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$_4$.

PbCuTe$_2$O$_6$ is considered as one of the rare candidate materials for a three-dimensional quantum spin liquid (QSL). This assessment was based on the results of various magnetic experiments, performed mainly on polycrystalline material. More recent measurements on single crystals revealed an even more exotic behavior, yielding ferroelectric order below $T_{\text{FE}}\approx 1\,\text{K}$, accompanied by distinct lattice distortions, and a somewhat modified magnetic response which is still consistent with a QSL. Here we report on low-temperature measurements of various thermodynamic, magnetic and dielectric properties of single crystalline PbCuTe$_2$O$_6$ in magnetic fields $B\leq 14.5\,\text{T}$. The combination of these various probes allows us to construct a detailed $B$-$T$ phase diagram including a ferroelectric phase for $B \leq$ $8\,\text{T}$ and a $B$-induced magnetic phase at $B \geq$ $11\,\text{T}$. These phases are preceded by or coincide with a structural transition from a cubic high-temperature phase into a distorted non-cubic low-temperature state. The phase diagram discloses two quantum critical points (QCPs) in the accessible field range, a ferroelectric QCP at $B_{c1}$ = $7.9\,\text{T}$ and a magnetic QCP at $B_{c2}$ = $11\,\text{T}$. Field-induced lattice distortions, observed in the state at $T>$ $1\,\text{K}$ and which are assigned to the effect of spin-orbit interaction of the Cu$^{2+}$-ions, are considered as the key mechanism by which the magnetic field couples to the dielectric degrees of freedom in this material.

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 present results of the magnetic susceptibility on high-quality single crystals of EuPd$_2$(Si$_{1-x}$Ge$_x$)$_2$ for Ge concentrations 0 $\leq x \leq$ 0.105 performed under varying hydrostatic (He-gas) pressure 0 $\leq p \leq$ 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 $T_{\text{N}}$ = 47 K for $x$ = 0.105. The valence-change-crossover temperature $T'_{\text{V}}$ shows an extraordinarily strong pressure dependence of d$T'_{\text{V}}$/d$p$ = +(80 $\pm$ 10) K/GPa. In contrast, a very small pressure dependence of d$T_{\text{N}}$/d$p \leq$ +(1 $\pm$ 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 $p_{\text{cr}} \approx$ 0.06 GPa and $T_{\text{cr}} \approx$ 45 K where intriguing strong-coupling effects between fluctuating charge-, spin- and lattice degrees of freedom can be expected.

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).

We report the realization of an advanced technique for measuring relative length changes ΔL/L of mm-sized samples under the control of temperature (T) and helium-gas pressure (P). The system, which is an extension of the apparatus described in the work of Manna et al. [Rev. Sci. Instrum. 83, 085111 (2012)], consists of two 4He-bath cryostats, each of which houses a pressure cell and a capacitive dilatometer. The interconnection of the pressure cells, the temperature of which can be controlled individually, opens up various modes of operation to perform measurements of ΔL/L under the variation of temperature and pressure. Special features of this apparatus include the possibility (1) to increase the pressure to values far in excess of the external pressure reservoir, (2) to substantially improve the pressure stability during temperature sweeps, (3) to enable continuous pressure sweeps with both decreasing and increasing pressure, and (4) to simultaneously measure the dielectric constant of the pressure-transmitting medium, viz., helium, εr He(T,P), along the same T-P trajectory as that used for taking the ΔL(T, P)/L data. The performance of the setup is demonstrated by measurements of relative length changes (ΔL/L)T at T = 180 K of single crystalline NaCl upon continuously varying the pressure in the range 6 ≤ P ≤ 40 MPa.

We report a detailed experimental and theoretical study on the effect of hydrostatic pressure on the structural and magnetic aspects of the layered honeycomb antiferromagent $\alpha$-RuCl$_{3}$. Magnetic susceptibility measurements performed under almost ideal hydrostatic-pressure conditions yield that the phase transition to zigzag-type antiferromagnetic order at $T_N$ = 7.3 K can be rapidly suppressed to about 6.1 K. A further suppression with increasing pressure is impeded due to the occurrence of a pressure-induced structural transition at $p \geq$ 104 MPa, accompanied by a strong dimerization of Ru-Ru bonds, which gives rise to a collapse of the magnetic susceptibility. Whereas the dimerization transition is strongly first order, as reflected by large discontinuous changes in $\chi$ and pronounced hysteresis effects, the magnetic transition under varying pressure and magnetic field also reveals indications for a weakly first-order transition. We assign this observation to a strong magnetoelastic coupling in this system. Measurements of $\chi$ under varying pressure in the paramagnetic regime ($T > T_N$) and before dimerization ($p <$ 100 MPa) reveal a considerable increase of $\chi$ with pressure. These experimental observations are consistent with the results of ab-initio Density Functional Theory (DFT) calculations on the pressure-dependent structure and the corresponding pressure-dependent magnetic model. Comparative susceptibility measurements on a second crystal showing two consecutive magnetic transitions instead of one, indicating the influence of stacking faults. Using different temperature-pressure protocols the effect of these stacking faults can be temporarily overcome, transforming the magnetic state from a multiple-$T_N$ into a single-$T_N$ state.

