Low-dimensional electron systems, as realized in layered materials, often tend to spontaneously break the symmetry of the underlying nuclear lattice by forming so-called density waves 1 ; a state of matter that at present attracts enormous attention 2-6 . Here we reveal a remarkable and surprising feature of charge density waves, namely their intimate relation to orbital order. For the prototypical material 1T-TaS 2 we not only show that the charge density wave within the two-dimensional TaS 2 layers involves previously unidentified orbital textures of great complexity. We also demonstrate that two metastable stackings of the orbitally ordered layers allow manipulation of salient features of the electronic structure. Indeed, these orbital e ects provide a route to switch 1T-TaS 2 nanostructures from metallic to semiconducting with technologically pertinent gaps of the order of 200 meV. This new type of orbitronics is especially relevant for the ongoing development of novel, miniaturized and ultrafast devices based on layered transition metal dichalcogenides 7,8 .Among the various transition metal dichalcogenides (TMDs), 1T-TaS 2 stands out because of its particularly rich electronic phase diagram as a function of pressure and temperature 9 . This phase diagram not only features incommensurate, nearly commensurate and commensurate charge density waves (CDWs), but also pressure-induced superconductivity below 5 K. In addition to this, it was proposed early on that the low-temperature commensurate CDW (C-CDW), which is illustrated in Fig. 1a,c, also features manybody Mott physics 10 . Experimental evidence for the presence of Mott physics in 1T-TaS 2 has indeed been obtained recently by timeresolved spectroscopies, which observed the ultrafast collapse of a charge excitation gap, which has been interpreted as a fingerprint of significant electron-electron interactions [11][12][13] . Even though the above scenario for the C-CDW is widely accepted, important experimental facts remain to be understood: the very strong suppression of the C-CDW with external pressure is puzzling. Already above 0.6 GPa, the C-CDW is no longer stable, although nesting conditions, band widths and lattice structure remain essentially unchanged. It is also not clear how ordered defects within the C-CDW, which emerge in the nearly commensurate phase (NC-CDW) on heating 14,15 and do not cause significant changes in the bandwidths, can render the electronelectron on-site interaction U completely ineffective 16 . In the following we will show that all these issues are explained consistently in terms of orbital textures that are intertwined with the CDW. Furthermore, we demonstrate that this new twist to the physics of

Superconductivity (SC) in so-called "unconventional superconductors" is nearly always found in the vicinity of another ordered state, such as antiferromagnetism, charge density wave (CDW), or stripe order. This suggests a fundamental connection between SC and fluctuations in some other order parameter. To better understand this connection, we used high-pressure x-ray scattering to directly study the CDW order in the layered dichalcogenide TiSe 2 , which was previously shown to exhibit SC when the CDW is suppressed by pressure [1] or intercalation of Cu atoms [2]. We succeeded in suppressing the CDW fully to zero temperature, establishing for the first time the existence of a quantum critical point (QCP) at P c = 5.1 ± 0.2 GPa, which is more than 1 GPa beyond the end of the SC region. Unexpectedly, at P = 3 GPa we observed a reentrant, weakly first order, incommensurate phase, indicating the presence of a Lifshitz tricritical point somewhere above the superconducting dome. Our study suggests that SC in TiSe 2 may not be connected to the QCP itself, but to the formation of CDW domain walls. *The term "unconventional superconductor" once referred to materials whose phenomenology does not conform to the Bardeen-Cooper-Schrieffer (BCS) paradigm for superconductivity. It is now evident that, by this definition, the vast majority of known superconductors are unconventional, notable examples being the copper-oxide, iron-arsenide, and iron-selenide high temperature superconductors, heavy Fermion materials such as CeIn 3 and CeCoIn 5 , ruthenium oxides, organic superconductors such as ϰ-(BEDT-TTF)2X, filled skutterudites, etc.Despite their diversity in structure and phenomenology, the phase diagrams of these materials all exhibit the common trait that superconductivity (SC) resides near the boundary of an ordered phase with broken translational or spin rotation symmetry. For example, SC resides near antiferromagnetism in CeIn 3 [3], near a spin density wave in iron arsenides [4], orbital order in ruthenates [5], and stripe and nematic order in the copper-oxides [6]. The pervasiveness of this "universal phase diagram" suggests that there exists a unifying framework -more general than BCS -in which superconductivity can be understood as coexisting with some ordered phase, and potentially emerging from its fluctuations.A classic example is the transition metal dichalcogenide family, MX 2 , where M=Nb, Ti, Ta, and X=Se, S, which exhibit a rich competition between superconductivity and Peierls-like charge density wave (CDW) order [7]. A recent, prominent case is 1T-TiSe 2 , which under ambient pressure exhibits CDW order below a transition temperature T CDW = 202 K [8]. This CDW phase can be suppressed either with intercalation of Cu atoms [2,9], or through the application of hydrostatic pressure [1,10], causing SC to emerge. These studies suggest that the emergence of SC coincides with a quantum critical point (QCP) at which T CDW goes to zero, suggesting that TiSe 2 exemplifies the universal phenomenon of superconductivity em...

