Unique spin dynamics and unconventional superconductivity in the layered heavy fermion compound CeIrIn5: NQR evidence

Study on superconducting properties of CeIrIn5thin films grown via pulsed laser deposition

We report the growth of CeIrIn₅thin films with different crystal orientations on a MgF₂(001) substrate using pulsed laser deposition technique. X-ray diffraction analysis showed that the thin films were either mainlya-axis-oriented (TF1) or a combination ofa- andc-axis-oriented (TF2). The characteristic features of heavy-fermion superconductors, i.e. Kondo coherence and superconductivity, were clearly observed, where the superconducting transition temperature (T_c) and Kondo coherence temperature (T_coh) are 0.58 K and 41 K for TF1 and 0.52 K and 37 K for TF2, respectively. The temperature dependencies of the upper critical field (H_c2) of both thin films and the CeIrIn₅single crystal revealed a scaling behavior, indicating that the nature of unconventional superconductivity has not been changed in the thin film. The successful synthesis of CeIrIn₅thin films is expected to open a new avenue for novel quantum phases that may have been difficult to explore in the bulk crystalline samples.

Single-crystal growth and magnetic anisotropy in PrFe2Ga8

Single crystals of PrFe₂Ga₈were successfully grown by using Ga self-flux. PrFe₂Ga₈crystallizes in the CaCo₂Al₈-type orthorhombic structure with the space groupPbam(no. 55). By combining the results from the magnetic-susceptibility, specific-heat, and resistivity measurements, we show that PrFe₂Ga₈exhibits a magnetic order at 14 K. ForH//c, the antiferromagnetic order can be suppressed by magnetic fields. However, the magnetic order is robust against magnetic fields forH⊥c. Our results provide basic physical properties of PrFe₂Ga₈and will help to further understand the magnetism in this system.

Foundations of heavy-fermion superconductivity: lattice Kondo effect and Mott physics

This article overviews the development of heavy-fermion superconductivity, notably in such rare-earth-based intermetallic compounds which behave as Kondo-lattice systems. Heavy-fermion superconductivity is of unconventional nature in the sense that it is not mediated by electron-phonon coupling. Rather, in most cases the attractive interaction between charge carriers is apparently magnetic in origin. Fluctuations associated with an antiferromagnetic (AF) quantum critical point (QCP) play a major role. The first heavy-fermion superconductor CeCu2Si2 turned out to be the prototype of a larger group of materials for which the underlying, often pressure-induced, AF QCP is likely to be of a three-dimensional (3D) spin-density-wave (SDW) variety. For UBe13, the second heavy-fermion superconductor, a magnetic-field-induced 3D SDW QCP inside the superconducting phase can be conjectured. Such a 'conventional', itinerant QCP can be well understood within Landau's paradigm of order-parameter fluctuations. In contrast, the low-temperature normal-state properties of a few heavy-fermion superconductors are at odds with the Landau framework. They are characterized by an 'unconventional', local QCP which may be considered a zero-temperature 4 f-orbital selective Mott transition. Here, as concluded for YbRh2Si2, the breakdown of the Kondo effect concurring with the AF instability gives rise to an abrupt change of the Fermi surface. Very recently, superconductivity was discovered for this compound at ultra-low temperatures. Therefore, YbRh2Si2 along with CeRhIn5 under pressure provide a natural link between the large group of about fifty low-temperature heavy-fermion superconductors and other families of unconventional superconductors with substantially higher T c, e.g. the doped Mott insulators of the perovskite-type cuprates and the organic charge-transfer salts.

