Session D8: Organo-metallics: Magnetic and other Properties

Polymorphism in Low-dimensional Molecule-based Materials: Synthetic Control of Magnetic and Electronic Structure

Competition for hydrogen bonds in molecule-based materials frequently leads to the stabilization of polymorphic structures with similar energies. Herein, we report that among magnetic coordination polymers selective crystallization of the [Cu(HF$_{2})$ (pyrazine)$_{2}$]SbF$_{6 }$and [Cu$_{2}$F(HF)(HF$_{2})$ (pyrazine)$_{4}$](SbF$_{6})_{2}$ polymorphs can be achieved through control of pressure and temperature during the crystallization process. Through this manner, orbital order and magnetic dimensionality (2D vs. 1D) can be controlled in this system. Similarly, in the analogous [Co(HF$_{2})$ (pyrazine)$_{2}$]SbF$_{6}$ system, selective crystallization of the [CoF$_{2}$ (pyrazine)$_{2}$]SbF$_{6}$(H$_{2}$O)$_{3}$(H$_{3}$O) polymorph has been achieved. While these cobalt materials exhibit a 2D magnetic structure, the interlayer coupling is diminished in the hydrated material. Among cation radical salts, five polymorphic phases in the [bis(ethylenedithio)tetrathiafulvalene]$_{2}$Ag(CF$_{3})_{4}$(1,1,2-trichloroethane) system have been crystallized, four of which have superconducting ground states. Among the superconducting salts, the ones with intercalated charge ordered layers exhibit a five-fold enhancement of T$_{c}$.   

Ground-state properties of EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ by many-variable variational Monte Carlo method

The organic Mott insulator EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ is a strongly correlated electron system on a nearly-regular triangular lattice and regarded as a spin liquid material. We investigate its effective low-energy model derived from first principles calculations and band+dimensional downfolding. The {\it ab initio} effective Hamiltonian is given in the form of the two-dimensional single-band extended Hubbard model on an anisotropic triangular lattice with short-ranged Coulomb and exchange interactions. Its ground state is calculated by the many-variable variational Monte Carlo method with quantum-number projection and multi-variable optimization. We draw the ground-state phase diagram as a function of scaling parameters for the interactions and the geometrical frustration by extending the {\it ab initio} model. We discuss Mott transitions and magnetic properties.   

Reduced Influence of Electronic Correlations in a Half-Filled System: Metallic Properties and Charge Order Transition in $\kappa$-(ET)$_2$Hg(SCN) $_2$Cl

The family of $\kappa-$(ET)$_2$Hg(SCN)$_x$Cl$_{2-x}$ can be described within a model of a metal with a half-filled conductance band. Our calculations indicate that the value of the on-site Columb repulsion U is relatively small in these system. We investigate the effects of lower U on the optical properties of $\kappa$-(ET)$_2$Hg(SCN)$_2$Cl in the metallic state and below the metal-insulator transition. Optical conductivity spectra in the metallic state above 35 K show a behavior already observed for half-filled metals on the edge of the Mott insulating state: an optical transition between Hubbard bands at frequencies equal to U is present in spectra at all temperatures, being a precursor of a Mott transition, while a coherent carriers response develops below T*=150 K. We note an influence of the lower U value both on the position of the ``U-band'' and on a relatively high T*. Untypically for a half-filled system, a metal-insulator transition at 35 K is found that is of a charge-order nature. A possibility of a charge-order ground state in a half-filed system in case of low U and relatively high nearest-neighbor Columb repulsion V is discussed.   

Non-monotonic magnetic field dependence of the staggered-moment in two-dimensional frustrated antiferromagnets

We present an efficient method to identify the degree of frustration in quasi-two-dimensional (2D) antiferromagnets described by the $J_1$-$J_2$ quantum Heisenberg model. The frustration ratio $J_2/J_1$ is commonly obtained from the analysis of magnetic susceptibility, specific heat, and saturation field. We show that the field-induced suppression of quantum fluctuations causes a nonlinear and non-monotonic magnetic field dependence of the staggered moment which depends strongly on the frustration ratio. This serves as a powerful criterion to determine $J_2/J_1$ using a combination of the exact diagonalization method on finite clusters and the spin-wave analysis. We apply this method to the quasi-2D compound $\rm{Cu(pz)_2(ClO_4)_2}$ obtaining an intermediate ratio of $J_2/J_1 \simeq 0.2$ for the frustration.   

