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Bad metallic behavior or incoherent charge dynamics is characteristic of the normal state of high-transition-temperature copper-oxide superconductors. A parent compound of iron-based superconductors, BaFe2As2, is also a bad metal in the high-temperature phase, whereas its phosphorous counterpart, BaFe2P2, is a good metal. Combined measurements of in-plane resistivity and optical conductivity for BaFe2(As1-xPx)2 make it possible to disentangle the coherent channel from the incoherent one in the charge transport. The isovalent P substitution promotes coherent motion of carriers without changing the balance of the electron and hole Fermi-surface volume and hence transforms a bad metal into a good metal [ABSTRACT FROM AUTHOR]

We report 29Si nuclear magnetic resonance measurements in single crystals and aligned powders of URu2Si2 in the hidden order and paramagnetic phases. The spin-lattice relaxation data reveal evidence of pseudospin fluctuations of U moments in the paramagnetic phase. We find evidence for partial suppression of the density of states below 30 K and analyze the data in terms of a two-component spin-fermion model. We propose that this behavior is a realization of a pseudogap between the hidden-order transition THO and 30 K. This behavior is then compared to other materials that demonstrate precursor fluctuations in a pseudogap regime above a ground state with long-range order. [ABSTRACT FROM AUTHOR]

Time-resolved measurement of photoemission spectroscopy was made to trace a change of surface potential after the surface photovoltage effect on a Si(111) 7×7 surface. Two relaxation processes were found with decay times of nanoseconds and hundreds of nanoseconds, which are explained in terms of the tunneling and the thermionic relaxation schemes, respectively. At the high laser power density, the relaxation has become oscillatory with a temporal period of several tens of nanoseconds. [ABSTRACT FROM AUTHOR]

The Fe1+y,Te1-xSex series of materials is one of the prototype families of Fe-based superconductors. To provide further insight into these materials, we present systematic inelastic neutron scattering measurements of the low-energy spin excitations for x = 0.27, 0.36, 0.40, and 0.49. These measurements show an evolution of incommensurate spin excitations towards the (1/2,1/2,0) wave vector with doping. Concentrations (x = 0.40 and 0.49) which exhibit the most robust superconducting properties have spin excitations closest to (1/2,1/2,0) and also exhibit a strong spin resonance in the spin excitation spectrum below Tc. The resonance signal appears to be closer to (1/2,1/2,0) than the underlying spin excitations. We discuss the possible relationship between superconductivity and spin excitations at the (1/2,1/2,0) wave vector and the role that interstitial Fe may play. [ABSTRACT FROM AUTHOR]

Realizing Majorana modes (MMs) in condensed-matter systems is of vast experimental and theoretical interests, and some signatures of MMs have been measured already. To facilitate future experimental observations and to explore further applications of MMs, generating many MMs at ease in an experimentally accessible manner has become one important issue. This task is achieved here in a one-dimensional p-wave superconductor system with the nearest- and next-nearest-neighbor interactions. In particular, a periodic modulation of some system parameters can induce an effective long-range interaction (as suggested by the Baker-Campbell-Hausdorff formula) and may recover time-reversal symmetry already broken in undriven cases. By exploiting these two independent mechanisms at once we have established a general method in generating many Floquet MMs via periodic driving. [ABSTRACT FROM AUTHOR]

In Fe-based superconductors, electron doping is often realized by the substitution of transition-metal atoms for Fe. In order to investigate how the electronic structure of the parent compound is influenced by Zn substitution, which supplies nominally four extra electrons per substituted atom but is expected to induce the strongest impurity potential among the transition-metal atoms, we have performed an angle-resolved photoemission spectroscopy measurement on Ba(Fe1-xZnx)2As2 (Zn-122). In Zn-122, the temperature dependence of the resistivity shows a kink around T ~ 135 K, indicating antiferromagnetic order below the Néel temperature of TN ~ 135 K. In fact, folded Fermi surfaces (FSs) similar to those of the parent compound have been observed below TN. The hole and electron FS volumes are, therefore, different from those expected from the rigid-band model. The results can be understood if all the extra electrons occupy the Zn 3d state ~10 eV below the Fermi level and do not participate in the formation of the FSs. [ABSTRACT FROM AUTHOR]

We present magnetotransport and 209Bi nuclear magnetic resonance (NMR) data on a series of single crystals of Bi2Se3, Bi2Te2Se, and CuxBi2Se3 with varying carrier concentrationn. The Knight shift of the bulk nuclei is strongly correlated with the carrier concentration via a hyperfine coupling of 27 μeV, which may have important consequences for scattering of the protected surface states. Surprisingly we find that the NMR linewidths and the spin-lattice relaxation rate appear to be dominated by the presence of localized spins, which may be related to the presence of Se vacancies. [ABSTRACT FROM AUTHOR]

