Narrow Field Range Research Articles

Use of High-Voltage Cyclic Voltammetry to Characterize Bulk and Interfacial Conduction Processes in Anodic Alumina Films

A high-voltage cyclic voltammetry method is introduced that enables separate determination both the bulk conduction kinetics of anodic films and kinetics associated with interface processes. The results confirm that ionic conduction in the oxide follows a high-field exponential dependence on the electric field with a field coefficient of approximately 33nm/V, consistent with reported values for barrier-type films formed in neutral solutions. The correlation of the pre-exponential current density with the incorporation level of different acid anions from the anodizing bath suggests that conduction is enhanced by anionic contaminants. Current densities associated with self-ordered pore formation in different solutions seem to be associated with a common narrow range of electric fields. The large size of the interface potential drop suggests that it derives from the elevated conduction resistance of an anion-contaminated oxide layer close to the solution interface. Such resistive surface layers are thought to be the source of compressive stress that drives oxide flow during self-organized pore formation.

Sign change of the vortex Hall effect in superconducting YBCO thin films with a square pattern of ion-irradiated defect columns

The Hall effect in the mixed state of thin films of the high-temperature superconductor YBa2Cu3O7−δ (YBCO) that were patterned with a square array of regions with suppressed superconducting order parameter is investigated. Cylindrical defect columns penetrating the entire thin YBCO film along its crystallographic c−axis have been created by irradiation with He+ ions through a silicon stencil mask. Distinct peaks of the critical current at commensurate arrangements of magnetic flux quanta with the artificial defect lattice confirm enhanced vortex pinning. Vortex motion not only leads to a dissipative voltage along the current direction but also to a transverse voltage, termed vortex Hall effect. We report on the observation of a novel commensurability effect in the transverse Hall signal. A sign change and a positive peak of the Hall coefficient appear in a narrow magnetic field range around the matching field. The feature appears in the temperature range below the critical temperature, where the Hall effect usually is negative in underdoped and optimally-doped cuprate superconductors. The results indicate that the Hall matching effect originates from enhanced pinning of the vortices along the regular defect columns.

North Carolina Women: Their Lives and Times, Vol. 2, ed. by Michele Gillespie and Sally G. McMillen

Carolina Women: Their Lives and Times. Volume 2. Edited by Michele Gillespie and Sally G. McMillen. Southern Women: Their Lives and Times. (Athens, Ga., and London: University of Georgia Press, 2015. Pp. [x], 414. Paper, $26.95, ISBN 978-0-8203-4005-5; cloth, $79.95, ISBN 978-0-8203 3999-3.) The authors of this collection of essays stretch the conventional boundaries of biography to narrate the events, actions, and values associated with selected whose lives intersected with Carolina history from the late nineteenth century through the late twentieth century. Rather than fixing the lives of to the usual landmark moment of southern history (the Civil War), the editors begin this second volume of essays with the Progressive era and the political reform movements that impacted the daily lives of American women. As Charlotte A. Haller also pointed out in her review of Volume 1 in the August 2015 issue of this journal, the conventional edge of biography toward accomplished, influential women shapes the very selection of whose stories get told (p. 1). While the editors achieve diversity in socioeconomic and racial backgrounds with this second volume, the accomplishments of the profiled fall within a narrow range of fields. Most fit within the bounds of education, politics, civil rights, and organized labor. The editors, Michele Gillespie and Sally G. McMillen, succeed in weaving a larger story of in the South through the lives of these selected individuals (p. 3). Perhaps the strongest essay, North Carolina's Farm Women: Plowing around Obstacles, by Lu Ann Jones, is also the one that strays furthest from the confines of conventional biography. Jones opens her story with the accomplishments of Edna Anderson Harris, who made her way into the public eye by organizing a local chapter of the United Farmers Organization. But hers is not a typical story of achievement. Instead, Jones tells Harris's story from the perspective of the survival skills honed by rural just to make it through daily life on the farm (p. 215). While Harris's story is accessible through the pages of the Raleigh Times, Jones draws from extensive oral history fieldwork completed in the 1980s to document the lives of other rural who came of age in the 1920s and 1930s (p. 218). Jones also captures the vivid details of Nellie Stancil Langley's everyday life and pays careful attention to the meaning of gender in Bessie Smith Pender's childhood memories of life on a tenant farm growing cotton and peanuts. And, using stories of women's chickens and eggs, Jones provides insights into exactly how products of women's labor entered the channels of commerce (p. 222). Only at the end of this collective biography does Jones turn to the stories of like Jane Simpson McKimmon, whose work with rural as Carolina's first state home agent associated with agricultural extension programs can be traced through government records. …

