Latest Papers in Condensed Matter Physics

Mesoscale And Nanoscale Physics

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Iterative path-integral summations for the tunneling magnetoresistance in interacting quantum-dot spin valves (1903.11426v2)

S. Mundinar, P. Stegmann, J. König, S. Weiss

2019-03-27

We report on the importance of resonant-tunneling processes on quantum transport through interacting quantum-dot spin valves. To include Coulomb interaction in the calculation of the tunneling magnetoresistance (TMR), we reformulate and generalize the recently-developed, numerically-exact method of iterative summation of path integrals (ISPI) to account for spin-dependent tunneling. The ISPI scheme allows us to investigate weak to intermediate Coulomb interaction in a wide range of gate and bias voltage and down to temperatures at which a perturbative treatment of tunneling severely fails.

Identification of point defects on Co-Ni co-doping in SnO nanocrystals and their effect on the structural and optical properties (1905.02255v2)

S. Roy, Brijmohan Prajapati, A. Singh, Amish G. Joshi, S. Chatterjee, Anup K. Ghosh

2019-05-06

SnCoNiO (0 0.04) nanocrystals, with average crystallite size in the range of 7.3 nm (=0.00) to 5.6 nm (=0.04), have been synthesized using pH-controlled chemical co-precipitation technique. The non-stoichiometric Sn related defects and the O related stoichiometric Frenkel defects arising in the nanocrystals because of co-doping have been identified and their effect on the structural and optical properties of the nanocrystals have been extensively studied. It has been observed, using XPS that on increasing the Ni co-doping concentration (), the non-stoichiometric Sn defect Sn increases in compensation of existing defect Sn for = 0.00 nanocrystals. High resolution transmission electron microscopy (HR-TEM) also confirms the existence of Sn. Regarding the Frenkel defect, XPS results indicate that the concentration of and O, manifested in the form of dangling bond related surface defect states,increases with increase in . Temperature dependent magnetisation measurement of the nanocrystals confirm the charge state of . The point defects have been found to affect the structural properties in a way that distortion in octahedral geometry of complete Sn-O octahderon effectively reduces whereas distortion in the trigonal planar coordination geometry of O increases. The investigation of Urbach edge indicates an enhancement in the disorder in the nanocrystals on co-doping. The optical band gap of the nanocrystals has been found to be red shifted upto =0.02 and then a gradual blue shift has been observed. A direct effect of the O related defect has been observed on the blue luminescence of the nanocrystals such that the spectral contribution of blue luminescence in the total emission intensity increases by 72% for =0.04 as compared to =0.00.

Extracting the Dynamic Magnetic Contrast in Time-Resolved X-ray Transmission Microscopy (1905.12497v2)

T. Schaffers, T. Feggeler, S. Pile, R. Meckenstock, M. Buchner, D. Spoddig, V. Ney, M. Farle, H. Wende, S. Wintz, M. Weigand, H. Ohldag, K. Ollefs, A. Ney

2019-05-29

Using a time-resolved detection scheme in scanning transmission X-ray microscopy (STXM) we measured element resolved ferromagnetic resonance (FMR) at microwave frequencies up to 10,GHz and a spatial resolution down to 20,nm at two different synchrotrons. We present different methods to separate the contribution of the background from the dynamic magnetic contrast based on the X-ray magnetic circular dichroism (XMCD) effect. The relative phase between the GHz microwave excitation and the X-ray pulses generated by the synchrotron, as well as the opening angle of the precession at FMR can be quantified. A detailed analysis for homogeneous and inhomogeneous magnetic excitations demonstrates that the dynamic contrast indeed behaves as the usual XMCD effect. The dynamic magnetic contrast in time-resolved STXM has the potential be a powerful tool to study the linear and non-linear magnetic excitations in magnetic micro- and nano-structures with unique spatial-temporal resolution in combination with element selectivity.

Impurity-induced triple point fermions in twisted bilayer graphene (1902.05862v4)

Aline Ramires, J. L. Lado

2019-02-15

Triple point fermions are elusive electronic excitations that generalize Dirac and Weyl modes beyond the conventional high energy paradigm. Yet, finding real materials naturally hosting these excitations at the Fermi energy has remained challenging. Here we show that twisted bilayer graphene is a versatile platform to realize robust triple point fermions in two dimensions. In particular, we establish that the introduction of localized impurities lifts one of the two degenerate Dirac cones, yielding triple point fermions at charge neutrality. Furthermore, we show that the valley polarization is preserved for certain impurity locations in the moire supercell for both weak and strong impurity potentials. We finally show that in the presence of interactions, a symmetry broken state with local magnetization can develop out of the triple point bands, which can be selectively controlled by electrostatic gating. Our results put forward twisted bilayer graphene as a simple solid-state platform to realize triple point fermions at charge neutrality, and demonstrate the non-trivial role of impurities in moire systems.

Nanowires: A route to efficient thermoelectric devices (1901.03046v2)

F. Dominguez-Adame, M . Martin-Gonzalez, D. Sanchez, A. Cantarero

2019-01-10

Miniaturization of electronic devices aims at manufacturing ever smaller products, from mesoscopic to nanoscopic sizes. This trend is challenging because the increased levels of dissipated power demands a better understanding of heat transport in small volumes. A significant amount of the consumed energy is transformed into heat and dissipated to the environment. Thermoelectric materials offer the possibility to harness dissipated energy and make devices less energy-demanding. Heat-to-electricity conversion requires materials with a strongly suppressed thermal conductivity but still high electronic conduction. Nanowires can meet nicely these two requirements because enhanced phonon scattering at the surface and defects reduces the lattice thermal conductivity while electric conductivity is not deteriorated, leading to an overall remarkable thermoelectric efficiency. Therefore, nanowires are regarded as a promising route to achieving valuable thermoelectric materials at the nanoscale. In this paper, we present an overview of key experimental and theoretical results concerning the thermoelectric properties of nanowires. The focus of this review is put on the physical mechanisms by which the efficiency of nanowires can be improved. Phonon scattering at surfaces and interfaces, enhancement of the power factor by quantum effects and topological protection of electron states to prevent the degradation of electrical conductivity in nanowires are thoroughly discussed.

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