V. A. Yampol'skii

Terahertz Josephson plasma waves in layered superconductors: spectrum, generation, nonlinear and quantum phenomena

The recent growing interest in terahertz (THz) and sub-THz science and technology is due to its many important applications in physics, astronomy, chemistry, biology and medicine, including THz imaging, spectroscopy, tomography, medical diagnosis, health monitoring, environmental control, as well as chemical and biological identification. We review the problem of linear and nonlinear THz and sub-THz Josephson plasma waves in layered superconductors and their excitations produced by moving Josephson vortices. We start by discussing the coupled sine-Gordon equations for the gauge-invariant phase difference of the order parameter in the junctions, taking into account the effect of breaking the charge neutrality, and deriving the spectrum of Josephson plasma waves. We also review surface and waveguide Josephson plasma waves. The spectrum of these waves is presented, and their excitation is discussed. We review the propagation of weakly nonlinear Josephson plasma waves below the plasma frequency, ωJ, which is very unusual for plasma-like excitations. In close analogy to nonlinear optics, these waves exhibit numerous remarkable features, including a self-focusing effect and the pumping of weaker waves by a stronger one. In addition, an unusual stop-light phenomenon, when ∂ω/∂k ≈ 0, caused by both nonlinearity and dissipation, can be observed in the Josephson plasma waves. At frequencies above ωJ, the current-phase nonlinearity can be used for transforming continuous sub-THz radiation into short, strongly amplified, pulses. We also present quantum effects in layered superconductors, specifically, the problem of quantum tunneling of fluxons through stacks of Josephson junctions. Moreover, the nonlocal sine-Gordon equation for Josephson vortices is reviewed. We discuss the Cherenkov and transition radiations of the Josephson plasma waves produced by moving Josephson vortices, either in a single Josephson junction or in layered superconductors. Furthermore, the expression for the Cherenkov cone of the excited Josephson plasma waves is derived. We also discuss the problem of coherent radiation (superradiance) of the THz waves by exciting uniform Josephson oscillations. The effects reviewed here could be potentially useful for sub-THz and THz emitters, filters and detectors.

Superposition of currents in hard superconductors placed into crossed ac and dc magnetic fields

The superposition of currents in YBCO melt-textured samples placed into crossed ac and dc magnetic fields is predicted and observed. This superposition is a direct consequence of the critical state model. The dc magnetic field distribution is shown to become uniform wherever the ac field has penetrated. Owing to this nonlinear process, the area of the dc magnetization loop diminishes and eventually disappears completely with an increase of the ac field magnitude. This means that under the action of the external ac field, the static magnetic properties of hard superconductors change and tend to the well known properties of soft ones.

Collapse of the magnetic moment in a hard superconductor under the action of a transverse ac magnetic field

Abstract The suppression of the static magnetic moment M of a superconducting plate by an alternating magnetic field applied perpendicularly to a dc magnetic field has been studied both experimentally and theoretically. Complete disappearance of the hysteresis of the magnetization curve in YBCO melt-textured samples was observed. Nonmonotonous behavior of M with the increase of the ac magnetic field amplitude was found. The dynamics of the collapse of the magnetic moment was studied in the pulse regime of the transverse field. The experimental results indicate homogeneity of the static magnetic induction in the sample regions where the ac field penetrates. The data obtained are interpreted within the framework of a two-liquid hydrodynamic model.

Lamb-Dicke spectroscopy of atoms in a hollow-core photonic crystal fibre

Unlike photons, which are conveniently handled by mirrors and optical fibres without loss of coherence, atoms lose their coherence via atom–atom and atom–wall interactions. This decoherence of atoms deteriorates the performance of atomic clocks and magnetometers, and also hinders their miniaturization. Here we report a novel platform for precision spectroscopy. Ultracold strontium atoms inside a kagome-lattice hollow-core photonic crystal fibre are transversely confined by an optical lattice to prevent atoms from interacting with the fibre wall. By confining at most one atom in each lattice site, to avoid atom–atom interactions and Doppler effect, a 7.8-kHz-wide spectrum is observed for the 1S0−3P1(m=0) transition. Atoms singly trapped in a magic lattice in hollow-core photonic crystal fibres improve the optical depth while preserving atomic coherence time.

