G. G. Lonzarich

Magnetically mediated superconductivity in heavy fermion compounds

In a conventional superconductor, the binding of electrons into the paired states that collectively carry the supercurrent is mediated by phonons — vibrations of the crystal lattice. Here we argue that, in the case of the heavy fermion superconductors CePd2Si2 and CeIn3, the charge carriers are bound together in pairs by magnetic spin–spin interactions. The existence of magnetically mediated superconductivity in these compounds could help shed light on the question of whether magnetic interactions are relevant for describing the superconducting and normal-state properties of other strongly correlated electron systems, perhaps including the high-temperature copper oxide superconductors.

Superconductivity on the border of itinerant-electron ferromagnetism in UGe2

The absence of simple examples of superconductivity adjoining itinerant-electron ferromagnetism in the phase diagram has for many years cast doubt on the validity of conventional models of magnetically mediated superconductivity. On closer examination, however, very few systems have been studied in the extreme conditions of purity, proximity to the ferromagnetic state and very low temperatures required to test the theory definitively. Here we report the observation of superconductivity on the border of ferromagnetism in a pure system, UGe 2, which is known to be qualitatively similar to the classic d-electron ferromagnets. The superconductivity that we observe below 1 K, in a limited pressure range on the border of ferromagnetism, seems to arise from the same electrons that produce band magnetism. In this case, superconductivity is most naturally understood in terms of magnetic as opposed to lattice interactions, and by a spin-triplet rather than the spin-singlet pairing normally associated with nearly antiferromagnetic metals.

A multi-component Fermi surface in the vortex state of an underdoped high-Tc superconductor

To understand the origin of superconductivity, it is crucial to ascertain the nature and origin of the primary carriers available to participate in pairing. Recent quantum oscillation experiments on high-transition-temperature (high-Tc) copper oxide superconductors have revealed the existence of a Fermi surface akin to that in normal metals, comprising fermionic carriers that undergo orbital quantization. The unexpectedly small size of the observed carrier pocket, however, leaves open a variety of possibilities for the existence or form of any underlying magnetic order, and its relation to d-wave superconductivity. Here we report experiments on quantum oscillations in the magnetization (the de Haas-van Alphen effect) in superconducting YBa2Cu3O6.51 that reveal more than one carrier pocket. In particular, we find evidence for the existence of a much larger pocket of heavier mass carriers playing a thermodynamically dominant role in this hole-doped superconductor. Importantly, characteristics of the multiple pockets within this more complete Fermi surface impose constraints on the wavevector of any underlying order and the location of the carriers in momentum space. These constraints enable us to construct a possible density-wave model with spiral or related modulated magnetic order, consistent with experimental observations.

P-WAVE AND D-WAVE SUPERCONDUCTIVITY IN QUASI-TWO-DIMENSIONAL METALS

We compare predictions of the mean-field theory of superconductivity for nearly antiferromagnetic and nearly ferromagnetic metals in two dimensions. The calculations are based on a parametrization of the effective interaction arising from the exchange of magnetic fluctuations. The Eliashberg equations for the transition temperature are solved including the full momentum dependence of the self-energy. The results show that for comparable parameters d-wave singlet pairing in nearly antiferromagnetic metals is generally much stronger than p-wave triplet pairing in nearly ferromagnetic metals in quasi two dimensions. The relevance to the layered materials, and in particular Sr2RuO4 that exhibits p-wave triplet pairing, is discussed.

