Publications

• Diminishing topological Faraday effect in thin layer samples. Berger, Christian; Bayer, Florian; Molenkamp, Laurens W.; Kiessling, Tobias. In Phys. Rev. Res., 6(1), p. 013068. American Physical Society, 2024.
  • [ Abstract ]
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  A striking feature of three-dimensional (3D) topological insulators (TIs) is the theoretically expected topological magnetoelectric (TME) effect, which gives rise to additional terms in Maxwell's laws of electromagnetism with an universal quantized coefficient proportional to half-integer multiples of the fine-structure constant \($\ensuremath{\alpha}$\). In an ideal scenario one therefore expects also quantized contributions in the magnetooptical response of TIs. We review this premise by taking into account the trivial dielectric background of the TI bulk and potential host substrates, and the often present contribution of itinerant bulk carriers. We show (i) that one obtains a nonuniversal magnetooptical response whenever there is impedance mismatch between different layers and (ii) that the detectable signals due to the TME rapidly approach vanishingly small values as the impedance mismatch is detuned from zero. We demonstrate that it is methodologically impossible to deduce the existence of a TME exclusively from an optical experiment in the thin film limit of 3D TIs at high magnetic fields.

• Observation of cnoidal wave localization in nonlinear topolectric circuits. Hohmann, Hendrik; Hofmann, Tobias; Helbig, Tobias; Imhof, Stefan; Brand, Hauke; Upreti, Lavi K.; Stegmaier, Alexander; Fritzsche, Alexander; Müller, Tobias; Schwingenschlögl, Udo; Lee, Ching Hua; Greiter, Martin; Molenkamp, Laurens W.; Kießling, Tobias; Thomale, Ronny. In Physical Review Research, 5(1), p. L012041. 2023.
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  We observe a localized cnoidal (LCn) state in an electric circuit network. Its formation derives from the interplay of nonlinearity and the topology inherent to a Su-Schrieffer-Heeger (SSH) chain of inductors. Varicap diodes act as voltage-dependent capacitors, and create a nonlinear on-site potential. For a sinusoidal voltage excitation around midgap frequency, we show that the voltage response in the nonlinear SSH circuit follows the Korteweg-de Vries equation. The topological SSH boundary state, which relates to a midgap impedance peak in the linearized limit is distorted into the LCn state in the nonlinear regime, where the cnoidal eccentricity decreases from edge to bulk.
• Fast low bias pulsed DC transport measurements for the investigation of low temperature transport effects in semiconductor devices. Fuchs, C.; Hofer, M.; Fürst, L.; Shamim, S.; Kießling, T.; Buhmann, H.; Molenkamp, L. W. In Journal of Applied Physics, 134(17), p. 175702. 2023.
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  We present a setup for fast, low-bias ( ≤ 1 mV) DC transport measurements with μs time resolution for high ohmic resistance ( ≈ 20 k Ω) semiconducting samples. We discuss the circuitry and instrumentation for the measurement approach that can be applied to any kind of semiconductor device or (gated) two-dimensional material and demonstrate the main measurement artifacts in typical measurements by means of circuit simulation. Based on the latter, we present a simple two-step protocol for eliminating the measurement artifacts reliably. We demonstrate the technique by measuring the transitions between quantum Hall plateaus in the HgTe quantum wells and resolve plateaus as short-lived as 100 μs.
• Hyperbolic matter in electrical circuits with tunable complex phases. Chen, Anffany; Brand, Hauke; Helbig, Tobias; Hofmann, Tobias; Imhof, Stefan; Fritzsche, Alexander; Kießling, Tobias; Stegmaier, Alexander; Upreti, Lavi K.; Neupert, Titus; Bzdušek, Tomáš; Greiter, Martin; Thomale, Ronny; Boettcher, Igor. In Nature Communications, 14(1), p. 622. 2023.
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  Curved spaces play a fundamental role in many areas of modern physics, from cosmological length scales to subatomic structures related to quantum information and quantum gravity. In tabletop experiments, negatively curved spaces can be simulated with hyperbolic lattices. Here we introduce and experimentally realize hyperbolic matter as a paradigm for topological states through topolectrical circuit networks relying on a complex-phase circuit element. The experiment is based on hyperbolic band theory that we confirm here in an unprecedented numerical survey of finite hyperbolic lattices. We implement hyperbolic graphene as an example of topologically nontrivial hyperbolic matter. Our work sets the stage to realize more complex forms of hyperbolic matter to challenge our established theories of physics in curved space, while the tunable complex-phase element developed here can be a key ingredient for future experimental simulation of various Hamiltonians with topological ground states.
• Giant THz Nonlinearity in Topological and Trivial HgTe-Based Heterostructures. Uaman Svetikova, Tatiana A.; de Oliveira, Thales V. A. G.; Pashkin, Alexej; Ponomaryov, Alexey; Berger, Christian; Fürst, Lena; Bayer, Florian B.; Novik, Elena G.; Buhmann, Hartmut; Molenkamp, Laurens W.; Helm, Manfred; Kiessling, Tobias; Winnerl, Stephan; Kovalev, Sergey; Astakhov, Georgy V. In ACS Photonics, 10(10), pp. 3708–3714. 2023.
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  Nonlinear phenomena in the THz spectral domain are important for understanding the optoelectronic properties of quantum systems and provide a basis for modern information technologies. Here, we report a giant THz nonlinearity in high-mobility 2D topological insulators based on HgTe quantum wells, which manifests itself in a highly efficient third harmonic generation. We observe a third harmonic THz susceptibility several times higher than that in bare graphene and many orders of magnitude higher than that in trivial quantum well structures based on other materials. To explain the strong nonlinearity of HgTe-based heterostructures at the THz frequencies, we consider the acceleration of free carriers with a high mobility and variable dispersion. This acceleration model, for which the nonparabolicity of the band dispersion is key, in combination with independently measured scattering time and conductivity, is in good agreement with our experimental data in a wide temperature range for THz fields below the saturation. Our approach provides a route to material engineering for THz applications based on frequency conversion.
• Efficient terahertz harmonic generation in topological metamaterials. Kovalev, S.; Tielrooij, K.; Ilyakov, I.; Deinert, J.-C.; De Oliveira, T.; Ponomaryov, O.; Principi, A.; Reig, D.; Block, A.; Varghese, S.; Schreyeck, S.; Brunner, K.; Karczewski, G.; Carbonell, C.; Valenzuela, S.; Molenkamp, L.; Kiessling, T.; Astakhov, G. V. In 2023 48th International Conference on Infrared, Millimeter, and Terahertz Waves ({IRMMW}-{THz}), pp. 1–2. 2023.
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  Graphene, with its massless Dirac surface states, exhibits large THz nonlinear susceptibilities leading to efficient THz high harmonic generation (HHG). However, the THz HHG efficiency in graphene strongly saturates with increasing incident field strength, limiting the HHG field conversion efficiency to a level of 1%. Here, we show that the THz HHG efficiency can be significantly enhanced at high THz driving fields by mitigating the overheating problem, making use of the efficient heat exchange between surface and bulk states of topological insulators. By implementing topological-based metamaterials for field enhancement, THz third harmonic generation with textbackslashsim 10% field conversion efficiency is demonstrated. These results are of particular interest for efficient on-chip optoelectronic devices operating at ultrahigh frequencies.

