Search

We systematically investigate the spin-pumping effect in a series of yttrium-iron-garnet (YIG)-platinum (Pt) bilayers, comparing both broadband ferromagnetic resonance and electrically detected spin-pumping experiments. We infer the effective spin-mixing conductance as a function of YIG and Pt layer thickness from ferromagnetic resonance measurements on pure YIG layers as well as on bilayer YIG/Pt samples in a frequency range from 2 to 20 GHz. In bare YIG films, we determine a Gilbert damping constant of approximately α=0.001. With an additional platinum layer on top of the YIG samples, the damping is significantly enhanced. The effective spin-mixing conductance extracted from the ferromagnetic resonance experiments is in agreement with that determined from independent electrically detected spin-pumping experiments. [ABSTRACT FROM AUTHOR]

We report that the spin current generated by the spin Seebeck effect (SSE) in yttrium iron garnet (YIG) can be detected by a ferromagnetic metal (NiFe). By using the ferromagnetic/antiferromagnetic (FM/AFM) exchange bias structure (NiFe/IrMn), the inverse spin Hall effect (ISHE) and planar Nernst effect (PNE) of NiFe can be unambiguously separated, allowing us to observe a pure ISHE signal. After eliminating the in-plane temperature gradient in NiFe, we can even observe a pure ISHE signal without PNE from NiFe itself. It is worth noting that a large spin Hall angle (0.098) of NiFe is obtained, which is comparable with Pt. This work provides a kind of FM/AFM exchange bias structure to detect the spin current by charge signals, and highlights that ISHE in ferromagnetic metals can be used in spintronic research and applications. [ABSTRACT FROM AUTHOR]

We have investigated the vibrational properties of van der Waals heterostructures of monolayer transition metal dichalcogenides (TMDs), specifically MoS2/WSe2 and MoSe2/MoS2 heterobilayers and twisted MoS2 bilayers, by means of ultralow-frequency Raman spectroscopy. We discovered Raman features (at 30-40 cm-1) that arise from the layer-breathing mode (LBM) vibration between the two incommensurate TMD monolayers in these structures. The LBM Raman intensity correlates strongly with the suppression of photoluminescence that arises from interlayer charge transfer. The LBM is generated only in bilayer areas with direct layer-layer contact and an atomically clean interface. Its frequency also evolves systematically with the relative orientation between the two layers. Our research demonstrates that the LBM can serve as a sensitive probe to the interface environment and interlayer interactions in van der Waals materials. [ABSTRACT FROM AUTHOR]

Quantization of spin-wave excitations necessitates the reconsideration of the classical fluctuation-dissipation relation (FDR) used for temperature control in spin-lattice dynamics simulations of ferromagnetic metals. In this paper, Bose-Einstein statistics is used to reinterpret the Langevin dynamics of both lattice and spins, allowing quantum statistics to be mimicked in canonical molecular dynamics simulations. The resulting quantum heat baths are tested by calculating the specific heats and magnetization over a wide temperature range, from 0 K to above the Curie temperature, with molecular dynamics (MD), spin dynamics (SD), and spin-lattice dynamics (SLD) simulations. The results are verified with experimental data and available theoretical analysis. Comparison with classical results also shows the importance of quantization effects for spin excitations in all the ferromagnetically ordered configurations. [ABSTRACT FROM AUTHOR]

We study, theoretically and numerically, the onset and development of modulational instability in an incoherently pumped spatially homogeneous polariton condensate. Within the framework of mean-field theory, we identify regimes of modulational instability in two cases: (1) strong feedback between the condensate and reservoir, which may occur in scalar condensates, and (2) parametric scattering in the presence of polarization splitting in spinor condensates. In both cases we investigate the instability-induced textures in space and time including nonequilibrium dynamics of phase dislocations and vortices. In particular we discuss the mechanism of vortex destabilization and formation of spiraling waves. We also identify the presence of topological defects, which take the form of half-vortex pairs in the spinor case, giving an "eyelet" structure in intensity and dipole-type structure in the spin polarization. In the modulationally stable parameter domains, we observe formation of the phase defects in the process of condensate formation from an initially spatially incoherent low-density state. In analogy to the Kibble-Zurek-type scaling for nonequilibrium phase transitions, we find that the defect density scales with the pumping rate. [ABSTRACT FROM AUTHOR]

We observe a bistability in an electrically driven polariton condensate, which is manifested by a memory dependent threshold characteristic. In contrast to the polariton bistabilities previously observed in resonantly optically pumped microcavities, our effect occurs under nonresonant electric pumping and is triggered by the current injection scheme. We explain the origin of the bistability by a dependence of the electron-hole tunneling lifetime on the carrier density in the embedded quantum wells. The field screening effect creates a positive feedback loop, which yields the bistable behavior of the condensate. We develop a rate-equation-based model which qualitatively explains the occurrence of the hysteresis under current injection, its reduction with increased magnetic field, and the absence of bistability under optical pumping. [ABSTRACT FROM AUTHOR]

