Search Results (35 total)

Particle-in-cell (PIC) and fluid simulations of two-plasmon decay (TPD) instability under conditions relevant to inertial confinement fusion show the importance of convective modes. Growing at the lower density region, the convective modes can cause pump depletion and are energetically dominant in the nonlinear stage. The PIC simulations show that TPD saturates due to ion density fluctuations, which can turn off TPD by raising the instability threshold through mode coupling.

We report a systematic investigation, together with a theoretical analysis, of the resistivity and Hall effect in single crystals of Ba(Fe_1−xCo_x)₂As₂ over a wide doping range. We find a surprisingly great disparity between the relaxation rates of the holes and the electrons in excess of one order of magnitude in the low-doping, low-temperature regime. The ratio of the electron to hole mobilities diminishes with temperature and doping (away from the magnetically ordered state) and becomes more conventional. We also find a straightforward explanation of the large asymmetry (compared to cuprates) of the superconducting dome: in the underdoped regime the decisive factor is the competition between antiferromagnetism and superconductivity, while in the overdoped regime the main role is played by degradation of the nesting that weakens the pairing interaction. Our results indicate that spin fluctuations due to interband electron-hole scattering play a crucial role not only in the superconducting pairing but also in the normal transport.

Using high-vacuum annealing treatment, the superconducting transition temperature T_c of the electron-doped Pr_0.88LaCe_0.12CuO_4±δ single crystals was successfully driven to a regime in which the T_c value continuously decreases with oxygen removal. This regime of oxygen-reduction process is hard to be understood according to the previous knowledge of this material. In addition, a remarkable relaxation of T_c over time was observed at room temperature, indicating that the variation in T_c is closely related to the rearrangement of oxygen in the sample. In combination with previous studies, it can be concluded that, in the whole process of oxygen reduction applied on Pr_0.88LaCe_0.12CuO_4±δ, although the strength of antiferromagnetic (fluctuation) correlations does play a role, T_c is dominated by the disorder effect of CuO₂ plane either on copper sites or oxygen sites, which leads to a strong depairing effect.

We have measured the magnetic field and temperature dependence of specific heat on Bi₂Sr_2-xLa_xCuO_6+δ single crystals in wide doping and temperature regions. The superconductivity related specific-heat coefficient γ_sc and entropy S_sc are determined. It is found that γ_sc has a humplike anomaly at T_c and behaves as a long tail which persists far into the normal state for the underdoped samples, but for the heavily overdoped samples the anomaly ends sharply just near T_c. Interestingly, we found that the entropy associated with superconductivity is roughly conserved when and only when the long tail part in the normal state is taken into account for the underdoped samples, indicating the residual superconductivity above T_c.

By substituting the Fe with the 4d- and 5d-transition metals Rh, Ir, and Pd in SrFe₂As₂, we have successfully synthesized a series of superconductors SrFe_2−xM_xAs₂ (M=Rh, Ir, and Pd) and explored the phase diagrams of them. The systematic evolution of the lattice constants indicated that part of the Fe ions were successfully replaced by the transition metals Rh, Ir, and Pd. By increasing the doping content of Rh, Ir, and Pd, the antiferromagnetic (AF) state of the parent phase is suppressed progressively and superconductivity is induced. The general phase diagrams were obtained and found to be similar to the case of doping Co and Ni to the Fe sites. However, the detailed structure of the phase diagram, in terms of how fast to suppress the antiferromagnetic order and induce the superconductivity, varies from one kind of doped element to another. Regarding the close values of the maximum superconducting transition temperatures in doping Co, Rh, and Ir which locate actually in the same column in the periodic table of elements but have very different masses, we argue that the superconductivity is intimately related to the suppression of the AF order, rather than the electron-phonon coupling.

