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1.	Bhalla, Pankaj, et al. (författare) Light-induced nonlinear spin Hall current in single-layer WTe2 2024 Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 26:2 Tidskriftsartikel (refereegranskat)abstract In this theoretical investigation, we analyze light-induced nonlinear spin Hall currents in a gated single-layer 1T ′ -WTe2, flowing transversely to the incident laser polarization direction. Our study encompasses the exploration of the second and third-order rectified spin Hall currents using an effective low-energy Hamiltonian and employing the Kubo’s formalism. We extend our analysis to a wide frequency range spanning both transparent and absorbing regimes, investigating the influence of light frequency below and above the optical band gap. Additionally, we investigate the influence of an out-of-plane gate potential on the system, disrupting inversion symmetry and effectively manipulating both the strength and sign of nonlinear spin Hall responses. We predict a pronounced third-order spin Hall current relative to its second-order counterpart. The predicted nonlinear spin currents show strong anisotropic dependence on the laser polarization angle. The outcomes of our study contribute to a generalized framework for nonlinear response theory within the spin channel will impact the development of emerging field of opto-spintronic.
2.	Bhalla, Pankaj, et al. (författare) Pseudogauge field driven acoustoelectric current in two-dimensional hexagonal Dirac materials 2022 Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 105:12 Tidskriftsartikel (refereegranskat)abstract Using a diagrammatic scheme, we study the acoustoelectric effects in two-dimensional (2D) hexagonal Dirac materials due to the sound-induced pseudogauge field. We analyze both uniform and spatially dispersive currents in response to copropagating and counterpropagating sound waves, respectively. In addition to the longitudinal acoustoelectric current, we obtain an exotic transverse charge current flowing perpendicular to the sound propagation direction owing to the interplay of transverse and longitudinal gauge field components j(T) proportional to A(L)A(T)*. In contrast to the almost isotropic directional profile of the longitudinal uniform current, a highly anisotropic transverse component j(T) similar to sin(6 theta) is achieved that stems from the inherited threefold symmetry of the hexagonal lattice. However, both longitudinal and transverse parts of the dispersive current are predicted to be strongly anisotropic similar to sin(2)(theta) or cos(2)(3 theta). We quantitatively estimate the pseudogauge field contribution to the acoustoelectric current that can be probed in future experiments in graphene and other 2D hexagonal Dirac materials.
3.	Bhalla, Pankaj, et al. (författare) Second harmonic helicity and Faraday rotation in gated single-layer 1T ' -WTe2 2022 Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 105:23 Tidskriftsartikel (refereegranskat)abstract A single layer of the 1T' phase of WTe2 provides a rich platform for exotic physical properties such as the nonlinear Hall effect and high-temperature quantum spin Hall transport. Utilizing a continuum model and the diagrammatic method, we calculate the second harmonic conductivity of monolayer 1T'-WTe2 modulated by an external vertical electric field and electron doping. We obtain a finite helicity and Faraday rotation for the second harmonic signal in response to linearly polarized incident light in the presence of time-reversal symmetry. The second harmonic signal's helicity is highly controllable by altering the bias potential and serves as an optical indicator of the nonlinear Hall current. Our study motivates future experimental investigation of the helicity spectroscopy of two-dimensional materials.
4.	Cappelluti, Emmanuele, et al. (författare) Flat-band optical phonons in twisted bilayer graphene 2023 Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 108:12 Tidskriftsartikel (refereegranskat)abstract Twisting bilayer sheets of graphene have been proven to be an efficient way to manipulate the electronic Dirac-like properties, resulting in flat bands at magic angles. Inspired by the electronic model, we develop a continuum model for the lattice dynamics of twisted bilayer graphene and we show that a remarkable band flattening applies to almost all the high-frequency in-plane lattice vibration modes, including the valley Dirac phonon, valley optical phonon, and zone-center optical phonon bands. Utilizing an approximate approach, we estimate small but finite magic angles at which a vanishing phonon bandwidth is expected. In contrast to the electronic case, the existence of a restoring potential prohibits the emergence of a magic angle in a more accurate modeling. The predicted phonon band flattening is highly tunable by the twist angle and this strong dependence is directly accessible by spectroscopic tools.
