Daniel Hernangómez Pérez

Articles

Spin-orbit torque in single-molecule junctions from ab initio
M. Camarasa-Gómez, D. Hernangómez-Pérez, and F. Evers
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The use of electric fields applied across magnetic heterojunctions that lack spatial inversion symmetry has been previously proposed as a non-magnetic mean of controlling localized magnetic moments through spin-orbit torques (SOT). The implementation of this concept at the single-molecule level has remained a challenge, however. Here, we present first-principle calculations of SOT in a single-molecule junction under bias and beyond linear response. Employing a self-consistency scheme invoking density functional theory and non-equilibrium Green’s function theory, we compute the current-induced SOT. Responding to this torque, a localized magnetic moment can tilt. Within the linear regime our quantitative estimates for the SOT in single-molecule junctions yield values similar to those known for magnetic interfaces. Our findings contribute to an improved microscopic understanding of SOT in single molecules.
Designable exciton mixing through layer alignment in WS₂-graphene heterostructures
A. Kleiner, D. Hernangómez-Pérez, and S. Refaely-Abramson
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Optical properties of heterostructures composed of layered 2D materials, such as transition metal dichalcogenides (TMDs) and graphene, are broadly explored. Of particular interest are light-induced energy transfer mechanisms in these materials and their structural roots. Here, we use state-of-the-art first-principles calculations to study the excitonic composition and the absorption properties of WS₂-graphene heterostructures as a function of interlayer alignment and the local strain resulting from it. We find that Brillouin zone mismatch and the associated energy level alignment between the graphene Dirac cone and the TMD bands dictate an interplay between interlayer and intralayer excitons, mixing together in the many-body representation upon the strain-induced symmetry breaking in the interacting layers. Examining the representative cases of the 0° and 30° interlayer twist angles, we find that this exciton mixing strongly varies as a function of the relative alignment. We quantify the effect of these structural modifications on exciton charge separation between the layers and the associated graphene-induced homogeneous broadening of the absorption resonances. Our findings provide guidelines for controllable optical excitations upon interface design and shed light on the importance of many-body effects in the understanding of optical phenomena in complex heterostructures.
Low-scaling GW algorithm applied to twisted transition-metal dichalcogenide heterobilayers
M. Graml, K. Zollner, D. Hernangómez-Pérez, P. E. Faria Junior, and J. Wilhelm
J. Chem. Theory Comput. (2024).
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The GW method is widely used for calculating the electronic band structure of materials. The high computational cost of GW algorithms prohibits their application to many systems of interest. We present a periodic, low-scaling and highly efficient GW algorithm that benefits from the locality of the Gaussian basis and the polarizability. The algorithm enables G₀W₀ calculations on a MoSe₂/WS₂ bilayer with 984 atoms per unit cell, in 42 hours using 1536 cores. This is four orders of magnitude faster than a plane-wave G₀W₀ algorithm, allowing for unprecedented computational studies of electronic excitations at the nanoscale.
Photooxidation driven formation of Fe-Au linked ferrocene-based single-molecule junctions
W. Lee, L. Li, M. Camarasa-Gómez, D. Hernangómez-Pérez, X. Roy, F. Evers, M. S. Inkpen, and L. Venkataraman
Nat. Commun. 15, 1439 (2024).
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Metal-metal contacts, though not yet widely realized, may provide exciting opportunities to serve as tunable and functional interfaces in single-molecule devices. One of the simplest components which might facilitate such binding interactions is the ferrocene group. Notably, direct bonds between the ferrocene iron center and metals such as Pd or Co have been demonstrated in molecular complexes comprising coordinating ligands attached to the cyclopentadienyl rings. Here, we demonstrate that ferrocene-based single-molecule devices with Fe-Au interfacial contact geometries form at room temperature in the absence of supporting coordinating ligands. Applying a photoredox reaction, we propose that ferrocene only functions effectively as a contact group when oxidized, binding to gold through a formal Fe³⁺ center. This observation is further supported by a series of control measurements and density functional theory calculations. Our findings extend the scope of junction contact chemistries beyond those involving main group elements, lay the foundation for light switchable ferrocene-based single-molecule devices, and highlight new potential mechanistic function(s) of unsubstituted ferrocenium groups in synthetic processes.

@article{Lee2024,
author = {Lee, Woojung and Li, Liang and Camarasa-G{\'o}mez, Mar{\'i}a and Hernang{\'o}mez-P{\'e}rez, Daniel and Roy, Xavier and Evers, Ferdinand and Inkpen, Michael S. and Venkataraman, Latha},
title = {Photooxidation driven formation of Fe-Au linked ferrocene-based single-molecule junctions},
journal = {Nature Communications},
year = {2024},
month = {Feb},
day = {16},
volume = {15},
number = {1},
pages = {1439},
doi = {10.1038/s41467-024-45707-z},
url = {https://doi.org/10.1038/s41467-024-45707-z}
}
Photovoltage and photocurrent absorption spectra of sulfur vacancies locally patterned in monolayer MoS₂
A. Hötger, W. Männer, T. Amit, D. Hernangómez-Pérez, T. Taniguchi, K. Watanabe, U. Wurstbauer, J. J. Finley, S. Refaely-Abramson, C. Kastl, A. Holleitner
Nano Lett. 23, 11655 (2023).
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We report on the optical absorption characteristics of selectively positioned sulfur vacancies in monolayer MoS₂, as observed by photovoltage and photocurrent experiments in an atomistic vertical tunneling circuit at cryogenic and room temperature. Charge carriers are resonantly photoexcited within the defect states before they tunnel through an hBN tunneling barrier to a graphene-based drain contact. Both photovoltage and photocurrent characteristics confirm the optical absorption spectrum as derived from ab initio GW and Bethe-Salpeter equation approximations. Our results reveal the potential of single-vacancy tunneling devices as atomic-scale photodiodes.

