| 1. |
- Biswas, Deepnarayan, et al.
(författare)
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Ultrafast Triggering of Insulator-Metal Transition in Two-Dimensional VSe2
- 2021
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 21:5, s. 1968-1975
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Tidskriftsartikel (refereegranskat)abstract
- The transition-metal dichalcogenide VSe2 exhibits an increased charge density wave transition temperature and an emerging insulating phase when thinned to a single layer. Here, we investigate the interplay of electronic and lattice degrees of freedom that underpin these phases in single-layer VSe2 using ultrafast pump-probe photoemission spectroscopy. In the insulating state, we observe a light-induced closure of the energy gap, which we disentangle from the ensuing hot carrier dynamics by fitting a model spectral function to the time-dependent photoemission intensity. This procedure leads to an estimated time scale of 480 fs for the closure of the gap, which suggests that the phase transition in single-layer VSe2 is driven by electron-lattice interactions rather than by Mott-like electronic effects. The ultrafast optical switching of these interactions in SL VSe2 demonstrates the potential for controlling phase transitions in 2D materials with light.
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| 2. |
- Curcio, Davide, et al.
(författare)
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Ultrafast electronic linewidth broadening in the C 1s core level of graphene
- 2021
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 104:16
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Tidskriftsartikel (refereegranskat)abstract
- We show that the presence of a transiently excited hot electron gas in graphene leads to a substantial broadening of the C 1s line probed by time-resolved x-ray photoemission spectroscopy. The broadening is found to be caused by an exchange of energy and momentum between the photoemitted core electron and the hot electron gas, rather than by vibrational excitations. This interpretation is supported by a quantitative line-shape analysis that accounts for the presence of the excited electrons. Fitting the spectra to this model directly yields the electronic temperature of the system, in good agreement with electronic temperature values obtained from valence band data. Furthermore, we show how the momentum change of the outgoing core electrons leads to a detectable but very small change in the time-resolved photoelectron diffraction pattern and to a nearly complete elimination of the core level binding energy variation associated with the presence of a narrow σ band in the C 1s state.
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| 3. |
- Frank, Marius S., et al.
(författare)
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Quantum embedding description of the Anderson lattice model with the ghost Gutzwiller approximation
- 2021
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 104:8
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Tidskriftsartikel (refereegranskat)abstract
- We present benchmark calculations of the Anderson lattice model based on the recently developed "ghost Gutzwiller approximation". Our analysis shows that, in some parameters regimes, the predictions of the standard Gutzwiller approximation can be incorrect by orders of magnitude for this model. We show that this is caused by the inability of this method to describe simultaneously the Mott physics and the hybridization between correlated and itinerant degrees of freedom-whose interplay often governs the metal-insulator transition in real materials. Finally, we show that the ghost Gutzwiller approximation solves this problem, providing us with results in remarkable agreement with dynamical mean-field theory throughout the entire phase diagram, while being much less computationally demanding. We provide an analytical explanation of these findings and discuss their implications within the context of ab-initio computation of strongly correlated matter.
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| 4. |
- Holt, Ann Julie U., et al.
(författare)
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Electronic properties of single-layer CoO2/Au(111)
- 2021
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Ingår i: Current Opinion in Chemical Engineering. - : IOP Publishing. - 2211-3398. ; 8:3
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Tidskriftsartikel (refereegranskat)abstract
- We report direct measurements via angle-resolved photoemission spectroscopy (ARPES) of the electronic dispersion of single-layer (SL) CoO2. The Fermi contour consists of a large hole pocket centered at the (Gamma) over bar point. To interpret the ARPES results, we use density functional theory (DFT) in combination with the multi-orbital Gutzwiller Approximation (DFT+GA), basing our calculations on crystalline structure parameters derived from x-ray photoelectron diffraction and low-energy electron diffraction. Our calculations are in good agreement with the measured dispersion. We conclude that the material is a moderately correlated metal. We also discuss substrate effects, and the influence of hydroxylation on the CoO2 SL electronic structure.
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| 5. |
- Jones, Alfred J. H., et al.
