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Research at the 2nd Institute of Physics A

Graphene

High quality CVD Graphene

Graphene has numerous properties such as its optical transparency, excellent electrical conductivity and remarkable mechanical properties that make it well suited for a large variety of potential applications. Amongst the most promising applications are high-frequency electronics, transparent electrodes and ultra-precise sensors for magnetic fields. For any of these applications to be realized, it is crucial to high quality graphene for a reasonable price. We work on large area graphene growth using chemical vapor deposition (CVD) on copper and on a dry and contamination-free process that allows to transfer the graphene on any substrate, while preserving its very high electronic quality. In order to reduce the cost of production for graphene, the copper substrate can be reused for multiple growth cycles.

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High quality CVD Graphene

 

Graphene Quantum Electromechanical Systems

The Extraordinary mechanical and electromechanichal properties of graphene – truly 2d material – make it one of the best candidate for future ultra-high frequency resonators, nano sensors or radio transducers. A detailed understanding of the interplay between mechanical and electronic properties of graphene is of high importance. For this reason a complete separation of mechanical degrees of freedom from all other adjustable parameters - i.e. temperature, charge carrier density, external pressure etc. - is considered as an important step towards exploring the fundamental properties of graphene nano-electromechanical systems. We are using an integrated system, where a graphene flake is transferred over a suspended movable silicon-based comb-drive actuator. The comb-drives are designed such that they can induce significant mechanical forces to strain graphene and the amount of strain is controlled independently. By using highly doped silicon the integrated comb-drive devices can be actuated at cryogenic temperatures, where a higher energy resolution will provide potentially more insights into the fundamental electromechanical properties of graphene. Further device functionality is introduced by a local bottom gate that enables to control the carrier concentration in graphene by tuning the Fermi level.

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Graphene Quantum Electromechanical Systems

 

Confocal Raman Spectroscopy

Raman spectroscopy has emerged as a key characterization tool in graphene research. Charge carrier concentration, strain, impurities and chemical functionalization can be probed fast and nondestructively with this method, allowing detailed analysis of fabrication processes. Moreover, fundamental physics can be accessed and studied as well. For example, key insight in the electron-phonon coupling in graphene or the interplay between lattice vibrations and Landau level excitations has been gained in previous years. Our investigations address both areas. To improve the usability in process and sample characterization we study Raman spectra of graphene on different substrates. In this regard we investigate the effect of dielectric screening on the Kohn anomaly, which strongly affects the Raman 2D line. Moreover, we study the interplay of strain fluctuations and the life time of electronic states onto the graphene Raman spectrum. Our second focus lies on the investigation of fundamental physical effects with optical methods. Here, we employ electronic and magnetic fields to probe the fine interplay of optical phonons and electronic excitons in exfoliated graphene flakes. Aside from graphene research, our Raman activities extend to the investigation of other material systems such as hexagonal boron nitride and bismuth selenide.

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Confocal Raman Spectroscopy

 

Spin Transport in Graphene

Graphene is an ideal material for spintronics. It has small spin-orbit coupling, weak hyperfine coupling and high mobility keeping the electron spins coherent over microns, enough to build spintronic devices and platforms for spin-based quantum computing. Our research activities focus on the understanding and improving of spin transport properties in engineered graphene structures. We address several unresolved fundamental issues including the role of the electrodes in spin injection and the various mechanisms behind spin dephasing and spin relaxation.

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Spin Transport in Graphene

 

Graphene Quantum Dots

Graphene has outstanding properties for electronic applications. Especially the weak spin- orbit and hyperfine interaction makes graphene interesting for hosting quantum dots (QDs) with potentially long-living spin states.

We fabricate and characterize etched single and double quantum dot devices with all- graphene charge detectors on SiO2 and hexagonal boron nitride (hBN). In low-temperature measurements we investigate the electronic excitation spectra in single-layer and bilayer graphene QDs [1]. Highly sensitive graphene charge sensors allow detecting currents through the QD in the fA regime [2]. RF pulsed-gate measurements are used to explore the relaxation dynamics of graphene quantum dots. By this technique charge relaxation times on the order of 60 to 100 ns can be determined [3]. In a comparative study of graphene QDs on SiO2 and hBN we investigate the influence of different substrates on the device performance. The results indicate a substantially reduced substrate induced disorder potential on hBN [4,5].

