Theory Meets Experiment in Low-Dimensional Structures with Correlated Electrons

We present a density-functional theory study of the exchange coupling between molecular/single atom spin systems and a magnetic substrate across a graphene spacer. We concentrate on cobaltocene (CoCp₂) molecules and Co adatoms adsorbed on graphene decorated Ni(111) substrates. The role of graphene is to preserve the adsorbate magnetic moments, acting as an electronic decoupling layer, while allowing effective spin communication between the adsorbate and the substrate. In fact, sizable magnetic coupling is predicted by the theory, and in some fortunate cases can be also rationalized in term of spatial and energy matching between adsorbate and substrate orbitals.

This is the case of cobaltocene, where in virtue of the peculiar magnetic properties of the molecule, the coupling mechanism is more accessible, and the description of the employed theoretical methodology suffices to rationalize it, and even tailor it by, for instance, intercalation of different ferromagnetic metal monolayers, such as Fe and Co, between graphene and the Ni substrate [S. Marocchi et al., Phys. Rev. B. 88, 144407 (2013)].

Concerning single atom spin adsorbates, we first concentrate on Co adatoms, where a thorough experimental analysis by STM and XMCD experiments has been also conducted, and where a consistent description between theory and experiments can be found only by considering the presence of more adsorption sites characterized by different magnetic properties [A. Barla et al., ACS Nano 10, 1101 (2016)]. If we extend the analysis to the other elements of the 3d series, i.e. from Sc to Fe, although some trends across the series could be found [V. Bellini et al., to be submitted], no clear understanding of the magnetic coupling can be set forward. Moreover, the employed methodology fails in the comparison with XMCD experiments performed on V adatoms, sign that more refined multi-reference methods might be necessary to correctly describe the ground state magnetic properties of these systems.

We studied the structural, electronic and magnetic properties of recently synthesized Ni(II)-quinonoid molecule adsorb on magnetic Co(001) substrate by means of density functional theory (DFT)+U calculations. A strong covalent interaction at the interface increases the Ni-O(N) bond lengths in the chemisorbed molecule leads to a spin state switching in the molecule from S = 0 state (in the gas phase) to S ≈ 1 state upon adsorption on the Co(001) substrate. Our DFT+U calculations shows that the molecule is ferromagnetically coupled with substrate. The exchange mechanism between the Co surface and metal centre in the molecule (Ni atom) was carefully investigated. It has been found that an indirect exchange interaction via quinonid ligands of the molecule, stabilizes the spin moment in the molecule in ferromagnetic alignment with the Co surface magnetization.

Electronic structure of antiferromagnetic Mn₂Au has been studied by utilizing first-principles density-functional theory calculations based on the Green’s function technique. The total density of states reveals contributions from the Au4f and two Mn3d states with collinear antiferromagnetic (AF) order with localized magnetic moments. our ground state band structure calculations representing the Bloch spectral functions shows that the essence of the AF-switching induced by Néel spin-orbit torques in Mn₂Au is in the ΓX direction of the Brillouin zone indicating a band crossing for Neel vector oriented along [100] direction. The one step model angle resolved photoemission calculations in a k_z band map for photon energies ranging from 400−1000 eV shows dispersive spectral weight at Fermi level along ΓΣ direction of the Brillouin zone.

The interplay of strong spin-orbit coupling, electron electron correlation, and crystal-field splitting in 5d transition metal oxides have recently attracted much attention both theoretically and experimentally. Here we investigate the electronic and magnetic properties of d^3.5 iridate Ba₃LiIr₂O₉ using first-principles electronic structure calculations. The results of the calculations reveal that the system lies in an intermediate spin-orbit coupling (SOC) regime. There is strong covalency of Ir-5d and O-2p orbitals. SOC, together with covalency, conspires to reduce the magnetic moment at the Ir site. By calculating the hopping interactions and exchange interactions, it is found that there is strong antiferromagnetic intradimer coupling within an Ir₂O₉ unit and other antiferromagnetic interdimer interactions make the system frustrated. The anisotropic magnetic interactions are also calculated. The calculations reveal that the magnitude of the Dzyaloshinskii-Moriya (DM) interactions parameter is small for this system. The effects of small electron and hole doping on DM interactions are also investigated here. The magnetocrystalline anisotropy energy is significant for this system, and the easy axis lies on the ab plane.

Metallic surfaces are an ideal playground for exploring correlation physics in magnetic chains made of ad-atoms or molecular networks. I will discuss manifestations of Kondo-lattice phenomenology in such chains and the possibility to observe heavy-fermion physics.

[1] M. Moro-Larages, R. Korytár, M. Piantek, R. Robles, N. Lorente, J. Pascual, M. R. Ibarra and D. Serrate, Nature Communications, to appear, (2019).

In 1959, physicist P. W. Anderson conjectured that superconductivity can exist only in objects where superconducting gap energy is larger than its electronic energy level spacing. In our work [1,2], we were able to address this limit in single nanoparticles of various sizes without suffering from inverse proximity effect. Here we report on a scanning tunneling spectroscopy study of superconducting lead nanocrystals grown on the (110) surface of InAs. The electronic transmission at the InAs/nanocrystal interface is weak; this leads to Coulomb blockade and enables the extraction of electron addition energy of the nanocrystals. The addition energy displays superconducting parity effect, a direct consequence of Cooper pairing. Due to the electrons forming Cooper pairs when an electron is added to the nanocrystal superconductor, the additional energy is different whether there is an even or odd number of electrons. Studying this parity effect as a function of nanocrystal volume, we find the suppression of Cooper pairing when the mean electronic level spacing overcomes the superconducting gap energy, thus demonstrating unambiguously the validity of the Anderson criterion.

