Exchange-bias via nanosegregation in novel Fe2-x Mn1+x Al (x = -0.25, 0, 0.25) Heusler films.

Exchange-bias has been reported in bulk nanocrystalline Fe2MnAl, but individual thin films of this Heusler alloy have never been studied so far. Here we study the structural and magnetic properties of nanocrystalline thin films of Fe2-x Mn1+x Al (x = -0.25, 0 and 0.25) obtained by sputtering and ex situ post-deposition annealing. We find that Fe2MnAl films display exchange-bias, and that varying Mn concentration determines the magnitude of the effect, which can be either enhanced (in Fe1.75Mn1.25Al) or suppressed (in Fe2.25Mn0.75Al). X-ray diffraction shows that our films present a mixed L21-B2 Heusler structure where increasing Mn concentration favors the partial transformation of the L21 phase into the B2 phase. Scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX) reveal that this composition-driven L21 → B2 transformation is accompanied by phase segregation at the nanoscale. As a result, the Fe2-x Mn1+x Al films that show exchange-bias (x = 0, 0.25) are heterogeneous, with nanograins of an Fe-rich phase embedded in a Mn-rich matrix (a non-negative matrix factorisation algorithm was used to give an indication of the phase composition from EDX data). Our comparative analysis of XRD, magnetometry and X-ray magnetic circular dichroism (XMCD), shows that the Fe-rich nanograins and Mn-rich matrix are composed of a ferromagnetic L21 phase and an antiferromagnetic B2 phase, respectively, thus revealing that exchange-coupling between these two phases is the cause of the exchange-bias effect. Our work should inspire the development of single-layer, environmentally friendly spin valve devices based on nanocomposite Heusler films.

Pure second harmonic current-phase relation in spin-filter Josephson junctions.

Higher harmonics in current-phase relations of Josephson Junctions are predicted to be observed when the first harmonic is suppressed. Conventional theoretical models predict higher harmonics to be extremely sensitive to changes in barrier thickness, temperature, and so on. Here we report experiments with Josephson junctions incorporating a spin-dependent tunnelling barrier, revealing a current-phase relation for highly spin polarized barriers that is purely second harmonic in nature and is insensitive to changes in barrier thickness. This observation implies that the standard theory of Cooper pair transport through tunnelling barriers is not applicable for spin-dependent tunnelling barriers.

Electric-field-dependent spin polarization in GdN spin filter tunnel junctions.

Tunnel junctions incorporating GdN ferromagnetic semiconductor barriers show a spin polarization exceeding 90% and a high conductance. These devices show an unusual low-bias conductance peak arising from a strong bias-dependence of the spin polarization. This originates from a strong magneto-electric coupling within a double Schottky barrier formed with the NbN electrodes.

A superconducting nanowire single photon detector on lithium niobate.

Superconducting nanowire single photon detectors (SNSPDs) are a key enabling technology for optical quantum information science. In this paper we demonstrate a SNSPD fabricated on lithium niobate, an important material for high speed integrated photonic circuits. We report a system detection efficiency of 0.15% at a 1 kHz dark count rate with a maximum of ~1% close to the critical current at 1550 nm wavelength for a parallel wire SNSPD with front side illumination. There is clear scope for improving on this performance with further materials optimization. Detector integration with a lithium niobate optical waveguide is simulated, demonstrating the potential for high single photon detection efficiency in an integrated quantum optic circuit.

Breast reconstruction with deep inferior epigastric perforator flaps.

