cumulant of $G$ is independent of $Q$ for $p<N/d$ (with $d=2$ or $d=1$ in the presence or in the absence of time-reversal symmetry). A large-$N$ effect such as weak localization cannot, therefore, probe the topological quantum number. For small $N$ we calculate the full distribution $P(G)$ of the conductance and find qualitative differences in the topologically trivial and nontrivial phases.

The Hall coefficient $R_{\mathrm{H}}$ of the cuprate superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ was measured in magnetic fields up to 60 T for a hole concentration $p$ from 0.078 to 0.152 in the underdoped regime. In fields large enough to suppress superconductivity, $R_{\mathrm{H}}(T)$ is seen to go from positive at high temperature to negative at low temperature, for $p>0.08$. This change of sign is attributed to the emergence of an electron pocket in the Fermi surface at low temperature. At $p<0.08$, the normal-state $R_{\mathrm{H}}(T)$ remains positive at all temperatures, increasing monotonically as $T\to 0$. We attribute the change of behavior across $p=0.08$ to a Lifshitz transition, namely a change in Fermi-surface topology occurring at a critical concentration $p_{\mathrm{L}}=0.08$, where the electron pocket vanishes. The loss of the high-mobility electron pocket across $p_{\mathrm{L}}$ coincides with a tenfold drop in the conductivity at low temperature, revealed in measurements of the electrical resistivity $\rho$ at high fields, showing that the so-called metal-insulator crossover of cuprates is in fact driven by a Lifshitz transition. It also coincides with a jump in the in-plane anisotropy of $\rho$, showing that without its electron pocket, the Fermi surface must have strong twofold in-plane anisotropy. These findings are consistent with a Fermi-surface reconstruction caused by a unidirectional spin-density wave or stripe order.

We present bias-dependent micro-photoluminescence ($\mu$-PL) spectroscopy of the neutral ($X^0$) and singly negatively-charged ($X^{-}$) excitons in single InAs/GaAs self-assembled quantum dots (QDs) embedded in the intrinsic region of an $n$-$i$-Schottky diode based on a two-dimensional electron gas (2DEG), which was obtained from a Si $\delta$-doped GaAs layer. Using such a device structure, we demonstrate bias-controlled single-electron charging of a single QD as the QD $s$-shell electron state is tuned below the Fermi level. This is verified experimentally by the sequential appearance of energetically-distinct PL emission lines from the two excitons and supported by theoretical calculations. In addition, it is shown both experimentally and theoretically that simultaneous PL emission from the $X^0$ and $X^{-}$ excitons within a particular bias range is the result of a long-lived charge-nonequilibrium state due to weak tunnel-coupling between the QDs and 2DEG in our device. Further, the ability to tune the exciton transition energies via the quantum-confined Stark effect is observed, offering insight into the carrier wave function distributions in the QD and the QD material structure. Finally, we propose a number of spintronic device concepts that may be made feasible as a result of this investigation into bias-controlled carrier tunneling between a self-assembled QD and a 2DEG.

We investigate the effect of surface termination on the charge state of nitrogen-vacancy (NV) centers, which have been ion-implanted a few nanometers below the surface of diamond. We find that, when changing the surface termination from oxygen to hydrogen, previously stable NV${}^{-}$ centers convert into NV${}^0$ and, subsequently, into an unknown nonfluorescent state. This effect is found to depend strongly on the implantation dose. Simulations of the electronic band structure confirm the disappearance of NV${}^{-}$ in the vicinity of the hydrogen-terminated surface. The band bending, which induces a $p$-type surface conductive layer, leads to a depletion of electrons in the nitrogen-vacancies close to the surface. Therefore, hydrogen surface termination provides a chemical way to control the charge state of nitrogen-vacancy centers in diamond. Furthermore, it opens the way to electrostatic control of the charge state with the use of an external gate electrode.

Substitution of more than $33\%$ of the naturally abundant ${}^{16}\mathrm{O}$ in strontium titanate ${\mathrm{SrTiO}}_3$ by ${}^{18}\mathrm{O}$ causes the system to become ferroelectric at low temperatures. The ferroelectricity has been observed via susceptibility measurements, but to date the details of the ferroelectric phase and the phase transition are unclear. Using ab initio density functional theory and lattice-dynamics calculations, we find that the stable structure of the ferroelectric phase is orthorhombic with Ima2 symmetry. The Ima2 point group is noncentrosymmetric and the proposed structure exhibits an electric dipole moment of (0.57 0 0) eÅ. The Ima2 symmetry is consistent with the limited structural details that are reported using neutron diffraction and Raman spectroscopy.