From Kondo lattices to Kondo superlattices

The realization of new classes of ground states in strongly correlated electron systems continues to be a major issue in condensed matter physics. Heavy fermion materials, whose electronic structure is essentially three-dimensional, are one of the most suitable systems for obtaining novel electronic states because of their intriguing properties associated with many-body effects. Recently, a state-of-the-art molecular beam epitaxy technique was developed to reduce the dimensionality of heavy electron systems by fabricating artificial superlattices that include heavy fermion compounds; this approach can produce a new type of electronic state in two-dimensional (2D) heavy fermion systems. In artificial superlattices of the antiferromagnetic heavy fermion compound CeIn3 and the conventional metal LaIn3, the magnetic order is suppressed by a reduction in the thickness of the CeIn3 layers. In addition, the 2D confinement of heavy fermions leads to enhancement of the effective electron mass and deviation from the standard Fermi liquid electronic properties, which are both associated with the dimensional tuning of quantum criticality. In the superconducting superlattices of the heavy fermion superconductor CeCoIn5 and nonmagnetic metal YbCoIn5, signatures of superconductivity are observed even at the thickness of one unit-cell layer of CeCoIn5. The most remarkable feature of this 2D heavy fermion superconductor is that the thickness reduction of the CeCoIn5 layers changes the temperature and angular dependencies of the upper critical field significantly. This result is attributed to a substantial suppression of the Pauli pair-breaking effect through the local inversion symmetry breaking at the interfaces of CeCoIn5 block layers. The importance of the inversion symmetry breaking in this system has also been supported by site-selective nuclear magnetic resonance spectroscopy, which can resolve spectroscopic information from each layer separately, even within the same CeCoIn5 block layer. In addition, recent experiments involving CeCoIn5/YbCoIn5 superlattices have shown that the degree of the inversion symmetry breaking and, in turn, the Rashba splitting are controllable, offering the prospect of achieving even more fascinating superconducting states. Thus, these Kondo superlattices pave the way for the exploration of unconventional metallic and superconducting states.

Infrared properties of heavy fermions: evolution from weak to strong hybridizations

In this article, we review the charge excitations of heavy fermion compounds probed by infrared spectroscopy. The article is not meant to be a comprehensive survey of experimental investigations. Rather it focuses on the dependence of charge excitations on the hybridization strength. In this context, the infrared properties of the Ce m M n In3m+2n family are discussed in detail since the hybridization strengths differ dramatically in different members despite their similar lattice structures. Investigations on some mixed valent compounds are also presented, aiming to elucidate the generic trend of the evolution. In particular, we address the scaling between hybridization energy gap [Formula: see text] and hybridization strength [Formula: see text]([Formula: see text]) in a wide range of heavy fermion compounds, which demonstrates that the periodic Anderson model can generally and quantitatively describe the low-energy charge excitations.

Nuclear magnetic resonance in Kondo lattice systems

Nuclear magnetic resonance has emerged as a vital tool to explore the fundamental physics of Kondo lattice systems. Because nuclear spins experience two different hyperfine couplings to the itinerant conduction electrons and to the local f moments, the Knight shift can probe multiple types of spin correlations that are not accessible via other techniques. The Knight shift provides direct information about the onset of heavy electron coherence and the emergence of the heavy electron fluid.

Ferromagnetic Spin Fluctuation and Unconventional Superconductivity in Rb2Cr3As3 Revealed by 75As NMR and NQR

We report (75)As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies on the superconductor Rb(2)Cr(3)As(3) with a quasi-one-dimensional crystal structure. Below T∼100  K, the spin-lattice relaxation rate (1/T(1)) divided by temperature, 1/T(1)T, increases upon cooling down to T(c)=4.8  K, showing a Curie-Weiss-like temperature dependence. The Knight shift also increases with decreasing temperature. These results suggest ferromagnetic spin fluctuation. In the superconducting state, 1/T(1) decreases rapidly below T(c) without a Hebel-Slichter peak, and follows a T(5) variation below T∼3  K, which points to unconventional superconductivity with point nodes in the gap function.

Thermal conductivity evidence for a dx2-y2 pairing symmetry in the heavy-fermion CeIrIn5 superconductor

The phase diagram of the quasi-2D Ce(Ir,Rh)In5 system contains two distinct superconducting domes. By the thermal transport measurements in rotating magnetic fields H, we pinned down the superconducting gap structure of CeIrIn5 in the second dome, located distant from the first dome in proximity to an antiferromagnetic quantum critical point. Clear fourfold oscillation was observed when H is rotated within the ab plane, while no oscillation was observed within the bc plane. In sharp contrast to previous reports, our results are most consistent with dx2-y2 symmetry, implying that the superconductivity in the second phase is also mediated by antiferromagnetic spin fluctuations.

