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@PHDTHESIS{Safari:1038874,
      author       = {Safari, Mohammad Reza},
      title        = {{S}pin selectivity of chiral molecules on surfaces},
      volume       = {108},
      school       = {Köln},
      type         = {Dissertation},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2025-01689},
      isbn         = {978-3-95806-810-0},
      series       = {Schriften des Forschungszentrums Jülich Reihe Information
                      / Information},
      pages        = {xiv, 165},
      year         = {2025},
      note         = {Dissertation, Köln, 2024},
      abstract     = {The purpose of this thesis is to investigate the spin
                      selectivity of chiral molecules
                      adsorbedonmagneticsubstrates,
                      withtheaimofimprovingourunderstandingofthecomplexinter-actions
                      between chirality and magnetism. This research centers on
                      the chirality-inducedspin selectivity (CISS) effect, an
                      emerging phenomenon that has captured the interest ofthe
                      scientific community due to its potential applications in
                      spintronics, efficient quantumcomputing, enantioseparation,
                      and selective chemical processes. Since the initial
                      identi-fication of the CISS effect, extensive research on
                      various molecules and substrates hasyielded significant
                      outcomes. Observations have been made that electrons
                      transmittedthrough chiral molecules at room temperature
                      exhibit spin polarizations exceeding sev-eral tens of
                      percent. The findings also include the enantiospecific
                      adsorption of chiralmolecules on perpendicularly magnetized
                      ferromagnetic substrates.This area of research, which
                      explores both theoretical and experimental aspects of
                      theCISS effect, remains a topic of scientific interest.
                      Despite substantial experimental
                      evi-dencesupportingCISS,acomprehensivetheoreticalunderstandingremainselusive.
                      Presenttheoretical approaches often fail to bridge the gap
                      between the magnitudes of experimen-tal results and
                      theoretical predictions, such as spin polarization values or
                      enantiospecificadsorption energies.A significant challenge
                      in bridging experimental and theoretical studies stems from
                      thecomplexity of real-world experiments. Often, these
                      involve molecular ensembles and yielddata that reflect
                      averages of various configurations, such as adsorption sites
                      and pathwaysof electric current. Additionally, experimental
                      conditions may necessitate
                      modificationslikeprotectivecoatingstopreventoxidation(e.g.,
                      Aucoatingonferromagneticsubstrates)and can introduce other
                      variables such as water from ambient humidity. Meanwhile,
                      the-oretical models typically overlook these complexities.
                      In response, this PhD researchproject is designed to
                      thoroughly investigate the CISS effect under ultra-high
                      vacuum(UHV) conditions, with precise control over geometric
                      configurations at the atomic scale.This approach could
                      facilitate a closer alignment between experimental
                      observations andtheoretical calculations. This thesis
                      investigates chiral heptahelicene (7[H]) molecules adsorbed
                      on various single-crystalline substrates, ranging from the
                      noble metal Cu(111) to more reactive substrateslike
                      ferromagnetic Co bilayer nanoislands on Cu(111) and Fe
                      bilayers on W(110). Low-temperature spin-polarized scanning
                      tunneling microscopy (SP-STM) and spectroscopy(SP-STS) are
                      employed to examine molecules that have been deposited on
                      these surfacesthrough sublimation under UHV
                      conditions.Themoleculesremainstructurallyintactafterdeposition,
                      withtheproximalphenanthrenegroup aligned parallel to the
                      substrate surface. The high-resolution STM topographydata
                      provide a precise method for accurately determining the
                      chirality of individual hep-tahelicene molecules on these
                      crystalline substrates. Additionally, detailed SP-STS
                      mea-surements from individual molecules indicate distinct
                      adsorption properties: moleculesundergo physisorption on
                      Cu(111) and chemisorption on Co and Fe bilayers.
