High-resolution Scanning Tunneling Spectroscopy Research Articles

Probing the Phase Transition to a Coherent 2D Kondo Lattice.

Kondo lattices are systems with unusual electronic properties that stem from strong electron correlation, typically studied in intermetallic 3D compounds containing lanthanides or actinides. Lowering the dimensionality of the system enhances the role of electron correlations providing a new tuning knob for the search of novel properties in strongly correlated quantum matter. The realization of a 2D Kondo lattice by stacking a single-layer Mott insulator on a metallic surface is reported. The temperature of the system is steadily lowered and by using high-resolution scanning tunneling spectroscopy, the phase transition leading to the Kondo lattice is followed. Above 27K the interaction between the Mott insulator and the metal is negligible and both keep their original electronic properties intact. Below 27K the Kondo screening of the localized electrons in the Mott insulator begins and below 11K the formation of a coherent quantum electronic state extended to the entire sample, i.e., the Kondo lattice, takes place. By means of density functional theory, the electronic properties of the system and its evolution with temperature are explained. The findings contribute to the exploration of unconventional states in 2D correlated materials.

Determining the temperature of a millikelvin scanning tunnelling microscope junction

Cooling the junction of a scanning tunneling microscope to millikelvin temperatures is fundamental for high-resolution scanning tunneling spectroscopy. However, accurately determining the junction temperature has proven elusive, due to the microscopic dimension of the junction and its continuous energy exchange with the surrounding environment. Here, we employ a millikelvin scanning tunnelling microscope cooled by an adiabatic demagnetization refrigerator. Using normal-metal and superconducting tips, we perform scanning tunnelling spectroscopy on an atomically clean surface of Al(100) in a superconducting state. By varying the refrigerator temperatures between 30 mK and 1.2 K, we show that the temperature of the junction is decoupled from the temperature of the surrounding environment. To corroborate our findings, we simulate the scanning tunnelling spectroscopy data with P(E) theory and determine that the junction has a temperature of 77 mK, despite its environment being at 1.5 K.

Electron-Exciton Coupling in 1T-TiSe2 Bilayer

Excitons in solid state are bosons generated by electron-hole pairs as the Coulomb screening is sufficiently reduced. The exciton condensation can result in exotic physics such as super-fluidity and insulating state. In charge density wave (CDW) state, 1T-TiSe2 is one of the candidates that may host the exciton condensation. However, to envision its excitonic effect is still challenging, particularly at the two-dimensional limit, which is applicable to future devices. Here, we realize the epitaxial 1T-TiSe2 bilayer, the two-dimensional limit for its 2 × 2 × 2 CDW order, to explore the exciton-associated effect. By means of high-resolution scanning tunneling spectroscopy and quasiparticle interference, we discover an unexpected state residing below the conduction band and right within the CDW gap region. As corroborated by our theoretical analysis, this mysterious phenomenon is in good agreement with the electron-exciton coupling. Our study provides a material platform to explore exciton-based electronics and opto-electronics.

Inducing Open-Shell Character in Porphyrins through Surface-Assisted Phenalenyl π-Extension.

Organic open-shell compounds are extraordinarily attractive materials for their use in molecular spintronics thanks to their long spin-relaxation times and structural flexibility. Porphyrins (Pors) have widely been used as molecular platforms to craft persistent open-shell structures through solution-based redox chemistry. However, very few examples of inherently open-shell Pors have been reported, which are typically obtained through the fusion of non-Kekulé polyaromatic hydrocarbon moieties to the Por core. The inherent instability and low solubility of these radical species, however, requires the use of bulky substituents and multistep synthetic approaches. On-surface synthesis has emerged as a powerful tool to overcome such limitations, giving access to structures that cannot be obtained through classical methods. Herein, we present a simple and straightforward method for the on-surface synthesis of phenalenyl-fused Pors using readily available molecular precursors. In a systematic study, we examine the structural and electronic properties of three surface-supported Pors, bearing zero, two (PorA2), and four (PorA4) meso-fused phenalenyl moieties. Through atomically resolved real-space imaging by scanning probe microscopy and high-resolution scanning tunneling spectroscopy combined with density functional theory calculations, we unambiguously demonstrate a triplet ground state for PorA2 and a charge-transfer-induced open-shell character for the intrinsically closed-shell PorA4.