We study the reversal mechanism between opposite closed flux states of ferromagnetic nanorings driven by an azimuthal magnetic field. The reversal proceeds via the formation of 360° domain walls, and we show that the role of interacting nucleation sites is essential for the process to take place. Such nucleation is seen to create domain walls with the right topological charge conditions for 360° domain-wall formation. Given the symmetry of the system, we utilize an energetic description as a function of the azimuthal field magnitude, which clearly reveals the different stages of this reversal process. The annihilation of the 360° domain walls that is necessary for the reversal process to complete is controlling the field value at the final stage of the process. Such a fundamental mechanism for ring reversal has several implications and will guide the design of the various data-storage-device proposals based on nanorings. [ABSTRACT FROM AUTHOR]

We observed in superconducting (Tl,Rb)2Fe4Se5 spin-wave branches that span an energy range from 6.5 to 209 meV. Spin dynamics are successfully described by a Heisenberg localized spin model whose dominant in-plane interactions include only the nearest (J1 and J1') and next nearest neighbor (J2 and J2') exchange terms within and between the tetramer spin blocks, respectively. These experimentally determined exchange constants would crucially constrain the theoretical viewpoints on magnetism and superconductivity in the Fe-based materials. [ABSTRACT FROM AUTHOR]

Plasmons dispersion and nonvertical interband transitions in Bi2Se3 single crystals were investigated by electron energy-loss spectroscopy in conjunction with (scanning) transmission electron microscopy [(S)TEM-EELS]. Both volume plasmons (π plasmon at 7 eV and π + σ plasmon at 17 eV) and surface plasmons (~5.5 and 10 eV) were demonstrated in STEM-EELS spectra and the corresponding spectral imaging in real space. In further EELS experiments in reciprocal space, the momentum-dependent spectra reveal very different dispersion behavior between π and π + σ plasmons, with π + σ plasmons showing a typical quadratic dependence, whereas the n plasmon exhibited a linear dispersion analogous to what was reported for graphene. Furthermore, a low energy excitation 0.7-1.6 eV was also observed which is attributed to direct nonvertical interband transitions along the Γ F direction. [ABSTRACT FROM AUTHOR]

Using atomically resolved electron energy-loss spectroscopy, the atomic-plane-by-atomic-plane, unit-cell-by-unit-cell stoichiometry, and charge characteristics of the oxide interface (Nd0.35Sr0.65)MnO3/SrTiO3, with a primitive polar discontinuity of (Nd0.35Sr0.65O)0.35+-(TiO2)0, were thoroughly investigated. (Nd0.35Sr0.65O)MnO3 is a strongly correlated insulator and the interface was characterized to be insulating. The cell-specific stoichiometric evaluation unveiled an extensive interdiffusion across the interface. The plane-specific charge characterization revealed that the interdiffusion grades the primitive polar discontinuity. Despite the graded polar discontinuity, a charge transfer inversely into (Nd0.35Sr0.65O)MnO3 was firmly resolved with a length scale of ∼2 nm and a charge density on the order of ∼1013/cm2 and is effectively mediated by an asymmetric Ti interdiffusion. The intricate electronic correlations of the interfacial (Nd0.35Sr0.65O)MnO3 unit cells and the interdiffusion-induced chemical disorder tend to render the charges localized, resulting in a localized two-dimensional electron density and thus the insulating interface, in distinct contrast to the conventional understanding of a vanishing charge density for an insulating interface and the metallic two-dimensional electron gas found at other classical polar-discontinuous interface systems. A potential strain manipulation on the electronic localization of the electron density was also proposed. [ABSTRACT FROM AUTHOR]

The diluted magnetic semiconductor (Ga,Mn)As is investigated with respect to its damping behavior using frequency dependent ferromagnetic resonance techniques. In particular we investigate the homogeneity of the spin relaxation mechanisms from a submicrometer length scale to several millimeters using time-resolved scanning Kerr microscopy in combination with ferromagnetic resonance. We find magnetic inhomogeneities on submillimeter and submicrometer length scales which lead to artificially broadened ferromagnetic resonance (FMR) lines in conventional experiments. [ABSTRACT FROM AUTHOR]

The normal incidence x-ray standing wave (NIXSW) technique is used to determine the adsorption geometry of submonolayer 3,4,9,1 0-perylene tetracarboxylic dianhydride (PTCDA) adsorbed on the Ag( 110) surface. An accurate analysis of both C1s and O1s photoemission (PE) spectra allows the respective adsorption heights of carbon and oxygen atoms in different chemical environments within PTCDA to be distinguished. Due to the intricacy of the PE fitting models, a systematic error analysis of NIXSW structural parameters was developed and employed. Based on the adsorption geometry of PTCDA on Ag( 110) a bonding mechanism is discussed. [ABSTRACT FROM AUTHOR]

