M. Hesselberth - Academia.edu (original) (raw)
Papers by M. Hesselberth
There is much interest in interfaces between insulating oxides, such as SrTiO3 (STO) and LaAlO3 (... more There is much interest in interfaces between insulating oxides, such as SrTiO3 (STO) and LaAlO3 (LAO), where a two dimensional electron gas can form due to the so-called polar discontinuity. These interfaces are mostly fabricated by Pulsed Laser Deposition (PLD). We have investigated such interfaces by reactive RF sputtering in a high-pressure oxygen atmosphere. The films are smooth and crystalline. Transmission Electron Microscopy indicates that the interfaces are sharp and continuous while Electron Energy Loss Spectroscopy data indicate some slight intermixing. However, we find these interfaces to be non-conducting. It appears that the sputtered interface is not electronically reconstructed in the way reported for films grown by PLD, notwithstanding the good structural quality and composition of the films. [DOI: 10.1380/ejssnt.2012.619]
Physical Review Letters, 2004
We report a new nonlocal effect in vortex matter, where an electric current confined to a small r... more We report a new nonlocal effect in vortex matter, where an electric current confined to a small region of a long and sufficiently narrow superconducting wire causes vortex flow at distances hundreds of inter-vortex separations away. The observed remote traffic of vortices is attributed to a very efficient transfer of a local strain through the one-dimensional vortex lattice, even in the presence of disorder. We also observe mesoscopic fluctuations in the nonlocal vortex flow, which arise due to "traffic jams" when vortex arrangements do not match a local geometry of a superconducting channel.
Physical Review B, 2009
The electrodynamic properties of Au 1−x Fe x films are measured for various Fe contents x = 0, 6,... more The electrodynamic properties of Au 1−x Fe x films are measured for various Fe contents x = 0, 6, 17, and 22 at. %. From room-temperature optical measurements over a broad spectral range ͑10-3.3ϫ 10 4 cm −1 ͒, we obtain the frequency-dependent conductivity of the films. As the iron concentration increases, a stronger decrease in the magnetic contribution to the dc conductivity compared to ac conductivity ͑at frequency 10 cm −1 ͒ is detected. In addition, a broad absorption peak is discovered at frequency 1000-3000 cm −1 for concentrations of 6-22 at. %. Both observations are associated with ferromagnetic clusters that cause partial localization of conduction electrons.
Physica C: Superconductivity, 2002
Under certain conditions, the order parameter induced by a superconductor (S) in a ferromagnet (F... more Under certain conditions, the order parameter induced by a superconductor (S) in a ferromagnet (F) can be inhomogeneous and oscillating, which results e.g. in the socalled π-coupling in S/F/S junctions. In principle, the inhomogeneous state can be induced at T c as function of the F-layer thickness d F in S/F bilayers and multilayers, which should result in a dip-like characteristic of T c (d F). We show the results of measurements on the S/F system Nb/Cu 1−x Ni x , for Ni-concentrations in the range x = 0.5-0.7, where such effects might be expected. We find that the critical thickness for the occurrence of superconductivity is still relatively high, even for these weak ferromagnets. The resulting dip then is intrinsically shallow and difficult to observe, which explains the lack of a clear signature in the T c (d F) data.
Physica B: Condensed Matter, 2000
We have used the low-energy μ+-beam set-up developed at PSI to perform exploratory μSR measuremen... more We have used the low-energy μ+-beam set-up developed at PSI to perform exploratory μSR measurements in thin films of colossal-magnetoresistance La0.7Ca0.3MnO3 and of AuFe spin glass. The transverse field (100 G) signals clearly indicate the transitions to the ferromagnetic and spin-glass states, respectively.
