Christoph Geibel

Heavy-fermion superconductivity atT c =2K in the antiferromagnet UPd2Al3

UPd2Al3 is a new heavy-fermion superconductor with a recordTc of 2 K. In addition, it shows a transition to long-range antiferromagnetic order atTN=14 K. Its Sommerfeld coefficient is reduced from γp=210mJ/K2 mole in the paramagnetic to γ0=150mJ/K2 mole in the antiferromagnetic phase.

A new heavy-fermion superconductor: UNi2Al3

Measurements of the resistivity, susceptibility and specific heat are reported which clearly show that the hexagonal compound UNi2Al3 is a heavy-fermion magnet belowTN=4.6 K and a heavy-fermion superconductor belowTc=1 K.

Strong coupling between local moments and superconducting ’heavy‘ electrons in UPd2Al3.

The electronic structure of heavy-fermion compounds arises from the interaction of nearly localized 4f- or 5f-shell electrons (with atomic magnetic moments) with the free-electron-like itinerant conduction-band electrons. In actinide or rare-earth heavy-fermion materials, this interaction yields itinerant electrons having an effective mass about 100 times (or more) the bare electron mass. Moreover, the itinerant electrons in UPd2Al3 are found to be superconducting well below the magnetic ordering temperature of this compound, whereas magnetism generally suppresses superconductivity in conventional metals. Here we report the detection of a dispersive excitation of the ordered f-electron moments, which shows a strong interaction with the heavy superconducting electrons. This ‘magnetic exciton’ is a localized excitation which moves through the lattice as a result of exchange forces between the magnetic moments. By combining this observation with previous tunnelling measurements on this material, we argue that these magnetic excitons may produce effective interactions between the itinerant electrons, and so be responsible for superconductivity in a manner analogous to the role played by phonons in conventional superconductors.

NaV2O5 as a Quarter-Filled Ladder Compound

A new X-ray diffraction study of the one-dimensional spin-Peierls compound \alpha-NaV_2O_5 reveals a centrosymmetric (Pmmn) crystal structure with one type of V site, contrary to the previously postulated non-centrosymmetric P2_1mn structure with two types of V sites (V^{+4} and V^{+5}). Density functional calculations indicate that NaV_2O_5 is a quarter-filled ladder compound with the spins carried by V-O-V molecular orbitals on the rungs of the ladder. Estimates of the charge-transfer gap and the exchange coupling agree well with experiment and explain the insulating behavior of NaV_2O_5 and its magnetic properties.

A new antiferromagnetic heavy Fermion compound : CePd2Al3

We report the first measurements of the electrical resistivity, the magnetic susceptibility and the specific heat of the new hexagonal compound CePd 2 Al 3 . The results indicate that CePd 2 Al 3 h is a new heavy fermion system with an electronic specific heat coefficient (γ) of 380 mJ/mol·K 2 and antiferromagnetic ordering below T N =2.8 K.

NMR and NQR Studies of Magnetism and Superconductivity in the Antiferromagnetic Heavy Fermion Superconductors UM2Al3 (M=Ni and Pd)

Both magnetic and superconducting characteristics of the new heavy fermion superconductors (HFS), UNi 2 Al 3 and UPd 2 Al 3 have been investigated extensively by means of 27 Al NMR and NQR. Systematic studies of the nuclear-spin-lattice relaxation time T 1 , the Knight shift and the spectrum of 27 Al have revealed different types of magnetic fluctuations and spin structure between these two compounds. UNi 2 Al 3 shows a band-type itinerant antiferromagnetic behavior, while UPd 2 Al 3 belongs to a heavy fermion antiferromagnet with an ordinary size of U-derived moment. In the superconducting state, the relaxation behavior in UPd 2 Al 3 is similar to those observed in the other HFS reported so far, providing an evidence for an unconventional anisotropic superconductivity with line of gap zeros on the Fermi surface as well. Knight shift has been found to decrease considerably below T c , showing that the pseudo spin is well defined, not affected by the impurity scattering.

d-Wave Superconductivity in Antiferromagnetic Heavy-Fermion Compound UPd2Al3 –Evidence from27Al NMR/NQR Studies–

