Charge ordering

Charge ordering (CO) is a (first- or second-order) phase transition occurring mostly in strongly correlated materials such as transition metal oxides or organic conductors. Due to the strong interaction, the charge is localized on different sites leading to a disproportionation and an ordered superlattice. It appears in different patterns ranging from vertical to horizontal stripes to a checkerboard–like pattern, and it is not limited to the two-dimensional case. The charge order transition is accompanied by symmetry breaking and ferroelectricity. It is often brought in context with superconductivity and colossal magnetoresistance.

Charge order patterns

This long range order phenomena was first discovered in the magnetite Fe₃O₄ by Verwey in 1939.^[1][2] He observed an increase of the electrical resistivity by two orders of magnitude at T_CO=120K, suggesting a phase transition which is now well-known as the Verwey transition. He was the first bringing up the idea of an ordering process in this context.

Theoretical Description

The extended one-dimensional Hubbard model delivers a good description of the charge order transition with the on-site and nearest neighbor Coulomb repulsion U and V. It emerged that V is a crucial parameter and important for developing the charge order state. Further model calculations try to take the temperature and an interchain interaction into account.^[3] The extended Hubbard model for a single chain including inter-site and on-site interaction V and U as well as the parameter δ_d for a small dimerization which can be typically found in the (TMTTF)₂X compounds is presented as follows:

$H = -t \sum_{i} \sum_{\sigma} \left ( \left [ 1+ \left(-1 \right)^i \delta_d \right ]c^{\dagger}_{i,\sigma}c_{i+1,\sigma}+ h.c \right)+ U \sum_i n_{i,\uparrow}n_{i,\downarrow} + V \sum_{i}n_i, n_{i+1}$

where t describes the transfer integral or the kinetic energy of the electron and $c^{\dagger}_{i,\sigma}$ and c_{i + 1,σ} are the creation respectively the annihilation operator for an electron with the spin $\sigma = \uparrow , \downarrow$ at the ith or i + 1th site. $n_{i,\downarrow, \uparrow}$ denotes the density operator. For non-dimerized systems, δ_d can be set to zero Normally, the on-site Coulomb repulsion U stays unchanged only t and V can vary with pressure.

Examples of Charge Ordering

Organic conductors

Organic conductors consist of donor and acceptor molecules building separated planar sheets or columns. The energy difference in the ionization energy acceptor and the electron affinity of the donor leads to a charge transfer and consequently to free carriers whose number is normally fixed. The carriers are delocalized throughout the crystal due to the overlap of the molecular orbitals being also reasonable for the high anisotropic conductivity. That is why it will be distinct between different dimensional organic conductors. They possess a huge variety of ground states, for instance, charge ordering, spin-Peierls, spin-density wave, antiferromagnetic state, superconductivity, charge-density wave to name only some of them.^[4][5]

Quasi One-dimensional Organic Conductors

The model system of one-dimensional conductors is the Bechgaard-Fabre salts family, (TMTTF)₂X and (TMTSF)₂X, where in the latter one sulfur is substituted by selenium leading to a more metallic behavior over a wide temperature range and exhibiting no charge order. While the TMTTF compounds depending on the counterions X show the conductivity of a semiconductor at room temperature and are expected to be more one-dimensional than (TMTSF)₂X.^[6] The transition temperature T_CO for the TMTTF subfamily was registered over two order of magnitudes for the centrosymmetric anions X = Br, PF₆, AsF₆, SbF₆ and the non-centrosymmetric anions X= BF₄ and ReO₄.^[7] In the middle of the eighties, a new "structureless transition" was discovered by Coulon et al.^[8] conducting transport and thermopower measurements. They observed a suddenly rise of the resistivity and the thermopower at T_CO while x-ray measurements showed no evidence for a change in the crystal symmetry or a formation of a superstructure. The transition was later confirmed by ¹³C-NMR^[9] and dielectric measurements.
Different measurements under pressure reveal a decrease of the transition temperature T_CO by increasing the pressure. According to the phase diagram of that family, an increasing pressure applied to the TMTTF compounds can be understood as a shift from the semiconducting state (at room temperature) to a higher dimensional and metallic state as you can find for TMTSF compounds without a charge order state.

Anion X	TCO (K)
(TMTTF)2Br	28
(TMTTF)2PF6	70
(TMTTF)2AsF6	100.6
(TMTTF)2SbF6	154
(TMTTF)2BF4	83
(TMTTF)2ReO4	227.5
(DI-DCNQI)2Ag	220
TTM-TTPI3	120

Quasi Two-dimensional Organic Conductors

A dimensional crossover can be induced not only by applying pressure, but also be substituting the donor molecules by other ones. From a historical point of view, the main aim was to synthesize a organic superconductor with a high T_C. The key to reach that aim was to increase the orbital overlap in two dimension. With the BEDT-TTF and its huge π-electron system, a new family of quasi two-dimensional organic conductors were created exhibiting also a great variety of the phase diagram and crystal structure arrangements.
At the turn of the 20th century, first NMR measurements on the θ-(BEDT-TTF)₂RbZn(SCN)₄ compound uncovered the known metal to insulator transition at T_CO= 195 K as an charge order transition.^[10]

Compound	TCO (K)
α-(BEDT-TTF)2I3	135
θ-(BEDT-TTF)2TlCo(SCN)4	240
θ-(BEDT-TTF)2TlZn(SCN)4	165
θ-(BEDT-TTF)2RbZn(SCN)4	195
θ-(BEDT-TTF)2RbCo(SCN)4	190