We introduce and model spin-Rabi oscillations based on exciton-polaritons in semiconductor microcavities. The phase and polarization of oscillations can be controlled by resonant coherent pulses and the propagation of oscillating domains gives rise to phase-dependent interference patterns in real space. We show that interbranch polariton-polariton scattering controls the propagation of oscillating domains, which can be used to realize logic gates based on an analog variable phase. [ABSTRACT FROM AUTHOR]

We consider adsorbates on metallic armchair-graphene nanoribbons (AGNRs). An adsorbed atom on a nodal site of the gapless subband in the AGNR is found to induce a bound state. The nodal-adsorbate bound state alone does not cause reflection in the AGNR quantum transport in the one propagating-channel regime. Yet its manifestation, as a Fano resonance in G, is brought forth by the presence of a non-nodal adsorbate (or a scatterer) located on a longitudinally separated site. Both the adsorbate-dependent nature and the significantly enhanced energy resolution of the Fano structure in G invite adsorbate recognition. The Fano peak is shown to be robust against weak disorder such as from edge vacancies, due to the localized nature of the Fano resonant state. Adsorbates F and OH, described within an extended Hiickel model, are considered as examples. [ABSTRACT FROM AUTHOR]

Polarized neutron scattering experiments on the slightly overdoped superconductor Ba0.5K0.5Fe2As2 reveal broad magnetic resonance scattering peaking at approximately 15 meV. In spite of doping far beyond the suppression of magnetic order, this compound exhibits dispersive and anisotropic magnetic excitations. At energies below the resonance maximum, magnetic correlations polarized parallel to the layers but perpendicular to the propagation vector are reduced by a factor of 2 compared to those in the two orthogonal directions; in contrast, correlations at the peak maximum are isotropic. [ABSTRACT FROM AUTHOR]

The crystal structure and charge and orbital order of magnetite below the Verwey temperature are calculated using a first-principles hybrid density functional theory (DFT) method. The initial atomic positions in the crystal-structure calculation are those recently refined from x-ray diffraction data for the Cc space-group unit cell [Senn, Wright, and Attfield, Nature (London) 481, 173 (2012)]. Fermi contact and magnetic dipolar contributions to hyperfine fields at 57Fe nuclei obtained from hybrid DFT calculations are used to obtain NMR resonance frequencies for magnetite for a range of external magnetic field directions in a relatively weak field. NMR frequencies from hybrid density functional theory calculations are compared to NMR data [M. Mizoguchi, J. Phys. Soc. Jpn. 70, 2333 (2001)] for a range of applied magnetic field directions. NMR resonance frequencies of B-site Fe ions show large relative variations with applied field direction owing to anisotropic hyperfine fields from charge and orbital ordered Fe 3d minority-spin electrons at those sites. Good agreement between computed and measured NMR resonance frequencies confirms the pattern of charge and orbital order obtained from calculations. The charge and orbital order of magnetite in its low-temperature phase obtained from hybrid DFT calculations is analyzed in terms of one-electron bonds between Fe ions. The Verwey transition in magnetite therefore resembles Mott-Peierls transitions in vanadium oxides which undergo symmetry-breaking transitions owing to electron-pair bond formation. [ABSTRACT FROM AUTHOR]

We report dielectric properties and dielectric relaxation behaviors of Mn-substituted CaCu3Ti4O12 (CCTO) on Cu sites. While CCTO exhibits the giant dielectric constant and low dielectric loss in a wide temperature range, drastic suppression of the dielectric constant in Mn-doped CCTO (CCMTO) samples have been observed in temperature and frequency dependencies of dielectric properties with two possible origins as Mn doping increases. The observed suppression of dielectric response in the low Mn doping differs from the heavy doping of Mn in CCMTO samples. The low-Mn-doped CCMTO samples (JC = 0.01 and 0.02) show that the relaxation time and the activation energy Ea were slightly reduced due to a decreased contribution from the density of the dipolar effect. However, in heavily doped CCMTO samples (x = 0.03,0.04, and 0.05), the dielectric response, relaxation time, and Ea were significantly decreased, suggesting Mn doping plays a significant role in the destruction of the intrinsic dipolar effect. [ABSTRACT FROM AUTHOR]

