Strong Rf Field Research Articles

Dissociation of composite Abrikosov vortices in two-band superconductors in a strong rf field

In several theoretical works, it was argued that under certain conditions Abrikosov vortices in multiband superconductors can split and exist in the form of fractional vortices, formed separately in superfluid condensates of different electron bands. Such vortices possess a fractional flux quantum, and these fractional vortices attract each other, trying to join into a composite vortex with the whole flux quantum ϕ0=h/2e. In the present work, we solve numerically the nonlinear dynamic equation for the composite vortex, settled in the pinning potential well of the columnar defect within a two-band superconductor, and exerted the rf Lorentz force action. We demonstrate that at high enough rf current amplitudes such composite Abrikosov vortices will dissociate into fractional ones and escape from the pinning potential well. The sequence of these events depends on the character of the pinning potential well, e.g., the radius of the pinning potential well. The possible manifestation of such kind transitions in rf electrodynamic characteristics, such as a complex rf resistivity and harmonics generation is calculated.

Cross‐Polarization Schemes for Improved Heteronuclear Transfers Involving Labile Protons in Biomolecular Solution NMR

AbstractINEPT‐based experiments are widely used for 1H→15N transfers, but often fail when involving labile protons due to solvent exchanges. J‐based cross polarization (CP) strategies offer a more efficient alternative to perform such transfers, particularly when leveraging the Hwater HN exchange process to boost the 1H→15N transfer process. This leveraging, however, demands the simultaneous spin‐locking of both Hwater and HN protons by a strong 1H RF field, while fulfilling the γHB1,H=γNB1,N Hartmann‐Hahn matching condition. Given the low value of γN/γH, however, these demands are often incompatible—particularly when experiments are executed by the power‐limited cryogenic probes used in contemporary high field NMR. The present manuscript discusses CP alternatives that can alleviate this limitation, and evaluates their performance on urea, amino acids, and intrinsically disordered proteins. These alternatives include new CP variants based on frequency‐swept and phase‐modulated pulses, designed to simultaneously fulfill the aforementioned conflicting conditions. Their performances vis‐à‐vis current options are theoretically analyzed with Liouville‐space simulations, and experimentally tested with double and triple resonance transfer experiments.

Multi-photon multi-quantum transitions in the spin- 32 silicon-vacancy centers of SiC

Silicon vacancy centers in silicon carbide are promising candidates for storing and manipulating quantum information. Implementation of fast quantum gates is an essential requirement for quantum information processing. In a low magnetic field, the resonance frequencies of silicon vacancy spins are in the range of a few MHz, the same order of magnitude as the Rabi frequencies of typical control fields. As a consequence, the rotating wave approximation becomes invalid and nonlinear processes like the absorption and emission of multiple photons become relevant. This work focuses on multi-photon transitions of negatively charged silicon vacancies driven by a strong RF field. We present continuous-wave optically detected magnetic resonance (ODMR) spectra measured at different RF powers to identify the 1-, 2-, and 3-RF photon transitions of different types of the silicon vacancy in the 6$H$-SIC polytype. Time-resolved experiments of Rabi oscillations and free induction decays of these multiple RF photon transitions were observed for the first time. Apart from zero-field data, we also present spectra in magnetic fields with different strength and orientation with respect to the system's symmetry axis.

A single electron transistor charge sensor in strong rf fields

We measure the charge sensitivity, Se, of a single electron transistor (SET) in the presence of strong (Vrf ∼ e/Cg) spurious radio frequency (rf) signals at frequencies up to 50 MHz, where Cg is the gate capacitance. Although Se appears to degrade when exposed to Vrf, we find that broadening of conduction peaks is largely due to the measurement technique and show that Se is maintained even with strong Vrf present. We show cancellation of a known Vrf signal at 1 MHz, demonstrating that a stable bias point in the presence of rf signals is possible.

