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The spin Seebeck effect is studied across a charge insulating magnetic junction, in which thermal-spin conjugate transport is assisted by the exchange interactions between the localized spin in the center and electrons in metallic leads. We show that, in contrast with bulk spin Seebeck effect, the figure of merit of such nanoscale thermal-spin conversion can be infinite, leading to the ideal Carnot efficiency in the linear response regime. We also find that in the nonlinear spin Seebeck transport regime the device possesses the asymmetric and negative differential spin Seebeck effects. In the last, the situations with leaking electron tunneling are also discussed. This nanoscale thermal spin rectifier, by tuning the junction parameters, can act as a spin Seebeck diode, spin Seebeck transistor, and spin Seebeck switch, which could have substantial implications for flexible thermal and information control in molecular spin caloritronics.

We show how to engineer enhanced skew scattering and gaplike openings in the surface state of three-dimensional topological insulators using only nonmagnetic impurities. Enhanced skewscattering off nonmagnetic impurities is obtained as a finite size effect of the scattering potential. Intimately related to the generated skew scattering is the emergence of a gaplike density of electron states locally around the impurities and surrounded by sharp resonances, with an extended energy gap appearing in engineered impurity structures.

We investigate the far-from-equilibrium nature of magnetic anisotropy and exchange interactions between molecular magnets embedded in a tunnel junction. By mapping to an effective spin model, these magnetic interactions can be divided into three types: isotropic Heisenberg, anisotropic Ising, and anisotropic Dzyaloshinski-Moriya contributions, which are attributed to the background nonequilibrium electronic structures. We further demonstrate that both the magnetic self- and exchange interactions can be controlled either electrically by gating and tuning the voltage bias, or thermally by adjusting the temperature bias. We show that the Heisenberg and Ising interactions scale linearly, while the Dzyaloshinski-Moriya interaction scales quadratically, with the molecule-lead coupling strength. The interactions scale linearly with the effective spin polarizations of the leads and the molecular coherence. Our results pave a way for smart control of magnetic exchange interactions at atomic and molecular levels.

Recent experimental conductance measurements performed on paramagnetic molecular adsorbates on a superconducting surface, using superconducting scanning tunneling microscopy techniques, are theoretically investigated. For low temperatures, we demonstrate that tunneling current assisted excitations of the local magnetic moment cannot occur for voltage biases smaller than the superconducting gap of the scanning tunneling microscope. The magnetic moment is only excited for voltages corresponding to the sum of the superconducting gap and the spin excitation energies. In excellent agreement with experiment, we show that pumping into higher excitations gives additional current signatures by accumulation of density in the lower ones. Using external magnetic fields, we Zeeman-split the possible degeneracy and thereby resolve all excitations comprised in the magnetic moment.

We present theoretical results on molecular structures for realistic spin-pump applications. Taking advantage of the electron spin resonance concept, we find that interesting candidates constitute triplet biradicals with two strongly spatially and energetically separated singly occupied molecular orbitals (SOMOs). Building on earlier reported stable biradicals, particularly bis(nitronyl nitroxide) based biradicals, we employ density functional theory to design a selection of potential molecular spin-pumps which should be persistent at ambient conditions. We estimate that our proposed molecular structures will operate as spin-pumps using harmonic magnetic fields in the MHz regime and optical fields in the infrared to visible light regime., KoF U3MEC

Inelastic electron tunneling spectroscopy is a powerful spectroscopy that allows one to investigate the nature of local excitations and energy transfer in the system of interest. We study inelastic electron tunneling spectroscopy for topological insulators and investigate the role of inelastic scattering on the Dirac node states on the surface of topological insulators. Local inelastic scattering is shown to significantly modify the Dirac node spectrum. In the weak coupling limit, peaks and steps are induced in second derivative d(2)I/dV(2). In the strong coupling limit, the local negative-U centers are formed at impurity sites, and the Dirac cone structure is fully destroyed locally. At intermediate coupling, resonance peaks emerge. We map out the evolution of the resonance peaks from weak to strong coupling, which interpolate nicely between the two limits. There is a sudden qualitative change of behavior at intermediate coupling, indicating the possible existence of a local quantum phase transition. We also find that, even for a simple local phonon mode, the inherent coupling of spin and orbital degrees in topological insulators leads to the spin-polarized texture in inelastic Friedel oscillations induced by the local mode., QC 20130211

