The phase diagram of the quasi two-dimensional antiferromagnet BaNi2V2O8 is studied by specific heat, thermal expansion, magnetostriction, and magnetization for magnetic fields applied perpendicular to c. At µ0H * ≃ 1.5 T, a crossover to a high-field state, where TN (H) increases linearly, arises from a competition of intrinsic and field-induced in-plane anisotropies. The pressure dependences of TN and H * are interpreted using the picture of a pressure-induced in-plane anisotropy. Even at zero field and ambient pressure, in-plane anisotropy cannot be neglected, which implies deviations from pure Berezinskii-Kosterlitz-Thouless behavior.PACS numbers: 75.30. Gw, 75.30.Kz, 75.50.Ee The study of quasi two-dimensional (2D) magnetic systems [1] continues to be a focus of theoretical and experimental investigations, motivated in large part by the discovery of high-temperature superconductivity in the quasi 2D cuprates. Further, the search for a magnetic system exhibiting true Berezinskii-Kosterlitz-Thouless (BKT) behavior, initially proposed for 2D XY magnetic systems [2], has been elusive and has only been seen in superfluid and superconducting films [3]. Theoretical studies indicate that BKT behavior can also be expected for 2D Heisenberg systems with a small easy-plane XY anisotropy [4]. Two recent experimental papers suggest that BaNi 2 V 2 O 8 may in fact be a physical realization of such a system [5,6]. BaNi 2 V 2 O 8 has a rhombohedral structure (space group R3) and its magnetic properties arise from a honeycomb-layered arrangement of spins S = 1 at the Ni 2+ sites. The quasi 2D properties are due to a strong antiferromagnetic Heisenberg superexchange J in the NiO honeycomb layers. Long range antiferromagnetic ordering, which would be precluded in a purely 2D Heisenberg system, sets in below the Néel temperature T N ≃ 50 K [5] because of small additional energy scales, which we include in the following Hamiltonian:The planar XY anisotropy D XY ≃ 1 meV is a factor of 10 smaller than J [7] and confines the spins to lie within the honeycomb layers (easy plane). If this were the only additional term in Eq.(1), a true BKT transition could be expected within each 2D layer [4]. However, a real crystal is always three-dimensional (3D) and a very small interlayer exchange J ′ ultimately leads to a crossover from 2D to 3D correlations, and then to a 3D ordering transition [4]. The value of J ′ is unknown for the present case [7]; however, the extremely small signal at T N in the specific heat [5] suggests that J ′ /J is very small (typically, J ′ /J is in the range 10 −2 -10 −6 in quasi 2D systems [1]). The in-plane anisotropy D IP is estimated by 4*10 −3 meV [7] and acts to align the spins along one of the three equivalent hexagonal easy a-axes [5]. The last term in Eq. (1) includes the effect of a magnetic field H, which also acts as an effective anisotropy [1,8].In this Letter, we study the (T, H) phase diagram of BaNi 2 V 2 O 8 for magnetic fields applied within the honeycomb planes. The combination of specific he...

We report on ac-susceptibility and heat-capacity measurements of the superconductor PrOs 4 Sb 12 in magnetic fields. The resulting phase diagram reveals two distinguishable superconducting phases with transitions at the upper critical field B c2 and a slightly lower field B c2 * . Between B c2 * and 0.7ϫ B c2 * the ac-susceptibility data shows a region with enhanced pinning properties characterized by an extended peak effect. The heat-capacity data reveal an extremely strongly coupled superconductivity with a considerable contribution of heavy quasiparticles. This unusual strong-coupling behavior originates in a sharp increase of the superfluid density at T c . The decrease of the discontinuity of the specific heat at T c and the corresponding pronounced increase of the Ginzburg-Landau-Maki parameter 2 indicate that the superconductivity is most probably not Pauli limited in a large temperature range.

We report on the low-temperature properties of the ternary europium pnictide EuZn 2 Sb 2 . Bulk properties were characterized in terms of the electrical resistivity, magnetoresistance, Hall effect, magnetization, specific heat, and thermal expansion. The data are consistent with an antiferromagnetic order of divalent europium below T N = 13.3 K. The phase transition at T N is clearly visible in all properties measured. The magnetic field-temperature phase diagram was derived from the magnetization, specific heat, and resistivity. For the field parallel and perpendicular to the trigonal c axis the antiferromagnetism is suppressed above B ʈ Ϸ 4.7 T and B Ќ Ϸ 3.5 T, respectively. The critical exponent ␣ observed in the specific heat near T N is consistent with both the predictions of the three-dimensional ͑3D͒ XY as well as the 3D-Heisenberg model.