Journal of Applied Physics, 2000
Specular and diffuse x-ray scattering are used to probe the mesoscopic structure of interfaces wi... more Specular and diffuse x-ray scattering are used to probe the mesoscopic structure of interfaces within two 30-period Mo/Si superlattices, grown on silicon and glass substrates by ion beam sputtering. The data are evaluated qualitatively and quantitatively on the basis of a distorted-wave Born approximation, which includes a correlating behavior of interface roughness in both the lateral and vertical directions. Different
We report measurements of vortex dynamics in a single nanofabricated weak-pinning ratchet channel... more We report measurements of vortex dynamics in a single nanofabricated weak-pinning ratchet channel of a-NbGe with strong-pinning NbN channel edges. The channel is arranged in a circle on a Corbino disk geometry with a radial bias current, thus eliminating the influence of edge barriers to vortex entry on the dynamics and resulting in closed circular orbits for the vortices. An
Acta Physica Polonica Series B
ABSTRACT
The dynamics of vortex flow in confined geometries can be explored with weak-pinning channels of ... more The dynamics of vortex flow in confined geometries can be explored with weak-pinning channels of superconducting a-NbGe surrounded by strong-pinning NbN channel edges. Periodic constrictions of the channel walls lead to strong oscillations of the critical current, which we observe through transport measurements of the vortex dynamics. We explore the role of the shape and periodicity of the confining potential, as well as intervortex interactions, by fabricating a variety of samples that me measure over a range of temperatures. For small magnetic fields, the measured critical currents are completely reversible with magnetic field, indicating that all of the vortices are confined to the weak-pinning channels. Beyond a certain threshold field, typically after at least several periods of the critical current oscillations, the critical current begins to exhibit magnetic hysteresis, characteristic of vortex entry into the strong-pinning NbN banks.
The dynamics of vortex flow in confined geometries can be explored with structured weak-pinning c... more The dynamics of vortex flow in confined geometries can be explored with structured weak-pinning channels of superconducting a-NbGe surrounded by strong-pinning NbN channel edges. The lack of pinning allows the vortices to move through the channels with the dominant interaction determined by the shape of the channel walls. We have fabricated such weak-pinning channels with asymmetric sawtooth edges for controlling the motion of vortices. We present measurements of vortex dynamics in the channels and compare these with similar measurements on a set of channels with uniform width. While the uniform-width channels exhibit a symmetric response for both directions through the channel, the vortex motion through the asymmetric channels is considerably different, with substantial asymmetries in both the static depinning and dynamic flux flow. We report on the rich dependence on magnetic field and driving force amplitude for this vortex ratchet effect.
We report transport measurements of vortex flow dynamics in nanofabricated weak-pinning channels ... more We report transport measurements of vortex flow dynamics in nanofabricated weak-pinning channels of a-NbGe with strong-pinning NbN channel edges. The channels are arranged in circular patterns on a Corbino disk geometry with a radial bias current, thus eliminating the influence of edge barriers to vortex entry on the dynamics. We have developed a SQUID voltmeter arrangement for resolving the flux flow voltage from a small number of vortices moving through such a single mesoscopic channel. We discuss the configuration of this measurement scheme, along with potential applications of this system for measuring the flow of vortices around single weak-pinning channels free from edge barriers.
Physical Review B, 2001
We introduce a type of vortex entry edge barrier which controls the critical current in a perpend... more We introduce a type of vortex entry edge barrier which controls the critical current in a perpendicular magnetic field in thin-film weak-pinning superconducting strips. Measurements of the critical current in thinfilm amorphous-MoGe strips show a linear decrease with increasing magnetic field strength at low magnetic fields, and a crossover at a well-defined threshold field to an inverse power-law decay that is independent of the strip width. This behavior has not been observed previously due to bulk pinning, which only becomes dominant in our MoGe samples at high magnetic fields. To describe our results, we present calculations of the current distribution in thin superconducting strips with a finite penetration depth and negligible bulk pinning, and show that the measured critical currents in our MoGe samples correspond to a current density at the strip edge which approaches the Ginzburg-Landau depairing limit. Shape variations and defects along the strip edges influence the vortex entry conditions, leading to deviations from the ideal behavior, including offsets in the critical current maximum with respect to zero field.
