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Single crystals of intermediate valent EuPd$_2$Si$_2$ were grown from an Eu-rich melt by the Bridgman as well as the Czochralski technique. The chemical and structural characterization of an extracted single crystalline Czochralski-grown specimen yielded a slight variation of the Si-Pd ratio along the growth direction and confirms the existence of a finite Eu(Pd$_{1-m}$Si$_m$)$_2$ homogeneity range. The thorough physical characterization carried out on the same crystal showed that this tiny variation in the composition strongly affects the temperature T$_v$ at which the valence transition occurs. These experiments demonstrate a strong coupling between structural and physical properties in the prototypical valence-fluctuating system EuPd$_2$Si$_2$ and explain the different reported values of T$_v$.

In single crystals of the solid solution Cs2 CuCl 4 - x Br x ( 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, χ ( T , p ) , we determine the pressure dependence of the magnetic coupling constant ∂ J / ∂ p and estimate the magneto-elastic couplings ∂ J / ∂ ∊ v . These values are enhanced by almost one order of magnitude compared to Cs2CuCl2Br2 with tetrahedral Cu coordination.

We report an experimental and theoretical study of the low-temperature specific heat $C$ and magnetic susceptibility $\ensuremath{\chi}$ of the layered anisotropic triangular-lattice spin-$1/2$ Heisenberg antiferromagnets ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4\ensuremath{-}x}{\mathrm{Br}}_{x}$ with $x=0$, 1, 2, and 4. We find that the ratio ${J}^{\ensuremath{'}}/J$ of the exchange couplings ranges from 0.32 to $\ensuremath{\approx}0.78$, implying a change (crossover or quantum phase transition) in the materials' magnetic properties from one-dimensional (1D) behavior for ${J}^{\ensuremath{'}}/Jl0.6$ to two-dimensional (2D) behavior for ${J}^{\ensuremath{'}}/J\ensuremath{\approx}0.78$. For ${J}^{\ensuremath{'}}/Jl0.6$, realized for $x=0$, 1, and 4, we find a magnetic contribution to the low-temperature specific heat, ${C}_{m}\ensuremath{\propto}T$, consistent with spinon excitations in 1D spin-$1/2$ Heisenberg antiferromagnets. Remarkably, for $x=2$, where ${J}^{\ensuremath{'}}/J\ensuremath{\approx}0.78$ implies a 2D magnetic character, we also observe ${C}_{m}\ensuremath{\propto}T$. This finding, which contrasts the prediction of ${C}_{m}\ensuremath{\propto}{T}^{2}$ 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.

$BaCuSi_2O_6$ is a quasi-two dimensional spin dimer system and a model material for studying Bose-Einstein condensation (BEC) of magnons in high magnetic fields. The new $Ba_{1-x}Sr_xCuSi_2O_6$ mixed system, which can be grown with x < 0.3, and $BaCuSi_2O_6$, both grown by using a crystal growth method with enhanced oxygen partial pressure, have the same tetragonal structure ($I4_1/acd$) at room temperature. The mixed system shows no structural phase transition, so that the tetragonal structure is stable down to low temperatures. The oxygen partial pressure acts as control parameter for the growth process. A detailed understanding of the crystal structure depending on the oxygen content will enable the study of the spin dynamics of field-induced order states in this model magnetic compound of high current interest with only one type of dimer layers, which shows the same distance between the Cu atoms, in the structure.

We construct a many-body theory of magneto-elasticity in one dimension and show that the dynamical correlation functions of the quantum magnet, connecting the spins with phonons, involve all energy scales. Accounting for all magnetic states non-perturbatively via the exact diagonalisation techniques of Bethe ansatz, we find that the renormalisation of the phonon velocity is a non-monotonous function of the external magnetic field and identify a new mechanism for attenuation of phonons - via hybridisation with the continuum of excitations at high energy. We conduct ultrasonic measurements on a high-quality single crystal of the frustrated spin-1/2 Heisenberg antiferromagnet $\textrm{Cs}_{2}\textrm{CuCl}_{4}$ in its one-dimensional regime and confirm the theoretical predictions, demonstrating that ultrasound can be used as a powerful probe of strong correlations in one dimension., Comment: 17 pages, 9 figures

