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Register a new userQuasi-two-dimensional $S=1/2$ magnetism of Cu[C$_6$H$_2$(COO)$_4$][C$_2$H$_5$NH$_3$]$_2$
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Added by
Ramesh Chandra Nath
Publication date
2014
fields
Physics
and research's language is
English
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We report structural and magnetic properties of the spin-$frac12$ quantum antiferromagnet Cu[C$_6$H$_2$(COO)$_4$][C$_2$H$_5$NH$_3$]$_2$ by means of single-crystal x-ray diffraction, magnetization, heat capacity, and electron spin resonance (ESR) measurements on polycrystalline samples, as well as band-structure calculations. The triclinic crystal structure of this compound features CuO$_4$ plaquette units connected into a two-dimensional framework through anions of the pyromellitic acid [C$_6$H$_2$(COO)$_4$]$^{4-}$. The ethylamine cations [C$_2$H$_5$NH$_3]^+$ are located between the layers and act as spacers. Magnetic susceptibility and heat capacity measurements establish a quasi-two-dimensional, weakly anisotropic and non-frustrated spin-$frac12$ square lattice with the ratio of the couplings $J_a/J_csimeq 0.7$ along the $a$ and $c$ directions, respectively. No clear signatures of the long-range magnetic order are seen in thermodynamic measurements down to 1.8,K. However, the gradual broadening of the ESR line suggests that magnetic ordering occurs at lower temperatures. Leading magnetic couplings are mediated by the organic anion of the pyromellitic acid and exhibit a non-trivial dependence on the Cu--Cu distance, with the stronger coupling between those Cu atoms that are further apart.
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We report the crystal growth and structural and magnetic properties of quasi two-dimensional $S=1/2$ quantum magnet Cu[C$_6$H$_2$(COO)$_4$][H$_3$N-(CH$_2$)$_2$-NH$_3$]$cdot$3H$_2$O. It is found to crystallize in a monoclinic structure with space group $C2/m$. The CuO$_4$ plaquettes are connected into a two-dimensional framework in the $ab$-plane through the anions of [C$_6$H$_2$(COO)$_4$]$^{4-}$ (pyromellitic acid). The [H$_3$N-(CH$_2$)$_2$-NH$_3$]$^{2+}$$cdot$3H$_2$O groups are located between the layers and provide a weak interlayer connection via hydrogen (H...O) bonds. The temperature dependent magnetic susceptibility is well described by $S=1/2$ frustrated square lattice ($J_1-J_2$) model with nearest-neighbor interaction $J_1/k_{rm B} simeq 5.35$ K and next-nearest-neighbor interaction $J_2/k_{rm B} simeq -0.01$ K. Even, our analysis using frustrated rectangular lattice ($J_{1a,b}-J_2$) model confirms almost isotropic nearest-neighbour interactions ($J_{rm 1a}/k_{rm B} simeq 5.31$ K and $J_{rm 1b}/k_{rm B} simeq 5.38$ K) in the $ab$-plane and $J_2/k_{rm B}simeq-0.24$ K. Further, the isothermal magnetization at $T=1.9$ K is also well described by a non-frustrated square lattice model with $J_1/k_{rm B} simeq 5.2$ K. Based on the $J_2/J_1$ ratio, the compound can be placed in the N{e}el antiferromagnetic state of the $J_1 - J_2$ phase diagram. No signature of magnetic long-range-order was detected down to 2 K.
This work presents swarm parameters of electrons (the bulk drift velocity, the bulk longitudinal component of the diffusion tensor, and the effective ionization frequency) in C$_2$H$_n$, with $n =$ 2, 4 and 6, measured in a scanning drift tube apparatus under time-of-flight conditions over a wide range of the reduced electric field, 1 Td $leq,E/N,leq$ 1790 Td (1 Td = $10^{-21}$ Vm$^2$). The effective steady-state Townsend ionization coefficient is also derived from the experimental data. A kinetic simulation of the experimental drift cell allows estimating the uncertainties introduced in the data acquisition procedure and provides a correction factor to each of the measured swarm parameters. These parameters are compared to results of previous experimental studies, as well as to results of various kinetic swarm calculations: solutions of the electron Boltzmann equation under different approximations (multiterm and density gradient expansions) and Monte Carlo simulations. The experimental data are consistent with most of the swarm parameters obtained in earlier studies. In the case of C$_2$H$_2$, the swarm calculations show that the thermally excited vibrational population should not be neglected, in particular, in the fitting of cross sections to swarm results.
Specific heat measurements down to 0.5 K have been performed on a single crystal sample of a spin-ladder like compound Cu$_{2}$(C$_{5}$H$_{12}$N$_{2}$)$_{2}$Cl$_{4}$ under magnetic fields up to 12 T. The temperature dependence of the observed data in a magnetic field below 6 T is well reproduced by numerical results calculated for the S=1/2 two-leg ladder with $J_{rm{rung}}$/$J_{rm{leg}}$=5. In the gapless region above 7 T ($H_{rm{c1}}$), the agreement between experiment and calculation is good above about 2 K and a sharp and a round peak were observed below 2 K in a magnetic field around 10 T, but the numerical data show only a round peak, the magnitude of which is smaller than that of the observed one. The origin of the sharp peak and the difference between the experimental and numerical round peak are discussed.
We re-examine the thermodynamic properties of the coupled dimer system Cu$_2$(C$_5$H$_{12}$N$_2$)$_2$Cl$_4$ under magnetic field in the light of recent NMR experiments [Clemancey {it et al.}, Phys. Rev. Lett. {bf 97}, 167204 (2006)] suggesting the existence of a finite Dzyaloshinskii-Moriya interaction. We show that including such a spin anisotropy greatly improves the fit of the magnetization curve and gives the correct trend of the insofar unexplained anomalous behavior of the specific heat in magnetic field at low temperature.
The electron paramagnetic resonance study for an organic superconductor $beta$-(BEDT-TTF)$_{4}$[(H$_3$O)Ga(C$_2$O$_4$)$_3$]$cdot$C$_6$H$_5$NO$_2$ reveals that superconductivity coexists uniformly with the charge ordered state in one material. In the charge ordered state, the interplane spin exchange is gapped, while the in-plane conductivity is not significantly modified. This anisotropic behavior is explained by the exotic charge ordered state, in which molecular-site selective carrier localization coexists with conducting carriers on other molecules. Relationship between superconductivity and this conductive charge ordered state is investigated.
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