A. Remhof - Academia.edu (original) (raw)

Papers by A. Remhof

Advanced Engineering Materials, 2016

Springer Series in Materials Science, 2007

VI Preface the storage density on a disk drive from 1 to 20 Gbit per square inch, merely by the i... more VI Preface the storage density on a disk drive from 1 to 20 Gbit per square inch, merely by the incorporation of the new GMR materials. The market only in the field of GMR-nanotechnology is estimated over 100 billion dollars annually.

Acta Physica Polonica A, 2009

Artificial magnetic dipole arrays arranged on a square lattice exhibit a fascinating variety and ... more Artificial magnetic dipole arrays arranged on a square lattice exhibit a fascinating variety and complexity of configurations. Among the 16 possible configurations, six fulfill the spin ice rule of two dipoles pointing into a vertex and two point out. We present experimental realizations of magnetic dipole arrays and discuss the remanent state as well as the magnetization reversal in an external field.

Physica E: Low-dimensional Systems and Nanostructures, 2008

ABSTRACT A site-selective growth of self-organized InAs quantum dots (QDs) employing a combinatio... more ABSTRACT A site-selective growth of self-organized InAs quantum dots (QDs) employing a combination of in-situ focused ion beam (FIB) implantation and self-organized molecular beam epitaxy (MBE) growth has been successfully demonstrated. First, a buffer layer of GaAs was grown by MBE before a square lattice of shallow holes with a pitch of 1–2 μm was fabricated by FIB implantation of Ga and In ions. Before InAs deposition, different procedures to remove the implantation damage were tested. The best results were obtained after re-evaporation of 3–5 nm GaAs. Another critical parameter that was carefully optimized is the deposited InAs amount. For the optimized process it was possible to induce growth of single QD in the hole with more than 50% probability. Between the shallow holes of 2 nm depths, no QD growth was observed. The optical quality of positioned dots was investigated by photoluminescence spectroscopy, employing a separate sample where the QDs were overgrown by GaAs cap layer. Excited-state interband transitions up to n=5 were observed from positioned QDs.

Physical Review B, 2000

Structural phase transitions in thin, epitaxial yttrium hydride films have been studied by means ... more Structural phase transitions in thin, epitaxial yttrium hydride films have been studied by means of synchrotron x-ray diffraction topography ͑XDT͒, complemented by atomic force microscopy and by measurements of the electrical resistance. Depending on the applied hydrogen pressure during the gas-phase loading experiments and the chosen temperature, different phases exhibiting different symmetries and electronic properties could be established. Despite the small scattering volume of the 200-nm-thick film, the phase nucleation, the spatial distribution of domains as well as the lateral hydrogen diffusion could be observed. Those experiments demonstrate the feasibility of XDT to study structural properties like the dynamics of phase transitions in thin epitaxially grown films on the nanometer scale. From the progression of the domain boundary between the YH 2 and YH 3 phase the hydrogen mobility was determined to 5ϫ10 Ϫ6 cm 2 /s at 300°C. Comparing hightemperature topographies with room-temperature results confirms that the phase boundary is narrower at high temperatures. Finally resistance measurements of Y/Nb double layers with varying hydrogen concentration confirm clearly that the YH 3 phase is insulating and that the resistance of the yttrium layer increases proportionally to the degree of YH 3 phase precipitation. The layer system acts as a hydrogen concentration-dependent potentiometer and could be used as a hydrogen sensor.

Physical Review B, 1997

Yttrium can be loaded with hydrogen up to high concentrations causing dramatic structural and ele... more Yttrium can be loaded with hydrogen up to high concentrations causing dramatic structural and electronic changes of the host lattice. We report on the reversibility of hydrogen loading in thin, monocrystalline Y-films grown by MBE on Nb/Al_2O3 substrates. During hydrogen loading, the Yttrium film undergoes structural transitions from the cubic dihydride to the hexagonal trihydride phase, while the structural coherence and the in-plane epitaxial relation to the Nb buffer layer is maintained. The transition from YH2 to YH3 occurs at room temperature at a hydrogen pressure of 10 mbar and is completely reversible. Reversibility is also observed for deuteration of Y. However, the kinetics is more sluggish. Although the YH2 structure is chemically stable, isotope exchange with deuterium takes place rapidly. (A. Remhof, G. Song, K. Theis-Bröhl, H. Zabel, Phys. Rev. B 56) R2897 (1997)

