| 51. |
- Iurchuk, Vadym, et al.
(författare)
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All-Electrical Operation of a Curie Switch at Room Temperature
- 2023
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Ingår i: Physical Review Applied. - : American Physical Society (APS). - 2331-7019. ; 20:2
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Tidskriftsartikel (refereegranskat)abstract
- We present all-electrical operation of a FexCr1-x-based Curie switch at room temperature. More specifically, we study the current-induced thermally driven transition from ferromagnetic to antiferromagnetic Magnetometry measurements at different temperatures show that the transition from the ferromagnetic to the antiferromagnetic coupling at zero field is observed at approximately 325 K. Analytical modeling confirms that the observed temperature-dependent transition from indirect ferromagnetic to indirect antiferromagnetic interlayer exchange coupling originates from the modification of the effective interlayer exchange constant through the ferromagnetic-to-paramagnetic transition in the Fe17.5Cr82.5 spacer with minor contributions from the thermally driven variations of the magnetization and magnetic anisotropy of the Fe layers. Room-temperature current-in-plane magnetotransport measurements on the patterned Fe/Cr/Fe17.5Cr82.5/Cr/Fe strips show the transition from the "low-resistance" parallel to the "highresistance" antiparallel remanent magnetization configuration, upon increased probing current density. Quantitative comparison of the switching fields, obtained by magnetometry and magnetotransport, confirms that the Joule heating is the main mechanism responsible for the observed current-induced resistive switching.
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| 52. |
- Ivanov, Ruslan, et al.
(författare)
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Polarization-Resolved Near-Field Spectroscopy of Localized States in m -Plane InxGa1-x N/Ga N Quantum Wells
- 2017
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Ingår i: Physical Review Applied. - : American Physical Society. - 2331-7019. ; 7:6
-
Tidskriftsartikel (refereegranskat)abstract
- We present a polarization, spectrally, and spatially resolved near-field photoluminescence (PL) measurement technique and apply it to the study of wide m-plane InxGa1-xN/GaN quantum wells grown on on-axis and miscut GaN substrates. It is found that PL originates from localized states; nevertheless, its degree of linear polarization (DLP) is high with little spatial variation. This allows an unambiguous assignment of the localized states to InxGa1-xN composition-related band potential fluctuations. Spatial PL variations, occurring due to morphology features of the on-axis samples, play a secondary role compared to the variations of the alloy composition. The large PL peak wavelength difference for polarizations parallel and perpendicular to the c axis, the weak correlation between the peak PL wavelength and the DLP, and the temperature dependence of the DLP suggest that effective potential variations and the hole mass in the second valence-band level are considerably smaller than that for the first level. DLP maps for the long wavelength PL tails have revealed well-defined regions with a small DLP, which have been attributed to a partial strain relaxation around dislocations.
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| 53. |
- Jacewicz, Marek, et al.
(författare)
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Temperature-Dependent Field Emission and Breakdown Measurements Using a Pulsed High-Voltage Cryosystem
- 2020
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Ingår i: Physical Review Applied. - College Park USA : American Physical Society (APS). - 2331-7019. ; 14:6
-
Tidskriftsartikel (refereegranskat)abstract
- A variable-temperature pulsed high-voltage system has been constructed and a series of high-field measurements on copper electrodes have been carried out. The measurements are made at ambient to cryogenic temperatures and include conditioning, breakdown threshold, and field emission. A significant, up to 50%, increase in the breakdown threshold and remarkable stability of field emission are observed when cooled to cryogenic temperatures compared to room temperature. These results provide important experimental input for the development of quantitative theories and models of high-field processes as well as practical input for cryogenic radio-frequency systems.
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| 54. |
- Jansson, Mattias, et al.