5.	Girija, Aswathy V., et al. (författare) Highlights from Faraday Discussion : From optical to THz control of materials, London, UK, May 2022 2023 Ingår i: Chemical Communications. - : Royal Society of Chemistry (RSC). - 1359-7345 .- 1364-548X. ; 59:27, s. 3939-3947 Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract The Faraday Discussion 'From optical to THz control of materials' was held in London, UK, and online on the 23rd, 24th, and 25th May 2022. The meeting brought together established and early-career scientists, postgraduate students, scientific editors, and industrial researchers in the field of spectroscopy and materials science from over ten different countries interested in exploring the physical properties of materials at ultrafast timescales driven by optical and THz excitations. This conference report provides highlights of this meeting and we give summaries of the presentations including the introductory lecture, all the papers discussed, and the concluding remarks.
6.	Hofmann, Johannes, 1988, et al. (författare) Nonlinear Thermoelectric Probes of Anomalous Dynamics in Topological Fermi Liquids 2023 Ingår i: arXiv:2312.12520. Tidskriftsartikel (refereegranskat)abstract In two-dimensional Fermi liquids, odd-parity Fermi surface deformations have anomalously slow relaxation rates that are suppressed as T^4 with temperature T, distinct from the standard Fermi-liquid T^2 scaling. We demonstrate here that these long-lived modes, which are often hidden in linear response, have a significant impact on nonlinear transport by establishing a direct proportionality of nonlinear thermoelectric currents to the anomalously large relaxation time. These currents exist in topological time-reversal invariant Fermi liquids, and their magnitude is characterized by new topological heat capacitance terms that we refer to as the Berry curvature capacity and the velocity-curvature capacity. We quantify the effect in bismuth telluride, which is an efficient thermoelectric and a topological insulator with a hexagonal Fermi surface. Our findings demonstrate the potential to explore topological and many-body effects in Fermi liquids through the nonlinear thermoelectric response, urging further experimental studies.
7.	Klimmer, Sebastian, et al. (författare) Four-Wave Mixing at Excitonic Resonances in the Telecom Spectral Range 2023 Ingår i: 2023 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2023. - : Institute of Electrical and Electronics Engineers (IEEE). Konferensbidrag (refereegranskat)abstract The generation of entangled photons by spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing (SFWM) attracted enormous interest in the field of quantum optics. Depending on applications, entangled photon sources prioritize either bandwidth (e.g. for quantum imaging) or brightness (e.g. for quantum key distribution). Layered materials offer unique advantages for both. They have already been used to realize thinnest SPDC sources [1], which, like other layered materials such as transition-metal dichalcogenides (TMDs) [2], offer nearly unlimited bandwidth thanks to relaxed phase-matching constraints. Further, their easy integration on photonic platforms is promising for bright on-chip entangled photon sources. In this context, SPDC-based solutions are limited by phase-matching, whereas SFWM would be an almost phase-matching-free process, as pump, idler, and signal photons can be generated at similar wavelengths, thus propagating at the same group velocity in integrated devices. Moreover, exploiting excitonic resonances could enhance FWM even more. However, to date, experiments with resonant FWM in TMDs have been limited to signals in the visible [3], which is unsuitable for integrated photonics and telecom systems due to reabsorption during propagation in the photonic device.
