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struck_223763.pdf (4.129Mb)Struck, Philipp (2013), Dissertation[more][less]
Zusammenfassung: The scope of this thesis is the coherence of spins in carbon-based nanodevices. The motivation for this study are the promising spin-related properties of carbon-based materials, such as weak spin-orbit and hyperfine interaction, which are advantageous for achieving long spin coherence times. In addition, carbon based materials such as graphene and carbon nanotubes have a low mass density and high stiffness which make them well-suited for building nanomechanical devices.<br /><br /> This thesis consists of three parts. The first part is an introduction to spin-based quantum computing. We give an overview of the prerequisites for quantum computing in general and discuss basic concepts of spin quantum dots, both in conventional semiconductors and in carbon-based devices.<br /><br /> In the second part we study stationary quantum dots made of graphene. In particular we investigate a gate tunable single-layer graphene quantum dot. We calculate the spin-relaxation time T1 of an electron confined to the quantum dot. We find a behavior markedly different from the known results from quantum dots in conventional semiconductors such as GaAs. The presence of two independent K-valleys in graphene results in an effective breaking of time-reversal symmetry of the electronic states in the quantum dot. This leads to an absence of the so-called Van Vleck cancellation even for a vanishing magnetic field. As a result the spin-relaxation time depends only weakly on the magnetic field for low field strengths. At higher fields a cross over to 1/T1 ∝ B2 and 1/T1 ∝ B4 is predicted. A novel direct spin-phonon coupling involving the out-of-plane phonons in graphene is found to be an important contribution to the spin-relaxation.<br /><br /> We also study the coupling of non-neighboring quantum dots in an array of dots in a graphene nanoribbon. The electronic states in the conduction band are coupled indirectly via tunneling to a common continuum of delocalized states in the valence band. We model the system with a two-impurity Anderson Hamiltonian which is transformed into an effective spin Hamiltonian with the help of a two-stage Schrieffer-Wolff transformation. The result is compared to that from a calculation using a Coqblin-Schrieffer approach as well as to fourth-order perturbation theory. We discuss the ranges of validity of the different models and derive an expression for the long-distance coupling for the case of an array of quantum dots in a graphene nanoribbon.<br /><br /> The second part of this thesis delves into the study of nanomechanics. We study the coupling of an electron spin to vibrational motion due to spin-orbit coupling in suspended carbon nanotube quantum dots. First we show that with current capabilities, a quantum dot with an odd number of electrons can serve as a realization of the Jaynes-Cummings model known from cavity quantum electrodynamics. Using realistic experimental parameters we argue that the strong-coupling regime can be reached. In the proposed setup, a quantized flexural mode of the suspended tube plays the role of the optical mode in cavity quantum electrodynamics and we identify two distinct two-level subspaces, at small and large magnetic field, which can be used as qubits in this setup. Using the quantum master equation we show how the coupling of spin and mechanical motion is imprinted in the amplitude of the stationary state of the nanotube.<br /><br /> Furthermore we demonstrate how the spin of the electron in system described above can be read out by a charge sensing device in the vicinity of the charged nanotube. We calculate the response of the system to pulsed external driving of the mechanical motion using a Jaynes-Cummings model. We show how the spin can be read-out by measuring the current through the charge sensing device. Dateien zu dieser Publikation: 1
struck_223763.pdf (4.129Mb) -