(författare)
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N-induced Quantum Dots in GaAs/Ga(N, As) Core/Shell Nanowires: Symmetry, Strain, and Electronic Structure
- 2018
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Ingår i: Physical Review Applied. - : AMER PHYSICAL SOC. - 2331-7019. ; 10:4
-
Tidskriftsartikel (refereegranskat)abstract
- Nanowires (NWs) with embedded zero-dimensional (0D) quantum dots (QDs) have interesting fundamental properties attractive for a variety of applications. The properties of such embedded QDs can be controlled by 0D quantum confinement and also via strain engineering in axial or radial heterostructures of the nanowire system. We evaluate the electronic structure of QDs, which are formed in the Ga(N, As) shell of the GaAs/Ga(N, As) core-shell NWs due to alloy fluctuations. It is found that the principal quantization axis of the studied QDs is primarily oriented along the NW axis, based on the performed polarizationresolved magneto-photoluminescence measurements. We also show that the QDs exhibit a large spectrally dependent variation of the valence band character, which changes from pure heavy-hole states for the low-energy QD emitters to the mixed light-hole heavy-hole states for the QDs emitting at high energies. We ascribe these changes to combined effects of the uniaxial strain caused by the lattice mismatch between the GaAs core and the Ga(N, As) shell, and the local strain/lattice distortion within the short-range fluctuations in the N content. The obtained results underline the importance of the local strain for valence band engineering in hybrid NW structures with embedded QDs.
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| 55. |
- Jayaraman, Aditya, 1993, et al.
(författare)
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Loss and decoherence in superconducting circuits on silicon: Insights from electron spin resonance
- 2024
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Ingår i: Physical Review Applied. - 2331-7019. ; 22:1
-
Tidskriftsartikel (refereegranskat)abstract
- Solid-state devices used for quantum computation and quantum sensing applications are adversely affected by loss and noise caused by spurious, charged two-level systems (TLS) and stray paramagnetic spins. These two sources of noise are interconnected, exacerbating the impact on circuit performance. We use an on-chip electron spin resonance (ESR) technique, with niobium nitride (NbN) superconducting resonators, to study surface spins on silicon and the effect of postfabrication surface treatments. We identify two distinct spin species that are characterized by different spin-relaxation times and respond selectively to various surface treatments (annealing and hydrofluoric acid). Only one of the two spin species has a significant impact on the TLS-limited resonator quality factor at low-power (near-single-photon) excitation. We observe a three- to fivefold reduction in the total density of spins after surface treatments and demonstrate the efficacy of ESR spectroscopy in developing strategies to mitigate loss and decoherence in quantum systems.
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| 56. |
- Jiang, Sheng, et al.
(författare)
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Using Magnetic Droplet Nucleation to Determine the Spin Torque Efficiency and Asymmetry in Co-x(Ni,Fe)(1-x) Thin Films
- 2018
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Ingår i: Physical Review Applied. - : AMER PHYSICAL SOC. - 2331-7019. ; 10:5
-
Tidskriftsartikel (refereegranskat)abstract
- We demonstrate how to extract the material-dependent spin-torque efficiency (epsilon) and asymmetry (lambda) from the field-current nucleation boundaries of magnetic droplet solitons in orthogonal nano-contact spintorque oscillators with Co-x(Ni80Fe20)(1-x), (x = 0 -1), fixed layers. As the perpendicular component of the fixed-layer magnetization plays a central role in governing droplet nucleation, the nucleation boundaries exhibit monotonic shifts towards higher perpendicular magnetic fields when the fixed-layer magnetization mu M-0(s, p) is tuned from 1.04 to 1.7 T. We then extract epsilon and lambda from fits to the nucleation boundaries and find that while epsilon does not vary with composition,lambda increases from 1.5 to 3 with increasing Co content. The analysis of droplet nucleation boundaries is hence a useful tool for the systematic study of both epsilon and lambda as functions of material composition.
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| 57. |
- Jiang, Wei, et al.
(författare)
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Static Magnetic Cloak without a Superconductor
- 2018
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Ingår i: Physical Review Applied. - : American Physical Society. - 2331-7019. ; 9:5
-
Tidskriftsartikel (refereegranskat)abstract
- Similar to its electromagnetic counterpart, magnetic cloaking also has very important technological applications. However, the traditional method to build a static magnetic cloak requires the use of superconducting materials as the diamagnetic component, which seriously limits the practical potential because of the cryogenic condition. We show that a diamagnetic active current boundary combined with a high-permeability magnetic inner shell (MIS) can be designed to solve this problem, rendering an ideal magnetic cloaking effect at zero frequency. We first theoretically prove that a current boundary could magnetically behave as a superconductor to external observers. Based on this phenomena, we introduce a high-permeability MIS made of magnetically ultrasoft metallic sheets (permeability mu > 10(3)) and experimentally prove that the bilayer combination can exactly balance out the disturbance to the external probing field and, meanwhile, have a large invisible inner space. We also show that the active boundary currents can be accordingly configured to overcome the permeability and frequency band limits, leading to a robust cloak over the entire quasistatic frequency region. Our work creates an efficient way to circumvent the traditional limits of metamaterials to build magnetic cloaks for ultralow frequencies. The active-passive hybrid approach could be generally extended to yield other artificial magnetic devices or systems as well.