We report extensive measurements on a new compound (Yb0.24Sn0.76)Ru that crystallizes in the cubic CsCl structure. Valence-band photoemission (PES) and L3 x-ray absorption show no divalent component in the 4f configuration of Yb. Inelastic neutron scattering (INS) indicates that the eight-fold degenerate J-multiplet of Yb3+ is split by the crystalline electric field (CEF) into a Γ7-doublet ground state and a Γ8 quartet at an excitation energy 20 meV. The magnetic susceptibility can be fit very well by this CEF scheme under the assumption that a Γ6-excited state resides at 32 meV; however, the Γ8/Γ6 transition expected at 12 meV was not observed in the INS. The resistivity follows a Bloch-Grüneisen law shunted by a parallel resistor, as is typical of systems subject to phonon scattering with no apparent magnetic scattering. All of these properties can be understood as representing simple local moment behavior of the trivalent Yb ion. At 1 K there is a peak in specific heat that is too broad to represent a magnetic-phase transition, consistent with absence of magnetic reflections in neutron diffraction. On the other hand this peak also is too narrow to represent the Kondo effect in the Γ7-doublet ground state. On the basis of the field dependence of the specific heat, we argue that antiferromagnetic (AF) short-range order (SRO) (possibly coexisting with Kondo physics) occurs at low temperatures. The long-range magnetic order is suppressed because the Yb site occupancy is below the percolation threshold for this disordered compound. [ABSTRACT FROM AUTHOR]

We use optical spectroscopy to investigate the electron-phonon interaction in Sr2IrO4, a well-known 5d transition metal oxide with spin-orbit entangled states. The temperature evolution in the optical spectra is well described by the Fröhlich polaron model, indicating a large electron-phonon interaction. We further find that electrons in different orbitals selectively couple with different phonon modes. While Jeff = 3/2 holes do not seem to couple with any phonons, Jeff = 1/2 and 3z²-r² electrons mainly couple with in-plane and out-of-plane Ir-O bending modes, respectively. The symmetries of the orbitals and phonons are consistent with our observations. [ABSTRACT FROM AUTHOR]

We present a time-resolved photoluminescence (PL) study in real and momentum space of a polariton condensate switch in a quasi-one-dimensional semiconductor microcavity. The polariton flow across the ridge is gated by excitons inducing a barrier potential due to repulsive interactions. A study of the device operation dependence on the power of the pulsed gate beam obtains a satisfactory compromise for the on-off signal ratio and switching time of the order of 0.3 and ~50 ps, respectively. The opposite transition is governed by the long-lived gate excitons, consequently, the off-on switching time is ~200 ps, limiting the overall operation speed of the device to ~3 GHz. The experimental results are compared to numerical simulations based on a generalized Gross-Pitaevskii equation, taking into account incoherent pumping, decay, and energy relaxation within the condensate. [ABSTRACT FROM AUTHOR]

The role of quasicylindrical waves and surface plasmon polaritons in beam shaping with resonant nanogratings is investigated. It is shown that the field on the grating surface can be strongly influenced by plasmons and quasicylindrical waves in the infrared. A method that combines far-field measurements with the fast Fourier transform to map the field amplitude at the grating surface is demonstrated. For samples with a small degree of geometric asymmetry, it is shown that the imaginary part of the transform (with null zeroth-order component) can better map the amplitude of the resonant surface waves than the full complex-valued transform. Our results will impact the study, design, and footprint of resonant nanogratings. [ABSTRACT FROM AUTHOR]

The tetragonal rare-earth transition-metal silicide system with three-dimensional crystallographic structure R5T4Si10, where R is Dy, Ho, Er, Tm, and Lu, and T = Ir and Rh, has been shown to exhibit fascinating chargedensity- wave (CDW) phase transitions, a phenomenon largely found in otherwise low-dimensional systems. In this study, we report the investigations of CDW in Dy5Ir4Si10 at different temperatures using transmission electron microscopy techniques including electron diffraction and dark-field imaging. Incommensurate superlattice spots along the c axis were observed in the electron-diffraction patterns when the sample was cooled below the CDW transition temperature at ~208 K. CDW becomes commensurate with further cooling and configurations of CDW dislocations convincingly show that the CDW phase transition is accompanied by a concomitant cell-doubling crystallographic structural phase transition. Intriguingly, the cell-doubling transition is featured by a broken inversion symmetry along the c axis and a disparity in the CDW-modulation vectors with opposite signs, which gives rise to two sets of CDW domains with reversed contrasts. The profound physics underlining this notable domain-contrast behavior is discussed. [ABSTRACT FROM AUTHOR]

We examined the temperature (T) evolution of the optical conductivity spectra of Sr3Ir2O7 over a wide range of 10-400 K. The system was barely insulating, exhibiting a small indirect bandgap of ⩽0.1 eV. The low-energy features of the optical d-d excitation (ħω < 0.3 eV) evolved drastically, whereas such evolution was not observed for the O K-edge x-ray-absorption spectra. This suggests that the T evolution in optical spectra is not caused by a change in the bare (undressed) electronic structure, but instead presumably originates from an abundance of phonon-assisted indirect excitations. Our results showed that the low-energy excitations were dominated by phonon-absorption processes which involved, in particular, the optical phonons. This implies that phonon-assisted processes significantly facilitate the charge dynamics in barely insulating Sr3Ir2O7. [ABSTRACT FROM AUTHOR]

