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.     

New Results for Superconductivity in κ-(BEDT-TTF)2Cu(NCS)2 When an Applied Magnetic Field is Aligned Parallel to the Conducting Planes

Using a Tunnel Diode Oscillator technique, we have measured the effect of a parallel magnetic field on the in-plane rf penetration depths in organic [α- (BEDT-TTF)₂NH₄Hg(SCN)₄ and κ-(BEDT-TTF)₂Cu(NCS)₂] and heavy fermion (CeCoIn₅) superconductors. We show that in this particular ge- ometry, the effects due to vortex activity are minimized. The penetration depth is then governed by the density of superconducting carriers. It is shown in many experiments including rf penetration depth measurement that α-(BEDT-TTF)₂NH₄Hg(SCN)₄ and CeCoIn₅ have s-wave and d-wave pairing symmetries, respectively. The pairing symmetry of κ-(BEDT-TTF)₂-Cu(NCS)₂, however, is still an unsolved matter, showing inconsistent results. In this paper, the penetration depth of κ-(BEDT-TTF)₂Cu(NCS)₂ is shown to be more similar to α-(BEDT-TTF)₂NH₄Hg(SCN)₄ than to CeCoIn₅, suggesting the pairing is nodeless.     

Local moment relaxation in heavy-fermion compounds

The Korringa relaxation rate for a local moment of an impurity in a heavy fermion compound is calculated using the model of Yoshimori and Kasai. Consistent with the recent ESR data for local moments in UBe₁₃, the relaxation rate is found to be unaffected by the heavy fermion renormalizations. This result can be traced to the single-site approximation and the weakk dependence of the conduction electron self-energy.     

Possible Fulde–Ferrel–Larkin–Ovchinnikov Inhomogeneous Superconducting State in CeCoIn5: Cd- and Hg-doping Studies

CeCoIn₅ undergoes a phase transition within the superconducting (SC) state. A new superconducting state occupies a high field–low temperature (HFLT) corner of the SC state in the H–T plane, and is a strong candidate for realization of the spatially inhomogeneous Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) superconducting state predicted theoretically in 1960s. We present specific heat data on Cd- and Hg-doped CeCoIn₅ that shows that the HFLT phase is suppressed by minute amount of impurities, few hundred parts per million. At this concentration, the average distance between impurities is close to twice the superconducting coherence length ξ, suggesting that ξ is the characteristic length of the HFLT state. We conclude that our investigations support the superconducting, posibly FFLO, origin of the HFLT state.      

Nuclear relaxation in the superconducting state of (MDT-TTF)2AuI2

We report the results of our attempt to measure the proton nuclear relaxation rate, 1/T ₁, in the superconducting state of the title material. The relaxation rate in the superconducting state at a field of 1 T was found much longer than that in the normal state, but it became clear that the dominant contribution came from the normal core region. The nuclear relaxation at zero field was examined by using the field cycling technique. An ln(t) term in the relaxation curve was observed at low temperatures, suggesting the contribution of the creeping motion of vortices. We discuss the possibility to determine the intrinsic temperature dependence of 1/T ₁ in the superconducting state.     

Feedback Spin Exciton Formation in Unconventional Superconductors

The superconducting feedback resonance in inelastic neutron scattering (INS) has now been found in numerous unconventional superconductors of the cuprate, ferropnictide, and heavy fermion classes. The collective spin excitation appears below T _c at an energy less than the quasiparticle threshold with momentum Q provided the gap changes sign under translation by Q. The resonance has been found in the heavy fermion (HF) superconductors CeCu₂Si₂, CeCoIn₅, and UPd₂Al₃, and recently in Fe-pnictide Ba_1−x K _x Fe₂As₂, BaFe_2−x Co _x As₂, BaFe_2−x Ni _x As₂, and FeSe_1−x Te _x compounds and may be a more general phenomenon. Of particular interest is the interaction of the 3d spin exciton with the 4f crystalline electric field (CEF) excitations in rare earth based unconventional superconductors like CeFeAsO_1−x F _x pnictide and Nd_2−x Ce _x CuO₄ cuprate where a coupling between 3d spin resonance and 4f CEF excitations leads to intriguing interaction effects observed experimentally by INS.     

Tuning Heavy Fermion Systems into Quantum Criticality by Magnetic Field

We discuss a series of thermodynamic, magnetic, and electrical transport experiments on the two heavy fermion compounds CeNi₂Ge₂ and YbRh₂Si₂ in which magnetic fields, B, are used to tune the systems from a non-Fermi liquid (NFL) into a field-induced FL state. Upon approaching the quantum-critical points from the FL side by reducing B we analyze the heavy quasiparticle (QP) mass and QP-QP scattering cross sections. For CeNi₂Ge₂ the observed behavior agrees well with the predictions of the spin-density wave (SDW) scenario for three-dimensional (3D) critical spin-fluctuations. By contrast, the observed singularity in YbRh₂Si₂ cannot be explained by the itinerant SDW theory for neither 3D nor 2D critical spinfluctuations. Furthermore, we investigate the magnetization M(B) at high magnetic fields. For CeNi₂Ge₂ a metamagnetic transition is observed at 43 T, whereas for YbRh₂Si₂ a kink-like anomaly occurs at 10 T in M vs B (applied along the easy basal plane) above which the heavy fermion state is completely suppressed.     