lebecki_212074.pdf (1.000Mb)Lebecki, Krzysztof; Nowak, Ulrich (2013), Artikel[more][less]
Erschienen in: Journal of Applied Physics ; 113 (2013), 2. - 023906 Zusammenfassung: Thermal properties of steady-state magnetic vortices in soft materials are numerically evaluated using the recently proposed Landau-Lifshitz-Bloch approach. Circular samples with permalloy-like parameters are simulated. Relevant properties of the vortex core, as its radius, the magnetization drop in its center, and the radius of this magnetization drop are extracted. The dependence of the vortex core radius on temperature agrees well with the theoretical predictions, if only temperature-dependent parameters are taken into account. A new effect is found, which we call magnetization squeezing, resulting from the thermodynamic nature of the Landau-Lifshitz-Bloch approach. Our results show, however, that this squeezing in vortices is a rather weak effect in permalloy. Dateien zu dieser Publikation: 1
lebecki_212074.pdf (1.000Mb) -
Holmqvist_224736.pdf (733.4Kb)Holmqvist, Cecilia; Fogelström, Mikael; Belzig, Wolfgang (2013), Preprint[more][less]
Zusammenfassung: We investigate the charge and spin transport of a voltage-biased superconducting point contact coupled to a nanomagnet. The magnetization of the nanomagnet is assumed to precess with the Larmor frequency, $\omega_L$, due to ferromagnetic resonance. The interplay between the ac Josephson current and the magnetization dynamics leads to spin-polarized Shapiro steps at voltages $|V|=\hbar \omega_L/2en$ for $n=1,2,...$ and the subharmonic steps with $n>1$ are a consequence of multiple Andreev reflection (MAR). Moreover, the spin-precession-assisted MAR generates quasiparticle scattering amplitudes that, due to interference, lead to current-voltage characteristics of the dc charge and spin currents with subharmonic gap structures displaying an even-odd effect. Dateien zu dieser Publikation: 1
Holmqvist_224736.pdf (733.4Kb) -
fleischhaker_224413.pdf (876.0Kb)Fleischhaker, Robert; Krauß, Nico; Schättiger, Farina; Dekorsy, Thomas (2013), Artikel[more][less]
Erschienen in: Optics Express ; 21 (2013), 6. - S. 6764-6776 Zusammenfassung: We study the comparability of the two most important measurement methods used for the characterization of semiconductor saturable absorber mirrors (SESAMs). For both methods, single-pulse spectroscopy (SPS) and pump-probe spectroscopy (PPS), we analyze in detail the time-dependent saturation dynamics inside a SESAM. Based on this analysis, we find that fluence-dependent PPS at complete spatial overlap and zero time delay is equivalent to SPS. We confirm our findings experimentally by comparing data from SPS and PPS of two samples. We show how to interpret this data consistently and we give explanations for possible deviations. Dateien zu dieser Publikation: 1
fleischhaker_224413.pdf (876.0Kb) -
kneier_207413.pdf (518.8Kb)Kneier, Fabian; Geldhauser, Tobias; Scheer, Elke; Leiderer, Paul; Boneberg, Johannes (2013), Artikel[more][less]
Erschienen in: Applied Physics : A, Materials science & processing ; 110 (2013), 2. - S. 321-327 Zusammenfassung: The vertical movement of a 40 nm thin Au film on a silicon substrate during intense nanosecond (ns) laser irradiation is determined on the nm vertical and ns time scales using an optimized Michelson interferometer. The balanced setup with two detectors uses the inverse interference signal and accounts for transient reflectivity changes during irradiation. We show that a change in phase shift upon reflection must be taken into account to gain quantitative results. Three distinct fluence regimes can be distinguished, characterized by transient reflectivity behavior, dewetting processes and film detachment. Maximum displacement velocities are determined to be 0.6 m s and 1.9 ms below and above the melting threshold of the metal, respectively. Flight velocities of detaching liquid lms are found to be between 30 and 70 ms for many nanoseconds. Dateien zu dieser Publikation: 1
kneier_207413.pdf (518.8Kb) -
