Differential Tunneling Conductance Research Articles

Electronic States of Single Perovskite Quantum Dots in Weak and Strong Interaction Regimes: Implications in Electrically Pumped Quantum Emitters.

We investigate the effect of Coulomb interactions on the electronic states of a single perovskite quantum dot (PQD), CsPbBr3, through scanning tunneling microscopy/spectroscopy (STM/S). Under a weak interaction regime, where the time-averaged occupation of electrons in a PQD remains zero, the peaks observed in the differential tunneling conductance (dI/dV) spectrum correspond to the single-particle density of states (DOS) without any electron-electron correlation. However, with a shorter tunnel distance between the STM tip and PQD, additional electrons are trapped in the QD, leading to a strong interaction regime with well-defined electronic fine structures due to the lifting of spin degeneracy in the conduction bands. Interestingly, we observe that the strong Coulomb interaction can modify the spin-orbit coupling (SOC) strength in the PQDs. We have concluded that the energy levels under a strong electron-electron interaction regime are of utmost importance since they will be applicable to electrically pumped PQD-based single photon quantum emitters.

Ultra-low-noise transimpedance amplifier in cryogenic STM for studying novel quantum states by measuring shot noise

An ultra-low-noise large-bandwidth transimpedance amplifier (TIA) for cryogenic scanning tunneling microscope (CryoSTM) is proposed. The TIA connected with the tip-sample component in CryoSTM is called as CryoSTM-TIA. Its transimpedance gain is as high as 1 GΩ, and its bandwidth is over 300 kHz, but its equivalent input noise current power spectral density is less than 4 (fA)2/Hz at 100 kHz. The low inherent noise for the CryoSTM-TIA is due to its special design: (1) its pre-amplifier is made of a pair of low-noise cryogenic high electron mobility transistors (HEMTs); (2) the noise generated by one HEMT is eliminated by a large capacitor; (3) the capacitance of the cable connected the gate of the other HEMT to the tip is minimized; (4) thermal noise sources, such as the feedback resistor, are placed in the cryogenic zone. The dc output voltage drift of the CryoSTM-TIA is very low, as 5 μV/°C. The apparatus can be used for measuring the scanning tunneling differential conductance spectra, especially the scanning tunneling shot noise spectra (STSNS) of quantum systems, even if the shot noise is very low. It provides a universal tool to study various novel quantum states by measuring STSNS, such as detecting the Majorana bound states.

Visualizing Large Facet-Dependent Electronic Tuning in Monolayer WSe2 on Au Surfaces.

Two-dimensional transition metal dichalcogenides (TMDs) have shown great importance in the development of novel ultrathin optoelectronic devices owing to their exceptional electronic and photonic properties. Effectively tuning their electronic band structures is not only desired in electronics applications but also can facilitate more novel properties. In this work, we demonstrate that large electronic tuning on a WSe2 monolayer can be realized by different facets of a Au-foil substrate, forming in-plane p-n junctions with remarkable built-in electric fields. This facet-dependent tuning effect is directly visualized by using scanning tunneling microscopy and differential conductance (dI/dV) spectroscopy. First-principles calculations reveal that the atomic arrangement of the Au facet effectively changes the interfacial coupling and charge transfer, leading to different magnitudes of charge doping in WSe2. Our study would be beneficial for future customized fabrication of TMD-junction devices via facet-specific construction on the substrate.

Topological Defects Induced High-Spin Quartet State in Truxene-Based Molecular Graphenoids

Topological Defects Induced High-Spin Quartet State in Truxene-Based Molecular Graphenoids

Low-noise large-bandwidth transimpedance amplifier for measuring scanning tunneling shot noise spectra in cryogenic STM and its applications

Shot noise is a powerful tool to study quantum systems. In this work, a design of transimpedance amplifier (TIA) for a cryogenic scanning tunneling microscope (CryoSTM) is proposed to meet the requirements of the shot noise measurements for quantum systems. In the TIA, the preamplifier is made of the low-noise low-power cryogenic high electron mobility transistors. With the high transimpedance gain of 1 GΩ, the bandwidth of the proposed TIA is larger than 300 kHz. In the CryoSTM, the TIA with the tip-sample component is called as CryoSTM-TIA. The bandwidth of the proposed CryoSTM-TIA is still larger than 300 kHz. Its equivalent input noise current power spectral density is less than 30(fA)2/Hz at 100 kHz. It is detailed, for quantum systems, by using the CryoSTM-TIA, how to measure scanning tunneling current spectra, scanning tunneling differential conductance spectra, and scanning tunneling noise current power spectra, in atomic scale, and then extract their scanning tunneling shot noise spectra. Thus, it is possible to study novel quantum phenomena in various quantum systems by measuring shot noise with the CryoSTM-TIA, such as the Andreev reflection in atomic scale, the Kondo effect in a single molecular magnet, and the existence of Majorana bound states, etc.

