31 research outputs found
Reducing the hydrogen content in liquid helium
Helium has the lowest boiling point of any element in nature at normal atmospheric pressure. Therefore, any unwanted substance like impurities present in liquid helium will be frozen and will be in solid form. Even if these solid impurities can be easily eliminated by filtering, liquid helium may contain a non negligible quantity of molecular hydrogen. These traces of molecular hydrogen are the causes of a known problem worldwide: the blocking of fine capillary tubes used as flow resistors in helium evaporation cryostats to achieve temperatures below 4.2 K. This problem seriously affects a wide range of cryogenic equipment used in low temperature physics research and leads to a dramatic loss of time and costs due to the high price of helium. Here, we present first the measurement of molecular hydrogen content in helium gas. Three measures to decrease this molecular hydrogen are afterward proposed; (i)improving the helium quality, (ii) release of helium gas in the atmosphere during purge time for the regeneration cycle of the helium liquefierĂąEurotms internal purifier, and (iii) installation of two catalytic converters in a closed helium circuit. These actions have eliminated all blockages of capillaries at low temperatures now for more than two years
Magnetic cooling for microkelvin nanoelectronics on a cryofree platform
We present a parallel network of 16 demagnetization refrigerators mounted on
a cryofree dilution refrigerator aimed to cool nanoelectronic devices to
sub-millikelvin temperatures. To measure the refrigerator temperature, the
thermal motion of electrons in a Ag wire -- thermalized by a spot-weld to one
of the Cu nuclear refrigerators -- is inductively picked-up by a
superconducting gradiometer and amplified by a SQUID mounted at 4 K. The noise
thermometer as well as other thermometers are used to characterize the
performance of the system, finding magnetic field independent heat-leaks of a
few nW/mol, cold times of several days below 1 mK, and a lowest temperature of
150 microK of one of the nuclear stages in a final field of 80 mT, close to the
intrinsic SQUID noise of about 100 microK. A simple thermal model of the system
capturing the nuclear refrigerator, heat leaks, as well as thermal and Korringa
links describes the main features very well, including rather high refrigerator
efficiencies typically above 80%.Comment: 4 color figures, including supplementary inf
Stretchable persistent spin helices in GaAs quantum wells
The Rashba and Dresselhaus spin-orbit (SO) interactions in 2D electron gases
act as effective magnetic fields with momentum-dependent directions, which
cause spin decay as the spins undergo arbitrary precessions about these
randomly-oriented SO fields due to momentum scattering. Theoretically and
experimentally, it has been established that by fine-tuning the Rashba
and Dresselhaus couplings to equal strengths
, the total SO field becomes unidirectional thus rendering the
electron spins immune to dephasing due to momentum scattering. A robust
persistent spin helix (PSH) has already been experimentally realized at this
singular point . Here we employ the suppression of weak
antilocalization as a sensitive detector for matched SO fields together with a
technique that allows for independent electrical control over the SO couplings
via top gate voltage and back gate voltage . We demonstrate for the
first time the gate control of and the of
the SO fields at , i.e., we are able to vary both and
controllably and continuously with and , while keeping them
locked at equal strengths. This makes possible a new concept: "stretchable
PSHs", i.e., helical spin patterns with continuously variable pitches over
a wide parameter range. The extracted spin-diffusion lengths and decay times as
a function of show a significant enhancement near
. Since within the continuous-locking regime quantum transport
is diffusive (2D) for charge while ballistic (1D) for spin and thus amenable to
coherent spin control, stretchable PSHs could provide the platform for the much
heralded long-distance communication m between solid-state
spin qubits.Comment: 5 color figures, with supplementary info available on arXiv. arXiv
admin note: substantial text overlap with arXiv:1403.351
Anisotropic Etching of Graphite and Graphene in a Remote Hydrogen Plasma
We investigate the etching of a pure hydrogen plasma on graphite samples and
graphene flakes on SiO and hexagonal Boron-Nitride (hBN) substrates. The
pressure and distance dependence of the graphite exposure experiments reveals
the existence of two distinct plasma regimes: the direct and the remote plasma
