Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Thermal expansion of organic superconductor a-(BEDT-TTF)2 NH4Hg(SCN)4   The temperature dependence of the linear thermal expansion coefficients
(LTEC) of a single crystal of a-(BEDT-TTF)2NH4Hg(SCN)4 where BEDT-TTF is
bis(ethylenedithio)tetrathiafulvalene was studied by the method of precision
capacitive dilatometry in the temperature range 2-250 K along the
crystallographic direction b* (perpendicular to the crystal layers). Negative
values of LTEC were found below 4 K. Probably it is due to charge fluctuations
as the temperature approaches the temperature of transition to the
superconducting state Tc. It has been suggested that the bends on temperature
dependence of LTEC observed in the temperature range 25-45 K are related to
order-disorder arrangement of NH4+ ions. Weak maximum of the LTEC, detected at
a temperature of about 200-220 K, can be caused by the processes of charge
redistribution and the associated intermolecular interaction fluctuations.

22
The categories of this paper are: chao-dyn
33
The categories of this paper are: plasm-ph
25
The categories of this paper are: patt-sol
31
The categories of this paper are: ao-sci
0
The categories of this paper are: hep-ph
18
The categories of this paper are: econ
24
The categories of this paper are: dg-ga
20
The categories of this paper are: acc-phys
5
The categories of this paper are: gr-qc
7
The categories of this paper are: hep-th
27
The categories of this paper are: comp-gas
15
The categories of this paper are: nucl-ex
10
The categories of this paper are: q-bio
34
The categories of this paper are: chem-ph
28
The categories of this paper are: alg-geom
13
The categories of this paper are: quant-ph