S. Wenderich | University of Twente (original) (raw)

Papers by S. Wenderich

We use a scanning Hall probe microscope with single vortex resolution to study the dynamics of si... more We use a scanning Hall probe microscope with single vortex resolution to study the dynamics of single vortices on grain boundaries in YBa2Cu3O6+x thin films with 10 kHz bandwidth. In the presence of an applied current, we observe individual vortices hopping between pinning sites. Detecting the motion of individual vortices allows us to probe the very- low-voltage regime of the

The study of new physical phenomena in the solid state requires the use of clean, stoichiometric ... more The study of new physical phenomena in the solid state requires the use of clean, stoichiometric materials. For example, the manifestation of the wave nature of two-dimensional electrons in semiconductor heterostructures and its progressive visibility in systems with higher mobility went hand-in-hand with the efforts to remove the effect of impurity scattering. The most important step was achieved by modulation

Physical Review B, 2015

The transport and thermoelectric properties of the interface between SrTiO3 and a 26-monolayer th... more The transport and thermoelectric properties of the interface between SrTiO3 and a 26-monolayer thick LaAlO3-layer grown at high oxygen-pressure have been investigated at temperatures from 4.2 K to 100 K and in magnetic fields up to 18 T. We observe two different electron-like charge carriers originating from two electron channels which contribute to transport. We apply a two-carrier model to our transport-data and develop a consistent model for the termoelectric tensor. We attribute the deviation from our model in magnetoresistance and thermopower at low temperatures to magnetism arising from the Ti 3+ -atoms at the interface.

APL Materials, 2014

We have performed high field magnetotransport measurements to investigate the interface electron ... more We have performed high field magnetotransport measurements to investigate the interface electron gas in LaAlO 3 /SrTiO 3 heterostructures. Shubnikovde Haas oscillations reveal several 2D conduction subbands with carrier effective masses between 1 and 3 m e , quantum mobilities of order 3000 cm 2 /V s, and band edges only a few millielectronvolts below the Fermi energy. Measurements in tilted magnetic fields confirm the 2D character of the electron gas, and show evidence of inter-subband scattering.

We use a scanning Hall probe microscope with single vortex resolution to study the dynamics of si... more We use a scanning Hall probe microscope with single vortex resolution to study the dynamics of single vortices on grain boundaries in YBa2Cu3O6+x thin films with 10 kHz bandwidth. In the presence of an applied current, we observe individual vortices hopping between pinning sites. Detecting the motion of individual vortices allows us to probe the very- low-voltage regime of the current -voltage (I-V) characteristic, at voltage levels of 2.10-15 V. By scanning the grain boundary we show local I-V curves and investigate the statistical processes at the onset of vortex motion.

Physical Review B, 2013

A combined experimental and theoretical investigation of the electronic structure of the archetyp... more A combined experimental and theoretical investigation of the electronic structure of the archetypal oxide heterointerface system LaAlO3 on SrTiO3 is presented. High-resolution, hard x-ray photoemission is used to uncover the occupation of Ti 3d states and the relative energetic alignment -and hence internal electric fields -within the LaAlO3 layer. Firstly, the Ti 2p core level spectra clearly show occupation of Ti 3d states already for two unit cells of LaAlO3. Secondly, the LaAlO3 core levels were seen to shift to lower binding energy as the LaAlO3 overlayer thickness, n, was increased -agreeing with the expectations from the canonical electron transfer model for the emergence of conductivity at the interface. However, not only is the energy offset of only ∼300meV between n = 2 (insulating interface) and n = 6 (metallic interface) an order of magnitude smaller than the simple expectation, but it is also clearly not the sum of a series of unit-cell by unit-cell shifts within the LaAlO3 block. Both of these facts argue against the simple charge-transfer picture involving a cumulative shift of the LaAlO3 valence bands above the SrTiO3 conduction bands, resulting in charge transfer only for n ≥ 4. We discuss effects which could frustrate this elegant and simple charge transfer model, concluding that although it cannot be ruled out, photodoping by the x-ray beam is unlikely to be the cause of the observed behavior. Turning to the theoretical data, our density functional simulations show that the presence of oxygen vacancies at the LaAlO3 surface at the 25% level reverses the direction of the internal field in the LaAlO3. Therefore, taking the experimental and theoretical results together, a consistent picture emerges for real-life samples in which nature does not wait until n = 4 and already for n = 2, mechanisms other than internal-electric-field-driven electron transfer from idealized LaAlO3 to near-interfacial states in the SrTiO3 substrate are active in heading off the incipient polarization catastrophe that drives the physics in these systems.

