Josephson Plasma Resonance Research Articles

C-Electron transport in Bi 2Sr 2CaCu 2O 8+ δ and Bi 2Sr 2CuO 6+ δ probed by Josephson plasma resonance

We have measured the Josephson plasma resonance for underdoped Bi 2Sr 2CaCu 2O 8+ δ and Bi 2Sr 2CuO 6+ δ by a novel scanning microwave spectrometer in a wide frequency range (21–147 GHz). The resonance frequency and the line width enable us to determine a superfluid density and quasiparticle conductivity σ qp c in the c-axis direction accurately. In the slightly underdoped crystals, the superfluid density shows very little change at low temperature, which is consistent with a d-wave coherent tunneling model. Below T c, σ qp c falls to low values, then decreases gradually with decreasing T. This behavior is very different from that in the ab plane.

Josephson plasma resonance in Bi 2Sr 2CaCu 2O 8+ y with partially introduced columnar defects

We study the Josephson plasma resonance in Bi 2Sr 2CaCu 2O 8+ y single crystals with columnar defects (CDs) which are partially introduced in a controlled way. When the CDs are introduced into the crystal, either into the half of the ab-plane or the half along the c-axis, we observe multiple resonance peaks, one from the unirradiated and the others from the irradiated part. The resonance field, however, depends on which half part is irradiated. As the size of the irradiated and unirradiated part decreases, the resonance from the unirradiated part is strongly suppressed. These results are interpreted by assuming the phase correlation length, which determines the spatial extent over which the phase coherence is averaged, is semi-macroscopic in the ab-plane and shorter than 20 μm along the c-axis.

Effect of thermal and quantum fluctuations on superfluid density in layered superconductors

We calculate suppression of inter- and intra-layer superconducting currents due to equilibrium phase fluctuations and find that, in contrast to a recent prediction, the effect of thermal fluctuations cannot account for linear temperature dependence of the superfluid density in high- T c superconductors at low temperatures. Quantum fluctuations are found to dominate over thermal fluctuations at low temperatures due to hardening of their spectrum caused by the Josephson plasma resonance. Near T c sizeable thermal fluctuations are found to suppress the critical current in the stack direction stronger than in the direction along the layers. Fluctuations of quasi-particle branch imbalance make the spectral density of voltage fluctuations at small frequencies non-zero, in contrast to what may be expected from a naive interpretation of Nyquist formula.

Influence of Josephson current on vortex lattice melting

To study the Josephson coupling between CuO 2 layers, the c-axis critical current I c( H) as a function of the c-axis field was measured on the intrinsic Josephson junctions formed from Bi 2Sr 2CaCu 2O 8+ y single crystals. The junctions were fabricated by focused ion beam to form superconducting electrodes on the junctions. We have first observed a step in I c( H), which is considered as the first order vortex lattice melting. The step height of I c, however, is much smaller than the value obtained by the measurements of Josephson plasma resonance. Furthermore, the field at the step is smaller than the melting field ( H m) determined by resistance measurements with a lower c-axis current. Resistance measurements show that H m decreases as the c-axis current becomes large. There are some possibilities, i.e., the suppression of the effective Josephson coupling or the formation of spiral structure by the existence of Josephson current.

New devices based on an HTS intrinsic Josephson junction

The frequency range of HTS intrinsic Josephson plasma resonance covers from 100 GHz to several THz depending on the materials of the high- T c superconductors. Using THz Josephson plasma frequency, it is possible to develop new high-frequency devices such as THz generators and detectors. This paper surveys the static and dynamic quantum-size interference effect, Josephson plasma detection, ac intrinsic Josephson effect, and single Cooper-pair tunneling of sub-μm stack junctions. Comparing with existing competing technologies, extremely high performances of THz generators and single Cooper-pair memory shall be emphasized.

C-axis Josephson plasma resonance observed in Tl_2Ba_2CaCu_2O_8 superconducting thin films by use of terahertz time-domain spectroscopy

We have unambiguously observed the c -axis Josephson plasma resonance (JPR) in high-critical-temperature (T(c)) cuprate (Tl(2)Ba(2)CaCu(2)O(8)) superconducting thin films, employing terahertz time-domain spectroscopy in transmission as a function of temperature in zero magnetic field. These are believed to be the first measurements of the JPR temperature dependence of a high-T(c) material in transmission. With increasing temperature, the JPR shifts from 705 GHz at 10 K to ~170 GHz at 98 K, corresponding to an increase in c-axis penetration depth from 22.4+/-0.6mum to 94+/-9mum . The linewidth of the JPR peak increases with temperature, which indicates an increase in the quasi-particle scattering rate. We have probed the onset of the c -axis phase coherence to ~0.95T(c) . The JPR vanishes above T(c) as expected.

