We describe instrumentation for conducting high sensitivity millimeter-wave cavity perturbation measurements over a broad frequency range (40 − 200 GHz) and in the presence of strong magnetic fields (up to 33 tesla). A Millimeter-wave Vector Network Analyzer (MVNA) acts as a continuously tunable microwave source and phase sensitive detector (8 − 350 GHz), enabling simultaneous measurements of the complex cavity parameters (resonance frequency and Q−value) at a rapid repetition rate (∼ 10 kHz). We discuss the principal of operation of the MVNA and the construction of a probe for coupling the MVNA to various cylindrical resonator configurations which can easily be inserted into a high field magnet cryostat. We also present several experimental results which demonstrate the potential of the instrument for studies of low-dimensional conducting systems.

We use a multi-high-frequency resonant cavity technique to obtain EPR spectra for single crystal samples of the biaxial molecular magnet Fe 8 [(tacn) 6 O 2 (OH) 12 ]Br 8 ·9H 2 O (Fe 8 ). By performing measurements at many closely spaced frequencies, we are able to extrapolate data back to zero magnetic field and, thereby, obtain accurate estimates of the zero-field splittings. Furthermore, from the (low-) field dependence of these splittings, with the magnetic field parallel to the easy axis, we can directly measure the g z -value. Measurements performed with the magnetic field parallel to the intermediate and hard axes may be used to constrain further the Hamiltonian parameters. Our results are in broad agreement with recent inelastic neutron scattering data. In addition, analysis of individual resonances (which we can assign to known transitions) reveals a pronounced M S dependence of the resonance line widths. Furthermore, the line positions exhibit complex (again M S dependent) temperature dependences that cannot be reconciled with the standard spin Hamiltonian.

We report torque magnetization measurements in regions of the mixed state phase diagram ( B approximately mu(o)H(c2) and T(c)/10(3)) of the organic superconductor kappa-(ET)2Cu(NCS)(2), where quantum fluctuations are expected to dominate thermal effects. Over most of the field range below the irreversibility line ( B(irr)), magnetothermal instabilities are observed in the form of flux jumps. The abrupt cessation of these instabilities just below B(irr) indicates a quantum melting transition from a quasi-two-dimensional vortex lattice phase to a quantum liquid phase.

A cavity perturbation technique is used to study the microwave response of the organic superconductor -(BEDT-TTF) 2 Cu(NCS) 2 . Observation of a Josephson plasma resonance, below T c (ϳ10 K), enables investigation of the vortex structure within the mixed state of this highly anisotropic, type-II superconductor. Contrary to previous assumptions, frequency-dependent studies ͑28-153 GHz͒ indicate that the squared plasma frequency ( p 2 ) depends exponentially on the magnetic-field strength. Such behavior has been predicted for a weakly pinned quasi-two-dimensional vortex lattice ͓Bulaevskii et al., Phys. Rev. Lett. 74, 801 ͑1995͔͒. Our data also suggests a transition in the vortex structure near the irreversibility line.

A microwave cavity perturbation technique is used to probe the interlayer electrodynamics within the vortex state of the organic superconductor κ−(BEDT−TTF) 2 Cu(NCS) 2 . A Josephson plasma mode is observed which is extremely sensitive to correlations in the locations of vortices in adjacent layers and may, therefore, be used to gauge collective effects between vortices and crystal pinning sites in the title compound. Our previous investigations [M. M. Mola et al., Phys. Rev. B 62 (2000) 5965] revealed a transformation from a correlated quasi-two-dimensional pinned vortex phase, to either a depinned or liquid state. In this study, we carry out a detailed analysis of the magnetic field dependence of the Josephson plasma frequency within the two phases. Our findings agree favorably with recent theoretical models: within the liquid state, the squared plasma frequency (ω 2 p ) decays with the inverse of the magnetic field strength, B; whereas, in the pinned phase, a much slower decay is observed (ω 2 p ∝ B −0.35 ), which is indicative of weak pinning.