We present detailed low-temperature scanning tunneling spectroscopy (STS) investigations of the local electronic density of states (DOS) on the fivefold and twofold $i$-Al${}_{70}$Pd${}_{21}$Mn${}_9$; pseudo tenfold ξ′-Al${}_{72}$Pd${}_{25}$Mn${}_3$; tenfold $d$-Al${}_{77}$Ni${}_{17}$Co${}_6$; twofold-(12110) $d$-Al${}_{72.9}$Ni${}_{10.4}$Co${}_{16.7}$; (100) Y-Al${}_{75.8}$Ni${}_{2.1}$Co${}_{22.1}$; and (111) Al surface in the range of +/$-$ 1 eV around the Fermi energy. All these quasicrystals and approximants exhibit the theoretically predicted electronic pseudogap. We observe no evidence for a correlation between the depth of this pseudogap, as measured by STS, and the remarkably high electrical resistivity of the investigated quasicrystals and approximants. A parametrization of the spatial variability of STS spectra is introduced as a measure of the magnitude of the electron localization for a given system. The resulting $\widehat{\mathrm{S}}$ parameter is found to be proportional to the square root of the electrical resistivity. This finding supports the validity of interpreting spatial variations of fine-structure features in the STS spectra of Al-based quasicrystals and approximants in terms of electron localization.

We report the observation of zero resistivity (ρ) in a hydrocarbon superconductor, and describe the temperature dependence of ρ in metal-doped hydrocarbons. The resistivity of K-doped picene (K${}_x$picene) has been recorded from pellet samples in a four-terminal measurement mode. A drop in $\rho$ is observed below 7 K for K${}_{3.1}$picene and below 11 K for K${}_{3.5}$picene, which clearly displays zero resistivity. The resistivity drop at 7 K is consistent with the superconducting critical temperature ($T_{\mathrm{c}}$) obtained from the magnetic susceptibility of the 7 K phase of K${}_3$picene, while the drop at 11 K is inconsistent with the $T_{\mathrm{c}}$'s of both the 7 and 18 K phases of K${}_3$picene reported previously. The temperature dependence of ρ for both samples exhibits granular-metal-like behavior in the normal state.

In this paper, we present systematic measurements of the temperature and magnetic field dependencies of the thermodynamic and transport properties of the Yb-based heavy fermion YbPtBi for temperatures down to 0.02 K with magnetic fields up to 140 kOe to address the possible existence of a field-tuned quantum critical point. Measurements of magnetic-field- and temperature-dependent resistivity, specific heat, thermal expansion, Hall effect, and thermoelectric power indicate that the AFM order can be suppressed by an applied magnetic field of $H_c\sim 4$ kOe. In the $H-T$ phase diagram of YbPtBi, three regimes of its low-temperature states emerge: (I) AFM state, characterized by a spin density wave-like feature, which can be suppressed to $T=0$ by the relatively small magnetic field of $H_c\sim 4$ kOe; (II) field-induced anomalous state in which the electrical resistivity follows $\Delta \rho \left(T\right)\propto T^{1.5}$ between $H_c$ and $\sim$8 kOe; and (III) Fermi liquid (FL) state in which $\Delta \rho \left(T\right)\propto T^2$ for $H\ge 8$ kOe. Regions I and II are separated at $T=0$ by what appears to be a quantum critical point. Whereas region III appears to be a FL associated with the hybridized 4$f$ states of Yb, region II may be a manifestation of a spin liquid state.

Crystallization of amorphous germanium (a-Ge) by laser or electron beam heating is a remarkably complex process that involves several distinct modes of crystal growth and the development of intricate microstructural patterns on the nanosecond to ten microsecond time scales. Here we use dynamic transmission electron microscopy (DTEM) to study the fast, complex crystallization dynamics with 10 nm spatial and 15 ns temporal resolution. We have obtained time-resolved real-space images of nanosecond laser-induced crystallization in a-Ge with unprecedentedly high spatial resolution. Direct visualization of the crystallization front allows for time-resolved snapshots of the initiation and roughening of the dendrites on submicrosecond time scales. This growth is followed by a rapid transition to a ledgelike growth mechanism that produces a layered microstructure on a time scale of several microseconds. This study provides insights into the mechanisms governing this complex crystallization process and is a dramatic demonstration of the power of DTEM for studying time-dependent material processes far from equilibrium.

