Quantum Imaging with Incoherently Scattered Light from a Free-Electron Laser

The advent of accelerator-driven free-electron lasers (FEL) has opened new avenues for high-resolution structure determination via diffraction methods that go far beyond conventional x-ray crystallography methods. These techniques rely on coherent scattering processes that require the maintenance of first-order coherence of the radiation field throughout the imaging procedure. Here we show that higher-order degrees of coherence, displayed in the intensity correlations of incoherently scattered x-rays from an FEL, can be used to image two-dimensional objects with a spatial resolution close to or even below the Abbe limit. This constitutes a new approach towards structure determination based on incoherent processes, including Compton scattering, fluorescence emission or wavefront distortions, generally considered detrimental for imaging applications. Our method is an extension of the landmark intensity correlation measurements of Hanbury Brown and Twiss to higher than second-order paving the way towards determination of structure and dynamics of matter in regimes where coherent imaging methods have intrinsic limitations

Oblique-incidence deposition of ferromagnetic thin films and their application in magnetoresistive sensors

Magnetic field sensors can be used for compass applications, to measure electrical cur-rents or to determine position and speed of objects. Layered thin-film sensors based onthe giant or tunneling magnetoresistance (GMR or TMR) are predestined for portabledevices where their small size, high effect strength and low power consumption are crucial.Through the use of oblique-incidence deposition (OID) it is possible to tailor the magneticand magnetoresistive properties of the sensors in an easy-accessible and flexible way tothe needs of an application. The goal of this thesis is to investigate the feasibility of OIDin GMR and TMR sensors.Building on the work presented in [Sch16] with extended GMR film samples, fundamentalquestions regarding magnetic coupling and sensor stability are answered. It is shown thatmicrostructuring and the OID-characteristic wavy surface morphology do not interferewith sensor performance. Overall, around 10% GMR are achieved in microstructureddevices, making OID-GMR sensors a suitable and uncomplicated solution for a variety ofapplications.TMR sensors are based on the material combination of FeCoB and insulating MgO astunnel barrier. Upon thermal treatment, FeCoB crystallizes guided by the MgO struc-ture as template. Obliquely deposited FeCoB exhibits a sensitive balance of differentanisotropy terms that is surprisingly found to induce a rotation of the preferred magneti-zation axis by 90◦in-plane. After synchrotron-based investigations to correlate structuraland magnetic properties, a model is developed that explains the additional OID-inducedanisotropy contributions.Despite the complex transition of FeCoB and the combination of wavy interfaces witha sensitive ultra-thin tunnel barrier, the OID-approach is for the first time successfullytransferred to TMR systems. A high TMR effect strength of up to 60% is achieved inmicrostructured sensors. By changing the layer thicknesses and deposition angles, sensorparameters are tuned and novel functionalities are implemented. Overall, it is shown thatoblique-incidence deposition can be used to prepare TMR systems based on MgO andFeCoB that exhibit unique and adjustable functionalities while maintaining a high effectstrength, temperature stability and compatibility to common fabrication processes. Thispaves the way for customizable and simplified magnetoresistive sensors

Barley seed proteomics from spots to structures

Thin films prepared of semiconductor nanoparticles are promising for low-cost electronic applications such as transistors and solar cells. One hurdle for their breakthrough is their notoriously low conductivity. To address this, we precisely decorate CdSe nanoparticles with platinum domains of one to three nanometers in diameter by a facile and robust seeded growth method. We demonstrate the transition from semiconductor to metal dominated conduction in monolayered films. By adjusting the platinum content in such solution-processable hybrid, oligomeric nanoparticles the dark currents through deposited arrays become tunable while maintaining electronic confinement and photoconductivity. Comprehensive electrical measurements allow determining the reigning charge transport mechanisms

Coulomb blockade based field-effect transistors exploiting stripe-shaped channel geometries of self-assembled metal nanoparticles