Impact of band-engineered polymer-interlayers on heterojunction solar cells based on 2D-layered perovskites (Cs3Sb2ClxI9−x)

• Solar cells based on 2D-layered perovskites (Cs 3 Sb 2 Cl x I 9− x ) fabricated. • A hybrid interlayer introduced to reduce the recombination loss . • Energy-matched MoS 2 and WS 2 nanosheets added to facilitate collection of photogenerated carriers . • Band-diagram of heterojunctions formed through scanning tunneling spectroscopy. Thin-film formation process of Cs 3 Sb 2 Cl x I 9− x perovskite in its 2D layered-form is complex and provides a poor control over film-morphology and results in a rough film-surface. Such a surface prompts formation of recombination centers for photogenerated carriers at the interface between the active and carrier-transport layers. We report a process of interface-engineering to decrease recombination loss and thereby improve photovoltaic parameters. Apart from using an inert polymer layer to improve the surface morphology of Cs 3 Sb 2 Cl x I 9− x thin-films, we have introduced transition metal dichalcogenide (TMD) nanoflakes into the polymer; the hybrid interlayer (polymer:TMD) between the active perovskite layer and an electron-transport layer (ETL) could increase the recombination resistance without affecting the series resistance of devices, leading to an increase in photovoltaic properties. We have employed atomic force microscopy and Kelvin probe force microscopy to probe the perovskite surface. Current-voltage characteristics and impedance spectroscopy of the devices were recorded to derive their recombination and series resistances upon the interface-engineering process. Band-diagram of the devices based on Cs 3 Sb 2 Cl x I 9− x /polymer:TMD/ETL have been drawn from scanning tunneling spectroscopy and differential tunnel conductance thereof. The heterojunction devices with a type-II band-alignment at the interfaces have resulted in an enhanced photovoltaic performance in Cs 3 Sb 2 Cl x I 9− x -based solar cells with a PCE of 3.42%.

Observation of Hidden Parts of Dislocation Loops in Thin Pb Films by Means of Scanning Tunneling Spectroscopy

Local electronic properties of quasi-two-dimensional Pb(111) islands with screw dislocations of different types on their surfaces were experimentally studied by means of low-temperature scanning tunneling microscopy and spectroscopy in the regime of constant current. A comparison of the topography map, the maps of tunneling current variation and the differential tunneling conductance acquired simultaneously allows one to visualize the hidden parts of the dislocation loops under the sample surface. We demonstrate that two closely-positioned screw dislocations with the opposite Burgers vectors can either (i) connect to each other by the sub-surface dislocation loop or (ii) generate independent hidden edge dislocation lines which run towards the perimeter of the Pb island. In addition, we found a screw dislocation, which does not produce outcoming sub-surface dislocation loops. Screw dislocations and the hidden dislocations lines are the source of the non-quantized variation of the local thickness of the Pb terraces, affecting the local electronic properties.

Comparative study of adsorptions, reactions and electronic properties of U atoms on Cu(111), Ag(111), Au(111) and Ru(0001) surfaces

The mysterious properties of individual U atoms on transition metal surfaces play indispensable parts in supplementing our understanding of uranium-transition metal systems, which are important subjects for both nuclear energy applications and fundamental scientific studies. By using scanning tunneling microscopy and density functional theory calculations, the adsorptions, reactions and electronic properties of individual U atoms on Cu(111), Ag(111), Au(111) and Ru(0001) surfaces were comparatively studied for the first time in this work. Upon the deposition of a small amount of U onto Cu(111) or Ag(111) at 8 K, individual U atoms show relatively high activity and can either be adsorbed on intact substrate surfaces or induce various surface vacancies surrounded by clusters of substrate atoms. By contrast, the majority of U atoms tend to dispersedly adsorb on intact surfaces of Au(111) and Ru(0001) rather than producing surface vacancies at the same temperature. In all cases, Kondo resonance manifested as asymmetric dip feature around Fermi energy is only observed in the differential tunneling conductance spectra of single U adatoms on Ag(111).

