Pure as well as Ag- and Au-substituted YbInCu$_4$ single crystals were structurally and chemically characterized and investigated by means of heat capacity, magnetization, resistivity and ultrasonic measurements. We studied the influence of different compositions of the initial melt as well as of Au and Ag substitutions on the valence change and investigated whether this change occurs via a first-order phase transition or via crossover. We constructed a phase diagram of YbInCu$_4$ as a function of various substitutions and show that the position of the critical endpoint of the valence transition depends on the substituent and on the conditions under which the samples were grown. Multiple thermal cycles through the first-order transition lead to a significant modification of the physical properties which clearly demonstrated the influence of defects in substituted YbInCu$_4$.

Geometric frustration is a key ingredient in the emergence of exotic states of matter, such as the quantum spin liquid in Mott insulators. Although there has been intense interest in experimentally tuning frustration in candidate materials, achieving precise and continuous control has remained a major hurdle—particularly in accessing the properties of the ideally frustrated lattice. Here, we show that large, finely controlled anisotropic strains can effectively tune the degree of geometric frustration in the Mott insulating κ ‐ ( ET ) 2 Cu 2 ( CN ) 3 —a slightly anisotropic triangular-lattice quantum magnet. Using thermodynamic measurements of the elastocaloric effect, we experimentally map out a temperature-strain phase diagram that captures both the ground state of the isotropic lattice and the less frustrated parent state. Our results provide a benchmark for calculations of the triangular-lattice Hubbard model as a function of frustration and highlight the power of lattice engineering as a route to realizing perfectly frustrated quantum materials.

The universality class of the Mott metal-insulator transition in electronic systems strongly coupled to lattice degrees of freedom has been the subject of recent debate. While transport scaling exponents as well as critical slowing down of resistance fluctuations point towards Ising-type criticality, a strong involvement of lattice elasticity suggests a mean-field scenario instead. We investigate the low-frequency resistance noise in model system kappa-(ET)2Cu[N(CN)2]Cl (kappa-Cl)-selected because it is known to exhibit critical elasticity-tuned to the Mott end point by hydrostatic pressure. Contrary to previous reports for the related, deuterated kappa compound, we do not find any indication of a critical slowing down, but observe a strong and anisotropic change of the relative resistance noise level in the metallic compared to the insulating phase. We speculate within framework of isostructural criticality that a critical slowing down might emerge in the presence of structural disorder.

Geometric frustration is a key ingredient in the emergence of exotic states of matter, such as the quantum spin liquid in Mott insulators. While there has been intense interest in experimentally tuning frustration in candidate materials, achieving precise and continuous control has remained a major hurdle – particularly in accessing the properties of the ideally frustrated lattice. Here, we show that large, finely controlled anisotropic strains can effectively tune the degree of geometric frustration in the Mott insulating κ-(ET)_2Cu_2(CN)_3 – a slightly anisotropic triangular-lattice quantum magnet. Using thermodynamic measurements of the elastocaloric effect, we experimentally map out a temperature-strain phase diagram that captures both the ground state of the isotropic lattice and the less frustrated parent state. Our results provide a new benchmark for calculations of the triangular-lattice Hubbard model as a function of frustration and highlight the power of lattice engineering as a route to realizing perfectly frustrated quantum materials.

We present results of the coefficient of thermal expansion for the frustrated quasi -two-dimensional molecular conductor 0-(BEDT-TTF)2RbZn(SCN)4 for temperatures 1.5 T 290 K. A pronounced first -order phase transition anomaly is observed at the combined charge-order/structural transition at 215 K. Furthermore, clear evidence is found for two separate glasslike transitions at Tg = 90-100 K and Tg dagger = 120-130 K, similar to previous findings for 0-(BEDT-TTF)2CsZn(SCN)4 and 0-(BEDT-TTF)2CsCo(SCN)4 reported by Thomas et al. [Phys. Rev. B 105, L041114 (2022)], both of which lack the charge-order/structural transition. Our findings indicate that these glasslike transitions are common features for the 0-(BEDT-TTF)2MM'(SCN)4 family, with M = (Rb, Cs) and M' = (Co, Zn), irrespective of the presence or absence of charge order. These results are consistent with our model calculations on the glasslike dynamics associated with the flexible ethylene end groups of the BEDT-TTF molecules for various 0-(BEDT-TTF)2MM'(SCN)4 salts, predicting two different conformational glass transitions. Moreover, calculations of the hopping integrals show a substantial degree of dependence on the end groups' conformation, suggesting a significant coupling to the electronic degrees of freedom. Our findings support the possibility that the glassy freezing of the ethylene end groups could drive or enhance glassy charge dynamics.