Superlattices and Microstructures, 2007

We have investigated the magnetization reversal of two regular arrays of Fe triangular microwire ... more We have investigated the magnetization reversal of two regular arrays of Fe triangular microwire rings (base 7.3 µm and 2.8 µm, periodicity 11.5 µm and 5.6 µm, respectively) using the magneto-optical Kerr effect in vector-MOKE and Bragg-MOKE configuration. The measurements are compared with the results of micromagnetic simulations, which allow a detailed interpretation of the experimental data. We find that the magnetization reversal in an external magnetic field depends on the size of the triangles. Domain formation is more pronounced in the large than in the small triangular rings, and micromagnetic simulations show a vortex state in the small triangular rings.

Review of Scientific Instruments, 2003

Hydrogen electrodiffusion waves are forced oscillations of the H concentration within a host meta... more Hydrogen electrodiffusion waves are forced oscillations of the H concentration within a host metal, driven by an electric field. Simulations show that they suffer less from the drawbacks of ordinary diffusion waves such as heavy damping. H in Y/V bilayers fulfills all the requirements to generate and to detect H electrodiffusion waves. We demonstrate the possibility to spatially modulate the H concentration in a thin V film and to drive H ''pulses'' via an applied electric field. The electric field is also used to control the hydrogen uptake of the sample. We visualize the temporal and spatial evolution of the H distribution in the V film using the switchable mirror material YH x as an optical hydrogen indicator.

Time-resolved diraction patterns down to a sub-nanosecond scale obtained with a high-resolution d... more Time-resolved diraction patterns down to a sub-nanosecond scale obtained with a high-resolution diractometer for high-energies X-rays on ultrasonically excited samples are presented. The time resolution gives direct insight into the purity of the excited sound waves and reveals the density of states for the lattice parameter at any point of time in a MHz oscillation period. The combination of the

The Journal of Physical Chemistry C, 2015

ABSTRACT For different hydrogenated metal intercalated fullerides (Na10C60-H, Li12C60-H, and Li28... more ABSTRACT For different hydrogenated metal intercalated fullerides (Na10C60-H, Li12C60-H, and Li28C60-H) the activation energies for hydrogen desorption were determined by DSC. The Vyazovkin advanced method (VA) was used for the calculation of the reaction model free activation energy as a function of the extent of conversion α. Activation energies are highest for low α and decrease for increasing α, between around 200−145 and 245−175 kJ/mol for the Na and Li compounds, respectively. The decrease of activation energy as a function of the extent of conversion can be explained by an increasing charge transfer to the C60H36+y cage during desorption. Na intercalation leads to a significant thermodynamic destabilization for hydrogen desorption. Dehydro- genation enthalpies of 52 (Na10C60-H), 66 (Li12C60-H), and 69 kJ/mol H2 (Li28C60-H) were determined. These values are lower compared to literature values for desorption of pure C60H36 (74 kJ/mol H2). The onsets of hydrogen desorption are 185 °C (Na10C60-H), 260 °C (Li12C60-H), and 250 °C (Li28C60-H) compared to >400 °C for pure C60H36

Physical Review B, 1999

Hydrogen in yttrium is of fundamental interest as a model system for driving metal-insulator tran... more Hydrogen in yttrium is of fundamental interest as a model system for driving metal-insulator transitions including switchable optical properties from reflecting to transparent in the visible region. We report on the structural properties of hydrogenated and deuterated thin, monocrystalline Y͑0001͒ films grown by molecular beam epitaxy on Nb/Al 2 O 3 substrates. X-ray diffraction reveals the response of the host metal lattice upon hydrogen loading. The structural coherence in all three spatial directions as well as the epitaxial relation to the substrate are maintained, though the sample undergoes structural phase transitions between the different hydride phases. With neutron reflectivity measurements we have determined the hydrogen and deuterium content since the critical angle for total reflection depends on their concentration in the sample. Measurements on hydrogenated and deuterated films show that H and D are completely interchangeable within the trihydride phase. Neutron scattering also allows us to determine the position of the deuterium atoms within the yttrium matrix. All structural information gained on thin films is in agreement with the space group P3 c1 which was previously determined from powder samples.