                      Notably,both Co and Fe bilayers maintain their ferromagnetic
                      properties and exhibit out-of-plane(OOP)
                      magnetization.Building on our preliminary observations, our
                      investigation delved deeper into the in-teractions between
                      chiral molecules and magnetic substrates, focusing on two
                      principalaspects.First, we explored the enantiospecific
                      adsorption of [7]H molecules onto OOP magne-tized cobalt
                      nanoislands, using both racemic and enantiopure samples. Our
                      investiga-tions revealed a magnetization-dependent
                      enantiomeric excess upon deposition of thesemolecules. In
                      the experiment using a racemic mixture, a detailed
                      statistical analysis ofover 740 molecules across 110 islands
                      revealed an enantiomeric excess ratio of 0.7. Thisratio,
                      when expressed by a Boltzmann factor, corresponds to an
                      energy difference of ap-proximately 10 ±2 meV. Further
                      experiments revealed that this energy difference is
                      notrelated to differences in adsorption energy. This finding
                      was further supported by sub-sequent experiments with more
                      than 2100 molecules on 225 islands, using
                      enantiopuremolecules, demonstrating a consistent picture of
                      enantioselectivity.The results of our experiment were then
                      compared with state-of-the-art density functionaltheory
                      (DFT) calculations performed by our colleagues at the Peter
                      Grünberg Institute(PGI-1), Jülich Research Center.
                      Interestingly, these advanced spin-resolved ab
                      initiosimulations showed no significant differences in
                      enantio-dependent chemisorption ener-gies. This discrepancy
                      between the experimental results and the simulations, along
                      withfurther experimental findings that molecular mobility
                      decreases significantly when
                      reach-ingthechemisorbedstateonthecobaltislands,
                      ledustohypothesizethatenantioselectionprimarily occurs
                      during an earlier, physisorbed state. These observations
                      suggest that van der Waals interactions, which are critical
                      for molecular magnetochiral processes, shouldalso take spin
                      fluctuations into account.Secondly, spin-selective electron
                      transport through chiral [7]H molecules at a low
                      temper-ature of 5K is investigated using a spin-sensitive
                      STM tip. These molecules are depositedon two distinct types
                      of ferromagnetic bilayer substrates: racemic mixtures are
                      depositedon Co/Cu(111), while enantiopure molecules are
                      deposited on Fe/W(110). This
                      experi-mentalapproachenablesthedirectmeasurementoftunnellingcurrentsthroughindividualmolecules
                      under precisely controlled conditions. In this setup, the
                      magnetization directionof either the STM tip or the
                      substrate can be systematically reversed, and the
                      chiralityof the molecules can be selectively chosen,
                      especially in experiments involving racemicmixtures.This
                      methodology enables accurate assessments of magnetochiral
                      conductance asymmetry(MChA) by comparing the tunneling
                      current measured through two different
                      enantiomersdepositedonthesamemagneticdomain. Additionally,
                      itallowsfortheevaluationofenan-tiospecific magnetic
                      conductance asymmetry (EMA) by comparing the tunneling
                      currentmeasured through molecules of the same handedness on
                      two magnetic domains with op-posing OOP magnetization. As a
                      result, there is significant conductance asymmetry for[7]H
                      molecules across both types of magnetic substrates, with EMA
                      values on Co islandsreaching as high as $50\%.$ This
                      detailed investigation also allows us to effectively
                      excludeensemble effects and electron-phonon coupling as
                      primary contributing factors, therebytaking a significant
                      step forward in clarifying the underlying mechanisms
                      influencing spin-selective transport through chiral
                      molecules.},
      cin          = {PGI-6},
      cid          = {I:(DE-Juel1)PGI-6-20110106},
      pnm          = {5213 - Quantum Nanoscience (POF4-521)},
      pid          = {G:(DE-HGF)POF4-5213},
      typ          = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
      doi          = {10.34734/FZJ-2025-01689},
      url          = {https://juser.fz-juelich.de/record/1038874},
}