Search results for "mesoscale and nanoscale physics"
showing 10 items of 720 documents
Coherent Excitation of Heterosymmetric Spin Waves with Ultrashort Wavelengths
2017
In the emerging field of magnonics, spin waves are foreseen as signal carriers for future spintronic information processing and communication devices, owing to both the very low power losses and a high device miniaturisation potential predicted for short-wavelength spin waves. Yet, the efficient excitation and controlled propagation of nanoscale spin waves remains a severe challenge. Here, we report the observation of high-amplitude, ultrashort dipole-exchange spin waves (down to 80 nm wavelength at 10 GHz frequency) in a ferromagnetic single layer system, coherently excited by the driven dynamics of a spin vortex core. We used time-resolved x-ray microscopy to directly image such propagati…
Laser-driven quantum magnonics and terahertz dynamics of the order parameter in antiferromagnets
2017
The impulsive generation of two-magnon modes in antiferromagnets by femtosecond optical pulses, so-called femto-nanomagnons, leads to coherent longitudinal oscillations of the antiferromagnetic order parameter that cannot be described by a thermodynamic Landau-Lifshitz approach. We argue that this dynamics is triggered as a result of a laser-induced modification of the exchange interaction. In order to describe the oscillations we have formulated a quantum mechanical description in terms of magnon pair operators and coherent states. Such an approach allowed us to} derive an effective macroscopic equation of motion for the temporal evolution of the antiferromagnetic order parameter. An impli…
Quantized adiabatic quantum pumping due to interference
2002
Recent theoretical calculations, demonstrating that quantized charge transfer due to adiabatically modulated potentials in mesoscopic devices can result purely from the interference of the electron wave functions (without invoking electron-electron interactions) are reviewed: (1) A new formula is derived for the pumped charge Q (per period); It reproduces the Brouwer formula without a bias, and also yields the effect of the modulating potential on the Landauer formula in the presence of a bias. (2) For a turnstile geometry, with time-dependent gate voltages V_L(t) and V_R(t), the magnitude and sign of Q are determined by the relative position and orientation of the closed contour traversed …
Persistent spin and charge currents and magnification effects in open ring conductors subject toRashba coupling
2007
We analyze the effect of Rashba spin-orbit coupling and of a local tunnel barrier on the persistent spin and charge currents in a one-dimensional conducting Aharonov-Bohm (AB) ring symmetrically coupled to two leads. First, as an important consequence of the spin-splitting, it is found that a persistent spin current can be induced which is not simply proportional to the charge current. Second, a magnification effect of the persistent spin current is shown when one tunes the Fermi energy near the Fano-type antiresonances of the total transmission coefficient governed by the tunnel barrier strength. As an unambiguous signature of spin-orbit coupling we also show the possibility to produce a p…
S-matrix formulation of mesoscopic systems and evanescent modes.
2009
The Landauer-Butikker formalism is an important formalism to study mesoscopic systems. Its validity for linear transport is well established theoretically as well as experimentally. Akkermans et al [Phys. Rev. Lett. {\bf 66}, 76 (1991)] had shown that the formalism can be extended to study thermodynamic properties like persistent currents. It was earlier verified for simple one dimensional systems. We study this formula very carefully and conclude that it requires reinterpretation in quasi one dimension. This is essentially because of the presence of evanescent modes in quasi one dimension.
Quantized current source with mesoscopic feedback
2011
We study a mesoscopic circuit of two quantized current sources, realized by nonadiabatic single-electron pumps connected in series with a small micron-sized island in between. We find that quantum transport through the second pump can be locked onto the quantized current of the first one by a feedback due to charging of the mesoscopic island. This is confirmed by a measurement of the charge variation on the island using a nearby charge detector. Finally, the charge feedback signal clearly evidences loading into excited states of the dynamic quantum dot during single-electron pump operation. © 2011 American Physical Society.
Bulk-free topological insulator Bi 2 Se 3 nanoribbons with magnetotransport signatures of Dirac surface states
2018
Many applications for topological insulators (TIs) as well as new phenomena require devices with reduced dimensions. While much progress has been made to realize thin films of TIs with low bulk carrier density, nanostructures have not yet been reported with similar properties, despite the fact that size confinement should help reduce contributions from bulk carriers. Here we demonstrate that Bi2Se3 nanoribbons, grown by a simple catalyst-free physical-vapour deposition, have inherently low bulk carrier densities, and can be further made bulk-free by size confinement, thus revealing the high mobility topological surface states. Magneto transport and Hall conductance measurements, in single n…
Thermal disorder driven magnetic phases in van der Waals magnet CrI3
2020
Abstract Magnetic phase transitions often occur spontaneously at specific critical temperatures and are instrumental to understand the origin of long-range spin order in condensed matter systems. The presence of more than one critical temperature (Tc) has been observed in several compounds where the coexistence of competing magnetic orders highlights the importance of phase separation driven by different factors such as pressure, temperature and chemical composition. However, it is unknown whether recently discovered two-dimensional (2D) van der Walls (vdW) magnetic materials show such intriguing phenomena that can result in rich phase diagrams with novel magnetic features to be explored. H…
Exciton Gas Compression and Metallic Condensation in a Single Semiconductor Quantum Wire
2008
4 páginas, 5 figuras.-- PACS numbers: 78.67.Lt, 71.30.+h, 71.35. -y.-- Comunicación presentada a la International Conference on the Physics of Semiconductors (ICPS) celebrada en Rio de Jqaneiro (Brasil/2008).
Two topologically distinct Dirac-line semimetal phases and topological phase transitions in rhombohedrally stacked honeycomb lattices
2018
Three-dimensional topological semimetals can support band crossings along one-dimensional curves in the momentum space (nodal lines or Dirac lines) protected by structural symmetries and topology. We consider rhombohedrally (ABC) stacked honeycomb lattices supporting Dirac lines protected by time-reversal, inversion and spin rotation symmetries. For typical band structure parameters there exists a pair of nodal lines in the momentum space extending through the whole Brillouin zone in the stacking direction. We show that these Dirac lines are topologically distinct from the usual Dirac lines which form closed loops inside the Brillouin zone. In particular, an energy gap can be opened only by…