arXiv subjects

Giriraj Jnawali

Publications and source records attributed to Giriraj Jnawali.

12 recordsLinked to original sources

arXiv2024

Critical behavior of the dimerized Si(001) surface: Continuous order-disorder phase transition in the two-dimensional Ising universality class

The critical behavior of the order-disorder phase transition in the buckled dimer structure of the Si(001) surface is investigated both theoretically by means of first-principles calculations and experimentally by spot profile analysis low-energy electron diffraction (SPA-LEED). We use density functional theory (DFT) with three different functionals commonly used for Si to determine the coupling constants of an effective lattice Hamiltonian describing the dimer interactions. Experimentally, the phase transition from the low-temperature $c(4 {\times} 2)$- to the high-temperature $p(2 {\times} 1)$-reconstructed surface is followed through the intensity and width of the superstructure spots within the temperature range 78-400 K. Near the critical temperature $T_c = 190.6\,\mathrm{K}$, we observe universal critical behavior of spot intensities and correlation lengths which falls into the universality class of the two-dimensional (2D) Ising model. From the ratio of correlation lengths along and across the dimer rows we determine effective nearest-neighbor couplings of an anisotropic 2D Ising model, $J_\parallel = (-24.9 \pm 0.9_\mathrm{stat} \pm 1.3_\mathrm{sys})\,\mathrm{meV}$ and $J_\perp = (-0.8 \pm 0.1_\mathrm{stat})\,\mathrm{meV}$. We find that the experimentally determined coupling constants of the Ising model can be reconciled with those of the more complex lattice Hamiltonian from DFT when the critical behavior is of primary interest. The anisotropy of the interactions derived from the experimental data via the 2D Ising model is best matched by DFT calculations using the PBEsol functional. The trends in the calculated anisotropy are consistent with the surface stress anisotropy predicted by the DFT functionals, pointing towards the role of surface stress reduction as a driving force for establishing the $c(4 {\times} 2)$-reconstructed ground state.

cond-mat.mes-hall↗

arXiv2023

Dimer Coupling Energies of the Si(001) Surface

The coupling energies between the buckled dimers of the Si(001) surface were determined through analysis of the anisotropic critical behavior of its order-disorder phase transition. Spot profiles in high-resolution low-energy electron diffraction as a function of temperature were analyzed within the framework of the anisotropic two-dimensional Ising model. The validity of this approach is justified by the large ratio of correlation lengths, $ξ_\parallel^+/ξ_\perp^+ = 5.2$ of the fluctuating $c(4 {\times} 2)$ domains above the critical temperature $T_\mathrm{c} = (190.6 \pm 10)$ K. We obtain effective couplings $J_\parallel = (-24.9 \pm 1.3)$ meV along the dimer rows and $J_\perp = (-0.8 \pm 0.1)$ meV across the dimer rows, i.e., antiferromagnetic-like coupling of the dimers with $c(4 {\times} 2)$ symmetry.

cond-mat.mes-hall↗

arXiv2022

Band Structure and Polarization Effects in Photothermoelectric Spectroscopy of a Bi2Se3 Device

Bi2Se3 is a prototypical topological insulator which has a small band gap (~0.3 eV) and topologically protected conducting surface states. This material exhibits quite strong thermoelectric effects. Here we show in a mechanically exfoliated thick (~100 nm) nanoflake device that we can measure the energy dependent optical absorption through the photothermoelectric effect. Spectral signatures are seen for a number of optical transitions between the valence and conduction bands, including a broad peak at 1.5 eV which is likely dominated by bulk band-to-band optical transitions but is at the same energy as the well known optical transition between the two topologically protected conducting surface states. We also observe a surprising linear polarization dependence in the response of the device that reflects the influence of the metal contacts.

cond-mat.mtrl-sci↗

arXiv2021

Evidence for Topological Band-to-Band Transitions in a Type-II Weyl Semimetal

The ternary van der Waals material NbIrTe$_4$ is a Type-II Weyl semimetal. We use a tunable circularly polarized mid-infrared laser to investigate the existence of band-to-band excitations using transient reflectivity in an exfoliated nanoflake and photothermoelectric and photogalvanic signals in a device. Unpolarized photothermoelectric spectroscopy shows that the absorption in the Weyl semimetal increases rapidly above 0.3 eV as expected from the increase in the density of states from DFT calculations. However, the reflectivity shows a sign change in the circular dichroism of the reflected light which is dominated by $σ^-$ light for energies below 0.5 eV and $σ^+$ light for energies above that energy. Using an intense pulse to perturb the electrons near the Fermi level, we show that this 0.5 eV energy is associated with a band-to-band transition from a band slightly below the Fermi energy to a higher lying empty band. We use spectroscopy of the circular photogalvanic effect (CPGE) from 0.3 to 1 eV to show a strong peak near 0.5 eV which lies on top of a two orders-of-magnitude increase at the lowest energies as the Weyl points are approached. We conclude that this band-to-band optical transition enhances the Berry Curvature responsible for the CPGE, and may involve the Weyl points just below the Fermi energy.