8.	Lozano, Rafael, et al. (författare) Measuring progress from 1990 to 2017 and projecting attainment to 2030 of the health-related Sustainable Development Goals for 195 countries and territories: a systematic analysis for the Global Burden of Disease Study 2017 2018 Ingår i: The Lancet. - : Elsevier. - 1474-547X .- 0140-6736. ; 392:10159, s. 2091-2138 Tidskriftsartikel (refereegranskat)abstract Background: Efforts to establish the 2015 baseline and monitor early implementation of the UN Sustainable Development Goals (SDGs) highlight both great potential for and threats to improving health by 2030. To fully deliver on the SDG aim of “leaving no one behind”, it is increasingly important to examine the health-related SDGs beyond national-level estimates. As part of the Global Burden of Diseases, Injuries, and Risk Factors Study 2017 (GBD 2017), we measured progress on 41 of 52 health-related SDG indicators and estimated the health-related SDG index for 195 countries and territories for the period 1990–2017, projected indicators to 2030, and analysed global attainment. Methods: We measured progress on 41 health-related SDG indicators from 1990 to 2017, an increase of four indicators since GBD 2016 (new indicators were health worker density, sexual violence by non-intimate partners, population census status, and prevalence of physical and sexual violence [reported separately]). We also improved the measurement of several previously reported indicators. We constructed national-level estimates and, for a subset of health-related SDGs, examined indicator-level differences by sex and Socio-demographic Index (SDI) quintile. We also did subnational assessments of performance for selected countries. To construct the health-related SDG index, we transformed the value for each indicator on a scale of 0–100, with 0 as the 2·5th percentile and 100 as the 97·5th percentile of 1000 draws calculated from 1990 to 2030, and took the geometric mean of the scaled indicators by target. To generate projections through 2030, we used a forecasting framework that drew estimates from the broader GBD study and used weighted averages of indicator-specific and country-specific annualised rates of change from 1990 to 2017 to inform future estimates. We assessed attainment of indicators with defined targets in two ways: first, using mean values projected for 2030, and then using the probability of attainment in 2030 calculated from 1000 draws. We also did a global attainment analysis of the feasibility of attaining SDG targets on the basis of past trends. Using 2015 global averages of indicators with defined SDG targets, we calculated the global annualised rates of change required from 2015 to 2030 to meet these targets, and then identified in what percentiles the required global annualised rates of change fell in the distribution of country-level rates of change from 1990 to 2015. We took the mean of these global percentile values across indicators and applied the past rate of change at this mean global percentile to all health-related SDG indicators, irrespective of target definition, to estimate the equivalent 2030 global average value and percentage change from 2015 to 2030 for each indicator. Findings: The global median health-related SDG index in 2017 was 59·4 (IQR 35·4–67·3), ranging from a low of 11·6 (95% uncertainty interval 9·6–14·0) to a high of 84·9 (83·1–86·7). SDG index values in countries assessed at the subnational level varied substantially, particularly in China and India, although scores in Japan and the UK were more homogeneous. Indicators also varied by SDI quintile and sex, with males having worse outcomes than females for non-communicable disease (NCD) mortality, alcohol use, and smoking, among others. Most countries were projected to have a higher health-related SDG index in 2030 than in 2017, while country-level probabilities of attainment by 2030 varied widely by indicator. Under-5 mortality, neonatal mortality, maternal mortality ratio, and malaria indicators had the most countries with at least 95% probability of target attainment. Other indicators, including NCD mortality and suicide mortality, had no countries projected to meet corresponding SDG targets on the basis of projected mean values for 2030 but showed some probability of attainment by 2030. For some indicators, including child malnutrition, several infectious diseases, and most violence measures, the annualised rates of change required to meet SDG targets far exceeded the pace of progress achieved by any country in the recent past. We found that applying the mean global annualised rate of change to indicators without defined targets would equate to about 19% and 22% reductions in global smoking and alcohol consumption, respectively; a 47% decline in adolescent birth rates; and a more than 85% increase in health worker density per 1000 population by 2030. Interpretation: The GBD study offers a unique, robust platform for monitoring the health-related SDGs across demographic and geographic dimensions. Our findings underscore the importance of increased collection and analysis of disaggregated data and highlight where more deliberate design or targeting of interventions could accelerate progress in attaining the SDGs. Current projections show that many health-related SDG indicators, NCDs, NCD-related risks, and violence-related indicators will require a concerted shift away from what might have driven past gains—curative interventions in the case of NCDs—towards multisectoral, prevention-oriented policy action and investments to achieve SDG aims. Notably, several targets, if they are to be met by 2030, demand a pace of progress that no country has achieved in the recent past. The future is fundamentally uncertain, and no model can fully predict what breakthroughs or events might alter the course of the SDGs. What is clear is that our actions—or inaction—today will ultimately dictate how close the world, collectively, can get to leaving no one behind by 2030.