Resonance enhancement of two photon absorption by magnetically trapped atoms in strong rf-fields

Applying a many mode Floquet formalism for magnetically trapped atoms interacting with a polychromatic rf-field, we predict a large two photon transition probability in the atomic system of cold 87Rb atoms. The physical origin of this enormous increase in the two photon transition probability is due to the formation of avoided crossings between eigen-energy levels originating from different Floquet sub-manifolds and redistribution of population in the resonant intermediate levels to give rise to the resonance enhancement effect. Other exquisite features of the studied atom-field composite system include the splitting of the generated avoided crossings at the strong field strength limit and a periodic variation of the single and two photon transition probabilities with the mode separation frequency of the polychromatic rf-field. This work can find applications to characterize properties of cold atom clouds in the magnetic traps using rf-spectroscopy techniques.

Gyromagnetic RF source for interdisciplinary research.

We demonstrate a source of high power nanosecond RF pulses based on gyromagnetic nonlinear transmission line. The source is designed to explore the exposure of different biological objects to strong RF fields in an air filled rectangular waveguide loaded onto ethanol RF load. The RF pulse amplitude can be varied by 52 dB, reaching a maximum value of nearly 40 kV/cm and decreasing to tens of V/cm. The RF pulse amplitude is controlled by decreasing the incident pulse amplitude from the high voltage driver. The duration of RF pulses lies in the range from 4 to 25 ns and the frequency from 0.6 to 1.0 GHz.

Superradiant Ka-band Cherenkov oscillator with 2-GW peak power

The generation of a 2-GW microwave superradiance (SR) pulses has been demonstrated at 29-GHz using a single-mode relativistic backward-wave oscillator possessing the beam-to-wave power conversion factor no worse than 100%. A record-breaking radiation power density in the slow-wave structure (SWS) of ∼1.5 GW/cm2 required the use of high guiding magnetic field (7 T) decreasing the beam losses to the SWS in strong rf fields. Despite the field strength at the SWS wall of 2 MV/cm, a single-pass transmission mode of a short SR pulse in the SWS allows one to obtain extremely high power density in subnanosecond time scale due to time delay in the development of the breakdown phenomena.

A toroidal trap for cold atoms using an rf-dressed quadrupole trap

We demonstrate the trapping of cold atoms in a toroidal geometry using a radio frequency (rf) dressed quadrupole magnetic trap formed by superposing a strong rf-field on a quadrupole trap. This rf-dressed quadrupole trap has the minimum potential away from the quadrupole trap centre on a circular path which facilitates trapping in toroidal geometry. In these experiments, the laser cooled atoms were first trapped in a quadrupole trap, then cooled evaporatively using a weak rf-field, and finally trapped in an rf-dressed quadrupole trap. The radius of the toroid could be varied by varying the frequency of the dressing rf-field. It has also been demonstrated that a single rf source and an antenna can be used for the rf-evaporative cooling as well as for the rf-dressing of atoms. The atoms trapped in the toroidal trap may have applications in the realization of an atom gyroscope as well as in studying the quantum gases in low dimensions.

Proton–nitrogen-14 overtone two-dimensional correlation NMR spectroscopy of solid-sample at very fast magic angle sample spinning

1H–14N overtone (OT) heteronuclear multiple quantum coherence (HMQC) experiment at very fast magic angle spinning (MAS) is reported. The 14N OT coherence is excited and reconverted by 14N OT pulses at twice the 14N Larmor frequency. The OT coherence is free from the first order quadrupolar broadening. MAS further removes the broadening due to chemical shift anisotropy (CSA). With a small 0.75mm MAS rotor and coil system, very fast MAS up to 90kHz and very strong rf field are achieved, enhancing the sensitivity of indirect 14N OT observation via protons. In comparison with 1H–14N double-quantum HMQC, an enhancement factor of 1.8 is obtained for glycine with the 14N OT irradiation. The bandwidth in the 14N OT dimension is limited due to long 14N OT pulses.