[1] Superconducting parity effect across the Anderson limit, S. Vlaic, S. Pons, T. Zhang, A. Assouline, A. Zimmers, C. David, G. Rodary, J.-C. Girard, D. Roditchev, H. Aubin, Nature Comm. 8, 14549 (2017).

[2] Quantum confinement effects in Pb nanocrystals grown on InAs, T. Zhang, S. Vlaic, S. Pons, A. Assouline, A. Zimmers, D. Roditchev, H. Aubin and G. Allan, C. Delerue, C. David, G. Rodary, J.-C. Girard, Phys. Rev. B 97, 214514 (2018).

A quantum dot attached to BCS superconducting leads exhibits a 0−π impurity quantum phase transition, which can be experimentally controlled either by the gate voltage or by the superconducting phase difference. From the theoretical point of view, the system is described by the single-impurity Anderson model, solving of which relies on heavy numerical methods. To enhance understanding and speed up characterization of the system, we aim to provide semi-analytical insight.

First, in the common case of the two leads having the same superconducting gap size, we explain the influence of asymmetric dot-lead coupling by relating the asymmetric system to a symmetric one. Surprisingly, it is the symmetric case, which is the most general, meaning that physical quantities in the case of asymmetric coupling are fully determined by their symmetric counterparts. We give ready-to-use conversion formulas for the 0−π phase transition boundary, on-dot quantities, and the Josephson current [1].

Next, we study the 0−π impurity quantum phase transition, clearly visible by a jump in the supercurrent in the ground state but smeared out to a crossover by finite temperature. For zero temperature, we present two simple analytical formulae describing the position of the phase boundary in parameter space for two opposing limits (the weakly correlated and Kondo regime). Furthermore, we show that the two-level approximation provides an excellent description of the low temperature physics near the phase transition. We discuss reliability and mutual agreement of available finite temperature numerical methods (the numerical renormalization group and quantum Monte Carlo) and suggest a novel approach for efficient determination of the quantum phase boundary from measured finite temperature data [2].

[1] A. Kadlecová, M. Žonda, T. Novotný, Phys. Rev. B 95(19), 195114 (2017).

[2] A. Kadlecová, M. Žonda, V. Pokorný, and T. Novotný, Phys. Rev. Applied 11, 044094 (2019).

We calculate charge transport properties of molecular nano-junctions employing a combination of the density-functional theory (DFT) and a transport calculation based on the non-equilibrium Green functions technique, using our effective implementation of the non-equilibrium density matrix calculation. We focus on the effect of the spin-orbit coupling on the transport properties of short metallic nanowires and small organic molecules connected to large metallic leads. The thermodynamic limit of infinite leads is achieved using absorbing boundary conditions. The DFT solver and the transport code run inside a charge self-consistency loop, providing reliable results on the transmission function and magnetic properties.

One-dimensional structures offer a rich ecosystem for realizing quantum states with potential application for advanced information technologies. Surface confined molecular self-assembly is one avenue for creating 1D systems, where the extant structure is controlled by the precursor shape, and functional group interlinking chemistry. As an example, recent studies of properly designed graphene nanoribbons demonstrated the presence of topologically protected end-states [1].

Here we study self-assembled 1D chains of zwitterionic molecule 2,5-diamino-1,4-benzoquinonediimines (DABQDI) [1] on Au(111) in ultrahigh vacuum at 5K using combined scanning tunneling and non-contact atomic force microscopies (STM/nc-AFM) supported by theoretical analysis. Sub-molecular resolution achieved with a CO-functionalized tip hints structural information, specifically regarding the hydrogen bonds linking the precursor units. This is further supported by density functional theory (DFT) simulations and calculations of proton tunneling in between the unit cells, that enhances electronic communication across the chain.

On top of this, scanning tunneling spectroscopy (STS) measurements reveal a presence of in-gap electronic states near the Fermi energy, strongly localized to the chain ends. To rationalize the presence of these states, we propose an extension of the seminal Su, Schrieffer, Heeger (SSH) [2] model by mimicking the chemical structure of 1D DABQDI chain. A detailed analysis of the proposed model shows possible solutions with non-trivial topological phase, that exist within intuitively reasonable range of model parameters.

[1] O. Groning et al., Nature 560, 209 (2018); D. J. Rizzo et al., Nature 560, 204 (2018).

[2] S. Pascal, O. Siri, Coordination Chemistry Reviews, Volume 350, 178-195, (2017).

[3] W. P. Su, J. R. Schrieffer, and A. J. Heeger, Phys. Rev. Lett. 42, 1698, (1979).