@article{doi:10.1021/acs.nanolett.3c03517,
author = {H{\"o}tger, Alexander and M{\"a}nner, Wolfgang and Amit, Tomer and Hernang{\'o}mez-P{\'e}rez, Daniel and Taniguchi, Takashi and Watanabe, Kenji and Wurstbauer, Ursula and Finley, Jonathan J. and Refaely-Abramson, Sivan and Kastl, Christoph and Holleitner, Alexander W.},
title = {Photovoltage and Photocurrent Absorption Spectra of Sulfur Vacancies Locally Patterned in Monolayer MoS₂},
journal = {Nano Letters},
volume = {23},
number = {24},
pages = {11655-11661},
year = {2023},
doi = {10.1021/acs.nanolett.3c03517},
url = {https://doi.org/10.1021/acs.nanolett.3c03517}
}
Reduced absorption due to defect-localized interlayer excitons in transition metal dichalcogenide-graphene heterostructures
D. Hernangómez-Pérez, A. Kleiner, and S. Refaely-Abramson
Nano Lett. 23, 5995 (2023).
| | Link | Download | Download SI

Associating atomic vacancies to excited-state transport phenomena in two-dimensional semiconductors demands a detailed understanding of the exciton transitions involved. We study the effect of such defects on the electronic and optical properties of WS₂–graphene and MoS₂–graphene van der Waals heterobilayers, employing many-body perturbation theory. We find that chalcogen defects and the graphene interface radically alter the optical properties of the transition-metal dichalcogenide in the heterobilayer, due to a combination of dielectric screening and the many-body nature of defect-induced intralayer and interlayer optical transitions. By analyzing the intrinsic radiative rates of the subgap excitonic features, we show that while defects introduce low-lying optical transitions, resulting in excitons with non-negligible oscillator strength, they decrease the optical response of the pristine-like transition-metal dichalcogenide intralayer excitons. Our findings relate excitonic features with interface design for defect engineering in photovoltaic and transport applications.

@article{doi:10.1021/acs.nanolett.3c01182,
author = {Hernang{\'o}mez-P{\'e}rez, Daniel and Kleiner, Amir and Refaely-Abramson, Sivan},
title = {Reduced Absorption Due to Defect-Localized Interlayer Excitons in Transition-Metal Dichalcogenide–Graphene Heterostructures},
journal = {Nano Letters},
volume = {23},
number = {13},
pages = {5995-6001},
year = {2023},
doi = {10.1021/acs.nanolett.3c01182},
url = {https://doi.org/10.1021/acs.nanolett.3c01182},
}
Spin-defect characteristics of single sulfur vacancies in monolayer MoS₂
A. Hötger, T. Amit, J. Klein, K. Barthelemi, T. Pelini, A. Delhomme, S. Rey, M. Potemski, C. Faugeras, G. Cohen, D. Hernangómez-Pérez, T. Taniguchi, K. Watanabe, C. Kastl, J. J. Finley, S. Refaely-Abramson, A. W. Holleitner, and A. V. Stier
npj 2D Mater. Appl. 7, 30 (2023).
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Single spin defects in 2D transition-metal dichalcogenides are natural spin-photon interfaces for quantum applications. Here we report high-field magneto-photoluminescence spectroscopy from three emission lines (Q1, Q2 and Q^∗) of He-ion induced sulfur vacancies in monolayer MoS₂. Analysis of the asymmetric PL lineshapes in combination with the diamagnetic shift of Q1 and Q2 yields a consistent picture of localized emitters with a wavefunction extent of ∼3.5 nm. The distinct valley-Zeeman splitting in out-of-plane B-fields and the brightening of dark states through in-plane B-fields necessitates spin-valley selectivity of the defect states and lifted spin-degeneracy at zero field. Comparing our results to ab-initio calculations identifies the nature of Q1 and Q2 and suggests that Q^∗ is the emission from a chemically functionalized defect. Analysis of the optical degree of circular polarization reveals that the Fermi level is a parameter that enables the tunability of the emitter. These results show that defects in 2D semiconductors may be utilized for quantum technologies.