Operation of a Six-Cavity <inline-formula> <tex-math notation="LaTeX">$S$ </tex-math></inline-formula>-Band Relativistic Magnetron at Frequencies in the Range of Its Resonant Response

Experimental research on the influence of the frequency characteristic of the anode block on the parameters of relativistic $S$ -band magnetron operation is described. A magnetron with a six-vane anode block was powered by a linear induction accelerator (350 kV, 2.5 kA, and 150 ns). It was found that the shift in the resonance frequency of the magnetron anode block cavity into the range of the frequencies obtained during the magnetron operation is affected by the change in the design of the magnetron output and leads to highly efficient ( $\eta \ge 40\% )$ microwave generation in a relatively narrow magnetic field range. This efficiency remains high during the entire duration ( $\tau \approx 100~\textrm {ns})$ of the microwave pulse, and the decrease in the electric field anode–cathode gap during the fall time of the applied voltage does not cause a drift in the frequency of the generated microwaves.

Transformation of the physical properties of PbIn1/2Nb1/2O3–PbMg1/3Nb2/3O3–PbTiO3 crystals in an electric field

The influence of the composition and different modes of applying a direct-current (dc) electric field (0 < E < 4 kV/cm) on the behavior of the temperature dependences of the dielectric permittivity, optical transmission, as well as the velocity and attenuation of sound, has been investigated in the [001]-oriented PbIn1/2Nb1/2O3–PbMg1/3Nb2/3O3–PbTiO3 single crystals located both far from the morphotropic phase boundary and in its vicinity. It has been found that, in the phase transition region in a narrow field range for any mode of applying an electric field, all the crystals under investigation are characterized by abrupt changes in their physical properties. It has been shown that the phase transition rate and the attenuation of sound are different in the crystals located at different distances from the morphotropic phase boundary. The E–T phase diagrams have been constructed for different modes of applying an electric field. It has been demonstrated that the phase diagrams constructed for the same crystal but different modes of applying an electric field differ from each other. Possible reasons for the observed phenomena have been discussed.

Stochastic jumps of magnetization in [Mn{(R/S)-pn}]2[Mn{(R/S)-pn}2(H2O)][Cr(CN)6]2 molecular magnet

Series of stochastic jumps of the magnetic moment (up to five individual jumps) have been observed at the demagnetization of single crystals of [Mn{(R/S)-pn}]2[Mn{(R/S)-pn}2(H2O)][Cr(CN)6]2 molecular magnet in a narrow range of magnetic fields near the coercive force (Hc = 7.5 Oe). The magnetic field at which jumps of the magnetization arise decreases with an increase in the temperature.

Broadscale resolving power performance of a high precision uniform field ion mobility-mass spectrometer.

An extensive study of two current ion mobility resolving power theories ("conditional" and "semi-empirical") was undertaken using a recently developed drift tube ion mobility-mass spectrometer. The current study investigates the quantitative agreement between experiment and theory at reduced pressure (4 Torr) for a wide range of initial ion gate widths (100 to 500 μs), and ion mobility values (K0 from 0.50 to 3.0 cm(2) V(-1) s(-1)) representing measurements obtained in helium, nitrogen, and carbon dioxide drift gas. Results suggest that the conditional resolving power theory deviates from experimental results for low mobility ions (e.g., high mass analytes) and for initial ion gate widths beyond 200 μs. A semi-empirical resolving power theory provided close-correlation of predicted resolving powers to experimental results across the full range of mobilities and gate widths investigated. Interpreting the results from the semi-empirical theory, the performance of the current instrumentation was found to be highly linear for a wide range of analytes, with optimal resolving powers being accessible for a narrow range of drift fields between 14 and 17 V cm(-1). While developed using singly-charged ion mobility data, preliminary results suggest that the semi-empirical theory has broader applicability to higher-charge state systems.

