ANNUAL REPORT - MTA SzFKI

G. ELECTRON CRYSTALS
G. Kriza, P. Matus # , Gy. Mihály + , L. Németh # , Á. Pallinger # , B. Sas, F.I.B. Williams
Dissipation in high-T c superconductors. — Most high-magnetic-field applications of
superconductors are limited by dissipative mechanisms. To investigate this important
problem, we have collected and analyzed a large body of data on the dissipation at high
transport currents in single crystals of the cuprate superconductor Bi 2 Sr 2 Ca 2 CuO 8 . We
have found that several aspects of the dissipative processes are in disagreement with the
current theoretical understanding of this important model system of high-temperature
superconductors. We have discovered a new scaling law of the resistance valid sufficiently
close to the critical temperature where the superconductivity sets on. According to this
law, the resistance depends on temperature only through the upper critical magnetic field
H c2 , one of the fundamental parameters of the superconductor related to the size of Cooper
pairs. Moreover, the resistance varies logarithmically with magnetic field, rather than
following a power law as in conventional superconductors. Nevertheless by a re-analysis
of experimental results in the literature, we have shown that for transport currents flowing
parallel to the superconducting planes of this layered material, a power law is still valid,
although the exponent ¾ is different from that described by the well known Bardeen-
Stephen law for conventional superconductors. We have also found an analytic form for
the resistance perpendicular to the superconducting layers, which is in good agreement
with experimental results.
Microwave Spectroscopy. — Two-dimensional systems, be they electrons, holes or
superconducting vortices, lend themselves well to investigation by finite-wavevector nearfield
microwave spectroscopy. It is planned to study holes at GaAs/GaAlAs
heterojunctions in the Wigner solid phase, electrons/holes in graphene in the quantum Hall
regime and Kelvin modes of quasi 2-D vortices in BSCCO (Bi 2 Sr 2 CaCu 2 O 8 , an extremely
anisotropic high temperature superconductor). To this end we have prepared a 0.1-10 GHz
swept-frequency low-level source and low noise (noise temperature ~200K)
superheterodyne detector which will be used with our 30 mK dilution refrigerator in the
mixing chamber of which is a microstrip meander line coupling device connected through
low thermal loss, high-transmittance Cu-Kapton based transmission lines. This alone
enables us to do microwave spectroscopy up to wavevectors of ~ 10 5 cm -1 , but with the
adjunction of a nanolithography-manufactured grid of 300 nm periodicity the range should
be extended to ~ 10 6 cm -1 . The refrigerator has been designed to be placed in high
magnetic field (28 T) at the Grenoble high magnetic field facility when necessary;
otherwise it is run in our local 8 T coil.
E-Mail:
György Kriza
László Németh
Péter Matus
György Mihály
Ágnes Pallinger
Bernadette Sas
F.I.B. Williams
kriza@szfki.hu
lnemeth@szfki.hu
matus@szfki.hu
mihaly@phy.bme.hu
pagnes@szfki.hu
sas@szfki.hu
willia@szfki.hu
# Ph.D. student
+ Permanent position: Budapest University of Technology and Economics
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