The quantum-spin S = 1=2 chain system Cs$_2$CuCl$_4$ is of high interest due to competing anti-ferromagnetic intra-chain J and inter-chain exchange J' interactions and represents a paramount example for Bose-Einstein condensation of magnons [R. Coldea et al., Phys. Rev. Lett. 88, 137202 (2002)]. Substitution of chlorine by bromine allows tuning the competing exchange interactions and corresponding magnetic frustration. Here we report on electron spin resonance (ESR) in single crystals of Cs$_2$CuCl$_{4-x}$Br$_x$ with the aim to analyze the evolution of anisotropic exchange contributions. The main source of the ESR linewidth is attributed to the uniform Dzyaloshinskii-Moriya interaction. The vector components of the Dzyaloshinskii-Moriya interaction are determined from the angular dependence of the ESR spectra using a high-temperature approximation. The obtained results support the site selectivity of the Br substitution suggested from the evolution of lattice parameters and magnetic susceptibility dependent on the Br concentration.

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.

A highly modular synthesis of BNB- and BOB-doped phenalenyls is presented. Treatment of the 1,8-naphthalenediyl-bridged boronic acid anhydride 1 with LiAlH4/Me3SiCl afforded the corresponding 1,8-naphthalenediyl-supported diborane(6) 2, which served as the starting material for all subsequent transformations. Upon addition of MesMgBr/Me3SiCl, 2 was readily converted to the tetraorganyl diborane(6) 5. The further heteroatoms were finally introduced through the reaction of 2 with (Me3Si)(2) NR' or 5 with H2NR' or H2O (R' = H, Me, p-Tol). A helically twisted, fully BNB-embedded PAH 11 was prepared by combining 2 with a dibrominated m-terphenylamine, followed by a Grignard-mediated double ring-closure reaction. All compounds devoid of B-H bonds show favorable optoelectronic properties, such as luminescence and reversible reduction behavior. In the case of the BNB-phenalenyl 7 (BMes, NMe), the radical-anion salt K[7(center dot)] was generated through chemical reduction with K metal and characterized by EPR spectroscopy. K[7(center dot)] is not long-term stable in a THF/c-hexane solution, but abstracts an H atom with formation of the diamagnetic BNB-doped 1H-phenalene K[7H].

An antiferromagnetically (AFM) coupled biradical based on oxoverdazyl and nitronylnitroxide was synthesized in 46 % yield using Sonogashira coupling. The obtained heterobiradical evidenced distinct properties of both radical entities in solution. Depending on the solvent, the prepared biradical crystallized in two different forms. SQUID magnetization measurements on Form II showed coupling constants J(intraII)/k(B)=-2.1 K and zJ(interII)/k(B)=-11.5 K. Consequently, total intermolecular exchange interactions are five times larger than the intramolecular ones. Further, DFT calculations explained this phenomenon and indicated the advantage of Form I for further in-depth investigations.

We report an experimental and theoretical study of the low-temperature specific heat C and magnetic susceptibility χ of the layered anisotropic triangular-lattice spin-1/2 Heisenberg antiferromagnets Cs_{2}CuCl_{4-x}Br_{x} with x=0, 1, 2, and 4. We find that the ratio J^{'}/J of the exchange couplings ranges from 0.32 to ≈0.78, implying a change (crossover or quantum phase transition) in the materials' magnetic properties from one-dimensional (1D) behavior for J^{'}/J<0.6 to two-dimensional (2D) behavior for J^{'}/J≈0.78. For J^{'}/J<0.6, realized for x=0, 1, and 4, we find a magnetic contribution to the low-temperature specific heat, C_{m}∝T, consistent with spinon excitations in 1D spin-1/2 Heisenberg antiferromagnets. Remarkably, for x=2, where J^{'}/J≈0.78 implies a 2D magnetic character, we also observe C_{m}∝T. This finding, which contrasts the prediction of C_{m}∝T^{2} made by standard spin-wave theories, shows that Fermi-like statistics also plays a significant role for the magnetic excitations in spin-1/2 frustrated 2D antiferromagnets.

In single crystals of the solid solution Cs2CuCl4-xBrx (0 <= x <= 4), depending on the growth conditions, two structural modifications with a tetrahedral or an octahedral Cu environment can be realized. The border compounds (x = 0 and 4) with a tetrahedral Cu coordination have been recognized as model systems for quasi-2D frustrated anisotropic triangular quantum antiferromagnets. This holds true also for the newly discovered stoichiometric compounds with x = 1 and 2. In contrast, the systems exhibiting an octahedral Cu environment can be classified as quasi-2D ferromagnets characterized by ferromagnetic layers with a weak inter-layer coupling. Here we study the magnetic and magneto-elastic couplings for the low-dimensional Heisenberg quantum magnet Cs2CuCl2Br2 with octahedral Cu coordination. By applying magnetic susceptibility measurements under varying hydrostatic (He-gas) pressure, chi(T , p), we determine the pressure dependence of the magnetic coupling constant partial derivative J /partial derivative p and estimate the magneto-elastic couplings partial derivative J / partial derivative (is an element of nu). These values are enhanced by almost one order of magnitude compared to Cs2CuCl2Br2 with tetrahedral Cu coordination.

