Structural and electronic properties of grain boundaries in graphite: Planes of periodically distributed point defects

We report on scanning tunneling microscopy and spectroscopy of grain boundaries in highly oriented pyrolytic graphite. Grain boundaries showed a periodic structure and an enhanced charge density compared to the bare graphite surface. Two possible periodic structures have been observed along grain boundaries. A geometrical model producing periodically distributed point defects on the basal plane of graphite has been proposed to explain the structure of grain boundaries. Scanning tunneling spectroscopy on grain boundaries revealed two strong localized states at -0.3 V and 0.4 V.Comment: 5 pages, 5 figure

Verification of the Thomson-Onsager reciprocity relation for spin caloritronics

We investigate the Thomson-Onsager relation between the spin-dependent Seebeck and spin-dependent Peltier effect. To maintain identical device and measurement conditions we measure both effects in a single Ni$_{80}$Fe$_{20}$/Cu/Ni$_{80}$Fe$_{20}$ nanopillar spin valve device subjected to either an electrical or a thermal bias. In the low bias regime, we observe similar spin signals as well as background responses, as required by the Onsager reciprocity relation. However, at large biases, deviation from reciprocity occurs due to dominant nonlinear contribution of the temperature dependent transport coefficients. By systematic modeling of these nonlinear thermoelectric effects and measuring higher order thermoelectric responses for different applied biases, we identify the transition between the two regimes as the point at which Joule heating start to dominate over Peltier heating. Our results signify the importance of local equilibrium for the validity of this phenomenological reciprocity relation.Comment: 5 pages, 5 figure

Giant inelastic tunneling in epitaxial graphene mediated by localized states

Local electronic structures of nanometer-sized patches of epitaxial graphene and its interface layer with SiC(0001) have been studied by atomically resolved scanning tunneling microscopy and spectroscopy. Localized states belonging to the interface layer of a graphene/SiC system show to have an essential influence on the electronic structure of graphene. Giant enhancement of inelastic tunneling, reaching 50% of the total tunneling current, has been observed at the localized states on a nanometer-sized graphene monolayer surrounded by defects.Comment: 6 pages, 5 figures, accepted for publication in Phys. Rev.

Comparison of the magneto-Peltier and magneto-Seebeck effects in magnetic tunnel junctions

Understanding heat generation and transport processes in a magnetic tunnel junction (MTJ) is a significant step towards improving its application in current memory devices. Recent work has experimentally demonstrated the magneto-Seebeck effect in MTJs, where the Seebeck coefficient of the junction varies as the magnetic configuration changes from a parallel (P) to an anti-parallel (AP) configuration. Here we report the study on its as-yet-unexplored reciprocal effect, the magneto-Peltier effect, where the heat flow carried by the tunneling electrons is altered by changing the magnetic configuration of the MTJ. The magneto-Peltier signal that reflects the change in the temperature difference across the junction between the P and AP configurations scales linearly with the applied current in the small bias but is greatly enhanced in the large bias regime, due to higher-order Joule heating mechanisms. By carefully extracting the linear response which reflects the magneto-Peltier effect, and comparing it with the magneto-Seebeck measurements performed on the same device, we observe results consistent with Onsager reciprocity. We estimate a magneto-Peltier coefficient of 13.4 mV in the linear regime using a three-dimensional thermoelectric model. Our result opens up the possibility of programmable thermoelectric devices based on the Peltier effect in MTJs

High sensitive quasi freestanding epitaxial graphene gassensor on 6H-SiC

We have measured the electrical response to NO$_2$, N$_2$, NH$_3$ and CO for epitaxial graphene and quasi freestanding epitaxial graphene on 6H-SiC substrates. Quasi freestanding epitaxial graphene shows a 6 fold increase in NO2 sensitivity compared to epitaxial graphene. Both samples show a sensitivity better than the experimentally limited 1 ppb. The strong increase in sensitivity of quasi freestanding epitaxial graphene can be explained by a Fermi-energy close to the Dirac Point leading to a strongly surface doping dependent sample resistance. Both sensors show a negligible sensitivity to N$_2$, NH$_3$ and CO

Spin-dependent Seebeck coefficients of Ni_{80}Fe_{20} and Co in nanopillar spin valves

We have experimentally determined the spin-dependent Seebeck coefficient of permalloy (Ni_{80}Fe_{20}) and cobalt (Co) using nanopillar spin valve devices. The devices were specifically designed to completely separate heat related effects from charge related effects. A pure heat current through the nanopillar spin valve, a stack of two ferromagnetic layers (F) separated by a non-magnetic layer (N), leads to a thermovoltage proportional to the spin-dependent Seebeck coefficient S_{S}=S_{\uparrow}-S_{\downarrow} of the ferromagnet, where S_{\uparrow} and S_{\downarrow} are the Seebeck coefficient for spin-up and spin-down electrons. By using a three-dimensional finite-element model (3D-FEM) based on spin-dependent thermoelectric theory, whose input material parameters were measured in separate devices, we were able to accurately determine a spin-dependent Seebeck coefficient of -1.8 microvolt/Kelvin and -4.5 microvolt/Kelvin for cobalt and permalloy, respectively corresponding to a Seebeck coefficient polarization P_{S}=S_{S}/S_{F} of 0.08 and 0.25, where S_{F} is the Seebeck coefficient of the ferromagnet. The results are in agreement with earlier theoretical work in Co/Cu multilayers and spin-dependent Seebeck and spin-dependent Peltier measurements in Ni_{80}Fe_{20}/Cu spin valve structures

Observation of the spin Peltier effect

We report the observation of the spin Peltier effect (SPE) in the ferrimagnetic insulator Yttrium Iron Garnet (YIG), i.e. a heat current generated by a spin current flowing through a Platinum (Pt)|YIG interface. The effect can be explained by the spin torque that transforms the spin current in the Pt into a magnon current in the YIG. Via magnon-phonon interactions the magnetic fluctuations modulate the phonon temperature that is detected by a thermopile close to the interface. By finite-element modelling we verify the reciprocity between the spin Peltier and spin Seebeck effect. The observed strong coupling between thermal magnons and phonons in YIG is attractive for nanoscale cooling techniques.Comment: 5 pages, 3 figures, 4 pages supplementary information, 4 supplementary figure