Thermal expansion of organic superconductor κ-(D₄-BEDT-TTF)₂Cu{N(CN)₂}Br. Isotopic effect
Linear thermal expansion coefficient (LTEC) of single crystal κ-(D₄-BEDT-TTF)₂Cu[N(CN)₂]Br was studied across the crystal layers in the temperature range 2–290 K using the method of precise capacitive dilatometry. Below Tc = 11.6 K the LTEC of the sample had a small negative value, which is apparent...
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irk-123456789-1753572021-02-02T01:26:25Z Thermal expansion of organic superconductor κ-(D₄-BEDT-TTF)₂Cu{N(CN)₂}Br. Isotopic effect Dolbin, A.V. Khlistuck, M.V. Eselson, V.B. Gavrilko, V.G. Vinnikov, N.A. Basnukaeva, R.M. Konstantinov, V.A. Nakazawa, Y. Низкоразмерные и неупорядоченные системы Linear thermal expansion coefficient (LTEC) of single crystal κ-(D₄-BEDT-TTF)₂Cu[N(CN)₂]Br was studied across the crystal layers in the temperature range 2–290 K using the method of precise capacitive dilatometry. Below Tc = 11.6 K the LTEC of the sample had a small negative value, which is apparently due to the transition from the paramagnetic metal in the superconducting state. There was a bend of temperature dependence of the LTEC, which shows broad peak around 40 K and can be attributed to the elastic lattice anomaly around the end-point of Mott boundary. A sharp jump in the LTEC values and hysteresis was observed in the area of Tg ∼ 75–77 K, what is likely explained by the transition in a glass-like state. The isotope effect in the thermal expansion is discusses, which manifested itself in a shift of the phase transitions in comparison with fully deuterated BEDT-TTF sample. 2017 Article Thermal expansion of organic superconductor κ-(D₄-BEDT-TTF)₂Cu{N(CN)₂}Br. Isotopic effect / A.V. Dolbin, M.V. Khlistuck, V.B. Eselson, V.G. Gavrilko, N.A. Vinnikov, R.M. Basnukaeva, V.A. Konstantinov, Y. Nakazawa // Физика низких температур. — 2017. — Т. 43, № 12. — С. 1740-1744. — Бібліогр.: 16 назв. — англ. 0132-6414 PACS: 65.40.De, 63.22.Np http://dspace.nbuv.gov.ua/handle/123456789/175357 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Низкоразмерные и неупорядоченные системы Низкоразмерные и неупорядоченные системы |
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Низкоразмерные и неупорядоченные системы Низкоразмерные и неупорядоченные системы Dolbin, A.V. Khlistuck, M.V. Eselson, V.B. Gavrilko, V.G. Vinnikov, N.A. Basnukaeva, R.M. Konstantinov, V.A. Nakazawa, Y. Thermal expansion of organic superconductor κ-(D₄-BEDT-TTF)₂Cu{N(CN)₂}Br. Isotopic effect Физика низких температур |
| description |
Linear thermal expansion coefficient (LTEC) of single crystal κ-(D₄-BEDT-TTF)₂Cu[N(CN)₂]Br was studied across the crystal layers in the temperature range 2–290 K using the method of precise capacitive dilatometry. Below Tc = 11.6 K the LTEC of the sample had a small negative value, which is apparently due to the transition from the paramagnetic metal in the superconducting state. There was a bend of temperature dependence of the LTEC, which shows broad peak around 40 K and can be attributed to the elastic lattice anomaly around the end-point of Mott boundary. A sharp jump in the LTEC values and hysteresis was observed in the area of Tg ∼ 75–77 K, what is likely explained by the transition in a glass-like state. The isotope effect in the thermal expansion is discusses, which manifested itself in a shift of the phase transitions in comparison with fully deuterated BEDT-TTF sample. |
| format |
Article |
| author |
Dolbin, A.V. Khlistuck, M.V. Eselson, V.B. Gavrilko, V.G. Vinnikov, N.A. Basnukaeva, R.M. Konstantinov, V.A. Nakazawa, Y. |
| author_facet |
Dolbin, A.V. Khlistuck, M.V. Eselson, V.B. Gavrilko, V.G. Vinnikov, N.A. Basnukaeva, R.M. Konstantinov, V.A. Nakazawa, Y. |
| author_sort |
Dolbin, A.V. |
| title |
