We have measured the magnetoresistance in a series of Ga1−xMnxAs samples with 0.033≤ x ≤ 0.053 for three mutually orthogonal orientations of the applied magnetic field. The spontaneous resistivity anisotropy (SRA) in these materials is negative (i.e. the sample resistance is higher when its magnetization is perpendicular to the measuring current than when the two are parallel) and has a magnitude on the order of 5% at temperatures near 10K and below. This stands in contrast to the results for most conventional magnetic materials where the SRA is considerably smaller in magnitude for those few cases in which a negative sign is observed. The magnitude of the SRA drops from its maximum at low temperatures to zero at TC in a manner that is consistent with mean field theory. These results should provide a significant test for emerging theories of transport in this new class of materials. PACS: 75.50.Pp, 72.20.My, 72.80.Ey, 73.50.Jt The recent discovery of ferromagnetism at temperatures as high as 110K in Ga 1−x Mn x As has greatly broadened the interest in diluted magnetic semiconductors over the past few years [1][2][3]. A large number of groups are now investigating these materials which could form the basis for a wide variety of new magneto-electronic devices that may be grown pseudo-morphically on GaAs. The utility of similar devices has already been established using nano-composite structures based on metallic magnetic materials (the so-called giant magnetoresistance and tunnel junction magnetoresistance [4,5]). The semiconducting materials open up new opportunities as they also introduce the prospects of optical or electronic control of the magnetic properties [6,7].These new materials present many interesting challenges as we embark on efforts to make use of their properties in real devices. These challenges come from both the fundamental point of view (trying to understand their properties and what factors control them) and from the desire to manufacture high quality materials in the face of severe constraints imposed by the possibility of nucleating unwanted phases during growth [2]. These two challenges are intimately connected since the density of carriers is expected to influence such material properties as the Curie temperature and magnetic anisotropy [8,9], but this density is strongly influenced by defects included in the structure as a result of the constraints on the growth conditions.In view of our limited understanding of these materials it is prudent to spend some effort exploring the transport properties of individual material layers in preparation for constructing multilayered structures to form devices. Toward that end, in this article we report on magnetotransport measurements in a series of Ga 1−x Mn x As films (x= 0.033 to 0.053). We concentrate on the dependence of the film resistivity on the orientation of an applied magnetic field in the hope that this dependence might be less sensitive to the details of the disorder in the films than is the resistivity itself.The Mn-alloyed films were...

Since the beginning of measurement of pressure in the 17th century, the unit of pressure has been defined by the relationship of force per unit area. The present state of optical technology now offers the possibility of using a thermodynamic definition-specifically the ideal gas law-for the realization of the pressure unit, in the vacuum regime and slightly above, with an accuracy comparable to or better than the traditional methods of force per area. The changes planned for the SI in 2018 support the application of this thermodynamic definition that is based on the ideal gas law with the necessary corrections for real-gas effects. The paper reviews the theoretical and experimental foundations of those optical methods that are considered to be most promising to realize the unit of pressure at the highest level of metrology.

We investigate using optically trapped linear polyatomic molecules as probes of nuclear spindependent parity violation. The presence of closely spaced, opposite-parity ℓ-doublets is a general feature of such molecules, allowing parity-violation-sensitive pairs of levels to be brought to degeneracy in magnetic fields typically 100 times smaller than in diatomics. Assuming laser cooling and trapping of polyatomics at the current state-of-the-art for diatomics, we expect to measure nuclear spin-dependent parity-violating matrix elements iW with 70 times better sensitivity than the current best measurements. Our scheme should allow for 10 % measurements of iW in nuclei as light as Be or as heavy as Yb, with averaging times on order the of 10 days and 1 second, respectively.