Controlled Creation of Structural Defects in the Heavy Fermion Superconductor UPt3 and Its Influence on the Superconducting Properties

The superconducting properties of the heavy fermion UPt ₃ have been changed by irradiation with high energy electrons which creates point defects in a reproducible and controled way. Measurements of the residual resistivity, critical temperature, upper critical field and thermal conductivity have been realized on these irradiated samples. The strong suppresion of superconductivity with increasing defect concentration is in agreement with the theory of unconventional superconductivity. However, our thermal conductivity data contradicts the simple predictions derived from the most popular modelsE _1g and E _2u) of the superconducting order parameter in UPt ₃ .     

Evidence for polarons and itinerant carriers in superconductivity from time-resolved optical measurements on YBa2Cu3O7-△

Time-resoived optical relaxation measurements on time scales from 10^-13 to 10^-8 s as a function of energy (0.8–3 eV) and temperature give evidence for the existence of two different carrier relaxation processes, indicating the presence of two distinct electronic subsystems in the normal state in high-temperature superconductors, one exhibiting band-like relaxation properties and the other those of localized, polaronic states. Both components of the electrodynamic response exhibit significant changes atT _c , indicating that both carrier types may be involved in superconductivity. The experiments also show that both polarons and band-like carriers exist predominantly in the CuO planes, with an out-of-plane component of the order of 10–20% which is close to the experimentally observed occupancy ratio of holes of O ions of the CuO5 pyramids. Spectrally, the superconducting condensate is also found to be coupled to two distinct components in frequency (for both carriers): one below 0.1 eV and one in the range 1.5-2 eV, but with different anisotropies. A natural explanation of these datas is that the two types of carriers correspond to the two phases suggested by the structural data and as such present the first direct evidence of two-component superconductivity. The underdoped sample data can be interpreted as evidence of pre-formed pairs aboveT _c .     

Effect of Cd Addition on Superconducting Fluctuations and Mechanical Properties of Bi1.82Pb0.36Sr2Ca2Cd x Cu3O y System

A study was made of the effect of Cd additions on the superconducting and mechanical properties of Bi_1.82Pb_0.36Sr₂Ca₂ Cd_xCu₃O_y (x = 0.0, 0.15, 0.25, 0.35 and 0.55). Characterization of the Cd-samples using XRD, DTA, and SEM techniques, has confirmed that remarkably formation of low-T _c phase (2212) by the addition of Cd up to 0.35. High-resolution electrical resistivity ρ (T) data on the composition of Cd = 0.35 have been taken for investigating critically the superconducting fluctuations. Using the Aslamazov and Larkin (AL) and Lawrence and Doniach (LD) models of excess conductivity. Excess conductivity analysis shows that this composition (Cd = 0.35) is 2D in the temperature range 137.8–163.7 K and a 3D one below 137.8 K. Thus, a crossover from 2D to 3D is observed at 137.8 K. Sample microhardness and density are greatly improved by Cd-additions (0.35). This trend is probably due to the intercalation of cadmium between superconducting grains in compositions may provide a plastic-flow region that allows relaxation of undesirable stresses resulting from the grain anisotropy of superconductors.     

Nodal Structure of Unconventional Superconductors Determined by Thermal Conductivity

To identify the gap structure of unconventional superconductors, spin-triplet Sr₂RuO₄, heavy fermion CeCoIn₅, organic -(BEDT-TTF)₂Cu(NCS)₂ and borocarbide YNi₂B₂C, we measured the thermal conductivity in magnetic field rotated within the 2D plane with very high precision. On the basis of these results, we show that the gap functions of Sr₂RuO₄, CeCoIn₅ and -(BEDT-TTF)₂Cu(NCS)₂ are most likely to be d(k)=₀ ^z(k _x+ik _y)(cos ck _z+), d_x ^{2-y 2} and d _xy, respectively. We also demonstrate the presence of point nodes along the a- and b-axes in YNi₂B₂C.     

Voltage Relaxation and Its Influence on Critical Current Measurements

Based on the collective creep model, we numerically studied evolution of electric field and current density in superconductors and its influence on transport measurements of critical current. It is shown that many experimental facts, such as the dependence of V-I curves on sweeping rate of applied current and voltage relaxation are the results of this evolution. The simulation results are confirmed by electric transport measurements on Ag-sheathed Bi_2–x Pb _x Sr₂Ca₂Cu₃O _y tapes. Discussions on influences of the voltage relaxation on electric transport measurements including superconducting critical current are made.     

