Fermi systems in the cross-over regime between weakly coupled Bardeen-Cooper-Schrieffer (BCS) and strongly coupled Bose-Einstein-condensate (BEC) limits are among the most fascinating objects to study the behavior of an assembly of strongly interacting particles. The physics of this cross-over has been of considerable interest both in the fields of condensed matter and ultracold atoms. One of the most challenging issues in this regime is the effect of large spin imbalance on a Fermi system under magnetic fields. Although several exotic physical properties have been predicted theoretically, the experimental realization of such an unusual superconducting state has not been achieved so far. Here we show that pure single crystals of superconducting FeSe offer the possibility to enter the previously unexplored realm where the three energies, Fermi energy e F , superconducting gap Δ, and Zeeman energy, become comparable. Through the superfluid response, transport, thermoelectric response, and spectroscopic-imaging scanning tunneling microscopy, we demonstrate that e F of FeSe is extremely small, with the ratio Δ=e F ∼ 1(∼ 0:3) in the electron (hole) band. Moreover, thermal-conductivity measurements give evidence of a distinct phase line below the upper critical field, where the Zeeman energy becomes comparable to e F and Δ. The observation of this field-induced phase provides insights into previously poorly understood aspects of the highly spin-polarized Fermi liquid in the BCS-BEC cross-over regime.BCS-BEC cross-over | Fermi energy | quasiparticle interference | iron-based superconductors | exotic superconducting phase S uperconductivity in most metals is well explained by the weak-coupling Bardeen-Cooper-Schrieffer (BCS) theory, where the pairing instability arises from weak attractive interactions in a degenerate fermionic system. In the opposite limit of Bose-Einstein condensate (BEC), composite bosons consisting of strongly coupled fermions condense into a coherent quantum state (1, 2). In BCS superconductors, the superconducting transition temperature is usually several orders of magnitude smaller than the Fermi temperature, T c =T F = 10 −5 -10 −4 , whereas in the BEC limit T c =T F is of the order of 10 −1 . Even in the high-T c cuprates, T c =T F is merely of the order of 10 −2 at optimal doping. Of particular interest is the BCS-BEC cross-over regime with intermediate coupling strength. In this regime the size of interacting pairs (∼ ξ), which is known as the coherence length, becomes comparable to the average distance between particles (∼ 1=k F ), i.e., k F ξ ∼ 1 (3-5), where k F is the Fermi momentum. This regime is expected to have the highest values of T c =T F = 0:1 − 0:2 and Δ=« F ∼ 0:5 ever observed in any fermionic superfluid.One intriguing issue concerns the role of spin imbalance: whether it will lead to a strong modification of the properties of the Fermi system in the cross-over regime. This problem has been of considerable interest not only in the context of superconductivity but also in ultraco...

By using a molecular beam epitaxy technique, we fabricate a new type of superconducting superlattices with controlled atomic layer thicknesses of alternating blocks between heavy fermion superconductor CeCoIn5, which exhibits a strong Pauli pair-breaking effect, and nonmagnetic metal YbCoIn5. The introduction of the thickness modulation of YbCoIn5 block layers breaks the inversion symmetry centered at the superconducting block of CeCoIn5. This configuration leads to dramatic changes in the temperature and angular dependence of the upper critical field, which can be understood by considering the effect of the Rashba spin-orbit interaction arising from the inversion symmetry breaking and the associated weakening of the Pauli pair-breaking effect. Since the degree of thickness modulation is a design feature of this type of superlattices, the Rashba interaction and the nature of pair-breaking are largely tunable in these modulated superlattices with strong spin-orbit coupling.PACS numbers: 71.27.+a, 74.70.Tx, 74.78.Fk, 81.15.Hi Among the existing condensed matter systems, the metallic state with the strongest electron correlation effects is achieved in heavy fermion materials with 4f or 5f electrons. In these systems, a very narrow conduction band is formed at low temperatures through the Kondo effect. In particular, in Ce(4f )-based compounds, strong electron correlations within the narrow band strikingly enhance the quasiparticle effective mass. As a result of notable many-body effects, a plethora of fascinating physical phenomena including unconventional superconductivity with non-s-wave pairing symmetry appears [1]. The unconventional pairing symmetry and the associated exotic superconducting properties have mystified researchers over the past quarter century.Recently, it has been suggested that the inversion symmetry breaking (ISB) together with strong spin-orbit interaction can dramatically affect the superconductivity, giving rise to a number of novel phenomena such as anomalous magneto-electric effects [2] and topological superconducting states [3][4][5]. It has also been pointed out that such phenomena are more pronounced in strongly correlated electron systems [6]. The inversion symmetry imposes important constraints on the pairing states: In the presence of inversion symmetry, Cooper pairs are classified into a spin-singlet or triplet state, whereas in the absence of inversion symmetry, an asymmetric potential gradient ∇V yields a spin-orbit interaction that breaks parity, and the admixture of spin singlet and triplet states is possible [7,8]. For instance, asymmetry of the potential in the direction perpendicular to the two-dimensional (2D) plane ∇V [001] induces Rashbathe Fermi wave number, and σ is the Pauli matrix. Rashba interaction splits the Fermi surface into two sheets with different spin structures: the energy splitting is given by α R , and the spin direction is tilted into the plane, rotating clockwise on one sheet and anticlockwise on the other. When the Rashba splitting exceeds the superc...