Low temperature specific heat (SH) was measured on the FeAs-based superconducting single crystals Ba_0.6K_0.4Fe₂As₂. It is found that the sharp SH anomaly ΔC/T∣_Tc in our samples reaches an unexpected high value of 98 mJ/mol K², about 1 order of magnitude larger than that of the F-doped system LnFeAsO (Ln=rare-earth elements), suggesting a very high normal-state quasiparticle density of states in the FeAs-122 system than in the FeAs-1111 system. Furthermore, we found that the electronic SH coefficient γ_e(T) of Ba_0.6K_0.4Fe₂As₂ is weakly temperature dependent and increases almost linearly with the magnetic field in the low temperature region, which may indicate that the hole-doped FeAs-122 system contains a dominant component with a full superconducting gap, although we cannot rule out the possibility of a small component with an anisotropic or nodal gap. A detailed analysis reveals that the γ_e(T) of Ba_0.6K_0.4Fe₂As₂ cannot be fitted with a single gap of s-wave symmetry, probably due to the multigap effect. These results indicate a clear difference between the properties of the superconducting state of the holed-doped Ba_0.6K_0.4Fe₂As₂ and the F-doped LnFeAsO (Ln=rare-earth elements) system, which we believe originated from the complex Fermi-surface structures in different systems.

The collisional effects on the current-filamentation instability (CFI) and the two-stream instability (TSI), which appear as a relativistic intense electron beam penetrating into a cold dense plasma, are investigated. It is shown that the growth rate of the CFI mode is first attenuated and then enhanced by the collisional effects as the density ratio of the background plasma to the beam increases. Meanwhile, the maximum CFI growth rate is shifted to the long-wavelength region due to both the bulk plasma density increase and the collisional effects, resulting in larger filaments formation. On the other hand, collisional effects mainly attenuate the TSI and finally stabilize it. Numerical solutions under parameters close to the fast ignition scenario (FIS) are given, which show that the CFI growth rate can be enhanced by 2 orders of magnitude instead of being suppressed in the dense region. Therefore, the CFI-induced electron filaments formation and the resultant kinetic anomalous heating are potentially significant for the target heating in the FIS.

Physical properties have been comprehensively investigated on the noncentrosymmetric superconductor Ru₇B₃ with T_C=3.3 K. It is found that the superfluid comprises a major component with a full-gap feature instead of a nodal-like pairing symmetry as revealed by specific-heat and lower-critical-field measurements. Combining with resistivity (ρ_xx and ρ_xy) and electronic band-structure calculation, the superconducting and normal-state physical parameters were determined by a self-consistent analysis. It is found that Ru₇B₃ may belong to a single band superconductor with energy gap of 0.5 meV and could be categorized into type-II superconductor with weak electron-phonon coupling. An unusual “kink” feature is clearly observed in the field induced broadening of the resistivity curves. Possible reasons are given for this unusual two-step transition.

We report a detailed investigation on the lower critical field H_c1 of the superconducting Ba_0.6K_0.4Fe₂As₂ (122) single crystals. A pronounced kink is observed on the H_c1(T) curve, which is attributed to the existence of two superconducting gaps. By fitting the data H_c1(T) to the two-gap BCS model in the full temperature region, a small gap of Δ_a(0)=2.0±0.3  meV and a large gap of Δ_b(0)=8.9±0.4  meV are obtained. The in-plane penetration depth λ_ab(0) is estimated to be 105 nm corresponding to a rather large superfluid density, which points to the breakdown of the Uemura plot in 122 superconductors.

The temperature-dependent resistivity of Ba_1−xK_xFe₂As₂ (x=0.23, 0.25, 0.28, and 0.4) single crystals and the angle-dependent resistivity of superconducting Ba_0.6K_0.4Fe₂As₂ single crystals were measured in magnetic fields up to 9 T. The data measured on samples with different doping levels revealed very high upper critical fields, which increase with the transition temperature, and a very low superconducting anisotropy ratio Γ=H_c2^ab/H_c2^c≈2. By scaling the resistivity within the framework of the anisotropic Ginzburg-Landau theory, the angle-dependent resistivity of the Ba_0.6K_0.4Fe₂As₂ single crystal measured with different magnetic fields at a certain temperature collapsed onto one curve. As the only scaling parameter, the anisotropy Γ was alternatively determined for each temperature and was found to be between two and three.

By measuring the dynamic and traditional magnetization relaxations we investigate the vortex dynamics of the recently discovered superconductor SmFeAsO_0.9F_0.1 with T_c=55 K. It is found that the relaxation rate is rather large reflecting a small characteristic pinning energy. Moreover it shows a weak temperature dependence in wide temperature region, which resembles the behavior of the cuprate superconductors. Combined with the resistivity data under different magnetic fields, a vortex phase diagram is obtained. Our results strongly suggest that the model of collective vortex pinning applies to this superconductor very well.