Thermal expansion of organic superconductor κ-(D₄-BEDT-TTF)₂Cu{N(CN)₂}Br. Isotopic effect |
| title_short |
Thermal expansion of organic superconductor κ-(D₄-BEDT-TTF)₂Cu{N(CN)₂}Br. Isotopic effect |
| title_full |
Thermal expansion of organic superconductor κ-(D₄-BEDT-TTF)₂Cu{N(CN)₂}Br. Isotopic effect |
| title_fullStr |
Thermal expansion of organic superconductor κ-(D₄-BEDT-TTF)₂Cu{N(CN)₂}Br. Isotopic effect |
| title_full_unstemmed |
Thermal expansion of organic superconductor κ-(D₄-BEDT-TTF)₂Cu{N(CN)₂}Br. Isotopic effect |
| title_sort |
thermal expansion of organic superconductor κ-(d₄-bedt-ttf)₂cu{n(cn)₂}br. isotopic effect |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2017 |
| topic_facet |
Низкоразмерные и неупорядоченные системы |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/175357 |
| citation_txt |
Thermal expansion of organic superconductor κ-(D₄-BEDT-TTF)₂Cu{N(CN)₂}Br. Isotopic effect / A.V. Dolbin, M.V. Khlistuck, V.B. Eselson, V.G. Gavrilko, N.A. Vinnikov, R.M. Basnukaeva, V.A. Konstantinov, Y. Nakazawa // Физика низких температур. — 2017. — Т. 43, № 12. — С. 1740-1744. — Бібліогр.: 16 назв. — англ. |
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Физика низких температур |
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Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 12, pp. 1740–1744
Thermal expansion of organic superconductor
κ-(D4-BEDT-TTF)2Cu{N(CN)2}Br. Isotopic effect
A.V. Dolbin, M.V. Khlistuck, V.B. Eselson, V.G. Gavrilko, N.A. Vinnikov,
R.M. Basnukaeva, and V.A. Konstantinov
B. Verkin Institute for Low Temperature Physics and Engeneering of the National Academy of Sciences of Ukraine
47 Nauky Ave., Kharkiv 61103, Ukraine
E-mail: dolbin@ilt.kharkov.ua
Y. Nakazawa
Department of Chemistry, Graduate School of Science, Osaka University
1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan
Received June 23, 2017, published online October 25, 2017
Linear thermal expansion coefficient (LTEC) of single crystal κ-(D4-BEDT-TTF)2Cu[N(CN)2]Br was
studied across the crystal layers in the temperature range 2–290 K using the method of precise capacitive
dilatometry. Below Tc = 11.6 K the LTEC of the sample had a small negative value, which is apparently due
to the transition from the paramagnetic metal in the superconducting state. There was a bend of temperature
dependence of the LTEC, which shows broad peak around 40 K and can be attributed to the elastic lattice
anomaly around the end-point of Mott boundary. A sharp jump in the LTEC values and hysteresis was ob-
served in the area of Tg ~ 75–77 K, what is likely explained by the transition in a glass-like state. The isotope
effect in the thermal expansion is discusses, which manifested itself in a shift of the phase transitions in compari-
son with fully deuterated BEDT-TTF sample.
PACS: 65.40.De Thermal expansion; thermomechanical effects;
63.22.Np Layered systems.
Keywords: organic molecular conductors, κ-(D4-BEDT-TTF)2Cu[N(CN)2]Br, thermal expansion, isotopic effect.
1. Introduction
Low-dimensional organic molecular conductors and mag-
netic materials are a new type of conductive compounds,
which are characterized by a delicate balance between the
various types of electronic instabilities [1,2]. Their study
yielded some interesting results in various areas of solid state
physics (metal-insulator and metal-superconductor transitions
due to electron correlations, nesting and reconstruction of
Fermi surface with low-dimensional characters, drastic
changes in transport properties induced by a magnetic field,
the angular magnetoresistance oscillations through interlayer
coupling, etc.) There are currently over 100 low-dimensional
synthesized organic superconductors, which are inherently
radical cation salts. The maximum temperature of the super-
conducting transition in them reaches 13 K.