Josephson plasma resonance in Nd 1.85Ce 0.15CuO 4− δ single crystals

The Josephson plasma resonance (JPR) was measured in the microwave experiment in electron-doped superconductor Nd 1.85Ce 0.15CuO 4− δ single crystals as a function of temperature and magnetic field in microwave frequency ranging from 20 to 179 GHz. We used JPR as a tool to study the Josephson coupling in zero field and in the vortex state. We also report JPR in the vicinity of critical field H c2⊥ .

Josephson plasma resonance in Josephson vortex states

Josephson plasma resonance of under-doped Bi 2Sr 2CaCu 2O 8+ δ single crystals in magnetic fields parallel to the ab-plane has been studied at microwave frequencies ν between 10 and 52 GHz. Two resonance lines are discovered with distinctly different characters as functions of different physical parameters such as magnetic fields, microwave frequencies, temperatures, etc. Those two resonances behave entirely differently from the ones for H∥c and are attributed to the mixed resonance plasma modes with collective motion of the Josephson vortices.

Josephson plasma resonance crossing the second peak; first order nature of the Bragg-to-vortex glass transition

Here we report the detailed and quantitative study of the Josephson coupling energy in the vortex liquid, Bragg glass and vortex glass phases of Bi 2Sr 2CaCu 2O 8+ δ by the Josephson plasma resonance. The measurements revealed distinct features in the T- and H-dependencies of the Josephson plasma frequency ω pl for each of these three vortex phases. When going across either the Bragg-to-vortex glass or the Bragg-to-liquid transition line, ω pl shows a dramatic change. We provide a quantitative discussion on the properties of these phase transitions.

Macroscopic interference of interlayer phase coherence in Bi 2Sr 2CaCu 2O 8+ y with inhomogeneous columnar defects

By measurements of Josephson plasma resonance (JPR), we study the interlayer phase coherence in Bi 2Sr 2CaCu 2O 8+ y single crystals with partially introduced columnar defects. In the half-irradiated Bi 2Sr 2CaCu 2O 8+ y , we observe several resonances, which cannot be regard as a superposition of each constituent parts. The frequency dependence of the highest resonance field, which is considered to be originated from the irradiated part, shows a cyclotron behavior which is different from the ordinary JPR. These facts indicate that the resonance field in a region is affected by the neighboring region with different phase coherence over a macroscopic scale.

Transverse-optical Josephson plasmons: Equations of motion

A detailed calculation is presented of the dielectric function in superconducttors consisting of two Josephson coupled superconducting layers per unit cell, taking into account the effect of finite compressibility of the electron fluid. From the model it follows, that two longitudinal, and one transverse optical Josephson plasma resonance exist in these materials, for electric field polarization perpendicular to the planes. The latter mode appears as a resonance in the transverse dielectric function, and it couples directly to the electrical field vector of infrared radiation. A shift of all plasma frequencies, and a reduction of the intensity of the transverse optical Josephson plasmon is shown to result from the finite compressibility of the electron fluid.

Effects of vortex pinning and thermal fluctuations on the Josephson plasma resonance in Tl2Ba2CaCu2O8 and YBa2Cu3O6.5.

We investigated the temperature dependence and c-axis magnetic field dependence of the Josephson plasma resonance in optimally doped Tl2Ba2CaCu2O8 thin films and underdoped YBa2Cu3O6.5 (YBCO) ortho-II single crystals using infrared spectroscopy. We obtained the c-axis penetration depths, at low temperature, in zero fields of about 20 and 7 microm, respectively. While the temperature dependencies of the resonances in the two compounds are very similar, the magnetic field dependence in YBCO is much weaker. We attribute this weak magnetic field dependence to the lower anisotropy of YBCO and interpret the observed behaviors in terms of thermal fluctuations and pinning of pancake vortices.

Critical current and Josephson plasma resonance in the vortex glass phase ofBi2Sr2CaCu2O8+δ

We calculate the field dependence of the critical current and of the Josephson plasma resonance (JPR) frequency in the macroscopically uniform vortex glass phase in highly anisotropic layered superconductors and in standard pointlike Josephson junctions when the field is applied perpendicular to the junction. In these calculations we assume that, in the single-vortex pinning regime, at low fields B, vortex positions are weakly adjusted to the Josephson coupling and we account only for the adjustment of the phase difference. Our results are in agreement with experimental data for the JPR frequency measured in the ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{x}$ superconductor in the field range 0.03--0.6 T after field cooling.

Determination of the vortex structure in κ-(ET) 2Cu(NCS) 2 by Josephson plasma resonance

Observation of a Josephson plasma resonance in κ-(ET) 2 Cu(NCS) 2 , below T c (∼10 K), enables investigation of the vortex phase diagram within the mixed state of this highly anisotropic type-II superconductor. Temperature dependent studies seem to indicate a transition in the vortex structure near the irreversibility line, not previously observed in this material using this technique. We also show that the frequency dependence of the resonance between 28-153 GHz is best described by a week pinning theory previously derived by Bulaevskii et al.