Resonant soft x-ray scattering experiments with photon energies near the O K and the Cu L3 edge on the system La1.8−xEu0.2SrxCuO4 for 0.1 ≤ x ≤ 0.15 are presented. A phase diagram for stripe-like charge ordering is obtained together with information on the structural transition into the low-temperature tetragonal phase. A clear dome for the charge ordering around x = 1 8 is detected well below the structural transition. This result is quite different from other systems in which static stripes are detected. There the charge order is determined by the structural transition appearing at the same temperature. Furthermore we present results for the coherence length and the incommensurability of the stripe order as a function of Sr concentration. Later on static stripe order was also detected in the compounds La 2−x Ba x CuO 4 (LBCO) [3][4][5] and La 1.8−x Eu 0.2 Sr x CuO 4 (LESCO). 6-8In all these systems it is believed that static stripe order is stabilized by a structural transition from a low temperature orthorhombic (LTO) to a low temperature tetragonal (LTT) phase in which the CuO 6 octahedra are tilted along the [110] HT T and [100] HT T directions of the high temperature tetragonal (HTT) phase, respectively. Generally, the stripe order is accompanied by a suppression of coherent superconductivity and the amount of this suppression is increasing with increasing tilt angle Φ in the LTT phase. 9The tilt angle increases with decreasing ionic radius of substitutes on the La sites due to a chemical pressure along the CuO 2 layers. In LESCO with the small Eu ions the antiferromagnetic stripe order almost completely replaces the superconducting phase for x < ∼ 0.2. This apparent anticorrelation between stripe order and superconductivity, however, was recently questioned by the interpretation of transport data in LBCO in terms of a layer decoupled stripe superconductor with no phase coherence perpendicular to the CuO 2 layers. 10So far complete phase diagrams for the structural, the charge, and the spin order in the systems LBCO, LNSCO, and LESCO were proposed in Ref. In this Brief Report we complete our previous resonant soft x-ray scattering (RSXS) studies on the stripe like charge order in LESCO. 8While there we have reported data only for doping concentrations x ≥ 1 8 , in the present contribution we present measurements on both sides of the concentration x = 1 8 . Furthermore we present more information on the correlation length and on the incommensurability wave vector as a function of doping concentration. Finally we show an extended phase diagram for the lattice and the charge order in LESCO.Traditionally, charge order was detected by measuring superstructure reflections with x-ray or neutron scattering. Both methods were used to study stripe-like charge order in LNSCO and LBCO. 2,11,17 In the case of LESCO, only hard x-ray scattering was successful. 18RSXS at the O K and Cu L edges is a particular sensitive method to detect charge order in doped cuprates. 5,8At the prepeak of the O K edge, the form factor ...

No abstract

No abstract