regime. Graphite surfaces exposed directly to the hydrogen plasma exhibit
numerous etch pits of various size and depth, indicating continuous defect
creation throughout the etching process. In contrast, anisotropic etching
forming regular and symmetric hexagons starting only from preexisting defects
and edges is seen in the remote plasma regime, where the sample is located
downstream, outside of the glowing plasma. This regime is possible in a narrow
window of parameters where essentially all ions have already recombined, yet a
flux of H-radicals performing anisotropic etching is still present. At the
required process pressures, the radicals can recombine only on surfaces, not in
the gas itself. Thus, the tube material needs to exhibit a sufficiently low H
radical recombination coefficient, such a found for quartz or pyrex. In the
remote regime, we investigate the etching of single layer and bilayer graphene
on SiO and hBN substrates. We find isotropic etching for single layer
graphene on SiO, whereas we observe highly anisotropic etching for graphene
on a hBN substrate. For bilayer graphene, anisotropic etching is observed on
both substrates. Finally, we demonstrate the use of artificial defects to
create well defined graphene nanostructures with clean crystallographic edges.Comment: 7 pages, 4 color figure
A spin qubit in a fin field-effect transistor
Quantum computing's greatest challenge is scaling up. Several decades ago,
classical computers faced the same problem and a single solution emerged:
very-large-scale integration using silicon. Today's silicon chips consist of
billions of field-effect transistors (FinFETs) in which current flow along the
fin-shaped channel is controlled by wrap-around gates. The semiconductor
industry currently employs fins of sub-10nm width, small enough for quantum
applications: at low temperature, an electron or hole can be trapped under the
gate and serve as a spin qubit. An attractive benefit of silicon's advantageous
scaling properties is that quantum hardware and its classical control circuitry
can be integrated in the same package. This, however, requires qubit operation
at temperatures greater than 1K where the cooling is sufficient to overcome
the heat dissipation. Here, we demonstrate that a silicon FinFET is an
excellent host for spin qubits that operate even above 4K. We achieve fast
electrical control of hole spins with driving frequencies up to 150MHz and
single-qubit gate fidelities at the fault-tolerance threshold. The number of
spin rotations before coherence is lost at these "hot" temperatures already
matches or exceeds values on hole spin qubits at mK temperatures. While our
devices feature both industry compatibility and quality, they are fabricated in
a flexible and agile way to accelerate their development. This work paves the
way towards large-scale integration of all-electrical and ultrafast spin
qubits
G-factor of electrons in gate-defined quantum dots in a strong in-plane magnetic field
We analyze orbital effects of an in-plane magnetic field on the spin
structure of states of a gated quantum dot based in a two-dimensional electron
gas. Starting with a Hamiltonian, we perturbatively calculate these
effects for the conduction band of GaAs, up to the third power of the magnetic
field. We quantify several corrections to the g-tensor and reveal their
relative importance. We find that for typical parameters, the Rashba spin-orbit
term and the isotropic term, , give the largest contributions in magnitude. The in-plane
anisotropy of the g-factor is, on the other hand, dominated by the Dresselhaus
spin-orbit term. At zero magnetic field, the total correction to the g-factor
is typically 5-10% of its bulk value. In strong in-plane magnetic fields, the
corrections are modified appreciably.Comment: 24 pages, 8 figures; v2 is in content identical to the version
published in PRB. Compared to v1, the minor changes adopted in v2 are
reflecting the PRB referees' suggestion
Breakdown of the Korringa Law of Nuclear Spin Relaxation in Metallic GaAs
We present nuclear spin relaxation measurements in GaAs epilayers using a new
pump-probe technique in all-electrical, lateral spin-valve devices. The
measured T1 times agree very well with NMR data available for T > 1 K. However,
the nuclear spin relaxation rate clearly deviates from the well-established
Korringa law expected in metallic samples and follows a sub-linear temperature
dependence 1/T1 ~ T^0.6 for 0.1 K < T < 10 K. Further, we investigate nuclear
spin inhomogeneities.Comment: 5 pages, 4 (color) figures. arXiv admin note: text overlap with
arXiv:1109.633
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