66 research outputs found
Spin-torque driven magnetic vortex self-oscillations in perpendicular magnetic fields
We have employed complete micromagnetic simulations to analyze dc current
driven self-oscillations of a vortex core in a spin-valve nanopillar in a
perpendicular field by including the coupled effect of the spin-torque and the
magnetostatic field computed self-consistently for the entire spin-valve. The
vortex in the thicker nanomagnet moves along a quasi-elliptical trajectory that
expands with applied current, resulting in blue-shifting of the frequency,
while the magnetization of the thinner nanomagnet is non-uniform due to the
bias current. The simulations explain the experimental magnetoresistance-field
hysteresis loop and yield good agreement with the measured frequency vs.
current behavior of this spin-torque vortex oscillator.Comment: 10 pages, 3 figures, to be appear on AP
Spectroscopy of spin-orbit quantum bits in indium antimonide nanowires
Double quantum dot in the few-electron regime is achieved using local gating
in an InSb nanowire. The spectrum of two-electron eigenstates is investigated
using electric dipole spin resonance. Singlet-triplet level repulsion caused by
spin-orbit interaction is observed. The size and the anisotropy of
singlet-triplet repulsion are used to determine the magnitude and the
orientation of the spin-orbit effective field in an InSb nanowire double dot.
The obtained results are confirmed using spin blockade leakage current
anisotropy and transport spectroscopy of individual quantum dots.Comment: 5 pages, supplementary material available at
http://link.aps.org/supplemental/10.1103/PhysRevLett.108.16680
Suppression of Zeeman gradients by nuclear polarization in double quantum dots
We use electric dipole spin resonance to measure dynamic nuclear polarization
in InAs nanowire quantum dots. The resonance shifts in frequency when the
system transitions between metastable high and low current states, indicating
the presence of nuclear polarization. We propose that the low and the high
current states correspond to different total Zeeman energy gradients between
the two quantum dots. In the low current state, dynamic nuclear polarization
efficiently compensates the Zeeman gradient due to the -factor mismatch,
resulting in a suppressed total Zeeman gradient. We present a theoretical model
of electron-nuclear feedback that demonstrates a fixed point in nuclear
polarization for nearly equal Zeeman splittings in the two dots and predicts a
narrowed hyperfine gradient distribution
Selective Control of Conductance Modes in Multi-terminal Josephson Junctions
The Andreev bound state spectra of multi-terminal Josephson junctions form an
artificial band structure, which is predicted to host tunable topological
phases under certain conditions. However, the number of conductance modes
between the terminals of multi-terminal Josephson junction must be few in order
for this spectrum to be experimentally accessible. In this work we employ a
quantum point contact geometry in three-terminal Josephson devices. We
demonstrate independent control of conductance modes between each pair of
terminals and access to the single-mode regime coexistent with the presence of
superconducting coupling. These results establish a full platform on which to
realize tunable Andreev bound state spectra in multi-terminal Josephson
junctions.Comment: 15 pages, 4 figure
Gate-tunable Superconducting Diode Effect in a Three-terminal Josephson Device
The phenomenon of non-reciprocal critical current in a Josephson device,
termed the Josephson diode effect, has garnered much recent interest.
Realization of the diode effect requires inversion symmetry breaking, typically
obtained by spin-orbit interactions. Here we report observation of the
Josephson diode effect in a three-terminal Josephson device based upon an InAs
quantum well two-dimensional electron gas proximitized by an epitaxial aluminum
superconducting layer. We demonstrate that the diode efficiency in our devices
can be tuned by a small out-of-plane magnetic field or by electrostatic gating.
We show that the Josephson diode effect in these devices is a consequence of
the artificial realization of a current-phase relation that contains higher
harmonics. We also show nonlinear DC intermodulation and simultaneous
two-signal rectification, enabled by the multi-terminal nature of the devices.
Furthermore, we show that the diode effect is an inherent property of
multi-terminal Josephson devices, establishing an immediately scalable approach
by which potential applications of the Josephson diode effect can be realized,
agnostic to the underlying material platform. These Josephson devices may also
serve as gate-tunable building blocks in designing topologically protected
qubits
Ultrafast switching of a nanomagnet by a combined out-of-plane and in-plane polarized spin-current pulse
We report on spin valve devices that incorporate both an out-of-plane
polarizer (OPP) to quickly excite spin torque (ST) switching and an in-plane
polarizer/analyzer (IPP). For pulses < 200 ps we observe reliable precessional
switching due largely to ST from the OPP. Compared to a conventional spin
valve, for a given current in the short pulse regime the addition of the OPP
can decrease the pulse width necessary for switching by a factor of 10 or more.
The influence of the IPP is most obvious at longer, smaller pulses, but also
has beneficial ST consequences for short pulse switching.Comment: 14 pages, 2 figure
Spin-orbit interaction in InSb nanowires
We use magnetoconductance measurements in dual-gated InSb nanowire devices
together with a theoretical analysis of weak antilocalization to accurately
extract spin-orbit strength. In particular, we show that magnetoconductance in
our three-dimensional wires is very different compared to wires in
two-dimensional electron gases. We obtain a large Rashba spin-orbit strength of
corresponding to a spin-orbit energy of
. These values underline the potential of InSb nanowires in
the study of Majorana fermions in hybrid semiconductor-superconductor devices.Comment: Version as accepted for publication as a Rapid in Phys. Rev.
"Smoking gun" signatures of topological milestones in trivial materials by measurement fine-tuning and data postselection
Exploring the topology of electronic bands is a way to realize new states of
matter with possible implications for information technology. Because bands
cannot always be observed directly, a central question is how to tell that a
topological regime has been achieved. Experiments are often guided by a
prediction of a unique signal or a pattern, called "the smoking gun". Examples
include peaks in conductivity, microwave resonances, and shifts in interference
fringes. However, many condensed matter experiments are performed on relatively
small, micron or nanometer-scale, specimens. These structures are in the
so-called mesoscopic regime, between atomic and macroscopic physics, where
phenomenology is particularly rich. In this paper, we demonstrate that the
trivial effects of quantum confinement, quantum interference and charge
dynamics in nanostructures can reproduce accepted smoking gun signatures of
triplet supercurrents, Majorana modes, topological Josephson junctions and
fractionalized particles. The examples we use correspond to milestones of
topological quantum computing: qubit spectroscopy, fusion and braiding. None of
the samples we use are in the topological regime. The smoking gun patterns are
achieved by fine-tuning during data acquisition and by subsequent data
selection to pick non-representative examples out of a fluid multitude of
similar patterns that do not generally fit the "smoking gun" designation.
Building on this insight, we discuss ways that experimentalists can rigorously
delineate between topological and non-topological effects, and the effects of
fine-tuning by deeper analysis of larger volumes of data.Comment: Data are available through Zenodo at DOI: 10.5281/zenodo.834930
- …