Effect of impurities on the superheating field of type-II superconductors

We consider the effect of nonmagnetic and magnetic impurities on the superheating field $H_s$ in a type-II superconductor. We solved the Eilenberger equations, which take into account the nonlinear pairbreaking of Meissner screening currents, and calculated $H_s(T)$ for arbitrary temperatures and impurity concentrations in a single-band s-wave superconductor with a large Ginzburg-Landau parameter. At low temperatures nonmagnetic impurities suppress a weak maximum in $H_s(T)$ which has been predicted for the clean limit, resulting instead in a maximum of $H_s$ as a function of impurity concentration in a moderately clean limit. It is shown that nonmagnetic impurities weakly affect $H_s$ even in the dirty limit, while magnetic impurities suppress both $H_s$ and the critical temperature $T_c$. The density of quasiparticles states $N(\epsilon)$ is strongly affected by an interplay of impurity scattering and current pairbreaking. We show that a clean superconductor at $H=H_s$ is in a gapless state, but a quasiparticle gap $\epsilon_g$ in $N(\epsilon)$ at $H=H_s$ appears as the concentration of nonmagnetic impurities increases. As the nonmagnetic scattering rate $\alpha$ increases above $\alpha_c=0.36$, the quasiparticle gap $\epsilon_g(\alpha)$ at $H=H_s$ increases, approaching $\epsilon_g\approx 0.32\Delta_0$ in the dirty limit $\alpha\gg 1$, where $\Delta_0$ is the superconducting gap parameter at zero field. The effects of impurities on $H_s$ can be essential for the nonlinear surface resistance and superconductivity breakdown by strong RF fields.

Very fast magic angle spinning 1H-14N 2D solid-state NMR: Sub-micro-liter sample data collection in a few minutes

Substantial resolution and sensitivity enhancements of solid-state 1H detected 14N HMQC NMR spectra at very fast MAS rates up to 80kHz, in a 1mm MAS rotor, are presented. Very fast MAS enhances the 1H transverse relaxation time and efficiently decouples the 1H-14N interactions, both effects leading to resolution enhancement. The micro-coil contributes to the sensitivity increase via strong 14N rf fields and high sensitivity per unit volume. 1H-14N HMQC 2D spectra of glycine and glycyl-l-alanine at 70kHz MAS at 11.7T are observed in a few minutes with a sample volume of 0.8μL.

Rotational echo double resonance without proton decoupling under fast spinning condition

We show that rotational echo double resonance (REDOR) experiments can be carried out without proton decoupling under the conditions of fast spinning and strong rf field. Numerical simulations on a five-spin systems show that no significant attenuation of the reference signal ( S 0) is observed at a spin rate of 25 kHz, provided that the rf power is larger than 100 kHz. This approach has been validated by 31P{ 13C} REDOR measurements on isotopically labeled glyphosate. The obtained van Vleck’s second moment is in favorable agreement with the value calculated based on the crystal structure.

Canceling evanescent waves in high-energy superconducting rf linacs

Many future projects plan using high-current, high-energy, multipass energy-recovery linacs that are based on superconducting rf (SRF) cavities. The necessity of ensuring the transverse stability of the beam in such accelerators imposes strict limits on the high order modes (HOMs) impedances, and demands effective HOM damping. The latter requirement often precludes achieving a high real-estate accelerating gradient in such structures. The modular structure of long SRF linacs also requires incorporating vacuum flanges; however, these flanges have surface contacts and cannot tolerate strong rf fields. Locating them in the low-field areas of the linac structure may involve considerably elongating the intermodular interfaces, a change that would reduce the linac’s real-estate accelerating gradient. In this paper, I propose a novel method to resolve this issue, using compact interconnects between the SRF cavities wherein to locate effective HOM dampers and vacuum flanges.

Simulations of multipactor thresholds in shielded microstrip lines

A particular software ‘MuSLi’ has been developed and applied for simulations of the multipactor effect in shielded microstrip lines with cross-sections that are partially filled with dielectric material. The software combines an electromagnetic field solver, determining the electric field structure in the microstrip line, and a Monte Carlo algorithm, calculating the corresponding electron trajectories taking into account a spread of the electron initial velocity and different secondary emission properties of the metal and dielectric surfaces. The simulations were carried out for a number of structures of interest for communication systems, currently being discussed for application in coming space missions. The microstrip line system is shown to be strongly resistant to multipactor growth and the main reason for this is identified as the ponderomotive (or Miller) force, which governs the average electron motion in non-uniform rf fields and which tends to push the electrons out of regions with strong rf fields.

