Unconventional Heavy Fermion Superconductors Research Articles

Muon spin rotation and relaxation in Pr1−xNdxOs4Sb12 : Superconductivity and magnetism in Pr-rich alloys

The Pr-rich end $0\ensuremath{\le}x\ensuremath{\le}0.25$ of the alloy series ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Nd}}_{x}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$ has been studied using muon spin rotation and relaxation. The end compound ${\mathrm{PrOs}}_{4}{\mathrm{Sb}}_{12}$ is an unconventional heavy-fermion superconductor, which exhibits a spontaneous magnetic field associated with broken time-reversal symmetry (TRS) in the superconducting phase. No such field is observed in the Nd-doped alloys for $x\ensuremath{\ge}0.05$, indicating that TRS has been restored. The superfluid density from the vortex-lattice field distribution is insensitive to Nd concentration for $x\ensuremath{\lesssim}0.2$, indicating that TRS restoration does not have a strong effect on the superconducting state. No ${\mathrm{Nd}}^{3+}$ static magnetism, ordered or disordered, is found down to the lowest temperatures of measurement.

Resonances in the Field-Angle-Resolved Thermal Conductivity of CeCoIn_{5}.

The thermal conductivity measurement in a rotating magnetic field is a powerful probe of the structure of the superconducting energy gap. We present high-precision measurements of the low-temperature thermal conductivity in the unconventional heavy-fermion superconductor CeCoIn_{5}, with the heat current J along the nodal [110] direction of its d_{x^{2}-y^{2}} order parameter and the magnetic field up to 7T rotating in the ab plane. In contrast to the smooth oscillations found previously for J∥[100], we observe a sharp resonancelike peak in the thermal conductivity when the magnetic field is also in the [110] direction, parallel to the heat current. We explain this peak qualitatively via a model of the heat transport in a d-wave superconductor. In addition, we observe two smaller but also very sharp peaks in the thermal conductivity for the field directions at angles Θ≈±33° with respect to J. The origin of the observed resonances at Θ≈±33° at present defies theoretical explanation. The challenge of uncovering their source will dictate exploring theoretically more complex models, which might include, e.g., fine details of the Fermi surface, Andreev bound vortex core states, a secondary superconducting order parameter, and the existence of gaps in spin and charge excitations.

Spin Exciton Formation inside the Hidden Order Phase ofCeB6

The heavy fermion metal CeB(6) exhibits a hidden order of the antiferroquadrupolar (AFQ) type below T(Q)=3.2 K and a subsequent antiferromagnetic (AFM) order at T(N)=2.3 K. It was interpreted as an ordering of the quadrupole and dipole moments of a Γ(8) quartet of localized Ce 4f(1) electrons. This established picture has been profoundly shaken by recent inelastic neutron scattering (G. Friemel et al., arXiv:1111.4151) that found the evolution of a feedback spin exciton resonance within the hidden order phase at the AFQ wave vector which is stabilized by the AFM order. We develop an alternative theory based on a fourfold degenerate Anderson lattice model, including both order parameters as particle-hole condensates of itinerant heavy quasiparticles. This explains in a natural way the appearance of the spin exciton resonance and the momentum dependence of its spectral weight, in particular, around the AFQ vector and its rapid disappearance in the disordered phase. Analogies to the feedback effect in unconventional heavy fermion superconductors are pointed out.

Pressure Evolution of the Magnetic Field induced Ferromagnetic Fluctuation through the Pseudo-Metamagnetism of CeRu2Si2

Resistivity measurements performed under pressure in the paramagnetic ground state of CeRu2Si2 are reported. They demonstrate that the relative change of effective mass through the pseudo metamagnetic transition is invariant under pressure. The results are compared with the first order metamagnetic transition due to the antiferromagnetism of Ce0.9La0.1Ru2Si2 which corresponds to the "negative" pressure of CeRu2Si2 by volume expansion. Finally, we describe the link between the spin-depairing of quasiparticles on CeRu2Si2 and that of Cooper pairs on the unconventional heavy fermion superconductor CeCoIn5.