We present results of the coefficient of thermal expansion for the frustrated quasi-two-dimensional molecular conductor $\ensuremath{\theta}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}\mathrm{RbZn}{(\mathrm{SCN})}_{4}$ for temperatures $1.5\ensuremath{\le}T\ensuremath{\le}290$ K. A pronounced first-order phase transition anomaly is observed at the combined charge-order/structural transition at 215 K. Furthermore, clear evidence is found for two separate glasslike transitions at ${T}_{\mathrm{g}}=90$--100 K and ${T}_{\mathrm{g}}^{\ifmmode\dagger\else\textdagger\fi{}}=120$--130 K, similar to previous findings for $\ensuremath{\theta}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}\mathrm{CsZn}{(\mathrm{SCN})}_{4}$ and $\ensuremath{\theta}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}\mathrm{CsCo}{(\mathrm{SCN})}_{4}$ reported by Thomas et al. [Phys. Rev. B 105, L041114 (2022)], both of which lack the charge-order/structural transition. Our findings indicate that these glasslike transitions are common features for the $\ensuremath{\theta}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}M{M}^{\ensuremath{'}}{(\mathrm{SCN})}_{4}$ family, with $M=(\mathrm{Rb}, \mathrm{Cs})$ and ${M}^{\ensuremath{'}}=(\mathrm{Co}, \mathrm{Zn})$, irrespective of the presence or absence of charge order. These results are consistent with our model calculations on the glasslike dynamics associated with the flexible ethylene end groups of the BEDT-TTF molecules for various $\ensuremath{\theta}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}M{M}^{\ensuremath{'}}{(\mathrm{SCN})}_{4}$ salts, predicting two different conformational glass transitions. Moreover, calculations of the hopping integrals show a substantial degree of dependence on the end groups' conformation, suggesting a significant coupling to the electronic degrees of freedom. Our findings support the possibility that the glassy freezing of the ethylene end groups could drive or enhance glassy charge dynamics.

Ferroelectricity, where electronic degrees of freedom determine the polar order—thereby enabling fast switching and phase control—is an important research field in current condensed-matter physics. Using a combination of resistance noise and dielectric spectroscopy we investigate the nature of relaxor-type electronic ferroelectricity in the organic conductor κ-(BETS)2Mn[N(CN)2]3, a system that represents a wider class of materials of correlated electron systems for which functionalities for organic spintronics recently have been discussed. The two complementary spectroscopies reveal a distinct low-frequency dynamics on different length scales, namely (i) an intrinsic relaxation that is typical for relaxor ferroelectrics which classifies the system as a possible new multiferroic, and (ii) two-level processes which we identify as fluctuating polar nanoregions (PNR), i.e., clusters of quantum electric dipoles that fluctuate collectively. The PNR preform above the metal insulator (MI) transition. Upon cooling through TMI, a drastic increase of the low-frequency 1/f-type fluctuations and slowing down of the charge carrier dynamics is accompanied by the onset of strong non-equilibrium dynamics indicating a glassy transition of interacting dipolar clusters. The freezing of PNR and non-equilibrium dynamics is suggested to be a common feature of organic relaxor-type electronic ferroelectrics.