Quantum transport in a single molecular transistor at finite temperature

We study quantum transport in a single molecular transistor in which the central region consists of a single-level quantum dot and is connected to two metallic leads that act as a source and a drain respectively. The quantum dot is considered to be under the influence of electron–electron and electron–phonon interactions. The central region is placed on an insulating substrate that acts as a heat reservoir that interacts with the quantum dot phonon giving rise to a damping effect to the quantum dot. The electron–phonon interaction is decoupled by applying a canonical transformation and then the spectral density of the quantum dot is calculated from the resultant Hamiltonian by using Keldysh Green function technique. We also calculate the tunneling current density and differential conductance to study the effect of quantum dissipation, electron correlation and the lattice effects on quantum transport in a single molecular transistor at finite temperature.

Visualizing delocalized correlated electronic states in twisted double bilayer graphene

The discovery of interaction-driven insulating and superconducting phases in moiré van der Waals heterostructures has sparked considerable interest in understanding the novel correlated physics of these systems. While a significant number of studies have focused on twisted bilayer graphene, correlated insulating states and a superconductivity-like transition up to 12 K have been reported in recent transport measurements of twisted double bilayer graphene. Here we present a scanning tunneling microscopy and spectroscopy study of gate-tunable twisted double bilayer graphene devices. We observe splitting of the van Hove singularity peak by ~20 meV at half-filling of the conduction flat band, with a corresponding reduction of the local density of states at the Fermi level. By mapping the tunneling differential conductance we show that this correlated system exhibits energetically split states that are spatially delocalized throughout the different regions in the moiré unit cell, inconsistent with order originating solely from onsite Coulomb repulsion within strongly-localized orbitals. We have performed self-consistent Hartree-Fock calculations that suggest exchange-driven spontaneous symmetry breaking in the degenerate conduction flat band is the origin of the observed correlated state. Our results provide new insight into the nature of electron-electron interactions in twisted double bilayer graphene and related moiré systems.

Surface Morphology and Differential Tunneling Conductivity in Multicomponent Solid Solutions (Bi, Sb, Sn, Ge)2(Te, Se)3

The morphology of the interlayer surface and the spectra of differential tunneling conductivity in solid solutions (Bi, Sb, Sn, Ge)2(Te, Se)3 has been studied by scanning tunneling microscopy and scanning tunneling spectroscopy. Surface defects associated with distortions of surface electronic states as a result of atomic substitution and with the formation of intrinsic defects formed during the growth of solid solutions are systematized. The position of the Dirac point and its fluctuations relative to the mean value are determined, the levels of impurity surface defects are found, and their energy depending on the composition of the solid solution, the value of the Seebeck coefficient, and the power parameter are calculated. The surface concentration of Dirac fermions is calculated and the compositions are established in which the contribution of surface states increases due to an increase in the concentration of fermions and due to an increase in the Fermi velocity and mobility of fermions in the surface layer.

Local Electronic Properties of Coherent Single-Layer WS2/WSe2 Lateral Heterostructures.

Lateral single-layer transition metal dichalcogenide (TMD) heterostructures are promising building blocks for future ultrathin devices. Recent advances in the growth of coherent heterostructures have improved the structural precision of lateral heterojunctions, but an understanding of the electronic effects of the chemical transition at the interface and associated strain is lacking. Here we present a scanning tunneling microscopy study of single-layer coherent TMD heterostructures with nearly uniform strain on each side of the heterojunction interface. We have characterized the local topography and electronic structure of single-layer WS2/WSe2 heterojunctions exhibiting ultrasharp coherent interfaces. Uniform built-in strain on each side of the interface arising from lattice mismatch results in a reduction of the bandgap of WS2. By mapping the tunneling differential conductance across the interface, we find type-II band alignment and an ultranarrow electronic transition region only ∼3 nm in width that arises from wave function mixing between the two materials.