The nuclear-spin-lattice relaxation rate, 27 (1/ T 1 ), of the high-quality polycrystalline UPd 2 Al 3 with a record T c =2 K and the narrowest Al NQR linewidth to date of 12 kHz has been measured in zero magnetic field by the NQR technique. 27 (1/ T 1 ) below T c has been found to obey the T 3 - law at least down to 0.18 K, giving strong evidence that the energy gap vanishes along lines on the Fermi surface. From the analysis of 27 Al Knight shift below T c , it is shown that the anisotropic residual Knight shift originates from the f -electron susceptibility in the antiferromagnetic state, whereas the isotropic reduction of the shift below T c is due to a singlet pairing nature of the superconductivity. Both the results of 27 (1/ T 1 ) and 27 Al Knight shift in UPd 2 Al 3 can be reproduced consistently by a d-wave pairing model characterized by lines of vanishing gap on the Fermi surface.

Fermi-surface collapse and dynamical scaling near a quantum-critical point

Quantum criticality arises when a macroscopic phase of matter undergoes a continuous transformation at zero temperature. While the collective fluctuations at quantum-critical points are being increasingly recognized as playing an important role in a wide range of quantum materials, the nature of the underlying quantum-critical excitations remains poorly understood. Here we report in-depth measurements of the Hall effect in the heavy-fermion metal YbRh2Si2, a prototypical system for quantum criticality. We isolate a rapid crossover of the isothermal Hall coefficient clearly connected to the quantum-critical point from a smooth background contribution; the latter exists away from the quantum-critical point and is detectable through our studies only over a wide range of magnetic field. Importantly, the width of the critical crossover is proportional to temperature, which violates the predictions of conventional theory and is instead consistent with an energy over temperature, E/T, scaling of the quantum-critical single-electron fluctuation spectrum. Our results provide evidence that the quantum-dynamical scaling and a critical Kondo breakdown simultaneously operate in the same material. Correspondingly, we infer that macroscopic scale-invariant fluctuations emerge from the microscopic many-body excitations associated with a collapsing Fermi-surface. This insight is expected to be relevant to the unconventional finite-temperature behavior in a broad range of strongly correlated quantum systems.

Anisotropy in a Heavy Fermion Superconductor: UPd2Al3

We report on the anisotropy of magnetization, electrical resistivity and upper critical magnetic fields ( H C2 ) of UPd 2 Al 3 . The magnetization curve in the antiferromagnetically ordered state shows large anisotropy, indicating that the sublattice moments lie in the basal hexagonal plane. The value of -( d H C2 / d T ) at T C is estimated to be about 36 (kOe/K) for the magnetic field parallel and perpendicular to the hexagonal c -axis, and the upper critical field seems to be rather isotropic.

Heavy fermions: Typical phenomena and recent developments

A survey is given of typical phenomena, new materials and recent developments in heavy-fermion physics. In particular, the following topics are addressed: (i) YbNiAl, a new heavy-fermion local-moment antiferromagnet (LMM) with Néel temperature Tn = 3 K, (ii) “non-Fermi-liquid” behavior at the magnetic instability in two heavy-fermion systems with intact f-ion sublattice, i.e. orthorhombic CePt(Si1−xGex) and tetragonal U(Cu4+xAl8−x), (iii) the low-temperature properties of the anisotropic “Rondo insulator” CeNiSn, and (iv) some of the most unusual observations made on “low-carrier-density” rare-earth systems like Sm3Te4 and Sm3Se4. While the exotic symmetry-broken (superconducting and magnetic) ground states of heavy-fermion metals are discussed in several other contributions to this volume, we focus in the remainder of this paper on the relationship between LMM ordering and heavy-fermion superconductivity: Firstly, the LMM ordered compound CeCu2Ge2 (Tn = 4.1K) is addressed which was recently found to become a non-magnetic heavy-fermion superconductor under high hydrostatic pressure, p ≥ 70 kbar (D. Jaccard et al., Phys. Lett. A163,475 (1992)). Point-contact spectroscopy is used to investigate in more detail the high-pressure superconducting phase of CeCu2Ge2. Secondly, we summarize high-pressure results on UPd2Al3, the first compound to show homogeneous coexistence between LMM ordering and heavy-fermion superconductivity.