Two dimensional particle-in-cell simulations show that laser channeling in millimeter-scale underdense plasmas is a highly nonlinear and dynamic process involving longitudinal plasma buildup, laser hosing, channel bifurcation and self-correction, and electron heating to relativistic temperatures. The channeling speed is much less than the linear group velocity of the laser. The simulations find that low-intensity channeling pulses are preferred to minimize the required laser energy but with an estimated lower bound on the intensity of I≈5×10¹⁸  W/cm² if the channel is to be established within 100 ps. The channel is also shown to significantly increase the transmission of an ignition pulse.

We use in-plane tunneling spectroscopy to study the temperature dependence of the local superconducting gap Δ(T) in electron-doped copper oxides with various T_c’s and Ce-doping concentrations. We show that the temperature dependence of Δ(T) follows the expectation of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, where Δ(0)∕k_BT_c≈1.72±0.15 and Δ(0) is the average superconducting gap across the Fermi surface, for all the doping levels investigated. These results suggest that the electron-doped superconducting copper oxides are weak-coupling BCS superconductors.

We report measurements of the spin polarization (P) of the concentrated magnetic semiconductor EuS using both zero-field and Zeeman-split Andreev reflection spectroscopy (ARS) with EuS∕Al planar junctions. The zero-field ARS spectra are well described by the modified (spin-polarized) Blonder-Tinkham-Klapwijk (BTK) model with expected superconducting energy gap and actual measurement temperature (no additional spectral broadening). The fittings consistently yield P close to 80% regardless of the barrier strength. Moreover, we performed ARS in the presence of a Zeeman splitting of the quasiparticle density of states in Al. To describe the Zeeman-split ARS spectra, we develop a theoretical model which incorporates the solution to the Maki-Fulde equations into the modified BTK analysis. The method enables the determination of the magnitude as well as the sign of P with ARS, and the results are consistent with those from the zero-field ARS. The experiments extend the utility of field-split superconducting spectroscopy from tunnel junctions to Andreev junctions of arbitrary barrier strengths.

We have measured ac magnetic susceptibility (χ=χ^′+iχ^″) of the electron-doped superconducting Pr_0.88LaCe_0.12CuO_4−δ with applied magnetic field (H) either parallel or perpendicular to the CuO₂ plane (H‖ab-plane or H‖c-axis). For H‖ab-plane, a peak in the temperature dependence of the screening current is revealed as a dip in the real part of the ac susceptibility χ. The temperature at which this peak anomaly occurs decreases with increasing field and lies well below the irreversibility line in the H-T phase diagram. This peak effect may arise from the phase transition of Josephson vortices from a quasiordered vortex lattice to a disordered glass phase.

We use near-field scanning optical microscopy (NSOM) operating in collection mode to map the optical field distribution of continuous wave infrared light propagating along an air-bridged W3 silicon photonic crystal (PC) slab multimode waveguide in a subwavelength resolution. The detected near-field optical intensity distribution patterns show very different longitudinal propagation features and transverse field profiles at different wavelength ranges. We have analyzed the dispersion of the guided modes in the PC waveguide and the corresponding eigenfield profile for each guided mode by means of the three-dimensional plane-wave expansion simulations. It is found that the experimentally observed complex while interesting near-field pattern features can be well explained by the field profiles of the multiple waveguide modes and their superposition at different excitation wavelengths. The detection and analysis of this multimode PC slab waveguide via NSOM could help us to understand complex wave propagation behavior and to further design novel PC devices.

The temperature and field dependence of reversible magnetization has been measured on a YBa₂Cu₃O_7−δ single crystal at six different doping concentrations. It is found that the data above 2 T can be described by a scaling law based on critical fluctuation theory in a lowest-Landau-level approach in Ginzburg-Landau theory yielding the values of the slope of upper critical field −dH_c2(T)∕dT near T_c. A universal scaling has been found in the five underdoped samples. Based on a simple Ginzburg-Landau approach, we determined the doping dependence of the upper critical field H_c2(0) and the coherence length ξ. Our results may indicate a growing coherence length with increasing underdoping.