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| 58. |
- Jonsson, Mattias, et al.
(författare)
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Current Crowding in Nanoscale Superconductors within the Ginzburg-Landau Model
- 2022
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Ingår i: Physical Review Applied. - : American Physical Society (APS). - 2331-7019. ; 17:6
-
Tidskriftsartikel (refereegranskat)abstract
- The current density in a superconductor with turnarounds or constrictions is nonuniform due to a geometrical current-crowding effect. This effect reduces the critical current in the superconducting structure compared to a straight segment and is of importance when designing superconducting devices. We investigate the current-crowding effect in numerical simulations within the generalized time-dependent Ginzburg-Landau (GTDGL) model. The results are validated experimentally by measuring the magnetic field dependence of the critical current in superconducting-nanowire structures, similar to those employed in single-photon detector devices. Comparing the results with London theory, we conclude that the reduction in critical current is significantly smaller in the GTDGL model. This difference is attributed to the current redistribution effect, which reduces the current density at weak points of the superconductor and counteracts the current-crowding effect. We numerically investigate the effect of the fill factor on the critical current in a meander and conclude that the reduction of the critical current is low enough to justify fill factors higher than 33% for applications where the detection efficiency is critical. Finally, we propose a meander design that can combine a high fill factor and low current crowding.
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| 59. |
- Karjalainen, Netta, et al.
(författare)
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Hamiltonian Inference from Dynamical Excitations in Confined Quantum Magnets
- 2023
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Ingår i: Physical Review Applied. - 2331-7019. ; 20:2
-
Tidskriftsartikel (refereegranskat)abstract
- Quantum-disordered models provide a versatile platform to explore the emergence of quantum excitations in many-body systems. The engineering of spin models at the atomic scale with scanning tunneling microscopy and the local imaging of excitations with electrically driven spin resonance has risen as a powerful strategy to image spin excitations in finite quantum spin systems. Here, focusing on S = 1/2 lattices as realized by Ti in MgO, we show that dynamical spin excitations provide a robust strategy to infer the nature of the underlying Hamiltonian. We show that finite-size interference of the dynamical many-body spin excitations of a generalized long-range Heisenberg model allows the underlying spin couplings to be inferred. We show that the spatial distribution of local spin excitations in Ti islands and ladders directly correlates with the underlying ground state in the thermodynamic limit. Using a supervised-learning algorithm, we demonstrate that the different parameters of the Hamiltonian can be extracted by providing the spatially dependent and frequency-dependent local excitations that can be directly measured by electrically driven spin resonance with scanning tunneling microscopy. Our results put forward local dynamical excitations in confined quantum spin models as versatile witnesses of the underlying ground state, providing an experimentally robust strategy for Hamiltonian inference in complex real spin models.
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| 60. |
- Kennedy, O. W., et al.
(författare)
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Tunable Nb Superconducting Resonator Based on a Constriction Nano-SQUID Fabricated with a Ne Focused Ion Beam
- 2019
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Ingår i: Physical Review Applied. - 2331-7019. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- Hybrid superconducting-spin systems offer the potential to combine highly coherent atomic quantum systems with the scalability of superconducting circuits. To fully exploit this potential requires a high-quality-factor microwave resonator, tunable in frequency and able to operate at magnetic fields optimal for the spin system. Such magnetic fields typically rule out conventional Al-based Josephson-junction devices that have previously been used for tunable high-Q microwave resonators. The larger critical field of Nb allows microwave resonators with large field resilience to be fabricated. Here we demonstrate how constriction-type weak links, patterned in parallel into the central conductor of a Nb coplanar resonator with a neon focused ion beam, can be used to implement a frequency-tunable resonator. We study transmission through two such devices and show how they realize high-quality-factor, tunable, field-resilient devices that hold promise for future applications coupling to spin systems.
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