Magnetic Field Regime Research Articles

Coherent control of an NV− center with one adjacent 13C

We investigate the theoretically achievable fidelities when coherently controlling an effective three-qubit system consisting of a negatively charged nitrogen vacancy (15NV−) center in diamond with an additional nearby carbon 13C spin via square and two frequency component radio and microwave frequency pulses in different magnetic field regimes. Such a system has potentially interesting applications in quantum information-related tasks such as distributed quantum computation or quantum repeater schemes. We find that the best fidelities can be achieved in an intermediate magnetic field regime. However, with only square pulses it will be challenging to reach the fidelity threshold(s) predicted by current models of fault-tolerant quantum computing.

Influence of composition and heat treatment on damping and magnetostrictive properties of Fe–18%(Ga + Al) alloys

The structure, magnetostriction and damping properties of Fe82Ga(18−x)Alx (x=0, 5, 8, 12) alloys were analyzed. The anelastic response of Fe–18(Ga+Al) alloys was studied as a function of temperature (from 0 to 600°C), frequency (from 0.01 to 200Hz) and amplitude (from 0.0004% to 0.2%) of forced vibrations. The origin of the relatively high damping capacity of Fe–Ga–Al alloy at room temperature was determined by applying a magnetic field and different heat treatment regimes. The substitution of Ga by Al in Fe–18% Ga alloys was found to decrease magnetostriction and damping. The heat treatment of alloys influences the damping capacity of alloys more than variations of their chemical compositions. Thermally activated frequency and temperature-dependent anelastic effects in Fe–Ga–Al alloys were analyzed and the corresponding activation parameters for relaxation processes were evaluated. Internal friction effects caused by structural transformations were recorded and were found to be consistent with the A2→D03→L12 reaction. The physical mechanisms for all anelastic effects are discussed.

Interaction mechanisms between dust grains in the presence of asymmetric ion flow and an external magnetic field in complex plasma

We have reported a theoretical study on the interaction mechanism between dust particles in the presence of asymmetric ion flow and an external magnetic field in complex plasma. The recent experimental and numerical results on the particle-wake interaction ensures the dominance of the wake effect in the subsonic regime of plasma flow using the cold ion approximation. The recent developments in dusty plasma research and its growing interest towards more realistic magnetized dusty plasma scenarios also demand serious attention to study the wake effect both in the sub and supersonic regimes in the presence of a magnetic field. It is a challenging task to develop a correct, quantitative theory of wake potential for different regimes of magnetic field and ion flow velocity. Analytic expressions for the wake potential have been reported in this paper for both subsonic and supersonic regimes in the presence of an external magnetic field along with Debye-Huckel type potentials. The results show that the wake potential plays a dominant role in the subsonic regime and its strength increases with an increase in magnetic field. The behaviour of the wake potential is found to have an interesting effect on the Coulomb crystallization of dust grains and is studied with the help of molecular dynamic (MD) simulation.

Nonlinear magnetotransport theory and Hall induced resistance oscillations in graphene

The quantum oscillations of nonlinear magnetoresistance in graphene that occur in response to a dc current bias are investigated. We present a theoretical model for the nonlinear magnetotransport of graphene carriers. The model is based on the exact solution of the effective Dirac equation in crossed electric and magnetic fields, while the effects of randomly distributed impurities are perturbatively added. To compute the nonlinear current effects, we develop a covariant formulation of the migration center theory. The current is calculated for short- and large-range scatterers. The analysis of the differential resistivity in the large magnetic field region, shows that the extrema of the Shubnikov de Hass oscillations invert when the dc currents exceed a threshold value. These results are in good agreement with experimental observations. In the small magnetic field regime, corresponding to large filling factors, the existence of Hall induced resistance oscillations are predicted for ultra clean graphene samples. These oscillations originate from Landau–Zener tunneling between Landau levels, that are tilted by the strong electric Hall field.

Non-monotonic magnetoresistance in an AlGaN/GaN high-electron-mobility transistor structure in the ballistic region

In this report, we will discuss the nonmonotonic magnetoresistance (MR) in an AlGaN/GaN high-electron-mobility transistor (HEMT) in a perpendicular magnetic field B in the ballistic region (kBTτ/ħ < 1) and in the weakly-disordered limit (kFl = 159 ≫ 1), where kB, T, τ, ħ, k F, and l represent the Boltzmann constant, temperature, elastic scattering time, reduced Planck constant, Fermi wave vector and mean free path, respectively. The MR shows a local maximum between the weak localization (WL) and the Shubnikov-de Haas regions. In the low magnetic field regime, the quantum correction to the conductivity is proportional to T −3/2, which is consistent with a recent theory [T. A. Sedrakyan, and M. E. Raikh, Phys. Rev. Lett. 100, 106806 (2008)]. According to our results, as the temperature is increased, the position of the MR maximum in B increases. These results cannot be explained by present theories. Moreover, in the high-magnetic-field regime, neither the magnetic and nor the temperature dependences of the observed MR is consistent with present theories. We, therefore, suggest that while some features of the observed nonmonotonic MR can be successfully explained, further experimental and theoretical studies are necessary to obtain a thorough understanding of the MR effects.