Дифференциальная туннельная проводимость в многокомпонентных твердых растворах Bi-=SUB=-2-x-=/SUB=-Sb-=SUB=-x-=/SUB=-Te-=SUB=-3-y-z-=/SUB=-Se-=SUB=-y-=/SUB=-S-=SUB=-z-=/SUB=-

Differential tunnel conductance of the interlayer van der Waals surface (0001) was investigated in n-Bi2-xSbxTe3-y-zSeySz multicomponent solid solutions at x = 0.2, y = z = 0.09, optimized for near room temperatures with a high power factor, and at x = 0.4, y = 0, z = 0.06 with optimal thermoelectric properties for low temperatures and a high Seebek coefficient. It is shown that the intensity of the fluctuations of the Dirac point energy, a shift the top of the valence band and presence of surface levels in the energy gap formed by impurity defects is determined with composition and thermoelectric properties of solid solutions. The contribution of the surface states of Dirac fermions increases in n-Bi1.8Sb0.2Te2.82Se0.09S0.09 solid solution with a high power factor due to significant decrease of the surface concentration near the charge neutrality point and the growth of fermion mobility.

Violation of the Time-Reversal and Particle-Hole Symmetries in Strongly Correlated Fermi Systems: A Review

In this review, we consider the time reversal T and particle-antiparticle C symmetries that, being most fundamental, can be violated at microscopic level by a weak interaction. The notable example here is from condensed matter, where strongly correlated Fermi systems like heavy-fermion metals and high Tc superconductors exhibit C and T symmetries violation due to so-called non-Fermi liquid (NFL) behavior. In these systems, tunneling differential conductivity (or resistivity) is a very sensitive tool to experimentally test the above symmetry break. When a strongly correlated Fermi system turns out to be near the topological fermion condensation quantum phase transition (FCQPT), it exhibits the NFL properties, so that the C symmetry breaks down, making the differential tunneling conductivity to be an asymmetric function of the bias voltage V. This asymmetry does not take place in normal metals, where Landau Fermi liquid (LFL) theory holds. Under the application of magnetic field, a heavy fermion metal transits to the LFL state, and σ(V) becomes symmetric function of V. These findings are in good agreement with experimental observations. We suggest that the same topological FCQPT underlies the baryon asymmetry in the Universe. We demonstrate that the most fundamental features of the nature are defined by its topological and symmetry properties.

Tunneling Spectroscopy in Carbon Nanotube-Hexagonal Boron Nitride-Carbon Nanotube Heterojunctions.

Electron tunneling spectroscopy is a powerful technique to probe the unique physical properties of one-dimensional (1D) single-walled carbon nanotubes (SWNTs), such as the van Hove singularities in the density of states or the power-law tunneling probability of a Luttinger liquid. However, little is known about the tunneling behavior between two 1D SWNTs over a large energy spectrum. Here, we investigate the electron tunneling behavior between two crossed SWNTs across a wide spectral window up to 2 eV in the unique carbon nanotube-hexagonal boron nitride-carbon nanotube heterojunctions. We observe many sharp resonances in the differential tunneling conductance at different bias voltages applied between the SWNTs. These resonances can be attributed to elastic tunneling into the van Hove singularities of different 1D subbands in both SWNTs, and they allow us to determine the quasi-particle bandgaps and higher-lying 1D subbands in SWNTs on the insulating substrate.

Two-dimensional nanohybrid of MoS2 and Rose Bengal: Facile solution growth and band structure probing

Abstract We report a facile growth of 2D layered nanohybrid (LNH) of liquid-phase exfoliated MoS2 flakes and Rose Bengal (RB) molecules as charge-transfer complex, evidenced with several methods including UV–vis absorption and photoluminescence. Raman line-broadening and red-shift to lower-wavenumber side of A1g peak of MoS2 in nanohybrid confirms adsorption of p-type RB as n-type dopant including lattice-strain modulation. Nanohybrid also shows better stability in solution than its constituent components as evidenced by Zeta potential results. The crystal phase invariance of MoS2 after forming nanohybrid has been confirmed from TEM and Raman studies. The resulting 2D-0D nanohybrid is found to suppress the intrinsic n-type nature of MoS2, which reflects as reduction of rectification ratio from 3.1 to 2.6, as observed in STM I-V measurements. The band edges are identified by measuring the local density-of-states (LDOS) as differential tunnel conductance (dI/dV) at room-temperature on an UHV-STM system, which strongly emphasizes the formation of type-II heterojunction of 2D MoS2 as acceptor and 0D RB as donor. The strategy explored here is the facile growth of 2D-0D nanohybrid by assembling organic molecules on 2D MoS2 flakes to achieve a distinct electronic structure and provide unprecedented capabilities to engineer the band structure of 2D layered materials.