Using a combination of resistance fluctuation (noise) and dielectric spectroscopy we investigate the nature of relaxor-type electronic ferroelectricity in the organic conductor $\kappa$-(BETS)$_2$Mn[N(CN)$_2$]$_3$, a system representative for a wider class of materials, where strong correlations of electrons on a lattice of dimerized molecules results in an insulating ground state. The two complementary spectroscopies reveal a distinct low-frequency dynamics. By dielectric spectroscopy we detect an intrinsic relaxation that is typical for relaxor ferroelectrics below the metal-to-insulator transition at $T_{\rm{MI}}\sim 25\,$K. Resistance noise spectroscopy reveals fluctuating two-level processes above $T_{\rm MI}$ which strongly couple to the applied electric field, a signature of fluctuating polar nanoregions (PNR), i.e. clusters of quantum electric dipoles fluctuating collectively. The PNR preform above the metal insulator transition. Upon cooling through $T_{\rm MI}$, a drastic increase of the low-frequency $1/f$-type fluctuations and slowing down of the charge carrier dynamics is accompanied by the onset of strong non-equilibrium dynamics indicating a glassy transition of interacting dipolar clusters, the scaling properties of which are consistent with a droplet model. The freezing of nano-scale polar clusters and non-equilibrium dynamics is suggested to be a common feature of organic relaxor-type electronic ferroelectrics and needs to be considered in theoretical models describing these materials.

We present results of the magnetic susceptibility on high-quality single crystals of EuPd_2(Si_1-xGe_x)_2 for Ge concentrations 0 ≤ x ≤ 0.105 performed under varying hydrostatic (He-gas) pressure 0 ≤ p ≤ 0.5 GPa. The work extends on recent studies at ambient pressure demonstrating the drastic change in the magnetic response from valence-change-crossover behavior for x = 0 and 0.058, to long-range antiferromagnetic (afm) order below T_N = 47 K for x = 0.105. The valence-change-crossover temperature T'_V shows an extraordinarily strong pressure dependence of dT'_V/dp = +(80 ± 10) K/GPa. In contrast, a very small pressure dependence of dT_N/dp ≤ +(1 ± 0.5) K/GPa is found for the afm order upon pressurizing the x = 0.105 crystal from p = 0 to 0.05 GPa. Remarkably, by further increasing the pressure to 0.1 GPa, a drastic change in the ground state from afm order to valence-change-crossover behavior is observed. Estimates of the electronic entropy, derived from analyzing susceptibility data at varying pressures, indicate that the boundary between afm order and valence-change crossover represents a first-order phase transition. Our results suggest a particular type of second-order critical endpoint of the first-order transition for x = 0.105 at p_cr≈ 0.06 GPa and T_cr≈ 45 K where intriguing strong-coupling effects between fluctuating charge-, spin- and lattice degrees of freedom can be expected.

We present results of the magnetic susceptibility on high-quality single crystals of EuPd2(Si1-xGex)2 for Ge concentrations 0 x 0.105 performed under varying hydrostatic (He-gas) pressure 0 p 0.5 GPa. The work extends recent studies at ambient pressure demonstrating the drastic change in the magnetic response from valence-crossover behavior for x = 0 and 0.058, to long-range antiferromagnetic (AFM) order below TN = 47 K for x = 0.105. The valence-crossover temperature TV ⠂ shows an extraordinarily strong pressure dependence of dTV ⠂/dp = +(80 & PLUSMN; 10) K/GPa. In contrast, a very small pressure dependence of dTN/dp +(1 & PLUSMN; 0.5) K/GPa is found for the AFM order upon pressurizing the x = 0.105 crystal from p = 0 to 0.05 GPa. Remarkably, by further increasing the pressure to 0.1 GPa, a drastic change in the ground state from AFM order to valencecrossover behavior is observed. Estimates of the electronic entropy related to the Eu 4 f electrons, derived from analyzing susceptibility data at varying pressures, indicate that the boundary between AFM order and valence crossover represents a first-order phase transition. Our results suggest a particular type of second-order critical end point of the first-order transition for x = 0.105 at pcr & AP; 0.06 GPa and Tcr & AP; 39 K where intriguing strongcoupling effects between fluctuating charge, spin, and lattice degrees of freedom can be expected.

