Behavior Of Persistent Current Research Articles

Pulse Magnetization of Jointless Superconducting Loops for Magnetic Bearings Height Control

Superconductors can be used for applications such as magnetic levitation. The Maglev Cobra project, developed at the Universidade Federal do Rio de Janeiro, uses superconducting bulks, which are produced in a partially industrial process, which does not guarantee homogeneity, and operate passively, making height control impossible. Replacing these bulks with jointless 2G wires superconducting loops will guarantee a homogenous production and enables active control of levitation height through current pulse magnetization. In this work, superconducting loops were exposed to different values of current pulse width and intensity. The induced current in the tape was then measured. With this, the persistent current behavior in the superconductor could be observed, showing a correlation between the persistent current and the applied current pulse parameters. Saturation of the current in the superconductor was observed above certain values of applied current, due to the critical current value of the superconducting tape. Furthermore, as the width of the current pulse increases, the persistent current increment decreases. According to these results, an optimum point can be observed, in which the ratio between the persistent current and the energy used is maximized. To operate at the optimum point, the current pulse width must decrease as its intensity increases.

Controlled parity switch of persistent currents in quantum ladders

We investigate the behavior of persistent currents for a fixed number of noninteracting fermions in a periodic quantum ladder threaded by Aharonov-Bohm and transverse magnetic fluxes $\mathrm{\ensuremath{\Phi}}$ and $\ensuremath{\chi}$. We show that the coupling between ladder legs provides a way to effectively change the ground-state fermion-number parity, by varying $\ensuremath{\chi}$. Specifically, we demonstrate that varying $\ensuremath{\chi}$ by $2\ensuremath{\pi}$ (one flux quantum) leads to an apparent fermion-number parity switch. We find that persistent currents exhibit a robust $4\ensuremath{\pi}$ periodicity as a function of $\ensuremath{\chi}$, despite the fact that $\ensuremath{\chi}\ensuremath{\rightarrow}\ensuremath{\chi}+2\ensuremath{\pi}$ leads to modifications of order $1/N$ of the energy spectrum, where $N$ is the number of sites in each ladder leg. We show that these parity-switch and $4\ensuremath{\pi}$ periodicity effects are robust with respect to temperature and disorder, and outline potential physical realizations using cold atomic gases and photonic lattices, for bosonic analogs of the effects.

Electronic States and Persistent Currents in Nanowire Quantum Ring

The new model of a quantum ring (QR) defined inside a nanowire (NW) is proposed. The one-particle Hamiltonian for electron in [111]-oriented NW QR is constructed taking into account both Rashba and Dresselhaus spin-orbit coupling (SOC). The energy levels as a function of magnetic field are found using the exact numerical diagonalization. The persistent currents (both charge and spin) are calculated. The specificity of SOC and arising anticrossings in energy spectrum lead to unusual features in persistent current behavior. The variation of magnetic field or carrier concentration by means of gate can lead to pure spin persistent current with the charge current being zero.

Performance of an HTS Persistent Current System for REBCO Pancake Coil

A superconducting joint and a persistent current switch are the key technologies in persistent current operation for MRI and NMR magnets to obtain higher resolution and sensitivity. We prepared a persistent current system including a superconducting joint, a persistent current switch, and a double pancake coil, which were fabricated with a REBCO tape. The magnetic field decay behavior in persistent current operation were observed in a self-field and the external magnetic field of 1 T at 20 K. In the self-field, the decay rate of persistent current excited up to 170 A reached to 1.7 ppm/h after five days operation. This means that the practical persistent current operation has been achieved successfully using the REBCO superconducting joint. From the voltage–current characteristics estimated from the magnetic field decay, it was found that the decay behavior of persistent current is dominated by the joint resistance in 1 T, but by the shielding current in the self-field.

Electronic states and persistent currents in nanowire quantum ring

AbstractA new model of a quantum ring defined inside a nanowire is proposed. The one-particle Hamiltonian for electron in [111]-oriented nanowire quantum ring is constructed taking into account both Rashba and Dresselhaus spin-orbit coupling. The energy levels as a function of magnetic field are found using the exact numerical diagonalization. The persistent currents (both charge and spin) are calculated. The specificity of spin-orbit coupling and arising anticrossings in energy spectrum lead to unusual features in persistent current behavior. The variation of magnetic field or carrier concentration by means of gates can lead to pure spin persistent current with the charge current being zero.

Persistent current in 2D topological superconductors

A junction between two boundaries of a topological superconductor (TSC), mediated by localized edge modes of Majorana fermions, is investigated. The tunneling of fermions across the junction depends on the magnetic flux. It breaks the time-reversal symmetry at the boundary of the sample. The persistent current is determined by the emergence of Majorana edge modes. The structure of the edge modes depends on the magnitude of the tunneling amplitude across the junction. It is shown that there are two different regimes, which correspond to strong and weak tunneling of Majorana fermions, distinctive in the persistent current behavior. In a strong tunneling regime, the fermion parity of edge modes is not conserved and the persistent current is a 2π-periodic function of the magnetic flux. When the tunneling is weak the chiral Majorana states, which are propagating along the edges have the same fermion parity. They form a 4π-phase periodic persistent current along the boundaries. The regions in the space of parameters, which correspond to the emergence of 2π- and of 4π-harmonics, are numerically determined. The peculiarities in the persistent current behavior are studied.