Electron optics exploits the analogies between rays in geometrical optics and electron trajectories, leading to interesting insights and potential applications. Graphene, with its two dimensionality and photon-like behavior of its charge carriers, is the perfect candidate for the exploitation of electron optics. We show that a circular gate- or doping-controlled region in the presence of Rashba spin-orbit interaction in graphene may indeed behave as a Veselago electronic lens but with two different indices of refraction. We demonstrate that this birefringence results in complex caustics patterns for a circular gate, selective focusing of different spins, and the possible direct measurement of the Rashba coupling strength in scanning-probe experiments.

The quasi-one-dimensional (Q1D) molecular conductors (DIETSe)${}_2$$M$Cl${}_4$ [$M=\text{Fe}$, Ga] undergo a spin density wave (SDW) transition below 12 K. The SDW ground state is suppressed by applying high pressure, recovering the Q1D Fermi surface which is confirmed by the appearance of Lebed resonance in the angle-dependent magnetoresistance (MR). Above the critical pressure of SDW, MR shows kink structures at high magnetic fields, reminiscent of field-induced spin density wave (FISDW) transition in both salts. The $\pi$-$d$ hybrid (DIETSe)${}_2$FeCl${}_4$ also exhibits an antiferromagnetic (AF) transition of $d$-electron spins at 2.5 K, below which the spin-flop-induced positive large MR are observed. The change in the interlayer MR reaches 130$\%$ at 10.5 kbar. The resistance anomalies associated with spin flop are also observed in the angle-dependent MR at low magnetic fields below 5 T, associated with clear hysteresis. A polar plot of these anomalies reveals the presence of hidden spin canting. Two magnetic easy axes of $d$-electron spins are found to be tilted $\pm$16 degrees from the $b$ axis towards the $c$ axis. The interplay between the SDW instability of Q1D $\pi$ electrons and the local moments of AF $d$-electron spins is considered as the origin of the anomalous transport behaviors.

We use broadband ferromagnetic resonance spectroscopy and x-ray diffraction to investigate the fundamental origin of perpendicular anisotropy in Co${}_{90}$Fe${}_{10}$/Ni multilayers. By careful evaluation of the spectroscopic $g$-factor, we determine the orbital moment along the out-of-plane and in-plane directions. For the multilayers, we find a direct relationship between the orbital moment asymmetry and the perpendicular anisotropy, consistent with the theory of Bruno [Bruno, Phys. Rev. B 39, 865 (1989)]. A systematic x-ray diffraction study revealed the presence of a trigonal strain as high as 0.7$\%$ in some samples. However, we found no direct correlation between the strain and the anisotropy, indicating that the anisotropy is not dominated by magnetoelastic effects. To further study the interface structure on the anisotropy, we prepared a set of equivalent alloy samples. The strain in the alloy samples was comparable to that of the multilayer samples; however, the orbital moment asymmetry in the alloy samples showed a very different trend, allowing us to isolate the effect of the interfaces in the multilayers.

The effect of crystalline order on the anisotropy of spin accumulation in Si/oxide/ferromagnet tunnel devices has been investigated. The spin accumulation induced electrically in the silicon changes when the magnetization of the ferromagnet is rotated either from in-plane to perpendicular to the tunnel interface or when it is rotated within the plane of the magnetic layer. A fourfold in-plane anisotropy, which reflects the crystalline nature of the tunnel contact, is observed not only for crystalline MgO/Fe contacts, but also for devices with amorphous Al${}_2$O${}_3$ tunnel barrier and polycrystalline ferromagnetic electrode. The in-plane anisotropy is attributed to the direct coupling of states from the ferromagnet to those in the Si, as in coherent tunneling, causing anisotropy in devices in which only the nonmagnetic (Si) electrode is crystalline.