9.	Principi, Alessandro, et al. (författare) Pseudo-Euler equations from nonlinear optics : Plasmon-assisted photodetection beyond hydrodynamics 2019 Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 99:7 Tidskriftsartikel (refereegranskat)abstract A great deal of theoretical and experimental efforts have been devoted in the last decades to the study of long-wavelength photodetection mechanisms in field-effect transistors hosting two-dimensional (2D) electron systems. A particularly interesting subclass of these mechanisms is intrinsic and based on the conversion of the incoming electromagnetic radiation into plasmons, which resonantly enhance the photoresponse, and subsequent rectification via hydrodynamic nonlinearities. In this paper, we show that such a conversion and subsequent rectification occur well beyond the frequency regime in which hydrodynamic theory applies. We consider the nonlinear optical response of generic 2D electron systems and derive pseudo-Euler equations of motion for suitable collective variables. These are solved in one- and two-dimensional geometries for the case of graphene and the results are compared with those of hydrodynamic theory. Significant qualitative differences are found, which are amenable to experimental studies. Our theory expands the knowledge of the fundamental physics behind long-wavelength photodetection.
10.	Rostami, Habib, et al. (författare) Dominant role of two-photon vertex in nonlinear response in two-dimensional Dirac systems 2021 Ingår i: npj 2D Materials and Applications. - : Springer Nature. - 2397-7132. ; 5:1 Tidskriftsartikel (refereegranskat)abstract We show that the standard concepts of nonlinear response to electromagnetic fields break down in two-dimensional Dirac systems, like graphene, in the quantum regime close to the Dirac point. We present a compelling many-body theory for nonlinear transport focusing on disorder scattering as a benchmark example. We show that, although the diamagnetic two-photon vertex is absent at the non-interacting level, disorder effects give rise to a self-generation of such two-photon vertex surviving even in the clean limit. We predict that the two-photon vertex self-generation is present only in two dimensions. The impact of such a striking scenario on the nonlinear quantum transport is discussed, predicting a huge enhancement of third-order dc conductivity comparing to the common models.
11.	Rostami, Habib, et al. (författare) Gauge invariance and Ward identities in nonlinear response theory 2021 Ingår i: Annals of Physics. - : Elsevier BV. - 0003-4916 .- 1096-035X. ; 431 Tidskriftsartikel (refereegranskat)abstract We present a formal analysis of nonlinear response functions in terms of correlation functions in real-and imaginary-time domains. In particular, we show that causal nonlinear response functions, expressed in terms of nested commutators in real time, can be obtained from the analytic continuation of time ordered response functions, which are more easily amenable to diagrammatic calculation. This generalizes the well-known result of linear response theory. We then use gauge invariance arguments to derive exact relations between second-order response functions in density and current channels. These identities, which are non-perturbative in the strength of inter-particle interactions, allow us to establish exact connections between nonlinear optics calculations done in different electromagnetic gauges.
12.	Rostami, Habib, et al. (författare) Helical metals and insulators : Sheet singularity of the inflated Berry monopole 2018 Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 98:24 Tidskriftsartikel (refereegranskat)abstract We study phases of interacting Dirac matter that host Berry signatures. We predict a topological Lifshitz phase transition caused by the changes of a Dirac cone intersection from a semimetallic phase to helical insulating or metallic phases. These helical phases provide examples of a gapless topological phase where the spectral gap is not required for a topological protection. To realize nodal helical phases one would need to consider isotropic infinite-range interparticle. interaction. This interaction could emerge because of a momentum conserving scattering of electrons from a bosonic mode. For repulsive/attractive inter-particle interaction in density/pseudospin channel, the system undergoes a transition to the helical insulator phase. For an attractive density-density interaction, a metallic phase forms that hosts a nodal circle and a nodal sphere in two and three dimensions, respectively. A sheet singularity of Berry curvature is highlighted as a peculiar feature of the nodal sphere phase in three dimensions and represents the extension of the Berry monopole singularities into an inflated monopole. To illustrate the properties of these helical phases we investigate Landau levels in both metallic and insulating phases. Our study provides an extension of the paradigm in the interacting Dirac matter and makes an interesting connection to inflated topological singularities in cosmology.