@article{Hotger2023,
author={H{\"o}tger, A. and Amit, T. and Klein, J. and Barthelmi, K. and Pelini, T. and Delhomme, A. and Rey, S. and Potemski, M. and Faugeras, C. and Cohen, G. and Hernang{\'o}mez-P{\'e}rez, D. and Taniguchi, T. and Watanabe, K. and Kastl, C. and Finley, J. J. and Refaely-Abramson, S. and Holleitner, A. W. and Stier, A. V.},
title={Spin-defect characteristics of single sulfur vacancies in monolayer MoS\textsubscript{2}},
journal={npj 2D Materials and Applications},
year={2023},
month={Apr},
day={08},
volume={7},
number={1},
pages={30},
doi={10.1038/s41699-023-00392-2},
url={https://doi.org/10.1038/s41699-023-00392-2}
}
Charge quenching at defect states in transition metal dichalcogenide-graphene van der Waals heterobilayers
D. Hernangómez-Pérez, A. Donarini, and S. Refaely-Abramson
Phys. Rev. B, 107, 075419 (2023). Editors' Suggestion.
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We study the dynamical properties of pointlike defects, represented by monoatomic chalcogen vacancies, in WS₂–graphene and MoS₂–graphene heterobilayers. Employing a multidisciplinary approach based on the combination of ab initio, model Hamiltonian and density matrix techniques, we propose a minimal interacting model that allows for the calculation of electronic transition times associated to population and depopulation of the vacancy by an additional electron. We obtain the “coarse-grained” semiclassical dynamics by means of a quantum master equation approach and discuss the potential role of virtual charge fluctuations in the internal dynamics of impurity quasidegenerate states. The interplay between the symmetry of the lattice and the spin degree of freedom through the spin-orbit interaction and its impact on charge quenching is studied in detail.

@article{PhysRevB.107.075419,
title = {Charge quenching at defect states in transition metal dichalcogenide--graphene van der Waals heterobilayers},
author = {Hernang\'omez-P\'erez, Daniel and Donarini, Andrea and Refaely-Abramson, Sivan},
journal = {Phys. Rev. B},
volume = {107},
issue = {7},
pages = {075419},
numpages = {16},
year = {2023},
month = {Feb},
publisher = {American Physical Society},
doi = {10.1103/PhysRevB.107.075419},
url = {https://link.aps.org/doi/10.1103/PhysRevB.107.075419}
}
Tunable magneto-optical properties in MoS₂ via defect-induced exciton transitions
T. Amit, D. Hernangómez-Pérez, G. Cohen, D. Y. Qiu, and S. Refaely-Abramson
Phys. Rev. B 106, L161407 (2022).
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The presence of chalcogen vacancies in monolayer transition metal dichalcogenides (TMDs) leads to excitons with mixed localized-delocalized character and to reduced valley selectivity. Recent experimental advances in defect design in TMDs allow for a close examination of such mixed exciton states as a function of their degree of circular polarization under external magnetic fields, revealing strongly varying defect-induced magnetic properties. A theoretical understanding of these observations and their physical origins demands a predictive, structure-sensitive theory. In this work, we study the effect of chalcogen vacancies on the exciton magnetic properties in monolayer MoS₂. Using many-body perturbation theory, we show how the complex excitonic picture associated with the presence of defects—with reduced valley and spin selectivity due to hybridized electron-hole transitions—leads to a structurally controllable exciton magnetic response. We find a variety of g-factors with changing magnitudes and sign depending on the exciton energy and character. Our findings suggest a pathway to tune the nature of the excitons—and by that their magneto-optical properties—through defect architecture.

@article{PhysRevB.106.L161407,
title = {Tunable magneto-optical properties in ${\mathrm{MoS}}_{2}$ via defect-induced exciton transitions},
author = {Amit, Tomer and Hernang\'omez-P\'erez, Daniel and Cohen, Galit and Qiu, Diana Y. and Refaely-Abramson, Sivan},
journal = {Phys. Rev. B},
volume = {106},
issue = {16},
pages = {L161407},
numpages = {6},
year = {2022},
month = {Oct},
publisher = {American Physical Society},
doi = {10.1103/PhysRevB.106.L161407},
url = {https://link.aps.org/doi/10.1103/PhysRevB.106.L161407}
}
Mixed excitonic nature in water-oxidized BiVO₄ surfaces with defects
R. Steinitz-Eliyahu, D. Hernangómez-Pérez, F. S. Hegner, P. Nikačevič, N. López, and S. Refaely-Abramson
Phys. Rev. Mat. 6, 065402 (2022).
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BiVO₄ is a promising photocatalyst for efficient water oxidation, with surface reactivity determined by the structure of active catalytic sites. Surface oxidation in the presence of oxygen vacancies induces electron localization, suggesting an atomistic route to improve the charge transfer efficiency within the catalytic cycle. In this paper, we study the effect of oxygen vacancies on the electronic and optical properties at BiVO₄ surfaces upon water oxidation. We use density functional theory and many-body perturbation theory to explore the change in the electronic and quasiparticle energy levels and to evaluate the electron-hole coupling as a function of the underlying structure. We show that while the presence of defects alters the atomic structure and largely modifies the wave-function nature, leading to defect-localized states at the quasiparticle gap region, the optical excitations remain largely unchanged due to the substantial hybridization of defect and nondefect electron-hole transitions. Our findings suggest that defect-induced surface oxidation supports improved electron transport, both through bound and tunable electronic states and via a mixed nature of the optical transitions, expected to reduce electron-hole defect trapping.