We present results of magnetic susceptibility and thermal expansion measurements performed on high-quality single crystals of EuPd _2 2 (Si _{1-x} 1−x Ge _x x ) _2 2 for 0 \leq x \leq ≤x≤ 0.2 and temperatures 2 K \leq T \leq ≤T≤ 300 K. Data were taken at ambient pressure and finite He-gas pressure p p \leq ≤ 0.5 GPa. For x = 0 x=0 and ambient pressure we observe a pronounced valence-change crossover centred around T'_V T′V \approx ≈ 160 K with a non-magnetic ground state. This valence-change crossover is characterized by an extraordinarily strong pressure dependence of d T'_V T′V /d p p = (80 \pm 10) ±10) K/GPa. We observe a shift of T'_V T′V to lower temperatures with increasing Ge-concentration, reaching T'_V T′V \approx ≈ 90 K for x = 0.1 x=0.1 , while still showing a non-magnetic ground state. Remarkably, on further increasing x x to 0.2 we find a stable Eu ^{(2+\delta)+} (2+δ)+ valence with long-range antiferromagnetic order below T_N TN = (47.5 \pm ± 0.1) K, reflecting a close competition between two energy scales in this system. In fact, by the application of hydrostatic pressure as small as 0.1 GPa, the ground state of this system can be changed from long-range antiferromagnetic order for p p < < 0.1 GPa to an intermediate-valence state for p p \geq ≥ 0.1 GPa.

We apply optical-pump terahertz-infrared-probe re- flection spectroscopy to study the non-equilibrium dynamics in the Mott insulator $\kappa-(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})_{2}\mathrm{C}\mathrm{u}[\mathrm{N}(\mathrm{C}\mathrm{N})_{2}]\mathrm{C}1(\kappa-\mathrm{C}1)$, following intra-molecular electronic excitation of the organic dimers. The perturbation and subsequent relaxation of the correlated electronic system is treated with a global spectral analysis including a bleach of the inter-Hubbard-band transition and a photoinduced low-frequency contribution from the excitations above the Mott gap, providing a more complete picture of the nature of the excited states.

We investigate experimentally the ultrafast changes in the spectral response of the Mott insulator $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl ($\kappa$-Cl) upon photodoping with intense excitation at 1.6 eV and probing with continuum pulses simultaneously covering both the terahertz and infrared (IR) ranges (from 0 to 0.6 eV). A quantitative analysis of the differential reflectivity using a multi-band Lorentzian model provide absolute changes in spectral weights and objective global time constants for the relaxation vs. temperature. The transient conductivity spectra deduced from the analysis suggest that the transient photoinduced spectral weight is dominated by a progressive closure of the Mott gap with increasing excitation density, i.e. due to changes in the inter-Hubbard-band absorption by the remaining singly occupied states. We critically examine this scenario compared to that proposed previously, whereby the low-energy spectral weight is attributed to a Drude-like response of photoexcited doublons/holons. We also consider the observed slowing down of the relaxation rate with increasing excitation density, and temperature dependence of the initial doublon/holon density in terms of the phonon-mediated gap recombination model.

EuPd_2Si_2 is a valence-fluctuating system undergoing a temperature-induced valence crossover at T'_V≈160K. We present the successful single crystal growth using the Czochralski method for the substitution series EuPd_2(Si_1-xGe_x)_2, with substitution levels x≤ 0.15. A careful determination of the germanium content revealed that only half of the nominal concentration is build into the crystal structure. From thermodynamic measurements it is established that T'_V is strongly suppressed for small substitution levels and antiferromagnetic order from stable divalent europium emerges for x≳ 0.10. The valence transition is accompanied by a pronounced change of the lattice parameter a of order 1.8 In the antiferromagnetically ordered state below T_N = 47 K, we find sizeable magnetic anisotropy with an easy plane perpendicular to the crystallographic c direction. An entropy analysis revealed that no valence fluctuations are present for the magnetically ordered materials. Combining the obtained thermodynamic and structural data, we construct a concentration-temperature phase diagram demonstrating a rather abrupt change from a valence-fluctuating to a magnetically-ordered state in EuPd_2(Si_1-xGe_x)_2.

EuPd2Si2 is a valence-fluctuating system undergoing a temperature-induced valence crossover at T-V' approximate to 160 K. We present the successful single-crystal growth using the Czochralski method for the substitution series EuPd2(Si1-xGex)(2), with substitution levels x <= 0.15. A careful determination of the germanium content revealed that only half of the nominal concentration is built into the crystal structure. From thermodynamic measurements it is established that T-V' is strongly suppressed for small substitution levels and antiferromagnetic order from stable divalent europium emerges for x greater than or similar to 0.10. The valence transition is accompanied by a pronounced change of the lattice parameter a of order 1.8%. In the antiferromagnetically ordered state below T-N = 47 K, we find sizable magnetic anisotropy with an easy plane perpendicular to the crystallographic c direction. An entropy analysis revealed that no valence fluctuations are present for the magnetically ordered materials. Combining the obtained thermodynamic and structural data, we construct a concentration-temperature phase diagram demonstrating a rather abrupt change from a valence-fluctuating to a magnetically ordered state in EuPd2(Si1-xGex)(2).