Physical Review B, 2013

Magnetotransport measurements of charge carriers at the interface of a LaAlO3/SrTiO3 heterostruct... more Magnetotransport measurements of charge carriers at the interface of a LaAlO3/SrTiO3 heterostructure with 26 unit cells of LaAlO3 show Hall resistance and magnetoresistance which at low and high temperatures is described by a single channel of electron-like charge carriers. At intermediate temperatures, we observe non-linear Hall resistance and positive magnetoresistance, establishing the presence of at least two electron-like channels with significantly different mobilities and carrier concentrations. These channels are separated by 6 meV in energy and their temperature dependent occupation and mobilities are responsible for the observed transport properties of the interface. We observe that one of the channels has a mobility that decreases with decreasing temperature, consistent with magnetic scattering in this channel.

Journal of the Korean Physical Society, 2013

We have measured the Hall resistance and magneto-resistance (MR) of LaAlO3/SrTiO3 heterojunctions... more We have measured the Hall resistance and magneto-resistance (MR) of LaAlO3/SrTiO3 heterojunctions at magnetic fields up to 30 T in a temperature range T = 4 K to 70 K. For temperature below 7 K and above 50 K the devices display linear Hall resistance, indicating that one type of charge carriers dominate the transport. For temperatures between 10 K and 40 K, the Hall resistance is strongly non-linear, and is accompanied by a large positive MR, which is governed by the component of magnetic field normal to the interface. This behaviour in the intermediate temperature regime can be related to thermally activated high-mobility carriers.

Under certain growth and preparation conditions, the interface between the perovskite oxides LaAl... more Under certain growth and preparation conditions, the interface between the perovskite oxides LaAlO3 and SrTiO3 can support a 2-dimensional electron gas (2deg) with diverse and remarkable electronic properties. When the mobility of this 2deg becomes high enough, quantum oscillations appear in the magnetoresistance and provide important information about the origin of the electronic behavior. Here we present an angle-dependent magnetotransport

The synthesis of materials with well-controlled composition and structure improves our understand... more The synthesis of materials with well-controlled composition and structure improves our understanding of their intrinsic electrical transport properties. Recent developments in atomically controlled growth have been shown to be crucial in enabling the study of new physical phenomena in epitaxial oxide heterostructures. Nevertheless, these phenomena can be influenced by the presence of defects that act as extrinsic sources of both doping and impurity scattering. Control over the nature and density of such defects is therefore necessary, are we to fully understand the intrinsic materials properties and exploit them in future device technologies. Here, we show that incorporation of a strontium copper oxide nano-layer strongly reduces the impurity scattering at conducting interfaces in oxide LaAlO 3 -SrTiO 3 (001) heterostructures, opening the door to high carrier mobility materials. We propose that this remote cuprate layer facilitates enhanced suppression of oxygen defects by reducing the kinetic barrier for oxygen exchange in the hetero-interfacial film system. This design concept of Submitted to 2 controlled defect engineering can be of significant importance in applications in which enhanced oxygen surface exchange plays a crucial role.

Applied Physics Letters, 2009

We employed a scanning Hall probe microscope to detect the hopping of individual vortices between... more We employed a scanning Hall probe microscope to detect the hopping of individual vortices between pinning sites along grain boundaries in YBa 2 Cu 3 O 6+δ thin films in the presence of an applied current. Detecting the motion of individual vortices allowed us to probe the current-voltage (I-V) characteristics of the grain boundary with voltage sensitivity below a femto-volt. We find a very sharp onset of dissipation with V∝I n with an unprecedented high exponent of n≈290 that shows essentially no dependence on temperature or grain boundary angle. Our data have no straightforward explanation within the existing grain boundary transport models.