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Graphene Quantum Dots

 

Graphene Nanoribbons

Due to its unique electrical properties, graphene is a promising candidate for future nanoelectronics and solid-state quantum computation. However, the lack of a band gap makes graphene difficult to implement state-of-the-art electronic devices and replace current silicon-based technology.

One approach to overcome its gapless nature is to tailor the bulk graphene into narrow ribbons and therefore obtain an energy gap. Unlike in common semiconductor materials, graphene nanoribbons show local resonances within the gap region. The transport mechanism can be then understood as a charge carrier hopping through a series of quantum dots. This Coulomb blockade dominated transport mechanism confirms the existence of a non-uniform potential landscape which, together with the quantum confinement of the charge carriers along the nanoribbon, prevents Klein tunneling processes and allows a region of reduced conductance. Impurities, substrate interaction and edge roughness are assumed to contribute to the arrangement of these charged puddles and thus define the transport gap characteristics.

Our group focuses on low temperature and magnetotransport measurements to investigate the influence of localized states and edge channels. Samples are fabricated by micromechanical exfoliation of graphite and patterned via reactive ion etching techniques.

Our aim is to control the opening of the energy gap in order to use graphene in electronic devices and tunable tunnel barriers.

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Graphene Nanoribbons

 

Topological Insulators

Transport in Bismuth Selenide

Topological insulators are insulating in the bulk with conducting states at the surface where the spin is locked to the momentum. In addition, the surface states are topologically protected which means that these states cannot be destroyed by impurities or imperfections. We focus on the synthesis by vapour phase deposition of Bi2Se3, a strong 3D topological insulator. The resulting single crystal flakes are characterised by Raman spectroscopy and used for the fabrication of micro structures to probe their electronic properties.

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Transport in Bismuth Selenide

 

Semiconductor Spintronics

All-Electrical Time-Resolved Spin Generation and Manipulation in InGaAs

Spin-orbit interaction in bulk semiconductors and semiconductor nanostructures provides a variety of useful applications in spintronic devices and can fulfill basic tasks such as electrical initialization, manipulation, and detection of electron spin polarizations or spin currents. We address these tasks in the III-V semiconductor InGaAs by combining ultrafast electrical with magneto-optical pump probe techniques. Electric field pulses act as spin-orbit induced local magnetic field pulses. These pulses can initilize a coherent spin polarization along the effective magnetic field direction. They furthermore can be used for time-resolved spin manipulation. By the temporal control of the local magnetic field pulses, we can turn on and off electron spin precession and thereby rotate the spin direction into arbitrary orientations in a two-dimensional plane.

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All-Electrical Time-Resolved Spin Generation and Manipulation in InGaAs

 

Spin dynamics of donor-bound excitons in ZnO - Toward donor-based spin qubits

In spintronics, the search for quantum systems with versatile properties aims to replace the electric charge as a carrier of information by spin degrees of freedom. We use ultra-short ps laser pulses to pump and probe coherent spin polarizations in the semiconductor ZnO. By electronically timing both laser pulse trains on picosecond-timescales, we are able to achieve time-resolved measurements of ultrafast spin dynamics including spin precssion and dephasing. In particular, we focus on creating spin-polarized donor-bound excitons (electron-hole pairs), which are bound to natural donor atoms like aluminum or indium. In this process spin-polarization of the excitons can be transferred to conducting electrons as well as donor-electrons. Above all, we are able to probe the nuclear spin state of Indium via the hyperfine splitting of its donor-electron, rendering it as an interisting candidate for a spin-qubit.

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Spin dynamics of donor-bound excitons in ZnO - Toward donor-based spin qubits

 

Spin Transport in III-V Semiconductors

Spin transport is essential in the field of spintronics as spin injection, manipulation and detection usually take place in different areas of a device (e.g. in a spin transistor). Besides, it also reveals a series of fascinating spin-orbit coupling effects such as the spin-Hall effect and the spin-Seebeck effect. We investigate spin transport in III-V semiconductors using ultrafast optical pump-probe experiments (i.e. time-resolved Kerr/Faraday microscopy) under applied electric fields and temperature gradients, giving rise to spin drag and spin-polarized Seebeck transport (expected) of optically generated spin packets. As a part of the DFG program "Spin-Caloric Transport" (SpinCaT), our main goals are to study lateral spin-polarized Seebeck transport and the spin-Nernst effect (the thermally-driven analogon of the spin-Hall effect), which have been predicted theoretically but not yet been demonstrated experimentally in non-magnetic semiconductors.

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Spin Transport in III-V Semiconductors