Metallic nanoparticles offer possibilities to build basic electric devices with new functionality and improved performance. Due to the small volume and the resulting low self-capacitance, each single nanoparticle exhibits a high charging energy. Thus, a Coulomb-energy gap emerges during transport experiments that can be shifted by electric fields, allowing for charge transport whenever energy levels of neighboring particles match. Hence, the state of the device changes sequentially between conducting and non-conducting instead of just one transition from conducting to pinch-off as in semiconductors. To exploit this behavior for field-effect transistors, it is necessary to use uniform nanoparticles in ordered arrays separated by well-defined tunnel barriers. In this work, CoPt nanoparticles with a narrow size distribution are synthesized by colloidal chemistry. These particles are deposited via the scalable Langmuir-Blodgett technique as ordered, homogeneous monolayers onto Si/SiO2 substrates with pre-patterned gold electrodes. The resulting nanoparticle arrays are limited to stripes of adjustable lengths and widths. In such a defined channel with a limited number of conduction paths the current can be controlled precisely by a gate voltage. Clearly pronounced Coulomb oscillations are observed up to temperatures of 150 K. Using such systems as field-effect transistors yields unprecedented oscillating current modulations with on/off-ratios of around 70 %.Comment: 20 pages, 5 figures, Nanoscale (2016

Novel TMR sensor functionalities via oblique-incidence deposition

Tailoring magnetic properties of ultra-thin films remains one of the main issues forfurther innovations in tunnel magnetoresistive (TMR) sensor applications. The magnetic response insuch devices is mostly governed by an extension of the primary TMR trilayer with a selection of suitablecontact materials. The transfer of magnetic anisotropy to the ferromagnetic electrodes, e.g. CoFeBlayers, results in a field-dependent TMR response that is determined but also limited by the magneticproperties of CoFeB and the contact materials. We flexibly apply oblique-incidence deposition (OID)to introduce arbitrary intrinsic in-plane anisotropy profiles into the primary TMR trilayer. The OIDinduced anisotropy shapes its magnetic response and eliminates the requirement of additional magneticcontact materials. Functional control is achieved via the adjustable shape anisotropy that can beselectively configured to the ultra-thin CoFeB layers. This approach removes former limitations andallows the design of new sensing functionalities that can be precisely customized to the specificapplication, even in the high field regime. Even though this striking gain in functional TMR designcapabilities is based on an anisotropic interfacial roughness, the resulting sensors maintain the typicalTMR signal strength as well as superb thermal stability of the tunnel junction

Novel Tunnel Magnetoresistive Sensor Functionalities via Oblique-Incidence Deposition

Controlling the magnetic properties of ultrathin films remains one of the main challenges to the further development of tunnel magnetoresistive (TMR) device applications. The magnetic response in such devices is mainly governed by extending the primary TMR trilayer with the use of suitable contact materials. The transfer of magnetic anisotropy to ferromagnetic electrodes consisting of CoFeB layers results in a field-dependent TMR response, which is determined by the magnetic properties of the CoFeB as well as the contact materials. We flexibly apply oblique-incidence deposition (OID) to introduce arbitrary intrinsic in-plane anisotropy profiles into the magnetic layers. The OID-induced anisotropy shapes the magnetic response and eliminates the requirement of additional magnetic contact materials. Functional control is achieved via an adjustable shape anisotropy that is selectively tailored for the ultrathin CoFeB layers. This approach circumvents previous limitations on TMR devices and allows for the design of new sensing functionalities, which can be precisely customized to a specific application, even in the high field regime. The resulting sensors maintain the typical TMR signal strength as well as a superb thermal stability of the tunnel junction, revealing a striking advantage in functional TMR design using anisotropic interfacial roughness

Rabi oscillations of X-ray radiation between two nuclear ensembles

The realization of the strong coupling regime between a single cavity mode and an electromagnetic resonance is a centrepiece of quantum optics. In this regime, the reversible exchange of a photon between the two components of the system leads to so-called Rabi oscillations. Strong coupling is used in the optical and infrared regimes, for instance, to produce non-classical states of light, enhance optical nonlinearities and control quantum states. Here, we report the first observation of Rabi oscillations of an X-ray photon between two resonant Fe-57 layers embedded in two coupled cavities. The system is described by an effective Hamiltonian, in which the two layers couple strongly. We observe sinusoidal beating as the signature of the Rabi oscillations in the system's temporal evolution, as well as the splitting of nuclear resonances in the reflected light spectrum. Our results significantly advance the development of the new field of X-ray quantum optics