The hydrogen dynamics in solid and in liquid LiBH 4 was studied by means of incoherent quasielast... more The hydrogen dynamics in solid and in liquid LiBH 4 was studied by means of incoherent quasielastic neutron scattering. Rotational jump diffusion of the BH 4 subunits on the picosecond scale was observed in solid LiBH 4 . The characteristic time constant is significantly shortened when the system transforms from the lowtemperature phase to the high-temperature phase at 383 K. In the molten phase of LiBH 4 above 553 K, translational diffusion of the BH 4 units is found. The measured diffusion coefficients are in the 10 -5 cm 2 /s range at temperatures around 700 K, which is in the same order of magnitude as the self-diffusion of liquid lithium or the diffusion of ions in molten alkali halides. The temperature dependence of the diffusion coefficient shows an Arrhenius behavior, with an activation energy of E a ) 88 meV and a prefactor of D 0 ) 3.1 × 10 -4 cm 2 /s.

Scripta Materialia, 2009

... Ji Woo Kim a , b , Oliver Friedrichs a , Jae-Pyoung Ahn c , Do Hyun Kim b , Seul Cham Kim b ,... more ... Ji Woo Kim a , b , Oliver Friedrichs a , Jae-Pyoung Ahn c , Do Hyun Kim b , Seul Cham Kim b , Arndt Remhof a , Hee-Suk Chung b , Jehyun Lee b , Jae-Hyeok Shim d , Young Whan Cho d , Andreas ... [2] P. Mauron, F. Buchter, O. Friedrichs, A. Remhof, M. Bielmann, CN Zwichy ...

Physical Review B, 2002

We show that the switchable mirror material YHx can be used both as an indicator to monitor and a... more We show that the switchable mirror material YHx can be used both as an indicator to monitor and as an agent to control hydrogen diffusion in thin films. The applicability of the optical-indicator technique is demonstrated for VHx thin films. The diffusion coefficient is typically 10-5 cm2/s at concentrations around 0.7 H/V at temperatures between 373 and 473 K. Deposition

Nuclear Instruments & Methods in Physics Research Section a-Accelerators Spectrometers Detectors and Associated Equipment, 1997

A longitudinal sound wave modulates the regular arrangement of the atomic planes of a crystal in ... more A longitudinal sound wave modulates the regular arrangement of the atomic planes of a crystal in two ways: first, the spacing between the atomic planes is modified in the regions of compression and expansion introducing a macroscopic strain and, second, the lattice planes acquire a velocity in the oscillating strain field. Bragg reflection in a strained crystal maintains the energy of the radiation, whereas Bragg reflection by a moving lattice provokes a Doppler shift of the radiation. In a diffraction experiment both these effects lead to an enlarged bandwidth of the reflection curve. The relative importance of strain and Doppler depends mainly on the radiation used. For thermal neutron scattering the profile of the rocking curve of a Bragg reflection may permit to separate the two effects. Atomic amplitudes of the sound field of 136 A peak to peak in the bulk of the crystal can be deduced from the rocking profile. The enlarged bandwidth of a sound-excited crystal opens a possibility for diffraction-based optical elements where the trade-off between resolution and intensity can be readily modified.

Time Structure of X-Ray Sources and Its Applications, 1998

Time resolved detection in both detector and synchrotron resolution mode are presented. Using thi... more Time resolved detection in both detector and synchrotron resolution mode are presented. Using this technique the response of the Bragg profile in perfect Silicon to a MHz ultrasonic wave field has been studied on the high resolution triple crystal diffractometer at the ESRF high energy beamline ID15A. High photon energies up to 500 keV have been used for the analysis of a longitudinal acoustic wave in the 10 mm bulk. First, conventional time averaged rocking curves show intensity gains by up to a factor of 50, and a reciprocal space mapping reveals full information on the acoustic wave vector. Secondly, time and space resolved reflection curves have been taken. They give detailed insights into the properties of the acoustic wave field in space and time. In particular, they allow us to identify uniquely true standing waves without parasitic strain components or higher harmonic excitation. Time averaged and snapshots of the time resolved rocking curves represent different aspects of the density of states for the lattice parameter distribution. They are understood analytically by the elliptic integral K and the inverse circle function, respectively.