We study the spin Hall conductance fluctuations in ballistic mesoscopic systems. We obtain universal expressions for the spin and charge current fluctuations, cast in terms of current-current autocorrelation functions. We show that the latter are conveniently parametrized as deformed Lorentzian shape lines, functions of an external applied magnetic field and the Fermi energy. We find that the charge current fluctuations show quite unique statistical features at the symplectic-unitary crossover regime. Our findings are based on an evaluation of the generalized transmission coefficients correlation functions within the stub model and are amenable to experimental test. [ABSTRACT FROM AUTHOR]

Dynamic magnetization reversal in the mesofrequency range is studied by ac magneto-optical Kerr effect (ac-MOKE) and ac anisotropic magnetoresistance (ac-AMR) magnetometry in a series of epitaxial and polycrystalline thin magnetic films. The dynamic coercive field was found to scale as a power law with scaling exponents ≤1/2 depending on the ferromagnetic material. In addition, there is a low sweep rate regime in which the dynamic coercivity reaches a minimum. These findings are explained in the context of reversal proceeding by motion of a few domain walls (~1). At dc and low field sweep rates the reversal proceeds between local pinning sites via Barkhausen avalanches and the overall reversal speed is strongly dependent on the field sweep rate. At higher field sweep rates a continuous motion regime is entered in which the reversal velocity depends linearly on the applied field sweep rate and only an average pinning force is experienced by the wall. The fit of the dynamic coercivity vs applied field sweep rate allows the determination of the average nonlocal pinning field. The nonuniversal scaling exponent can be explained using recently developed models and introducing a field rate-dependent number of active domain walls. [ABSTRACT FROM AUTHOR]

We find that the Hall effect resistivity (ρxy) of thin films of La0.8Sr0.2MnO3 varies as a function of the angle θ between the applied magnetic field and the film normal as ρxy = a cosθ + b cos 3θ, where |b| increases with increasing temperature and decreases with increasing magnetic field. We find that the angular dependence of the longitudinal resistivity and the magnetization cannot fully explain the surprising term b, suggesting it is a manifestation of an intrinsic transport property. [ABSTRACT FROM AUTHOR]

We performed a laser angle-resolved photoemission spectroscopy (ARPES) study on a wide doping range of Ba1-xKxFe2As2 (BaK) and precisely determined the doping evolution of the superconducting gaps in this compound. The gap size of the outer hole Fermi-surface (FS) sheet around the Brillouin zone (BZ) center shows an abrupt drop with overdoping (for x ≳ 0.6) while the inner and middle FS gaps roughly scale with Tc. This is accompanied by the simultaneous disappearance of the electron FS sheet with similar orbital character at the BZ corner. These results indicate the different contributions of X²-Y² and XZ/YZ orbitals to superconductivity in BaK and can hardly be completely reproduced by the available theories on iron-based superconductors. [ABSTRACT FROM AUTHOR]

Flux-grown single crystals of PuPt2In7 are characterized and found to be both nonsuperconducting and nonmagnetic down to 2 K. The Sommerfeld specific heat coefficient of 250 mj/mol K² indicates heavy fermion behavior. We report the results of generalized gradient approximation (GGA) + U calculations of PuPt2In7 and as yet unsynthesized isovalent PuPt2Ga7. The strength of the c-f hybridization of PuPt2In7 is similar to the PuCoIn5 superconductor. The bare and /-weighted susceptibility within the constant-matrix-element approximation is calculated, showing a maximum along the qz direction at qx = qy = 0.5. A similar and slightly stronger maximum is also found in the structurally related heavy-fermion materials PuCoGa5 and PuCoIn5. The absence of superconductivity in PuPt2In7 is examined based on the results of our calculations. [ABSTRACT FROM AUTHOR]

The spin-torque switching of metallic nanopillar spin valves showing strong perpendicular anisotropy are studied. The magnetic states of the layers depend on extrinsic parameters such as the magnetic field and the dc current applied to the device. A state diagram presents a comprehensive graph of the role of those parameters on the spin-valve magnetic response. After explaining how state diagrams can be built and the different possible representation, experimental state diagrams are studied for perpendicular devices and the influence of lateral size, temperature, and field orientation are shown. An analytical model of a purely uniaxial system is presented. It is shown that this simple model does not properly reflect the experimental results, whereas if the symmetry is broken a qualitative agreement is obtained. Finally, the possible origins of the symmetry break are discussed in light of an analytical model and numerical simulations. [ABSTRACT FROM AUTHOR]

