A large number of experimental discoveries especially in the heavy quarkonium sector that did not at all fit to the expectations of the until then very successful quark model led to a renaissance of hadron spectroscopy. Among various explanations of the internal structure of these excitations, hadronic molecules, being analogues of light nuclei, play a unique role since for those predictions can be made with controlled uncertainty. We review experimental evidences of various candidates of hadronic molecules, and methods of identifying such structures. Nonrelativistic effective field theories are the suitable framework for studying hadronic molecules, and are discussed in both the continuum and finite volumes. Also pertinent lattice QCD results are presented. Further, we discuss the production mechanisms and decays of hadronic molecules, and comment on the reliability of certain assertions often made in the literature.

The observation of Zc(3900) by the BESIII collaboration in the invariant mass spectrum of J/ψπ ± in e + e − → J/ψπ + π − at the center of mass 4.260 GeV suggests the existence of a chargedDD * +DD * molecular state with I(J P ) = 1(1 + ), which could be an isovector brother of the famous X(3872) and an analogue of Z b (10610) claimed by the Belle Collaboration. We demonstrate that this observation provides strong evidence that the mysterious Y (4260) is aDD1(2420) + DD1(2420) molecular state. Especially, we show that the decay of this molecule naturally populates low momentumDD * pairs and leads unavoidably to a cusp at theDD * threshold. We discuss the signatures that distinguish such aDD * cusp from the presence of a true resonance.PACS numbers: 14.40. Rt, 13.75.Lb, 13.20.Gd During the past years, the experimental observation of a large number of so-called X, Y , Z states has initiated tremendous efforts to unravel their nature beyond the conventional quark model. Especially, the confirmation of signals in charged channels would be a direct evidence for exotic states. For instance, the Belle Collaboration reported signals for Z(4430) in ψ ′ π ± , and Z 1 (4050) andHowever, an enhancement in the same mass range was interpreted as a reflection by BaBar [2]. The more recent experimental results for charged bottomonium states Z b (10610) and Z b (10650), located close to theBB * andB * B * thresholds, respectively, by the Belle Collaboration [3] seem to be the first strong evidence for QCD "exotics" in the heavy quark sector. In this context the recent report of an enhancement in the J/ψπ ± invariant mass distribution around 3900 MeV, right at theDD * threshold, by the BESIII collaboration [4] clearly reinforces the existence of such an unusual phenomenon. This state, called Z c (3900) below, might be an isovector partner of the well established 1 ++ isoscalar X(3872) [5], but with I G (J P C ) = 1 + (1 +− ) for the neutral state.In this work we demonstrate that, if Y (4260) is ā DD 1 + c.c. molecule (below we useDD 1 as a short notation), the appearance of an enhancement around 3900 MeV in the J/ψπ invariant mass distribution can be shown to be natural. Here D 1 refers to the narrow axial vector D 1 (2420) (Γ = 27 ± 3 MeV) with I(J P ) = 1 2 (1 + ) [5]. In this sense the observation of the charged Z c (3900) state by BESIII in Y (4260) → J/ψππ provides a very strong evidence for the molecular nature of Y (4260). We also discuss whether the observed en- * Email address: q.wang@fz-juelich.de † Email address: c.hanhart@fz-juelich.de ‡ Email address: zhaoq@ihep.ac.cn hancement can be interpreted purely as a cusp or whether the inclusion of explicit poles in theDD * system is necessary. Before we go into details of our calculations we first briefly review the status of Y (4260).The most mysterious fact about Y (4260) is not that its mass does not agree to what is predicted by the potential quark model. Instead, as a charmonium state with J P C = 1 −− , it is only "seen" as a bump in its two pion transitions to J/ψ,...

Well-aligned arrays of ZnO nanoneedles were fabricated using a simple vapor phase growth. The diameters of the nanoneedle tips are as small as several nanometers, which is highly in favor of the field emission. Field-emission measurements using the nanoneedle arrays as cathode showed emission current density as high as 2.4 mA/cm2 under the field of 7 V/μm, and a very low turn-on field of 2.4 V/μm. Such a high emission current density is attributed to the high aspect ratio of the nanoneedles. The high emission current density, high stability, and low turn-on field make the ZnO nanoneedle arrays one of the promising candidates for field-emission displays.