Maret, Georg (2013), Rezension[more][less]
Erschienen in: Physik Journal ; 12 (2013), 2. - S. 46-47 Dateien zu dieser Publikation: 0
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heinecke_223927.pdf (3.188Mb)Heinecke, Dirk C.; Kliebisch, Oliver; Flock, Johanna; Bruchhausen, Axel; Köhler, Klaus; Dekorsy, Thomas (2013), Artikel[more][less]
Erschienen in: Physical Review B ; 87 (2013), 7. - 075307 Zusammenfassung: The selective optical excitation of coherent zone-folded acoustic phonon modes in a semiconductor superlattice is demonstrated. By using a time-delayed control pulse, we show the modification of phonon amplitudes within a phonon triplet and demonstrate that the phase relation of phonon modes can be mapped over time delays of several tens of picoseconds. This method with high spectral resolution has the potential to tailor specific amplitudes of phonon modes that are otherwise only collectively excited. Dateien zu dieser Publikation: 1
heinecke_223927.pdf (3.188Mb) -
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Zusammenfassung: The dissertation studies the coarsening dynamics in aging liquid foams. The foam samples are stabilized against drainage by means of diamagnetic levitation and the bulk dynamics are followed using light scattering techniques such as diffusing wave spectroscopy and measurement of the diffuse transmitted intensity. The experimental findings reveal scaling behaviours for the mean bubble size in the aging foam that depends on liquid fraction. The results agree well with theoretic predictions in the dry and wet regime. Dateien zu dieser Publikation: 1
isert_214725.pdf (15.58Mb) -
Yang_223916.pdf (272.6Kb)Yang, Kejian; Heinecke, Dirk; Kölbl, Christoph; Zhao, Shengzhi; Zheng, Lihe; Xu, Jun; Zhao, Guangjun (2013), Artikel[more][less]
Erschienen in: Optics Express ; 21 (2013), 2. - S. 1574-1580 Zusammenfassung: A passively mode-locked Tm,Ho:YAP laser around 2.1 μm wavelength employing a semiconductor saturable absorber mirror is demonstrated. Stable continuous wave mode-locking operation was achieved at variable center wavelengths of 2036.5 nm, 2064.5 nm, 2095.5 nm, 2103.5 nm, and 2130 nm, respectively. Pulses as short as 40.4 ps were obtained at 2064.5 nm with a spectral FWHM of 0.5 nm at output powers of 132 mW and a repetition rate around 107 MHz. A maximum output power of 238 mW was obtained at 2130 nm with a pulse duration of 66 ps. Dateien zu dieser Publikation: 1
Yang_223916.pdf (272.6Kb) -
pacile_220159.pdf (1.171Mb)Pacilé, Daniela; Leicht, Philipp; Papagno, Marco; Sheverdyaeva, Polina M.; Moras, Paolo; Carbone, Carlo; Krausert, Konstantin; Zielke, Lukas; Fonin, Mikhail; Dedkov, Yuriy S.; Mittendorfer, Florian; Doppler, Jörg; Garhofer, Andreas; Redinger, Josef (2013), Artikel[more][less]
Erschienen in: Physical Review B ; 87 (2013), 3. - 035420 Zusammenfassung: We report on the structural and electronic properties of an artificial graphene/Ni(111) system obtained by the intercalation of a monatomic layer of Ni in graphene/Ir(111). Upon intercalation, Ni grows epitaxially on Ir(111), resulting in a lattice-mismatched graphene/Ni system. By performing scanning tunneling microscopy measurements and density functional theory calculations, we show that the intercalated Ni layer leads to a pronounced buckling of the graphene film. At the same time, an enhanced interaction is measured by angle-resolved photoemission spectroscopy, showing a clear transition from a nearly undisturbed to a strongly hybridized graphene π-band. A comparison of the intercalation-like graphene system with flat graphene on bulk Ni(111), and mildly corrugated graphene on Ir(111), allows us to disentangle the two key properties which lead to the observed increased interaction, namely lattice matching and electronic interaction. Although the latter determines the strength of the hybridization, we find an important influence of the local carbon configuration resulting from the lattice mismatch. Dateien zu dieser Publikation: 1
pacile_220159.pdf (1.171Mb)
