Brian C. Watson

Magnetic Spin Ladder (C5H12N)2CuBr4: High Field Magnetization and Scaling Near Quantum Criticality

The magnetization, M(H< or =30 T,0.7< or =T< or =300 K), of (C5H12N)2CuBr4 has been used to identify this system as an S = 1/2 Heisenberg two-leg ladder in the strong-coupling limit, J( perpendicular) = 13.3 K and J( parallel) = 3.8 K, with H(c1) = 6.6 T and H(c2) = 14.6 T. An inflection point in M(H,T = 0.7 K) at half saturation, M(s)/2, is described by an effective XXZ chain. The data exhibit universal scaling behavior in the vicinity of H(c1) and H(c2), indicating that the system is near a quantum critical point.

Pulsed Fourier-Transform Ultrasonic Spectroscopy for Ultralow Temperature Applications

Pulsed Fourier-Transform (FT) ultrasonic spectroscopy has been developed for the purpose of studying 3He at ultralow temperatures. Our method is analogous to pulsed FT NMR and possesses several advantages over conventional time-of-flight acoustic techniques. For use at low temperatures, the most significant advantage is the ability to observe the frequency spectrum while the temperature, pressure, and magnetic field may be independently tuned. More specifically, using longitudinal LiNbO3 transducers operating both on- and off-resonance, we are able to study several broadband windows of frequency, namely 16–25 MHz, 60–70 MHz, and 105–111 MHz. The determination of the energy gap of 3He-B is presented as an effective application, and other novel uses are discussed.

Zero Sound Attenuation Near the Quantum Limit in Normal Liquid 3He Close to the Superfluid Transition

The zero sound attenuation of normal liquid 3He has been studied over a range of temperatures from slightly above the superfluid transition temperature, Tc, to approximately 10 mK at the constant pressures of 1 and 5 bar. Using longitudinal LiNbO3 transducers, operating both on and off resonance, the experiment was performed at 15 discrete frequencies located in several broadband frequency windows, including 16–25, 60–70, and 105–111 MHz. The results are compared to Landaus prediction for the attenuation of zero sound in the quantum limit, (kBT≪ħω≪kBTF), where α0(P,T, ω)= α′(P) T2{1+(ħω/2πkBT)2}. Calibration of the received zero sound signals was performed by measuring the temperature dependence of the first sound attenuation from 30 to 800 mK at those same frequencies and pressures. The data are compared to previous results.

Raman scattering measurements of the spin ladder compound (C5H12N)2CuBr4

We report Raman scattering measurements on the spin ladder compound (C5H12N)2CuBr4. Pronounced multiphonon scattering is observed up to eighth-order, arising from the Franck?Condon process. Furthermore, a two-magnon continuum is seen with frequencies around 18?cm?1 at low temperatures. Its symmetric line shape suggests a substantial hopping of triplets along the leg of the ladder. Thus, the studied system is considered to represent a two-leg ladder with coupling constants in the intermediate regime between the strong and the isotropic limit.

Magnetic phase diagram of the quasi-2D mixed metal phenylphosphonates

Abstract The magnetic properties of pure and mixed (0⩽x⩽1) metal phenylphosphonates, MnxM1−x(O3PC6H5)·H2O, where M is Co or Zn, have been investigated. The pure Mn system consists of quasi-two-dimensional Heisenberg spins that experience long-range canted antiferromagnetic order at T N ≈12 K . The MnxM1−x systems form solid solutions for all values of x, and the corresponding magnetic phase diagrams (TN versus x) are presented.

ESR study of (C5H12N)2CuBr4

Abstract ESR studies at 9.27, 95.4, and 289.7 GHz have been performed on (C5H12N)2CuBr4 down to 3.7 K . The 9.27 GHz data were acquired with a single crystal and do not indicate the presence of any structural transitions. The high frequency data were collected with a polycrystalline sample and resolved two absorbances, consistent with two crystallographic orientations of the magnetic sites and with earlier ESR studies performed at 300 K . Below B C1 =6.6 T , our data confirm the presence of a spin singlet ground state.

A Magnetic Manganese Phosphonate Langmuir-Blodgett Film Containing a Tetrathiafulvalene Amphiphile

An amphiphilic tetrathiafulvalene derivative, with two alkylphosphonic acid substituents, has been prepared and used to form a mixed organic/inorganic Langmuir-Blodgett (LB) film containing a TTF-based organic network and a manganese phosphonate continuous lattice inorganic network. The inorganic network is isostructural with known manganese phosphonate layered solids, and undergoes magnetic ordering to a canted antiferromagnetic ground state. A model TTF derivative containing the alkylphosphonic acids, but without hydrophobic alkyl tails, was also synthesized and a manganese phosphonate solid, isostructural with the LB film, was prepared and characterized.