Bascom S. Deaver

Magnetic susceptibility measurements using a superconducting magnetometer

The design and performance of an apparatus for measuring magnetic susceptibility using a superconducting magnetometer and superconducting magnet and shields are described. Measurements of static susceptibility and of nuclear magnetic resonance have been made with this system. Volume susceptibility changes of 10−10 cgs can be measured with a 1 sec response time, a 100 Oe applied field, and a 1 cm3 sample. Noise limitations and potential for improvement of sensitivity by several orders of magnitude are discussed.

Crystal structures of A2FeCl5⋅H2O (A=Rb+, Cs+) and field dependent superconducting susceptometer measurements

Crystal structures and low field principal crystal axis magnetic measurements are reported for the antiferromagnetic iron (III) salts A2FeCl5⋅H2O; A=Rb+, Cs+. The two compounds undergo a zero field paramagnetic to antiferromagnetic phase transition at 6.5 K (A=Cs+) and 10.05 K (A=Rb+). The spin flop magnetic phase is observed in high field measurements. The magnetic field‐temperature phase diagrams are reported. All magnetic measurements were recorded on a superconducting susceptometer. The bicritical points are 6.25 K, 14.5 kOe (A=Cs+), 9.75 K, 17.8 KOe (A=Rb+). Crystal data for Cs2FeCl5⋅H2O: space group Cmcm, Z=4, a=7.442(3) A, b=17.307(7) A, c=8.077(7) A, V=1040 A3, R=5.2%, 561 reflections. Crystal data for Rb2FeCl5⋅H2O, space group Pnma, Z=4, a=13.825(4) A, b=9.918(3) A, c=7.100(2) A, V=4.9 A3, 5.7%, 712 reflections.

Raman and far-infrared spectra of proustite (Ag3AsS3) and pyrargyrite (Ag3SbS3)

The polarized Raman spectra of large (1 cm × 1 cm diameter) single crystal specimens of proustite (Ag3AsS3) and pyrargyrite (Ag3SbS3) have been obtained The number of irreducible representations of the unit cell, C3μ6 factor group, was calculated and the nineteen fundamental Raman and infrared-active modes (K ≈ 0) were accounted for in both materials. The far-infrared spectra were obtained from powder samples.The vibrational modes were characterized qualitatively via symmetry coordinates calculated from internal coordinates of the unit cell. It is concluded that the low frequency A1 mode (37 cm-1) seen in both materials involves essentially torsional motion of the AsS3 ≡ and SbS3 ≡ pyramids about the c-axis.

rf SQUID’s in the nonhysteretic mode: Detailed comparison of theory and experiment

A simple analytic theory for the operation of rf SQUID’s in the regime where the current I in the superconducting ring is a continuous function of the applied flux (nonhysteretic mode) has been extended to include the effects of screening flux LI, where L is the inductance of the ring. A comparison of calculated results with detailed measurements of the response of a rf SQUID using a toroidal Nb ring containing a Nb point contact shows excellent agreement both in cases when screening flux is significant and when it can be neglected.

Microwave mixing with niobium variable thickness bridges

Niobium thin‐film bridges 300 A thick, 1 μm wide, and 0.5 μm long joining two bulk films 5000 A thick and having normal resistance of ∼1 Ω have been fabricated and used for microwave mixing at 10 GHz. They exhibit Josephson, bolometric, and multiple‐flux‐flow mixing and have useful response at 100–200 GHz. The data show in a direct way limitations imposed by flux flow and heating.

Photomagnetism of the triplet state of coronene

Abstract A new technique using a superconducting susceptometer to record the transient paramagnetism of excited states induced by irradiation of molecules with a pulsed laser has been used to measure the magnetic susceptibility of the lowest excited triplet state of coronene, yielding a triplet lifetime τ T = 9.2 s and quantum yield Φ T = 0.61, consistent with a pure spin state.

An experimental demonstration of winding number dependence of the Aharonov-Bohm effect☆

Abstract Measurements on a superconducting ring which twice encircles a long solenoid containing flux show quantized flux states with δφ = φ 0 2 = h 4e . This explicity demonstrates the winding number dependence of the Aharonov-Bohm effect.

Heating and flux flow in niobium variable‐thickness bridges

The properties of niobium superconducting microbridges consisting of a thin‐film bridge 300 A thick, 1 μm wide, and 0.5 μm long joining two bulk films 5000 A thick have been studied by measurements of I‐V and dV/dI versus V as functions of temperature and microwave power and by microwave mixing experiments. Near Tc these bridges behave as nearly ideal Josephson junctions but at lower temperature their characteristics appear to be determined by flux flow. Hysteresis of the I‐V curves, depression of the energy gap, a feature interpreted as the onset of a self‐sustaining hot spot, and bolometric mixing can all be described very well by a simple heating model.

Temperature Dependence of Structure on the I‐V Curves of Superconducting Point Contacts

In addition to the structure at V=2Δ/ne (2Δ is the energy gap, n is an integer), we observe prominent structure on the I‐V curves of superconducting point contacts at V≫2Δ/e. We have studied the temperature dependence of all the structure and find some features vary like Δ(T) and others vary linearly with T4. These variations are interpreted in terms of a simple equivalent circuit model of the point contact and indicate that all of the features for clean point contacts are related to the Josephson Effect.

Measurements of the penetration depth in the superconducting layer crystals NbSe2 and TaS2 (pyridine) 12

Abstract The superconducting penetration depth λ for a magnetic field applied parallel to the layers has been measured for the superconducting layer crystals NbSe2 and TaS2 (py) 1 2 in fields ranging from 1 to 100 mGauss near Tc. For NbSe2 these data together with the anisotropy parameter ϵ = λ ⊥ λ ″ of 0.25 (λ″ and λ⊥ denote the penetration depths parallel and perpendicular to the crystal layers, respectively) yield λ⊥ (0) = 0.124μm and λ″ (0) = 0.496μm. For TaS2 (py) 1 2 the value of λ″ (0) = 130μm is found. Comparison to results from upper critical field measurements and to a Josephson coupling model for layer crystals are made.