• Simulating hyperbolic space on a circuit board. Lenggenhager, Patrick M.; Stegmaier, Alexander; Upreti, Lavi K.; Hofmann, Tobias; Helbig, Tobias; Vollhardt, Achim; Greiter, Martin; Lee, Ching Hua; Imhof, Stefan; Brand, Hauke; Kießling, Tobias; Boettcher, Igor; Neupert, Titus; Thomale, Ronny; Bzdušek, Tomáš. In Nature Communications, 13(1), p. 4373. 2022.
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  The Laplace operator encodes the behavior of physical systems at vastly different scales, describing heat flow, fluids, as well as electric, gravitational, and quantum fields. A key input for the Laplace equation is the curvature of space. Here we discuss and experimentally demonstrate that the spectral ordering of Laplacian eigenstates for hyperbolic (negatively curved) and flat two-dimensional spaces has a universally different structure. We use a lattice regularization of hyperbolic space in an electric-circuit network to measure the eigenstates of a ‘hyperbolic drum’, and in a time-resolved experiment we verify signal propagation along the curved geodesics. Our experiments showcase both a versatile platform to emulate hyperbolic lattices in tabletop experiments, and a set of methods to verify the effective hyperbolic metric in this and other platforms. The presented techniques can be utilized to explore novel aspects of both classical and quantum dynamics in negatively curved spaces, and to realise the emerging models of topological hyperbolic matter.
• Milliwatt terahertz harmonic generation from topological insulator metamaterials. Tielrooij, Klaas-Jan; Principi, Alessandro; Reig, David Saleta; Block, Alexander; Varghese, Sebin; Schreyeck, Steffen; Brunner, Karl; Karczewski, Grzegorz; Ilyakov, Igor; Ponomaryov, Oleksiy; de Oliveira, Thales V. A. G.; Chen, Min; Deinert, Jan-Christoph; Carbonell, Carmen Gomez; Valenzuela, Sergio O.; Molenkamp, Laurens W.; Kiessling, Tobias; Astakhov, Georgy V.; Kovalev, Sergey. In Light: Science & Applications, 11(1), p. 315. 2022.
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  Achieving efficient, high-power harmonic generation in the terahertz spectral domain has technological applications, for example, in sixth generation (6G) communication networks. Massless Dirac fermions possess extremely large terahertz nonlinear susceptibilities and harmonic conversion efficiencies. However, the observed maximum generated harmonic power is limited, because of saturation effects at increasing incident powers, as shown recently for graphene. Here, we demonstrate room-temperature terahertz harmonic generation in a Bi2Se3 topological insulator and topological-insulator-grating metamaterial structures with surface-selective terahertz field enhancement. We obtain a third-harmonic power approaching the milliwatt range for an incident power of 75 mW—an improvement by two orders of magnitude compared to a benchmarked graphene sample. We establish a framework in which this exceptional performance is the result of thermodynamic harmonic generation by the massless topological surface states, benefiting from ultrafast dissipation of electronic heat via surface-bulk Coulomb interactions. These results are an important step towards on-chip terahertz (opto)electronic applications.

• Topological Defect Engineering and \($\mathcal{P}\mathcal{T}$\) Symmetry in Non-Hermitian Electrical Circuits. Stegmaier, Alexander; Imhof, Stefan; Helbig, Tobias; Hofmann, Tobias; Lee, Ching Hua; Kremer, Mark; Fritzsche, Alexander; Feichtner, Thorsten; Klembt, Sebastian; Höfling, Sven; Boettcher, Igor; Fulga, Ion Cosma; Ma, Libo; Schmidt, Oliver G.; Greiter, Martin; Kiessling, Tobias; Szameit, Alexander; Thomale, Ronny. In Physical Review Letters, 126(21), p. 215302. 2021.
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  We employ electric circuit networks to study topological states of matter in non-Hermitian systems enriched by parity-time symmetry PT and chiral symmetry anti-PT (APT). The topological structure manifests itself in the complex admittance bands which yields excellent measurability and signal to noise ratio. We analyze the impact of PT-symmetric gain and loss on localized edge and defect states in a non-Hermitian Su-Schrieffer-Heeger (SSH) circuit. We realize all three symmetry phases of the system, including the APT-symmetric regime that occurs at large gain and loss. We measure the admittance spectrum and eigenstates for arbitrary boundary conditions, which allows us to resolve not only topological edge states, but also a novel PT-symmetric Z2 invariant of the bulk. We discover the distinct properties of topological edge states and defect states in the phase diagram. In the regime that is not PT symmetric, the topological defect state disappears and only reemerges when APT symmetry is reached, while the topological edge states always prevail and only experience a shift in eigenvalue. Our findings unveil a future route for topological defect engineering and tuning in non-Hermitian systems of arbitrary dimension.
• Active topolectrical circuits. Kotwal, Tejas; Moseley, Fischer; Stegmaier, Alexander; Imhof, Stefan; Brand, Hauke; Kießling, Tobias; Thomale, Ronny; Ronellenfitsch, Henrik; Dunkel, Jörn. In Proceedings of the National Academy of Sciences, 118(32), p. e2106411118. 2021.
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  The transfer of topological concepts from the quantum world to classical mechanical and electronic systems has opened fundamentally different approaches to protected information transmission and wave guidance. A particularly promising emergent technology is based on recently discovered topolectrical circuits that achieve robust electric signal transduction by mimicking edge currents in quantum Hall systems. In parallel, modern active matter research has shown how autonomous units driven by internal energy reservoirs can spontaneously self-organize into collective coherent dynamics. Here, we unify key ideas from these two previously disparate fields to develop design principles for active topolectrical circuits (ATCs) that can self-excite topologically protected global signal patterns. Realizing autonomous active units through nonlinear Chua diode circuits, we theoretically predict and experimentally confirm the emergence of self-organized protected edge oscillations in one- and two-dimensional ATCs. The close agreement between theory, simulations, and experiments implies that nonlinear ATCs provide a robust and versatile platform for developing high-dimensional autonomous electrical circuits with topologically protected functionalities.

