Inhomogeneous Magnetic Field Profile Research Articles

Thermal QCD in a non-uniform magnetic background

Off-central heavy-ion collisions are known to feature magnetic fields with magnitudes and characteristic gradients corresponding to the scale of the strong interactions. In this work, we employ equilibrium lattice simulations of the underlying theory, QCD, involving similar inhomogeneous magnetic field profiles to achieve a better understanding of this system. We simulate three flavors of dynamical staggered quarks with physical masses at a range of magnetic fields and temperatures, and extrapolate the results to the continuum limit. Analyzing the impact of the field on the quark condensate and the Polyakov loop, we find non-trivial spatial features that render the QCD medium qualitatively different as in the homogeneous setup, especially at temperatures around the transition. In addition, we construct leading-order chiral perturbation theory for the inhomogeneous background and compare its prediction to our lattice results at low temperature. Our findings will be useful to benchmark effective theories and low-energy models of QCD for a better description of peripheral heavy-ion collisions.

Effects of Different Magnetic Field Profiles on Output Power and Efficiency of a Second-Harmonic Gyrotron

As the magnetic field used in nuclear magnetic resonance (NMR) research increases (up to 23 T), the irradiation frequency of the electromagnetic field used for dynamic nuclear polarization (DNP) needs to be in the terahertz band (140–600 GHz). This article analyzed and compared the influence of different external magnetic field profiles on start current and beam–wave interaction of a 394-GHz gyrotron. As the simulation results show, the minimum start current corresponding to the linear and parabolic external magnetic field profile is lower than that corresponding to the uniform magnetic field. The minimum start current in the case of parabolic magnetic field profile is a little lower than that in the case of a linear magnetic field profile. Based on the nonlinear self-consistent calculation, it is observed that the optimum output powers in both linear and parabolic magnetic field profile cases are nearly equal when the beam current is 0.25 A and both of them are larger than the optimum output power when the magnetic field is uniform. In the calculation of orbital efficiency <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\eta _{\bot } $ </tex-math></inline-formula> , the maximum orbital efficiency in the soft-excitation region in the inhomogeneous magnetic field profile cases is larger than that in the case of uniform magnetic field profile. After comprehensive consideration of beam voltage, beam current, and efficiency, the final operation point is located in the place where the beam current is 0.25 A and the beam voltage is 15 kV. The output power is more than 300 W and meets the requirements of DNP-NMR experiment.

The Low-Energy State of an Electron Beam in a Coaxial Diode with a Homogeneous Anode and Inhomogeneous Magnetic Field Profile

The possible formation of an extended low-energy state of electron beam in a coaxial diode with homogeneous cylindrical anode and moderate magnetic field with inhomogeneous profile is demonstrated for the first time. It is established that, depending on the magnetic field configuration, virtual cathodes (VCs) of two types can be formed: (i) a stationary VC with a localized reflection plane and (ii) a moving VC with a two-stream low-energy state of the electron beam.

Direction-dependent tunneling through nanostructured magnetic barriers in graphene

We evaluate the transmission through magnetic barriers in graphene-based nanostructures. Several particular cases are considered: a magnetic step, single and double barriers, and $\ensuremath{\delta}$-function barriers. A separate class of magnetic-barrier structures are those with inhomogeneous magnetic-field profiles, such that the average magnetic field vanishes, which can be realized by nanostructured ferromagnetic stripes placed on top of the graphene layer. Quantum bound states that are localized near or in the barrier are predicted for a magnetic step and some structures with finite-width barriers but none for $\ensuremath{\delta}$-function barriers. When a bound state is localized close to the barrier edge, it has a nonzero velocity parallel to this edge. The transmission depends strongly on the direction of the incident electron or hole wave vector and gives the possibility to construct a direction-dependent wave vector filter. In general, the resonant structure of the transmission is significantly more pronounced for (Dirac) electrons with linear spectrum than for the usual electrons with a parabolic spectrum.

Hall magnetometer in the ballistic regime

A Monte Carlo simulation of the ballistic motion of electrons in a mesoscopic Hall bar containing a local inhomogeneous magnetic field profile is presented. We find that in the regime of low magnetic fields, the Hall resistance is determined by the average magnetic field in the cross junction and is independent of the shape and position of this profile in the junction. Information on the size of the local magnetic field profile can also be obtained from the Hall measurement. The bend resistance, on the other hand, is much more precise on the exact details of the local magnetic field in the cross junction.

An exact QED3+1 effective action

We compute the exact QED_{3+1} effective action for fermions in the presence of a family of static but spatially inhomogeneous magnetic field profiles. An asymptotic expansion of this exact effective action yields an all-orders derivative expansion, the first terms of which agree with independent derivative expansion computations. These results generalize analogous earlier results by Cangemi et al in QED_{2+1}.

Scattering on circular inhomogeneous magnetic field profiles in an electron waveguide junction

The low magnetic field Hall resistance resulting from a circular inhomogeneous magnetic field profile situated in the cross junction of a Hall bar is calculated. Models for the magnetic field profile resulting from: (1) a superconducting dot, (2) a magnetic dipole, and (3) a magnetic vortex are considered. We find that the inhomogeneous nature of the magnetic field influences the Hall resistance drastically. In particular, in the low field regime the Hall resistance is completely determined by the average magnetic field in the cross junction of the Hall bar.

An All-Orders Derivative Expansion

We evaluate the exact QED2+1 effective action for fermions in the presence of a family of static but spatially inhomogeneous magnetic field profiles. This exact result yields an all-orders derivative expansion of the effective action, and indicates that the derivative expansion is an asymptotic, rather than a convergent, expansion.