Search for a spin-nematic phase in the quasi-one-dimensional frustrated magnetLiCuVO4

We have performed NMR experiments on the quasi one-dimensional frustrated spin-1/2 system LiCuVO$_4$ in magnetic fields $H$ applied along the c-axis up to field values near the saturation field $H_{\rm sat}$. For the field range $H_{\rm c2}<H<H_{\rm c3}$ ($\mu_0H_{\rm c2}\approx 7.5$T and $\mu_0H_{\rm c3} = [40.5 \pm 0.2]$T) the $^{51}$V NMR spectra at $T$ = 380mK exhibit a characteristic double-horn pattern, as expected for a spin-modulated phase in which the magnetic moments of Cu$^{2+}$ ions are aligned parallel to the applied field $H$ and their magnitudes change sinusoidally along the magnetic chains. For higher fields, the $^{51}$V NMR spectral shape changes from the double-horn pattern into a single Lorentzian line. For this Lorentzian line, the internal field at the $^{51}$V nuclei stays constant for $\mu_0 H > 41.4$T, indicating that the majority of magnetic moments in LiCuVO$_4$ are already saturated in this field range. This result is inconsistent with the previously observed linear field dependence of the magnetization $M(H)$ for $H_{\rm c3}<H<H_{\rm sat}$ with $\mu_0H_{\rm sat}=45$T [L. E. Svistov {\it et al}., JETP Letters {\bf 93}, 21 (2011)]. We argue that the discrepancy is due to non-magnetic defects in the samples. The results of the spin-lattice relaxation rate of $^7$Li nuclei indicate an energy gap which grows with field twice as fast as the Zeeman energy of a single spin, therefore, suggesting that the two-magnon bound state is the lowest energy excitation. The energy gap tends to close at $\mu_0H \approx 41$T. Our results suggest that the theoretically predicted spin-nematic phase, if it exists in LiCuVO$_4$, can be established only within the narrow field range $40.5 < \mu_0 H < 41.4$T .

Optical and acoustic properties of 33PbIn1/2Nb1/2O3-35PbMg1/3Nb2/3O3-32PbTiO3 single crystals in an electric field

The influence of different conditions of applying a dc electric field (0 < E < 4 kV/cm) on the behavior of optical transmission and on the acoustic parameters (sound velocity and attenuation) in the [001]- and [011]-oriented single crystals 33Pb(In1/2Nb1/2)O3-35PbMg1/3Nb2/3O3-32PbTiO3 existing near the morphotropic phase boundary has been investigated. It has been found that the optical transmission, sound velocity, and attenuation sharply change near the phase transition in a narrow range of electric fields for any method of their applying. In the field applied along [001], the change in the sound velocity due to the phase transition is 1.5 times greater than that in the field applied along [011]. This is caused by a larger contribution of the piezoelectric effect to the elastic modulus determining the sound velocity along [001] as compared to [011]. It has been shown that the number, symmetry, and stability of the phases formed in the field depend on the conditions of applying the field.

Tunable rotons in square-lattice antiferromagnets under strong magnetic fields

Excitation spectra of square lattice Heisenberg antiferromagnets in magnetic fields are investigated by the spin-wave theory. It is pointed out that a rotonlike structure appears in a narrow range of magnetic fields as a result of strong nonlinear effects. It is shown that the energy gap and the mass of the ``roton'' are quite sensitive to the magnetic field: the roton gap softens rapidly and eventually closes as a precursor of a quantum phase transition. The possibility of the experimental observation of the roton and a new ground state after its softening are discussed.

Highly sensitive magnetometer based on the off‐diagonal GMI effect in Co‐rich glass‐coated microwire

The design and performance of a magnetometer based on the off‐diagonal GMI effect in Co‐rich glass‐coated microwire are presented. The sensing element of the magnetometer is a 10‐mm long piece of Co–Fe–Ni–B–Si–Mo microwire with a small pick‐up coil of 85 turns wounded around the microwire. The electronics with a feedback circuit is used to register an electromotive force proportional to the external magnetic field applied along the wire axis. In the absence of the feedback current the magnetometer is capable of measuring a narrow range of magnetic fields, ±3.5 μT, in the frequency range of 0–1 kHz, the level of the equivalent magnetic noise being about 10 pT Hz−1/2 at a frequency of 300 Hz. The use of the feedback circuit increases the range of the measured magnetic fields up to ±250 μT. Photo of the giant magnetoimpedance magnetometer based on the off‐diagonal GMI effect in Co‐rich glass‐coated microwire.

Stark effect of optical properties of excitons in a quantum nanorod with parabolic confinement

We study the quantum Stark effect of optical properties of a quasi-one-dimensional quantum rod with parabolic confinement. Interplays between the competing/cooperative forces from confinement, electron–hole (e–h) attraction, and an external field are examined by studying the binding energy, the oscillator strength, and the root-mean-square (RMS) average of the e–h separation in a nonlinear electric field. In a long rod with weak confinement, the e–h interaction dominates over the confinement effect, yielding an abrupt drop of the exciton binding energy, oscillator strength, and a sudden increase of the RMS average e–h separation as the excitons are dissociated at the threshold field as the field increases. The exciton-dissociation transition is gradual in a short rod, where the confinement force dominates over the e–h attraction. We show that a DC field can induce an optically active excited exciton state in a narrow field range, causing a sharp peak in the oscillator strength and a dip in the RMS average of the e–h separation as the field increases. The Stark effects are also investigated as a function of the linear confinement length (i.e., rod length) at fixed fields.