13.	Rostami, Habib, et al. (författare) Layer and orbital interference effects in photoemission from transition metal dichalcogenides 2019 Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 100:23 Tidskriftsartikel (refereegranskat)abstract In this work, we provide an effective model to evaluate the one-electron dipole matrix elements governing optical excitations and the photoemission process of single-layer (SL) and bilayer (BL) transition metal dichalcogenides. By utilizing a k . p Hamiltonian, we calculate the photoemission intensity as observed in angle-resolved photoemission from the valence bands around the (K) over bar valley of MoS2. In SL MoS2, we find a significant masking of intensity outside the first Brillouin zone, which originates from an in-plane interference effect between photoelectrons emitted from the Mo d orbitals. In BL MoS2, an additional interlayer interference effect leads to a distinctive modulation of intensity with photon energy. Finally, we use the semiconductor Bloch equations to model the optical excitation in a time- and angle-resolved pump-probe photoemission experiment. We find that the momentum dependence of an optically excited population in the conduction band leads to an observable dichroism in both SL and BL MoS2.
14.	Rostami, Habib (författare) Light-induced shear phonon splitting and instability in bilayer graphene 2023 Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 107:16 Tidskriftsartikel (refereegranskat)abstract Coherent engineering of landscape potential in crystalline materials is a rapidly evolving research field. Ultrafast optical pulses can manipulate low-frequency shear phonons in van der Waals layered materials through the dynamical dressing of electronic structure and photoexcited carrier density. In this work, we provide a diagrammatic formalism for nonlinear Raman force and implement it to shear phonon dynamics in bilayer graphene. We predict a controllable splitting of double degenerate shear phonon modes due to light-induced phonon mixing and renormalization according to a coherent nonlinear Raman force mechanism. Intriguingly, we obtain a light-induced shear phonon softening that facilitates structural instability at a critical field amplitude for which the shear phonon frequency vanishes. The phonon splitting and instability strongly depend on the laser intensity, frequency, chemical potential, and temperature of photoexcited electrons. This study motivates future experimental investigation of the optical fine tuning and regulation of shear phonons and layer stacking order in layered van der Waals materials.
15.	Rostami, Habib, et al. (författare) Many-body effects in third harmonic generation of graphene 2021 Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 103:12 Tidskriftsartikel (refereegranskat)abstract The low-energy (intraband) range of the third harmonic generation of graphene in the terahertz regime is governed by the damping terms induced by the interactions. A controlled many-body description of the scattering processes is thus a compelling and desirable requirement. In this paper, using a Kadanoff-Baym approach, we systematically investigate the impact of many-body interaction on the third harmonic generation of graphene, taking elastic impurity scattering as a benchmark example. We predict the onset in the mixed inter- and intraband regime of incoherent features driven by the interaction at four- and five-photon transition frequencies in the third harmonic optical conductivity with a spectral weight proportional to the scattering rate. We also show that in spite of the complex many-body physics, the purely intraband term governing the limit omega -> 0 resembles the constraints of the phenomenological model. We ascribe this agreement to the fulfilling of the conservation laws enforced by the conserving approach. However, the overlap with incoherent features and the impact of many-body-driven multiphoton vertex couplings severely limit the validity of phenomenological description.
