Giant spin Seebeck effect in a non-magnetic material.

The spin Seebeck effect is observed when a thermal gradient applied to a spin-polarized material leads to a spatially varying transverse spin current in an adjacent non-spin-polarized material, where it gets converted into a measurable voltage. It has been previously observed with a magnitude of microvolts per kelvin in magnetically ordered materials, ferromagnetic metals, semiconductors and insulators. Here we describe a signal in a non-magnetic semiconductor (InSb) that has the hallmarks of being produced by the spin Seebeck effect, but is three orders of magnitude larger (millivolts per kelvin). We refer to the phenomenon that produces it as the giant spin Seebeck effect. Quantizing magnetic fields spin-polarize conduction electrons in semiconductors by means of Zeeman splitting, which spin-orbit coupling amplifies by a factor of ∼25 in InSb. We propose that the giant spin Seebeck effect is mediated by phonon-electron drag, which changes the electrons' momentum and directly modifies the spin-splitting energy through spin-orbit interactions. Owing to the simultaneously strong phonon-electron drag and spin-orbit coupling in InSb, the magnitude of the giant spin Seebeck voltage is comparable to the largest known classical thermopower values.

Exchange-Driven Spin Relaxation in Ferromagnet-Oxide-Semiconductor Heterostructures.

We demonstrate that electron spin relaxation in GaAs in the proximity of a Fe/MgO layer is dominated by interaction with an exchange-driven hyperfine field at temperatures below 60 K. Temperature-dependent spin-resolved optical pump-probe spectroscopy reveals a strong correlation of the electron spin relaxation with carrier freeze-out, in quantitative agreement with a theoretical interpretation that at low temperatures the free-carrier spin lifetime is dominated by inhomogeneity in the local hyperfine field due to carrier localization. As the regime of large nuclear inhomogeneity is accessible in these heterostructures for magnetic fields <3 kG, inferences from this result resolve a long-standing and contentious dispute concerning the origin of spin relaxation in GaAs at low temperature when a magnetic field is present. Further, this improved fundamental understanding clarifies the importance of future experiments probing the time-dependent exchange interaction at a ferromagnet-semiconductor interface and its consequences for spin dissipation and transport during spin pumping.

Ferromagnetic Resonance Spin Pumping and Electrical Spin Injection in Silicon-Based Metal-Oxide-Semiconductor Heterostructures.

We present the measurement of ferromagnetic resonance (FMR-)driven spin pumping and three-terminal electrical spin injection within the same silicon-based device. Both effects manifest in a dc spin accumulation voltage V_{s} that is suppressed as an applied field is rotated to the out-of-plane direction, i.e., the oblique Hanle geometry. Comparison of V_{s} between these two spin injection mechanisms reveals an anomalously strong suppression of FMR-driven spin pumping with increasing out-of-plane field H_{app}^{z}. We propose that the presence of the large ac component to the spin current generated by the spin pumping approach, expected to exceed the dc value by 2 orders of magnitude, is the origin of this discrepancy through its influence on the spin dynamics at the oxide-silicon interface. This convolution, wherein the dynamics of both the injector and the interface play a significant role in the spin accumulation, represents a new regime for spin injection that is not well described by existing models of either FMR-driven spin pumping or electrical spin injection.

Electrical spin injection from an organic-based ferrimagnet in a hybrid organic-inorganic heterostructure.

We report the successful extraction of spin-polarized current from the organic-based room temperature ferrimagnetic semiconductor V[TCNE](x) (x∼2, TCNE: tetracyanoethylene; T(C)∼400 K, E(G)∼0.5 eV, σ(300 K)∼10(-2) S/cm) and its subsequent injection into a GaAs/AlGaAs light-emitting diode. The spin current tracks the magnetization of V[TCNE](x∼2), is weakly temperature dependent, and exhibits heavy-hole-light-hole asymmetry. This result has implications for room temperature spintronics and the use of inorganic materials to probe spin physics in organic and molecular systems.

Ferromagnetic imprinting of nuclear spins in semiconductors.

