Session A33: 2D Materials and Transition Metal Chalcogenides

Ultrafast nonlocal collective dynamics of Kane plasmon-polaritons in a narrow-gap semiconductor

The observation of ultrarelativistic Dirac, Weyl, and Kane fermions in condensed-matter systems has uncovered a cornucopia of novel phenomenology as well as a potential for effective ultrafast light engineering of novel states of matter. While the non-equilibrium properties of two- and three-dimensional (3D) hexagonal crystals have been studied extensively, our understanding of the photoinduced dynamics in 3D singlevalley ultrarelativistic materials is lacking. Here, we employ ultrafast scanning near-field optical spectroscopy to access and control non-equilibrium finite-momentum plasmon-polaritons in thin films of a prototypical narrow-band-gap semiconductor Hg0.81Cd0.19Te. We demonstrate that these collective excitations exhibit distinctly non-classical scaling with electron density characteristic of the ultrarelativistic Kane regime and experience mode splitting due to the coupling of charge oscillations at the top and bottom surface of the thin film. Our observation and ultrafast control of Kane plasmon-polaritons in a semiconducting material using light sources in the standard telecommunications fiber-optics window open a new avenue towards high-bandwidth coherent information processing in next-generation plasmonic circuits.   

Quantum nonlinear Hall effect in two-dimensional materials with time-reversal symmetry

We study the quantum nonlinear Hall effect in two-dimensional (2D) materials with time-reversal symmetry. When only one mirror line exists, a transverse charge current occurs in the second-order response to an external electric field, as a result of the Berry curvature dipole in momentum space. Candidate 2D materials to observe this effect are two-dimensional transition metal dichalcogenides (TMDCs)¹. First, we use an ab initio based tight-binding approach to demonstrate that monolayer T_d-structure TMDCs exhibit a finite Berry curvature dipole. In the 1H and 1T′ phase of TMDCs, we show the emergence of a finite Berry curvature dipole with the application of strain and an electrical displacement field, respectively.

1. Jhih-Shih You, Shiang Fang, Su-Yang Xu, Efthimios Kaxiras, and Tony Low, Phys. Rev. B 98, 121109(R)   

Quasiparticle Interference on a natural mineral: Mapping the bands of argentiferous Galena

Galena (PbS) is the main ore of lead, and in the early days of wireless technology, it played a fundamental part in crystal radios. High-quality samples, that cleave perfectly, are common natural occurrences. It belongs to the lead chalcogenides PbX (X=Te, Se, S), narrow direct bandgap semiconductors which have high thermoelectric figures of merit and unusual semiconducting properties. Their electronic band structure has been studied theoretically and experimentally, revealing a bandgap which decreases with decreasing temperature and under pressure.

Previous Scanning tunnelling microscopy (STM) studies on PbS crystals reported atomically-resolved surfaces. In this study, a natural argentiferous PbS crystal (Ag impurities, p-doped) was cleaved in situ and measured at 20K, using a home-built low-temperature STM. We observe high-quality surfaces with a large variety of defects, surrounded by clear patterns due to quasiparticle interference (QPI). We will show results from QPI to map its electronic structure in the occupied and unoccupied states.   

Emphanitic anharmonicity in PbSe at high temperature and anomalous electronic properties in the PbQ (Q=S,Se,Te) system

The temperature dependence of the local structure of PbSe has been investigated using pair distribution function (PDF) analysis of x-ray and neutron powder diffraction data and density functional theory (DFT) calculations. Observation of non-Gaussian PDF peaks at high temperature indicates the presence of significant anharmonicity, which can be modeled as Pb off-centering along [100] directions that grows on warming similar to the behavior seen in PbTe and PbS and sometimes called emphanisis. Interestingly, the emphanitic response is smaller in PbSe than in both PbS and PbTe indicating a nonmonotonic evolution with chalcogen atomic number in the PbQ (Q = S, Se, Te) series. The DFT calculations indicate a correlation between band gap and the amplitude of [100] dipolar distortion, suggesting that emphanisis may be behind the anomalous composition and temperature dependencies of the band gaps in this series.
[1] R. Yu et al., Phys. Rev. B 98, 144108 (2018).   

Nonreciprocity in Thermal and Thermoelectric Transport of Electrons in Noncentrosymmetric Crystals

A system with broken inversion symmetry shows direction-dependent conduction phenomena beyond linear response regime, which is referred to as nonreciprocal transport. In a single crystalline system, noncentrosymmetric crystal structure serves as a source of the nonreciprocal transport of the electrons. In this study, the nonlinear thermal and thermoelectric conductivities of electronic systems are calculated up to the second order in an electric field and a temperature gradient by using the Boltzmann equation with the relaxation time approximation. All the second-order conductivities are described by a small number of functions including the Berry curvature dipole. The second-order conductivities are estimated for the 1H monolayer of the transition metal dichalcogenides and a polar semiconductor BiTeX(X=I,Br).   