@article{PhysRevMaterials.6.065402,
title = {Mixed excitonic nature in water-oxidized ${\mathrm{BiVO}}_{4}$ surfaces with defects},
author = {Steinitz-Eliyahu, Rachel and Hernang\'omez-P\'erez, Daniel and Hegner, Franziska S. and Nika\ifmmode \check{c}\else \v{c}\fi{}evi\ifmmode \acute{c}\else \'{c}\fi{}, Pavle and L\'opez, N\'uria and Refaely-Abramson, Sivan},
journal = {Phys. Rev. Materials},
volume = {6},
issue = {6},
pages = {065402},
numpages = {6},
year = {2022},
month = {Jun},
publisher = {American Physical Society},
doi = {10.1103/PhysRevMaterials.6.065402},
url = {https://link.aps.org/doi/10.1103/PhysRevMaterials.6.065402}
}
Atomically resolved single-molecule triplet quenching
J. Peng, S. Sokolov, D. Hernangómez-Pérez, F. Evers, L. Gross, J. Lupton and J. Repp
Science 373, 452 (2021).
| | Link | featured in Phys.org, Chemistry World, Science News

The nonequilibrium triplet state of molecules plays an important role in photocatalysis, organic photovoltaics, and photodynamic therapy. We report the direct measurement of the triplet lifetime of an individual pentacene molecule on an insulating surface with atomic resolution by introducing an electronic pump-probe method in atomic force microscopy. Strong quenching of the triplet lifetime is observed if oxygen molecules are coadsorbed in close proximity. By means of single-molecule manipulation techniques, different arrangements with oxygen molecules were created and characterized with atomic precision, allowing for the direct correlation of molecular arrangements with the lifetime of the quenched triplet. Such electrical addressing of long-lived triplets of single molecules, combined with atomic-scale manipulation, offers previously unexplored routes to control and study local spin-spin interactions.

@article{doi:10.1126/science.abh1155,
author = {Jinbo Peng and Sophia Sokolov and Daniel Hernangómez-Pérez and Ferdinand Evers and Leo Gross and John M. Lupton and Jascha Repp},
title = {Atomically resolved single-molecule triplet quenching},
journal = {Science},
volume = {373},
number = {6553},
pages = {452-456},
year = {2021},
doi = {10.1126/science.abh1155},
url = {https://www.science.org/doi/abs/10.1126/science.abh1155},
eprint = {https://www.science.org/doi/pdf/10.1126/science.abh1155}
}
The role of chalcogen vacancies for atomic defect emission in MoS₂
E. Mitterreiter, B. Schuler, A. Micevic, D. Hernangómez-Pérez, K. Barthelmi, K. A. Cochrane, J. Kiemle, F. Sigger, J. Klein, E. Wong, E. S. Barnard, K. Watanabe, T. Taniguchi, M. Lorke, F. Jahnke, J. J. Finley, A. M. Schwartzberg, D. Y. Qiu, S. Refaely-Abramson, A. W. Holleitner, A. Weber-Bargioni and C. Kastl
Nat. Commun. 12, 3822 (2021).
| | Link | Download | Download SI

For two-dimensional (2D) layered semiconductors, control over atomic defects and understanding of their electronic and optical functionality represent major challenges towards developing a mature semiconductor technology using such materials. Here, we correlate generation, optical spectroscopy, atomic resolution imaging, and ab initio theory of chalcogen vacancies in monolayer MoS₂. Chalcogen vacancies are selectively generated by in-vacuo annealing, but also focused ion beam exposure. The defect generation rate, atomic imaging and the optical signatures support this claim. We discriminate the narrow linewidth photoluminescence signatures of vacancies, resulting predominantly from localized defect orbitals, from broad luminescence features in the same spectral range, resulting from adsorbates. Vacancies can be patterned with a precision below 10 nm by ion beams, show single photon emission, and open the possibility for advanced defect engineering of 2D semiconductors at the ultimate scale.

@article{Mitterreiter2021,
author = {Mitterreiter, Elmar and Schuler, Bruno and Micevic, Ana and Hernang{\'o}mez-P{\'e}rez, Daniel and Barthelmi, Katja and Cochrane, Katherine A. and Kiemle, Jonas and Sigger, Florian and Klein, Juliana nd Wong, Edward and Barnard, Edward S. and Watanabe, Kenji and Taniguchi, Takashi and Lorke, Michael and Jahnke, Frank and Finley, Johnathan J. and Schwartzberg, Adam M. and Qiu, Diana Y. and Refaely-Abramson, Sivan and Holleitner, Alexander W. and Weber-Bargioni, Alexander and Kastl, Christoph},
title = {The role of chalcogen vacancies for atomic defect emission in MoS\textsubscript{2}},
journal = {Nature Communications},
year = {2021},
month = {Jun},
day = {22},
volume = {12},
number = {1},
pages = {3822},
doi = {10.1038/s41467-021-24102-y},
url = {https://doi.org/10.1038/s41467-021-24102-y}
}
Quartic multifractality and finite-size corrections at the spin quantum Hall transition
M. Puschmann, D. Hernangómez-Pérez, B. Lang, S. Bera, and F. Evers
Phys. Rev. B 103, 235167 (2021).
| | Link | Download