• Polarization-Assisted Vector Magnetometry with No Bias Field Using an Ensemble of Nitrogen-Vacancy Centers in Diamond. Münzhuber, F.; Bayer, F.; Marković, V.; Brehm, J.; Kleinlein, J.; Molenkamp, L. W.; Kiessling, T. In Physical Review Applied, 14(1), p. 014055. 2020.
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  Nitrogen-vacancy (N-V) centers in diamond are among the best-studied quantum-based sensors for a wide range of applications. Here, we demonstrate an optical polarization-based method for full vector magnetometry using ensembles of N-V centers. The anisotropy of the electric dipole moments of the N-V centers enables the assignment of the different magnetic field split transitions in the optically detected magnetic resonance spectra by appropriately setting the optical polarization in the excitation and/or detection of the N-V photoluminescence signal. By further employing the optical-selection rules for the driving microwave field, we show how the full vector of the investigated magnetic field can be restored without the need to apply external bias fields. We discuss different N-V-center orientation implementations and limitations toward common approximations used in the N-V magnetometry literature for our approach.
• Generalized bulk–boundary correspondence in non-Hermitian topolectrical circuits. Helbig, T.; Hofmann, T.; Imhof, S.; Abdelghany, M.; Kiessling, T.; Molenkamp, L. W.; Lee, C. H.; Szameit, A.; Greiter, M.; Thomale, R. In Nature Physics, 16(7), pp. 747–750. 2020.
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  The study of the laws of nature has traditionally been pursued in the limit of isolated systems, where energy is conserved. This is not always a valid approximation, however, as the inclusion of features such as gain and loss, or periodic driving, qualitatively amends these laws. A contemporary frontier of metamaterial research is the challenge open systems pose to the characterization of topological matter1,2. Here, one of the most relied upon principles is the bulk–boundary correspondence (BBC), which intimately relates the surface states to the topological classification of the bulk3,4. The presence of gain and loss, in combination with the violation of reciprocity, has been predicted to affect this principle dramatically5,6. Here, we report the experimental observation of BBC violation in a non-reciprocal topolectric circuit7, which is also referred to as the non-Hermitian skin effect. The circuit admittance spectrum exhibits an unprecedented sensitivity to the presence of a boundary, displaying an extensive admittance mode localization despite a translationally invariant bulk. Intriguingly, we measure a non-local voltage response due to broken BBC. Depending on the a.c. current feed frequency, the voltage signal accumulates at the left or right boundary, and increases as a function of nodal distance to the current feed.
• Reciprocal skin effect and its realization in a topolectrical circuit. Hofmann, Tobias; Helbig, Tobias; Schindler, Frank; Salgo, Nora; Brzezińska, Marta; Greiter, Martin; Kiessling, Tobias; Wolf, David; Vollhardt, Achim; Kabaši, Anton; Lee, Ching Hua; Bilušić, Ante; Thomale, Ronny; Neupert, Titus. In Physical Review Research, 2(2), p. 023265. 2020.
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  A system is non-Hermitian when it exchanges energy with its environment and nonreciprocal when it behaves differently upon the interchange of input and response. Within the field of metamaterial research on synthetic topological matter, the skin effect describes the conspiracy of non-Hermiticity and nonreciprocity to yield extensive anomalous localization of all eigenmodes in a (quasi) one-dimensional geometry. Here, we introduce the reciprocal skin effect, which occurs in non-Hermitian but reciprocal systems in two or more dimensions: Eigenmodes with opposite longitudinal momentum exhibit opposite transverse anomalous localization. We experimentally demonstrate the reciprocal skin effect in a passive RLC circuit, suggesting convenient alternative implementations in optical, acoustic, mechanical, and related platforms. Skin mode localization brings forth potential applications in directional and polarization detectors for electromagnetic waves.
• Efficiency of ultrafast optically induced spin transfer in Heusler compounds. Steil, Daniel; Walowski, Jakob; Gerhard, Felicitas; Kiessling, Tobias; Ebke, Daniel; Thomas, Andy; Kubota, Takahide; Oogane, Mikihiko; Ando, Yasuo; Otto, Johannes; Mann, Andreas; Hofherr, Moritz; Elliott, Peter; Dewhurst, John Kay; Reiss, Günter; Molenkamp, Laurens; Aeschlimann, Martin; Cinchetti, Mirko; Münzenberg, Markus; Sharma, Sangeeta; Mathias, Stefan. In Physical Review Research, 2(2), p. 023199. 2020.
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  Optically induced spin transfer (OISTR) is a pathway to control magnetization dynamics in complex materials on femto- to attosecond timescales. The direct interaction of the laser field with the material creates transient nonequilibrium states, which can exhibit an efficient spin transfer between different magnetic subsystems. How far this spin manipulation via OISTR is a general phenomenon or restricted to a subset of materials with specific properties is an open experimental and theoretical question. Using time-resolved magneto-optical Kerr measurements and time-dependent density functional theory we investigate OISTR in Heusler compounds. We show that the half-Heusler materials NiMnSb and CoMnSb exhibit strong signatures of OISTR, whereas this is less pronounced in the full-Heusler compounds Co2MnSi, Co2FeSi, and Co2FeAl in agreement with ab initio calculations. Our work opens up a systematic path for coherent manipulation of spin dynamics by direct light-matter interaction.
• High-resolution resonance spin-flip Raman spectroscopy of pairs of manganese ions in a CdTe quantum well. Cherbunin, R. V.; Litviak, V. M.; Ryzhov, I. I.; Koudinov, A. V.; Elsässer, S.; Knapp, A.; Kiessling, T.; Geurts, J.; Chusnutdinow, S.; Wojtowicz, T.; Karczewski, G. In Physical Review B, 101(24), p. 241301. 2020.
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  We report the observation of tens of minor lines of the combinational spin-flip Raman scattering in a CdTe:Mn quantum well by means of the high-resolution optical spectroscopy. Classification of this manifold leads to four characteristic values of energy, that correspond to four different types of pair clusters of Mn ions: the nearest, second, third, etc., neighbors. All the four energies show up in a single experiment with a very high precision, providing experimental grounds for a deeper understanding of the d−d exchange interactions in a diluted magnetic semiconductor and demonstrating the capacity of the employed method. The major (nearest-neighbor) exchange constant J1=6.15±0.05 K was found to consent with its previously reported value. Other detected characteristic energies are as follows (±0.05 K): J(2)=1.80 K, J(3)=1.39 K, J(4)=0.81 K.

• Band structure engineering and reconstruction in electric circuit networks. Helbig, Tobias; Hofmann, Tobias; Lee, Ching Hua; Thomale, Ronny; Imhof, Stefan; Molenkamp, Laurens W.; Kiessling, Tobias. In Physical Review B, 99(16), p. 161114. 2019.
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  We develop an approach to design, engineer, and measure band structures in a synthetic crystal composed of electric circuit elements. Starting from a nodal analysis of a circuit lattice in terms of currents and voltages, our Laplacian formalism for synthetic matter allows us to investigate arbitrary tight-binding models in terms of wave-number-resolved Laplacian eigenmodes, yielding an admittance band structure of the circuit. For illustration, we model and measure a honeycomb circuit featuring a Dirac cone admittance bulk dispersion as well as flat band admittance edge modes at its bearded and zigzag terminations. We further employ our circuit band analysis to measure a topological phase transition in the topolectrical Su-Schrieffer-Heeger circuit.