16.	Rostami, Habib, et al. (författare) Piezoelectricity and valley chern number in inhomogeneous hexagonal 2D crystals 2018 Ingår i: npj 2D Materials and Applications. - : Nature Publishing Group. - 2397-7132. ; 2, s. 1-6 Tidskriftsartikel (refereegranskat)abstract Conversion of mechanical forces to electric signal is possible in non-centrosymmetric materials due to linear piezoelectricity. The extraordinary mechanical properties of two-dimensional materials and their high crystallinity make them exceptional platforms to study and exploit the piezoelectric effect. Here, the piezoelectric response of non-centrosymmetric hexagonal two-dimensional crystals is studied using the modern theory of polarization and k-p model Hamiltonians. An analytical expression for the piezoelectric constant is obtained in terms of topological quantities, such as the valley Chern number. The theory is applied to semiconducting transition metal dichalcogenides and hexagonal Boron Nitride. We find good agreement with available experimental measurements for MoS2. We further generalize the theory to study the polarization of samples subjected to inhomogeneous strain (e.g., nanobubbles). We obtain a simple expression in terms of the strain tensor, and show that charge densities greater than or similar to 10(11)cm(-2) can be induced by realistic inhomogeneous strains, epsilon approximate to 0.01-0.03.
17.	Rostami, Habib, et al. (författare) Probing quantum criticality using nonlinear Hall effect in a metallic Dirac system 2020 Ingår i: Physical Review Research. - : American Physical Society (APS). - 2643-1564. ; 2:1 Tidskriftsartikel (refereegranskat)abstract Interaction-driven symmetry breaking in a metallic (doped) Dirac system can manifest in the spontaneous gap generation at the nodal point buried below the Fermi level. Across this transition linear conductivity remains finite, making its direct observation difficult in linear transport. We propose the nonlinear Hall effect as a direct probe of this transition when inversion symmetry is broken. Specifically, for a two-dimensional Dirac material with a tilted low-energy dispersion, we predict a transformation of the characteristic interband resonance peak into a non-Lorentzian form in the collisionless regime. Furthermore, we show that inversion-symmetry-breaking quantum phase transition is controlled by an exotic tilt-dependent line of critical points. As this line is approached from the ordered side, the nonlinear Hall conductivity is suppressed owing to the scattering between the strongly coupled incoherent fermionic and bosonic excitations. Our results should motivate further studies of nonlinear responses in strongly interacting Dirac materials.
18.	Rostami, Habib, et al. (författare) Strain-driven chiral phonons in two-dimensional hexagonal materials 2022 Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 105:19 Tidskriftsartikel (refereegranskat)abstract Hexagonal two-dimensional materials with broken inversion symmetry (as BN or transition metal dichalcogenides) are known to sustain chiral phonons with finite angular momentum, adding a further useful degree of freedom to the extraordinary entangled (electrical, optical, magnetic, and mechanical) properties of these compounds. However, because of lattice symmetry constraints, such chiral modes are constrained to the corners of the Brillouin zone, allowing little freedom for manipulating the chiral features. In this paper, we show how the application of uniaxial strain leads to the existence of unique chiral modes in the vicinity of the zone center. We also show that such strain-induced chiral modes, unlike the ones pinned at the K points, can be efficiently manipulated by modifying the strain itself, which determines the position of these modes in the Brillouin zone. The present paper results add a technique for the engineering of the quantum properties of two-dimensional lattices.
19.	Rostami, Habib (författare) Theory for shear displacement by light-induced Raman force in bilayer graphene 2022 Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 106:15 Tidskriftsartikel (refereegranskat)abstract Coherent excitation of shear phonons in van der Waals layered materials is a nondestructive mechanism to fine-tune the lattice structure and the electronic state of the system. We develop a diagrammatic theory for the displacive Raman force and apply it to the shear phonon's dynamics. We obtain a noticeable light-induced Raman force of the order of F similar to 10-100 nN/nm(2) leading to a large rectified shear displacement Q(0) in bilayer graphene. In analogy to the photogalvanic effect, we decompose the Raman force to circular and linear components where the former vanishes due to the lattice symmetry in bilayer graphene. We show that the laser frequency and polarization can effectively tune Q(0) in different electronic doping, temperature, and scattering rates. The finite rectified shear displacement induces a Dirac crossing pair in the low-energy dispersion that photoemission spectroscopy can probe. Our systematic formalism of Raman force can simulate the coherent manipulation of stacking order in the heterostructures of layered materials by laser irradiation.
