High resolution ultrasonic velocity measurements have been used to determine the temperaturemagnetic-field phase diagram of the monoclinic multiferroic CuO. A new transition at TN3 = 230 K, corresponding to an intermediate state between the antiferromagnetic non-collinear spiral phase observed below TN2 = 229.3 K and the paramagnetic phase, is revealed. Anomalies associated with a first order transition to the commensurate collinear phase are also observed at TN1 = 213 K. For fields with B b, a spin-flop transition is detected between 11 T -13 T at lower temperatures. Moreover, our analysis using a Landau-type free energy clearly reveals the necessity for an incommensurate collinear phase between the spiral and the paramagnetic phase. This model is also relevant to the phase diagrams of other monoclinic multiferroic systems.PACS numbers: 75.30.Kz, 75.85.+t, 75.30.Gw Multiferroic phenomena have been a subject of intense interest in recent decades arising from opportunities to explore new fundamental physics as well as possible technological applications [1][2][3]. Coupling between different ferroic orders has been proven to be driven by several different types of mechanisms. In particular, multiferroics with a spiral spin-order-induced ferroelectricity have revealed high spontaneous polarization and strong magnetoelectric coupling [4,5]. Cupric oxide (CuO), the subject of this letter, was characterized as a magnetoelectric multiferroic four years ago when it was shown that its ferroelectric order is induced by the onset of a spiral antiferromagnetic (AFM) order at an unusually high temperature of 230 K [3]. Thus far, two AFM states have been reported, a low temperature (T N 1 ∼ 213 K) AF1 commensurate collinear state with the magnetic moments along the monoclinic b axis and an AF2 incommensurate spiral state with half of the magnetic moments in the ac plane (T N 2 ∼ 230 K) [3,6,7]. However, the authors of the neutron diffraction measurements [6] Encouraged by recent experiments on other multiferroic systems using ultrasonic measurements [11], we measured the temperature and field dependence of the velocity of transverse modes in order to determine the magnetic phase diagram of CuO. A new transition is detected at T N 3 = 230 K just above the AF2 spiral phase observed at T N 2 = 229.3 K, while the first order transition is observed at T N 1 = 213 K. Furthermore, dielectric constant measurements confirm that only the spiral phase (between T N 1 and T N 2 ) supports a spontaneous electric polarization. In addition, we report on a spin-flop transition in the low temperature AF1 collinear phase when B b. Thus, based on these findings, a new magneticfield vs temperature phase diagram is proposed for CuO. In order to elucidate the possible nature of the AFM states observed in CuO, a non-local Landau-type free energy is also developed for CuO and similar monoclinic multiferroics. This approach has been very successful in explaining the magnetic phase diagrams of other multiferroic systems [12][13][14]. In contrast w...

We report the incorporation of substitutional Mn atoms in high-quality, epitaxial 1 graphene on Cu(111), using ultra-low energy ion implantation. We characterize in detail the atomic structure of substitutional Mn in a single carbon vacancy and quantify its concentration. In particular, we are able to determine the position of substitutional Mn atoms with respect to the Moiré superstructure (i.e. local graphene-Cu stacking symmetry) and to the carbon sublattice; in the out-of-plane direction, substitutional Mn atoms are found to be slightly displaced towards the Cu surface, i.e. effectively underneath the graphene layer. Regarding electronic properties, we show that graphene doped with substitutional Mn to a concentration of the order of 0.04%, with negligible structural disorder (other than the Mn substitution), retains the Dirac-like band structure of pristine graphene on Cu(111), making it an ideal system in which to study the interplay between local magnetic moments and Dirac electrons. Our work also establishes that ultra-low energy ion implantation is suited for substitutional magnetic doping of graphene; given the flexibility, reproducibility and scalability inherent to ion implantation, our work creates numerous opportunities for research on magnetic functionalization of graphene and other 2D materials.

We provide the first systematic characterization of the structural and photoluminescence properties of optically active centers fabricated upon implantation of 30–100 keV Mg+ ions in synthetic diamond. The structural configurations of Mg-related defects were studied by the electron emission channeling technique for short-lived, radioactive 27Mg implantations at the CERN-ISOLDE facility, performed both at room temperature and 800 °C, which allowed the identification of a major fraction of Mg atoms (∼30 to 42%) in sites which are compatible with the split-vacancy structure of the MgV complex. A smaller fraction of Mg atoms (∼13 to 17%) was found on substitutional sites. The photoluminescence emission was investigated both at the ensemble and individual defect level in the 5–300 K temperature range, offering a detailed picture of the MgV-related emission properties and revealing the occurrence of previously unreported spectral features. The optical excitability of the MgV center was also studied as a function of the optical excitation wavelength to identify the optimal conditions for photostable and intense emission. The results are discussed in the context of the preliminary experimental data and the theoretical models available in the literature, with appealing perspectives for the utilization of the tunable properties of the MgV center for quantum information processing applications.

Note: numbers of references are with respect to those in the main paper Additional info on Emission Channeling (EC) experimental parameters and data analysisBoth 121 Sn (t 1/2 =27.06 h) and 121m Sn (t 1/2 =55 y) were produced at CERN's ISOLDE on-line isotope separator facility [39][40] by means of bombarding a W neutron converter next to a UC 2 target with 1 GeV protons, thus causing neutron-induced fission in the UC 2 . Following out-diffusion from the ~2000°C hot target, Sn atoms were selectively ionized to Sn + by means of resonant laser ionization [41], followed by 60 kV electrostatic acceleration and high-resolution (M/∆M~6000) separation of mass 121 using a 90° and 60° magnet. The implanted beam thus consisted of a mixture of 121 Sn and 121m Sn. The number of implanted 121 Sn+ 121m Sn atoms was determined as 1.8×10 10 by integrating the implantation current of ~3pA (corrected for emission of secondary electrons), resulting in the fluence of 2.3×10 12 cm −2 in the beam spot with 1 mm diameter collimation. The β − activity of the sample was determined from the count rate measured in the pad detector (cf. below) as 56370 Bq. Note that due to the large half-life ratio of 121m Sn and 121 Sn (17804:1), the activity of the sample was almost entirely due to the short-lived isotope 121 Sn, from which one estimates a number of 7.9×10 9 121 Sn atoms, or 44% of the total number of implanted atoms, the remaining 56% being due to 121m Sn. Note that the 121m Sn activity calculated from this ratio is 4.0 Bq only. The EC measurements in the asimplanted state were completed 36.5 h after the implantation, and the sample was then annealed at 920°C in vacuum better than 10 −5 mbar for 10 min. At this point in time around 27% of the implanted 121 Sn+ 121m Sn atoms had decayed to 121 Sb.

We report the formation of nanobubbles on graphene with a radius of the order of 1 nm, using ultralow energy implantation of noble gas ions (He, Ne, Ar) into graphene grown on a Pt(111) surface. We show that the universal scaling of the aspect ratio, which has previously been established for larger bubbles, breaks down when the bubble radius approaches 1 nm, resulting in much larger aspect ratios. Moreover, we observe that the bubble stability and aspect ratio depend on the substrate onto which the graphene is grown (bubbles are stable for Pt but not for Cu) and trapped element. We interpret these dependencies in terms of the atomic compressibility of the noble gas as well as of the adhesion energies between graphene, the substrate, and trapped atoms.