The spin quantum Hall transition (or class C transition in two dimensions) represents one of the few localization-delocalization transitions for which some of the critical exponents are known exactly. Not known, however, is the multifractal spectrum τ_q, which describes the system-size scaling of inverse participation ratios P_q, i.e., the q moments of critical wave-function amplitudes. We here report simulations based on the class C Chalker-Coddington network and demonstrate that τ_q is (essentially) a quartic polynomial in q. Analytical results fix all prefactors except the quartic curvature that we obtain as γ = (2.22 ± 0.15) x 10⁻³. In order to achieve the necessary accuracy in the presence of sizable corrections to scaling, we have analyzed the evolution with system size of the entire P_q-distribution function. As it turns out, in a sizable window of q values this distribution function exhibits a (single-parameter) scaling collapse already in the preasymptotic regime, where finite-size corrections are not negligible. This observation motivates us to propose new, original approach for extracting τ_q based on concepts borrowed from the Kolmogorov-Smirnov test of mathematical statistics. We believe that our work provides the conceptual means for high-precision investigations of multifractal spectra also near other localization-delocalization transitions of current interest, especially the integer (class A) quantum Hall effect.

@article{PhysRevB.103.235167,
title = {Quartic multifractality and finite-size corrections at the spin quantum Hall transition},
author = {Puschmann, Martin and Hernang\'omez-P\'erez, Daniel and Lang, Bruno and Bera, Soumya and Evers, Ferdinand},
journal = {Phys. Rev. B},
volume = {103},
issue = {23},
pages = {235167},
numpages = {16},
year = {2021},
month = {Jun},
publisher = {American Physical Society},
doi = {10.1103/PhysRevB.103.235167},
url = {https://link.aps.org/doi/10.1103/PhysRevB.103.235167}
}
Photoinduced electronic and spin properties of two-dimensional electron gases with Rashba spin-orbit coupling under perpendicular magnetic fields
D. Hernangómez-Pérez, J. D. Torres, and A. López
Phys. Rev. B 102, 165414 (2020).
| | Link | Download

We theoretically investigate photoinduced phenomena induced by time-periodic driving fields in two-dimensional electron gases under perpendicular magnetic fields with Rashba spin-orbit coupling. Using perturbation theory, we provide analytical results for the Floquet-Landau energy spectrum appearing due to THz radiation. By employing the resulting photomodulated states, we compute the dynamical evolution of the spin polarization function for an initially prepared coherent state. We find that the interplay of the magnetic field, Rashba spin-orbit interaction, and THz radiation can lead to nontrivial beating patterns in the spin polarization. The dynamics also induces fractional revivals in the autocorrelation function due to interference of the photomodulated quantum states. We calculate the transverse photoassisted conductivity in the linear response regime using Kubo formalism and analyze the impact of the radiation field and Rashba spin-orbit interaction. In the static limit we find that our results reduce to well-known expressions of the conductivity in nonrelativistic and quasirelativistic (topological insulator surfaces) two-dimensional electron gas thoroughly described in the literature. We discuss the possible experimental detection of our theoretical prediction and their relevance for spin-orbit physics at high magnetic fields.

@article{PhysRevB.102.165414,
title = {Photoinduced electronic and spin properties of two-dimensional electron gases with Rashba spin-orbit coupling under perpendicular magnetic fields},
author = {Hernang\'omez-P\'erez, Daniel and Torres, Juan Daniel and L\'opez, Alexander},
journal = {Phys. Rev. B},
volume = {102},
issue = {16},
pages = {165414},
numpages = {12},
year = {2020},
month = {Oct},
publisher = {American Physical Society},
doi = {10.1103/PhysRevB.102.165414},
url = {https://link.aps.org/doi/10.1103/PhysRevB.102.165414}
}
Reorganization energy and polaronic effects of pentacene on NaCl films
D. Hernangómez-Pérez, J. Schlör, D. A. Egger, L. L. Patera, J. Repp, and F. Evers
Phys. Rev. B 102, 115419 (2020).
| | Link | Download

Due to recent advances in scanning-probe technology, the electronic structure of individual molecules can now also be investigated if they are immobilized by adsorption on nonconductive substrates. As a consequence, different molecular charge states are now experimentally accessible. Thus motivated, we investigate as an experimentally relevant example the electronic and structural properties of a NaCl(001) surface with and without pentacene adsorbed (neutral and charged) by employing density-functional theory. We estimate the polaronic reorganization energy to be E_reog ≃ 0.8−1.0 eV, consistent with experimental results obtained for molecules of similar size. To account for environmental effects on this estimate, different models for charge screening are compared. Finally, we calculate the density profile of one of the frontier orbitals for different occupations and confirm the experimentally observed localization of the charge density upon charging and relaxation of molecule-insulator interface from ab initio calculations.

