By applying measurements of the dielectric constants and relative length changes to the dimerized molecular conductor κ-(BEDT-TTF)_{2}Hg(SCN)_{2}Cl, we provide evidence for order-disorder type electronic ferroelectricity that is driven by the charge order within the (BEDT-TTF)_{2} dimers and stabilized by a coupling to the anions. According to our density functional theory calculations, this material is characterized by a moderate strength of dimerization. This system thus bridges the gap between strongly dimerized materials, often approximated as dimer-Mott systems at 1/2 filling, and nondimerized or weakly dimerized systems at 1/4 filling, exhibiting a charge order. Our results indicate that intradimer charge degrees of freedom are of particular importance in correlated κ-(BEDT-TTF)_{2}X salts and can create novel states, such as electronically driven multiferroicity or charge-order-induced quasi-one-dimensional spin liquids.

We investigate the structural, electronic and magnetic properties of the newly synthesized mineral barlowite Cu4(OH)6FBr which contains Cu 2+ ions in a perfect kagome arrangement. In contrast to the spin-liquid candidate herbertsmithite ZnCu3(OH)6Cl2, kagome layers in barlowite are perfectly aligned due to the different bonding environments adopted by F − and Br − compared to Cl − . We perform density functional theory calculations to obtain the Heisenberg Hamiltonian parameters of Cu4(OH)6FBr which has a Cu 2+ site coupling the kagome layers. The 3D network of exchange couplings together with a substantial Dzyaloshinskii-Moriya coupling lead to canted antiferromagnetic ordering of this compound at TN = 15 K as observed by magnetic susceptibility measurements on single crystals.

We present a combined study of zero-field 51 V and 127 I NMR at ambient pressure and specific heat and magnetization measurements under pressure up to 2.08 GPa on bulk single crystals of the van-der-Waals ferromagnet VI3. At ambient pressure, our results consistently demonstrate that VI3 undergoes a structural transition at Ts ≈ 78 K, followed by two subsequent ferromagnetic transitions at TF M 1 ≈ 50 K and TF M 2 ≈ 36 K upon cooling. At lowest temperature (T < TF M 2), two magnetically-ordered V sites exist, whereas only one magnetically-ordered V site is observed for TF M 1 < T < TF M 2. Whereas TF M 1 is almost unaffected by external pressure, TF M 2 is highly responsive to pressure and merges with the TF M 1 line at p ≈ 0.6 GPa. At even higher pressures (p ≈ 1.25 GPa), the TF M 2 line merges with the structural transition at Ts which becomes moderately suppressed with p for p < 1.25 GPa. Taken together, our data point towards a complex magnetic structure and an interesting interplay of magnetic and structural degrees of freedom in VI3. PACS numbers: xxxarXiv:1907.08550v1 [cond-mat.str-el]

We report on measurements of the magnetic susceptibility and electrical resistance under He-gas pressure on single crystals of Ca(Fe1−xCox)2As2. We find that for properly heat-treated crystals with modest Co-concentration, x = 0.028, the salient ground states associated with iron-arsenide superconductors, i.e., orthorhombic/antiferromagnetic (o/afm), superconducting, and nonmagnetic collapsed-tetragonal (cT) states can be accessed all in one sample with reasonably small and truly hydrostatic pressure. This is possible owing to the extreme sensitivity of the o/afm (for T ≤ Ts,N ) and superconducting (T ≤ Tc) states against variation of pressure, disclosing pressure coefficients of dTs,N /dP = -(1100 ± 50) K/GPa and dTc/dP = -(60 ± 3) K/GPa, respectively. Systematic investigations of the various phase transitions and ground states via pressure tuning revealed no coexistence of bulk superconductivity (sc) with the o/afm state which we link to the strongly firstorder character of the corresponding structural/magnetic transition in this compound. Our results, together with literature results, indicate that preserving fluctuations associated with the o/afm transition to low enough temperatures is vital for sc to form.