155 research outputs found
Nonequilibrium Relaxations and Aging Effects in a Two-Dimensional Coulomb Glass
The relaxations of conductivity have been studied in the glassy regime of a
strongly disordered two-dimensional electron system in Si after a temporary
change of carrier density during the waiting time t_w. Two types of response
have been observed: a) monotonic, where relaxations exhibit aging, i.e.
dependence on history, determined by t_w and temperature; b) nonmonotonic,
where a memory of the sample history is lost. The conditions that separate the
two regimes have been also determined.Comment: 4 pages; published versioi
Observation of Mass Transport through Solid 4He
By use of a novel experimental design, one that provides for superfluid
helium in contact with bulk hcp 4He off the melting curve, we have observed the
DC transport of mass through a cell filled with solid 4He in the hcp region of
the phase diagram. Flow, which shows characteristics of a superflow, is seen to
be independent of the method used to grow the solid, but depends on pressure
and temperature. The temperature dependence suggests the possibility of
hysteresis.Comment: 1 zipped file, produces 16 page paper, with 20 figures; resubmitted
with typos corrected, a figure corrected, some discussion improved, and
additional references - still 16 pages and 20 figure
Radio-Frequency Single-Electron Refrigerator
We propose a cyclic refrigeration principle based on mesoscopic electron
transport. Synchronous sequential tunnelling of electrons in a
Coulomb-blockaded device, a normal metal-superconductor single-electron box,
results in a cooling power of at temperature
over a wide range of cycle frequencies . Electrostatic work, done by the
gate voltage source, removes heat from the Coulomb island with an efficiency of
, where is the superconducting gap. The
performance is not affected significantly by non-idealities, for instance by
offset charges. We propose ways of characterizing the system and of its
practical implementation.Comment: 5 pages, 4 figures; corrected typos, language improve
Adiabatic Magnetization of Superconductors as a High-Performance Cooling Mechanism
The adiabatic magnetization of a superconductor is a cooling principle
proposed in the 30s, which has never been exploited up to now. Here we present
a detailed dynamic description of the effect, computing the achievable final
temperatures as well as the process timescales for different superconductors in
various regimes. We show that, although in the experimental conditions explored
so far the method is in fact inefficient, a suitable choice of initial
temperatures and metals can lead to unexpectedly large cooling effect, even in
the presence of dissipative phenomena. Our results suggest that this principle
can be re-envisaged today as a performing refrigeration method to access the
microK regime in nanodevices.Comment: 4 pages, 3 color figure
Hot electron cooling by acoustic phonons in graphene
We have investigated the energy loss of hot electrons in metallic graphene by
means of GHz noise thermometry at liquid helium temperature. We observe the
electronic temperature T / V at low bias in agreement with the heat diffusion
to the leads described by the Wiedemann-Franz law. We report on
behavior at high bias, which corresponds to a T4 dependence
of the cooling power. This is the signature of a 2D acoustic phonon cooling
mechanism. From a heat equation analysis of the two regimes we extract accurate
values of the electron-acoustic phonon coupling constant in monolayer
graphene. Our measurements point to an important effect of lattice disorder in
the reduction of , not yet considered by theory. Moreover, our study
provides a strong and firm support to the rising field of graphene bolometric
detectors.Comment: 5 figure
Magnetocaloric Study of Spin Relaxation in `Frozen' Dipolar Spin Ice Dy2Ti2O7
The magnetocaloric effect of polycrystalline samples of pure and Y-doped
dipolar spin ice Dy2Ti2O7 was investigated at temperatures from nominally 0.3 K
to 6 K and in magnetic fields of up to 2 T. As well as being of intrinsic
interest, it is proposed that the magnetocaloric effect may be used as an
appropriate tool for the qualitative study of slow relaxation processes in the
spin ice regime. In the high temperature regime the temperature change on
adiabatic demagnetization was found to be consistent with previously published
entropy versus temperature curves. At low temperatures (T < 0.4 K) cooling by
adiabatic demagnetization was followed by an irreversible rise in temperature
that persisted after the removal of the applied field. The relaxation time
derived from this temperature rise was found to increase rapidly down to 0.3 K.
The data near to 0.3 K indicated a transition into a metastable state with much
slower relaxation, supporting recent neutron scattering results. In addition,
magnetic dilution of 50 % concentration was found to significantly prolong the
dynamical response in the milikelvin temperature range, in contrast with
results reported for higher temperatures at which the spin correlations are
suppressed. These observations are discussed in terms of defects and loop
correlations in the spin ice state.Comment: 9 figures, submitted to Phys. Rev.
Specific Heat of the Dilute Ising Magnet LiHoYF
We present specific heat data on three samples of the dilute Ising magnet
\HoYLF with , 0.045 and 0.080. Previous measurements of the ac
susceptibility of an sample showed the Ho moments to remain
dynamic down to very low temperatures and the specific heat was found to have
unusually sharp features. In contrast, our measurements do not exhibit these
sharp features in the specific heat and instead show a broad feature, for all
three samples studied, which is qualitatively consistent with a spin glass
state. Integrating , however, reveals an increase in residual entropy with
lower Ho concentration, consistent with recent Monte Carlo simulations showing
a lack of spin glass transition for low x.Comment: 10 pages, 3 figurs, accepted for publication in Phys. Rev. Let
High-Temperature Superconducting Level Meter for Liquid Argon Detectors
Capacitive devices are customarily used as probes to measure the level of
noble liquids in detectors operated for neutrino studies and dark matter
searches. In this work we describe the use of a high-temperature
superconducting material as an alternative to control the level of a cryogenic
noble liquid. Lab measurements indicate that the superconductor shows a linear
behaviour, a high degree of stability and offers a very accurate determination
of the liquid volume. This device is therefore a competitive instrument and
shows several advantages over conventional level meters.Comment: 13 pages, 11 figures. Accepted for publication in JINS
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