Effect of Disorder Induced by Heavy-Ion Irradiation on CeCoIn5 Superconductivity

We present a study of the effect of heavy-ion irradiation on a thinned single crystal of the unconventional heavy fermion superconductor CeCoIn5. Magnetic susceptibility and low-temperature specific heat results show that the superconducting transition temperature (T c) changed only 4% with irradiation of 1×1012 ions/cm2, Energy=1 GeV, while the specific heat jump at T c, ΔC, divided by C n, where C n is the normal state specific heat just above T c, was reduced to 3.6 from ΔC/C n=4.5 for unirradiated CeCoIn5. The increase of low-temperature magnetic susceptibility and the saturation in magnetization suggest that the defects induced by heavy-ion irradiation are magnetic in nature, as was seen in the case of neutron irradiation on the heavy fermion superconductors, UBe13 and UPt3. The non-Fermi liquid behavior of the irradiated sample, based on the temperature dependence of the low temperature magnetic susceptibility, is significantly altered.

Anisotropic spin fluctuations in CeCoIn 5

CeCoIn 5 is an unconventional heavy fermion superconductor with a relatively large transition temperature T c = 2.3 K . NMR measurements of the spin lattice relaxation rate at the In(1) site reveal a significant anisotropy of the fluctuating hyperfine fields, which reverses below 30 K. These results suggest that two-dimensional fluctuations of the Ce 4f moments are relevant for the superconducting pairing.

Heavy Electron Unconventional Superconductivity in UPd2Al3and PrOs4Sb12

The unconventional Heavy Fermion superconductors UPd 2 Al 3 and PrOs 4 Sb 12 are discussed within interaction models involving exchange of bosonic modes related to the excitation of 4f/5f CEF transitions. It is proposed that both mass enhancement and pair formation are due to this mechanism. The gap functions have line nodes or point nodes in the two cases respectively. Recent field-angle resolved thermal conductivity measurements in the vortex phase of UPd 2 Al 3 are analyzed within the Doppler shift approximation and the evidence for specific superconducting gap functions is discussed.

195Pt NMR study on noncentrosymmetric heavy-fermion superconductor CePt3Si

We report on magnetic and novel superconducting properties in heavy-fermion (HF) noncentrosymmetric compound CePt 3 Si via the measurement of 195 Pt nuclear spin-lattice relaxation rate 1/ T 1 . This compound reveals the uniform coexistence of antiferromagnetism ( T N =2.2 K) and superconductivity ( T c =0.75 K). CePt 3 Si is the first HF superconductor that reveals a peak in 1/ T 1 just below T c and, additionally, does not follow the T 3 law that was observed for most unconventional HF superconductors. The novel relaxation behavior found below T c may be relevant with a possible novel SC state being realized in the noncentrosymmetric CePt 3 Si

Preparation of superconducting thin films of UNi [formula omitted]Al [formula omitted

Epitaxial thin films of the unconventional heavy fermion superconductor UNi 2 Al 3 we prepared by coevaporation of the elementary components in a molecular beam epitaxy system (MBE). The phase purity and structural quality of the films deposited on (0 1 0)- or (1 1 2)-oriented YAlO 3 substrates were studied by X-ray diffraction and RHEED. The observed R ( T ) behavior is consistent with data obtained from bulk samples and proves the purity of the films. Superconductivity was found with transition temperature T c =0.97 K.

Low-field magnetic investigation of the unconventional heavy-fermion superconductor PrOs 4Sb 12

From measurements of dc isothermal magnetization curves for a collection of PrOs 4Sb 12 single crystals, we have obtained the temperature dependence of the lower critical field H C 1 ( T ) . At temperatures as low as 0.05 K, the value of H C 1 does not exceed 20 Oe. A comparison of the Meissner slope, d M / d H in the limit H → 0 , with ac-magnetic susceptibility for the same sample allowed us to investigate in detail the region where two distinct superconducting anomalies have been reported. d M / d H H → 0 is temperature independent only below about 1.5 K, i.e., well below T C 2 ≃ 1.75 K .

Magnetic excitations in the heavy-fermion superconductor [formula omitted

The low-lying magnetic excitations of the unconventional heavy-fermion superconductor PrOs 4 Sb 12 were studied by inelastic neutron scattering on single-crystalline sample. The crystal field excitation first evidenced at 0.7 meV on powder samples is found to have a minimum of energy at the zone boundary corresponding to the wave vector of antiferro-quadrupolar instability. The evolution of the magnetic excitation spectrum under magnetic field confirms the singlet ground state.

