David Pappas - Academia.edu (original) (raw)
Papers by David Pappas
Applied Physics Letters, 2015
We demonstrate photon-noise limited performance at sub-millimeter wavelengths in feedhorn-coupled... more We demonstrate photon-noise limited performance at sub-millimeter wavelengths in feedhorn-coupled, microwave kinetic inductance detectors (MKIDs) made of a TiN/Ti/TiN trilayer superconducting film, tuned to have a transition temperature of 1.4 K. The lumped-element detector design enables dual-polarization sensitivity.
MRS Proceedings, 1993
With spin polarized electron spectroscopies, we have invest.igatrd ortleretl (:t1(0001) films dep... more With spin polarized electron spectroscopies, we have invest.igatrd ortleretl (:t1(0001) films deposited on W(110). The photoemission featilres of the gadoli~liu~n 5d sllrlacr state, the 4f levels, and the background exhibit considerable spin polarization alor~g tllc same direction in the plane of the film, indicative of ferromagnetic coupli~ig brlurrrlt III(. surface and the bulk. The 4f spin polarized photoemission data provides strorig rvidriirr that the surface 4f polarization differs from the bulk 4f polarization for Ctl(0001). Our temperature dependent measurements with spin polarized secondary electron sl)rrtrosropv conclusively establishes that the surface of clean Gd(0001) possesses a prr11~1ldic11Iar 1)olarization component which persists to an enhanced surface Curie temperature. Slnall amounts of contamination at the surface result in the disappeara~ice of the ~) c r o r t~( l i~~~I~r component and, therefore, a more perfect ferromagnetic coupling between tlic suriarr atitl the bulk.
Physical review letters, 2014
We present measurements of a topological property, the Chern number (C_{1}), of a closed manifold... more We present measurements of a topological property, the Chern number (C_{1}), of a closed manifold in the space of two-level system Hamiltonians, where the two-level system is formed from a superconducting qubit. We manipulate the parameters of the Hamiltonian of the superconducting qubit along paths in the manifold and extract C_{1} from the nonadiabatic response of the qubit. By adjusting the manifold such that a degeneracy in the Hamiltonian passes from inside to outside the manifold, we observe a topological transition C_{1}=1→0. Our measurement of C_{1} is quantized to within 2% on either side of the transition.
Laser Diodes and LEDs in Industrial, Measurement, Imaging, and Sensors Applications II; Testing, Packaging, and Reliability of Semiconductor Lasers V, 2000
ABSTRACT
Quantum Computing in Solid State Systems, 2006
At present, the performance of superconducting qubits is limited by decoherence. Strong decoheren... more At present, the performance of superconducting qubits is limited by decoherence. Strong decoherence of phase qubits is associated with spurious microwave resonators residing within the Josephson junction tunnel barrier . In this work, we investigate three different fabrication techniques for producing tunnel junctions that vary the properties of the superconductor-insulator interface. Through experimental measurements, we characterize the junction and corresponding qubit quality. We find that there is a strong correlation between the morphology of oxidized base electrodes and the lowering of subgap currents in the junction I-V characteristics, while there is no noticeable improvement in the performance of fabricated phase qubits. Thus, "traditional" indicators of junction performance may not be enough to determine qubit performance. However, truly crystalline insulating barriers may be the key to improving Josephson junction based qubits.
Recent renewed interest in the origin of satellites in core level photoemission from magnetic sur... more Recent renewed interest in the origin of satellites in core level photoemission from magnetic surfaces reflects a more general interest in the potential information one can obtain from the effects of magnetic interactions in spin-integrated photoemission spectroscopy. In the case of 3s core level photoemission from ferromagnets like Fe it has long been known that core- hole interactions with the near-Fermi level magnetic states gives rise to mulitplet structures, although a full appreciation of the processes involved remains lacking. In this contribution we present high resolution 3s core level photoemission data recorded from a range of ultrathin magnetic surface alloys comprising the basic ferromagnetic metals Fe, Co and Ni. Comparisons with relevant theoretical calculations are presented, in particular configuration interaction calculations performed on finite clusters.
ABSTRACT Measurement and control of magnetic nanostructure is essential for the development of lo... more ABSTRACT Measurement and control of magnetic nanostructure is essential for the development of low frequency, room temperature magnetic sensors with high sensitivity and low noise. In this study, we use scanning electron microscopy with polarization analysis (SEMPA) to characterize the magnetic nanostructure in single film anisotropic magnetoresistance sensors. The sensors are fabricated using patterned NiFe microstructures that are designed so that the shape anisotropy biases the magnetization relative to the current. This study focused on "zig-zag" shaped sensors which are composed of multiple repeats of overlapped rectangular unit cells, aligned at 45 degrees from the direction of replication (the easy- or long-axis of the device). SEMPA images show geometric and size dependent magnetic structures, such as vortices that may be responsible for measured magnetoresistance instabilities. SEMPA measurements also provide a direct, quantitative comparison to micromagnetic calculations. This comparison has been made for remanent sates after applying magnetic fields in various directions. In the optimized structures the measured magnetization along the axis of the device oscillates by approximately +/- 25 degrees from the direction of the current flow. For the magnetic simulations, the angle of modulation is somewhat larger at +/- 30 degrees. Detailed comparisons will be presented.
