Relaxation dynamics in the one-dimensional organic charge-transfer salt $\ensuremath{\delta}\ensuremath{-}{(\mathrm{EDT}\ensuremath{-}\mathrm{TTF}\ensuremath{-}{\mathrm{CONMe}}_{2})}_{2}\mathrm{Br}$

Relaxation dynamics in the one-dimensional organic charge-transfer salt $δ - {(EDT - TTF - {CONMe}_{2})}_{2} Br$

J. K. H. Fischer, P. Lunkenheimer, C. Leva, S. M. Winter, M. Lang, C. Mézière, P. Batail, A. Loidl, and R. S. Manna

Phys. Rev. B 97, 235156 – Published 29 June 2018

Abstract

A detailed investigation of the charge-ordered charge-transfer salt $δ - {(EDT - TTF - {CONMe}_{2})}_{2} Br$ by thermal-expansion measurements and dielectric spectroscopy reveals three dynamic processes of relaxational character. The slowest one exhibits the characteristics of glassy freezing and is ascribed to the conformational dynamics of terminal ethylene groups of the organic molecules. Such a process was previously found for related charge-transfer salts where, however, the anions form polymerlike chains, in contrast to the spherical anions of the present material. Dielectric spectroscopy reveals two additional relaxational processes. The characteristics of the faster one are consistent with excitations of a one-dimensional Wigner lattice as recently observed in this material by infrared spectroscopy, which are also accompanied by conformational changes of the molecules. However, at low temperature the ethylene-group relaxation exhibits the cooperativity-induced dramatic slowing down that is typical for glassy freezing, while the defect-related Wigner-lattice excitation follows thermally activated behavior as expected for single-dipole relaxations.

Received 11 April 2018
Revised 25 May 2018

DOI:https://doi.org/10.1103/PhysRevB.97.235156

Physics Subject Headings (PhySH)

Charge order Dielectric properties Glass transition Thermal expansion

Charge-transfer insulators Wigner crystal

Dielectric spectroscopy Thermal techniques

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

J. K. H. Fischer^1,*, P. Lunkenheimer¹, C. Leva², S. M. Winter^2,3, M. Lang², C. Mézière⁴, P. Batail⁴, A. Loidl¹, and R. S. Manna^2,5

¹Experimental Physics V, Center for Electronic Correlations and Magnetism, University of Augsburg, 86159 Augsburg, Germany
²Institute of Physics, Goethe University Frankfurt (M), Max-von-Laue-Strasse 1, 60438 Frankfurt (M), Germany
³Institute for Theoretical Physics, Goethe University Frankfurt (M), Max-von-Laue-Strasse 1, 60438 Frankfurt (M), Germany
⁴MOLTECH-Anjou, UMR 6200, CNRS-Université d’Angers, Bâtiment K, Angers F-49045, France
⁵Department of Physics, IIT Tirupati, Tirupati 517506, India

^*Corresponding author: Jonas.Fischer@Physik.Uni-Augsburg.de

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Vol. 97, Iss. 23 — 15 June 2018

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Figure 1
(a) EDT-TTF- $CONM e_{2}$ molecular unit. (b) Crystal structure of $δ - {(EDT - TTF - {CONMe}_{2})}_{2} Br$ at 295 K: parallel stacks along the $a$ direction. (c) Transverse interstacks pattern along the $b$ axis (two successive layers are shown) [26]. The largest spheres denote the ${Br}^{-}$ ions.
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Figure 2
Temperature dependence of the dielectric constant $ɛ^{'} (T)$ (a) and conductivity $σ^{'} (T)$ (b) of $δ$ -Br as measured for various frequencies along the $a$ axis. The two observed relaxation processes are marked by I and II; the development of the corresponding loss-peak positions is indicated by the dashed lines in (b). For clarity, $ɛ^{'} (T)$ is only presented for seven frequencies instead of the 20 shown in $σ^{'} (T)$ . The conductivity curves were cut at low and high temperatures where data scatter and systematic errors start to dominate the data. For selected frequencies, the arrows in (b) indicate the expected peak positions arising from the glasslike relaxation as calculated from the VFT curve shown in Fig. 5. The inset shows a zoomed view of $ɛ^{'} (T)$ revealing the superimposed step associated with the high-temperature relaxation $I$ . The arrows in the inset indicate the points of inflection.
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Figure 3
Temperature dependence of the uniaxial thermal expansion coefficient $α_{a}$ of $δ$ -Br measured along the $a$ axis under heating after cooling with 10 K/h. The structural and glasslike transitions are indicated by the arrows. The inset shows the data across the antiferromagnetic transition, expected at 12 K, on an expanded scale.
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Figure 4
Thermal expansion coefficient along the $a$ axis across the glasslike transition on an expanded scale, measured with a heating rate of $+ 3.0 K / h$ after cooling with three different rates. The inset shows an Arrhenius plot of the cooling rate $| q_{c} |$ vs the temperature of the glasslike anomaly determined by reading off the midpoint of the broad rise in $α_{a} (T)$ . The line is a linear fit, corresponding to thermally activated behavior.
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Figure 5
Arrhenius plot of the temperature-dependent relaxation times determined by dielectric spectroscopy (squares: process I, triangles: process II) and thermal expansion (circles: process III). For the dielectric data, the filled symbols show the results obtained from fits to the frequency-dependent data; the empty symbols were deduced from the peak positions in the temperature-dependent graph [Fig. 2]. The dashed lines indicate Arrhenius behavior with activation energies of 70 and 100 meV for the slower process I and faster process II, respectively. The solid line indicates VFT behavior with a fixed attempt frequency of $3 \times 10^{- 13} s$ . The structural and glasslike transitions are indicated by the arrows and labeled $T_{s}$ and $T_{g}$ , respectively.
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Physical Review B

covering condensed matter and materials physics

Relaxation dynamics in the one-dimensional organic charge-transfer salt $δ - {(EDT - TTF - {CONMe}_{2})}_{2} Br$

J. K. H. Fischer, P. Lunkenheimer, C. Leva, S. M. Winter, M. Lang, C. Mézière, P. Batail, A. Loidl, and R. S. Manna

Phys. Rev. B 97, 235156 – Published 29 June 2018

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Physical Review B

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Relaxation dynamics in the one-dimensional organic charge-transfer salt δ−(EDT−TTF−CONMe2)2Br

J. K. H. Fischer, P. Lunkenheimer, C. Leva, S. M. Winter, M. Lang, C. Mézière, P. Batail, A. Loidl, and R. S. Manna

Phys. Rev. B 97, 235156 – Published 29 June 2018

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Relaxation dynamics in the one-dimensional organic charge-transfer salt $δ - {(EDT - TTF - {CONMe}_{2})}_{2} Br$