Flux pinning behaviors of a textured Gd–Ba–Cu–O superconductor mediated by the addition of BaTiO3, TiO2 and BaO2

We present a study on the investigation of the proper secondary phases for engineering magnetic flux pinning centers in textured Gd–Ba–Cu–O bulk superconductors. Compared with the joint TiO2 and BaO2 doping methods, the addition of BaTiO3 shows a relevant advantage due to its perovskite structure. Both TiO2 and BaO2 doping tends to enter the crystal lattice of the superconducting matrix by ionic substitution, which leads to the variable peak effect in the Jc–B diagram. The BaTiO3 doping method shows a unique pinning behavior, which is attributed to the distinctive precipitations generated by the additions. The composition of the BaTiO3 doping-related precipitation appears to be controlled under different growth temperatures. The self-field Jc improved by nearly double at 77K in a sample grown with a low growth temperature. In addition, the second peaks caused by various types of additions are located in a narrow field range of 1.25–1.5T in the Jc–B diagram, indicating that the dilute doping follows a unique tendency, which is likely relevant to anisotropic crystal growth. The different observed peak heights suggest the existence of several average pinning potentials provided by various types of additions. BaTiO3 doping exhibited a stabilization of the unique onset superconducting transition temperature. Therefore, BaTiO3 can be classified as a useful dopant to control flux pinning behavior in Gd–Ba–Cu–O bulk superconductors.

Singlet-triplet crossover in the two-dimensional dimer spin system YbAl3C3

Low-temperature magnetization (M) measurements down to 0.1 K have been performed in magnetic fields up to 14.5 T for a single piece of a tiny single-crystalline sample (0.2 mg weight) of the spin-gap system YbAl3C3. At the base temperature of 0.1 K, several metamagnetic transitions were clearly observed for H // c in the range 6 T < H < 9 T whereas only two transitions were observed, one at 4.8 T and the other at 6.6 T, for H // a. At fields above 9 T, the magnetization becomes almost saturated for both H // a and H // c. The present results indicate that a singlet-triplet crossover occurs in a relatively narrow field range, suggesting a rather weak interdimer interaction in spite of the nearly triangular lattice of Yb ions.

Creating an Artificial Two-Dimensional Skyrmion Crystal by Nanopatterning

A Skyrmion crystal typically arises from helical spin structures induced by the Dzyaloshinskii-Moriya interaction. Experimentally its physical exploration has been impeded because it is a rarity and is found only within a narrow temperature and magnetic field range. We present a method for the assembly of a two-dimensional Skyrmion crystal based upon a combination of a perpendicularly magnetized film and nanopatterned arrays of magnetic vortices that are geometrically confined within nanodisks. The practical feasibility of the method is validated by micromagnetic simulations and computed Skyrmion number per unit cell. We also quantify a wide range in temperature and field strength over which the Skyrmion crystal can be stabilized without the need for any intrinsic Dzyaloshinskii-Moriya interactions, which otherwise is needed to underpin the arrangement as is the case in the very few known Skyrmion crystal cases. Thus, our suggested scheme involves a qualitative breakthrough that comes with a substantial quantitative advance.

Magnetization reversal in a preferred oriented (111) L10 FePt grown on a soft magnetic metallic glass for tilted magnetic recording

L10 FePt is an important material for the fabrication of high density perpendicular recording media, but the ultrahigh coercivity of L10 FePt restricts its use. Tilting of the magnetic easy axis and the introduction of a soft magnetic underlayer can solve this problem. However, high temperature processing and the requirement of epitaxial growth conditions for obtaining an L10 FePt phase are the main hurdles to be overcome. Here, we introduce a bilayered magnetic structure ((111) L10 FePt/glassy Fe71Nb4Hf3Y2B20/SiO2/Si) in which the magnetic easy axis of L10 FePt is tilted by ∼36° from the film plane and epitaxial growth conditions are not required. The soft magnetic underlayer not only promotes the growth of L10 FePt with the preferred orientation but also provides an easy cost-effective micro/nanopatterning of recording bits. A detailed magnetic characterization of the bilayered structure in which the thickness of (111) L10 FePt with the soft magnetic Fe71Nb4Hf3Y2B20 glassy underlayer varied from 5 to 60 nm is carried out in an effort to understand the magnetization switching mechanism. The magnetization switching behavior is almost the same for bilayered structures in which FePt layer thickness is >10 nm (greater than the domain wall thickness of FePt). For FePt film ∼10 nm thick, magnetization reversal takes place in a very narrow field range. Magnetization reversal first takes place in the soft magnetic underlayer. On further increase in the reverse magnetic field, the domain wall in the soft magnetic layer compresses at the interface of the hard and soft layers. Once the domain wall energy becomes sufficiently large to overcome the nucleation energy of the domain wall in L10 FePt, the magnetization of the whole bilayer is reversed. This process takes place quickly because the domain walls in the hard layer do not need to move, and the formation of a narrower domain wall may not be favorable energetically. Our results showed that the present bilayered structure is very promising for the fabrication of tilted bit-patterned magnetic recording media.