Scanning SQUID susceptometry images the local magnetization and susceptibility of a sample. By accurately modeling the SQUID signal we can determine physical properties such as the penetration depth and permeability of superconducting samples. We calculate the scanning SQUID susceptometry signal for a superconducting slab of arbitrary thickness with isotropic London penetration depth λ on a nonsuperconducting substrate, where both slab and substrate can have a paramagnetic response that is linear in the applied field. We derive analytical approximations to our general expression in a number of limits. Using our results, we fit experimental susceptibility data as a function of the sample-sensor spacing for three samples: (1) δ-doped SrTiO3, which has a predominantly diamagnetic response, (2) a thin film of LaNiO3, which has a predominantly paramagnetic response, and (3) the two-dimensional electron layer at a SrTiO3/LaAlO3 interface, which exhibits both types of response. These formulas will allow the determination of the concentrations of paramagnetic spins and superconducting carriers from fits to scanning SQUID susceptibility measurements. [ABSTRACT FROM AUTHOR]

The transition between Kondo and Coulomb blockade effects in discontinuous double magnetic tunnel junctions is explored as a function of the size of the CoPt magnetic clusters embedded between AlOx mnnel barriers. A gradual competition between cotunneling enhancement of the tunneling magnetoresistance (TMR) and the TMR suppression due to the Kondo effect has been found in these junctions, with both effects having been found to coexist even in the same sample. It is possible to tune between these two states with temperature (at a temperature far below the cluster blocking temperature). In addition, when further decreasing the size of the CoPt clusters, another gradual transition between the Kondo effect and direct tunneling between the electrodes takes place. This second transition shows that the spin-flip processes found in junctions with impurities in the barrier are in fact due to the Kondo effect. A simple theoretical model able to account for these experimental results is proposed. [ABSTRACT FROM AUTHOR]

Using piezoresponse force microscopy on epitaxial ferroelectric thin films, we have measured the evolution of domain wall roughening as a result of heat-quench cycles up to 735 °C, with the effective roughness exponent ζ changing from 0.25 to 0.5. We discuss two possible mechanisms for the observed ζ increase: a quench from a thermal one-dimensional configuration and from a locally equilibrated pinned configuration with a crossover from a two- to one-dimensional regime. We find that the postquench spatial structure of the metastable states, qualitatively consistent with the existence of a growing dynamical length scale whose ultraslow evolution is primarily controlled by the defect configuration and heating process parameters, makes the second scenario more plausible. This inteϕretation suggests that pinning is relevant in a wide range of temperatures and, in particular, that purely thermal domain wall configurations might not be observable in this glassy system. We also demonstrate the crucial effects of oxygen vacancies in stabilizing domain structures. [ABSTRACT FROM AUTHOR]

Recently, it has been observed that thin ferromagnetic Fe films deposited on top of (Ga,Mn)As layers induce a significant proximity polarization in the (Ga,Mn)As film even at room temperature. Furthermore, it was found that a thin interfacial region of the (Ga,Mn)As film is coupled antiferromagnetically to the Fe layer. Here we report a series of combined x-ray magnetic dichroism and superconducting quantum interference device magnetometer measurements for Fe/(Ga,Mn)As bilayers where the (Ga,Mn)As layer thickness is varied between 5 and 50 nm. We find a reorientation transition of the magnetic proximity polarization as a function of the (Ga,Mn)As thickness. The data are compared to results obtained performing ab initio calculations. A varying concentration of Mn interstitials as a function of (Ga,Mn)As layer thickness is responsible for this reorientation. Furthermore, exchange bias is studied in the fully epitaxial bilayer system. We find a rather strong ferromagnetic exchange bias. The strength of the exchange bias can be estimated by using a simple partial domain wall model. [ABSTRACT FROM AUTHOR]

The magnetization reversal process within the first two iron layers at the Fe/GaAs(001) interface is found to be different and independent from the Fe thin film bulk as measured by magnetic second-harmonic generation and magneto-optical Kerr effect. The interface magnetization is largely noncollinear from the bulk with an abrupt magnetic boundary and an anisotropic exchange coupling stiffness, weak interlayer coupling but relatively strong intralayer stiffness. In contrast, Fe/GaAs(l 10) exhibits a rigid coupling between interface and bulk magnetization suggesting that the interfacial bonding structure can dramatically change the nature of the exchange coupling. Moreover, the uniaxial magnetic anisotropy in Fe/GaAs(001) extends from the interface to the first 5 nm in the Fe film and is induced by stress. These results are also relevant to other magnetic/nonmagnetic interfaces with abrupt chemical bond structures. [ABSTRACT FROM AUTHOR]

