On the implementation of the coexistence phase of antiferromagnetism and superconductivity in heavy-fermion intermetallides

The region of the state diagram in which the pressure-induced quantum phase transition occurs with the destruction of the antiferromagnetic ordering and the appearance of the superconductivity has been described within the periodic Anderson model. It has been shown that a microscopically homogeneous coexistence phase of antiferromagnetism and superconductivity is implemented in the vicinity of the critical point, which was experimentally found in the heavy-fermion compound CeRhIn₅. In this region, the pressure increase is accompanied by the experimentally observed strong growth of the effective fermion mass.     

The coexistence of superconductivity and antiferromagnetism

It is shown that the exchange interaction in an ensemble of itinerant electrons gives rise to a super-conducting phase and a mixed phase in which the Cooper instability occurs in the presence of the long-range antiferromagnetic order. A phase diagram determining the regions of existence of the phases mentioned above is constructed.     

Dual model for magnetic excitations and superconductivity in UPd 2 Al 3

The Heavy Fermion state in UPd₂Al₃ may be approximately described by a dual model where two of the three U-5 f electrons are in a localized state split by the crystalline electric field into two low lying singlets with a splitting energy Δ≃ 6 meV. The third 5 f electron has itinerant character and forms the Heavy Electron bands. Inelastic neutron scattering and tunneling experiments suggest that magnetic excitons, the collective propagating crystal field excitations of the localized 5 f electrons, mediate superconducting (sc) pairing in UPd₂Al₃. A theory for this novel mechanism is developed within a nonretarded approach. A model for the magnetic exciton bands is analyzed and compared with experiment. The sc pair potential which they mediate is derived and the gap equations are solved. It is shown that this mechanism favors an odd parity state which is nondegenerate due to the combined symmetry breaking by the crystalline electric field and the AF order parameter. A hybrid model including the spin fluctuation contribution to the pairing is also discussed. Received 22 October 2001 and Received in final form 28 February 2002     

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

UPd₂Al₃ is a new heavy-fermion superconductor with a recordT _c of 2 K. In addition, it shows a transition to long-range antiferromagnetic order atT _N =14 K. Its Sommerfeld coefficient is reduced from _p =210mJ/K² mole in the paramagnetic to ₀=150mJ/K² mole in the antiferromagnetic phase.     

Quantum effects on the competition between antiferromagnetism and superconductivity in heavy-fermion systems

We study a Ginzburg-Landau theory of two coupled fields describing superconductivity and antiferromagnetism in a metal. A coupling between the two-components superconductor and the antiferromagnetic (AF) field is included in the classical action. The classical results are improved calculating the quantum corrections to one-loop order with the method of the effective potential near the AF phase, but in the paramagnetic side. We discuss the influence of these corrections, including the possibility of fluctuation induced first order transitions. A scaling approach is used to obtain the critical and shift exponents at a quantum bicritical point.     

Symmetry properties of the phase of coexistence of superconductivity and antiferromagnetism in 2D systems with strong electron correlations

Using the diagram technique for Hubbard operators, the effect of quasi-two-dimensionality and hybridization of the 4f electrons of cerium ions and p electrons of indium ions on the properties of the antiferromagnetic, superconducting, and mixed phases in heavy-fermion intermetallic compounds of cerium is studied. It is shown that taking into account quasi-two-dimensionality, low-energy hybridization processes renormalize the antiferromagnetic and superconducting order parameters in the broken time-reversal symmetry phase. Estimates of the critical temperatures of antiferromagnetic ordering and Cooper instability, obtained by the developed approach, are in good agreement with experimental data for cerium-based intermetallic compounds.     

Coexistence of antiferromagnetism and superconductivity near the quantum criticality of the heavy-fermion compound CeRhIn5

We report a study on the interplay between antiferromagnetism (AFM) and superconductivity (SC) in a heavy-fermion compound CeRhIn5 under pressure P=1.75 GPa. The onset of the magnetic order is evidenced from a clear split of 115In nuclear quadrupole resonance spectrum due to the spontaneous internal field below the Néel temperature T(N)=2.5 K. Simultaneously, bulk SC below T(c)=2.0 K is demonstrated by the observation of the Meissner diamagnetism signal whose size is the same as in the exclusively superconducting phase. These results indicate that the AFM coexists homogeneously with the SC at a microscopic level.     

