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Register a new userQuantum well states in fractured crystals of the heavy fermion material CeCoIn$_5$
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Quantum well states appear in metallic thin films due to the confinement of the wave function by the film interfaces. Using angle-resolved photoemission spectroscopy, we unexpectedly observe quantum well states in fractured single crystals of CeCoIn$_5$. We confirm that confinement occurs by showing that these states binding energies are photon-energy independent and are well described with a phase accumulation model, commonly applied to quantum well states in thin films. This indicates that atomically flat thin films can be formed by fracturing hard single crystals. For the two samples studied, our observations are explained by free-standing flakes with thicknesses of 206 and 101 r{A}. We extend our analysis to extract bulk properties of CeCoIn$_5$. Specifically, we obtain the dispersion of a three-dimensional band near the zone center along in-plane and out-of-plane momenta. We establish part of its Fermi surface, which corresponds to a hole pocket centered at $Gamma$. We also reveal a change of its dispersion with temperature, a signature that may be caused by the Kondo hybridization.
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We report a systematic study of temperature- and field-dependent charge ($boldsymbol{rho}$) and entropy ($mathbf{S}$) transport in the heavy-fermion superconductor CeIrIn$_5$. Its large positive thermopower $S_{xx}$ is typical of Ce-based Kondo lattice systems, and strong electronic correlations play an important role in enhancing the Nernst signal $S_{xy}$. By separating the off-diagonal Peltier coefficient $alpha_{xy}$ from $S_{xy}$, we find that $alpha_{xy}$ becomes positive and greatly enhanced at temperatures well above the bulk $T_c$. Compared with the non-magnetic analog LaIrIn$_5$, these results suggest vortexlike excitations in a precursor state to unconventional superconductivity in CeIrIn$_5$. This study sheds new light on the similarity of heavy-fermion and cuprate superconductors and on the possibility of states not characterized by the amplitude of an order parameter.
The tilted balance among competing interactions can yield a rich variety of ground states of quantum matter. In most Ce-based heavy fermion systems, this can often be qualitatively described by the famous Doniach phase diagram, owing to the competition between the Kondo screening and the Ruderman-Kittel-Kasuya-Yoshida exchange interaction. Here, we report an unusual pressure-temperature phase diagram beyond the Doniach one in CeCuP2. At ambient pressure, CeCuP2 displays typical heavy-fermion behavior, albeit with a very low carrier density. With lowering temperature, it shows a crossover from a non Fermi liquid to a Fermi liquid at around 2.4 K. But surprisingly, the Kondo coherence temperature decreases with increasing pressure, opposite to that in most Ce-based heavy fermion compounds. Upon further compression, two superconducting phases are revealed. At 48.0 GPa, the transition temperature reaches 6.1 K, the highest among all Ce-based heavy fermion superconductors. We argue for possible roles of valence tuning and fluctuations associated with its special crystal structure in addition to the hybridization effect. These unusual phase diagrams suggest that CeCuP2 is a novel platform for studying the rich heavy fermions physics beyond the conventional Doniach paradigm.
Heavy fermion materials gain high electronic masses and expand Fermi surfaces when the high-temperature localized f electrons become itinerant and hybridize with the conduction band at low temperatures. However, despite the common application of this model, direct microscopic verification remains lacking. Here we report high-resolution angle-resolved photoemission spectroscopy measurements on CeCoIn5, a prototypical heavy fermion compound, and reveal the long-sought band hybridization and Fermi surface expansion. Unexpectedly, the localized-to-itinerant transition occurs at surprisingly high temperatures, yet f electrons are still largely localized at the lowest temperature. Moreover, crystal field excitations likely play an important role in the anomalous temperature dependence. Our results paint an comprehensive unanticipated experimental picture of the heavy fermion formation in a periodic multi-level Anderson/Kondo lattice, and set the stage for understanding the emergent properties in related materials.
Orbital degrees of freedom in condensed matters could play important roles in forming a variety of exotic electronic states by interacting with conduction electrons. In 4f electron systems, because of strong intra-atomic spin-orbit coupling, an orbitally degenerate state inherently carries quadrupolar degrees of freedom. The present work has focussed on a purely quadrupole-active system PrIr2Zn20 showing superconductivity in the presence of an antiferroquadrupole order at TQ = 0.11 K. We observed non-Fermi liquid (NFL) behaviors emerging in the electrical resistivity and the 4f contribution to the specific heat, C_4f, in the paramagnetic state at T > TQ. Moreover, in magnetic fields below 6 T, all data set of the electrical resistivity and C_4f(T) are well scaled with characteristic temperatures T0s. This is the first observation of the NFL state in the nonmagnetic quadrupole-active system, whose origin is intrinsically different from that observed in the vicinity of the conventional quantum critical point. It implies possible formation of a quadrupole Kondo lattice resulting from hybridization between the quadrupoles and the conduction electrons. Below 0.13 K, the electrical resistivity and C_4f(T) exhibit anomalies as B approaches 5 T. This is the manifestation of a field-induced crossover toward a Fermi-liquid ground state in the quadrupole Kondo lattice.
Exotic phenomenon can be achieved in quantum materials by confining electronic states into two dimensions. For example, relativistic fermions are realised in a single layer of carbon atoms, the quantized Hall effect can result from two-dimensional (2D) systems, and the superconducting transition temperature can be enhanced significantly in a one-atomic-layer material. Ordinarily, 2D electronic system can be obtained by exfoliating the layered materials, growing monolayer materials on substrates, or establishing interfaces between different materials. Herein, we use femtosecond infrared laser pulses to invert the periodic lattice distortion sectionally in a three-dimensional (3D) charge density wave material, creating macroscopic domain walls of transient 2D ordered electronic states with exotic properties. The corresponding ultrafast electronic and lattice dynamics are captured by time- and angle-resolved photoemission spectroscopy and MeV ultrafast electron diffraction. Surprisingly, a novel energy gap state, which might be a signature of light-induced superconductivity, is identified in the photoinduced 2D domain wall near the surface. Such optical modulation of atomic motion is a new path to realise 2D electronic states and will be a new platform for creating novel phases in quantum materials.
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