Peter Thalmeier

Magnetic-Field Effects on Quadrupolar Ordering in a Γ8-Quartet System CeB6

A theory of the antiferro-quadrupolar ordering in CeB 6 is developed in the framework of the Γ 8 -quartet basis. Possible ordered phases under the external magnetic field are classified in terms of their symmetry properties, and the relations between quadrupolar order parameters and field-induced moments are established. Then we formulate a mean-field theory of Ohkawas RKKY model based on the group-theoretical consideration. The stability of the possible phases and the resulting phase diagrams are analyzed in connection with the character of quadrupolar moments, the influence of induced moments and the form of Zeeman term. Considering available experimental results, we introduce an improved model with asymmetric interactions and discuss the phase diagram of CeB 6 . The connection between magnetostriction and uniform quadrupolar moments is discussed.

Multiple superconducting phases in new heavy fermion superconductor PrOs4Sb12

The superconducting gap structure of recently discovered heavy fermion superconductor PrOs4Sb12 was investigated by using thermal transport measurements in magnetic field rotated relative to the crystal axes. We demonstrate that a novel change in the symmetry of the superconducting gap function occurs deep inside the superconducting state, giving a clear indication of the presence of two distinct superconducting phases with twofold and fourfold symmetries. We infer that the gap functions in both phases have a point node singularity, in contrast to the familiar line node singularity observed in almost all unconventional superconductors.

A New Interpretation of NMR in Quadrupolar Ordering Phase of CeB6 - Consistency with Neutron Scattering -

The hyperfine coupling between the multipolar moments of the Ce ion and the nucleus of B ion is discussed phenomenologically for the antiferro-quadrupolar ordering (AFQ) phase of CeB 6 . The longstanding mutual inconsistency between neutron diffraction and NMR in phase II can be resolved by considering the induced octupolar moment with the T x y z type. The experimental facts indicate that the AFQ of the Γ 5 + symmetry (i.e. O x y , O y z and O z x types) is realized in CeB 6 .

Gap function with point nodes in borocarbide superconductor YNi2B2C

To determine the superconducting gap function of YNi2B2C, the c-axis thermal conductivity kappa(zz) was measured in H rotated in various directions. The angular variation of kappa(zz) in H rotated within the ab plane shows a peculiar fourfold oscillation with narrow cusps. The amplitude of this fourfold oscillation becomes very small when H is rotated conically around the c axis with a tilt angle of 45 degrees. These results provide the first compelling evidence that the gap function has point nodes located along the a and b axes. This unprecedented gap structure challenges the current view on the pairing mechanism.

Interplay of Field-Induced Multipoles in CeB6

We propose a theory of CeB 6 in a magnetic field by extending and slightly modifying a previous paper. The antiferro-quadrupolar ordering induces dipoles and octupoles when the magnetic field is applied. The nature of those field-induced multipoles is studied by changing the direction of the field. It is shown that the contradiction between NMR and neutron diffraction is resolved by taking into account the influence of octupoles. Thus it is concluded that CeB 6 is the first example in which the octupoles play a crucial role on the anomalous f -electron ordered phase.

Theoretical Model for the Semi-Metal Yb4As3

We present a model which can explain semi-quantitatively a number of the unusual properties of Yb4As3. The structural phase transition at Tc 300 K is described by a band Jahn-Teller effect of correlated electrons and is interpreted as a charge ordering of the Yb ions. The low carrier concentration in the low-temperature phase follows from the strong electron correlations of the 4f-holes on the Yb sites and can be viewed as self-doping of charge-ordered chains. The observed heavy-fermion behaviour is on a scale of T* 50 K and is due to spinon-like excitations in the Yb3+-chains. The appearance of a second low-energy scale around 0.2 K is due to the Fermi energy of the low-density carriers.

Anisotropic s-wave superconductivity in borocarbides LuNi2B2C and YNi2B2C

The symmetry of superconductivity in borocarbides

Finite temperature properties and frustrated ferromagnetism in a square lattice Heisenberg model

{\mathrm{LuNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}

Elastic constant anomalies and the superconducting B-T phase diagrams of UPt3 and URu2Si2

and

Dynamics of Multipoles and Neutron Scattering Spectra in Quadrupolar Ordering Phase of CeB6

{\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}