Precursor State to Unconventional Superconductivity in CeIrIn5

We present Hall effect and magnetoresistance measurements in the heavy fermion superconductor CeIrIn(5). At low temperature, a Kondo coherent state is established. Deviations from Kohler's rule and a quadratic temperature dependence of the cotangent of the Hall angle are reminiscent of properties observed in the high-temperature superconducting cuprates. A striking observation pertains to the presence of a precursor state--characterized by a change in the Hall mobility--that precedes the superconductivity in this material, in similarity to the pseudogap in the cuprate superconductors.

Electronic structure of Ce(n)M(m)In(2m+3n), where n = 1, 2; m = 0, 1;M = Co, Rh or Ir: experiment and calculations

We present a detailed study of the electronic structure of the Ce(n)M(m)In(2m+3n) (M = Co, Rh, Ir; n = 1, 2 and m = 0, 1) series of Ce intermetallic compounds and of the reference LaIn(3) compound. The ground state of these heavy-fermion (HF) materials can be tuned between antiferromagnetic (AF) and superconducting (SC), with pressure or doping as the tuning parameter. Performing the x-ray photoelectron spectroscopy (XPS) measurements on the Ce 3d core levels as well as on the valence band states we analyse the dependence of both the Ce 4f band character and physical properties on the kind of transition metal atom M and on the number of CeIn(3) layers intervened by the MIn(2) layers in the investigated family of compounds. We draw a parallel between the XPS valence band spectra and ab initio band structure calculations based on the full-potential linearized augmented plane-wave (FP-LAPW) method. We analyse changes in valence band states within the whole family of materials. We compare the experimental magnetic moments on Ce atoms with the theoretical ones calculated within different approximations for exchange-correlation potential. Finally, we have shown that the Ce 4f electrons participate in bonding formation for all investigated compounds. Our study indicates that the observed changes in the 4f band on-site hybridization energy result from the reconstruction of the charge density distribution driven by transition metal atoms inserted into the CeIn(3) structure.

Hybrid gap structure of the heavy-fermion superconductor CeIrIn5

The thermal conductivity kappa of the heavy-fermion superconductor CeIrIn5 was measured as a function of temperature down to T(c)/8, for current directions parallel (J parallel c) and perpendicular (J parallel a) to the tetragonal c axis. For J parallel a, a sizable residual linear term kappa(0)/T is observed, as previously, which confirms the presence of line nodes in the superconducting gap. For J parallel c, on the other hand, kappa/T-->0 as T-->0. The resulting precipitous decline in the anisotropy ratio kappa(c)/kappa(a) at low temperature rules out a gap structure with line nodes running along the c axis, such as the d-wave state favored for CeCoIn5, and instead points to a hybrid gap of E(g) symmetry.

Theoretical study on superconductivity in CeMIn(5) (M = Co, Rh, Ir): pairing symmetry and pressure dependence

We study theoretically heavy fermion superconductors CeMIn(5) (M = Co, Rh, Ir). CeCoIn(5) and CeIrIn(5) that become superconducting at ambient pressure with T(c) = 2.3 K and 0.4 K, respectively. On the other hand, CeRhIn(5) is an antiferromagnet at ambient pressure and becomes superconducting under pressures greater than 1.6 GPa. With regards to the superconductivity, the existence of line nodes is indicated by nuclear-quadrupole-resonance (NQR), thermal conductivity, specific heat and electrical resistivity measurements. However, the pairing symmetry between d(x(2)-y(2)) and d(xy) is controversial. Therefore, we investigate the gap structure of CeMIn(5) by a detailed calculation. We introduce a three-dimensional periodic Anderson model (3D-PAM) in order to reproduce the band characteristics of CeMIn(5). Thus, we identify the gap structure of CeMIn(5) as the d(x(2)-y(2)) symmetry by solving the Èliashberg equations. In addition, we discuss the pressure dependence of T(c) and show that two factors determine T(c). One factor is the momentum dependence of quasi-particle interaction and the other factor is the wavefunction renormalization factor. Thus, we have explained the superconductivity in CeMIn(5) using the Fermi liquid theory.

