M. M. Mola

Instrumentation for millimeter-wave magnetoelectrodynamic investigations of low-dimensional conductors and superconductors

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 T). 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 principle 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.

Single crystal EPR determination of the spin Hamiltonian parameters for Fe8 molecular clusters

Abstract 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.

Quantum Melting of the Quasi-Two-Dimensional Vortex Lattice inκ-(ET)2Cu(NCS)2

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.

Josephson plasma resonance in κ − ( BEDT − TTF ) 2 Cu ( NCS ) 2

A cavity perturbation technique is used to study the microwave response of the organic superconductor k-(BEDT-TTF)2Cu(NCS)2. Observation of a Josephson plasma resonance, below Tc (approx. 10 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 GHz) indicate that the squared plasma frequency 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 (1995)], but has not so far been observed experimentally. Our data also suggests a transition in the vortex structure near the irreversibility line not previously reported for an organic superconductor using this technique.

Josephson plasma resonance in k-(BEDT-TTF)2Cu(NCS)2

A cavity perturbation technique is used to study the microwave response of the organic superconductor k-(BEDT-TTF)2Cu(NCS)2. Observation of a Josephson plasma resonance, below Tc (approx. 10 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 GHz) indicate that the squared plasma frequency 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 (1995)], but has not so far been observed experimentally. Our data also suggests a transition in the vortex structure near the irreversibility line not previously reported for an organic superconductor using this technique.

Interlayer electrodynamics in the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 (BEDT-T T F ≡ bis- ethylenedithio-tetrathiafulvalene): evidence for a transformation within the vortex state

A microwave cavity perturbation technique is used to probe the interlayer electrodynamics within the vortex state of the organic superconductor κ-(BEDT-TTF)2Cu(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 (Mola M M et al2000 Phys. Rev. B 62 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 favourably with recent theoretical models: within the liquid state, the squared plasma frequency (ωp2) decays with the inverse of the magnitude of the magnetic field strength, B; whereas, in the pinned phase, a much slower decay is observed (ωp2 ∝ B−0.35), which is indicative of weak pinning.

MAGNETO-THERMAL INSTABILITIES IN AN ORGANIC SUPERCONDUCTOR

Angle and temperature dependent torque magnetization measurements are reported for the organic superconductor κ-(ET)2Cu(NCS)2, at extremely low temperatures (~ Tc/103). Magneto-thermal instabilities are observed in the form of abrupt magnetization (flux) jumps. We carry out an analysis of the temperature and field orientation dependence of these flux jumps based on accepted models for layered type-II superconductors. Using a simple Bean model, we also find a critical current density of 4 × 108A/m2 from the remnant magnetization, in agreement with previous measurements.

Electron magnetic resonance imaging of the Fermi surface of Sr2RuO4

Abstract We have carried out detailed angle dependent studies of the normal state microwave (40–112 GHz) magneto-conductivity of several single crystal samples of the perovskite superconductor Sr 2 RuO 4 . As previously reported [Phys. Rev. Lett. 84 (2000) 3374], we observe a series of resonant absorptions which we attribute to cyclotron resonance of quasiparticles belonging to the three well-known Fermi surfaces for this material. From the angle dependence, we confirm the two-dimensional character of these resonances, i.e. the cyclotron frequencies scale as the inverse cosine of the angle between the magnetic field and the normal to the conducting layers. Furthermore, by performing measurements on several samples, and in different electromagnetic field configurations, we are able to couple to different cyclotron modes (+ harmonics) which derive from deformations (warpings) of the Fermi surfaces from perfect cylinders. These mode couplings will be discussed in the light of recent angle dependent de Haas–van Alphen measurements.

Flux jumps and melting of the vortex lattice in κ-(ET)2Cu(NCS)2

The angular and temperature dependence of the DC magnetization of the organic superconductor κ-(ET) 2 Cu(NCS) 2 has been investigated in high magnetic fields (up to 20 T) and at low temperatures (25-200 mK). In moderate fields, the mixed state of this layered superconductor exhibits a series of flux jumps, indicative of critical surface screening currents. At higher fields, a melting of the quasi-two-dimensional vortex lattice is observed below the irreversibility line.

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

Abstract 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 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.