Effects of Pressure on the Magnetic Properties of Prussian Blue Analogues

Prussian blue analogues (PBAs) have been shown to exhibit interesting magnetic characteristics under various external stimuli: charge transfer induced spin transition (CTIST),\footnote{N. Shimamoto \textit{et al.} Chem. Lett. \textbf{31}, (2002) 486.} a temperature effect; persistent photo-induced magnetism (PPIM),\footnote{O. Sato \textit{et al.} Science \textbf{272} (1996), 704-705.} an optical effect; and pressure induced electron transfer (PIET).\footnote{V. Ksenofontov \textit{et al.} Phys. Rev. B \textbf{68} (2003) 024415.} Our research has established that photocontrol of the magnetic response of CoFe-PBA, can be extended to higher temperatures, accompanied by a decrease in magnetization, using heterostructured PBA particles composed of Rb$_a$Co$_b$[Fe(CN)$_6$]$_c\cdot$$m$H$_2$O cores surrounded by shells of K$_j$Ni$_k$[Cr(CN)$_6$]$_l\cdot$$n$H$_2$O.\footnote{M. F. Dumont \textit{et al.} Inorg. Chem. \textbf{50} (2011) 4295.} This effect is attributed to strain in the shell, induced by a lattice expansion in the core during illumination. We report a similar effect in the core-shells accompanied by PIET under application of external pressure and probe the similarities between optical and pressure effects.   

Effect of hydrostatic pressure on a low dimensional S=1/2 gapped quantum antiferromagnet by inelastic neutron scattering

We report an inelastic neutron scattering study of a quasi-two-dimensional $S$= 1/2 dimer system piperazinium hexachlorodicuprate (PHCC) under hydrostatic pressure up to $P$=9.0 kbar. The spin gap $\Delta $ decreases with increasing pressure. The observation of three fold degenerate triplet excitation at $P$=6.0 kbar is consistent with the theoretical prediction and the bandwidth of the dispersion relation is unaffected within the experimental uncertainty. At $P$=9.0 kbar the spin gap is reduced to $\Delta $=0.55 meV from $\Delta $=1.0 meV at ambient pressure [1]. \\[4pt] [1] T. Hong \textit{et. al}., Physical Review B 82, 184424 (2010)   

Density functional theory study of alkali doped picene

We employ density functional theory methods to determine the equilibrium structures of $A_x$picene where $A={\rm Na}$, K, Rb, Cs and $x=1,2,3$. We find that alkali doping with one, two and three alkali ions per picene molecule leads to subsequent filling of the LUMO and LUMO+1 derived bands of picene, leading to quarter, half and three quarter filled systems. We analyse the electronic structures using tight binding methods to derive the kinetic energy part of the underlying Hubbard Hamiltonian. As the interaction strength $U$ on the picene molecules is expected to be large compared to the band width, we also employ manybody methods on the resulting Hamiltonian. We compare our results to photoemission experiments.   

First principles investigation of the Fabre salts

The Fabre salts (TMTTF)$_2X$ are one dimensional charge transfer salts with a very rich phase diagram including charge ordered, Mott insulating, antiferromagnetic and spin Peierls phases. The structure involves stacks of TMTTF molecules with varying degree of dimerization. Open questions involve the role of the anions in the phase transitions. We investigate the electronic structure of compounds with a number of different anions like $X={\rm AsF}_6$, PF$_6$ and Br. We discuss the properties of the underlying Hubbard Hamiltonian for these materials.   

Scaling Theory of the Mott Transition and Breakdown of the Gr\"uneisen Scaling Near a Finite-Temperature Critical End Point

We discuss a scaling theory of the lattice response in the vicinity of a finite-temperature critical end point. The thermal expansivity is shown to be more singular than the specific heat such that the Gr\"uneisen ratio diverges as the critical point is approached, except for its immediate vicinity. More generally, we express the thermal expansivity in terms of a scaling function which we explicitly evaluate for the two-dimensional Ising universality class. Recent thermal expansivity measurements on the layered organic conductor $\kappa$-(BEDT-TTF)$_2 X$ close to the Mott transition are well described by our theory.\\[2mm] [1] Lorenz Bartosch, Mariano de Souza, and Michael Lang, Physical Review Letters {\bf 104}, 245701 (2010).   

Tuning the magnetic dimensionality by charge ordering in the molecular TMTTF salts

Low-dimensional (D) organic conductors TMTTF$_{2}$X are considered as a prototypical system for investigations of D-crossover phenomena, since the transfer integrals connecting the conduction chains are readily controlled by applied pressure [1]. In addition, recent NMR experiments in TMTTF salts imply the existence of another type of D-crossover which accompanies a multiferroic property; decreasing the ferroelectric-type charge ordering (FCO) by pressure is associated with the suppression of the antiferromagnetic (AF) transition temperature [2]. In this work, we theoretically investigate the interplay between FCO and magnetic states in TMTTF salts [3], and show that FCO increases 2-D AF spin correlation, whereas in the 1-D regime two different spin-Peierls states are stabilized. By performing first-principles band calculations for different salts and comparing our results with experiments, we identify the controlling parameters in the experimental phase diagram to be not only the inter-chain transfer integrals but also the amplitude of the FCO. [1] D. Jerome, Science 252, 1509 (1991). [2] W. Yu \textit{et al., }Phys. Rev. B. \textbf{70 }121101 (2004). [3] K. Yoshimi, H. Seo, S. Ishibashi, and S. E. Brown, arXiv:1110.3573 and arXiv:1110.3575.   