We study dynamic polarization and resistive detection of nuclear spins in a semiconductor quantum dot (QD) under the Kondo effect regime. We find that the differential conductance spectra of the QD exhibit hysteresis under the Kondo effect regime in magnetic fields. Relevance of nuclear spins to the hysteresis is confirmed by the detection of nuclear magnetic resonance signals by monitoring the differential conductance. We attribute the origin of the hysteresis to the dynamic nuclear spin polarization (DNP) induced in the QD. Using the DNP, we demonstrate nuclear spin relaxation rate measurements in the QD under the Kondo effect regime.

Graphitic carbon nitride ($g$-C${}_3$N${}_4$) has recently triggered extensive investigations due to its potential applications, such as in direct photochemical water splitting, CO${}_2$ activation, and transition-metal-free spintronics. However, electronic, and particularly the optical properties of $g$-C${}_3$N${}_4$ still have not been well established. Based on one of the state-of-the-art approaches—many-body Green's function theory (i.e., $GW$ $+$ BSE)—absorption of ultraviolet light by $g$-C${}_3$N${}_4$ is found to be determined by strong excitonic effects with a significantly large binding energy assigned to the bound excitons. Dark states have also been found in $g$-C${}_3$N${}_4$, which can affect the photoluminescence yield of $g$-C${}_3$N${}_4$. We find that the band gap of $g$-C${}_3$N${}_4$ probably can be tuned by adjusting the condensation (dimensionality) to initiate excitonic absorption in the visible light region, which might help improve the solar energy conversion efficiency.

We use polarized inelastic neutron scattering (INS) to study spin excitations of optimally hole-doped superconductor Ba${}_{0.67}$K${}_{0.33}$Fe${}_2$As${}_2$ ($T_c=38$ K). In the normal state, the imaginary part of the dynamic susceptibility, ${\chi }^{\prime \prime }(Q,\omega )$, shows magnetic anisotropy for energies below $\sim$7 meV with $c$-axis polarized spin excitations larger than that of the in-plane component. Upon entering into the superconducting state, previous unpolarized INS experiments have shown that spin gaps at $\sim$5 and 0.75 meV open at wave vectors $Q=(0.5,0.5,0)$ and $(0.5,0.5,1)$, respectively, with a broad neutron spin resonance at $E_r=15$ meV. Our neutron polarization analysis reveals that the large difference in spin gaps is purely due to different spin gaps in the $c$ axis and in-plane polarized spin excitations, resulting in a resonance with different energy widths for the $c$-axis and in-plane spin excitations. The observation of spin anisotropy in both optimally electron- and hole-doped BaFe${}_2$As${}_2$ is due to their proximity to the AF ordered BaFe${}_2$As${}_2$ where spin anisotropy exists below $T_N$.

Atomic resolution imaging using secondary electrons (emitted as a result of the interaction of incident fast electrons with a specimen) was only achieved recently. There has been considerable speculation as to the physical mechanisms underpinning the imaging. In this paper we use a quantum mechanical model to show that the image contrast is due to electrons ejected in inner-shell ionization events initiated by the primary beam, an atomic scale, and focused coherent electron probe. The angular probability distribution of the ejected electrons is key in understanding the (relative) contrast from different atomic species within the specimen. For a given species of atom, this angular probability distribution is predominantly determined by the angular momentum quantum number of the bound electron prior to ionization. The model is compared to experiment and reproduces the essential features in the data.