e-Journal of Surface Science and Nanotechnology, 2012
There is much interest in interfaces between insulating oxides, such as SrTiO3 (STO) and LaAlO3 (... more There is much interest in interfaces between insulating oxides, such as SrTiO3 (STO) and LaAlO3 (LAO), where a two dimensional electron gas can form due to the so-called polar discontinuity. These interfaces are mostly fabricated by Pulsed Laser Deposition (PLD). We have investigated such interfaces by reactive RF sputtering in a high-pressure oxygen atmosphere. The films are smooth and crystalline. Transmission Electron Microscopy indicates that the interfaces are sharp and continuous while Electron Energy Loss Spectroscopy data indicate some slight intermixing. However, we find these interfaces to be non-conducting. It appears that the sputtered interface is not electronically reconstructed in the way reported for films grown by PLD, notwithstanding the good structural quality and composition of the films.
We have investigated the properties of interfaces between LaAlO3 films grown on SrTiO3 substrates... more We have investigated the properties of interfaces between LaAlO3 films grown on SrTiO3 substrates singly terminated by TiO2. We used RF sputtering in a high-pressure oxygen atmosphere. The films are smooth, with flat surfaces. Transmission Electron Microscopy shows atomically sharp and continuous interfaces while EELS measurements show some slight intermixing. The elemental ratio of La to Al measured by EDX is found to be 1.07. Importantly, we find these interfaces to be non-conducting, indicating that the sputtered interface is not electronically reconstructed in the way reported for films grown by Pulsed Laser Deposition because of the different interplay between stoichiometry, mixing and oxygen vacancies.
Physica C: Superconductivity, 2000
We have studied the flux dynamics in superconducting strips patterned from both Nb and weak-pinni... more We have studied the flux dynamics in superconducting strips patterned from both Nb and weak-pinning amorphous MoGe films using a Scanning SQUID Microscope (SSM). The unparalleled flux sensitivity of the SSM allows us to image the vortices in the strip under a variety of field and cooling conditions with single vortex resolution for low flux density. We are able to apply transport currents while imaging the strip and observe the shift of the vortex distributions due to the Lorentz force. Surface steps etched into the strips significantly alter the flux patterns and introduce asymmetry in the vortex motion under applied transport currents. Both the change in vortex line energy across the step and the screening currents which flow along the step influence the vortex distributions and flux dynamics. We are studying the relationship between the vortex distributions from the SSM images and the transport characteristics of the strips.
ABSTRACT We report transport measurements of vortex flow dynamics in mesoscopic weak-pinning chan... more ABSTRACT We report transport measurements of vortex flow dynamics in mesoscopic weak-pinning channels of a-NbGe with strong-pinning NbN channel edges. The channels are arranged in circular patterns on a Corbino disk geometry, thus eliminating the influence of edge barriers to vortex entry on the dynamics. The number of vortices which can be detected at particular flow velocities is limited by the method for measuring the flux flow voltage and the channel configuration. We discuss potential applications of this system for guiding vortices around nanofabricated structures free from edge barriers.
ABSTRACT Nanofabricated pinning structures can be used to guide vortices in superconductors throu... more ABSTRACT Nanofabricated pinning structures can be used to guide vortices in superconductors through various potential energy landscapes. We report transport measurements of vortex flow dynamics in structured weak-pinning channels of a-NbGe with strong-pinning NbN channel edges. By arranging the channels in circular patterns on a Corbino disk geometry, we eliminate the influence of edge barriers to vortex entry on the dynamics. Patterning channel edges with different shapes allows us to explore the influence of the confinement potential on the vortex dynamics. We discuss one such pattern with channel edges in an asymmetric sawtooth configuration for investigations of vortex ratchet dynamics.