• Topolectrical-circuit realization of topological corner modes. Imhof, Stefan; Berger, Christian; Bayer, Florian; Brehm, Johannes; Molenkamp, Laurens W.; Kiessling, Tobias; Schindler, Frank; Lee, Ching Hua; Greiter, Martin; Neupert, Titus; Thomale, Ronny. In Nature Physics, 14(9), pp. 925–929. 2018.
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  Quantized electric quadrupole insulators have recently been proposed as novel quantum states of matter in two spatial dimensions. Gapped otherwise, they can feature zero-dimensional topological corner mid-gap states protected by the bulk spectral gap, reflection symmetries and a spectral symmetry. Here we introduce a topolectrical circuit design for realizing such corner modes experimentally and report measurements in which the modes appear as topological boundary resonances in the corner impedance profile of the circuit. Whereas the quantized bulk quadrupole moment of an electronic crystal does not have a direct analogue in the classical topolectrical-circuit framework, the corner modes inherit the identical form from the quantum case. Due to the flexibility and tunability of electrical circuits, they are an ideal platform for studying the reflection symmetry-protected character of corner modes in detail. Our work therefore establishes an instance where topolectrical circuitry is employed to bridge the gap between quantum theoretical modelling and the experimental realization of topological band structures.
• Topolectrical Circuits. Lee, Ching Hua; Imhof, Stefan; Berger, Christian; Bayer, Florian; Brehm, Johannes; Molenkamp, Laurens W.; Kiessling, Tobias; Thomale, Ronny. In Communications Physics, 1(1), pp. 1–9. 2018.
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  Invented by Alessandro Volta and Félix Savary in the early 19th century, circuits consisting of resistor, inductor and capacitor (RLC) components are omnipresent in modern technology. The behavior of an RLC circuit is governed by its circuit Laplacian, which is analogous to the Hamiltonian describing the energetics of a physical system. Here we show that topological insulating and semimetallic states can be realized in a periodic RLC circuit. Topological boundary resonances (TBRs) appear in the impedance read-out of a topolectrical circuit, providing a robust signal for the presence of topological admittance bands. For experimental illustration, we build the Su-Schrieffer–Heeger circuit, where our impedance measurement detects the TBR midgap state. Topolectrical circuits establish a bridge between electrical engineering and topological states of matter, where the accessibility, scalability, and operability of electronics synergizes with the intricate boundary properties of topological phases.

• Thermodynamic origin of the slow free exciton photoluminescence rise in GaAs. Beck, M.; Hübner, J.; Oestreich, M.; Bieker, S.; Henn, T.; Kiessling, T.; Ossau, W.; Molenkamp, L. W. In Physical Review B, 93(8), p. 081204. 2016.
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  We use time-resolved photoluminescence (TRPL) spectroscopy to unequivocally clarify the microscopic origin of the nanosecond free exciton photoluminescence rise in GaAs at low temperatures. In crucial distinction from previous work, we examine the TRPL of the GaAs free exciton second LO-phonon replica. This enables us to simultaneously monitor the unambiguous time evolution of the total exciton population and the cooling dynamics of the initially hot free exciton ensemble. We demonstrate by a model based on the Saha equation and the experimentally determined cooling behavior that the long-debated slow photoluminescence rise is caused by time-dependent shifts in the thermodynamic quasiequilibrium between free excitons and the uncorrelated electron-hole plasma.
• NV-center diamond cantilevers: Extending the range of available fabrication methods. Kleinlein, J.; Borzenko, T.; Münzhuber, F.; Brehm, J.; Kiessling, T.; Molenkamp, L. W. In Microelectronic Engineering, 159, pp. 70–74. 2016.
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  We present an alternative fabrication process for single crystal diamond cantilevers that consist of a platform carrying a nanopillar containing nitrogen vacancy centers close to its surface. These cantilevers are suitable for vector magnetometry by fitting them to the scanning unit of an atomic force microscope. Our methods extend existing technology to lower electron voltages (30kV) and standard reactive ion etching. Special procedures are presented to overcome both restrictions due to charging of the diamond during electron beam lithography and limitations of the etch depth due to the low plasma density in reactive ion etching systems. The presence of nitrogen vacancy centers in the fabricated nanostructures is confirmed by photoluminescence measurements.

• Optical power-driven electron spin relaxation regime crossover in Mn-doped bulk GaAs. Münzhuber, F.; Kiessling, T.; Ossau, W.; Molenkamp, L. W.; Astakhov, G. V. In Physical Review B, 92(11), p. 115208. 2015.
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  We demonstrate tunability of the electron spin lifetime in Mn-doped GaAs by purely optical means. The observed behavior stems from a crossover of the electron spin relaxation rate with increasing excitation density, first decreasing due to the exchange interaction of Mn bound holes with Mn ions, and then increasing again as the valence band is populated and Bir-Aranov-Pikus relaxation sets in. On this account, we explain the complex spatial spin polarization profiles emerging from inhomogeneous optical excitation, which are the result of the combined action of this nonmonotonic spin relaxation characteristics and the intricate photocarrier decay dynamics.
• Spatially Resolved Thermodynamics of the Partially Ionized Exciton Gas in GaAs. Bieker, S.; Henn, T.; Kiessling, T.; Ossau, W.; Molenkamp, L. W. In Physical Review Letters, 114(22), p. 227402. 2015.
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  We report on the observation of macroscopic free exciton photoluminescence (PL) rings that appear in spatially resolved PL images obtained on a high purity GaAs sample. We demonstrate that a spatial temperature gradient in the photocarrier system, which is due to nonresonant optical excitation, locally modifies the population balance between free excitons and the uncorrelated electron-hole plasma described by the Saha equation and accounts for the experimentally observed nontrivial PL profiles. The exciton ring formation is a particularly instructive manifestation of the spatially dependent thermodynamics of a partially ionized exciton gas in a bulk semiconductor.
• Polytypism and band alignment in ZnSe nanowires revealed by photoluminescence spectroscopy of embedded ({Zn},{Cd})Se quantum dots. Bieker, S.; Pfeuffer, R.; Kiessling, T.; Tarakina, N.; Schumacher, C.; Ossau, W.; Molenkamp, L. W.; Karczewski, G. In Physical Review B, 91(12), p. 125301. 2015.
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  We report on the optical characterization of single (Zn,Cd)Se quantum dots (QDs) embedded in vapor-liquid-solid-grown ZnSe nanowires (NWs). The temperature dependent quenching of the QD luminescence demonstrates that their electronic structure is comparable to that of self-assembled (Zn,Cd)Se QDs in ZnSe matrices. The photoluminescence excitation (PLE) spectrum of single nanowire QDs reveals the presence of both zinc blende (ZB) and wurtzite (WZ) crystal modifications of ZnSe in the NW shafts. PLE provides, therefore, a complementary technique to transmission electron microscopy imaging to reveal polytypism in ZnSe NWs. A transient quenching of the PL emission suggests a type II staggered band alignment at the ZB/WZ interface in our ZnSe NWs.
• Correct determination of low-temperature free-exciton diffusion profiles in GaAs. Bieker, S.; Kiessling, T.; Ossau, W.; Molenkamp, L. W. In Physical Review B, 92(12), p. 121201. 2015.
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  We report on low-temperature spatially resolved photoluminescence (SRPL) experiments to study the diffusion of free excitons in a 1.5−μm-thick layer of high-purity epitaxial GaAs. Extending previous SRPL experiments, we analyze the stationary diffusion profiles detected on the second LO-phonon replica of the free exciton. This allows us to circumvent the inherent interpretation ambiguities of the free-exciton zero-phonon line. Moreover, a spatially resolved line shape analysis of the (FX)−2ℏΩLO replica provides direct experimental access to the pump-induced exciton temperature profile. We demonstrate that only resonant optical excitation prevents the buildup of a temperature gradient in the carrier system, which otherwise severely distorts the stationary and time-resolved free-exciton diffusion profiles.
• Dimensional crossover of free exciton diffusion in etched GaAs wire structures. Bieker, S.; Stühler, R.; Kiessling, T.; Ossau, W.; Molenkamp, L. W. In Applied Physics Letters, 107(12), p. 122106. 2015.
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  We report on low-temperature spatially resolved photoluminescence spectroscopy to study the diffusion of free excitons in etched wire structures of high-purity GaAs. We assess the stationary diffusion profiles by the free exciton second LO-phonon replica to circumvent the inherent interpretation ambiguities of the previously investigated free exciton zero-phonon line. Moreover, strictly resonant optical excitation prevents the distortion of the diffusion profiles due to local heating in the carrier system. We observe a dimensional crossover from 2D to 1D exciton diffusion when the lateral wire width falls below the diffusion length.