Feshbach resonance in a synthetic non-Abelian gauge field

We study the Feshbach resonance of spin-1/2 particles in the presence of a uniform synthetic non-Abelian gauge field that produces spin orbit coupling along with constant spin potentials. We develop a renormalizable quantum field theory that includes the closed channel boson which engenders the Feshbach resonance, in the presence of the gauge field. By a study of the scattering of two particles in the presence of the gauge field, we show that the Feshbach magnetic field, where the apparent low energy scattering length diverges, depends on the conserved centre of mass momentum of the two particles. For high symmetry gauge fields, such as the one which produces an isotropic Rashba spin orbit coupling, we show that the system supports two bound states over a regime of magnetic fields for a negative background scattering length and resonance width comparable to the energy scale of the spin orbit coupling. We discuss the consequences of these findings for the many body setting, and point out that a broad resonance (width larger than spin orbit coupling energy scale) is most favourable for the realization of the rashbon condensate.

Dynamic behavior of magnetoelectric coupling of CuFeO2 induced by a high magnetic field

Magnetoelectric effects and their dynamic behavior in a CuFeO2 single crystal grown by the floating zone technique are investigated at low temperature in pulsed high magnetic fields. Experimental results show that the magnetization, dielectric polarization, and magnetoelectric coupling are anisotropic. In the magnetization, a field-induced multi-step-like transition with hysteresis is observed, in which the critical magnetic fields are independent of the field sweep rate. In the dielectric polarization, the field-induced dielectric polarization with hysteresis (or partial irreversible) is only observed in the incommensurate-noncollinear phase. In particular, no obvious spontaneous dielectric polarization is observed in the lower magnetic field regime. Using a pulsed high magnetic field with various magnetic field strength and field sweep rates, the coupling behavior between the magnetic moment and the dielectric polarization is investigated. Experimental results show that the dielectric polarization is weakly related to the field sweep rate. In the field-decreasing branch, the magnetic field is higher, the magnetic field region of the dielectric polarization remains is lower, indicating that the pulsed high magnetic field has an effect on the enhancement of the spontaneous dielectric polarization.

Magnetoresistive conductive polymer-tungsten trioxide nanocomposites with ultrahigh sensitivity at low magnetic field

The effect of conductive polymer matrix including polyaniline (PANI) and polypyrrole (PPy) on the magnetoresistance (MR) behaviors in the variable range hopping (VRH) regime has been investigated in the disordered polymer nanocomposites containing tungsten trioxide (WO3) nanoparticles. These nanocomposites have demonstrated ultrahigh MR sensitivity at low magnetic field regime. The observed positive MR has been well explained by the wave-function shrinkage model. The conductive polymer matrix has shown different effects on the MR behaviors of the nanocomposites. The WO3/PANI nanocomposites have a lower localization length (a0) and density of states at the Fermi level (N(EF)), and higher average hopping distance (Rhop) and average hopping energy (W) compared with those of the WO3/PPy nanocomposites.

Magnetic field-tuned localization of the5f-electrons in URu2Si2

We report Shubnikov-de Haas oscillation measurements within the high magnetic field (${\ensuremath{\mu}}_{0}H\phantom{\rule{0.16em}{0ex}}&gt;\phantom{\rule{0.16em}{0ex}}39$ T) magnetically polarized regime of URu${}_{2}$Si${}_{2}$, made possible using mesoscopic samples prepared by means of focused ion beam lithography. A significant change in the Fermi surface topology relative to the ``hidden-order'' phase is observed, signaling a transformation into a high magnetic field regime in which $5f$-electrons are removed from the Fermi surface. URu${}_{2}$Si${}_{2}$ is therefore a rare example of an actinide compound in which a transformation of $5f$-electrons can be directly observed at low temperatures, setting the stage for the unconventional ordering and high magnetic field quantum criticality in this material.