Josephson plasma and vortex modes in layered superconductors

The Josephson-plasma and vortex modes in layered superconductors have been studied theoretically for low magnetic fields parallel and perpendicular to layers. The two modes belong to the same lowest-frequency branch of collective-mode spectrum localized near vortices. This is the Josephson-plasma resonance if pancakes are strongly pinned and cannot move. Otherwise the lowest-frequency mode is a vortex mode governed by pancake pinning and the vortex mass. The latter is strongly enhanced by wandering of a vortex line around its average direction. The recently observed jump of the magnetoabsorption-resonance frequency at the vortex phase transition line is interpreted as a transition from the Josephson-plasma to the vortex mode.

Josephson plasma resonance lineshape in the vortex liquid state at high frequency

Josephson plasma resonance lineshape in the vortex liquid state at high frequency

Vortex structure and dynamics in κ-(ET) 2Cu(NCS) 2

We use Josephson plasma resonance (JPR) and DC torque magnetization measurements to investigate the vortex structure and dynamics in the layered organic superconductor κ-(ET) 2Cu(NCS) 2. JPR studies probe the AC response of this extreme type-II superconductor for currents driven along the low conductivity axis. An observed crossover in the magnetic field and temperature dependence of the JPR frequency, in close proximity to the irreversibility line, is attributed either to a melting or a depinning transition in the quasi-two-dimensional (Q2D) vortex structure. Angle and temperature dependent magnetization studies reveal the rich dynamical process by which magnetic flux enters this highly anisotropic superconductor, as well as information on the structure of the vortices once inside. At extremely low temperatures ( T<100 mK), the sudden cessation of avalanching flux jumps above an angle- and temperature-dependent critical field B *(θ, T) , is indicative of Q2D vortex lattice melting. Thus, using these two very different techniques, we are able to probe the vortex phase diagram over a wide temperature and magnetic field range.

Josephson coupling, phase correlations, and Josephson plasma resonance in vortex liquid phase

Josephson plasma resonance (JPR) has been introduced recently as a powerful tool to probe interlayer Josephson coupling in different regions of the vortex phase diagram in layered superconductors. In the liquid phase, the high-temperature expansion with respect to the Josephson coupling connects the Josephson plasma frequency with the phase correlation function. This function, in turn, is directly related to the pair distribution function of the liquid. We develop a recipe to extract the phase and density correlation functions from the dependencies of the plasma resonance frequency ${\ensuremath{\omega}}_{p}(\mathbf{B})$ and the c-axis conductivity ${\ensuremath{\sigma}}_{c}(\mathbf{B})$ on the ab component of the magnetic field at fixed c component. Using Langevin dynamic simulations of two-dimensional vortex arrays we calculate density and phase correlation functions at different temperatures. Calculated phase correlations describe very well the experimental angular dependence of the plasma resonance field. We also demonstrate that in the case of weak damping in the liquid phase, broadening of the JPR line is caused mainly by random Josephson coupling arising from the density fluctuations of pancake vortices. In this case the JPR line has a universal shape, which is determined only by parameters of the superconductors and temperature.

Interaction between columnar defects and pancake vortices in Bi 2Sr 2CaCu 2O 8+ y studied by Josephson plasma resonance

Interlayer pancake vortex arrangement is studied by Josephson plasma resonance in Bi 2Sr 2CaCu 2O 8+ y with columnar defects in a wide range of matching field B Φ (5 G ∼ 40 kG). The c-axis coherence is found to be drastically enhanced above a characteristic field H* in the vortex liquid state, which we interpreted as recoupling of pancake vortices by columnar defects. The recoupling field H* scales as (0.2–0.3) B Φ at high doses ( B Φ > 500 G), but at lower doses it is almost constant ( H* ∼ 60 Oe), suggesting that the recoupling occurs when vortex-vortex interactions are significant at low doses. At high doses we found that (1) H* does not depend on oxygen content of the crystal and (2) when columnar defects are introduced at an angle θ from the c axis, H* is scaled as (0.2–0.3) B Φ cos θ. These results indicate that the recoupling at high doses is controlled only by the density of defects in each CuO 2 layers.

Josephson plasma resonance in Bi 2Sr 2CaCu 2O 8+ y crystals with macroscopic inhomogeneities

The Josephson plasma resonance (JPR) is studied in Bi 2Sr 2CaCu 2O 8+ y (BSCCO) crystals with macroscopic inhomogeneities, which are introduced by two methods; fabricating the crystal in the form having arrays of mesa structures (20×20 μm 2 ∼ 100×100 μm 2) or bu irradiating heavy-ions in half of the crystal. The JPR in BSCCO with mesa array shows an additional resonance peak indicating that the length scale where phase coherence is averaged is smaller than the size of the mesas. On the other hand, the resonance fields in the half-irradiated BSCCO and those after cutting the crystal into two are different, suggesting the presence of some kind of interference between regions which have different macroscopically averaged phase coherence properties.