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Spectroscopy That Doesn’t Scratch the Floor


    Atsuo Kawasuso

    • Nationwide Institutes for Quantum Science and Know-how, Takasaki, Japan

• Physics 15, 129

Researchers have demonstrated a approach of measuring the digital states of a fabric’s floor whereas avoiding sign contaminations from deeper layers.

Determine 1: Sketch of the Auger-mediated positron floor sticking approach demonstrated by Fairchild and colleagues. (Left) A low-energy positron (pink) is directed towards a single layer of graphene (SLG) on a copper (Cu) substrate. Interacting through a digital photon (inexperienced), the positron causes an electron (blue) to be emitted from the SLG. (Middle) The digital photon has an power equal to the sum of the positron’s preliminary kinetic power and the surface-state binding power. (Proper) By absorbing the digital photon, an electron is ejected from the valence band with an power equal to that of the digital photon minus the electron binding power and the electron work operate.Sketch of the Auger-mediated positron floor sticking approach demonstrated by Fairchild and colleagues. (Left) A low-energy positron (pink) is directed towards a single layer of graphene (SLG) on a copper (Cu) substrate. Interacting through a digital ph… Present extra

The digital states of a fabric’s floor would possibly solely be 2D, however they provide a depth of attention-grabbing physics. Such states, that are distinct from these of the fabric’s bulk, dominate many phenomena, comparable to electrical conduction, magnetism, and catalysis, and they’re accountable for nontrivial floor results present in topological supplies and methods with sturdy spin-orbit interplay. Floor digital states additionally management the properties of so-called 2D supplies, comparable to graphene. To know floor phenomena and harness them in sensible units, researchers mainly depend on photoemission spectroscopy, which measures the power and momentum of electrons emitted when photons hit the fabric. The excessive decision with which electron power and momentum may be characterised permits physicists to measure each the band construction and the density of states (DOS) within the few floor layers the place escaping photoelectrons originate. Nonetheless, the contribution of bulk electrons to photoemission indicators makes it troublesome to acquire the digital states of solely the outermost layer of a pattern. Now Alexander Fairchild on the College of Texas at Arlington (UTA) and colleagues have invented a brand new methodology—primarily based on a modification of a way that liberates electrons utilizing positrons reasonably than photons—to selectively decide the electron DOS of the primary floor layer with none contributions from the majority [1].

The group’s approach, known as Auger-mediated positron floor sticking (AMPS), shouldn’t be the primary that may decide the digital states of a fabric’s top-most layer. To date, a number of such strategies have been created, one of the best recognized being spectroscopic-imaging scanning tunneling microscopy [2]. On this methodology, Fourier transformation of the real-space picture permits researchers to measure the floor native DOS and to map the digital states in k-space. A limitation is that the pattern should be electrically conductive. A second methodology for probing floor digital states is metastable-atom deexcitation spectroscopy, by which the fabric floor is probed utilizing atoms (sometimes, helium) which have been excited to a metastable digital state [3]. These metastable atoms decide up electrons on the first floor layer, which causes the atoms to decay to the bottom state. The surplus power liberated via the decay supplies the DOS of the primary floor layer. A limitation of this system is the truth that in metals the decay happens via a so-called Auger course of involving two electrons, making it troublesome to reconstruct the digital bands. New strategies are required to compensate for the weaknesses of those methods.

A promising method is positronium spectroscopy. On this methodology, a positron beam scanned throughout the pattern implants positrons into the subsurface area. These positrons diffuse again to the floor and, after choosing up electrons, they’re emitted as positronium, an electron-positron sure state. Within the pattern’s bulk, the excessive electron density screens the Coulomb interplay between a positron and an electron, which means positronium is fashioned solely within the low-electron-density area past the fabric’s outer floor, the place the screening impact vanishes. This ensures that the detected positronium particles carry power and momentum details about electrons in solely the primary floor layer. Not too long ago, a bunch on the College of California, Riverside, succeeded in acquiring the DOS of a copper floor by measuring the positronium power spectrum with meV power decision [4], whereas my very own group noticed the spin-polarized digital states of sure ferromagnet surfaces [5]. Nonetheless, this methodology is just relevant to methods with a destructive positronium work operate, that’s, methods by which positronium atoms are spontaneously emitted. For instance, bismuth-based topological insulators—that are extensively used fashions for analysis into topological supplies—have a constructive positronium work operate and therefore can’t be inspected utilizing positronium spectroscopy. The brand new AMPS methodology has no such restriction.

The brand new methodology grows out of labor carried out by the UTA group to develop magnetically guided, low-energy positron beams for positron annihilation-induced Auger electron spectroscopy, by which positrons projected on the pattern kind sure states on the pattern’s floor [6]. Finally, these positrons annihilate with core electrons of the floor atoms, leaving core holes. As electrons from increased states make the transition to fill the core holes, the surplus power is emitted through Auger electrons, the energies of which convey details about the floor digital state. If the width of the valence band is giant sufficient (10–20 eV), then Auger electrons from the valence band are additionally emitted through the transition of valence electrons into deeper valence holes [7]. (The spectrum of this “VVV” Auger course of has lengthy been neglected in standard Auger electron spectroscopy.)