A magnetoresistance method is proposed which extracts both the growth direction mobility and the planar mobility of an anisotropic thin film, such as a superlattice. A magnetic field which varies in magnitude and orientation is employed. Errors appearing in previous magnetotransport studies on superlattices are resolved. In particular, it is shown that the contact layers used to send currents through an active region do not have a negligible resistance, even if they are much more conductive than the active region. The technique is demonstrated on an InAs/GaSb-based structure where the growth axis to planar mobility anisotropy ratio is found to drop rapidly with increasing temperature. [ABSTRACT FROM AUTHOR]

We study theoretically the ground states of topological defects in a spinor four-component condensate of cold indirect excitons. We analyze possible ground-state solutions for different configurations of vortices and half vortices. We show that if only Rashba or Dresselhaus spin-orbit interaction (SOI) for electrons is present the stable states of topological defects can represent a cylindrically symmetric half vortex or half-vortex-antivortex pairs or a nontrivial pattern with warped vortices. In the presence of both Rashba and Dresselhaus SOI the ground state of a condensate represents a stripe phase and vortex type solutions become unstable. [ABSTRACT FROM AUTHOR]

We study the dynamics of polariton condensate wave trains that propagate along a quasi-one-dimensional waveguide. Through the application of tuneable potential barriers the propagation can be reflected and multiple reflections used to confine and store a propagating state. Energy-relaxation processes allow the delayed relaxation into a long-living coherent ground state. Aside from the potential routing of polariton condensate signals, the system forms an AND-type logic gate compatible with incoherent inputs. [ABSTRACT FROM AUTHOR]

Polariton condensates have proved to be model systems to investigate topological defects, as they allow for direct and nondestructive imaging of the condensate complex order parameter. The fundamental topological excitations of such systems are quantized vortices. In specific configurations, further ordering can bring the formation of vortex lattices. In this work we demonstrate the spontaneous formation of ordered vortical states, consisting in geometrically self-arranged vortex-antivortex pairs. A mean-field generalized Gross-Pitaevskii model reproduces and supports the physics of the observed phenomenology. [ABSTRACT FROM AUTHOR]

Building on spin-modified Coulomb blockade theory, we derive an approximate analytical solution which matches closely to the existing numerical models in the limit of a large difference in the resistance of the two tunnel junctions. Using this, we are able to explore the properties of an I(V) measurement for a double tunnel junction incorporating a nanoparticle in various magnetic configurations. It is shown that this information allows, under certain conditions, seven of the device's physical parameters to be determined in a single I(V) sweep, including individual junction capacitances and spin-dependent resistances. The tunnel magnetoresistance and spin polarizations are also examined in the light of this solution and are shown to be similar in nature and dominated by the magnetic properties of the most resistive barrier. It is shown that spin injection and detection are theoretically possible in a single device containing a nonmagnetic nanoparticle, suggesting a novel means of measuring the tunnel spin polarization of ferromagnetic materials. [ABSTRACT FROM AUTHOR]

Epitaxial Y3Fe5O12 thin films deposited by off-axis sputtering exhibit excellent crystalline quality enabling observation of large spin pumping signals in Pt/Y3Fe5O12 and W/Y3Fe5O12 bilayers driven by cavity ferromagnetic resonance. The inverse spin Hall voltages reach 2.10 and --5.26 mV in 5-mm-long Pt/Y3Fe5O12 and W/Y3FesO12 bilayers, respectively, excited by a radio-frequency magnetic field of 0.3 Oe. From the ferromagnetic resonance linewidth broadening, we obtain high interfacial spin mixing conductances of 4.56 × 1014 and 2.30 × 1014 Ω-1 m-2 for Pt/Y3Fe5O12and W/Y3Fe5O12 bilayers, respectively. [ABSTRACT FROM AUTHOR]

The valley-orbit coupling in a few-electron Si quantum dot is expected to be a function of its occupation number N. We study the spectrum of multivalley Si quantum dots for 2≤N≤4, showing that, counterintuitively, electron-electron interaction effects on the valley-orbit coupling are negligible. For N=2 they are suppressed by valley interference, for N=3 they vanish due to spinor overlaps, and for N=4 they cancel between different pairs of electrons. To corroborate our theoretical findings, we examine the experimental energy spectrum of a few-electron metal-oxide-semiconductor quantum dot. The measured spin-valley state filling sequence in a magnetic field reveals that the valley-orbit coupling is definitively unaffected by the occupation number. [ABSTRACT FROM AUTHOR]

