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Figure 2 | Imaging silicon atoms with terahertz-driven scanning tunnelling microscopy (TD-STM). a. Topographic images of the Si(111)-(7 x 7) surface measured in constant-current mode with standard STM at V_d.c. = -0.5 V, E_THz,pk = 0 V cm^-1, I_d.c. = -20 pA b. and terahertz-driven STM (TD-STM) of the same region with V_d.c. = 0 V, E_THz,pk = -200 V cm^-1, I_THz,avg = -20 pA

Abstract
Ultrafast control of current on the atomic scale is essential for future innovations in nanoelectronics. Extremely localized transient electric fields on the nanoscale can be achieved by coupling picosecond duration terahertz pulses to metallic nanostructures. Here, we demonstrate terahertz scanning tunnelling microscopy (THz-STM) in ultrahigh vacuum as a new platform for exploring ultrafast non-equilibrium tunnelling dynamics with atomic precision. Extreme terahertz-pulse-driven tunnel currents up to 10⁷ times larger than steady-state currents in conventional STM are used to image individual atoms on a silicon surface with 0.3 nm spacial resolution. At terahertz frequencies, the metallic-like Si(111)-(7 x 7) surface is unable to screen the electric field from the bulk, resulting in a terahertz tunnel conductance that is fundamentally different than that of the steady state. Ultrafast terahertz-induced band bending and non-equilibrium charging of surface states open new conduction pathways to the bulk, enabling extreme transient tunnel currents to flow between the tip and sample.

Public Reference:
NATURE PHYSICS: DOI: 10.1038/NPHYS4047

Credits:
Vedran Jelic^1*, Krzysztof Iwaszczuk², Peter H. Nguyen¹, Christopher Rathje³, Graham J. Hornig¹, Haille M. Sharum¹, James R. Hoffman¹, Mark R. Freeman¹ and Frank A. Hegmann^1*

¹Department of Physics, University of Alberta, Edmonton, Alberta T6G 2E1, Canada. ²DTU Fotonik – Department of Photonics Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark. ³4th Physical Institute, University of Gottingen, 37077 Gottingen, Germany.
*e-mail: jelic@ualberta.ca;, hegmann@ualberta.ca

Microscope:
RHK UHV3500 STM/AFM with Inverted Viewport

Control System:
RHK SPM1000

Figure 2 | Charge density wave (CDW) transition in MTBs. (a) STM images of a single MTB at low temperatures (120 K) exhibit three times the periodicity than the atomic corrugation imaged at room temperature. In (b) a larger scale low-T STM image and the corresponding cross-section along the indicated MTB is shown that measured the periodicity of the CDW as B1.0 nm. The schematic in (c) illustrates the relationship between CDW period and nesting vector q = 2k_F. Also the opening of a band gap at k_F is illustrated. Temperature dependent resistance measurements, shown in (d), indicate two CDW transitions. The transitions at 235 and 205 K correspond to incommensurate and commensurate CDW transitions, respectively. Depending on the applied bias voltage we also observe a drop in resistance below the CDW transition temperatures, which is attributed to CDW-sliding. The inset shows the control measurement on a bare MoS₂ substrate and shows no transitions.

Material line defects are one-dimensional structures but the search and proof of electron behaviour consistent with the reduced dimension of such defects has been so far unsuccessful. Here we show using angle resolved photoemission spectroscopy that twin-grain boundaries in the layered semiconductor MoSe₂ exhibit parabolic metallic bands. The one-dimensional nature is evident from a charge density wave transition, whose periodicity is given by k_F/π, consistent with scanning tunnelling microscopy and angle resolved photoemission measurements. Most importantly, we provide evidence for spin- and charge-separation, the hallmark of one-dimensional quantum liquids. Our studies show that the spectral line splits into distinctive spinon and holon excitations whose dispersions exactly follow the energy-momentum dependence calculated by a Hubbard model with suitable finite-range interactions. Our results also imply that quantum wires and junctions can be isolated in line defects of other transition metal dichalcogenides, which may enable quantum transport measurements and devices.

Credits:
Yujing Ma¹, Horacio Coy Diaz¹, José Avila^2,3, Chaoyu Chen^2,3, Vijaysankar Kalappattil¹, Raja Das¹, Manh-Huong Phan¹, Čadež^4,5, Josè M.P. Carmelo^4,5,6, Maria C. Asensio^2,3 & Matthias Batzill¹

¹ Department of Physics, University of South Florida, Tampa, Florida 33620, USA. ² Synchrotron SOLEIL, L’Orme des Merisiers, Saint Aubin-BP 48, Gif sur Yvette Cedex 91192, France. ³ Universite ́ Paris-Saclay, L’Orme des Merisiers, Saint Aubin-BP 48, Gif sur Yvette Cedex 91192, France. ⁴ Beijing Computational Science Research Center, Beijing 100193, China. ⁵ Center of Physics of University of Minho and University of Porto, Oporto P-4169-007, Portugal. ⁶ Department of Physics, University of Minho, Campus Gualtar, Braga P-4710-057, Portugal.

Microscope:
PanScan Freedom UHV

Control System:
RHK R9 Control System