• Removal of GaAs growth substrates from II–VI semiconductor heterostructures. Bieker, S.; Hartmann, P. R.; Kießling, T.; Rüth, M.; Schumacher, C.; Gould, C.; Ossau, W.; Molenkamp, L. W. In Semiconductor Science and Technology, 29(4), p. 045016. 2014.
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  We report on a process that enables the removal of II–VI semiconductor epilayers from their GaAs growth substrate and their subsequent transfer to arbitrary host environments. The technique combines mechanical lapping and layer selective chemical wet etching and is generally applicable to any II–VI layer stack. We demonstrate the non-invasiveness of the method by transferring an all-II–VI magnetic resonant tunneling diode. High resolution x-ray diffraction proves that the crystal integrity of the heterostructure is preserved. Transport characterization confirms that the functionality of the device is maintained and even improved, which is ascribed to completely elastic strain relaxation of the tunnel barrier layer.
• Time and spatially resolved electron spin detection in semiconductor heterostructures by magneto-optical Kerr microscopy. Henn, Tobias; Kießling, Tobias; Molenkamp, Laurens W.; Reuter, Dirk; Wieck, Andreas D.; Biermann, Klaus; Santos, Paulo V.; Ossau, Wolfgang. In physica status solidi (b), 251(9), pp. 1839–1849. 2014.
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  We review on time and spatially resolved two-color pump–probe magneto-optical Kerr effect (MOKE) microscopy studies of electron spins in bulk n-GaAs and GaAs (110) quantum wells (QWs) at low lattice temperatures. The influence of photocarrier heating by above-bandgap optical spin excitation on the spatially resolved magneto-optical spin detection is considered in detail. We demonstrate that a continuous-wave (cw) measurement of the local Kerr rotation at a fixed arbitrary probe wavelength in general does not correctly reveal the local spin polarization when hot electrons are present. For bulk GaAs we determine the true lateral electron spin polarization profile from cw measurements of the spatial dependence of the full excitonic Kerr rotation spectrum. For the (110) QWs, we directly obtain the electron spin diffusion coefficient from picosecond real space imaging of the time evolution of an optically excited electron spin packet, which we observe with a spectrally broad probe pulse.
• Exciton decay dynamics controlled by impurity occupation in strongly Mn-doped and partially compensated bulk GaAs. Münzhuber, F.; Henn, T.; Kiessling, T.; Ossau, W.; Molenkamp, L. W.; Gieseking, B.; Astakhov, G. V.; Dyakonov, V. In Physical Review B, 90(12), p. 125203. 2014.
  • [ Abstract ]
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  We report on a pronounced prolongation of the exciton decay in strongly p-doped and partially compensated direct band-gap semiconductor GaAs:Mn with increasing optical excitation power. Using time-resolved photoluminescence we show that the intricate interplay of excitons, shallow and deep impurity centers in GaAs:Mn results in a complex recombination behavior that cannot be characterized in terms of simple rates. The decay can be precisely described by a model based on Shockley-Read-Hall recombination, which shows that the observed dynamics arise from a varying neutralization of shallow and deep recombination centers. This enables the investigation of the carrier dynamics in the impurity system by measuring only the exciton decay time.
• Excitonic ring formation in ultrapure bulk GaAs. Bieker, S.; Henn, T.; Kiessling, T.; Ossau, W.; Molenkamp, L. W. In Physical Review B, 90(20), p. 201305. 2014.
  • [ Abstract ]
  • [ DOI ]
  We report on spatially resolved low-temperature photoluminescence (PL) measurements of excitons in ultrapure bulk GaAs. At moderate excitation densities we observe butterfly-shaped luminescence images in the wavelength–radial distance plane with a pronounced quench of the exciton PL intensity at the excitation center. The shapes of the PL images show a delicate dependence on excitation wavelength and pump power. We present a model that quantitatively explains the PL intensity quench by a localized overheating of the exciton ensemble due to nonresonant optical excitation. Our model allows us to extract absolute exciton temperatures and to trace the influence of excitation excess energy on the spatial dependence of the exciton energy relaxation. We observe temperature gradients in the exciton system which persist over distances ≥10μm away from the excitation spot.
• Hot electron spin diffusion in n-type GaAs. Henn, T.; Quast, J.-H.; Beck, M.; Kiessling, T.; Ossau, W.; Molenkamp, L. W. In The European Physical Journal Plus, 129(6), p. 118. 2014.
  • [ Abstract ]
  • [ DOI ]
  We use two-color magneto-optical Kerr (MOKE) microscopy to study low-temperature electron spin diffusion in bulk n-type GaAs (n = 1.4 × 1016 cm−3). We show that simple drift-diffusion models do not describe the spin diffusion for above-bandgap optical spin injection correctly. By variation of the excitation energy and lattice temperature we demonstrate that this discrepancy is caused by the influence of the pump-induced local overheating of the electron system with respect to the lattice, which persists over length scales comparable to the spin diffusion length. Consideration of this hot carrier effect is crucial for a reliable extraction of spin propagation parameters from optical experiments.

