Tuning Charge Order in $κ$-(BEDT-TTF)$_2$Hg(SCN)$_2$X (X=Br, Cl) via Uniaxial Strain

TL;DR

The study tackles reversible control of charge order in the organic Mott insulators κ-(BEDT-TTF)$_2$Hg(SCN)$_2$X (X=Br,Cl) by applying uniaxial strain to traverse the proposed phase boundary. Using tensile strain, they induce charge order at T$_{CO}$ ≈ 33 K in κ-(BEDT-TTF)$_2$Hg(SCN)$_2$Br with 0.4% along the c-axis (Δn ≈ 0.24e) and suppress charge order in κ-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl down to T$_{CO}$ ≈ 10 K with 1.6% along the b-axis (Δn ≈ 0.26e), complemented by a soft-mode signature in Raman scattering near 50 cm$^{-1}$. The results are interpreted through an extended Hubbard model mapped to a transverse field Ising model, with the key control parameter being the ratio of interdimer to intradimer interactions, effectively K′/K$_⊥$ (or V′/t_d), which strain tunes to drive the transition. This constitutes the first robust experimental demonstration of the proposed phase diagram and establishes a mechanism to control electronic ferroelectric/dipole order in these materials, with implications for switching multiferroic behaviors. Key theoretical relationships include $K_⊥ = 2t_d$ and $H = K_⊥\sum_i S_i^x + \tfrac{1}{2}\sum_{i\neq j} K^{ij} S_i^z S_j^z$, with $K^{ij}$ influenced by $V′$ and $t_d$, and strain-induced changes modulating the competition between $K_⊥$ and $K′$, thereby tuning $V′/t_d$ across the phase boundary.

Abstract

In condensed matter physics, experimental control of the properties of materials realizes the aspiration to physically govern the properties of materials and demonstrates an understanding of their underlying physics. In recent years, meaningful progress has been made towards a description of the physics of correlated electron systems, but examples of control of these systems remain rare. In this work, we confirm a phase diagram theoretically proposed for organic Mott insulators. We use $κ$-(BEDT-TTF)$_2$Hg(SCN)$_2$X (X=Br,Cl) (BEDT-TTF = bis(ethylenedithio)tetrathiafuvalene) materials as experimental realization of the proposed model and demonstrate the ability to tune them both ways across a phase border between a Mott insulator with a uniformly distributed charge and a charge ordered state through the application of uniaxial strain. We induce charge order at 33 K in the quantum dipole liquid material $κ$-(BEDT-TTF)$_2$Hg(SCN)$_2$Br through the application of tensile strain of 0.4% along the c-axis. We suppress charge order down to 10 K in $κ$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl by applying a tensile strain of 1.6% along the b-axis. We use Raman scattering spectroscopy to probe both the charge state and a soft mode of collective dipole fluctuations close to the phase border.

Figures (7)

• Figure 1: Phase diagram suggested for organic Mott insulators Hotta2010Naka2010 with experimental results for $\kappa$-(BEDT-TTF)$_2$X, X=Cl, Br mapped on it. Large values of t$_d$, left: The hole is equally distributed within (BEDT-TTF)$_2^{1+}$ dimers, the material is in the dimer Mott insulator phase and can realize quantum spin liquid and quantum dipole liquid. Small values of t$_d$, right: charge order phase. A proposed tuning between the dimer Mott insulator and charge order phases is shown via arrows, with corresponding values of strain.
• Figure 2: Effects of the application of strain as detected by the charge sensitive molecular vibrations of $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$X (X=Br, Cl). For $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Br: (a) Raman scattering spectra under 0.4% tensile strain along the c-axis, the (b) frequency and (c) full-width-half-maximum (FWHM) of $\nu_2$ without strain from Ref. Hassan2018, and the (d) frequency and (e) full-width-half-maximum (FWHM) of $\nu_2$ under 0.4% tensile strain along the c-axis. For $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl: (f) Raman scattering spectra of under 1.6% tensile strain along the b-axis, the (g) frequency and (h) full-width-half-maximum (FWHM) of $\nu_2$ under 1.6% tensile strain along the b-axis, and the (i) frequency and (j) full-width-half-maximum (FWHM) of $\nu_2$ without strain from Ref. Hassan2020.
• Figure 3: Low frequency Raman scattering spectra of $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Br in the (b,b) polarization, with phonons subtracted. Spectra without phonons subtracted are shown in the insets.
• Figure 4: Fitted resonant frequency of collective dipole fluctuations in $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Br. (Inset) Integrated Intensity of the fit. Dashed lines are guides to the eye.
• Figure 5: Phase Diagram of TFIM parameters from Ref. Jacko2020, with $\langle S_z^2 \rangle$ estimated via Monte Carlo Simulation.