Superfluid response in heavy fermion superconductors

Motivated by a recent London penetration depth measurement [H. Kim, et al., Phys. Rev. Lett. 114, 027003 (2015)] and novel composite pairing scenario [O. Erten, R. Flint, and P. Coleman, Phys. Rev. Lett. 114, 027002 (2015)] of the Yb-doped heavy fermion superconductor CeCoIn₅, we revisit the issue of superfluid response in the microscopic heavy fermion lattice model. However, from the literature, an explicit expression for the superfluid response function in heavy fermion superconductors is rare. In this paper, we investigate the superfluid density response function in the celebrated Kondo–Heisenberg model. To be specific, we derive the corresponding formalism from an effective fermionic large-N mean-field pairing Hamiltonian whose pairing interaction is assumed to originate from the effective local antiferromagnetic exchange interaction. Interestingly, we find that the physically correct, temperature-dependent superfluid density formula can only be obtained if the external electromagnetic field is directly coupled to the heavy fermion quasi-particle rather than the bare conduction electron or local moment. Such a unique feature emphasizes the key role of the Kondo-screening-renormalized heavy quasi-particle for low-temperature/energy thermodynamics and transport behaviors. As an important application, the theoretical result is compared to an experimental measurement in heavy fermion superconductors CeCoIn5 and Yb-doped Ce_1?xYb_xCoIn₅ with fairly good agreement and the transition of the pairing symmetry in the latter material is explained as a simple doping effect. In addition, the requisite formalism for the commonly encountered nonmagnetic impurity and non-local electrodynamic effect are developed. Inspired by the success in explaining classic 115-series heavy fermion superconductors, we expect the present theory will be applied to understand other heavy fermion superconductors such as CeCu₂Si₂ and more generic multi-band superconductors.     

Upper critical field of heavy fermion superconductors

The upper critical field in a model with dispersion in the f-band of a periodic Anderson model has been calculated.     

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.     

London field penetration in heavy fermion superconductors

We present results of a series of high resolution, low fielddc-magnetization measurements on the heavy fermion superconductors UPt₃, UBe₁₃, U_1–x Th _X Be₁₃ and CeCu₂Si₂, from which values of the magnetic penetration depth can be extracted. A study of the temperature variation reveals aT ² power law in all cases. This can not be reconciled with a BCS-like isotropic energy gap but may be explained by the presence of low energy quasiparticle states inside the gap. In the case of very pure superconductors, one such possibility is the assumption of point-nodes in the gap function. We argue, however, that an interpretation in terms of resonant impurity scattering in various anisotropic superconducting states is more likely to explain a broad range of experimental data. The results on differently oriented single crystals of UBe₁₃ and UPt₃ reveal no pronounced anisotropy related to the crystal lattice. Absolute values of atT=0 are deduced by a novel method in which the field is first screened out from the sample by means of an evaporated superconducting film (of lowerT _c ). Above this lowerT _c the sudden penetration of field into the sample can then be measured absolutely.     

Nuclear magnetic resonance study of strongly correlated superconductors

This paper introduces nuclear magnetic resonance works in the strongly correlated super-conductors: heavy-Fermion, high-T _C superconductors and Sr₂RuO₄. The analyses strongly support the spin-fluctuation-mediated superconductivity model in a high-T _C superconductor.     

Upper critical field in the heavy fermion superconductors

The upper critical field H_c2 (T) and its temperature derivative at T_c in the heavy fermion superconductors have been calculated by using the simplified Kondo lattice model.     

Nuclear quadrupole resonance of norephedrine

Toward searching for illegal drugs, we investigated the pulsed nuclear quadrupole resonance (NQR) response of ¹⁴N in (1R,2S)-(-)-norephedrine, based on the predictions of quantum chemical calculations. Two pairs of spectral lines (ν₊=3.089, 3.093 MHz and ν₋=2.594, 2.608 MHz) were observed despite its molecule structure having only a single nitrogen atom. This indicates that the molecular crystal has two nonequivalent nitrogen atoms in the unit cell. The temperature dependence of the NQR frequencies and relaxation properties were investigated for the purpose of accurate remote sensing of the drugs. The NQR frequency shift was approximately 0.23 kHz/K around room temperature. The spin-lattice relaxation and spin-phase memory times were 5.2–10.2 ms and 0.6–1.5 ms, respectively.     

Observability of order parameter collective modes in heavy fermion superconductors

The effect of impurity scattering on the dynamics of the order parameter ofa “bnon-s-wave” anisotropic superconducting state is considered. It is found that near T_c and for realistic scattering rates the dynamics of all non-Goldstone order parameter components is purely relaxational. At sufficient low temperatures pair vibration modes with frequency ω∼δ are found to be weakly damped even though ωτ may be much less than unity.     

Nuclear quadrupole resonance in arsenic metal

The nuclear quadrupole resonance in arsenic metal, in the temperature range 4.2 K – 396 K, has been studied. e²qQ/h at 4.2 K is 47.123 MHz. It is in good agreement with the one predicted by the low temperature specific heat measurements.     

Nuclear quadrupole resonance echoes from hexamethylenetetramine

We investigated the echo phenomenon of nuclear quadrupole resonance (NQR) from hexamethylenetetramine (HMT). We detected the pure NQR echo signal of HMT with a short pulse interval. The intensity of the echo signal increased as the pulse interval time was decreased. We observed that a clean echo signal was generated even when the pulse interval was shorter than the decay time constant T₂^*. Since the short interval time gives a strong echo, our result insists that shorter interval time is preferred for the NQR detection.     