We report on measurements of vortex ratchets fabricated from weak-pinning superconducting a-NbGe ... more We report on measurements of vortex ratchets fabricated from weak-pinning superconducting a-NbGe channels bounded by strong-pinning NbN banks with asymmetric sawtooth edges. This configuration for the vortex confinement potential results in an asymmetric response for the vortex dynamics in the channels. Interactions between vortices, both within a channel and between neighboring channels, have a substantial influence on the ratchet behavior,
Physical Review B, 2001
ABSTRACT
There is much interest in interfaces between insulating oxides, such as SrTiO3 (STO) and LaAlO3 (... more There is much interest in interfaces between insulating oxides, such as SrTiO3 (STO) and LaAlO3 (LAO), where a two dimensional electron gas can form due to the so-called polar discontinuity. These interfaces are mostly fabricated by Pulsed Laser Deposition (PLD). We have investigated such interfaces by reactive RF sputtering in a high-pressure oxygen atmosphere. The films are smooth and crystalline. Transmission Electron Microscopy indicates that the interfaces are sharp and continuous while Electron Energy Loss Spectroscopy data indicate some slight intermixing. However, we find these interfaces to be non-conducting. It appears that the sputtered interface is not electronically reconstructed in the way reported for films grown by PLD, notwithstanding the good structural quality and composition of the films. [DOI: 10.1380/ejssnt.2012.619]
Physical Review Letters, 2004
We report a new nonlocal effect in vortex matter, where an electric current confined to a small r... more We report a new nonlocal effect in vortex matter, where an electric current confined to a small region of a long and sufficiently narrow superconducting wire causes vortex flow at distances hundreds of inter-vortex separations away. The observed remote traffic of vortices is attributed to a very efficient transfer of a local strain through the one-dimensional vortex lattice, even in the presence of disorder. We also observe mesoscopic fluctuations in the nonlocal vortex flow, which arise due to "traffic jams" when vortex arrangements do not match a local geometry of a superconducting channel.
Physical Review B, 2009
The electrodynamic properties of Au 1−x Fe x films are measured for various Fe contents x = 0, 6,... more The electrodynamic properties of Au 1−x Fe x films are measured for various Fe contents x = 0, 6, 17, and 22 at. %. From room-temperature optical measurements over a broad spectral range ͑10-3.3ϫ 10 4 cm −1 ͒, we obtain the frequency-dependent conductivity of the films. As the iron concentration increases, a stronger decrease in the magnetic contribution to the dc conductivity compared to ac conductivity ͑at frequency 10 cm −1 ͒ is detected. In addition, a broad absorption peak is discovered at frequency 1000-3000 cm −1 for concentrations of 6-22 at. %. Both observations are associated with ferromagnetic clusters that cause partial localization of conduction electrons.
Physica C: Superconductivity, 2002
Under certain conditions, the order parameter induced by a superconductor (S) in a ferromagnet (F... more Under certain conditions, the order parameter induced by a superconductor (S) in a ferromagnet (F) can be inhomogeneous and oscillating, which results e.g. in the socalled π-coupling in S/F/S junctions. In principle, the inhomogeneous state can be induced at T c as function of the F-layer thickness d F in S/F bilayers and multilayers, which should result in a dip-like characteristic of T c (d F). We show the results of measurements on the S/F system Nb/Cu 1−x Ni x , for Ni-concentrations in the range x = 0.5-0.7, where such effects might be expected. We find that the critical thickness for the occurrence of superconductivity is still relatively high, even for these weak ferromagnets. The resulting dip then is intrinsically shallow and difficult to observe, which explains the lack of a clear signature in the T c (d F) data.
Physica B: Condensed Matter, 2000
We have used the low-energy μ+-beam set-up developed at PSI to perform exploratory μSR measuremen... more We have used the low-energy μ+-beam set-up developed at PSI to perform exploratory μSR measurements in thin films of colossal-magnetoresistance La0.7Ca0.3MnO3 and of AuFe spin glass. The transverse field (100 G) signals clearly indicate the transitions to the ferromagnetic and spin-glass states, respectively.