The fractional quantum Hall (FQH) regime of the Si two-dimensional electron system (2DES) in enhancement-mode field-effect transistors of Si/SiGe heterostructures was probed via electrical transport measurements. At n ∼ 2.6 x 1011 cm2 with µ = 1.6 x 106 cm2/Vs, signatures of FQH states at filling factors v = 4/5, 6/5, and 10/7 were observed, in addition to the FQH states reported in previous studies. The temperature dependence of the FQH states is investigated and comparison is made with previous work done on modulation-doped samples. Results indicate that robustness of the FQH states is dependent on the nature of disorder in the 2DES. [ABSTRACT FROM AUTHOR]

The upper critical fields Hc2(T) of single crystals of Sr1-xEux(Fe0.89Co0.11)2As2 (x -- 0.20 and 0.46) were determined by radio-frequency penetration depth measurements in pulsed magnetic fields. Hc2(T) approaches the Pauli limiting field but shows an upward curvature with an enhancement from the orbital limited field, as inferred from the Werthamer-Helfand-Hohenberg theory. We discuss the temperature dependence of the upper critical fields and the decreasing anisotropy using a two-band BCS model [ABSTRACT FROM AUTHOR]

Local electronic structures of ruthenocuprate RuSr2EuCu2O8 (RuEu-1212) were investigated by using x-ray absorption near-edge structure (XANES) and valence-band photoemission (VB-PES) measurements at room temperature. 80 K, and 25 K. The XANES results indicate that when RuEu-1212 is below Curie temperature, TM, electrons are transferred not only from Cu 3d states, but also from Ru 4 d states, to O 2p orbitals. Additionally, the partial spectral weight distributions derived from VB-PES measurements provide evidence of a weak overlapping between Ru 4 d states and strongly Cu 3d--O 2p hybridized states below the Fermi level, which results in a weak coupling between the CuO2 and RuO2 layers. The analysis of extended x-ray absorption fine structure shows that the appearance of weak ferromagnetism and superconductivity is accompanied by a significant decrease of dynamic local lattice distortions of high-shell Cu-Sr and Ru-Sr bonding in RuEu-1212. [ABSTRACT FROM AUTHOR]

The magnetocrystalline anisotropy (MCA) of (Ga,Mn)As films has been studied on the basis of ab initio electronic structure theory by performing magnetic torque calculations. An appreciable contribution to the in-plane uniaxial anisotropy can be attributed to an extended region adjacent to the surface. Calculations of the exchange tensor allow to ascribe a significant part to the MCA to the exchange anisotropy, caused either by a tetragonal distortion of the lattice or by the presence of the surface or interface. [ABSTRACT FROM AUTHOR]

We investigated the transport properties of BaFe2As2 single crystals before and after annealing with BaAs powder. The annealing remarkably improves transport properties, in particular, the magnitude of residual resistivity, which decreases by a factor of more than 10. From the resistivity measurement on detwinned crystals, we found that the anisotropy of the in-plane resistivity is remarkably diminished after annealing, indicative of dominant contributions to the charge transport from the carriers with isotropic and high mobility below magnetostructural transition temperature Ts and the absence of nematic state above Ts. We found that the Hall resistivity shows strong nonlinearity against magnetic field, and the magnetoresistance becomes very large at low temperatures. These results give evidence for the manifestation of multiple carriers with distinct characters in the ordered phase below Ts. By analyzing the magnetic-field dependences, we found that at least three carriers equally contribute to the charge transport in the ordered phase, which is in good agreement with the results of recent quantum oscillation measurements. [ABSTRACT FROM AUTHOR]

All-electrical spin-wave spectroscopy and frequency-resolved magneto-optical Kerr-effect measurements are combined to study spin waves propagating through a magnetic antidot lattice nanopatterned from a Ni80Fe20 thin film. Spin waves are injected from a plain film into the antidot lattice and the transmission across the interface is explored in detail for different wavelengths. We find that spin waves with a wavelength much greater than the lattice periodicity are not described well by recently discussed approaches. Instead the spin-wave dispersion is consistent with an effective magnetization smaller than the saturation magnetization measured on the unstructured ferromagnetic material. Consistently, we find that the transmission coefficients are modeled well by assuming an effectively continuous metamaterial for spin waves characterized by the reduced magnetization. The experimental data and interpretation are substantiated theoretically using the plane-wave method and micromagnetic modeling. The results are interesting for the development of frequency-selective mirrors in magnonics through lateral nanopatterning. [ABSTRACT FROM AUTHOR]