• Hot carrier effects on lateral electron spin diffusion in \\($n\$\)-type GaAs. Quast, J.-H.; Henn, T.; Kiessling, T.; Ossau, W.; Molenkamp, L. W.; Reuter, D.; Wieck, A. D. In Physical Review B, 87(20), p. 205203. 2013.
  • [ Abstract ]
  • [ DOI ]
  We report on spatially resolved two-color pump-probe Hanle–magneto-optical Kerr effect studies of low-temperature electron spin diffusion in bulk n-type GaAs (nD=1.4×1016cm−3). The influence of the lattice temperature on electron spin diffusion is investigated and it is shown that simple drift-diffusion models do not describe the spin diffusion for above-band-gap optical spin injection correctly. Variation of lattice temperature and excitation energy allow us to trace this discrepancy to the influence of the pump-induced local overheating of the electron system with respect to the lattice which persists over length scales comparable to the spin diffusion length. Consideration of this hot carrier effect is crucial for a reliable extraction of spin propagation parameters from optical experiments.
• Ultrafast supercontinuum fiber-laser based pump-probe scanning magneto-optical Kerr effect microscope for the investigation of electron spin dynamics in semiconductors at cryogenic temperatures with picosecond time and micrometer spatial resolution. Henn, T.; Kiessling, T.; Ossau, W.; Molenkamp, L. W.; Biermann, K.; Santos, P. V. In Review of Scientific Instruments, 84(12), p. 123903. 2013.
  • [ Abstract ]
  • [ DOI ]
  We describe a two-color pump-probe scanning magneto-optical Kerr effect microscope which we have developed to investigate electron spin phenomena in semiconductors at cryogenic temperatures with picosecond time and micrometer spatial resolution. The key innovation of our microscope is the usage of an ultrafast “white light” supercontinuum fiber-laser source which provides access to the whole visible and near-infrared spectral range. Our Kerr microscope allows for the independent selection of the excitation and detection energy while avoiding the necessity to synchronize the pulse trains of two separate picosecond laser systems. The ability to independently tune the pump and probe wavelength enables the investigation of the influence of excitation energy on the optically induced electron spin dynamics in semiconductors. We demonstrate picosecond real-space imaging of the diffusive expansion of optically excited electron spin packets in a (110) GaAs quantum well sample to illustrate the capabilities of the instrument.
• Picosecond real-space imaging of electron spin diffusion in GaAs. Henn, T.; Kiessling, T.; Ossau, W.; Molenkamp, L. W.; Reuter, D.; Wieck, A. D. In Physical Review B, 88(19), p. 195202. 2013.
  • [ Abstract ]
  • [ DOI ]
  We use time-resolved two-color Kerr microscopy to study low-temperature electron spin diffusion in bulk GaAs with high spatial and temporal resolution. This method enables direct real-space imaging of the advancing spread of an optically excited electron spin packet by spin diffusion. We observe a high initial expansion rate of the spin packet, which is strongly reduced with increasing time. Comparison with continuous-wave Kerr spectroscopy demonstrates that this decrease is caused by the influence of the transient cooling of hot photocarriers on the electron spin diffusion coefficient.
• Hot carrier effects on the magneto-optical detection of electron spins in GaAs. Henn, T.; Heckel, A.; Beck, M.; Kiessling, T.; Ossau, W.; Molenkamp, L. W.; Reuter, D.; Wieck, A. D. In Physical Review B, 88(8), p. 085303. 2013.
  • [ Abstract ]
  • [ DOI ]
  We report on spatially resolved pump-probe MOKE spectroscopy measurements of electron spins in bulk n-type GaAs. For low lattice temperatures these measurements are significantly compromised by local changes in the excitonic magneto-optical response which are not related to the electron spin polarization. These local changes in the excitonic Kerr spectrum are due to carrier heating caused by above-band-gap optical excitation. When hot electrons are present, a measurement of the local Kerr rotation at a fixed arbitrary probe wavelength does not necessarily correctly reveal the local electron spin polarization. By analyzing the spatial dependence of the full excitonic Kerr rotation spectrum we determine the local excitonic spin splitting energy from which we obtain the true lateral electron spin polarization profile.

• Spatially resolved photocarrier energy relaxation in low-doped bulk GaAs. Kiessling, T.; Quast, J.-H.; Kreisel, A.; Henn, T.; Ossau, W.; Molenkamp, L. W. In Physical Review B, 86(16), p. 161201. 2012.
  • [ Abstract ]
  • [ DOI ]
  We report on spatially resolved cw-photoluminescence studies of very-low-doped n- and p-type GaAs, which demonstrate that optically excited electrons retain significant excess energy on length scales of several tens of microns away from the excitation point. In contrast, the lattice is heated only negligibly outside the optical excitation area even for moderate excitation intensities. When diffusing away from the excitation site the photoelectrons are therefore not in thermal equilibrium with the lattice. Our results imply that it is inappropriate to describe low temperature photocarrier diffusion in GaAs with a spatially uniform diffusion constant.

• Induced magnetic anisotropy in lifted ({Ga},{Mn})As thin films. Greullet, F.; Ebel, L.; Münzhuber, F.; Mark, S.; Astakhov, G. V.; Kießling, T.; Schumacher, C.; Gould, C.; Brunner, K.; Ossau, W.; Molenkamp, L. W. In Applied Physics Letters, 98(23), p. 231903. 2011.
  • [ Abstract ]
  • [ DOI ]
  We demonstrate the ability to release the growth-induced strain in (Ga,Mn)As layers and (In,Ga)As/(Ga,Mn)As bilayers by lifting them from the GaAs substrate. The lifted (bi)layers are then deposited back onto various substrates. The change in strain before and after processing has been studied by means of x-ray diffraction. Magnetic characterization demonstrates the efficiency of our lift-off process to reorient the magnetization to the direction normal to the layer plane.
• Nonthermal Photocoercivity Effect in Low‐Doped ({Ga},{Mn})As Ferromagnetic Semiconductor. Kiessling, T.; Astakhov, G. V.; Hoffmann, H.; Korenev, V. L.; Schwittek, J.; Schott, G. M.; Gould, C.; Ossau, W.; Brunner, K.; Molenkamp, L. W. In AIP Conference Proceedings, 1399(1), pp. 741–742. 2011.
  • [ Abstract ]
  • [ DOI ]
  We report a photoinduced change of the coercive field of a low doped Ga1‐xMnxAs ferromagnetic semiconductor under very low intensity illumination. This photocoercivity effect (PCE) is local and reversible, which enables the controlled formation of localized magnetization domains. The PCE arises from a light induced lowering of the domain wall pinning energy as confirmed by test experiments on high doped, fully metallic ferromagnetic Ga1‐xMnxAs.

• Nonthermal Photocoercivity Effect in a Low-Doped ({Ga},{Mn})As Ferromagnetic Semiconductor. Astakhov, G. V.; Hoffmann, H.; Korenev, V. L.; Kiessling, T.; Schwittek, J.; Schott, G. M.; Gould, C.; Ossau, W.; Brunner, K.; Molenkamp, L. W. In Physical Review Letters, 102(18), p. 187401. 2009.
  • [ Abstract ]
  • [ DOI ]
  We report a photoinduced change of the coercive field, i.e., a photocoercivity effect (PCE), under very low intensity illumination of a low-doped (Ga,Mn)As ferromagnetic semiconductor. We find a strong correlation between the PCE and the sample resistivity. Spatially resolved dynamics of the magnetization reversal rule out any role of thermal heating in the origin of this PCE, and we propose a mechanism based on the light-induced lowering of the domain wall pinning energy. The PCE is local and reversible, allowing writing and erasing of magnetic images using light.

• Suppression of Electron Spin Relaxation in Mn-Doped GaAs. Astakhov, G. V.; Dzhioev, R. I.; Kavokin, K. V.; Korenev, V. L.; Lazarev, M. V.; Tkachuk, M. N.; Kusrayev, Yu. G.; Kiessling, T.; Ossau, W.; Molenkamp, L. W. In Physical Review Letters, 101(7), p. 076602. 2008.
  • [ Abstract ]
  • [ DOI ]
  We report a surprisingly long spin relaxation time of electrons in Mn-doped p-GaAs. The spin relaxation time scales with the optical pumping and increases from 12 ns in the dark to 160 ns upon saturation. This behavior is associated with the difference in spin relaxation rates of electrons precessing in the fluctuating fields of ionized or neutral Mn acceptors, respectively. For the latter, the antiferromagnetic exchange interaction between a Mn ion and a bound hole results in a partial compensation of these fluctuating fields, leading to the enhanced spin memory.