The UTA group found an extra Auger electron spectrum—totally different from each the core-hole-mediated peaks and the VVV spectrum—that emerges when the positron power exceeds a sure threshold. This spectrum, which AMPS exploits, displays the contribution of a definite Auger-emission course of by which the positrons don’t penetrate the pattern however are captured within the mirror potential fashioned outdoors of the floor. As a result of there isn’t any want for the spontaneous emission of positronium atoms, the approach can be utilized to characterize supplies with a constructive positronium work operate. And since the positrons work together with floor electrons through “digital” photons that don’t penetrate the pattern, the power spectrum of the Auger electrons emitted through the AMPS course of symbolize the DOS of the outermost floor layer solely. The group confirmed the floor specificity of AMPS by making use of the approach to a clear copper floor and to a single layer of graphene on a copper substrate (Fig. 1). They discovered that the AMPS spectrum form differed considerably for the 2 surfaces: the spectrum for the clear copper floor confirmed a broad characteristic decided by the contribution of the fabric’s 3d and 4s electrons, whereas the graphene exhibited the anticipated “Dirac cone” DOS form with no contribution from the underlying copper layer.

There are numerous potential functions for a spectroscopy methodology that’s delicate to solely the floor DOS. Whereas Fairchild and colleagues confirmed that the pure graphene on copper maintained the attribute linear dispersion of the Dirac cone, it’s predicted that the Dirac cone of graphene on different 3D metals, comparable to cobalt and nickel, will probably be destroyed due to higher lattice matching between graphene and people metals [8]. A scientific survey of the modulation of graphene’s band construction relying on the substrate materials could be necessary to verify this prediction [9]. Topological floor states may also be intriguing in an identical context. The power decision of the AMPS methodology is roughly 0.2–0.5 eV for electron energies of 1–5 eV [10]. Enhancing this decision will probably be important to watch the floor DOS in additional element. Such an enchancment will make the statement of an power hole in some 2D superconductors a sensible prospect, doubtlessly confirming that Cooper pairs exist at a superconductor’s top-most layer. Moreover, if spin-polarized AMPS spectroscopy turns into accessible, extra functions in spintronics may be envisioned.

References

  1. A. J. Fairchild et al., “Photoemission spectroscopy utilizing digital photons emitted by positron sticking: A complementary probe for top-layer floor digital buildings,” Phys. Rev. Lett. 129, 106801 (2022).
  2. S. Edkins, Spectroscopic-imaging STM (SI-STM), in Visualising the Cost and Cooper-Pair Density Waves in Cuprates, Springer Theses. (Springer, Cham. 2017).
  3. Y. Yamauchi and M. Kurahashi, “Spin-polarized metastable deexcitation spectroscopy research of iron movies,” Appl. Surf. Sci. 169-170, 236 (2001).
  4. A. C. L. Jones et al., “Angle-resolved spectroscopy of positronium emission from a Cu(110) floor,” Phys. Rev. Lett. 117, 216402 (2016).
  5. M. Maekawa et al., “Spin-polarized positronium time-of-flight spectroscopy for probing spin-polarized floor digital states,” Phys. Rev. Lett. 126, 186401 (2021).
  6. A. Weiss et al., “Auger-electron emission ensuing from the annihilation of core electrons with low-energy positrons,” Phys. Rev. Lett. 61, 2245 (1988).
  7. V. A. Chirayath et al., “Auger electron emission initiated by the creation of valence-band holes in graphene by positron annihilation,” Nat. Commun. 8, 16116 (2017).
  8. A. Miyashita et al., “Spin polarization of graphene and h-BN on Co(001) and Ni(111) noticed by spin-polarized floor positronium spectroscopy,” Phys. Rev. B 97, 195405 (2018).
  9. Y. Fukaya et al., “Spacing between graphene and metallic substrates studied with total-reflection high-energy positron diffraction,” Carbon 103, 1 (2016).
  10. S. Mukherjee et al., “Time of flight spectrometer for background-free positron annihilation induced Auger electron spectroscopy,” Rev. Sci. Instrum. 87, 035114 (2016).

Concerning the Writer

Image of Atsuo Kawasuso

Atsuo Kawasuso is a senior analysis Fellow on the Nationwide Institutes for Quantum Science and Know-how, Japan. Since 1995, he has labored on creating positron beam probes and making use of them to supplies science issues. In 1998, he succeeded in demonstrating high-energy positron diffraction from a silicon floor utilizing an isotope-based electrostatic positron beam. He’s at present engaged on the event of spin-polarized positron beam methods for spintronics functions.


Topic Areas

Condensed Matter PhysicsAtomic and Molecular Physics

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