cond-mat.mtrl-sci↗

arXiv2021

Band Splitting and Long-lived Carrier Recombination in Ferromagnetic CrSiTe3 Nanosheets

Magnetic layered ternary chalcogenides hold great promise for future spin-optoelectronic devices in the two dimensional limit. Understanding how the properties of the materials are impacted by magnetic ordering and the spin-orbit interactions is critically needed information for the development of applications. Ultrafast transient reflectance (TR) and photocurrent (PC) spectroscopies are combined with ab initio density functional theory (DFT) calculations to investigate the band structure and photoresponse of a layered magnetic semiconductor CrSiTe3 (CST) nanosheet in the paramagnetic (PM, 300K) and ferromagnetic (FM, 10 K) phases. We observe both a decrease of the direct bandgap and emergence of a 120 meV splitting of the optical transition when the FM phase is present. DFT band structure calculations suggest that the band modifications are driven by a FM ordering-induced band splitting between the Te p and the Cr d states at the valence and conduction band edges. We find that the majority of carriers photoexcited at the direct gap recombine within picoseconds through defect-mediated recombination, but that 2-3 % of the electrons scatter into indirect conduction band valleys resulting in very long-lived electrons and holes. Those long-lived carriers contribute to the broadband PC response of CST devices that also features indirect absorption. These results provide critical insights into the dynamics and energy landscape of photoexcited electrons and holes, and how they are impacted by spin-ordering effects in layered ferromagnets.

cond-mat.mtrl-sci↗

arXiv2021

A Raman Probe of Phonons and Electron-phonon Interactions in NbIrTe4

The semimetal NbIrTe4 has been proposed to be a Type-II Weyl semimetal with 8 pairs of opposite Chirality Weyl nodes which are very close to the Fermi energy. This topological electronic structure is made possible because of the broken inversion symmetry of NbIrTe4 which is an orthorhombic crystal with Td symmetry. Using micro-Raman scattering as a probe, we observe the frequencies and symmetries of 19 phonon modes (ranging from 40 to 260 cm-1) in this material and compare to Density Functional Theory calculations. Using angular and polarization resolved Raman scattering for green (514 nm) and red (633 nm) laser excitation, we show that it is possible to extract the excitation energy dependence of the Raman tensor elements associated with each measurable phonon mode. We show that these tensor elements vary substantially in a small energy range which reflects a strong variation of the electron-phonon coupling for these modes.

cond-mat.mtrl-sci↗

arXiv2020

Ultrafast Photoinduced Band Splitting and Carrier Dynamics in Chiral Tellurium Nanosheets

Trigonal tellurium (Te) is a chiral semiconductor that lacks both mirror and inversion symmetries, resulting in complex band structures with Weyl crossings and unique spin textures. Detailed time-resolved polarized reflectance spectroscopy is used to investigate its band structure and carrier dynamics. The polarized transient spectra reveal optical transitions between the uppermost spin-split H4 and H5 and the degenerate H6 valence bands (VB) and the lowest degenerate H6 conduction band (CB) as well as a higher energy transition at the L-point. Surprisingly, the degeneracy of the H6 CB (a proposed Weyl node) is lifted and the spin-split VB gap is reduced upon photoexcitation before relaxing to equilibrium as the carriers decay. Using ab initio density functional theory (DFT) calculations we conclude that the dynamic band structure is caused by a photoinduced shear strain in the Te film that breaks the screw symmetry of the crystal. The band-edge anisotropy is also reflected in the hot carrier decay rate, which is a factor of two slower along c-axis than perpendicular to it. The majority of photoexcited carriers near the band-edge are seen to recombine within 30 ps while higher lying transitions observed near 1.2 eV appear to have substantially longer lifetimes, potentially due to contributions of intervalley processes in the recombination rate. These new findings shed light on the strong correlation between photoinduced carriers and electronic structure in anisotropic crystals, which opens a potential pathway for designing novel Te-based devices that take advantage of the topological structures as well as strong spin-related properties.

cond-mat.mtrl-sci↗

arXiv2020

Exploring the Band Structure of Wurtzite InAs Nanowires Using Photocurrent Spectroscopy