Anomalous properties and coexistence of antiferromagnetism and superconductivity near a quantum critical point in rare-earth intermetallides

Mechanisms of the appearance of anomalous properties experimentally observed at the transition through the quantum critical point in rare-earth intermetallides have been studied. Quantum phase transitions are induced by the external pressure and are manifested as the destruction of the long-range antiferromagnetic order at zero temperature. The suppression of the long-range order is accompanied by an increase in the area of the Fermi surface, and the effective electron mass is strongly renormalized near the quantum critical point. It has been shown that such a renormalization is due to the reconstruction of the quasiparticle band, which is responsible for the formation of heavy fermions. It has been established that these features hold when the coexistence phase of antiferromagnetism and superconductivity is implemented near the quantum critical point.     

Calculated electronic and magnetic structure of UNi2Al3 and UPd2Al3

The electronic band structure of UNi₂Al₃ and UPd₂Al₃ is calculated in the local spin-density functional approximation using the self-consistent scalar relativistic ASW-method including spin-orbit coupling. The chemical bond is discussed qualitatively and an explanation is given for the small observed magnetic moments ofU in terms of nearly compensating spin and orbital moments. Calculated total energies are used to discuss a number of possible ground state magnetic-moment configurations.     

Topology of the Fermi surface and the coexistence of orbital antiferromagnetism and superconductivity in cuprates

It has been shown that the strong coupling model taking into account a rise in the spin antiferromagnetic insulating state explains the doping dependence of the topology and shape of the Fermi contour of superconducting cuprates. Hole pockets with shadow bands in the second Brillouin zone form the Fermi contour with perfect ordinary and mirror nesting, which ensures the coexistence of orbital antiferromagnetism and superconductivity with a large pair momentum for T < T_C. The weak pseudogap region (T_* < T < T*) corresponds to the orbital antiferromagnetic ordering, which coexists with the incoherent state of superconducting pairs with large momenta in the strong pseudogap region (T_C < T < T_*).     

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 belowT _N=4.6 K and a heavy-fermion superconductor belowT _c=1 K.On leave from Institute of Low Temperature and Structure Research, Polish Academy of Sciences PL-50-950, Wroclaw 2, Poland     

Thin-film preparation,superconductivity and transport properties of the heavy-fermion system CeCu2Si2

The preparation and superconducting and transport properties of thin films of the heavy fermion superconductor CeCu₂Si₂ are reported. Superconductivity with onset temperatures above 600 mK was observed. The resistively measured temperature dependence of the upper critical field could be self-consistently fitted by Werthamer-Helfand-Hohenberg-theory with a Maki parameter α ≈ 20 and a spin-orbit scattering parameter λ_SO ≈ 6. The electrical resistivity was measured up to 300 K revealing characteristic structures slightly more pronounced than is known from bulk material and a clear T² behavior below 2 K. The formation of heavy quasiparticles at low temperature appears to be also the origin of the measured unusual temperature dependences of the Hall-effect and magnetoresistance.     

Microscopic coexistence of antiferromagnetism and superconductivity in hole-doped Iron-pnictides Ba0.77K0.23Fe2As2 revealed by NMR

We have performed nuclear magnetic resonance (NMR) measurements on an underdoped single-crystal Ba_0.77K_0.23Fe₂As₂ with T _c?=?16.5 K. Below T _N?= 46 K, an internal magnetic field splits the NMR peaks of H?∥?c and shifts those of H?∥?a to higher frequencies. The nuclear spin-lattice relaxation rate 1/T ₁ measured at the central peak with H?∥?a shows a distinct decrease below T _c(μ ₀ H?=?12 T)?= 16 K. Our results clearly show that antiferromagnetic order and superconductivity coexist at a microscopical length scale.     

Evidence for unconventional s-wave superconductivity in the heavy-fermion compound UPt3

We have discussed the ultrasonic attenuation and specific heat in the superconducting state of the heavy-fermion compound UPt₃ in a quasiparticle picture. Coherence and pair-breaking effect have been taken into account for this system. Our results show that UPt₃ is most probably an unconventional s-wave superconductor.