A study of sub-terahertz and terahertz gyrotron oscillators

We present a theoretical study on some of the key physics issues associated with over-moded, sub-terahertz, and terahertz (THz) gyrotron oscillators. Simulations of a large number of fundamental and second cyclotron harmonic modes yield a broad database for physics observations as well as a test of relevant scaling laws. Linear properties over a broad magnetic field range exhibit a number of interesting trends. Nonlinearly, despite the possibility of multimode excitation, each mode is found to exist as a dominant single mode in a narrow magnetic field range, and a significant fraction of these modes are due to second cyclotron harmonic interactions. The wall resistivity, while a relatively minor concern for sub-terahertz gyrotrons, is shown to play a radically different role in the THz regime. It affords a linear advantage to the harmonic modes while also significantly degrading their output efficiencies. These results are interpreted in terms of the nature of harmonic mode competition and the scaling laws for the cavity quality factors.

Thermodynamics of phase formation in the quantum critical metal Sr 3 Ru 2 O 7

The behavior of matter near zero temperature continuous phase transitions, or "quantum critical points" is a central topic of study in condensed matter physics. In fermionic systems, fundamental questions remain unanswered: the nature of the quantum critical regime is unclear because of the apparent breakdown of the concept of the quasiparticle, a cornerstone of existing theories of strongly interacting metals. Even less is known experimentally about the formation of ordered phases from such a quantum critical "soup." Here, we report a study of the specific heat across the phase diagram of the model system Sr(3)Ru(2)O(7), which features an anomalous phase whose transport properties are consistent with those of an electronic nematic. We show that this phase, which exists at low temperatures in a narrow range of magnetic fields, forms directly from a quantum critical state, and contains more entropy than mean-field calculations predict. Our results suggest that this extra entropy is due to remnant degrees of freedom from the highly entropic state above T(c). The associated quantum critical point, which is "concealed" by the nematic phase, separates two Fermi liquids, neither of which has an identifiable spontaneously broken symmetry, but which likely differ in the topology of their Fermi surfaces.

Spin-polarized multiexcitons in quantum dots in the presence of spin-orbit interaction

An efficient electron spin-relaxation mechanism has been observed in InAs quantum dots (QDs) that manifests itself as a sharp drop in the circular polarization of the light emitted by Fe spin-light emitting diodes, which incorporate a single layer of InGaAs QDs, for a narrow range of magnetic fields around 5 T. The underlying mechanism occurs when the QDs are occupied by three-electron-hole pairs forming a tri-exciton (3X) and is a two-step process. The first step involves the spin flip of one of the three electrons mediated by the spin-orbit interaction; in the second step the 3X relaxes to its ground state via phonon emission.

Characterization of the ion beam formed in a low magnetic field helicon mode

An experimental characterization of the ion beam formed during a low magnetic field (<3 mT) helicon mode has been performed using a number of retarding field energy analysers, for pressures between 0.03 Pa < p0 < 0.4 Pa in argon. The low-field helicon mode is formed over a narrow magnetic field range (1 mT < B0 < 3 mT) using a single solenoid giving a strong diverging field, and is characterized by a distinct density peak. For pressures below about 0.3 Pa, an ion beam is observed in the downstream region of the plasma reactor, and its presence is directly correlated with that of the low-field mode, suggesting that the ion beam formation is induced by the appearance of the helicon mode. For the same pressure and rf power input, as the magnetic field is further increased, a second ion beam regime is observed for magnetic fields above 5 mT. In this second regime the plasma density is less than half that for the low-field mode, and the system seems most likely to be inductively coupled. A simple analytical model is developed to describe the general features and behaviour of the observed ion energy distribution functions as a function of the plasma potential profile and neutral gas pressure.