The existence of the delocalized-localized quantum phase transition (QPT) in the ohmic spin-boson model has been commonly recognized. While the physics in the localized regime is relatively simple, the delocalized regime shows many interesting behaviors. Here we reveal that in this regime there exists a novel QPT: namely, from a phase without a bound state to a phase with a bound state, which leads to completely different dynamical behaviors in these two phases. If the reservoir is initially in the displaced vacuum state (i.e., the coherent state), the spin dynamics exhibits lossless oscillation when the bound state exists; otherwise, the oscillation will decay completely. This result is compatible with the coherence-incoherence transition occurring in the small-tunneling limit. Our work indicates that the QPT physics in the spin-boson model needs further exploration. [ABSTRACT FROM AUTHOR]

Silica nanowires (SiOx-NWs) embedded with Au peapods have been studied by energy-filtered scanning transmission electron microscopy (EFTEM), O K- and Au L3-edge x-ray absorption near-edge structure (XANES), and extended x-ray absorption fine structure (EXAFS), x-ray emission spectroscopy (XES) and scanning photoelectron microscopy. XANES and XES data show that band gaps of Au-peapod-embedded and pure SiOx-NWs were 6.8 eV. In additional, XANES and EXAFS results indicate illumination-induced electron transfer from Au peapod to SiOx-NWs and does not show any feature attributable to the formation of Au-Si bonding in the Au peapod embedded in SiOx-NWs with or without illumination. Photoresponse and EFTEM measurements show that green light has more significant enhancement of photoconductivity than red and blue light due to surface plasmon resonance and suggest that transport of electrons across SiOx-NWs is via Mott-variable-range hopping mechanism through localized or defect states. [ABSTRACT FROM AUTHOR]

We report the results of time- and spin-resolved photoemission (TR-SPES) and time-resolved magneto-optical Kerr effect experiments on iron thin films. In particular, the extracted demagnetization times for both techniques are compared. It is shown that while for the Kerr measurements the demagnetization times are always limited by our time resolution (250 ± 30 fs), for the TR-SPES measurements this situation occurs only for relative quenching below 30%. Above this value, the measured TR-SPES demagnetization time exceeds 500 fs. Different demagnetization probes can hence track different demagnetization times. [ABSTRACT FROM AUTHOR]

We present evidence for a concomitant structural and ferroelectric transformation around Ts ~ 360 K in multiferroic BiFeO3/LaAlO3 thin films close to the tetragonal phase. Phonon excitations are investigated by using Raman scattering as a function of temperature. The low-energy phonon modes at 180-260 cm-1 related to the FeO6 octahedron tilting show anomalous behaviors upon cooling through Ts: (i) a large hardening amounting to 15 cm-1, (ii) an increase of intensity by one order of magnitude, and (iii) an appearance of a dozen new modes. In contrast, the high-frequency modes exhibit only weak anomalies. This suggests an intimate coupling of octahedron tilting to ferroelectricity leading to a simultaneous change of structural and ferroelectric properties. [ABSTRACT FROM AUTHOR]

The properties of Shockley-type interface states between ir-conjugated organic molecular layers and metal surfaces are investigated by time-resolved two-photon photoemission experiments and density functional theory. For perylene- and naphthalene-tetracarboxylic acid dianhydride (PTCDA and NTCDA) adsorbed on Ag(111), a common mechanism of formation of the interface state from the partly occupied surface state of the bare Ag(111) is revealed. The energy position is found to be strongly dependent on the distance of the molecular carbon rings from the metal and their surface density. Bending of the carboxyl groups enhances the molecular overlap of the interface state. [ABSTRACT FROM AUTHOR]

The first step required for oxide heteroepitaxy with atomically abrupt interfaces on Si and Ge(100) is the formation of an alkaline-earth template layer. The atomic structure of this template layer on Ge(100) was characterized using scanning tunneling microscopy and electron diffraction. At elevated temperatures, Sr immediately roughens the surface; a transition that can be associated with Sr displacing Ge from the surface. With increasing Sr coverage a series of ordered (3 × 4), (3 × 2), (9 × 1), and (6 × 1) phases were observed. Transitions between these phases were accompanied by morphological changes: formation of the (3 × 4) phase smoothed surface; transition to a local (3 × 2) ordering was accompanied by trench formation; ordering of the trenches led to the (9 × 1) structure; and finally, the (6 × 1) structure was characterized by atomic rows. For both Sr and Ba, highly ordered arrays of one-dimensional islands could be produced with double-height steps preventing orthogonal domain formation. We associate the morphological transitions with strain relief of the surface phases and interactions of step ledges. [ABSTRACT FROM AUTHOR]