We have studied the superlattices with alternating block layers (BLs) of heavy-fermion superconductor CeCoIn5 and conventional-metal YbCoIn5 by site-selective nuclear magnetic resonance(NMR) spectroscopy, which uniquely offers spatially-resolved dynamical magnetic information. We find that the presence of antiferromagnetic fluctuations is confined to the Ce-BLs, indicating that magnetic degrees of freedom of f -electrons are quenched inside the Yb-BLs. Contrary to simple expectations that the two-dimensionalization enhances fluctuations, we observe that antiferromagnetic fluctuations are rapidly suppressed with decreasing Ce-BL thickness. Moreover, the suppression is more prominent near the interfaces between the BLs. These results imply significant effects of local inversion-symmetry breaking at the interfaces.

The effects of reduced dimensions and the interfaces on antiferromagnetic quantum criticality are studied in epitaxial Kondo superlattices, with alternating n layers of heavy-fermion antiferromagnet CeRhIn 5 and 7 layers of normal metal YbRhIn 5 . As n is reduced, the Kondo coherence temperature is suppressed due to the reduction of effective Kondo screening. The Néel temperature is gradually suppressed as n decreases and the quasiparticle mass is strongly enhanced, implying dimensional control toward quantum criticality. Magnetotransport measurements reveal that a quantum critical point is reached for n = 3 superlattice by applying small magnetic fields. Remarkably, the anisotropy of the quantum critical field is opposite to the expectations from the magnetic susceptibility in bulk CeRhIn 5 , suggesting that the Rashba spin-orbit interaction arising from the inversion symmetry breaking at the interface plays a key role for tuning the quantum criticality in the two-dimensional Kondo lattice. 1In Kondo lattices consisting of a periodic array of localized spins which are coupled to conduction electrons, a very narrow conduction band is formed at sufficiently low temperatures through the Kondo effect [1]. Such systems are realized in intermetallic heavy-fermion metals, which contain a dense lattice of certain lanthanide (4f ) and actinide (5f ) ions. In particular, in Ce(4f )-based compounds, strong electron correlations strikingly enhance the quasiparticle (QP) effective mass to about 100 times or more of the bare electron mass, resulting in a heavy Fermi liquid state. In the strongly correlated electron systems, nonFermi liquid behavior, associated with the quantum fluctuations near a quantum critical point (QCP), a point at which a material undergoes a second-order transition from one phase to another at absolute zero temperature [2], has been one of the central issues. The heavy-fermion systems are particularly suitable for this study, because the ground state can be tuned readily by control parameters other than temperature, such as magnetic field, pressure, or chemical substitution [3]. As a result of the many-body effects within the narrow band in these heavy-fermion compounds, a plethora of fascinating properties have been reported in the vicinity of a QCP.Recently, a state-of-the-art molecular beam epitaxy (MBE) technique has been developed to fabricate an artificial Kondo superlattice, a superlattice with alternating layers of Ce-based heavy-fermion compounds and nonmagnetic conventional metals with a few atomic layers thick [4]. These artificially engineered materials provide a new platform to study the properties of two-dimensional (2D) Kondo lattices, in contrast to the three-dimensional bulk materials. In the previously studied CeCoIn 5 /YbCoIn 5 superlattices [4], where CeCoIn 5 is a heavy-fermion superconductor and YbCoIn 5 is a conventional metal, each Ce-block layer (BL) is magnetically decoupled from the others, since the Ruderman-Kittel-Kasuya-Yoshida interaction between the spatially se...

We discuss the charge carrier dynamics of the heavy-fermion compound CeCoIn 5 in the metallic regime measured by means of quasi-optical THz spectroscopy.The transmittance of electromagnetic radiation through a CeCoIn 5 thin film on a dielectric substrate is analyzed in the single-particle Drude framework. We discuss the temperature dependence of the electronic properties, such as the scattering time and dc-conductivity and compare with transport measurements of the sheet resistance. Towards low temperatures, we find an increasing mismatch between the results from transport and Drude-analyzed optical measure- ments and a growing incapability of the simple single-particle picture describing the charge dynamics, likely caused by the evolving heavy-fermion nature of the correlated electron system.