Transition metal oxides

The most prominent transition metal oxide revealing a CO transition is the magnetite Fe₃O₄ being a mixed-valence oxide where the iron atoms have a statistical distribution of Fe³⁺ and Fe²⁺ above the transition temperature. Below 122 K, the combination of 2+ and 3+ species arrange themselves in a regular pattern, whereas above that transition temperature (also referred to as the Verwey temperature in this case) the thermal energy is large enough to destroy the order.^[11]

Compound[12]	TCO (K)
Y0.5NiO3	582
YBaCo2O5	220
CaFeO3	290
TbBaFe2O5	282
Fe3O4	122
Li0.5MnO2	290
LaSrMn3O7	210
Na0.25Mn3O6	176
YBaMn2O6	498
TbBaMn2O6	473
PrCaMn2O6	230
α'-NaV2O5	34

Detection of charge order

• NMR spectroscopy is a powerful tool to measure the charge disproportionation. To apply this method to a certain system, it has to be doped with a nuclei, for instance ¹³C as it is the case for TMTTF compounds, being active for NMR. The local probe nuclei is very sensitive to the charge on the molecule observable in the Knight shift K and the chemical shift D. The Knight shift K is proportional to the spin susceptibility χ_Sp on the molecule. The charge order or charge disproportionation appear as a splitting or broadening of the certain feature in the spectrum.
• The X-ray diffraction technique allows to determine the atomic position, but the extinction effect hinders to receive a high resolution spectrum. In the case of the organic conductors, the charge per molecule is measured by the change of the bond length of the C=C double bonds in the TTF molecule. A further problem arising by irradiating the organic conductors with x-rays is the destruction of the CO state.^[13]
• In the organic molecules like TMTTF, TMTSF or BEDT-TFF, there are charge-sensitive modes changing their frequency depending on the local charge. Especially the C=C double bonds are quite sensitive to the charge. If a vibrational mode is infrared active or only visible in the Raman spectrum depends on its symmetry. In the case of BEDT-TTF, the most sensitive ones are the Raman active ν₃ , ν₂ and the infrared out of phase mode ν₂₇.^[14] Their frequency is linearly associated to the charge per molecule giving the opportunity to determine the degree of disproportionation.
• The charge order transition is also a metal to insulator transition being observable in transport measurements as a sharp rise in the resistivity. Transport measurements are therefore a good tool to get first evidences of a possible charge order transition.

References

1. ^ Verwey, E.J.W. (1939). "Electronic conduction of magnetite (Fe₃O₄) and its transition point at low temperatures". Nature 144 (3642): 327–328. Bibcode 1939Natur.144..327V. doi:10.1038/144327b0. 
2. ^ Verwey, E.J.W.; Haayman, P.W. (1941). "Electronic conductivity and transition point of magnetite (Fe₃O₄)". Physica 8 (9): 979–987. doi:10.1016/S0031-8914(41)80005-6. 
3. ^ Yoshioka, H.; Tsuchuuzu, M; Seo, H. (2007). "Charge-Ordered State versus Dimer-Mott Insulator at Finite Temperatures". Journal of the Physical Society of Japan 76 (10): 103701. doi:10.1143/JPSJ.76.103701. 
4. ^ Ishiguro, T. (1998). Organic Superconductors. Berlin Heidelberg New York: Springer-Verlag. ISBN 3-540-63025-2. 
5. ^ Toyota, N. (2007). Low-dimensional Molecular Metals. Berlin Heidelberg: Springer-Verlag. ISBN 978-3-540-49574-1. 
6. ^ Jeromé, D. (1991). "THE PHYSICS OF ORGANIC SUPERCONDUCTORS". Science 252 (5012): 1509–1514. doi:10.1126/science.252.5012.1509. PMID 17834876. 
7. ^ Nad, F.; Monceau, P. (2006). "Dielectric response of the charge ordered state in quasi-one-dimensional organic conductors". Journal of Physical society of Japan 75 (5): 051005. doi:10.1143/JPSJ.75.051005. 
8. ^ Coulon, C.; Parkin, S.S.P.; Laversanne, R. (1985). "STRUCTURELESS TRANSITION AND STRONG LOCALIZATION EFFECTS IN BIS-TETRAMETHYLTETRAHTHIAFULVALENIUM SALTS [(TMTTF)₂X]". Physical Review B 31 (6): 3583–3587. doi:10.1103/PhysRevB.31.3583. 
9. ^ Chow, D.S.; et al. (2000). "Charge Ordering in the TMTTF Family of Molecular Conductors". Physical Review Letters 85 (8): 1698–1701. doi:10.1103/PhysRevLett.85.1698. PMID 10970592. 
10. ^ Miyagawa, K.; Kawamoto, A.; Kanoda, K. (2000). "Charge ordering in a quasi-two-dimensional organic conductor". Physical Review B 62 (12): R7679. doi:10.1103/PhysRevB.62.R7679. 
11. ^ Rao, C. N. R. (1997). "Materials Science: Charge Ordering in Manganates". Science 276 (5314): 911–912. doi:10.1126/science.276.5314.911. 
12. ^ Attfield, J.P. (2006). "Charge ordering in transition metal oxides". Solid State Sciences 8 (8): 861–867. doi:10.1016/j.solidstatesciences.2005.02.011. 
13. ^ Coulon, C.; Lalet, G.; Pouget, J.-P.; Foury-Leylekian, P.;Moradpour, A. (2007). "Anisotropic conductivity and charge ordering in (TMTTF)(₂)X salts probed by ESR". Physical Review B 76 (8): 085126. doi:10.1103/PhysRevB.76.085126. 
14. ^ Dressel, M.; Drichko, N. (2004). "Optical Properties of Two-Dimensional Organic Conductors: Signatures of Charge Ordering and Correlation Effects". Chemical Review 104 (11): 5689–5715. doi:10.1021/cr030642f.