We investigated the electronic structure of Na2IrO3 using optical spectroscopy, first-principles calculation, and x-ray absorption spectroscopy. We found that the electronic structure of Na2IrO3 is mainly determined by anisotropic hopping interactions and spin-orbit coupling. Due to the hopping interaction, the orbital character of the bands near the Fermi level deviates from the spin-orbit coupling-induced Jeff = 1/2 states. Polarization-dependent O 1s x-ray absorption spectroscopy showed that the Jeff = 1/2 state of an Ir atom can be mixed with the Jeff = 3/2 state of the neighboring Ir atom. This result implies that mixing between the Jeff = 1/2 and 3/2 states in the valence state should be carefully considered in proposed exotic states of Na2IrO3, such as topological insulator and quantum spin liquid states. [ABSTRACT FROM AUTHOR]

We present a time-resolved study of energy relaxation and trapping dynamics of polariton condensates in a semiconductor microcavity ridge. The combination of two nonresonant, pulsed laser sources in a GaAs ridge-shaped microcavity gives rise to profuse quantum phenomena where the repulsive potentials created by the lasers allow the modulation and control of the polariton flow. We analyze in detail the dependence of the dynamics on the power of both lasers and determine the optimum conditions for realizing an all-optical polariton condensate transistor switch. The experimental results are interpreted in the light of simulations based on a generalized Gross-Pitaevskii equation, including incoherent pumping, decay, and energy relaxation within the condensate. [ABSTRACT FROM AUTHOR]

We investigate theoretically the effect of an external magnetic field on polarization patterns appearing in quantum microcavities due to the optical spin Hall effect. We show that increase of the magnetic field perpendicular to the plane of the cavity resulting in the increase of the Zeeman splitting leads to the transition from azimuthal separation of polarizations to their radial separation. This effect can be straightforwardly detected experimentally. [ABSTRACT FROM AUTHOR]

We present a comparison between experimental and theoretical values of transport τt and quantum τq scattering lifetimes in high-quality undoped Al0.34Ga0.66As/GaAs heterostructures. We obtain excellent agreement between the experimental and modeled scattering times using three scattering processes: background impurity, interface roughness, and remote ionized impurity scattering from surface states. We show that despite the high mobility μpeak = 5.6 × 106 cm² V-1 s-1), the quantum lifetime τq is significantly reduced by small-angle scattering from remote surface charge. We further show that in shallow devices scattering from surface charges will be a limiting factor for both transport and quantum lifetimes. [ABSTRACT FROM AUTHOR]

We present a complete photonic logic gate architecture in a compact solid-state system, making use of the nonlinear and spintronic properties of exciton polaritons in semiconductor microcavities. The dynamics of the system is modeled using the spinor Gross-Pitaevskii equations, and it is shown that the proposal fulfills all the necessary criteria for fully functioning information processing devices without the use of any external electric fields. [ABSTRACT FROM AUTHOR]

We present a detailed report of microwave irradiation of ultranarrow superconducting nanowires. In our nanofabricated circuits containing a superconducting NbSi nanowire, a dc blockade of current flow was observed at low temperatures below a critical voltage Vc, a strong indicator of the existence of quantum phase-slip (QPS) in the nanowire. We describe the results of applying microwaves to these samples, using a range of frequencies and both continuous-wave and pulsed drive, in order to search for dual Shapiro steps which would constitute an unambiguous demonstration of quantum phase-slip. We observed no steps, and our subsequent thermal analysis suggests that the electron temperature in the series CrO resistors was significantly elevated above the substrate temperature, resulting in sufficient Johnson noise to wash out the steps. To understand the system and inform future work, we have constructed a numerical model of the dynamics of the circuit for dc and ac bias (both continuous-wave and pulsed drive signals) in the presence of Johnson noise. Using this model, we outline important design considerations for device and measurement parameters which should be used in any future experiment to enable the observation of dual Shapiro steps at experimentally accessible temperatures and, thus, lead to the development of a QPS-based quantum current standard. [ABSTRACT FROM AUTHOR]

The asymmetric spin-orbit interactions play a crucial role in realizing topological phases in a noncentrosym-metric superconductor (NCS). We investigate the edge states and the vortex core states in the .s-wave NCS with Dresselhaus (110) spin-orbit coupling by both numerical and analytical methods. In particular, we demonstrate that there exists a semimetal phase characterized by the flat Andreev bound states in the phase diagram of the s-wave Dresselhaus NCS which supports the emergence of Majorana fermions. The flat dispersion implies a peak in the density of states which has a clear experimental signature in the tunneling conductance measurements and the Majorana fermions proposed here should be experimentally detectable. [ABSTRACT FROM AUTHOR]

A concept of complex reflectivity modulation is proposed based on the electrical control of quantum well exciton resonances that influence the propagation of light in a layered semiconductor structure. By variation in exciton energies, both the intensity and the phase of reflected light can be fully controlled. Unlike previous devices, for full complex light modulation, the design is based on a single device in a single structure. The device allows complete 100% intensity contrast and allows for the construction of small pixel sizes with fast response times. [ABSTRACT FROM AUTHOR]