Nuclear quadrupole resonance in C-nitro compounds

The nitrogen-14 nuclear quadrupole resonance spectra of CH₃NO₂ and of C₆H₅NO₂ were measured at 77°K. The results are: CH₃NO₂: e²qQ/h = 1694.6 kHz, η = 0.4240 and C₆H₅NO₂: e²qQ/h = 1424.9 kHz, η = 0.4039.     

重费米子超导

重费米子材料作为典型的强关联电子体系,具有丰富的物理内涵。重费米子超导也因复杂的电子间相互作用而具有多种不同的超导配对机理。文章以几个典型的重费米子超导体(CeCu₂Si₂、CeMIn₅、UTe₂)为例,介绍其基本物理性质,其中重点讨论超导与反铁磁、铁磁之间密不可分的竞争/共存关系。另外,文章还讨论空间反演中心/时间反演对称性破缺对重费米子超导体序参量的影响。最后,简单介绍了几种比较特殊的竞争序诱导的重费米子超导态。     

Nuclear acoustic quadrupole resonance in CsBr

Nuclear acoustic resonance is used to measure the deformation in single crystal of CsBr. The measured relative deformation is estimated to be 0.1 per cent and the ratio of the bromine and cesium anti-shielding factors is 1.6.     

Nuclear quadrupole resonance echoes from hexamethylenetetramine

We investigated the echo phenomenon of nuclear quadrupole resonance (NQR) from hexamethylenetetramine (HMT). We detected the pure NQR echo signal of HMT with a short pulse interval. The intensity of the echo signal increased as the pulse interval time was decreased. We observed that a clean echo signal was generated even when the pulse interval was shorter than the decay time constant T₂^*. Since the short interval time gives a strong echo, our result insists that shorter interval time is preferred for the NQR detection.     

238U Mössbauer study on the magnetic properties of uranium-based heavy fermion superconductors

We have performed ²³⁸U Mössbauer spectroscopy of uranium-based heavy fermion superconductors, UPd₂Al₃ and URu₂Si₂, in order to investigate their physical properties, mainly their magnetic properties. The slow relaxation of magnetic hyperfine interaction in a paramagnetic state and the static hyperfine field has been observed in an antiferromagnetic ordered state for each compound. The line-widths have maximum at their characteristic temperatures where their magnetic susceptibilities have maximum values.     

Nuclear quadrupole resonance studies of transparent conducting oxides

We report ^63,65Cu spin–lattice relaxation rates measured by nuclear quadrupole resonance (NQR) in the delafossite compound CuYO₂ and CuYO₂:Ca over a temperature range from 200 to 450 K. CuYO₂:Ca is a prototype transparent oxide exhibiting p-type electrical conductivity. Relaxation rates in CuYO₂:Ca are enhanced by one to two orders of magnitude relative to undoped material, exhibit much stronger temperature dependence, and contain contributions from magnetic and quadrupolar relaxation mechanisms with roughly equal strengths. Relaxation in undoped CuYO₂ is of purely quadrupolar origin and is attributed to interactions with lattice phonons. The main focus of this paper is the magnetic contribution to the relaxation rate in CuYO₂:Ca which is attributed to the hyperfine fields of carriers. It is argued that the dynamics of the hyperfine field are dominated by the hopping rate for carrier transfers between neighboring atoms in the copper planes of the delafossite structure. Comparison of the magnetic relaxation rates with the DC conductivity permits an estimate of the carrier concentration and mobility.     

Nuclear quadrupole resonance of bromine in some tribromides

The NQR of bromine in some tribromides was observed at various temperatures. Ammonium, rubidium, and cesium tribromides yielded three widely separated frequencies, indicating that the Br₃- ion has a considerably asymmetric structure in crystals. These resonance frequencies were observed for tetra-n-propylammonium tribromide at liquid N₂ temperature, indicating the presence of nearly symmetric Br₃- ions in crystals. Tetra-n-butylammonium tribromide showed two frequencies attributable to the central and terminal bromine atoms of a symmetric Br₃- ion. From the observed frequencies, charge distribution in Br₃- was evaluated. The lowest-frequency line of ammonium tribromide showed an extraordinarily large positive temperature coefficient of resonance frequencies. This anomaly was interpreted in terms of the effect of neighboring cations and the formation of weak hydrogen bonds.     

Antiferromagnetism in metals: from the cuprate superconductors to the heavy fermion materials

The critical theory of the onset of antiferromagnetism in metals, with concomitant Fermi surface reconstruction, has recently been shown to be strongly coupled in two spatial dimensions. The onset of unconventional superconductivity near this critical point is reviewed: it involves a subtle interplay between the breakdown of fermionic quasiparticle excitations on the Fermi surface and the strong pairing glue provided by the antiferromagnetic fluctuations. The net result is a logarithm-squared enhancement of the pairing vertex for generic Fermi surfaces, with a universal dimensionless coefficient independent of the strength of interactions, which is expected to lead to superconductivity at the scale of the Fermi energy. We also discuss the possibility that the antiferromagnetic critical point can be replaced by an intermediate 'fractionalized Fermi liquid' phase, in which there is Fermi surface reconstruction but no long-range antiferromagnetic order. We discuss the relevance of this phase to the underdoped cuprates and the heavy fermion materials.