Scanning tunneling microscopy studies of monolayer templates: alkylthioethers and alkylethers

Scanning tunneling microscopy has been used to determine the molecular ordering in stable, ordered monolayers formed from long-chain normal and substituted alkanes in solution on highly oriented pyrolytic graphite surfaces. Monolayers were initially formed using an overlying solution of either a symmetrical dialkylthioether or a symmetrical dialkylether. Initially pure thioether solutions were then changed to nearly pure solutions of the identical chain-length ether, and vice versa. The direct application of a pure solution of long-chain symmetrical ethers onto graphite produced a lamellate monolayer within which the individual molecular axes were oriented at an angle of ~65° to the lamellar axes. In contrast, a pure solution of long-chain symmetrical thioethers on graphite produced a monolayer within which the molecular axes were oriented perpendicular to the lamellar axes. When ethers were gradually added to solutions overlying pure thioether monolayers, the ethers substituted into the existing monolayer structure. Thus, the ether molecules could be forced to orient in the perpendicular thioether-like manner through the use of a thioether template monolayer. Continued addition of ethers to the solution ultimately produced a nearly pure ether monolayer that retained the orientation of the thioether monolayer template. However, a monolayer of thioether molecules formed by gradual substitution into an ether monolayer did not retain the 65° orientation typical of dialkylethers, but exhibited the 90° orientation typical of dialkylthioether monolayers. The thioethers and ethers were easily distinguished in images of mixed monolayers, allowing both an analysis of the distribution of the molecules within the mixed monolayers and a comparison of the monolayer compositions with those of the overlying solutions. Substitution of molecules into the template monolayer did not proceed randomly; instead, a molecule within a monolayer was more likely to be replaced by a molecule in the overlying solution if it was located next to a molecule that had already been replaced

Room Temperature Intrinsic Ferromagnetism in Epitaxial Manganese Selenide Films in the Monolayer Limit

Monolayer van der Waals (vdW) magnets provide an exciting opportunity for exploring two-dimensional (2D) magnetism for scientific and technological advances, but the intrinsic ferromagnetism has only been observed at low temperatures. Here, we report the observation of room temperature ferromagnetism in manganese selenide (MnSe$_x$) films grown by molecular beam epitaxy (MBE). Magnetic and structural characterization provides strong evidence that in the monolayer limit, the ferromagnetism originates from a vdW manganese diselenide (MnSe$_2$) monolayer, while for thicker films it could originate from a combination of vdW MnSe$_2$ and/or interfacial magnetism of $\alpha$-MnSe(111). Magnetization measurements of monolayer MnSe$_x$ films on GaSe and SnSe$_2$ epilayers show ferromagnetic ordering with large saturation magnetization of ~ 4 Bohr magnetons per Mn, which is consistent with density functional theory calculations predicting ferromagnetism in monolayer 1T-MnSe$_2$. Growing MnSe$_x$ films on GaSe up to high thickness (~ 40 nm) produces $\alpha$-MnSe(111), and an enhanced magnetic moment (~ 2x) compared to the monolayer MnSe$_x$ samples. Detailed structural characterization by scanning transmission electron microscopy (STEM), scanning tunneling microscopy (STM), and reflection high energy electron diffraction (RHEED) reveal an abrupt and clean interface between GaSe(0001) and $\alpha$-MnSe(111). In particular, the structure measured by STEM is consistent with the presence of a MnSe$_2$ monolayer at the interface. These results hold promise for potential applications in energy efficient information storage and processing

Imaging Spin Reorientation Transitions in Consecutive Atomic Co layers

By means of spin-polarized low-energy electron microscopy (SPLEEM) we show that the magnetic easy-axis of one to three atomic-layer thick cobalt films on ruthenium crystals changes its orientation twice during deposition: one-monolayer and three-monolayer thick films are magnetized in-plane, while two-monolayer films are magnetized out-of-plane, with a Curie temperature well above room temperature. Fully-relativistic calculations based on the Screened Korringa-Kohn-Rostoker (SKKR) method demonstrate that only for two-monolayer cobalt films the interplay between strain, surface and interface effects leads to perpendicular magnetization.Comment: 5 pages, 4 figures. Presented at the 2005 ECOSS conference in Berlin, and at the 2005 Fall meeting of the MRS. Accepted for publication at Phys. Rev. Lett., after minor change

Using magnetic stripes to stabilize superfluidity in electron-hole double monolayer graphene

Experiments have confirmed that double monolayer graphene cannot generate finite temperature electron-hole superfluidity. This has been shown to be due to very strong screening of the electron-hole pairing attraction. The linear dispersing energy bands in monolayer graphene prevent attempts to reduce the strength of the screening. We propose a new hybrid device in which the two sheets of monolayer graphene are placed in a modulated periodic perpendicular magnetic field. Such a magnetic field preserves the isotropic Dirac cones of the original monolayers but it reduces the slope of the cones so that the monolayer Fermi velocity $v_F$ is smaller. We show that with current experimental techniques, this reduction in $v_F$ can sufficiently weaken the screening to permit electron-hole superfluidity at measurable temperatures.Comment: Revised version. MultiSuper collaboration: http://www.multisuper.or