Search results for "MAGNETIC FIELD"
showing 10 items of 1488 documents
Experimental investigation of the breakdown of the Onsager-Casimir relations
2006
We use magnetoconductance fluctuation measurements of phase-coherent semiconductor billiards to quantify the contributions to the nonlinear electric conductance that are asymmetric under reversal of magnetic field. We experimentally determine that the average asymmetric contribution is linear in magnetic field (for magnetic flux much larger than one flux quantum) and that its magnitude depends on billiard geometry. In addition, we find an unexpected asymmetry in the power spectrum characteristics of the magnetoconductance with respect to reversal of magnetic field and bias voltage.
Spin and rotational symmetries in unrestricted Hartree–Fock states of quantum dots
2007
Ground state energies are obtained using the unrestricted Hartree Fock method for up to four interacting electrons parabolically confined in a quantum dot subject to a magnetic field. Restoring spin and rotational symmetries we recover Hund first rule. With increasing magnetic field, crossovers between ground states with different quantum numbers are found for fixed electron number that are not reproduced by the unrestricted Hartree Fock approximation. These are consistent with the ones obtained with more refined techniques. We confirm the presence of a spin blockade due to a spin mismatch in the ground states of three and four electrons.
Geometrically enhanced closed-loop multi-turn sensor devices that enable reliable magnetic domain wall motion
2017
We experimentally realize a sophisticated structure geometry for reliable magnetic domain wall-based multi-turn-counting sensor devices, which we term closed-loop devices that can sense millions of turns. The concept relies on the reliable propagation of domain walls through a cross-shaped intersection of magnetic conduits, allowing for the intertwining of loops of the sensor device. As a key step to reach the necessary reliability of the operation, we develop a combination of tilted wires called the syphon structure at the entrances of the cross. We measure the control and reliability of the domain wall propagation individually for cross-shaped intersections, the syphon geometries, and fin…
Single-parameter quantized charge pumping in high magnetic fields
2008
We study single-parameter quantized charge pumping via a semiconductor quantum dot in high magnetic fields. The quantum dot is defined between two top gates in an AlGaAs/GaAs heterostructure. Application of an oscillating voltage to one of the gates leads to pumped current plateaus in the gate characteristic, corresponding to controlled transfer of integer multiples of electrons per cycle. In a perpendicular-to-plane magnetic field the plateaus become more pronounced indicating an improved current quantization. Current quantization is sustained up to magnetic fields where full spin polarization of the device can be expected.
Aharonov-Bohm effect in many-electron quantum rings
2010
The Aharonov-Bohm effect is investigated in two-dimensional, single-terminal quantum rings in magnetic fields by using time-dependent density-functional theory. We find multiple transport loops leading to the oscillation periods of $h/(en)$, where $n$ is the number of loops. We show that the Aharonov-Bohm oscillations are relatively weakly affected by the electron-electron interactions, whereas the ring width has a strong effect on the characteristics of the oscillations. Our results propose that in those experimental semiconductor quantum-ring devices that show clear Aharonov-Bohm oscillations the electron current is dominated by a few states along narrow conduction channels.
Many-body spectrum and particle localization in quantum dots and finite rotating Bose condensates
2001
The yrast spectra (i.e. the lowest states for a given total angular momentum) of quantum dots in strong magnetic fields, are studied in terms of exact numerical diagonalization and analytic trial wave functions. We argue that certain features (cusps) in the many-body spectrum can be understood in terms of particle localization due to the strong field. A new class of trial wavefunctions supports the picture of the electrons being localized in Wigner molecule-like states consisting of consecutive rings of electrons, with low-lying excitations corresponding to rigid rotation of the outer ring of electrons. The geometry of the Wigner molecule is independent of interparticle interactions and the…
Theory of radiative recombination from the metastable excited states of quantum dots
1998
The radiative recombination of an exciton ~electron-hole pair! confined in a semiconductor quantum dot is studied within a general model based on the effective-mass approximation. The dependence of the photoluminescence spectrum on the size of the dot and the magnetic field describe well a series of recent experimental results. In particular, a characteristic splitting of the main photoluminescence peak into a doublet or triplet is observed at the critical size and magnetic field, as a consequence of the appearance of metastable states in the exciton spectrum. @S0163-1829~98!06915-X#
Magnetic-field-enhanced outgoing excitonic resonance in multi-phonon Raman scattering from polar semiconductors
1996
A combined scattering mechanism involving the states of free electron-hole pairs (exciton continuum) and discrete excitons as intermediate states in the multi-phonon Raman scattering leads to (1) a strong increase of the scattering efficiency in the presence of a high magnetic field and to (2) an outgoing excitonic resonance: the two features are not compatible when only free pairs (leading to a strong increase of the scattering efficiency under the applied magnetic field) or discrete excitons (resulting in the outgoing resonance at the excitonic gap) are taken into account.
Quantum knots in Bose-Einstein condensates created by counterdiabatic control
2017
We theoretically study the creation of knot structures in the polar phase of spin-1 BECs using the counterdiabatic protocol in an unusual fashion. We provide an analytic solution to the evolution of the external magnetic field that is used to imprint the knots. As confirmed by our simulations using the full three-dimensional spin-1 Gross-Pitaevskii equation, our method allows for the precise control of the Hopf charge as well as the creation time of the knots. The knots with Hopf charge exceeding unity display multiple nested Hopf links.
Engineering Ising-XY spin models in a triangular lattice via tunable artificial gauge fields,
2013
Emulation of gauge fields for ultracold atoms provides access to a class of exotic states arising in strong magnetic fields. Here we report on the experimental realisation of tunable staggered gauge fields in a periodically driven triangular lattice. For maximal staggered magnetic fluxes, the doubly degenerate superfluid ground state breaks both a discrete Z2 (Ising) symmetry and a continuous U(1) symmetry. By measuring an Ising order parameter, we observe a thermally driven phase transition from an ordered antiferromagnetic to an unordered paramagnetic state and textbook-like magnetisation curves. Both the experimental and theoretical analysis of the coherence properties of the ultracold g…