Magnetism and superconductivity in heavy fermion systems

We discuss some consequences of the interplay between magnetism and superconductivity in the two heavy fermion systems URu₂Si₂ and UPt₃, notably on the temperature dependence of the specific heat, on possible observation of Larkin-OvchinnikovFulde-Ferrel phase, and on the anisotropy of the upper critical field. We demonstrate that in UPt₃, a clear double steep superconducting transition can be obtained reversibly.     

63,65Cu NQR Evidence for Low-Temperature Charge Ordering in the Superconducting State of YBa2Cu3O7?δ

Measurements of the nuclear quadrupole resonance (NQR) transverse relaxation rate and of the NQR spectral linewidths of ^63,65Cu in the superconducting state of YBa₂Cu₃O_7– reveal unusual features below T = 35 K. A narrow peak in the transverse relaxation rate and an increased quadrupolar line broadening with decreasing temperature are attributed to a charge density wave (CDW) ordered state below T = 35 K.     

Pseudogap and Superconducting Energy Gap in Single Crystals of URu2Si2 by Point Contact Spectroscopy

We performed point contact spectroscopy measurements in the superconducting and normal state of a single crystal of the heavy fermion URu₂Si₂. The differential resistance as a function of bias voltage shows the features of the superconducting energy gap up to 1.37 K, above T _C we observe another feature that we identify as a pseudogap that persists up to 2 K. The superconducting gap does not fit a BCS behavior.     

Microwave measurements on the heavy fermion superconductors UPt3 and UBe13

The complex microwave impedance at 11.5 GHz was measured in the superconducting state of two well characterized heavy fermion single crystals, UPt ₃ and UBe ₁₃ . This microwave photon energy is about 30% and 20% of the zero-temperature energy gap, respectively, and is higher than previous measurements. The results for UBe ₁₃ seem to be well described by the Mattis-Bardeen theory in the dirty-limit for a conventional superconductor. However, the behavior in UPt ₃ deviates from this conventional theory even when paramagnetic impurity scattering effects are taken into account.     

Surface Superconducting State of Heavy-Fermion Superconductor CeIrIn5 Probed by CeIrIn5-Ag Junctions

The CeIrIn₅-Ag junctions of about 2×10^–9 cm^–2 area have been made using microfabrication techniques, and the surface superconducting state of CeIrIn₅, which has two characteristic temperatures T ₀ and T _c, has been investigated, where T ₀ and T _c are the transition temperature to zero-resistivity state and the bulk, thermodynamic transition temperature, respectively. The temperature, below which superconducting anomalies are observed, varies from junction to junction, and yet it is always well above T _c=0.4 K. This result, together with no indication of transition at T _c, suggests that at least the surface of CeIrIn₅ is in the superconducting state above T _c. The data on the critical current I _c in superconducting anomalies point to the possibility to define a local transition temperature for each junction.     

Ultrasonic studies of the heavy fermion compounds

A review of ultrasonic measurements on the three heavy fermion compounds UPt ₃ , URu ₂ Si ₂ and CeCu ₆ is given. These compounds show one or more of the three types of co-operative behavior; Kondo-like effect, antiferromagnetism and superconductivity; all these features are observed as ultrasonic anomalies. Both UPt ₃ and CeCu ₆ show a strong coherence effect which is akin to a Kondo-like transition in the electronic system. Velocity anomalies are found at the antiferromagnetic transition for URu ₂ Si ₂ . UPt ₃ also has an antiferromagnetic transition which is believed to influence the subsequent superconducting ordering. For UPt ₃ , the superconductivity is of an unconventional nature; multiple superconducting phases have been observed and a complete phase diagram has been obtained from high resolution velocity measurements.     

Unconventional Superconductivity and Heavy Fermion Behavior in PrOs4Sb12

Research carried out during the last several years on the heavy fermion superconductor PrOs₄Sb₁₂ is reviewed. The compound PrOs₄Sb₁₂ exhibits unconventional strong coupling superconductivity below a superconducting critical temperature T _c = 1.85 K. The superconducting state breaks time reversal symmetry, apparently consists of several distinct superconducting phases, some of which may have point nodes in the energy gap, and may involve triplet spin pairing of electrons. The determination of the Pr³⁺ energy level scheme in the crystalline electric field, as well as the discovery and characterization of the high field-ordered phase (HFOP), which has been identified with antiferroquadrupolar order, are described. Studies of the substitutional systems Pr(Os_1−x Ru _x )₄Sb₁₂, Pr_1−x La _x Os₄Sb₁₂, and Pr_1−x Nd _x Os₄Sb₁₂ are summarized and compared. PACS 71.27.+a; 74.62.Dh; 74.70.Tx.     

Debye temperatures of uranium chalcogenides from their lattice dynamics

Phonon dispersion relations in uranium chalcogenides have been investigated using a modified three-body force shell model. From the phonon frequencies, their Debye temperatures are evaluated. Further, on the basis of the spin fluctuation in the heavy fermion uranium compounds, UPt₃ and UBe₁₃, the possible superconducting transition temperatures of chalcogenides are theoretically predicted. The Tc values are in the same range as of those in UPt₃ and UBe₁₃.