INTRODUCTION: Approximately 45,000 women are diagnosed with breast cancer in the UK each year. The success of screening and the introduction of adjuvant therapies have meant that prognosis is improving and an increasing number of patients are seeking reconstruction following mastectomy. The purpose of this study was to evaluate the deep inferior epigastric perforator (DIEP) flap reconstructions performed in Stoke Mandeville Hospital and, through analysis of complications, detail the evolution of the current care pathway. METHODS: A retrospective analysis was performed of all the DIEP flap reconstructions performed by the senior author (MT) between July 2003 and December 2010. RESULTS: Overall, 159 flaps were performed on 141 patients (including 36 bilateral flaps). The average patient age was 49 years (range: 28-70 years) and 13% of flaps were risk reducing for BRCA1/2. Twenty-six per cent of patients suffered one or more complication post-operatively, including systemic complications (pulmonary embolism 2%) and flap specific complications (partial flap necrosis 9%, reanastomosis 3%, fat necrosis 9%). Seventy-four per cent had further elective operations including nipple reconstruction (72%), contralateral breast reduction (36%) and scar revision (21%). CONCLUSIONS: DIEP flaps are a safe and reliable option for breast reconstructions. This series illustrates the significant leaning curve, with complications, operative time and ischaemic time reducing through the series and post-operative haemoglobin increasing. The complications experienced in this series of 159 flaps with no total flap loss provide the framework for the evolution of the current care pathway including pre-operative imaging, peri-operative deep vein thrombosis prophylaxis and analgesia.

Magnetic exchange hardening in polycrystalline GdN thin films.

We report the observation of intrinsic exchange hardening in polycrystalline GdN thin films grown at room temperature by magnetron sputtering. We find, in addition to the ferromagnetic phase, that a fraction of GdN crystallizes in a structural polymorphic form which orders antiferromagnetically. The relative fraction of these two phases was controlled by varying the relative abundance of reactive species in the sputtering plasma by means of the sputtering power and N(2) partial pressure. An exchange bias of ∼ 30 Oe was observed at 10 K. The exchange coupling between the ferromagnetic and the antiferromagnetic phases resulted in an order of magnitude enhancement in the coercive field in these films.

Spin-1/2 optical lattice clock.

We experimentally investigate an optical clock based on ;{171}Yb (I = 1/2) atoms confined in an optical lattice. We have evaluated all known frequency shifts to the clock transition, including a density-dependent collision shift, with a fractional uncertainty of 3.4 x 10;{-16}, limited principally by uncertainty in the blackbody radiation Stark shift. We measured the absolute clock transition frequency relative to the NIST-F1 Cs fountain clock and find the frequency to be 518 295 836 590 865.2(0.7) Hz.

Cardiac cholinergic NO-cGMP signaling following acute myocardial infarction and nNOS gene transfer.

Myocardial infarction (MI) is associated with oxidative stress, which may cause cardiac autonomic impairment. We tested the hypothesis that acute MI disrupts cardiac cholinergic signaling by impairing nitric oxide (NO)-cGMP modulation of acetylcholine (ACh) release and whether the restoration of this pathway following cardiac neuronal NO synthase (nNOS) gene transfer had any bearing on the neural phenotype. Guinea pigs underwent four ligature coronary artery surgery (n = 50) under general anesthesia to induce MI or sham surgery (n = 32). In a separate group, at the time of MI surgery, adenovirus encoding nNOS (n = 29) or enhanced green fluorescent protein (eGFP; n = 30) was injected directly into the right atria, where the postganglionic cholinergic neurons reside. In vitro-evoked right atrial [3H]ACh release, right atrial NOS activity, and cGMP levels were measured at 3 days. Post-MI 24% of guinea pigs died compared with 9% in the sham-operated group. Evoked right atrial [3H]ACh release was significantly (P < 0.05) decreased in the MI group as was NOS activity and cGMP levels. Tetrahydrobiopterin levels were not significantly different between the sham and MI groups. Infarct sizes between gene-transferred groups were not significantly different. The nNOS transduced group had significantly increased right atrial [3H]ACh release, right atrial NOS activity, cGMP levels, and decreased cAMP levels. Fourteen percent of the nNOS transduced animals died compared with 31% mortality in the MI + eGFP group at 3 days. In conclusion, cardiac nNOS gene transfer partially restores the defective NO-cGMP cholinergic pathway post-MI, which was associated with a trend of improved survival at 3 days.

