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Publications (10 of 20) Show all publications
Bhalla, P. & Rostami, H. (2024). Light-induced nonlinear spin Hall current in single-layer WTe2. New Journal of Physics, 26(2), Article ID 023042.
Open this publication in new window or tab >>Light-induced nonlinear spin Hall current in single-layer WTe2
2024 (English)In: New Journal of Physics, E-ISSN 1367-2630, Vol. 26, no 2, article id 023042Article in journal (Refereed) Published
Abstract [en]

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.

Keywords
two-dimensional materials, spin current, Green's function approach
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:su:diva-227808 (URN)10.1088/1367-2630/ad2822 (DOI)001174536700001 ()2-s2.0-85185963500 (Scopus ID)
Available from: 2024-03-27 Created: 2024-03-27 Last updated: 2024-03-27Bibliographically approved
Cappelluti, E., Silva-Guillén, J. A., Rostami, H. & Guinea, F. (2023). Flat-band optical phonons in twisted bilayer graphene. Physical Review B, 108(12), Article ID 125401.
Open this publication in new window or tab >>Flat-band optical phonons in twisted bilayer graphene
2023 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 108, no 12, article id 125401Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:su:diva-223009 (URN)10.1103/PhysRevB.108.125401 (DOI)001080542900004 ()2-s2.0-85172665064 (Scopus ID)
Available from: 2023-10-23 Created: 2023-10-23 Last updated: 2023-10-30Bibliographically approved
Girija, A. V., Basini, M. & Rostami, H. (2023). Highlights from Faraday Discussion: From optical to THz control of materials, London, UK, May 2022. Chemical Communications, 59(27), 3939-3947
Open this publication in new window or tab >>Highlights from Faraday Discussion: From optical to THz control of materials, London, UK, May 2022
2023 (English)In: Chemical Communications, ISSN 1359-7345, E-ISSN 1364-548X, Vol. 59, no 27, p. 3939-3947Article in journal, Editorial material (Other academic) Published
Abstract [en]

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.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:su:diva-216339 (URN)10.1039/d3cc90092d (DOI)000953002300001 ()36929932 (PubMedID)2-s2.0-85151018475 (Scopus ID)
Available from: 2023-04-18 Created: 2023-04-18 Last updated: 2023-05-04Bibliographically approved
Rostami, H. (2023). Light-induced shear phonon splitting and instability in bilayer graphene. Physical Review B, 107(16), Article ID 165418.
Open this publication in new window or tab >>Light-induced shear phonon splitting and instability in bilayer graphene
2023 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 107, no 16, article id 165418Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:su:diva-218035 (URN)10.1103/PhysRevB.107.165418 (DOI)000981079800004 ()2-s2.0-85158830474 (Scopus ID)
Available from: 2023-07-26 Created: 2023-07-26 Last updated: 2023-07-26Bibliographically approved
Wang, Y., Iyikanat, F., Rostami, H., Bai, X., Hu, X., Das, S., . . . Sun, Z. (2022). Probing Electronic States in Monolayer Semiconductors through Static and Transient Third-Harmonic Spectroscopies. Advanced Materials, 34(3), Article ID 2107104.
Open this publication in new window or tab >>Probing Electronic States in Monolayer Semiconductors through Static and Transient Third-Harmonic Spectroscopies
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2022 (English)In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 34, no 3, article id 2107104Article in journal (Refereed) Published
Abstract [en]

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.

Keywords
electronic states, monolayer transition metal dichalcogenides, static third-harmonic spectroscopy, third-harmonic generation, transient third-harmonic spectroscopy
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:su:diva-199505 (URN)10.1002/adma.202107104 (DOI)000721484000001 ()34743375 (PubMedID)2-s2.0-85119667671 (Scopus ID)
Available from: 2021-12-13 Created: 2021-12-13 Last updated: 2022-10-25Bibliographically approved
Bhalla, P., Vignale, G. & Rostami, H. (2022). Pseudogauge field driven acoustoelectric current in two-dimensional hexagonal Dirac materials. Physical Review B, 105(12), Article ID 125407.
Open this publication in new window or tab >>Pseudogauge field driven acoustoelectric current in two-dimensional hexagonal Dirac materials
2022 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, no 12, article id 125407Article in journal (Refereed) Published
Abstract [en]

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 jTALAT. In contrast to the almost isotropic directional profile of the longitudinal uniform current, a highly anisotropic transverse component jT∼sin(6θ) 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 ∼sin2(3θ) or cos2(3θ). 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.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:su:diva-203696 (URN)10.1103/PhysRevB.105.125407 (DOI)000771483700003 ()2-s2.0-85126441231 (Scopus ID)
Available from: 2022-04-08 Created: 2022-04-08 Last updated: 2022-04-08Bibliographically approved
Bhalla, P. & Rostami, H. (2022). Second harmonic helicity and Faraday rotation in gated single-layer 1T ' -WTe2. Physical Review B, 105(23), Article ID 235132.
Open this publication in new window or tab >>Second harmonic helicity and Faraday rotation in gated single-layer 1T ' -WTe2
2022 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, no 23, article id 235132Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:su:diva-208214 (URN)10.1103/PhysRevB.105.235132 (DOI)000823048000006 ()2-s2.0-85133682458 (Scopus ID)
Available from: 2022-08-25 Created: 2022-08-25 Last updated: 2022-08-25Bibliographically approved
Rostami, H., Guinea, F. & Cappelluti, E. (2022). Strain-driven chiral phonons in two-dimensional hexagonal materials. Physical Review B, 105(19), Article ID 195431.
Open this publication in new window or tab >>Strain-driven chiral phonons in two-dimensional hexagonal materials
2022 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, no 19, article id 195431Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:su:diva-206873 (URN)10.1103/PhysRevB.105.195431 (DOI)000809554000007 ()
Available from: 2022-06-30 Created: 2022-06-30 Last updated: 2022-06-30Bibliographically approved
Rostami, H. (2022). Theory for shear displacement by light-induced Raman force in bilayer graphene. Physical Review B, 106(15), Article ID 155405.
Open this publication in new window or tab >>Theory for shear displacement by light-induced Raman force in bilayer graphene
2022 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 106, no 15, article id 155405Article in journal (Refereed) Published
Abstract [en]

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∼10–100 nN/nm2 leading to a large rectified shear displacement Q0 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 Q0 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.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-211095 (URN)10.1103/PhysRevB.106.155405 (DOI)000870984800006 ()2-s2.0-85139798612 (Scopus ID)
Available from: 2022-11-10 Created: 2022-11-10 Last updated: 2022-11-10Bibliographically approved
Rostami, H. & Cappelluti, E. (2021). Dominant role of two-photon vertex in nonlinear response in two-dimensional Dirac systems. npj 2d materials and applications, 5(1), Article ID 50.
Open this publication in new window or tab >>Dominant role of two-photon vertex in nonlinear response in two-dimensional Dirac systems
2021 (English)In: npj 2d materials and applications, ISSN 2397-7132, Vol. 5, no 1, article id 50Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-195587 (URN)10.1038/s41699-021-00217-0 (DOI)000650046800001 ()
Available from: 2021-08-24 Created: 2021-08-24 Last updated: 2022-02-25Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-9521-1008

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