Advanced Functional Materials, 2013

The study of new physical phenomena in the solid state requires the use of clean, stoichiometric ... more The study of new physical phenomena in the solid state requires the use of clean, stoichiometric materials. For example, the manifestation of the wave nature of twodimensional electrons in semiconductor heterostructures and its progressive visibility in systems with higher mobility went hand-in-hand with the efforts to remove the effect of impurity scattering. The most important step was achieved by modulation doping, in which the carrier mobility is significantly increased by spatially separating the twodimensional electron gas from the ionized impurity scattering sites. Here we show highmobility two-dimensional electron gases (2DEGs) in complex oxides, free from impurity scattering. Interface doping with a remote defect scavenging layer containing copper dioxide in a SrTiO 3 -SrCuO 2 -LaAlO 3 -SrTiO 3 (001) heterostructure, 2DEG with enhanced electron mobility is achieved, which is similar to a modulation doping scheme successfully used in classic semiconductors. Having made this technologically important step, oxide high mobility 2DEG electronics becomes feasible.

Applied Physics Letters, 2013

Quantum magnetotransport properties of ultrathin topological insulator films J. Appl. Phys. 113, ... more Quantum magnetotransport properties of ultrathin topological insulator films J. Appl. Phys. 113, 043720 (2013) Electrical properties and thermal stability of Pd-doped copper nitride films J. Appl. Phys. 113, 043705 Photocarrier generation in CuxO thin films deposited by radio frequency sputtering Appl. Phys. Lett. 102, 032101 Controlling interfacial states in amorphous/crystalline LaAlO3/SrTiO3 heterostructures by electric fields Appl. Phys. Lett. 102, 021602 (2013) A comparative study of transport properties in polycrystalline and epitaxial chromium nitride films

We use a scanning Hall probe microscope with single vortex resolution to study the dynamics of si... more We use a scanning Hall probe microscope with single vortex resolution to study the dynamics of single vortices on grain boundaries in YBa2Cu3O6+x thin films with 10 kHz bandwidth. In the presence of an applied current, we observe individual vortices hopping between pinning sites. Detecting the motion of individual vortices allows us to probe the very- low-voltage regime of the

The study of new physical phenomena in the solid state requires the use of clean, stoichiometric ... more The study of new physical phenomena in the solid state requires the use of clean, stoichiometric materials. For example, the manifestation of the wave nature of two-dimensional electrons in semiconductor heterostructures and its progressive visibility in systems with higher mobility went hand-in-hand with the efforts to remove the effect of impurity scattering. The most important step was achieved by modulation

Physical Review B, 2015

The transport and thermoelectric properties of the interface between SrTiO3 and a 26-monolayer th... more The transport and thermoelectric properties of the interface between SrTiO3 and a 26-monolayer thick LaAlO3-layer grown at high oxygen-pressure have been investigated at temperatures from 4.2 K to 100 K and in magnetic fields up to 18 T. We observe two different electron-like charge carriers originating from two electron channels which contribute to transport. We apply a two-carrier model to our transport-data and develop a consistent model for the termoelectric tensor. We attribute the deviation from our model in magnetoresistance and thermopower at low temperatures to magnetism arising from the Ti 3+ -atoms at the interface.

APL Materials, 2014

We have performed high field magnetotransport measurements to investigate the interface electron ... more We have performed high field magnetotransport measurements to investigate the interface electron gas in LaAlO 3 /SrTiO 3 heterostructures. Shubnikovde Haas oscillations reveal several 2D conduction subbands with carrier effective masses between 1 and 3 m e , quantum mobilities of order 3000 cm 2 /V s, and band edges only a few millielectronvolts below the Fermi energy. Measurements in tilted magnetic fields confirm the 2D character of the electron gas, and show evidence of inter-subband scattering.