Field-angle-dependent specific heat in the unconventional heavy-fermion superconductor [formula omitted

We have measured the field-angle-dependent specific heat, C ( H , θ ) , in the unconventional heavy-fermion superconductor CeCoIn 5 down to ∼ 0.29 K . In a magnetic field rotating in the tetragonal c-plane, a clear fourfold angular oscillation is observed in C ( H , θ ) with the minima (maxima) occurring along the [1 0 0] ([1 1 0]) directions. Oscillation persists down to low fields H ⪡ H c 2 , thus directly proving the existence of gap nodes. Position of the minima in C ( H , θ ) indicates that the gap node is located along the [1 0 0] and [0 1 0] directions, implying that the superconducting gap symmetry of CeCoIn 5 is most probably of d xy type.

Novel superconductivity in noncentrosymmetric heavy-fermion compound CePt 3Si: a 195Pt-NMR study

We report on a 195Pt-NMR study of the antiferromagnetic heavy-fermion (HF) superconductor CePt 3Si without an inversion center. The measurements of the nuclear spin-lattice relaxation rate 1 / T 1 has revealed a uniform coexistence of antiferromagnetism and superconductivity. CePt 3Si is the first HF superconductor that reveals a peak in 1 / T 1 just below T c and, additionally, does not follow the T 3 law that was observed for most unconventional HF superconductors. We remark that this unexpected superconducting characteristic may be related to the lack of an inversion center in its crystal structure.

Evidence for a novel state of superconductivity in noncentrosymmetric CePt3Si: a 195Pt-NMR study.

We report on novel antiferromagnetic (AFM) and superconducting (SC) properties of noncentrosymmetric CePt3Si through measurements of the 195Pt nuclear spin-lattice relaxation rate 1/T(1). In the normal state, the temperature (T) dependence of 1/T(1) unraveled the existence of low-lying levels in crystal-electric-field multiplets and the formation of a heavy-fermion (HF) state. The coexistence of AFM and SC phases that emerge at T(N)=2.2 K and T(c)=0.75 K, respectively, takes place on a microscopic level. CePt3Si is the first HF superconductor that reveals a peak in 1/T(1) just below T(c) and, additionally, does not follow the T3 law that used to be reported for most unconventional HF superconductors. We remark that this unexpected SC characteristic may be related to the lack of an inversion center in its crystal structure.

Tcfor non-s-wave pairing superconductors correlated with coherence length and effective mass

For unconventional heavy fermion superconductors, typified by ${\mathrm{UBe}}_{13},$ the superconducting transition temperatures ${T}_{c}$ are shown to correlate with a characteristic energy ${\ensuremath{\Elzxh}}^{2}{/(m}^{*}{\ensuremath{\xi}}^{2}{),m}^{*}$ being the effective mass, and $\ensuremath{\xi}$ the coherence length. For four of the six materials for which ${T}_{c},$ ${m}^{*},$ and $\ensuremath{\xi}$ are available, ${k}_{B}{T}_{c}\ensuremath{\sim}20{\ensuremath{\Elzxh}}^{2}{/(m}^{*}{\ensuremath{\xi}}^{2}).$ One heavy fermion material, ${\mathrm{UPd}}_{2}{\mathrm{Al}}_{3},$ reveals a tendency for the above linear behavior to saturate at substantially larger ${\ensuremath{\Elzxh}}^{2}{/(m}^{*}{\ensuremath{\xi}}^{2})$ than for ${\mathrm{UBe}}_{13}.$ The sixth material considered, ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2},$ falls between ${\mathrm{UBe}}_{13}$ and ${\mathrm{UPd}}_{2}{\mathrm{Al}}_{3}.$ To embrace d-wave pairing in cuprates, a $\mathrm{log}\ensuremath{-}\mathrm{log}$ plot reveals that ${k}_{\mathrm{B}}{T}_{c}\ensuremath{\sim}{\ensuremath{\Elzxh}}^{2}{/(m}^{*}{\ensuremath{\xi}}^{2}),$ but more materials for which ${m}^{*}$ and $\ensuremath{\xi}$ are measured will be required to substantiate the correlation in these high-${T}_{c}$ substances.

Josephson effects in unconventional heavy fermion superconductors

We discuss the Josephson effect for pairing states which break crystal symmetries in addition to gauge symmetry. We consider theE 1g andE 2u models for the low-temperature phase ofUPt 3, with order parameters Δ(E 1g )∼p z (p x +ip y ) and Δ(E 2u )∼p z (p x +ip y )2. We report calculations of Josephson critical currents, taking into account the effects of depairing at the interface. For singlet-triplet junctions the critical current is non-zero only for spin-orbit, spin-flip tunneling, and is found to be much smaller than the Ambegaokar-Baratoff value even when the spin-orbit tunneling amplitude is comparable to the spin-independent amplitude.