A variety of planar π-conjugated hydrocarbons such as heptauthrene, Clar's goblet and, recently synthesized, triangulene have two electrons occupying two degenerate molecular orbitals. The resulting spin of the interacting ground state is often correctly anticipated as S = 1, extending the application of Hund's rules to these systems, but this is not correct in some instances. Here we provide a set of rules to correctly predict the existence of zero mode states, as well as the spin multiplicity of both the ground state and the low-lying excited states, together with their open- or closed-shell nature. This is accomplished using a combination of analytical arguments and configuration interaction calculations with a Hubbard model, both backed by quantum chemistry methods with a larger Gaussian basis set. Our results go beyond the well established Lieb's theorem and Ovchinnikov's rule, as we address the multiplicity and the open-/closed-shell nature of both ground and excited states.

We monitor the conductance evolution by varying the coupling between an organic molecule and the Ag(111) surface by lifting the molecule from the surface with the tip of a scanning tunneling microscope. Numerical renormalization group theory analysis of the junction conductance reveals that the system is tuned from the strongly coupled Kondo singlet to the free spin 1/2 ground state. In the crossover between the strong and weak coupling regimes, where the Kondo temperature T_K is similar to the experimental T_exp, fitting procedures generically provide ambiguous estimates of T_K that tend to vary by an order of magnitude. Measuring the dependence of the junction conductance in external magnetic fields resolves this ambiguity if the Zeeman splitting due to the imposed magnetic field overcomes the thermal energy k_BT_exp. We demonstrate that Frota analysis overestimates T_K by several orders of magnitude in the transient and weak coupling regimes. The numerical analysis also allows us to determine molecular coupling to surface and tip and their evolution during the lifting process.

Nitrogen doping of graphene significantly affects its chemical properties, which is particularly important in molecular sensing and electrocatalysis applications. However, detailed insight into interaction between N-dopant and molecules at the atomic scale is currently lacking. Here we demonstrate control over the spin state of a single iron(II) phthalocyanine molecule by its positioning on N-doped graphene. The spin transition was driven by weak intermixing between orbitals with z-component of N-dopant (p_z of N-dopant) and molecule (d_xz, d_yz, d_z²) with subsequent reordering of the Fe d-orbitals. The transition was accompanied by an electron density redistribution within the molecule, sensed by atomic force microscopy with CO-functionalized tip. This demonstrates the unique capability of the high-resolution imaging technique to discriminate between different spin states of single molecules. Moreover, we present a method for triggering spin state transitions and tuning the electronic properties of molecules through weak non-covalent interaction with suitably functionalized graphene.

Porphyrins and phtalocyanines with transition-metal cores make the ideal building blocks for functional thin films in nanotechnology. Although they were intensively studied in the past decades, some of their characteristics can only be described computationally. In this respect, we investigated the interaction characteristics between some transition-metal phthalocyanines/porphyrins and metallic surfaces i.e. Au(111) and Ag(111), with focus on simulated STM images to be further compared to experimental data. Our approach is based on the vdW-DF-cx exchange–correlation functional of Berland and Hyldgaard as implemented in SIESTA and includes DFT+U corrections. The simulation of the STM images was performed using the Tersoff-Hamman approximation as a "constant current" experiment. We emphasize the role of the transition metals (Cr, Mn, Fe, Co, Ni) as well as that of the organic moiety, and thus provide a powerful tool for discriminating between various experimental STM images.

Acknowledgements: Experimental data is provided by Nanolab team from UniBasel, Switzerland. The computational work was funded by the UEFISCDI, project PN-III-P4-IDPCE-2016-0217.

Despite the formal similarity, the structure, stability and chemical properties of unsaturated systems containing higher main group elements are strikingly different from their first row counterparts. This is particularly evident in group IV, since the chemical flexibility of carbon is not shared by its congeners. In fact, the nature and structure of multiple bonds between higher main group elements remain elusive, thereby hindering the development of their low-dimensional systems. Here, focusing on the Si vs. C comparison, we shed light on this issue with the help of analytical modeling and first-principles calculations on both finite-sized and extended systems. We show that trans-bending in disilenes and buckling in low-dimensional silicon nanostructures (e.g., silicene) have a common origin, as both result from a tug of war between the σ and the π bonds in which the former favor distortion and the latter oppose to it. In carbon, π bonds are stiffer and effectively resist to the distortion, but in silicon they are softer and typically succumb. As a consequence, Si structures display weaker π bonds than they could otherwise do if undistorted. Correlation between π electrons plays a major role in this context, since Coulomb repulsion moves π bonds apart from the molecular orbital limit and renders them sensitive to doping charges. Hence, upon weakening the effective e-e repulsion in the p shell, one may completely remove any structural instability, strengthen the π bonds, and turn Si into a closer relative of C.

The stacking of monolayers in the form of van der Waals heterostructures is a useful strategy for band gap engineering and the control of dynamics of excitons for potential nano-electronic devices. We performed first-principles calculations to investigate the structural, electronic, optical and photocatalytic properties of the SiC–MX₂ (M = Mo, W and X = S, Se) van der Waals heterostructures. The stability of most favorable stacking is confirmed by calculating the binding energy and phonon spectrum. SiC–MoS₂ is found to be a direct band gap type-II semiconducting heterostructure. Moderate in-plane tensile strain is used to achieve a direct band gap with type-II alignment in the SiC–WS₂, SiC–MoSe₂ and SiC–WSe₂ heterostructures. A difference in the ionization potential of the corresponding monolayers and interlayer charge transfer further confirmed the type-II band alignment in these heterostructures. Furthermore, the optical behaviour is investigated by calculation of the absorption spectra in terms of ε₂(ω) of the heterostructures and the corresponding monolayers. The photocatalytic response shows that the SiC–Mo(W)S₂ heterostructures can oxidize H₂O to O₂. An enhanced photocatalytic performance with respect to the parent monolayers makes the SiC–Mo(W)Se₂ heterostructures promising candidates for water splitting.

