We have experimentally investigated the density of states (DOS) in Nb/Ni (S/F) bilayers as a function of Ni thickness, dF . Our thinnest samples show the usual DOS peak at ±∆0, whereas intermediate-thickness samples have an anomalous "double-peak" structure. For thicker samples (dF ≥ 3.5 nm), we see an "inverted" DOS which has previously only been reported in superconductor/weak-ferromagnet structures. We analyze the data using the self-consistent nonlinear Usadel equation and find that we are able to quantitatively fit the features at ±∆0 if we include a large amount of spin-orbit scattering in the model. Interestingly, we are unable to reproduce the sub-gap structure through the addition of any parameter(s). Therefore, the observed anomalous sub-gap structure represents new physics beyond that contained in the present Usadel theory.The co-existence of superconductivity and ferromagnetism was first proposed by Fulde and Ferrell [1] and Larkin and Ovchinnikov [2] more than forty years ago. While some unusual materials have since been found with both superconducting and ferromagnetic transitions (e.g. Qualitative evidence for the first two of these effects is convincing, but definitive quantitative agreement with theory has been problematic. The evidence for triplet superconductivity is less certain, although a recent report by Keizer et al [7] provides tantalizing evidence for such an effect. One reason for the difficulty in achieving quantitative agreement with theory is the proliferation of physical effects that now have been incorporated into the theory, leading to a concomitant proliferation of fitting parameters, which makes discriminating fits to limited data sets nearly impossible.In order to obtain more discriminating data sets and to further explore the SF proximity effect in the case of strong ferromagnets, we have undertaken superconducting tunneling densities of state (DOS) measurements on Nb/Ni thin-film bilayers. By varying the Ni thickness, d F , we can track the spatial evolution of the behavior of the Cooper pairs diffusing into the ferromagnet. This approach gives us much more information per sample (the entire DOS spectrum) than T c or J c measurements, and is less sensitive to variations in boundary parameters. Analyzing these results with the most complete forms of the Usadel theory available has allowed us to discriminate critically for the first time the relative importance of the various physical effects now incorporated into the theory. We find that by far the most important parameter beyond the exchange field, E ex , is the degree of spin-orbit scattering (first suggested by Demler et al [8]). In addition, we find an anomalous double-peak structure in the DOS that has not been reported previously and that we have been unable to account for theoretically.We use planar tunnel junctions of the form normalinsulator-ferromagnet-superconductor. A schematic of our sample geometry is shown in the inset of Fig. 2. The deposition of our samples and characterization of the tunnel junctions has be...

We present in vivo images of the human brain acquired with an ultralow field MRI (ULFMRI) system operating at a magnetic field B 0 ∼ 130 μT. The system features prepolarization of the proton spins at B p ∼ 80 mT and detection of the NMR signals with a superconducting, second-derivative gradiometer inductively coupled to a superconducting quantum interference device (SQUID). We report measurements of the longitudinal relaxation time T 1 of brain tissue, blood, and scalp fat at B 0 and B p , and cerebrospinal fluid at B 0 . We use these T 1 values to construct inversion recovery sequences that we combine with Carr-Purcell-Meiboom-Gill echo trains to obtain images in which one species can be nulled and another species emphasized. In particular, we show an image in which only blood is visible. Such techniques greatly enhance the already high intrinsic T 1 contrast obtainable at ULF. We further present 2D images of T 1 and the transverse relaxation time T 2 of the brain and show that, as expected at ULF, they exhibit similar contrast. Applications of brain ULFMRI include integration with systems for magnetoencephalography. More generally, these techniques may be applicable, for example, to the imaging of tumors without the need for a contrast agent and to modalities recently demonstrated with T 1ρ contrast imaging (T 1 in the rotating frame) at fields of 1.5 T and above. . 3D magnetic field gradients specify a unique magnetic field and thus an NMR frequency or phase in each voxel of the subject, so that with appropriate signal decoding one can acquire a 3D image (4).Clinical MRI systems with B 0 = 1:5 T achieve a spatial resolution of typically 1 mm; 3-T systems are becoming increasingly widespread in clinical practice (5), offering a higher signal-tonoise ratio (SNR) and thus higher spatial resolution. Nonetheless, there is ongoing interest in less expensive MRI systems operating at lower fields. Commercially available 0.2-to 0.5-T systems based on permanent magnets offer both lower cost and wider patient aperture than their higher field counterparts, at the expense of spatial resolution. At the still lower field of 0.03 T maintained by a room temperature solenoid, Connolly and coworkers (6, 7) obtained clinically useful SNR and spatial resolution by prepolarizing the protons in a field B p of 0.3 T. Prepolarization (8) enhances the magnetization of the proton ensemble over that produced by the lower precession field; after the polarizing field is removed, the higher magnetization produces a correspondingly larger signal during its precession in B 0 . Using the same method, Stepisnik et al. (9) obtained MR images in the Earth's magnetic field (∼ 50 μT).In recent years there has been increasing interest (10-36) in NMR and MRI at fields ranging from a few nanotesla to the order of 100 μT. The enormous reduction in the detected signal amplitude compared with the high field value is overcome partly by using prepolarization and partly by detecting the signal with an untuned superconducting input circuit inductively coupl...

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