Charge dynamics in Mott-insulating and charge-ordered organic crystals: a possibility of the Berezinskii-Kosterlitz-Thouless transition

The organic Mott insulator $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl exhibits power-law current-voltage characteristics in a low current range at temperatures below $\approx$ 10 K. The power-law current-voltage characteristics are similar to those observed in the charge-ordered organic crystals $\theta$-(BEDT-TTF)$_2$MZn(SCN)$_2$ (M = Cs and Rb) and are accounted for in the same way in terms of exciton excitations: The electric field assists the thermal unbinding of pairs of a doublon and a holon that attract each other due to the two-dimensional long-range Coulomb interaction. The two-dimensional Coulomb interaction arises from the layered structure of the organic crystals and their large dielectric anisotropy. The in-plane dielectric constant can be understood within the context of the same model in terms of polarization of the bound pairs. We discuss a possibility of the Berezinskii-Kosterlitz-Thouless transition in these layered organic crystals. $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl has a much smaller magnetoresistance (25 $\%$ in 10 T at 0.3 K) than $\theta$-(BEDT-TTF)$_2$CsZn(SCN)$_4$ ($\approx$10000 $\%$ in 10 T at 0.1 K), which probably reflects different spin states of the excitations for the Mott and charge-ordered states. Yamaguchi Takahide et al. PRB 84, 035129 (2011)   

$^{1}$H NMR study on X-ray irradiated $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br

The $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br ($\kappa$-Br) is a quasi-two dimensional superconductor with $T_{c}$ of 12 K. By substituting Cl for Br in the insulating layer, the system becomes a Mott insulator, k-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl ($\kappa$-Cl), with the Neel temperature of 25 K. So, $\kappa$-Br salt is situated close to the Mott transition. Recently, Sasaki {\it et al.,} have reported the transformation from a metallic (superconducting) state to an insulating state by the x-ray irradiation. The 500h-irradiated thin sample shows insulating behavior in the temperature dependence of resisitivity. We report the $^{1}$H NMR studies on the 500h-irradiated $\kappa$-Br salt. Below 150 K, where the nuclear relaxation is dominated by electron spins, 1/$T_{1}$ of the irradiated sample increases from that of the non-irradiated sample and is even larger than the value of $\kappa$-Cl. Nevertheless, there is no manifestation of magnetic ordering; that is, neither a line broadening nor a divergent peak in 1/$T_{1}$ down to 1.5 K. We will discuss the electronic state of irradiated $k$-Br salt based on experimental results.   

TCP, a new quasi-one-dimensional conducting platinate: DC and NMR studies

Cs$_{4}$[Pt(CN)$_{4}$](CF$_{3}$SO$_{3})_{2}$, or TCP, is the newest member of the family of quasi-one-dimensional conducting platinates that includes the widely studied K$_{2}$[Pt(CN)$_{4}$]Br$_{0.30}$3(H$_{2}$O) (KCP) -- best-known for its metal-insulator transition consistent with a Peierls instability. Unlike KCP, however, we have found properties unique to TCP. X-ray diffraction shows longer Pt-Pt separations, and undergoes subtle change with cooling. DC resistivity measurements presented technical challenges that had to be resolved, but in the end revealed a more highly insulating phase at room temperature, and the temperature dependence of resistivity has an anomalous ``peak'' at around 150 K. NMR also presented a technical challenge in that the $^{195}$Pt nucleus, which had been successfully used for NMR studies on KCP, did not produce a usable signal in TCP, wherein we utilized the peripheral $^{133}$Cs nuclei instead. Quadrupole splitting of spin states of $^{133}$Cs measured as a function of orientation showed consistency with the angular dependence expected of the known symmetry axes of $^{133}$Cs. Preliminary measurements of longitudinal relaxation time T1 also reveal an anomalous temperature dependence in the vicinity of 150 K. All these considerations point to a possible structural transformation, as will be discussed.   

Experimental and theoretical study of the temperature dependent structural and electronic properties of the spin liquid candidate $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$

The organic charge transfer salt $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ is of great experimental as well as theoretical interest due to the interplay of the effects of electronic correlations, low dimensionality and spin frustration. Besides the spin liquid property at very low temperatures, the material has interesting anomalies at 6K, 60K and 150K. Even though the structure of the material has been determined several times over the past 20 years, complete structural data are only available at room temperature. In our work, we fill this gap by precisely determining the structure at 300K, 250K, 200K, 150K, 100K and 20K. We analyze the structures using density functional theory and tight binding methods. We show that the triangular lattice Hubbard Hamiltonian parameters are temperature dependent, with the interaction strength increasing with decreasing temperature and with the frustration going through a minimum at 150 K. Our results point to the fact that even in the absence of structural phase transitions, experimental determination of structures at various temperatures may be important as a starting point for realistic many-body theoretical investigation of complex materials.