48 research outputs found
Magnetic Switching of Phase-Slip Dissipation in NbSe2 Nanobelts
The stability of the superconducting dissipationless and resistive states in
single-crystalline NbSe2 nanobelts is characterized by transport measurements
in an external magnetic field (H). Current-driven electrical measurements show
voltage steps, indicating the nucleation of phase-slip structures. Well below
the critical temperature, the position of the voltage steps exhibits a sharp,
periodic dependence as a function of H. This phenomenon is discussed in the
context of two possible mechanisms: the interference of the order parameter and
the periodic rearrangement of the vortex lattice within the nanobelt.Comment: 4 figure
The Principles of Social Order. Selected Essays of Lon L. Fuller, edited With an introduction by Kenneth I. Winston
The electron spins of semiconductor defects can have complex interactions with their host, particularly in polar materials like SiC where electrical and mechanical variables are intertwined. By combining pulsed spin resonance with ab initio simulations, we show that spin-spin interactions in 4H-SiC neutral divacancies give rise to spin states with a strong Stark effect, sub-10(-6) strain sensitivity, and highly spin-dependent photoluminescence with intensity contrasts of 15%-36%. These results establish SiC color centers as compelling systems for sensing nanoscale electric and strain fields
Theoretical model of the dynamic spin polarization of nuclei coupled to paramagnetic point defects in diamond and silicon carbide
Dynamic nuclear spin polarization (DNP) mediated by paramagnetic point
defects in semiconductors is a key resource for both initializing nuclear
quantum memories and producing nuclear hyperpolarization. DNP is therefore an
important process in the field of quantum-information processing,
sensitivity-enhanced nuclear magnetic resonance, and nuclear-spin-based
spintronics. DNP based on optical pumping of point defects has been
demonstrated by using the electron spin of nitrogen-vacancy (NV) center in
diamond, and more recently, by using divacancy and related defect spins in
hexagonal silicon carbide (SiC). Here, we describe a general model for these
optical DNP processes that allows the effects of many microscopic processes to
be integrated. Applying this theory, we gain a deeper insight into dynamic
nuclear spin polarization and the physics of diamond and SiC defects. Our
results are in good agreement with experimental observations and provide a
detailed and unified understanding. In particular, our findings show that the
defects' electron spin coherence times and excited state lifetimes are crucial
factors in the entire DNP process
High fidelity bi-directional nuclear qubit initialization in SiC
Dynamic nuclear polarization (DNP) is an attractive method for initializing
nuclear spins that are strongly coupled to optically active electron spins
because it functions at room temperature and does not require strong magnetic
fields. In this Letter, we demonstrate that DNP, with near-unity polarization
efficiency, can be generally realized in weakly coupled hybrid registers, and
furthermore that the nuclear spin polarization can be completely reversed with
only sub-Gauss magnetic field variations. This mechanism offers new avenues for
DNP-based sensors and radio-frequency free control of nuclear qubits
Polytype control of spin qubits in silicon carbide
Crystal defects can confine isolated electronic spins and are promising
candidates for solid-state quantum information. Alongside research focusing on
nitrogen vacancy centers in diamond, an alternative strategy seeks to identify
new spin systems with an expanded set of technological capabilities, a
materials driven approach that could ultimately lead to "designer" spins with
tailored properties. Here, we show that the 4H, 6H and 3C polytypes of SiC all
host coherent and optically addressable defect spin states, including spins in
all three with room-temperature quantum coherence. The prevalence of this spin
coherence shows that crystal polymorphism can be a degree of freedom for
engineering spin qubits. Long spin coherence times allow us to use double
electron-electron resonance to measure magnetic dipole interactions between
spin ensembles in inequivalent lattice sites of the same crystal. Together with
the distinct optical and spin transition energies of such inequivalent spins,
these interactions provide a route to dipole-coupled networks of separately
addressable spins.Comment: 28 pages, 5 figures, and supplementary information and figure
Spheromak Merging And Field Reversed Configuration Formation At The Swarthmore Spheromak Experiment
The initial results of coaxial co- and counter-helicity spheromak merging studies at the Swarthmore Spheromak Experiment (SSX) [M. R. Brown, Phys. Plasmas 6, 1717 (1999)] are reported. In its new configuration, SSX is optimized to study field reversed configuration (FRC) formation and stability by counter-helicity spheromak merging. A pair of midplane coils magnetically restricts the merging process to determine how the stability of the resulting magnetic configuration depends upon the quantity of toroidal flux remaining from the initial spheromaks. The diagnostic set at SSX, featuring the capability of measuring up to 600 magnetic field components at 800 ns time resolution, permits detailed studies of the dynamic three-dimensional magnetic structures produced during these experiments. A compact array of magnetic probes examines the local reconnection process, while a distributed array of probes examines global magnetic structure. Counter-helicity merging produces a FRC that persists for several Alfvén times, although the oppositely directed toroidal field of the initial spheromaks does not completely annihilate. The m=1 (toroidal) mode dominates late in the evolution, and is consistent with the tilt instability. Co-helicity merging produces a single elongated spheromak that rapidly tilts
Optical polarization of nuclear spins in silicon carbide
We demonstrate optically pumped dynamic nuclear polarization of 29-Si nuclear
spins that are strongly coupled to paramagnetic color centers in 4H- and
6H-SiC. The 99 +/- 1% degree of polarization at room temperature corresponds to
an effective nuclear temperature of 5 microKelvin. By combining ab initio
theory with the experimental identification of the color centers' optically
excited states, we quantitatively model how the polarization derives from
hyperfine-mediated level anticrossings. These results lay a foundation for
SiC-based quantum memories, nuclear gyroscopes, and hyperpolarized probes for
magnetic resonance imaging.Comment: 21 pages including supplementary information; four figures in main
text and one tabl