Manipulating spin textures through electric fields is a timely issue due to the potential for applications in low power electronics. Recently, a novel class of ferroelectric materials combining ferroelectric order with Rashba type switching of spin textures at room temperatures [1-3] opened an intriguing route for a non-volatile static electrical control of the spin degrees of freedom. In this respect, GeTe(111) is a model system which appears to possess an extra bonus: fully spin-polarized bulk states. We extended the experimental verification of the ferroelectric switching of these fully spin-polarized states in spin and momentum-resolved ways [3] with operando x-ray photoelectron-diffraction experiments [4]. We found that the ferroelectric response feature systematic displacive response with characteristic fatigue effects consistent with operando spin-resolved measurements [3], proving that static electric fields indeed couple to GeTe domain polarizations and thus to Rashba-type splitting.

[1] D. Di Sante et al., Electric Control of the Giant Rashba Effect in Bulk GeTe, Adv. Mater. (2012).

[2] J. Krempaský et al., Disentangling bulk and surface Rashba effects in ferroelectric α-GeTe, PRB 94, 205111 (2016).

[3] J. Krempaský et al., Operando imaging of all-electric spin texture manipulation in ferroelectric and multiferroic Rashba semiconductors, PRX 8, 021067 (2018).

[4] J. Krempaský et al., in preparation

The choice of the transition metal atom in an organometallic complex decisively affects its physiochemical properties such as its color or magnetic moment. Towards rationalization of the impact of the choice of transition metal atom on the electronic and spintronic properties of organometallic wires, we study the electric transport through Fe, Co and Ni containing 1D coordination polymers contacted by the tip of a scanning probe microscope (SPM). The coordination polymers are synthesized in-situ by co-deposition of the metal atoms and the quinonediimine (2,5-diamino-1,4-benzoquinone-diimine) ligand onto Au(111). Scanning tunneling microscopy combined with atomic force microscopy is used for characterization as well as lifting and transport experiments. All three coordination polymers are found to be structurally equivalent, with a coordination geometry consistent with the expected formal M(II) oxidation states. This implies dehydrogenation of the quinonediimine ligand. Wires with lengths over 100 nm can be obtained. Transport measurements are performed by contacting one end of the surface-adsorbed polymer with the SPM tip. The combination of STM and AFM allows for simultaneous measurement of the current, conductance and force gradient as a function of bias voltage and lifting height. The transition metal element contained in the organometallic wire is found to have a profound impact on its electronic properties: lifting of Fe and Ni wires leads to the opening of a band gap at heights of a few nanometers. In contrast, Co containing polymers do not exhibit no gap. In addition, conductance measurements reveal pronounced steps at bias voltages of a few millivolts for Fe and Co wires but not for Ni ones. These conductance steps are tentatively attributed to spin excitations.

The analysis of the magnetic coupling and the magnetic anisotropy of localized 4f magnetic moments connected via organic ligands in molecular networks is a highly topical field of research owing to potential applications in organic spintronics. Our on-surface synthesis method allows us to synthesize sandwich-molecular nanowires that consist of alternating ring-like cyclooctatetraene (Cot) molecules and varying rare-earth ions. Using thulium hereby offers not only various phases but may also exhibit interesting properties due to its strong single-ion anisotropy compared to e. g. europium. The electronic and magnetic properties of these compounds were studied by X-ray absorption spectroscopy and X-ray magnetic circular dichroism accompanied by a theoretical approach based on multiplet calculations. The X-ray absorption spectroscopy of the systems indicate a 4f¹² electronic state of the thulium. In contrast to Eu-cyclooctatetraene nanowires, the XMCD of the Tm systems suggest, that they are not saturated even in high fields and low temperatures. We acknowledge financial support by DFG through the project WE 2623/17-1.

Based on the first-principles calculations, we investigate the structural, electronic, and magnetic properties of defects in monolayer SnS. We study the formation and migration of vacancies at both Sn- and S-sites. In comparison to the S-site vacancy, our calculations show that creating a vacancy at the Sn-site requires lesser energy, indicating that the vacancy at the Sn-site is more likely to be formed in experiments with energetic particle irradiation. For the Sn-rich (S-rich) environment, the vacancy at the S-site (Sn-site) is more likely to be found than the vacancy at the Sn-site (S-site). Reducing the formation of vacancy clusters, the S vacancy remains at the position where it is created because of the high vacancy migration barrier. Both types of vacancies remain nonmagnetic. To induce magnetism in monolayer SnS, we also study the transition metal (TM=Mn, Fe, and Co) doping at the Sn-site and find a significant influence on the electronic and magnetic properties of monolayer SnS. The doping of TM changes non-magnetic monolayer SnS to magnetic one and keeps it semiconducting. Additionally, long-range ferromagnetic behavior is observed for all the doped system. Hence, doping TM atoms in monolayer SnS could be promising to realize a two-dimensional diluted magnetic semiconductor. More interestingly, all the doped TM configurations show a high spin state, which can be used in nanoscale spintronic applications such as spin-filtering devices.