The low-temperature state of the quantum spin liquid candidate κ-(BEDT-TTF)_{2}Cu_{2}(CN)_{3} emerges via an anomaly at T^{*}∼6 K. Although signatures of this anomaly have been revealed in various quantities, its origin has remained unclear. Here we report inelastic neutron scattering measurements on single crystals of κ-(BEDT-TTF)_{2}Cu_{2}(CN)_{3}, aiming at studying phonon renormalization effects at T^{*}. A drastic change was observed in the phonon damping across T^{*} for a breathing mode of BEDT-TTF dimers at E=4.7 meV. The abrupt change in the phonon damping is attributed to a phase transition into a valence bond solid state based on an effective model describing the spin-charge coupling in this dimer-Mott system.

We present a combined study of thermal expansion and resistance fluctuation spectroscopy measurements exploring the static and dynamic aspects of the charge-glass formation in the quasi-twodimensional organic conductors θ-(BEDT-TTF)2MM (SCN)4 with M = Cs and M ′ = Co,Zn. In these materials, the emergence of a novel charge-glass state so far has been interpreted in purely electronic terms by considering the strong frustration of the Coulomb interactions on a triangular lattice. Contrary to this view, we provide comprehensive evidence for the involvement of a structural glass-like transition at Tg ∼ 90− 100 K. This glassy transition can be assigned to the freezing of structural conformations of the ethylene endgroups in the donor molecule with an activation energy of Ea ≈ 0.32 eV, and the concomitant slowing down of the charge carrier dynamics is well described by a model of non-exponential kinetics. These findings discolse an important aspect of the phase diagram and renders the current understanding of the charge-glass state in the whole family of θ-(BEDT-TTF)2MM (SCN)4 incomplete. Our results suggest that the entanglement of slow structural and charge-cluster dynamics due to the intimate coupling of lattice and electronic degrees of freedom determine the charge-glass formation under geometric frustration.

Inelastic neutron scattering measurements have revealed an abrupt change in the phonon damping at the 6 K anomaly in the quantum spin liquid candidate $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu${}_{2}$(CN)${}_{3}$. This is interpreted as a phase transition to the valence bond solid state on the basis of an effective model describing the spin-charge coupling in the dimer-Mott system.

Organic salts represent an ideal experimental playground for studying the interplay between magnetic and charge degrees of freedom, which has culminated in the discovery of several spin-liquid candidates such as κ-(ET)_{2}Cu_{2}(CN)_{3} (κ-Cu). Recent theoretical studies indicate the possibility of chiral spin liquids stabilized by ring exchange, but the parent states with chiral magnetic order have not been observed in this material family. In this Letter, we discuss the properties of the recently synthesized κ-(BETS)_{2}Mn[N(CN)_{2}]_{3} (κ-Mn). Based on analysis of specific heat, magnetic torque, and NMR measurements combined with ab initio calculations, we identify a spin-vortex crystal order. These observations definitively confirm the importance of ring exchange in these materials and support the proposed chiral spin-liquid scenario for triangular lattice organics.

Geometrical frustration among interacting spins combined with strong quantum fluctuations destabilize long-range magnetic order in favor of more exotic states such as spin liquids. By following this guiding principle, a number of spin liquid candidate systems were identified in quasi-two-dimensional (quasi-2D) systems. For 3D, however, the situation is less favorable as quantum fluctuations are reduced and competing states become more relevant. Here we report a comprehensive study of thermodynamic, magnetic and dielectric properties on single crystalline and pressed-powder samples of PbCuTe2O6, a candidate material for a 3D frustrated quantum spin liquid featuring a hyperkagome lattice. Whereas the low-temperature properties of the powder samples are consistent with the recently proposed quantum spin liquid state, an even more exotic behavior is revealed for the single crystals. These crystals show ferroelectric order at TFE ≈ 1 K, accompanied by strong lattice distortions, and a modified magnetic response—still consistent with a quantum spin liquid—but with clear indications for quantum critical behavior.