The classical problem of magnetic stripe domain behavior in films and plates with uniaxial magnetic anisotropy is addressed. Exact analytical results are derived for the stripe domain widths as a function of applied perpendicular field $H$, in the regime where the domain period becomes large. The stripe period diverges as ${(H_c-H)}^{-1/2}$, where $H_c$ is the critical (infinite period) field, an exact result confirming a previous conjecture. The magnetization approaches saturation as ${(H_c-H)}^{1/2}$, a behavior that compares excellently with experimental data obtained for a $4-\mu$m thick ferrite garnet film. The exact analytical solution provides a new basis for precise characterization of uniaxial magnetic films and plates, illustrated by a simple way to measure the domain wall energy. The mathematical approach is applicable for similar analysis of a wide class of systems with competing interactions where a stripe domain phase is formed.

We study the unitary dynamics and the thermalization properties of free-fermion-like Hamiltonians after a sudden quantum quench, extending the results of S. Ziraldo et al. [Phys. Rev. Lett. 109, 247205 (2012)]. With analytical and numerical arguments, we show that the existence of a stationary state and its description with a generalized Gibbs ensemble (GGE) depend crucially on the observable considered (local versus extensive) and on the localization properties of the final Hamiltonian. We present results on two one-dimensional (1D) models, the disordered 1D fermionic chain with long-range hopping and the disordered Ising/$XY$ spin chain. We analytically prove that, while time averages of one-body operators are perfectly reproduced by GGE (even for finite-size systems, if time integrals are extended beyond revivals), time averages of many-body operators might show clear deviations from the GGE prediction when disorder-induced localization of the eigenstates is at play.

On the basis of extensive experimental studies of Pb${}_{1-x}$Ba${}_x$(Fe${}_{1/2}$Nb${}_{1/2}$)O${}_3$ (PFN-BFN) and Pb(Fe${}_{1/2}$Nb${}_{1/2}$)${}_{1-x}$Ti${}_x$O${}_3$ (PFN-PT) single crystals by several experimental methods, we have proposed phase diagrams describing the magnetic properties of these solid solutions. The comprehensive consideration of the magnetic properties of the PFN-based solid solutions has shown that these phase diagrams can be explained on the basis of a model suggested earlier for pure PFN [Phys. Rev. Lett. 105, 257202 (2010)]. This model assumes the coexistence in the crystal lattice of the long-range antiferromagnetic (AFM) cluster, which defines the Néel order parameter, with the finite-size mixed ferromagnetic-AFM clusters, responsible for the spin-glass order parameter. We state that one of these parameters, the Néel temperature, linearly decreases with the increasing dopant concentration and eventually disappears at some critical concentration as a result of the percolation phase transition. The other parameter survives until the maximal concentrations studied. We have also found a phase which can be related to the super-AFM order. These data can have important implications and provide the basis for the development of novel fundamental theory of multiferroics with the site, charge, and spin disorder.

The oxygen-saturated Fe(001)-$p(1\times 1)$O surface has been used as a template to stabilize two-dimensional Cr oxides on Fe(001). Cr deposition at 400 ${}^{\circ }$C leads to two different well-ordered phases, depending on the amount of Cr deposited. In the submonolayer regime a novel $c(4\times 2)$ overlayer self-assembles on the Fe(001)-$p(1\times 1)$O surface, saturating for a coverage of about $0.75$ monolayers. This phase becomes unstable for higher coverages, when a $(\sqrt{5}\times \sqrt{5})R{27}^{\circ }$ superstructure emerges. The structural and electronic details of the two one-layer-thick oxides are studied by combining high-resolution scanning tunneling microscopy, low-energy electron diffraction, Auger electron spectroscopy, and density functional theory.