The development of on-demand individual deep impurities in silicon is motivated by their employment as a physical substrate for qubits [1], nanoscale transistors [2], single photon emitters [3] and Hubbard simulators [4]. Single-atom silicon devices based on conventional doping elements such as phosphorous [5-7] can operate only at cryogenic temperature due to shallow weakly localized ground state impurity levels.

Differently, the implantation of Ge dopants in silicon and the subsequent annealing generate stable Ge-vacancy defects [8] that are promising candidates to achieve single-atom quantum effects at room temperature. These hybrid complexes combine indeed the properties of the silicon vacancy, which carries deep states in the band-gap, with the accurate spatial controllability of the defect obtainable through state of the art single-ion implantation of Ge atom.

We characterize GeV_n complexes in silicon by means of accurate ab initio density functional theory calculations with a hybrid functional. We get insight on the defect local arrangement and on their electronic properties, demonstrating their suitability for the purposed application. We show that due to the deep defect levels the electrons bound to the defects are strongly correlated and more localized than in conventional dopants [9]. Through a multi-scale theoretical approach that combines ab initio DFT and an extended Hubbard model we analyze also the electronic transport through a chain of GeV dopants. By mapping the ab initio results into the model Hamiltonian we are able to describe the electronic conductance of the array as a function of the temperature and to determine the activation energy for hopping processes.

We compare the theoretical results with the experimental data showing a notable agreement between the calculated and measured excited state energies and temperature activated transport properties [10].

This work has been performed in the context of a user access request within the Nanoscale Facilities and Fine Analysis project [11], funded by European Community.

[1] J. J. Pla et al., Nat. 489, 541 (2012).

[2] T. Shinada et al., Silicon Nanoelectronics Workshop 1–2 (IEEE, 2014).

[3] I. Aharonovich et al., Nature Photonics 10, 631 (2016).

[4] L. Fratino, Phys. Rev. B 95, 235109 (2017).

[5] E. Prati, et al., Nature Nanotech. 7, 443 (2012).

[6] E. Prati, et al., Sci. Rep. 6, 19704 (2016).

[7] M. Khalafalla, et al., Appl. Phys. Lett. 91, 263513 (2007).

[8] Y. Suprun-Belevich and L. Palmetshofer, Nucl. Instr. Methods Phys. Res. B 96, 245–248 (1995).

[9] S. Achilli et al., Sci. Rep. 8, 18054 (2018).

[10] S. Achilli et al. (unpublished, in preparation)

[11] https://nffa.eu

Magnetism of diluted low-dimensional systems and nanostructures is often studied by means of x-ray magnetic circular dichroism (XMCD). The analysis of XMCD experiments is greatly aided by the XMCD sum rules which, in principle, give access to the spin and orbital magnetic moments of the chemically specific photoabsorbing atom. Unfortunately, the XMCD spin sum rule allows for the spin magnetic moment m_spin to be determined only in combination with the magnetic dipole term T_z. This T_z term is not just a minor factor that affects the sum rules analysis; on the contrary, it may change the overall picture completely. We present few examples demonstrating that neglecting the T_z term could in some cases lead to completely wrong conclusions about the trends of m_spin with the size of the system or with the direction of the magnetization.

Earlier it was shown that the magnetic dipole term T_z can be eliminated from the sum rule analysis altogether by employing a special experimental setup. We demonstrate that for supported nanostructures this can be done only if certain conditions concerning the strength of the spin-orbit coupling and the hybridization between the states of the nanostructure and of the substrate are satisfied.

By means of density functional theory calculations, we explain the anomalous thermally induced peaks in the XPS spectra of the core electrons previously reported for a metalloporphyrin molecule adsorbed on TiO₂ surface [JCP 146, 184704 (2017)]. The shift of such levels is routinely interpreted as a signature of charge exchange with the substrate. However, in this case we show that some atoms with no chemical bonding to the surface also experience significant core level shifts. Our interpretation based by simulations is that the electronic structure of this dye synthesizer-semiconductor complex is partly originated from its core geometry deformation. In particular, the next neighbors are also affected. One should therefore use the valuable photoelectron spectroscopy outputs with caring such effects. Our theoretical prediction is in fair agreement with experiment, based on which we are able to disclose the mechanism of the splitting the degenerate core states in terms of the symmetry breaking rather than the dye-substrate chemical bonding. This, additionally, shows that how one can facilitate the high sensitivity of photoelectron spectroscopy to help identification the adsorption site and orientation of molecules on atomic surfaces.

We present a computational study of L-edge resonant inelastic x-ray scattering (RIXS) in 3d high-valence transition-metal (TM) oxides: RNiO₃ (R: rare-earth element); LaCuO₃ and NaCuO₂ [1]. We employ a theoretical approach based on local density approximation (LDA) and dynamical mean-field theory (DMFT), where the Anderson impurity model with DMFT hybridization function is extended to include TM 2p core orbitals. The heart of the present approach is the configuration-interaction-based exact-diagonalization impurity solver that allows to include the low-energy electron-hole pairs accurately, necessary to describe RIXS in the high-valence materials with negative charge-transfer energy. The LDA+DMFT approach enables to describe both bound and unbound electron-hole pair excitations in the RIXS spectra. In this talk, we discuss the behavior of the fluorescence-like (FL) feature and how it is connected to the low-energy valence states and crystal structure. Special attention will be paid to the selection rules for the visibility of the FL RIXS feature, that reflect the lattice geometry around the x-ray excited TM site. Our results reproduce the recent experimental data for RNiO₃ [2].