WS2 atomically thin nanodot array via self-organized templates

Tungsten Disulphide (WS₂) atomic layer nanodot arrays were fabricated with self- organized porous alumina templates as evaporation masks. First tungsten oxide (WO₃) was deposited on sapphire substrates through the alumina template to create WO₃ nanodot arrays, then WS₂ nanodots were formed by the reaction of sulfur with WO₃ using chemical vapor deposition (CVD) technique. The sizes of the observed two-dimensional WS₂ nanodots varies from 45nm to 70nm. These nanodots exhibit a layer thickness ranging from monolayers to a few atomic layers. This is confirmed by atomic force microscopy (AFM) measurements. Monolayer dots dominated region produces characteristic Raman peaks and high intensity photoluminescence (PL) due to direct band gap transition. Also it has more edge sites which can increase the PL of WS₂ dots. A region dominated by multilayer dots produces WS₂ Raman peaks but negligible photoluminescence due to indirect transition in multilayers. These novel atomically thin nanodots are attractive for both fundamental studies and future device applications, for e.g. ultrasensitive and ultrafast sensors.
Scanning Electron Microscopy and Atomic Force Microscopy of WS2 nanodots is shown in figure
  

Piezochromism in the magnetic chalcogenide MnPS3

We bring together diamond anvil cell techniques, optical spectroscopy, and first principles electronic structure calculations to reveal the piezochromic properties of MnPS₃. Color changes (green - red - black) take place with pressure as the charge gap systematically redshifts across the visible regime and into the near infrared, moving toward closure at the rate of 0.05 eV/GPa. Strikingly, this extrapolated value for potential band gap closure (55 GPa) corresponds to the theoretically-predicted crossover to the dimerized phase.   

Epitaxial growth and collective electronic states of monolayer 1T-NbSe2

At the single layer limit, transition metal dichalcogenides (TMDs) can adopt two different structural variants, i.e., polymorphs, depending on the anionic environment around the metal ions: the anions arrange in trigonal prismatic fashion in the 1H polymorph, whereas in 1T the arrangement is octahedral. While bulk 1T NbSe₂ doesn’t exist in nature, here we show that single layer 1T NbSe₂ polymorph can be grown by molecular beam epitaxy on epitaxial graphene/SiC(0001) substrates. A (Ö13xÖ13) Star-of-David charge density waves is observed by in situ scanning tunnelling microscopy, which persists above room temperature. A gap of 0.50 eV are further observed by tunnelling spectroscopy and angle resolved photoemission spectroscopy, indicating that this monolayer 1T phase of NbSe₂ is also a Mott insulator, similar to that of bulk 1T TaS₂. Our findings indicate that the presence of epitaxial constraints can generate structural configurations that are prohibited in fully-bonded TMD crystals. These findings and their implication on the collective electronic states of single layer 1T-NbSe₂ will be discussed at the meeting.   

Low-Pressure Chemical Vapor Deposition Growth of Iron-Doped MoS2 Monolayers

Dilute magnetic semiconductors have drawn researchers’ attention over the last decade, driven by the advent of spintronics. Transition metal dichalcogenides (TMDs) have demonstrated great potential for new generation spintronics owing to their unique structural and electronic properties. Experimental and theoretical efforts have been made to understand the role of magnetic impurities in TMDs, such as Mn, Fe, Co, and Ni.¹ Theoretical studies show that Fe-substitution in Mo site leads to spin-polarized states and can achieve room temperature ferromagnetic ordering and clustering.²

Here, we demonstrate direct growth of Fe-doped monolayer (ML) MoS₂ via the chemical vapor deposition. We obtained Fe-doped ML MoS₂ with an average crystal size of 40 μm. We observed a PL peak at 1.78 eV, a 70 MeV eV redshift from 1.85 eV obtained from undoped ML MoS₂ synthesized in our lab. Scanning transmission electron microscopy images show Fe atoms substituted Mo atoms. X-ray photoelectron spectroscopy will be performed to quantitatively analyze the doping concentration. We will then correlate the growth recipe with its doping concentration for the optimized growth of Fe-doped MoS₂ monolayers.