We calculate the spatial structure of four-component spinor systems of mixed bright and dark exciton condensates in coupled quantum wells. The spin-dependent bright-dark exciton conversion and Dresselhaus spin-orbit coupling is found to generate a rich variety of topological elements. By propagating the Gross-Pitaevskii equation in imaginary time, we observe the following: single and multiple polarized vortices; the phase separation of bright and dark excitons; and exotic spatial structures in density and spin polarization. [ABSTRACT FROM AUTHOR]

Understanding interactions between orbital and valley quantum states in silicon nanodevices is crucial in assessing the prospects of spin-based qubits. We study the energy spectra of a few-electron silicon metal-oxide-semiconductor quantum dot using dynamic charge sensing and pulsed-voltage spectroscopy. The occupancy of the quantum dot is probed down to the single-electron level using a nearby single-electron transistor as a charge sensor. The energy of the first orbital excited state is found to decrease rapidly as the electron occupancy increases from Ν = 1 to 4. By monitoring the sequential spin filling of the dot we extract a valley splitting of ~230µβν, irrespective of electron number. This indicates that favorable conditions for qubit operation are in place in the few-electron regime. [ABSTRACT FROM AUTHOR]

We have performed elastic and inelastic neutron scattering experiments on single crystal samples of the coordination polymer compound CuF2(H2O)2(pyz) (pyz = pyrazine) to study the magnetic structure and excitations. The elastic neutron diffraction measurements indicate a collinear antiferromagnetic structure with moments oriented along the [0.7 0 1] real-space direction and an ordered moment of 0.60 ± 0.03μB/Cu. This value is significantly smaller than the single-ion magnetic moment, reflecting the presence of strong quantum fluctuations. The spin wave dispersion from magnetic zone center to the zone boundary points (0.5 1.5 0) and (0.5 0 1.5) can be described by a two-dimensional Heisenberg model with a nearest-neighbor magnetic exchange constant J2D = 0.934 ± 0.0025 meV. The interlayer interaction Jperp in this compound is less than 1.5% of J2D The spin excitation energy at the (0.5 0.5 0.5) zone boundary point is reduced when compared to the (0.5 1 0.5) zone boundary point by ~10.3% ± 1.4%. This zone boundary dispersion is consistent with quantum Monte Carlo and series expansion calculations for the S = 1/2 Heisenberg square lattice antiferromagnet, which include corrections for quantum fluctuations to linear spin wave theory. [ABSTRACT FROM AUTHOR]

Several physical properties of the superconducting Heusler compounds, focusing on two systems (Se, Y, Lu)Pd2Sn and APd2M, where A = Hf, Zr and M = Al, In, are summarized and compared. The analysis of the data shows the importance of the electron-phonon coupling for superconductivity in this family. We report the superconducting parameters of YPd2Sn, which has the highest Tc among all known Heusler superconductors. [ABSTRACT FROM AUTHOR]

We have investigated the mechanisms of photoelectron tunneling into gold from tipless, nitridized, gallium arsenide microcantilevers through thin polymers. A study of the tunnel bias, tunnel distance, and light excitation power dependence of the tunneling photo-current reveals the tunneling process to be Fowler-Nordheim like. Both the value of the dielectric constant and the sign of the tunneling photocurrent at low excitation power suggest the important role of polymer defects. [ABSTRACT FROM AUTHOR]

We present a direct observation of a flipping magnetization of a uniform Fe film/wedged-Mn bilayer induced by a biquadratic-type exchange coupling established at the interface. The element-resolved magnetic imaging shows that the Fe film exhibited a flipping of magnetization between the [&1horbar00] and [010] directions with a periodicity of one monolayer Mn thickness as below a critical temperature. The enhancement of the variation angle with the increase of Mn thickness follows the tendency of the finite-size effect of antiferromagnetism on interface exchange coupling. The flipping magnetization emerging coincidentally with the uncompensated-compensated transition of Mn magnetic surface indicates a frustration-induced biquadratic-type interface exchange coupling, and suggests a layered-like uncompensated AFM ordering for the Mn layer. [ABSTRACT FROM AUTHOR]

The dynamics of electrons in image-potential resonances on the Ag(111) surface, i.e., image-potential states that are resonant with bulk bands, have been studied by time-resolved two-photon photoemission in combination with quantum-beat spectroscopy. Energies and lifetimes of these resonances were determined up to a quantum number n = 7. Both quantities show a hydrogen-like scaling with quantum number n. The measured decay time of the first image-potential state (n = 1), which is still located in the projected band gap, is 31.5(1.5) fs. The decay time decreases to 23(2) fs for the resonance n = 2 and then increases up to 1 ps for n = 7. It is concluded that the elastic decay of the resonances due to elastic electron transfer into the bulk is nearly one order of magnitude faster than the inelastic electron-hole pair decay. Nevertheless, the elastic decay is found to be slower than theoretically predicted. The pure dephasing times of the resonances n = 3-7 are longer than the decay times. This suggests that electron-phonon scattering is weak despite a large bulk penetration of the resonance wave functions. [ABSTRACT FROM AUTHOR]

