Gap Symmetry of the Heavy Fermion Superconductor CeCu_{2}Si_{2} at Ambient Pressure

Spin-flip-driven anomalous Hall effect and anisotropic magnetoresistance in a layered Ising antiferromagnet

The influence of magnetocrystalline anisotropy in antiferromagnets is evident in a spin flip or flop transition. Contrary to spin flops, a spin-flip transition has been scarcely presented due to its specific condition of relatively strong magnetocrystalline anisotropy and the role of spin-flips on anisotropic phenomena has not been investigated in detail. In this study, we present antiferromagnet-based functional properties on an itinerant Ising antiferromagnet Ca_0.9Sr_0.1Co₂As₂. In the presence of a rotating magnetic field, anomalous Hall conductivity and anisotropic magnetoresistance are demonstrated, the effects of which are maximized above the spin-flip transition. Moreover, a joint experimental and theoretical study is conducted to provide an efficient tool to identify various spin states, which can be useful in spin-processing functionalities.

An emerging global picture of heavy fermion physics

Recent progresses using state-of-the-art experimental techniques have motivated a number of new insights on heavy fermion physics. This article gives a brief summary of the author's research along this direction. We discuss five major topics including: (1) development of phase coherence and two-stage hybridization; (2) two-fluid behavior and hidden universal scaling; (3) quantum phase transitions and fractionalized heavy fermion liquid; (4) quantum critical superconductivity; (5) material-specific properties. These cover the most essential parts of heavy fermion physics and lead to an emerging global picture beyond conventional theories based on mean-field or local approximations.

Pressure-stabilized graphene-like P layer in superconducting LaP2

MgB₂-type superconductors are of great interest in chemistry and condensed matter physics due to their superconductivity dominated by the structural unit of graphene-like B. However, this kind of material is absent in phosphides resulting from the inherent lone pair electrons of phosphorus. Here, we report that a pressure-stabilized LaP₂, isostructural to MgB₂, shows superconductivity with a predicted T_c of 22.2 K, which is the highest among those of already known transition metal phosphides. Besides the electron-phonon coupling of graphene-like P, alike the role of the B layer in MgB₂, La 5d/4f electrons are also responsible for the superconducting transition. The distinct P atomic arrangement is attributed to its sp² hybridization and out-of-plane symmetric distribution of lone pair electrons. On the other hand, its dynamically stabilized pressure reaches as low as 7 GPa, a desirable feature of pressure-induced superconductors. Although P is isoelectronic to N and As, we hereby find the different stable stoichiometries, structures, and electronic properties of La phosphides compared with La nitrides/arsenides at high pressures.

Prediction of Novel Boron-carbon Based Clathrates

Clathrates are inclusion compounds featured with host framework cages and trapped guest atoms or small molecules. Recently, the first boron-carbon (B-C) clathrate SrB3C3 was successfully synthesized at high pressures near...

Revealing the Heavy Quasiparticles in the Heavy-Fermion Superconductor CeCu_{2}Si_{2}

The superconducting order parameter of the first heavy-fermion superconductor CeCu_{2}Si_{2} is currently under debate. A key ingredient to understand its superconductivity and physical properties is the quasiparticle dispersion and Fermi surface, which remains elusive experimentally. Here, we present measurements from angle-resolved photoemission spectroscopy. Our results emphasize the key role played by the Ce 4f electrons for the low-temperature Fermi surface, highlighting a band-dependent conduction-f electron hybridization. In particular, we find a very heavy quasi-two-dimensional electron band near the bulk X point and moderately heavy three-dimensional hole pockets near the Z point. Comparison with theoretical calculations reveals the strong local correlation in this compound, calling for further theoretical studies. Our results provide the electronic basis to understand the heavy-fermion behavior and superconductivity; implications for the enigmatic superconductivity of this compound are also discussed.