Spin Triplet Superconducting State due to Broken Inversion Symmetry in Li(2)Pt(3)B

We report (11)B and (195)Pt NMR measurements in noncentrosymmetric superconductor Li(2)Pt(3)B. We find that the spin susceptibility measured by the Knight shift remains unchanged across the superconducting transition temperature T(c). With decreasing temperature (T) below T(c), the spin-lattice relaxation rate 1/T(1) decreases with no coherence peak and is in proportion to T3. These results indicate that the Cooper pair is in the spin-triplet state and that there exist line nodes in the superconducting gap function. They are in sharp contrast to those in the isostructural Li(2)Pd(3)B which is a spin-singlet, s-wave superconductor, and are ascribed to the enhanced spin-orbit coupling due to the lack of spatial inversion symmetry. Our finding points to a new paradigm where exotic superconductivity arises in the absence of electron-electron correlations.

Enhancing the superconducting transition temperature of CeRh 1-x IrxIn5 due to the strong-coupling effects of antiferromagnetic spin fluctuations: an 115In nuclear quadrupole resonance study

We report on systematic evolutions of antiferromagnetic (AFM) spin fluctuations and unconventional superconductivity (SC) in heavy-fermion (HF) compounds CeRh(1-x)Ir(x)In(5) via an (115)In nuclear-quadrupole-resonance experiment. The nuclear spin-lattice relaxation rate 1/T(1) has revealed the marked development of AFM spin fluctuations as approaching an AFM ordered state. Concomitantly, the superconducting transition temperature T(c) and the energy gap Delta0 increase drastically from T(c)= 0.4K and 2Delta0/k(B)T(c)=5 in CeIrIn(5) up to T(c) =1.2K and 2Delta0/k(B)T(c) =8.3 in CeRh(0.3)Ir(0.7)In5 , respectively. The present work suggests that the AFM spin fluctuations in close proximity to the AFM quantum critical point are indeed responsible for the strong-coupling unconventional SC in HF compounds.

Enhancing the superconducting transition temperature of the heavy fermion compound CeIrIn5 in the absence of spin correlations

We report on a pressure- (P-)induced evolution of superconductivity and spin correlations in CeIrIn(5) via the (115)In nuclear-spin-lattice-relaxation rate measurements. We find that applying pressure suppresses dramatically the antiferromagnetic fluctuations that are strong at ambient pressure. At P = 2.1 GPa, T(c) increases to T(c) = 0.8 K, which is twice T(c) (P = 0 GPa), in the background of Fermi-liquid state. This is in sharp contrast to the previous case in which a negative, chemical pressure (replacing Ir with Rh) enhances magnetic interaction and increases T(c). Our results suggest that multiple mechanisms work to produce superconductivity in the same compound CeIrIn(5).

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.

Unconventional superconductivity and electron correlations in the cobalt oxyhydrate Na0.35CoO2.yH2O from nuclear quadrupole resonance

We report a careful 59Co nuclear quadrupolar resonance measurement on the recently discovered cobalt oxyhydrate Na0.35CoO2.yH(2)O superconductor from T=40 K down to 0.2 K. We find that in the normal state the spin-lattice relaxation rate 1/T(1) follows a Curie-Weiss type temperature (T) variation, 1/T(1)T=C/(T-theta), with theta=-42 K, suggesting two-dimensional antiferromagnetic spin correlations. Below T(c)=3.9 K, 1/T(1) decreases with no coherence peak and follows a T(n) dependence with n approximately 2.2 down to approximately 2.0 K but crosses over to a 1/T(1) proportional to T variation below T=1.4 K, which suggests non-s-wave superconductivity. The data in the superconducting state are most consistent with the existence of line nodes in the gap function.

Two fluid description of the Kondo lattice

We present a two-fluid description of the Kondo lattice that is based on an analysis of CeCoIn5 at various levels of dilution with La. We show that thermal and transport measurements provide evidence for the survival of the Kondo impurity component in the lattice as T-->0 K, and that the evolution of the low temperature properties of the Kondo lattice can be viewed as the partial condensation of the lattice of Kondo centers into a heavy fermion fluid. The resulting two-fluid model is shown to be applicable to the general problem of the ground state of the Kondo lattice.