In this study, we investigate the electronic structure and electron self-energy of palladium single crystals using polarization-dependent high-resolution angle-resolved photoemission spectroscopy. The observed Fermi surfaces and energy-band dispersions agree with those given by the band-structure calculation. A detailed comparison between the observed and theoretical band dispersions of the ${\Sigma }_1$ band forming the electronlike Fermi surface indicates an electron-electron coupling parameter of ${\lambda }_{ee}\sim 0.02$. Near the Fermi level, a kink structure in the energy-band dispersion exists at $\sim -20$ meV, in agreement with the Debye energy. The electron-phonon coupling parameter is estimated to be ${\lambda }_{ep}=0.39\pm 0.05$ at 8 K for the ${\Sigma }_1$ band, which is consistent with the theoretical values of ${\lambda }_{ep}=0.35–0.41$. Furthermore, analyses of the self-energy indicate a possible contribution from the electron-paramagnon interaction in the energy range of $-50\sim -150$ meV. The evaluated electron-paramagnon coupling parameter is ${\lambda }_{em}\sim 0.06$ for the ${\Sigma }_1$ band. We found that the magnitudes of ${\lambda }_{ep}$ and ${\lambda }_{em}$ depend on the Fermi surface points. The total effective mass enhancement factor is estimated to be $1+{\lambda }_{ep}+{\lambda }_{ee}+{\lambda }_{em}\sim 1.5$ for the ${\Sigma }_1$ band, which is close to the values $m^{*}/m_b\sim 1.5–1.7$ given by the de Haas–van Alphen and electron specific-heat measurements.

It has recently been realized that a general class of non-Abelian defects can be created in conventional topological states by introducing extrinsic defects, such as lattice dislocations or superconductor-ferromagnet domain walls in conventional quantum Hall states or topological insulators. In this paper, we begin by placing these defects within the broader conceptual scheme of extrinsic twist defects associated with symmetries of the topological state. We explicitly study several classes of examples, including $Z_2$ and $Z_3$ twist defects, where the topological state with $N$ twist defects can be mapped to a topological state without twist defects on a genus $g\propto N$ surface. To emphasize this connection we refer to the twist defects as genons. We develop methods to compute the projective non-Abelian braiding statistics of the genons, and we find the braiding is given by adiabatic modular transformations, or Dehn twists, of the topological state on the effective genus $g$ surface. We study the relation between this projective braiding statistics and the ordinary non-Abelian braiding statistics obtained when the genons become deconfined, finite-energy excitations. We find that the braiding is generally different, in contrast to the Majorana case, which opens the possibility for fundamentally novel behavior. We find situations where the genons have quantum dimension 2 and can be used for universal topological quantum computing (TQC), while the host topological state is by itself nonuniversal for TQC.

The $p$-$n$ junction has provided the basis for the semiconductor-device industry. Investigations of $p$-$n$ junctions based on Mott insulators is still in its infancy. Layered Mott insulators, such as cuprates or other transition metal oxides, present a special challenge since strong in-plane correlations are important. Here we model the planes carefully using plaquette cellular dynamical mean field theory with an exact diagonalization solver. The energy associated with interplane hopping is neglected compared with the long-range Coulomb interaction that we treat in the Hartree-Fock approximation. Within this new approach, dynamical layer theory, the charge redistribution is obtained at the final step from minimization of a function of the layer fillings. A simple analytical description of the solution, in the spirit of the Thomas-Fermi theory, reproduces quite accurately the numerical results. Various interesting charge reconstructions can be obtained by varying the Fermi energy differences between both sides of the junction. One can even obtain quasi-two-dimensional charge carriers at the interface, in the middle of a Mott insulating layer. The density of states as a function of position does not follow the simple band bending picture of semiconductors.

We report on the measurement of phase coherence length in a high-mobility two-dimensional electron gas patterned in two different geometries, a wire and a ring. The phase coherence length is extracted both from the weak localization correction in long wires and from the amplitude of the Aharonov-Bohm oscillations in a single ring, in a low-temperature regime when decoherence is dominated by electronic interactions. We show that these two measurements lead to different phase coherence lengths, namely $L_{\Phi }^{\mathrm{wire}}\propto T^{-1/3}$ and $L_{\Phi }^{\mathrm{ring}}\propto T^{-1/2}$. This difference reflects the fact that the electrons winding around the ring necessarily explore the whole sample (ergodic trajectories), while in a long wire the electrons lose their phase coherence before reaching the edges of the sample (diffusive regime).