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تسجيل مستخدم جديدBandwidth-tuning from insulating Mott quantum spin liquid to Fermi liquid via chemical substitution in $kappa$-[(BEDT-TTF)$_{1-x}$(BEDT-STF)$_x$]$_2$Cu$_2$(CN)$_3$
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The electronic properties of molecular conductors can be readily varied via physical or chemical pressure as it increases the bandwidth W; this enables crossing the Mott insulator-to-metal phase transition by reducing electronic correlations U/W. Here we introduce an alternative path by increasing the molecular orbitals when partially replacing sulfur by selenium in the constituting bis-(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) molecules of the title compound. We characterize the tuning of the insulating quantum spin liquid state via a Mott transition to the metallic Fermi-liquid state by transport, dielectric, and optical measurements. At this first-order phase transition, metallic regions coexist in the insulating matrix leading to pronounced percolative effects most obvious in a strong enhancement of the dielectric constant at low temperatures.
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We have in detail characterized the anisotropic charge response of the dimer Mott insulator $kappa$-(BEDT-TTF)$_2$-Cu$_2$(CN)$_3$ by dc conductivity, Hall effect and dielectric spectroscopy. At room temperature the Hall coefficient is positive and cl
ose to the value expected from stoichiometry; the temperature behavior follows the dc resistivity $rho(T)$. Within the planes the dc conductivity is well described by variable-range hopping in two dimensions; this model, however, fails for the out-of-plane direction. An unusually broad in-plane dielectric relaxation is detected below about 60 K; it slows down much faster than the dc conductivity following an Arrhenius law. At around 17 K we can identify a pronounced dielectric anomaly concomitantly with anomalous features in the mean relaxation time and spectral broadening. The out-of-plane relaxation, on the other hand, shows a much weaker dielectric anomaly; it closely follows the temperature behavior of the respective dc resistivity. At lower temperatures, the dielectric constant becomes smaller both within and perpendicular to the planes; also the relaxation levels off. The observed behavior bears features of relaxor-like ferroelectricity. Because heterogeneities impede its long-range development, only a weak tunneling-like dynamics persists at low temperatures. We suggest that the random potential and domain structure gradually emerge due to the coupling to the anion network.
Geometrical frustration, quantum entanglement and disorder may prevent long-range order of localized spins with strong exchange interactions, resulting in a novel state of matter. $kappa$-(BEDT-TTF)$_2$-Cu$_2$(CN)$_3$ is considered the best approxima
tion of this elusive quantum-spin-liquid state, but its ground-state properties remain puzzling. Here we present a multi-frequency electron-spin resonance study down to millikelvin temperatures, revealing a rapid drop of the spin susceptibility at $T^*=6,mathrm{K}$. This opening of a spin gap, accompanied by structural modifications, suggests the enigmatic `$6,mathrm{K}$-anomaly as the transition to a valence-bond-solid ground state. We identify an impurity contribution that becomes dominant when the intrinsic spins form singlets. Only probing the electrons directly manifests the pivotal role of defects for the low-energy properties of quantum-spin systems without magnetic order.
The electrodynamic response of the organic spin-liquid candidate $kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ has been measured in an extremely wide energy range ($10^{-13}$ to 2 eV) as a function of temperature (5 to 300 K). Below the Mott gap, excitations
from the un-gapped spinon continuum cause a considerable contribution to the infrared conductivity, as suggested by the U(1) gauge theory. At THz frequencies we can identify a power-law behavior $sigma(omega) propto omega^{beta}$ with two distinct exponents $beta$ that change from 0.9 to 1.3 at low temperatures. The corresponding crossover scales with temperature: $hbaromega_c approx k_B T$. The observed exponents differ by more than a factor of 2 from the theoretically predicted ones. The findings are compared with those obtained on Herbertsmithites.
The recently proposed multiferroic state of the charge-transfer salt {kappa}-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl [P. Lunkenheimer et al., Nature Mater., vol. 11, pp. 755-758, Sept. 2012] has been studied by dc-conductivity, magnetic susceptibility and meas
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Recent experiments on the organic compound $kappa-(BEDT-TTF)_2 Cu_2 (CN)_3$ raise the possibility that the system may be described as a quantum spin liquid. Here we propose a pairing state caused by the `Amperean attractive interaction between spinon
s on a Fermi surface mediated by the U(1) gauge field. We show that this state can explain many of the observed low temperature phenomena and discuss testable consequences.
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