The push to advance efficient, renewable, and clean energy sources has brought with it an effort to generate materials that are capable of storing hydrogen. Metal-organic framework materials (MOFs) have been the focus of many such studies as they are categorized for their large internal surface areas. We have addressed one of the major shortcomings of MOFs (their processibility) by creating and 3D printing a composite of acrylonitrile butadiene styrene (ABS) and MOF-5, a prototypical MOF, which is often used to benchmark H 2 uptake capacity of other MOFs. The ABS-MOF-5 composites can be printed at MOF-5 compositions of 10% and below. Other physical and mechanical properties of the polymer (glass transition temperature, stress and strain at the breaking point, and Young's modulus) either remain unchanged or show some degree of hardening due to the interaction between the polymer and the MOF. We do observe some MOF-5 degradation through the blending process, likely due to the ambient humidity through the purification and solvent casting steps. Even with this degradation, the MOF still retains some of its ability to uptake H 2 , seen in the ability of the composite to uptake more H 2 than the pure polymer. The experiments and results described here represent a significant first step toward 3D printing MOF-5-based materials for H 2 storage.3

The National Institute of Standards and Technology has recently begun a program to develop a primary pressure standard that is based on ultra-cold atoms, covering a pressure range of 1 × 10−6 Pa to 1 × 10−10 Pa and possibly lower. These pressures correspond to the entire ultra-high vacuum (UHV) range and extend into the extreme-high vacuum (XHV). This cold-atom vacuum standard (CAVS) is both a primary standard and absolute sensor of vacuum. The CAVS is based on the loss of cold, sensor atoms (such as the alkali-metal lithium) from a magnetic trap due to collisions with the background gas (primarily H2) in the vacuum. The pressure is determined from a thermally-averaged collision cross section, which is a fundamental atomic property, and the measured loss rate. The CAVS is primary because it will use collision cross sections determined from ab initio calculations for the Li + H2 system. Primary traceability is transferred to other systems of interest using sensitivity coefficients.

The temperature dependence of the Hall coefficient of a series of ferromagnetic Ga 1Ϫx Mn x As samples is measured in the temperature range 80 KϽTϽ500 K. We model the Hall coefficient assuming a magnetic susceptibility given by the Curie-Weiss law, a spontaneous Hall coefficient proportional to xx 2 (T), and including a constant diamagnetic contribution in the susceptibility. For all low resistivity samples this model provides excellent fits to the measured data up to Tϭ380 K and allows extraction of the hole concentration (p). The calculated p are compared to alternative methods of determining hole densities in these materials: pulsed high magnetic field ͑up to 55 T͒ technique at low temperatures ͑less than the Curie temperature͒, and electrochemical capacitance-voltage profiling. We find that the anomalous Hall effect ͑AHE͒ contribution to xy is substantial even well above the Curie temperature. Measurements of the Hall effect in this temperature regime can be used as a testing ground for theoretical descriptions of transport in these materials. We find that our data are consistent with recently published theories of the AHE, but they are inconsistent with theoretical models previously used to describe the AHE in conventional magnetic materials.

We demonstrate a compact (0.25 L) system for laser cooling and trapping atoms from a heated dispenser source. Our system uses a nanofabricated diffraction grating to generate a magnetooptical trap (MOT) using a single input laser beam. An aperture in the grating allows atoms from the dispenser to be loaded from behind the chip, increasing the interaction distance of atoms with the cooling light. To take full advantage of this increased distance, we extend the magnetic field gradient of the MOT to create a Zeeman slower. The MOT traps approximately 10 6 7 Li atoms emitted from an effusive source with loading rates greater than 10 6 s −1 . Our design is portable to a variety of atomic and molecular species and could be a principal component of miniaturized cold-atom-based technologies.

Cold atoms are excellent metrological tools; they currently realize SI time and, soon, SI pressure in the ultra-high (UHV) and extreme high vacuum (XHV) regimes. The development of primary, vacuum metrology based on cold atoms currently falls under the purview of national metrology institutes. Under the emerging paradigm of the “quantum-SI”, these technologies become deployable (relatively easy-to-use sensors that integrate with other vacuum chambers), providing a primary realization of the pascal in the UHV and XHV for the end-user. Here, we discuss the challenges that this goal presents. We investigate, for two different modes of operation, the expected corrections to the ideal cold-atom vacuum gauge and estimate the associated uncertainties. Finally, we discuss the appropriate choice of sensor atom, the light Li atom rather than the heavier Rb.