A charge-density wave (CDW) state has a broken symmetry described by a complex order parameter with an amplitude and a phase. The conventional view, based on clean, weak-coupling systems, is that a finite amplitude and long-range phase coherence set in simultaneously at the CDW transition temperature Tcdw. Here we investigate, using photoemission, X-ray scattering and scanning tunnelling microscopy, the canonical CDW compound 2H-NbSe2 intercalated with Mn and Co, and show that the conventional view is untenable. We find that, either at high temperature or at large intercalation, CDW order becomes short-ranged with a well-defined amplitude, which has impacts on the electronic dispersion, giving rise to an energy gap. The phase transition at Tcdw marks the onset of long-range order with global phase coherence, leading to sharp electronic excitations. Our observations emphasize the importance of phase fluctuations in strongly coupled CDW systems and provide insights into the significance of phase incoherence in ‘pseudogap’ states.
In order to understand the origin of high-temperature superconductivity in copper oxides, we must understand the normal state from which it emerges. Here, we examine the evolution of the normal state electronic excitations with temperature and carrier concentration in Bi 2 Sr 2 CaCu 2 O 8þδ using angle-resolved photoemission. In contrast to conventional superconductors, where there is a single temperature scale T c separating the normal from the superconducting state, the high-temperature superconductors exhibit two additional temperature scales. One is the pseudogap scale T Ã , below which electronic excitations exhibit an energy gap. The second is the coherence scale T coh , below which sharp spectral features appear due to increased lifetime of the excitations. We find that T Ã and T coh are strongly doping dependent and cross each other near optimal doping. Thus the highest superconducting T c emerges from an unusual normal state that is characterized by coherent excitations with an energy gap.cuprates | photoelectron spectroscopy G eneral features of the phase diagram of the copper oxide superconductors have been known for some time. The superconducting transition temperature T c has a dome-like shape in the doping-temperature plane with a maximum near a doping δ ∼ 0.167 electrons per Cu atom. Although in conventional metals the electronic excitations for T > T c are (i) gapless and (ii) sharply defined at the Fermi surface (1), the cuprates violate at least one of these conditions over much of their phase diagram. These deviations from conventional metallic behavior are most easily described in terms of two energy scales T Ã (2, 3) and T coh (4), which correspond to criteria (i) and (ii), respectively.To address the role of these energy scales in defining the phase diagram, we concentrate on spectra where the superconducting energy gap is largest, the antinode [ðπ;0Þ → ðππÞ Fermi crossing], where the spectral changes with doping and temperature are most pronounced (the SI Appendix has further details). Spectral changes at the node have been previously studied by Valla et al.(5) and such spectra remain gapless for all doping values (6). In Fig. 1, we show spectra at fixed temperature as a function of doping. Data points are indicated in Fig. 1A (See SI Appendix for experimental conditions and sample details). Initially, we show spectra at fixed momenta as a function of energy (energy distribution curves, or EDCs) that have been symmetrized (7) about the Fermi energy to remove the effects of the Fermi function. Later, we show that equivalent results are obtained from division of the EDCs by a resolution-broadened Fermi function. In the following figures, because two values of the doping can result in the same T c , samples are labeled as OP for optimally doped, OD for overdoped, and UD for underdoped.The spectra at the antinode at the highest temperature (approximately 300 K) in Fig. 1D show two remarkable features: They are extremely broad in energy, exceeding any expected thermal broadening, and their line...
A key question in condensed-matter physics is to understand how high-temperature superconductivity emerges on adding mobile charged carriers to an antiferromagnetic Mott insulator. We address this question using angle-resolved photoemission spectroscopy to probe the electronic excitations of the non-superconducting state that exists between the Mott insulator and the d-wave superconductor in Bi 2 Sr 2 CaCu 2 O 8+δ . Despite a temperature-dependent resistivity characteristic of an insulator, the excitations in this intermediate state have a highly anisotropic energy gap that vanishes at four points in momentum space. This nodal-liquid state has the same gap structure as that of the d-wave superconductor but no sharp quasiparticle peaks. We observe a smooth evolution of the excitation spectrum, along with the appearance of coherent quasiparticles, as one goes through the insulator-tosuperconductor transition as a function of doping. Our results suggest that high-temperature superconductivity emerges when quantum phase coherence is established in a nonsuperconducting nodal liquid.High-temperature superconductivity in the cuprates occurs by doping a Mott insulator for which the antiferromagnetic ground state and low-energy excitations are well understood 1 . By adding carriers, the parent insulator turns into a superconductor for dopings that exceed 0.05 holes per CuO 2 plane. The d-wave nature of the superconducting ground state 2 and its low-lying excitations are also well understood. Between these phases lies an electronic ground state that is poorly understood. As the temperature is raised, this intermediate 'pseudogap' state occupies a larger and larger region of the phase diagram (Fig. 1a). It is from this phase that superconductivity emerges for all but the most highly doped samples. Consequently, the nature of this phase holds the key to the origin of high-temperature superconductivity.Whereas the electronic excitations in the high-temperature pseudogap region have been studied extensively, there is little spectroscopic data at low temperatures, as there is only a very narrow window of dopings where neither superconducting nor antiferromagnetic order occurs. Here, we present angleresolved photoemission spectroscopy (ARPES) data on single crystals and thin films 3 with doping levels that range all the way from the insulator to the over-doped superconductor. We focus in particular on non-superconducting thin films, just to the left of the superconducting transition temperature T c dome (see Fig. 1a), with an estimated hole doping ∼0. 04 (ref. 3). It is normally quite difficult to span the insulator-superconductor transition in Bi 2 Sr 2 CaCu 2 O 8+δ single crystals. However, it is possible to obtain very underdoped thin films by removing oxygen without film decomposition, as their large surface/volume ratio allows much lower annealing temperatures than those required for crystals. The integrity of the insulating films was confirmed by re-oxygenating them and checking their resistivity R(T ) and X-ray diffracti...
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