A study of the magnetic susceptibility under variable hydrostatic (He gas) pressure on single crystals of Cs2CuCl4−xBrx is presented. This work includes the border compounds x = 0 and 4, known as good realizations of the distorted triangular‐lattice spin‐1/2 Heisenberg antiferromagnet, as well as the recently discovered well‐ordered isostructural systems Cs2CuCl3Br1 and Cs2CuCl2Br2. For the determination of the exchange coupling constants J and J′ of their anisotropic triangular lattice, the susceptibility data are fitted by the recently proposed J–J′ model [Schmidt and Thalmeier, New J. Phys. 2015, 17, 073025]. Its application on magnetic susceptibility data, validated for the border compounds, yields a degree of frustration J′/J = 0.47 for Cs2CuCl3Br1 and J′/J ≃ 0.63–0.78 for Cs2CuCl2Br2, making these systems particular interesting representatives of this family. From the evolution of the magnetic susceptibility under pressure up to about 0.4 GPa, the maximum pressure applied, two observations were made for all the compounds investigated here. First, it has been found that the overall energy scale, given by Jc = (J2 + J′2)1/2, increases under pressure, whereas the ratio J′/J remains unchanged in this pressure range. These experimental observations are in accordance with the results of DFT calculations performed for these materials. Secondly, for the magnetoelastic coupling constants, extraordinarily small values are obtained, about two orders of magnitude smaller compared to other Cu‐based quantum magnets. These observations have been assigned to a structural peculiarity of this class of materials, consisting of well‐isolated magnetic units not sharing any common coordination element.

Dynamic susceptibility, commonly referred to as AC susceptibility, χAC, is a powerful tool to characterize a material's magnetic properties in the presence of a magnetic field B, such as magnetic ordering or spin-relaxation phenomena. The standard technique for accessing χAC is based on measurements of the voltage which is induced in a coil by changes of a sample's magnetization in response to a small oscillating magnetic field. Importantly, this setup allows for a phase-sensitive detection of the susceptibility, thereby providing information on the magnetization dynamics. This method is frequently used in the low-field range, up to several Tesla, where DC fields are readily available. However, there is a growing demand for such magnetic measurements at higher fields, conveniently accessible by the use of pulse-field technology. Whereas various techniques are available for measuring the in-phase part of the susceptibility χAC ', none of them allows for the determination of the out-of-phase part χAC ″. Here we present a realization of an AC susceptometer for measurements of both the in- and out-of-phase component at frequencies up to 47 kHz and an oscillating amplitude of 1.5 mT in a pulsed magnetic field. With this setup, a magnetic signal of 3.6 × 10-6 emu can be resolved. As the setup also enables susceptibility measurements to be performed before and after the field pulse, it can be used to detect temperature changes in response to changes of the magnetic field. This will be demonstrated by measurements on the low-dimensional spin-1/2 system CuSO4·5H2O.

R. HassanAbadi, R. M. Eremina, 3 M. Hemmida, A. Dittl, B. Wolf, W. Assmus, A. Loidl, and H.-A. Krug von Nidda Experimentalphysik V, Center for Electronic Correlations and Magnetism, Institute for Physics, Augsburg University, D-86135 Augsburg, Germany Zavoisky Physical-Technical Institute, Federal Research Center ”Kazan Scientific Center of RAS”, 420029 Kazan, Russia Institute for Physics, Kazan (Volga region) Federal University, 420008 Kazan, Russia Physikalisches Institut, Goethe-Universität Frankfurt, Max-von-Laue-Strasse 1, 60438 Frankfurt am Main, Germany (Dated: May 28, 2019)

Four weakly antiferromagnetic interacting biradicals of benzo[1,2- b:4,5- b']dithiophene (BDT) and BDT extended with two thiophenes (BDTTh2) linked with nitronyl and imino nitroxides (NN and IN) as BDT-NN, BDT-IN, BDTTh2-NN, and BDTTh2-IN were designed, synthesized, and characterized. Short intermolecular π-π distances were found (3.42 Å) for BDT-NN, whereas larger ones were found for BDT-IN (3.54 Å) and BDTTh2-NN (3.67 Å), respectively. Intramolecular magnetic interaction ( Jintra,exp/ kB) of BDT-NN (-26 K) is much larger than for BDT-IN (-5.3 K), while it is reduced for the dithiophene-extended molecule BDTTh2-NN (-2.3 K). Intermolecular interactions ( zJinter,exp/ kB) of BDT-NN (-6.5 K) and BDT-IN (-6.0 K) are stronger than for BDTTh2-NN (-4.6 K). Such large intermolecular couplings resulting from good π-stacking mark BDT-IN and BDTTh2-NN as promising crystalline materials with similar sized Jintra and Jinter. In addition, we also extracted a coupling within the chain of Jchain/ kB = -2.2 K and a coupling between the chains of zJinterchain = -1.5 K for BDTTh2-NN by a Heisenberg chain model. Intra- and intermolecular interactions and spin densities were examined by DFT studies.