[1] A. Hariki, M. Winder, and J. Kuneš, Phys. Rev. Lett. 121, 126403 (2018).

[2] V. Bisogni, S. Catalano, R. J. Green, M. Gibert, R. Scherwitzl, Y. Huang, V. N. Strocov, P. Zubko, S. Balandeh, J.-M. Triscone, G. Sawatzky, and T. Schmitt, Nat. Commun. 7, 13017 (2016).

Scattering from the impurities and the resulting Friedel oscillations (FO) in the Fermi liquid phase, Mott insulating phase, and at the Mott transition is studied in finite lattice systems using the one-band Hubbard model, both at zero and finite temperatures in the presence of a single impurity potential [1,2,3]. The problem is also extended for the cases of multiple impurities. These impurities model the effects of defects and adsorbed atoms on the surfaces of real systems with correlated electrons. Electronic correlations are accounted for by solving the real-space dynamical mean-field theory (R-DMFT) equations, and also using other reliable yet computationally cheap self-energy approximations based on the R-DMFT. It is seen that the FO is damped with the interaction and the Fermi liquid renormalization factor is primarily responsible for this damping. FO almost disappears at the Mott transition and completely beyond it in the Mott insulating phase. The screening charge decreases with the interactions and approaches zero towards the Mott transition. It is left as an open question if any signatures of FO can be observed in real Mott systems, e.g. transition metal oxides which are the potential functional materials for the Mott transistors [4,5]. Moreover, it is interesting to probe if these quantum oscillations would affect the different transport properties, e.g. transport current, resistivity etc. in such systems.

[1] B. Chatterjee, J. Skolimowski, K. Makuch, K. Byczuk, arXiv:1807.08566 (under review)

[2] B. Chatterjee, K. Byczuk, JPCS 592, 012059 (2015).

[3] K. Byczuk, B. Chatterjee, D. Vollhardt, Eur. Phys. J. B 92, 23 (2019).

[4] P. Maier, F. Hartmann et. al, App. Phys. Lett. 110, 093506 (2017).

[5] P. Scheiderer, M. Schmitt et.al, Adv. Mat. 30, 25 (2018).

Over the last decade molecular spins have been object of intense research as potential candidates for quantum technologies [Ghirri et al., Magnetochem. 3, 12 (2017)]. The interest is given by the possibility of tailoring their magnetic properties at synthetic level as well as their relatively easy integration on surfaces. Another key aspect is the presence of both electronic and nuclear spin degrees of freedom which, in principle, might open wide possibilities for the implementation of protocols for quantum information processing. Here, two of the main challenges are addressing the transitions with external microwave and radiofrequency stimuli and developing suitable protocols for the manipulation of the spins.

In this work, we deal with the microwave excitation of molecular spin ensembles at low temperatures. To achieve this goal, we have first developed a set of planar superconducting resonators at microwave frequency to couple to the ensembles via magnetic dipolar interaction in an electron spin resonance-like transmission spectroscopy [Bonizzoni et al., Adv. in Phys. X 3, 1435305 (2018)]. The coupling conditions of several molecular ensembles are first studied under the continuous-wave excitation regime. Remarkably, experimental evidence of coherent spin-photon coupling between the ensembles and the resonators at low temperatures is found within two different approaches. In the first one the number of coupled spins of a diluted vanadyl phthalocyanine is maximized by using an optimized resonator [Bonizzoni et al., Sci. Rep. 7, 13096 (2017)], while in the second one the exchange narrowing effect of the spin linewidth in concentrated organic radicals is exploited [Ghirri et al., Phys. Rev. A 93, 063855 (2016)]. We then focus to experiments in the pulsed-wave regime. Here an arbitrary waveform generator is used for the synthesis of the microwave pulses which are then injected into the resonant geometry to drive the evolution of the spins. Our preliminary results concerning the measure of the phase memory time on an ensemble of vanadyl phthalocyanines are then given.

Molybdenum dichalcogenides are probably the most studied single layer TMDCs by virtue of being appealing for sundry possible applications suchlike transistors, diodes, solar cells or more fundamental studies of spin or valley pseudospin and their interactions. Tungsten-based counterparts are on the other hand evincing much stronger spin-orbit coupling due to which all the spin-related effects are more stable at room temperature and thus more feasible for application. WTe₂, the type-II Weyl semimetal is in particular interesting due to having two pairs of spin-differentiated Weyl points above Fermi energy. We have conducted several experiments following the evolution of the band dispersion in the vicinity of X and Y points of the Brillouin zone of WTe₂ which is substantial for understanding the fundamental properties of the structure-property relation of the system. Ab-initio set of photoemission calculations was performed using SPR-KKR package and compared to photoemission experimental results. In addition, TR-SARPES studies were carried out by the group on such materials to determine the ultrafast dynamics of the carrier density and to disentangle the subsequent relaxation processes using pump-probe experiments.