Depending on the crystal growth conditions, an orthorhombic (O-type) or a tetragonal (T-type) structure can be found in the solid solution ${\rm Cs}_{2}{\rm CuCl}_{4-x}{\rm Br}_{x}~(0\leq x\leq 4)$ . Here, we present measurements of the temperature-dependent magnetic susceptibility and isothermal magnetization on the T-type compounds $x=1.6$ and 1.8 and compare these results with the magnetic properties recently derived for the O-type variant. The systems were found to exhibit quite dissimilar magnetic properties, which can be assigned to differences in the Cu coordination in these two structural variants. Whereas the tetragonal compounds can be classified as quasi-2-D ferromagnets, characterized by ferromagnetic layers with a weak antiferromagnetic interlayer coupling, the orthorhombic materials, notably the border compounds $x=0$ and 4, are model systems for frustrated 2-D Heisenberg antiferromagnets.

The crystal structures of two crown-ether-coordinated caesium halogen salt hydrates, namely di-μ-bromido-bis[aqua(1,4,7,10,13,16-hexaoxacyclooctadecane)caesium(I)] dihydrate, [Cs2Br2(C12H24O6)2(H2O)2]·2H2O, (I), and poly[[diaquadi-μ-chlorido-μ-(1,4,7,10,13,16-hexaoxacyclooctadecane)dicaesium(I)] dihydrate], {[Cs2Cl2(C12H24O6)(H2O)2]·2H2O} n , (II), are reported. In (I), all atoms are located on general positions. In (II), the Cs+ cation is located on a mirror plane perpendicular to the a axis, the chloride anion is located on a mirror plane perpendicular to the c axis and the crown-ether ring is located around a special position with site symmetry 2/m, with two opposite O atoms exactly on the mirror plane perpendicular to the a axis; of one water molecule, only the O atom is located on a mirror plane perpendicular on the a axis, while the other water molecule is completely located on a mirror plane perpendicular to the c axis. Whereas in (I), hydrogen bonds between bromide ligands and water molecules lead to one-dimensional chains running along the b axis, in (II) two-dimensional sheets of water molecules and chloride ligands are formed which combine with the polymeric caesium–crown polymer to give a three-dimensional network. Although both compounds have a similar composition, i.e. a Cs+ cation with a halogen, an 18-crown-6 ether and a water ligand, the crystal structures are rather different. On the other hand, it is remarkable that (I) is isomorphous with the already published iodide compound.

We present a study of the magnetic susceptibility $\chi_{mol}$ under variable hydrostatic pressure on single crystals of Cs$_2$CuCl$_{4-x}$Br$_x$. This includes the border compounds \textit{x} = 0 and 4, known as good realizations of the distorted triangular-lattice spin-1/2 Heisenberg antiferromagnet, as well as the isostructural stoichiometric systems Cs$_2$CuCl$_{3}$Br$_1$ and Cs$_2$CuCl$_{2}$Br$_2$. For the determination of the exchange coupling constants $J$ and $J^{\prime}$, $\chi_{mol}$ data were fitted by a $J-J^{\prime}$ model \cite{Schmidt2015}. Its application, validated for the border compounds, yields a degree of frustration $J^{\prime}$/$J$ = 0.47 for Cs$_2$CuCl$_3$Br$_1$ and $J^{\prime}$/$J$ $\simeq$ 0.63 - 0.78 for Cs$_2$CuCl$_2$Br$_2$, 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, we find that the overall energy scale, given by $J_c = (J^2$ + $J^{\prime 2}$)$^{1/2}$, increases under pressure, whereas the ratio $J^{\prime}$/$J$ 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. We assign these observations to a structural peculiarity of this class of materials.

Today, event processing (EP) is the first choice technology for analyzing massive event streams in a timely manner. EP allows to detect user-defined situations of interest, like in streaming position events for example, in near real-time such that actions can be taken immediately. Unfortunately, each specific EP system has its very own API and query language because there are no standards. The exchange of EP systems as well as their use within a federation is challenging, error-prone, and expensive. To overcome these problems, we introduce the Java Event Processing Connectivity (JEPC) that is a middleware for uniform EP functionality in Java. JEPC provides always the same API and query language for EP completely independent of the EP system beneath. Furthermore, we show in detail how JEPC can integrate database systems besides EP systems and evaluate the performance of EP powered by databases systems.