Multiple In-Gap States Induced by Topological Surface States in the Superconducting Topological Crystalline Insulator Heterostructure Sn1−xPbxTe−Pb

Superconducting topological crystalline insulators (TCIs) have been proposed to be a new type of topological superconductor where multiple Majorana zero modes may coexist under the protection of lattice symmetries. The bulk superconductivity of TCIs has been realized, but it is quite challenging to detect the superconductivity of topological surface states inside their bulk superconducting gaps. Here, we report high-resolution scanning tunneling spectroscopy measurements on lateral Sn_{1-x}Pb_{x}Te-Pb heterostructures using superconducting tips. Both the bulk superconducting gap and the multiple in-gap states with energy differences of ∼0.3 meV can be clearly resolved on TCI Sn_{1-x}Pb_{x}Te at 0.38K. Quasiparticle interference measurements further confirm the in-gap states are gapless. Our work demonstrates that the unique topological superconductivity of a TCI can be directly distinguished in the density of states, which helps to further investigate the multiple Dirac and Majorana fermions inside the superconducting gap.

Band Bending and Valence Band Quantization at Line Defects in MoS2

The variation of the electronic structure normal to 1D defects in quasi-freestanding MoS2, grown by molecular beam epitaxy, is investigated through high resolution scanning tunneling spectroscopy at 5 K. Strong upward bending of valence and conduction bands toward the line defects is found for the 4|4E mirror twin boundary and island edges but not for the 4|4P mirror twin boundary. Quantized energy levels in the valence band are observed wherever upward band bending takes place. Focusing on the common 4|4E mirror twin boundary, density functional theory calculations give an estimate of its charging, which agrees well with electrostatic modeling. We show that the line charge can also be assessed from the filling of the boundary-localized electronic band, whereby we provide a measurement of the theoretically predicted quantized polarization charge at MoS2 mirror twin boundaries. These calculations elucidate the origin of band bending and charging at these 1D defects in MoS2. The 4|4E mirror twin boundary not only impairs charge transport of electrons and holes due to band bending, but holes are additionally subject to a potential barrier, which is inferred from the independence of the quantized energy landscape on either side of the boundary.

(Invited) Microscopic Studies of Black Phosphorus and Its Field-Effect Transistors

In this manuscript, we report atomic scale studies of electronic direct bandgap measurement of the puckered honeycomb structure of freshly cleaved black phosphorus using a high-resolution scanning tunneling spectroscopy (STS) survey. Through a combination of STM/S measurements and first-principles calculations, a model for edge reconstruction and the related edge states are also determined. Besides, we also investigate electrical transport and optoelectronic properties of field effect transistors (FETs) made from few-layer black phosphorus (BP) crystals down to a few nanometers. In particular, we explore the anisotropic nature and photocurrent generation mechanisms in BP FETs through spatial- and polarization-dependent photocurrent measurements. Our results reveal that the anisotropic feature primarily results from the directional-dependent absorption of BP crystals.

(Invited) Microscopic Studies of Black Phosphorus and Its Field-Effect Transistors

An increasing number of alternative two-dimensional (2D) materials are being explored in the “post-graphene age”, such as transition metal dichalcogenides (TMDs), silicene, and germanene. These materials are studied with the hope of overcoming graphene’s deficiencies such as its zero bandgap. Among these 2D layered materials, layered black phosphorus (phosphorene) is expected to exhibit superior mechanical, electrical, and optical properties due to an intrinsic and tunable bandgap. It is, besides graphene, the only stable elemental 2D material that can be mechanically exfoliated. Its high hole mobility and direct semiconducting bandgap hold the hope for the development of new electronic devices in the postsilicon era. However, before phosphorene can be integrated in everyday electronics, many aspects of the material still remain to be elucidated. Here we report atomic scale studies of electronic variation related to strain-induced anisotropic deformation of the puckered honeycomb structure of freshly cleaved black phosphorus using a high-resolution scanning tunneling spectroscopy (STS) survey along the light (x) and heavy (y) effective mass directions. Through a combination of STS measurements and first-principles calculations, a model for edge reconstruction and the related edge states are also determined. Besides, we also investigate electrical transport and optoelectronic properties of field effect transistors (FETs) made from few-layer black phosphorus (BP) crystals down to a few nanometers. In particular, we explore the anisotropic nature and photocurrent generation mechanisms in BP FETs through spatial-, polarization-, gate-, and bias-dependent photocurrent measurements. Our results reveal that the photocurrent signals at BP-electrode junctions are mainly attributed to the photovoltaic effect in the off-state and photo-thermoelectric effect in the on-state, and their anisotropic feature primarily results from the directional-dependent absorption of BP crystals. 1. Liangbo Liang, Jun Wang, Bobby G. Sumpter, Vincent Meunier, and Minghu Pan, “Electronic Bandgap and Edge Reconstruction in Phosphorene Materials”, Nano Letters, 2014, 14 (11), 6400–6406 2. Tu Hong, Bhim Chamlagain, Wenzhi Lin, Hsun-Jen Chuang, Minghu Pan, Zhixian Zhou, and Ya-Qiong Xu, “Polarized Photocurrent Response in Black Phosphorus Field-Effect Transistors”, Nanoscale, 6, 8978-8983(2014).