We use a scanning Hall probe microscope with single vortex resolution to study the dynamics of si... more We use a scanning Hall probe microscope with single vortex resolution to study the dynamics of single vortices on grain boundaries in YBa2Cu3O6+x thin films with 10 kHz bandwidth. In the presence of an applied current, we observe individual vortices hopping between pinning sites. Detecting the motion of individual vortices allows us to probe the very- low-voltage regime of the current -voltage (I-V) characteristic, at voltage levels of 2.10-15 V. By scanning the grain boundary we show local I-V curves and investigate the statistical processes at the onset of vortex motion.

Physical Review B, 2013

A combined experimental and theoretical investigation of the electronic structure of the archetyp... more A combined experimental and theoretical investigation of the electronic structure of the archetypal oxide heterointerface system LaAlO3 on SrTiO3 is presented. High-resolution, hard x-ray photoemission is used to uncover the occupation of Ti 3d states and the relative energetic alignment -and hence internal electric fields -within the LaAlO3 layer. Firstly, the Ti 2p core level spectra clearly show occupation of Ti 3d states already for two unit cells of LaAlO3. Secondly, the LaAlO3 core levels were seen to shift to lower binding energy as the LaAlO3 overlayer thickness, n, was increased -agreeing with the expectations from the canonical electron transfer model for the emergence of conductivity at the interface. However, not only is the energy offset of only ∼300meV between n = 2 (insulating interface) and n = 6 (metallic interface) an order of magnitude smaller than the simple expectation, but it is also clearly not the sum of a series of unit-cell by unit-cell shifts within the LaAlO3 block. Both of these facts argue against the simple charge-transfer picture involving a cumulative shift of the LaAlO3 valence bands above the SrTiO3 conduction bands, resulting in charge transfer only for n ≥ 4. We discuss effects which could frustrate this elegant and simple charge transfer model, concluding that although it cannot be ruled out, photodoping by the x-ray beam is unlikely to be the cause of the observed behavior. Turning to the theoretical data, our density functional simulations show that the presence of oxygen vacancies at the LaAlO3 surface at the 25% level reverses the direction of the internal field in the LaAlO3. Therefore, taking the experimental and theoretical results together, a consistent picture emerges for real-life samples in which nature does not wait until n = 4 and already for n = 2, mechanisms other than internal-electric-field-driven electron transfer from idealized LaAlO3 to near-interfacial states in the SrTiO3 substrate are active in heading off the incipient polarization catastrophe that drives the physics in these systems.

Physical Review B, 2013

Magnetotransport measurements of charge carriers at the interface of a LaAlO3/SrTiO3 heterostruct... more Magnetotransport measurements of charge carriers at the interface of a LaAlO3/SrTiO3 heterostructure with 26 unit cells of LaAlO3 show Hall resistance and magnetoresistance which at low and high temperatures is described by a single channel of electron-like charge carriers. At intermediate temperatures, we observe non-linear Hall resistance and positive magnetoresistance, establishing the presence of at least two electron-like channels with significantly different mobilities and carrier concentrations. These channels are separated by 6 meV in energy and their temperature dependent occupation and mobilities are responsible for the observed transport properties of the interface. We observe that one of the channels has a mobility that decreases with decreasing temperature, consistent with magnetic scattering in this channel.

Journal of the Korean Physical Society, 2013

We have measured the Hall resistance and magneto-resistance (MR) of LaAlO3/SrTiO3 heterojunctions... more We have measured the Hall resistance and magneto-resistance (MR) of LaAlO3/SrTiO3 heterojunctions at magnetic fields up to 30 T in a temperature range T = 4 K to 70 K. For temperature below 7 K and above 50 K the devices display linear Hall resistance, indicating that one type of charge carriers dominate the transport. For temperatures between 10 K and 40 K, the Hall resistance is strongly non-linear, and is accompanied by a large positive MR, which is governed by the component of magnetic field normal to the interface. This behaviour in the intermediate temperature regime can be related to thermally activated high-mobility carriers.