@article{PhysRevB.102.115419,
title = {Reorganization energy and polaronic effects of pentacene on NaCl films},
author = {Hernang\'omez-P\'erez, Daniel and Schl\"or, Jakob and Egger, David A. and Patera, Laerte L. and Repp, Jascha and Evers, Ferdinand},
journal = {Phys. Rev. B},
volume = {102},
issue = {11},
pages = {115419},
numpages = {13},
year = {2020},
month = {Sep},
publisher = {American Physical Society},
doi = {10.1103/PhysRevB.102.115419},
url = {https://link.aps.org/doi/10.1103/PhysRevB.102.115419}
}
Mechanically-tunable quantum interference in ferrocene-based molecular junctions
M. Camarasa-Gómez^∗, D. Hernangómez-Pérez^∗, M. S. Inkpen, G. Lovat, E-Dean Fung, X. Roy, L. Venkataraman, and F. Evers (∗ = equally contributing authors)
Nano Lett. 20, 6381 (2020).
| | Link | Download SI

Ferrocenes are ubiquitous organometallic building blocks that comprise a Fe atom sandwiched between two cyclopentadienyl (Cp) rings that rotate freely at room temperature. Of widespread interest in fundamental studies and real-world applications, they have also attracted some interest as functional elements of molecular-scale devices. Here we investigate the impact of the configurational degrees of freedom of a ferrocene derivative on its single-molecule junction conductance. Measurements indicate that the conductance of the ferrocene derivative, which is suppressed by 2 orders of magnitude as compared to a fully conjugated analogue, can be modulated by altering the junction configuration. Ab initio transport calculations show that the low conductance is a consequence of destructive quantum interference effects of the Fano type that arise from the hybridization of localized metal-based d-orbitals and the delocalized ligand-based π-system. By rotation of the Cp rings, the hybridization, and thus the quantum interference, can be mechanically controlled, resulting in a conductance modulation that is seen experimentally.

@article{doi:10.1021/acs.nanolett.0c01956,
author = {Camarasa-Gómez, María and Hernangómez-Pérez, Daniel and Inkpen, Michael S. and Lovat, Giacomo and Fung, E-Dean and Roy, Xavier and Venkataraman, Latha and Evers, Ferdinand},
title = {Mechanically Tunable Quantum Interference in Ferrocene-Based Single-Molecule Junctions},
journal = {Nano Letters},
volume = {20},
number = {9},
pages = {6381-6386},
year = {2020},
doi = {10.1021/acs.nanolett.0c01956},
url = {https://doi.org/10.1021/acs.nanolett.0c01956},
eprint = {https://doi.org/10.1021/acs.nanolett.0c01956}
}
Solitonics with polyacetylenes
D. Hernangómez-Pérez, S. Gunasekaran, L. Venkataraman, and F. Evers
Nano Lett. 20, 2615 (2020).
| | Link | Download SI | featured in Phys.org

Polyacetylene molecular wires have attracted a long-standing interest for the past 40 years. From a fundamental perspective, there are two main reasons for the interest. First, polyacetylenes are a prime realization of a one-dimensional topological insulator. Second, long molecules support freely propagating topological domain-wall states, so-called “solitons,” which provide an early paradigm for spin-charge separation. Because of recent experimental developments, individual polyacetylene chains can now be synthesized on substrates. Motivated by this breakthrough, we here propose a novel way for chemically supported soliton design in these systems. We demonstrate how to control the soliton position and how to read it out via external means. Also, we show how extra soliton–antisoliton pairs arise when applying a moderate static electric field. We thus make a step toward functionality of electronic devices based on soliton manipulation, that is, “solitonics”.

@article{doi:10.1021/acs.nanolett.0c00136,
author = {Hernangómez-Pérez, Daniel and Gunasekaran, Suman and Venkataraman, Latha and Evers, Ferdinand},
title = {Solitonics with Polyacetylenes},
journal = {Nano Letters},
volume = {20},
number = {4},
pages = {2615-2619},
year = {2020},
doi = {10.1021/acs.nanolett.0c00136},
url = {https://doi.org/10.1021/acs.nanolett.0c00136}
}
Near length-independent conductance of polymethine molecular wires
S. Gunasekaran, D. Hernangómez-Pérez, I. Davydenko, S. Marder, F. Evers, and L. Venkataraman
Nano Lett. 18, 6387 (2018).
| | Link | Download SI

Polymethine dyes are linear π-conjugated compounds with an odd number of carbons that display a much greater delocalization in comparison to polyenes that have an even number of carbon atoms in their main chain. Herein, we perform scanning tunneling microscope based break-junction measurements on a series of three cyanine dyes of increasing length. We demonstrate, at the single molecule level, that these short chain polymethine systems exhibit a substantially smaller decay in conductance with length (attenuation factor β = 0.04 Å^-1) compared to traditional polyenes (β ≈ 0.2 Å^-1). Furthermore, we show that by changing solvent we are able to shift the β value, demonstrating a remarkable negative β value, with conductance increasing with molecular length. First principle calculations provide support for the experimentally observed near-uniform length dependent conductance and further suggest that the variations in β with solvent are due to solvent-induced changes in the alignment of the frontier molecular orbitals relative to the Fermi energy of the leads. A simplified Hückel model suggests that the smaller decay in conductance correlates with the smaller degree of bond order alternation present in polymethine compounds compared to polyenes. These findings may enable the design of molecular wires without a length-dependent decay for efficient electron transport at the nanoscale.