Characterization of transverse tensile stress response of critical current and delamination behaviour in GdBCO coated conductor tapes by anvil test

Recent problems in superconducting devices operating at the high magnetic field regime include the possibility of substantial critical current, Ic, degradation due to the delamination of the coated conductor (CC) tapes. The delamination may originate from excessive tensile radial stress and cleavage stress developed due to epoxy curing, cool-down and Lorentz force generated. Therefore, it is necessary to investigate the Ic response under transverse loading and the delamination behaviour in REBCO CC tapes to predict its performance in practical device applications. In this study, using the anvil test method, the delamination strength and the transverse tensile stress response of Ic in GdBCO CC tapes were investigated. Under transverse tensile stress, the Ic degradation in GdBCO CC tapes showed both abrupt and gradual behaviours. A reversible Ic degradation behaviour was also observed which was similar to the case under uniaxial tension. The electro-mechanical and mechanical delamination strengths were determined, wherein both increased when a smaller-sized anvil was used during the delamination test. The electro-mechanical delamination strength was approximately half of the mechanical delamination strength in the GdBCO CC tapes tested. Four delamination modes were observed and well correlated with electro-mechanical delamination strength and Ic degradation behaviour. Lastly, delamination sites within the CC tapes were also correlated with the Ic degradation behaviour that had been observed.

Noise Spectral Density and Vortex Velocity in YBCO Thin Film at High Critical Temperature

We have studied the voltage noise Vnoise and the voltage noise spectral density Sv of YBaCuO superconductor in the regime of high magnetic field. Our measurements were taken for different temperatures covering the vortex-glass and vortexliquid transition which represents the Tg temperature glass, The voltage noise spectral density Sv(f, T, H) as a function of frequency exhibits 1/f behaviour according to a lorentzian shape A((1+ πf/f0) ), where the constants A, f0, B and C are all determined, and temperature has a strong influence. in addition, we study the variation in the velocity of the vortex as a function of temperature.

Intraband absorption in GaAs-(Ga,Al)As variably spaced semiconductor superlattices under crossed electric and magnetic fields

A theoretical study of the intraband absorption properties of GaAs-Ga1−xAlxAs variably spaced semiconductor superlattices under crossed magnetic and electric fields is presented. Calculations are performed for the applied electric field along the growth-axis direction, whereas the magnetic field is considered parallel to the heterostructure layers. By defining a critical electric field so that the heterostructure energy levels are aligned in the absence of the applied magnetic fields, one finds that, in the weak magnetic-field regime, an abrupt red shift of the absorption coefficient maxima is obtained at fields equal to or larger than the critical electric field, a fact which may be explained from the localization properties of the electron wave functions. Results in the strong magnetic-field regime reveal a rich structure on the intraband absorption coefficient which may be explained from the strong dispersion exhibited by both the energy levels and transition strengths as functions of the generalized orbit-center position. Moreover, the possibility of occurrence of absorption in a wide frequency range is also demonstrated. Present calculated results may be of interest for future design and improvement of multilayered-based photovoltaic and solar-cell devices.

Structural mechanisms of anelasticity in Fe–Ga-based alloys

Abstract Thermally activated frequency and temperature dependent anelastic effects in Fe–Ga and Fe–Ga–Al alloys with 13, 17%Ga and 5%Al are analyzed. Activation parameters for thermally activated relaxation peaks are evaluated. Frequency independent effects caused by structural transformations (ordering and disordering) are studied. Physical mechanisms are proposed for most of these effects. Origin of high damping capacity of Fe–Ga alloy is checked by applying magnetic field and different heat treatment regimes.

Spin decoherence due to a randomly fluctuating spin bath

We study the decoherence of a spin in a quantum dot due to its hyperfine coupling to a randomly fluctuating bath of nuclear spins. The system is modelled by the central spin model with the spin bath initially being at infinite temperature. We calculate the spectrum and time evolution of the coherence factor using a Monte Carlo sampling of the exact eigenstates obtained via the algebraic Bethe ansatz. The exactness of the obtained eigenstates allows us to study the non-perturbative regime of weak magnetic fields in a full quantum mechanical treatment. In particular, we find a large non-decaying fraction in the zero-field limit. The crossover from strong to weak fields is similar to the decoherence starting from a pure initial bath state treated previously. We compare our results to a simple semiclassical picture [Merkulov et al., Phys. Rev. B 65, 205309 (2002)] and find surprisingly good agreement. Finally, we discuss the effect of weakly coupled spins and show that they will eventually lead to complete decoherence.

Orthogonal Polynomials on the Unit Circle with Fibonacci Verblunsky Coefficients, II. Applications

We consider CMV matrices with Verblunsky coefficients determined in an appropriate way by the Fibonacci sequence and present two applications of the spectral theory of such matrices to problems in mathematical physics. In our first application we estimate the spreading rates of quantum walks on the line with time-independent coins following the Fibonacci sequence. The estimates we obtain are explicit in terms of the parameters of the system. In our second application, we establish a connection between the classical nearest neighbor Ising model on the one-dimensional lattice in the complex magnetic field regime, and CMV operators. In particular, given a sequence of nearest-neighbor interaction couplings, we construct a sequence of Verblunsky coefficients, such that the support of the Lee-Yang zeros of the partition function for the Ising model in the thermodynamic limit coincides with the essential spectrum of the CMV matrix with the constructed Verblunsky coefficients. Under certain technical conditions, we also show that the zeros distribution measure coincides with the density of states measure for the CMV matrix.