Advanced Engineering Materials, 2016

Springer Series in Materials Science, 2007

VI Preface the storage density on a disk drive from 1 to 20 Gbit per square inch, merely by the i... more VI Preface the storage density on a disk drive from 1 to 20 Gbit per square inch, merely by the incorporation of the new GMR materials. The market only in the field of GMR-nanotechnology is estimated over 100 billion dollars annually.

Acta Physica Polonica A, 2009

Artificial magnetic dipole arrays arranged on a square lattice exhibit a fascinating variety and ... more Artificial magnetic dipole arrays arranged on a square lattice exhibit a fascinating variety and complexity of configurations. Among the 16 possible configurations, six fulfill the spin ice rule of two dipoles pointing into a vertex and two point out. We present experimental realizations of magnetic dipole arrays and discuss the remanent state as well as the magnetization reversal in an external field.

Physica E: Low-dimensional Systems and Nanostructures, 2008

ABSTRACT A site-selective growth of self-organized InAs quantum dots (QDs) employing a combinatio... more ABSTRACT A site-selective growth of self-organized InAs quantum dots (QDs) employing a combination of in-situ focused ion beam (FIB) implantation and self-organized molecular beam epitaxy (MBE) growth has been successfully demonstrated. First, a buffer layer of GaAs was grown by MBE before a square lattice of shallow holes with a pitch of 1–2 μm was fabricated by FIB implantation of Ga and In ions. Before InAs deposition, different procedures to remove the implantation damage were tested. The best results were obtained after re-evaporation of 3–5 nm GaAs. Another critical parameter that was carefully optimized is the deposited InAs amount. For the optimized process it was possible to induce growth of single QD in the hole with more than 50% probability. Between the shallow holes of 2 nm depths, no QD growth was observed. The optical quality of positioned dots was investigated by photoluminescence spectroscopy, employing a separate sample where the QDs were overgrown by GaAs cap layer. Excited-state interband transitions up to n=5 were observed from positioned QDs.

Physical Review B, 2000

Structural phase transitions in thin, epitaxial yttrium hydride films have been studied by means ... more Structural phase transitions in thin, epitaxial yttrium hydride films have been studied by means of synchrotron x-ray diffraction topography ͑XDT͒, complemented by atomic force microscopy and by measurements of the electrical resistance. Depending on the applied hydrogen pressure during the gas-phase loading experiments and the chosen temperature, different phases exhibiting different symmetries and electronic properties could be established. Despite the small scattering volume of the 200-nm-thick film, the phase nucleation, the spatial distribution of domains as well as the lateral hydrogen diffusion could be observed. Those experiments demonstrate the feasibility of XDT to study structural properties like the dynamics of phase transitions in thin epitaxially grown films on the nanometer scale. From the progression of the domain boundary between the YH 2 and YH 3 phase the hydrogen mobility was determined to 5ϫ10 Ϫ6 cm 2 /s at 300°C. Comparing hightemperature topographies with room-temperature results confirms that the phase boundary is narrower at high temperatures. Finally resistance measurements of Y/Nb double layers with varying hydrogen concentration confirm clearly that the YH 3 phase is insulating and that the resistance of the yttrium layer increases proportionally to the degree of YH 3 phase precipitation. The layer system acts as a hydrogen concentration-dependent potentiometer and could be used as a hydrogen sensor.

Physical Review B, 1997

Yttrium can be loaded with hydrogen up to high concentrations causing dramatic structural and ele... more Yttrium can be loaded with hydrogen up to high concentrations causing dramatic structural and electronic changes of the host lattice. We report on the reversibility of hydrogen loading in thin, monocrystalline Y-films grown by MBE on Nb/Al_2O3 substrates. During hydrogen loading, the Yttrium film undergoes structural transitions from the cubic dihydride to the hexagonal trihydride phase, while the structural coherence and the in-plane epitaxial relation to the Nb buffer layer is maintained. The transition from YH2 to YH3 occurs at room temperature at a hydrogen pressure of 10 mbar and is completely reversible. Reversibility is also observed for deuteration of Y. However, the kinetics is more sluggish. Although the YH2 structure is chemically stable, isotope exchange with deuterium takes place rapidly. (A. Remhof, G. Song, K. Theis-Bröhl, H. Zabel, Phys. Rev. B 56) R2897 (1997)

