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Biomedical subjects

K H Ploog

Publications and source records attributed to K H Ploog.

15 recordsLinked to original sources
PubMed2005

Colossal magnetic moment of Gd in GaN.

We investigate the magnetic properties of epitaxial GaN:Gd layers as a function of the external magnetic field and temperature. An unprecedented magnetic moment is observed in this diluted magnetic semiconductor. The average value of the moment per Gd atom is found to be as high as 4000 micro(B) as compared to its atomic moment of 8 micro(B). The long-range spin polarization of the GaN matrix by Gd is also reflected in the circular polarization of magnetophotoluminescence measurements. Moreover, the materials system is found to be ferromagnetic above room temperature in the entire concentration range under investigation (7 x 10(15) to 2 x 10(19) cm(-3)). We propose a phenomenological model to understand the macroscopic magnetic behavior of the system. Our study reveals a close connection between the observed ferromagnetism and the colossal magnetic moment of Gd.

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PubMed2004

Spin filtering in a hybrid ferromagnetic-semiconductor microstructure.

We fabricated a hybrid structure in which cobalt and permalloy micromagnets produce a local in-plane spin-dependent potential barrier for high-mobility electrons at the GaAs/AlGaAs interface. Spin effects are observed in ballistic transport in the range of tens of mT of the external field and are attributed to switching between Zeeman and Stern-Gerlach modes--the former dominating at low electron densities.

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PubMed2004

Quantum confinement in monatomic Cu chains on Cu(111).

The existence of one-dimensional (1D) electronic states in Cu/Cu(111) chains assembled by atomic manipulation is revealed by low-temperature scanning tunneling spectroscopy and density functional theory (DFT) calculations. Our experimental analysis of the chain-localized electron dynamics shows that the dispersion is fully described within a 1D tight-binding approach. DFT calculations confirm the confinement of unoccupied states to the chain in the relevant energy range, along with a significant extension of these states into the vacuum region.

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PubMed2004

Phase-resolved nonlinear response of a two-dimensional electron gas under femtosecond intersubband excitation.

Strong electric-field transients resonant to intersubband transitions in n-type modulation-doped GaAs/AlGaAs quantum wells induce coherent Rabi oscillations, which are demonstrated by a phase-resolved measurement of the light emitted by the sample. The time evolution of the intersubband polarization is influenced by Coulomb-mediated many-body effects. The subpicosecond period and the phase of the Rabi oscillations are controlled by the properties of the midinfrared driving pulse.

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PubMed2003

Programmable computing with a single magnetoresistive element.

The development of transistor-based integrated circuits for modern computing is a story of great success. However, the proved concept for enhancing computational power by continuous miniaturization is approaching its fundamental limits. Alternative approaches consider logic elements that are reconfigurable at run-time to overcome the rigid architecture of the present hardware systems. Implementation of parallel algorithms on such 'chameleon' processors has the potential to yield a dramatic increase of computational speed, competitive with that of supercomputers. Owing to their functional flexibility, 'chameleon' processors can be readily optimized with respect to any computer application. In conventional microprocessors, information must be transferred to a memory to prevent it from getting lost, because electrically processed information is volatile. Therefore the computational performance can be improved if the logic gate is additionally capable of storing the output. Here we describe a simple hardware concept for a programmable logic element that is based on a single magnetic random access memory (MRAM) cell. It combines the inherent advantage of a non-volatile output with flexible functionality which can be selected at run-time to operate as an AND, OR, NAND or NOR gate.

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PubMed2003

Magnetologic with alpha-MnAs thin films.

Taking advantage of the spin information in present day computing is expected to yield an enormous increase in efficiency. A promising ferromagnetic material compatible with semiconductors for room temperature applications is MnAs. By sensitive cantilever beam magnetometry, we discovered that alpha-MnAs films on GaAs(001) exhibit an additional small out-of-plane component of the magnetization which is magnetically coupled with the dominant in-plane magnetization. We demonstrate that by regarding the two components as independent inputs, the alpha-MnAs layer can be utilized as a logic gate with nonvolatile output. The logic functionality of the layer can be preselected to be AND or OR at run time, thus offering the perspective for programmable magnetologic devices.

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PubMed2003

Ferromagnetism of MnAs studied by heteroepitaxial films on GaAs(001).

Thin epitaxial films of MnAs--promising candidates for the spin injection into semiconductors--are well known to undergo simultaneously a first-order structural and magnetic phase transition at 10-40 degrees C. The evolution of stress and magnetization of MnAs/GaAs(001), both measured quantitatively with our cantilever beam magnetometer at the coexistence region of alpha-MnAs and beta-MnAs, reveal an orthorhombically distorted unit cell of the ferromagnetic phase, which provides important clues on the origin of ferromagnetism in MnAs.