• CdSe/ZnSe heteroepitaxy: Aspects of growth and self organization of nanostructures. Mahapatra, S.; Kiessling, T.; Margapoti, E.; Astakhov, G. V.; Renner, J.; Bass, U.; Bougerol, C.; Schmidt, T.; Bendounan, A.; Schmitt, F.; Schumacher, C.; Worschech, L.; Ossau, W.; Geurts, J.; Molenkamp, L. W.; Reinert, F.; Forchel, A.; Brunner, K. In physica status solidi c, 4(9), pp. 3129–3149. 2007.
  • [ Abstract ]
  • [ DOI ]
  Formation and properties of quantum dots (QDs) and related nanostructures by conventional molecular beam epitaxy (MBE) and two other variants of the technique, have been systematically investigated. While MBE growth of CdSe on ZnSe at 300 °C results in the formation of rough CdSe quantum-well-like structures, by one of the variants, which combines epitaxial growth of a CdSe layer at 230 °C and subsequent in-situ annealing at 310-340 °C, tiny but distinct three-dimensional (3D) QDs are formed. We demonstrate that QD formation by this method, developed originally by Rabe et al. (J. Cryst. Growth 184/185, 248 (1998) [30]), essentially takes place during the annealing step, by a process of “controlled roughening” of the as-grown quasi-two-dimensional (2D) CdSe layer. In comparison to conventional MBE, this process results in reduction of the areal density of the self-organized QDs by up to two orders of magnitude, thus making them distinctly discernible. In the second variant technique, similar to that applied originally for the self-organization of CdTe/ZnTe quantum dots by Tinjod et al. (Appl. Phys. Lett. 82, 4340 (2003) [34], QD-formation takes place as a result of deposition of an amorphous group-VI layer onto a strained 2D CdSe layer at ∼ 50 °C, and its subsequent re-desorption at temperatures of 280-320 °C. While deposition and desorption of amorphous Se leads to the formation of large isolated QDs, we show that use of amorphous Te results in the self-assembly and ordering of a rich variety of nanostructures. Luminescence, morphology, and composition of QDs and the underlying formation mechanism of the three processes are discussed in detail. (© 2007 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim)
• Layer-by-layer growth and island formation in CdSe/ZnSe heteroepitaxy. Mahapatra, S.; Kiessling, T.; Margapoti, E.; Astakhov, G. V.; Ossau, W.; Worschech, L.; Forchel, A.; Brunner, K. In Journal of Crystal Growth, 301-302, pp. 310–314. 2007.
  • [ Abstract ]
  • [ DOI ]
  Layer-by-layer epitaxial growth of CdSe on ZnSe has been observed at temperatures as low as 170–240°C. Beyond this range, Reflection high energy electron diffraction (RHEED) specular spot intensity oscillations disappear, signifying a multilayer growth mode. While conventional epitaxial growth of CdSe at ∼300°C results in formation of a rough layer-like structure, a method combining the epitaxial growth of a quasi-two-dimensional (2D) CdSe layer at 230°C and subsequent annealing at 310°C induces the formation of discrete CdSe quantum dots (QDs). Here we show that the formation of QDs by this method relies essentially on the layer-by-layer growth of CdSe at 230°C, which allows an induced roughening of the as-grown 2D layer, during the subsequent annealing step. The layer-by-layer growth mode results possibly due to the nucleation of small and irregular precursor 2D clusters during growth at low temperatures, at the kink sites of which potential barriers for downward inter-layer transport of adatoms is low/absent.
• Optical studies of structural and magnetic anisotropies in epitaxial CdSe/ZnSe quantum dots. Kiessling, T.; Astakhov, G. V.; Platonov, A. V.; Mahapatra, S.; Slobodskyy, T.; Ossau, W.; Schmidt, G.; Brunner, K.; Molenkamp, L. W. In physica status solidi c, 4(9), pp. 3324–3333. 2007.
  • [ Abstract ]
  • [ DOI ]
  We summarize experimental studies on the optical polarization properties of MBE-grown shape-asymmetric self-assembled CdSe/ZnSe quantum dots (QDs). The zero magnetic field fine structure splitting of excitonic transitions in structurally anisotropic QDs gives rise to an effective linear-to-circular and circular-to-linear optical polarization conversion upon resonant excitation of the QD ensemble. The effect is qualitatively modelled in terms of a pseudospin formalism. Furthermore, we show how an in-plane magnetic field leads to a complex rotation behavior of the axis of the optical polarization. Our analysis based on the exciton pseudospin Hamiltonian unambiguously displays that these effects are induced by isotropic and anisotropic contributions to the heavy hole Zeeman term. Likewise we show that the latter mechanism can be used to tune the optical respone of shape-anisotropic QDs such that the optical polarization properties are equal to QDs of perfect D2d symmetry. (© 2007 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim)