The focus of this review is recent work in which we have demonstrated a highly-flexible approach to the study of Fano-resonance phenomena, by making use of the mesoscopic devices known as quantum point contacts (QPCs). Utilizing the ability of these structures to function as an on-demand quantum state, we demonstrate a highly-flexible system for the investigation of Fano resonances. Our approach involves making measurements of non-locally coupled pairs of QPCs, one of which is used to form the discrete state needed for the Fano resonance, while the other serves as a detector whose conductance is sensitive to the energy of this state. As a demonstration of the flexibility of this approach, we show how it can be used to implement a multi-state Fano resonance, in which two discrete states undergo a robust interaction that is achieved by coupling them to each other through a common continuum.

We address local inelastic scattering from the vibrational impurity adsorbed onto graphene and the evolution of the local density of electron states near the impurity from a weak to strong coupling regime. For weak coupling the local electronic structure is distorted by inelastic scattering developing peaks or dips and steps. These features should be detectable in the inelastic electron tunneling spectroscopy d(2)I/dV(2) using local probing techniques. Inelastic Friedel oscillations distort the spectral density at energies close to the inelastic mode. In the strong coupling limit, a local negative U center forms in the atoms surrounding the impurity site. For those atoms, the Dirac cone structure is fully destroyed, that is, the linear energy dispersion as well as the V-shaped local density of electron states is completely destroyed. We further consider the effects of the negative U formation and its evolution from weak to strong coupling. The negative U site effectively acts as a local impurity such that sharp resonances appear in the local electronic structure. The main resonances are caused by elastic scattering off the impurity site, and the features are dressed by the presence of vibrationally activated side resonances. Going from weak to strong coupling, changes the local electronic structure from being Dirac-cone-like including midgap states, to a fully destroyed Dirac cone with only the impurity resonances remaining.

The recent experimental observations of designer Dirac fermions and topological phases in molecular graphene are addressed theoretically. Using scattering theory, we calculate the electronic structure of finite lattices of scattering centers dual to the honeycomb lattice. In good agreement with experimental observations, we obtain a V-shaped electron density of states around the Fermi energy. By varying the lattice parameter we simulate electron and hole doping of the structure, and by adding and removing scattering centers we simulate, respectively, vacancy and impurity defects. Specifically, for the vacancy defect we verify the emergence of a sharp resonance near the Fermi energy for increasing strength of the scattering potential.

Syftet med denna undersökning har varit att studera gymnasieelevers förståelse för funktionsbegreppet.Den teoretiska utgångspunkten har varit att studera elevernas förmåga att växla mellanolika representationsformer och studera deras förståelse för relationen mellan de olika representationerna.Elevernas förmåga att resonera har också varit en väsentlig del och därför har forskningenkring matematiskt resonemang också varit en viktig del i arbetet. För att nå ett resultat har treklasser på gymnasiet fått skriva ett test konstruerat för att svara mot uppsatsens syfte. Utövertestet intervjuades tre av eleverna för att det skulle finnas möjlighet att fånga elevernas förmågaatt resonera. Resultatet kom att visa att vissa elever har svårt att utföra växlingar mellan vissaolika representationer, men mellan andra är det betydligt enklare. Det framkom att eleverna varskolade i ett algoritmiskt tänkande, och att eleverna hade svårt att resonera fritt och kreativt kringfunktionsbegreppet, samt att eleverna hade svårt att skilja de närliggande begreppen ekvation ochfunktion., The aim with this thesis has been to study Swedish high school students’ comprehension for theconcept of a function. The theoretical starting point has been to study the students’ ability tochange between different forms of representations of a function and study their comprehensionfor the relation between the different representations. The students’ ability to use mathematicalreasoning has also been a crucial part for the thesis, and therefore research on mathematicalreasoning has also been an important part. Three high school classes were given a test, whichsought to test their ability to change between different forms of representation. In addition to thetest three students were interviewed, so that the students’ ability to reason would be possible toanalyze. The results came to show that some students have difficulties to change between someof the representations but between others it’s far easier for them. The results also showed thatthe students were trained in algorithmically thinking, and that it was difficult to reason creativelyabout the concept of a function. The students also had trouble separating a function from therelated concept of equations.