We use polarized photocurrent spectroscopy in a nanowire device to investigate the band structure of hexagonal Wurtzite InAs. Signatures of optical transitions between four valence bands and two conduction bands are observed which are consistent with the symmetries expected from group theory. The ground state transition energy identified from photocurrent spectra is seen to be consistent with photoluminescence emitted from a cluster of nanowires from the same growth substrate. From the energies of the observed bands we determine the spin orbit and crystal field energies in Wurtzite InAs. This information is essential to the development of crystal phase engineering of this important III-V semiconductor.

cond-mat.mes-hall↗

arXiv2019

Strong Hot Carrier Effects Observed in a Single Nanowire Heterostructure

We use Transient Rayleigh Scattering to study the thermalization of hot photoexcited carriers in single GaAsSb/InP nanowire heterostructures. By comparing the energy loss rate in single bare GaAsSb nanowires which do not show substantial hot carrier effects with the core-shell nanowires, we show that the presence of an InP shell substantially suppresses the LO phonon emission rate at low temperatures leading to strong hot carrier effects.

cond-mat.mes-hall↗

arXiv2018

Revealing Optical Transitions and Carrier Recombination Dynamics within the Bulk Band Structure of Bi2Se3

Bismuth selenide (Bi2Se3) is a prototypical three-dimensional topological insulator whose Dirac surface states have been extensively studied theoretically and experimentally. Surprisingly little, however, is known about the energetics and dynamics of electrons and holes within the bulk band structure of the semiconductor. We use mid-infrared femtosecond transient reflectance measurements in a thick exfoliated nanoflake to study the ultrafast thermalization and recombination dynamics of photoexcited electrons and holes within the extended bulk band structure over a wide energy range (0.3-1.2 eV). Theoretical modeling of the reflectivity spectral lineshapes at 10 K demonstrates that the electrons and holes are photoexcited within a dense and cold electron gas with a Fermi level positioned well above the bottom of the lowest conduction band. Direct optical transitions from the first and the second spin-orbit split valence bands to the Fermi level above the lowest conduction band minimum are identified. The photoexcited carriers thermalize rapidly to the lattice temperature within a couple of picoseconds due to optical phonon emission and scattering with the cold electron gas. The minority carrier holes recombine with the dense electron gas within 150 ps at 10 K and 50 ps at 300 K. Such knowledge of interaction of electrons and holes within the bulk band structure provides a foundation for understanding of how such states interact dynamically with the topologically protected Dirac surface states.

cond-mat.mes-hall↗

arXiv2018

Graphene-Complex-oxide Nanoscale Device Concepts

The integration of graphene with complex-oxide heterostructures such as LaAlO$_3$/SrTiO$_3$ offers the opportunity to combine the multifunctional properties of an oxide interface with the electronic properties of graphene. The ability to control interface conduction through graphene and understanding how it affects the intrinsic properties of an oxide interface are critical to the technological development of novel multifunctional devices. Here we demonstrate several device archetypes in which electron transport at an oxide interface is modulated using a patterned graphene top gate. Nanoscale devices are fabricated at the oxide interface by conductive atomic force microscope (c-AFM) lithography, and transport measurements are performed as a function of the graphene gate voltage. Experiments are performed with devices written adjacent to or directly underneath the graphene gate. Unique capabilities of this approach include the ability to create highly flexible device configurations, the ability to modulate carrier density at the oxide interface, and the ability to control electron transport up to the single-electron-tunneling regime, while maintaining intrinsic transport properties of the oxide interface. Our results facilitate the design of a variety of nanoscale devices that combine unique transport properties of these two intimately coupled two-dimensional electron systems.

cond-mat.mes-hall↗

arXiv2016

Room-temperature quantum transport signatures in graphene/LaAlO3/SrTiO3 heterostructures

The pseudospin quantum degree of freedom is one of the most remarkable properties of graphene that distinguishes it from ordinary two-dimensional metals and semiconductors. Pseudospin quantum interference leads to weak antilocalization (WAL) and is influenced strongly by point defects and thermal perturbations that break chirality and destroy phase coherence. Preserving and manipulating quantum transport properties up to room temperature is key to realizing practical pseudospin-based graphene devices. Here we report fabrication and transport characterization of graphene field-effect devices on a complex-oxide heterostructure, LaAlO3/SrTiO3. Signatures of WAL, a consequence of pseudospin quantum interference, persist up to room temperature. The LaAlO3/SrTiO3 substrate plays a critical role in suppressing short-range impurity scattering and electron-phonon coupling. The observation of quantum transport signatures at room temperature is unique to this system and presents new opportunities for the development of pseudospin-based devices and novel multifunctional devices that couple to the complex-oxide interface.

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