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Register a new userMagnetoelastic Coupling in the Spin-Dimer System TlCuCl$_3$
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We present high-resolution measurements of the thermal expansion and the magnetostriction of TlCuCl$_{3}$ which shows field-induced antiferromagnetic order. We find pronounced anomalies in the field and temperature dependence of different directions of the lattice signaling a large magnetoelastic coupling. The phase boundary is extremely sensitive to pressure, e.g. the transition field would change by about +/- 185$%/GPa under uniaxial pressure applied along certain directions. This drastic effect can unambiguously be traced back to changes of the intradimer coupling under uniaxial pressure. The interdimer couplings remain essentially unchanged under pressure, but strongly change when Tl is replaced by K.
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Neutron elastic scattering experiments have been performed on the spin gap system TlCuCl$_3$ in magnetic fields parallel to the $b$-axis. The magnetic Bragg peaks which indicate the field-induced N{e}el ordering were observed for magnetic field higher than the gap field $H_{rm g}approx 5.5$ T at $Q=(h, 0, l)$ with odd $l$ in the $a^*-c^*$ plane. The spin structure in the ordered phase was determined. The temperature and field dependence of the Bragg peak intensities and the phase boundary obtained were discussed in connection with a recent theory which describes the field-induced N{e}el ordering as a Bose-Einstein condensation of magnons.
Single-crystal neutron inelastic scattering was performed in order to investigate the magnetic excitations in the spin gap system TlCuCl$_3$. The constant-${bf Q}$ energy scan profiles were collected in the $a^*-c^*$ plane. Three excitations are observed for $E{leq}15$ meV. One of the excitations is identified to be magnetic excitation. The lowest magnetic excitation with $Esim 0.5$ meV occurs at ${bf Q}=(1, 0, 1)$, as observed in KCuCl$_3$. The dispersion relation of the magnetic excitation can be fitted to the dispersion formula derived from the weakly coupled dimer model. The intradimer interaction is evaluated as $J=5.23$ meV, which coincides with the value estimated from the susceptibility data. However, one of the interdimer interactions obtained is so large that the weakly coupled dimer model is broken down.
Neutron diffraction, magnetization, resistivity, and heat capacity measurements on the 6H-perovskite Ba3BiRu2O9 reveal simultaneous magnetic and structural dimerization driven by strong magnetoelastic coupling. An isostructural but strongly displacive first-order transition on cooling through T*=176 K is associated with a change in the nature of direct Ru-Ru bonds within Ru2O9 face-sharing octahedra. Above T*, Ba3BiRu2O9 is an S=1 magnetic dimer system with intradimer exchange interactions J0/kB=320 K and interdimer exchange interactions J/kB=-160 K. Below T*, a spin-gapped state emerges with Deltaapprox220 K. Ab initio calculations confirm antiferromagnetic exchange within dimers, but the transition is not accompanied by long range-magnetic order.
High-quality single crystals of CoTiO$_3$ are grown and used to elucidate in detail structural and magnetostructural effects by means of high-resolution capacitance dilatometry studies in fields up to 15 T which are complemented by specific heat and magnetization measurements. In addition, we refine the single-crystal structure of the ilmenite ($Rbar{3}$) phase. At the antiferromagnetic ordering temperature $T_mathrm{N}$, pronounced $lambda$-shaped anomaly in the thermal expansion coefficients signals shrinking of both the $c$ and $b$ axes, indicating strong magnetoelastic coupling with uniaxial pressure along $c$ yielding six times larger effect on $T_mathrm{N}$ than the pressure applied in-plane. The hydrostatic pressure dependency derived by means of Gruneisen analysis amounts to $partial T_mathrm{N}/ partial papprox 2.7(4)$~K/GPa. The high-field magnetization studies in static and pulsed magnetic fields up to 60~T along with high-field thermal expansion measurements facilitate in constructing the complete anisotropic magnetic phase diagram of CoTiO$_3$. While the results confirm the presence of significant magnetodielectric coupling, our data show that magnetism drives the observed structural, dielectric, and magnetic changes both in the short-range ordered regime well-above $T_mathrm{N}$ as well as in the long-range magnetically ordered phase.
The interactions between elementary excitations such as phonons, plasmons, magnons, or particle-hole pairs, drive emergent functionalities and electronic instabilities such as multiferroic behaviour, anomalous thermoelectric properties, polar order, or superconductivity. Whereas various hybrid excitations have been studied extensively, the feed-back of prototypical elementary excitations on the crystal electric fields (CEF), defining the environment in which the elementary excitations arise, has been explored for very strong coupling only. We report high-resolution neutron spectroscopy and ab-initio phonon calculations of {ceaual}, an archetypal fluctuating valence compound. The high resolution of our data allows us to quantify the energy scales of three coupling mechanisms between phonons, CEF-split localized 4f electron states, and conduction electrons. Although these interactions do not appear to be atypically strong for this class of materials, we resolve, for the first time, a profound renormalization of low-energy quasiparticle excitations on all levels. The key anomalies of the spectrum we observe comprise (1) the formation of a CEF-phonon bound state with a comparatively low density of acoustic phonons reminiscent of vibronic modes observed in other materials, where they require a pronounced abundance of optical phonons, (2) an anti-crossing of CEF states and acoustic phonons, and (3) a strong broadening of CEF states due to the hybridization with more itinerant excitations. The fact that all of these features are well resolved in CeAuAl$_3$ suggests that similar hybrid excitations should also be dominant in a large family of related materials. This promises a predictive understanding towards the discovery of new magneto-elastic functionalities and instabilities.
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