Abstract. We report the multiferroic behaviour of MnWO 4 , a magnetic oxide with monoclinic crystal structure and spiral long-range magnetic order. Based upon recent theoretical predictions MnWO 4 should exhibit ferroelectric polarization coexisting with the proper magnetic structure. We have confirmed the multiferroic state below 13 K by observing a finite electrical polarization in the magnetically ordered state via pyroelectric current measurements.Multiferroic materials which combine magnetism and ferroelectricity currently attract considerable attention [1][2][3][4]. There are already several multiferroic materials recently discovered among transition metal oxides:. Nevertheless, the search for novel systems with multiferroic properties presents a definite interest. In this letter we report that yet another transition metal oxide, MnWO 4 , belongs to the same class of materials and develops spontaneous electric polarization in a spiral magnetically ordered state [8].There exist several different microscopic mechanisms which may cause multiferroic behavior [3]. One of the most interesting cases is when a spontaneous polarization exists only in a magnetically ordered phase with a particular type of ordering. This is e.g. the case in TbMnO 3 and TbMn 2 O 5 . Microscopic [9] and phenomenological [10] treatments have shown that this happens particularly in spiral magnetic structures with the spin rotation axis − → e not coinciding with the magnetic propagation vector − → Q : theoretical treatment shows that in this case a finite spontaneous polarization perpendicular to the plane spanned by − → e and − → Q may appearThis is not the only source for a magnetically driven ferroelectricity [11,12], but perhaps the most common one. Accordingly, one strategy to search for new multiferroic materials is to look for magnetic systems with proper magnetic structures. MnWO 4 (also known as the mineral hübnerite) appears to be just such a system. Detailed studies of the magnetic ordering in this material have shown [13,14] that below 12.3 K a spiral magnetic ordering develops which seems to satisfy the criterion of Eq. (1). In order to test this we carried out measurements of the dielectric response and of spontaneous polarization of MnWO 4 using single-crystalline samples. The crystals of MnWO4 were grown from melt solution. On the basis of earlier work [15] we applied a modified flux technique, using a melt solvent from the system Na 2 WO 4 -WO 3 . The resulting crystals are of dimensions up to 15 x 5 x 3 mm 3 and of 3 Author to whom correspondence should be addressed (khomskii@ph2.uni-koeln.de).

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The phase transitions of the strongly correlated tetrahedral V 4 -cluster compound GeV 4 S 8 have been studied by low-temperature powder x-ray diffraction, magnetic susceptibility, and specific heat measurements. The crystal structure is cubic at room temperature ͑space group F43m͒ and transforms to orthorhombic ͑space group Imm2͒ at T S = 30 K. A Jahn-Teller distortion reduces the symmetry of the V 4 -cluster from 43m to mm2. The second transition at 18 K is the onset of antiferromagnetic ordering without symmetry change but with a certain increase in the distortion. The latter reflects a strong magnetoelastic coupling at T N . Specific heat anomalies at 30 and 18 K confirm the two phase transitions.

LiFeAs is unique among the broad family of FeAs-based superconductors, because it is superconducting with a rather large Tc 18 K under ambient conditions although it is a stoichiometric compound. We studied the electrical transport on a high-quality single crystal. The resistivity shows quadratic temperature dependence at low temperature giving evidence for strong electronelectron scattering and a tendency towards saturation around room temperature. The Hall constant is negative and changes with temperature, what most probably arises from a van Hove singularity close to the Fermi energy in one of the hole-like bands. Using band structure calculations based on angular resolved photoemission spectra we are able to reproduce all the basic features of both the resistivity as well as the Hall effect data.