Journal of Applied Physics, 2000
Specular and diffuse x-ray scattering are used to probe the mesoscopic structure of interfaces wi... more Specular and diffuse x-ray scattering are used to probe the mesoscopic structure of interfaces within two 30-period Mo/Si superlattices, grown on silicon and glass substrates by ion beam sputtering. The data are evaluated qualitatively and quantitatively on the basis of a distorted-wave Born approximation, which includes a correlating behavior of interface roughness in both the lateral and vertical directions. Different
We report measurements of vortex dynamics in a single nanofabricated weak-pinning ratchet channel... more We report measurements of vortex dynamics in a single nanofabricated weak-pinning ratchet channel of a-NbGe with strong-pinning NbN channel edges. The channel is arranged in a circle on a Corbino disk geometry with a radial bias current, thus eliminating the influence of edge barriers to vortex entry on the dynamics and resulting in closed circular orbits for the vortices. An
Acta Physica Polonica Series B
ABSTRACT
The dynamics of vortex flow in confined geometries can be explored with weak-pinning channels of ... more The dynamics of vortex flow in confined geometries can be explored with weak-pinning channels of superconducting a-NbGe surrounded by strong-pinning NbN channel edges. Periodic constrictions of the channel walls lead to strong oscillations of the critical current, which we observe through transport measurements of the vortex dynamics. We explore the role of the shape and periodicity of the confining potential, as well as intervortex interactions, by fabricating a variety of samples that me measure over a range of temperatures. For small magnetic fields, the measured critical currents are completely reversible with magnetic field, indicating that all of the vortices are confined to the weak-pinning channels. Beyond a certain threshold field, typically after at least several periods of the critical current oscillations, the critical current begins to exhibit magnetic hysteresis, characteristic of vortex entry into the strong-pinning NbN banks.
The dynamics of vortex flow in confined geometries can be explored with structured weak-pinning c... more The dynamics of vortex flow in confined geometries can be explored with structured weak-pinning channels of superconducting a-NbGe surrounded by strong-pinning NbN channel edges. The lack of pinning allows the vortices to move through the channels with the dominant interaction determined by the shape of the channel walls. We have fabricated such weak-pinning channels with asymmetric sawtooth edges for controlling the motion of vortices. We present measurements of vortex dynamics in the channels and compare these with similar measurements on a set of channels with uniform width. While the uniform-width channels exhibit a symmetric response for both directions through the channel, the vortex motion through the asymmetric channels is considerably different, with substantial asymmetries in both the static depinning and dynamic flux flow. We report on the rich dependence on magnetic field and driving force amplitude for this vortex ratchet effect.
We report transport measurements of vortex flow dynamics in nanofabricated weak-pinning channels ... more We report transport measurements of vortex flow dynamics in nanofabricated weak-pinning channels of a-NbGe with strong-pinning NbN channel edges. The channels are arranged in circular patterns on a Corbino disk geometry with a radial bias current, thus eliminating the influence of edge barriers to vortex entry on the dynamics. We have developed a SQUID voltmeter arrangement for resolving the flux flow voltage from a small number of vortices moving through such a single mesoscopic channel. We discuss the configuration of this measurement scheme, along with potential applications of this system for measuring the flow of vortices around single weak-pinning channels free from edge barriers.
Physical Review B, 2001
We introduce a type of vortex entry edge barrier which controls the critical current in a perpend... more We introduce a type of vortex entry edge barrier which controls the critical current in a perpendicular magnetic field in thin-film weak-pinning superconducting strips. Measurements of the critical current in thinfilm amorphous-MoGe strips show a linear decrease with increasing magnetic field strength at low magnetic fields, and a crossover at a well-defined threshold field to an inverse power-law decay that is independent of the strip width. This behavior has not been observed previously due to bulk pinning, which only becomes dominant in our MoGe samples at high magnetic fields. To describe our results, we present calculations of the current distribution in thin superconducting strips with a finite penetration depth and negligible bulk pinning, and show that the measured critical currents in our MoGe samples correspond to a current density at the strip edge which approaches the Ginzburg-Landau depairing limit. Shape variations and defects along the strip edges influence the vortex entry conditions, leading to deviations from the ideal behavior, including offsets in the critical current maximum with respect to zero field.