We consider inelastic scattering of localized magnetic moments coupled with the electrons on the surface. We argue that spin-inelastic transitions of the magnetic impurities generate oscillations at a momentum k, corresponding to the inelastic mode, in the second derivative of the current with respect to voltage d(2)I/dV(2). These oscillations are similar in nature to Friedel oscillations. Inelastic Friedel oscillations, which were previously proposed for a spin-unpolarized setup, are here extended for spin-polarized systems. We propose to use scanning tunneling microscopy to measure spin-inelastic scattering generated at the impurity spin by imaging the d(2)I/dV(2) oscillations on the metal surface.

We consider spin dynamics for implementation in an atomistic framework and we address the feasibility of capturing processes in the femtosecond regime by inclusion of moment of inertia. In the spirit of an s-d-like interaction between the magnetization and electron spin, we derive a generalized equation of motion for the magnetization dynamics in the semiclassical limit, which is nonlocal in both space and time. Using this result we retain a generalized Landau-Lifshitz-Gilbert equation, also including the moment of inertia, and demonstrate how the exchange interaction, damping, and moment of inertia, all can be calculated from first principles.

We consider the motion of and Josephson current through a mechanically oscillating superconducting island asymmetrically embedded in a Josephson junction. The electromechanical coupling is provided by distance-dependent tunneling rates between the electrodes and the island. The system asymmetry, resulting from the geometrical configuration, leads, for weak coupling, to an equation of the mechanical motion that reduces to the well-known Duffing equation. At zero bias voltage the island motion is determined by the homogenous Duffing equation that opens up two separate regions of solutions depending on the superconducting phases. The island either moves under influence of an anharmonic single-well potential or is governed by a double-well potential that allows for off-center oscillations. Under applied bias voltage the island equation of motion turns into a modified Duffing equation, with time-dependent coefficients, that demonstrate both quasiperiodic and chaotic behavior.

We demonstrate a fully tunable realization of a multistate Fano resonance, in which a pair of remote quantum states experience an effective coupling due to their mutual overlap with a continuum. Our mesoscopic implementation of this system exploits the ability of the semiconductor nanostructures known as quantum point contacts (QPCs) to serve, in the low-density limit close to pinch-off, as an on-demand localized state. By coupling the states formed on two separate QPCs, through a two-dimensional electron gas that serves as a continuum, we observe a robust effective interaction between the QPCs. To explain this result, we develop a theoretical formulation, based on the ideas of the Schrieffer-Wolff transformation, which is able to reproduce our key experimental findings. According to this model, the robust character of the interaction between the two remote states arises from the fact that the interaction is essentially mediated by a large number of degenerate continuum states. While the continuum is often viewed as a source of decoherence, our experiment therefore instead suggests the possibility of using this medium to support the interaction of quantum states, a result that may allow new approaches to coherently couple nanostructures in extended geometries.

In this paper two methods are used, analysis of web access logs together with a web survey, to connect sessions with task context and intention. The dual methods approach connects information seeking behavior with actual IR-interactions to get a more holistic view of the information seeking and retrieval (IS&R) process. The studied collection is a cultural heritage web site containing digitalized material and artist information. Based on different navigation strategies task context and intention was related to session length and arrival level in the site. Some statistically significant relationships were found between the factors. Users in a work context more often access the site by direct navigation for looking up facts. Users in a hobby or leisure context more often access the site by links or topical searches in search engines and they arrive further down in the site structure and their sessions are shorter. How and to what extent survey data and log data can be combined is discussed.