20.	Shahnazaryan, V, et al. (författare) Exciton routing in the heterostructure of a transition metal dichalcogenide monolayer on a paraelectric substrate 2019 Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 100:16 Tidskriftsartikel (refereegranskat)abstract We propose a scheme for the spatial exciton energy control and exciton routing in a transition-metal dichalcogenide (TMD) monolayer which lies on a quantum paraelectric substrate. It relies on the ultrasensitive response of the substrate dielectric permittivity to temperature changes, allowing for spatially inhomogeneous screening of Coulomb interaction in a monolayer. As an example, we consider the heterostructure of TMD and strontium titanate oxide SrTiO3, where large dielectric screening can be attained. We study the impact of substrate temperature on the characteristic electronic features of TMD monolayers such as the particle band gap and exciton binding energy, Bohr radius, and nonlinearity (an exciton-exciton interaction). The combination of particle band gap and exciton binding energy modulation results in the shift of the exciton resonance energy. Applying local heating, we create spatial patterns with varying exciton resonant energy and an exciton flow toward the energetically lower region of the sample.
21.	Shahnazaryan, Vanik, et al. (författare) Nonlinear exciton drift in piezoelectric two-dimensional materials 2021 Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 104:8 Tidskriftsartikel (refereegranskat)abstract The noncentrosymmetric nature of single-layer (SL) transition-metal dichalcogenides (TMD) manifests itself in the finite piezoelectricity and valley-Zeeman coupling. We microscopically model nonlinear exciton transport in nanobubbles of SL TMDs. Thanks to the giant piezoelectric effect, we obtain an enormous internal electric field, E-piezo similar to 10(7) V/m, resulting in a built-in dipole moment of excitons. We demonstrate that the piezo-induced dipole-dipole interaction provides a novel channel for the nonlinear exciton transport distinct from the conventional isotropic funneling of excitons and leads to the formation of a hexagon-shaped exciton droplet on the top of circularly symmetric nanobubbles. Moreover, we found that the hexagonal distribution of exciton density is preserved even for strongly elliptic nanobubbles. The effect is tunable via the bubble-size dependence of the piezoelectric field E-piezo similar to h(max)(2)/R-3 with h(max) and R being the bubble height and radius, respectively.
22.	Shubnic, A., et al. (författare) Exciton spin Hall effect in arc-shaped strained WSe2 2024 Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 109:20 Tidskriftsartikel (refereegranskat)abstract Generating a pure spin current using electrons, which have degrees of freedom beyond spin, such as electric charge and valley index, presents challenges. In response, we propose a mechanism based on intervalley exciton dynamics in arc-shaped strained transition metal dichalcogenides (TMDs) to achieve the exciton spin Hall effect in an electrically insulating regime, without the need for an external electric field. The interplay between strain gradients and strain-induced pseudomagnetic fields results in a net Lorentz force on long-lived intervalley excitons in WSe2, carrying nonzero spin angular momentum. This process generates an exciton-mediated pure spin Hall current, resulting in opposite-sign spin accumulations and local magnetization on the two sides of the single-layer arc-shaped TMD. We demonstrate that the magnetic field induced by spin accumulation, at approximately ∼mT, can be detected using techniques such as superconducting quantum interference magnetometry or spatially resolved magneto-optical Faraday and Kerr rotations.