(författare)
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Visualizing band structure hybridization and superlattice effects in twisted MoS2/WS2 heterobilayers
- 2022
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Ingår i: Current Opinion in Chemical Engineering. - : IOP Publishing Ltd. - 2211-3398. ; 9:1
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Tidskriftsartikel (refereegranskat)abstract
- A mismatch of atomic registries between single-layer transition metal dichalcogenides (TMDs) in a two-dimensional (2D) van der Waals heterostructure produces a moire superlattice with a periodic potential, which can be fine-tuned by introducing a twist angle between the materials. This approach is promising both for controlling the interactions between the TMDs and for engineering their electronic band structures, yet direct observation of the changes to the electronic structure introduced with varying twist angle has so far been missing. Here, we probe heterobilayers comprised of single-layer MoS2 and WS2 with twist angles ranging from 2 degrees to 20 degrees and determine the twist angle-dependent evolution of the electronic band structure using micro-focused angle-resolved photoemission spectroscopy. We find strong interlayer hybridization between MoS2 and WS2 electronic states at the (Gamma) over bar -point of the Brillouin zone, leading to a shift of the valence band maximum in the heterostructure. Replicas of the hybridized states are observed at the center of twist angle-dependent moire mini Brillouin zones. We confirm that these replica features arise from the inherent moire potential by comparing our experimental observations with density functional theory calculations of the superlattice dispersion. Our direct visualization of these features underscores the potential of using twisted heterobilayer semiconductors to engineer hybrid electronic states and superlattices that alter the electronic and optical properties of 2D heterostructures for a wide range of twist angles.
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| 6. |
- Lanata, Nicola, et al.
(författare)
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Efficient implementation of the Gutzwiller variational method
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121. ; 85:3
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Tidskriftsartikel (refereegranskat)abstract
- We present a self-consistent numerical approach to solve the Gutzwiller variational problem for general multi-band models with arbitrary on-site interaction. The proposed method generalizes and improves the procedure derived by Deng et al., Phys. Rev. B. 79 075114 (2009), overcoming the restriction to density-density interaction without increasing the complexity of the computational algorithm. Our approach drastically reduces the problem of the high-dimensional Gutzwiller mini- mization by mapping it to a minimization only in the variational density matrix, in the spirit of the Levy and Lieb formulation of DFT. For fixed density the Gutzwiller renormalization matrix is deter- mined as a fixpoint of a proper functional, whose evaluation only requires ground-state calculations of matrices defined in the Gutzwiller variational space. Furthermore, the proposed method is able to account for the symmetries of the variational function in a controlled way, reducing the number of variational parameters. After a detailed description of the method we present calculations for multi-band Hubbard models with full (rotationally invariant) Hund’s rule on-site interaction. Our analysis shows that the numerical algorithm is very efficient, stable and easy to implement. For these reasons this method is particularly suitable for first principle studies – e.g., in combination with DFT – of many complex real materials, where the full intra-atomic interaction is important to obtain correct results.
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| 7. |
- Lanatà, Nicola
(författare)
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Operatorial formulation of the ghost rotationally invariant slave-boson theory
- 2022
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 105:4
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Tidskriftsartikel (refereegranskat)abstract
- We propose an operatorially exact formalism for describing the equilibrium and quantum-dynamical properties of many-electron systems interacting locally on a lattice, called "ghost rotationally invariant slave-boson theory" (g-RISB). We demonstrate that our theoretical framework reduces to the recently developed ghost Gutzwiller approximation (g-GA) at the mean-field level. Furthermore, we introduce the time-dependent mean-field g-RISB action, generalizing the time-dependent GA theory. Since the g-RISB is based on exact reformulation of the many-body problem, it may pave the way to the development of practical implementations for adding systematically quantum-fluctuation corrections towards the exact solution, in arbitrary dimension.
