Pulsed electron paramagnetic resonance spectroscopy powered by a free-electron laser.

Electron paramagnetic resonance (EPR) spectroscopy interrogates unpaired electron spins in solids and liquids to reveal local structure and dynamics; for example, EPR has elucidated parts of the structure of protein complexes that other techniques in structural biology have not been able to reveal. EPR can also probe the interplay of light and electricity in organic solar cells and light-emitting diodes, and the origin of decoherence in condensed matter, which is of fundamental importance to the development of quantum information processors. Like nuclear magnetic resonance, EPR spectroscopy becomes more powerful at high magnetic fields and frequencies, and with excitation by coherent pulses rather than continuous waves. However, the difficulty of generating sequences of powerful pulses at frequencies above 100 gigahertz has, until now, confined high-power pulsed EPR to magnetic fields of 3.5 teslas and below. Here we demonstrate that one-kilowatt pulses from a free-electron laser can power a pulsed EPR spectrometer at 240 gigahertz (8.5 teslas), providing transformative enhancements over the alternative, a state-of-the-art ∼30-milliwatt solid-state source. Our spectrometer can rotate spin-1/2 electrons through π/2 in only 6 nanoseconds (compared to 300 nanoseconds with the solid-state source). Fourier-transform EPR on nitrogen impurities in diamond demonstrates excitation and detection of EPR lines separated by about 200 megahertz. We measured decoherence times as short as 63 nanoseconds, in a frozen solution of nitroxide free-radicals at temperatures as high as 190 kelvin. Both free-electron lasers and the quasi-optical technology developed for the spectrometer are scalable to frequencies well in excess of one terahertz, opening the way to high-power pulsed EPR spectroscopy up to the highest static magnetic fields currently available.

Long-lived spin coherence in silicon with an electrical spin trap readout.

Pulsed electrically detected magnetic resonance of phosphorous (31P) in bulk crystalline silicon at very high magnetic fields (B0>8.5 T) and low temperatures (T=2.8 K) is presented. We find that the spin-dependent capture and reemission of highly polarized (>95%) conduction electrons by equally highly polarized 31P donor electrons introduces less decoherence than other mechanisms for spin-to-charge conversion. This allows the electrical detection of spin coherence times in excess of 100 mus, 50 times longer than the previous maximum for electrically detected spin readout experiments.

Electron paramagnetic resonance shift in II1-xMnxVI diluted magnetic semiconductors in the presence of strong exchange coupling.

Electron paramagnetic resonance (EPR) has been investigated in two II1-xMnxVI alloys--Cd1-xMnxSe and Cd1-xMnxS--for a series of high Mn concentrations and at low temperatures T, i.e., under conditions where the spin subsystems in these materials are strongly coupled. We have observed a very significant shift of the resonance field from the EPR position of Mn2+ ions that increases with increasing x and with decreasing T. Furthermore, the use of multiple frequencies has allowed us to attribute the observed shift to an internal field that originates from the spin sublattice within the II1-xMnxVI host.

Low temperature magnetic instabilities in triply charged fulleride polymers

The electronic properties of the C3-60 polymer in Na2Rb0.3Cs0.7C60 were studied by X-band and high field (109.056 GHz) ESR. They are characteristic of a strongly correlated quasi-one-dimensional metal down to 45 K. On further cooling, a pseudogap of magnetic origin opens at the Fermi level below 45 K with three-dimensional magnetic ordering occurring below T(N) approximately 15 K, as confirmed by the observation of an antiferromagnetic resonance mode. The Na2Rb1-xCsxC60 family of polymers offers a unique way to chemically control the electronic properties, as the opening of the gap in this system of predominantly itinerant electrons is an extremely sensitive function of the interchain separation.

Multifrequency EPR spectra of molecular oxygen in solid Air

Multifrequency EPR spectra in the 94 to 550 GHz range were performed on solid air samples condensed at 5 K in the waveguide of a single pass probe. The spectra of molecular oxygen were observed and interpreted in the frame of the spin Hamiltonian model as axial S = 1 spectra with a zero field splitting parameter D = 3.572(3) cm(-1). The result of this study is relevant in the field of high field-high frequency EPR application in which solid air O(2) is a common paramagnetic impurity. Copyright 2000 Academic Press.