We report that small amounts of metal atom nonstoichiometry are possible in the ZnCr${}_2$O${}_4$ and MgCr${}_2$O${}_4$ spinels. The nonstoichiometry, though less than 2%, significantly impacts $T_{\mathrm{N}}$ and the nature of the magnetic correlations above $T_{\mathrm{N}}$. The ${\mathrm{Zn}}_{1+x}{\mathrm{Cr}}_{2-x}{\mathrm{O}}_4$ spinel is particularly sensitive. While stoichiometric ZnCr${}_2$O${}_4$ displays antiferromagnetic short-range correlations in the susceptibility above $T_{\mathrm{N}}$, ferromagnetic correlations are observed in nonstoichiometric, hole-doped ${\mathrm{Zn}}_{1+x}{\mathrm{Cr}}_{2-x}{\mathrm{O}}_4$. The ${\mathrm{Mg}}_{1+x}{\mathrm{Cr}}_{2-x}{\mathrm{O}}_4$ spinels are less profoundly affected by non-stoichiometry, though significant changes are also observed. We contrast the magnetic properties of ${\mathrm{Zn}}_{1+x}{\mathrm{Cr}}_{2-x}{\mathrm{O}}_4$ and ${\mathrm{Mg}}_{1+x}{\mathrm{Cr}}_{2-x}{\mathrm{O}}_4$ ($x$ $=$ 0, 0.02, 0.04) with those of materials with the equivalent amounts of isovalent nonmagnetic Ga${}^{3+}$ substituted on the Cr${}^{3+}$ site to separate the effects of static site disorder and hole doping.

The recently discovered topological insulators exhibit topologically protected metallic surface states which are interesting from the fundamental point of view and could be useful for various applications if an appropriate electronic gating can be realized. Our photoemission study of Cu-intercalated ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ shows that the surface-state occupancy in this material can be tuned by changing the photon energy and understood as a photoemission-induced gating effect. Our finding provides an effective tool to investigate the new physics coming from the topological surface states and suggests intercalation as a recipe for synthesis of a material suitable for electronic applications.

We present a systematic numerical approach to compute the eigenmodes and the related eigenfrequencies of a disordered photonic crystal, characterized by small fluctuations of the otherwise periodic dielectric profile. The field eigenmodes are expanded on the basis of Bloch modes of the corresponding periodic structure, and the resulting eigenvalue problem is diagonalized on a truncated basis including a finite number of Bloch bands. The Bloch-mode expansion is very effective for modeling modes of very extended disordered structures in a given frequency range, as only spectrally close bands must be included in the basis set. The convergence can be easily verified by repeating the diagonalization for an increased band set. As illustrations, we apply the method to the W1 line-defect waveguide and to the L3 coupled-cavity waveguide, both based on a photonic crystal slab with a triangular lattice of circular holes. We compute and characterize the eigenfrequencies, spatial field profiles, and radiation loss rates of the localized modes induced by disorder. For the W1 waveguide, we demonstrate that radiation losses, at the bottom of the spatially even guided band, are determined by a small hybridization with Bloch modes of the spatially odd band, induced by disorder in spite of their frequency separation. The Bloch-mode expansion method has a very broad range of applications: It can be also used to accurately compute the modes of structures with systematic modulations of the periodic dielectric constant, as in several designs of high-$Q$ cavities.

We report the thermodynamic and transport properties of the newly synthesized Ce-intermetallic compound CeRuSn. This ternary stannide possesses an unconventional structure with two Ce sites at room temperature which exhibit different valencies. Just below room temperature there are large thermal hysteretic effects in the magnetic susceptibility, in the specific heat, as well as in electronic and heat transport properties suggesting the formation of an incommensurate charge density wave modulation whose $q$ vector changes as a function of temperature. Our measurements indicate that one site displays magnetic Ce${}^{3+}$ behavior while the other is a valence fluctuator. At $2.7$ K antiferromagnetic long-range order occurs within one-half of the Ce sites, e.g., the magnetic entropy of the transition is $\frac{1}{2}R\ln 2$. Below $T_N$ a series of metamagnetic transitions takes place in rather small fields ($~$1–2 T), leaving a magnetically fluctuating background. Such behavior is unique among the many Ce–transition-metal compounds.

This study was conducted with the aim of investigating the effect of magnetic fields on the carbon diffusion in pure iron. The Fe/Fe-0.87 mass% C diffusion couples prepared by explosive welding were annealed at temperatures from 850 to 1200 K without and with a uniform magnetic field up to 6 T and with a magnetic-field gradient of 45 T/m. We evaluated the carbon diffusivity in iron from the penetration profiles of carbon into iron measured by secondary ionization mass spectroscopy. The diffusivity of carbon in $\alpha$-Fe decreased under a magnetic field, while it increased by twice under a magnetic-field gradient.

