Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 03/05

In der vorliegenden Doktorarbeit werden die elektronischen Eigenschaften von elektrostatisch definierten Quantenpunktsystemen in GaAs/AlGaAs Heterostrukturen untersucht.

Dazu werden gepulste Hochfrequenzexperimente zur Untersuchung der Dynamik von einzelnen Elektronen in Doppelquantenpunkten durchgeführt. Der im Laufe dieser Arbeit entwickelte Messaufbau ermöglicht zeitaufgelöste Messungen im sub-Nanosekunden Bereich. Verschiedene Kopplungsregime der Quantenpunkte an die Zuleitungen wurden untersucht. Durch das Anwenden eines Ratengleichungsmodells wurden aus den durchgeführten Messungen Relaxationsraten und Tunnelraten von Elektronen bestimmt.

Des weiteren wurden die Rückkopplungen eines Quantenpunktkontakts, der als Ladungsdetektor betrieben wird, auf einen Doppelquantenpunkt untersucht. Ein Quantenpunktkontakt, über den eine Spannung angelegt wird, emittiert Ladungsträger, die eine Überschussenergie tragen. Diese Energie kann von einem Elektron im Doppelquantenpunkt absorbiert werden und Übergänge zwischen den Quantenpunkten (oder den Quantenpunkten und den Zuleitungen) induzieren. Solche Übergänge finden zwar häufig statt, können jedoch nur unter bestimmten Voraussetzungen im Doppelquantenpunkt detektiert werden.

Es wurden zwei Mechanismen zum Austausch von Energie identifiziert, nämlich akustischen Phononen und indirekte Coulomb-Wechselwirkung.

The scope of this work is to gain insight and a deeper understanding of exploring and controlling molecular devices like proteins and rotors by fine tuned manipulation via mechanical or electrical energies.

I focus on three main topics.

First, I investigate vectorial forces as a tool to explore the energy landscape of protein complexes.

Second, I apply this method to a biologically important force transduction complex, the integrin-talin complex.

Third, I use Terahertz electric fields to manipulate the energy landscape of a molecular rotor on a gold surface and drive their effective rotation bidirectionally.

Force is by nature a vector and depends on its three parameters: magnitude, direction and attachment point. Here, the impact of different force protocols varying these parameters is shown for an antibody-antigen complex and the ribonuclease-inhibitor complex barnase-barstar.

Antibodies are essential for our adaptive immune system in their function to bind specific antigens. Here, the binding of an antibody to a peptide is probed with varying attachment points. Different attachment points clearly change the dissociation pathways. The barriers identified using experimental atomic force microscopy (AFM) and molecular dynamics (MD) simulations are in excellent agreement. I determine the molecular interactions of two main barriers for each setup. This results in a common outer barrier of the complex and different inner barriers probed by AFM.

The ribonuclease barnase and its inhibitor barstar form an evolutionary optimized complex. Different force protocols are shown to determine the hierarchy of relative stability within a protein complex.

For the barnase-barstar complex, the internal fold of the barstar is identified to be less stable than the barnase-barstar binding interaction. High velocities probe the lability or barriers of the system while low velocities probe the stability or energy wells of this system.

Forces impact biological life on totally different length scales which range from whole organisms to individual proteins. Integrins are the major cell adhesion receptors binding to the extracellular matrix and talin. Talin activates the integrins and creates the initial connection to the actin cytoskeleton of the cell. Here, I have chosen to investigate the integrin-talin complex as a biologically important force transduction complex. The force dependence of the system is probed by constant force MD simulations. The two main results include the activation of the complex and its force response. I demonstrate, that the binding of talin to integrin does not disrupt the integrin's transmembrane helix interactions sterically. Since, this disruption is necessary for integrin activation, a modified activation mechanism requiring a small force application is proposed. The response of the integrin-talin complex normal and parallel to the cell membrane is analyzed. The complete dissociation pathways generated for both directions identify a force-induced formation of a stabilizing beta strand between integrin and talin only for normal forces. Furthermore, the complex tries to rotate such that the external force aligns with the more force resistant axis of the complex.

In nature, molecular rotors are essential building blocks of many molecular machines and brownian motors like the F1-ATPase or the flagellum of a bacterium. The direction of rotation often steers different processes in clockwise and counterclockwise directions. Rotation on the nanoscopic level in artificial devices is still very limited and requires a deeper understanding.