23.	Soavi, G., et al. (författare) Hot electrons modulation of third harmonic generation in graphene 2019 Ingår i: Optics InfoBase Conference Papers. - : OSA - The Optical Society. Konferensbidrag (refereegranskat)abstract Hot-electrons dominate the ultrafast (∼fs-ps) optical and electronic properties of metals and semiconductors [1-2] and they are exploited in a variety of applications including photovoltaics and photodetection. Here we perform power-dependent third harmonic generation (THG) measurements on gated single layer graphene (SLG) and we show that hot-electrons modulate significantly the power-law dependence of THG, inducing a large deviation from the expected cubic power-law. We use a Chemical Vapor Deposition (CVD) SLG sample transferred on Fused Silica (FS) and gated by ionic liquid (IL), Fig.1(a). We excite the sample with the idler beam of an Optical Parametric Oscillator (OPO, Coherent) at a photon energy of ħω0=0.69eV. The OPO is seeded by a mode-locked Ti:Sa laser (Coherent) with 150fs pulse duration and 80MHz repetition rate. The OPO idler spot-size is∼4.7µm and the pulse duration ∼300fs.
24.	Sukhachov, Pavlo O., et al. (författare) Acoustogalvanic Effect in Dirac and Weyl Semimetals 2020 Ingår i: Physical Review Letters. - : American Physical Society (APS). - 0031-9007 .- 1079-7114. ; 124:12 Tidskriftsartikel (refereegranskat)abstract The acoustogalvanic effect is proposed as a nonlinear mechanism to generate a direct electric current by passing acoustic waves in Dirac and Weyl semimetals. Unlike the standard acoustoelectric effect, which relies on the sound-induced deformation potential and the corresponding electric field, the acoustogalvanic one originates from the pseudoelectromagnetic fields, which are not subject to screening. The longitudinal acoustogalvanic current scales at least quadratically with the relaxation time, which is in contrast to the photogalvanic current where the scaling is linear. Because of the interplay of pseudoelectric and pseudomagnetic fields, the current could show a nontrivial dependence on the direction of sound wave propagation. Being within the experimental reach, the effect can be utilized to probe dynamical deformations and corresponding pseudoelectromagnetic fields, which are yet to be experimentally observed in Weyl and Dirac semimetals.
25.	Volckaert, Klara, et al. (författare) Momentum-resolved linear dichroism in bilayer MoS2 2019 Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 100:24 Tidskriftsartikel (refereegranskat)abstract In solid state photoemission experiments it is possible to extract information about the symmetry and orbital character of the electronic wave functions via the photoemission selection rules that shape the measured intensity. This approach can be expanded in a pump-probe experiment where the intensity contains additional information about interband excitations induced by an ultrafast laser pulse with tunable polarization. Here, we find an unexpected strong linear dichroism effect (up to 42.4%) in the conduction band of bilayer MoS2, when measuring energy- and momentum-resolved snapshots of excited electrons by time- and angle-resolved photoemission spectroscopy. We model the polarization-dependent photoemission intensity in the transiently populated conduction band using the semiconductor Bloch equations. Our theoretical analysis reveals a strongly anisotropic momentum dependence of the optical excitations due to intralayer single-particle hopping, which explains the observed linear dichroism.
26.	Wang, Y., et al. (författare) Probing Electronic States in Monolayer Semiconductors through Static and Transient Third-Harmonic Spectroscopies 2022 Ingår i: Advanced Materials. - : Wiley. - 0935-9648 .- 1521-4095. ; 34:3 Tidskriftsartikel (refereegranskat)abstract Electronic states and their dynamics are of critical importance for electronic and optoelectronic applications. Here, various relevant electronic states in monolayer MoS2, such as multiple excitonic Rydberg states and free-particle energy bands are probed with a high relative contrast of up to ≥200 via broadband (from ≈1.79 to 3.10 eV) static third-harmonic spectroscopy (THS), which is further supported by theoretical calculations. Moreover, transient THS is introduced to demonstrate that third-harmonic generation can be all-optically modulated with a modulation depth exceeding ≈94% at ≈2.18 eV, providing direct evidence of dominant carrier relaxation processes associated with carrier–exciton and carrier–phonon interactions. The results indicate that static and transient THS are not only promising techniques for the characterization of monolayer semiconductors and their heterostructures, but also a potential platform for disruptive photonic and optoelectronic applications, including all-optical modulation and imaging.

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