Ultrahigh Magnetic Field Study of Layer Split Bands in Graphite

We report studies of the magnetospectroscopy of graphite into a new regime of high energies and ultrahigh magnetic fields which allows us to perform the first spectroscopic studies of the interlayer split-off bands, E1 and E2. These bands can be well described by an asymmetric bilayer model and have only a small interlayer band gap asymmetry. We show that all of the properties of the electrons and holes can be described by a simple relativistic behavior determined by γ0 and γ1.

Classical percolation fingerprints in the high temperature regime of the quantum Hall effect

We have performed magnetotransport experiments in the high-temperature regime (up to 50 K) of the integer quantum Hall effect for two-dimensional electron gases in semiconducting heterostructures. While the magnetic field dependence of the classical Hall law presents no anomaly at high temperatures, we find a breakdown of the Drude–Lorentz law for the longitudinal conductance beyond a crossover magnetic field Bc ≃ 1 T, which turns out to be correlated with the onset of the integer quantum Hall effect at low temperatures. We show that the high magnetic field regime at B > Bc can be understood in terms of classical percolative transport in a smooth disordered potential. From the temperature dependence of the peak longitudinal conductance, we extract scaling exponents which are in good agreement with the theoretically expected values. We also prove that inelastic scattering on phonons is responsible for dissipation in a wide temperature range going from 1 to 50 K at high magnetic fields.

Enhanced Flux Pinning Properties in Self-Assembled Magnetic $\hbox{CoFe}_{2}\hbox{O}_{4}$ Nanoparticles Doped $ \hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7 - \delta}$ Thin Films

In this work, self-assembled magnetic CoFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> (CFO) nanoparticles are introduced into YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-δ</sub> (YBCO) thin films as both cap and buffer layers to enhance the flux pinning properties. CFO nanoparticles are incorporated into the YBCO matrix by alternative laser ablation of the YBCO and CFO targets by employing the pulsed laser deposition method, which is compatible with the commercial processing method of coated conductors. By introducing ferromagnetic nanoinclusions, both magnetic and defect pinning can be incorporated into YBCO thin films to achieve stronger pinning effects under higher applied magnetic field regimes. Detailed microstructural studies are conducted to verify the morphologies of all doped samples. Both self-field and in-field performance (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sf</sup> and J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in-field</sup> (H//c)) measured at various temperatures (65, 40, and 5 K) are studied. The results suggest that the CFO nanoparticle cap is an ideal approach to introduce ordered magnetic pinning centers in the YBCO thin films, which shows enhanced superconducting properties both at self-field and under high applied field regimes without degrading YBCO's intrinsic properties.

Properties of 1D Classical and Quantum Ising Models: Rigorous Results

In this paper, we consider one-dimensional classical and quantum spin-1/2 quasi-periodic Ising chains, with two-valued nearest neighbor interaction modulated by a Fibonacci substitution sequence on two letters. In the quantum case, we investigate the energy spectrum of the Ising Hamiltonian, in presence of constant transverse magnetic field. In the classical case, we investigate and prove analyticity of the free energy function when the magnetic field, together with interaction strength couplings, is modulated by the same Fibonacci substitution (thus proving absence of phase transitions of any order at finite temperature). We also investigate the distribution of Lee–Yang zeros of the partition function in the complex magnetic field regime, and prove its Cantor set structure (together with some additional qualitative properties), thus providing a rigorous justification for the observations in some previous works. In both, quantum and classical models, we concentrate on the ferromagnetic class.

Anisotropic low field magnetoimpedance in (001) oriented La0.7Sr0.3MnO3 thin films

We report the evidence for the anisotropic magnetoimpedance behavior in (001) oriented La0.7Sr0.3MnO3 (LSMO) thin films, in low frequency–low magnetic field regime. (001) oriented LSMO thin films were deposited using pulsed laser deposition and characterized with X-ray diffraction and temperature dependent magnetization studies. In the in-plain configuration, an ac magnetoresistance (MRac) of ∼−0.5% was observed at 1000 Oe, at 100 Hz frequency in these films. The MRac was found to decrease with increase in frequency. We observe increases in MRac at low frequency, indicating major contribution for change of permeability from domain wall motion. At higher frequencies, it decreases due to decrease in transverse permeability, resulting from dampening of domain wall motion. Out-of-plane configuration showed MRac ∼ 5.5% at 1000 Oe, at 100 Hz frequency. The MRac turned from positive to negative with increase in frequency in out-of-plane configuration. These changes are attributed to the change in permeability of the film with the frequency and applied magnetic field.