Interplay of weak ferromagnetism, ferroelasticity and shape-memory effects in the spin-orbit coupled antiferromagnet K2_2ReCl6_6

The magnetic and structural phase transitions occurring in K$_2$ReCl$_6$ were studied by macroscopic and microscopic techniques. Structural phase transitions associated with rotations of the ReCl$_6$ octahedra lower the symmetry from cubic to monoclinic, form ferroelastic domains, and are visible in susceptibility, specific heat and thermal expansion measurements. In the antiferromagnetically ordered state slightly below $T_{\rm N}$=12\,K these domains can be rearranged by a magnetic field inducing a relative elongation of the polydomain crystal parallel to the field of 0.6\%. At zero field the magnetic structure in K$_2$ReCl$_6$ does not exhibit a weak ferromagnetic component, but at large magnetic field a distinct magnetic structure with a finite weak ferromagnetic component is stabilized. High magnetic fields rearrange the domains in the crystal to align the weak ferromagnetic moment parallel to the field. The altered domain structure with the crystal elongation is abruptly suppressed at lower temperature but persists upon heating to well above $T_{\rm N}$. However, heating above the lowest structural phase transition and successive cooling restore the initial shape, i.e. a magnetic shape memory effect.Comment: 11 pages, 9 figure

Crystal growth, crystal structure and optical properties of calcium antimony tartrate nonahydrate, Ca[Sb-2((+)C4H2O6)(2)]center dot 9H(2)O

Large single crystals of optical quality of the non-centrosymmetric orthorhombic (space group P 2(1)2(1)2(1)) calcium antimony tartrate nonahydrate, Ca[Sb-2((+)C4H2O6)(2)]center dot 9H(2)O, were grown from aqueous solution. The crystal structure and thermal stability of the crystals are evaluated and refractive indices and their dispersion in the wavelength range 404 nm - 1083 nm are reported. Based on the linear optical data phase matching conditions for second harmonic generation (SHG) are analysed. This new non-centrosymmetric crystal allows phase matched collinear SHG processes of both, type I and type II, in a broad wavelength range in the visible and near IR regions

Linear electrooptic effect of the monoclinic polar bismuth triborate, BiB3O6

For the monoclinic (point group 2) nonlinear optical crystal BiB3O6 the linear electrooptic properties (Pockels effect), i.e. the 8 independent coefficients of the linear electrooptic tensor [r(ijk)(o)] (at constant stress s) at the wavelength lambda = 632.8 nm are determined by a method based on Michelson interferometry. The electrooptic tensor is analysed with respect to the longitudinal and transverse effects and a classification of BiB3O6 as electrooptic material is given

Pyroelectric properties of the monoclinic rare earth nitrates A(2)Ln(NO3)(5)center dot 4H(2)O (A = NH4, Rb; Ln = La, Ce)

The pyroelectric effect of four isomorphic monoclinic (space group Cc), non-ferroelectric rare earth nitrates A(2)Ln(NO3)(5)center dot 4H(2)O (A = NH4, Rb; Ln = La, Ce) was investigated in the temperature range between 100 K and 300 K, using a home-made continuous-flow cryostat for measurements of pyroelectric currents. The symmetry-allowed temperature-dependent change of orientation of the pyroelectric vector p within the mirror plane is unusually large, showing a rotation of p of 148 degrees, 129 degrees, 36 degrees and 40 degrees for (NH4)(2)La(NO3)(5)center dot 4H(2)O, (NH4)(2) Ce(NO3)(5)center dot 4H(2)O, Rb2La(NO3)(5)center dot 4H(2)O and Rb2Ce(NO3)(5)center dot 4H(2)O, respectively, while changing the temperature from 100 K to 300 K in each case. The pyroelectric coefficients are up to ten times larger than p(3) of tourmaline. In addition, new data of the pyroelectric coefficients of Li2SO4 center dot H2O and BiB3O6 and their temperature dependence are given

Triclinic Pedial Sarcosinium Hydrogen L(+)-Tartrate-Crystal Growth, Pyroelectricity, Thermal Expansion, and Linear Optical Properties

Single crystals of optical quality and dimensions up to 25 x 25 x 40 mm(3) of sarcosinium hydrogen L(+)-tartrate and up to 15 x 20 x 30 mm(3) of sarcosinium hydrogen D(-)-tartrate are grown from aqueous solution during growth periods of 10-14 weeks. For sarcosinium hydrogen L(+)-tartrate refractive indices and their dispersion in the wavelength range 365-1083 nm are determined. The biaxial positive crystals show marked optical anisotropy with maximum birefringence at 365 nm of Delta n = 0.089 and only small orientation dispersion of the principal axes of the optical indicatrix. Based on the linear optical data, phase matching conditions for collinear second harmonic generation (SHG) are analyzed. The optical properties allow SHG phase matching for both, type I and type II processes. Temperature-dependent coefficients of the tensor of thermal expansion [alpha(ij)] between 145 and 300 K show pronounced anisotropy and moderate orientation dispersion of thermal expansion, with principal values at 293 K of alpha(0)(11) = -3.3(1) x 10(-6) K-1, alpha(0)(22) = 35.7(1) x 10(-6) K-1, and alpha(0)(33) = 66.6(1) x 10-6 K-1. The absolute value of the pyroelectric vector vertical bar p(sigma)vertical bar of sarcosinium hydrogen L(+)-tartrate amounts to about 20 mu C K-1 m(-2) and varies only slightly with temperature in the temperature range 100-320 K