• Anomalous in-plane magneto-optical anisotropy of self-assembled quantum dots. Kiessling, T.; Platonov, A. V.; Astakhov, G. V.; Slobodskyy, T.; Mahapatra, S.; Ossau, W.; Schmidt, G.; Brunner, K.; Molenkamp, L. W. In Phys. Rev. B, 74(4), p. 041301. American Physical Society, 2006.
  • [ DOI ]
• Nanosecond spin memory of electrons in CdTe/CdMgTe quantum wells. Astakhov, G. V.; Kiessling, T.; Yakovlev, D. R.; Zhukov, E. A.; Bayer, M.; Ossau, W.; Zakharchenya, B. P.; Karczewski, G.; Wojtowicz, T.; Kossut, J. In physica status solidi (b), 243(4), pp. 858–862. 2006.
  • [ Abstract ]
  • [ DOI ]
  We report on spin memory relaxation time τ s of free electrons in CdTe-based quantum wells which is found to be in the nanosecond range. In these studies two different techniques have been exploited: (i) the Hanle effect under cw excitation and (ii) the time-resolved pump-&-probe Kerr rotation measured under excitation by 1.8-ps optical pulses. These independent techniques give very close results τ s ns and τ s = 14 ns, respectively. To our knowledge this is by two orders of magnitude longer than the electron spin relaxation times reported for CdTe-based quantum wells so far. (© 2006 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim)
• Energy relaxation in CdSe/ZnSe quantum dots under the strong exciton-phonon coupling regime. Platonov, A. V.; Kiessling, T.; Astakhov, G. V.; Maksimov, A. A.; Larionov, A. V.; Yakovlev, D. R.; Slobodskyy, T.; Mahapatra, S.; Ossau, W.; Schmidt, G.; Brunner, K.; Bayer, M.; Molenkamp, L. W. In physica status solidi c, 3(4), pp. 924–927. 2006.
  • [ Abstract ]
  • [ DOI ]
  Resonant photoluminescence and photoluminescence excitation spectroscopy have been applied to CdSe/ZnSe quantumdots where separation between the quantized levels is close to the optical phonon energy. Astrong relaxation enhancement induced by the optical phonon is observed. The fine structure of the phonon replicas is resolved. It is attributed to the optical phonons with large wave vectors from the symmetrical points of the Brillouin zone. (© 2006 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim)
• Phonon replica fine structure in CdSe/ZnSe quantum dots. Kiessling, T.; Platonov, A. V.; Astakhov, G. V.; Mahapatra, S.; Slobodskyy, T.; Ossau, W.; Schmidt, G.; Brunner, K.; Molenkamp, L.W. In Proc. 14th Int. Symp. „Nanostructures: Physics and Technology“, p. 269. 2006.
• Anomalous in-plane magneto-optical anisotropy of self-assembled quantum dots (N:PaT). Platonov, A. V.; Kiessling, T.; Astakhov, G. V.; Slobodskyy, T.; Mahapatra, S.; Ossau, W.; Schmidt, G.; Brunner, K.; Molenkamp, L. W. In Proc. 14th Int. Symp. „Nanostructures: Physics and Technology“, pp. 146–147. 2006.
  • [ Abstract ]
  We report on a complex nontrivial behavior of the optical anisotropy of quantum dots that is induced by a magnetic field in the plane of the sample. We find that the optical axis either rotates in the opposite direction to that of the magnetic field or remains fixed to a given crystalline direction. A theoretical analysis based on the exciton pseudospin Hamiltonian unambiguously demonstrates that these effects are induced by isotropic and anisotropic contributions to the heavy-hole Zeeman term, respectively. The latter is shown to be compensated by a built-in uniaxial anisotropy in a magnetic field BC=0.4T, resulting in an optical response typical for symmetric quantum dots.
• Comparative study of self-assembled CdSe/ZnSe quantum dots grown by variants of conventional MBE. Mahapatra, S.; Brunner, K.; Schumacher, C.; Kiessling, T.; Astakhov, G. V.; Bass, U.; Margapoti, E.; Ossau, W.; Geurts, J.; Worschech, L.; Forchel, A.; Molenkamp, L. W. In physica status solidi c, 3(4), pp. 928–932. 2006.
  • [ Abstract ]
  • [ DOI ]
  The morphology, composition, and optical properties of CdSe/ZnSe quantum dots (QDs), grown by conventional molecular beam epitaxy (MBE) and two other variants of the same (low temperature MBE and in-situ annealing), have been compared based on the results of high resolution X-ray diffraction (HRXRD), low temperature micro- and ensemble-photoluminescence (PL), atomic force microscopy (AFM), and Raman spectroscopy (RS) studies. While conventional MBE results in the formation of no discernible QD-like features, by the alternative methods reported here, discrete CdSe QDs, 1.5 to 15 nm high and with a lower density, could be realised. Shape anisotropy of the dots has also been investigated. (© 2006 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim)
• CdSe/ZnSe Quantum Dots Formed by Low Temperature Epitaxy and In-Situ Annealing: Properties and Growth Optimization. Mahapatra, S.; Schumacher, C.; Kiessling, T.; Astakhov, G. V.; Bass, U.; Ossau, W.; Geurts, J.; Brunner, K. In Acta Physica Polonica A, 108, p. 769. 2006.
  • [ Abstract ]
  The formation of CdSe/ZnSe quantum dots by a method combining a low temperature MBE growth of a CdSe layer and its subsequent in-situ annealing at temperatures between 280-340ºC has been studied. The thermal treatment results in a re-organization of the surface from a nearly two-dimensional layer to an ensemble of three-dimensional dot-like features. In this work we optimized the different growth and annealing parameters of this process and compared the properties of the resultant dots with those of dots grown by conventional MBE at 300ºC. It is demonstrated that the luminescence properties of the dots for both growth techniques are comparable but the areal density achieved by the in-situ annealing technique is an order of magnitude lower. From high resolution X-ray diffraction results, it could be established that no desorption takes place despite significantly long annealing duration. Beyond a nominal coverage of 3.5 ML CdSe, stacking faults are generated, leading to a gradual decrease in luminescence intensities and an overlap of pendellösung fringes in X-ray diffractograms.
• Circular-to-Linear and Linear-to-Circular Conversion of Optical Polarization by Semiconductor Quantum Dots. Astakhov, G. V.; Kiessling, T.; Platonov, A. V.; Slobodskyy, T.; Mahapatra, S.; Ossau, W.; Schmidt, G.; Brunner, K.; Molenkamp, L. W. In Physical Review Letters, 96(2), p. 027402. 2006.
  • [ Abstract ]
  • [ DOI ]
  We report circular-to-linear and linear-to-circular conversion of optical polarization by semiconductor quantum dots. The polarization conversion occurs under continuous wave excitation in the absence of any magnetic field. The effect originates from quantum interference of linearly and circularly polarized photon states, induced by the natural anisotropic shape of the self-assembled dots. The behavior can be qualitatively explained in terms of a pseudospin formalism.
• Optical spin pumping of modulation-doped electrons probed by a two-color Kerr rotation technique. Hoffmann, H.; Astakhov, G. V.; Kiessling, T.; Ossau, W.; Karczewski, G.; Wojtowicz, T.; Kossut, J.; Molenkamp, L. W. In Physical Review B, 74(7), p. 073407. 2006.
  • [ Abstract ]
  • [ DOI ]
  We report on optical spin pumping of modulation electrons in CdTe-based quantum wells with low intrinsic electron density (by 1010cm−2). Under continuous-wave excitation, we reach a steady-state accumulated spin density of about 108cm−2. Using a two-color Hanle-MOKE technique, we find a spin-relaxation time of 34ns in the nearly unperturbed electron gas. Independent variation of the pump and probe energies demonstrates the presence of additional electrons in the quantum well, whose spin-relaxation time is substantially shorter.
• Formation mechanism and properties of CdSe quantum dots on ZnSe by low temperature epitaxy and in situ annealing. Mahapatra, S.; Kiessling, T.; Margapoti, E.; Astakhov, G. V.; Ossau, W.; Worschech, L.; Forchel, A.; Brunner, K. In Applied Physics Letters, 89(4), p. 043102. 2006.
  • [ Abstract ]
  • [ DOI ]
  The formation mechanism of CdSe quantum dots (QDs) on ZnSe by low temperature epitaxial growth of a CdSe layer and its subsequent in situ annealing at a higher temperature has been studied. By this method, a small postgrowth residue of mobile adatoms nucleates distinct, stable, three-dimensional islands atop preexisting two-dimensional (2D) precursors. High temperature annealing activates an up-climb of these adatoms onto the 2D precursors, which, however, predominantly climb down at the low growth temperature (TG). A growth interruption at TG, therefore, smoothens the as-grown CdSe layer, and when introduced for appropriate durations, enables reducing the QD areal density.
• Optical anisotropy of CdSe/ZnSe quantum dots. Kiessling, T.; Astakhov, G. V.; Platonov, A. V.; Slobodskyy, T.; Mahapatra, S.; Ossau, W.; Schmidt, G.; Brunner, K.; Molenkamp, L. W. In physica status solidi c, 3(4), pp. 912–915. 2006.
  • [ Abstract ]
  • [ DOI ]
  We present comprehensive studies of in-plane optical anisotropy in CdSe/ZnSe quantum dot ensembles. The anisotropy reveals itself as circular-to-linear polarization conversion and as an alignment occurring at resonant excitation without any applied magnetic fields. The variation of the anisotropy in different dots has been analyzed based on angle scans. (© 2006 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim)