A pulse tube cryocooler operating at 120Hz with 3.5MPa average pressure achieved a no-load temperature of about 49.9K and a cooldown time to 80K of 5.5min. The net refrigeration power at 80K was 3.35W with an efficiency of 19.7% of Carnot when referred to input pressure-volume (PV or acoustic) power. Such low temperatures have not been previously achieved for operating frequencies above 100Hz. The high frequency operation leads to reduced cryocooler volume for a given refrigeration power, which is important to many applications and can enable development of microcryocoolers for microelectromechanical system applications.
Single crystals of two-dimensional
2H-NbSe2, which undergoes a superconducting transition at
Tc = 7.2 K and a charge-density wave (CDW) transition at
TCDW = 30 K, were synthesized. Measurements of the resistivity, Hall coefficient
and magnetoresistance (MR) versus temperature were performed on
NbSe2
crystals with different resistance residual ratios
(RRR = R(300 K)/R(8 K)), chosen from different batches. The superconducting transition temperature hardly
changes with the RRR, while the MR and Hall coefficient are strongly dependent on the
RRR values of the samples. Moreover, the temperature and field dependence
of the MR violate Kohler’s rule, indicating that the scattering times of the
charge carriers are no longer isotropic. A steep decrease of the Hall coefficient
RH
below 50 K is only observed for the high quality sample; this can be interpreted in terms of
a drastic increase of the mean free path for the electron-type charge carriers. The
effect of the CDW transition on the Hall coefficient is discussed using a two-band
model and a sharp change in the scattering rate on an electron-like orbit below
TCDW
is suggested.
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