Under certain growth and preparation conditions, the interface between the perovskite oxides LaAl... more Under certain growth and preparation conditions, the interface between the perovskite oxides LaAlO3 and SrTiO3 can support a 2-dimensional electron gas (2deg) with diverse and remarkable electronic properties. When the mobility of this 2deg becomes high enough, quantum oscillations appear in the magnetoresistance and provide important information about the origin of the electronic behavior. Here we present an angle-dependent magnetotransport

The synthesis of materials with well-controlled composition and structure improves our understand... more The synthesis of materials with well-controlled composition and structure improves our understanding of their intrinsic electrical transport properties. Recent developments in atomically controlled growth have been shown to be crucial in enabling the study of new physical phenomena in epitaxial oxide heterostructures. Nevertheless, these phenomena can be influenced by the presence of defects that act as extrinsic sources of both doping and impurity scattering. Control over the nature and density of such defects is therefore necessary, are we to fully understand the intrinsic materials properties and exploit them in future device technologies. Here, we show that incorporation of a strontium copper oxide nano-layer strongly reduces the impurity scattering at conducting interfaces in oxide LaAlO 3 -SrTiO 3 (001) heterostructures, opening the door to high carrier mobility materials. We propose that this remote cuprate layer facilitates enhanced suppression of oxygen defects by reducing the kinetic barrier for oxygen exchange in the hetero-interfacial film system. This design concept of Submitted to 2 controlled defect engineering can be of significant importance in applications in which enhanced oxygen surface exchange plays a crucial role.

Applied Physics Letters, 2009

We employed a scanning Hall probe microscope to detect the hopping of individual vortices between... more We employed a scanning Hall probe microscope to detect the hopping of individual vortices between pinning sites along grain boundaries in YBa 2 Cu 3 O 6+δ thin films in the presence of an applied current. Detecting the motion of individual vortices allowed us to probe the current-voltage (I-V) characteristics of the grain boundary with voltage sensitivity below a femto-volt. We find a very sharp onset of dissipation with V∝I n with an unprecedented high exponent of n≈290 that shows essentially no dependence on temperature or grain boundary angle. Our data have no straightforward explanation within the existing grain boundary transport models.

Advanced Functional Materials, 2013

The study of new physical phenomena in the solid state requires the use of clean, stoichiometric ... more The study of new physical phenomena in the solid state requires the use of clean, stoichiometric materials. For example, the manifestation of the wave nature of twodimensional electrons in semiconductor heterostructures and its progressive visibility in systems with higher mobility went hand-in-hand with the efforts to remove the effect of impurity scattering. The most important step was achieved by modulation doping, in which the carrier mobility is significantly increased by spatially separating the twodimensional electron gas from the ionized impurity scattering sites. Here we show highmobility two-dimensional electron gases (2DEGs) in complex oxides, free from impurity scattering. Interface doping with a remote defect scavenging layer containing copper dioxide in a SrTiO 3 -SrCuO 2 -LaAlO 3 -SrTiO 3 (001) heterostructure, 2DEG with enhanced electron mobility is achieved, which is similar to a modulation doping scheme successfully used in classic semiconductors. Having made this technologically important step, oxide high mobility 2DEG electronics becomes feasible.

Applied Physics Letters, 2013

Quantum magnetotransport properties of ultrathin topological insulator films J. Appl. Phys. 113, ... more Quantum magnetotransport properties of ultrathin topological insulator films J. Appl. Phys. 113, 043720 (2013) Electrical properties and thermal stability of Pd-doped copper nitride films J. Appl. Phys. 113, 043705 Photocarrier generation in CuxO thin films deposited by radio frequency sputtering Appl. Phys. Lett. 102, 032101 Controlling interfacial states in amorphous/crystalline LaAlO3/SrTiO3 heterostructures by electric fields Appl. Phys. Lett. 102, 021602 (2013) A comparative study of transport properties in polycrystalline and epitaxial chromium nitride films