@article{doi:10.1021/acs.nanolett.8b02743,
author = {Gunasekaran, Suman and Hernangómez-Pérez, Daniel and Davydenko, Iryna and Marder, Seth and Evers, Ferdinand and Venkataraman, Latha},
title = {Near Length-Independent Conductance in Polymethine Molecular Wires},
journal = {Nano Letters},
volume = {18},
number = {10},
pages = {6387-6391},
year = {2018},
doi = {10.1021/acs.nanolett.8b02743},
url = {https://doi.org/10.1021/acs.nanolett.8b02743},
eprint = {https://doi.org/10.1021/acs.nanolett.8b02743}
}
Silver makes better electrical contacts to thiol-terminated silanes than gold
H. Li, T. A. Su, M. Camarasa-Gómez, D. Hernangómez-Pérez, S. E. Henn, V. Pokorný, C. D. Caniglia, M. S. Inkpen, R. Korytár, R. Steigerwald, C. Nuckolls, F. Evers, and L. Venkataraman
Angew. Chem. Int. Ed. 56, 14145 (2017).
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We report that the single-molecule junction conductance of thiol-terminated silanes with Ag electrodes are higher than the conductance of those formed with Au electrodes. These results are in contrast to the trends in the metal work function ɸ(Ag) < ɸ(Au). As such, a better alignment of the Au Fermi level to the molecular orbital of silane that mediates charge transport would be expected. This conductance trend is reversed when we replace the thiols with amines, highlighting the impact of metal–S covalent and metal–NH₂ dative bonds in controlling the molecular conductance. Density functional theory calculations elucidate the crucial role of the chemical linkers in determining the level alignment when molecules are attached to different metal contacts. We also demonstrate that conductance of thiol-terminated silanes with Pt electrodes is lower than the ones formed with Au and Ag electrodes, again in contrast to the trends in the metal work-functions.

@article{https://doi.org/10.1002/anie.201708524,
author = {Li, Haixing and Su, Timothy A. and Camarasa-Gómez, María and Hernangómez-Pérez, Daniel and Henn, Simon E. and Pokorný, Vladislav and Caniglia, Caravaggio D. and Inkpen, Michael S. and Korytár, Richard and Steigerwald, Michael L. and Nuckolls, Colin and Evers, Ferdinand and Venkataraman, Latha},
title = {Silver Makes Better Electrical Contacts to Thiol-Terminated Silanes than Gold},
journal = {Angewandte Chemie International Edition},
volume = {56},
number = {45},
pages = {14145-14148},
keywords = {metal–molecule interactions, silanes, silver, platinum, single-molecule electronics},
doi = {10.1002/anie.201708524},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201708524},
year = {2017}
}
Transformation optics approach to plasmon-exciton strong coupling in nanocavities
R.-Q. Li, D. Hernangómez-Pérez, F. J. García-Vidal, and A. I. Fernández-Domínguez
Phys. Rev. Lett. 117, 107401 (2016).
| | Link | Download | Download SI

We investigate the conditions yielding plasmon-exciton strong coupling at the single emitter level in the gap between two metal nanoparticles. Inspired by transformation optics ideas, a quasianalytical approach is developed that makes possible a thorough exploration of this hybrid system incorporating the full richness of its plasmonic spectrum. This allows us to reveal that by placing the emitter away from the cavity center, its coupling to multipolar dark modes of both even and odd parity increases remarkably. This way, reversible dynamics in the population of the quantum emitter takes place in feasible implementations of this archetypal nanocavity.

@article{PhysRevLett.117.107401,
title = {Transformation Optics Approach to Plasmon-Exciton Strong Coupling in Nanocavities},
author = {Li, Rui-Qi and Hern\'angomez-P\'erez, D. and Garc\'{\i}a-Vidal, F. J. and Fern\'andez-Dom\'{\i}nguez, A. I.},
journal = {Phys. Rev. Lett.},
volume = {117},
issue = {10},
pages = {107401},
numpages = {5},
year = {2016},
month = {Aug},
publisher = {American Physical Society},
doi = {10.1103/PhysRevLett.117.107401},
url = {https://link.aps.org/doi/10.1103/PhysRevLett.117.107401}
}
Topologically protected entanglement of electron-pair cyclotron motions
T. Champel, D. Hernangómez-Pérez, and S. Florens
Phys. Rev. B 93, 075112 (2016).
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Considering two-dimensional electron gases under a perpendicular magnetic field, we pinpoint a specific kind of long-range bipartite entanglement of the electronic motions. This entanglement is achieved through the introduction of bicomplex spinorial eigenfunctions admitting a polar decomposition in terms of a real modulus and three real phases. Within this bicomplex geometry the cyclotron motions of two electrons are intrinsically tied, so that the highlighted eigenstates of the kinetic energy operator actually describe the free motion of a genuine electron pair. Most remarkably, these states embody phase singularities in the four-dimensional (4D) space, with singular points corresponding to the simultaneous undetermination of the three phases. Because the entanglement between the two electrons forming a pair, as well as the winding and parity quantum numbers characterizing the 4D phase singularity, are topological in nature, we expect them to manifest some robustness in the presence of a smooth disorder potential and an electron-electron interaction potential. The relevance of this effective approach in terms of 4D vortices of electron pairs is discussed in the context of the fractional quantum Hall effect.