Although the search for new low-dimensional molecular
superconductors in class of ion-radical salts is continued, the
interest in this area in recent years largely shifted to the crea-
tion of multi-functional materials combining the (over) con-
ductivity with other physical properties. Design and synthesis
of hybrid molecular systems, that combine two or more phys-
ical properties, such as (super) conductivity, magnetism, pho-
tochromism, ionic conductivity, nonlinear optical properties,
etc., is currently among the most rapidly developing areas in
the chemistry and physics of new materials. The combination
of these properties in a crystal lattice and their synergism can
lead to new physical phenomena and applications in molecu-
lar electronics. The unusual properties of molecular conduc-
tors and magnetic materials are due to the peculiarities of the
crystal structure of these substances. Organic (super) con-
ductors and magnets on the basis of the cation-radical salts
are quasi one-dimensional or quasi-two-dimensional sys-
tems, whose electronic structure is characterized by the
presence of conductive packages or layers of organic (metal
organic) π-electron donors linked by weak electrostatic in-
termolecular interactions [1,2].
© A.V. Dolbin, M.V. Khlistuck, V.B. Eselson, V.G. Gavrilko, N.A. Vinnikov, R.M. Basnukaeva, V.A. Konstantinov, and Y. Nakazawa, 2017
http://scitation.aip.org/search?option1=pub_affiliation&value1=B.I.%20Verkin%20Institute%20for%20Low%20Temperature%20Physics%20and%20Engeneering,%20NAS%20of%20Ukraine,%2061103%20Kharkov,%20Ukraine&option912=resultCategory&value912=ResearchPublicationContent
http://scitation.aip.org/search?option1=pub_affiliation&value1=B.I.%20Verkin%20Institute%20for%20Low%20Temperature%20Physics%20and%20Engeneering,%20NAS%20of%20Ukraine,%2061103%20Kharkov,%20Ukraine&option912=resultCategory&value912=ResearchPublicationContent
mailto:dolbin@ilt.kharkov.ua
Thermal expansion of organic superconductor κ-(D4-BEDT-TTF)2Cu{N(CN)2}Br. Isotopic effect
The basis of one of the largest groups of organic conduc-
tors is bis (ethylenedithio) tetrathiafulvalene (BEDT-TTF),
which was obtained by modifying a TTF in 1978 [3], and is a
good electron donor. BEDT-TTF molecule consists of two
five-membered and two six-membered rings, each of which
contains two sulfur atoms (see Fig. 1(a)). The same figure also
shows the crystal structure of the compound.
Unpaired π-electrons of carbon and sulfur atoms of
BEDT-TTF molecules form a set of π-orbitals, perpendicular
to the plane of the molecule [1–2]. Due to such configuration
BEDT-TTF molecules can form dense dimers, stacks and
other aggregates in crystal in which overlapping π-orbitals
provides stability of such formations. In the present study we
investigated partially deuterated single crystal κ-(D4-BEDT-
TTF)2Cu[N(CN)2]Br, where the molecules forming dimers
deployed with respect to one another at an angle of approxi-
mately 80°. In turn, dimers in the crystal form a layer in which
they are packed in a triangular anisotropic lattice.
Organic salt κ-(BEDT-TTF)2Cu[N(CN)2]Br has an bi-
layer orthorhombic lattice (space group Pnma), wherein ani-
ons layers which are located in the ac plane are perpendicular
to the b axis (see Fig. 1(c)), along which there are flat BEDT-
TTF dimers (four per unit cell of the crystal lattice). The di-
mer planes are tilted along a axis in opposite directions for
adjacent layers. BEDT-TTF molecules have two stable
conformers differing in orientation direction of the end of
ethylene groups (eclipsed configuration if hydrogen atoms
at the vertices of the hexagon are in the same directions,
and staggered configuration in opposite case). At room
temperature, these two conformations are experiencing
continuous mutual transformation due to thermal fluctua-
tions. By lowering the temperature the thermal motion of
ethylene groups freezes, and BEDT-TTF molecules form a
glassy phase. The speed of the cooling in the region of the
glass transition (Tg ≈ 65–85 K) plays a decisive role in the
residual disordering of molecules BEDT-TTF, which is
stored then in the crystal to very low temperatures [5,6].