Chemisorption of a single atomic layer of oxygen on the Fe(001) surface yields a highly ordered and reproducible benchmark substrate [1] for theoretical and experimental studies, and for the epitaxial growth of metal oxides, including atom-thick Cr_xO_y layers, and hybrid interfaces with foreseen applications e.g. in organic spintronics.

This talk initially presents ab initio investigations that have supplemented microscopy and spectroscopy experiments of the electronic and magnetic properties of two-dimensional Chromium oxides of Cr₃O₄ and Cr₄O₅ stoichiometry grown on Fe(001), featuring antiferromagnetic magnetic configurations with underlying Fe(001) [2,3]. Despite Cr / CrO systems are notoriously difficult for mean field approaches, generalized-gradient results are found to explain most experimental findings, with a rigid shift of oxygen bands accounting for electronic correlation effects.

We eventually consider the effect of inserted Cr₄O₅ layers at the interface between the prototypical C₆₀ organic semiconductor and Fe(001), which is shown to enhance the magnetic hybridization between the molecule and the surface through x-ray magnetic circular dichroism (XMCD) [4,5]. By means of ab initio calculation we characterize the local interface morphology, the magnetic configuration of the surface and the induced spin dependent electronic properties of the molecule, the latter reflecting the magnetic electronic properties of the surface at the relevant energy range. As seen from the substrate, adsorbates can influence the magnitude and even orientation of surface Cr magnetic moments. The interest in this interface is then twofold: on one side the thin magnetic oxide allows tailoring the magnetic properties of the organic layer, on the other side the adsorption of C₆₀ can be envisioned as a tool to control the magnetic ordering of Cr atoms at the interface.

[1] A. Picone, M. Riva, A. Brambilla, A. Calloni, G. Bussetti, M. Finazzi, F. Ciccacci, L. Duò, Surface Science Reports 71, 32 (2016).

[2] A. Picone, G. Fratesi, M. Riva, G. Bussetti, A. Calloni, A. Brambilla, M. I. Trioni, L. Duò, F. Ciccacci, and M. Finazzi, Phys. Rev. B 87, 085403 (2013).

[3] A. Calloni, G. Fratesi, S. Achilli, G. Berti, G. Bussetti, A. Picone, A. Brambilla, P. Folegati, F. Ciccacci, and L. Duò, Phys. Rev. B 96, 085427 (2017).

[4] A. Brambilla, A. Picone, D. Giannotti, A. Calloni, G. Berti, G. Bussetti, S. Achilli, G. Fratesi, M. I. Trioni, G. Vinai, P. Torelli, G. Panaccione, L. Duò, M. Finazzi, and F. Ciccacci, Nano Lett. 17, 7440 (2017).

[5] A. Brambilla, A. Picone, S. Achilli, G. Fratesi, A. Lodesani, A. Calloni, G. Bussetti, M. Zani, M. Finazzi, L. Duò, and F. Ciccacci, Journal of Applied Physics 125, 142907 (2019).

Large magnetic anisotropy (MA) of the material is a keystone to the high-density magnetic storage. It defines the easy magnetization direction and ensures information storage by preventing the magnetization from switching. In bilayers, MA can be controlled via pinning of a ferromagnetic (F) layer by an antiferromagnet (AF) in the exchange bias (EB) [1] systems [2]. In this work we aim at evaluating the anisotropy constant in the antiferromagnetic uranium dioxide by making use of exchange bias effect study in the UO₂/Fe₃O₄ bilayers deposited on various substrates. The shift of the magnetic hysteresis curve along the field direction of the magnetic bilayer field-cooled below the Néel temperature (T_N) of the antiferromagnet is related to its anisotropy constant K_AF [3,4] via the critical thickness of the AF layer, above which the antiferromagnet is strong enough to pin the magnetization of a ferromagnet. Using reactive sputtering from metallic U and Fe targets, we prepared sets of the UO₂/Fe₃O₄ samples with the varied thickness of UO₂ (50-300 Å) and with a constant thickness of magnetite (~270±20 Å). The stoichiometry of each deposited layer was controlled by x-ray photoelectron spectroscopy (XPS). The samples were further characterized by Rutherford back-scattering spectrometry (RBS) and x-ray diffraction (XRD). Magnetization study revealed a large EB in the samples [5]. Using the approach of Ref. 4, we find the anisotropy constants of UO₂ in the samples ~0.1-0.3 MJ/m³. We further compare the experimental results with those obtained theoretically using a first-principles density-functional+dynamical-mean-field-theory (DFT+DMFT) method [6] in conjunction with a quasi-atomic (Hubbard-I) treatment of U 5f states. We predict a strong easy-axis anisotropy to originate from the substrate-induced in-plain tetragonal compression of the UO₂ layer.

The samples were prepared in the framework of the TALISMAN project of the European Commission Joint Research Centre, ITU Karlsruhe. RBS measurements have been carried out at the CANAM (Centre of Accelerators and Nuclear Analytical Methods) infrastructure LM 2015056 and supported by OP RDE, MEYS, Czech Republic under the project CANAM OP, CZ.02.1.01/0.0/0.0/16_013/0001812 and by the Czech Science Foundation (GACR 18-03346S and 18-02344S). Part of the work was supported by “Nano-materials Centre for Advanced Applications,” Project CZ.02.1.01/0.0/0.0/15_003/0000485, financed by ERDF.