In-situ observation of transition between surface relief and wrinkling in thin film shape memory alloys.

Significant surface morphology evolution between relief and wrinkling was observed on a 3.5 microm thick TiNiCu film sputter-deposited on a silicon substrate. At room temperature, variation in surface relief morphology (from separated martensite crystals embedded in amorphous matrix to fully interweaved martensite plates) was observed with slight change in film composition. The phenomenon was attributed to variations in crystallization temperatures of as-deposited amorphous films during annealing because of the compositional difference. During thermal cycling between room temperature and 100 degrees C, reversible surface morphology changes can be observed between surface relief and wrinkling patterns. The formation of the surface wrinkling is attributed to the large compressive stress in the film during high temperature post-annealing and crystallization, whereas surface relief is caused by the martensitic transformation to relieve the large tensile stress in the film. Compositional effect on this surface morphology evolution is discussed. Results also indicate that there is a critical dimension for the wrinkling to occur, and a small circular island can only relax by in-plane expansion.

Optical lattice induced light shifts in an yb atomic clock.

We present an experimental study of the lattice-induced light shifts on the (1)S(0) --> (3)P(0) optical clock transition (nu(clock) approximately 518 THz) in neutral ytterbium. The "magic" frequency nu(magic) for the 174Yb isotope was determined to be 394 799 475(35) MHz, which leads to a first order light shift uncertainty of 0.38 Hz. We also investigated the hyperpolarizability shifts due to the nearby 6s6p(3)P(0) --> 6s8p(3)P(0), 6s8p(3)P(2), and 6s5f(3)F(2) two-photon resonances at 759.708, 754.23, and 764.95 nm, respectively. By measuring the corresponding clock transition shifts near these two-photon resonances, the hyperpolarizability shift was estimated to be 170(33) mHz for a linear polarized, 50 microK deep, lattice at the magic wavelength. These results indicate that the differential polarizability and hyperpolarizability frequency shift uncertainties in a Yb lattice clock could be held to well below 10(-17).

Sr lattice clock at 1 x 10(-16) fractional uncertainty by remote optical evaluation with a Ca clock.

Optical atomic clocks promise timekeeping at the highest precision and accuracy, owing to their high operating frequencies. Rigorous evaluations of these clocks require direct comparisons between them. We have realized a high-performance remote comparison of optical clocks over kilometer-scale urban distances, a key step for development, dissemination, and application of these optical standards. Through this remote comparison and a proper design of lattice-confined neutral atoms for clock operation, we evaluate the uncertainty of a strontium (Sr) optical lattice clock at the 1 x 10(-16) fractional level, surpassing the current best evaluations of cesium (Cs) primary standards. We also report on the observation of density-dependent effects in the spin-polarized fermionic sample and discuss the current limiting effect of blackbody radiation-induced frequency shifts.

Magnetic field-induced spectroscopy of forbidden optical transitions with application to lattice-based optical atomic clocks.

We develop a method of spectroscopy that uses a weak static magnetic field to enable direct optical excitation of forbidden electric-dipole transitions that are otherwise prohibitively weak. The power of this scheme is demonstrated using the important application of optical atomic clocks based on neutral atoms confined to an optical lattice. The simple experimental implementation of this method--a single clock laser combined with a dc magnetic field--relaxes stringent requirements in current lattice-based clocks (e.g., magnetic field shielding and light polarization), and could therefore expedite the realization of the extraordinary performance level predicted for these clocks. We estimate that a clock using alkaline-earth-like atoms such as Yb could achieve a fractional frequency uncertainty of well below 10(-17) for the metrologically preferred even isotopes.

Direct excitation of the forbidden clock transition in neutral 174Yb atoms confined to an optical lattice.