We study the density of states and localization properties of the lowest Landau levels of graphene at high magnetic fields. We focus on the effects caused by correlated long-range hopping disorder, which, in exfoliated graphene, is induced by static ripples. We find that the broadening of the lowest Landau level shrinks exponentially with increasing disorder correlation length. At the same time, the broadening grows linearly with magnetic field and with disorder amplitudes. The lowest Landau level peak shows a robust splitting, the origin of which we identify as the breaking of the sublattice (valley) degeneracy. [ABSTRACT FROM AUTHOR]

The local structure and 4f orbital occupancy have been investigated in Ce1-xYbxCoIn5 via Yb LIII-edge extended x-ray absorption fine structure (EXAFS), Ce and Yb LIII-edge x-ray absorption near-edge structure (XANES), and angle-resolved photoemission spectroscopy (ARPES) measurements. Yb(III) (4f13) is the hole analog of Ce(III) (4f1). Yb is found to be strongly intermediate-valent in Ce1-xYbxCoIn5 throughout the entire doping range, including pure YbCoIn5, with an f-hole occupancy for Yb of nf≃0.3 (i.e., Yb2.3+), independent of Yb concentration and independent of temperature down to T=20 K. In contrast, the f-electron orbital occupancy for Ce remains close to 1 for all Yb concentrations, suggesting that there is no mutual influence on nf between neighboring Ce and Yb sites. Likewise, ARPES measurements at 12 K have found that the electronic structure along Γ-X is not sensitive to the Yb substitution, suggesting that the Kondo hybridization of Ce f electrons with the conduction band is not affected by the presence of Yb impurities in the lattice. The emerging picture is that in Ce1-xYbxCoIn5 there are two networks, interlaced but independent, that couple to the conduction band: one network of Ce ions in the heavy-fermion limit, and one network of Yb ions in the strongly intermediate-valent limit. The robustness of the local and electronic structure to doping suggests the absence of charge transfer between the Ce and Yb ions, and may explain the relative robustness of superconductivity for this Ce-site substitution as compared to the In-site substitution. [ABSTRACT FROM AUTHOR]

We have performed unpolarized and polarized neutron diffraction experiments on monodisperse 8- and 13-nm antiferromagnetic MnO nanoparticles. For the 8-nm sample, the antiferromagnetic transition temperature TN (114 K) is suppressed compared to that in the bulk material (119 K), while for the 13-nm sample TN (120 K) is comparable to that in the bulk. The neutron diffraction data of the nanoparticles is well described using the bulk MnO magnetic structure but with a substantially reduced average magnetic moment of 4.2±0.3 μB/Mn for the 8-nm sample and 3.9±0.2 μB/Mn for the 13-nm sample. An analysis of the polarized neutron data on both samples shows that in an individual MnO nanoparticle about 80% of Mn ions order. These results can be explained by a structure in which the monodisperse nanoparticles studied here have a core that behaves similar to the bulk with a surface layer which does not contribute significantly to the magnetic order. [ABSTRACT FROM AUTHOR]

We report the investigation of charge density waves (CDW's) in Lu2Ir3Si5 by electron diffraction and dark-field imaging in transmission electron microscopy using superlattice diffraction spots. The CDW state is confirmed by the presence of superlattice reflections. Most interestingly, the CDW state at low temperatures is found to be electronically phase separated, with the coexistence of CDW domains and low-temperature normal phase domains. With a change in temperature, unlike other typical incommensurate CDW systems in which commensurability varies with temperature, we find that commensurability remains unchanged in the present case and that the predominant change is in the redistribution of the area ratio of the two coexisting phases, which is clearly revealed in the dark-field images obtained from the CDW superlattice reflections. The electronic phase separation in the CDW state of Lu2Ir3Si5 is unprecedented in CDW systems, and its temperature dependence is also anomalous. [ABSTRACT FROM AUTHOR]

An optical study of fully strained ultrathin LaNiO3 films is presented and compared with LDA?+?DMFT calculations. LaNiO3 films were grown by pulsed laser deposition on LaAlO3 and SrTiO3 substrates which provide compressive and tensile strain, respectively. Optical conductivity data show a Drude peak with a spectral weight that is significantly reduced compared to that obtained from LDA calculations. The extended Drude analysis reveals the presence of a pseudogap around 80 meV for the film on SrTiO3 and near 40 meV, at low temperature only, for the film on LAO. An unusual temperature dependence of the optical conductivity is observed, with the Drude plasma frequency increasing by up to 20% at low temperature due to spectral weight transfer from bands lying 2-4 eV below the Fermi energy. Such a strong temperature dependence of the Drude spectral weight has previously been reported for correlated electron systems in which a phase transition is present. In LaNiO3, however, no phase transition is observed. [ABSTRACT FROM AUTHOR]

