Magnetism and Superconductivity in (RE)Ni2B2C: The Case of TmNi2B2C

The recently reported coexistence of an oscillatory magnetic order with the wave vector Q=0.241 \AA^{-1} and superconductivity in TmNi2B2C is analyzed theoretically. It is shown that the oscillatory magnetic order and superconductivity interact predominantly via the exchange interaction between localized moments (LM's) and conduction electrons, while the electromagnetic interaction between them is negligible. In the coexistence phase of the clean TmNi2B2C the quasiparticle spectrum should have a line of zeros at the Fermi surface, giving rise to the power law behavior of thermodynamic and transport properties. Two scenarios of the origin of the oscillatory magnetic order in TmNi2B2C are analyzed: a) due to superconductivity and b) independently on superconductivity. Experiments in magnetic field are proposed in order to choose between them.Comment: 12 pages with 2 PS figures, RevTe

Conventional Magnetic Superconductors: Coexistence of Singlet Superconductivity and Magnetic Order

The basic physics of bulk magnetic superconductors (MS) related to the problem of the coexistence of singlet superconductivity (SC) and magnetic order is reviewed. The interplay between exchange (EX) and electromagnetic (EM) interaction is discussed and argued that the singlet SC and uniform ferromagnetic (F) order practically never coexist. In case of their mutual coexistence the F order is modified into a domain-like or spiral structure depending on magnetic anisotropy. It turns out that this situation is realized in several superconductors such as $ErRh_{4}B_{4}$, $HoMo_{6}S_{8}$, $HoMo_{6}Se_{8}$ with electronic and in $AuIn_{2}$ with nuclear magnetic order. The later problem is also discussed here. The coexistence of SC and antiferromagnetism is more favorable than with the modified F order. Very interesting physics is realized in systems with SC and weak-ferromagnetism which results in an very reach phase diagram. The properties of magnetic superconductors in magnetic field are very peculiar, especially near the (ferro)magnetic transition temperature where the upper critical field becomes smaller than the thermodynamical critical field. The extremely interesting physics of Josephson junctions based on MS with spiral magnetic order is also discussed. The existence of the triplet pairing amplitude $F_{\uparrow \uparrow}$ ($F_{\downarrow \downarrow}$) in MS with rotating magnetization (the effect recently rediscovered in SFS junctions) gives rise to the so called $\pi$-contact. Furthermore, the interplay of the superconducting and magnetic phase in such a contact renders possibilities for a new type of coupled Josephson-qubits.Comment: 28 pages, 3 figures; submitted for the Special Issue Comptes de l'Academie des Sciences: Problems of the Coexistence of Magnetism and Superconductivity, edited by A. Buzdi

Radiation Due to Josephson Oscillations in Layered Superconductors

We derive the power of direct radiation into free space induced by Josephson oscillations in intrinsic Josephson junctions of highly anisotropic layered superconductors. We consider the super-radiation regime for a crystal cut in the form of a thin slice parallel to the c-axis. We find that the radiation correction to the current-voltage characteristic in this regime depends only on crystal shape. We show that at large enough number of junctions oscillations are synchronized providing high radiation power and efficiency in the THz frequency range. We discuss crystal parameters and bias current optimal for radiation power and crystal cooling.Comment: 4 pages, 1 figure, to be published in Phys. Rev. Let

Nondemolition measurements of a single quantum spin using Josephson oscillations

We consider a Josephson junction containing a single localized spin 1/2 between conventional singlet superconducting electrodes. We study the spin dynamics and measurements when a dc-magnetic field ${\bf B}\parallel z$ acts on the spin and the junction is embedded into a dissipative circuit. We show that when tunneling or a voltage are turned on at time $t=0$ the Josephson current starts to oscillate with an amplitude depending on the initial ($t=0$) value of the spin $z$-component, $S_z= \pm 1/2$. At low temperatures, when effects of quasiparticles may be neglected, this procedure realizes a quantum-non-demolition (QND) measurement of $S_z$.Comment: 4 pages, 1 figure; average value of spin z operator changed to eigenvalue S_

Superconducting Plasma Excitation at Microwave Frequencies in Parallel Magnetic Fields in Bi2Sr2CaCu2O8+δ\mathrm{\mathbf{Bi_2Sr_2CaCu_2O_{8+\delta}}}

Josephson plasma resonance has been studied in a wide microwave frequency range between 10 and 52 GHz in a magnetic field parallel to the $ab$-plane in under-doped \BI. Above about 30 GHz two resonance modes were observed: one (LT mode) appears at low temperatures and another (HT mode) at higher temperatures, leaving a temperature gap between two regions. These two resonance modes exhibit a sharp contrast each other both on temperture and magnetic field dependences and show distinct characters different entirely from the c-axis Josephson plasma resonance. From temperature and field scan experiments at various frequencies it is suggested that the LT mode can be attributed to the coupled Josephson plasma mode with Josephson vortices, while the HT mode is a new plasma mode associated possibly with the periodic array of Josephson vortices.Comment: submitted to Physica C (Prceedings of Plasma2000, Sendai

Ballistic Four-Terminal Josephson Junction: Bistable States and Magnetic Flux Transfer

The macroscopic quantum interference effects in ballistic Josephson microstructures are investigated. The studied system consists of four bulk superconductors (terminals) which are weakly coupled through the mesoscopic rectangular normal metal (two dimensional electron gas). We show that nonlocal electrodynamics of ballistic systems leads to specific current-phase relations for the mesoscopic multiterminal Josephson junction. The nonlocal coupling of supercurrents produces the "dragging" effect for phases of the superconducting order parameter in different terminals. The composite Josephson junction, based on this effect, exhibits the two -level system behaviour controlled by the external magnetic flux. The effect of magnetic flux transfer in a system of nonlocally coupled superconducting rings is studied.Comment: 12pages LaTex, 6 figures; e-mail: [email protected]

I-V characteristics of short superconducting nanowires with different bias and shunt: a dynamic approach

We derived the I-V characteristics of short nanowire in the circuit with and without resistive and inductive shunt. For that we used numerical calculations in the framework of time-dependent Ginzburg-Landau equations with different relaxation times for the amplitude and phase dynamics. We also derived dependence of the I-V characteristics on flux in superconducting quantum interference device (SQUID) made of such two weak links.Comment: 5.4 pages and 7 figure

Enhancement of critical current density in superconducting/magnetic multi-layers with slow magnetic relaxation dynamics and large magnetic susceptibility

We propose to use superconductor-magnet multi-layer structure to achieve high critical current density by invoking polaronic mechanism of pinning. The magnetic layers should have large magnetic susceptibility to enhance the coupling between vortices and magnetization in magnetic layers. The relaxation of the magnetization should be slow. When the velocity of vortices is low, they are dressed by nonuniform magnetization and move as polarons. In this case, the viscosity of vortices proportional to the magnetic relaxation time is enhanced significantly. As velocity increases, the polarons dissociate and the viscosity drops to the usual Bardeen-Stephen one, resulting in a jump in the I-V curve. Experimentally the jump shows up as a depinning transition and the corresponding current at the jump is the depinning current. For Nb and proper magnet multi-layer structure, we estimate the critical current density $J_c\sim 10^{9}\ \rm{A/m^2}$ at magnetic field $B\approx 1$ T.Comment: 4 pages, 2 figure