Surface josephson plasma waves in layered superconductors above the plasma frequency: Evidence for a negative index of refraction

V.A. Golick, D.V. Kadygrob, V.A. Yampol’skii, 2, 3 A.L. Rakhmanov, 4 B.A. Ivanov, 5, 6 and Franco Nori 7 V.N. Karazin Kharkov National University, 61077 Kharkov, Ukraine A.Ya. Usikov Institute for Radiophysics and Electronics, Ukrainian Academy of Sciences, 61085 Kharkov, Ukraine Advanced Science Institute, The Institute of Physical and Chemical Research (RIKEN), Saitama, 351-0198, Japan Institute for Theoretical and Applied Electrodynamics Russian Acad. Sci., 125412 Moscow, Russia Institute of Magnetism, Ukrainian Academy of Sciences, 03142 Kiev, Ukraine National T. Shevchenko University of Kiev, 03127 Kiev, Ukraine Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA Abstract We predict a new branch of surface Josephson plasma waves (SJPWs) in layered superconductors for frequencies higher than the Josephson plasma frequency. In this frequency range, the permittivity tensor components along and transverse to the layers have different signs, which is usually associated with negative refraction. However, for these frequencies, the bulk Josephson plasma waves cannot be matched with the incident and reflected waves in the vacuum, and, instead of the negative-refractive properties, abnormal surface modes appear within the frequency band expected for bulk modes. We also discuss the excitation of high-frequency SJPWs by means of the attenuated-total-reflection method.

Voltage-driven quantum oscillations in graphene

We predict unusual (for non-relativistic quantum mechanics) electron states in graphene, which are localized within a finite-width potential barrier. The density of localized states in the sufficiently high and/or wide graphene barrier exhibits a number of singularities at certain values of the energy. Such singularities provide quantum oscillations of both the transport (e.g. conductivity) and thermodynamic properties of graphene—when increasing the barrier height and/or width, similarly to the well-known Shubnikov–de-Haas (SdH) oscillations of conductivity in pure metals. However, here the SdH-like oscillations are driven by an electric field instead of the usual magnetically driven SdH-oscillations.

Conversion of terahertz wave polarization at the boundary of a layered superconductor due to the resonance excitation of oblique surface waves.

We predict a complete TM↔TE transformation of the polarization of terahertz electromagnetic waves reflected from a strongly anisotropic boundary of a layered superconductor. We consider the case when the wave is incident on the superconductor from a dielectric prism separated from the sample by a thin vacuum gap. The physical origin of the predicted phenomenon is similar to the Wood anomalies known in optics and is related to the resonance excitation of the oblique surface waves. We also discuss the dispersion relation for these waves, propagating along the boundary of the superconductor at some angle with respect to the anisotropy axis, as well as their excitation by the attenuated-total-reflection method.

Paramagnetism of a hard superconductor in crossed magnetic fields

The d.c. magnetic moment along the d.c. magnetic field is observed to be suppressed under the action of the perpendicular a.c. magnetic field. Moreover, this moment changes its sign at some value of the a.c. magnetic field amplitude and becomes paramagnetic. This behavior can be explained in the framework of a two velocity hydrodynamic model of the critical state of a hard superconductor in crossed magnetic fields. The physical nature of this phenomenon is connected with the homogeneity of the vortex density inside a superconductor at these condition.

Shape Waves in 2D Josephson Junctions: Exact Solutions and Time Dilation

We predict a new class of excitations propagating along a Josephson vortex in two-dimensional Josephson junctions. These excitations are associated with the distortion of a Josephson vortex line and have an analogy with shear waves in solid mechanics. Their shapes can have an arbitrary profile, which is retained when propagating. We derive a universal analytical expression for the energy of arbitrary shape excitations, investigate their influence on the dynamics of a vortex line, and discuss conditions where such excitations can be created. Finally, we show that such excitations play the role of a clock for a relativistically moving Josephson vortex and suggest an experiment to measure a time dilation effect analogous to that in special relativity.

Suppression of magnetic relaxation processes in melt-textured YBa2Cu3Ox superconductors by a transverse ac magnetic field

The effect of the transverse AC magnetic field on relaxation process in YBa