24 research outputs found
Nonlinear Hall effect as a signature of electronic phase separation in the semimetallic ferromagnet EuB6
This work reports a study of the nonlinear Hall Effect (HE) in the
semimetallic ferromagnet EuB6. A distinct switch in its Hall resistivity slope
is observed in the paramagnetic phase, which occurs at a single critical
magnetization over a wide temperature range. The observation is interpreted as
the point of percolation for entities of a more conducting and magnetically
ordered phase in a less ordered background. With an increasing applied magnetic
field, the conducting regions either increase in number or expand beyond the
percolation limit, hence increasing the global conductivity and effective
carrier density. An empirical two-component model expression provides excellent
scaling and a quantitative fit to the HE data and may be applicable to other
correlated electron systems.Comment: 15 Pages, 4 Figures. Accepted for publication in Phys. Rev. Let
Orbital effects of a strong in-plane magnetic field on a gate-defined quantum dot
We theoretically investigate the orbital effects of an in-plane magnetic
field on the spectrum of a quantum dot embedded in a two-dimensional electron
gas (2DEG). We derive an effective two-dimensional Hamiltonian where these
effects enter in proportion to the flux penetrating the 2DEG. We quantify the
latter in detail for harmonic, triangular, and square potential of the
heterostructure. We show how the orbital effects allow one to extract a wealth
of information, for example, on the heterostructure interface, the quantum dot
size and orientation, and the spin-orbit fields. We illustrate the formalism by
extracting this information from recent measured data [L.~C.~Camenzind, et al.,
arXiv:1804.00162; Nat. Commun. 9, 3454 (2018)].Comment: 14 pages, 9 figures; minor changes resulting from refereeing and
proof
Probing Hundreds of Individual Quantum Defects in Polycrystalline and Amorphous Alumina
Quantum two-level systems (TLSs) are present in the materials of qubits and are considered defects because they limit qubit coherence. For superconducting qubits, the quintessential Josephson junction barrier is made of amorphous alumina, which hosts TLSs. However, TLSs are not understood generally -- either structurally or in atomic composition. In this study, we greatly extend the quantitative data available on TLSs by reporting on the physical dipole moment in two alumina types: polycrystalline γ−AlO and amorphous a−AlO. To obtain the dipole moments p, rather from the less-structural coupling parameter g, we tune individual TLSs with an external electric field to extract the p of the TLSs in a cavity QED system. We find a clear difference in the dipole moment distribution from the film types, indicating a difference in TLS structures. A large sample of approximately 400 individual TLSs are analyzed from the polycrystalline film type. Their dipoles along the growth direction p have a mean value of 2.6±0.3 Debye (D) and standard deviation σ = 1.6±0.2 D . The material distribution fits well to a single Gaussian function. Approximately 200 individual TLSs are analyzed from amorphous films. Both the mean p =4.6±0.5 D and σ =2.5±0.3 D are larger. Amorphous alumina also has some very large p, > 8.6 D, in contrast to polycrystalline which has none of this moment. These large moments agree only with oxygen-based TLS models. Based on data and the candidate models (delocalized O and hydrogen-based TLSs), we find polycrystalline alumina has smaller ratio of O-based to H-based TLS than amorphous alumina
Signatures of electronic phase separation in the Hall effect of anisotropically strained La0.67Ca0.33MnO3 films
Systematic transport measurements have been performed on a series of
La0.67Ca0.33MnO3 (LCMO) thin films with varying degrees of anisotropic strain.