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PubMed2003

Two-dimensional coarsening kinetics of reconstruction domains: GaAs(001)-beta(2 x 4).

We study the nonconserved coarsening kinetics of a reconstructed semiconductor surface. The domain size evolution is obtained in situ by time-resolved surface x-ray diffraction. The system exhibits four equivalent domain types with two nonequivalent types of domain boundaries. Small domains are prepared by molecular beam epitaxy deposition of one GaAs layer. We find the correlation lengths of the domain size distribution to depend on time as l is proportional to t(0.42+/-0.05) in the half-order reflections and l is proportional to t(0.22+/-0.05) in the quarter-order reflections. The fraction of the higher energy domain boundaries increases as lnt.

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PubMed2001

Transport and lifetime enhancement of photoexcited spins in GaAs by surface acoustic waves.

We demonstrate spin transport and spin lifetime enhancement in GaAs quantum wells induced by the traveling piezoelectric field of a surface acoustic wave (SAW). Spin transport lengths of about 3 microm corresponding to spin relaxation times during transport over 1 ns are observed, which are considerably longer than the exciton spin diffusion lengths in the absence of a SAW. The slow spin relaxation is attributed to a reduced electron-hole exchange interaction, when the carriers are spatially separated by the lateral potential modulation induced by the SAW.

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PubMed2001

Quantum mechanical repulsion of exciton levels in a disordered quantum well.

Spatially resolved photoluminescence spectra of a single quantum well are recorded by near-field spectroscopy. A set of over four hundred spectra displaying sharp emission lines from localized excitons is subject to a statistical analysis of the two-energy autocorrelation function. An accurate comparison with a quantum theory of the exciton center-of-mass motion in a two-dimensional spatially correlated disordered potential reveals clear signatures of quantum mechanical energy level repulsion, giving the spatial and energetic correlations of excitons in disordered quantum systems.

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PubMed2001

Room-temperature spin injection from Fe into GaAs.

Injection of spin polarized electrons from a metal into a semiconductor is demonstrated for a GaAs/(In,Ga)As light emitting diode covered with Fe. The circular polarization degree of the observed electroluminescence reveals a spin injection efficiency of 2%. The underlying injection mechanism is explained in terms of a tunneling process.

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PubMed2001

Low temperature near-field luminescence studies of localized and delocalized excitons in quantum wires.

Excitons in a GaAs quantum wire were studied in high-resolution photoluminescence experiments performed at a temperature of about 10 K with a spatial resolution of 160 nm and a spectral resolution of 100 microeV. We report the observation of quasi-one-dimensional excitons which are delocalized over a length of up to several micrometres along the quantum wire. Such excitons give rise to a 10 meV broad luminescence band, representing a superposition of transitions between different delocalized states. In addition, we find a set of sharp luminescence peaks from excitons localized on a sub150 nm length scale. Theoretical calculations of exciton states in a disordered quasi-one-dimensional potential reproduce the experimental results.

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PubMed2001

Femtosecond near-field spectroscopy: carrier relaxation and transport in single quantum wires.

Quasi-two-colour femtosecond pump and probe spectroscopy and near-field scanning optical microscopy are combined to study the carrier dynamics in single semiconductor nanostructures. In temporally, spectrally and spatially resolved measurements with a time resolution of 200 fs and a spatial resolution of 200 nm, the non-linear change in reflectivity of a single quantum wire is mapped in real space and time. The experiments show that carrier relaxation into a single quantum wire occurs on a 100 fs time scale at room temperature. Evidence is given for a transient unipolar electron transport along the wire axis on a picosecond time and 100 nm length scale.

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PubMedDate not supplied

Time-resolved near-field optics: exciton transport in semiconductor nanostructures.

We present a systematic, temperature-dependent study of excitonic real-space transfer into single GaAs quantum wires using time-resolved low-temperature near-field luminescence spectroscopy. Excitons generated by local short pulse optical excitation in a 250 nm spot undergo diffusive transport over a length of several micrometres and are subsequently trapped into the quantum wire by optical phonon emission. The effect of local energy barriers in the vicinity of the quantum wire on the real-space transfer dynamics is monitored directly by mapping the time-resolved quantum wire luminescence. Experiments at variable temperatures are compared to numerical simulations based on drift-diffusive model calculations, and the spatio-temporal evolution of the two-dimensional exciton distribution within the nanostructure is visualized.

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