We report real space imaging measurements of inelastic Friedel oscillations. The inelastic electron tunneling spectroscopy, using scanning tunneling microscopy, around dimers of dichlorobenze adsorbates on Au(111) surface display clear spatial modulations that we attribute to inelastic scattering at the molecular sites caused by molecular vibrations. Due to local interactions between the adsorbate and the surface states, the molecular vibrations generate a redistribution of the charge density at energies in a narrow range around the inelastic mode. Our experimental findings are supported by theoretical arguments., KoF U3MEC

We predict a transition to metallicity when a sufficient amount of disorder is induced in graphene. Calculations were performed by means of a first-principles stochastic quench method. The resulting amorphous graphene can be seen as nanopatches of graphene that are connected by a network of disordered small and large carbon rings. The buckling of the lattice is minimal and is a result of averaging of counteracting random in-plane stress forces. The linear response conductance is obtained by a model theory as function of lattice distortions, and results in a similar behavior as the first-principles calculation.

Within a generalized version of previously considered model of a molecular spin pump controlled by an external electric field [J. Fransson and M. Galperin, Phys. Rev. B, 2010, 81, 075311] we discuss thermal properties of such spintronic devices. The spin Seebeck coefficient of a molecular spin pump is introduced, and several possible definitions of a nonequilibrium Seebeck coefficient are discussed. The influence of inelastic effects in spin transport on its thermospintronic characteristics are demonstrated within numerical examples.

The article discusses the relationship between users, information objects, information systems and the Web, combined with the concepts findability and information literacy. An important aspect in all information literacy definitions is the ability to effectively gain access to the information needed. But the information is not equally available, even if it is published and freely available on the Web. Each Web page, or other information object, has a degree of findability. The degree of the findability depends on the relationships between object, system and the Web. Six sub concepts are identified: the object's characteristics, accessibility, internal navigation, internal search, reachability and web prestige. Both information literacy and findability are defined as query independent aspects of the search, working on a structural level.

In this Ph.D. project three different cultural heritage resources on the web are studied with a triangulation of methods. 1) The users’ navigational strategies to reach the resources and their usage of them are examined by web log analysis. 2) User attitude and experiences are collected through resource specific web surveys. 3) A findability analysis is made to measure how easy the resources are found on the web. Finally, the results from the different angles are compared for correlations.

Transport measurements have revealed several exotic electronic properties of graphene. The possibility to influence the electronic structure and hence control the conductivity by adsorption or doping with adatoms is crucial in view of electronics applications. Here, we show that in contrast to expectation, the conductivity of graphene increases with increasing concentration of vacancy defects, by more than one order of magnitude. We obtain a pronounced enhancement of the conductivity after insertion of defects by both quantum mechanical transport calculations as well as experimental studies of carbon nano-sheets. Our finding is attributed to the defect induced mid-gap states, which create a region exhibiting metallic behaviour around the vacancy defects. The modification of the conductivity of graphene by the implementation of stable defects is crucial for the creation of electronic junctions in graphene-based electronics devices.

We report on a theoretical study of the electronic-structure and transport properties of single and bilayer graphene with vacancy defects, as well as N-doped graphene. The theory is based on first-principles calculations as well as model investigations in terms of real-space Green's functions. We show that increasing the defect concentration increases drastically the conductivity in the limit of zero applied gate voltage, by establishing carriers in originally carrier-free graphene, a fact which is in agreement with recent observations. We calculate the amount of defects needed for a transition from a nonconducting to a conducting regime (i.e., a metal-insulator transition) and establish the threshold of the defect concentration where the increase in impurity scattering dominates over the increase in carrier-induced conductivity.

