Charge order, dynamics, and magneto-structural transition in multiferroic LuFe2_2O4_4

We investigated the series of temperature and field-driven transitions in LuFe$_2$O$_4$ by optical and M\"{o}ssbauer spectroscopies, magnetization, and x-ray scattering in order to understand the interplay between charge, structure, and magnetism in this multiferroic material. We demonstrate that charge fluctuation has an onset well below the charge ordering transition, supporting the "order by fluctuation" mechanism for the development of charge order superstructure. Bragg splitting and large magneto optical contrast suggest a low temperature monoclinic distortion that can be driven by both temperature and magnetic field.Comment: 4 pages, 3 figures, PRL in prin

Two-gap superconductivity in Ba_1-xK_xFe_2As_2: A complementary study of the magnetic penetration depth by \muSR and ARPES

We investigate the magnetic penetration depth \lambda in superconducting Ba_1-xK_xFe_2As_2 (T_c\simeq32K) with muon-spin rotation (\muSR) and angle-resolved photoemission (ARPES). Using \muSR, we find the penetration-depth anisotropy \gamma_\lambda=\lambda_c/\lambda_{ab} and the second-critical-field anisotropy \gamma_{H_c2} to show an opposite T-evolution below T_c. This dichotomy resembles the situation in the two-gap superconductor MgB_2. A two-gap scenario is also suggested by an inflection point in the in-plane penetration depth \lambda_ab around 7K. The complementarity of \muSR and ARPES allows us to pinpoint the values of the two gaps and to arrive to a remarkable agreement between the two techniques concerning the full T-evolution of \lambda_ab. This provides further support for the described scenario and establishes ARPES as a tool to assess macroscopic properties of the superconducting condensate.Comment: Accepted for publication by Phys. Rev. Let

Anisotropy and internal field distribution of MgB2 in the mixed state at low temperatures

Magnetization and muon spin relaxation on MgB2 were measured as a function of field at 2 K. Both indicate an inverse-squared penetration depth strongly decreasing with increasing field H below about 1 T. Magnetization also suggests the anisotropy of the penetration depth to increase with increasing H, interpolating between a low Hc1 and a high Hc2 anisotropy. Torque vs angle measurements are in agreement with this finding, while also ruling out drastic differences between the mixed state anisotropies of the two basic length scales penetration depth and coherence length.Comment: 4 pages, 4 figure

Temperature-dependent Hc2H_{c2} anisotropy in MgB2_2 as inferred from measurements on polycrystals

We present data on temperature-dependent anisotropy of the upper critical field of MgB$_2$ obtained from the analysis of measurements on high purity, low resistivity polycrystals. The anisotropy decreases in a monotonic fashion with increase of temperature

Distinct order of Gd 4f and Fe 3d moments coexisting in GdFe4Al8

Single crystals of flux-grown tetragonal GdFe4Al8 were characterized by thermodynamic, transport, and x-ray resonant magnetic scattering measurements. In addition to antiferromagnetic order at TN ~ 155 K, two low-temperature transitions at T1 ~ 21 K and T2 ~ 27 K were identified. The Fe moments order at TN with an incommensurate propagation vector (tau,tau,0) with tau varying between 0.06 and 0.14 as a function of temperature, and maintain this order over the entire T<TN range. The Gd 4f moments order below T2 with a ferromagnetic component mainly out of plane. Below T1, the ferromagnetic components are confined to the crystallographic plane. Remarkably, at low temperatures the Fe moments maintain the same modulation as at high temperatures, but the Gd 4f moments apparently do not follow this modulation. The magnetic phase diagrams for fields applied in [110] and [001] direction are presented and possible magnetic structures are discussed.Comment: v2: 14 pages, 12 figures; PRB in prin

Anisotropy of the upper critical field in superconductors with anisotropic gaps. Anisotropy parameters of MgB2

The upper critical field Hc2 is evaluated for weakly-coupled two-band superconductors. By modeling the actual bands and the gap distribution of MgB2 by two Fermi surface spheroids with average parameters of the real material, we show that H_{c2,ab}/H_{c2,c} increases with decreasing temperature in agreement with available data.Comment: 4 pages, 2 figure

Compact hot-nozzle fourier-transform microwave spectormeter

A newly constructed pulsed nozzle, Fourier-transform microwave spectrometer utilizes a Fabry-Perot cavity consisting of spherical resonators having diameters of only 10 cm. Tests of this very compact-cavity system show that its sensitivity is only slightly lower than that of a comparably configured system of the Balle-Flygare design having resonators with diameters of 36 cm. With a volume 50 times smaller than in conventional systems, the compact cavity also requires a much smaller vacuum chamber which can be pumped by a relatively small 6 in. diffusion pump. The system includes an integral ceramic nozzle which can be heated to temperatures above 1000 degrees C. Spectrometer characteristics have been demonstrated by means of experiments on OCS isotopomers in ground and excited vibrational states, ArOCS complexes, and chloroketene, a reactive intermediate formed by pyrolysis of chloroacetylchloride. (C) 1995 American Institute of Physics

Lattice dynamical probe of charge order and antipolar bilayer stacking in LuFe2O4

We investigated the infrared response of LuFe2O4 through the series of charge, magnetic, and structural transitions. All vibrational modes couple strongly to the charge order, whereas the LuO zone-folding modes are also sensitive to magnetic order and structural distortion. The dramatic splitting of the LuO2 layer mode is attributed to charge-rich/poor proximity effects and its temperature dependence reveals the antipolar nature of the W layer pattern

Coherence lengths and anisotropy in MgB2 superconductor

Field and temperature microwave measurements have been carried out on MgB2 thin film grown on Al2O3 substrate. The analysis reveals the mean field coherence length xi_{MF} in the mixed state and a temperature independent anisotropy ratio gamma_{MF} = xi_{MF}^{ab} / xi_{MF}^c approximately 2. At the superconducting transition, the scaling of the fluctuation conductivity yields the Ginzburg-Landau coherence length with a different anisotropy ratio gamma_{GL} = 2.8, also temperature independent.Comment: submitted to PR