Unpaired π-electrons of skeleton of BEDT-TTF molecules
provide the ability to charge transfer both within the dimer
and between dimmers. A prerequisite for the charge
transport in the crystal is the presence of anionic
(Cu[N(CN)2]Br) layers. Anion layer is composed of poly-
mer zigzag chains, elongated in the direction of a, which
consist of copper ions Cu+, linked by two dicyanamide
bridges [(NC)N(CN)]–(DCA) and a halogen atom (Br) at
the end (see Fig. 1(b)). Anions, performing the function of
Fig. 1. (Color online) BEDT-TTF molecule (a); polymer anions Cu[N(CN)2]Br, arrows indicate the direction in the crystal lattice of
κ-(ET)2Cu[N(CN)2]Br (b); crystal structure of κ-(ET)2Cu[N(CN)2]Br, side view showing the sequence of conducting ET layers and insu-
lating anion sheets (c); schematic view of the relative orientations of the ET molecules ethylene end groups (EEGs) in the staggered (d)
and eclipsed (e) conformation, respectively [4].
Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 12 1741
A.V. Dolbin, et al.
an electron acceptor, can affect the packing of the donor
molecule BEDT-TTF, which determines the nature of the
transport properties, but do not directly participate in the
conduction process. The electronic state of cation–anion
system is interpreted as a simple effectively half-filled Mott–
Hubbard state, and is determined by a subtle balance of band
energy W, indoor/outdoor Coulomb energy of dimer (U and
V, respectively), and the electron–phonon interaction be-
tween π-electrons existing in the low-dimensional lattice. If
U is greater than W, the system goes into Mott’s insulating
state (AFM ground state). On the other hand, in the region
where W exceeds U, the ground state is strongly correlated
metallic state. A relatively high temperature superconductiv-
ity (Тс ≈ 10 K) is observed in this area [7–9]. A large impact
on this ratio can have pressure applied to the crystal, which
helps to reduce the ratio of U/W, and the type of halogen in
the anion molecule, which increasing in mass has an effect
comparable to the application of external pressure [10].
Small changes in pressure, temperature, chemical composi-
tion, have a decisive effect on the properties of such sys-
tems. For such objects with many degrees of freedom, the
proper interpretation of the experimental results is often
complex and ambiguous. The combination of structural and
thermal methods of investigation, in particular, studies of
thermal expansion substantially able to facilitate the inter-
pretation of results, since various processes of magnetic and
structural ordering appear differently in the physical proper-
ties of these substances. Precision measurement of thermal
expansion and the specific heat at low temperatures can also
provide information about the specific phase transitions in-
duced by quantum phenomena at low energies. The aim of
this work was to study the thermal expansion features of the
single crystal κ-(D4-BEDT-TTF)2Cu[N(CN)2]Br in a wide
temperature range.
2. The thermal expansion of the sample
κ-(D4-BEDT-TTF)2Cu[N(CN)2]Br
We curried out the thermal expansion measurement of
monocrystalline sample κ-(D4-BEDT-TTF)2Cu[N(CN)2]Br
in the temperature range 2–290 K using a low-temperature
high-sensitivity capacitive dilatometer [11]. The rate of
sample cooling from room temperature to 2 K amounted to
an average 0.74 K⋅min–1. Measurements of thermal expan-
sion were carried out along the crystallographic direction b
(Fig. 2), perpendicular to the crystal plane.
The temperature dependence of the linear thermal ex-
pansion coefficient (LTEC) of the investigated sample is
shown in the Fig. 3. It has a number of features. Below
Tc = 11,6 K [6] the LTEC of the sample has a small nega-
tive values (~ 10–7⋅K–1, Fig. 3(c)), which is apparently due
to the transition from the paramagnetic metal in the super-
conducting state (see Fig. 4). Such behavior of LTEC when
passing through Tc had been observed previously in
cuprates [13].