Superlattices and Microstructures, 2007

We have investigated the magnetization reversal of two regular arrays of Fe triangular microwire ... more We have investigated the magnetization reversal of two regular arrays of Fe triangular microwire rings (base 7.3 µm and 2.8 µm, periodicity 11.5 µm and 5.6 µm, respectively) using the magneto-optical Kerr effect in vector-MOKE and Bragg-MOKE configuration. The measurements are compared with the results of micromagnetic simulations, which allow a detailed interpretation of the experimental data. We find that the magnetization reversal in an external magnetic field depends on the size of the triangles. Domain formation is more pronounced in the large than in the small triangular rings, and micromagnetic simulations show a vortex state in the small triangular rings.

Review of Scientific Instruments, 2003

Hydrogen electrodiffusion waves are forced oscillations of the H concentration within a host meta... more Hydrogen electrodiffusion waves are forced oscillations of the H concentration within a host metal, driven by an electric field. Simulations show that they suffer less from the drawbacks of ordinary diffusion waves such as heavy damping. H in Y/V bilayers fulfills all the requirements to generate and to detect H electrodiffusion waves. We demonstrate the possibility to spatially modulate the H concentration in a thin V film and to drive H ''pulses'' via an applied electric field. The electric field is also used to control the hydrogen uptake of the sample. We visualize the temporal and spatial evolution of the H distribution in the V film using the switchable mirror material YH x as an optical hydrogen indicator.

Time-resolved diraction patterns down to a sub-nanosecond scale obtained with a high-resolution d... more Time-resolved diraction patterns down to a sub-nanosecond scale obtained with a high-resolution diractometer for high-energies X-rays on ultrasonically excited samples are presented. The time resolution gives direct insight into the purity of the excited sound waves and reveals the density of states for the lattice parameter at any point of time in a MHz oscillation period. The combination of the

The Journal of Physical Chemistry C, 2015

ABSTRACT For different hydrogenated metal intercalated fullerides (Na10C60-H, Li12C60-H, and Li28... more ABSTRACT For different hydrogenated metal intercalated fullerides (Na10C60-H, Li12C60-H, and Li28C60-H) the activation energies for hydrogen desorption were determined by DSC. The Vyazovkin advanced method (VA) was used for the calculation of the reaction model free activation energy as a function of the extent of conversion α. Activation energies are highest for low α and decrease for increasing α, between around 200−145 and 245−175 kJ/mol for the Na and Li compounds, respectively. The decrease of activation energy as a function of the extent of conversion can be explained by an increasing charge transfer to the C60H36+y cage during desorption. Na intercalation leads to a significant thermodynamic destabilization for hydrogen desorption. Dehydro- genation enthalpies of 52 (Na10C60-H), 66 (Li12C60-H), and 69 kJ/mol H2 (Li28C60-H) were determined. These values are lower compared to literature values for desorption of pure C60H36 (74 kJ/mol H2). The onsets of hydrogen desorption are 185 °C (Na10C60-H), 260 °C (Li12C60-H), and 250 °C (Li28C60-H) compared to >400 °C for pure C60H36

Physical Review B, 1999

Hydrogen in yttrium is of fundamental interest as a model system for driving metal-insulator tran... more Hydrogen in yttrium is of fundamental interest as a model system for driving metal-insulator transitions including switchable optical properties from reflecting to transparent in the visible region. We report on the structural properties of hydrogenated and deuterated thin, monocrystalline Y͑0001͒ films grown by molecular beam epitaxy on Nb/Al 2 O 3 substrates. X-ray diffraction reveals the response of the host metal lattice upon hydrogen loading. The structural coherence in all three spatial directions as well as the epitaxial relation to the substrate are maintained, though the sample undergoes structural phase transitions between the different hydride phases. With neutron reflectivity measurements we have determined the hydrogen and deuterium content since the critical angle for total reflection depends on their concentration in the sample. Measurements on hydrogenated and deuterated films show that H and D are completely interchangeable within the trihydride phase. Neutron scattering also allows us to determine the position of the deuterium atoms within the yttrium matrix. All structural information gained on thin films is in agreement with the space group P3 c1 which was previously determined from powder samples.

