†These authors contributed equally to this work. Ultrashort light pulses can selectively excite charges, spins and phonons in materials, providing a powerful approach for manipulating their properties. Here we use femtosecond laser pulses to coherently manipulate the electron and phonon distributions, and their couplings, in the charge density wave (CDW) material 1T-TaSe2. After exciting the material with a short light pulse, spatial smearing of the electrons launches a coherent lattice breathing mode, which in turn modulates the electron temperature. This indicates a bi-directional energy exchange between the electrons and the strongly-coupled phonons. By tuning the laser excitation fluence, we can control the magnitude of the electron temperature modulation, from ~ 200 K in the case of weak excitation, to ~ 1000 K for strong laser excitation. This is accompanied by a switching of the dominant mechanism from anharmonic phonon-phonon coupling to coherent electronphonon coupling, as manifested by a phase change of  in the electron temperature modulation.Our approach thus opens up possibilities for coherently manipulating the interactions and properties of quasi-2D and other quantum materials using light.

We utilized high-resolution resonant angle-resolved photoemission spectroscopy (ARPES) to study the band structure and hybridization effect of the heavy-fermion compound Ce2IrIn8. We observe a nearly flat band at the binding energy of 7 meV below the coherent temperature Tcoh~40 K, which characterizes the electrical resistance maximum indicating the onset temperature of hybridization. However, the Fermi vector kF and the Fermi surface (FS) volume have little change around Tcoh, challenging the widely believed evolution from a high-temperature small FS to a low-temperature large FS. Our experimental results of the band structure fit well with the density functional theory plus dynamic mean-field theory (DFT+DMFT) calculations. INTRODUCTION:Heavy-fermion compounds, first discovered in CeAl3 in 1975 [1], are some of the most exotic materials in condensed matter physics. The name originates from the largely enhanced effective mass of the heavy quasi-particles, which can be 2 or 3 orders of magnitude higher than that in a normal metal [2]. These compounds usually contain some of Ce, Sm, Yb, U, Pr, Pu, Np elements, which possess an unfilled 4f or 5f shell. It is widely believed that 4f/5f electrons are local moments at high temperatures and become itinerant after hybridized with the conduction electrons at low temperatures. Varieties of phenomena, e.g., antiferromagnetism [3], ferromagnetism [4], superconductivity [5], quantum critical point [6], quadrupole order [7], hidden order [8-9], topological property [10], have been discovered in heavy-fermion compounds. Central to understanding these exotic phenomena is the interplay of itinerancy and localization. However, the low energy scales (critical temperature, hybridization gap, superconducting gap) in heavy-fermion systems have brought major challenges to many experimental techniques.CemTnIn3m+2n (m = 1, 2; n = 1, 2 and T: Co, Rh, Ir, Pd, Pt) family is a good platform of heavy-fermion materials for studying the interplay between c-f hybridization, magnetism, superconductivity, quantum criticality, and etc. CemTnIn3m+2n crystallizes with a tetragonal unit cell that can be viewed as m-layers of CeIn3 unit stacked sequentially with intervening n-layers of TIn2 along the c-axis. Among them, the spin-glass state observed in Ce2IrIn8 indicates partially delocalized Ce 4f electron [11]. The magnetism in Ce2IrIn8 depends on Ce-Ir hybridization and local Ce environment. The small but finite onset temperature for spin freezing rules out the quantum critical point (QCP) scenario in Ce2IrIn8 [11]. High Sommerfeld coefficient (γ~700 mJ/mol‧K 2 ) [12] and the absence of long-range magnetic order indicate an itinerant behavior of Ce 4f electron. μSR observed a 'Knight-shift anomaly' in which the Knight shift constant K no longer scales linearly with the susceptibility below a characteristic temperature Tcoh, in agreement with the "two-fluid" model of heavy-fermion formation [15]. Resistivity measurements showed a broad maximum near 40 -50 K [13,16], manifesting the development of a...

The cuprate superconductors distinguish themselves from the conventional superconductors in that a small variation in the carrier doping can significantly change the superconducting transition temperature ( ), giving rise to a superconducting dome where a pseudogap 1,2 emerges in the underdoped region and a Fermi liquid appears in the overdoped region. Thus a systematic study of the properties over a wide doping range is critical for understanding the superconducting mechanism. Here, we report a new technique to continuously dope the surface of Bi 2 Sr 2 CaCu 2 O 8+x through an ozone/vacuum annealing method. Using in-situ ARPES, we obtain precise quantities of energy gaps and the coherent spectral weight over a wide range of doping. We discover that the d-wave component of the quasiparticle gap is linearly proportional to the Nernst temperature that is the onset of superconducting vortices 3 , strongly suggesting that the emergence of superconducting pairing is concomitant with the onset of free vortices, with direct implications for the onset of superconducting phase coherence at and the nature of the pseudogap phenomena.Bi 2 Sr 2 CaCu 2 O 8+x (Bi2212) single crystals have been extensively studied by angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy (STS) 4,5 , two of the major experimental techniques for probing the cuprates. However, high-quality Bi2212 crystals can be only obtained within a narrow doping range. Moreover, surface cleaving, necessary for surface techniques such as ARPES and STS, posses a serious problem for quantitative comparisons from sample to sample due to variation of surface conditions. Realizing that the doping level in this material is solely controlled by the excess oxygen concentration, we use ozone/vacuum annealing to continuously change the doping level of the surface layers, which are subsequently measured by in-situ ARPES (Figs. 1a-c).