The strain is induced via epitaxial growth on NdGaO3(001) substrates and varied
by controlling the thermal annealing time. An antiferromagnetic insulating
(AFI) state, possibly associated with charge ordering, emerges upon thermal
annealing. The Hall effect in these materials exhibits features that are
indicative of a percolative phase transition and correlate closely with the
emergence of the AFI state. In the paramagnetic phase, the Hall resistivity
takes on two slopes in all samples: a decreasing negative slope with increasing
temperature at low fields, which is attributed to the carrier hopping motion,
and an almost temperature independent positive slope at high fields due to
diffusive transport of holes. Significantly, the crossover fields of the Hall
resistivity slope at different temperatures correspond to the same
magnetization, which is interpreted as the critical point of a magnetic
field-driven percolative phase transition. At lower temperatures near the
zero-field metal-insulator transition, pronounced enhancement of the Hall
coefficient with the development of the AFI state is observed. The enhancement
peaks near the magnetic field-driven percolation; its magnitude correlates with
the strength of the AFI state and is suppressed with the melting of the AFI
state by an in-plane magnetic field. The observations resemble many features of
the enhancement of the Hall coefficient in granular metal films near the
composition-driven percolation
Spectroscopy of Quantum-Dot Orbitals with In-Plane Magnetic Fields
We show that in-plane-magnetic-field assisted spectroscopy allows extraction
of the in-plane orientation and full 3D shape of the quantum mechanical
orbitals of a single electron GaAs lateral quantum dot with sub-nm precision.
The method is based on measuring orbital energies in a magnetic field with
various strengths and orientations in the plane of the 2D electron gas. As a
result, we deduce the microscopic quantum dot confinement potential landscape,
and quantify the degree by which it differs from a harmonic oscillator
potential. The spectroscopy is used to validate shape manipulation with gate
voltages, agreeing with expectations from the gate layout. Our measurements
demonstrate a versatile tool for quantum dots with one dominant axis of strong
confinement.Comment: 4 pages, 3 color figures, including supplementary on arXi
Hyperfine-phonon spin relaxation in a single-electron GaAs quantum dot
Understanding and control of the spin relaxation time T-1 is among the key challenges for spinbased qubits. A larger T-1 is generally favored, setting the fundamental upper limit to the qubit coherence and spin readout fidelity. In GaAs quantum dots at low temperatures and high inplane magnetic fields B, the spin relaxation relies on phonon emission and spin-orbit coupling. The characteristic dependence T-1 alpha B-5 and pronounced B-field anisotropy were already confirmed experimentally. However, it has also been predicted 15 years ago that at low enough fields, the spin-orbit interaction is replaced by the coupling to the nuclear spins, where the relaxation becomes isotropic, and the scaling changes to T-1 alpha B-3. Here, we establish these predictions experimentally, by measuring T-1 over an unprecedented range of magnetic fields-made possible by lower temperature-and report a maximum T-1 = 57 +/- 15 s at the lowest fields, setting a record electron spin lifetime in a nanostructure
Experimentally revealing anomalously large dipoles in the dielectric of a quantum circuit
Quantum two-level systems (TLSs) intrinsic to glasses induce decoherence in many modern quantum devices, such as superconducting qubits. Although the low-temperature physics of these TLSs is usually well-explained by a phenomenological standard tunneling model of independent TLSs, the nature of these TLSs, as well as their behavior out of equilibrium and at high energies above 1 K, remain inconclusive. Here we measure the non-equilibrium dielectric loss of TLSs in amorphous silicon using a superconducting resonator, where energies of TLSs are varied in time using a swept electric field. Our results show the existence of two distinct ensembles of TLSs, interacting weakly and strongly with phonons, where the latter also possesses anomalously large electric dipole moment. These results may shed new light on the low temperature characteristics of amorphous solids, and hold implications to experiments and applications in quantum devices using time-varying electric fields
Machine learning enables completely automatic tuning of a quantum device faster than human experts
Variability is a problem for the scalability of semiconductor quantum devices. The parameter space is large, and the operating range is small. Our statistical tuning algorithm searches for specific electron transport features in gate-defined quantum dot devices with a gate voltage space of up to eight dimensions. Starting from the full range of each gate voltage, our machine learning algorithm can tune each device to optimal performance in a median time of under 70 minutes. This performance surpassed our best human benchmark (although both human and machine performance can be improved). The algorithm is approximately 180 times faster than an automated random search of the parameter space, and is suitable for different material systems and device architectures. Our results yield a quantitative measurement of device variability, from one device to another and after thermal cycling. Our machine learning algorithm can be extended to higher dimensions and other technologies