Electronic bandgap and edge reconstruction in phosphorene materials.

Single-layer black phosphorus (BP), or phosphorene, is a highly anisotropic two-dimensional elemental material possessing promising semiconductor properties for flexible electronics. However, the direct bandgap of single-layer black phosphorus predicted theoretically has not been directly measured, and the properties of its edges have not been considered in detail. Here we report atomic scale electronic variation related to strain-induced anisotropic deformation of the puckered honeycomb structure of freshly cleaved black phosphorus using a high-resolution scanning tunneling spectroscopy (STS) survey along the light (x) and heavy (y) effective mass directions. Through a combination of STS measurements and first-principles calculations, a model for edge reconstruction is also determined. The reconstruction is shown to self-passivate most dangling bonds by switching the coordination number of phosphorus from 3 to 5 or 3 to 4.

Ex situ elaborated proximity mesoscopic structures for ultrahigh vacuum scanning tunneling spectroscopy

We apply ultrahigh vacuum Scanning Tunneling Spectroscopy (STS) at ultra-low temperature to study proximity phenomena in metallic Cu in contact with superconducting Nb. In order to solve the problem of Cu-surface contamination, Cu(50 nm)/Nb(100 nm) structures are grown by respecting the inverted order of layers on SiO2/Si substrate. Once transferred into vacuum, the samples are cleaved at the structure-substrate interface. As a result, a contamination-free Cu-surface is exposed in vacuum. It enables high-resolution STS of superconducting correlations induced by proximity from the underlying superconducting Nb layer. By applying magnetic field, we generate unusual proximity-induced superconducting vortices and map them with a high spatial and energy resolution. The suggested method opens a way to access local electronic properties of complex electronic mesoscopic devices by performing ex situ STS under ultrahigh vacuum.

Evidence of a Spin Resonance Mode in the Iron-Based SuperconductorBa0.6K0.4Fe2As2from Scanning Tunneling Spectroscopy

We used high-resolution scanning tunneling spectroscopy to study the hole-doped iron pnictide superconductor Ba(0.6)K(0.4)Fe(2)As(2) (T(c)=38 K). Features of a bosonic excitation (mode) are observed in the measured quasiparticle density of states. The bosonic features are intimately associated with the superconducting order parameter and have a mode energy of ~14 meV, similar to the spin resonance measured by inelastic neutron scattering. These results indicate a strong electron-spin excitation coupling in iron pnictide superconductors, similar to that in high-T(c) copper oxide superconductors.

High-Resolution Scanning Tunneling Spectroscopy of Magnetic Impurity Induced Bound States in the Superconducting Gap of Pb Thin Films

Tunneling spectra for individual atoms and dimers of Mn and Cr adsorbed on superconducting Pb thin films were measured by a low temperature scanning tunneling microscope. Multiple-resonance structures within the superconducting gap on the adsorbates were resolved and interpreted as the magnetic impurity-induced bound states associated with different scattering channels. The experiment demonstrates a spectroscopic approach to characterizing the spin states of magnetic structures and exploring the competition between superconductivity and magnetism at the nanometer scale.

High resolution scanning tunneling spectroscopy of ultrathin Pb on Si(1 1 1)-(6 × 6) substrate

The electronic structure of Si(1 1 1)–(6 × 6)Au surface covered with submonolayer amount of Pb is investigated using scanning tunneling spectroscopy. Already in small islands of Pb with thickness of 1 ML Pb (1 1 1) and with the diameter of only about 2 nm we detected the quantized electronic state with energy 0.55 eV below the Fermi level. Similarly, the I( V) characteristics made for the Si(1 1 1)–(6 × 6)Au surface reveal a localized energy state 0.3 eV below the Fermi level. These energies result from fitting of the theoretical curves to the experimental data. The calculations are based on tight binding Hubbard model. The theoretical calculations clearly show prominent modification of the I( V) curve due to variation of electronic and topographic properties of the STM tip apex.