We report direct single-laser excitation of the strictly forbidden (6s2)1S0 <--> (6s6p)3P0 clock transition in 174Yb atoms confined to a 1D optical lattice. A small (approximately 1.2 mT) static magnetic field was used to induce a nonzero electric dipole transition probability between the clock states at 578.42 nm. Narrow resonance linewidths of 20 Hz (FWHM) with high contrast were observed, demonstrating a resonance quality factor of 2.6 x 10(13). The previously unknown ac Stark shift-canceling (magic) wavelength was determined to be 759.35 +/- 0.02 nm. This method for using the metrologically superior even isotope can be easily implemented in current Yb and Sr lattice clocks and can create new clock possibilities in other alkaline-earth-like atoms such as Mg and Ca.

Silicon-substituted hydroxyapatite thin films: effect of annealing temperature on coating stability and bioactivity.

The effect of annealing temperature on the physicochemical and biological characteristics of magnetron cosputtered silicon-substituted hydroxyapatite (SiHA) thin coatings was studied. Annealing is required to transform as-sputtered amorphous films into crystalline coatings. A nanocrystalline, single-phase apatite structure was achieved for coatings heated to 600 or 700 degrees C and, with increasing annealing temperature, the crystallite size increased. Small crystallites were found to be more soluble in the physiological environment but, at the same time, were able to induce early formation of a new apatite layer. A human osteoblast-like (HOB) cell model was used to evaluate the performance of these annealed SiHA coatings. HOB cells attached and grew well on coatings and, after 42 days in culture, a mineralization process was observed to be taking place, with evidence of calcium phosphate minerals throughout the extracellular matrix. Our findings indicated that an annealing temperature of 600 degrees C is sufficient to achieve crystalline SiHA coatings and exhibiting good chemical stability and bioactivity.

Novel silicon-doped hydroxyapatite (Si-HA) for biomedical coatings: an in vitro study using acellular simulated body fluid.

Magnetron co-sputtering was used to produce silicon-doped hydroxyapatite (Si-HA) as coatings intended for potential applications such as orthopedic and dental implants. It was found that the crystallinity of the as-sputtered coatings increased after annealing, resulting in a nanocrystalline apatite structure. Subsequently, the bioactivity of the coatings was evaluated in an acellular simulated body fluid (SBF). Physicochemical evaluation demonstrated that a carbonate-containing apatite layer, which is essential for bonding at the bone/implant interface, was formed on the coating surfaces after immersion in SBF between 4 and 7 days. The annealed coatings exhibited enhanced bioactivity and chemical stability under physiological conditions, as compared with the as-sputtered coatings. It is proposed that the rate at which the carbonate-containing apatite layer forms is dependent on the scale factor of the structure. A nanocrystalline structure can provide a higher number of nucleation sites for the formation of apatite crystallites, leading to a more rapid precipitation of carbonate-containing apatite layer. This work shows that Si-HA coatings offer considerable potential for applications in hard tissue replacement, owing to their ability to form a carbonate-containing apatite layer rapidly.

Observation and absolute frequency measurements of the 1S0-3P0 optical clock transition in neutral ytterbium.

We report the direct excitation of the highly forbidden (6s2) 1S0 <--> (6s6p) 3P0 optical transition in two odd isotopes of neutral ytterbium. As the excitation laser frequency is scanned, absorption is detected by monitoring the depletion from an atomic cloud at approximately 70 microK in a magneto-optical trap. The measured frequency in 171Yb (F=1/2) is 518,295,836,591.6 +/- 4.4 kHz. The measured frequency in 173Yb (F=5/2) is 518,294,576,847.6 +/- 4.4 kHz. Measurements are made with a femtosecond-laser frequency comb calibrated by the National Institute of Standards and Technology cesium fountain clock and represent nearly a 10(6)-fold reduction in uncertainty. The natural linewidth of these J=0 to J=0 transitions is calculated to be approximately 10 mHz, making them well suited to support a new generation of optical atomic clocks based on confinement in an optical lattice.

