Strongly disordered Hubbard model in one dimension: spin and orbital infinite randomness and Griffiths phases

We study by the strong disorder renormalization group (RG) method the low-energy properties of the one-dimensional Hubbard model with random-hopping matrix-elements $t_{min}<t<t_{max}$, and with random on-site Coulomb repulsion terms $0 \le U_{min}<U<U_{max}$. There are two critical phases, corresponding to an infinite randomness spin random singlet for strong interactions ($U_{min} > t_{max}$) and to an orbital infinite randomness fixed point for vanishing interactions ($U_{max}/t_{max} \to 0$). To each critical infinite randomness fixed point is connected a Griffiths phase, the correlation length and dynamical exponent of which have well defined asymptotic dependences on the corresponding quantum control parameter. The theoretical predictions for the scaling in the vicinity of the critical points compare well to numerical RG simulations.Comment: 9 pages, 7 figure

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_

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

Plasma resonance at low magnetic fields as a probe of vortex line meandering in layered superconductors

We consider the magnetic field dependence of the plasma resonance frequency in pristine and in irradiated Bi$_2$Sr$_2$CaCu$_2$O$_8$ crystals near $T_c$. At low magnetic fields we relate linear in field corrections to the plasma frequency to the average distance between the pancake vortices in the neighboring layers (wandering length). We calculate the wandering length in the case of thermal wiggling of vortex lines, taking into account both Josephson and magnetic interlayer coupling of pancakes. Analyzing experimental data, we found that (i) the wandering length becomes comparable with the London penetration depth near T$_{c}$ and (ii) at small melting fields ($< 20$ G) the wandering length does not change much at the melting transition. This shows existence of the line liquid phase in this field range. We also found that pinning by columnar defects affects weakly the field dependence of the plasma resonance frequency near $T_c$.Comment: RevTex, 4 pages, 2 PS figures, Submitted to Phys. Rev.

Localized mode interactions in 0-pi Josephson junctions

A long Josephson junction containing regions with a phase shift of pi is considered. By exploiting the defect modes due to the discontinuities present in the system, it is shown that Josephson junctions with phase-shift can be an ideal setting for studying localized mode interactions. A phase-shift configuration acting as a double-well potential is considered and shown to admit mode tunnelings between the wells. When the phase-shift configuration is periodic, it is shown that localized excitations forming bright and dark solitons can be created. Multi-mode approximations are derived confirming the numerical results.Comment: 4 pages, to appear in Phys. Rev.

Ferromagnetic Film on a Superconducting Substrate

We study the equilibrium domain structure and magnetic flux around a ferromagnetic (FM) film with perpendicular magnetization M_0 on a superconducting (SC) substrate. At 4{\pi}M_0<H_{c1} the SC is in the Meissner state and the equilibrium domain width in the film, l, scales as (l/4{\pi}{\lambda}_{L}) = (l_{N}/4{\pi}{\lambda}_{L})^{2/3} with the domain width on a normal (non-superconducting) substrate, l_{N}/4\pi\lambda_L >> 1. Here \lambda_L is the London penetration length. For 4{\pi}M_0 > H_{c1} and l_{N} in excess of about 35 {\lambda}_{L}, the domains are connected by SC vortices. We argue that pinning of vortices by magnetic domains in FM/SC multilayers can provide high critical currents.Comment: 4 pages, 2 figures, submitted to PR

Inhomogeneous LOFF phase revisited for surface superconductivity

We consider 2D surface superconductivity in high magnetic fields parallel to the surface. We demonstrate that the spin-orbit interaction at the surface changes the properties of the inhomogeneous superconducting Larkin-Ovchinnikov-Fulde-Ferrell state that develops above fields given by the paramagnetic criterion. Strong spin-orbit interaction significantly broadens the range of existence of the LOFF phase, which takes the form of periodic superconducting stripes running along the field direction on the surface, leading to the anisotropy of its properties. In connection with experiments by J.H. Schon et al. [Nature 914, 434 (2001)] on superconductivity of electrically doped films of the cuprate material CaCuO2, we also discuss this problem for the d-wave pairing to indicate the possibility of a re-orientation transition as the magnetic field direction is rotated in the plane parallel to the surface. Our results provide a tool for studying surface superconductivity as a function of doping.Comment: 4 pages, 1 fig, revtex