Magnetically driven second-harmonic generation with phase matching in MnWO4

Phase matching is known to enhance the nonlinear optical response in materials with a non-centrosymmetric crystallographic or electronic structure. In contrast, phase-matched frequency doubling driven by non-centrosymmetric magnetism that induces acentricity in otherwise centrosymmetric structures has not been reported yet. In our study we demonstrate the emergence of magnetically driven second-harmonic generation (SHG) with phase matching in MnWO4. The phase-matched wavelength for SHG can be tuned continuously between 450 nm to 630 nm with the conversion efficiency being determined by the refractive indices and their dispersion. Our findings reveal a new strategy towards magnetismbased conversion-materials and a route for controlling the nonlinear signal yield by acting primarily on the material's spin degree of freedom rather than employing its electronic or structural properties. (C) 2015 Optical Society of Americ

Structural and magnetic phase transitions in Cs-2[FeCl5(H2O)]

The compound Cs-2[FeCl5(H2O)] is magnetoelectric but not multiferroic with an erythrosiderite-related structure. We present a comprehensive investigation of its structural and antiferromagnetic phase transitions by polarization microscopy, pyroelectric measurements, x-ray diffraction and neutron diffraction. At about (157.6 +/- 0.1) K, the compound changes its symmetry from Cmcm to I2/c, with a doubling of the original c-axis. This transformation is associated with rotations of the [FeCl5O] octahedra and corresponds to an ordering of the H2O molecules and of the related H bonds. A significant ferroelectric polarization can be excluded for this transition by precise pyrocurrent measurements. The antiferromagnetic phase transition occurring at 6.4K results in the magnetic space group I2'/c, which perfectly agrees with previous measurements of the linear magnetoelectric effect and magnetization

Stimulated Raman Scattering (SRS) in alpha-AlOOH (Diaspore)

In single crystals of orthorhombic alpha-AlOOH, known also as mineral diaspore, chi((3))-nonlinear lasing by stimulated Raman scattering (SRS) and Raman-induced four-wave mixing (RFWM) is investigated. Picosecond pumping at 1.064 mu m wavelength produces a broadband Stokes and anti-Stokes frequency comb with up to 25 SRS- and RFWM-generated emission lines. All observed Stokes and anti-Stokes lasing components in the visible and near-IR are identified and attributed to a single SRS-promoting vibration mode with omega(SRS) approximate to 445 cm(-1). The first Stokes steady-state Raman gain coefficient in the visible spectral range is estimated to a value not less than 0.36 cm GW(-1)

Observation of chiral solitons in LiCuVO4

Quantum spin liquids represent a magnetic ground state arising in the presence of strong quantum fluctuations that preclude ordering down to zero temperature and leave clear fingerprints in the excitation spectra. While theory bears a variety of possible quantum spin liquid phases their experimental realization is still scarce. Here, we report experimental evidence for chiral solitons in the S = 1/2 spin chain compound LiCuVO4 from measurements of the complex permittivity epsilon* in the GHz range. In zero magnetic field our results show short-lived thermally activated chiral fluctuations above the multiferroic phase transition at T-N = 2.4 K. In epsilon* these fluctuations are seen as the slowing down of a relaxation with a critical dynamical exponent nu(xi)z approximate to 1.3 in agreement with mean-field predictions. When using a magnetic field to suppress T-N towards 0 K the influence of quantum fluctuations increases until the thermally activated fluctuations vanish and only an excitation can be observed in the dielectric response in close proximity to the phase transition below 400 mK. From direct measurements we find this excitation's energy gap as E-SE approximate to 14.1 mu eV, which is in agreement with a nearly gapless chiral soliton that has been proposed for LiCuVO4 based on quantum spin liquid theory. Quantum spin liquids describe a system where any type of long-range or local ordering is absent even at absolute zero but despite prominent theoretical studies definitive experimental evidence of such a system is difficult to obtain. Here, by analysing the complex permittivity of LiCuVO4 the authors find evidence of chiral solitons suggesting the presence of a quantum spin liquid state

Crystal growth, thermal expansion, pyroelectricity and vibrational spectroscopy of barium antimony tartrate, Ba[Sb-2((+)C4H2O6)(2)]center dot 3H(2)O

The tetragonal polar barium antimony tartrate trihydrate, Ba[Sb-2((+)C4H2O6)(2)]center dot 3H(2)O, was grown to large single crystals of optical quality and several cm dimensions. A vibrational spectroscopy study, thermal expansion measurements and a determination of the pyroelectric properties were performed and possibilities for phase matching for second harmonic generation (SHG) were analysed. The crystals show a remarkable high pyroelectric coefficient, a moderate anisotropy of thermal expansion and allow for type I and type II phase matching for collinear SHG. A redetermination of the crystal structure of Ba[Sb-2((+)C4H2O6)(2)]center dot 3H(2)O at 150 K is given in addition, together with the vibrational spectra and crystal structures of two new related compounds, Ba [As-2((+)C4H2O6)(2)]center dot 3H(2)O and Ba[Sb-2((+)C4H2O6)(2)]center dot 4H(2)O, determined at the same temperature