Fig. 2. Photopicture of the monocrystal used for this work.
Lines show the crystallographic axes direction, obtained from
the x-ray study.
Fig. 3. The dependence of LTEC on temperature 2–290 K (a);
65–95 K (b); 2–30 K (c).
1742 Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 12
Thermal expansion of organic superconductor κ-(D4-BEDT-TTF)2Cu{N(CN)2}Br. Isotopic effect
In the temperature range of T*~30–45 K (Fig. 3(a)) there
is a bend of the temperature dependence of the LTEC with a
peak of 35–40 K. This is not considered as a phase transition
but reflecting a gradual change in the lattice. In the vicinity of
the temperature Tg ~ 75–77 K there is a thermal expansion
anomaly (Fig. 3(b)), typical for the phase transition, a sharp
jump in the LTEC values and hysteresis: the values obtained
during the heating and cooling of the sample differed signifi-
cantly. This anomaly is likely explained by the transition
upon cooling of the sample in a glass-like state. A single
point of view on the nature of this state is absent. Number
of authors attributed this anomaly to the conformational
freezing of the terminal ethylene moiety in BEDT-TTF
molecules (see Fig. 1(d), (f)) [4–6]. An alternative view is
that structural transformations of the polymeric chain will
translate into changes of the orbital overlap determining the
electronic properties within the BEDT-TTF layers [14].
At higher temperatures in the range 160–190 K, there is a
maximum of LTEC (Fig. 3(a)), likely due to the influence of
several competing mechanisms. Positive contribution to the
thermal expansion makes the anharmonicity of the vibrations
of BEDT-TTF dimers. The negative contribution may be re-
lated to the excitation of transverse optical vibrations of Cu-
dicyanamide-Cu anionic polymer molecules chains. Reduc-
tion of the LTEC values with increasing temperature above
190 K, is apparently due to the increase of this contribution.
3. Influence of deuteration on the thermal expansion
Investigated κ-(D4-BEDT-TTF)2Cu[N(CN)2]Br sample
was partially deuterated - four of the eight hydrogen atoms in
ethylene BEDT-TTF groups have been replaced with deuteri-
um (Fig. 5). It is known that deuteration has a strong effect on
the electronic state. κ-(D4-BEDT-TTF)2Cu[N(CN)2]Br is
located in the border region of the superconducting and anti-
ferromagnetic phase, while D8-compound and H8-compound
is in the antiferromagnetic and superconductiing phase,
respectively [12].
Figure 6 indicates that the deuteration degree of the sample
(d [2;2] blank circle — this sample, d [4;4] triangles – data
[6]) strongly affects the glass transition temperature Tg. For
our sample d [2; 2] region of the glass phase transition was
shifted toward lower temperatures (Fig. 6(b)), which is likely
due to the smaller total angular momentum of the ethylene
groups of molecules BEDT-TTF in the case of the sample
d [2;2] compared with fully deuterated sample d [4;4], result-
ing in disinhibition of the synchronized motion of ethylene
groups in both side of BEDT-TTF molecules. At the same
time, changes in the deuteration degree weakly affected the
transition temperature to the superconducting state Tc. It is
further interesting that in the sample d [2;2] the anomaly at
T* is much more pronounced and becomes sharper than for
the sample d [4;4] (Fig. 6). This is considered as a change
Fig. 4. Phase diagram of the dimer-Mott system based on κ-type
salts consisting of BEDT-TTF molecules and effect of the
deuteration degree (based on the data [12]).
Fig. 5. Schematic image of the molecule d [2,2] BEDT-TTF [12].