The hydrogen dynamics in solid and in liquid LiBH 4 was studied by means of incoherent quasielast... more The hydrogen dynamics in solid and in liquid LiBH 4 was studied by means of incoherent quasielastic neutron scattering. Rotational jump diffusion of the BH 4 subunits on the picosecond scale was observed in solid LiBH 4 . The characteristic time constant is significantly shortened when the system transforms from the lowtemperature phase to the high-temperature phase at 383 K. In the molten phase of LiBH 4 above 553 K, translational diffusion of the BH 4 units is found. The measured diffusion coefficients are in the 10 -5 cm 2 /s range at temperatures around 700 K, which is in the same order of magnitude as the self-diffusion of liquid lithium or the diffusion of ions in molten alkali halides. The temperature dependence of the diffusion coefficient shows an Arrhenius behavior, with an activation energy of E a ) 88 meV and a prefactor of D 0 ) 3.1 × 10 -4 cm 2 /s.

Scripta Materialia, 2009

... Ji Woo Kim a , b , Oliver Friedrichs a , Jae-Pyoung Ahn c , Do Hyun Kim b , Seul Cham Kim b ,... more ... Ji Woo Kim a , b , Oliver Friedrichs a , Jae-Pyoung Ahn c , Do Hyun Kim b , Seul Cham Kim b , Arndt Remhof a , Hee-Suk Chung b , Jehyun Lee b , Jae-Hyeok Shim d , Young Whan Cho d , Andreas ... [2] P. Mauron, F. Buchter, O. Friedrichs, A. Remhof, M. Bielmann, CN Zwichy ...

Physical Review B, 2002

We show that the switchable mirror material YHx can be used both as an indicator to monitor and a... more We show that the switchable mirror material YHx can be used both as an indicator to monitor and as an agent to control hydrogen diffusion in thin films. The applicability of the optical-indicator technique is demonstrated for VHx thin films. The diffusion coefficient is typically 10-5 cm2/s at concentrations around 0.7 H/V at temperatures between 373 and 473 K. Deposition

Nuclear Instruments & Methods in Physics Research Section a-Accelerators Spectrometers Detectors and Associated Equipment, 1997

A longitudinal sound wave modulates the regular arrangement of the atomic planes of a crystal in ... more A longitudinal sound wave modulates the regular arrangement of the atomic planes of a crystal in two ways: first, the spacing between the atomic planes is modified in the regions of compression and expansion introducing a macroscopic strain and, second, the lattice planes acquire a velocity in the oscillating strain field. Bragg reflection in a strained crystal maintains the energy of the radiation, whereas Bragg reflection by a moving lattice provokes a Doppler shift of the radiation. In a diffraction experiment both these effects lead to an enlarged bandwidth of the reflection curve. The relative importance of strain and Doppler depends mainly on the radiation used. For thermal neutron scattering the profile of the rocking curve of a Bragg reflection may permit to separate the two effects. Atomic amplitudes of the sound field of 136 A peak to peak in the bulk of the crystal can be deduced from the rocking profile. The enlarged bandwidth of a sound-excited crystal opens a possibility for diffraction-based optical elements where the trade-off between resolution and intensity can be readily modified.

Time Structure of X-Ray Sources and Its Applications, 1998

Time resolved detection in both detector and synchrotron resolution mode are presented. Using thi... more Time resolved detection in both detector and synchrotron resolution mode are presented. Using this technique the response of the Bragg profile in perfect Silicon to a MHz ultrasonic wave field has been studied on the high resolution triple crystal diffractometer at the ESRF high energy beamline ID15A. High photon energies up to 500 keV have been used for the analysis of a longitudinal acoustic wave in the 10 mm bulk. First, conventional time averaged rocking curves show intensity gains by up to a factor of 50, and a reciprocal space mapping reveals full information on the acoustic wave vector. Secondly, time and space resolved reflection curves have been taken. They give detailed insights into the properties of the acoustic wave field in space and time. In particular, they allow us to identify uniquely true standing waves without parasitic strain components or higher harmonic excitation. Time averaged and snapshots of the time resolved rocking curves represent different aspects of the density of states for the lattice parameter distribution. They are understood analytically by the elliptic integral K and the inverse circle function, respectively.