e-Journal of Surface Science and Nanotechnology, 2012
There is much interest in interfaces between insulating oxides, such as SrTiO3 (STO) and LaAlO3 (... more There is much interest in interfaces between insulating oxides, such as SrTiO3 (STO) and LaAlO3 (LAO), where a two dimensional electron gas can form due to the so-called polar discontinuity. These interfaces are mostly fabricated by Pulsed Laser Deposition (PLD). We have investigated such interfaces by reactive RF sputtering in a high-pressure oxygen atmosphere. The films are smooth and crystalline. Transmission Electron Microscopy indicates that the interfaces are sharp and continuous while Electron Energy Loss Spectroscopy data indicate some slight intermixing. However, we find these interfaces to be non-conducting. It appears that the sputtered interface is not electronically reconstructed in the way reported for films grown by PLD, notwithstanding the good structural quality and composition of the films.
We have investigated the properties of interfaces between LaAlO3 films grown on SrTiO3 substrates... more We have investigated the properties of interfaces between LaAlO3 films grown on SrTiO3 substrates singly terminated by TiO2. We used RF sputtering in a high-pressure oxygen atmosphere. The films are smooth, with flat surfaces. Transmission Electron Microscopy shows atomically sharp and continuous interfaces while EELS measurements show some slight intermixing. The elemental ratio of La to Al measured by EDX is found to be 1.07. Importantly, we find these interfaces to be non-conducting, indicating that the sputtered interface is not electronically reconstructed in the way reported for films grown by Pulsed Laser Deposition because of the different interplay between stoichiometry, mixing and oxygen vacancies.
Physica C: Superconductivity, 2000
We have studied the flux dynamics in superconducting strips patterned from both Nb and weak-pinni... more We have studied the flux dynamics in superconducting strips patterned from both Nb and weak-pinning amorphous MoGe films using a Scanning SQUID Microscope (SSM). The unparalleled flux sensitivity of the SSM allows us to image the vortices in the strip under a variety of field and cooling conditions with single vortex resolution for low flux density. We are able to apply transport currents while imaging the strip and observe the shift of the vortex distributions due to the Lorentz force. Surface steps etched into the strips significantly alter the flux patterns and introduce asymmetry in the vortex motion under applied transport currents. Both the change in vortex line energy across the step and the screening currents which flow along the step influence the vortex distributions and flux dynamics. We are studying the relationship between the vortex distributions from the SSM images and the transport characteristics of the strips.
ABSTRACT We report transport measurements of vortex flow dynamics in mesoscopic weak-pinning chan... more ABSTRACT We report transport measurements of vortex flow dynamics in mesoscopic weak-pinning channels of a-NbGe with strong-pinning NbN channel edges. The channels are arranged in circular patterns on a Corbino disk geometry, thus eliminating the influence of edge barriers to vortex entry on the dynamics. The number of vortices which can be detected at particular flow velocities is limited by the method for measuring the flux flow voltage and the channel configuration. We discuss potential applications of this system for guiding vortices around nanofabricated structures free from edge barriers.
ABSTRACT Nanofabricated pinning structures can be used to guide vortices in superconductors throu... more ABSTRACT Nanofabricated pinning structures can be used to guide vortices in superconductors through various potential energy landscapes. We report transport measurements of vortex flow dynamics in structured weak-pinning channels of a-NbGe with strong-pinning NbN channel edges. By arranging the channels in circular patterns on a Corbino disk geometry, we eliminate the influence of edge barriers to vortex entry on the dynamics. Patterning channel edges with different shapes allows us to explore the influence of the confinement potential on the vortex dynamics. We discuss one such pattern with channel edges in an asymmetric sawtooth configuration for investigations of vortex ratchet dynamics.
We report on measurements of vortex ratchets fabricated from weak-pinning superconducting a-NbGe ... more We report on measurements of vortex ratchets fabricated from weak-pinning superconducting a-NbGe channels bounded by strong-pinning NbN banks with asymmetric sawtooth edges. This configuration for the vortex confinement potential results in an asymmetric response for the vortex dynamics in the channels. Interactions between vortices, both within a channel and between neighboring channels, have a substantial influence on the ratchet behavior,
Physical Review B, 2001
ABSTRACT