Electrooptic properties of aqueous suspensions of nanotubes based on magnesium hydrosilicate

Nanotubes suspended in a fluid medium have been investigated by the electrooptic method. This method is based on measuring the relative changes in the intensity of light transmitted through a suspension of nanotubes with application of an external electric field. The electrooptic effect (difference in these changes for transmitted light plane-polarized along the external field and perpendicularly to it) is analyzed. For aqueous suspensions of Mg3Si2O5(OH)4 nanotubes, the field and relaxation dependences of this effect are investigated. An analysis of the results obtained suggests that nanotubes, even when their concentration in water is low, form strong stable aggregates, which, however, can be elongated and enlarged under long-term action of strong rf fields.

Dynamics of vortex penetration, jumpwise instabilities, and nonlinear surface resistance of type-II superconductors in strong rf fields

We consider the nonlinear dynamics of a single vortex in a superconductor in a strong rf magnetic field ${B}_{0}\phantom{\rule{0.2em}{0ex}}\mathrm{sin}\phantom{\rule{0.2em}{0ex}}\ensuremath{\omega}t$. Using the London theory, we calculate the dissipated power $Q({B}_{0},\ensuremath{\omega})$ and the transient time scales of vortex motion. For the linear Bardeen-Stephen viscous drag force, vortex velocities reach unphysically high values during vortex penetration through the oscillating surface barrier. It is shown that penetration of a single vortex through the ac surface barrier always involves penetration of an antivortex and the subsequent annihilation of the vortex-antivortex pairs. Using the nonlinear Larkin-Ovchinnikov (LO) viscous drag force at higher vortex velocities $v(t)$ results in a jumpwise vortex penetration through the surface barrier and a significant increase of the dissipated power. We calculate the effect of dissipation on the nonlinear vortex viscosity $\ensuremath{\eta}(v)$ and the rf vortex dynamics and show that it can also result in the LO-type behavior, instabilities, and thermal localization of penetrating vortex channels. We propose a thermal feedback model of $\ensuremath{\eta}(v)$, which not only results in the LO dependence of $\ensuremath{\eta}(v)$ for a steady-state motion, but also takes into account retardation of the temperature field around a rapidly accelerating vortex and a long-range interaction with the surface. We also address the effect of pinning on the nonlinear rf vortex dynamics and the effect of trapped magnetic flux on the surface resistance ${R}_{s}$ calculated as a function of rf frequency and field. It is shown that trapped flux can result in a temperature-independent residual resistance ${R}_{i}$ at low $T$ and a hysteretic low-field dependence of ${R}_{i}({B}_{0})$, which can decrease as ${B}_{0}$ is increased, reaching a minimum at ${B}_{0}$ much smaller than the thermodynamic critical field ${B}_{c}$. We propose that cycling of the rf field can reduce ${R}_{i}$ due to rf annealing of the magnetic flux which is pumped out by the rf field from a thin surface layer of the order of the London penetration depth.

超伝導クラブ空洞

Complete overlapping of the electron and positron bunches at colliding point can be attained by using the strong time-depending electromagnetic RF field in the superconducting crab cavity. Commissioning of the crab cavities started in February 2007 at KEKB. Effective head-on collision of electron and positron has been achieved successfully. After introduction of crab crossing and crab cavity, the structure and the fabrication of the KEKB superconducting crab cavity are discussed.

A large volume flat coil probe for oriented membrane proteins

15N detection of mechanically aligned membrane proteins benefits from large sample volumes that compensate for the low sensitivity of the observe nuclei, dilute sample preparation, and for the poor filling factor arising from the presence of alignment plates. Use of larger multi-tuned solenoids, however, is limited by wavelength effects that lead to inhomogeneous RF fields across the sample, complicating cross-polarization experiments. We describe a 600MHz 15N–1H solid-state NMR probe with large (580mm3) RF solenoid for high-power, multi-pulse sequence experiments, such as polarization inversion spin exchange at the magic angle (PISEMA). In order to provide efficient detection for 15N, a 4-turn solenoidal sample coil is used that exceeds 0.27λ at the 600MHz 1H resonance. A balanced tuning-matching circuit is employed to preserve RF homogeneity across the sample for adequate magnetization transfer from 1H to 15N. We describe a procedure for optimization of the shorted 1/4λ coaxial trap that allows for the sufficiently strong RF fields in both 1H and 15N channels to be achieved within the power limits of 300W 1H and 1kW 15N amplifiers. The 8×6×12mm solenoid sustains simultaneous B1 irradiation of 100kHz at 1H frequency and 51kHz at 15N frequency for at least 5ms with 265 and 700W of input power in the respective channels. The probe functionality is demonstrated by 2D 15N–1H PISEMA spectroscopy for two applications at 600MHz.