A new way of incorporating silicon in hydroxyapatite (Si-HA) as thin films.

Bioactive silicon-containing hydroxyapatite (Si-HA) thin films that can be used as coatings for bone tissue replacement have been developed. A magnetron co-sputtering technique was used to deposit Si-HA films up to 700 nm thick on titanium substrates, with a silicon level up to 1.2 wt%. X-ray diffraction demonstrated that annealing transformed the as-deposited Si-HA films which were amorphous, into a crystalline HA structure. A human osteoblast-like (HOB) cell model was used to determine the biocompatibility of these films. HOB cells were seen to attach and grow well on the Si-HA films, and the metabolic activity of HOB cells on these films was observed to increase with culture time. Furthermore, mineralisation of the cell layers was observed after 8 weeks of culture. Based on the present findings, Si-HA of different film compositions demonstrate bioactive properties in-vitro, and indicate the potential as biocoatings for a wide variety of medical implants including load-bearing applications such as the femoral stem of hip replacement implants.

Magnetron co-sputtered silicon-containing hydroxyapatite thin films--an in vitro study.

The use of silicon-substituted hydroxyapatite (Si-HA) as a biomaterial has been reported recently. In vivo testing has shown that Si-HA promotes early bonding of the bone/implant interface. In order to extend its usage to major load-bearing applications such as artificial hip replacement implants, it has been proposed that the material could be used in the form of a coating on implant surfaces. This paper reports a preliminary study of the biocompatibility of magnetron co-sputtered silicon-containing hydroxyapatite (Si-HA) coatings on a metallic substrate. Magnetron co-sputtered Si-HA films of thickness 600 nm with a Si content of approximately 0.8 wt% were produced on titanium substrates. X-ray diffraction analysis showed that the as-deposited Si-HA films were either amorphous or made up of very small crystals. The crystallinity of Si-HA films was increased after post-deposition heat treatment at 700 degrees C for 3 h, and the principal peaks were attributable to HA. The formation of nano-scale silicon-calcium phosphate precipitates was noted on the heat-treated films. In vitro cell culture has demonstrated that human osteoblast-like cells attached and grew well on all films, with the highest cell growth and signs of mineralisation observed on the heat-treated Si-HA films. In addition, many focal contacts were produced on the films and the cells had well-defined actin cytoskeletal organisation. This work shows that as-deposited and heat-treated Si-HA films have excellent bioactivity and are good candidates when rapid bone apposition is required. Furthermore, heat-treated Si-HA films have improved biostability compared to as-deposited films under physiological conditions.

Critical current of YBa(2)Cu(3)O(7-delta) low-angle grain boundaries.

Transport critical current measurements have been performed on 5 degrees [001]-tilt thin film YBa(2)Cu(3)O(7-delta) single grain boundaries with the magnetic field rotated in the plane of the film, phi. The variation of the critical current has been determined as a function of the angle between the magnetic field and the grain boundary plane. In applied fields above 1 T the critical current j(c) is found to be strongly suppressed only when the magnetic field is within an angle phi(k) of the grain boundary. Outside this angular range the behavior of the artificial grain boundary is dominated by the critical current of the grains. We show that the phi dependence of j(c) in the suppressed region is well described by a flux cutting model.

Temporally overlapped linear frequency-chirped pulse programming for true-time-delay applications.

A novel technique for programming broadband true-time delays that uses two frequency-offset temporally overlapped linear frequency-chirped pulses to produce periodic spectral gratings in an inhomogeneously broadened absorber is presented. Advantages of this technique include its ability to use chirped pulses that are longer than the coherence time of the crystal, less stringent laser frequency-stability requirements for grating accumulation, lower power requirements, a simplified system design, and the ability to tune broadband (multigigahertz) delays over a wide dynamic range (picoseconds to microseconds).