We have studied how the magnetic properties of oxygen-deficient EuO sputtered thin films vary as a function of thickness. The magnetic moment, measured by polarized neutron reflectometry, and the Curie temperature are found to decrease with reducing thickness. Our results indicate that these surface-induced effects are caused by the reduced number of nearest neighbors, band bending, and the partial depopulation of the 4f states of Eu. [ABSTRACT FROM AUTHOR]

The elastic moduli of pure polycrystalline beta plutonium were measured over its full range of existence (417-491 K) using resonant ultrasound spectroscopy. The Debye temperature (138 K), Poisson's ratio (0.28), Gruneisen parameter (2.3), and the zero-temperature atomic volume (21.2 Å3) were computed from the measurements. Both bulk and shear moduli decrease smoothly on warming with expected discontinuities at the phase boundaries. The shear modulus is surprisingly nearly the same for beta and gamma Pu. The temperature dependence of bulk moduli for beta Pu is, like gamma Pu, unusually small. Poisson's ratio shows very strong differences among alpha, beta, and gamma Pu indicating they are entirely different metals. The zero-temperature elastic moduli were computed for the three phases as well as for gallium-stabilized delta Pu (also measured by us) and compared to calculations. [ABSTRACT FROM AUTHOR]

An efficient electron spin-relaxation mechanism has been observed in InAs quantum dots (QDs) that manifests itself as a sharp drop in the circular polarization of the light emitted by Fe spin-light emitting diodes, which incorporate a single layer of InGaAs QDs, for a narrow range of magnetic fields around 5 T. The underlying mechanism occurs when the QDs are occupied by three-electron-hole pairs forming a tri-exciton (3X) and is a two-step process. The first step involves the spin flip of one of the three electrons mediated by the spin-orbit interaction; in the second step the 3X relaxes to its ground state via phonon emission. [ABSTRACT FROM AUTHOR]

By neutron scattering, we have observed a well-ordered magnetic vortex lattice (VL) in KFe2As2 single crystals. With the field along the c axis, a nearly isotropic hexagonal VL is formed, with no symmetry transitions up to high fields, indicating a small anisotropy of the superconducting state around this axis. This rules out line nodes parallel to the c axis, and thus d-wave or anisotropic s-wave pairing. However, the strong temperature dependence of the signal at T « Tc shows that extremely small gap values exist; these may arise from nodal lines perpendicular to the c axis. [ABSTRACT FROM AUTHOR]

Describing the origin of uniaxial magnetic anisotropy (UMA) is generally problematic in systems other than single crystals. We demonstrate an in-plane UMA in amorphous CoFeB films on GaAs(001) which has the expected symmetry of the interface anisotropy in ferromagnetic films on GaAs(001), but strength which is independent of, rather than in inverse proportion to, the film thickness. We show that this volume UMA is consistent with a bond-orientational anisotropy, which propagates the interface-induced UMA through the thickness of the amorphous film. It is explained how, in general, this mechanism may describe the origin of in-plane UMAs in amorphous ferromagnetic films. [ABSTRACT FROM AUTHOR]

We report on observation of coherent longitudinal acoustic phonons in Fe/AlGaAs (001) heterostructure by time-resolved second-harmonic generation (TRSHG). The phonon induces not only a fast and highly damped oscillatory signal but also a long-lived one that exhibits an oscillating period identical to that obtained from the reflectivity measurement. The long-lived oscillatory second-harmonic generation is generated by an electric field modulated at 42.5 GHz because of the interference of the incident beam and the reflected light by the propagating phonon. TRSHG lets us probe the internal field modulated by phonons and differentiate their impacts at the interface and in the bulk. [ABSTRACT FROM AUTHOR]

Annealing conditions are critical to the properties of epitaxial graphene formed by thermal decomposition of silicon carbide. Here, we report the evolution of coherent electronic transport with increasing anneal temperatures, combined with low energy electron micrographs of equivalent surfaces showing corresponding structural coherence. Ultrahigh vacuum conditions and temperatures in the range of 1250-1300 °C produce granular films with a lateral grain size of ~20 nm, while temperatures of 1400 °C or higher result in grains with progressively larger lateral dimensions in the micron range. Transport measurements show how the electronic coherence length increases as a result of the more coherent physical structure, with a crossover from two-dimensional variable range hopping to the weak localization regime. Here, we show that while the duration of the anneal affects coverage and clustering of grains, the size of individual grains is determined by anneal temperature, with evidence of coalescence of smaller grains into larger domains, suggesting that multistage anneals at different temperatures may yield high-quality graphene. [ABSTRACT FROM AUTHOR]