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[2] S. van Dijken et al., J. Appl. Phys. 97 063907 (2005).

[3] D. Mauri et al., J. Appl. Phys. 57, 3047 (1987).

[4] Ch. Binek et al., J. Magn. Magn. Mater. 234, 353 (2001).

[5] E. A. Tereshina et al., Appl. Phys. Lett. 105 122405 (2014).

[6] M. Aichhorn et al., Comp. Phys. Comm. 204, 200 (2016).

The ordered adsorbate layer Sn/Si(111) (√3 x √3) with coverage of one third of a monolayer is considered as a realization of strong electronic correlation in surface physics. Our theoretical analysis shows that electron-hole pair excitations in this system can be long-lived, up to several hundred nanoseconds, since the decay into surface phonons is found to be a highly non-linear process.

We combine first-principles calculations with help of a hybrid functional (HSE06) with modeling by a Mott-Hubbard Hamiltonian coupled to phononic degrees of freedom. The calculations show that the Sn/Si(111) (√3 x √3) surface is insulating and the two Sn-derived bands inside the substrate band gap can be described as the lower and upper Hubbard band in a Mott-Hubbard model with U=0.75eV. Furthermore, phonon spectra are calculated with particular emphasis on the Sn-related surface phonon modes. The calculations demonstrate that the adequate treatment of electronic correlations leads to a stiffening of the wagging mode of neighboring Sn atoms; thus, we predict that the onset of electronic correlations at low temperature should be observable in the phonon spectrum, too. The deformation potential for electron-phonon coupling is calculated for selected vibrational modes and the decay rate of an electron-hole excitation into multiple phonons is estimated, substantiating the very long lifetime of these excitations.

Using the first-principle methods and deformation theory, we detail investigate the two-dimensional semiconductor materials with monolayer compounds which suggest a novel playground to implement nanoscale mechanical, thermoelectric and electronics devices to improved their functionality. Using the combination of approaches to compute the electronic and phonon structures with Green’s function-based transport techniques, we report the thermometric performance of the group-III ternary monochalcogenides compounds. Our outcomes show strong mechanical properties and high carrier mobility ( ~ 2.18x10⁵ cm²V^-1s^-1). Our materials also show the high figure of merit at room temperatures ZT ~1.12. Thus, indicating at the high potential of these new materials in thermoelectric application.

[1] Bell, L. E. Cooling, Heating, Generating Power, and Recovering Waste Heat with Thermoelectric Systems. Science 2008, 321 (5895), 1457–1461.

[2] Zebarjadi, M.; Esfarjani, K.; Dresselhaus, M. S.; Ren, Z. F.; Chen, G. Perspectives on Thermoelectrics: From Fundamentals to Device Applications. Energy Environ. Sci. 2012, 5 (1), 5147–5162.

[3] Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl. Acad. Sci. 2005, 102 (30), 10451–10453.

[4] Demirci, S.; Avazlı, N.; Durgun, E.; Cahangirov, S. Structural and Electronic Properties of Monolayer Group III Monochalcogenides. Phys. Rev. B 2017, 95 (11), 115409.

[5] Vineis, C. J.; Shakouri, A.; Majumdar, A.; Kanatzidis, M. G. Nanostructured Thermoelectrics: Big Efficiency Gains from Small Features. Adv. Mater. 2010, 22 (36), 3970–3980.

The goal of my master's thesis is to implement the LDA+Hubbard-I method in the framework of the FLAPW code FLEUR, which is developed at the IAS-1 Institute at the FZ Jülich. For this purpose, it is also necessary to implement functionality for calculating green's functions in the FLAPW basis. In the following, I want to apply this method to systems like Gd on Cu.

FLEUR is a full potential linearized augmented plane wave code which is applicable to a wide range of structures and elements. The current development of the code is aimed at high-throughput computations [1].

The LDA+Hubbard-I method works in the atomic limit of DMFT and is specifically developed for 4f systems, where we can neglect the hybridization effects of the 4f orbitals [2]. The advantage we gain from this method is that it is computationally relatively cheap when compared to other DMFT methods while including dynamical effects from the correlated electrons. The latter are ignored in the LDA+U method.

[1] www.flapw.de

[2] Phys. Rev. B 80, 085106 (2009).

Strontium titanate (SrTiO₃) has been a subject of intensive discussion in recent years both experimentally as well as theoretically. Strontium titanate is an insulator with a large band gap (Eg=3.2 eV), but it can become conductive by doping with transition metals or oxygen vacancies [1]. Here we report the fabrication and electronic properties of Ni doped SrTiO₃ polycrystalline and crystalline films. The polycrystalline films were prepared by reactive magnetron co-sputtering with different Ni concentration. Whereas the crystalline films were grown by pulsed laser deposition (PLD). In this experimental investigation, we used high energy ARPES and core level spectroscopy to perform the electronic properties of the films as a function of doping concentration. Our experimental results were supported by ab-initio calculations using SPR-KKR's one-step model of photoemission [2,3].

[1] F. Alarab et al., AIP Conference Proceedings 1996, 020001 (2018).

[2] J. Braun, Rep. Prog. Phys. 59, 1267-1338 (1996).

[3] H. Ebert, D. Kodderitzsch and J. Minar, Rep. Prog. Phys. 74, 096501 (2011).