In my last project, I study the switching and driving of a molecular diethylsulfid rotor on a gold (111) surface by Terahertz electric fields. The response of the rotational energy landscape to static and oscillation electric fields is analyzed. Varying the Terahertz driving frequency, the rotation direction and frequency are controlled. A theoretical framework is presented to describe the b

In this dissertation we use gauge/gravity duality to investigate various phenomena of strongly coupled field theories. Of special interest are quantum phase transitions,

quantum critical points, transport phenomena of charges and the thermalization process of strongly coupled medium. The systems studied in this thesis might be used as models

for describing condensed matter physics in a superfluid phase near the quantum critical point and the physics of quark-gluon plasma (QGP), a deconfinement phase of QCD,

which has been recently created at the Relativistic Heavy Ion Collider (RHIC). Moreover, we follow the line of considering different gravity setups whose dual field descriptions show interesting phenomena of systems in thermal equilibrium, slightly out-of-equilibrium and far-from-equilibrium.

We first focus on systems in equilibrium and construct holographic superfluids at finite baryon and isospin charge densities. For that we use two different approaches, the bottom-up with an U(2) Einstein-Yang-Mills theory with back-reaction and the top-down approach with a D3=D7 brane setup with two coincident D7-brane probes. In both cases we observe phase transitions from a normal to a superfluid phase at finite and also at zero temperature. In our setup, the gravity duals of superfluids are Anti-de Sitter black holes which develop vector-hair. Studying the order of phase transitions at zero temperature, in the D3=D7 brane setup we always find a second order phase transition, while in the Einstein-Yang-Mills theory, depending on the strength of the back-reaction, we obtain a continuous or first order transition.

We then move to systems which are slightly out-of-equilibrium. Using the D3/D7 brane setup with Nc coincident D3-branes and Nf coincident D7-brane probes, we compute transport coefficients associated with massive N = 2 supersymmetric hypermultiplet fields propagating through an N = 4 SU(Nc) super Yang-Mills plasma inthe limit of Nf << Nc. Introducing a baryon number density and arbitrary constant electric and magnetic fields, we compute all components of the conductivity tensor associated with transport of baryon number charge. Determining the contribution that the flavor degrees of freedom make to the stress-energy tensor, we are able to identify the rates of energy and momentum loss of the flavor field to the plasma.

Going towards systems far-from-equilibrium, we investigate a time-dependent geometry consisting of a mirror moving in the bulk of the Anti-de Sitter space. This geometry can be seen as a toy model which is relevant to the formulation of holographic thermalization in strongly coupled field theory. For this configuration, we establish a procedure for calculating time-dependent two-point functions of scalar fluctuations, based on a WKB approximation. We test our method on two sample trajectories for the mirror, and find that the singularity structure of the two-point functions is in agreement with the geometric optics.

In this thesis ultrafast processes occuring in photolabile protecting groups of the ortho-nitrobenzyl type have been studied. These protecting groups enable the spatially and temporarily defined release of chemically or biologically active molecules.

Due to these properties they find widespread applications, besides technical ones mainly in biochemical research. Molecules of the o-nitrobenzyl type represent the major part of photolabile protecting groups used today. Therefore, in-depth knowledge of the reaction and release mechanism is desired to improve and adapt the properties of these molecules to their applications. Despite this relevance experiments on the ultrafast behaviour of these molecules are scarce. Here, femtosecond pump probe experiments with UV/VIS, IR and Raman probing have been employed to elucidate the early processes occuring during de-protection. Model systems as well as a "real" protecting group have been investigated. Thereby, a general picture

of the ultrafast processes occuring in the photoreaction of these molecules could be obtained. Excitation with uv-light to an upper singlet state leads to ultrafast internal conversion to the S1 state. Three competing processes, internal conversion, inter system crossing and the photoreaction, contribute to the decay of this state on the

time scale of 1 ps. For the reaction path the formation of the first intermediate could be traced. It results from a hydrogen transfer from the substituent in o-position to the nitro group and exhibits aci-nitro like structure. In addition to the direct formation from the S1 state this intermediate is also formed via a second channel on the

nanosecond time scale. This channel involves a triplet state. The "local" triplet state undergoes a hydrogen transfer reaction resulting in a triplet phased bi-radical. The recombination of this biradical then yields the aci-nitro tautomer. Once formed the aci-nitro tautomer transforms into the final nitroso product and the released functionality in a yield of 100 %. Furtheron, the similarity between the reaction mechanisms of all molecules investigated leads to the conclusion that it is universal for the group of o-nitrobenzenes. Additionally, for oNBA the influence of vibrational excitation on the stability of a ground state intermediate could be traced. The ultrafast formation of the first intermediate is about one order of magnitude faster than typical cooling times for these molecules. Therefore, the dynamics of vibrational excitation and its impact on the subsequent reaction could be studied. The vibrational excitation results in bi-phasic kinetics: a fast phase during vibrational excitation of the molecule and a slower one for the thermalized molecule.