Temperature dependences of piezoelectric, elastic and dielectric constants of L-alanine crystal.

Temperature changes in the components of piezoelectric, elastic and dielectric tensors were studied in L-alanine crystals in the range 100-300 K. A jumpwise increase in the c(55) component of the elastic stiffness accompanied by maxima in damping of all face-shear modes observed at 199 K in L-alanine crystal were interpreted as a result of changes in the NH(3)(+) vibrations occurring through electron-phonon coupling. All components of the piezoelectric tensor show small anomalies in this temperature range. The components of the electromechanical coupling coefficient determined indicate that L-alanine is a weak piezoelectric.

Various ferroic orderings of triclinic tetrachloro-metallate dihydrate crystals.

In the triclinic crystals Rb(2)MnCl(4)·2H(2)O, Cs(2)MnCl(4)·2H(2)O and Cs(2)CaCl(4)·2H(2)O the ferroelastic domain structure has been observed for temperatures from 100 K to the dehydration point. On heating at a few degrees before the dehydration point the structure has been found to reorganize. On heating in Rb(2)MnCl(4)·2H(2)O and Cs(2)MnCl(4)·2H(2)O (but not in Cs(2)CaCl(4)·2H(2)O) a dielectric anomaly typical of a weak ferroelectric phase transition has been noted. The low-frequency dielectric dispersion in the low-temperature phase is attributed to the oscillations of the domain walls. The activation energy of this motion is 73 and 67 kJ mol(-1) in Rb(2)MnCl(4)·2H(2)O and Cs(2)MnCl(4)·2H(2)O, respectively. In the phase transition region the complex permittivity has been found to show oscillations as a function of temperature.

Bose-Einstein condensation of magnons in Cs2CuCl4.

We report on results of specific heat measurements on single crystals of the frustrated quasi-2D spin-1/2 antiferromagnet Cs2CuCl4 (T(N)=0.595 K) in external magnetic fields B<12 T and for temperatures T>30 mK. Decreasing B from high fields leads to the closure of the field-induced gap in the magnon spectrum at a critical field Bc approximately = 8.51 T and a magnetic phase transition is clearly seen below Bc. In the vicinity of Bc, the phase transition boundary is well described by the power law Tc(B) proportional, variant (Bc-B)(1/phi), with the measured critical exponent phi approximately =1.5. These findings are interpreted as a Bose-Einstein condensation of magnons.

Direct measurement of the spin Hamiltonian and observation of condensation of magnons in the 2D frustrated quantum magnet Cs2CuCl4.

We propose a method for measuring spin Hamiltonians and apply it to the spin- 1/2 Heisenberg antiferromagnet Cs2CuCl4, which shows a 2D fractionalized resonating valence bond state at low fields. By applying strong fields we fully align the spin moment of Cs2CuCl4, transforming it into an effective ferromagnet. In this phase the excitations are conventional magnons and their dispersion relation measured using neutron scattering give the exchange couplings directly, which are found to form an anisotropic triangular lattice with small Dzyaloshinskii-Moriya terms. Using the field to control the excitations we observe Bose condensation of magnons into an ordered ground state.

Experimental realization of a 2D fractional quantum spin liquid.

The ground-state ordering and dynamics of the two-dimensional S = 1/2 frustrated Heisenberg antiferromagnet Cs(2)CuCl(4) are explored using neutron scattering in high magnetic fields. We find that the dynamic correlations show a highly dispersive continuum of excited states, characteristic of the resonating valence bond state, arising from pairs of S = 1/2 spinons. Quantum renormalization factors for the excitation energies (1.65) and incommensuration (0.56) are large.