The point-contact Andreev reflection spectroscopy for a simple BCS type-II superconductor was investigated in pure niobium (Nb). The small broadening factor of Γ∕Δ(0)<0.09 was achieved by careful preparation of the junctions. Then we attempted to use the recently proposed two-channel model [Phys. Rev. B 72, 012502 (2005)] to explain the spectra measured in various magnetic fields for such low-Γ case. It was found that this rigid vortex core model is appropriate below a crossover field H^*, above which it is not adequate due to the onset of substantial vortex overlapping effect.

We consider how an unmagnetized plasma responds to an incoming flux of energetic electrons. We assume a return current is present and allow for the incoming electrons to have a different transverse temperature than the return current. To analyze this configuration we present a nonrelativistic theory of the current-filamentation or Weibel instability for rigorously current-neutral and nonseparable distribution functions, f₀(p_x,p_y,p_z)≠f_x(p_x)f_y(p_y)f_z(p_z). We find that such distribution functions lead to lower growth rates because of space-charge forces that arise when the forward-going electrons pinch to a lesser degree than the colder, backward-flowing electrons. We verify the growth rate, range of unstable wave numbers, and the formation of the density filaments using particle-in-cell simulations.

We present a systematic characterization of fluctuations in submicron Hall devices based on GaAs/AlGaAs two-dimensional electron gas heterostructures at temperatures between 1.5 to 60 K. A large variety of noise spectra, from 1/f to Lorentzian, are obtained by gating the Hall devices. The noise level can be reduced by up to several orders of magnitude with a moderate gate voltage of 0.2 V, whereas the carrier density increases less than 60% in the same range. The significant dependence of the Hall noise spectra on temperature and gate voltage is explained in terms of the switching processes related to impurities in n-AlGaAs.

A comprehensive examination of the interaction of a picosecond-long ignition pulse on high-density (40 times critical density) pellets using a two-dimensional particle-in-cell model is described. The global geometry consists of a 50   μm diameter pellet surrounded by a corona which is isolated by a vacuum region from the boundary. For cone-attached targets, as much as 67% of the incident laser energy is absorbed with 12% sent forward as fast electrons in a 23° cone. The current filaments are driven by the Weibel instability of the forward-going fast electron flux and its return current with the ions playing an important role of neutralizing the space charge. No global current filament coalescence has been observed. The electron distribution function obeys a power law, which begins at E∼0.2  MeV and falls off as E^-(2–3).

We studied low-frequency noise in NiFe-Al₂O₃-NiFe based magnetic tunnel junctions (MTJ’s) with and without a hard-axis bias field. The 1/f noise is observed to be magnetic-field dependent and reduced with the application of hard-axis bias fields, attributed to thermally activated magnetization fluctuations in the magnetic electrodes. A linear dependence of noise on derivative of magnetoresistance has been observed, and the magnetic-field noise for MTJ sensing elements is defined and evaluated to be as low as 60 nT μm/Hz^1/2.

The formation of strong, high Mach number (2–3), electrostatic shocks by laser pulses incident on overdense plasma slabs is observed in one- and two-dimensional particle-in-cell simulations, for a wide range of intensities, pulse durations, target thicknesses, and densities. The shocks propagate undisturbed across the plasma, accelerating the ions (protons). For a dimensionless field strength parameter a₀=16 (Iλ²≈3×10²⁰  W cm^-2 μm², where I is the intensity and λ the wavelength), and target thicknesses of a few microns, the shock is responsible for the highest energy protons. A plateau in the ion spectrum provides a direct signature for shock acceleration.

The interaction between a low-density electron cloud in a circular particle accelerator with a circulating charged particle beam is considered. The particle beam’s space charge attracts the cloud, enhancing the cloud density near the beam axis. It is shown that this enhanced charge and the image charges associated with the cloud charge and the conducting wall of the accelerator may have important consequences for the dynamics of the beam propagation. The tune shift due to the electron cloud is obtained analytically and compared to a new numerical model (QuickPIC) that is described here. Sample numerical results are presented and their significance for current and planned experiments is discussed.

This Letter examines the electron-hosing instability in relation to the drivers of current and future plasma-wakefield experiments using fully three-dimensional particle-in-cell simulation models. The simulation results are compared to numerical solutions and to asymptotic solutions of the idealized analytic equations. The measured growth rates do not agree with the existing theory and the behavior is shown to depend sensitively on beam length, shape, and charge. We find that even when severe hosing occurs the wake can remain relatively stable.