Low-frequency spin dynamics in the CeMIn5 materials

We measure the spin lattice relaxation of the planar In(1) nuclei in the CeMIn5 materials, extract quantitative information about the low energy spin dynamics of the lattice of Ce moments in both CeRhIn5 and CeCoIn5, and identify a crossover in the normal state. Above a temperature T(*) the Ce lattice exhibits "Kondo gas" behavior characterized by local fluctuations of independently screened moments; below T(*) both systems exhibit a "Kondo liquid" regime in which interactions between the local moments contribute to the spin dynamics. Both the antiferromagnetic and superconducting ground states in these systems emerge from the Kondo liquid regime. Our analysis provides strong evidence for quantum criticality in CeCoIn5.

Electronic structure and the Fermi surface of PuCoGa5 and NpCoGa5

Using a relativistic linear augmented-plane-wave method, we clarify energy band structures and Fermi surfaces of recently discovered plutonium-based superconductor PuCoGa5 and the isostructural material NpCoGa5. For PuCoGa5, we find several cylindrical sheets of Fermi surfaces with large volume, similar to CeMIn5, while for NpCoGa5, the Fermi surfaces are found to be similar to those of UMGa5. These similarities are discussed based on the j-j coupling scheme, suggesting some hints for the superconducting mechanism in HoCoGa5-type f-electron compounds.

Fermi-liquid ground state in the n-type Pr(0.91)LaCe(0.09)CuO(4-y) copper-oxide superconductor

We report nuclear magnetic resonance studies on the low-doped n-type copper-oxide Pr(0.91)LaCe(0.09)CuO(4-y) (T(c)=24 K) in the superconducting state and in the normal state uncovered by the application of a strong magnetic field. We find that when the superconductivity is removed the underlying ground state is the Fermi liquid state. This result is at variance with that inferred from previous thermal conductivity measurement and appears to contrast with that in p-type copper oxides with a similar doping level where high-T(c) superconductivity sets in within the pseudogap phase. The data in the superconducting state are consistent with the line-node gap model.

Superconductivity caused by the pairing of plutonium 5f Eelectrons in PuCoGa5

On the basis of electronic structure calculations we identify the superconductivity in the novel, high-temperature superconductor PuCoGa5 to be caused by the pairing of Pu 5f electrons. Assuming delocalized Pu 5f states, we compute theoretical crystallographic constants very near to the experimental ones, and the calculated specific heat coefficient compares reasonably to the measured coefficient. The theoretical Fermi surface is quasi-two-dimensional and the material appears to be close to a magnetic phase instability.

Microwave conductivity and penetration depth in the heavy fermion superconductor CeCoIn5

The electrodynamic properties of the quasi-two-dimensional heavy fermion superconductor CeCoIn5 have been investigated by microwave surface impedance measurements over a wide range of microwave frequencies and temperatures. We derive a value penetration depth lambda(0) approximately 190 nm, with a strong linear term in the temperature dependence of lambda(T) below T(c)/3, consistent with a superconducting gap with line nodes. The real part of the conductivity displays a broad peak at low temperatures consistent with a decreased scattering rate of almost 2 orders of magnitude below T(c). CeCoIn5 has remarkably similar properties to those of the high-T(c) cuprates.

Angular position of nodes in the superconducting gap of quasi-2D heavy-fermion superconductor CeCoIn5

The thermal conductivity of the heavy-fermion superconductor CeCoIn5 has been studied in a magnetic field rotating within the 2D planes. A clear fourfold symmetry of the thermal conductivity which is characteristic of a superconducting gap with nodes along the ( +/- pi,+/- pi) directions is resolved. The thermal conductivity measurement also reveals a first-order transition at H(c2), indicating a Pauli limited superconducting state. These results indicate that the symmetry most likely belongs to d(x(2)-y(2)), implying that the anisotropic antiferromagnetic fluctuation is relevant to the superconductivity.