Mutually coupled modes of a pair of active LRC circuits, one with amplification and another with an equivalent amount of attenuation, provide an experimental realization of a wide class of systems where gain/loss mechanisms break the Hermiticity while preserving parity-time PT symmetry. For a value γPT of the gain/loss strength parameter the eigen-frequencies undergo a spontaneous phase transition from real to complex values, while the normal modes coalesce acquiring a definite chirality. The consequences of the phase-transition in the spatiotemporal energy evolution are also presented. PACS numbers: 11.30.Er, 03.65.Vf Parity (P) and time -reversal (T ) symmetries, as well as their breaking, belong to the most basic notions in physics. Recently there has been much interest in systems which do not obey P and T -symmetries separately but do exhibit a combined PT -symmetry. Examples of such PT -symmetric systems range from quantum field theories and mathematical physics [1-3] to atomic [4], solid state [5,6] and classical optics [7][8][9][10][11][12][13][14][15]. A PTsymmetric system can be described by a phenomenological "Hamiltonian" H. Such Hamiltonians may have a real energy spectrum, although in general are nonHermitian. Furthermore, as some parameter γ that controls the degree of non-Hermiticity of H changes, a spontaneous PT symmetry breaking occurs. The transition point γ = γ PT show the characteristic behaviour of an exceptional point (EP) where both eigenvalues and eigenvectors coallesce (for experimental studies of EP singularities of lossy systems see Ref.[16]). For γ > γ PT , the eigenfunctions of H cease to be eigenfunctions of the PToperator, despite the fact that H and the PT -operator commute [1]. This happens because the PT -operator is anti-linear, and thus the eigenstates of H may or may not be eigenstates of PT . As a consequence, in the broken PT -symmetric phase the spectrum becomes partially or completely complex. The other limit where both H and PT share the same set of eigenvectors, corresponds to the so-called exact PT -symmetric phase in which the spectrum is real. This result led Bender and colleagues to propose an extension of quantum mechanics based on nonHermitian but PT -symmetric operators [1,2]. The class of non-Hermitian systems with real spectrum has been extended by Mostafazadeh in order to include Hamiltonians with generalized PT (antilinear) symmetries [17].While these ideas are still debatable, it was recently suggested that optics can provide a particularly fertile ground where PT -related concepts can be realized [7] and experimentally investigated [8,9]. In this framework, PT symmetry demands that the complex refractive index obeys the condition n( r) = n * (− r). PTsynthetic materials can exhibit several intriguing features. These include among others, power oscillations and non-reciprocity of light propagation [7,9,11], absorption enhanced transmission [8], and unidirectional invisibility [15]. In the nonlinear domain, such nonreciprocal effects can be used to realiz...

Beam dynamics in synthetic optical media with PT symmetries imposed by a balanced arrangement of gain or loss is investigated. We find that the beam power evolution is insensitive to microscopic details of the system and that it follows three distinct universal laws which depend only on the magnitude of the gain or loss parameter. Our theoretical calculations are confirmed numerically for the experimentally realizable case of a lattice consisting of coupled PT -symmetric dimers.

Oxygen vacancy (VO) is a common native point defects that plays crucial roles in determining the physical and chemical properties of metal oxides such as ZnO. However, fundamental understanding of VO is still very sparse. Specifically, whether VO is mainly responsible for the n-type conductivity in ZnO has been still unsettled in the past fifty years. Here we report on a study of oxygen self-diffusion by conceiving and growing oxygen-isotope ZnO heterostructures with delicately-controlled chemical potential and Fermi level. The diffusion process is found to be predominantly mediated by VO. We further demonstrate that, in contrast to the general belief of their neutral attribute, the oxygen vacancies in ZnO are actually +2 charged and thus responsible for the unintentional n-type conductivity as well as the non-stoichiometry of ZnO. The methodology can be extended to study oxygen-related point defects and their energetics in other technologically important oxide materials.

The conservation status of small cetaceans has significantly worsened since the 1980s, when the baiji was the only species of small cetacean listed as Endangered by IUCN. Now the baiji is almost certainly extinct and 13 other species, subspecies, or populations (hereafter units-to-conserve or units) of small cetaceans are listed as Critically Endangered (CR) on the IUCN Red List. Bycatch is the main threat to 11 of the CR units. Entanglement in gillnets contributed to the extinction of the baiji and is responsible for the imminent extinction of the vaquita. Unfortunately, there is no simple technical solution to the problem of bycatch of small cetaceans. If the 8 CR units with 100 or fewer remaining individuals are to be saved, conservation zones must be established where gillnets are eliminated and bans on their use are strictly enforced. Recent experience with the vaquita in Mexico demonstrates that enforcement of such conservation zones can be very difficult. Ineffective enforcement is also a problem for at least 4 of the other CR units. Time is very short and, unless major efforts are made now to address the bycatch problem, the prospects for CR small cetaceans and other at-risk aquatic megafauna are grim. The ultimate long-term solution to the bycatch problem is the development of efficient, inexpensive, alternative fishing gear that can replace gillnets without jeopardizing the livelihoods of fishermen. Good fishery governance and the direct involvement of fishing communities are also essential to the successful conservation of most threatened populations of small cetaceans.