High-energy magnetic excitations from heavy quasiparticles in CeCu2Si2

Magnetic fluctuations is the leading candidate for pairing in cuprate, iron-based, and heavy fermion superconductors. This view is challenged by the recent discovery of nodeless superconductivity in C e C u 2 S i 2 , and calls for a detailed understanding of the corresponding magnetic fluctuations. Here, we mapped out the magnetic excitations in superconducting (S-type) C e C u 2 S i 2 using inelastic neutron scattering, finding a strongly asymmetric dispersion for E ≲ 1.5 m e V , which at higher energies evolves into broad columnar magnetic excitations that extend to E ≳ 5 m e V . While low-energy magnetic excitations exhibit marked three-dimensional characteristics, the high-energy magnetic excitations in C e C u 2 S i 2 are almost two-dimensional, reminiscent of paramagnons found in cuprate and iron-based superconductors. By comparing our experimental findings with calculations in the random-phase approximation,we find that the magnetic excitations in C e C u 2 S i 2 arise from quasiparticles associated with its heavy electron band, which are also responsible for superconductivity. Our results provide a basis for understanding magnetism and superconductivity in C e C u 2 S i 2 , and demonstrate the utility of neutron scattering in probing band renormalization in heavy fermion metals.

Nature of the spin resonance mode in CeCoIn5

Spin-fluctuation-mediated unconventional superconductivity can emerge at the border of magnetism, featuring a superconducting order parameter that changes sign in momentum space. Detection of such a sign-change is experimentally challenging, since most probes are not phase-sensitive. The observation of a spin resonance mode (SRM) from inelastic neutron scattering is often seen as strong phase-sensitive evidence for a sign-changing superconducting order parameter, by assuming the SRM is a spin-excitonic bound state. Here we show that for the heavy fermion superconductor CeCoIn5, its SRM defies expectations for a spin-excitonic bound state, and is not a manifestation of sign-changing superconductivity. Instead, the SRM in CeCoIn5 likely arises from a reduction of damping to a magnon-like mode in the superconducting state, due to its proximity to magnetic quantum criticality. Our findings emphasize the need for more stringent tests of whether SRMs are spin-excitonic, when using their presence to evidence sign-changing superconductivity.

Quasi-Two-Dimensional Fermi Surfaces and Unitary Spin-Triplet Pairing in the Heavy Fermion Superconductor UTe_{2}

We report first-principles and strongly correlated calculations of the newly discovered heavy fermion superconductor UTe_{2}. Our analyses reveal three key aspects of its magnetic, electronic, and superconducting properties that include (i) a two-leg ladder-type structure with strong magnetic frustrations, which might explain the absence of long-range orders and the observed magnetic and transport anisotropy, (ii) quasi-two-dimensional Fermi surfaces composed of two separate electron and hole cylinders with similar nesting properties as in UGe_{2}, which may potentially promote magnetic fluctuations and help to enhance the spin-triplet pairing, and (iii) a unitary spin-triplet pairing state of strong spin-orbit coupling at zero field, with point nodes presumably on the heavier hole Fermi surface along the k_{x} direction, in contrast to the previous belief of nonunitary pairing. Our proposed scenario is in excellent agreement with latest thermal conductivity measurement and provides a basis for understanding the peculiar magnetic and superconducting properties of UTe_{2}.

Evidence for the Presence of the Fulde-Ferrell-Larkin-Ovchinnikov State in CeCu_{2}Si_{2} Revealed Using ^{63}Cu NMR

Nuclear magnetic resonance measurements were performed on CeCu_{2}Si_{2} in the presence of a magnetic field close to the upper critical field μ_{0}H_{c2} in order to investigate its superconducting (SC) properties near pair-breaking fields. In lower fields, the Knight shift and nuclear spin-lattice relaxation rate divided by temperature 1/T_{1}T abruptly decreased below the SC transition temperature T_{c}(H), a phenomenon understood within the framework of conventional spin-singlet superconductivity. In contrast, 1/T_{1}T was enhanced just below T_{c}(H) and exhibited a broad maximum when magnetic fields close to μ_{0}H_{c2}(0) were applied parallel or perpendicular to the c axis; although the Knight shift decreased just below T_{c}(H). This enhancement of 1/T_{1}T, which was recently observed in the organic superconductor κ-(BEDT-TTF)_{2}Cu(NCS)_{2}, suggests the presence of high-density Andreev bound states in the inhomogeneous SC region, a hallmark of the Fulde-Ferrell-Larkin-Ovchinnikov phase.