Fig. 6. Effect of the deuteration degree on thermal expansion of
κ-(BEDT-TTF)2Cu[N(CN)2]Br.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 12 1743
A.V. Dolbin, et al.
of elastic feature occurs in the Mott–Hubbard physics in
organic systems. Kagawa et al observed by the transport
measurements under precisely controlled helium gas pres-
sures that the Mott boundary in the dimer-Mott system
gives unusual criticality [15]. They further indicated that
the end-point of the Mott boundary exits at about 33–38 K
in the case of κ-(BEDT-TTF)2Cu[N(CN)2]Cl by the gas
pressure controlled experiments to adjust U/W ratio at the
boundary. The higher temperature region than this critical
end point temperature, a supercritical region like liquid-gas
system under high pressure can exist. Since the d [2,2]
sample exists just close to the boundary at ambient pres-
sure, the observed pronounced anomaly may be related to
the criticality around this end point. The elastic change oc-
curs around the boundary region for κ-(BEDT-
TTF)2Cu[N(CN)2]Cl compound under gas pressure is re-
ported in [16]. More detail experiments around this region
for d [2,2] sample and other deutrated samples with different
ratio are necessary to investigate this point more clearly. The
formation of SDW like anomaly previously reported in this
temperature region in κ-(BEDT-TTF)2Cu[N(CN)2]Cl may
be related to this criticality and elastic feature. The observed
hump like structure of LTEC and the difference of d [2,2]
and d [4,4] are consistent with picture of the two-di-
mensional Mott boundary (Fig.4).
In the temperature range 11–66 K the LTEC values for
partially deuterated sample were systematically lower than
for fully deuterated sample. In the case of deuterated sam-
ple, hydrogen bonding between donors and anions may be
suppressed by the deuteration and the thermal expansion
may become easier.
4. Conclusion
Thermal expansion of monocrystalline sample κ-(D4-
BEDT-TTF)2Cu[N(CN)2] Br was studied along the crys-
tallographic direction b (perpendicular to the plane of the
crystal) in the temperature range 2–290 K. The temperature
dependence of the linear thermal expansion coefficient
(LTEC) of the investigated sample had a number of fea-
tures. Below Tc = 11.6 K the LTEC of the sample had a
small negative values which is apparently due to the transi-
tion from the paramagnetic metal in the superconducting
state. In the temperature range T* ~ 35–40 K there was the
bend of the temperature dependence of the LTEC, which
can be explained by the criticality of the Mott–Hubbard
physics in 2D dimer-Mott state. A sharp jump in the
LTEC values and hysteresis was observed in the vicinity
of the temperature Tg ~ 75–77 K, what is likely explained
by the transition upon cooling of the sample in a glass-
like state. A broad maximum of LTEC took a place in the
range 160–190 K. Presumably, this feature is associated
with the competing influence of the positive contribution
to the thermal expansion caused by anharmonicity of vi-
brations of the BEDT-TTF dimers, and the negative con-
tribution associated with the excitation of transverse opti-
cal vibrations of molecules of anionic polymer chains Cu-
dicyanamide-Cu. Lowering the LTEC values with increa-
sing temperature above 190 K is most likely due to in-
creased role of the last contribution.
The thermal expansion of the sample κ-(D4-BEDT-
TTF)2Cu[N(CN)2]Br clearly manifested isotope effect in
comparison with fully deuterated sample. In κ-(D4-BEDT-
TTF)2Cu [N(CN)2]Br the area of the glass phase transition
was shifted toward lower temperatures. This can be due to
smaller total rotational momentum of BEDT-TTF ethylene
groups in the case of partially deuterated sample. In the tem-
perature range 11–66 K values of LTEC for partially
deuterated sample were systematically smaller than LTEC of
fully deuterated sample. This can be attributed to the pres-
ence of a negative contribution due to quantum effects avail-
able in the energy spectrum of the partially deuterated mole-
cule BEDT-TTF. For heavier fully deuterated BEDT-TTF,
this effect is less pronounced.
Acknowledgement
The authors thank Prof. A. Kawamoto at Hokkaido
University and Prof. H. Taniguchi at Saitama University
for their synthesis of partially deuterated molecules. The
authors are grateful Prof. A. Prokhvatilov for fruitful dis-
cussion of result.
1. M. Sorai, Y. Nakazawa, M. Nakano, and Y. Miyazaki,
Chem. Rev. 113, PR41 (2013).
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1. Introduction
2. The thermal expansion of the sample κ-(D4-BEDT-TTF)2Cu[N(CN)2]Br
3. Influence of deuteration on the thermal expansion
4. Conclusion
Acknowledgement
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