Towards resolution of the Fermi surface in underdoped high-Tc superconductors

We survey recent experimental results including quantum oscillations and complementary measurements probing the electronic structure of underdoped cuprates, and theoretical proposals to explain them. We discuss quantum oscillations measured at high magnetic fields in the underdoped cuprates that reveal a small Fermi surface section, comprising quasiparticles that obey Fermi-Dirac statistics, unaccompanied by other states of comparable thermodynamic mass at the Fermi level. The location of the observed Fermi surface section at the nodes is indicated by a body of evidence including the collapse in Fermi velocity measured by quantum oscillations, which is found to be associated with the nodal density of states observed in angular resolved photoemission, the persistence of quantum oscillations down to low fields in the vortex state, the small value of density of states from heat capacity and the multiple frequency quantum oscillation pattern consistent with nodal magnetic breakdown of bilayer-split pockets. A nodal Fermi surface pocket is further consistent with the observation of a density of states at the Fermi level concentrated at the nodes in photoemission experiments, and the antinodal pseudogap observed by photoemission, optical conductivity, nuclear magnetic resonance (NMR) Knight shift, as well as other complementary diffraction, transport and thermodynamic measurements. One of the possibilities considered is that the small Fermi surface pockets observed at high magnetic fields can be understood in terms of Fermi surface reconstruction by a form of small wavevector charge order, observed over long lengthscales in experiments such as NMR and x-ray scattering, potentially accompanied by an additional mechanism to gap the antinodal density of states.

Magnetic and superconducting phases of CePd2Si2

Abstract The cross-over from a magneticallyordered to a non-magnetic spin liquid state has been investigated in a series of resistance measurements under hydrostatic pressures of up to 30 kbar and at temperatures down to below 200 mK in the heavy fermion antiferromagnet CePd2Si2. The electrical resistivity changes dramatically with increasing pressure. Near the critical pressure, at which the magnetic ordering temperature is extrapolated to zero, it exhibits a quasi-linear variation over two orders of magnitude in temperature. This non-Fermi liquid formof π(T) extends down to the onset of a new superconducting transition below 430 mK.

Metamagnetic Quantum Criticality in Metals

We present a renormalization group treatment of metamagnetic quantum criticality in metals. We show that for clean systems the universality class is that of the overdamped, conserving (dynamical exponent z = 3) Ising type. We obtain detailed results for the field and temperature dependence of physical quantities including the differential susceptibility, resistivity, and specific heat. Our results are shown to be in quantitative agreement with data on Sr3Ru2O7 except very near to the critical point itself.

Magnetic oscillations and the quasiparticle bands of heavy electron systems

Abstract Studies of the quasiparticle band structure in a series of heavy electron metals have been performed by means of angle resolved measurements of the de Haas-van Alphen effect in pure crystals. A review is presented of the principal results of these investigations in the magnetic transition metal MnSi, the incipient antiferromagnet Pr, the actinide heavy fermion superconductor UPt3 and rare earth compounds CeAl2 and CeRu2Si2.

Fermi-liquid breakdown in the paramagnetic phase of a pure metal.

Fermi-liquid theory (the standard model of metals) has been challenged by the discovery of anomalous properties in an increasingly large number of metals. The anomalies often occur near a quantum critical point—a continuous phase transition in the limit of absolute zero, typically between magnetically ordered and paramagnetic phases. Although not understood in detail, unusual behaviour in the vicinity of such quantum critical points was anticipated nearly three decades ago by theories going beyond the standard model. Here we report electrical resistivity measurements of the 3d metal MnSi, indicating an unexpected breakdown of the Fermi-liquid model—not in a narrow crossover region close to a quantum critical point where it is normally expected to fail, but over a wide region of the phase diagram near a first-order magnetic transition. In this regime, corrections to the Fermi-liquid model are expected to be small. The range in pressure, temperature and applied magnetic field over which we observe an anomalous temperature dependence of the electrical resistivity in MnSi is not consistent with the crossover behaviour widely seen in quantum critical systems. This may suggest the emergence of a well defined but enigmatic quantum phase of matter.

The normal and superconducting states of CeIn3 near the border of antiferromagnetic order

Abstract Close to the border of antiferromagnetic order at high pressure the cubic stoichiometric compound CeIn 3 displays an unconventional normal-state resistivity followed by superconductivity at low temperatures. The superconducting transition temperature varies rapidly with pressure near the critical pressure where the Neel temperature collapses towards absolute zero.