One of the greatest challenges in the development of a practical solid-state quantum computer is ... more One of the greatest challenges in the development of a practical solid-state quantum computer is to overcome decoherence due to coupling between the environment and the qubit. Superconducting quantum computers based on Josephson phase qubits are susceptible to decoherence due to charge noise in both the tunnel barrier and crossover insulators. We have demonstrated that the usage of crystalline tunnel
Applied Physics Letters, 2015
We demonstrate photon-noise limited performance at sub-millimeter wavelengths in feedhorn-coupled... more We demonstrate photon-noise limited performance at sub-millimeter wavelengths in feedhorn-coupled, microwave kinetic inductance detectors (MKIDs) made of a TiN/Ti/TiN trilayer superconducting film, tuned to have a transition temperature of 1.4 K. The lumped-element detector design enables dual-polarization sensitivity.
MRS Proceedings, 1993
With spin polarized electron spectroscopies, we have invest.igatrd ortleretl (:t1(0001) films dep... more With spin polarized electron spectroscopies, we have invest.igatrd ortleretl (:t1(0001) films deposited on W(110). The photoemission featilres of the gadoli~liu~n 5d sllrlacr state, the 4f levels, and the background exhibit considerable spin polarization alor~g tllc same direction in the plane of the film, indicative of ferromagnetic coupli~ig brlurrrlt III(. surface and the bulk. The 4f spin polarized photoemission data provides strorig rvidriirr that the surface 4f polarization differs from the bulk 4f polarization for Ctl(0001). Our temperature dependent measurements with spin polarized secondary electron sl)rrtrosropv conclusively establishes that the surface of clean Gd(0001) possesses a prr11~1ldic11Iar 1)olarization component which persists to an enhanced surface Curie temperature. Slnall amounts of contamination at the surface result in the disappeara~ice of the ~) c r o r t~( l i~~~I~r component and, therefore, a more perfect ferromagnetic coupling between tlic suriarr atitl the bulk.
Physical review letters, 2014
We present measurements of a topological property, the Chern number (C_{1}), of a closed manifold... more We present measurements of a topological property, the Chern number (C_{1}), of a closed manifold in the space of two-level system Hamiltonians, where the two-level system is formed from a superconducting qubit. We manipulate the parameters of the Hamiltonian of the superconducting qubit along paths in the manifold and extract C_{1} from the nonadiabatic response of the qubit. By adjusting the manifold such that a degeneracy in the Hamiltonian passes from inside to outside the manifold, we observe a topological transition C_{1}=1→0. Our measurement of C_{1} is quantized to within 2% on either side of the transition.
Laser Diodes and LEDs in Industrial, Measurement, Imaging, and Sensors Applications II; Testing, Packaging, and Reliability of Semiconductor Lasers V, 2000
ABSTRACT
Quantum Computing in Solid State Systems, 2006
At present, the performance of superconducting qubits is limited by decoherence. Strong decoheren... more At present, the performance of superconducting qubits is limited by decoherence. Strong decoherence of phase qubits is associated with spurious microwave resonators residing within the Josephson junction tunnel barrier . In this work, we investigate three different fabrication techniques for producing tunnel junctions that vary the properties of the superconductor-insulator interface. Through experimental measurements, we characterize the junction and corresponding qubit quality. We find that there is a strong correlation between the morphology of oxidized base electrodes and the lowering of subgap currents in the junction I-V characteristics, while there is no noticeable improvement in the performance of fabricated phase qubits. Thus, "traditional" indicators of junction performance may not be enough to determine qubit performance. However, truly crystalline insulating barriers may be the key to improving Josephson junction based qubits.
Recent renewed interest in the origin of satellites in core level photoemission from magnetic sur... more Recent renewed interest in the origin of satellites in core level photoemission from magnetic surfaces reflects a more general interest in the potential information one can obtain from the effects of magnetic interactions in spin-integrated photoemission spectroscopy. In the case of 3s core level photoemission from ferromagnets like Fe it has long been known that core- hole interactions with the near-Fermi level magnetic states gives rise to mulitplet structures, although a full appreciation of the processes involved remains lacking. In this contribution we present high resolution 3s core level photoemission data recorded from a range of ultrathin magnetic surface alloys comprising the basic ferromagnetic metals Fe, Co and Ni. Comparisons with relevant theoretical calculations are presented, in particular configuration interaction calculations performed on finite clusters.
ABSTRACT Measurement and control of magnetic nanostructure is essential for the development of lo... more ABSTRACT Measurement and control of magnetic nanostructure is essential for the development of low frequency, room temperature magnetic sensors with high sensitivity and low noise. In this study, we use scanning electron microscopy with polarization analysis (SEMPA) to characterize the magnetic nanostructure in single film anisotropic magnetoresistance sensors. The sensors are fabricated using patterned NiFe microstructures that are designed so that the shape anisotropy biases the magnetization relative to the current. This study focused on "zig-zag" shaped sensors which are composed of multiple repeats of overlapped rectangular unit cells, aligned at 45 degrees from the direction of replication (the easy- or long-axis of the device). SEMPA images show geometric and size dependent magnetic structures, such as vortices that may be responsible for measured magnetoresistance instabilities. SEMPA measurements also provide a direct, quantitative comparison to micromagnetic calculations. This comparison has been made for remanent sates after applying magnetic fields in various directions. In the optimized structures the measured magnetization along the axis of the device oscillates by approximately +/- 25 degrees from the direction of the current flow. For the magnetic simulations, the angle of modulation is somewhat larger at +/- 30 degrees. Detailed comparisons will be presented.
One of the greatest challenges in the development of a practical solid-state quantum computer is ... more One of the greatest challenges in the development of a practical solid-state quantum computer is to overcome decoherence due to coupling between the environment and the qubit. Superconducting quantum computers based on Josephson phase qubits are susceptible to decoherence due to charge noise in both the tunnel barrier and crossover insulators. We have demonstrated that the usage of crystalline tunnel