Low-temperature specific heat (SH) has been measured in KxFe2-ySe2 single crystals with Tc=32 K. The SH anomaly associated with the superconducting transition is moderate and sharp, yielding a value of ΔC/T∣Tc=11.6±1.0 mJ/mol K². The residual SH coefficient γ(0) in the superconducting state at T→0 is very small with a value of about 0.39 mJ/mol K². The magnetic-field-induced enhancement of the low-T SH exhibits a rough linear feature indicating a nodeless gap. This is further supported by the scaling based on the nodeless-gap approach of the low-T data at different magnetic fields. A rough estimate indicates that the normal-state SH coefficient γn is about 6±0.5 mJ/mol K², leading to ΔC/γnT∣Tc=1.93 and placing this superconductor in the strong coupling region. [ABSTRACT FROM AUTHOR]

Threshold photoemission magnetic circular dichroism of perpendicularly magnetized Ni films on Cu(001) was measured in total electron yield and used to observe the magnetic domain structure in a photoemission electron microscope. Spin-polarized relativistic Korringa-Kohn-Rostoker Green's function calculations including a dynamical mean-field theory approach within the one-step-photoemission model reproduce the measured asymmetry in the photocurrents for left and right circularly polarized light. In addition, a three-step photoemission model calculation based on the same ab initio calculation is used to quantitatively explain the MCD effect near the photoemission threshold. [ABSTRACT FROM AUTHOR]

The spin dependence of the photoelectron tunnel current from free-standing GaAs films into out-of-plane magnetized cobalt films is demonstrated. The measured spin asymmetry (A), resulting from a change in light helicity, reaches ±6% around zero applied tunnel bias and drops to ±2% at a bias of -1.6 V applied to the GaAs. This decrease is a result of the drop in the photoelectron-spin polarization that results from a reduction in the GaAs surface-recombination velocity. The sign of A changes with that of the cobalt magnetization. In contrast, A is negligible on nonmagnetic gold films. [ABSTRACT FROM AUTHOR]

Using x-ray photoemission electron microscopy and the magneto-optical Kerr effect, we have demonstrated a perpendicular magnetic anisotropy that could be due to exchange coupling between the ferromagnetic and antiferromagnetic layers. The results of magnetic imaging and hysteresis loops show that the magnetization of Fe and permalloy (Py) films orients from the in-plane to perpendicular direction, as an Mn underlayer is above a threshold value that depends on the Fe or Py layer thickness. Their thickness-dependent behaviors can be quantitatively described by a phenomenological model that takes into account the finite-size effect of the antiferromagnet on exchange coupling. The anisotropy energy extracted from the model and the thermal stability of perpendicular magnetization enhanced with the increase of the Mn underlayer further demonstrate the exchange coupling nature. [ABSTRACT FROM AUTHOR]

We report magnetic quantum oscillations in magnetic fields extending to ∼60 T in single crystals of the body-centered tetragonal antiferromagnet CePt2In7--recently discovered to exhibit pressure-induced superconductivity at Tc≈ 2.1 K. Despite the two-dimensionality of its Fermi surface, the microscopic electronic properties of layered CePt2In7 are revealed to be more similar to those of cubic CeIn3 than those of layered CeRhIn5. A significant field-induced change in the Fermi surface occurs below Hm≈ 45 T in both CePt2In7 and CeIn3, where it is broken into small pockets with field-dependent effective masses-signaling partial 4f-electron involvement in the Fermi surface for H

CePt2In7 is a heavy fermion system with an antiferromagnetic transition at TN = 5.2 K at ambient pressure. 195Pt Knight shift measurements reveal anomalous behavior below T* ~40 K. By comparing the susceptibility and Knight shift data, we extract the individual contributions of the local moments and the heavy electron components to the magnetic susceptibility. Our results confirm the scaling predictions of the standard model, but reveal a surprising result, namely, that below 14 K the hybridized quasiparticles begin to "relocalize," an important precursor to their joining the local moments in becoming antiferromagnetically ordered at TN. [ABSTRACT FROM AUTHOR]

In multiferroic TbMn2O5, the behavior of the terbium ions forms a crucial part of the magnetoelectric coupling. The result is a magnetically induced reversal of the electric polarization at 2 T. In this article we present the first direct measurement of the terbium magnetic structure under applied magnetic fields. Contrary to the current interpretation of the magnetic properties of TbMn2O5, we show that upon the electric polarization reversal the terbium sublattice adopts a canted antiferromagnetic structure with a large component of magnetic moment parallel to the a axis. Furthermore, we provide evidence for a coupling between the manganese 3d magnetic structure and the terbium 4∫ magnetism, which is of great significance in the elusive magnetoelectric mechanisms at play. [ABSTRACT FROM AUTHOR]