1. C. Jia et al., RSC Adv., 2018, 8, 18837
2. Y. Wang et al., Mat. & Des., 2017, 121, 77   

Direct observation of ultrafast free carrier dynamics in InSe

The optical properties of semiconductors are specified by the dynamics of free carriers and bound excitons, both originating from dynamics of electron-hole pair excitations. However, the contribution of free carrier to the optical properties is masked by the excitons-related features. We overcome this issue by assessing the free carrier dynamics in bulk InSe with TR-ARPES measurements.
Snapshots of conduction band pumped by 1.55 eV photons at different time delays is explored. The peak excitation is reached at t = 0.2 ps. The parabolic dispersion around the conduction band edges can be extracted from the peak intensity difference at t = 0.2 ps, where the full parabolic dispersions of the band extrema emerge due to the increased population of carriers. This then relaxes at later time delays. During interband excitons, a non-thermal distribution of excited electron-hole pairs is generated at the earliest time delays, relaxing into a quasi-equilibrium state with a temperature higher than that of the lattice.
Our measurements provide a direct observation of population inversion occurs immediately after interband excitation. By directly measuring the band occupancy at all energies and times, we have made a quantitative evaluation of the carrier dynamics.   

ReS2 double gate field-effect transistors with h-BN as dielectric layers

Transition metal dichalcogenides (TMDs) are regarded as promising 2-dimensional materials for semiconducting devices and optoelectronic applications. Unlike group VI-TMDs, the rhenium-based dichalcogenides crystallize in a distorted triclinic (1T’) structure due to an extra electron in the d orbital. Because of this structure, the interlayer coupling effect is weaker than other TMDs. Here, we demonstrated electrical characteristics of double gate multilayer ReS₂ FETs, which is passivated with h-BN as gate dielectric layers. The device were measured in double gate mode by sweeping the bottom gate voltage with fixed the top gate bias. The field effect mobilities can be modulated from 26.1 to 40.2 cm²V^-1s^-1. The second peak in transconductance (g_m) of bottom gate sweep is appeared depending on changing top gate potential indicating that conduction path separates in a multilayer system. Also, the threshold voltage shift of the bottom gate depends on the top gate voltage with two linear relationships due to top gate screening effect.   

The thermoelectric and electron transport properties on the charge density wave(CDW) transition of (PbSe)1(VSe2)1,2,3 heterostructure materials as well as 1-T VSe2 2D materials.

A series of (PbSe)₁(VSe₂)_n with n = 1,2,3 turbostratic disorder heterostructure materials were synthesized through modulated elemental reactants (MER) method. The in-plane and cross-plane Seebeck coefficient, as well as electron transport properties and Hall coefficient, were measured from 80K to 300K. At the charge density wave (CDW) transition, the Seebeck coefficient decrease by approximately two-fold when the material transforms from CDW state to normal state. And the Raman shift spectra dependence on the temperature of the materials were also measured. All those properties were also compared to the 1-T VSe2 2D materials.   

Structure and Electronic Properties of InSe and GaSe Polytypes

Equilibrium crystal structures, electron band dispersions, and bandgap values of layered GaSe and InSe semiconductors, each being represented by four polytypes, have been investigated within the density functional theory [1]. Several dispersion correction methods to the DFT, from empirical Grimme corrections to many-body dispersion schemes, have been assessed in comparison with experimental cell parameters [2]. Due to the high technical accuracy achieved, nearly degenerate energy-volume curves of different polytypes are resolved, and the conclusions concerning the relative stability of competing polytypes drawn [3]. The band structures are inspected using the modified Becke-Johnson (mBJ) scheme and hybrid HSE06 functional. Both methods nicely agree with the experimental results and with state-of-the-art GW calculations. Some discrepancies are identified in cases of close competition between the direct and indirect gap (e.g., in GaSe). The placement of bands are slightly differents for mBJ and HSE06.

[1] J. Srour, M. Badawi, F. El Haj Hassan, A. Postnikov, J. Chem. Phys. 149 (2018)
[2] J. Srour, M. Badawi, F.E.H. Hassan, A.V. Postnikov, Physica Status Solidi (B). 253 (2016) 1472
[3] J. Srour, A. Postnikov, M. Badawi, F.E.H. Hassan, Physica Status Solidi (B) 254 (2017) 1700120   

Interplay between Rashba Spin-Orbit Coupling, Magnetic Field, and Topological Superconductivity in Monolayer NbSe_2

The persistence of superconductivity above the Pauli limit in monolayer 1H-NbSe_2 is indicative of strong Ising spin-orbit coupling (SOC). It has been suggested that the interplay between Ising SOC and strong in-plane magnetic fields can lead to a topological nodal superconducting phase protected by a time reversal-like symmetry. An important ingredient not fully considered, however, is Rashba SOC. Not only is it expected to be significant due to substrate effects, but it is also tunable by electrostatic gating. In this talk, we present a microscopic model for superconductivity of NbSe_2 driven by repulsive electronic interactions as a function of magnetic field and Rashba SOC. For large enough magnetic fields, a small Rashba SOC lifts the nodes unless the magnetic field is aligned along certain directions, in which case a residual mirror symmetry protects some of the nodes and corresponding flat band edge modes. In the opposite limit of small magnetic fields, a topological chiral phase is found to be stabilized by large Rashba SOC.