Tunneling in two-layer systems with electron-hole coupling (Review article)

A phase-coherent state of electron–hole pairs may emerge in two-layer n–p systems, which is generated by the Coulomb attraction of electrons of the n-layer to holes of the p-layer. Unlike a Josephson junctions, the order parameter phase in n–p bilayers is locked by interlayer tunneling matrix elements T12. The phase locking determines the response of the electron–hole condensate to the electric voltage between the layers: the phase is constant at low voltages V Vc. The change in the system dynamics at V = Vc results in a peak along the differential tunneling conductance. The width of the Vc peak is proportional to the absolute value of the tunneling matrix element |T12|, while its height does not depend on |T12|. Thus, for small |T12| the peak is tall and narrow. In the case of long two-layer systems, a magnetic field parallel to the layers significantly reduces the peak height. In small two-layer systems, the height of the tunneling conductance peak as a function of a parallel magnetic field is similar to the Fraunhofer diffraction pattern. The interlayer differential tunneling conductance peak is also strongly suppressed by temperature, due to thermal interlayer voltage fluctuations.

Tunneling and fluctuating electron-hole Cooper pairs in double bilayer graphene

A strong low-temperature enhancement of the tunneling conductance between graphene bilayers has been reported recently, and interpreted as a signature of equilibrium electron-hole pairing, first predicted in bilayers more than forty years ago but previously unobserved. Here we provide a detailed theory of conductance enhanced by fluctuating electron-hole Cooper pairs, which are a precursor to equilibrium pairing, that accounts for specific details of the multi-band double graphene bilayer system which supports several different pairing channels. Above the equilibrium condensation temperature, pairs have finite temporal coherence and do not support dissipationless tunneling. Instead, they strongly boost the tunneling conductivity via a fluctuational internal Josephson effect. Our theory makes predictions for the dependence of the zero bias peak in the differential tunneling conductance on temperature and electron-hole density imbalance that capture important aspects of the experimental observations. In our interpretation of the observations, cleaner samples with longer disorder scattering times would condense at temperatures $T_c$ up to $\sim 50 {\rm K}$, compared to the record $T_c \sim 1.5 $K achieved to date in the experiment.

Suppression of dynamic disorder in fullerenes at metal-organic interfaces.

Organic semiconductors are prone to strong disorder effects that often exhibit significant dynamic characteristics. In this study, static disorder and dynamic disorder of fullerene molecules at the interface of Au(111) are directly distinguished using Scanning Tunneling Microscopy (STM) and Scanning Tunneling Spectroscopy. We consider an STM image consisting of hundreds of different molecules, as an ensemble which exhibits conductance fluctuations due to both dynamic disorder and static disorder. On the other hand, local conductance measurement of single molecules over time reflects only dynamic disorder. We demonstrate that dynamic disorder is always smaller than static disorder at the C60/Au(111) interface due to structural constraints of molecules at the interface. Dynamic disorder in our experiment is due to small librations of individual molecules that are restricted by the metal surface, while static disorder is related to different bonding orientations of the C60 cage that are frozen-in at room temperature. Our experimental results can be modeled with direct simulations of differential tunneling conductance.

Universal power-law exponents in differential tunneling conductance for planar insulators near Mott criticality at low temperatures

We consider the low-temperature differential tunneling conductance G for interfaces between a planar insulating material in the Mott-class and a metal. For values of the applied potential difference that are not very small, there is a experimentally observed universal regime in which , where m is a universal exponent. We consider the theoretical prediction of the values of m by using the method of effective field theory (EFT), which is appropriate for discussing universal phenomena. We describe the Mott material by the EFT pertaining the long-distance behavior of a spinless Hubbard-like model with nearest neighbors interactions previously considered. At the Mott transition, the EFT is known to be given by a double Abelian Chern–Simons theory. The simplest realization of this theory at the tunneling interface yields a Conformal Field Theory with central charges and Jain filling fraction describing a pair of independent counter-propagating chiral bosons (one charged and one neutral). Tunneling from the material into the metal is, therefore, described by this EFT at the Mott critical point. The resulting tunneling conductance behaves as , yielding the prediction m = 1/2, which compares well (within a deviation) with the results for this exponent in two experimental studies considered here.