Search results for "TUNNEL"
showing 10 items of 576 documents
Coherent quantum phase slip
2012
A hundred years after discovery of superconductivity, one fundamental prediction of the theory, the coherent quantum phase slip (CQPS), has not been observed. CQPS is a phenomenon exactly dual to the Josephson effect: whilst the latter is a coherent transfer of charges between superconducting contacts, the former is a coherent transfer of vortices or fluxes across a superconducting wire. In contrast to previously reported observations of incoherent phase slip, the CQPS has been only a subject of theoretical study. Its experimental demonstration is made difficult by quasiparticle dissipation due to gapless excitations in nanowires or in vortex cores. This difficulty might be overcome by usin…
Coulomb Blockade and Bloch Oscillations in Superconducting Ti Nanowires
2012
Quantum fluctuations in quasi-one-dimensional superconducting channels leading to spontaneous changes of the phase of the order parameter by $2\pi$, alternatively called quantum phase slips (QPS), manifest themselves as the finite resistance well below the critical temperature of thin superconducting nanowires and the suppression of persistent currents in tiny superconducting nanorings. Here we report the experimental evidence that in a current-biased superconducting nanowire the same QPS process is responsible for the insulating state -- the Coulomb blockade. When exposed to RF radiation, the internal Bloch oscillations can be synchronized with the external RF drive leading to formation of…
Tunneling spectroscopy of persistent currents in superconducting microrings
2007
It is shown that in a structure consisting of a superconducting ring-shaped electrode overlapped by a normal metal contact through a thin oxide barrier, measurements of the tunnel current in magnetic field can probe persistent currents in the ring. The effect manifests itself as periodic oscillations of the tunnel current through the junction at a fixed bias voltage as function of perpendicular magnetic field. The magnitude of oscillations depends on bias point. It reaches maximum at energy eV which is close to the superconducting gap and decreases with increase of temperature. The period of oscillations dF in units of magnetic flux is equal neither to h/e nor to h/2e, but significantly exc…
Tunneling spectroscopy of giant vorticity states in superconducting micro- and nanorings at ultra-low temperatures
2008
Abstract When a superconducting contour is exposed to a magnetic field screening currents are induced. At temperatures well below the critical temperature Tc periodicity of the persistent currents can significantly exceed the superconducting flux quantum h / 2 e due to formation of metastable energy states with high quantum winding numbers (vorticity). We have studied the effect in normal metal-insulator-superconductor (NIS) and SIS’IS tunnel structures with the superconducting (S or S′) electrode in a shape of a loop. The tunnel current oscillates due to the modulation of the superconducting density of states by the persistent currents reaching the sub-critical values. In the limit of loop…
Microrefrigeration by NIS tunnel junctions
1996
By using a normal metal-insulator-superconductor (NIS) tunnel junction one can manipulate the Fermi-Dirac distribution of the electrons in the normal electrode. If the junction is biased close to the superconducting gap, Δ, only “hot electrons” above Fermi level can tunnel from the normal electrode to the superconductor. Thus, due to the decoupling of the conduction electrons from the lattice at low temperatures, there exists a possibility to decrease the electronic temperature by this method. Because of the symmetry with bias voltage, two NIS tunnel junctions in series can form an efficient microrefrigerator. Temperature can be measured with two additional junctions by considering the vari…
Microrefrigeration by normal-metal/ insulator/superconductor tunnel junctions
1997
Abstract A normal-metal/insulator/superconductor (NIS) tunnel junction can be applied to cool electrons by biasing the junction suitably with external voltage. Because of the symmetry with bias voltage, two NIS junctions in series can form an efficient microrefrigerator. So far our SINIS microrefrigerator has been capable of reaching electronic temperatures of about 100 mK starting from 300 mK. To achieve appreciable refrigeration of the underlying lattice, microrefrigerator must be thermally decoupled from the bulk substrate. We have demonstrated experimentally the reduction of lattice temperature of a few mK at 200 mK by extending the normal electrode on a thin dielectric membrane. Method…
Probing the superconducting state of UPd2Al3 thin films by tunneling spectroscopy
1997
Abstract Giaever-type planar cross junctions of thin films of the heavy fermion superconductor UPd2Al3 and counter-electrodes of Au, Ag and Al were prepared. Tunneling barriers consisting of the native oxide layer of UPd2Al3 and artificial barriers of AlOx and UOx were investigated. The junctions without artificial barrier show a lack of reproducebility. On some junctions a BCS-like tunneling conductivity with a ratio of 2Δ 0 /k B T c ⋍ 3.8 was observed. Using oxidized Al deposited at room temperature as a barrier no substantial increase of the junction resistance occured. With Uraniumoxide a much higher resistance was obtained, but an association of the bias dependent junction conductivity…
Microrefrigeration by quasiparticle tunnelling in NIS and SIS junctions
2000
Abstract A solid-state refrigeration method at sub-kelvin temperatures has been developed. It is based on quasiparticle tunnelling between a superconductor and a normal metal, or, between two dissimilar superconducting metals. The refrigerator is fabricated by combining nanolithography and micromachining methods. This technique has been demonstrated in both electron cooling from 0.3 to 0.1 K and in refrigeration of a dielectric platform. We describe a new fabrication method of tunnel junctions in a shadow evaporation configuration using a mechanical mask of silicon nitride.
Response time of a thermometer based on normal metal–insulator–superconductor (NIS) tunnel junctions
2003
Abstract We have measured the thermal response of a superconductor–normal metal–superconductor (SINIS) tunnel junction structure at substrate temperature ∼60 mK by directly heating the electron system in the normal metal island. In our structure, we find the response time is determined by the electron–phonon coupling in the electron temperature range 300– 600 mK . By using AC heating, the cut-off frequency caused by this response time has been measured, showing that SINIS structures operate as a thermometer up to a few MHz in this temperature range.
Cooling of a superconductor by quasiparticle tunneling
1999
We have extended the cryogenic cooling method based on tunneling between a superconductor and another metal to the case when both metals are superconducting but when their energy gaps are different; earlier, this method was applied between a superconductor and a normal metal. The electron system of a titanium strip with the superconducting transition temperature Tc2=0.51 K has been cooled from 1.02Tc2 to below 0.7Tc2 by this method, using aluminum as the other superconductor.