The effect of interactions on topological insulators and superconductors remains, to a large extent, an open problem. Here, we describe a framework for classifying phases of one-dimensional interacting fermions, focusing on spinless fermions with time-reversal symmetry and particle number parity conservation, using concepts of entanglement. In agreement with an example presented by L. Fidkowski and A. Kitaev [Phys. Rev. B 81, 134509 (2010)], we find that in the presence of interactions there are only eight distinct phases which obey a ${\mathbb{Z}}_8$ group structure. This is in contrast to the $\mathbb{Z}$ classification in the noninteracting case. Each of these eight phases is characterized by a unique set of bulk invariants, related to the transformation laws of its entanglement (Schmidt) eigenstates under symmetry operations, and has a characteristic degeneracy of its entanglement levels. If translational symmetry is present, the number of distinct phases increases to 16.

In this paper we show how the classification of topological phases in insulators and superconductors is changed by interactions, in the case of one-dimensional systems. We focus on the time-reversal-invariant Majorana chain (BDI symmetry class). While the band classification yields an integer topological index $k$, it is known that phases characterized by values of $k$ in the same equivalence class modulo 8 can be adiabatically transformed one to another by adding suitable interaction terms. Here we show that the eight equivalence classes are distinct and exhaustive, and provide a physical interpretation for the interacting invariant modulo 8. The different phases realize different Altland-Zirnbauer classes of the reduced density matrix for an entanglement bipartition into two half chains. We generalize these results to the classification of all one-dimensional gapped phases of fermionic systems with possible antiunitary symmetries, utilizing the algebraic framework of central extensions. We use matrix product state methods to prove our results.

Superconducting pnictides are widely found to feature spin-singlet pairing in the vicinity of an antiferromagnetic phase, for which nesting between electron and hole Fermi surfaces is crucial. LiFeAs differs from the other pnictides by its (i) poor nesting properties and (ii) unusually shallow hole pockets. Investigating magnetic and pairing instabilities in an electronic model that incorporates these differences, we find antiferromagnetic order to be absent. Instead we observe almost ferromagnetic fluctuations which drive an instability toward spin-triplet $p$-wave superconductivity.

We study the non-Abelian statistics of quasiparticles in the Ising-type quantum Hall states which are likely candidates to explain the observed Hall conductivity plateaus in the second Landau level, most notably the one at filling fraction $\nu =5/2$. We complete the program started in V. Gurarie and C. Nayak, [Nucl. Phys. B 506, 685 (1997)]. and show that the degenerate four-quasihole and six-quasihole wave functions of the Moore-Read Pfaffian state are orthogonal with equal constant norms in the basis given by conformal blocks in a $c=1+\frac{1}{2}$ conformal field theory. As a consequence, this proves that the non-Abelian statistics of the excitations in this state are given by the explicit analytic continuation of these wave functions. Our proof is based on a plasma analogy derived from the Coulomb gas construction of Ising model correlation functions involving both order and (at most two) disorder operators. We show how this computation also determines the non-Abelian statistics of collections of more than six quasiholes and give an explicit expression for the corresponding conformal block-derived wave functions for an arbitrary number of quasiholes. Our method also applies to the anti-Pfaffian wave function and to Bonderson-Slingerland hierarchy states constructed over the Moore-Read and anti-Pfaffian states.

The pseudogap state of cuprate high-temperature superconductors has been often viewed as either a yet unknown competing order or a precursor state to superconductivity. While awaiting the resolution of the pseudogap problem in cuprates, we demonstrate that local pairing fluctuations, vortex liquid dynamics, and other precursor phenomena can emerge quite generally whenever fermionic excitations remain gapped across the superconducting transition, regardless of the gap origin. Our choice of a tractable model is a lattice band insulator with short-range attractive interactions between fermions in the $s$-wave channel. An effective crossover between Bardeen-Cooper-Schrieffer (BCS) and Bose-Einstein condensate (BEC) regimes can be identified in any band insulator above two dimensions, while in two dimensions only the BEC regime exists. The superconducting transition is “unconventional” (non-pair-breaking) in the BEC regime, identified by either the bosonic mean field or $\mathit{XY}$ universality class. The insulator adjacent to the superconductor in the BEC regime is a bosonic Mott insulator of Cooper pairs, which may be susceptible to charge density wave ordering. We construct a function of the many-body excitation spectrum whose nonanalytic changes define a sharp distinction between band and Mott insulators. The corresponding “second-order transition” can be observed out of equilibrium by driving a Cooper pair laser in the Mott insulator. We explicitly show that the gap for charged bosonic excitations lies below the threshold for Cooper pair breakup in any BEC regime, despite quantum fluctuations. Our discussion ends with a view of possible consequences for cuprates, where antinodal pair dynamics has certain features in common with our simple $s$-wave picture.