We provide a theory for scanning tunneling microscopy and spectroscopy using a spin-polarized tip. It it shown that the tunneling conductance can be partitioned into three separate contributions, a background conductance which is independent of the local spin, a dynamical conductance which is proportional to the local-spin moment, and a conductance which is proportional to the noise spectrum of the local-spin interactions. The presented theory is applicable to setups with magnetic tip and substrate in noncollinear arrangement, as well as for nonmagnetic situations. The partitioning of the tunneling current suggests a possibility to extract the total spin moment of the local spin from the dynamical conductance. The dynamical conductance suggests a possibility to generate very high-frequency spin-dependent ac currents and/or voltages. We also propose a measurement of the dynamical conductance that can be used to determine the character of the effective exchange interaction between individual spins in clusters. The third contribution to the tunneling current is associated with the spin-spin correlations induced by the exchange interaction between the local-spin moment and the tunneling electrons. We demonstrate how this term can be used in the analysis of spin excitations recorded in conductance measurements. Finally, we propose to use spin-polarized scanning tunneling microscopy for detailed studies of the spin-excitation spectrum.

We consider a model for a spin field-effect molecular transistor, where a directed pure spin current is controlled by an external electric field. Inelastic scattering effects of such molecular device are discussed within a framework of full counting statistics for a multilevel molecular system. We propose that the heating of the molecular junction can be controlled by external electric and magnetic fields. Characteristic features of the model are demonstrated by numerical calculations.

We address quantum invisibility in the context of electronics in nanoscale quantum structures. We make use of the freedom of design that quantum corrals provide and show that quantum mechanical objects can be hidden inside the corral, with respect to inelastic electron scattering spectroscopy in combination with scanning tunneling microscopy, and we propose a design strategy. A simple illustration of the invisibility is given in terms of an elliptic quantum corral containing a molecule, with a local vibrational mode, at one of the foci. Our work has implications to quantum information technology and presents new tools for nonlocal quantum detection and distinguishing between different molecules.

We investigate dynamical transport aspects of a combined nanomechanical-superconducting device in which Cooper pair tunneling interfere with the mechanical motion of a vibrating molecular quantum dot embedded in a Josephson junction. Six different regimes for the tunneling dynamics are identified with respect to the electron level and the charging energy in the quantum dot. In five of those regimes new time scales are introduced which are associated with the energies of the single-electron transitions within the quantum dot while there is one regime where the internal properties of the quantum dot are static.

The electron mediated exchange interaction between local spins adsorbed on two- dimensional surface is studied under nonequilibrium conditions. The effective spin-spin interaction is found to depend both on the spin polarization of the substrate and the excitation spectrum of the local spins. For spatially anisotropic spin polarization of the substrate, the spatial dependence of the interaction comprise components decaying as sin (2k(F)R)/(2k(F)R) and sin (2k(F)R)/(2k(F)R)(2).

The dynamics of a single spin embedded in a tunnel junction between ferromagnetic contacts is strongly affected by the exchange coupling to the tunneling electrons. Moment reversal of the local spin induced by the bias voltage across the junction is shown to have a measurable effect on the tunneling current. Furthermore, the frequency of a harmonic bias voltage is picked up by the local spin dynamics and transferred back to the current, generating a double frequency component.

The dynamics of a single spin that is embedded in a tunnel junction between ferromagnetic contacts is strongly affected by the exchange coupling to the tunneling electrons, By using time-dependent equations of motion for the spin, which is under the influence of a spin-polarized tunneling current, it is shown that the magnetic field induced by bias voltage pulses allows for a subnanosecond switching of the local spin and the possibility of spin reversal is illustrated. Furthermore, it is shown that the time evolution of the Larmor frequency sharply peaks around the spin-flip event and it is argued that this feature can be used as an indicator for the spin flip.

The magnetic ordering of a series of samples consisting of ultrathin Fe layersembedded in Pd was investigated using the magneto-optical Kerr effect. Thesamples consisted of a single Fe layer with nominal thickness 0.2 dFe 1.6monolayers sandwiched between two 20 monolayer Pd layers. A dimensionalitycrossover from two dimensions to three dimensions occurs as dFe is increasedfrom 0.4 to 1.0 monolayers. First-principles calculations were performed inorder to determine the magnetic profile, and we used a spin-wave quantum wellmodel for obtaining a qualitative description of the dimensionality crossover.The results clearly prove the existence of a dimensionality crossover in theinduced magnetization, opening new routes for addressing the influence ofextension on order.

Upprättat; 2007; 20141022 (dicsan)