Persistent current of relativistic electrons on a Dirac ring in presence of impurities

We study the behavior of persistent current of relativistic electrons on a one dimensional ring in presence of attractive/repulsive scattering potentials. In particular, we investigate the persistent current in accordance with the strength as well as the number of the scattering potential. We find that in presence of single scatterer the persistent current becomes smaller in magnitude than the scattering free scenario. This behaviour is similar to the non-relativistic case. Even for a very strong scattering potential, finite amount of persistent current remains for a relativistic ring. In presence of multiple scatterer we observe that the persistent current is maximum when the scatterers are placed uniformly compared to the current averaged over random configurations. However if we increase the number of scatterers, we find that the random averaged current increases with the number of scatterers. The latter behaviour is in contrast to the non-relativistic case.

Carbon nanotube in a threading magnetic field: Conductance oscillations persistent current and magnetization

AbstractThe magnetic response of electrons in a zigzag carbon nanotube (CN) threaded by an Aharonov Bohm flux φ is carefully analyzed by using a Hartree Fock approximation. The ground state energy as a function of the magnetic field, B, and the electron–electron repulsion strength, U, are obtained for Ne interacting electrons confined in short (∼100 nm) and long (∼1 μm) CNs. The spin and orbital magnetization responses to the magnetic field variations are discussed. The CN transport properties are investigated and periodic field dependent oscillations are predicted for both longitudinal ballistic and persistent currents. The behavior of persistent current is investigated as a function of U and a significant suppression in current amplitude is observed when the strength of the electron–electron repulsion increases.

Many electrons on a mesoscopic cylinder in a threading magnetic field: Persistent current and magnetization

AbstractThe behavior of Ne interacting electrons confined in a mesoscopic “quantum” cylinder, QC, threaded by an Aharonov–Bohm (AB) flux ϕ is carefully analyzed by using a Hartree–Fock approximation. The ground state energies of the QC as a function of the magnetic field, B, and the electron–electron repulsion strength, U, are obtained. A phase diagram is presented indicating a rich variety of ground states and the spin and orbital magnetization responses to the magnetic field's variations are discussed. The behavior of persistent current is investigated and a significant change in current amplitude is observed when the strength of the electron–electron repulsion increases.

Distribution of persistent currents in a multi-arm mesoscopic ring

We propose an idea to investigate persistent current in individual arms of a multi-arm mesoscopic ring. Following a brief description of persistent current in a traditional Aharonov-Bohm (AB) ring, we examine the behavior of persistent currents in separate arms of a three-arm mesoscopic ring. Our analysis may be helpful in studying magnetic response of any complicated quantum network.

Oscillatory persistent currents in quantum rings: Semiconductors versus superconductors

Persistent currents are a hallmark of superconductivity in metals. To observe those dissipationless currents in a non-superconducting ring, the circumference of the ring must be short enough so that the phase coherence of the electronic wave functions is preserved around the loop. Recent progress in the fabrication of self-assembled semiconductor quantum rings (SAQRs), which can be filled with only a few (1–2) electrons, has offered the unique possibility to study the magnetic-field-induced oscillations in the persistent current carried by a single electron. In this paper, we discuss similarities and distinctions between the behavior of persistent currents in semiconductor and superconductor samples and give an overview of the recent results for oscillatory persistent currents in SAQRs. Although the real SAQR shape differs strongly from an idealized circular-symmetric open ring structure, the Aharonov–Bohm oscillations of the magnetization survive, as observed in low temperature magnetization measurements on In x Ga 1− x As/GaAs SAQRs.

On the role of electron correlation and disorder on persistent currents in isolated one-dimensional rings

To understand the role of electron correlation and disorder on persistent currents in isolated 1D rings threaded by magnetic flux ϕ, we study the behavior of persistent currents in aperiodic and ordered binary alloy rings. These systems may be regarded as disordered systems with well-defined long-range order so that we do not have to perform any configuration averaging of the physical quantities. We see that in the absence of interaction, disorder suppresses persistent currents by orders of magnitude and also removes its discontinuity as a function of ϕ. As we introduce electron correlation, we get enhancement of the currents in certain disordered rings. Quite interestingly we observe that in some cases, electron correlation produces kink-like structures in the persistent current as a function of ϕ. This may be considered as anomalous Aharonov–Bohm oscillations of the persistent current and recent experimental observations support such oscillations. We find that the persistent current converges with the size of the rings.

Strange behavior of persistent currents in small Hubbard rings

We show exactly that small Hubbard rings exhibit unusual kink-like structures giving anomalous oscillations in persistent current. Singular behavior of persistent current disappears in some cases. In half-filled systems mobility gradually drops to zero with interaction, while it converges to some finite value in non-half-filled cases.

Behavior of Persistent Currents under Conditions of Strong Decay

We report observations of the decay of persistent currents of thin superfluid $^{4}\mathrm{He}$ films for which the superfluid velocity deviates strongly from the previously observed $logt$ behavior. We present data which document the evolution of this behavior as a function of film thickness. The Iordanskii-Langer-Fisher theory does not accurately describe our observations.