Materials close to a quantum-critical point – a zero-temperature phase transition – exhibit anomalous thermodynamic properties also at finite temperatures. Close to a magnetic field-induced quantum-critical point, for example, the finite-temperature entropy ST shows strong variations upon varying the magnetic field. Here we discuss the possibility to use this accumulation of entropy around a field-induced quantum-critical point for realizing an efficient magnetic cooling. Our proof-of-principle demonstration is based on measurements and theoretical calculations of the magnetocaloric properties of low-dimensional spin-1/2 antiferromagnets close to their field-induced quantum-critical points. We present results of the magnetocaloric effect ΓB = T−1(∂T/∂B)S ≈ const as a function of both field and temperature in the vicinity of the quantum-critical point and discuss various performance characteristics, such as range of operation, efficiency and hold time. These figures are compared with those of a state-of-the-art paramagnetic coolant.

The magnetic insulators Cs2CuCl4 and Cs2CuBr4 are model systems for a frustrated quasi-two-dimensional triangular-lattice spin-1/2 Heisenberg antiferromagnet with a weak interlayer coupling. The quasi-two-dimensional character manifests itself in a broad maximum at Tmax in the magnetic susceptibility, which for Cs2CuCl4 has been found to mark the upper bound for a spin-liquid regime (R. Coldea et al. in Phys. Rev. B 68:134424, 2003). Motivated by the observation of characteristic B2 dependencies in the elastic constants and ultrasound attenuation in the spin-liquid regime for this material, we look for corresponding signatures in the related Cs2CuBr4 compound at low temperatures T≤4.2 K and fields B≤50 T. We observe a softening of the elastic constant up to Bs∼32 T (B∥a) and a maximum in the sound attenuation at 0.86 Bs. Both quantities exhibit the same characteristic B2 dependence as found for Cs2CuCl4, suggesting the existence of a spin-liquid phase in Cs2CuBr4 as well.

We present a structural analysis of the substituted system (Ba$_{1-x}$Sr$_{x}$)CuSi$_{2}$O$_{6}$, which reveals a stable tetragonal crystal structure down to 1.5 K. We explore the structural details with lowtemperature neutron and synchrotron powder diffraction, room-temperature and cryogenic highresolution NMR, as well as magnetic- and specific-heat measurements and verify that a structural phase transition into the orthorhombic structure which occurs in the parent compound BaCuSi2O6, is absent for the x = 0.1 sample. Furthermore, synchrotron powder-diffraction patterns show a reduction of the unit cell for x = 0.1 and magnetic measurements prove that the Cu-dimers are preserved, yet with a slightly reduced intradimer coupling Jintra. Pulse-field magnetization measurements reveal the emergence of a field-induced ordered state, tantamount to Bose-Einsteincondensation (BEC) of triplons, within the tetragonal crystal structure of $I\,4_{1}/acd$. This material offers the opportunity to study the critical properties of triplon condensation in a simple crystal structure.

Magneto-acoustic investigations of the frustrated triangular-lattice antiferromagnet Cs2CuCl4 were performed for the longitudinal modes c11 and c33 in magnetic fields along the a-axis. The temperature dependence of the sound velocity at zero field shows a mild softening at low temperature and displays a small kink-like anomaly at TN. Isothermal measurements at T < TN of the sound attenuation reveal two closely-spaced features of different character on approaching the materials quantum-critical point (QCP) around Bs = 8.5 T for B // a. The peak at slightly lower fields remains sharp down to the lowest temperature and can be attributed to the ordering temperature TN(B). The second anomaly which is rounded and which becomes reduced in size upon cooling is assigned to the materials spin-liquid properties preceding the long-range antiferromagnetic ordering. These two features merge upon cooling suggesting a coincidence at the QCP. The elastic constant at lowest temperatures of our experiment at 32 mK can be well described by a Landau free energy model with a very small magnetoelastic coupling constant G/kB = 2.8 K. The applicability of this classical model indicates the existence of a small gap in the magnetic excitation spectrum which drives the system away from quantum criticality.