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| 8. |
- Lanata, Nicola
(författare)
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Variational approach to transport in quantum dots
- 2010
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Ingår i: PHYSICAL REVIEW B. - 1098-0121. ; 82:19
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Tidskriftsartikel (refereegranskat)abstract
- We have derived a variational principle that defines the nonequilibrium steady-state transport across a correlated impurity (mimicking, e.g., a quantum dot) coupled to biased leads. This variational principle has been specialized to a Gutzwiller’s variational space, and applied to the study of the simple single-orbital Anderson impurity model at half filling, finding a good qualitative accord with the observed behavior in quantum dots for the expected regime of values of the bias. Beyond the purely theoretical interest in the formal definition of a variational principle in a nonequilibrium problem, the particular methods proposed have the important advantage to be simple and flexible enough to deal with more complicated systems and variational spaces.
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| 9. |
- Lee, Tsung-Han, et al.
(författare)
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Efficient Slave-Boson Approach for Multiorbital Two-Particle Response Functions and Superconductivity
- 2021
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Ingår i: Physical Review X. - : AMER PHYSICAL SOC. - 2160-3308. ; 11:4
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Tidskriftsartikel (refereegranskat)abstract
- We develop an efficient approach for computing two-particle response functions and interaction vertices for multiorbital strongly correlated systems based on the rotationally invariant slave-boson framework. The method is applied to the degenerate three-orbital Hubbard-Kanamori model for investigating the origin of the s-wave orbital antisymmetric spin-triplet superconductivity in Hund's metal regime, previously found in the dynamical mean-field theory studies. By computing the pairing interaction considering the particleparticle and the particle-hole scattering channels, we identify the mechanism leading to the pairing instability around Hund's metal crossover arises from the particle-particle channel, which contains the local electron pair fluctuation between different particle-number sectors of the atomic Hilbert space. On the other hand, the particle-hole spin fluctuations induce the s-wave pairing instability before entering Hund's regime. Our approach paves the way for investigating the pairing mechanism in realistic correlated materials.
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| 10. |
- Majchrzak, P., et al.
(författare)
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Switching of the electron-phonon interaction in 1T-VSe2 assisted by hot carriers
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 103:24
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Tidskriftsartikel (refereegranskat)abstract
- We apply an intense infrared laser pulse in order to perturb the electronic and vibrational states in the three-dimensional charge density wave material 1T-VSe2. Ultrafast snapshots of the light-induced hot carrier dynamics and nonequilibrium quasiparticle spectral function are collected using time- and angle-resolved photoemission spectroscopy. The hot carrier temperature and time-dependent electronic self-energy are extracted from the time-dependent spectral function, revealing that incoherent electron-phonon interactions heat the lattice above the charge density wave critical temperature on a timescale of (200±40) fs. Density functional perturbation theory calculations establish that the presence of hot carriers alters the overall phonon dispersion and quenches efficient low-energy acoustic phonon scattering channels, which results in a new quasiequilibrium state that is experimentally observed.
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| 11. |
- Rogers, J., et al.
(författare)
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Bypassing the computational bottleneck of quantum-embedding theories for strong electron correlations with machine learning
- 2021
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Ingår i: Physical Review Research. - : American Physical Society. - 2643-1564. ; 3:1
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Tidskriftsartikel (refereegranskat)abstract
- A cardinal obstacle to performing quantum-mechanical simulations of strongly correlated matter is that, with the theoretical tools presently available, sufficiently accurate computations are often too expensive to be ever feasible. Here we design a computational framework combining quantum-embedding (QE) methods with machine learning. This allows us to bypass altogether the most computationally expensive components of QE algorithms, making their overall cost comparable to bare density functional theory. We perform benchmark calculations of a series of actinide systems, where our method accurately describes the correlation effects, reducing by orders of magnitude the computational cost. We argue that, by producing a larger-scale set of training data, it will be possible to apply our method to systems with arbitrary stoichiometries and crystal structures, paving the way to virtually infinite applications in condensed matter physics, chemistry, and materials science.
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| 12. |
- Rostami, Habib, et al.
(författare)
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Layer and orbital interference effects in photoemission from transition metal dichalcogenides
- 2019
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 100:23
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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.
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| 13. |
- Volckaert, Klara, et al.
(författare)
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Momentum-resolved linear dichroism in bilayer MoS2
- 2019
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 100:24
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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.
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