Spin qubits composed of either one or three electrons are realized in a quantum dot formed at a Si/SiO2 interface in isotopically enriched silicon. Using pulsed electron-spin resonance, we perform coherent control of both types of qubits, addressing them via an electric field dependent g factor. We perform randomized benchmarking and find that both qubits can be operated with high fidelity. Surprisingly, we find that the g factors of the one-electron and three-electron qubits have an approximately linear but opposite dependence as a function of the applied dc electric field. We develop a theory to explain this g-factor behavior based on the spin-valley coupling that results from the sharp interface. The outer "shell" electron in the three-electron qubit exists in the higher of the two available conduction-band valley states, in contrast with the one-electron case, where the electron is in the lower valley. We formulate a modified effective mass theory and propose that intervalley spin-flip tunneling dominates over intravalley spin flips in this system, leading to a direct correlation between the spin-orbit coupling parameters and the g factors in the two valleys. In addition to offering all-electrical tuning for single-qubit gates, the g-factor physics revealed here for one-electron and three-electron qubits offers potential opportunities for different qubit control approaches. [ABSTRACT FROM AUTHOR]

We show theoretically that an open-dissipative polariton condensate confined within a trapping potential and driven by an incoherent pumping scheme gives rise to bistability between odd and even modes of the potential. Switching from one state to the other can be controlled via incoherent pulsing which becomes an important step towards construction of low-powered optoelectronic devices. The origin of the effect comes from modulational instability between odd and even states of the trapping potential governed by the nonlinear polariton-polariton interactions. [ABSTRACT FROM AUTHOR]

A spin-spin coupling mechanism that occurs during the transport of spin-polarized minority electrons in semiconductors is described. Unlike the Coulomb spin drag, this coupling arises from the ambipolar electric field, which is created by the differential movement of the photoelectrons and the photoholes. Like the Coulomb spin drag, it is a pure spin coupling that does not affect charge diffusion. Experimentally, the coupling is studied in p+-GaAs using polarized microluminescence. The coupling manifests itself as an excitation power-dependent reduction in the spin polarization at the excitation spot without any change of the spatially averaged spin polarization. [ABSTRACT FROM AUTHOR]

In p+ GaAs thin films, the effect of photoelectron degeneracy on spin transport is investigated theoretically and experimentally by imaging the spin polarization profile as a function of distance from a tightly focused light excitation spot. Under degeneracy of the electron gas (high concentration, low temperature), a dip at the center of the polarization profile appears with a polarization maximum at a distance of about 2 μm from the center. This counterintuitive result reveals that photoelectron diffusion depends on spin, as a direct consequence of the Pauli principle. This causes a concentration dependence of the spin stiffness while the spin dependence of the mobility is found to be weak in doped material. The various effects which can modify spin transport in a degenerate electron gas under local laser excitation are considered. A comparison of the data with a numerical solution of the coupled diffusion equations reveals that ambipolar coupling with holes increases the steady-state photoelectron density at the excitation spot and therefore the amplitude of the degeneracy-induced polarization dip. Thermoelectric currents are predicted to depend on spin under degeneracy (spin Soret currents), but these currents are negligible except at very high excitation power where they play a relatively small role. Coulomb spin drag and band-gap renormalization are negligible due to electrostatic screening by the hole gas. [ABSTRACT FROM AUTHOR]

The π-plasmon dispersion in graphene was scrutinized by momentum-resolved electron energy-loss spectroscopy with an improved momentum q resolution and was found to display the square root of the q dispersion characteristic of the collective excitation of two-dimensional electron systems, in contrast to previous experimental and theoretical studies which reported a linear q dispersion. Our theoretical elaborations on the q-dependent spectra affirm this square root of q relation and further unveil an in-plane electronic anisotropy. The physical property of the π plasmon is thoroughly compared to that of the two-dimensional plasmon due to carriers of the Dirac fermions. A clear distinction between the π plasmon and the two-dimensional Dirac plasmon is demonstrated, clarifying the common notion about correlating the linearly dispersed Dirac cones with the linear dispersion of the n plasmon previously reported. [ABSTRACT FROM AUTHOR]

We study the emergence of strongly correlated states and Kondo physics in disordered graphene. Diluted short-range disorder gives rise to localized midgap states at the vicinity of the system charge neutrality point. We show that long-range disorder, ubiquitous in graphene, allows for the coupling of these localized states to an effective (disorder averaged) metallic band. The system is described by an Anderson-like model. We use the numerical renormalization group method to study the distributions of Kondo temperatures P(TK). The results show that disorder can lead to long logarithmic tails in P(TK), consistent with a quantum Griffiths phase. [ABSTRACT FROM AUTHOR]