We examine how a ferromagnetic layer affects the coherent electron spin dynamics in a neighboring gallium arsenide semiconductor. Ultrafast optical pump-probe measurements reveal that the spin dynamics are unexpectedly dominated by hyperpolarized nuclear spins that align along the ferromagnet's magnetization. We find evidence that photoexcited carriers acquire spin-polarization from the ferromagnet, and dynamically polarize these nuclear spins. The resulting hyperfine fields are as high as 9000 gauss in small external fields (less than 1000 gauss), enabling ferromagnetic control of local electron spin coherence.

Membrane mechanics govern spatiotemporal heterogeneity of endocytic clathrin coat dynamics.

Dynamics of endocytic clathrin-coated structures can be remarkably divergent across different cell types, cells within the same culture, or even distinct surfaces of the same cell. The origin of this astounding heterogeneity remains to be elucidated. Here we show that cellular processes associated with changes in effective plasma membrane tension induce significant spatiotemporal alterations in endocytic clathrin coat dynamics. Spatiotemporal heterogeneity of clathrin coat dynamics is also observed during morphological changes taking place within developing multicellular organisms. These findings suggest that tension gradients can lead to patterning and differentiation of tissues through mechanoregulation of clathrin-mediated endocytosis.

Native defects in ultra-high vacuum grown graphene islands on Cu(1 1 1).

We present a scanning tunneling microscopy (STM) study of native defects in graphene islands grown by ultra-high vacuum decomposition of ethylene on Cu(1 1 1). We characterize these defects through a survey of their apparent heights, atomic-resolution imaging, and detailed tunneling spectroscopy. Bright defects that occur only in graphene regions are identified as C site point defects in the graphene lattice and are most likely single C vacancies. Dark defect types are observed in both graphene and Cu regions, and are likely point defects in the Cu surface. We also present data showing the importance of bias and tip termination to the appearance of the defects in STM images and the ability to achieve atomic resolution. Finally, we present tunneling spectroscopy measurements probing the influence of point defects on the local electronic landscape of graphene islands.

Nanogram calorimetry using microscale suspended SiNx platforms fabricated via focused ion beam patterning.

Comprehensive characterization of thermal properties in nanoscale heterostructures requires microscale thermally isolated platforms combined with sensitive thermometry in order to measure small heat accumulations. Amorphous SiNx membranes are often used for these measurements due to their low thermal conductivity and compatibility with standard fabrication techniques. The total thermal conductance of such SiNx membranes is typically microwatts per kelvin or higher. Here, we further reduce this thermal coupling to 120 nW/K by using a focused ion beam (FIB) to remove large portions of commercially available amorphous SiNx membranes, leaving a 100 µm × 100 µm square platform suspended by 10 µm wide by 325 µm long support legs. We demonstrate the capability of these platforms by measuring the heat capacity of a 6.2 ng Au sample and show that it matches well with established specific heat of bulk Au.

Anisotropic thermopower and planar Nernst effect in Ga1-xMnxAs ferromagnetic semiconductors.

We present the first experimental study of the thermopower in Mn-doped GaAs ferromagnetic semiconductors. Large magnetothermopower effects in both longitudinal and transverse directions have been observed below the ferromagnetic transition temperature. Unlike magnetoresistance, neither the transverse thermopower (planar Nernst effect) nor the longitudinal thermopower explicitly depend on the strength of the in-plane magnetic field, but rather are intimately related to each other through the magnetization. These newly discovered effects can be satisfactorily explained by an extension of anisotropic magnetotransport model and place important constraints on potential microscopic descriptions of the scattering mechanisms in these materials.

Element resolved spin configuration in ferromagnetic manganese-doped gallium arsenide.

We report induced Ga and As moments in ferromagnetic Ga(1-x)MnxAs detected using x-ray magnetic circular dichroism at the Mn, Ga, and As L(3,2) edges. Across a broad composition range, we find As and Ga dichroism signals which indicate an As 4s moment coupled antiparallel to the Mn 3d moment, and a smaller parallel Ga 4s moment. The Ga moment follows that of Mn in both doping and temperature dependence. These results are consistent with recent predictions of induced GaAs host moments and support the model of carrier-mediated ferromagnetic ordering involving As-derived valence band states.