## Projects

### P1: Coherent spin transport in III-V semiconductors

Beschoten (AC), Schäpers (JÜL)

Within this project, we will perform time- and spatially resolved studies of coherent spin currents in strained InGaAs epilayers and nanowires. Spin currents shall be generated by electric fields. We will use a novel time-resolved electrical pump optical probe schema to study coherent spin transport. Here, ultrafast current pulses are used to create spin polarized electron packets in the InGaAs channel. Phase-locked time-resolved magneto-optical Kerr effect probes the coherence of this current-induced spin polarization. We will use this method in lateral devices to obtain spatio-temporal maps of the coherent spin currents with both spin and phase information in order to extract spin diffusion and accumulation times and length, which will be compared to transport experiments.

### P2: Spin states in carbon based quantum dots

Meyer (JÜL), Stampfer (AC)

Carbon nanotubes filled with magnetic organic molecules such as endohedral fullerenes and metallocences form a model system for studying one-dimensional spin chains coupled to a ballistic conductor or, for metallic tubes, a Tomonaga-Luttinger liquid. Spin scattering on carbon nanotubes is expected to be small, while endohedral fullerenes have long spin relaxation and coherence times. Information can thus be stored in the spin states, and processed along the nanotube opening a charge-less information channel. In this project we want to resolve the main spin coupling mechanisms of spin chains inside carbon nanotubes, both within the chain as well as between chain and carbon nanotube host, and explore whether and how coherent spin processing can be achieved.

More information: Spintronics in Carbon Nanostructures | Low Dimensional Quantum Solids

### P3: Magnetic resonance of long living spin states

Kataev, Büchner (DD)

The objective of the proposed project is to study by means of ESR and NMR the properties of the long-living spin coherent electron states in III/V semiconductors and semiconductor heterostructures, such as GaAs, which can be optically pumped over the band gap. The ultimate goal is to understand the spin-dephasing and spin relaxation mechanisms that affect the long-time spin coherence. In particular, the role of the interaction of conduction electrons with electrons weakly localized at the donor states, interaction with different kinds of impurities and dislocations, the hyperfine interaction with the nuclei spins, as well as the influence on the spin coherence of the carrier concentration (metal-insulator transition), magnetic field and spin-phonon relaxation has to be elucidated.

More information: Electron Paramagnetic Resonance

### P4: Spin and current dynamics in low-dimensional quantum magnets and electronic structures

Brenig (BS), Heidrich-Meisner (M)

Recently, several types of low dimensional quantum magnets with localized spin degrees of freedom have been observed to allow for purely magnetic transport phenomena with almost no dissipation. Using numerical as well as analytical tools of many body physics, namely exact diagonalization, density matrix renormalization group, and quantum Monte-Carlo as well as projection techniques, this project deals with the theory of spin transport and spin dynamics in microscopic models such as spin ladders and non-integrable spin chains. In particular, the frequency and time dependence of the longitudinal and transverse relaxation of spins, spin currents, and spin heat currents will be studied as a function of temperature, exchange coupling parameters, and in external magnetic fields. Both intrinsic as well as extrinsic scattering mechanisms will be considered. The results will be used to shed light on high-field magnetic resonance experiments and time dependent spin transport data.

### P5: Spin and heat transport in one-dimensional magnets

Heß, Klauß, Büchner (DD)

For one-dimensional quantum spin systems recent experimental and theoretical studies indicate unexpectedly large, in some cases diverging, spin and heat transport coefficients. This can be the basis of a radically new concept of spin transport â€“ the coherent transport of magnetic information. In the proposed research, pure magnetic excitations like magnons will be used to carry spin information and energy on magnetic lattices. This transport phenomenon is independent of charge motion, which is the prerequisite of usual investigations of spin transport using spin-polarized electron currents. In the proposed work, we want to clarify the relaxation mechanisms and interdependence of spin and heat currents in combined heat transport and magnetic resonance studies. Extending conventional transport and magnetic resonance measurement concepts, we want to develop new techniques for generating pulsed spin and heat currents based on thermal and optical pumping.

### P6: Spin properties and spin transport in graphene

Morgenstern, Beschoten, Güntherodt, (AC)

Within project P6, we explore extrinsic and intrinsic spin properties of graphene. On the one hand, “non-local” spin valve structures using MgO-barriers for spin injection will be fabricated in order to study the spin relaxation lengths and relaxation times of the bare material. These parameters can be explored as a function of carrier density by means of a top-gate electrode. Pulsed electrical spin injection into “non-local” spin valve structures of graphene will be used for an all-electrical proof of the spin coherence. On the other hand, spin defects at graphene edges and introduced by ion bombardment will be explored in detail on the local scale by spin-polarized STM and scanning tunnelling spectroscopy (STS).

More information: Spin transport graphene | STM Graphene

### P8: Coherent few-electron spin-states in graphene nanoribbons from ab-initio

Blügel (JÜL)

The goal of project P8 is the development an ab-initio-based approach for the realistic numerical simulation of coherent many-body spin-states in graphene-nanoribbon structures under the influence of external gate electrodes. The main idea behind the proposed algorithm is the usage of density-functional theory (DFT) as an ab-initio approach to calculate single-particle basis wavefunctions and associated microscopic matrix elements and, in turn, to employ a relevant subset of resulting Slater-determinants for the diagonalization (configuration interaction, CI) of the many-body Hamiltonian. As compared to highly idealized models, the advantage of the envisioned approach is the combination of an ab-initio theory with a generic many-body model. This enables a realistic calculation of coherent many-body spin-states and their real-time evolution.

Team: T. Burnus, Y. Mokrousov, D. Wortmann, G. Bihlmayer, S. Blügel, K. M. Indlekofer.

### P9: Spin transport and spin coherence in quantum wires and dots

Team: Schoeller, Meden, Andergassen (AC),Wegewijs (JÜL)

Goal of project P9 is the theoretical investigation of spin relaxation and spin dephasing in one-dimensional quantum wires. Fingerprints of spin-dependent interactions will be analysed in transport properties, spectral densities and in the real-time evolution of local spin excitations. Specific subtopics are:

- The calculation of the magnetoresistance of a quantum wire between two ferromagnetic leads
- The study of Kondo physics through a quantum dot coupled to a Luttinger liquid lead or a lead with strong spin-orbit interaction
- The investigation of the dynamics of local spins in endohedral fullerenes in a carbon nanotube and their backaction on the dynamics of the itinerant electrons
- The study of the time evolution of locally excited spin packets in one-channel quantum wires and quantum spin chains using TD-DMRG

### P10: Spin coherence, spin qubits and spin transport in carbon-based nanostructures

Burkard (K), Honerkamp (AC)

This theory project deals with the spin coherence of electrons localized in quantum dots fabricated in one- and two-dimensional semiconductor and carbon structures will be studied. In contrast to GaAs, InGaAs exhibits much stronger spin-orbit coupling that allows for electrical control of the spin via the Rashba effect but also strongly affects the spin coherence and spin relaxation processes. Carbon is very different from both GaAs and InGaAs, with spin-orbit coupling expected to be very weak, and thus the spin relaxation times T1 in carbon-based structures are expected to be quite long. This makes carbon a promising material for spin qubits (see schematic figure).