@article{PhysRevB.93.075112,
title = {Topologically protected entanglement of electron-pair cyclotron motions},
author = {Champel, T. and Hernang\'omez-P\'erez, D. and Florens, S.},
journal = {Phys. Rev. B},
volume = {93},
issue = {7},
pages = {075112},
numpages = {10},
year = {2016},
month = {Feb},
publisher = {American Physical Society},
doi = {10.1103/PhysRevB.93.075112},
url = {https://link.aps.org/doi/10.1103/PhysRevB.93.075112}
}
Signatures of Rashba spin-orbit interaction in charge and spin properties of quantum Hall systems
D. Hernangómez-Pérez, S. Florens, and T. Champel
Phys. Rev. B 89, 155314 (2014).
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We study the local equilibrium properties of two-dimensional electron gases at high magnetic fields in the presence of random smooth electrostatic disorder, Rashba spin-orbit coupling, and the Zeeman interaction. Using a systematic magnetic length (l_B) expansion within a Green's function framework we derive quantum functionals for the local spin-resolved particle and current densities which can be useful for future studies combining disorder and mean-field electron-electron interaction in the quantum Hall regime. We point out that the spin polarization presents a peculiar spatial dependence which can be used to determine the strength of the Rashba coupling by local probes. The spatial structure of the current density, consisting of both compressible and incompressible contributions, also essentially reflects the effects of Rashba spin-orbit interaction on the energy spectrum. We show that in the semiclassical limit l_B→0 the local Hall conductivity remains, however, still quantized in units of e²/h for any finite strength of the spin-orbit interaction. In contrast, it becomes half-integer quantized when the latter is infinite, a situation which corresponds to a disordered topological insulator surface consisting of a single Dirac cone. Finally, we argue how to define at high magnetic fields a spin Hall conductivity related to a dissipationless angular momentum flow, which is characterized by a sequence of plateaus as a function of the inverse magnetic field (thus free of resonances).

@article{PhysRevB.89.155314,
title = {Signatures of Rashba spin-orbit interaction in charge and spin properties of quantum Hall systems},
author = {Hernang\'omez-P\'erez, Daniel and Florens, Serge and Champel, Thierry},
journal = {Phys. Rev. B},
volume = {89},
issue = {15},
pages = {155314},
numpages = {16},
year = {2014},
month = {Apr},
publisher = {American Physical Society},
doi = {10.1103/PhysRevB.89.155314},
url = {https://link.aps.org/doi/10.1103/PhysRevB.89.155314}
}
Spectral properties and local density of states of disordered quantum Hall systems with Rashba spin-orbit coupling
D. Hernangómez-Pérez, J. Ulrich, S. Florens, and T. Champel
Phys. Rev. B 88, 245433 (2013).
| | Link | Download

We theoretically investigate the spectral properties and the spatial dependence of the local density of states (LDOS) in disordered two-dimensional electron gases (2DEG) in the quantum Hall regime, taking into account the combined presence of electrostatic disorder, random Rashba spin-orbit interaction, and finite Zeeman coupling. To this purpose, we extend a coherent-state Green's function formalism previously proposed for spinless 2DEG in the presence of smooth arbitrary disorder, that here incorporates the nontrivial coupling between the orbital and spin degrees of freedom into the electronic drift states. The formalism allows us to obtain analytical and controlled nonperturbative expressions of the energy spectrum in arbitrary locally flat disorder potentials with both random electric fields and Rashba coupling. As an illustration of this theory, we derive analytical microscopic expressions for the LDOS in different temperature regimes which can be used as a starting point to interpret scanning tunneling spectroscopy data at high magnetic fields. In this context, we study the spatial dependence and linewidth of the LDOS peaks and explain an experimentally noticed correlation between the spatial dispersion of the spin-orbit splitting and the local extrema of the potential landscape.

@article{PhysRevB.88.245433,
title = {Spectral properties and local density of states of disordered quantum Hall systems with Rashba spin-orbit coupling},
author = {Hernang\'omez-P\'erez, Daniel and Ulrich, Jascha and Florens, Serge and Champel, Thierry},
journal = {Phys. Rev. B},
volume = {88},
issue = {24},
pages = {245433},
numpages = {16},
year = {2013},
month = {Dec},
publisher = {American Physical Society},
doi = {10.1103/PhysRevB.88.245433},
url = {https://link.aps.org/doi/10.1103/PhysRevB.88.245433}
}

Peer-reviewed Conference Proceedings

G. Rizelli, M. Camarasa-Gómez, D. Hernangómez-Pérez, and J. D. Ania-Castañón
Multi-level Approach to the Optimization of Unrepeatered WDM Systems with Distributed Amplification
European Conference on Lasers and Electro-Optics - European Quantum Electronics Conference
Munich, June 21-25 (2015).
D. Hernangómez-Pérez, S. Martín-López, F. Rodríguez-Barrios, P. Corredera-Guillén, M. González-Herráez, and J. D. Ania-Castañón
Sensado óptico distribuido Brillouin asistido por amplificación Raman de primer orden
XXV National Symposium of URSI - International Union of Radio Science
Bilbao, September 15-17 (2010).

Referee Activities

I have acted as referee for the following journals: Phys. Rev. (A, B, Letters, Materials), Phys. Chem. Chem. Phys, ACS Omega.