A new approach in 1D and 2D 13C high-resolution solid-state NMR spectroscopy of paramagnetic organometallic complexes by very fast magic-angle spinning.

Novel 1D and multidimensional solid-state NMR (SSNMR) methods using very fast magic-angle spinning (VFMAS) (spinning speed > 20 kHz) for performing 13C high-resolution SSNMR of paramagnetic organometallic complexes are discussed. VFMAS removes a majority of 13C-1H and 1H-1H dipolar couplings, which are often difficult to remove by RF pulse techniques in paramagnetic complexes because of large paramagnetic shifts. In the first systematic approach using the unique feature of VFMAS for paramagnetic complexes, we demonstrate a means of obtaining well-resolved 1D and multidimensional 13C SSNMR spectra, sensitivity enhancements via cross polarization, and signal assignments, and applications of dipolar recoupling methods for nonlabeled paramagnetic organometallic complexes of moderate paramagnetic shifts ( approximately 800 ppm). Experimental results for powder samples of small nonlabeled coordination complexes at 1H frequencies of 400.2-400.3 MHz show that highly resolved 13C SSNMR spectra can be obtained under VFMAS, without requirements of 1H decoupling. Sensitivity enhancement in 13C SSNMR via cross polarization from 1H spins was demonstrated with an amplitude-sweep high-power CP sequence using strong RF fields ( approximately 100 kHz) available in the VFMAS probe. 13C CPMAS spectra of nonlabeled Cu(II)(dl-alanine)2.(H2O) and V(III)(acetylacetonate)3 (V(acac)3) show that it is possible to obtain high-resolution spectra for a small quantity ( approximately 15 mg) of nonlabeled paramagnetic organometal complexes within a few minutes under VFMAS. Experiments on Cu(II)(dl-alanine)2.(H2O) demonstrated that 1H-13C dipolar recoupling for paramagnetic organometal complexes can be performed under VFMAS by application of rotor-synchronous pi-pulses to 1H and 13C spins. The results also showed that signal assignments for 13CH, 13CH3, and 13CO groups in paramagnetic complexes are possible on the basis of the amount of 13C-1H dipolar dephasing induced by dipolar recoupling. Furthermore, the experimental 2D 13C/1H chemical-shift correlation NMR spectrum obtained for nonlabeled V(acac)3 exhibits well-resolved lines, which overlap in 1D 13C and 1H spectra. Signals for different chemical groups in the 2D spectrum are distinguished by the 13C-1H dipolar dephasing method combined with the 2D 13C/1H correlation NMR. The assignments offer information on the existence of nonequivalent ligands in the coordination complex in solids, without requiring a single-crystal sample.

Radiofrequency sustained double layer in a plasma reactor

This article presents experimental evidence of a coupling of power between the rf supply and a collisionless plasma in a capacitively coupled plasma reactor. The reactor is driven in plasma sheath resonance to let high rf currents circulate in the system. These currents allow the formation of strong rf fields inside the nonuniform plasma leading to a double layer with a mechanism similar to the rf plasma sheath at the electrodes. Electrons accelerated by the double layer are very effective in ionizing, because of the increased cross section, and create a dense plasma at low pressures. The power is consequently transferred by the ions accelerated in the electrode sheath to the substrate to be processed with high improvement in both performance and process rate. Diagnostics include a retarding field analyzer on the grounded electrode to measure the energy spectrum of ions hitting the substrate and a capacitive probe to obtain the rf potential variations in the plasma.